PAPERmaking! Vol.7 No.2 2021

PAPERmaking!

The e-magazine for the Fibrous Forest Products Sector

Produced by:

The Paper Industry Technical Association

Volume 7 / Number 2 / 2021

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 2, 2021

CONTENTS: FEATURE ARTICLES: 1. Strength: Polymeric strength aids for foam-formed paper.

2. Green Chemistry: Pulping using carbonate hydrolysis then mild kraft process.

3. Mill Waste: Utilisation for high value-added biomaterials.

4. Papermaking: PCC-CTMP composites and their effects on sheet properties.

5. Fibre Properties: Predicting tensile strength of cellulose nanopaper.

6. Wood Panel: Assessing ignition behaviour of OSB. 7. Corrugated: Estimating compressive strength of Corrugated Boxes.

8. Paper Drying: Estimating evaporation heat in the paper drying process.

9. Polymers: Dry and wet tensile strength of paper sheets.

10. Fire Safety: Enforcing fire safety policy with warning signs.

11. Leadership: Approaching today’s new hybrid workforce.

12. Remote Working: 10 Top Tips for a healthy remote working office system.

SUPPLIERS NEWS SECTION: News / Products / Services: Section 1 – PITA Corporate Members: ABB / PILZ / VALMET Section 2 – PITA Non-Corporate Members VOITH Section 3 – Other Suppliers Mycon / SKF / Toscotec DATA COMPILATION: Installations: Overview of equipment orders and installations since May 2021 Research Articles: Recent peer-reviewed articles from the technical paper press Technical Abstracts: Recent peer-reviewed articles from the general scientific press The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International and the PITA Annual Review, both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating.

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Contents

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 2, 2021

On the strength improvement of lightweight fibre networks by polymers, fibrils and fines Tiina Pöhler, Jukka A. Ketoja, Timo Lappalainen, Veli-Matti Luukkainen, Ilkka Nurminen, Panu Lahtinen & Katariina Torvinen Novel lightweight cellulose fibre materials containing various strength enhancing polymeric and fibrillar components were formed with the help of foam technology. Increasing inter-fibre bond strength and local material density was attempted with unique lignin-containing wood fines (V-fines), cellulose microfibrils (CMF), TEMPO-oxidized cellulose nanofibrils (TCNF), and macromolecules such as cationic starch, polyvinyl alcohol (PVA), and locust bean gum (LBG). The investigated fibres included both long hemp bast fibres and northern bleached softwood Kraft pulp. In the low-density range of 38–52 kg/m3, the compression stress and modulus were highly sensitive to inter-fibre bond properties, the multi-scale features of the fibre network, and the foaming agent employed. Still, the compression-stress behaviour in most cases approached the same theoretical curve, derived earlier by using a mean-field theory to describe the deformation behaviour. At 10% addition level of fine components, the specific compression stress and compression modulus increased in the order of V-fines\CMF\TCNF. A tremendous increase in the compression modulus was obtained with LBG, leading to a material surface that was very hard. In general, the foams made with PVA, which acts both as foaming agent and reinforcing macromolecule, led to better strength than what was obtained with a typical anionic sodium dodecyl sulphate surfactant. Strength could be also improved by refining the softwood pulp.

Contact information: Solutions for Natural Resources and Environment, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, 02044 Espoo, Finland Cellulose (2020) 27:6961–6976 https://doi.org/10.1007/s10570-020-03263-x Creative Commons Attribution 4.0 International License

The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International and the PITA Annual Review, both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating.

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Article 1 – Foam Forming & Strength Improvement

Cellulose (2020) 27:6961–6976 https://doi.org/10.1007/s10570-020-03263-x

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ORIGINAL RESEARCH

On the strength improvement of lightweight fibre networks by polymers, fibrils and fines Tiina Pöhler . Jukka A. Ketoja . Timo Lappalainen . Veli-Matti Luukkainen .

Ilkka Nurminen . Panu Lahtinen . Katariina Torvinen

Received: 28 June 2019 / Accepted: 21 May 2020 / Published online: 28 May 2020 Ó The Author(s) 2020

Abstract Novel lightweight cellulose fibre materials containing various strength enhancing polymeric and fibrillar components were formed with the help of foam technology. Increasing inter-fibre bond strength and local material density was attempted with unique lignin-containing wood fines (V-fines), cellulose microfibrils (CMF), TEMPO-oxidized cellulose nanofibrils (TCNF), and macromolecules such as cationic starch, polyvinyl alcohol (PVA), and locust bean gum (LBG). The investigated fibres included both long hemp bast fibres and northern bleached softwood Kraft pulp. In the low-density range of 38–52 kg/m3, the compression stress and modulus were highly sensitive to inter-fibre bond properties, the multi-scale features of the fibre network, and the foaming agent employed. Still, the compression-stress

behaviour in most cases approached the same theoretical curve, derived earlier by using a mean-field theory to describe the deformation behaviour. At 10% addition level of fine components, the specific compression stress and compression modulus increased in the order of V-fines \ CMF \ TCNF. A tremendous increase in the compression modulus was obtained with LBG, leading to a material surface that was very hard. In general, the foams made with PVA, which acts both as foaming agent and reinforcing macromolecule, led to better strength than what was obtained with a typical anionic sodium dodecyl sulphate surfactant.

Strength could be also improved by refining the softwood pulp.

Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10570-020-03263-x) contains supplementary material, which is available to authorized users.

T. Pöhler (&) J. A. Ketoja T. Lappalainen V.-M. Luukkainen I. Nurminen P. Lahtinen K. Torvinen Solutions for Natural Resources and Environment, VTT Technical Research Centre of Finland Ltd, P.O. Box 1000, 02044 Espoo, Finland e-mail: tiina.pohler@vtt.fi

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Graphic abstract

Keywords Foam forming Cellulose nanofibril Wood fines Fibre Softwood Hemp Polyvinyl alcohol Locust bean gum Compression test Stress Strain Model

Introduction New low-density cellulose-based fibre materials made with the foam forming technique have recently been introduced as potential solutions to replace oil-based fibrous or foamed materials (Poranen et al. 2013; Madani et al. 2014). Feasible applications can be found in air filtration (Jahangiri et al. 2014), thermal insulation (Jahangiri et al. 2014; Pöhler et al. 2017), in acoustic control as sound absorption material (Jahangiri et al. 2016; Pöhler et al. 2016), and as cushioning material in packaging (Luo et al. 2017; Paunonen et al. 2018). These new lightweight materials are made by mixing fibres with aqueous wet foam generated with the help of a foaming agent (AlQararah et al. 2015a). The bubbles in the foam support fibres until drying so that a highly porous, deformable fibre network is formed. Rather little is known about the structure-strength relationship of these ‘‘fibre foams’’, despite several recent experimental studies (Alimadadi and Uesaka 2016; Burke et al. 2019; Ketoja et al. 2019) and model simulations using

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idealized fibre systems (Alimadadi et al. 2018; Hossain et al. 2019). The material characterization is challenging as strength can depend on the structural features in several size scales. These include the overall material network geometry, fibre properties (size distribution and flexibility) and the bonding ability of the fibres (total bond area and bond strength) (Alava and Niskanen 2006). Moreover, strength is a physical property that is not necessarily determined by the average structure but rather by structural heterogeneity affecting both stress concentration and local load-carrying capacity. For that reason, the deformation behaviour can be sensitive to the size distribution of fibres and fine components. With optimal combination and distribution of raw materials, it is possible to increase compression strength through local loadbearing regions without deteriorating large-scale material homogeneity. Current paper investigates this question applying a selected set of raw materials that have an exceptionally wide variety of characteristic sizes.

There is vast literature (Alava and Niskanen 2006; Picu 2011) on the strength properties of fibre networks in the medium density range of 300–1000 kg/m3, where the number of inter-fibre joints per fibre is large.

The number of joints is proportional to the square of total fibre length in unit volume (Komori and Makishima 1977). Thus, with decreasing density, the number of joints per fibre and the relative bonded area

Cellulose (2020) 27:6961–6976

are greatly reduced. In such a situation, network deformations become non-affine (Picu 2011), so that local strains can differ greatly from the average behaviour. Moreover, the role of structural variations increases with reduced material density. With a low relative bonded area of a fibre network, heterogeneity of fibres becomes essential as well. The segment lengths between inter-fibre joints become longer [mean segment length can be 5–40 times greater than the fibre diameter (Ketoja et al. 2019)], and weak points along a fibre are not necessarily supported by other neighbouring fibres. Such features become increasingly important for compressive loads, for which a local network failure can simply mean that the associated fibre segment loses partly its ability to carry stress. This may happen not only by fibre bending, as shown in many previous models (Subramanian and Picu 2011; Bergström et al. 2019), but also by sudden buckling or displacement of an individual fibre under compressive stress (Ketoja et al. 2019).

This was noticed experimentally by Ketoja et al.

(2019), who found abrupt displacements in fibre networks using an image cross-correlation technique.

The possibility of a localizing failure in a heterogeneous fibre has been omitted in previous simulation models (Alimadadi et al. 2018; Hossain et al. 2019; Bergström et al. 2019), which are based on uniform fibre segments without inherent structural unevenness inside a single segment between two joints. Ketoja et al. (2019) introduced a mean-field theory where the buckling of fibre segments was postulated to be an important deformation mechanism describing mean stress build-up during network compression. The theory explained the stress increase for intermediate compressive strains in varied well-bonded fibre materials in the density range of 20–100 kg/m3. Mäkinen et al. (2020) provided further evidence for the theory by showing that acoustic energy emission could be directly correlated with the stress-compression behaviour. Their cyclic measurements revealed another source of acoustic events than fibre bending. However, the theory was not able to explain the behaviour at very large strains because of collective softening of the material (Picu and Subramanian 2011). Moreover, the theory overestimated the stress when the strain was below 10% (Ketoja et al. 2019; Mäkinen et al. 2020) and when large amounts of cellulose microfibrils (CMF) were added to a fibre network (Ketoja et al.

2019). Added CMF seemed to increase strength at

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intermediate compression levels by forming cellulose micro-scale sheets within the fibre network (Bossu et al. 2019).

In the current study, we have taken the above considerations into account when improving the strength of lightweight natural fibre materials. The bond strength and total inter-fibre contact area (Sormunen et al. 2019) can be increased with polymeric, fibrillar and fines materials. The strengthening mechanisms are twofold: Firstly, fine components can build loose material bridges between fibres, which, during drying with the help of surface tension forces, drag fibres in contact with one another thus increasing the number of bonds and reducing the mean fibre segment length. Usually, this is associated with a higher density of the whole material as well. Secondly, polymeric and fibrillar components can strongly affect the strength of individual bonds by (a) enhancing the mechanical entanglement of the inter-fibre fibrils, (b) creating a larger microscopic contact area where short-range interactions become effective, or (c) by changing these chemical forces (Schmied et al. 2013). Overall material strength is further improved if the formed network is able to distribute local stresses evenly over the fibre network. For this reason, we explored multi-scale network structures by combining wood and hemp fibres with varying dimensions and flexibilities. The studied fibre and fine components covered the broad length (width) range from * 40 lm (* 3 nm) up to * 15 mm (* 50 lm). In addition to strength measurements, the micro-scale homogeneity of the structures was followed as well.

Materials and methods Fibres and fine components Hemp bast fibres were selected as the long fibre component in the furnish. The fibre length of a single hemp bast fibre can vary from 1 to 34 mm (average 8–14 mm) with a diameter between 17 and 23 lm (Shahzad 2012). The typical value of the longitudinal Young’s modulus of a hemp fibre is 30–60 GPa (Shahzad 2012). Finnish hemp bast fibres (Cannabis sativa, delivered by HempRefine Oy) with the chemical composition of 81% cellulose, 11% hemicellulose, and 5% lignin were treated with a Kvarn BAHS30 hammer mill (Kamas Industri AB). Fibres were

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inserted into the mill and let to rotate until they fell through a 15 mm slot screen. The milling process enhanced the separation of fibres from fibre bundles, see Fig. 1. The treated fibres were further processed to increase their bonding ability with a soda cooking process. The cooking time was 90 min at 165 °C. The total alkali (NaOH) was 4 mol/kg. The liquid to solid ratio was 7.5, the yield was 70.5%, and the kappa number was 6.0.

The medium-size fibre component was Northern Bleached Softwood Kraft (NBSK) pulp (Metsä-Fibre Oyj, Äänekoski, Finland) with an average lengthweighted fibre length of 2.1 mm and fibre width 28 lm. Both unrefined (ur) and refined (r) pulps were used. The refining was done with a Voith Sulzer laboratory refiner at KCL Oy, Finland, using conical discs. The pulp consistency in the refining was 4%, and the specific edge load was 2.5 J/m. The refining was stopped at the specific energy consumption of 100 kWh/t. The SR number of the refined pulp was 18.7.

The chemical composition of NBSK pulp has been reported to be 85% cellulose, 14% hemicellulose, and 1% lignin (Lahtinen et al. 2014a) and Young’s modulus of NBSK fibre * 40 GPa by Neagu et al.

(2006).

Three different wood- and Kraft-cellulose-originated fine components were employed in the furnishes: V-fines, cellulose microfibrils (CMF), and TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl) oxidized cellulose nanofibrils (TCNF). Lignocellulosic V-fines were produced directly from fresh

Cellulose (2020) 27:6961–6976

softwood (Picea Abies) using a laboratory grinder equipped with a unique grinding stone designed for fines production in atmospheric conditions (Nurminen et al. 2018). A grinding stone with 70 mesh grits (Norton A701-L12VGP) and a serrated V-shape surface was employed. The angle between the apparent fibre alignment and the serrated surface was 15 degrees. The wood feed was 0.9 mm/s and stone velocity 20 m/s. The estimated energy consumption was 2 MWh/t. The fibrillated material was screened through a 14-mesh wire. The population average fibre length was 0.18 mm, and over 70% of the measured particles had a length below 0.2 mm.

The two different fibrillated cellulose grades were produced from a never-dried NBSK. The production of the coarser CMF was started by soaking the pulp at 2% consistency and dispersing it with a high-shear Ystral X50/10 Dispermix mixer for 30 min at 2000 rpm. The pulp suspension was fibrillated in a grinder (Supermasscolloider MKZA10-15J, Masuko Sangyo Co., Japan) at 1500 rpm. Fibrillated material was obtained after six passes through the grinder. The fibre slurry turned into a homogeneous, white and viscous gel after the grinding with a final solid content of 2.2%. The apparent viscosity at a shear rate of 10 1/ s was 0.68 Pas, measured according to the method described by Lahtinen et al. (2014b).

A more anionic and nano-sized TCNF was produced according to the method described by Skogberg et al. (2017). The outcome was a viscous and transparent hydrogel with a final dry matter content

Fig. 1 Photographs of a untreated hemp bast fibres, b milled hemp bast fibres, and c optical microscope image of macerated and stained (Herzberg stain) hemp bast fibres

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of 1.08% and a charge value of approximately 1 mmol/g dry pulp. The apparent viscosity at the shear rate 10 1/s was 2.48 Pas. Optical micrographs of the different fine components are shown in Fig. 2 and scanning electron microscope (SEM) micrographs in Fig. 3.

Figure 4 shows an estimated size scale of the employed fibre and fine components based on microscopy. The fibre width was spread over 5 and the fibre length over 4 orders of magnitude. The foamaided material formation was able to handle the large size differences among the raw materials.

Hydrocolloidal and water-soluble polymers Locust bean gum (LBG), a galactomannan obtained from the seed endosperm of the carob tree (i.e.

Ceratonia silique), was obtained from Sigma-Aldrich.

LBG is widely utilized as an additive in various industries such as food, pharmaceuticals, paper, textile, oil well drilling, and cosmetics (Barak and Mudgil 2014). It is a straight chain polymer of mannose with one galactose branch on every fourth mannose and has an average molecular weight of approx. 310 000 g/mol (Barak and Mudgil 2014). Industrial applications of LBG are based on its ability to form hydrogen bonding with water molecules. LBG was added as a powder into the foam at room temperature.

According to Barak and Mudgil (2014), LBG is partially soluble in cold water.

The conventional cationic wet end starch Raisamyl 50021 (DS = 0.035) was employed mainly for ensuring high retention of the CMF and the strongly negatively charged TCNF in particular.

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Foaming agents Two different foaming agents were applied: anionic sodium dodecyl sulphate, NaC12H25SO4, SDS (Sigma-Aldrich) with 90% purity, and polyvinyl alcohol PVA (Poval Kuraray). PVA is a linear water-soluble synthetic polymer that inherently creates a suitable type of foam for the foam-forming process. On the other hand, PVA is a strong binder: it is 3–4 times stronger than starch and 2–3 times stronger than acrylic or PVAc-based latexes (Miller et al. 1998; Finch 1992). The molecular weight of PVA usually varies between 15,000 and 250,000 g/mol. This weight is much higher than that of small anionic surfactants (e.g. the molecular weight of SDS is about 288 g/mol) but smaller than the molecular weight of LBG. PVA grades 6–88 (partially hydrolysed, Mw * 37,000 g/mol) and 28–99 (fully hydrolysed, Mw * 145,000 g/mol) were applied.

Since the foaming of grade 28–99 was fairly poor, a blend with better foaming 6–88 was employed to optimize foaming efficiency and binding effect. Both SDS and PVA foaming agents are readily biodegradable.

Foam generation and forming Water, a foaming agent (SDS or PVA blend), and the furnish components were mixed in a cylindrical vessel. CMF and TCNF were mixed with the fibre suspension before the surfactant was added and before foaming. Cationic starch, when applied, was mixed with the pulp slurry approximately 30 s before the foaming agent was added. Foam generation was continued until the air content of the fibre foam

Fig. 2 Optical micrograph images of a V-fines, b CMF, and c TCNF. V-fines and CMF were stained with Congo red, and TCNF was stained with toluidine blue. The measure bar shows the length of 200 lm

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Fig. 3 SEM images of a V-fines, b CMF, and c TCNF using equal magnification to illustrate the size difference between the components. The measure bar shows the length of 1 lm

Fig. 4 Estimated size scale of the fibre and fine components employed based on microscopy

reached the target of 50–60%. The fibre foam was poured along a plate into a deep mould with a wire bottom, drained by gravity for 15 min, and removed from the mould on the wire. The wet material (dry matter content around 12%) was inserted into a laboratory oven and dried at 70 °C. Drying to a remaining moisture content below 10% was complete within 24 h. In a special trial point (TP11) made with LBG, the drainage was extremely slow. Therefore, the samples were dried in the oven at a higher temperature (100 °C). Table 1 shows the most essential foaming parameters for the trial points.

The final material density level was adjusted by pressing the wet foam after the drainage of the free water and/or after the drying of the material. The oncedried materials were rewetted with water to the dry matter content of 50% and pressed between two plates with spacers to the desired final thickness and dried again in an oven (70 °C). Both panel surfaces were densified due to the dewatering in one direction and the pressing procedures. The bulk density values presented in the Tables 2 and 3 are the average density

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values of the materials. Photographs of foam-formed fibre materials are shown in Fig. 5.

In thin foam-formed sheets obtained by removing the foam by vacuum, fibres usually orient themselves mainly to the planar direction, in parallel to the foam flow (Järvinen et al. 2018). It is not obvious that similar preferred orientation could be found inside thick fibre networks drained by gravity. In addition to foam flow and drainage, the orientation may also depend on the air content and bubble size and employed raw materials together with boundary effects caused by the mould walls. Tomography images were produced to capture possible preferred fibre orientations in the formed materials.

Material testing methods Basis weight was measured by weighing the material at 50% RH and 23 °C and by measuring the material dimensions. Thickness was measured with a laser beam. Compression stress was measured with universal material tester (Lloyd) at 10% deformation, according to EN 826 (2013). The compression test was continued to 50% deformation level after 1 min of relaxation. Compression modulus was defined from the slope of the stress–strain curve in the elastic (linear) deformation region. Specific compression stress was defined by dividing the compression stress with density. Materials were imaged with a tomographic scanner (RX Solutions, France). Tube voltage 40 kV and current 300 lA were used. The voxel size was 25 lm.

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Table 1 Foaming parameters for the trial points

Diameter of the cylindrical foaming tank (cm)

TP1–TP10

TP11 (LBG)

32

16

Mixing speed (rpm)

3000

3500

Initial volume of the pulp suspension (l) Fibre consistency in pulp suspension before foaming (%)

6.8 2

0.93 2.7

Air content after foaming (%)

58–62 (SDS)

63

SDS concentration (%)

0.12

PVA concentration (%)

0.1 (6–88) and 0.5 (28–99)

–

Mould dimensions (cm)

42 9 42

9.8 (diameter)

Density adjustment

Pressing of wet foam ? rewetting and pressing

Pressing of wet foam

56–63 (PVA blend) 0.06

The furnish composition of TP1–TP6 are specified in Table 2 and TP7–TP11 in Table 3

Fig. 5 Photographs of the foam-formed materials, a 100% unrefined NBSK foamed with PVA (TP4) and b refined NBSK and hemp foamed with SDS (TP3), see Table 2. The sawn material pieces are of size 30 9 30 9 20 mm

Theoretical analysis The analysis of the experimental results is based on the following postulates on relevant mechanisms: (1) under compressive load, deformation of very porous material begins with bending of surface and other slender fibre segments (Subramanian and Picu 2011) and closing up of the largest voids and gaps of the material. (2) Gradually, at higher strains, local axial stresses build up (Subramanian and Picu 2011), especially in regions where a fibre or several of them resist bending. This resistance may be caused by the local orientation of fibres, their geometry (e.g. large cross-sectional area), internal stresses or by high local network density. (3) At certain points, the stress can

become high enough to trigger a sudden (buckling) failure in a fibre segment. Because of the inherent heterogeneity of the fibres, their failures can deviate significantly from the classical Eulerian buckling.

Besides buckling of the whole segment, a localized failure in a fibre wall is possible as well (Mäkinen et al.

2020). In any case, if such a failure happens, the stress is re-distributed quickly to other resisting regions.

Thus, the development of local deformations can be quite heterogeneous.

Ketoja et al. (2019) derived simple equations for a situation where free fibre segments buckle (with critical load proportional to the inverse square of segment length) and describe the stress-compression behaviour. Their theory assumed exponentially

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Table 2 Basic material properties of the foam-formed materials without added fine components TP#

Foaming agent

Density (kg/m3)

Furnish NBSK ur

NBSK r

1

SDS

2

SDS

3

SDS

4

PVA

5

PVA

100

6

PVA

80

Stress (kPa, 50%)

1 dr r d

at 50%

Hemp

100 100 80

Stress (kPa, 10%)

20

100 20

41.0

3.1 ± 0.1

18.3 ± 0.1

4.39 ± 0.08

43.6

4.7 ± 0.1

24.5 ± 0.1

4.24 ± 0.26

38.7

3.1 ± 0.5

20.6 ± 0.8

3.77 ± 0.13

43.4

5.2 ± 0.9

22.3 ± 2.6

3.79 ± 0.26

44.5

9.8 ± 0.7

31.8 ± 2.1

2.96 ± 0.19

45.0

12.6 ± 1.9

37.8 ± 2.5

2.79 ± 0.10

Equation (5) predicts the value 3.80 for the scaled slope of the stress–strain curve r1 dr d at 50% compression. The experimental values in the last column can be compared with this prediction

Table 3 Basic material properties of the foam-formed materials with added fine components TP#

Fine component

Density (kg/m3)

Stress (kPa, 10%)

Stress (kPa, 50%)

1 dr r d

7

No added fine components

48.6

3.3 ± 0.5

21.0 ± 2.7

4.52 ± 0.19

at 50%

8

V-fines (10%)

44.3

5.4 ± 0.4

27.4 ± 1.0

4.00 ± 0.14

9

CMF (10%)

45.8

6.9 ± 0.8

34.4 ± 2.0

3.86 ± 0.18

10

TCNF (10%)

51.5

8.1 ± 0.9

42.2 ± 4.7

3.88 ± 0.15

11

V-fines (10%) and LBG (25%)

38.5

17.1

37.4

3.52

In all cases, the furnish was unrefined NBSK (80%) and hemp fibres (20%) with 1% addition of cationic starch, and the surfactant was SDS. The added fine component amounts are given as a weight proportion of the fibre component (NBSK ? hemp, 100%).

Equation (5) predicts the value 3.80 for the scaled slope of the stress–strain curve r1 ddr at 50% compression. The experimental values in the last column can be compared with this prediction. There is no error estimate for the special LBG trial point TP11 because of a lack of parallel samples in this case

distributed segment lengths (Sampson 2008; Subramanian and Picu 2011), and that the changes in mean supporting structural element follow the macroscopic strain. In such a case, the material stress rð Þ at the compressive strain can be written as rð Þ ¼

r0 ½sð Þ 2

;

½sð Þ þ 1 e sð Þ ¼

ð1Þ ð2Þ

where the function sð Þ describes the mean relative length of fibre segments that buckle at strain , and r0 is a constant corresponding to the stress at strain 1 ¼ 2e 1 at which sð Þ ¼ 1. In other words, the constant r0 sets the absolute level of compression stress, which depends not only on the fibre type but also on bonding properties. According to the theory, load–displacement behaviour is described by the same function sð Þ

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for all random fibre networks with exponentially distributed segment lengths. The mean-field argument used to derive Eqs. (1, 2) is geometric, and bucklings are used only to connect local deformations with the applied stress. Thus, deriving the equations does not require any assumption on the proportion of fibre segments that undergo buckling nor on their postbuckling behaviour. However, the acoustic emission measurements by Mäkinen et al. (2020) indicate that a significant amount of acoustic energy during compression is released by events whose number is described explicitly by the function s defined by Eq. (2).

In an earlier study (Ketoja et al. 2019), the buckling theory predicted the quantitative compression behaviour surprisingly well for the studied fibre materials.

This was seen by comparing the prediction of Eq. (1) with experimental compression-stress curves and

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considering the statistical distribution of the ratio of stresses measured at 50% and 10% compression levels with a theoretical prediction,

rð0:5Þ sð0:1Þ 2 5:374 ð3Þ ¼ rð0:1Þ sð0:5Þ In reality, other deformation modes like the bending of fibres can be significant as well (Hossain et al.

2019; Bergström et al. 2019). Moreover, especially the initial deformation behaviour can be very non-uniform (Mäkinen et al. 2020). However, Eqs. (1, 2) provide an interesting reference curve which helps the comparison of the data. In the current study, we consider the slope of the stress–strain curve at high strain levels, drð Þ 2r0 esð Þ 2rð Þ½sð Þ þ 1 ¼ ¼ d ½sð Þ 2 ½sð Þ 4

ð4Þ

instead of the ratio given by Eq. (3). This slope is less sensitive to the changes at small strains, which are poorly described by the simplified model lacking the effect of fibre bending or closing up of large initial voids at small strains. At 50% compression, we obtain the prediction 1 dr 4½sð0:5Þ þ 1 3:804 ð ¼ 0:5Þ ¼ r d ½sð0:5Þ 2

ð5Þ

for the slope of the stress–strain curve scaled by the stress level. We will compare this value with the corresponding experimental results in addition to plotting the complete stress–strain curves against the theoretical Eq. (1).

Results and discussion Effects of fibre size distribution and the type of foaming agent The foam-formed fibre materials differ significantly from conventional water-formed random fibre networks. Perhaps, the most striking feature is the significant proportion of large pores, which are left as traces of air bubbles. This is seen clearly in the x-ray scans of the fibre network, viewed over a series of planar sections in the thickness direction (see Supplementary material). The traces are not as clear in the cross-sectional side views because of the partial collapse of the pores during the density adjustment.

Still, the overall pore size distribution is affected by the mean bubble size also in denser foam-formed sheets than those studied here (Al-Qararah et al.

2015b).

The above bubble traces cause the material distribution to be heterogeneous in the micro scale so that fibres are confined in narrow gaps between bubbles.

Thus, the effective local density can become quite large in these densified regions. This has an important impact on the compression properties of the materials.

Besides the normal pores, almost all samples included also larger voids, which may have originated from either very big bubbles or local opening of the fibre network because of drying stresses. The other typical heterogeneity came from the denser material surfaces, which was caused by the rewetting and pressing procedure used in sample making.

Table 2 shows the measured initial material density and compression stress at two deformation levels, 10% and 50%, for the various trial points without added fine components. We expect the structure to become more uniform when the compression proceeds and possible voids are closed. Therefore, it is of interest to compare the slope of the stress–strain curve at the highest studied compression level (50%) with the theoretical prediction. This comparison is also included in Table 2.

Compression stress (see Fig. 6a) and especially the compression modulus (see Fig. 6b) increased when polymeric water-soluble and film-forming PVA was applied as the foaming agent instead of anionic SDS. The refining of NBSK, which caused external fibrillation of the fibres and the generation of cellulosic fines, increased the relative material strength of the PVA foamed samples (TP5) more than the strength of the SDS foamed samples (TP2). Despite material nonuniformity (see Fig. 7b), PVA increased bonding so that both types of fibres, as well as formed cellulosic fines, contributed more actively to the strength of a highly porous fibre network. The material may have densified locally because of the large voids in the material obtained using PVA, see Fig. 7b. Subsequently, the mean free segment length in the fibre network was shorter in the densified regions. This could make a significant contribution to strength, which behaves like the square of density in these types of materials (Ketoja et al. 2019). The effect of improved bonding was seen, especially when 20% of the refined NBSK pulp was replaced with poorly

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bonding but stiff and long hemp fibres. The compression stress and modulus of the PVA trial point were further improved with the addition of hemp (TP6). In contrast, a slight drop in these properties was seen for the SDS trial point (TP3) where fibres distributed more evenly and the local densification was not as strong, see Fig. 7a.

When looking at the stress–strain curves of Fig. 8, the refined and unrefined NBSK samples obtained with SDS follow the predicted theoretical behaviour (Ketoja et al. 2019) of random fibre networks [refer to Eqs. (1, 2)] quite well, except below 10% strain (Mäkinen et al. 2020). A slight deviation was observed with the added hemp component in Fig. 8c, which could have resulted from either voids in the structure or altered fibre segment distribution for intermediate span lengths. Interestingly, similar compression-stress behaviour was produced by unrefined NBSK with the PVA foaming agent (see Fig. 8d), which produced an uneven fibre network with large voids (Fig. 7b). It is possible that PVA forms films (Bossu et al. 2019) that, together with increased bonding and locally increased density, interfere with the distribution of intermediate segment lengths in a fashion similar to when refining and hemp are added. The deviation from the model behaviour for the PVA samples appeared to increase with NBSK refining (Fig. 8e) and added hemp component (Fig. 8f). Both factors probably drove the formed complex multi-scale material network away from the simple random fibre network that had exponentially distributed free segment lengths. The scaled slope of stress–strain curve r1 dr d at 50% compression, when the large voids can be expected to be closed, varies in the range of 2.8–4.4, see Table 2. All

Cellulose (2020) 27:6961–6976

results with SDS surfactant are relatively close (average deviation 9%) to the theoretical prediction 3.8 given by Eq. (5). The larger deviations from the theory with PVA foaming agent for refined NBSK cause a clear drop in the slope at 50% compression.

Buckling of fibre segments has been postulated as the failure mechanism underlying Eqs. (1, 2) (Ketoja et al. 2019; Mäkinen et al. 2020). Thus, the recovery of sample thickness after the load has been removed should depend mainly on the recovery of fibres from buckling deformation and not on bonding properties.

Figure 9 shows the thickness recovery of samples exposed to 50% compression recorded 1 min after the load was removed. The recovery was roughly equal for both surfactant types despite their highly different bonding properties. This agrees with the above theoretical postulate. Unrefined curly Kraft fibres led to better recovery than straighter refined Kraft fibres.

Addition of hemp fibres (TP3 and TP6) did not change this picture.

Effects of lignocellulosic fines, cellulose nanofibrils, and LBG The results presented in the previous chapter suggest that the fines and fibrillation coming from refining, together with added strengthening polymers like PVA, can ‘‘activate’’ the multi-scale fibre network and improve strength properties significantly. Thus, the contribution to strength of fines, fibrils, and macromolecules was studied more thoroughly. This was done for the fibre composition that included 80% NBSK and 20% long hemp pulp fibres. The testing plan (see Table 3) included the reference with starch

Fig. 6 a Specific compression stress (compression stress divided by bulk density) of the materials (Table 2) made with SDS (TP1–3) and PVA (TP4–6) foams at 10% and 50% deformation. b The compression modulus of the same materials

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Fig. 7 Typical cross-sectional images obtained with x-ray tomography for a refined NBSK and hemp with SDS (TP3), and b unrefined NBSK with PVA (TP4). Both samples include voids which cause deviation from the theoretical stress–strain curve

below 40% strain in Fig. 8c, d. The images are averages of 60 slices taken from a central part of the samples. Wire side is at the bottom

Fig. 8 Comparison of the measured compression behaviour (solid curve) to the theoretical prediction (dashed curve) for a unrefined Kraft (TP1), b refined Kraft (TP2), c refined Kraft and hemp (TP3), d unrefined Kraft and PVA (TP4), e refined Kraft and PVA (TP5), and f refined Kraft, hemp, and PVA (TP6). The

experimental stress–strain curves are averages of 5 parallel measurements from the 1st (0–10%) and 2nd (10–50%) cycles.

The foaming agent was SDS for the data in a–c. The parameter r0 of Eq. (1) was fitted to the experimental curve at 50% strain

(TP7), and the trial points with added V-fines (TP8), CMF (TP9) and TCNF (TP10). The addition of fine components was 10% of the long fibre amount.

Cationic starch was added to ensure high retention of the negatively charged fibrils. Retention of the fine

materials was not measured. However, based on the work of Turpeinen et al. (2018) and the realized grammages, we assume high, over 90% retention. This trial set was extended with a special trial point that included a partly soluble natural polymer, colloidal

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Fig. 9 Recovery from 50% compression 1 min after the load was removed. The recovery depended more on the fibre properties than on network bonding. TP1–3 are samples made with SDS and TP4-6 samples made with PVA foam

LBG (TP11) with addition level of 25% of the long fibre amount. With its high molecule weight, this component could be viewed to describe the bonding behaviour for an additive having size scale between PVA and TCNF. Assuming complete retention, the amounts of fine components (V-fines, CMF, and TCNF) and LBG in the whole material were 7% and 18%, respectively. Table 3 shows the measured material properties with densities varying in the range of 38–51 kg/m3.

The addition of fine components caused an apparent increase in all the measured strength properties, as shown in Fig. 10. Within experimental accuracy, the finest and highly charged TCNF did not increase compression stress much more than CMF. The clearly coarsest fine component, fibrillar V-fines, also had potential as a strength additive. The addition of LBG clearly led to the highest compression modulus and compression stress at the small 10% deformation level (see Fig. 10). The formed material was very hard and

Cellulose (2020) 27:6961–6976

brittle so that the fractures formed at higher compressive strains caused a decay of the compression stress.

At small strains, the addition of LBG or similar hydrocolloidal natural polysaccharide can improve the strength and stiffness of fibre foams made with SDS tremendously. Values that are similar to or improved compared with foaming with PVA can be achieved.

Figure 11 shows the stress–strain behaviour for unrefined kraft and hemp with added fine components.

Earlier without added fines, this furnish (TP3) led to significant deviation from the theoretical behaviour of Eqs. (1, 2) at strains below 40% (Fig. 8c). Curiously, with added fine components, the agreement was much better over the whole strain range of 10–50%, as shown in Fig. 11b–d. The same was observed with added cationic starch (Fig. 11a). As shown by Fig. 12, both the fine components and the starch made the network more uniform and tightly bonded, which might have brought the distribution of segment lengths closer to the assumed exponential distribution of the theoretical model. The same theoretical form was well maintained, even when the fitted factor r0 of Eq. (1) increased significantly when finer particles were used.

This increase was probably at least partly caused by the higher local density (Ketoja et al. 2019) achieved with more effective bonding.

In Table 3, the scaled slope of stress–strain curve 1 dr r d at 50% compression varies in the range of 3.5–4.5.

Thus, the deviation from the theoretical value 3.8 is rather small. Moreover, the experimental slope is not correlated with material density (also compare with Table 2). Similar quantitative agreement was observed with a broader range of densities earlier (Ketoja et al. 2019). A correlation with material

Fig. 10 a Specific compression stress at 10% and 50% deformation, and b compression modulus for the samples with added fine components (Table 3). Note that the modulus obtained with added LBG exceeded the scale of the y-axis

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Fig. 11 Comparison of the measured compression behaviour (solid curve) to the theoretical prediction (dashed curve) for structures made with unrefined Kraft (80%) and hemp (20%) with a cationic starch, b V-fines and starch, c CMF and starch, d TCNF and starch. The experimental stress–strain curves are

averages of 5 parallel measurements from the 1st (0–10%) and 2nd (10–50%) cycles. The last figure e shows the plot for the special LBG trial point TP11. The foaming agent was SDS for all samples

density would be expected if the stress increase at high compression was caused mainly by the appearance of new inter-fibre contacts as proposed earlier (Alimadadi et al. 2018). With higher initial density, the proposed densification mechanism should take place earlier during compression, and the scaled slope at 50% compression should increase with initial density.

Our data does not support this idea for the compression levels studied. Even at 50% compression, densities of the studied materials are still rather low, and the mean contact stress (Subramanian and Picu 2011) remains small.

when the density of the final dried materials is kept on a similar level. Interestingly, the most significant impacts were from the smallest polymer components, such as the PVA and LBG. Using PVA to make wet foam leads to an uneven fibre network but to a stronger material than the one obtained using SDS foam. PVA densified the network locally and formed a film substructure, which affected fibre segment buckling. This was visible as a deviation from the theoretical stress– strain curve based on exponentially distributed segment lengths. A similar deviation was observed when hemp bast fibres that bonded poorly with NBSK were added. On the other hand, after improving bonding and material homogeneity with fine components, the experimental stress–strain curves approached the theoretical prediction in all cases with the SDS surfactant beyond 40% strain. Among all studied samples, the average scaled slope of the stress–strain curve at 50% compression was 3.79 ± 0.33, which

Conclusions The mechanical behaviour of foam-formed lightweight materials can be greatly affected by the multi-scale raw material components chosen, even

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Fig. 12 Typical cross-sectional images obtained with x-ray tomography for samples with fine components. a Reference with unrefined NBSK and hemp with starch (TP7). The added fine

components included b V-fines (TP8), c CMF (TP9), and d TCNF (TP10). The corresponding stress–strain curves are shown in Fig. 11. Wire side is at the bottom

can be compared with theoretical prediction of 3.80.

Moreover, the slope was not correlated with material density. This gives further evidence of a buckling mechanism describing the observed stress–strain behaviour at intermediate compression levels, before collective phenomena soften the structure (Mäkinen et al. 2020).

The resilience of the samples after compression was not affected by the bonding properties but by the dominant fibre type. This suggests that resilience is related to the recovery of fibres from earlier buckling; poor resilience does not necessarily mean that interfibre bonds would have failed.

These findings will be useful in developing new applications for lightweight cellulose materials, where separate tuning of mechanical behaviour and material density are essential.

Acknowledgments Open access funding provided by Technical Research Centre of Finland (VTT). This article was based on the results achieved in the Novel Structural Materials with Multi-scale Fibre Components (NoMa) project. The authors are grateful for the financial support from Tekes (now Business Finland, the Finnish innovation funding, trade, investment, and travel promotion organization), VTT, the participating companies, and Lahti University of Applied Sciences. We are also grateful for the support by the FinnCERES Materials Bioeconomy Ecosystem. We would like to thank Kirsi Immonen and Jari Sirviö for useful discussions. Tuomas Turpeinen, Annika Ketola, Päivi Sarja, Meiju Sinkkonen, and Timo Rantanen are appreciated for their laboratory and analysis work.

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Funding

Funding was provided by Tekes and VTT.

Compliance with ethical standards Conflict of interest The authors declare that they have no conflicting interests.

Cellulose (2020) 27:6961–6976 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Colloids Surf A Physicochem Eng Asp 482:544–553 Barak S, Mudgil D (2014) Locust bean gum: processing, properties and food applications—a review. Int J Biol Macromol 66:74–80 Bergström P, Hossain S, Uesaka T (2019) Scaling behavior of strength of 3D-, semi-flexible-, cross-linked fibre network.

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6975 based materials. Nord Pulp Pap Res J (Biorefinery) 29(4):584–591 Jahangiri P, Logawa B, Korehei R, Hodgson M, Martinez DM, Olson JA (2016) On acoustical properties of novel foamformed cellulose-based material. Nord Pulp Pap Res J 31(1):14–19 Järvinen M, Pihko R, Ketoja JA (2018) Density development in foam forming: wet pressing dynamics. Nord Pulp Pap Res J 33:226–236 Ketoja JA, Paunonen S, Jetsu P, Pääkkönen E (2019) Compression strength mechanisms of low-density fibrous materials. Materials 12:384 Komori T, Makishima K (1977) Numbers of fiber-to-fiber contacts in general fiber assemblies. Text Res J 47:13–17 Lahtinen P, Liukkonen S, Pere J, Sneck A, Kangas HA (2014a) Comparative study of fibrillated fibers from different mechanical and chemical pulps. BioResources 9(2):2115–2127 Lahtinen P, Torvinen K, Kangas H, Liukkonen S, Sneck A, Peresin MS, Pere J, Hänninen T, Meyer V, Tammelin T (2014b) Effect of fibrillated cellulosic additives on paper strength properties. In: PaperCON 2014 proceedings, TAPPI Luo Y, Xiao S, Li S (2017) Effect of initial water content on foaming quality and mechanical properties of plant fibre porous cushioning materials.

BioResources 12(2):4259–4269 Madani A, Zeinoddini S, Varahmi S et al (2014) Ultra-lightweight paper foams: processing and properties. Cellulose 21:2023–2031 Mäkinen T, Koivisto J, Pääkkönen E, Ketoja JA, Alava MJ (2020) Crossover from mean-field compression to collective phenomena in low-density foam-formed fiber material.

Soft Matter (submitted) Miller GD, Boylan JR, Jones RB (1998) Synthetic coating adhesives. Poly(vinyl alcohol)—a versatile polymer for paper and paperboard applications. Tappi Press Neagu C, Gamstedt K, Berthold F (2006) Stiffness contribution of various wood fibers to composite materials. J Compos Mater 40:663–699 Nurminen I, Saharinen E, Sirviö J (2018) New technology for producing fibrillar fines directly from wood. BioResources 13(3):5032–5041 Paunonen S, Timofeev O, Torvinen K, Turpeinen T, Ketoja JA (2018) Improving compression recovery of foam-formed fiber materials. BioResources 13:4058–4074 Picu RC (2011) Mechanics of random fiber networks—a review.

Soft Matter 7:6768–6785 Picu RC, Subramanian G (2011) Correlated heterogeneous deformation of entangled fiber networks. Phys Rev E 84(3):031904 Pöhler T, Jetsu P, Isomoisio H (2016) Benchmarking new woodfibre based sound absorbing material made with a foamforming technique. Build Acoust 23(3–4):131–143 Pöhler T, Jetsu P, Fougerón A, Barraud V (2017) Use of papermaking pulps in foam-formed thermal insulation materials. Nord Pulp Pap Res J 32(3):367–374 Poranen J, Kiiskinen H, Salmela J, Asikainen J, Keränen J, Pääkkönen E (2013) Breakthrough in papermaking resource efficiency with foam forming. In Proceedings of the TAPPI PaperCon, Atlanta, GA, 28 April–1 May 2013

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Sci Rep 9:4258 Subramanian G, Picu CR (2011) Mechanics of three-dimensional, nonbonded random fiber networks. Phys Rev E 83:056120 Turpeinen T, Lappalainen T, Kenttä E, Torvinen K (2018) Detection of iron and iron-cobalt labeled cellulose nanofibrils using ICP-OES and XlCT. Nord Pulp Pap Res J 33(4):610–617 Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Volume 7, Number 2, 2021

Lignocellulosic Fibers from Renewable Resources Using Green Chemistry for a Circular Economy Khandoker S. Salem, Ved Naithani, Hasan Jameel, Lucian Lucia & Lokendra Pal. The sustainable development of lignocellulose fibers exhibits significant potential to supplant synthetic polymer feedstocks and offers a global platform for generating sustainable packaging, bioplastics, sanitary towels, wipes, and related products. The current research explores the dynamics of fiber production from wood, non-wood, and agro-residues using carbonate hydrolysis and a mild kraft process without bleaching agents. With respect to carbonate hydrolysis, high yield, and good coarseness fibers are attained using a simple, low-cost, and ecofriendly process. Fibers produced using a mild kraft process have lower Klason lignin, carboxyl content, surface charges, and higher fiber length, and crystallinity. Eucalyptus fibers show the highest crystallinity while softwood carbonate fibers show the lowest crystallinity. Hemp hurd fibers contain the highest concentration of hard-to-remove water, and thus, suffer maximum flattening visualized by the microscopic images. The relatively high yield sustainable fibers with versatile properties can provide a significant economic benefit since fiber is the dominant cost for producing various bioproducts to meet society’s current and future needs.

Contact information: Department of Forest Biomaterials, NC State University, Raleigh, NC 27695–8005, USA Global Challenges 2020, 2000065 https://doi.org/10.1002/gch2.202000065 Creative Commons Attribution 4.0 International License

The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International and the PITA Annual Review, both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating.

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Lignocellulosic Fibers from Renewable Resources Using Green Chemistry for a Circular Economy Khandoker S. Salem, Ved Naithani, Hasan Jameel, Lucian Lucia, and Lokendra Pal* which are rich in forestry resources. But, in places where the availability of forest is limited, non-wood species and agro-industrial waste can be possible alternatives and equally valuable sources of biomass.

However, the foremost challenge in producing sustainable lignocellulosic fibers is ensuring a sustainable source of biomass supply, since lignocellulosic resources are prima facie feedstocks for pulp, lignin, nanocrystals nanofibers, and biopolymers, which have versatile industrial applications, for example, paper products, bioplastics, biocomposites, biofuels, and bioenergy production, etc. Indeed, lignin has emerged as a potential sustainable bioresource for advanced applications due to high carbon content and renewable character.[5] Researchers have developed techniques to valorize lignin for applications in polymers and composites[6,7] and for producing sustainable thermo–electric materials based on carbon nanofibers (CNFs).[8,9] These CNFs have been successfully used as an alternative to carbon nanotubes (CNTs) to create high performance anodes for better thermo-mechanical and electrical properties.[10,11] However, to continue to increase its utilization, the circular economy is pivotal though it has been very difficult to efficiently recycle lignocellulosic fibers due to limitations in collection and its high degree of contamination. Thus, extensive research is being carried out to produce sustainable feedstock by introducing alternative pulping methods and/or alternative raw materials that would conserve the usage of chemicals, energy, and water.[12–14] Despite considerable research effort, wood is still the predominant resource for pulp production because it has been used to produce 89% of the world’s paper, whereas, only 11% is based on non-woody plants.[15] However, non-woody and agro-residues such as hemp hurds have been emerging as a viable resources to meet growing demand for economic and sustainable paper products.[16] Presently, hemp hurd has been used to meet 5% of the total global paper supply for tissue papers, bio-plastics etc., which has been made possible due to the valorization of the hemp hurd from its original use as animal bedding and hemp-lime construction materials.[16] The high cellulose content ≈ 50%–70%, quick harvesting time (4 months) and requirement of fewer harsh chemicals for processing, with respect to the hardwoods and softwoods, have made hemp hurd a promising candidate for sustainable lignocellulosic resource.[16] Another growing challenge for sustainable fiber sources is the introduction of an environment friendly, low chemical pulping

The sustainable development of lignocellulose fibers exhibits significant potential to supplant synthetic polymer feedstocks and offers a global platform for generating sustainable packaging, bioplastics, sanitary towels, wipes, and related products. The current research explores the dynamics of fiber production from wood, non-wood, and agro-residues using carbonate hydrolysis and a mild kraft process without bleaching agents. With respect to carbonate hydrolysis, high yield, and good coarseness fibers are attained using a simple, low-cost, and ecofriendly process. Fibers produced using a mild kraft process have lower Klason lignin, carboxyl content, surface charges, and higher fiber length, and crystallinity. Eucalyptus fibers show the highest crystallinity while softwood carbonate fibers show the lowest crystallinity. Hemp hurd fibers contain the highest concentration of hard-to-remove water, and thus, suffer maximum flattening visualized by the microscopic images. The relatively high yield sustainable fibers with versatile properties can provide a significant economic benefit since fiber is the dominant cost for producing various bioproducts to meet society’s current and future needs.

1. Introduction The world has been moving towards a sustainable, bio-based and circular economy due to social awareness and governmental regulations against single plastic use which could save up to $26 trillion by 2030.[1,2] Global environmental awareness has propelled the bio-based industries, like paper, tissue paper products, bioplastics, packaging, renewable lubricants, etc., more so than many other industries to play an important role in sustainable development because among their chief raw materials are renewable wood and non-wood fibers.[3,4] Appropriate and sustainable utilization of bioresources are the major requirements to expand the bio-based economy to produce biochemicals, biofuels, bioplastics, and related products to their petroleum analogues. Wood has been a major source of biomass for the places K. S. Salem, Dr. V. Naithani, Prof. H. Jameel, Dr. L. Lucia, Dr. L. Pal Department of Forest Biomaterials NC State University Raleigh, NC 27695–8005, USA E-mail: lokendra_pal@ncsu.edu The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/gch2.202000065. © 2020 The Authors. Published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/gch2.202000065

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2000065 (1 of 10)

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Table 1. Chemical composition of feedstock.

Cellulose [%]

Klason lignin [%]

Extractives [%]

Ash [%]

Eucalyptus

Raw material

46.7

27.9

4.3

1.3

19.8

Hemp hurds

43.0

24.4

2.2

1.4

29.0

Bamboo

43.2

26.7

2.2

0.8

27.1

Hardwood

44.6

26.9

3.7

1.1

23.7

Softwood

46.0

28

3.0

1.0

22.0

**

Hemicelluloses [%]

Constituent content calculated on a dry basis by subtracting sum of cellulose, lignin, extractives, and ash contents from 100%.

methods to raise pulp yield. Despite considerable research effort, kraft and soda are still the predominant processes for producing fibers with low lignin content suitable for papermaking.[17] Almost 80% of the total chemical pulping industry employees kraft pulping, which involves digestion of wood chips in a solution of sodium sulfide and sodium hydroxide at elevated temperatures and pressures.[18] It offers the benefit of recovering the cooking chemicals and heat, but also has some drawbacks including emissions of sodium and calcium salts particulates (flue gases) and volatilized reduced sulfur compounds.[19–24] In contrast to kraft, soda and organosolv are sulfur-free processes and thus alkali such as NaOH, KOH, and Ca(OH)2 are widely used as pretreatment steps for pulping.[25] They can be performed at ambient conditions, but require longer pretreatment times.[26,27] Alkali pretreatment is most effective for biomass having low lignin contents such as leftover lignocellulosic feedstocks, corn stover, switchgrass, bagasse, wheat, and rice straw.[28] Organosolv has been proposed based methanol, ethanol, acetic acid, acetone, etc., to remove lignin.[29] Although this technique comes with reduced emissions of sulfur dioxide and odorous gases because of the use of sulfur-free technology, it is not ready yet to be implemented on an industrial scale.[30] Thus, there is a great opportunity for an alternative pulping system to reduce ecological damage by minimizing waste in conventional pulping.[24] The current work discusses new types of chemical pulping where significant efforts have been made to attain sustainable utilization of natural resources. The new processes were carried out using very mild chemical conditions and resulted in higher yield of pulp compared to conventional methods (34%– 55%).[31] Non-conventional pulping using 4% sodium carbonate based on Na2O for carbonate hydrolysis and 12% active alkali (NaOH + Na2S, based on Na2O) for mild kraft were studied for five feedstocks: eucalyptus, hemp hurd, bamboo, hardwood and softwood. Fiber yield, lignin content, brightness, surface charges, hard to remove water, and crystallinity were determined. Hence, the present work demonstrates how carbonate hydrolysis and mild kraft pulping can produce higher value fibers. This study will enable acceleration of the biorefinery to tune the properties different feedstock fibers.

the Netherlands and Bamboo (Henon) supplied by National Bamboo, LLC. Raleigh, NC, Southern pine, eucalyptus (E. grandis), and mixed hardwood and softwoods from local mills. The chemical compositions of these fibers were determined according to the guidance of U.S. Department of Energy, National Renewable Energy Laboratory (NREL) analytical procedure TP-510-42618 or the ASTM E1758-01 (2015) in agreement with previous studies[32,33] presented in Table 1. All chemical reagents were purchased from Fisher Scientific.

2.2. Methods 2.2.1. Defibration Laboratory defibration highlighting the features of sodium carbonate and mild kraft defibration are shown in Figure 1. The defibration process conditions are in Table 2. A higher water-tosolids ratio of 8:1 was used for hemp due to its low bulk density compared to hardwood. The carbonate hydrolysis was carried out using 4% carbonate (based on Na2O). Kraft pulping was done using 12% active alkali and 25% sulfidity (NaOH+Na2S) (all on an Na2O basis). Both processes were conducted in a stainless-steel reactor under controlled temperature (160 °C) for 3 h using a mild concentration of sodium carbonate (carbonate) and a high concentration of alkali charge (kraft), respectively. The pulp was washed and refined on the laboratory disc refiner (The Bauer Bros Co., Springfield, Ohio, Model 148-2, rpm 3600) at disc gaps of (0.2– 0.1–0.05) mm with three passes for hard wood pulp and disc gaps of (0.1–0.05) mm with two passes for hemp pulp before screening on a 0.15 mm slotted laboratory screen. Pulp yield, Klason lignin content, and freeness were determined using the TAPPI T222 and TAPPI T227 om-09 standard methods, respectively.

For simplicity, the following sample IDs will be mentioned in the later part of the article: EU_C and EU_K (Eucalyptus fiber produced by Carbonate and Kraft process); HM_C and HM_K (Hemp fiber produced by Carbonate and Kraft process); BMB_C and BMB_K (Bamboo fiber produced by Carbonate and Kraft process); HW_C and HW_K (Hardwood fiber produced by Carbonate and Kraft process) and SW_C and SW_K (Softwood fiber produced by Carbonate and Kraft process)

2. Experimental Section 2.1. Materials

2.2.2. Characterization of Different Fibers

Five types of feed stocks were selected: Futura 75 cultivar hemp hurds, dew retted, and decorticated procured from

Fiber Quality Analysis: Fiber length (lw), fine contents and other physical properties of the different samples were determined

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Figure 1. The pulping scheme for different feed stocks and subsequent processing of fibers.

by using a high-resolution fiber quality analyzer: HiRes FQA, OpTest Equipment Inc, Hawkesbury, ON, Canada. Before testing, fiber quality analysis (FQA) was calibrated and used according to the manufacturers’ specifications. The whole pulp for each sample was taken and disintegrated before the measurement was done.

Fourier Transform Infrared Spectroscopy (FTIR): Fourier transform infrared (FTIR) spectra of the different fibers were recorded using a PerkinElmer Spectrum One FTIR spectrometer. For each sample, the diamond crystal of an attenuated total reflectance (ATR) accessory was brought into contact with the area to be analyzed. The contact area was a circle of about 1.5 mm in diameter. All spectra were recorded between 4000 and 500 cm−1 with a resolution of 4 cm−1 with 32 scans per sample.

For comparison, spectra were adjusted to the same baseline.

Brightness Measurement: The brightness of the fibers was measured by following the ISO 2470 standard method using the color spectrophotometer Color Touch X, Technidyne, Indiana, USA. This procedure was repeated for ten samples for each of the fibers, and the mean value was taken.

Total Carboxyl Content Analysis: Acid-base titration was used to determine the total carboxyl content of the pulps.[34,35] 100 mg of pulp was dissolved in 20 mL 0.1 n NaOH and stirred with a magnetic stirrer for 2 h. An excess amount of 0.1 n NaOH was titrated with 0.1 n HCl using phenolphthalein as an indicator.

The carboxyl content in milliequivalents per 100 g of the pulp slurry was calculated as: Carboxyl content = (Vb − Va ) × N × 100/W

(1)

Where, N is the normality of HCl (Equation (l)), Vb and Va are the volumes of HCl in absence and presence of sample, and W is the weight of sample (g).

Charge Determination: Charge estimation on the fibers was carried out using the colloidal titration method.[36] The fiber samples were disintegrated into pulp at a 0.075% consistency (mass of solids relative to total mass + water) in deionized water.

The colloidal charge of 200 mL of the resulting pulp suspension was evaluated by titration using a CHEMTRAC ECA 2000 P streaming current analyzer. The charge neutralization points of fibers were determined by the addition of cationic polymers (poly(diallyldimethylammonium chloride) (polyDADMAC)) to the suspension.

Hard-to-Remove Water Measurement: Hard to remove (HR) water was determined using TGA.[37] The sample was taken in platinum sample pans. The temperature was then raised to 110 °C from 30 °C at 10 °C min−1. Drying experiments were run isothermally at 110 °C for 15 min until no more drying occurred. The non-freezing bound water was calculated by subtracting the total freezable water from the moisture ratio in the

Table 2. Process condition.

Process Carbonate hydrolysis

Feedstock

pH

Water–solid ratio

Pulping chemicals

Hemp

11.2

8

4% sodium carbonate based on Na2Oa)

Eucalyptus, bamboo, hardwood, softwood

11.3

4

4% sodium carbonate based on Na2O

Hemp

12.8

8

12% active alkali 25% sulfidity (NaOH+Na2S) based on Na2O

Eucalyptus, bamboo, hardwood, softwood

13.3

4

12% active alkali 25% sulfidity (NaOH + Na2S) based on Na2O

Mild kraft

a)

Reporting on a Na2O basis indicates the actual chemical relationship between the pulping chemicals used. It is a standardized way of normalizing the concentration strengths (based on Na2O).

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Figure 2. FTIR spectra of different fibers prepared by A) Carbonate Hydrolysis and B) Mild Kraft.

sample determined gravimetrically using a TGA microbalance (110 °C until weight did not change).

X-Ray Diffraction (XRD) Analysis: A Rigaku SmartLab X-Ray Diffractometer, operated at 45 kV and 40 mA with a Ni-filtered CuK radiation, was used to determine the crystallinity index of the samples. X-ray diffractograms were recorded at 0.02° s-1 over a 2 scan in the range 5°–60°. Segal Peak height method was used to calculate the crystallinity index (CI) of the samples using the following equation: Crystallinity Index ( CI) = (I 002 − I AM ) × 100/I 002

(2)

Scanning Electron Microscopy (SEM): Handsheets of the different fibers produced using carbonate and mild kraft pulping process were made at a target basis weight of ≈40 g m−2 as per the TAPPI T205 method with a light weight (≈0.15 kg) foam roller instead of a standard heavy (13 kg) brass roller, without any pressing, and dried twice on a drum dryer at 220°F. The morphological characterization of samples was carried out using Hitachi S3200N variable pressure scanning electron microscope (VPSEM). Samples were sputter coated with AuPd coating for 10 minutes and images were taken at 10kV.

3. Results and Discussions 3.1. Fourier Transform Infra-Red Spectroscopic Analysis The chemical structure of the different fibers prepared by carbonate hydrolysis and kraft process were compared in Figure 2 to examine the effect of different pulping on the fiber structures. The appearance of peak in the region of 1030–1170 cm−1 is due to COC and CO stretch of primary and secondary hydroxide groups of the carbohydrates and hydroxyphenyl, guaiacyl, and syringyl groups of lignin.[38] The peaks at 1281, 1370, and 1427 cm−1 may be due to aromatic esters, ether, and

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phenol compounds.[39] The peak corresponding to 1730 cm−1 is from unconjugated CO stretching which is due to vibration of aliphatic carboxylic acids and ketones of hemicellulose or lignin groups and that near 1650 cm−1 is due to conjugated carbonyl found in hemicellulose and lignin groups.[40] Moreover, CH symmetric and asymmetric stretching bands appeared at 2900 cm−1 and hydrogen bonded stretching bands of OH groups at 3400 cm−1 are characteristics peaks of hydroxyl groups of cellulose.[39] All the fibers, including the hemp hurds, had similar FTIR spectra since they had similar lignocellulosic compositions as shown in Table 1 and none of the pulping resulted in significant change in the chemical structure of the fibers.

3.2. Pulp Yield The yield of the carbonate and kraft pulps (total pulp wt% obtained after defibration) were calculated and found to vary from 49%–75% depending on the source of fibers and pulping, which were comparatively higher than the pulp yield produced by conventional methods as shown in Figure 3A. The yield of carbonate hemp pulps was 71.3%, which was slightly less than the carbonate eucalyptus and bamboo, but almost the same as carbonate hardwood and softwood pulps. However, the yield of mild kraft hemp was similar to eucalyptus but higher than the other kraft pulps. This might be due to the morphology of the hemp hurds allowing easier penetration of pulping chemicals compared to hardwood and softwood.[16] The pulp yield was higher for the carbonate than the kraft for all the fiber types because carbonate was operated at lower pH. Higher pH resulted in higher hydroxyl concentration which controls the intensity of fiber defibration and consequently causes random chain scission in cellulose and hemicelluloses. This promotes peeling and results in lower pulp yield because the pulp defibration rate is proportional to the OH concentration.[16,41,42] Thus, kraft pulping facilitated removal of higher

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Figure 3. Graphical representation of A) Total yield and B) Klason lignin content of different fibers produced using two different methods.

fines showed an opposite trend, that is, decreased when compared with the fibers produced by carbonate. The production of longer fiber and lower fines during kraft can be attributed to higher pH. Higher pH resulted in longer fibers, lower coarseness, and lower fines by removing more hemicelluloses, lignin, and extractives as reported earlier.[16,43] Studies of fiber properties have shown that the fiber strength increases with increases in fiber length and decrease in coarseness. Longer fibers have more fiber joints and a higher area for bonding and therefore create a stronger network compared to shorter fibers. On the other hand, thinner fibers collapse more easily than a coarser fiber with thicker cell walls. The collapsed fibers will create a network with much higher density and lower bulk. The flexibility of the fibers and the number of fiber joints will therefore increase the tensile strength. The uncollapsed fibers will create a network with higher porosity and higher bulk compared with the collapsed ones.[44] Thus, fibers produced in kraft will form stronger sheets and possess better tensile properties.[45] However, fiber curliness negatively affects tensile and tensile

amounts of lignin at high pH as shown in Figure 3B and supported by FTIR (Figure 2B). The peaks corresponding to 1730 and 1650 cm−1 arise from lignin functional groups from carbonate which is less visible in mild kraft because lignin content is relatively low in the latter. Thus, carbonate hydrolysis and mild kraft produced pulps with different yields, lignin percentages, and different compositions.

3.3. Fiber Morphology The fiber structure was further investigated by comparing the fiber morphology of the pulps produced by the two processes. Fiber morphology is related to fiber structure, which is primarily regulated by feedstock and pulping. Generally, fiber length, coarseness, kink, curl and fines are used to describe fiber morphology. Table 3 provides data on the morphology of fibers.

The length of all the fibers produced by kraft pulping increased; but the other properties: coarseness, kink, curl, and Table 3. Physical Properties of Different fibers.

Fiber length Lw, [mm]

Coarseness [mg m−1]

Curl index

Kink index [1 mm−1]

Fines [%]

EU_C

1.06

0.20

0.05

0.67

2.29

10.9

EU_K

1.09

0.08

0.05

0.08

2.12

22.5

HM_C

0.69

0.18

0.07

1.27

7.30

15.7

HM_K

0.77

0.07

0.05

0.08

3.93

23.4

BMB_C

1.22

0.36

0.07

0.80

17.73

10.9

BMB_K

1.41

0.11

0.06

0.62

15.09

23.3

HW_C

0.8

0.20

0.08

0.84

5.81

12.7

HW_K

1.17

0.10

0.05

0.62

2.33

19.7

SW_C

1.94

0.45

0.06

0.65

4.29

12.6

SW_K

2.45

0.27

0.05

0.55

1.25

13.2

Sample ID

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Brightness ISO

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stiffness indices, while it increases tear and fracture toughness indices.[46] The curl index gives the maximum elongation of the fiber up to breakage, which indicates the stability of the paper web in the open draw. The deformed fibers contribute uneven distribution of stress along the length of a curled fiber in a fracture zone, transferring more stresses to the bonds, which increases the tear strength of the sheets. Thus, fibers produced by carbonate form sheets having lower tensile strength, but higher tear strength. The fiber curl, in addition, increases the bulk and porosity of the pulp sheet but decreases the drainage resistance.[46] Curly fibers cause more scattered reflection of light, which results in a matte appearance and opacity. Thus, fibers produced by carbonate have higher coarseness and lower brightness than kraft. The presence of lower lignin content in kraft also affected the brightness. Thus, carbonate pulps with higher coarseness, curl index and fines could be used to make paper or tissue products with high bulk and tear strength, whereas, kraft pulp generates longer, brighter fibers to give stronger, flexible and brighter paper products.

3.4. Carboxyl Content and Surface Charge In addition to the above mentioned properties, the charge on fibers is a well-known parameter in papermaking and plays a vital role in their performance through electronic interactions of the charged soluble and particulate fractions.[43] The electrical charges at the surfaces of the cellulose fibers are a significant parameter strongly affecting the swelling ability of cellulose fibers and provides a driving force for adsorption of retention aids, sizing agents, and strength enhancing chemicals.[47] Fiber charge is a significant characteristic of cellulose fibers, which strongly affects post-processing such as enzymatic modification.[48] The carboxyl content and surface charge of different fibers are shown in Figure 4. The carbonate pulps have relatively higher carboxyl content and surface charge than kraft. The carboxyl content of the fibers is formed due to con-

Figure 4. Carboxyl content and surface charge of different fibers produced using two different methods.

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version of the 4-O-methylglucuronic acid side groups (MeGlcA) of xylan to hexenuronic acids (HexA) by elimination of the 4-Omethoxyl group and deprotonation during peeling at alkaline conditions.[47] The carboxyl content of hemp pulp produced by carbonate was similar compared to carbonate hardwood pulp, but lower than the other carbonate pulps. However, the kraft hemp pulp contained almost similar carboxyl content like eucalyptus and hardwood but lower than the bamboo and softwood pulps produced by mild kraft. The lower carboxyl content of the kraft pulp is due to higher alkaline conditions at which the pulping was carried out which decreased reprecipitation of lignin and hemicellulose onto the surface of the fibers.[47] The relatively higher surface charge of the carbonate pulps was due to its high anionic charge density contributed by resins, fatty acids, carboxylic acid groups of the hemicelluloses, and lignin’s phenolic groups.[49] The hydrophilic carboxyl group in the fiber interacts with water and enables fibers to absorb more water to increase swelling capacity, important for the tissue paper industry.[50] The presence of greater numbers of carboxyl groups and surface charge in the fiber facilitates better entanglement by inducing hydrogen bonding to increase the tensile and burst properties of the fibers.[44,51] The carboxyl groups act as binding sites and also enables controlled chemical loading and release by regulating the interactions between the carboxyl group and chemicals.[34,35] Thus, tissue, towels and wipes with varying surface charge and carboxyl content can be produced by using different fiber types or pulping (Figure 4).

3.5. Crystallinity Indices The effect of the carbonate and mild kraft pulping on the crystalline structure was evaluated further to study the changes in crystallinity index of different fibers. It was found that the crystallinity indices of the kraft fibers were higher than that of carbonate fibers as shown in Figure 5. The amorphous nature

Figure 5. The Crystallinity Index of different fibers produced using two different methods.

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of the fiber is imparted by the hemicelluloses which is a short, linear and highly branched chain of sugars and possesses shorter side chains.[52] Thus, they induce a branched network through extensive crosslinking to lignin and cellulose by cinnamate acid ester linkages and hydrogen, respectively.[53] The kraft fibers has higher crystallinity since the process was operated at higher pH which facilitated dissolution of a higher level of amorphous cellulose. pH range at which carbonate ruptured the structural linkages between lignin and other carbohydrate fractions in the lignocellulosic biomass, but could not remove lignin as much as kraft.[54,55] As a result, the carbonate treatment of the fibers resulted in a carbohydrate heterogeneous matrix with lower crystallinity compared to kraft and a lower degree of polymerization.[47]

3.6. Hard-to-Remove Water The Hard-to-Remove water is defined as trapped water, freezing bound water, and non-freezing bound water which requires a higher energy for drying.[37] It is an important parameter as it affects the different properties of the paper products like water absorption, tensile, drying energy, and the reactivity cellulose towards chemicals.[37,44,56] Thus, the effect of the different pulping processes on the HR water content of the different feedstocks were investigated graphically represented in Figure 6.

It can be observed form Figure 6 that the hemp fiber produced using carbonate and mild kraft contained the highest levels of HR water compared to other fibers, whereas the carbonate and kraft softwood fibers contained the least amount of HR water. The hemp fiber had higher HR water than the others since it has a much higher hemicellulose content. Hemicellulose is amorphous in nature and allows more water penetration which facilitates more fiber-water interaction increasing the HR water content.[52,56] The presence of relatively higher lignin content in softwood reduces the HR water content as lignin being hydrophobic diminishes fiber-water interactions.[57] However, HR water content was higher for fibers produced

using kraft (Figure 6). The carbonate fibers, though they have less crystallinity, are associated with less HR water. This might be due to the presence of higher lignin content which limits the ability of water molecules to form hydrogen bonds with the hydroxyls of the cellulose.[58] On the contrary, kraft fibers have relatively much less lignin content which allows the cellulosic hydroxyls to interact with the water forming hydrogen bonds and increasing HR water content. Thus, kraft fibers would collapse more as shown in Figure 7 because the HR water regulates extent of fiber collapsing. The reactivity of the kraft fibers will be negatively affected as well, since HR water hinders the facile diffusion of chemical reagents by forming a hydration barrier which limits accessibility to the hydroxyl groups and reduces the reaction kinetics.[56] Thus, lower HR content of the carbonate fibers indicates that they will suffer less hornification than kraft fibers, supported by the coarseness data of the fibers as shown in Table 3 and SEM images in Figure 7.

3.7. Scanning Electron Microscopic Image Analysis SEM image analysis was done to visualize the effect of carbonate and mild kraft on fiber morphology. It can be observed form the images of the different fibers that the handsheets made from kraft suffer greater flattening than carbonate (Figure 7). These results directly relate to the coarseness and HR water of the fibers. Higher bulk of carbonate handsheets can be attributed to higher coarseness of the pulps produced by carbonate. Thus, kraft hemp pulp having the least coarseness (Table 3) yielded sheets with the least thickness (Figure 7B), whereas, the carbonate softwood showed highest thickness owing to its highest coarseness.

The kraft fibers exhibited higher HR water and as HR water leaves during drying, more hydroxyls of cellulose become available for bonding. These hydroxyls interact with each other forming hydrogen bonds which leads to irreversible flattening of the fibers resulting in higher flattening of the fibers.[56] Therefore, hemp fibers suffered the highest flattening due to its least coarseness (Table 3) and highest HR water value (Figure 6).

4. Conclusions

Figure 6. Hard to remove water content of different fibers.

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This study reports production of pulp from several fiber feedstocks after sodium carbonate and mild kraft pulping processes. The pulp yield was higher for sodium carbonate, which is a significant economic benefit because fiber is the dominant cost for producing paper products. The high yield of hemp fibers using both pulping processes and its low coarseness, low fines, and high brightness value qualify it an emerging potential resource for paper and tissue paper industries because it improves tensile, burst resistance, and softness of tissue handsheets. The lignin content was lower for pulp produced using mild kraft because it removes more lignin, which results in decreased coarseness. Higher coarseness and lower HR water content using carbonate led to less fiber collapsing, very useful for maintaining higher bulk for tissue paper products which has a direct correlation with water absorbency and bulk

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Figure 7. SEM images of different fibers produced using A) Carbonate Hydrolysis and B) Mild Kraft techniques.

softness. Moreover, a lower HR water content of carbonate also facilitates chemical treatment of the pulp at much less chemical consumption compared to kraft because the HR water acts

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as a barrier for chemical modification. The kraft pulps, on the other hand, have longer fibers, lower curl and kink index, and higher brightness, important for producing strong sheets. The

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carbonate pulps possessed higher carboxyl and surface charge which increases water uptake, a very important requirement for paper towels. Thus, carboxyl content can be controlled using different pulping methods and fiber types, which can be manipulated to control the binding capacity of the fibers and have potential use in antimicrobial tissue wipes.

Acknowledgements The work was carried out in the Department of Forest Biomaterials at NC State University. This work was performed in part at the Analytical Instrumentation Facility (AIF) at NC State University, which is supported by the State of North Carolina and the National Science Foundation (Award Number ECCS-1542015).

Conflict of Interest The authors declare no conflict of interest.

Keywords bamboo, carbonate hydrolysis, green chemistry, hemp, high-kappa kraft pulping, sustainable fibers Received: July 29, 2020 Revised: September 8, 2020 Published online:

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© 2020 The Authors. Published by Wiley-VCH GmbH

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 2, 2021

Pulp and paper mill wastes: utilizations and prospects for high valueǦadded biomaterials Adane Haile, Gemeda Gebino Gelebo, Tamrat Tesfaye, Wassie Mengie, Million Ayele Mebrate, Amare Abuhay & Derseh Yilie Limeneh.

A wide variety of biomass is available all around the world. Most of the biomass exists as a by-product from manufacturing industries. Pulp and paper mills contribute to a higher amount of these biomasses mostly discarded in the landfills creating an environmental burden. Biomasses from other sources have been used to produce different kinds and grades of biomaterials such as those used in industrial and medical applications. The present review aims to investigate the availability of biomass from pulp and paper mills and show sustainable routes for the production of high value-added biomaterials. The study reveals that using conventional and integrated biorefinery technology the ample variety and quantity of waste generated from pulp and paper mills can be converted into wealth. As per the findings of the current review, it is shown that high-performance carbon fiber and bioplastic can be manufactured from black liquor of pulping waste; the cellulosic waste from sawdust and sludge can be utilized for the synthesis of CNC and regenerated fibers such as viscose rayon and acetate; the mineral-based pulping wastes and fly ash can be used for manufacturing of different kinds of biocomposites. The different biomaterials obtained from the pulp and paper mill biomass can be used for versatile applications including conventional, high performance, and smart materials. Through customization and optimization of the conversion techniques and product manufacturing schemes, a variety of engineering materials can be obtained from pulp and paper mill wastes realizing the current global waste to wealth developmental approach..

Contact information: Biorefinery Research Centre, Ethiopian Institute of Textile and Fashion Technology, Bahir Dar University, Bahir Dar, Ethiopia Bioresour. Bioprocess. (2021) 8:35 https://doi.org/10.1186/s40643-021-00385-3 Creative Commons Attribution 4.0 International License

The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International and the PITA Annual Review, both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating.

Page 1 of 23

Article 3 – Valorization of Mill Waste

(2021) 8:35 Haile et al. Bioresour. Bioprocess. https://doi.org/10.1186/s40643-021-00385-3

Open Access

REVIEW

Pulp and paper mill wastes: utilizations and prospects for high value-added biomaterials Adane Haile*, Gemeda Gebino Gelebo, Tamrat Tesfaye, Wassie Mengie, Million Ayele Mebrate, Amare Abuhay and Derseh Yilie Limeneh

Abstract A wide variety of biomass is available all around the world. Most of the biomass exists as a by-product from manufacturing industries. Pulp and paper mills contribute to a higher amount of these biomasses mostly discarded in the landfills creating an environmental burden. Biomasses from other sources have been used to produce different kinds and grades of biomaterials such as those used in industrial and medical applications. The present review aims to investigate the availability of biomass from pulp and paper mills and show sustainable routes for the production of high value-added biomaterials. The study reveals that using conventional and integrated biorefinery technology the ample variety and quantity of waste generated from pulp and paper mills can be converted into wealth. As per the findings of the current review, it is shown that high-performance carbon fiber and bioplastic can be manufactured from black liquor of pulping waste; the cellulosic waste from sawdust and sludge can be utilized for the synthesis of CNC and regenerated fibers such as viscose rayon and acetate; the mineral-based pulping wastes and fly ash can be used for manufacturing of different kinds of biocomposites. The different biomaterials obtained from the pulp and paper mill biomass can be used for versatile applications including conventional, high performance, and smart materials. Through customization and optimization of the conversion techniques and product manufacturing schemes, a variety of engineering materials can be obtained from pulp and paper mill wastes realizing the current global waste to wealth developmental approach.

Keywords: Paper mill, Biorefinery, Pulping waste, Biomass, Biomaterials, High value-added

Introduction The demand and use of pulp and paper have marked the levels of civilization and development of many societies (Armstrong et al. 1998; Lwako et al. 2013). The pulp and paper mills mainly utilize wood sources for the production of pulp and paper. Though appropriate quantitative analysis of waste produced from pulp and paper mills has not been done yet; in general only a few percent of the wood sources are utilized for the actual pulp and paper production (Bajpai 2015b). The rest is discarded as solid *Correspondence: adaneh77@gmail.com Biorefinery Research Centre, Ethiopian Institute of Textile and Fashion Technology, Bahir Dar University, Bahir Dar, Ethiopia

and liquid waste. Considering the developing countries’ scenario both quantitative and qualitative analysis of biomass generated from the mills has not been done yet. Except for a few percent of wastepaper utilized in paper recycling the rest of the waste is left in the landfill or incinerated with no commercial significance; besides no satisfactory waste recycling scheme is implemented.

Furthermore, in developing countries, the trend towards the conversion of the wastes into valuable materials is not practiced appreciably (Bajpai 2015d).

Environmental sustainability is a priority issue as far as sustainable development of nations is progressive.

It is also a global concern recently to get rid of the factors that counteract environmental sustainability such

© The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativeco mmons.org/licenses/by/4.0/.

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as climate change, natural sources depletion, ecosystem prevention, and environmental degradation (McKinnon 2010; Bajpai 2018b; Carroll and Turpin 2002). The wastes from pulp mills one way or the other have a contributory effect on these factors and a feasible remedy needs to be designed for the overall protection of the environment for its sustainability. The pulp industry wastewater is generated from several sources like washing of raw wood materials before pulping, washing of cooked pulp and bleaching pulp, and finally, chemical recovery system (Song et al. 2019). Solid waste is produced primarily from the rejection of screening, primary and secondary sludge from wastewater management, and lime sludge from the chemical recovery system (Đurđević et al. 2020).

Currently, a large amount of solid and liquid wastes are generated by the pulp and paper mills and environmental aspects are becoming a major concern that needs intervention.

The waste generated from pulp and paper mills adversely affects the environment from different perspectives. The emissions from pulp and paper industries have a significant effect on the environment. The generated waste from the pulp and paper industry causes severe harm to aquatic life, disturbs the food chain, and also causes various health implications (Gupta et al.

2019). The effect of pollutants on the environment has been assessed many times, but their mitigation still has a significant challenge (Gupta et al. 2019). Besides, many of the developing countries do not produce pulp. These paper mills use imported pulp and waste paper as raw materials.

It is vital from an environmental and socio-economic social point of view to utilize appropriate technology to convert the wastes from pulp and paper mills to highly

Fig. 1 Yearly trend and future forecast on world paper production

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valuable products that will have an impact on everyday life. This requires a thorough analysis of generated biomass quantitatively and qualitatively and the current review is limited to envisaging the availability of biomass from the said mills, the current utilization scheme, and the potentiality for biomaterial production and diversified application. Besides protection of the environment from undesired waste impacts, utilization of the wastes will greatly support the product diversity scheme and economic growth potency (Landrigan and Fuller 2015).

Overview of pulp and paper industry Status of pulp and paper industry at a global perspective

As one of the most important industries worldwide pulp and paper mills supply varied products for around 5 billion people for versatile applications (Bajpai, 2015c). The paper is manufactured from pulp and a huge amount of trees is consumed for the pulp production with different shares by different countries in the world (Brännvall 2009, Cabreha 2017). The overall trend in the consumption of paper and paperboard by different countries and the forecasted consumption is shown in Fig. 1.

The pulp and paper industry is one of the largest industries in the world. The consumption of paper is dominated by North American, Northern European, and East Asian countries (Fig. 2) (Diesen et al. 1998; Gopal et al.

2019; Hammett et al. 2001). Other dominant countries with significant pulp and paper industries are Australia and South America. A massive and key contribution is expected from China and India in the upcoming few years.

Considering the significant variation in consumption of paper products per person from the country to country the global consumption of paper and paper board is

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Fig. 2 Global consumption of paper by major countries

estimated on average around 55 kg/person per year. The utilization of paper and board is increasing year to year though digitalization is expected to influence production and consumption in different ways. The dominant countries like North America, Europe, and Asia and low producers and consumers like African countries are still utilizing huge amounts of paper.

The key components in the paper are cellulose sheets.

There are also other constituents derived from the woody and related raw materials. The raw materials for manufacturing pulp and paper are ample and mainly consist of cellulose. Besides wood which is a chief source of cellulose, recycled paper and other cellulose bearing agricultural residues can be used as a raw material. The scenario in developing countries showed around 60% cellulose is obtained from jute, sisal, bamboo, bagasse, and similar non-wood resources.

Wood is considered the primary raw material and the major source of pulp in paper production (Eugenio et al.

2019). The gross characteristic composition of wood indicates its major constituents are cellulose, lignin, and hemicellulose. It also contains a trace amount of extractives of different types.

Numerical observation revealed that wood contains around 40–50% of cellulose (Table 1) (Sjöström and Westermark 1999). Considering ultimate properties such as strength and printability of paper it is possible to use hardwood and softwoods separately or combined in the manufacturing process. (Liu et al. 2018; Asmare 2017).

An onsite availability study has been conducted on paper mills in Ethiopia. According to the study report, 25 paper enterprises are assessed and of which 12 companies

Table 1 Composition of wood S. N

Component

% Composition

1

Cellulose

40–50%

2

Hemicellulose

25–30

3

Lignin

25–30

4

Extractives

Minor

are effective in producing different kinds of paper products with measurable production losses (Table 2).

The main aim of the preliminary study was to determine the availability of waste and possible ingredients for biomaterial synthesis. The production loss indicated the potential availability of wastes and allied potential ingredients from the mills. The availability study in the specified country ensured a potential for conversion of pulp and paper mill biomass into biomaterials. Limitation to produce pulp indigenously can also be solved for the satisfaction of the demand for paper and paper products.

Overview on pulp and paper manufacturing process

The availability of raw materials for pulp and paper manufacturing is versatile and mainly cellulosic fibers from wood and selected plants are utilized. Besides wood used most widely in many countries of the world plant resources such as sugar cane residues, residues on cottonseed, and remnants from flax and rags are also used (Osman Khider et al. 2012; Asmare 2017; Enayati et al.

2009; Jahan et al. 2004; Tutus et al. 2010). Besides, in case of scarcity of virgin raw material resources recycled paper after proper after-treatment, through appropriate

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Table 2 Status of paper manufacturing in Ethiopia Company Name

Planed production (tonnes per year)

Production loss/year (%)

Product type

Ethiopia Pulp and Paper S.C 6000

5–15

Printing and Writing Paper, Stationery Paper, Paper board, Corrugated box, Wood free and manila papers, Kraft liners, Test liners, Fluting papers

Anmol Products Ethiopia

12,000–14,000

6.5–10.8

Printing writing papers, Wood free and manila papers, Kraft liners, Test liners, Fluting papers

Yekatit Paper Converting

1400

5–10

Different paper products

DA Packaging

2000

5–10

Kraft liners, Test liners, Fluting papers, Corrugated cartons

Barguba Trading

8000

7.2–13.5

Different paper products

Mamco Paper Products

200

2–10

Paper and tissue products

Burayu Development

10,000–11,500

15–18.5

Carton boxes, Trays, Packets

Minaye Packaging

3500

2.8–12.3

Carton

Balaje Packaging

1800

3–11.2

Carton

Unlimited Packaging

20,000

4.2–15

Carton

Seven Hills Packaging

7000

2–7.6

Carton

Akless Paper Product

350

5–8.6

Different Paper products

blending and recycling process, is used in some paper manufacturing trends.

The different pulping operations and the integrated paper manufacturing process are shown in Fig. 3. Two steps are involved in the manufacturing of paper. First cellulose sheets are converted into pulp which is the intermediate material for paper production. The second step involves the conversion of pulp into the paper with the necessary after-treatment depending on the targeted end-use. The overall manufacturing process is accompanied by multiple stages which mainly involve raw material preparation and handling, controlled pulping, and discrete or continuous paper manufacturing. The pulping involves the actual making of the pulp followed by pulp washing and screening. After proper chemical recovery and bleaching, the stock is prepared. Finally, the papermaking is carried out which itself has different stages.

Pulp mills and paper mills may exist separately or as integrated operations (Bajpai 2018b; de Alda 2008; Ragauskas 2002).

Operational mechanism of pulping

Pulp manufacturing starts with raw material preparation, which includes debarking, chipping, chip screening, chip handling, and storage (Bajpai 2011, Brännvall 2009).

The pulp needs to be free of dirt and superficial materials such as bark, and debarking is done for making the pulp physically clean. The next operation in raw material preparation is chipping to ensure better handling of the logs which is done through cutting to get the required size of logs for the subsequent pulping process. Pulping requirements need to be considered in optimizing chip

handling and determining the suitable chip storage conditions (Bajpai 2015a).

The pulping operation involves the separation of the wood chips to get the fibrous materials. In the process, lignin is removed from the lignocellulosic wood and the individual cellulose fibers are isolated. Though there are different pulping methods chemical and mechanical methods are the most widely used in paper mills. Either of the three techniques namely kraft, soda, or sulfite are used in the chemical pulping approach. In all the cases the cellulose is extracted via cooking of the chips in the aqueous solutions of the reagents at elevated pressure and temperature (Bajpai, 2011, 2015a). Physical energy is used in the mechanical pulping approach which mainly involves the removal of cellulose through grinding or shredding.

The most widely used pulping process is kraft pulping and uses alkaline cooking liquor of sodium hydroxide (NaOH) and sodium sulfide (Na2S) to digest wood. In the process, a digester is used for mixing the chips with the kraft cooking solution. The kraft pulping process uses an alkaline cooking liquor of sodium hydroxide and sodium sulfide to digest the wood. The process involves the digestion of wood chips at high temperature in the range of 145–170 °C and pressure in “white liquor,” which is a water solution of sodium sulfide (Na2S) and sodium hydroxide (NaOH), for a few hours (Huber et al. 2014).

During this treatment, the hydroxide and hydrosulfide anions react with the lignin, causing the polymer to fragment into smaller water/alkali-soluble fragments and isolating the cellulose fibers.

After the wood chips have been cooked, the contents of the digester are discharged under pressure into a blow

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Fig. 3 Process flow in pulp and paper manufacturing mills

tank. As the mass of softened, cooked chips impacts the tangential entry of the blow tank, the chips disintegrate into fibers or “pulp.” The pulp and spent cooking liquor (black liquor) are subsequently separated in a series of brown stock washers (Alén 2019). After the required cooking is carried out the cooking liquor is discharged under pressure into a blow tank. The mechanistic tangential flow of the cooked chips in the blow tank isolates the chips into pulp (Yang and Liu 2005; Al-Dajani and Tschirner 2008).

After pulp production, after-treatment of pulp is carried out through washing to ensure removal of

contaminants which include uncooked chips, and recycling of residual liquor is carried out via pulp washing operations. The pulp washers separate the pulp from the spent cooking liquor during chemical pulping (Dimmel and Gellerstedt 2009, Chakar and Ragauskas 2004). After washing, screening is done to remove remaining off-size particles such as fragments from barky matters, bigger size chips, and chips that remained uncooked, and the material is delivered to the pulp bleaching process.

A very important process in the pulp mill is bleaching used for the removal of coloring and allied impurities in the raw pulp. Bleached pulp grades are used to produce

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white papers. Nearly half of the Kraft production is in bleached grades. The bleaching of pulp is carried out to increase the whiteness and brightness of the pulp (Bajpai 2013; Hammett et al. 2001). This is important for making the paper suitable for selected products such as those produced in tissue and printing enterprises. Enhancement of physical and optical qualities of the pulp is achieved by removing impurities such as entangles of fibers, barky fragments, or decolorizing the lignin making it a potential benefit in the chemical pulping process (AlDajani and Tschirner 2008, Chakar and Ragauskas 2004).

Kraft bleaching has been refined into a stepwise progression of chemical reaction, evolving from a singlestage hypochlorite treatment to a multi-stage process, involving chlorine), chlorine dioxide, hydrogen peroxide, and ozone. The optimized and controlled condition must be used in the bleaching process. If bleaching conditions are too severe there will be fiber damage, leading to a lower strength of the paper (Brännvall 2009; Angevine, 1998). After bleaching stock preparation is conducted to convert raw stock into finished stock to be used in the paper machine. Raw stock can be available in different forms. In batch processes, the delivery can be in loose pulp or bale and the integrated pulp to paper conversion suspensions can be used as well (Dimmel and Gellerstedt 2009, 2009).

In the chemical pulping process, it is necessary to recover the used cooking chemicals, and a proper chemical recovery system is needed (Gellerstedt 2009; Walker 2004). At the pulping mills, the weak black liquor also called spent cooking liquor, from the different rinsing stages is transferred to the chemical recovery which as well is situated alongside the mills (Santos et al. 2011; Al-Dajani and Tschirner 2008). The chemical recovery process is conducted at different stages to ensure proper reconstitution of the cooking liquor. Initially, the weak black liquor is concentrated followed by combustion of organic matters and reduction of inorganic matters. Finally, the cooking liquor is reconstituted in the required concentration. All the stages need critical control on the parametric allowances. The quality of paper produced from the pulp is affected by the quality of water and different companies have incorporated water treatment schemes along with the pulping process. To prevent deposits on pulp, water treatment chemicals such as antiscale agents and pitch control agents are incorporated during the route of manufacturing.

As part of pulping process dregs, lime mud, and grits are generated in the chemical recovery process. Green liquor contains sodium sulfide and sodium carbonate; and insoluble unburned carbon and inorganic impurities called dregs, which are removed in a series of clarification tanks. The green liquor is a residue of dissolution of

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the molten inorganic salts (smelt) from the black liquor concentration process (Angevine 1998, Brännvall 2009).

The lime mud is generated in the causticization process of chemical recovery in the decantation of the green liquor using calcium oxide (lime) (Ai et al. 2007). The grits are originated from the white liquor slurry in the slacker by gravity and classified as unreacted lime (Kinnarinen et al. 2016). Grits are insoluble compounds, most of which come from purchased lime, which collect in the slacker and need to be removed and disposed of.

Final pulping operations are involved in the boiler unit which causes the generation of large content of ashes.

In particular, fly ash is produced as by-products of the biomass combustion process which is generated during the high-temperature combustion of hog fuel in power boiler units (Cherian and Siddiqua 2019). As the flue gas approaches the low-temperature zones, the fused substances solidify to form fly ash. The fly ash consists of fine particulates and precipitated volatiles, typically with a high specific surface area, whereas bottom ashes tend to be coarser in texture (Cherian and Siddiqua 2019; Scheepers and du Toit 2016).

Most of the pulping operations so far especially those involving chemicals are not ecofriendly. Nowadays new pulping methods are used for manufacturing different grade pulps that utilize enzymes (Lin et al. 2018).

These enzymatic pulping techniques are being used for the preparation of dissolving pulp for the production of fibrous materials. In the enzymatically assisted pulping process xylanase, cellulase, and hemicellulase enzymes are utilized for segregation of the pulp from the rest of lignocellulosic mass and after-treatment (Lin et al. 2018; Rashmi and Bhardwaj Nishi 2010; Yang et al. 2019b). It is reported that the purity of enzymatically produced pulp is better than conventional kraft pulping processes.

Papermaking process

The process adopted for the making of paper is identical for all pulp types (Bajpai 2018b; Woiciechowski et al.

2020). The pulp from the chest compartment is screened which when required is refined to the required level. The pulp slurry is formed by mixing the refined pulp in water in a wet end operation. This slurry at the headbox is put through a paper machine and pressed by the press compartment. In the press section, the sheet forming process is commenced through the draining of the water. The next operation is passing of the formed sheet in a dryer which involved hierarchically arranged compartments of the dry end for coating and drying. The final dried and finished product is transferred to calendaring operation for reducing the thickness of paper as required and sheet surface smoothing before winding on to the take-up reels.

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Availability of by-products from pulp and paper mill

A global review of manufacturing sectors divulged that 17% of the total global waste comes from paper industries (Fig. 5) (Karak et al. 2012; Sakai et al. 1996). Pulp and paper mills contribute to air, water, and land pollution and discarded paper and paperboard make up roughly 26% of solid municipal waste in landfill sites (Pati et al. 2008; Ritchlin 2012). Waste data based on relative waste analysis from a specific global company revealed the large contribution of pulp and paper mill waste to the total solid waste indicative of a need to set up a platform for strategic intervention for the realization of a green environment (Mladenov and Pelovski 2010; Ince et al. 2011). The study reveals that from a total solid waste generation of 238,771 thousand tonnes, around 186 thousand tonnes is generated from pulp and paper industries.

Different types of solid wastes and sludge are generated in the pulp and paper mills at different sites of the process (Fig. 4). Potential wastes are generated from wood preparation and the actual pulp manufacturing and paper-making stages. Furthermore, an ample amount of different types of wastes are generated from chemical recovery, effluent treatment, and paper manufacturing methods that utilize recycled paper routes (Camberato et al. 2006; Akbari et al. 2018).

Fig. 4 Sites of waste generation in pulp and paper manufacturing

Page 7 of 22

Among different factors that affect the identity and amount of generated waste from pulp and paper, mills is the type of manufacturing process amplified with the wastewater treatment technologies adopted. The pulp and paper mill waste mainly includes rejects at different stages such as woody and barky residues and sand particles, black liquor, and wastewater sludges. Inorganic sludges are isolated from the chemical recovery station and are composed mainly of calcite lime mud, the slacker grits, and green liquor dregs (Bird and Talberth 2008; Environmental 2007; Leponiemi 2008).

The sources of wastewater treatment residues as part of potential sludge are conveyed from two sources. The major part is the primary sludge from the entire manufacturing route and those generated from the secondary clarifier are categorized as biological sludge (Simão et al. 2019). Also, the sludge from the water treatment is regarded as chemical flocculation sludge. Deinking sludge from recycled paper production which contains mainly tiny fibers or fragmented fines and additives is another source of waste (Simão et al. 2018). The major source of waste from different stages of pulp and paper manufacturing is summarized in Table 3.

The regions and different recycling processes and allied recycling rates determine the quantity of biomass generated from paper production. The variation within regions

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Table 3 Potential pulp and paper mill waste types Type of waste

Source of waste

Black liquor

Chemical pulp manufacture (kraft process)

Wood/bark residues [Saw dust and others]

All types of pulping mainly chemical and mechanical methods of pulp manufacturing

Screening and other rejects

Stock preparation for paper machine, kraft pulping, and recycling route for paper manufacturing

Semi-mechanical sludge

Effluent and water treatment

Biological sludge

Effluent treatment utilizing biological technique

Sludge from de-inking treatment

Recycled paper de-inking treatment

Table 4 Generation of solid waste from kraft mill (Gavrilescu 2004; Bajpai 2015b; Quina and Pinheiro 2020) Type of waste

Yield (kg/t of dry pulp)

Sawdust

10–30

Bark

100–300

Pins and fine

50–100

Woody fragments/wood yard/

0–20

Knot

25–70

Recovery boiler salts

5–10

Dregs

10–30

Grits

15–40

is also very wide. Due to internally established treatment and utilization routes for generated waste along with the manufacturing process by different pulp and paper mills, there is limited quantitative data on the total amount of waste generated from the mills. Estimated data on the quantity of solid waste from sample pulp and paper mill is reported. The potential woody wastes and rejects are from the preparation of pulpwood and contribute to solid waste generation (Table 4).

The predominant biomass available in large quantities in pulp mills is the black liquor. It is reported that kraft mill producing bleached paper engenders around 1.7–1.8 tonnes of black liquor per one tonne of pulp on a bone dry basis (Leponiemi 2008). This highly viscous liquor is a result of the conversion of almost half of the raw chips used in the hopper of the pulping operation. A report of IEA revealed about 170 million tonnes of black liquor per year is released from pulp and paper mills worldwide.

Another portion of the waste involves sludge of different types namely primary, secondary, and deinking sludge. From a total sludge of 40–50 kg per tonne of the virgin paper production, the primary sludge accounts for 70% and the rest 30% is the secondary sludge. On a glance approximation, the sludge generated from the

manufacturing of different paper products varies from 20 to 40% on a dry mass. The grade of paper and the specific type of process adopted for the manufacturing of the paper product determines the amount of sludge generated. (Quina and Pinheiro 2020). In comparison, the amount of waste sludge generated from industries that utilize the recycled paper approach produces a large amount of sludge than the conventional approach through virgin raw materials. From each one tonne of reclaimed paper, a total of 300 kg sludge is generated.

Depending on production capacity and the type of pulping a large amount of fly ash is released annually from pulp and paper mills (Environmental 2007; Mikkanen 2000). A recent report in Finland indicated a release of 240,000 tonnes of ash every year. As an illustration of the availability of waste biomass from pulp and paper mills, it is reported that the collective fly ash and wood ash produced from Canadian mills is 1 million tonnes per year.

Most of the pulp and paper mill wastes have an impact on the environment and create health problems. The major sources of pollutants at different stages of pulping and paper manufacturing are summarized (Table 5) (Gavrilescu et al. 2012). The environmental impact generated by the manufacture of pulp and paper results mainly from wood pulping and pulp bleaching.

In pulping processes, sulfur compounds and nitrogen oxides are emitted into the air, and during pulp bleaching, chlorinated and organic compounds and nutrients are discharged to the wastewaters (Gavrilescu et al.

2012; Singh and Chandra 2019).

The pulp paper industries release very complex organic and inorganic pollutants mainly from pulping and bleaching stages. Particularly, sulfur compounds and nitrogen oxides are emitted into the air, and chlorinated and organic compounds and nutrients are discharged to the wastewaters (Gavrilescu et al. 2012; Bank 1999). The pollutants are gaseous, inorganic, and organic type. The major pollutants include hydrogen

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Table 5 Potential pollutants from pulp and paper mill waste S. N

Main input

Process step

Pollutants

1

Raw material/wood

Wood preparation

Solid wastes and wastewater

2

Chemicals/energy

Pulp manufacture

Air emissions and used water

3

Process water

Pulp washing/screening

Dissolved material, residual chemicals, and wastewater

4

Chemicals/energy

Pulp bleaching

Air emissions, dissolved material, residual chemicals, and wastewater

5

Energy

Pulp drying

Air emissions

6

Energy/water/chemicals

Paper manufacture

Solid wastes, dissolved material, residual chemicals, and wastewater

sulfides, sodium sulfide, sulfur, chlorine dioxide, ferrous, copper, hexadecanoic acids, octacosane, phenol, and β-sitosterol. It has been reported that the different pollutants cause environmental and health impacts in terms of respiratory disorder and irritation to the skin, neurotoxicity, and direct toxicity of effluent to the reproductive system in aquatic flora and fauna are reported. Several of these pollutants are reported to contain endocrine-disrupting chemicals. Furthermore, the environmental pollution caused by the pollutants poses a threat to aquatic life as well as to plants and human beings (Bank 1999; Gavrilescu et al. 2012; Singh and Chandra 2019).

Current utilization of pulp and paper mill waste Characteristic components of major by-products

The wastes from pulp and paper mills are both of organic and inorganic types. Most of the biomass generated from pulp and paper mills is organic residues. Black liquor consists of organic polymers (lignin, polysaccharides, and resinous compounds of a low molar mass) and inorganic

compounds mainly soluble salt ions (Fig. 5a) (Viel et al.

2020).

Black liquor is separated from the pulp in washing (Fig. 5b). The biorefinery approach can be used for the conversion of lignin and hemicellulose in black liquor into biomaterials (Vakkilainen and Välimäki 2009; Vakkilainen 2016).

Black liquor being the major waste other potential wastes include sawdust, sludges of different types, waste paper, and fly ash. The major inorganic type wastes from pulping and after-treatment stage include the dregs, lime mud, and grits. The different wastes are obtained as extracted from different pulp and paper industries are summarized (Fig. 6).

The sawdust (Fig. 6a) as woody material is composed of cellulose, lignin, and hemicelluloses (Bajpai 2018a). The cellulose composition of sawdust is by far higher than the lignin content. Accordingly, the interest in biomaterial synthesis relies on the cellulose component of the sawdust. The current preferably adapted extraction of cellulose from sawdust involves the preparation of sawdust (drying, cutting, and conditioning); dissolution of wood

Fig. 5 Black liquor: a Gross structure of black liquor; b Black liquor separation

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Fig. 6 Major wastes generated from pulp and paper mills: a sawdust; b sludge; c waste paper; d inorganic wastes (dregs, lime mud, grits); e Fly ash

in appropriate ionic liquid, and precipitation of cellulose through the removal of lignin.

Another potential source of cellulose is wastewater treatment plants and recycled paper converters. The virgin or waste recycling sludge (Fig. 6b) constitutes cellulosic materials. The mill sludge accounts for the main waste from pulp and recycled paper production. Pulp and paper mill sludge is an organic residual generated from wastewater treatments (Faubert et al. 2016). Per unit of paper production 23.4% sludge is generated. The major components of the paper mill sludge are inorganic residues and organic cellulosic fines and fragments (Albuquerque et al. 2015; Abdullah et al. 2015). The organic constitutes in primary sludge mainly consist of cellulosic fibers.

A noticeable quantity of cellulose can also be obtained from waste paper (Fig. 6c) which is a release of every paper utilizer with a huge amount from office and related service providers. Directly or as part of recycling residue, the waste paper is considered as a major source of cellulose, and using appropriate cleaning and recycling procedures the cellulose can be made use of as a raw material for biomaterial synthesis (Kim and Kim 2019).

Inorganic wastes (Fig. 6d) namely liquor dregs, calcite mud, and slacker grits are also considered as the major portion of solid waste in the mills (Quina and Pinheiro 2020). These mineral wastes are generated during the chemical recovery process at the pulping mill. The green liquor dregs are obtained The inorganic biomasses contain different types of minerals (Bird and Talberth 2008; Leponiemi 2008). The mineral Gipsite and Calcite are present in the dregs and the mud, whereas the minerals

such as brucite, larnite, pirssonite, portlandite, and calcite are ingredients of the biomass grits. The mineral content of these wastes can be made administered and used for making biomaterials.

The successive development in the pulp and paper mill has headed to the release of a large quantity of fly ash (Fig. 6e) as a result of combustion processes of waste biomass. The elemental composition survey on fly ash from pulp and paper mills revealed that the ash consists of different minerals predominantly iron oxide, silica, and alumina. The fly ash also consists of other metal oxides such as oxides of sodium, calcium, magnesium, potassium sulfur, and titanium (Na2O, CaO, MgO, K2O, SO3, and TiO2) in variable quantities. Through polymerization, the metal oxides can be converted to biopolymers for diversified end-use. The widely and adopted and conventionally established way of valorization of fly ash is geo-polymerization. In the polymerization process demand-driven three-dimensional aluminosilicate and similar materials so-called geopolymers are synthesized (Cherian and Siddiqua 2019; Sjöström and Westermark 1999).

Disposal technique Waste management in pulp and paper mill

The pulp and paper mill generates a large volume of wastes with an estimate of around 100 tonnes per 550 tonnes of pulp production. Though the toxicity of pulp and paper wastes is minimal appropriate disposal technique is required for appropriate management of the land, environment, and allied issues (Monte et al. 2009).

As per the environmental policies recommend based on directives of the waste framework the landfill must be

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reduced by avoiding disposal of waste from industries. In certain cases, an obligation is placed for selected wastes to terminate as a waste resource. The wastes from pulp and paper mills have a tremendous negative and adverse impact on the environment. Inherently high consumption of water is the major cause, and the problems are wider in terms of generation of effluent and other wastes of solid and liquid type. These solid and liquid wastes along with air emissions require effective disposal and treatment approach (Bajpai 2015d).

Three different approaches can be used in pulp and paper mill waste management. The first choice is to minimize waste in ensuring product efficiency, higher yield of materials, and lower waste management (Mladenov and Pelovski 2010). The second choice is to find a suitable way for reuse to maintain cost improvement and lower environmental approach. This could be done by material or energy valorization of the generated wastes. The final option is landfilling which shall only be done when a choice is not an option.

In minimizing waste generation, it is necessary to cope up with established concrete environmental legislations. In line with this concerns related to cost of disposal, treatment technologies, and opt for new utilization schemes need to be taken into consideration. Different techniques are adopted for the reuse of the mill wastes which include burning or incineration, biofuel production, gasification, pyrolysis, and anaerobic digestion (Bajpai 2015d; Mladenov and Pelovski 2010). As far as the application is concerned; for solid wastes incineration techniques is widely used, whereas anaerobic digestion is widely practiced for wastewater. Landfilling is still the most widely adopted disposal technique.

and burnt for energy recovery (Bahar et al. 2011; Naqvi et al. 2010a). Other wastes like sawdust and woody matters are also discarded in the landfill. The destination of liquor dregs, grits, lime mud, and mill ash is the landfill after dewatering and drying.

The wastewater sludge is disposed of through a burning approach. The conventional sludge treatment process includes the thickening of the sludge waste which is dehydrated by mechanical means. The prepared sludge is incinerated at optimal conditions. In the pretreatment process blending of sludges is done with the addition of polymer and dewatering is carried out to obtain a dry solid content of 25–40%. Incineration which is the widely practiced internal route for biofuel production in pulp and paper mills is carried out. The deinking sludge from recycled paper production can be incinerated or reused in other mills (Faubert et al. 2016).

Current trends in the disposal of pulp and paper mill wastes

Table 6 Current uses of pulp and paper major wastes

In the current world pulp and paper mill practice, both landfill and incineration techniques are applied for the disposal of waste. The opt for incineration and other reuse options differ from country to country and are based on technology availability and economic growth hierarchy (Ince et al. 2011; Bahar et al. 2011). Most of the solid wastes generated from pulp and paper mills are disposed of as landfills. Landfilling with its critical limitations regarding increasing volumes and the possibility of hazardous substances is still the most widely used disposal technique by pulp and paper mills. The hazardous matters in the watercourse impose environmental dangers as well. As far as the context in most of the developing countries is concerned most of the waste is still landfilled and some are incinerated.

The black liquor is incinerated or gassed as a source of fuel energy and where the technology is not available it as well is discarded as a landfill. Rejects are dewatered

Current uses of pulp and paper mill waste

The pulp and paper mill waste have different chemical constituents and physicochemical characteristics. Based on their chemical and physical properties the wastes are widely used in conventional and industrial applications.

As most of these wastes are mainly based on woody features comprising lignocellulosic behavior they are used as fuel or energy sources for pulp and paper mill or other industries (Simão et al. 2018; Sarkar et al. 2017). Many are used for construction and building as potential and economic substitutes. The major schemes and sectorial applications followed in the current utilization of wastes from pulp and paper mill are summarized (Table 6).

S. N.

Type of waste

Utilization scheme

Industry

1

Black liquor

Energy

Pulp and paper

2

Sawdust

Furniture and building

Construction

3

Sludge

Compost

Agriculture

4

5

Fly ash

Dregs

Energy

Pulp and paper

Cement base

Construction

Binder Components

Construction

Soil Amendments

Agriculture/Forestry

Cementitious Material

Construction

Fertilizer

Agriculture

Wastewater treatment

Environmental technology

6

Grits

Building

Construction

7

Lime mud

Fertilizer

Agriculture

Building

Construction

Stabilization

Environmental technology

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The incinerated black liquor in recovery boilers is used in pulp and paper mills as an optional energy source as it provides steam. It is also possible to reclaim pulping chemicals for reuse. The current and alternative technology for reuse is the gasification of black liquor. This process has the potency to replicate the amount of production energy used in the pulp and paper mill (Naqvi et al. 2010b). The sawdust as well can be used as a secondary raw material for pulp production and recycled for paper manufacturing (Srinivasakannan and Bakar 2004). It can also be used as filler for the manufacturing of bricks and cement material in the field of construction.

The sludge from pulp and paper mills can be recycled via different routes. Mainly the sludge can be used in construction and energy and as a chief source of energy.

The suitability of sludge for utilization in power generation is the inherent compounds present in the sludge that are organic with a high degree of combustibility.

Commonly cement plants use fly ash as their raw material together with other ingredients. Fly ash is also used as fertilizer in modern agricultural practice. The suitability especially in cement application is due to the inherent high strength to weight ratio of fly ash as compared with other cement materials. This is also supplemented by carbon footprint and minimal energy consumption which makes fly ash a replacement for cement in cost-effective construction and eco-efficient geotechnical applications (Cherian and Siddiqua 2019). It can also be used as an immobilizer of pollutants. Their physical properties such as mechanical resistance and durability make

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the fly ashes used in concrete systems utilized in hostile environments.

The inorganic wastes from pulp and paper mills have been engineered for different applications (Fig. 7). Green liquor dregs have shown promising effects for correcting soil acidity and fertilizer. Dregs have also been used in wastewater treatment. The inorganic grits are mainly composed of calcium carbonate and can be used for the replacement of calcareous raw materials such as in building and construction (Quina and Pinheiro 2020).

Agriculturalists already investigated the lime mud can be used in soil healing such as for fertilizing and remedial agents. Compositionally lime mud is similar to the commercially available calcium carbonate and can be used as a potential substitute and replacement in building materials. The crude carbonate component of lime mud provides high alkalinity which makes the use of lime mud in precipitation and immobilization of heavy metals in watery streams, removal of phosphorus, and stabilization of sewage sludge.

Utilization of pulp and paper waste: future prospects

Most of the pulp and paper mill wastes in developing countries adopt recycling for lower grade paper; recycling being the only used pulp and paper mill waste utilization method. The scope and technology for utilization of the different wastes for other industrial applications are practically not available and not attempted (Raut et al. 2012; de Alda 2008). The biomass from pulp and paper mill is composed of different ingredients. The major wastes of biomaterial concern from these mills are

Fig. 7 Current utilization of pulp and paper mill inorganic wastes (GLD green liquor dregs, SG slacker grits, LM lime mud, BFA boiler fly ash)

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black liquor, woody residues including sawdust, sludges of different types, and fly ash.

An abundant quantity of the mill wastes is disposed to landfill, and some are incinerated. Instead using an appropriate biorefinery approach, it is possible to extract different ingredients from the biomass for versatile utilization as a biomaterial in different fields of applications.

Through the biorefinery approach environmental pollution is reduced or eliminated, product substitution is enhanced. Furthermore, the allied spinoff company for biomaterials and the industrial-scale prospect will boost the economic growth. The potential ingredients of current concern from the biomass are identified (Table 7).

Based on the characteristics of ingredients in each biomass potential beneficiation routes are proposed for high value-added materials (Simão et al. 2018; Rajput et al.

2012).

The general challenges with regards to the forecasted application of pulp and paper industry wastes as high value-added materials can be broadly classified into three patterns. One major challenge is the lower yield from each biomass in terms of the continuous supply of certain

Table 7 Potential ingredients available from pulp and paper mill waste S. N

Type of waste

Potential ingredients

1

Black liquor

Lignin, xylan

2

Sawdust /woody residues

Cellulose

3

Sludges

Cellulose

4

Fly ash

Minerals

Fig. 8 Major precursors of lignin (a) typical structural model of lignin (b)

waste types. The lower yield of biomaterial ingredients from selected biomass can be enhanced by the integration of pulp and paper industries especially in developing countries like Ethiopia. Another challenge which is still prominent in developing countries is the availability of suitable and integrated waste to energy and related biorefinery system (Ismail and Nizami, 2016; Diep et al.

2012). This directly affects the mass production of high valued-added materials from pulp and paper industry biomass. The other potential challenge is the adequacy of labor in terms of skill as the production of the high valueadded biomaterials requires stringent control allied with health and environmental safety concerns and regulations. There are also specific challenges in terms of each high-value-added biomaterial addressed in the review.

These challenges need critical investigation and can be amended with versatile possibilities as far as continuous marketability of the biomaterials and their high valueadded application is targeted.

Beneficiation of black liquor for high value-added materials High-performance carbon fiber from black liquor lignin

There is an estimated 70 million tonnes of lignin from pulping processes worldwide (Dessbesell et al. 2020; Bajwa et al. 2019). Structurally lignin is a heterogeneous aromatic polymer and consists of mainly three precursors: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (Fig. 8) (Chen 2012). It is a natural macromolecular with multiple aromatic ring compounds consisting of hydrophobic non-polar substructures of phenyl propane and polar groups like carboxyl.

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Several approaches have been used to purify the spent black liquor for the production of pure lignin (Hubbe et al. 2019; Xinde 1996). Acidification via precipitation is the most prominent. The extraction involved precipitation of the black liquor in acid solution, coagulation, and removal of lignin (Norberg 2012; Olsson et al.

2017; Bengtsson et al. 2018).

The biomaterial options for lignin are versatile. The modern interest in carbon fibers is very crucial, and lignin is one of the important biomasses for the production of carbon fibers. Carbon fiber is known for its extremely high strength-to-weight ratio which makes it an ideal engineering material for a load-bearing element in lightweight high-performance composites.

The versatile application of carbon fiber in the modern industry especially in composites is related to its low density and excellent mechanical properties.

High-performance carbon fibers are currently produced primarily from polyacrylonitrile (PAN). Carbonrich precursor fiber with a carbon content of more than 90% is required for the manufacturing of carbon fibers.

Once the precursor is availed the transformation is carried out in a two-stage thermal process. The delicate and rigorous web spinning process used for polyacrylonitrile-based precursors is one of the bottlenecks in the cost-effective manufacturing of carbon fiber. Half the cost of manufacturing carbon fiber is spent in making the PAN precursor (Bengtsson et al. 2019). Another potential limitation of using PAN precursors is the raw materials are fossil-based which entails a high cost.

The high price of carbon from the perspective of raw

Fig. 9 Spinning of lignin into carbon fiber (Gbenebor and Adeosun 2019)

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materials and production cost is the driving force of finding cheaper and renewable alternatives.

A potential and interesting substitute precursor for carbon fiber production is lignin. It can be utilized for the production of a high volume of carbon fibers for a multitude of applications. This is possible, because especially the lignin from kraft pulp consists of high carbon content of around 60–65% and the lignin is available in huge quantity. This also makes lignin a promising alternative precursor based on high yield after processing into carbon fiber (Olsson et al. 2017; Hubbe et al. 2019).

The lignin can be used as a potential precursor for carbon fiber production by incorporating appropriate spinning and carbonization processes. The most common way of manufacturing carbon fibers from lignin involves melt spinning to produce precursor fibers, thermo-stabilization (200–350 °C), and carbonization (over 1000 °C under nitrogen atmosphere) to produce carbon fibers (Fig. 9). Wet spinning can also be used when rheological problems are encountered in melt spinning. It is reported that greater than 35% of manufacturing cost is reduced using renewable lignin as a precursor in carbon fiber production (Bengtsson et al. 2018; Gbenebor and Adeosun 2019). The challenges in regards to lignin to carbon fiber conversion are the availability of fiber spinning technologies (Souto et al. 2018) in some countries which limit the product diversification scheme.

Bioplastics from black liquor hemicellulose

The hemicellulose from black liquor consists of xylan among other ingredients (Yang et al. 2019a; Liu et al.

2012). The polysaccharide-based xylans are made

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up of β-1,4-linked xylose residues. They consist of α-arabinofuranose and α-glucuronic acids pendant branches that contribute to cross-linking of cellulose microfibrils and lignin through ferulic acid residues.

Xylan can be extracted from several locations in the pulp mill. During kraft pulping, xylan is partly dissolved in the cooking liquor but part of it will be redeposited onto the fibers in the later parts of the cook. The dissolved xylan can be isolated from black liquor for its utilization in the production of different biomaterials. Among the different methods used for xylan extraction, those utilizing alkali are most commonly and widely used.

Xylan is constituted by the black liquor along with fractions lignin fractions. The extraction of xylan from black liquor requires two-step precipitation. First, black liquor acidification is conducted and this is followed by precipitation by ethanol which ultimately provides the fractions of xylan (Stoklosa 2014). The xylan hemicellulose separation is done using potassium hydroxide (KOH) for hardwood and sodium hydroxide for softwood in aqueous media. The mechanism of separation of the hemicellulose is achieved through hydrolysis which involves the breakage of ester linkages between xylan and other alkaline black liquor components.

Xylan can be biorefined into lactic acid (Fig. 10).

First, xylan is converted to low molecular weight sugars by hydrolysis using enzymes or chemically using acid hydrolysis. Then the resulting low molecular weight sugars can be converted to lactic acid by fermentation or alkali oxidation. Biomass-derived lactic acid is an important renewable chemical building block for synthesizing bioplastics (Fernández-Rodríguez et al. 2019; Chen

Fig. 10 Xylan to PLA route

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2012). Polylactic acid (polylactide) used as a precursor for plastics can be obtained by polymerization of a lactic acid refinery of xylan.

Polylactic acid (polylactide) [PLA] is biodegradable as well as recyclable polyester made from renewable feedstock. PLA is synthesized by the condensation polymerization of lactic acid or ring-opening polymerization of the corresponding lactide. As one of the highly biodegradable and crystalline polymers the lactic acid polymer (PLA) has a high melting point and outstanding mechanical characteristics (Gordobil et al. 2015; Singhvi et al.

2019). It can be used for bioplastic manufacturing used for different engineering applications. Biomaterials of versatile need can be obtained from PLA using advanced manufacturing techniques such as electrospinning, especially for medical products. The major challenges in conversion and utilization of xylan are of two ways. One is the difficulty in obtaining fully purified xylan with the grade required for polymer production and the other is the lower melting pint of the PLA produced which limits its application for high-performance materials. Different researches are ongoing especially in maximizing extraction effectiveness of xylan and usage of modified polymerization and spinning technologies for manufacturing higher grade PLA products.

Beneficiation of cellulose for high value-added materials Production of cellulose nanocrystals [CNC] from cellulosic residues

Nanocellulose is a unique and promising natural compound derived from ordinary biomass. It is currently the most environmentally friendly compound that is

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techno-feasible and cost-effective, and also reduces effluent production (Clemons 2016). Nanocellulose has retained significant attention due to its tremendous functionality, i.e., greater surface chemistry, extraordinary biotic possessions, low toxicity, low cost, lower density, and significant mechanical properties. Cellulose Nanocrystals have many important physical properties Nanocrystalline cellulose has high heat stability which makes it suitable as a potential engineering material for aggressive temperature environments and its morphology with regards to small size and shape can be managed for different applications in solutions (Feng et al. 2015; Aguayo et al. 2018). The cellulose from the different waste sources in pulp and paper mills can be converted into cellulose nanocrystals (CNC) for wider application as a biomaterial in diversified fields of application (Fig. 11).

CNC is needle-shaped and highly crystalline material produced from cellulose pulp (Clemons 2016). The outlooked application of CNC is very wide and mostly its use in medical and industrial fields is tremendous. CNC can be used for the fabrication of medical products such as artificial skin, breast implants, versatile hygiene products, tissue-engineered materials, wound dressings, and drug delivery biomaterials. Industrially CNC can be used for the automotive interior, cosmetics industry, acoustics/ photonics, build-tech, and different packaging materials.

Cellulose fiber is a key and characteristic component in pulp and paper mill sludge and is also constituted by sawdust and other woody residues (Aguayo et al.

2018). Cellulose is an important biopolymer that consists of semicrystalline regions which are responsible

Fig. 11 Multitude application of CNC biomaterial

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for the outstanding mechanical properties of the polymer. Cellulose as macromolecule is composed of many cellobiose repeat units which themselves are made up of glucose monomeric units (β-glucose) via β-1,4glycosidic linkages. The individual extended cellulose chains are parallel to each other and the inherent stability and crystallization of cellulose polymers are due to the presence of extensive intramolecular and intermolecular hydrogen bonds.

In pulp and paper mills different conventional processes are used to recover cellulose from wastewater.

The process of separation of cellulose from dried sludge is done using ionic liquid-based segregation techniques which involve cellulose precipitation from the sludge (Gibril et al. 2018). Direct extraction and utilization of cellulose from sawdust and woody matters are also possible.

The production of CNC from cellulose is an emerging possibility for the diversified utilization of trees for biomaterials (Fig. 12). A biorefinery approach can be used to convert cellulose from pulp and paper mill waste into nanocrystalline cellulose (Souza et al. 2017). The production of nanocellulose is attained by a two-step process (Clemons 2016). In the first step, the pretreatment process of native cellulose biomass is done which yields treated cellulose fibers. While in the second step, pretreated cellulose fibers are converted into nanocellulose using various routes, e.g., high-pressure homogenization, micro fluidization, micro grinding, high-intensity ultra-sonication, electrospinning, and steam explosion.

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Fig. 12 Extracting CNC from trees

Method of synthesis of CNC generally adopts breakdown of cellulose chains to the desired nanoconfiguration via different techniques. The chemical and biological techniques utilize different chemicals and enzymes, while the physical technique involves the mechanical breakdown of the cellulose chains. The chemical process mainly involves hydrolysis using acids and alkalis or a series of reagents by the partial breaking of glucosidic bonds for obtaining the CNC. The four steps in the synthesis of CNC via mechanical method are high-pressure homogenization, fluidization to micro-level, grinding to fine particles, and finally smashing under freezing conditions. Conventionally combined methods are utilized for optimized processes and characteristics of the cellulose nanocrystal (Song et al. 2019). The biological and combined methods of the CNC synthesis method employed for hydrolysis minimize concerns with regards to environmental pollution. The production technique of CNC is well-practiced; ensuring high-grade purity of the cellulose with well-advanced extraction technology could eliminate the associated challenges in terms of high value-added products especially those used for medical items.

Synthesis of textile fibers from dissolving pulp

Dissolving pulp has versatile applications. Dissolving pulp can be used as a source of pulp for the paper mill (Kihlman 2012). This is particularly important as economic support for developing countries that are entirely based on imported pulp. Besides many chemicals used

in washing and value additions in wet and chemical processing industries such as detergents, softeners, and binders can be produced using dissolving pulp as a precursor. These chemicals are cellulose ether-based and can be extracted from the dissolving pulp.

Dissolving pulp besides its utilization as raw material for the paper mill and surfactant synthesis it can be used for the manufacturing of textile fibers (Ma et al. 2011).

The dissolving pulp can be prepared from cellulosic wastes using the same pulping route described in the current review and can be utilized for the making of viscose rayon and cellulose acetate fibers (Woodings 2001). Once the required grade of dissolving pulp is availed viscose rayon can be manufactured using wet spinning technology and cellulose acetate is manufactured using dry spinning technology. The manufactured viscose rayon and acetate fibers have diversified applications (Fig. 13).

The use of dissolving pulp as a precursor for the manufacture of commodity textile fibers has an economic impact on developing countries that entirely depend on using a single fiber such as cotton for the production of textile materials. In the region of concern, where the supporting data for availability study is conducted it has been described that the paper mill entirely depends on the imported pulp and it is not possible to produce and market the important textile fibers. It is vital to consider the economic advantage of producing potential fibers from indigenous pulp and paper wastes. Modern manufacturing techniques such as dry jet wet spinning and electrospinning techniques can be utilized for the

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Fig. 13 Dissolving pulp beneficiation: regenerated fibers

manufacturing of different grades of fibers, especially for high-performance applications. The major challenge in the utilization of pulping waste as a precursor for textile fibers is the limited availability of fiber manufacturing units in areas, where resources are abundant; and associated yield-related problems for producing textile fibers as per the global demand.

Biocomposites from pulp and paper mill waste

The global concerns for a safe environment and risk minimized livelihood of society are high. With this regard issues that need immediate intervention are the highest priorities of global nations. As far as feasible interventions are concerned focus needs to be given on increasing cost of petroleum and the allied depletion and frameworks in line with new environmental regulations.

To this end engineering materials which have a vital impact in replacing existing petroleum-based materials and which are capable of addressing the environmental legislations are required. This was the rationale behind seeking eco-friendly green substitutes especially biocomposites for modern engineering applications (Soucy et al.

2014; Manesh 2012).

Renewable lignocellulosic materials are suitable for the reinforcement of polymers and provide biocomposites for the relevant industry. Such kinds of production trends provide relief with regards to problems encountered in using petroleum-based composites. These frequently available resources provide a remedial solution with regards to the production of attractive, sustainable, cost-effective, and eco-friendly materials so-called

biocomposites with a safe environment as a result of preferred disposal and reuse options (Schorr et al. 2014).

The fly ash and other mineral-based wastes from pulp and paper mill are underutilized and fully discarded especially in developing countries (Novais et al. 2018).

The fly ash is entirely composed of metal oxide minerals.

Though it is not a concern of the present review the grits, dregs, and lime muds have also plenty of mineral content for specific biomaterial applications. The biomaterial which can be obtained by a suitable biorefinery technique from fly ash can be used for versatile engineering applications as a biocomposite. The prospective application of fly ash as a biomaterial is by converting the minerals into their polymer counterpart via geo-polymerization which normally are referred to as geopolymers (Mohammadkazemi 2018; Yoon-moon and Naik 2005).

Depending on the end-use the geopolymers can be incorporated in manufacturing composite materials mainly as the reinforcing component of the biocomposite. A polymer of fly ash that can be used for biocomposite is synthesized using predefined steps. The major step involves treatment with highly alkaline liquors such as using aqueous caustic soda in combination with silicate compounds such as sodium silicate in a process called alkali activation (Saeli et al. 2019; Rajamma et al. 2012).

The highly alkaline environment will lead to the breaking of silica and alumina bonds in the fly ash which provides conditions for the dissolution of free silicon and aluminum ions. The reaction between the free silicon and aluminum ions with active alkali ions leads to the formation of an intermediate precursor which up on

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precipitation reorganize into the polymeric three-dimensional aluminosilicate structures.

Besides fly ash, sawdust from the pulp and paper mill waste is another potential source for the manufacturing of biocomposite (Zhang et al. 2020). Sawdust which mainly constitutes cellulose can be used in blending for synergistic enhancement of the reinforcement of the composite structures. The inherent biodegradability made it a preferred raw material in biocomposite applications (Zhang et al. 2020; Jiang et al. 2009).

Through advanced composite engineering and characterization, different biocomposites can be manufactured from sawdust and fly ash and modeled for a multitude of applications. The diverse applications of biocomposite from fly ash and saw dust-based biomaterials are summarized (Fig. 14). Both sawdust and fly ash-based biomaterial provide high heat resistant biocomposite suitable for high-performance applications. The biocomposite from sawdust cellulose can be used in the manufacturing of automotive interior, different boards, furniture, packaging, and so on, whereas fly ash based biocomposite can be used in the production of civil engineering materials mainly in construction technology (Zhang et al. 2020; Akampumuza et al. 2017; El‐Meligy et al. 2004). In both fly ash and sawdust systematic collection of sample raw material for biorefinery needs attention as they mostly are lightweight and interfere with a feasible collection.

Improper management of sample collection could result in disturbance of the working environment and personal safety through inhalation needing high priority of conditioning at work stations.

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Conclusions Pulp and paper mills release a large amount of waste globally. Most of these wastes are directly disposed to landfills with minimal incineration and nil recycling.

Though there are trials for the reuse of these wastes in some of the developed world the trend is almost none in developing countries. The present review has shown that the pulp and paper mill biomass; besides their conventional use, can be converted into biomaterials that have high value for versatile applications. It is revealed that the biomass generated from the mills provides ingredients for the synthesis of biomaterials such as lignin, hemicellulose, cellulose, and various minerals. The paramount importance of biomass ingredients is that they can be converted into high value-added biomaterials using an appropriate biorefinery system. With this regard, a possibility is observed that potentially important engineering materials such as carbon fiber, bioplastic, and fibers, CNC, and biocomposites can be manufactured from waste biomass for diversified engineering applications.

The synthesized biomaterials through appropriate and feasible technologies will be useful for the manufacturing of versatile bio-based products that are used in widespread conventional, high-performance, and smart applications. Future study will entail extensive research and development work to develop appropriate technologies for their full utilization and commercialization as a source of some of the proposed applications mentioned in this review. In so doing dual the impact of the newly investigated materials is realized both from the economic point of view through product diversification

Fig. 14 Possible applications of biocomposites from pulp and paper mill waste

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and environment aspects in protection and hazard minimization.

Acknowledgements The authors would like to acknowledge the Higher Education and TVET program Ethiopia-Phase 3, PE479-Higher Education, KFW project No. 51235, and BMZ No. 201166305 for the financial support of this research.

Authors’ contributions AH, GGG, TT, WM, MAM, AA and DYL of this review paper have directly participated in the planning, execution, or analysis of this study with the due role of the corresponding author in overall coordination as well. All authors read and approved the final manuscript.

Funding The prerequisite funding support is from the Higher Education and TVET program Ethiopia, KFW Project, and Ethiopian Institute of Textile and Fashion Technology, Bahir Dar University. No other funding.

Availability of data and materials All data and materials are availed in the manuscript and no additional input is required.

Declarations Ethics approval and consent to participate Not applicable.

Consent for publication Not applicable.

Competing interests The authors declare that they have no competing interests.

Received: 28 January 2021 Accepted: 19 April 2021

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PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 2, 2021

Fractionation of pulp and precipitated CaCO3–pulp composites: effects on sheet properties of selective CaCO3 precipitation onto fiber size fractions Teija Laukala, Kaj Backfolk & Isto Heiskanen.

CaCO3-pulp composite was prepared via precipitation of calcium hydroxide in the presence of pulp. In order to investigate the precipitation selectivity and mechanism, the substrate pulps and the obtained composites were fractionated (R30, R100, R200, R400 and a sedimented fraction that passed the 400 mesh wire) using a Bauer-McNett unit. The main fractionation criterion was therefore fiber length. The pulp used was CTMP (chemithermomechanical pulp), yielding a precipitated calcium carbonate-chemithermomechanical pulp (PCC-CTMP) composite with a targeted PCC-to-CTMP ratio of 1:1. The PCC consisted primarily of nanosized primary particles which formed aggregates and clusters on the fibers. When the fiber morphology, zeta potential and surface charge density of the fractions were determined, a correlation was found between the surface charge density of the CTMP and the ash content of the corresponding PCCCTMP fractions. This supports the hypothesis that the precipitation on the CTMP fiber is driven by the charge interparticle interaction. The use of refined CTMP furnishes and fractionation of the PCC-CTMP furnishes demonstrates that PCC is preferably fixed on fines and fibrils since it appears at a higher content in the fines fractions.

Fiber activation via fiber split, removal of primary wall and surface defibrillation enhanced the affinity of the PCC for the fibrils. The laboratory handsheets prepared from the material demonstrated the importance of controlling the substrate fiber properties for the mineral-fiber composite, e.g. via refining, as differences between the refining levels and fractions were found to lead to differences in both optical properties and bonding.

Contact information: T. Laukala: Lappeenranta-Lahti University of Technology Packaging technology of LUT School of Energy Systems, P.O. Box 20, 53850 Lappeenranta, Finland K. Backfolk & I. Heiskanen: Stora Enso OyjResearch Center Imatra, 55800 Imatra, Finland Cellulose (2021) 28:5807–5826 https://doi.org/10.1007/s10570-021-03856-0 Creative Commons Attribution 4.0 International License

The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International and the PITA Annual Review, both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating.

Page 1 of 21

Article 4 – PCC-pulp Composites

Cellulose (2021) 28:5807–5826 https://doi.org/10.1007/s10570-021-03856-0

(0123456789().,-volV) ( 01234567 89().,-volV)

ORIGINAL RESEARCH

Fractionation of pulp and precipitated CaCO3–pulp composites: effects on sheet properties of selective CaCO3 precipitation onto fiber size fractions Teija Laukala

. Kaj Backfolk . Isto Heiskanen

Received: 22 December 2020 / Accepted: 29 March 2021 / Published online: 11 May 2021 Ó The Author(s) 2021

Abstract CaCO3-pulp composite was prepared via precipitation of calcium hydroxide in the presence of pulp. In order to investigate the precipitation selectivity and mechanism, the substrate pulps and the obtained composites were fractionated (R30, R100, R200, R400 and a sedimented fraction that passed the 400 mesh wire) using a Bauer-McNett unit. The main fractionation criterion was therefore fiber length. The pulp used was CTMP (chemithermomechanical pulp), yielding a precipitated calcium carbonate-chemithermomechanical pulp (PCC-CTMP) composite with a targeted PCC-to-CTMP ratio of 1:1. The PCC consisted primarily of nano-sized primary particles which formed aggregates and clusters on the fibers. When the fiber morphology, zeta potential and surface charge density of the fractions were determined, a correlation was found between the surface charge density of the

CTMP and the ash content of the corresponding PCCCTMP fractions. This supports the hypothesis that the precipitation on the CTMP fiber is driven by the charge interparticle interaction. The use of refined CTMP furnishes and fractionation of the PCC-CTMP furnishes demonstrates that PCC is preferably fixed on fines and fibrils since it appears at a higher content in the fines fractions. Fiber activation via fiber split, removal of primary wall and surface defibrillation enhanced the affinity of the PCC for the fibrils. The laboratory handsheets prepared from the material demonstrated the importance of controlling the substrate fiber properties for the mineral-fiber composite, e.g. via refining, as differences between the refining levels and fractions were found to lead to differences in both optical properties and bonding.

T. Laukala (&) Lappeenranta-Lahti University of Technology Packaging technology of LUT School of Energy Systems, P.O. Box 20, 53850 Lappeenranta, Finland e-mail: teija.laukala@lut.fi K. Backfolk I. Heiskanen Stora Enso OyjResearch Center Imatra, 55800 Imatra, Finland

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5808

Cellulose (2021) 28:5807–5826

Graphic abstract

Keywords PCC Precipitated calcium carbonate CTMP Improved filler Composite filler In-situ precipitation

Introduction Calcium carbonate, CaCO3, is a commonly used filler or pigment in many industrial applications including ceramics, composites, paints, plastics and paper.

While the effects of typical fillers and filling mechanisms on paper properties are fairly well understood and widely discussed in the literature, e.g. by Hubbe and Gill (2016), work on different filler dosing concepts and modified fillers is still in progress and requires more attention. In a review paper by Shen et al. (2009), different approaches for filler modification were explored and discussed, for example the use of inorganic substances to improve acid tolerance, the use of organic polymers to enhance filler-fiber bonding, and filler modification with cellulose and cellulose derivatives. The traditional way of adding fillers has its limitations, and alternative concepts such as in-situ precipitation onto fibers or in-line precipitation have therefore recently gained interest.

One approach where CaCO3 is used as a filler is through in-situ precipitation (IS-precipitation), i.e. the synthesis and precipitation of calcium carbonate (PCC) in the presence of the cellulosic fiber, where the conditions of precipitation can be controlled and adjusted so that the PCC is precipitates directly onto cellulosic material (Rantanen 2016; Silenius 2002; Subramanian 2008) or into the lumen and cell wall

123

pores (Ciobanu et al. 2010; Fortuna et al. 2003).

Mechanisms have been proposed to explain the PCC attachment onto the cellulose, but results and theories are controversial and there is still a lack of understanding of the fixation and adhesion mechanism.

Subramanian (2008) suggested that the precipitation proceeds via an amorphous calcium carbonate (ACC) precursor which forms line-like aggregates on the cellulosic particles due to inter-particle interactions (due to polarization of the ACC particles), and is subsequently precipitated as calcite. Silenius (2002), on the other hand, suggested a heterogeneous nucleation based on classical crystallization theory. The crystallization on cellulose has been associated with the functional groups capable of binding metal ions (including Ca2?), which is thought to lead to subcritical nuclei being formed on the cellulosic substrate, and subsequent nucleation (Dalas et al. 2000).

Silenius (2002) pointed out that the PCC precipitates mainly on the cellulosic fines due to the large surface area of the fines in comparison to that of the larger fibers. ‘Fines’ is usually defined as the material that passes the 200 mesh wire of Bauer McNett classifier (Heinemann and Vehniäinen 2009). Silenius’ (2002) result has been supported by Seo et al.

(2017). Wood pulp fines also contain more carboxyl groups, metal ions and negative charges than the larger fibers (Odabas et al. 2016). This suggests that the preferential precipitation of PCC onto fines may be partially due to the charged, metal-binding groups.

The importance of functional groups has also been observed with synthetic polymers. For example, Lakshminarayanan et al. (2003) precipitated PCC insitu onto both untreated and chemically treated

Cellulose (2021) 28:5807–5826

synthetic polyamide fibers (Nylon 66 and Kevlar 29) and reported that the nucleation density was greater on the treated fibers, confirming the importance of charged sites. On the other hand, recent findings have demonstrated that PCC can also be precipitated on essentially uncharged fibers such as bacterial nanocellulose (Stoica-Guzun et al. 2012) or polyvinyl alcohol fiber (Park et al. 2017).

The formation of the PCC-fiber composite makes it possible to create a novel platform of engineered fillers which may provide new features for the papermaking process and end products. The surface morphology and chemistry of the fibers are altered by the mineral precipitation process, which can be used to enhance the material property space and to control optical properties, fiber-fiber contacts and bonding in sheets.

The effects on sheet properties have often been contradictory when filling is done using IS-precipitated CaCO3. For example, sheet opacity has been reported to increase by Ciobanu et al. (2010), and to decrease by Klungness et al. (2000) and Kumar et al.

(2009). The brightness has been reported to increase

by Mohamadzadeh-Saghavaz et al. (2014) and to decrease by Klungness et al. (2000). Similarly, the tensile index has been reported to increase by Silenius (2002) and to decrease by Mohamadzadeh-Saghavaz et al. (2014).

One possible reason for the contradictory findings and statements is the use of different raw materials. A thorough review of the role of fiber source and particularly of the dependence on size fractions and fiber size in the precipitation mechanism and nucleation is lacking. Also identically performed ISprecipitation has been found to result in different PCC-fiber properties depending on the fiber source (Klungness et al. 2000).

In general, the presence of fines increases inter-fiber bonding in sheets. Fines affect many strength properties and especially the tensile strength (Odabas et al.

2016), although the strength-enhancing effect is greater with kraft fines than with mechanical fines (Retulainen et al. 1998). Light scattering, important for optical development of sheets, is instead increased by the presence of mechanical fines, while kraft fines tend to reduce it (Odabas et al. 2016). This is explained by the non-bonded surface of the mechanical fines (Leskelä 1998). The assumed preferential precipitation of PCC onto fines can be expected to alter the properties of the fines fraction more than those of the

5809

coarser fiber fractions. The preferential precipitation might reduce the strength-enhancing properties of the fines fraction, although the optical properties would be positively affected.

In the present work, PCC was precipitated onto refined and unrefined chemithermomechanical pulp (CTMP) in order to study the role of CTMP composition and the precipitation of PCC onto the CTMP fines. The CTMP pulp, at three different refining levels, was then used for in-situ PCC precipitation and for making PCC-CTMP composite filler. The PCCCTMP composite fillers were subsequently fractionated in order to gain a further understanding of the selectivity and precipitation efficiency of PCC onto each fraction. The effect of the different PCC-CTMP fractions on sheet properties were evaluated. The CTMP fiber was chosen because of its broad fiber size distribution and sufficiently high fines content, while primary wall parts still attached to some of the fibers.

Materials and methods The CTMP was provided by Stora Enso (Kaukopää mill Imatra, Finland) as air dry sheets, which were disintegrated and refined using a Valley beater (SCAN-C 25:96). The Schopper Riegler freeness values (SCAN C 19:65) of the pulps were 23 (0 min refined), 42 (30 min refined) and 58° SR (60 min refined). The fiber length distributions of the CTMP materials are shown in Fig. 1. The calcium hydroxide (Ca(OH)2) was prepared from calcium oxide (CaO, Honeywell reagent, purity [ 96 %).

Sample preparation PCC-CTMP preparation The three grades of PCC-CTMP were prepared by beating the pulp to three different levels. Each grade was prepared by reacting Ca(OH)2 and carbon dioxide (CO2 (AGA, purity C 99.8 %)) in the presence of CTMP fibers in a 10 l open batch reactor. The reactor height was 50.6 cm and inner diameter 16.2 cm. The reactor was equipped with a gas outlet at the bottom and a stirrer with three impellers, 8.4 cm Roushton which was the lowest and two 8.5 cm pitched blade

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5810

Cellulose (2021) 28:5807–5826 CTMP 0 min

CTMP 30 min

CTMP 60 min

9 8

Frequency [%]

7 6 5 4 3 2 1 0 50

500 Fiber length [μm]

5000

Fig. 1 Fiber length distributions of CTMP: 0 min refined (23° SR), 30 min refined (42° SR) and 60 min refiner (58° SR). The values given are weighted with respect to fiber length

impellers. The impellers were placed 16 cm distance from each other.

The batch size was 9.5 l and the fiber consistency 1.3 wt% before precipitation. For each batch, the depletion of Ca(OH)2 was determined by the decrease in pH. At the beginning of the reaction, the temperature was ca. 20 °C, but it was allowed to change freely during the exothermic precipitation process.

The CO2 feed was 3 l/min and the target Ca(OH)2 dose (before precipitation) was 1.0 wt%, corresponding to 50 wt% CaCO3 filler (ash) content assuming 100 % conversion.

The Ca(OH)2 used was prepared from CaO. Slaking was carried out for 4 h. The CaO-to-H2O ratio used was 1:9. The initial temperature was 40 °C, but the temperature was allowed to change freely.

Pulp fractionation The pulps (CTMPs and PCC-CTMPs) were fractionated using a Bauer-McNett classifier according to the SCAN-M 6:96 standard, with the exception of the dosed fiber weight. In case of PCC-CTMP, 20 g of solids (CTMP and PCC combined, containing approximately 10 g of CTMP), were used in the fractionation. In the case of the CTMP, the amount was equal to that used for the corresponding PCC-CTMP, i.e. close

123

to 10 g. The wires used during fractionation were 30, 100 and 200 of the ASTM series (nominal openings 595, 149 and 75 lm, respectively) and 400 mesh wire with a measured opening of 25 lm. The fractions were labelled to indicate residuals. For example R30 refers to fraction retained by 30 mesh wire. The scheme of experiments is shown in Fig. 2.

A minimum of seven fractionations were carried out for each pulp. Five were done to collect fractions in separate vessels for further use, and two to weigh the fractions and measure their ash. To do so the fractions were collected on ashless filter paper (MachereyNagel MN 640 m) and dried in an oven at 105 °C prior the ash content determination. TAPPI standards T 211 om-02 and T 413 om-11 were used in conjunction to determine the ash content.

Handsheet preparation Handsheets were prepared according to the SCAN C 26:99 standard with the exception of the drying (drum, surface temperature ca. 60 °C) and the use of an auxiliary wire (90 gsm polyester satin, thread width approximately 300 lm). The auxiliary wire was placed on the sheet mould wire and removed after wet-pressing.

Analytical procedures Fiber analysis The CTMPs, PCC-CTMPs and fractions were analyzed using the L&W Fiber Tester (Lorentzen and Wettre, Kista, Sweden) using a dilution of approximately 0.1 g fibers in 100 ml of water. Fiber length, fiber width, shape factor, the amount of fines and the kink index were determined and recorded as lengthweighted averages.

The ash and CaCO3 contents of the samples were measured according to TAPPI standards T 211 om-02 and T 413 om-11 in conjunction.

Zeta potential The zeta potentials of the pulps and fractions were measured using the streaming potential method (Mütek SZP-06, BTG Instruments GmbH, Herrsching, German). The conductivity of the samples was increased to 0.30 ± 0.01 mS/cm with 0.1 M NaCl

Cellulose (2021) 28:5807–5826

5811

Fig. 2 Scheme of experiments. In the fractionation, the values refer to wire meshes used (ASTM series). The R30, R100, R200, and R400 fractions were collected from the wires and vessels

after the fractionation. An additional sedimentation phase was added after the fractionation, and this yielded the ‘pass’ fraction.

The bold text indicates the samples taken for further analysis

solution at least 18 h before the measurements were made.

Some of the coarsest fractions could not be measured according to the recommended procedure due to the poor water retention of the fiber cake and resulting water overflow. A filter paper (MachereyNagel MN 616) was therefore used, as instructed in the manual. Use of the filter paper was however found to affect the absolute zeta potential value.

Scanning electron microscopy

Surface charge density (cationic-demand titration) The charge density of the pulps and fractions was measured using a cationic-demand titration procedure (PCD 02, BTG Mütek GmbH and Mettler DL25, GWB for titrant dosing). Titration was done as back titration as recommended by the manufacturer, although the amount of sample was modified.

The amount of fiber slurry containing approximately 0.2 g of fiber was filtered on a polycarbonate membrane (GE Water & Process technologies, 3.0 lm) and rinsed into a decanter using deionized water to obtain a 5.0 g sample. The amount of poly(diallyldimethylammonium chloride) (PDADMAC, 1.0 mN, M = 107 000 g/mol, BTG) solution added was 20 g. Fibers were removed from the residual PDADMAC by centrifugation (6400 g).

Titration was carried out using polyethene sodium sulfonate (PES-Na, 1.0 mN and M = 19 100 g/mol, BTG). After the titration, the fiber weight was determined by drying the fiber overnight at 105 °C. The results are given as averages of three or four measurements.

The PCC-CTMP samples were characterized by scanning electron microscopy (SEM) using a Hitachi SU3500 equipped with a wolfram filament and backscatter electron (BSE) detector to capture micrographs of the samples. Composition (COMP) mode was used in order to obtain good contrast between PCC and the cellulosic fiber; the PCC particles or PCC rich areas therefore show lighter than areas free from PCC. The imaging was done on non-coated samples using the variable pressure (VP-SEM) mode at 60 Pa, 15 kV acceleration voltage and a working distance of 10 mm. These images are labeled with the text VPSEM. The coated samples (sputter with an Au/Pd target) were imaged using secondary electron imaging (labeled SEM in the images), an acceleration voltage 15 kV and a working distance of 10 or 15 mm.

Handsheet testing The handsheets were conditioned (23 °C and 50 % RH) and tested according to the SCAN standards SCAN P6:75 (grammage), SCAN P 7:96 (density and thickness), and SCAN P 38:80 (tensile index, tensile stiffness, and elastic modulus). The tensile and tensile stiffness indices were additionally calculated using a fiber-weight-based index, i.e. the ash weight was subtracted from the grammage used in the calculation.

When the fiber-weight-based index is discussed, this is indicated in the text.

Brightness and opacity were measured using Elrepho (L&W) according to standards ISO 2470-1

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Cellulose (2021) 28:5807–5826

(brightness) and ISO 9416 (light scattering coefficient (sr), light absorption coefficient (kr) and calculated opacity for 65 g/m2 sheet), with the exception of measured samples (4 test pieces instead of 10).

Results Fractionation of CTMP and PCC-CTMP The different CTMP and PCC-CTMP furnishes were fractionated and characterized as shown in Table 1.

With increasing refining, the fines content increased in both CTMP and PCC-CTMP. In the case of the PCCCTMP, the ‘pass’ fraction corresponded to 40–50 % of the total dry material of the sample, which was significantly greater than for the CTMP. The ‘pass’ fraction was mainly PCC, as indicated by the high ash

Table 1 Proportions of unfractionated pulps and pulp fractions as mean values with 95% confidence intervals. The amount of CaCO3 was calculated based on the 525 °C and 900 °C ash content measurements, i.e. the approximate percentage of ash that was CaCO3. Values are also given for CTMP, although the samples contained no added CaCO3

content (80–85 wt% of all the ‘pass’ fraction samples).

Most (84–89 wt%) of the total PCC was found to pass the finest 400 mesh wire.

When the cumulative residual was determined, excluding the fraction passing the 400 mesh wire (see Fig. 3), precipitation was found to affect the distribution: the fractions of smaller fiber sizes were emphasized. This was especially obvious when the fibers were refined. This was associated mainly with the weight of PCC in the finer fiber fractions, which reduced the relative weight of the less PCC-rich fractions (see Fig. 4), and to a lesser degree with the increasing tendency of material to pass the wires.

Figure 4 shows the distribution of total ash between the PCC-CTMP fractions and the ash content in each fraction. Most of the total ash that did not pass the 400 mesh wire was found in R100 (0 min refined sample) or R200 (30 and 60 min refined samples), but the ash

CTMP 0 min

PCC-CTMP 30 min

60 min

0 min

30 min

60 min

Distribution in fractionation (wt%) R30

20.4 ± 2.6

12.2 ± 0.9

7.5 ± 0.4

8.6 ± 1.5

4.9 ± 0.5

3 ± 0.4

R100

50.4 ± 2.3

50.9 ± 2.0

50.8 ± 1.1

30.6 ± 5.6

16.3 ± 2.4

16.3 ± 3.9

R200

17.9 ± 1.3

23.2 ± 2.0

26.9 ± 1.9

14.4 ± 1.4

28.9 ± 2.1

25.7 ± 2.8

R400

4.6 ± 0.2

5.4 ± 0.2

6.3 ± 0.7

4.6 ± 0.2

6.1 ± 0.9

5.7 ± 0.6

Pass

6.7 ± 1.2

8.2 ± 0.8

8.4 ± 0.5

41.8 ± 7.1

43.9 ± 3.3

49.3 ± 1.9

Ash 525 ± 25 °C (%) R30

1.2 ± 0.0

1.2 ± 0.2

1.3 ± 0.1

6.4 ± 0.9

8.7 ± 1.1

7.9 ± 2.3

R100

1.1 ± 0.1

1.1 ± 0.0

1.1 ± 0.0

8.1 ± 0.9

8.5 ± 1.5

8.7 ± 3.6

R200

1.3 ± 0.1

1.2 ± 0.1

1.2 ± 0.3

15.0 ± 1.4

11.2 ± 2.1

11.4 ± 4.5

R400 Pass

1.8 ± 1.1

1.9 ± 0.4

1.7 ± 0.3

42.2 ± 3 89 ± 0.1

29.4 ± 2.9 86.3 ± 0.4

29.0 ± 7.7 84.0 ± 0.4

Not fract

1.0 ± 0.0

47.9 ± 0.7

47.3 ± 0.2

48.5 ± 0.6

3.8 ± 0.5

5.1 ± 0.6

4.7 ± 1.2

Ash 900 ± 25 °C (%) R30

0.8 ± 0.0

0.8 ± 0.2

0.9 ± 0.1

R100

0.8 ± 0.2

0.8 ± 0.1

0.7 ± 0.0

4.8 ± 0.5

5 ± 0.7

5.1 ± 2.1

R200

0.9 ± 0.0

0.9 ± 0.1

0.8 ± 0.2

8.6 ± 0.9

6.5 ± 1.2

6.6 ± 2.5

R400

1.4 ± 0.9

1.5 ± 0.6

1.3 ± 0.2

24 ± 0.0

17 ± 1.8

16.8 ± 4.3

50.2 ± 0.1

48.7 ± 0.1

47.4 ± 0.2

Pass CaCO3 (% of ash) R30

69.9

75.9

68.2

92.3

93.8

91.6

R100

66.5

66.1

70.4

94.0

94.2

92.5

R200

63.8

63.0

62.7

96.7

95.5

96.0

R400

49.3

55.2

60.4

98.1

96.3

96.0

99.1

99.0

99.3

Pass

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Cellulose (2021) 28:5807–5826

5813

CTMP 0 min

content of the fraction increased with increasing mesh number. The ash contents of the samples refined for 30 and 60 min were essentially identical, although the 30 min sample fractions had a slightly higher percentage of total ash. This was explained by slightly better retention of ash in non-pass fractions in 30 min refined sample, and the slightly higher total ash of the 60 min refined sample.

PCC-CTMP 0 min PCC-CTMP 30 min PCC-CTMP 60 min

CTMP 30 min CTMP 60 min

Residuals, cumulative [%]

100 80 60 40

Fiber analysis

20

The average fiber length, fiber width, shape factor, amount of fines and kink index are shown in Table 2.

Refining of the unfractionated CTMP reduced the fiber length, fiber width and kink index, but increased the shape factor and amount of fines. Similar changes were detected for the PCC-CTMP, although the change in fiber width was negligible.

0 R30

R100

R200

R400

0 min (of tot. ash)

30 min (of tot. ash)

60 min (of tot. ash)

0 min (ash cont.)

30 min (ash cont.)

60 min (ash cont.)

7

70

6

60

5

50

4

40

3

30

2

20

1

10

0

Ash content of fraction [%]

Of total ash in composite [%]

Fig. 3 Cumulative residuals on the wires, calculated by weight.

The ‘pass’ fraction has been excluded to facilitate the comparison

0 R30

R100

R200

R400

Fig. 4 Ash distribution in the fiber fractions. The bars (indicated on the left-hand ordinate, ‘‘Of tot ash in composite’’) show the percentage of ash in the fractions of the unrefined and refined PCC-CTMP fillers after fractionation. The lines (indicated on the right-hand ordinate, ‘‘Ash content of fraction’’) show the ash content in each fraction

Comparison of the unfractionated CTMP and PCCCTMP showed that the fiber length was unaffected by precipitation process of PCC. The fiber width, shape factor and amount of fines increased, but the kink index decreased. The changes caused by precipitation in width and shape factor were however very small: the maximum change detected for these values was 6.4 % increase in width.

The differences between the unfractionated PCC and PCC-CTMP were similar to those observed between the fractions. The fiber length in each fraction decreased slightly with refining, but no differences were seen between the CTMP and PCC-CTMP fractions. The main difference was in the fines content of the fractions. The CTMP fractions had more fines than the PCC-CTMP fractions, although the fines content of the unfractionated PCC-CTMP was larger than that of the CTMP. The fiber length, fiber width and shape factor distributions of the fractions were essentially unaffected by the precipitation process, as the fines were not included in the distributions. This is illustrated in Figs. 5 and 6, which show the length and width distributions, respectively, in fractions of samples refined for 30 min.

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5814 Table 2 Physical fiber properties. Shape refers to shape factor. The amount of fines is given as a percentage of non-fines material. The values given are weighted with respect to fiber length

Cellulose (2021) 28:5807–5826

Sample

Length (mm)

Width (lm)

Shape (%)

Fines (%)

Kink index

R30

2.65

42.8

91.2

0.6

0.19

R100

1.65

41.1

91.8

0.8

0.36

R200

1.00

36.8

91.6

11.3

0.48

R400

0.50

38.7

90.8

212.2

0.52

Not fract

1.49

39.3

90.8

10.9

0.46

30 min R30

2.52

44.1

93.0

0.5

0.24

R100

1.48

40.9

92.9

1.5

0.26

R200

0.95

37.1

92.4

12.2

0.33

R400

0.57

32.3

91.8

211.1

0.39

Not fract

1.26

38.2

91.0

14.5

0.33

R30

2.40

44.1

96.6

0.4

0.10

R100

1.34

40.8

92.9

1.0

0.19

R200

0.87

37.2

92.6

11.9

0.17

R400

0.66

33.9

92.1

171.6

0.24

Not fract

1.15

38.2

91.1

15.3

0.30

R30

2.73

43.8

92.3

0.7

0.12

R100

1.65

41.5

92.1

0.9

0.30

R200 R400

1.06 0.64

37.8 32.8

91.8 91.3

9.3 184.3

0.38 0.59

Pass

0.58

31.7

92.3

601.0

1.29

Not fract

1.50

40.4

91.1

13.7

0.38

R30

2.63

45.3

93.1

0.3

0.08

R100

1.49

41.9

93.0

0.7

0.21

R200

0.94

37.8

92.6

10.0

0.31

R400

0.57

32.2

92.2

194.6

Pass

0.77

30.6

92.6

617.6

0.50

Not fract

1.27

40.6

91.9

16.8

0.30

R30

2.50

45.5

92.4

0.4

0.08

R100

1.33

42.0

92.9

0.8

0.21

R200 R400

0.87 0.51

38.1 31.5

92.7 91.9

10.0 270.5

0.27

Pass

0.61

31.7

90.7

433.7

1.13

Not fract

1.12

40.3

92.2

17.0

0.25

CTMP 0 min

60 min

0 min

PCC-CTMP 30 min

60 min

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Cellulose (2021) 28:5807–5826

5815

CTMP R30 CTMP R100 CTMP R200 CTMP R400

PCC-CTMP R30 PCC-CTMP R100 PCC-CTMP R200 PCC-CTMP R400

25

Frequency [%]

20

15

10

Zeta potential and surface charge density The zeta potential and surface charge density data are shown in Table 3; Fig. 7. Although the samples were either anionic (CTMP and most PCC-CTMP samples) or essentially non-charged (some of the PCC-CTMP samples), the unfractionated PCC-CTMPs showed positive zeta potentials. Refining of the CTMP increased the surface charge density but reduced the zeta potential, presumably due to the creation of more cellulose rich areas on the CTMP fiber surfaces (Hubbe 2006). This was observed particularly for the R30 CTMP samples. Increasing refining did not affect the surface charge of the R400 fractions in an obvious manner.

5

0 0.05 50

500

0.1

0.2

0.5

5000

1

2

5

Fiber length class [mm] Fig. 5 Fiber length frequency distributions for CTMP and CTMP-PCC refined for 30 min. Note the logarithmic x-axis

CTMP R30

PCC-CTMP R30

CTMP R100

PCC-CTMP R100

CTMP R200

PCC-CTMP R200

CTMP R400

PCC-CTMP R400

25

Frequency [%]

20

15

10

5

0 5

10

20

30 40 50

100

200

Fiber width class [μm] Fig. 6 Fiber width frequency distribution for CTMP and CTMP-PCC refined for 30 min. Note the logarithmic x-axis

The charge development detected for CTMP is in good agreement with the literature. Sundberg and Holmbom (2004) reported that flake-like CTMP fines had a higher total charge than fibril-like fines, both of which had a total charge more than twice that of the CTMP fibers. Mosbye and Laine (2002) reported that the primary fines of mechanical pulp had a higher total charge than secondary fines, although the difference was greatly diminished by alkali treatment used to imitate the effect of bleaching. In our case the pulp was bleached.

Although the charge of PCC is commonly cationic due to an excess of positive calcium (Ca2?) (Laine 2007), all fractions of PCC-CTMP, including the extremely PCC rich ‘pass’ fraction, had negative or essentially neutral surface charges. This may have been due to adsorbed anionic dissolved (and colloidal) material without significant incorporation of the materials into crystal as described by Jada and Verraes (2003) for a different polymer or the ‘‘cleaning effect’’ of the in-situ precipitation of PCC, in which dissolved (and perhaps colloidal) material is absorbed onto the PCC as reported by Matula et al. (2018).

The positive zeta potentials of unfractionated PCCCTMP were probably an artifact caused by small, PCC-rich particles found on the ‘pass’ fraction of the unfractionated pulps, as they were poorly immobilized during the zeta potential measurement and were able to pass the wire and disturb the measurement. Another possible explanation was over-emphasizing of possibly cationic PCC residing on fiber surfaces of unfractionated samples due to the flow conditions and ion

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Cellulose (2021) 28:5807–5826

Table 3 Zeta potential and surface charge mean values with 95% confidence limit. Note that R30 zeta potential absolute value was affected by the use of a filter paper CTMP

PCC-CTMP

Zeta potential (mv)

pH

EC (mS/cm)

Surface charge density (lekv/g)

Zeta potential (mV)

pH

EC (mS/cm)

CTMP

PCC-CTMP

0 min R30

–5.8 ± 4.3

–1.2 ± 3.9

R100

–7.1 ± 2

–3.4 ± 5.2

R200

–52.2 ± 1

7.3

0.30

–50.3 ± 2.9

8.0

0.29

–9.7 ± 0.9

–3.8 ± 3.2

R400

–29.1 ± 0.6

7.5

0.29

–26.1 ± 0.6

8.0

0.29

–46.9 ± 2.4

–41 ± 2.2

–64.9 ± 1.6

6.5

0.30

16.8 ± 0.1

8.1

0.30

–18 ± 0.6

–26 ± 1.6

–20.4 ± 4.4

7.3

0.3

–8.3 ± 3.1

–1.9 ± 5.2

–20.2 ± 2.5

Pass Not fract 30 min R30 R100

–47.4 ± 1.1

7.6

0.30

–63 ± 2.3

7.9

0.29

–8.2 ± 2.1

2.1 ± 6.4

R200 R400

–48.8 ± 2.2 –26.8 ± 0.6

7.4 7.4

0.31 0.30

–54.6 ± 1.7 –22.8 ± 0

7.9 7.9

0.30 0.28

–11.3 ± 3.6 –47.3 ± 1.7

–5.2 ± 3 –42.1 ± 1.2 –31.1 ± 4.1

Pass Not fract

–55 ± 1.6

6.3

0.3

–13.6 ± 1.1

7.7

0.29

14.2 ± 1.7

8.5

–21.2 ± 2.7

–20.5 ± 1.2

–10.7 ± 4.2

–10.9 ± 4.3

60 min R30 R100

–47.3 ± 1.4

7.5

0.31

–60.8 ± 1.2

8.0

0.30

–12.1 ± 0.5

–9.7 ± 1.5

R200

–51.8 ± 1.6

7.5

0.30

–32.1 ± 0.8

8.0

0.30

–10.8 ± 2.7

–16.6 ± 1.7

R400

–26.8 ± 9.5

7.4

0.30

–23.7 ± 0.4

0.29

–45.9 ± 5.5

Not fract

–44.6 ± 1.7

0 min

6.7

12.4 ± 0.5

0.30

30 min

8.5

0.30

–27.8 ± 0.4

–15.5 ± 3.4

flux. In addition, the streaming potential method used in this study is affected by fiber pad compression (Hubbe 2006), and the fiber pad compression was visibly dependent on the fraction measured, which may explain why the zeta potential was closer to zero for R30 and R400 than for R100 and R200.

60 min

-60

Surface charge density [μekv/g]

–57.6 ± 1.1 –38.2 ± 1.6

Pass

-50 -40 -30

Scanning electron microscopy -20 -10 0 C P-C C P-C C P-C C P-C C P-C C P-C R30

R100

R200

R400

pass

n/f

Fig. 7 Surface charge densities of the samples. C refers to CTMP and P-C to PCC-CTMP. The error bars represent 95 % confidence values. n/f = not fractionated

123

Scanning electron microscopy of the fractionated PCC-CTMP samples further confirmed that the ash content increased with decreasing fiber size, see Fig. 8. The PCC was in the form of typically nanosized roundish particles and their aggregates or clusters. No variation in PCC morphology or primary particle size was detected between the different fiber fractions, but the larger amount of PCC in the fines fraction suggested that the PCC nanoparticle aggregate size perhaps increased.

Cellulose (2021) 28:5807–5826

5817

Fig. 8 SEM micrographs of fractionated PCC-CTMP samples. a (30 min R200): a band of outer fiber layers essentially free from PCC (indicated with the arrow), surrounded by more PCC rich areas. b (0 min R200): heavily fibrillated fiber with high

content of PCC and a more intact fiber with substantially no PCC. c (0 min ‘pass’ fraction): fibrils and fines extremely rich in PCC. Image d (60 min R200): split or cleaved fiber with PCC deposited in lumen and outer layer essentially free from PCC

The PCC was located mainly on fiber regions where the outer layer of the fibers had been removed (Fig. 8a) or where the fiber surface was damaged or subjected to fibrillation (Fig. 8b). The tendency of the PCC to attach to fibrils or to damaged regions of the fiber was clearly evident in the fractionated samples with the highest amount of fines, i.e. the R400 and in ‘pass’ fractions (Fig. 8c). Split fibers had enriched PCC deposits in the lumen, while the outer surface appeared to be essentially free from PCC (Fig. 8d). This type of finding was usually seen in R100 and R200, but occasionally also in R30.

When the fractions of PCC-CTMP were compared with the unfractionated PCC-CTMP (Fig. 9), it was evident that a larger portion of the fibers were substantially free from PCC in the fractionated pulps than in the unfractionated sample. Some of this difference could be explained by the hypothesis that the fines in the unfractionated pulp were attached or otherwise located on larger fibers (thus appearing a

part of them), and these objects containing more PCC were removed during fractioning. The surfaces of the larger CTMP fibers were however also affected by fractionation: some of the PCC residing on the fiber surface was removed during fractionation.

Handsheet properties Physical properties The physical properties of the handhseets are shown in Table 4 and in Figs. 10, 11, 12 and 13. The sheets had an average grammage of 68.6 g/m2 (standard deviation 5.8 g/m2). The ash content of the sheets was assumed to be close to that of the pulp or fraction used in sheet preparation (Table 1), since the auxiliary wire improved mechanical retention.

The density of PCC-CTMP handsheets from unfractionated pulps was higher than that of CTMP handsheets free from PCC. The difference in density

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5818

Cellulose (2021) 28:5807–5826

Fig. 9 Unfractionated pulp (a) and fractionated pulp (b) R30. Both images were taken on PCC-CTMP refined for 60 min

was 16, 14 and 7 % for samples refined for 0, 30 and 60 min, respectively. The handsheet strength increased with decreasing fiber size and increasing pulp refining for both CTMP and PCC-CTMP. The coarse, stiff fibers, as in R30, are known to give high bulk and a highly porous sheet, but poorer bonding than the more flexible and conformable finer fibers (Retulainen et al. 1998). This result was, therefore, expected.

A more relevant comparison between CTMP and PCC-CTMP sheets reveals the effect of the PCC. The increase in density of the PCC-CTMP fraction sheets with increasing refining level was not as pronounced as that of the CTMP sheets. PCC-CTMP sheets from the 60 min refined pulp fractions had lower density than the corresponding CTMP sheets. This was associated with the debonding effect of nanosized particles and agglomerates of PCC on the fiber. The debonding effect is supported by the tensile index and tensile stiffness index results.

There was a good linear correlation between tensile index, elastic modulus, tensile stiffness and density for both CTMP and PCC-CTMP sheets made of fractions (Fig. 14). The slopes of the regression lines differed greatly for CTMP and PCC-CTMP sheets in the case of tensile index versus density (0.130 and 0.061, respectively) and tensile stiffness index versus density (0.015 and 0.010, respectively). The difference in slope was significantly less for elastic modulus versus density (0.0075 and 0.0062, respectively).

When the fiber-weight-based indices were calculated, the tensile index values of the fractionated PCCCTMP and the corresponding CTMP sheets were similar, although the tensile index of the PCC-CTMP

123

sheets was lower than that of the CTMP sheets.

Similarly, fiber-weight-based tensile stiffness index for PCC-CTMP was only slightly lower for R100 and R200. PCC-CTMP R400 sheets instead had a clearly higher tensile stiffness index than the corresponding CTMP R400 sheets.

Optical properties Both the brightness (Fig. 15) and the opacity (Fig. 16) of the PCC-CTMP handsheets were greater than those of the CTMP sheets when unfractionated pulps were used. This was not, however, the case for the individual fractions. The opacity of all the fractions increased when PCC was added, but the brightness decreased for fractions R30, R100 and R200 sheets, and increased for R400 sheets.

The scattering coefficient (sr) and absorption coefficient (kr) (Table 5), showed that the PCC-CTMP fraction sheets had, on average, a slightly higher sr and kr than the CTMP sheets, except for the R400 fraction sheets. In other words, although the light scattering improved, the change in the opacity of the fractions was due to an increase in light absorption rather than due to an increase in light scattering. The increase in opacity and brightness of the unfractionated PCCCTMP sheets over those of the unfractionated CTMP

68.0 52.0

R200 R400

n/f

70.5

67.0

61.0

66.5

R100

R200

R400 Pass

n/f

75.0

R30

60 min

62.5

79.0 69.0

66.5

66.5

68.5

65.0

81.5

65.0

67.5

73.0

R400

65.5

69.0

R200

67.0

67.0

n/f

71.0

R100

68.0

67.5

68.0 81.0

67.5

69.5

Pass

69.0

R30

30 min

67.0

69.0

R100

Pass

73.0

R30

0 min

284 ± 22 237 ± 18 154 ± 13 68 ± 0 172 ± 8

286 ± 21

224 ± 14

139 ± 9

196 ± 11

321 ± 17

327 ± 50

216 ± 17

68 ± 0

166 ± 15

171 ± 10

249 ± 22

250 ± 29

251 ± 12

241 ± 30

275 ± 18

305 ± 31

74 ± 7

74 ± 7

366 ± 18

253 ± 12 179 ± 19

254 ± 8 134 ± 16

329 ± 30

333 ± 22

337 ± 36

390 ± 28

359 ± 26

382 ± 80

PCC-CTMP

CTMP

CTMP

PCC-CTMP

Thickness (lm)

Grammage (g/m2)

339

441

298

246

229

263

427

274

216

177

243

267 387

205

192

CTMP

363

512 1043

280

234

213

300

991

492

260

220

183

282

914

268 454

202

194

PCC-CTMP

Density [kg/m3]

20.9 ± 2.5

34.7 ± 1.7

12 ± 1.4

7.1 ± 0.6

5.4 ± 0.3

18.9 ± 0.7

29.5 ± 1.5

10.3 ± 0.4

5.5 ± 0.6

n/d

10.7 ± 0.9

8.1 ± 0.6 24.7 ± 1.6

n/d

n/d

CTMP

n/d

23 ± 0.7

8.2 ± 0.5

5.5 ± 0.4

5.7 ± 0.5

n/d

21.5 ± 1.1

7.7 ± 0.6

4.3 ± 0.1

n/d

n/d

6.4 ± 0.5 11.8 ± 0.4

n/d

n/d

PCC-CTMP

Tensile index [Nm/g]

1.08 ± 0.08

1.92 ± 0.03

0.66 ± 0.05

0.33 ± 0.02

0.21 ± 0.01

0.74 ± 0.04

1.61 ± 0.04

0.49 ± 0.02

0.2 ± 0.02

n/d

0.44 ± 0.03

0.38 ± 0.02 1.34 ± 0.04

n/d

n/d

CTMP

n/d

2.04 ± 0.05

0.46 ± 0.02

0.24 ± 0.01

0.21 ± 0.01

n/d

1.76 ± 0.08

0.39 ± 0.02

0.16 ± 0.02

n/d

n/d

0.31 ± 0.02 1 ± 0.03

n/d

n/d

PCC-CTMP

Elastic modulus [MPa]

3.2 ± 0.24

4.36 ± 0.07

2.21 ± 0.17

1.34 ± 0.07

0.92 ± 0.05

2.81 ± 0.15

3.77 ± 0.10

1.8 ± 0.06

0.91 ± 0.11

n/d

1.81 ± 0.14

1.4 ± 0.03 3.48 ± 0.11

n/d

n/d

CTMP

n/d

3.97 ± 0.09

1.65 ± 0.06

1.04 ± 0.06

0.96 ± 0.08

n/d

3.57 ± 0.17

1.49 ± 0.09

0.73 ± 0.03

n/d

n/d

1.15 ± 0.10 2.21 ± 0.08

n/d

n/d

PCC-CTMP

Tensile stiffness index [kNm/g]

Table 4 Physical properties of the handsheets. n/f = not fractionated pulp. n/d = not detected (in this case due to poor sheet strength). For the ash contents of the pulps, see Table 1.

Cellulose (2021) 28:5807–5826 5819

123

5820

Cellulose (2021) 28:5807–5826 CTMP (0 min) CTMP (30 min) CTMP (60 min)

PCC-CTMP

100 R30 R100 R200 R400 n/f R30 R100 R200 R400 n/f R30 R100 R200 R400 n/f

0

0 min

30 min

60 min

3 2 1 0 R30

Fig. 10 Densities of the CTMP and PCC-CTMP sheets. n/f = not fractionated CTMP (0 min) CTMP (30 min) CTMP (60 min)

4

PCC-CTMP (0 min) PCC-CTMP (30 min) PCC-CTMP (60 min)

60 min

200

5

30 min

300

Tensile stiffness index [kNm/g]

Density [kg/m³]

400

PCC-CTMP (0 min) PCC-CTMP (30 min) PCC-CTMP (60 min)

0 min

CTMP 500

R100

R200

R400

n/f CTMP

Fig. 12 Tensile stiffness indexes of the CTMP and PCC-CTMP sheets. The category n/f refers to the sheets made using unfractionated pulp. Tensile stiffness indexes of unfractionated PCC-CTMP sheets were below test limit CTMP (0 min) CTMP (30 min) PCC-CTMP (60 min)

30

PCC-CTMP (0 min) PCC-CTMP (30 min) CTMP (60 min)

R30

R100

R200

R400

n/f CTMP

Fig. 11 Tensile index of CTMP and PCC-CTMP sheets. n/f = not fractionated. The tensile indexes of unfractionated PCC-CTMP sheets were below the test limit

sheets was therefore mainly due to the presence of the mineral-rich ‘pass’ fraction.

Discussion Effect of CTMP refining on the in-situ precipitation of calcium carbonate The current work confirms that mechanical refining changes the precipitation efficiency and thus the composition and properties of the PCC-fiber composite. The CTMP has a large particle size distribution in

123

1.0 0.5

60 min

1.5

30 min

0

2.0

0 min

60 min

30 min

10

Elastic modulus [MPa]

2.5 20

0 min

Tensile index [Nm/g]

40

0.0 R30

R100

R200

R400

n/f CTMP

Fig. 13 Elastic moduli of the CTMP and PCC-CTMP sheets. n/f = not fractionated. The elastic moduli of unfractionated PCC-CTMP sheets were below the test limit

which fines have physical and chemical properties different from those of the coarse fraction. Refining introduces changes, including fiber damage and removal of the fiber layer and the formation of debris.

Calcium carbonate precipitation onto the fibers can be affected by refining. PCC was found to be more prone to attach to the fiber parts where the outer layer (P1) had been removed or damaged, and SEM images showed that a prolonged refining and formation of higher content of split fibers means that PCC can be found on the inner lumen wall, despite the fact that the outer layers of CTMP fibers (primary wall and

Cellulose (2021) 28:5807–5826

5821

CTMP

PCC-CTMP

CTMP

PCC-CTMP

100

30

Opacity [%-ISO]

y = 0.13x - 24.9 R² = 0.981

20

10

y = 0.062x - 8.8 R² = 0.989 200

400

80 70 60

0 0

90

R30 R100 R200 R400 Pass n/f R30 R100 R200 R400 Pass n/f R30 R100 R200 R400 Pass n/f

Tensile index [Nm/g]

40

600

0 min

30 min

60 min

Sheet density [kg/m³] Fig. 14 Tensile index versus sheet density of fractionated sheets. The lines show the linear regression

CTMP

PCC-CTMP

Brightness [%-ISO]

80 70 60 50

R30 R100 R200 R400 Pass n/f R30 R100 R200 R400 Pass n/f R30 R100 R200 R400 Pass n/f

40

0 min

30 min

60 min

Fig. 15 Brightness (R457C) of the handsheets prepared using fractions of CTMP and CTMP-PCC. n/f indicates sheets made using unfractionated pulp

secondary wall S1) are known to be richer in charged species (such as lignin) than the inner layers of the fibers, which in turn are richer in less charged cellulose (Hubbe 2006). The study showed an approximately linear correlation between the ash content of the PCCCTMP fractions and the anionic surface charge density of the corresponding CTMP fraction (R2 of liner fit was 0.849, see Fig. 17).

Assuming that the metal-binding ability of the functional groups correlates with their charge, this is partly in conflict with the hypothesis that the functional metal-binding groups steer the precipitation onto the fibers. The situation is however more complex. The dominating interaction is stated to be

Fig. 16 Opacity of the handsheets prepared using fractions of CTMP and CTMP-PCC. n/f indicates sheets made using unfractionated pulp

the interaction with Ca2? and the partial negative charge of the hydroxyl group (due to polarization) (Dalas et al. 2000), but the hydroxyl groups on the fibers require alkaline conditions to ionize (Sjostrom 1989) and contribute to the fiber surface charge. The fiber charge at neutral pH range is commonly attributed to carboxyl groups found e.g. in hemicelluloses instead of cellulose (Laine 2007; Sjostrom 1989), and CaCO3 has been reported to precipitate onto surfaces via an ACC precursor that attaches onto the surface containing carboxyl groups (Stephens et al.

2011).

Lignin is known to form complexes with metals (Schmidt 2010) and has been reported to have a high affinity towards Ca2? (Torre et al. 1992). The mechanism of metal uptake of lignin is however not well understood (Guo et al. 2008; Suhas et al. 2007). It has been linked with proton displacement (Christ et al.

2002; Guo et al. 2008) and carboxylic and phenolic sites, of which phenolic groups are thought to be dominant (Guo et al. 2008). The hydroxyl groups of cellulose may undergo metal-ion facilitated deprotonation especially at high pH (Al-Sogair et al. 2011), whereas it has been reported that lignin exhibits the opposite behavior, i.e. a decreasing deprotonation with increasing pH (Christ et al. 2002). This could explain why cellulose was ‘‘more effective than its charge’’ in initiating PCC precipitation under the conditions used in our work, especially if the charges are compared at neutral pH. Another possible reason for the increase in ash content with increasing beating and increasing surface

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5822 Table 5 Optical properties of CTMP and PCC-CTMP handsheets

Cellulose (2021) 28:5807–5826

R457C

sr (m2/kg)

kr (m2/kg)

Opacity (YC/2)

R30

50.0

22.42

1.92

81.3

R100

53.3

27.11

1.82

84.0

R200

54.4

37.63

2.34

90.4

R400

55.7

69.69

4.18

97.9

Not fract

64.7

35.71

1.09

84.8

30 min R30

51.2

23.22

1.86

81.6

R100

53.3

26.85

1.84

83.9

R200

55.1

35.51

2.18

89.2

R400

55.5

64.78

4.01

97.5

Not fract

63.0

40.94

1.49

88.7

R30

49.8

23.24

1.98

82.1

R100

53.2

27.54

1.90

84.6

R200

55.2

34.80

2.10

88.7

R400

56.1

59.83

3.60

96.7

Not fract

65.8

39.54

1.17

86.8

R30

43.8

22.47

2.63

84.2

R100 R200

46.9 50.7

28.95 36.91

2.81 2.92

88.4 91.7

R400

58.8

78.23

4.13

98.3

Pass

75.7

60.70

1.11

91.8

Not fract

65.8

47.86

1.48

90.6

R30

43.9

24.15

2.86

86.1

R100

47.5

29.28

2.79

88.5

R200

50.4

36.39

2.94

91.5

R400

57.1

74.73

4.37

98.3

Pass

74.5

60.25

1.29

92.4

Not fract

68.9

52.03

1.33

91.0

R30

43.0

23.51

2.95

85.9

R100

47.1

28.35

2.78

88.1

R200 R400

50.0 57.9

34.68 79.36

2.86 4.51

90.8 98.5

Pass

70.7

47.24

1.39

90.1

Not fract

69.0

54.70

1.41

91.8

CTMP 0 min

60 min

PCC-CTMP 0 min

30 min

60 min

charge density was the increase in surface area and fiber damage (resulting in high energy sites). It can be argued that the surface created in refining has, for the

123

most part, a charge similar to that of pre-existing surface because the newly created surface has a chemical composition similar to that of the pre-

Cellulose (2021) 28:5807–5826

5823

50 45

Ash content [%]

40 35 30 25

R² = 0.8487

20 15 10 5 0 0

20

40

60

Surface charge [μekv/g] Fig. 17 Ash content of PCC-CTMP fractions plotted against the surface charge of the corresponding CTMP fraction. The error bars represent 95 % confidence values

existing surface. In this case the surface charge would be indicative of an approximately proportional increase in surface area, and the observed correlation with the surface charge density could indicate a correlation with surface area. The changes in the surface area alone do not, however, explain why PCC appeared to discriminate against the areas with intact primary wall, and it does not explain the obvious difference in fiber coverage by PCC between fines and larger fibers.

It is tempting to think that the correlation between ash content and surface charge supports a precipitation mechanism similar to that described by Subramanian (2008), i.e. that the precursor of PCC aggregates on the cellulosic particles due to electrostatic interactions, and that it adheres and subsequently crystallizes as CaCO3. The precursor was expected to be cationic due to an excess of Ca2? in compared to the CO32- during precipitation. The ‘‘charge neutralization by CaCO3 precursor’’ concept may be partially supported by the data, as the PCC-CTMP R30 and R100 fractions refined for 0 and 30 min had essentially no anionic surface charge and that of R200 was also extremely low. On the other hand, the highly anionic surface charge of R400 and the ‘pass’ fraction, although the ‘pass’ fraction had a high mineral content, do not fully support the simple ‘‘charge neutralization by CaCO3 precursor’’ mechanism.

The surface charge of a pulp commonly depends on the production process (Laine 2007). Therefore, the

relationship between the surface charge or the surface concentration of a functional groups and precipitated filler could be useful for controlling the formation of PCC-fiber composite material, if such a correlation were universally established. It was shown that the fiber surface and morphology plays a role in PCC precipitation, and that this affects the homogeneity of the PCC-fiber composite, although the precipitation was carried out in a simplified system free from many susbstances, such as cationic polymers and inorganic salts such as sulphates at present in paper mill waters (Laine 2007). Some of these substances may be designed to alter fiber surfaces (such as cationic polymers) and some, for example sulphonates (Tang et al. 2012), may affect the precipitation of PCC. While the concentration of a single substance in PCCfiber composite preparation may be below the concentration it alone would significantly affect the precipitation and steering of particles onto the fiber, the net effect of different substances is difficult to predict.

Papermaking potential of PCC-CTMP: influence on fiber bonding and optical properties As the fiber morphology remained essentially the same, the differences between the CTMP and PCCCTMP sheets were associated with the presence of the mineral. The information regarding the role of different fractions may help to explain why the same precipitation process on different pulps may result in differences in the relative development of optical and strength properties, as was reported by Kumar et al.

(2009) to happen.

The physical and optical properties of the CTMP and PCC-CTMP sheets made using the fractionated pulps suggested that the enrichment of PCC onto the desired fractions of the pulp (or onto the desired parts of the fiber) is important when tailoring the PCC-fiber composite. The differences observed between CTMP and CTMP-PCC sheets were primarily associated with differences in surface characteristics, i.e. mainly in light scattering but also in debonding as indicated by the tensile strength and density development of the sheets. The presence of a salt such as sodium or calcium chloride has been associated with changes in fiber swelling and tensile index (Fält and Wågberg 2003), but it is not known whether the PCC precipitation can alter fiber swelling. Another factor

123

5824

affecting the bonding was that the PCC was precipitated in (split fiber) lumens and perhaps inside the fibrillated structures of the more well-fibrillated CTMP, which is expected to stiffen the fibers, to make them less conformable and less collapsible (Kumar et al. 2011). Such factors are desirable when high bulk is required, but they tend to have a negative effect on bonding.

There was significantly less debonding with the separate fractions than with the unfractionated PCCCTMP. The large-fiber fractions retained their strength properties better than the small-fiber fractions. A comparison of the fiber-weight-based indices for PCC-CTMP and CTMP sheets showed only a slight decrease on strength for most fractions, and the tensile stiffness index of R400 even increased although filler is known to disturb the formation of a fiber network (Hubbe and Gill 2016). The conserved strength could depend on the distribution of PCC in the sheets (different perhaps to that of traditionally filled sheets, as reported by Silenius (2002)), or due to differences in segment activation compared with a typical sheet. The results suggest, however, that the fines fractions may be capable of containing a considerable amount of PCC without losing their bonding potential to a proportional degree.

Based on the SEM micrographs, most the PCC present was expected to form a PCC-CTMP composite, i.e. the yield of CTMP and Ca(OH)2 to PCCCTMP was considered to be high. This suggests excellent retention during sheet-making, but this may not be so since the fines easily passed wires during fractionation.

Fines were expected to grow in size due to attachment of PCC. This may partially explain why the fines content increased during precipitation of PCC onto CTMP (i.e. they were better detected by the fiber analyzer), but the increase in size does not explain the increased tendency for the fines to pass the wires. We suggest that the loss of fines during fractionation was due to the precipitated PCC that reduced the tendency of the fines to attach onto other particles. Such result has been reported by Klungness et al. (1996): In their pilot trial, when using a PCC-fiber composite the total first pass retention was negatively affected, though first-pass retention of filler increased.

The PCC attached onto the fines appeared to prevent the fines from forming a gel-like, waterremoval-resisting structure, which was observed on

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Cellulose (2021) 28:5807–5826

the 400 mesh wire when the CTMP was fractionated.

Reduced interparticle interactions can explain the increase in fines content during precipitation of PCC and the loss of fines during fractionation. This is in good agreement with the report that water removal from a MNFC-containing furnish is increased when PCC-fiber composite is created on the MNFC (Rantanen et al. 2015), although the increase in the amount of mobile fines is expected to decrease the water removal from the sheet (Hubbe and Heitmann 2007).

Since the PCC was found to be attached mainly onto the fines, which increases the light scattering of mechanical pulp (Leskelä 1998), the increase in sr was probably also low for the fines fraction.

Therefore, assuming that PCC is enriched on the fibrils of externally fibrillated fibers similarly than it attaches onto fines and damaged CTMP fibers, a carefully executed refining of the pulp prior to precipitation phase could be beneficial for pulp intended for PCC-fiber composite preparation. By controlling external fibrillation it could be possible to tailor PCC-fiber composites, for example by steering the precipitation onto larger fiber fractions by the use of external fibrillation. The use of selected fractions in pulp blends could also be of interest, although fractionation as such may not be the most alluring industrial approach.

Conclusions Fractionation of the PCC-CTMP composite showed that the ash content increased with decreasing fraction size. The PCC was attached well onto the fiber, as shown by its tendency to remain in the samples and by SEM micrographs in case of the ‘pass’ fraction.

The reason for the preferred precipitation onto fines is unclear, but a correlation was found between the anionic surface change density of the CTMP fraction and the ash content of the PCC-CTMP fraction. SEM micrograph data suggests that the PCC prefers to precipitate to the areas typically associated a lower charge than the primary wall. This may, however, be explained by chemical changes such as metal-ion facilitated deprotonation in cellulose due the higher pH. The correlation is therefore thought to support the suggestion that the interparticle (fiber-ACC) interactions, perhaps partially originating from cationic-

Cellulose (2021) 28:5807–5826

anionic interactions, steer the ACC onto the cellulosic fibers, onto which it subsequently precipitates as PCC. The PCC-CTMP handsheets based on the coarser fiber fractions were more prone for brightness loss, probably due to a combined effect alkali yellowing and less enhanced light scattering due to lower PCC content, whereas sheets comprising higher content of the fine fractions provided positive effects especially on light scattering. The fines also contained significant amount of PCC without proportional loss in fiberweight based tensile index, suggesting that the substrate fiber properties can greatly affect the PCCfiber composite and its effects on paper. These findings demonstrate the importance of controlling the substrate fiber properties e.g. by ensuring that the pulp contains the different fiber fractions in desirable proportion, or by favorable external fibrillation of larger fibers. The findings also suggest that careful, target-oriented refining of the fiber is an important pretreatment when attempting to control the properties of PCC-fiber composite material.

Acknowledgments Stora Enso Oyj is acknowledged for financial support of the project. Dr Anthony Bristow is thanked for linguistic revision of the work.

Funding Open access funding provided by LUT University (previously Lappeenranta University of Technology (LUT)).

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declarations Conflict of interest The authors declare that they have no conflict of interest.

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Heinemann S, Vehniäinen A, The character and properties of mechanical pulps, in ‘Papermaking chemistry’ 2nd ed, Papermaking science and technology vol. nbsp;5, Lönnberg B. (ed), 2009, p. and nbsp;458–514 Hubbe MA (2006) Sensing the electrokinetic potential of cellulosic fiber surfaces. BioResources 1(1):116–119 Hubbe MA, Gill RA (2016) Fillers for papermaking: a review of their properties, usage practices, and their mechanistic role,. BioResources 11(1):2886–2963 Hubbe MA, Heitmann JA (2007) Review of factors affecting the release of water from cellulosic fibers during paper manufacture. BioResources 2(3):500–533 Jada A, Verraes A (2003) Preparation and microelectrophoresis characterisation of calcium carbonate particles in the presence of anionic polyelectrolyte. Colloid Surf A 2019:7–15. https://doi.org/10.1016/S09277757(03)00010-4 Klungness JH, Sykes MS, Tan F, Abubakr S, Eisenwasser JD (1996) Effect of fiber loading on paper properties. Tappi J 79(3):297–301. https://doi.org/10.1177/ 1687814016664258 Klungness J, Ahmed A, Ross-Sutherla N, AbuBakr S (2000) Lightweight, high-opacity paper by fiber loading: filler comparison. Nord Pulp Pap Res J 15(5):345–350. https:// doi.org/10.3183/npprj-2000-15-05-p345-350 Krogerus B (2007) Papermaking additives. In: Alén R (ed) Papermaking chemistry, 2nd edn. Gummerus Oy, Jyväskylä, pp 56–121 Kumar P, Gautam S, Kumar V, Singh S (2009) Enhancement of optical properties of bagasse pulp by in-situ filler precipitation. BioResources 4(4):1635–1646 Kumar P, Negi YS, Singh SP (2011) Filler loading in the lumen or/and cell wall of fibers -a literature review. BioResources 6(3):526–3546

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Gummerus oy, Jyväskylä, pp 28–53 Lakshminarayanan R, Valiyaveettil S, Luan Loy G (2003) Selective nucleation of calcium carbonate polymorphs: role of surface functionalization and poly(vinyl alcohol) additive. Cryst Growth Des 3(6):953–958. https://doi.org/ 10.1021/cg034022j Leskelä M (1998) Optical properties. In: Niskanen K (ed) Paper physics. Gummerus Oy, Jyväskylä, pp 117–137 Matula J, Räsänen J, Tahkola K (2018) In-line PCC technology cleans up circulation water from dissolved materials, metals and bio based microorganisms resulting to clean process and high runnability. In: PaperCon 2018. Charlotte, North Carolina, April 15–18 Mohamadzadeh-Saghavaz K, Resalati H, Ghasemian A (2014) Cellulose-precipitated calcium carbonate composites and their effect on paper properties. Chem pap 68(6):774–778. https://doi.org/10.2478/s11696-013-0513-7 Mosbye J, Laine J (2002) The charge and chemical composition of fines in mechanical pulp. Nord Pulp Pap Res J 17(3):352–356. https://doi.org/10.3183/npprj-2002-17-03p352-356 Odabas N, Henniges U, Potthast A, Rosenau T (2016) Cellulosic fines: properties and effects. Prog Mater Sci 83:574–594. https://doi.org/10.1016/j.pmatsci.2016.07.006 Park Y, Rawat P, Ford E (2017) Role of Polymerized Micelles on the Calcium Carbonate Mineralization of Nanofibers.

Ind Eng Chem Res 56:8241–8250. https://doi.org/10.1021/ acs.iecr.7b00902 Rantanen J (2016) The manufacturing potential of micro and nanofibrillated cellulose composite papers Ph.D. Dissertation (Tech.), Aalto University Rantanen J, Dimic-Misic K, Kuusisto J, Maloney TC (2015) The effect of micro and nanofibrillated cellulose water uptake on high filler content composite paper properties and furnish dewatering. Cellulose 22:4003–4015. https://doi.org/ 10.1007/s10570-015-0777-x Retulainen E, Niskanen K, Nilsen N (1998) Fibers and bonds.

In: Niskanen K (ed) Paper physics. Gummerus Oy, Jyväskylä, pp 55–83 Schmidt JA (2010) Electronic spectroscopy of lignins. In: Heither C, Dimmel DR, Schmidt JA (eds) Lignin and lignans: advances in chemistry. CRC Press, Miami, pp 49–102 Seo BS, Ahn JH, Lee HL (2017) Upgrading waste paper by insitu calcium carbonate formation. J Clean Prod

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003 Shen J, Song Z, Qian X, Liu W (2009) Modification of papermaking grade fillers: a brief review. BioResources 4(3):1190–1209 Silenius P (2002) Improving the combinations of critical properties and process parameters of printing and writing papers and paperboards by new paper-filling methods.

Dissertation PhD (Tech.), Helsinki University of Technology Sjostrom E (1989) The origin of charge on cellulosic fibers, Nord. Pulp Paper Res J 4(2):90–93. https://doi.org/10.

3183/npprj-1989-04-02-p090-093 Stephens CJ, Kimy Y-Y, Evans SD, Meldrum FC, Christenson HK (2011) Early stages of crystallization of calcium carbonate revealed in picoliter droplets. J Am Chem Soc 133:5210–5213. https://doi.org/10.1021/ja200309m Stoica-Guzun A, Stroescu M, Jinga S, Jipa I, Dobre T, Dobre L (2012) Ultrasound influence upon calcium carbonate precipitation on bacterial cellulose membranes. Ultrason Sonochem 19:909–915. https://doi.org/10.1016/j.ultsonch.

2011.12.002 Subramanian R (2008) Engineering fine paper by utilising the structural elements of the raw materials. Ph.D. Dissertation (Tech.), Helsinki University of Technology Suhas, Carrott PJM, Ribeiro Carrott MML (2007) Lignin – from natural adsorbent to activated carbon: a review. Bioresource Technol 98:2301–2312. https://doi.org/10.1016/j. biortech.2006.08.008 Sundberg A, Holmbom B (2004) Fines in spruce TMP, BTMP and CTMP - chemical composition and sorption of mannans, Nord. Pulp Pap Res J 19(2):176–182. https://doi.org/ 10.3183/npprj-2004-19-02-p176-182 Tang Y, Zhang F, Cao Z, Jing W, Chen Y (2012) Crystallization of CaCO3 in the presence of sulfate and additives: experimental and molecular dynamics simulation studies.

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1016/j.jcis.2012.02.069 Torre M, Rodrigues AR, Saura-Calixto F (1992) Study of the interactions of calcium Ions with lignin, cellulose, and pectin. J Aric Food Chem 40:1762–1766. https://doi.org/ 10.1021/jf00022a007 Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 2, 2021

Using fibre property measurements to predict the tensile index of microfibrillated cellulose nanopaper Lewis Taylor, Jonathan Phipps, Stuart Blackburn, Richard Greenwood & David Skuse A wide variety of wood and non-wood cellulosic fibre sources were used as a feed to produce microfibrillated cellulose (MFC) using a grinding process. Nanopaper was formed using this product, and the tensile index was measured. The hemicellulose content of the feed fibres was measured, and was found to correlate with the production of finer microfibrils and a higher MFC tensile strength. The correlation with tensile strength was improved by the inclusion of a measurement of the MFC particle lengths as measured by a fibre image analyser, with the resulting relation fitting a modified Page Equation. It was hypothesised that the frequency of flaws in the feed fibre cross-section influences the length of the MFC particles produced, and so the zerospan tensile index of the fibres was measured as a proxy for this since it forces cross-sectional fibre breakage. The fibre zerospan tensile index was found to correlate with MFC particle length and so was used in its place in the equation. The resultant equation can predict MFC tensile strength from zero-span tensile index and hemicellulose content measurements of cellulosic fibres and can aid in optimising feedstock selection for mechanical MFC production processes.

Contact information: L. Taylor,S. Blackburn, R. Greenwood & D. Skuse: School of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK L. Taylor, J. Phipps & D. Skuse: Par Moor Centre, FiberLean Technologies, Par Moor Road, Par PL24 2SQ, UK Cellulose (2020) 27:6149–6162 https://doi.org/10.1007/s10570-020-03226-2 Creative Commons Attribution 4.0 International License

The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International and the PITA Annual Review, both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating.

Page 1 of 15

Article 5 – Predicting Strength from Fibre Properties

Cellulose (2020) 27:6149–6162 https://doi.org/10.1007/s10570-020-03226-2

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ORIGINAL RESEARCH

Using fibre property measurements to predict the tensile index of microfibrillated cellulose nanopaper Lewis Taylor David Skuse

. Jonathan Phipps . Stuart Blackburn . Richard Greenwood .

Received: 1 December 2019 / Accepted: 9 May 2020 / Published online: 25 May 2020 Ó The Author(s) 2020

Abstract A wide variety of wood and non-wood cellulosic fibre sources were used as a feed to produce microfibrillated cellulose (MFC) using a grinding process. Nanopaper was formed using this product, and the tensile index was measured. The hemicellulose content of the feed fibres was measured, and was found to correlate with the production of finer microfibrils and a higher MFC tensile strength. The correlation with tensile strength was improved by the inclusion of a measurement of the MFC particle lengths as measured by a fibre image analyser, with the resulting relation fitting a modified Page Equation. It was hypothesised that the frequency of flaws in the feed fibre cross-section influences the length of the MFC particles produced, and so the zero-span tensile index of the fibres was measured as a proxy for this since it forces cross-sectional fibre breakage. The fibre zerospan tensile index was found to correlate with MFC particle length and so was used in its place in the Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10570-020-03226-2) contains supplementary material, which is available to authorized users.

L. Taylor S. Blackburn R. Greenwood D. Skuse School of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK L. Taylor (&) J. Phipps D. Skuse Par Moor Centre, FiberLean Technologies, Par Moor Road, Par PL24 2SQ, UK e-mail: lxt189@bham.ac.uk; lewis.taylor@fiberlean.com

equation. The resultant equation can predict MFC tensile strength from zero-span tensile index and hemicellulose content measurements of cellulosic fibres and can aid in optimising feedstock selection for mechanical MFC production processes.

Keywords Microfibrillated cellulose Hemicellulose Zero span tensile strength Tensile strength

Introduction Microfibrillated cellulose Cellulose is an essential structural component in higher plants and is the most abundant polymer on Earth, with an almost inexhaustible supply of around 1.5 9 1012 tonnes per year produced by biomass (Klemm et al. 2005). Cellulose fibres can be separated from other biomass components by pulping and bleaching processes, producing a cellulose fibre pulp that is used primarily in the production of paper and cellulose-based packaging to impart mechanical strength properties.

These mechanical properties are a consequence of the fibre microstructure. Cellulose fibres have a fractal-like structure with several hierarchies of fibre-like sub-units that extend down to the nanoscopic scale. A cellulose fibre consists of parallel chains of

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long fibrils with micron-scale diameters, which are similarly made up of microfibrils around 30 nm in diameter. These microfibrils in turn consist of elementary fibrils around 3.5 nm in diameter (ChingaCarrasco 2011).

In 1982, Turbak et al. (1983) and Herrick et al.

(1983) reported that cellulose fibres could be disintegrated to liberate these finer structural components, greatly increasing the specific surface area and hydrogen bonding capability. This product was termed microfibrillated cellulose (MFC), and has been found to have many applications that exploit its hydrophilicity, renewability, biodegradability, and high specific strength and surface area (for reviews regarding applications, see Klemm et al. 2011; Shatkin et al.

2014).

The original production process involved disintegrating cellulose pulp fibres into MFC using a homogenizer (Turbak et al. 1983). In the decades since, other equipment has been used as an alternative, such as microfluidizers and friction grinders, and chemical pre-treatment methods including enzymatic degradation and 2,2,6,6-tetramethylpiperidine-1-oxl radical (TEMPO) oxidation have been applied to reduce the energy input (for a review on production methods, see Siró and Plackett 2010).

FiberLean Technologies has adapted minerals grinding technology to mechanically disintegrate cellulose fibres into MFC cost-effectively at large scales, without requiring cellulose pre-treatment. This method uses stirred media detritor technology (Husband et al. 2015), which disintegrates fibres into MFC by agitating grinding media beads. In this process, a paper filler mineral such as calcium carbonate is added as a grinding aid, greatly reducing the energy required.

A laboratory scale equivalent of this process is used to produce the MFC that is the subject of this research.

MFC has been made from numerous sources including hardwoods (e.g. birch, eucalyptus, and acacia), softwoods (various species of coniferous trees) and non-wood sources (e.g. cotton, abaca, flax, bamboo, and sugar beet (Jonoobi et al. 2015). The fibre source can have a strong influence on MFC quality.

For example Alila et al. (2013) found that abaca and sisal fibres produced finer MFC than flax and hemp.

Others have investigated the influence of fibre species on MFC quality (e.g. Chaker et al. 2013; Desmaisons et al. 2017) but these are limited to comparisons of a small number of species at a time, making the drawing

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of conclusions applicable to most fibre sources problematic.

Fibres can differ from each other in many ways, so it cannot be taken for granted that differences in MFC quality between two fibre sources are due to a difference in one parameter rather than another. This work therefore analyses twenty-four different fibre species in order to have a large enough data set that such comparisons are more valid, and so the relative importance of a fibre property can be assessed with a greater degree of confidence.

Paper tensile strength theory The MFC produced by the FiberLean process is in the form of a MFC-mineral composite that was originally created to service the paper filler additive market. In this application, the addition of MFC to a paper formulation increases the specific strength, allowing for a disproportionate amount of native fibres to be removed and replaced with paper filler mineral; this reduces the cost of the paper considerably and improves optical properties. The MFC quality is assessed by forming nanopaper sheets and testing their tensile strength; this strength appears proportional to the specific strength increase that this MFC would impart on paper when added to a fibre furnish.

Page (1969) introduced what is today the most commonly used theory for predicting the tensile strength of paper. This postulates that the failure of a sheet occurs partly by bonds between fibres being broken, and partly by the fibres themselves breaking across their cross-sections, and that the tensile index of the sheet is more dependent on the weakest of these two mechanisms. The Page Equation describing this is stated below: 1 9 12Aq ¼ þ T 8Z sB PLðRBAÞ

ð1Þ

where T is the sheet tensile index (Nm/g), Z is the zero-span tensile index (Nm/g), A is the fibre crosssectional area (m2), P is the fibre cross-section perimeter (m), q is the fibre density (kg/m3), L is the fibre length (m), sB is the shear bond strength per unit area (Pa), and RBA is the relative bonded area.

Zero-span tensile index is a measure of individual fibre strength and is discussed later in this work. RBA is a measure of the fraction of the fibre surface area that is used for inter-fibre bonding. The first term on

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the right-hand side represents the weakness of the individual fibres, whereas the second term represents the weakness of the bonds between fibres. Usually, a sheet of paper fails due to bonds between fibres breaking rather than the fibres themselves breaking, so the second term is limiting. Adding MFC to a fibre furnish greatly increases relative bonded area (Lindström et al. 2016), and so tensile index tends to improve considerably.

Hemicellulose Fibre chemical composition is known to be important in papermaking. Most raw plant material from which cellulose fibres are extracted are rich in hemicellulose.

Though pulping and bleaching removes much of the hemicellulose, there is still typically a large residual fraction within the fibre cell wall, with the amount dependent on fibre species and pulping conditions.

Hemicellulose is a broad term for a wide variety of polysaccharides with differing monomer sugars, functional groups, and degrees of branching, but for woods and many non-woods, there are only two important families; xylans and glucomannans. Xylans are found in the vast majority of plants, and account for almost all the hemicellulose in hardwoods, whereas glucomannans are found in large quantities in softwoods (in comparable amounts to xylans) (Ebringerová 2005).

Compared to cellulose, hemicellulose is always amorphous, whereas cellulose is partly crystalline, and hemicellulose molecules have relatively short chain lengths of 70–200 units (Fengel and Wegener 1983), compared to 300–1700 units typical for cellulose (Klemm et al. 2005).

Within a fibre cell wall, hemicellulose closely associates with the cellulose microfibril surface, forming a layer separating neighbouring microfibrils.

NMR spectroscopy work by Liitiä et al. (2003) indicates that both xylan and glucomannan do this, and are comparable in function. Hemicellulose has a branched, amorphous structure, and readily swells in water, as shown by work investigating the change in zeta potential during this process (Uetani and Yano 2012). This hydrophilicity also aids in the plasticity of the fibre to deformation (Bolam 1965), which would be expected to facilitate disintegration into MFC. NMR studies by several authors using fibres that have undergone different pulping conditions have shown that reducing the hemicellulose content appears

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to increase the fibril aggregate dimension size appreciably (Hult et al. 2001; Virtanen et al. 2008; Duchesne et al. 2001). This supports the notion that hemicellulose inhibits the spontaneous coalescence of neighbouring microfibrils.

Hemicellulose has long been understood to aid in conventional fibre refining for papermaking; chemically removing the hemicellulose content prior to refining reduces the tensile strength of the refined fibres (Bolam 1965). Sorbing hemicellulose onto fibres prior to refining has been found to improve sheet tensile strength, primarily by reducing the ‘kink’ deformations induced in the fibres (Mäkelä et al.

2010).

It is therefore plausible that a higher hemicellulose content would lead to high quality MFC. Existing work in the literature has supported this idea. Iwamoto et al. (2008) showed that drying a pulp after removing hemicellulose by alkali treating results in irreversible microfibril aggregation, inhibiting fibrillation compared to an untreated pulp. Numerous authors have found that a high hemicellulose content coincides with a high microfibril yield and better individualisation.

This appears true whether comparing fibres from different plant species (Alila et al. 2013; Desmaisons et al. 2017; Chaker et al. 2013) or from the same plant species but with different pulping conditions (Chaker et al. 2013; Petroudy et al. 2015; Spence et al. 2010).

There are two major mechanisms thought to explain this. The presence of surface hemicellulose itself improves inter-fibre bonding (or inter-fibril bonding in the case of MFC), since amorphous hemicellulose chains extend out from the microfibrils when immersed in water, and form bridges between neighbouring microfibrils when dried (Bolam 1965). Therefore, when disintegrating a high hemicellulose pulp into MFC, the liberated surface area has a higher surface concentration of hemicellulose, and so this strengthening effect is enhanced compared to a low hemicellulose pulp. Arola et al. (2013) removed hemicellulose from nanocellulose with xylanase enzymes, which resulted in poorer tensile properties, even with similar nanocellulose geometry, clearly demonstrating this effect.

The second mechanism is that a high hemicellulose pulp produces finer microfibrils with better individualisation, as microscopy images in various studies has demonstrated (e.g. Alila et al. 2013; Iwamoto et al.

2008; Chaker et al. 2013). Given similar microfibril

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lengths, this increases particle aspect ratio, improving tensile strength. Hemicellulose forms an amorphous layer between microfibrils that readily swells in water, and so this would be expected to provide a preferred plane of breakage parallel to the microfibril lengths, thereby facilitating the production of finer microfibrils. Additionally, xylan develops a surface charge due to carboxyl group dissociation under typical processing conditions, causing mutual microfibril repulsion, enhancing this effect to some degree.

Due to the importance of hemicellulose indicated so far in the literature, and its expected influence on MFC geometry and bonding, the hemicellulose content was investigated for all fibre species tested in this study.

Zero-span tensile strength It is later suggested that the fibril lengths of the MFC correlate with a high MFC tensile strength. It is desirable to be able to predict the resultant MFC fibril length from intrinsic fibre properties. It is reasonable to believe that having long fibrils within the fibre structure will, all other things being equal, lead to long liberated fibrils. Also, having fibrils with few preexisting defects should reduce the degree of fibril length degradation during processing. Both of these factors are expected to affect the individual fibre strength; intrinsically long fibrils result in fewer discontinuities at fibril endpoints, and having undamaged fibrils means fewer microscopic weak points in the fibre. Both can be seen to influence the ‘quality’ of the fibre cross-sectional area, i.e. having long, undamaged fibrils should result in the cross-sectional area having few flaws.

A measurement that could assess the specific strength of the fibre cross-sectional area could therefore be useful for indicating the frequency of fibril flaws and intrinsic fibril length; and is therefore expected to correlate with long fibril lengths of the MFC product. The zero-span tensile index of a fibre sheet prior to MFC production is believed to be such a measurement. In the zero-span tensile test, the two clamps are essentially touching (within microns of each other), forcing the vast majority of the fibres between the clamps to be held by both clamps at once, since the separation distance between clamps is a small fraction of typical fibre lengths. When the sample is broken under tensile stress, these clamped fibres will fail by cross-sectional breakage, rather than

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the bonds between fibres failing as with conventional tensile testing. Since the zero-span tensile test is normalised by weight, the thickness of the fibre cell wall and fibre diameter are accounted for.

The notion that zero-span tensile strength is a measure of fibre damage is well-supported. Zeng et al.

(2012) damaged fibres by refining, finding that the zero-span tensile strength was inversely proportional to the frequency of fibre kinks induced (i.e. sharp bends in the fibre). Joutsimo et al. (2005) induced cell wall damage (dislocations) by homogenization, which decreased fibre strength; they attributed this to inducing non-uniform load distributions across the crosssection.

Research has shown that fibril and microfibril-scale damage is also important. Nevell and Nugawela (1987) treated fibres with acids, causing localised damage to microfibrils that substantially reduced zerospan tensile strength. Wathén (2006) damaged fibrils homogeneously throughout the fibre by thermal ageing degradation, and found that the zero-span strength decreased.

Definition of research objectives MFC is produced by a mechanical process with a large and diverse range of fibre species, and attempts are made to correlate the tensile index of this MFC with the feed fibre properties, including the fibre hemicellulose content, and the fibre zero-span tensile index.

Materials and methods Fibre sources In total, twenty-four separate fibre species were investigated, from a variety of wood and non-wood sources. These are listed in Table 1 below.

Pulp preparation Fibres were added to the grinding process as a 30% solids cake. Miscanthus, sorghum, bagasse, enzymetreated Nordic pine, and Giant Reed were supplied in this form, whereas most other fibre sources were supplied as dry pulp boards. These dry pulps were soaked in water and broken down into individual fibres using a pulp disintegrator for 10 min. Excess water

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Table 1 Fibre species used in this study Softwoods

Hardwoods

Non-woods

Nordic pine

Birch #1

Cotton (linters)

Black spruce

Birch #2

Jeansà

Radiata pine Southern pine

Eucalyptus Acacia

Abaca Sisal

Douglas Fir

Mixed South Asian Hardwood

Kenaf

Enzyme-treated Nordic Pine

Mixed European Hardwood

Giant Reed (Arundo Donax)

Dissolving Pulp*

Tissue Dust

Bagasse (sugar cane) Miscanthus Sorghum Flax

* Dissolving Pulp is a Nordic pine/spruce blend (Domsjö Fabriker AB—Domsjö Cellulose Plus) that was subjected to the sulphite bleaching process, rather than the kraft (sulphate) process which all other softwoods and hardwoods listed underwent Waste short fibres collected from the dust extraction system of a tissue mill àCotton fibres from recycled jeans, following disintegration into individual fibres and formation into a pulp board

was removed by a vacuum filter, forming a cake. To avoid choking of the grinder due to entanglement of very long fibres, the flax fibres were cut with a guillotine into fragments around 3 mm in length.

Sweeps tab in the Supplementary Material. Consequently, a single energy input was chosen which was applied to all fibre species listed in Table 1 to produce MFC.

MFC production by stirred media detritor grinding

Nanopaper sheet preparation

MFC was produced from these fibres using a lab-scale version of the FiberLean production process. This involves a grinder that uses the motion of media beads to disintegrate the fibres. The presence of micron-scale mineral particles greatly improves the efficiency of this process, so the fibre charge was complemented with an equal amount (on a dry mass basis) of IC60 (60 wt% \ 2 lm) ground calcium carbonate mineral (Imerys). Sufficient water was also added to form a slurry with the target fibre solids content. The precise operating conditions, including the energy input, are proprietary. When a grind was completed, the slurry was separated from the media with a vibrating screen, or by a washing method when particles were too coarse for the screen.

Previous experience with this process has shown that almost all fibre sources reach a peak tensile strength at around the same energy, and an unpublished investigation with seven of the fibre species in listed in Table 1 has confirmed this. The relationships between energy input and MFC tensile strength for these seven fibre species are displayed in the Energy

The MFC-mineral composite product was collected as an aqueous slurry. The solids content was measured by weighing a sample before and after oven drying. The mineral content was determined from the change in weight of an oven-dried sample after burning off the cellulose in a 450 °C furnace for 2 h. Using this information, sufficient IC60 calcium carbonate mineral was added to the sample to dilute the MFC content to 20 wt% on a dry mass basis. These samples were used to form a nanopaper sheet in a vacuum sheet former and dried using a Rapid-Köthen dryer, targeting a weight of 220 gsm. These sheets were conditioned in a controlled environment at 23 °C and 50% relative humidity overnight prior to tensile testing.

Nanopaper tensile testing The nanopaper samples were weighed and cut into strips 15 mm wide. These strips were clamped in a Testometric M350 tensile tester and strained at a constant rate until failure. The force applied at break was divided by the strip width and gsm to obtain the

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tensile index in Nm/g. The tensile index of this MFCmineral composite nanopaper is much weaker than pure MFC due to the mineral disrupting inter-fibril bonding. However, work with MFC made from this process by Phipps et al. (2017) has shown that the tensile indices of FiberLean MFC with or without a given fraction of mineral added are roughly proportional (i.e. breakage is mainly bonding-limited in both cases), so the conclusions are still believed to be applicable to pure MFC. Three nanopaper sheets were produced for each sample, each cut into five strips. The mean tensile index of these fifteen strips is reported as the sample tensile index here. Fibre content was verified to be on target by measuring the ash content after burning off the cellulose in a 450 °C furnace for 2 h (these results are displayed in the Tensile Sheet Mineral Content tab in the Supplementary Material).

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glucomannan (Ebringerová 2005). Total hemicellulose content was determined by adding together the xylan and glucomannan fractions. The hemicellulose content of cotton linters (which is negligible) was also used as the hemicellulose content of the jeans cotton, since it was expected that the presence of dye and other additives in the jeans cotton would give misleading results. This is a reasonable assumption as textile-grade cotton contains negligible hemicellulose (Prado and Spinacé 2015).

Lignin content Acid-soluble and acid-insoluble lignin content of the pulps were measured by Labtium Oy according to a modified TAPPI standard T 222 om-88 test.

Fibre zero-span tensile testing

Fibre image analysis A Valmet FS5 fibre image analyser was used to measure geometric parameters of the original fibres before grinding, and the MFC particles after grinding.

This equipment operates by pumping a sample suspension past a camera that takes thousands of images, and parameters such as length and width distributions are determined by a computer algorithm.

Though designed for fibre measurement, this is sensitive enough to measure MFC particles with lengths around a hundred microns and above.

Hemicellulose content Hemicellulose measurements were subcontracted to Labtium Oy, who used the SCAN-CM 71:09 method (Scandinavian Pulp, Paper and Board Testing Committee 2009). Fibre samples underwent sulphuric acid hydrolysis, and the resultant concentration of various monomer sugar residues were detected by chromatography. This included glucose, xylose, mannose, arabinose, and galactose. To convert these sugar contents into xylan and glucomannan hemicellulose contents, it was assumed that the xylan content was the sum of all the xylose and arabinose, and the glucomannan content consisted of all the galactose, mannose, and 1 unit of glucose for every 4 units of mannose, since a ratio of 1:4 glucose to mannose is typical in softwood

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Zero-span tensile tests were carried out on the initial fibre samples rather than the MFC. The TAPPI standard T 231 cm-07 (Technical Association of the Pulp and Paper Industry 2007) was followed, using 60 gsm mineral-free handsheets for each fibre species produced according to TAPPI standard T 205 sp-12 (Technical Association of the Pulp and Paper Industry 2012). Each handsheet was cut into five strips, which were clamped in a Pulmac zero-span tensile tester and strained until broken. As with conventional tensile testing, the force measured was converted to a zero-span tensile index.

Four dry sheets were tested for each fibre species, and a mean average result is reported here.

Scanning electron microscopy A Jeol 6700 Scanning Electron Microscope was used to image several MFC samples at 10,000 x magnification. Preparation involved filtering a dilute MFC sample through a 0.2 lm nucleopore membrane. The samples were sputter coated with platinum to form a conductive monolayer. Secondary electrons emitted from the sample during measurement were used to form the images.

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Results and discussion Influence of hemicellulose content Hemicellulose within bleached chemical pulp resides within the fibre cell wall, where it forms a layer around the microfibrils, separating them from their neighbours. It was expected both from mechanistic considerations and from results previously reported in the literature that it would positively correlate with MFC quality. The hemicellulose content for each fibre species was measured, and is plotted against the MFC tensile index as Fig. 1.

Figure 1 shows that a high hemicellulose content of the fibre correlates with a high MFC tensile index.

Fitting the data to a general straight line equation gave an R2 value of around 0.63, which is moderately good for a single parameter, considering that cellulose is a natural product with a complex hierarchy of structures with different scales and chemistries between plant species.

As discussed in the introduction, there are two mechanisms by which hemicellulose is thought to improve MFC tensile index. One mechanism is that a high hemicellulose fibre when disintegrated to form

14 R² = 0.63

MFC Tensile Index (Nm/g)

12 10 8 6 4 2 0 0% Nordic Pine Enzyme Nordic Pine Birch #2 South Asian HW Abaca Miscanthus

5% 10% 15% 20% 25% Hemicellulose Content (mass frac on) Black Spruce Douglas Fir Eucalyptus Tissue Dust Sisal Sorghum

Radiata Pine Dissolving Pulp Acacia Co on Bagasse Giant Reed

30%

Southern Pine Birch #1 European HW Jeans Kenaf Flax

Fig. 1 Fibre hemicellulose content (wt%) versus tensile index at the selected energy input. For this figure and all subsequent plots, crosses represent softwoods, circles represent hardwoods, diamonds represent cotton fibres, triangles represent leaf fibres, and squares represent other miscellaneous non-wood sources

MFC has more of this liberated surface area coated in hemicellulose, which is thought to form bridges between particles upon drying and thereby improve relative bonded area and specific bonding strength.

The other mechanism is that the presence of hemicellulose, being a relatively mechanically weak amorphous layer, provides a preferred plane of breakage along the microfibril lengths, thereby facilitating disintegration into finer microfibrils, increasing the aspect ratio. To assess whether the latter mechanism is taking place, SEM images were taken of samples with a range of hemicellulose contents to assess fibril and microfibril diameters. Four of such images are displayed as Fig. 2.

Figure 2a shows bagasse MFC, which had the highest hemicellulose content of any fibre tested, at 28%. There appears to be a much higher frequency of finer microfibrils in this case than dissolving pulp and cotton MFC, shown as 2c, d, which have 4% and 0% hemicellulose respectively. Nordic pine MFC with a hemicellulose content of 17% is displayed as 2b, and also shows a high abundance of fine microfibrils, though to a lesser extent than bagasse MFC. A comparison of Fig. 2b, c is important; the dissolving pulp used in the latter is a Scandinavian pine/spruce blend, similar to the Nordic pine used in the former; the major difference is in their processing.

The Nordic pine pulp underwent a kraft pulping process, which cannot penetrate well into the fibres to remove hemicellulose, whereas the dissolving pulp was pulped with the sulphite process, which is capable of this, removing the vast majority of the hemicellulose.

Consequently, the fact that the liberated microfibrils in Nordic pine MFC tend to be much finer than with dissolving pulp MFC implies that the correlation of hemicellulose content with finer microfibrils is causal.

There is also the question of whether the two dominant types of hemicellulose found in these fibres, xylan and glucomannan, are equivalent in terms of enhancing microfibrillation. Correlations between MFC tensile strength and xylan or glucomannan alone, or a weighted sum of the two were investigated, but the best fit was found by treating the two as identical and correlating tensile strength with the sum of the two species. These data are displayed in the Pulp Chemical Composition tab in the Supplementary Material.

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Fig. 2 Secondary electron SEM images of MFC-mineral composite made from a bagasse (28% hemicellulose), b Nordic pine (17% hemicellulose), c dissolving pulp (4%

hemicellulose) and d cotton (0% hemicellulose). The low hemicellulose fibres give coarser and more strongly defined fibrils and microfibrils

Influence of lignin content

Especially considering the numerous relatively novel non-wood pulps of unknown chemistry used in the present study, it was initially conceivable that differences in lignin content could be contributing to the noise seen in Fig. 1. The lignin contents of the fibre species used in this study were measured, and the results are shown in the Pulp Chemical Composition tab in the Supplementary Material. Lignin was found to be present only at very low concentrations in all samples, except Giant Reed, which contained 12.7% lignin and does not seem to perform unusually compared to other fibre species. Therefore, for this study, the influence of lignin on these results can be neglected.

Given that the influence of hemicellulose is established, it was an open question as to what the influence of the lignin content would be. For thermomechanical or chemithermomechanical pulp, there is an extensive lignin coating on the fibre surface, which seriously inhibits the ability of the fibre to fibrillate and form high quality MFC, at least for entirely mechanical processes (e.g. Meyer et al. 2018; Lahtinen et al.

2014). However the role of lignin within the fibre cell wall is less clear, since good quality MFC can still be made at high lignin contents with unbleached chemical pulp (e.g. Spence et al. 2010).

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Influence of feed fibre geometry Fibre geometry has a strong influence in tensile strength in paper made from cellulose fibres; all other things being equal, longer and thinner fibres result in higher tensile strengths because they maximise aspect ratio. Longer fibres increase the number of connections that each individual fibre can make, and thereby distribute stress over a larger area. Finer fibres (with a thinner cell wall) result in more fibres per unit mass.

The fact that a high fibre aspect ratio leads to high sheet tensile strength has been found experimentally for both softwoods (Horn 1974) and hardwoods (Horn 1978). The Page Equation, shown as Eq. (1) earlier in this paper, makes use of such fibre dimensions to model paper tensile strength.

Though established as true for fibres, the influence of feed fibre dimensions on MFC properties was an open question. It stands to reason that in the case of partially processed MFC where the initial fibre structure is partly intact that this relationship would hold somewhat, but it is intuitive that as the fibre structure degrades into fully processed MFC, such a correlation would disappear. It was not ruled out, however, that the dimensions of the fibre such as length or cell wall thickness affect how the fibre disintegrates into MFC. Geometric parameters of the fibres were measured using a fibre image analyser, including length and length distributions, widths, and coarseness (a measure of fibre wall cross sectional area). Attempts were made to plot such geometric parameters against tensile index, and no clear correlation was apparent. The geometry of the feed fibres is therefore not thought to have any obvious influence on the quality of fully processed MFC. The geometric parameters measured by the fibre analyser and their near-zero correlations with MFC tensile strength are displayed in the Pulp Geometry tab in the Supplementary Material.

Influence of MFC length Stirred media detritor grinding produces MFC mostly in the form of fibril ‘aggregate’ particles rather than fully individualised fibrils, i.e. the fibrils are liberated from the fibre, but are imperfectly separated from their neighbours and so are physically rooted to other fibrils to form a network (The Microscopy tab in the Supplementary Material gives several examples of

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what this looks like for different fibre species). These particles are large enough to have some geometric properties inferred from measurement with the fibre analyser. In this study, the length-weighted fibre length of these MFC particles, called from here onwards MFC length was found to be useful. The fibre analyser interprets the ‘fibre length’ in this case as the longest dimension of an MFC particle. When plotting MFC length against MFC tensile index for all fibre species, there is no general correlation, but for fibres of a similar hemicellulose content, there appears to be a clear positive trend (see Figure D1 in the MFC Correlation Investigation tab in the Supplementary Material). Therefore, using the product of hemicellulose and MFC length to predict tensile index appeared promising.

In order to have some theoretical grounding for this correlation, the Page Equation was used as a starting point. The Page Equation, shown as Eq. (1) in the introduction, was formulated to predict tensile strength for straight, individualised fibres with lengths on the scale of millimetres, when formed into a mineral-free sheet. It is not obvious whether this would be directly applicable to MFC nanopaper, which is in the form of highly entangled and conformable, physically connected networks of fibrils, which in this study forms a composite that is 80% paper filler mineral. However, the Page Equation is conceptually useful since parameters such as fibre length and relative bonded area are expected to be applicable in a similar way to this MFC-mineral composite nanopaper. This equation will be used to justify a semi-empirical model to predict MFC tensile strength using hemicellulose and MFC length.

It stands to reason that as relative bonded area increases greatly with MFC production, the bonding term of the Page Equation becomes less limiting, and strength would eventually be dominated by the zerospan term representing fibre or fibril breakage if fibrillation is extensive enough. For chemical-treated nanocellulose (i.e. TEMPO MFC) with very fine microfibrils and very extensive bonding, this may be the case, but at least for the relatively coarse mechanically processed FiberLean MFC under consideration, further work not published here shows that this limit is not reached. It was found that the tensile strength of MFC produced using this method varies strongly with mineral content, demonstrating that bonding failure is still important.

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Additionally, the zero-span tensile strength of the MFC at 50 dry wt% mineral was measured, and for most fibre species was over 2.5 times the long span tensile strength of the MFC, when normalised by fibre content. This MFC zero-span tensile strength was also found to vary strongly with mineral content, implying that bonding failure was not completely suppressed (likely because smaller MFC particles are more prone of slipping out of the gap between the clamps). The notion that MFC fails mainly by network failure even in a zero-span tensile test is supported by electron microscopy of breakage sites by Varanasi et al. (2012).

This makes the measured MFC zero-span strength a gross underestimate of the true value, and so fibril failure is responsible for much less than 40% of the breakage.

Therefore, the fibre weakness term will be assumed to be zero as the MFC zero-span tensile strength is believed to be much higher than the bonding strength.

The Page Equation is therefore reduced to: 1 12Aq ¼ T sB PLðRBAÞ

ð2Þ

Hemicellulose on microfibril surfaces would be expected to increase relative bonded area since extended hemicellulose chains allows for more extensive and intimate contact between microfibrils. Additionally, hemicellulose appears to result in finer microfibrils when MFC is produced. Finer diameter microfibrils could be expected to be more flexible, and so be more capable of deforming for a more intimate contact with other particles. It is also expected that finer microfibrils would be more susceptible to capillary forces drawing them in contact with other surfaces during drying. These effects would imply that a high hemicellulose content would lead to a high relative bonded area. It is conceivable that these same factors could lead to a higher specific bond strength sB within a given region of bonded area (i.e. more hemicellulose chains likely means a higher fraction of the bonded area in molecular contact).

The perimeter to cross-sectional area ratio, P/A, would also correlate with increased bonding; since this increases as fibril diameter decreases, this would also correlate with hemicellulose content. Though P/A, RBA, and sB are all believed to be influenced by hemicellulose content, their relative influences cannot be distinguished. If hemicellulose correlated linearly with all three, then it would be expected that, all other

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things being equal, tensile index would be proportional to the cube of the hemicellulose content; however, Fig. 1 shows that this correlation is linear, and the maximum R2 value of the fit between hemicellulose and MFC tensile strength was obtained with a hemicellulose exponent value close to unity.

Therefore, rather than distinguishing between the three, the RBA term, the P/A term, and the sB term in the denominator of the Page Equation are replaced with the hemicellulose content H in Eq. (3) below.

Since the larger MFC particles are the primary loadbearing particles, the corresponding MFC length term LMFC is expected to be analogous to the fibre length term L in the Page Equation, and so this substitution is also made: 1 12q ¼k T LMFC H

ð3Þ

where k is a proportionality constant. Density q is constant for all MFC, so can be combined with k under a single coefficient B1: 1 1 ¼ T B1 LMFC H

ð4Þ

which can be inverted to obtain the MFC tensile index: T ¼ B1 LMFC H

ð5Þ

The product of LMFC and H was calculated for all fibre sources and plotted against MFC tensile index in Fig. 3. As can be seen, the fit to a general straight line is much better when MFC length is included in this analysis compared to just hemicellulose content alone, with the R2 improving from 0.63 to 0.87. However, this fit requires an intercept that is not accounted for in Eq. (5). This intercept is at least in part because fibrillation can take place in the absence of hemicellulose, as the cotton linter and jeans MFC demonstrate.

Other contributions to this intercept may be due to some of the assumptions being inaccurate, for example if the contribution of fibre weakness to the strength was significant, or if the tensile strength at 80% mineral loading was not directly proportional to mineral-free tensile strength for all samples. Despite this, the fit is good, and so an empirical residual strength term, r0, is added to Eq. (5), representing at least in part the MFC tensile strength when hemicellulose is absent.

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R² = 0.87

MFC Tensile Index (Nm/g)

12 10 8 6

4 2 0 0.00

0.01 0.02 0.03 0.04 0.05 0.06 Hemicellulose Content (mass frac on) * MFC Length (mm)

Nordic Pine Enzyme Nordic Pine Birch #2 South Asian HW Abaca Miscanthus

Black Spruce Douglas Fir Eucalyptus Tissue Dust Sisal Sorghum

Radiata Pine Dissolving Pulp Acacia Co on Bagasse Giant Reed

Southern Pine Birch #1 European HW Jeans Kenaf Flax

Fig. 3 Tensile index versus the product of the hemicellulose content and the MFC length

T ¼ B1 LMFC H þ r0

ð6Þ

Figure 3 gives r0 a value of 4.1 N m/g on a basis of the fit with the twenty-four fibre sources tested.

Despite the good fit, using the MFC length in order to shortlist which fibre sources are worth using as a feed source is impractical; the MFC must first be created to obtain this length value, and so this only reduces the need of the tensile test, saving little effort overall. Instead, it is more useful to identify the reasons behind why different fibres give MFC of different MFC particle lengths, and relate this to a parameter that can be measured in the unprocessed fibres.

Influence of fibre zero-span tensile index It stands to reason that if the fibrils that form the cell wall are long, that once the fibre is disintegrated the liberated fibrils will also be long. Additionally, a greater number of flaws in the fibril structure would make breaking the fibril (and the larger scale fibre) across the cross-section easier. Therefore, a fibre that has intrinsically long fibrils with few discontinuities and cross-sectional flaws would be expected to produce long fibrils when ground down into MFC, and would therefore give a relatively high MFC

particle lengths as measured by the fibre analyser. As discussed in the introduction, the zero-span tensile test gives an indication of the frequency of these flaws by forcing fibres to break across their cross section, with a high value indicating a lack of flaws and discontinuities.

The zero-span tensile test is not solely a measure of fibre and fibril damage. A good correlation has been found between the microfibril angle and the zero-span tensile strength, at least with fibres that are relatively undamaged by processing (Courchene et al. 2006).

This agrees with data and theoretical modelling from El-Hosseiny and Page (1975). However, the microfibril angle is not expected to directly influence intrinsic fibril lengths or frequency of fibril damage, and it would therefore be surprising if it influences fibril length in MFC. Although easy to measure in wood, the microfibril angle is difficult to measure accurately in pulped fibres, and so was not attempted in this study, particularly because it is impractical in an industrial setting. This is likely a significant cause of some of the data spread seen in Fig. 4.

Figure 4 shows the correlation between the fibre zero-span tensile index and the fibre length of the resultant MFC. There appears to be a linear relation between these two parameters when considering most fibre species, although there are several exceptions

200

Fibre Zero Span Tensile Index (Nm/g)

14

150

100

50

0

0

0.1

0.2

0.3

0.4

MFC Length (mm) Nordic Pine Enzyme Nordic Pine Birch #2 South Asian HW Abaca Miscanthus

Black Spruce Douglas Fir Eucalyptus Tissue Dust Sisal Sorghum

Radiata Pine Dissolving Pulp Acacia Co on Bagasse Giant Reed

Southern Pine Birch #1 European HW Jeans Kenaf Flax

Fig. 4 The relationship between the zero-span tensile index of unground fibres and the resultant MFC length

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that deviate significantly, including flax, sisal, and kenaf.

Despite this complication, the fit between MFC length and zero-span tensile index for most fibre species appears sufficient for most of the other fibre species to use zero-span tensile index as a replacement for MFC length in Eq. (6). This equation is therefore modified below: T ¼ B2 ZH þ r0

ð7Þ

where Z represents the zero-span tensile index of the fibre and B2 is a proportionality coefficient. Figure 5 shows the correlation between the product of the fibre zero-span tensile index and hemicellulose content, with the MFC tensile index.

As Fig. 4 shows, the assumption that fibre zero-span tensile index correlates with MFC length is not completely accurate, so it is not surprising that the fit shown in Fig. 5 for Eq. (7) (R2 = 0.78) is poorer than when MFC length was used (R2 = 0.87). Although this relationship fits well considering the extremes, towards the centre of the graph certain fibre species such as miscanthus, abaca, and sisal deviate considerably from the best fit curve.

14 R² = 0.78

MFC Tensile Index (Nm/g)

12 10 8 6 4 2 0

0

10

20

30

40

Hemicellulose Content (mass frac on) * Fibre Zero Span Tensile Index (Nm/g) Nordic Pine Enzyme Nordic Pine Birch #2 South Asian HW Abaca Miscanthus

Black Spruce Douglas Fir Eucalyptus Tissue Dust Sisal Sorghum

Radiata Pine Dissolving Pulp Acacia Co on Bagasse Giant Reed

Southern Pine Birch #1 European HW Jeans Kenaf Flax

Fig. 5 The correlation between the product of the hemicellulose content and zero-span tensile index of the unground fibres, with the resultant MFC tensile index when ground to the standard energy input

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A particularly notable comparison can be made between Nordic pine and enzyme-treated Nordic pine fibres, which are essentially identical in all ways measured except the zero-span tensile index, which has been greatly degraded in the case of the latter. The enzyme-treated variant consequently has much lower MFC particle length and lower MFC tensile strength.

This is evidence that the correlation seen with zerospan tensile index is causal.

Despite the increase in data spread compared to when MFC length is used directly, Fig. 5 shows a good fit for most fibre species, and this relationship comes with the important advantage that both predictive parameters are fibre properties that can be measured relatively easily without having to produce the MFC first. This relationship would be a practical tool for shortlisting a large number of fibre species, in order to determine which are worth pursuing to use as a feed for MFC production.

Conclusions In this work, microfibrillated cellulose was produced from twenty-four fibre species using a stirred media detritor. The product was used in a composite with calcium carbonate mineral to form nanopaper sheets, which were tested to assess the tensile index. The hemicellulose content of the fibres prior to MFC production was measured and was found to correlate moderately with the tensile index of the respective MFC produced. Additionally, SEM images show that higher hemicellulose fibres result in the liberation of microfibrils with finer widths, which is consistent with what others have reported using alternative MFC production methods.

Various geometric parameters of the fibres such as length and width were measured with a fibre analyser and did not correlate with the MFC tensile index.

However, measurements of the apparent length of the MFC product particles were taken with this equipment, and multiplying this length by the hemicellulose content resulted in a parameter that correlated strongly with MFC tensile index. The Page Equation was applied and modified to give this correlation some theoretical basis, with some of the parameters in the bonding term being substituted with hemicellulose content and MFC length. It was found that the addition of a constant r0 to represent the residual strength in the

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absence of hemicellulose was required for the model to fit the data.

In order to make this model useful, the zero-span tensile strength of the initial fibres was used as a proxy for the MFC length, since it appears that the frequency of flaws in the fibril structures that is represented by the zero-span tensile strength affects the particle lengths when MFC is produced. This replacement resulted in a weaker fit, but still a substantial improvement over hemicellulose content alone. From this work, it is shown that measurements of the hemicellulose content and zero-span tensile strength of pulp fibres can be used for a reasonably accurate prediction of the resultant MFC tensile strength, providing a facile method to aid in the selection of cellulose sources for use as a feedstock for microfibrillated cellulose production.

Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest.

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.

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Carbohydr Polym 73(1):156–163 Wathén R (2006) Studies on fiber strength and its effect on paper properties. PhD Thesis, King’s College London. ISSN 1457–6252 Zeng X, Retulainen E, Heinemann S, Fu S (2012) Fibre deformations induced by different mechanical treatments and their effect on zero-span strength. Nord Pulp Pap Res J 27(2):335–342 Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 2, 2021

Experimental Study of Oriented Strand Board Ignition by Radiant Heat Fluxes Ivana Tureková, Iveta Marková, Martina Ivanovičová & Jozef Harangózo Wood and composite panel materials represent a substantial part of the fuel in many building fires. The ability of materials to ignite when heated at elevated temperatures depends on many factors, such as the thermal properties of materials, the ignition temperature, critical heat flux and the environment. Oriented strand board (OSB) without any surface treatment in thicknesses of 12, 15 and 18 mm were used as -2 experimental samples. The samples were gradually exposed to a heat flux of 43 to 50 kW.m , with an -2 -2 -2 increase of 1 kW.m . At heat fluxes of 49 kW.m and 50 kW.m , the ignition times are similar in all OSB thicknesses, in contrast to the ignition times at lower heat fluxes. The influence of the selected factors (thickness and distance from the heat source) was analysed based on the experimentally obtained data of -2 ignition time and weight loss. The experimentally determined value of the heat flux density was 43 kW.m , which represented the critical heat flux. The results show a statistically significant effect of OSB thickness on ignition time.

Contact information: Ivana Tureková, Martina Ivanovičová & Jozef Harangózo: Department of Technology and Information Technologies, Faculty of Education, Constantine the Philosopher University in Nitra, Tr. A. Hlinku 1, 949 74 Nitra.

Iveta Marková: Department of Fire Engineering, Faculty of Security Engineering, University of Žilina, Univerzitná 1, 010 26 Žilina, Slovakia.

Polymers 2021, 13, 709. https://doi.org/10.3390/polym13050709 Creative Commons Attribution 4.0 International License

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Article 6 – Ignition of OSB

polymers Article

Experimental Study of Oriented Strand Board Ignition by Radiant Heat Fluxes Ivana Tureková 1 , Iveta Marková 2, * , Martina Ivanovičová 1 and Jozef Harangózo 1 1

2

*

Citation: Tureková, I.; Marková, I.; Ivanovičová, M.; Harangózo, J. Experimental Study of Oriented

Department of Technology and Information Technologies, Faculty of Education, Constantine the Philosopher University in Nitra, Tr. A. Hlinku 1, 949 74 Nitra, Slovakia; ivana.turekova@ukf.sk (I.T.); mabumb@gmail.com (M.I.); jozef.harangozo@ukf.sk (J.H.) Department of Fire Engineering, Faculty of Security Engineering, University of Žilina, Univerzitná 1, 010 26 Žilina, Slovakia Correspondence: iveta.markova@fbi.uniza.sk; Tel.: +421-041-513-6799

Abstract: Wood and composite panel materials represent a substantial part of the fuel in many building fires. The ability of materials to ignite when heated at elevated temperatures depends on many factors, such as the thermal properties of materials, the ignition temperature, critical heat flux and the environment. Oriented strand board (OSB) without any surface treatment in thicknesses of 12, 15 and 18 mm were used as experimental samples. The samples were gradually exposed to a heat flux of 43 to 50 kW.m−2 , with an increase of 1 kW.m−2 . At heat fluxes of 49 kW.m−2 and 50 kW.m−2 , the ignition times are similar in all OSB thicknesses, in contrast to the ignition times at lower heat fluxes. The influence of the selected factors (thickness and distance from the heat source) was analysed based on the experimentally obtained data of ignition time and weight loss.

The experimentally determined value of the heat flux density was 43 kW.m−2 , which represented the critical heat flux. The results show a statistically significant effect of OSB thickness on ignition time.

Keywords: OSB; heat flux density; ignition time; weight loss

Strand Board Ignition by Radiant Heat Fluxes. Polymers 2021, 13, 709. https://doi.org/10.3390/polym1305 0709 Academic Editor: Antonios N. Papadopoulos Received: 1 February 2021 Accepted: 21 February 2021 Published: 26 February 2021

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Copyright: © 2021 by the authors.

Licensee MDPI, Basel, Switzerland.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

1. Introduction Composite panel materials are important wood products [1]. Their production encompasses the utilization of wood of lower quality classes to obtain suitable materials with improved physical and mechanical properties [2]. Oriented strand board (OSB) belong to this group of products but essentially, these products are input materials in the furniture and construction industries [3].

The production of wood-based sheet materials utilizes wood of lower quality classes and chemically safe recyclates and generates materials with improved physical and mechanical properties compared to raw wood [4]. Oriented strand boards (OSBs) are defined in [5] as multilayer boards made of wood strands of a specific shape, thickness and adhesiveness. Strands in the outer layers are oriented parallel to the length or width of the boards. Strands in the middle layer or layers may be oriented randomly or generally perpendicular to the strands of the outer layers [5].

Oriented strand board, also known as OSB, waferboard, Sterling board or Exterior board and SmartPly, is a widely used engineered wood product formed by strands (flakes) of wood, often layered in specific orientations [6]. In appearance, it may have a rough and variegated surface with the individual strands (typically around 2.5 by 15 cm each) lying unevenly across each other. OSBs are cheap and strong boards, and this makes them excellent building material [7].

OSB is produced from thinner wood strands, which can be arranged better with respect to the direction of the wood fibres when adding the layers in longitudinal and transverse directions in the production flow. Strands have the longest length in the direction of the fibres [8]. Thinner debarked forest wood assortments, predominantly soft deciduous

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woods but also coniferous woods, are used as a raw material for OSB production. Great emphasis is placed on the preparation of strands. They are mainly produced by disc and drum chippers [9]. Thinner and longer strands are used for the surface layer, thicker and slightly shorter strands are used for the middle layers. Fine particles and dust are carefully sorted out. The strands are pressed under high pressure and temperature, using formaldehyde-based synthetic resins [10].

Most commonly used adhesives are conventional synthetic adhesives, both liquid and powder, including isocyanate-based adhesives [11]. OSB is categorized based on the purpose of its application. General-purpose boards (manufactured with a thickness of 9, 11, 15 or 18 mm) and boards for indoor furnishing (including furniture) in dry conditions are classified as OSB/1; load-bearing boards for use in dry conditions are classified as OSB/2; load-bearing boards for use in humid conditions are OSB/3; and heavy-duty load-bearing boards for use in humid conditions are OSB/4 [12].

OSB panel product in terms of physical and health hazards is unclassified according to safety data sheets [13]. OSB is the flammable composite material. OSB can be in contact with heat sources, and it will react to the effect of heat and temperature rise in its structure [14,15].

Wood materials are thermal insulators and do not conduct heat; hence there is a gradual process of thermal degradation, which can result in ignition and fire [16,17]. In the case of thermal stress, the strength of OSB decreases with increasing temperature and time of its action, while at higher temperatures the rate of this change is higher [18].

Flammability is defined as the ability of a sample to ignite under the action of an external thermal initiator and under defined test conditions according to [19]. According to International Organization for Standardization (ISO) 3261 [20], flammability is the ability of a material to ignite. Flammability is characterized by the ignition time of substances and materials, which depends on the ignition temperature, thermal properties of materials, sample conditions (size, humidity, orientation) and critical heat flux [21]. The definition of the term “ignition temperature” can be interpreted as the minimum temperature to which the air must be heated so that the sample placed in the heated air environment ignites, or the surface temperature of the sample just before the ignition point [22,23]. This interpretation was used as the basis of our research. The results were realized on tested equipment without a small burner flame, having only a radiated heat loading.

The aim of this article is to monitor the significant effect of heat flux density (from 43 to 50 kW.m−2 ) and thickness (12 mm, 15 mm, 18 mm) of OSB on the ignition time and change in the weight of the sample. The change in heat flux was previously monitored and recorded for the purpose of validating equipment for the research of heat release loading.

At the same time, the critical temperature of the ignition point of the OSB was experimentally determined depending on the time of action of the radiant heat source and the intensity of the heat flux.

2. Materials and Methods 2.1. Experimental Samples Samples of oriented strand board (OSB) without surface treatment, produced by Kronospan Jihlava (KRONOSPAN CR, spol. s r.o., Jihlava, Czech Republic) under the title OSB/3 SUPERFINISH ECO (Figure 1), were selected for the experiments. The OSBs used were multilayer boards made of flat strands of a specific shape and thickness. The strands in the outer layers were oriented parallel to the length or width of the board, and the strands in the middle layers were oriented randomly or were generally perpendicular to the lamellas of the outer layers. They were bonded with melamine formaldehyde resin and polymeric diphenylmethane diisocyanate (PMDI) (Shandong Shanshi Chemical Co Ltd, Zhangdian District, Zibo City, Shandong Province, China) and they were flat-pressed.

The boards contained mainly a mixture of coniferous wood. Table 1 shows the physical and chemical properties and fire technical characteristics of the OSB with thickness of 10 mm to 18 mm [10,13].

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(a) Example of OSB

(b) Example of experimental sample of OSB

Figure 1. Oriented strand board in (a) full piece and (b) prepared for measurement according to [24].

Samples of OSB were cut to specific dimensions (165 × 165 mm) according to ISO 5657: 1997 [24]. The density of the strand boards was determined according to Standard EN 323: 1996 [25].

Table 1. Physical and chemical properties and fire technical characteristics of the OSB with thickness of 10–18 mm [13].

Parameters

Notes

(kg.m−3 )

Density Humidity (%) Bending Strength (N.mm−2 ) Modulus of Elasticity (N.mm−2 ) Swelling (%) Thermal Conductivity (W.m−2 .K−1 ) Formaldehyde Content (mg.100 g−1 ) Flame Spread Index 1 Reaction to Fire

Main Axis Secondary Axis Main Axis Secondary Axis

Thickness 9 mm 2 Thickness 18 mm 3

Class of Fire Reaction [26]

Values 630 ± 10% 5 ± 12% 20 10 3500 1400 15 0.13 8 83.3 D-s2, d2 D-s1, d0 E—Eoderately Flammable

1

Flame spread index was calculated from peak heat release rate (HRR), total heat release and time to sustained ignition by ASTM E 84 ([27] and [28]; 2 [29]; 3 [10].

The selected board materials were stored at a specific temperature (23 ◦ C ± 2 ◦ C) at relative humidity (50 ± 5%). Rantuch et al. [30] state that OSB moisture does not have a significant effect on the critical conditions under which they can be ignited, but significantly affects the time at which it occurs.

2.2. Experimental Procedures The methodology can be divided into three parts: 1.

2.

3.

Verification of test equipment; a small burner flame was not used as the secondary ignition source.

Determination of ignition time and weight loss depending on the selected level of heat flux density and thickness of board materials and on the distance of selected board materials from the ignition source.

Determination of the critical temperature during ignition of an OSB with a thickness of 15 mm.

2.2.1. Verification of Test Equipment The beginning of the experimental measurement entailed verification of the test equipment according to ISO 5657: 1997 [24]. The aim of this verification was to monitor

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the temperature parameters of the cone calorimeter. The cone calorimeter must provide a heat flux in the range of 10 to 70 kW.m−2 in the centre of the hole in top plate and in a base plate which matches with the bottom side of the top plate (Figure 1b).

Verification of the actual condition of the cone calorimeter was performed using a SBG01-100 radiometer with a cooling sensor (Hukseflux, Delft, The Netherlands) and a METEX M-3890D digital multimeter (Manufacturer: Zebronics, India) with a USBVIEV program (SOFTONIC INTERNATIONAL S.A., Barcelona, Spain). Several experimental measurements, which included collecting voltage values from the multimeter and evaluating the results using the calibration curve specified in the standard, confirmed that at the selected temperature, the cone calorimeter indicated the corresponding heat flux densities in accordance with the previous measurements performed during test equipment calibration.

Based on the obtained values and compared results, a graphical dependence of the heat flux on the temperature of the cone calorimeter was constructed (Figure 2). The measurements were repeated with each thickness of the board material 5 times.

Figure 2. Heat flux dependence on cone calorimeter temperature.

2.2.2. Methodology for Determining Ignition Time and Weight Loss The ignition time and weight loss depending on the selected level of heat flux density and thickness of the board materials, as well as on the distance of the selected board materials from the ignition source, were determined according to a modified procedure based on ISO 5657: 1997 [24]. This modification included a change of igniter. Ignition was caused only by heat flux, without the use of a direct flame (Figure 3a).

The samples were placed horizontally and exposed to a heat flux of 43 to 50 kW.m−2 by an electrically heated conical radiator (Beijing Global Trade Software Technology Co., Ltd., Chaoyang District, Beijing, China). Orientation experiments determined the minimum heat flux required to maintain flame combustion. The time-to-ignition value was recorded, while considering only the permanent ignition of the surface of the analysed sample when exposed to a selected level of heat flux density.

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(a)

(b)

Figure 3. (a) Scheme of the equipment for determination of flammability of materials at a heat flux of radiant heat of 10–50 kW.m−2 according to [24]; (b) a photo of the real test equipment [31].

Legend: 1, heating cone; 2, board for sample; 3, movable arm; 4, connection point for recording experimental data.

The observed factors influencing the ignition time and weight loss were

• •

the thickness of the board material; and the density of the radiant heat flux.

2.2.3. Determination of the Critical Ignition Temperature Determination of the critical ignition temperature was performed on a sample of OSB with a thickness of 15 mm. Test equipment according to [24] was supplemented by two thermocouples. These thermocouples were placed in the middle of the top and bottom surfaces of the board.

Test samples were exposed to four selected levels of heat flux density (44, 46, 48 and 50 kW.m−2 ) for 300 s, which was repeated 5 times.

2.2.4. Statistical Processing of Data and Evaluation of Results To evaluate the influence of the above-mentioned factors on the ignition temperature and weight loss, the obtained results were subjected to statistical analysis. The obtained results of the ignition and weight loss temperatures were statistically evaluated by oneway analysis of variance (ANOVA) using the Least Significant Difference LSD test (95%, 99% detectability level) (STATGRAPHICS software version 18/19, The Plains, VA, United States), with the use of board material thickness (12, 15 and 18 mm) and radiant heat flux density (from 43 to 50 kW.m−2 ) as influence factors.

3. Results and Discussion The course of the experiment (Figure 4) according to [24] confirmed the verified behaviour of the material in terms of the classification “reaction to fire D-s1, d0 by [10]”. Ds1, d0 means “combustible materials – medium contribution to fire, with speed of emission absent or weak during combustion” by [26]. The priority of the experiment was to monitor the critical parameters of the ignition based on the change in board thickness (Table 2).

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Figure 4. Demonstration of the course of burning of oriented strand boards (OSBs) after their ignition by radiant heat (a) ignition (100 s); (b) burning time of 160 s; and (c) 300 s. (d) A demonstration of the combustion process by 44 kW.m−2 . (e) A sample selected from the measuring device; specific distance of 12 mm, time of ignition of 57 s and density heat flux of 50 kW.m−2 . (f) Cooled sample 10 min after the experiment; sample thickness of 15 mm and ignition time of 108 s, 44 kW.m−2 .

3.1. Determination of Ignition Temperature and Weight Loss For the purposes of our research, the initial value of the radiant heat flux, to which the selected OSBs were exposed, was experimentally set to 43 kW.m−2 . This value represented the critical heat flux for the selected samples. Critical heat flux is the heat flux between the minimum incident heat fluxes at which ignition occurs and maximum incident heat flux at which ignition does not occur. It can be used to evaluate the ignition ability. The critical heat flux is obtained experimentally by gradually exposing the samples to decreasing heat flux until ignition ceases [30]. The maximum value of the radiant heat flux, to which selected board materials were exposed, was 50 kW.m−2 . The heat flux gradually increased by 1 kW.m−2 (Table 2).

The effect of external heat flux on a sample is the incident energy upon its surface. Part of the energy is reflected (depending on the emissivity of the surface), part is transferred by conduction to deeper layers of the material (depending on its thermal conductivity) and the rest is absorbed by a thin layer on the surface, i.e., heating takes place. As the temperature of the sample increases, pyrolysis and thermal oxidation occur, as a result of which gaseous products are released into the environment [30]. Therefore, the time to ignition of the sample decreases with increasing external heat flux [31], which is confirmed by our results.

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Table 2. Ignition time and weight loss in samples with different thicknesses using heat fluxes of 40 to 50 kW.m−2 at a distance of 20 mm.

1

Density of Radiant Heat Flux (kW.m−2 )

Corresponding Temperature (◦ C) 1

Thickness (mm)

Ignition Time (s)

Weight Loss (%)

43

700

44

710

45

720

46

724

47

727

48

730

49

735

50

742

12 15 18 12 15 18 12 15 18 12 15 18 12 15 18 12 15 18 12 15 18 12 15 18

107.4 ± 32.927 172.8 ± 68.271 170.0 ± 19.279 80.80 ± 14.372 108.0 ± 31.093 140.0 ± 31.698 86.4 ± 10.442 100.2 ± 21.673 111.2 ± 24.235 84.4 ± 9.002 93.4 ± 21.767 98.8 ± 12.592 67.08 ± 5.403 71.0 ± 8.671 103.6 ± 18.391 58.60 ± 5.953 63.40 ± 7.116 77.60 ± 25.881 62.20 ± 3.2497 63.20 ± 3.187 65.0 ± 11.436 56.80 ± 2.039 59.40 ± 5.607 60.20 ± 5.741

19.018 ± 0.742 16.528 ± 1.103 12.436 ± 0.402 20.188 ± 1.210 16.092 ± 0.885 13.256 ± 0.745 20.87 ± 0.889 17.026 ± 0.541 13.716 ± 0.303 21.868 ± 0.879 17.272 ± 0.647 13.504 ± 0.228 22.026 ± 0.908 17.5 ± 0.455 13.818 ± 0.266 23.206 ± 0.505 18.366 ± 0.910 14.222 ± 0.826 23.578 ± 0.858 18.764 ± 0.571 14.678 ± 0.899 24.302 ± 0.814 19.402 ± 0.586 14.846 ± 1.033

Based on the graphical dependence in Figure 3.

An interesting result is the reduction of time differences in individual thicknesses at higher heat fluxes. Heat fluxes of 49 and 50 kW.m−2 offered almost equal times in all thicknesses. Differences in ignition times decreased with increasing heat flux, and the ignition differences between the individual thicknesses decreased. The maximum difference of ignition time between thicknesses (67.4 s) was at 43 kW.m−2 and the minimum (3.4 s) was at 50 kW.m−2 .

It can therefore be concluded that with increasing heat flux value, the thickness of the OSB has no significant effect on the ignition time. The increase in weight loss was with increasing heat flux, namely in 12 mm by 5.3%, in 15 mm by 2.9% and in 18 mm by 2.41%.

With increasing heat flux, there was an increase in weight loss of 12 mm by 5.3%, 15 mm by 2.9% and 18 mm by 2.41%. It is possible to make the following assumption: smaller thicknesses of OSB due to the increase of heat flow have more intensive thermodegradation processes, which manifest in higher weight losses.

Mitterová and Garaj [32] studied the weight loss and ignition time of an OSB caused by the action of radiant heat. Test samples were exposed to an infrared heater with a power of 1000 W for 600 s, while the distance of the samples from the surface of the radiating body was 30 mm.

The weight loss of the OSB reached 58.92%, and the ignition time was 53.4 s. In comparison with the values obtained by our research, the results are significantly different, because the distance between the emitters and the sample was relatively high, which affected the ignition time. The surprising result is more than 50% weight loss under these conditions.

The use of one-way ANOVA confirms the significant dependence of the ignition time on the board thickness (Table 3, Figure 5). The ANOVA table decomposes the variance of Col_4: Time- to-ignition into two components: a between-group component and a within-group component. The F-ratio, which in this case equals 4.31448, is a ratio of the between-group estimate to the within-group estimate. Since the P-value of the F-test is less than 0.05, there is a statistically significant difference between the mean Col_4 from one level of Col_2 to another at the 5% significance level. To determine which means are significantly different from which others, select Multiple Range (Table 3).

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Table 3. ANOVA table for Col_4 Time-to-ignition by Col_2 thickness.

Source

Sum of Squares

Df

Mean Square

F-Ratio

P-Value

Between Groups Within Groups Total (Corr.)

12,608.1 170,953 183,561

2 117 119

6304.03 1461.13

4.31

0.0156

Figure 5. Graphical representation of the statistical evaluation of the influence of the board thickness on the ignition time under the action of the radiant heat source on the OSB. Legend: Col_2, thickness; Col_4, Time-to-ignition.

Concurrently, a significant dependence of the ignition time on the heat flux was confirmed (Table 4, Figure 6). The ANOVA table decomposes the variance of Col_4 into two components: Time-to-ignition and heat flux. The F-ratio, which in this case equals 17.5456, is a ratio of the between-group estimate to the within-group estimate. Since the P-value of the F-test is less than 0.05, there is a statistically significant difference between the means Col_4 Time-to-ignition from one level of Col_1 heat flux to another at the 5% significance level. To determine which means are significantly different from which others, select Multiple Range (Table 4).

Figure 6. Graphical representation of the statistical evaluation of the influence of heat flux on ignition time under the action of a radiant heat source on the OSB. Legend: Col_1, heat flux; Col_4, Time-to-ignition.

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Table 4. ANOVA table for Col_4 Time-to-ignition by Col_1 heat flux.

Source

Sum of Squares

Df

Mean Square

F-Ratio

P-Value

Between Groups Within Groups Total (Corr.)

96,009.1 87,551.5 183,561

7 112 119

13,715.6 781.71

17.55

0.000

Weight loss caused by thermal degradation of the board surface, which was exposed to radiant heat (Table 2), maintained the same course. Trend lines for the individual board thicknesses have a parallel direction, so it can be assumed that the weight loss is uniform per each unit of thickness (Figure 7). Difference can be seen in the samples with a thickness of 12 mm.

Figure 7. Graphical dependence of the weight loss ΔM and sample thickness H ratio on the heat flux.

Legend: black point, 12 mm thickness; blue point, 15 mm thickness; red point, 18 mm thickness.

The results clearly show the following dependence: with increasing thickness, the weight loss decreases, but the ratio of weight loss and sample thickness ΔM/H is rather similar in all heat fluxes (Figure 7). A similar scenario is observed at a thickness of 15 mm.

The highest weight loss is observed at a thickness of 12 mm, where an area of light weight of the material burns down and it is possible to observe a slight increase in weight loss with increasing heat flux. There are several studies that confirm the reduction of the weight loss and prolongation of time-to-ignition by appropriate fire-retardant treatment of the boards [3,33,34].

Statistical analysis confirmed this fact. The heat flux (Table 5) and material thickness (Table 6) have no statistically significant effect on the change in sample weight.

Table 5. ANOVA table for Col_3 Weight loss by Col_1 heat flux.

Source

Sum of Squares

Df

Mean Square

F-Ratio

P-Value

Between Groups Within Groups Total (Corr.)

5.11703E7 4.00277E8 4.51448E8

7 112 119

7.31005E6 3.5739E6

2.05

0.0554

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Table 6. ANOVA table for Col_3 Weight loss by Col_2 thickness.

Source

Sum of Squares

Df

Mean Square

F-Ratio

P-Value

Between Groups Within Groups Total (Corr.)

5.78793E6 4.4566E8 4.51448E8

2 117 119

2.89396E6 3.80906E6

0.76

0.4701

The ANOVA table decomposes the variance of Col_3 Weight loss (%) into two components: a between-group component and a within-group component. The F-ratio, which in this case equals 2.0454, is a ratio of the between-group estimate to the within-group estimate. Since the P-value of the F-test is greater than or equal to 0.05, there is not a statistically significant difference between the mean Col_3 Mass loss (%) from one level of Col_1 (heat flux) to another at the 5% significance level.

Additionally, since the P-value of the F-test is greater than or equal to 0.05, there is not a statistically significant difference between the mean Col_3 Weight loss (%) from one level of Col_2 thickness to another at the 5% significance level.

3.2. Determination of Critical Ignition Temperature Critical ignition temperature was monitored by means of thermocouples placed below and above the test sample. The increase in temperature was monitored on the upper surface of the 15 mm OSB, which was exposed to a direct radiant heat flux of 44, 46, 48 and 50 kW.m−2 for 300 s. The OSB boards with 15 mm thickness are the most commonly used building material in the construction of soffits.

The first set of experiments with a heat flux density of 44 kW.m−2 for the period of 300 s resulted in an ignition time of 142 s (Table 7). The temperature at the bottom surface of the board rose minimally (Figure 8). The time to ignition (142 s) was the longest compared to other values obtained at higher heat fluxes (Table 3). The ignition temperatures obtained from the experiments varied. It is not possible to find the correlation with other parameters.

Table 7. Time-to-ignition and ignition temperature at the upper and lower surfaces of a 15 mm thick OSB sample corresponding to the individual radiant heat flux densities.

Heat Flux (kW.m−2 )

Time to Ignition (◦ C)

44 46 48 50

142 70 64 58

Temperature (◦ C) Direct Side of Heat

Opposite Side

287 358 252 319

34 25 26 27

Figure 8. Temperature dependence of the upper surface of an OSB with a thickness of 15 mm on the exposure time to the cone calorimeter.

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The temperature at the upper surface of the sample rose sharply to about 700 ◦ C after ignition (Figure 8). The temperature at the bottom surface of the sample rose slowly and had a comparable course in all heat fluxes (Figure 8).

The temperature curves have a smooth linear course and show dependence of the temperature at the upper surface on the time of exposure to radiant heat with heat fluxes of 44 kW.m−2 and 46 kW.m−2 (Figure 8). After ignition, there was a sharp rise in temperature at the upper surface of the sample (Figure 8). The sharp rise in temperature was likely caused by two effects. The first effect was a higher heat flux from the cone calorimeter to the surface of the tested sample and the second effect was a higher reverse heat radiation from flares [35].

Rantuch et al. [21] divided the OSB combustion process into five phases based on extensive research on OSB (14 mm thickness) using a cone calorimeter with heat fluxes of 20, 30, 40, 50 and 60 kW.m−2 . The first phase entails a period prior to the sample ignition.

The heat received by the samples is consumed while heating them, but the concentration of gaseous combustible products during thermal degradation is not sufficient to initiate the flame combustion. The second phase occurs after ignition of the sample. The amount of released flammable gases is sufficient to maintain a steady-flame combustion and at the same time a carbonized layer has not yet formed on the surface of the OSB. The mass burning rate as well as the rate of heat release have, therefore, high values. After the formation of a carbonized layer (Figure 4e) on the exposed surface of the samples, a steady burning phase occurs. The rate of heat release is almost constant, and the weight loss is uniform. This is followed by the burning phase of the superheated sample, characterized by significant pyrolysis in its entire volume (Figure 4d).

The increase in the concentration of pyrolysis products is manifested by a more pronounced combustion and, thus, also by an increase in the rate of heat release. After complete burnout of the gaseous pyrolysis products, flameless combustion of the sample occurs with low weight loss [21].

OSB is often a part of the structural elements in the exterior, and therefore it is also crucial to carry out large-scale fire tests [36–38]. Sultan [36] showed that the effect of insulation types on the fire resistance of exterior wall assemblies with OSB sheathing can be considered significant for specific conditions.

4. Conclusions The ignition process cannot be defined by a single characteristic. Our research monitored important parameters, specifically type of the sample (OSB), thickness, heat flux density, weight loss, ignition temperature and critical ignition temperature, and modified conditions specified in the test procedures. OSBs with a thickness of 12, 15 and 18 mm were analysed with a focus on the critical heat flux and the ignition temperature. The experimentally determined value of the heat flux density was 43 kW.m−2 , which represented the critical heat flux. In all cases, dependences of the ignition time on the external heat flux were confirmed and there was a correlation between the ignition time, weight loss and intensity of radiant heat.

The following results were obtained from the conducted experiments:

• • •

•

As the heat flux density increased, the ignition time decreased in all thicknesses of analysed OSB. The ignition time increased with increasing thickness of the OSB, and the weight loss decreased with increasing thickness of the OSB at a constant heat flux.

With increasing board thickness, the weight loss decreased. The largest average weight loss of 24.31% was recorded in a 12 mm OSB that was exposed to a radiant heat flux of 50 kW.m−2 ; the lowest average weight loss of 12.44% was recorded in a sample of 18 mm OSB that was exposed to a radiant heat flux of 43 kW.m−2 .

The ignition time is significantly dependent on the thickness of the OSB sample and on the value of the heat flux. As the heat flux increases, the ignition time shortens; as the thickness of the OSB increases, the ignition time extends. The largest weight

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• •

loss of 27.22% was recorded in an OSB with a thickness of 12 mm and a density in the range of 500 to 550 kg.m−3 , which was exposed to a radiant heat flux of 50 kW.m−2 .

The lowest weight loss of 11.91% was found in an OSB with a thickness of 18 mm and a density in the range of 550–600 kg.m−3 , which was exposed to a radiant heat flux of 43 kW.m−2 .

The weight loss is not significantly dependent on the ignition time and the thickness of the OSB. Critical temperature of an OSB with a thickness of 15 mm that was exposed to heat flux densities of 44 kW.m−2 and 46 kW.m−2 had a linear character; at the heat flux densities of 48 kW.m−2 and 50 kW.m−2 , it had an initially linear course, but due to ignition, a sharp rise in temperature was noted at the upper surface of the sample.

The sharp rise in temperature was caused by two effects. The first effect was a higher heat flux from the cone calorimeter to the surface of the tested sample, and the second effect was a higher reverse heat radiation from flares.

Analysed parameters, such as the time-to-ignition parameter, related weight loss of the OSBs, the density and thickness of the OSBs, radiant heat flux density, distance of the ignition source from the material and determination of the critical ignition temperature with a modified arrangement of the test equipment and horizontal placement of the sample, confirmed the importance and complexity of these parameters for a better understanding of the critical ignition conditions of the OSB, as well as the combustion process.

Author Contributions: Conceptualization, I.T. and M.I.; methodology, I.T.; software, I.M.; validation, I.T., M.I. and J.H.; formal analysis, I.M.; investigation, I.T.; resources, I.T. and M.I.; data curation, M.I.; writing—original draft preparation, I.T.; writing—review and editing, I.M.; project administration, I.M.; funding acquisition, I.T. All authors have read and agreed to the published version of the manuscript.

Funding: This article was supported by the Cultural and Educational Grant Agency of the Ministry of Education, Science, Research and Sport of the Slovak Republic on the basis of the project KEGA 0014UKF-4/2020 Innovative learning e-modules for safety in dual education.

Data Availability Statement: Not applicable for studies not involving humans or animals.

Acknowledgments: This article was supported by the Project KEGA 0014UKF-4/2020 Innovative learning e-modules for safety in dual education.

Conflicts of Interest: The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript or in the decision to publish the results.

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PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 2, 2021

Estimation of the Compressive Strength of Corrugated Cardboard Boxes with Various Perforations Tomasz Garbowski, Tomasz Gajewski & Jakub Krzysztof Grabski This paper presents a modified analytical formula for estimating the static top-to-bottom compressive strength of corrugated board packaging with different perforations. The analytical framework is based here on Heimerl’s assumption with an extension from a single panel to a full box, enhanced with a numerically calculated critical load. In the proposed method, the torsional and shear stiffness of corrugated cardboard, as well as the panel depth-to-width ratio is implemented in the finite element model used for buckling analysis.

The new approach is compared with the successful though simplified McKee formula and is also verified with the experimental results of various packaging designs made of corrugated cardboard. The obtained results indicate that for boxes containing specific perforations, simplified methods give much larger estimation error than the analytical – numerical approach proposed in the article. To the best knowledge of the authors, the influence of the perforations has never been considered before in the analytical or analytical – numerical approach for estimation of the compressive strength of boxes made of corrugated paperboard. The novelty of this paper is to adopt the method presented to include perforation influence on the box compressive strength estimation.

Contact information: Tomasz Garbowski: Department of Biosystems Engineering, Poznan University of Life Sciences, Wojska Polskiego 50, 60-627 Poznań, Poland Tomasz Gajewski: Institute of Structural Analysis, Poznan University of Technology, Piotrowo Street 5, 60965 Poznań, Poland Jakub Krzysztof Grabski: Institute of Applied Mechanics, Poznan University of Technology, Jana Pawła II Street 24, 60-965 Poznań, Poland Energies 2021, 14, 1095. https://doi.org/10.3390/en14041095 Creative Commons Attribution 4.0 International License

The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International and the PITA Annual Review, both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating.

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Article 7 – Corrugated Strength

energies Article

Estimation of the Compressive Strength of Corrugated Cardboard Boxes with Various Perforations Tomasz Garbowski 1 , Tomasz Gajewski 2 1

2

3

*

Citation: Garbowski, T.; Gajewski, T.;

and Jakub Krzysztof Grabski 3, *

Department of Biosystems Engineering, Poznan University of Life Sciences, Wojska Polskiego 50, 60-627 Poznań, Poland; tomasz.garbowski@up.poznan.pl Institute of Structural Analysis, Poznan University of Technology, Piotrowo Street 5, 60-965 Poznań, Poland; tomasz.gajewski@put.poznan.pl Institute of Applied Mechanics, Poznan University of Technology, Jana Pawła II Street 24, 60-965 Poznań, Poland Correspondence: jakub.grabski@put.poznan.pl

Abstract: This paper presents a modified analytical formula for estimating the static top-to-bottom compressive strength of corrugated board packaging with different perforations. The analytical framework is based here on Heimerl’s assumption with an extension from a single panel to a full box, enhanced with a numerically calculated critical load. In the proposed method, the torsional and shear stiffness of corrugated cardboard, as well as the panel depth-to-width ratio is implemented in the finite element model used for buckling analysis. The new approach is compared with the successful though the simplified McKee formula and is also verified with the experimental results of various packaging designs made of corrugated cardboard. The obtained results indicate that for boxes containing specific perforations, simplified methods give much larger estimation error than the analytical– numerical approach proposed in the article. To the best knowledge of the authors, the influence of the perforations has never been considered before in the analytical or analytical–numerical approach for estimation of the compressive strength of boxes made of corrugated paperboard. The novelty of this paper is to adopt the method presented to include perforation influence on the box compressive strength estimation.

Grabski, J.K. Estimation of the Compressive Strength of Corrugated Cardboard Boxes with Various

Keywords: corrugated board; McKee formula; buckling; orthotropic panels; laboratory tests; box strength; crease; perforation

Perforations. Energies 2021, 14, 1095. https://doi.org/10.3390/en14041095 Academic Editor: José A.F.O. Correia Received: 31 December 2020 Accepted: 18 February 2021 Published: 19 February 2021

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Copyright: © 2021 by the authors.

Licensee MDPI, Basel, Switzerland.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

1. Introduction Shelf-ready packaging (SRP) is playing an increasingly important role in the modern industry, including the supply and sales market. The main task of SRP is to ensure safe transportation and storage of products in packaging and then putting them directly on shelves without unpacking individual goods. Application of SRP leads to the reduction of costs related to the work of employees and helps to save time. Therefore, tedious unpacking of products on the shelf has been reduced to a few seconds. The SRPs are made of corrugated cardboard with different perforations, which enable easy separation of the lower part of the packaging, called the tray, and the upper part of the packaging, called the cover. On the other hand, perforations can significantly influence the mechanical strength of boxes made of corrugated paperboards [1]. It reduces the integrity of the wall. Thus, the final compression strength of the box would also decrease. In this paper, the strength of boxes with perforations is investigated.

Performing physical experiments allows for the design and development of paperboard or corrugated cardboard boxes, to ensure their proper mechanical strength. It strictly depends on two characteristic in-plane directions of the corrugated cardboard. They are related to fluting; the first direction, called machine direction (MD), is perpendicular to the main axis of it and parallel to paperboard fiber alignment. The second one, called cross direction (CD), is parallel to the fluting.

Energies 2021, 14, 1095. https://doi.org/10.3390/en14041095

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In the packing industry, the strength of corrugated cardboard boxes can be examined by carrying out some leading physical tests. The most important, from a practical point of view, are the compressive, tensile, or bursting strength tests. The most common tests of packaging are the box compression test (BCT) and the edge crush test (ECT) for corrugated cardboard. Performing physical experiments is always time- and cost-consuming. Thus, recently, there have been some alternatives for the examination of corrugated cardboard boxes to determine their strength only by physical testing.

The first one is the prediction of compressive strength based on the formulas published in the literature. One advantage of such an approach is its simplicity, which makes it very easy and fast to apply in the real world, without the need of performing any additional experiments. Three groups of parameters used in these formulas proposed in the literature over the years can be distinguished. They include parameters of paper, board and box, respectively [2]. The most common formula in the packaging industry is the approach presented by McKee, Gander, and Wachuta in 1963 [3]. The authors formulated a relationship between the box perimeter and two corrugated board parameters, which are ECT and flexural stiffness. Two earlier approaches also took into account parameters of the paperboard [4,5]. However, one can notice that the formula proposed by McKee et al. is accurate only for some relatively simple containers. In the literature, there were some attempts for improving accuracy and extending the applicability of the analytical formulas for prediction of compressive box strength. Allerby et al. proposed a modification of constants and exponents in the original McKee’s relationship [6]. Schrampfer et al. modified the McKee’s approach, allowing to extend the applicability of the formula for a wide range of cutting methods and equipment [7]. Batelka et al. also took into account dimensions of the box [8], while Urbanik et al. included the Poisson’s ratio [9]. The constants in the original McKee’s formula was later analyzed for more complicated cases and was recently modified

by Aviles et al. [10] and later by Garbowski et al. [11,12]. The compression strength of boxes made of corrugated paperboard [13] can be affected by many factors, including the moisture content of the box [14,15], the existence of openings [12,16], storage time, stacking conditions [17], and many others.

The other alternative is a purely numerical method, which may be applied to investigate box compression strength. The most popular method in engineering is the finite element method (FEM). It was applied in many problems related to the strength of corrugated boxes. They include research on transverse shear stiffness of corrugated cardboards [18–21] or buckling and post-buckling phenomena [22].

One important step in the numerical analysis of corrugated cardboards is homogenization [23–26]. It allows simplifying the analyzed models, saving time on calculations by ensuring adequate accuracy of the results. As a consequence of the homogenization process of layered materials, one can obtain a single layer described by effective properties of the composite instead of the structure of layers made of different materials. Hohe proposed a method of homogenization based on strain energy [27]. It is suitable for sandwich panels.

In his approach, there is an assumption of equivalence of a representative element of the homogenized and heterogeneous elements. Buannic et al. proposed a periodic homogenization method. In this approach, an equivalent membrane and pure bending characteristic of period plates are obtained [28]. They also modified their approach for the analysis of sandwich panels to take into consideration the transfer shear effect. Biancolini obtained the stiffness properties of the sandwich panel using the energy equivalency between the considered model and the plate, and applying the FEM in the analysis of a micromechanical part of the plate [29]. In the approach presented by Abbès and Guo, the considered plate is decomposed into two beams in directions of the plate [30]. It allows calculating the torsion rigidity of the orthotropic sandwich plates. Another approach to a homogenization of composite materials may be the measurement of the effective properties based on a correctly designed or a selected set of laboratory experiments performed directly on the composite. Such a technique is applied in this study.

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In the literature, one can also find investigations on the influence of creasing on the mechanical strength of corrugated paperboard. Creasing is a process, in which fold and perforation lines are manufactured. These lines reduce the strength of corrugated paperboard. In 2008, Thakkar et al. compared the results from the experiments and numerical simulations based on the FEM to investigate the influence of the creasing on the local strength of corrugated paperboard [31]. Later, Beex and Peerlings performed physical and numerical experiments related to influence of creasing and subsequent folding on the mechanical properties of the laminated paperboard [32]. Giampieri et al. [33] proposed a constitutive model for the mechanical response of creased paperboard after folding. The FEM simulations of paperboard creasing and folding useful for the packing industry and important from a practical point of view has been presented by Domaneschi et al. [34].

Awais et al. [35] and Leminena et al. [1] investigated, experimentally and numerically, the influence of the crease on the mechanical properties of the paperboard during the press forming process. However, to the best knowledge of the authors, there are no papers on the influence of perforations on the strength of the whole boxes made of corrugated cardboard.

In this paper, a modification of the analytical–numerical approach is proposed for corrugated boxes with perforations. The article verifies the influence of perforation on the compressive strength of corrugated cardboard boxes with a modified analytical formula, through a numerically aided calculation of the critical load. Furthermore, the applicability of the approach presented in [11,12,20,21] for corrugated cardboard boxes has been extended to cases of the SRPs with different perforations.

2. Materials and Methods 2.1. Ultimate Compressive Strength of a Plate In order to compute the ultimate load on the box in a static top-to-bottom compression test, see Figure 1c, one can follow an idea presented by Heimerl [36], later used by McKee et al. [3] and recently extended by Garbowski et al. [11,12]. The basis of this approach was the calculation of the ultimate load, Pf , of each separated panel (see Figure 1a) using the edge-loaded compressive strength of corrugated board, ECT, and a critical load, Pcr , resulting from the buckling phenomenon:

Pf ECT r =k , Pcr Pcr

(1)

where k is a constant, r is an exponent, r ∈ (0, 1) (most often due to empirical fitting r ∈ [0.5, 0.75]; [9]), ECT is given in N/mm, and Pcr is given in N/mm.

Figure 1. (a) A panel b × a separated from corrugated box as a pined plate under in-plane compression; (b) corrugated board packaging with perforations; (c) box compression test (BCT) press used in laboratory tests [37].

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In Equation (1), only elastic buckling is took into account. This approach was later modified by Urbanik et al. [9,18] in order to also take into consideration the nonlinear material effects by the following:

Pf Pcr un =k , ECT ECT

(2)

where n = 1 − r. If ECT > Pcr , than u = 1, which corresponds to the situation when elastic buckling is activated first, otherwise, u = 0, which corresponds to the situation when the ultimate load Pf depends only on ECT, namely: Pf = k ECT.

(3)

In any case, the critical load for a rectangular orthotropic plate (see Figure 1a) can be assumed as [38–40]: √ π 2 D11 D22 b , (4) Pcr = k cr b2 b is the critical load of the panel of dimensions a × b (height and width, respecwhere Pcr tively), k cr is a dimensionless buckling coefficient, which depends, e.g., on the ratio a/b, boundary conditions applied to the plate edges, material characteristics, the buckling mode, etc.; D11 is the bending stiffness in the MD, in Nmm, D22 is the bending stiffness in the CD in Nmm and b is the plate width in mm.

Mechanical properties of the corrugated cardboard depend on the direction (CD or MD), which is typical for fibrous materials. Therefore, the buckling coefficient can be assumed as [39]:

D11 mb 2 D22 a 2 ( D12 + 2D33 ) k cr = +2 √ + , (5) D22 a D11 mb D11 D22

where m is the number of half-waves in the direction of loading; a is panel height in mm; D33 is the torsional stiffness in Nmm; D12 = ν21 D11 = ν12 D22 in Nmm; ν12 and ν21 are the effective in-plane Poisson’s ratios of the panel.

2.2. Buckling—Finite Element Method In our case, the panel may contain various types of perforations with different positions, slopes, etc. Such a situation requires the definition of the buckling coefficient of a panel-by-panel basis, which leads to difficult and time-consuming development of analytical formulas. In such a case, the FEM is much more general because it allows determining the critical load of panels with any shape and boundary conditions, including perforations, even with complex shapes. In this study, the critical load of each panel was calculated using a three-dimensional (3D) finite element (FE) model, where all outer edges are fixed in the out-of-plane direction (see dashed lines in Figure 1a). Moreover, the bottom edge was fixed in the vertical direction. The remaining translations and all rotations were inactive.

The critical load in the general case reads [40]: b Pcr = λq,

(6)

where λ is a critical loading multiplier, i.e., the smallest eigenvalue and q is a distributed load on a panel upper edge [Nmm].

In order to calculate the critical force multiplier, the typical formulation of initial buckling problem is required: (7) [K0 + λKσ ]v = 0, where K0 is the global stiffness matrix of the whole panel and Kσ is an initial geometrical stress matrix of the whole panel.

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By solving the eigenvector problem, it is possible to determine the pairs (λ1 , v1 ), . . . , (λ N , v N ), where N is the number of degrees of freedom, λi is i-th eigenvalue, vi = Δdi is i-th eigenvector (post-buckling deformation mode). The minimum eigenvalue and the corresponding eigenvector define the critical load multiplier and the buckling mode of the panel. More details are provided in our previous article [12].

In our case, the geometry of each panel does not have to be regular, as well as the position and shape of the perforations can be chosen arbitrarily. Therefore, the triangular shell FE was adopted, see Figure 2a. Due to shear locking that can occur in FE formulation of thick plates, most of the Reissner–Mindlin FE triangular shells were developed using the assumed transverse shear strain techniques [41–43]. Here, the 6-noded quadratic triangle FE with a standard quadratic interpolation for the deflections and rotations, and the assumed linear transverse shear strain field was utilized. More details on the implementation of this FE, including shear locking description, can be found in our previous article [12].

Figure 2. (a) Triangular mesh on a single panel with perforation; (b) spring connection between two parts separated by a perforation; (c) thickness reduction of elements adjacent to perforation.

2.3. Finite Element Model of a Single Panel Used in the Buckling Analysis A new issue, not addressed in our previous article [12], concerns perforations, i.e., crease lines and partial cuts on vertical panels of the box, which is a non-trivial task when they need to be included in numerical calculations. The computational model used in our study to estimate the critical load of each panel is based on the FEM. Therefore, it is required to define the panel geometry, its boundary conditions, and the constitutive model of the corrugated boards, as well as to define the perforation lines behavior.

In order to compute the critical load of each vertical panel of the box, we used the 6-node FE described in the previous section and in [12]. An in-house routine, written in MATLAB software, was used for all analyses. Because the eigenvalue buckling prediction belongs to the linear perturbation procedures, we need to define only a linear elastic behavior. To correctly identify whole elastic material properties of corrugated cardboard, it is necessary to perform some laboratory tests, e.g., ECT, 4-point bending test, torsional stiffness test (TST), etc. In this study, we used a box strength estimation (BSE) system, which allows performing four independent tests in two main directions of corrugation and is able to identify all elastic parameters of corrugated board (www.fematsystems.pl/systems/BSE).

The following parameters describe the behavior of corrugated board in the elastic range:

• • • • • • • •

D11 —bending stiffness in the MD; D22 —bending stiffness in the CD; D66 —twisting bending stiffness; A11 —compression stiffness in the MD; A22 —compression stiffness in the CD; A33 —compression stiffness in the z direction (out of plane); A44 —transverse shear stiffness in the 1–3 (x–z) plane; A55 —transverse shear stiffness in the 2–3 (y–z) plane.

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These stiffnesses are used in the constitutive equations that describe shell FE [21]: ⎡

A11 A = ⎣ A12 0

⎤ 0 0 ⎦, A33

A12 A22 0

(8)

⎡

⎤ D11 D12 0 D = ⎣ D12 D22 0 ⎦, 0 0 D33

0 A44 , Γ= 0 A55

(9)

(10)

and are represented by the following formulas: D11 = E1 ×

t3 , 12w

D22 = E2 ×

t3 , 12w

D12 = D21 = D22 ν12 = D11 v21 , D33 = G12 × A11 = E1 t,

t3

, 12 A22 = E2 t,

(11) (12) (13) (14)

A12 = A21 = A11 ν21 = A22 ν12 ,

(15)

A33 = G12 t

(16)

5 G t, 6 13 5 A55 = G23 t, 6 w = 1 − ν12 ν21 A44 =

(17) (18) (19)

where:

• • • • • • • •

t—effective cardboard thickness; E1 —effective stiffness modulus in the MD; E2 —effective stiffness modulus in the CD;

ν12 = 0.293 ( E1 /E2 )—effective Poisson’s ratio in the 1–2 (x–y) plane; v21 = v12 E2 /E1 —effective Poisson’s ratio in the 1–2 (x–y) plane, G12 —effective shear modulus in 1–2 (x–y) plane, G13 —effective transverse shear modulus in the 1–3 (x–z) plane, G23 —effective transverse shear modulus in the 2–3 (y–z) plane.

Having the geometry of all panels, the adopted boundary conditions and the corrugated board constitutive model defined above, all that remains is to define the behavior of perforation lines. In order to include the mechanical properties of perforations in the numerical model, two techniques are used here, namely: (a) spring connection between the parts separated by perforation, see Figure 2b, and (b) thickness reduction of the elements adjacent to the perfection, see Figure 2c. The first technique is to divide the panel with the perforation line, duplicate the mesh nodes, and add spring joints at each of the 6 degrees of freedom at each duplicated node. The second technique is to search for all elements adjacent to the perforation line, sort them by the number of nodes lying on the perforation line, and reduce the thickness of these elements (due to creasing techniques used to produce perforations). Figure 2c shows the elements, in which the thickness was reduced. Element color indicates the amount of reduced thickness; the darker the color, the greater the reduction. Both spring stiffness and thickness reduction depends on the definition of perforation, its geometrical pattern, and shape. In each case, the influence

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of the perforation on the calculation results can be observed both in the decrease of the critical force, as well as in the compression force/reaction.

No specific mesh study was performed here, as buckling analysis is a linear perturbation procedure, where the sensitivity of the critical load concerning mesh size is low. In all computations, we used the seed value 1/10 of the mean edge length of the analyzed panel (see Figure 2a).

2.4. Box Compression Strength—McKee’s Formula In order to compute failure load of a single panel, we substitute the buckling force derived from Equation (4) into Equation (1), after [3] we have: Pf = k1 ECT r

D11 D22

1−r

Z −2(1−r ) ,

(20)

∼ 2.028, while r = 0.746 and k = 0.4215. where k1 = 1.33 k (4π)2(1−r) = To obtain the failure load for the whole corrugated box, BCT, it is sufficient to multiply the Equation (20) by the box perimeter Z. This is related to the Mckee et al. assumption [3], details can be found in [11,12]. Thus, the ultimate compressive strength, also known as the long McKee formula, reads: BCTMK1 = k1 ECT r

D11 D22

1−r

Z2r−1 .

(21)

A further simplification, due to the empirical observations of McKee et al., leads to a final form of well-known short McKee’s formula: BCTMK2 = 5.874 ECT h0.508 Z0.492 .

(22)

It is the most widely used formula. However, its accuracy is only acceptable for the simplest boxes. If the ratio of length to width or height to length of the box is relatively high, the McKee formula is no longer valid [11]. The same limitations apply if the box contains any perforations. To overcome this limitation, it is sufficient to derive in Equation (1) the critical load calculated by numerical methods instead of analytical formulas. Both will be shown in the following sections.

2.5. Box Compression Strength—General Case In order to obtain the compressive strength of the box, BCT, the ultimate loads for each panel must be summed up, whereas the compressive load of each panel (see Figure 1a) can be calculated by multiplying the value of the force Pf described by the Equation (1) by the width of the i-th panel b or c. Thus, in a general case, we get the following:

1−r

2 2 r b b c 1−r b γp γ p γc c , (23) γb b + ∑ ( Pcr )i BCT = k ECT ∑ Pcr i =1

i

i

i =1

i

b and Pc are the critical where (·)i indicates the i-th panel of width b or c (i = 1, . . . , 2). Pcr cr b forces of panels of width b and c, respectively (see Figure 1a). γ p and γcp are the reduction factors taking into account the ratio of the compressive strength of the plate with and without the perforations for plate width b and c, respectively. γb and γc are the reduction coefficients due to the in-plate aspect ratio of the box dimensions, defined as: √ γb = b/c, γc = 1 if b ≤ c, (24) √ γc = c/b, γb = 1 i f b > c.

(25)

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3. Results In this section, the results of boxes are presented for both physical tests and numerical simulations. The boxes selected in the study represent a common flap box design of FEFCO 201, although with perforations included on the front and sidewalls. One box design was considered with dimensions of 300 × 200 × 300 mm. All boxes considered here had the same overall design. However, it was considered with three types of different perforations of the SRP type, see Figure 3 (cf. see Figure 1b). The perforations were on three walls, in which, on two opposite walls they were diagonally inclined (30 mm from both corners), and on one wall, in-between wall, the perforation was horizontally positioned (30 mm from the bottom).

Figure 3. Schematic designs of FEFCO 201 box with different perforations considered in the study: (a) 25 × 75, (b) 50 × 50 and (c) 75 × 25.

In the study, the perforations were described by two values, namely, A × B, where A means the percentage of the length of 10 mm, in which the knife cut the corrugated cardboard, and B means the percentage of the length of 10 mm, in which the corrugated cardboard was intact. Thus, the following perforations were considered in the study: 25 × 75, 50 × 50 and 75 × 25 (in decreasing order of perforation/box strength).

The boxes selected in the study were subjected to compression in a strength testing machine, namely BCT-19T10 from FEM at [37]; see Figure 1c. According to the specification, the testing machine has a 0.1 N resolution of force control and may induce the force up to 10 kN. The displacement may be controlled with 0.001 mm accuracy. For each type of perforation, five box samples were tested to acquire a representative result. For 50 × 50 perforation, one of the test results was rejected due to unusual deformation mode.

Thus, finally, 14 measurements of ultimate loads of boxes with perforations were used in the study, see Table 1. In Table 1, the mean values computed with two standard deviations, which, for the normal distribution, accounts for 95.5% of cases for each box design, are also presented.

The samples were produced from the same corrugated cardboard, namely single-side three-layer quality with E flute of 450 g/m2 (marked as 3E450-1). Cardboard thickness measured was from 1.58 mm to 1.60 mm, with the mean equal to 1.59 mm. Each box sample was cut by plotter and manually folded; at the top and bottom, the flaps were taped. Since the plotter was used, no converted corrugated cardboard, i.e., raw cardboard material without printing, lamination, etc., was taken into consideration in this study. Before and after the test, the cardboard/box samples were visually inspected for any damages or unusual issues. Boxes were subjected to a displacement control test. The resultant force was registered by the testing machine (no additional strain gauges were used). Cardboard was laboratory conditioned with a temperature of 23◦ C ± 1◦ C and relative humidity

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of 50% ± 2%, TAPPI standard T402 was used. In Figure 4, the full histories of the box measurements are presented. The histories represent the force versus displacement data obtained from the strength testing machine [37]. Note that good repeatability of the histories of the measurements was observed.

Table 1. Experimental ultimate loads of boxes tested in the study with different perforation types.

No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Type of Perforation

Ultimate Load (N)

Mean Ultimate Load ± 2 Standard Deviations (N)

25 × 75

1033.0 1078.0 1003.0 974.7 1009.0

1020 ± 77.8

50 × 50

1050.0 933.6 1017.3 1007.0

1002.0 ± 98.4

75 × 25

987.8 979.6 944.9 899.1 880.2

939.3 ± 95.6

Figure 4. Measurements from a box compression test machine [37] for FEFCO 201 box with different perforations: (a) 25 × 75, (b) 50 × 50 and (c) 75 × 25.

The experimental results, namely, the mean ultimate loads computed from the peak forces (presented in Figure 4 and Table 1), were used to verify the approach proposed in the paper, see Equation (23). According to the proposed method, the critical buckling loads from the first buckling mode of all box panels (computed separately; with and without perforations) were computed via the FE model to use them in Equation (23). In Supplementary Materials, the data were attached, which feed the Equation (23), and served to compute the estimations of the ultimate loads of boxes with perforations. In Table 2, the material constants were demonstrated, which were the input for the constitutive law used in the FEM computations. Apart from the mean values, in order to show the uncertainty of the data, the minimum and maximum values were also presented. Moderate scatter of the material constants was observed.

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Table 2. Material characteristics for 3E450-1 cardboard quality, which were used in the finite element method (FEM) to determine the critical buckling loads of the panels [44].

3E450-1

Thickness (mm)

ECT

D11 (Nmm)

D22 (Nmm)

D33 (Nmm)

A44 (N/mm)

A55 (N/mm)

mean min max

1.589 1.582 1.596

5.299 5.028 5.574

1261 1206 1318

507.8 469.2 547.3

1036 967 1115

3.904 3.743 4.077

4.074 3.691 4.498

The data used in Table 2 and the selected equations presented earlier may be used to determine the typical set of elastic orthotropic material constants. One of the issue to address is to compute w, which, according to Equation (19), depends on the Poisson’s ratios, v12 and v21 . Here, the Baum et al. [45] formula was used to compute v12 , namely:

E1 , E2

ν12 = 0.293

(26)

In the above equation, the relation E1 /E2 was obtained from Equation (11); thus, we may write the following formula: D E1 = 11 .

(27) E2 D22 On the other hand, the ratio v21 was computed from the classic formula, namely: ν21 = ν12 ×

E2 , E1

(28)

In the above equation, the following relation, derived again from Equation (11), may be used: D E2 = 22 .

(29) E1 D11 Finally, having w, allows us to compute E1 and E2 from the Equation (11). Similarly, G12 was computed from Equation (13); G13 and G23 were obtained from Equation (17) and Equation (18), respectively. The computed material constants are presented in Table 3; mean, minimal, and maximal values are demonstrated.

Table 3. Elastic orthotropic parameters for 3E450-1 cardboard quality computed from stiffnesses, see Table 2.

3E450-1

E1 (MPa)

E2 (MPa)

v12 (-)

v21 (-)

mean min max

3451 3340 3560

1390 1301 1478

0.461 0.450 0.470

0.184 0.183 0.189

G12 (MPa) G13 (MPa) G23 (MPa) 3099 2931 3291

2.95 2.84 3.07

3.08 2.80 3.38

In Figure 5, the mean data from the physical measurements, and estimated counterpart values were presented by the bar plot. Black bars represent the mean values measured at the laboratory, see the last column in Table 1. The values obtained from the analytical– numerical approach proposed are presented by light blue plot, see Equation (23). In the analytical–numerical approach presented, k and r used were k = 0.4 and r = 0.75.

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Figure 5. Box compression strength obtained within different approach for shelf-ready boxes considered in the study with various perforations, i.e., 25 × 75, 50 × 50 and 75 × 25.

The comparison between the measured and estimated values of ultimate loads of the boxes with perforations may be represented by the mean error. It was computed for all three cases according to the following formula:

BCT − BCTest exp

× 100% , (30) error =

BCT exp where BCT exp is an experimental value of the box compression strength averaged for four or five samples with peculiar perforation, and BCTest is its counterpart computed by the analytical–numerical approach proposed in this paper. The average error, while using the analytical–numerical approach for the boxes analyzed in this paper was 3.5%.

A similar comparison with the experimental values may be considered for the estimation via McKee formula—the approach, which is easy, and thus in common use in the packaging industry. The results were presented by red bars in Figure 5, see Equation (30).

Similarly, the mean error for the McKee formula was computed, it was equal to 19.5%. The summary of the mean errors is presented in Table 4.

Table 4. The mean error for estimations of McKee formula and the method proposed.

Case

k (-)

r (-)

Mean Error (%)

McKee formula method proposed

0.4215 0.4

0.746 0.75

19.5 3.5

4. Discussion The analytical–numerical method proposed here shows the correct trend while comparing it with the experimental results. Namely, with decreasing stiffness of the perforation, the strength of the box also decreases. Experimental results show that, if the box with 50 × 50 perforation would be the reference, the strength increase for 25 × 75 is 2.0%, while for 75 × 25 the decrease equals 6.3%. For the analytical–numerical approach proposed, the counterpart values are 8.4% and 6.5%, respectively. For the McKee formula, the values are constant for all three boxes. Moreover, note that the average error computed for the method proposed here was more than five times smaller than for the McKee formula, i.e., 3.5% vs. 19.5%, respectively. This is the most important conclusion from our study. The proposed approach is much more accurate than a simplified method.

In the study presented, all designs of the boxes, due to the same overall geometry, had 1000 mm of the in-plane circumference. This caused the strength of the boxes to be the same for all three box designs with perforations, while considering the McKee formula approach, see Figure 5. The McKee formula does not include the information about perforations in the box design.

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Many challenges are still actual in the mechanics of corrugated cardboards and packaging due to the nonlinear material and the nonlinear/discontinuous geometry of the designs. Previously, the box strength was computed with the simplified formulas, which currently does not provide sufficient accuracy to the increasing needs of the packaging industry. This was proved by our recent series of publications, namely in [11,12,20,21].

5. Conclusions In this study, the boxes made of corrugated cardboards with knife-cut perforations were analyzed. The box designs selected for the study represented the SRPs. Three types of perforations were considered, namely, 25 × 75, 50 × 50, and 75 × 25, in which the first value represents the percentage of the length of 10 mm, at which the knife cut the corrugated cardboard, and the second value represents the percentage of the length of 10 mm, at which the corrugated cardboard was intact.

In the first part of the study, the series of top-to-bottom compression strength tests of boxes were conducted to determine their experimental ultimate loads. In the second part of the study, the analytical–numerical approach was proposed to take into consideration the different properties of perforations in modeling. In the third part of the study, the boxes with their peculiar design of perforations were modeled with simplified (McKee formula) and proposed the analytical–numerical approach. The tests conducted in the first part served to verify the modeling approach proposed in the second part of the study. In the third part of the study, it appeared that the approach proposed in the paper gave very low mean estimation error in comparison with the reference simplified (McKee formula) approach; the mean error was only a few percent. The McKee formula approach was presented to contrast the influence of the perforation on the estimation of the box ultimate load, according to the classical approach still widely used by the lab technicians. Thus, it can be concluded that the method presented in the study is a promising tool for determining the strength of perforated boxes, which are commonly used in the supply and sales market.

To the best knowledge of the authors, the influence of the perforations has never been considered before in the analytical or analytical–numerical approach for estimation of the compressive strength of boxes made of corrugated cardboard. The novelty of the presented work is the use of the analytical–numerical method, which finally takes into account the impact of perforation on the evaluation of the compressive strength of the SRP packaging.

Supplementary Materials: The following are available online at https://www.mdpi.com/1996-107 3/14/4/1095/s1. In the TXT file, the computational results based on the method proposed, in which Figure 5 was generated are presented. In this file, the 1st and 2nd columns are k and r parameters; the 3rd column is the ECT value for particular cardboard quality; the 4th to 7th columns are critical loads of the i-th panel (obtained from FEM); the 8th to 11th columns are the reduction factors due to the in-plate aspect ratio of the box panel, γi , where i = {1, 2, 3, 4} and denotes the number of the panel. The 12th to 15th columns are the reduction factors, taking into account the ratio of the compressive strength of the plate with and without a perforation γ pi , where i = {1, 2, 3, 4} and denotes the number of the panel. The 16th to 19th columns are the widths of the i-th panel of the box.

Author Contributions: T.G. (Tomasz Garbowski): conceptualization, methodology, software, writing— original draft, writing—review and editing, supervision, project administration, funding acquisition; T.G. (Tomasz Gajewski): software, validation, formal analysis, investigation, writing—original draft, writing—review and editing, visualization; J.K.G.: writing—original draft, writing—review and editing, data curation, supervision, funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding: The APC was funded by the Ministry of Science and Higher Education, Poland, grant from Poznan University of Technology; grant number 0612/SBAD/3567.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

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Acknowledgments: Special thanks to the FEMAT company (www.fematsystems.pl) for the financial support of this research project. The authors also thank AQUILA VPK Września for providing samples of corrugated cardboard for the study.

Conflicts of Interest: The authors declare no conflict of interest.

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PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 2, 2021

Theoretical Estimation of Evaporation Heat in Paper Drying Process Based on Drying Curve Xiaobin Chen, Qifu Zheng & Yunyuan Dong At present, the theoretical estimation of paper web’s evaporation heat is based on sorption isothermals. The measuring conditions are harsh, and the test speed is slow. This paper attempts to explore a theoretical method which can quickly determine the evaporation heat of paper web. During the new method, based on the measurement of the paper drying curve, the theoretical estimation model of paper evaporation heat was obtained by deducing the mechanism of heat and mass transfer. Compared with the traditional method based on sorption isothermals, the new model based on the drying curve has some advantages in measurement speed and easy access to basic data. Finally, the paper verifies the reliability of the model from two application scenarios of the laboratory and production line.

Contact information: Xiaobin Chen: College of Chemical and Material Engineering, Quzhou University, Quzhou 324000, China; Qifu Zheng: Postdoctoral Workstation, Shanying International Holding Co., Ma’anshan 243000, China Yunyuan Dong: Guangdong Provincial Key Lab of Green Chemical Product Technology, South China University of Technology, Guangzhou 510640, China Processes 2021, 9, 1117. https://doi.org/10.3390/pr9071117 Creative Commons Attribution 4.0 International License

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Article 8 – Paper Drying

processes Article

Theoretical Estimation of Evaporation Heat in Paper Drying Process Based on Drying Curve Xiaobin Chen 1,2,3, *, Qifu Zheng 1, * and Yunyuan Dong 1 1

2 3

*

College of Chemical and Material Engineering, Quzhou University, Quzhou 324000, China; yunyuandong@qzc.edu.cn Postdoctoral Workstation, Shanying International Holding Co., Ma’anshan 243000, China Guangdong Provincial Key Lab of Green Chemical Product Technology, South China University of Technology, Guangzhou 510640, China Correspondence: xiaobinchen@qzc.edu.cn (X.C.); zqf@qzc.edu.cn (Q.Z.)

Abstract: At present, the theoretical estimation of paper web’s evaporation heat is based on sorption isothermals. The measuring conditions are harsh, and the test speed is slow. This paper attempts to explore a theoretical method which can quickly determine the evaporation heat of paper web. During the new method, based on the measurement of the paper drying curve, the theoretical estimation model of paper evaporation heat was obtained by deducing the mechanism of heat and mass transfer.

Compared with the traditional method based on sorption isothermals, the new model based on the drying curve has some advantages in measurement speed and easy access to basic data. Finally, the paper verifies the reliability of the model from two application scenarios of the laboratory and production line.

Keywords: paper drying process; modeling; evaporation heat; energy

Citation: Chen, X.; Zheng, Q.; Dong, Y. Theoretical Estimation of Evaporation Heat in Paper Drying Process Based on Drying Curve.

Processes 2021, 9, 1117. https:// doi.org/10.3390/pr9071117 Academic Editors: Evangelos Tsotsas, Zhiqiang Sun, Sheng Yang and Yi Ma Received: 3 May 2021 Accepted: 25 June 2021 Published: 28 June 2021

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1. Introduction Paper is a kind of hygroscopic material with a porous structure, mainly composed of fibers and other solid particles, such as fillers, sizing materials, additives and so on.

Paper products play a significant role in every area of human activity, such as the recording, storage and dissemination of information, wrapping and packaging, writing and printing and so on. Papermaking is a basic raw material industry, closely related to the national economy.

The paper-making process is essentially a very large dewatering operation. The major sections of the paper machine consist of: the forming section, press section and dryer section.

The dryer section removes between 1.1–1.3 kg of water per kg of paper production, as compared with the 200 and 2.6 kg removed in the forming and press section [1]. Although the dryer section is responsible for a small fraction of total dewatering, it is the major energy consumer in the paper mill because porous and hygroscopic pulp fibers have hardto-remove water that is considered to be located in the fiber cell wall and trapped in the fiber network geometry. According to the report prepared by the Institute of Paper Science and Technology (IPST), 61.9% of the total energy required for paper making is consumed in the paper drying process [2], and about 65% of the thermal energy is used for water evaporation based on Chen’s investigation [3,4]. In spite of its key role in papermaking and its high energy consumption—taking approximately 60% of the total physical length and accounting for almost 40% of the total capital cost of a common paper machine—paper drying is arguably the least understood papermaking operation. Perhaps the reason is the complexity of the paper drying process that involves heat transfer, evaporation, and the water removal process where steam pressure, air conditions, and condensate removal play key roles in determining the drying capacity and final product quality. The papermakers

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often treat the dryer section as a “black box”. Nevertheless, rising energy costs are forcing papermakers to pay more attention to the dryer section, and especially steam usage.

The thermal energy consumption during paper drying is mainly used to evaporate the water in the moist paper. According to the diverse binding force, the water in the moist paper can be divided into different types, such as free water and bound water, of which thermodynamic and physical properties are different. For the bound water, evaporation heat is used not only to vaporize water, but also to overcome the interaction between cellulose fibers and water. The quantitative measurement of evaporation heat is important to understand the interaction between fibers and water (drying mechanism) and the resulting impact on the paper drying behavior for optimizing the capital and operating costs of the dryer section.

At present, there are few studies on the quantitative measurement of evaporation heat, mainly because heat energy flow is difficult to measure directly. Heikkilä [5] studied the evaporation heat of coated paper and found that the evaporation heat of bound water was composed of two parts, latent heat of vaporization and an extra amount of energy called heat of sorption. The latent heat of evaporation is the same as that of pure water, which can be easily obtained by consulting the physical property data of water, but the data of sorption heat is difficult to obtain. At present, the traditional method to obtain the sorption heat is to establish its theoretical model based on the Clausius–Clapeyron equation, and the theoretical model estimation needs to use the data of paper sorption isotherm [1]. Taking mechanical pulp as an example, the theoretical estimation equation of sorption heat is as follows: 2

1− ϕ ΔHsorp = Rv c3 uc4 Tp + 273.15 (1) ϕ

ϕ = 1 − exp −c1 uc2 − c3 Tp uc4 (2) where Equation (1) is the calculation model of sorption heat (ΔHsorp ), Rv is the gas constant, Tp is the paper temperature, and u is the paper moisture, Equation (2) is an example of fitting equation of sorption isotherm(ϕ), c1 , c2 , c3 and c4 are the equation parameters.

This model had been widely used in modeling and simulation of the paper drying process. For instance, Slätteke [6] established a complete simulation model for the drying section that is implemented in the object-oriented modeling language Modelica. The purpose is to study the feedback control of paper drying. Roonprasang [7] made a theoretical study on the modeling and simulation of the impact of new design configuration geometry on the drying process, with means of theoretical estimation model of evaporation heat.

Karlsson [8,9] presented several models for the drying section to describe the phenomena at a steady state. New control strategies applicable for both steady-state control and for grade changes were derived. Heo [10] used the theoretical estimation model of evaporation heat to model and simulate the state change of paper in the drying section, trying to solve the problem of the “black box” in dryer section. Åkesson [11] used the theoretical estimation model of evaporation heat to present a Modelica library, DryLib, which enables users to rapidly develop complex models of dryer section. In addition, parameter optimization by means of non-linear model predictive control is treated.

Nevertheless, there were still many inconveniences in the practical application in the process of using the above model based on the sorption isotherm. The main reason is that the data of sorption isotherm curves of different paper grades in the industry are very few. The sorption isotherm is a correlation between relative humidity and the equilibrium moisture content. The measurement of the sorption isotherm is time-consuming and difficult, requiring strict test conditions such as constant temperature and humidity [12,13].

Measurement of sorption isotherms is a common practice in many fields of science and engineering, such as food science, material science, and so on. Traditional methods to determine sorption isotherms usually rely on conditioning above saturated salt solutions that keep constant relative humidity and gravimetric methods to determine the moisture content in the sample. In order to reduce the systematic error caused by the change of relative humidity around the sample during sampling and weighing, the whole experiment

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process, including water balance and weighing, needs to be kept under the same relative humidity. Many different relative humidity are needed to determine a complete sorption isotherm, as at each relative humidity the water balance of the sample takes a long time.

Therefore, the measurement of sorption isotherm line is very time-consuming.

The sorption isotherm of paper depends on the structure of the paper and the fibers and other raw materials used to manufacture the paper. Keränen [14] has studied handmade paper’s sorption isotherms with different filler contents. Fitting the experimental data with the Soininen model, it was found that the model parameters was obviously different corresponding to the different filler content. Popescu [15] studied the adsorption isotherm of wood and got the similar conclusion. There are more than 1000 kinds of paper in the current paper-making industry; according to market demand, each kind of paper’s manufacturing process has been frequently adjusted in the actual production process. How to rapidly estimate the evaporation heat of the frequently changing paper, and then guide the dryer section to optimize the process parameters in time, is a new problem encountered in the process of paper enterprises in pursuit of energy saving and consumption reduction.

The traditional estimation method based on sorption isotherm obviously can not achieve the purpose of rapid measurement, and can not adapt to the rapidly changing market demand. Therefore, developing a simple approach for rapidly measuring the evaporation heat of paper is urgent.

The drying curve is an overall indication of the paper drying process, covering all the information during paper drying, and including paper’s evaporation heat [16,17].

The current study attempts to explore a simple method for rapidly measuring evaporation heat based on the drying curve measurement. Generally, the drying curve is a curve that changes in the moisture content of the material with the drying time. In the papermaking industry, it was also expressed by a correlation between paper moisture content and drying position (cylinder number) [18]. The new method starts from the measurement of the drying curve, and no longer needs to measure the sorption isotherm, which makes the measurement of evaporation heating time-saving and convenient.

2. Methodology 2.1. Theoretical Model Based on Drying Curve The moist paper consists of water in different fractions, free water in large pores and bound water in micro pores. They have different thermodynamic and physical properties, such as different vapor partial pressures, enthalpies and so on. The thermodynamic and physical properties of free water being same as that of bulk water. The binding force of free water is small, generally, it evaporates first in the drying process. Drying decreases the level of fiber swelling by closing the pores in a fiber cell wall irreversibly. The removal of bound water needs to overcome such resistance, so with the passage of time, it is more and more difficult to remove water by drying, so it also needs to consume more heat energy.

During the drying process, wet paper passes through three distinct phases, including the preheating phase, c, and the falling drying rate phase. In the preheating phase, part of the heat of the heating medium is used to heat the paper, and the paper temperature increases rapidly, but the moisture content of the paper changes little with the drying time.

Then, the paper drying enters the constant drying rate phase, and the moisture content of the paper is basically linear with the drying time. The slope is constant, that is, the drying rate is constant. At this time, the heat transferred by the heating medium to the paper and the heat required by the evaporation of the water in the paper are in balance, and the surface temperature of the paper remains unchanged, which is just equal to the wet bulb temperature of the hot air around the paper surface. After a period of time, when the moisture content of the paper is lower than the critical moisture content (CMC), the paper drying enters the deceleration stage. In this stage, the drying characteristic curve tends to be flat, gradually approaching the equilibrium moisture content (EMC), and the drying rate decreases. Part of the heat provided by the heating medium is used to evaporate water, and part is used to heat the paper, and the surface temperature of the paper continues to

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rise. The above idealized scheme occurs if drying conditions are similar over the entire drying process. In commercial dryer sections, the constant rate phase does not often exist.

The schematic diagram of the evaporation heat measurement based drying curve is shown in Figure 1. It shows that the one-to-one correspondence function between the evaporation heating curve (Figure 1a) and the drying curve (Figure 1b) can be established theoretically during the paper drying. The specific derivation process is as follows: 'H

'E 'W

Assumptions: 1) Drying conditions are consistent; 2) Ignoring the energy loss, the energy provided by the drying system is only used for paper drying.

'H

'E 't |K 'W 'W

'H

'W

u

M dp 'u

u

u

(a) Evaporation heat curve (əH ~ u)

K 1 M dp 'u / 't

Assumptions: 3)The absolute dry weight of paper does not change during drying

'E | K 't

u

'E 't |K 'W 'W

(a) Evaporation heat curve (əH ~ u)

(a) Evaporation heat curve (əH ~ u)

Theoretical analysis: 1) It is feasible to estimate the heat of evaporation by measuring the drying curve.

2) Another key problem is to determine the parameters K and Mdp.

t

(b) Drying curve (u ~ t)

Figure 1. Schematic diagram of evaporation heat measurement based drying curve: (a) Evaporation heat curve; (b) Drying curve.

The evaporation heat (ΔH) in the paper drying process refers to the energy consumed by the evaporation of water inside the paper sheet and the final release. As shown in Figure 1a, it is a curve where the y-axis is evaporation heat (ΔH) and the x-axis is the moisture of the paper (u), u is a state variable, which can be easily obtained by weighing the paper sheet. ΔH is a process variable, which can be calculated by Equation (3). ΔH =

ΔE ΔW

(3)

Energy consumption(ΔE) is difficult to measure directly. According to the principle of energy conservation, it can theoretically be measured indirectly by measuring the energy consumption of the drying system. As shown in Equation (4), K is the effective horsepower of drying system. In a period of time (t1 ~t2 ), energy consumption (ΔE) can be calculated by an integral formula. The right side approximation holds in the assume that K is a constant. ΔE =

t2 t1

Kdt ≈ K ·Δt

(4)

Water evaporation (ΔW) is also a process variable, which is a function of the drying rate (R) and time (t), as shown in Equation (5). Mdp is the mass of the dry paper. Δu ΔW = R = Mdp Δt Δt

(5)

From Equations (3)–(5), evaporation heat (ΔH) can be calculated by Equation (6), under the following assumptions: (1) drying conditions are consistent; (2) ignoring the

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energy loss, the energy provided by the drying system is only used for paper drying; (3) the absolute dry weight of the paper does not change during drying. ΔH ≈

K ·Δt K 1 = · ΔW Mdp Δu/Δt

(6)

A relationship model between evaporation heat (ΔH) and the first derivative of paper moisture (Δu/Δt) has been established in Equation (7). That means the evaporation heat curve can be obtained as long as the drying curve (Figure 1b) was measured. K/Mdp is the model parameter.

Through theoretical analysis, it is found that it is feasible to theoretically estimate the heat of evaporation by measuring the drying curve, but there is still a key problem to be solved, which is to determine the constants K and Mdp . The method for determination of the model parameter K/Mdp is introduced in Figure 2. In the initial drying period, free water is preferentially evaporated. The thermodynamic and physical properties of free water are the same as that of bulk water. Its evaporation heat is just the latent heat of bulk water (ΔHlat ). Therefore, Equation (7) can be obtained according to the law of energy conservation. u∞ is the paper moisture in the initial drying period, mainly referring to the constant drying rate phase.

K 1

M dp 'u / 't

'H lat u

uf Figure 2. Determination of model parameter K/Mdp .

ΔH ≈

K 1 · Mdp du dt

= ΔHlat

(7)

u=u∞

The model parameter (K/Mdp ) can be calculated as following:

K du = ΔHlat · Mdp dt u=u∞

(8)

At atmospheric pressure, ΔHlat is a single variable function of evaporation temperature that is approximately equal to paper temperature (Tp ). By inquiring the physical property data of water, the following fitting Equation (9) can be utilized to calculate the latent heat of water vaporization (ΔHlat ), which is obtained by regressing the physical property data of water at atmospheric pressure.

Tp ΔHlat = 2504.7 − 2.4789 ◦ kJ/kg C

(9)

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Finally, we find that the evaporation heat is related to the drying curve (Figure 1b) and temperature of paper at atmospheric pressure. It can be calculated as following Equation (10), which is obtained by substituting Equation (8) into Equation (6):

1 du (10) · du ΔH = ΔHlat · dt u=u∞ dt

2.2. Advantages of the New Method In this paper, a convenient and rapid method to measure the evaporation heat of paper is proposed. The new method is based on the drying curve, while the old method is based on the sorption isotherm. Because the measurement of the drying curve is more convenient than that of sorption isotherm, the new method has two great advantages.

As shown in Table 1, one is the measuring time, in the same laboratory environment, the old method based on the sorption isotherm get each data point generally needs to be in constant temperature and humidity environment for 24 h to balance water, however, the new method based on the drying curve only takes 1–2 h to complete the measurement of the whole drying curve, or even less than 15 min when using the moisture rapid analyzer. During on-line measurement at the production site, in the on-line measurement of production site, it takes 10 min for the new method to measure a data point, but the old method can not measure directly, so it can only use the data measured in the laboratory.

Table 1. Comparison of new and old methods.

Item

The New Method

The Old Method

Measuring principle

Based on the measurement of drying curve

Based on the measurement of sorption isotherm

Measuring time

(1) Laboratory: The traditional method is 1–2 h, if the moisture rapid analyzer is used, it will take about 15 min (2) On-line measurement: One data point is about 10 min

(1) Laboratory: Each data point generally needs to be in constant temperature and humidity environment for 24 h to balance water, so it usually takes several weeks (2) On-line measurement: Can’t measure

Measuring conditions

(1) Conventional laboratory conditions

(1) Conventional laboratory conditions (2) Constant temperature and humidity environment

Another advantage is the measuring conditions. The measurement of the drying curve only needs the conventional experimental environment, while the measurement of the sorption isotherm needs the special experimental environment of constant temperature and humidity.

3. Results and Discussion 3.1. Laboratory Verification In the laboratory, mechanical pulp was used as raw material to make hand paper.

The specific steps are as follows: Step1: Through the Wally beater (VB-42F, China pulp and Paper Research Institute Co., Ltd., Beijing, China), the raw material of mechanical pulp was dredged and beat to a specific beating degree (30◦ SR).

Step2: The pulp concentration was measured, and a certain amount of pulp was weighed based on the paper with a weight of 60 g/m2 .

Step3: The pulp was dissociated with a fiber standard dissociator (ZY-XW, Shandong Zhongyi Instrument Co., Ltd., Shandong, China), and formed into wet paper on a rapid Kaiser sheet former (ASM-32N2F, China pulp and Paper Research Institute Co., Ltd., Beijing, China).

Step4: The wet paper was pressed on the multi-functional extruder (MASP22H803, China pulp and Paper Research Institute Co., Ltd., Beijing, China) by 50 kPa.

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Step5: It was sent to the moisture rapid analyzer (MB120, Aohaus Changzhou Co., Ltd., Jiangsu, China) to determine the drying curve (Figure 3), obtain and record the test data of paper moisture (u) changing with time(t).

Figure 3. Measurement and analysis of drying curve in laboratory.

The experimental model, Gaussian model and Fourier model were used to fit the test data. The fitting curves were shown in Figure 3, and the model structure and model parameter results are shown in Table 2. The Gaussian model has the best simulation effect and R2 value is 0.996. Therefore, the Gaussin model is used to estimate the subsequent evaporation heat.

t + 78.25 2 (11) u = 2.563· exp − 445.6 Table 2. Fitting results of drying curve data in laboratory.

Models

Model Structure

Model Parameter

R2

Experimental Model

u = a· exp(b·t)

a = 2.87; b = −0.00322

0.953

Gaussian Model

2

u = a·exp − t−c b

a = 2.563; b = −78.25; c = 445.6

0.996

u = a + b· cos(c·t) + d· sin(c·t)

a = 1.59; b = 0.953; c = −0.00282; d = −1.27

0.995

Fourier Model

Figure 4 shows a comparison of calculation results of the evaporation heat between the new and old methods in laboratory. The results calculated by the new method based on the test of drying curve are similar to the old method based on the measurement of the sorption isotherm. The MRE (average relative error) was calculated as 0.81% by Equation (12).

1 MRE = N

ΔH1,i − ΔH2,i

∑ ΔH2,i × 100% i =1 N

(12)

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where, i represents the number of data groups, N represents the number of data groups, ΔH1 represents the evaporation heat obtained by the new method, ΔH2 represents the evaporation heat obtained by the old method.

Figure 4. Comparison of results between the new and old methods in the laboratory.

The laboratory verification results show that the new method can achieve the credibility of the old method. However, the new method can greatly simplify the test process of the water evaporation heat in paper drying process.

3.2. Industrial Verification The verification object is a pre-drying process of high-strength corrugated paper with an annual output of 80,000 tons. The paper machine has 48 dryer cylinders, a speed of 500 m/min, a width of 4 m, and can produce high-strength corrugated paper with basis weight of 100 g/m2 .

Figure 5 shows the schematic diagram of paper drying process. The material flow in and out of the pre-dryer section is clearly marked. In the actual production process, the paper drying is completed in the dryer-section, that is, a group of horizontally arranged heating dryer cylinders. The paper is pasted on the surface of the dryer cylinders to evaporate and dry. The production is continuous and the paper is moving forward dynamically, generally, the speed of the paper machine is 1000~2000 m/min. In this case, it is difficult to obtain the data of the moisture content of paper with time. Therefore, the drying curve of the production process is usually recorded as the change of the moisture content of the paper with different drying positions(u~N). The moisture content of the paper can be measured by hand-held portable sensor, and the drying position is usually indicated by the cylinder number of different positions.

Figure 6 shows measurement and analysis of drying curve in industrial production.

The basis weight of paper web was measured by a portable sensor (NDC 8110-F/104 from NDC infrared Engineering Co., Ltd., Los Angeles, CA, USA). This kind of small single-sided sensor can be extended into the narrow gap between the dryer cylinders, and it can be used to measure quantitatively on-line on the surface of the wet web without affecting the normal production. The drying curve data represented by paper moisture (u) and cylinder number (N) are recorded online as shown in Figure 6. The experimental model, Gaussian model and Fourier model were used to fit the test data. The fitting curves

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are shown in Figure 6, and the model structure and model parameter results are shown in Table 3. The Gaussian model has the best simulation effect and the R2 value is 0.986.

Therefore, the Gaussian model is used to estimate the subsequent evaporation heat.

N + 15 2 (13) u = 1.19· exp − 32.2 To atmosphere

To atmosphere

Steam for air heating

Supply air

Steam for air heating

Condensate

Condensate

Supply air

+5

Exhaust air

$+

Supply air

+5

Exhaust air

$+

Supply air

Wet paper

Dry paper

Steam for cylinders Pre-dryer section

Flash steam for condenser

Condensate

Leakage air

Figure 5. Schematic diagram of paper drying process.

Figure 6. Measurement and analysis of drying curve in industrial production.

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Table 3. Fitting results of drying curve data in industrial production.

Models

Model Structure

Model Parameter

R2

Experimental Model

u = a· exp(b· N )

a = 1.03; b = −0.0550

0.967

Gaussian Model

2

b u = a·exp − N − c

a = 1.19; b = −15.0; c = 32.2

0.986

u = a + b· cos(c· N ) + d· sin(c· N )

a = 0.964; b = 0.00159; c = 0.0346; d = −0.927

0.985

Fourier Model

Figure 7 shows comparison of calculation results of evaporation heat between the new and old methods in industrial production. Like the results of the laboratory verification, the results calculated by the new method based on the test of the drying curve are similar to the old method based on the measurement of the sorption isotherm in industrial production.

The MRE was calculated as 0.80%. It shows that the new method can achieve the credibility of the old method in industrial production too.

Figure 7. Comparison of results between the new and old methods in industrial production.

4. Conclusions In this paper, a mathematical model for rapid estimation of water evaporation heat in the paper drying process was proposed. During modeling, the drying curve is selected as the model input variables, and then evaporation heat (ΔH) can be calculated. Compared with the old method using adsorption isotherm as model input, the new method has the advantages of easily obtained model inputs, short measurement time, and less harsh measurement environment.

The current paper also compares and analyzes the calculation results of the new and old methods in laboratory and industrial production. The results calculated by the new method based on the test of the drying curve are similar to the old method based on the measurement of the sorption isotherm in both scenarios. It shows that the new method can achieve the credibility of the old method.

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This paper provides a simple tool for estimating the heat of evaporation in the drying process, and it can also be used to estimate the total energy consumption in the drying process, taking the total energy consumption as the goal, an optimal drying curve can be found through the optimization calculation, and then the drying process can be optimized and adjusted. The focus of our next study is the quantitative study of this model applied for optimal operation during paper drying.

Author Contributions: Conceptualization, X.C. and Q.Z.; methodology, X.C.; software, X.C.; validation, X.C., Q.Z. and Y.D.; formal analysis, X.C.; investigation, X.C. and Y.D.; resources, X.C.; data curation, X.C.; writing—original draft preparation, X.C.; writing—review and editing, X.C.; visualization, Y.D.; supervision, Q.Z.; project administration, Q.Z.; funding acquisition, Q.Z. All authors have read and agreed to the published version of the manuscript.

Funding: This work was supported from the Basic Public Research Project of Zhejiang Province (LQ18B060003), the Research Fund Program of Guangdong Provincial Key Lab of Green Chemical Product Technology (GC201809), the Quzhou science and technology plan project (2018ASA90140), the Key R & D plan of Zhejiang Province (2021C02031).

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: The study did not report any data.

Conflicts of Interest: The authors declare no conflict of interest.

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Popescu, C.-M.; Hill, C.A.S.; Curling, S.; Ormondroyd, G.; Xie, Y. The water vapour sorption behaviour of acetylated birch wood: How acetylation affects the sorption isotherm and accessible hydroxyl content. J. Mater. Sci. 2014, 49, 2362–2371. [CrossRef] Stenström, S. Drying of paper: A review 2000–2018. Dry. Technol. 2019, 4, 1–21. [CrossRef] Chen, X.; Man, Y.; Zheng, Q.; Hu, Y.; Li, J.; Hong, M. Industrial verification of energy saving for the single-tier cylinder based paper drying process. Energy 2019, 170, 261–272. [CrossRef] Li, J.; Kong, L.; Liu, H. Dryer section energy system measurement and energy-saving potential analysis for a paper machine.

Meas. Control. 2012, 45, 239–243. [CrossRef]

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 2, 2021

Accessibility of fiber surface sites for polymeric additives determines dry and wet tensile strength of paper sheets J.-L. Schäfer, S. Schölch, O. Prucker, T. Brandstetter, J. Rühe, A. Ritter v. Stockert, T. Meckel & M. Biesalski This contribution focuses on understanding of paper wet-strength properties, by taking a closer look at the spatial distribution of wet-strengthening polymers inside the cellulosic fiber network deposited under different treatment conditions using confocal laser scanning microscopy as in situ imaging tool. We compare the behavior of paper samples treated with a photochemically cross-linkable copolymer using an impregnation process employing three different solvents, namely water, 2-propanol (IPA) and 1-butanol (BuOH), respectively. As these solvents swell paper fibers to quite different extents, the deposition of the polymer, on, in or in-between the cellulosic fibers varies quite strongly, as is shown by in-depth analysis using confocal laser scanning microscopy. The difference in accessibility of distinct surface sites exclusively on or also in and between the fibers controls the macroscopic tensile strength under both dry and wet conditions.

Contact information: J.-L. Schäfer, A. R. v. Stockert, T. Meckel & M. Biesalski: Department of Chemistry, Macromolecular Chemistry & Paper Chemistry, Technical University Darmstadt, Alarich-Weiss-Straße 8, 64287 Darmstadt, Germany S. Schölch, O. Prucker, T. Brandstetter & J. Rühe: Department of Microsystems Engineering (IMTEK), Laboratory for Chemistry & Physics of Interfaces, Albert-Ludwigs-Universita¨t Freiburg, Georges-Ko¨hlerAllee 103, 79110 Freiburg, Germany Cellulose (2021) 28:5775–5791 https://doi.org/10.1007/s10570-021-03817-7 Creative Commons Attribution 4.0 International License

The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International and the PITA Annual Review, both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating.

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Article 9 – Polymeric Strength Additives

Cellulose (2021) 28:5775–5791 https://doi.org/10.1007/s10570-021-03817-7

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ORIGINAL RESEARCH

Accessibility of fiber surface sites for polymeric additives determines dry and wet tensile strength of paper sheets J.-L. Schäfer . S. Schölch . O. Prucker . T. Brandstetter . J. Rühe .

A. Ritter v. Stockert . T. Meckel . M. Biesalski

Received: 4 November 2020 / Accepted: 5 March 2021 / Published online: 18 April 2021 Ó The Author(s) 2021

Abstract This contribution focuses on understanding of paper wet-strength properties, by taking a closer look at the spatial distribution of wet-strengthening polymers inside the cellulosic fiber network deposited under different treatment conditions using confocal laser scanning microscopy as in situ imaging tool. We compare the behavior of paper samples treated with a photochemically cross-linkable copolymer using an impregnation process employing three different solvents, namely water, 2-propanol (IPA) and 1-butanol J.-L. Schäfer, S. Schölch contributed equally.

(BuOH), respectively. As these solvents swell paper fibers to quite different extents, the deposition of the polymer, on, in or in-between the cellulosic fibers varies quite strongly, as is shown by in-depth analysis using confocal laser scanning microscopy. The difference in accessibility of distinct surface sites exclusively on or also in and between the fibers controls the macroscopic tensile strength under both dry and wet conditions.

Keywords Cellulose Paper Wet strength Spatial distribution Swelling Photo-crosslinking

Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/ s10570-021-03817-7.

J.-L. Schäfer A. R. v. Stockert T. Meckel M. Biesalski (&) Department of Chemistry, Macromolecular Chemistry & Paper Chemistry, Technical University Darmstadt, Alarich-Weiss-Straße 8, 64287 Darmstadt, Germany e-mail: markus.biesalski@tu-darmstadt.de

S. Schölch O. Prucker T. Brandstetter J. Rühe Freiburg Institute for Interactive Materials and Bioinspired Technologies (FIT), Albert-LudwigsUniversität Freiburg, Georges-Köhler-Allee 105, 79110 Freiburg, Germany

S. Schölch O. Prucker T. Brandstetter J. Rühe (&) Department of Microsystems Engineering (IMTEK), Laboratory for Chemistry & Physics of Interfaces, AlbertLudwigs-Universität Freiburg, Georges-Köhler-Allee 103, 79110 Freiburg, Germany e-mail: ruehe@imtek.uni-freiburg.de

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Introduction The tensile strength of paper sheets in their dry or wet state is one of the most important relevant properties for a large number of paper grades progressing from packaging, to hygienic paper, and specialty paper, such as filter or bank notes, respectively. In addition to such classical applications of paper, in recent years, fueled by the ongoing effort for environmentally friendly products, the use of paper has expanded into a variety of new sectors, such as paper for construction materials (Auslender et al. 2017) and the use in diagnostics, microfluidic and lab-on-paper devices (Credou and Berthelot 2014).

Due to its wet-layed production, the strength of the paper sheets is a complex interplay of the properties of individual lignocellulosic paper fibers (length, kink, width, degree of polymerization, fibrillation, defects) and the bonds formed between them, constituting a non-woven fiber network. Tejado and van de Ven (2010) conducted a detailed analysis of the processes in paper formation and the associated forces. They argue, that the fibers are held together via entanglement between smaller cellulose fibrils on the macroscopic fiber surfaces and the main reason for the wetweb strength of undried paper are friction forces. Belle et al. (2015) analyzed freeze-dried wet webs of different solids content with field emission-scanning electron microscopy (FE-SEM) and were able to show such entanglement between fibrils of adjacent fibers and the inter-fiber bonds produced thereby. Another mechanism leading to inter-fiber bond strength can be described by the diffusion of macromolecular cellulose chains into adjacent fibers in close contact (Lindström et al. 2005; Hubbe 2006). While interfiber bonding is the subject of many research projects, the exact mechanism of it is still not fully understood.

However, it is widely accepted that hydrogen bonding, van der Waals and Coulomb forces, mechanical interlocking of fibrils and diffusion of cellulose chains on the surface of fibers, all play a crucial role in this complex interaction, leading to the intrinsic macroscopic tensile strength of a paper sheet (Hirn and Schennach 2015).

A first attempt describing the tensile strength of paper in a quantitative manner was done by Page (1969). Considering that the strength of a single cellulose fiber is significantly higher compared to the strength of a respective paper sheet, it becomes

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apparent that in order to increase the strength of a sheet, the inter-fiber bonds must be strengthened.

Strengthening of the tensile properties in the dry state is traditionally done with so called dry strength agents.

Additionally, pulp refining (Lindström et al. 2016) and wet pressing (He et al. 2003) of sheets can be used to adjust and control sheet tensile properties. A variety of macromolecules have been used in order to increase the dry tensile strength of paper. Examples are biobased polymeric additives, such as starch, chitosan, or carboxy methyl cellulose (CMC), as well as synthetic polymeric systems, such as polyacrylamide (PAM), polyvinylamine, polyethyleneimine or polyelectrolyte multilayers (Lindström et al. 2005). There are at least three proposed mechanisms that explain the dry strengthening effect of additives, including increased inter-fiber bond strength by the reinforcement of existing and/or the contribution via additional bonds, decreased stress concentrations in the sheet while drying and by enhancing the consolidation of the sheet (Lindström et al. 2016).

Upon rewetting, cellulose fiber networks loose nearly their entire strength. This can be explained by the hydrophilic nature of cellulose fibers and their swelling behavior in contact with water (Lindner 2018). Water molecules penetrate the cellulose fibers and fiber walls and consecutively weaken the attractive hydrogen bonds between cellulose chains and fibrils. In addition to that, the swollen fibers are highly malleable, and the aforementioned frictional forces holding fibers together are weakened as well. It is widely discussed, that in order to increase the wet strength, the introduction of covalent bonds between adjacent fibers is necessary, however, other treatments such as wet pressing or the use of microfibrillated cellulose (MFC) have also been shown to increase the wet strength of paper. In the industrial application so called wet strength agents are used to increase the tensile strength of paper in wet conditions. The two main working models of those additives are the protection and the reinforcement mechanism, respectively (Lindström et al. 2005). With the protection mechanism, a firm non-polar polymeric sleeve wraps around fiber crossing points thereby limiting the access of water molecules to the area of fiber contact.

Typically, such polymers are cross-linked hydrophobic resins such as for example phenol–formaldehyde resins, where the used polymeric precursors typically react with one another but do not significantly bind to

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the cellulosic fiber (homo-cross-linking additives).

The other category are additives that are (also) able to form covalent bonds with the cellulose chains, so called hetero-cross-linking, like polyamidoamine epichlorohydrin (PAE). An important difference in comparison to the aforementioned formaldehyde resins is that cross-linking occurs between the polymeric additive as well as between the additive and the fiber, respectively, thereby acting towards wetstrengthening of the sheet through a reinforcement mechanism. Finally, such reinforcement additives can be of polar nature, and may even swell in water, which is necessary for applications, where paper-water contact is crucial, such as with hygienic paper grades.

Note, although there are clear evidences for these postulated mechanisms, there are still a number of open and fundamental questions that have not yet been answered to validate these models or to even precalculate which polymer-intrinsic and/or paper-related parameters determine the wet-strength action of (pre-)polymeric additives. For example, with respect to the latter, knowledge on the exact spatial distribution of the wet-strength agent in the non-woven sheet becomes crucial. Mangiante et al. (2018) prepared paper samples with alkyne-functionalized cellulose fibers, cross-linking them with difunctional PEG chains, leading to significantly improved wet tensile strength. They furthermore analyzed the spatial distribution via Raman confocal microscopy and found, that the fiber core had rich amounts of the alkyne. However, these observations weren’t further discussed with regard to the effect on the macroscopic wet strength.

In previous work of our own groups, we used for the first time polymers that can cross-link within paper sheets under the influence of light (Jocher et al. 2015; Bump et al. 2015). We were able to show that this treatment increases the macroscopic tensile strength, and we analyzed the binding of the polymers to the fibers by Raman spectroscopy and imaging via confocal laser scanning microscopy, respectively.

However, a systematic analysis of the spatial distribution of strengthening agents in the cellulosic fiber network and their impact on dry and wet strengths is still missing. Finally, it is not yet understood how fiber swelling may affect the interaction of the fiber with the polymeric additive and how the latter leads to different deposition scenarios of the polymer on/in the fiber.

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The latter may also influence the macroscopic wetstrength.

As we will show in this contribution, taking a closer look at the spatial distribution inside the fiber network and the fibers themselves can improve our understanding of the mechanisms involved in dry and wet strengthening of paper. In particular, as will be shown, fiber swelling leads to very different deposition mechanisms of a polymeric wet-strength agent and therefore affects largely the macroscopic tensile strength in wet conditions. We compare the treatment of paper samples with a light-sensitive copolymer (Toomey et al. 2004; Janko et al. 2015; Jocher et al.

2015; Bump et al. 2015) using an impregnation process with three different solvents. The solvents were chosen based on literature-data of cellulose fiber swelling (El Seoud et al. 2008; Karppinen et al. 2004) because we were particularly interested in the use of solvents, that don’t lead to significant fiber swelling, in comparison to water. The latter is known for its capability to significantly swell paper fibers thereby altering the morphology. The paper topology includes pores or grooves in which surrounding polymer molecules can be taken up during impregnation. On this basis water, 2-propanol (IPA) and 1-butanol (BuOH) were used to apply the copolymer to labmade paper samples. By using confocal laser scanning microscopy in combination with tensile measurements, we study how this difference in swelling leads to different spatial deposition of the copolymer inside the paper samples and, ultimately, controls dry and wet paper sheet tensile strength.

Experimental section Materials All chemicals and solvents were purchased from Merck, Alfa Aesar, Alberdingk Boley, Fisher Scientific, Fluka, Covestro and TIB Chemicals, respectively, and were used as received, unless otherwise specified. For impregnation and extraction pure distilled water was used, which is denoted as H2O throughout this work.

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Pulp and handsheet preparation All paper samples, modified with the copolymer as described below, where lab-engineered using bleached eucalyptus-sulfate pulp [median fiber length (length-weighted): 0.76 mm; curl: 15.9%; fibrillation degree: 5.1%; fines content: 9.1%]. The paper samples having a grammage of 80 ± 1.6 g m-2 where prepared using a Rapid-Köthen sheet former according to DIN 54358 and ISO 5269/2. In order to prevent any influence on the physical properties, no additives or filler materials other than the copolymer were used.

Prior to impregnation, the paper was conditioned for at least 24 h under standard conditions (23 °C, 50% r.h.).

Polymer synthesis The photo-cross-linking monomer 4-benzoylphenyl methacrylate (MABP) was synthesized according to the procedure developed in one of our groups and described by Toomey et al. (2004). First, the fluorescent monomer rhodamine B methacrylamide (RhBMa) was prepared in two steps from the fluorescing rhodamine B base according to previously published literature (Schäfer et al. 2013). The copolymer poly(dimethylacrylamide-co-4-benzoylphenyl-2methacrylate-co-rhodamine B-methacrylic acid (P(DMAA-co-MABP-co-RhBMA)) carrying the fluorescent monomer, the photo-cross-linking monomer and the matrix monomer, respectively, was synthesized using free radical copolymerization according to the procedure described by Janko et al. (2015). The structure is schematically illustrated in Fig. 1. The

Fig. 1 Molecular structure of the synthesized copolymer poly(dimethylacrylamide-co-4-benzoylphenyl-2-methacrylateco-rhodamine B-methacrylic acid (P(DMAA-co-MABP-coRhBMA)) carrying the fluorescent monomer (m), the photocross-linking monomer (o) and the matrix monomer (n), respectively. Molar composition as determined by 1H-NMR (see supporting information): m ? o = 3.5%; n = 96.5%

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chemical analysis of the molar composition of the copolymer was done using 1H-NMR (see supporting information), which showed that the copolymer consists of about 96.5 mol% of the matrix DMAA, and 3.5 mol% of MABP and RhBMA, respectively. Using size exclusion chromatography (SEC) with a GRAM VS/GRAM linear 10 HS/100lL (DMF 0.002 LiCl) column and a narrow dispersed poly(methyl methacrylate) standard, the molar mass was determined to be around Mn = 27 000 g mol-1 (Ð * 5.7). The copolymer was stored inside a plastic container in a refrigerator at 6 °C until further use.

Copolymer treatment of the paper samples Before impregnation, the lab-engineered paper was cut into samples with the dimensions of 15 9 120 mm. Those dimensions were chosen because tensile testing was carried out according to DIN ISO 1924-2, where a gauge length of 100 mm is specified. To determine the amount of cross-linked copolymer in the samples, a gravimetric approach was chosen. Before each weighing/measurement on a scale under climate controlled conditions (23 °C and 50% r.h.), the samples were conditioned for at least 24 h.

Three measurements were carried-out, once before impregnation (m before), after drying the impregnated samples (m after) and finally after the extraction and consecutive drying (m extraction), in order to calculate the amount (wt%) of copolymer in relation to the paper weight as well as the amount of extractable (unbound) copolymer. For the three solvents the amount of copolymer in the paper samples was 11.4 (H2O), 8.0 (IPA) and 7.3 wt% (1-BuOH). For the reference experiments the paper samples were impregnated in H2O without any copolymer, but otherwise the procedure was carried out in the same way as described before. Here, the measurement after swelling and drying (m delta) was used to calculate the wt% change induced by the treatment without copolymer. Complete details of these calculations and results can be found in the supplementary information. The copolymer was dissolved in H2O, in 2-propanol, or in 1-butanol, respectively, at a concentration of 25 mg mL-1 and stored in flasks in a refrigerator at 6 °C under exclusion of light until use. The polymermodification of the paper samples is illustrated in Fig. 2 and was conducted as follows.

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Fig. 2 Schematic illustration of the impregnation process of paper samples with the copolymer dissolved in three different solvents H2O, 2-propanol (IPA) and 1-butanol (BuOH)

In brief, (1) a PTFE (Teflon)-dish was filled with the given copolymer solution (H2O, IPA or BuOH) and the pre-weighed paper samples were submerged one at a time. A petri dish on top (of the Teflon-dish) was used to prevent large amount of solvent evaporating and thus changing the concentration over time.

After at least 60 s, the paper samples were pulled out and laid flat on a 3D-printed frame with Teflon yarn (see supporting information for details) for drying overnight under climate-controlled conditions. (2) After equilibration and consecutive weighing of the impregnated paper samples, ambient dried samples were pressed with a defined weight (* 15 kg) for about four hours to ensure flat samples and a uniform UV-excitation. By illumination of the paper samples with UV-light, the benzophenone groups were excited and thus the cross-linking of the copolymer was induced. The UV source used was a Newport 1000 W Oriel Flood Exposure Source with a wavelength of k = 365 nm. Five samples were illuminated at a time, each with an energy density of E = 16 J cm-2, at which approx. 94% of the present benzophenone groups reacted, as reported previously (Toomey et al.

2004; Jocher et al. 2015). (3) After the UV-excitation, extraction with cold H2O was carried out, in order to remove any unbound copolymer from the paper samples. For this, the paper samples were extracted in an extraction apparatus for four cycles, which is sufficient for complete removal of unbound copolymer, after which the paper samples were dried on the Teflon-sieve under climate-controlled conditions.

After equilibration the dried paper samples were weighed one more time, before they were pressed again to guarantee flat samples for the tensile measurements. Note, for the reference experiments, paper samples were impregnated in H2O, IPA or BuOH for at least 60 s and dried afterwards, all other steps were carried out as described above.

Preparation of thin slices for confocal microscopy To improve the visibility of cellulose fibers under fluorescence microscopic investigation, paper sheets were stained in a 100 lM aqueous Calcofluor White (CW) solution for ten minutes, washed in H2O for another 10 min to remove unbound CW and dried overnight. A commercially available polyurethane system consisting of an aliphatic polyisocyanate (Desmodure 3200), a polyol (Albodur 956), and a catalyst (TIB Kat 318) in a ratio of 1:1:5 9 10–4 was used to embed the samples. After a few vacuum cycles at room temperature to improve the penetration of the embedding medium inside the paper samples, they were cured overnight and consecutively cut into 120 lm slices using a microtome.

This technique has been used to analyze the spatial modification of paper and the functionalization of fiber surfaces, where details can be found in recent publications (Bump et al. 2015; Janko et al. 2015; Nau et al.

2019).

Despite an increased resolution and contrast that confocal detection provides over standard widefield

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microscopy, light scattering limits axial resolution, especially with highly scattering samples, such as paper. The latter to some extent can be circumvented, by imaging thin cross-sections of paper to assess the distribution of chemical modifications across the paper width. As lateral resolution of microscopy applies, which, for confocal detection, exceeds the axial resolution by a factor of around 3, this method becomes highly suitable for distinguishing, e.g. whether the copolymer has adsorbed onto or within a fiber wall, this resolution gain is required. Additionally, image stacks of paper samples were made with traditional confocal detection and combined into 3Dimages in order to analyze the spatial distribution of the copolymer between the fibers and inside the whole fiber network. It should be noted, that this approach reintroduces some of the challenges, which were avoided by using thin cross-sections.

For confocal investigation, the prepared slices or the paper samples were mounted between two round coverslips with Type F Immersion liquid from Leica or H2O, respectively, as immersion medium. Using a Leica TCS SP8 (Leica Microsystems, Mannheim, Germany) CW fluorescence was excited at 405 nm and fluorescence emission was detected between 420 and 480 nm. Rhodamine B labelled copolymer, in turn, was excited at 552 nm and detected between 570 and 650 nm. Image stacks of * 50 slices with widths of 0.6 to 1 lm were acquired and processed within the Leica Application Suite X to render 3D-stacks of paper samples for further analysis.

It should be noted, that during image acquisition the settings for laser intensity, detector sensitivity (gain and offset) and later on during image processing, the brightness and contrast of the images were adjusted, in order to achieve the highest detail and images where the spatial distribution of the copolymers can best be compared. Thus, no quantitative information about copolymer amount in the different samples can be derived from the images provided/shown in this work.

However, quantitative analysis through calibration via a concentration series can be done as was shown previously by our group (Bump et al. 2015), however it was not in focus in this work here.

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to DIN ISO 1924-2 with a Zwick Z1.0 with a 1 kN load cell using the software testXpert II V3.71 (ZwickRoell GmbH & Co. Kg) in a controlled environment with 23 °C and 50% r.h. Wet tensile strength measurements were conducted in analogy, after the paper samples had been submerged in H2O for at least five minutes.

To remove any excess water, the paper samples were sandwiched between tissue paper and slightly pressed.

A 20 N load cell was used for those measurements, to improve the precision of the tensile tests in the wet state. Along the measured dry and wet tensile indices, the relative wet strength is a relevant quantity, which is used in the paper formation industry and can be calculated according to this equation: rel: wet strength ¼

wet tensile index 100 dry tensile index

ð1Þ

The definition of the dry/wet tensile index is shown in the following equation: dry=wet tensile index ¼

Fmax 100 b grammage

ð2Þ

Here Fmax is the maximum force at break in N, b is the width of the sample in mm and the grammage of the paper is given in g m-2.

Setup for fiber failure analysis via Zwickvideography A commercially available full frame mirrorless camera from Panasonic (DC S1) with a macro lens from Canon (MP-E 65 mm f/2.8 1–5 9 Macro Photo) and an adapter from Novoflex (SL/EOS) was used. The camera was mounted on a manual x/y/z-stage on a table that was decoupled from vibrations of the tensile testing equipment. The aperture was set to 5.6, the shutter speed was 1/30 s and the ISO was set to 800.

The videos were recorded with a resolution of 3840 9 2160 px at a frame rate of 29.97 frames/ second. A UV-lamp (365 nm) was used from the backside of the paper samples to excite the fluorophore (cationic dye—Pergasol Yellow F6-GZ) of the labelled fibers (0.1 wt% of all fibers).

Tensile strength analysis

Results and discussion

Dry tensile strength of the paper samples was determined as an average of at least five samples according

As water is by far the predominant processing solvent used in the paper formation industry, we used this

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solvent in conjunction with copolymer treatments of paper in previous studies (Jocher et al. 2015). However, while it was possible to prepare paper samples with high wet tensile indices with the copolymer dissolved in H2O, we also became interested in the use of other solvents in combination with copolymer immobilization. We were particularly interested in the use of solvents, that don’t lead to significant fiber swelling, in comparison with water, which is known to swell paper fibers. As model paper sheets eucalyptussulfate paper samples with a grammage of 80 g m-2 were prepared and impregnated with the respective copolymer solution in the chosen solvent. In order to prevent any mechanical agitation of the cellulose fibers during treatment with the copolymer solution, an impregnation approach was used in this work. This also enables the analysis of the diffusion process of the copolymer in the different solvents throughout the fiber network and the fibers themselves, without any influence from external forces. A sieve with thin Teflon-yarn was designed for non-contact drying, to prevent further influence on the spatial distribution by capillary or gravitational forces. The copolymer modified paper sheets were illuminated by UV-light in order to cross-link the copolymer inside the sheet.

Non-bound macromolecules were removed by solvent extraction. The amount of paper sheet attached, crosslinked polymer was determined by gravimetric analysis and was typically on the order of 10 wt% in relation to untreated paper samples (see supporting information for details). After cross-linking and extraction, respectively, we first investigated possible failure-mechanisms by time-resolved tensile microscopy. Secondly, the macroscopic tensile properties in the dry and the wet state were determined via tensile strength analysis and the spatial distribution of the macromolecules inside the fiber network was finally analyzed in detail using confocal laser scanning microscopy (CLSM).

Tensile properties The tensile properties of paper sheets are very sensitive to a wide variety of environmental factors, e.g. humidity, temperature, UV-exposure, swelling and drying. Since the copolymer-application includes multiple swelling and drying cycles in addition to significant UV-exposure, the influence of this treatment without the copolymer on the tensile and wet

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tensile strength was analyzed first as a reference. It should be noted that paper without any wet strengthening is intrinsically very weak in the presence of water and it is therefore challenging to analyze such wet paper sheets in a conventional tensile testing device, as such measurements generally lead to significant errors in the range of 10–13% (SD) for eucalyptus-sulfate paper with a grammage of 80 g m-2. While the influence of grammage or fiber type was not the focus of the present study, this may well be addressed in a follow-up work.

Figure 3 shows examples of tensile curves in (a) the dry and (b) the (aqueous) wet-state, respectively, for paper samples that have been modified and crosslinked with the same copolymer from different solvents mentioned in the figure. The dry and wet tensile index, as derived from these measurements using Eq. (2) is shown in Fig. 4.

Failure mechanisms of functionalized paper sheets in the wet state It is believed that the main failing mechanism of cellulose paper in the wet state is governed by fiber pull out. This would be intuitive, because in the wet state the fiber–fiber-bonds are weakened, and the fibers themselves become more flexible provided that the fibers do swell significantly. The failure of single fibers, on the other hand, should occur less frequently, but should be observable nevertheless (Siqueira 2012).

In order to get a better understanding of the failuremechanisms for the eucalyptus polymer-modified paper samples used in this study here, an experimental setup for observing single cellulose fibers during the tensile straining was assembled. Paper sheets, where 0.1 wt% of all fibers have been stained with fluorophore Pergasol Yellow F6-GZ, were analyzed in a time-resolved fashion.

Figure 5 shows snapshots of tensile-test videos taken from polymer-modified paper sheets. Note, the complete video sections can be found in the supporting information. As shown in the first example in Fig. 5a, we frequently observed tearing of single fibers, while being under significant bending load. By looking at the combined length of the fiber fragments and comparing it to the initial fiber length, it becomes apparent, that the fiber-wall delaminates (Fig. 5a). Delamination of the fiber cell wall has been shown to occur in other eucalyptus paper samples with the help of electron

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Fig. 3 Exemplary stress–strain curves measured during dry (a) and wet (b) tensile testing, from which the maximum force at break (Fmax) and subsequently the tensile indices were calculated Fig. 4 Dry and wet tensile indices of eucalyptus-sulfate paper samples with a grammage of 80 g m-2 shown side by side with the amount of copolymer inside the paper samples, as determined by gravimetric analysis for: pure cellulose paper samples not subjected to any treatment (Ref), samples subjected to the procedure for copolymer application, without any copolymer in the solution (RefSwell), and samples where the copolymer was applied out of H2O, IPA and BuOH (for more details, see supplementary information)

microscopy by other groups as well. (Siqueira 2012) In addition, fiber pull-out was observed as a second typical failure mechanism, shown in Fig. 5b, respectively. Both mechanisms, fiber pull-out and fiber delamination driven breakage, have been reported before by others, studying the tensile properties of wet-strengthened paper sheets. (Siqueira 2012)

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Studying single-fiber behavior in more depth is of high interest, however, also requires an extensive statistical analysis. Note, at present we did not intend to draw such quantitative information from the tensiletest videos in order to judge which kind of failure mechanism determines the breakage of the wet paper samples most. We instead moved our attention to a

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Fig. 5 Examples of two frequently observed mechanisms (a fiber failure and b fiber pull-out) during the failure of polymer-modified paper sheets during tensile straining in the wet state, captured with a self-made Zwick-videography setup

quantitative macroscopic tensile behavior of the paper sheets, and in particular, of polymer-functionalized paper sheets, which allows for higher throughput of various samples.

Impact of swelling-drying-cycles combined with UVtreatment In order to compare the tensile strength of paper samples treated with the copolymer, we first analyzed the influence of the solvent treatment without any copolymer. When comparing the tensile-straining behavior of the Ref- and RefSwell-samples in the dry state in Fig. 3, it is apparent, that the treatment without copolymer leads to a significantly lower maximum force at break, while the elongation (strain at break) is increased. In the wet state, however, differences in the elongation are not as significant. In Fig. 4 the dry and wet tensile index, as derived from the stress–strain curves for the different paper samples are shown. If a reference paper sheet (Ref) is analyzed with respect to the tensile index, we observe typical values of about

17.6 N m g-1 and 0.3 N m g-1 for the dry and wet state, respectively. If the same paper is now being treated with one of the solvents without any copolymer and a similar swelling/drying protocol in combination with UV-exposure is used as with the copolymer impregnation process, we observe a decrease of both, dry and wet tensile index, to 9.8 and 0.2 N m g-1 (RefSwell—H2O), 12.1 and 0.2 N m g-1 (RefSwell—IPA) and 12.2 and 0.2 N m g-1 (RefSwell—BuOH), respectively.

There are multiple factors which have to be taken into account, when analyzing the effect of such a swelling/drying treatment with UV-exposure. Gravimetric experiments show, that drying under climate controlled conditions (23 °C, 50% r.h.) leads to a significant increase in moisture content (for all three solvents—detailed information in table S4), which can weaken the forces described earlier, being responsible for the intrinsic strength of paper, and thus the tensile strength. Further on, it has been found that unrestrained drying, which is comparable to drying on a mesh, significantly lowers the tensile strength (Strand

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et al. 2017; McKenzie and Higgins 1955), which further explains our results with the reference paper.

Another factor is the extraction of fiber fragments known as ‘‘fines’’ of the fiber network, which is also known to decrease the tensile strength. Although the paper samples in this study haven’t been dry or wet pressed after paper making, the additional swelling and drying cycles during the treatment of paper samples can also lead to hornification, which could explain the observed decrease in measured tensile strength. In addition, it is safe to assume, that the intensive UV-exposure also has an influence on the tensile properties. It is known, that such UV-exposure significantly reduces the degree of polymerization (DP) of cellulose fibers (Kolar et al. 2000), and the group of Fang et al. (2020) observed that such a decrease of cellulose DP leads to a significant decrease of the tensile strength of nanocellulose films prepared thereof. Considering, that for the three reference experiments without any copolymer addition, the extraction process was carried out in H2O and the UV-treatment was identical, the observed similar values for dry and wet tensile index can be explained.

Impregnation using H2O versus IPA versus BuOH We next investigated the stress–strain behavior of paper sheets modified with the copolymers in water, 2-propanol and 1-butanol, respectively. Examples of the data are shown in Fig. 3. It can be inferred that the dry tensile behavior for all solvents chosen are (almost) similar, whereas the stress–strain behavior of the respective samples in the wet state differs significantly. In particular, the maximum force at break progresses as BuOH \ IPA \ H2O. Dry and wet tensile index were further calculated from the measurements and are shown for all samples in Fig. 4.

As can be inferred from the figure, the dry tensile index was increased by all three impregnation-treatments from 17.6 and 9.8 N m g-1 for the Ref and RefSwell, respectively, to 36.8, 46.3 and 44.5 N m g-1 for H2O-, IPA- and BuOH-impregnation, respectively. Comparing the wet tensile indices, it becomes clear that the H2O-impregnated paper samples possessed significantly higher wet strength.

The latter increased from 0.3 and 0.2 N m g-1 for RefSwell and Ref, respectively, to 8.9 N m g-1, while IPA and BuOH-impregnation led to less increased values of 2.2 N m g-1 and 1.4 N m g-1 respectively.

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Considering, that the coefficients of variation are all below 5% (see supplementary information), these results are reproducible over many test samples. Only measurements for the wet tensile index of IPAimpregnated samples have a significantly higher coefficient of variation, indicating that IPA introduces a broader range of slightly different impregnation results, even though this can’t be observed for tensile tests in the dry state where the coefficient of variation is significantly lower.

The amount of cross-linked copolymer inside the paper samples was evaluated by a gravimetric approach under norm climate conditions. The results (see supplementary information) show that more copolymer adsorbs during impregnation with H2O (11.4 wt%) if compared to IPA- (8.0 wt%) and BuOHimpregnation (7.3 wt%). However, given the low absolute difference of the impregnated amount combined with the similar dry tensile values, we do not believe that this parameter is the determining factor for the observed difference in tensile properties.

In Fig. 6 the calculated values for relative wet strength of the different paper samples are shown. It is immediately evident, that applying the copolymer from H2O yields the highest relative wet strength paper samples. The high relative wet strength of 24.2% is directly related to the significant increase in wet tensile index, while at the same time the dry tensile index increases only by a significantly lower factor.

Because the relative wet strength (Eq. 1) is calculated from the ratio of wet to dry tensile index, this overstates/exaggerates the wet strength of the paper samples, in comparison to the IPA- and BuOHimpregnated variants, where the increase in wet tensile strength was lower, while the increase in dry tensile strength was significantly higher. Therefore, the values for relative wet strength should always be treated with caution, when comparing paper samples and the data used for the calculation should be taken into account.

Spatial distribution—impregnation in H2O versus IPA versus BuOH Next, we addressed the question why different solvents lead to significant differences in tensile strength. To this, the spatial distribution of copolymer inside the cellulose fiber network was analyzed using CLSM. By incorporating a fluorescing group,

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Fig. 6 Relative wet strength of eucalyptus-sulfate paper samples with a grammage of 80 g m-2 shown side by side for: pure cellulose paper samples not subjected to any treatment (Ref), samples subjected to the procedure for copolymer application, without any copolymer in the solution (RefSwell), samples where the copolymer was applied out of H2O, IPA and BuOH (for more results, see supplementary information)

Rhodamine B, into the copolymer and subsequently staining the treated cellulose fibers with the fluorescent brightening agent CW, before embedding them in an epoxy-resin and preparing thin cross-sections, the spatial distribution throughout the fiber network was analyzed.

Cellulose fibers were stained after the copolymertreatment, in order to enhance their visibility while fluorescence imaging. Although cellulose fibers from eucalyptus-sulfate pulp contain considerable amounts of lignin detectable with fluorescence imaging, the importance of the staining procedure can be seen in Fig. S4 in the supplementary information. Staining cellulose fibers after impregnation was done in order to rule out any effect of the staining on the spatial distribution of the copolymer and on the development

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of tensile properties of the samples. Due to this, some images may show inhomogeneous staining (e.g.

Fig. 7g), probably caused by interactions between the copolymer and the dye.

In consideration of the ability of H2O to swell cellulose fibers, it can be assumed, that the dissolved copolymer is able to penetrate the cellulose fiber network as a whole, which includes the fiber wall and the lumen, respectively. Taking a closer look at the fluorescence-channel from the fluorescently-labelled copolymer in Fig. 7b, this hypothesis can be confirmed. The fluorescence can be observed across the whole paper width, pointing towards the complete and homogeneous penetration on the scale of the fibers.

Furthermore, we find copolymer inside the fiber lumen and upon higher magnification (see Fig. 7c) even within the fiber walls. In contrast, as both IPA and BuOH do not account for high degrees of swelling of the paper fibers, polymers dissolved in these solvents and brought into contact with the fiber are not expected to access all spaces within the non-woven sheets.

Taking a closer look at the superimposed image of the fluorescence channels of the IPA- and BuOH-impregnated cellulose fibers (CW—cyan) and the copolymer (RhB—magenta) in Fig. 7f, i, respectively, this hypothesis can be confirmed. The copolymer seems to be mainly accumulated on the outside of the fibers and scattered rather inhomogeneously across the paper width. Upon higher magnification, Fig. 7f, i, respectively, show, that the copolymer is not able to penetrate inside the fiber or into the fiber walls and only a few of the fiber lumens are partially filled.

To further analyze the spatial distribution between the fibers, image-stacks of the cellulose fiber network were taken with the CLSM, and combined to 3Dimages. The fluorescent brightening agent CW was once again used to stain the cellulose fibers.

The stacked images of the cellulose fiber network in Fig. 8 reveal the distribution of the copolymer in between the fibers. In analogy to the observations in the cross-sectional images, the overlay of the fluorescence channels in Fig. 8c shows, that the copolymer dissolved in H2O seems to be distributed homogeneously across the paper width. In comparison, applying the copolymer out of IPA and BuOH seems to lead to a more scattered, inhomogeneous distribution across the paper width, as can be inferred from Fig. 8f, i, respectively. However, BuOH-impregnation seems to be more homogeneous, especially when

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Fig. 7 CLSM cross-sectional images of an embedded paper sample, with the fluorescently labelled copolymer applied out of H2O (a–c), IPA (d–f) and BuOH (g–i), respectively, beforehand.

The images show the cellulose fibers stained by CW (a, d, g), PDMAA labelled by RhB (b, e, h) and an overlay of the latter (c,

f, i). The insets each show a magnification to highlight the spatial distribution of the RhB-labeled PDMAA across the fiber width and inside the fiber lumen. Scale bars are 100 and 20 lm for the overlay and insets, respectively. Overlays of both fluorescent signals appear as white

comparing the fluorescence channels of the copolymer for IPA- and BuOH-impregnated samples in Fig. 8e, h, respectively. It is also apparent, that in particular with H2O as a solvent there are almost no sleeves around fiber crossing points, which could yield a significant reinforcement of these fiber crossingpoints, if in contact with water. Note, the use of dimethylacrylamide as the matrix monomer leads to a

highly hydrophilic polymer, which can readily swell in water, even in the cross-linked state. Thus, we do see clear evidence for action of the copolymer/H2O system by the reinforcement mechanism, rather than by a protection mechanism. However, as is also evident from the structure analysis, the copolymers do not act purely on the reinforcement of fiber-crossing points but rather on the complete paper fiber. In the

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Fig. 8 CLSM image stacks of paper samples combined to 3Dimages, with the fluorescently labelled copolymer applied out of H2O (a–c), IPA (d–f) and BuOH (g–i), respectively, beforehand.

The images show a top view of cellulose fibers stained by CW (a, d, g), PDMAA labelled by RhB (b, e, h) and overlays of the latter (c, f, i). For H2O-impregnation the images show the homogeneous spatial distribution of copolymer across the whole

fiber network. Overlays of both fluorescent signals appear as blue in 3D-renderings. In contrast, the IPA -impregnated samples show an inhomogeneously distributed copolymer with agglomeration at fiber–fiber crossings, while the BuOHimpregnated samples show a more homogeneous distribution, but not on the single fiber scale comparable to H2O-impregnation. Scale bar is 100 lm

case of IPA and BuOH, the copolymer can be observed as sleeve-like-aggregates both at as well as away from fiber–fiber crossings, as can be seen in Fig. 8f, i, respectively. However, as they are not fully wrapping around the fiber crossing points and due to the hydrophilic nature of the cross-linked polymer, the

protection of those crossing points from moisture according to a protection mechanism can be neglected.

While those aggregates are able to form cross-links between adjacent fibers, the remaining fiber surface, without any fibers in close proximity, is virtually free of adsorbed copolymer—at least as far as fluorescence

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imaging allows to state this. In contrast, when comparing the fluorescence channels of the fibers (cyan) and the copolymer (magenta) of the H2Oimpregnated paper samples (Fig. 8a, b), it is difficult to see significant differences. It seems as though the fluorescence of the CW-labelled fibers is at the outermost part and the fluorescence of the copolymer is beneath it, which can be observed in the crosssectional images in Fig. 7c, too. This suggests, that the copolymer is not outside of the fibers or on the surface, but rather inside the fiber wall and the lumen, respectively. The presence of the copolymer inside the fibers can also be observed when looking at the inplane projection of the stacked images (see supplementary information).

When comparing different solvents, the hydrodynamic properties, and, in particular, size exclusion effects as well as thermodynamic behavior of the polymers in solution can become critical and can impact the deposition and resulting spatial distribution of the adsorbed copolymers in/on the fibers. In order to learn more about the thermodynamic behavior of the polymers in dilute and concentrated solutions, we analyzed the polymer solutions by dynamic light scattering (at concentrations of 5 mg mL-1) and additional turbidity measurements at higher concentrations (5–45 mg mL-1), in order to determine the phase behavior at variable temperatures (5–50 °C).

Data can be found in the supporting information. In brief, hydrodynamic radii of the copolymers in H2O, and IPA, show very similar values in the range of 7–11 nm, respectively.

The turbidity experiments (see supplementary information) do not show any aggregation for the solvents IPA and BuOH over a wide temperaturerange even at highest chosen concentrations of up to 45 mg mL-1. However, for the copolymer-solution in H2O, clouding can be observed at around 13 °C which becomes more pronounced with increasing temperature. Lowering the concentration to 25 mg mL-1 (impregnation) and further to 5 mg mL-1 (DLS), the temperature, where clouding is first observed, increases significantly to 21 and 36 °C, respectively.

In particular, in alcoholic solutions under impregnation-conditions chosen, no aggregates are present.

To elucidate how accessible the adsorption sites are for the copolymer in the cellulose fiber network, information on the pore size (distribution) is important. The pore sizes of the paper sheets used in our

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study range from tens of nanometers to 30 lm and was analyzed by mercury-intrusion (for an example of the latter see supporting information). With respect to the adsorption of macromolecules on/in paper fibers, smaller pores in the cell walls were studied by various methods and values ranging from 10 to 100 nm were reported (Wu et al. 2009). Pores and grooves that are on the order of the size of an unperturbed polymer chain may not be trivial to be accessed by the macromolecules due to confinement effects (i.e. the polymer chain can only enter the pore if segments are stretching, which is thermodynamically not favored).

However, polymers may easily enter pores that are larger than the molecular size of the macromolecule.

With respect to this, it therefore makes a large difference, if fibers are pre-swollen in the solvent or if the polymer solution is transferred onto a dry paper sample. Submerging a dried paper sample in a given solvent leads to sudden imbibition into the sample, due to strong capillary forces. (Alava and Niskanen 2006) The work of El Seoud et al. (2008) allows for a quantitative comparison of the used solvents in this regard, by determining the extent of swelling using a simple gravimetric approach. The results show a significant difference of the ability to swell cellulose fibers: H2O * 62.7%, IPA * 4.7% and BuOH * 7.2%, respectively. From these values it can be inferred, that impregnating the paper with the copolymer dissolved in H2O, a significantly higher volume of the copolymer solution, i.e. around ten-fold more, is spontaneously pulled inside the fiber network and inside the fibers by capillary forces and fiber swelling, as compared to impregnation of paper with IPA and BuOH solutions.

Without the possibility of fiber swelling, IPA and BuOH can only transport the copolymer to the outer surface of fibers, or inside the fiber lumen through larger defects and pinholes in the fiber wall. While drying, the solvent accumulates at fiber–fiber crossings, due to increased capillary forces, leading to precipitation of the copolymer at such spots. In addition, IPA has a lower boiling point and a higher vapor pressure [42.6 hPa (20 °C) (Lide 2004)] compared to H2O and BuOH, leading to a more sudden evaporation of the solvent and hence yielding an inhomogeneous distribution of the copolymer across the paper width, which again is in accordance with our structure analysis.

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The copolymer inside the fibers of H2O-impregnated samples can form homo-cross-links with its own backbone (Toomey et al. 2004; Körner et al. 2016; Prucker et al. 2018), but is also able to yield covalent hetero-cross-links with CH-groups of cellulose chains (Jocher et al. 2015). In this way, cellulose chains, penetrating into the fiber wall from adjacent fibers, can covalently link with each other and with the fibers under the influence of UV-light, thereby also strengthening fiber–fiber bonds. Hence, the latter, may be one explanation for the increase in dry and wet tensile strength, even though most of the copolymer is located inside the fibers, ostensibly not contributing to the reinforcement of fiber–fiber bonds. Another explanation, how a cross-linking polymer within the fiber wall may contribute to a higher wet strength can be deduced from our video observations. Single fiber failure and in particular delamination would directly benefit from a strengthened fiber wall, i.e. reinforced by the crosslinked copolymer on and in the fiber wall. While the slipping mechanism may strongly be affected by fiber–fiber connections, it is reasonable to assume, that fiber flexibility in the dry and the wet state plays a significant role if fibers glide/slide past one another during tensile load.

Combining the results of the spatial analysis and the tensile tests, it may be assumed, that the copolymer works as a bulking wet strength agent. Luner and Zhou (1993) reported that depositing or introducing chemicals of molecular size, small enough to penetrate the cell wall, is a method to achieve wet strengthening. By doing this, the moisture regain is reduced and therefore the dimensional stability is increased, inhibiting swelling of the cellulose fibers. Swelling of cellulose fibers has a major effect on the morphology especially the length and width (Lindner 2018). A possible explanation for the significant impact decreased swelling and increased dimensional stability of single cellulose fibers have on the wet tensile strength of the fiber network inside paper sheets is, that the relative bonded area (RBA) isn’t decreased when the paper comes into contact with moisture. The RBA is one of the parameters of the Page equation and is a key factor influencing the dry tensile strength, next to the single fiber bond strength (Page 1969). Even though introducing moisture inside a fiber network changes the situation and probably the failure mechanism under tensile load, one may still consider the RBA as an

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important factor, if describing the tensile strength of paper in the wet state.

Studies looking at the enhancement of dry strength by using carboxymethylcellulose-grafted (CMC) pulp, cationic starch (C-starch) and microfibrillated cellulose (MFC) found, that the enhancement was mainly due to the increase in RBA on the microscale (Lindström et al. 2016). These results are supported by experiments on the influence of the applicationmethod of polyacrylamide (PAM) on the enhancement of dry tensile strength of paper (Mihara et al. 2008).

They found, that an external application method by impregnating finished paper samples in an aqueous PAM solution yielded significantly higher dry tensile index values, compared to the mass application during paper formation. Additionally, the spatial adsorption of PAM was analyzed by controlled etching of the cellulose and concurrent ATR-FTIR measurements, which showed, that the external application method led to the deposition of PAM mainly on the fiber surface and around fiber–fiber bonds, respectively.

This can explain the difference in the observed dry tensile indices of H2O-, IPA- and BuOH-impregnated samples, since impregnation from H2O yields densely homo- and hetero-cross-linked networks of copolymer inside the cellulose fibers. Therefore, the amount of copolymer contributing to cross-linking outside and in between fibers, possibly increasing the RBA and thus the dry tensile strength, can’t be as high.

Conclusions The preparation of paper samples impregnated with a photo-cross-linkable fluorescent copolymer P(DMAA-co-MABP-co-RhBMA) was achieved using three different solvents H2O, IPA and BuOH, respectively. Concurrently the spatial distribution of the copolymer inside the fiber network and the fibers themselves was analyzed with confocal microscopy (CLSM) of thin cross-sections and 3D-images. This method, in contrast to commonly used imaging methods in paper science and technology, does not require sophisticated sample preparation nor any specialized very expensive equipment and would thus be of interest for many more analytical tasks in the context of paper structure analysis.

Using either of the three solvents for impregnation, we were able to significantly increase the dry and wet

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tensile indices, by using the aforementioned copolymer yielding relative wet strengths of up to 25%.

Because H2O has the ability to readily swell cellulose fibers inside the paper network, a homogeneous distribution of the copolymer in and on the fibers can be observed. This reinforces both the fiber crossing points and the fibers themselves, which increases the tensile index, especially in the wet state.

By using solvents which do not swell the cellulose fibers, e.g. IPA or BuOH, the copolymer can’t penetrate into the fibers, the distribution of the copolymer on the surface of the fibers is less homogeneous and wet tensile (relative wet-) strength is much lower.

Overall, we can thus conclude that impregnation with IPA and BuOH solutions of the copolymer is preferable for the dry tensile strength, while impregnation with H2O dissolved macromolecules followed by photocrosslinking allows for increased wet tensile strength of paper samples.

To gain an even deeper understanding of the underlying mechanisms, in particular when using the photo-reactive wet-strength agents, in future, we will focus in more detail on the influence of paper-extrinsic parameters, such as the source of light, illumination conditions and solvent content during illumination, as well as paper/paper technology intrinsic parameters such as a comparison of the here reported impregnated papers vs. the use of (charged) copolymers in wet end processes.

Acknowledgments The authors thank Heike Herbert and Martina Ewald for technical support. Funding by the DFG under Grants #405422473, and #406026418 is gratefully acknowledged. This study was in part also supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy – EXC2193/1–390951807 (livMatS).

Authors’ contributions J-LS and SS have contributed equally to this work. SS contributed the expertise and synthesized and provided the materials for photochemically cross-linkable copolymers and performed the measurements of the hydrodynamic radii. J-LS contributed the expertise in cellulose (paper) modification and prepared the functionalized paper samples, carried-out all tensile tests as well as confocal laser scanning microscopy measurements. ARvS prepared sample paper sheets as well as he provided specifications of the used lab-paper. OP and TB planned and supervised the synthesis of the copolymers, and TM planned and supervised the measurements by various microscopy techniques. JR and MB planned and supervised the whole project and are equally

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Funding Open Access funding enabled and organized by Projekt DEAL. Funding of this work is provided by the German Research Council, DFG under grants #405422473 and #406026418 is gratefully acknowledged. This study was in part also supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy – EXC-2193/1–390951807 (livMatS).

Availability of data and material The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request. All data generated or analysed during this study are included in this published article (and its supplementary information files).

Declarations Conflict of interest competing interests.

There are no conflicts of interest nor

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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PAPERmaking!

FROM THE TH PUBLISHERS PUB SH RS OF PAPER PAP R TECHNOLOGY T INTERNATIONAL

Volume 7, Number 2, 2021

How fire warning signs can help you enforce your fire safety policy Fire warning signs are a key part of your health and safety requirements and legal responsibilities. No matter the type of business or environment, you need to provide clear, unambiguous information for staff, visitors and customers to allow them to respond quickly and safely in the event of an emergency, as well as advise them on how to behave to minimise the risks.

There are a lot of different fire safety signs on the market, so it can be tricky to get a clear picture of what you need, both legally and from a safety perspective. Here’s our overview of the different types of signs, best practice for using them and your legal obligations.

The different types Fire safety signs can be broadly split into four main categories: prohibition signs, warning signs, mandatory signs and equipment/exit signs.

Prohibition signs are used to show that a behaviour or activity is not permitted. These include the commonly seen ‘No Smoking’ sign, as well as signs such as ‘No Unauthorised Personnel’ and ‘No Naked Flames’.

Warning signs indicate that there is a particular risk or hazard present — for example, it’s a high voltage area or a flammable liquid store.

Mandatory signs give instructions to ensure that risks are minimised. These include signs like ‘Fire Door Keep Shut’ and ‘Fire Door Keep Clear’.

Equipment and exit signage is used to show the quickest way out of the building, as well as the location of, and route to, first aid equipment, alarm call points and extinguishers.

These categories are usually further broken down with colours: ● ● ● ●

Red indicates prohibited activities Yellow advises caution Green denotes positive actions Blue shows mandatory information

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Shapes are also used to immediately convey meaning: ● ● ● ●

A circle with a diagonal bar indicates that something is forbidden Triangles are used for warnings Green square/rectangle is used for safety information Red square/rectangle is used for fire-specific safety information

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Placement Signs should be placed frequently and visibly. If a visitor is wandering around any part of your site they should never be too far from a sign pointing them towards the nearest emergency exit, alarm and/or extinguisher, making sure that any junctions or changes of direction are clearly marked.

Don’t put your sign in an obscure place like behind a piece of machinery, make sure they are clearly and easily visible (not too high, not too low) and ideally can be read in low or no light environments (dark corridors, power cuts etc) — so look for luminous signs or models with independent emergency lighting.

Emergency and safety signs should be kept separate from other signs and notices — placing them on a notice board that is full of other posters, pictures and information can cause them to be significantly harder to see as they are lost in the crowd.

As a guide, signs placed on walls should be 1.7 metres from the floor so that they are both conspicuous and easy to read.

You’ll also want to make sure that your signs are of a good quality. Signs that fade, curl or fall down are going to need replacing to ensure that they are still both useful and compliant with the law. Make sure to conduct regular maintenance as well, ensuring that the sign is clean and legible. Lastly, make sure that any changes to the site, whether temporary or permanent, are reflected in your use of signs — arrows pointing to a door that is jammed or blocked could prove to be disastrous.

Legal responsibilities The laws regarding fire safety are far ranging and can seem quite daunting. At the core of them all, though, is good health and safety practice — both in reducing the likelihood of a fire and how to deal with one if it breaks out. ● ● ● ● ●

Evacuation routes must be readily available to anyone on site, and must be easy to understand.

Fire response equipment like alarms and extinguishers must be easy to find, access and use.

First aid equipment must be similarly easy to find, access and use.

Fire safety instructions also have to be displayed and understood by anyone on site.

Behavioural signs like ‘Fire Door Keep Shut’ must be used to reduce risks and possible escalations in the event of an emergency.

The key law to be familiar with is the Regulatory Reform (Fire Safety) Order, 2005 (available on the UK Government website). Non-compliance can result in criminal proceedings against directors, safety managers and proprietors. Punishments include fines and up to two years’ imprisonment. In the event of a fatality, charges can include manslaughter or murder.

No Smoking signs are mandated under the Health Act 2006 Part 1 and Smoke Free Signs Regulation 2012.

Regulations covering the quality, design and content of the signs themselves are now grouped under BS ISO EN 7010:2011 — Graphical symbols, safety colours and safety signs, registered safety signs. This regulation supersedes and incorporates the previous standard BS 5499, relating to safety signage.

Make sure that you have reviewed these general laws as well as any site, location, industry or equipmentrelated specific laws before you conduct your fire safety review.

Author Bio: First Mats started life as safety matting specialists, but have since expanded to become a complete industrial and commercial supplies company. The focus of First Mats is to provide safety-focused products that improve the wellbeing of staff through quality approved products, backed up by extensive knowledge. www.firstmats.co.uk

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How Leaders Should Approach Today’s New Hybrid Workforce Before the global pandemic, remote workers who were regularly telecommuting often expressed frustration at conference calls where 80% of attendees were in one room, while they were among the 20% calling in from other locations. The 20% often reported that they felt like outliers: it could be challenging to hear, difficult to get the group’s attention, and hard to read the room or contribute to the discussion.

For many of these already-remote employees, the global COVID pandemic was an equalizer: Everybody was calling in from somewhere, and so the meeting experience suddenly felt more inclusive.

With unprecedented levels of virtual team collaboration, more flexible work hours, and less time spent commuting, it’s no wonder that, as has been widely reported, global productivity increased dramatically during the pandemic…although these gains undoubtedly came at a high cost for some individuals and organizations.

Now, as vaccination rates increase and pandemic restrictions are lifting in some places, leaders are facing the new reality of leading a hybrid workforce.

Every business or organization has its own timetable for when and how to return to in-person work. And on a broader scale, many organizations are eager for a return to normal, yet want to maintain productivity gains and stem the tide of widespread burnout.

Transitioning to the New Normal of the Hybrid Workforce While the shift to virtual work may have been precipitated by the emergency work-from-home measures in 2020, plans for the new hybrid workforce are becoming less reactionary and more deliberate now.

The details aren’t clear yet, but one thing is certain: The post-pandemic world of work is definitely going to look different.

According to a McKinsey survey, 9 in 10 organizations plan to combine remote and on-site work going forward. While there’s no one-size-fits-all model for a hybrid workforce or team, the term usually refers to a setting where some employees work remotely, some work on-site, and others work a combination of both.

This new way of working can be challenging — especially at first.

Leaders must balance differing employee preferences with a desire for continued productivity and collaboration. Some team members may be clamoring to get back to the office, but many others want to continue to work from home.

In fact, the majority of employees say they want flexible and remote work options to remain — one study even found that 64% would prefer a permanent work-from-home situation over a $30K pay raise.

That’s why hybrid workforce policies and how effectively managers can lead hybrid and virtual teams will be key for attracting and retaining talent in many industries going forward.

Leading a Hybrid Workforce: 5 Tips How will this shift towards a more hybrid workforce affect leaders and leadership?

Leading a hybrid team requires adaptability to change and an openness to experimentation. In this new and changing environment, we believe that leaders will be most successful when they take the following 5 steps.

1. Acknowledge what’s new and different about the hybrid workforce.

Leaders should start by acknowledging that this environment is new and different, and that what works for one person will not work for everyone. Leaders must reassure people that whether they’re working in person, Page 1 of 3

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remotely, or a combination of those, their choice is not career-limiting, and their contributions are valued.

They must also set the stage for effective team collaboration.

Work to create a psychologically safe work environment where people can be open about their uncertainties and their frustrations. Ensure team members have the skills they need to hold candid conversations with one another, no matter where they are.

It requires truth and courage to develop a coaching culture at your organization, but will help foster more innovative solutions, greater morale, and improved productivity among members of your newly hybrid workforce. (It’s also essential for building a strong organizational culture, when some at your organization are remote.)

2. Foster empathy, equity, and inclusion in your hybrid workforce.

Work to maintain or establish inclusive practices for things like team huddles or group calls. In 2020, when everyone was working from home, many experienced firsthand the challenges of contributing in virtual meetings — something already-remote workers knew all too well.

As offices reopen with newly hybrid workforces and teams, remember those lessons learned to ensure meetings remain inclusive. Without intentionality and clearly defined processes, it could be easy for those returning to work in-person to fall back into old habits, unintentionally excluding or overlooking colleagues who are working offsite.

Also take time to help your newly hybrid workforce to develop an identity by crafting a set of shared team norms and expectations. Everyone should understand the team’s purpose and goals, work processes, meeting frequency, and how decisions will be made. Consider the following questions: x x x

If individuals plan to work part-time remote and part-time in person, which days will they come to the office?

How can team meetings accommodate the needs and leverage the contributions of both in-person and remote participants effectively?

How will communication tools and technology platforms be used to support the team’s goals and outputs?

Working through these questions allows leaders to foster inclusion and connectedness among members of their hybrid workforce. And agreeing on use of technology tools and platforms can help remote individuals feel less detached and ensure that everyone’s needs are being met. (That’s one of our recommended best practices for managing virtual teams and meetings, but really applies in a hybrid workforce situation as well.)

Leaders of highly effective teams enable full participation of all their members, no matter where they are.

3. Cultivate the mindsets of learning agility and resilience.

Here’s the reality of the new hybrid workforce: Plans will change.

In the same McKinsey study as mentioned above, researchers also found that organizations willing to take a “test-and-learn” approach to designing — and redesigning — their processes for a more remote workforce realize higher levels of productivity.

Learning agility, or the ability to adapt and thrive in new situations, is crucial to your organization’s hybrid workforce success. Leaders should schedule time every couple of weeks to talk with their team members about what’s working and what isn’t. In addition, they should connect with other leaders of hybrid teams throughout your organization. What are their successes and challenges?

In this uncharted territory, resilience is more important than ever and burnout continues to be a problem. So leaders should set an example for the hybrid workforce by prioritizing their own mental and physical health.

Whenever possible: x x x x

Nurture your personal and professional relationships; Prioritize regular exercise and sleep each night; Make time for mindfulness; and Respect downtime and avoid contacting employees during their off hours.

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4. Pay close attention to direction, alignment, and commitment (DAC).

At CCL, we know that leadership is a social process. When it’s working well, individuals collaborate to achieve results they never could’ve achieved working alone. The DAC framework for leadership can help you gauge the success of your new hybrid workforce, too: x x x

When you look around, do you see strong direction? In other words, are team members in agreement on the group’s overall goals?

Ask yourself whether your group is aligned — whether members with different tasks and roles are coordinating their work.

Finally, is there commitment, or mutual responsibility for the group? When teams have strong commitment, they feel responsible for the success and well-being of the group. There’s a high level of trust and psychological safety.

Compare current levels of DAC among your hybrid workforce to pre-pandemic times. Organizations that switch into hybrid mode and see DAC levels hold steady or improve should take it as a sign that things are going well. On the other hand, if DAC has decreased, recognize that your group’s shared interactions aren’t what they should be.

Remember that DAC is ever-evolving, similar to your hybrid model. As you discover what works best for your organization, expect to encounter bumps along the way. The most important thing leaders can do is maintain perspective by embracing change and seeing challenges as learning opportunities.

5. Focus on boundary spanning — within your hybrid team and across the organization.

On a typical day at work, leaders must collaborate across boundaries throughout their organizations. They collaborate with peers in different departments and hierarchical levels; they work with external stakeholders; they unite diverse groups. They also traverse geographic boundaries by managing remote employees — one of the most important focus areas when some team members are working on-site and others are working remotely.

In establishing what a hybrid workforce will look like for your organization, think consciously about your individual team’s network within the organization. Ask yourself: x x x x

How is my team connected with other teams?

How do my team members and I best interact with those teams?

How is my team currently spanning boundaries?

How can we foster interaction, better conversations, and effective collaboration between departments in a hybrid workforce environment?

While a more hybrid team may present new challenges to collaborate and navigate across boundaries, the solutions to today’s most pressing business challenges are often found at the intersection of multiple boundaries.

When members of your hybrid workforce see leaders recalibrate, adapt, and embrace the future with an agile and collaborative mindset, they’ll likely do the same. https://www.ccl.org/articles/leading-effectively-articles/how-leaders-should-approach-todays-new-hybrid-workforce/

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Ten Top Tips: Healthy remote working One major result of COVID has been the increase in remote working. For those used to working in offices, or with teams of others, this can be a difficult transition. The attached sheet gives 10 top tips from The Chartered Institute of Personnel and Development. www.cipd.co.uk/coronavirus

The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International and the PITA Annual Review, both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating.

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Volume 7, Number 2, 2021

Products & Services & News PITA CORPORATE SUPPLIER MEMBERS Page 2 ABB QCS Colour Control Upgrade Page 4 Pilz PSENscan and PSENmech safety Page 5 Valmet Recovery Ash Analyzer

PITA NON-CORPORATE SUPPLIER MEMBERS Page 6 Voith Duoshake 800

OTHER SUPPLIERS Page 7 Mycon Page 8 SKF Page 9 Toscotec

Cleaning Service for Heat Exchangers Oil Conditioning Unit New Shoe Press

The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International and the PITA Annual Review, both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating.

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Volume 7, Number 2, 2021

ABB UPGRADES QCS COLOR CONTROL TO STABILIZE SHEET COLOR FASTER AND MITIGATE VARIABILITY New features on ABB’s Color Control for QCS800xA take guesswork out of color control and help customers reach shade targets faster ABB has launched its latest generation of Color Control with new features that improve overall quality and enable faster color matching, making it easier for operators of all levels to match specifications and eliminate the guesswork normally associated with shading.

ABB’s upgraded automated Color Control for QCS800xA is a powerful tool for paper mills that make colored board, paper and tissue products – from whites and pastels to dark shades – to help reduce color variability, dye usage and costly rejects associated with off-shade production.

The system utilizes the full power of ABB’s unique High-Performance Color Measurement – the first LED-based continuous color measurement device on the market – and combines it with the fastest Multivariable Color Control to establish full shade property insight and traceablity. It stabilizes sheet color faster by using control algorithms, based on advanced models rooted in the Kubelka-Munk theory of paper coloring.

The new functionalities build on a renowned control package, particularly when it comes to more powerful shade change capabilities, including improved dye state visualization, shade change selection and boost control. It features an enhanced operator user interface for faster responsiveness, with less system lag, improved data graphic interactions and faceplate control-state and interlock-condition awareness.

Designed for customers already using ABB’s High Performance Color Sensor and those considering a new/replacement color sensor for Smart Color, this is an important tool for mills facing a challenge in Color Management, whether that be reaching shade targets, facing excessive recalls or reject levels, or spending too much manual resource on color management.

“Our latest innovation in color measurement and control capability allows mills to benefit from a robust and smart system that delivers the best color in all operating conditions,” said CarlMagnus Tjeder, Global Product Manager at ABB. “Because it only requires one calibration for each dye, which is integrated into the dye management handling system, it also places fewer demands on operators to intervene manually, leaving more time for other, more pressing tasks.”

ABB’s system capability makes for even higher color property awareness because of the higher sampling rate for control. If paired with ABB’s latest color measurement, it provides feedback control with a high signal to noise ratio, hence feedback error is reduced and less noise is fed back through the controller, achieving truer color process control and less noise influence.

ABB in pulp and paper is working in more than 50 countries to provide its comprehensive portfolio of integrated digital solutions, automation and electrification systems, industryfocused products and services to help customers optimize all phases of the papermaking process. The company works across packaging, paper, tissue and pulp disciplines to help drive availability, performance, cost and quality improvements.

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ABB is a leading global technology company that energizes the transformation of society and industry to achieve a more productive, sustainable future. By connecting software to its electrification, robotics, automation and motion portfolio, ABB pushes the boundaries of technology to drive performance to new levels. With a history of excellence stretching back more than 130 years, ABB’s success is driven by about 105,000 talented employees in over 100 countries. www.abb.com Process Automation: ABB’s Process Automation business is a leader in automation, electrification and digitalization for the process and hybrid industries. We serve our customers with a broad portfolio of products, systems, and end-to-end solutions, including our #1 distributed control system, software, and lifecycle services, industry-specific products as well as measurement and analytics, marine and turbocharging offerings. As the global #2 in the market, we build on our deep domain expertise, diverse team and global footprint, and are dedicated to helping our customers increase competitiveness, improve their return on investment and run safe, smart, and sustainable operations.

Contact: ABB Ltd Affolternstrasse 44 8050 Zurich Switzerland

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Volume 7, Number 2, 2021

PILZ SAFETY LASER SCANNER PSENSCAN: VERSATILE THANKS TO THE NEW C++ LIBRARY The safety laser scanner PSENscan offers productive area monitoring and dynamic navigation of automated guided vehicles (AGV). Applications with PSENscan can be adapted flexibly to customised tasks.

Previously, ready-made ROS modules (Robot Operation System) have been available for navigation. Now we have the PSENscan Standalone C++ Library, which makes it easy to use the laser scanner also in Windows or Linux production environments. PSENscan provides simple support for the safe, dynamic implementation of your mobile applications.

More about PSENscan

NEW FROM PILZ: EXPAND PSENMECH WITH THE SAFETY BOLT PSENBOLT Combine the mechanical safety gate system PSENmech with the safety bolt PSENbolt and the result is a complete solution comprising mechanical bolt, handle and safety gate switch.

The safety bolt is available in several versions. You can choose between standard versions and versions with a key. The latter enable the bolt to be locked in a certain position, thus preventing an unintended restart of the machine, during maintenance for example.

More about the safety bolt PSENmech Contact: Pilz Automation Ltd Pilz House, Little Colliers Field Corby, Northants, NN18 8TJ United Kingdom T: +44 1536 460766 E: sales@pilz.co.uk

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Volume 7, Number 2, 2021

VALMET’S NEW ONLINE ASH ANALYZER OFFERS SIGNIFICANT OPERATIONAL SAVINGS AND IMPROVED RECOVERY BOILER AVAILABILITY Valmet has developed an innovative solution to reduce forced boiler shutdowns, caused by superheater corrosion or boiler plugging, and provide guidance for ash treatment and sootblowing need.

The new Valmet Recovery Ash Analyzer measures the ash composition at the electrostatic precipitator (ESP) and determines the ash melting temperatures which describe the corrosiveness and stickiness of ash.

A new Valmet Industrial Internet (VII) application, Valmet Ash Balance Advisor, is an additional option to maximize the Valmet Recovery Ash Analyzer’s advantages and optimize ash treatment for significant cost savings in sodium make-up. Operators are provided with an online view of ash balance and boiler operation with real-time indicators of fouling and corrosion risks, which helps to achieve cost-effective ash treatment amount or dump volume.

Optimized operation Until now, ash treatment has typically been a batch operation based on infrequent laboratory tests with no standard criteria of when to start treating or dumping ash.

"The problem with laboratory measurements is that samples are taken so seldom, and the results are often incomplete and cannot be used for optimal control. Now, with the online analyzer, control of actual ash chemistry is possible and boiler cleaning together with operational costs can be optimized on a continuous basis," says Timo Laurila, Business Manager, Recovery Analyzers and Advanced Process Controls, Automation, Valmet.

Technical information about Valmet Recovery Ash Analyzer The Valmet Recovery Ash Analyzer measures the complete chemical balance including ESP ash pH, carbonate (CO3), sulfate (SO4), chloride (Cl), potassium (K) and sodium (Na) concentrations as well as calculating ash melting temperatures T0, T15 and T70. The ESP ash sample is taken from the ash drop shoot after the ash conveyor by a robust stainlesssteel sampler, dissolved in water, and transferred to the analyzer where chemical analyses and ash melting temperature calculations are made. Comprehensive diagnostics and secure Valmet Industrial Internet (VII) ready capabilities ensure reliable and dependable results with intuitive access to measurement operation and remote services from Valmet.

For further information, please contact: Timo Laurila, Automation, Valmet, T: +358400922442

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Volume 7, Number 2, 2021

VOITH’S NEW COMPACT AND POWERFUL DUOSHAKE 800 DG OPTIMIZES CROSS DIMENSIONAL STRENGTH AS WELL AS FORMATION OF ALL PAPER GRADES • DuoShake 800 DG contributes to excellent formation of all paper grades and a low break length ratio • Reduced basis weight due to improved strength values and increased production speed due to optimized dewatering • Compact and reliable design with state-of-the-art automation and control technologies as well as efficient digitization solutions The shaking unit DuoShake, already established on the market for decades, is now available with an 800 nM drive motor and the latest control and automation technologies. This complements the DuoShake DG designed for 250 nM, with both variants now achieving a stroke length of up to 35 mm. The DuoShake 800 DG optimizes the alignment of the fibers in the cross direction and contributes to a homogeneous fiber distribution through a highfrequency shaking of the breast roll. This guarantees excellent formation of all paper grades and higher paper strength in the cross direction. The innovative shaking unit builds on several design innovations that enable increased paper quality and production speed. For example, the base only has to support the unit's own weight. In addition, the design avoids reaction forces being transmitted to the foundation, headbox or wire section. The DuoShake 800 DG system is extremely robust, compact and requires very low maintenance.

Customers worldwide have been convinced by the shaking system, including an American recycled linerboard producer who wanted to optimize the sheet strength and basis weight of the paper – at maximum machine speed.

Cutting-edge technology meets digitalization The DuoShake DG combines in-depth technical know-how for an efficient manufacturing process with optimized digitalization technologies from the Voith Papermaking 4.0 portfolio.

Future paper production requirements are safely met and supported.

As required, the DuoShake also generates a monthly performance and maintenance report, giving customers a practical overview of their plant.

Two sizes for individual requirements The DuoShake DG is available in two sizes with drive motors of 250 or 800 Nm. The most suitable type and size can be determined roughly with the aid of the operating characteristic diagram and then on site. The DuoShake 800 DG, in particular, is characterized by its very compact design, combined with high performance.

To learn more about the new DuoShake 800 DG, find information, including a product video, at https://voith.com/duoshake-digital-generation.

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Volume 7, Number 2, 2021

MYCON - OPTIMAL CLEANING SERVICE FOR HEAT EXCHANGERS, BASED ON DIGITAL MONITORING The outstanding cleaning delivered by the JetMaster AS fully automated cleaning system is made possible by digital heat exchange monitoring. This automated cleaning replaces previous cleaning intervals based on set times or visual assessment.

Normally, large-surface cooling systems such as air condensers used in energy production, are cleaned 1-3 times per year. Most of the time, cleaning is semi-automated; a motor moves the nozzles vertically, while the ladders of the cleaning system are moved manually. This is a time-consuming process, and difficult for employees due to the high temperatures present in this area.

No employees are needed to conduct the fully-automated, patented JetMaster process. The system’s electronic controller offers multiple cleaning programs that can be selected easily at the push of a button.

The JetMaster AS is environmentally-friendly, with jet widths of 500mm, for instance, and a water usage of over 240 l per hour. Approx. 100m2 of the exchanger’s surface can be cleaned in just one hour. The system also produces only a small amount of waste water. The JetMaster AS is chemical-free, using only 5-7 bar compressed air and water. The JetMaster process delivers outstanding quality cleaning.

What applications are suited to using the JetMaster AS automated cleaning system?

The ControlMaster mycon algorithm calculates the optimal times to clean the heat exchangers based on data available from the customer. This data covers at least the last year, and preferably the last three years. Optimizing cleaning intervals during this period also results in cost savings. Because of this, purchasing a fully-automated cleaning system offers an excellent ROI. Based on mycon's previous experience, the system begins delivering a ROI in 6 – 36 months. Customers can decide on this basis whether a fully-automated cleaning system would be an economical choice for them.

Data transmission / cost calculations with ControlMaster ControlMaster reviews and processes data received from the customer in a fully automated procedure. The required data and transmission are coordinated with the customer in advance. Calculations are completed quickly, even for large quantities of data. This allows mycon to offer the above calculation at a fixed price of 380.00 euros (three hundred and eighty).

ControlMaster can also be used by the customer to determine the optimal time for cleaning, even independently from mycon. They can then notify the control room of this optimal cleaning time. However, ControlMaster can also start the cleaning process autonomously, with all necessary additional measures if desired.

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Volume 7, Number 2, 2021

SKF CONDITIONING UNIT RAISES QUALITY OF LUBRICATION OIL SKF’s Oil Conditioning Unit (OCU) helps to prolong bearing lifetimes by improving the performance of lubricants in service.

The plug-and-play unit cools and filters lubrication oil as it circulates. Cooling helps to improve lubricant performance, while filtration removes contaminants. Filtration levels can be selected from coarse to ultra-fine.

“All this helps to reduce bearing failures and ensure reliable operations,” says Tommi Pitkaaho, Product Manager at SKF. “It can give a five-fold extension to oil service life.”

Around half of all premature bearing failures are caused by inappropriate lubrication and contamination. Cleaning lubrication oil, and bringing it to the optimum temperature, helps to reduce the risk of failure. This leads to increased machinery uptime and service life, plus a reduction in maintenance needs and repair costs.

An added benefit is that oil does not have to be replaced so frequently, which helps to reduce consumption. In addition, this helps to reduce the cost of oil disposal.

The OCU range includes a number of options, including stainless steel models for demanding environments, and add-on sensors to allow oil condition monitoring. It can be bundled with other offerings, such as SKF’s RecondOil DST oil treatment, which filters out nanoparticles – to further enhance machine performance. This can be extended to an ‘oil as a service’ agreement.

The OCU is widely used in machinery such as gearboxes, fans and other large bearing housings. Typical applications are in heavy industries such as pulp & paper, steel and mining – in machinery from drives and pumps to conveyors and rotating kilns.

The system can be connected to existing hydraulic and lubrication reservoirs when extra oil recovery is needed. It is typically aimed at OEMs and aftermarket users whose equipment runs at an oil temperature of 10-80°C and ambient temperatures of -10 to 40°C.

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Volume 7, Number 2, 2021

TOSCOTEC INTRODUCES NEW SHOE PRESS GENERATION DESIGN FOR TISSUE Toscotec has launched the new generation design of its shoe press TT NextPress, featuring superior drying performances and energy savings, resulting from R&D activities in cooperation with Voith. TT NextPress delivers a stable after-press dryness rate of more than 47% on a tissue line operating above 2,000 mpm speed. This dryness level ensures a substantial reduction of energy consumption, as the Yankee and the hoods use significantly less thermal energy to complete the paper drying process.

Superior Dewatering delivers higher Dryness A new shoe shape has been introduced to enhance the nip width and increase the peak pressure at the nip outlet, reshaping the outward pressure into a steeper curve. This eliminates the risk of paper re-wetting at the exit of the nip, and increases sheet dryness, without affecting the belt’s life.

Continuous Nip Control for maximum Reliability Machine operators can now visualize on the DCS (Distributed Control System) the actual nip pressure of TT NextPress in real time. The operator can regulate the nip pressure at which the TT NextPress is loaded onto the Yankee resulting from both a theoretical and empirical system, as the nip pressure is calculated based on the shoe press loading system and also measured by the load cells installed on the press mechanical stop.

Higher Precision: the perfect fit for Rebuilds In rebuilds, the installation of a shoe press on an old-style cast iron Yankee can compromise the lifetime of both the felt and the belt. Toscotec has designed a dedicated solution on the TT NextPress consisting of multiple independent pressure zones, up to 14 in total, of which 12 are located on the edges. This design ensures a perfect fit between the shoe and the Yankee deformation, and maximizes the clothing lifetime.

Improved Safety and Yankee Protection TT NextPress nip pressure lines are fitted with a pressure accumulator pre-charged with nitrogen, which is installed inside the belt as close as possible to the loading pistons. The accumulators absorb instantaneous overloads or pressure increases on the line due to the presence of external elements or paper wads in the nip area. This eliminates the risk of any possible damage to the Yankee dryer.

The new TT NextPress builds on the features of its previous version which have been widely appreciated by tissue producers around the globe. These include: x x x

the hydraulic loading system, which ensures top pressure uniformity and reliability the extreme flexibility offered by the shoe tilting regulation during operations the maximization of the belt lifetime both through the continuous digital monitoring of the heads position with regular lateral movements of the belt, and through the sleeve tensioning device actuated by hydraulic cylinders, which delivers a constant and adjustable stretching force.

Toscotec, together with Voith, has sold more than 60 shoe presses for tissue machines all over the world.

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FROM THE PUBLISHERS OF PAPER TECHNOLOGY Volume 7, Number 2, 2021

Installations The following pages contain a summary of the various installations and orders from around the world of papermaking, wood panel and saw mills, and bio-power generation, received between the start of May 2021 and end of July 2021.

The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International and the PITA Annual Review, both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating.

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Installations

g PAPERmaking!

FROM THE PUBLISHERS OF PAP PER TECHNOLOGY

Volume 7, Number 2, 2021

COMPANY, SITE Aktül Kagit Turkey Arboreal Tacuarembó Uraguay Arctic Paper Grycksbo Mill Sweden Anon Far East Asia Symbol Rugao site north-west of Shanghai China Avelino Bragagnolo Faxinal dos Guedes Santa Catarina Brazil BioPappel Veracruz Mill Mexico Bracell (Project “STAR”) Lençóis Paulista São Paulo state Brazil Brigl & Bergmeister Niklasdorf Mill, Austria, & Vevče Mill, Slovenia C&S China CMPC Santa Fe Pulp Mill Chile Daio Paper Corporation Mishima Mill Japan Domtar Kingsport Tennessee USA

SUPPLIER Projet BV

Domtar Kingsport Tennessee USA

Projet BV

ORDER DESCRIPTION High pressure felt showers (TM1) Cross Laminated Timber (CLT) plant (pine sawmill)

URL link

ABB

New drive and automation for winder modernisation project

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FMW

Largest biomass power plant in region (106MWth) Boxboard production machine (BM13)

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Hergen

New rewinder for PM3

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Projet BV

Two power cleaners for dryer fabrics (PM1)

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Andritz

Major pulp production technologies and key process equipment for the world’s largest Dissolving Pulp Mill

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Valmet

Two Paper Lab automated test lines

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Valmet

Tissue line (12 such line supplied to company) Maintenance agreement

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Conversion of paper machine (PM15) to fluff pulp drying machine Conversion of a power boiler into a natural gas-fired power boiler with back-up fuel oil capability, and a recovery boiler to change fuels from wood and sludge firing to corrugated cardboard (OCC) rejects, natural gas, and fuel oil firing (for mill conversion into packaging facility) High pressure showers

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Ledinek

Voith

Valmet

Valmet

Andritz

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Installations

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FROM THE PUBLISHERS OF PA APER TECHNOLOGY

Volume 7, Number 2, 2021

COMPANY, SITE Dunn Paper Michigan USA Egypt Kuwait Holding Sadat City Egypt Fajar Surya Wisesa Tbk. Bekasi Indonesia GO Lab, Inc. Madison Maine USA Greif Board Massilon OH, USA Hayat Kimya Russia Huafon Microfiber Co. Ltd. Qidong Jiangsu Province China Hyuga Miyazaki Prefecture Japan Iggesund Iggesund Mill Sweden

SUPPLIER Toscotec

ORDER DESCRIPTION Wet end rebuild (PM2)

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Dieffenbacher

Woodchip cleaning line

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Projet BV

High pressure showers (PM1)

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Dieffenbacher

Relocation of wood fibre insulation board plant from Berga, Germany

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Projet BV

Two drier fabric cleaner systems (PM1)

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Valmet

New tissue line (capacity 70,000tpy) (TM10) Three needling lines (nonwovens)

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Mitsubishi Power

Biomass power plant (50MW)

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Valmet

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Instroret Co and Renovo of Mantua Bioedil Sardinia Italy Jiangxi Five Star Paper Hukou county Jiujiang city China Kartonsan Karton Sanayi Ve Ticaret A.S. Turkey Klabin Ortigueira Mill Brazil Klabin (“PUMA II Project”) Brazil

Imal-Pal Group

Rebuild of hardwood pulp line In the first stage, a vacuum filter will be replaced with a new TwinRoll press and in a second phase, remaining filters in the bleach plant will be replaced with TwinRoll presses.

Technology for cork-based insulation board plant

Andritz

Four jet coaters for board lines

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Runtech

Rebuild vacuum system (BM2)

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Andritz

Sulphuric acid plant

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Valmet

Boardmaking line (PM28) and fibreline

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Dilogroup

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Installations

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FROM THE PUBLISHERS OF PAP PER TECHNOLOGY

Volume 7, Number 2, 2021

COMPANY, SITE Kochavinska Paper Factory JSC Ukraine Lemit Papers LPP Ahmedabad India Mayr-Melnhof Karton Frohnleiten Mill Austria Meghna Pulp & Paper Mills Ltd Bangladesh Mercer Rosenthal Germany Merino Industries Ltd. Halol Gujarat State India Metsä Fibre Kemi Bioproduct Mill Finland Metsä Fibre Kemi Bioproduct Mill Finland Metsä Fibre Rauma Mill Finland Mirae Paper Jeonju Paper Mill South Korea Nine Dragons Paper China Norske Skog Golbey Mill France Palm Paper Neukochen Germany Papelera Internacional Rio Hondo Zacapa Guatemala Papeles Ultra Mexico City Papresa Paper Mill Spain Paraibuna Embalagens Juiz de For a Brazil PT. Riau Andalan Kertas (RAK) Indonesia

SUPPLIER A.Celli

ORDER DESCRIPTION Tissue production line

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Cellwood

Disperser for new duplex board machine

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Andritz

Rebuild board making machine (KM3)

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Recard

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Dieffenbacher

New tissue and MG paper production plant IQ QCS with scanner and related measurements Particleboard line

Eltel

Construct 110kV power line

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Kelvion BV

Cooling towers

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Oy Botnia Mill Service AB

Maintenance contracts (sawmill)

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Toscotec

Steel Yankee dryer (PM2)

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Voith

Three testliner paper machines

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Voith

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Hüttenwerke Königsbronn

Rebuild PM1 for grade conversion from publication to packaging papers Roll shells for film press (cast on behalf of Valmet)

Toscotec

Complete tissue line (PM7)

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Projet BV

High pressure showers

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Greycon

Opt-Studio and X-Trim software

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Voith

Quality Control System (QCS) scanners and sensors (PM8)

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ABB

The first order for its recentlylaunched Sheet Break Performance system)

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Valmet

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Installations

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FROM THE PUBLISHERS OF PA APER TECHNOLOGY

Volume 7, Number 2, 2021

COMPANY, SITE Perlen Papier AG Perlen Switzerland Sappi Ltd (worldwide) Sappi Stanger Mill South Africa Salzburg AG Salzburg Austria Saudi Paper Group Dammam Mill Saudi Arabia Sekab Örnsköldsvik Sweden

SUPPLIER Heimbach

ORDER DESCRIPTION Press fabrics for Tandem Shoe Press (PM4)

URL link

ABB

Manufacturing Execution System (MES) Steel Yankee dryer

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Valmet

Biopower plant (green electricity power plant 4MWe and 17MWth)

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Toscotec

Tissue machine

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Afry

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Shah Paper India

Projet BV

Shandong Huatai Paper Industry Shareholding Dongying Shandong province China

Andritz

Shanying Paper Guangdong Zhaoqing China SLDK Syktywkar Komi Republic Russia Smurfit Kappa Nervion Iurreta Spain Södra Värö, Mörrum, and Mönsterås Pulp Mills Sweden Spinnova-Suzano JV textile fibre factory Jyväskylä Finland ST Paper Duluth Minnesota USA

Andritz

Engineering and project partner assignment for production facility to produce green chemicals from forestry industry residues Two new dry end High Pressure Tail Cutters and two Dryer Fabric Cleaners To upgrade an existing BCTMP plant to become the world’s largest mechanical pulping line (processing a mixture of eucalyptus, poplar and acacia wood chips) for P&W paper production Approach flow and broke handling systems for two board lines

Urbas Energietechnik

30MW hot water boiler and heating equipment (saw and pellet mill)

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Andritz

Industrial maintenance services

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SPM

One thousand (1000) Airius vibration sensors with temperature measurements

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Valmet

Drying technology

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Andritz

New tissue machine

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Toscotec

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Installations

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FROM THE PUBLISHERS OF PAP PER TECHNOLOGY

Volume 7, Number 2, 2021

COMPANY, SITE Stora Enso Anjala-Ingerois sites Finland Stora Enso Imatra Mills Finland Umka Cardbord Mill Serbia United Paper Public Company Limited Prachinburi Thailand Takasago Paper Japan Tolko Slave Lake AB, Canada United States Gypsum Gypsum OH, USA UPM Schongau Germany Veracel Celulose S.A. Eunapólis Brazil Zhejiang Huajiang Science and Technology Co., Ltd. China

SUPPLIER Vamet

ORDER DESCRIPTION Boiler upgrade and flue gas cleaning system

URL link

Raumaster

Chip handling and other equipment

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Valmet

Rebuild of board machine (PM1)

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Andritz

Main equipment – from pulping to the storage tower – for OCC line

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Projet BV

Two dryer fabric cleaners (PM1)

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Projet BV

Two high pressure belt cleaners (OSB plant)

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Projet BV

Three suction roll cleaners (plasterboard line)

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Voith

QCS (PM6)

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Voith

To rebuild and upgrade the QCS Scanner (PDM 1)

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Dilogroup

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Zhejiang Jingxing Paper Joint Stock Co., Ltd. China) Zhejiang Kingsafe Hygiene Materials Technology Co., Ltd. Hezhou China Zhejiang Saintyear Textile Co., Ltd. China Zhejiang Shanying Paper Jiaxing City Zhejiang province China

Valmet

Complete web-forming and needling line to process blends of glass and polypropylene fibre (nonwovens) Two tissue lines with automation and three rewinders

Andritz

Spunlace line (nonwovens)

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Andritz

Spunlace line (nonwovens)

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Valmet

Two board making lines with extensive packages of automation, services and industrial internet applications

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Installations

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PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL L

Volume 7, Number 2, 2021

Research Articles Most journals and magazines devoted to the paper industry contain a mixture of news, features and some technical articles. Very few contain research items, and even fewer of these are peer-reviewed.

This listing contains the most recent articles from three of the remaining specialist English language journals and one Korean journal that publish original peer-reviewed research: x x x x

IPPITA PAPYRUS 360° JOURNAL OF KOREA TAPPI (English abstract only) NORDIC PULP & PAPER RESEARCH JOURNAL TAPPI JOURNAL

Notes: 1. IPPTA PAPYRUS 360° has taken the place of IPPTA JOURNAL as of late 2020.

2. JOURNAL OF KOREA TAPPI is an excellent open-access research journal – abstracts are in English but articles in Korean.

3. TAPPI JOURNAL went open-access in 2020.

The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International and the PITA Annual Review, both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating.

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Research Articles

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FROM THE PUBLISHERS OF PAP PER TECHNOLOGY INTERNATIONAL

Volume 7, Number 2, 2021

IPPTA PAPYRUS 360°, Vol.32(3), 2020 1.

Advanced Analytics for Optimization of stage-wise ISO Brightness gain in Kraft Pulp Bleaching 2.

An Efficient Agro based Integrated Chemical Recovery Plant at M/s Bindals Papers Mills Ltd. 3.

Application of Data Mining in Pulp & Paper Industry 4.

Case Study of Consistent Pulp Quality with Optimization utilising advanced algorithms 5.

Controlling Microbiological Growth and Foul Odors in Pulp& Paper Industry 6.

Impact of Storage Time on Chemical Constituents, Pulp, and Papermaking Properties of Bamboo 7.

Innovative Solution to Address Cracking Problem in Kraft Paper 8.

Kinetic Study of Wheat Straw Acid Hydrolysis using Phosphoric Acid to Produce Furfural 9.

Kolhu’s Bagasse: A New Resource of Agro based Raw Material for Writing & Printing Grade Paper JOURNAL OF KOREA TAPPI, Vol.53(2), April 2021 1.

Paper Defects Classification Based on VGG16 and Transfer Learning 2.

Study on the Manufacturing and Applicability of Natural Glues for Woodcrafts Using Shark Skin 3.

Wet Pressing Effects of Hybrid Calcium Carbonate in Papermaking 4.

Application of Organic and Inorganic Additive Addition for Linerboard Drying Energy Reduction 5.

Characteristics of Cellulose Nanofibrils (CNF) Film from Red Algae Pulp 6.

Fundamental Study of the Production of Paper Ash Made from Paper Sludge II: Development of Paper Ash Application Technology to Improve the Bulk and Save Drying Energy of Paperboard 7.

Polymer Film Separation Method for the Recycling of Polymer Laminated Paper Carton 8.

Preparation of Lignin-based Poly(m -aminobenzenesulfonic acid) Copolymers 9.

Survey on Recyclability of Take-out Cups in Korea JOURNAL OF KOREA TAPPI, Vol.53(3), June 2021 1.

Introduction to the Novel Plate for Manufacturing Thermomechanical Pulp 2.

Facile Acetylation of Woody Biomass Using a Glycol Ether Solvent 3.

Analysis of Ink for the Woodblock Printing of Samguk Yusa (I): Ink with Low Solid Content 4.

Exploring Deep Eutectic Solvents for the Extraction of Lignocellulosic Materials (II): Extraction Tests for Lignin- and Cellulose-based Standard Materials 5.

New Eco-Friendly Coating Formulations to Improve Strength and Physical Properties of Recycled Paper Boards for Sustainable Packaging Applications 6.

Effect of Polyelectrolyte-Cationized Cellulose Nanofibril on the Properties of Paper NORDIC PULP & PAPER RESEARCH JOURNAL, Vol.36(2), June 2021 1.

Review: Aspects on bond strength in sheet structures from TMP and CTMP – a review (OPEN ACCESS)

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Research Articles

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FROM THE PUBLISHERS OF PAP PER TECHNOLOGY INTERNATIONAL

Volume 7, Number 2, 2021

2.

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Chemical pulping: The impact of using different wood qualities and wood species on chips produced using a novel type of pilot drum chipper (OPEN ACCESS) Chemical pulping: Temperature and effective alkali effect on brown pulp kraft cooking Mechanical pulping: On the modeling of pulp properties in CTMP processes (OPEN ACCESS) Paper technology: Pulp pumping efficiency I – A critical review on loss and its estimation Paper technology: Pulp pumping efficiency II – Designing of a pulp pump Paper technology: Three-dimensional scanning electron microscopy used as a profilometer for the surface characterization of polyethylene-coated paperboard (OPEN ACCESS) Paper chemistry: Ageing mechanisms of the papers with zeolite and PCC fillers Paper chemistry: Synthesis of macro-RAFT agent containing M13-10 and its application in surface sizing agent Coating: The role of MFC and hydrophobically modified ethyl(hydroxyethyl)cellulose in film formation and the barrier properties of methyl nanocellulose film Printing: Fabrication of rosemary essential oil microcapsules and using in active packaging Packaging: Development of a nonwetting coating for packaging substrate surfaces using a novel and easy to implement method Recycling: Wet route pellets production using primary sludge from kraft pulp mill Chemical technology/modifications: Modification of cellulosic filler with diisocyanates – volatile organic compounds emission assessment and stability of chemical structure over time Chemical technology/modifications: Characterization and application of woodZrO2 sorbent for simultaneous removal of chromium (III) and chromium (VI) from binary mixture

TAPPI JOURNAL, April 2021 1.

Guest Editorial: TAPPI Journal names two Best Research Paper Award winners for 2020 2.

Assessing variation in package modelling 3.

Mechanical modification of softwood pulp fibers using a novel lightweight vertical bar plate 4.

Use of fines-enriched chemical pulp to increase CTMP strength 5.

An experimental method to study the absorption capacity of paper towels 6.

Application of fuzzy TOPSIS in the Analyze phase of the DMAIC cycle to aid decision-making TAPPI JOURNAL, May 2021 1.

Editorial: Research, conferences, and COVID-19 2.

Peracetate/singlet oxygen chemistry used in post-bleaching of kraft pulp as a practical oxidant for paper machines 3.

Gas dispersion in the oxygen delignification process 4.

The role of gas dispersion in the oxygen delignification process 5.

Impact and feasibility of a membrane pre-concentration step in kraft recovery

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Research Articles

g PAPERmaking!

FROM THE PUBLISHERS OF PAP PER TECHNOLOGY INTERNATIONAL

Volume 7, Number 2, 2021

TAPPI JOURNAL, June 2021 1.

Guest Editorial: Research and development in the post COVID-19 era 2.

Rethinking the paper cup — beginning with extrusion process optimization for compostability and recyclability 3.

A guide to eliminating baggy webs 4.

Probing the molecular weights of sweetgum and pine kraft lignin fractions 5.

Rheological behavior of magnetic pulp fiber suspensions 6.

New learnings and strategies for meeting future recovery boiler particulate emission limits with existing electrostatic precipitators

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Research Articles

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY Volume 7, Number 2 2021

Technical Abstracts The general peer-reviewed scientific and engineering press consists of several thousand journals, conference proceedings and books published annually. In among the multitude of articles, presentations and chapters is a small but select number of items that relate to papermaking, environmental and waste processing, packaging, moulded pulp and wood panel manufacture. The abstracts contained in this report show the most recently published items likely to prove of interest to our readership, arranged as follows:

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Agri-Fibres

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Coating

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Environment Moulded Pulp

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Nano-Science Novel Products

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Packaging Technology Papermaking

Page 8

Testing

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Waste Treatment

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Wood Panel

The Paper Industry Technical Association (PITA) is an independent organisation which operates for the general benefit of its members – both individual and corporate – dedicated to promoting and improving the technical and scientific knowledge of those working in the UK pulp and paper industry. Formed in 1960, it serves the Industry, both manufacturers and suppliers, by providing a forum for members to meet and network; it organises visits, conferences and training seminars that cover all aspects of papermaking science. It also publishes the prestigious journal Paper Technology International and the PITA Annual Review, both sent free to members, and a range of other technical publications which include conference proceedings and the acclaimed Essential Guide to Aqueous Coating.

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Technical Abstracts

g PAPERmaking!

FROM THE PUBLISHERS OF PA APER TECHNOLOGY

Volume 7, Number 2, 2021

AGRI-FIBRES “Camellia oleifera Shell as a Potential Agricultural By-product for Paper Production”, Wenhua Gao, Luyao Huang, Zhihui Lei & Zhiwei Wang, BioResources, 2021, Vol.16(2), pp.3734-3745. The aim of this study was to investigate a potential biorefinery process to realize the high utilization of Camellia oleifera shell (COS), which is an agricultural by-product mainly composed of cellulose, hemicellulose, and lignin. Before treatment by steam explosion, the COS was impregnated with water, 3.0 wt% NaOH solution, or 3.0 wt% H2SO4 solution. The morphological structure and chemical composition of the steam-exploded COS pulp were investigated. The results indicated that the impregnation treatments increased the cellulose content of the steam-exploded COS pulp and decreased the hemicellulose and lignin content. The morphology of steamexploded COS fiber was short, coarse and stiff. Hydrophobic and colorful handsheets were fabricated by mixing proportional bleached softwood fiber. This study demonstrated that COS was a potential material for the papermaking industry, and the combination of water impregnation and steam explosion treatment for COS was a good pulp process.

“Review on The Paper Making Process From Bamboo As A Paper Product”, Muhammad Akie Shamsuri, Progress in Engineering Application and Technology, Vol.1(1), (2021). Bamboo is a grass that can grow to 25 m in six months. Every culm emerges from the ground at its final diameter (i.e. its girth does not extend over its lifetime), tapering as it rises in height, and increasing vertically by "telescopically" cell division between nodes (i.e., the distance between nodes increases as it grows). When fully cultivated, it takes normally three to five years to grow to full strength, during which they are silified and lignified. This study aims to review the papermaking process from bamboo as a paper product. The density (for bamboos), fiber dimensions, morphological indices, and chemical composition of the raw materials studied were determined. The Kraft, pulping processes were applied and some of the pulps produced were bleached according to the D0EpD1 bleaching sequences (where D represents chlorine dioxide and Ep represents peroxide reinforced alkaline extraction) bleaching methods. The fiber dimensions were microscopically evaluated. The average fiber length of bamboo is around 2.29 mm for length, 16.4 μm for total width, 6.4 μm for wall thickness, and 3.5 μm for lumen diameter. According to flexibility coefficients, bamboo has higher bonding strength (tensile, burst, double folds). The thicker wall of Gigantochloa scortechinii (9 μm) compared to other bamboo indicated very different morphological indices and behavior.

The chemical composition of the raw materials was typical for non-woody plants. They have high ash content and low lignin contents. The very good cellulose content of bamboo can make very good pulp yield. The higher cellulose content (52.7%) of Melocanna Baccifera predicted better yields. The overall chemical composition indicates the suitability of alkaline methods for pulping with reasonable alkali charges and predicted good to excellent yields. This was demonstrated by cooking with different alkali charges.

Nonwood Plant Fibers for Pulp and Paper (Book), Pratima Bajpai, Elsevier, 2021.

Nonwood Plant Fibers for Pulp and Paper examines the use of nonwood plant fibers for pulp and paper, worldwide pulping capacity of nonwood fibers, categories of non-wood raw materials, problems associated with the utilization of non-wood fibers, pulping, bleaching, chemical recovery and papermaking of nonwood raw materials, the use of nonwood plant fibers in specific paper and paperboard grades, and the advantages and drawbacks of using nonwood fiber for papermaking and future prospects. This book gives professionals in the field the most up-to-date and comprehensive information on the stateof- the-art techniques and aspects involved in pulp and paper making from nonwood plant fibers.

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Technical Abstracts

g PAPERmaking!

FROM THE PUBLISHERS OF PA APER TECHNOLOGY

Volume 7, Number 2, 2021

“Assessment of the papermaking potential of processed Miscanthus × giganteus stalks using alkaline pre-treatment and hydrodynamic cavitation for delignification”, Dimitrios Tsalagkas, Zoltán Börcsök, Zoltán Pásztory, Parag Gogatec & Levente Csóka, Ultrasonics Sonochemistry, Vol.72, April 2021. One way of satisfying increased market demand and simultaneously achieving a reduced environmental load in the industrial paper production is the use of fibrous agricultural residues. The aims of this study were i) to investigate the effect of alkaline – hydrodynamic cavitation (HC) pretreatments on the delignification of Miscanthus × giganteus stalks (MGS) and ii) establishing the suitability of MGS as feedstock and their exploitation in pulp and paper manufacturing. It was demonstrated that the proposed treatment is an efficient delignification method for the non-wood fiber sources, such as miscanthus. A significant outcome of this work was the observation that HC treatment preserved the fibres lengths and surface quality of raw MGS, but at the same time increased the amount of kinked and curled fibers present in cavitated miscanthus fibers. The average miscanthus fiber length was found to be relatively short at 0.45 (±0.28) mm, while the slenderness ratio, the flexibility coefficient and Runkel ratio values were calculated to be 28.13, 38.16 and 1.62, respectively. The estimated physical properties of MGS pulp hand-sheets were 24.88 (±3.09) N m g−1 as the tensile index, 0.92 (±0.06) kPa m2 g−1 as the burst index and 4.0 (±0.37) mN m2 g−1 as the tear index. Overall the current work demonstrated effective use of hydrodynamic cavitation for improving the processing in pulp and paper manufacturing.

COATING “The fluorine-free coating has excellent hydrophobic and oleophobic properties for porous cellulose-based materials”, Peng Du, Qijun Ding, Chuanshan Zhao, Yifei Jiang, Wenjia Han & Xia Li Cellulose, Vol.28, pp.6133–6146 (2021). Nanofibrillated cellulose is obtained from cellulose resources (e.g., Natural cellulose, bacterial cellulose) and is deemed to be environmentally friendly, green, and inexhaustible. Accordingly, we reported an innovative method for fluorine-free water- and oil/grease-repellent surface based on porous cellulose-based materials. In order to improve oil/grease resistance, sulfonated nanofibrillated cellulose (SNFC) was coated on the surface to fill the pores, the obtained paper was soaked in chitosan aqueous solution as the second layer. The-SNFCchitosan-coated paper was applied with modified polyvinyl alcohol (MPVA) as the third layer to increase water resistance and water vapour transmittance. The experimental results showed 5.2 g/m2 SNFC, 2.5 g/m2 chitosan and 7% MPVA demonstrated excellent kit rating value of 12/12 (maximum grease resistance) (oil resistance contact angle is 103.93°), good water resistance (water contact angle is 108.6°) and high water vapour transmission rate (4.8 × 10–14 g cm/cm2 s Pa) under the condition of 90% relative humidity. This non-fluorinated and practical method is an excellent alternative to existing fluorinated materials, which will further promote the development of cellulose-based packaging.

“Investigation of Industrial Paper Coating with Recycled Kaolin”, Journal of Color Science and Technology, F. Mortazavi, H. Resalati, S. Rasouli & Gh. Asadpour, Journal of Color Science and Technology, Vol.15 (2), Summer 2021, pp.117-129. The paper coating process has a special place in the paper industry in order to reduce production costs and improving quality. In this study, kaolin was first extracted from waste paper and XRD, FTIR, SEM/EDX analysis are used to investigate the microstructural properties of kaolin recycled from waste paper. Then, the coating solution of Recycled kaolin was coated on paper at four levels of thickness 30, 60, 90, and 120 micrometers.

Based on the study of permeability properties and mechanical properties, the superior treatment was related to coated paper with 120 micrometers thickness.

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Technical Abstracts

g PAPERmaking!

FROM THE PUBLISHERS OF PA APER TECHNOLOGY

Volume 7, Number 2, 2021

“Hydrophobic, moisture resistant and biorenewable paper coating derived from castor oil based epoxy methyl ricinoleate with repulpable potential”, P.A. Parvathy & Sushanta K. Sahoo, Progress in Organic Coatings, Vol.158, September 2021. After ban on single-use plastics, completely bio-based coating on paper is in great demand to replace the plastic liner and other organic-inorganic hybrid coating. To resolve the issues like moisture, hydrophilicty and repulpability after use, a green, sustainable and hydrophobic bio-coating material for paper substrate has been developed. In this work, silanized castor oil (SCO) and silanized methyl ricinoleate (SMR) bio-resins are synthesized by structural modifications of epoxidized castor oil through transesterification and silanization process, Subsequently, the silylated bio-resins are hydrolysed and subjected to crosslinking with cellulosic paper substrate through condensation process.

The coated papers showed hydrophobicity with water contact angle (WCA) up to 97° and strongly moisture resistant (moisture content of 2–3%) nature. Scanning electron microscopy analysis with energy dispersive X-ray spectra confirms the uniform coating of SMR resin with better texture and higher degree of silane grafting. The water vapor transmission rate (WVTR) decreased by 77.5% after coating because of blockage of pores and strong bonding with the cellulosic fibers. Further the coated papers showed adequate thermal stability and better tensile strength for commercial packaging. These coatings are removed by exposing to strong alkali solution at 90 °C and the recyclability potential of the paper is confirmed for reuse. These renewable resourced and eco-friendly coating materials are found to be suitable alternative for paper packaging and other coating applications with circular economy approach.

ENVIRONMENT BioǦbased Packaging: Material, Environmental and Economic Aspects, Chapter 22 “Life Cycle Assessment of Bio-Based Packaging Products”, H.N. Salwa, S.M. Sapuan, M.T. Mastura, M.Y.M Zuhri & R.A. Ilyas, (Book Editor(s): S.M. Sapuan, R.A. Ilyas). Life cycle assessment (LCA) can be utilized to evaluate the technical solutions employed in the production process to identify impacts originating not only from the production itself but also from the phases of use and end-of-life (EOL) issues. This chapter discusses thoroughly each phase of the LCA outline and reviews recent different LCA studies conducted on packaging products constructed from bio-based materials. The main elements in the first phase of LCA operation are illustrated. Life cycle inventory (LCI) is the second phase of LCA where usage of resources and materials and the consumption of fuels and energy need to be quantified. Packaging materials go through different life cycles, which then generate certain environmental impacts at different stages of the supply chain and for different environmental indicators. The third phase of the LCA framework is the life cycle impact assessment, which focuses on evaluating and understanding the environmental impacts established by the LCI analysis.

MOULDED PULP “Design and development of eco-friendly cutlery out of paper waste through molding next generation waste management”, Richa Pandey, Rahul Singh, Phuleshwar Baitha & Roshan Topno, Materials Today Proceedings, 43(3), 2021, pp.1557-1561. The waste of paper or the paper dump is a serious problem and its occurrence is widespread in various institutions. Wastepaper can be recycled to produce some important and usable products that are ecological and biodegradable. For example, it is used to extract new paper from the old by chemical decomposition. Other products such as paper plates, paper cups, paper spoons, tiles, toys, crafts, etc. can be formed from the slurry. This project also initiates a mould that makes cups and plates from the wastepaper management circuit. With learning institutions being the main consumers,

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Technical Abstracts

g PAPERmaking!

FROM THE PUBLISHERS OF PA APER TECHNOLOGY

Volume 7, Number 2, 2021

most paper are usually eliminated after its use. The waste remains undeveloped and unused, although is a valuable resource. Studying the design of a manual, economical and efficient paper recycling machine, it can be surmised that the design uses the integration of the knowledge gained about the recycling technology that exists manually from the paper recycling machines used to form a cheap but effective paper recycling system. The advantages of the machine are not only of recycled paper, but also the advantages of the interaction of the manual drive system, which will also reduce the high cost. As the design is not 100% effective, the transmission, belt and chain transmission correspond to the estimated 90% efficiency using the 90% for the design. The power consumption for the design is 450 W and since an average user can produce 100 W constant, it takes 5 people to drive the machine.

NANO-SCIENCE “Preparation of cellulose nanocrystals from deinked waste newspaper and their usage for papermaking”, Ying Guan, Wenqi Li, Hui Gao, Liping Zhang, Liang Zhou & Feng Peng, Carbohydrate Polymer Technologies and Applications, Vol.2, 25 December 2021. Cellulose nanocrystals (CNC) were successfully prepared from waste newspapers through a simple pretreatment and hydrolysis process. The Fourier transform infrared spectroscopy (FTIR) result for the CNC suggested that no new chemical bonds were formed during the preparation of CNC. X-ray diffraction results indicated that the crystallinities of the deinked and untreated waste newspapers were 66% and 73.8%, respectively. Greater smoothness of the deinked waste newspapers fibers could be clearly observed by scanning electron microscope (SEM), which was attributed to the removal of ink from the chemical treatment. The transmission electron microscopy (TEM) and atomic force microscopy (AFM) results of the CNC revealed a needle-like natural fiber with a length of 300 nm to 600 nm, a diameter of 10 nm to 30 nm, and an aspect ratio in the 10 to 60 range. The obtained CNC particles were found to improve the physical properties of paper sheets, specifically that of the tear, tensile, and burst indices. The maximum increase in the physical properties was achieved with a 5.5% CNC addition.

“A review of bacterial cellulose: sustainable production from agricultural waste and applications in various fields”, Leire Urbina, María Ángeles Corcuera, Nagore Gabilondo, Arantxa Eceiza & Aloña Retegi, Cellulose, online (2021). Bacterial cellulose (BC) is a polymer with interesting conformation and properties. BC can be obtained in different shapes and is easily modified by chemical and physical means, so its applications in the production of new materials and nanocomposites for different purposes have been in the focus of many research projects. However, one of the major challenges to address in bacterium-derived polymer technology is to find suitable carbon sources as substrates that are cheap and do not compete with food production for achieving large scale industrial applications. Agricultural wastes are defined as the residues from the growing and processing of raw agricultural products such as crops, fruits, vegetables and dairy products. Their composition can vary depending on the type of agricultural activity and harvesting conditions, but these residues are suitable for the production of BC. The aim of this review is to give insight into the production of BC using agro-wastes and an overview of the most interesting and novel applications of this biopolymer in different areas i.e. environmental applications, optoelectronic and conductive devices, food ingredients and packaging, biomedicine, and 3D printing technology.

NOVEL PRODUCTS “A simple method of simultaneously endowing paper or fluff pulp with both high softness or appropriate fluffing properties and antimicrobial properties”, Yongjian

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Xu, Yun Shi, Xuyong Chen, Fenfen Liu & Wei Zhao, Cellulose, Vol.28, pp.7327–7339 (2021). As the living standard improves, disposable sanitary and household paper requires not only softness or easy to fluff in a dry state, but also good antibacterial property. In this study, a series of alkyl quaternary ammonium salts (AQAS), a kinds of antibacterialdebonding agents, were synthesized by using a two-step process. The SN2 nucleophilic substitution reaction was designed between triethanolamine and sulfoxide chloride, and followed subsequent quaternization by tertiaryamines with different long alkyl chains. The obtained products can be used as an antibacterial and softening/debonding agent for improving the performance of paper or fluff pulp. The results showed that these new compounds can endow antibacterial function and high softness to paper or control appropriate burst strength to fluff pulp. These AQAS products can be not only used in daily disposable sanitary products, but also have potential applications in other products such as glass spacer paper, advanced household paper, or antibacterial tissue products to prevent microbial contamination.

PACKAGING TECHNOLOGY “Rheological and anti-microbial study of silica and silver nanoparticles-reinforced kcarrageenan/hydroxyethyl cellulose composites for food packaging applications”, Balasubramanian Rukmanikrishnan, Srinivasan Ramalingam, Sam Soo Kim & Jaewoong Lee, Cellulose, Vol.28, pp.5577–5590 (2021). Sustainable food packaging films were developed using a combination of k-Carrageenan (k-C), hydroxyl ethyl cellulose (HEC), silicon dioxide (SiO2), and silver (Ag) nanoparticles. The CH-SiO2/Ag nanocomposites showed promising results, mainly due to their transparency, flexibility, low cost, and environmental friendliness. The structure and uniform morphology of the CHSiO2/Ag nanocomposites were determined by FT-IR, XRD, and SEM analysis. Barrier properties (water vapor permeability-WVP), thermal properties (T5% loss, char yield), and mechanical properties determined for the k-C/HEC and k-C/HEC-SiO2/Ag nanocomposites, which improved by 3.3–1.9 × 10−9 gm/m2 Pa s (WVP), 59.1–115.7 °C (T5%), 13.4–29.3% (char yield), 23.8–41.5 MPa (tensile strength), and 22.3–28.9 (EB), respectively. The contact angle of the k-C/HEC-SiO2/Ag nanocomposites were in the range of 60.1°–76.4°. The UV transmittance of the k-C/HEC composites decreased with the addition of SiO2 and Ag nanoparticles. However, the transparency of the composites was not affected, and it inhibited UVA and UVB rays by the addition of Ag nanoparticles.

The viscosity of the k-C/HEC composites increased with the SiO2 content and decreased with the shear rate. All the composites exhibited shear-thinning behavior. The storage modulus of the prepared composites is higher than the loss modulus in the entire frequency region. Overall, SiO2 and Ag nanoparticles improved the hydrophobic nature of the k-C/HEC-SiO2/Ag films and showed significant activity against six common food pathogens, Staphylococcus aureus, Bacillus cereus, Listeria monocytogenes, Bacillus subtilis, Salmonella typhi, and Cronobacter sakazakii. The synergistic combination of kC/HEC-SiO2/Ag nanocomposite has potential candidate for packaging and other biomedical applications.

PAPERMAKING “Energy Saving in Papermaking by Application of Hybrid Calcium Carbonate”, Min Woo Lee, Sun Young Jung & Yung Bum Seo, BioResources; Raleigh Vol.16 (3), (Aug 2021), pp.5011-5023. Hybrid calcium carbonate (HCC) is prepared by pre-flocculating calcium oxide and ground calcium carbonate (GCC) with ionic polymers, and by injecting carbon dioxide to the pre-flocculated flocs until termination of the reaction at neutral pH. When used in papermaking, HCC gave higher bulk, higher stiffness, and higher tensile strength than GCC did. Furthermore, when higher wet pressing pressure was applied to

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the paper containing HCC, higher solid content was achieved with still better stiffness, smoothness, and tensile strength in comparison with the paper containing GCC. Actually, very high bulk of the HCC containing paper was compromisingly reduced to obtain high solid content by applying higher wet pressing pressure. Higher solid content achieved by combination of HCC technology and higher wet pressing pressure resulted in paper drying energy savings of more than 10%, while a high quality of all the essential printing paper properties was maintained. Drying energy savings may contribute to greenhouse gas (GHG) reduction and lowering of the paper production cost.

“Analysis and control of the paper moisture content variability by using fuzzy and traditional individual control charts”, José L.Rodríguez-Álvarez, Rogelio LópezHerrera, Iván E. Villalon-Turrubiates, Rey D. Molina-Arredondo, Jorge L. GarcíaAlcaraz & Óscar D. Hernández-Olvera, Chemometrics and Intelligent Laboratory Systems, Vol.208, 15 January 2021. For several decades, the fuzzy theory has been used to develop the so-called fuzzy control charts to analyze process data in a more reliable and flexible way than traditional control charts. Control charts are more commonly used in the papermaking processes to check the stability of the data source. These charts are conceived as devices that enhance routine time-by-time adjustments during the production process, so their value is incomparable. A critical variable that must be controlled during the papermaking process is the moisture content of the paper. However, due to the paper’s hygroscopic characteristics, its moisture content values may carry some uncertainty derived from measurement systems and the nature of the papermaking process. Therefore, an alternative approach that includes fuzz control charts to handle these uncertainties is proposed in this paper. The method to convert individual data to fuzzy numbers is based on the sigma level process as a first stage, and then, the fuzzy individual and moving range control charts are introduced using the cut fuzzy midrange approach. Data from the moisture content of a 240 g coated paper sample is used to prove the performance of the fuzzy individual control charts. According to the proposed methodology, findings show that fuzzy individual and moving range control charts have greater flexibility than traditional individual control charts by reflecting a larger amplitude of their control limits, in addition to a lower number of “out of control” values.

“Recycling cellulose nanofibers from wood pulps provides drainage improvements for high strength sheets in papermaking”, Shaun Ang, Debjani Ghosh, Victoria Haritos & Warren Batchelor, Journal of Cleaner Production, Vol.312, Online, 20 August 2021. The recyclability of cellulose nanofibers (CNF) is one of its most promising features. However, the performance of recycled CNF is not fully established. In this work, we engineered CNF sheets from refined and homogenized industrial virgin pulp (BEK), industrial recycled pulp (DIP) and lab-scale recycled pulp (LRC). These original CNF sheets were then soaked and recycled using a standard disintegrator to produce recycled CNF sheets. The key changes with recycled CNF sheet properties were tracked and compared to their original CNF sheets. Fiber retention and sheet uniformity measurements revealed a higher proportion of fiber aggregates with original CNF from DIP and LRC, compared to BEK. The proportion of fiber aggregates increased even further with recycled CNF. These results are consistent with the well-established formation of strong irreversible hydrogen bonds which hinder complete refibrillation. The tensile index of original 10K1P (10,000 PFI revolutions refined and 1 high-pressure homogenized pass) CNF sheets made from recycled DIP and LRC fibers at 110 Nm/g was only 10% lower compared to those made from virgin BEK fibers. Interestingly, all recycled CNF sheets from homogenized fibers successfully retained about 80% of their original CNF sheet strength.

Remarkably, all recycled CNF sheets also had significant reductions in sheet drainage

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times, with the standout being the reduction from LRC 10K1P at 84 min to R-LRC 10K1P at 1.14 min of drainage time. The combination of high sheet strength and sharply reduced drainage times provide a rare example of the recycled product being superior to the original product.

TESTING “Research on determination of total ash and its components of paper by continuous X-ray method”, Lianhua Hu & Xinyu Lu, Chemical Papers (2021). The X-ray diffraction internal standard method can quantitatively determine the ash content of paper, but the requirements for the experimental environment are high, the determination is not convenient and the cost is high, and it is difficult to be widely used in the papermaking enterprises. In order to determine the total ash and its components of the single filler paper samples, and meet the ash determination requirements of papermaking enterprises, the continuous X-ray method is proposed. In this paper, an X-ray tube is used as the radiation source and an ionization chamber is used as the detector. Through polynomial fitting of the total ash and voltage logarithm ratio of the single filler paper samples, the mathematical model of total ash is established; the total ash and its components content of paper are calculated according to the mathematical model of total ash and the Beer– Lambert law deformation formula; the total ash and its components content of paper are verified by the burning method. The research results show that the total ash error of the single filler paper samples is less than 4%, and the ash component error is less than 5%.

“Liquid droplet imbibition into a thin coating layer: Direct pore-scale modeling and experimental observations”, H. Aslannejad, S.V. Loginov, B. van der Hoek, E.M. Schoonderwoerd, H.C. Gerritsen and S.M. Hassanizadeh, Progress in Organic Coatings, Vol.151, February 2021. In order to control ink droplet movement into the printing-paper layer, a set of pore-scale two-phase flow simulations were performed. The high-resolution three-dimensional pore space of the paper was obtained using focused ion beam scanning electron microscopy (FIB-SEM). Solving Navier-Stokes equations yielded details about dynamic movement of a droplet into the layer. To evaluate simulation results, for the first time, confocal laser microscopy imaging technique was integrated into a FIBSEM chamber. Doing so, high resolution imaging of the droplet penetration inside paper was conducted and computed volume of penetrated ink at final stage was compared to the imaged volume. The ink penetration and spreading extent showed a good agreement with simulation results. Therefore, the developed simulation case was further investigated to study impact of liquid contact angle, real ink properties, and droplet arrival velocity on paper surface on final print quality. A faster penetration into the paper coating was observed for smaller equilibrium contact angles; meanwhile, more radial wicking was observed. In case of velocity of impact, higher velocity caused creation of irregular shapes of the ink footprint on paper surface. In addition to that, higher velocity caused ink splash which consequently created satellite droplets and lowered the print quality. Comparing inklike liquid (representing real ink liquid properties) with water, water moves faster than inklike liquid into the paper. This is mainly due to the higher viscosity and lower surface tension of the ink-like liquid.

“Correlation between Dusting Gauge Method and Paper Quality Values”, Elina Asikainen, Tampere University of Applied Sciences, Bachelor's thesis, June 2021, Dusting of paper can be caused by different factors and can occur in many ways. This makes the measuring of the dusting tendency challenging. One method developed is Emerson’s Dusting Gauge, which can be used to take dusting samples of paper on a paper machine. Emerson ́s method does not give standards for dust results. Those must

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be determined by the user. Dusting is said to be a result of poor surface strength which can be caused by poor bonding of particles on paper web or poor adhesion of surface size. The purpose of this thesis was to find out whether the results of dust measurements have a correlation with surface strength, tensile strength, filler content, surface size amount, porosity and surface roughness. The filler content of paper affects the dusting of paper. In addition to filler amount, the particle shape, size and chemical behaviour are factors that affect dusting. Distribution of fillers and fines in paper network is a factor affecting dusting and those can be affected on paper machine forming section. Dusting of paper can be reduced with surface sizing.

WASTE TREATMENT “Neighborhood component analysis for modeling papermaking wastewater treatment processes”, Yuchen Zhang, Jie Yang, Mingzhi Huang & Hongbin Liu, Bioprocess and Biosystems Engineering, online (2021). It is of great importance to obtain accurate effluent quality indices in time for pulping and papermaking wastewater treatment processes. However, considering the complex characteristics of industrial wastewater treatment systems, conventional modeling methods such as partial least squares (PLS) and artificial neural networks (ANN) cannot achieve satisfactory prediction accuracy. As a supervised metric learning method, neighborhood component analysis (NCA) is able to significantly improve the prediction performance by training an appropriate model in metric space using the distance between samples for papermaking wastewater treatment processes. The results on two data sets show that NCA has a higher prediction accuracy compared with PLS and ANN. Specifically, NCA has the highest determination coefficient (R2) and the lowest root mean square error in a benchmark simulation data set.

On the other hand, the results on the data from an industrial wastewater process indicate that NCA has better modeling accuracy and its R2 increases by 32.80% and 29.08% compared with PLS and ANN, respectively. NCA provides a feasible way to realize online monitoring and automatic control in wastewater treatment processes.

“Application of aluminum sulfate in the treatment of papermaking white water”, X Ming, Q. Li & W. Jiang, BioResources; Raleigh Vol.16 (1), pp.1382-1393. Physical chemical methods were used to treat papermaking white water used to produce plant fiber mulch sheet that contained fine fibers and inorganic fillers as suspended solids. The ordinary chemical oxygen demand (CODCr) was obviously reduced after the papermaking white water was treated by the flocculant. By comparing three different coagulants (aluminum sulfate (Al2(SO4)3), poly-aluminum chloride (PAC), and poly(diallyldimethylammonium chloride) (PDADMAC)) and flocculant (poly-acrylamide copolymer (PAM)) to process papermaking white water, it was found that Al2(SO4)3 had the best coagulation effect and the lowest cost. The best flocculation conditions were 2,733 mg/L of Al2(SO4)3 and 4.52 mg/L of PAM to treat the papermaking white water.

Under the best flocculation conditions, the CODCr was less than 300 mg/L. The goal of closed recycling and zero discharge of white water in the production process of plant fiber mulch sheet was realized.

“Quality-related monitoring of papermaking wastewater treatment processes using dynamic multiblock partial least squares”, Jie Yang, Yuchen Zhang, Lei Zhou, Fengshan Zhang, Yi Jing, Mingzhi Huang & Hongbin Liu, Journal of Bioresources and Bioproducts, online 7 April 2021. Environmental problems have attracted much attention in recent years, especially for papermaking wastewater discharge. To reduce the loss of effluence discharge violation, quality-related multivariate statistical methods have been successfully applied to achieve a robust wastewater treatment system. In this work, a

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new dynamic multiblock partial least squares (DMBPLS) is proposed to extract the timevarying information in a large-scale papermaking wastewater treatment process. By introducing augmented matrices to input and output data, the proposed method not only handles the dynamic characteristic of data and reduces the time delay of fault detection, but enhances the interpretability of model. In addition, the DMBPLS provides a capability of fault location, which has certain guiding significance for fault recovery. In comparison with other models, the DMBPLS has a superior fault detection result. Specifically, the maximum fault detection rate of the DMBPLS is improved by 35.93% and 12.5% for bias and drifting faults, respectively, in comparison with partial least squares (PLS).

WOOD PANEL “Variability in moisture sorption isotherms of plywood and oriented strand board with accelerated ageing”, Kevin Zhang and Russell Richman, Canadian Journal of Civil Engineering, online 22 July 2020. Simulation of heat and mass transfer in buildings can be a powerful tool in analyzing overall durability but is dependent on accurate input data. This study is a part of a larger study on the effects of moisture storage in a commercially available hygrothermal modelling software. Moisture storage has not been well measured historically due to time requirements and necessity, and the effects of moisture storage on modelling have not been studied extensively. This study measures sorption isotherms in plywood and oriented strand board (OSB) sheathing that has undergone accelerated ageing. Results have shown that ageing samples may not have a significant effect on sorption isotherms, however, this may be due to the accelerated ageing method. Common methods used in accelerated ageing of wood products were primarily designed for testing mechanical properties and were too destructive for measuring hygrothermal properties. Further refinement of the accelerated ageing methodology may yield improved data.

“Experimental study on correlation between adhesive penetration pattern and mechanical performances in oriented strand board”, Hamidreza Edalat, Volker Thole & Mehdi Faezipour, European Journal of Wood and Wood Products, Vol.79, pp.59– 74 (2021). The aim of the experimental study presented here is to investigate the correlation between adhesive penetration and mechanical performances of wood-based composites. For this purpose, oriented strand boards were made with poplar uni-strands and PF resin. To make differences in resin penetration, strand moisture content (MC) and resin solid content (SC) were selected as variables of the experiment. The adhesive penetration pattern was studied by the elemental counting method (ECM) in two layers of the boards’ thickness. Mechanical properties including MOE, MOR, IB, tensile strength parallel to the grain, impact resistance, and creep bending were evaluated as well as resin droplets distribution. The results showed that strand MC and resin SC influence the mechanical performance and resin penetration parameters. The penetration parameters increased in the middle layer where the bondlines are close to heat and hydrodynamic pressure sources. The composites made with higher concentration resin and drier strands expressed weak penetration causing a reduction in mechanical properties except for tensile strength where the shear stress took place between bonded strands. Resin droplet size analysis proved that resin SC could change the droplet size distribution and influence the strengths. Strand MC could act as a modifying factor to develop the penetration and balance it between the layers. The results of the correlation analysis proved a significant relationship between mechanical properties and penetration parameters, in particular, those obtained from fiber quantity.

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Volume 7, Number 2, 2021

“The Removal of Cured Urea-Formaldehyde Adhesive towards Sustainable Medium Density Fiberboard Production: A Review”, Muhammad Adly Rahandi Lubis, Sumit Manohar Yadav, Raden Permana Budi Laksana, Widya Fatriasari, Maya Ismayati, Faizatul Falah, Nissa Nurfajrin Solihat, Fahriya Puspita Sari & Wahyu Hidayat, Jurnal Sylva Lestari, Vol.9 (1) (2021). Medium density fiberboard (MDF) is an engineered wood product that has density and specific gravity similar to solid wood, ranging from 600 to 800 kg/m3 of density and 0.6 to 0.8 of specific gravity. This makes MDF suitable to partially replace solid wood, particularly for interior application. Approximately more than 100 million m3 of MDF were produced in 2020, resulting in a large amount of waste MDF will be generated in the next 20 years. MDF is produced using urea-formaldehyde (UF) resins adhesive. UF resins adhesive is a poly-condensation product of urea and formaldehyde via an alkaline acid two-step reaction. Sustainable MDF production is required as the world is facing climate change and deforestation. Recycling is a way to support sustainable production in the engineered wood products manufacturing. Many attempts have been done to find ways to recycle waste MDF. The main problem is UF resins, which bond the MDF panel fibers. In order to re-manufacture the waste MDF into new recycled MDF, UF resins should be eliminated from the waste MDF before being used. The presence of UF resins in MDF can interfere with the utilization of the recycled fibers, whether it will be used as a raw material for new MDF or other composite products. This paper reviews the process of removal of cured UF resins from waste MDF panel by considering the hydrolytic stability of cured UF resins for MDF recycling, providing a comprehensive review of how cured UF resins can be removed from waste MDF and characterization of recycled fibers obtained from recycling prior to re-manufacturing of recycled MDF panel.

“Understanding moisture durability of wood-based composites using crack propagation fracture toughness”, Sweta Mahapatra, Arijit Sinha & John A. Nairn, Wood Materials Science & Engineering, Published online: 4 Apr 2021. A persistent issue in adhesively bonded wood-based composites is their moisture durability. A key question, therefore, is: what is the best approach to assessing moisture durability of wood composites? Previous work has focused on failure initiation (e.g. internal bond strength) and therefore, could not consider effects of moisture on subsequent damage propagation.

This research addressed that knowledge gap by using crack propagation fracture experiments to study moisture effects on medium-density fiberboard (MDF), oriented strand board (OSB), and particleboard (PB). Fracture toughness vs. crack growth curves (i.e. material R curves) were measured for MDF, OSB, and PB after immersing them in water for immersion times up to one week. Experiments were done immediately after immersion to record crack propagation in states of increasing moisture content. Trends in experimental results could be characterized by a three-point model for MDF and PB or a linear model for OSB. Experiments and modeling showed that all composites degraded in both initiation toughness and in their ability to inhibit crack growth by fiber bridging mechanisms. Summary plots compare how R curves for MDF, OSB, and PB changed from control conditions to water saturation conditions and compare their moisture degradation rates.

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