Polímeros: Ciência e Tecnologia (Polimeros) 3rd. issue, vol. 29, 2019

ISSN 0104-1428 (printed) ISSN 1678-5169 (online)

P o l í m e r o s - I ss u e I I I - V o l u m e X X I X - 2 0 1 9 I n d e x e d i n : “ C h e m ic a l A b s t r a c t s ” — “ RA P RA A b s t r a c t s ” — “A l l - R u s s i a n I n s t i t u t e o f S ci e n c e ­T e c h n ic a l I n f o r m a t i o n ” — “ R e d d e R e v i s t a s C i e n t i f ic a s d e A m e r ic a L a t i n a y e l C a r i b e ” — “ L a t i n d e x ” — “ W e b o f S ci e n c e ”

and

Polímeros E d i t o r i a l C o u nci l Antonio Aprigio S. Curvelo (USP/IQSC) - President

Editorial Committee Sebastião V. Canevarolo Jr. – Editor-in-Chief

Members Adhemar C. Ruvolo Filho (UFSCar/DQ) Ailton S. Gomes (UFRJ/IMA) Alain Dufresne (Grenoble INP/Pagora) Antonio Aprigio S. Curvelo (USP/IQSC) Bluma G. Soares (UFRJ/IMA) César Liberato Petzhold (UFRGS/IQ) Cristina T. Andrade (UFRJ/IMA) Edson R. Simielli (Simielli - Soluções em Polímeros) Edvani Curti Muniz (UEM/DQI) Elias Hage Jr. (UFSCar/DEMa) Eloisa B. Mano (UFRJ/IMA) João B. P. Soares (UAlberta/DCME) José Alexandrino de Sousa (UFSCar/DEMa) José António C. Gomes Covas (UMinho/IPC) José Carlos C. S. Pinto (UFRJ/COPPE) Júlio Harada (Harada Hajime Machado Consutoria Ltda) Laura H. de Carvalho (UFCG/DEMa) Luiz Antonio Pessan (UFSCar/DEMa) Luiz Henrique C. Mattoso (EMBRAPA) Marco-Aurelio De Paoli (UNICAMP/IQ) Osvaldo N. Oliveira Jr. (USP/IFSC) Paula Moldenaers (KU Leuven/CIT) Raquel S. Mauler (UFRGS/IQ) Regina Célia R. Nunes (UFRJ/IMA) Richard G. Weiss (GU/DeptChemistry) Rodrigo Lambert Oréfice (UFMG/DEMET) Sadhan C. Jana (UAKRON/DPE) Sebastião V. Canevarolo Jr. (UFSCar/DEMa) Silvio Manrich (UFSCar/DEMa)

A ss o ci at e E d i t o r s Adhemar C. Ruvolo Filho Alain Dufresne Bluma G. Soares César Liberato Petzhold José António C. Gomes Covas José Carlos C. S. Pinto Paula Moldenaers Richard G. Weiss Rodrigo Lambert Oréfice

Sadhan C. Jana

D e s k t o p P u b l is h in g

www.editoracubo.com.br

“Polímeros” is a publication of the Associação Brasileira de Polímeros São Paulo 994 St. São Carlos, SP, Brazil, 13560-340 Phone: +55 16 3374-3949 emails: abpol@abpol.org.br / revista@abpol.org.br http://www.abpol.org.br Date of publication: September 2019

Financial support:

Available online at: www.scielo.br

Polímeros / Associação Brasileira de Polímeros. vol. 1, nº 1 (1991) -.- São Carlos: ABPol, 1991Quarterly v. 29, nº 3 (July/Sept. 2019) ISSN 0104-1428 ISSN 1678-5169 (electronic version)

Website of the “Polímeros”: www.revistapolimeros.org.br

1. Polímeros. l. Associação Brasileira de Polímeros.

Polímeros, 29(3), 2019

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E E E E E E E E E E E E E E E E E E E E E E E E E E E E

I I I I I I I I I I I I I I I I I

Editorial Section News....................................................................................................................................................................................................E3 Agenda.................................................................................................................................................................................................E4 Funding Institutions.............................................................................................................................................................................E5

O r i g in a l A r t ic l e System to measure torsion modulus of polymers using the deformation energy method Carlos Alberto Fonzar Pintão, Lucas Pereira Piedade and Edgar Borali....................................................................................................... 1-8

Silver nanoparticles incorporated PVC films: evaluation of structural, thermal, dielectric and catalytic properties Ganesh Shimoga, Eun-Jae Shin and Sang-Youn Kim....................................................................................................................................... 1-9

Controlled release fertilizer encapsulated by a κ-carrageenan hydrogel Gladys Rozo, Laura Bohorques and Johanna Santamaría............................................................................................................................... 1-7

Effect of moringa and bagasse ash filler particles on basalt/epoxy composites Prakash Sampath and Senthil Kumar Velukkudi Santhanam............................................................................................................................ 1-7

Plastics floatability: effect of saponin and sodium lignosulfonate as wetting agents Fernando Pita and Ana Castilho...................................................................................................................................................................... 1-9

Effects of miniemulsion operation conditions on the immobilization of BSA onto PMMA nanoparticles Izabella Campos, Thamiris Paiva, Helen Ferraz and José Carlos Pinto....................................................................................................... 1-10

Grafting polypropylene over hollow glass microspheres by reactive extrusion Carlos André Baptista and Sebastião Vicente Canevarolo............................................................................................................................... 1-9

Influence of water absorption on glass fibre reinforced IPN composite pipes Suresh Gopi, Ganesh Babu Loganathan, Bharani Kumar Sekar, Rajesh Kanna Krishnamoorthy, Vivek Sekaran and Akash Rajendran Mohan................................................................................................................................................................................... 1-8

In vitro evaluation of PVA gels loaded with Copaiba Oil and Duotrill

Ingrid Cristina Soares Pereira, Natália Rodrigues Rojas dos Santos, Antonieta Middea, Edlene Ribeiro Prudencio, Rosa Helena Luchese, Ana Paula Duarte Moreira and Renata Nunes Oliveira................................................................................................................................... 1-8

Crosslinking starch/oat hull mixtures for use in composites with PLA Thamires da Silva Peixoto, Fabio Yamashita, Ana Paula Bilck, Gizilene Maria Carvalho and Maria Victoria Eiras Grossmann................. 1-8

Influence of ZnO on the properties of elastomeric compositions and their leached extract Daiane Torani, Janaina da Silva Crespo and Rosmary Nichele Brandalise.................................................................................................... 1-8

Synthesis and characterization of microalgae fatty acids or Aloe vera oil microcapsules Luiza Brescovici Badke, Bruno Campos da Silva, Agne Roani de Carvalho-Jorge, Dhyogo Mileo Taher, Izabel Cristina Riegel-Vidotti and Cláudia Eliana Bruno Marino.......................................................................................................................................................................... 1-9

Influences of the mesh in the CAE simulation for plastic injection molding Felipe Marin, Adriano Fagali de Souza, Rodolfo Gabriel Pabst and Carlos Henrique Ahrens..................................................................... 1-10

Extraction and analysis of the parietal polysaccharides of acorn pericarps from Quercus trees Moubarek Mébarki, Kadda Hachem, Céline Faugeron-Girard, Riad el Houari Mezemaze and Meriem Kaid-Harche.................................. 1-4

Bionanocomposites of PLA/PBAT/organophilic clay: preparation and characterization Josiane Dantas Viana Barbosa, Joyce Batista Azevedo, Edcleide Maria Araújo, Bruna Aparecida Souza Machado, Katharine Valéria Saraiva Hodel and Tomas Jefferson Alves de Mélo........................................................................................................................................ 1-10

Cover: SEM images of cryogenically fractured composites: uncompatibilized PP/HGM composite showing clean detached HGM particle from the PP matrix. Overview of the mesh density according to the maximum segment length. Arts by Editora Cubo.

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Polímeros, 29(3), 2019

New vulcanization accelerator from LANXESS The specialty chemicals company LANXESS has developed a universally suitable vulcanization accelerator for tires and technical rubber goods. The product is a sulfenamide based on aromatic amines, and is suitable for all types of rubber.

“We are now inviting our industrial customers to try our new high-performance accelerator for themselves. We are already producing pilot-scale samples,” says Dr. Jens-Hendrik Fischer, who is responsible for the global antioxidants and accelerators business in the Advanced Industrial Intermediates (AII) business unit. At K 2019, the international trade show for plastics and rubber taking place in Düsseldorf from October 16 to 23, LANXESS has showcased the new accelerator for the first time.

The trial product has an impressively long scorch time.

“There is no premature onset of the vulcanization process for the rubber, which has to pass through pipelines in the mixing process, and consequently it does not stick to sections of the production plant. This particular property of our trial product thus ensures a smooth production process and homogeneous cross-linking, which means that the structure can cure evenly,” explains Melanie Wiedemeier-Jarad, Technical Service Manager for the AII Antioxidants & Accelerators (AXX) unit. For all mixtures produced using a sulfenamide vulcanization accelerator, the cure times tend to be short in relation to the scorch times – making this group of accelerators suitable not only for press heating, but also for injection and transfer molding processes, which are used primarily for complex geometries.

Source: LANXESS AG - lanxess.com

Celanese Delivers Broadest Engineered Materials Portfolio to Support Automakers’ Targets Polymers from Celanese enable automakers to create solutions for emerging EV, autonomous driving and flexibility‑in‑interior design platforms. With electric and hybrid vehicles gaining consumer attention and momentum, auto manufacturers are challenged to attract buyers using better systems and designs. Partnering with a global materials supplier with the technical and application expertise is critical for the development of these mobility, EV and hybrid auto platforms.

Celanese, a global chemical and specialty materials company, is able to provide the broadest portfolio of polymers that help original equipment manufacturers (OEMs) secure a competitive edge with more efficient, automated, lightweight, and better‑designed vehicles. “The transition to emission-free mobility is gaining attention from both consumers and legislators.

Automakers’ answer to regulators, investors and drivers who collectively seek improving energy efficiency, automation, design and performance – all while keeping costs down – will be critical to their success of attracting global car buyers. Celanese, having the broadest portfolio in the industry, provides solutions

independent of product.” said Stefan Kutta, Vice President of EMEA Commercial Operations, Celanese. Automotive OEMs, tier suppliers and molders worldwide use Celanese polymers as effective and sustainable materials of choice for a variety of under-the-hood, interior and exterior parts. These OEMs are seeking an engineered materials partner who can achieve four key manufacturing strategies: • Solutions for Electric Vehicles (EV): Efficient energy consumption is critical to emerging EV platforms. From improving battery performance, to light-weighting battery housings, to thermal management, Celanese provides high performance polymers to enable electrification of the automotive powertrain and provide higher energy output, but also production efficiency even in smaller electric engines that allow manufacturers to increase the range of an electric vehicle and to reduce cost and weight.

• Solutions for Passive Automation Driver Assistance Systems (ADAS): With increasing automation level in vehicles, new systems like cameras, radar, LIDAR (light detection and ranging), and high speed data connection systems come into play which places extreme requirements on polymer materials. Celanese offers precision fit and low CLTE materials for key technologies which meet demanding requirements like high dimension stability, chemical resistance, specific di-electric properties, as well as flame retardancy and low emissions for reliable connectivity solutions.

• Solutions for Driver Experience and Flexibility of Interior Design: As vehicles see increasing and more severe usage of interior control and communication systems due to autonomous driving and ridesharing, automakers require durable surface materials at lower cost while not sacrificing appearance and quality. Celanese offers polymers which reduce interior cabin noise and weight for an experience friendly to occupants and beneficial to fuel economy, with solutions ranging from low-friction modified copolymers, composite materials for metal replacement, and high-flow elastomers for complex liners and seals.

• Solutions for recycled demands: In addition to the ongoing trend for light-weighting solutions with the shift from metal to plastic, there is the need to meet OEM’s recyclability goals with the demand for polymer solutions to replace prime material with eco-friendly recycled grades. Celanese’s new polyamide solutions provide an excellent balance of strength, ductility, temperature stability and a wide processing window. “We’re our auto customers’ first choice solution source for engineered materials because we fulfill their immediate needs to improve their environmental profile while meeting demanding applications requirements,” concluded Kutta. “Experience across automotive systems and polymer profiles enables Celanese to partner with customers to ‘rethink’ how to design critical, complex systems with chemical resistant, dimensionally stable and sustainable solutions.”

Source: Celanese - celanese.com

Polímeros, 29(3), 2019 E3

N N N N N N N N N N N N N

A A A A A A A A A A A A A A A A A A A A A

February Layered Polymeric Systems Date: February 23-26, 2020 Location: Windsor, United States Website: www.polyacs.net/20lps 3rd World Congress on Bio-Polymers and Polymer Chemistry Date: February 24-25, 2020 Location: Rome, Italy Website: polymerchemistrycongress.alliedacademies.com 10th Edition of International Conference on Biopolymers & Bioplastics Date: February 24-25, 2020 Location: Dubai, United Arab Emirates Website: biopolymers-bioplasticsconference.euroscicon.com 2nd International Conference on Plastic and Polymers, Robotics, Applied Sciences, Design Engineering & Artificial Intelligence Date: February 26-27, 2020 Location: Kuala Lumpur, Malaysia Website: aet-forum.com/prada-feb-2020

Mach 14th Expo Plasticos Date: March 10-13, 2020 Location: Guadalajara, Mexico Website: http://expoplasticos.com.mx/2020 4th International Conference on Materials Engineering and Nano Sciences (ICMENS 2020) Date: March 13-15, 2020 Location: Pattaya, Thailand Website: www.icmens.org World Congress on Carbon and Advanced Energy Materials Date: March 16-17, 2020 Location: Sydney, Australia Website: global.materialsconferences.com 5th International Conference and Exhibition on Polymer Chemistry Date: March 23-24, 2020 Location: London, United Kingdom Website: polymerchemistry.euroscicon.com 10th Edition of International Conference on Biopolymers & Bioplastics Date: March 23-25, 2020 Location: London, United Kingdom Website: biopolymers-bioplastics.euroscicon.com Advanced Functional Materials Congress (AFMC 2020) Date: March 23-25, 2020 Location: Stockholm, Sweden Website: www.advancedmaterialscongress.org/mar20/

April International Symposium on Nanostructured, Nanoengineered and Advanced Materials (ISNNAM 2020) Date: April 30 – May 3, 2020 Location: Johannesburg, South Africa Website: isnnam.org

May International Conference on Biomedical Engineering, Bioinformatics, IT, Polymers and Plastics & Engineering Management Date: May 16-17, 2020 Location: Istanbul, Turkey Website: society-eas.com/bbpp2020 Sustainable Polymers Date: May 17-20, 2020 Location: Safety Harbor, United States Website: www.polyacs.net/20sustainablepolymers

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36th International Conference of the Polymer Processing Society (PPS-36) Date: May 31- June 4, 2020 Location: Montreal, Canada Website: www.polymtl.ca/pps-36/en

June Plastic Closure Innovations Conference Date: June 1-3, 2020 Location: Barcelona, Spain Website: www.ami.international/events/event?Code=C1054 8th Polymer Foam Date: June 28 – July 1, 2020 Location: Denver, United States Website: www.polyacs.net/20fluoropolymer Fluoropolymer Date: June 23-24, 2020 Location: Pittsburgh, United States Website: www.polyacs.net/20fluoropolymer International Conference on Advanced Nanomaterials (ICANM 2020) Date: June 23-25, 2020 Location: Perth, Australia Website: www.icanm.net

July 48th World Polymer Congress (IUPAC - MACRO2020) Date: July 5-9, 2020 Location: Jeju Island, South Korea Website: www.macro2020.org Frontiers of Polymer Colloids (FPCOL 2020) Date: July 12-16, 2020 Location: Prague, Czech Republic Website: www.imc.cas.cz/sympo/84pmm

August Interplast Date: August 11-14, 2020 Location: Joinville, Brazil Website: www.interplast.com.br

September 9th International Conference on Fracture of Polymers, Composites and Adhesives Date: September 6–10, 2020 Location: Les Diablerets, Switzerland Website: www.elsevier.com/events/conferences/esistc4conference 11th Conference on Modification, Degradation and Stabilization of Polymers (MoDeSt 2020) Date: September 6–10, 2020 Location: Berlin, Germany Website: www.bam.de/Content/EN/Events/2020/2020-09-06-10modest.html

October 59th Tutzing Symposion 2020: Polymers for a better life and circular economy Date: October 26–28, 2020 Location: Tutzing, Germany Website: dechema.de/en/tusy59.html

November Wood-Plastic Composites Date: November 4–5, 2020 Location: Vienna, Austria Website: www.ami.international/events/event?Code=C1071 Controlled Radical Polymerization Date: November 15–18, 2020 Location: Charleston - United States Website: www.polyacs.net/crp2020

Polímeros, 29(3), 2019

ABPol Associates Sponsoring Partners

Collective Members Master Polymers Ltda.

Nexo International Ltda.

Nitriflex S/A Ind. e Com. Radici Plastics Ltda.

Uniflon - Fluoromasters Polimeros Ind .Com. Imp. Export.Ltda

PolĂ­meros, 29(3), 2019

E5

UBE lança ETERNATHANE®, pré-polímeros de poliuretano à base de policarbonato-diol para elastômeros de alto desempenho e durabilidade A UBE é uma indústria multinacional Japonesa que atua nos setores de químicos, máquinas, fármacos, energia e construção. Com escritórios ao redor do mundo e fábricas no Japão, Tailândia e Espanha, há um destaque na produção de caprolactama, poliamidas, fertilizantes e produtos químicos nos.

O poliuretano para elastômeros tornou-se cada vez mais soosticado para atender às exigências do mercado atual. Neste contexto, a UBE desenvolveu o ETERNACOLL® e o ETERNATHANE®, uma grande plataforma de soluções que oferecem possibilidades personalizáveis aos materiais de poliuretano, bem como retenção de desempenho superior e a longo prazo, como estabilidade térmica, resistência a óleo, estabilidade hidrolítica, resistência à intempéries e resistência química.

retenção das propriedades mecânicas após exposição a altas temperaturas

redução da absorção de água

retenção das propriedades originais após severa agressão hidrolííca e química

redução da perda de volume quando exposto à abrasão extrema

Os pré-polímeros de poliuretano ETERNATHANE®, à base de policarbonato-dióis ETERNACOLL® e terminados em isocianatos, são aplicados em elastômeros de alto desempenho. Através do aprimoramento das propriedades de resistência mecânica, química e térmica dos poliuretanos tradicionais, os novos elastômeros obtidos podem ser aplicados a novos usos e funções não disponíveis até o momento para novos mercados e clientes, tais como: petróleo e mineração, revestimento de rolos, membranas elastoméricas, pisos, elastômeros fundidos, TPU, rodas e pneus, compostos de poliuretano, selantes, eletrônicos e encapsulamento, entre outros. poliu

https://www.ube.com/contents/pcd/index.html

Rua Iguatemi, 192, cj. 134 Itaim Bibi - São Paulo +55 11 30785424

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.01019

System to measure torsion modulus of polymers using the deformation energy method Carlos Alberto Fonzar Pintão1* , Lucas Pereira Piedade1  and Edgar Borali1  Departamento de Física, Faculdade de Ciências, Universidade Estadual Paulista – UNESP, Bauru, SP, Brasil

1

*carlos.fonzar@unesp.br

Abstract This paper presents an alternative method to measure the torsion modulus, G, for samples of polymers. We constructed a measurement system with a force sensor (FS) and a rotational movement sensor (RMS) to obtain a relationship between force (F) and torsion angle (θ). An expression that could return the value of G was deduced using the deformation energy method. This technique is nondestructive and independent of knowing the value of Poisson’s ratio. Samples with different diameters of polytetrafluoroethylene (PTFE) were submitted to quasi-static torsion at the same aspect ratio. The aim was to present and validate the use of the technique for a known polymer. The approximate value of 350 MPa of the torsion modulus G was found for PTFE samples. As the values obtained are within the limits found in the literature, the technique can be used to study samples of polymers and other materials.

Keywords: deformation energy, force sensor, polytetrafluoroethylene (PTFE), rotational movement sensor, torsion modulus.

How to cite: Pintão, C. A. F., Piedade, L. P., & Borali, E. (2019). System to measure torsion modulus of polymers using the deformation energy method. Polímeros: Ciência e Tecnologia, 29(3), e2019031. https://doi.org/10.1590/01041428.01019

1. Introduction In order to predict the behavior of materials when subjected to stresses or loads, it is necessary to know their characteristics, such as rigidity (elastic modulus). This paper uses a specially designed system to determine the torsion modulus (G) of polymers. We have chosen the polymer polytetrafluoroethylene (PTFE), a well-known fluoropolymer, to validate the use of the technique. The properties of this polymer make it suitable for use in aerospace applications[1] and the biomedical industry, for implants[2-4], making it essential to know its elastic properties, such as the torsion modulus. In 1946, Renfrew and Lewis[5] was one of the first to report a few mechanical parameters of PTFE, followed later by Thomas et al.[6], who published its tensile properties related to crystallinity. Brown and Parrish[7] investigated the tensile behavior of PTFE in a liquid nitrogen environment, showing a decrease in strength, and Kletschkowski et al.[8] explored the elastic behavior of filled PTFE to develop models for seal materials. Rae and Brown[9] found that some grades of PTFE are sensitive to strain-rate, temperature, and crystallinity under tension.

Dynamic torsion tests can also be used to characterize the viscoelastic properties of polymers[10]. Andreozzi et al.[11] proposed a new procedure for measuring the shear modulus of laminated glass interlayers using a rheometer as a simpler and more reliable test than those presently in use. In a recent article[12], the authors use a dynamic technique based on mechanical spectroscopy knowledge[13] and apply it to biomaterials, determining the torsion modulus. The same

Polímeros, 29(3), e2019031, 2019

technique can be used to measure G in polymeric materials.

However, this technique showed some limitations regarding the sample sizes, which were limited, and difficulty in obtaining relaxation curves of ductile polymer materials.

Therefore, we developed an apparatus that is fixed to the same torsion pendulum used in the dynamic measure[12] and which will be used as a means to overcome the limitations related to the dynamic method. In this work, we intend to introduce and apply a quasi-static technique to measure the shear modulus (G) of PTFE and, further on, of other polymers and materials. By using a force sensor (FS) attached to a coordinated table which is driven by Stepper motor, and a rotational motion sensor (RMS) capable of recording the torsion angle (θ), we obtain the curve force (F) versus torsion angle (θ). Circular cross-sections of PTFE samples with different diameters are subject to a quasi-static torsion and a linear adjustment from their curves provides a way to calculate the value of the torsion modulus. Although this technique uses concepts from the classical torsion test to determine the shear behavior of a material[14-16], in this work we present a new approach for obtain G, which uses concepts such as torsion deformation energy and force work, with equation deduction especially for the system and measure. Also, the technique is consider nondestructive and different from the traditional quasi-static tensile test, where the information on the Poisson coefficient is needed to obtain G, this technique does not require knowledge of Poisson’s ratio, making it of great practical interest.

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2. Materials and Methods

2.1 Density, ρ

Cylindrical bars extruded of pure, commercial PTFE supplied by DuPont with dimensions of 10.85x500 mm, 8.00x500 mm, 6.50x500 mm, and 5.00x500 mm were used. Theses bars with different diameters d were cut and fixed in the measuring system under the same ratio L/d = 10.64, where L is effective length and then subjected to quasi-static torsion. One end of the sample was fixed to the pendulum with screws while the other was attached to a static three-jaw chuck, which allows adjustment of the clamping for different sample diameters (Figure 1). A detailed description of the system and measures is provided in section 2.4. For scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) measurements, the samples were cleaned in the ultrasonic bath for 600s at 50W. These samples were only sawed and did not undergo any sanding or polishing process to avoid adhesion of abrasive grains or other impurities to the material, which could lead to misinterpretations of the results.

PTFE is a semi-crystalline fluoropolymer used as a high-performance material, with a low coefficient of friction, chemical inertia, and thermal stability properties.

The carbon-fluorine binding force, together with the protection of the fluorine atoms surrounding its carbon chain, is responsible for these properties[17]. The material exhibits four phases dependent on temperature and pressure, with the configuration of orthorhombic crystal as the only one not reached at atmospheric pressure[18,19]. Its high melt viscosity (1011 Pa at 380°C) prevents pieces of this material being obtained by traditional methods of injection and molding, with extrusion being one of the processes available for its production[20-22]. Depending on the processing route (thermal history) used to obtain PTFE, the crystalline percentage of the material can be changed, which has a known influence on its mechanical properties[9,18,23]. Thus, it is essential to estimate the degree of crystallinity of the material when subjected to mechanical testing. Several methods to estimate crystallinity can be found in the literature[1,24-27], of which DSC was used in our work.

The PTFE’s density was obtained through the Archimedes principle[28], a method based on volume displacement with liquid (vF − v0) and sample mass (m), according to Equation 1: ρ=

m (1) v f − v0

A 10.0 ml graduated glass cylinder was used with 5.0 ml of deionized water. The mass of the polymer was measured through an analytical balance in an air-conditioned environment at 25°C. By inserting a piece of extruded PTFE into the graduated cylinder and obtaining the volume of the displaced liquid, the density of the polymer is calculated

by Equation 1. Three samples of each extruded bar were measured, and their mean density value is ρ = 2156.3 kg m−3.

2.2 Scanning electron microscopy SEM was performed on a Zeiss, model EVO LS1. Samples were coated with gold for visualization. Measurements were made in high vacuum (10−3 Pa) with magnifications of 250x, 1000x, and 2000x of the internal region in longitudinal section of the PTFE bar. The aim was to visualize defects in the extrusion process, such as bubbles and cracks, which could compromise test results.

2.3 Differential scanning calorimetry The DSC curve was obtained in the Mettler-Toledo model DSC 1 Staree System, using closed 40 µl crucibles with a hole in the cover. The sample mass was 14.46 mg, heated from 25°C to 350°C at a ratio of 10°C min−1 under dry air atmosphere with 50 ml min−1 flow.

The crystalline percentage can be estimated by calculating the ratio of the heat of fusion (∆HF) obtained from the DSC measurements and the heat of fusion of a theoretical 100% crystalline sample (∆H0F), Equation 2:

Figure 1. (a) System for fixing samples: (1) shaft of the pendulum with fixing screws; (2) PTFE sample; and (3) three-jaw chuck. (b) Schematic drawing of the measuring system: (c) schematic drawing of the sample showing the effective length L and the oriented axis.

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Polímeros, 29(3), e2019031, 2019

System to measure torsion modulus of polymers using the deformation energy method

XC =

∆H f ( sample) ∆H 0f

(2)

A wide range of values for the PTFE heat of fusion can be found in the literature, as shown by Lehnert et al.[27].

To calculate the crystalline percentage of our material, we use a mean of these values (80 J g−1), as did Rae and Brown[9] and Jordan et al.[1] Also, the DSC was used to determine the melt temperature (Tm) and the PTFE phase transitions at ambient pressure.

2.4 Measurement of torsion modulus G The system used to apply the torque (Mt) on one of the pendulum arms is shown in Figure 2. It consists of a coordinated table attached to a plate, which is fixed in relation to the structure of the torsion pendulum (Figure 2a and b).

The sample is attached to the system as described in section 2, with the system previously positioned at the desired effective length L. Using an inextensible wire attached at one end to the pendulum upper rod and the other to the FS (FS-PASCO CI6537, 0.0305 newtons of resolution), it is possible to apply and measure the force. We used a 12V Stepper motor (60 Hz, 4 W, 3.3 RPM) to move the coordinated table in the two required directions: one for approximation and the other to draw it away from the pendulum (Figure 2d), twisting the sample at a rate of 3 x 10-4 s-1 (estimated value of (θ* x 0.222)/120 s). The pendulum was placed at an initial position perpendicular to the sensor set to be pulled (Figure 2a and b). Using an RMS (RMS-PASCO CI6538, 1º and 0.25º of resolution ± 0.09 degree accuracy) capable of recording the torsion angle of the axis (θ) that holds one end of the sample, the force curves (FS) versus torsion

angle (θ) are obtained. The two sensors were connected at an interface to a computer. Using the software PASCO, the experimental points for F (N) and θ (rad) were obtained simultaneously in real time. We can thus adjust the straight line and obtain the slope of B, because the points have linear behavior in a region where the material shows elastic behavior. The slope B has the units N/rad. A correction must be made in relation to the angle measured in the rotation sensor (θ*) and the torsion angle in the sample (θ), due to the difference in diameters between the pendulum’s metal rod (which transmits motion) and the RMS, as shown in Figure 2c. The ratio (θ/θ*) of these angles is obtained experimentally and is 0.222 ± 0.001. This means that the angle measured in the RMS is greater than that of the torsion in the sample studied. Thus, slope B should also be calculated as B*/0.222. All samples were twisted at room temperature up to approximately 0.16 rad (9º) on the RMS, which equates as 0.0355 rad (2º) in the PTFE samples by the ratio correction. Three measurements were made for each sample diameter, with the RMS configuration in high resolution (division/revolution = 1440) and the sample rate of 1 Hz and the FS configuration in high resolution (100x).

To carry out these measurements we had to obtain a calibration factor, f, because the value of measured force (FMEASURED = FS) in the FS is different from that of applied force (FAPPLIED). Factor f can be obtained experimentally by applying known forces (FAPPLIED) and measuring the forces with the FS (FMEASURED). For this, we used known masses: 0.020, 0.050, 0.100, 0.200, 0.500, and 1.00 kg. They were fixed at one end of an inextensible cord passing over a pulley at the other end and attached to the FS. The local gravity is g = 9.79 ± 0.01 m s-2.

Figure 2. (a) System for measuring G: (1) interface (PASCO: CI7650-750); (2) force sensor (FS-PASCO: CI6537); (3) motor for moving coordinated table; (4) torsion pendulum structure for coordinated table attachment; (5) pendulum’s metal rod; (6) shaft pendulum attached to the sample; (7) rotational movement sensor (RMS-PASCO: CI6538); (8) coordinated table; and (9) torsion pendulum. (b) Photo of the coordinated table attached to a plate, fixed in relation to the structure of the torsion pendulum. (c) Photo demonstrating the difference in diameters to calculate the ratio θ/θ*. (d) Schematic drawing showing some of the parameters used in section 2.5.

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Pintão, C. A. F., Piedade, L. P., & Borali, E. 2.5 Deformation energy method for obtaining G Consider an elastic structure subjected to applied loads and deformed elastically. In this deformation process, the principle of energy conservation[29] applies, expressed as: WE + Q = ∆E

(3)

WE is the work carried out by applied external forces, Q is the heat exchanged by the structure with its surrounding area and ΔE is the variation in the associated deformation energies of the structure: kinetic energy of the particles (K) and internal energy (U). Considering that the increase in these loads is gradual (rate of 3 x 10-4 s-1) and that a state of equilibrium is maintained in this process, then the variation of kinetic energy is zero, because there is no movement of either the amorphous or crystalline regions that occurs during plastic deformation[30]. Although there is a slight movement of adjacent molecules, the variation is so small that it can be neglected. We assume the hypothesis that there is only elastic deformation due to the small deformation at which the test is performed (0.0355 rad). Thus, ΔE is due only to the variation in internal energy, U. In these conditions, Equation 1 is reduced to: WE = ∆U (4)

The work, which can be considered as energy stored in the structure due to torsion in an element of infinitesimal volume[29,31], is represented by strain and stress tensors, σij and eij, respectively. The energy, dU, stored in this element when the deformation has reached its final value eij is: eij

dU = ∫ σ ij deij (5) 0

By integrating the entire volume, V, of the structure, we obtain the total internal energy, U, due to torsion, which is expressed as: eij

U = ∫ ( ∫ σ ij deij ) dV (6) 0

In cases where the elastic structure has linear behavior, it is isotropic, and it is subjected to pure torsion (Figure 1c).

Using Hooke’s law[29], it is established that: 1 2 2 U ∫ = (σ 12 + σ 13 ) dV (7) 2G

where G is the modulus of elasticity of torsion.

For measuring G, a prismatic bar with a uniform circular cross-section of area A and length L is subjected to applied torque (Mt) by an FS attached to a coordinated table at one end and at the other a torsion pendulum (see Figure 2a, b, and d).

With regard to Mechanics of materials[31], it is known that the stress state on an internal point of a polymer, xi, under the experimental conditions shown in Figure 1c, is expressed as:

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σ 12 =

M t x3 (8) J

σ 13 =

M t x2 (9) J

The polar moment of the cross-sectional area is represented 4

by J, which is given by J = π d . Substituting these stress 32

components into Equation 7 yields the internal strain energy in the structure for this specific application: U=

M t 2 L (10) 2GJ

The torque of torsion is: M t = FS a cos θ (11)

The external work conducted by Mt is: WE =

1 M t θ (12) 2

Equating Equations 10 and 12, and for a very small θ, we find: FS =

GJ = θ Bθ La

(13)

If we replace the values of the polar moment of the cross-sectional area, J, and use B and the calibration factor of the FS, f, we obtain an equation for the calculation of G: G=

B L a (14) f J

Knowing the values of the polar moment of the cross-sectional area of sample J, the slope obtained in the torsion test B, the calibration factor of the FS f, the pendulum arm length a, and the effective length at which the sample is positioned L, we can calculate the torsion modulus G by using Equation 14. In this work, to determine B*, the correlation coefficient r was used to best fit the line to the experimental points (the adjustment is better when r is closer to 1). The error associated with B* refers to the standard deviation value. We used the error theory to calculate the error propagation relative to the magnitude G, where the standard deviation uncertainty in G is, in the first approximation, given by:  ∂G 

 ∂G 

 ∂G 

 ∂G 

 ∂G 

2 2 2 2 2 σ G2 =   ²σ f (15)  ²σ B +   ²σ L +   ²σ a +   ²σ J +   ∂B   ∂L   ∂a   ∂J   ∂f 

3. Results and Discussions Figure 3 shows the inner region of the extruded PTFE bar in longitudinal section. The marks on the material surface are from the saw used to make the sample. No bubbles or voids were observed in the internal structure of the material.

Neither cracks nor microcracks, which may appear due to the state of tension or the presence of voids, were seen. These types of defects may occur during extrusion, by incorrect use of temperature, pressure, or velocity, or the presence of impurities[17,32,33]. As the extrusion process is continuous, the defects propagate along the entire length of the bar, which did not occur in this case.

Figure 4 shows the characteristic DSC curve for pure PTFE. At 31.6°C, an endothermic peak indicates the phase transition β from hexagonal crystal to pseudo-hexagonal.

Because the measurement starts at room temperature it is not possible to visualize the acute peak of β transition at 19°C from triclinic to hexagonal crystal. In addition, Polímeros, 29(3), e2019031, 2019

System to measure torsion modulus of polymers using the deformation energy method

Figure 3. Images of PTFE by scanning electron microscopy (SEM): beam energy = 16.0 keV; vacuum of 10-3 Pa; magnifications of (a) 250 X, (b) 1000 X, and (c) 2000 X.

Figure 4. Results of exploratory differential calorimetry (DSC) of PTFE at 25°C and atmospheric pressure.

the phase transition γ is not clearly seen and the glass transition of the material is very small and could not be detected, probably due to the heat flow value established for measurement[34]. The wider endothermic peak indicates the melting of the material, with Tm= 330.6°C. The heat of fusion was 35.5 J g−1 and the crystalline percentage was estimated according to Equation 2. These values and that of density are given in Table 1.

The density of PTFE in the literature is between 2140.0 and 2200.0 kg m−3[17,35,36], and 2156.3 kg m−3 was the value calculated for our material. Mechanical properties may undergo significant changes depending on the degree of crystallinity.

The reason for this is that it affects the secondary bonds, or Van der Waals bonds, between the intermolecular chains.

For regions of higher crystallinity these chains are closer to each other, intensifying the interaction between them by means of these bonds. Thus, for a specific polymer with more amorphous regions, this interaction will be smaller, and in turn there will be changes in the mechanical properties[35].

Furthermore, the modulus rises as both the secondary bond strength and chain alignment increase.

The value obtained in the calibration curve is f = 1.10697 ± 0.00006. Thus, f > 1 means that the value measured at the sensor is smaller than the applied force.

Polímeros, 29(3), e2019031, 2019

This is due to an intrinsic characteristic of the force sensor purchased from the manufacturer and must be corrected.

Most likely the manufacturer calibrated the sensor with a gravity value different from g = 9.79 ± 0.01 m s-2. This value was used in Equation 14 to obtain G. Typical curves, F as a function of θ* for PTFE, are shown in Figure 5a, b, c, and d.

The slope B* of the curves of Figure 5 was obtained by fitting a straight line to each of the curves when they showed linear behavior characteristic of an elastic regime. For each PTFE sample with different diameters (d1 = 5.00 mm; d2 = 6.50 mm; d3 = 8.00 mm and d4 = 10.85 mm), we determined the value of B*, measured by the FS and RMS, relative to the one measured of the curve as a function of the angular position.

It should be noted that the ratio θ/θ* is 0.222 ± 0.001, so that B is calculated as B*/0.222. The B* value corresponds to the average of the three values found in the linear fit for each set of measurement considering the fixed diameter.

Equation 14 was then used to calculate the value of G and Equation 15 to calculate σG. The correlation coefficient r was smaller for d1 = 5.00 mm, and its value is r = 0.969, the mean value of the three coefficients. As the diameter of the samples increased, it approached the value 1 (Figure 5a-d).

A smaller correlation coefficient means that the experimental points are more dispersed in respect to the adjusted line, reflecting that the force applied, in this case, is very close to the resolution of the force sensor, which did not happen for the other samples with higher diameters.

Figure 6 shows the final results of G, which is expressed in MPa, for all the studied samples with different diameters.

Using this method, the G value of the PTFE is close to the expected value[9,23]. The linear ratio L/d = 10.64 provides the same torsion modulus value for samples with different diameters, considering the error. Actual properties may change due to processing method, compound type, extruded dimensions, and other variables. For these reasons, there is in the literature a much more extensive range of values for the tensile modulus (E) of PTFE, from 0.4 to 1.6 GPa[9,23,35], whereas the values of G are more rare to find in the literature.

Even if we consider that the relation G = E/2(1+υ), only valid for isotropic solids, is within the elastic region and can be used to evaluate G, we must resort to a Poisson ratio. However, the Poisson ratio at small strains was found to differ in tension[9] (~0.36) and compression[23] (~0.46).

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Pintão, C. A. F., Piedade, L. P., & Borali, E. Table 1. Physical characteristics of extruded PTFE. Material

Density (kg m-3)

∆Hf (J g-1)

DSC (% crist.)

Tm (°C)

pure PTFE (extruded)

2156.3 ± 0.1

35.5

44 ± 1

330.6

Figure 5. Typical curves of force F (N) as a function of the angular position θ* (rad). (a) Sample of PTFE: L = (53.20 ± 0.05) mm; d1 = (5.00 ± 0.05) mm; B* = 0.387 ± 0.006 N/rad; r= 0.969; G is calculated by Equation 14, G = (351 ± 15) MPa. (b) Sample of PTFE: L = (69.20 ± 0.05) mm; d2 = (6.50 ± 0.05) mm; B*= 0.856 ± 0.007 N/rad; r= 0.992; G = (354 ± 11) MPa. (c) Sample of PTFE: L = (85.15 ± 0.05) mm; d3 = (8.00 ± 0.05) mm; B*= 1.584 ± 0.008 N/rad; r= 0.997; G = (351 ± 9) MPa. (d) Sample of PTFE: L = (115.50 ± 0.05) mm; d4 = (10.85 ± 0.05) mm; B*= 3.98 ± 0.02 N/rad; r= 0.997; G = (354 ± 7) MPa.

Figure 6. Values of G obtained for the samples of PTFE for different diameters.

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Therefore, it was necessary to consider these two values of the Poisson ratio to compare with our values. Using the values of 0.36 and 0.46, the values evaluated for G are within the ranges that follow: 147 MPa < G < 588 MPa and 137 MPa < G < 548 MPa respectively. If we verify the obtained results (Figures 5 or 6) by the method proposed in this article, independent of the distinct values of the Poisson coefficient, they are contained within these calculated intervals. Also, some authors[37-39] report that depending on the applied strain for PTFE extrudates, the Poisson’s ratio could vary between −14 and 0. Therefore, the advantage of this technique lies in the fact that the Poisson’s ratio is not necessary. Taking advantage of the previous discussion, we verified that using the dynamic method[12], the values obtained for G are very close to those found with this technique. For samples with the diameters of 5.00, 6.50, and 8.00 mm used in this work, the following values were Polímeros, 29(3), e2019031, 2019

System to measure torsion modulus of polymers using the deformation energy method obtained: G = (352 ± 14) MPa, G = (354 ± 11) MPa, and G = (351 ± 9) MPa respectively. These values were obtained respecting the ratio L/d = 10.64. As it was not possible to perform the measurement with d4 = 10.85 mm, the dynamic method was limited to these three sample diameters. However, with the method applied in this article, it was possible to carry out the measurement of G for d4 = 10.85 mm. Further details of the dynamic method applied to polymers will be provided in the near future.

4. Conclusions The system described herein is an alternative method to obtain G in polymers. It was applied to PTFE samples with different diameters with a fixed linear L/d ratio. The linear aspect ratio provided an approximate value of 350 MPa of G for all the PTFE samples and showed smaller deviations at higher diameters. Based on our results, the studied polymer showed values of G congruent with those of the literature, validating the use of the technique. In a competitive market, it is always interesting to know alternative methods to produce or find the parameters necessary for the development of products. Having been validated, the technique can now be applied to other types of materials, particularly those recently discovered, for which the Poisson’s ratio is not yet known.

5. Acknowledgements The authors thank Fundação de Amparo à Pesquisa do Estado de São Paulo - FAPESP, proc. 2007/04094-9, proc.

2017/08820-8, and proc. 2018/12463-9, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES proc. 024/2012 and 011/2009 Pro-equipment. Also, we would like to thank the group “Laboratório de Análise Térmica e Polímeros, UNESP-Bauru”.

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33. Patil, P. M., & Sadaphale, P. D. B. (2018). A study of plastic extrusion process and its defects. International Journal of Latest Technology in Engineering, Management & Applied Sciences (Basel, Switzerland), 7(9), 13-20.

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Received: Jan. 23, 2019 Revised: Apr. 15, 2019 Accepted: Apr. 15, 2019

Polímeros, 29(3), e2019031, 2019

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.08218

Silver nanoparticles incorporated PVC films: evaluation of structural, thermal, dielectric and catalytic properties Ganesh Shimoga1 , Eun-Jae Shin1  and Sang-Youn Kim1*  1

Interaction Laboratory of Advanced Technology Research Center, Korea University of Technology and Education, Chungjeollo, Byeongcheon-Myeon, Cheonan City, South Korea *sykim@koreatech.ac.kr

Abstract In this work, silver nanoparticle – polyvinylchloride (SNC-PVC) composites were synthesized by loading 2.5% to 10.0% silver ions to PVC using simple solution casting technique. Material properties including dielectric, thermal stability were discussed in some detail. Incorporation of silver nanoparticles (SNPs) in the PVC matrix was confirmed by UV-Visible spectroscopy (UV-Vis), X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDX) and Field Emission Scanning Electron Microscopy (FE-SEM). FE-SEM confirms the shape of the SNPs are roughly spherical with average size of the SNPs in the range of 60 - 80 nm. The thermal degradation studies were analysed via sensitive graphical Broido’s method using Thermogravimetric analysis (TGA). The resulting SNC-PVC films, especially with 10% silver loading showed improved catalytic performance during the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of aqueous sodium borohydride with apparent rate constant 1.956 × 10-3 sec-1 at ambient temperature.

Keywords: dielectric, heterogeneous catalysis, polyvinylchloride, silver nanocomposites, thermal analysis.

How to cite: Shimoga, G., Shin, E.-J., & Kim, S.-Y. (2019). Silver nanoparticles incorporated PVC films: evaluation of structural, thermal, dielectric and catalytic properties. Polímeros: Ciência e Tecnologia, 29(3), e2019032. https:// doi.org/10.1590/0104-1428.08218

1. Introduction Polymer nanocomposites provided exciting new insights in academia and industrial research[1-3]. There have been a great improvement in the synthesis of metal nanocomposites incorporated in fascinating polymer matrix and used in several practical applications[4,5]. Reinforcement of noble metal nanostructures into the polymer can improve the macroscopic properties of the bulk polymer, and the applications preceded to new directions[6,7]. From qualitative and quantitative point of view, the properties of nanoscale materials differ significantly in compare to those of bulk materials[8]. The remarkable change in physical and chemical properties including catalytic, electronic, and optical, can be observed with the reinforcement of nanosize materials into the bulk of the polymer matrix[9-11]. Among all the noble metal nanoparticles, nowadays silver nanoparticles are predominantly used in a wide range of potential applications in consumer products, electronic equipments, cancer treatment, medical imaging, drug delivery, and in the removal of organic pollutants from water (i.e. removal of nitro phenols by heterogeneous catalysis)[12,13].

There have been some successful attempts to develop varieties of polymer/copolymer matrices incorporated by SNPs and exploring their applications in industrial catalysis, hydrogen gas storage, toxic gas capture, water purification and etc[14,15]. Due to exceptional versatility and applicability, PVC is considered as one of the most widely used plastics in the global market. Also, excellent chemical stability, ease of modification and non flammability characters of PVC made them to use as a matrix for nanocomposites[16,17].

Polímeros, 29(3), e2019032, 2019

The applications of PVC ranges from construction materials to health care, electronics, automobile and other industrial sectors[18-20]. Recently, Braga et al., reported the synthesis of SNPs incorporated PVC films with the silver loading of 1.0 to 8.0%, the results revealed that the synthesized SNC-PVC composite films exhibited the highest antimicrobial activities and used for food packaging applications[21]. Thabet et al., reported incorporating clay and metal oxide nanoparticle fillers in the PVC can enhance the surface energy properties and tuned the dielectric properties for improved electronic applications[22].

To best of our knowledge, no systematic empirical research exists addressing the simple synthesis of SNC-PVC films for heterogeneous catalytic applications. In this work, simple solution casting method was adopted to fabricate SNC-PVC composite films with preferred thickness by reducing silver ions to SNPs using ecofriendly trisodium citrate dihydrate solution. Thermal degradation of SNC-PVC composite films were systematically characterized using TGA and a sensitive graphical Broido’d method[23] was used to characterize each degradation steps. The variation of dielectric properties (dielectric constant and loss factor) were measured using impedance analyzer. Morphology of SNC-PVC films are examined using FE-SEM, which exemplifies homogeneous distribution of silver nanoparticles in the PVC matrix and are roughly spherical with size of SNPs in the range of 60-80 nm. SNC-PVC composite films were subjected to catalytic performances for the reduction of 4-nitrophenol to 4-aminophenol in presence of sodium borohydride and the results were reported.

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O O O O O O O O O O O O O O O O

Shimoga, G., Shin, E-J & Kim, S-Y

2. Materials and Methods 2.1 Materials Silver nitrate, polyvinylchloride (Mw ~233000), and trisodium citrate dihydrate were procured from Sigma Aldrich, Seoul, South Korea. All the other chemicals and solvents are of reagent grade and used without any further purification. Double distilled HPLC grade water was used throughout the study.

2.2 SNC-PVC Nanocomposite films preparation PVC films doped with silver (II) nitrate (AgNO3) in various concentrations were prepared at room temperature by simple solution casting method. The desired concentration of silver nitrate solutions (2.5, 5.0, 7.5 and 10.0 weight %) were prepared by using cold distilled water (400 - 800 ÂľL).

PVC (1g) was dissolved in 20 mL tetrahydrofuran (THF) at room temperature (22 °C), a known amount silver nitrate solutions were loaded into the polymer solutions separately (For Ag-PVC-2.5, 25 mg of silver nitrate was dissolved in 500 ¾L of water and loaded to 1 g of PVC solution in 25 mL of THF). The solution was stirred for 10 h at room temperature and the resulting homogeneous solution was cast onto a glass plate with the aid of a casting knife. The thin films were allowed to dry at room temperature for 72 h and vacuum dried at 50 °C for 10 h, the completely dried films were subsequently peeled off. The stoichiometric mass ratio of silver nitrate with respect to PVC was varied as 0.0, 2.5, 5.0, 7.5 and 10.0, and the resulting thin films were designated as PVC, Ag-PVC-2.5, Ag-PVC-5.0, Ag-PVC-7.5 and Ag-PVC-10.0, respectively. The thickness of the thin films was measured at different points using peacock dial thickness gauge (Model G, Ozaki MFG. Co. Ltd., Japan) with an accuracy of ¹2 ¾m and the average thickness was considered for calculation. The thickness of the membranes was found to be 350¹2 ¾m.

The Ag+2 doped PVC films (Ag-PVC-2.5, Ag-PVC-5.0, Ag-PVC-7.5 and Ag-PVC-10.0) were cut in to the dimensions of 250 mm x 250 mm and suspended in 50 mL of 1 mM solution of trisodium citrate solution for 30 min with gentle stirring. The obtained SNC-PVC composite films were washed repeatedly with distilled water, gently wiped with clothing tissue paper and vacuum dried at 50 °C for 10 h. The resulting composite films were designated as SNC-PVC-2.5, SNC-PVC-5.0, SNC-PVC-7.5 and SNC-PVC-10.0, respectively.

The percentage catalytic convertion efficiency of 4-NP into 4-AP was calculated by the following Equation (1): Percentage ( % ) conversion =

C0 â&#x2C6;&#x2019; Ct (1) C0

where Ct is the concentration of 4-NP measured at time t, C0 is the initial concentration of 4-NP measured at time zero[24, 25].

2.4 Instrumentation SNC-PVC composite films were investigated using solid state electronic absorption spectra on a Perkin-Elmer UV-Vis spectrometer, model UV/VIS-35 (PerkinElmer, Inc., MA, USA). To understand the effect of doping of Ag+2 and SNPs content in PVC, thin films were subjected to powder X-ray diffraction study using Bruckerâ&#x20AC;&#x2122;s D-8 advanced X-ray diffractometer. The X-ray source was Ni filtered Cu KÎą radiation. The diffraction was scanned in the reflection mode at an angle 2θ over a range of 5 to 90° at a constant speed of 8°/min. Similarly, the surface morphology of PVC silver nanocomposites were analyzed by using Ultra-High-Resolution Field-Emission Scanning Electron Microscope (FESEM, FEI, & Nova NanoSEM450) instrument operating at 25 kV. Thermal stability of SNC-PVC films were investigated using thermogravimetric analyzer (Q500, TA instruments, DE, USA) in the range from 25 °C to 600 °C in a 50 mL/min flow of N2 gas at a heating rate of 10 °C/min.

SNC-PVC composite films were subjected to the dielectric measurements; samples were sandwiched between two gold plated electrodes and analysed by impedance analyzer, model HIOKI 3352-50 HiTESTER Version 2.3. The electrical contacts were checked to verify the ohmic connection.

The measurements were carried out at room temperature in between the 50 Hzâ&#x20AC;&#x201C;5 MHz. The dielectric constant (đ?&#x153;&#x20AC;â&#x20AC;&#x2122;) was calculated using Equation (2) and dielectric loss (tan δ) was calculated using Equation (3). Îľ ' = ( C p d ) / ( ξο A ) (2)

tanδ = Îľ â&#x20AC;˛â&#x20AC;˛ / Îľ â&#x20AC;˛

(3)

where â&#x20AC;&#x2DC;đ?&#x2018;&#x2018;â&#x20AC;&#x2122; is the thickness of the polymer film and â&#x20AC;&#x2DC;đ??´â&#x20AC;&#x2122; is the cross-section area, â&#x20AC;&#x2DC;đ?&#x153;&#x20AC;Oâ&#x20AC;&#x2122; is the permittivity of the free space, â&#x20AC;&#x2DC;Îľ â&#x20AC;˛â&#x20AC;˛â&#x20AC;&#x2122; and â&#x20AC;&#x2DC;Îľ â&#x20AC;˛â&#x20AC;&#x2122; is the permittivity (imaginary part) and permittivity (real part) of the material respectively. All these measurements were performed under dynamic vacuum.

2.3 Catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP)

3. Results and Discussions

Catalytic performance of SNC-PVC-10.0 was evaluated for the model reduction reaction of 4-NP to 4-AP in a standard 10 mL quartz cell. Typically, 10 mL of aqueous 4-NP (0.1 mM) solution was mixed with 0.05 g of SNC-PVC-10.0, followed by the addition of 5 mL aqueous NaBH4 solution (50 mM), and the time dependent UV-vis absorption spectra was recorded by VARIANEL08043361 UV- vis spectrophotometer (Varian, USA). the conversion of 4-NP to 4-AP processes was monitored every 5 min interval in a scanning range from 250 to 600 nm at room temperature (22 °C).

Solid state UV-Vis spectroscopy was performed for Ag-PVC-10.0 and SNC-PVC-10.0 thin films. As can be seen in Figure 1 (a), spectra confirms the characteristic surface plasmon resonance band for SNC-PVC-10.0, which was observed in the range of 440 â&#x20AC;&#x201C; 460 nm, confirms the formation of SNPs on the PVC matrix.

Figure 1 (b) shows the XRD pattern of PVC, Ag-PVC-10.0 and SNC-PVC-10.0. The diffraction peaks of PVC locate at 17.2° and 24.5°, which was the characteristic pattern of PVC. A low percolation threshold of Ag+ doping to the bulk PVC

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3.1 Characterization of SNC-PVC nanocomposite films

PolĂ­meros, 29(3), e2019032, 2019

Silver nanoparticles incorporated PVC films: evaluation of structural, thermal, dielectric and catalytic properties was found in the pattern of Ag-PVC-10.0. The crystalline nature of the SNPs was confirmed for SNC-PVC-10.0 by the arrival of 2θ values at 38.2°, 44.8°, 64.3°, 76.8°and 82.6° for characteristic crystal planes (111), (200), (220), (311) and (222) respectively[26].

The FE-SEM of all the SNC-PVC composite surfaces are displayed in Figure 2 (a) to (d) and EDX spectrum of SNC-PVC-10.0 was displayed in Figure 2 (e), The surface morphology for SNC-PVC-2.5 and SNC-PVC-5.0 were shown in Figure 2 (a) and (b) respectively, from which the average size of the SNPs were found to be distributed in the

Figure 1. (a) Solid state UV-Vis spectra of Ag-PVC-10.0 and SNC-PVC-10.0 films (b) XRD pattern of PVC, Ag-PVC-10.0 and SNC-PVC-10.0 films.

Figure 2. FE-SEM micrograph of (a) SNC-PVC-2.5 surface (b) SNC-PVC-5.0 surface (c) SNC-PVC-7.5 surface (d) SNC-PVC-10.0 surface (e) EDX Spectrum of SNC-PVC-10.0.

Polímeros, 29(3), e2019032, 2019

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Shimoga, G., Shin, E-J & Kim, S-Y range of 20 nm to 140 nm, it seems that the size of the SNPs are not uniform. Where as, from the Figure 2 (d) and (e), the surface morphology of SNC-PVC-7.5 and SNC-PVC-10.0 were seems to be uniform and roughly spherical in shape with average size of the SNPs distributed in the range of 60-80 nm. Elemental composition analysis by EDX presented in Figure 2 (e), which shows strongest signal near to 3 keV, which is the typical absorption pattern of metallic nanocrystalline silver surface, also strongest signals for carbon and chlorine atoms of PVC were obtained it indicates the presence of SNPs in PVC matrix.

The average particle size, size distribution of SNC-PVC films in colloidal THF suspensions were evaluated by using dynamic light scattering (DLS) experiments. It measures the Brownian motion and relates this to the size of the particles. The nanosize measurements were performed using the Zetasizer (Nano ZS, Malvern, UK). Each SNC-PVC composite films (100 mg) were dissolved in 100 mL of THF and the measurement was repeated in triplicate at a fixed angle of 173° at 22 °C. Representative histogram displayed in Figure 3 (a) (b) (c) and (d), showed that the particles are distributed in the range of 20â&#x20AC;&#x201C;140 nm and majority of the particles are in the range of 60â&#x20AC;&#x201C;80 nm. This was further evidenced by FESEM analysis.

From the TG plot shown in Figure 4 (a), it was observed that the thermogram of PVC and SNC-PVC showed two major degradation steps with onset decomposition at 240 °C. But, PVC suffers the degradation at 90 ºC due to trapped

THF molecules inside the PVC chains. The initial weight loss step started at around 240â&#x20AC;&#x201C;415 °C was attributed to the dehydrochlorination in the PVC chains, leading to the formation of long sequences of polyenes. Second degradation step is in the range of 415â&#x20AC;&#x201C;475 °C can be accounted for the main PVC chains with conjugated double bonds resulted from dehydrochlorination[27, 28].

However, PVC had a initial slight reduced thermal stability due to trapped THF solvent inside the PVC chains, because of this PVC suffers degradation below 100 °C. Practical weight loss in PVC was observed with onset decomposition at 240 °C, and slight thermal stability of SNC-PVC compare to PVC was due to the SNPs covering the PVC surface. which resists partially for the better diffusion of hydrocholoric acid (HCl) gas produced during dehydrochlorination of PVC. Kinetic and thermodynamic parameters were calculated using Broidoâ&#x20AC;&#x2122;s method[21]. Broido has developed a model and the activation energy associated with each stage of decomposition and was evaluated by this method[29].

Plots of â&#x2C6;&#x2019;ln(ln(â&#x2C6;&#x2019;1/đ?&#x2018;Ś)) versus 1/đ?&#x2018;&#x2021; (Figures 4(b), 4(c) and 4(d)) were developed for the decomposition segments of PVC and SNC-PVC films. From the plots, the activation energy (đ??¸đ?&#x2018;&#x17D;) and frequency factor (lnđ??´) were evaluated. The enthalpy (Î&#x201D;đ??ť), entropy (Î&#x201D;đ?&#x2018;&#x2020;), and free energy (Î&#x201D;đ??ş) have been calculated using standard equations and are summarized in Tables 1, Tables 2 and Tables 3.

The results indicated in Table 1, the activation energies for SNC-PVC-2.5 and SNC-PVC-5.0 compared to PVC,

Figure 3. Histogram of particle size distribution of (a) SNC-PVC-2.5 (b) SNC-PVC-5.0 (c) SNC-PVC-7.5 (d) SNC-PVC-10.0.

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Silver nanoparticles incorporated PVC films: evaluation of structural, thermal, dielectric and catalytic properties

Figure 4. (a) TG analysis of PVC and SNC-PVC films (b) Plots of –Ln(ln(-1/y)) vs 1/T×10-3 for the first decomposition step in the range 240 – 340 °C (c) Plots of –Ln(ln(-1/y)) vs 1/T×10-3 for the decomposition step in the range 340 – 415 °C (d) Plots of –Ln(ln(-1/y)) vs 1/T×10-3 for the second decomposition step in the range 415 – 475 °C. Table 1. Kinetic and thermodynamic parameters of PVC and SNC-PVC at the decomposition range 240 – 340 °C. Ea

Title

(kJ/mol)

lnA

PVC SNC-PVC-2.5 SNC-PVC-5.0 SNC-PVC-7.5 SNC-PVC-10.0

x 10-3 16.031 14.642 15.926 16.382 16.382

-6.551 -6.787 -6.635 -6.555 -6.555

∆H

∆S

∆G

(kJ/mol)

(kJ/K)

(kJ/mol)

-4.665 -4.666 -4.665 -4.664 -4.664

-160.214 -161.565 -162.066 -161.532 -161.532

90.200 90.961 91.243 90.942 90.942

Table 2. Kinetic and thermodynamic parameters of PVC and SNC-PVC at the decomposition range 340 – 415 °C. Ea

Title

(kJ/mol)

lnA

PVC SNC-PVC-2.5 SNC-PVC-5.0 SNC-PVC-7.5 SNC-PVC-10.0

x 10-3 1.368 1.433 1.657 1.671 1.752

-9.489 -9.464 -9.911 -9.969 -9.847

∆H

∆S

∆G

(kJ/mol)

(kJ/K)

(kJ/mol)

-5.406 -5.406 -5.406 -5.407 -5.406

-162.580 -162.742 -162.792 -162.761 -162.782

105.760 105.865 105.898 105.878 105.891

Table 3. Kinetic and thermodynamic parameters of PVC and SNC-PVC at the decomposition range 415 – 475 °C. Ea

Title

(kJ/mol)

lnA

PVC SNC-PVC-2.5 SNC-PVC-5.0 SNC-PVC-7.5 SNC-PVC-10.0

x 10-3 30.465 16.853 22.718 24.363 26.233

-4.447 -6.228 -5.138 -4.936 -5.469

Polímeros, 29(3), e2019032, 2019

∆H

∆S

∆G

(kJ/mol)

(kJ/K)

(kJ/mol)

-5.939 -5.953 -5.945 -5.943 -5.947

-160.989 -161.327 -160.645 -160.757 -160.894

115.594 115.837 115.347 115.427 115.526

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Shimoga, G., Shin, E-J & Kim, S-Y SNC-PVC-7.5 and SNC-PVC-10.0 are lower, signposting that the decomposition step is faster in case of SNC-PVC-2.5 and SNC-PVC-5.0. Similarly, from the Table 2. it reveals that, gradual increase in the activation energy of PVC to SNC-PVC-10.0 indicates the thermal stability of SNC-PVC composite films over the temperature range 340 – 415 °C. From the Table 3. we can easily recognize that the rate of second major degradation step in the decomposition range 415 – 475 °C of PVC was comparibly slower to their respective SNC-PVC composite films. This is due to the presence of Ag+ and nitrate ions in the vicinity of PVC chains. During the reduction process of Ag+ ions by trisodium citrate solution, more Ag+ ions on the surface of PVC was reduced for form SNPs. Since the formation of SNPs on the surface of SNC-PVC samples from 2 weight % to 10 weight % increases, due to the reduction of more Ag+ ions on the surface. The stability of SNC-PVC films gradually increses from 2 weight % to 10 weight %, consequently the rate of decomposition is slower with the increase of SNPs on the surface of SNC-PVC composite films.

The plot of variation of dielectric constant as a function of log (frequency) at room temperature is shown in Figure 5 (a).

The plot illustrates that it is generally followed by almost all the dielectric and ferroelectric materials. The dielectric constant drops at high frequencies; this is due to the fact that the dipoles can no longer follow the high frequencies; the dielectric constant of 2.2 is achieved for SNC-PVC-2.5.

In the middle frequency region the dielectric constant is approximately constant for all SNC-PVC and PVC films except SNC-PVC-2.5. Minimum dielectric constant value of 1.22 was obtained for SNC-PVC-10.0.

Higher the content of SNPs in the PVC matrix upshot the lower dielectric constant and it may also be due to affecting factor of restricted motion of SNPs. Also, increasing of the frequencies cause decreasing of space charge polarization (interfacial polarization) to the total polarization[30]. At lower frequencies, this interfacial polarization is most contributing type of polarization, and less contributing with the increase of frequency; this caused the decreasing of dielectric constant values for increasing the SNPs. Frequency dependence variations of dielectric loss at room temperature were presented in Figure 5 (b). As per the plot, tan δ decreases with increase of frequency and attains the constant value. The dielectric loss of all composite films

are lower than 0.08; at 1.5 log F (Hz) values, tan δ was well below 0.05 and maintaining the constant value. But the SNC-PVC-2.5 films display higher dielectric loss compared to other SNC-PVC films, which is due to high ion drifting and freedom of movement in dipole polarisation[31,32] due to the available free space for SNPs with respect to PVC.

3.2 Catalytic activity The catalytic performance of SNC-PVC-10.0 was investigated for the aqueous reduction of 4-NP to 4-AP in the presence of NaBH4 as a model reaction was depicted in Figure 6 (a). After the addition of 50 mM aqueous solution of NaBH4 to the aqueous 4-NP solution, the light yellow color of 4-NP solution changed to intense yellow, due to the immediate formation of sodium phenolate. The reaction was not proceeded without the addition of SNC-PVC-10.0 catalyst even after 9 days of monitering by UV-Vis spectroscopy.

Also, the visible persistence of yellow color confirms very slow conversion rate of 4-NP into 4-AP. Addition of finely chopped (1 mm × 10 mm) SNC-PVC-10.0 films into the mixture of aqueous 4-NP and NaBH4 solution, drastically changes the rate of reaction. The intense yellow color the reaction mixture (aqueous 4-NP and NaBH4 solution) was diminished to colorless in 25 min (Table 4).

The catalytic reaction was monitored by time dependant UV-visible spectrophotometer and also by visual appearance. The mechanism involved in the reduction reaction can be explained as follows: initialy, BH4̄ ions reacts with SNPs present on the surface of PVC matrix to form silver hydride, the formed silver hydrides reacts with nitro (-NO2) groups to convert it into amino (-NH2) groups. The electron transfer from donor (BH4̄ ions) to acceptor (4-NP ions) was facilitated by SNC-PVC-10.0 (heterogeneous solid catalyst). Figure 6 (b), shows the characteristic time dependant absorption peak at 400 nm and 300 nm for 4-NP and 4-AP respectively.

SNC-PVC-10.0 composite films shows appreciable catalytic performances for the model reduction reaction of 4-NP. Esumi et al.[33] in 2004, reported the catalytic activities of silver, platinum and palladium composites of poly(amidoamine) and poly(propyleneimine) dendrimenrs, the apparent rate constant (kAPP) reported was 5.9 × 10-4 sec-1 and 1.22 × 10-3 sec-1 respectively. The value reported by Chang et al.[25] for polypyrrole encased SNPs was 1.1 × 10−3 s−1.

Figure 5. Room temperature variation of (a) dielectric constant (b) dielectric loss with log (frequency) for PVC and SNC-PVC nanocomposite films.

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Silver nanoparticles incorporated PVC films: evaluation of structural, thermal, dielectric and catalytic properties

Figure 6. (a) Pictograph representing the catalytic reduction of 4-NP into 4-AP by SNC-PVC-10.0 (b) Time dependant UV-Vis spectra representing the catalytic reduction of 4-NP into 4-AP in aqueous medium in the presence of 5 mg mL-1 SNC-PVC-10.0 [(4-NP = 0.1 mM), (NaBH4 = 50 mM; 22 °C)].

Table 4. Catalytic reduction of 4-NP to 4-AP using SNC-PVC-10.0 films.

Time (min) 0 5 10 15 20 25

% Conversion of 4-NP to 4-AP 0 7.948 79.374 83.970 88.152 94.085

Murugan et al.[34] reported solid catalysts composed of polystyrene functionalized with polyvinylimidazole immobilized SNPs with kAPP = 5.12 × 10−4 s−1. The apparent rate constant (kAPP) calculated from our study using PVC as polymer matrix was 1.956 × 10-3 sec-1, which was comparatively higher than the reported values from the literature for same catalytic reduction reaction by polymer supported SNPs. Also, it was considerably close to the value 2.12 × 10-3 sec-1 for SNPs encapsulated amphiphilic copolymer micelles of poly(2-ethyl-2-oxazoline) and poly(ε-caprolactone)s reported by Safari et al.[12].

4. Conclusions In summary, SNC-PVC films of silver doping from 2.5 to 10.0% (stochiometric) were successfully prepared using simple solution casting technique. The thermal stability of all the composites films were studied using TGA. The sensitive graphical Broido’s method was employed to study the thermodynamical parameters at each stage of thermal degradation steps. The SNPs on the PVC surface was confirmed by UV-Vis, XRD, EDX. The SNPs on SNC-PVC-10.0 were studied using FE-SEM and DLS measurements, it was found that SNPs were uniformly distributed with size in the range of 60-80 nm.

EDX elemental composition at any place of the SNC-PVC films clears the uniform distribution of SNPs on the PVC surface. It was noted that, the thermal stability of SNC-PVC films are increased from 2.5 weight % to 10 weight %, confirming that positive effect on the thermal stability of the SNC-PVC films. Dielectric studies of all SNC-PVC films were performed using impedance analyser, it was found that Polímeros, 29(3), e2019032, 2019

dielectric constant and loss factor for all the composites were reduced drastically at higher content of SNPs loading due to either reduced atomic polarizability or density within the SNC-PVC films[35]. The article also highlights the catalytic performances of a model reduction reaction of 4-NP using aqueous NaBH4 solution. The catalytic reaction in the first and second run starts immediately after the addition of NaBH4 to 4-NP solution with SNC-PVC-10.0. So the catalytic performances of SNC-PVC-10.0 was briefly studied out of all SNC-PVC composites. The calculated apparent rate constant for SNC-PVC-10.0 was 1.956 × 10-3 sec-1 at ambient temperature. Our results show that these SNC-PVC-10.0 composites are highly recommeded as nanocatalysts and ideal materials as heterogeneous catalysts to demonstrate their potential applications in industrial catalysis[36], also tenability of dielectric properties of these materials opens up sensing and electronic applications including microelectronics[37].

5. Acknowledgements This work was supported by the Technology Innovation Program (10077367, Development of a film-type transparent /stretchable 3D touch sensor /haptic actuator combined module and advanced UI/UX) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

This work was also supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1A6A1A03025526). Thanks for Cooperative Equipment Center at KoreaTech for assistance with UV-Vis, TGA, DLS, XRD, FESEM analysis.

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Simple synthesis of active films based on PVC incorporated with silver nanoparticles: evaluation of the thermal, structural and antimicrobial properties. Food Packaging and Shelf Life, 15, 122-129. http://dx.doi.org/10.1016/j.fpsl.2017.12.005.

22. Thabet, A., & Ebnalwaled, A. A. (2017). Improvement of surface energy properties of PVC nanocomposites for enhancing electrical applications. Measurement, 110, 78-83. http://dx.doi. org/10.1016/j.measurement.2017.06.023.

23. Broido, A. (1969). A simple, sensitive graphical method of treating thermogravimetric analysis data. Journal of Polymer Science. Part A-2, Polymer Physics, 7(10), 1761-1773. http:// dx.doi.org/10.1002/pol.1969.160071012.

24. Palem, R. R., Ganesh, S. D., Saha, N., Kronek, J., & Sáha, P. (2018). ‘Green’ synthesis of silver polymer Nanocomposites of poly(2-isopropenyl-2-oxazoline-co-N vinylpyrrolidone) and its catalytic activity. Journal of Polymer Research, 25(7), 152. http://dx.doi.org/10.1007/s10965-018-1548-9.

25. Chang, M., Kim, T., Park, H.-W., Kang, M., Reichmanis, E., & Yoon, H. (2012). Imparting chemical stability in nanoparticulate silver via a conjugated polymer casing approach. ACS Applied Materials & Interfaces, 4(8), 4357-4365. http://dx.doi. org/10.1021/am3009967. PMid:22860984.

26. Kora, A. J., & Rastogi, L. (2013). Enhancement of antibacterial activity of capped silver nanoparticles in combination with antibiotics, on model gram-negative and gram-positive bacteria.

Bioinorganic Chemistry and Applications, 2013, 871097. http:// dx.doi.org/10.1155/2013/871097. PMid:23970844.

27. Aouachria, K., Massardier-Nageote, V., & Belhaneche-Bensemra, N. (2014). Thermal stability and Kinetic Study of rigid and plasticized Poly(vinyl chloride)/Poly(methylmethacrylate) blends. Journal of Vinyl & Additive Technology, 21(2), 102110. http://dx.doi.org/10.1002/vnl.21372.

28. Van Der Ven, S., & De Wit, W. F. (1969). Thermal degradation of poly(vinyl chloride): the accelerating effect of hydrogen chloride. Die Angewandte Makromolekulare Chemie, 8(1), 143-152. http://dx.doi.org/10.1002/apmc.1969.050080110.

29. Ganesh, S. D., Pai, V. K., Kariduraganavar, M. Y., & Jayanna, M. B. (2014). Fluorinated poly(arylene ether-1,3,4-oxadiazole)s containing a 4-bromophenyl pendant group and its phosphonated derivatives: synthesis, spectroscopic characterization, thermal and dielectric studies. Polymer-Plastics Technology and Polímeros, 29(3), e2019032, 2019

Silver nanoparticles incorporated PVC films: evaluation of structural, thermal, dielectric and catalytic properties Engineering, 53(1), 97-105. http://dx.doi.org/10.1080/0360 2559.2013.843694.

30. Peiris, T. A. N. (2014). Microwave-assisted processing of solid materials for sustainable energy related electronic and optoelectronic applications. Loughborough: Loughborough University.

31. Daněk, J., Klaiber, M., Hatsagortsyan, K. Z., Keitel, C. H., Willenberg, B., Maurer, J., Mayer, B. W., Phillips, C. R., Gallmann, L., & Keller, U. (2018). Interplay between Coulombfocusing and non-dipole effects in strong-field ionization with elliptical polarization. Journal of Physics. B, Atomic, Molecular, and Optical Physics, 51(11), 114001. http://dx.doi. org/10.1088/1361-6455/aaba42.

32. Sidebottom, D. L., Roling, B., & Funke, K. (2000). Ionic Conduction in Solids: Comparing Conductivity and Modulus Representations With Regard to Scaling Properties. Physical Review B: Condensed Matter, 63(2), 024301. http://dx.doi. org/10.1103/PhysRevB.63.024301.

33. Esumi, K., Isono, R., & Yoshimura, T. (2004). Preparation of PAMAM− and PPI−Metal (Silver, Platinum, and Palladium) Nanocomposites and Their Catalytic Activities for Reduction of 4-Nitrophenol. Langmuir, 20(1), 237-243. http://dx.doi. org/10.1021/la035440t. PMid:15745027.

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34. Murugan, E., & Jebaranjitham, J. N. (2012). Synthesis and characterization of silver nanoparticles supported on surfacemodified poly(N-vinylimidazale) as catalysts for the reduction of 4-nitrophenol. Journal of Molecular Catalysis A Chemical, 365, 128-135. http://dx.doi.org/10.1016/j.molcata.2012.08.021.

35. Liu, Y., Zhang, Y., Lan, Q., Liu, S., Qin, Z., Chen, L., Zhao, C., Chi, Z., Xu, J., & Economy, J. (2012). High-Performance Functional Polyimides Containing Rigid Nonplanar Conjugated Triphenylethylene Moieties. Chemistry of Materials, 24(6), 1212-1222. http://dx.doi.org/10.1021/cm3003172.

36. Geng, Q., & Du, J. (2014). Reduction of 4-nitrophenol catalyzed by silver nanoparticles supported on polymer micelles and vesicles. RSC Advances, 4(32), 16425-16428. http://dx.doi. org/10.1039/C4RA01866D. 37. Roy, A. S., Gupta, S., Seethamraju, S., Ramamurthy, P. C., & Madras, G. (2014). Fabrication of Poly(Vinylidene ChlorideCo-Vinyl Chloride)/TiO2 Nanocomposite Films and Their Dielectric Properties. Science of Advanced Materials, 6(5), 946-953. http://dx.doi.org/10.1166/sam.2014.1858.

Received: Dec. 07, 2018 Revised: May 24, 2019 Accepted: June 09, 2019

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.02719

Controlled release fertilizer encapsulated by a κ-carrageenan hydrogel Gladys Rozo1* , Laura Bohorques2 and Johanna Santamaría3 Departmento de Ciencias Básicas y Modelado, Universidad Jorge Tadeo Lozano, Bogotá, DC, Colombia 2 Proyecto Curricular de Licenciatura en Química, Universidad Francisco José de Caldas, Bogotá, Colombia 3 Departmento de Ciencias Biológicas y Ambientales, University Jorge Tadeo Lozano, Bogotá, Colombia

1

*gladys.rozo@utadeo.edu.co

Abstract The release kinetics of nitrogen and phosphorous of a granulated fertilizer, encapsulated in a κ-carrageenan-based hydrogel (CBH), was evaluated in order to determine its release mechanism given the potential this hydrogel has as coating material for controlled release fertilizers (CFRs). The effect of pH on the release properties was also investigated. The relationship between the NH4+, NO3-, and PO43- release of encapsulated fertilizers and time was determined by short- and long-term laboratory incubations. The mechanism of the release of nutrient ions was determined by comparing the release data with the zero-order, first-order, Higuchi, Hixon-Crowell and Korsmeyer-Peppas kinetic models. The findings showed that the Korsmeyer-Peppas model could be used to describe the release characteristics of the nutrients in the encapsulated fertilizers and that non-Fickian diffusion is the main release mechanism. The experimental hydrogel showed a high water retention capacity able to absorb 300 times its weight water.

Keywords: carrageenan, controlled release fertilizers, hydropolymers, release behavior.

How to cite: Rozo, G., Bohorques, L., & Santamaría, J. (2019). Controlled release fertilizer encapsulated by a κ-carrageenan hydrogel . Polímeros: Ciência e Tecnologiam 29(3), e2019033. https://doi.org/10.1590/0104-1428.02719

1. Introduction The use of fertilizers since the end of the World War II significantly increased the production per unit area, allowing the agricultural industry to meet the demand for food of the increasing human populations[1-3]. Despite the positive impact these inputs have on productivity, their use is inefficient.

Less than 50% of the nitrogen (N) and phosphorus (P) in the fertilizers is taken up by the plants[4,5]. The non-assimilated nutrients are lost by leaching or by volatilization into the atmosphere[4,6,7], causing serious environmental problems and economic losses[8-11].

In coated controlled release fertilizers (CRFs), the mechanism and the factors influencing the speed and duration of the nutrient release are known and controllable, and they are currently being used as a strategy to decrease the rate at which nutrients are released from the fertilizer to the soil solution, allowing synchronization between the onset of nutrient uptake by the crop and the availability of nutrients[12]. Another important aspect of CRFs is the rate of reduction in nutrient removal from soil by rain or irrigation water[13], which can help mitigate the eutrophication and greenhouse gases flow to the atmosphere as a result of reduction of the N and P output from the productive systems.

Among the materials being used commercially as fertilizer coatings are minerals and synthetic polymers[14-16]. Although these materials have proven to be useful for increasing nutrient use efficiency and for decreasing nutrient losses

Polímeros, 29(3), e2019033, 2019

from the agricultural production system[17-19], they may be toxic and / or nonbiodegradable; additionally, they raise the fertilizer price[12,14], which prevents a more frequent use by producers. For this reason, the search has begun for a low cost, nontoxic, biodegradable material within the different organic polymers of natural origin that can be used as a coating for fertilizer grains[16,19-21].

In the search for alternative coating materials, the use of Carrageenan, a family of sulfated polysaccharides found in the wall of red algae of the Rhodophyceae class, has begun to be tested for the preparation of high water retention hydrogels with good mechanical properties regarding processability and end use applications. Kappa-carrageenan hydrogels have been synthesized by copolymerization with N-vinyl formamide[22] and polyacrylamides[23], which although have shown to have good hydration capacity are not biodegradable.

Additionally, hydrogels containing polyacrylamides, when added to the soil, could cause environmental damage due to the accumulation of nitrogen[24].

To continue exploring the possibilities of this natural polymer in the construction of fertilizers with a lower environmental impact and promoting the development of products based on raw materials available in the Colombian Caribbean, we have designed a carrageenan based hydrogel (CBH) less complex in structure and chemical composition in order to encapsulate fertilizer granules[25]. This hydrogel

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O O O O O O O O O O O O O O O O

Rozo, G., Bohorques, L., & Santamaría, J. is porous, semipermeable, biodegradable, nontoxic[26-27], able to absorb up to ten thousand times its weight in water without dissolving or losing its integrity, and its raw material, carrageenan, can be easily obtained[28]. Its chemical structure is composed of alternative units of D-galactose and 3,6-anhydrogalactose (3,6 AG) that are bound through α-1,3 and β-1,4 glycosidic bonds. It is an anionic polymer, with high molecular weight. The presence of a cross-linking agent allows carrageenan to form hydrogels in which polysaccharides form three-dimensional networks that swell in water[29,30]. The wetting and swelling behavior of the hydrophilic polymers allow the hydrogels to control water penetration, and thus the rate of nutrient dissolution present in the core of the coated fertilizer[31-33]. As these nutrients dissolve, they migrate from their initial position in the polymer system to the outer surface of the polymer and are subsequently released into the environment. Laboratory soil column experiments have shown that fertilizer granules encapsulated with the CBH release nutrients more slowly in comparison to the non-encapsulated fertilizer[25]. Although, CBH proves to be useful in delaying the release of nutrients to the environment, nothing is yet known about its soil release mechanism.

Therefore, the objective of this work is to evaluate the release behavior of fertilizer granules encapsulated within the CBH by interpreting its N and P release profile with various mathematical models in order to obtain insight into its release mechanism accompanied by swelling. These mathematical models let us associate the properties of the materials used to build a CRF and its release mechanism, which makes it possible to improve its design. The mathematical models chosen for this study include the mechanisms of penetration of water within the CFR, diffusion[31-33] and erosion[34]. These are important for the CRF design based on the fact that these mechanisms can be controlled through the design of the hydrogel in order to get the greatest release of nutrients to the soil solution when crop plants begin their greatest peak of nutrients absorption.

Considering that the CBH has been designed to be used in yellow potato crops (Solanum pureja) which are frequently located in soils whose soil pH vary 4.5 and 6.0[35-37], and since the stability of the three-dimensional structure of the hydrogel depends on the pH, the nutrient release in the potato crop could be produced by the breakdown of the gel in an environment with acidic pH[38]. Therefore, the effect of an acidic pH (4.5) and a pH close to neutrality (6.3) will be evaluated as release controlling factors. This study will provide relevant information for the future development and field use of a CRF with a carrageenan-based hydrogel as a coating material in yellow potato crops.

2. Materials and Methods 2.1 Synthesis of CBH and preparation of the coated fertilizer An aqueous κ-carrageenan powder solution was prepared by dissolving a commercial food grade κ-carrageenan powder (Cimpa MCH 5722) in distilled water to obtain a final concentration of 3% (w/v). Next, 1.5 mL of glycerol (Chemi Laboratory) was added as a plasticizing agent, and the mixture was stirred continuously until a clear solution 2/7

was obtained. This mixture was poured into silicone molds and allowed to cool slowly. Before gelation was completed, NPK fertilizer granules (13-26-6 Nutrimon) were added to the silicone mold at a final concentration of 14% (p/p) in order to obtain cylindrical capsules of 1.5 cm3, which were used to analyze the kinetic release behavior through the CBH.

2.2 Water swelling capacity of the CBH CBH capsules (25) were immersed in 140 ml of DH2O for 1, 2, 4, 24 h and 30 days at 25 °C. At each time, five capsules were taken out of the solution and the equilibrium water content (EWC) was calculated with Equation 1: %EWC =

Wh-Wd *100 (1) Wd

Where Wh is the hydrated weight of the sample and Wd is the dry weight of the sample. An average value was obtained from five parallel measurements. The obtained values were used to calculate the nutrient concentration in the aqueous solution that was released from the CBH encapsulated fertilizer with Equation 2: ICS =

ion in the aqueous phase (mg) (2) Vo-dWh t

Where Vo is the initial volumen of the aqueous phase and Veq is the Volume of the aqueous phase at the hydrogel absorption equilibrium at time t. Veqt =Vo – dWh, where d is the water density at 20 oC and Wh.

2.3 Release of NO3-, NH4+ and PO43- from a granulated fertilizer encapsulated with CBH The relationship between time and the ammonium (NH4+), nitrate (NO3-), and phosphate (PO43-) ion release, was determined by placing one capsule of coated NPK fertilizer (13-26-6 Nutrimon) in a stainless-steel basket support inside a 150 mL beaker with 140 mL dH2O (pH 5.5). The beaker was covered with parafilm and incubated at 25 °C in a rotatory shaker. Nutrient release was measured at regular sampling times up to 40320 min. At each sampling time, nutrients were measured in the aqueous solution of three replicates.

Samples were analyzed for NH4+, NO3- and PO4-3 by the Indophenol blue[39], sodium salicylate[40], and Canterbury colorimetric methods[41], respectively, using a Thermo Scientific UV-Visible Analyzer EV-300. The concentrations of the ions in the solutions were measured at wavelengths of 640 nm for NH4+, 415 for NO3- nm and 650 nm for PO4-3.

The calibration curves for all the evaluated ions were linear in the tested ranges, going from 0.05 to 5.5 mmol L-1 for NH4+, from 0.0008 to 0.037 mmol L-1 for NO3- and from 0.0005 to 0.024 mmol L-1 for PO4-3, and they had correlation coefficients (R2) better than 0.997. The equations for the regression lines were y = 0.787x + 0.0349 for NH4+, y = 0.3114x + 0.0015 for NO3- and y = 0.0189x + 0.0132 for PO4-3, where y is the absorbance and x is the concentration of the ion.

The same procedure described above was carried out with shorter incubation times up to 240 and 300 min, in order to evaluate the effect that pH has on the release of the NH4+, NO3- and PO43- ions. Besides using dH2O with a pH of 5.5, solutions of pH 4.5 and 6.3, prepared with acetate buffer and citrate buffer were also used in the release experiments[42, 43].

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Controlled release fertilizer encapsulated by a κ-carrageenan hydrogel All the reagents used for preparation of the buffer solutions were provided by Sigma-Aldrich. Phosphate buffer pH 6.3 was prepared with NaH2PO4 and Na2HPO4 solutions; therefore, PO43- concentration was not measured at pH 6.3 The percentage of released nutrient (%RN) was calculated

by Equation 3: %RN =

Rt *100 Co

model[44-46], which allows us to identify the type of diffusion of the nutrients incorporated in polymeric matrices with spherical, cylindrical or tablet shapes. In order to compare the experimental results to this model a graph is plotted between log cumulative percentage of nutrient release vs. log time. Model equations are presented in Table 1. The model fitting analysis was applied to the data from the incubation experiments at pH 4.5, 5.5 (dH2O) and 6.3. To assess the fit of each model to the data, the correlation coefficient was determined for each case. The best model to explain the nutrient release mechanism from CBH, was the one with the highest adjusted correlation coefficient. Data analysis was carried out with the Statgraphics Centurion XVII program (Version 17.0.16, Statpoint 2009).

(3)

where Rt is the nutrient concentration in the solution at time t and Co is the initial concentration of nutrients in the encapsulate fertilizer.

Nutrient release results from solutions at different pH values were compared by means of a one-way analysis of variance (ANOVA) using the Statgraphics Centurion XVII program (Version 17.0.16, Statpoint 2009).

3. Results and Discussions

2.4 Evaluation of nutrient release data on mathematical models

3.1 Water swelling capacity of the CBH

The physical mechanism of nutrient release was determined by comparing the release data with the following five mathematical models. The first is a zero-order model[43], which describes a system where the nutrient release rate is independent of its concentration. To compare the experimental results to this model, the nutrient release data are plotted vs. time. The second is a first-order model[44], where the nutrient release is directly proportional to the concentration of the nutrient in the fertilizer. To compare the experimental results to this model, the log of cumulative percentage of nutrient remaining data is plotted vs. time.

The third is the Higuchi model[44], where the prime mechanism of nutrient release is diffusion. To compare the experimental results to this model the data obtained in the incubation experiments is plotted as cumulative percentage of the nutrient released vs. the square root of time. The fourth model is the Hixon-Crowell model[44], which describes the nutrient release from systems where there is a change in surface area and diameter of particles and tablets. To compare the experimental results to this model, a graph is plotted between the cube root of nutrient percentage remaining in the fertilizer vs. time. The final model is the Korsmeyer-Peppas

The CBH absorbs water quickly during the first 8 h and then, the value remains constant (Figure 1). The absorption of water by the gel is approximately 4 g in the last 2700 minutes, which allows the calculation of nutrient release taking into account the swelling effect. Another important observation is that the gel manages to absorb 320 times its weight in water and maintain it during the 28 days of the test. During this time and at room temperature, no fungal contamination or deterioration of the hydropolymer matrix was observed.

Figure 1. Sewelling behavior of the CBH hydrogel.

Table 1. Coefficients obtained to evaluate the kinetic model that best fit the experimental nutrient release data.

Nutrient

pH

NO3-

4.5 5.5 6.3 4.5 5.5 6.3 4.5 5.5

NH4+

PO4-3

Zero Order

First Order

Q t = Qo + K o t

ln Q = ln Qo + K 1t

K 8.99 7.70 18.11 5.95 4.93 11.21 2.00 6.38

R2 0.95 0.71 0.58 0.92 0.95 0.82 0.92 0.73

K 0.93 0.84 1.02 0.75 0.70 0.84 0.71 0.26

R2 0.53 0.44 0.84 0.61 0.73 0.52 0.77 0.38

Kinetic Models Higuchi

Qt = K H √ t K 5.09 3.32 9.55 6.00 2.51 1.07 11.18 0.97

R2 0.97 0.91 0.77 0.96 0.95 0.96 0.86 0.94

Hixson

Korsmeyer-Peppas

3 1/ 3 Q1/ o - Qt = K s t

Q t / Q∞ = K k t n

K 0.19 0.12 0.32 0.15 0.07 0.18 0.01 0.10

R2 0.55 0.74 0.63 0.49 0.94 0.87 0.94 0.78

n 0.61 0.44 0.54 0.64 0.55 0.62 0.56 0.67

K 0.35 0.65 0.38 0.11 0.33 0.21 0.31 0.40

R2 0.99 0.95 0.92 0.98 0.99 0.98 0.82 0.97

K = Release constant, R2 = Adjusted Correlation coefficient, n = Indicator of nutrient release mechanism from cylindrical shapes. Kinetic Models Parameters: Qt = amount of nutrient released at time t, Q0 = initial concentration of the nutrient at t =0, Ko = zero order rate constant, Q = percent of nutrient remaining at time t, K1 = first order rate constant, KH = Higuchi dissolution constant, Ks = Hixson release constant, Kk = Korsmeyer release constant.

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Rozo, G., Bohorques, L., & Santamaría, J. During the swelling process, there were no major changes in the cylindrical shape of the capsules.

3.2 Release mechanism The results of the released nutrients as a function of time were adjusted to the different kinetic models, and as expected, according to the correlation coefficient obtained for each nutrient and pH condition (Table 1), the best fit to the data is presented by the Korsmeyer-Peppas model, confirming diffusion, and not the fracture mechanism, as the mechanism of nutrient release from the CBH encapsulated fertilizer, even at acidic pH. Therefore, the Korsmeyer-Peppas model was used to elucidate the type of diffusion from the encapsulated fertilizer. This model describes the nutrient release from a polymeric system with cylindrical shapes, as represented

by Equation 4[39-40], Mt =Kt n (4) M∞

where Mt/M∞ is the fraction of nutrient released at time t; K is the release constant, which includes characteristics of the polymer matrix; and n is the diffusion exponent or nutrient release exponent, an indicator of the nutrient transport mechanism through the polymer. The value of n[38] characterizes the release mechanism of the nutrients as described in Table 2 in cylindrical shapes.

3.3 Rate and percentage of release Figures 2 a, b, c show the release of nutrients in dH2O (pH 5.5) in a long-term incubation. It is observed that 17% of NH4+ was released after a one-hour incubation and 95% was released after 28 days. For nitrate, 23% was released in 24 hours and 68% in 28 days. In the case of PO43-, 1.8% of this ion was released in the first ten minutes of incubation and 62% by day 28. These in vitro results show that the prototype of a CRF coated with the CBH does not meet quite well the criteria as a CRF in regard to the European Standardization Committee (CEN), according to which, nutrients released from a CRF in 24 hours must exceed 15% but not more than 75% in 28 days[51,52]. In spite of this, these study results do not mean that the CBH cannot be used as a CRF coating material. The in vitro release results are an input that contribute to the tailoring of material for optimal controlled release based on the structure-function relationship of the CRF’s building materials. Our experimental data indicate that a significant percentage of the nutrients are being released before 30 days, which means that the structure of the hydrogel must be modified in order to retain the nutrients for a longer time. With a slower nutrient release, it will be more feasible to synchronize the CRF’s nutrient release and the crop’s onset of the nutrients uptake. In particular, this study is interested in the development of a CRF for potato crops, given the worldwide importance of this crop, with close to 19.3 million hectares planted[53], and more than

The graphs of the fraction of nutrients released over time and Equation 5 were used to determine the diffusion coefficient (n) and the release constant (K) log (

Mt ) = log (K) + n log (t) (5) M∞

Table 1 shows that the values of n are between the range of 0.45 to 0.67, therefore the nutrients follow a diffusion release of the anomalous type, which means that the nutrient release from the CRF coated with CBH takes place by non-Fickian diffusion[47]. This anomalous diffusion occurs when the relaxation time of the polymer’s chains is of the same order as the diffusion time of the penetrant solution.

Non-Fickian diffusion is also the result of hydropolymer matrix erosion, which is due to chain disentanglement that occurs in physically crosslinked matrices induced by the matrix swelling fluid, and the hydrodynamic conditions imposed in the release environment[47]. As the diffusing penetrant enters the polymer, it causes a deformation, which induces a stress driven diffusion. What causes Fick’s law to not represent the diffusion in the CBH polymeric matrix is that it does not take into consideration the viscoelastic nature of polymers[48-50].

Figure 2. Release behavior of (a) NH4+, (b) NO3-, and (c) PO43- in a long term incubation in dH2O, pH= 5.5.

Table 2. Different release mechanisms for swelling polymeric matrices with cylindrical shape.

Release exponent (n) 0.45 >0.45 -0.89 0.89 >0.89

Nutrient transport mechanism Fickian diffusion Non Fickian transport Case II transport Super case II transport

Rates as a function of time t-0.5 tn-1 Zero order release tn-1

Source Gouda et al.[44]

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Controlled release fertilizer encapsulated by a κ-carrageenan hydrogel 246.000 tons of NPK fertilizers used in this crop annually in Latin America, in addition to potato being one of the crops with the highest fertilizer application rates around the world[54]. The greatest assimilation time for the potato plant is 35 - 45 days after being sown[55]. It means that in order to use the CRF coated with CBH in the potato crop, it would be necessary to modify the coating material to ensure the greatest outflow of nutrients between 35 and 45 days after planting. To diminish the CBH nutrient release during the first 30 days, the polymer synthesis ratios could be modified to ensure noncovalent spatial networks with three-dimensional conformations that slow down the diffusion of nutrients to the medium. These modifications require the integration of more inner charges to the non covalent spatial networks by cross-linking through covalent chemical bonds, which stabilizes the hydrogel and improves the nutrients retention properties. To integrate more inner charges it is suggested to increase the concentration of any of the following: calcium salts (from 2 to 5% w / v), glycerol (from 5 to 10% w / v) or self-assembling peptides[56]. Additionally, the hydrogel synthesis pH could be modified to guarantee the reduction of hydrolysis and thus ensure the formation of stronger networks. On the other hand, the concentration of fertilizer in the capsule could be reduced to produce a longer nutrient diffusion process. The diffusivity could also be adjusted by changing the concentration of plasticizer additives (or solvents) that increase the mobility of the polymer chains in order to reduce the ions mobility, or by adding fillers such as chitosan or starch. The ultimate refinement of the CRF requires field tests, because the CRF performance will depend on many controlled release factors associated with soil type and rainfall precipitation.

treatments for the N release from the encapsulated fertilizer to the aqueous medium (P> 0.05). The release curves of PO43- in aqueous solution with pH values of 4.5 and 5.5 are presented in Figure 3 c. The release of PO43- towards the aqueous medium is greater at pH 4.5 (P <0.05). After 300 minutes of starting the experiment, 80% of PO43- in the fertilizer coated with CBH, had already been released. In contrast, at pH 5.5, after 300 minutes of incubation, only 22% of the ion had been released into the solution. This difference can be explained because aqueous solutions with low pH values have a higher positive charge of ions in the medium, which attracts the negatively charged phosphate ions found in the hydrogel matrix, which has a negative charge due to the sulfate and hydroxyl groups present in carrageenan.

3.4 Effect of solution pH

This work was carried out with the support of the Departamento Administrativo de Ciencia y Tecnología de la República de Colombia (COLCIENCIAS, Grant No. 1202-669-45888) and Universidad Jorge Tadeo Lozano.

The release curves of NH4+ and NO3- in solutions with different pH values are presented in Figures 3 a and b. In this case, no significant differences were observed between pH

4. Conclusions A CRF encapsulated with κ-carrageenan hydrogel plasticized with glycerol was synthesized. It does not fracture at acidic pH and is able to reduce the speed at which the NO3-, PO43- and NH4+ ions of the fertilizer migrate into the medium. The hydrogel has excellent water retention.

The Korsmeyer-Peppas model was applied successfully, and it was determined that the ions leave the polymeric matrix by Non-Fickian diffusion without polymer fracture. The results of this study suggest that the fertilizer encapsulated with CBH may have potential as a CRF in order to reduce the environmental impact imposed by the excessive use of fertilizers, without problems of toxicity or biodegradability.

5. Acknowledgements

6. References

Figure 3. Effect of different pH values on the (a) NH4+, (b) NO3-, and (c) PO43- release behavior.

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46. Ritger, P. L., & Peppas, N. A. (1987). A simple equation for description of solute release II. Fickian and anomalous release from swellable devices. Journal of Controlled Release, 5(1), 37-42. http://dx.doi.org/10.1016/0168-3659(87)90035-6.

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Received: Apr. 09, 2019 Revised: June 12, 2019 Accepted: June 13, 2019

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.01219

Effect of moringa and bagasse ash filler particles on basalt/epoxy composites Prakash Sampath1  and Senthil Kumar Velukkudi Santhanam1*  1

Department of Mechanical Engineering, Anna University, Chennai, Tamil Nadu, India *vssk70@gmail.com

Abstract This study evaluated the effect of moringa and nano ash filler particles on mechanical, chemical corrosion and moisture absorption of basalt fiber/epoxy composites. Epoxy resin and Araldite HY951 hardener was used as matrix along with the inclusion of filler particles while the bi-directional woven basalt fiber mat was used as the reinforcement in fabricating composites using hand layup technique. Three different composites i.e. Plain BF + matrix with no filler particles, BF + matrix with 10 wt, % moringa ash and BF + matrix with 10 wt, % bagasse ash were fabricated and tested according to the ASTM standards to determine the mechanical properties and chemical corrosion resistance. According to the experimental test results, the composites with moringa ash inclusion showed better properties than the bagasse ash inclusion and with no filler added composites.

Keywords: bagasse ash, basalt fiber, chemical corrosion, Moringa ash.

How to cite: Sampath, P., & Santhanam, S. K. V. (2019). Effect of moringa and bagasse ash filler particles on basalt/epoxy composites. Polímeros: Ciência e Tecnologia, 29(3), e2019034. https://doi.org/10.1590/0104-1428.01219

1. Introduction Basalt fiber based polymer composites play a major role in the automobile and aircraft industries and are seen as better substitutes for glass and carbon fiber with advantages in terms of cost, eco-friendly, seawater resistance, sufficient interfacial adhesion, resistance to chemical and better mechanical properties. Basalt fiber composites have found extensive applications in fuselage, wind turbine blades, pipelines in petrochemical industries, a fine line of front car bumpers, and ship body etc.[1-4]. Natural filler particle’s have become alternative reinforcing in various areas of polymer matrix composites due to their advantages over synthetic fibers, e.g. low density, less tool wear during processing, low cost, non-toxic, easy process, environmentally friendly, and biodegradability[5].

The determination of carbonized bone charcoal particles used in the polypropylene composite enhanced the resistance of the material to surface damage[6]. The Basalt/epoxy composites had improved young’s modulus and impact behaviour in comparison with the Glass/epoxy composites[7-9].

The composite strength by coating of nano-sio2 on basalt fiber was improved interface bonding resistance[10].

The agriculture-plant waste, prepared as bio organic particles, is currently the best suitable, inexpensive and eco-friendly material for the fabrication of advanced composite materials. Many researchers have reported the use of bagasse, dust, rice husk, nano fly ash and coconut shells as filler materials in fabricating polymer composites[11-16], in nature mineral fillers were added in the resin and applied on the fiber, showed increase in the mechanical behavior[17,18].

The determination of Sugarcane bagasse in the form of ash

Polímeros, 29(3), e2019034, 2019

particles and suggested that it be used as fillers in cement and clay bricks, due to its being rich in crystalline silica[19].

The experimental on rigid (Al2O3) as nano filler in a glass fiber composite found that the mechanical properties improved up to (5-10) wt% filler content ratio in a glass fiber composite[20].

Many investigations on oil palm ash’s (OPA) have rigid nano filler content in the unsaturated polyester polymer matrix with 5 wt% to 35 vol% filler content ratios, and observed that the mechanical properties were improved[21].

In nature the latent of maize stalk charcoal particles as reinforced in polyester composites enhanced the tensile properties of composites[22].

In most situations, fiber reinforced composites are widely used in automobile components and aircraft structure applications: therefore testing a polymer matrix composite after creation a drilled hole develops unavoidable. The big size of an open hole leads to high stress absorption in the open hole composite. The open hole tensile (OHT) strength of maleic anhydride mixed polypropylene (MAPP) dried polymer matrix composites had increased their tensile properties by 4-7%[23]. Investigated the tensile and impact performance of coal ash strengthened (at various wt %) glass fiber composites and then concluded. The 20% and 16% of coal ash strengthened glass fabric laminates exhibits better performance during tensile and impact tests than compared to other (0%, 4%, 8% and 12% wt coal ash) composite laminates[24].

An investigation on Chemical corrosion on the polymer matrix composites is necessary in some applications.

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O O O O O O O O O O O O O O O O

Sampath, P., & Santhanam, S. K. V. For example, some containers, off-shore, vessels, tubes, platforms and equipment’s in marine and chemical industrial applications may be corroded after long-term service in an alkali environment. It is supposed that one of the difficulties precluding the extensive use of polymer composites is the lack of long‑term durability and performance data when servicing in critical environments. So, it is necessary to understand how the materials behave during chemical corrosion on long-term applications[25,26].

The determination of HNO3 exhibited the stronger corrosion to ECR glass fiber than that in different HCl and H2SO4. For HCl, it mainly destroys the network structure of [AlO4], while H2SO4 and HNO3 destroy the structure of [SiO4]. Moreover, the sizing effect on acid resistance of ECR glass fiber is also discussed here. The sizing agent has limited protection time about 70 h on glass fiber[27].

The determination of no natural filler fibres have very low water absorption followed by samples with chemically treated natural fiber and untreated natural fiber exhibit maximum water absorption. Specimens with chemically treated fibres exhibit lower water absorption than specimens fabricated using untreated fiber as a result of surface of natural fiber getting modified by alkaline treatment which aids in good interface bonding between matrix and fiber. The specimen reinforced with banana fiber exhibit increased water absorption as the volume of banana fiber in the composite is more than that of bagasse[28].

To the best of author’s knowledge, no scientific report was made to investigate the effect of MOA ash particles as filler particles in the basalt fiber reinforced polymer composites. Only a few researchers have focused on natural filler particles influence on the polymer composites. The goal of this research was to investigate the usage of agro waste ash particles as fillers in basalt/epoxy composites. Furthermore, the effects of moringa and bagasse ash filler particles on the chemical composition, mechanical, chemical corrosion and water absorption properties of fabricated composites have determined.

for 10 hr in an oven. Table 1 shows the chemical composition of moringa and bagasse ash particles.

2.2 Method Hand lay-up method was used for the composite fabrication process. Addition of 10 gms of natural fillers Moringa and Bagasse was mixed in 200 ml of epoxy resin and mechanically stirred for 15 min. 100 mL hardener was added in the resin and stirred for 30 min. The resin/hardner mixture was taken in the ratio of 2:1 to obtain a curing time of 3 hr. A layer of basalt fiber mat was spread over the mould and resin mixture was introduced over the surface of the fiber mat. The resin evenly spread all over the matrix using a roller. This procedure was repeated for 8 fiber laminas to obtained at specimen thickness of 3.5mm. Table 2 shows the fabricated composite/specimen notations used.

Water absorption test was conducted on composites of dimensions 3×3×3 mm. The composite samples were fully immersed in a beaker containing distilled water maintained at atmospheric temperature and then removed at periodic intervals. The water on the specimen surface was removed and was weighed immediately. The water absorption percentage is calculated using the Equation 1.

Wt − Wd ∆W% =

Wd

where, Wt - weight of wet composite, Wd - weight of dry composite Fabricated composites were cut into small pieces weighing 5 – 10 gm and immersed in 150 mL acid solution for varying time durations from 24 to 120 hr at room temperature.

The three different acid solutions are sulfuric acid, nitric acid and hydrochloric acid. The corroded composite samples were sanitized with the help of distilled water, and dried in Table 1. Chemical composition of moringa and bagasse ash particles.

2. Materials and Methods 2.1 Materials Bagasse waste was collected from sugarcane industry and Moringa from green farm house. Chemicals used were hydrochloric acid, sodium hydroxide and ethanol. Basalt fiber in the form of bi-directional woven mat was applied as the reinforcement with a matrix mixture (LY556 Epoxy resin + Hardener) in the ratio of 1:10. The Moringa and bagasse were chemically treated by submerging the strands in NaOH solution for 6 hr at room temperature and dried.

Strands were converted into powder using a mill grinder.

Bagasse and Moringa oleifera were heated using an electric furnace at a temperature of 500 °C for 5 hr to obtain ash.

Ash particles were mixed with 2.5 mL NaOH solution, boiled at 75 °C for 3 hr with magnetic stirrer. The precipitate was mixed with distilled water and filtered. The process was continued 3-4 times for the deletion of NaOH solution from ash. The resultant was filtered, mixed with 2% HCl solution and boiled at 75 °C for 4 hr. The collected precipitate was finally filtered, mixed with 1% ethanol and dried at 110 °C 2/7

(1)

Elements

% composition Moringa

SiO2 Al2O3 Fe2O3 TiO2 P2O5 CaO MgO Na2O K2O SO3

67.22 2.4 7.45 1.9 0.87 5.35 1.24 0.91 7.07 0.64

% composition Bagasse 65.4 1.92 7.36 1.46 0.98 5.0 1.17 0.87 6.22 0.42

Table 2. Specimen notation for fabricated composites and respective matrix fraction.

No.

Composite/ specimen

Hardener (mL)

Epoxy Resin (mL)

Ash filler particles 10 wt. %

1 2 3

A B C

100 100 100

200 200 200

Bagasse Moringa

SI.

Polímeros, 29(3), e2019034, 2019

Effect of moringa and bagasse ash filler particles on basalt/epoxy composites oven. Further the weight loss of acidic treated composites was measured using an electronic weigh balance with an accuracy of 0.0001 gm. The weight loss ratio was calculated using Equation 2.

W% =

M 0 − M1

M1

(2)

where, M0 – Initial mass of composite, M1– Final mass after acid treated The impact (charpy) test specimens with dimensions 75×10×10 mm having a groove penetration of 1.5 mm and a groove point radius of 0.02 mm at an angle of 45° was used.

The tensile test specimens with dimensions 165 mm × 19 mm (dog bone shape) were prepared according to the ASTM D638 standard.

3. Results and Discussions 3.1 Open hole tensile test Open hole tensile tests were carried out using a universal testing machine (make: INSTRON 50 KN) and the results are shown in Table 3. From Figure 1 it is evident that the composite C has higher tensile strength compared to the other composites. The composite A exhibited 30% lesser tensile strength than the ash filler particles included composites.

The addition of ash filler particles enhances the matrix adhesion strength and increases interfacial bond strength between matrix and fiber. The chemical treatment of ash filler particles removed the impurities by neutralizing the potential of hydrogen i.e. acid and base treatment and this helps in improved wettability with the matrix and increases the elastic behaviour of the matrix. The ash filler particles Table 3. Open hole tensile test results.

S.no

Composite/ Displacement, Stress, Elongation Load, kN specimen mm MPa %

1

A

5.78

4.15

84.7

2.12

2

B

7.27

5.01

113.39

2.56

3

C

8.51

5.20

138.6

2.66

have a globular structure and provide more surface contact area with the matrix increasing bond strength.

The presence of silica in Moringa ash contributes to the increase in tensile strength and this result agrees well with the results of Bin wei[29] that the SiO2 nano-filler increased the bonding rigidity of basalt fiber/epoxy composite.

Figure 1 shows the stress-strain plot which clearly reveals that composite C had higher stress values and elongation % compared to Composite A and composite B. The matrix failure in the composite A occurred in early stage due to the poor wettability with the fiber. Composite A suffered failure at 84.7 MPa due to disintegration of matrix and fiber interface. Composite B and C showed improved strength due to better bonding between the matrix and fiber which consequently resulted in increased interfacial shear strength.

The composite C had higher elastic limit due to the presence of Moringa ash particles in the matrix increasing it yield strength. This is because as the filler content increased, the interfacial bonding area also increased between the filler (hydrophilic) and the epoxy matrix (hydrophobic), which increases the tensile strength[30-32].

3.2 Impact test Figure 2, shows the total energy absorbed by the composite specimen A, B and C during the impact test and the results are shown in Table 4. Composite C had an energy absorption of 24 J and this was 16% higher than the composite B (20.6 J) and 47% higher than composite A (16.3 J).

The higher percentage of silica particles present in moringa ash filler particles in the composite C improved the fracture toughness of composites compared to composite B as shown in Figure 2. The same results have found that the uniform distribution of the ash filler particles led to the improvement in fiber-matrix adhesion properties as the existence of rigid fillers particles reduced the matrix brittle behaviour. The similar result have found with the impact performance of 20% and 16% of coal ash filled glass fiber composite exhibits better performance during the impact test than compared to other (0%, 4%, 8% and 12% wt coal ash) composite laminates[24].

Figure 2. Impact test results.

Table 4. Energy Absorption of specimen.

Figure 1. Stress vs strain plot for open hole tensile test of fabricated composites.

Polímeros, 29(3), e2019034, 2019

S.NO 1 2 3

Composite/Specimen A B C

Energy Absorption, Joules 16.3 20.6 24

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Sampath, P., & Santhanam, S. K. V. 3.3 Chemical corrosion From Figure 3. Corrosion test specimens immersed in three different acid solutions and Table 5 shows the weight loss calculation of each composite specimen using sulphuric acid, nitric acid, hydrochloric acid. Deterioration in composite A was found to be higher when compared to other composites when treated in sulphuric acid. The Sulphuric acid caused weight losses by destroying the network structure of SiO4 present in filler particles and fibers. The weight loss of composite A is due to chemical reaction of matrix with the acid solution. From Figure 4, it is observed that the weight loss % increases with increasing time duration. For the composites B and C with the inclusion of filler particles, the composites underwent observable changes beyond the 90 hr as the acidic resistance is limited.

The composite A shows linear increase in wt. % loss beyond 90 hr. Composites treated in sulphuric acid vs weight loss is shown in Figure 4, for nitric acid in Figure 5 and for hydrochloric acid in Figure 6. The results showed that ion precipitation rate of composite A are lesser than that of composite B and C. The rigid filler particles on the basalt/epoxy composite can reduce the acid corrosion, which leads to the low weight loss rate in the initial stage of

Figure 3. Corrosion test – specimen immersed in acid solution.

corrosion. When the chemical reaction between epoxy and acid occurs, the protective effect on basalt fiber weakens.

Therefore, the ash filler particles act a barrier in limiting the corrosion of epoxy matrix in all the acid solutions considered. The similar results were found from the glass fiber composite with three acid solutions Therefore, basalt fiber composites has high acid resistance compare than ECR glass fiber and has been used in the manufacture of basalt fiber reinforced plastic pipeline[27-31].

3.4 Water absorption Basalt fiber contains plenty of hydroxyl groups which react with water molecules and form hydrogen bonds.

The water molecules easily penetrate in the composite A due to weak bonding of the matrix and basalt fiber.

The chemically treated filler particles have better interfacial bond strength with the matrix and the fiber which reduced the penetration of water molecules to react with the fiber. From Table 6 the composite C has improved the water absorption capacity. The water absorption results are shown in Figure 7. The similar results have found from the water absorption of the composites increases at higher filler particles content, although there was not much difference

Figure 4. Weight loss ratio of Sulphuric acid treated samples.

Table 5. Weight loss test result of different composite into different acid solutions.

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S.NO

Composite

Time (hr)

1

A

2

B

3

C

24 48 72 96 120 24 48 72 96 120 24 48 72 96 120

H2SO4

HNO3

HCL

Mass loss, wt % 2.3 4.8 6.09 7.4 8.5 1.1 2.8 3.6 4.8 5.1 1.3 2.5 3.5 4.5 4.9

Mass loss, wt % 2 4.6 6 6.7 7.9 1.48 3 3.79 4.8 5.25 1.37 3.2 3.8 5 5.29

Mass loss, wt % 3 5.29 6.2 6.3 8.6 0.72 1.8 2.7 4.3 4.5 0.58 2.19 2.4 3.2 4.7

Polímeros, 29(3), e2019034, 2019

Effect of moringa and bagasse ash filler particles on basalt/epoxy composites Table 6. Calculation of water absorption ratio by different composites.

Composite A B C

24hrs 0.24 0.171 0.197

48hrs 0.502 0.402 0.419

ΔW% 72hrs 0.698 0.598 0.617

96hrs 0.76 0.62 0.64

120hrs 0.79 0.64 0.67

in water absorption between composites with banana filled basalt fiber composite have exhibit increases water absorption as the volume of banana fiber in the composite is more than that of bagasse filled composites[28].

3.5 Failure analysis using SEM

Figure 5. Weight loss ratio of Nitric acid treated samples.

From SEM images, the following failures were identified: (a) fiber crack, (b) crack propagation, (c) fiber debonding, (d) matrix crack and fiber breakage, (e) fiber pullout, (f) porosity, (g) Matrix crack and fiber failure, (h) Fiber breakage and fiber crack. From Mechanical properties (Open hole tensile and Impact test) test result, the material absorbs the energy through plastic deformation, and it could be observed from the formation of surface crack by failure mechanism.

Figure 8 (a-d) Shows that with the addition of natural filler in the basalt fiber composite exhibited improved mechanical properties as observed in the fiber failure in the depressed zone when compared to the plain basalt fiber composite. Intra layer failures and matrix cracking have been identified from the SEM images.

Figure 6. Weight loss ratio of hydrochloric acid treated samples.

Figure 7. Water absorption result.

Polímeros, 29(3), e2019034, 2019

Figure 8 (e-j) Indicates the morphological feature of natural fillers reinforced basalt fiber composite immersed into the three different acid solutions as examined using scanning electron microscopy (SEM). The agglomeration of particles, resulting in poor interfacial bonding between the particle-fiber-epoxy interactions created different modes of failure on the basalt fiber composite laminates. SEM images provide a better explanation of the damage modes of composites like matrix breakage, fiber breakage, delamination, etc. Presence of Moringa ash filler particles on the surface of basalt and in the epoxy matrix confirms the homogeneous distrubtion of ash particles. This is a qualitative indication of a greater interfacial strength between the fiber-filler and the matrix-filler.

Figure 8 (k-l) Shows the SEM images of composite surface after the water absorption test. If voids content is higher in the natural fillers reinforced basalt fiber composite, there is a more chance for higher amount of water absorption and if voids are less, the composite absorb less amount of water in the particle-fiber-epoxy system. Higher void content in the composites causes degradation of composites due to higher absorption and also creates poor interfacial bonding between the natural filler in the epoxy and the matrix. The effect would degrade the composite strength.

Water absorption of moringa filler reinforced basalt fiber composite is less than that of bagasse filler reinforced composite due to minimum voids in the moringa filled composites.

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4. Conclusions The composite materials were successfully fabricated using hand lay-up technique with basalt fiber as the reinforcement and inclusion of fillers (moringa and bagasse ash particles) as the matrix. From the experimental study, the following conclusions have been drawn: • Open hole tensile strength of the composites C had higher ultimate strength (138.6 MPa) which is 17% higher than composite B (113.39 MPa) and 47% of composite A (84.7 MPa) and this was due to the presence of ash filler particles in strengthening the matrix yield strength.

• The impact strength of the composite C was 16% higher than composite B and 47% of composite A due to the strengthening effect of silica particles present in the moringa ash.

• The resistance to chemical corrosion was higher in the composites B & C with the inclusion of ash filler particles when treated under sulphuric acid, nitric acid, hydrochloric acid solutions.

• The water absorption of the composite C exhibited the better resistance than that of composite B and A due to the increase in interfacial bond resistance between the matrix and the fiber.

• The microstructural analysis of SEM image suggested that the fractured surfaces, internal cracks and interfacial properties are observed the tested composite.

5. Acknowledgements The authors gratefully acknowledge the assistance from Department of Science and Technology, New Delhi in the financial support to carry out this research work under Promotion of University Research and Scientific Excellence - II scheme. One of the authors, Mr. Prakash Sampath is thankful to Department of Science and Technology, New Delhi for the award of Department of Science and Technology - Promotion of University Research and Scientific Excellence fellowship.

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Figure 8. Microstructural image for Tested composites (a-l).

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Received: Mar. 12, 2019 Revised: July 03, 2019 Accepted: July 12, 2019 7/7

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.01419

Plastics floatability: effect of saponin and sodium lignosulfonate as wetting agents Fernando Pita1*  and Ana Castilho1  Centro de Geociências – CGEO, Departamento de Ciências da Terra – DCT, Faculdade de Ciências e Tecnologia – FCTUC, Universidade de Coimbra – UC, Coimbra, Portugal

1

*fpita@ci.uc.pt

Abstract Froth flotation is the most common process in mineral processing. For the separation of plastic mixtures by flotation, the use of appropriate wetting agents is mandatory. The floatability of six post-consumer plastics was studied at different concentrations of the wetting agents, saponin and sodium lignosulfonate. Also, the influence of size and shape of the particles were analyzed. Contact angle and floatability of the six plastics decreased with increasing wetting agents concentration. The order of floatability is similar to the order of the contact angles values. However, the influence of the wetting agents in the plastics floatability is more pronounced than in the contact angle. Floatability decreased with the increase of particle density, particle size and spherical shape. For fine particles floatability is fundamentally conditioned by the contact angle, while for coarse particles floatability is fundamentally conditioned by the particles weight.

Keywords: flotation, particle size, plastic, wetting agent.

How to cite: Pita, F., & Castilho, A. (2019). Plastics floatability: effect of saponin and sodium lignosulfonate as wetting agents. Polímeros: Ciência e Tecnologia, 29(3), e2019035. https://doi.org/10.1590/0104-1428.01419

1. Introduction Plastics have become widely used materials because of their advantages, such as cheapness, durability, lightness, and hygiene. Global production of plastics has been continuously rising, gradually replacing materials like glass and metal.

In the last decade, the world production of plastics has been growing around 3.5% per year, increasing from 230 million tonnes in 2005 to 348 million tonnes in 2017, with an European production of 64.4 million tonnes (18.5% of the world plastic production)[1]. In 2017, Europe demand for plastic materials was 51.2 million tonnes, but about 39% of the demand is concentrated in two countries: Germany and Italy[1]. To take full advantage of the benefits of plastics, their products require a proper recovery and management when they reach the end of their service life. Recycling is the preferred option for plastics waste. In Europe, in 2016, 31.1% of post-consumer plastic was recycled and 41.6% was recovered through energy recovery processes, being the landfill disposal the main form of plastics disposal (27.3%)[1]. In the last decade, recycling has increased by 79%, incineration has increased by 61% and landfilling has decreased by 43%.

It should be noted that, in some countries, such as Austria and the Netherlands, less than 1% of plastics are deposited in landfills, while in other countries, like Malta and Greece, about 80% of plastics are deposited in landfills.

Plastics recycling require the separation from other constituents and also the separation of plastic mixtures into individual plastics in order to achieve a good recycled plastic quality. Over the recent years, some separation technologies developed in mineral processing engineering have been applied in the separation of plastic mixtures into their individual components. One of them is the froth flotation method.

Polímeros, 29(3), e2019035, 2019

Flotation is the most common concentration process used by the mineral industry, allowing the separation of hydrophobic material from hydrophilic one. In a mixture of hydrophobic and hydrophilic particles suspended in water, with air bubbled through the suspension, the hydrophobic particles attach to the air bubbles and by buoyancy are transported from the suspension to the froth zone. The hydrophilic particles do not attach to air bubbles, thus remaining in the pulp.

The use of flotation for plastics separation is particularly challenging because, unlike most minerals, most plastics are hydrophobic in their natural state. Thus, in order to separate plastic mixtures by froth flotation, one or more plastic type must become hydrophilic by the addition of selective wetting agents, while the others are maintained hydrophobic[2-5]. Several wetting agents of plastics have been tested for the selective flotation of plastic mixtures.

Methyl cellulose, polyvinyl alcohol, polyethylene glycol, gelatin, tannic acid, saponin, terpineol, triton X-100, calcium lignosulfonate and sodium lignosulfonate have been used as wetting agents by several authors[6-17]. Plastic flotation is controlled not only by the hydrophobicity, but also by the shape and size of the plastic particles[18-22].

This work aimed to study the influence of saponin and sodium lignosulfonate on the floatability of six plastics, and also, to compare it with the effect of tannic acid[22]. Moreover, it tried to establish the ideal concentrations of these two wetting agents to separate plastic mixtures and evaluate the relations of the floatability of plastics with contact angles and gravity factors, such as particle density, particle shape and particle size.

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2. Experimental 2.1 Materials and methods This work used six types of post-consumer plastic: Polystyrene (PS, black), Polymethyl methacrylate (PMMA, white), Polyethylene Terephthalate (PET-S, blue), Polyethylene Terephthalate (PET-D, transparent), Polyvinyl Chloride (PVC-M, light green) and Polyvinyl Chloride (PVC-D, gray) (Figure 1). The density of these plastics, measured by an Ultra Pycnometer (AccuPyc 1330), are as follows: PS: 1.047 g/cm3; PMMA: 1.204 g/cm3; PET-S: 1.372 g/cm3; PET-D: 1.364 g/cm3; PVC-M: 1.326 g/cm3 and PVC-D: 1.209 g/cm3. To study the influence of the particle size, the material was classified by sieving in five size fractions: +1-1.4 mm, +1.4-2 mm, +2-2.8 mm, +2.8-4 mm and +4-5.6 mm. The shape factor, defined by the ratio between the thickness (Dmin) and length (Dmax), of the six plastics versus size fractions was already assessed[22]. PMMA, PS and PVC-D particles presented more spherical shapes, while PET-D particles showed lamellar shapes. Particle shape factor proportionally decreased with particle size in PET-D, PET-S and PVC-M, whereas in the other three plastics, particle shape factor increased with particle size.

Wetting agents used in the flotation experiments were saponin (84510 Sigma Aldrich) and sodium lignosulfonate (471038 Sigma Aldrich). Methyl isobutyl carbinol (MIBC) (109916 Sigma Aldrich) was used as frothing reagent.

2.2 Contact angle measurements The interaction between particles and air bubbles is a key element to effectively recover valuable minerals via flotation process. The easiest way to determine the hydrophobicity of a substance is to measure the contact angle, that is, the angle

formed by a water droplet in contact with a solid surface.

There is a positive correlation between the hydrophobicity and the floatability, i.e., the flotation recovery increases with the increase of the contact angle. Contact angles were measured in the Data Physics Instruments OCA20 equipment, using the sessile drop method. A water drop was placed onto the surface of plastic particles (before and after their treatment with the wetting agents) and the different contact angles were measured. This process was repeated seven times for each plastic and the average value was considered to be the contact angle of the plastic.

2.3 Flotation experiments Flotation experiments were performed in a Denver cell with a capacity of 3 dm3. Each test used 40 g of plastics and was conditioned with different concentrations of wetting agents (saponin and sodium lignosulfonate) for about 5 minutes and later with the frother (MIBC) for about 2 minutes before the flotation, at the constant concentration of 30 × 10-3 g/L in all experiments. After conditioning, floated product was collected over 6 minutes. Both the floated and the sunk (non-floated) were dried, screened and weighed. Flotation tests were carried out previously with one-component plastic samples at different wetting agents concentrations. Then, flotation separation of plastic mixtures was done using bi-component mixtures, with each plastic type contributing with 20 g. The effectiveness of the flotation tests was evaluated by the separation efficiency, defined by Schulz[23]. In the separation tests of the plastic mixtures, the plastic types presented in floated and sunk were separated from each other by manual sorting, since the various types of plastics have different colour and shape. Experiments were done three times under similar operating conditions.

Figure 1. Original pictures of six plastics.

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Plastics floatability: effect of saponin and sodium lignosulfonate as wetting agents

3. Results and Discussion 3.1 Effect of the wetting agents concentration on the contact angle of plastics Contact angle of the plastics decreases with increasing wetting agents concentration (Figure 2). In the absence of wetting agent, all plastics have large contact angle, with PS having the largest contact angle (97º) and PET-D the lowest (73º). The contact angles measured of the six plastics are in agreement with previous studies[2,6,24-27].

The contact angle of the six plastics decreases with increasing saponin concentration (Figure 2a). The contact angle of PVC-M is slightly smaller than that of the other five plastics. The decrease was stronger for concentrations of saponin below 10 mg/L and was more pronounced for PS, PVC-M and PVC-D plastics. The effect of saponin on the contact angle of PS and PVC is slightly different from what was observed by Ma[25].

The contact angle of the six plastics also decreases with increasing sodium lignosulfonate concentration (Figure 2b).

The reduction of the contact angle was more pronounced for PS, PVC-M and PVC-D plastics. The effect of sodium lignosulfonate on the contact angle of PS is similar from what was observed by Ma[25], but is different from what was observed by Wang et al.[28]. The effect of sodium lignosulfonate on the contact angle of PET follows what was observed by Florido and Torem[29]. The effect of sodium lignosulfonate on the PVCs contact angle is similar from what was observed

by Shibata et al.[6], Ma[25] and Wang et al.[28], but is different from what was observed by Pascoe and Hou[18].

Pita and Castilho[22] verified that the decrease of contact angles of these six plastics occurs for small concentrations of tannic acid. A concentration of 5 mg/L of tannic acid caused similar decreases of the contact angle to those obtained for concentrations of 100 mg/L of saponin and 300 mg/L of sodium lignosulfonate.

3.2 Effect of the wetting agent concentration on the floatability of plastics The recovery of the six plastics in the floated decreases with an increase of the two wetting agents concentration (saponin and sodium lignosulfonate) (Figure 3). It is verified that all the studied plastics are naturally floatable, because in the absence of wetting agents the flotation recovery is about 100%. The same behaviour was observed in other studies[6,19,24,25,27-29]. Even PET-D and PMMA, with contact angle below 80º in the absence of a wetting agent, both naturally float. Similarly, other studies[25,27,28] using PS, PMMA, PET and PVC, found that these plastics with contact angles of about 75º, also float naturally.

Pita and Castilho[22] verified that depression of these six plastics occurs in smaller concentrations of tannic acid.

The wetting ability order of these wetting agents for these six plastics is tannic acid > saponin > sodium lignosulfonate.

Analogously, in the flotation of PS, PET and PVC, Shen et al.[2], Wang et al.[13] and Wang et al.[28] found that the depressing effect of tannic acid is stronger than sodium lignosulfonate.

In the presence of saponin, PVC-M is the plastic with lower floatability (Figure 3a). The lower floatability of PVC-M can be justified by its lower contact angle. However, in spite

Figure 2. Contact angle of six plastics versus saponin (a) and sodium lignosulfonate (b) concentration (with standard errors less than 1.67).

Figure 3. Floatability of six plastics versus saponin (a) and sodium lignosulfonate (b) concentration (with standard errors less than 1.74) (MIBC: 30 × 10-3 g/L).

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Pita, F., & Castilho, A. of PS having one of the smallest contact angles (Figure 2a), it does not present low recoveries, perhaps explained by its low density. Also, PET-D with one of the smallest contact angles (Figure 2a), have not had small recoveries, justified by its lamellar shape. The largest difference in floatability between PVC-M and the other plastics was obtained for a concentration of saponin between 10 mg/L and 30 mg/L. The depression of plastics with sodium lignosulfonate can only be achieved with high concentrations (Figure 3b).

PVC-M and PVC-D are the plastics with lower floatability.

PMMA and PET-D plastics present similar behaviour and high floatability for high concentrations of sodium lignosulfonate.

The low floatability of PVC-M can be justified by its lower contact angle. Better floatability of PS could be justified by its low density of this plastic. On the other hand, although PET-S has the greatest contact angle, it does not present the greatest recovery, perhaps explained by its greater density.

PVC-M is depressed selectively from the other plastics.

Maximum difference between the recovery of PVC-M and

the other plastics is obtained with concentrations of sodium lignosulfonate between 100 and 300 mg/L. For the two wetting agents, the difference between the floatability of the plastics is much greater than the difference between the respective contact angles.

Pita and Castilho[22] verified that in the presence of tannic acid, PS is the plastic with higher floatability, and the other five plastics show similar variation of the floatability, being depressed at very low tannic acid concentration. So, the largest difference of the floatability between PS and the other five plastics is obtained using tannic acid[22], but the largest difference of the floatability between the other five plastics is obtained using saponin or sodium lignosulfonate.

3.3 Effect of the particle size on the floatability of plastics Particle size is an important parameter in the flotation process. For all size fractions, the floatability of the six plastics decreases with the increase of the two wetting agents concentration (Figure 4-5). In the presence of

Figure 4. Influence of saponin concentration and particle size on the floatability of six plastics (with standard errors less than 1.70) (MIBC: 30 × 10-3 g/L).

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Plastics floatability: effect of saponin and sodium lignosulfonate as wetting agents

Figure 5. Influence of sodium lignosulfonate concentration and particle size on the floatability of six plastics (with standard errors less than 1.62) (MIBC: 30 × 10-3 g/L).

saponin or sodium lignosulfonate, for the three finer size fractions, PMMA presents the greatest floatability, and for the two larger size fractions, PET-D presents the greatest floatability. PVC-M presents the smallest floatability for all size fractions.

For the six plastics and for the two wetting agents, the flotation recovery decreased with increasing particles size. It was also observed that plastics with a more lamellar shape, such as PET-D, show less influence of the particles size in the floatability. On the other hand, the floatability of plastics with a more regular shape (PMMA and PVC-D) is more influenced by the particles size. The decrease of the floatability of plastics with the increasing particles size, mainly plastics with a regular shape, is a consequence of the greater weight of the coarse particles, which require a larger number of air bubble carriers in order to float, that is, to form particle-bubbles aggregates with a density lower than the density of the water.

Considering the plastic particles weight, the density of the particle-bubbles aggregates of the six plastics has the order followed in Table 1. Also, in Table 1 it is presented the order of contact angle of the six plastics and the order Polímeros, 29(3), e2019035, 2019

of floatability in the presence of the saponin, sodium lignosulfonate and tannic acid[22] for the finer and coarser fractions. In the presence of saponin, for the finer fraction, PVC-M has the lowest floatability as consequence of its lower contact angle. On the contrary, PMMA has the greatest floatability as consequence of its greater contact angle.

Since for the finer fraction the particles are lightweight, the floatability is mainly affected by hydrophobicity. For the coarser fraction, the greatest floatability of PET-D can result from the lower weight of the particle-bubbles aggregates, therefore, they require the attachment of less air bubbles to float, whereas the lowest floatability of PVC-M can result from the greatest weight of the particle-bubbles aggregates or the lowest contact angle. For the coarser fraction, the low floatability of PVC-D and PMMA can result from the high density of the particle-bubbles aggregates.

In the presence of sodium lignosulfonate, for the finer fraction, PVC-M has once again the lowest floatability as consequence of its lower contact angle; PMMA has the greatest floatability as consequence of its greater contact angle. For the coarser fraction, the greatest floatability of PET-D can result from the lower weight of the PET-D 5/9

Pita, F., & Castilho, A. Table 1. Ranking of the six plastics in the finer and coarser fractions: density of particle-bubbles aggregates; contact angle and floatability in the presence of the three wetting agents.

Particle-bubble aggregates density

Ascendant order of the six plastic

1/1.4 mm

PS < PET-D ≈ PMMA < PVC-D < PET-S ≈ PVC-M

4/5.6 mm

PS < PET-D < PET-S < PVC-M ≈ PMMA ≈ PVC-D

Saponin (10 mg/L) Contact angle

PVC-M < PET-D < PVC-D < PS ≈ PET-S < PMMA

Experimental floatability 1/1.4 mm

PVC-M< PET-S < PVC-D < PET-D < PS < PMMA

4/5.6 mm

PVC-M ≈ PVC-D < PMMA < PET-S < PS < PET-D

Sodium lignosulfonate (300 mg/L) Contact angle

PVC-M < PS < PET-D < PET-S < PVC-D < PMMA

Experimental floatability 1/1.4 mm

PVC-M < PVC-D < PS < PET-D < PET-S < PMMA

4/5.6 mm

PVC-M ≈ PVC-D < PS < PMMA < PET-S < PET-D

Tannic acid (3 mg/L)[22] Contact angle

PET-D < PVC-M < PMMA < PVC-D < PET-S < PS

Experimental floatability 1/1.4 mm

PET-D < PVC-M < PET-S < PVC-D < PMMA < PS

4/5.6 mm

PVC-M ≈ PMMA < PVC-D ≈ PET-S < PET-D < PS

particles, whereas the low floatability of PVC-D can result from the higher weight of the PVC-D. In the presence of tannic acid, for the finer fraction, PET-D has the lowest floatability as consequence of its lower contact angle[22]. For the coarser fraction, the lowest floatability of PVC-M and PMMA can result from the highest weight of the particles, whereas the greatest floatability of PS can result from the greater contact angle or from the lower density of the particle-bubble aggregates.

It is verified that the plastics floatability depends on their hydrophobicity (contact angle) and particle weight (size, shape and density). Plastics with larger contact angle, lower density, smaller size and lamellar shape, have achieved higher floatability. In the presence of the three agents and for finer particles, the floatability order is similar to the contact angle order, and for the coarser fraction, the floatability order is similar to the density order. For the finer fraction, the influence of hydrophobicity (contact angle) is stronger, whereas for the coarser fraction, the influence of particle weight is stronger.

3.4 Separation of bi-component mixtures of plastics In the presence of saponin or sodium lignosulfonate, PVC-M has lower floatability than the others plastics. So, in the presence of saponin or sodium lignosulfonate, further flotation tests were developed using bi-component plastic mixtures of PVC-M and other plastic in equal proportions, to obtain a selective separation of PVC-M. For saponin flotation tests, the concentration of 10 mg/L was used, because it led to the largest difference in floatability between PVC-M and the other plastics. Sodium lignosulfonate concentration of 100 mg/L led to the best separation for PVC-M/PS, whereas concentration of 300 mg/L led to the best separation for PVC-M//PMMA, PVC-M/PET-S and PVC-M/PET-D. 6/9

Saponin led to a better separation of the PVC-M/PS mixture than sodium lignosulfonate (Table 2 and 3). However, sodium lignosulfonate led to a better separation of PVC-M/PMMA, PVC-M/PET-S and PVC-M/PET-D than saponin. The best results were obtained in the PVC-M/PMMA and PVC-M/PET-D mixtures separation with sodium lignosulfonate, having the highest separation efficiency (near 70%) (Table 3).

The worst results were obtained in the PVC-M/PET-S mixture with saponin, having the lowest separation efficiency (53.2%). The separation efficiency of PVC-M/PS mixture with saponin and sodium lignosulfonate is worse than that observed by Pita and Castilho[22] who used tannic acid and verified a separation efficiency of PVC-M/PS mixture in the presence of tannic acid of 94.5%.

The influence of the particle size in the separation of PVC-M and other plastics with saponin and sodium lignosulfonate was also analyzed (Table 4 and 5). In the presence of saponin, the separation of the four mixtures presented the worst results for the finer particles. However, in the presence of sodium lignosulfonate, PVC-M/PMMA and PVC-M/PET-S mixtures presented the worst results for the coarser particles.

The PVC-M/PMMA mixture in the presence of sodium lignosulfonate was the one that led to the best results (Table 5).

For the +1.4-2 mm fraction, the separation efficiency was maximum (96.5%), where the PVC-M recovery in the non-floated was 96.9% with a grade of 99.6% and the PMMA recovery in the floated was 99.6% with a grade of 97.0%.

In the presence of saponin, for the finer fraction, the separation efficiency order of the four plastic mixtures (PVC-M/PET-S < PVC-M/PET-D ≈ PVC-M/PMMA ≈ PVC-M/PS) is similar to the contact angle order (PVC-M < PET-D < PS ≈ PET-S < PMMA). For the coarser fraction, the separation efficiency order of the five plastic Polímeros, 29(3), e2019035, 2019

Plastics floatability: effect of saponin and sodium lignosulfonate as wetting agents Table 2. Results of the flotation tests on the mixtures of PVC-M with PS, PMMA, PET-S and PET-D in the presence of saponin (average and standard error of three samples).

Recovery (%) PVC-M OP* PVC-M/PS Non-Floated 72.8 (1.15) 3.3 (0.45) Floated 27.2 96.7 PVC-M/PMMA Non-Floated 72.1 (1.07) 11.5 (0.78) Floated 27.9 88.56 PVC-M/PET-S Non-Floated 71.7 (0.88) 18.5 (0.95) Floated 28.3 81.5 PVC-M/PET-D Non-Floated 71.9 (0.97) 6.0 (0.64) Floated 28.1 94.0 OP* denotes the other plastics, namely PS, PMMA, PET-S or PET-D. Plastic Mixtures

Products

Grade (%) PVC-M OP* 95.7 4.3 21.9 78.1 86.2 13.8 24.0 76.0 79.5 20.5 25.8 74.2 92.23 7.7 23.0 77.0

Separation Efficiency (SE) (%) 69.5 60.6 53.2 65.9

Table 3. Results of the flotation tests on the mixtures of PVC-M with PS, PMMA, PET-S and PET-D in the presence of sodium lignosulfonate (average and standard error of three samples).

Recovery (%) PVC-M OP* PVC-M/PS Non-Floated 77.4 (1.21) 18.3 (1.02) Floated 22.7 81.7 PVC-M/PMMA Non-Floated 97.1 (1.32) 25.7 (1.26) Floated 2.9 74.3 PVC-M/PET-S Non-Floated 97.6 (0.98) 37.3 (1.12) Floated 2.4 62.7 PVC-M/PET-D Non-Floated 97.0 (1.07) 27.3 (1.15) Floated 3.1 72.7 OP* denotes the other plastics, namely PS, PMMA, PET-S or PET-D. Plastic Mixtures

Products

Grade (%) PVC-M 80.9 21.7 79.1 3.7 72.4 3.7 78.0 4.0

OP* 19.1 78.3 20.9 96.3 27.7 96.3 22.0 96.07

Separation Efficiency (SE) (%) 59.1 71.4 60.3 69.7

Table 4. Influence of the particle size in the recovery and grade of the non-floated (concentrated of PVC-M) and floated products, in the separation of bi-component mixtures in the presence of saponin.

Size fraction (mm) PVC-M/PS

+1-1.4 +1.4-2 +2-/2.8 +2.8-4 +4-5.6

PVC-M/PMMA

+1-1.4 +1.4-2 +2-/2.8 +2.8-4 +4-5.6

PVC-M/PET-S

+1-1.4 +1.4-2 +2-/2.8 +2.8-4 +4-5.6

PVC-M/PET-D

+1-1.4 +1.4-2 +2-/2.8 +2.8-4 +4-5.6

Polímeros, 29(3), e2019035, 2019

Non-Floated PVC-M PVC-M Recovery (%) Grade (%) 46.4 100.0 61.9 100.0 77.5 96.7 86.6 94.8 91.9 91.0 PVC-M PVC-M Recovery (%) Grade (%) 45.6 100.0 63.5 99.2 75.2 97.0 85.9 86.5 90.3 68.6 PVC-M PVC-M Recovery (%) Grade (%) 44.4 90.8 59.8 87.9 76.7 78.9 86.6 76.0 91.2 74.1 PVC-M PVC-M Recovery (%) Grade (%) 47.2 96.4 61.0 94.9 74.4 92.9 86.5 91.1 90.6 89.2

Floated PS PS Recovery (%) Grade (%) 100.0 65.1 100.0 72.4 97.3 81.2 95.3 87.6 90.9 91.8 PMMA PMMA Recovery (%) Grade (%) 100.0 64.8 99.5 73.2 97.7 79.7 86.5 86.0 58.6 85.8 PET-S PET-S Recovery (%) Grade (%) 95.5 63.2 91.7 69.5 79.4 77.3 72.6 84.4 68.1 88.5 PET-D PET-D Recovery (%) Grade (%) 98.2 65.0 96.7 71.3 94.3 78.6 91.5 87.1 89.0 90.4

Separation efficiency (SE) (%) 46.4 61.9 74.8 81.8 82.8 SE (%) 45.6 63.0 72.9 72.5 48.9 SE (%) 39.9 51.5 56.1 59.2 59.3 SE (%) 45.4 57.7 68.7 78.0 79.6

7/9

Pita, F., & Castilho, A. Table 5. Influence of the particle size in the recovery and grade of the non-floated (concentrated of PVC-M) and floated products, in the separation of bi-component mixtures in the presence of sodium lignosulfonate.

Size fraction (mm) PVC-M/PS

PVC-M/PMMA

PVC-M/PET-S

PVC-M/PET-D

+1-1.4 +1.4-2 +2-/2.8 +2.8-4 +4-5.6

+1-1.4 +1.4-2 +2-/2.8 +2.8-4 +4-5.6

+1-1.4 +1.4-2 +2-/2.8 +2.8-4 +4-5.6

+1-1.4 +1.4-2 +2-/2.8 +2.8-4 +4-5.6

Non-Floated PVC-M PVC-M Recovery (%) Grade (%) 39.7 98.0 62.6 97.5 87.3 90.9 97.8 78.6 99.5 65.0 PVC-M PVC-M Recovery (%) Grade (%) 88.9 100.0 96.9 99.6 99.8 91.9 100.0 71.1 100.0 56.0 PVC-M PVC-M Recovery (%) Grade (%) 90.2 86.0 97.8 78.5 100.0 71.0 100.0 66.9 100.0 64.7 PVC-M PVC-M Recovery (%) Grade (%) 89.3 81.2 95.5 79.4 100.0 77.6 100.0 76.9 100.0 75.8

mixtures (PVC-M/PMMA < PVC-M/PET-S ≈ PVC-M/PET-D < PVC-M/PS) is similar to the order of the bubble-particle aggregates density (PS < PET-D, < PET-S < PMMA ≈ PVC-M).

In the presence of sodium lignosulfonate, for the finer fraction, the separation efficiency order of the four plastic mixtures (PVC-M/PS < PVC-M/PET-D ≈ PVC-M/ PET-S < PVC-M/PMMA) is similar to the contact angle order (PVC-M < PS < PET-D < PET-S < PMMA). For the coarser fraction, the separation efficiency order of the four plastic mixtures (PVC-M/PMMA < PVC-M/PET-S ≈ PVC-M/PS < PVC-M/PET-D) is similar to the order of the bubble-particle aggregates density (PS < PET-D, < PET-S < PMMA ≈ PVC-M).

So, in the presence of saponin or sodium lignosulfonate, for the fine particles, the separation is mainly influenced by hydrophobicity difference, and for the coarse particles, the flotation is mainly influenced by the bubble-particle aggregates density and to a lesser extent by the hydrophobicity difference.

4. Conclusions The six plastics are naturally floatable in the absence of a wetting agent. Thus, to separate plastic mixtures by froth flotation, saponin and sodium lignosulfonate have been tested to render one component of the mixture more hydrophilic.

Contact angle and floatability of the six plastics decreased 8/9

Floated PS PS Recovery (%) Grade (%) 99.2 62.2 98.4 72.4 91.3 87.8 73.4 97.1 46.3 98.9 PMMA PMMA Recovery (%) Grade (%) 100.0 90.0 99.6 97.0 91.2 99.8 59.4 100.0 21.3 100.0 PET-S PET-S Recovery (%) Grade (%) 85.3 89.7 73.2 97.1 59.2 100.0 50.4 100.0 45.4 100.0 PET-D PET-D Recovery (%) Grade (%) 79.4 88.1 75.2 94.3 71.1 100.0 69.9 100.0 68.0 100.0

Separation efficiency (SE) (%) 38.9 61.0 78.6 71.2 45.8 SE (%) 88.9 96.5 91.0 59.4 21.3 SE (%) 75.5 71.0 59.2 50.4 45.4 SE (%) 68.7 70.7 71.1 69.9 68.0

with the increase of the saponin and sodium lignosulfonate concentration.

In the presence of the two agents, plastic floatability is not only dominated by hydrophobicity/wettability but also by particle weight (size, shape and density). Flotation recovery decreased with the increase of the particle size, an influence that was more noted in the plastics with more spherical shapes (PMMA and PVC-D). For fine particles, the floatability was mainly influenced by the hydrophobicity, and for coarse particles the floatability was strongly influenced by the weight of the particles.

In the presence of saponin or sodium lignosulfonate, PVC-M is the plastic with lower floatability. For separation of the PVC-M mixture with PS, saponin led to better results than sodium lignosulfonate, but for separation of PVC-M mixture with the other three plastics, sodium lignosulfonate led to better results than saponin. The best results were obtained in the PVC-M/PMMA mixture with sodium lignosulfonate, having the highest separation efficiency (71.4%). For the +1.4-2 mm fraction the separation efficiency was the maximum (96.5%).

Separation efficiency of plastic mixtures changes according to the mixture and also depends on the size, shape and density of the particles. Separation quality improves when the most hydrophobic plastic has lower density, lamellar shape and smaller size.

Polímeros, 29(3), e2019035, 2019

Plastics floatability: effect of saponin and sodium lignosulfonate as wetting agents

5. Acknowledgements This work was supported by the Portuguese Foundation for Science and Technology (FCT-MEC) through national funds and, when applicable, co-financed by FEDER in the ambit of the partnership PT2020, through the following research projects: UID/Multi/00073/2013 of the Geosciences Center of the University of Coimbra.

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Received: Apr. 12, 2019 Revised: July 01, 2019 Accepted: July 15, 2019 9/9

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.05818

Effects of miniemulsion operation conditions on the immobilization of BSA onto PMMA nanoparticles Izabella Campos1 , Thamiris Paiva1 , Helen Ferraz1  and José Carlos Pinto1*  Programa de Engenharia Química – COPPE, Universidade Federal do Rio de Janeiro – UFRJ, Rio de Janeiro, RJ, Brasil

1

*pinto@peq.coppe.ufrj.br

Abstract Polymer nanoparticles have been widely used in many biomedical applications, constituting a major incentive for immobilization of proteins. Poly(methyl methacrylate) nanoparticles were synthesized through miniemulsion polymerizations and used as supports for bovine serum albumin immobilization. Particularly, the effects of surfactant type (anionic sodium dodecyl sulfate and cationic cetyl trimethyl ammonium bromide) surfactant concentration and monomer holdup on some of the final nanoparticle properties (particle sizes, zeta potential and protein load) were characterized with help of statistical experimental designs for the first time. Results showed that the characteristics of the surfactant controlled the BSA adsorption efficiency, with enhanced rates of adsorption on the anionic particle surfaces, showing that the surfactant exerts fundamental effect on functionalization of emulsified polymer particles, which must be explicitly acknowledged in studies of polymer particle functionalization with proteins. Finally, BSA adsorption was shown to follow a multilayer process, given the better fitting with the Freundlich model.

Keywords: methyl methacrylate, miniemulsion polymerization, surfactant, protein adsorption, statistical experimental design.

How to cite: Campos, I., Paiva, T., Ferraz, H., & Pinto, J. C. (2019). Effects of miniemulsion operation conditions on the immobilization of BSA onto PMMA nanoparticles. Polímeros: Ciência e Tecnologia, 29(3), e2019036. https:// doi.org/10.1590/0104-1428.05818

1. Introduction The use of polymer nanoparticles (NPs) constitutes an interesting alternative for treatment of several diseases, particularly when it is intended to increase the absorption efficiency of encapsulated drugs and to provide protection for the bioactive principle in contact with body fluids.

Furthermore, NPs can ensure the controlled release of the encapsulated drug through the polymer matrix. The possibility to develop site-specific targeted drug delivery systems through bioconjugation procedures constitutes an additional advantage of using polymer NPs in biomedical applications[1-3].

Different techniques can be used to produce polymer NPs loaded with biomedical compounds, including miniemulsion polymerizations[4,5]. It must be emphasized that advances in the process control field make possible the production of polymer NPs with controlled properties, such as shape, size, surface charge and functionalization[6]. During the production of polymer NPs in miniemulsion processes, the addition of surfactants is mandatory to stabilize the high amount of energy stored in the interfaces[7]. It is important to observe that surfactants exert significant impact on the properties of the polymer NPs, affecting the applicability of the final product and modulating the therapeutic response in drug delivery applications[8]. Despite that, quantitative analyses of surfactant effects of the immobilization of proteins onto nanoparticle surfaces produced through miniemulsion polymerizations have not been performed yet.

Polímeros, 29(3), e2019036, 2019

The production of polymer NPs loaded with drugs through miniemulsion polymerizations has already been reported by many authors. For instance, Lorca et al.[9] produced PMMA NPs loaded with benzophenone-3 to be used in sunscreen formulations. This technique was also used by Fonseca et al.[10] to encapsulate the drug praziquantel (used in schistosomiasis treatments) in PMMA NPs. Moreira et al.[11] encapsulated the drug tamoxifen (used in cancer treatment) in PMMA NPs through in situ miniemulsion polymerizations. However, in these previously published studies, detailed analyses of the surfactant effects on the final polymer NPs properties were not performed.

Based on the previous paragraphs, the main objective of the present study was to perform a quantitative investigation of protein immobilization onto PMMA NPs produced through miniemulsion polymerizations, with help of experimental design tools. To the best of our knowledge, this has never been performed in previous publications. In order to do that, two different surfactants were employed: the anionic surfactant SDS (sodium dodecyl sulfate) and the cationic surfactant CTAB (cetyl trimethyl ammonium bromide), frequently used to perform miniemulsion polymerizations[12], as it is well known that proteins are sensitive to the local surface charges during the immobilization process[13].

Initially, the influence of surfactant type and concentration on the polymer NPs properties (particle sizes and zeta

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O O O O O O O O O O O O O O O O

Campos, I., Paiva, T., Ferraz, H., & Pinto, J. C. potential) were evaluated, in order to provide benchmark information for posterior quantitative analyses related to protein immobilization.

2. Materials and Methods 2.1 Materials Monomer methyl methacrylate (MMA, with minimum purity of 99.5 wt%), surfactant SDS (with minimum purity of 99 wt% and water content of 10 wt%), initiator potassium persulfate (KPS, with minimum purity of 99 wt%), co-stabilizer hexadecane (minimum purity of 99.5 wt%), pH buffer sodium bicarbonate (minimum purity of 99.7 wt%) and inhibitor hydroquinone (minimum purity of 99 wt%) were supplied

by VETEC Chemistry (Rio de Janeiro, Brazil). Surfactant CTAB (minimum purity of 99 wt%) was purchased from REAGEN (Colombo, Brazil). Protein BSA (minimum purity of 98 wt%) was supplied by Sigma-Aldrich (Missouri, USA). All chemicals were used as received, without further purification.

2.2 Synthesis of PMMA NPs PMMA NPs were produced by miniemulsion polymerization. The aqueous phase was prepared by solubilizing the surfactant (SDS or CTAB) and sodium bicarbonate (0.1 wt% in respect to water) in distilled water under magnetic stirring for 10 min. The organic phase, containing MMA and hexadecane (0.4 wt% in relation to the monomer), was prepared separately and added into the previously prepared aqueous phase under magnetic stirring for about 10 min, in order to form a pre-emulsified mixture. The miniemulsion was obtained through sonication for 5 min with amplitude of 20% (80 W) using a Branson Sonifier (Model 102C, Danbury, USA). To prevent the temperature increase during sonication, the vessel used for emulsification was placed inside an ice bath. The miniemulsion was then transferred to a mini-reactor (Mettler Toledo, Model EasyMaxTM 102, São Paulo, Brazil) and heated. The initiator potassium persulfate (0.5 wt% in respect to the monomer) was added when the desired reaction temperature was reached, and then it was assumed that the reaction started. Reaction runs were carried out at 80 °C under stirring of 500 rpm for 2 hours. At the end of the reaction step, samples of the reaction medium were collected for evaluation of monomer conversion by gravimetry. The polymerization was halted with the addition of 1 wt% aqueous hydroquinone solution to collected samples.

In order to evaluate the effects of the operation parameters on the final properties of the produced PMMA NPs, a 23 factorial design was proposed, with replicates at the central points of the continuous variables (Table 1).

Table 1. Experimental levels of analyzed parameters.

Variable Surfactant (x1)

Minimum (-1) SDS

Maximum (+1) CTAB

Surfactant concentration (wt%)a (x2)

1

5

Mass ratio of O/Wb phases (%) (x3)

15

25

Relative to the monomer. bO/W: oil/water.

a

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As the type of surfactant is a discrete variable and was one of the analyzed design variables (x1), replicates were performed at the central point of the continuous variables for both discrete variables (SDS and CTAB). This type of experimental plan allows for independent evaluation of the experimental effects and proper characterization of the experimental reproducibility, assumed to be constant in the analyzed experimental grid[14]. Particle sizes, zeta potential and BSA adsorption were used as experimental responses.

The obtained experimental results were analyzed and the significance of each process parameter was determined with help of statistical models.

2.3 Characterization Average particle sizes, polydispersity index (PdI) and zeta potential were measured through dynamic light scattering (DLS), using a Zetasizer Nano ZS (Malvern, Worcestershire, UK). The size distributions of both PMMA NPs and emulsified MMA droplets were analyzed. Characterization tests were performed with dilution of 1:500 (v/v) in distilled water at 25 °C. BSA adsorption tests were conducted with the produced latex, using 25 mg of PMMA NPs per assay.

10 mL of 1 mg.mL-1 BSA solution, previously prepared in acetate buffer (pH = 4.5), were added into tubes containing the latex and kept under mild stirring for 24 hours at room temperature. This time is sufficient to achieve the maximum BSA adsorption, as previously determined through more detailed kinetic studies[15]. Afterwards, the solid phase was separated through centrifugation (ThermoScientific, Model Megafuge 16R, Waltham, USA) using an AMICON 100K device (Millipore, Darmstadt, Germany). The BSA concentration in the filtrate was evaluated with standard Bradford assays[16]. The adsorbed BSA was quantified through mass balance and reported as the mass of BSA per surface area of the NPs, assuming the spherical geometry, as reported in previous studies[17]. The BSA adsorption isotherms were evaluated by varying the initial BSA concentration in the range from 0.1 to 5.0 mg.mL-1. Experimental data were fitted using the standard Langmuir (Equation 1), Freundlich (Equation 2) and Langmuir-Freundlich (Equation 3) models[18-20]: q mL k L Ceq (1) qe = 1+ k L Ceq F q e = q mF k F C1/n eq (2)

LF q mLF k LF C1/n eq qe =

1/n LF 1 + k LF Ceq

(3)

where qe (mg.g-1) is the adsorption capacity; qmL, qmF and qmLF (mg.L-1) are the maximum adsorption capacities; kL, kF and kLF (L.mg-1) are the Langmuir, Freundlich and Langmuir-Freundlich adsorption constants, respectively; nF and nLF are the Freundlich and Langmuir-Freundlich adsorption intensity constants, respectively; and Ceq (mg.L-1) is the protein concentration at equilibrium. Parameter estimation was performed with help of the standard least squares method[21] and was implemented in Mathematica 10.1 software, using the Nelder Mead method for minimization of the objective function[22] and using a confidence level of Polímeros, 29(3), e2019036, 2019

Effects of miniemulsion operation conditions on the immobilization of BSA onto PMMA nanoparticles 95%. Statistical analyses and estimation of model parameters for the empirical statistical models were performed with the software STATISTICA 8.0 (StatSoft), with help of the standard least squares method, using a confidence level of 95%. In order to facilitate the reading, obtained model representations and its correlation matrixes are presented in Supplementary Materials.

3. Results and Discussions 3.1 Preliminary analyses Initially, preliminary correlation analyses (Table 2) were performed considering the variables (surfactant type, surfactant concentration and O/W ratio) and the responses (size, zeta potential and BSA adsorption). As one can see, the type of surfactant was one of the analyzed design variables (x1).

As variable x1 has a discrete nature, replicates were performed at the central point of the continuous variables for both discrete variables (SDS and CTAB) and statistical analyses were conducted considering all experiments simultaneously and blocks generated by each discrete variable.

As one can observe in Table 2, BSA adsorption and zeta potential responses are correlated significantly with the surfactant type. This behavior can be explained in terms of the surfactant nature (anionic or cationic), since the ionic characteristics of the surfactant affects the surface charge of the particle and the adsorption process[23]. In addition, Table 2. Correlation matrix for preliminary analyses.

Correlation Matrix x1

x2

x3

Size

ZP

BSA

x1

1.00

0.29

0.00

0.04

0.99

-0.76

x2

0.29

1.00

0.00

-0.86

0.26

-0.78

x3

0.00

0.00

1.000

0.02

-0.01

-0.02

Size

0.03

-0.86

0.02

1.00

0.08

-0.61

ZP

0.99

0.26

0.01

0.08

1.00

-0.73

BSA

-0.76

-0.79

-0.02

-0.61

-0.73

1.00

BSA adsorption is also significantly correlated with surfactant concentration, particle size and zeta potential responses. This indicates that the amount of surfactant can significantly affect the protein adsorption process and the functionalization of the obtained polymer particles. Such behaviour has been usually overlooked in most publications in the field, which tend to concentrate on the reaction mechanism of functionalization.

As a matter of fact, protein adsorption onto polymer NPs can constitute a complex process, depending on the size and surface charge of the particles, rendering the role of the surfactant very important[15]. Regarding the average particle sizes, it presented strong negative correlation with the surfactant concentration, as it might already be expected[24].

Table 3 presents the experimental responses for average sizes, PdI and zeta potential for the miniemulsion droplets and polymer NPs. Conversion and BSA adsorption results are also presented in Table 3. Most tests achieved maximum conversions after 2 hours of reaction, as one might expect given the small average particle sizes and the high reactivity of the MMA monomer[25]. However, reactions performed with lower organic loads (15% of O/W) and higher surfactant concentrations (5 wt%) (Tests 3 and 7) exhibited reduced conversions for both surfactants, indicating that the use of very high amounts of surfactant could negatively affect the evolution of the miniemulsion reaction, an aspect that has not been discussed in the literature.

Table 3 presents the experimental responses for average sizes, PdI and zeta potential for the miniemulsion droplets and polymer NPs. Conversion and BSA adsorption results are also presented in Table 3. Most tests achieved maximum conversions after 2 hours of reaction, as one might expect given the small average particle sizes and the high reactivity of the MMA monomer[25]. However, reactions performed with lower organic loads (15% of O/W) and higher surfactant concentrations (5 wt%) (Tests 3 and 7) exhibited reduced conversions for both surfactants, indicating that the use of very high amounts of surfactant could negatively affect the evolution of the miniemulsion reaction, an aspect that has not been discussed in the literature.

Table 3. Experimental responses.

Run

Surfactant

1 2 3 4 5 6 7 8 9 10 11 12 13 14

-1 -1 -1 -1 +1 +1 +1 +1 -1 -1 -1 +1 +1 +1

Surfactant O/W phase Conversion Concentration ratio (%) -1 -1 +1 +1 -1 -1 +1 +1 0 0 0 0 0 0

-1 +1 -1 +1 -1 +1 -1 +1 0 0 0 0 0 0

100 100 84.94 100 2.69 2.68 69.37 99.88 100 100 99.89 99.55 100 99.66

Zeta Potential BSA (mV) adsorption Droplet Particle Droplet Particle Droplet Particle (mg/m2) 98.63 157.37 0.092 0.013 -48.87 -43.33 7.16 158.03 149.07 0.316 0.023 -48.53 -49.63 6.66 106.30 55.51 0.137 0.064 -44.77 -52.87 2.88 125.97 64.48 0.207 0.068 -34.20 -43.70 3.44 94.15 -a 0.194 -a +26.37 -a -a 89.59 -a 0.284 -a +13.07 -a -a 71.05 72.01 0.101 0.019 +15.97 +42.53 1.12 81.73 75.66 0.355 0.012 +35.60 +34.60 0.71 91.43 72.19 0.299 0.011 -42.10 -49.67 3.91 75.43 71.39 0.183 0.033 -55.07 -54.27 3.77 70.34 76.59 0.331 0.019 -52.60 -56.90 4.09 115.87 109.77 0.108 0.195 +25.73 +44.13 1.74 90.86 105.20 0.176 0.008 +33.93 +41.70 2.41 98.03 111.57 0.160 0.152 +37.67 +40.17 2.13 Size (nm)

PdI

not measured.

a

Polímeros, 29(3), e2019036, 2019

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Campos, I., Paiva, T., Ferraz, H., & Pinto, J. C. As exposed in Table 3, some reaction runs (using SDS as surfactant) showed a significant reduction of the average particle sizes, suggesting the occurrence of micellar nucleation, which can indicate the existence of significant mass transfer resistance of monomer from previously formed nanoparticles (containing hexadecane co-surfactant) to newly nucleated particles. Apparently, this very interesting reaction effect has been neglected in previous publications, as surfactant excess is avoided on purpose in miniemulsion polymerizations (see Supplementary Material). It is important to report that monomer conversion is normally expected to increase when the surfactant concentration is increased[26], due to the increase of the number of polymer particles through micellar nucleation, loci of the polymerization reactions. However, classical models neglect the presence of co-stabilizers in the initially dispersed monomer droplets (usually prepared in absence of co-stabilizers in classical emulsion polymerizations), which prevent mass transfer of monomer from dispersed droplets to newly formed polymer particles.

In the case of CTAB, the reduction of the organic load caused significant reduction of the zeta potential of droplets, indicating the preferential formation of micelles, which could lead to similar mass transfer limitations. Finally, tests 5 and 6 indicated the loss of emulsion stability, showing the lower coverage efficiency provided by CTAB and explaining the very low achieved monomer conversions.

According to Table 3, monomer droplets with average diameters in the nanometric region were obtained in all

cases, validating the use of the sonicator for the initial droplet dispersion. Zeta potential results showed negative values for samples prepared with SDS and positive values for samples prepared with CTAB. This result might already be expected, given the ionic nature of each particular surfactant, as SDS is an anionic surfactant and CTAB is a cationic surfactant. In addition, the obtained values were higher than 30 mV (in absolute values) in most experimental conditions, suggesting the high concentration of surfactants on the NPs surfaces and indicating that the dispersion stability was controlled by charge repulsion and charge distribution on the NPs surfaces. High dispersion stability is very important for actual applications, ensuring the preservation of the morphological properties of the final latex at rest for long periods of time.

Figure 1 shows the comparison between produced droplets and nanoparticles, for the two surfactants used, cationic and anionic. These results are consistent with the expected miniemulsion polymerization mechanism, as the initial monomer droplets are nucleated by free radicals and behave as ‘nanoreactors’, keeping the original size and interfacial charge essentially constant throughout the reaction course[27]. However, when extended to the other analyzed experimental conditions, the obtained results were somewhat different, as shown in Figure 1. Particle sizes (Figure 1-A) were equivalent to the droplet sizes when CTAB was used as surfactant, confirming the stability of the emulsion. However, when SDS was used as surfactant, growth of particle sizes was observed when the surfactant

Figure 1. Comparison between produced droplets and nanoparticles (cationic and anionic). (A) Size; (B) PdI; (C and D) Zeta potential.

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Polímeros, 29(3), e2019036, 2019

Effects of miniemulsion operation conditions on the immobilization of BSA onto PMMA nanoparticles concentration was smaller (due to mild agglomeration) and reduction of particle sizes was observed when the SDS was larger (due to micellar nucleation). It is interesting to observe that the reactor operation seemed to be more sensitive to modification of the SDS concentrations than to modification of the CTAB concentration, making the use of CTAB more advantageous in respect to control of the reaction conditions.

In general, it is important to emphasize that size distributions of the droplets were more dispersed (Figure 1-B) (higher PdI) than the polymer NPs (Figure S1 - Supplementary Material), indicating the occurrence of mass transfer effects during the reaction, possibly because of the large range of analyzed surfactant concentrations[24].

Regarding the zeta potential (Figure 1-C and Figure 1-D), polymer particles usually presented higher electrical charges (in absolute values) than the droplets. As this charge concentration effect was very significant in some cases, this cannot be assigned only to volume reduction caused by the higher density of the polymer material. This effect probably indicates that the particle-surfactant interaction changed during the course of the reaction, due to the continuous modification of the properties of the particle surface and of the solution. This complex dynamic feature of the particle-surfactant interaction, which certainly depends on the chemical, thermodynamic and electronic characteristics of the involved chemical species, has yet to be analyzed in detail in the literature, as it has been implicitly assumed that the charge distribution remains constant throughout the reaction course.

Regarding the BSA adsorption, polymer particles produced with SDS showed higher adsorption efficiencies than particles produced with CTAB at the analyzed conditions, confirming that the amount of adsorbed protein is controlled mainly by the surface charge. This behaviour makes the initial emulsion formulation fundamental for proper understanding of the final protein functionalization of the particle, an issue that has been frequently overlooked in the open literature, which tends to concentrate the analyses on the chemical characteristics of the produced polymer materials and to neglect the relevant role of the surfactant on the performance of the functionalization step.

3.2 Statistical analyses for polymer NPs The statistical models used to describe average sizes, zeta potential and BSA adsorption as functions of the process parameters had the general form (Equation 4): NX

NX NX

i=1

i=1 j=i+1

yc = a 0 + ∑ a i x i + ∑ . ∑ bijx i x j + cx i2 (4)

where yc is the model response, a0 is the response bias, ai are the main variable effects, bij are the synergetic interaction effects and c is a nonlinear quadratic effect. Statistical models were built iteratively. Initially, all analyzed effects were assumed to exist. Then, insignificant (within the 95% confidence level) parameters were discarded and the model was rebuilt. The procedure was repeated until attaining a full set of statistically significant parameters. Then, the model quality was evaluated, using the minimum value of the objective function and the experimental variance at the central point to perform the Chi-square test, with confidence Polímeros, 29(3), e2019036, 2019

level of 95%. Besides, the experimental variance and the model prediction variance were compared with the F-test, with confidence level of 95%. Models were accepted only when all these conditions were satisfied.

One must observe that variable x1 is discrete, meaning that x1 can only assume the values (-1) or (+1). Therefore, the coefficients that multiply the variable x1 indicate the effect of changing the type of surfactant in the particularly analyzed statistical effect. As a consequence, one may assume that Equation (4) summarizes two distinct equations: one for SDS (when x1 is made equal to -1) and one for CTAB (when x1 is made equal to +1). The simultaneous analyses of all available experimental data is advantageous because leads to higher number of degrees of freedom and more precise estimation of the model parameters.

Regarding the average sizes of the PMMA NPs, the obtained model (Equation 5) showed that surfactant concentration was the most significant variable (a2), exerting a negative effect on the average particle size, indicating that the increase of the surfactant concentration caused the reduction of average particle sizes, as well documented in the literature[28,29]. It is important to observe that the surfactant type also exerted a significant effect on average particle sizes, as the cationic surfactant shifted the average sizes towards higher values (a1)[30]. Despite that, the use of CTAB made the system even more sensitive to modification of the surfactant concentration (b12), indicating the less efficient stabilization of the NPs promoted by CTAB, despite the apparent higher sensitivity of particle diameters to change of the SDS concentrations, as described in the previous section. Finally, it must be observed the negligible effect exerted by the organic load on the average particle sizes (b23) and the existence of quadratic nonlinear effects (c) associated with the modification of the process parameters. Although the proposed experimental design does not allow for unambiguous identification of the quadratic effect, it was assumed that this effect was related to the surfactant concentration, given its much larger influence on the analyzed process response.

Size = ( 91 ± 3) + ( 18 ± 3) ⋅ x1 - ( 57 ± 4 ) ⋅ x 2 -

(11 ± 4 ) ⋅ x1. x 2 + ( 4 ± 3) ⋅ x 2 . x 3 + ( 33 ± 5) ⋅ x 22

(5)

The analysis of the zeta potential showed that this process response was influenced mainly by the type of surfactant (a1), as shown in Equation 6. The surfactant molecules are located on the surfaces of the particles and define the observed charge. As expected, particles produced with SDS were negative, while the ones prepared with CTAB showed a positive zeta potential[31]. It is important to observe the lower absolute surface charge provided by CTAB, which can possibly be related to its lower stabilizing effect, when compared to SDS. Zeta Potential = − ( 5 ± 3) + ( 45 ± 3) ⋅ x1 (6)

A statistical model was also used to evaluate the influence of the process variables on the BSA adsorption onto PMMA NPs. Equation 6 shows that both variables, surfactant type and concentration, exerted significant effects on the adsorption process. This can be regarded as a very important result, since the role of surfactant on bioconjugation processes has 5/10

Campos, I., Paiva, T., Ferraz, H., & Pinto, J. C. been largely neglected, as discussed previously. Adsorption assays were performed in acetate buffer pH = 4.5, where BSA presents null charge[15,32]. This condition was selected on purpose, considering that the protein presents better adsorptive properties in the isoelectric point and the obtained results should not be influenced by the protein charge[33].

According to Equation 7, the surfactant concentration was once more the most influential process parameter (a2).

Besides, the cationic surfactant shifted protein adsorption towards lower values (a1), a result that can possibly be related to its lower stabilizing capacity (and lower absolute surface charges). It should be noted the occurrence of less important nonlinear effects (b23 and c).

BSA = ( 2.9 ± 0.2 ) − (1.0 ± 0.2 ) ⋅ x1 − (1.9 ± 0.2 ) ⋅ x 2 +

( 0.3 ± 0.2 ) ⋅ x 2 . x 3 + (1.1 ± 0.2 ) ⋅ x 22

(7)

BSA adsorption using CTAB were smaller than when SDS was used, indicating that the amount of adsorbed protein can be controlled by the surface charge of the polymer particles.

These results show that protein adsorption is significantly influenced by the electrostatic interaction between the protein and the adsorbent, as reported by Patil et al.[23], as the surface charge can modify the electrostatic interaction between nanoparticles and the proteins. Nandhakumar et al.[34] found that poly(ε-caprolactone) nanoparticles positively charged by CTAB showed higher adsorption efficiency of HSA (human serum albumin) than negatively charged particles by SDS. However, these authors performed the experiments above the isoelectric point of the protein, making it negatively charged, leading to larger affinity with the positively charged nanoparticles. Thus, an important factor to be taken into account during protein bioconjugation through adsorption is the protein charge (which is influenced by its isoelectric point and by the pH of the medium) and the polymer NP charge, affected mostly by the surfactant.

3.3 Statistical analyses for polymer NPs for each surfactant The discussion presented above showed that the type of surfactant exerted a dominant and pronounced effect on the analyzed properties, which can eventually hide and confound the less important effects exerted by the remaining variables. For this reason, analyses of variable effects on average sizes, zeta potential and BSA adsorption data were performed for each analyzed surfactant independently, following the very same procedures described previously.

As one can see in Equation 8 and Equation 9, built with SDS data, surfactant concentration (parameter a2) was the variable that exerted the highest influence on average particle sizes and BSA adsorption. Once again, the existence of significant nonlinear effects (parameters b23 and c) could be observed.

These results are consistent with the previously described models, as presented in Equations 5 and 7. The correlation among parameters (Supplementary Material) were equal to zero, with exception of the quadratic parameters, as expected. However, it was not possible to obtain a significant statistical model for zeta potential, which means that the type of surfactant controls this property, which is not affected by the remaining variables. This means that the particle 6/10

surfaces were saturated with SDS in all analyzed conditions, showing that SDS molecules are preferentially adsorbed on the particle-water interfaces, regardless the particularly analyzed SDS concentrations and monomer loads.

( 74 ± 3) − ( 47 ± 2 ) ⋅ x 2 + (8) ( 4 ±2 ) ⋅ x 2 ⋅ x 3 + ( 33 ± 4 ) ⋅ x 22

SizeSDS =

( 3.9 ± 0.1) − (1.9 ± 0.1) ⋅ x 2 + (9) ( 0.26 ± 0.1) ⋅ x 2 ⋅ x 3 + (1.1 ± 0.2 ) ⋅ x 22 BSASDS =

Regarding CTAB data, Equations 10 and 11 represent the adjusted models for average particle sizes and BSA adsorption. As expected, the surfactant concentration was once more the most influential parameter (a2) to explain the average sizes and BSA adsorption[34]. For particle sizes, this result can explain the reaction behaviour in tests 5 and 6, when massive agglomeration of polymer particles took place. Once again, it was not possible to obtain a significant statistical model for zeta potential, which means that the type of surfactant controls this property, which is not affected by the remaining variables. As already described, this means that the particle surfaces were saturated with CTAB in all analyzed conditions, showing that CTAB molecules are preferentially adsorbed on the particle-water interfaces, regardless the particularly analyzed CTAB concentrations and monomer loads.

SizeCTAB=

(109 ± 4 ) − ( 35 ± 6 ) ⋅ x 2 (10)

BSA CTAB = ( 2 ± 4 ) − (1.2 ± 0.6 ) ⋅ x 2 (11)

3.4 Adsorption analyses The zeta potential of the BSA-modified polymer NPs was evaluated after the adsorption process, as shown in Figure 2. Particles produced with SDS (initially charged negatively) presented positive charges, while particles produced with CTAB (initially charged positively) presented lower positive zeta potentials after adsorption. These results clearly indicate the modification of the PMMA NPs surfaces after adsorption of BSA.

Figure 2. Zeta potentials of PMMA NPs before and after the BSA adsorption process.

Polímeros, 29(3), e2019036, 2019

Effects of miniemulsion operation conditions on the immobilization of BSA onto PMMA nanoparticles

Figure 3. Fits of BSA adsorption isotherms for nanoparticles produced with SDS and CTAB. (A) Freundlich isotherm; (B) Langmuir isotherm; and (C) Langmuir-Freundlich isotherm.

Table 4. Estimated parameters for the analyzed adsorption models.

Isotherm Langmuir

Constants R2 qML

Freundlich

LangmuirFreundlich

SDS 0.8871 198.101

CTAB 0.9725 230.633

kL

8.1991

0.0028

R2

0.9292

0.9766

qMF

2.7202

1.8826

kF

39.9292

2.5420

nF

6.7046

1.8692

R2

0.9290

qMLF

6018.64

0.9762 1259.43

kLF

0.0184

0.0033

nLF

6.5118

1.7418

In order to better understand the BSA adsorption behaviour on the surface of the synthesized PMMA NPs, adsorption isotherms were built for PMMA NPs obtained at the central point of the proposed factorial design. Modeling the adsorption isotherms can be important when it is desired to compare the relative adsorption performances of different NPs[35].

As shown in Figure 3, the surface charge represents a significant variable for analysis of BSA adsorption in PMMA NPs. Although both nanoparticles present favorable adsorption (concave profiles)[36], nanoparticles prepared with SDS show higher adsorption capacity, as also described in Equation 7.

All analyzed isotherm models provided satisfactory fits for the available data, with exception of the Langmuir model when applied to NPs prepared with SDS. Table 4 shows the estimated parameters for all studied adsorption models. It can be observed that Freundlich isotherm provided the best fits for both nanoparticles, suggesting a multilayer adsorption process[37]. This can be regarded as an important feature of protein immobilization onto nanoparticle surfaces, as the formation of protein monolayers has been frequently assumed in the open literature for interpretation of experimental data[38].

4. Conclusions PMMA NPs were produced through miniemulsion polymerizations at different reaction conditions, in order to evaluate the influence of the anionic surfactant SDS and the cationic CTAB on protein immobilization through Polímeros, 29(3), e2019036, 2019

adsorption. Obtained results showed that both surfactant type and surfactant concentration are the most significant variables to explain not only the size of the nanoparticles, but also the rate and efficiency of protein adsorption, exerting fundamental influence on functionalization of PMMA nanoparticles with proteins.

Particularly, PMMA NPs presented average sizes ranging from 55 to 157 nm, with narrower polydispersities than the initial monomer droplets. The produced NPs presented high values of zeta potential, ranging from -57 to +44 mV, controlled strictly by the type of surfactant, indicating that the particle-water interfaces were fully saturated with surfactant in all analyzed conditions and also indicating the preferential adsorption of both surfactants on the interfacial surfaces of the dispersed media. Regarding the BSA adsorption, the characteristics of the surfactant also controlled the adsorption efficiency, with enhanced rates of adsorption on the anionic particle surfaces, showing that the surfactant exerts fundamental effect on functionalization of emulsified polymer particles, which must be explicitly acknowledged in studies of polymer particle functionalization with proteins.

Finally, BSA adsorption on PMMA NPs was shown to follow a multilayer adsorption process, given the better fitting of available data obtained with the Freundlich model.

5. Acknowledgments The authors thank CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brazil) for providing scholarships.

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11. Moreira, T. S., Oliveira, M. A. M., Lima, L. M. T. R., Souza, M. N., & Pinto, J. C. C. S. (2014). Synthesis of nanoparticles loaded with tamoxifen by in situ miniemulsion RAFT polymerization. Macromolecular Symposia, 344(1), 101-107. http://dx.doi.org/10.1002/masy.201400025.

12. Schork, F. J., Luo, Y., Smulders, W., Russum, J. P., Butté, A., & Fontenot, K. (2005). Miniemulsion polymerization. In M. Okubo (Ed.), Polymer particles (pp. 129-255). Berlin: Springer. http://dx.doi.org/10.1007/b100115.

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14. Carlson, R. (1992). Two-level factorial designs. In R. Carlson (Ed.), Design and optimization in organic synthesis (pp. 89122). New York: Elsevier Science 15. Yasuda, M., & Ono, K. (2015). BSA adsorption and immobilization onto charged monodisperse polymer nanoparticles. Journal of Biosensors & Bioelectronics, 6(4), 4-11. http://dx.doi. org/10.4172/2155-6210.1000183.

16. Bradford, M. M. (1976). A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Analytical Biochemistry, 72(12), 248-254. http://dx.doi.org/10.1016/0003-2697(76)90527-3.

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18. Langmuir, I. (1918). The adsorption of gases on plane surfaces of glass, mica and platinum. Journal of the American Chemical Society, 40(9), 1361-1403. http://dx.doi.org/10.1021/ja02242a004.

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Adsorption of BSA on highly carboxylated microspheres: quantitative effects of surface functional groups and interaction forces. Journal of Colloid and Interface Science, 177(2), 613620. http://dx.doi.org/10.1006/jcis.1996.0075.

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Protein adsorption and cellular uptake of cerium oxide nanoparticles as a function of zeta potential. Biomaterials, 28(31), 46004607. http://dx.doi.org/10.1016/j.biomaterials.2007.07.029.

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24. Hecht, L. L., Wagner, C., Landfester, K., & Schuchmann, H. P. (2011). Surfactant concentration regime in miniemulsion polymerization for the formation of MMA nanodroplets by high-pressure homogenization. Langmuir, 27(6), 2279-2285. http://dx.doi.org/10.1021/la104480s. PMid:21314152.

25. Peixoto, A. C. B., Campos, I. M. F., Ferraz, H. C., & Pinto, J. C. (2016). Use of Hydrophilic Monomers to Avoid Secondary Particle Nucleation in Miniemulsion Polymerizations of Methyl Methacrylate. Journal of Research Updates in Polymer Science, 5(2), 60-71. http://dx.doi.org/10.6000/1929-5995.2016.05.02.2.

26. Fontenot, K., & Schork, F. J. (1993). Batch polymerization of methyl methacrylate in mini/macroemulsions. Journal of Applied Polymer Science, 49(4), 633-655. http://dx.doi. org/10.1002/app.1993.070490410.

27. Asua, J. M. (2002). Miniemulsion polymerization. Progress in Polymer Science, 27(7), 1283-1346. http://dx.doi.org/10.1016/ S0079-6700(02)00010-2.

28. Antonietti, M., & Landfester, K. (2002). Polyreactions in miniemulsions. Progress in Polymer Science (Oxford), 27(4), 689-757. http://dx.doi.org/10.1016/S0079-6700(01)00051-X. 29. Pichot, R., Spyropoulos, F., & Norton, I. T. (2010). O/W emulsions stabilised by both low molecular weight surfactants and colloidal particles: the effect of surfactant type and concentration. Journal of Colloid and Interface Science, 352(1), 128-135. http://dx.doi.org/10.1016/j.jcis.2010.08.021.

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30. Qian, C., & McClements, D. J. (2011). Formation of nanoemulsions stabilized by model food-grade emulsifiers using high-pressure homogenization: factors affecting particle size. Food Hydrocolloids, 25(5), 1000-1008. http://dx.doi. org/10.1016/j.foodhyd.2010.09.017.

31. Visaveliya, N., & Köhler, J. M. (2014). Control of shape and size of polymer nanoparticles aggregates in a single-step microcontinuous flow process: a case of flower and spherical shapes. Langmuir, 30(41), 12180-12189. http://dx.doi. org/10.1021/la502896s. PMid:25251615.

32. Yoon, J. Y., Lee, J. H., Kim, J. H., & Kim, W. S. (1998). Separation of serum proteins with uncoupled microsphere particles in a stirred cell. Colloids and Surfaces. B, Biointerfaces, 10(6), 365-377. http://dx.doi.org/10.1016/S0927-7765(97)00068-4.

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Effects of miniemulsion operation conditions on the immobilization of BSA onto PMMA nanoparticles 33. Gelamo, E. L., Silva, C. H. T. P., Imasato, H., & Tabak, M. (2002). Interaction of bovine (BSA) and human (HSA) serum albumins with ionic surfactants: spectroscopy and modelling.

Biochimica et Biophysica Acta, 1594(1), 84-99. http://dx.doi. org/10.1016/S0167-4838(01)00287-4. PMid:11825611.

34. Nandhakumar, S., Dhanaraju, M. D., Sundar, V. D., & Heera, B. (2017). Influence of surface charge on the in vitro protein adsorption and cell cytotoxicity of paclitaxel loaded poly(ε-caprolactone) nanoparticles. Bulletin of Faculty of Pharmacy, Cairo University, 55(2), 249-258. http://dx.doi.org/10.1016/j.bfopcu.2017.06.003.

35. Rabe, M., Verdes, D., & Seeger, S. (2011). Understanding protein adsorption phenomena at solid surfaces. Advances in Colloid and Interface Science, 162(1–2), 87-106. http://dx.doi. org/10.1016/j.cis.2010.12.007. PMid:21295764.

36. Sun, C. J., Sun, L. Z., & Sun, X. X. (2013). Graphical evaluation of the favorability of adsorption processes by using conditional

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langmuir constant. Industrial & Engineering Chemistry Research, 52(39), 14251-14260. http://dx.doi.org/10.1021/ie401571p.

37. Foo, K. Y., & Hameed, B. H. (2010). Insights into the modeling of adsorption isotherm systems. Chemical Engineering Journal, 156(1), 2-10. http://dx.doi.org/10.1016/j. cej.2009.09.013.

38. Dominguez-Medina, S., Blankenburg, J., Olson, J., Landes, C. F., & Link, S. (2013). Adsorption of a protein monolayer via hydrophobic interactions prevents nanoparticle aggregation under harsh environmental conditions. ACS Sustainable Chemistry & Engineering, 1(7), 833-842. http://dx.doi.org/10.1021/ sc400042h. PMid:23914342.

Received: Sept. 01, 2018 Revised: Mar. 24, 2019 Accepted: July 11, 2019

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Campos, I., Paiva, T., Ferraz, H., & Pinto, J. C.

Supplementary Material Supplementary material accompanies this paper.

Legenda S.1 CMC Calculations Legenda S.2 Comparisons between particle and droplet size distributions.

Legenda S.3 Statistical analyses This material is available as part of the online article from http://www.scielo.br/po

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PolĂ­meros, 29(3), e2019036, 2019

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.06118

Grafting polypropylene over hollow glass microspheres by reactive extrusion Carlos André Baptista1,2 and Sebastião Vicente Canevarolo2*  3M do Brasil, Sumaré, SP, Brasil Programa de Pós-graduação em Ciência e Engenharia de Materiais – PPG-CEM, Universidade Federal de São Carlos – UFSCar, São Carlos, SP, Brasil 1

2

*caneva@ufscar.br

Abstract Hollow glass microspheres HGM are light, round, hollow, hydrophilic microspheres used to produce polyolefin composites with reduced density. To maintain mechanical strength, it is necessary to improve the adhesion between the polymer matrix and the microspheres, which is done by a compatibilizer. For polypropylene composites a maleic anhydride grafted polypropylene copolymer PP-g-MAH is employed. The melt mixing is done in a reactive extrusion when the maleic group of the compatibilizer reacts with hydroxyl groups present at the microspheres’ surface, grafting a long PP chain. The aim of this work is to quantify the esterification grafting conversion and its efficiency during the reactive extrusion to the formation of PP/HGM composites compatibilized with PP-g-MAH. Techniques like TGA, FTIR and SEM were used to quantify the grafted PP content formed and the efficiency of the esterification reaction, which is mainly dependent of the compatibilizer concentration and reactive extrusion temperature.

Keywords: hollow glass microspheres, maleic anhydride, grafted polypropylene, reactive extrusion.

How to cite: Baptista, C. A., & Canevarolo, S. V. (2019). Grafting polypropylene over hollow glass microspheres by reactive extrusion. Polímeros: Ciência e Tecnologia, 29(3), e2019037. https://doi.org/10.1590/0104-1428.06118

1. Introduction Reactive extrusion takes place when the synthesis or modification of a polymeric material happens at the same time of its processing and shaping into an intermediate or finished plastic product[1]. A common way to promote polymer compatibility in a blend or composite is through copolymers with segments capable of specific interactions and/or chemical reactions with the other components.

The copolymers may be added separately or formed in situ by blending suitably functionalized polymers[2].

Functionalized polyolefin, and more particularly functionalized polypropylene (PP), were used to compatibilize a large number of polar polymers such as polyesters and polyamides, thus improving the properties and stability of immiscible polymer blends[3-5]. Polypropylene compatibilization with polar polymers can be due to specific interactions, such as hydrogen bonding, or as the formation of graft copolymers by reactions between the functions grafted over the PP and others existing in polar polymers. These reactions can be made through reactive extrusion processing.

Radical functionalization of PP by reactive extrusion is one of the common ways to graft functional groups to the PP chains. Among possible reactive monomers, one of the most important for PP is maleic anhydride (MAH).

It generally improves the adhesion of PP to metals, glass fibers, or other polymers[6-8]. The grafting of maleic anhydride to PP has been prepared successfully in the

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melt state, by reactive extrusion[9-11], and in the presence of liquid solvents [7,12], which creates the problem of removing the organic solvent after reaction. It is generally accepted that chain scission occurs during the peroxide initiated functionalization, and the maleic anhydride is grafted to the PP backbone[10].

The use of this grafted PP as a compatibilizer on fiber composites has been widely reported in the literature[8,13-15], and also specially on glass fibers[16,17]. Works done on natural fibers, such as from Felix et al.[18], show that esterification reactions can occur between the maleic anhydride grafted to the PP chains and the hydroxyl groups present on the surface of such fibers. These applications lead to the polypropylene grafted to maleic anhydride (PP-g-MAH) also to be used as a compatibilizing agent on hollow glass microspheres composites.

The use of hollow glass microspheres (HGM) in different polymeric composites has been continuously explored by both scientific and industrial means. Its influence in a wide array of properties, such as reducing density[19], thermal conductivity[20-22], dielectric properties[23,24], and increasing sound dampening[25] have created important benefits to industrial sectors such as automotive, electronics, mechanical and aerospace.

Despite its versatility and capacity to tinker several properties, using HGM in thermoplastic compounds usually diminishes the mechanical properties of the final product,

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O O O O O O O O O O O O O O O O

Baptista, C. A., & Canevarolo, S. V. such as tensile strength and impact resistance[26,27], unless either a surface treatment and/or compatibilizers are used to enhance the interaction between HGMs and the polymeric matrix[19,21,28]. The works of Patankar et al.[21,29], show that the use of 1% in weight of PE-g-MAH in a PE/HGM composite can positively influence toughness, yield strength and maximum strain of such composites. Kumar et al.[15], reported that the use of surface treated HGMs on PP and fiber composites improve mechanical properties while also reducing density. Patent literature of 3M[28], shows an increase of approximately 40% on the tensile strength of PP/PP-g-MAH/HGM composites in comparison to composites without the compatibilizing agent. Even though these works show such effects, the content of PP grafted over the HGM and its efficiency has not been fully studied in PP/HGM composites compatibilized with maleic anhydride grafted polypropylene PP-g-MAH. Given the need to create light and mechanically strong composites, and the economic advantage of the high yield provided by reactive extrusion, the goal of the present work is to assess both conversion and efficiency of the grafting reaction during reactive extrusion of a PP/PP-g-MAH/HGM composite, while analyzing the effect of the extrusion temperature in the process.

2. Materials and Methods 2.1 Materials The major component of the composite is a commercial isotatic homopolypropylene (H301) with medium flow rate (MFI 10 g/10 min), and normal molecular weight distribution, produced by Braskem. Hollow glass microsphere (3MTM iM16k) is an injection molding grade HGM, with high pressure resistance, produced by 3M Company and donated by 3M do Brasil LTDA. The PP-g-MAH (Polybond PB 3200), produced and donated by AddivantTM has a nominal content of maleic anhydride of 1% weight, and MFI of 115 g/10 min. Materials were used as received without any further purification.

2.2 Methods 2.2.1 Reactive extrusion The reactive extrusion was carried out in a co-rotating twin-screw extruder from HAAKE, Rheomex OS PTW24 model, with a 24 mm screw, L/D = 25 and a side feeder at L/D = 12. The screw was designed to have only conveying screw elements between the side feeder and die exit, to help diminish the breakage of HGMs[19]. The rotation speed was 50 rpm and extrusion temperatures, kept constant along the whole barrel, were set at 200 °C, 220 °C and 240 °C. All components were oven dried for 12 hours at 80 °C prior to melt mixing. Manually mixed formulations of PP and PP-g-MAH pellets were fed through the main feeder and HGMs were fed on the side feeder. The content of HGM was fixed at 10% (w/w) in all formulations, the content of PP-g-MAH varied from 2% up to 14% (w/w), adding up with the PP main component. The polymer melt while flowing inside an extruder spreads over many different path, with different length and shear histories, which results in a distribution of residence times. The shear level encountered 2/9

by the melt and the residence time it stays inside the extruder directly affect the reaction conversion and so the breath of the residence time distribution RTD curve is important to be known, particularly when reactive extrusion is under study. For that the extrusion RTD curve was measured by adding a single pellet of colored PP tracer in the aperture of zone #4, while the compositions were being prepared.

To represent the RTD curve three main residence times were measured: initial time corresponding to the moment of the first sign of color exits the extruder which represents the fastest route the polymer melt flows, the peak time that of maximum color intensity when the biggest fraction of molten polymer exit the extruder and the final time when the slowest melt fraction exits the extruder, represented by the disappearance of the color in the extruded strands.

This last fraction have taken the longest rote and probably faced the highest shear. A control composition containing only PP-g-MAH and HGMs (10% w/w) was prepared in a HAAKE torque rheometer at 200 °C for 10 minutes.

2.2.2 Solution extraction of reacted hallow glass microspheres After reactive extrusion, 1.5 g of each compound was solubilized in 300 ml hot xylene at 90 °C for two hours, enough to dissolve completely all soluble components.

The suspension was then filtered with the help of a vacuum pump, leaving the PP grafted HGMs on the filter. The whole solution/filtering process was repeated twice for each sample.

2.2.3 Quantification of the polypropylene grafted over the HGM microspheres using thermo-gravimetric analysis, TGA The PP grafted HGM samples were pyrolyzed in a TGA model Q50 from TA Instruments, available at the laboratories of 3M do Brasil LTDA, Sumaré, SP. The heating scans went up to 600 °C at 10º/min under a flow of compressed air. At least two samples were tested for each composition.

2.2.4 Quantification of the polypropylene grafted over the HGM microspheres using infrared analysis, FTIR-ATR FTIR-ATR was performed in a Thermo Scientific, model Nicolet iS50, under Attenuated Total Reflectance, available at 3M do Brasil LTDA. Extruded strands were chopped and the fresh surface measured. Each composition was measured ten times, discarding the outliers, in order to attenuate the intrinsic limitations of the ATR technique.

Reference PP/HGM compositions were prepared to produce the calibration curve by dissolving known amount of pristine PP in a round flask with hot xylene at 90 °C, until complete dissolution. Enough amount of HGM was added to prepare compositions with PP content varying from 1% to 12.5% (w/w) and well stirred. The mixture was poured on a glass plate and dried at 75 °C for 12 hours. Portions of the dried mixture was squeezed over the ATR crystal window and the spectra taken. The known added weight’s ratio between both components were related to the area under the absorbance bands present between 2970 cm-1 and 2840 cm-1 (referring to the symmetric and asymmetric stretch of the aliphatic CH2 and CH3, taken here as an indication the organic PP content) and the area of the bands present between 1300 cm-1 and 880 cm-1 (referring to the hollow glass microspheres inorganic phase content) as Peak Area Ratio = (Peak Area from 2970 to 2840)/(Peak Area from 1300 to 880). Area Polímeros, 29(3), e2019037, 2019

Grafting polypropylene over hollow glass microspheres by reactive extrusion instead of the absorbance at peak maxima was chosen because the PP content in the extrude grafted compound is low and the silica band is quite broad. A straight line was obtained and used to quantify the grafted PP content on the solvent extracted HGM after the reactive extrusion.

2.2.5 Electron microscopy SEM and elemental analysis EDS Micrographs of the extruded compounds were taken by a scanning electron microscope FEI Inspect, model S50, available at 3M do Brasil LTDA. Extruded strands of PP/PP-g-MAH/HGM composites were cryogenically fractured, silver glued, gold sputtered and examined under various magnifications. The atomic concentration of C, O, Na, Si and Ca on the surface of the HGM particles in some selected areas was quantified by energy-dispersive X-ray spectroscopy EDS analysis.

3. Results and Discussions 3.1 Simulation of maximum PP grafting content over glass microspheres To simulate the maximum amount of polypropylene that could be attached to the surface of the HGM spheres a simple model was built. The area to be covered can be obtained from the mean diameter of the spheres (20 µm) and the density of the spheres (0.46 g/cm3). The maximum number of PP chains that could graft at the surface of one HGM can be estimated assuming that the minimum average cross section area of an extended single PP chain is that of the maximum packing of its crystalline unit cell. Knowing that its unit cell parameters are a = 6.65Å and b = 20.96Å and that there are four PP chains per unit cell we reach to the minimum cross section area of a PP chain is 34.85Å2, i.e. a diameter of 6.66Å. From this we get that the maximum weight content of grafted PP chains that could fit over the surface of −4 HGM is w max PPgMAH ( % ) = 3.11*10 * M n. By assuming that the number average molecular weight of the PP-g-MAH is in the range of M n = 70, 000 g / mol, one can find that the maximum weight content of grafted PP chains is w max PPgMAH ≅ 22% w / w .

3.2 Average residence times during extrusion Table 1 shows the average residence times during the extrusion of the composites measured from the entrance port (feeding hoper) of the HGM particles (L/D = 12) until the die exit, at three different extrusion temperatures. The increase of extrusion temperature reduces the melt viscosity and so facilitates the particle/polymer melt mixing, which, in turn, reduces the average residence time. This shorten the time the reactive processing has to graft the PP chains over the HGM surface. The neat final time/temperature effect in the grafting efficiency will be discussed later and is one of the objectives of this paper.

3.3 PP grafting content quantified by thermo-gravimetric TGA measurements The dried HGMs after solvent extraction were pyrolyzed in a TGA to quantify their organic content, which is assumed to be made solely by grafted PP chains. Pyrolyzed dried pristine HGM shows a maximum weight reduction of less than 0.4% w/w. Figure 1 shows TGA curves for PP grafted HGM after solvent extraction, that have being made by reactive extrusion of PP/PP-g-MAH/HGM composites at 200 °C, with varying PP-g-MAH content, ranging from 2% up to 14% w/w. The drop in weight starts at approximately 300 °C spanning the pyrolysis up to almost 500 °C. The burning process happens continuously and in only one degradation mechanism, above 500 °C the residue weight keeps constant indicating that all organic component has being pyrolyzed.

The higher the PP-g-MAH content in the initial extruded formulation the higher the weight loss, i. e. the higher the grafted PP chains content.

Figure 2 shows curves of the amount of grafted PP chains over the extracted HGM as a function of the initial content of PP-g-MAH compatibilizer (in terms of 1 / w PPgMAH ) used during reactive extrusion, at three different temperatures.

The content of PP chains grafted to the HGM’s particles increases with the content of PP-g-MAH compatibilizer

Figure 1. TGA/DTG curves of PP grafted onto hollow glass microsphere HGM compound, after solvent extraction. The organic pyrolyzed content correspond to the grafted PP content. The grafted compounds were extracted from PP/PP-g-MAH/HGM composites extruded at 200 °C, with varying PP-g-MAH content, as indicated.

Table 1. Average residence times measured from the HGM’s extrusion entrance port (zone #4).

Extrusion temperature Initial residence time (s) Peak residence time (s) Final residence time (s)

200 °C 65 90 180

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220 °C 60 85 140

240 °C 55 75 110

Figure 2. Grafted PP content over the extracted HGM measured by thermo-gravimetric analysis as a function of the initial content of PP-g-MAH compatibilizer used during reactive extrusion.

The extrapolated value of 21.6% is the maximum PP grafting content.

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Baptista, C. A., & Canevarolo, S. V. added to the reactive extruded compound, indicating that more ester bonds are created as PP-g-MAH compatibilizer chains becomes more available. For reactive extrusion at 200ºC the data follows a straight line which extrapolates to the maximum weight of PP chains that can be grafted over the HGM particles, which is w max PPgMAH = 21.6% , a value very close to the simulated data using M PPgMAH = 70, 000 g / mol. n To reach this limiting condition, it is necessary to use samples of PP-g-MAH in which all PP chains are grafted with maleic anhydride, which does not happens in commercial samples, given that not all chains get grafted during its reaction[7].

Upon rising the reactive extrusion temperature of the composite to 220 °C and 240 °C the efficiency of the PP grafting content on the HGM particles reduces. It is known that the monoesterification reaction between styrene-maleic anhydride and aliphatic alcohols decreases significantly with increasing temperature[30]. Another effect is that, at higher reactive extrusion temperatures the time available to the reaction, i. e. the average residence time during extrusion is greatly reduced, as shown in Table 1. Together both effects reduce the grafting efficiency of the reaction leading to a lower total conversion.

To prepare a formulation as close as possible to the maximum compatibilizer content in the composite, HGM particles with pure PP-g-MAH were mixed in a HAAKE rheometer. In doing so it was possible to get the datum at 1 / w PPgMAH = 1.05 which reaches to a PP grafted content of w PPgMAH = 18.72%. The HAAKE mixing time was 600s, well above the average residence time of the extrusion, allowing plenty of time for the reaction to occur.

The presence of organic material in the HGMs after the hot solvent extraction indicates that it is covalently bounded (some hydrogen bonds may be formed as well) to the surface of the microspheres, grafted during the reactive extrusion.

Figure 3 shows such a reaction, proposed by Felix et al.[18] for grafting PP-g-MAH over cellulose fibers. The maleic anhydride functional groups in the PP-g-MAH chains react with the hydroxyl groups present in the HGM’s surface forming ester bonds, grafting the copolymer. The layer of PP chains over the HGM particles more efficiently anchor them in the polymer matrix producing composites with improved mechanical properties, as mentioned in various works[28,29], by improving the load transfer on the composite’s interface[31].

Figure 3. Proposed grafting mechanisms of the maleic anhydride functional group present in the PP-g-MAH compatibilizer and hydroxyl groups present on the surface of the HGMs particles (adapted[18]).

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3.4 Efficiency of PP grafting over HGM measured by TGA The PP grafting over the hollow glass microspheres is needed for a better anchoring which leads to an efficient load transfer during mechanical stress. The mixing method used here is by reactive extrusion in which the polymer matrix and the compatibilizer are added to the extruder in the first feeding zone and the HGM downstream by a side feeder located at L/D = 12 to reduce, as much as possible, the breakage of the HGM microspheres. In addition, any high shearing screw elements, like kneading elements, should be added in the screw profile only prior to the side feeder entrance and having none downstream. In doing so the PP-g-MAH compatibilizer is mixed and diluted in the PP matrix prior to be able to get in contact with the HGM. When it does the amount of compatibilizer is proportionally reduced according to the initial feeding composition.

The grafting efficiency, i.e. how much the fed PP-g-MAH content actually reacts, producing the grafted layer over the glass microspheres, should be calculated. We choose to quantify the efficiency by calculating the weight percentage of grafted PP over the HGM microspheres measured by the TGA analysis, in relation to the original total mass of PP/PP-g-MAH/HGM fed to the extruder. The experiment covered five different compatibilizer concentrations and three reactive extrusion temperatures.

Figure 4 shows bar curves of PP-g-MAH compatibilizer content (in gray) present in the initial composite formulation and the correspondent grafted PP content (in blue), at reactive extrusion temperatures of 200ºC, 220ºC and 240ºC. All contents are in %w/w, and are relative to the initial composition fed into the extruder. Finally, a linear fitting curve is added to the PP grafted content, obtained from the TGA analysis.

Analyzing Figure 4 one immediately observes that the amount of PP-g-MAH grafted over the HGM is very low compared to the amount initially added. The diluting effect of the PP-g-MAH in the PP matrix along the extruder, until reaching the side feeder, reduces the reagent content for the esterification reaction to occur. As already observed the grafting conversion reduces at higher extrusion temperatures.

Knowing that the HGM content in the original formulation is 10% then the simulated maximum grafting content would reach to w max PPgMAH = 2.16% . The highest grafting content obtained was 1.66%, reacting at the minimum extrusion temperature 200 °C and at the maximum PP-g-MAH compatibilizer content used here of 14%. This level of compatibilizer content is already too high, and lowering the extrusion temperature even further would increase the melt viscosity and so the extrusion torque, hindering the process.

In order to remedy the low efficiency various strategies should be applied, such as: design the screw profile to improve microspheres dispersion as well as avoiding their breakage, feed the compatibilizer downstream as close as possible to the feeding microspheres’ port, premix the microspheres with the compatibilizer as a master concentrate, set the extrusion temperature profile in the reaction zone Polímeros, 29(3), e2019037, 2019

Grafting polypropylene over hollow glass microspheres by reactive extrusion

Figure 4. Efficiency curves from TGA analysis in terms of added PP-g-MAH compatibilizer and grafted PP contents, at various reactive extrusion temperatures, as indicated. Gray bars represent PP-g-MAH in the initial composite formulation, blue bars show the PP grafted content. All contents are in %w/w and relates to the initial composition fed in the extruder. Linear fitting curves.

as low as possible, i.e. downstream after the microspheres feeding port, and so on.

3.5 FTIR-ATR spectra of PP/HGM compound and its calibration curve Figure 5 shows the infrared spectra of a PP grafted HGM after solvent extraction from an extruded original formulation with 14% of PP-g-MAH and 10% HGM at 220 °C. Two main sets of bands are present: i) bands between 2970 cm-1 and 2840 cm-1, referring to the symmetric and asymmetric stretch of aliphatic CH2 and CH3[32] bands, and ii) a broad band, between 1300 cm-1 and 880 cm-1, referring to the 1100 cm-1 SiO2 silica bonds of the HGM. A 10x enlarged portion of the spectra, in the 4000 – 2300 cm-1 range, is shown to highlight the PP band, present in low quantities in the PP grafted HGM solvent extracted compound.

The quantification is done by measuring the area under each band, employing a horizontal straight base-line, as shown in Figure 5.

The sensitivity of the FTIR-ATR method to quantitatively measure the expected low levels of the grafted PP content over the solvent extracted HGM was checked by obtaining a calibration curve from mixtures of PP homopolymer and HGM pristine microspheres, in the expected grafted PP content range of the composites.

The areas under the PP and silica bands was measured and their ratio calculated resulting the calibration curve presented in Figure 6. The data points fit quite well in a straight line, with R2 = 0.997, good enough to allow the use of the FTIR-ATR method to quantitatively characterize the content of the grafted PP over the solvent extracted HGM microspheres.

Polímeros, 29(3), e2019037, 2019

Figure 5. FTIR spectra of a PP grafted HGM after solvent extraction from an extruded original formulation with 14% of PP-g-MAH and 10% HGM w/w at 220 °C. The spectra above 2300 cm-1 is shown enlarged 10x in the upper shifted curve. The quantification is done taking the area under each band, as shown.

Figure 6. FTIR calibration curve of PP/HGM reference mixtures as curves of Peak Area Ratio =

( Peak Area from 2970 to 2840 ) as ( Peak Area from1300 to 880 )

a function of the PP/HGM weight content (%).

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Baptista, C. A., & Canevarolo, S. V. 3.6 PP grafting content quantified by FTIR-ATR measurements The amount of grafted PP over the HGM microspheres was also quantified by FTIR/ATR. Figure 7 shows the same set of curves for the three extrusion temperatures as used during the TGA quantification. The behavior, as expected, is almost the same. Both techniques can quantify almost equally the grafting yield. Again data for 200 °C follow a straight line which extrapolate to w max PPgMAH = 19.4%, within the experimental error of the value obtained by TGA. The behavior at higher extrusion temperature (220 °C and 240 °C) also shows a reduction in the PP grafting conversion over the HGM particles, agreeing with the expected reduction in the

conversion of the esterification reaction between maleic anhydride and alcohols with increasing temperature[30].

The already mentioned reduction in the average residence time during extrusion at higher temperatures also contributes to the reduction in grafting efficiency.

The FTIR-ATR measurements gives higher data dispersion, partially due to the great difference between the band area of the aliphatic CH2 and CH3 of PP and the band area of silica, when calculating their ratio. The FTIR-ATR spectra (Figure 5) also does not show peaks at 1780 cm-1, which would correspond to the carbonyl group in cyclic anhydrides, nor at 1746 cm-1, which would correspond to ester bonds on the HGM’s surface[18]. This indicates that the amount of ester bonds is below the detection, just as indicated by Patankar et al.[29], and expected by the slow reaction kinetics of maleic anhydride to hydroxyl groups[33].

Even though, it is still possible to detect the polypropylene chain grafted to these bonds, hence TGA and FTIR technique were used to quantify it.

3.7 Efficiency of PP grafting over HGM measured by FTIR

Figure 7. Grafted PP content over the extracted HGM measured by FTIR-ATR method as a function of the initial content of PP-g-MAH compatibilizer used during reactive extrusion. The maximum PP grafting content is the extrapolated value of 19.8%.

The efficiency of the total reaction was calculated the same way as done for the TGA samples. Figure 8 shows how much of the initial compounded PP-g-MAH reacted. Again the low grafting efficiency is also revealed by the FTIR-ATR analysis, the reaction consumes only a small portion of the initially fed content of PP-g-MAH compatibilizer. Even if this technique presents a slightly more dispersed data, the conclusions drawn from both analysis are the same, no more than 10% of the added PP-g-MAH in the original extruded formulation is grafted onto the HGM microspheres.

Figure 8. Efficiency curves from FTIR analysis in terms of added PP-g-MAH compatibilizer and grafted PP contents, at various reactive extrusion temperatures, as indicated. Gray bars represent PP-g-MAH in the initial composite formulation, blue bars show the PP grafted content. All contents are in %w/w and relates to the initial composition fed in the extruder. Linear fitting curves.

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Polímeros, 29(3), e2019037, 2019

Grafting polypropylene over hollow glass microspheres by reactive extrusion 3.8 SEM micrographs and EDS analysis of the composite fractured surface As seen so far, the content of the grafted PP over the surface of the HGM particles measured by TGA and FTIR techniques is dependent of the concentration of the PP-g-MAH compatibilizer added in the composite formulation.

The anchoring effect of this polymeric layer is expected to improve the mechanical properties of the composite[28].

Figure 9 shows SEM micrographs of cryogenically fractured extruded strands of the uncompatibilized PP/HGM (Figure 9a) and compatibilized with 14% of PP-g-MAH (Figure 9b) composites. The difference of the interface in both cases is clearly evidenced, on the uncompatibilized composite the interaction between the PP polymer matrix and the HGM particle is almost inexistent, the round glass particles are detached from the matrix, very little load transfer during mechanical deformation can be expected. On the contrary, in the PP-g-MAH compatibilized composite there is a smooth change from the matrix to the glass particle, no clear interface is seen. The particle is well immersed in the matrix, with layers of the organic matrix covering it, an interaction that gives a good load transfer during mechanical deformation.

Figure 9 also shows selected points (crosses) on the surface of the hollow glass microspheres in which weight and atomic concentrations of C, O, Na, Si and Ca were measured by EDS analysis. The data are presented in Table 2. The particle surface of uncompatibilized PP/HGM

composite seen in Figure 9a show low levels of carbon and high levels of silicon and calcium atoms (marked #1).

On the other hand, the compatibilized PP/PP-g-MAH/HGM composite (Figure 9b) shows a much higher concentration of carbon and lower for silicon and calcium atoms (#2), indicating the presence of an organic layer covering the entire glass particle surface. Moreover, selecting another area in this same compatibilized composite (#3) in which a visible extra layer was left over the glass particle, while the crack was propagating, the carbon concentration increases even further, in detriment to the silicon and calcium atoms content (see Table 2). This is a quantitative indication that there are PP chains adhered to the HGM surface, result of the grafting reaction occurred during the compatibilized reactive extrusion.

4. Conclusions This paper quantifies the conversion and efficiency of the esterification reaction between the maleic anhydride from a maleate polypropylene (PP-g-MAH) to graft onto hollow glass microspheres HGM. They are dependent on both, the initial content of available PP-g-MAH in the composite formulation and the reactive extrusion temperature.

The reaction conversion increases with the PP-g-MAH content in the formulation, topping near the theoretical limit of approximately 20% w/w of grafted PP over the HGM

Figure 9. SEM images of cryogenically fractured composites: (a) uncompatibilized PP/HGM composite showing clean detached HGM particle from the PP matrix, (b) compatibilized PP/PP-g-MAH/HGM composite with well-anchored interface matrix/HGM particle.

Crosses show approximate locations were the concentration of carbon, silicon and other atoms was measured, with data shown in Table 2.

Note that location #3 is over an extra layer of PP. Table 2. Weight and atomic concentrations of C, O, Na, Si and Ca over the surface of the HGM particles in selected areas, as indicated in Figure 9.

Atom C O Na Si Ca

w(%) 14.1 34.8 2.4 37.2 11.5

Uncomp. 1 Atom.(%) 23.2 42.9 2.1 26.1 5.7

Polímeros, 29(3), e2019037, 2019

Graf 2 w(%) 23.0 33.8 2.2 31.9 9.0

Graf 3 Atom.(%) 34.9 38.5 1.8 20.7 4.1

w(%) 36.9 32.9 2.1 22.3 5.8

Atom.(%) 49.8 33.4 1.5 12.9 2.3

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Baptista, C. A., & Canevarolo, S. V. microspheres. As expected, an increase in the extrusion temperature reduces the esterification reaction conversion, reducing the grafted PP content.

The traditional way of adding the PP-g-MAH compatibilizer with the polymer matrix promotes its dilution in a large amount of material and so most of it remains unreacted in the PP matrix, reducing its efficiency. The results also highlights the importance in choosing the right screw profile to get a good balance between be mild enough to avoid the HGM’s breakage, but enough shearing to grant high efficiency to the esterification reaction and so the grafting conversion.

5. Acknowledgements This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001, to Conselho Nacional de Desenvolvimento Científico e Tecnológico, CNPq (for granting the research scholarship PQ 311790/2013-5 to S.V. Canevarolo) and to the Programa de Pós-Graduação em Ciência e Engenharia de Materiais (PPG-CEM) of UFSCar for providing its research facilities. C. A. Baptista acknowledges 3M do Brasil LTDA, Sumaré, SP, for allowing him the develop this research, donating the materials and lending their facilities.

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Received: Jan. 22, 2019 Revised: June 21, 2019 Accepted: Aug. 01, 2019

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.02818

Influence of water absorption on glass fibre reinforced IPN composite pipes Suresh Gopi1* , Ganesh Babu Loganathan2, Bharani Kumar Sekar1, Rajesh Kanna Krishnamoorthy1, Vivek Sekaran1 and Akash Rajendran Mohan1 Department of Mechanical Engineering, Rajalakshmi Institute of Technology, Chennai, India 2 Department of Mechatronics Engineering, ISHIK International University, Iraq

1

*saisuresh1979@gmail.com

Abstract Glass fibre reinforced composite pipes were fabricated by using an IPN blend of 0%, 10%, 20%, 30%, 40%, 50% PU prepolymer (Polyurethane) with VER(Vinylester) resin. The prepared IPN (Interpenetrating polymer networks) composite pipes were subjected to boiling water immersion tests in order to study the effect of moisture absorption characteristics.

The Burst strength and Hoop strength of water immersed specimens were evaluated (3, 6, 9, 12 months) and compared along with dry composite specimens. It was found that, percentage of moisture uptake was significantly reduced with increase in PU loading into the IPN system as well the Burst strength and Hoop strength of the specimens were found to be diminishing with raise in percentage of moisture uptake and raise in temperature; contrarily, the addition of PU significantly reduced the moisture intake, in addition to that the PU added IPN pipe offers better mechanical strength while compared with neat Vinylester pipes.

Keywords: interpenetrating polymer networks, vinylester, polyurethane, hygrothermal, GFRP pipes.

How to cite: Gopi, S., Loganathan, G. B., Sekar, B. K., Krishnamoorthy, R., K., Sekaran, V., & Mohan, A. R. (2019).

Influence of water absorption on glass fibre reinforced IPN composite pipes. Polímeros: Ciência e Tecnologia, 29(3), e2019038. https://doi.org/10.1590/0104-1428.02818

1. Introduction Nowadays glass fibre reinforced polymer composite materials have been very extensively used in most of the application like manufacturing of pipes, tanks and other products used in humid conditions. In general GFRPs normally have more durability when compared with other types of conventional materials[1]. It provides high strength, chemical stability, stiffness to weight ratio, chemical resistance and have the ability to be tailored for different requirements[2].

More over the GRP Pipes are often considered as an alternative to conventional metallic pipes where corrosion, environmental effect and weight limit are considered. Whenever these pipes are subjected to hygrothermal conditions, it absorbs moisture from the surrounding environment[3]. This kind of frequent moisture uptake has played an important influence on the mechanical strength of these materials. Apart from that environmental factors are also very much important when the GFRP are exposed to moist environments in order to forecast their long term properties[4]. Complex phenomenon like plasticizing effect of the matrix, rearrangement of the chemical structure, de-bonding might occur on GFRP when they come across hygrothermal environment. The strength between the fibre-matrix (interface) also degraded in the high temperature environment. It has been concluded that rather than creating diffusion through the matrix (plasticization), the moisture diffuse itself more easily along the fibre and

Polímeros, 29(3), e2019038, 2019

destroy the fibre-matrix interface. Although the fibre-matrix interface plays a crucial role in strength by load transferring, without a tough bond at the interface sometimes the matrix strength dominate in all environments[5]. Normally the water absorption by the matrix takes place by capillarity action along the fibre-matrix interface, through some cracks, void present in the resin. Many diffusion models have been predicted over the years to model the hygrothermal effect in FRP pipes. The first diffusion model has been proposed

by Fick, by the analogy of heat conduction model. Most of the researchers completely rely on Fick’s second law to find out the moisture absorption models[6-8]. The rapid study and development in the area of GFRP manufacturing promulgates to understand the behavioral change of the material in different environmental conditions in order to certify for their long-term durability. Hence, the new product needs numerous investigations to make sure their sustainability in different hygrothermal conditions by comparing dry and aged specimens. Moreover, polymeric matrix material often plasticized, swelled and softenend when subjected into hygrothermal environment. The primary degradation or deterioration of polymeric resin leads to weak interface bond between the fibre-resin bonds[9,10].

Among all kind of thermoset resins, vinylester (VER) resins are widely used and considered as the best one

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O O O O O O O O O O O O O O O O

Gopi, S., Loganathan, G. B., Sekar, B. K., Krishnamoorthy, R. K., Sekaran, V., & Mohan, A. R. kind of thermoset resin in the area of pipe, tanks, ducts manufacturing. Because vinylester resins have the combined properties of epoxies and unsaturated polyesters. They possess better chemical resistance than all kind of polyester resins. Basically vinylester resin are based on the reactant group of an epoxy and ethylenically unsaturated carboxylic acid, which adversely result in a polymer with chain of end unsaturation. The mechanical strength mainly depends on the structure of the matrix resins, properties of fibres, volumetric fraction of fibres (Vf) and interphase strength[11-13].

Similarly another class matrix material was polyurethane (PU), famous in the field of rubber cum coating industry for their adaptable, versatile and attractive properties. It possess very good abrasion resistance property, tear strength, shock absorption property and renowned for their elastic property[14,15]. These kinds of polymer are generally used as adhesives, coatings for infrastructural applications as well as used as matrix resin for composite manufacturing. Besides that, it has a very good (low viscosity) processability and excellent bonding characteristics with different substrates.

Moreover, when polyurethanes are subjected into hygrothermal environment, it undergoes plasticizing effect by degrading it, this was because of the intrusion of the water into the free volume of the polymer during absorption. The splintering of the hydrogen bond intrachains also reason behind that of the plasticizing effect (increase in mobility chain macromolecular) of the polyurethane. Hence with exclusive properties of both the resins, a special class of polymer blend matrix material has been formed, which is called as interpenetrating polymer networks (IPNs). Instead of spending valuable time in the area of altering the chemistry of the styrene and other functional group of vinylester polymer, a blend of mixing of two or more polymers to extract specific superior quality of individual polymers has been carried out. This kind of blends does not chemically react with each other rather it maintains the mutual entanglement between two of them[16,17].

2. Experimental

2.2.2 Fabrication of composite pipes The IPN pipes were fabricated by hand lay-up technique.

First, the mandrel with the diameter of 124 mm was placed over the two steel roller supports as shown in the Figure 1.

Secondly the mandrel was wetted with the releasing agent, before the coating of IPNs. Thirdly surface mat was wounded over the entire circumference of the mandrel in order to get the glossy surface finish inside the pipe as shown in Figure 2.

Following this plain woven fabric (350 gsm) was wounded over the entire mandrel after wetting the surface mat with IPN blend. This wounding process was continued until an average thickness of 3 mm was obtained.

Besides that the pipe was post cured at 80 °C for 2 hours, after it was removed from the mandrel. All the samples were subjected to visual inspection method in order to find out the scratches, flaws, voids and other imperfections. The pipes were made with different proportions of blends with VER and PU as per the Table 1.

The total length of the final composite pipe was one meter. With help of the special saw, the composite pipe was cut into different specifications according ASTM D 570, ASTM D1599 & ASTM D2290 standards. Utmost care was given in order to avoid the cutting edge effects. In addition to that the following defects were avoided in the specimens at the cutting edges, like air bubble, accumulation of resin, buckling, de-lamination, wrong laying of the fibre. Specimens were completely wiped dry in order to remove surface

Figure 1. Terphane releasing film wounded mandrel.

2.1 Materials Vinyl ester resin VBR 4508 used in this study was procured from Vasavi Bala Resins (P) Ltd. Chennai. Polyurethane (CG-60A Commercial Grade based on TDI System) was provided by Cross Link Technology. The E-glass fibre used in this study was plain – woven mat (350 gsm). All chemicals were used as purchased.

2.2 Sample preparation 2.2.1 VER/PU blend preparation The ratio of polyurethane (Polyether polyol) pre polymer (PU) and hardener was 100:7 (mocca – methylene bis-ortho chloroaniline) wt/wt was taken initially. The entire mixture was kept in the degassing chamber for a period of 5 minutes till most of the bubbling got ceased. Following that, Vinylester (VER) was added with cobalt naphthenate accelerator, promoter and catalyst as per the manufacturer recommendation of 100/2/2/2. Both the resins were thoroughly mixed[6-8].

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Figure 2. Wrapping of surface mat.

Table 1. Typical combination of IPN - Formulation.

Sample. No 1 2 3 4 5 6

VER (g) 100 90 80 70 60 50

PU (g) 0 10 20 30 40 50

Polímeros, 29(3), e2019038, 2019

Influence of water absorption on glass fibre reinforced IPN composite pipes moisture (moisture absorption subjected specimen) before weighing. Cut edge surfaces were also sealed to avoid the water uptake through the fibre broken region for moisture absorption test[6-8].

3. Experimental Methodology

‘h’ is considered as the thickness of the specimen and D is the diffusion coefficient. The diffusion coefficient can be calculated by drawing the curve of the test specimens’ weight gain versus the square root of aging time, through the slope of the initial linear region of the curve. The value of D can be calculated with help of the following equation[16].  h  D = π.    4M m 

3.1 Moisture absorption characterization Water absorption test was performed according to the ASTM D570 standard, by immersing the various test specimens in distilled water at room temperature for twelve months for various temperatures (45°C, 55°C, 65°C). In order to examine the moisture absorption behavior of the composite pipe, the weight gain of the composite specimens were measured with a precision of 1 mg repeatedly over the immersion period.

The percentage weight gain at any time as a effect of moisture absorption was determined by the Equation (1): Mt =

Ww − Wd ×100 ( % ) (1) Wd

Where Ww and Wd denotes weight of the specimen in wet condition and dry condition respectively, with respect to time t. To calculate the moisture diffusion process in polymer matrix composite pipes, the Fick’s law was acceptable one to demonstrate the diffusivity. The same equation was used to identify the moisture uptake in composite pipes, as well the following solution was considered as the viable one.

Mt 8 1 = 1− 2 ∑ Mm π n = 0 ( 2n + 1)2 ∞

 − ( 2n + 1)2 π2 Dt   (2) exp    h2  

Where Mt and Mm are the percentage of the absorbed moisture at time t and the saturation stage respectively,

2

2

 M 2 − M1     t −√t  1  2

(3)

3.2 Burst strength test The length, thickness and inner diameter of the specimens were maintained as 300 mm, 3 mm, and 124 mm respectively.

One end of the test specimen was completely sealed where as another end was provided with nozzle at end of the pipe.

To do the burst test, first the aged specimen as shown in Figure 3(a) was completely filled with water in order to generate the hydrostatic pressure inside the test specimen.

By the way, the burst pressure was calculated by constantly increasing the pressure inside the tube. To maintain safety for the doer all the tests were conducted under the closed environment condition as shown in Figure 3(b). As well to find out the outdoor exposure, the pipe was filled with water and exposed to hygrothermal environment for the period of 3, 6, 9, 12 months and later pressure was tested[6-8].

3.3 Hoop strength test The specimens’ size of the hoop test specimens were maintained as 124 mm inner diameter, width as 25 mm and thickness as 3 mm as shown in Figure 4(a). Five samples for each (Ring test specimens) proportion (0%PU, 10%PU, 20%PU, 30%PU, 40%PU and 50%PU) were randomly selected and

Figure 3. (a) Burst pressure test specimens (b) Specimen in connector & placed in closed location.

Figure 4. (a) shaped specimens (before test) (b) specimen in ring test kit.

Polímeros, 29(3), e2019038, 2019

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Gopi, S., Loganathan, G. B., Sekar, B. K., Krishnamoorthy, R. K., Sekaran, V., & Mohan, A. R. immersed in water at elevated temperatures of 45°C, 55°C and 65°C well below the service temperature of the matrix.

As similar to that of the burst test all the specimens were tested (ageing test) at equal intervals like 3, 6, 9, 12 months respectively. The testing was performed as per the ASTM D2290 standard. The hoop test was conducted on mechanically driven test equipment with 100kN capacity. The feed was maintained as 5 mm/min. As shown in the Figure 4(b), the inner steel half disks were used to load the specimens. After the test, the fractured surfaces of the specimens were analyzed by visual inspection to indentify the mode of failure. All the samples were weighed before and after immersion[6-8].

4. Results and Discussion 4.1 Characteristics of GFRP IPN pipe specimen in water When the composite is immersed in the fluid medium and maintained at various temperatures for different time period; the matrix material (resin) in the composite should not get detach from the fibre and react with the fluid medium. The moisture absorption of the different proportions (0%PU, 10%PU, 20%PU, 30%PU, 40%PU & 50%PU) specimen graphs were plotted with the square root of day as X axis, against the various moisture absorption level as Y axis. The similar kind of graphs were plotted for various temperatures like 45°C, 55°C and 65 °C as shown in Figures 5(a) to 5(b).

From the graph of 0%PU (100%VER), it can be seen that during the initial time of study for all the proportions (5 samples), the specimen’s moisture intake was very high

with respect to all the temperature variations, includes of 45 °C, 55 °C and 65 °C. In this context as shown in graph when the temperature of water increases, the specimen absorbs more water than the specimens maintained at room temperature. The specimens maintained at 55 °C was found to absorb 27.58% higher than 45 °C specimen during the initial period of time. Similarly the specimen maintained at 65°C exhibited a similar trend. The 65 °C kept specimen absorbs 21.62% of moisture higher than that of the 55 °C kept specimens. As the square root of day increased the moisture intake or absorption level got saturated. During the period of √7 days the variation found between the 45 °C to 55 °C was 12.95%. Similarly the variation between 65 °C to 55 °C was 20.57%, this was quite high as compared to the variation between 45 °C to 55 °C. Whereas during the √19 the moisture absorption level got almost saturated and there was no variation between the √18 and √19 days. Similarly when we consider the uptake of the moisture level during the initial days it was found to be 0.105%, of total weight. Also the graph depicts that for 55 °C the moisture level was 0.145%. All the values were found to be higher for short span of time for √7. This shows that the temperature rise in the water plays a major role in due respect of absorption peaks in all the specimens.

But it gradually decreases and almost gets saturated to the downtrodden level irrespective of temperature when it reaches the level of √19 days. From the Figure 5 revealed that all the specimens shown good agreement between the data and the theory, because all the proportionate graphs follows the Fick’s behavior. It was found that 45 °C absorbs only 0.92% of its total weight with the corresponding diffusion

Figure 5. Water absorption curves of specimens (a) 0% PU, (b) 10% PU, (c) 20% PU, (d) 30% PU, (e) 40%, (f) 50% PU. 4/8

Polímeros, 29(3), e2019038, 2019

Influence of water absorption on glass fibre reinforced IPN composite pipes coefficient of 1.8124 × 10-6 (m2/s), similarly the 55 °C specimens absorbs 1% of total weight with the corresponding diffusion coefficient of 2.9543 × 10-6(m2/s), whereas the 65 °C absorbs 1.2% of total weight with diffusion coefficient of 4.1232 × 10-6(m2/s). The diffusion coefficient as well shows much difference, this was completely based upon the increase in temperature of boiling water. As diffusion increases it leads to much weight again on the specimens.

From the Table 2 it was proven that diffusion coefficient is directly proportional to the moisture gain of the IPN system.

This actually shows that the temperature played a major factor for all the moisture absorption studies.

Whereas the 10% PU specimens exhibited a different trend from that of the 0%PU specimens. During the initial period of time, it absorbs (20.45%) or intake more water than that of the 0% PU specimens. But as the square root of days increase, the amount of water intake gets saturated at lower temperature than that of the 0%PU. During the study of √7 days of absorption the absorption value found was 0.57%, 0.69%, 0.8% for 45 °C, 55 °C, 65 °C respectively.

The percentage of variation between this was 17.39%, 13.75% for 55 °C, 65 °C respectively. The found value was much lower than that of the 0%PU, and the variation between 0%PU to 10%PU was 0.102, 0.082, 0.172 for 45 °C, 55 °C & 65 °C respectively. It seems that the addition of PU into the IPN system remarkably reduced the water intake in to some extent. Besides that, during the study √19 days the variation again seemed to be similar as discussed for √7 days. The moisture uptake was 0.72(%), 0.84(%), 0.95(%) of total absorption rate for corresponding 45 °C, 55 °C & 65 °C. The addition of 10% PU shows significant reduction in moisture absorption. This was mainly because of the PU prepolymer addition into VER system. As far as the diffusion coefficient study concerned, the Table 2 showed 5.4537 × 10-7(m2/s), 6.7654 × 10-7(m2/s), 7.7654 × 10-7(m2/s) for the corresponding temperature of 45 °C, 55 °C & 65 °C Table 2. Diffusion coefficient values for corresponding temperatures.

Specimens 0% PU

10% PU

20% PU

30% PU

40% PU

50% PU

Immersion Temperature (°C) 45 55 65 45 55 65 45 55 65 45 55 65 45 55 65 45 55 65

Diffusion coefficient (m2/s) 1.8 ± 0.5 × 10-6 2.9 ± 0.8 × 10-6 4.1 ± 0.9 × 10-6 5.4 ± 0.2 × 10-7 6.7 ± 0.9 × 10-7 7.7 ± 0.3 × 10-7 6.3 ± 0.4 × 10-8 7.8 ± 0.8 × 10-8 9.1 ± 0.7 × 10-8 7.3 ± 0.4 × 10-9 8.8 ± 0.6 × 10-9 10.2 ± 0.3 × 10-9 8.2 ± 0.2 × 10-10 9.9 ± 0.3 × 10-10 10.5 ± 0.1 × 10-10 7.4 ± 0.6 × 10-11 8.5 ± 0.4 × 10-11 9.8 ± 0.7 × 10-11

Polímeros, 29(3), e2019038, 2019

M∞ (%) 0.92 1.00 1.20 0.72 0.84 0.95 0.64 0.76 0.88 0.58 0.70 0.84 0.48 0.59 0.71 0.42 0.55 0.65

respectively. Again it proves that the moisture absorption was directly proportional to the water temperature. The 20% PU showed a similar trend as that of 0% and 10% PU systems.

The difference of moisture absorption percentage was 26.7% for initially. However for √7 days the difference of variations were 18.38-12.6% respectively.

At last 6.3876 × 10-8(m2/s), 7.8745 × 10-8(m2/s) & 9.1652 × 10-8(m2/s) respectively (45 °C, 55 °C & 65 °C) for the corresponding (20% PU) water temperatures. The PU prepolymer addition into VER system again proved that there was a significant reduction of water absorption for all the specimens. This can be proved from the absorption percentage of 0.64, 0.76 & 0.88% respectively for the corresponding temperatures. This shows the 80% of VER present into the IPN system absorbs the much moisture as compared with the remaining 20% of PU. For 50%PU, the diffusion coefficient was 7.4863 × 10-11 (m2/s), 8.5432 × 10-11(m2/s) & 9.8745 × 10-11(m2/s) with the corresponding moisture gain of 0.42, 0.55 & 0.65% for the water temperature of 45 °C, 55 °C & 65 °C. From these values, it can be concluded that that with increase in PU prepolymer content in IPN systems the moisture absorption was found to decrease.

4.2 Influence of water absorption (diffusion) on hoop strength In general, all the engineering products require good and reliable mechanical properties, even after exposure to various hygrothermal environments like temperature, pressure and moisture etc. This is also applicable for composite pipes, which are mostly known to be used under high temperature and moist environments. Hence their mechanical property was studied under artificially created hygrothermal conditions, where as the dry specimens without hygrothermal ageing, have shown the hoop strength of 95.49, 91.15, 84.64, 77.69, 69.40, 60.33 MPa of strength for respective proportions[7]. In the study the mechanical behavior of (hoop strength) IPN pipes when subjected to hygrothermal (pertains to moisture and heat) condition was studied. The Hoop strength for different proportions of PU (0%PU, 10%PU, 20%PU, 30%PU, 40%PU & 50%PU) loaded pipes after moisture ageing treatment are shown in Figure 6. The overall view obtained from the study is that, increase in temperature decreases the Hoop strength of the IPN pipes. While considering the 0%PU loaded pipe the hoop strength showed a decrease with hygrothermal ageing. The hoop strength of the pipe showed a decrease of 16% compared with the specimens exposed to water at 45 °C for 3 and 6 months. A similar trend is also depicted in PU loaded pipes. For the PU loaded pipes, increase in PU content caused a decrease in the Hoop stress. In addition, exposures to various (45 °C, 55 °C & 65 °C) temperatures with different time period also cause a decrease in hoop stress. On considering the 30%PU loaded pipe, the dry specimen had the hoop strength value of 77.7 MPa[6], specimen exposed to temperature of 55 °C had the value of 63 MPa for 90 days.

Subsequently the decline of hoop stress was noted as 58 MPa for 180 days and 44 MPa for 270 days. This showed that, the exposure to high temperature weakens the bond in between the fibre and matrix system causing fracture 5/8

Gopi, S., Loganathan, G. B., Sekar, B. K., Krishnamoorthy, R. K., Sekaran, V., & Mohan, A. R.

Figure 6. Ring test results after water boiling treatment of 0%, 10%, 20%, 30%, 40%, 50% PU loaded IPN composite pipe.

Figure 7. Ring test (Hoop test) specimens after test (a) 45°C (b) 55°C (c) 65°C.

at reduced loads. At 65 °C of 50% PU loaded IPN composite pipe shown very lower moisture absorption.

The corresponding diffusion (Table 2) coefficient value of 65 °C (50% PU) also comparatively very less as similar to that of the remaining proportionate of IPNs. When the same specimen subjected to hoop strength, the stress value was 37 MPa. This phenomenon was due to the plasticizing effect of PU (soft segment), which lost its shear force with VER (hard segment) at higher temperature. It was found that, fibre/resin binding was completely disturbed in the fractured specimens this was because of leathery state of the polyurethane at considerable temperature[. The fracture was mainly because of fibre/matrix interface debonding. During the hygrothermic test, water penetrates into the interface region through the micro voids and the micro cracks present in the specimen. This trapped water partially damages the interface region. This leads to fibre breakage in most of the specimens as dominant failure mode. Figure 7. Clearly shows the fibre pullout phenomenon of all the specimens in 6/8

various temperatures. This was the main failure mode in all the specimens irrespective of the temperature. Similarly the effect of the temperature was key phenomenon for all the matrix degradation, thus the way; the matrix degradation was the evident for the early failure during the test.

Surprisingly the another notable study was found during this research, that was, as much as the PU loading is increased into the system, it remarkably reduces the diffusion coefficient to the much higher stage, by the way it decreases the moisture absorption value to the higher stages.

Form the diffusion table, it can be very clearly seen. As we discussed, the difference of hoop strength between the dry to the PU loaded specimen reduces as we increase the PU loading into the IPN system. Besides that, the moisture absorption characteristics were limited to the boiling water aged (PU loading) specimens while comparing with the pure VER made specimens. Figure 8 completely describes the TGA value of the IPNs after the hygrothermal ageing test.

From the Figure 8, it was observed that PU loaded VER Polímeros, 29(3), e2019038, 2019

Influence of water absorption on glass fibre reinforced IPN composite pipes IPN have had an very lesser thermal stability as compared with the pure VER system.

The onset of degradation for 0% PU system was 365 °C. With increasing PU content from 10% to 50%, there was a gradual and steady decrease in thermal stability from 300 °C to 200 °C. Irrespective of PU loading the IPN system exhibited a maximum degradation temperature around 410 °C after ageing. The reason behind this degradation was that, the breakage of the interchain bond by the gain of the water into the polymer, this phenomenon results the increase in mobility of the macromolecular this leads to the decrease in the TGA after the hygrothermal test.

4.3 Influence of moisture on burst pressure Burst pressure test also known as hydrostatic pressure test was performed under room temperature as per ASTM D1599 Standard. The PU loaded IPN composite test specimens were immersed in the de-ionized water for various period of time e.g. 3, 6, 9 and 12 months and then tested for burst pressure. The initial pure virgin dry sample was tested, and the value was found to be 25.5 bar (2.55 MPa)[6]. With increase in ageing period the burst pressure showed a decreasing trend, due to the absorbed moisture. The values obtained through burst pressure test after moisture absorption were recorded and plotted in Figure 8. The 0%PU in the figure showed a nominal decrease in the value of burst pressure during the first 3 months. After 3 months exposure period, the percentage decrease to be 15.7%. This drastic drop in burst pressure might be due to of the absorption of moisture.

After 6 to 9 months, there is no significant change in the burst pressure values. There was a minor increase in the value of the burst pressure at 12 month exposure period, which was considered as saturation point. When the (0% PU) dry specimen comes across the burst test, known that, there was proper bonding as well adhesion between the matrix and the fibre, but when it exposed to certain humid condition there was degradation of fibre-matrix interface. Similarly when the ageing time increases, the significant reduction of resin around the fibre leading to weakening the fibre-matrix interface. Various proportions of PU loaded glass fibre reinforced VER IPN composites were subjected to burst pressure test and the values obtained were plotted and compared with the 0% PU pipe as in the Figure 9.

A similar trend was observed for all the proportions (10%PU, 20%PU, 30%PU, 40%PU & 50%PU). For 10%PU it was observed that, the burst pressure decreased greatly for the first 3 month exposure period to moisture. The values at 6 and 9 month exposure period showed a marginal difference in the value with a decrease of 2.6%. The similar trend was not seen in 12 months, because no significant change in the values. The 12 month period had a percentage increase of 0.52% in burst pressure. The values of burst pressure for 0%PU and 10%PU, only has a small change in the burst pressure, showing that the moisture absorption in the pipes were similar. Comparing the pipes of other compositions for burst pressure against moisture absorption, the values for 20%PU had a decrease in the percentage from 7.69% - 3.12% for 3 to 9 months. For 10%PU, the value was steadily maintained for 12 months as well.

In 30%PU, 40%PU, 50%PU the percentage reduction in the Polímeros, 29(3), e2019038, 2019

Figure 8. TGA Curve of IPNs (a) 0%PU, (b) 10%PU, (c) 20%PU, (d) 30%PU, (e) 40%PU, (f) 50%PU (After Hygrothermal Ageing Test).

Figure 9. Burst strength analysis of different proportion of IPN composite pipe.

values of burst pressure for moisture absorbed pipes were similar and had a slight change with percentage differences from 4% to 1.1% for 3 to 9 months, whereas the pipes with exposure period of 12 month showed slight increase in percentage of burst pressure from 1.6% to 2.5%. Figure 9 shows that, the moisture absorption characteristic abruptly decreases the burst pressure for all the proportions for the corresponding immersion period of 3 to 9 months. The values on burst pressure and moisture absorption have not shown any significant changes for the remaining period up to 12 months. In addition to this, the significant observation was that, the PU loaded VER pipes showed very less effect to moisture absorption because PUs offers good resistance against moisture absorption.

5. Conclusions The following observations were drafted from the experimental analysis. Hygrothermal behavior of Glass fibre reinforced IPN composite pipe was studied for different proportions (0%PU, 10%PU, 20%PU, 30%PU, 40%PU & 50%PU) by subjecting the pipe at 45 °C, 55 °C & 65 °C in water testing chamber for a maximum duration of 365 days.

The maximum moisture uptake eventually reduced the mechanical properties of the GFRP pipes, when the specimens were subjected to tests like Burst pressure test, Hoop strength test. From the study it was observed that, the plasticization effect of resin material created the de-bonding effect between the fibre and matrix. This effect was very common phenomenon for all the composite pipes. Besides that, the 7/8

Gopi, S., Loganathan, G. B., Sekar, B. K., Krishnamoorthy, R. K., Sekaran, V., & Mohan, A. R. diffusion coefficient was evaluated for all the composition.

The diffusion coefficients with their corresponding moisture uptake was that, the 0%PU gains1.2% of weight%, towards the corresponding diffusion coefficient of 4.1232 × 10-6 at the water temperature of 65 °C. Eventually for 50%PU, significant achievement of 0.65% of moisture gain was seen for the corresponding temperature of 65 °C. This was due to the loading of PU into the VER system. Whereas the same parameter was very high for neat resin E-glass fibre reinforced composite pipes. It was found that the moisture absorption increased with increase of temperature for all the proportions. In the contrary, PU loaded specimens were found to be superior ones as far as the moisture absorption behavior concerned.

6. References 1. Al-Sulaiman, F., Khan, J., Merah, N., Kounain, M. A., & Mehdi, M. (2010). Effects of weathering on failure pressure of filamentwound GFRP thermoset pipes. Journal of Composite Materials, 45(6), 645-655. http://dx.doi.org/10.1177/0021998310377933.

2. Chang, D. J. (2003). Burst tests of filament-wound graphiteepoxy tubes. Journal of Composite Materials, 37(9), 811-882. http://dx.doi.org/10.1177/002199803031032.

3. Yao, J., & Ziegmann, G. (2007). Water absorption behavior and its influence on properties of GRP Pipe. Journal of Composite Materials, 41(8), 993-1008. http://dx.doi. org/10.1177/0021998306067265.

4. Ghorbel, I. (1995). Durability of closed-end pressurized GRP Filament wound pipes under hygrothermal aging conditions.

Part II: creep Tests. Journal of Composite Materials, 30(14), 1581-1595. http://dx.doi.org/10.1177/002199839603001404.

5. Shahram, E., Abbas, H. R., & Shiva, E. (2015). Effects of moisture absorption on degradation of E-glass fiber reinforced Vinyl Ester composite pipes and modelling of transient moisture diffusion using finite element analysis. Corrosion Science, 90(1), 168-175.

6. Suresh, G., Jayakumari, L. S., & Dinesh, K. S. (2017). Finite element analysis of IPN reinforced woven fabric composite.

Revistamateria, 22(4), 1-6. http://dx.doi.org/10.1590/s1517707620170004.0216.

7. Suresh, G., & Jayakumari, L. S. (2016). Analyzing the mechanical behavior of E-glass fibre reinforced interpenetrating polymer network composite pipe. Journal of Composite Materials, 50(22), 3053-3061. http://dx.doi.org/10.1177/0021998315615408.

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8. Lima Sobrinho, L., Ferreira, M., & Bastian, F. L. (2009). The effects of water absorption on an ester vinyl resin system.

Materials Research, 12(3), 353-361. http://dx.doi.org/10.1590/ S1516-14392009000300017.

9. Suresh, G., & Jayakumari, L. S. (2015). Evaluating the mechanical properties of E-Glass fiber/carbon fiber reinforced interpenetrating polymer networks. Polimeros: Ciência e Tecnologia, 25(1), 49-57. http://dx.doi.org/10.1590/0104-1428.1650.

10. Guoqiang, L., Su-Seng, P., & Jack, E. H. (2001). Stiffness degradation of FRP strengthened RC beams subjected to hygrothermal and aging attacks. Journal of Composite Materials, 36(7), 795-812.

11. Wang, G. Y., Wang, Y. L., & Hu, C. P. (2000). Interpenetrating polymer networks of polyurethane and graft vinyl ester resin: polyurethane formed with toluene diisocyanate. European Polymer Journal, 36(4), 735-742. http://dx.doi.org/10.1016/ S0014-3057(99)00113-5.

12. Cristea, M., Ibanescu, S., Cascaval, C. N., & Rosu, D. (2009). Dynamic mechanical analysis of polyurethane-epoxy interpenetrating polymer networks. High Performance Polymers, 21(5), 608-623. http://dx.doi.org/10.1177/0954008309339940.

13. Fan, L. H., Hu, C. P., & Ying, S. K. (1996). Thermal analysis during the formation of polyurethane and vinyl ester resin interpenetrating polymer networks. Polymer, 37(6), 887-1058. http://dx.doi.org/10.1016/0032-3861(96)87280-6.

14. Qin, C. L., Cai, W. M., Cai, J., Tang, D.-Y., Zhang, J.-S., & Qin, M. (2004). Damping properties and morphology of polyurethane/vinyl ester resin interpenetrating polymer network. Materials Chemistry and Physics, 85(3), 402-409. http://dx.doi.org/10.1016/j.matchemphys.2004.01.019.

15. Mezghani, K. (2012). Long term environmental effects on physical properties of Vinylester composite pipes. Polymer Testing, 31(1), 76-82. http://dx.doi.org/10.1016/j.polymertesting.2011.10.001.

16. Feih, S., Mathys, Z., Mathys, G., Gibson, A. G., Robinson, M., & Mouritz, A. P. (2008). Influence of water content on failure of phenolic composites in fire. Polymer Degradation & Stability, 93(2), 376-382. http://dx.doi.org/10.1016/j. polymdegradstab.2007.11.027.

17. Suresh, G., Abdul Munaf, A., Akash, R. M., Bharani Kumar, S., & Kanagaraja, K. (2019). Analyzing the mechanical behaviour of carbon fiber reinforced spur gear with IPN as matrix material. SSRN, 1(11), 1-4.

Received: Sept. 24, 2018 Revised: July 26, 2019 Accepted: Aug. 01, 2019

Polímeros, 29(3), e2019038, 2019

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.03719

In vitro evaluation of PVA gels loaded with Copaiba Oil and Duotrill Ingrid Cristina Soares Pereira1 , Natália Rodrigues Rojas dos Santos1, Antonieta Middea2, Edlene Ribeiro Prudencio3, Rosa Helena Luchese3, Ana Paula Duarte Moreira4* and Renata Nunes Oliveira1 Departamento de Engenharia Química – DEQ, Instituto de Tecnologia – IT, Universidade Federal Rural do Rio de Janeiro – UFRRJ, Seropédica, RJ, Brasil 2 Centro de Tecnologia Mineral – CETEM, Rio de Janeiro, RJ, Brasil 3 Departamento de Tecnologia de Alimentos – DTA, Instituto de Tecnologia – IT, Universidade Federal Rural do Rio de Janeiro – UFRRJ, Seropédica, RJ, Brasil 4 Programa/Departamento de Engenharia Metalúrgica e de Materiais, Escola Politécnica – POLI, Instituto Alberto Luiz Coimbra de Pós-Graduação e Pesquisa de Engenharia – COPPE, Universidade Federal do Rio de Janeiro – UFRJ, Rio de Janeiro, RJ, Brasil

1

*duarteap@gmail.com

Abstract Enrofloxacin can be slowly delivered through polymeric systems and the addition of oil could increase the polymeric gels hydrophobicity and help the continuous release. The present work intended to develop and characterize microstructurally (XRD and FTIR) and in vitro (swelling and antimicrobial tests) the PVA hydrogels loaded with copaiba oil and Duotrill (enrofloxacin) to treat bacterial infections, as pyelonephritis, in the veterinary field. Duotrill and oil combined diminished the gels degree of crystallinity and it was observed interaction between phases due to a new band found only in PVA hydrogels loaded with copaiba oil and Duotrill (PVA-D-O) FTIR spectrum. The samples with oil swelled less than samples without it, where copaiba oil altered the samples’ hydrophilicity. PVA-D-O presented lower weight loss and higher gel fraction than PVA, indicating the loaded material increased the gels stability. All samples containing oil and Duotrill inhibited S. aureus.

Keywords: PVA, hydrogel, copaiba oil, enrofloxacin, in vitro.

How to cite: Pereira, I. C. S., Santos, N. R. R, Middea, A., Prudencio, E. R., Luchese, R. H., Moreira, A. P. D., & Oliveira, R. N. (2019). In vitro evaluation of PVA gels loaded with Copaiba Oil and Duotrill. Polímeros: Ciência e Tecnologia, 29(3) e2019039. https://doi.org/10.1590/0104-1428.03719

1. Introduction Pyelonephritis is a name used to describe an inflammatory process of the pelvis and renal parenchyma originated by bacterial infections all over the lower urinary tract.

These bacterial infections are generally caused by aerobic bacteria, e.g. Escherichia coli and Staphylococcus sp., and rarely by species of Proteus, Streptococcus, Klebsiella and Enterobacter[1,2]. There is a wide variety of antibiotics to treat bacterial diseases in animals, specially dogs and cats.

Consequently, there is also an increase of intoxication due to the incorrect use (overdose) of this medicine. Some cases, the drugs’ collateral effects and toxins could lead to death[3].

One possible alternative to avoid poisoning is the use of drug delivery systems (DDS), e.g. hydrogels[4].

groups, which form crystallites through of intra‑ and inter‑chain hydrogen bonding[7].

Stauffer and Peppast[5] developed polyvinyl alcohol (PVA) hydrogels (3D networks of hydrophilic polymers) with structural integrity by physical crosslinking[5], using freeze‑thawing method[6]. The development of PVA hydrogels by freeze-thawing are based on the polymer’s hydroxyl

PVA hydrogels are usually combined it with antimicrobial agents to grant them this characteristic. Regarding natural materials with antimicrobial properties, Oliveira et al.[4] had successfully loaded propolis (bee-based material) to PVA hydrogels[4]. In addition, bioactive oils are also able of

Polímeros, 29(3), e2019039, 2019

The PVA physical hydrogels are biocompatible, stable at room temperature, ease form film by solution casting and suffer natural biodegradation under physiological conditions[7-9] Biodegradation or erosion mechanism of PVA physical hydrogels is essential to drug delivery by implantable biomaterials[9]. Among PVA gels used as DDS, there are: Jensen et al.[9], who developed gels with high potential for drug delivery through spontaneous erosion[9]; Marques[10] studied hydrogels loaded with ibuprofen and obtained gels with excellent mechanical propriety and an efficient controlled ibuprofen delivery[10].

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O O O O O O O O O O O O O O O O

Pereira, I. C. S., Santos, N. R. R, Middea, A., Prudencio, E. R., Luchese, R. H., Moreira, A. P. D., & Oliveira, R. N. delay the microbial activity, due to phenolic and terpenoids groups to which is attributed their antimicrobial activities[11].

Brandelero et al.[12] added copaiba and lemongrass oils directly to starch-polyvinyl alcohol-alginate device, which presented improved antimicrobial properties[12,13].

Kavoosi et al.[14] developed gelatin/PVA hydrogel loaded with Zataria essential oil (ZO) for wound-dressing, obtaining increased the antimicrobial activities due to the addition of ZO, which also decrease the films’ swelling ability.

ZO probably contributed to the gels hydrophobicity due to its characteristics[12,14]. The essential oils have a nature hydrophobic due to substances that stimulate create region non-polar in the polymeric matrix. This efficiency is linked with rate of proportion between hydrophilic and hydrophobic of film, and with characteristics of the compounds added such as polarity or structural chemical[12,15].

2. Materials and Methods 2.1 Materials Polyvinyl alcohol - PVA, Mw 85000-124000 Da and degree of hydrolysis 99%, was purchased from Sigma Aldrich. The copaiba oil, natural product, was obtained commercially from Ashram Aquarius. Ethyl alcohol, 95% purity, was purchased from Vetec. The Duotril (drug) was obtained commercially from Laboratory Duprat in Brazil. All reagents described were used without further purification.

2.2 Preparation of the samples The method employed to preparation of four samples distinct was based on Oliveira et al.[4] The PVA pristine and PVA hydrogels containing duotril, oil and both oil and duotril were labelled PVA, PVA-D, PVA-O, PVA-D-O, respectively. PVA aqueous solution (10% w/v) was prepared by dissolution in 90°C for 4h, under mechanical stirring and it was named ‘PVA’. The duotril was dissolved in distilled H2O at room temperature, under magnetic stirring and mixed to PVA solution. This sample was named ‘PVA-Duotril’ (PVA-D). The copaiba oil was associated to ethyl alcohol (molar ratio 1:1) at room temperature under magnetic stirring and after mixed to PVA solution. This sample was named ‘PVA-Oil’ (PVA-O). The samples composition is displayed in Table 1.

Copaiba oil, obtained from copaiba trees (Copaifera sp., Fabaceae), is a natural antimicrobial agent native from western Africa and South America (specifically from the Amazon, north of Brazil). The Amazon indigenous people use copaiba oil for treatment of various diseases, e.g. stomach ulcers and tonsillitis[16-18]. Antimicrobial studies of the copaiba oleoresin found its high potential as medicine[19].

Sachetti et al.[16] observed that the oleoresin did not cause negative effects (toxicity) to rats, but further studies are necessary[16]. Copaiba oil could add the antimicrobial property and hydrophobicity to PVA hydrogels.

Among the hydrophilic drugs that could be added to PVA, there is Duotrill (commercial name), an enrofloxacin based antibiotic[20]. The enrofloxacin is a fluoroquinolone used in the veterinary medicine. According to Vancutsem et al.[21] the fluoroquinolones is efficient in the treatment of bacterial diseases in several animals, including birds, except to juvenile dogs and horses, since it effects their cartilage[21,22].

The enrofloxacin is excellent to treat pyelonephritis, since this antibiotic has a wide spectrum of action against Gram‑negative (E. coli, Pseudomonas sp and Enterobacter sp) and some Gram-positive bacteria (Streptococcus sp and Staphylococcus sp) and Mycoplasma and Chlamydia[2,20,22].

Enrofloxacin can be loaded to polymers to be slowly delivered, e.g. enrofloxacin loaded to Poly(3-hydroxybuty rate‑co‑3‑hydroxyvalerate) (PHBV) microspheres showed delivery for 13 days and when inserted intramuscular, it was detected in rats’ blood for 3 days. Nonetheless it could be inferred that the therapeutic concentration was maintained for long periods when enrofloxacin was delivered through PHBV microspheres[23].

The duotril and the copaiba oil were mixed to PVA solution when it reached room temperature under stirring and it was named ‘PVA-Duotril-Oil’ (PVA-D-O). 10 mL of each final solution were poured in petri dishes (diameter of 90 mm), and the samples were freeze-thawed (1 cycle of 16 h at -16°C and 30 min at 25°C followed by 4 cycles of 1h at -16°C and 30 min at 25°C). The samples were dried in room temperature afterwards.

2.3 Microstructural analysis Microstructural characterization of dry samples was performed using Fourier Transform Infrared Spectroscopy (FTIR, PerkinElmer equipment, Spectrum 100 (COPPE/UFRJ), in the ATR mode, wavenumber range of 4000 cm-1 and 600 cm-1, 32 scans per samples and a spectral resolution of 4 cm-1) and X-ray diffraction analysis (XRD, Bruker-AXS D8 Advance Eco diffractometer (CETEM/UFRJ), Cu kα radiation (40 kV/25 mA), in the 2Ɵ angle range of 10º - 60º, with a 0.01º step size and a position-sensitive Lynx Eye XE detector with energy discrimination). The degree of crystallinity (Xc) of the samples evaluated by XRD was based on the area of crystalline peaks per area of crystalline and amorphous phases[24].

The goal of present work was to develop and characterize microstructurally and in vitro the PVA hydrogels loaded with copaiba oil and Duotrill (enrofloxacin) intended to treat bacterial infections in the veterinary field.

Table 1. Hydrogels samples composition.

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Sample

PVA (g)

H2O (mL)

Oil (mL)

Duotril (mg)

PVA PVA-D PVA-O PVA-D-O

10 10 10 10

100 95 95 90

0 0 5 5

0 50 mg/5 mL H2O 0 50 mg/5 mL H2O

Polímeros, 29(3), e2019039, 2019

In vitro evaluation of PVA gels loaded with Copaiba Oil and Duotrill 2.4 In vitro analysis Swelling/Weight loss tests were adapted according to Oliveira et al.[4,25] and Costa[26]. Each sample composition (n=5) was evaluated, where the samples remained immersed in 10mL saline solution (SS) for 4 days at room temperature, being weighed periodically (30 min, 1h, 2h, 3h, 4h, 24h, 48h, 72h and 96h). The samples were dried and weighted afterwards. The swelling degree (SD) and weight loss (WL) were calculated according to Equations (1) and (2), respectively. Furthermore, the samples’ gel fraction (GF) percentage, was calculated, Equation (3).

SD = 100 ×

WS − WD

WD WD − WDS

WD

(2)

WDS

WD

(3)

WL = 100 ×

GF = 100 ×

(1)

The WS is the samples’ weight at each interval time.

Whereas, WD is the dry weight prior to swelling test and WDS is the dry weight after swelling[25-27].

Antimicrobial activity of hydrogels was evaluated according to standard ASTM E2180-07 with some changes, using Staphylococcus aureus. In the initial step, a cell suspension of S. aureus (ATCC 6538) was prepared, adjusting the turbidity on the MacFarland scale to 5, that is equivalent to 108 colony forming units per mL (CFU/mL). Afterwards,

one (1) mL of this suspension was diluted in 100 mL of agar paste to obtain concentration of 106 CFU/mL. The samples were placed on 24-well plates and each sample was added of 200 microliters of agar inoculated paste. The plates were incubated at 30°C for 24h. Thereafter, incubated samples were moved to Falcon tubes and it was added 1,8 mL of buffer solution. Subsequent decimal dilutions were prepared up to 10-4 and S. aureus survivability was evaluated on PCA agar using the micro-drop plate technique.

2.5 Statistical analysis The statistical analysis was performed using the one-way ANOVA analysis and Tukey test. The ANOVA one-way analysis, 95% significance level, was used to evaluate the parameter amount of drug and/or oil, with four levels: PVA, PVA-Drug, PVA-Oil and PVA-Oil-Drug. The gels’ swelling capacity, weight loss and gel fraction were used as response data. Tukey test, α=0.05, was conducted to determine if the difference between each pair was significant.

3. Results and Discussions 3.1 Microstructural analysis The FTIR spectra of all samples, Figure 1, shows the comparison between PVA-D, PVA-O and PVA, as well as PVA-D-O with PVA-D and PVA-O. Table 2 summarizes the FTIR band assignments of the hydrogels produced as shown in Figure 1. PVA presents bands at: 3626 cm-1, ν(-OH), regarding inter- and intramolecular hydrogen bonds; 2942 cm-1, ν(-CH) from alkyl groups; 2915 cm-1, νas(CH2);

Figure 1. FTIR spectra of the samples: (a) PVA and PVA-D; (b) PVA and PVA-D; (c) PVA-D and PVA-D-O; (d) PVA-O and PVA-D-O. Polímeros, 29(3), e2019039, 2019

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Pereira, I. C. S., Santos, N. R. R, Middea, A., Prudencio, E. R., Luchese, R. H., Moreira, A. P. D., & Oliveira, R. N. Table 2. FTIR wavenumbers and respective vibration modes of samples.

Wavenumber (cm-1) 3626 2942 2915 1652 1560 1413 1380 1236 1142 1089 947 831 2922 2854 1743 1415 3274 1700-1600 1652 1236 900-600

Assignments

combination of (CH + OH) groups C – H bending stretching vibration related to crystallites formation C – O streching of secondary alcohols; C – O out-of-plane bonding C – O and C – C streching C – C bonding C – H streching CH2 asymmetric streching C = O streching C – H bending O – H streching of alcohol C = O streching aromatic C = C streching C – O streching; C – O – H bending C – H streching of aromatic ring

1652 cm-1, ν(-CH) from alkyl groups; 1560 cm-1, ν(C=C); 1413 cm-1, hydroxyl group δ(OH); 1380 cm-1, (CH + OH) group; 1236 cm-1, ν(C-H); 1142 cm-1, stretching vibration related to crystallites formation; 1089 cm-1, ν(C-O) of secondary alcohols, C-O out-of-plane bonding; 917 cm-1, ν(CO and CC groups) and 831 cm-1, C-C bonding[28-34]. Some of the PVA bands present lower intensity due to presence of ‘oil’ (PVA-O and PVA-D-O). However, sample PVA-O presented also bands related to copaiba oil. The copaiba oil bands were observed at: 2922 cm-1, ν(-CH); 2854 cm-1, ν(-CH2); 1743 cm-1, ν(-C=O); 1415 cm-1, δ(-CH)[35].

Spectra of the samples PVA and PVA-D (Figure 1(a)) did not show remarkable difference between them, where the only difference would be the PVA bands intensity. The active antibiotic in Duotrill would be enrofloxacin. Its main bands would be at: 3274 cm-1 ν(-OH of alcohol); 1700-1600 cm-1 ν(C=O); 1652 cm-1 ν(C=C); 1236 cm-1 ν(CO), ν(COH); and bands between 900-600 cm-1 ν(-CH aromatic)[36,37].

Samples PVA-D-O displayed the main bands of PVA and copaiba oil, similar to sample PVA-O, although the bands intensity varied (Figure 1(d)). It was expected to observe high intensity of the bands between 3400–2900 cm-1 and 1800–1100 cm-1 with the addition of enrofloxacin[37]. This effect was not observed, but there is a band at 871 cm-1 in PVA-D-O sample, that was not observed in PVA, PVA-D or PVA-O samples, indicating a possible interaction between of copaiba oil and Duotrill.

The XRD analysis revealed probable overlapped peaks and they were deconvoluted to distinguish the crystalline amount from the amorphous one, Figure 2. The addition of copaiba oil to PVA altered the XRD spectrum, although the main peak of all spectra is at 2θ ~39°, a wide peak that apparently is the overlap of different peaks (peaks at 2θ = ~32°, ~39° and ~47°). The main PVA peak (2θ ~20°) was not identified in the samples[38]. Nonetheless, the 4/8

Sample PVA and PVA compounds[28-34]

O – H streching C – H streching CH2 asymmetric streching C – H streching C = C streching O – H bending

Copaiba oil and PVA-O[35]

Duotril and PVA-D[36,37]

addition of Duotrill led to the presence of another peak at 2θ ~19°, which could be related to the main enrofloxacin peak at 2θ ~25°[39,40].

The XRD curves deconvolution, Figure 2, revealed the peaks at 2θ = ~32°, ~39° (both probably related to crystalline phase) and a wide peak at 2θ = ~47° (possibly related to the amorphous phase). The addition of copaiba oil or duotril to PVA altered the chains packing (decreasing the Xc of the samples), but the addition of both revealed a considerable synergic effect on the samples Xc. The presence of enrofloxacin peak in the XRD spectra of samples could indicate incomplete incorporation of the drug in the PVA gel[41], or even simple physical presence of the loaded material between PVA chains[42]. Although the position of the peaks remained similar after loading, altering the samples Xc could indicate interaction between the materials[43]. Duotrill and oil combined diminished the gels degree of crystallinity and there is an interaction between phases revealed by a new band found only in PVA-D-O sample’s FTIR.

3.2 In vitro analysis The swelling tests revealed the all samples swelled at least 180%. There was a peak of media uptake at the onset of all curves and the equilibrium swelling degree (ESD) was reach after 1 day of immersion, Figure 3.

The condition to occur the ESD is when the swelling forces (media entrance stretches the network) and elastic forces of the network (chains relaxation and crosslinking are responsible for a partial network’s contraction) reach the equilibrium[44]. PVA gels usually reaches ESD at 37°C of approximately 400% and at least 300%[4]. The samples in this work presented relatively low ESD (evaluated at room temperature). The temperature could have influenced the ESD, increasing it when evaluated at 37°C[4,25]. The ANOVA Polímeros, 29(3), e2019039, 2019

In vitro evaluation of PVA gels loaded with Copaiba Oil and Duotrill

Figure 2. XRD deconvoluted curves of the samples: (a) PVA; (b) PVA-O; (c) PVA-D and (d) PVA-D-O.

Figure 3. (a) Samples swelling degree and (b) Gel fraction and Weight loss.

analysis on the ESD revealed that samples with oil swelled less than samples without it (p < 0.05). Thereby, copaiba oil alters the samples’ hydrophilicity due to its hydrophobic characteristic / substances [45].

The PVA samples weight loss was higher than the PVA-D-O weight loss, as well as PVA presented lower gel fraction than PVA-D-O(p < 0.05), Figure 3 (b). It seems Polímeros, 29(3), e2019039, 2019

that the PVA chains presented lower mobility and higher inter / intra-connections in PVA-D-O samples. This is an indication that Duotrill and copaiba oil combined increased the structural stability of the PVA gels[12].

Table 3 shows the results obtained by inhibition of S. aureus. The samples loaded with oil reduced the S. aureus proliferation considerably, but the highest values 5/8

Pereira, I. C. S., Santos, N. R. R, Middea, A., Prudencio, E. R., Luchese, R. H., Moreira, A. P. D., & Oliveira, R. N. Table 3. Antimicrobial activity against S. aureus.

Samples PVA PVA-O PVA-D PVA-D-O

Counting mean (CFU/g) 5.5 × 106 1.1 × 105 1.4 × 103 7.5 × 103

Reduction (%) 0 (reference) 98 99.97 99.86

of inhibition (total inhibition) was observed in samples containing Duotrill. Classically, copaiba oil[46], as well as enrofloxacin[47], inhibits S. aureus, although some organisms could develop resistance to enrofloxacin[48]. The gels of the present work seem to have incorporated ‘oil’ and Duotrill presenting activity against S. aureus.

4. Conclusions Duotrill and copaiba oil combined diminished the gels degree of crystallinity and there is an interaction between phases revealed by a new band found only in PVA-D-O sample’s FTIR. The samples with oil swelled less than samples without it, where copaiba oil altered the samples’ hydrophilicity. PVA-D-O presented lower weight loss and higher gel fraction than PVA, indicating the loaded material increased the gels stability. All hydrogels containing copaiba oil and Duotrill reduced S. aureus load, but the combination of both did not result in a greater reduction. Gels loaded with copaiba oil or Duotrill are potential materials to treat bacterial infections in the veterinary field.

5. Acknowledgements The authors thank CNPq, CAPES and FAPERJ for the support. This work was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico [grant number 405922/2016-7].

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Pereira, I. C. S., Santos, N. R. R, Middea, A., Prudencio, E. R., Luchese, R. H., Moreira, A. P. D., & Oliveira, R. N. 45. Brandelero, R. P. H., Grossmann, M. V., & Yamashita, F. (2013). Hidrofilicidade de filmes de amido/poli(butileno adipato co-tereftalato) (Pbat) adicionados de tween 80 e óleo de soja. Polímeros Ciência e Tecnologia, 23(2), 270-275. http:// dx.doi.org/10.4322/S0104-14282013005000011.

46. Pereira, N. C. M., Mariscal, A. G., Nepoceno, K. L. P. C., Silva, V. C. C. R., Fernandes, H. M., & Vivi, V. K. (2018).

Antimicrobial activity of natural/commercial copaiba oil-resin against standard strains. Journal Health NPEPS, 3(2), 527-539. http://dx.doi.org/10.30681/252610103189.

47. Nunes, S. F., Bexiga, R., Cavaco, L. M., & Vilela, C. L. (2007).

Technical note: antimicrobial susceptibility of Portuguese isolates of Staphylococcus aureus and Staphylococcus

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epidermidis in subclinical bovine mastitis. Journal of dairy science, 90(7), 3242-3246. http://dx.doi.org/10.3168/jds.2006739. PMid:17582107.

48. Wang, W., Lin, X., Jiang, T., Peng, Z., Xu, J., Yi, L., Li, F., Fanning, S., & Baloch, Z. (2018). Prevalence and characterization of Staphylococcus aureus cultured from raw milk taken from dairy cows with mastitis in Beijing, China.

Frontiers in Microbiology, 9, 1123. http://dx.doi.org/10.3389/ fmicb.2018.01123. PMid:29988423.

Received: May 14, 2019 Revised: Aug. 26, 2019 Accepted: Aug. 26, 2019

Polímeros, 29(3), e2019039, 2019

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.02519

Crosslinking starch/oat hull mixtures for use in composites with PLA Thamires da Silva Peixoto1 , Fabio Yamashita1 , Ana Paula Bilck1 , Gizilene Maria Carvalho2  and Maria Victoria Eiras Grossmann1*  Departamento de Ciência e Tecnologia de Alimentos, Universidade Estadual de Londrina – UEL, Londrina, PR, Brasil 2 Departamento de Química, Universidade Estadual de Londrina – UEL, Londrina, PR, Brasil

1

*mvgrossmann@gmail.com

Abstract Modification of composite components has been proposed as a tool to improve their functional properties. The present work studied crosslinking of a starch/oat hull mixture by reactive extrusion with sodium trimetaphosphate (STMP), for application in composites with polylactic acid (PLA). The treated mixture was characterized regarding degree of substitution, FT-IR, and thermal properties. The native and modified mixtures were processed by injection molding, together with PLA and glycerol. The microstructure, mechanical properties, shrinkage, density, and thermal properties of the composites were determined. The FT-IR results, increase in phosphorus content and thermal stability after starch/fiber mixture treatment with STMP suggested the occurrence of crosslinking. Better interfacial adhesion between oat hulls and the polymeric matrix was observed in electron micrographs of the composites containing the modified components.

Slight decreases in tensile strength and modulus were observed in the modified composites, suggesting that extrusion and subsequent milling may have broken some structures/linkages.

Keywords: biocomposites, reactive extrusion, lignocellulosic fibers, injection molding, reticulation.

How to cite: Peixoto, T. S., Yamashita, F., Bilck, A. P., Carvalho, G. M., & Grossmann, M. V. E. (2019). Crosslinking starch/oat hull mixtures for use in composites with PLA. Polímeros: Ciência e Tecnologia, 29(3), e2019040. https:// doi.org/10.1590/0104-1428.02519

1. Introduction Several studies have been performed aiming to improve the mechanical and barrier properties of biodegradable packaging materials based on starch. Among them, the blend of starch with other biodegradable polymers is one of the most commonly used options[1]. The commercial availability of some blends, especially starch/PLA, demonstrates the impact of this solution in the substitution of non-degradable and/or non-renewable polymers[2-4]. Another strategy used with the same objective is to reinforce the structure of the polymeric matrix by adding lignocellulosic fibers, which can be used in the native, purified, or modified form[5-7]. Native fibers from coconut[8], sisal[9], jute[10,11], sugarcane[12], and oat hulls[13-15] were added to composites containing different biodegradable polymeric blends.

The advantages of lignocellulosic fibers, when compared with traditional fibers such as, for example, glass fibers, are mainly their low cost, availability, diversity of morphologies, and biodegradability. This last feature is of vital importance when looking at the end-of-life scenario of composite materials[16]. However, due to their hydrophilic structure, natural fibers may not demonstrate good interaction with the polymeric matrices[11,13,17]. Furthermore, this hydrophilic character may contribute to the increase in water absorption

Polímeros, 29(3), e2019040, 2019

of the composites, negatively affecting their mechanical properties and dimension stability[2].

Chemical modification of fibers and/or matrices is one of the tools used to improve compatibility with most of biodegradable polyesters and their blends. These modifications allow better interfacial adhesion between the fiber and matrix, providing efficient stress transfer so that the reinforcing effect of the material is achieved.

There are several types of chemical treatments (mercerization, oxidation, crosslinking, grafting, etc.) applied to the fibers, as well as studies on their influence on the properties of the composites prepared with the treated fiber[18-24].

In the same manner, chemically modified starch, mainly in the form of crosslinked starch, has been used with the aim of improving the functional properties of biodegradable polymeric blends[25-28]and fiber composites[29,30].

A new approach was used by Kumar et al.[31] and Wang et al.[17], which promoted crosslinking directly in composites of starch/cellulosic fibers, through UV irradiation and reactive extrusion with sodium trimetaphosphate, respectively.

The process in these systems can form homogeneous or heterogeneous crosslinking (starch-starch, starch-cellulose, and cellulose-cellulose), which will reinforce the structure of the composites. Similar results were observed by Niu et al.[32]

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O O O O O O O O O O O O O O O O

Peixoto, T. S., Yamashita, F., Bilck, A. P., Carvalho, G. M., & Grossmann, M. V. E. using a mechanochemical approach to crosslink cellulose fibers and poly(vinyl alcohol) (PVA) through pan-milling with succinic anhydride as a crosslinking agent.

at 60 °C for 12 h and ground in a laboratory IKA-A 11 Basic Mill (Ika, São Paulo, Brazil).

Based on the above, the objective of this investigation was to evaluate the impact of crosslinking starch and fiber on the properties of starch/oat hull/PLA composites.

660 nm (AJX 1600 spectrophotometer, Micronal, São Paulo, Brazil) following the method described by Walinga et al.[35] The phosphorus content of the native mixture (NM) of starch/hulls was also determined.

2. Materials and Methods

2.3.2 Fourier Transform Infrared Spectroscopy (FT-IR)

2.1 Material

The sample was dried in a desiccator containing CaCl2 (~ 0% RH) for one week and analyzed in a Fourier transform infrared spectrophotometer (Bruker, model Vertex 70, Germany) using an attenuated total reflection (ATR) module operating over the spectral range of 4000 cm-1 - 400 cm-1, with a 4 cm-1 resolution and 10 scans per sample. For comparison, this analysis was also performed for the native mixture of starch/hulls (NM) and in the crosslinked mixture without washing (CM).

2.3.1 Phosphorus content and Degree of Substitution (DS) Among the long list of fiber sources, oats hulls, which The degree of substitution (DS) was calculated by are a rich fiber by-product from oat processing, are a means of Equation 1. promising option as fillers in composites, and have been used in both natural[13-15] and modified form[23]. According= SD 162 P / ( 3100 − 102 P ) Eq. 1 to Cardoso et al.[23], although superficial fiber modification [24] using alkaline hydrogen peroxide solution has improved where P is the phosphorus content (%); 162 is the molecular interfacial adhesion between oat fibers and the matrix weight of the monomeric starch unit; 3100 refers to the (starch/poly(butylene adipate co-terephtalate)(PBAT), the atomic weight of the phosphorus multiplied by 100; and improvement was not enough to promote a significant effect 102 is the mass of the phosphate substituent group[34]. on composite reinforcement.

The phosphorus content was determined by colorimetry at

Cassava starch (Indemil Ltda., Guaíra, Brazil), PLA - REVODE201, (Hisun,Taizhou, China), micronized oat hulls (4.6% ash, 3.9% protein, 2.1% lipid, 23.1% cellulose, 26.2% hemicellulose, and 3.8% lignin- w/w, db) provided by SL Alimentos Ltda. (Mauá da Serra, Brazil), and glycerol (Dinâmica, Diadema, Brazil) were used for composite production. Sodium trimetaphosphate and sodium sulfate from Sigma-Aldrich (St. Louis, USA), and sodium hydroxide (Synth, Diadema, Brazil) were used for starch/fiber modification.

2.2 Starch/fiber modification A mixture of oat hulls and cassava starch containing 35.7wt% (db) of hulls was modified by reactive extrusion, based on the methodologies of Wurtzburg[33] and Wang et al.[17] After homogenization, sodium trimetaphosphate, and sodium sulfate, both in the proportion of 3 wt% (starch/hull mass basis) were added. The material was conditioned to 32% moisture and pH 10.5 by adding the required amount of NaOH dissolved in the conditioning water. Furthermore, prior to extrusion, the sample was maintained under refrigeration (7-10 °C) for 12 h to achieve moisture balance and allowed to stand for 1 h to reach ambient temperature. Reactive extrusion was performed in a single-screw extruder (AX Plasticos, Diadema, Brazil) with D =1.6 cm, L/D=40, four heating zones, and a 0.8 mm die diameter. The temperature profile was 90/110/110/110°C and the screw speed was 100 rpm. These conditions were established in preliminary tests. The extruded material was cut into pieces of approximately 1 cm, dried at 60 °C for 3 h, and ground (particle size < 0.250 mm) in a Marconi MA090 mill (Marconi, Piracicaba, Brazil).

2.3 Characterization of the modified starch/hull mixture.

The phosphorus content, substitution degree, and Fourier Transform Infrared (FT-IR) spectra of the modified starch/hull mixture were determined. Aiming to remove the remaining free reagents, a portion of 50 g was washed with ethanol/water (1:1), followed by three washes with distilled water, until neutral pH. Furthermore, the material was dried 2/8

2.3.3 Thermogravimetric Analysis (TGA) The analysis was conducted using TGA – 4000 equipment (PerkinElmer, Cleveland, USA). The samples were scanned from 30 to 800 °C, with a heating rate of 10 °Cmin-1 under a nitrogen atmosphere (flow rate 20 mL min-1). The native mixture (NM) and the washed crosslinked mixture (WCM) were analyzed.

2.4 Composites production by injection molding The components of the formulations (starch, oats hulls, glycerol, PLA) were manually mixed and the mixtures dried at 60 °C for 3 h. The compounding was performed using a single-screw extruder model EL-25 (BGM, Taboão da Serra, Brazil) with a screw diameter of 25 mm and barrel length of 700 mm (L/D = 28). The temperature profile was set at 90/150/150/140 °C from feeder to die zone, the screw speed was set at 35 rpm, and a two-hole cylindrical die (2 mm) was used. The obtained materials were further pelletized and stored in sealed bags. Before injection molding, the pellets were dried (50 °C/1 h), to avoid problems with moisture during processing, performed in a lab-scale injection molding machine AX16 II (AX-Plasticos, Diadema, Brazil).

The processing parameters, defined in preliminary tests, were: temperature profile 150/175/175 °C; mold temperature 20 ºC; injection time 9.5 s; cooling time 30 s; and mold opening time 1 s. Specimens in a dog bone shape were obtained. The performance of two different formulations of composites was compared, both containing 20% oat hulls, 36% starch, 12% glycerol, and 32% PLA (w/w), one being formulated with the raw mixture of starch/hulls and the other containing the mixture modified as explained in 2.2.

Polímeros, 29(3), e2019040, 2019

Crosslinking starch/oat hull mixtures for use in composites with PLA The proportion of oat hull was selected in preliminary tests as the highest level that can be added in composites with PLA/starch with adequate dispersion and without failures in the injected specimens.

3. Results and Discussions

2.5 Composites characterization

The phosphorus content of the native mixture (NM) of starch/oat hulls was 0.34 ± 0.05% (w/w), which essentially came from the hulls. The crosslinked mixture (CM) presented 1.57 ± 0.12% phosphorus and the increase in this parameter represents the level of phosphorus linked to the starch and/or cellulose chains, since the sample was washed to eliminate the non-reacted STMP. The degree of substitution (DS) represents the average number of substituent groups attached per monomeric unit.

Thus, in the case of starch and cellulose, this represents the average of substituent groups per anhydroglucose unit.

The DS calculated for the crosslinked mixture (CM) was 6.1 × 10-2. Although this is a low value, studies have proven that when modifications of similar DS were promoted in starch, a large impact on its properties was observed[38,39].

At the same time, Wang et al.,17 using a similar process to crosslink a mixture of starch/purified sisal fiber, reported similar DS (6 × 10-2, transforming their data to percent of substituents per monomeric unit). Observing that the DS increased steadily with increasing fiber content from 2.5 to 25%, the authors suggested the fiber could have participated in the crosslinking reaction together with the starch. The results of our work can be considered good, since in the purified fiber the cellulose is more exposed, while in the oat hulls it is covered by other components such as hemicellulose and lignin.

2.5.1 Scanning Electron Microscopy (SEM) The microstructure of the composites was recorded using a FEI Quanta 200 scanning electron microscope (FEI Company, Tokyo, Japan). The specimens were dried in a desiccator containing CaCl2 for ten days, fractured in liquid nitrogen, and coated with gold using a BAL-TEC SCD 050 (Leica Microsystems, Germany) sputter coater. Images of the surface and fracture were obtained.

2.5.2 Longitudinal shrinkage The linear shrinkage measured along the flow direction was calculated as the difference between the dimension of the mold cavity and the molded specimen using Equation 2, according to the method ASTM D955-00[36].

Lm − Ls  = Shrinkage ( % )   × 100  Lm 

Eq. 2

where: Lm = mold length and Ls = specimen length.

The analysis was performed 48 h after injection and ten replicates were evaluated.

2.5.3 Density The density was determined as the relation of weight and volume of samples previously kept in a desiccator with anhydrous CaCl2 (~ 0% RH) for 2 weeks. The volume was calculated by measuring the length, width, and thickness of the sample. Three replications were used.

2.5.4 Mechanical properties Tensile tests were realized using a universal testing machine model DL2000 (EMIC, São Jose dos Pinhais, Brazil) according to the ASTM D 638-03 method[37], with some modifications. Ten test specimens of each composite were previously conditioned in a desiccator at 53±2% RU (Mg(NO3)2 saturated solution) and 25 °C for ten days.

Each specimen was fitted in the tensile grips, which had an initial distance of 60 mm. The crosshead speed of the test was 50 mm min-1.

3.1 Characterization of crosslinked mixture of starch/hulls 3.1.1 Phosphorus content and Degree of Substitution (DS)

3.1.2 Fourier Transform Infrared Spectroscopy (FT-IR) The FT-IR analysis was performed aiming to detect chemical alterations in the structure of the blend components promoted by the modification process. The spectra of the native (NM) and modified mixtures, with (WCM) and without washing (CM), were similar (Figure 1).

The band around 3500-3200 cm-1 (with peak at 3288 cm-1) is due to stretching of the O-H groups. A band around 2900 cm-1 (peak at 2926 cm-1), attributed to C-H stretching of aliphatic groups, is also observed[40]. Related to the band at 3288 cm-1, the decreased intensity observed in the spectra

2.5.5 Thermogravimetric Analysis (TGA) The thermal stability was determined as explained in 2.3.3, comparing composites containing the native starch/hull mixture (NMC) with those containing crosslinked mixture (CMC).

2.6 Statistical analyses Results are expressed as means ± standard deviation.

Differences among samples were tested by analysis of variance (ANOVA) followed by the Tukey’s test (p <0.05).

Analyses were performed using the software STATISTICA 7.0 (Statsoft Inc., Oklahoma, USA).

Polímeros, 29(3), e2019040, 2019

Figure 1. FT-IR spectra of native (NM) and crosslinked mixtures, without (CM) and with washing (WCM).

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Peixoto, T. S., Yamashita, F., Bilck, A. P., Carvalho, G. M., & Grossmann, M. V. E. of CM and WCM compared with NM could indicate a decrease in the number of hydrogen bonds. The same effect was observed by Shalviri et al.[41] in starch/xanthan gum mixtures crosslinked by STMP. In the region of 1640 cm (1639 cm ) another band is observed, which is firstly attributed to the presence of water in the samples. In addition, in this region comprising bands from 1640 – 1424 cm-1, other nearby bands may be superimposed, such as those originated from carboxyl-conjugated carbonyl stretches from fiber components.[17] -1

-1

The absorption bands at 1149 cm-1 and 1075 cm-1 are attributable to C-O-C and C-OH stretching, respectively[40].

However, specific bands involving phosphate groups (as 1298 and 997 cm-1 ascribed to P=O and P-O, respectively) were not detected in the WCM and CM samples, even when, in the latter case, all the added phosphorus was present.

This is probably due to the low level of phosphorus added, as already reported by Sechi and Marques[34]. Another possible reason is that these bands may be being covered up by others, nearby, with an overlap occurring. Therefore, the only evidence of crosslinking via phosphate groups in the FTIR spectra would be a decrease in the intensity of the band corresponding to the hydrogen bonds in the CM and WCM samples.

3.1.3 Thermogravimetric Analysis (TGA) When comparing thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of the untreated mixture (NM) with that of the crosslinked mixture (CM), conformational similarities were observed (Figure 2).

In the two samples, the loss mass was initiated at approximately 100 °C, being due to the loss of water and volatiles[13]. A more expressive loss mass was also observed, showing peaks of maximum degradation rates in the DTG curves of NM and CM at 333 and 340 °C, respectively.

These temperatures, which are included in the range from 300 to 400 °C, correspond to the degradation of starch, cellulose, hemicellulose, and lignin[19]. The fact that the peak has moved to a higher temperature in the case of CM may be attributed to the formation of crosslinks, giving higher thermal stability to the material. Yoon et al.[28] and Niu et al.[32]

observed similar behavior promoted by crosslinking in a starch/PVA blend and cellulose/PVA composites, respectively.

3.2 Composites characterization 3.2.1 Morphology- Scanning Electron Microscopy (SEM) Some differences can be observed when comparing the micrographs of composites elaborated with the native mixture of starch/hulls, coded NMC, with those elaborated with the crosslinked mixture (CMC) (Figure 3).

The surface of NMC (left side) is rough, making it appear that the components were only compacted. The fibers are poorly inserted in the polymeric matrix and there seems to be little interaction between them. On the other hand, CMC presented a smoother surface (left side), with the fiber better wrapped by polymers.

In the fracture images (Figure 3, right side), gaps and fissures were observed in the two composites, but with lower dimensions in the case of CMC. This fact, associated with the coating of oat hulls observed in CMC indicate the better adhesion between fiber and matrix in the sample containing the modified mixture. It is possible that crosslinking via covalent or non-covalent bonds may have occurred, resulting in better interaction. Other authors reported the enhancement of fiber/matrix adhesion when the fiber was modified by different treatments[19-23].

Wang et al.[23] observed poor adhesion between sisal fiber and starch in composites in which STMP was used as the crosslinking agent. The authors presented only the micrographs of the reticulated samples and reported the presence of holes and smooth grooves, indicating the modification was not sufficient to impart the desired compatibilization.

Another point to highlight is the presence of remnant non-gelatinized starch granules, in both samples, indicating that the operational conditions were not sufficient to promote the complete disruption of granular structure. Even though the CMC underwent additional extrusion processing pre-treatment, to crosslink the starch/hull mixture, several starch granules retained their shape. This result is closely related to the higher thermal stability of CM (item 3.1.3). Kaewtatip and Thongme[26] also observed remnant starch granules in fractured surfaces of crosslinked starch/thermoplastic

Figure 2. Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of the untreated (NM) and crosslinked (CM) oat hull/starch mixtures.

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Polímeros, 29(3), e2019040, 2019

Crosslinking starch/oat hull mixtures for use in composites with PLA

Figure 3. SEM micrographs of composites containing the native (NMC) and the crosslinked (CMC) starch/hull mixtures.

starch (TPS) materials, attributing this result to the higher resistance conferred by crosslinking.

3.2.2 Longitudinal shrinkage Shrinkage is an undesirable phenomenon in the injection molding process, as it causes dimensional changes and, consequently, low quality specimens[42]. There was no significant difference between the linear shrinkage of composites containing raw or modified mixtures of starch/hulls.

The median value was 0.39%, which is considered low, indicating good dimensional stability.

While crystalline and semi-crystalline materials normally present high shrinkage, ranging from 1.0 to 4.0% in traditional polymers, such as LDPE and HDPE, less shrinkage is observed in amorphous materials[43]. Müller et al.[18] reported that TPS injected specimens present high shrinkage that increases with storage time. The addition of wood fibers up to 15 wt% promoted a fast decrease in shrinkage, which was almost negligible. This behavior of fibers as fillers in composites is a general rule, in different polymer matrices[43]. In the present study, both fibers and PLA must have helped to preserve the original dimension of the molded pieces. Although only the primary shrinkage was evaluated (the measurement was performed 48 h after processing), it not being possible to predict the behavior during storage, based on the literature data[18,43] it is possible to wait for the protective action of oat hulls, ensuring dimensional stability over time.

3.2.3 Density There was a significant difference between the densities of the samples (Table 1), with those of NMC being lower.

This can be justified by the microstructure of the sample Polímeros, 29(3), e2019040, 2019

Table1. Density and mechanical properties of the composites.

Sample Density NMC 1.29 ± 0.015b CMC 1.33 ± 0.016a

σ (MPa) ε (%) YM (MPa) 4.65 ± 0.18a 5.75± 0.40b 144.75± 4.91a 4.31 ± 0.17b 6.16 ± 0.17a 133.55± 7.30b

σ: tensile strength, ε: elongation, YM: Young modulus. Results expressed as mean ± standard deviation. Means followed by same letters in a column do not differ significantly (Tukey’s test p≤0.05).

(Figure 3), in which low compaction and a higher incidence of free spaces were observed, contributing to the low density.

The higher compaction in the case of CMC, which contributed to its high density, may have been caused by several factors, such as: the physical effects of extrusion and milling which the mixture of starch/oat hulls was subjected to in the pre-processing; the reticulation promoted by STMP during pre-processing; the better compatibility between PLA and the other components promoted by these physical and chemical modifications.

Ayoub and Rizvi[44] explained that the occurrence of crosslinking in starch, during extrusion, increases the viscosity of melted material, resulting in denser extrudates.

3.2.4 Mechanical properties The composite containing the crosslinked mixture of starch/oat hulls (CMC) presented slightly lower tensile strength and Young’s modulus and higher elongation than those of NMC (Table 1). Although the crosslinking process was used to increase the strength and stiffness of the composites, the opposite result obtained was not totally unexpected. Depending on the composition of the polymer matrix, the degree of crosslinking, and the crosslinking agent 5/8

Peixoto, T. S., Yamashita, F., Bilck, A. P., Carvalho, G. M., & Grossmann, M. V. E.

Figure 4. TG and DTG curves of composites.

employed, contrary results can be observed. Das et al.[25], using borax, formaldehyde, and epichlorohydrin as cross-linkers in starch/poly(vinyl alcohol) films, observed that only borax promoted increases in strength and modulus, while epichlorohydrin increased the strain and formaldehyde did not affect the mechanical properties.

Thus, it is possible that the level of crosslinking achieved by the reaction with STMP (item 3.1.1) was not sufficient to promote the expected effect on the mechanical properties of the composite, despite the improvements promoted in its morphology (Figure 3). In addition to the degree of substitution being low, it is necessary to remember that not all the phosphate groups attached to the chains of starch or cellulose form crosslinks.

Wang et al.[17] did not obtain improvements in mechanical properties from the reticulation of TPS/sisal fiber composites, although they used the purified fiber, which could have facilitated its interaction with the starch.

The authors justified the results considering that the extent of crosslinking (similar to that obtained in our work) was low to structurally reinforce the polymeric matrix. On the other hand, Niu et al.[32] obtained a remarkable improvement in the strength of PVA/cellulose composites after crosslinking with succinic anhydride, applying a pan-milling process. In this case, the better fiber dispersion and chemical modification promoted could have compensated the possible effects of milling (reduction in aspect ratio of fibers and/or polymer chain scission) that harm the strengthening.

3.2.5 Thermogravimetric Analysis (TGA) The TG and DTG curves of the composites are shown in Figure 4. Both samples presented three degradation peaks. The first occurred at temperatures around 100 °C and is related to vaporization of water and other volatiles[13].

The other two peaks occurred at 336 and 371 °C for NMC and at 324 and 364 °C for CMC. As previously discussed (item 3.1.3), the degradation curves observed in the DTG figures represent the overlap of curves corresponding to the degradation of the fiber components (cellulose, hemicellulose, lignin) and starch.

Similar results were observed by Cardoso et al.[23] and Rosa et al.[19], in composites containing oat and coconut fibers, respectively. Although in the case of CMC the peaks of maximum degradation have occurred at slightly lower 6/8

temperatures, the higher residual weight indicated relatively high thermal stability.

TGA analysis of the mixture of starch/PLA, without oat hulls, was also performed (figure not shown), with two degradation peaks being observed at 339 and 398 °C. While the first peak is attributed to the starch degradation, the second corresponds to the degradation of PLA[45]. In the case of DTG curves of composites (Figure 4), the peak related to PLA degradation was shifted to lower temperatures, being overlapped with the curves of the other components.

The lower level of PLA in the composites, together with the lower degradation temperatures of the other components explains the decrease in PLA thermal stability. Other authors observed similar results in other fiber composites[19,45].

4. Conclusions Reactive extrusion using STMP as a crosslinking agent was an efficient process to modify starch/oat hull mixtures.

The higher thermal stability of the treated mixture was an indication of the occurrence of reticulation. When the crosslinked mixture was processed together with PLA, denser composites with a smooth surface were obtained, due to the better interfacial adhesion between fibers and matrix. The resistance of the composites containing the modified mixture were slightly lower than that of the control sample. It is possible that the additional processing applied to promote the crosslinking, consisting of extrusion and grinding, ruptured structures in the fiber and/or starch, which weakened the performance of the final material.

5. Acknowledgements The authors would like to thank the ESPEC and LMEM Laboratories from the Universidade Estadual de Londrina (UEL) for the SEM and TGA analyses, and CNPq and Fundação Araucária for the financial support.

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Polímeros, 29(3), e2019040, 2019

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.07218

Influence of ZnO on the properties of elastomeric compositions and their leached extract Daiane Torani1, Janaina da Silva Crespo1 and Rosmary Nichele Brandalise1*  1

Universidade de Caxias do Sul – UCS, Caxias do Sul, RS, Brasil *rnbranda@ucs.br

Abstract The reductionin zinc oxide (ZnO) content in elastomeric compositions has become a subject due to the deleterious effect of zinc ions on aquatic organisms. The purpose of this study was to develop elastomeric compositions containing 0, 1, 3 and 5 phr of ZnO aiming at assessing the influence of the different contents on the rheometric, mechanical and thermal properties. The release of the zinc (Zn) content by leaching, at each step of the production and after the ageing was also assessed. All ZnO-containing compounds had similar rheometric, thermal and mechanical properties. Also, during and after exposure to accelerated ageing the released contents were similar for all compositions regarding the residual ZnO percent. In conclusion, the utilization of the 3 phr ZnO content is viable for the replacement of the usual amount employed.

Keywords: leaching, properties, vulcanization activation, zinc oxide.

How to cite: Torani, D., Crespo, J. S., & Brandalise, R. N. (2019). Influence of ZnO on the properties of elastomeric compositions and their leached extract. Polímeros: Ciência e Tecnologia, 29(3), e2019041. https://doi.org/10.1590/01041428.07218

1. Introduction The reduction in ZnO content in elastomeric compositions has become a subject of technological and industrial interest due to the deleterious effect of zinc ions on aquatic organisms[1]. Zn release to the environment can occur during production, elimination and recycling of elastomeric products, however it is relevant to keep low ZnO levels in elastomeric formulations, not only for environmental, but also for economic reasons[2]. Zinc is a metal easily encountered in the environment, however, amounts which exceed the safe limit that an organism can absorb and eliminate may cause a lack of equilibrium in the environment. According to the 2004/73/CE European Norm, Zn is classed as deleterious and toxic for aquatic organisms with lasting effects[3].

In this sense, Heideman[4] studied the reduction of ZnO levels during sulfur vulcanization of elastomeric compositions based on poly(ethylene propylene diene) (EPDM) and poly(butadiene styrene) (SBR). The author tried to develop novel activators which would contain only traces of Zn. The results of the studied Zn complexes were similar to those of the current vulcanization system, without harm to the vulcanization process and further increasing the density of the elastomeric composition with half the amount of Zn. In another utilization of zinc stearate as a substitute for ZnO and stearic acid, it was concluded that crosslinking was not effective, compromising vulcanization[5].

The Zn release and its toxicity on algae and frogs’ embryos was studied in NR compositions[6]. The toxicity test evaluated EC50 which means growth inhibition of 50% of the tested organisms and LC50, the mortality of 50% of

Polímeros, 29(3), e2019041, 2019

the organisms present in solution. By evaluating Zn release to the eluate with pH varying between 3 and 7, a reduction in release with pH increase could be observed. The authors concluded that Zn release to the environment can occur by means of natural phenomena such as rain and wind, and this can cause toxicity in aquatic species.

Moresco et al.[7] studied the utilization of an organic Zn composition as compared with the usual ZnO in NR-based elastomeric compositions. The composition of organic Zn exhibited lower Zn content as compared with ZnO. Even at the lower Zn content of the composition, the authors concluded that organic Zn is more reactive, enabling it to be utilized at 3 phr organic Zn, the results obtained being like those obtained when 5 phr of ZnO are used.

In the studies by Pysklo et al.[1] the influence of particle size on the values of Zn in the leached extract was evaluated.

Authors made use of cars’ and trucks’ tire samples, the particle size distribution varying between 0.5 and 5 mm, as well as the amount of ZnO in phr contents. Samples were assessed after the two extraction forms: with deionized water and with CO2-saturated water. The ZnO particle size distribution did not influence the Zn release into the leached extract, rather, this resulted from the acidic medium where leaching was performed. When extraction was performed with particles larger than 1 mm, Zn levels in the eluate were reduced of approximately 20% relative to the samples prepared from lower particle size distribution. The authors concluded that not only particle size distribution and surface area, but also the particle shape influences Zn release during leaching.

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O O O O O O O O O O O O O O O O

Torani, D., Crespo, J. S., & Brandalise, R. N. The amount of leached Zn is reduced with the increase in sample particle dimension.

2.4 Preparation of test specimens Test specimens were prepared by compression molding in accordance with ASTM D3182-89, in a hydraulic Copé press model Lab Press 215 (Brazil), at the temperature of 50 °C, pressure of 3.5 MPa, the vulcanization time being the optimum vulcanization time (t90) as obtained from the rheometric test.

The objetive of this study was to assess the influence of contents of 0, 1, 3 and 5 phr of ZnO for natural rubber‑based (NR) compositions on the rheometric, mechanical and thermal properties and on the leached extract of the elastomeric compositions before vulcanization and after vulcanization. The leached extract was also evaluated after thermal-oxidation of the samples.

2.5 Characterization of the elastomeric compositions Thermal properties were evaluated by thermogravimetry (TGA) in a TGA equipment by TA Instruments (USA) model Q5000, at a heating rate of 20 ºC⋅min-1, in the range of 25°C to 800 °C, under 25 mL⋅min-1 N2 flow as reported

by Carli et al.[8].

2. Materials and Methods 2.1 Materials For the development of this work the following raw materials were employed: GEB 10 NR (Borrachas Quirino/Brazil), stearic acid from animal fatty acid (Sabões Fontana/Brazil) 62.0% purity, ZnO (Votarantin Metais/Brazil) 99.5% purity, sulfur (Intercuf/Brazil), cyclohexyl-2-benzothiazol‑sulfenamide accelerator (CBS) (Lanxess/Brazil), toluene (F. Maia/Brazil), glacial acetic acid (Dinâmica) and micro-pearl NaOH (Neon).

The crosslinking density of the vulcanized compositions was obtained by adapting the experiment by Khalaf and coworkers[9] with toluene as solvent. Samples (20 x 20 x 0.2 mm) were submersed in toluene in the dark and at the temperature of 23 ± 2 ºC. The samples’ mass was registered before and after solvent immersion, with daily mass records, for 7 days, until stabilization. Samples were evaluated in triplicate.

Crosslinking density of the elastomeric compositions was determined by equilibrium swelling in toluene on the basis of the Flory-Rehner Equation 1[10].

Table 1 lists the elastomeric formulations of the present study in accordance with ASTM D312-89, the ZnO content being varied (0, 1, 3 and 5 phr) while the other raw materials contents are fixed.

2.2 Development of the elastomeric compositions

[X ] =

The mixing operation of the raw materials was performed in a Banbury Copé (Brazil) closed mixer with the NR remaining in the mixer for 60 seconds and then vulcanization activators being added. After 24 hours, the mixture was transferred to an open mixer Copé Model Lab Mill 350 (Brazil) with friction ratio between cylinders of 1:1.25. At this step, the mixture was added of accelerator and sulfur. The process of incorporation of the acceleration system was performed at a temperature of 70 °C, for 3 minutes, in accordance with ASTM D312-89.

− ln ( 1 − υr ) + υr + χυr2   

 13  Vο υr − υr / 2   

(1)

Where ʋr is the volumetric fraction of the swollen rubber (cm3), χ is the elastomer-solvent interaction and Vo is the solvent molar volume (cm3·mol-1).

For evaluating mechanical properties, tensile strength, modulus at 300% and elongation at break were determined in a universal testing machine EMIC model DL-300 (Brazil) in accordance with ASTM D412-06. Dumbbell shaped test specimens type C, charge cell 5 N at clamp separation speed of 500 ± 50 mm⋅mim-1 was used. Data refer to the average of 5 test specimens with the respective standard deviation. Tear strength of the formulations was determined in accordance with ASTM D624-00, the test being run under the same parameters as the tensile strength test, and in the same equipment. Test specimens were type C and the results refer to the average of 5 test specimens with the respective standard deviation. Hardness of the vulcanized compositions was evaluated in a Shore A durometer by Bareiss (Germany), in accordance with ASTM 2240-05.

2.3 Determination of the vulcanization parameters Vulcanization parameters for the formulations were evaluated by means of an oscillating disk rheometer from Alpha Technologies (USA) model MDR 2000, at the temperature of 150°C, deformation amplitude of 1 and frequency of 1 Hz, in accordance with ASTM D5289-12.

The Mooney viscosity of the formulations was obtained with the aid of the Alpha Technologies (USA) Mooney MV 2000 instrument, at the temperature of 100 °C, for 4 minutes, in accordance with ASTM D1646-07.

Table 1. Composition and coding for the elastomeric formulations.

Codification FC /AE /0 F1b/AE/1 F1/AE/3 F1/AE/5 a

c

g

NRd 100 100 100 100

ZnOe 0 1 3 5

Raw material (phr content) sulfur 2 2 2 2

stearic acid 2 2 2 2

CBSf 0.6 0.6 0.6 0.6

FC- control formulation; bF1-formulation; cAE-estearic acid; dNR-natural rubber; eZnO-zinc oxide; fCBS-accelerator; gThe number indicates the ZnO content in the composition. a

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Influence of ZnO on the properties of elastomeric compositions and their leached extract Accelerated ageing of the vulcanized compositions was run in a Marconi Model MA 035 (Brazil) air circulating oven, in accordance with ASTM D573-04. Test specimens were placed in an oven at the temperature of 70 °C for 14 days.

Elastomeric compositions were submitted to a leaching test in an acidic medium in accordance with ASTM D6234‑13 before vulcanization, after vulcanization and after heat accelerated ageing for a 14-days period. Samples of ≤ 9 mm dimensions were used for assessment. For determining the Zn content in the leached extract, the acidic medium sample was submitted to a glass fiber filtration system and further analysis by atomic absorption spectrometry in a Perkin Elmer Analyst 200 (USA) spectrometer. Before the analysis by atomic absorption spectrometry Zn was digested in nitric acid (10% v/v) at a temperature of 90 °C and for a time of 3 to 4 hours. The atomic absorption reading is given in mg of Zn⋅L- 1.

The reaction between stearic acid and ZnO with zinc stearate formation[11] is represented in Figure 1. Considering the molar masses of each chemical substance in the reaction (g⋅mol-1), (Zn 81.4 g⋅mol-1, stearic acid 284.5 g⋅mol-1 and water 18 g⋅mol-1) and the respective stoichiometric coefficients, it is possible to calculate the amounts, by mass, required for the reaction to occur without excess reagents.

The stoichiometric ratio for the calculation of the reacting amounts of ZnO and stearic acid is shown in Equation 2.

Y=

X .MM Ac MM ZnO

3. Results and Discussions 3.1 Characterization of the rheological and rheometric properties Rheological properties and rheometric parameters of the elastomeric compositions are listed in Table 2. For the compositions with different ZnO phr contents the highest values of MH could be observed for the 3 and 5 ZnO phr contents relative to the 1 phr content. As for ML which is indicative of processability, the highest value was obtained for the FC1/AE/0 sample, without ZnO, which is coherent with the Mooney values for viscosity. The higher the Mooney viscosity, the higher will be the plasticization (mastication) times for further incorporation of additives, besides high elastomer processing cost[12]. The different ZnO contents neither exhibited differences in t90 nor in the process (ts1) safety time however it was expected that the absence of ZnO would promote high values of t90 for the FC1/AE/0 composition, however this was not observed. Because of this, the CRI of this composition presented the highest value.

Possibly, the vulcanization mechanism without the presence of ZnO implies higher cure rates. The other CRI values were similar and are in agreement with the observed t90 values.

Figure 2 illustrates data for crosslinking density and the difference between ML and MH (ΔM) for the studied elastomeric compositions.

Since ΔM is directly related to the crosslinking density[13] a correlation was observed between the two properties evaluated (Figure 2). It could also be seen that the compositions containing 5 phr (F1/AE/5) and 3 phr (F1/AE/3) of ZnO had similar crosslinking densities, the values being higher than those observed for the 1 phr of ZnO compound and for the compound free from ZnO. Compositions containing 5 phr (F1/AE/5) and 3 phr (F1/AE/3) of ZnO had similar crosslinking densities, the values being higher than those observed for the 1 phr of ZnO composition and for the composition free from ZnO. This was attributed to the fact that their formulation had lower or no content of ZnO, which hindered activation in the vulcanization process by not forming zinc stearate.

(2)

Y being the amount (in grams) of stearic acid required for reacting under stoichiometric ratio with ZnO in grams, without excess reagents, X the different amounts of ZnO added to the formulation, MMAC the stearic acid molar mass and MMZnO the ZnO molar mass.

The Variance Analysis Method (ANOVA) of an alpha factor of 0.05 and with a confidence degree of 95% was employed to validate the significance of the Zn content in mg of Zn·L-1 for the elastomeric compositions released in the leached extract. For the calculation of the variance analysis the Excel software year 2010 was used.

Figure 1. Reaction between ZnO and stearic acid forming zinc stearate and the molar masses of each chemical substance in the reaction.

Table 2. Rheological and rheometric properties of the elastomeric compositions developed for different ZnO contents in phr.

Codification FC1/AE/0 F1/AE/1 F1/AE/3 F1/AE/5

Mooney viscosity [ML 1+4 100 °C] 49.68 37.98 42.01 37.83

ML (dN·m)

MH (dN·m)

ts1 (min)

t90 (min)

CRI* (min-1)

1.32 1.26 1.21 1.09

4.04 6.99 7.84 7.86

6.06 5.31 6.03 5.18

7.18 9.21 11.06 10.18

89.3 25.6 19.9 20.0

*Cure rate index - CRI = 100/(t90-ts1).

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Torani, D., Crespo, J. S., & Brandalise, R. N. This feature was also observed by Heideman[4]. Zinc stearate forms complexes with accelerators this in turn promoting, during vulcanization, a crosslinking network, resulting in improved sulfur reaction kinetics[2]. Data obtained for crosslinking density confirm the values obtained for MH. Figure 3 depicts the reaction between CBS, sulfur and zinc stearate[11-14].

At first ZnO reacts with stearic acid forming zinc stearate.

The interaction between the accelerator and the activator in the presence of Zn enables the formation of the accelerator active complex. The so-formed complex reacts with sulfur by means of the S8 ring, forming a sulfurizing agent. Such sulfurizing agent reacts with the elastomer chains to form a crosslinking precursor. The precursor is a polysulfide linked to a moiety of the accelerator molecule. Polysulfide crosslinks are formed through this precursor[15].

3.2 Characterization of thermal properties The compositions were evaluated by TGA with only one event of mass loss being observed for NR 99.8%.

The different ZnO contents in the compositions did not

promote significant differences in TGA curves, exception made to the percentage of residues, the values of which were attributed to the different ZnO contents incorporated into the compositions. Table 3 lists the thermogravimetric data for the studied compositions.

3.3 Characterization of mechanical properties before and after thermo-oxidative ageing Figure 4 exhibits results for tear strength and hardness for the developed elastomerics. As for tear strength, it could be observed that the results for composition FC1/AE/0 were lower than those for the other studied compositions, and this could be related to its lower crosslinking densities since given the absence of ZnO in the composition, crosslinking formation is compromised.

The lower hardness value of the FC1/AE/0 composition can be explained by the absence of crosslinks formation.

Hardness is a measure of the resistance offered by the material to the penetration of a body of higher hardness[16].

Figure 5 exhibits results for tensile strength, elongation at break and modulus at 300% for the compositions.

For the F1/AE/1, F1/AE/3 and F1/AE/5 compositions, considering standard deviations, tear strength and hardness data were similar, no significant influence resulting from the ZnO content variation in the compositions.

Figure 2. Crosslinking density and vulcanization rate of the elastomeric compositions developed with different ZnO contents in phr.

Tensile strength and modulus at 300% of the F1/AE/1, F1/AE/3 and F1/AE/5 compositions exhibited, when standard deviation values are considered, similar results. They are however higher than those for the ZnO- free composition (FC1/AE/0). Tensile strength is directly related to crosslinking formation therefore, data corroborate the crosslinking results found for the compositions. The low result for the modulus at 300% for the FC1/AE/0 composition is related to the incomplete vulcanization resulting from the lack of ZnO in the formulation. The presence of ZnO in the formulation leads to crosslinking, which limits the compositions’ deformation capacity. Modulus at 300% and hardness are indicative of

Figure 3. Zinc stearate formation and vulcanization mechanisms for Step 1 compositions. Adapted from Joseph et al.[14] and Coran[11].

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Polímeros, 29(3), e2019041, 2019

Influence of ZnO on the properties of elastomeric compositions and their leached extract Table 3. Decomposition temperatures of the elastomeric compositions developed with different ZnO contents in phr.

Compositions

Tonset (°C)

Tendset (°C)

TMax (°C)

Mass loss (%)

Residues (%)

FC1/AE/0 F1/AE/1 F1/AE/3 F1/AE/5

358.8 359.2 360.2 361.8

411.9 416.0 416.7 417.5

385.7 386.2 386.5 388.2

99.8 98.8 97.2 95.4

0.2 1.2 2.8 4.6

Figure 4. Tear strength and hardness for the FC1/AE/0, F1/AE/1, F1/AE/3 and F1/AE/5 compositions.

Figure 5. Tensile strength, elongation at break and modulus at 300% for the FC1/AE/0, F1/AE/1, F1/AE/3 and F1/AE/5 compositions.

Figure 6. Tensile strength and elongation at break retention for the FC1/AE/0, F1/AE/1, F1/AE/3 and F1/AE/5 compositions after thermo-oxidative ageing during a period of 7 and 14 days exposure (the dotted line corresponds to 100%).

Polímeros, 29(3), e2019041, 2019

crosslinking formation which confirms crosslinking density data found for the studied compositions. Elongation at break for the ZnO-containing compositions was lower than that for the ZnO-free composition. Elongation at break is related to the elasticity or flexibility of the composition[17]. In this way, when ZnO is withdrawn from the formulation, crosslinking does not occur in an effective manner, damaging this property since crosslinking formation is diminished. Figure 6 exhibits data for retention of tensile strength for the compositions after 7 and 14 days of thermo-oxidative ageing.

As relates to the results of the tensile strength retention property, it could be observed that all the ZnO-containing compositions exhibited reduction in the property, however, the better performance for the FC1/AE/0 and F1/AE/3 compositions after 14 days ageing should be highlighted.

Without ZnO, at first the accelerator reacts with sulfur leading to a polysulfidic structure; this reaction yields an active sulfurous agent. Then the accelerator S-N link is broken, forming a pair of free radicals. One of these radicals reacts with one sulfur molecule yielding a new sulfur atom[11-14].

At the start of an ageing process of elastomeric compositions, additional crosslinks can be formed because of residual crosslink, rearrangement of the crosslinked sulfur with the residual accelerator molecules and oxidative coupling[18,19].

Such sulfur rearrangement in a shorter exposure period combined to the post-cure and oxidative crosslinking reactions could explain the increase observed in the tensile strength retention of the compositions.

By checking Figure 6 related to elongation at break retention it was possible to observe that all the compositions exhibited reduction in this property, with the exception of F1/AE/1. Degradation of the elastomeric composition caused by-thermo-oxidative ageing causes reduction in elongation at break[12-18]. Polysulfidic crosslinkings are more unstable to thermo-oxidative degradation (less binding energy), converting themselves and keeping the mono- and disulfidic links. Such links can impart higher stiffness to the elastomeric system[20].

The crosslinking density of the F1/AE/1 composition with 1 phr ZnO is lower than that of the other ZnO-containing compositions (F1/AE/3 and F1/AE/5), and as a consequence tolerates better the breaking of the polysulfidic links, its post-ageing deformation ability being more robust.

By examining the lower exposure time, 7 days, smaller losses in elongation at break were observed relative to 14 days losses. One of the possible explanations for the observed pattern is that ageing promotes weakening of the elastomeric matrix by scission of the elastomer main chain besides new crosslinks, which contribute to reduced elongation at break[12-21]. A further aspect to be considered is that the new crosslinks promote the stabilization of the polymeric chain leading to increased stiffness of the composition[22]. Upon higher ageing exposure, it is expected that sulfur rearrangement reduces the polysulfide and 5/8

Torani, D., Crespo, J. S., & Brandalise, R. N. increase the monosulfide crosslinks which usually reduce the compositions’ mechanical properties. Oxidation-induced crosslinking results in main chain scission and interruption of crosslinks formation[18,19].

3.4 Characterization of the ZnO content in accordance with the reaction stoichiometry and effectively used mass in phr Mass contents, considering ZnO and stearic acid purities utilized in the formulations related to the phr employed are presented in Table 4, 13.1 g being the stearic acid mass used in the formulations considering the 2 phr content as fixed.

By replacing x in Equation 2 by the amounts in grams equivalent to the 1, 3 and 5 phr ZnO, the required values in Y are calculated for reacting under the stoichiometric reaction conditions (Table 4).

Considering the stoichiometric coefficients and the molar masses of the stearic acid reaction with ZnO (Figure 1) it could be seen that for the 1 phr ZnO composition 10.7 g ZnO were added to the formulation while according to stoichiometry 74.7 g of stearic acid would be required to react with the whole added ZnO. For the 3phr ZnO composition 31.5 g of ZnO were added while 219.9 g of stearic acid would be needed. The 5 phr ZnO composition would require 360.1 g of stearic acid to react with the 51.5 g of ZnO. In this way, it could be seen that the amount of stearic acid added to formulations (13.1g) was not sufficient to react with all the amount of ZnO added to the evaluated compositions, stearic acid being the limiting reagent, and in view of this it is probable that there is an excess of ZnO in the compositions.

The assessment of the relationship between the stoichiometric amounts in mass and the amounts employed in the development of the elastomeric composition can enable the evaluation of the ZnO contents of the leached extract after each step under study.

Results of Zn content obtained by ASTM 6234-13 followed by data by atomic absorption spectrometry (mg of Zn⋅L-1)

considering purity and the ZnO release capacity are listed in Table 5.

A gradual increase in Zn release was observed for the leached extract in the evaluated steps. By relating the data for Zn released during leaching with the required amount of stearic acid to react with ZnO (Table 4), it was possible to understand that the amounts of Zn released in leaching are lower than expected. This can be explained in te light of the stability of the salt formed between stearic acid and ZnO. It being a stable salt, it is able to retain Zn in the elastomeric matrix, hindering release.

The highest values were found after thermal oxidative ageing and statistical differences were observed as can be seen in Table 5, this result being explained by chain scission as well as by chemical and physical changes occurring during the elastomer ageing process[23], influencing Zn release. The Zn percentage released in this study, for the several process steps, was lower than expected. As stated in Table 4, the ZnO amount in the elastomeric matrix is in excess, with the consequence of high probability to release such amount by leaching. However, the salt formed between ZnO and stearic acid is stable[5] so that release is hindered, besides the fact that ZnO has high thermal stability as stated

by TGA in a test performed at the temperature of 800 °C. Zn release in the leached extract of the present study exhibited lower values than those reported in the studies

by Gualtieri et al.[6] (44,7 mg⋅L-1) and like those found by Pysklo et al.[1] (0.04 up to 1.69 mg⋅L-1) however the particle size employed by the authors was smaller (0.5 and 5 mm) than those of the present study (9 mm). In this way, it is possible that the particle size influenced Zn release.

Lower dimension particles have smaller contact areas, releasing higher Zn amounts to the eluate. Zn release to the environment, through small particles generated by the effect of rolling of tires in roads is potentialized because of the huge number of vehicles, which can impart toxicity to the aquatic environment caused by the leaching effect occurring naturally.

Table 4. Stoichiometric ratio of the elastomeric compositions developed with different ZnO contents in phr.

Composition

Xa (g)

Yb (g)

F1/AE/1 F1/AE/3 F1/AE/5

10.7 31.5 51.5

74.7 219.9 360.1

Stearic acid amount employed in the formulation (g) 13.1 13.1 13.1

X – ZnO amount utilized in the formulations considering 99.5% purity; bY – amount calculated by Equation 2, required for reaction with the ZnO amount employed considering the stoichiometric ratio. a

Table 5. Zn content in mg of Zn⋅L-1 for the elastomeric compositions released in the leached extract in accordance with ASTM 6234-13 with statistical analysis of the samples studied after each step under study.

Composition F1/AE/1 F1/AE/3 F1/AE/5 Source of variation Before Cure After Cure After Aging

6/8

Before Cure (mg⋅L-1) 0.18 0.79 1.15 SQ 0.86 0.81 3.09

gl 1 1 1

ANOVA MQ 0.86 0.81 3.09

After cure (mg⋅L-1) 0.20 0.69 1.19 F 26.6123 12.9392 23.8828

P Value 0.035 0.069 0.039

After aging (mg⋅L-1) 0.45 1.44 2.16 F critical 18.5128 18.5128 18.5128

Significant Yes No Yes

Polímeros, 29(3), e2019041, 2019

Influence of ZnO on the properties of elastomeric compositions and their leached extract

4. Conclusions All the ZnO-containing compounds exhibited similar rheometric, thermal and mechanical properties, except the property of hardness which was 8% higher for the 3 and phr ZnO formulations as compared with the 1 phr ZnO composition. The 3 phr ZnO formulation exhibited the best performances relative to the crosslinking density and consequently to ΔM. Formulations with 0 and 3 phr ZnO after ageing by thermo-oxidation retained 85% of the tensile strength, on the other hand the compound with 1 phr ZnO maintained 100% of the elongation property, indicating lower stiffness in the system. By evaluating the stoichiometry of the stearic acid and ZnO reaction, it was possible to observe that the amount of stearic acid in the elastomer compounds was not sufficient to react with all the ZnO added to the formulations. Upon considering the error in the determination of the leaching assay as 8%, ZnO-containing formulations released similar contents in residual ZnO percent during and after accelerated ageing exposure. Under the point of view of the properties of interest, time of production process and formulation cost the 3 phr ZnO compound is highlighted as the best performing one of this study.

5. Acknowledgements The authors are thankful to Vipal Rubber S.A and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for financial sponsor.

6. References 1. Pysklo, L., Pawlowski, P., Parasiewicz, W., & Piaskiewicz, M. (2006). Influence of the zinc oxide level in rubber compositions on the amount of zinc leaching. KGK - Kautschuk Gummi Kunstsoffe, 59(6), 328-331. Retrieved in 2019, September 6, from https://www.kgk-rubberpoint.de/forschung/influence-ofthe-zinc-oxide-level-in-rubber-compositions-on-the-amountof-zinc-leaching/ 2. Heideman, G., Noordermeer, J. W. M., Datta, R. N., & van Baarle, B. (2006). Various ways to reduce zinc oxide levels in S-SBR rubber compositions. Macromolecular Symposia, 245246(1), 657-667. http://dx.doi.org/10.1002/masy.200651393.

3. Kadlcak, J., Kuritka, I., Konecny, P., & Cermak, R. (2011). The effect of ZnO modification on rubber compound properties.

In Proceedings of the 4th WSEAS international conference on Energy and development - Environment – Biomedicine (pp.

347-352). Wisconsin, USA: World Scientific and Engineering Academy and Society.

4. Heideman, G. (2004). Reduced zinc oxide levels in sulphur vulcanization of rubber compositions (Doctoral thesis).

University of Twente, Enschede, The Netherlands.

5. Helaly, F. M., El Sabbagh, S. H., El Kinawy, O. S., & El Sawy, S. M. (2011). Effect of synthesized zinc stearate on the properties of natural rubber vulcanizates in the absence and presence of some fillers. Materials & Design, 32(5), 2835-2843. http://dx.doi.org/10.1016/j.matdes.2010.12.038.

6. Gualtieri, M., Andrioletti, M., Vismara, C., Milani, M., & Camatini, M. (2005). Toxicity of tire debris leachates.

Environment International, 31(5), 723-730. http://dx.doi. org/10.1016/j.envint.2005.02.001. PMid:15910969.

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7. Moresco, S., Giovanela, M., Carli, L. N., & Crespo, J. S. (2016). Development of passenger tire treads: reduction in zinc content and utilization of a bio-based lubricant.

Journal of Cleaner Production, 117, 199-206. http://dx.doi. org/10.1016/j.jclepro.2016.01.013.

8. Carli, L. N., Bianchi, O., Mauler, R. S., & Crespo, J. S. (2011). Crosslinking kinetics of SBR composites containing vulcanized ground scraps as filler. Polymer Bulletin, 67(8), 1621-1631. http://dx.doi.org/10.1007/s00289-011-0521-0.

9. Khalaf, A. I., Yehia, A. A., Ismail, M. N., & El-Sabbagh, S. H. (2012). High performance oil resistant rubber. Open Journal of Organic Polymer Materials, 2(4), 88-93. http:// dx.doi.org/10.4236/ojopm.2012.24013.

10. Flory, P. J. (1953). Principles of polymer chemistry. New York: Cornel University.

11. Coran, A. Y. (2013). Vulcanization. In: Mark, J. E., Erman, B., & Roland, M. The science of rubber compounding (pp.

337-338). Boston: Elsevier. http://dx.doi.org/10.1016/B9780-12-394584-6.00007-8.

12. Bilgili, E., Arastoopour, H., & Bernstein, B. (2011).

Pulverization of rubber granulates using the solid state shear extrusion (SSSE) process: part I. Process concepts and characteristics. Powder Technology, 115(3), 265-276. http://dx.doi.org/10.1016/S0032-5910(00)00353-3.

13. Movahed, S. O., Ansarifar, A., & Mirzaie, F. (2015). Effect of various efficient vulcanization cure systems on the compression set of a nitrile rubber filled with different fillers.

Journal of Applied Polymer Science, 132(8), 1-10. http:// dx.doi.org/10.1002/app.41512.

14. Joseph, A. M., George, B., Madhusoodanan, K. N., & Rosamma, A. (2015). Current status of sulphur vulcanization and devulcanization chemistry: process of vulcanization.

Rubber Science, 28(1), 82-121. Retrieved in 2019, September 6, from htps://www.researchgate.net/publication/275519885 15. Heideman, G., Datta, R. N., Noordermeer, J. W. M., & van Baarle, B. (2004). Activators in accelerated sulfur vulcanization.

Rubber Chemistry and Technology, 77(3), 512-541. http:// dx.doi.org/10.5254/1.3547834.

16. White, J. L. (1995). Rubber processing: technology, materials, and principles. Cincinnati: Hanser/Gardner Publications.

17. Nabil, H., Ismail, H., & Azura, A. R. (2014). Optimisation of accelerators and vulcanising systems on thermal stability of natural rubber/recycled ethylene–propylene–dienemonomer blends. Materials & Design, 53, 651-661. http://dx.doi. org/10.1016/j.matdes.2013.06.078.

18. Hamed, G. R., & Zhao, J. (1999). Tensile behavior after oxidative aging of gum and black-filled vulcanizates of SBR and NR. Rubber Chemistry and Technology, 72(4), 721-730. http://dx.doi.org/10.5254/1.3538829.

19. South, J. T., Case, S. Q., & Reifsnider, K. L. (2003). Effects of thermal aging on the mechanical properties of natural rubber. Rubber Chemistry and Technology, 76(4), 785-802. http://dx.doi.org/10.5254/1.3547772.

20. Dijkhuis, K. A. J., Noordermeer, J. W. M., & Dierkes, W. K. (2009). The relationship between crosslink system, network structure and material properties of carbon black reinforced EPDM. European Polymer Journal, 45(11), 3302-3312. http://dx.doi.org/10.1016/j.eurpolymj.2009.06.029.

21. Oliani, W. L., Parra, D. F., & Lugão, A. B. (2010). UV stability of HMS-PP (high melt strength polypropylene) obtained by radiation process. Radiation Physics and 7/8

Torani, D., Crespo, J. S., & Brandalise, R. N. Chemistry, 79(3), 383-387. http://dx.doi.org/10.1016/j. radphyschem.2009.08.037.

22. Bussière, P. O., Gardette, J. L., Lacoste, J., & Baba, M. (2005).

Characterization of photodegradation of polybutadiene and polyisoprene: chronology of crosslinking and chain-scission.

Polymer Degradation & Stability, 88(2), 182-188. http:// dx.doi.org/10.1016/j.polymdegradstab.2004.02.013.

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23. Somers, A. E., Bastow, T. J., Burgar, M. I., Forsyth, M., & Hill, A. J. (2000). Quantifying rubber degradation using NMR. Polymer Degradation & Stability, 70(1), 31-37. http:// dx.doi.org/10.1016/S0141-3910(00)00076-8.

Received: Mar. 06, 2019 Revised: July 09, 2019 Accepted: Aug. 14, 2019

PolĂ­meros, 29(3), e2019041, 2019

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.01119

Synthesis and characterization of microalgae fatty acids or Aloe vera oil microcapsules Luiza Brescovici Badke1 , Bruno Campos da Silva1,2, Agne Roani de Carvalho-Jorge3, Dhyogo Mileo Taher4, Izabel Cristina Riegel-Vidotti2* and Cláudia Eliana Bruno Marino1 Grupo de Biomateriais e Eletroquímica, Departamento de Engenharia Mecânica, Universidade Federal do Paraná – UFPR, Curitiba, PR, Brasil 2 Grupo de Pesquisa em Macromoléculas e Interfaces, Departamento de Química, Universidade Federal do Paraná – UFPR, Curitiba, PR, Brasil 3 Instituto Senai de Inovação em Eletroquímica, Curitiba, PR, Brasil 4 Núcleo de Pesquisa e Desenvolvimento de Energia Autossustentável, Universidade Federal do Paraná – UFPR, Curitiba, PR, Brasil 1

*izabel.riegel@ufpr.br

Abstract It’s proposed a single methodology for the encapsulation of Aloe vera oil or microalgae fatty acids using the complex coacervation process between gelatin and gum arabic. Although a very recurrent method, it is not trivial to establish a single coacervation methodology to encapsulate different compounds. The optimal synthesis conditions, that resulted in the best yield and encapsulation efficiency, are 1:1 (m/m) wall-to-core ratio, a temperature of 40°C and agitation speed of 10,000 rpm. Optical microscopy analysis revealed that the microcapsules are spherical, have average diameters of 112 μm (A. vera) and 118 μm (microalgae) and do not form agglomerates. The microcapsules were characterized by the osmotic pressure at which they ruptured, allowing the calculation of their mechanical resistance, which resulted in 392 MPa (A. vera) and 425 MPa (microalgae). The presented optimized methodology to encapsulate both compounds aims to contribute to their efficient and rational use, especially in cosmeceutical applications.

Keywords: cosmeceutics, gelatin, gum arabic, morphology, osmotic pressure.

How to cite: Badke, L. B., Silva, B. C., Carvalho-Jorge, A. R., Taher, D. M., Riegel-Vidotti, I. C., & Marino, C. E. B. (2019).

Synthesis and characterization on microalgae fatty acids or Aloe vera oil microcapsules. Polímeros: Ciência e Tecnologia, 29(3), e2019042. https://doi.org/10.1590/0104-1428.01119

1. Introduction Microencapsulation is a promising technology that encapsulates active agents in a protective boundary, thus increasing the stability of the encapsulated material and extending its lifetime. Microencapsulation inhibits chemical reactions between the core material and external species, such as moisture, oxygen, and protects the encapsulated molecules against temperature variations, and other environmental stresses[1,2]. Microcapsules have several applications, for example, in smart coatings (self-healing coatings), and also in pharmaceuticals, food, textiles, and cosmetics[1,2].

Nowadays, there are several encapsulation methods. Among the techniques used for the microencapsulation of active components, complex coacervation, which is considered the oldest method, stands out as a fast and simple method.

The coacervation technique involves the interaction between hydrocolloids of opposite charges. Driven by the neutralization of the charges, the polyelectrolytes undergo phase separation. Thus, a two-phase system, comprising of a colloid-rich phase and a colloid-poor phase is formed.

The colloid-rich phase coalesces around the dispersed material (an oil, for example), thus forming the microcapsule[3,4].

Complex coacervation allows the production of microcapsules

Polímeros, 29(3), e2019042, 2019

using charged biomacromolecules with high efficiency, as well as the encapsulation of hydrophobic materials at mild temperatures[3,5-7].

The complex coacervation method has recurrently being used in food and cosmetics industries, to encapsulate a diversity of materials. Gelatin/gum arabic microcapsules, containing broccoli particles, were prepared by complex coacervation; aiming to preserve the encapsulated components, increase the chemical stability, and mask the aroma[8]. Peppermint oil and poppy seed oil were also encapsulated by complex coacervation of gelatin/gum arabic[7,9]. Nevertheless, not all materials are prone to be encapsulated, therefore the establishment of proper methods is required. Also, from the industrial perspective, the development of methods that can be applied for the encapsulation of different compounds is advantageous from the economical point of view.

In the cosmetic industry, cosmeceuticals are gaining much attention since they are an alternative to deliver active ingredient in cosmetics formulations[10]. In particular, some oils that have antioxidant, bactericidal, anti-inflammatory, and analgesic properties, have been the focus of various studies. For example, Aloe vera[11] is a plant of African

1/9

O O O O O O O O O O O O O O O O

Badke, L. B., Silva, B. C., Carvalho-Jorge, A. R., Taher, D. M., Riegel-Vidotti, I. C., & Marino, C. E. B. origin that was used for many years in traditional medicine.

It is known for its foliage, which contains a mucilage-like gel. A. vera oil is obtained from the gel, which contains carbohydrates (glucomannan and acemannan), mineral salts (calcium, sodium, zinc, magnesium, chrome and copper) and vitamins (A, B12, C, E, choline and folic acid). When applied topically to the skin, the oil has beneficial properties preventing scaring and also having anti-inflammatory, hydrating, bactericidal, antiviral, and antifungal effects[11-14].

Another interesting source of compounds of cosmeceutical interest are microalgae, which are photosynthetic organisms whose size can vary between 5 and 50 μm and are found in both fresh and salt water. Due to their high nutritional value, they have been prominent in the economic area besides their ecological benefit[15]. Among the various species of microalgae, Acutodesmus obliquus, also known as ‘Curitibana’ because of its origin in Curitiba (state of Paraná, southern Brazil), is known to adapt rapidly to different climatic conditions, for example, the absence of sunlight and average temperatures of 17 C. A. obliquus contains proteins, pigments (chlorophyll, carotenoids), carbohydrates (β 1,3 – glucan, alginates, carrageenans, cellulose), and lipids, whose content is particularly high[16]. Microalgae pigments have been used in the pharmaceutical and cosmetic industries because of their antioxidant properties in addition to their use as dyes in paper, food and textile industries[17]. The proteins present in microalgae have their most common application in food and nutritional supplementation because they have antioxidant, nutritive and immunostimulatory properties[15,17].

In addition, the lipids in their composition are rich in fatty acids of high commercial value. In this way, researches have been seeking different applications for microalgae and its derivatives, covering areas such as food, cosmetics, medicinal and agriculture[16,18-20].

To the best of our knowledge, there is only one study about the encapsulation of A. vera and none aiming the encapsulation of A. obliquus fatty acids using the complex coacervation method. Considering the promising application of both compounds and the simplicity and economic feasibility of the coacervation method, this study sought the best experimental conditions to encapsulate A. vera or A. obliquus fatty acids using gelatin and gum arabic as the constituent wall polymers. The studied variables were the wall-to-core ratio, temperature and agitation speed. The best experimental condition was selected according to the polymers yield and encapsulation efficiency. The capsules obtained at the best condition were evaluated with respect to their size, morphology and mechanical resistance. In addition, a detailed discussion is presented about the parameters and mechanisms involved in the coacervation process, thus this study contributes to the better performance of encapsulation processes from both the economic and efficiency points of view.

2. Materials and Methods 2.1 Materials Commercial gum arabic (GA), type-A gelatin, and glutaraldehyde were purchased from Sigma–Aldrich. GA and type-A gelatin were previously characterized as described elsewhere[5]. A. vera oil was purchased from Brazilian industry, and fatty acids from A. obliquus were donated by NPDEAS (Núcleo de Pesquisa e Desenvolvimento de Energia 2/9

Autossustentável – UFPR, Brazil). All other chemicals and reagents were of analytical grade.

2.2 Study of the complex coacervation experimental parameters The most relevant experimental parameters for the complex coacervation method are temperature, concentration, agitation speed and pH. Regarding pH, it was previously demonstrated that the optimal pH for the coacervation of the studied materials is 4.00, so this was the adopted value[2,5]. In this way, the studied variables were wall-to-core ration (m/m) (1:1 or 1:2), temperature (40°C or 60°C) and agitation speed (900 or 10,000 rpm). A. vera oil was employed as the model core material.

Microparticles were produced as described in the literature[5]. Briefly, GA (2.5 wt%) was dissolved in distilled water with magnetic stirring at room temperature (25°C) overnight with continuous stirring. Gelatin (2.5 wt%) was dissolved in distilled water at 40°C with continuous stirring.

The pH of each solution was measured to check that a pH of around 5.6 was achieved. The pH was adjusted with 0.1 mol L-1 sodium hydroxide solution when necessary. The GA and gelatin solutions were mixed at a selected mass ratio, and a certain amount of core material was added. Then, the resulting dispersion was stirred for 5 min at 40°C or 60°C. Subsequently, 80 mL of deionized water was added (at the same temperature), and the solution was stirred for 5 min.

The pH was adjusted to 4.00 by adding HCl (0.1 mol L-1), and the resulting suspension was slowly cooled to 10°C in an ice bath. Finally, the dispersions were transferred to a refrigerator and left for 4 hours for complete precipitation and storage before further use. The microcapsules were crosslinked using 1 mmol L-1 of glutaraldehyde for each gram of protein. The reaction was carried out at 25°C for 12 hours under magnetic stirring. The microcapsules were washed three times with distilled water and acetone to remove some residual core material. The drying process was performed under reduced pressure.

2.3 Encapsulation efficiency and yield percentage The encapsulation efficiency (EE%) was determined by the quantitative determination of the encapsulated material, based on the procedure described in the literature[21].

A known mass of the synthesized dried microcapsules was macerated for 15 minutes, washed with acetone, and dried at room temperature. The resulting mass indicates the mass of the coating material of the capsules. Thus, it is possible to calculate the amount of encapsulated core material extracted with acetone.  extracted oil  EE % =   x100 (1)  total oil 

To calculate the yield percentage (Y%), the dried microcapsules were weighed, and the mass ratio was calculated with respect to the initial total mass, as shown in Equation 2.

Y% =

mass of dry microcapsules x100 (2) total initial mass

2.4 Characterization of the microcapsules The morphology of the microcapsules was observed by optical microscopy (Alphaphot YS2, Nikon). The diameter and size distribution of microcapsules were obtained by laser Polímeros, 29(3), e2019042, 2019

Synthesis and characterization of microalgae fatty acids or Aloe vera oil microcapsules granulometry using a Microprocessor S3500 Bluewave on a wet sample.

The chemical characterization was carried out using a Fourier transform infrared (FTIR) Vertex 70 spectrometer with HTS-XT microplate extension (Bruker OPTIK GmbH).

Attenuated total reflectance and transmission modes with a diamond crystal were used in the observation range of 4000 to 400 cm-1 with a resolution of 4 cm-1.

The net surface electric charge and stability of the microcapsules, in water, were analyzed using a zeta-potential analyzer (Brookhaven Zeta PALS). The electrophoretic mobility was converted into zeta potential values based on the Smoluchowski model. The measurements were performed in triplicate at 25°C.

2.5 Mechanical resistance of the microcapsules The microcapsules dispersions were mixed with an equal volume amount of glucose solution, prepared at different concentrations, and left stirring for 1 hour. Optical microscopy (Alphaphot YS2, Nikon) was employed to determine the number of deformed capsules. In this calculation, at least 100 capsules were counted at each glucose concentration.

The minimum concentration necessary to induce the deformation of 50% of the microcapsules was defined as the critical glucose concentration and was used to determine the critical osmotic pressure based on the calibration curve available in the literature[22]. The modulus of elasticity of the polymer wall was calculated using Equation 3. π c = 4µ

δ R2

(3)

Here, πc is the critical osmotic pressure (Pa), µ is the modulus of elasticity (Pa), δ thickness of the microcapsule wall (m), and R the radius of the capsule (m). The mean wall thickness values were obtained from the optical micrographs and scanning electron microscopy (SEM) images. SEM images were taken of cross-sectioned dried microcapsules, and thickness measurements were made using ImageJ. The radius of the microcapsule was calculated from the average diameters obtained.

3. Results and Discussion 3.1 Determination of the best synthesis parameters The formation of polyelectrolyte complexes occurs upon the electrostatic interactions, which depend on the degree of ionization of the polymers that is determined according to the environmental pH[5]. The pH adjustment is a fundamental step that promotes the equilibrium of the charges of the polymers present in the medium leading to the interaction between the polymers allowing the formation of the coacervates. Also, the precise selection of the pH is important to achieve the maximum coacervation yield. To confirm that the ideal pH for the formation of the polysaccharide/protein complex found is the same as indicated in the literature[5] and to evaluate the behavior of the wall materials, the zeta potential of the solutions of starting polymers was determined as a function of pH, as shown in Figure 1.

As shown in Figure 1, the polyelectrolytic complexation between gum arabic and gelatin is favorable because gum arabic has negatively charged surface for the whole studied pH range. Thus, the deprotonation of the carboxylic groups in gum arabic yields an anionic polyelectrolyte that will interact Polímeros, 29(3), e2019042, 2019

with positively charged species. Gelatin is an amphoteric macromolecule; that is, it has basic and acidic functional groups and, thus, an isoelectric point. Based on the graph, gelatin has zero zeta potential at pH 5.8. This result evidences that the acidic and basic groups of gelatin are equivalently ionized at this pH, indicating charge neutralization, thus giving the isoelectric point. Because the encapsulation efficiency depends on the interaction of oppositely charged species, the optimal complexation condition is when both concentrations are concomitantly maximized[5,23]. As depicted in Figure 1, at pH 4.0, gelatin and gum arabic have the greatest difference between their surface charges, therefore indicating that the ideal interaction condition is at this pH. This result is in perfect agreement with previously reported results[5].

As already explained, coacervation is based on a complex mechanism since it involves variables such as wall-to-core mass ratio, temperature, agitation speed and pH adjustment.

At the selected pH 4.0, different synthesis variables were employed and the results in terms of the yield percentage (Y%) and encapsulation efficiency (EE%) are displayed in Table 1.

Based on Table 1, it was found that samples 2, 3, 4, 7 and 8 had yields above 80%. Analyzing the studied variables, it was observed that the best yield results were for the samples performed at 40°C, thus indicating that the temperature influences the phase separation (coacervation). Since high temperatures promote the increase of the diffusion coefficient of the macromolecules and their internal energy, the formed complexes can be destabilized, thus reducing the yield.

Conversely, low temperatures favor the hydrogen bonds, aiding in the formation of stable polyelectrolyte complexes[3].

Samples 1, 3, 5 and 7, both synthesized at an agitation speed of 900 rpm, exhibited a greater amount of free oil at the end of the coacervation process. Among them, samples 5 and 7 showed the highest amount of free residual oil, since for these samples the wall-to-core ratio was 1: 2.

These results corroborate with the values of the encapsulation efficiency, demonstrating that at 1:2 not enough material is available to form the capsule’s wall, thereby resulting in a higher concentration of free core material in the medium, thus reducing the encapsulation efficiency[24].

The agitation speed is a parameter which influences, in particular, the characteristic of the formed emulsion. Table 1 shows that samples 2, 4, 6 and 8, in which the stirring speed of 10,000 rpm was used, when compared to the samples obtained at a speed of 900 rpm, exhibits a difference in both

Figure 1. Zeta potential (mV) of the initial biopolymer’s dispersions, gelatin and gum arabic, both at 2.5 wt%.

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Badke, L. B., Silva, B. C., Carvalho-Jorge, A. R., Taher, D. M., Riegel-Vidotti, I. C., & Marino, C. E. B. encapsulation efficiency and yield. This is because lower agitation values (<2,000 rpm) make it difficult to break the droplets of the material to be encapsulated and to form a stable emulsion [25].

In this way, it was possible to verify that sample 4 displayed the better results, since it presented an encapsulation efficiency of 79.6% and a yield of 89.8%. Therefore, the condition selected as the most favorable to achieve the higher yielding and encapsulation efficiency was 1:1 (m/m) wall-to-core ratio, 40°C and 10,000 rpm.

3.2. Preparation of the microcapsules containing A. vera and microalgae fatty acids Considering an oil-in-water (o/w) emulsion, the high interfacial energy between water and oil is the driving force that promotes the formation of the coacervates around the oil droplets, thus reduction of the total energy of the system.

For this to occur, the coacervates must exhibit a hydrophobic

region, which will interact with the microcapsule nucleus (oil), and a hydrophilic region that will interact mainly with the solvent (water). When the core material is highly hydrophobic, it is necessary to use surfactants which will increase the affinity of the core with the shell, enhancing the encapsulation efficiency.

The A. vera oil contains several active components, among them the fatty acids (salicylic acid, gamma-linolenic acid, arachidonic acid and cholesterol). Equally, A. obliquus fatty acids are described as containing arachidonic acid, linoleic, oleic and alpha linoleic acids. Due to their chemical composition, these compounds exhibit amphiphilic character[14,20]. Therefore, both compounds were successfully encapsulated by the gelatin-gum arabic complex without the need to use surfactants.

Figure 2 shows photomicrographs of the gelatin/gum arabic microcapsules filled with microalgae fatty acids and A. vera oil. As observed, microcapsules containing both

Table 1. Experimental parameters of the coacervation between gelatin (2.5 wt%) and gum arabic (2.5 wt%), using calendula oils as the core material.

Sample 1 2 3 4 5 6 7 8

Wall-to-core mass ratio 1:1 1:1 1:1 1:1 1:2 1:2 1:2 1:2

Temperature (°C) 60 60 40 40 60 60 40 40

Speed (rpm) 900 10,000 900 10,000 900 10,000 900 10,000

Y%

EE%

72.0 ± 1.0* 81.6 ± 0.7* 83.0 ± 2.0* 89.8 ± 0.5* 69.6 ± 0.7* 78.4 ± 1.3* 84.9 ± 1.2* 85.3 ± 1.0*

28.7 ± 1.6* 43.4 ± 1.2* 34.4 ± 1.0* 79.6 ± 0.6* 36.4 ± 0.7* 58.6 ± 1.0* 41.3 ± 0.9* 50.7 ± 1.3*

*Average values determined in triplicate.

Figure 2. Optical microscopy images of synthesized microcapsules: (a1) gelatin/gum arabic microcapsules containing fatty acids from microalgae 40X and (a2) 100X; (b1) gelatin/gum arabic microcapsules containing A. vera oil 40X and (b2) 100X. Scale bars = 100 µm.

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Polímeros, 29(3), e2019042, 2019

Synthesis and characterization of microalgae fatty acids or Aloe vera oil microcapsules compounds could be successfully synthesized by the complex coacervation technique. All microcapsules have a spherical morphology with well-defined shells and a multinuclear structure. Spherical particles are desirable because they exhibit higher fluidity and a lower surface/volume ratio, allowing the complete coverage of the core material, thus favoring material retention[26]. The formation of continuous shells is very important for the microcapsules, ensuring material retention and the protection of the encapsulated material.

During the observations, the microcapsules remained stable, and no rupturing was observed.

Figure 3 shows the SEM image of the cross section of a microcapsule containing A. vera oil. Based on this image and on the optical microscopy images and using ImageJ, the calculus of the average shell thickness is 3.5 μm.

The particle size (diameter) distributions for the capsules containing fatty acids and those containing A. vera oil are displayed in Figure 4.

Figure 3. SEM image of a ruptured microcapsule containing A. vera oil showing the wall thickness. Scale bar: 10 μm.

As shown in Figure 4, the average diameter of the microcapsules obtained were 118 μm for microalgae fatty acids and 112 μm for A. vera oil and both types of microcapsules had a broad particle size distribution, ranging from 37 to 275 μm. The size of the microparticles determines their application, since larger capsules contain larger amounts of encapsulated material.

Furthermore, the yields obtained for A vera and A. obliquus microcapsules were all higher than 80% (80.2 ± 0.7 and 88.6 ± 0.3 for A. obliquus and A. vera, respectively) and the efficiencies of encapsulation were 78.3% (A. vera) and 77.9% (A. obliquus), indicating that the optimized conditions successfully produced microcapsules.

3.3 Chemical composition of the microcapsules containing A. vera and microalgae fatty acids The FTIR spectra of gelatin (Gel), gum arabic (GA), microalgae fatty acids, A. vera oil, empty microcapsules (MC) and microcapsules filled with the materials are shown in Figure 5.

In the coacervation process, interactions between the positive and negative charges of the amino and carboxylic acid groups of the coating materials are expected. Based on the FTIR spectra, we verified that the gelatin/gum arabic wall of the capsule was formed because characteristics bands of gelatin and gum arabic are present in the spectrum of the empty microcapsules. In contrast, in the spectrum of the filled microcapsules, characteristic bands of the organic compounds present as the core materials are observed. Based on the spectra of the synthesized microcapsules, characteristic absorption bands are identified indicating contribution of both the wall and core materials. The broad band around 3290 cm- 1 refers to NH and OH groups of amide A and to the OH groups of the gum backbone. The bands at 2923 cm-1 and 2853 cm-1 are relative to the stretching of CH bonds.

The presence of carbonyl (C=O) groups of carboxylic acid

Figure 4. Particle size distribution obtained by laser granulometry: (a) microcapsules containing fatty acids from microalgae and (b) microcapsules containing A. vera oil.

Polímeros, 29(3), e2019042, 2019

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Badke, L. B., Silva, B. C., Carvalho-Jorge, A. R., Taher, D. M., Riegel-Vidotti, I. C., & Marino, C. E. B.

Figure 5. FTIR spectra of the core materials (Aloe vera oil and fatty acids), the shell polymers (Gel and GA), empty microcapsules (MC), and filled microcapsules with the respective material (M. Aloe vera oil and M. fatty acids).

Table 2. Materials characteristic bands used in the microcapsule’s formation[7,27].

Band Assignment (cm-1) Arabic gum 3290 O-H stretching 2923 C-H stretching 1650 COO- symmetric stretching 1018 C-O carbohydrates Gelatin 3290 N-H stretching 2923 C-H asymmetric stretching 1650 C=O stretching 1549 N-H bend coupled with C-N stretching Microalgae 2923 =C-H stretching fatty acids 2853 -CH2 asymmetric stretching and 1743 C=O stretching C=O stretching A. vera oil 1710 1160 C-O group stretching

Component Shell materials

Core materials

(gum) and of amide I (gelatin) are evident at 1743 cm-1 and 1650 cm- 1, respectively. The stretching vibration of the C-O bond of esters present in the core materials can be seen at 1160 cm-1 and the band at 1018 cm-1 is consistent with the vibration of C-O groups in the gum.

It can be assumed that the desired compounds were effectively encapsulated in the gelatin/gum arabic microcapsules because the spectra of the empty microcapsules, the pure materials for encapsulation, and the filled microcapsules show significant similarities. Furthermore, we determined that there were no interactions between the core materials because no changes in the characteristic bands were observed when they were encapsulated. Thus, the complex coacervation process can be considered efficient for the synthesis and preparation of these microcapsules. The main characteristic IR bands of the core and shell materials are listed in Table 2.

3.4 Zeta potential determination and stability considerations Most of the particulate materials acquire a surface electric charge when they are dispersed in solution, forming an electric layer on the surface of the particle. The first 6/9

charged layer is called the Stern layer and can be charged positively or negatively depending on the characteristics of the particle and the medium. The next charged layer, which is more external than the Stern layer, is the diffusion layer, where the counter ions are strongly contributing to the total surface net charge. Since the particles diffuse in the medium along with the ions that are strongly attached to their surface, a shear plane can be defined as the location where the electric potential reduces exponentially as a function of the distance from the Stern layer. The value of the electric potential in the shear plane is called Zeta Potential (ζ) and is an indicative of colloidal stabilization[28].

In the case of capsules produced by polyelectrolytic complexation, where positive and negative charges interact by attractive forces, thus forming the capsule shell, the excess charge of this interaction results in the capsule charge. Thus, the zeta potential is an indicator of the electrostatic interactions that occurred between the polyelectrolyte molecules. The zeta potential values of the microcapsules of A. vera and microalgae fatty acids are −6.93 ± 0.69 mV and −6.51 ± 0.45 mV, respectively.

Since the method formed particles with charges very close to zero, through neutralization of the constituent polymers, it is possible to infer that the coacervation process was very efficient and the mass ratio was properly selected. However, surface charges close to zero are very unfavorable from the stability point of view. Nevertheless, the microcapsules did not show aggregation and remained stable within the observation time of 15 days, then, the colloidal stability may be attributed to factors different from electrostatic repulsion.

Sterically stabilized particles are an important class of polymer particles. When two microcapsules approach each other, the macromolecules meet at the particle interface. As a result, the interpenetration of these macromolecules occurs, and the degree of organization of the system increases, decreasing the entropy. Thus, the thermodynamics of aggregation is unfavorable, and particle separation is preferred, resulting in easy resuspension and preventing irreversible flocculation.

Concerning practical applications, because the obtained microcapsules have a slightly negative zeta potential, they could be applied in neutral to anionic cosmetic formulations without electrostatic interactions that would negatively affect the functionality of the final formulation.

3.5 Analysis of the mechanical properties of the microcapsules The mechanical properties of the particles, such as rigidity, must be controlled for the processing of the final product.

Thus, it was determined the mechanical properties of the capsules, specifically, the elastic modulus, also known as Young’s modulus, which provides a measure of the stiffness of a solid material[29].

The micrographs of the capsules subjected to an osmotic pressure that caused their rupture are presented in Figure 6 and the values of the parameters related to the elastic modulus are shown in Table 3. After the minimum concentration of glucose necessary to promote the rupture of 50% of the microcapsules had been achieved, it was possible to calculate the elastic modulus of the polymer shell using Equation 3[30]. The obtained values of the elastic modulus Polímeros, 29(3), e2019042, 2019

Synthesis and characterization of microalgae fatty acids or Aloe vera oil microcapsules

Figure 6. Optical microscopy images of gelatin/gum arabic microcapsules containing fatty acids from microalgae ((a1) 40X and (a2–a3) 100X) and microcapsules containing A. vera oil ((b1) 40X and (b2–b3) 100X) in 60% glucose solution for 1 h. Scale bars =100 µm.

Table 3. Glucose concentration, osmotic pressure, capsule thickness and radius, and percentage of capsules ruptured.

Glucose concentration (%) Osmotic pressure (MPa) Shell thickness (µm) Radius (µm) Capsules rupture (%)

20 3.0 4.5 59 12

Microalgae fatty acids 40 6.1 4.5 59 28

at the critical glucose concentration for microcapsules containing A. vera oil and for those containing microalgae fatty acids are 392 MPa and 425 MPa, respectively.

The chemical crosslinking following the synthesis of the gelatin and gum arabic microparticles by complex coacervation changes the release characteristics of the modified particles[31], as well as affects the strength of the capsules. Many crosslinking agents have reactive groups for protein immobilization, which can produce stable bonds with specific residues. The most common reagent for the formation of microcapsules using polysaccharides and/or proteins is glutaraldehyde because aldehydes react rapidly and form strong bonds with the polymeric microcapsule walls[31]. The crosslinking degree is proportional to the crosslinking reagent concentration. At low concentrations, intermolecular crosslinking is dominant. In contrast, at high crosslinking concentrations, more transverse intermolecular bonds are formed, resulting in low protein solubility[32].

It is known that crosslinked particles prepared with a high concentration of glutaraldehyde are more resistant to the spray drying process[2].

Considering that, in a typical process of packing, the material may be subjected to a pressure ranging from 0.065 MPa to 160 MPa[33], the magnitude of the elastic modulus found for the microcapsules prepared in this work demonstrates that they can be submitted to real conditions of transportation, packaging and storage without disruption.

It is worth mentioning that the mechanical strength of the Polímeros, 29(3), e2019042, 2019

60 10.1 4.5 59 59

20 3.0 4.5 56 10

Aloe vera oil 40 6.1 4.5 56 37

60 10.1 4.5 56 53

microcapsules can be properly varied according to the desired application by modifying the nature and concentration of the crosslinking agent.

4. Conclusions Microencapsulation is a technology used to provide protection to the active agents so increasing their stability, performance and safe use. Nevertheless, not all agents are prone to be encapsulated. The complex coacervation process is a suitable method for the synthesis of microcapsules of gelatin / gum arabic containing microalgae fatty acids or A. vera oil. In this study, the best parameters for the complex coacervation method was determined, being obtained yields > 80% and encapsulation efficiency > 77%.

The microcapsules were found to be spherical and multinuclear.

The chemical analysis confirmed the formation of the polymeric microcapsule wall and the encapsulation of the core material, which did not evidence interactions between the wall and the core materials. The synthesized microcapsules are sterically stabilized and have a slightly negative surface charge. The mean particle diameter was 112 μm and 118 μm (A. vera oil and microalgae fatty acid microcapsules, respectively), suggesting that the capsules may encapsulate a sufficient amount of oil for different applications. Therefore, the proposed method of encapsulation of A. vera oil and fatty acids from the “Curitibana” microalgae by complex coacervation is efficient and feasible for industrial use.

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Badke, L. B., Silva, B. C., Carvalho-Jorge, A. R., Taher, D. M., Riegel-Vidotti, I. C., & Marino, C. E. B. Finally, osmotic pressure tests showed that the synthesized capsules presented good mechanical resistance allowing a wide variety of possible applications. Thus, the crosslinking conditions used were appropriate for the capsules studied.

5. Acknowledgements Authors are grateful to PIPE-UFPR (Graduate Program of Engineering and Materials Science), NPDEAS-UFPR, and SENAI-PR. L.B. Badke and B.C. da Silva thank the CNPq and CAPES (Code 001) for the scholarships, respectively. C.E. B. Marino and I.C. Riegel-Vidotti thank the CNPq for the productivity fellowships (grants 301989/2016-8 and 309800/2014-5).

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Synthesis and characterization of microalgae fatty acids or Aloe vera oil microcapsules 27. Cebi, N., Durak, M. Z., Toker, O. S., Sagdic, O., & Arici, M. (2016). An evaluation of Fourier transforms infrared spectroscopy method for the classification and discrimination of bovine, porcine and fish gelatins. Food Chemistry, 190, 1109-1115. http://dx.doi.org/10.1016/j.foodchem.2015.06.065.

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gelatin–gum arabic microparticles containing a fluorescent probe plus vetiver essential oil. Colloids and Surfaces. B, Biointerfaces, 67(2), 171-178. http://dx.doi.org/10.1016/j. colsurfb.2008.08.014. PMid:18835139.

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Received: Feb. 07, 2019 Revised: Aug. 18, 2019 Accepted: Aug. 26, 2019

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.05019

Influences of the mesh in the CAE simulation for plastic injection molding Felipe Marin1* , Adriano Fagali de Souza2, Rodolfo Gabriel Pabst2 and Carlos Henrique Ahrens2 Centro de Fabricación Avanzada Aeronáutica – CFAA, Departamento de Ingeniería Mecánica, Universidad del País Vasco/Euskal Herriko Unibertsitatea – UPV/EHU, Bilbao, Vizcay, Espanã 2 Grupo de Pesquisa de Manufatura Auxiliada por Computador – GPCAM, Núcleo de Inovação em Moldagem e Manufatura Aditiva – NIMMA, Programa de Pós-graduação em Engenharia Mecânica, Universidade Federal de Santa Catarina – UFSC, Florianópolis, SC, Brasil 1

*flpmarin@gmail.com

Abstract Although computer-aided engineering (CAE) software has been used for many years in the plastic industry, identifying the most appropriate mesh geometry and density remains a challenge. It can affect the accuracy of the simulation, the time and the costs. The evaluation of the most suitable mesh is not easy because the difficulties to obtain the real the values of the pressure and temperature inside the mold. The current work investigates this issue. A mold was manufactured and sensors were installed in its interior. CAE simulations using different mesh geometries and densities were evaluated against the experimental data. The results showed that the computational time was mostly influenced by the mesh geometry. The use of 2D mesh and lower density can lead to a faster and more precise simulation of pressure inside the mold and 3D mesh with lower density can provide a faster and precise simulation of the temperature.

Keywords: CAE simulation precision, pressure monitoring, plastic molding process, sensor.

How to cite: Marin, F., Souza, A. F., Pabst, R. G., & Ahrens, C. H. (2019). Influences of the mesh in the CAE simulation for plastic injection molding. Polímeros: Ciência e Tecnologia, 29(3), e2019043.

1. Introduction Plastics have been used for many years to manufacture technical products to supply the automotive, aeronautic, medical and electrometric industries. Injection molding is the main plastic transformation process and several parameters influence the product quality and the cycle time, such as temperature, pressure and cooling time. In many cases, the outcome of the injection molding process is difficult to predict and the definition of the most suitable parameters is obtained empirically. Nowadays, computer-aided engineering (CAE) software can be used to assist the plastics industry, to reduce the costs and the cycle time and improving the product quality[1,2] by assisting in two of the production phases: i) Simulating the plastic product: In CAE, defects in the plastic parts, such as, welding lines and shrinkage, can be simulated[3]. CAE can also be used to detect critical regions with heat accumulation, sink marks, residual stress built-up and product warpage[4,5]. In such cases, the accuracy of the CAE simulation can be easily verified by comparison, accessing the distortion of the manufactured plastic part using a CAD/CAI/ 3D scanner or microscopy for structural analysis and measuring the residual stress.

Padilla et al.[6] evaluated the warpage and shrinkage of a product and compared the alterations observed in the experimental injected part with the simulation results.

The simulation resulted in some differences in the regions close to edges of the product.

Polímeros, 29(3), e2019043, 2019

ii) Simulating the molding process: To evaluate the CAE simulation of the plastic injection molding process, an adequate data acquisition system has to be developed and sensors need to be installed inside the mold cavity to obtain the experimental behavior of the plastic during the molding phase. Thus, the accuracy of this type of simulation is not addressed in depth in the current literature

In the CAE software, a mesh is used to perform the calculations. The mesh geometry can vary and the mesh can be generated with different densities (number of elements).

Theoretically, the higher the mesh density the higher the accuracy of the simulation will be, because there will be more elements to describe the simulated part. However, a longer computational time will be required. Identifying the most appropriate mesh geometry and density to perform the CAE calculation still represents a challenge. The CAE software user guide provides only general suggestions regarding the geometry of the mesh to be used and no specific information on the density. According to the Moldflow Manual[7], a 3D mesh requires a longer computational time, could provide better accuracy and is preferable for parts with thickness variation or mass accumulation (chunky regions). No further details regarding the most suitable mesh and density are given.

Miranda and Nogueira[8] studied the influence of gas entrapment on the plastic injection process using CAE and

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O O O O O O O O O O O O O O O O

Marin, F., Souza, A. F., Pabst, R. G., & Ahrens, C. H. experimental analysis. Four thermocouples were used to measure the real mold wall temperatures with variations in the injection temperatures and pressure. For the best combination of parameters with an appropriate venting outlet the cycle time was 35% lower than without and there was a good fit between the experimental and simulation temperature data.

The authors evaluated the mesh refinement and found that the smaller the element size the more reliable the geometry of the simulation domain will be.

The results of the simulation can be influenced by the quality of the mesh (in general, by the homogeneity of the distribution of the mesh elements) and its density (distance between two adjacent nodes). Both of these factors can affect the computation time and the precision of the simulation[9]. According to Kovács and Sikló[10], a high number of nodes can cause a fluctuation in the results, probably due to truncation and rounding during the calculation. On the other hand, the use of a low-density mesh can lead to a deviation in the results since a low number of nodes does not represent adequately the geometry.

Although it cannot be considered a new technology, a scarcity of studies on the geometry and density of the mesh used to simulate the plastic injection molding process was verified during a review of the literature. Thus, the effects of these factors were investigated in this study. The aim of the research was to understand and quantify the influence of the mesh geometry and density in the CAE software used to simulate the plastic injection process. Critical variables inside the mold (pressure and temperature) were simulated and the results compared with experimental data.

2. Plastic Injection Molding Simulated Using CAE Software The simulation process can be divided into three steps: i) pre-processing, where the mesh is generated and boundary conditions are applied; ii) processing, where the CAE software solve several equations in the mesh domain; and iii) post-processing: where the user interacts with the results, performing an evaluation and implementing changes if necessary. The main aspects of CAE software, the mesh generation (pre-processing) and the solving of equations (processing), will be discussed ahead.

2.1 Mesh generation In the pre-processing stage, the CAE software (or other mesh generating software) generates a bidimensional or tridimensional mesh to describe the mold and/or the part

domains of the plastic molding process[3]. In the case of a bidimensional mesh, the quality of the mesh is related to the distribution of the elements along the surface and the homogeneity on its length. The quality of a tetrahedral element is commonly measured in terms of minimal and maximal dihedral angles, since small or large angles result in less accuracy of the solution. The numerical solution stability is mainly affected by the worst tetrahedral element[11].

Different approaches have been developed to generate a mesh as follows: a) The Delaunay approach attempts to distribute a set of vertices in the domain and additional vertices can be iteratively added as needed. In this method a 2D triangular mesh is usually applied, and it is not easily extended to the 3D complex domains[11,12]. b) Octree-based methods subdivide the domain enclosing the given mesh recursively until a certain stopping criterion is reached[11]. In the modified-Octree technique, a CAD model is divided into various sizes of solid cubes and converted into tetrahedral elements[13]. c) The advancing front technique (AFT) is based on the generation of a tetrahedron mesh from the front toward the interior, creating new elements until the entire solid geometry is filled with elements. In this approach, new nodes are inserted inside the domain to achieve the specified shapes and sizes for the mesh generation[11].

This is widely applied in 3D tetrahedral meshing and allows a better control of the elements generated[13].

Jin and Tanner[14] summarized the 3 main steps to run the AFT method, as follows: i) Discretize lines which form surfaces to generate the initial front. ii) Discretize triangular faces on the surface to form the initial front. iii) Discretize the triangular domain to create tetrahedral elements, only for elements on the advance front, checking the volume and upgrading mesh elements as necessary.

As described in Figure 1, to generate volumetric elements from a planar triangle it is necessary to create a new node (d) (offset from the face) using the existing nodes (a, b and c).

Jin and Tanner[14] suggest limiting the new node location to within a sphere, with the center at d and a radius of 1.25δ, where δ is the edge reference dimension of the element to

Figure 1. a) Representation of equilateral element and possible locations for a new node and b) tetrahedral element and normal vectors.

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Polímeros, 29(3), e2019043, 2019

Influences of the mesh in the CAE simulation for plastic injection molding be created (Figure 1a). New nodes should not be too close to existing nodes, line segments or faces, and only new nodes whose distance from the other nodes is not less than 0.55δ can be selected. To avoid inconsistency in the calculation Jin and Tanner[14] sugestes that nabd . nabc > cos 75 and[15] that nabd . nabc > cos 60ᵒ, thus tetrahedron abcd is reasonably well-shaped (Figure 1b).

Ito et al.[15] proposed a mathematical model to analyze the normal vector of the element and create the mesh as smooth as possible. To do so, the author used a Laplacian method with AFT to find the radius of all neighboring nodes to evaluate the triangles and correct them, in order to create a tetrahedral element which is as equilateral as possible. Parmar and Kaiser[16] evaluated the results for the temperature and the warpage obtained from simulations with an imported mesh compared with the use of 3D elements generated directly in commercial CAE software.

The meshing method showed minimal variations and no influence on the simulation results. This study highlights the robustness of the mathematical mesh methods currently used in CAE software.

2.2 Solver equations In this phase, using the mesh generated with proper boundary conditions, the software models the process and performs mathematical calculations. The main calculus on CAE software are the mold cooling (COOL), polymer injection (FILL) and packing (PACK), as well as dimensional calculations associated with shrinkage and warpage (WARP).

In some commercial software programs, mechanical analysis of the mold and inserts can also be conducted.

For more than a decade, computer software has been available to solve the relevant equations for a Newtonian fluid flowing in a cold cavity. CAE software uses numerical methods, such as the finite element method (FEM) or finite difference method (FDM), to solve structural, thermal, fluid-dynamics and rheological equations. The predictions of the pressure curves inside the mold cavity can be significantly improved by introducing the effect of viscosity on pressure and the mold cavity deformation[17]. To solve thermal, stress and fluid-dynamics issues, the CAE software uses the elements domain, generated in the pre-processing step.

To ensure accuracy, the boundary conditions applied must be close to experimental process conditions.

In the injection molding processing, the fluid is non-Newtonian with high viscosity (103 to 104 Pa.s) and the pressure is as high as 106 to 1010 Pa[18]. In the generalized Newtonian model, the process can be described by mass, momentum and energy conservation equations. Although these equations describe different phenomenon, they come from the general transport equation (Equation 1): ∂ ( ρϕ ) ∂  ∂ϕ  ∂  ∂ϕ  ∂  ∂ϕ  +  ρϕ u j − Γ +  ρϕ ui − Γ +  ρϕ uk − Γ  − Sϕ =0 ∂t ∂xi  ∂xi  ∂x j  ∂x j  ∂xk  ∂xk 

In Table 1, η and k are the viscosity and the thermal conductivity of the fluid respectively. The source terms for the momentum equations include the effects of pressure gradients and inertial forces. The source term of the energy equation is composed by the sum of the dissipated power due to viscous stress and other heat sources Se. The characteristic variable of the energy equation is the internal energy e of the fluid. However, it is common for this equation to be rewritten as a function of fluid temperature T .

To deduce this expression, a relationship between internal energy, enthalpy h and measurable properties of the fluid is used. Zhou et al.[19] propose the thermodynamic relation: e= h +

p

ρ

(2)

In energy equations the fluid is generally considered incompressible[19]. Considering this and the enthalpy as a linear function of temperature, we can obtain  ∂ 2T ∂ 2T ∂ 2T  ∂T ∂T ∂T ∂T  + ui +u j + uk −k 2 + 2 + 2  ∂t   ∂xi x x x ∂ ∂ ∂ ∂x j ∂xk i j k   

ρc p 

  −η ( γ )2 = 0 (3)  

where c p is the specific heat at constant pressure. During COOL there is a zero velocity field, so the energy equation can be simplified to: ρc p

 ∂ 2T ∂ 2T ∂ 2T ∂T −k 2 + 2 + 2  ∂t ∂x j ∂xk  ∂xi

 = 0 (4)  

γ is a scalar variable derived The effective shear rate = from the shear rate tensor γ through the equation: γ =

1 = =   γ :γ  (5) 2 

where (:) is the double dot product.

The relation between the shear rate γ and the shear stress τ is given by a rheological equation. According to Fernandes et al.[20], the viscosity η ( γ,T , p ) is a function of the shear rate, the temperature and the pressure as show the Equation 6. τ =η ( γ,T , p ) γ (6) Table 1. Selection of variables to deduce the generic Newtonian model from the generic transport equation.

Equation Mass conservation equation

ϕ 1

Sϕ

Γ 0

0

xi -Component momentum equation

ui

η

∂p − + ρ fi ∂xi

x j-Component momentum equation

uj

η

−

∂p +ρ fj ∂x j

xk-Component momentum equation

uk

η

−

∂p + ρ fk ∂xk

Energy conservation equation

e

k

η ( γ ) + Se

(1)

where ϕ and Γ are called characteristic variable and diffusivity, respectivally, and Sϕ is the source term[19]. The variables ui, u j and uk are the components of the velocity field in the three-dimensional Euclidian space with coordinates xi, x j and xk .

Polímeros, 29(3), e2019043, 2019

Zhou et al.[19] shown that by the appropriate choice of ϕ , Γ and Sϕ it is possible to obtain the conservative equations of the generalized Newtonian model. The alternatives to ϕ , Γ and Sϕ in the general transport equation are presented in the Table 1.

2

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Marin, F., Souza, A. F., Pabst, R. G., & Ahrens, C. H. The rheological behavior has still not been fully defined and the equations are theoretically explained by the principles of continuum mechanics. However, extensive empirical data needs to be collected for the material characterization.

In recent years, several models have been developed to describe the non-linear behavior of non-Newtonian liquids, such as polymers. The models used include the Cross-WLF Viscosity and Matrix viscosity model.

According to Li and Shen[21], the calculations can be simplified by considering as incompressible Newtonian flow and neglecting the surface tension during FILL. During PACK the compressibility of the melt shall be taken into account.

Therefore, a dependency model of the specific volume with temperature and pressure it’s necessary. The modified Tait model is considered more suitable, because it can predict de abrupt volumetric change in the liquid-solid transition for amorphous polymers[20]. The modified Tait model is given by Equation 7.   p  v ( p,T= ) v (0,T ) 1 − Cln  1 +   + vt ( p,T ) (7) B (T )    

where v ( 0,T ) is the specific volume at zero gauge pressure, C is a constant (equal to 0,0894) and B (T ) is the pressure sensitivity of the material. The transition temperature Tt divides the temperature domain into two regions. In the lower temperature region (T < Tt), vt ( p,T ) = 0. The functions v ( 0,T ), vt ( p,T ) and B (T ) are data-fitted[20].

According to Zhou et al.[19] simplifications can be made in the generalized Newtonian model to fit majority of injection cases, thin shell structures. Furthermore, due to the long molecular chain structure of polymers the viscous shear stress is much bigger than the inertial forces.

As a result, velocity in the thickness direction and inertial forces can be neglected and the pressure is a function of planar coordinates[20]. Under these assumptions, we have the Hele-Shaw model: ∂ρ ∂ ∂ ∂ + ρu j + 0 (8) ( ρ ui ) + ( ρ uk ) = ∂t ∂xi ∂x j ∂xk

(

)

∂p ∂  ∂ui  ∂p ∂  ∂u j  = = η , η  (9) ∂xi ∂xk  ∂xk  ∂x j ∂xk  ∂xk   ∂T ∂T ∂T  ∂ 2T 2 + ui +u j 0 (10)  − k 2 −η ( γ ) =  ∂t  x x ∂ ∂ x ∂ i j k  

To solve the boundary conditions problems described by the Hele-Shaw or the generalized Newtonian models, Moldflow’s software uses the FEM. In terms of the general transport equation, the FEM approximates the unknown function ϕ over the domain of a finite element to obtain a system of algebraic equations. The approximation by a weighted procedure: 

ϕˆ = ∑N a ϕ a (12) a

and creates a residuo R: R=

∂ ( ρϕˆ ) ∂  ∂ϕˆ  ∂ +  ρϕˆui − Γ + ∂t ∂xi  ∂xi  ∂x j

∂  ∂ϕˆ   ρϕˆuk − Γ  − Sϕ ∂xk  ∂xk 

 ∂ϕˆ   ρϕˆu j − Γ +  ∂ x j  

(13)

which should be zero at exact solution. The terms N a are known functions of local coordinates (basis functions) of the finite elements, while ϕa are unknown parameters. The best estimative of ϕa ensures that: Ω 0, = b 1, 2,…, n (14) ∫ Wb Rd =

Ω

where Wb is an arbitrary function and n is the number of nodes in the finite element domain Ω . In order to avoid high-order derivatives, and achieve a weak formulation of the problem, integration by parts can be used[22]. The general transport equation is in the strong formulation due to the second-order derivative in the diffusion therm.

The basis functions N a are equal to one in the vertices of the finite element and are subject to the following restriction: ∑N a = 1 (15) a

so, if there are convergence to the real solution, the FEM provides exact values of the characteristic variable ϕ at the nodes. In other points of the finite element domain Ω , ϕ̂ is a interpolation of the nodal values of ϕ [23].

Finite element methods differ from each other in terms of the function Wb. Moldflow uses a Petrov-Galerkin method, where Wb is the sum of a basis function Nb and an artificial diffusion term that ensures the convergence for partial differential equations that not admit a weak formulation[22].

ρc p 

where:

2

γ =

2

 ∂ui   ∂u j  (11)    +  ∂xk   ∂xk 

The boundary and initial conditions of the generalized Newtonian model and of the Hele-Shaw model include zero velocity on the normal and tangential directions of the mold cavity walls. Similarly, we have zero gradient pressure along the normal direction of the mold cavity walls. At the flow front and at the surface where melt enters the cavity (the gate) the pressure boundary conditions are p = 0 and p = pinj, respectivally. The injection pressure pinj and the gate speed are determined by the melt flow rate at the gate.

The temperature boundary conditions can be prescribed in each boundary[19].

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3. Materials and Methods To achieve the aims of this research, a mold was designed and manufactured. Temperature and pressure sensors were installed inside the mold cavity and a data acquisition system was developed to acquire the data (in real-time) during the injection molding process. Systematic CAE simulations were conducted in Moldflow Plastics Insight 2014 and analysis was carried out with the simulated results and the experimental data. The computation time was also recorded for all simulations.

The workpiece geometry designed for this study had a thickness of 2 mm, diameter of 140 mm and five equidistant cavities (Figure 2). The cold sprue dimensions were: length of 60 mm, entrance with a diameter of 6.5 mm and draft angle of 2°. The cavity has a volume of 50.7 g/cm3. The cooling Polímeros, 29(3), e2019043, 2019

Influences of the mesh in the CAE simulation for plastic injection molding system had a diameter of 8 mm and was at a distance of 19 mm from the workpiece (U shape). The design of the mold was also aided by CAE. Three CAE mesh geometries were evaluated: i) 2D midplane (MP); ii) 2D dual-domain (DD); and iii) 3D tetrahedral.

The influence of the density of the mesh, which is the result of the maximum segment length defined by the user in the CAE software, was evaluated using four different values.

The maximum segment length was evaluated with values from 1 mm to 6 mm using the same CAE tool tolerance of 0.1 mm for mesh generation. Table 2 shows the input variables and the number of elements generated in each mesh (density) according to the mesh geometry and maximum segment length.

On observing the extremes of the lengths evaluated for the 2D dual-domain and 3D meshes, it can be noted that the number of elements generated increases by around a factor of 7. Therefore, these two geometries are expected to have the greatest influence on the calculation time. The density for the 2D midplane geometry varied little as a function of the maximum segment length. Thus, using this mesh

geometry, it is possible to obtain a higher precision without a large increase in the number of elements and, probably, little change in the computational time. Figure 3 shows an example of the mesh density for the 2D midplane geometry according to the maximum segment length.

The satisfactory quality of all meshes generated was verified by a computational tool available in the CAE software called ‘Aspect Ratio’, which calculates the level of distortion of the segment length of the meshes, and no significant distortions were found. The advancing Table 2. Number of elements generated according to mesh geometry and maximum segment length.

Mesh geometry 2D midplane 2D dual-domain 3D

Maximum Segment Length 1 (mm) 2 (mm) 4 (mm) 6 (mm) Number of elements generated (103) 64 34 18 19 138 34 18 19 1,420 345 240 222

Figure 2. (a) CAD of the mold and (b) the injected workpiece.

Figure 3. Overview of the mesh density according to the maximum segment length.

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Marin, F., Souza, A. F., Pabst, R. G., & Ahrens, C. H. front technique (AFT) was used to generate the 3D mesh.

The simulation of the injection molding process was carried out in CAE software Moldflow Plastics Insight 2014, run on an up-to-date personal computer. In the simulation was selected the Cross-WLF model and the mold expansion was taken into account. A preliminary analysis to determine the COOL, FILL, PACK and WARP values was then conducted.

The mold transient temperature regime was also evaluated for the 3D mesh.

The experimental molding of the workpieces was conducted in a HAITIAN SA1200/410 machine. The switchover was setted for 98% of the volume filled in the simulations for the mold design, in the experiment short shoting technique was carried out and checked by the volume filled. The injection molding process parameters are shown in Table 3.

A piezoelectric sensor (Kistler, model 6190CA) was installed inside the mold cavity to acquire the pressure and temperature signals in real-time during the injection cycles (Figure 4a). This sensor has a T-type thermocouple to measure the temperature of the melted material (in contact).

The sensor was positioned 22 mm from the feed channel as shown in Figure 4b.

The sensor was calibrated for this application and the signals were amplified by a Kistler Amplifier 5039A221 and captured by an Agilent 34970 board.

The results for the CAE computation time and the temperature and pressure data were analyzed.

4. Results and Discussion 4.1 Computation time The time required to compute the cases investigated can be seen in Figure 5 . As expected, the computation time increases with the mesh density (more elements). In the case of the 2D midplane mesh, using a maximum segment length of 1 mm, the simulation did not converge. A truncation might have led to an infinite calculation loop. Figure 5 also shows that the mesh density has a stronger influence on the computational time in the case of 3D elements compared with the 2D counterpart.

CAE users usually reduce the mesh density to shorten the computation time. However, in contrast to practical use, the results of Figure 5 show that, for all three mesh geometries, on using a mesh with over 2 mm of maximum segment length the computation time did not reduce significantly.

In general, the computation time for the 3D mesh was much longer (by up to a factor of 6) compared with the other meshes. However, the number of elements was around 10 times higher. This means that the calculation time for each element must be shorter for the 3D mesh. Endorsing this fact, varying the maximum segment length from 4 mm to 2 mm, the computation time increased by 21% and the number of elements increased by 43%. The accuracy of the simulated values according to the mesh are discussed below.

Figure 4. (a) Piezoelectric sensor (Kistler 6190CA) and (b) its position on the workpiece.

Table 3. Injection molding parameters.

Material H 503 Braskem Density (g/cm3) 0.905 Injection time (s) 1.0 Part weight (witout inlet) (g) 40.8 Holding time (s) 4.7 Switchover 98% of filling Holding pressure (bar) Start with 80% of the injection pressure decreasing to zero after 4.7seg.

Process temperature (°C) 230 Mold temperature (°C) 40 Coolant flow - water (L/min) 9 Coolant temperature (°C) 25

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Figure 5. Computational time according to mesh geometry and number of elements.

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Influences of the mesh in the CAE simulation for plastic injection molding In Figure 6, two important moments of the injection process can be identified: the maximum pressure point[7] and the peak at the end of the injection[13]. These specific moments are used to evaluate the pressure. Table 4 reports the simulated results for the pressure, the experimental pressure and the percentage of error between them.

4.2 Analysis of the pressure data Figure 6 shows the profiles for the dynamic pressure during the practical injection cycle and the simulated results.

Firstly, it can be noted that for all cases the values for the simulated pressures increase before the experimental (real) values. This could be because of a delay in the drive of the injection machine (that could be up to 0.1 seconds according machine datasheet), when the control on the machine changes from the volumetric (speed) to pressure domain. This could not be properly considered in the simulation.

The results show that there is no direct relation between the mesh geometry and density with the precision of simulation, which was an unexpected finding. Highlighting this situation, the most sophisticated mesh (3D) using the shortest maximum segment length (1 mm) generated the highest number of elements, but it resulted in the highest error (117%). Also, notably, this mesh required the longest computation time (around 6 h). The 2D dual-domain mesh results had the lowest deviations from the experimental data, with a minimum error of -3% and maximum of -12% (for the maximum pressure).

The machine switchover delay, as can be observed in the Figure 6, resulted in a decrease of the preasure after the point 2 when compared to the simulation. This could also resulted in thiker frozen layers what explain the bigger preassure peaks comparing to midplane and dual-domain mesh simulation. Besides, the PP crystallization it’s very sensible and can influence the pressure inside the cavity during cooling phase.

The 2D midplane mesh was associated with the shortest computation time and the lowest deviation of the experimental pressure (12% to 35%). This mesh calculated some peaks in the pressure at the end of the injection time (point 2) that were not present in the experimental injection molding.

The computation for the 2D midplane with a maximum segment length of 1 mm did not converge. On analyzing the data error report of the simulation, this appears to be due to rounding and truncation, as suggested in the literature[10].

It is also observed that the simulated pressure drops faster than the experimental pressure, that is, in about half the time. This can be attributed to the viscoelastic behavior of the injected material. At the end of the packing phase, the material is subjected to an abrupt pressure variation followed by a relatively long relaxation period during the cooling phase, and the elastic component results in a delay to responses such as a drop in pressure. The simulation was not able to identify this phenomenon, which could affect the cycle time and the product quality.

The greater errors observed for the 2D midplane meshes compared to the 2D dual-domain meshes can be attributed to the behavior of the thin-walled injection flow. The melt can

Figure 6. Simulated vs. experimental pressure inside the mold cavity for (a) midplane mesh; (b) dual-domain mesh and (c) tetrahedral mesh.

Table 4. Analysis of pressure data.

Midplane 2mm 4mm 6mm 6.5 7.4 6.4

Pressure Point 1 (MPa) Error (%) -32.0 Pressure Point 2 7.5 (MPa) Error (%) 14.5

1mm 8.4

Simulated Dual-Domain 2mm 4mm 6mm 8.9 9.2 8.9

1mm 20.6

Tetrahedral 2mm 4mm 19.5 19.2

6mm 19.1

Experimental pressure (MPa) 9.5

-22.3 8.9

-33.0 7.3

-11.9 6.8

-6.7 7.1

-3.0 7.7

-5.9 7.4

117.0 9.5

105.4 8.4

101.4 8.9

100.8 8.8

6.5

35.5

12.1

4.1

8.3

17.1

12.5

45.9

29.0

35.9

34.2

-

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Marin, F., Souza, A. F., Pabst, R. G., & Ahrens, C. H.

Figure 7. Measured temperature and values simulated with midplane and dual-domain meshes.

Figure 8. Transient temperature simulation and the experimental data.

higher. The 2D midplane mesh was more accurate in this case, but the minimal error was around 14%, compared with the experimental (real) temperature.

Figure 8 shows the dynamic temperature, obtained using the 3D mesh, during three injection cycles. It is possible to identify the start of the cycle, the end of the filling of the plastic material and the opening time.

In the case of the 3D mesh no significant influence of the maximum segment length was observed and the error between the simulated and the experimental temperature was lower than 2%. Therefore, the use of a 3D mesh with a longer segment length can save considerable computational time without loss of accuracy in the temperature simulation.

Another point observed in Figure 8 is that the simulated temperature rises sharply, in contrast to the experimental values. An angle α can be observed between the line of the simulated temperature and the line formed with the data obtained with the sensor, representing the divergence in the values. With the simulation is possible to evaluate, independently, the plastic mesh domain and mold mesh domain at the same instant of time. Thus, is possible to evaluate the effects of the thermal exchange on the temperature at specific points, where the mold and the plastic touch each other, as shown in Figure 9.

For the simulation results for the plastic and mold domain profiles, the temperature increase forms a vertical line, but differing in magnitude. The angle α represents the divergence between the simulation and experimental curves for the temperature. The resistance between the plastic material and the mold, and between the frozen layers of the polymer (results of the fountain flow phenomenon), does not allow instantaneous changes in temperature. The divergence of the simulated results from the sensor data can induce an error or a lack of convergence in a closed loop control, resulting in the need for manual adjustment of the parameters.

The angle α identified in this study will be of use in further investigations. It should be noted that this divergence was observed only for the temperature simulation.

5. Conclusions Figure 9. Temperature of the first 10 cycles at the sensor position and 3D mesh simulations in the plastic and mold domain.

be regarded as a general Hele-Shaw flow, which neglects the “fountain flow” phenomenon at the front of the melt[24].

4.3 Analysis of the temperature data Since the 2D meshes (midplane and dual-domain) are not able to simulate the dynamic temperature during the cycle, the temperature analysis was split into two parts: i) static analysis at the end of the cycle; and ii) dynamic temperature during the cycle using a 3D mesh. In order to obtain a steady regime of the mold temperature, 10 batches of injection moldings were required. Figure 7 shows the results of the static analysis of the temperature.

The results obtained with the 2D dual-domain mesh did not vary with the mesh density and the error value was 8/10

This paper reports the computational time and the temperature and pressure results obtained in simulations carried out with different CAE meshes. Experimental data was also acquired in real time. The main conclusions of this study can be detailed as follows: 1. The results show that the computational time was mainly influenced by the geometry of the mesh rather than the mesh density and it varied by up to a factor of 1,000 depending on the mesh geometry and density. In the case investigated, the mesh with the highest density (1 mm maximum segment length) was associated with the longest computational time.

2. In general, the use of a lower density mesh did not reduced significantly the accuracy of the simulation, which was unexpected, and the computation time was shorter.

3. The CAE simulation can reach a relatively good accuracy for the temperature parameter, when compared with the Polímeros, 29(3), e2019043, 2019

Influences of the mesh in the CAE simulation for plastic injection molding temperature measured experimentally inside the mold.

For the 3D tetrahedral mesh the divergence was less than 2%. Therefore, in cases where high accuracy is required, the use of a 3D mesh is recommended.

4. Although the simulated temperature results showed good accuracy, the shape of the curves presented some divergence. The simulated temperature increased sharply, along a vertical line, during the injection period.

However, in the real process the temperature increases in a nonlinear manner, as expected. This divergence was identified as angle α. This angle can affect the accuracy of the simulation and vary according to the material and the process parameters.

5. For the pressure parameter, the 3D mesh results show a significant divergence (of up to 117%) from the measured data. The closest results were obtained for the pressure simulated with the 2D dual-domain mesh, with a maximum error of 3%.

6. The pressure curves generated by the simulations were accurate up to the end of the speed control domain of the machine. The switchover to the pressure control domain resulted in errors in the simulations. The software was not able to properly identify the decompression during the process, indicating around half of the actual time required until the end of the packing phase, which can affect the filling and the integrity of the plastic part.

7. To have a more accurate simulation of the injection molding process by CAE systems, new developments should include the dynamic limitations of the process and the machine, such as the response time for temperature alterations, and input it into the software database.

To summarize the conclusions, for the pressure parameter, the 2D dual domain resulted in the most precise simulation and it was not influenced by the mesh density. Therefore, the use of this type of mesh and a lower density can provide a faster and more precise simulation of the pressure inside the mold. For the temperature parameter, the 3D mesh was more precise and it was also not influenced by the mesh density. Therefore, the use of a 3D mesh together with a lower density can allow a faster and more precise simulation of the temperature inside the mold.

6. Acknowledgements The authors are grateful to the Coordination of Superior Level Staff Improvement (CAPES), National Council for Scientific and Technological Development (CNPq), Santa Catarina Research Foundation (FAPESC - project number 04/2011) and the enterprises Villares Metals; Polimold and SOKIT.

7. References 1. Mathivanan, D., & Parthasarathy, N. S. (2009). Prediction of sink depths using nonlinear modeling of injection molding variables. International Journal of Advanced Manufacturing Technology, 43(7-8), 654-663. http://dx.doi.org/10.1007/ s00170-008-1749-1.

2. Park, H. S., & Dang, X. P. (2010). Optimization of conformal cooling channels with array of baffles for plastic injection mold. International Journal of Precision Engineering and Polímeros, 29(3), e2019043, 2019

Manufacturing, 11(6), 879-890. http://dx.doi.org/10.1007/ s12541-010-0107-z.

3. Kiam, T. M., & Pereira, N. C. (2007). Study of injectioncompression molded part using CAE analysis. Polímeros.

Ciência e Tecnologia, 17(1), 16-22. http://doi.org/10.1590/ S0104-14282007000100007.

4. Hassan, H., Regnier, N., Lebot, C., Pujos, C., & Defaye, G. (2009). Effect of cooling system on the polymer temperature and solidification during injection molding. Applied Thermal Engineering, 29(8), 1786-1791. http://dx.doi.org/10.1016/j. applthermaleng.2008.08.011.

5. Hsu, F. H., Wang, K., Huang, C. T., & Chang, R. (2013).

Investigation on conformal cooling system design in injection molding. Advances in Production Engineering & Management, 8(2), 107-115. http://dx.doi.org/10.14743/apem2013.2.158.

6. Padilla, A., Baselga, J., & Bravo, J. (2006). Comparison of gauge deformation determined by simulation of the injection process with real process values. Información Tecnológica, 17(4), 53-58. http://dx.doi.org/10.4067/S0718-07642006000400010.

7. Autodesk Moldflow Insight (2017). User guide. Retrieved in 2019, June 24, from knowledge.autodesk.com/support/moldflow -insight/troubleshooting/caas/sfdcarticles/sfdcarticles/Whento-use-Dual-Domain-or-3D-mesh-in-Simulation-Moldflow. html?st=dd 8. Miranda, D. A. D., & Nogueira, A. L. (2019). Simulation of an injection process using a CAE tool: assessment of operational conditions and mold design on the process efficiency. Materials Research, 22(2), e20180564. http://dx.doi.org/10.1590/19805373-mr-2018-0564.

9. Yang, D., Zhao, P., Zhou, H., & Chen, L. (2014). Computer determination of weld lines in injection molding based on filling simulation with surface model. Journal of Reinforced Plastics and Composites, 33(15), 1403-1415. http://dx.doi. org/10.1177/0731684414535277.

10. Kovács, J. G., & Sikló, B. (2010). Experimental validation of simulated weld line formation in injection moulded parts.

Polymer Testing, 29(7), 910-914. http://dx.doi.org/10.1016/j. polymertesting.2010.06.003.

11. Wang, J., & Yu, Z. (2012). Feature-sensitive tetrahedral mesh generation with guaranteed quality. Computer Aided Design, 44(5), 400-412. http://dx.doi.org/10.1016/j.cad.2012.01.002.

PMid:22328787.

12. Ito, Y., Shih, A. M., & Soni, B. K. (2004). Reliable isotropic tetrahedral mesh generation based on an advancing front method. In Proceedings of The 13th International Meshing Roundtable – IMR (pp. 95-106). Williamsburg, Virginia: IMR. 13. Choi, W. Y., Kwak, D. Y., Son, I. H., & Im, Y. T. (2003).

Tetrahedral mesh generation based on advancing front technique and optimization scheme. International Journal for Numerical Methods in Engineering, 58(12), 1857-1872. http://dx.doi. org/10.1002/nme.840.

14. Jin, H., & Tanner, R. I. (1993). Generation of unstructured tetrahedral meshes by advancing front technique. International Journal for Numerical Methods in Engineering, 36(11), 18051823. http://dx.doi.org/10.1002/nme.1620361103.

15. Ito, Y., Murayama, M., Yamamoto, K., Shih, A. M., & Soni, B. K. (2013). Efficient hybrid surface/volume mesh generation using suppressed marching-direction method. AIAA Journal, 51(6), 1450-1461. http://dx.doi.org/10.2514/1.J052125.

16. Parmar, K. C., & Kaiser, H. (2017). Comparison of simulation results when using two different methods for mold creation in moldflow simulation. International Journal of Scientific & Technology Research, 6(4), 128-131.

17. Vietri, U., Sorrentino, A., Speranza, V., & Pantani, R. (2011).

Improving the predictions of injection molding simulation software. Polymer Engineering and Science, 51(12), 25422551. http://dx.doi.org/10.1002/pen.22035.

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Marin, F., Souza, A. F., Pabst, R. G., & Ahrens, C. H. 18. Fan, X. J., Tanner, R. I., & Zheng, R. (2010). Smoothed particle hydrodynamics simulation of non-Newtonian moulding flow.

Journal of Non-Newtonian Fluid Mechanics, 165(5), 219-226. http://dx.doi.org/10.1016/j.jnnfm.2009.12.004.

19. Zhou, H., Hu, Z., & Li, D. (2013). Mathematical models for the filling and packing simulation. In H. Zhou (Ed)., Computer modeling for injection molding: simulation, optimization and control (chap.

3). Wiley. http://dx.doi.org/10.1002/9781118444887.ch3.

20. Fernandes, C., Pontes, A. J., Viana, J. C., & Gaspar-Cunha, A. (2016). Modeling and optimization of the injection molding: a review. Advances in Polymer Technology, 37(2), 21683-21704. https://doi.org/10.1002/adv.21683.

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Finite element methods for fluid dynamics. Elsevier.

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24. Wang, X., Li, H., Gu, J., Li, Z., Ruan, S., Shen, C., & Wang, M. (2017). Pressure analysis of dynamic injection molding and process parameter optimization for reducing warpage of injection molded products. Polymers, 9(3), 85. http://dx.doi. org/10.3390/polym9030085. PMid:30970773.

Received: June 24, 2019 Revised: Aug. 29, 2019 Accepted: Aug. 30, 2019

PolĂ­meros, 29(3), e2019043, 2019

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.06119

Extraction and analysis of the parietal polysaccharides of acorn pericarps from Quercus trees Moubarek Mébarki1 ⃰ , Kadda Hachem1,2 , Céline Faugeron-Girard3 , Riad el Houari Mezemaze1 and Meriem Kaid-Harche1  Laboratoire des Productions, Valorisations Végétales et Microbiennes – LP2VM, Faculté des Sciences de la Nature et de la Vie, Université des Sciences et de la Technologie d’Oran Mohamed Boudiaf, Oran, Algérie 2 Département de Biologie, Faculté des Sciences, Université Dr. Moulay Tahar de Saida, Saida, Algérie 3 Laboratoire PEIRENE, Faculté des Sciences et Techniques, Université de Limoges, Limoges, France 1

*moubarek.mebarki@univ-usto.dz

Abstract Acorns produced by Quercus trees are currently underexploited and undervalued. To evaluate the commercial and health benefits of acorns, we examined the cell wall components of acorn pericarps from Quercus suber and Quercus ilex, growing in North-Western Algeria. Acorn pericarps were sequentially extracted and the polysaccharide fractions were analyzed by gas liquid chromatography and Fourier-transform infrared spectroscopy. The lignocellulosic fraction was the major component of Q. suber and Q. ilex cell walls (37.19% and 48.95%, respectively). Lower amounts of pectins and hemicelluloses were also found in both species. Hemicellulose extracts from the two species contained xylose as the major monosaccharide (ranging from 36.7% to 49.4%). Galacturonic acid was the major component of hot water- or ammonium oxalate-extracted pectins from both species (ranging from 20.6% to 46.8%). The results reported in this paper reveal that acorn pericarp cell walls from these two oak could be potential sources of bioactive compounds.

Keywords: Quercus sp., pericarp, polysaccharides.

How to cite: Mébarki, M., Hachem, K., Faugeron-Girard, C., Mezemaze, R. H., & Kaid-Harche, M. (2019). Extraction and analysis of the parietal polysaccharides of acorn pericarps from Quercus trees. Polímeros: Ciência e Tecnologia, 29(3), e2019044. https://doi.org/10.1590/0104-1428.06119

1. Introduction The genus Quercus spp. is one of the most species-rich genus among forest trees. This genus consists of several hundred species which grow in temperate, as well as in Mediterranean climates, particularly in America, Europe, and Asia[1].

In Algeria, oak trees represent an important forest resource, as they account for nearly 40% of the Algerian forest, and play important ecological, economic roles. In Algeria the local population uses the fruit of Quercus as a traditional food resource[2]. Numerous studies have recently explored the health benefits of natural plant compounds including bioactive compounds such as polyphenols, proteins, lipids, vitamins, polysaccharides, and other constituents[3].

Cell walls are important for the growth and the development of plants; they provide a significant barrier to diseases, making them targets for improving the post-harvest storage and processing of the fruits. Thus, studies evaluating how plants synthesize and remodel their cell walls constitute an important and expanding area of research, particularly in the renewable energy field[4] .

Identifying, isolating, and evaluating new sources of bioactive polysaccharides, in order to promote their use in technological applications or food formulations has recently

Polímeros, 29(3), e2019044, 2019

been widely examined[5]. Although a few works have been dedicated to acorns[6], no single study exists which has been specifically devoted to acorn pericarps.

This study was conducted to evaluate the cell wall of acorn pericarps from two Quercus species growing in different regions of North-Western Algeria, in the Saida (high plateau region) and Oran (coastal region) areas. To the best of our knowledge, the present study is the first report on cell wall polysaccharides extracted from acorn pericarps.

2. Materials and Methods 2.1 Plant material The two different oak species, Q. suber and Q. ilex, were selected because of their high prevalence among North-Western Algerian forests. Acorn samples, from approximately 100 year old trees, were collected in December 2016, Q. ilex in the Saida region (34°48’45.5”N 0°09’43.5”E) and Q. suber in the Oran region (35°38’20.3”N 0°50’22.6”W) and were identified by Pr. Meriem Kaid Harche. Two voucher specimens (QS 8409 and QI 8410) have been deposited at the Herbarium of the Department of Biotechnology, Mohamed

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O O O O O O O O O O O O O O O O

Mébarki, M., Hachem, K., Faugeron-Girard, C., Mezemaze, R. H., & Kaid-Harche, M. Boudiaf University of Sciences and Technology, Oran, Algeria. After cleaning, pericarps were manually detached from acorns and dried in a ventilated oven (40 °C). Pericarps were then ground (particle size <200 µm), and the resulting powder was stored in desiccators at room temperature.

2.2 Sequential extraction of parietal components The sequential and selective extraction of parietal polysaccharides present in acorn pericarps from Q. suber, and Q. ilex, was carried out according to Hachem et al.[7].

All extraction procedures were carried out using magnetic stirring. All extracts were filtered through a porous glass frit (Porosity number 3) and then transferred to pre-soaked dialysis tubing (Spectra/Por; molecular weight cutoff 6000–8000 Da). After dialysis, polymers were precipitated by addition of 3 volumes of 96% ethanol, centrifuged, and finally freeze-dried. The subsequent fractionation procedure is summarized in Figure 1.

2.3 Colorimetric assay of total sugars Sugars in the polysaccharide fraction were identified using the sulfuric phenol methodfor neutral sugars with glucose as standard[8], and the meta-hydroxydiphenyl (m-HDP) method for uronic acids with glucuronic acid as standard[9]. Because of the interference of uronic acids in the determination of neutral sugars and vice versa, it was necessary to apply the correction method established by Montreuil et al.[10].

Figure 1. Extraction and isolation of cell wall polysaccharides.

2.4 Qualitative analysis by gas liquid chromatography The monosaccharide composition of the extracted fractions was determined after methanolysis (MeOH/HCl 1 M, 24 h, 80 °C) by gas-liquid chromatography of pertrimethylsilylated methylglycosides as previously described by Hachem et al.[7].

2.5 Fourier-transform Infrared spectroscopy (FT-IR) The different fractions from the acorn pericarps of Q. suber and Q. ilex were compressed into KBr pellets.

The FTIR spectra of these pellets were obtained using a Cary 600 FTIR spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) over the 400–4000 cm-1 range.

3. Results and Discussions 3.1 Polysaccharide yields at each extraction step The freeze-dried cell wall residue contained 82.11 ± 2.54% and 88.93 ± 2.30% of the dry mass of the Q. suber and Q. ilex pericarp, respectively (Table 1).

The lignocellulosic fraction comprised the main part, representing 37.19 ± 8.63% and 48.95 ± 8.51%, respectively.

The KOH and NaOH hemicellulose extracts represented 10.16±1.24% and 3.40 ± 1.05% of the Q. suber cell wall residue, respectively, whereas these same hemicellulose fractions from Q. ilex represented 7.38 ± 1.67%, and 4.81±0.64%, respectively. The hot water pectin extract of Q. suber and Q. ilex represented 6.47 ± 3.04% and 3.88 ± 0.78% of the cell wall residues and was composed of 9.6% and 20% uronic acid respectively. While ammonium oxalate pectin extracts of both species (4.19 ± 1.73%, 2.61 ± 1.61%) contained more uronic acid than hot water extracts (68.5% and 39.3%). This difference may be the result of different environmental conditions and/or genetic factors that can affect the cell wall structure and also affect the levels of its various components[11].

Comparing the composition of the two cell walls, it can be seen that the Q. ilex acorn pericarp appears to contain much more lignocellulose, but less pectin and hemicellulose polysaccharides, than Q. suber. This difference could be due to different environmental conditions and/or genetic factors, since these two distinct species of Quercus have different provenances; Saïda belongs to the high plateau, while Oran is situated on the Northwestern Mediterranean

Table 1.Yields of differentially extracted fractions prepared from Q. suber and Q.ilex acorn pericarps.

Fractions Cell wall residue* Pectins H2O** Pectins oxalate** Hemicellulosic KOH** Hemicellulosic NaOH** Lignocellulosic fraction**

Q. suber Extraction yield (%)*** Uronic acid (%) 82.11 ± 2.54 Nd 6.47 ± 3.04 9.6 4.19 ± 1.73 68.5 10.16 ± 1.24 19.6 3.40 ± 1.05 10.6 37.19 ± 8.63 Nd

Q. ilex Extraction yield (%)*** Uronic acid (%) 88.93 ± 2.30 Nd 3.88 ± 0.78 20 2.61 ± 1.61 39.3 7.38 ± 1.67 8 4.81 ± 0.64 10.2 48.95 ± 8.51 Nd

*Percentage of 15 g starting dry weight of pericarp powder; **% Weight of cell wall residue, Nd: not detected; ***values are means ± standard deviation of three samples.

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Polímeros, 29(3), e2019044, 2019

Extraction and analysis of the parietal polysaccharides of acorn pericarps from Quercus trees coast of Algeria. Numerous studies have shown that many abiotic and biotic factors, such as geographic location, soil salinity, light intensity, levels of water nutrition, plant species, time of harvest, and stage of life cycle can cause changes in cell wall structure and can significantly affect the levels of various cell wall components[11,12] .

3.2 Monosaccharide composition of cell walls The amounts of pectins extracted with hot water or ammonium oxalate confirmed the high levels of pectin (Table 2), in agreement with the high levels of galacturonic acid (20.6 - 46.8%). Rhamnose levels in the pectin extracts ranged from 5.2% to 8.3%, suggesting that these fractions also contain rhamnogalacturonans that can be substituted with arabinan, galactan, and/or arabinogalactan side chains.

The detection of fucose in pectins extracted from Q. ilex pericarps suggests the presence of rhamnogalactoronan II. Moreover, high levels of glucuronic acid in the pectins extracted with hot water from Q. ilex pericarps (19.1%) suggest differences in the pectic components between the two species. According to Alba and Kontogiorgos[13], the diversity of pectin structures depends on the botanical source, plant ripening stage, and extraction procedure as well. Hemicelluloses extracted with KOH or NaOH were found to be rich in xylose (Table 2), suggesting the presence of xylans. The Ara/Xyl ratio is interesting because it allows to compare the degrees of substitution of the polymer. In the fraction extracted with KOH this ratio is 0.37, and 0.59 in Q. ilex and Q. suber pericarps respectively, compared to

0.40 and 0.42 for the fraction extracted with NaOH. This suggests that the main xylan chain of Q. suber pericarps is more substituted than that of Q. ilex. The results obtained also show that the pericarps of both species contain xylans with significant degrees of substitution, compared to the results obtained by Habibi et al.[14,15] with the seed pericarps of Opuntia ficus-indica and Argania spinosa. In the case of arabinoxylans, a low Ara / Xyl ratio corresponds to a low degree of polymer branching, making it less water soluble, while water soluble arabinoxylans are characterized by a higher Ara / Xyl ratio[16,17]. However, the presence of arabinoglucuronoxylans in the fraction extracted with KOH cannot be excluded given the presence of glucuronic acid (2.4 to 2.7%). Glucose (5.5 to 15.2%) was also found, suggesting the presence of hemicelluloses of the xyloglucan type. According to Hu et al.[18], xyloglucans are the predominant family of hemicelluloses and are mainly found in dicotyledons, but at lower levels in monocotyledons.

The results obtained in our study are consistent with previous studies on the same tissues[19].

3.3 FT-IR spectra FT-IR spectra of pectins and hemicelluloses are presented in Figures 2 and 3. The large intense band between 3200 and 3500 cm-1 can be attributed to the elongation vibration of hydroxyl groups (-OH)[20]. Small vibration bands indicating C-H bonds were observed between 2800 and 3000 cm-1. Other signals at 1746 and 1756–1760 cm-1 suggest the presence of acetyl groups in pectic residues[21]. Absorption bands around 1600 and 1400 cm-1 can be attributed to carboxylate

Table 2. Monosaccharide composition of acorn pericarps from Q. suber, and Q. ilex assessed by gas liquid chromatography.

Species Pectins H2O Pectins Oxalate Hemicelluloses KOH Hemicelluloses NaOH

Q.ilex Q.suber Q.ilex Q.suber Q.ilex Q.suber Q.ilex Q.suber

Ara 16.1 36.9 13 18.6 18.5 21.9 18.3 20.5

Rha 6.9 8.3 5.2 7.2 4.8 5.8 4.7 6.2

Fuc 1.5 Nd 0.9 Nd Nd Nd Nd Nd

Monosaccharide composition (%mol) Xyl Gal A Man 4.5 24.3 2.1 3.1 20.6 0.8 3.5 46.8 5.6 1.6 23.5 1.7 49.4 8.5 Nd 36.7 8.6 Nd 45.7 13.8 1.4 48.5 13 Nd

Gal 11 5.9 12.1 11 9.8 9.4 9.5 6.3

Glc 14.5 21.5 4.4 24.3 6.3 15.2 6.5 5.5

Glc A 19.1 2.9 8.5 12 2.7 2.4 Nd Nd

Ara: arabinose; Rha: rhamnose; Fuc: fucose; Xyl: xylose; Gal A: galacturonic acid; Man: mannose; Gal: galactose; Glc: glucose; Glc A: glucuronic acid; Nd: not detected.

Figure 2. Infrared spectrum of the different pectin extracts; (PH2O): pectins extracted with hot water. (Poxa): pectins extracted with ammonium oxalate.(Q.s): Q. suber. (Q.i):Q. ilex.

Polímeros, 29(3), e2019044, 2019

Figure 3. Infrared spectrum of different hemicellulose extracts; H NaOH: hemicelluloses extracted with NaOH. H KOH: hemicelluloses extracted with KOH. (Q.s): Q. suber. (Q.i):Q. ilex.

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Mébarki, M., Hachem, K., Faugeron-Girard, C., Mezemaze, R. H., & Kaid-Harche, M. groups (COO-). The large band at 1610 to 1637 cm-1 and the band near 1430 cm-1 were attributed to asymmetric and symmetric stretching of C=O, respectively[22]. Finally, the bands observed between 890 and 1200 cm-1 are specific of the vibrations of the C-O-C and C-O-H bonds present in polysaccharide structures[23].

4. Conclusions The results reported in this paper reveal that the cell wall of acorn pericarps from two Quercus species could be potential sources of bioactive constituents, mainly polysaccharides (pectins, hemicelluloses, celluloses) and lignin. These include xylans, xyloglucan type hemicelluloses, and homogalacturonan and rhamnogalacturonan pectins.

These constituents are non-toxic, biocompatible, and biodegradable and hold a high potential for their broad application in food or for their pharmacological effects, which have yet to be exploited.

5. Acknowledgements The authors acknowledge Pr. Vincent Sol, head of the PEIRENE laboratory (EA 7500 - France) for his warm and friendly welcome, and Dr. Michel Guilloton for his help in manuscript editing.

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20. Fernando, I. P. S., Sanjeewa, K. K. A., Samarakoon, K. W., Lee, W. W., Kim, H.-S., Kim, E.-A., Gunasekara, U. K. D. S. S., Abeytunga, D. T. U., Nanayakkara, C., de Silva, E. D., Lee, H.-S., & Jeon, Y.-J. (2017). FTIR characterization and antioxidant activity of water soluble crude polysaccharides of Sri Lankan marine algae.

Algae - Korean Phycological Society, 32(1), 75-86. http://dx.doi. org/10.4490/algae.2017.32.12.1.

21. Brito, A. C. F., Silva, D. A., Paula, R. C. M., & Feitosa, J. P. A. (2004). Sterculia striata exudate polysaccharide: characterization, rheological properties and comparison with Sterculia urens (karaya) polysaccharide. Polymer International, 53(8), 1025-1032. http:// dx.doi.org/10.1002/pi.1468.

22. Pasandide, B., Khodaiyan, F., Mousavi, Z. E., & Hosseini, S. S. (2017). Optimization of aqueous pectin extraction from Citrus medica peel. Carbohydrate Polymers, 178, 27-33. http://dx.doi. org/10.1016/j.carbpol.2017.08.098. PMid:29050593.

23. Morais, E. S., Mendonça, P. V., Coelho, J. F. J., Freire, M. G., Freire, C. S. R., Coutinho, J. A. P., & Silvestre, A. J. D. (2018).

Deep eutectic solvent aqueous solutions as efficient media for the solubilization of hardwood xylans. ChemSusChem, 11(4), 753-762. http://dx.doi.org/10.1002/cssc.201702007. PMid:29345423.

Received: July 23, 2019 Revised: Sept. 16, 2019 Accepted: Sept. 18, 2019 Polímeros, 29(3), e2019044, 2019

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.09018

Bionanocomposites of PLA/PBAT/organophilic clay: preparation and characterization Josiane Dantas Viana Barbosa1 , Joyce Batista Azevedo2, Edcleide Maria Araújo3, Bruna Aparecida Souza Machado1* , Katharine Valéria Saraiva Hodel1 and Tomas Jefferson Alves de Mélo3 Laboratório de Formulações Farmacêuticas, Instituto de Tecnologias da Saúde (ITS), Centro Universitário SENAI CIMATEC, Salvador, BA, Brasil 2 Departamento de Materiais, Universidade Federal do Recôncavo da Bahia – UFRB, Feira de Santana, BA, Brasil 3 Departamento de Materiais, Universidade Federal de Campina Grande – UFCG, Campina Grande, PA, Brasil

1

*brunam@fieb.org.br

Abstract The objective of this study was to develop bionanocomposites from blends of poly(lactic acid) (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) and from 3% and 6% bentonite clay. Initially, the bentonite clay was treated with Praepagen salt, and the properties of the modified clay were evaluated. After the organophilization of the clay was completed, 50:50 blends of PLA/PBAT were prepared, and 3 and 6% clay was added. To test the dispersion of the system, the blending sequence was performed using eight different sequences for the addition of clay to the PLA/PBAT matrices. The mixtures were prepared in a twin screw extruder, and the specimens were subsequently injection molded.

The investigated mechanical and morphological properties included the yield strength, yield strain, tensile and bending elastic modulus, and scanning and transmission electron microscopy analyses. The results of this study showed increases of the mechanical properties when nanoparticles were added and the formation of bionanocomposites with intercalated structures.

Keywords: bionanocomposite, nanoclay, PBAT, PLA, bionanocomposites.

How to cite: Barbosa, J. D. V., Azevedo, J. B., Araújo, E. M., Machado, B. A. S., Hodel, K. V. S., & Mélo, T. J. A. (2019). Bionanocomposites of PLA/PBAT/organophilic clay: preparation and characterization. Polímeros: Ciência e Tecnologia, 29(3), e2019045. https://doi.org/10.1590/0104-1428.09018

1. Introduction In the last 50 years, polymers derived from petroleum have been used extensively due to their versatility, mechanical properties, and relatively low cost. However, its extensive use has had environmental impacts because a large volume of waste is disposed of in the environment, especially disposable plastics packaging[1,2]. As a result, plastic packaging has become one of the main contributors to the various environmental impacts since over a third of current plastics production is used to make them, and also because its relativity short life cycle[3,4]. In addition, recent increases in the cost of raw petroleum have led to a dramatic increase in the cost of plastics. Technologies for recovering plastic have also improved in recent years but are not totally free from environmental damage[5].

As a result, society has pressured the industrial sector to adopt innovative environmentally friendly policies, such as the rational use of natural resources, especially in the production of resins for the productive sector. In this context, several materials have been researched in the search for environmentally favorable solutions[6-10]. As an alternative to reduce environmental impacts, a new class of materials,

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biopolymers, has emerged and have motivated a significant number of studies due to the large environmental interest and possible lack of fossil resources[2,11-15].

Compared to conventional thermoplastics, biopolymers exhibit poor performance in several specific properties and therefore must be modified to become more competitive[16-19].

This process resulted in different biopolymers with smart behavior and a significant change in one property upon an external trigger[20]. Biodegradable polymers are not easily classified because they can be organized according to their chemical composition, synthesis method, processing method, economic relevance, and application[21-27].

These biopolymers are commonly blending with other biopolymers or with conventional polymers and/or inorganic particles. The blending of biopolymers with other polymers provides a way to modify the properties of biopolymers and can reduce the overall cost of the material.

These blends form a new class of materials: biocomposites that are obtained from based on biodegradable polymers as matrix[6,28-30]. Poly(lactic acid) (PLA) and poly(butylene adipate-co-terephthalate) (PBAT) are among the polymers

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O O O O O O O O O O O O O O O O

Barbosa, J. D. V., Azevedo, J. B., Araújo, E. M., Machado, B. A. S., Hodel, K. V. S., & Mélo, T. J. A. most investigated as possible matrices for the formation of these blends, on account of chemical and mechanical properties, besides being are both biodegradables[30-32].

The development of polymer blends using nanotechnology has emerged as a possible solution that may be adopted in different fields of technology to improve mechanical, barrier, flammability, thermal, electric, and cosmetic properties[33-35].

Nanoclay is among the most studied nanomaterials, because its potential of use is variable fields, thanks to properties such as the highly oriented nanoclay structures showed a tortuous path that was responsible for the reduced gas and vapor transmission[36]. Besides that, nanoclay provides increased of the nanocomposite tensile strength and the compressive, fracture and Young’s modulus were related to the dispersion of the clay, degree of delamination, form factor of the clay, and polymer-clay interfacial interactions[37-40].

Correa et al.[15] and Kumar et al.[41] analyzed the properties of Polyhydroxybutyrate/Polycaprolactone (PHB/PCL) and PLA/PBAT blends, respectively, incorporated with nanoclays.

The results showed that the blends with the nanoclays presented better thermal stability, mechanical, and barrier properties when compared to their respective controls.

In this context, the aim of this study was to develop bionanocomposites from two polymer matrices, PLA and PBAT with bentonite clay, using the melt intercalation technique with different blending sequences and to then evaluate the mechanical and morphological properties of the bionanocomposites obtained.

2. Materials and Methods 2.1 Materials Brasgel PA clay, supplied by Indústria Bentonit União Nordeste (BUN), located in Campina Grande, Paraíba, Brazil, was used to obtain organophilic clay. Industrialized clay was treated with Praepagen WB® salt and the following procedure was adopted: dispersions containing 768 mL of distilled water and 32.0 g of clay were prepared. The clay was slowly added with concomitant mechanical stirring and after the addition of all the clay the stirring was maintained for 20 minutes. Then, a solution containing 20 mL of distilled water and 20.4 g of the quaternary ammonium salt was added. Stirring was continued for another 20 minutes.

Sequentially, the containers were closed and kept at room temperature for 24 h. After that, the material obtained was washed and filtered to remove excess salt. The washing was done with 2000 mL of distilled water, using Buchner Funnel with kitassato, coupled to a vacuum pump with a pressure of 635 mmHg. The agglomerates obtained were oven dried at 60 °C ± 5 °C for a period of 48 h. Finally, the dry agglomerates were disaggregated with the aid of a mortar until powdery materials were obtained, which were passed in ABNT No. 200 sieve for further characterization.

Two polymers were used as polymer matrices: i) poly(lactic acid) (PLA), which was supplied by Cargill-Dow and is commercially known as Nature Works 2002D and ii) poly(butylene adipate-co-terephthalate) (PBAT), a biodegradable polyester that was supplied by BASF and is commercially known as ECOFLEX® F BX 7011.

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2.2 Preparation of the PLA/PBAT/Clay (OMMT) systems Eight PLA/PBAT blend samples with 50/50 weight ratios that contained clay at concentrations of 3% and 6% were investigated. These polymer/clay systems were prepared in a modular co-rotating twin screw extruder (model DRC 30:40 IF, Imacom, Barretos, Brazil) with a thread diameter of 30 mm and an L/D ratio of 30 that was fitted with a degassing system. Before processing, the organophilic clay and the polymer matrices were dried at 100 ± 5 °C for three hours in a forced air oven. First, the materials were weighed and pre-blended by tumbling. The samples were subsequently dosed at the main feed zone of the extruder (beginning of the thread) using a volume doser from Brabender.

To investigate the influence of the blending sequence of the material, four procedures were used to obtain 50% PLA and 50% PBAT blends for both clay concentrations (3% and 6%) as described: (1) For the first blending sequence, a blend of 50% PLA and 50% PBAT was prepared with the extrusion technique, and 3% and 6% by weight of clay was then added to the PLA/PBAT blend, which was extruded again; (2) For the second sequence, concentrates that contained PBAT and 3% and 6% by weight of clay were prepared by the extrusion technique, and PLA was then added to the systems with the extrusion technique; (3) For the third sequence, concentrates that contained PLA and 3% and 6% by weight of clay were prepared with the extrusion technique, and PBAT was then added to the systems with the extrusion technique; and, (4) For the fourth sequence, the three components (PLA/PBAT/clay) were added simultaneously with 3% and 6% concentrations by weight of clay and processed using the extrusion technique.

The temperatures in the 10 zones in the twin screw extruder for the PLA and PBAT blends were Z1 = 140 °C, Z2 = 150 °C, Z3 = 175 °C, Z4 = 180 °C, Z5 = 185 °C, Z6 = 185 °C, Z7 = 185 °C, Z8 = 175 °C, Z9 = 175 °C and Z10 = 170 °C[42]. A screw speed of 80 rpm was used for the 3% (PLA + clay) and (PBAT + clay) concentrations, and 60 rpm was used for the 6% clay concentrations. The measured temperature of the molten polymer was 185°C, and the same screw configuration was used for all the investigated samples[43]. The extruded material was granulated and dried, and the injected specimens were subsequently prepared as will be described throughout the text. The thread profile is illustrated in Figure 1.

Figure 1. Thread profile used for processing the PLA/PBAT/clay systems (Redrawn from Ref.[44]).

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Bionanocomposites of PLA/PBAT/organophilic clay: preparation and characterization First, the blends described above were dried for 4 h at 50 °C. Specimens were then produced using the injection molding process. The samples were prepared in injection machine with a mold clamping force of 100 tons, manufactured by ROMI model PRIMAX 120 (Santa Bárbara d’Oste, Brazil).

The injection conditions were: injection pressure of 250 bar, injection velocity of 120cm3/s, settling of 1s / 100 bar and tube temperature profile of T1 = 170 °C, T2 = 170 °C T3 = 175 °C and (no) nozzle T4 = 160 °C (injection nozzle). Samples were produced for tensile/bending strength and Izod impact strength tests according to standards ISO 527 and ISO 180.

2.3 Mechanical properties Tensile/bending measurements were performed according to standard ISO 527 at a temperature of 25 ± 2 °C and a relative humidity of 55 ± 5%. Five specimens were tested on average for each sample. The tests were performed in an universal testing machine (model DL200, EMIC) at a constant speed of 10 mm/min. Notched Izod impact strength tests were performed according to standard ISO 180 on an EMIC impact tester.

2.4 X-ray diffraction X-ray diffraction (XRD) analyses of the 8 samples were performed on a diffractometer (model XRD 6000, Shimadzu) that operated with copper Kα radiation (λ=1.5406), 40 kV and 30 mA. Diffraction patterns were collected at a scanning rate of 20(2Ɵ)/min in the interval of 1.50<2Ɵ<300 with exposition the 60sec.

2.5 Scanning electron microscopy Scanning electron microscopy (SEM) analyses of the 8 samples were performed on a microscope (model SSX-550, Shimadzu, Kyoto, Japan) that operated under different conditions, which can be observed in the captured images. The injected specimens underwent brittle fracture in liquid nitrogen, and the fracture surfaces were analyzed.

2.6 Transmission electron microscopy For the transmission electron microscopy (TEM) analyses, samples were prepared by reducing the cross-sectional area of the sample (“trimming”), and the edge of the sample to be ultramicrotomed was sized into a trapezoidal shape for a

better stress distribution when cutting thin slices (sections) with a surface area of approximately 0.5 mm2.

The samples were cut with an ultramicrotome (model Reichert Ultracut S, Leica) using a diamond knife (Diatome) at a cutting temperature of -40 °C under liquid nitrogen at a cutting speed of 0.1 mm.s-1 and a slice thickness of 25 nm.

The cryo-ultramicrotomed samples were observed in a TEM (model CM120, Philips) with a voltage of 120 kV.

3. Results and Discussions 3.1 Mechanical characterization Table 1 presents the values obtained for the yield strength, yield strain, and tensile and bending elastic moduli for the PLA/PBAT blend and the PLA/PBAT/clay systems investigated. The maximum yield strength of the systems with clay was 24.3 MPa for sample 7 (PLA 6% clay concentrate + PBAT), and the minimum was 8.3 MPa for sample 6 (PLA 3% clay concentrate + PBAT). The maximum yield strain was 2.0% in sample 5, and the minimum yield strain was 1.0% in sample 6 (PLA 6% clay concentrate + PBAT).

The results indicate that different blending conditions and the addition of different concentrations of clay cause changes in the mechanical properties. It is important to emphasize that this system is composed of two polymer matrices with distinct behaviors. Different studies with PLA/PBAT blends, showed the use additives to improve properties mechanical and morphology such as Signori et al.[45] and Jiang et al.[46].

These studies suggested that adding PBAT to the PLA matrix increased the ductility of the PLA. In the other study, Ko et al.[47] made PLA/PBAT blends and carbon nanotubes 2% (MNWT) in the presence the antioxidant additive. It has verified that the PLA/PBAT blends are immiscible, and that the MWNT has a (referential) affinity for the PBAT phase and this phenomenon makes unique morphological properties of the nanocomposite system.

Such a strong affinity of the MWNT to the PBAT phase might be related to chemical structure of the PBAT which possesses aromatic molecules in its main chain, as many groups reported that MWNT prefers aromatic molecules.

Therefore, having as reference these results, the increases in the properties resulted from the greater dispersion of the clay in the PLA/PBAT blend, which provided greater interaction between the two matrices and consequently improved the results.

Table 1. Mechanical properties of the PLA/PBAT blends and PLA/PBAT/clay systems.

Samples 1 2 3 4 5 6 7 8 9

PLA + PBAT pure blend PLA + PBAT blend + 3% clay PLA + PBAT blend + 6% clay PBAT 3% clay concentrate + PLA PBAT 6% clay concentrate + PLA PLA 3% clay concentrate + PBAT PLA 6% clay concentrate + PBAT PLA + PBAT + 3% clay PLA + PBAT + 6% clay

Yield Strength (MPa) 4.0 ± 1.0 15.5 ± 1.4 15.1 ± 0.3 12.7 ± 1.0 16.2 ± 2.0 8.3 ± 0.5 24.3 ± 1.8 11.7 ± 0.5 9.5 ± 2.0

Yield Strain (%) 1.8 ± 0.70 1.71 ± 0.20 1.75 ± 0.03 1.42 ± 0.20 2.00 ± 0.40 1.00 ± 0.08 1.46 ± 0.15 1.32 ± 0.06 1.14 ± 0.50

Flexure Elast. Mod. (MPa) 1248 ± 52.5 1222 ± 37.2 1263 ± 15.0 1443 ± 65.5 1388 ± 177 1249 ± 38.3 1485 ± 53.5 1232 ± 45.6 1224 ± 60.5

The reported data represents arithmetic mean values and the error bars refer to the standard deviation of the mean.

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Barbosa, J. D. V., Azevedo, J. B., Araújo, E. M., Machado, B. A. S., Hodel, K. V. S., & Mélo, T. J. A. It is worth mentioning that the polymer chains that form the PBAT/PLA system exhibit a dipole moment as a function of the chain configuration (slight polarity difference), which most likely contributed to the increase of the properties. This finding can be confirmed when the mechanical performance obtained for the blend in sample 7 (PLA 6% clay concentrate + PBAT) is observed individually.

Similar behavior was observed by Kumar et al.[41] for PLA/PBAT/Glycidyl methacrylate nanocomposites with 5% clay, which exhibited a significant increase in the elastic modulus, in this case the additive plastificant (glycidyl) was essential to improve these properties because it provided better compatibility the matrices PLA/PBAT. This finding confirms that a change in the morphology of the system most likely occurred. When researching PLA/PBAT and acetyl tributyl citrate blends, Coltelli et al.[48] showed that the plasticizer exhibited higher solubility in the PBAT phase because the dipole moment of this phase is on the order of 4132 Debye, while the dipole moment of the PLA is on the order of 3223 Debye.

The formation of the morphology of immiscible polymer blends is the result of interaction between process variables (temperature, deformation types and rate) and blend components properties (composition, viscosity ratio, interfacial tension, continuous phase viscosity and elasticity of the components).

Thus, the final morphology is the combination of these factors. The initial particle size, the polymer elasticity, the dispersed phase percentage and the draw ratio are the main factors affecting the morphology formed during drawing.

When the dispersed phase concentration gets close to 1:1, complex structures, such as ribbon- or sheet-like, platelet, stratified and continuous structures, are formed. However, the prevalence of one or other structure is basically controlled by factors such as the flow type and the intensity during its processing in the molten state, the viscosity ratio and the interfacial tension[49-51].

Table 2 shows the results of the Izod impact strength tests for the PLA/PBAT blend and for the PLA/PBAT/clay systems. The results showed that the blending conditions and the increased clay concentration influenced the dispersion of the clay in the blends such that the samples with 6% clay had lower impact strengths regardless of the blending sequence.

Nishida et al.[52] showed that the addition of a crosslinking agent [5-Dimethyl 2,5-di(tert-butylperoxy)hexane] in the PLA/PBAT blend increased the fracture toughness of the Izod impact tests. Furthermore, it is important to highlight that polymer research has shown that the addition of conventional reinforcements can increase the stiffness of the material and simultaneously reduce the impact strength due to nucleating properties. This affects crystal growth and crystallization also acting as a stress concentrator. Defects that begin to form around the reinforcement will quickly create cracks that will cause fracturing or failure of the material. In general, studies have shown that the greatest challenge is to modify the stiffness of the PLA by adding more flexible polymers[53]. The results of the mechanical properties of the PLA/PBAT/clay systems indicated the possibility of obtaining materials with good mechanical performance. A comparative analysis shows that it is possible to increase the mechanical properties (strength, tensile and bending elastic moduli, and impact strength) by adding organophilic clay to the PLA/PBAT system, a behavior similar to that found by Adrar et al.[54].

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3.2 X-Ray diffraction Table 3 shows the interplanar basal distances for the PLA/PBAT/clay systems as well as the d001 values of the systems in relation to the d001 distance of the organophilic clay. Increases in the d001 interlamellar spacings were observed for all the systems, which indicates the formation of nanocomposites with intercalated structures. The largest values were for samples 4, 6 and 7. These results show that the different blending conditions influenced the polymer/clay interaction and consequently the degree of exfoliation of the clay in the polymer matrix. A similar behavior was found by Nofar et al.[55], which analyzed blends of PLA with 25 wt% PBAT containing 1 and 5 wt% Cloisite 30B nanoclay that were prepared using an internal batch mixer with three mixing strategies. However, they conclude that this difference is not significant.

3.3 Scanning electron microscopy and transmission electron microscopy SEM observations were performed for the morphological analyses of the PLA/PBAT/clay systems, and photomicrographs of the fracture surface of the specimens were obtained.

These photomicrographs indicate that the blending sequence is directly responsible for the morphological changes in the bionanocomposites that formed. Possible clay clusters are observed in these photomicrographs (circled).

Figures 2 to 6 show photomicrographs of the different PLA/PBAT/clay systems at concentrations of 3% and 6%.

Table 2. Impact properties of the PLA/PBAT blend and the PLA/PBAT/clay systems.

Samples 1 2 3 4 5 6 7 8 9

PLA + PBAT pure blend PLA + PBAT blend + 3% clay PLA + PBAT blend + 6% clay PBAT 3% clay concentrate + PLA PBAT 6% clay concentrate + PLA PLA 3% clay concentrate + PBAT PLA 6% clay concentrate + PBAT PLA + PBAT + 3% clay PLA + PBAT + 6% clay

Izod Impact Strength (KJ/m2) 5.7 ± 0.5 5.8 ± 0.2 4.0 ± 0.3 6.5 ± 0.1 5.8 ± 0.2 4.8 ± 0.3 3.9 ± 0.2 5.2 ± 0.2 4.0 ± 0.1

The reported data represents arithmetic mean values and the error bars refer to the standard deviation of the mean.

Table 3. Interplanar basal distances for the PLA/PBAT/clay systems and the changes in the systems with organophilic clay.

System Evaluated 1 2 3 4 5 6 7 8 9

PLA + PBAT pure blend PLA + PBAT blend + 3% clay PLA + PBAT blend + 6% clay PBAT 3% clay + PLA PBAT 6% clay + PLA PLA 3% clay + PBAT PLA 6% clay + PBAT PLA + PBAT + 3% clay PLA + PBAT + 6% clay

*d001(Å)

**Δd (Å)

40.84 41.86 45.04 39.05 45.04 42.84 40.84 42.40

4.16 5.18 8.36 2.37 8.36 6.16 4.16 5.72

*d001 of 1346 (OMMT) clay = 36.68(Å); **Δd (Å) = changes in the interplanar basal distances (system - clay).

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Bionanocomposites of PLA/PBAT/organophilic clay: preparation and characterization

Figure 2. (a) SEM of the PLA/PBAT blend at 100X and (b) 1000X.

Figure 3. SEM at 1000X magnification of (a) PLA/PBAT blend + 3% clay and (b) PLA/PBAT blend + 6% clay.

Figure 4. SEM at 500X magnification of (a) PBAT 3% clay concentration + PLA and (b) PBAT 6% clay concentration + PLA. Polímeros, 29(3), e2019045, 2019

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Barbosa, J. D. V., Azevedo, J. B., Araújo, E. M., Machado, B. A. S., Hodel, K. V. S., & Mélo, T. J. A.

Figure 5. SEM at 500X magnification of (a) PLA 3% clay concentration + PBAT and (b) PLA 6% clay concentration + PBAT.

Figure 6. SEM at 500X magnification of (a) PLA + PBAT + 3% clay concentration and (b) PLA + PBAT + 6% clay concentration.

Figure 2 shows photomicrographs of the PLA/PBAT 50/50 blend.

A distinct morphology is observed that is characteristic of an immiscible blend, where circular PBAT particles are dispersed in the PLA matrix. Kumar et al.[41] and Zhang & Sun[56] showed similar morphologies of blends composed of 70%PLA/30%PBAT. According to Zhang & Sun[56], using glycidyl methacrylate as a compatibilizer resulted in better blend compatibility and dispersion.

The morphology of nanocomposites with intercalated and/or exfoliated structures cannot be observed by SEM; however, as a preliminary investigation, it is possible to evaluate the degree of dispersion of the clays. Therefore, the following photomicrographs present the morphologies of the PLA/PBAT/clay systems under different blending conditions as well as the degree of dispersion of the clays.

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Freitas et al.[57] demonstrated that PLA/PBAT blends containing MMT showed a dispersed phase covered by the matrix and Adrar et al.[54] observed that fibrillar morphologies in the OMMT-PLA/PBAT blends. This morphology change may be an indication of an improvement of PLA/PBAT blending miscibility after the incorporation of OMMT. Furthermore, the morphologies indicated that adding clay caused a change in the PLA and/or PBAT crystallization.

Research by Xiao et al.[58] on the kinetics of PLA crystallization showed that the PLA chain is sensitive to the presence of another phase and to the processing conditions. Therefore, we can conclude that adding clay disturbed the PLA/PBAT/clay system, which modified the growth of the spherulitic crystals and hindered the organization of the PLA chains.

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Bionanocomposites of PLA/PBAT/organophilic clay: preparation and characterization

Figure 7. TEM of the PLA 6% clay concentrate + PBAT sample.

Correlating the results of the mechanical properties and the morphologies of the PLA/PBAT/clay systems showed that the higher values of the properties occurred in the samples that exhibited better PLA/PBAT/clay compatibility. For example, the morphology of sample 7 (PLA 6% clay concentrate + PBAT), which is shown in Figure 5b, shows the formation of a very dispersive PBAT phase and small clay clusters impregnated in the PBAT/PLA matrix, which caused its high mechanical performance.

Based on the mechanical properties, XRD and SEM properties of sample 7 (PLA 6% clay concentrate + PBAT) and taking into account the TEM data in Figure 7, good dispersion of the PBAT phase and the clay in the PLA matrix with partially intercalated regions was observed.

This finding probably contributed to the good performance of these properties.

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4. Conclusions The current study investigated the influence of the different conditions blending (contend) 50%PLA/50%PBAT with 3 and 6% clay. In this study was prepared 8 samples PLA/PBAT/Clay bionanocomposites and analyzed mechanical properties and morphology. The addition of clay associated with the sequence used caused a change in the PLA/PBAT blends morphology, resulting in changes in the mechanical properties.Furthermore, the results transmission microscopy (TEM) to sample with PLA 6% clay concentrate + PBAT (sample 7) showed regions intercaled PBAT and clay in the PLA phase. The results may contribute to a better elucidation regarding the characteristics of the PLA/PBAT and OMMT blends, since the two polymers have gained great importance in the area of materials and with the addition of OMMT their properties can be improved.

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5. Acknowledgements The authors would like to thank the National Service for Industrial Training (Serviço Nacional de Aprendizagem Industrial) – SENAI (Bahia - Brazil), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) and CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior).

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Received: Jan. 29, 2019 Revised: Aug. 28, 2019 Accepted: Sept. 21, 2019

Polímeros, 29(3), e2019045, 2019

Polímeros VOLUME XXVIII - Issue II - Apr./May, 2018

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