PAPERmaking! Vol.7 No.3 2021

PAPERmaking!

The e-magazine for the Fibrous Forest Products Sector

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Paper Technology International

Volume 7 / Number 3 / 2021

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 3, 2021

CONTENTS: FEATURE ARTICLES: 1. Strength: Locust bean gum as a strength aid.

2. Coating: Influence of coat weight on paper properties.

3. Pulp: Impact of shredding on papermaking potential of recycled pulp.

4. 3D Forming: Tray forming using paperboard: a case study using FEA. 5. Environment: Maintenance in the drying section and CO2 emissions.

6. Theory: Cellulose and the role of hydrogen bonding.

7. Water Treatment: Applications of cellulose-based flocculation agents.

8. Wood Panel: Mercury intrusion porosimetry investigation of OSB panels.

9. Corrugated: Crush testing study of single-walled corrugated board.

10. Management: Leadership skills for middle managers.

11. Leadership: Coaching and mentoring programs to develop new leaders.

12. Safer Driving: Safety tips for driving at Christmas.

SUPPLIERS NEWS SECTION: News / Products / Services: Section 1 – PITA Corporate Members: ABB / VALMET Section 2 – PITA Non-Corporate Members VOITH Section 3 – Other Suppliers Ametek / SKF DATA COMPILATION: Installations: Overview of equipment orders and installations since August 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 3, 2021

Locust bean gum adsorption onto softwood kraft pulp fibres: isotherms, kinetics and paper strength 1,2

3

1,2

JINGQIAN CHEN , RODGER P. BEATSON & HEATHER L. TRAJANO .

The adsorption of locust bean gum (LBG) onto Northern Bleached Softwood Kraft (NBSK) pulp improved paper tensile and burst strength and lowered refining energy by strengthening inter-fibre bonding. Adsorption kinetics and isotherms were investigated to develop a fundamental understanding of the adsorption mechanism. The adsorption rate followed pseudo-second-order kinetics and the activation energy was 99.34 -1 kJmol , suggesting chemisorption. The adsorption rate constant increased rapidly with temperature from 25 to 45 °C (k = 1.93 to 24.03 gmg-1min-1), but the amount adsorbed at equilibrium decreased (qe = 1.91 to 0.48 mgg-1 o.d. fibre). LBG adsorption to NBSK at 25 °C was consistent with the Langmuir adsorption model for LBG<2.1 wt% of o.d. fibre, suggesting reversible, homogenous adsorption to a finite number of sites on the fibre surface. Refining to 3000 rev increased the heterogeneity of the NBSK pulp surface leading to multilayer Freundlich adsorption with adsorption constant n = 5.00, and the equilibrium constant Kf = 2.57 mgg 1 -1 -1/n (mgL ) at 25 °C. Favorable adsorption conditions for negatively charged LBG were identified: 25 °C for 10 min, low dosage level (<2 wt%), lightly refined (<3000 rev) NBSK pulp at low fibre consistency (<0.5 wt%), high agitation rate (>150 r.p.m.), acidic or neutral conditions (pH 2–7) without salt addition.

Contact information: 1. Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC V6T 1Z3, Canada.

2. BioProducts Institute, University of British Columbia, Vancouver, BC V6T 1Z3, Canada 3. Chemical and Environmental Technology, British Columbia Institute of Technology, 3700 Willingdon Avenue, Burnaby, BC V5G 3H2, Canada Cellulose (2021) 28:10183–10201 https://doi.org/10.1007/s10570-021-04133-w 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 20

Article 1 – Locust Gum Bean & Strength

Cellulose (2021) 28:10183–10201 https://doi.org/10.1007/s10570-021-04133-w

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ORIGINAL RESEARCH

Locust bean gum adsorption onto softwood kraft pulp fibres: isotherms, kinetics and paper strength Jingqian Chen

. Rodger P. Beatson . Heather L. Trajano

Received: 7 October 2020 / Accepted: 3 August 2021 / Published online: 1 October 2021 The Author(s) 2021

Abstract The adsorption of locust bean gum (LBG) onto Northern Bleached Softwood Kraft (NBSK) pulp improved paper tensile and burst strength and lowered refining energy by strengthening inter-fibre bonding.

Adsorption kinetics and isotherms were investigated to develop a fundamental understanding of the adsorption mechanism. The adsorption rate followed pseudo-second-order kinetics and the activation energy was 99.34 kJ mol-1, suggesting chemisorption. The adsorption rate constant increased rapidly with temperature from 25 to 45 C (k = 1.93 to 24.03 g mg-1 min-1), but the amount adsorbed at equilibrium decreased (qe = 1.91 to 0.48 mg g-1 o.d. fibre). LBG adsorption to NBSK at 25 C was

consistent with the Langmuir adsorption model for LBG \ 2.1 wt% of o.d. fibre, suggesting reversible, homogenous adsorption to a finite number of sites on the fibre surface. Refining to 3000 rev increased the heterogeneity of the NBSK pulp surface leading to multi-layer Freundlich adsorption with adsorption constant n = 5.00, and the equilibrium constant Kf = 2.57 mg g-1 (mg L-1)-1/n at 25 C. Favorable adsorption conditions for negatively charged LBG were identified: 25 C for 10 min, low dosage level (\ 2 wt%), lightly refined (\ 3000 rev) NBSK pulp at low fibre consistency (\ 0.5 wt%), high agitation rate ([ 150 r.p.m.), acidic or neutral conditions (pH 2–7) without salt addition.

Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/ s10570-021-04133-w.

J. Chen H. L. Trajano (&) Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC V6T 1Z3, Canada e-mail: heather.trajano@ubc.ca J. Chen H. L. Trajano BioProducts Institute, University of British Columbia, Vancouver, BC V6T 1Z3, Canada R. P. Beatson Chemical and Environmental Technology, British Columbia Institute of Technology, 3700 Willingdon Avenue, Burnaby, BC V5G 3H2, Canada

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Graphic abstract

Keywords Northern bleached softwood kraft pulp Paper strength additive Hemicellulose Locust bean gum Adsorption isotherms Adsorption kinetics

Introduction Paper strength, essential to many applications, relies on the number of interfibre bonds, the strength of the bonds, fibre properties (e.g. fibre strength, length, and coarseness), and the distribution of fibres and bonds (sheet formation) (Niskanen 1998; Lindström et al.

2005; Leech 1953, 1954). Paper strength can be modified through mechanical refining and the use of additives such as polysaccharides. Softwood grown in the Northern hemisphere (e.g. British Columbia, Canada) produces high-strength bleached kraft pulp; finding ways to further enhance strength properties while reducing refining energy is a key objective for Northern Bleached Softwood Kraft (NBSK) producers.

Increasing paper strength through refining consumes large amounts of mechanical energy. Strength improvement through application of additives after refining reduces the energy required to achieve target

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paper strength (Bhaduri et al. 1995; Silva et al. 2010).

The earliest paper strength additives were polysaccharides with structural affinity for cellulose such as locust bean gum (LBG) and guar gum (Leech 1953; Lindström et al. 2005; Most 1957; Russo 1959; Swanson 1950). Leech (1954) concluded that 0.5 wt% LBG dosage doubled the bonding strength of paper. Swanson (1950) reported that tensile strength increased by 33% following sorption of 2 wt% LBG to coniferous sulphite pulp. Burst strength increased 32% with 0.5 wt% LBG in beaten pulp; this corresponded to a 70% reduction in beating time (Swanson 1950).

Paper strength enhancement by hemicellulose adsorption results from an increased number of bonds plus increased bonded area and bond strength (Leech 1954). The primary bonding between fibres and between hemicellulose and fibres is hydrogen bonding (Niskanen 1998; Leech 1954; Hannuksela et al. 2002).

Many factors have been shown to influence adsorption of hemicellulose onto pulp fibre: adsorption conditions (Russo 1959; Gruenhut 1953; Leech 1953; Most 1957; Swanson et al. 1949), fibre properties (Zakrajšek et al. 2009), and hemicellulose properties (Hannuksela et al. 2002, 2004; Lindqvist et al.

2013). Given the multitude of factors and interactions,

Cellulose (2021) 28:10183–10201

there are many contradictory reports regarding the effects of changing variables on adsorption results.

Adsorption conditions include temperature, time, hemicellulose dosage, pH, salt addition, fibre consistency and agitation rate. Temperature and time are the most commonly examined factors for hemicellulose adsorption, but the reported effects vary greatly.

Adsorption of partially methylated LBG on bleached sulfite pulp increased with temperature from 5 to 61 C (Russo 1959). However, Gruenhut (1953) concluded that LBG adsorption to kraft fibre increased with decreasing temperature; maximum adsorption was observed at 4.2 C. Most (1957) and Leech (1953) reported a greater amount of hemicellulose was retained by pulp fibre with increasing time, and further concluded that adsorption equilibrium was not obtained even after 10 days. However, Swanson et al. (1949) obtained 76–96% LBG adsorption to bleached sulfite pulp and reported that equilibrium was reached within 30 min.

Salts, process chemicals and pH also strongly influence the process (Hedborg and Lindström 1993; Shirazi et al. 2003; van de Steeg 1989; van de Steeg et al. 1993a, b; Zakrajšek et al. 2009) since adsorption occurs by electrostatic interaction of polyelectrolytes (polymers with electrolyte groups) with negativelycharged cellulose fibres (Niskanen 1998; van de Ven 2000; Sjostrom 1989). Cellulose fibres are negatively charged due to carboxyl groups and hydroxyl groups.

Addition of salts decreases the attractive electrostatic forces between cationic starch and cellulose fibre thus adsorption decreases (van de Steeg 1989; van de Steeg et al. 1993a, b; Hedborg and Lindström 1993). When pH increases, carboxyl groups deprotonate and generate more negative charge on fibre surface (Hedborg and Lindström 1993). As a result, adsorption of cationic polymers increases with rising pH (Shirazi et al. 2003; van de Steeg 1992; van de Steeg et al.

1993a, b). However, for polymers with negative charge, low pH facilitates adsorption by converting carboxyl groups to their undissociated state (Scallan 1983). High pH leads to a high electrostatic repulsion between fibres and negatively charged polymer, thus reducing the adsorption. Gruenhut (1953) concluded that LBG adsorption to kraft pulp fibre was higher at pH 4 than at pH 6.5. Keen and Opie (1957) found that maximum guar gum adsorption to bleached kraft pulp was obtained at pH 6.7 and minimum adsorption occurred at pH 11.5. In contrast, Most (1957) found

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hemicellulose from slash pine adsorbed more to bleached sulfite pulp at pH 10 than at pH 4.5. Finally, Hannuksela et al. (2002) reported that adsorption of guar gum on bleached kraft pulp was independent of refining severity, pH, temperature and salt concentration. de Jong and van de Velde (2007) determined that the charge density, defined as mol negative charge/mol of monosaccharide, of native LBG was less than 0.3.

Thus, LBG is a weakly, negatively charged polymer, and salts might have a relatively low impact on adsorption. The results of the limited number of studies on the influence of ionic strength and pH on hemicellulose adsorption to cellulose are contradictory.

Mass transfer, another important adsorption condition, is influenced by agitation rate and fibre consistency. Turbulence, created by strong agitation, reduces mass transfer resistance by disrupting the boundary layer at the interface of the fibre and bulk solution (Russo 1959; Zakrajšek et al. 2009). Fibre consistency negatively correlates to extent of adsorption. Zakrajšek et al. (2009) and Most (1957) showed low fibre consistency increased adsorption of starch and hemicellulose to pulp fibres due to high concentration gradient and greater fibre surface availability.

Fibre properties such as surface area and fines content change the availability of adsorption sites (Zakrajšek et al. 2009). Several scholars attributed the increase of adsorption as a function of refining due to fibrillation, generation of fines, increased surface area and total pore volume (Zakrajšek et al. 2009; Russo 1959; Keen and Opie 1957; Hannuksela et al. 2002).

To elucidate the contradictory effects of factors, fundamental analysis including adsorption isotherms and kinetics are needed. Adsorption isotherms describe adsorption of a substance to a solid surface from an aqueous phase under isothermal conditions (Foo and Hameed 2010). Langmuir isotherms and Freundlich isotherms are commonly used to describe dye or chemical adsorption to cellulosic fibres (Langmuir 1916; Li et al. 2018; Roy et al. 2013; Urruzola et al. 2013; Vučurović et al. 2012; Zakrajšek et al.

2009).

Adsorption kinetics describe the variation of amount adsorbed with time and can guide how to most effectively apply additives during papermaking (Zakrajšek et al. 2009). Adsorption rates of polymers are related to the collision rate. For small particles, collision rate is dependent on Brownian motion, while

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for large particles or systems with flow motion, the rate is dependent on flow conditions (van de Ven 1994). Pseudo-first-order and pseudo-second-order models are commonly used to describe the kinetics of adsorption of dyes or chemicals to pulp fibres. They were proposed (Lagergren 1898; Ho 1995, 2006; Ho et al. 1996; Blanchard et al. 1984; Ho and McKay 1998, 2000) and applied in many cellulose/fibre adsorption studies (Li et al. 2018; Roy et al. 2013; Vučurović et al. 2012; Pan et al. 2016). However, reports on application to polysaccharides adsorption to pulp fibre are rare.

Currently, starch is the most widely used strength additive thus its adsorption is well studied (Hedborg and Lindström 1993; Wågberg and Bjorklund 1993; van de Steeg 1989; van de Steeg et al. 1993a, b; van de Steeg 1992; Shirazi et al. 2003; Zakrajšek et al. 2009).

However, hemicelluloses recovered from wood, such as O-acetyl-galactoglucomannans (GGM), could also be used as strength additives. These could be isolated from process streams in the pulp and paper mills as part of an integrated biorefinery. There is limited fundamental understanding of application of hemicellulose polysaccharides such as GGM, or the closelyrelated LBG. GGM recovered from pulp mill wastes has high polydispersity (Chen et al. 2020), are contaminated with other biomass components (e.g. extractives) and thus are is not well-suited to studies investigating fundamental adsorption mechanisms. In contrast, LBG is a well-defined, commercially available galactomannan-type polysaccharide: a backbone of (1–4)-b-D-mannopyranosyl units with side chains of (1–6)-a-D-galactopyranosyl units having a 1:4 ratio of galactose to mannose (BeMiller and Whistler 2012; Roller and Jones 1996). The composition, molecular structure and charge density of starch and LBG differ considerably thus past starch research cannot be transferred to LBG adsorption.

The goals of this work are to identify the favorable adsorption conditions for LBG on NBSK pulp, understand the underlying mechanisms, and the resulting effects on paper properties. LBG adsorption is analyzed using pseudo-first-order and pseudo-second-order kinetics with respect to LBG concentration.

The adsorption isotherms are analyzed using both the Langmuir model and the Freundlich model. The effects of temperature, refining, sodium chloride addition, and pH on LBG adsorption are investigated.

Changes in paper strength due to LBG adsorption to

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unrefined and refined NBSK pulp are investigated; refining and LBG dosage are varied.

Materials and methods Materials Never dried NBSK pulp was supplied by Canfor Pulp Products. The NBSK pulp was washed with deionized water until the UV–vis absorption of filtrate was less than 0.005 abs at 200 nm wavelength before use. LBG with purity greater than 90% was purchased from Sigma Aldrich. Sodium chloride ([ 99%), potassium chloride ([ 99%), hydrochloric acid (37%), sodium acetate ([ 99%), acetic acid ([ 99%), sodium bicarbonate ([ 99%), sodium carbonate ([ 99%), potassium chloride ([ 99%) and sodium hydroxide ([ 98%) were purchased from Sigma Aldrich. Sodium phosphate monobasic ([ 98%) and dibasic ([ 99%) were purchased from Fisher Scientific. Sulfuric acid (98 wt%) was purchased from Sigma Aldrich and diluted to desired concentration. The carbohydrates kit (CAR10-1KT) used to calibrate the high-performance liquid chromatography (HPLC) was purchased from Sigma Aldrich and contained mannose, glucose, galactose, xylose and arabinose. The purity of the standards was greater than 98%.

LBG adsorption LBG powder was hydrolyzed in deionized water at a concentration of 0.5 wt% at 98–100 C for 45 min with continuous agitation to produce a solution of polysaccharides with a narrow molar mass distribution. Undissolved gum particles in hydrolyzed LBG stock solution were removed by two rounds of vacuum filtration. The filtrate was further diluted and centrifuged twice at 3500 r.p.m. for 15 min, and the supernatant was recovered for adsorption experiments. The weight-average molar mass of hydrolyzed LBG was measured by a Waters Alliance HPLC coupled with refractive index (RI) detector and Ultrahydrogel 120, 250, and 1000 columns. The calibration standard was a pullulan standard kit (WAT034207). The weight-average molar mass of LBG was 1215 kDa (± 89 kDa standard deviation) in this work.

Cellulose (2021) 28:10183–10201

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Adsorption experiments were conducted in capped 150 mL Erlenmeyer flasks. Control flasks containing only LBG or only fibre were run in parallel. The galactomannan detected in the supernatant of the fibreonly control was 0.14 mg g-1 o.d. fibre (standard deviation of 0.05 mg g-1). All conditions were tested as 2–4 replicates. After adsorption, all samples were centrifuged at 3500 r.p.m. for 15 min. The supernatant was preserved for compositional analysis.

The experimental design for LBG adsorption kinetics and isotherm is summarized in Table 1. The kinetics study was conducted by adding 0.12–0.21 wt% LBG relative to o.d. pulp fibre (0.21wt% at 25 C, 0.14wt% at 35 C, and 0.12wt% at 45 C); the fibre consistency of the slurry was 0.5 wt%. Adsorption was conducted at 25 C, 35 C, and 45 C in an incubator shaker with continuous agitation (150 r.p.m.) for 0.5 to 120 min for NBSK unrefined pulp.

Adsorption isotherm experiments were conducted by varying LBG dosage from 0.1 to 2.1 wt% of o.d. pulp fibre. The PFI mill (Noram Quality Control & Research Equipment Limited) was used to refine the pulp to 3000 rev to compare with unrefined NBSK pulp. Adsorption was conducted at 25 C and 35 C with continuous agitation (150 r.p.m.) for 10 min in the incubator shaker for unrefined NBSK pulp. The isotherm experiments for refined NBSK pulp were conducted by varying LBG dosage from 0.2 to 0.6 wt% of o.d. pulp fibre at 25 C. Table 1 also summarizes the conditions tested to determine the effect of LBG concentration, temperature, salt addition, and pH on adsorption. All trials were performed for 10 min with continuous agitation of 150 r.p.m. Sodium chloride was varied from 0–1 mol L-1 and pH was varied from 2 to 13 in order to test the full range of conditions previously reported

in the literature. The buffer (0.1 M) was prepared with potassium chloride and hydrochloric acid (pH 2), sodium acetate and acetic acid (pH 5), sodium phosphate monobasic and dibasic (pH 7), sodium bicarbonate and sodium carbonate (pH 10), and potassium chloride and sodium hydroxide (pH 13).

LBG solution compositional analysis The galactomannans were hydrolyzed from polysaccharides to monosaccharides with sulfuric acid in an autoclave at 121 C for 1 h according to National Renewable Energy Laboratory Analytical Procedures (Sluiter et al. 2008, 2006; Hames et al. 2008). The galactomannan monomer content in the supernatant was analyzed by Dionex AS50 HPLC (Thermo Scientific) coupled with an ion exchange PA1 column (Dionex), an ED50 electrochemical detector (pulsed amperometric detector) with a gold electrode, and an AS50 autosampler (Dionex). Deionized water was used as eluent with a flow rate of 1 mL min-1. The auxiliary pump added 0.2 M NaOH at 0.5 mL min-1.

The samples were filtered through a 0.22 lm nylon syringe filter before injection. The injection volume was 10 lL. The fraction of LBG adsorbed to the pulp, fL, was determined by the difference of galactomannan content in supernatant, relative to LBG control after adsorption: fL ¼

C0F þ C0L CSL 100% C0L

ð1Þ

where C0F is the galactomannan concentration (mg L-1) detected in supernatant of fibre control flask, C0L is initial LBG concentration (mg L-1),

Table 1 LBG adsorption kinetics and isotherm experimental design for unrefined and refined NBSK pulp. Factors investigated for LBG adsorption: LBG dosage, temperature, NaCl addition, and pH. Note: standard deviation after ± Use

Refining (rev)

Dosage relative to o.d. pulp (wt%)

Adsorption temperature ( C)

Adsorption time (min)

NaCl addition (mol L-1)

pH of sample suspension

Kinetics

0

0.12–0.21

25, 35, 45

0.5–120

0

5.33 ± 0.35

Isotherm

0

0.1–2.1

25, 35

10

0

5.43 ± 0.35

Isotherm

3000

0.2–0.6

25

10

0

5.04 ± 0.41

NaCl effect

0

0.2

25

10

0–1

5.08 ± 0.56

pH effect

0

0.2

25

10

0

2–13

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calculated from LBG control flask and CSL is LBG concentration in supernatant (mg L-1) after adsorption.

The absolute amount of LBG, mL, adsorbed to the pulp (mg g-1 o.d. fibre) was calculated as: mL ¼

C0F þ C0L CSL C0P

ð2Þ

where C0P is the initial o.d. pulp fibre concentration (g L-1).

Adsorption kinetics and isotherms The Langmuir isotherm model assumes ideal monolayer chemisorption on a smooth surface with a finite number of sites (Langmuir 1916; Laidler 1987; Foo and Hameed 2010). The Langmuir isotherm is derived from the equilibrium adsorption reaction of LBG to substrate NBSK pulp fibre:

Ke ½Ce ¼ ð1 þ Ke ½Ce Þh

ð9Þ

Ke ½Ce q ¼ e 1 þ Ke ½Ce Qmax

ð10Þ

h¼

where qe (mg g-1 o.d. fibre) is equilibrium LBG adsorption capacity and Qmax (mg g-1 o.d. fibre) is maximum adsorption capacity. From Eq. 10, when Ce is large, h is approximately equal to 1, representing full coverage of substrate. In contrast when Ce is small, h approaches zero, suggesting limited adsorption and surface coverage. Nonlinear regression of Eq. 10 was conducted by OriginLab 2016 to determine the Qmax and Ke (Tran et al. 2017).

The rate equation for the Freundlich isotherm (Freundlich 1906; Foo and Hameed 2010) is: 1

qe ¼ Kf Cne

ð11Þ

Defining substrate surface coverage as h, then Se can be expressed as Eq. 6:

where Kf is the Freundlich equilibrium constant (mg g-1 (mg L-1)-1/n), qe is the concentration of LBG (mg g-1 o.d. fibre) adsorbed at equilibrium state, n is the Freundlich constant (dimensionless) related to adsorption intensity and Ce is concentration of LBG (mg L-1) in the aqueous phase (Foo and Hameed 2010; Bergmann and Machado 2015). Nonlinear regression of Eq. 11 was conducted by OriginLab 2016 to determine the Kf and n (Tran et al. 2017). The isotherm fitting was assessed by the reduced chisquared (v2) and R2adj as described in Eq. 12 and 13 (Bergmann and Machado 2015).

2 N qi;exp qi;model X ð12Þ v2 ¼ N p i

½Ce Se h¼ ½Se þ ½Ce Se

ð5Þ

R2adj

½Se ¼ ð1 hÞð½Se þ ½Ce Se Þ

ð6Þ

where qi,model is model fitted value of q, qi,exp is experimental value of q, N is the total number of experiments, and p is the number of parameters in the model.

Pseudo-first-order and pseudo-second-order adsorption kinetics (Lagergren 1898; Ho 1995, 2006; Ho et al.

1996; Blanchard et al. 1984; Ho and McKay 2000) are described by Eq. 14 and Eq. 15, respectively:

Ce þ Se $ Ce Se

ð3Þ

where Ce is the equilibrium concentration of LBG (mg L-1) in the aqueous phase, Se is the concentration of empty sites at equilibrium (mg g-1 o.d. fibre) on the surface of pulp fibres, and CeSe is the equilibrium concentration of adsorbed LBG (mg g-1 o.d. fibre) on fibre surface.

When at equilibrium, k1 is the adsorption rate constant (L g mg-2) and k-1 is the desorption rate constant (g mg-1). k1 ½Ce ½Se ¼ k 1 ½Ce Se

ð4Þ

Equations 4, 5 and 6 can be combined to yield Eq. 7: k1 ½Ce ð1 hÞ ¼ k 1 h

ð7Þ

The equilibrium constant Ke (L mg-1) is defined as: Ke ¼

k1 ½Ce Se ¼ k 1 ½Ce ½Se

ð8Þ

Equation 7 and 8 can be combined and further rearranged as Eq. 9. Thus, h could be solved as Eq. 10:

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¼1 1 R

dqt ¼ kð qe qt Þ dt

2

N 1

N p 1

ð13Þ

ð14Þ

Cellulose (2021) 28:10183–10201

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dqt ¼ kð qe qt Þ 2 dt

ð15Þ

where k is rate constant (min-1 in Eq. 14 and g mg-1 min-1 in Eq. 15), qe is the concentration of LBG (mg g-1 o.d. fibre) adsorbed at equilibrium, same as in Eq. 10, and qt is concentration of LBG (mg g-1 o.d. fibre) adsorbed at any time, t (min).

Separating variables of the pseudo-first-order model yields: dqt ¼ kdt qe qt

ð16Þ

Using the boundary condition qt (0 s) = 0 mg g-1 o.d. fibre and integrating, the pseudo-first-order model yields: lnðqe qt Þ ¼ kt þ lnqe

ð17Þ

Separating variables of the pseudo-second-order model yields: dqt ð qe qt Þ 2

¼ kdt

ð18Þ

Once again, using the boundary condition qt (0 s) = 0 mg g-1 o.d. fibre and integrating, the pseudo-second-order model yields: 1 1

¼ kt qe qt qe

ð19Þ

Equation 19 can be rearranged as: t 1 t ¼ þ 2 qt kð qe Þ qe

ð20Þ

For pseudo-first-order adsorption kinetics, plotting ln(qe-qt) as a function of t will yield a line with slope of - k and intercept of lnqe. For pseudo-second-order adsorption kinetics, plotting qt as a function of t will

LBG adsorption for strength analysis An aqueous LBG solution of 0.5 wt% consistency was hydrolyzed at 85 C for 10 min with constant stirring to produce a transparent viscous solution. The PFI mill was used to refine the pulp to 3000–9000 rev at 10wt% fibre consistency. A NBSK pulp suspension of 1.5 wt% fibre consistency was prepared after refining in the pulp disintegrator for 600 counts, which is equivalent to 1500 rev. The LBG solution was then added to NBSK pulp suspension to the desired dosage with manual stirring for 10 min at 25 C. Table 2 summarizes the combinations of LBG addition and refining tested. The treated pulp was next diluted to a fibre consistency of 0.3 wt%. A 2 L sample was collected for freeness testing, and the remaining suspension was used for handsheet making. Two to three replicates were conducted for each condition.

Freeness testing, handsheet preparation and strength analysis Freeness (Canadian standard method) was tested according to Tappi Method T 227. Handsheets with an average grammage of 60 g m-2 were prepared on a wire of 200 cm2 according to Tappi Method T 205. The following handsheet properties were tested: weight and thickness (L&W micrometer), tensile strength (L&W Tensile Strength Tester, Tappi Method T 494), tear index (Elmendorf Tearing Tester, Tappi Method T 414) and burst index (Mullen Tester, Tappi Method T 403). Brightness and scattering coefficient were tested

by Technidyne ColorTouch PC according to ISO 2470–1 and TAPPI T 525.

t

yield a line with slope q1 and intercept of kðq1 Þ2 . e

e

Table 2 Experimental design for investigation into the effects of LBG dosage and pulp refining on paper strength Refining (rev)

Dosage relative to o.d. pulp (wt%)

0

0, 0.1, 0.5, 1

3000

0, 0.1, 0.5, 1

6000 9000

0, 0.1, 0.5, 1 0, 0.1, 0.5, 1

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Results and discussion Adsorption kinetics The fraction of LBG adsorbed to NBSK pulp (Eq. 1) at 25 C is plotted as a function of time with LBG dosage of 0.2 wt% relative to o.d. pulp (Fig. 1). The initial adsorption rate was high with more than 52% adsorption in 0.5 min and 82% adsorption within 5 min. After 10 min, the adsorption fraction plateaued at approximately 93%. Adsorption equilibrium was achieved in 10 min as the LBG adsorption fraction was constant from 10 to 120 min. This result is consistent with previous research that found initial adsorption of hemicellulose is rapid and achieves equilibrium in a few minutes (Zakrajšek et al. 2009; Swanson et al.

1949) when low dosages are applied. Adsorption residence time was maintained at 10 min in all subsequent studies.

Kinetic plots for LBG adsorption are presented in Fig. 2. The poor fit of the pseudo-first-order model (R2 = 0.635) at 25 oC suggests this model cannot describe LBG adsorption kinetics. The pseudo-secondorder kinetics model fit well at all tested temperatures (R2 [ 0.997). The standard error on slopes are relatively small at all temperatures. Slope is used to calculate qe (Eq. 10). The standard error on intercepts increase with increasing temperature. The intercept and qe determined from the slope are used to determine 110

LBG adsorption fraction (fL, %)

100 90 80 70 60 50 40 30 0

20

40

60

80

100

120

Time (min)

Fig. 1 The fraction of LBG adsorbed to NBSK pulp at 25 C as function of time. LBG dosage was 0.2 wt% relative to o.d. pulp and fibre consistency was 0.5 wt%; all runs were conducted with an agitation rate of 150 r.p.m

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the rate constant. The good fit of the pseudo-secondorder model indicates that LBG adsorption is strongly influenced by the concentration of LBG in solution.

The second-order reaction may reflect the potential of a single, high molar mass LBG polymer to form multiple bonds. Pseudo-second-order kinetics have also been observed for adsorption of several dyes and chemicals to pulp fibres (Li et al. 2018; Urruzola et al. 2013; Roy et al. 2013; Vučurović et al. 2012).

The adsorption rate constant and equilibrium adsorption amount at 25–45 C are summarized in Table 3. The rate constant (k) at 45 C is 12 times larger than at 25 C, and 3.7 times larger than at 35 C, indicating that LBG adsorption to NBSK pulp is strongly temperature dependent. The adsorption rate in an agitated pulp suspension depends on turbulent transport and Brownian motion (Zakrajšek et al. 2009; van De Ven 1994). When temperature increases, the collision frequency of particle and fibre increases thus increasing adsorption rate with temperature (Table 3).

The large confidence intervals associated with the rate constant are due to propagation of the uncertainty of the intercepts predicted in Fig. 2.

The equilibrium adsorption capacity (qe), however, decreased with increasing temperature from 25 to 45 C (Table 3). The equilibrium adsorption capacity at 45 C is 25% of that at 25 C. Since the amount adsorbed is a result of competition between adsorption and desorption, the decrease of qe indicates an increase in the escaping capacity of LBG at elevated temperature. To further investigate this temperature effect, adsorption isotherms are discussed below. The activation energy was determined by linear regression of the Arrhenius equation. For LBG adsorption at 25 C to 45 C, the activation energy was 99.34 kJ mol-1 (± 9.85 kJ mol-1, 95% confidence interval) with a pre-exponential factor of 4.76 9 1017 L mol-1 min-1 (Supplementary information Fig. 9). The high activation energy suggests that LBG adsorption to NBSK pulp is a chemisorption process (Laidler 1987). Russo (1959) studied partially methylated LBG adsorption to bleached sulfite pulp and determined the activation energy of adsorption to be 18.4 kJ mol-1 leading Russo (1959) to propose that adsorption is a physical process dominated by diffusion or adsorption via van der Waals forces. The difference between this work and Russo (1959)’s might lie in the agitation. Russo (1959) applied a low agitation rate of 12 r.p.m., while this study applied an agitation rate of 150 r.p.m. When

Cellulose (2021) 28:10183–10201

10191

(b)

(a)

(c)

(d)

Fig. 2 Linear fit of a pseudo-first-order kinetics and b pseudo-second-order kinetics of LBG adsorption to NBSK pulp at 25 C. c Linear fit of pseudo-second-order kinetics of LBG adsorption to NBSK pulp at 35 C and d at 45 C

mass transfer limits are high the activation energy will be low reflecting the diffusion process. However, when agitation rate is sufficiently high, the activation energy will reflect the chemical interaction between LBG and cellulose.

Adsorption isotherms LBG adsorption isotherms were investigated by varying initial dosage of LBG relative to the weight of o.d. pulp fibre. In Fig. 3a the fraction of LBG adsorbed is plotted as a function of dose while Fig. 3b plots the equilibrium concentration of LBG adsorbed as a function of dose. Increasing dosage causes the fraction of LBG adsorbed to decrease but a greater

mass of LBG is retained on the fibre up to a dosage of 0.5 wt%. Given the plateau in mass of LBG adsorbed for dosage between 0.5 and 2.1 wt%, it can be inferred that there is a finite number of adsorption sites on the fibre surface and that LBG adsorption is limited to the fibre surface. This inference is also supported by Wågberg and Hägglund (2001)’s conclusion that polymers with molar mass greater than 48 kDa can only adsorb on the external fibre surface; the weightaverage molar mass of LBG was 1215 kDa (± 89 kDa standard deviation) in this work. Hannuksela et al.

(2002) also observed that the fraction of guar gum adsorbed decreased with increasing concentration of guar gum; they attributed this to slow diffusion.

123

10192

Cellulose (2021) 28:10183–10201

Table 3 LBG adsorption capacity at equilibrium (qe) and adsorption rate constant (k) at temperature range of 25–45 C as determined by fitting pseudo-second-order kinetics.

Equilibrium adsorption capacity, qe, (mg g-1 o.d. fibre)

Temperature ( C)

Rate constant, k, (g mg-1 min-1)

25

1.91 ± 0.08

1.93 ± 1.46

35

1.16 ± 0.03

6.51 ± 7.20

45

0.48 ± 0.01

24.03 ± 45.05

95% confidence limits after ± ; all runs were conducted with an agitation rate of 150 r.p.m.

6

LBG adsorption amount (qe , mg/g pulp)

120

LBG adsorption fraction ( fL, %)

(a) 100

80

60

40

20

0 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Dosage of LBG to o.d. pulp (%)

(b) 5

4

3

2

1

0 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Dosage of LBG to o.d. pulp (%)

Fig. 3 LBG adsorbed to unrefined NBSK pulp fibre as a function of LBG dosage to o.d. fibre after 10 min at 25 C. a The fraction of LBG adsorbed and b the equilibrium concentration of LBG adsorbed. Error bars represent standard deviation of 2–4 trials

The nonlinear regression results of the Langmuir isotherm model and Freudlich isotherm model at 25 C and 35 C with unrefined and refined NBSK pulp are summarized in Table 4. To better illustrate the isotherm fitting, the experimentally determined and predicted adsorption amount, qe, was plotted as a function of aqueous phase LBG concentration at equilibrium (Ce) in Fig. 4. Based on Bergmann and Machado (2015), the model with the best fit will have the lowest reduced chi-squared (v2) and highest R2adj .

From Table 4, Langmuir isotherm fits better at 25 C for unrefined NBSK pulp. At 35 C, both isotherm models fit well with R2adj values close to unity. The temperature effect is discussed in Sect. 3.3.1. For LBG adsorption to refined NBSK pulp (3000 rev) at 25 C, the Freundlich isotherm model better fit the data as demonstrated by v2&0 and R2adj =0.99. This result is discussed in Sect. 3.3.2.

From Table 4 and Fig. 4, it was concluded that LBG adsorption to unrefined NBSK pulp at 25 C is

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consistent with Langmuir adsorption principles, indicating that LBG adsorption is a chemisorption process limited to a finite number of sites. According to the Langmuir model, the maximum adsorption capacity (Qmax) is 2.34 mg g-1 (± 0.20 mg g-1, 95% confidence interval) at 25 C. Similar adsorption capacities to pulp fibre (Table 5) were observed for native LBG (1.8–5.0 mg g-1, Gruenhut 1953) and partially methylated LBG (0.61–12.34 mg g-1, Russo 1959).

These values are much lower than adsorption capacity for cationic starch (20–66 mg g-1, Table 5). However, the capacity for native corn starch adsorption to pulp was 1.1–4.5 mg g-1 (Cushing and Schuman 1959).

Since the charge density of native starch is almost as low as that of LBG, it can be concluded that high positive charge density leads to high adsorption capacity.

The equilibrium constant (Ke) calculated from the Langmuir model is approximately 2.82 L mg-1 (± 1.73 L mg-1, 95% confidence interval) at 25 C. The large confidence interval reflects the limited

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Table 4 Summary of nonlinear regression results for Langmuir isotherm and Freundlich isotherm at 25 C and 35 C with unrefined and refined NBSK pulp.

Isotherm

Temperature ( C)

Freundlich

R2adj

Qmax (mg g-1)

Ke (L mg-1)

Reduced chi-squared v2 (mg2 g-2)

25

0

0.46

2.34 ± 0.20

2.82 ± 1.73

0.13

35

0

0.95

6.31 ± 0.75

0.16 ± 0.06

0.16

25

3000

0.73

3.87 ± 1.28

3.75 ± 6.50

0.33

Temperature ( C)

Refining (rev)

R2adj

n

Kf (mg g (mg L-1)-1/n)

Reduced chi-squared v2 (mg2 g-2)

Langmuir

Isotherm

Refining (rev)

-1

25

0

0.38

12.82 ± 7.09

1.70 ± 0.25

0.15

35

0

0.94

2.71 ± 0.50

1.48 ± 0.31

0.19

25

3000

0.99

5.00 ± 0.04

2.57 ± 0.01

0.00

95% confidence limits after ±

7

3.5

(b) 35oC unrefined

3.0

6

2.5

5

qe (mg/g pulp)

qe (mg/g pulp)

(a) 25oC unrefined

2.0 1.5

4 3 2

1.0

Experimental Langmuir Freundlich

0.5

Experimental Langmuir Freundlich

1 0

0.0 0

20

40

60

80

0

100

10

20

30

40

50

Ce (mg/L)

Ce (mg/L) 5

(c)

25 oC refined

qe (mg/g pulp)

4

3

2

Experimental Langmuir Freundlich

1

0 -2

0

2

4

6

8

10

12

14

Ce (mg/L)

Fig. 4 Nonlinear fit of Langmuir model and Freundlich model with experimental results at a 25 C and b 35 C of LBG adsorption to unrefined NBSK pulp, and at c 25 C of LBG adsorption to refined NBSK pulp (3000 rev)

123

123

Cushing and Schuman

Russo

Gruenhut

1959

1959

1953

Locust bean gum

Partially methylated LBG

Native corn starch

Cationic starch

Rosin sized kraft pulp

Bleached sulfite pulp

Bleached sulphite pulp

Unbleached black spruce TMP

Hardwood sulphate short fibres

Northen bleached softwood kraft pulp

Adsorbent

1.8–5.0*

0.61–12.34*

1.1–4.5*

20

N.A

N.A

N.A

N.A

40

2.82 ± 1.73

2.34 ± 0.20 66

Equilibrium rate constant, Ke (L mg-1)

Maximum adsorption capacity, Qmax (mg g-1)

*No isotherm fitting, therefore, adsorption amount range is indicated

95% confidence limits after ±

Shirazi et al

2003

Cationically modified starch

Locust bean gum

This work

Zakrajšek et al

Adsorbate

Author

2009

Year

Table 5 Summary of adsorption capacity and isotherms of polysaccharides adsorption to pulp fibre.

22

25

91–96

N.A

25

25

Temperature ( C)

N.A

6.5

5.0–5.1

N.A

12

N.A

200

500

7.5 ± 0.2

5.2

150

Agitation rate (r.p.m.)

5.2–5.8

pH

N.A

N.A

N.A

Langmuir

Modified Langmuir

Langmuir

Isotherm model

10194 Cellulose (2021) 28:10183–10201

Cellulose (2021) 28:10183–10201

10195

temperature. At high LBG dosage levels, the amount of LBG adsorbed to fibre at 35 C is much higher than that at 25 C. The amount of LBG adsorbed to refined pulp at 25 C is comparable to the amount adsorbed to unrefined pulp at 35 C. When LBG concentration in solution increased ([ 0.5wt%), the adsorption amount differentiates depending on temperature and surface condition of NBSK pulp.

From Table 4 and Fig. 4, it was found that adsorption at 25 C is best described by the Langmuir model while adsorption at 35 C can be described by either the Langmuir model or Freundlich model. From the Langmuir model, the maximum adsorption capacity (Qmax) of pulp fibres at 35 C was calculated to be 6.31 mg g-1 (± 0.75 mg g-1, 95% confidence interval) o.d. pulp. This is greater than the Qmax determined at 25 C, 2.34 mg g-1, suggesting that more surface sites are accessible at higher temperature. However, increasing temperature could hardly increase the total surface area of pulp fibres. Given the good fit of both models, neither can be definitively selected. Instead, we suggest that an alternate mechanism may be involved. There may be multi-layer adsorption at 35 C, and the adsorption occurs layer-by-layer (a pseudo-Langmuir mechanism). By this reasoning, the number of sites in each adsorption layer is constant and the increase in predicted Qmax at 35 C reflects the presence of multiple layers.

number of data points used to prepare the model. The equilibrium constant for cationic starch adsorption to pulp fibre is much higher, for example Zakrajšek et al.

(2009) reported Ke = 40 L mg-1. This high equilibrium constant could be due to the high concentration of adsorbed cationic starch resulting from the natural attraction between negatively charged pulp fibres and cationic starch. Unlike cationic starch, LBG is a natural carbohydrate polymer with a negative surface charge (de Jong and van de Velde 2007). The repulsive forces between LBG and NBSK pulp fibre will reduce adsorption capacity. Consequently, LBG adsorption to unrefined NBSK pulp is characterized by low adsorption capacity and a relatively low equilibrium constant.

Factors influencing adsorption Effect of temperature The influence of temperature on LBG adsorption to NBSK pulp fibre at two temperatures, 25 C and 35 C on o.d. fibre, is reported in Fig. 5a. At low dosage levels (\ 0.5wt%), LBG adsorption amount is comparable at both temperatures; the adsorption amount to refined pulp at 25 C is similar to the unrefined materials. At low dosage, it appears that LBG affinity to NBSK pulp fibre is not affected by

1.4

6

(a)

(b)

25oC unrefined 35oC unrefined 25oC refined

5

1.2

Site coverage

qe (mg/g pulp)

1.0 4

3

0.8 0.6 0.4

2

0.2

25oC unrefined 35oC unrefined

1

0.0 0.0

0.5

1.0

1.5

2.0

LBG dosage of o.d. pulp (%)

2.5

0.0

0.5

1.0

1.5

2.0

2.5

LBG dosage of o.d. pulp (%)

Fig. 5 a Amount of LBG (mg g-1 o.d. pulp) adsorbed to NBSK pulp after 10 min as a function of LBG concentration in aqueous solution (mg L-1) at 25 C, 35 C for unrefined and refined NBSK pulp. b Fibre surface site coverage (h) calculated from Langmuir model at 25 C and 35 C for unrefined NBSK pulp

123

10196

Effect of refining

heterogeneous surface. The calculated adsorption constant (n) is 5.00 (± 0.04, 95% confidence interval), and Kf the equilibrium constant is 2.57 mg g-1 (mg L-1)-1/n (± 0.01 mg g-1 (mg L-1)-1/n, 95% confidence interval). This change in adsorption isotherm relative to the unrefined NBSK pulp is likely a result of the heterogeneity of the fibre surface created by fibrillation during refining.

Effect of salt addition LBG adsorption to NBSK pulp fibre in response to sodium chloride addition was investigated at 25 C for 10 min with LBG dosage 0.2 wt% relative to o.d. fibre; sodium chloride concentration was varied from 0–1 mol L-1 (Fig. 6). The LBG adsorption fraction was approximately 85–100% and independent of sodium chloride concentration at concentrations from 0–1.0 mol L-1 (Fig. 6). The small effect of sodium chloride might due to the small electrostatic repulsion between LBG and fibre and the low negative charge density on LBG (de Jong and van de Velde 2007). This trend is consistent with Hannuksela et al. (2002)’s research that guar gum and GGM adsorption were unaffected by addition of less than 0.1 mol L-1 sodium chloride.

110 100

LBG adsorption fraction (fL, %)

Adsorption–desorption equilibrium is a dynamic process. Surface site coverage will vary with adsorption conditions. The Langmuir equilibrium constant (Ke) was determined to be 0.16 L mg-1 (± 0.06 L mg-1, 95% confidence interval) at 35 C, which is lower than Ke = 2.82 L mg-1 at 25 C. This decrease could be caused by lower concentration of adsorbed LBG ([CeSe], Eq. 8) and/or increased surface site concentration ([Se], Eq. 8). From Fig. 5a, the equilibrium adsorption amount (qe) increased with increasing temperature at higher dosage levels, indicating that the adsorbed LBG concentration (CeSe) increases with increasing temperature. Thus, the lowered Ke value at 35 C could due to the increased surface site concentration.

Site coverage on unrefined NBSK pulp increased rapidly at LBG dosages less than 0.2wt% at 25 C (Fig. 5b). However, the dependence on dosage diminishes as full coverage is approached. At 35 C, the reduced dependence on dosage could result from increased maximum adsorption capacity (Qmax). Past studies have shown paper tensile strength improvement is not proportional to hemicellulose dosage, and the increase in paper tensile strength diminishes as dosage is increased. Hannuksela et al. (2004) reported GGM dosage of 0.8 wt% o.d. fibre increased tensile strength by 13% but a higher dosage of 1.6 wt% o.d. fibre caused only a small additional increase in tensile strength. The most significant improvement in tensile strength was achieved at GGM dosage less than 0.1 wt% dried fibre (Hannuksela et al. 2004). Our results help explain Hannuksela et al. (2004)’s observations.

When LBG dosage was approximately 0.12 wt%, the coverage of fibre sites at 25 C was 0.52 (Fig. 5b). Full coverage was obtained when the dosage was increased to 2.12 wt% of o.d. pulp fibre. As the fibre surface becomes saturated, it is likely that improvement in inter-fibre bonding will be limited leading to small gains in paper tensile strength, even after addition of excess LBG.

Cellulose (2021) 28:10183–10201

90 80 70 60 50 40 30

Table 4 and Fig. 4c clearly demonstrate that the adsorption to lightly refined pulp at 25 C is best described by the Freundlich model. Adsorption to unrefined pulp at 25 C was best described by the Langmuir model. The Freundlich model is used to describe multi-layer adsorption on a non-uniform and

123

20 0.000

0.005

0.010

0.2

0.4

0.6

0.8

1.0

NaCl concentration (mol/L)

Fig. 6 The adsorbed fraction of LBG on NBSK pulp fibre with varying sodium chloride concentration 0–1.0 mol L-1 after 10 min at 25 C, LBG dosage 0.2 wt% of o.d. fibre

Cellulose (2021) 28:10183–10201

10197

LBG adsorption amount (mL, mg/g pulp)

3.2 2.8 2.4 2.0 1.6 1.2 0.8 0.4 0.0 2

4

6

8

10

12

14

Adsorption pH

Fig. 7 The adsorbed amount of LBG on NBSK pulp fibre with varying pH of pulp suspension after 10 min at 25 C, LBG dosage 0.2 wt% of o.d. fibre

Effect of pH LBG adsorption to NBSK pulp was investigated after 10 min at 25 C with LBG dosage 0.2 wt% of o.d. fibre (Fig. 7) while buffer pH varied from 2 to 13. The amount of LBG adsorbed ranged from 1.8–2.7 mg g-1 of o.d. fibre with maximum standard deviation of 0.26 mg g-1 of o.d. fibre (Fig. 7), indicating adsorption was not strongly affected by pH. Adsorption was slightly higher at pH 2 and 5 due to the undissociated hydroxyl and carboxyl groups on LBG and reduced repulsive forces. At high pH, hydroxyl and carboxyl groups deprotonate more easily increasing repulsive forces between fibre and LBG thus lowering adsorption. However, the differences are small due to LBG’s low negative charge density on LBG. Hannuksela et al. (2002) reported that pH (5 and 8) had no influence on guar gum adsorption to softwood kraft pulp fibre. This observation is probably due to guar gum’s weak negative charge and the limited range of pH tested.

Paper strength enhancement by LBG adsorption Pulp and paper properties including tensile, burst, tear index, scattering coefficient, brightness, and pulp freeness were plotted as a function of PFI refining revolutions and LBG dosage in Fig. 8. Adsorption was conducted after refining. Refining and dosage positively influenced tensile strength and burst strength.

Tear strength, freeness, scattering coefficient, and brightness, in contrast, decreased with increasing refining or dosage level.

It is well-established that refining increases NBSK paper strength (Fig. 8). Tensile index and burst index increased with refining level until 6000 rev and then plateaued. Refining to 9000 rev without LBG adsorption doubled the tensile index of unrefined NBSK paper. However, as highlighted by Leech (1954), paper strength does not increase continuously with bonding strength as at higher levels of bonding, handsheet strength becomes more dependent on individual fibre strength. Leech (1954) concluded that tensile and burst strength of bleached sulfite pulp plateau after refining to approximately 6000 rev, which is consistent with the results in Fig. 8a, b.

Mechanical refining had a much greater effect on tensile strength than LBG adsorption. Miletzky et al.

(2015) similarly reported that the effects of hemicellulose addition are minimal if high refining is applied.

In this work, the maximum increase in tensile index due to adsorption of LBG to unrefined pulp was 20.1%. In comparison, tensile index after refining was approximately double that of unrefined pulp. However, LBG adsorption can reduce the degree of refining and energy required to reach a target tensile index. For example, pulp must be refined to 2000 rev to reach a tensile index of 80 N m g-1 without LBG adsorption but only 1000 rev were required to achieve the same index if the pulp is treated with 1wt% LBG. Swanson (1950) likewise reported that refining time was reduced by approximately 70% with addition of 0.5 wt% LBG. Burst strength demonstrated a similar trend as tensile strength (Fig. 8b). The maximum burst index also doubled after applying 1 wt% LBG to highly refined pulp (9000 rev). Enhanced tensile index and burst index could be due to either increased bonding area or an increased number of bonds and bond strength (Leech 1954). Similar trends were reported by Swanson (1950).

As expected, tear index decreased with refining and LBG addition possibly due to stronger bonding between fibres (Fig. 8c). At 9000 rev, tear index decreased 62.8% after adsorption with 1 wt% LBG. A drop in tear index normally accompanies increased tensile strength (Hannuksela et al. 2004; Leech 1954; Swanson 1950). Tearing occurs due to the breakage of individual fibres or individual fibres being pulled from

123

10198 Fig. 8 NBSK pulp and paper properties as a function of LBG dosage and PFI refining level: a tensile index, b burst index, c tear index, d freeness, e scattering coefficient and f brightness. LBG adsorption was conducted at 25 C for 10 min

Cellulose (2021) 28:10183–10201

(a)

(c)

(d)

(e)

(f)

the sheet matrix. At higher bonding levels, individual fibres will break and this requires less energy than removing fibres from the sheet matrix (Hannuksela et al. 2004; Leech 1954; Swanson 1950).

Freeness is a measurement of pulp drainage (Smook and Kocurek 1982) and reflects fines content, flexibility and the degree of external fibrillation (Niskanen 1998). With increasing refining energy and LBG adsorption, freeness decreases (Niskanen 1998). With the application of 1 wt% LBG to NBSK pulp refined at 3000 rev a tensile strength of 108 N m g-1 could be achieved at a freeness of 531 mL (Fig. 8d). To achieve a similar tensile

123

(b)

strength solely by refining, required 9000 rev and resulted in the lower freeness of 434 mL. Thus, LBG adsorption resulted in better drainage properties at equal tensile strength with less refining energy.

Scattering coefficient decreased with refining level and LBG dosage (Fig. 8e), which suggests more bonding occurs after LBG adsorption and refining.

The greatest change in scattering coefficient on LBG adsorption, a 14% decrease, occurs between unrefined pulp without LBG and unrefined pulp with 1wt% LBG. Increasing refining level diminished the impact of LBG dosage on scattering coefficient (Fig. 8e), indicating that refining is the dominant factor for

Cellulose (2021) 28:10183–10201

bonding. At 6000 rev and 9000 rev, the scattering coefficients are independent of LBG dosage, thus indicating there is a finite degree to which bonding can be improved. The overall trends of scattering coefficient agree with the trend in tensile strength with respect to refining and LBG adsorption. Consequently, NBSK paper tensile strength enhancement is mainly due to bonding formation.

Brightness decreased with increased refining and LBG dosage level (Fig. 8f). However, as the chromophores in the pulp are not changed and the LBG is colourless, the main reason for the decrease in brightness must be the reduction in the scattering coefficient arising from increased bonded area (Ek et al. 2009; Parsons 1941; Simmonds and Coens 1967).

Conclusion Locust bean gum adsorption and its performance as a strength additive was studied. The adsorption rate followed pseudo-second-order chemisorption kinetics. The adsorption rate constant increased rapidly with temperature from 25 to 45 C, but the amount adsorbed at equilibrium decreased.

Langmuir and Freundlich models were used to describe LBG adsorption with equal fit and both were used to gain physical insight. The maximum LBG adsorption capacity of NBSK pulp fibre was comparable to that of native starch to pulp. The mechanism of LBG adsorption to cellulose fibres is complex and may involve multi-layer adsorption to a finite number of sites. Refining to 3000 rev increased surface heterogeneity of NBSK pulp as evidenced by the excellent fit of the Freundlich model.

Increasing temperature from 25 to 35 C caused LBG adsorption to increase at dosage level higher than 0.5wt%. Sodium chloride addition (0–1.0 mol L-1) had little effect on adsorption and adsorption increased slightly at pH 2–5. Both observations are likely due to the low negative charge density on LBG. Refining and LBG dosage increased NBSK paper tensile strength and burst strength. Tensile and burst strength plateaued when refining over 6000 rev, and strength gains were small for LBG dosage greater than 0.5 wt%. However, addition of LBG enabled a reduction in refining revolutions to achieve a target tensile strength with higher freeness compared to

10199

solely refining. Tear index, brightness and scattering coefficient decreased, likely due to greater inter-fibre bonding.

Acknowledgments The authors acknowledge Canfor Pulp University Grants Program and the Natural Sciences and Engineering Research Council of Canada Collaborative Research & Development Grant (Grant CRDPJ 462081-13) for financial support. This research was conducted as part of the UBC BioProducts Institute portfolio administered from the University of British Columbia.

Declarations Conflicts of interest financial interest.

The authors declare no competing

Human and animal rights This article does not contain any studies with human or animal subjects performed by any of the authors.

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 PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 3, 2021

Influence of coating grammage on the utility properties of coated papers 1 2 ALEKSANDRA NISKA & EDYTA MAŁACHOWSKA .

The paper coating offers the opportunity to create a product with high added value, the potential of which has been recognized by both paper mills and polygraphs. Indeed, paraffin coatings have excellent barrier, sliding and strength properties, and also increase the quality and durability of the print.

This paper presents the results of the research on the influence of the coating weight on the functional properties of the coated paper. To this end, a commercially available paper was coated with a paraffin emulsion using various Mayer rods and then tested using standard mechanical, surface, and water absorption tests. It was found that the coating of the base paper, regardless of the amount of the applied mixture, significantly influences its hydrophobic, surface, and strength properties. Papers with the highest coating weight allowed to obtain a paper with increased strength and high surface smoothing. The completed coatings significantly increased the water barrier, regardless of their thickness Contact information: 1. Faculty of Wood Technology, Warsaw University of Life Sciences - SGGW, Warsaw, Poland 2. Institute of Wood Sciences and Furniture, Warsaw University of Life Sciences – SGGW, Warsaw, Poland Annals of Warsaw University of Life Sciences – SGGW Forestry and Wood Technology № 113, 2021: 5-12 (Ann. WULS - SGGW, For. and Wood Technol. 113, 2021: 5-12) Creative Commons license - Non-commercial use 3.0 Poland

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 9

Article 2 – Coat Weight & Paper Properties

Annals of Warsaw University of Life Sciences – SGGW Forestry and Wood Technology № 113, 2021: 5-12 (Ann. WULS - SGGW, For. and Wood Technol. 113, 2021: 5-12)

ALEKSANDRA NISKA1, EDYTA MAŁACHOWSKA2

Influence of coating grammage on the utility properties of coated papers 1 2

Faculty of Wood Technology, Warsaw University of Life Sciences - SGGW, Warsaw, Poland Institute of Wood Sciences and Furniture, Warsaw University of Life Sciences – SGGW, Warsaw, Poland

Abstract: Influence of coating grammage on the utility properties of coated papers.The paper coating offers the opportunity to create a product with high added value, the potential of which has been recognized by both paper mills and polygraphs.Indeed, paraffin coatings have excellent barrier, sliding and strength properties, and also increase the quality and durability of the print.

This paper presents the results of the research on the influence of the coating weight on the functional properties of the coated paper. To this end, a commercially available paper was coated with a paraffin emulsion using various Mayer rods and then tested using standard mechanical, surface, and water absorption tests. It was found that the coating of the base paper, regardless of the amount of the applied mixture, significantly influences its hydrophobic, surface, and strength properties. Papers with the highest coating weight allowed to obtain a paper with increased strength and high surface smoothing. The completed coatings significantly increased the water barrier, regardless of their thickness Keywords: paper coating; paper hydrophobic; paper utility properties; Mayer rod; papermaking technology

INTRODUCTION The coating process consists of applying a coating mixture to the surface of the paper product to improve the functional properties of the paper or for other specialized applications, depending upon the type of mixture. The coating mixture on the surface of paper leads to improvement in the printability of the paper, it also enables the temporary exposure of packaging papers to the weather conditions or reduces the penetration of liquids and fats into packages intended for contact with food. The composition of coating mixtures may differ for various applications it depends mainly on the printing technique for which the coated paper products are intended and the coating technique, and in particular the blend concentration, that can be used in a given coating technique.Depending on the intended use of the paper, it can be coated single or both sides.

In the paper industry, for the coating of cartons, ready-made packages, and cardboard, emulsions based on high-melting polyethylene waxes or ethylene homopolymers and copolymers are used.These emulsions show excellent stability, a high degree of fragmentation of the wax particles to a size below 0.5 μm [1], as well as appropriate utility parameters, consisting in the production of wax coatings with an excellent barrier, sliding, and strength properties.The produced coatings are transparent, which increases the quality and durability of the print [2,3,4].

The paraffin emulsion coatingis a certain barrier to water, as it has a much lower porosity than the substrate. In the case of printing papers, excessive liquid absorption is most often associated with poor image reproduction. Such a layer, therefore, enables a clear and defect-free print to be obtained with a much lower ink requirement than in the case of paper without such a coating. Paper-based materials produced without hydrophobising agents exhibit natural hydrophilicity, i.e. the ability to absorb water, both in liquid form and from water vapor.

The rate of absorption and the amount of water that the paper can absorb in a given time depends, among others, on the ingredients of the paper and its structure, the moisture content of the paper, the degree of water purity, or its temperature [5]. Most often, this is a disadvantageous phenomenon, because the absorption of liquid by the paper causes changes in 5

paper dimensions (deformation) and deterioration of mechanical properties, which is important, especially for packaging papers. From the utility point of view, depending on the intended use, the coated paper should therefore have a strictly defined hydrophobicity, and thus the assessment of water absorption by the paper is one of the key methods of assessing the performance of paper.

The appearance of the coating and its thickness determine the final appearance of the paper product.Coating grammage primarily determines the functional properties of the coated paper, which undoubtedly include its hydrophobicity, but also strength properties. Therefore, this study investigated how the coating grammage influences the hydrophilicity as well as the surface and strength properties of the coated paper.

MATERIALS AND METHODS For the tests, the commercially available Mondi ProVantage Kraftliner paper with a basis weight of 125 g/m² and the coating mixture in the form of a paraffin-water emulsion was used. Before the coating process, paper samples were cut from the rolls and subjected to air conditioning at 23°C and 50% relative humidity, according to ISO 187:1990, for a minimum of 24 hours.

The coating process was carried out with the use of Mayer rods of different numbers: 4, 5, 6, 7, 8, 9, 12, and 15, to vary the coating grammage. The thickness of the coating increased with the developing number of rods. The photo of one of used rods is shown in the drawing (Fig. 1). After the coating process, the coated paper samples were dried in a thermal research chamber (Wamed KBC-65W) at a temperature of 60°C. Thereafter, the coated paper samples were conditioned at 23°C and 50% relative humidity according to ISO 187:1990 for a minimum of 24 hours.

Figure 1.Mayer Rod

After the samples were conditioned, the following utility properties of the paper were tested: 1. The Degree of hydrophobicity and/or hydrophilicity of paper, according to ISO 535:2014 (water absorption by Cobb60 method) - Cobb apparatus (Danex, Katowice, Poland); 2. The Air permeability, according to ISO 5636-3:2013 - Bendtsen apparatus (Kontech, Lodz, Poland); 3. The Roughness of the coated paper surface, in accordance with ISO 8791-2:2013) – Bendtsen apparatus (Kontech, Lodz, Poland); 4. The Priority strength properties of the base papers, in accordance with ISO 1924-2:2010 - Zwick 005 ProLine testing machine (Zwick-Roell, Ulm, Germany), coupled with testXpert III software.

The tensile paper properties were examined as follows: – breaking length [m]; • IB b • VT – width related force with break [N/m]; W • VT – force at break index [Nm/g]; 6

• HT – strain at break [%]; • WTb – energy absorption [J/m2]; • WTW – energy absorption index [J/g]; – tensile stiffness [kN/m]; • Eb w – tensile stiffness index [Nm/g]; • E • E* – Young’s modulus [MPa]; – tensile force at break [N].

• FB The detailed statistical analysis was performed for individual research series, determining the basic indicators - arithmetic mean, extended deviation and percentage relative error.

RESULTS AND DISCUSSION The obtained results of the hydrophobicity of the tested samples are shown in the drawing (Fig. 2). The initial Cobb60 water absorption value for the base paper prior to the coating process was 47 g/m². On the other hand, for the tested samples of papers coated with a paraffin mixture, Cobb's value was in the range of 0.33 ÷ 5.12 g/m² and decreased with the the increasing thickness of the coating applied to the base paper. Consequently applying a paraffin coating to the paper allowed to reduce water absorption by as much as 99 ÷ 89%, depending on the coating weight. Interestingly, changing the rod number by 11 units causes only a 10% difference in the water absorption value. Therefore, regardless of the number used in the Mayer rod research, coating the paper with a mixture in the form of a paraffin-water emulsion reduces the rate of water penetration into the coated substrate. Such a significantly increased hydrophobicity causes a significant development in the usability of the paper, especially in the packaging and printing sectors.

Cobb Water Absorption, g/m²

6.0 5.0 4.0 3.0 2.0 1.0 0.0 3

4

5

6

7

8

9

10

11

12

13

14

15

16

Mayer Rod Number Figure 2. Influence of coating grammage on the Cobb water absorption

Printing papers should also be suitable for printing on the surface. The roughness of the paper (or its smoothness) determines the susceptibility of the surface to printing. The roughness/smoothness significantly affects also many other functional and processing values of the finished product. Table 1 shows the results obtained for the coated surface of the tested papers. The initial roughness value for the base paper before the coating process was 489 ml/min, while for the coated samples it ranged from 149 to 272 ml/min. Thus, the obtained results indicate that the increase in the thickness of the coating, as a result of using the Mayer rod with a higher number, causes the expected surface smoothing of the coated papers by 70 ÷ 44%. It is important in the case of printing because the increase in smoothness allows to make

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prints with a greater degree of accuracy and sharpness. The practical conclusion is that only onigh-smoothness papers it is possible to accurately print an image with a high screen ruling.

Table 1.Structural properties of paper

Roughness ml/min 272 4 (15) 230 5 (21) 295 6 (11) 213 7 (16) 180 8 (10) 183 9 (13) 176 12 (8) 149 15 (11) Note: Extended deviations are given in brackets.

Mayer Rod number

Air permeability ml/min 80 (7) 68 (4) 47 (4) 20 (3) 12 (4) 11 (4) 7 (2) 7 (2)

The thickness of the coating significantly influenced another, extremely important structural property of the paper, which is air permeability. The high air permeability is a desirable property in dustproof cartons and filter papers. However, many wrapping papers, especially food packaging paper should have a very low air permeability.

On the basis presented results, it can be observed that the coating process with the use of the coating mixture significantly influences the structure of the tested base paper. Due to the fact that air permeability depends on the porosity and density of the article structure, an increase in the thickness of the coating caused a decrease in air permeability. The starting value of the parameter for the base paper before the coating process was 202 ml/min. The coated paper samples were characterized by air permeability within the range of 7 ÷ 80 ml/min, which is as much as 97 ÷ 60% lower. Therefore, the change of the Mayer rod number by several numbers caused in this case significant differences in the value of the considered parameter.

The level of air permeability indicates not only the porosity of the product but also other properties of the paper, including strength properties. The results of testing the strength parameters of the coated papers are presented in Table 2. It should be emphasized that the presented results were determined for the machine direction (MD). The strength parameters for this direction are of real practical value because in this direction the processing processes are carried out.

Te mesurement of breaking length of paper is one of the most popular methods recommended for comparing the static strength properties of the tested papers. The parameter is generally used in the paper trade to describe the inherent strength of paper. Futhemore it constitutes a very good basis for comparing the strength of papers made from different materials and of different basis weight.The breaking length is a widely used indicator because it allows 8

the estimate the usable properties of many products. This is particularly important for the evaluation of the usefulness of packaging and newsprint papers.

Figure 3 shows a comparison of the breaking length of papers with different thicknesses of the coating. The initial value of the breaking point for the base paper before the coating process was 12 650 m. In the grounds on the presented results (Fig. 3), it can be observed that the coating process using most of the Mayer rods used (No. 4 - 9) caused a decrease in the value of the breaking length of the paper by 1 ÷ 14%. Only two samples with the highest coating weight increased the parameter value by an average of 6%. On this basis, it can be concluded that the breaking length capacity of the coated paper decreases to a certain coating thickness. Only a relatively high development in the coating grammage causes an increase in the tear resistance properties.

14000

Breaking length, m

13500 13000 12500 12000 11500 11000 10500 3

4

5

6

7

8

9

10

11

12

13

14

15

16

Mayer Rod Number Figure 3. Influence of coating grammage on the breaking length of paper

Another analyzed index characterizing the functional properties of coated paper samples was the width related force with break (VTb), is one of the basic static strength properties and determines the value of the breaking load causing a 1 m wide sample of a paper to break.

The value of the VTb for the base paper before the coating process was 15 490 N/m, while after coating it was within the range of 13 700 ÷ 18 570 N/m. Interestingly, applying the coating with some Mayer rods resulted in a decrease in the parameter by 4 ÷ 12% (rods No. 4 7), or its increase to 20% (rods No. 8 - 15). Regardless of that, the increase in the coating grammage caused a commensurate increase in the width related force with break (Tab. 2).

The force at break index (VTW) was also determined, i.e. the value of the width related force with break that a hypothetical 1 g/m² paper sample would achieve. The force at break index is an extremely practical parameter because it allows to carry out free and reliable comparison of coated paper samples, both for a wide range of base papers of different weights and for paper with coatings of different weights. Therefore, the presented property allows the coated sample of the paper to be assessed in terms of its material properties.

The base paper used for the coating process had an initial value of the force at break index at the level of 123.8 N/mg, while the coated samples were 105.9 ÷ 133.2 N/mg. Thus, depending on the grammage of the coating, an increase or decrease of the considered parameter was observed (Tab. 2).

The same characteristics of the test specimen as the width related force with break represent the tensile force at break (FB). This parameter is also one of the basic static properties that characterize the performance of paper products. It specifies the limit value of the stress which, if exceeded, will break the tested sample of the coated paper. As in the case of breaking 9

length, this property is important Because it has a decisive influence on the behaviour of the paper web during its high-speed processing (e.g. during the coating process on individual sections of the coater). A desirable feature for coated paper is the tensile force at break parameter as high as possible. The lower the value of the paper tensile force at break, the greater the probability of a break during the operation of a drying process or a coated web winding operation.

The uncoated backing paper had an initial tensile force at break of 155.6 N. This parameter for the coated samples was in the range of 136.4 ÷ 183.0 N. Similarly to the width related force with break, the coatings applied with some Mayer rods caused a decrease in the tensile force at break of the paper to 12% (rods no. 4 - 7) or an increase of the parameter value by 3 ÷ 18% (rods no. 8 - 15) (Tab. 2).

The strain at break of paper (HT), defined as a measure of deformation under tensile stress, up to the point where the paper sample breaks, significantly affects the behaviour of coated paper under production conditions paper.

The initial strain at break for the base paper prior to the coating process was 2.53%.

Coating the samples, regardless of the thickness of the applied coating, increased the stretchability of the paper by 6 ÷ 14% (Tab. 2).

The consensus between tensile force at break and strain at break is the energy absorption parameter (WTb). The energy absorption describes the resistance of the test paper to the dynamic load acting on it. The higher the values of the energy absorption the tested paper achieves, the more resistant it is to the dynamic loads acting on it.

The initial value of the energy absorption for the base paper before the coating process was 245.4 J/m². On the basis of obtained results, it can be observed that only the use of the lowest number rod resulted in a slight decrease in the parameter. The remaining samples coated with rods no. 5 - 15, were characterized by higher values of the energy absorption by 2 ÷ 31% (Tab. 2). The increase in the value of the energy absorption with the increase of the coating grammage was also observed.

Taking into account different weights of coatings, the analysis of the energy absorption of the samples was extended to include the energy absorption index, i.e. the work necessary to break the tested paper sample with respect to its weight. The initial value of the considered parameter for the base paper before the coating process was 1.96 J/g. The analysis of the results showed an increase in the energy absorption index for the samples with the highest grammage of the coating by 9 ÷ 22%. The samples with lower coating thicknesses (coated with rods no. 4 - 8) showed the decrease of the considered parameter not exceeding 9% (Tab. 2).

For the analyzed samples, the tensile stiffness, i.e. the resistance of the paper samples to deformations caused by external tensile forces, was also determined. The initial value of the tensile stiffness for the base paper was 1,290 kN/m, while for the coated samples it was within the range of 1,098 ÷ 1,438 kN/m (Tab. 2). Depending on the Mayer rod used, the application of the coating compound to the paper resulted in a decrease or increase in the parameter value.

It was also observed that with the increase of the coating grammage, the tensile stiffness of the sample increased.

In the case of the tensile stiffness index, a decrease in the parameter was observed for most of the tested samples by 6 ÷ 18%. Only for the papers with the highest grammage of the coating, coated with rods no. 12 and 15, a very slight increase in the value of the considered parameter was observed (Tab. 2).

The last parameter tested for papers coated with various mixtures was Young's modulus of elasticity. This parameter characterizes the ability to distribute stresses acting on the material and describes the ability of an elastic body to resist deformation when stretched. The lower the value of Young's modulus, the better the tested paper has the ability to distribute stresses acting on it.

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The base paper used for the tests was characterized by the initial value of Young's modulus at the level of 11,650 MPa. The coating of the paper caused an increase or decrease in the value of the considered parameter, depending on the grammage of the coating. The use of rods with numbers 4 - 7 contributed to a decrease in Young's modulus by 7 ÷ 14%. On the other hand, for rods 8 – 15 an increase in the parameter by 2 ÷ 12% was observed. Importantly, the increase in the coating grammage caused a proportional increase in Young's modulus (Tab. 2).

Table 2. Tensile properties of paper

Mayer Rod number

V Tb

V TW

FB

HT

WTb

WTW

Eb

Ew

E*

N/m

Nm/g

N

%

J/m2

J/g

kN/m

Nm/g

MPa

232,0 (20,4) 249,7 (19,1) 250,4 (16,3) 249,7 (19,1) 261,6 (17,3) 293,5 (18,1) 322,6 (19,6) 321,4 (25,4)

1,79 (0,16) 1,91 (0,15) 1,87 (0,12) 1,91 (0,15) 1,91 (0,12) 2,13 (0,13) 2,40 (0,15) 2,31 (0,18)

1098 (26) 1187 (15) 1145 (25) 1187 (15) 1303 (29) 1339 (31) 1394 (26) 1438 (61)

8487 (194) 9056 (99) 8545 (173) 9056 (99) 9499 (216) 9729 (228) 10360 (217) 10316 (424)

9990 (237) 10790 (137) 10420 (230) 10790 (137) 11850 (276) 12170 (291) 12650 (237) 13070 (546)

13700 105,9 136,4 2,76 (380) (3,0) (4,9) (0,17) 14870 113,8 145,4 2,75 5 (445) (3,4) (4,7) (0,12) 14840 110,5 145,6 2,77 6 (320) (2,5) (2,8) (0,11) 14870 113,8 145,4 2,75 7 (445) (3,4) (4,7) (0,12) 16250 117,4 159,6 2,68 8 (519) (3,7) (4,8) (0,10) 16960 123,2 166,5 2,78 9 (366) (2,8) (5,4) (0,11) 17520 130,3 172,1 2,89 12 (368) (2,8) (4,7) (0,09) 18570 133,2 183,0 2,83 15 (492) (3,3) (6,3) (0,15) Note: Extended deviation are given in brackets.

4

CONCLUSIONS On the basis of conducted research, it was found that the basic functional properties of paper depend on the pigment-adhesive mixture applied as a coating on the paper. The characteristics of the Cobb water absorption curve, as well as the results of tensile and structural parameters, indicate that the highest hydrophobicity, surface smoothing, and highest mechanical properties are achieved in papers with the highest coating weight. Interestingly, not all coatings increase the tensile parameters of the paper. The decrease in strength was observed for samples with the thinnest coatings, which may be caused by the uneven distribution of the mixture on the surface, resulting in local weaknesses in the paper structure. Therefor, the results of the test, suggest that the most effective migration of binding agents takes place in the thickest coatings. This leads to a homogeneous distribution of the binders in the dry film. Thus, not only increasing the number of coating agents but also controlling the uniformity of the formed coating structure is decisive in improving the performance of coated papers.

Acknowledgment.This work was financially supported by the National Centre of Research and Development in Poland (Project NumberPOIR.01.01.01-00-0084/17 and POIR.04.01.04-00-0022/18). The authors are also grateful to Natural Fibers Advanced Technologies for allowing access to the company's R&D laboratory.

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REFERENCES 1. SYREK H., ANTOSZ A., PIROWSKI A., KACZOR T.: Badania nad wytwarzaniem mikroemulsji woskowych w reaktorze ciśnieniowym, http://archiwum.inig.pl/INST/nafta-gaz/nafta-gaz/Nafta-Gaz-2012-12-28.pdf.

2. KUMAR P., MITTAL K.L. (red.), 1999:Handbook of Microemulsion Science and Technology. Marcel Dekker, Inc., New York.

3. BASF information materials, http://www.performancechemicals.basf.com/ev-wcmsin/internet/en_GB/function/conversions:/publish/upload/EV/EV5/products/waxes_and_ wax_emulsions.

4. Sumika Chemtex Co. Ltd. Emulsions group, Functional Polymer Division, http://www.chemtex.co.jp/english/division/emulsion/product_grade/.

5. KOZIELEC T., 2006: Wpływ wody na papiery o różnym składzie. Cz. I. Składniki i struktura papieru – rys historyczny. Przegląd papierniczy, 62; 393-398.

Streszczenie: Wpływ gramatury powłoki na właściwości użytkowe papierów powlekanych.

Powlekanie papieru daje możliwość stworzenia produktu o wysokiej wartości dodanej, którego potencjał dostrzegli zarówno papiernicy, jak i poligrafowie. Powłoki parafinowe posiadają bowiem doskonałe właściwości barierowe, poślizgowe i wytrzymałościowe, a także zwiększają jakość i trwałość druku.

W niniejszej pracy przedstawiono wyniki badań wpływu gramatury powłoki na właściwości użytkowe powleczonego papieru. W tym celu komercyjnie dostępny papier został powleczony emulsją parafinową, przy użyciu różnych prętów Mayera, a następnie przebadany przy użyciu standardowych testów mechanicznych, powierzchniowych i absorpcji wody. Stwierdzono, że powleczenie papieru bazowego, niezależnie od ilości nałożonej mieszanki, istotnie wpływa na jego właściwości hydrofobowe, powierzchniowe i wytrzymałościowe. Papiery o najwyższej gramaturze powłoki pozwoliły uzyskać papier o zwiększonej wytrzymałości i wysokim wygładzeniu powierzchni. Wykonane powłoki znacznie zwiększyły także barierowość względem wody, niezależnie od ich grubości.

Corresponding author: Aleksandra Niska Faculty of Wood Technology, Warsaw University of Life Sciences - SGGW Warsaw University of Life Sciences (SGGW) 159 Nowoursynowska St., 02-776 Warsaw, Poland e-mail: aaleksandraniska1997@gmail.com

12

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 3, 2021

Impact of shredding degree on papermaking potential of recycled waste 1

1,2

1

1,2

ANETA LIPKIEWICZ , EDYTA MAŁACHOWSKA , MARCIN DUBOWIK & PIOTR PRZYBYSZ .

The properties of paper products depend on the structure of the cellulose fibres therein. Although fibre properties in virgin pulps can be modified by a refining process, this is more difficult in pulp from recovered fibre, particularly waste from office shredders that tend to shorten fibres during shredding. The shorter fibres in shredded paper make it difficult to easily reconstitute them into high quality paper products. Moreover, because of high energy usage during the recycling process and transportation inefficiencies, there is a need to determine how to responsibly shred paper to alleviate this environmental burden. With this in mind, the influence of initial fibre length on the tensile properties of paper was investigated. Changes in initial fibre length significantly influenced many pulp and paper properties. It was found that cutting the paper into pieces 2 with an area less than 25 mm caused significant changes in the important morphological parameters of the fibres and a sharp decrease in the tensile properties of the reconstituted paper.

Contact information: 1. Natural Fibers Advanced Technologies, 42A Blekitna Str., 93Ǧ322 Lodz, Poland.

2. Institute of Wood Sciences and Furniture, Warsaw University of Life Sciences - SGGW , 159 Nowoursynowska Str., 02Ǧ787 Warsaw, Poland.

Scientific Reports | (2021) 11:17528 | https://doi.org/10.1038/s41598-021-96325-4 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 10

Article 3 – Effect of Shredding on Pulp Properties

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ͷǡ Ï ͷǡ͸*ǡ ͷ Ƭ ͷǡ͸ Ƥ Ǥ Ƥ Ƥ Ƥ ǡ ƥ Ƥ ǡ ƥ Ƥ Ǥ Ƥ ƥ Ǧ Ǥ ǡ ƥ ǡ Ǥ ǡ ƪ Ƥ Ǥ Ƥ Ƥ ƪ Ǥ ͸ͻ

͸ Ƥ Ƥ Ǥ Wastepaper, for both ecological and economic reasons, is a good raw material for the production of paper or cardboard. Paper recycling reduces the use of wood, and thus, helps preserve forest resources, saves energy, reduces littering, the amount of waste going to landfills, air pollution, and wastewater generation, and instils ecological attitudes in society.

However, the properties of fibres in wastepaper can deteriorate not only during processing, but also at the collection stage due to shredding, which can cause excessive shortening of the fibres. In addition to fibre bonding1, fibre length and strength are basic factors influencing the tensile and structural properties of paper products2–4.

Fibre and pulp properties also affect the cost of producing paper products. Hence, the ability to control fibre properties during the recycling stage is a determining factor in effective quality control and the cost of paper production from waste paper.

In industrial practice, the fibres are shortened as a direct result of the fibre refining process or by high-shear processing of the fibrous suspension in the refining zone5,6. Therefore, the process of pulp refining has a direct influence on fibre properties, and consequently the properties of the final product7–9. Through refining, the properties of the refined pulp can be modified to obtain paper with the desired properties. The refining process, aside from affecting paper properties, also has a decisive impact on the unit energy consumption in this process10–13. Owing to the increasing global growth of the market for paper products14,15, it is extremely important to minimise the unit energy consumption in this process and optimise the development of useful properties of the paper during processing.

Changes taking place in the structure of the refined fibres during the refining process determine how the pulp behaves during web formation and the basic properties of the paper that is produced16,17. Fibre shortening has a direct negative impact on the dynamic properties of paper, including its tear resistance18,19. Often, refining is consciously carried out to improve the conditions of web forming and improve its transparency.

When paper is thrown into shredders, the objective is to destroy documents; however, the process also unconsciously shortens the fibres. Paper shredders should facilitate the preparation of pulped materials for further production, rather than render the paper useless. Cut fibres in the shredded paper make it difficult to carry out easy reconstitution into high-quality paper products. Moreover, most recycling centres do not handle small strips or bits of paper. Large-scale recycling facilities use large screens to dry pulped paper on, and finely shredded paper is not well retained and can fall through the screens. Reports claim that the utilisation or recycling of shredded paper is much more problematic than that of mixed unshredded paper. The Environmental Paper Network, a worldwide association of 140 civil society groups and NGOs concerned with the sustainability of pulp and paper

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Specific security levels

Paper media requirements

P-1

12 mm strips or maximum particle surface area of 2.000 mm2

P-2

6 mm strips or maximum particle surface area of 800 mm2

P-3

2 mm strips or maximum particle surface area of 320 mm2

P-4

Maximum cross cut particle surface area 160 mm2 with a maximum strip width of 6 mm = 6 × 25 mm

P-5

Maximum cross cut particle surface area 30 mm2 with a maximum strip width of 2 mm = 2 × 15 mm

P-6

Maximum cross cut particle surface area 10 mm2 with a maximum strip width of 1 mm = 1 × 10 mm

P-7

Maximum cross cut particle surface area 5 mm2 with a maximum strip width of 1 mm = 1 × 5 mm

Table 1. Requirements for machines and processes shredding documents (DIN 66399).

practices suggests shredding paper only when necessary20, especially since destruction also increases the bulk density of the paper, reducing its transportation efficiency. The office waste problem is particularly significant because of the high amount generated. For example, Japan has a paper collection rate of around 81.6%21, which is higher than that of other countries. However, this high collection rate is mainly constituted by a high recycling rate for paper grades such as cardboard and newspapers. The collection rate for office paper and shredded paper remains low (less than 60%)21 because office paper, on which confidential information is often printed, is generally disposed of. In U.S. offices, 50% of business waste is composed of paper. Offices use approximately 12.1 trillion sheets of paper per year, and paper accounts for 25% of landfill waste and 33% of municipal waste. It was found that each tonne of recycled paper allows for 64% energy savings, 58% water savings, and 60 pounds less air pollution22. Security issues aside, one should consider how to efficiently destroy documents while still allowing fibres to be efficiently used in further processing, which will enable the above savings; this paper attempts to address this issue.

Paper destruction categories are assigned based on the standard German Deutsches Institut für Normung (DIN) classification for paper shredding machines. DIN 66,399 classifications, in which ‘P’ refers to ‘Paper-Based’ material, are based on the size and type of particle (Table 1).

The aim of this study is to examine the effect of the surface area of shredded particles on paper properties. The study therefore estimates the extent to which waste paper can be shredded while still producing a high-quality product. Pulp samples with different initial particle sizes treated under constant papermaking conditions were used for this purpose.

Ǥ Industrial air-dried and bleached kraft pine pulp in the form of sheets (Arctic Paper Kostrzyn S. A.) was used in this study. In order to keep all other parameters constant, samples of the pulp were cut manually into squares of different areas (1–400 mm2) to reduce and diversify the fibre length.

Hand-cut samples were compared with strips from shredding machines (destroyed in accordance with DIN 66399). The following shredders were used for the tests: Kobra 240.1 S2 ES (for strips with P-3 specific security levels), HSM Shredstar S5 (for strips with P-4 specific security levels), HSM Securio C18 (for strips with P-5 specific security levels), and HSM Securio B26 (for strips with P-6 and P-7 specific security levels). All cutting processes in the shredders were performed without the addition of oil. Ǥ Sheets of paper from the cut samples were produced under laboratory conditions from rewetted pulp samples (22.5 g dry weight samples were soaked in water for 24 h) that were subjected to disintegration using a laboratory JAC SHPD28D propeller pulp disintegrator (Danex, Katowice, Poland) for 23.000 revolutions, according to ISO 5263-1 (2004). The disintegrated pulps were concentrated to a dry weight content of 10% and refined in a JAC PFID12X PFI mill (Danex, Katowice, Poland) under standard conditions [ISO 5264-2 (2011)]. All the samples were refined for a constant time of 120 s. After the model pulp recycling processes, including refining, the following properties of the pulps were evaluated:

• Schopper-Riegler freeness parameter (SR) was measured using a Schopper–Riegler apparatus (Danex, Katowice, Poland) in accordance with PN-EN ISO 5267-1 (2002); • The water retention value (WRV) was determined according to ISO 23714 (2014); • The dimensions of the fibre parameters were measured according to ISO 16065-2 (2016) using a Morfi Compact Black Edition apparatus (Techpap, Grenoble, France).

In the next step, sheets of paper were formed in a Rapid-Koethen apparatus in accordance with PN-EN ISO 5269-2 (2007). Each paper sheet had a basis weight of 80 g/m2 (according to ISO 536:2012). Only sheets with basis weights between 79 and 81 g/m2 were used for further investigation. The sheets were conditioned for 24 h at a relative humidity of 50 ± 2% and a temperature of 23 ± 1°C [ISO 187 (1990)] before determining their properties. The properties of the paper sheets were examined as follows. A ZwickRoell Z005 TN ProLine tensile testing machine (Zwick-Roell, Ulm, Germany) was used to measure the mechanical properties of the paper in accordance with PN-EN ISO 1924-2 (2010). The roughness and air permeability were measured using a Bendtsen apparatus (Messmer Buchel, Veenendaal, The Netherlands). Ƥ | Vol:.(1234567890)

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Sample dimensions (mm)

Sample area (mm2)

Freeness

WRV

(°SR)

(%)

1×1

1

51

214.5 (3.4)

2×2

4

42

212.5 (2.2)

1 × 5 (P-7)

5

39

211.1 (1.2)

3×3

9

31

208.5 (1.6)

1 × 10 (P-6)

10

31

208.1 (1.3)

4×4

16

30

207.2 (1.4)

5×5

25

29

206.8 (1.2)

2 × 15 (P-5)

30

28

206.3 (1.5)

6×6

36

28

206.1 (1.9)

8×8

64

26

205.2 (2.5)

10 × 10

100

25

205.6 (1.8)

6 × 25 (P-4)

150

25

204.8 (2.2)

P-3

320

25

203.6 (2.8)

20 × 20

400

25

202.8 (1.6)

Reference

–

25

201.2 (3.1)

Table 2. Characteristics of refined cellulose pulps. Standard deviation are given in brackets.

The average degree of polymerisation (DP) of cellulose in each pulp was determined using the viscometric method described in ISO 5351 (2010).

In industrial conditions, the freeness index of pulp is a commonly used parameter to assess the degree of refining of pulp based on how easily it dewaters. The dewatering of pulp on a paper machine screen is a very useful indicator in industrial practice; therefore, in this study, the effect of the initial fibre length on changes in this indicator was examined. Table 2 shows that the freeness of the refined pulp decreases as the initial fibre length increases, which is attributed to the decreasing amount of fine material in the pulp, in line with the current state of knowledge23–28. It is assumed that a freeness index of about 30°SR is optimal for most papermaking properties29,30. However, obtained results do not indicate that a freeness of ~ 30°SR necessarily achieves a maximum tensile strength (Table 2). This confirms that there is no straightforward relationship between freeness and paper properties. Therefore, the freeness of the pulp is not useful to compare the papermaking potential of pulp with different initial parameters.

The impact of initial fibre length on internal fibrillation, one of the basic effects of refining31–35, was also studied. Progress in achieving internal fibrillation of refined fibres is commonly assessed based on an increase in their swelling36,37, usually measured by the WRV38–40. The analysis in Table 2 indicates that the WRV increases as the initial fibre length decreases, reaching a maximum value of 214.5%. This increase in fibre swelling is accompanied by an increase in the density of the paper, which in turn increases the resistance of the paper to air permeability (Table 2), consistent with previous studies8,41–43.

The initial fibre length did not affect the DP of the produced pulps. The DP was 931 ± 0.89 regardless of the initial dimensions of the samples tested. This confirms previous findings in the literature that mechanical treatment has little effect on the DP of cellulose44,45.

Table 3 and Fig. 1 show the morphological characteristics of the fibres of the examined pulps. The fibre length for the refined pulps is characterised by lower values compared to the unrefined pulps, which confirms that one of the basic effects of refining is fibre shortening46–48. It should be noted that the use of mean weighted or mean geometric fibre length eliminates the influence of the fines fraction on the analysis result49,50.

The results presented in the Table 3 show that the examined parameter values of fibres and pulps (mean fibre width, mean fibre coarseness, macro fibrillation index, broken fibre content, and fine content) tend to decrease with decreasing length of the initial fibres, and consequently, the mean weighted fibre length increases. However, an initial sample length greater than 5 mm does not significantly affect the examined fibre properties and fine content. It is therefore likely that a strip width of 5 mm is the limit above which significant changes in the pulp do not occur. The results are similar before and after the refining process (Fig. 1).

Microscopic images of the refined pulps, recorded using a Morfi Compact Black Edition camera, are shown in Fig. 2. The decrease in fibre length after the model pulp recycling process is proportional to the decrease in the dimensions of the pre-cut pulp samples. The most significant fibre shortening is noticed for the 1 × 1 mm sample. There is no significant difference in fibre dimensions for the 10 × 10 mm samples and the reference sample (Fig. 2). Therefore, it is possible to shred paper in a shredder to a specific level of fragmentation without fear of excessive shortening of the fibres, which would make the production of high-quality paper more difficult.

According to previous findings, fibre length has a significant impact on the paper stretch index51, and excessive fibre shortening and a high fines content cause the paper product to become rigid and reduce its deformability49,52,53. Our results, however, indicate that the stretch of the examined papers is similar, irrespective of the fibre length and fines fraction content (Table 4), in contrast to previously reported results. However,

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Sample area

Mean fibre width

Mean fibre coarseness

Macro fibrillation index

Broken fibre content

Fine content

mm2

μm

mg/m

%

%

% in area

1

32.9 (0.1)

0.207 (0.017)

1.135 (0.047)

52.3 (1.1)

11.70 (0.54)

4

32.4 (0.3)

0.199 (0.009)

1.085 (0.007)

50.0 (0.5)

9.42 (0.13)

5

32.5 (0.2)

0.197 (0.008)

1.059 (0.009)

49.3 (0.6)

9.20 (0.19)

9

31.9 (0.1)

0.190 (0.011)

0.890 (0.023)

45.8 (0.4)

6.85 (0.24)

10

31.8 (0.2)

0.195 (0.007)

0.903 (0.020)

46.1 (0.3)

6.78 (0.18)

16

31.5 (0.1)

0.196 (0.006)

0.861 (0.011)

44.9 (0.8)

5.98 (0.18)

25

31.7 (0.4)

0.195 (0.001)

0.832 (0.007)

43.8 (1.5)

5.51 (0.14)

30

31.5 (0.3)

0.193 (0.002)

0.819 (0.006)

43.6 (1.1)

5.26 (0.16)

36

31.2 (0.2)

0.191 (0.008)

0.814 (0.005)

43.0 (0.6)

5.08 (0.28)

64

31.5 (0.1)

0.187 (0.009)

0.799 (0.012)

41.5 (0.2)

4.26 (0.14)

100

31.1 (0.1)

0.182 (0.012)

0.764 (0.022)

39.3 (0.6)

3.25 (0.21)

150

31.0 (0.3)

0.184 (0.011)

0.769 (0.015)

39.8 (0.4)

3.04 (0.18)

320

30.8 (0.1)

0.183 (0.010)

0.752 (0.012)

38.4 (0.4)

2.95 (0.10)

400

30.9 (0.4)

0.184 (0.001)

0.758 (0.058)

39.2 (0.5)

3.00 (0.60)

Reference

30.7 (0.3)

0.180 (0.010)

0.777 (0.024)

35.4 (0.6)

2.81 (0.60)

Table 3. Fibre and pulp properties after model pulp recycling process including refining. Standard deviation are given in brackets.

Figure 1. Average fibre length dependency on the cut surface area of samples (before and after model pulp recycling process including refining).

the results listed in Table 4 show that fibre length exerts a significant impact on the dynamic tensile properties of paper54–56. Importantly, in the case of fibre properties (Table 3 and Fig. 1), changes in the area of the shredded samples above 25 mm2 did not significantly affect the tensile paper properties (Table 4). Based on this, it can be concluded that samples can be cut at 5 mm or larger without significant shortening of the fibres and no significant changes in pulp and paper properties. Therefore, the paper shredding provides useful wastepaper when performed in devices up to class P-6.

Research has shown that the fines fraction produced from fibres is responsible for slowing pulp-dewatering in the forming section of a paper machine57,58. The results obtained are fully consistent with those of earlier studies, in that the pulps with the highest fines content also have the highest SR freeness values (Table 2).

The air permeability of paper decreases as the SR freeness level increases59–62, in agreement with previous results. The pulps characterised by lower average fibre length, and therefore, have improved barrier properties to gases even though their tensile properties are reduced (Table 4).

The roughness of paper increased with increasing initial fibre length, in agreement with previous results34,61,63,64. Therefore, the best smoothness results (230 mL/min) were obtained for paper produced from the pulp with the lowest fibre length. From the data provided, it can be concluded that this paper would likely have the best printability.

Microscopic images of the paper sheets, recorded using a Keyence VHX-6000 microscope equipped with a VH-Z100UR lens, are presented in Fig. 3. The fibres in the reference pulp (Fig. 3a) appear undamaged. The paper obtained from samples cut into 5 × 5 mm pieces (Fig. 3b) shows both undamaged fibres and cut fibres. Ƥ | Vol:.(1234567890)

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Figure 2. Microscopic images of tested fibrous materials.

This observation clearly indicates that initial shortening of fibres damages the fibre structure to some extent, with an effect on average fibre length. However, these are local symptoms of shortening to a certain size. The vast majority of the fibres remain intact, as in the virgin pulp. Therefore, it can be concluded that shortening of fibres can be tolerated to a certain extent if the overall change to the fibre mixture is small.

An image from the sample with an initial size of 1 × 1 mm shows many damaged fibres (Fig. 3c), i.e., disintegration into finer material. Therefore, excessive fragmentation of the fibrous material causes considerable

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Sample area

Force at break index

Stretch

Energy absorption index

Tensile stiffness index

Roughness

mm2

Nm/g

%

J/g

Nm/g

ml/min

ml/min

1

47.7 (0.8)

4.38 (0.01)

1.53 (0.07)

5116 (134)

230 (5)

156 (2)

Air permeability

4

57.9 (1.0)

4.43 (0.05)

1.80 (0.05)

5813 (146)

243 (3)

256 (2)

5

58.7 (0.9)

4.42 (0.03)

1.77 (0.04)

5866 (132)

251 (5)

252 (3)

9

69.0 (0.6)

4.30 (0.09)

2.05 (0.04)

5969 (118)

263 (8)

527 (4)

10

68.8 (0.5)

4.39 (0.08)

2.11 (0.03)

5963 (122)

269 (7)

533 (6)

16

70.6 (0.7)

4.21 (0.05)

2.15 (0.04)

6124 (147)

281 (6)

629 (3)

25

72.6 (1.1)

4.53 (0.02)

2.23 (0.05)

6334 (154)

290 (10)

857 (5)

30

73.2 (1.0)

4.44 (0.03)

2.20 (0.03)

6371 (139)

298 (8)

876 (6)

36

73.5 (0.3)

4.38 (0.06)

2.18 (0.02)

6384 (111)

301 (3)

954 (4)

64

74.0 (0.5)

4.23 (0.03)

2.24 (0.03)

6412 (125)

304 (7)

1201 (10)

100

74.5 (0.7)

4.45 (0.04)

2.20 (0.05)

6301 (108)

321 (8)

1315 (9)

150

74.6 (0.3)

4.36 (0.04)

2.16 (0.01)

6358 (111)

343 (5)

1354 (7)

320

74.6 (0.3)

4.42 (0.03)

2.19 (0.02)

6294 (130)

339 (6)

1418 (11)

400

74.5 (0.6)

4.32 (0.03)

2.15 (0.07)

6215 (149)

351 (10)

1415 (13)

Reference

74.6 (0.9)

4.61 (0.03)

2.20 (0.04)

6344 (151)

344 (8)

1421 (22)

Table 4. Mechanical properties of paper sheets produced from refined pulps. Standard deviation are given in brackets.

Figure 3. Microscopic images of paper sheets derived from (a) reference and (b, c) selected refined pulps produced from the tested fibrous materials.

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damage to the fibrous fraction, and hence, deteriorates the potential of this fraction, which ultimately results in less desirable paper properties.

Based on the results obtained, the degree to which the shortened pieces of office waste paper can be converted into high-quality paper without any issues that reduce the value of the finished product can be predicted to a certain extent. From an ecological point of view, it is necessary to consider at the very least the issue of pulp freeness, and in turn, the energy needed for dewatering of the pulp. Excessive shortening of the paper samples worsens their dewatering capacity, as confirmed by the obtained results (Table 2). Therefore, the practice seeks to maintain a compromise and reduction of the refining range. Achieving efficiency in energy usage is considered the most cost-effective way to reduce CO2 emissions65. Therefore, by producing waste office paper without undue fragmentation in shredder (if data security issues need not be considered), the properties of recycled paper products can be improved and the energy consumption can be reduced concurrently. Thus, the recycling of shredded paper can be carried out in a more ecological, environmentally friendly, and economic manner.

It was found in this study that the initial fibre length affects paper properties, the morphological characteristics of fibres, and the fine content of pulp, which in practice affects the cost of paper production. With other papermaking conditions held constant, paper samples produced from pulps with different initial fibre lengths showed various tensile properties. It is important to note that cutting paper into pieces with an area higher than 25 mm2 in the shredding process does not significantly influence the properties of pulp or the tensile properties of paper made from the pulp. Cutting samples into pieces smaller than 25 mm2, however, causes significant changes in the most important morphological parameters of the fibres and a sharp decrease in the dynamic and static strength properties of the paper with decreasing size of the shredded paper. Such a level of cutting is therefore unprofitable from a technological and an economic point of view because it required difficult pulp-dewatering processes and thus involves increased costs. The obtained results are therefore of great practical value because they demonstrate that the method of shredding paper in the shredder determines the processing potential of this pulp and the papermaking utility of the product. These conclusions can serve as a guide on how to responsibly shred paper, as well as to efficiently recycle fibres to manage biomass consumption.

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Received: 22 May 2021; Accepted: 5 August 2021

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This work was supported by the National Center of Research and Development in Poland (Project Number POIR.01.01.01-00-0084/17).

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A.L. conceived the experiment(s), A.L. and M.D. conducted the experiment(s), P.P., E.M. and A.L. analysed the results, M.D. literature review, E.M. writing—original draft preparation, E.M. writing—review and editing, P.P. funding acquisition. All authors reviewed the manuscript.

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PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 3, 2021

Tray forming operation of paperboard: A case study using implicit finite element analysis GUSTAV LINDBERG & ARTEM KULACHENKO. The possibility to perform advanced forming operations of initially plane paperboard paves the way to making products like food trays, plates, cups and other containers as a part of shifting towards a circular bioeconomy. As a part of the ongoing efforts of expanding the product range using paperboard, we performed analyses of the forming operation using simulations. An implicit non-linear finite element model is built to more accurately than previous studies simulate the tray forming process of paperboard. Two different commercial paperboards are investigated. The use of an implicit solver enabled the inclusion of the creasing pattern into the geometry of the paperboard blank resolving the formation of wrinkles during forming. The material data is extracted from tensile test curves of the investigated paperboards and was fitted accurately using Hill's plasticity with difference in tension and compression accounted for with subsequent failure evaluation. The results showed that the inclusion of the creases in the geometry is vital for getting a correct shape of the formed tray and important for decreasing the risk of failure. The results also showed that friction has a big impact on the formed shape and hence on the stress levels, and therefore supports the means of lowering friction between the blank holder and the blank during the tray forming operation. A stochastic approach is proposed to determine the probability of failure for the boards. The performed failure evaluation is consistent with the field observations. The developed approach enables more precise simulations of paperboard tray forming.

Contact information: Solid Mechanics, Department of Engineering Mechanics, Royal Institute of Technology, Stockholm, Sweden Packag Technol Sci. 2021;1–16.

DOI: 10.1002/pts.2619 Creative Commons Attribution 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 17

Article 4 – Tray Forming of Paperboard

Received: 1 March 2021

Revised: 9 August 2021

Accepted: 5 November 2021

DOI: 10.1002/pts.2619

RESEARCH ARTICLE

Tray forming operation of paperboard: A case study using implicit finite element analysis Gustav Lindberg

|

Artem Kulachenko

Solid Mechanics, Department of Engineering Mechanics, Royal Institute of Technology, Stockholm, Sweden

Abstract The possibility to perform advanced forming operations of initially plane paperboard

Correspondence Gustav Lindberg, Solid Mechanics, Department of Engineering Mechanics, Royal Institute of Technology, Stockholm, Sweden.

Email: gulindbe@kth.se

paves the way to making products like food trays, plates, cups and other containers

Funding information Swedish National Infrastructure for Computing (SNIC), Grant/Award Number: SNIC 2020/5-428 and SNIC 2021/6-51

to more accurately than previous studies simulate the tray forming process of paper-

as a part of shifting towards a circular bioeconomy. As a part of the ongoing efforts of expanding the product range using paperboard, we performed analyses of the forming operation using simulations. An implicit non-linear finite element model is built board. Two different commercial paperboards are investigated. The use of an implicit solver enabled the inclusion of the creasing pattern into the geometry of the paperboard blank resolving the formation of wrinkles during forming. The material data is extracted from tensile test curves of the investigated paperboards and was fitted accurately using Hill's plasticity with difference in tension and compression accounted for with subsequent failure evaluation. The results showed that the inclusion of the creases in the geometry is vital for getting a correct shape of the formed tray and important for decreasing the risk of failure. The results also showed that friction has a big impact on the formed shape and hence on the stress levels, and therefore supports the means of lowering friction between the blank holder and the blank during the tray forming operation. A stochastic approach is proposed to determine the probability of failure for the boards. The performed failure evaluation is consistent with the field observations. The developed approach enables more precise simulations of paperboard tray forming.

KEYWORDS

creases, Hill's plasticity, non-linear finite elements, paperboard, tray forming

1

|

I N T RO DU C T I O N

time, paperboard is an important part of shifting towards a circular bioeconomy and the effort to increase formability and the processes

Today, the possibility to perform advanced forming operations of

enabling the forming operation is ongoing.

paperboard has a big interest in the industry to create products like

There are several types of forming operations for paperboard,

food trays, plates, cups and other containers.1–3 However, compared

such as hydroforming, press (tray) forming, deep drawing and molding.

to plastics and sheet metal, paperboard has lower formability. Deep

The first three of them are all variants of pressing down a sheet of

4–6

and plas-

paper or paperboard, called the blank, into a die. Hydroforming pres-

tics are used to form a variety of trays and containers.7,8 At the same

ses the blank downwards utilizing air pressure. The press forming uses

drawing of sheet metal is widely used in many industries,

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. © 2021 The Authors. Packaging Technology and Science published by John Wiley & Sons Ltd. Packag Technol Sci. 2021;1–16.

wileyonlinelibrary.com/journal/pts

1

2

LINDBERG AND KULACHENKO

a punch, also called male die, to press the blank into the bottom of a

methods, and the considerably higher number of elements com-

die, while deep drawing uses the punch to press the blank into a bot-

pared to earlier studies, allowing for more exact results. This

tomless die. The methods are described in e.g. Hagman et al.3 Östlund and Söderberg9 and Lowe et al.10 Complex products such as egg pack-

enables to study the effect of creases on the frictional interaction.

4. The use of implicit time integration during the simulations, which

ages are molded.11 As a part of the product development process using paperboard,

has a direct impact on the accuracy of the test.

5. The analysis of the frictional conditions required to achieve the desired shape.

simulations of the forming operations are used. The behaviour of the paperboard can then be studied, and it can be evaluated if and how material properties should be altered to minimize the risk of failure of

With all the aforementioned parts included at once in the model,

the paperboard during the forming operation. The present study

the possibility to simulate advanced forming operations is enhanced.

focuses on the tray forming operation of paperboard. A numerical

The objective is to increase the precision of the results coming from a

approach is presented where the simulation of the operation is further

simulation model of tray forming, giving more exact results for

developed compared to what has been done previously.

stresses, strains and the risk of failure compared to previous models.

Deep drawing and tray forming of paperboard has been simulated

The verification is based on achieving the correct shape of the tray,

in earlier studies,12–14 with different numerical approaches and objec-

something not fully attained in earlier studies of tray forming where

tives. In Wallmeier et al.12 the effects of varying the blank holder

the flange of the corners has been larger than what is seen in produc-

force, the die temperature, and the thickness of the paperboard were

tion, even for failed trays. The risk of failure is also determined and

investigated. They showed, amongst other things, that the friction of

compared to reality.

the blank holder and die have significant effects on the stress in the

The paperboard is commercially produced under the name

blank, implying that low-friction dies and blank holders can consider-

Inverform by Iggesund paperboard, which is a part of the Holmen

ably reduce the failure probability. In Awais et al.13 the effect of the

Group. The paperboard is produced on two board machines, but dif-

number of creases on the strain levels was numerically investigated

ferences in converting performance have been observed and to

for paperboard. The creases were modelled with hinge connector ele-

understand this difference the present investigation was performed.

ments, and not explicitly included in the geometry. It was seen that

The paperboard coming from the first board machine is here called

the number of creases aided in lowering the strain levels. It could also

Board A, and the paperboard coming from the second machine is here

be seen that this effect was greater for the first paperboard having a

called Board B.

higher Young's modulus compared to the second investigated paper-

In Figure 1, the failure mode of the studied tray using Board A is

board, which had a larger strain to failure. The approach with hinge

shown, where the material fails in the corner under the creases. With

connector elements for the creases is investigated more in detail in

the new simulation approach, the tray forming operation is simulated

Livill et al.,14 where the approach allows for spontaneous wrinkling,

using the different paperboards, and the difference in results is evalu-

that is, the number of creases or the position of them do not need to

ated and compared to reality.

be known by forehand. The approach is included in deep drawing simulations and shows that the number of creases is about the same as in experiments. The disadvantage of these approaches is the inability to

2

MATERIALS AND METHOD

|

assess the effect of creases on the frictional interaction between the paperboard and the forming unit as the wrinkles were not resolved

2.1

|

The tray

physically. The inclusion of the creases in a numerical model is important not only to simulate the correct shape, friction interaction and

Paper and paperboard are anisotropic, heterogeneous, and hygroscopic

strain levels, but also to investigate the possibilities to add a lid onto

materials. They are built up by a network of cellulose fibers from soft-

the tray to seal it. For the sake of successful sealing operations, the

wood or hardwood, or from a combination of the two. The fibers are

upper edge of the formed tray should be as smooth as possible.

randomly distributed over the sheet with, partly, random orientations.

The fibers are connected via fibre bonds which form spontaneously

The advances in the current study are as follows:

when water disappears from the web in the papermaking process. The anisotropic material behaviour must be considered in numerical simula-

1. The use of an orthotropic material model with isotropic hardening

tions of paperboard. A common way is to model the paperboard as

according to Hill plasticity, fitted to actual tensile tests and com-

orthotropic,3,12–16 where material properties are specified in the

pared with two sets of data from papers with different failure

paperboard machine-direction (MD), cross-direction (CD), and in the

properties. The fitting accuracy was improved, which allowed a

Z-direction (ZD), that is, through the thickness of the paperboard.

reliable comparison of different materials.

2. Accounting for the difference in tension and compression for yielding and failure.

In Figure 2 the paperboard blank is shown as it is prepared by the tray manufacturer for the forming operation. The blank is laminated with a polymer that is extruded over the blank since the tray must

3. The detailed resolution of the creases which are introduced

withstand moisture during usage. The creasing pattern with 30 creases

through geometrical features rather than through the ad-hoc

in each corner has been pressed into the paperboard so that it folds

LINDBERG AND KULACHENKO

F I G U R E 1 The failure mode occurring for the studied tray using Board A. In (A) seen from the inside the tray, and in (B) seen from the outside

FIGURE 2

The paperboard blank before forming operation. (A) The full blank with dimensions and (B) close-up of the creases

FIGURE 3

The studied tray after a successful forming operation. In (A) seen slightly from above and in (B) seen from the side

3

4

LINDBERG AND KULACHENKO

and shapes correctly. The grammage is 330 g/m2 and the thickness of

2.3

|

Elastic material properties

the paperboards, including the thin (30 μm) and compliant PET coating, is 0.5 mm.

In the following theory, the principal material directions MD, CD and

Figure 3 shows the studied tray after a successful forming operation, that is, without detectable failure. The linear dimensions of the formed tray are 185 125 25 mm.

ZD are described with indices 1, 2 and 3, respectively.

The elastic part of the paperboard is described using Hooke's law ε ¼ Cσ: For an orthotropic material the full expression reads 2

2.2

|

Material data

Paperboard is an anisotropic material, which may be approximated as an orthotropic material. The material shows different responses in tension and compression which may not always be captured by the materials models available in the standard libraries in the commercial finite element tools. The source for the input data was the physical tensile tests of the two paperboard types considered in this study.

1 ν21 ν31 0 0 6 E1 E2 E3 6 6 ν 2 3 6 12 1 ν32 0 0 6 ε11 E2 E3 6 E 6ε 7 6 1 6 22 7 6 ν13 ν23 1 6 7 6 0 0 6 ε33 7 6 E1 E3 E2 6 7¼6 6ε 7 6 1 6 23 7 6 0 0 0 0 6 7 6 2G23 4 ε31 5 6 6 1 6 ε12 6 0 0 0 0 6 2G31 6 4 0 0 0 0 0

The tensile tests are performed under ISO standard conditions in

3 0 7 7 7 2 3 0 7 7 σ 11 7 76 σ 7 76 22 7 6 7 0 7 76 σ 33 7 7 76 76 σ 7 : 6 23 7 0 7 7 76 74 σ 31 5 7 7 0 7 σ 12 7 7 1 5

ð1Þ

2G12

three in-plane directions, MD, CD and 45 , and are shown in Figure 4.

As observed, the biggest difference between the boards is the

The paperboards are modelled with 3D shells with plane stress

tensile properties in the 450-direction. The tensile strain in the MD is

assumption. In the case of plane stress, the expression in Equation 1

about 1.9% for Board A, and 2.2% for Board B. The tensile stress in

reduces to

the MD is about 60 MPa for Board A and 70 MPa for Board B. For the CD, Board A has a tensile strain of about 5.0% at 30 MPa, while for Board B the tensile strain is 6.4% at 32 MPa. The distribution of the tensile test results is discussed later in the paper.

Paperboard exhibit a reduction in yield limit and strength in compression compared to the corresponding values in tension.17,18 This is

2

1 ν21 2 3 6 Ex Ey 6 ε11 6 6 7 6 ν12 1 4 ε22 5 ¼ 6 6 Ex Ey 6 ε12 4 0 0

3 0 72 3 7 σ 11 7 7 76 0 74 σ 22 5 : 7 7 σ 1 5 12 2G12

ð2Þ

taken into account by assuming the yield stress in compression being 70% of that in tension. For the failure evaluation, the compressive

The two in-plane values for the Young's modulus Ei in Equation 2 are

strength is reduced by 50% from the tensile value. The chosen values

determined by the fitting procedure, and the in-plane shear modulus

for yield and failure stress levels in compression are based on the

G12 and Poisson's ratio ν12 are then calculated using the two separate

reported values in Xia et al.18

relations for commercially produced papers19

FIGURE 4

Uni-axial tensile tests in the three directions for the two paperboards as received for this study: (A) Board A; (B) Board B

5

LINDBERG AND KULACHENKO

TABLE 1

! ! !

1 1 1 1 1 1 1 1 1 1 1 1 þ

þ

þ

; G ¼ ; H ¼ ; 2 R212 R233 R222 2 R233 R211 R222 2 R211 R222 R233 ! ! !

3 1 3 1 3 1 ; M¼ ; N¼ : L¼ 2 R223 2 R213 2 R212

Elastic material parameters

F¼

Young's modulusE1 , E2 [MPa]

Shear ModulusG12 [MPa]

Poisson's ratioaν12 [ ]

Board A

7143, 3078

2258

0.446

Board B

7501, 2948

1511

0.467

ð5Þ The Hill's parameters Rij in Equation 5 are defined as

ν21 is determined from the above parameters due to the symmetry of the stiffness matrix. a

pffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffi E1 E2 G12 ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffi and ν12 ν21 ¼ 0:293 : 2 1 þ ν12 ν21

σ 11 y σ 22 y σ 33 y ; R22 ¼ ; R33 ¼ ; σy σy σy y y pffiffiffi σ 23 pffiffiffi σ 13 ¼ 3 ; R13 ¼ 3 , σy σy

R11 ¼

pffiffiffi σ 12 y R12 ¼ 3 ; R23 σy ð6Þ

ð3Þ and determine the shape of the yield surface, which initial size is

Equation 3 along with the symmetry condition of the compliance

determined by the initial yield stresses σ ij y and the isotropic yield

matrix, giving ν21 =E2 ¼ ν12 =E1 , give the in-plane elastic material

stress σ y . The material parameters in Equation 6 are found with the

parameters which are listed in Table 1.

previously mentioned fitting procedure. The stress–strain curve mea-

The out-of-plane strain ε33 can be derived as ε33 ¼ νE113 σ 11 νE232 σ 22

sured in the CD is used as a master curve for the multilinear hardening

as given by Equation 1 with σ zz ¼ 0, but requires estimations of the

and the rest of the parameters are fitted in Matlab using the fmincon-

Poisson's ratios ν31 and ν23 .

function to minimize the error between the measured and the calculated tensile test curves. The paperboard is modelled to yield at Rp ¼ 0.0001, that is, the initial yield stress is in this study the stress

2.4

|

Hardening model

giving a permanent deformation of 0.01% strain. Such a low value is required to get a good fit between the experimental and the numerical

Plasticity in paperboard has been modelled in many different studies.

tensile test curves. The quality of the fit is shown in Figure 5. The cur-

In Harrysson and Ristinmaa20 a large strain orthotropic elasto-plastic

ves on the compressive side are only from the numerical tests since

model was developed with a yielding surface based on the Tsai–Wu

no data is given from experiments. In compression, the two paper-

failure surface,21 which made it possible to directly introduce the dif-

boards have a 30% reduction of the yield stress and a 50% reduction

ference in tensional and compressive yield behaviour for paperboard.

of the ultimate stress compared to the tensional side, which renders

Several models based on the complex anisotropic yield surface intro-

in the curves on the compressive side in Figure 5.

duced by Xia et al.18 have been developed, such as the in-plane paper-

The shape of the Hill yield surfaces for the two boards is shown

board models established in Li et al.22 and Tjahjanto et al.23 The latter

in Figure 6, plotted for zero shear stress, τ12 ¼ 0. As seen in Figure 6,

model is a viscoelastic-viscoplastic small strain approach developed to

the modelled difference in tension and compression for the paper-

capture creep and relaxation for transient uniaxial loading. One of the

boards render in two yield surfaces per board, one for compression

latest publications on the subject is the one by Robertsson et al.24

and one for tension. Which surface that applies for the current point

where the continuum model is based on previous models15,25 using

is determined by the sign of the hydrostatic stress. With a positive

24

results from simula-

sign, the hardening in the current point is evaluated towards the sur-

tions using solid continuum elements and shell elements are compared

face for tension, and a negative sign evaluates the evolution of the

for some forming operations. They showed, amongst other things,

plastic strains towards the surface for compression. If a sign change

that the shell elements had a better performance compared to the

would occur during the analysis, the point remains on the initially

continuum elements. For the example simulating an actual forming

assigned surface which is shown with dotted lines in Figure 6. This

operation from the industry, frictionless contacts, an explicit solver

approach of using two surfaces has an advantage in avoiding the diffi-

scheme and ideal plasticity were used.

culties in simultaneous fitting compressive and tensile behaviour with

numerous sub-surfaces. In Robertsson et al.,

The evolution of the plastic strains in the current study is 26

non-symmetric surfaces. The disadvantage is an abrupt change in the

which is suitable for composites and

second and fourth quadrants. As the largest part of the paperboard

a common way to model plasticity for orthotropic composite such as

appears in the first and third quadrant and the surface is not swapped

paperboard. Hill's plasticity is defined as

during the simulations, this does not present a problem with conver-

described using Hill's plasticity,

gence or thermodynamic inconsistency.

2

2

2

f ðσ, σ f Þ ¼ Fðσ 22 σ 33 Þ þ Gðσ 33 σ 11 Þ þ Hðσ 11 σ 22 Þ þ2Lσ 223 þ 2Mσ 231 þ 2Nσ 212 σ y 2 ¼ 0

ð4Þ

The fitting procedure resulted in different yield surfaces for the two paperboards (see Figure 6) Board B has a yield surface close to circular and an earlier yield point, compared to Board A. In Table 2,

where F, G, H, L, M and N are defined as

the complete set of parameters for the Hill's plasticity used in this

6

LINDBERG AND KULACHENKO

FIGURE 5

Numerical tensile tests compared to the mean values of the tensile test curves for the respective direction

F I G U R E 6 The Hill's surfaces for the two paperboards for zero shear stress. (A) Board A, (B) Board B and (C) Board A and B normalized in the same figure for comparison of shapes

TABLE 2

σ TW ¼ F 1 σ 11 þ F 2 σ 22 þ F 11 σ 11 2 þ F22 σ 22 2 þ 2F12 σ 11 σ 22 þ F66 σ 12 2 < 1, ð7Þ

Hill's parameters R11 [ ]

R22 [ ]

R33 [ ]

R12 [ ]

R23 [ ]

R13 [ ]

Board A

2.3248

1.0

1.2198

1.69

1.0

1.0

Board B

2.576

1.0

1.0512

1.3

1.0

1.0

where F i and Fij are defined as 1 1 1 1 1 ; F2 ¼ t - c ; F 11 ¼ ; σ 11 t σ c11 σ t11 σ c11 σ 22 σ 22 pffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 ; F 66 ¼ 2 ; F 12 ¼ k F 11 F22 , F22 ¼ t σ 22 t σ c22 σ F1 ¼

study is presented. Note that the fit of R33 is important even though

ð8Þ

12

plane stress is approximated, since R33 influences the shape of the and the indices ‘t’ and ‘c’ are for ultimate tensile stress and ultimate

yield surface in the MD-CD plane.

compressive stress respectively. In Equation 8, F 12 and the in-plane ultimate shear stress σ t12 require some extra attention. These cannot

2.5

|

Failure evaluation

be directly determined from tensile and compressive tests and require shear testing where the failure envelope is studied. For the current

Failure is not included in the numerical model but will be evaluated as

study, no such data is given for the two paperboards. Some estima-

a part of the post-processing of the final results using the Tsai–Wu

tions from the literature are required. For F 12 the constant k is chosen

21

stress failure criterion.

27,28

For plane stress, it reads

as k ¼ 0.5, which is suitable for most composites.27,29 In Li et al.,27

7

LINDBERG AND KULACHENKO

for the interested reader, F 12 is analysed not only for closed failure

2.6

|

Finite element model

surfaces but also for open surfaces. The ultimate shear stress σ t12 in Equation 8 may be estimated by using the geometrical mean of the

The simulations are performed with the finite element solver Ansys

tensile strength values in MD and CD, as done by Fellers et al.30 for

2019R1 in a quasi-static regime using an implicit time-integration

evaluation of the compressive modes. This study utilizes the

method. It is common to use explicit solver schemes for models

geometrical mean for the tensile modes as

exhibiting large non-linearities to avoid convergence problems. The

qffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ σ t11 σ t22 :

use of explicit solvers introduces a limitation of the time step, which is

σ t12

ð9Þ

limited by the size of the smallest element in the model and often requires the use of increase rate, mass-scaling, and the use of reduced-integration elements to make the solution solvable. In addi-

In Figure 7 the failure surface is shown for the two boards for zero

tion, to achieve the accuracy for the spring-back, damping or implicit

shear stress. The shape of the two surfaces is very similar, but Board

solver should be used. The motivation for choosing the implicit solver

A fails earlier compared to Board B. It is obvious that the combined

was to avoid these limitations.

stress state allows for considerably higher stress levels than can be

The model consists of the paperboard blank, the blank holder, the punch and the die. The blank is modelled with shell element 181, here

concluded if only the uni-axial tensile tests are studied.

Strain failure is also evaluated, here using maximum strain theory

fully integrated and with five integration points through the thickness.

The tools are modelled as rigid bodies. Due to the symmetry, a quarter

according to

ε11 ε22 ε12 εF ¼ max t ; t ; abs t < 1: ε11 ε22 ε12

model is simulated, as seen in Figure 8. Initially, the blank is located so ð10Þ

that the MD is parallel with the global x-axis and CD with the global y-axis, compare Figure 2A. The blank is meshed with 0.5-mm quad elements over the area

For zero shear strain, the failure envelop is a rectangle limited by the

with the creases, and then up to 1 mm towards the centre of the

uni-axial tensile and compressive strains.

model, as seen in Figure 9. In total, the model consists of about

In Equation 10 the tensile shear strain must be estimated and is in this project estimated in the same way as the tensile shear stress as εt12

qffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ εt11 εt22 :

50 300 elements. This is the finest mesh among those used to address similar problems. The creases are included in the geometry, in total 30 with a depth of 0.05 mm. The depth is based on the average depth

ð11Þ

measured on the actual blank. The depth is considerably lower than for creases used for folding corners of, for instance, boxes but it sufficient for this kind of converting operation. The thickness of the blank

In Table 3 the material parameters for the failure evaluation are listed, which are given by the expression in Equation 8 along with the end value of the MD and CD curves in Figure 5.

F I G U R E 7 The Tsai–Wu failure envelops for the two boards for zero shear stress

in the model is 0.5 mm.

The blank holder is force controlled, with a force of 700N acting on it (a quarter of 2800 N due to the use of quarter symmetry).

8

LINDBERG AND KULACHENKO

TABLE 3

Tsai–Wu parameters F 1 [1/MPa]

F2 [1/MPa]

F 11 [1/MPa2]

F 22 [1/MPa2]

F 66 [1/MPa2]

F12 [1/MPa2]

Board A

1.6e 2

3.3e 2

5.5e 4

2.2e 3

5.5e 4

5.5e 4

Board B

1.4e 2

3.1e 2

4.1e 4

1.9e 3

4.4e 4

4.4e 4

FIGURE 8

The quarter model used in the finite element simulation

It corresponds to the actual force applied on the tray with the dead

coefficient for tools with room temperature was higher, from 0.3 and

weight during the tray forming operation. The influence of the blank

upwards. However, they also showed that for tools holding 60 C the

holder force has been investigated in previous studies. In Wallmeier

friction coefficient was about 0.2, and for 120 C 0.1. Similar findings

31

it was seen that the number of formed wrinkles for blank

were seen in Wallmeier et al.12 but with even lower values; 0.1 and

paperboard depend highly on the blank holder force, but also on the

0.075 respectively. Further, in the manufacturing process, it is possi-

height of the formed product. In Awais et al.13 an optimum blank

ble to apply wax over the corner area to lower the friction when prob-

holder force was found to avoid failure during the forming process,

lems with failure are seen. With this background, the friction

et al.

32

the use of advanced force control was stud-

coefficient is chosen to 0.3 for all contacts, but the critical area where

ied. The current study focuses on the difference between the two

the creases are located, see Figure 10, are modelled with a 0.3, 0.1

paperboards, and so the blank holder force will not be further evalu-

and 0 coefficient of friction to study the influence of the friction in

ated here. The punch is displacement controlled, pressing down the

this area.

and in Tanninen et al.

blank to form the final tray.

During the real tray forming operation the temperature of the die is in the range of 140–180 C, and the punch 30–55 C. Paperboard is

3

|

RESULTS AND DISCUSSION

affected by temperature and highly affected by moister change.9,33 In this case, the operation takes about 1 s, and the temperature and

In the following, the trays are evaluated for stresses, strains, failure

moisture change of the paperboard blank during this time is not fully

and the effect of creases and friction. All results are displayed at the

known. Hence the paperboard material properties are based on the

end of the punch stroke step, that is, when the punch is pressing the

tensile tests performed under ISO standard conditions. The tempera-

blank towards the very bottom of the die. The results are displayed

ture of the forming tools also has a great effect on the paperboard-

for the case of zero friction in the area shown in Figure 10 since this

metal friction coefficient.9,12,34,35 In the current finite element model,

gave the best shape of the tray. The effect of altering the friction

all contacts are modelled as frictional contacts. In Huttel et al.34 the

coefficient in the corner is shown later as a separate section.

friction coefficient was determined to be in the range of 0.2–0.3 for 35

tools holding room temperature. In Lenske et al.

the friction

In Figure 11 the shape of the simulated tray is shown and compared to the real tray. The shape of the simulated tray is to a high

9

LINDBERG AND KULACHENKO

FIGURE 9

The geometry and mesh of the blank. (A) the geometry, (B) the mesh and (C) close-up of the creases

F I G U R E 1 0 Area used for investigating the effect of changing the friction coefficient

extent agreeing with the shape of the real tray. At position ‘A’ in

model uses homogenous material properties, whereas in reality, the

Figure 11, the material is buckling slightly inwards, which is seen in

paperboard consists of several plies through the thickness with vari-

both the real and simulated tray. The same thing is seen at position

ous stiffness and the possibility to delaminate in the folding process.

‘C’. At position ‘D’, a small outgoing ‘buckle’ is seen in both the real

The interplay delamination and subsequent relaxation of stresses are

and simulated tray. At position ‘B’, some difference is seen. Here the

not resolved with the current model. However, for these specific

material in the simulated tray has not folded enough to form the fully

trays, the failure is seen in the lower corner and the current model

correct corner, but it is only a small difference. Further, some more

outside the creased area, and therefore the limitation of not modelling

wrinkles on the edges can be seen in the simulated tray. The appear-

delamination should affect the stresses in this area.

ance of the wrinkles along the edge is undesired as it hinders sealing

In the MD, the maximum stress outside the region of the creases

the tray. The quantification of the wrinkles may be yet another verifi-

occurs in the lower corner, see Figure 12A,B. For Board A, the MD

cation possibility.31

stress here is 85 MPa, and for Board B about 65 MPa. For CD, the stress in the corner becomes about 50 MPa for Board A and 35 for Board B, see Figure 12C,D. The maximum stress in CD and MD does

3.1

|

Normal stresses

not occur in the exact same location.

In Figure 12 the normal stresses in MD and CD are displayed. The stresses are sampled from the mid-plane of the shell elements. For

3.2

|

Total normal strains

both boards, the highest tensile and compressive stress levels, in both MD and CD, arise between the creases. Although high stresses can

The total normal strains in the MD and CD, sampled from the mid-

occur over these areas, the numerical model used in this study possi-

plane, are shown in Figure 13. At the observed failure location, that is,

bly overestimates the stresses in that region. The reason is that the

the lower corner, the strain in MD is about 2% and 1.3% for Board A

10

LINDBERG AND KULACHENKO

F I G U R E 1 1 Virtual twin of the tray. To the left the actual tray after a successful forming operation. To the right, the simulated tray

F I G U R E 1 2 Normal stress distribution sampled from the midplane. In (A) Board A MD stress, (B) Board B MD stress, (C) Board A CD stress and (D) Board B CD stress

and Board B respectively. For the CD, the strain for Board A is high,

would allow for higher strain levels before failure. Hence the Maxi-

almost 12% close the lower part of the creases, and 8% in the same

mum Strain Theory is a conservative approach, something seen in the

location for Board B. The strain levels may seem high but, as shown in

results in Figure 14. In both models, there are many locations where

for instance Hagman et al.,36 can be considerably higher under com-

the strain state is outside of the failure surface, that is, εF > 1, also in

plex loading states than seen in a standard tensile test.

the lower corner for Board B where failure is not seen in production.

In Figure 15 the results using the Tsai–Wu Theory for failure evaluation are shown, see Equation 7. Here it can be seen that Board A

3.3

|

Failure results

experiences a stress state outside of the failure surface (σ TW > 1) in the lower corner, and Board B stays below σ TW ¼ 1 in the lower cor-

The stress and strain levels for the analyses are used to study failure

ner, which is in line with what has been observed in production. For

of the trays. In Figure 14 the failure evaluation using Maximum Strain

both Board A and Board B, the area over the creases has several loca-

Theory is shown. The expression is shown in Equation 10 and, as seen

tions with high values of σ TW . Although failure can occur over these

there, only one component at a time of the in-plane strain compo-

areas as well, the use of homogenous material properties and ignoring

nents is evaluated. In other words, the contribution from the total

delamination leads to an overestimation of the stresses in these

strain state is not considered which in most locations (but not all)

regions.

11

LINDBERG AND KULACHENKO

F I G U R E 1 3 Total normal strain distribution sampled from the midplane. In (A) Board A MD strain, (B) Board B MD strain, (C) Board A CD strain and (D) Board B CD strain

FIGURE 14

3.4

|

Failure evaluation with Maximum Strain theory. In (A) Board A and in (B) Board B

Failure evaluation

numbers of the tensile and the compressive stresses are picked from their respective distribution.

The Maximum Strain Theory was shown to be too conservative for this study. The Tsai–Wu stress σ TW for Board A implies that the risk of fail-

6. Analyse the results to see the probability for failure, that is, probability to pass σ TW = 1, in the critical location.

ure in the lower corner is very high, as seen in Figure 15A, where σ TW reaches 1.7 [-] which is far beyond the failure limit. Board B, however,

So far, point 1 to 4 has been performed. The tensile tests from point

has an area where σ TW reaches 0.8 [-] in the lower corner. In the fol-

2 are shown in Figure 4 and consist of 10 curves in each direction

lowing, these values are further analysed to show the probability of

(MD, CD and 45). The number of points is too low to make a reliable

failure in these locations. The analysis is performed as follows:

estimate of how the tensile strengths spread. However, it has been seen in previous studies37 that the strength parameters for paper can

1. Run FE analysis.

be well described with the Weibull distribution. Hence, the Weibull

2. Identify the distributions for the tensile and compressive strengths

probability density function, PDF, is fitted to the tensile strengths

given by tensile tests.

3. Use the modes, that is, the most frequent value, of the tensile and compressive strength distributions to post-process the FE-model

from the tensile tests performed for this study. The fit is performed utilizing the Matlab wblfit-function. The two-parameter Weibull PDF reads

for the Tsai–Wu stress σ TW .

4. Identify critical locations in the model.

5. With the stress state (σ 11 , σ 22 , σ 12 ) in the critical location, run Monte Carlo simulations of the σ TW (Equation 7) where random

f ðxÞ ¼

β x β 1 ðxηÞβ e η η

ð12Þ

12

LINDBERG AND KULACHENKO

FIGURE 15

Failure evaluation with Tsai–Wu theory. In (A) Board A; and in (B) Board B

Paperboard

Strength

Scale η [MPa]

Shape β [MPa]

Mode [MPa]

Board A

MD tension

60.0

39.6

60.0

MD compression

30.0

39.6

( ) 30.0a

CD tension

29.4

45.3

29.4

CD compression

14.7

45.3

( ) 14.7a

MD tension

70.6

57.0

70.6

MD compression

35.3

57.0

( ) 35.3a

Board B

CD tension

32.3

60.3

32.3

CD compression

16.2

60.3

( ) 16.1a

T A B L E 4 Weibull parameters for the tensile and compressive strengths from Figure 4

a

The negative sign must be added after the fitting procedure.

where the scale η and shape β must be determined. The results of the

1 f ðxÞ ¼ tðxÞβþ1 e tðxÞ η

x μ 1=β tðxÞ ¼ 1 þ β if β ≠ 0 η

fit of Equation 12 to the tensile tests in Figure 4 are shown in Table 4.

Since the Weibull distribution returns 0 for a negative variable, the distributions for the compressive stresses were created by divid-

ð13Þ

ing the tensile test data by two (since the failure in compression is 50% of that in tension) for the MD and the CD respectively, which were then used for fitting of the Weibull function.

The analysis continues with Monte Carlo simulations of the σ TW in the studied locations. As seen in Equation 7, the current stress state

Equation 13 is fitted to the Monte Carlo simulations for the two boards using the Matlab gevfit-function, and the results are shown in Figure 17. The fitted GEV distribution parameters and the modes are shown in Table 5.

(σ 11 , σ 22 , σ 12 ) in the studied location is a part of the σ TW expression. In

With the cumulative distribution function, CDF, for the

Figure 16, a close-up of the corners of the trays is shown. For

fitted GEV functions, the probability of staying under the failure

Board A, the σ TW is about 1.7 [-] in the most critical point, and for

value σ TW = 1 can now be derived. The CDF for the GEV function

Board B the σ TW is about 0.8 [-]. Note that the locations are not the

reads

same. The stress state in the studied location is σ 11 ¼ 75 MPa, σ 22 ¼ 50 MPa and σ 12 ¼ 1:5 MPa for Board A, and for Board B the stress

F ðxÞ ¼ e tðxÞ

ð14Þ

state is σ 11 ¼ 9:5 MPa, σ 22 ¼ 34 MPa and σ 12 ¼ 2:7 MPa. Now Monte Carlo simulations are performed of Equation 7 where

and derives the probability of staying below x. With Equation 14 the

the Fi and F ij coefficients are re-calculated each time with values ran-

probability of passing x ¼ 1, that is, σ TW ¼ 1, becomes for Board A in

domly picked from the Weibull tensile strength distributions. The

practice 100%. For Board B, this probability is about 1%, or 1 out of

values picked from the distributions for the compressive stresses are

100 trays can be expected to fail in this location.

multiply by 1 to make the stresses negative. The distribution of the

The parameters in Table 5 only apply for the specific stress state

σ TW Monte Carlo simulations is expected to have a skewness, partly

(σ 11 , σ 22 , σ 12 ) since these are included in the expression for the

due to the Weibull distributed input strength parameters, and partly

Tsai–Wu stress, and hence affect the results of the Monte Carlo simu-

due to the non-linearity of the Tsai–Wu stress equation itself and its

lations. Hence, both point 5 and 6 must be performed each time a

parameters, see Equations 7 and 8. The distribution with the best fit

new location in the model is evaluated. That is, new Monte Carlo sim-

to the Monte Carlo simulations was the Generalized Extreme Values

ulations must be performed for the studied stress state and then the

(GEV) distribution. The GEV PDF reads

risk of getting failure must be evaluated again. Other examples of how

13

LINDBERG AND KULACHENKO

FIGURE 16

Close-up of critical points in the lower corner using the two different boards. In (A) Board A, and in (B) Board B

F I G U R E 1 7 Histograms of the Monte Carlo simulations of the Tsai–Wu stress and fitted GEV PDF. (A) Plot of results for Board A, and in (B) for Board B the Tsai–Wu stress deviates depending on the stress state can be 38

seen in, for instance, Mukherjee et al.

It should be emphasized that at σ TW ¼ 1 failure is initiated, and is not necessarily the same thing as visible failure. The number of trays with visible failure can be expected to be fewer than what has been

TABLE 5 Figure 17

GEV distribution parameters for the fitted PDF seen in Scale η [ ]

Shape β [ ]

Mode μ [ ]

Board A

0.154

0.067

1.71

Board B

0.053

0.003

0.78

Paperboard

calculated in this study. Further, the stochastic approach can be extended to include a variation of the stress state. This means having a variation of the load or the material parameters in the constitutive

considered to be quite a controlled process, but friction possibly adds

equations. The assumption of zero variation of the stress state is of

a small variation of the stress levels in the paperboard blank. The non-

course non-conservative. The pressing of the punch can be

uniformities of paperboard result in a variation of constitutive material

14

LINDBERG AND KULACHENKO

parameters, and the drying constraints during the manufacturing of

advantage of keeping the friction low in the corner. The shape closest

paperboard will further contribute to a variation, as numerically

to reality is given by the simulation using zero friction in the corner,

proved in Alzweighi et al.39 To include a variation of constitutive

shown in Figure 18C, which is also the model used for the above

parameters requires further testing but, if acquired, is then straightfor-

presented results.

ward to include in the analysis.

The approach may also be more exact if testing of the materials

In Figure 19, the effect of the friction in the corner on the Tsai–Wu stress σ TW is shown. Here it is seen that the area where

size dependency is performed. As mentioned earlier, the failure

σ TW > 1 is very large for the cases with a friction coefficient of 0.3 and

stresses and strains for paperboard have been shown36 to have a size

0.1. In Figure 19C the friction coefficient is zero, and this is the same

dependency, that is, locally allowing for higher stresses and strains

results as shown in Figure 15A. The results strongly support means of

than seen in standard tensile tests. Data for material size dependency

lowering the friction, such as the use of lubricants or increased die

was not accessible for this study and is hence not included, and in that

tool temperature, in the forming operation to aid problems with

sense the approach is conservative.

failure and bad shapes.

The effect of the polymer extruded on the paperboard blank

In Figure 20 the influence of the creases is shown. The model has

before the tray forming operation could also be investigated. This

zero friction in the corner area and can be compared with the results

coating may affect the upper ply allowing for higher failure strain.40 Finally, the use of shell elements leaves the contribution of the out-of-plane (ZD) stress outside of the analysis. The ZD stress will influence the stress-based failure surfaces, locally allowing for both lower and higher stress levels.

3.5

|

Effect of friction and creases

In the following the numerical model is used for investigating the influence of friction and the importance of including the creases in the geometry. We used only Board A for this evaluation as the results are similar to Board B. The effect on the shape of changing the friction coefficient in the corner (the area shown in Figure 10) is shown in Figure 18, where the friction coefficient in (a) is 0.3, in (b) 0.1 and in (c) 0. It is obvious that the friction makes the forming operation harder, and that it is an

FIGURE 20 creases

The shape of the formed tray without the use of

FIGURE 18

Effect of friction coefficient in the corner on the shape. (A) Friction coefficient 0.3, (B) 0.1 and (C) 0

FIGURE 19

Effect of friction coefficient in the corner on the Tsai–Wu stress. (A) Friction coefficient 0.3, (B) 0.1 and (C) 0

15

LINDBERG AND KULACHENKO

in Figure 18C. The model with no creases renders in a shape very dif-

statistical distributions of the paper properties on the relevant scales.

ferent to that observed in the physical samples even for low friction

The latter is outside the scope of this work.

and shows how important the creases are to avoid unwanted shapes.

Without the creases, the material is having a hard time to fold leading

AC KNOWLEDG EME NT S

to a large amount of material left on the upper edge at the end of the

The authors would like to thank Johan Lindgren, Brita Timmermann

forming process.

and Tommy Ström at Iggesund Paperboard for their inputs and support during this study. The authors are also grateful to Hannes Womhoff at Holmen AB for his input.

4

|

CONCLUSIONS

This work has been carried out within the national platform Treesearch and is funded through the strategic innovation programme

An implicit non-linear finite element model with full-integrated shell

BioInnovation, a joint effort by Vinnova, Formas and the Swedish

elements is built to simulate the tray forming process of paperboard.

Energy Agency.

Two different boards with different failure propensities are investi-

The computations were performed on resources provided by the

gated. The creases are included in the geometry of the paperboard

Swedish National Infrastructure for Computing (SNIC) at HPC2N

blank which is a new approach compared to earlier studies. The mate-

(projects SNIC 2020/5-428 and SNIC 2021/6-51).

rial data are extracted from tensile test curves of the investigated

The authors are grateful for the financial support.

paperboards, and the used material model includes different behaviour in tension and compression. The numerical tensile test curves

DATA AVAILABILITY STATEMEN T

have a very good fit with the experimental tensile test curves.

The data that support the findings of this study are available from the

The results from the tray forming simulation show a very good

corresponding author upon reasonable request.

agreement of the shape between the real tray and the simulated trays, which is a clear improvement compared to earlier studies of the

OR CID

paperboard tray forming process. The results show that including the

Gustav Lindberg

https://orcid.org/0000-0001-5033-7611

creases in the geometry is very important to acquire the correct shape observed in reality. The incorrect shape is associated with an increased risk of failure of the tray during the converting operation.

Further, the model shows that the friction between the paperboard blank and the die and blank holder has a great effect on the shape of the tray. High friction leads to incorrect shape and an increased risk of failure. The model supports the measures used in the industry to lower the friction during the converting operation. The area with the creases has several locations with large stresses. Although failure can occur over these areas as well, the use of homogenous material properties and ignoring delamination leads to an overestimation of the stresses in these regions. The failure evaluation using Maximum Strain Theory shows small differences between the two paperboards but is deemed too conservative. The failure evaluation using Tsai–Wu theory shows that Board A has a very high risk of failure, and that the risk of failure using Board B is lower, something that is in full agreement with what has been seen in production. The results from the stochastic failure evaluation based on the numerical model more precisely suggest that the risk of failure in the lower corner using Board A is close to 100%, and for Board B the risk of failure is about 1%. No known problems with Board B are reported from production and the here calculated failure risk is probably conservative.

The model may be further used to estimate the most critical material properties for the tray forming such as elongation, strength, and friction. Among these, in this study, we only probed the impact of the friction in the corner. A stochastic approach is used as a part of the post processing to study the impact of strength. However, the stochastic nature of paper makes it difficult to include the elongation and strength directly in the model as these parameters can vary locally and therefore require characterization and modelling which includes

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How to cite this article: Lindberg G, Kulachenko A. Tray forming operation of paperboard: A case study using implicit finite element analysis. Packag Technol Sci. 2021;1-16. doi:10.1002/pts.2619

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 3, 2021

Influence of Maintenance Actions in the Drying Stage of a Paper Mill on CO2 Emissions 1

LUIS MIGUEL CALVO AND ROSARIO DOMINGO

2

Greenhouse gases from industrial activities have become a global problem. Emissions management is being developed to raise awareness of the importance of controlling pollution in general and atmospheric emissions in particular. In 2017, the deficit of the rights of issuance in the industrial sectors increased up to 8.3% (verified emissions in 2017 versus allocation in 2017). This trend will increase more at the end of Phase III due to a progressive reduction in allocation. Phase IV will be much more restrictive in allocating emission rights than Phase III. The extra cost of this deficit reinforces the need for industry in general to reduce CO2 and for the paper industry to reduce GHG emissions and generate credits. Old factories are typically identified as sources of pollution in addition to being inefficient compared to new factories. This article discusses the possibilities offered by maintenance actions, whose integration into a process can successfully reduce the environmental impact of industrial plants, particularly by reducing the CO2 equivalent emissions (CO2-eq units henceforth CO2) they produce. This case study analyzes the integration of maintenance rules that enable significant thermal energy savings and consequently CO2 emissions reduction associated with papermaking. Managing Key Performance Indicators (KPIs), such as the amount of cold water added to the boiler circuit and the conditions of the air blown into the dryer section hood, can be used as indicators of CO2 emissions generated. The control of the water and temperature reduces these emissions. A defined measure—in this case, t CO2/t Paper—indicates an achievement of a 21% reduction in emissions over the past 8 years.

Contact information: 1. Department de Mechanical Engineering, Energy and Materials, Universidad Pública de Navarra, C/Tarazona, km 2, 31500 Tudela, Spain 2. Department of Construction and Manufacturing Engineering, Universidad Nacional de Educación a Distancia (UNED), C/Juan del Rosal 12, 28040 Madrid, Spain Processes 2021, 9, 1707. https://doi.org/10.3390/pr9101707 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 5 – Drying Section & CO2 Emissions

processes Article

Influence of Maintenance Actions in the Drying Stage of a Paper Mill on CO2 Emissions Luis Miguel Calvo 1 and Rosario Domingo 2, * 1

2

*

Citation: Calvo, L.M.; Domingo, R. Influence of Maintenance Actions in the Drying Stage of a Paper Mill on CO2 Emissions. Processes 2021, 9, 1707.

Department de Mechanical Engineering, Energy and Materials, Universidad Pública de Navarra, C/Tarazona, km 2, 31500 Tudela, Spain; luismiguel.calvo@unavarra.es Department of Construction and Manufacturing Engineering, Universidad Nacional de Educación a Distancia (UNED), C/Juan del Rosal 12, 28040 Madrid, Spain Correspondence: rdomingo@ind.uned.es

Abstract: Greenhouse gases from industrial activities have become a global problem. Emissions management is being developed to raise awareness of the importance of controlling pollution in general and atmospheric emissions in particular. In 2017, the deficit of the rights of issuance in the industrial sectors increased up to 8.3% (verified emissions in 2017 versus allocation in 2017). This trend will increase more at the end of Phase III due to a progressive reduction in allocation. Phase IV will be much more restrictive in allocating emission rights than Phase III. The extra cost of this deficit reinforces the need for industry in general to reduce CO2 and for the paper industry to reduce GHG emissions and generate credits. Old factories are typically identified as sources of pollution in addition to being inefficient compared to new factories. This article discusses the possibilities offered by maintenance actions, whose integration into a process can successfully reduce the environmental impact of industrial plants, particularly by reducing the CO2 equivalent emissions (CO2 -eq units henceforth CO2 ) they produce. This case study analyzes the integration of maintenance rules that enable significant thermal energy savings and consequently CO2 emissions reduction associated with papermaking. Managing Key Performance Indicators (KPIs), such as the amount of cold water added to the boiler circuit and the conditions of the air blown into the dryer section hood, can be used as indicators of CO2 emissions generated. The control of the water and temperature reduces these emissions. A defined measure—in this case, t CO2 /t Paper—indicates an achievement of a 21% reduction in emissions over the past 8 years.

https://doi.org/10.3390/pr9101707

Keywords: industrial process; CO2 emissions; maintenance; papermaking Academic Editor: Dominic C. Y. Foo Received: 2 June 2021 Accepted: 18 September 2021 Published: 23 September 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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 In recent years, the relationship between greenhouse gas (GHG) emissions from industrial activities and global warming has been highlighted by the United Nations Climate Change Conferences [1] and the European Commission [2]. GHG emissions from the use of fossil fuels, such as natural gas, often occur because of manufacturing processes, which is considered necessary and inevitable. Most industrial plants have reduced GHG emissions by replacing equipment with more modern, energy-efficient technology as can be seen in the application of additive manufacturing technology to a radiant tube used to improve its radiant heat efficiency [3], in the use of innovative design solutions in drive unit control systems of mobile wood-chipping machines [4], or in the use of finite element methods to optimize parameters of piercing punch [5]. Equipment deteriorates with use and, after some time, does not run at its designed parameters. The activity linked to maintenance is fundamental in the industry. Alsyouf [6] finds that approximately 13% of his time is spent planning maintenance activities and 33% is dedicated to unplanned tasks. The implementation of periodic maintenance tasks is related to the improvement in production indicators [7,8]. Domingo and Aguado [9] also investigated the relationship of sustainability with the main indicator of the TPM, the OEE (Overall Equipment

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https://www.mdpi.com/journal/processes

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Effectiveness). This deterioration may be due to various reasons (e.g., waste increases [10], changes in the operating parameters [11], and deficient maintenance [12]), resulting in far lower and less efficient performance than initially desired. These GHGs may be grouped according to the Kyoto Protocol into carbon dioxide (CO2 ), methane (CH4 ), nitrogen oxide (N2 O), hydrofluorocarbons (HFC), perfluorocarbons (PFC), and sulfur hexafluoride (SF6 ) (IPCC 2014). The GHG emissions can be determined by aggregation into CO2 equivalent (CO2 -eq) units (henceforth CO2 in this article).

In the European Union (EU), the paper industry uses a fuel mixture consisting of 32% renewable energy from biomass, 3% fuel oil, and 65% natural gas [13,14]. Biomass is considered to be carbon neutral by the Intergovernmental Panel on Climate Change (IPCC) [15]. Almost all pulp and paper mills are a part of the EU Emissions Trading System (ETS), which has been in place since 2005. The direct emissions falling under the EU ETS mainly come from natural gas combustion and must be covered by credits, some of which are received for free and the remainder of which are bought at government auctions. This is a burden that competition in the pulp and paper industry in non-European regions does not have to bear. Among the GHG emissions by the European paper industry, the most important indicator in terms of amount and effect on global warming is CO2 emissions.

The great concern regarding the effects of CO2 emissions in the intermediate and long term has led to awareness that such emissions must be curtailed to achieve a sustainable society [13,14].

Austin [16] showed that it is possible to reduce power consumption by controlling the variables associated with processes. Virtanen et al. [17] demonstrated the relationship between energy efficiency and productivity. In the pulp and paper industry, the current concepts of energy efficiency apply only to fossil fuels and are based on optimizing the use of energy to reduce consumption. Siitonen et al. [18] and Moya and Pardo [19] discussed the relationship between energy efficiency and CO2 emissions in terms of the paper industry.

Moya and Pardo [19] also observed how the adoption of the Best Available Technologies (BAT) enhanced by energy efficiency policies can help to reduce CO2 emissions and be economically viable through the savings produced. However, the papermaking sector is very reluctant to prove new techniques due to the large investment required and the long life of the production facilities [20].

There are process variables that affect both energy efficiency and emissions as indicators for the paper industry [18], but they are not typically used to establish emissions reduction strategies as a result of planned maintenance measures or to address CO2 emissions. Calvo and Domingo [21] demonstrated the relationship between the effectiveness of machinery processes and the influence of process conditions, equipment maintenance and operational parameters, and the generation of CO2 emissions. Supervision and knowledge of the actual state of machinery operation and maintenance can lead to significant energy savings. This may result in a reduction in CO2 emissions associated with the process.

Investments in plant maintenance and improvement in energy efficiency can result in reduced CO2 emissions [22] and can be profitable by themselves.

Due to the pulp and papermaking process, which is an energy-intensive process, a study on installations and the reductions in CO2 emissions are required. Papermaking has established indicators that relate CO2 emissions to the energy efficiency of the drying process, called product benchmarks, which record emissions levels of 10% for the more efficient production processes for each grade of paper within the European Union [23–25].

This benchmark provides a valid measure of the effectiveness of the process indicators and allows us to compare the studied process to those of other factories in the same sector that produce the same products. The proper control of emissions can provide a competitive advantage for a particular industrial plant over others or could compromise its continuity due to inadequate management.

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1.1. Sustainability and Efficiency in Papermaking Sustainability and efficiency are difficult to implement simultaneously in manufacturing plants [26]. This difficulty arises from identifying tools needed and quantifying the improvements achieved. This issue was addressed by the indicator identified by Calvo and Domingo [21], ‘t CO2 /t Paper’. The identified indicator was considered during the drying stage of the papermaking process and indicates the relationships among energy consumption, process efficiency, CO2 emissions, and machine availability variables.

Calvo and Domingo [27] studied the influence of several other factors, including web paper input to the thermal drying section, how it affects the indicator ‘t CO2 /t Paper’, and whether it affects the measures of efficiency and sustainability in papermaking.

Moya and Pardo [19] investigated how adopting the BATs can be a cost-effective contribution to reducing CO2 emissions and achieving European Union targets through energy-efficient policies. However, Del Río González [28] noted how difficult it was for the papermaking industry to introduce the BATs or cleaner technologies due to the large investments required and the technical complexity associated with the papermaking process. The intention of the European Authorities with Decision 2010/2/UE [29] is to create a costly EU ETS to force industries to significantly reduce their emissions levels.

Ghose and Chinga-Carrasco [30] reviewed CO2 emissions through a life cycle assessment and found that up to 85% of the total energy in papermaking, depending on the paper product, is used for paper drying. The drying process in papermaking affects paper characteristics and cannot be designed by considering only the energy efficiency. Karlsson [31] reviewed the main parameters that affect the evaporation process in the drying phase, and Hostetler et al. [32] studied web temperature throughout the drying process to determine the drying conditions that ensure optimal paper quality at minimal cost.

There is considerable information regarding the drying phase. Laurijssen et al. [33] studied the influence of dryer elements on the drying process and proposed actions to decrease heat use in conventional multicylinder drying sections and calculated their effect on energy use. The main optimization measures to be implemented in the drying phase include decreasing the heat used to evaporate water by increasing the air dew point temperature of the dryer section, as noted by Laurijssen et al. [33]. This measure is difficult to implement due to the poor insulation condition of the drying hood.

Other measures increase the amount of heat recovered by using the exhaust air to preheat the blown air and water. Ruohonen et al. [34] described the energy required to heat air in the papermaking process and the steam needed to heat the inlet air provided to the drying hood after heat recovery, highlighting the importance of heat recovery systems. The influence in this case is clear; Calvo and Domingo [21] identified the relationship between the external air conditions and energy utilization in the drying stage and found that recovering the energy to preheat blown air requires less steam energy to heat the air hood.

By focusing on the conditions in the drying stage, Sivill et al. [35] found that the humidity of the hood exhaust air affects the efficiency of the heat recovery rate and inlet blown air temperature. Heat recovery is used in the drying stage to reduce the energy used to dry the paper. Sivill and Ahtila [36] studied the recovery of energy from the exhaust air, which directly results in reduced CO2 emissions. Zvolinschi et al. [37] found that regulating the temperature of steam in each dryer section can reduce the energy demand by up to 3%, and they also discussed the effect of humidity in the exhaust air, which can achieve energy savings of up to 35%. Ruohonen et al. [34] indicated that all previously considered options can be used to reduce the CO2 emissions of a mill. Kong et al. [38] compiled the available information on energy savings, environmental costs, and commercialization status for 25 emerging technologies to reduce the energy use and CO2 emissions in the paper production process. Including four drying sections (gas-fired dryers, boost drying, and microwaves) may be an alternative in the future for improving the multicylinder drying efficiency; however, this measure is not widely used. In line with the previous study, Kong et al. [39] used the conservation supply curve to measure the potential savings, from both the engineering and economic perspectives of energy, to show potential opportunities

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for achieving savings. We conclude that the drying section has three of the most important fuel-saving technologies: cost-effective and significant energy and CO2 emissions savings in the steam and condensate loop and enclosure hood. Calvo and Domingo [40] analyzed the importance of the monitoring and control of the drying parameters to achieve an efficient use of the energy in the drying process of the paper and the savings obtained in both energy used and CO2 emissions by improving the control of the parameters involved in drying enclosure hood sections.

A stricter environmental legislative framework can achieve emissions reductions if it considers the implications of the regulatory framework on industrial activity. Silvo et al. [41] studied the influence of European IPPC regulations in Finnish pulp and paper mills with regard to BATs. Wang et al. [42] studied the effect of stricter environmental regulations in Shandong Province and found that most of the efficiency indicators (except CO2 emissions) improved significantly with the implementation of stricter regulations.

These facts necessitate a review of the equipment and operating parameters associated with papermaking, particularly with regard to the energy efficiency of the drying process.

Control of the CO2 emissions associated with energy consumption is even more necessary.

As illustrated above, CO2 emissions are significantly affected by the condition and operating parameters of the drying process. Placing a limit on CO2 emissions may represent an actual limit on facility use. Thus, the importance of monitoring and controlling such emissions is clear. In addition, we need to know the influence of maintenance tasks in this type of facility, which has been studied by analyzing the evolution of some typical indicators such as the overall equipment efficiency and mean time to failure [43] or the influence of downtimes on CO2 emissions [21]. Although Nakajima [44] showed the advantages of involving workers in maintenance tasks, the effect of the involvement of workers on the definition of new indicators and the analysis of its evolution has not been considered in the literature on papermaking.

1.2. Objectives In 2017, the deficit of the rights of issuance in the industrial sectors, the verified emissions in 2017 versus the allocation in 2017 increased to 8.3% according to the European Commission [45]. This trend will increase due to a progressive reduction in allocation.

Reductions in emissions have been frequently associated with the adoption of new technologies and/or the replacement of fossil fuels with renewable energy sources.

Approximately 70% of the total primary energy in China comes from coal according to Zhang and Liu [46]. In China, Peng et al. [47] pointed out that the savings in total energy consumption was due mainly to technology updates, policy changes, and fuel substitution.

Another measure for reducing emissions and improving energy efficiency is declaring many plants obsolete and decommissioning production facilities. Regarding the pulp and paper industry in China, the authors indicate the potential for CO2 reductions through energy efficiency improvements and the application of wide-scale development of the BAT [48]. The predominant fuel used in China is coal. A priority strategy for reducing CO2 emissions is fuel substitution.

The implementation of maintenance related to the improvement in production indicators [7,8] and the relationship of sustainability with OEEs analyzed by Domingo and Aguado [9] do not specify maintenance guidelines that jointly improve the efficiency and sustainability of the production process.

This article adopts the methodology of the case study to identify methods for reducing emissions without substituting machinery or fuel sources. The case study is widely accepted in the scientific literature, and Fidel [49] recommends its use when there are many factors and relationships in the phenomenon to be studied and when the factors or relationships can be directly observed or measured, and without these premises, you can determine its importance. The systematic collection of data provides rigor to research and avoids biases in interpretation, the lack of which is a weakness sometimes attributed to this methodology, but this can be addressed, according to Flyvbjerg [50].

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The current work investigates options for reducing emissions without machinery replacement or fuel substitution by focusing only on the maintenance of machinery and minor reforms of maintenance guidelines.

The study does not seek to determine OEEs, characterized by relating the availability of equipment, their performance, and the quality rate [44], but analyzes some of these elements together with CO2 emissions and the identification of patterns that facilitate their increase while reducing emissions.

The main aim of this study is to analyze the management of environmental indicators, such as ‘t CO2 /t Paper’, and its decomposition into several subcomponents to identify their effects and the variables involved in the drying process on CO2 emissions.

This work does not consider indirect emissions, such as those from other companies, including purchased electrical power, raw materials, the transport of finished products, or wastewater treatment.

2. Methodology The methodology is based on the case study, which is focused on papermaking. The facility under analysis uses 77% of its total energy on drying paper using steam. This section includes a description of the papermaking process, the methodology used to determine the CO2 emissions, the data collection strategy, and the maintenance levels.

This work considers the CO2 -eq emissions produced by the plant from the combustion of natural gas used to generate steam for drying paper (in the studied factory, this is the only source of thermal energy used in the dryer section). These emissions are included in the ETS scheme.

2.1. Levels of Maintenance To manage these indicators, TPM and techniques such as total employee involvement (TEI) and continuous performance improvement (CPI) are analyzed and implemented [51].

TEI is implemented to motivate maintenance and production personnel, creating mixed work teams to analyze conditions where participation is facilitated and encouraged to generate action guidelines. Three levels of action are proposed depending on the frequency of analysis, follow-up, and/or action required.

CPI actions are small changes; in particular, employee observations are considered, and initially, major reforms or new installations are not, making improvements that are measurable and repeatable and that are as inexpensive as possible. Employees are made accountable for improvements.

This methodology begins to be implemented at the end of 4 years, and regarding maintenance, three levels are defined to verify the influence of several indicators, called subindicators, on the indicator ‘t CO2 /t Paper’: The first level of action involves maintenance and manufacturing workers, who collect and monitor the process data that relate directly or significantly to the parameters they can control. These records are collected frequently enough that maintenance or production workers can involve themselves in follow-up.

The second level reviews the monthly data collected by the first action level; this involves technical personnel and environmental management staff, who check whether the values obtained are compatible with the historical consumption and recent measures taken to reduce the indicator.

The third level of action features bimonthly meetings involving a small group of people from the second and third levels. This level analyzes the variation in the identified subindicators that affect the main indicator ‘t CO2 /t Paper’. This third level takes the necessary actions to adjust the desired value of each subindicator to reduce the main indicator, ‘t CO2 /t Paper’.

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2.2. Description of the Papermaking Process Pulp and paper can be separated into two clearly differentiated and interconnected subsectors: pulp production and papermaking, which can be in a single location or separate.

The first, pulp production, uses mainly wood as a raw material or recycled paper to transform into pulp. The pulp or furnish is screened to remove impurities.

The papermaking plant can also be divided into four main phases: the sheet section, press section, drying section, and finishing section.

As seen in Figure 1, the pulp is converted into paper. In the first section (forming section), a sheet is formed by feeding the pulp in a fourdrinier machine, with a moving belt of fine mesh screening to remove water initially by gravity and later by applying a progressive vacuum, reaching a fiber content of 25%. Subsequently, progressive mechanical pressing begins (press section), eliminating the water until a dryness of approximately 52% is reached. A final dryness of 6% is reached in the drying section, which is composed of a series of steam-heated drying cylinders, and the water is removed by thermal means. The process can have a section of smoothing and/or calendering; later, the paper is cut to the measure requested by the client and packed for later issue.

Figure 1. Process flow.

This study is focused on the drying section, in which the water remaining in the incoming paper sheet is eliminated by thermal means; the initial moisture coming into this section is close to 52%, which the drying process reduces to approximately 6%, depending on the type of paper and the customer’s requirements. The drying section of the studied facility has 38 drying cylinders internally heated by steam, which is supplied by only one steam generator. The steam energy is transferred to the sheet when the paper contacts the drying cylinders. The energy transferred to the sheet causes water evaporation and consequently dries the sheet. A ventilation hood system is used to remove this evaporated water from the sheet; this system controls the main variables associated with air circulation inside the hood. The air extracted from the hood passes through a heat recovery system to heat the air blown into the hood (which compensates for the air previously extracted, maintaining a slight reduction in air). This system ensures energy recovery and directly

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affects emissions. This section produces direct emissions of CO2 , which are included in the EU ETS scheme.

The facility under study produces 55,000 tons per year of uncoated carton board using 100% recycled paper as a raw material. The grammage ranges from 200 to 650 g/m2 , and the unit used for grammage is the surface density, according to ISO 536 [52], the international reference in this industrial sector. The machine speed varies between 1.30 m/s for heavy paper and 4.16 m/s for light paper. Figure 1 shows the installation studied.

Note that the emission level of the plant under study is close to the corresponding product benchmark. The emissions associated with the factory are in accordance with the benchmark or even lower than the benchmark.

Before the study, no special actions were carried out to maintain the isolation facilities of the enclosure hood. Due to maintenance operations, some panels had deteriorated, and the adjustment between them due to disassembly and assembly operations was far from optimal. Regarding the cold water system, the maintenance was outsourced, and control and tracking of the associated KPI or main parameters were carried out only monthly by the external company.

The air exchangers of the enclosure hood were checked only once a year without cleaning and maintenance routines.

2.2.1. System of Cold Water Added to the Boiler Circuit The drying section heats the paper sheet to remove the remaining moisture by thermal processes. The energy is supplied from steam generated in the boiler. When the latent heat of the steam is transferred to heat the paper sheet, the steam condenses into water, which is also known as ‘condensate’. The factory has a closed circuit to recover all the condensate generated in the dryer section.

Most of this condensate is used to heat the air blown to the dryer hood and is then fed to the boiler feedtank. The feedtank is the major location where cold reserve water and condensate return meet. The feedtank provides a reserve of water to cover any interruption of the cold water supply and should have sufficient capacity above its normal working level to accommodate any surge in the rate of condensate return. In the dryer section, the steam passes through various regulation valves to adjust the dryer temperature and enters the cylinder section through a rotating joint; here, the steam changes state back to condensate, which is high-quality hot water.

Condensate is an ideal boiler feedwater. From an economic perspective, reuse is desirable; however, in practice, reuse is not possible due to rotating joint and valve losses, and there will typically be water loss from the boiler via blowdown.

A high condensate return rate can occur at start-up when the condensate is lying in the plant and pipework is suddenly returned to the tank, where it may be lost to the drain through the overflow.

This subindicator affects the boiler efficiency, hood heat recovery, condensate recovery, and steam transport.

The condensate return rate represents the addition of cold water needed by the feedtank to maintain the feedtank level and to supply the steam demanded by the dryer section. The return of condensate represents considerable potential for energy savings in energy and CO2 emissions.

Blowdown is necessary for the correct quality and amount of steam to be generated by the boiler. The boiler generates steam, and any impurities in the boiler feedwater that do not boil off with the steam concentrate in the boiler water. The dissolved solids become increasingly concentrated, and the steam bubbles tend to become more stable, failing to burst as they reach the water surface of the boiler. There comes a point (depending on the boiler pressure, size, and steam load) at which a substantial part of the steam space in the boiler becomes filled with bubbles, and foam is carried over into the main steam. This is undesirable because this steam is excessively wet as it leaves the boiler and contains a high level of dissolved and perhaps suspended solids, which can contaminate and possibly

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damage control valves and heat exchangers. The control of the total dissolved solids (TDS) in the boiler water level ensures that the risks of foaming and carryover are minimized.

The rate of theoretical blowdown Bdr (m3 ) is calculated as the balance of the TDS by Equation (1). The difference between them and the total cold water added to the feedtank represents the installation losses. Cw is an indirect indicator of these installation losses.

Bdr = (Feed water TDS· · · Steam generation)/(Required boiler water TDS − Feed water TDS),

(1)

where: Feed water TDS refers to the current TSD contained in the water (ppm).

Steam generation corresponds to the steam flow rate generated by the boiler (kg/h).

Required boiler water TDS is measured according to EN 12953 [53].

The difference between the theoretical blowdown rate and cold water added in Equation (2) represents the total installation losses Il (m3 ) due to leakage from control valves, drying cylinders, and gaskets.

Il = Cw − Bdr, (2) To control these parameters, the plant implements a system to monitor the basic boiler, steam consumption, and condensate recovery parameters. To manage these indicators, the first-level-of-action personnel are assigned the task of tracking the parameters three times a week instead of laboratory personnel. This approach involves the maintenance staff in preventing the overconsumption of water Il and reviewing steam production and distribution leakages.

TPM and techniques such as TEI and CPI are analyzed and implemented [51].

The personnel are trained in TEI and CPI techniques to perform these checks; every fortnight, the analyzed values are contrasted by laboratory personnel to verify that the parameter measurements and the interpretation of the results are correct. New forms are created to record the main parameters of the steam and condensate installation at least three times a week.

If the TDS in the boiler water decreases (Equation (2)), the blowdown is higher than needed, and the blowdown frequency must be adjusted. In contrast, the blowdown flow must be increased if the TDS increases.

When the blowdown is correctly adjusted, Il comes from the distribution and consumption system. Then, the first-level-of-action personnel look for other losses and act to correct them.

Historical maintenance data indicate that major losses come from rotary joints in the drying cylinders installed in the dryer section. Rotary joints are responsible for introducing steam into the dryer cylinder and removing the condensate formed due to the energy transferred by the steam to the web. Assuming proper installation, the major causes of steam losses in the rotary joints come from carbon seal wear. Initially, the factory rules said to change the entire rotary joint (to repair it) only when steam leaks were detected. If the lost steam had no greater importance, the joint was generally kept running as long as the leak was small. Its replacement was postponed until the next technical or maintenance shutdown. The supply of steam to the dryer cylinder was closed only when the leak affected the quality of the paper produced. A closed dryer cylinder resulted in a smaller heating area, lower drying capacity, and inefficiency in the dryer section.

A previously unused maintenance strategy is developed to control carbon seal wear.

Instead of applying the manufacturer’s maintenance rules, which require qualified personnel to disassemble part of the rotary joint, a new rule is developed based on taking the absolute and angular positions of the item. The establishment of this rule provides a preventive maintenance program as an alternative to waiting to detect steam leakage to reduce steam spills and maximizes the amount of condensate recovery. This rule identifies the state of wear in the carbon seal, thus providing the necessary time to organize appropriate maintenance.

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This action controlling the wear state of the rotary joint has been performed since the responsibility of controlling the fresh water (Cw) boiler transferred to maintenance personnel. The maintenance personnel now understand the entire circuit of steam and condensate and are aware of the amount of steam lost by rotating joints in poor condition and the benefits of controlling its maintenance state. This control is fundamental for reducing the scheduled maintenance of the drying section recommended by the manufacturer and preventing steam losses.

2.2.2. Temperature of the Air Blown into the Dryer Section Hood The drying section enclosure hood has two exhausts—hot air from inside the hood and a fan that blows hot air at the same flow rate to compensate for the extracted air. This air stream is passed through a series of three heat exchangers that harness the heat energy contained in the exhaust air stream, generate flash steam in the last condensate collection tank, and collect water condensate, which is returned to the boiler.

To determine the airflow, all the technical fan characteristics, including the consumption curve airflow (m3 ) through which the flow of air is introduced into the hood, must be known.

The motor power consumption is determined by the operating parameters of the fan.

2.3. Data Collection The calculation of the subinstallation output production (Pp) to set the indicator in reference periods is based on the following guidelines indicated in methodological guidelines for ECOFYS for the European Commission [54–56]. The study is based on a representative period of production in which there are neither changes in machine conditions nor alterations that change the production capacity. The following parameters are collected to determine the amount of paper produced and energy consumption:

• • • • • • • • • • • • • • • • • • • • •

Pp: Paper production (t).

Ms: Machine speed (m/min).

Pd: Manufactured paper density (g/m2 ) (area density, according to ISO 536 [52].

Pwi: Paper width entering the drying section (mm).

Paper conditions before and after the drying section: Twt: Water temperature at the entrance to the drying section (◦ C).

Pti: Paper temperature entering the drying section (◦ C).

Pmi: Moisture content of the paper entering the drying section (%).

Pto: Paper temperature leaving the drying section (◦ C).

Pmo: Moisture of the paper leaving the drying section (%).

Drying section conditions: Sf: Drying section feed steam flow rate (kg).

St: Drying section feed steam temperature (◦ C).

Sp: Drying section feed steam pressure (bar).

Ct: Temperature of the condensates extracted from the drying section (◦ C) Cp: Pressure of the condensates extracted from the drying section (bar).

Eat: Temperature of the exhaust air (◦ C).

Eam: Moisture content of the exhaust air (% saturation).

Ebt: Temperature of the blown air (◦ C).

Ebm: Moisture content of the blown air (% saturation).

Ot: Outside temperature (◦ C)

The parameters indicated above are used in this work as follows: Ms, Pwi, and Pd are used to calculate the subfacility output Pp in a period and later to calculate the indicator under study.

Sf, St, Sp, Ct, and Cp are used to calculate the activity data (Equation (3)).

To perform this analysis, we use the machine’s control instrumentation based on Beckhoff hardware with the SCADA Wonderware display system ‘Smart Control’ Quality Control System (QCS) and Process Control System.

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The study begins by analyzing the historical available data of the indicators related to ‘t CO2 /t Paper’, the patterns for maintenance and minor reforms that have been proposed and implemented. The introduced indicators have been detailed in terms of their relation with energy-efficiency saving opportunities in the subsections identified by Kong et al. [57] on paper machines (boiler efficiency, the implementation of heat recovery, and exhaust humidity control). Energy-saving opportunities are directly related to the CO2 emissions.

In addition, other energy-saving opportunities such as steam transport and condensate recovery are included in these subsections. The enclosure hood of the drying section is an important part of the paper drying process in terms of energy efficiency but is beyond the scope of this article.

For the analysis of CO2 emissions, the indicators are defined in terms of the main physical variables involved in the process of drying paper identified by Karlsson [31] and related to the consumption of thermal energy, the energy aspects from the theoretical perspective of paper drying [35,37], and the maximization of energy recovery [33,36]. The reforms and maintenance guidelines proposed, based on data provided by the defined indicators, are those that reduce the indicator ‘t CO2 /t Paper’.

The drying process significantly affects the characteristics of the paper produced and the efficiency of the process; thus, it is necessary to analyze whether the improvements and maintenance guidelines are favorable for maintaining product quality and reducing internal rejections.

Data are collected from the machine’s control instrumentation and are supplemented by daily collection data from the environmental management system. The factory has only one energy supplier, and all of its energy is used by the paper dryer. In this case, it is possible to assume that the difference between the energy used and theoretical needs are energy losses that can be saved.

2.4. Determination of ‘t CO2 /t Paper’ This study is focused on a paper manufacturing plant. The data are collected mainly from the paper drying section. The facility has only one natural gas supply source, which is used only in one boiler to generate the demanded steam for thermal paper drying. The direct emissions of CO2 produced by the plant come only from the combustion of the natural gas mentioned above. This natural gas combustion is included in the EU ETS scheme and, in this case, generates all the plant’s direct CO2 emissions. Figure 1 shows a schematic process flow in which the boiler (steam generator) can be identified as the only point of natural gas consumption, and the dryer section is the only point that consumes the steam produced by the boiler. We consider the direct emissions of CO2 produced in the installation from the combustion of natural gas, which is under the EU ETS. The analysis of data availability, paper-produced air dry tons (ADTs) and emissions, are collected with regular frequency and measured in accordance with Spanish law [58–60].

Other variables associated with the processes and the drying section were collected. The variables regarding the temperatures of the paper, steam, and condensate system and balance of air in the enclosure hood (identifying each flow, temperature, moisture, and other associated enthalpy characteristics) are analyzed to find the relations among them and the CO2 emissions through the considered indicator, ‘t CO2 /t Paper’.

The conditions of the dryer hood, mainly steam pressure in dryer cylinders and inside hood air conditions, affect the evaporation capacity of the drying system and the speed of the process. This issue determines the difference between the theoretical capacity and actual production yielded. This also affects the indicator ‘t CO2 /t Paper’ and thus the CO2 emissions.

The indicator ‘t CO2 /t Paper’ is obtained as a direct ratio between CO2 emissions, determined according to Spanish law, and the tons of paper produced at the plant in the same period. A month is taken as a period to determine the effect of each change in the operating parameters of papermaking. The period of one month provides sufficient data to study and compare to other periods.

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The CO2 emissions are calculated following the methodology outlined in the ‘Calculation of emissions and emission factors’ from the ‘GHG Inventory Report for the Implementation of the Trading Directive’ [61]. The calculation is performed as shown in Equation (3), where Activity Data represents the energy consumed in the period under consideration.

CO2 (t) = Activity Data (TJ)· · · Emission Factor (t CO2 /TJ)· · · Oxidation Factor,

(3)

The calculation of Activity Data considers the total energy supplied during the period, in terajoules, calculated with the invoiced energy data from the external natural gas supplier.

The emission factor and oxidation factor correspond to natural gas [61], the fuel used by the factory to produce the steam required to dry the paper.

The calculation of paper manufactured requires an indicator in the periods of reference based on European Commission [54,55] methodological guidelines.

The CO2 emissions have been checked with annual statements of emissions by the factory; this is public data that can be checked according to the European Climate Registry Rules (the EU ETS registry) approved by the Climate Change Committee as well as other annual statements, such as the European Pollutant Release and Transfer Register (E-PRTR); this Europe-wide register provides information about environmental data from industrial facilities in European Union Member States, replacing and improving on the previous European Pollutant Emission Register (EPER), PRTR. The study is based on historical papermaking data from 10 years, which is a representative period in which there were neither major changes in machine conditions nor alterations that changed their production capacity.

3. Results and Discussion The most important subindicators are detailed in this section, and the influence of each subindicator on ‘t CO2 /t Paper’ is analyzed. These subindicators are cold water (Cw) added to the boiler circuit and the temperature of blown air in the drying section enclosure hood (Ebt), the two elements with most significant thermal losses of this process.

3.1. Cold Water Added to the Boiler Circuit The main sources of loss in the steam and condensate loop (Il) that cause more water (Cw) to be needed by the circuit than that required by Bdr are as follows: excess blowdown in steam generators, steam traps, rotary joints in drying cylinders installed in the dryer section, losses through pump mechanical seals, valve seals, and other components, and steam flash produced in atmospheric tanks.

Figure 2 shows the daily average steam production and cold water added to the circuit due to boiler blowdown in each year considered. As expected, the evolution of the consumption of steam, cold water, and blowdown follow the same trend. A similar trend can be seen for the energy losses, which decrease starting in period 4 such as the other variables mentioned, while the paper production increases (see Figure 3). The latter is important because although paper production has increased over the years, in this dryer section and in particular in the boiler circuit, the energy losses are reduced. The amount of cold water added to the boiler circuit could be a good environmental indicator that could also be used to check the efficiency of maintenance routines.

In Figure 4, the factory CO2 emissions and CO2 losses due to the boiler circuit can be observed; starting in period 4, the CO2 emissions and CO2 losses decrease; the latter yields percentages of 15.7, 11.2, 7.5, 6.3, 4.8, 2.6, and 4.0 in periods 4, 5, 6, 7, 8, 9, and 10, respectively. The losses of total emissions indicated in Figure 4 represent the associated emissions regarding energy losses due to the energy contained in the blowdown water stream and the reduction in steam losses from rotary joints. The minimum reached in the last three periods indicates that the point of stability has been reached.

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Figure 2. Daily average steam production and cold water added to the circuit due to boiler blowdown.

Figure 3. Daily and drying steam losses.

Figure 4. Yearly CO2 emissions and CO2 losses.

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With the data in Figure 4, the conclusion that the total energy consumption and the energy losses display similar tendencies is drawn. As we observed in Figure 3, lower losses imply a greater availability of energy and consequently an increase in production.

Note in Figure 4 that the % of total losses increase with the % of factory emissions due to global factory emissions reduction.

3.2. Temperature of Air Blown (Ebt) into the Dryer Section Hood The maintenance state of the drying enclosure hood (Figure 1, process flow) and the characteristics of the airflows within it significantly affect both the energy consumption and total amount of water that can be evaporated from the paper, and consequently, considering that the drying section is the bottleneck of paper production, the production obtained is affected.

The enclosure hood must maintain optimum thermal conditions to favor water evaporation from the incoming sheet. The elimination of water occurs at a temperature lower than that of the water evaporation point (85 ◦ C) by transferring the moisture contained in the paper to the circulating air inside the hood. When the air reaches a sufficient moisture content, it is expelled from the hood, sending the water content to the atmosphere. The extracted air must be replaced by another equal volume from the atmosphere, which is preheated by air–air exchangers with the extracted air stream, taking advantage of the condensation energy. The energy contained in the airflow depends on its temperature and moisture content. The energy extracted from the extraction flows comes from the energy contained in the water vapor, of which most of the water is contained as condensate in the exchangers.

Following the methodology described in Section 2, the main characteristics of each airflow and its energy before and after maintenance actions are obtained, as shown in Table 1.

Table 1. Data of air before and after the repair of the exchangers.

Input and/Output Air

Type

Before maintenance

Extraction Blown air Compensation

Outlet Inlet Inlet

After maintenance

Extraction Blown air Compensation

Outlet Inlet Inlet

RH (%)

g H2 O/kg Dry Air

Barometric Pressure (Pa)

49 97 21

93 3 18

78.1 15 8

101,325.0 101,325.0 101,325.0

0.9 0.9 1.0

64,506 32,892 31,000

238 124 60

15,322,110 4,078,279 1,844,500

54.7 92.4 21

66 2 18

72.5 15 8

101,325.0 101,325.0 101,325.0

0.9 0.9 1.1

85,000 59,500 25,500

244 111 60

20,706,000 6,583,675 1,530,000

Temperature (◦ C)

Density (kg/m3 )

Flow Rate (m3 /h)

Enthalpy (kJ/kg)

Energy (kJ)

The main thermal streams involved in the drying process into the enclosure hood include the paper from the press section (32 ◦ C, moisture content 52%) and paper leaving the enclosure hood (85 ◦ C, moisture content 6%); the water content is extracted by airflow to the atmosphere. Table 1 shows the characteristics of each airflow.

The initial engineering designed flow conditions of the enclosure hood streams are 85,000 m3 /h of extraction and 59,500 m3 /h of blowing, and the difference between the extraction and blowing may be 25,500 m3 /h.

Inside the hood, as indicated by Calvo and Domingo [62], despite the external temperature conditions of the blowing air and the temperature of the incoming paper into the dryer section, the outlet air and paper temperature are constant at approximately 95 ◦ C. Considering that the extracted air temperature is 85 ◦ C, the energy added to all streams (paper and air) comes from the radiation of the dryer cylinders heated by steam supplied to the dryer section (steam in Figure 1).

To determine the actual state of the dryer system and enclosure hood, the data of the main associated variables of the airflow are collected to determine the air flow enthalpy before (taken in February of year 8) and after the cleaning and conditioning of the exchangers. The difference between the enthalpies determined by the Mollier diagram [63] gives us the energy saved after repairing and cleaning the exchanger.

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Inlet air passes through three heat exchangers, each of which has a separate power source that provides energy to the blown airflow. The first-level-of-action personnel, who are responsible for cleaning the equipment, are assigned the task of tracking the parameters once a day. This approach is taken to involve the manufacturing staff in the prevention of the decrease in Ebt, which results in increased consumption of steam, and to review the cleanliness of the exchangers listed above. The maintenance actions resulting from the measurements of the temperature values are performed on July 8 on the following equipment: The air–air exchanger harnesses the energy extracted from the airflow in the hood to warm blown air. This exchanger heats the air up to 45 ◦ C above the external temperature.

The water–air exchanger, which harnesses the energy of the condensate, returns to the boiler to heat the blown air to 75 ◦ C. The flash steam exchanger harnesses flash steam generated in the last low-pressure condensate collection tank and heats the blown air to 95 ◦ C. The temperature reached at the end of the exchanger system for blown air is 97 ◦ C, but only 32,892 m3 /h is obtained. The difference between the air flow compensation comes from the inside of the manufacturing hall where the enclosure hood is located.

The CO2 emissions associated with energy losses are calculated through the energy supplied to the flow of hot air blowing from the actual temperature to the desired temperature. This energy comes exclusively from the boiler to generate steam supplied to the dryer section. This boiler steam generation exclusively uses natural gas as an energy source. The emissions can then be calculated using Equation (2), which considers the energy provided to the air and the emissions factors associated with the consumption of natural gas needed to heat that air.

Subsequently, the ‘energy losses/emissions’ are calculated according to Equation (3) and compared with the difference in the temperature of the blown air Ebt into the hood with an indicator defined by Calvo and Domingo [21,62], ‘t CO2 /t Paper’, to determine the relevance of the losses.

The average ADT of paper produced in the factory is 5.75 t/h, and the only source of energy used in drying the paper comes from the steam generated in a single boiler. Natural gas is supplied to the plant by a single source, and the only gas consumption occurs in the boiler steam generation.

The average energy consumption for each paper ton is 1190 kWh. With these data, as shown in Table 2, the portion of energy lost in the blown air corresponds to 10.17% of the total energy used in the thermal drying of the paper. The support guidelines for the exchangers are established in months M7-M12 of year 8.

Table 2. Emissions balance.

Energy (kWh) Hour Difference in blowing air Total energy consumption

695.94 6842.50

CO2 Emissions (t) Hour Total Year 0.125821 1.237069

1011.60 9946.04

% 10.17

3.3. Evolution of the CO2 Emissions The first action (Cw) is performed continuously, and the second (Ebt) is performed after 6 months. Table 3 shows the quantification of savings blow-down energy, savings exchangers energy, and total energy savings over 10 years.

The losses or reductions always refer to days of operation, a very important parameter in this type of plant, in which the start-up and shutdown processes are periods of intensive heating energy consumption and energy released not directly applied to production. These types of situations are beyond the scope of this paper. On the other hand, the impact of the measurement of the exchangers is much more significant than that of the purges, due to the amount of air conveyed in the drying installation that is expelled to the atmosphere in a more continuous manner.

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Table 3. Total energy savings over 10 years.

Year

1

2

3

4

Production days Paper production (t) Blowdown loses MJ Savings Blowdown energy MJ Savings Exchangers energy MJ Total Energy consumption Total Energy Savings

344.5 121.3 9833

351.0 124.2 11,523

350.2 127.6 13,647

345.0 133.2 10,686

0 0 144,347 147,798

0 151,844

0 158,508

5

6

341.0 313.7 131.3 133.5 7111 4404 4310 7017 0 0 150.995 152.857 4310 7017

7

8

9

10

339.0 134.9 3951 7470 0 153,786 7470

335.0 135.6 2917 8504 24,191 137,172 32,695

334.0 134.1 1449 9972 23,923 135,655 33.895

317.0 137.6 2129 9292 25,547 139,196 34,839

Figure 5 shows the evolution of the annual average of the indicator, which gradually declines in value over time. The indicator ‘t CO2 /t Paper’ decreases by an average of 21% from year 2 to 10 and by approximately 30% since year 1.

Figure 5. Evolution of the indicator—“t CO2 /t Paper” over 10 years.

In Figure 5, there is a stable zone indicator value in the first four years that represents a period in which the indicator remains stable because there are no actions for reducing emissions.

After applying TPM techniques such as TEI to motivate maintenance and production personnel and CPI actions (small changes, especially considering employee observations) at the end of 4 years and regarding maintenance, at three levels, from year 5 onward, the indicator is starts to improve until it reaches 0.2300 in years 9 and 10.

Figure 6 is divided into three sections to clarify the data interpretation and facility analysis. The first section contains the data for the years preceding year 1 and year 1 inclusive, a period that had no active EU ETS emissions scheme. The second dataset contains data from years 2–4, which corresponds to the implementation of the first period of the EU ETS. The third section includes the data from year 5 to the present, including the second period of the EU ETS and the first year of the third period of the EU ETS. Figure 6 shows that the indicator under consideration declines gradually each year, which becomes apparent starting in year 8. Figure 6a–c also shows the variation in the index over the twelve months of the year; the outside temperature is colder in the winter months (1, 2, 11, and 12) than in the remaining months, which increases the importance of controlling the blown air temperature (Ebt).

The indicator progressively decreases in each period, as seen by the first year (white) in Figure 6a to the last year (black) in Figure 6c. In all graphs of Figure 6, the difference between the first and last years is evident. Reviewing the three graphs reveals that the same indicator level is maintained on the ordinate and decreases significantly and steadily until year 5, where we see a further decline; this coincides with the start of the leading indicator ‘t CO2 /t Paper’.

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Figure 6. Monthly evolution of the indicator “t CO2 /t Paper” over 10 years: (a) Years 1, 2, 3 and 4; (b) Years 5, 6 and 7; (c) Years 8, 9 and 10.

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Figure 6a shows that the energy consumption and emissions in the coldest months of the year (M1, M2, M11, and M12) are higher than those in the warmer months (M7 and M8). This fact is due to the strong initial influence with respect to the outside temperature (Ot), due to the poor conditions of the exchangers.

As the monitoring and continuous improvement techniques are applied, in Figure 6b, it is observed that in years 5, 6, and 7 there is a significant decrease in the global energy consumption and consequently the CO2 emissions, decreasing the initial difference between the cold and hot months. Figure 6c shows that the overall level decreases, making the difference between all months very small; the losses decrease, and the exchangers work properly, making the system almost independent of the Ot at the end of year 10.

4. Conclusions As seen in the methodology, the emissions in this case are calculated as an emission balance depending on the primary energy consumed, which in this study is natural gas.

The improvement in the energy efficiency due to the reduction in energy process losses and/or the better use of available energy results in a significant reduction in the ratio of the associated CO2 emissions.

The greater availability of energy in the process bottleneck, the drying section, allows an increase in production (Figure 3) so that the energy and CO2 emissions ratio is reduced.

The reduction in CO2 emissions indicates that it is feasible to achieve significant emissions reduction through the control and maintenance of installations, as well as daily rules and processes. Establishing maintenance guidelines and minor renovations helps facilities meet their emissions reduction targets without first having to make costly investments.

This study revealed that the involvement of TPM techniques such as TEI and CPI in production and maintenance workers in controlling the process variables is critical.

Moreover, it shows how introducing CO2 emissions as a principal indicator and identifying subindicators led to a significant emissions reduction in the last six years.

This method can also identify the parts of the installation in which it is possible or necessary to take urgent action to reduce emissions and predict the reduction potential, which may allow new investments in the facilities to be planned more effectively.

There is a direct relationship between the defined indicator, ‘t CO2 /t Paper’, and the facility efficiency. An increase in this ratio due to the deterioration of the facilities can be considered to assess the capacity of papermaking and find the causes of a decline in output paper due to process inefficiencies.

This manuscript identifies the effect of industrial variables involved in the drying process on CO2 emissions, such as the amount of cold water added to the boiler circuit and the temperature of the blown air into the drying section (enclosure hood).

Author Contributions: Conceptualization, L.M.C. and R.D.; methodology, L.M.C.; formal analysis, L.M.C., R.D.; investigation, L.M.C.; resources, L.M.C., R.D.; writing—original draft preparation, L.M.C.; writing—review and editing, L.M.C., R.D.; supervision, R.D.; funding acquisition, R.D. All authors have read and agreed to the published version of the manuscript.

Funding: The authors thank the Spanish Ministry of Science, Innovation and Universities for supporting through RTI2018-102215-B-I00 project.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Acknowledgments: The author thanks ASPAPEL for its support.

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 3, 2021

Cellulose and the role of hydrogen bonds: not in charge of everything 1 2 2,3 3,4 MALIN WOHLERT , TOBIAS BENSELFELT , LARS WÅGBERG , ISTVÁN FURÓ , LARS A. 2,3 2,3 BERGLUND , JAKOB WOHLERT .

In the cellulose scientific community, hydrogen bonding is often used as the explanation for a large variety of phenomena and properties related to cellulose and cellulose based materials. Yet, hydrogen bonding is just one of several molecular interactions and furthermore is both relatively weak and sensitive to the environment. In this review we present a comprehensive examination of the scientific literature in the area, with focus on theory and molecular simulation, and conclude that the relative importance of hydrogen bonding has been, and still is, frequently exaggerated.

Contact information: 1. Division of Applied Mechanics, Department of Materials Science and Engineering, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden 2. Department of Fiber and Polymer Technology, School of Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden 3. Wallenberg Wood Science Center, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden 4. Department of Chemistry, School of Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden Cellulose https://doi.org/10.1007/s10570-021-04325-4 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 24

Article 6 – Hydrogen Bonds

Cellulose https://doi.org/10.1007/s10570-021-04325-4

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REVIEW PAPER

Cellulose and the role of hydrogen bonds: not in charge of everything Malin Wohlert . Tobias Benselfelt . Lars Wågberg . István Furó .

Lars A. Berglund . Jakob Wohlert

Received: 13 August 2021 / Accepted: 6 November 2021 The Author(s) 2021

Abstract In the cellulose scientific community, hydrogen bonding is often used as the explanation for a large variety of phenomena and properties related to cellulose and cellulose based materials. Yet, hydrogen bonding is just one of several molecular interactions and furthermore is both relatively weak and sensitive to the environment. In this review we present a comprehensive examination of the scientific literature in the area, with focus on theory and molecular simulation, and conclude that the relative importance of hydrogen bonding has been, and still is, frequently exaggerated.

Keywords Cellulose Hydrogen bonding Computer modeling Nanomaterials M. Wohlert Division of Applied Mechanics, Department of Materials Science and Engineering, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden T. Benselfelt L. Wågberg L. A. Berglund J. Wohlert Department of Fiber and Polymer Technology, School of Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden L. Wågberg I. Furó L. A. Berglund J. Wohlert (&) Wallenberg Wood Science Center, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden e-mail: jacke@kth.se I. Furó Department of Chemistry, School of Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden

Introduction and background Consider a wet nanocellulose film that is rolled up and left to air dry, resulting in a nanopaper roll that is sufficiently stiff and wet-stable to be used as a drinking straw. No adhesives are used to seal the roll, but it nevertheless holds together. This mechanism is attributed to the formation of hydrogen bonds (Hbonds) between the nanopaper surfaces (Wang et al.

2020). But can this really be the full story?

The year 2020 marked the 100-year anniversary of the H-bond concept (Gibb 2020; Pauling 1939), which has, since then, been central for explaining structureproperty relationships in biological matter (Jeffrey and Saenger 1994), including cellulose. Starting with the discovery of nanocellulose and promises of a bright future as sustainable load-bearing component in high performance materials (Berglund and Peijs 2010; Benı́tez and Walther 2017), the last decade has seen an exponential growth of the interest in cellulose research. Rapid development in cellulose chemistry, processing and characterization has led to a property range of cellulose-based materials that expanded beyond imagination and to new areas of application that are continuously discovered. But where do all these intriguing and, indeed, extraordinary properties originate from? Although justified in some cases, there is a tendency in the cellulose field to invoke H-bonding as an almost magical explanation. For example, unique characteristics of cellulose such as high axial

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modulus and strength of fibrils, strong fiber-fiber bonding, or forming of paper from fibers are commonly explained based on ‘‘hydrogen bonding effects’’. Sometimes this explanation is wrong (high modulus and strength of fibrils) or very often incomplete (fiber-fiber bonding and forming of paper), and in most cases the effect of moisture is neglected.

However, simplistic explanations to complex problems are convenient and thereby tend to survive.

Thus, there is a need from time to time to reexamine the claims made with respect to H-bonding in cellulose and cellulose-based materials, and this is the purpose of this review. To this end, we discuss H-bonds in the context of the research on cellulose and their role at different length scales (Fig. 1) including: • Molecular level (conformation of a cellulose molecule).

• Intermolecular level (how cellulose molecules interact with each other).

• Fibril level (how cellulose is arranged into crystals).

• Interfacial level (how cellulose fibrils interact with other molecules).

Fig. 1 Structure of cellulose at various length scales and organizational levels. The dihedral angles u and w are defined by the atomic sequences O5’-C1’-O4-C4 and C1’-O4-C4-C3,

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• Interfibril level (how fibrils aggregate into larger structures).

• Fiber and interfiber level (how fibril aggregates are assembled in fibers and how fibers form joints in paper).

Based on a critical survey of suitable literature, both old and recent, our aim is to present a more nuanced description of the role of H-bonds in cellulose research. Molecular modeling such as molecular dynamics (MD) simulations has played an important role over the years for the understanding of molecularscale phenomena in cellulose (Zhou et al. 2020) since they offer both a level of detail that surpasses what can be reached by experimental methods and the ability to quantitatively extract almost any experimental parameter from the simulated ensemble. However, how simulated microscale properties are related to the macroscale is not always clear. A molecular simulation typically represents less than one millionth of a real sample that is observed during less than one microsecond, and great care must naturally be taken when transferring simulation results to larger scales.

Nevertheless, simulated molecular interactions are a strong qualitative and quantitative tool to understand

respectively. The figure depicts a tentative model for elementary fibrils from wood, although its exact dimensions and shape are still matters of debate

Cellulose

interaction mechanisms. Thus, results from modeling studies are central to this text. Our most important message is that H-bonding is just one of several such mechanisms, and the main reason why it stands out is because its importance is frequently exaggerated sometimes to the extent as if it was the only reason behind a variety of complex cellulose-related phenomena.

One example for this interpretational havoc is the debate concerning the physical mechanisms behind cellulose dissolution. Owing to the abundance of accessible hydroxyl groups on its surface, cellulose is rightfully considered a hydrophilic molecule with a pronounced hygroscopic character and wetting. At the same time, as distinct molecules it is completely insoluble in water at ambient conditions, which has been attributed to the formation of H-bonds between the cellulose molecules themselves leading to large and readily precipitating aggregates. However, more than a decade ago MD simulations were used to show that the contribution from H-bonding to the insolubility of cellulose in an aqueous environment was an order of magnitude smaller than hydrophobic solvation energies (Bergenstråhle et al. 2010). About the same time, in a series of papers, Björn Lindman and co-workers argued that H-bonding can hardly be the driving force for aggregation in water, and that one should treat cellulose as an amphiphilic molecule dressed with several interactions (Medronho et al.

2012; Lindman et al. 2021) which at the time was coined the ‘‘Lindman hypothesis’’ (Glasser et al.

2012). A few years later Nishiyama (2018) showed that London dispersion interaction is the dominating contribution to the total cohesive energy of cellulose.

Today, it seems a large fraction of the cellulose community concurs with the notion that H-bonds play only a minor role for the precipitation of cellulose chains in aqueous environments, but solubility is not the only area where H-bonding effects are being exaggerated.

In this context, it is important to consider that all biological processes take place in water, the hydrogenbonding liquid par excellence. This means that H-bonds within or between biomolecules always have to compete with H-bonds to water, and in this competition the relatively high mobility, both translational and rotational, of the water molecules with the unparalleled capacity of forming four hydrogen bonds for a puny molar mass of 18 gives the latter an

advantage. This hints to that H-bonds themselves cannot be the main driving force for biomolecular assembly in water, as the total number of H-bonds in a hydrated system will remain more or less constant. In fact, we are probably lucky that H-bonds between biomolecules in water are weak, reversible, and dynamic, since the molecules of life would otherwise be strongly associated in uninteresting lumps. However, their directionality can make them into a significant steering force and thereby proficient organizers of three-dimensional structures (Jeffrey and Saenger 1994). Many processes involving cellulose are non-equilibrium processes. This applies to the biosynthesis of cellulose in the plant cell walls, to the mechanical treatments of fibers by which the elementary fibrils are liberated, and to the application of shear forces (stirring) for efficient dissolution. With that in mind, it makes sense to differentiate between the making and the breaking of H-bonds, and to realize that it may require a large activation energy to both make or break, even if the net effect in the free energy from an equilibrium point of view is zero.

A single H-bond is rather unspecific and relatively weak, and thereby can form and break on short (nanosecond) timescales, activated by thermal fluctuations alone at biological temperatures. However, consider N H-bonds defining a molecular complex.

These bonds are subject to N 1=2 kB T thermodynamic fluctuations. If N is large the kinetic stability of this complex is decided by the relation of that term to NCkB T, where CkB T is the average activation energy to break an intermolecular H-bond. In other words, since the total activation energy scales with N but the random force with N 1=2 , massively H-bonded molecular complexes are not easy to disassemble since many bonds have to break simultaneously without reforming. Once formed they can thermally be prone to remain assembled, even in the case of a favorable free energy for dissolution. Thus, their strength lies in their number, which combined with ordering in specific patterns can lead to considerable specificity. When H-bonds are broken, they can readily re-form provided that the geometry is right. This allows for flexible structures which is utilized in, e.g., spider silk (Nova et al. 2010), and can be exploited in self-healing biomaterials (Brochu et al. 2011). Thus, H-bonds is utilized by nature both to create kinetically stable, highly ordered structures, and to dissipate energy in

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flexible and ductile materials. Both aspects are relevant to cellulose.

The physical chemistry of hydrogen bonds Intermolecular interactions are complex and often – even in leading textbooks – their description is heavily simplified and sometimes even incorrect (Truhlar 2019). Hydrogen bonding is a simple name for a complex situation, even in the case of two isolated molecules. Its contributing molecular elements are conventionally named as ‘‘donor’’ (D) and ‘‘acceptor’’ (A) where, provided that both A and D are sufficiently electronegative, D supposedly donates a proton that A accepts, forming the H-bond system D-H A. For two isolated molecules, the experimental standpoint is simple: in this system there is (i) a positive charge density on H, negative charge density on A and therefore there will be an electrostatic term contributing, and (ii) a nonzero electron density between H and A as unequivocally shown by the non-zero hyperfine coupling between D and A (Grzesiek et al. 2004; Dračı́nský and Hodgkinson 2015). Compton scattering experiments concur (Isaacs et al. 1999). This latter feature suggests that there is charge transfer or, if one wishes, covalency over the H A bond. In addition, induction and dispersion terms are contributing as well. There is some argument going on over the relative importance of the different contributions, and the extent of the covalent nature of the bond (Grabowski 2011). However, the clear correlation between the electron density between H and A and the bond strength suggests that the covalency cannot be entirely negligible (Shahi and Arunan 2014). Even with the difficulties of uniquely defining H-bonding (the IUPAC definition from 2011 is quite vague (Arunuan et al. 2011)) it undoubtedly exists, and its energies spread from weak (about 1 kB T in C-H C bonds), through moderate (5-15 kB T, ‘‘normal’’ OH O bonds) to very strong ([50 kB T, in HF). A final complication is that H-bonding is cooperative (Mahadevi and Sastry 2016). This means that a bond which is part of an extended H-bond network may be different in strength from the isolated bond. Specifically, for the case of cellulose the intermolecular H-bond strength was shown to increase with the number of glucose units, and to plateau at a degree of polymerization of four (Qian 2008). In addition to H-bonding, ionic interaction, electrostatic multi-pole

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interaction and London dispersion (van der Waal’s) interaction contribute to the intermolecular forces in cellulose materials. Notably, even if the dispersion force is the weakest and most short-ranged of them when considering just a pair of atoms, their additivity makes the total contribution to the force between supra-molecular objects both considerably larger in magnitude and also significantly more long-ranged (Hamaker 1937), to the end that they can become the dominating interaction between two (uncharged) molecular surfaces.

Modeling and simulation of hydrogen bonds In quantum chemistry (QC), some profound issues concerning hydrogen bonds remain unsettled and controversies seem to prevail about methodology, specifically the choice of base functions and its consequences for covalency, namely that some models predict significant charge transfer, whereas some predict much less (Stobe 2017; Weinhold and Glendening 2018; Stone and Szalewicz 2018). A related problem is that QC estimates of H-bond strengths remain uncertain, and so does the relative importance of the electrostatic, charge transfer, induction and dispersion terms. Moreover, QC cannot tell exactly what the angular dependence of the bond strength is, that is how directional are the H-bonds (Gilli and Gilli 2009). Probably, this depends on the exact D-H A system.

Molecular dynamics (MD) simulations are routinely used to study molecular-scale structure and dynamics in most fields of material science, and there are several optimized parameter sets (force fields) specifically designed for simulations of carbohydrates, such as GLYCAM06 (Kirshner and Woods 2001), CHARMM 36 (Guvench et al. 2008, 2009), and GROMOS 56CARBO (Hansen and Hünenberger 2010).

Most contemporary empirical biomolecular force fields do not employ any special H-bonding potential.

The non-bonded interactions are usually limited to the Coulomb potential, which acts pairwise between fractional charges that are distributed over the atoms with a distance dependence of r 1 , and the LennardJones potential, which describes repulsion with an r 12 , term, and dispersion attraction with a r 6 , term.

Nevertheless, H-bond configurations form anyway as a consequence of a favorable combination of Coulomb

Cellulose

and Lennard-Jones energies for atoms in the H-bonding geometries. Although it may seem so, this fact is in no way lending support for the non-covalent side in the controversy discussed above but is rather indicative of successful parameterization of the models. It has been noted that quantum mechanical contributions are required to capture the fine details of the structure of liquid water in simulations (Chen et al. 2003), and that inclusion of explicit polarizability affects both H-bond strength and kinetics (Xu et al. 2002), but in general empirical force fields are accurate enough to reproduce structure and dynamics of massively H-bonded systems, such as hydrated polysaccharides (Chen et al.

2018). Since H-bonds in MD do not possess a distinct and fundamental identity, they are usually identified in the post-processing of the molecular trajectories based on a set of geometrical criteria. A common choice is a donor-acceptor distance less than 0.35 nm, and a hydrogen-donor-acceptor angle below 30 . The actual H-bond definition will affect the number of detected bonds at a certain instant, but the H-bond dynamics(Luzar 2000) can be analyzed in a way such that it becomes independent of the cutoffs.

Cellulose from the inside The cellulose polymer Cellulose is a linear polymer composed of D-glucopyranose units linked by b-1,4-glycosidic bonds. The native polymer has high molecular weight with a degree of polymerization sometimes exceeding 10,000 (Gralén and Svedberg 1943). It is also rather inflexible with a persistence length that has been estimated to *15 nm (or *30 D-glucopyranose residues) from molecular modeling (Kroon-Batenburg et al. 1997). Since the sugar rings are relatively stiff, conformational freedom around the glycosidic bonds, commonly described by the u and w torsional angles (Fig. 1), lends flexibility to cellulose, and polysaccharides in general. As a consequence of having its hydroxyl groups equatorially positioned a cellulose polymer can form intra-molecular hydrogen bonds between sequential glucose units: between the hydroxyl group on C3 and the ring oxygen (O3H3 O5) and between the hydroxyl groups on C2 and C6 respectively (O2H2 O6 or O6H6 O2). Due to the geometry of the molecule, these hydrogen bonds can only

form if two consecutive glucose units attain a 21-fold (or close to) conformation, meaning that the two glucose units are twisted 180 with respect to each other around the chain axis (Fig. 1). This conformation is indeed the most dominant one in water soluble cellooligomers both in solution (Kroon-Batenburg et al. 1993) and in the solid state (Chu and Jeffrey 1968; Ham and Williams 1970). One question that arises is whether these hydrogen bonds play an active role in driving the molecular conformation, or if they are merely a consequence of favorable geometry.

The influence of trans-glycosidic H-bonding was investigated by several computer modeling studies where the H-bonding capability was modulated either by chemical substitution of the hydroxyls (French et al. 2005, 2021), by using glucose epimers having the hydroxyl groups in axial position as opposed to equatorial (e.g., mannose or allose) (Wang et al.

2013), or by looking at hemicelluloses (Berglund et al.

2016, 2019, 2020), which, due to its variation in chemical structure, can be viewed as a combination of both approaches. Indeed, both Density Functional Theory (DFT) calculations with solvent effects included and MD simulations in explicit water show that the conformation of the b-1,4 linkage is very consistent regardless of the number of H-bonds. The exceptions to this are chemical substitutions that have a relatively large effect on the steric environment (Yu et al. 2019), which can both shift the lowest energy conformation and influence the rigidity of the structure. Another example is xylan, where the hydroxymethyl group is removed altogether, which favors a twisted conformation in solution but has a relatively low free energy barrier for conversion to 21-fold (Berglund et al. 2016; Ling et al. 2020). The conclusion is that since the intra-molecular H-bonds can be effectively replaced by H-bonds to water molecules they do not contribute to the energy of the different conformations. In the gas phase on the other hand, trans-glycosidic H-bonding may indeed have profound effects on the conformation of celloligomers, as revealed by both modeling(Strati et al. 2002; French 2012) and experiments (Anggara et al. 2021). In this context, using a combination of vibrational spectroscopy and QM calculations (Cocinero et al. 2009), it was shown that the cis (untwisted) conformation produced the lowest energy conformers of phenyl bcellobioside in vacuum, which were stabilized by H-bonding between O2’ and O3. Furthermore, using

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cellobiose analogues that lack H-bonding, French (2012) showed that the region of 2-fold conformations was the most stable one, also in vacuum. Thus, in an environment where intermolecular H-bonds are easily exchanged (e.g., in water) the propensity for the (near) 21-fold conformation in b-1,4-linked carbohydrates such as cellulose seems to be caused mainly by steric forces.

Intermolecular H-bonds and the formation of fibrils During the biosynthesis, glucan chains coalesce into extended structures – the elementary fibrils. These fibrils are often perceived as being constituted by a crystalline core covered by more disordered surface chains and, moreover, occasional regions along the fibril where the crystalline order may be lacking. From X-ray, neutron diffraction, and NMR studies, four major crystalline allomorphs have been reported – cellulose I, II, III and IV. Cellulose I is the native form and also the most widely studied. It is present in all plant cell walls and is further divided into two sub forms, Ia (Nishiyama et al. 2003) and Ib (Nishiyama et al. 2002). Native cellulose is most commonly a combination of these two allomorphs (Atalla and VanderHart 1984) in proportions that depend on the source. Cellulose II is irreversibly obtained from cellulose I upon regeneration or alkali treatment (Langan et al. 1999, 2001). Cellulose III (Wada et al.

2004a, b) can be obtained from both cellulose I (in that case called cellulose IIII) and cellulose II (cellulose IIIII), by soaking in liquid ammonia or organic amine at low temperatures, whereas cellulose IV can be formed by thermal treatment of cellulose III in glycerol (Wada et al. 2004a, b). All these allomorphs are recognized by conformational differences, various packing arrangements and, importantly, their different intra- and inter-molecular hydrogen bonding patterns (Kovalenko 2010) (Fig. 2).

The exact location of hydrogens in the crystal structure is difficult to determine experimentally due to their small X-ray scattering power. Furthermore, even if heavier atoms are ordered, the hydrogens may be disordered in the structure, as for instance seen in hexagonal ice (Peterson and Levy 1957). Consequently, the H-bonding in crystalline cellulose is not necessarily fixed to a single pattern but may be prone to fluctuations. Indeed, based on X-ray and neutron

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fiber diffraction several different H-bonding arrangements were concluded leading to two, mutually exclusive, H-bonding pattern models existing alongside each other in (local) equilibrium in both cellulose Ib (Nishiyama et al. 2002) and cellulose Ia (Nishiyama et al. 2003). Based on MD and DFT, alternate H-bonding patterns for cellulose II and IIII were also proposed (Chen et al. 2015).

From computational modeling it was shown that for Ib specifically, one of the two suggested patterns (denoted pattern A) was energetically more stable than the other one (pattern B) but that (presumably) static domains exhibiting pattern B still existed within real samples, possibly as disordered regions (Nishiyama et al. 2008). The preference for pattern A was also revealed by comparing DFT calculations with dichroism data from polarized IR spectroscopy of aligned cellulose samples (Lee et al. 2015). For cellulose II, however, the energy difference between the different patterns was not large enough to exclude the possibility of co-existence in dynamic equilibrium (Hayakawa et al. 2017). Interestingly, the notion that the cellulose crystal structure is not entirely static and homogeneous, but prone to structural disorder and possibly fluctuations may be coupled to the dynamical heterogeneity of the glucan chains that is manifested in broad distributions of 13 C NMR T1 relaxation times, also within the supposedly crystalline fibril core (Terenzi et al. 2015; Chen et al. 2019).

The sensitivity to small variations in the cellulose crystal structure is also seen in numerous MD simulations. Using the experimentally determined atomic coordinates as input structure, including the hydroxyl groups’ hydrogen positions, MD simulations employing common force fields typically yield structures that are stable in periodic crystal models, i.e. where the cellulose chains are covalently bonded to their own periodic image (Mazeau and Heux 2003; Bergenstråhle et al. 2007). Such structures can to reproduce crystallographic lattice parameters with reasonable deviations (within 1–8%). On the contrary, significant structural disorder including deviations from the experimental H-bonding patterns is obtained in simulations of finite fibrils, i.e. models including cellulose chain ends and interfaces to water, especially in long (microsecond) simulations (Matthews et al.

2012). Further, at high temperature a structural transition, initiated by a change in the conformation of hydroxymethyl groups from tg to gg has been

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Fig. 2 H-bonding system in two cellulose allomorphs, cellulose Ib and cellulose II. Based on the location of the heavy atoms, several patterns are possible. The figure depicts the one for Ib that is lowest in energy based on simulations, denoted

pattern A. For cellulose II, the sole difference between the patterns is whether OH2 and OH6 act as donor or acceptor, respectively. The orientation of the hydroxymethyl groups is highlighted, denoted tg in cellulose Ib, and gt in cellulose II

observed in simulations of cellulose Ib (Bergenstråhle et al. 2007; Matthews et al. 2011). The intermolecular H-bonds at high temperature were more dynamic and less regular than the relatively stable pattern at room temperature, but remarkably, the intramolecular H-bond O3H3 O5 was found to persist even at 500 K. The regularity of the interchain H-bond arrangements in native cellulose suggests that H-bonds have an organizing role in creating the fibril structures. That would not be unique to cellulose since hydrogen bonds define some of the most important structures known to molecular biology: the secondary structure in proteins (Pauling and Corey 1951; Pauling et al. 1951), and the

formation of base-pairs in DNA (Watson and Crick 1953). Yet, it has been argued that even for the structure of DNA, hydrophobic interactions have a strong contributing and perhaps dominant role (Lindman et al. 2021). Hydrophobic interactions are, however, not specific. This means that even if they constitute a strong thermodynamic driving force to compact moieties together to minimize the total amount of non-polar (‘‘hydrophobic’’) surface area that is exposed to water, hydrophobicity does not, for the same reason, concern the interactions between those molecular surfaces. To create crystalline order, additional interactions that are specific to the type of atoms involved are needed, such as ionic interaction,

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van der Waal’s interaction, or H-bonding, in combination with a regular chemical structure.

However, the cellulose I structure is not merely a result of spontaneous processes. It is well known that cellulose, when precipitated from solution, crystallizes in the form of cellulose II. This indicates that this structure is lower in energy than cellulose I, and thereby that native cellulose is in a metastable state (Stöckmann 1972). Moreover, over the scale of organisms, cellulose fibrils have, with retained interchain arrangements within the fibrils, an extreme variation in cross-sectional fibril dimensions that are regular and reproducible within a given species (Tsekos 1999). Fibril dimensions vary from thin (two to three nanometers) and isotropic (i.e. the width approx. equal to the height) in most land plants, via large (up to 20 nm) isotropic shapes in some species (e.g. Oocystis, Valonia and Hypoglossum), to the highly anisotropic ribbon-like structures produced by, e.g., Acetobacter and Erythrocladia. The cross-sectional variation is correlated with the spatial arrangement of the linear terminal complexes (TC) that secrete the glucan chains and hence the fibrils: lateral TC sizes in land plants are much smaller than those in Valonia or Oocystis and in Acetobacter the TC shape is elongated. The only possible way of having such correlation is that cellulose association into fibrils and ribbons is not random but directed by the regular arrangement of cellulose synthase units. Curiously, cyanobacteria, which seems to lack regularly arranged

TCs, create fibrils that have very small cross-sectional size (Nobles et al. 2001). The organization of fibrils outside the TC has been suggested to be a selforganizing process (Emons and Mulder 2000) possibly guided by the microtubules (Paradez et al. 2006). The environment into which the glucan chains are secreted will influence the microfibril architecture. Specifically, it was shown that it is possible to influence the ratio between cellulose Ia and Ib produced by Acetobacter by the addition of polymers to the culture medium (Yamamoto et al. 1996), or even to make it produce cellulose II (Shibazaki et al. 1998).

These observations suggest that native cellulose is not created by spontaneous self-organization, but rather that during biosynthesis the glucan chains are guided to associate into a regular intermolecular arrangement within which they remain trapped.

Hence, H-bonds do not alone cause the regular intermolecular arrangement of as-biologically-produced cellulose chains within fibrils but are merely consistent (in form of a local free energy minima) with the chains being guided into a specific spatial arrangement, thereby contributing to the stabilization of the metastable state. But even in this respect, they are not working alone. An often-neglected fact in the case of cellulose is that the molecular packing of chains is to a large extent a joint result of steric repulsion and attractive dispersion interactions and their total contribution to the cohesive energy of cellulose may be as high as 70% (Nishiyama 2018).

Fig. 3 The size and spatial arrangement of the cellulose-synthesizing complexes are responsible for the large variation in fibril dimensions among different species

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Axial stiffness of the cellulose crystal Cellulose has an axial elastic modulus which is often compared to that of steel or Kevlar (Moon et al. 2011).

The many hydrogen bonds within and between cellulose chains have been proposed to be important for the mechanical properties of cellulose crystals and highly crystalline cellulose materials. Specifically, it has been suggested that the intramolecular transglycosidic H-bonds (O3HO3 O5 and O2HO2 O6 in cellulose Ib) contributes to the axial stiffness of cellulose, since they have a large component in the chain direction. Quantum mechanics (Santiago Cintrón et al. 2011), molecular mechanics modeling(Eichhorn and Davies 2006) and MD simulations(Wohlert et al. 2012; Wu et al. 2013) are generally fairly successful in reproducing the experimental axial crystal modulus of 138 GPa (Nishino et al. 1995).

Thus, computer modeling is relevant and suitable for investigating the contribution of H-bonds to the stiffness, both regarding nature and extent.

As mentioned above, the intramolecular O2H2 O6 H-bond is on average broken in glucan chains residing in surfaces exposed to water. Owing to the 21-fold symmetry, this means that surface chains have, on average, 1.5 intramolecular H-bonds per glucose unit, as opposed to 2 in the crystalline core.

Thus, if H-bonds contributed significantly to the axial modulus, one would expect a lateral size dependence of the stiffness, since the proportion of surface to core becomes smaller as the fibril cross sections become larger. A nano-scale three-point-bending experiment using atomic force microscopy (AFM) on bacterial cellulose fibrils between 35 and 90 nm thick did not detect any difference (Guhados et al. 2005), but that range was probably too small to see any such effect.

Moreover, the measured moduli were low, around 76 GPa, which could indicate large contributions from non-crystalline material within their samples. However, also in MD simulations using fibrils of lateral size between 2.3 and 6.8 nm (Wohlert et al. 2012), no such size dependence was found.

Cellulose II and cellulose IIII are both different from native cellulose with respect to their dominating H-bonding pattern. Due to that the hydroxymethyl group has rotated with respect to the conformation in cellulose I (from tg to gt, see Fig. 2), the transglycosidic hydrogen bond to the hydroxyl in position two is no longer possible. Therefore, these allomorphs

have only one intramolecular H-bond per glucose (Hayakawa et al. 2017), although this conformation does permit a bifurcated O3H3 O6 H-bond as reported for Me-cellobioside (French 2012). Based on literature values, cellulose II and IIII are generally 10-40% less stiff than cellulose I, athough experimental variability makes direct comparisons difficult. This variation was reproduced in MD simulations of all three allomorphs (Djahedi et al. 2015). However, in the only experimental study that compared elastic moduli of all three allomorphs using similar conditions it was concluded that the stiffnesses of cellulose I and IIII were similar, while it was lower for cellulose II (Ishikawa et al. 1997).

Fig. 4 A molecular scale leverage effect, proposed by Altaner et al. (Altaner et al. 2014), amplifies the relative contribution of the O3H3 O5 H-bond to the axial stiffness of cellulose (top).

MD simulations show that the O3H3 O5 bond is stretched during axial deformation, whereas the O2H2 O6 bond remains unchanged (bottom, from Djahedi et al. (2016))

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Both dynamic FTIR and Raman spectroscopy (Hinterstoisser and Salmén 1999; Kong and Eichhorn 2005) have shown that intramolecular H-bonds shift their frequencies during axial deformation of cellulose, and there was also evidence that they may disassociate if the strain was sufficiently high. However, a H-bond is approximately ten times easier to stretch than a C-O-C bond angle and hundred times less stiff than an ordinary C-O covalent bond. Glucan chains in cellulose are both aligned and extended, which leads to a dominant part of the axial deformation being associated with covalent degrees of freedom (Djahedi et al. 2015). Completely removing hydrogen bonds within any cellulose crystalline allomorph (Wohlert et al. 2012; Eichhorn and Davies 2006) causes a significant reduction of its stiffness, but also a loss in structural organization of the chains.

Therefore, cooperative effects between hydrogen- and covalent bonding were suggested, and also investigated spectroscopically by Altaner et al. (Altaner et al.

2014). They proposed a molecular-scale leverage mechanism by which the deformation of the O3H3 O5 H-bond was enhanced since FTIR spectroscopy showed that the O2H2 O6 bond on the opposite side of the glycosidic linkage was not deforming during chain extension (Fig. 4). This effect was analyzed in a simplified spring model with parameters based on MD data (Djahedi et al. 2016), which showed that the leverage effect was indeed present within MD simulations, although mitigated by the fact that a large part of the total deformation took place in the sugar rings. Nevertheless, this study showed a relative effect of intramolecular hydrogen bonds of about 15-20% of the total stiffness, similar to what has been seen in MD simulations of crystalline cellulose where H-bonds were artificially turned off (Wu et al. 2013). However, analysis of the respective energy contributions to the total MD potential showed that the major part of the axial stiffness comes from deformations of bonds, angles and dihedrals in combination with a large contribution from dispersion interactions (Djahedi et al. 2015). In the transverse directions on the other hand, the situation is different, since covalent interactions can be expected to contribute less. DFT calculations (Chen et al. 2021) show that dispersion interactions contribute around 50% of the stiffness in the direction perpendicular to the H-bonded planes, and about 30% in the direction parallel to the H-bonded planes (Fig. 2). In that

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direction, removal of inter-chain H-bonds would likely have a large effect on the stiffness. However, the specific effect of H-bonds on the stiffness, in any direction, is not easy to isolate, both because removal of the H-bonds in simulations simultaneously changes other parameters as well (chain packing, etc.) and because different contributions to the stiffness (Hbonds, dispersion interactions, covalent bonds, etc.) are not necessarily additive.

At the surfaces Accessible OH groups are abundant on cellulose crystallite surfaces. The ‘‘hydrophilic’’ (110) and (1-10) crystallographic planes (Fig. 1) are most likely the dominant planes exposed in native fibrils from wood (Nishiyama 2009; Daicho et al. 2018), although alternative models have larger exposure of the (010) or the ‘‘hydrophobic’’ (200) planes (Fernandes et al.

2011; Yang and Kubicki 2020). The two hydrophilic surfaces are similar, exposing about 5.4 OH per nm2 that are potentially accessible to the environment.

However, both modeling (Heiner and Teleman 1997; Heiner et al. 1998) and experiments (Lindh et al. 2016) show that the intra-chain H-bond O3H3 O5 is so stable that, in practice, the O3H3 hydroxyl does not act as a donor. This leaves 3.6 OH groups per nm2 that are available for H-bonding with other fibrils, macromolecules, or solvent molecules. In addition, their high reactivity makes them serve as points for chemical surface modification such as acetylation (Sassi and Chanzy 1995), TEMPO-mediated oxidation (Saito et al. 2006), or polymer grafting (Rol et al.

2019), which naturally affect the fibril’s H-bonding capability.

Adsorption to cellulose To explain adsorption, it is common to look for favorable specific interactions, such as hydrogen bonds, electrostatic attraction, or p p interactions, and seeking explanations in the chemical structure of the surface and the adsorbing molecule. Since both cellulose and molecules that adsorb to cellulose are often decorated with polar groups, such as hydroxyl groups, it is assumed that hydrogen bonds are important. This explanation is so common in the literature

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that it is frequently used without scientific evidence.

What is often forgotten or actually neglected is the chemical structure and adsorption energy of the solvent. This is especially true in studies from the early days of MD simulation where the solvent was often left out due to computational limitations. It is useful to view adsorption as a process in which solvent molecules close to the surface are replaced by the adsorbing species, and that adsorption will only occur if the total free energy balance of this exchange process is negative (Fleer et al. 1993). In this context, it is important to emphasize that the adsorption energy is a free energy, which besides enthalpic contributions from specific molecular interactions also contains a (often significant) contribution from entropy: DG ¼ DH TDS. One example of this is the wellstudied case of polyelectrolyte adsorption to charged surfaces. Despite the strong Coulombic interactions that are present between the adsorbing molecule and the surface, the net result is an ion-exchange process driven by the increased entropy of releasing counterions and associated water (Fu and Schlenoff 2016; Michaels 1965). This becomes relevant for most colloidally stable nanocelluloses that are decorated with charged groups.

For the adsorption of nonionic molecules, the literature is quite ambiguous concerning the role of H-bonds. Hydrogen-bonded polymer association in water is indeed a frequent description, such as hydrogen-bonded multilayer assembly or hydrogenbonded polymer complexation (Kharlampieva et al.

2009; Tsuchida and Abe 1982). However, even in the highly cited work by Tsuchida and Abe the message is incomplete. They describe the interaction as driven by hydrogen bonds, but then show that DHand DS are both positive upon complexation with water as solvent, meaning that H-bonds alone could not be responsible for the adsorption process. However, just as oppositely charged groups pair up in polyelectrolyte association, hydrogen bonds do indeed form between the adsorbing molecule and the cellulose surface, but that does not automatically contribute favorably to the adsorption energy. Namely, in a simplified picture, breaking one water-surface H-bond and one watersolute H-bond to form one solute-surface and one water-water H-bond is a net zero process in terms of enthalpy (DH). If DH ¼ 0 then a favorable change in entropy is required to drive the adsorption.

Fig. 5 Adsorption of xyloglucan (XG) to native or charged cellulose. a Schematic of the adsorption process. b Simulated entropy of a single water molecule at different distances from charged and native cellulose, and XG, which determine the free energy gain upon aggregation from decreasing the total solventaccessible surface area. These thermodynamic principles govern the adsorption of many molecules to cellulose (Lombardo and Thielemans 2019)

Xyloglucan (XG) is a good example in the discussion about hydrogen bond-driven adsorption. XG is a nonionic hemicellulose found in the primary cell wall of all vascular plants. It adsorbs strongly to cellulose, which is not surprising since, to put it simply, XG is a cellulose chain decorated with xylose or xylosegalactose residues. The interaction between XG and cellulose has historically been ascribed to hydrogen bonds, but this perception has recently started to change due to measurements that indicate an endothermic process (Lopez et al. 2010; Benselfelt et al. 2016).

Recently, MD simulations were used to show that adsorption of XG to cellulose is driven by the increased translational entropy of releasing interfacial water (Fig. 5) from reducing the total solvent-accessible surface area and that the number of hydrogen bonds was the same before and after adsorption (Kishani et al. 2021). The interaction was endothermic at room temperature but turned exothermic as the temperature increased due to the less favorable hydration of cellulose and XG. However, such considerations are not unique to XG, and it is clear that reduction of solvated interfaces leading to the

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release of (relatively) constrained water molecules, i.e. hydrophobic effects, is the main driving force for all hemicellulose adsorption to cellulose in water.

The analogy with cellulose association is obvious.

Although H-bonds do not drive adsorption (making) from an equilibrium thermodynamics point of view, they can still affect how easy it is to remove an adsorbed molecule (breaking), either by force or desorption, by contributing to the kinetic stability (Stuart and Fleer 1996), since the probability of many bonds being spontaneously broken at the same time becomes low. They can also contribute to specificity, namely how hemicelluloses organize on the cellulose surface. Specifically, recent experimental results in combination with MD simulation suggest that both xylan and glucomannan exhibit an abundance of structural motifs that allow them to adsorb to cellulose in conformations where they become almost seamless extensions to the cellulose crystal structure, including the H-bond network (Busse-Wicher et al. 2014, 2016; Martinez-Abad et al. 2017, 2020; Grantham et al.

2017; Simmons et al. 2016; Pereira et al. 2017).

However, while the simulations generally show that hemicellulose molecules adsorbed in this manner can be remarkably stable against fluctuations, they also show that in terms of adsorption free energy, hemicelluloses prefer the essentially non-H-bonded association to hydrophobic (200) surfaces (Fig. 1), if such surfaces are present (Martinez-Abad et al. 2017; Pereira et al. 2017; Zhang et al. 2011).

Nanocelluloses are suitable substrates for experimental determination of thermodynamic quantities of adsorption using isothermal titration calorimetry (ITC). A recent review (Lombardo and Thielemans 2019) that compiles the findings from ITC studies shows that adsorption of biomacromolecules, dyes, or drugs to cellulose or partially modified cellulose, is entropy driven and endothermic in most cases. In addition, MD simulations have shown that both urea (Chen et al. 2017) and the poorly soluble drug furosemide (Lombardo et al. 2018) adsorb to cellulose driven by a combination of dispersion interactions and translational entropy when they are exchanged with surface water. The exothermic exceptions, where H-bonding is a possibility, are the adsorption of cellulose binding modules (CBMs) of carbohydrate active enzymes or small and uncharged organic molecules. The binding site of CBMs is rich in aromatic amino acids such as tyrosine or tryptophane,

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placed at a distance so that they can stack on top of glucose rings along the cellulose backbone (Beckham et al. 2010; Ponyi et al. 2000). This suggests that the main mechanism for the adsorption is hydrophobic association by the release of interfacial water from both CBM and cellulose. At these less hydrophilic locations, there are no polar groups for water interaction, which means that the free energy of the hydrated state is high. Reducing the exposure of these areas to solvent thus results in an exothermic response upon release of water. Aromatic groups are relatively polar and interact strongly with water, compared to for example cyclohexane, which leads to a more favorable interface to water (Raschke and Levitt 2005). In addition, exchange of O-H O H-bonds to O-H N which have slightly lower energy (Pandey et al. 2017) would also lead to an exothermic response. Indeed, amines or amides are found in both CBMs and aromatic-rich molecules such as methylene blue (Lombardo and Thielemans 2019) that undergo exothermic adsorption.

Fig. 6 The continuous interface between two crystalline domains (left) can be perturbed by molecular scale ‘‘defects’’ such as chemical surface modification (right), in this case surface acetylation. This leads to sub-nanometer sized cavities that can harbor water molecules. Cellulose is shown in black for carbon and yellow for oxygen, surface acetyl groups in green (carbon) and red (oxygen). Water is shown in light blue.

Reproduced from Chen et al. (2020) with permission from the Royal Society of Chemistry

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Fibril-fibril aggregation and hornification Fibrils both in suspension and in the plant cell wall form self-associated structures. However, under ambient conditions fibrils in plant cell walls do not fuse completely to form larger crystallites, which makes them distinguishable as separate entities using experimental methods such as X-ray and neutron scattering (Jarvis 2018). This can in part be explained by the fibrils being partly covered by hemicelluloses, although small-angle neutron scattering show fibrils that are in direct contact in conifers (Fernandes et al.

2011), bamboo (Thomas et al. 2015), and spruce wood (Thomas et al. 2020). The presence of a structurally disordered (as compared to crystalline order) cellulose-cellulose interface was also identified by spectral fitting of the C4 region in the 13 C NMR spectrum, which also showed that these surfaces exhibited significantly different polymer dynamics based on their T1 relaxation times (Wickholm et al. 1998). This was later replicated in MD simulations of fibril aggregates (Chen et al. 2019).

Wherever the molecular structure prevents a perfect fit between fibrils, such as anti-parallel arrangement (Chen et al. 2019), fibril twist (Paajanen et al. 2019), adsorbed hemicelluloses(Thomas et al. 2020) or the presence of a chemically modified surface (Chen et al.

2020), small sub-nano-sized cavities are present, which can harbor water molecules (Fig. 6). These waters are confined to the interface between the fibrils and substantially restricted with respect to their translational and rotational degrees of freedom.

Indeed, both 2 H NMR(Lindh et al. 2017) and neutron scattering(O’Neill et al. 2017) reveal a population of water in hydrated fibril systems that has significantly slower dynamics than those normally associated with surface-bound water.

Computer simulation of fibril aggregates in excess water show that water molecules between fibrils tend to stay in place on MD (100 ns) timescales (Chen et al.

2019; Paajanen et al. 2019). This indicates that these water molecules are either in thermodynamic equilibrium with the water outside the aggregate or kinetically trapped due to their slow dynamics. The first case is supported by the observation that these water molecules can lower the total enthalpy of the system by saturating potential H-bonds that are lost due to defects. This will lower the total free energy provided that the entropic penalty associated with being

confined is not too large. MD simulations further show that the inter-fibrillar water molecules will spontaneously leave their confinement at high (160 C) temperature (Langan et al. 2014). This could be due to either the thermal energy becoming high enough to overcome the activation energy for diffusion, or the entropic term for the confined water becoming larger than the enthalpy gain. Interestingly, X-ray diffraction shows that hydrothermal treatment of wood does induce co-crystallization of fibrils, although not into a regular Ib structure (Kuribayashi et al. 2016). Instead a structure was obtained that was consistent with the fusion of anti-parallel fibrils.

When cellulose is dried from air it will form large, micrometer-sized aggregates (Peng et al. 2012). Such drying-induced association is a technical problem of great significance since the dry fibrils can be difficult to redisperse. Thus, from a practical perspective, the association is described as irreversible, a phenomenon usually referred to as hornification in the pulp and paper community. This has large consequences for the industrial use of nanocellulose since fibrils have to be kept in their dispersed state, with large transportation costs as a consequence (Posada et al. 2020). The extent of hornification depends strongly on drying methods (Peng et al. 2012; Nodenström 2020) and can also be mitigated by additives such as glycerol (Moser et al.

2018), which presumably act as spacers between fibrils. It also depends on surface chemistry of the fibrils (Benselfelt et al. 2019), where for instance acetylation was shown to reduce the work of adhesion between fibrils in water (Chen et al. 2020) due to the surface acetyl groups preventing tight association leading to the interpenetration of water molecules at the interface (Fig. 6). Although hornification is often explained as irreversible H-bonding between the fibrils, H-bonding is not sufficient to explain this process. With respect to making, hornification must be driven by a force that is both sufficiently long ranged and sufficiently strong to force fibrils and fibril aggregates into close contact. The best candidate is capillary forces that arise from the water seeking to minimize its liquid/air interface (Fig. 7). The magnitude of the capillary force depends on the geometry of the problem and the solid/liquid work of adhesion, WA. The crystalline cellulose surfaces are strongly hydrophilic and their WA to water was calculated to 118 mJ m-2 from MD simulations (Karna et al. 2020).

This leads to the idea that the forces can become

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Fig. 7 Drying-induced deformation of nanofibrils driven by capillary effects. The ‘‘dry’’ fibril-fibril joint contains structured and immobile water, which can contribute to both total adhesion and deformation mechanisms

substantial at the nanoscale and are strong enough to deform fibrils plastically (Ogawa et al. 2020). Moreover, MD simulations have shown that the capillary force can act over large distances (several nanometers) through liquid capillary bridges (Sinko and Keten 2014; Zhang et al. 2021) and thus fulfils both criteria.

When two cellulose surfaces finally are in contact there will of course be H-bonds. However, with respect to breaking, since the H-bond energy is smaller than the dispersion energy even for the best possible interface with respect to H-bond formation (i.e., the crystal structure), H-bonding likely plays a minor role for the stability also here. Thus, if the surfaces are close enough to be in molecular contact (i.e., van der Waal’s and/or H-bonding) the London dispersion will dominate over the H-bond contribution to the interaction energy. However, there is undoubtedly water present even in cellulose material that is perceived as dry and this will complicate the picture further.

Even if the porosity of cellulose nanomaterials can be surprisingly low, unless fibrils are perfectly aligned, they are prohibited from fusing completely into one continuous phase. Thus, as mentioned above, subnanometer sized cavities are formed, for instance around fibril-fibril joints, where surface hydroxyl groups are available for H-bonding. This space can be effectively filled by water molecules which will saturate the H-bonds, as long as the entropic penalty of confinement is not too high (Fig. 7). Thus, it is not only difficult to remove the water fraction that is most tightly attached to cellulose, but it is even more difficult to prepare and maintain samples (either as a specimen to be studied or a starting point for controlled hydration) that are devoid of all water (Lindh et al. 2017), because the initial water uptake of dry cellulose from ambient air is extremely rapid and amounts to about 8 wt% (based on cellulose dry mass)

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at 50% relative humidity. The emerging picture is that the extent to which H-bonding contributes to the adhesion between fibrils, even in ‘‘dry’’ conditions, is limited and an indirect effect of the high surface tension of liquid water.

Interfacial water also affects the friction between fibrils. In completely dry conditions, which can only be realized in simulations, the traction is dominated by a stick-slip behavior, in part related to breaking and reformation of interfacial H-bonds (Zhang et al. 2021).

In that respect, the small size and relatively high mobility of water molecules make them into an effective lubricant, which can significantly decrease the resistance to interfacial shear stress (Sinko and Keten 2014), and possibly also serve as a toughening mechanism since a weaker interface reduces brittleness (Hou et al. 2021).

Cellulose nanopaper films A unique feature of cellulose nanofibrils is that ‘‘nonporous’’ nanopaper films can be formed by drying from colloidal dispersions of the nanofibers in water.

Filtration and drying results in nanofibrils which are oriented random-in-plane, with slightly swirled conformation (Henriksson et al. 2008). The formation process can be compared to the formation of photonic crystals where capillary force through liquid bridges (Fig. 7) is the main adhesion mechanism, not hydrogen bonding (Zhou et al. 2006). Such forces can become very large, enough to deform the nanofibers plastically (Ogawa et al. 2020), which explains the low porosity of cellulose nanopaper dried from water.

The Young’s modulus of dry wood cellulose nanofibril films was recently reported to be 24.9 GPa by careful strain field measurements (Yang et al.

2021). This result is much higher than for any polymer

Cellulose

film with comparable molecular orientation distribution, for instance 5.2 GPa for biaxially oriented PETfilms (Breil 2010). From modeling work, the H-bonding between nanofibrils was erroneously stated to be critical for mechanical performance (Meng et al.

2017), and such ideas were further developed in a recent review paper (Meng and Wang 2019). Although the suggested model is interesting, the statement needs correction.

The ultimate strength of CNF nanopaper depends on cellulose molecular weight, indicating that it depends critically on cellulose nanofibril strength (Henriksson et al. 2008) and nanofibril length (Fukuzumi et al. 2013), but defects and local stress transfer mechanisms are also important. High hemicellulose content has a positive contribution to stressstrain behavior of nanopaper, which is related to interfibril bonding (Kontturi et al. 2021; Yang et al.

2021). However, in contrast to typical polymer films, even dry cellulose films (in the example above vacuum dried at 75 C for three days) will unavoidably contain water due to the finite, however brief, time it is exposed to moist air during the actual mechanical testing (Lindh et al. 2017). The large majority of the moisture is located in the interfibril region, since the fibril center is inaccessible to water (Sakurada et al.

1962). At 50% relative humidity, the moisture content of cellulose nanopaper is 8-10% (Yang et al. 2021).

There is strong dependence of both modulus and yield strength on moisture content (Benı́tez et al. 2013; Yang et al. 2021). The yield strength in nanopaper is probably related to interfibril shearing in the interphase region, and is lowered by increasing moisture content, a problem that has been analyzed at a molecular scale (Sinko and Keten 2014; Zhang et al.

2021). In the dry state, no apparent yielding is observed (Yang et al. 2021). The mechanisms for low strain deformation (modulus) and plastic yielding (yield strength) in practice thus depend primarily on moisture-related effects. Thus, cellulose nanopaper does not obtain its excellent properties thanks to H-bonding but rather despite the abundant H-bonding sites on the nanofibril surfaces. On one hand, they lead to formation of dense structures through capillary forces during drying, but on the other hand H-bonding sites increase moisture sorption, even at low relative humidity, with reduced properties as consequence.

The main reason for the superior modulus of nanopaper to polymer films, however, is the high axial

modulus of the cellulose nanofibril, which, even for random-in-plane orientation, dominates in-plane modulus for paper structures (Page 1965) and composites.

The cellulose fiber and paper making The scientific background to the excellent mechanical properties of different paper qualities has attracted substantial research interest over the years. This is partly due to the possibilities to form numerous materials from different types of fibers via waterbased processing routes, and partly since the excellent inherent mechanical properties of the fibers are utilized (Bolam 1961). A large focus has been on the contact zone between the fibers and how the properties of this contact zone can be understood and manipulated (Lindström et al. 2005; Hirn and Schennach 2017). To a large extent, the extensive work by Nissan (Nissan 1955, 1976a, b; Nissan and Batten 1997) has dominated the view among paper scientists that hydrogen-bonding between the fibers is the major interaction responsible for the mechanical properties of the paper. This is appealing at a first glance, since cellulose and hemicelluloses, the dominating components of most delignified, papermaking fibers, contain a large number of OH-groups that are known to be able to engage in H-bonds. However, the process of rough cellulose-rich fibers approaching each other during drying and consolidation of paper in the presence of water is complex and a more elaborate description of the mechanisms behind the making and the breaking of fiber-fiber joints is needed, especially considering the very short-ranged nature of H-bond interaction and formation (Santiago Cintrón et al. 2017). The development of new measuring techniques and the development of model materials mimicking the components of the fiber wall (Gustafsson et al. 2012; Li et al. 2021) provide new experimental information to separate the different molecular mechanisms responsible for fiberfiber interactions, but still, there is no quantitative description of their relative influence on the formation of a strong fiber-fiber joint (Lindström et al. 2005; Hirn and Schennach 2017).

The making and the breaking of a fiber-fiber joint are schematically described in Fig. 8. As the water between the fibers is evaporating the capillary pressure increases, pulling the fibers together. Under wet conditions, the delignified fiber wall is highly hydrated

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Cellulose

Fig. 8 Schematic description of the making and breaking of fiber-fiber joints. The wet fibers are pulled together by the capillary pressure and the wet fiber wall will yield in the wet zone. In the dry joint there will be areas in molecular contact and areas not in molecular contact but still close enough to allow for

an interaction between the surfaces. Of the proposed mechanisms, only van der Waals and ionic interactions are significant when there is no direct molecular contact (during the making of the joints in the wet state)

and the water has a strong plasticizing effect. Since the wet fiber wall has a modulus (E) of around one to ten MPa (Nilsson et al. 2001) a capillary radius (r) of one micrometer (a reasonable dimension considering the dimensions of the fiber) would lead to a strain ( ) of the wet fiber wall,

cellulose-hemicellulose, and cellulose-lignin interactions as evaluated by contact adhesion testing (Gustafsson et al. 2012). Considering the function of these components in trees, this result is also very logical since there should be a good adhesive interaction between these components to create a strong structure.

As the dry joint is strained, all the separate interactions across the interface will act to resist the load and breakage can occur either through adhesive failure of the joint or cohesive failure of the fiber wall (Fig. 8). The cohesive interactions inside the fiber wall are similar to the interactions that exist between fibers, which means that in an ideal joint it is difficult to identify the actual interface. The presence of water, in the form of moist air, will have several effects. First, it will soften the fiber wall due to moisture adsorption.

Further, as water adsorbs at the interface, specifically into those regions where the molecular contact between the fibers is poor, the water will weaken the dispersion interaction, enable disentanglement of molecules that might have moved across the interface and naturally also disrupt possible H-bonds.

The making and breaking of fiber-fiber joints is an interplay between the deformation of the macroscopic fiber wall due to massive capillary forces which allows close contact between dry fibers, and the development of molecular interactions at those contact points,

¼

2ccosh Er

between 1.5 and 15% assuming a fully wetted fiber surface (cosh ¼ 1) and using the surface tension of pure water c ¼ 72 mN m-1, and as the capillaries narrow, the deformation will be even higher. This means that just as for the fibrils (see above) the fibers are pulled closely together during drying. However, unlike the elementary fibril, the fiber surface is not molecularly smooth, which means that only a part of the interface between the fibers will be in molecular contact whereas other parts will be separated. As mentioned earlier, H-bonds are specific and shortranged, and practically require direct contact to form, whereas other forces (ionic and van der Waals) can be significantly long-ranged at this scale to act over the separation distance.

It has been shown that the migration of polymers across the interface has a significant influence on the dry adhesive properties of the fiber-fiber joints (Johansson et al. 2009) and that the thermodynamic work of adhesion is similar for cellulose-cellulose,

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Cellulose Table 1 Importance of H-bonding for various physical and chemical properties of cellulose and cellulose based materials, at normal conditions Property

Importance

Comment

Solubility

None

Dominated by solvent effects

Molecular conformation

Depends

Negligible in solution but important in the gas-phase

Crystal structure

Some

Defines H-bonding pattern, but structure is to a large extent the result of efficient packing, i.e. steric/dispersion interactions

Elastic properties

Some

Axial modulus is dominated by covalent interactions

Adsorption of polymers/molecules

None

Dominated by solvent effects

Fibril aggregation Nanopaper prop.

Indirect Some

Mediated by water Contributes to dry strength, responsible for high moisture sorption which affects ductility

Strength of paper

Minor

One among many contributions

including H-bonds. Once formed, these interactions will play different roles during the loading of the joints and also the combination of different interactions will contribute. H-bonding naturally influences the strength of these joints, as they will inevitably form wherever a favorable molecular geometry arises, but to what extent they contribute at the fiber scale in comparison to other relevant molecular forces is not possible to determine using any technique available today. However, considering their short range and directional sensitivity, contributions from other interactions are most likely dominating.

Perspective and outlook In this paper we have reviewed the role of H-bonding in cellulose-based materials at different scales, from H-bonds within small molecular fragments up to macroscopic fiber-fiber bonds. It is clear that the role of H-bonds has in many cases been exaggerated, at all levels in the structural hierarchy of cellulose. Considering that cellulose H-bonds in most relevant systems can exchange with H-bonds to water molecules it becomes evident that their net effect on the overall energetics is relatively small compared to other factors such as dispersion interactions and hydrophobic effects. A misplaced focus on H-bonding is an oversimplification that has indeed slowed down the development of our fundamental understanding of cellulose-rich materials and impeded their use in more advanced applications. On the other hand, to ignore

H-bonding completely in the analysis of cellulose is not a solution; they play an important role for defining the crystal structure of cellulose and give a surprisingly large contribution to its elastic modulus.

H-bonds can also be expected to contribute to molecular adhesion in dry conditions although completely dry cellulose hardly exist except in theory, and the role of water for interactions at both fibril- and fiber level under normal conditions cannot be stressed enough. This observation also applies to drinking straws.

Molecular modeling has indeed been instrumental for our understanding of the molecular-scale properties of cellulose. But at the same time the atomistic picture that permits us to actually count H-bonds has almost been a curse. Furthermore, it is important to realize that when we discuss experimental observations, we almost exclusively limit ourselves to a material that has been taken out of its native environment and has been mechanically and/or chemically modified. This undoubtedly affects its physico-chemical properties, particularly so when comparing to computer modeling studies, which are restricted to highly idealized models.

The emerging conclusion is that H-bonding is and should be viewed as one interaction among several, and its relative contribution is highly dependent on the specific conditions and cannot easily be determined by intuition alone or, indeed, in some cases not even by careful analysis. Based on our combined experience from working with cellulose and from the review of the scientific literature in this field presented in this

123

Cellulose

paper, we suggest a ranking of the relative importance of H-bonding for different physical and chemical properties of cellulose and cellulose based materials (Table 1). We hope that this work will inspire other scientists in this field to see beyond the standard explanations, and to not be content with descriptions of H-bonding as the sole defining feature for celluloserelated phenomena. This, we believe, would be of great benefit for the cellulose community, as a whole.

Acknowledgments The authors thank Dr. Alfred D. French and Dr. Tomas Larsson for helpful discussions during the writing of this manuscript. Benselfelt SciArt is acknowledged for the illustrations.

Funding Open access funding provided by Royal Institute of Technology.

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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PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 3, 2021

Applications of cellulose-based agents for flocculation processes: a bibliometric analysis 1

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1

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ALEJANDRO BARRERO-FERNÁNDEZ , ROBERTO AGUADO , ANA MORAL , CELESTE BRINDLEY , 1 MENTA BALLESTEROS .

Not surprisingly, cellulose-based agents for wastewater treatments, and more precisely for coagulationflocculation processes, raise growing interest, boosted not only by the high availability, functionality, renewability, and biodegradability of cellulose, but also by the outstanding performance of their derivatives.

The analysis of 460 publications including review papers, research articles and book chapters, among others, reveals a multidisciplinary approach, where the fields Materials Science, Chemistry, Chemical Engineering and Environmental Science play a major role. In terms of institutions, some of the most relevant contributors are the Wuhan University, Zhejiang Sci-Tech University, Universidad Complutense de Madrid, to name a few. The most relevant journals were found to be Carbohydrate Polymers, International Journal of Applied Polymer Science and Cellulose. An analysis of 332 keywords allowed us to classify works into three major clusters (besides two minor ones): one mostly defined by cellulose and coagulation; a second one championed by flocculation and cellulose derivatives; and a third one including wastewater treatment and polysaccharides. While the evolution of the scientific production leaves little doubt about it, as depicted in this bibliometric study, this is the first work providing an in-depth assessment and classification of the literature on cellulose for particle aggregation purposes.

Contact information: 1. Molecular Biology and Biochemical Engineering Department, Universidad Pablo de Olavide, Ctra. De Utrera km 1, 41013 Seville, Spain 2. Department of Chemistry, CQC, Universidade de Coimbra, Rua Larga, 3004-535 Coimbra, Portugal 3. Department of Chemical Engineering, Universidad de Almería, La Canada de San Urbano s/n, 04120 Almería, Spain Cellulose (2021) 28:9857–9871 https://doi.org/10.1007/s10570-021-04122-z 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 – Flocculation & Wastewater Treatment

Cellulose (2021) 28:9857–9871 https://doi.org/10.1007/s10570-021-04122-z

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ORIGINAL RESEARCH

Applications of cellulose-based agents for flocculation processes: a bibliometric analysis Alejandro Barrero-Fernández . Roberto Aguado . Ana Moral .

Celeste Brindley . Menta Ballesteros

Received: 21 May 2021 / Accepted: 28 July 2021 © The Author(s) 2021, corrected publication 2021

Abstract Not surprisingly, cellulose-based agents for wastewater treatments, and more precisely for coagulation-flocculation processes, raise growing interest, boosted not only by the high availability, functionality, renewability, and biodegradability of cellulose, but also by the outstanding performance of their derivatives. The analysis of 460 publications including review papers, research articles and book chapters, among others, reveals a multidisciplinary approach, where the fields Materials Science, Chemistry, Chemical Engineering and Environmental Science play a major role. In terms of institutions, some of the most relevant contributors are the Wuhan University, Zhejiang Sci-Tech University, Universidad Complutense de Madrid, to name a few. The Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/s10570-021-04122-z.

most relevant journals were found to be Carbohydrate Polymers, International Journal of Applied Polymer Science and Cellulose. An analysis of 332 keywords allowed us to classify works into three major clusters (besides two minor ones): one mostly defined by cellulose and coagulation; a second one championed by flocculation and cellulose derivatives; and a third one including wastewater treatment and polysaccharides. While the evolution of the scientific production leaves little doubt about it, as depicted in this bibliometric study, this is the first work providing an in-depth assessment and classification of the literature on cellulose for particle aggregation purposes.

Keywords Bibliometric study · Cellulose · Coagulation-flocculation · Wastewater treatment

Introduction A. Barrero-Fernández · A. Moral · M. Ballesteros (&) Molecular Biology and Biochemical Engineering Department, Universidad Pablo de Olavide, Ctra. de Utrera km 1, 41013 Seville, Spain e-mail: mmbalmar@upo.es R. Aguado Department of Chemistry, CQC, Universidade de Coimbra, Rua Larga, 3004-535 Coimbra, Portugal C. Brindley Department of Chemical Engineering, Universidad de Almerı́a, La Cañada de San Urbano s/n, 04120 Almerı́a, Spain

Every day, manufacturing industries produce large volumes of wastewater that, due to the presence of barely biodegradable and persistent contaminants, alter the physical, chemical and biological properties of natural water bodies (Teles et al. 2020). Some of these contaminants are also harmful to human health (Kasonga et al. 2021). Remediation techniques, whose choice depends on the type of wastewater, include, among others, sedimentation, ionic exchange, biodegradation, adsorption on activated

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carbon, filtration, and chemical oxidation. Usually, none of such methods can offer a stand-alone solution and they need to be continuously revised, complemented and improved to address the new emerging contaminants (Verma et al. 2012; Teles et al. 2020).

Undoubtedly, coagulation-flocculation is one of the most widely used wastewater treatment techniques, aiming to reduce turbidity, colour and organic matter levels by aggregation of small or colloidal particles (Matilainen et al. 2010). These terms, coagulation and flocculation are sometimes used interchangeably and/or ambiguously (Xiao et al.

2015). For the sake of accuracy and in agreement with other authors (Al-Risheq et al. 2021) we henceforth define coagulation as the destabilization of a suspension of particles, whereas flocculation is the aggregation of destabilized particles. By reducing the electrostatic repulsion between microparticles, their size geometrically increases, and thus sediment with ease or become readily treatable by other kinds of solid–liquid separation (Matilainen et al. 2010).

Depending on the nature of the agent, this loss of electrostatic repulsion can be attained by chemical or organic coagulation. Chemical coagulation, based on the use of inorganic compounds, strongly depends on many variables such as the dosing time, the pH, and the concentration of the inorganic salts (Li et al.

2021). Hence, it is difficult to predict the behaviour of a certain chemical coagulant in real complex environments.

Organic flocculation, on the other hand, usually involves biomolecules of natural origin and attains denser and bigger flocs, which may imply a more cost-effective solution (Renault et al. 2009). Besides improving the coagulation process in itself, some other advantages include more biodegradability and less toxicity to the environment (Bolto and Gregory 2007). It is easy to see why, among the biological macromolecules that can be used as main agents or at least as adjuvants in coagulation-flocculation processes, cellulose derivatives play a major role. Firstly, cellulose is highly available, being the most abundant biopolymer on Earth (Peng et al. 2020). Secondly, like any other hydroxy polymer, it can be chemically modified towards different functional groups, cationic or anionic, hydrophilic or hydrophobic, etc. (Moral et al. 2015; Aguado et al. 2019). Thirdly, their glycosidic bonds are chemically stable, but cleavable

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by many microorganisms, granting cellulose versatility and biodegradability.

An in-depth bibliometric analysis will allow us to estimate the extent of interest in cellulose derivatives in wastewater treatments. This valuable tool, which has proved its usefulness in the context of coagulation-flocculation processes and other water remediation techniques (Zhang et al. 2020; Demir and Sharma 2021), helps researchers in relating scientific production and industrial development, evaluating the main trends in the generation of knowledge, and identifying the main contributors (Zhang et al. 2020). While the timely bibliometric study by Zhang et al. (2020) on agents for sludge removal mentions polysaccharides such as starch and chitosan (Zhang et al. 2020), cellulose is left out the picture and, for that matter, the vast literature on cellulose-based agents for coagulation-flocculation is yet to be analysed.

We aim to provide researchers and manufacturers with a thorough and systematic analysis of the scientific production on cellulose and its derivatives in wastewater treatments, particularly for particle aggregation purposes. In order to show the growing interest in this topic, trends of cellulosic coagulants, flocculants and/or adjuvants are analysed. More specifically, the methodology encompasses Scopusindexed publications (studying both journals and papers therein), books or book chapters and conferences, if they are accessible through the keywords that define the scope of this article. This paper seeks to assess the evolution of the scientific production on the topic, to classify it by type of publication and subject area, and to identify the main authors and institutions involved. These tasks manifest the potential of the knowledge gathered to evaluate and improve the current industrial processes, their environmental impact, and the social perception thereof.

Methodology Searching process The strategy chosen to develop this bibliometric analysis involves the search engine of Elsevier’s database Scopus, as chosen elsewhere (Mongeon and Paul-Hus 2016), given that it encompasses more publications than ISI Web of Knowledge and, at the

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same time, grants the fulfilment of peer review and all other essential requirements of scientific practice.

Search was limited to dates comprised between 1975 and 2020, using the keyword string TITLE-ABSKEY (cellulose AND (coagulant OR flocculant)), obtaining 460 matches. We excluded non-related areas, e.g., medicine and odontology, which fall far from the scope of this work, even though they fulfilled the search requirements.

Data analysis Scopus matches were processed to extract the data that was required to analyse the number of publications by year, the distribution of publications by affiliation and country, the universities and funding institutions accounting for most publications, the journals containing most publications and related citations, and the most relevant authors. Furthermore, the present work particularly emphasised the importance of the keywords used in the scientific literature.

For the analysis of keywords, we acknowledged that some terms can be used with the same meaning, such as bacteria and bacterium. Moreover, terms that could not contribute to the study were discarded (e.g., article). For the same reason, while the use of paper as material was accepted, the use of paper as document was obviously rejected. This analysis encompassed keywords that appeared at least 5 times, obtaining a map which comprised a total of 332 terms. These terms could be classified into 5 groups or clusters.

In order to consider an association of institutions as a scientific community, we inquired whether or not such association implied a tighter collaboration than the ones between each of those institutions and the rest of the research network. Scientific communities, attending to specific keywords in each case, were determined by means of two software packages: VOSviewer (Leiden University and CWTS), which allows for editing graphs in such a way that links between institutions or countries are shown as nodes, meaning tight collaboration. Likewise, the software OpenRefine (open source) was generally used to process the raw data provided by the search engine.

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Results and discussion Evolution of scientific production The number of matches from the search engine, 460, is deemed suitable for a bibliometric analysis, given that it belongs to the usual range of results in bibliometric studies on similar topics (Zhang et al 2020; Malik et al. 2020). The year 1975 is a proper starting point to assess the progression of the scientific production, as therefrom publications occur on a continuous basis. During the two following decades there are few variations in the publications (Fig. 1). However, from 1995 onwards a change of trend can be observed as references gradually increase, and from 2010 growth is faster, evidencing the rising interest in research on cellulose applications as flocculant and coagulant. These publications are mainly research articles (82%) written in English (89%), as can be seen in Figs. S1 and S2 (supplementary material).

Analysis by subject area As can be seen from Fig. 2, the branch of scientific knowledge accounting for the highest number of publications is Materials Science (22% of total). Over 80% was published within five subject areas, which, besides Materials Science, include Chemistry (19%), Chemical Engineering (16%), Engineering (14%) and Environmental Sciences (11%).

Figure 2 shows how the study of cellulose-based coagulants and flocculants benefits from a

Fig. 1 Number of publications on cellulose based flocculants and coagulants from 1975 to 2020

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Fig. 2 Subject areas with most articles related to cellulose based flocculants and coagulants

multidisciplinary approach. Strategies often involve physical combinations with other materials (Lee et al.

2012), chemical modifications of cellulose (Koshani et al. 2020) and/or both. As an example of hybrid material, cellulose can be bound to magnetite powder to obtain a valuable flocculation agent for a palm oil mill effluent (Mohamed Noor et al. 2018). Likewise, the presence of Ag2O or TiO2 along the cellulose backbone provides usefulness for simultaneous photodegradation (Koshani et al. 2020). As for chemical modifications, they include not only the typical etherifications of hydroxyl groups towards anionic (carboxymethylation) or cationic (quaternary ammonium moieties) derivatives, but also more inspired solutions. For instance, seeking the performance of polyacrylamides, which are poorly biodegradable, and at the same time the biodegradability of polysaccharides, acrylamide units can be grafted onto carboxymethyl cellulose (Miyata et al. 1975).

All things considered, Materials Science and Chemistry are mainly found in terms of strategy, while Environmental Science and Engineering encompass frequent objectives of coagulation-flocculation, namely environmental concerns and the need for treating industrial effluents. We particularly remark the abundance of reviews within the area of Environmental Science. Such reviews mainly study the application of different wastewater treatments to grant the safe disposal of effluents or to make water drinkable, comparing innovative proposals to current methods (Matilainen et al. 2010; Verma et al. 2012; Lee et al. 2014; Teh et al. 2016; Wei et al. 2018).

Allegedly, and considering they can be prepared in a wide variety of ways (powder, suspension, solution, etc.), cellulose-based agents are compatible with the

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existing wastewater treatment technologies (Oyewo et al. 2020).

While most research and review papers deal with flocculation in the context of wastewater treatments, the importance of cell flocs in microbiological systems should not be disregarded. It has been shown, for instance, that the flocculation of cellulose-producing bacteria is enhanced by cellulose itself, and thus the biotechnologist often needs to choose an additive, such as carboxymethyl cellulose, to disperse fibers (Andrade et al. 2019). This kind of consideration partly explains the contribution of Biology to the topic.

Ranking of countries, institutions, authors, and journals by number of publications As can be seen from Fig. 3, the main contributor to the scientific production in relation to the use of cellulose as coagulant and flocculant is China, with over 100 publications, which accounts for more than twice the number of publications of the second most prolific country: Japan. The USA, Canada and India come next, with over 25 publications each. China is also the country with the highest number of institutions that carry out research in this area as well as the biggest funder of this research area, as can be seen in figures S3 and S4, respectively.

To analyse the main contributors to research on cellulosic flocculants and coagulants, the ten most prolific authors in the field have been identified. The classification of the authors and their most relevant contributions are listed in Table 1. As can be seen from Table 1, most of the authors with most publications are in China, which is to be expected

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Fig. 3 Worldwide distribution of publications on cellulose-based flocculants and coagulants in countries with more than 10 publications Table 1 Top-10 authors with the highest number of publications (N) on cellulose-based flocculants and coagulants Author

N

Institution

Zhang, Lina

14 Wuhan University

Country

h

Most contributed topics

ID

China

87 Purified Rayon; Cellulose Films

55917992100

Zhang, Yong

9 Zhejiang Sci-Tech University

China

12 Flocculants; Polyacrylic Acid; Sludge Dewatering

57211127995

Zhou, Jinping

9 Wuhan University

China

35387227300

Negro, Carlos

8 Universidad Complutense de Madrid

Spain

44 Purified Rayon; Cellulose Films 32 Nanocellulose; Oxidized Cellulose; Nanowhiskers; Papermaking

Okajima, Kunihiko

8 Tokusihima Bunri University

Japan

26 Cellulose; Membranes; Regenerated Fibers

7202754455

Yang, Xiaogang

7 Zhejiang Sci-Tech University

China

13 Flocculants; Polyacrylic Acid; Sludge Dewatering

56146642400

Liimatainen, Henrikki

7 Oulun Yliopisto

Finland 31 Nanocellulose: Cellulose; Oxidation; Deep Eutectic Solvents; Water Chemicals

55961077200

Yao, Juming

6 Zhejiang Sci-Tech University

China

33 Nanocellulose; Oxidized Cellulose; Nanowhiskers

56387954400

Blanco, Ángeles

6 Universidad Complutense de Madrid

Spain

17 Nanocellulose; Oxidized Cellulose; Nanowhiskers

56374315900

Yu, Jianyong

6 Donghua University

China

75 Air Filters; Fibrous Membranes; Nanofibers

35185851400

7005434014

h: h-index

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seen as China is the country with most publications and citations. Table 1 comes to show that in China there are not only many researchers with an interest in the topic, but also that they have authored many publications. Among the institutions, the Chinese universities of Zhejiang Sci-Tech University and Wuhan University stand out, with several authors on the list. Japan is also present in the table, and the presence of Spain and Finland is also noteworthy.

The two latter countries count with some very prolific authors, suggesting that in these countries publications originated from a few research groups; the lack of out-standing authors in the USA or India seems to suggest the opposite, that is, publications originated from many disperse research groups.

Figure 4 presents the nine Scopus- and JCRindexed journals with most publications in this area of study. The journals Carbohydrate Polymers, Journal of Applied Polymer Science and Cellulose constitute the top three journals with most publications, with more than 15 each, followed closely by Journal of Membrane Science, with more than 10 publications. The aforementioned journals belong to the first quartile of the area of study that encompasses the publications considered in the year 2019, except for Journal of Applied Polymer Science, which belongs to the second quartile, although it maintains a high h index, over 150 at the end of 2020 (SJR).

Cellulose (2021) 28:9857–9871 Fig. 5 Clusters of keywords that appear with a frequency c greater than 5 (a), and evolution of such keywords over time in the period 2005–2020 (b). The color scale indicates the frequency according to the year of publication. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article)

Their impact factors cannot be related to the number of publications, in general terms. In any case, in the same range of impact factors, the number of publications is higher for the journal whose scope addresses the topic more specifically.

Analysis of keywords A thoughtful evaluation of keywords implied revising carefully the data, manually discarding vague or unrelated terms. Likewise, synonyms were removed or merged. Figure 5 displays a network of the main keywords in publications dealing with cellulosebased coagulation and/or flocculation. The resulting map contains 169 terms, of which the most frequent are cellulose, flocculation and coagulation. Each node in Fig. 5 presents a keyword in such way that terms in bigger rectangles are more frequent than those in smaller ones. In Fig. 5a, keywords are classified in groups (or clusters) attending to the relations among them, which are shown as lines joining terms that tend to appear in the same work. Broad lines mean a more frequent coexistence than thin lines. Thence 5 clusters were identified. Furthermore, the evolution of their relevance over the period of study (2005–2020) is depicted in Fig. 5b. It is not meaningless that the term flocculation was found with higher frequency than coagulation. Although less frequent, watersoluble polyelectrolytes, including those derived from biopolymers such as cationic cellulose and cellulose sulphonate, may be capable of “direct flocculation” (Chong 2012; Lee et al. 2014), thus not needing a previous destabilizing step, i.e., not needing to rely on the coagulation-flocculation process.

Group 1 (red cluster): materials and target pollutants

Fig. 4 Journals with most publications on the use of cellulose as coagulant and flocculant, highlighting their impact factor (as of 2020)

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The most remarkable keywords in the red cluster were cellulose (274 appearances) and coagulation (127). This is also the cluster in which most terms

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related to Materials Science are included—among others, tensile strength, cellulose films (24), membranes (33), and fibers. Membranes are significantly linked with ultrafiltration (18), in which case those membranes can even discriminate by molecular weight. Interestingly enough, even the keywords solutions, aqueous solution and spinning are often framed in the context of the preparation of different materials, as can be easily seen from a search with the string TITLE-ABS-KEY (cellulose AND coagulation AND (aqueous solution OR solutions OR spinning)).

Some of the most relevant and recent papers matching this requirement report the fabrication of permeable membranes (Makarov et al. 2021), protein-repellent coatings (Bračič et al. 2021), antimicrobial packaging (Oliva et al. 2020), and composite fibres with great tensile properties (Song et al. 2018). In these cases, coagulation is not conceived as a wastewater treatment, but as a step in the production process of films or other solid products from aqueous dispersions of cellulose. This also explains the finding of the name of several chemicals as keywords. Sodium hydroxide (17) and urea (22) constitute the most popular system for the dissolution or amorphization of cellulose in aqueous media, while sulphuric acid (10) generates nanocellulose dispersions through the hydrolysis of the least crystalline domains of fibers. Ethanol (10) is the most widely used anti-solvent in the regeneration of cellulose from its dispersed or dissolved form.

The publications with environmental aims that fall into this group, being as valuable as they undoubtedly are, do not usually address real, complex systems.

Generally speaking, they target individual pollutants such as methylene blue (Hossain et al. 2021) and different metallic ions including copper(II) (Maaloul et al. 2021), lead(II) (Li et al. 2020) and chromium (VI) (Wang et al. 2020). For such purposes, the contaminant is dissolved in distilled water or in aqueous buffer solutions. On one hand, this is necessary to attain a deep understanding of the physicochemical interaction between pollutants and cellulose. On the other hand, actual wastewater samples for any manufacturing industry do not share this simplicity. Unpredictable interferences come from the presence of many different compounds, the competition between the potential targets, and the difficulties of adjusting factors such as the pH and the ionic strength. Therefore, we may identify not only a

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knowledge gap, but also an opportunity arising from the unbalance between the excellent performance reported for these cellulosic materials, frequently hydrogels or membranes (Kanmani et al. 2017; Peng et al. 2020), and the uncertainty of their applicability in real wastewaters.

In any case, looking at the evolution of these keywords with time (Fig. 5b), it is clear that research on most of these topics began before 2010. From this data, it is reasonable to infer that knowing the behaviour of cellulose in aqueous dispersions, even if this knowledge was directed towards sorbents and films (Syverud and Stenius 2009), boosted the research onto the synthesis of soluble cellulose derivatives and their use as flocculants. This concept is key in Group 2, which generally involves more recent insights.

Group 2 (green cluster): flocculation using cellulose derivatives Whereas in Group 1 coagulation was not framed in coagulation-flocculation treatments as much as in procedures to prepare materials, Group 2 encompasses publications which include flocculation as an essential feature. Whereas in Group 1 we remarked the appearance of solutions and aqueous solution, in Group 2 we found suspensions. Whereas cellulose was highlighted as the key material for membranes, films and other materials, now we must notice the high frequency of cellulose derivatives (42) since, when it comes to flocculation, the insertion of cationic or anionic functional groups is extremely useful for the aims of charge neutralization, electrostatic patching, and bridging (Koshani et al. 2020).

Furthermore, pH-independent ionization, such as that achieved with quaternary ammonium groups, broadens the applicability of the potential flocculants.

The appearance of flocculation without coagulation in an article does not necessarily mean that coagulation was not involved. Generally speaking, coagulation and flocculation are found simultaneously in particle aggregation processes. Often, coagulation is deemed a constant factor and only flocculation is evaluated and mentioned, even when in the case of study there are inorganic species that act as coagulants. Cellulose derivatives may be used as flocculants in the presence of inorganic compounds such as calcium chloride, ferric sulphate or

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aluminium chloride (Liu et al. 2014; Peng et al.

2020), i.e., very well-known destabilizers. Other works of similar nature actually include coagulation in the list of keywords, in the abstract or in the title (Suopajärvi et al. 2013; Liu et al. 2014). Noticeably, particle aggregation may involve Ostwald ripening (25) as mechanism.

Dyes (10) are, generally speaking, very popular pollutants in studies seeking to estimate the performance of a flocculation system (Kono and Kusumoto 2015). However, while treating dye containing waters is necessary, the choice of dye as model pollutant has more to do with the ease of quantitative analysis, as its concentration can be simply calculated from the absorbance at a certain wavelength, than with a social or scientific demand for those dyehouse waters.

Likewise, the abundance of studies on kaolin (20), which is not the most common filler in papermaking, probably arises from its strongly negative zeta potential, ensuring the successful application of cationic cellulose derivatives (Bratskaya et al. 2006; Aguado et al. 2017).

Nevertheless, many articles framed in Group 2 do not study cellulose-based flocculants for decontamination purposes, but within the scope of the wet end of a paper machine, as additives to the pulp stock or, more precisely, retention aids (Li et al. 2015; Aguado et al. 2017). Thus, papermaking (17), one of the major keywords, is of utmost importance, and pulp, additives and retention aids are strongly related to it.

While the main goals of flocculation in wet-end chemistry are filler retention and drainage rate, it also has a decisive impact on the composition of the filtrate or white water. Like any other effluent, this stream requires treatment before being discarded, but synthetic flocculants often persist after such treatment.

While biodegradability is alleged as an advantage of cellulose-derived flocculants in 16 articles, only those from the research groups of Yao and Zhang, working with polyacrylamide-grafted cellulose, include biodegradability assays in their experimental section (Zhu et al. 2016; Chen et al. 2020). Most often, however, the biodegradability of cellulose derivatives is taken for granted ignoring the fact that many chemical modifications, such as silanization, cationization and oxidation, lower the biodegradation rate or narrow down the range of suitable conditions for microorganism activity (Kargarzadeh et al. 2017;

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Frank et al. 2018). Even in the absence of chemical modifications, researchers must take into account that it is the structure, not the origin of the raw material, what determines biodegradability (Witt et al. 1999).

The isolation of the crystalline domains of cellulose, as in the production of nanocrystals, hinders biodegradability in comparison to materials that keep hemicellulose and amorphous regions, which are more prone to microbial attack (Barreto et al. 2010).

All things considered, biodegradation studies benefit from discussion on the supramolecular structure of the cellulosic product proposed in each time, and saline, acidic and alkaline media should be considered. Finally, we suggest assessing not only the biodegradability of cellulosic flocculants, but also the toxicity of the products that result from their aerobic or anaerobic decomposition (Vikman et al. 2015).

Group 3 (blue cluster): parameters of wastewater treatments This keyword cluster is mostly characterised by the occurrence of water treatment (39), turbidity (38), polymers (33), pH (27), particle size (24), filtration (23), effluents (19), pollutant removal (16) and chemical oxygen demand (15), among others. Therefore, publications falling into this cluster aim at water decontamination techniques involving the use of polymers. These polymers include carboxymethyl cellulose (10) and derivatives thereof, but this derivatization, once again, often implies the use of poorly biodegradable polyacrylamides (21) (Feng et al. 2020). It is worth clarifying, however, that this term is more commonly found as a conventional flocculant, chosen for comparison purposes, than as a derivatizing agent for cellulose (Vuoti et al. 2018). In fact, charged flocculants outweigh non-ionic flocculants all along this search. Since the most common cellulose-based agents are carboxymethyl cellulose (anionic), cationic cellulose, cellulose sulphonate (anionic) and combinations with acrylamides (anionic or cationic), it is common to find measurements of zeta potential (12).

A critical approach to these keywords demands some discussion on turbidity, the most widely used parameter to indicate contamination or removal. This can be explained because the most obvious effect of particle aggregation, caused either by coagulationflocculation or by direct flocculation, is a reduction of

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light scattering, i.e., an increase of transparency of the liquid phase. Moreover, legislation and standards on water quality make a prominent use of turbidity as a key indicator (Boyd 2015). However, the absence of pathogens is also a legal requirement in drinking water and, despite the usefulness of flocculation to generate microbial flocs that could be physically separated, the extent of disinfection is seldom reported. In this direction, particle size and chemical oxygen demand are undoubtedly helpful to monitor and evaluate the extent of flocculation, but as unspecific as turbidity. The three of them are quantitative indicators. Therefore, it is difficult to judge if cellulose-based flocculation, as a decontamination technique, is successful when it comes to the removal of emergent pollutants, which are increasingly raising concern among lawmakers. This bibliometric finding encourages the use of chromatography coupled to mass spectrometry (or another kind of detector for qualitative purposes), which is available in most laboratories, to track the removal efficiency for each compound in a real system (Ma et al. 2021).

Most likely, in the elimination of persistent contaminants, particle aggregation complements an advanced technique (Suarez et al. 2009), since wastewater treatment plants are typically unable to target such contaminants through filtration, flotation or sedimentation only. Instead, the pollutant may be adsorbed onto activated carbon (6), oxidized with ferric chloride (5), or exchanged with acrylic resins (7). In those cases, like when working with biological membranes or ultrafiltration devices, coagulationflocculation generally precedes the advanced technique (Zahrim et al. 2011), freeing those methods from suspended solids.

Publications included in Group 3 tend to study more real wastewater streams, frequently industrial waste (5) or sewage (8), than those in the previous keyword groups. This is clearer in the most recent publications (Vuoti et al. 2018). As for the pollutants targeted, there is a wide variety of them, remarkably encompassing phenols (5), azo dyes (5) and metal ions (5), but not addressing pharmaceuticals and other emerging pollutants (PhEPs) which include pharmaceutical compounds and their metabolites, hormones, steroids, personal care products, etc., contained in water at concentrations below ppm level, with harmful effects on human health and, in

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general, for all ecosystems. Further research is necessary, thus, not only to identify greener alternatives to the functionalization with acrylamides with no major loss of performance, but also to broaden the spectrum of contaminants to be efficiently removed.

Group 4 (yellow cluster): biological applications A relatively low number of publications featuring cellulose-based flocculation, or even where cellulose and flocculation are only tangentially related, show biological or biochemical terms: protein (6), enzyme (5), metabolism (11), nonhuman (17), human (7), bacteria (8) and animal (11), along with animal experiment. Less intuitively, ion-exchange chromatography (5) must also be understood in the frame of biological macromolecules, since it has been applied as a high-performance separation technique for protein samples. However, that method is less commonly used in recent papers (Fig. 5b), giving way to affinity chromatography as a more selective technique.

In these cases, a cellulose derivative, such as cellulose acetate phthalate, is used as microbicide or as another kind of biological agent for disease prevention or treatment of animal organs (Otten et al. 2005). Then, the role of flocculation across this cluster ranges from recovering an animal protein from an aqueous suspension (e.g., livestock waters) to synthesizing bacterial cellulose.

Group 5 (purple cluster): micro- and nanotechnological applications In what should be labelled as another minor cluster, many keywords are related to micro- and nanotechnology: microscopy (5), microfiltration (6), microorganisms (7), nanocrystals (5), nanoparticles (7). In publications which belong to Group 5, interestingly, flocculation may be something to be avoided, rather than promoted. This is the case of Li et al.’s (2017) cellulose films with silver nanoparticles, whose antimicrobial potential is hindered by flocculation (Li et al. 2017).

Ultimately, there is a wide variety of applications fitting this context. Nonetheless, the main one is harvesting microalgal biomass by means of flocculants, which is indicated by the appearance of harvesting (7), biofuels (6) and microalgae (6), three

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keywords that are found tightly related. In this case, those flocculants are cellulose derivatives or even cellulose nanocrystals (Eyley et al. 2015). It is worth mentioning that papers on this application are relatively recent, thus showing a current research trend (Fig. 5b).

Trends and missing keywords Other than cellulose, coagulation and flocculation, i. e., the terms of the search string, the nine main keywords are presented in Fig. 6, showing their key statistics. Scanning electron microscopy is the most frequently mentioned technique along the 460 publications (Fig. 6b) and the most popular method to analyze morphology and the structure of membranes.

Besides turbidity, which is closely related to solutions and polymers (Fig. 6c), the use of light scattering

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across particle suspensions is outweighed by direct imaging.

Figure 6a displays the year of highest occurrence of each of the main keywords. Interestingly, a shift from solutions (2006), frequently meaning laboratorial aqueous systems, to wastewater (2014) can be observed. Also, remarkably, even considering the consistent increase in the number of publications on the subject, cellulose derivatives were mostly mentioned back in 2007. In contrast, current trends and future research seem to aim more at nanocelluloses, not necessarily involving chemical modifications, than at etherification or esterification of anhydroglucose units. These seem to be displaced by crosslinking and/or grafting of side chains.

Bibliometric studies are effective in identifying not only the current and past research trends, but also which keywords do not appear in the literature, and thus hinting which fields are not being studied and

Fig. 6 Selection of nine major keywords, highlighting the year of highest popularity (a), their individual occurrence (b) and their most usual combinations (c)

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which ones deserve special attention. For example, stimuli-responsive polymers or “smart polymers” are macromolecules that are sensitive to a variety of signals, including temperature, pH, light, electrical or magnetic fields, chemicals, etc. In spite of being widely studied in the last decade, they do not appear in any of the clusters in Fig. 5. They have been used in medical applications and they show convincing potential for flocculation processes (Maćczak et al.

2020). Oechsle et al. (2018) prepared a CO2-switchable hydrogel from a suspension of cellulose nanocrystals (CNCs) and its properties make it a promising candidate to be used as flocculant in water treatment (Oechsle et al. 2018). However, much remains to be investigated in this field, which rises as an alternative way to obtain novel cellulose-derived bio-flocculants.

Bioflocculants of cellulosic origin have been used in a wide variety of systems, including turbidity removal from drinking water and municipal wastewater treatment plants, water decolorization in textile industry effluent treatment, flocculation in pulp slurries, treatment of water contaminated by oil spills, etc. (Maćczak et al. 2020). However, in coagulation-flocculation processes in wastewater, there is a lack of pilot plant experiments and techno-economic assessments with coagulants/flocculants from cellulose, as evidenced by the absence of keywords like cost, economic, feasibility or pilot plant. Those studies are necessary to address the (sometimes unpredictable) problems associated with upscaling. In parallel, pilot plant assays allow the accomplishment of a reliable economic analysis of the process. In this sense, the authors believe that future studies should be accompanied by an economic study on the viability of the process, detailing the cost of treatment per cubic meter as a function of the application of the treated water.

Conclusions The results of this bibliometric study show that cellulose-based agents for flocculation-coagulation processes is a research area of growing interest with a multidisciplinary approach. There is a wide variety of applications for cellulosic flocculants, but the main one by use and interest is the treatment of wastewater.

China, Japan, and the US are the most prolific

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countries, although Spanish and Finnish authors also stand out on the European continent that deserve special attention when establishing new scientific collaborations in this area. In this field, high-level journals stand out, including Carbohydrate Polymers, Journal of Applied Polymer Science and Cellulose, which gives an idea of the impact of the research.

The analysis of the keywords establishes several scientific communities that indicate the main areas of research being conducted: (1) fabrication of cellulose-based membranes, hydrogels and materials for decontamination; (2) chemical modification of cellulose towards bioflocculants; (3) control and monitoring of wastewater treatment. The extraction of biological macromolecules and applications on nanotechnology account for minor contributions.

Moreover, this analysis also reflects some gaps that must be overcome and that can point out for the scientific community which areas need greater efforts to be focused on, such as evaluating in each case the biodegradability of cellulose derivatives instead of taking it for granted. Also, the high functionality of cellulose allows for its modification towards promising, and fortunately trendy as of today, stimuliresponsive polymers. Despite the usefulness of such polymers in solid–liquid systems, including flocculation (e.g., for reversible aggregation as a function of temperature or pH), the lack of those terms shows that researchers have not taken that opportunity yet.

One opportunity that is being tackled and that will probably constitute a major research trend in the short term, is the use of bioflocculants to harvest microalgae biomass, particularly as feedstock for biofuel generation.

Most water treatment documents focus on the study of environmentally concerning parameters of quantitative nature, such as chemical oxygen demand, total suspended solids, turbidity and color.

However, it is necessary to make efforts to study the effectiveness of this type of compounds in eliminating PhEPs in order to meet the environmental legislation before the wastewater is discharged. In many articles a high-cost effectiveness and low cost of cellulose derivatives is assumed due to the great availability of the biopolymer. Nonetheless, there are no studies detailing the total cost of treatment to make it comparable with existing data in the literature on conventional coagulants/flocculants. Although laboratory-scale studies carried out with cellulosic

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derivatives suggest that this area has great potential, limitations in terms of applicability in drinking water or wastewater treatment plant effluents have not yet been resolved, and extensive research is required to increase viability of cellulose-based flocculants production in a large scale.

Authors' contributions All authors made substantial contributions to the conception of the work, the acquisition and interpretation of data, and writing. All authors approve the manuscript. All authors agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Funding Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. Universidad Pablo de Olavide/CBUA.

Availability of data and material The authors made all data public in the international repository Zenodo.

Code availability

Not applicable.

Declarations Conflict of interest The authors declare that there is no conflict of interest and that they do not have competing interests.

Ethics approval Not applicable. No studies involving humans and/or animals.

Consent to participate Not applicable. No studies involving humans and/or animals.

Consent for publication Not applicable. No studies involving humans and/or animals.

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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FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 3, 2021

Investigation of Pore Size Distribution

by Mercury Intrusion Porosimetry (MIP) Technique Applied on Different OSB Panels 1 1 FABIANE SALLES FERRO , FELIPE NASCIMENTO ARROYO , EDSON FERNANDO CASTANHEIRA 1 1 RODRIGUES , IURI FAZOLIN FRAGA , JOÃO PAULO BOFF ALMEIDA1, HELOIZA CANDEIA RUTHESQ1, 2 3 VINÍCIUS BORGES DE MOURA AQUINO , ELEN APARECIDA MARTINES MORALES , MATHEUS 1 4 HENRIQUE MORATO DE MORAES , FRANCISCO ANTÔNIO ROCCO LAHR , AND ANDRÉ LUIS CHRISTOFORO1

Mercury intrusion porosimetry (MIP) is a technique used to characterize the pore size distribution and resin penetration in lignocellulosic materials, such as oriented strand board specimens (OSB), a multilayer panel utilized in structural applications. The method is based on the isostatic injection, under very high pressure, of a non-wetting fluid (mercury) into the porous material to determine parameters such as pore size distribution and percentage of porosity of the specimens. In this study, five different OSB were analyzed; they contained different wood species, resin type, and resin content. The panels manufactured with castor oil polyurethane resin showed porosity values in the range of 54.7 and 27.8%. This was a promising result compared with those obtained for panels made with phenolic resins, which are currently commercialized in Brazil.

Contact information: 1. Department of Civil Engineering, Federal University of São Carlos, São Carlos, Brazil; 2. Department of Civil Engineering, Federal University of the South and Southeast of Pará, Santana do Araguaia, Brazil; 3. Department of Materials Science and Engineering, Paulista State University, Itapeva, Brazil; 4. Structural Engineering Department, University of São Paulo, São Carlos School of Engineering, São Carlos, Brazil Ferro et al. (2021). “Strandboard Hg porosimetry,” BioResources 16(4), 6661-6668.

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Investigation of Pore Size Distribution by Mercury Intrusion Porosimetry (MIP) Technique Applied on Different OSB Panels Fabiane Salles Ferro,a Felipe Nascimento Arroyo,a,* Edson Fernando Castanheira Rodrigues,a Iuri Fazolin Fraga,a João Paulo Boff Almeida,a Heloiza Candeia Ruthes,a Vinícius Borges de Moura Aquino,b Elen Aparecida Martines Morales,c Matheus Henrique Morato de Moraes,a Francisco Antônio Rocco Lahr,d and André Luis Christoforo a Mercury intrusion porosimetry (MIP) is a technique used to characterize the pore size distribution and resin penetration in lignocellulosic materials, such as oriented strand board specimens (OSB), a multilayer panel utilized in structural applications. The method is based on the isostatic injection, under very high pressure, of a non-wetting fluid (mercury) into the porous material to determine parameters such as pore size distribution and percentage of porosity of the specimens. In this study, five different OSB were analyzed; they contained different wood species, resin type, and resin content. The panels manufactured with castor oil polyurethane resin showed porosity values in the range of 54.7 and 27.8%. This was a promising result compared with those obtained for panels made with phenolic resins, which are currently commercialized in Brazil.

Keywords: Mercury porosimetry; Oriented strand board; Castor oil polyurethane resin Contact information: a: Department of Civil Engineering, Federal University of São Carlos, São Carlos, Brazil; b: Department of Civil Engineering, Federal University of the South and Southeast of Pará, Santana do Araguaia, Brazil; c: Department of Materials Science and Engineering, Paulista State University, Itapeva, Brazil; d: Structural Engineering Department, University of São Paulo, São Carlos School of Engineering, São Carlos, Brazil. Corresponding author: lipe.arroyo@gmail.com

INTRODUCTION The commercial exploitation of forest resources is growing globally as a result of the many applications of wood-based products (Santos et al. 2014). From an economic point of view, the activities of the lumber industry are of paramount importance. However, logging must be done in a controlled and sustainable manner, looking for ways that lead to more appropriate use of wood (González-Garcia et al. 2014). For example, wood can be utilized as a feedstock for the production of reconstituted wood panels (Mattos et al. 2008), including oriented strand board (OSB).

The European standard EN 300 (2006) defines OSB as a multilayer panel, generally composed of three to five layers and made from strand of wood with a length greater than 50 mm and less than 2 mm thick, joined together by an adhesive. The wood strands in the external layers tend to align parallel to the length of the panel and in the internal layers may be randomly distributed or aligned generally perpendicular to the external layers (Souza 2012). OSB products are used for structural applications such as walls, ceilings, floors, beams, structural components, etc., due to their mechanical strength and good dimensional stability, competing directly with the plywood market (Mendes 2011).

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The OSB market is growing worldwide, with an expected growth of 28% until 2022 (Grand View Research 2015). This increasing consumption in different sectors (mainly construction, furniture, and packing) is related to improved panel properties such as strength, workability, and versatility (Grand View Research 2015). Moreover, the growing substitution of OSB for plywood is expected to continue due to factors such as reduced availability of good quality logs for lamination, making the OSB an advantageous option, as it can be produced from lower quality logs and low commercial value species (Mendes 2013).

In Brazil the OSB is predominantly manufactured with Pinus sp. wood species (Pinus elliotti and Pinus taeda). However, with the growing demand for this product and the fact that Pinus species are widely used for various other purposes such as manufacture of plywood boards, cellulose industry, and sawmills (Vidal and Hora 2014), the amount of wood stored may not be enough to supply the market. Other species need to be studied for their use in these products. There have been some studies of the viability of OSB production with other species found in Brazil such as Croton sonderianus Muell. Arg, Piptadenia stipulacea, Croton sonderianus Muell. Arg (Nascimento et al. 2015), Schizolobium amazonicum (Ferro 2015), Eucalyptus grandis, and Eucalyptus dunnii (Iwakiri et al. 2004).

The common properties investigated in studies about OSB performance are bending strength, modulus of elasticity in bending in major and minor axis, internal bond, and thickness swelling after 24 h of immersion in water, as defined by the EN 300 (2006) standard. Bertolini (2014), Bertolini et al. (2014), and Varanda (2016) examined the structural porosity of particleboards. Jin et al. (2021) analyzed the porosity of wood species. The presence of pores may influence the mechanical properties, dimensional stability, thermal conductivity, permeability, and acoustical properties of particleboards.

The determination of pore size through mercury intrusion porosimetry (MIP) involves the forced intrusion of mercury into the pores and the measurement of the amount of liquid spent in the procedure (Zhao et al. 2021). According to Varanda (2014), this technique can be used to determine several important parameters in the characterization of porous materials, including total intrusion volume, total pore area, average pore diameter, real and apparent density, and porosity of the sample.

Because OSB is used for structural applications, the porosity must be included in the performance evaluation of these materials. The present study evaluated the porosity and pore distribution in different types of OSB manufactured in Brazil using mercury intrusion porosimetry.

EXPERIMENTAL Materials This study used five OSBs made with different wood species, resin type, and resin content. Four of them were produced in the Wood and Timber Structures Laboratory (LaMEM), Department of Structures Engineering (SET), São Carlos Engineering School (EESC), São Paulo University (USP), using wood species particles such as Schizolobium amazonicum sp., Pinus sp., and Corymbia citriodora, which were bonded with castor oilbased polyurethane. The other OSB analyzed in this study was found in the Brazilian market, manufactured by Louisiana Pacific Corporation (LP Brazil) with Pinus sp. wood and phenol-formaldehyde resin. For OSB manufactured, wooden beams were sectioned into pieces approximately 90 mm wide and 35 mm thick, which defined the length and Ferro et al. (2021). “Strandboard Hg porosimetry,” BioResources 16(4), 6661-6668.

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width of the particles respectively. The particles, generated in a disk chipper, were obtained with a thickness of approximately 0.7 mm. These parameters were selected, once that are the parameters used by OSB industries. Table 1 shows the differences in the panels.

Table 1. OSB Types Subjected to MIP Test OSB type

Wood species

Resin

OSB density (kg/m³)

Resin content (%)

OSB1

Pinus sp.

Phenol-formaldehyde

600

8

OSB 2

Schizolobium amazonicum Schizolobium amazonicum

Castor oil-based polyurethane Castor oil-based polyurethane Castor oil-based polyurethane Castor oil-based polyurethane

650

10

650

12

700

12

750

12

OSB 3 OSB 4

Pinus sp.

OSB 5

Corymbia citriodora

Methods The mercury intrusion porosimetry method is based on the isostatic injection, under very high pressure (several hundred megapascals), of a non-wetting fluid (mercury) into the porous material (Zhao et al. 2021). For this study, it is important to highlight that the mercury intrusion porosimetry technique was developed based on the studies of Bertolini (2004) and Varanda (2016), which analyzed this property for wood-based panels.

The pore size distribution is determined from the volume intruded at each pressure increment, and total porosity is determined from the total volume intruded (Abell et al.

1999). MIP tests were performed with a Micromeritics Poresizer (model 9320, Sao Calos, Brazil) with a maximum 200 MPa injection pressure. Parameters such as total intrusion volume (mL/g), total pore area (m2/g), average pore diameter (μm), bulk density (mL/g), skeletal density (mL/g), and porosity (%) were determined.

The specimens had dimensions of 14 mm width and 23 mm length. In total, it was used two samples for each OSB type. They were dried in an oven with air circulation at 50 °C for 24 h before the test. The MIP parameters for were as follows: mercury with a surface tension of 0.494 g/cm2 and density of 13.533 g/mL, advancing and rewind contact angle of 130°, and equilibrium time between the low and high pressure of 10 seconds.

RESULTS AND DISCUSSION Figure 1 shows the pore size distribution of the specimens obtained by plotting the log differential intrusion volume dV/dP versus the pore diameter. OSB 1 had a greater number of pores with diameters smaller than 0.05 μm, and there was a small pore amount in the range of 0.2 and 0.8 μm. There were few pores above 11 μm. For OSB 2, there were more pores with diameters below 0.04 μm and a considerable number with dimensions between 7 and 12 μm. In OSB 3, there were two pore size distribution ranges, the predominant band at less than 0.04 μm and another between 8 and 16 μm. For OSB 4, the most pores were below 0.02 μm; there was a small number of pores in the range of 0.2 to 1 μm, and others in the range 8 to 12 μm. Finally, for OSB 5, there were more pores in the

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size distribution ranges below 0.01 μm and a small number of pores between 0.2 and 0.5 μm.

Regarding differential intrusion volume, all specimens presented values between approximately 0.20 and 0.90 mL/g. Table 2 shows the results obtained in the porosity tests.

Table 2. Porosity Features of the Panels Porosity Features Total intrusion volume (mL/g) Total pore area (m2/g) Average pore diameter (μm) Bulk density (mL/g) Skeletal density (mL/g) Porosity (%)

OSB 1 0.492 8.66 0.227 0.716 1.107 35.29

OSB 2 0.756 22.91 0.132 0.724 1.599 54.73

OSB 3 0.752 32.30 0.093 0.724 1.593 54.52

OSB 4 0.312 33.71 0.037 1.079 1.630 33.77

OSB 5 0.330 11.15 0.119 0.910 1.306 27.85

According to Bertolini et al. (2019), porosity of the panel is associated with voids between particles and with wood microstructural elements. Table 2 shows that OSB 2 and OSB 3, both filled with castor oil polyurethane resin and produced with Schizolobium amazonicum wood, exhibited the highest intruded mercury volumes and porosity values.

OSB 1 and 4, both manufactured with Pinus sp., showed similar porosity values, even though they were manufactured with different resin types and different resin content.

However, OSB 4 presented a slightly lower value, which may be related to the higher density of the panel, as well as the higher amount of resin. OSB 5 had the lowest porosity value. In the other hand, the increase in the amount of resin from 10 (OSB 2) to 12% (OSB 3), both made with Schizolobium amazonicum, did not significantly influence the porosity performance of the panels.

According to Vidaurre (2010), Schizolobium amazonicum species has a lumen diameter of about of 25.6 μm. On the other hand, Corymbia citriodora and Pinus sp. had lumen diameters of about 5.4 μm and 27.84 μm, respectively (Amaral 2014; Segura 2015).

However, it is important to highlight that the anatomy of wood of Corymbia citriodora and Pinus sp. is totally different, once the first one is hardwood and has fibriform fibers and vessels, whereas the second one is softwood and has tracheids cell. So, the anatomy characteristics of wood significantly contributed to the porosity results in OSB specimens.

The OSB 4 and 5 specimens showed a total intrusion volume of 0.31 and 0.33 mL/g, respectively, but OSB 4 presented a greater percentage of porosity. This could be explained by the pore geometry (Fig. 2); when the pore is open and interconnected (c, d, e and f in Fig. 2), the mercury fills the space even before applying pressure (Santos et al. 2014).

Fig. 2. Representative scheme of pores with different sizes and shapes: (a) and (b) closed pores; (c) open pore; (d) and (e) interconnected pores; (f) surface roughness. (Santos et al. 2014)

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The empty spaces in the microstructure of a system are classified by the International Union of Pure and Applied Chemistry (IUPAC) as micropores, mesopores, and macropores (Santos et al. 2014), as shown in Table 3.

Table 3. Pore Classification Classification Micropores Mesopores Macropores

Pore size diameter < 0.002 μm 0.002 μm<diameter < 0.05 μm diameter > 0.05 μm

According to Table 3, all OSB types present macropores. The presence of macropores in each type of OSB is related to anatomical aspects of the wood. Because of the stochastic manner in which OSB is formed, as well as the differences in sizes of the strands, one can expect there to be a high frequency of empty spaces within the structure.

The amount of resin applied, in combination with the applied pressure, is not likely to be large enough, during practical manufacturing conditions, to fill eliminate such pores between the strand elements. All panels in this study were pressed with 4.0 MPa. Nonetheless, Bertolini et al. (2019) analyzed the porosity for Pinus sp. particleboards pressed with 4.5 and 2.5 MPa. Porosity of panels were influenced by intensity of pressure during their manufacturing, being large for panel of 2.5 MPa. The use of uniform particles in the production of panels enables the occurrence of a larger voids, as these ordinarily would be filled with smaller particles.

Schizolobium amazonicum, used in the production of OSB 1 and 2, is a low-density wood (390 kg/m³). It shows no difference between heartwood and sapwood, and lacks knots. The low density contributes for more workability of the species, and these factors collaborate to generate particles with more uniform dimensions. Pinus sp. and the Corymbia citriodora (with a density of 490 kg/m³ and 700 kg/m³, respectively), show differences between the heartwood, sapwood, and knots, thus making particles with less uniformity. The utilization of smaller particles in the manufacture of the panels reduces the empty spaces; thus, the number of pores in the panel decreases. Allied to this are the pore diameters related to each species of wood, as already commented. It is also noteworthy that in addition to the reduced pore diameter compared to the other species under study, Corymbia citriodora has pores filled with tyloses. Expansion of the cell wall of a parenchyma cell adjacent to a vessel element, through the opening of a puncture, partially or completely blocking the lumen diameter. Tyloses in wood cause effects such as vessel closure causing low wood permeability.

The obtained results were compared to the literature. Varanda (2016), for example, analyzed the porosity of particleboard produced with Pinus sp. and peanut shell, glued with oil-based polyurethane resin beaver. Results showed a porosity of 33.7%, a value very similar to obtained in this study for treatments OSB 1 and OSB 4 (both produced with Pinus sp.). Bertolini et al. (2019) obtained approximate value of 54% for particleboard produced with Pinus sp. waste materials, which were treated with copper chrome boric oxide preservative and glued with castor oil-based polyurethane resin. This result is similar to that obtained for OSB 2 and OSB 3.

This result is very important for wood construction industry, especially in Brazil, where most of the houses are manufactured with OSB. Characteristics such as thermal conductibility and acoustic comfort are very important, and are used improved with the addition of other material as glass wool, pet wool. The porosity of the panel can improve Ferro et al. (2021). “Strandboard Hg porosimetry,” BioResources 16(4), 6661-6668.

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theses performances of the construction system and enable the reduced usage of other materials.

According to Luamkanchanaphan et al. (2012), thermal and acoustic comfort are related to density of the panel: higher density leads to higher thermal conductivity. In insulation materials, heat transfers occur in the materials and voids filled with air.

CONCLUSIONS 1. The pore size distribution for oriented strand board specimens OSB 2 and OSB 3 indicated a large presence of macropores in the microstructure of the panels, contributing to a higher total porosity. This could be related to panel manufactured, as well as, microstructure of wood.

2. The OSB produced with Schizolobium amazonicum and castor oil polyurethane resin showed higher total porosity in the process conditions used in this study.

3. The commercial OSB found in the Brazilian market showed smaller structural porosity.

REFERENCES CITED Abell, A. B., Willis, K. L., and Lange, D. A. (1999). “Mercury intrusion porosimetry and image analysis of cement-based materials,” Journal of Colloid and Interface Science 211(1), 39-44. DOI: 10.1006/jcis.1998.5986 Amaral, D. (2014). “Anatochemistry of Pinus elliottii var. Elliottii,” Ph.D. Dissertation, Federal University of Paraná, Brazil. (in Portuguese) Bertolini, M. S. (2014). Wood Waste and Tire Rubber Panels Associated with Castorbased Polyurethane Foam for Application as a Thermo-Acoustic Composite, Ph.D. Dissertation, University of São Paulo, São Carlos, Brazil. (in Portuguese) Bertolini, M. S. Nascimento, M. F., Christoforo, A. L., and Lahr, F. A. R. (2014).

“Particle boards from Pinus sp. treated with CCA preservatives and biomass-derived resin,” Revista Árvore 38, 339-346. DOI: 10.1590/S0100-67622014000200014 (in Portuguese) Bertolini, M. S., de Morais, C. A. G., Christoforo, A. L., Bertoli, S. R., Santos, W. N., and Lahr, F. A. R. (2019). “Acoustic absorption and thermal insulation of wood panels: Influence of porosity,” BioResources 14(2), 3746-3757. DOI: 10.15376/biores.14.2.3746-3757 EN 300 (2006). “Oriented strand boards (OSB) – Definitions, classification and specifications,” European Committee for Standardization, Brussels, Belgium.

Ferro, F. S. (2015). Oriented Particle Boards (OSB) with Alternative Inputs: Technical Feasibility and Proposal for Improving Environmental Performance, Ph.D. Dissertation, University of São Paulo, São Carlos, Brazil. (in Portuguese) Gonzalez-Garcia, S. Dias, A. C., Feijoo, G., Moreira, M. T., and Arroja, L. (2014).

“Divergences on the environmental impact associated to the production of maritime pine wood in Europe: French and Portuguese case studies,” Science of the Total Environment 472, 324-337. DOI: 10.1016/j.scitotenv.2013.11.034

Ferro et al. (2021). “Strandboard Hg porosimetry,” BioResources 16(4), 6661-6668.

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Grand View Research (2015). “Oriented strand board (OSB) market worth $17 million by 2012,” (https://www.grandviewresearch.com/press-release/global-oriented-strandboard), Accessed on March 20, 2021.

Iwakiri, S., Mendes, L. M., Saldanha, L. K., and Santos, J. C. (2004). “Use of eucalyptus wood for oriented strand board (OSB) manufacturing,” Cerne 10, 4652. (in Portuguese) Jin, Z., Ma, J. F., and Fu, Y. J. (2021). “Research on the distribution of cell wall components and porosity in Populus nigra tension wood fiber based on Raman imaging data,” Science Press 41(3), 801-806. DOI: 10.3964/j.issn.1000-0593 Luamkanchanaphan, T., Chotikaprakhan, S., and Jarusombati, S. (2012). “A study of physical, mechanical and thermal properties for thermal insulation from narrow-leaved cattail fibers,” APCBEE Procedia 1, 46-52. DOI: 10.1016/j.apcbee.2012.03.009 Mattos, R. L. G., Gonçalves, R. M., and Chagas, F. B. (2008). Wood Panels in Brazil: Panorama and Perspective, (http://web.bndes.gov.br/bib/jspui/handle/1408/2526), BNDES Setorial, Rio de Janeiro. (in Portuguese) Mendes, R. F. (2011). Effect of Heat Treatment on the Properties of OSB Panels, Master’s Thesis, University of São Paulo, Piracicaba, Brazil. (in Portuguese) Mendes, N. C. (2013). Characterization Methods and Models for Assessing the Impact of the Life Cycle: Analysis and Subsidies for Application in Brazil, Master’s Thesis, University of São Paulo, São Carlos, Brazil. (in Portuguese) Nascimento, M. F., Bertolini, M. S., Panzera, T. H., Christoforo, A. L., and Lahr, F. A. R. (2015). “OSB panels made with wood species from the Brazilian Northeast's caatinga,” Ambiente Construído 15, 41-48. DOI: 10.1590/S167822586212015000100005. (in Portuguese) Santos, W. L. F., Silva, A. J. P., Cabral, A. A., and Mercury, J. M. R. (2014).

“Particleboard manufactured from Tauari (Couratari oblongilofia) wood waste using castor oil-based polyurethane resin,” Materials Research 17(3), 657-663. DOI: 22910.1590/S1516-14392014005000013 Segura, T. E. S. (2015). Evaluation of the Woods of Corymbia citriodora, Corymbia torelliana and their Hybrids for Bleached Kraft Pulp Production, Ph.D. Dissertation, University of São Paulo, Piracicaba, Brazil. (in Portuguese) Souza, A. M. (2012). Production and Performance Evaluation of Oriented Particle Board (OSB) from Pinus sp. with the Inclusion of Metallic Screens, Master’s Thesis, University of São Paulo, São Carlos, Brazil. (in Portuguese) Varanda, L. D., Christoforo, A. L., Almeida, D. H., Silva, D. A. L., Panzera, T. H., and Lahr, F. A. R. (2014). “Evaluation of modulus of elasticity in static bending of particleboards manufactures with Eucalyptus grandis wood and oat hulls,” Acta Scientiarum 36, 405-411. DOI: 10.4025/actascitechnol.v36i3.21077 Varanda, L. D. (2016). High Density Panels for Flooring Application: Production and Performance Evaluation, Ph.D. Dissertation, University of São Paulo, São Carlos, Brazil. (in Portuguese) Vidal, A. C. F., and Hora, A. B. D. (2014). Market Overview: Wood Panels, (http://web.bndes.gov.br/bib/jspui/handle/1408/3023), BNDES Setorial, Rio de Janeiro. (in Portuguese) Vidaurre, G. B. (2010). Anatomic, Chemical and Physico-mechanical Characterization of Paricá (Schizolobium amazonicum) Wood for Energy and Cellulosic Pulp Production, Ph.D. Dissertation, Federal University of Viçosa, Viçosa, Brazil.

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Zhao, J., Yang, L., and Cai, Y. (2021). “Combining mercury intrusion porosimetry and fractal theory to determine the porous characteristics of wood,” Wood Science and Technology 55(1),1-16. DOI: 10.1007/s00226-020-01243-9 Article submitted: April 26, 2021; Peer review completed: July 11, 2021; Revisions accepted: July 28, 2021; Published: August 9, 2021.

DOI: 10.15376/biores.16.4.6661-6668

Ferro et al. (2021). “Strandboard Hg porosimetry,” BioResources 16(4), 6661-6668.

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PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 3, 2021

Crushing of Single-Walled Corrugated Board during Converting: Experimental and Numerical Study 1

TOMASZ GARBOWSKI , 4 JĘDRZEJCZAK .

TOMASZ

2

GAJEWSKI ,

DAMIAN

MRÓWCZYŃSKI

3

&

RADOSŁAW

Corrugated cardboard is an ecological material, mainly because, in addition to virgin cellulose fibers also the fibers recovered during recycling process are used in its production. However, the use of recycled fibers causes slight deterioration of the mechanical properties of the corrugated board. In addition, converting processes such as printing, die-cutting, lamination, etc. cause microdamage in the corrugated cardboard layers. In this work, the focus is precisely on the crushing of corrugated cardboard. A series of laboratory experiments were conducted, in which the different types of single-walled corrugated cardboards were pressed in a fully controlled manner to check the impact of the crush on the basic material parameters. The amount of crushing (with a precision of 10 micrometers) was controlled by a precise FEMat device, for crushing the corrugated board in the range from 10 to 70% of its original thickness. In this study, the influence of crushing on bending, twisting and shear stiffness as well as a residual thickness and edge crush resistance of corrugated board was investigated. Then, a procedure based on a numerical homogenization, taking into account a partial delamination in the corrugated layers to determine the degraded material stiffness was proposed. Finally, using the empirical-numerical method, a simplified calculation model of corrugated cardboard was derived, which satisfactorily reflects the experimental results.

Contact information: 1. Department of Biosystems Engineering, Poznan University of Life Sciences, Wojska Polskiego 50, 60-627 Poznań, Poland.

2. Institute of Structural Analysis, Poznan University of Technology, Piotrowo 5, 60-965 Poznań, Poland.

3. R&D Department, FEMat Sp. z o.o., Romana Maya 1, 61-371 Poznań, Poland.

4. Werner Kenkel Sp. z o.o., Mórkowska 3, 64-117 Krzycko Wielkie, Poland.

Energies 2021, 14, 3203. https://doi.org/10.3390/en14113203 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 – Crushing of Corrugated Board

energies Article

Crushing of Single-Walled Corrugated Board during Converting: Experimental and Numerical Study Tomasz Garbowski 1 , Tomasz Gajewski 2, * , Damian Mrówczyński 3 and Radosław J˛edrzejczak 4 1

2 3

4

*

Citation: Garbowski, T.; Gajewski, T.; Mrówczyński, D.; J˛edrzejczak, R. Crushing of Single-Walled Corrugated Board during Converting: Experimental and Numerical Study.

Energies 2021, 14, 3203. https://

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 5, 60-965 Poznań, Poland R&D Department, FEMat Sp. z o.o., Romana Maya 1, 61-371 Poznań, Poland; damian.mrowczynski@fematproject.pl Werner Kenkel Sp. z o.o., Mórkowska 3, 64-117 Krzycko Wielkie, Poland; radoslaw.jedrzejczak@wernerkenkel.com.pl Correspondence: tomasz.gajewski@put.poznan.pl

Abstract: Corrugated cardboard is an ecological material, mainly because, in addition to virgin cellulose fibers also the fibers recovered during recycling process are used in its production. However, the use of recycled fibers causes slight deterioration of the mechanical properties of the corrugated board. In addition, converting processes such as printing, die-cutting, lamination, etc. cause microdamage in the corrugated cardboard layers. In this work, the focus is precisely on the crushing of corrugated cardboard. A series of laboratory experiments were conducted, in which the different types of single-walled corrugated cardboards were pressed in a fully controlled manner to check the impact of the crush on the basic material parameters. The amount of crushing (with a precision of 10 micrometers) was controlled by a precise FEMat device, for crushing the corrugated board in the range from 10 to 70% of its original thickness. In this study, the influence of crushing on bending, twisting and shear stiffness as well as a residual thickness and edge crush resistance of corrugated board was investigated. Then, a procedure based on a numerical homogenization, taking into account a partial delamination in the corrugated layers to determine the degraded material stiffness was proposed. Finally, using the empirical-numerical method, a simplified calculation model of corrugated cardboard was derived, which satisfactorily reflects the experimental results.

doi.org/10.3390/en14113203 Academic Editor: Peter Foot

Keywords: corrugated cardboard; converting; numerical homogenization; strain energy equivalence; finite element method; shell structures; transverse shear

Received: 6 May 2021 Accepted: 27 May 2021 Published: 30 May 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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 Paper and cardboard are made of cellulose fibers that mainly come from trees. Some of the fibers circulate repeatedly in the production-recycling loop. The material is, therefore, environmentally friendly, but the quality of the produced material from recycled fibers iteratively declines. This requires a deeper understanding if one wants to optimize the product and at the same time keep the material eco-friendly. It becomes even more important if the final product is a corrugated cardboard, which consists of two to seven alternating flat (liners) and corrugated (fluting) layers of paperboard.

The particular orientation of the fibers resulting from the cardboard production process causes the material to have different mechanical properties along the mutually perpendicular directions. Such materials are called orthotropic materials, as opposed to isotropic ones, which exhibit the same physical properties independent of the direction. The material orientation along the fibers that follow the direction of the web during production is called the machine direction (MD), the direction perpendicular to it is called the cross direction (CD).

As the material is much stiffer and stronger in MD (along fibers), the corrugated board production method compensates for its poorer behavior in CD by using the corrugated

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layers. The main task of which is to increase the load capacity and stiffness in the CD, but most importantly, to keep the flat layers at an appropriate distance from each other, which allows obtaining significantly increased bending stiffness in both directions.

The optimal corrugated board should be characterized by maximum strength and stiffness, and at the same time be light and cheap to produce. In assessing the quality of corrugated board, the compressive or tensile and flexural or torsional stiffnesses/strengths are important; the former due to the load-bearing capacity, while the latter due to the resistance to loss of stability. The maximum stiffness or strength can be obtained by selecting the appropriate materials for the individual layers and/or by selecting the appropriate geometry (wave height and period) for the corrugated layers. There are hundreds of papers on the market with different grammages and mechanical properties, produced in a different proportion of virgin to recycled fibers. This gives an enormous number of combinations of possible layer arrangements and layer geometry of the corrugated cardboard, which does not stop producers from constantly trying to find the best solution. Unfortunately, their efforts may be wasted if the carefully designed quality of the corrugated cardboard does not achieve the assumed strength parameters after production, i.e., converting processes.

Before corrugated board becomes a typical transportation box, or a color, branded shelf-ready box (SRP) or a display-ready box it goes through a number of converting processes, e.g., printing, lamination, die-cutting, etc. All these processes cause crushing of the corrugated layers, which in turn leads to a reduction of strength and stiffness of the material and therefore affects the performance of structures made of corrugated boards.

The deterioration of the mechanical properties can be observed in all crucial laboratory test results, e.g., four-point bending test, torsional test, shear test, edge crush test, etc.

Even though, the degraded stiffnesses and strengths could easily be measured by cutting specimens from the converted cardboard, this is rarely done in practice. Typically, this effect is accounted for by adding safety factors to the equations that estimate the load capacity of the package.

Since the middle of the last century, scientists and engineers have tried to find robust and simple tools for estimating the strength of corrugated board boxes. Among them the analytical tools [1–10] are the simplest, but unfortunately less precise and limited only to the typical box structures. Numerical models of corrugated board packaging [11–16], although much more precise, require specific knowledge and a full set of material parameters to correctly simulate the behavior of the box.

In the finite element-based tools, a procedure called homogenization [17–25] is very often used, which allows for significant time savings in the analysis and at the same time guarantees the correct behavior of simplified models. This is especially important when the computational models are complex (they consist of many layers of cardboard) or the analysis concerns geometries with complex shapes, such as cardboard furniture [26].

In our previous works, we presented analytical-numerical methods [27–29] for assessing the strength of corrugated board boxes, which allow to obtain quick and accurate results for the packaging with different geometries (ventilation/hand holes [28] and perforations [29]). In order to properly estimate the strength of a corrugated box, special attention should be paid to transverse shear properties [30–32]. This is because the decrease in this material parameter significantly affects the load-bearing capacity of the package. In general, any deteriorated stiffness (i.e., flexural, torsional or transversal) and the strength of the corrugated board can be readily implemented in any estimation method. The only requirement is to determine the relationship between the amount of damage (stiffness and strength degradation) and the amount of physical crushing of the cardboard.

The article presents the results of the research on the crushing of various single-walled corrugated boards. In the first step, the relationship between intentionally induced crushing with very high accuracy to different types of corrugated boards and the measured loss of stiffness in various laboratory tests was checked. Since the crushing of the sample takes place in the thickness direction, the greatest decrease in bending and twisting stiffness (due to the reduction of the corrugated board thickness and, therefore, a significant reduction of

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the section moment of inertia) was expected. Less load capacity reduction was expected in the edge crush test. However, due to the high resilience of the corrugated board, in some cases, especially for a small amount of crushing, the sample recovers its original thickness (because of the relaxation), which allow the observation of interesting findings. All issues will be discussed in the Results section.

In the second step, a numerical model based on the homogenization that can easily be used to predict all the stiffness parameters of the corrugated board with different degrees of crushing was proposed. The numerical model, based on the finite element method (FEM), was used to build a global stiffness matrix of the full detailed 3D finite element (FE) model of the corrugated board. The model (based on the numerical homogenization procedure presented in [25]) takes into account the crushing of the corrugated board. This extension of the homogenization procedure allows to determine the degraded corrugated board stiffness matrix, which ultimately enables a robust simulation of the real laboratory tests using the simplified analytical formulas. The results obtained from the simulation of four-point bending and torsion (twist) tests [31,32] are in good agreement with the results obtained from the laboratory tests.

2. Materials and Methods 2.1. Mechanical Tests of Corrugated Cardboard The strength and stiffness of a corrugated board can be determined by performing various types of tests. The most commonly used include: (a) BNT—stiffness in the fourpoint bending test; (b) ECT—edge crush test, (c) SST—shear stiffness testing and (d) TST— torsional stiffness test.

The measurement of the bending stiffness is a laboratory test which is based on the four-point bending method (BNT), see Figure 1. This test is usually carried out on a sample with a dimension of 50 × 250 mm. It is important that in this test there is a constant moment and zero shear force in the sample between the internal supports. However, there is still a shear force between the outer and inner supports—this allows to take into consideration the shear stiffness aspect. It is worth noting that a bending stiffness measurement is very sensitive to sample damage (crushing or creasing), so the results for samples crushed by more than 50% are usually unreliable.

(a)

(b)

(c)

Figure 1. The four-point bending test: (a) cardboard testing device; (b) loaded sample; (c) deformed sample.

The sample compressive strength in the Edge Crush Test (ECT) is obtained for relatively stocky samples (conventionally thicker than 1 mm) with dimensions of 25 × 100 mm, see Figure 2. In the case of slender specimens, the main failure mechanism is the loss of stability, and not the crushing of a sample. The ECT of the corrugated cardboard is one of the most known and important (from the practical point of view) parameter, often used in an analytical [1,10] or an analytical-numerical [27–29] determination of the load capacity of a corrugated board packaging.

The shear stiffness (SST) of a corrugated board is measured on a sample 80 × 80 mm loaded with a pair of forces at opposite corners, see Figure 3. Measuring the displacements and reaction forces at the other two corners allows to determine the cardboard shear stiffness. Only the linear part of the load-displacement curve is used in an identification of the sample shear stiffness. The SST parameter is very sensitive to a sample

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crushing. The results obtained in the SST laboratory tests are valid also for highly crushed or broken samples.

(a)

(b)

(c)

Figure 2. The edge crush test: (a) corrugated board testing device; (b) loaded sample; (c) deformed sample.

(a)

(b)

(c)

Figure 3. The shear stiffness testing: (a) cardboard testing device; (b) loaded sample; (c) deformed sample.

In the torsional stiffness (TST) measurement, a twisting of a 25 × 150 mm sample by a few degrees in both directions is conducted, see Figure 4. The results obtained are valid even for highly crushed or broken samples. Therefore, the TST parameter has a high sensitivity to crushing of the corrugated board sample. Only the linear part of a diagram (i.e., the angle of rotation vs bending moment) is used to determine the sample torsional stiffness. The reliable measurements are assured by: (1) a stable method of holding the sample, (2) a static method of measuring the angle of rotation and torque and (3) a relatively large width of the sample, thanks to which the sample behaves as a homogenized material.

(a)

(b)

(c)

Figure 4. The torsional stiffness test: (a) cardboard testing device; (b) loaded sample; (c) deformed sample.

The crushing device (CRS) is used to assess the impact of converting processes such as laminating, stamping, creasing or printing on the quality and load-bearing capacity of the corrugated board, see Figure 5. In this research, a fully controlled manner of crushing cardboard in the range from 10% to 70% was precisely obtained by using the CRS laboratory device (fematsystems.pl/services/crs [33]), which assured the crushing accuracy of ±10 μm.

In Figures 1–5, the devices for different testing methods of corrugated cardboards are presented. Furthermore, the loading scheme of the sample (Figure 5b), as well as the

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deformed shape of the sample (Figure 5c) with exemplary displacement fields (obtained via finite element method modelling and analytical approximation) are illustrated.

(a)

(b)

(c)

Figure 5. The cardboard crushing: (a) cardboard testing device; (b) loaded sample scheme; (c) deformed sample scheme.

2.2. Estimation Error by Coefficient of Determination To explore the relationship between a crushing and a decrease of the corrugated board stiffness values, the coefficient of determination was computed for each board quality, defined by the formula: 2 ∑n ( xi − ŷi ) , (1) R2 = 1 − i =1 (n − 1)·var( x ) where: xi —the expected ratio of the measured value of the crushed sample to the initial value ( CRS = 0% → xi = 1.0, CRS = 10% → xi = 0.9, etc.), ŷi —the values computed on the basis of the formula (2) describing the linear regression, var( x ) — the variance of the expected ratio of the measured value of the crushed sample to the initial value: ŷi = a( xi − x ) + y,

(2)

where: x—the mean value of the expected ratio of the measured value of the crushed sample to the initial value, y—the mean value of the measured quantities (SST-MD, TST-CD etc.), a—the slope of the linear regression: a=

∑in=1 ( xi − x )(yi − y) ∑in=1 ( xi − x )

2

.

(3)

The higher the value of the coefficient of determination, R2 , the better the fit of the regression line and the estimation error is considered to be smaller.

2.3. Numerical Approach for Modelling Crushing In this paper, apart from laboratory tests on crushed corrugated cardboards, also a simple numerical approach to consider the crushed properties of the corrugated board is proposed. The derived method does not require the modelling of the plasticization of the fluting, because its analytical equivalent (presented later) in FE model can be used. The aim of this part of the study is to validate the approach by using torsion test modelling [31,32].

The numerical study consists of several steps, illustrated by scheme in Figure 6:

• • • • •

Building initial geometry of the intact corrugated cardboard (stage a).

Performing FE analysis of corrugated cardboard crushing with plasticity included (stage b)—substituted with an analytical, simplified crushed flute shape approximation.

Using the crushed geometry to build the classical material stiffness matrix of corrugated cardboard (stage c,d).

Homogenizing the crushed corrugated cardboard by composite properties, based on Garbowski and Gajewski method [25] (stage d,e).

Computing torsion/bending response of crushed corrugated cardboard sample using composite properties (stage f).

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Figure 6. The scheme representing the steps of numerical study conducted in this study: (a) undeformed RVE, (b) loaded and deformed RVE, (c) crushed geometry extracted, (d) material stiffness matrix of RVE, (e) the representative shell stiffness matrix and (f) tests outcomes from analytical estimation.

Initial geometry of the corrugated cardboard used in the numerical study represents an intact (i.e., unconverted or uncrushed) geometry of the cardboard and it was assumed from the literature [19,25]. The fraction of a single wall corrugated cardboard was simulated, namely, the in-plane section of 8 × 8 mm. The fluting period was also 8 mm; the fluting wave “starts” from the liner. The thickness of the liners and fluting are 0.29 mm and 0.30 mm, respectively. The axial spacing between the liners is 3.51 mm.

The material parameters of intact corrugated cardboard were also taken from the literature [19,25]. The classical orthotropic constitutive law was assumed for each layer with a perfect plasticity (no hardening). The orthotropic material data (E1 , E2 , v12 , G12 , G13 and G23 , i.e., Young moduli in both directions, Poisson’s ratio and 3 shear moduli, respectively) and yield stress, σ0 , for liners and fluting are presented in Table 1.

Table 1. Material properties of liners and fluting of intact corrugated cardboard.

Layers liners fluting

E1

E2

ν12

G12

G13

G23

σ0

(MPa)

(MPa)

(-)

(MPa)

(MPa)

(MPa)

(MPa)

3326 2614

1694 1532

0.34 0.32

859 724

429.5 362

429.5 362

2.5 2.5

To acquire the crushed geometry of the corrugated cardboard the static FE analysis was performed. In the numerical study, five cases were considered, see Figure 7, in which the induced crushing of the cardboard were 10%, 20%, 30%, 40% and 50%. For instance, 10% of crushing means here that the corrugated cardboard was enforced by kinematic constraint to decrease its thickness to be 90% of the intact geometry (see Figure 6a,b). In the numerical model (to the upper and lower liner surfaces) the kinematic constraints were applied assuming that 50% of the crushed deformation is elastic and the other 50% comes from the plastic and/or damage deformations. Therefore, in the numerical analysis, to obtain the geometry from plastic deformation only (i.e., after unloading), the actual kinematic constraints were 5%, 10%, 15%, 20% and 25%. The output geometries of the FE analysis using those constraints were later considered to be the ones coming from the crushing of 10%, 20%, 30%, 40% and 50%.

For the FE analysis the Abaqus Unified FEA from Dassault Systems was used, in which 4-node general-purpose shell finite elements were utilized (S4 according to [34]).

Single model had about 3280 shell elements with linear shape functions and about 3160 nodes. The fluting was represented by 64 segments, since this value is important to retrieve

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the correct transvers shear stiffnesses of a representative volume element (RVE) as shown by our recent work [25]. Here, the number of segments was doubled due to modelling contact between the top liner vs. fluting and fluting vs. bottom liner. In tangential direction, the frictionless contact was assumed; and in normal direction the Herz type contact was assumed. Boundary conditions allowed to deform the RVE in out of plane direction, blocking from movement the external (side) nodes.

(a)

(d)

(b)

(c)

(e)

(f)

Figure 7. The crushed geometries of the corrugated cardboard obtained from the static finite element analysis: (a) 0%, (b) 10%, (c) 20%, (d) 30%, (e) 40% and (f) 50%.

Based on FEM computations performed to obtain different crushing levels, the alternative approach may be used to determine the crushing shapes of fluting to reconstruct its crushed geometry (this is valid for different flute amplitudes and periods). The analytical formula is proposed here, which accounts for the vertical coordinates of a half-wave fluting:

1 1 f ( x ) = ts − (4) 2 1 + e−2wx/L in which t is the amplitude of intact fluting, L is the period length of intact fluting, x is the horizontal coordinate, w is the parameter related to inclination and curviness of the fluting vertical wall, while s is the parameter scaling the crushing thickness; w and s should be used to fit the fluting shape to particular level of crushing. The parameters of w and s for cases used in this study are summarized in Table 2. The fluting shapes of half-waves for different crushing levels obtained from the formula proposed are presented in Figure 8a.

It should be noted that the fluting length in the analytical approach was preserved by reproducing the geometry from FE analyses. The example of comparison between the fluting shape computed with the FE model and the analytical formula for 40% of crushing is presented in Figure 8b; a perfect agreement can be observed.

In the next stage of the study, the output geometries (without any residual stresses) were imported to Abaqus software to build the initial material stiffness matrix of the structure, see Figure 6c–d. Before this, for each case, namely, crushing of 10%, 20%, 30%, 40% and 50%, the geometries were inspected in order to determine which regions of the fluting were actually plasticized. For those finite element models (along CD), all elastic properties (apart Poisson’s ratio) were deteriorated by scaling factor. Two regions of fluting were distinguished for each case; thus, two scaling factors were considered, see Figure 9.

The first region is the contacting area of liners and fluting (region A) and the second

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region is in span, which clearly would evolve into the plastic joint (region B) for larger crushing loads. The elements, in which the material was identified to be plasticized, i.e., regions A and B, were obtained from FE computations, see Figure 9. Since 50% of the deformation was assumed to be elastic, new geometries for further computations with 50% less crushing were generated by f function, see Equation (4), but in A and B regions the material properties were deteriorated.

Table 2. Geometrical parameters of w and s for fluting to determine analytically the crushed geometries of corrugated cardboard used in the numerical part of this study.

Crushing (%)

w (-)

s (-)

0 (intact) 10 20 30 40 50

3.5 4.0 4.8 6.2 9.0 15.5

1.06 0.98 0.92 0.86 0.80 0.75

(a)

(b)

Figure 8. Half-waves of fluting due to crushing obtained from the analytical formula (4) and parameters presented in Table 2: (a) shapes of the flute corresponding to different levels of crushing and (b) the comparison for 40% of crushing: FEM (magenta circles) vs analytical formula (solid line).

Figure 9. A and B regions for all geometries of corrugated cardboard considered, i.e., with crushing of (a) 10%, (b) 20%, (c) 30%, (d) 40% and (e) 50%.

The local material deterioration was defined independently for two regions: A and B. Acquired material stiffness matrix of RVE with embedded orthotropic and locally

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deteriorated properties (due to decreasing of elastic properties for two regions by scaling factor) were subjected to Garbowski and Gajewski homogenization method [25], see Figure 6d,e. The method based on the stiffness matrix of the RVE enables computing Ak matrix, which is the overall stiffness matrix for the laminate shell element. The great advantage of the method is that it captures also the effective transvers shear stiffnesses in CD and MD of the input RVE structure, i.e., A44 and A55 , respectively.

To summarize, the overall algorithm presented in this subsection enables to compute the representative transvers shear stiffnesses for corrugated cardboard samples with different intensity of its crushing included. Results section will prove that the torsion test may be used to effectively validate this algorithm to determine the local deterioration of a corrugated board sample with particular intensity of the crushing.

3. Results 3.1. Experimental Study In the experimental part of our study, four corrugated boards of different grammage were selected and tested, namely, two B flutes: 285 g/m2 , (B-285), 410 g/m2 (B-410) and two C flutes: 340 g/m2 (C-340), 440 g/m2 (C-440). Those corrugated boards were subjected to a series of measurements and laboratory tests to check: (a) sample thickness before and after crushing—THK and THK2; (b) sample resistance to edge crushing–ECT; (c) bending stiffness in machine direction (BNT–MD) and in cross-direction (BNT–CD); (d) shear stiffness in machine direction (SST–MD) and in cross-direction (SST–CD); and (e) torsion stiffness in machine direction (TST–MD) and in cross-direction (TST–CD). In most cases, each type of corrugated board was subjected to three to four series of tests at the same crushing level. Each corrugated board was crushed in the range of 10% to 70% of its original thickness with 10% increments. Due to the elastic relaxation of the corrugated cardboard, the measurement of the crushed sample thickness was performed a few minutes after crushing, which guaranteed initial stabilization of the relaxation of the material.

In Figures 10–13, the results of the measured parameters for the four analysed corrugated boards are presented. Based on the data, the regression lines for each test, according to Equation (2) and (3), were determined. The BNT, SST and TST values were presented in a normalized manner, the values demonstrated are the ratio of the value obtained for crushed sample to the value obtained from an intact corrugated cardboard (i.e., for CRS = 0%).

(a)

(b)

(d)

(c)

(e)

Figure 10. The decreases in measured values for B-285 corrugated cardboard in: (a) BNT; (b) SST; (c) TST; (d) ECT and (e) THK2.

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(a)

(b)

(d)

(c)

(e)

Figure 11. The decreases in measured values for B-410 corrugated cardboard in: (a) BNT; (b) SST; (c) TST; (d) ECT and (e) THK2.

(a)

(b)

(d)

(c)

(e)

Figure 12. The decreases in measured values for C-340 corrugated cardboard in: (a) BNT; (b) SST; (c) TST; (d) ECT and (e) THK2.

The relationship between crushing and decrease of parameter values was investigated by computing the coefficient of determination for each quantity, according to Equation (1).

In Table 3, the values obtained are presented.

Determining the crushing level from the stiffness tests is more precise when the average values from the MD and CD are used to compute the linear regression. This fact may be observed in Figure 14, in which the normalized parameter values were averaged from two directions and the crushing lines are shown. In Table 4, the corresponding coefficients of determination for the averaged values from two directions are presented.

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(a)

(b)

(d)

(c)

(e)

Figure 13. The decreases in measured values for C-440 corrugated cardboard in: (a) BNT; (b) SST; (c) TST; (d) ECT and (e) THK2.

Table 3. The coefficient of determination R2 between crushing and normalized values.

Cardboard Index

THK2

ECT

BNT-MD

BNT-CD

SST-MD

SST-CD

TST-MD

TST-CD

B-285 B-410 C-340 C-440

0.154 0.001 0.111 0.000

0.481 0.596 0.016 0.452

0.928 0.986 0.517 0.967

0.705 0.687 0.861 0.841

0.995 1.000 0.719 0.855

0.964 0.992 1.000 0.999

0.914 0.535 0.542 0.290

0.932 0.988 0.993 0.968

(a)

(b)

(c)

(d)

Figure 14. The decreases in measured values (average from two directions) for corrugated board with an index: (a) B-285; (b) B-410; (c) C-340 and (d) C-440.

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Table 4. The coefficient of determination R2 between crushing and average normalized values.

Cardboard Index

BNT

SST

TST

B-285 B-410 C-340 C-440

0.823 0.885 0.815 0.972

0.983 0.998 0.970 0.957

0.996 0.846 0.876 0.742

We found that there is a relationship between the decreased ECT and THK2 value. A reference line, which capture the relationship between the decrease of these normalized parameters and crushing of the corrugated cardboard was established. The equation describing the reference line has a following form: y = 1 − 0.53x

(5)

where: y—the normalized parameter value and x—the crushing value.

In Figure 15, the normalized ECT and THK2 values for the analysed corrugated cardboards fitting to the reference lines represented by the Equation (5) are presented.

(a)

(b)

(c)

(d)

Figure 15. The decreases in normalized ECT and THK2 values for corrugated board with an index: (a) B-285; (b) B-410; (c) C-340 and (d) C-440.

For the reference line and normalized ECT and THK2 values, the coefficients of determination R2 were computed (Table 5), according to Equation (1) for the two sets of data simultaneously, where: xi —reference line value calculated from the Equation (5), ŷi —normalized ECT and THK2 values and var( x )—variance of the reference line value.

Table 5. The coefficient of determination R2 between reference line and normalized ECT and THK2 values.

Cardboard Index

R2

B-285 B-410 C-340 C-440

0.985 0.982 0.965 0.995

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3.2. Numerical Appraoch for Modelling Crushing The shell stiffnesses (Ak matrix) were computed for five crushing levels of corrugated cardboard considered, i.e., with different amount of crushing: 10%, 20%, 30%, 40% and 50%. The local deterioration factor for region A was assumed 0.5 and for region B was assumed 0.9 (due to severe delamination in these regions as shown by [35,36]). Selected values of Ak matrices for different level of crushing were presented in Table 6.

Table 6. The stiffnesses of the representative shell element computed for different crushing of corrugated cardboard sample for local deterioration scaling factors, separately for region A and region B. Stiffness

10% Crushing

20% Crushing

30% Crushing

40% Crushing

50% Crushing

A11 , (kPa · m) A22 , (kPa · m) A12 , (kPa · m) A33 , ( kPa · m ) D11 , Pa · m3 D22 , Pa · m3 D12 , Pa · m3 D33 , Pa · m3 A44 ,(Pa · m) A55 , (Pa · m)

2101.4 1591.5 371.3 603.0 5.79 3.6 1.01 1.50 45.18 82.13

2092.8 1568.1 367.8 586.3 5.19 3.23 0.90 1.34 31.12 72.55

2088 1548.5 365.5 573.4 4.64 2.89 0.81 1.19 22.01 65.58

2085.3 1531.6 363.8 562.3 4.11 2.57 0.71 1.06 15.41 60.11

2078.7 1477.4 361.5 543.9 3.61 2.21 0.63 0.92 10.14 51.45

Having computed Ak stiffnesses which represents the overall material properties of RVE, the values were used to determine the behavior of the corrugated cardboard samples from different tests. Torsion and bending stiffness tests in both directions were considered here, see its analytical formulas derived in [31,32]. The dimension of the torsion sample was 80 × 80 mm, in bending the sample had 100 mm length between the internal supports.

The decreases of stiffness of the corrugated sample in the cases of torsion, bending in CD and bending in MD for assumed scaling factors (see non deteriorated values in Table 1) are presented in Table 7. It may be observed that the values are close to the induced crushing values (first column), especially for torsion test (second column), what proves that the method was validated. This method may be used in other applications for modelling crushed corrugated cardboard samples for deriving its material and mechanical properties.

Table 7. The decreases in stiffness of torsion, bending in CD and bending in MD for scaling factors separately identified for region A and region B – obtained for different input geometry due to induced crushing (see Figure 9).

Induced Crushing (%)

Torsion Stiffness Decrease (%)

CD Bending Stiffness Decrease (%)

MD Bending Stiffness Decrease (%)

10 20 30 40 50

12 21 30 38 46

10 19 28 36 44

12 21 29 37 46

4. Discussion In the first part of our work, we investigated the relationship between the intentional level of flat corrugated cardboard crushing and the drops in the measured parameters of various laboratory tests. The results in Tables 4 and 5 show the coefficients of determination for linear trends (y = x) across all tests. Values closer to 1 indicate the better correlation between the experiment result and the regression lines. It has been observed that the decrease in both bending and torsional stiffness is more or less correlated with the amount of crush.

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The results obtained show that the SST parameters best catch the amount of intentional crushing, the TST parameters are slightly worse and the BNT parameters are the least accurate. Mean coefficients of determination for all four corrugated board types were: SST— 0.977; TST—0.874, BNT—0.865. The correlation of crush amount with the results of the SST tests at the level of 97.7% means that if there is a need to check a posteriori the amount of the unknown damage caused by the converting machines, it is sufficient to measure the SST parameter before and after the converting process and its ratio will indicate the crushing level. This is because the amount of decrease of the SST parameter precisely reflects the degree of corrugated board crushing. The decrease in the parameter measured in SST does not depend on the residual thickness of the crushed corrugated cardboard, which often returns to its original value due to relaxation. Therefore, organoleptic inspection (e.g., measuring thickness) often does not reveal the problem hidden inside the delaminated cardboard fibers. Therefore, the new insight from this part of study is the following: one may use shearing test (SST) for determining the crushing level of the single wall cardboard, other tests such as bending or torsion are less indicative. This conclusion opens new possibilities to design similar test/machines in other types of materials in which crushing is observed and unwanted, for instance in civil engineering or automotive industry.

On the other hand, the drop in ECT parameter and the residual thickness, THK2 correlate well with each other. The measured thickness after crushing decreases almost by the same value as the measured ECT value (again after crushing). The amount of deterioration of measured ECT and the permanent thickness reduction is approximately 50% of the actual crush amount of the corrugated board. Thus, thickness/ECT reduction may be used to roughly estimate the amount of crushing in a single wall corrugated boards but is not recommended for cases in which high accuracy is expected.

In the numerical part of the study, the results show that the numerical procedure presented can reproduce the deterioration of the samples stiffnesses due to crushing by decreasing the elastic parameters of the RVE, see Table 6. The regions for which the deterioration must be included were identified as the contacting area of liners and fluting (region A) and in the span area (region B), see Figure 9, also the deterioration factors were acquired, i.e., 0.5 for region A and 0.9 for region B. This property seems to be significantly important if one would like to compute the deteriorated properties of the cardboard, basing only on its crushed shape and knowing intact properties of the corrugated cardboard (Ak matrix). As shown, by introducing the analytical formulation, Equation (4), with adequate parameters, see Table 2, to determine the crushed shape one does not have to perform costly FE analysis [37,38], nor have to use advanced (plastic) material constitutive models, see Figure 6. The analytical formulation may be therefore adopted in optimization or inverse problem frameworks, in which computational cost should be limited.

It should be underlined that the numerical study presented enables obtaining the transversal shear stiffness properties of the sample by using the torsion test for different levels of crushing, see Table 6. This is extremely important since the crushing of the cardboard is often suspected to cause the biggest amount of unintended decrease of the load capacity of corrugated cardboard boxes. However, up to this point, there are no systematic and implemented in the industry methods that would be able to determine how the crushing influences the deterioration of the transversal shear stiffnesses. Thus, if this phenomenon may be modelled easily, by the procedure proposed (without using FE computations, nor advanced material models, but adopting an analytical/algebraic rapid and versatile approach), we are one step closer to deliver the scientific-based methodology for dealing with the crushing issue for the corrugated board packaging industry.

5. Conclusions The article presents extended laboratory tests of single-walled corrugated cardboard, consisting in checking the impact of crushing on its mechanical properties. The intentional crushing of the cross-section from 10 to 70% of the original height was fully controlled and initiated with high precision. During the tests, a number of parameters of the corrugated

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board were measured, i.e., (a) residual thickness after elastic relaxation, (b) decrease in bending stiffness, (c) decrease in torsional stiffness, (d) decrease in shear stiffness and (e) decrease in edge crush strength. The reduction in stiffness was checked in two mutually perpendicular directions (i.e., in machine direction and cross direction), while the strength reduction was checked in cross direction only. A correlation was found between intentional and controlled crushing and the corresponding reduced stiffness. It was observed that the best match between the amount of crush and the reduction in stiffness for all specimens tested occurred in the shear stiffness test (SST). The paper also presents numerical and analytical tools for quick and reliable calculations of degraded stiffness of crushed corrugated cardboard samples. For selected crushing levels of the corrugated cardboards the stiffnesses of the representative volume elements were computed. In further research, the impact of the crushing of corrugated board for double-walled structures on the packaging performance will be studied.

Author Contributions: Conceptualization, T.G. (Tomasz Garbowski) and T.G. (Tomasz Gajewski); methodology, T.G. (Tomasz Garbowski); software, T.G. (Tomasz Gajewski) and D.M.; validation, T.G. (Tomasz Garbowski), T.G. (Tomasz Gajewski) and D.M.; formal analysis, T.G. (Tomasz Garbowski), T.G. (Tomasz Gajewski) and D.M.; investigation, T.G. (Tomasz Garbowski), T.G. (Tomasz Gajewski), D.M. and R.J.; writing—original draft preparation, T.G. (Tomasz Garbowski), T.G. (Tomasz Gajewski) and D.M.; writing—review and editing, T.G. (Tomasz Garbowski), T.G. (Tomasz Gajewski) and R.J.; visualization, T.G. (Tomasz Gajewski) and D.M.; supervision, T.G. (Tomasz Garbowski); project administration, T.G. (Tomasz Garbowski); funding acquisition, R.J. All authors have read and agreed to the published version of the manuscript.

Funding: The APC was funded by the National Center for Research and Development, Poland, grant at Werner Kenkel Sp z o. o., grant number POIR.01.01.01–00-1306/15–00.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: The data presented in this study are available on request from the corresponding author.

Acknowledgments: The authors thank AQUILA VPK Wrzesnia for providing samples of corrugated cardboard for the study. The authors also thank to Femat Sp. z o. o. for providing the laboratory equipment and commercial software.

Conflicts of Interest: The authors declare no conflict of interest. The funders 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 TH PUBLISHERS PUB SH RS OF PAPER PAP R TECHNOLOGY T INTERNATIONAL

Volume 7, Number 3, 2021

The Leadership Skills That Managers in the Middle Need to Advance Overcoming the Challenges of Leading from the Middle Working and leading from the middle is tough.

Your boss has priorities. Your direct reports have questions. Peers and colleagues ask you for help and toss extra projects your way. The result: You regularly get pulled in different directions.

Working hard and helping others has gotten you this far in your career, but now, new skills are required in order to keep advancing.

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According to Sinclair, middle managers often take these competing demands personally. But they shouldn’t.

“The truth is, that’s often the system — you just happen to be in the middle of it,” she says.

Those leading in the middle may include general managers, plant managers, regional managers, divisional managers, directors, and sometimes even vice presidents.

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The 6 Leadership Skills Middle Managers Need Based on decades of our research and work with over 100,000 middle managers around the world — we know that the 6 leadership skills middle managers most need are: x x x x x x

Thinking and acting systemically Resiliency Communication Influence Learning agility Self-awareness

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2. Resiliency Leadership resiliency is about handling stress, uncertainty, and setbacks well — learning to maintain equilibrium under pressure. In our leadership programs, we spend a lot of time helping participants find tools for building resiliency for themselves and for others in their organization.

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3. Communication Communication is a core leadership function, requiring the ability to think with clarity and to express ideas and information to a multitude of audiences. Effective communication is also about listening, asking questions, and aligning words and actions.

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Developing Middle Managers Through Leadership Training Managers who spend significant time leading from the middle must give up the need to constantly please. As you’re pulled from all directions, it’s important to stay focused on thinking and acting systemically by seeing the big picture and understanding how the various parts of the organization function together.

As middle managers learn how to get things done with the help of others, they become more effective leaders. “The higher up you go, the more you have to learn to work through other people and influence the system,” Sinclair says. https://www.ccl.org/articles/leading-effectively-articles

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How to Use Coaching and Mentoring Programs to Develop New Leaders If you’re like most HR professionals, you’re familiar with this common workplace scenario: A first-time manager feels overwhelmed and frustrated. The skills and talents that led to his or her success as an individual contributor now feel insufficient. They’re not sure how to make the leap from “friend” to “boss,” and seem to be drowning in work they don’t know how to delegate.

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Mentors can also leverage their positions to sponsor mentees for developmental experiences, advocate on their behalf for promotions, and survey the environment for threatening forces and opportunities. They can leverage their expertise to transfer knowledge and help expand networks for their “mentee.”

It’s important for leaders to develop both coaching and mentoring skills in order to increase employee engagement and develop a robust talent pipeline.

Recommendations for HR Leaders Implementing Coaching and Mentoring Programs Learning to lead is an intensely personal experience, we note in our recent emerging leaders research insights report. As a result, it’s important for emerging leaders to have access to coaches and mentors who can provide context for their personal development journey.

Coaching and mentoring programs can be a formal part of an organization-wide initiative, or they can be an informal process agreed to by both parties.

How to Create a Culture of Coaching When an organization has a “culture of coaching,” it has a culture that encourages giving feedback and honest conversations across functions and leader levels that amplify collaboration, agreement, and alignment.

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Article 11 – Leadership

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FROM THE PUBLISHERS OF PA APER TECHNOLOGY

Volume 7, Number 3, 2021

As we explain in our white paper, Truth and Courage: Develop Conversational Skills to Implement a Coaching Culture, any conversation can be a leadership development opportunity when it’s candid. Senior leaders and managers can apply the following foundational skills: x x x

x

Listen to understand. When supervisors listen to colleagues, they should be aware of their own agenda. Instead of trying to promote that agenda, listening to understand involves listening with an open mind for facts, feelings, and values.

Ask powerful questions. As 2 people delve into a conversation, they can uncover new insights by making inquiries that stretch the other person’s thinking. Encourage “coaches” to begin their questions with “what” or “how” to tap into feelings and values that encourage reflection.

Strike a balance between challenge and support. Listening to understand doesn’t mean listening to agree. Supervisors can show their support by restating the facts and values they hear. When 2 people have a shared trust built on psychological safety, they are able to ask tough, challenging questions that uncover unexamined assumptions.

End your conversation with clear next steps. Supervisors can establish a sense of accountability by agreeing to next steps. That can be as easy as committing to one small action item that moves the issue forward and demonstrates the supervisor values the facts and emotions shared by the individual being coached.

Our research shows that when people are in the early stages of their careers, they often feel it’s risky to speak up. When supervisors and informal coaches throughout the organization use these conversational skills, they demonstrate that they value the thoughts and perspectives of even the youngest members of their teams.

Our emerging leaders report also encourages senior leaders to facilitate meaningful discussions about sensitive issues like diversity, equity, and inclusion in order to glean an accurate picture of their team and the challenges and opportunities they face.

What Makes Mentoring Successful Whereas coaching is intended to address a current challenge, mentoring looks to the future. Therefore, the most successful mentoring programs include careful, strategic planning.

According to our guidebook, Seven Keys to Successful Mentoring, mentoring is an intentional, developmental relationship between a more experienced, knowledgeable person and a less experienced, less knowledgeable person. Often, but not always, this means an older person mentoring a younger one, although reverse mentoring arrangements flip this model around, but work in much the same way.

When creating or improving an organizational mentoring initiative, use these strategies and questions as a guide: x

x

x

x

x

Be purposeful and strategic. Before you begin pairing mentors and mentees, consider your goals and how these goals fit into your overall development efforts. Think about how your demographics might change in the next 5 years: Who will retire, and who will backfill those roles? How will this mentoring program fit into your overall business plan and human resources strategies?

Engage leaders. The most effective mentorship programs have buy-in at the executive level. Once you’ve outlined your goals, clearly articulate and communicate those goals. What role can the CEO and senior team play in the process? Who else in the organization will help make the formal mentoring program work?

Start small. It takes time to recruit and brief the right mentors and mentees, and lessons learned from the beginning of the program can prove beneficial when it’s time to extend it to more people. Be sure your program includes a diverse group of leaders (all genders, people of color, different levels/career stages, etc.) and establishes clear rules about confidentiality to establish trust.

Train mentors and mentees on skills for developing the relationship and holding mentor conversations. You can’t assume senior people will have the right skills for mentoring. Investing time and resources in training also shows that the company leadership values the program. Along the way, offer support for mentors; this support should be included in the program’s design.

Measure and share. What is most important for the organization and those participating? Consider the specific needs of the mentoring partners, HR, and business leaders. How can you publicize any early wins in order to build momentum?

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Article 11 – Leadership

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FROM THE PUBLISHERS OF PA APER TECHNOLOGY

Volume 7, Number 3, 2021

Coaching and Mentoring Are Key for First-Time Managers When individual contributors or professionals are promoted into their first formal leadership positions, many don’t expect the transition to be as difficult as it is. Worse, they often lack the support and development needed to help make that transition successfully.

Without support, new managers can suffer — along with their teams and direct reports. By extension, this affects the organization’s retention levels and leadership pipeline, which ultimately can negatively impact the bottom line.

Given the important role that first-time managers play in talent development and succession management, organizations should help ease their transition by providing them with access to leadership development — especially courses targeted to the needs of new mangers — and by exploring formal organizational mentoring programs to support them.

Leading Effectively Staff, November 8, 2021 https://www.ccl.org/articles/leading-effectively-articles/

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Article 11 – Leadership

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 3, 2021

Safely driving home for Christmas We’re on the downward slope to Christmas, hopefully your shopping is done, presents are wrapped and under the tree and you’ve sent out all your Christmas cards.

Now all that’s left is to finish the last few days at work and you’re free for the holidays.

There’s no denying it, Christmas is a busy time of the year, there are more vehicles on the road, people will be in a hurry and the road conditions are likely not to be the best, with gritters out in force and the likelihood that the weather is not going to be warm and sunny (unless you’re in the Southern Hemisphere) is strong.

Taking all of these factors into account, it’s incredibly important that you take care when you are visiting friends and family.

In this article we’re going to look at the best ways for you to keep safe on the roads and ensure you arrive at your Christmas destination (wherever that may be) safely.

How to drive safely during the winter As with anything, whether it’s making a cake or going on a journey, preparation is key. Having a checklist will make this even easier.

Check the weather In the UK, we’re known for our weather. It’s become a global joke that a key topic of English smalltalk is our unpredictable and changeable weather conditions. One day it can be sunny and warm and the next a torrential downpour. That goes double for the winter with the weather becoming less conducive to safe driving conditions, with wind, rain, hail, sleet and snow all possible in a single day.

It’s for that reason it’s vital you check the weather forecast regularly when planning to travel, especially if you’re going on a longer journey.

If you’re thinking about visiting family over the Christmas period, then you should always check the weather not only of your home location, but also your intended destination. It may well be sunny and warm where you are in the South of England, but get a couple of hundred miles North of the Watford Gap and you could be faced with two feet of snow and a storm!

Plan your journey The roads are busy all year round, but come Christmas, it can seem as though the roads are even busier, because they are. This increased number of vehicles on the road can cause congestion and sometimes lead to accidents, which can cause further congestion and delays.

It’s vital that you plan your journey in advance, to ensure that you take the best route possible and, maybe even have a secondary route as a back-up.

We recommend that you do a search using local and national council websites and the Highways England website to find out about planned roadworks or road closures that may affect you on your travels. Carrying out your planning a few days in advance will give you time to find an alternative route, should it be necessary.

Things can change in a matter of moments, so it’s a very good idea to check the traffic situation a few hours before you leave, ensuring that nothing that could impact on your journey has occurred. Also, make sure that you have set your radio to interrupt your in-car entertainment when traffic updates are on.

Many sat-nav systems get live traffic updates and will reroute you accordingly. It may seem, sometimes, that these suggested routes are going to take you longer, or via a route that looks as though it’s going to take you down unfamiliar roads. However, the chances are the sat-nav is leading you via a quicker, alternative route, so we recommend you follow the instructions it gives you.

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Article 12 – Safe Driving Tips

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FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

If you’re driving a long distance then you need to take regular breaks. Some vehicles will prompt you to take breaks when travelling a long way. However, you should always take these breaks into account when planning the length of your journey and ensure that you not only add the time into your plan, but also tie it in with appropriate locations for a safe stop.

Make sure your car is prepared Just like we have to prepare ourselves for the winter (swap the summer and winter wardrobes, check the heating, find the hot water bottles), it’s also vital you ensure your car is prepared for colder weather.

Below are a few of the checks you should carry out to ensure that your vehicle is in good shape and ready to be driven over the colder winter months: x x x x x x

Fill up your tank Replace the windshield wipers Ensure all the fluids are topped up Check the tyre pressure Change to winter tyres if possible Check the battery cables (for corrosion)

If you don’t feel confident in carrying out these checks yourself, there are several places that offer a free winter check, including Halfords, Kwik Fit and ProTyre. The tests offered vary between five and ten key checkpoints, but they will ensure that your vehicle is prepared for winter road conditions.

The checks offered will check: car lights, wiper blades, car battery, tyre tread and oil levels.

There are some garages and service centres that will offer a winter service that is similar to the full service you should be getting every year, focused specifically on the areas that are important to the cold weather health of your vehicle. These services will include oil changes, a top-up of the antifreeze screen wash, tyre depth check, and starter motor check. The checks vary depending on the garage you are visiting.

How to prepare for a travel emergency Accidents happen!

We know it’s not something you like to think about, but, just like at any other time of year, emergencies can occur, even at Christmas.

The last thing you want to be, if you breakdown or get stuck in traffic, is unprepared. It’s sensible to ensure that you have a preparedness kit with you when you go on car journeys whether they’re hundreds of miles or just somewhere you’re unfamiliar with. No, we’re not talking full-on apocalypse prepper, but enough of an emergency kit in that if you are in a situation where you are unable to get your car started, or there is a very long tailback, you aren’t going to be battling with hungry and agitated passengers or shivering in the car when the temperature drops.

So, in case of emergency, we recommend you keep the following in your vehicle – whether in a bag in your boot, or under the car seats: x x x x x

x

Blankets o Keep a few of these in the car, it can get cold when you’re not moving for a while Winter boots o Not the best thing to drive in, but necessary to keep your feet warm if you need to change a tyre or walk somewhere if you breakdown Gloves o Extra layers are important when it’s cold A fully-charged mobile phone o It goes without saying that most people have their phones with them all the time, but make sure to remember it when you’re driving long-distance Phone charger pack (plus cables) o Batteries do die, so as a backup, make sure you have a fully-charged battery pack for your phone (and other electronic equipment) with you. These are small, easy to find and relatively low in price, but invaluable when your phone battery starts getting low Non-perishable food

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Volume 7, Number 3, 2021

o o

x

x x x x x

x

Hunger = anger and frustration when stuck in one place with no escape The food could be in the car for months without being eaten (unlikely), but there is nothing worse than being stuck in a car for hours, unable to find somewhere to stop for food and feeling hungry.

Foods such as cereal bars, packets of crisps, dried fruit or pretzels are ideal

o Water o It goes without saying really that you need to keep hydrated. Don’t drink so much that you need a toilet break when you’re miles from the nearest rest stop, but keep some bottles of water in the car A torch o We recommend a wind-up one unless you have spare batteries in the glove box Engine oil Washer fluid Coolant Spare tyre kit o Repair kit/spare tyre o Car jack o Locking wheel nut key A pack of playing cards or games o We’ve said it before (Road Trip Tips) and we’ll say it again, if you’re travelling with kids and there is a chance that you’re going to be stuck in traffic for hours (as is the possibility when travelling over Christmas) then you’ll need something to distract/amuse them.

Winter vehicle tips and tricks Keeping your car roadworthy and also safe from the elements in the winter is important, especially if you don’t tend to drive it much over the colder months. So, we’ve put together a few tips and tricks that will help you to maintain your vehicle and keep it in good condition in the winter.

Where should I store my car in the winter?

The winter weather can be harsh and if you are wondering where to store your car to keep it in the best condition, we would recommend storing it in a safe and dry building (such as your garage).

Before you store it, you should ensure that the oil and fluid levels are topped up, tyres are inflated to the recommended level and the battery is corrosion free. If you aren’t planning on driving your car over the winter then you may want to remove the battery – though you will need to store it in the house on a piece of wood and connected to a maintainer/tender.

Condensation is a problem during the winter, so in order to prevent any moisture build-up inside your vehicle, roll the windows down around an inch.

Fill any holes, such as the exhaust with steel wool balls or the fabric sheets you use in the washing machine to prevent mice or rats camping out in your car Of course, not everyone has access to a garage, especially with so many living in blocks of flats or new developments where parking is at a premium. The next best option is a cover that you can easily put on the car and take off when you’re heading off to work.

A cover will protect your car from snow and ice, however, condensation can get trapped underneath it and, if it’s cold enough this will make the cover brittle. If this happens then the cover could crack when you try and remove it, it could also stick to the car.

Some companies, like Autocovers, have now started to produce covers specifically designed for cold weather, protecting against frost and/or snow. A cover that is designed to fit your car properly can help to protect it better from the environment.

Should I wash my car during the winter?

The salt used to grit roads in the UK is actually corrosive to car exteriors so if you are driving anywhere over the holidays then be prepared to stand outside at some point and give your car a wash to protect it from the damage that could occur. You will need to make sure that you can dry it off well if it’s not stored in a garage though, so it doesn’t freeze.

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Article 12 – Safe Driving Tips

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Volume 7, Number 3, 2021

Should I idle my car?

Time was that you used to have to idle your car for 10-15 minutes first thing in order to warm up the engine.

But that was due to the fact that old cars with carburated engines needed to be warmed. If you do this with a newer car you could be doing it more harm than good.

Not only could you be opening yourself up to coming out to find that your car has been driven off by an opportunistic thief, but you could also be damaging your car.

Modern vehicles only need a few minutes (at most) to warm up before you start driving. If you leave it idling for any longer than you’ll be stripping vital oil from your engine pistons and cylinders.

Once you start driving, we would recommend that you go slowly for the first 5-10 minutes until everything has been properly warmed up.

Should I fill my car with fuel more frequently?

If you want to avoid problems such as ice in the fuel lines then you should fill your car up more frequently in the winter to keep your tank in good shape.

If your tank is almost empty in the winter this can lead to the moist air to freeze and crystallise, causing ice.

If you fill your tank more frequently then you’re reducing the risk of ice in your fuel lines. You can purchase antifreeze (such as Heet) for your fuel line and getting an annual winter service can be the perfect check to ensure that your antifreeze levels are right.

Conclusion: Be prepared There are many things that you can do to keep safe when driving home for Christmas, or just travelling over the winter months. Planning, checking the weather, packing an emergency kit, getting a winter service.

If you follow these tips then, whatever happens, you’ll be warm and have something to occupy you and your car passengers when you’re stuck for any length of time in traffic.

Citations x Motorgeek x The AA x Drifted x The RAC https://www.osv.ltd.uk/driving-home-for-christmas/

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 3, 2021

Products & Services & News PITA CORPORATE SUPPLIER MEMBERS Page 2 ABB Brining AI to Asset Performance Management Page 3 Valmet Winder Diagnostics

PITA NON-CORPORATE SUPPLIER MEMBERS Page 4 Voith Design study for Future Paper Machines

OTHER SUPPLIERS Page 6 Ametek Page 7 SKF

AMECare Performance Services Mounted Tapered Roller Bearings

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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Products & Services

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FROM THE PUBLISHERS OF PAP PER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

ABB BRINGS AI TO ASSET PERFORMANCE MANAGEMENT FOR IMPROVED EFFICIENCIES AND INCREASED UPTIME ACROSS INDUSTRIES ABB has strengthened its existing digital offerings with the launch of ABB Ability Genix Asset Performance Management (APM) Suite for condition monitoring, predictive maintenance and 360-degree asset performance insights for the process, utility and transportation industries.

The Genix APM Suite makes it easy to add asset condition monitoring to existing operational technology (OT) landscapes, enables prioritization of maintenance activities based on AIinformed predictions, and provides a comprehensive overview of asset performance.

Genix APM Suite also empowers significant improvements in operational sustainability. By assessing the remaining useful life of industrial assets, Genix APM generates a plan for preventive maintenance, which can extend equipment uptime by as much as 50 percent and increase asset life by up to 40 percent[1].

With reliable data insights, decision makers are provided with the information required in order to identify gaps and areas of improvement for energy efficiency and tighter control of operations, increasing asset availability and improving profit potential.

“Poor asset availability and reliability is a major problem that results in unplanned downtime and unexpected maintenance costs, and also impedes strategic planning and procurement,” said Rajesh Ramachandran, Chief Digital Officer at ABB Process Automation. “It’s not that industrial customers lack data; it’s that many lack effective ways to use their data to improve operational and business performance.”

Genix APM is built on the ABB Ability™ Genix Industrial Analytics and AI Suite. ABB Ability Genix is a modular, IIoT and analytics suite, which integrates IT, OT and other enterprise data in a contextualized manner, applying advanced industrial AI capabilities that support new insights to optimize operations.

[1] https://www.mckinsey.com/business-functions/operations/our-insights/manufacturinganalytics-unleashes-productivity-and-profitability ABB (ABBN: SIX Swiss Ex) 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

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Products & Services

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FROM THE PUBLISHERS OF PAP PER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

VALMET INTRODUCES PERFORMANCE

WINDER

DIAGNOSTICS

FOR

OPTIMAL

WINDER

As part of the Industrial Internet offering, Valmet has developed winder diagnostics to help board and paper customers optimize the overall performance of their winder. Winder diagnostics is an essential part of a winder reliability agreement.

The service ensures better maintenance predictability which in turn means fewer unplanned stops and improved performance of the winder. It provides information from the winder enabling quick reactions to sudden abnormalities and prompt scheduling of maintenance actions.

Collaboration drives development forward Valmet Winder Diagnostics has been developed in close collaboration with selected customer mills to ensure its usability and usefulness.

“The collaboration with mills was started at a very early stage of the development process. It has been very important to exchange ideas and development needs and receive feedback from genuine user experiences all along the way. We also used the Valmet Customer Portal as the common communication platform,” says Markku Savioja, Global Product Manager, Board and Paper Service Technology at Valmet.

Technical information about Valmet Winder Diagnostics Valmet Winder Diagnostics is available for all types of winders, and any board or paper mill can be connected to the service. The application always comes with a Valmet Performance Center (VPC) remote support agreement as a minimum. Through the agreement, customers can consult Valmet’s experts and receive remote support to ensure winder runnability. The experts will take action based on the results of the data analysis as well as giving recommendations on value-adding services.

Winder Diagnostics runs in Valmet’s cloud environment and analyzes and visualizes the data received from the winder. The application is accessed through the Valmet Customer Portal, and both the customer and Valmet have the same view.

Valmet is continuously looking for new ways to utilize data efficiently to take the customers’ operations forward. Winder Diagnostics is part of Valmet Industrial Internet (VII) solutions for Machine Diagnostics.

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FROM THE PUBLISHERS OF PAP PER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

PAPERMAKING VISION: VOITH CREATES VISIONARY DESIGN STUDY FOR THE PAPER PRODUCTION LINE OF THE FUTURE With a visionary design study, technology group Voith is creating the paper production line of the future. The design particularly focuses on improved efficiency and ease of maintenance.

As a full-line supplier, the company has considered all aspects of the paper production process for an integrated concept. Maintenance and operation of the facility are simplified, a higher degree of interconnectedness reduces interfaces, and an appealing design ensures a consistent overall look, while at the same time improving efficiency, safety and sustainability.

“The goal of our long-term design study is to significantly improve efficiency and ease of maintenance across the entire production line,” says Dr. Michael Trefz, President Division Projects at Voith Paper. “We are considering a wide variety of aspects, such as the degree of automation of the machine, the interconnectivity of various sections, better accessibility and greater safety as well as optimized availability. However, the materiality and an appealing design also play an important role in the concept.”

‘Clean design’ for greater efficiency and a smooth production process Special attention was paid to the implementation of ‘clean design’ principles, for example by increasing cleanliness in the wet end. Less contamination reduces the risk of unscheduled downtimes due to web breaks. Consequently, operational reliability and machine efficiency are improved and the production process runs smoothly. Also, scheduled shutdowns are completed more efficiently thanks to more accessible work areas and simpler maintenance procedures. The design concept is a long-term project and will be implemented step by step over the coming months and years.

Optimized human-machine interface improves ease of maintenance “The smartphone has shown how simple and intuitive communication between humans and machines makes many areas of everyday life more efficient. In the industrial environment too, the information and all relevant functions a user needs will in future be presented in one platform,” says Oliver Kunerth, Digital Product Manager at Voith Paper. As part of the vision for the future, a standardized and intuitive user interface will therefore be introduced from stock preparation through to the reeling. In this context, the human-machine interface uses a role-based operating concept to adapt to the individual task of users, which at the same time improves ease of maintenance. In the future, more efficiency will also be ensured by distinctive SmartLights on individual components of the system. They show the machine status at a glance, and if necessary, whether operator intervention is required.

Automatic data analysis replaces pure monitoring Papermaking 4.0 solutions, sensors, field devices, scanners and actuators already monitor the condition of machines and the quality of production in real time. In the coming years, automatic data analysis will become even more important and replace pure monitoring.

Intelligent algorithms will then ensure that in a very short time, the system autonomously creates the optimal conditions for resource-saving production. In the near future, new apps on the Voith OnCumulus IIoT platform will create complete transparency about all conditions in a paper mill and give paper producers far-reaching optimization potential. “For example, one of these apps, the AI-based OnEfficiency.BreakProtect system, can already provide users with recommendations about how to avoid web breaks,” says Oliver Kunerth. Timeconsuming and cost-intensive downtimes can thus be minimized, while maintenance and servicing are optimized in a forward-looking manner.

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FROM THE PUBLISHERS OF PAP PER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

In the future, the OnCumulus cloud platform will also provide a networked interface to all service functions, such as for ordering spare parts via the web store or to the remote service center OnPerformance.Lab, enabling more efficient and sustainable production.

Appealing design for a calm overall appearance In future, Voith’s expertise as a full-line supplier is also set to be reflected in the design of a plant. In this context, clear structures, high-quality surface finishes and modern materials underline the special quality of a Voith BlueLine stock preparation and XcelLine paper machine.

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Volume 7, Number 3, 2021

AMETEK SURFACE VISION INTRODUCES NEW PERFORMANCE SERVICES TO ENHANCE INSPECTION SYSTEMS, REDUCE DOWNTIME AND MAXIMIZE RETURN ON INVESTMENT Leading automated surface inspection solutions provider, AMETEK Surface Vision, now offers a range of expert system services to ensure optimum levels of operation for the lifetime of its inspection and monitoring products.

Customers will benefit from AMECare Performance Services throughout their system lifecycles. AMECare — which features a range of global services available 24/7 — offers assistance for a variety of needs, from initial set up to trainings and spare parts.

Starting from purchase, AMECare provides tailored support for the installation, alignment, calibration, and configuration of new systems. On-site engineers will ensure the system is designed for individual requirements, using unique recipes and configured communications to ensure seamless integration with existing plant systems.

AMECare also provides service contracts to ensure customers receive dedicated on-site and remote support in response to any critical system problems, as well as general maintenance and software updates.

Yamina Lansari, Global Manager of Technical Services for Surface Vision, said: “We understand our customers often have mission-critical systems where downtime is not an option. AMECare service and support engineers understand the importance of remote and on-site services to ensure the most appropriate action in the event of any problems.”

Preventative maintenance is included in the AMECare package to identify and solve potential problems early, before they cause significant, costly process downtime. This service can also lead to less general maintenance, fewer part failures, and optimal system performance.

Other services offered as part of AMECare include expert technical support, training to ensure operators get the best out of their systems, and advice on stocking the right spare parts.

Paul Stuyt, Global Manager Projects and Service at Surface Vision added: “With over 2,500 installations worldwide, we understand how important it is to keep your systems working. Our global team of service and project engineers are constantly reviewing and updating our processes, ensuring customers always receive best-in-class service and support to get the most out of their systems.”

Other benefits of using AMECare include reduced material waste, improved product quality, enhanced defect detection, and process optimization.

AMETEK Surface Vision provides products and solutions for a wide range of industries, including metals, paper, plastic film, and nonwovens.

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Products & Services

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FROM THE PUBLISHERS OF PAP PER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

ROBUST SEAL PERFORMANCE

RAISES

NEW

MOUNTED

TAPERED

ROLLER

BEARING

SKF has vastly extended the performance, and product life, of its new mounted tapered roller bearing by pairing it with a superior seal.

The mounted tapered roller bearing, also known as a Type E bearing, incorporates SKF’s CR Seal. This extends service life by virtually eliminating ingress of contamination, even under harsh conditions.

“In mud-slurry tests, our seal withstood more than 600 hours with zero contamination ingress,” says Eric Brubaker, Director of Product Management at SKF North America. “Rival products saw ingress at 50 hours or earlier.”

The mounted tapered roller bearing is used when an increased thrust load is needed with limited axial movement. This makes it appropriate for demanding industries such as mining, aggregates and cement. Potential applications include material handling equipment, such as belt feeders and screw conveyors, and process machinery including grinders, shredders and mixers.

These units typically break down for one reason: seal failure. By improving seal performance, they can be designed to last much longer – extending uptime and increasing reliability.

Customers running this type of machine usually accept that contamination ingress will inevitably lead to premature failure. By combining its bearing and sealing expertise into a single unit, SKF has managed to overcome this.

“The seal performance of this unit surpasses that of our competitors by more than 10 times,” says Brubaker.

CR Seals offer several lines of defence against contamination. The use of hydrogenated nitrile rubber (HNBR) resists both wear and high temperatures, while a full rubber outside diameter improves static sealing within the housing. Multiple sealing lips exclude all types of contamination including dirt, mud, water, sand, and powder. The seal also allows the bearing to be relubricated without risking damage to the sealing lips.

Other areas of use for the Type E bearing include conveyors and process equipment in lumberyards, pulp & paper, metals and food processing.

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FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL L

Volume 7, Number 3, 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 end of July 2021 and end of November 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

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FROM THE PUBLISHERS OF PAP PER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

COMPANY, SITE Alizay Papier SASU France Anon Indonesia

SUPPLIER Andritz

Anon Asia Arkhbum Tissue Group LLC Vorsino industrial park Kaluga region Russia Asia Symbol (Guangdong) Paper Jiangmen Xinhui China Asia Symbol China Astrabel Belišće Croatia Astrabel Croatia BioCMPC Guaíba Mill Brazil

Valmet

BioPappel Tres Valles Mexico Cartiera Capostrada Italy

Projet BV

Cartiere Saci Verona Mill Italy Century Sunshine Paper Group Ltd Weifang City Shandong province China Domtar Hawesville Pulp and Paper Mill USA

Toscotec

Valmet

URL Alizay - Andritz Anon (Indonesia) Valmet Anon (Asia) Valmet Arkhbum Valmet

Andritz

coated board making and BCTMP production lines complete tissue production line including stock preparation, automation system and a rewinder tissue production line

Projet BV

tail cutter (order via Andritz)

Andritz

new tissue line (the country’s first)

Asia Symbol Projet Astrabel – Andritz

Projet BV

tail cutter (order via Andritz)

Astrabel - Projet

Valmet

modernisation of Pulp Line 2, including rebuild of the pulp drying, fiberline, evaporation and white liquor plant, a new recovery boiler and new ash treatment, extended distributed control system including advanced industrial internet features two dryer fabric cleaners (PM1)

Valmet

Replus Tissue and EIL

equipment upgrade project including changing the iron Yankee for a steel model and replacing the old boiler replacement of cast-iron Yankee with steel Yankee (PM2)

A.Celli

four winders for PM4 & PM5

Valmet

to supply the 100th latest generation Valmet Recovery Liquor Analyzer, to be used in the Valmet Causticizing Optimizer to increase cooking liquor strength

Page 2 of 6

ORDER DESCRIPTION pulping and storage system, and rejects system grade conversion from fine paper to brown grades

Installations

Asia Symbol Andritz

BioCMPC Valmet

Biopappel Projet Capostrada – Replus

Saci - Toscotec

Century Sunshine ACelli

Domtar - Valmet

g PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

COMPANY, SITE Eczacibasi Consumer Products Manisa Plant Turkey Egger Brilon Mill Germany Graphic Packaging Kalamazoo MI and Middletown OH USA Grasim Industries Ltd. Harihar Polyfibers Pulp Mill India Grupo Bignardi Jundiaí São Paulo Brazil Hayat Kimya five sites Henan Xinyaxin New Technology Packaging Material Xinxiang City Henan Province China IRANI Papel e Embalagem Vargem Bonita SC Brazil Jeonju Paper Korea Jiangsu Jintian Paper Yancheng Mill China JK Paper Limited Fort Songadh Gujarat India JSC Arkhbum Ulyanovsk Russia Kastamonu Entegre AS Balıkesir Turkey Kipaş Kağit Sanayi Isletmeleri A.Ş.

Kahramanmaraş Paper Mill Turkey

SUPPLIER Valmet

ORDER DESCRIPTION new tissue production line

URL Eczac - Valmet

Büttner

replacement of steam-heated drum dryer (MDF line)

Egger - Buttner

Projet BV

dryer fabric cleaning system

GraphicP Projet

Valmet

rebuild to improve the evaporator reliability and to handle the increased plant capacity CCM grade production machine

Voith

Grasim - Valmet

Bignardi - Voith

IBS Paper Performance Group Andritz

web inspection systems for 10 tissue machines two calenders and a complete paper machine approach flow system (PM9)

Hergen

replacement of drying section, including Yankee

Cellwood Machinery

Algas microfilter test unit

Kadant

two OCC systems and two approach flow systems

AFT

POM approach flow and stock preparation system (PM5)

JKPaper - AFT

BW Papersystems

Three flexo-rotary die-cut converting lines

JSC Arkhbum BWP

Siempelkamp

MDF / LDF production line

Kastamonu Siempelkamp

Valmet

multifuel boiler and auxiliary process equipment (for future PM4)

Page 3 of 6

Installations

Hayat - IBS Henan - Andritz

IRANI - Hergen

Jeonju Cellwood Jiangsu Kadant

Kahramanmaraş - Valmet

g PAPERmaking!

FROM THE PUBLISHERS OF PAP PER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

COMPANY, SITE Kipas Kagit Söke Mill Turkey Koehler Paper Oberkirch Germany Kyiv Cardboard and Paper Mill Verkhnedneprovsk Ukraine Ledesma Libertador Argentina Lee & Man Chongqing Mill China Metsä Board Husum Mill Sweden

SUPPLIER Valmet

ORDER DESCRIPTION recycled CCM production line (PM3)

URL Soke - Valmet

Voith

rebuild the transfer process from the press to the dryer section of PM5 corrugator

Koehler - Voith

Metsä Fibre Bioproduct Mill Kemi Finland MG TEC Romania

Endress+Hauser

MM Board & Paper Frohnleiten Mill Austria

ABB

two L&W Autolines

MMB&P – ABB

Muda Paper Mills Sdn. Bhd. Malaysia

Valmet

key paper technology to improve MG machine PM9

Muda - Valmet

Naturheld GmbH Grafenwöhr Germany Navigator Group Figueira Mill Portugal Neenah Paper Inc. Stevens Point WI Neenah Nine Dragons Beihai Mill China Nine Dragons Malaysia and elsewhere

Andritz

two fiber preparation systems (insulation material)

Naturheld – Andritz

Raumaster

wood processing plant

Navigator Raumaster

Projet BV

Dual Cutter, also called Tail & Deckle Cutter, for the Wire Section of PM1

Valmet

Norske Skog Golbey Mill France

Valmet

two complete fibrelines, two BCTMP lines, a recovery boiler and DeNOx scrubbers five BlueLine OCC stock preparation lines and two wet end process (WEP) systems high capacity winder (PM1)

BHS Corrugated

Voith

service agreement including OnCall video system

Valmet

recovery boiler upgrade service

L&M - Valmet

AFRY

the engineering assignment for the investment to increase their folding boxboard production capacity supply field devices top cover flow, pressure, differential pressure, level measurement and liquid analysis wet end tail cutter (order via Andritz)

MetsaB - AFRY

Projet BV

Voith

Page 4 of 6

KCPM - BHS

Installations

Ledesma - Voith

MetsaF – E&H

MGTEC - Projet

Neenah – Projet

9Dragons Valmet 9Dragons Voith NorskSkog Valmet

g PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

COMPANY, SITE Oji Paper Saga Mill Japan Owens Corning Fort Smith Arkansas USA PEGAS d.s.

Czech Republic

SUPPLIER Projet Japan Co

ORDER DESCRIPTION two dryer fabric cleaning systems

Voith

production machine and automation for glass mat production (nonwovens)

Owens – Voith

Projet BV

PEGAS - Projet

Pehlivanoglu Kagit Cerkezkoy Turkey Pölkky Oy's Kajaani Finland

Runtech

two power cleaners for Spunbond and Meltblown line (nonwovens) vacuum system rebuild

Porto Feliz Ltda Porto Feliz City São Paulo Brazil Pratt Industries Henderson Kentucky USA Renewcell Kristinehamn and Sundsvall plants Sweden Saudi German Co. Dammam Kingdom of Saudi Arabia SCA Obbola Mill Umeå Sweden SCA Ortviken Pulp Mill Sundsvall Sweden SCA Sundsvall Mill Sweden

Hergen

Schumacher Packaging Myszków site Poland Segezha Group Lesosibirsk Sawmills and Woodworking Plant Siberia

Voith

Jartek Invest Oy

green sorting and sticker stacker machinery with peripherals and automation packaging paper machine (PM3)

URL Oji – Projet

Pehlivanoglu – Runtech POK - Jartek

Hergen – Porto Feliz

Valmet

recycled board production line (PM18)

AFRY

Industrial IT solutions to manage production information

Projet BV

power cleaner system (nonwovens)

Saudi - Projet

ABB

to deliver drives and electrification support to the new PM2 (being supplied by Voith)

Obbola – ABB

Andritz

rebuild a disc filter

ABB

automation, electrification, quality control systems, motors and drives for Renewcell’s new industrial textile recycling production line rebuild PM2 in order to double capacity

AS Hekotek

equipment for wood pellet production (LDK No.1)

Page 5 of 6

Installations

Pratt – Valmet

Renewcell – AFRY

Ortviken – Andritz

Sundsvall – ABB

Schumacher – Voith Lesosibirsk – ASH

g PAPERmaking!

FROM THE PUBLISHERS OF PAP PER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

COMPANY, SITE Shandong Huatai Paper Shandong China Softys Caieiras Brazil ST Paper USA Starwood Orman Urunleri Sanayi A.S. İnegöl Turkey Starwood Orman Urunleri Sanayi A.S. İnegöl Turkey Suzano “Cerrado project” Ribas do Rio Pardo Mato Grosso do Sul Brazil Tolko Slave Lake AB Canada Toyo Engineering Corporation Niigata East Port Honshu Island Japan Tufropes Gujarat India UPM Platting Mill Germany Vajda Papír Dunaföldvár Mill near Budapest Hungary Visy Pulp & Paper Tumut Kraft Mill Australia Volga Pulp and Paper Mill Balakhna Nizhnij Novgorod Region Russia Xinxiang Xinya Paper Co., Ltd. China Zhejiang Forest United Paper Taizhou Zhejiang province China

SUPPLIER Voith

Voith Projet BV Andritz

supply the third complete fibre preparation system for its MDF production lines

Dieffenbacher

MDF line

Andritz

state-of-the-art and resourceefficient technologies for all main process islands in the fibre production and chemical recovery plant two full width high pressure belt cleaners (wood panel)

Projet BV

URL Shandong – Voith Softys – Voith STPaper Projet Starwood – Andritz

Starwood – Dieffenbacher

Cerrado – Andritz

Tolko - Projet

Andritz

circulating fluidised bed boiler for new biomass power plant

Toyo-Andritz

Voith & Truetzschler

production line for biodegradable hygiene wipes (nonwovens)

Tufropes – Voith

Valmet

xxtends maintenance operation agreement for an additional three years install a second tissue line

UPM_Valmet

Andritz

woodchip handling and storage system

Visy – Andritz

Andritz

a new OCC line as well as to convert PM6 from newsprint to a packaging paper machine

Volga – Andritz

Valmet

IQ Scanner and IQ Dilution Profiler (PM2) new containerboard machine (PM6)

Xinxiang-Valmet

Toscotec

Valmet

Page 6 of 6

ORDER DESCRIPTION Newsprint Machine Conversion Project (PM11) to high highquality graphic paper virtualization project for tissue paper machine TM 11 tail cutter (order via Andritz)

Installations

Vajda – Toscotec

Zhejiang Valmet

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 3, 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 publishes 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 are 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.

Page 1 of 4

Research Articles

g PAPERmaking!

FROM THE PUBLISHERS OF PAP PER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

IPPTA PAPYRUS 360°, Vol.33(1), 2021 1.

Modification of conventional chlorine dioxide bleaching to hot chlorine dioxide to reduce the fatty and resin acids in bleaching effluent 2.

Mathematical Modelling of Displacement Washing of Packed Bed of Cellulosic Fibers 3.

Mineral Deposits in Pulp Bleaching 4.

Performance Analysis of Different Pulp Consistency Models Using Particle Swarm Optimization Based Proportional Integral Derivative Controller 5.

Production, Brightness and Environmental Dynamics in modern Pulp making 6.

Strength, Odour and Hygiene in the face of increased reuse of fibre and water 7.

Surface sizing of paper with polyvinyl alcohol 8.

Technical advancements in manufacturing of packaging grades of paper as a substitute to plastic films in flexible packaging 9.

Water-Based Coating Solutions: A sustainable alternative to Plastic JOURNAL OF KOREA TAPPI, Vol.53(4), August 2021 1.

Characteristics of Hybrid Calcium Carbonate Prepared from Ground and Precipitated Calcium Carbonate 2.

Study on the Effects of Raw Materials and Impregnation Treatment on the Paper Strain 3.

Effects of Strength Additives on the Properties of High Bulk Pulp Mold 4.

A Study on the Physical Properties and Surface Characteristics of Hanji by the Treatment of Soybean Juice 5.

Characterization of the Paper Coated with Polyhydroxybutyrate/Ethyl Cellulose Blends 6.

Evaluation of the Disintegration in Water of Tissue Papers Distributed in Korea 7.

Characterization of the Pressure Drop during Condensation in Channels of MultiChannel Cylinder Dryer Using Homogeneous and Separated Flow Models 8.

Aging Analysis of Paper Depending on Light Source and Paper Color 9.

Decoupling Control Strategy of Alkali Recovery Furnace 10. Quality Evaluation of Cellulose Nanofiber Manufactured with a Prototype Grinder for the Development of a Taylor-Flow Nanogrinding System 11. Characteristics of Surface-Sizing Solutions Pigmentized with Ground Calcium Carbonate and Clay 12. Characteristics of Pigment Distribution in the Pigmentized Surface-Sizing Films 13. Effect of Pulp on the Strength of Coloring Paper Used for the Production of Fruit Bags 14. Growth Responses of Aster Yomena Based on Cellulose Nanofiber Mixing Soils JOURNAL OF KOREA TAPPI, Vol.53(5), October 2021 1.

Combined Enzymatic Pretreatment of Pulp for Production of CNF 2.

Establishment of NOx Concentration Model at the Outlet of SCR Denitrification System for Alkali Recovery Furnace Flue Gas Based on Improved LSSVM 3.

Separation of Cellulose from Rice Hull 4.

Analysis of Ink for Woodblock Printing of Samguk Yusa (II): Ink with High Solid Content 5.

Effects of Wood and Paper Products on National Economy in Korea: An Inputoutput Analysis 6.

Data Reconstruction Method for CD Basis Weight Analysis of Paper by using Compressed Sensing Technology 7.

Dilute Acid Hydrolysis of Jute Bast Fiber for Selected Xylan Removal

Page 2 of 4

Research Articles

g PAPERmaking!

FROM THE PUBLISHERS OF PAP PER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

8.

9.

10.

11.

12.

13.

14.

A Study on the Application of Deep Learning Models for Real-time Defect Detection in the Manufacturing Process: Cases of Defect detection in the Label Printing Process Development of a Cellulose-Based Freshness Sensor (II): Manufacture of Multilayer Freshness Sensor Using Cellulose-based Materials and Evaluation of Its Color Expression Hydrophobization of Cationic Cellulose Nanofiber Using Rosin Sizing Agent Study on the Use of Coating Pigment for Inkjet Printing (I): Effect of Zeolite Particle Size on the Rheological Properties of Coating Color and Coated Paper Quality for Inkjet Printing Effect of the PVA Impregnation Treatment Conditions on the Strain and Physical Properties of Paper Basic Study on Manufacturing of Inkjet Paper by High-Speed Coating (II): Effects of Binder Composition on Rheological Property of Inkjet Coating Colors Effects of SDS Dosages on Properties of Foam-formed HwBKP Sheets and Drainage

NORDIC PULP & PAPER RESEARCH JOURNAL, Vol.36(3), September 2021 1.

Chemical Pulping: Mapping variation of handsheet properties within loblolly pine trees 2.

Chemical Pulping: A simplified kinetic model for modern cooking of aspen chips 3.

Bleaching: Bleaching of bagasse-pulp using short TCF and ECF sequence 4.

Mechanical Pulping: Energy efficiency in mechanical pulping – definitions and considerations 5.

Paper Technology: Development of ash condensation performance of paper materials via saccharides and Nano HAP application 6.

Paper Technology: Effects of calcium silicate synthesized in situ on Fiber loading and paper properties 7.

Paper Technology: Production of fines from refined kraft pulp by fractionation with micro-perforated screens 8.

Paper Technology: Dynamic-head space GC-MS analysis of volatile odorous compounds generated from unbleached and bleached pulps and effects on strength properties during ageing 9.

Paper Technology: Composite paper from an agricultural waste of bagasse sugarcane and pineapple leaf fibre: a novel random and multilayer hybrid fibre reinforced composite paper 10. Paper Physics: Phenomenological analysis of constrained in-plane compression of paperboard using micro-computed tomography Imaging 11. Paper Chemistry: Preparation and characterization of tung oil-rosin-based polyester internal sizing agent 12. Coating: Research on brightening modification of molecular sieves coated fly ash based on alkaline melting hydrothermal method 13. Coating: Application of modified cellulose nanofibrils as coating suspension on recycled paper using size press 14. Chemical Technology/Modifications: Novel calcium carbonate filler for cellulose industry TAPPI JOURNAL, July 2021 1.

Editorial: TAPPI Standards development: Authors and reviewers are welcome 2.

Three-dimensional visualization and characterization of paper machine felts and their relationship to their properties and dewatering performance

Page 3 of 4

Research Articles

g PAPERmaking!

FROM THE PUBLISHERS OF PAP PER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

3.

4.

5.

Preparation of regenerated cellulose from rice straw lignocellulosic waste and its use for reinforced paper products Production of antimicrobial paper using nanosilver, nanocellulose, and chitosan from a coronavirus perspective Addressing production bottlenecks and brownstock washer optimization via a membrane concentration system

TAPPI JOURNAL, August 2021 1.

Guest Editorial: Still working (overtime): Life-saving nonwovens and continued innovation 2.

Formic acid pulping process of rice straw for manufacturing of cellulosic fibers with silica 3.

Technological evaluation of Pinus maximinoi wood for industrial use in kraft pulp production 4.

Multifunctional starch-based barrier materials 5.

Can carbon capture be a new revenue opportunity for the pulp and paper sector?

TAPPI JOURNAL, September 2021 1.

Editorial: Collaboration: A necessary recipe for technological growth 2.

Displacement washing of softwood pulp cooked to various levels of residual lignin content 3.

Effects of phosphogypsum whiskers modification with calcium stearate and their impacts on properties 4.

Tetraethyl orthosilicate-containing dispersion coating — water vapor and liquid water barrier proper 5.

Commercially relevant water vapor barrier properties of high amylose starch acetates: Fact or fiction?

TAPPI JOURNAL, October 2021 1.

Editorial: Seshadri Ramkumar: Nonwovens specialist and TTU professor joins TJ Editorial Board 2.

Control of malodorous gases emission from wet-end white water with hydrogen peroxide 3.

Kraft recovery boiler operation with splash plate and/or beer can nozzles — a case study 4.

Application of spruce wood flour as a cellulosic-based wood additive for recycled paper applications — A pilot paper machine study 5.

Corrosion damage and in-service inspection of retractable sootblower lances in recovery boilers TAPPI JOURNAL, November 2021 1.

Editorial: Industry coating expert Gregg Reed joins TAPPI Journal editorial board 2.

NovInfluence of pallet pattern on top-to-bottom compression performance of unitized loads 3.

Temperature profile measurement applications of moving webs and roll structures with intelligent roll embedded sensor technology 4.

Determining operating variables that impact internal fiber bonding using Wedge statistical analysis 5.

Evaluation of rice straw for purification of lovastatin

Page 4 of 4

Research Articles

PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY TE INTERNATIONAL

Volume 7, Number 3, 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:

Page 2

Coating

Page 4

Moulded Pulp

Page 5

Nano-Science

Page 6

Novel Products

Page 7

Packaging Technology

Page 8

Papermaking

Page 9

Pulp

Page 10

Testing

Page 11

Tissue Waste Treatment

Page 12

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.

Page 1 of 13

Technical Abstracts

g PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

COATING “Synergetic effect of cationic starch (ether/ester) and Pluronics for improving inkjet printing quality of office papers”, Mohit Sharma, Roberto Aguado, Dina Murtinho, Artur J. M. Valente & Paulo J. T. Ferreira, Cellulose, Vol.28, pp.10609–10624 (2021).

Improving the printability of paper is still a relevant challenge, despite the fast development of digital communications. While it is well-known that cationic starches enhance ink density, their commercial paper-grade forms are limited to ethers with low degree of substitution. This work addresses the underexplored potential of highly substituted cationic starch for paper coating and its combination with tri-block polymers, namely Pluronics (P123 and F127), taking advantage of their supramolecular interactions with amylose chains. For that purpose, cationic starch ether and ester (starch betainate), both with a degree of substitution of 0.3, were synthesized by alkaline etherification and by transesterification, respectively. Paper without any surface treatment was subjected to one-side bar coating with suspensions encompassing those products and Pluronics, besides other common components. Black, cyan, yellow and magenta inks were printed on all coated papers through an inkjet printer. Key properties of printing quality such as the gamut area, gamut volume, optical density, print-through, inter-color bleed and circularity were measured in a controlled temperature-humidity environment. For instance, a formulation with cationic starch (ether/ester) and P123 improved the gamut area by 16– 18% in comparison to native starch-coated paper sheets. Interestingly, the individual assessment of each component showed that cationic starch ether, starch betainate and P123 only improved the gamut area by 5.6%, 8.9% and 6.8%, respectively. Finally, but not less importantly, starch betainate was found to quench optical brightening agents to a lesser extent than cationic starch ethers.

“Fabrication of high mechanical properties papers coated with CMC-based nanocomposites containing nanominerals synthesized from paper waste”, Faegheh Alsadat Mortazavi Moghadam, Hossein Resalati, Sousan Rasouli & Ghasem Asadpour, Cellulose, Vol.28, pp.11153–11164 (2021). In this work, nanokaolin (K) and nanometakaolin (MK) were synthesized from waste paper via planetary ball milling and characterized by means of X-ray diffraction (XRD), Fourier transform infrared vibrational spectroscopy (FTIR), and scanning electron microscopy (SEM). Suspensions of carboxymethyl cellulose (CMC) and the synthesized nanominerals at 1.5, 3, 6, and 9% led to nanocomposites 30, 60, 90, and 120 μm thick. Nanocomposites coated papers were analyzed by means of XRD, FTIR, SEM, tensile index, strain rate, burst index, and air passage resistance. The highest tensile index, burst index, and air permeability resistance values of 80.78 N.mg, 5.24 kPa.m2g, and 286.66 ml/min, respectively, were obtained for CMC-9%MK-120. These results indicated that paper coated with CMC nanometakaolin composite is suitable for packaging.

“Effects of Chitin Nanocrystals on Coverage of Coating Layers and Water Retention of Coating Color”, Ruoshi Gao, Yi Jing, Yeyan Ni & Qiwen Jiang, Journal of Bioresources and Bioproducts, published online. This study assessed the applicability of chitin nanocrystals prepared by 2, 2, 6, 6-Tetramethyl-1-Piperidine-1-oxyl radical (TEMPO)-mediated oxidation in traditional papermaking coating color systems. The αchitin nanocrystals (CTNCs) with different carboxyl content, size, and morphology were prepared from crab shells by alkali pretreatment and TEMPO-mediated oxidation in the water at pH 10, and then the ratio of CTNCs to latex was applied to traditional coating color system to replace part of latex. The results showed that when the amount of NaClO added as co-oxidant in the oxidation was 15.0 mmol/g of chitin, the carboxyl content of alkali-pretreated CTNCs was up to 0.76 mmol/g. The amount of carboxyl groups presented

Page 2 of 13

Technical Abstracts

g PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

a linear relation with the degree of individualization of nanocrystals and dispersion. When the ratio of latex to CTNCs was 90꞉10, the water retention value of the coating was 92% lower than that of the pure latex system, and the rheological property was better. The relationship between the addition amount of CTNCs and the surface strength and the coverage of coating layers were also studied, and results showed that when the ratio of latex to CTNCs was 95꞉5, the surface strength was the highest of 1.45 m/s, and the coverage of coating layers rate reached the highest of 78%.

“Study on flame retardancy of ammonium polyphosphate/montmorillonite nanocompound coated cellulose paper and its application as surface flame retarded treatment for polypropylene”, Peifan Qin, Deqi Yi, Jun Xing, Mingzhu Zhou & Jianwei Hao, Journal of Thermal Analysis and Calorimetry, Vol.146, pp.2015–2025 (2021).

Natural cellulose paper is flame retarded using ammonium polyphosphate/montmorillonite (APP/MMT) nanocompound through coating method. Their morphologies are assessed by scanning electron microscopy, chemical components by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, chemical structure by Fourier transform infrared, thermal stability by thermo-gravimetric analysis and fire retardancy by 45° horizontal burning test. The results show that the thermal stability and flame retardancy of coated paper samples are enhanced obviously, and all samples are self-extinguishing during the tests due to the char formation reaction between APP/MMT and cellulose. Then the treated paper, AM-18.3, as surface flame retardant treatment for low specific surface area (SSA) polypropylene (PP) specimens (PP@AM18.3) also is investigated using cone calorimeter. Compared with two controlled samples, PP@AM-18.3 shows significantly increased time to ignition, the lowest fire growth index and highest fire performance index. This may give a new flame retarded approach for low SSA polymer materials with little amount flame retardants.

“Best pigment coating for a dual-purpose coated paper”, Jae Y. Shin, Paul Dan Fleming & Hak Lae Lee, Journal of Coatings Technology and Research, Vol.18, pp.1281–1294 (2021). Dual-purpose coated paper, which enables in-line inkjet printing with web offset, can be used for both offset and inkjet prints. This product has potential to meet the ever-increasing demand for printing individualized information on commercial printing materials. In this work, commercial samples were printed in the two presses. It was deduced that low permeability correlates with large color gamut volume. The correlation would be useful to design coating formulations for good printability. Also, we examined the packing of coating layers to identify a coating formulation that provides good printing quality in both offset and inkjet printing. Many combinations of pigments were tested, and the effects of particle shape on viscosity and packing were examined. For all coating colors, pigment mixtures with a weight ratio of 3:1 gave the highest packing density. The mixture of GCC 90 and delaminated clay with a weight ratio of 3:1 was the best selection studied for dual-purpose coated paper. Among commercial paper samples, a web offset coated paper showed the best overall performance, indicating that coating formulations with minimal blistering are suitable for dual-purpose coated paper.

“Preparation of polyaniline/cellulose nanocrystal composite and its application in surface coating of cellulosic paper”, Xiaoyu Wanga, Peng Zhua, Tianying Chena & Yiming Zhoua, Progress in Organic Coatings, Vol.159, Oct. 2021. Imparting electroconductivity to cellulosic paper may allow this conventional material to hold great promise for a wide range of applications. In this work, polyaniline/cellulose nanocrystal (PANI/CNC) composite was prepared via emulsion polymerization and subsequently used as a conductive and reinforcing pigment for paper coating fabrication. Initially, the

Page 3 of 13

Technical Abstracts

g PAPERmaking!

FROM THE PUBLISHERS OF PAPER TECHNOLOGY INTERNATIONAL

Volume 7, Number 3, 2021

microstructure and properties of the fabricated PANI/CNC composite were comprehensively characterized. Furthermore, the effect of the PANI/CNC composite on the properties of paper coatings and coated paper was systematically investigated. The results showed that the viscosity and viscoelasticity of paper coatings were increased as a function of the PANI/CNC composite addition. Meanwhile, the increased CNC ratio in PANI/CNC composite led to the decreased viscosity and viscoelasticity of paper coatings.

Furthermore, surface coating application was found to impart desired electro-conductivity and enhanced mechanical properties to coated paper. Compared to the coated paper without PANI/CNC composite, the coated paper with 4 wt% CNC added PANI/CNC composite maintained an electro-conductivity of 4.0 S·m−1 and exhibited increases of 14.6%, 30.7% in tensile index and folding strength, respectively. In particular, the electroconductivity and mechanical properties of coated paper were also enhanced with the increased ratio of CNC in PANI/CNC composite.

MOULDED PULP “A thermal insulator manufacturing, using solid residuals from pulp and paper factories in Missan City”, Waleed Khalaf Jabbar Allamy & Ibrahim Ali Hameed AlNajati, AIP Conference Proceedings, 2338, 040022 (2021). Residuals from pulping process of Iraqi pulp and papermaking factories was taken to produce a thermal insulator by wetting, crushing, and pressing to a certain shape to get a board called by the authors as “Cellulose Board” that can be used as a thermal insulator for air conditioning and piping systems carrying hot water for heating. It was found that thermal conductivity of cellulose board is 0.0311 W/m.c hence it can be compared with common thermal insulators such as rock wool and fiber glass which is expensive from economic point of view. It is recommended to use vapor barriers to prevent board from getting moisture. Additionally, it is recommended not to use Cellulose Board in a high temperature application.

“Molded Fiber and Pulp Products as Green and Sustainable Alternatives to Plastics: A Mini Review”, Yanling Zhang, Chao Duan, Swetha Kumari Bokk, Zhibin He & Yonghao Ni, Journal of Bioresources and Bioproducts, published online 14 October 2021. There are significant incentives/pressures on decreasing the use of plastics and their related products in the packaging industry, correspondingly, strong demands are emerging for clean, renewable, recyclable/ biodegradable packaging products. In this context, molded fiber/pulp products have attracted increasing attention, due to their green/sustainable advantages, simply because the raw materials used are plant-based and/or recycled fibers. Many companies have switched their packing practices from plastics to more environmentally friendly products, such as molded fiber products, which already have had and will continue to have obvious effect on packaging industries. This paper initially provides an overview on the general concept of molded pulp products, and further summarizes the different types of molded fiber products in terms of natural fiber sources, manufacturing processes, current and emerging applications as well as the environmental sustainability of molded products.

“Effect of Mono- and Bilayer Coating of Nanofibrillated Cellulose, its Modification, and Shellac on Properties of Molded Pulps”, Supattra Klayya, Thawan Chotimarnon, Nattaya Tawichai, Uraiwan Intatha & Nattakan Soykeabkaew, Key Engineering Materials, Vol.889, pp.79-84. A molded pulp is increasingly used as eco-packaging, but it has poor water resistance. Therefore, surface coating is common to perform on pulp or paper packaging to overcome this shortcoming. In this study, the bagasse (BG) molded pulp sheets were mono-and bilayer coated with nanofibrillated cellulose (NFC), modified NFC (mNFC), and shellac (S) by using a spin coating technique. Surface morphology,

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surface wettability, water absorption, and mechanical properties of the coated sheet samples were evaluated and compared to the uncoated sample. It was found that mNFC could effectively provide an even and complete coverage coating layer on the BG-based sheet (BG/mNFC), thanks to the partially substituted ester groups. On the contrary, NFC could not be coated evenly on the BG-based sheet surface (BG/NFC) due to its tendency towards agglomeration. The homogeneity of surface obtained from the first layer coating

by NFC or mNFC affected the surface quality of the second layer coating by shellac. As a result, the BG/mNFC/S bilayer coated sample showed the smoothest surface and also the highest water resistance confirmed by SEM, contact angle measurement, and water absorption results. Furthermore, the tensile properties of both bilayer coated samples (BG/NFC/S and BG/mNFC/S) were significantly improved (p<0.05) as compared to the uncoated BG sample. This results suggested that the current bilayer coating system is very promising for advancing the performance of molded pulps in novel packaging uses.

“Environmental Protection Status and Analysis of Pulp Molded Products Based on LCA”, Xueqin Ni, Zimin Li, Zijie Cui, Danfei Liu & Yunfei Zhong, Advances in Graphic Communication, Printing and Packaging Technology and Materials, pp.466-470.

Based on the life cycle assessment system, this paper summarized the environmental load caused by the three processes of raw material source, production and processing, waste and recycling. According to the data obtained from consulting literature, it is analyzed by Simopro7.1 software database, and the results show that the molded pulp products are not completely friendly materials. Finally, according to the results, this paper analyzes the improvement measures of the environmental hazards in the whole life cycle of pulp molded products, aiming at providing clear sustainable development direction and environmental protection guidance suggestions for the workers in environmental protection departments and pulp molded manufacturers.

NANO-SCIENCE “Role of cellulose nanofibrils in improving the strength properties of paper: a review”, Thabisile Brightwell Jele, Prabashni Lekha & Bruce Sithole, Cellulose, published online (2021). The pursuit for sustainability in the papermaking industry calls for the elimination or reduction of synthetic additives and the exploration of renewable and biodegradable alternatives. Cellulose nanofibrils (CNFs), due to their inherent morphological and biochemical properties, are an excellent alternative to synthetic additives. These properties enable CNFs to improve the mechanical, functional, and barrier properties of different types of paper. The nanosize diameter, micrometre length, semicrystalline structure, high strength, and modulus of CNFs have a direct influence on the mechanical properties of paper, such as tensile index, burst index, Scott index, breaking length, tear index, Z-strength, E-modulus, strain at break, and tensile stiffness.

This review details the role played by CNFs as an additive to improve strength properties of paper and the factors affecting the improvement in paper quality when CNFs are added as additives. The paper also includes techno-economic aspects of the process and identifies areas that need further research.

“Cellulose Nanofibrils as Reinforcement in the Process Manufacture of Paper Handsheets”, Lívia Ribeiro Costa, Luiz Eduardo Silva, Lays Camila Matos, Gustavo Henrique Denzin Tonoli & Paulo Ricardo Gherardi Hein, Journal of Natural Fibres, published online: 17 Aug 2021. The objective of this study was to better understand the effect of micro/nanofibrils added as reinforcement on paper handsheets, their physicalmechanical performance and barrier properties, and to determine the ideal cellulose nanofibrils (CNF) proportion to increase such paper properties. Mechanically produced

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CNF were added at increasing amounts (0%, 2%, 5%, 8% and 10%) to commercial Eucalyptus pulp to produce paper handsheet samples. Morphology, crystallinity, physicalmechanical and air barrier properties of the paper handsheets were evaluated. The results suggested that adding CNF has decreased the presence of empty spaces inside and on the surface of the paper handsheets by up to 61% due to the interaction between fibers and nanofibrils. The paper handsheets became denser, more compact and resistant to passage of air, as well as with greater mechanical performance with higher CNF content (10%). The bursting index is approximately 5 times higher on paper with addition of 10% of CNF compared to control paper handsheets. There were significant gains in the studied properties without any change in CNF/fibers surface charge or the use of any cationic polymer to assist the retention of nanofibrils and fibers. This study highlights the potential of CNF as additives in papermaking process, increasing its properties.

NOVEL PRODUCTS “On the role of fibre bonds on the elasticity of low-density papers: a micromechanical approach”, L. Orgéas, P.J.J. Dumont, F. Martoïa, C. Marulier, S. Le Corre & D. Caillerie, Cellulose, Vol.28, pp.9919–9941 (2021). Fine prediction of the elastic properties of paper materials can now be obtained using sophisticated fibre scale numerical approaches. However, there is still a need, in particular for low-density papers, for simple and compact analytical models that enable the elastic properties of these papers to be estimated from the knowledge of various structural information about their fibres and their fibrous networks. For that purpose, we pursued the analysis carried out in Marulier et al. (Cellulose 22:1517–1539, 2015. https://doi.org/10.1007/s10570-015-0610-6) with low-density papers that were fabricated with planar random and orientated fibrous microstructures and different fibre contents. The fibrous microstructures of these papers were imaged using X-ray synchrotron microtomography. The corresponding 3D images revealed highly connected fibrous networks with small fibre bond areas. Furthermore, the evolutions of their Young’s moduli were non-linear and evolved as power-laws with the fibre content. Current analytical models of the literature do not capture these trends. In light of these experimental data, we developed a fibre network model for the in-plane elasticity of papers in which the main deformation mechanisms of the micromechanical model is the shear of the numerous fibre bonds and their vicinity, whereas the fibre parts far from these zones were considered as rigid bodies. The stiffness tensor of papers was then estimated both numerically using a discrete element code and analytically using additional assumptions. Both approaches nicely fit the experimental trends by adjusting a unique unknown micromechanical parameter, which is the shear stiffness of bonding zones. The estimate of this parameter is relevant in light of several recently reported experimental results.

“Bio-based materials for nonwovens”, A.S. Santos, P.J.T. Ferreira & T. Maloney, Cellulose, Vol.28, pp.8939–8969 (2021). The nonwoven industry is one of the most innovative and important branches of the global fiber products industry. However, the use of petrochemical-based materials in many nonwoven products leads to severe environmental issues such as generation of microplastics. Synthetic material use in nonwovens is currently around 66%. This review covers potential technologies for the use of bio-based materials in nonwoven products. The current generation of nonwoven products relies heavily on the use of synthetic binders and fibers. These materials allow for products with high functional properties, such as permanence, strength, bulk, and haptic properties. The next generation of nonwoven products will have a higher fraction of natural and renewable materials as both binders and fiber elements. There are a wide range of materials under investigation in various nonwoven product categories. Especially,

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lignocellulosic materials are of interest. This includes traditional pulp fibers, regenerated cellulose fibers, lignin binders and nanomaterials derived from wood. The development of water stable, strong interfiber bonding concepts is one of the main problems to be solved for advancing bio-based nonwoven products.

PACKAGING TECHNOLOGY “One-step Coassembled Nanocoatings on Paper for Potential Packaging Applications”, Sonia E. Chavez, Hao Ding, Brandon L. Williams, Sunghyun Nam, Zaili Hou, Dongqiao Zhang & Luyi Sun, ES Materials & Manufacturing, Vol.15, (2022).

Plastics films have been widely used in food packaging. But due to the environmental concerns of plastics films, there is a trend of replacing plastics films with paper for food packaging. To meet the requirements of packaging, the paper must be modified to improve its barrier properties. In this report, a sonication and dip coating method was developed to deposit a PVA/MMT nanocoating on two representative paper substrates: regular paper and cotton paper. The coated paper substrates were characterized by X-ray diffraction, scanning electron microscopy (SEM), water vapor transmission rate (WVTR), and microscale combustion calorimetry (MCC). The XRD results support the formation of wellaligned MMT nanosheets on paper substrates and the SEM images show that most pores on the substrates were covered by the nanocoatings, which leads to a drastic decrease in WVTR of the coated substrates. The nanocoatings also led to a minor improvement in flame retardancy. The results suggest that applying nanocoating is a promising approach to improve the barrier properties of paper for potential packaging applications.

“Preparation of eco-friendly wax-coated paper and its rheological and waterresistant characteristics”, Eun Ju Lee & Kwang-Hee Lim, Korean Journal of Chemical Engineering, published online (2021). The blend (wax M) of crude by-product polyolefin wax (wax K) and a fractionated commercial paraffin wax (wax J) was suggested to replace the wax J as a coating agent for wax-coated papers. The rheological properties of waxes J, K, and M were examined and compared. The correlation between viscosity and shear rate applied on these waxes maintained at 90 °C and 130 °C was identified. In particular, this paper, for the first time, presented non-Newtonian shear thinning behavior of not only wax K but also its blend of wax M in terms of their viscosity affected by shear rate at an operating temperature below their melting temperature of higher-meltingtemperature DSC endothermic peaks (HMTEPs). They showed non-Newtonian behavior, so-called shear thinning behavior, at 90 °C in the light of characteristics of both suspension systems and polymer systems. In addition, the profiles of viscosity at 130 °C of all the waxes versus the shear rate exhibited Newtonian fluid behavior. Wax J also showed the behavior of a dilatant fluid. Then, the physical properties including water vapor transmission rates (WVTR), surface roughness, and coated weights, of thin papers coated with waxes J (WJP), K (WKP), and M (WMP) were evaluated, characterized, and compared. As a result, WMP had an equivalent value to that of WJP or the lowest value among wax-coated papers in terms of WVTR. The surface roughness and the barrier property of WVTR were minimized and enhanced, respectively, by blending waxes J and K. The additional physical properties, including dynamic contact angles, surface tension, wet and dry tensile strength, optical examination of the wax-coated fiber structure, and antimicrobial properties of the wax-coated papers, were evaluated. The excellent antimicrobial properties of clinoptilolite added to wax J or wax M appeared.

“The Effect of Different Virgin Fibers to the Properties of Paper Products, Tibor Czene & László Koltai, Proceedings of the 2nd International Conference on Circular Packaging, 2021. The products from paper are widely used materials with several

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benefits. The corrugated paper keeps items protected through long-distance logistic processes and constant shipping and handling. The corrugated boxboards are ideal options for any industry’s shipping, packaging and storage needs. Papers and cardboards are quite a low cost and also provide environmental-friendly solutions, using recyclable materials such as used corrugated cartons and old newspapers. Recycling offers a reduction in environmental impact in densely populated regions and a large production of paper and board products. Generally, the use of recycled fiber produces paper with poorer mechanical properties due to the decrease in interfiber bonding. The recycled pulp must be treated to restore its bonding strength, for which there are six methods possible: mechanical treatment, chemical additives, chemical treatment, fractionation, papermaking process modification, and blending with virgin fiber. Although some mills produce 100% recycled paper, the majority augment their used pulp with some virgin fiber. Paper properties can be tailored within some ranges by modifying the properties of fibers, but the influence of fine quality on the structure, strength, and optical properties of paper can be even greater. The properties of papers are essentially determined by their raw materials.

Most of these raw materials are made from 100% recycled fiber, but as the quality of the waste fiber varies, different chemicals must be used to provide the desired or expected properties. From an environmental and economic point of view, the use of primary fibers can be an alternative.

PAPERMAKING “Development of Post Hybrid Calcium Carbonate for High Loaded Paper”, Min Woo Lee, Dong Suk Kang & Yung Bum Seo, BioResources, Vol.16(4), pp.7716-7728 (2021). In papermaking, pre-flocculation of fillers such as ground calcium carbonate (GCC) improves the tensile strength of paper sheets. However, the pre-flocculated fillers mostly suffer from the instability of the floc shape such as the decrease in floc diameter with time elapse after preparation and no improvement of bulk and stiffness. The addition of calcium compounds such as calcium oxide or calcium hydroxide to the preflocculated GCC, and injection with carbon dioxide caused pre-flocculated GCC flocs to be covered with newly formed calcium carbonate. This product, called post hybrid calcium carbonate (pHCC), was found to be more stable in size and gave better sheet strength than the preflocculated ones. Furthermore, pHCC gave remarkably higher bulk and stiffness than the pre-flocculated flocs did without impairing smoothness that was essential in printing paper.

The proper use of pHCC in papermaking could allow the production of high loaded paper with more than 10% higher filler contents, which could reduce paper production cost and save drying energy. The proportion of the newly formed calcium carbonate in pHCC, turbulence intensity at preparation stage, and the effect of storage time were investigated.

“Data-driven method for monitoring and diagnosis in energy system of papermaking process”, Yanzhong Zhang, Zhiqiang Zeng, Jigeng Li & Huanbin Liu, Drying Technology, online 23 Aug 2021. This study proposes a data-driven model for monitoring and diagnosis in energy system of papermaking process based on kernel component analysis (KPCA) and the kernel slow feature analysis (KSFA). Four different abnormal patterns are designed, while the false alarm rate (FAR) and the missed detection rate (MDR) are used to evaluate the validity of the proposed model. The performance of KPCA is better than that of conventional PCA and KSFA. The online monitoring and diagnosis analysis of energy utilization are achieved based on the historical data and the proposed model. The production status and the energy consumption level across the whole process can be acquired. The results demonstrate that the proposed method is effective. It can provide the valuable reference foundation for further energy analysis and optimization in papermaking process.

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“Characterization of retention chemicals and their effect on the paper forming process on machine PM4, Billerud Korsnäs Gävle Mill”, Therese Alm, 2021 (English) Independent thesis Basic level (degree of Bachelor). In the papermaking industries spots in the finished product is a recurring problem. Billerud Korsnäs have in earlier studies identified poorly optimized retention systems as one of the reasons for these spots.

Poorly optimized retention systems will allow for detrimental substances to flow freely in the system, which may cause agglomeration into larger particles that could end up as darker spots in the finished product. The aim of this thesis is to investigate a number of retention systems, consisting of a retention polymer and retention microparticles, and characterize the polymers. The retention aid systems task is to flocculate fibres, fines and fillers along with the colloidal material to improve process parameters. To investigate the retention systems three different retention polymers with different charge densities have been investigated alongside one microparticle. The parameters investigated in this thesis were the retention systems effect on drainage time, turbidity, charge demand and zeta potential. Pulp and white water from PM4 was used to imitate mill conditions. The results showed that the drainage time and turbidity was most effected by the retention aid systems. The polymer with the highest charge yielded the best results. Only minor effects could be detected on charge demand and Zeta potential.

PULP “How Different Carryover Pitch Extractive Components are Affecting Kraft Paper Strength”, Jussi Lahti, Roman Poschner, Werner Schlemmer, Andrea Hochegger, Erich Leitner, Stefan Spirk, & Ulrich Hirn, ACS Omega 2021, 6, 44, 29350–29359. We present how harmful different wood extractives carried over to paper mill with unbleached softwood Kraft pulp are for the strength of packaging papers and boards. The investigations were done by simulating industrial papermaking conditions in laboratoryscale trials for handsheet production. It was found that fatty acids are the most relevant compounds in the carryover pitch extractives (CPEs), as they readily interfere in fiber–fiber bonding strength, control the properties of CPE micelles, and are furthermore the most abundant compounds. Addition of cationic starch improved strength and evened out the strength differences of handsheets with different CPE compounds. Oleic acid (unsaturated fatty acid) was an exception, as it was above average harmful for paper strength without cationic starch and also heavily impaired the functioning of cationic starch. As a whole, these findings demonstrate that fatty acids, especially unsaturated ones, are the most relevant CPE compounds contributing to the reduced efficiency of cationic starch and decreased strength of unbleached softwood Kraft paper. This makes the cleaning of process waters by precipitating CPEs on the pulp fibers harmful for paper strength.

“Utilization of corn husk for tissue papermaking”, Natalia Suseno, Marisca E. Gondokesumo, & Puspita R. Permatasari, AIP Conference Proceedings 2338, 040019 (2021); https://doi.org/10.1063/5.0067417. The demand of tissue papers is increasing with the population increase. This will definitely increase the need of wood fibers as the main raw material. However, due to the wood shortages, there have been many attempts to use non-wood fibers as substitutes for papermaking. In Indonesia, corn production has gradually increased for the last 5 years, hence it also has an impact on the raising in the amount of corn husk waste. Corn husk has a high cellulose content which suitable to be used as a raw material for tissue papermaking. In this experiment, soda pulping process was conducted to remove out lignin. The resulting tissue paper will be added with additives that have antimicrobial properties of chitosan and mangosteen peel for the purpose of increasing the tensile strength or absorption of water. The aim of this research is to study the effect of depending variables (temperature and NaOH concentration) on

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chemical composition (cellulose and lignin content), and physical properties including water absorption and tensile strength.The research was started with the initial process of removing the lignin content in the pulp by pretreating delignification using the sodium hydroxide (NaOH) process with several variations in concentration (4–10%), and temperature (60–90°C) for 1.5 hours. To obtain tissue with a good physical condition, it has been influenced by the optimum chemical composition containing high cellulose and low lignin content, high tensile strength and water absorption. The optimum conditions for tissue paper in this study were at 90°C and 4% of NaOH concentration. The next step will be to vary the composition of the additive in order to obtain the effect of physical properties (tensile strength and water absorption).

“A Comparative Fiber Morphological Analysis of Major Agricultural Residues (Used or Investigated) as Feedstock in the Pulp and Paper Industry”, Dimitrios Tsalagkas, Zoltán Börcsök, Zoltán Pásztory, Vladimír Gryc, Levente Csóka & Kyriaki Giagli, BioResources, Vol.16(4), (2021). The suitabilities of major agricultural residues were assessed as papermaking feedstocks. All the examined agricultural residues were assumed as potential candidates for substituting hardwood fibers in mixed pulp blends from a fiber morphological perspective. Wheat, barley, rice, rapeseed, maize, sunflower, sugarcane bagasse, coconut husk, and two genotypes of miscanthus grass underwent identical maceration. The fiber length, fiber width, cell wall thickness, and lumen diameter were measured to calculate the slenderness ratio, flexibility coefficient, and Runkel ratio.

The average fiber length ranged from 0.50 mm ± 0.32 mm (MG-S-02-V) to 1.15 mm mm ± 0.58 mm (sugarcane bagasse). The fiber width ranged from 10.77 μm ± 3.28 μm (rice straw) to 22.99 mm ± 5.20 mm (sunflower stalk). The lumen diameter ranged from 4.52 μm ± 2.52 μm (rice straw) to 13.23 μm ± 4.87 μm (sunflower stalk). The cell wall thickness ranged from 3.02 μm ± 0.95 μm (rice straw) to 4.80 μm ± 1.48 μm (sunflower stalk). The slenderness ratio, flexibility coefficient, and Runkel ratio values ranged between 28.08 to 58.11, 37.97 to 60.8, and 0.62 to 1.68, respectively. Wheat, maize, rapeseed, sugarcane bagasse, and coconut husk were found to be appropriate residue sources for papermaking feedstocks.

TESTING “Failure prediction of waterborne barrier coatings during folding”, Yaping Zhu, Douglas Bousfield & William Gramlich, Journal of Coatings Technology and Research, Vol.18, pp.1117–1129 (2021). Adding pigments into waterborne barrier coatings improves barrier properties and cost-effectiveness but increases the risk of crack formation during folding. Crack formation is affected by pigment shape, aspect ratio, and concentration; however, the exact mechanism for these effects is still not well understood.

In this work, a systematic model was used to understand the influence of the paper and coating thickness, the latex and pigment modulus, the pigment shape and aspect ratio, and pigment concentration on the failure of waterborne barrier coatings during folding. A finite element method-based model was solved with a commercial package to simulate the folding process. These simulations were compared to experimental results to verify the key parameters that affect coating failure. High paper and coating thickness, pigment loadings, pigment aspect ratios, and modulus differences between latex and pigment increased the likelihood of failure. Experiments and models using lower modulus spherical plastic pigments were more difficult to fail than coatings made with higher modulus kaolin. The maximum strain for coatings bent to a set curvature was the smallest when the modulus of latex and pigment were similar. The model agreed closely with experimental results for two pigment types at various pigment loadings.

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“Experimental quantification of differences in damage due to in-plane tensile test and bending of paperboard”, Gustav Marin, Mikael Nygårds & Sören Östlund.

Packaging Technology and Science, published online (2021). Creasing is an essential process to convert paperboards into packages since it enables folding along well-defined lines. The creasing process relies on purpose-made damage that is initiated in the paperboard structure: delamination. However, creasing might also cause in-plane cracks, which must be avoided. In this laboratory study, three paperboards were creased at six different depths, respectively. Two mechanical tests were performed to characterize the creases at standard climate (23°C and 50% RH): 2-point folding, to examine the bending force and short-span in-plane tensile test to evaluate the strength. The results were normalized with the values for the uncreased boards, which gave the relative strength ratios: relative creasing strength (RCS) and relative tensile strength (RTS). When the relative strengths were evaluated against the normative shear strains, a creasing window was formed. This window has an upper limit given by the RTS values, corresponding to the in-plane cracks, and a lower limit given by the RCS values, corresponding to the delamination damage initiated in the paperboard during creasing. It was observed that both the RCS and RTS values exhibit a linear relation against normative shear strain.

From this, it was concluded that performing tests at two creasing depths might be sufficient to estimate the lower, and upper, limits for the creasing window in future studies. Finally, the effect of moisture was investigated by creasing, folding and tensile testing at 23°C and 90% RH, which showed that moisture had no clear effect on the RCS or the RTS values.

TISSUE “Dust Exposures in Swedish Soft Tissue Paper Mills”, Richard L Neitzel, Marianne Andersson, Susanna Lohman, Gerd Sällsten, Kjell Torén & Eva Andersson, Annals of Work Exposures and Health, 2021, https://doi.org/10.1093/annweh/wxab063. Paper dust has previously been linked to adverse health effects. However, a comprehensive dataset of paper dust exposures does not appear to have been published previously. Our study was intended to address this need by describing a large dataset of measurements made in Swedish soft tissue paper mills. Our analysis of measured paper dust exposures may be useful for historical and contemporary exposure assessment in our own and other epidemiological studies. We have identified specific characteristics (i.e. papermaking operations and mill) and time trends that are important data features to consider, and documented continuing overexposure situations. Our results highlight the ongoing need for application of exposure controls to reduce paper dust exposures in the soft tissue paper industry.

WASTE TREATMENT “A review of process and wastewater reuse in the recycled paper industry”, Ngoc Han, Jianhua Zhang & Manh Hoang, Environmental Technology & Innovation, Vol.24, November 2021, 101860. Water plays several essential roles in paper manufacturing. It serves as a suspending medium and a swelling agent for the fibres, dispersing and forming them into a uniform sheet during the initial stage of the papermaking process. It also serves as the solvent for a variety of chemicals and additives to adjust product quality. Water reclamation has always been a momentous task in the pulp and paper (P&P) industry. The main driving forces for the adoption of process water and wastewater treatment technologies are environmental regulations, costs of wastewater discharge and the high cost of freshwater. Recent developments have made it possible to not only reduce water consumption and environmental impacts, but also to recover treated water and valuable compounds such as fibres, making water recycling technologies cost-efficient. Thus, the economic viability of these technologies has played

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an important role in their application. The technologies applied, the level of reduction in water consumption and the extent of water recycling are different for each mill, since the quality of whitewater and wastewater varies depending on the raw materials and products.

The maintenance of the balance between partial equilibria of process variables such as water flowrates, pulp consistency, physiochemical, thermal and microbiological properties through water management is important for maintaining efficient water use and lowering the need for consumption of additional fresh water.

“Monitoring of papermaking wastewater treatment processes using t-distributed stochastic neighbor embedding”, Xiaobo Ma, Yuchen Zhang, Fengshan Zhang & Hongbin Liu, Journal of Environmental Chemical Engineering, Vol.9(6), 2021, 106559. A combination of t-distribution stochastic neighbor embedding with a Gaussian mixture model (t-SNE-GMM) is proposed for sensor fault detection in wastewater treatment processes. The proposed method can be used to handle the non-Gaussian and nonlinear characteristics of wastewater treatment processes simultaneously. The t-SNE method is first used to reduce the dimension of process data, and then GMM only uses the normal process data to accomplish fault detection. With manifold learning, the hybrid model can reduce computation complexity and improve detection accuracy. Two methods that combined GMM with principal component analysis (PCA-GMM) and kernel PCA-GMM (KPCA-GMM) are used for comparing with t-SNE-GMM. The fault detection performance was verified by simulating sensor faults in the wastewater treatment process. Among them, the fault detection rates of bias fault, drifting fault, and complete failure fault using tSNE-GMM are increased by 67.8%, 5%, and 109.52%, respectively, compared with KPCA-GMM. Although the improvement of the fault detection rate of drifting faults is not obvious, it has an excellent performance in the false alarm rate. The combined method has the capability of detecting the sensor faults in the wastewater treatment process.

WOOD PANEL “Recent applications of nanoparticles in wood-based panels”, Nadir Ayrilmis, Machines. Technologies. Materials. Vol.15(7), pp.287-290, pp.287-290, (2021).

Nanocellulose applications in the wood-based panels have gained a great deal in the scientific researches and industrial applications. Utilization of natural and synthetic nanoparticles as reinforcement in the wood-based panels has considerably increased in the last two decades due to their unique properties. The main property of the nanocellulose is its very high surface area. Hereby, the very small use of nanoparticles such as 1-2 wt% in the composites is enough at a relatively low-cost. Nanoparticles are presently considered to be high-potential reinforcing fillers for the enhancement of the physical, mechanical, electrical/electronic properties, thermal resistivity, fire, durability properties of wood-based panels such as particleboard, fibreboard, oriented strandboard, and plywood. The nanoparticles are applied to wood based panels during the manufacture and after production. The raw materials such as wood or resin can be treated with nanoparticles or the finished panels can be treated with nanoparticles. In this study, the recent developments in the nano particles, their applications in the wood based panels, and their effects on the panel properties were reviewed “Structural Insulation Materials from Plant Resources for Building”, Aung Htut Thu & Alexander I. Zakharov, Macromolecular Symposia, online. Nowadays, researchers are focusing on the methods of using recycled industrial/agricultural wastes as raw materials, which are not only economically but also environmentally useful. In the recent years, many scholars have conducted research on the rational use of rice husks (RHs) and found that RHs are rich in inorganic/organic components and used in combination with

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Volume 7, Number 3, 2021

the certain building materials to produce new green composite building materials. This paper studies the method of using RH processed products and the sodium silicate solution (liquid glass) made from RH ash to produce cheap wood substitute composite materials, which can be improved the insulation performance of the wall greatly. Obtained results are determined by comparing with the properties of standard polymer composite materials such as particleboards, chipboards, and medium-density fiberboards (MDF). The significance of the work is pointing out by solving the problem of recycling large-tonnage agricultural waste and reproduce products with the consumer's value.

“Preparation of an environment-friendly fiberboard with high mechanical strength using delignified wood fiber”, Tianxiang Yuan, Wenxin Du, Kaiwen Bai, Dongxuan Huang, Tat Thang Nguyen, Jingjing Li & Xiaodi Ji, Vacuum, online (2021). This paper reports environment-friendly fiberboard with high mechanical strength using delignified wood fibers and chitosan adhesive. The influence of delignification on chemical structure and microstructure of wood fibers and mechanical strength and water resistance of fiberboards was studied. The results showed delignification effectively removed hemicellulose and a part of lignin, decomposed some large-sized lignin into small fragments distributed on wood fiber surface, and exposed cellulose bundles. The mechanical strength and water resistance of fiberboards using wood fibers delignified for 3 h reached optimal and were far higher than the requirement of the Chinese national standard. The large improvement in internal bonding strength (IB) and thickness swell (24 h TS) was mainly attributed to the delignification, while that in modulus of rupture (MOR) and modulus of elasticity (MOE) was principally due to the addition of chitosan adhesive.

This study can open a new path for production of scalable environment-friendly highperformance wooden composite.

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Technical Abstracts