Polímeros: Ciência e Tecnologia (Polimeros) 4th. issue, vol. 30, 2020

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VOLUME XXX - Issue IV - Oct./Dec., 2020

No treatment

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Banana fiber untreated (UTBF) and treated (TBF)

SEBS (copolymer TPE)

K-Mixer

São Paulo 994 St. São Carlos, SP, Brazil, 13560-340 Phone: +55 16 3374-3949 Email: abpol@abpol.org.br 2020

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Banana fiber SEBS

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

P o l í m e r o s - I ss u e I V - V o l u m e X X X - 2 0 2 0 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 a n d ­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 ”

Polímeros E d i t o r i a l C o u nci l

Editorial Committee

Antonio Aprigio S. Curvelo (USP/IQSC) - President

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

Members Adhemar C. Ruvolo Filho (UFSCar/DQ) Ailton S. Gomes (UFRJ/IMA) Alain Dufresne (Grenoble INP/Pagora) 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) 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) Luiz Antonio Pessan (UFSCar/DEMa) Luiz Henrique C. Mattoso (EMBRAPA) Marcelo Silveira Rabello (UFCG/UAEMa) 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) 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

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: December 2020

Financial support:

Available online at: www.scielo.br

Polímeros / Associação Brasileira de Polímeros. vol. 1, nº 1 (1991) -.- São Carlos: ABPol, 1991Quarterly v. 30, nº 4 (Oct./Dec. 2020) ISSN 0104-1428 ISSN 1678-5169 (electronic version)

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

1. Polímeros. l. Associação Brasileira de Polímeros.

Polímeros, 30(4), 2020

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

O r i g in a l A r t ic l e Mercerization effect on the properties of LDPE/PHB composites reinforced with castor cake Marisa Cristina Guimarães Rocha, Nancy Isabel Alvarez de Acevedo, Carlos Ivan Ribeiro de Oliveira, Maira Cunha Sanches and Natália Nogueira Coelho.................................................................................................................................................................................. 1-9

Influence of Nanoclay on the technical properties of Glass-Abaca hybrid Epoxy composite Sudhagar Manickam, Thanneerpanthalpalayam Kandasamy Kannan, Benjamin Lazarus Simon, Rajasekar Rathanasamy, and Sachin Sumathy Raj.......................................................................................................................................................................................... 1-7

Reactive processing of maleic anhydride-grafted ABS and its compatibilizing effect on PC/ABS blends Erick Gabriel Ribeiro dos Anjos, Juliano Marini, Larissa Stieven Montagna, Thaís Larissa do Amaral Montanheiro and Fabio Roberto Passador.................................................................................................................................................................................... 1-8

The incorporation of untreated and alkali-treated banana fiber in SEBS composites Letícia Cuebas, José Armando Bertolini Neto, Renata Tâmara Pereira de Barros, Alexandre Oka Thomaz Cordeiro, Derval dos Santos Rosa and Cristiane Reis Martins........................................................................................................................................ 1-9

The effect of extrusion processing on the physicochemical and antioxidant properties of fermented and non-fermented Jabuticaba pomace Eduardo Ramirez Asquieri, Jose de Jesus Berrios, Elaine Meire de Assis Ramirez Asquieri, James Pan, Aline Gomes de Moura e Silva and Rayssa Dias Batista.......................................................................................................................................................................... 1-8

Mechanical and water absorption properties and morphology of melt processed Zein/PVAl blends Sandro Junior Vessoni Torres, Gabriela Brunosi Medeiros, Francisco Rosário, Fabio Yamashita, Luiz Henrique Capparelli Mattoso and Elisângela Corradini........................................................................................................................................................................................ 1-8

Melt-mixed nanocomposites of SIS/MWCNT: rheological, electrical and structural behavior Ludimilla Barbosa Ferreira, Rayane de Souza Fernandes, Rosario Elida Suman Bretas and João Paulo Ferreira Santos............................ 1-9

Surface functionalization of polyvinyl chloride by plasma immersion techniques Péricles Lopes Sant’Ana, José Roberto Ribeiro Bortoleto, Nilson Cristino da Cruz, Elidiane Cipriano Rangel, Steven Frederick Durrant and Wido Herwig Schreiner..................................................................................................................................................................................... 1-7

Investigating the surface performance of impregnated and varnished Calabrian pine wood against weathering Türkay Türkoğlu, Ergün Baysal, Çağlar Altay, Hilmi Toker, Mustafa Küçüktüvek and Ahmet Gündüz........................................................... 1-6

Selection of appropriate reinforcement for nylon material through mechanical and damping characteristics Hari Bodipatti Subburamamurthy, Rajasekar Rathanasamy, Harikrishna Kumar Mohan Kumar, Moganapriya Chinnasamy, Gobinath Velu Kaliyannan and and Saravanan Natarajan.............................................................................................................................. 1-8

Commercial and potential applications of bacterial cellulose in Brazil: ten years review Luiz Diego Marestoni, Hernane da Silva Barud, Rodrigo José Gomes, Rebeca Priscila Flora Catarino, Natália Norika Yassunaka Hata, Jéssica Barrionuevo Ressutte and Wilma Aparecida Spinosa........................................................................................................................ 1-19

Antibacterial activity of polypyrrole-based nanocomposites: a mini-review Fernando Antonio Gomes da Silva Júnior, Simone Araújo Vieira, Sônia de Avila Botton, Mateus Matiuzzi da Costa and Helinando Pequeno de Oliveira........................................................................................................................................................................ 1-9 Cover: Composites preparation: (a) Banana fiber extraction, (b.1) Banana fiber untreated and (b.2) alkali-treated, (c) Knife milling of fibers, (d) K-Mixer processing, (e) Production of plates banana fiber-TPE composites in the compression molding. Arts by Editora Cubo.

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Polímeros, 30(4), 2020

Sabic and Plastic Energy Set to Start Construction of Pioneering Advanced Recycling Unit to Increase Production of Certified Circular Polymers In another significant contribution towards the development of a circular economy for plastics, SABIC and Plastic Energy are set to commence construction on the first commercial unit to produce its flagship certified circular polymers, part of the TRUCIRCLE™ portfolio, which are made from the upcycling of mixed and used plastic.

SABIC, a global leader in diversified chemicals, along with partner Plastic Energy, a pioneer in chemical plastics recycling, are set to start the construction phase for the unit, which will be based in Geleen, the Netherlands and is expected to become operational in the second half of 2022. The project will be realized under a 50-50 joint venture called SPEAR (SABIC Plastic Energy Advanced Recycling BV) and is being executed with a Top Sector Energy Subsidy from the Ministry of Economic Affairs in the Netherlands.

As part of the project’s market foundation stage, SABIC has worked together with Plastic Energy and leading customers and converters to produce and commercialise certified circular polymers since early 2019. The new unit will enable SABIC to significantly upscale the production of certified circular polymers to provide customers with greater access to sustainable materials which have been recycled, repurposed and produced in a way that can help protect our planet’s natural resources, whilst acting as a drop-in solution.

‘Advancements in this pioneering project take us one step closer to driving the change needed to become a circular global industry,’ said Fahad Al Swailem, Vice President, PE & Sales at SABIC. ‘We have overcome significant external, global challenges to reach this important milestone and remain fully committed to closing the loop on used plastic. We are continuing to collaborate on an unprecedented level with our partners upstream and downstream to achieve this.’

‘It has been an exciting journey in making our vision of building advanced recycling plants come to life, and we are delighted to announce the construction of this new facility with SABIC,’ said Carlos Monreal, Founder and CEO of Plastic Energy. ‘We have worked jointly with SABIC towards our common goal of making plastics more sustainable and moving towards a more circular economy for plastics.”

SABIC’s certified circular polymers are produced using Plastic Energy’s advanced recycling technology to convert low quality, mixed, and used plastic, otherwise destined for incineration or landfill, into TACOIL. The TACOIL produced in the new commercial unit will be used by SABIC in their production process as an alternative to traditional fossil materials to create new circular polymers.

The circular polymers form part of SABIC’s TRUCIRCLE portfolio and services for circular innovations. Launched in 2019, the TRUCIRCLE portfolio is a considerable milestone on the journey towards closing the loop and creating a circular economy for plastics and intends to provide manufacturers with access to more sustainable materials. The TRUCIRCLE portfolio spans design for recyclability, mechanically recycled products, certified circular products from feedstock recycling of used plastic and certified renewables products from bio-based feedstock.

Source: Sabic - www.solvay.com

Polímeros, 30(4), 2020

Braskem develops unique methodology for creating more sustainable packaging In order to help clients, brand owners and design professionals in all sectors to find solutions to the challenges of plastic packaging, Braskem has launched its own development methodology to help companies achieve their 2025 and 2030 sustainability commitments. This unique tool was developed for an open support service to the entire supply chain, by expanding deliveries to the circular economy packaging.

The methodology is based on the Design for Environment (DfE) concept, aimed at improving the environmental impact of packaging, creating a new way of conceiving these projects, from format selection and composition of material to definition of circular paths, consumer engagement and product end-of-life solution. The intended goal is to ensure this packaging follows the circular economy model and contributes towards driving solutions for the reuse, refill and recycling process, reducing environmental impact and generating value for the entire chain.

The differential in the company’s proposal is the use of a multidisciplinary and systems process focused on the challenges faced by each client, which goes through pre-set and well-defined phases: mapping of impacts, such as using the Life Cycle Assessment tool (LCA) and assessment of product journey, including circular paths; research to understand consumption trends and social behavior; definition of general application; design, manufacture and, finally, product launch.

In addition to this new methodology is all the expertise that Braskem has in circular economy and thermoplastic resins, accumulated over the years in servicing the market focused on innovation and sustainability.

“Considering that up to 80% of the environmental impact is set in the design phase, we believe it is essential that we help our partners and clients in creating more circular solutions that will help them achieve their sustainability goals for the coming years. This is the main focus of Braskem’s own methodology for developing more sustainable packaging” says Américo Bartilotti, Director of Packaging and Consumer Goods Business at Braskem.

Braskem Circular Economy director for South America, Fabiana Quiroga, believes this new methodology is fertile ground for future innovations in the packaging sector. “Circular economy is a commitment undertaken by the company through which all our internal initiatives are considered. Offering this Design for Environment support service to our partners makes the links of the plastics chain stronger as a whole and promotes this economic model as a tangible and feasible concept to all players.”

Braskem’s exclusive practices for circular packaging development are the result of more than one year’s research and application of the Design for Environment (DfE) concept on different fronts. One of them is Braskem Design Challenge, a program in which the company’s partners are invited to present an actual challenge so that designers, whether students or newly graduated professionals, develop an idea for solving that challenge using concepts such as Circular Economy, Design for Environment and Life Cycle Assessment. Braskem and the invited partner provide mentoring and guidance to those selected.

Source: Braskem - www.braskem.com

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June Gordon Research Conference — Polymers Date: June 5-6, 2021 Location: South Hadley, United States Website: www.grc.org/polymers-conference/2021 Biopolymer – Processing & Moulding Date: June 15-16, 2021 Location: Halle (Saale), Germany Website: polykum.de/en/biopolymer-mkt-2021 RosUpack - 25th International Exhibition for the Packaging Industry Date: June 15-18, 2021 Location: Moscow, Russia Website: www.rosupack.com Gordon Research Conference — Polyamines Date: June 27 - July 2, 2021 Location: Waterville Valley, United States Website: www.grc.org/polyamines-conference/2021

July 2nd PHA platform World Congress Date: July 06–07, 2021 Location: Cologne, Germany Website: www.bioplasticsmagazine.com/en/event-calendar/ termine/2nd-pha-world-congress-2020 25th IUPAC International Conference on Physical Organic Chemistry Date: July 10–15, 2021 Location: Hiroshima, Japan Website: icpoc25.jp 84th Prague Meeting on Macromolecules — Frontiers of Polymer Colloids Date: July 18–22, 2021 Location: Prague, Czech Republic Website: mns-20.com 2nd International Conference on Materials and Nanomaterials (MNs-20) Date: July 26–28, 2021 Location: Rome, Italy Website: www.imc.cas.cz/sympo/84pmm

August International Conference on Electronic Materials (2021 IUMRS-ICEM) XIX Brazilian Materials Research Society Meeting (XIX B-MRS) Date: August 30 – September 3, 2021 Location: Foz do Iguaçu, Brazil Website: www.sbpmat.org.br/19encontro

September Global Summit and Expo on Materials Science and Nanoscience Date: September 6-8, 2021 Location: Lisbon, Portugal Website: www.thescientistt.com/materials-science-nanoscience CIRM – Workshop — Directed Polymers and Folding Date: September 6-10, 2021 Location: Marseille, France Website: conferences.cirm-math.fr/2021-calendar.html 100 Years Macromolecular Chemistry Date: September 12-14, 2021 Location: Freiburg, Germany Website: veranstaltungen.gdch.de/tms/frontend/index. cfm?l=9162&sp_id=2

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7th Edition of International Conference on Polymer Science and Technology Date: September 13-14, 2021 Location: Berlin, Germany Website: polymerscience.euroscicon.com/ Fluoropolymer 2021 Date: September 19-22, 2021 Location: Denver, United States Website: www.polyacs.net/21fluoropolymer 9th International Conference on Fracture of Polymers, Composites and Adhesives Date: September 26-30, 2021 Location: Les Diablerets, Switzerland Website: www.elsevier.com/events/conferences/esistc4conference 36th International Conference of the Polymer Processing Society Date: September 26-30, 2021 Location: Montreal, Canada Website: www.polymtl.ca/pps-36/en 13th PVC Formulation Date: September 27-29, 2021 Location: Cologne, Germany Website: www.ami.international/events/event?Code=C1104

October International Conference on Materials Science and Engineering Date: October 11-14, 2021 Location: Brisbane, Australia Website: www.materialsconferenceaustralia.com Sustainable Polymers Date: October 17-20, 2021 Location: Safety Harbor, United States Website: www.polyacs.net/21sustainablepolymers 16th Brazilian Polymer Conference — (16thCBPol) Date: October 24-28, 2021 Location: Ouro Preto, Brazil Website: www.cbpol.com.br

November Performance Polyamides USA Date: November 2, 2021 Location: Cleveland, United States Website: www.ami.international/events/event?Code=C1149 Plástico Brasil Date: November 8-12, 2021 Location: São Paulo, Brazil Website: www.plasticobrasil.com.br Controlled Radical Polymerization Date: November 14-17, 2021 Location: Charleston, United States Website: www.polyacs.net/crp2021 Multilayer Flexible Packaging Date: November 23-25, 2021 Location: Barcelona, Spain Website: www.ami.international/events/event?Code=C1147 16th European Bioplastics Conference Date: November 30 – December 1, 2021 Location: Berlin, Germany Website: www.european-bioplastics.org/events/eubp-conference

December Polymers in Flooring Date: December 9-10, 2021 Location: Berlin, Germany Website: www.ami.international/events/event?Code=C1150

Polímeros, 30(4), 2020

ABPol Associates Sponsoring Partners

Polímeros, 30(4), 2020

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Polímeros, 30(4), 2020

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.07720

Mercerization effect on the properties of LDPE/PHB composites reinforced with castor cake Marisa Cristina Guimarães Rocha1* , Nancy Isabel Alvarez de Acevedo1, Carlos Ivan Ribeiro de Oliveira1, Maira Cunha Sanches1 and Natália Nogueira Coelho1 Universidade do Estado do Rio de Janeiro – UERJ, Rio de Janeiro, RJ, Brasil

1

*mrocha@iprj.uerj.br

Abstract The aim of this work was to investigate the effects of mercerization on the structure of castor oil cake (CC) and on the tensile properties of LDPE/PHB/CC composites. To achieve this goal, the fibers were treated with NaOH solutions (5 and 10 wt%). Characterization techniques such as: scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to investigate the structure of modified fibers. The composites were processed in a Haake mixer. Tensile tests of the composites were performed according to ASTM D638 standard. The analyzes revealed that mercerization promoted a partial conversion of cellulose I into cellulose II. Mercerization performed with 5% NaOH solution improved the tensile properties of the LDPE/PHB/CC composites, which were superior to those obtained with the 10% NaOH solution. This result suggests that the higher concentration of NaOH compromises the integrity of the fibers, deteriorating the mechanical properties.

Keywords: mercerization, castor oil cake, composites, fiber characterization, mechanical properties.

How to cite: Rocha, M. C. G., Acevedo, N. I. A., Oliveira, C. I. R., Sanches, M. C., & Coelho, N. N. (2020). Mercerization effect on the properties of LDPE/PHB composites reinforced with castor cake. Polímeros: Ciência e Tecnologia, 30(4), e2020037. https://doi.org/10.1590/0104-1428.07720.

1. Introduction Nowadays, the scientific community has boosted intense research and development efforts in order to exploit the potential of agricultural wastes and lignocellulosic materials as an alternative source of fibers in the polymer industry[1-3].

The reasons for this include growing interest in reducing the environment impact associated with discard of polymer-based materials; depletion of fossil fuel nonrenewable resources and better understanding of correlation between the structure and properties of natural materials[1-6].

Natural fibers are very attractive for composites due to their low-cost, biodegradable nature, low density, desirable fiber aspect ratio, minimum health hazards and excellent mechanical properties[2,3,5,7]. Besides, the agricultural wastes are renewable sources of natural fibers[1-3]. However, the use of lignocellulosic fibers as reinforcing agent for the hydrophobic matrices presents some drawbacks due to the low interfacial adhesion properties between fiber and matrix[7,8]. The obtained composite exhibits poor properties such as high uptake of moisture that leads to obtaining poor mechanical properties and low dimensional stability.

Chemical treatments on natural fiber are one of the alternative solutions to circumvent these problems[3,7].

Mercerization is a process of subjecting a vegetable fiber to an interaction with a concentrated aqueous solution of strong base, to produce great swelling, with resulting changes in the fine structure, dimension, morphology and mechanical properties[7]. The type of alkali and its

Polímeros, 30(4), e2020037, 2020

concentration have influence on the degree of swelling, and therefore, on the degree of lattice transformation from the monoclinic crystalline lattice of cellulose I, presented by native cellulose, to-cellulose II, which is an allomorph polymorph of cellulose. There is some indication that sodium hydroxide (NaOH) treatment results in a higher degree of swelling and leads to obtain the thermodynamically stable crystalline structure, cellulose II[7,8].

Besides cellulose, natural fibers consist of others constituents like hemicelluloses, lignin, pectin and impurities such as wax, ash and natural oil[7]. The removal of these substances promotes changes in the topography of the fiber surface, increased roughness and the ratio between the length and diameter of the fiber in addition to a better dispersion of the filler particles in the polymer matrix resulting in superior mechanical properties[3,8-12].

The degrees of phase conversion of cellulose depend on the concentration of sodium hydroxide, time and temperature of process[8,13]. According to some published studies, the conversion from cellulose I to II happens in NaOH solutions of concentration higher than 10%[8]. However, there are some studies showing better mechanical behavior of composites in concentrations lower than 10%[9].

The chemical composition of similar types of natural fibers, as well as of different types of natural fibers will vary for each fiber. Therefore, the optimal conditions of

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Rocha, M. C. G., Acevedo, N. I. A., Oliveira, C. I. R., Sanches, M. C., & Coelho, N. N. mercerization will be dependent on the type of lignocellulosic material[8].

In general, there is a positive effect of mercerization of natural fibers on the mechanical properties of the composites[7,10,14-17]. However, some authors reported better mechanical properties for systems containing cellulose I[8].

Castor bean (Ricinuscommunis L) is of great economic importance due to the oil extracted from its seeds[18-21]. Castor oil is a valuable raw material for biodiesel production.

Therefore, the valorization of the castor oil cake, co-product of biodiesel process production, contributes to improve the productive chain of biodiesel. Some few papers report the use of castor cake as filler for composites[22-28]. The addition of this filler seems to improve the mechanical behavior of the composites. There is also a patent claim for obtaining composite materials with enhanced biodegradability using castor oil cake as filler requested by a Brazilian Oil Industry[29].

In a previous work, blends of low-density polyethylene (LDPE) and poly(3-hydroxybutyrate) (PHB) filled with castor oil pressed cake (CC) were developed. The results showed that the addition of a certain amount of PHB or CC to LDPE makes possible obtaining LDPE based materials with increased susceptibility to biodegradation. However, the poor interfacial adhesion became visible in all samples[28].

This study aims to evaluate the effect of mercerization on the structure of castor oil cake. This investigation also seeks to assess the influence of this chemical treatment on the mechanical properties of LDPE/PHB blends filled with castor cake.

2. Materials and Methods 2.1 Materials Low density polyethylene (LDPE, TS O728), MFI = 2.20 g/10 min - ASTM D 1238, at 190 ºC, was donated by Petroquímica Triunfo S.A./BRASKEM (Brazil) and used as received. Poly(3-hydroxybutyrate) (PHB), MFI = 16 g/10 min - ASTM D 1238, at 190 ºC was obtained from PHB Industrial S.A. (Brazil) and used as received.

Bom-Brasil Óleo de Mamona Ltda. (Brazil) provided the castor oil pressed cake (CC). This material was detoxified by autoclaving at 120 ºC for 30 minutes. After a drying process, the filler was ground in a ball mill and passed through a set of sieves with a decreasing mesh size. Only the powdered material with a size less or equal to 0.25 mm was used in this work. Sodium hydroxide (NaOH) used for alkali treatment of castor oil cake was furnished by Vetec Química Fina Ltda (Brazil).

2.2 Methods The methods used in this work are described below 2.2.1 Alkali treatment of castor oil cakes The alkali treatment of castor oil cake was carried out by immersing the fibers in 5% and 10% NaOH solution respectively, for 1 hour. Subsequently, the fibers were thoroughly washed with water to remove the NaOH in excess, and kept in distilled water for 24 hours. After this time, the fibers were washed with water again to remove the 2/9

residual NaOH, and kept in distilled water for another 24 hours. This procedure was repeated until pH values of the solution equal to 7 were obtained. The fibers were then oven dried at 80 ºC for 24 hours to remove the water in excess.

2.2.2 Castor oil cake characterization Fourier transform spectroscopy (FTIR), X-ray diffraction (XRD) and Scanning electron microscopy (SEM) techniques were used to investigate the structure and morphology of the alkali treated castor oil cake.

Fourier Transform Spectroscopy (FTIR): FTIR spectra for KBr pellets containing untreated castor oil cake and alkali treated samples were recorded in the region of 600 - 4000 cm-1 with 120 scans and a 4 cm-1 resolution, using the Varian infrared spectrometer, Excalibur Series 3100, equipped with a diamond crystal attenuated total reflectance (ATR) accessory.

X-Ray Diffraction (XRD): X-Ray diffraction data of untreated castor oil cake and NaOH treated samples, in powder form, were obtained using a Rigaku, Miniflex instrument.

Ni-filtered CuKα radiation (λ = 1.542 Å) generated at a voltage of 30 kV and current of 15 mA, and a scan rate of 1º/min in the 2θ range from 2º to 40º was used.

Scanning Electron Microscopy (SEM): The morphology of treated and untreated samples of castor oil cake was examined by using a scanning electron microscope, Hitachi Benchtop model TM 3000 2.2.3 Mixtures Processing LDPE/PHB/CC, LDPE/PHB/CCM5 and LDPE/PHB/ CCM10 mixtures at 80/10/10 wt% were melt processed during 10 minutes in a Haake Rheomix OS, equipped with roller blades, at 150 ºC and 50 rpm. CCM5 means castor cake treated with 5% of NaOH and CCM10 means castor cake treated with 10% of NaOH. 2.2.4 Determination of the Composites Tensile Properties Tensile properties were measured using an EMIC Universal Test Machine, Model DL300 with a 10kN load cell.

Tests were conducted in accordance with ASTM D 638[30].

Type V test specimens were prepared using a Carver press at 69 x 10 N/m2 and 190 ºC for 15 minutes. A crosshead speed of 1 mm/min was employed. Seven (7) specimens were used to analyze each sample.

3. Results and Discussions 3.1 Characterization of the Fibers The effects of the alkali treatments on the surface of the castor oil cake were investigated by SEM (Figure 1).

All micrographs show the fibrous nature of castor oil cake.

However, the removal of hemicelluloses, lignin and wax by the alkali treatment gave rise to thinner cellulose fibers and increased roughness (Figure 1d), which may promote greater contact area between these fibers with polymer matrices. The micropores were also more visible in the samples submitted to mercerization procedure. These results are in agreement with those obtained by others researchers[2,3,8,9,11,16,17], which noted that the processes of mercerization determine the Polímeros, 30(4), e2020037, 2020

Mercerization effect on the properties of LDPE/PHB composites reinforced with castor cake character of the filler fibrillation, promoting better adhesion between fibers and polymer matrix.

Figure 2 presents the FTIR characteristic features of untreated castor oil cake (CC) and alkali treated castor cake samples: MCC5 – 5% NaOH solution; MCC10 – 10% NaOH solution.

The infrared indices (IR indices), (ratio of IR absorption band intensity at a given wavenumber to that at the reference wavenumber)[31], used in this work were based on the absorbance peaks at 1059 cm-1 (>CO/C-C/COH stretching vibration). These IR indices were chosen to analyse the possible changes of cellulose conformation, as well as the intensity changes of OH streching vibration and hydrogenbonds due to the alkali treatment of the castor cake. However, curve-fitting deconvolution methods must be used in order to avoiding erroneous results[32].

It is well known that absorbance and/or wavenumber of most of the FTIR characteristic bands of cellulose changes with the alkali treatment[33]. The FTIR spectra of the samples under study provide some evidence that the fibers mercerization process has promoted the removal of lignin and hemicellulose.

A band at 2858 cm-1 observed in the spectrum of raw castor cake and in the spectrum of the castor oil cake treated with 5% of NaOH is attributed to the –OH stretching vibration of inter and intramolecular H-bonding present in the cellulose, hemicellulose and lignin molecules of castor oil cake making the resulting structure as a network. This strucure was gradually destroyed and disappered, when a 10% alkali concentration was used in the mercerization process.

Similar finding was obtained by Das and Chakraborty[33] in their study of the effect of alkali treatment on the strucure and morphology of bamboo fibers.

Figure 1. SEM micrographs of (a) untreated castor oil cake; (b) castor oil cake treated with 5% NaOH solution; (c) castor oil cake treated with 10% NaOH solution; and (d) thinner cellulose fibers in the castor oil cake treated with 5% NaOH solution.

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Rocha, M. C. G., Acevedo, N. I. A., Oliveira, C. I. R., Sanches, M. C., & Coelho, N. N.

Figure 2. FTIR spectra of the materials under study: CC, MCC5 and MCC10.

The peak at 1639 cm-1 may be ascribed to the absorbed water [33]. This band may also be attributed to the carboncarbon streching vibrations in the aromatic ring and may be superimposed with the absorption related to the carbonyl group of ester linkage of lignin[31,34]. The intensity of this peak decreased as the alkali concentration increased. Castor oil cake is dark brown. As the fibers were submitted to the mercerization processes, the color of the castor cake became light brown due to the partially removal of lignin.

Therefore, the decrease of the intensity of 1639 cm-1 may be attributed to the predominant effect of the removal of lignin due to the alkali treatment.

The IR index for the absorption band at 596 cm-1 is 0.58 for the raw castor cake sample. The IR indices at 5 and 10% alkali treatment are respectively, 0.56 and 0.62.

The intensity of this absorption band is influenced by the torsional vibration of pyranose ring. Das and Chakraborty[33], in a study of treatment of bamboo fibers with alkali attributed the increase in the IR index value with the increasing of alkali concentration to the removal of cementing material from bamboo fibers, which allows the pyranose ring to undergo more torsional vibration.

Lignin and hemicellulose work on ligocellulosic fibers as a natural barrier for the protection of cellulose. The removal of these substances promotes greater accessibility to cellulose enhancing mechanical anchorage with the polymer matrix.

There is no evidence of absorption bands due to the CO stretching of carboxilic acids or ester in the samples under study (1730-1734 cm-1). This result suggests that there is no hemicellulose in the castor oil cake or its concentration is very low.

It is worth noting that an unexpected result was obtained.

The FTIR pattern of raw castor cake shows strong broad 4/9

peak at 3399 cm-1, which is characteristic of the OH streching vibrations. The FTIR indices for this vibration mode corresponding to the CC, MCC5 and MCC10 samples were 1.2, 1.0, and 0.9, respectively. According to Das and Chakraborty[33], the increase of NaOH concentration should promote the removal of more and more alkali sensitive material. Therefore, an increase in the number of free OH groups with the increasing of alkali concentration was expected[33]. This result may be attributed to the poor resolution of the OH region. Deconvolution methods must be used in order to obtain a more reliable result.

The FTIR spectra also provide some evidence that the fibers alkali treatment led to the conversion of celullose I into the cellulose II. Figure 2 shows the maximum absorbance of the hydrogen-bonded OH streching band was shifted to a higher wavenumber, from 3399 to 3408 cm-1 at 10 wt% of NaOH. The CH streching mode was shifted to a lower wavenumber (from 2928 to 2924 cm-1). Similar results were obtained by Oh et al.[31]. These researchers noticed the absorbance of the hydrogen-bonded OH streching band shifted from 3352 to 3447 cm-1, when the cellulose was treated with 20 wt% NaOH. This result was attributed to the intramolecular bonding of O(2)H…O(6) of cellulose II, allomorph resulting from the mercerization process. According to Lee et al.[35], the characteristic peak of celullose I is at 3270 cm-1. Cellulose II presents two weak additional peaks at 3450 cm-1 and 3480 cm-1.However, it is difficult to identify these weak absorption bands by FTIR as the OH region is broad and poorly resolved.

The band at 901 cm-1 observed in the spectrum of the raw castor cake is attributed to the β-glucosidic linkage. This absorption band shifts to a lower wavenumber, 897 cm-1, in Polímeros, 30(4), e2020037, 2020

Mercerization effect on the properties of LDPE/PHB composites reinforced with castor cake the spectra of alkali treated fibers. The IR index for the 901 cm-1 with respect to the peak at 1059 cm-1 increases slightly as the alkali concentration increased. In the case of untreated cake, where cellulose occurs as the cellulose I lattice, the IR index is found to be 0.29. However, treating the castor cake with 5% and 10% NaOH solution increased slightly the IR values, from 0.29 to 0.33 and 0.45, respectively. These results suggest that there was some change in the lattice structure, when the castor oil cake is treated with 5% and 10% NaOH solution. According to some published studies, conformational changes occurring during decrystallization/conversion of cellulose I to cellulose II may promote alterations in the intensity and sharpness of the FTIR β-glucosidic linkage absorption band[31,34].

Cellulose I occurs in two allomorphic forms ( Iα and I β ).

The cellulose I β is the allomorph predominantly found in higher plants[36]. According to French[37], the diffractogram of cellulose I β presents three main peaks related to the crystallographic planes specified by Miller indices of ( 110 ), (110 ) and ( 200 ). These peaks occurs at 2θ = 15º, 17º and 23º, respectively. The diffractogram of cellulose II also presents three main peaks, located at 2θ around 12º, and 22º, corresponding to the crystallographic planes ( 110 ), (110 ) and (020 )[37,38].

The X-ray diffraction (XRD) pattern of lignocellulosic materials shows characteristic peaks around 16.0º, 22.5º and 34.5º[39,40]. The first two peaks are typical of cellulose I[39,40] and the last is attributed to the ( 004 ) crystallographic plane of cellulose I[41]. French[37] describes this peak as a composite of various peaks where the ( 004 ) crystallographic plane is not the predominant contributor.

Literature data show, that when there is a large amount of amorphous materials in the lignocellulosic fibers, the

peaks corresponding to the planes ( 110 ) and ( 110 ) appear as a single broad peak[42,43].

Figure 3 shows X-ray diffraction (XRD) patterns of untreated castor cake (CC) and alkali treated castor cake samples: MCC 5 and MCC10.

The XRD pattern of the raw castor cake (CC) shows five diffraction peaks: 15.5º, 19.5º, 20.8º, 21.4º and 29.5º. The first peak is attributed to the crystallographic planes ( 110 ) and ( 110 ) of cellulose I and the peak at 2θ of 21.4º is assigned to an ordered crystalline arrangement involving intra- or intermolecular hydrogen bond of cellulose. Guimarães et al.[44] found a peak at 2θ of 21º in the diffractogram of a castor oil cake. This peak was attributed to an intramolecular bonding of the amino proteins and, or cellulose, but it can be attributed to the crystallographic plane ( 021) of cellulose I[45]. Although, the presence of the characteristic peaks of cellulose I was detected, evidently, no peak corresponding to the ( 200 ) plane of cellulose I was observed. The presence of silica (quartz), which is often used as a fertilizer in agriculture, is evidenced by the peaks at 2θ of 20.8º and 26.6º [46].

According to Sánchez-Cantu et al.[47], the peak at 2θ of 19.5º is a characteristic signal of crystalline hemicellulose and the peak at 2θ of 29.5º is a representative reflection of magnesium calcium carbonates, compounds identified in oil extraction residues.

The mercerization of the castor oil cake caused visible changes in the X-ray pattern The MCC5 besides the peaks attributed to silica and magnesium calcium carbonates shows the characteristic peaks of cellulose I at 2θ of 16.5º and 22.5º corresponding to the ( 110 ) and ( 200 ) planes, respectively[38]. The peak at 2θ of 21.8º is attributed to the (020 ) plane of cellulose II[38]. The presence of one doublet

Figure 3. XRD patterns of untreated castor cake (CC) and MCC5 and MCC10 samples.

Polímeros, 30(4), e2020037, 2020

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Rocha, M. C. G., Acevedo, N. I. A., Oliveira, C. I. R., Sanches, M. C., & Coelho, N. N. at 2θ around 20º and 22º indicates the partial conversion of cellulose I to II[41,48]. The peak at 2θ of 12° is not always found due to the noise of the experiment[45]. The evidence of changing from cellulose I to cellulose II due to the mercerization process is also corroborated by the FTIR spectrum. Sample MCC10 presents a peak at 2θ of 22.4º that can be attributed to the ( 020 ) plane of cellulose II and to the ( 200 ) plane of cellulose I[37,38]. In our view, the mercerization process of castor cake in the experimental conditions used in this work promoted the partial conversion of cellulose I to cellulose II. The apparent XRD crystallinity index, CrI ( % ), of CC, MCC5 and MCC10 was calculated using the following Equation 1[49]: CrI= (%)

Acryst Atotal

× 100

(1)

where Acryst is the sum of crystalline band areas and Atotal is the total area under the diffractogram. The areas were obtained by using the Origin Lab 8.5 software.

The values obtained were 34.5%, 34.9% and 48.2% for CC, MCC5 and MCC10 samples, respectively.

Cellulose II is more reactive and more accessible to reagents than cellulose I due to its lower crystallinity, since there is an increased number of inter- and intra-planar hydrogen bonds compared to cellulose I[49]. However, the removal of lignin promotes the increase of crystallinity of the samples.

3.2 Processing and Characterization of the Composites The torque rheometer is commonly used in polymer processing laboratories as it can simulate mixing and shaping processes in nearly real processing conditions[50]. For a pure

polymer if the torque values were corrected to minimize the effect of the viscous dissipation, the steady-state torque can be related to the viscosity data obtained from capillary and parallel plate rheometers[51]. Here, as mentioned before, the torque rheometer was used to produce the LDPE/PHB/CC, LDPE/PHB/CCM5 and LDPE/PHB/CCM10 composites at 80/10/10 wt%. Figure 4 shows the torque versus time curves of the LDPE and those of the composite materiais.

Figure 4 also shows the torque versus time curve of LDPE/ PHB (90/10) blend. All curves have a similar profile. High torque values are initially observed, as the initial minutes of mixing involve the shearing of solid materials in addition to the effect of temperature. This continuos shearing action promotes the melting of the polymers. A lower resistance to rotor´s shearing action is observed, leading to lower torque values, which decrease with the stabilization of the the melting process.Then, a stable torque is obtained [52].

A slight increase in stable torque is observed when 10 wt% of PHB is added to LDPE. Some published studies on blends of LDPE/PHB and LLDPE/PHB show that the incorporation of 10 wt% of PHB into the polymer matrices does not cause significant changes in the values of melt flow index (MFI) of polyolefins[53,54]. Karami et al. [55] found that the complex viscosity of the LDPE samples with less than 50 wt% of PHB content were greater than that of the neat LDPE. However, this work does not give any indication as to whether the Cox-Merz rule is valid for the LDPE/PHB samples. This finding requires further investigation. Some interaction between these two polymers may be taking place under these experimental conditions.

In general, the mercerization of natural fibers promotes the obtaining of better mechanical properties due to the

Figure 4. Torque versus time curves of LDPE, LDPE/PHB blend, and LDPE/PHB/CC and LDPE/PHB/MCC composites.

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Polímeros, 30(4), e2020037, 2020

Mercerization effect on the properties of LDPE/PHB composites reinforced with castor cake Table 1. Elastic modulus and Tensile strenght of the LDPE/PHB/CC and LDPE/PHB/MCC composites.

Composites LDPE/PHB/CC LDPE/PHB/MCC5 LDPE/PHB/MCC10

Elastic modulus (MPa) 81.8 ± 5.3 199.1 ± 22.4 100.5 ± 14.9

modification of surface topography and diameter of the fibers resulting in improved interfacial adhesion properties[7,10,14].

Table 1 shows elastic modulus and tensile strength of the LDPE/PHB composites filled with the untreated castor oil cake (CC) and with the alkali treated fibers (MCC5 and MCC10).

The mercerization process promotes an increase of the surface area of the fiber contact with the polymeric matrix due to the increase in the aspect ratio and the roughness of the fiber. As a result, in general, the incorporation of mercerized fibers into the polymeric matrices increases the elastic modulus[8-12]. Table 1 shows that the mercerization process of the castor oil cake leads to the increasing of the tensile elastic modulus of the composites. However, this effect is more evident, when the fiber was treated with 5% NaOH solution. According to Kabir et al.[49], if the alkali concentration is higher than the optimum condition, an excess of delignification may occur, which weakens or damages the fibers. In the present work, as the concentration of NaOH increased from 5% to 10%, the fibers became whiter due to the more efficient removal of lignin.

The tensile strength is a function of the surface area of the fiber`s contact with the polymer matrix and the interfacial adhesion. In general, the hydrophilic nature of the fibers hinders its effective interaction with hydrophobic polymeric matrices. The mercerization process removes lignin and hemicelluloses from the surface of the fiber improving the interactions between the fiber and the polymer matrices[8].

In the treatment of the fibers with NaOH, there is a reaction between the hydroxyl groups (OH) of the fiber with the alkaline solution (Equation 2)[56], which reduces the water absorption of the fibers and improves the compatibility between them and the polymeric matrices[10]. As a result, there is an increase in the tensile strength.

Fiber − O − H + NaOH → Fiber − O + Na − + H 2O (2)

Table 1 shows that fiber mercerization with 5% NaOH solution generates higher tensile strength values of composites. However, the alkali treatment of the fibers with 10% NaOH solution does not promote a significant increase in the tensile strength.

There is some controversy about the effect of the mercerization of the fibers on the mechanical behavior of the composites obtained[8]. Some authors attributed the decrease in the tensile strength of the composites developed with alkali treated fibers to the changes from crystalline cellulose I into cellulose II, which is less crystalline than cellulose I[57]. In the present work, the mercerization process with both concentrations of NaOH solution results in the partial formation of cellulose II as evidenced by FTIR data.

However, the treatment of the fibers with 10% NaOH solution increases significantly the apparent XRD crystallinity index of the castor oil cake. This increase is due to the more efficient Polímeros, 30(4), e2020037, 2020

Tensile strenght(MPa) 4.21 ± 0.38 6.55 ± 0.24 4.50 ± 0.7

removal of lignin, waxes, and other impurities. Therefore, the partial breakdown of the fibers caused by the higher concentration of NaOH may be responsible by the lower tensile strength values of the composite filled with MCC10.

4. Conclusions In this work, the effects of mercerization on castor oil cake structure and tensile properties of LDPE/PHB/castor oil cake composites were investigated. FTIR and XRD data indicated that there was a partial formation of cellulose II, as well as the removal of lignin and other impurities. The morphological analysis of the mercerized fibers by SEM showed that the extraction of hemicelluloses, lignin and waxes gave rise to thinner cellulose fibers and increased roughness that in turn may promote greater contact area between these fibers with the polymeric matrix. The mercerization process performed with 5% NaOH solution improved the tensile properties of the LDPE/PHB/castor cake composites. The elastic modulus and tensile strength values of composites filled with the fibers treated with 5% NaOH solution were superior to those obtained with 10% NaOH solution. This result suggests that the higher NaOH concentration compromises the integrity of the fibers, deteriorating the mechanical properties.

5. Acknowledgments This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brazil (CAPES) – Finance Code 001. We also thank Conselho Nacional para o Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) for financial support and BRASKEM, PHB Industrial S.A. and Bom-Brasil Óleo de Mamona Ltda. for providing the LDPE resin, the PHB and castor oil cake, respectively.

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48. Goldberg, R. N., Schliesser, J., Mittal, A., Decker, S. R., Santos, A. F. L. O. M., Freitas, V. L. S., Urbas, A., Lang, B. E., Heiss, C., Ribeiro da Silva, M. D. M. C., Woodfield, B. F., Katahira, R., Wang, W., & Johnson, D. K. (2015). A thermodynamic investigation of the cellulose allomorphs: Cellulose (am), cellulose Iβ (cr), cellulose II (cr) and cellulose III (cr). The Journal of Chemical Thermodynamics, 81, 184-226. http:// dx.doi.org/10.1016/j.jct.2014.09.006.

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50. Ferreira, D. P., Cruz, J., & Fangueiro, R. (2019). Surface modification of natural fibers in biopolymer composites. In G. Koronis & A. Silva (Eds.), Woodhead Publishing series in Composites Science and Enginering, Green composites for automotive applications (pp. 3-41). Duxford, UK: Woodhead Publishing.

51. Izani, M. A., Paridah, M. T., Anwar, U. M., Nor, M. Y. M., & H’ng, P. S. (2013). Effects of fiber treatment on morphology, tensile and thermogravimetric analysis of oil palm empty fruit bunches fibers. Composites. Part B, Engineering, 45(1), 12511257. http://dx.doi.org/10.1016/j.compositesb.2012.07.027.

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Received: Aug. 11, 2020 Revised: Nov. 10, 2020 Accepted: Nov. 26, 2020

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.08520

Influence of Nanoclay on the technical properties of Glass-Abaca hybrid Epoxy composite Sudhagar Manickam1* , Thanneerpanthalpalayam Kandasamy Kannan2 , Benjamin Lazarus Simon1 , Rajasekar Rathanasamy3 , and Sachin Sumathy Raj2  Department of Mechanical Engineering, The Kavery Engineering College, Salem, Tamilnadu, India Department of Mechanical Engineering, Gnanamani College of Technology, Namakkal, Tamilnadu, India 3 Department of Mechanical Engineering, Kongu Engineering College, Erode, Tamilnadu, India 1

2

*sudhagar85@gmail.com

Abstract The blending of nanoclay in polymers has potential prospects in the recent development of composite technology. In this present research work, Nanoclay was added to Glass fiber and Abaca fiber reinforced hybrid epoxy composites to enhance the wear resistance of the material. Nanoclay at weight ratios of 2%, 4%, 6%, and 8% was reinforced and the composite was fabricated into laminates using compression moulding. Nanoclay reinforced composites were tested for mechanical characteristics and wear rate in comparison to the non nanoclay reinforced hybrid composites. Water absorption character and morphology were also studied. It was observed that the 4% nanoclay reinforced composites showed the optimum results, with an increase in tensile strength, flexural strength and impact strengths of 6.6%, 19.6%, and 22.6% respectively when compared with EGA composite. Similarly the wear rate of the 4% nanoclay reinforced composite also was better than the EGA composite, showing an increase of 22.1% improved resistance.

Keywords: epoxy, hybrid composite, nanoclay, mechanical characterization, wear behavior.

How to cite: Manickam, S., Kannan, T. K., Simon, B. L., Rathanasamy, R., & Raj, S. S. (2020). Influence of Nanoclay on the technical properties of Glass-Abaca hybrid Epoxy composite. Polímeros: Ciência Tecnologia, 30(4), e2020038. https://doi.org/10.1590/0104-1428.08520

1. Introduction The trend of hybrid composites comprising of both natural and synthetic fibers have been attracting much attention among researchers lately. The addition of natural fibers in epoxy based synthetic composites helps in shifting the final component towards a much more environmental friendly product that is partially degradable[1].

Glass fiber and Epoxy resin had been used previously to fabricate composites. Majority of these composites vary in reinfrocement weight fractions of 20%, 40%, 60% and upto 80%. Various tests conducted in finding the tensile strength, impact strength, and flexural strength of the composites concluded that the material with a lower percentage of glass fiber showed lower properties. Similarly, a higher percentage of glass fiber reinforcement in epoxy resin had produce better mechanical properties[2,3].

Epoxy-Glass fiber-Abaca composite had maximum flexural and impact strengths when compared with other hybrid composites namely, Epoxy-Glass fiber-Jute and Epoxy-Glass fiber-Jute fiber-Abaca fiber in a research carried out[4]. The glass fiber and abaca combination had showed better adhesion with epoxy than other combinations of natural and synthetic fibers. Studies on epoxy reinforced E-glass fiber and Abaca fiber hybrid composites prepared at varying ratios of (70, 20, 10) wt%, (60, 20, 20) wt% and (50, 20, 30) wt% showed that tensile properties were

Polímeros, 30(4), e2020038, 2020

best for the (70, 20, 10) wt% composite due to the good adhesion. Similarly flexural strength was found to be best for the (60, 20, 20) wt% composite[5] which was due to the improved stiffness that was provided to the composite due to the increased abaca content. The hybrid composite having ratios of 50% epoxy with 50% reinforcements showed the best modulus values. Mechanical properties of the epoxy polymer showed improvement with addition of the synthetic and natural fibers due to better internal bonding of the materials[6]. Impact strength of Epoxy-glassfiber-abaca fiber hybrid composite was studied and found that 40% epoxy combined with 60% equal amount of reinforcements proved to show the best impact strength due to good absorption of impact energy by the natural fiber[7].

However, these hybrid composites still have few drawbacks like poor wear resistance and moderate mechanical properties. The reinforcement of nanoparticles is found to be one of the promising methods to improvise the mechanical properties of a composite[8]. Nanoclay is one recent material that possesses good thermal properties, tensile strength, modulus, fire resistance and wear resistance characteristics[9].

Nanoclays are arrangements of stacked silicate layers or nanoplatelets with non-metric thickness and have diameters ranging between 50 to 200 nm. Different types of nanoclays are Montmorillonite (MMT), kaolinite, and saponite. MMT

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

Manickam, S., Kannan T. K., Simon B. L., Rathanasamy R., & Raj S. S. nanoclay is commonly preferred in industries because of its availability, cost-effectiveness and simple processing ability[10]. MMT has a structure of layered phyllosilicates which when arranged in the ratio of 2:1 gives an extract of Cloisite 30B and Cloisite 20A. Among these derivatives the Cloisite 20A is more economically available. It is an organically modified Montmorillonite (OMMT) which shows strong antimicrobial activity against all kinds of bacteria[11]. This factor also helps to support the natural fiber reinforcement in a hybrid composite material form bacterial decay. Nanoclay had shown good adherence with epoxy. compositions of 0.5, 1, 1.5, 2 parts per hundred resin (phr) of nanoclay with epoxy which showed an optimum increase of tensile strength, flexural strength and thermal properties of the polymer[8]. Nanoclay at both reinforcement levels of 5% and 10% proved to produce better mechanical properties when combined with epoxy than with polyester resin in a study carried out[12]. Mechanical and tribological behavior of nanoclay reinforced Glass fiber-epoxy composite at reinforcement ratios of 0%, 1%, 3%, 5% and 7% by weight resulted in the 5wt% of nanoclay reinforced composites showing good wear behavior due to the low specific wear rate[13].

Composites prepared with varying in different weight percentage of 3%, 5%, 7%, and 10% nanoclay reinforcements.

The results showed that an increase in the weight percentage beyond 5% reduced the strength of the final composite product[14]. Similarly, 3% of nanoclay loading had produced the maximum mechanical properties when compared to the other percentage of reinforcements[15].

In industries, hybrid polymer composites have increasing usage in sliding and rolling applications like bearings, seals, gears, rollers, wheels, clutches and cams[16]. Reducing wear rate is therefore an important factor while fabricating such components[17]. In this present work, Cloisite 20A MMT nanoclay was dispersed with Epoxy resin and hybrid composites were fabricated by reinforcing Glass fiber and Abaca fiber at equal weight ratios. The technical properties of the nanoclay reinforced hybrid composites were compared with that of plain Epoxy polymer, Epoxy-Glass fiber composite and Epoxy Glass fiber Abaca fiber composite to study the enhancement in the properties.

2.2 Fabrication method In the first step a mica sheet was placed at the bottom of a mold of size 300 X 300 mm, and a coat of wax was applied throughout the mold for easy removal of the composite.

Epoxy resin and the hardener were combined in a ratio of 10:1. To fabricate the S3 composite, in the first step Glass fiber mat was placed at the bottom of the mold and epoxy applied onto it. Secondly abaca fiber mat was placed, followed by applying epoxy. The procedure was followed until three alternative layers each of glass fiber and abaca fiber were sandwithced with the help of epoxy resin through handlay up method. The laminated composite was then compression molded for 4 hours under a compression load of 1000 N. Post compression, the composite was allowed to cure for 48 hours at room temperature. After curing, the test specimens were cut down from the laminated slabs using water jet machining to ASTM standard dimensions.

S1 and S2 specimens were fabricated in the similar fashion.

All the test specimens were fabricated to an even thickness of 4 mm as represented in Figure 1.

For the preparation of the nano clay reinforced hybrid composites, the nanoclay was initally dried in an oven at a temperature of 90 °C for 5 hours to remove all moisture.

Epoxy was heated to 80 °C to reduce its viscosity[8] and measured quantity of the dried nanoclay as represented in Table 1 was added to it and stirred at 600 rpm mechanically for about 30 minutes[13]. The mixture was then placed in a high-intensity ultrasonicator at 300 rpm for 30 minutes[13,18].

The curing reagent was then mixed at 1 part to 10 parts of the epoxy-nanoclay mixture. Fabrication of the Epoxy-GlassAbaca +Nano clay followed similar methods of fabrication of the S3 composite laminates, with the nanoclay content varying at 2, 4, 6 and 8 wt%.

2. Materials and Methods 2.1 Materials In this present work, an Epoxy resin - LY556 DGEBA of density 1.16 g/cm3, curing time and temperature are 8 hrs and 140°C, with Hardener- HY951 Triethylenetetramine of density 0.95 g/cm3 was used and purchased from Sri Sakthi Enterprises, Chennai, India. E-Glass fiber mat of thickness 1 mm and density 2.6 g/cm3 was purchased from Sri Sakthi Enterprises, Chennai, India. Abaca (Manila hemp) of fiber diameter 260 microns, thickness 1 mm, and its density 1.5 g/cm3 purchased from Go Green Products, Chennai, India in the form of plain fabric mat. Cloisite 20A MMT Nanoclay of density 1.7 g/cm3, and dry powder 500 mesh with pass rate ≥95% was purchased from Ultrananotech Pvt. Ltd, Bangalore, India.

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Figure 1. Thickness of the composite laminate.

Table 1. Sample code for all the specimens.

Sample Code S1 S2 S3 S4 S5 56 S7

Epoxy resin (wt%) 100 75 50 48 46 44 42

Glass Fibre (wt%) 0 25 25 25 25 25 25

Abaca fibre (wt%) 0 0 25 25 25 25 25

Nano clay (wt%) 0 0 0 2 4 6 8

Polímeros, 30(4), e2020038, 2020

Influence of Nanoclay on the technical properties of Glass-Abaca hybrid Epoxy composite 2.3 Experimentation and Methodology: All the test specimens were machined to dimensions and tested to ASTM standards considering ASTM D638 for tensile test, D790 for flexural test, and D256 for impact test[19,20]. Tensile and flexural tests were carried out at 10 KN load capacity and a Crosshead speed of 2 mm/min on a Universal Testing Machine (Model: INSTRON-3365). Unnotched test specimens were tested on an Izod impact testing machine using a hammer head of 25 Joules impact force and the test specimes underwent complete fracture. Wear test specimens were punched to a diameter of 15.8 mm. Pin on-Drum method was adopted to analyse the wear rate. The drum had a cylinder size of 150 mm diameter and 500 mm length. The coarser Abrasive sheet (Grade 60) layer on the drum was made to rotate at 40 rpm with a load of 1 kg to test the rate of wear on the specimens. Water uptake for all the samples was carried out by initially measuring their dry weight (Wd). The samples were then immersed in water for 48 hours at room temperature. After the immersion period the samples were measured for the change in their weight, which is the wet weight (Ww). Finally the percentage of water uptake (%WA) was calculated using the formula shown in Equation 1[21].

 W −Wd  = %WA  w  ×100  Wd 

(1)

Figure 2. (a) Variation of Tensile Strength with the different Compositions; (b) Variation of Tensile Modulus with the different Compositions.

Morphology was studied on the fractured surface of all the specimens using Scanning Electron Microscope (Model: MIRA 3 TESCAN) at 3500 X magnification and a scale of 10 microns.

3.Results and Discussions 3.1 Mechanical analysis Figure 2a and 2b show the variation of the tensile strength and tensile modulus of the various samples in this research. The tensile strength and tensile modulus of S2 were 144.67% and 105.47% higher than S1, due to the reinforcement of the glass fiber. S3 produced 167.60% and 131.70% higher values than S1, and 9.37% and 12.76% higher values than the S2 due to the hybridization of glass fiber and abaca fiber reinforcements as intimated in many literatures. Initial addition of nanoclay, in the S1 specimen showed marginal improvement in the tensile properties, this pattern followed for S5 which showed further increase in the tensile properties. Beyond 4 wt% reinforcement of nanoclay the tensile properties for the S6 and S7 composites showed reduced patterns.

Figure 3a and 3b shows that the Variation of flexural strength and flexural modulus with the different compositions of composites. The flexural strength and flexural modulus of S2 composite was 34.25% and 37.48% greater than S1.

S3 composite had produced flexural strength and flexural modulus of 54.20% and 57.42% higher than S1, 14.85% and 14.50% higher than S2 composite. Flexural properties of the nanoclay reinforced compsoties showed replicated similar fashion as obtained for the tensile properties. S5 Polímeros, 30(4), e2020038, 2020

Figure 3. (a) Variation of Flexural Strength with the different Compositions; (b) Variation of Flexural Modulus with the different Compositions.

composite showed the best trend in flexural strength and flexural modulus among the different nanoclay reinforced composites showing 91.83% and 95.41% higher values than S1, 42.88% and 42.14% higher than S2, and 24.40% and 24.13% higher than S3 composites respectively.

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Manickam, S., Kannan T. K., Simon B. L., Rathanasamy R., & Raj S. S. The tensile and flexural properties in common for S2 were higher than S1 because the reinforced glass fiber had acted as the main load carrying agent in the composite thereby improving the mechanical character of the Epoxy matrix[22]. Similarly, S3 composite had an increase in the trend of tensile and flexural properties when compared with S1 and S2. This was due to fact that synthetic fiber and natural fiber combination exhibits better results than individual type fiber reinforced composites[4,23]. Comparing the S4, S5, S6 and S7, the best tensile strength and tensile modulus were attained for the S5 composite. S5 showed 7.14% and 22.04% increase in the tensile strength and tensile modulus when compared with the values of S3. Commonly, the test results showed that the addition of nanoclay up to 4wt%, elevated the properties to the maximum level. The increase in the tensile strength, tensile modulus, flexural strength, and flexural modulus can be attributed due to the high surface area of nanoclay content that can lead to homogenous distribution in epoxy resin[13,24,25].

On the other hand, the properties of the composites had drop beyond 4wt% addition of nanoclay reinforcement, because the epoxy resin could not accommodate an impregnation of high nanoclay content. A few portion of nanoclay platelets were not homogenously dispersed and had led to agglomeration[18,26,27]. The agglomerated nanoclay platelets form weaker spots in the epoxy resin, and this leads to sudden crack initiation and propagation phenomenon, thereby causing a decrease in mechanical properties[12,24].

Variation of impact strength with the different compositions of composites is shown in Figure 4. Impact strength of S2 composite was 90.47% higher than S1. S3 composite produced 100.12% higher impact resistance than S1 and 5.00% higher than the S2 composite. S5 showed 158.50% higher impact strength than S1, 35.71% higher values than S2 and 29.25% higher impact resistance than S3 composite.

This was because of the better impact resistance nature of nanoclay and good stress energy transfer between the matrix and fiber materials[18,28]. S6 and S7 composites showed lower impact strengths due to brittle fracture that had occurred in the specimens. This led to lower impact strengths and reduced stiffness of the composite when compared to the S5 composite.

SEM image Figure 5a and 5b showed that the tensile and flexural properties of S2 were higher than S1 because of reinforcement of glass fiber. Comparing the SEM

image of Figure 5c with Figure 5a and 5b, the presence of synthetic fiber and natural fiber combination exhibits better results than individual type fiber reinforced composites.

The good bonding of the composite was also visible in the SEM image Figure 5d. The agglomerated distribution of high nanoclay content in between the matrix and fiber pores decreased the ability of the composite to transfer the impact energy efficiently by resulting in internal flaws as seen in the SEM image Figure 5e.

3.2 Wear rate Specimens of S1, S2, S3, S4, S5, S6, and S7 were dimensionally prepared for the wear testing. Figure 6 shows that the Variation of wear rate with the different compositions of composites.

From the tested results, wear rate of the S1 and S2 composite are 0.1882 g/m and 0.1638 g/m respectively.

When comparing the S1 with the S2 composite, S2 produced wear rate of 12.96% lower than S1 because of the Glass fiber reinforcement[17].The hardness of glass fiber was 94 RHN and the hardness of plain Epoxy was 76 RHN when measured using a Rockwell hardness testing machine.

The hardness parameters prove that the harder glass fiber that was reinforced into the matrix resulted in higher wear resistance of the composite.

The wear rate of S3 composite was 0.1266 g/m. When comparing the S3 composite with S1 and S2, the S3 showed 32.73% and 22.71% lower than S1 and S2 composites respectively. The chemical composition of natural fiber emphasizes fiber properties in the composites[29]. While comparing the chemical composition of abaca with the other commonly used natural fiber reinforcement like hemp fiber, areca fiber, jute, sisal, flax and other banana fibers, the abaca fiber inherits a higher content of 66.43% cellulose, 30% hemicellulose, and 13.6% lignin. Cellulose is one of the important factors which can be produced strengthens and stability to the cell walls and fiber. It is rigid, high crystalline, and un dissolved in an organic solvent[5].

This factor justifies that the chemical composition present in natural fibers may have supported in lowering the wear rate of the natural fiber reinforced hybrid composites.

Reinforcement of nanoclay in general, further reduced the wear rate of the hybrid composite. Addition of nanoclay lowered the wear rate of the hybrid composite to 0.1124 g/m.

The S5 composite which had the best performance in the mechanical properties had a wear rate of 0.0986 g/m. This was because nanoclay has the inherent property to withstand high frictional stress[17,27]. Increasing nanoclay content showed further reduction in wear rate of the composites. The S7 composite which had the highest nanoclay content resulted in a wear rate of 0.0864 g/m showing the highest resistance to abrasion among all the test specimens in this study[30].

3.3 Water absorption analysis

Figure 4. Variation of Impact Strength with the different Compositions.

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Figure 7 shows that the percentage of water absorbed by the different specimens. The water absorption of both the plain Epoxy polymer and the S2 composite were zero since they are both synthetic materials.

The addition of Abaca fiber showed water absorption of 0.75% since a natural fiber has the tendency to absorb Polímeros, 30(4), e2020038, 2020

Influence of Nanoclay on the technical properties of Glass-Abaca hybrid Epoxy composite

Figure 5. SEM images of (a) S1, (b) S2, (c) S3, (d) S5 and (e) S6 composites.

moisture[21]. The increase in nanoclay reinforcement showed further increase in moisture absorption. The S5 composite which had the best mechanical properties showed water absorption of 0.94%. S7 with the maximum amount of nanoclay content, resulted with a moisture absorption of 1.16%. This was due to the fact that nanoclay tends to absorb water[10].

4. Conclusions

Figure 6. Wear rate of the Specimen.

Figure 7. Percentage of water taken for different Specimen.

Polímeros, 30(4), e2020038, 2020

The addition of nanoclay to the hybrid composite containing Epoxy matrix with Glass fiber and Abaca fiber reinforcements, showed improved mechanical properties in general. The mechanical analysis carried out to determine the tensile, flexural and impact properties resulted in the 4% nanoclay reinforced hybrid composite having the best characteristics. The maximum increase in tensile strength, flexural strength and impact strength for EGA+4% showed an increase by 6.6%, 19.6% and 22.6% respectively when compared with EGA. Wear property of the nanoclay reinforced composites were also found to be better than the composites without nanoclay reinforcements. The wear resistance of the EGA+4% composite, that had the best mechanical properties in this research was 22.1% more efficient than the EGA composite. 4% nanoclay reinforcement provided the most optimum and improvised technical properties through this research.

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Manickam, S., Kannan T. K., Simon B. L., Rathanasamy R., & Raj S. S.

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21. Raj, S. S., Kannan, T. K., & Rajasekar, R. (2020). Influence of prosopis juliflora wood flourin poly lactic acid– developing a novel bio-wood plastic composite. Polímeros: Ciência e Tecnologia, 30(1), e2020012. http://dx.doi.org/10.1590/01041428.00120.

22. Abdel-Rahim, R. H., Hasan, A. M., & Hussein, A. K. (2015).

Mechanical properties of epoxy based hybrid composites reinforced by glass fibers and sic particles. The Iraqi Journal for Mechanical and Material Engineering, 15(1), 66-79.

Retrieved in 2020, September 15, from https://www.iasj.net/ iasj/download/b8781a4891e35f3e 23. John, K., & Naidu, S. V. (2004). Tensile properties of unsaturated polyester-based sisal fiber–glass fiber hybrid composites.

Journal of Reinforced Plastics and Composites, 23(17), 18151819. http://dx.doi.org/10.1177/0731684404041147.

24. Shokrieh, M. M., Kefayati, A. R., & Chitsazzadeh, M. (2012). Fabrication and mechanical properties of clay/epoxy nanocomposite and its polymer concrete. Materials & Design, 40, 443-452. http://dx.doi.org/10.1016/j.matdes.2012.03.008.

25. Saeed, K., & Khan, I. (2015). Characterization of clay filled poly (butylene terephthalate) nanocomposites prepared by solution blending. Polímero: Ciência e Tecnologia, 25(6), 591-595. http://dx.doi.org/10.1590/0104-1428.2039.

26. Alsagayar, Z. S., Rahmat, A. R., Arsad, A., & Binti Mustaph, S. N. H. (2015). Tensile and flexural properties of montmorillonite Polímeros, 30(4), e2020038, 2020

Influence of Nanoclay on the technical properties of Glass-Abaca hybrid Epoxy composite Nanoclay reinforced epoxy resin composites. Advanced Materials Research, 1112, 373-376. http://dx.doi.org/10.4028/ www.scientific.net/AMR.1112.373.

27. Shettar, M., Kowshik, C. S. S., Manjunath, M., & Hiremath, P. (2020). Experimental investigation on mechanical and wear properties of Nanoclay–epoxy composites. Journal of Materials Research and Technology, 9(4), 9108-9116. http:// dx.doi.org/10.1016/j.jmrt.2020.06.058.

28. Mylsamy, B., Palaniappan, S. K., Pavayee Subramani, S., Pal, S. K., & Aruchamy, K. (2019). Impact of Nanoclay on mechanical and structural properties of treated Coccinia indica fibre reinforced epoxy composites. Journal of Materials Research and Technology, 8(6), 6021-6028. http://dx.doi. org/10.1016/j.jmrt.2019.09.076.

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29. Sachin, S. R., Kannan, T. K., Babu, M., & Vairavel, M. (2019).

Processing and testing parameters of PLA reinforced with natural plant fiber composite materials – a brief review. International Journal of Mechanical and Production Engineering Research and Development, 9(2), 933-940.

30. Uygunoglu, T., Brostow, W., & Gunes, I. (2015). Wear and friction of composites of an epoxy with boron containing wastes. Polímeros: Ciência e Tecnologia, 25(3), 271-276. http://dx.doi.org/10.1590/0104-1428.1780.

Received: Sept. 24, 2020 Revised: Nov. 20, 2020 Accepted: Dec. 10, 2020

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.09220

Reactive processing of maleic anhydride-grafted ABS and its compatibilizing effect on PC/ABS blends Erick Gabriel Ribeiro dos Anjos1 , Juliano Marini2 , Larissa Stieven Montagna1 , Thaís Larissa do Amaral Montanheiro3  and Fabio Roberto Passador1*  1 Laboratório de Tecnologia em Polímeros e Biopolímeros – TecPBio, Departamento de Ciência e Tecnologia – DCT, Instituto de Ciência e Tecnologia – ICT, Universidade Federal de São Paulo – UNIFESP, São José dos Campos, SP, Brasil 2 Departamento de Engenharia de Materiais – DEMa, Universidade Federal de São Carlos – UFSCar, São Carlos, SP, Brasil 3 Laboratório de Plasmas e Processos – LPP, Instituto Tecnológico de Aeronáutica – ITA, São José dos Campos, SP, Brasil

*fabio.passador@unifesp.br

Abstract Polymer compatibilizer agents are crucial for industrial materials development. Compatibilizer agents may be prepared by melt-grafting in the reactive extrusion process which is cheaper and environmentally friendly. Maleic anhydride-grafted acrylonitrile-butadiene-styrene (ABS-g-MA) has emerged as a relevant compatibilizer agent for immiscible blends, like polycarbonate (PC)/ABS. In this work, ABS-g-MA was prepared by a simple reactive extrusion process using ABS, maleic anhydride (MA) and benzoyl peroxide (BPO). The MA:BPO ratios of 1:0.5 and 1:1 varying the content of MA by 1, 2 and 5 wt% were investigated. The grafting reaction was confirmed through Fourier transform infrared spectroscopy (FT-IR), grafted degree (GD%), thermal and rheological analysis. The effectiveness of the compatibilizer agent was evaluated in PC/ABS blends (70/30 and 85/15 blend ratios). The addition of 5 wt% of ABS-g-MA (5 MA:2.5 BPO) in the PC/ABS blends promoted an expressive reduction of ABS domain sizes and better dispersion in the PC matrix.

Keywords: ABS-g-MA, melt-grafting, blends, compatibilizer agent, reactive extrusion.

How to cite: Anjos, E. G. R., Marini, J., Montagna, L. S., Montanheiro, T. L. A., & Passador, F. R. (2020). Reactive processing of maleic anhydride-grafted ABS and its compatibilizing effect on PC/ABS blends. Polímeros: Ciência e Tecnologia, 30(4), e2020039. https://doi.org/10.1590/0104-1428.09220.

1. Introduction Polymeric compatibilizer agents are used to obtaining new materials by combining the best features of already commercial materials, as polymer blends and polymer composites[1].

In the specific case of polymer blends, most systems are composed by polymers of different chemical nature, which can cause a weak interface between two different phases and compromises its mechanical properties[2-5]. An efficient way to overcome this problem is the use of a compatibilizer agent to improve the dispersion and the distribution of the second phase in the matrix, thereby promoting a better interaction between these components[4-6].

Several types of compatibilizer agents can be used in polymer blends, especially polymers grafted with reactive groups, block copolymers and polymers with polar groups[7,8]. The first group is popular and used for reactive blending, and can be used in polycarbonate (PC)/ acrylonitrile-butadiene-styrene (ABS) blends[8-10], polyamide 6 (PA6)/ABS[11,12] and polyethylene (PE)/PA6 blends[13].

In these cases, the backbone of the compatibilizer agent usually is chemically compatible with the non-polar phase and the grafting group reacts with the other phase, providing

Polímeros, 30(4), e2020039, 2020

the copolymer formation at the interface[8]. This process promotes a stronger interface and decreases the interfacial energy[8] and as a consequence, there are improvements in the mechanical properties of the blends.

In this way, the grafting of maleic anhydride (MA) on the backbone of a rubber or a non-polar polymer is an important type of compatibilizer agent[6,14]. There are some commercial grades of MA-grafted polymer like polyethylene (PE-g-MA)[13], polypropylene (PP-g-MA)[15] and polystyrene (PS-g-MA)[14]. Among other monomers, used in grafting, MA is preferred due to its high reactivity[16], thermal stability[17,18] and difficulty in homopolymerization[19]. For polymer blends, MA is advantageous because may result in less mechanical properties loss when compared with other types of compatibilizer agent like for example the glycidyl acrylate (GA)[10] and is more efficient than methyl methacrylate (MMA) requiring smaller quantities in polymer blends compositions[9].

Several studies have been published [6] about different ways to prepare these graft polymers using vinyl monomers, like MA, GA, and MMA. However,

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

Anjos, E. G. R., Marini, J., Montagna, L. S., Montanheiro, T. L. A., & Passador, F. R. the most common method is by chemical initiation[20], also called free-radical grafting[14]. Under the industrial technologies available for free-radical grafting there are the solution, emulsion or suspension grafting methods, which can achieve high grafting degree (GD%) levels, but require high costs and some organic solvent, being environmentally dangerous [6,14]. The solid-state graft technology which is solvent-free and achieves high GD%, nevertheless, requires long reaction times and it is limited to semi-crystalline polymers with high molecular weight. Finally, the most applied technique is the melt-grafting technology that has the advantages of being simple, fast, solvent-free and not limited by polymers melting temperature[14].

MA melt-grafting by reactive extrusion is a wellknown and applied technology in the industry to produce compatibilizer agents[14], which is still being developed because it is strongly dependent on process parameters, like the temperature of the process and the screw configuration of the extruder, and the grafting levels could be improved by using comonomers and mixed initiators[21].

PC/ABS blends are an economical way to improve the balance of mechanical properties of PC[22,23] and overcome other limitations like being difficult to process due to high melt viscosity and sensitivity to UV degradation[2,24]. This balance of properties has made the PC/ABS blend largely used in technical applications as automotive parts[2], and electronic engineering products[23].

MA grafted-ABS (ABS-g-MA) has been used successfully for PC/ABS blends. Rao et al.[25] prepared ABS-g-MA by solution process using toluene as solvent, evaluating the effect of monomer and initiator concentrations and time reaction.

The grafting reaction occured onto butadiene regions of the ABS backbone. Balakrishnan et al.[26] prepared ABS-g-MA by reactive extrusion on a single screw extruder using benzoyl peroxide (BPO) as an initiator and then mixed with PC to verify the efficiency on PC/ABS blends. As a result, an improvement in impact strength and the compatibility of PC/ABS blends were observed with the addition of 2 wt% of MA in this process.

Aiming to improve the %GD thought the reactive extrusion process Qi et al.[21] prepared ABS-g-MA using two initiators (BPO and dicumyl peroxide - DCP) and styrene as a comonomer. The use of DCP as initiator and the use of a comonomer improved the %GD due to the longer half-life of DCP compared to BPO at the temperature utilized, and the presence of styrene with MA could form a charge-transfer complex which is more reactive than the individual monomers[21].

In this study, ABS-g-MA was produced through reactive melt-grafting method using BPO as the initiator and MA at 1:0.5 and 1:1 MA:BPO ratios and varying the content of MA by 1, 2 and 5 wt%. The effectiveness of the compatibility was verified in different PC/ABS blends (70/30 and 85/15). In addition, the mechanical and morphological properties of the PC/ABS/ABS-g-MA blends were studied.

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2. Materials and Methods 2.1 Materials Acrylonitrile-butadiene-styrene copolymer (ABS) was supplied by TRINSEO (specification Magnum 3404, Switzerland), with a PB content of 20%, MFI of 2.0 g/10 min (230 °C/3.8 kg) and density of 1.05 g/cm3.

Polycarbonate (PC) was supplied by UNIGEL (Durolon IR-2200, Brazil) with a density of 1.20 g/cm3 and a melt flow index (MFI) of 12.0 g/10 min (300 °C/1.2 kg).

Maleic anhydride (MA) was utilized as a monomer and supplied by Sigma-Aldrich (Saint Louis, MO. USA) with 99% purity. As initiator was used benzoyl peroxide (BPO) commercial grade supplied by Dinâmica Química Contemporânea (Indaiatuba, Brazil). Both reagents were used as received.

2.2 Reactive processing of ABS-g-MA Before the reactive processing, ABS was dried at 80 °C for at least 12 h in a vacuum oven to minimize the hydrolytic degradation during the processing. The reactive extrusion processes to produce ABS-g-MA by melt-grafting were carried out in a co-rotational twin-screw extruder, AX Plásticos, model AX16:40DR (L/D = 40, D = 16 mm), with screw rotation speed set in 80 rpm and feeding in 20 rpm. The temperature profile set was 190/200/200/200/210 °C from the first zone to the die, in these conditions the residence time was near to 2 min. After extrusion, the materials were pelletized and dried on a vacuum oven for 4 h at 80 °C. Table 1 presents the compositions studied with the variation contents of MA and BPO.

2.3. Characterization of the compatibilizer agent ABS-g-MA 2.3.1 FT-IR Spectroscopy ABS-g-MA with different MA:BPO ratios were analyzed

by Fourier transform infrared spectroscopy (FTIR) using universal attenuated total reflectance (UATR) in a PerkinElmer Frontier equipment and a scan range of 400-4000 cm-1.

2.3.2 Grafting degree (%GD) The grafting degree of MA in the melt-grafted compatibilizer agents was determined by a back-titration procedure based on the methodology described by Qi et al.[22]. First, after the reactive extrusion process, the compatibilizer agents were purified by dissolving in 5 mL of acetone and precipitated adding 5 mL of ethanol, twice, to remove the residual reactants (MA and BPO). Then, 0.5 g of each composition Table 1. Nomenclature and compositions studied.

Composition C1 C2 C3 C4 C5 C6

ABS (wt%) 98.5 98 97 96 92.5 90

MA (wt%) 1 1 2 2 5 5

BPO (wt%) 0.5 1 1 2 2.5 5

Polímeros, 30(4), e2020039, 2020

Reactive processing of maleic anhydride-grafted ABS and its compatibilizing effect on PC/ABS blends was dissolved in 50 mL of acetone and 5 mL of ethanol.

NaOH (1.0 mol/L) solution was added with 0.5 mL of Yamada Universal indicator. After 30 min with mechanical stirring, these solutions were back titrated with 0.1 mol/L of HCL. Then, the quantity of MA grafted was calculated as a percentage of ABS by the following: %GD

( VRABS − VS ) × 10−3 × 2WS

CHCl × MWMA

× 100 (1)

Where VRABS is the volume (mL) of HCl solution consumed with neat ABS as a reference, VS is the volume consumed by the sample (mL), CHCl is constant and corresponds to 0.1 mol/L of HCl titration solution, MWMA is the molecular weight of MA, also a constant used as 98.06 g/mol and Ws is the weight of each sample (g).

2.3.3 Rheological characterization The rheological behavior of the compositions were evaluated in a rotational controlled stress rheometer from TA Instruments, model ARG2. The steady-state viscosity was evaluated as a function of the shear rate in an inert nitrogen atmosphere at a temperature of 210 °C, using parallel-plates geometry with 25 mm of diameter and gap of 1 mm. The delay before measurement was determined from stress overshoot experiments performed at 210 °C and 0.01 s-1 until constant stress was attained.

2.4 Melt flow index (MFI) MFI values of the compositions were analyzed according to the ASTM D1238 standard[27], a Hebert Lambert plastometer was utilized at 200 °C and a load of 5 kg.

2.4.1 Differential Scanning Calorimetry (DSC) The glass transition (Tg) of SAN phase of ABS for the compositions were analyzed by DSC. The tests were performed in a TA Instruments equipment, model Q2000 under nitrogen atmosphere. Samples were sealed in an aluminum DSC pan and heated from room temperature to 220 °C at 10 °C/min, followed by an isothermal at 220°C for 3 min, a cooling cycle to 30°C at 10 °C/min and a second heating cycle up to 220 °C also at 10 °C/min.

2.4.2 Thermogravimetric Analysis (TGA) Thermogravimetric analysis (TGA) of the compositions were performed on a Netzsch model Iris® TG 209 F1 equipment from room temperature to 800 °C at a heating rate of 20 °C/min, under nitrogen atmosphere.

2.4. Effectiveness of the compatibilizer agent in PC/ABS blends The effectiveness of the ABS-g-MA as a compatibilizer agent was verified for PC/ABS blends. PC/ABS blends with different blend ratios (70/30 and 85/15) were prepared using 5 wt% of each ABS-g-MA produced replacing neat ABS. All blends were produced using an AX Plásticos, model AX16:40DR co-rotational twin-screw extruder with screw rotation speed set in 80 rpm and feeding in 20 rpm with the temperature profile of 225/235/245/245/255 °C from the first section to the die. The blends were pelletized and dried on a vacuum oven for 4 h at 80 °C. Polímeros, 30(4), e2020039, 2020

Specimens for Izod impact strength tests were pressed into 3.2 mm thick plates in a hydropneumatic press (MH Equipamentos Ltda, model PR8HP) at 260 °C with a pressure of 5 bar for 7 min.

2.5. Characterization of the PC/ABS blends 2.5.1 Impact Izod test Izod impact strength tests were performed on a CEAST/Instron Izod impactor test machine (model 9050) according ASTM D256-06[28]. All the test specimens were notched using a manual notched machine (CEAST/Instron) and the tests were performed using a 5.5 J hammer, the largest hammer available for testing, five samples of each composition were tested.

MINITAB®17 statistical software was used to the Izod impact strength data. First, the data distribution was analyzed

by Anderson-Darling test for each composition, then the Levane test was used to evaluate the homogeneity between variances of different compositions[29]. Analyses of variance one-way (ANOVA) were performed to compare the mean size distributions and mean impact strength among the groups, followed by post-HOC Tukey HSD test to compare each pair at α = 0.05[29,30].

Morphological characterization microscopy (SEM)

with

scanning

electron

The morphology of the PC/ABS blends were analyzed using a FEI Inspect S50 scanning electron microscope operating at 15 keV. The samples of each blend composition were taken immediately at the exit of the extruder die to verify the formation of phases during the extrusion process.

These samples were cryogenically fractured with liquid nitrogen, to preserve the morphology, perpendicularly to flow direction and the surfaces were coated with a thin gold layer. To obtain the average and distribution of ABS domains sizes the SEM images were analyzed with the software Image J. First, one image of each blend composition was selected on the same amplified magnitude (10000x), 50 ABS domains[11] were selected and measured a minimum of ten times each. Then these data were compiled to create a normal size distribution.

3. Results and Discussions 3.1. Characterization of ABS-g-MA Figure 1A shows the FTIR spectra of ABS and the compatibilizer agents (C1 – C6). For all compositions, it is possible to observe the main ABS characteristics bands.

The band at 2238 cm-1 corresponding the C≡N stretch of acrylonitrile[20,31], the bands at 1494 cm-1 and 1602 cm-1 are associated to styrene ring modes and bands at 911 and 966 cm-1 are associated to C-H in plane bending in polybutadiene (PB)[31,32]. The bands at 759 and 700 cm-1 are correlated to bends of C-H styrene rings and 1453 cm-1 indicating the scissoring mode of CH2 bonds. And some general peaks like 3200-3000 cm-1 which are associated to aromatic the C-H stretches and 3000-2800 cm-1 to the aliphatic ones[31]. For the ABS-g-MA curves (C1 to C6) it is possible to observe a new absorbance band at 1780 cm-1 3/8

Anjos, E. G. R., Marini, J., Montagna, L. S., Montanheiro, T. L. A., & Passador, F. R.

Figure 1. FTIR spectra (A) spectra of ABS and ABS-g-MA (C1 to C6) and (B) detail of 1780 cm-1 peak for all composition.

which corresponds to the C=O stretching from anhydride indicating that MA was grafted in the ABS backbone[20,32].

This peak also increases with the concentration of MA and BPO[32], as is sown in Figure 1B. As shown by Braga et al.[33] the C=O stretching band from MA is located at 1709 cm-1.

The new absorption band observed in the compatibilizer agents is located at 1780 cm-1 and refers to C=O stretching from anhydride. This shift in the wavelength means that MA was covalently bonded to the ABS backbone, and that the grafting reaction was effective[34,35] suggested by some authors the MA was preferentially grafted onto PB segments of ABS backbone[32].

The efficiency of the reactive extrusion process could be associated with grafting degree of MA (%GD) determined by back-titration method and exhibited in Figure 2. For an ease understand of the results, the compositions could be divided into two groups: First, C1, C3 and, C5 which were prepared with 1:0.5 MA:BPO ratio and second, C2, C4, and C6 with 1:1 MA:BPO ratio. First, the %GD increased with the MA concentration comparing composition into each group. This might be associated with the increase in the number of molecules diffusing in the reaction medium (the molten polymer), therefore increasing the probability of grafting reaction onto the polymer back-bone[32]. Then, an analogous behavior was observed for the initiator concentration when comparing compositions with 1:0.5 MA:BPO and 1:1 MA:BPO. It may be explained by an increase of free radicals formed from thermal decomposing of BPO[32].

However, even working with high contents of MA and BPO (5 wt% of each component) the grafting degree is still less than 2.0 wt%, as supported the literature on similar process conditions[14]. Another side effect of high initiator and monomer content is polymer chains scissions[21]. In order to understand this effect, the MFI was measured, and the rheological behavior was analyzed.

MFI for ABS-g-MA of all compositions is also shown in Figure 2. It was found that the MFI of ABS-g-MA was higher than the neat ABS resin (around 2.0 g/10min). The MFI of the compatibilizer agents increased with BPO and MA content. The presence of higher levels of BPO has a 4/8

Figure 2. Grafting degree of MA (%GD) and MFI of ABS-g-MA compositions.

greater influence on MFI values and the MFI behavior could explain some thermal degradation of ABS under processing conditions. Moreover, a lubricant effect caused by a residual amount of MA and BPO can increase MFI value. A decrease in MFI can occur if the grafting reaction was predominant than chain scission[32].

The rheological behavior under low shear rates (0.01 - 1.0 s-1) conditions is commonly used as an indirect way to understand some structural modifications on polymers. At this range of shear rates, a Newtonian behavior is expected for the majority of the thermoplastics and in this analysis the viscosity values for the first Newtonian plateau (η0) will be considered as those measured at 0.01 s-1 [33]. Figure 3 shows the viscosity as function of the shear rate of the samples at 210 °C. Comparing the rheological behavior of ABS pellet and the processed ABS it is possible to conclude that the processing conditions applied had no significant influence on the material molecular structure. For the ABS-g-MA compositions (C1 to C6) the η0 were lower than ABS. The extension of the Newtonian plateau for all samples is small, restricted to very low shear rates and the greater the MA and BPO contents (samples C5 and C6) the smaller the Polímeros, 30(4), e2020039, 2020

Reactive processing of maleic anhydride-grafted ABS and its compatibilizing effect on PC/ABS blends of ABS[14], so that supports a possible degradation process on higher contents of BPO and MA.

plateau extension. Thus ABS and ABS-g-MA presented a pseudo-plastic behavior even at low shear rates. Agreeing to MFI results, the viscosity reduction may be associated with molecular weight reduction, due to ABS chain scissions[34] and a possible lubricant effect of a residual amount of BPO and MA molecules.

Table 2 presents the values of glass transition temperature (Tg) of SAN present in ABS for first and second heating.

First, comparing Tg values between the heating cycles is clearly observed that the Tg is higher in the second heat cycle for all compositions. It might indicate the presence of some molecule acting as a plasticizer which exudates on the first heating cycle[24], and on the present context, could be non-reacted BPO and MA molecules or some molecules with little molecular weight formed between them.

Comparing the Tg values it was observed a decrease in Tg with the MA content. One hypothesis is that MA grafting reactions and the degradation of ABS occur simultaneously during reactive extrusion and both events affect the Tg value.

MA grafted on ABS chains could increase Tg making it more rigid[24,32]. However, the short MA branches[8] and more chain ends[24] resultant from the chains scissions decrease the Tg value. Even some authors had observed the opposite behavior for the Tg[32], but for ABS-g-MA prepared on the solvent approach which is less susceptible to degradation

The TGA results are expressed in terms of the temperature of 10, 50 and 90% mass loss and temperature of maximum degradation rate (Tmax), presented in Table 2.

For ABS-g-MA (C4 to C6), it is possible to observe a mass loss before the main degradation step, which is probably associated with residual molecules of MA and BPO. This mass loss increases gradually with the reactant´s content.

T10 for ABS-g-MA was lower than for ABS and decreases gradually for C4 to C6, this may be explained by the presence of residual molecules and a possible reduction in molecular weight by oxidative chain scissions on reactive extrusion, supporting the MFI and DSC results.

3.2. Effectiveness of ABS-g-MA as compatibilizer agent for PC/ABS blends.

The Izod impact strength values of PC/ABS blends with different blend ratios are shown in Table 3. During the impact test two different specimen failures were observed according to ASTM D256-06 standard. For the most composition occurred a partial break (P), which the specimen had an incomplete break higher than 90% of the sample length and still supporting itself vertically. On the other hand, for other compositions, the specimens had failed by a complete break (C) been separated into two parts[36].

The ANOVA results for impact strength proved a statistical difference between groups (p-value of 0.000 and F-Value of 228.64)[29,30,34,35], so the different compositions were classified in statistically different groups by the postHOC Tukey test exhibited in Table 3.

The addition of ABS increases the PC toughness for both PC/ABS blend ratios (70/30 and 85/15) with a synergistic effect, higher to PC/ABS (85/15), agreeing with Greco et al.[37] results for PC/ABS blends with ratios between 15-30 wt% of ABS. The higher impact strength for 85/15 than 70/30 ratio was explained for Lee et al.[36] as a change in the main toughening mechanism. The addition of compatibilizer agents C2 and C5 did not significantly change the impact strength in comparison with the non-compatibilized blends for both blend ratios. The compatibilizers C1, C3, and C4 significantly decreased the impact strength of PC/ABS (85/15), which may be associated with a non-homogeneous morphology[10].

Figure 3. Viscosity curve as a function of the shear rate obtained by deformational rheometry of non-processed ABS (ABS pellet), processed ABS (ABS) and ABS-g-MA (C1 – C6) compositions.

Table 2. Values of glass transition (Tg) of SAN phase of ABS on fisrt and second heating. TGA analys results in terms of the temperature of 10, 50 and 90% mass loss and temperature of maximum degradation rate.

Sample

1st

DSC 2nd

T10%

TGA T50%

T90%

DTG Tmax

ABS pellets ABS C1 C2 C3 C4 C5 C6

Tg* (°C) 108 115 107 102 105 99 84 86

Tg* (°C) 111 114 109 109 108 106 101 102

(°C) 395 389 393 388 391 383 380 360

(°C) 423 417 416 416 417 414 414 409

(°C) 458 448 447 446 448 445 447 455

(°C) 421 415 414 415 416 413 413 409

Polímeros, 30(4), e2020039, 2020

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Anjos, E. G. R., Marini, J., Montagna, L. S., Montanheiro, T. L. A., & Passador, F. R. The addition of C6 as a compatibilizer agent for PC/ABS (70/30) and PC/ABS (85/15) resulted in the lowest impact strength, comparable with the values of the neat materials PC and ABS. C6 is the ABS-g-MA with higher content of MA grafted and the degradation by severing chain scissions suffered during the melt-graft process probably generates a weaker interface on PC/ABS blends.

The micrographs of the blends are shown in Figure 4.

For all blends, it is possible to observe ABS domains on a PC matrix which are marked by interface lines.

Comparing PC/ABS (70/30) non-compatibilized blend (Figure 4A) with the compatibilized blend using C3 (Figure 4B) and C5 (Figure 4C) as compatibilizer agents it is possible to note a gradual reduction on the size of ABS domains. Moreover, a more homogeneous and thinner

dispersion for C5 is observed and is in accordance with other works[9]. However, for the PC/ABS blend using C6 as compatibilizer agent (Figure 4D) it is possible to observe that the ABS domains increase, suggesting a coalescence of ABS domains with a heterogeneous form.

Blends morphologies are strongly correlated to the content of each polymer[5]. Comparing the two blend ratios (70/30 and 85/15), the PC/ABS (85/15) presents a thinner dispersion and size of the ABS domains. Increasing the content of the higher viscosity phase (PC) causes a change in the viscosity ratio between the disperse phase and the matrix[5]. For PC/ABS blend (85/15) the addition of C3 as a compatibilizer agent results in an increase in the size of ABS domains, which may be correlated with the lower impact strength for blends PC/ABS (85/15) with the addition of

Table 3. Impact strength test for PC/ABS blends and neat materials (PC and ABS).

PC ABS B70 B70C1 B70C2 B70C3 B70C4 B70C5 B70C6 B85 B85C1 B85C2 B85C3 B85C4 B85C5 B85C6

Impact strength (J/m) Mean (SD) 73.8 (3.1) 124.8 (0.9) 428.8 (28.1) 436.6 (23.4) 427.4 (14.8) 358.2 (11.1) 319.4 (27.3) 403.8 (14.4) 150.5 (5.3) 533.0 (25.7) 445.0 (20.8) 514.4 (28.9) 138.0 (50.1) 90.4 (6.3) 522.3 (19.0) 70.1 (31.0)

Fracture type Complete break Complete break Partial Break Partial Break Partial Break Partial Break Partial Break Partial Break Complete break Partial Break Partial Break Partial Break Complete break Complete break Partial Break Complete break

p- value Anderson–Darling test 0.494 0.102 0.505 0.139 0.873 0.109 0.187 0.364 0.350 0.178 0.056 0.248 0.238 0.520 0.160 0.064

Groups* F EF B B B CD BC B F A B A E D A F

*By post-HOC Tukey’s test, mean values which do not share letters are significantly different from others. **P-value Levene test 0.336. *** ANOVA results p-Value = 0.000, F-Value = 228.64, S= 22.9979J/m, and R2=98.62%.

Figure 4. SEM micrographs of fractured surfaces of (A) PC/ABS (70/30), (B) PC/ABS/C3(70/25/5), (C) PC/ABS/C5(70/25/5), (D) PC/ABS/C6(70/25/5), (E) PC/ABS (85/15), (F) PC/ABS/C3(85/10/5), (G) PC/ABS/C5(85/10/5) and (F) PC/ABS/C6(85/10/5) with magnification of 15000x.

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Reactive processing of maleic anhydride-grafted ABS and its compatibilizing effect on PC/ABS blends

Figure 5. Size ABS domains distribution for PC/ABS with different blend ratio e ABS-g-MA composition (A) 70/30 and (A)85/15.

C3 and C4. A better dispersion and distribution of the ABS phase was observed for the PC/ABS blends that use C5.

Figure 5 shows the effect of compatibilizer addition in size distributions of ABS domains in blends PC/ABS (70/30) (Figure 5A) and PC/ABS (85/15) (Figure 5B), respectively.

The distributions of the blend are represented in bold lines.

Looking at both different blends compositions the thinner dispersion of PC/ABS (85/15) is confirmed quantitatively, agreeing with the qualitative analyses.

According to the Izod impact test, the compositions of ABS-g-MA (C2 and C5) could be used for PC/ABS blends of both blend ratio. However, based on the sizes distribution is observed that the addition of C5 as compatibilizer agent is promoted the thinnest and narrowest distribution for both PC/ABS blends compositions, indicating that it may be the best compatibilizer agent for that system.

4. Conclusions Maleic anhydride was successfully melt grafted into ABS by a reactive extrusion process. The effectiveness of the ABS-g-MA as a compatibilizer agent has been confirmed for PC/ABS blends with different blend ratios (70/30 and 85/15). FTIR spectra of ABS-g-MA of all compositions showed a new characteristic band at 1780 cm-1 confirming the graft reaction of MA onto the ABS chain. Grafting degree analysis indicated a proportional increase with the MA and BPO content, MFI and rheological behavior suggested a reduction in the molecular weight of ABS due to chain scissions. For the PC/ABS blends, the Izod impact tests confirmed that ABS improved the PC impact strength and that ABS-g-MA compatibilizer agents C2 and C5 could be used for both blend ratios. The morphologies analyzed by SEM and qualitative analyzes of size ABS domains distribution suggests that ABS-g-MA composition C5 might be the best compatibilizer agent for PC/ABS blends.

5. Acknowledgements The authors are grateful to Brazilian Funding institution FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo – 2019/11130-9) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico – 310196/2018‑3 and 405675/2018-6) for the financial support.

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Received: Oct. 19, 2020 Revised: Nov. 03, 2020 Accepted: Dec. 17, 2020

Polímeros, 30(4), e2020039, 2020

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.07520

The incorporation of untreated and alkali-treated banana fiber in SEBS composites Letícia Cuebas1, José Armando Bertolini Neto2, Renata Tâmara Pereira de Barros2, Alexandre Oka Thomaz Cordeiro2, Derval dos Santos Rosa1 and Cristiane Reis Martins2*  Centro de Engenharia, Modelagem e Ciências Sociais Aplicadas – CECS, Universidade Federal do ABC – UFABC, Santo André, SP, Brasil 2 Instituto de Ciências Ambientais, Químicas e Farmacêuticas – ICAQF, Universidade Federal de São Paulo – UNIFESP, Diadema, SP, Brasil 1

*cr.martins@unifesp.br

Abstract In this work, banana fiber was used as reinforcement for the preparation of a thermoplastic elastomer composite (TPE). Few studies are exploring the natural fiber incorporation on TPEs, with no one using banana fiber. The fiber was extracted from banana pseudostem and modified with an alkaline solution. The untreated and treated banana fibers were incorporated in 2%, 5%, and 10% in SEBS. The mixture was performed in a thermokinetic mixer (K-Mixer) and plates prepared by compression molding. The composites were characterized by Scanning Electron Microscopic (SEM), tensile testing, mechanical dynamical analysis (DMA). It can be observed that the samples with untreated fibers presented a higher tensile strength, except for the incorporation of 5% of BF. Young’s modulus increase as the fiber’s incorporation grows, indicating greater rigidity of the composite. It was found that the chemically treated banana fiber composites, e.g., TPE/TBF5 and TPE/TBF10, shows a 15.4% and 22.2% higher elongation.

Keywords: banana fiber, composite, SEBS, thermoplastic elastomer, thermokinetic-mixer.

How to cite: Cuebas, L., Bertolini Neto, J. A., Barros, R. T. P., Cordeiro, A. O. T., Rosa, D. S., & Martins, C. R. (2020).

The incorporation of untreated and alkali-treated banana fiber in SEBS composites. Polímeros: Ciência e Tecnologia, 30(4), e2020040. https://doi.org/10.1590/0104-1428.07520

1. Introduction Natural fibers are growing increase importance as reinforcing materials in composites due to some advantages, such as low density, inexpensive, no toxicity, biodegradable in nature, good mechanical properties, and provide a market reputation as an eco-friendly material[1]. Recent studies using natural fiber as reinforced material in rubbers offer an attractive, easy, and economic friendly approach to originate commercially feasible natural rubber composite[1-3].

Hashim et al.[4] investigate the incorporation of mengkuang (Pandanus tectorius) leaf fiber in blends of ethylene-vinyl acetate and natural rubber, and ethylene-vinyl acetate and epoxidized natural rubber. The authors showed better filler-matrix interaction between ethylene-vinyl acetate and epoxidized natural rubber with fiber filler causing stronger tensile properties than non-epoxidized rubber blend. Rice straw fiber was incorporated by Paran et al.[5] as a filler in nitrile butadiene rubber (NBR) and poly (vinyl chloride) (PVC) thermoplastic elastomer also containing an organoclay. Their work shows the higher rice straw concentration leads to an increase in thermal stability and higher thermal decomposition temperature. This improvement is believed that the physical structure of rice straw can act as an obstacle for the volatile products resulted from the thermal decomposition of the polymer. Miedzianowska et al.[3]. evaluated ethylene-octane

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copolymer in the presence of several fibers, as wheat, oat, rye, barley, and triticale fiber as well compared the size of particles as a filler. The results demonstrated that the smallest particle size filler was characterized as having the lowest viscosity, and the minimum torque increased with the increasing content of lignocellulosic material in the composite. The addition of the fiber increased the stiffness of the composites, as evidenced by the increase in the maximum torque value in compared to pure ethylene-octene rubber. They appointed the type of fiber, fiber dispersion, porosity, and interfacial strength as the main factors affecting mechanical performance.

Despite the improvements offered by natural fibers, incompatibility between the polymer matrix and fiber surface has been identified as a major difficulty in processing polymer composites containing natural fibers[6]. Alkaline treatment, or mercerization, is one of the chemical treatments of natural fibers used to modify the network structure, disrupting hydrogen bonds and increasing surface roughness. In this treatment, lignin, hemicellulose, and waxes are removed from the fiber, increasing fiber density. Many studies have explored the alkali treatment on natural fibers and the incorporation in polymer matrices. Yantaboot et al.[7] reported the improvement of the adhesion of pineapple leaf

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

Cuebas, L., Bertolini Neto, J. A., Barros, R. T. P., Cordeiro, A. O. T., Rosa, D. S., & Martins, C. R. fiber into natural rubber. They used in their work untreated fiber and 10% sodium hydroxide (w/v) treated fiber during 30 minutes. The moduli strains of the composites containing treated fiber are higher than those containing untreated fiber. Pineapple leaf fiber treated with 10% NaOH was compared with aramid fiber (Kevlar) in reinforcement of natural rubber[8]. At 25 ºC, as fiber content increases, Kevlar fiber has a slightly greater effect than pineapple leaf. As the temperature increased to 60ºC, 2 and 5 phr content, for both fibers, the effect of both fibers remains similar. However, at a fiber content of 10 phr, natural fiber demonstrates a much greater effect that does Kevlar becoming a good and cheap alternative for reinforcement filler.

In the literature, the concentration of alkali agents (NaOH, KOH, etc.) and the treatment time varies in a wide range from 0.03 wt% to 40 wt% and from a few minutes to 48 h[9].

The adjust of these parameters is essential to remove lignin, hemicellulose, and waxes partially, and if the treatment parameters are not optimized, the mercerization can cause fiber defibrillation and pore formation[10]. In Cai et al. work[11], abaca fibers was treated with 5, 10, and 15% NaOH solution and its effects on the interfacial adhesion. The results showed that mild treatment such as 5 wt % NaOH for 2 hours improves the fiber-matrix interfacial strength which ensures the good mechanical properties of developed composites. Oushabi et al.[12] investigated the effect of mild alkali treatment on palm fibers surface reinforced polyurethane composites. The treatment was carried out with NaOH, with low to high concentrations; 0 wt%, 2 wt%, 5 wt% and 10 wt% for 1 hour. The experimental results showed the optimal alkali concentration was reported at 5 wt% in terms of reduction of the amorphous parts of the fibers.

Thermoplastic elastomer (TPE) composites have attracted much attention because of their advantages over conventional elastomeric materials[4]. This type of material can be processed and recycled and replacing vulcanized rubber and PVC. They can also be processed using conventional thermoplastic processing equipment[13]. The copolymer styrene-(ethylene butylene)-styrene (SEBS) represents an important type of TPE. However, the systematic study based on the hybridized reinforcement of SEBS composites with natural fiber is very rare. Few studies have been done about the reinforcement of SEBS with natural fibers such as oil palm fiber[14], hemp fiber[15] and mango wood fiber[16], pineapple leaf fiber[17,18], curaua fiber[19], and wood fiber[20-22].

Banana harvest generates residue from the leaves, pseudostem, and roots. After the banana is harvested, the pseudostem is discarded, this residue is fibrous in nature and enriched in strands with cellulose content[23]. Balaji et al.[24] prepared hybrid polymer composites of epoxy reinforced with sisal, banana, coir, and a mix of the three fibers. The authors observed that the tensile strength of the pure epoxy composite is much lower than all composites. The composite with banana fiber presented higher impact strength, 1.31 kJ.m-2 because the fibers have introduced some brittleness as an increase in hardness, which led to a decrease in impact strength.

They found that the tensile strength of the banana fiber is 68 MPa and 1.0 to 3.5% of elongation. Zaki et al.[19] found 20.18 MPa for tensile strength in epoxy resin with 10% of banana fiber treated with 6 wt% NaOH. Polyester–banana 2/9

fiber composite prepared Kumari et al.[25] showed a tensile strength of 20.1 for untreated fiber and 26.0 for treated fiber.

Given the possible damage to the fiber caused by high concentrations of alkaline agents and the increasing interest in natural fibers as reinforceing fillers in rubbers, the main goal of this present study was to develop a natural fiberbased SEBS composite using banana fiber (BF), untreated and mild alkaline treated, as filler. The influence of the BF content in thermoplastic elastomer (TPE) composites and their chemical treatment was examined in terms of morphological, mechanical, and dynamic mechanical properties.

2. Materials and Methods 2.1 Materials Banana pseudostem of Musa sapientum was obtained from Agência Paulista de Tecnologia do Agronegócio (APTA, Registro, São Paulo, Brazil) and from Cooperativa de Artesãos de Miracatu (BANARTE, Vale do Ribeira, São Paulo, Brazil). The matrix was a thermoplastic elastomer (SEBS Fortiprene TPE-7140) donated by FCC®. NaOH (purity ≥99%) was supplied by Vetec®.

2.2 Methods 2.2.1 Fibers extraction from banana pseudostem Pseudostem was cut, and layers manually defibrillated.

Layers were dried at 55ºC in a hot air circulating oven. Fibers extraction from pseudostem was performed by the following method. The layers were immersed in distilled water for two days, were shredded with a wire brush, washed with water, and dried in an air oven until the complete drying, obtaining banana fiber (BF). Fibers were ground in a knife mill and sieved to obtain particles of about 87 µm in size.

2.2.2 Alkali treatment Figure 1 shows the banana fibers after extraction (untreated and treatment) used for the preparation of TPE-based composites. Untreated banana fiber (UTBF) was subject to alkali treatment by soaking in 5% sodium hydroxide solution (w/v) for 1 hour and washed with water up to neutral pH. Treated fiber (TBF) was dried in an oven at 55 ºC and sieved (Ø 85μm).

2.2.3 Preparation of thermoplastic elastomer/banana fiber composite Figure 2 describes the experimental aspects used for processing the composites. TPE/BF composites were made from untreated banana fiber (UTBF) and treated banana fibers (TBF). Composites were processed in a K-Mixer (MH 50H; MH Equip. Ltda., Unifesp Diadema, São Paulo, Brazil) and plates prepared by compression molding.

Appropriate quantities of thermoplastic elastomer (TPE) polymer and banana fiber were pre-weighed to the desired loading, combined, and added into the K-Mixer. The contents of banana fiber in the TPE/UTBF and TPE/TBF composites used were 2, 5, and 10 wt%, giving the samples names TPE/ UTBF2, TPE/UTBF5 and TPE/UTBF10 for untreated fibers, and TPE/TBF2, TPE/TBF5 and TPE/TBF10 for treated fibers, respectively. In the process, the increase of amperage Polímeros, 30(4), e2020040, 2020

The incorporation of untreated and alkali-treated banana fiber in SEBS composites

Figure 1. Banana fibers after extraction: (a) untreated and (b) after alkali treatment.

Figure 2. Composites preparation: (a) Banana fiber extraction, (b.1) Banana fiber untreated and (b.2) alkali-treated, (c) Knife milling of fibers, (d) K-Mixer processing, (e) Production of plates banana fiber-TPE composites in the compression molding.

is indicative of the polymer melt and ending of processing.

Then, each process occurred by 65 s at 1750 rpm when it is possible to hear when fluxing of the materials occurred followed at 3550 rpm for complete mixing with BF. Further, plates composites with dimensions of 120 x 80 x 2mm were prepared in compression molding equipment (Hidro MH-P8-MT; MH Equip. Ltda., Unifesp Diadema, São Paulo, Brazil) using molds at 160 °C for 3 minutes and 8 bar pressure, producing 2.0 mm thick plates. After the plates were prepared, the specimens were stamped using a manual press (Tecnal Equipamentos Científicos Ltda., São Paulo, Brazil) and specimens were obtained for application of mechanical tests.

between the holocellulose and α-cellulose content. The lignin fraction was calculated as the sum of non-soluble and soluble lignin. The results presented are the mean and standard deviations of three replicate determinations of each sample.

2.3.2 Fourier transform infrared spectroscopy (FT-IR)

2.3 Characterization

FTIR is performed to verify chemical compositional change after chemical treatment. Five percent of both untreated and treated banana fibers were carried out dispersing this powdered BF on KBr pellets and using a Shimadzu spectrophotometer (Model Prestige 21-IR, Unifesp, Diadema, São Paulo, Brazil) with a resolution of 4 cm−1, accumulation of 32 scans and in the range of 4000-500 cm−1. Triplicate data were collected for each sample.

2.3.1 Chemical composition analysis

2.3.3 Scanning electron microscopy

Compositional of fiber contents (extractives, cellulose, and lignin) was carried out with some modifications according to TAPPI’s protocols described in[26-28]. Holocellulose was determined according to the method described by Wise et al.

[29] . The hemicellulose fraction was calculated as the difference

Scanning electron microscope (SEM, JEOL JSM, Model 6610 L20, Unifesp, Diadema, São Paulo, Brazil) with an acceleration voltage of 10 kV was used for the study of the morphological behavior of the untreated and chemically modified fiber surface, and the analysis of the composite.

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Cuebas, L., Bertolini Neto, J. A., Barros, R. T. P., Cordeiro, A. O. T., Rosa, D. S., & Martins, C. R. Fracture surface of composites was prepared by placing samples in liquid nitrogen and breaking it. These surfaces were coated with gold coating.

2.3.4 Tensile Testing Tensile test measurement was carried out according to ASTM Standard D412[30], and dumbbell‐shaped samples (Die D) were cut from the molded sheets presented in the method. Samples were submitted in the universal testing machine (EMIC DL-10000, FATEC, São Paulo, Brazil).

A load cell of 5kN was used, and the cross-head speed was maintained at 10 mm/s. Six replicates were made for each sample.

Table 1. Lignocellulosic content of untreated and treated banana fibers.

Banana Fiber (BF) Untreated (UTBF) Treated (TBF)

Lignin (%) 6.3 2.1

Hemicellulose (%) 18.0 11.6

2.3.5 Dynamical mechanical properties For the determination of dynamic parameters of composites under cyclic external forces, dynamic mechanical analysis (DMA) was done. The dynamic parameters such as storage modulus (E′), loss modulus (E″), and damping factor (Tan δ) are temperature dependent and provide information about interfacial bonding between the reinforced fiber and polymer matrix of composite material. Specimens with a nominal size of 30 × 5 × 1 mm were cut from the compression molded plates and measures were carried out on a TA Instruments DMA (model Q800, UFABC, Santo André, São Paulo, Brazil) with a heating rate of 3 ºC/min, in strain mode. Duplicate samples were scanned over a temperature range of -100 to 120 ºC at a constant frequency of 1 Hz. Loss modulus and loss tangent (tan δ) were measured and analyzed as a function of temperature.

3. Results and Discussions 3.1 Chemical composition Chemical characterizations of the raw and treated banana fibers were done to determine the cellulose, hemicelluloses, lignin, and ash content. Results are presented in Table 1.

The treatment of natural fibers by sodium hydroxide (NaOH) is widely used to change the cellulosic molecular structure. This provides more access to penetrate chemicals, and hydrophilic hydroxyl groups are reduced and increase the fibers moisture resistance property. It also removes a certain portion of hemicelluloses, lignin, pectin, and waxes.

As can be seen in Table 1, the alkaline treatment carried out on the BF resulted in a decrease in the amount of lignin and hemicellulose present in about 66.7 and 35.5%, respectively, showing partial removal of these components. These results showed similarities with chemical compositions previously reported for banana fibers, as reported by Gonçalves et al.[31] that found 9.0% and 3.0% of lignin for untreated and treated banana fibers, as well as 25.1% and 16.1% of hemicellulose for untreated and treated fiber, respectively. Nery and José’s work[32] was also in accordance with the results. These authors reported 5.2% and 3.8% of lignin for untreated and treated banana fiber and 20.9% and 14.1% of hemicellulose.

Figure 3. FTIR spectra of banana fiber before (black line) and after alkali treatment (red line).

BF are presented in Figure 3, which seems much similar to banana fiber characterization reported earlier[33]. From left to right, a band in the region of 3400 cm-1 can be observed, indicating the presence of free hydroxyl groups (OH) present in the cellulosic structures in both fibers[34]. These hydroxyls refer to amorphous cellulose, hemicellulose, and lignin. The decreased presence of this group in the treated fiber shows that after mercerization, the sample became more hydrophobic and abler to interact with the matrix. The peak at 2930 cm-1 has been assigned to the C-H stretching vibration from the -CH2 group of cellulose and hemicellulose[35]. The peak at 1740 cm-1 in both samples corresponds to the carbonyl group (C=O) of hemicellulose or the ester bond of the carboxylic group of ferulic acid and p-coumeric acid of lignin and hemicellulose[34]. This peak has a lower intensity for the treated fiber due to the removal of lignin and hemicellulose from banana fibers by alkaline treatment. The band at 1640 cm-1 in both samples indicated the presence of C=O stretch of the acetyl group of hemicellulose[36]. The 1050 cm-1 stretch present in the spectrum shows the absorbance of the O-C-O stretch present in the cellulose and hemicellulose, and C-O/C-C stretching vibration[36].

3.4 Scanning electron microscopy Figure 4 presents the SEM micrographs of the untreated and treated banana fibers. For both treatments, at 2% loading, the appearance of the composite appears homogeneous (Figure 4a and 4b). In untreated banana fibers shown in Figure 4b, c cellulosic structures can be seen outside the matrix suggesting poor interfacial adhesion, which for samples with treated fibers (Figure 4d-f) does not occur.

3.2 Fourier transform infrared spectroscopy

3.5 Mechanical test

Fourier transform infrared spectroscopy (FTIR) was used to analyze the functional groups present in the untreated and treated fibers. FTIR spectra of the untreated and NaOH treated

Mechanical test was applied for the untreated and treated banana fibers composites. Figure 5a shows, comparatively, the tensile strength of all compositions. With increasing

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Polímeros, 30(4), e2020040, 2020

The incorporation of untreated and alkali-treated banana fiber in SEBS composites

Figure 4. SEM micrographs of (a) untreated banana fiber, (e) alkaline treated banana fiber and, cross-section of SEBS composites with different concentrations of untreated and treated banana fibers: (b) TPE/UTBF2, (c) TPE/UTBF5, (d) TPE/UTBF10, (f) TPE/TBF2, (g) TPE/TBF5 and (h) TPE/TBF10.

incorporation of banana fibers, the tensile strength of TPE/ BF composites decreases, indicating that the addition of BF to SEBS decreases the deformation capacity of the material.

Comparing the mechanical performance with the fiber treatment, it can be observed that the samples with untreated fibers presented a higher tensile strength, except for the incorporation of 5% of BF, where there was an inversion.

This can also be seen in Figure 5b, where Young’s modulus increases as the fiber’s incorporation grows, indicating greater rigidity of the composite. It is also possible to observe that the non-treatment of the fibers becomes the material slightly more rigid, 8 to 10%, in all compositions. The Polímeros, 30(4), e2020040, 2020

greater rigidity of the composite at higher concentrations can be explained by the difficulty in movement imposed by the presence of fibers. Few studies were found about the incorporation of natural fibers in SEBS blends, many of them with polyethylene and no one only with SEBS. Panaitescu et al.[37] have shown the reinforcement of hemp fibers on polypropylene/SEBS composite. In their work, hemp fibers were treated with a mild condition, 1% NaOH solution during 30 minutes and 1 hour. The soft treatment was chosen because the complete removal of lignin can suppress the action of π electron interactions between SEBS containing an aromatic portion and lignin, which may lead 5/9

Cuebas, L., Bertolini Neto, J. A., Barros, R. T. P., Cordeiro, A. O. T., Rosa, D. S., & Martins, C. R. the polymer. Except for the 2% sample, the BF treatment showed a longer elongation than the untreated fibers, this may be due to the amount of treated fiber added to the composite that caused a greater specific interaction between the banana fiber and the polymer. It was found that the chemically treated banana fiber composites, i.e., TPE/TBF5 and TPE/TBF10 show a 15.4% and 22.2% higher elongation, respectively, than the corresponding untreated samples. This might be attributed to better fiber-matrix interaction adhesion.

3.6 Dynamic mechanic test of TPE/BF composites The effect of BF on the viscoelastic behavior of SEBS was investigated by DMA. The loss modulus and tan delta resulting from the dynamic frequency scans of SEBS composites are shown in Figure 6a-d as a function of temperature. Loss modulus shows the maximum value, which corresponds to the viscoelastic behavior of the elastomers when the frequency of the material’s internal movements is comparable to the frequency at which the measurement is made. It is seen from Figure 6a-c that the incorporation of untreated BF in SEBS increased the loss modulus of composites between -60ºC to -40ºC. At a fiber loading of 10% (TPE/UTBF10), the most pronounced effect of the filler was found, 927 MPa of loss modulus, a value 56.5% higher than TPE/UTBF2 and 45.3% greater than TPE/UTBF5, reflecting the increased concentration of fiber with greater capacity to convert mechanical energy into thermal. All treated samples had a loss modulus lower than pure TPE, 278 and 222 MPa, respectively, TPE/TBF2 and TPE/TBF5, TPE/TBF10 obtained a loss modulus similar to TPE, 328 MPa.

Figure 5. Mechanical behavior of banana fiber-TPE composites. (a) Tensile strength, (b) Young’ modulus and (c) elongation at break of TPE/BF composites.

to better interfacial interaction, as shown in Szabó et al. work[38]. However, in Panaitescu et al. work, composite was formed majority by polypropylene (PP) mixed with 5% of a coupling agent (maleic anhydride grafted polypropylene), 15% of SEBS, and 30% of fiber. Although the increasing of mechanical properties caused by alkaline treatment of fibers was observed in many works with different natural fibers and polymer matrices[39-41], similar results were found[42,43].

Besides the alternative explanation of SEBS aromatic groups and the presence of lignin in UTBF, according Adeniyi et al., the fact may be attributed by adhesive effect of matrix bound favorably with the untreated plantain fiber.

Consequently, to Young’s modulus, the elongation (Figure 5c) decreases with the increase in the fiber loading.

This behavior suggests poor bonding of banana fiber with 6/9

Block copolymers such as SEBS have two glass transition temperatures, one for the styrene block and one for the ethylene-butylene block. In Figure 6d the damping properties are expressed by the plot of tan δ as a function of temperature. SEBS fiber composites showed two glass transition (TG), at -50ºC referred of ethylene-butylene, and a shoulder peak at 120 ºC, probably referred to styrene block.

Tjong et al.[44] found a peak related to ethylene-butylene of SEBS at -35 ºC and Hashemi[45] reported 3 relaxation peaks at -105 ºC, -36 ºC and 109 ºC. According to Tjong et al. the transitions are due to the relaxations of the hard polystyrene domains and soft ethylene-butylene block, respectively, whereas the third transition is caused by the crank-shaft mechanism involving the –(CH2)– units present in the elastomeric poly(ethylene-butylene) segments[44]. Compatibility of the fibers with the matrix can be assessed by observing the behavior of the composite over the temperature range.

Improvement in interfacial bonding in composites occurs as observed by the lowering in tan delta values between 20 and 60 ºC. The higher the damping at the interfaces, the poorer the interface adhesion. In this work, the peak of -105 ºC was not found as shown by Kumari[25], however, the analysis shown in Figure 6 was performed between -100 and 120 ºC. The maximum observed in the loss module in the glass transition region pointed in tan delta curves is due to the high conversion of mechanical energy into heat through the micro-Brownian movements of the main chain segments.

Maximum peak corresponds to the maximum dissipation of mechanical energy situation curve.

Polímeros, 30(4), e2020040, 2020

The incorporation of untreated and alkali-treated banana fiber in SEBS composites

Figure 6. Loss modulus (a-c) and tan delta curves (d) of TPE/BF composites with untreated and treated banana fibers.

4. Conclusions It was reported that alkali treatment promotes the removal of lignin and hemicellulose from the fiber. The breakage of hydrogen bonds creates many active hydroxyl groups that increase the fiber’s hydrophilicity, improving the compatibility of banana fiber with the matrix. It can be observed that the samples with untreated fibers presented a higher tensile strength, except for the incorporation of 5% of BF, where there was observed a reversal pattern. Moreover, the tensile strength of all compositions decreases with the incorporation of fiber, indicating that the addition of BF to SEBS decreases the rigidity of the composite. Hence, we can understand the reason for the increase of Young’s modulus of alkali-treated fiber composites for all compositions. It is also possible to observe that the non-treatment of the fibers becomes the material slightly more rigid, 8 to 10%, in all compositions. Finally, based on DMA results of SEBS composites, it is seen the incorporation of untreated BF in SEBS increased the loss modulus of composites between -60ºC to -40ºC and at a fiber loading of 10% it was obtained the most pronounced effect, 927 MPa of loss modulus, a value 56.5% higher than the 2% and 5% samples, what reflects the increased concentration of fiber with higher capacity to convert mechanical energy into thermal.

5. Acknowledgements The authors would thank Federal University of São Paulo - Núcleo de Instrumentação para Pesquisa e Ensino (NIPE) for FTIR analysis, Federal University of São Paulo Centro de Equipamentos e Serviços Multiusuários (CESM) for the SEM analysis, São Paulo State Technological College (FATEC-SP) for the mechanical test and MultiUser Experimental Center (CEM) of Federal University of ABC for the DMA analysis. The authors also thanks São Paulo Research Foundation (2018/11277-7), Engineering, Modeling and Applied Social Sciences Center (UFABC), and Núcleo de Revalorização de Resíduos (REVALORES) for the support.

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Received: July 30, 2020 Revised: Dec. 05, 2020 Accepted: Jan. 08, 2021

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.06620

O The effect of extrusion processing on the physicochemical O and antioxidant properties of fermented and non-fermented O Jabuticaba pomace O Eduardo Ramirez Asquieri * , Jose de Jesus Berrios , Elaine Meire de Assis Ramirez Asquieri , O James Pan , Aline Gomes de Moura e Silva  and Rayssa Dias Batista  O Faculdade de Farmácia – FF, Universidade Federal de Goiás – UFG, Goiânia, GO, Brasil Western Regional Research Center – WRRC, Agricultural Research Service – ARS, United States Department of Agriculture – USDA, Albany, CA, United States O Faculdade de Nutrição – FANUT, Universidade Federal de Goiás – UFG, Goiânia, GO, Brasil Escola de Agronomia – EA, Universidade Federal de Goiás – UFG, Goiânia, GO, Brasil O O Abstract O Previous studies have proven that the flour obtained from the residue of Jabuticaba (Jab) juice and wine industries is source of bioactive compounds and an option for the production of food such as extrudates. The objective of this work O was to produce extrudates with different concentrations (0, 5, 10, 15, 20%) of non-fermented and fermented Jab pomace flour, as well as to evaluate their physical, chemical and technological properties and the effect of the extrusion process O on the antioxidant capacity. Results showcased that the extrudates have low content of resistant starch (0.26 g/100 g) and extrusion conditions decreased the content of polyphenols and antioxidant potential. The addition of 20% non-fermented O Jab pomace reached an antioxidant activity of 2904 µg trolox/g in the DPPH method, and promoted rheological changes in the product, such as lower expansion index, higher density and hardness; while presenting higher phenolic content and antioxidant capacity.

O Keywords: extrudates, Myrciaria cauliflora, peels, polyphenols, seeds.

O How to cite: Asquieri, E. R., Berrios, J. de J., Asquieri, E. M. A. R., Pan, J., Silva, A. G. M., & Batista, R. D. (2020).

The effect of extrusion processing on the physicochemical and antioxidant properties of fermented and non-fermented O Jabuticaba pomace. Polímeros: Ciência e Tecnologia, 30(4), e2020041. https://doi.org/10.1590/0104-1428.06620 1

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*asquieri@gmail.com

1. Introduction Jabuticaba (Jab) is a highly perishable tropical fruit which is native to south-central Brazil. Amongst the known species of Jab are Myrciaria cauliflora (DC) Berg and Myrciaria Jabuticaba (Vell) Berg, whose ripe fruits showcase dark, thin and fragile peels, while the pulps present whitish color and slightly sour-sweet mouthfeel[1].

The industrialization of Jab results in products such as juices, jellies, ice cream and fermented beverages.

Regarding Jab wine production, its agroindustrial residue has a remarkably high volume due to the discard of half of the fruit content, which is mostly composed by peels and seeds[2].

Jab pomace contains large amounts of phenolics such as anthocyanins when compared to the whole fruit[3].

Leite-Legatti et al.[4] conducted an experiment to assess the nutraceutical effect of adding freeze-dried Jab peel in a high fat diet, and their results evidenced that Jab peel consumption increased HDL-cholesterol, therefore suggesting a protective effect against cardiovascular disease and improved insulin resistance. Moreover, extracts prepared with Jab peel

Polímeros, 30(4), e2020041, 2020

showcased antimutagenic (in vivo) and antiproliferative (in vitro) effects against leukemia and prostate cancer[5].

Extrusion is a processing technique that can be used in the development of new food products from food industry byproducts[6]. Besides, extrusion facilitates the inclusion of fibers in the starchy material and improves the sensorial and functional characteristics of the produced extrudates[7].

In a previous study, Morales et al.[8] analyzed the fermented and non-fermented Jab residues and found that the pomaces are a source of bioactive compounds such as tocopherols, polyunsaturated fatty acids and phenolic compounds with high antioxidant potential.

Owing to the fact that Jab pomace may be considered a functional ingredient in the fabrication of human and livestock food, the objective of this work was to prepare extrudates with different concentrations of fermented and non-fermented Jab pomace flour, and evaluate the physical, chemical and technological properties of the developed products.

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Asquieri, E. R., Berrios, J. de J., Asquieri, E. M. A. R., Pan, J., Silva, A. G. M., & Batista, R. D.

2. Materials and Methods 2.1 Sample preparation This study used the industrial byproducts of Jab wine and juice production, whose industrial plant is located in the region of Nova Fatima, GO, Brazil. These residues were constituted by fermented (JF) and non-fermented (JNF) peels and seeds. The industrial byproducts were subjected to 24 hours dehydration in a forced-air drying oven (Imperial IV Microprocessor Oven, Lab-Line Instruments, Melrose Park, USA) at 50 °C, and were grounded thereafter in a knife mill (Wiley, Thomas Scientific, New Jersey, USA). After grounding, the material was sieved in a 100-mesh stainless steel tamis with a 0.15 mm aperture and thereafter, packed under vacuum in plastic bags.

The mixture of lentil and rice flour (70:30) (L/R) was used a complementary input for extrudate elaboration. The proportion was established by the content of starch, lipids and fibers, as well as their effect on the blend processability in the extruder. Furthermore, this mixture also provides a balanced composition of nutrients and is a gluten-free product.

The raw materials were weighed and mixed in the following proportions: non-fermented/fermented Jab flour (0, 5, 10, 15, 20%); salt (1.25%); sugar (5%) and L/R (70:30), resulting in 10 formulations. The research was developed at the Western Regional Research Center (WRRC) of the United States, Department of Agriculture (USDA), Albany, California, USA.

2.2 Extrusion conditions A single screw extruder was employed, and the following conditions were set to each zone (n = 1 to 6): at first the material was water cooled (n = 1), then the temperature was raised up to 60 °C (n = 2). Thereafter, the temperature was once again raised, up to 80 °C (n = 3), and then kept steady at 90 °C (n = 4 and 5). At the final zone, the temperature reached 100 °C (n = 6). The screw speed was 500 rpm; the screw diameter was 2.5 mm; the screw length was 200 mm; the feed rate was 50 kg/h and the feed moisture content was 21%.

Then, 150 μL of 0.25 N Folin-Ciocalteu reagent was added and the solution was incubated for three-minutes at room temperature. The reaction was quenched by the addition of 300 μL 1N Na2CO3 and the mixture was incubated again for 25 min. Absorbance was measured at 725 nm using a spectrophotometer (Pharmaspec UV-1700, Shimadzu, Kyoto, Japan). The blank was prepared using methanol and a standard curve (0-0.375 mg/mL) developed with gallic acid (GA) was used for the quantification of phenolics. The content was expressed as mg of GA equivalentes per g of sample.

2.5 Antioxidant capacity The antioxidant capacity was determined for the mixes and extrudates using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) method adapted from Brand-Williams et al.[12] with some modifications. The analysis used the same methanolic extract that was prepared for the phenolic compounds quantification.

A 50 μL aliquot of the sample was reacted with 2950 μL of DPPH (103.2 μM in methanol, with an absorbance of ~ 1.2 at 515 nm). The reaction occurred at room temperature under stirring at 180 rpm x 22 hours on a horizontal shaker (HS 250 basic, IKA Labortechnik, Staufen, Germany)[13,14].

The spectrophotometer was calibrated with methanol and the absorbance was measured at 515 nm. A standard curve was assayed for 6-hydroxy-2,5,7,8-tetramethylchroman-2carboxylic acid (Trolox) (0-750 μg/mL) and results were expressed as µg of Trolox equivalents per g of sample.

2.6 Expansion index The expansion index was calculated by the ratio of the extrudate diameter (mm) to the extruder diameter (mm) using stick-shaped samples of 6 cm length[15,16].

2.7 Apparent density A Syntron Vibra-Flow Feeder equipment (F-T01A, Syntron Co., Homer City, USA) was used to determine the apparent density (g/cm3) of the extrudates[17]. The bulk volume that was occupied by the extrusion product was calculated from the weight of the displaced beads and the density by the ratio between weight and bulk solution volume.

2.3 Dietary fiber and Total, available and resistant starch

2.8 Pasting properties

Content of soluble and insoluble dietary fiber in JNF and JF was determined according to a standard enzymaticgravimetric method[9]. The content of total, available and resistant starch was determined for both of the investigated flours (JNF, JF), mixes and extrudates, using the MEGAZYME Kit following the methods 2002.02[9] and 32-40.01[10].

The viscosity was determined for the mixes and extrudates using a RVA (Rapid Visco Analyzer)[18]. Therefore, 3 g of sample were weighed in RVA crucibles, followed by addition of deionized water, according to the moisture of each sample, so that all samples reached 10% The stirring was carried out to prevent the formation of lumps in the RVA. The mixture was initially held at 25 °C for 2 min, the temperature was gradually raised at a rate of 5.83 °C/min up to 95 °C. When the system reached 95 °C, the temperature was kept for 4 min. Cooling was also gradual at a rate of 11.25 °C/min until reaching a final temperature of 50 °C, which was kept for 2 min.

2.4 Total soluble polyphenols Total soluble polyphenol content was determined in the mixtures and the extrudates using the methodology described by Swain and Hillis[11] with some modifications.

Five grams of sample were were homogenized with 20 mL of methanol using a shaker (Waring, Torrington, USA), wrapped in dry ice and shaken for one minute. The tubes were centrifuged (SA-600 rotor, 15600 rpm for 15 min at 4°C), and aliquots of 150 μL were withdrawn from the clear supernatant and diluted with 2400 μL of nanopure water.

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2.9 Texture analysis The texture of the extrudates was determined with a TATX2 Texture Analyzer (Stable Micro Systems, Goldaming, Surrey, UK) using the Texture Expert software (Stable Micro Polímeros, 30(4), e2020041, 2020

The effect of extrusion processing on the physicochemical and antioxidant properties of fermented and non-fermented Jabuticaba pomace System version 1.22). The conditions were the following: pre-test speed of 2.5 mm/s; test speed of 2.00 mm/s; posttest speed of 10.0 mm/s; distance of 3.0 mm; compression force of 20 g; and trigger (Self-20g). The hardness (N) and crunchiness (N/mm) were determined using 15 randomly selected samples of 6 cm length, and the force peak (hardness) and the area under the curve (crunchiness) were chosen to represent the textural properties of the extrudates.

2.10 Color evaluation The samples were grounded in a Cyclone Sample Mill (Udy Corporation, Fort Collins, USA) to obtain particles of 1 mm. The JF and JNF flours, mixes and extrudates were evaluated the instrumental parameters of color in the colorimeter (CM-3500d, Minolta, Tokyo, Japan)[19]. Color attributes were expressed according to the Cielab system with values of L* (black-white component, luminosity), a* (+red to −green component) and b* (+yellow to −blue component). The following Equation 1 was used to obtain the total color difference (ΔE*) between the extrudate and the mix. ∆E * =

( ∆L *

2

+ ∆a *2 + ∆b *2

)

1/ 2

(1)

2.11 Scanning electron microscopy A scanning electron microscope (Quanta-200, FEI Company, Netherlands, USA) was used to visualize the ultra-structural morphology of the materials. The extrudate flours were fixed in aluminum stubs using a double-sided adhesive tape, bathed in a thin gold film (10 nm) and examined under a 2 kV acceleration voltage. Micrographs were obtained at 30× magnification.

2.12 Statistical analysis The Minitab® 16 software was used for statistical analysis. For comparison of fiber contents a t-Student test was performed. The data of total, available and resistant starch of the raw materials (JF, JNF, L/R) was analized with one way Anova. For the analysis of polyphenols, color parameters and antioxidant capacity of mix and extrudates was performed Anova with 3 factors, being the factors the type of jab flour (JF or JNF), the % of Jab flour (0, 5, 10, 15 and 20%) and the type of sample (Mix or ext). For the analysis of density, expansion index and texture Anova was made with 2 factors being the factors Jab flour type in the extrudate (JF or JNF), and the percentage of Jab flour in the extrudate (0, 5, 10, 15, and 20%). Tukey’s test was used for comparing means.

The results were considered significant whenever p≤0.05.

The data was expressed as mean ± standard deviation and reported as means of triplicates (except expansion index, texture analysis and pasting properties which were means of ten replicates).

3. Results and Discussions 3.1 Dietary fiber and total, available and resistant starch The content of dietary fiber was determined only for JNF and JF flours in order to verify its effect on the Polímeros, 30(4), e2020041, 2020

extrusion process. For JNF, 26.3 g/100 g of insoluble fiber, 12.1 g/100 g of soluble fiber and 38.4 g/100 g of total fiber were quantified. The JF obtained a higher total fiber content (53.9 g/100 g) which was distributed in 42.4 g/100 g of insoluble and 11.5 g/100 g of soluble fiber.

The available starch content of the raw materials (JF, JNF) was lower than the contained in L/R (Table 1), in contrast, resistant starch presented higher values in JNF (p<0.05). A comparison of the mixes pointed out that the addition of either 20% JF or JNF increased the content of resistant starch (p<0.0053).

After the extrusion, we observed that the available starch in the extrudates decreased and Berrios et al.[20] observed a decrease in the total carbohydrate content after extrusion, suggesting that the cause would be the degradation of the starch into low molecular weight derivatives which were involved in Maillard and caramelization reactions.

Starch has a great importance in the extrusion process, influencing the texture and the expansion of the extrudates.

The mixes showcased significant contents (p<0.0002) of available starch (58.47 to 71.92 g/100 g); however, the resistant starch contents detected in the extrudates were low (0.06 to 0.26 g/100 g).

Morales et al.[21] revealed that the total fiber content decreased after the extrusion of lentil flour, which may also have occurred in the extrudate with 20% Jab flour, because the resistant starch has properties similar to fibers.

3.2 Total soluble polyphenols (TSP) and antioxidant activity Table 2 shows the content of TSP and antioxidant activity in mixes and extrudates. It was observed that the values increased with the addition of Jab flour. The comparison of the mixtures and their respective extrusions showed that the extrusion process decreased the phenolic content (p<0.011).

However, the loss was greater in JF extrudates (about half in relation to their respective mixture). This fact is probably related to the degradation of the polyphenol molecules after the yeast action during the fermentation, making these compounds fragile to the extrusion conditions[22].

Lohani and Muthukumarappan[23] reported that the extrusion process alters the phenolic content due to the polymerization or degradation of these compounds upon incidence of heat.

The addition of Jab flour increased the antioxidant activity (p<0.05). Regarding the extrudate without jabuticaba flour, antioxidant activity was observed even when phenolic compounds were not detected and this could be due to the presence of other compounds with antioxidant action, such as tocopherols and organic acids.

Table 2 shows that after extrusion, the antioxidant capacity decreased in formulations containing JF and JNF, with the exception of JNF 20% which remained stable (p<0.05).

A possible explanation for the stability of the antioxidant activity with 20% JNF could be the presence of sugars, gums and mucilages in the raw material, which crystallize and encapsulate antioxidant compounds, thus protecting them from the effects of temperature and pressure during extrusion. The decline of the antioxidant capacity may be 3/8

Asquieri, E. R., Berrios, J. de J., Asquieri, E. M. A. R., Pan, J., Silva, A. G. M., & Batista, R. D. Table 1.Total, available and resistant starch (g/100 g) of the flours, mixes and extrudates.

Available starch Resistant starch Total starch 57.91±0.12a 0.27±0.03c 58.19±0.11a 2.40±0.16c 1.78±0.11b 4.17±0.27c 6.88±0.25b 3.46±0.09a 10.34±0.33b Mix Ext. Mix Ext. Mix Ext. (%) 0 71.92±0.23aA 70.35±0.59aB 0.35±0.006cA 0.07±0.01bB 73.31±0.22aA 70.40±0.31aB JF 20 58.12±0.43dA 58.47±0.29dA 1.05±0.02bA 0.25±0.006aB 58.46±0.29dA 58.86±0.82dA 0 68.12±0.30bA 68.57±0.43bA 0.39±0.01cA 0.06±0.006bB 69.17±0.45bA 68.64±0.59bA JNF 20 63.86±0.82cA 61.86±0.85cB 1.39±0.02aA 0.26±0.01aB 64.11±0.44cA 62.12±0.85cB Mean and standard deviation of three replicates. For the Tukey test in raw materials, means with different lower case letters in the same column are significantly different (p≤0.05). For the Tukey test in mix and extrudates, means with different lower case letters in the same column and upper case letters in the same line are significantly different (p≤0.05). Ext: extrudates; L/R: lentil and rice flour (30:70). Non-fermented flour of Jab (JNF) and fermented flour of Jab (JF).

L/R JF JNF

Table 2. Total soluble polyphenols (TSP) and antioxidant capacity (AC) in the mixes and extrudates (Ext). Non-fermented flour of Jab (JNF) and fermented flour of Jab (JF).

(%)

0

Total soluble polyphenols

Antioxidant capacity

(mg of GA equivalentes per g of sample) Mix Ext 0.03 ± 0.01d -

(µg of Trolox equivalents per g of sample) Mix Ext 247.00 ± 16.8dA 189.26 ± 8.32eB

5 0.17 ± 0.03cA 0.06 ± 0.005dB 758.84 ± 25.31cA 323.26 ± 22.8dB bA cB bA 10 0.37 ± 0.01 0.15 ± 0.004 1779.9 ± 7.12 721.21 ± 11.9cB JF 15 0.72 ± 0.01aA 0.26 ± 0.006bB 2976.3 ± 0.67aA 1071.9 ± 26.8bB 20 0.79 ± 0.07aA 0.43 ± 0.004aB 2979.6 ± 0.42aA 1772.5 ± 32.7aB 0 0.02 ± 0.007e 252.14 ± 6.76dA 181.36 ± 4.59eA dA dA cA 5 0.15 ± 0.009 0.10 ± 0.003 708.40 ± 14.88 487.17 ± 18.7dB 10 0.35 ± 0.004cA 0.27 ± 0.005cA 1887.8 ± 16.46bA 1244.6 ± 27.3cB JNF 15 0.66 ± 0.008bA 0.38 ± 0.029bB 2971.2 ± 2.30aA 1859.8± 72.2bB aA aB aA 20 0.82 ± 0.033 0.64 ± 0.052 2981.7 ± 1.72 2898.1 ± 36.7aA Mean and standard deviation of three replicates. For the Tukey test, means with different lower case letters in the same column and upper case letters in the same line are significantly different (p≤0.05). Ext: extrudates, Non-fermented flour of Jab (JNF) and fermented flour of Jab (JF).

related to the decrease of phenolic compounds, organic acids and tocopherols, which are present in Jab’s flour[8], as a consequence of the extrusion conditions of temperature and shear. The same behavior was observed in lentil flour after extrusion[21].

Carbohydrates are the most frequently used compounds for sample encapsulation as exemplified by Azeredo[24]. This protection of antioxidant compounds through encapsulation does not occur in JF, because sugars and polysaccharides are consumed and hydrolyzed during fermentation.

3.3 Apparent density, expansion index and texture analysis The density of the extrudate products varied from 0.08 to 0.34 g/cm3 (Table 3). The extrudates with 20% JNF and JF were denser to other extrudates (p<0.014). In addition, the extrudates with JNF and JF presented significantly different densities between them (p<0.014). JF flour presented lower apparent density due to the fermentation process that allows microorganisms to convert the nutritional compositions of the substrate into organic acid, causing this reduction in density[25,26].

As showcased in Table 3, increasing concentrations of Jab flour lead to a decrease of the expansion index, thus reaching the lowest value (1.6) with 20% of JNF (p<0.05).

This possibly occurs because Jab flour has high fiber content.

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For JNF 38.4 g/100 g of total fiber were quantified and for JF obtained fiber content (53.9 g/100 g). It is reported that insoluble fiber decreases the proportion of starchy material and reduces the viscosity for expansion. The fiber also binds to water during extrusion, therefore reducing their availability for expansion[27,28].

The highest expansion index of 0% Jab flour in the extrudates may be attributed to the starch content, which influences the elastic characteristics of the mass due to the hygroscopic nature of this polysaccharide and also contributes to increase the expansion[7].

The hardness values varied from 495.0 to 1868.2 N. The 20% Jab flour extrudates presented the highest hardness values in addition to the lowest expansion rates and the highest densities (p<0.05). The reduction in the expansion and increase in hardness are characteristic of products with high fiber content, whose action decreases the elasticity of the molten material upon exiting the extruder[29]. The extrudates with 15 and 20% JF showed hardness values statistically higher (p<0.05) than JNF extrudates, possibly due to the higher fiber content in JF flour (53.9 g/100 g), compared to JNF (38.4 g/100 g) (p<0.05).

The crunchiness presented values from 0.22 to 0.66 N/mm, the least crunchy extrudates were the ones with 20% Jab flour addition (Table 3). As the % of Jab flour increased, the fiber content also increased and the crunchiness decreased. The incorporation of fibers in extrudates limits their expansion Polímeros, 30(4), e2020041, 2020

The effect of extrusion processing on the physicochemical and antioxidant properties of fermented and non-fermented Jabuticaba pomace Table 3. Apparent density, expansion index and instrumental texture analysis of the extrudates. Non-fermented flour of Jab (JNF) and fermented flour of Jab (JF).

0

5

% flour 10

15 20 Apparent density (g/cm3) JF 0.09 ± 0.001aD 0.09 ± 0.001bD 0.12 ± 0.006bC 0.19 ± 0.003bB 0.24 ± 0.005bA JNF 0.08 ± 0.003aE 0.12 ± 0.002aD 0.14 ± 0.003aC 0.28 ± 0.006aB 0.34 ± 0.010aA Expansion index JF 3.5 ± 0.31aA 3.2 ± 0.22aAB 2.9 ± 0.32aBC 2.8 ± 0.25aC 1.8 ± 0.16aD aA aB aBC aC JNF 3.5 ± 0.12 3.0 ± 0.18 2.8 ± 0.20 2.7 ± 0.14 1.6 ± 0.18aD Hardness (N) JF 495.0 ± 47.1aC 602.7 ± 27.9aC 691.1 ± 67.7aC 1599.5 ± 27aB 1868.2 ± 88aA aC aC aC bB JNF 536.9 ± 41 553.6 ± 40.5 590.0 ± 74.6 848.4 ± 81.6 1324.7 ± 72bA Crunchiness (N/mm) JF 0.61 ± 0.17aA 0.50 ± 0.26aAB 0.45 ± 0.24aAB 0.28 ± 0.05aB 0.28 ± 0.10aB JNF 0.78 ± 0.47aA 0.64 ± 0.31aAB 0.44 ± 0.24aBC 0.31 ± 0.16aC 0.20 ± 0.06aC Mean and standard deviation of three replicates. For the Tukey test, means with different lower case letters in the same column and upper case letters in the same line are significantly different (p≤0.05). Ext: extrudates, Non-fermented flour of Jab (JNF) and fermented flour of Jab (JF).

and reduces their crunchiness[30]. Both the hardness and the crunchiness of the extrudates are associated to the expansion and cellular structure of the product[28]. Therefore, an extrudate with low expansion results in a product with reduced crunchiness and high hardness.

3.4 Pasting properties Figure 1 shows the paste viscosity profiles of the mixes and extrudates as a function of time and temperature. The mixes showcased similar behavior regarding the time to achieve the maximum and final viscosity. The highest values of maximum and final viscosity were obtained with the mix without Jab and the lowest at 20% Jab. The addition of flours with high fiber content can contribute to the reduction of the viscosity values, by the fact of reducing the total accessible starch content that will gelatinize during the analysis[31]. This fact was observed with the addition of jabuticaba flour that presents high fiber content as previously mentioned.

The mix without Jab showed a breakdown of 522.5 cP and a setback of 1649.5 cP, differing from both JNF (20%, breakdown: 230.5 cP) and JF (20%, setback: 829.5cP). This means that the mix without Jab presented lower stability at higher temperatures under agitation (higher breakdown value) and a greater tendency to retrograde (high setback value).

The analysis of the extrudates (Figure 1c and 1d) revealed that the initial viscosities showcased the highest values reached during the RVA analysis. After 3 min, was observed a viscosity drop. The minimum viscosity was reached between 8 and 10 min at 95 ºC. From there, a slight increase of viscosity was observed until the final viscosity was recorded.

During the extrusion of the formulations the same conditions of temperature, moisture and rotation speed were maintained and because they remained constant, the different values of paste properties were attributed completely to the jab flour.

The absence of maximum peak viscosity demonstrates that heat treatment during extrusion may have destroyed Polímeros, 30(4), e2020041, 2020

the crystal structure of the starch granules resulting in low viscosity values during heating at RVA[27,31].

The viscosity profiles of the extrudates demonstrate that the starch was gelatinized in the extrusion process, because it showed the ability to increase the viscosity of the solution at room temperature and showed no tendency to retrograde. These findings therefore suggest the use of this extrudates flour in instant foods.

3.5 Color evaluation The results of the color analysis are presented in Table 4.

Regarding luminosity (L *), JNF flour is darker than JF flour (p<0.001). JF flour was obtained from the residue of wine production, probably during fermentation there was release of pigments such as anthocyanins to the must, making JF flour lighter[27].

As the Jab flour content increased, the mixtures became darker (decreasing the L* value), which was also observed in the extrudates. The mixes without Jab were lighter than the other samples (p<0.001).

The raw materials JNF and JF presented a red tone and yellow subtones (positive values for a* and b* color coordinates, respectively). While for the mixes, yellow tones and red subtones were observed, as well as for the extrudates (values of b* stood out from the values of a*). JF appeared to be more reddish than JNF (p<0.05), probably due to the detachment of the mesocarp during fermentation[27].

For ΔE*, the lowest values were found for the samples without Jab flour (p<0.05), indicating that the extrusion had little effect on the color parameters when the formulations did not present Jab flour. With the addition of Jab residues, the color difference between mix and extrudates was significant, possibly due to Maillard reaction that occurred during extrusion and also the degradation of anthocyanins present in jabuticaba flour.

The extrusion allows a greater interaction between sugars and proteins, triggering non-enzymatic browning (caramelization and Maillard reaction)[7] and the degradation of anthocyanins also contributes to the darkening of the extrudates, as reported for purple potato and pea flour extrudates[32].

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Asquieri, E. R., Berrios, J. de J., Asquieri, E. M. A. R., Pan, J., Silva, A. G. M., & Batista, R. D.

Figure 1. Paste viscosity of the mixes (a-b) and extrudates (Ext) (c-d). Non-fermented flour of Jab (JNF) and fermented flour of Jab (JF).

Lines in dark blue (0%); pink (5%); black (10%); green (15%); orange (20%) and brown (temperature).

Table 4. Color attributes of the flours, mixes and extrudates.

JF JNF

L* 48.85±0.75 53.37±0.67

a* 7.39±0.09 8.32±0.14

b* 6.03±0.25 6.75±0.28

ΔE* -

Mix Ext Mix Ext Mix Ext 0 90.21±0.5aA 85.00±0.48aB -0.42±0.1eB -0.29±0.01bA 9.24±0.52aB 14.70±0.02aA 7.55±0.99b bA bB dB aA bB bA 5 84.47±1.29 63.76±1.33 1.21±0.28 3.64±0.1 5.56±0.45 8.38±0.09 21.05±2.34a cA cB cB aA bB bA JF 10 79.16±0.62 59.10±0.65 2.10±0.21 3.71±0.1 5.36±0.82 7.79±0.06 20.29±1.17a 15 74.58±1.04dA 56.91±0.19cB 3.22±0.27bB 3.67±0.1aA 5.43±0.43bA 5.80±0.08cA 17.70±1.05a 20 73.58±0.31dA 54.37±1.91dB 3.52±0.03aB 4.07±0.1aA 5.38±0.12bA 5.56±0.21cA 19.23±1.91a aA B dB eA B A 0 90.48±0.35 85.61±0.12ª -0.38±0.01 -0.32±0.02 9.34±0.22ª 13.91±0.04ª 6.68±0.26c 5 83.03±0.25bA 64.64±0.49bB 1.96±0.05cB 3.65±0.11dA 6.28±0.22bB 10.27±0.06bA 18.89±0.37ª JNF 10 78.23±0.22cA 60.62±0.48cB 3.32±0.04bB 4.43±0.16cA 5.85±0.16cB 9.36±0.21cA 17.99±0.50ª 15 75.21±0.36dA 57.64±0.19dB 4.15±0.03ªB 4.86±0.11bA 6.25±0.11bB 8.77±0.06dA 17.76±0.54ab eA dB aB aA bB eA 20 72.43±0.89 56.59±0.59 4.45±0.09 5.40±0.15 6.22±0.11 7.29±0.18 15.91±1.44b Mean and standard deviation of three replicates. For the Tukey test, means with different lower case letters in the same column and upper case letters in the same line are significantly different (p≤0.05). Ext: extrudates, Non-fermented flour of Jab (JNF) and fermented flour of Jab (JF).

3.6 Scanning electron microscopy The micrographs showcased differences in the structural characteristics of the extrudates (Figure 2). Differentiated and irregular cell sizes were observed. Figure 2a illustrates the complete gelatinization of the starch. The extrudate has the appearance of a homogeneous amorphous mass in which it is not possible to distinguish starch granules.

The extrudate with 10% JF (Figure 2b) showed large holes, which were formed by the expansion of the product at the extruder outlet, while the extrudate with 10% JNF 6/8

(Figure 2c) exhibited a larger number of air bubbles, thinner walls and was brittle and less hard, as well.

The extrudate with 20% JNF had the lowest expansion index. Figure 2e displays a compact, thick-walled structure, with highest density, unlike the extrudate with 20% JF (Figure 2d), probably because this sample had undergone fermentation. As previously mentioned, the fermentation process allows microorganisms to convert the nutrient compositions of the substrate into organic acid, causing this reduction in density[26].

Polímeros, 30(4), e2020041, 2020

The effect of extrusion processing on the physicochemical and antioxidant properties of fermented and non-fermented Jabuticaba pomace

Figure 2. Micrographs of the extrudates (Ext). Non-fermented flour of Jab (JNF) and fermented flour of Jab (JF). JNF 0% (a); JF 10% (b); JNF 10% (c); JF 20% (d); JNF 20% (e).

4. Conclusion

5. Acknowledgements

Fermented and non-fermented Jab pomace flours are industrial byproducts that aggregate nutrients to the extrudates but increasing their concentration in the extrudate products decreases the expansion index, increases the density and renders darker and reddish tones. The content of polyphenols and the antioxidant activity decreased after extrusion, although the products with 20% of non-fermented Jab pomace flour showcased stable antioxidant activity, thence emphasizing that the flours behaved differently post-extrusion.

The results showed that JNF is the flour with the best potential for the elaboration of extrudates, for presenting polyphenols and other antioxidant compounds that have shown to be resistant, to a certain extent, to the conditions studied in this work for the extrusion process. It is necessary to deepen this study looking for a way to encapsulate the antioxidant compounds to minimize their losses during the extrusion, as well as to expand the complements that can be used (in this work was used the rice and lentil flours).

This work showed that the jabuticaba residues flour can be applied in order to enrich extrudates as snacks that have wide commercialization, as well as other food products that need enrichment with antioxidant compounds.

Acknowledgement The authors wish to acknowledge financial support from Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES, Brazil.

Polímeros, 30(4), e2020041, 2020

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Received: Jun. 19, 2020 Revised: Jan. 07, 2021 Accepted: Jan. 09, 2021 Polímeros, 30(4), e2020041, 2020

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.10619

Mechanical and water absorption properties and morphology of melt processed Zein/PVAl blends Sandro Junior Vessoni Torres1, Gabriela Brunosi Medeiros2, Francisco Rosário1, Fabio Yamashita3, Luiz Henrique Capparelli Mattoso4 and Elisângela Corradini1,2*  Departamento de Engenharia de Materiais, Universidade Tecnológica Federal do Paraná – UTFPR, Londrina, PR, Brasil 2 Programa de Pós-graduação em Ciência e Engenharia de Materiais, Universidade Tecnológica Federal do Paraná – UTFPR, Londrina, PR, Brasil 3 Departamento de Ciência e Tecnologia de Alimentos, Universidade Estadual de Londrina – UEL, Londrina, PR, Brasil 4 Laboratório Nacional de Nanotecnologia para Agronegócio, Empresa Brasileira de Pesquisa Agropecuária – EMBRAPA, São Carlos, SP, Brasil 1

*ecorradini@utfpr.edu.br

Abstract Blends of zein and poly(vinyl alcohol) (PVAl) were processed in an internal mixer (150ºC, 50 rpm) for 5-8 minutes.

Glycerol and oleic acid were used as plasticizers. The mixtures obtained were then compression molded and further characterized by Fourier transform infrared spectroscopy (FTIR), water-absorption experiments, mechanical tests, and scanning electron microscopy (SEM). FTIR analysis indicated the existence of hydrogen bonding interactions between zein and PVAl. Tensile tests showed that the addition of PVAl increased the flexibility of the blends. The tensile strength ranged from 1.7 to 5.7 MPa, elongation at break ranged from 2.7 to 32% and Young’s modulus ranged from 433 to 7371 MPa. Water absorption at equilibrium decreased with increasing zein content, which favored a brittle behavior in the zein/PVAl. The blends were immiscible in the composition studied and the presence of voids indicated poor interfacial interaction between the polymers.

Keywords: melt processing, oleic acid, glycerol, poly(vinyl alcohol) and zein.

How to cite: Torres, S. J. V., Medeiros, G. B., Rosário, F., Yamashita, F., Mattoso, L. H. C., & Corradini, E. (2020).

Mechanical and water absorption properties and morphology of melt processed Zein/PVAl blends. Polímeros: Ciência e Tecnologia, 30(4), e2020042. https://doi.org/10.1590/0104-1428.10619

1. Introduction Zein is a corn protein that represents about 80% of the total proteins in corn grains[1]. Zein is an amphiphilic protein that has both hydrophobic and hydrophilic characteristics.

More than 50% of its amino acid residues[2] are hydrophobic, including high percentages of leucine (20%), proline (10%) and alanine (10%)[3].

Several studies have shown the high potential of zein for the production of packaging and edible films because of its film-forming ability, good oxygen and carbon gas barrier and antioxidant properties, and others such as low water solubility, biocompatibility and biodegradability[4-6].

Zein films are produced by two technological processes: a wet process based on solubilization, and a dry process based on the thermoplastic properties of zein under low humidity conditions. Zein films are prepared by dissolution in aliphatic alcohols and further evaporation of the solvent on inert surfaces. Zein films can be obtained by processing in devices such as kneading, blowing and/or extrusion machines[7-9].

Polímeros, 30(4), e2020042, 2020

Plasticizers such as glycerol, polyols and fatty acids are added during the formation of zein films to improve film flexibility and manageability, since pure zein films are usually very brittle and fragile[10-12]. Glycerol is considered a secondary plasticizer for zein and, when used alone, its effect is limited due to its incompatibility with zein[13]. Oleic acid, as a hydrophobic molecule, is a primary plasticizer, meaning it can effectively plasticize the zein film on its own with a low level of water vapor barrier[14]. Xu et al.[15] studied the effect glycerol and oleic acid mixtures to plasticize zein films. The authors observed by combination of the two plasticizers a synergistic effect in the decreasing in the glass transition temperature (Tg).

Although the addition of plasticizers diminishes the brittleness of zein films, these plasticizers may facilitate the absorption of moisture from highly air humidity, impairing the barrier and mechanical properties of zein films. Blending zein with other polymers is a simple, rapid and low-cost method to overcome those drawbacks. The amphiphilic nature of zein makes it a highly versatile polymer to be

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

Torres, S. J. V., Medeiros, G. B., Rosário, F., Yamashita, F., Mattoso, L. H. C., & Corradini, E. combined with both hydrophilic and hydrophobic polymers in the production of compatible materials with better properties than the pure constituents. The literature reports on several studies about blends of zein with conventional synthetic polymers, such as polyethylene, nylon[16] and polyvinylpyrrolidone (PVP)[17], and with biodegradable (natural or synthetic) polymers such as starch[18-20], chitosan[21] poly(ε-caprolactone) (PCL)[22], poly(vinyl alcohol) (PVAl) [23] , and poly(propylene carbonate)[24], Poly(butylene adipateterephthalate) (PBAT)[9].

The combination of PVAl and zein might be an interesting alternative in the field of biodegradable plastics, since both are biodegradable and processable in the presence of plasticizers, similarly to most conventional synthetic thermoplastic polymers.

PVAl is a flexible material and its presence can favors the ductility of zein/PVAl blends. Additionally, the low solubility of zein in water could increase the hydrophobic character of the corresponding zein/PVAl compositions. However, to the best of author’s knowledge, only three papers dealing to zein/PVAl blends are reported in the literature[23,25,26]. In these studies, the zein/PVAl blends were prepared by casting and using glycerol as plasticizer. So, up to now there is no published report on the melt-processing of zein with PVAl. Efforts were dedicated in present work for obtaining polymer blends from zein and PVAl, looking optimizing the polymers properties and their processing characteristics.

The zein/PVAl blends were obtained by a melting processing.

Oleic acid and glycerol were investigated as plasticizers to process the zein/PVAl blends.

is a completely amorphous polymer, with Tg at about 165 °C, decreasing significantly in response to an increasing degree of plasticization. Lawton[27] showed that the addition of 20% oleic acid lowers the zeins’s Tg to 80 °C. Zein is thermally stable up to 280°C[28]. Glycerol well interacts with PVAl. The melting temperature of PVAl (Selvol™ 203) is in the range of 180–190°C. When plasticized with glycerol, its melting point reduces, enabling it to be melt processed at temperatures below 180oC.

2.3 FT-IR analysis FT-IR spectra of samples, after conditioning at 54 ± 3% of relative humidity (RH) and 25 ± 3ºC for 14 days, were recorded on a Perkin Elmer Spectrum Two FT-IR spectrometer with Universal Attenuated Total Reflectance accessory.

The experiments were recorded in the range of 4000 to 400 cm-1, at a resolution of 4 cm-1 and 64 scans.

2.4 Water absorption Circular samples (11mm in diameter and 2.5mm thick), pre-dried overnight at 105ºC, were weighed and placed in hermetically closed containers with 54 ± 3% of RH at 25 + 2ºC, using a saturated Mg(NO3) solution, as prescribed by the ASTM E 104 standard. The amount of water absorbed by the samples was determined by weighing them periodically until reaching constant weight. The water absorption (W) of the samples was = W (%)

2. Materials and Methods 2.1 Materials Corn zein (protein) was purchased from Sigma–Aldrich, USA, [9010–66-6, MW: ~40 kDa]. Poly(vinyl alcohol) (PVAl) Selvol™ 203 (degree of hydrolysis: 88%, MW ~18 kDa) was purchased from Sekisui Chemical, Japan. Analytical grade glycerol and oleic acid were purchased from Synth Reagents, Brazil.

2.2 Preparation of the blends Blends of zein with PVAl (zein/PVAl) were prepared in proportions of 0/100, 25/75, 50/50, 75/25 and 100/0 (%, w/w), using oleic acid and glycerol as plasticizers. The total content of plasticizers was kept at 20% by weight with respect to the total polymer mass (dry basis). The glycerol:oleic acid ratio (%, w/w) was 1:0; 0.25:0.75; 0.5:0,5; 0.75:0.25 and 0:1 for 0/100, 25/75, 50/50, 75/25 and 100/0 zein/PVAl blends, respectively.

The polymer powders and plasticizers were premixed in a beaker to ensure good homogenization. They were then processed at 160ºC for 5-8 minutes in an internal mixer coupled to a Haake Rheomix 600P torque rheometer operating at a rotor rotational speed of 50 rpm. Then, the mixtures were then compression molded at 150ºC for 5 minutes under a pressure of 5 tons to produce 150 x 120 x 2.5 mm molded sheets.

The processing conditions were defined from the thermal transitions of the polymers and from preliminary tests. Zein 2/8

Mt − M 0 × 100 M0

(1)

where MT is the weight at time t and MO is the dry weight before the exposure of samples to 54 ± 3% of RH.

2.5 Tensile tests Tensile tests were performed in an EMIC DL3000 universal testing machine equipped with a 50 kgf load cell. The samples, pre-conditioned at 54 ± 3% RH and 25 ± 3ºC for 14 days, were prepared according to the ASTM D638M standard, type II. At least 5 samples of each material formulation were tested at a crosshead speed of 2mm/min and room temperature (ca. 25ºC).

2.6 Scanning Electron Microscopy (SEM) The specimens were fractured after immersion in liquid nitrogen and sputter-coated with 20 nm thickness of gold in a Balzers model SCD 50 sputter-coater. SEM images were obtained using a Zeiss Digital Scanning Microscope Model DSM operating at 10 to 15 kV range.

3. Results and Discussion After processing, visual analysis indicated that the processing conditions were sufficient to promote changes in the original structure of the polymers and in their plastification, resulting in thermoplastic materials. It was observed that, as compared to neat zein, the plasticity of polymeric mass improved with the addition of PVAl. Polímeros, 30(4), e2020042, 2020

Mechanical and water absorption properties and morphology of melt processed Zein/PVAl blends Figure 1 shows the equilibrium torque curve as a function of the zein/PVAl blend compositions. In the PVAl/glycerol (0/100) mixture, the torque reached a stable value of around 0.3 ± 0.1 Nm, and no change was observed in the torque curve after the melting of polymer, indicating that no loss of plasticizer, crosslinking or degradation occurred during the processing of the PVAl / glycerol mixture.

In the case of zein plasticized with oleic acid, the torque increased progressively over time, reaching 16.7 ± 0.3 Nm after mixing for 5 minutes. The experiment was stopped at this point, when a stiff polymer paste was obtained.

The progressive increase in torque suggests the formation of crosslinking between zein chains and/or the formation of interactions between zein and oleic acid. According to Gerrard[29], protein crosslinking can increase the resistance to plastic flow or viscosity, making the polymer more difficult to process. The torque values of the zein/PVAl blends at equilibrium were 1.1 ± 0.3 Nm, 2.4 ± 0.3 Nm, and 3.8 ± 0.3 Nm, respectively, for compositions 25/75, 50/50 and 75/25. These values were much lower than that of the plasticized zein, indicating that blend processing was facilitated by the addition of PVAl and glycerol, which reduced the viscosity.

Figure 2 depicts the FTIR spectra of plasticized polymers (PVAl and zein) and their blends. The PVAl spectrum shows a broad band at around 3200-3570 cm-1,

Figure 1. Torque as a function of mixing time of Zein/PVAl blends.

which is associated with O-H stretching of intermolecular and intramolecular hydrogen bonds. The band at around 2850-3000 cm-1 denotes C-H stretching. The peak at about 1725cm-1 is due to C=O and C-O stretching. The peaks at around 1650 cm-1, 1370 cm-1 and 1260 cm-1 were attributed, respectively, to absorbed water, O-H bending and residual acetate[30]. The peak at 1094 cm–1 is related to the C=O stretching in the crystalline region of PVAl[31]. The band at 1039 cm-1 is attributed to the C–O stretching vibration of primary alcohol in glycerol[32].

The FTIR spectrum of zein shows four characteristic bands[28,33]. The band corresponding to stretching of the N-H and O-H bonds of the protein amino acids, which appears between 2800 and 3500 cm-1, is called amide A. Another band, which appears at 1650 cm-1, corresponds to carbonyl (C=O) stretching of amide groups belonging to the peptide groups (amide I). The band at 1540 cm-1 is called amide II and corresponds to the angular deformation vibrations of the N-H bond. The band at 1230 cm-1 corresponds to the axial deformation vibrations of C-N bond in the blends spectra.

One shoulder band observed at around 1710 cm-1 is attributed to the carboxylic C=O stretching of the oleic acid[34]. This signal was overlapped with band of PVAl in the spectrum of blends. In a study using 13C NMR by Forato et al.[35] showed that interactions between oleic acid molecules and zein occur mainly between carboxylic groups (–COOH) present in oleic acid and with groups (NH2) of arginine residues present in the structure of the zein. Gillgren et al.[36] studied the molecular interaction of water and glycerol with zein using FTIR. They reported that water and glycerol bind with the amide groups of zein through hydrogen bonds as they were used as plasticizers. In this work, the effect of water in structure of zein was investigated by the relative heights of the peaks at 1540 and 1515 cm-1 in amide II region for compositions with different moisture contents. It was verified an increase in heights with the water content (results not shown) and similar results were observed by Gillgren et al.[36].

Some slight shifts of the bands of the spectra of zein/ PVAl blends in relation to pure polymer, in the wavenumber range of 1700-1200 cm-1 (Figure 2b) and 3200-3570 cm-1, were observed and suggest possible specific chemical interactions between zein and PVAl and also between these polymers and plasticizers. Interactions between zein and PVAl may occur through the formation of hydrogen

Figure 2. FTIR spectra of Zein, PVAl and their blends.

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Torres, S. J. V., Medeiros, G. B., Rosário, F., Yamashita, F., Mattoso, L. H. C., & Corradini, E.

Figure 3. Water absorption at 54 ± 3% RH versus composition of Zein/PVAl blends.

bonds. The -OH, -NH2 and -C=O groups in zein are able to form hydrogen bonds with -OH in PVAl. The degree of interaction between the polymers (zein-PVAl) may have been reduced by polymer-plasticizer interactions. The band due to -OH stretching vibration observed at 3285 cm-1 for zein plasticized; and at 3290 cm-1 for PVAl plasticized was slightly shifted for zein/PVAl blends. The intensity of this band decreased with increasing ration of PVA in the blend. The lowering in intensity may be due to weaker polymer-water interactions at higher concentrations of zein, suggesting a slight dehydration of PVAl after the addition of zein, as reported by Lacroix et al.[25].

Figure 3 illustrates the results of the water uptake experiments. The addition of zein decreased the water uptake at equilibrium of blends with PVAl. This behavior was attributed to the difference in the hydrophilicity of zein and PVAl. Zein is composed of amino acids, many of which have nonpolar side groups, whereas PVAl is highly hydrophilic and interacts more strongly with water than zein.

The water absorbed of the zein-based materials depends also on the nature of plasticizer. Lawton[27] prepared zein films by casting using plasticizers with different degrees of hydrophilicity. He observed that films containing more hydrophilic plasticizers (e.g. glycerol and triethylene glycol) absorbed considerably more water than films containing more hydrophobic plasticizers (dibutyl tartrate and oleic acid).

Corradini et al.[37] studied the water absorption properties of starch/zein blends plasticized with glycerol. The results showed that starch/zein plasticized with 22% of glycerol exhibited higher water uptake value than the obtained for the zein/PVAl plasticized with glycerol and oleic acid. They also verified that the combination of glycerol and oleic acid to plasticize Zein/PVAl blends is better for reducing the water uptake of these blends compared to the use of glycerol alone. Zein/starch blends plasticized with glycerol[37] exhibited the water uptake at equilibrium values in the range from 8.5% to 10% when conditioned at 52 ± 2% RH. Comparing these values with those obtained for zein/PVAl plasticized with oleic acid and glycerol, it is observed that the water uptake is ca. 20% lower than zein/starch blends.

These differences are partially due to the lower amount of glycerol in the zein/PVAl blends.

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Figure 4 illustrates the Young modulus (E), ultimate tensile strength (σMax), and elongation at break (εMax), which were determined from the stress-strain curves at different zein/PVAl blend compositions. Figure 4 illustrates the Young modulus (E), ultimate tensile strength (σMax), and elongation at break (εMax), which were determined from the stress-strain curves at different zein/PVAl blend compositions. The mean values of σr for the 0/100, 25/75, 50/50, 75/25 and 100/0 zein/PVAl compositions varied in the range of 1.7 – 5.7 MPa. The addition of PVAl caused the σMax of the blends containing zein to decrease significantly. For example, the zein/PVAl blend with the 50/50 composition showed a 333% decrease in σMax in relation to the plasticized zein (composition 100/0).

The mean values of Young’s modulus (E) for the 0/100, 25/75, 50/50, 75/25 and 100/0 zein/PVAl compositions varied in the range of 433 – 7371 MPa. Zein was more rigid and brittle than PVAl. The value of E of zein was 1370% higher than that of PVAl, and the addition of zein produced blends that were more rigid than PVAl, with E values increasing to up to 980% in the 25/75 composition when compared to pure PVAl. The mean values of the elongation at break (εMax) for the 0/100, 25/75, 50/50, 75/25 and 100/0 zein/ PVAl compositions varied from 2.7 – 32%. PVAl presenting higher εMax than zein. The behavior of zein/PVAl at 50/50 and 75/25 compositions was similar to that of zein, indicating that the elongation of these blends was strongly reduced by the addition of zein when compared to that of pure PVAl. The 25/75 composition showed 49% lower elongation than that of pure PVAl. These results suggest that zein had stronger effect on the mechanical properties of the blends than PVAl, since low amount of the former (25%) affected this property significantly. The curves of the mechanical properties of the compositions showed a typical behavior of incompatible blends, indicating weak interaction between PVAl and zein. These results were similar to those reported

by Corradini et al.[22] for PCL/zein blends, which were also incompatible. Corradini et al.[37] reported that the addition of zein favors the rigidity of zein/starch blends plasticized with glycerol. For zein/starch blends (0/100, 25/75, 50/50, 75/25 and 100/0, wt%), containing 22 wt% of glycerol, the E values ranged from 77 to 1162 MPa; the σr values ranged from 4 to 12%; and εr ranged from 66 to 2. Leroy et al.[38] verified that, compared to glycerol, the use of (1-butyl-3methyl imidazolium chloride [BMIM]Cl) leads to a more efficient plasticization for starch/zein blends, indicating that a compatibilization between starch and zein blends takes place in presence of [BMIM]Cl. The composition 50/50 (wt%) of starch/zein plasticized with glycerol presented values of 2.5 MPa (σr), 50MPa (E) and 5% (εr), whereas the values for σr, E and εr for same composition plasticized with [BMIM]Cl were respectively, 20 MPa, 4.0 and 150%. Senna et al.[26] observed that the increase of PVAl ratio increased the toughness of the blends of zein/ PVAl. In another study, Giteru et al.[39] evaluated the effect of the composition of edible films containing zein, chitosan, poly(vinyl alcohol) and poly(ethylene glycol) (PEG400) in mechanical properties of the mixture. They verified that the incorporation of poly(vinyl alcohol) increased the ductility of the composite films. The σr ranged from 7.0 to 37.5 MPa, εr ranged from 26 to 233% and E ranged from 82 to 613 MPa. Wei et al.[40] prepared blends of zein and Polímeros, 30(4), e2020042, 2020

Mechanical and water absorption properties and morphology of melt processed Zein/PVAl blends

Figure 4. (a) Young’s Modulus (E); (b) elongation at break (εMax); (c) Ultimate tensile strength (σMax) as a function of composition of Zein/PVAl blends.

poly(butylene adipate-terephthalate) (PBAT) by reactive blending in the presence of poly(ethylene glycol diglycidyl ether) (PEGDGE). PEGDGE acted as plasticizer and reactive compatibilizer in the PBAT/zein blending system. The values for σr, E and εr for the composition 25/75 (wt%) without PEGDGE were, 6 MPa, 120 MPa and 80%, respectively.

When 5 phr (parts per hundreds of mixture PBAT and zein) of PEGDGE was added to 25/75 composition, an increasing of 50% and 33% in σr and εr, respectively and a decrease of 38% in E, were observed.

All the above-mentioned studies showed that the addition of zein favors the rigidity of the blends, increasing their modulus of elasticity and tensile strength and reducing their deformation. Comparing the mechanical properties of cited studies with the results obtained for the blends under study, it was observed that for the most compositions, zein/PVAl exhibited higher E values, although lower values of εMax and σMax. The E and σMax values increased with increasing zein content, whereas εMax values decreased. Based on the research reported by Wei et al.[40] and Leroy et al.[38], the utilization of compatilizers can be an alternative for improving mechanical properties of the zein/PVAl blends.

It was observed also that oleic acid and glycerol affected the mechanical property of zein/PVAl blends differently.

For compositions containing oleic acid content up to 0.5% (50/50, 75/25 and 100/0 zein/PVAl blends), the εMax values Polímeros, 30(4), e2020042, 2020

remained approximately constant, whereas E and σMax increased. This behavior was probably due to formation of crosslinking between zein chains, as mentioned before.

On the other hand, for composition with PVAl content up to 0.5%, εMax values increased sharply, E decreased and σMax slightly increased. This is probably explained by strong intermolecular interactions between PVAl and glycerol.

A similar effect was observed by Park et al.[41]. They added saturated fatty acids (lauric acid, palmitic acid, and stearicpalmitic acid blends) to methyl cellulose/corn zein films and observed a decrease in σMax but an increase in εMax. Figure 5 shows SEM images of fractured surfaces of PVAl, zein and their blends. The glycerol-plasticized PVAl presented an uniform and continuous surface with some roughness. The zein plasticized showed sheet-like structure, which are probably formed by confinement the orientation correlated protein molecules in the presence of oleic acid[15].

Heterogeneous morphologies, such as dispersed morphology and co-continuous morphology, are usually observed in two immiscible blend-melted polymers[42]. The morphology of the zein/PVAl blends changed in response to variations in blend composition. The blend containing 25% PVAl showed PVAl domains in the zein matrix, and the increase the zein content to 50% led to the formation of a co-continuous morphology, while the blend containing 80% zein showed 5/8

Torres, S. J. V., Medeiros, G. B., Rosário, F., Yamashita, F., Mattoso, L. H. C., & Corradini, E.

Figure 5. SEM micrographs of the surfaces of the 25/75, 50/50 and 75/25 Zein/PVAl blends.

a PVAl phase dispersed in the zein matrix. The morphology of the blends also revealed interfacial voids, indicating poor adhesion between PVAl and zein phases, which rendered the mechanical properties of the Zein/PVAl blends inferior to those of pure polymers. During processing, zein and PVAl structures undergo physical and chemical changes, depending on the processing conditions. These will dictate these materials’ final properties. The incompatibility of Zein/ PVAl blends is related to the extent of polymer-polymer intrachain interactions, which may have been reduced by strong interactions between plasticizers and polymers.

variations in blend composition, and the presence of voids indicated poor interfacial interaction between the polymers.

Despite the immiscibility of the blends, their flexibility was improved with addition of PVAl, indicating same degree of compatibility between the polymers. It is possible to combine the good processability and flexibility capabilities of PVAl and the lower water solubility of zein to produce biodegradable films with a promising potential for use as packaging materials. However, further work will be required to fully understand the relationship between processing and chemical structure and properties of zein/PVAl blends plasticized with glycerol and oleic acid.

4. Conclusions

5. Acknowledgements

The results showed the melt-processing feasibility of the zein/PVAl blends plasticized with glycerol and oleic acid. Increasing the concentration of PVAl reduced the viscosity of the blend, thereby improving its processability.

The morphology of the blends changed in response to

The authors gratefully acknowledge the Brazil’s Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (Grant No. 408717/2016-5) for the financial support and LNNA–EMBRAPA/CNPDIA for its technical support.

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Mechanical and water absorption properties and morphology of melt processed Zein/PVAl blends

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Received: Dec. 23, 2019 Revised: Jan. 26, 2021 Accepted: Jan. 28, 2021

Polímeros, 30(4), e2020042, 2020

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.08220

Melt-mixed nanocomposites of SIS/MWCNT: rheological, electrical and structural behavior Ludimilla Barbosa Ferreira1, Rayane de Souza Fernandes1, Rosario Elida Suman Bretas2 and João Paulo Ferreira Santos1*  Departamento de Engenharia de Materiais – DEMAT, Centro Federal de Educação Tecnológica de Minas Gerais – CEFET-MG, Belo Horizonte, MG, Brasil 2 Departamento de Engenharia de Materiais – DEMa, Universidade Federal de São Carlos – UFSCar, São Carlos, SP, Brasil 1

*joaopaulofsbrasil@gmail.com

Abstract In this work, nanocomposites based on the triblock copolymer polystyrene-b-polyisoprene-b-polystyrene (SIS) thermoplastic elastomer filled with multiwall carbon nanotubes (MWCNT) were obtained by melt mixing. The nanocomposites were characterized by oscillatory rheometry, electrical resistivity, small angle x-ray scattering (SAXS) and transmission electron microscopy (TEM). The results showed that both, rheological and electrical percolation were achieved at MWCNT loadings between 1-3 vol.%. Rheological tests revealed that the insertion of MWCNT into SIS significantly enhanced the process of relaxation of SIS blocks. Resistivity measurements revealed that conductive nanocomposites were obtained at MWCNT loadings ~1.6 vol.%. The electrical resistivity decreased eleven orders of magnitude from neat SIS to SIS/ 5 vol.% MWCNT. Finally, SAXS and TEM showed that the melt mixing process and the presence of MWCNT hampered the self-assembly of SIS into well-ordered domains.

Keywords: SIS, MWCNT, rheological percolation, electrical percolation, self-assembly.

How to cite: Ferreira, L. B., Fernandes, R. S., Bretas, R. E. S., & Santos, J. P. F. (2020). Melt-mixed nanocomposites of SIS/MWCNT: rheological, electrical and structural behavior. Polímeros: Ciência e Tecnologia, 30(4), e2020043. https://doi.org/10.1590/0104-1428.08220

1. Introduction The use of multiwall carbon nanotubes (MWCNT) in polymeric matrixes to produce conductive nanocomposites has been widely studied[1,2]. Among the broad range of polymer matrixes, the thermoplastic elastomers have the advantage of behaving as elastomers at room temperature, but being able to be processed as thermoplastics[3]. Their elastomeric properties make them good candidates for proton exchange membranes[4], sensors[5,6], photoactuators[7], among others.

Thermoplastic elastomers based on block copolymers have an additional advantage: their morphologies are based on rigid domains that anchor flexible or elastomeric domains.

The insertion of nanoparticles inside these domains constitutes one of the current approaches to produce nanocomposites for energy conversion and storage[8-10]; thus, due to the large variety of morphologies that these domains can assume, a myriad of possibilities exists. In a previous work of ours[3], flexible conductive nanocomposites based on the thermoplastic elastomer polystyrene-b-polybutadiene-bpolystyrene (SBS) filled with MWCNT were produced by solution casting followed by annealing during several days. These nanocomposites had a microstructure composed of hexagonally packed cylinders (HEX) co-existing with lamellar morphologies.

Polímeros, 30(4), e2020043, 2020

The triblock copolymer polystyrene-b-polyisoprene-bpolystyrene (SIS) belongs to the styrene based thermoplastics elastomers group and like SBS is extremely flexible and easily processable. SIS has a biphasic morphology, in which the polystyrene (PS) microdomains are dispersed through a flexible elastomeric matrix of polyisoprene (PI). Its flexible PI segments allow large deformations, while the PS domains act as physical cross-links anchoring the PI blocks, yielding an elastomeric response to the whole material[11,12].

Ilčíková et al.[7] produced nanocomposites of SIS filled with MWCNT by solution mixing. These authors found that these nanocomposites had photo-actuation abilities. Garate et al.[13] also produced nanocomposites of SIS/MWCNT by solution mixing. They improved the dispersion of the MWCNT into the SIS by grafting MWCNT with PS oligomers and using a surfactant. Brook et al.[14] produced conductive hybrid nanocomposites of SIS/MWCNT/Polyaniline (Pani) with improved thermal, mechanical and electrical properties.

Ponnamma et al.[15] developed flexible oil sensors based on SIS/MWCNT nanocomposites. The materials showed best oil sensing abilities above the percolation level.

All the studies above cited used solution casting or solution precipitation as the processing method to obtain the SIS/MWCNT nanocomposites. However, it is known that

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

Ferreira, L. B., Fernandes, R. S., Bretas, R. E. S., & Santos, J. P. F. melt mixing is the most common method of compounding in the thermoplastic industry. Melt mixing is scalable for large productions, being also environmental friendly as it does not use toxic solvents and it is free of carbon emissions.

Nevertheless, we were able to find only one study on melt mixing of SIS/MWCNT[16] and none on the analysis of the influence of MWCNT on the structure, relaxation and electrical properties of SIS after melt mixing.

Thus, in this work, nanocomposites with different volumetric fractions of MWCNT in SIS triblock copolymer were produced by melt mixing. To the best of our knowledge, this is the first work directed to perform a comprehensive study on the rheological, electrical and structural behavior of SIS/MWCNT nanocomposites produced by melt mixing.

2. Materials and Methods 2.1 Materials The SIS triblock copolymer was donated by Kraton South America (Kraton D1162P) with 43 m.% of polystyrene (PS), antioxidant content between 0.08 and 0.30 m.%, density of 0.92 g/cm3 and MFI of 45 g/10min, according to the manufacturer; the molar mass was not informed. The SIS was ground in a mill (Micron Powder Systems, model CF Bantam) at 100 rpm with injection of liquid nitrogen in order to obtain a micronized powder. Multiwall carbon nanotubes (MWCNTs), grade number NC7000, were supplied

by Nanocyl S.A. with 90% purity. The MWCNT were characterized in earlier works of ours[3,17] by transmission electron microscopy (TEM) using a FEI TECNAI G F20 HRTEM at 80 kV and Raman spectroscopy using a Renishaw spectrometer, with in via Raman microscope, NIR laser, in the regular mode. By TEM, their external diameters (d=9.5 nm) and their average lengths (l=1.5 μm) were measured. By Raman spectroscopy, the peaks at 1355 and 1594 cm-1 attributed to the characteristics D and G bands of the MWCNT, respectively, were observed. The D band can be correlated to structural defects of the nanotubes, while the G band to in plane vibration of a sp2 bonded structure.

The D and G intensities ratio (ID/IG) is used as an indicator of the degree of imperfections of the carbon nanotube; the higher, the more defectives are the MWCNT. Another band at 2781 cm-1, the Go band, is correlated to the electronic properties of the carbon nanotube; thus, the ratio of the Go and G intensities (IGo/IG) is used as an indicator of the metallicity of the carbon nanotube. The higher, the more metallic are the nanotubes. The calculated ratios were ID/IG =1.14 and IGo/IG=0.42[3], which indicated that the MWCNT were highly defectives and had a low metallicity[18]. White et al.[19] also found that the NC7000 Nanocyl grade had Al, Fe and Co as impurities and O on the surface.

Toluene (PA Synth) was used as a solvent.

2.2 Melt mixing Prior to the processing, SIS and MWCNT were dried for 12 hours at 60 ºC. First, a masterbatch of 5 vol.% of MWCNT and SIS was produced using a torque rheometer Haake, model Rheomix 600p, at 190ºC, 100rpm, during 5 min. The masterbatch was then diluted with SIS to yield the final concentrations of MWCNT in the nanocomposites 2/9

in the same Haake torque rheometer. The following MWCNT concentrations were used for this study: 0.1, 1.0, 3.0 and 5.0 vol.% MWCNT. These filler loadings were selected based on previous works[1,20,21], which pointed out a value of 0.31vol.% MWCNT for the theoretical percolation threshold of a generic polymer matrix filled with the MWCNT NC7000 from Nanocyl[1,3,22]. After melt mixing, films of the resulting nanocomposites were produced by compression molding at 190ºC under 8 MPa of pressure using a hot press. The samples were named SIS/MWCNTX, where x represents the volumetric fraction of MWCNT. As a benchmark, samples of neat SIS were produced by solvent casting to observe their quasi-equilibrium morphologies. First, the SIS powder was melted and later dissolved in toluene (concentration: 0.2 g/ml) under magnetic stirring for 2 h at 100ºC. The resulting solution was poured in Petri dishes to obtain films, which were maintained at room temperature during 4 h and then left in a vacuum oven at 110 ºC during eight days to enable the self-organization of SIS into well-ordered rigid and soft domains. This approach of annealing was done based on previous works[3,23,24].

Samples of the SIS/MWCNT nanocomposites were not prepared by solution, because as already reported in early work of ours on SBS/MWCNT nanocomposites[3] to allow complete solvent evaporation and to obtain self-standing films the casting process takes approximately 7 days to be completed.

2.3 Characterization The rheological behavior of the nanocomposites was evaluated using a stress-controlled rheometer AR-G2, from TA Instruments, with parallel plates, diameter of 25 mm, gap of 1 mm between the plates, frequencies ω between 0.1 and 100 rad/s, at a temperature of 190 ºC. The complex viscosity η*= (ω ) η' (ω ) − iη "(ω ) , where η' = dynamic viscosity and η " = imaginary viscosity was obtained. The measurements were done within the linear viscoelastic regime. The electrical resistivity measurements of the nanocomposites were made using two setups. For samples with resistivity lower than 103 Ωm, two probe measurements were done using a stabilized source-meter device (Keithley, model 237) and parallel Pt electrodes having 1 cm diameter; for samples with electrical resistivity higher than 103 Ωm, a Keithley 6517A electrometer was used.

The domains structure of SIS was characterized by small angle x-rays scattering (SAXS) using an equipment from Bruker (AXS 2D Nanostar, Cu, Kα radiation, 30 mA, 40 kV), at scanning angles below 5º. The morphology of the nanocomposites was analyzed by transmission electron microscopy (TEM) and atomic force microscopy (AFM).

TEM was done using an equipment from FEI company (Magellan 400L) at 20kV. AFM was done using an equipment from Veeco Digital Instruments, model MMA FM-2, with silicon cantilevers.

3. Results and Discussions 3.1 Rheological behavior Figure 1 shows the complex viscosity as a function of frequency of the samples. The neat SIS and the SIS/MWCNT Polímeros, 30(4), e2020043, 2020

Melt-mixed nanocomposites of SIS/MWCNT: rheological, electrical and structural behavior

Figure 1. Complex viscosity (η*) versus oscillatory frequency (ω) of the nanocomposites.

nanocomposites showed a shear-thinning behavior; if a power law model ( η = mω n −1 , where m=consistency and n=power law index) is fitted to the curves, a highly pseudoplastic behavior is observed (n→0). The nanocomposites containing higher amounts of MWCNT had higher η* than the others.

It is known that MWCNT have high aspect ratios and high modulus. Therefore, the MWCNT can easily form entangled and rigid networks with the polymer macromolecules, unable to relax along the flow direction, thus increasing the flow resistance of the nanocomposites. Accordingly, the higher the MWCNT content the more entangled are these networks, yielding high viscosities in the molten state[25,26].

As said before, the complex viscosity has two components: η’ and η”. η’ is proportional to the viscous response or energy dissipation of the material, while η” is proportional to the elastic response or energy storage.

Therefore, the relationships between these viscosities and the components of the complex modulus G* (ω ) = G′ + iG " are: η’ = G "/ ω and η " = G '/ ω , where G” is the loss modulus and G’ is the storage modulus. Figure 2 shows plots of η’ and η” versus ω. It can be noted that, at lower MWCNT loadings, the viscous response is more pronounced than the elastic response; however, between 1-3vol.% MWCNT the elastic response became more pronounced than the viscous response through the broadband frequency range.

That is, a transition from a predominantly liquid or viscous behavior to predominantly elastic or solid behavior occurred above 1 vol.% of MWCNT; i.e, the rheological percolation threshold was achieved.

Another way to obtain the value of the rheological percolation threshold is to use the slopes of the G’(ω) and G”(ω) curves at the terminal zone of the viscoelastic spectrum. It is worthwhile to recall that at the terminal zone, the frequencies are small; because the mechanical response of the macromolecules will be dependent on their relaxation times tr (ω = 1 / tr ) , at the terminal zone, the responses will be associated to the parts of the macromolecules which have long relaxation times (mainly their backbones)[27-29]. A linear polymer of high molecular weight has entanglement and disentanglement of their coils[28,29], with the formation of a temporary network structure due to inter and intra-molecular Polímeros, 30(4), e2020043, 2020

junctions derived from the twisting and bending of the coils.

Between these junctions (which are continually being formed and undone), the resistance to conformational changes will be low, and the elastic response, which is dependent upon the life time of these junctions will decay rapidly with the frequency, due to the macromolecules relaxation. The viscous response (which will reflect the flow resistance and energy dissipation) will also decay, but with less intensity.

All the macromolecular models predict, therefore, that, at the terminal zone, G’ ~ ω 2 and G ” ~ ω ; that is, the slope of the G’(ω ) curve at the terminal zone will be approximately 2 while the slope of the G ”(ω ) curve will be 1[28]. On the other hand, in a slightly crosslinked polymer, the junctions are permanent; therefore, the conformational changes between the crosslinks will be restrained and G’ → Ge= equilibrium modulus, while G” will flatten. That is, both G’ and G” will be independent of ω; therefore, if the slopes of the components of the complex modulus change towards zero at the terminal zone, a very restrained structure (a so called pseudo-solid behavior) will be attained. The change of slopes can, therefore, predicts the formation of a percolated network of MWCNT within the polymer, as others works have already shown[27,30-32].

The slopes at the terminal zone were calculated and are shown in Table 1.

The slopes of the G’(ω ) and G ”(ω ) curves of the pure SIS were low, not even close to 2 and 1, respectively; that is, SIS, being elastomeric and having rigid styrene domains anchoring the isoprene domains and thus acting as crosslinking, has already a quasi-solid behavior. However, when 1 vol.% of MWCNT were added to the SIS, both slopes changed drastically to almost 0, confirming the complex viscosity analysis, that above 1 vol%. MWCNT the rheological percolation occurred. From a rheological point of view, at MWCNT loadings above the percolation threshold, an entangled and well-established network between SIS backbones and MWCNT exists.

From the crossover frequency ωc (where G’=G”), the relaxation time at the beginning of the plateau region (t r,c) and consequently the influence of the MWCNT on the SIS relaxation can be analyzed[27]. Table 1 shows also these values. The relaxation time of the SIS macromolecules is around 0.018 s; when 0.1 vol.% of MWCNT was added, the relaxation of the SIS macromolecules increased to 0.025 s while another (very high) relaxation time at 10 s surged due to the presence of the MWCNT. That is, some SIS macromolecules were restrained by the MWCNT, but the MWCNT themselves did not form a percolated network with the SIS macromolecules (MWCNT high relaxation time).

However, when 1.0 vol.% of MWCNT is added, only one relaxation time at 0.13 s is observed, due to the formation of a restrained and percolated network. This result confirms the viscosity results, that is, the percolation concentration for the SIS/MWCNT was around 1 vol.% MWCNT.

3.2 Electrical behavior The theory of electrical percolation predicts the dependence of electrical resistivity (ρ) versus filler volumetric loading (V) according to Equation 1 [33]: −t

ρ (V ) ∝ (V − Vc ) , when V > Vc

(1) 3/9

Ferreira, L. B., Fernandes, R. S., Bretas, R. E. S., & Santos, J. P. F.

Figure 2. Real viscosity (η’) and imaginary viscosity (η”) versus oscillatory frequency (ω).

Table 1. Slopes (α) of the G’(ω ) and G ”(ω ) curves, crossover frequency (ωc) and relaxation time at crossover frequency (tr,c).

Sample

α G’

α G”

ωc (rad/s)

t r,c (s)

SIS/MWCNT0.0 SIS/MWCNT0.1 SIS/MWCNT1.0 SIS/MWCNT3.0 SIS/MWCNT5.0

0.89 0.92 0.19 0.01 0.01

0.32 0.30 0.02 0.01 0.02

55 40, 0.1 7.5 -

0.018 0.025,10 0.13 -

Table 2. Electrical resistivity of the samples.

Sample SIS/MWCNT0.0 SIS/MWCNT0.1 SIS/MWCNT1.0

4/9

Resistivity (Ω.m) 1.2x1011 1.2x1011 4.4x105

Sample SIS/MWCNT3.0 SIS/MWCNT5.0

Resistivity (Ω.m) 1.7x101 1.5x100

where Vc is the electrical percolation threshold concentration and t is denominated critical exponent. Figure 3 shows a graph of electrical resistivity versus filler volumetric loading and Table 2 summarize these values. It can be observed that the electrical resistivity decreased various orders of magnitude with increasing MWCNT loading. By definition[3] at the electrical percolation concentration, the electrical conductivity increases promptly several orders of magnitude; however, in our case, there was a gradual increase of the conductivity (or decrease in resistivity), between 0.1 and 3 vol.% MWCNT. As the percolation-dispersion staircase model[34,35] has pointed out, when an abrupt increase of the conductivity occurs, only one conductivity mechanism exists, while if a gradual increase occurs, then multiple local conductivity mechanisms are contributing to the global conductivity. This latter behavior seems to predominate in the SIS/MWCNT Polímeros, 30(4), e2020043, 2020

Melt-mixed nanocomposites of SIS/MWCNT: rheological, electrical and structural behavior

Figure 3. Volumetric resistivity versus MWCNT loading.

of filler for electrical conduction in a nanocomposite[38] is required. This minimum amount is called the electrical percolation threshold. The fillers must touch each other or being close enough to allow the establishment of conduction paths for the charge carriers (electrons, holes or ions).

Therefore, the higher the MWCNT loading the higher the probability of establishing these paths. As well, the higher the aspect ratio of the MWCNT, the higher the probability of the nanotubes to touch each other[18]. During melt mixing, however, a damage and breakage of the MWCNTs (or other rigid nanoparticles) due to the shear stresses can occur, as it has been observed in other works[30,39-41], Thus, for melt mixed nanocomposites, a shifting of the percolation threshold towards higher filler loadings compared with solution mixed nanocomposites[1,41] is expected.

The critical exponent t can be correlated with the system dimensionality and with conduction mechanisms. As pointed out by Bauhofer and Kovacs in their throughout review[2], values of t~3.0 were obtained using a Bethe lattice and Mean field theory. These theories correlated increasing values of t with conduction by tunneling effects. When the MWCNTMWCNT distances are below 10 nm, the electrons can jump from one nanotube to another[42,43]. The higher the MWCNT loading, the shorter the distance between the MWCNT, thus increasing the chance of electrons to surpass the barrier.

Probably, in these SIS/MWCNT nanocomposites there were individual and well-dispersed MWCNT coexisting with remaining MWCNT aggregates, as said before. By increasing the amount of MWCNT the individual MWCNT and MWCNT aggregates became closer enough to allow conduction by hopping and tunneling effects.

3.3 Structural characterization

Figure 4. (a) Intensity I measured in SAXS experiments using compression molded films samples as a function of the magnitude q of the scattering vector (b) 2D SAXS patterns.

nanocomposites, as observed in other work of ours[3]. The multiple local conductivity mechanisms can be credited to the co-existence of different sizes of MWCNT aggregates with single MWCNT in the SIS matrix. Therefore, electrical conduction will occur through the aggregates, through the single MWCNT and through electron tunneling between adjacent MWCNT. From Equation 1, Vc~1.6vol.% MWCNT and t~2.9.

Thus, these melt mixed SIS/MWCNT nanocomposites percolated at MWCNT loadings significantly above the theoretical value (0.31vol.% MWCNT). In other works, solution prepared nanocomposites achieved the electrical percolation at lower MWCNT loadings[2,3,36]. Accordingly, to the statistical theory of percolation[37], a minimum amount Polímeros, 30(4), e2020043, 2020

SAXS profiles and 2D patterns of some of the samples are shown in Figure 4a and 4b, respectively. The scattering vector is q = ( 4π / λ ) senθ , where λ is the wavelength and 2θ the scattering angle. As expected, the sample of pure SIS from annealing treatment (SIS/MWCNT0 A) displayed concentric rings and SAXS peaks at q =0.2 nm-1, 2q = 0.4 nm-1, 3q=0.6 nm-1 and 4q = 0.8 nm-1, which are ascribed to lamellar morphologies[3,44]. That is, eight days were enough time to allow the self-organization of SIS into well-ordered lamellar domains.

On the other hand, the sample of pure SIS produced by melt mixing (SIS/MWCNT0) showed less evident peaks and hallo rings; in the sample SIS containing 3 vol% MWCMT (SIS/MWCNT3), the peaks and hallo rings were absent. This result shows that both melt mixing and the presence of MWCNT hampered the self-assembly of SIS domains. Figure 5a depicts an AFM micrograph of the SIS/ MWCNT0 sample, where typical lamellar morphologies can be observed; the PS domains can be identified as the yellow ones (higher elastic moduli) while the PI domains are the brown (lower elastic moduli). PS and PI domains presented average thicknesses of 21.0 nm and 13.1 nm, respectively.

It is known that the stability of different equilibrium domain morphologies of a block copolymer depends on the relationship between enthalpy (interaction energies between dissimilar polymer blocks given by the Flory-Huggins interaction parameter, χ) and entropy (conformational energy 5/9

Ferreira, L. B., Fernandes, R. S., Bretas, R. E. S., & Santos, J. P. F.

Figure 5. (a) AFM micrograph of SIS/MWCNT0 A annealed during eight days; (b) Distributions of thickness of the lamellar PS and PI domains.

depending on the degree of polymerization, N). For di-blocks copolymers, it has been observed that when χ N > 10.5 different stable and even co-existing morphologies can be obtained depending on the values of χ N and f, the volumetric fraction of one of the co-monomers. Numerous studies have showed various types of equilibrium morphologies of di-blocks AB and tri-blocks ABA copolymers, which are spheres (S), cylinders (C), lamellas (L) and gyroids (G)[45-47]. When f is near 0.5, lamellar morphologies are usually expected after annealing during several days. In fact, the SIS employed in this work has a PS molar fraction ~43%, that is, f is near 0.5.

However, when χ N < 10.5 only a disordered melt is formed[48]. In general, long times are required to self-assembly domains from the melt or solution; therefore, in the melt compounding process employed to prepare SIS/MWCNT nanocomposites probably time was not enough to allow the self-assembly of PS and PI into well-ordered domains.

Moreover, it is known that even small changes in temperature or shearing can induce large changes in these equilibrium morphologies[49].

Figure 6a shows TEM micrographs of the MWCNT as received, prior to the mixing with SIS. Figure 6b shows a TEM micrograph of the nanocomposite containing 3 vol% MWCNT. This sample was selected because its MWCNT loading was above both, the rheological and electrical percolations. The MWCNT can be seen as long and entangled tubes through the SIS matrix. The measurement of the lengths of the MWCNT was difficult because they were highly entangled; however, comparison of the micrographs Figure 6a and Figure 6b clearly suggests that a reduction of 6/9

Figure 6. TEM images: (a) Pure MWCNT, (b) Nanocomposite SIS/ 3vol.% MWCNT. Polímeros, 30(4), e2020043, 2020

Melt-mixed nanocomposites of SIS/MWCNT: rheological, electrical and structural behavior the MWCNT lengths occurred during the process of melt mixing, as already pointed out in other works[1,50]. In addition, Figure 6b shows that the MWCNT touch each other and are close enough to allow the establishment of electrical conduction paths. As pointed out by SAXS measurements, there was a reduction of regularity of the SIS domains from the sample SIS/MWCNT0 to the sample SIS/MWCNT3, thus MWCNT hampered the self-assembly of SIS, which is in line with previous works[3,51]. The presence of MWCNT can change the correlation between enthalpy and entropy[10]; for example, π-π interactions can be established between the MWCNT graphitic structure, the phenyl groups from PS and the doubles bonds from the PI backbones[52,53]. Moreover, MWCNT are rigid structures having ultrahigh moduli[54], thus being physical barriers against molecular motion of the PS and PI blocks.

Other works have shown that modifying the surface of MWCNT can cause remarkable changes in the self-assembly of MWCNT/block copolymers composites. For example, grafting PS chains on MWCNT surfaces influenced the domains morphology of SBS/MWCNT composites[55]; instead of being inserted in the PS domains, the PS-g-MWCNT templates the lamellar PS domains morphology. Other studies have concluded that the MWCNT are inserted in both rigid and soft domains of SBS[56] or SIS[13].

4. Conclusions Nanocomposites with different volumetric fractions of MWCNT in SIS block copolymers were produced by melt mixing. Both, rheological and electrical percolation thresholds were achieved at MWCNT loadings between 1-3 vol.%. The complex viscosity increased two orders of magnitude from neat SIS to SIS/5vol.%MWCNT, while electrical resistivity decreased eleven orders of magnitude from neat SIS to SIS/5vol.%MWCNT. Therefore, conductive nanocomposites were obtained. Moreover, the presence of MWCNT significantly enhanced the relaxation of SIS blocks. Structural characterization revealed that both, the melt mixing process and the presence of MWCNT hampered the self-assembly of SIS into well-ordered domains.

5. Acknowledgements The authors are grateful to Programa de Educação Tutorial em Engenharia de Materiais (PET) of Centro Federal de Educação Tecnológica de Minas Gerais (CEFETMG), to Fundação de Amparo à Pesquisa do Estado de São Paulo - FAPESP (2013/07296-2, 2013/03118-2, 2014/17597-2, 2016/03667-4) and Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPQ (573636/2008-7, 141456/2013-2 and 10086/2019) for the financial support, to Kraton for the SIS donation, to Prof. Marcelo Orlandi (Unesp) for some of the electrical measurements and to Prof. Edson R. Leite, Ms. Renata Sala and Mr.William Leonel (LIEC-UFSCar) for the SAXS measurements.

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Received: Aug. 30, 2020 Revised: Jan. 11, 2021 Accepted: Feb. 02, 2021

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.06020

Surface functionalization of polyvinyl chloride by plasma immersion techniques Péricles Lopes Sant’Ana1* , José Roberto Ribeiro Bortoleto1 , Nilson Cristino da Cruz1 , Elidiane Cipriano Rangel1 , Steven Frederick Durrant1  and Wido Herwig Schreiner2 Laboratório de Plasmas Tecnológicos – LaPTec, Universidade Estadual Paulista – UNESP, Sorocaba, SP, Brasil 2 Departamento de Física – DF, Universidade Federal do Paraná – UFPR, Curitiba, PR, Brasil 1

*e-mail: drsantanapl@gmail.com

Abstract In this work we discuss the wettability, chemical composition, surface morphology and optical transmittance of polyvinyl chloride (PVC) samples treated by Plasma Immersion and by Plasma Immersion Ion Implantation. The total pressure of N2 or SF6 was 6.66 Pa, for treatments of 900 s, applied rf power of 25 and 100 W, and the substrate temperature was about 298 K. In PIII, high voltage pulses of -2400 V at a cycle time of 30 µs and a frequency of 300 Hz were used.

The wettability of the samples was assessed via contact angle measurements, which indicated either hydrophilicity or hydrophobicity, depending on the plasma composition. X-ray Photoelectron Spectroscopic analysis confirmed strong fluorine attachment to the surface after treatments using SF6 plasmas, and the presence of oxygen after treatments using nitrogen plasmas. Atomic Force Microscopy images showed that the roughness Rrms, depends on the plasma conditions. Optical transmittance in the visible region, T (λ), was increased by plasma immersion. The greatest contact angle observed was 142º (PI cathode), while the highest roughness was 213.2 nm. The highest optical transmittance in the visible region was around to 90% (PI anode).

Keywords: PVC, plasma immersion techniques, contact angle, XPS, AFM, optical transmittance.

How to cite: Sant’Ana, P. L., Bortoleto, J. R. R., Cruz, N. C., Rangel, E. C., Durrant, S. F., & Schreiner, W. H. (2020).

Surface functionalization of polyvinyl chloride by plasma immersion techniques. Polímeros: Ciência e Tecnologia, 30(4), e2020044. https://doi.org/10.1590/0104-1428.06020

1. Introduction Low-pressure plasma treatment, an environmentally friendly alternative to conventional methods of surface modification, can improve the surface properties of polymers without changing their bulk properties[1]. Surface modification in plasmas fed different gases has several advantages, such as a simple and rapid control of the process rate and economic efficiency[2]. Radiofrequency plasma technologies using fluorinated gases are currently employed in materials science, offering low-temperature reactions (often the treatment can be achieved at room temperature, which avoids the thermal degradation of the material).

Direct fluorination is an effective method for improving the surface properties of pristine polymer materials, including barrier, gas separation and bactericide properties, adhesion, printability, chemical resistance, and biocompatibility[3]. One of the purposes of fluorinating polymers is to increase their surface hydrophobicity for use in the polymer packaging of foodstuffs, leading to improvements in anti-sticking, friction, corrosion resistance, flammability, refractive index, dielectric constant, and water/oil repellence. Improved hydrophilization is also useful since it improves adhesion in such processes as painting, coating or gluing[4]. To our best knowledge, different PI electrical configurations for the

Polímeros, 30(4), e2020044, 2020

surface functionalization of PVC have not been previously reported. Other results concerning hydrophobic/hydrophilic surface treatments of PET[5] and PVC[6] in low-pressure plasmas have been reported. In those treatments, plasma immersion techniques (PI) and Plasma Immersion Ion Implantation (PIII) were employed, to modify surface wetting, roughness and optical transmission. Higher contact angles were observed after plasma fluorination and lower ones after plasma nitrogenation, while maintaining low surface roughness and high transparency[7,8]. Substrate cooling was used since high substrate temperatures can reduce optical transmittance in the visible range.

In the present study, plasma immersion techniques were used to modify the surface properties of PVC, to change the wettability from intermediate to hydrophobic using fluorine plasma or to hydrophilic using nitrogen plasmas.

These results are important since wettability influences the exclusion of water and the possibility of adhesion between different polymers. An additional objective was to maintain or improve the optical transparency of ‘blue’ PVC in the visible region, which is necessary in packaging applications, where the product must remain clearly visible.

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

Sant’Ana, P. L., Bortoleto, J. R. R., Cruz, N. C., Rangel, E. C., Durrant, S. F., & Schreiner, W. H.

2. Methods The experimental setup used consists of a stainless-steel vacuum chamber (30 cm in height and 25 cm in diameter) with two internal, horizontal, circular stainless-steel electrodes of 11 cm diameter. Substrates were placed on the driven or biased electrode and the system was evacuated by a rotary pump (18 m3/h) down to ~0.7 Pa. Needle valves were employed to control the gas feed (gases of purity 99.9995%), and a capacitive pressure sensor to monitor the chamber pressure. Samples were exposed directly to the plasma environment established by the application of radiofrequency power (13.56 MHz) at 25 and 100 W for both gases (N2 and SF6). Twenty four substrates of blue PVC (2.5 cm × 1.5 cm, and 1 cm of thickness) were used.

Eight substrates were treated in three different rf plasma immersion modes, as indicated in previous literature[6-9], and summarized as follows: (i) The sample holder and the chamber walls were grounded while rf power was connected to the opposite electrode (driven electrode): PI ‘anode’; (eight samples); (ii) The rf power was connected to the sample holder (driven electrode) while the other electrode and chamber walls were grounded: PI ‘cathode’; (eight samples); (iii) The rf power was connected to the upper electrode (driven electrode) while negative pulses of high voltage were applied to the substrate holder: PIII, (eight samples).

A high voltage source (model RUP-6) and an oscilloscope (TDS from Tektroniks) were employed for the PIII experiments. The negative pulses, -2400 V, were supplied at 300 Hz and period (duty cycle) was calculated as being: ton/(ton + toff), which value was ~3.3 ms for ton fixed to 30µs, for all PIII experiments. Twelve PVC samples were treated in SF6 plasmas and twelve in N2 plasmas. Fig. 1 shows the electrical configuration of the plasma immersion and plasma immersion ion implantation techniques used in our experiments.

As temperature changes can change the optical transmittance, during treatment the sample holder was cooled with water at room temperature. Immediately after removing the treated samples from the reactor, the samples were characterized. The surface contact angle, Ɵ, was measured using a Goniometer (100-00, Ramé- Hart) with three drops of deionized water.

Ten measurements were taken for each drop. The drops had a volume of 0.6 µl. Surface morphologies of four different samples were examined using an Atomic Force Microscope

(XE-100, Park Instruments) operating in air, generating 5 μm x 5 μm images. From the images acquired in the noncontact mode, the root mean square roughness, Rrms, was calculated using the PicoView 1.2 software.

The effect of the plasma treatment on the chemical composition of the PVC surface was evaluated from X-Ray Photoelectron Spectroscopy, XPS, survey spectra. Data were collected for the untreated PVC (one sample) and plasma-treated PVC (four samples). A Microtech - ESCA 3000 Spectrometer was employed, which achieved a base pressure of 2 × 10-8 Pa. The samples were studied using Mg Kα radiation. A resolution of about 0.8 eV was achieved.

The spectra show intensity as function of binding energy from zero to 800 eV[9]. The binding energies (BEs) of the peaks in the spectra were referenced to that of the 1s electrons in carbonaceous carbon (284.6 eV)[10]. Shirley background correction was used.

Optical transmittance, T (λ), of Blue PVC was measured for six samples, using a UV-vis-NIR Spectrometer (Perkin Elmer Lambda 750) over the wavelength range from 190 nm to 3300 nm. Table 1 shows the technical parameters.

3. Results Table 2 shows the values of contact angles, immediately after the treatment and after 30 days (ageing). Also, the optical transmittance of visible light (at 550 nm) was investigated for PVC after the different plasma treatments: PI cathode, PI anode and PIII, at low and high applied rf power using fluorine or nitrogen plasma[6-9].

Table 1. Plasma immersion procedure conditions for the PVC samples.

Substrate

PVC (ρ = 1.3 g/cm3) PI parameters Gas system SF6 and N2 Base pressure (Pa) 0.7 Work pressure (Pa) 6.66 Treatment time (s) 900 rf power (W) 25 and 100 Temperature (K) 298 PIII parameters High voltage (V) -2400 Cycle time (µs) 30 Frequency (Hz) 300

Figure 1. Electrical configuration of plasma immersion techniques applied in our experiments. (a) Plasma Immersion anode; (b) Plasma Immersion cathode; (c) Plasma Immersion Ion Implantation[6-9].

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Surface functionalization of polyvinyl chloride by plasma immersion techniques Table 2. Treatments took place at 25 W and 100 W and 6.66 Pa for 900 s.

Gas discharge SF6

N2

Treatment

Power RF

identification Virgin PVC Flu1 Flu2 Flu3 Flu4 Flu5 Flu6 Nit1 Nit2

(W) 0 25 100 25 100 25 100 100 100

Plasma configuration

Initial contact angle (°)

PIII PIII Cathode Cathode Anode Anode PIII Cathode

76[6,9] 112 ± 1 113 ± 1 100 ± 1 142 ± 2 103 ± 1 102 ± 1 9±2 1± 0.5

PVC is a slightly hydrophobic polymer with a water contact angle of around 76°[6,9]. According to the data of Table 2, wettability after treatment depends on the plasma configuration. Plasma immersion using nitrogen decreases contact angles[5], while plasma immersion using fluorine increases them[6], independent of the plasma technique.

Owing to the plasma action, the breaking of covalent chemical bonds, such as C-H or C-O and their subsequent recombination may lead to the formation of C-F bonds after the fluoride treatment by PI[5,6,9] which explains the increased hydrophobic character of the polymers undergoing this treatment. For treatment by PI, the increasing power supplied to the SF6 plasma enhances ion bombardment.

Thus, a greater number of fragmented species impinging on the surface offers a greater number of active sites for binding C-F, which justifies the higher θ values after fluorination[5-9]. In the reactor there are also the effects of ionic bombardment for all PI configurations[9]. In addition, as the ageing time increases, Ɵ gradually recovers its initial character, particularly after treatment in nitrogen plasmas.

The chemical composition of the treated material can change upon ageing. Furthermore, we believe that the structural reorganization of the polymer chains may play an important role in the modification of the polymer’s wettability[6-9,11].

An increase in optical transmittance was observed for the different plasma configurations, except for sample ‘Nit2’, which used a nitrogen plasma in the ‘cathode’ mode.

3.1 Effect of ageing on θ This section discusses the evolution of contact angles as a function of ageing time for samples treated by plasma immersion and plasma immersion ion implantation, using SF6 and N2, respectively, under different conditions. After fluorination treatments the contact angles were stable over time, while the nitrogen treatments presented a clear tendency to become hydrophobic. After treatment, short chain molecules reorient towards the surface and oxidized functional groups may diffuse into the interior of the material[12]. The literature also indicates that ageing causes the incorporation of oxygen as C-O and C=O, and also as -OH. This is typical of plasma polymers, which upon deposition usually have a high density of free-radicals that react with oxygen and water vapor. As mentioned previously, immediately after being withdrawn from the reactor, samples treated with nitrogen are highly hydrophilic. This wettability was linked to the chemical composition obtained by XPS. Incorporation of oxygencontaining polar groups explains the wettability of samples Polímeros, 30(4), e2020044, 2020

Contact angle (°)

Transmittance

30 days 76 ± 1 135 ± 2 131± 2 123 ± 1 124 ± 2 123 ± 1 120 ± 1 75 ± 1 74 ± 2

at 550 nm (%) 75 85 85 85 84 82 84 83 74

Figure 2. XPS survey spectra of untreated PVC and PVC treated under different conditions: PIII associated with fluorine or nitrogen treatment, and PI associated with fluorine treatment (cathode or anode electrical configuration).

treated with nitrogen[5-9,13]. The use of reactive nitrogen plasma surface modification promotes an increase in surface hydrophilicity. As a result of the high instability of the species generated during and after plasma modification, however, the hydrophilic properties achieved by plasma surface modification are quickly lost. This process is well known as hydrophobic recovery[14]. Changes in contact angle with ageing can be much smaller for polymers that contain many crosslinks since these limit the mobility of polymeric chains[15] and therefore impede the reorganization of polar groups[9].

3.2 Composition (XPS analysis) As is clear from Fig. 2, treatment in SF6 plasmas introduces fluorine into the treated polymer surfaces.

For virgin PVC the following peaks were observed in the spectra: C 1s located at 300 eV, Cl 2s at ~280 eV and O 1s at ~538 eV. After fluorination, C 1s was set to 300 eV, Cl 2s at 280 eV, O 1s at 540 eV and F 1s at 695 eV. After nitrogen ion implantation, C 1s was located at 292 eV, Cl 2s at 274 eV, O 1s at 538e V, and N 1s was located at 408 eV[9].

After the treatment, XPS analyses revealed the following: carbon, oxygen, nitrogen and chlorine at, respectively, 52 at.(%), 33 at.(%), 12 at.(%) and 3 at.(%). In this situation, Ɵ changed from 9º (immediately after the treatment) to 102° after 30 days. The corresponding atomic concentrations for the untreated and treated PVC are presented in Table 2 (which also shows the binding energies). PVC is composed of about (50 at.%) carbon, (38 at.%) chlorine and few percent hydrogen[9,16]. As hydrogen is not detected by XPS, the atomic concentrations obtained for the untreated PVC are consistent. A small amount (~11 at.%) of oxygen is detected 3/7

Sant’Ana, P. L., Bortoleto, J. R. R., Cruz, N. C., Rangel, E. C., Durrant, S. F., & Schreiner, W. H. even in the as-received material[17]. After the fluorination treatment the oxygen content did not change, except for the cathode configuration at 100 W, where it was only 3 at.(%). Nevertheless, substantial fluorine incorporation was observed, while chlorine was removed, (virgin PVC contained 38 at.(%) Cl, and after SF6 bombardment, this had decreased to about 2 at.(%). These results support the explanation of the wettability and the fluorination mechanism[5,6]. Substitution of fluorine, displacing chlorine or hydrogen atoms in polymers decreases their surface energy because of the strong covalence and small polarizability of the C-F bonds. The surface energy of a material depends on the character of terminal groups, and decreases from -CH2 → -CH → -CF → -CF3. Nevertheless, the presence of fluorinated groups (CF, CF2 and CF3) and the subsequent surface energy decrease are not enough to reach superhydrophobicity[5]. Flat surfaces terminated with –CF3 groups, which have the lowest free energy, exhibit a maximum contact angle of around 120º[16]. On the other hand, when nitrogen ions were implanted into the surface of PVC, the oxygen content increased, from 11 at.% to 33 at.%. It is believed that residual oxygen bonds with active sites on the surface caused by ion bombardment. After nitrogen plasma immersion ion implantation the surface composition was as follows: (52% C), (33%O), (12% N) and (3% Cl). For sample Nit1 (PIII), hydrophilization may be caused by the formation of oxygen-containing groups[5,7-9]. The high concentration of oxygen is consistent with the reduction in θ. The binding energy of O 1s observed for virgin PVC and PVC treated with nitrogen is the same: 538 eV. Under these conditions there is expected to be a high degree of bond fragmentation and the emission of species from the solid. As hydrogen and chlorine atoms are side groups in PVC, they are very prone

to be lost upon bombardment[6]. Carbon atoms can also be ejected and O incorporation can be observed. Free radicals generated by Cl and H emission can react with atmospheric H2O and O2, thereby incorporating oxygen groups.

3.3 Surface morphology Another property of great importance in the wettability of solids is surface roughness[18]. Roughness alters the contact area between the liquid drop and the surface, resulting in higher values of θ in hydrophobic materials and lower ones in hydrophilic surfaces[19]. To investigate the morphology of PVC, two-dimensional AFM images (5 µm × 5 µm) were obtained for the untreated and treated PVC, as shown in Fig. 3. Although the tip of equipment is very fine and the spatial resolution should be high, the images are not all as well-defined as was expected.

The roughness of the non-treated PVC (a) is 7.8 nm.

According to Fig. 3a, virgin PVC possesses a high density of pinholes[16], which is caused by the absence of polymeric chains in those regions. These pinholes have depths of a few nanometers, being of the same order as the Rrms of PVC (7.8 nm). In relation to PVC treated by plasma immersion, Fig. 3b, there is a considerable increase in roughness, to about 213.2 nm, which reinforces the explanation of the increase in hydrophobicity. Increased cross-linking between the neighboring polymer chains has reduced the presence of pinholes. This usually occurs when acquiring images of polymers. In relation to virgin PVC, there was a small increase in roughness for the sample treated by PIII; 27.3 nm at 25 W. As shown in Fig. 3d, there was no significant reduction in the density of pinholes, which implies a relatively low

Figure 3. AFM images of: (a) Virgin PVC surface, R = 7.8 nm [20]; Plasma treatments in SF 6 (b) Flu4: PI (cathode) 100 W, R = 213.2 nm (c) Flu1: PIII 25W, R = 27.3 nm and (d) Flu6: PI (anode) 100W R= 18.7 nm[9].

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Surface functionalization of polyvinyl chloride by plasma immersion techniques

Figure 4. Optical transmittance as function of wavelength for blue PVC treated using SF6 under different conditions: PIII 25 W or 100 W; PI cathode 25 W or 100 W; and PI anode 25 W or 100 W (Flu 1 to Flu6).

number of lateral links, or a lower degree of intertwining.

The roughness was 18.7 nm.

The polymer surface in the plasma is exposed to a broad spectrum of ions, electrons, excited neutrals, radicals, visible and ultraviolet radiation that cause etching and, consequently, increase surface roughness and change surface chemistry.

Plasma roughening, however, was not regarded as a method for producing useful patterns. Recent published work has changed this view, and in fact, interesting regular surface topographies after plasma treatment of polymer surfaces under defined conditions have recently been reported[20].

In all cases, we hypothesize that on the surface of treated PVC, pointed peaks are aligned, like a “grass”. For roughnesses of a few tens of nanometers, we characterize the surface as smooth. Both treated and untreated PVC have low roughnesses, producing low reflectance.

3.4 Optical transmittance – T (λ) Fig. 4 shows the optical transmittance spectra of blue PVC treated with SF6 under different conditions. Each spectrum was generated at high resolution, ~0.2 nm/pixel.

Whatever the treatment, the transmittance of all blue PVC samples in visible light increased. PIII is most appropriate for maintaining high transparency in the visible. Thus, plasma treatment distinctively influenced the transmittance of blue PVC. At high powers there is a greater fragmentation of molecules, and subsequent recombination between different plasma species, causing greater distortions of the polymer chains, thus producing voids among them, which increase the mean free path of the incident radiation. This effect, however, was limited owing to the low temperature treatment[7,8].

4. Discussions 4.1 Physico-chemical modifications After fluorine insertion into the polymer backbone, the surface becomes hydrophobic. Although C-F bonds are highly polar, when these species are present on the surface of a material, they will increase its hydrophobicity. The repulsion between the oxygen in the water and the fluoride surface is greater than attraction of the fluorine for hydrogen, which Polímeros, 30(4), e2020044, 2020

implies a non-chemical attraction between the fluorinated surface and the test fluid[21]. The incorporation of fluorine resulted in the formation of different functional groups on the surface, such as CF, CHF, CF2 and CF3[22]. Two factors were instrumental in the increase of hydrophobicity: (i) the degree of fluorination; (ii) the surface roughness[5]. The degree of wetting of the samples immediately after treatment has its effect amplified by the increased surface roughness, which increases the area available for ions or even for neutral species from the plasma to bind chemically with the surface atoms. On the other hand, the main mechanism responsible for the hydrophilic behavior of the samples treated with N2, is the incorporation of polar groups[5-9,16]. Although the treatment results in the incorporation of polar groups, polymer wettability does not involve only oxidative reactions.

Loss and incorporation of new species, other than O, may occur as ageing time increases[23]. XPS analysis shows an increase in [O] from 11 at.% to 33 at.% on PVC samples after N2 treatment by PIII. This leads to the hypothesis that the presence of oxygen in the reactor, deriving either from residual gas or gas released from the glass chamber near the electrode region, may also account for the hydrophilization process[12,13]. Chemical modifications can occur upon ageing and reorganize chemical groups of the polymeric chains, which favor the reduction of the surface free energy until chemical stabilization occurs. Thus, the stability of the treated surface is determined by the extent to which polar species are allowed to move. A way of restraining reorientation is by an increase in cross-linking[24], which may be produced by ion implantation. An increase in the density of covalent bonds among neighboring chains, known as anchor points, limits vibrational and rotational movements. As a consequence, the polymer structure becomes strongly connected and the backbone more rigid[9,16].

The change in the surface hydrophilicity is caused by the replacement of C–C or C–H group on the surface of PVC by C–O or C=O groups[25-29]. A small quantity of nitrogen was detected on the treated sample surface and may be caused by atmospheric N2 reacting with the active surface functional groups[30]. Interactions between the nitrogen and the surface can also result in the diffusion of the nitrogen on the substrate surface[31]. Indeed, the XPS analysis (sample Nit1), presents a low N concentration after PIII[32]. Hydrogen atoms may also be liberated from the polymer, causing cross-linking.

For engineering applications, the degree of cross-linking and scission as well the depth of the modified layer can be tailored by the electronic-to-nuclear interactions via a judicious choice of ion species and ion energy[33]. Experimental results suggest that unsaturation can occur when ion pairs in two neighboring chains overlap[34]. Although both electronic and nuclear processes cause cross-linking as well as scissions, it has been found that the most important parameter to achieve a high degree of cross-linking is electronic, while nuclear collisions tends to cause degradation[35].

Correlating the three different plasma immersion configurations with the wettability results, the key parameters are seen to be the ion flux and energy. When the sample is placed on the driven electrode (PI cathode), the self-bias will guarantee a high average negative potential relative to the plasma, so that the energy of the ions reaching the sample holder will be high, causing high recombination rates and 5/7

Sant’Ana, P. L., Bortoleto, J. R. R., Cruz, N. C., Rangel, E. C., Durrant, S. F., & Schreiner, W. H. therefore high production rates of C-F bonds in fluorinecontaining plasmas. However, when the substrate holder is grounded together with the chamber walls (PI anode), the average potential drop in electrode sheath is not so high. In PIII ion energy is controlled by the HV pulses, being much more intense than in PI configurations. Another important point is that PVC is an insulating material. Consequently, independent of the substrate holder potential, the sample will be at floating potential, and possibly the charged particle flux onto the sample is not so different in the three PI configurations when the sample is at floating potential.

Thus θ is influenced much more by ion energy than by the ion flux of charged particles.

4.2 Spectroscopy The increase in optical transmittance of polymers can be explained by changes in chemical structures and bonds that occur on the surface of samples owing to rf plasma treatment.

Under plasma treatment some bonds in the polymer structure will break and some new chains will form. In this case, plasma treatment may affect optical properties of the material[5-9,16].

Summarizing, PIII did not change optical transmittance, that is, the samples treated at the same rf power of 100 W presented high optical transmittance for SF6 plasma or N2 plasmas, affecting only the wettability (samples Nit1 vs. Flu2). On the other hand, PI cathode changed the wettability, presenting the most extreme values of Ɵ, (1º for nitrogen plasma; 142º for fluorine plasmas). In addition, the PI cathode treatment at rf = 100 W changed the optical transmittance only for the sample treated with fluorine-containing plasmas (Flu4), producing an increase in optical transmittance from 75% to 84% in the visible range. Moreover it is possible with fluorine ion implantation to maintain the low roughness, which presented a slight increase of 10 nm (sample Flu1), or even to considerably increase the roughness from 7.8 to 213.2 nm using PI cathode (sample Flu3).

5. Conclusions Plasma treatments of PVC samples can increase hydrophobicity by using SF6 or increase hydrophilicity using N2. AFM images showed that low energy ion implantation is more effective in maintaining the original low roughness of the polymer surface. Thus, roughness did not significantly affect the high transmittance of blue PVC. When plasma treatment increased the surface roughness, the incorporated surface species increased the change in wettability for all samples. The pinholes typically present in PVC substrates were only suppressed when the PI cathode (sample Flu4) was employed. Roughnesses were of a few nanometers, and influenced the optical properties. XPS analysis revealed the surface incorporation of fluorine for all techniques that used SF6. No significant nitrogenation of PVC was observed in nitrogen plasmas but considerable amounts of oxygen were introduced into the treated surface. Optical transmittance was increased for all plasma-treated samples.

More specifically, PI cathode and PIII plasmas with fluorine produced hydrophobicity and maintained high optical transparency. The chemical composition was influential in changing the transmittance, causing a decrease in the absorption coefficient.

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6. Acknowledgements The authors are grateful to the Brazilian agencies Fundação de Amparo à Pesquisa do Estado de São Paulo - FAPESP (Project 2017/15853-0) and Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq for financial support. This study was also financed in part by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil, CAPES - Finance Code 001.

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Received: Dec. 17, 2020 Revised: Jan. 25, 2021 Accepted: Feb. 05, 2021

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.10120

Investigating the surface performance of impregnated and varnished Calabrian pine wood against weathering Türkay Türkoğlu1* , Ergün Baysal2 , Çağlar Altay3 , Hilmi Toker2 , Mustafa Küçüktüvek4  and Ahmet Gündüz2  Department of Forestry, Koycegiz Vocational School, Muğla Sıtkı Koçman University, Muğla, Turkey Department of Wood Science and Technology, Faculty of Technology, Muğla Sıtkı Koçman University, Muğla, Turkey 3 Department of Furniture and Decoration, Aydin Vocational School, Aydin Adnan Menderes University, Aydın, Turkey 4 Department of Interior Architecture and Environmental Design, Fine Arts and Architecture Faculty, Antalya Bilim University, Antalya, Turkey 1

2

*turkayturkoglu@mu.edu.tr

Abstract The study investigates the gloss and color changes values of Calabrian pine (Pinus brutia Ten.) wood impregnated with some copper content impregnation chemicals such as Celcure C4, Korasit KS, and Tanalith E 8000 and then waterbased varnish (WBV) and polyurethane varnish (PV) coated after 6 months of weathering. The results of study showed that gloss values of PV coated Calabrian pine wood were higher than that of WBV coated Calabrian pine wood before weathering. The gloss values of all treatment groups decreased after weathering. Lightness values of Calabrian pine wood also decreased for all treatment groups after weathering. Pre-impregnation before PV coating resulted in lower 𝜟L* values of Calabrian pine wood. While all treatment groups tended to turn reddish by giving 𝜟a* positive values, they tended to turn bluish by giving negative 𝜟b* values. The lowest total color change was obtained with only WBV coated Calabrian pine wood.

Keywords: impregnation, weathering, varnish, color and gloss test, Calabrian pine wood.

How to cite: Türkoğlu, T., Baysal, E., Altay, Ç., Toker, H., Küçüktüvek, M., & Gündüz, A. (2020). Investigating the surface performance of impregnated and varnished Calabrian pine wood against weathering. Polímeros: Ciência e Tecnologia, 30(4), e2020045. https://doi.org/10.1590/0104-1428.10120

1. Introduction Wood is an environmentally friendly and sustainable natural material used for a wide variety of both structural and non-structural applications[1,2], especially in building and construction applications[3]. In addition to many positive properties of wood material, there are some undesirable negative properties. The wood material can be burned, absorb water, exposed to fungal and insect attack, unprotected against outside weathering factors.

Environmental conditions such as atmospheric pollutants, oxygen, moisture, sunlight, cold, heat, chemicals, and wind erosion can cause an economic depreciation by reduced service life of wood. Therefore, it is necessary to protect the wood material in certain ways in order to protect it against such weather conditions. The coating is applied to protect wooden surfaces from environmental influences, biological degradation, and corrosion[4]. Coating adds an aesthetic appearance to wooden surfaces, making it desirable for the users[5]. However, coating thickness becomes thinner over time. The tissues on the surface undergo deformation[6].

Wood should be impregnated with suitable material and varnish applied before it is used in structural or furniture

Polímeros, 30(4), e2020045, 2020

production. After these processes, the wood used outdoors is much more durable in terms of biological decomposition, dimensional changes and photochemical degradation[7].

Thus, modification techniques with impregnating agents can increase the resistance of wood to weather conditions[8].

One of the methods applied for the protection of outdoor degradation of wood material is impregnation with watersoluble salts such as chromium and copper. The use of various impregnating agents has become important in place of chromated copper arsenate (CCA) impregnation, which is a water-soluble impregnation agent and has been used in large quantities throughout the world. Most of them do not contain arsenic and can be listed as follows: Acid copper chromium (ACC); alkali copper quat (ACQ); copper azole (CA); copper citrate (CC); copper diethyldithiocarbamate (CDDC); copper HDO[9]. Since the carcinogenic structure of chromium compounds is well known, the new impregnation agent is generally seen as copper in alternative substances[10].

Zhang et al.[11] in their study, found that by impregnating with copper ethanolamine after the accelerated weather conditions test, they greatly prevented the change in the

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

Türkoğlu, T., Baysal, E., Altay, Ç., Toker, H., Küçüktüvek, M., & Gündüz, A. surface properties of the wood materials. Altay et al.[12] investigated color changes of Scots pine wood impregnated with copper content chemical before synthetic, cellulosic, polyurethane varnishes (PV) coating after weathering. They found that a good color stability in the Scots pine specimens treated with Wolmanit CX-8 (WCX-8) before PV coated of weathering conditions.

This study was presented at Calabrian pine (Pinus brutia Ten.) wood impregnated with a 2.5 and 5 percent aqueous solutions of copper-based chemicals such as Celcure C4, Korasit KS, and Tanalith E 8000. After impregnation, waterbased varnish (WBV) and PV were coated to the wood surface. After the preparation process the wood specimens were exposed to the weather condition for 6 months in Mugla Province of Turkey. Therefore, this study aims at investigating the effect of weathering on gloss and color changes of impregnated Calabrian pine wood with copper content impregnation chemicals before water-based and polyurethane varnishes coatings.

the wooden surfaces with 220 grit sandpaper, the varnish was taken to an empty container and coated to all surfaces and edges of the wood with a brush. During the application, the undiluted varnish was applied in 2 layers. Polyurethane and water-based varnish application values are as follows: Polyurethane varnish: Filling: 100 g/m2 – Topcoat: 100 g/m2 Water-based varnish: Filling: 100 g/m2 – Topcoat: 100 g/m2

2.4 Gloss test The gloss values Calabrian pine wood specimens were determined using a gloss meter (BYK Gardner, MicroTRIGloss) according to ASTM D523-14[15]. The measurement geometry was chosen a 60° incidence angle. Ten replicates were made for each treatment group. Gloss measurements were made parallel to the fibers.

2.5 Color test

2. Materials and Methods 2.1 Preparation of wood specimens The Calabrian pine (Pinus brutia Ten.) wood specimens were used with a dimension of 10 mm x 100 mm x 150 mm (radial x tangent x longitudinal). The test samples were conditioned for two weeks at 20 °C and 65% relative humidity before testing.

2.2 Impregnation process The Calabrian pine wood specimens were impregnated with 2.5% and 5% aqueous solutions of impregnation chemicals according to ASTM D1413-07e1[13]. The retention values of the test samples at the end of the impregnation were calculated by the following formula 1: Retention =

(

G.C X 10 3 Kg / m3 V

)

(1)

G = T2-T1 T1: Weight of specimen before impregnation (g) T2: Weight of specimen after impregnation (g) V: Volume of specimen volume (cm3) C: Concentration (%)

The color parameters a*, b* and L* were determined by the CIEL*a*b* method (Figure 1).

The L* axis shows brightness, while a* and b* are color coordinates. The parameters +a* and -a* show red and green, respectively. The parameter +b* shows yellow, while -b* shows. L* can range from 100 (white) to 0 (black). The total color changes (∆E*) for Calabrian pine wood specimens were determined by ASTM D 1536–58 T[16]. Equations 2-5:

∆a* = af * – ai *

(2)

∆b*= bf * −bi *

(3)

∆L*= Lf * − Li *

(4)

( ∆E *)

2

= (( ∆a *) +

( ∆b *)2 + ( ∆L *)2

(5)

Where: ∆a*, ∆b*, and ∆L* are the changes between the first and last range values. Ten replicates were made for each treatment group. Color measurements were made parallel to the fibers.

2.3 Application of varnish The Calabrian pine wood specimens were varnished with polyurethane and water-based varnishes after the impregnation process in this study. Polyurethane varnish application; all surfaces and edges of specimens were applied polyurethane varnish with a spray gun according to ASTM D3023-98 standard[14]. Before the application of polyurethane varnish, the surfaces were cleaned from dust by the suitable sanding process. After the polyurethane varnish was mixed thoroughly, it was thinned by the addition of thinner, and two cross-layers were applied on normal porous surfaces. Then, the wood specimens were sanded with sand number 220 and the topcoat application started. In water-based varnish application, after sanding 2/6

Figure 1. The CIEL*a*b* colour space[6].

Polímeros, 30(4), e2020045, 2020

Investigating the surface performance of impregnated and varnished Calabrian pine wood against weathering Varnished wood samples reflect the natural appearance of wood surfaces[18].

In this study, while the highest gloss value (99.52) was obtained with PV coated Calabrian pine wood, the lowest gloss value (44.56) was obtained with 5% Korasit KS+WBV treated Calabrian pine wood before weathering. Baysal et al.[18] found that that the glossiness of Scots pine impregnated with some copper content chemical before polyurethane varnish coating ranged from 96 to 103 depending on the treatment.

Another study, Toker et al.[19] reported that the gloss values of Calabrian pine pre-treated with borates before polyurethane varnish coating ranged from 89.1 to 97. Our results were similar to Baysal et al.[18] and Toker et al.[19]. Except for the Korasit KS+WBV and Tanalith E 8000 + WBV treatment groups at all concentrations, the glosses of the Calabrian pine wood increased with rising concentration levels of chemicals. Pre-impregnation with chemicals before the PV coating decreased the gloss values of Calabrian pine wood.

The chemicals can limit gloss to a certain degree in test samples before weathering. The reason for the experimental results reached may be due to the dispersion and absorption of rays reflected from salt crystals. There are photoactive ions on the wood surface. Varnished wood due to photoactive ions may loss of gloss before decomposition[7]. These results are similar to the gloss values of Scots pine and Oriental beech wood impregnated before varnishing[20,21]. Gündüz et al.[22] investigated gloss values of Scots pine pre-impregnated with some copper-based chemicals such as Wolmanit CX-8 and Celcure AC 500 before water-based varnish coating.

They found that pre-impregnation with chemicals before

2.6 Weathering test The Calabrian pine wood was exposed to weathering for 6 months from May to November in 2019. The wood panels were prepared for weathering according to ASTM D 358-55[17]. A test site has been established for practical evaluations near Muğla Regional Meteorological Observation Station in the South Aegean Region. The meteorological data of Muğla is provided in Table 1.

2.7 Statistical evaluations IBM SPSS® program was used to evaluate the test results.

Variance Analysis and Duncan tests were performed on the results. The Variance Analysis and Duncan test applied at 95% confidence level in these test result. Statistical evaluations were made on homogeneity groups (HG) where different letters reflect statistical significance according to test results.

Ten replicates were made for each treatment group and total 140 data were analysed.

3. Results and Discussions 3.1 Gloss test results The gloss and gloss losses values of impregnated and varnished Calabrian pine wood before and after weathering are demonstrated in Table 2. A clear coating application is the easiest and most common method to protect wood against weather conditions and increase its distinctive appearance.

Table 1. Meteorological data of Muğla.

Months Average temperature per month (°C) Humidity per month (%) Average wind speed per month (m/sn) Total rainfall per month (mm=kg/m-2) Number of rainy days

May 18.4 54.4 1.3 9. 0 5

June 23.4 55.4 1.4 76.5 12

July 26.1 42.7 1.8 43.2 1

August 27.8 40.0 1.7 16.4 1

September 22.6 53.3 1.4 51.4 5

October 18.0 65.1 0.9 78.6 5

Table 2. Gloss change values of Calabrian pine wood specimens before and after natural weathering.

Impregnation + Varnish

Concentration (%)

Retention (Kg/m3)

2.5 5.0 2.5 5.0 2.5 5.0 2.5 5.0 2.5 5.0 2.5 5.0

13.48 30.92 14.88 32.11 15.05 31.47 13.48 30.92 14.88 32.11 15.05 31.47

PV WBV Celcure C4+PV Celcure C4+PV Korasit KS+ PV Korasit KS+ PV Tanalith E 8000 +PV Tanalith E 8000 +PV Celcure C4+ WBV Celcure C4+ WBV Korasit KS+ WBV Korasit KS+ WBV Tanalith E 8000 + WBV Tanalith E 8000 + WBV

Before natural weathering gloss values Mean SD 99.52 9.72 46.26 6.50 97.60 11.19 98.36 10.06 97.93 9.76 98.66 8.87 93.12 12.49 96.36 13.11 50.43 6.89 54.68 8.05 52.90 12.61 44.56 6.39 52.62 7.90 52.62 5.72

After 6 months of natural weathering gloss values Mean SD 55.32 5.95 24.15 3.25 57.24 6.10 43.30 9.58 56.25 7.03 50.90 8.55 54.57 6.45 51.66 7.48 19.79 5.69 33.94 5.04 24.62 5.84 22.90 5.69 22.46 4.26 32.15 5.89

Gloss losses (%) Mean -44.41 -47.79 -41.35 -55.97 -42.56 -48.40 -41.39 -46.38 -60.75 -37.92 -53.45 -48.60 -57.31 -38.91

HG C DE B G B E B D H A F E G A

Note: Results reflect the average of 10 Calabrian pine wood specimens. PV: Polyurethane varnish; WBV: Water-based varnish; SD: Standard deviation; HG: Homogeneity groups; A-G letters: Statistical differences.

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Türkoğlu, T., Baysal, E., Altay, Ç., Toker, H., Küçüktüvek, M., & Gündüz, A. WBV coating caused increase in the gloss of Scots pine.

In addition, Gündüz et al.[23] studied gloss values of Scots pine pre-impregnated with some copper-containing chemicals before polyurethane varnish (PV) coating. They reported that pre-impregnation with chemicals before PV coating caused decrease in the gloss of Scots pine. In our study, after the weathering process, the glosses of all Calabrian pine wood decreased. Because erosion and abrasion on the wood surfaces cause gloss loss in the varnish layers after weathering[7,24].

Gloss losses values after 6 months of weathering range from -37.92% to -60.75%. Türkoglu et al.[20] found that gloss loss of some copper-based chemicals treated and polyurethane varnish coated Scots pine ranged from 59.91% to 69.90% after 6 months of weathering. Surprisingly, our results showed that lower concentration levels (2.5%) of chemicals for PV coated Calabrian pine wood caused decrease in gloss losses of Calabrian pine wood samples after weathering. For WBV coated Calabrian pine wood, higher concentration levels of chemicals resulted in lower gloss losses of Calabrian pine wood after weathering.

3.2 Color test results The color parameters, color changes, and total color change values of the Calabrian pine wood impregnated with copper content chemicals and varnished before and after weathering are given in Table 3. According to Table 3, while the highest L* value (70.89) was obtained with WBV coated Calabrian pine wood, the lowest L* value (43.13) was measured with 5% Tanalith E 8000+PV treated Calabrian pine wood before weathering. Our results showed that pre-impregnation with copper-based chemicals before varnishing caused decrease L* values of Calabrian pine wood to some extent.

Baysal[25] investigated the color characteristics of Scots pine wood impregnated with some copper content chemicals before polyurethane varnish coating after accelerated weathering.

They reported that pre-impregnation with chemicals before varnish coating reduced L* values of Scots pine wood before accelerated weathering. The results of our study are in good agreement with data Baysal[25]. Gündüz et al.[22] studied color characteristics of Wolmanit CX-8, Adolit KD-5 and Celcure AC 500 impregnated Scots pine wood before water-based varnish (WBV) coating after accelerated weathering. They reported that L* values of Scots pine wood decreased impregnated with chemicals before WBV coating before weathering. The results are in good agreement with Gündüz et al.[22]. Concentration levels of chemicals had no significant effect on the L* values of Calabrian pine wood before weathering. While a* values range from 0.87 to 6.32, b* values vary between 30.34 to 42.29 before weathering.

After 6 months of weathering, the 𝜟L* values of Calabrian pine wood decreased for all treatment groups. Since wood surfaces are delicate to UV light, the 𝜟L* values were negative in all treatments. Chemical changes ensue in wood constituent such as lignin on surfaces of wood during photo degradation[26]. Depolymerisation of lignin may cause the darkening of the wood surface[18,27,28]. Pre-impregnation with chemicals before PV coating gave lower 𝜟L* values than only PV coated Calabrian pine wood. Because 𝜟L* value was -7.75 for only PV coated Calabrian pine wood, it changed from -0.29 to -6.65 for impregnated and PV coated Calabrian pine wood. 𝜟L* values of Calabrian pine wood increased with increasing concentration levels of chemicals. All treatment groups tended to turn reddish and bluish, giving a positive 𝜟a* and negative 𝜟b* values, respectively after natural weathering. Ghosh et al.[28] studied color characteristics of a copper-containing chemical such

Table 3. Color change values of the Calabrian pine wood specimens before and after natural weathering.

Impregnation + Varnish

Conc. (%)

Color values before natural weathering

Color change values after natural weathering

Li*

ai*

Lf*

bi*

af*

bf*

𝜟L* Mean

𝜟a*

Total color change values after natural weathering

𝜟b*

HG

Mean

HG

Mean

HG

𝜟E* Mean HG

PV

-

67.92

6.32

41.51 60.17 14.18 37.10

-7.75

G

7.86

ABC

-4.41

A

11.88

WBV

-

70.89

4.79

33.03 66.00 11.01 30.84

-4.89

DE

6.22

DE

-2.19

A

8.20

DE G

Celcure C4+PV

2.5

49.78

4.15

42.29 44.36 12.15 23.51

-5.42

E

8.00

ABC -18.78

EF

21.12

AB

Celcure C4+PV

5.0

44.47

1.24

30.34 40.79

8.45 25.83

-3.68

C

7.21

BCD

-4.51

A

9.26

G

Korasit KS+ PV

2.5

50.36

3.82

40.50 43.71 12.32 23.17

-6.65

F

8.50

AB

-17.33

E

20.41

B

Korasit KS+ PV

5.0

46.90

2.09

33.53 43.36

8.55 23.59

-3.54

C

6.46

DE

-9.94

C

12.37

DE

Tanalith E 8000 +PV

2.5

50.46

3.04

39.34 50.17 11.95 29.22

-0.29

A

8.91

A

-10.12

C

13.48

D

Tanalith E 8000 +PV

5.0

43.13

3.07

37.93 42.67

9.49 16.51

-0.46

A

6.42

DE

-21.42

G

22.36

A

Celcure C4+ WBV

2.5

49.57

2.07

33.74 49.32 10.24 26.27

-0.25

A

8.17

ABC

-7.47

B

11.07

EF

Celcure C4+ WBV

5.0

44.01

2.00

31.34 39.64

9.04 17.17

-4.37

CD

7.04

CD

-14.17

D

16.41

C

Korasit KS+ WBV

2.5

56.12

3.59

39.60 46.75

9.46 19.80

-9.37

H

5.87

DE

-19.80

FG

22.67

A

Korasit KS+ WBV

5.0

49.99

2.18

33.07 48.40

9.44 24.41

-1.59

B

7.26

BCD

-8.66

BC

11.41

EF

Tanalith E 8000 + WBV

2.5

51.01

3.30

38.80 44.21

8.86 17.86

-6.80

F

5.56

E

-20.94

FG

22.70

A

Tanalith E 8000 + WBV

5.0

52.35

0.87

30.56 48.53

8.97 26.63

-3.82

C

8.10

ABC

-3.93

A

9.77

FG

Note: Results reflect the average of 10 Calabrian pine wood specimens. PV: Polyurethane varnish; WBV: Water-based varnish; Conc: Concentration (%); HG: Homogeneity groups; A-G letters: Statistical differences

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Polímeros, 30(4), e2020045, 2020

Investigating the surface performance of impregnated and varnished Calabrian pine wood against weathering as Wolmanit CX-8 impregnated and PV coated Oriental beech after 500 h of Scots pine specimens after accelerated weathering. They revealed that Scots pine specimens showed reddish, giving a positive 𝜟a* values and gave negative 𝜟b* values tended to turn bluish. Türkoğlu et al.[20] studied color changes of a copper-based chemical such as Adolit KD 5 impregnated and PV coated Oriental beech wood samples. They explained that Oriental beech wood gave negative 𝜟a* and 𝜟b* values, respectively after 6 months of weathering. In this study, while the highest total color change (𝜟E*) was obtained in 2.5% Tanalith E 8000+WBV treated Calabrian pine wood, the lowest total color change was determined in the only WBV coated wood specimens. The total color changes of Calabrian pine wood vary between 8.20 to 22.70 after weathering. In this study, as the concentration levels of chemicals increased, total color change of Calabrian pine wood decreased. In other words, the increase in concentration levels of chemicals has produced positive results in terms of total color changes of Calabrian pine wood. Yalinkilic et al.[7] investigated the weathering performance of Scots pine and chestnut wood impregnated with chromium-copper-boron (CCB) before polyurethane varnish coating. They reported that CCB impregnation reduced total color changes of Scots pine and chestnut wood. Türkoğlu et al.[20] reported that impregnation of wood some copper content chemicals before polyurethane varnish coating reduced total color changes of wood after weathering. Gündüz et al.[22] found that preimpregnation with copper-based chemicals such as Adolit KD 5, Wolmanit CX-8, and CAC 500 before water-based varnish coating reduced total color changes of Scots pine after 1000 h of accelerated weathering. Our results are similar to Yalinkilic et al.[7], Türkoğlu et al.[20], and Gündüz et al.[22].

4. Conclusions The gloss losses were observed for all treatment groups after weathering. The gloss values of PV coated Calabrian pine wood were higher than that of SBW coated Calabrian pine wood before weathering. While higher concentration levels for WBV coated Calabrian pine wood resulted in lower gloss losses, higher concentration levels of PV coated Calabrian pine wood resulted in higher gloss losses after weathering. 𝜟L* values of impregnated and coated Calabrian pine wood decreased. All treatment groups giving positive 𝜟a* and negative 𝜟b* values tended to turn reddish and bluish, respectively after weathering. Total color change (𝜟E*) was the lowest for only WBV coated Calabrian pine wood. Additionally, higher concentration levels of chemicals resulted in lower total color changes of Calabrian pine wood after weathering.

5. Acknowledgements This study has been granted by the Muğla Sıtkı Koçman University Research Projects Coordination Office through Project Grant Number (18/024).

6. References 1. Priadi, T., & Hiziroglu, S. (2013). Characterization of heat Treated wood species. Materials & Design, 49, 575-582. http:// dx.doi.org/10.1016/j.matdes.2012.12.067.

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2. Obata, Y., Takeuchi, K., Furuta, Y., & Kanayama, K. (2005).

Research on better use of wood for sustainable development: quantitative evaluation of good tactile warmth of wood. Energy, 30(8), 1317-1328. http://dx.doi.org/10.1016/j.energy.2004.02.001.

3. Woodard, A. C., & Milner, H. R. (2016). Sustainability of construction materials. United Kingdom: Woodhead Publishing.

4. Akbarian, M., Olya, M. E., Ataeefard, M., & Mahdavian, M. (2012). The Influence of NaNosilver on thermal and antibacterial properties of A 2K waterborne polyurethane coating. Progress in Organic Coatings, 75(4), 344-348. http:// dx.doi.org/10.1016/j.porgcoat.2012.07.017.

5. De Meijer, M. (2001). Review on the durability of exterior wood coatings with reduced VOC-content. Progress in Organic Coatings, 43(4), 217-225. http://dx.doi.org/10.1016/S03009440(01)00170-9.

6. Hunt, R. (1995). Measuring color. United Kingdom: Ellis Horwood Limited.

7. Yalinkilic, M. K., Ilhan, R., Imamura, Y., Takahashi, M., Demirci, Z., Yalınkilic, A. C., & Peker, H. (1999). Weathering durability of CCB-impregnated wood for clear varnish coatings. Journal of Wood Science, 45(6), 502-514. http://dx.doi.org/10.1007/ BF00538961.

8. Kamdem, D. P., Pizzi, A., & Jermannaud, A. (2002). Durability of heat-treated wood. Holz als Roh- und Werkstoff, 60(1), 1-6. http://dx.doi.org/10.1007/s00107-001-0261-1.

9. Lebow, S. T. (2004). Alternatives to Chromated Copper Arsenate (CCA) for residential construction. In Environmental Impacts of Preservative-Treated Wood Conference (pp. 156-168). Orlando, FL, USA: The Florida Center for Environmental Solutions.

10. Barceloux, D. G., & Barceloux, D. (1999). Chromium. Journal of Toxicology. Clinical Toxicology, 37(2), 173-194. http:// dx.doi.org/10.1081/CLT-100102418. PMid:10382554.

11. Zhang, J., Kamdem, D. P., & Temiz, A. (2009). Weathering of copper-amine treated wood. Applied Surface Science, 256(3), 842-846. http://dx.doi.org/10.1016/j.apsusc.2009.08.071.

12. Altay, C., Baysal, E., Toker, H., Türkoğlu, T., Küçüktüvek, M., Gündüz, A., & Peker, H. (2020). Effects of natural weathering on surface characterıstics of Scots pine impregnated with wolmanit CX-8 and varnished. Wood Research, 65(1), 87-100. http://dx.doi.org/10.37763/wr.1336-4561/65.1.087100.

13. American Society for Testing and Materials – ASTM. (2007).

ASTM 1413-07e1: Standard Test Method For Wood Preservatives

By Laboratory Soil-Block Cultures. West Conshohocken‎, PA, USA: ASTM International.

14. American Society for Testing and Materials – ASTM. (2017).

ASTM D3023- 98: Standard practice for determination of resistance of factory-applied coatings on wood products to stains and reagents. West Conshohocken‎, PA, USA: ASTM International.

15. American Society for Testing and Materials – ASTM. (2018).

ASTM D523-14: Standard test method for specular gloss. West Conshohocken‎, PA, USA: ASTM International.

16. American Society for Testing and Materials – ASTM. (1964).

ASTM D1536-58 T: Tentative method of test color difference using the colormaster differential colourimeter. West Conshohocken‎, PA, USA: ASTM International.

17. American Society for Testing and Materials – ASTM. (1970).

ASTM D358-55: Standard specification for wood to be used panels in weathering tests of paints and varnishes. West Conshohocken‎, PA, USA: ASTM International.

18. Baysal, E., Dizman Tomak, E., Ozbey, M., & Altin, E. (2014).

Surface properties of impregnated and varnished Scots pine wood after accelerated weathering. Coloration Technology, 130(2), 140-146. http://dx.doi.org/10.1111/cote.12070.

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Türkoğlu, T., Baysal, E., Altay, Ç., Toker, H., Küçüktüvek, M., & Gündüz, A. 19. Toker, H., Baysal, E., & Kesik, H. I. (2009). Surface characteristics of wood pre-impregnated with borates before varnish coating.

Forest Products Journal, 59(7-8), 43-46.

20. Türkoğlu, T., Baysal, E., Kureli, I., Toker, H., & Ergün, M. E. (2015). The effects of natural weathering on hardness and gloss of impregnated and varnished Scots pine and oriental beech. Wood Research, 60(5), 833-844.

21. Pori, P., Vilčnik, A., Petrič, M., Sever Škapin, A., Mihelčič, M., Šurca Vuk, A., Novak, U., & Orel, B. (2016). Structural studies of TiO2/wood coatings prepared by hydrothermal deposition of rutile particles from TiCl4 aqueous solutions on spruce (Picea abies) Wood. Applied Surface Science, 372, 125-138. http://dx.doi.org/10.1016/j.apsusc.2016.03.065.

22. Gündüz, A., Baysal, E., Türkoğlu, T., Küçüktüvek, M., Altay, C., Peker, H., & Toker, H. (2019). Accelerated weathering performance of Scots pine preimpregnated with copper based chemicals before varnish coating. Part II: coated with water based varnish. Wood Research, 64(6), 987-998. Retrieved in 2020, November 17, from http://www.woodresearch.sk/ wr/201906/06.pdf 23. Gündüz, A., Baysal, E., Türkoğlu, T., Altay, C., Küçüktüvek, M., Toker, H., & Peker, H. (2020). Accelerated weathering performance of Scots pine preimpregnated with copper based chemicals before varnish coating. Part: I coated with cellulosic and polyurethane varnishes. Coloration Technology, 136(1), 34-44. http://dx.doi.org/10.1111/cote.12435.

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24. Baysal, E., Degirmentepe, S., Toker, H., & Türkoğlu, T. (2014). Some mechanical and physical properties of AD-KD 5 impregnated and thermally modified Scots pine wood. Wood Research, 59(2), 283-296. Retrieved in 2020, November 17, from http://www.woodresearch.sk/wr/201402/07.pdf 25. Baysal, E. (2012). Surface characteristics of CCA treated Scots pine after accelerated weathering. Wood Research, 57(3), 375-382. Retrieved in 2020, November 17, from http://www. woodresearch.sk/wr/201203/04.pdf 26. Feist, W., & Hon, D. N.-S. (1984). Chemistry of weathering and protection. In R. Rowell (Eds.), The chemistry of solid wood (Chap. 11, pp. 401-451). USA: American Chemical Society. http://dx.doi.org/10.1021/ba-1984-0207.ch011.

27. Temiz, A., Yildiz, U. C., Aydin, I., Eikenes, M., Alfredsen, G., & Colakoglu, G. (2005). Surface roughness and color characteristics of wood treated with preservatives after accelerated weathering test. Applied Surface Science, 250(14), 35-42. http://dx.doi.org/10.1016/j.apsusc.2004.12.019.

28. Ghosh, S. C., Militz, H., & Mai, C. (2009). Natural weathering of Scots pine (Pinus sylvestris L.) boards modified with functionalised commercial silicone emulsions. BioResources, 4, 659-673. http://dx.doi.org/10.15376/BIORES.4.2.659-673.

Received: Nov. 17, 2020 Revised: Feb. 08, 2021 Accepted: Feb. 09, 2021

Polímeros, 30(4), e2020045, 2020

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.05520

Selection of appropriate reinforcement for nylon material through mechanical and damping characteristics Hari Bodipatti Subburamamurthy1 , Rajasekar Rathanasamy1* , Harikrishna Kumar Mohan Kumar1 , Moganapriya Chinnasamy1 , Gobinath Velu Kaliyannan2 and  and Saravanan Natarajan1  Department of Mechanical Engineering, Kongu Engineering College, Erode, Tamil Nadu, India Department of Mechatronics Engineering, Kongu Engineering College, Erode, Tamil Nadu, India 1

2

*rajasekar.cr@gmail.com

Abstract Nylon composites were developed using 5-20 wt.% of Talc, Kaolin, Mica and Calcium Carbonate (CaCO3) particulates.

Mechanical and free vibration characteristics of nylon composites were examined through experimental and analytical approach. Particulate filled nylon composites exhibited enhancement in tensile strength, specific stiffness, natural frequency and damping factor compared to pure nylon. As a whole, talc reinforced nylon composites especially 15 wt.% filler content (T15) portrayed significant performance in mechanical and vibrational characteristics. This is followed by nylon composites based on kaolin (K15) and mica (M20) compared to CaCO3 based nylon composites. T15 depicted 18.13%, 33.33%, 81.2% increment in tensile strength, natural frequency and damping factor compared to pure nylon.

The simulated ANSYS results are in agreement with experimental results. Among four different particulates, talc is proven as appropriate reinforcing agent for nylon owing to larger surface area of talc particles and polar-polar interaction between talc and nylon matrix.

Keywords: damping factor, mechanical testing, mineral fillers, nylon 6.

How to cite: Subburamamurthy, H. B., Rathanasamy, R., Kumar, H. K. M., Chinnasamy, M., Kaliyannan, G. V., & Natarajan, S. (2020). Selection of appropriate reinforcement for nylon material through mechanical and damping characteristics. Polímeros: Ciência e Tecnologia, 30(4), e2020046. https://doi.org/10.1590/0104-1428.05520

1. Introduction Nylon is made with combining appropriate monomers to form a long chain by condensation polymerization reaction followed by granulation. It contains oxygen, nitrogen and hydrogen atoms. Nylon is the widely used polymer due to its commendable mechanical properties and processability.

The nylon is used in many engineering applications due to its high abrasion resistance, noiseless operation and self-lubricating property, since it is used in places where lubrication cannot be provided (i.e., textile machines, washing machines, automobiles, aerospace, printers, etc…).

Pure nylon has moderate mechanical properties and vibrational characteristics and it is not preferred in some engineering applications due to its limitations such as poor resistance to heat, low dimensional stability, high water absorption[1-4]. Researchers overcome the stated limitations through the development of nylon composites by incorporating fibers or fillers[5-7]. Particulates like carbon nanotubes, graphene, zinc oxide, talc, mica, calcium carbonate, kaolin, etc., are used for the preparation of nylon composites / nanocomposites. It was evident that the improvement in physical and mechanical properties of particulate based polymer composites depends on the structural geometry of the particle (i.e., shape, size, surface area) and extent of distribution of particles in the polymer matrices[3,11]. Mineral fillers are available in different forms such as sphere, cube,

Polímeros, 30(4), e2020046, 2020

sheets, flakes, plates and fibers[7]. Mechanical properties like tensile strength, modulus, impact strength and stiffness were increased upon adding nano fillers to the nylon matrix, whereas the cost of nano fillers were high[7,17,20]. The mineral fillers with larger particle size (µm) also increased the mechanical properties with less cost[6,8-10]. The natural fibers like kenaf, flax, hemp etc., were also added to nylon to improve the properties. However, processing of natural fiber based nylon composites are challenging owing to low thermal stability of natural fibers. The properties of natural fiber polymer composites depends on the properties, orientation and concentration of fibers[11]. Natural fiber composites cannot be used for many applications like gears, bushings, fasteners, etc. The particulate filled composites can be processed easily and applied in many applications than fiber composites[13-21].

The present research work utilizes talc, kaolin mica and CaCO3. Talc contains magnesium - silica, kaolin contains aluminium – silica, mica contains aluminium-magnesiumiron-silica combinations[7,10,12]. Whereas CaCO3 contains calcium. Talc, kaolin, mica and CaCO3 have polar groups which can form chemical interaction with polar nylon matrix[7,12,14].

Based on the observation from literature survey, it is evident that the studies on vibration behaviour of nylon

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

Subburamamurthy, H. B., Rathanasamy, R., Kumar, H. K. M., Chinnasamy, M., Kaliyannan, G. V., & Natarajan, S. composites is not yet explored. This present research study stands unique in analysing the vibrational characteristics of four different particulate reinforced nylon composites in addition to mechanical performance.

Mineral fillers such as talc, kaolin, mica and CaCO3 particulates are reinforced into the Nylon 6 base matrix using melt mixing method[10,21-23]. Extruded nylon composites were palletized and the samples for vibration and tensile studies were prepared using injection moulding machine. Tensile strength and stiffness were determined using tensometer.

Natural frequency and damping factor were measured by performing modal analysis. The natural frequency and damping factor are considered as an important vibration parameter, when the material is subjected to both constant and cyclic loading[24-26]. The mechanical properties and modal analysis of the composites were also simulated using ANSYS 18.1 software and simulated results were compared with experimental results.

2. Materials and Methods 2.1 Materials The material used in this research work was nylon 6 (PA6) pellets, were procured from ARS polymers, Coimbatore, India. The technical grade mineral fillers (talc, kaolin, mica and CaCO3) were purchased from Astrra Chemicals, Chennai, India and its properties is listed in Table 1.

2.2 Sample preparation Initially fillers (talc, kaolin, mica and CaCO3) and Nylon 6 were dried at 80°C for 1 hour using hot air oven to remove the moisture content. The materials are allowed to cool inside the oven to prevent oxidization. The nylon 6 does not lead to any structural change up to 150°C. Fillers (talc, kaolin, mica and CaCO3) and nylon 6 were melt mixed and extruded using twin extruder according to the formulation table shown in Table 2. The extruder was operated at 50 rpm and at 240 °C. Due to corotation and intermeshing of the screws, the melt mixed nylon with filler were extruded. The extruded materials were palletized is used for preparing tensile and vibration samples with injection molding machine. Injection molding machine was operated at 280°C, injection and holding pressure was maintained at 78 x 105 Pa and 35 x 105 Pa respectively. Preparation method of the composite was described in Figure 1.

2.3 Characterization techniques 2.3.1 Mechanical testing Tensile specimens were prepared according to ASTM D638 standard. Tensile test were carried out using electronic tensometer (PC2000-20 kN capacity) and the tensometer was operated with a crosshead speed of 1 mm/min using the load cell of 5 kN. Tensile strength and modulus of elasticity were determined for three specimens in each category of composites. Average values of three samples were reported in this work.

Tensile strength was also verified using ANSYS R18.1 software. H-method structural analysis with Solid quad 2/8

Table 1. Material properties of fillers.

Talc Grain Size (µm) 1 to 1.5 Grain shape Platy

Density (g/cc)

2.76

Kaolin 1 to 10 Tetra/ octahedral sheet 2.59

Mica CaCO3 1 to 20 ≤50 No proper Orthorhombic shape 2.90

2.70

Table 2. Formulation of compounds.

Filler material wt. %

Sample code

Nylon 6 wt. %

Talc

Kaolin

Mica

CaCO3

N T5 T 10 T 15 T 20 K5 K 10 K 15 K 20 M5 M 10 M 15 M 20 C5 C 10 C 15 C 20

100 95 90 85 80 95 90 85 80 95 90 85 80 95 90 85 80

5 10 15 20 -

5 10 15 20 -

5 10 15 20 -

5 10 15 20

4 node 182 element type was used for analysis. A model was created according to the ASTM D638 standard using Creo 4.0 and the model is imported to ANSYS. The material properties obtained from experimental analysis is given as input to ANSYS specimen. Tetrahedral mesh was done to fit the tensile specimen geometry. The displacement was constrained at one end with all DOF fixed and force was defined along the X-axis on other end. Model was solved and the solutions were obtained.

2.3.2 Modal analysis Nylon composite was prepared with the size of 200 mm x 20 mm x 3 mm. Experimental setup of modal analysis used in this work is shown in Figure 2. Free vibration analysis technique was used to carry out modal analysis with cantilever beam. One end of the composite beam was fixed and piezoelectric sensor was fixed on the other end. Data acquisition card (NI USB 6008 DAQ) used as an interface between Lab VIEW 2016 software and the piezoelectric sensor.

The beam was subjected to free oscillation on the free end to obtain the time domain signals using Lab VIEW software. A circuit was designed using Lab VIEW software to convert the time domain signals into frequency domain signals. The signal was sampled with sampling rate n = 1000 samples/sec and sampling time T = 10 sec. Obtained frequency domain signals were stored in a computer for further analysis.

The half-power band width method was used to find the damping coefficient values of nylon composites through frequency response function (FRF) curves obtained from the Lab VIEW software. Band width values are obtained Polímeros, 30(4), e2020046, 2020

Selection of appropriate reinforcement for nylon material through mechanical and damping characteristics

Figure 1. Specimen preparation.

Figure 2. Experimental setup for modal analysis.

from the sample frequency curve shown in Figure 3. In accordance, the damping values were calculated based on Equation 1. Initial vibration amplitude (mode 1) is considered for calculating natural frequency and damping factor of the composite.

ζ = ∆ω / 2 ωn

(1)

Where, ζ – damping coefficient, ∆ω – bandwidth, and ωn – natural frequency.

Modal analysis was also carried out using ANSYS software. Beam was modelled in the software and the element type used was selected as beam 2 node 188. The material properties and the real constants were defined with reference to the specimen properties. Meshing was performed with 10 as element size. Modal analysis type was selected and three modes of frequency were extracted during the analysis. Degrees of freedom were arrested on one end of the beam and natural frequency was obtained on solving the above-mentioned criteria.

Polímeros, 30(4), e2020046, 2020

Figure 3. Sample frequency curve.

3. Results and Discussions 3.1 Mechanical testing Modulus of elasticity of the prepared composites are shown in Figure 4. It is evident that addition of particulates 3/8

Subburamamurthy, H. B., Rathanasamy, R., Kumar, H. K. M., Chinnasamy, M., Kaliyannan, G. V., & Natarajan, S.

Figure 4. Effect of particulates on modulus of elasticity.

enhanced the modulus of elasticity of nylon composites compared to pure nylon. Moreover, increase in concentration of particulates in the nylon matrix gradually increase the elastic modulus. Modulus of elasticity of talc, kaolin, mica and CaCO3 particulates reinforced composites (T20, K20, M20 and C20) was increased by 156%, 151.3%, 77.2% and 19.3% respectively compared to pure nylon. The possible reasons are depicted as follows. Addition of rigid particles in low stiffness polymer like nylon can cause increase in elastic modulus. Secondly, incorporated particulates create strong physical interaction with the nylon matrix, thereby restricting the movement of the polymer chains causing increment in modulus[27]. However, talc, kaolin and mica reinforced system showed pronounced effect of elastic modulus increment than CaCO3. In reference to base chemical structure of reinforcements and nylon (Table 3), it is affirmed that there exists hydrogen bonding between talc-nylon, kaolin-nylon and mica-nylon. Hence, it is evident that apart from physical interaction, the chemical interaction also plays a vital role in improvement of elastic modulus. On the other hand, there is no chemical interaction between CaCO3-nylon matrix and hence there was a marginal increase in elastic modulus due to physical interaction between the two.

Figure 5 shows the tensile strength of the pure and talc, kaolin, mica and CaCO3 filled nylon composites obtained through experimental method and ANSYS simulation method. Talc, kaolin and mica reinforced nylon composites shows increase in tensile strength compared to pure nylon matrix. Talc and Kaolin reinforced composites showed a similar trend, i.e., the tensile strength gradually increased with increase in addition of particulates upto 15 wt.% and further drops, while increasing the loading to 20 wt.%. The increase in tensile strength can be attributed to homogeneous distribution of particulates in nylon matrix and the drop in tensile strength (T 20 & K 20) can be ascribed to agglomeration of particulates due to higher loading beyond the acceptable limit. In case of mica reinforced composites, the tensile strength increased with gradual increase in loading of mica (5-20 wt.%). Platy fillers have high aspect ratio, which increases the wettability of fillers by the matrix. Further addition of filler reduces the mobility of polymer chains and affects the kinetics of crystallisation. However, CaCO3 based nylon composites showed consecutive drop in tensile strength with gradual increase in addition of particulate.

CaCO3 is known to promote craze formation in deformed 4/8

Figure 5. Effect of particulates on tensile strength.

polymers before fracture and low surface tension. This causes dewetting of CaCO3 particles from the nylon matrix.

Dewetting becomes more evident as the concentration of the filler increases. Also filler–matrix adhesion is relatively weak, allowing debonding to occur before fully developed plastic deformation and therefore, the formation of cavities.

The particle size of CaCO3 is comparatively higher than other three fillers.

Talc reinforced nylon composites showed superior tensile strength followed by kaolin and mica based systems compared to CaCO3 containing nylon composites. The structural backbone with presence of elements like Si, Mg, Al imparts strength and stiffness to talc, kaolin and mica. Hence, incorporation of the same in nylon leads to elevation in tensile strength. On the other hand, CaCO3 has a weak structural backbone bearing only calcium content and hence, incorporation of the same in nylon detoriate the tensile strength.

The increment in tensile strength of T 15, K 15 and M 20 was 18%, 14% and 10% compared to pure nylon.

Hence, T 15 was confirmed as the suitable particulate and optimized filler concentration for nylon.

Figure 6 shows the meshing of the tensile specimen using tetrahedral elements. Figure 7a and Figure 7b shows the tensile strength obtained from ANSYS software for Polímeros, 30(4), e2020046, 2020

Selection of appropriate reinforcement for nylon material through mechanical and damping characteristics Table 3. Interaction of polymer matrix and reinforcement.

Polymer Matrix – Nylon 6

Reinforcement

Chemical Interaction

Compatibility

1. Talc

H-bonding

Both physical and chemical interaction

2. Kaolin

H-bonding

Both physical and chemical interaction

3. Mica

H-bonding

Both physical and chemical interaction

4. CaCO3

No interaction

Only physical interaction

of the composite relies on tensile strength, modulus of elasticity and density of the composites. Specific stiffness and strength increases as weight content of filler increases in nylon matrix. Maximum specific stiffness was achieved for the composites (T20, K20, M20 and C20) with high filler content. Despite T20 and K20 achieved maximum specific stiffness, tensile strength of those composites was reduced when compared to composites T15 and K15. Among the T15, K15, M20 and C20 composite T15 composite achieved the maximum specific stiffness due to the density of the composite and the compatibility between filler and nylon matrix resulted in strong interfacial bonding[10].

3.2 Modal analysis Figure 6. Meshing of tensile specimen in ANSYS.

T15 and K15 specimens respectively. Difference between experimental and ANSYS results of tensile strength were calculated in terms of coefficient of variation and it was found to be 5.3%. Hence both experimental and ANSYS results were found to be similar.

Figure 8 shows the graph plotted against specific stiffness and specific strength. Specific strength and specific stiffness Polímeros, 30(4), e2020046, 2020

Modal analysis was performed for the prepared nylon composites using the experimental setup. Natural frequency and damping factor have been obtained for all the prepared composites to analyse the effect of addition of filler in nylon using free vibration technique. Modal analysis was also carried out using ANSYS software and the ANSYS images of T15 and K15 composites were shown in Figure 9a and Figure 9b respectively. Figure 10, shows the natural frequency values obtained experimentally and by simulation using ANSYS software.

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Subburamamurthy, H. B., Rathanasamy, R., Kumar, H. K. M., Chinnasamy, M., Kaliyannan, G. V., & Natarajan, S.

Figure 7. Tensile strength analysis using ANSYS of (a) T 15 (b) K 15.

Figure 9. Modal analysis using ANSYS of (a) T15 (b) K15.

Figure 10. Natural frequency of composite specimens.

Figure 8. Effect of fillers on specific stiffness and specific strength of composites.

Natural frequency increases as quantity of filler increases in base matrix. Enhancement in natural frequency was achieved based on the increase in stiffness of the beam, modulus of elasticity and interfacial bonding between filler - base matrix[24-26]. Natural frequency of the composite (T15, K15 and M20) was increased by 33.33%, 16.66% and 16.66% respectively when compared to pure nylon (N). Whereas natural frequency of CaCO3 composite decreases when 6/8

compared to pure nylon (N). Natural frequency obtained from modal analysis using ANSYS software follows the similar pattern of experimental results. Difference between experimental and ANSYS results were determined in coefficient of variation with 3.64% deviation.

Figure 11 shows the damping factor for the prepared composites. Damping factor increases as weight content of filler increases in nylon matrix. Among the prepared composites T15, K15 and M15 composite shows maximum damping factor. Damping factor of T15, K15 and M15 composite increases by 81.2%, 54.24% and 46.72% respectively when Polímeros, 30(4), e2020046, 2020

Selection of appropriate reinforcement for nylon material through mechanical and damping characteristics

Figure 11. Damping factor of the composite specimens.

compared to pure nylon (N). The T15 specimen has higher frequency, even though it has a high damping value due to the major difference in half power band width (∆ω). The higher damping factor implies that the material withstands more loads and shocks[24]. Maximum damping factor was achieved due to the major difference in half power band width (∆ω). Maximum damping factor indicates that nanocomposite can absorb amount of vibration energy[25].

4. Conclusions Nylon composites were prepared by incorporating mineral fillers (Talc, Kaolin, Mica and CaCO3) into nylon matrix.

Fillers were mixed with nylon using melt mixing method and specimens were moulded using injection moulding machine. Composites were prepared by reinforcing fillers with 5, 10, 15 and 20 wt. % into nylon matrix. Tensile strength, modulus of elasticity, specific stiffness, and specific strength were examined to study the mechanical behaviour of composite. Modal analysis were also carried out for the prepared composite to determine vibration characteristics of the composites. Tensile strength and natural frequency of the composite were also determined using ANSYS and both the results were compared for validation. Increase in filler (talc, kaolin and mica) content in nylon matrix increases the tensile strength, specific stiffness, natural frequency and damping factor. Larger surface area of the filler leads to the strong interfacial bonding between filler and nylon matrix.

Tensile strength was reduced during the addition of CaCO3 filler in nylon matrix. T15 (talc with 15 wt.%) composite shows superior mechanical and vibration characteristics when compared with all other composites. Tensile strength and natural frequency of ANSYS results also followed the similar pattern of experimental results. Tensile strength and natural frequency of both results were compared and difference in coefficient of variation was found to be 5.3% and 3.64% respectively.

5. References 1. Bose, S., & Mahanwar, P. (2004). Effect of particle size of filler on properties of nylon-6. Journal of Minerals & Materials Polímeros, 30(4), e2020046, 2020

Characterization & Engineering, 3(1), 23-31. http://dx.doi. org/10.4236/jmmce.2004.31003.

2. Bose, S., & Mahanwar, P. (2005). Influence of particle size and particle size distribution on MICA filled nylon 6 composite.

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3. Unal, H., Fındık, F., & Mimaroglu, A. (2003). Mechanical behavior of nylon composites containing talc and kaolin.

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4. Lapčík, L., Maňas, D., Lapčíková, B., Vašina, M., Staněk, M., Čépe, K., Vlček, J., Waters, K. E., Greenwood, R. W., & Rowson, N. A. (2018). Effect of filler particle shape on plasticelastic mechanical behavior of high density poly (ethylene)/ mica and poly (ethylene)/wollastonite composites. Composites.

Part B, Engineering, 141, 92-99. http://dx.doi.org/10.1016/j. compositesb.2017.12.035.

5. Abu Bakar, M., Leong, Y., Ariffin, A., & Mohd Ishak, Z. A. (2008). Effect of chemical treatments on the mechanical, flow, and morphological properties of talc-and kaolin-filled polypropylene hybrid composites. Journal of Applied Polymer Science, 110(5), 2770-2779. http://dx.doi.org/10.1002/app.28791.

6. Jang, K.-S. (2016). Mineral filler effect on the mechanics and flame retardancy of polycarbonate composites: talc and kaolin. e-Polymers, 16(5), 379-386. http://dx.doi.org/10.1515/ epoly-2016-0103.

7. Xanthos, M. (2010). Functional fillers for plastics. Germany: John Wiley & Sons. http://dx.doi.org/10.1002/9783527629848.

8. Ouchiar, S., Stoclet, G., Cabaret, C., Georges, E., Smith, A., Martias, C., Addad, A., & Gloaguen, V. (2015). Comparison of the influence of talc and kaolinite as inorganic fillers on morphology, structure and thermomechanical properties of polylactide based composites. Applied Clay Science, 116, 231-240. http://dx.doi.org/10.1016/j.clay.2015.03.020.

9. Zhang, Z.-X., Zhao, X.-P., Sun, B., Ma, Z.-G., Xin, Z. X., & Prakashan, K. (2017). Synergistic effects of kaolin and talc in a bromobutyl rubber compound for syringe plunger application.

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10. Leong, Y., Abu Bakar, M., Ishak, Z. M., Ariffin, A., & Pukanszky, B. (2004). Comparison of the mechanical properties and interfacial interactions between talc, kaolin, and calcium carbonate filled polypropylene composites. Journal of Applied Polymer Science, 91(5), 3315-3326. http://dx.doi.org/10.1002/app.13542.

11. Ozen, E., Kiziltas, A., Kiziltas, E. E., & Gardner, D. J. (2013).

Natural fiber blend-nylon 6 composites. Polymer Composites, 34(4), 544-553. http://dx.doi.org/10.1002/pc.22463.

12. Larrañaga, M. D., Lewis, R. J., & Lewis, R. A. (2016). Hawley’s condensed chemical dictionary. USA: John Wiley & Sons. http://dx.doi.org/10.1002/9781119312468.

13. Unal, H., Mimaroglu, A., & Alkan, M. (2004). Mechanical properties and morphology of nylon-6 hybrid composites.

Polymer International, 53(1), 56-60. http://dx.doi.org/10.1002/ pi.1246.

14. Bakar, M. A., Leong, Y., Ariffin, A., & Ishak, Z. M. (2007).

Mechanical, flow, and morphological properties of talc-and kaolin-filled polypropylene hybrid composites. Journal of Applied Polymer Science, 104(1), 434-441. http://dx.doi.org/10.1002/ app.25535.

15. Kumar, K. V. M., Krishnamurthy, K., Rajasekar, R., Kumar, P. S., Pal, K., & Nayak, G. C. (2019). Influence of graphene oxide on the static and dynamic mechanical behavior of compatibilized polypropylene nanocomposites. Materials Testing, 61(10), 986-990. http://dx.doi.org/10.3139/120.111411.

16. Kumar, M. K. H., Shankar, S., Rajasekar, R., Kumar, P. S., & Kumar, P. S. (2017). Partial replacement of carbon black by nanoclay 7/8

Subburamamurthy, H. B., Rathanasamy, R., Kumar, H. K. M., Chinnasamy, M., Kaliyannan, G. V., & Natarajan, S. in butyl rubber compounds for tubeless tires. Materials Testing, 59(11-12), 1054-1060. http://dx.doi.org/10.3139/120.111109.

17. Mohan Kumar, H. K., Subramaniam, S., Rathanasamy, R., Pal, S. K., & Palaniappan, S. K. (2020). Substantial reduction of carbon black and balancing the technical properties of styrene butadiene rubber compounds using nanoclay. Journal of Rubber Research, 23(2), 79-87. http://dx.doi.org/10.1007/s42464-020-00039-7.

18. Koo, J. H. (2006). Polymer nanocomposites: processing, characterization, and applications. USA: McGraw Hill Education.

19. Das, C., Rajasekar, R., Friedrich, S., & Gehde, M. (2011). Effect of nanoclay on vibration welding of LLDPE nanocomposites in presence and absence of compatibiliser. Science and Technology of Welding and Joining, 16(2), 199-203. http://dx.doi.org/10.

1179/1362171810Y.0000000017.

20. Araújo, E. M., Mélo, T. J. A., Santana, L. N. L., Neves, G. A., Ferreira, H. C., Lira, H. L., Carvalho, L. H., A’vila, M. M., Jr., Pontes, M. K. G., & Araújo, I. S. (2004). The influence of organobentonite clay on the processing and mechanical properties of nylon 6 and polystyrene composites. Materials Science and Engineering B, 112(2-3), 175-178. http://dx.doi.org/10.1016/j. mseb.2004.05.027.

21. Fornes, T., & Paul, D. (2003). Formation and properties of nylon 6 nanocomposites. Polímeros: Ciência e Tecnologia, 13(4), 212-217. http://dx.doi.org/10.1590/S0104-14282003000400004.

22. Huber, T., Misra, M., & Mohanty, A. K. (2014). Mechanical properties of compatibilized nylon 6/polypropylene blends; studies of the interfacial behavior through an emulsion model.

Journal of Applied Polymer Science, 131(18), 40792. http:// dx.doi.org/10.1002/app.40792.

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23. Sangroniz, L., Moncerrate, M. A., De Amicis, V. A., Palacios, J. K., Fernández, M., Santamaria, A., Sánchez, J. J., Laoutid, F., Dubois, P., & Müller, A. J. (2015). The outstanding ability of nanosilica to stabilize dispersions of nylon 6 droplets in a polypropylene matrix. Journal of Polymer Science. Part B, Polymer Physics, 53(22), 1567-1579. http://dx.doi.org/10.1002/ polb.23786.

24. Frulloni, E., Kenny, J. M., Conti, P., & Torre, L. (2007).

Experimental study and finite element analysis of the elastic instability of composite lattice structures for aeronautic applications. Composite Structures, 78(4), 519-528. http:// dx.doi.org/10.1016/j.compstruct.2005.11.013.

25. Arvinda Pandian, C., & Siddhi Jailani, H. (2019). Dynamic and vibrational characterization of natural fabrics incorporated hybrid composites using industrial waste silica fumes. International Journal of Polymer Analysis and Characterization, 24(8), 721-730. http://dx.doi.org/10.1080/1023666X.2019.1668141.

26. Rajesh, M., Pitchaimani, J., & Rajini, N. (2016). Free vibration characteristics of banana/sisal natural fibers reinforced hybrid polymer composite beam. Procedia Engineering, 144, 10551059. http://dx.doi.org/10.1016/j.proeng.2016.05.056.

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Received: Jun. 27, 2020 Revised: Feb. 06, 2021 Accepted: Feb. 09, 2021

Polímeros, 30(4), e2020046, 2020

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.09420

Commercial and potential applications of bacterial cellulose in Brazil: ten years review Luiz Diego Marestoni1 , Hernane da Silva Barud2 , Rodrigo José Gomes3 , Rebeca Priscila Flora Catarino3 , Natália Norika Yassunaka Hata3 , Jéssica Barrionuevo Ressutte3  and Wilma Aparecida Spinosa3*  Laboratório de Química, Departamento de Biotecnologia, Instituto Federal do Paraná – IFPR, Londrina, PR, Brasil 2 Laboratório de Biopolímeros e Biomateriais, Universidade de Araraquara – UNIARA, Araraquara, SP, Brasil 3 Laboratório de Prestação de Serviços, Departamento de Ciência e Tecnologia de Alimentos, Universidade Estadual de Londrina – UEL, Londrina, PR, Brasil

1

*wilma.spinosa@uel.br

Abstract In the last decade, bacterial cellulose (BC) has received considerable attention around the world, including in Brazil.

The unique properties of BC, such as mechanical stability, tensile strength, thermostability, crystallinity, purity and biocompatibility make it a promising candidate for commercial applications in different areas. This article provides a comprehensive synthesis of commercial applications and studies related to BC around the world and shows the importance and development of Brazilian research during the last decade. In this review we present an overview of BC structure, biosynthesis and possible applications of BC mainly in the food, electronics, bioengineering, cosmetics and biomedical areas. The most significant contributions of Brazilian researchers using BC have been carried out in the biomedical area. Despite the increase in BC reserch, Brazil also needs to develop strategies to expand the use and commercialization of BC products, for which government financial support is extremely necessary.

Keywords: bacterial cellulose, bacterial cellulose applications, biomedical, Brazil, electronics.

How to cite: Marestoni, L. D., Barud, H. S., Gomes, R. J., Catarino, R. P. F., Hata, N. N. Y., Ressutte, J. B., & Spinosa, W. A. (2020). Commercial and potential applications of bacterial cellulose in Brazil: ten years review. Polímeros: Ciência e Tecnologia, 30(4), e2020047. https://doi.org/10.1590/0104-1428.09420

1. Introduction During the last century, massive exploitation of fossil resources and pollution problems increased concerns related to the economy and the environment. In this context, polymers from renewable sources, like polysaccharides, among others, have received considerable and growing attention[1]. Cellulose (C6H10O5)n is the most abundant renewable biopolymer produced in the biosphere, being basically composed of glucose monomers connected by β (1-4) glycosidic bonds. It can be synthesized by plants, animals and microorganisms[2,3].

Cellulose derived from plants is commonly incorporated into other biopolymers as hemicellulose and lignin; therefore, aggressive chemical treatments are necessary to remove these impurities[4]. On the other hand, BC produced by microbial fermentation is characterized by higher purity, and its purification is relatively simple, not requiring energy or chemically intensive processes. Furthermore, due its unique physical and chemical properties, BC has been successfully applied in the food, biomedicine, textile, and papermaking fields, as well as in biosorbent material and acoustic diaphragms[5,6].

Polímeros, 30(4), e2020047, 2020

However, the interest in cellulose is not only limited to industrial fields; it has also become increasingly relevant and interesting in academic areas[7]. Figure 1 shows an explosive increase in the number of publications related to BC since 2011. The years 2017, 2018, 2019 and 2020 have been the most productive in terms of publications.

In Brazil, many groups have contributed and emerged in several areas of cellulose. In biomedicine, for example, Brazil was a pioneer, and gained prominence in employment of BC as an artificial skin to replace burned skin. Besides the commercial application in the biomedical area, potential applications of BC in Brazil include studies in the electronic/ electrochemical/magnetic field, food and food packaging, cosmetic area, bioengineering, as well as in reinforcement material to make blends, composites and nanocomposites.

The aim of this review is to open with a sketch of the major cellulose-producing microorganisms and to provide a comprehensive discussion of cellulose synthesis. We then move to commercial applications of BC and the potential applications of biopolymers in Brazil and around the world by considering their uses ranging from food to medical industries.

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R R R R R R R R R R R R R R

Marestoni, L. D., Barud, H. S., Gomes, R. J., Catarino, R. P. F., Hata, N. N. Y., Ressutte, J. B., & Spinosa, W. A.

Figure 1. Evolution of the number of bacterial cellulose-related publications around the world, including Brazil, between 2011 and 2020. The searches were performed with Google scholar using the topics: bacterial cellulose, biocellulose, microbial cellulose and Brazil.

1.1 Synthesis, properties and production of BC The BC consists of a transparent and gelatinous pellicle, produced in the vast majority by the Gram-negative bacterial cultures of Acetobacter, Agrobacterium, Achromobacter, Aerobacter, Sarcina, Azotobacter, Rhizobium, Pseudomonas, Salmonella and Alcaligenes[8]. Among them, the most efficient BC producer belongs to the group of acetic acid bacteria (AAB) previously known as Acetobacter xylinum[9], which was recently transferred to the newly proposed genus Komagataeibacter and named Komagataeibacter xylinus[10].

It is suggested that cellulose production is a bacterial defense mechanism that can provide protection from hazardous ultraviolet light radiation effect and can help bacteria move to the aerobic environment of surface. Furthermore, the pellicule retains moisture and confers mechanical, chemical, and physiological protection[11,12]. The biosynthesis of BC involves several biochemical processes containing a large number of enzymes, and the regulation of these key enzymes controls the cellulose production by means of three metabolic pathways: the pentose cycle, or branched hexose monophosphate pathway (HMP), for the oxidation of carbohydrates; the tricarboxylic acid cycle (TCA), for the oxidation of organic acids and other compounds; and the Embden-Meyerhof-Parnas pathway (EMP)[13,14].

Figure 2 briefly describes the biosynthetic pathway of K. xylinus cellulose production from glucose and fructose, although several other substrates can be used for this purpose[17].

Cellulose synthesis from glucose occurs initially through phosphorylation of hexose by GHK, thereby producing Glucose-6-Phosphate (G6P). G6P is metabolized by the Hexose monophosphate pathway, since Fructose-6-phosphate cannot be converted to Fructose-1,6-phosphate. Next, G6P is metabolized to UDP-glucose (UDPG), a direct precursor of the biopolymer, by PGM and UGP enzymatic action[15].

The fructose can be metabolized to cellulose following two paths: (I) phosphorylation of fructose to Fructose6-phosphate (F6P) by FHK, and (II) phosphorylation of fructose to Fructose-1-phosphate (F1P) by PTS. In the first, PGI changes F6P to G6P, which can be used for cellulose 2/19

production or can proceed to the Hexose monophosphate pathway. In (II) 1PFK transforms PF1 into Fructose-1,6diphosphate (FDP) and it is later dephosphorylated to F6P. Through the EMP, FDP can also be metabolized, starting the whole process of cellulose synthesis[18].

Microscopic analysis has shown that from one bacterial cell a single cellulose ribbon is produced (Figure 3). The single ribbon is constituted of a structure of cellulose microfibrils that are excreted from a row of a complex of proteins called the terminal complexes (TCs), located in the outer cell membrane[19,20]. During the conversion of glucose to UDPG in cytoplasm, a row of 60 TCs conducts the recognition and synthesis of UDPG, as well as the crystallization and extrusion of cellulose fibers[19].

Komagataeibacter species are able to produce two forms of cellulose: (I) cellulose I, the ribbon-like polymer; and (II) cellulose II, which is the most thermodynamically stable amorphous form of the polymer. While cellulose I is constituted of parallel β (1-4) glucan chains organized uniaxially with van der Waals forces, β (1-4) glucan chains of cellulose II are arranged in random way. The latter is mainly antiparallel, with a large number of hydrogen bonds that provide more stability to the structure[21]. According to Koizumi et al.[19], during the BC synthesis, the amorphous regions, interspersed among crystalline regions, occupy 90% of material total volume.

Due to its microfibrillar structure, several mechanical properties are attributed to BC, such as: tensile strength, which may vary between 200-300 MPa with Young’s modulus, up to 15-35 GPa; and thermal stability, reaching temperatures above 100 °C without changes in their biophysical properties[17,22,23]. Furthermore, BC presents high purity, high degree of polymerization, water holding capacity up to a hundred times its weight, excellent biodegradability and biological affinity and non-allergenicity[11,24-27]. The structure, physical, and mechanical properties of BC are directly related to the cultivation technique employed, of which, the main methods of production are static, agitating culture, and the airlift reactor [16,28].

Despite its unique properties, the production of BC is still limited due to difficulties in the large-scale production, which are related to long-time cultivation methods, low production yields, bacterial strain mutation and high cost of the process. The static method is the simplest and widely used to produce BC, mainly in lab-scale[29]. This technique presents high cost, long cultivation time (~7-20 days), low productivity, and uneven distribution of nutrients, oxygen and cells, resulting in a material with non-uniform thickness[30].

On the other hand, the agitation method favors the diffusion of oxygen and the availability of nutrients, promoting greater production and shorter cultivation time (~5 days).

However, it is associated with mutation and formation of non-cellulose-forming cells[31,32].

Studies have also been conducted to find strains with high capacity for BC synthesis, besides employing genetic engineering techniques to improve the production[33-35].

Alternatively, bioreactors can be employed for industrial-scale production of BC, since they show higher yield compared to static and agitated methods. Some bioreactors developed use oxygen, rotating disc and can produce BC through biofilm Polímeros, 30(4), e2020047, 2020

Commercial and potential applications of bacterial cellulose in Brazil: ten years review

Figure 2. Pathway of cellulose synthesis from glucose and fructose in K. xylinus, also containing the direct path from Glucose-6Phosphate to Glucose-1-Phosphate. GHK glucose hexokinase, FHK fructose hexokinase, 1PFK fructose-1-phosphate kinase, FBP fructose bisphosphatase, PGI phosphoglucose isomerase, PGM phosphoglucomutase, UGP UDP-glucose pyrophosphorylase, G6PD glucose-6-phosphate dehydrogenase, PTS phosphotransferase system; CS cellulose synthase; EMP Embden-Myerhoff pathway; HMP Hexose Monophosphate pathway[13,15,16].

Figure 3. Schematic diagram of microbial cellulose organization. Adapted from Picheth et al.[8] and Koizumi et al.[19].

support. This method still requires studies that allow the use of controlled parameters and low-cost substrates[29,36].

To find new cost-effective fermentation media to replace the expensive Hestrin-Schramm medium (standard medium), researchers have checked the use of media containing either sugary materials or even agricultural and industrial waste products[37]. In such media containing high sugar content, are included the fruit juices[38] and related by-products such as fruit peels[12,39] and rotten fruit[40]. Molasses derived from the industries of sugarcane[41], beet[42] and soybean[43] have also been tested for production. Other several agricultural Polímeros, 30(4), e2020047, 2020

and industrial residues already tested include waste beer yeasts, fruit bagasse, cheese whey, glycerol and textile wastes[44]. These industrial by-products, rich in sugars and other nutrients could be suitable fermentative substrates for the production of BC, since they have low-cost and good sources of carbon[45]. By using these strategies, it is possible not only to make the BC production more economically feasible, but also make it more ecologically sustainable, which could help to reduce the release of waste by industries. Other strategies include screening of high-yield strains, inhibition of mutation, controlled side product formation, selection 3/19

Marestoni, L. D., Barud, H. S., Gomes, R. J., Catarino, R. P. F., Hata, N. N. Y., Ressutte, J. B., & Spinosa, W. A. of suitable cultivation methods, optimization of medium composition, and searching for low-cost raw materials[31].

1.2 Overview of commercial applications of BC Considering all the properties of BC, it is expected that this polymer will have several industrial applications. In fact, the number of patent applications registered around the world has been quite impressive, but unfortunately, few commercial applications have been pursued[15].

Among the most known industrial applications of BC is the “nata de coco”, a traditional food consumed in the Philippines and other countries in Southeast Asia. The manufacturing process of this jelly-like product consists of using coconut water as a fermentation medium for cellulose production. The pellicule is then cleaned, washed, chopped and immersed in sugar syrup, to be served as a dessert.

Originally from Southeast Asia, nata de coco became a very popular food worldwide[15,46,47]. Japan and the USA are the largest markets for nata de coco; between 2009 and 2011, the export volume from the Philippines reached an average of 6000 MT (Metric Tons)[48].

Potential application of BC has also been reported in the acoustic transducers area; BC presents remarkable shape retention ability, measured as the Young’s Modulus, and also bears two essential properties for speaker diaphragms: high sonic velocity and low dynamics loss. Thus, novel diaphragms have been marketed by Sony Corp. as headphones[16,46].

Regarding applicability of BC in biomedicine, Brazil participated in one of the main applications associated with wound dressing, in which Fontana et al.[49] became pioneers in employing BC as an artificial skin to replace burned skin. BioFill® and Bioprocess® (BioFill Produtos Biotecnológicos – Curitiba, PR, Brazil; used as temporary skin in the treatment of burns and ulcers) and Gengiflex (BioFill Produtos Biotecnológicos; applied in periodontal diseases) are examples of Brazilian commercial products of BC-based wound healing systems that have been launched.

Although these launches are no longer on the market, other products as NEXFILL, DERMAFILL and CUTICELL EPIGRAFT (Seven Indústria de Produtos Biotecnológicos Ltda – Londrina, PR, Brazil), Biocel (DMC Importação e Exportação de Equipamentos LTDA - São Carlos, SP, Brazil; Florida USA), Bionext® (Bionext® Produtos Biotecnológicos Ltda – São Paulo, Brazil), and Membracel® (Vuelo Pharma – Curitiba, PR, Brazil), emerged, thereby demonstrating the relevance of BC application in this area. Among other products available around the world are CelMat® Wound (BOWIL Biotech sp. - Poland), Bio-skinG (Coreleader Biotech Co. Ltd. - New Taipei City Taiwan) and XCell (Xylos Corporation – US; used to maintain an ideal moisture balance). The prerequisites for BC to be applied as a wound dressing include: high mechanical strength in a wet state, water vapor permeability, good adherence to the wound, low cost, good biocompatibility, durability, transparence and easy handling[15,16,50].

The increase in cardiovascular diseases has also led the researchers to reflect about the need for replacement blood vessels[51]. Klemm et al.[52] have developed a clinical product from BC and patented it as BASYC®-tubes (BActerial SYnthesized Cellulose). Besides their suitability for different 4/19

inner diameter vascular conduits, studies show BASYC tubes have high mechanical strength in a wet state, great water retention properties and can be successfully used to replace carotid arteries in rats, pigs and sheep[50,53].

1.3 Reports applications of BC around the world and in Brazil The studies related to the potential applications of BC in Brazil, as on the world stage, involve the biomedical area, electronic/electrochemical/magnetic field, food and food packaging, bioengineering and the cosmetics area (Figure 4 and 5). The studies also involve the use of reinforcement material to make blends, composites and nanocomposites, with possible application in many fields.

For such applications, the properties of BC have usually been tailored by using various in situ techniques (such as addition of various substances and alteration of culture conditions) and ex situ strategies (physical and chemical modification)[54,55]. These applications are discussed in detail in the following sections.

1.3.1 Food and food packaging 1.3.1.1 World Considered a dietary fiber and classified as “generally recognized as safe” (GRAS) by the USA Food and Drug Administration in 1992, BC can offer several health benefits, reducing the risk of chronic diseases such as cardiovascular disease, diabetes, obesity and diverticulitis[17,47]. Most of the approaches use BC as a raw material for obtaining new products and explore its use as food additive.

In this area, BC has been used as a fat substitute; as potential gelling, thickening, suspending and emulsion stabilizer, as solid support to immobilize cells and as food packaging. As a fat substitute, reports are found in the literature of BC application in meatballs[56], surimi products[57], cheese[58], ice cream[59] and mayonnaise[60]. BC also has been applied as a potential gelling, thickening, suspending and emulsion stabilizer to produce meat products[61], whey protein isolate[62], olive oil[63] and edible foam[64].

Recently, BC has gained prominence in studies related to the immobilization processes of cells, enzymes and probiotics for application in food[65]. Some authors immobilize yeast in BC for wine production. Immobilized yeast reduced expenses for inoculum preparation, since the yeast was recovered and separated at the end of the fermentation process[66].

Similarly, Fijałkowski et al.[65] obtained promising results in immobilizing probiotic strains of Lactobacillus spp in BC. In the literature, the most explored food area worldwide is the development of films and packaging. BC has been incorporated into a wide variety of substances in order to increase the shelf life of food products[67]. Among the substances to which BC has been incorporated for the production of packaging can be mentioned: cotton fibers[68], postbiotics of lactic acid bacterium[69] and potato peel[70]. Other BC applications in the food field can be found in Table 1.

1.3.1.2 Brazil In Brazil, the research groups have focused more on the development of edible films and food packaging. Viana et al.

[108] produced films with different ratios of nanofibrillated Polímeros, 30(4), e2020047, 2020

Commercial and potential applications of bacterial cellulose in Brazil: ten years review

Figure 4. Main applications of bacterial cellulose in the world.

Figure 5. Main applications of bacterial cellulose in Brazil.

BC (NFBC) to pectin, with or without the addition of fruit purees. In their study, films (with or without purees) with higher NFBC contents showed improvement in physical properties and were proposed for use in food wrapping or coating.

Studies conducted by Malheiros et al.[109] show that the immobilization of antimicrobial peptides from Lactobacillus sakei subsp. sakei 2a in BC is a promising strategy for the control of Listeria monocytogenes in foods. Similarly, a new Polímeros, 30(4), e2020047, 2020

material composed of BC/poly(3-hydroxybutyrate) with the addition of clove essential oil demonstrated a reduction of 65% in microbial growth and attractive properties for active food packaging[110].

In order to develop active composite films from cashew by-products, Sá et al.[111] produced BC from cashew juice and then they added nanocrystals of lignin and cellulose (both from cashew pruning fiber) to produce the film. Although lignin gave brown color and opacity, the films showed good 5/19

Marestoni, L. D., Barud, H. S., Gomes, R. J., Catarino, R. P. F., Hata, N. N. Y., Ressutte, J. B., & Spinosa, W. A. Table 1. Exemples of BC applications in the world.

Type

Description BC can be used as a fat substitute; as solid support to immobilization of enzymes and cells; as food Food packaging and as potential gelling, thickening, suspending and emulsion stabilizer.

Due to its porous nanofibrous network structure, Electronics, electrochemical and magnetic fields BC can be used as a flexible matrix for developing biomaterials.

After being combined with substances with Antimicrobial activity antimicrobial effects, BC can be used to protect against infections and contaminations.

BC can be used for drug delivery due to its Drug delivery water holding capacity and controlled release of substances.

BC can be used to reduce pain, maintain moisture Wound dressing and serve as a bacteriological barrier in patients with burns and chronic wounds.

Since BC’s morphological structure has a high degree of purity, excellent biocompatibility and Tissue engineering high tensile strength, BC can be used to support newly formed tissues, interact with cells and release substances necessary for cell growth.

BC can be used as biosensors to detect and quantify certain analytes in several areas. The Bioengineering immobilization of enzymes and cells into BC has been promising for effluent treatment, biomedical area and food manufacturing.

Due to its controlled release of substances, water Cosmetics holding capacity and ability to stabilize emulsions, BC can be used to produce cosmetic products.

BC combined with materials specific materials Filtration/ adsorption can be used to remove contaminants, heavy metal and impurities from water.

mechanical properties and interesting antioxidant capacity.

Other works involve the preparation of biodegradable films made from blends of potato starch/BC/ glycerol[112] and BC/ polycaprolactone acetone solution[113].

1.3.2 Electronic, electrochemical and magnetic field 1.3.2.1 World In electronic field, BC has attracted great attention mainly due to depletion of non-renewable resources[114]. Moreover, due to its porous nanofibrous network structure, BC is used as a flexible matrix for developing biomaterials with desired properties. BC-based materials have been developed using conducting polymers, graphene, graphene oxide, carbon nanotube and carbon nanofiber (Table 1). Their application occurs especially in flexible supercapacitors, ion battery, fuel cell, and other electrochemical devices[115].

Actually, several works have combined BC nanofibers with conducting polymers as polyaniline and polypyrrole.

When Graphene/Carbon Nanotube/Bacterial Cellulose (RGO/CNT/BC) architecture was designed as substrate for loading polypyrrole (PPy), Bai et al.[116] showed that flexible supercapacitor exhibits stable electrochemical performance under bending and flat conditions. Similarly, Rebelo et al.[117] synthesized an electrically conductive BC/Polyvinylaniline/ Polyaniline (BC/PVAN/PANI) nanobiosensor for potential use in nerve regenerative medicine, which demands both electroactivity and biocompatibility. BC has also become a valuable for production of lithium-ion batteries (LIB).

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References Akoğlu et al.[71], Bandyopadhyay et al.[72], Jayani et al.[73], Razavi et al.[74]

Fei et al.[75], Guan et al.[76], Kim et al.[77], Xie et al.[78] Żywicka et al.[79], Adepu and Khandelwal[80], Horue et al.[81], Sajjad et al.[82] Badshah et al.[83], Beekmann et al.[84], Inoue et al.

[85] , Luo et al.[86], Weyell et al.[87] Gupta et al.[88], Faisul Aris et al.[89], Ye et al.[90], Moradi et al.[91]

Halib et al.[92], Frone et al.[93], Zhang et al.[94,95]

Hu et al.[96], Moradi et al.[97], Cai et al.[98]

Wang et al.[99], Muhsinin et al.[100], Amnuaikit et al.

[101] , Aramwit and Bang[102] Urbina et al.[103], Zhuang and Wang[104], Núñez et al.

[105] , Shoukat et al.[106], Luo et al.[107]

Yuan et al.[118] designed free-standing films of tin sulfide (SnS) nanosheets distributed uniformly on carbonized BC (CBC) nanofibers. LIB using SnS/CBC as anode could be a promising electrode material, since exhibited high capacity, excellent cycling stability and high-rate capability. In order to develop proton exchange membranes (PEMs) for application in polymer electrolyte fuel cells (PEFC), Vilela et al.[119] combined BC (support), fucoidan (polyelectrolyte) and tannic acid (cross-linker). The membranes presented thermaloxidative stability (180-200 °C), good dynamic mechanical performance and the protonic conductivity increased with the increase in relative humidity. The authors concluded that BC/fucoidan membranes have potential as eco-friendly alternatives to other PEMs for application in PEFCs. 1.3.2.2 Brazil In Brazil, Müller et al.[120] carried out an in situ oxidative polymerization of EDOT on nanocellulose fibers to obtain PEDOT-nanocellulose flexible composites. The nanocomposites showed an increase in electrical conductivity and in elongation at break, with a lower thermal stability when compared to pure BC. Therefore, the authors concluded that the flexible membranes have potential use for flexible organic electronics.

In a recent study, Legnani et al.[121] produced multifunctional membranes based on BC and an organic-inorganic sol, composed of boehmite (Boe) nanoparticles and epoxy modified siloxane (GTPS), to be used as substrates in organic Polímeros, 30(4), e2020047, 2020

Commercial and potential applications of bacterial cellulose in Brazil: ten years review light-emitting diodes (OLEDs). Then, they covered the BC/Boe-GTPS with silicon dioxide (SiO2) and indium tin oxide (ITO), thereby obtaining eco-friendly, biocompatible substrates comparable to fabricated commercial glass substrate.

Conductive composite membranes of BC and polyaniline, using different oxidizing agents like FeCl3.6H2O[122] and ammonium peroxydisulfate[123], may allow for important technological applications, such as sensors, electronic devices, intelligent clothes, flexible electrodes and tissue engineering scaffolds. Other interesting approachs exploit the intrinsic properties of BC, combined with the physical and chemical characteristics of compounds such as nanoparticulated Boe and GTPS[124], polypyrrole[125] and laponite[126], to produce new materials that may be suitable for biomedical and electronic applications. Other studies are shown in Table 2.

1.3.3 Biomedical area 1.3.3.1 World Most BC studies are currently focused in the biomedical field, where the multifunctionality of this polymer has been shown mainly through the development of biomaterials as wound dressing, scaffold for tissue engineering and drug delivery[135] (Table 1).

BC membranes have several advantages that allow it to be considered as an ideal wound dressing material. It forms a bacteriological barrier that allows for gas exchange and reduces pain, it maintains moisture of the environment, its transparency favors direct visualization of the wound, and it also reduces the treatment time and the costs of hospitalization of patients with burns and chronic wounds[8,136]. The direct use of BC itself has no antimicrobial activity to prevent wound infection[137]; therefore, recently, several studies have incorporated active compounds into its structure to improve the properties and functionalities.

Cao et al.[138] evaluated the potential use of a structured surface BC biomaterial incorporated with human urinederived stem cells for use as wound dressing. The in vivo results were promising for this combination, since the healing rate was significantly higher compared to the control. BC was also incorporated with sodium alginate, chitosan and copper sulfate, in order to provide an antimicrobial for use as wound dressing. The material obtained showed antimicrobial activity against methicillin-resistant Staphylococcus aureus and Escherichia coli[139]. In another study, BC was combined with poly(methylmethacrylate) (PMMA) in order to obtain biocompatible and biodegradable bandages to support wound healing. Preliminary analyses of swelling characteristics and mechanical properties indicated that this biomaterial is a promising candidate for biomedical applications[140].

Tissue cells, scaffold and growth factors are extremely important factors in the success of tissue engineering.

Scaffolding is used to support the newly formed tissues, interact with cells and release substances necessary for cell growth. Since BC’s morphological structure has a high degree of purity, excellent biocompatibility and high tensile strength, BC has been extensively studied as a candidate for tissue engineering[141]. Ahn et al.[142] developed a structure of BC coated with hydroxyapatite (HA) for bone tissue regeneration. The bone regeneration capacity was assessed using a rat calvary defect model. The results showed that both scaffold (BC coated with BC and HA) were effective in bone formation derived from existing bone, in addition, a new bone was found inside the scaffold.

Torgbo and Sukyai[143] synthesized a nanocomposite by combining BC, hydroxyapatite (HA) and magnetite nanoparticles (Fe3O4) through ultrasonic irradiation. The scaffold was not toxic to mouse fibroblast cells and was also biocompatible to osteoblasts attachment and proliferation.

Table 2. Summary of studies of BC applications in electronics/electrochemical/magnetic field.

Material

Main results of biomaterials based on BC The BC/PU composites offer new opportunities as substrates for Castor oil based polyurethane (PU) flexible electronic displays, since they exhibit great transparency and excellent mechanical properties.

Atomic force microscopy height and phase images of BC/Fe2O3 PEG (polyethylene glycol)-Fe2O3 magnetic nanocomposites confirmed that the addition of Fe2O3 nanoparticles nanoparticles did not change the ability of the BC to form nanofiber networks.

The impregnation of Boehmite through BC pores improves transparency Boehmite of pure BC, suggesting applications in optics and electronics fields.

BC–TEA membranes showed potential application as ion-conducting membranes, since the addition of TEA avoids complete dryness, Triethanola-mine (TEA) ensuring humidity necessary for maintaining high ion-conductivity of the membranes.

4’-(hexyloxy)-4-biphenyl carbonitrile, 4’-(hexyl)-4-biphenyl-carbonitrile Switchable photoluminescence liquid crystal and multicomponent nematic mixture were used to impregnate BC film. This resulted in a transparent film with 3D rigid structure.

BC membranes were polymerized with polyaniline in the presence Polyaniline and Ammonium persulfate (APS) of APS or FeCl3. The membranes with FeCl3 displayed higher or iron(III) chloride (FeCl3) conductivity and better mechanical performance than membranes pure or prepared with APS. BC/PU nanocomposites present higher thermal stability, a non-crystalline Polyurethane (PU) character and low water absorption when compared to control.

CdTe-GSH/BC membranes were tested as photoelectrodes and Cadmium tellurite quantum dots (CdTe) capped exhibited maximum photocurrent around 530nm, indicating potential with glutathione (CdTe-GSH) application as flexible electrodes, sensors and photovoltaic systems

Polímeros, 30(4), e2020047, 2020

References Pinto et al.[127]

Barud et al.[128] Salvi et al.[129]

Salvi et al.[130]

Tercjak et al.[131]

Marins et al.[132]

Pinto et al.[133] Pinheiro et al.[134]

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Marestoni, L. D., Barud, H. S., Gomes, R. J., Catarino, R. P. F., Hata, N. N. Y., Ressutte, J. B., & Spinosa, W. A. Aiming to promote the formation of blood vessels, Wang et al.[144] modified BC/gelatin (BC/G) scaffold with heparin and studied its skill of promoting angiogenesis in terms of vascular endothelial growth factor (VEGF) release.

The scaffold provided prolonged VEGF release. Proliferation and migration (in vitro cellular assay) were observed in the presence of VEGF. Furthermore, heparinized scaffolds loaded with VEGF (V-BC/G/H) improved the angiogenesis compared to BC/G scaffold.

During the past few years, the number of studies related to the incorporation of drugs into BC membranes has increased. The most common drugs to be loaded in BC are non-steroidal anti-inflammatory drugs (NSAIDs) and antibiotics[30]. Shao et al.[145] loaded BC membrane with tetracycline hydrochloride (TCH). The biomaterial showed biocompatibility, high antibacterial activity and it was not toxic in HEK293 cells. Similarly, BC, PVA and chitosan mono and multilayer films were effective for controlled release of ibuprofen sodium salt[146].

1.3.3.2 Brazil In Brazil, although considerable research has been devoted to wound dressings and tissue engineering, the incorporation and modification of BC with countless substances has further expanded the spectrum of applications in the biomedical area (Table 3). For example, BC and carboxymethylcelullose (CMC) biocomposites loaded with methotrexate were developed in order to evaluate their effectiveness as a drug delivery system, as an alternative for the topical treatment of psoriasis. The results showed that different amounts of DS-CMC can generate distinct biomaterials, to be applied through the cutaneous route at different stages of evolution of a pathology[160].

In the case of application for cancer therapeutics, photodynamic therapy (PDT) has emerged as an innovative therapeutic modality, focused mainly on skin cancer treatment. Peres et al.[161] utilized chloroaluminum phthalocyanine (ClAlPc) as a photosensitizer and impregnated BC membranes with it, with the goal of developing such applications as a drug delivery system for PDT skin cancer protocols. Photophysical studies of BC-ClAlPc showed that the properties of photosensitizer were not affected, and the result of the cell viability test using Chinese hamster ovary cells (CHO-K1) demonstrated their potential for safe biological use. Another study of great relevance is related to Leishmaniasis, a group of parasitic diseases caused by protozoa of the genus Leishmania. Brazil, together with nine other countries, is responsible for 70-75% of the global Cutaneous Leishmaniasis (CL) occurrence. In this context, Celes et al.[162] described a topical formulation by incorporating Diethyldithiocarbamate (DETC), a superoxide dismutase 1 inhibitor, into BC membranes for CL treatment. BC-DETC did not cause noticeable toxic effects and resulted in parasite killing. The topical formulation significantly reduced the lesion size, inflammatory response at the infection site and parasite load, highlighting its availability for treatment of CL. Dengue is an endemic disease widespread throughout the tropics, in which the regions most affected are the Americas, South-East Asia and Western Pacific[163,164].

According to the World Health Organization[164], in Brazil, the average of number of reported dengue infections was over 100 thousand between 2010-2016. Since misdiagnosis is 8/19

still a problem[165], Picheth et al.[163] proposed that diagnostic assays were more sensible, faster and less expensive for this disease. For this purpose, piezoelectric sensors were coated with thin BC nanocrystals (CN) in order to facilitate anchoring of monoclonal immunoglobulin G (IgGNS1) against NS1 dengue antigen. The authors observed that biosensors, compared to cellulosic surfaces, increased the total IgGNS1 immobilized mass by twofold and reduced the need for sample dilution by tenfold. Lastly, they concluded the sensors can be used qualitatively in clinical diagnosis after suitable validation.

In the ophthalmological area, Coelho et al.[166] developed and evaluated the cytoxicity, genotoxicity and mutagenicity of contact lenses based on BC, coated with either glycidoxypropyltrimethoxysilane (H) or chitosan (Q), incorporating ciclodextrin (CD) to release diclofenac sodium (DS) or ciprofloxacin (CP). Functionalized BC lenses safely allowed for the bioavailability of ophthalmic drugs, in which only BC-H-CD-DS presented cytotoxic and genotoxic effects and BC-Q-CD-DS showed cytotoxic effects. Therefore, the authors suggested other specific tests with corneal lineage to ensure safe ophthalmologic use.

Additionally, unmodified BC also proved to be a useful material in medical displacement procedures of the vocal folds.

Souza et al.[167] demonstrated that laryngeal medialization with BC in the larynx of rabbits did not cause rejection or absorption and it was stable over a long period.

In order to obtain antimicrobial properties, Brazilian researchers have incorporated different antimicrobial to BC. For example, silver nanoparticles composites[137,168] (cerium nitrate and silver nanoparticles[136], copper nitrate (Cu(NO3)2)[169] and ceftriaxone[170]) have been incorporated in BC membranes and exhibited strong antimicrobial activity against Gram-negative (Pseudomonas aeruginosa, Salmonella and Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria, which are commonly found in skin infections.

The antimicrobial activity and the wound healing properties of novel BC containing Brazilian propolis was also demonstrated by Wei et al.[171], Marquele-Oliveira et al.

[172] and Picolotto et al.[173]. The first two authors proved the antimicrobial efficacy of in vitro BC/propolis membranes against Gram-negative and Gram-positive bacteria; their in vitro studies suggested that the biomaterial may promote fast re-epithelization and tissue organization, setting up a potential therapy for infected wounds. The accelerated wound healing process in a diabetic mouse model was also evidenced by Picolotto et al.[173].

Other approaches favoring wound healing have also been developed. Picheth et al.[51] assembled a novel wound dressing sensitive to lysozyme by depositing nanopolymeric chitosan and alginate films onto oxidized BC membranes incorporated with epidermal growth factor (EGF). The proposed system proved to be effective as wound dressing and presented a local delivery mechanism to recognize infections and to respond with a burst of EGF release. Wound dressing based on BC/collagen hydrogel in rat dorsum stimulated better wound healing than commercial collagenase and control group (untreated wound)[174].

As previously commented, BC has also become a promising biopolymer for tissue engineering and regenerative Polímeros, 30(4), e2020047, 2020

Commercial and potential applications of bacterial cellulose in Brazil: ten years review Table 3. Summary of studies of potential applications of BC in the biomedical area.

Purpose

Therapeutic materials Main results of biomaterials based on BC Films developed from BC, hydrocolloids and collagen showed Hydrocolloids (guar gum and hyaluronic potential for applications as bioactive wound dressings, scaffolds acid) and collagen for cellular growth and sustained drug release systems.

BC from agro-waste juices were similar to the standard culture Bionanocomposites based on alternative medium. In vitro studies showed that materials were biocompatible sources (cashew juice and sisal liquid and proved to be suitable for the preparation of nanocomposites waste) and hydroxyapatite (HA) for applications in the biomedical field.

Presence of SF in BC nanostructure induced a significant increase in cell adhesion and could be an excellent option in Silk fibroin (SF) bioengineering, since the material was non-genotoxic and safe for medical applications.

Succinylation over the surface of the BC showed the potential Succinic anhydride for conjugations with molecules of medical interest, such as hydrolyzed collagen.

The incorporation of Aloe vera extract fractions during the BC General biomedical synthesis resulted in biocomposites with mechanical properties applications Aloe vera portions and chemical composition distinct from pure BC. The material could be used as a scaffold for skin substitution and regeneration and cell culture substrates.

The BC/PEG films showed more hydrophilic properties, Polyethylene glycol (PEG) supporting possible use as biomedical devices.

Incorporation of AG and/or XG into BC mechanically defibrillated membranes caused a change in the hydrophilic Arabinogalactan (AG) / Xyloglucan (XG) surface characteristics. AG inclusion also showed viability and cellular adhesion profiles similar to those of commercial BC. CB-RS/P nanocomposite films showed an increase in thermal stability. The films also enhanced the drug dissolution rates, Resistant starch/pectin (RS/P) and therefore may be a potent material used as a carrier of poor solubility drugs, which may enhance oral bioavailability.

Mimics the structural architecture and biological functions of BC membranes with IKVAV peptide the extracellular matrix for vasculogenic mimicry of human melanoma cells.

Modification of BC structure by acetylation and/or oxidation has Dimethylacetamide/lithium chloride developed films with more degradability in vivo and potential applications as scaffold material for tissue engineering.

O/C/CB new scaffolds should be applied as: inducers of vascularization; facilitators of deposition of otoliths in predefined Otoliths/collagen (O/C) regions; guides for the regeneration of tissue, allowing the development of different tissues; or inhibitors of calcification Tissue engineering and cell adhesion.

Glycidyltrimethylammonium chloride BC/GTMAC films showed a significant increase in cell attachment (GTMAC) and spreading compared to unmodified membranes.

The membranes promoted cell proliferation and alkaline BC and collagen associated with phosphatase activity in osteoblastic cell cultures, in addition to osteogenic growth peptide. better bone regeneration in vivo than the control group.

medicine applications; many studies have been conducted to synthesize new biomaterials based on BC (Table 3).

Saska et al.[175] developed and evaluated the biological properties of bacterial cellulose-hydroxyapatite (BC-HA) nanocomposite membranes in noncritical bone defects in rat tibiae at 1, 4, and 16 weeks. BC-HA composites have presented properties similar to that of physiological bone and have accelerated new bone formation of rat tibiae, without showing inflammatory reaction. Furthermore, the authors concluded that the membranes exhibited slow reabsorption, suggesting that the material takes longer to be completely reabsorbed. Afterwards, Saska et al.[176] demonstrated that the peptide (osteogenic growth peptide (OGP) and its C-terminal pentapeptide OGP (10-14)) incorporation did not change the BC properties. Furthermore, in vitro assays revealed BC membranes influenced osteogenic cell proliferation and do not present cytotoxic, genotoxic or mutagenic action.

Polímeros, 30(4), e2020047, 2020

References Woehl et al.[147]

Duarte et al.[148]

Barud et al.[149]

RibeiroViana et al.[150]

Godinho et al.[151]

Silva et al.[152]

Lucyszyn et al.[153]

Meneguin et al.[154]

Reis et al.[155]

Lima et al.[156]

Olyveira et al.[157]

Courtenay et al.[158] Saska et al.[159]

Similarly, Coelho et al.[177] associated BC, HA and anti-bone morphogenetic protein antibody (anti-BMP-2) (BC-HA-anti-BMP-2), and did not observe toxicity of the membranes in MC3T3-E1 cells. BC-HA-anti-BMP-2 increased the expression of genes related to bone repair, the mineralization nodules and the levels of alkaline phosphatase activity when compared to the control group. Biocompatibility tests of BC, BC-HA and PTFE (polytetrafluoroethylene) using rats (Wistar) complemented prior studies[175,176] which displayed that BC and BC/HA materials have the same inflammatory pattern when compared to PTFE, thus proving to be biocompatible materials[178].

Composites based on collagen have demonstrated improvement of the biological and mechanical properties in bone tissue engineering. This protein is plentiful in the natural extracellular matrix (ECM) and in the human body, in addition to stimulating the regeneration process[179]. Based 9/19

Marestoni, L. D., Barud, H. S., Gomes, R. J., Catarino, R. P. F., Hata, N. N. Y., Ressutte, J. B., & Spinosa, W. A. on this, Saska et al.[180] developed a composite based on BC and type I collagen (COL) and evaluated the in vitro bone regeneration. BC-COL presented a more flexible structure than BC membranes, and showed osteoblastic differentiation that was observed by way of higher levels of alkaline phosphatase activity. Aiming to functionalize BC-COL with other proteins and/or peptides and promote bone formation, the group later synthesized and evaluated in vitro the biomaterial based on BC, COL, apatite (Ap) and OGP or OGP(10-14).

The nanocomposites (OGP/OGP(10-14)-BC-COL-Ap) produced did not display cytotoxic, genotoxic or mutagenic action, and in vitro tests showed a synergism between the elements that provided cell growth regarding the BC-Ap nanocomposite. The authors consider that (BC-COL)-Ap associated with OGP peptides might be potential candidates for bone tissue engineering applications .

Recently, Birkheur et al.[181] prepared aminoaryl mannoside and conjugated it to a succinic group of BC without disrupting the microfibril network. The use of glycoconjugates to BC showed good fibroblast compatibility. Conversely, Souza et al.

[182] developed films from mechanical defibrillation of BC followed by the dry-cast generation and incorporation of the xyloglucan (XGT), extracted from tamarind seeds, at various percentages. According to the authors, both mannosylated cellulose[181] and BC combined with hydrocolloids[182] demonstrated promise as biomaterials for this area.

1.3.4 Bioengineering area 1.3.4.1 World In bioengineering area, BC has been used mainlny for bioanalysis, enzyme and cell immobilization and to produce biosensors. BC based biosensors have been explored for different applications. These biomaterials have been used in the food and biomedical areas and also to detect contaminants in the environment. Several studies have provided advantageous results to detect contaminants in aqueous matrices, such as bisphenol A in effluent[183] and heavy metal traces as Cd (II) and Pb (II) in drinking water[184]. Pollutant biosensors for detection of H2O2 in the environment[185] and formaldehyde vapors in houses and workplaces[186] were also developed showing low detection limits. In foods, BC has been used as an optical sensor for detection and determination of ethylene concentration in fruits[187] and potential biosensors for detecting and measuring the growth of pathogenic bacteria[188]. Other studies are related to the development of colorimetric pH indicators through the incorporation of anthocyanin from several sources into BC. These indicators demonstrate to be ideal candidates to monitor the freshness/ spoilage of foods and beneficial for further development of smart indicator films for practical use[189].

Many researchers have also applied BC as a carrier of enzymes and cells since the immobilization method provides greater stability, reusability, more tolerance to changes in environmental conditions and less vulnerability to toxic substances present in the surrounding[190]. In biomedical area, BC based biosensor for dopamine detection in human urine was used successfully[191]. When enzymes such as lysozyme were immobilized onto BC, Bayazidi et al.[192] verified the system obtained good antimicrobial activity against several microorganisms, with potential applications in water treatment or food industry.

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Aiming to develop an innovative treatment to remove the paracetamol from wastewater, Żur et al.[190] immobilized Pseudomonas moorei KB4 onto BC, since this strain is one of the few bacteria able to degrade the analgesic drug.

Using the Real-Time PCR technique, they verified that paracetamol exposure influenced the expression of the selected genes encoding the degradation enzymes and KB4 strain was able to degrade 150 mg L-1 of paracetamol in the three cycles. Żywicka et al.[193] immobilized rod-shaped bacteria Lactobacillus delbruecki, spherical-shaped yeast Saccharomyces cerevisiae and hyphae forms of Yarrowia lipolytica) onto BC. As a result, the authors concluded that carrier must be individually combined to the cell type, considering mainly the carrier’s porosity parameter.

1.3.4.2 Brazil In Brazil, studies in this area still need to be explored.

However, recently, Vasconcelos et al.[194] developed a new process of purification (via alkaline treatment with K2CO3) and chemical modification (NaIO4 oxidation) for covalent immobilization of papain into BC. Oxidized BC (OxBC) demonstrated no cytotoxicity and a greater amount of immobilized enzyme than BC alone, with recovered enzyme activity of 93.1%. The authors immobilized papain in BC by surface response methodology, exhibiting 53% of the generalized papain activity. They concluded that the biomaterial facilitate debridement of skin wounds[195].

In another study, Gomes et al.[196] developed a flexible biosensor for detection of lactate in artificial sweat by immobilizing lactate oxidase (Lox) into BC. The biosensor displayed excellent amperometric response to lactate in artificial sweat, high sensitivity, superior mechanical resistance and biocompatibility; offering new opportunities for the development of wearable devices.

1.3.5 Cosmetics 1.3.5.1 World In addition to the bioengineering area, scientists have studied the cosmetic application of BC in face masks for delivery of active compounds and increased skin hydration, as well as an emulsion stabilizer[197]. In vivo studies conducted

by Perugini et al.[198] demonstrated that BC masks loaded with different bioactive ingredients (peptides, natural extracts and biopolymers) can be used as an effective delivery method for revitalization of facial tissue.

Likewise, Stasiak-Różańska and Płoska[199] used BC as a biocarrier for 1,3-dihydroxy 2-propanone (DHA) also for cosmetic purposes. The biomaterial showed as an alternative in masking the effects of vitiligo, without leaving unpleasant odor, typical of commercial cosmetics containing DHA. The application of BC is attracting attention of the cosmetics industry and scientific community[197] (Table 1).

1.3.5.2 Brazil In Brazil, as well as in the world, this area has been few explored[200]. BC was successfully loaded with different cosmetic actives for skin treatment and then evaluated through sensorial tests carried out by humans. The sensory tests revealed that masks based on BC were effective for skin adhesion and handling, and the actives improved the skin moisture of the volunteers[201]. Amorim et al.[114] aiming to Polímeros, 30(4), e2020047, 2020

Commercial and potential applications of bacterial cellulose in Brazil: ten years review develop a biomask that helps in the healing of inflammations caused by acne, loaded BC with natural propolis extract.

The dermatological and cosmetic products improved the hydration and texture skin, accelerated the healing process and improved the self-esteem of acne patients.

2. Conclusions BC stands out as a versatile biomaterial that allows promising applications in the areas of food, electronics, bioengineering, cosmetics and biomedics. In Brazil, the areas of food, bioengeering and cosmectics are still scarce for economic reasons. In the food field, Brazilian studies are essentially focused on the development of food packaging.

Although a large amount of research have been developed worldwide in the electronic/ electrochemical/ magnetic fields, in Brazil, research in this area has only started to be more explored recently. Certainly, the most significant contributions of Brazilian researchers using BC have been made in the biomedical area. We highlight here the use of BC to treat psoriasis and cutaneous leishmaniasis, and the development of sensors in the clinical diagnosis of dengue. In addition, the incorporation of antimicrobials, polysaccharides, proteins / peptides and other compounds into BC has shown promising results in wound healing properties. In tissue engineering and regenerative medicine, the immobilization of biomolecules and their potential in vitro and in vivo is still being explored for greater activity and stability. In conclusion, Brazil is one of the countries that most develops research using BC. However, the expansion of the use and commercialization of BC products could be increased through improvements in its productivity, using, for example, residues generated in Brazilian agribusiness, which could contribute to an environmentally friendly society. Studies with new methods and technologies for the production of cellulose need to be explored, in addition to new biochemical and genetic investigations. Greater government financial support for Brazilian research is also sorely needed.

3. Acknowledgements The authors would like to thank the Universidade Estadual de Londrina, the Instituto Federal do Paraná, the Universidade de Araraquara, the Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES. H. S. Barud thanks CNPq (Grant No. 407822/2018-6; INCT-INFO), São Paulo Research Foundation (FAPESP) (Grants no. 2018/25512-8 and no.

2013/07793-6), and TA Instruments Brazil.

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Received: Oct. 15, 2020 Revised: Jan. 04, 2021 Accepted: Jan. 05, 2021

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.08020

R Antibacterial activity of polypyrrole-based nanocomposites: R a mini-review R Fernando Antonio Gomes da Silva Júnior , Simone Araújo Vieira , Sônia de Avila Botton , R Mateus Matiuzzi da Costa  and Helinando Pequeno de Oliveira  R Institute of Materials Science, Universidade Federal do Vale do São Francisco – UNIVASF, Juazeiro, BA, Brasil Postgraduate Program in Veterinary Medicine - PPGMV, Department of Preventive Veterinary Medicine DMVP, Universidade Federal de Santa Maria – UFSM, Santa Maria, RS, Brasil R R Abstract R The development of polypyrrole-based nanocomposites as alternative antibacterial agents represents a promising strategy to be applied against the prevailing multi-resistant bacteria. Herein, it is reported the most recent development R of antibacterial materials based on the combination of polypyrrole and different fillers (metal nanoparticles, carbon nanotubes, and polysaccharides) and strategies to improve their action (such as light and electrical stimulus). The synergistic interaction of electrostatic forces provided by charged polypyrrole combined with the permeation of nanoparticles through R the cell wall favors the leakage of cytoplasmic components and reinforces the antibacterial activity of the resulting material, observed in all-organic composites of polypyrrole and chitosan that reached superior performance against R Escherichia coli (10 CFU) or metal-polymer composites (polypyrrole-palladium) with an outstanding performance against different types of bacteria. The development of binary and ternary composites is explored to potentialize the R antibacterial synergy of components.

Keywords: antibacterial, carbon nanotubes, nanocomposites, polypyrrole, silver nanoparticles.

R How to cite: Silva Júnior, F. A. G., Vieira, S. A., Botton, S. A., Costa, M. M., & Oliveira, H. P. (2020). Antibacterial R activity of polypyrrole-based nanocomposites: a mini-review. Polímeros: Ciência e Tecnologia, 30(4), e2020048.

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*helinando.oliveira@univasf.edu.br

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https://doi.org/10.1590/0104-1428.08020

1. Introduction The resistance against antibiotics developed by microorganisms has been considered an emerging worldwide crisis in a scenario of the scarcity in the production of new antibiotics[1-5], one of the most relevant topics for world public health[6,7]. As a consequence, the development of strategies to circumvent the use of antibiotics becomes critically important[2,5,8-10]. The substitution of conventional drugs by chemical composites at nanoscale introduces important advantages to inactivate new mechanisms of resistance acquired by several microorganisms[1].

In this direction, the use of conducting polymers (isolated, combined, and in association with antibiotics) as antibacterial agents has been considered a promising methodology for new antibacterial systems[11-19]. In particular, polypyrrole has been considered as one of the most important organic materials in the literature, being successfully explored in plenty of applications, making use of its superior electrical properties[18], ease synthesis, high stability under ambient conditions, and good redox properties[20].

The most common applications of polypyrrole involve the development of electrodes for supercapacitors[21], sensors[22], anticorrosive surfaces[23], removal of heavy metal ions and traces from wastewater[24], adsorbent for dyes[25], selective adsorption of components[26], electromagnetic wave absorbers[27] and antibacterial agents[28-30].

Polímeros, 30(4), e2020048, 2020

In the last application, the use of antimicrobial agents is strongly motivated in different areas such as medical, food, and textiles due to the scarcity of conventional antibiotics.

The development of these alternative materials is guided by characteristic environmentally friendly behavior, high biodegradability, and the intrinsic activity against antibioticresistant organisms. These applications involve the disinfection of water[31], for food packing[32], for inhibition of methicillin-resistant Staphylococcus aureus[33], and antibacterial textiles[34] based on poly(lactic acid) incorporated chitosan nanocomposites[35].

The intrinsic antibacterial activity of polypyrrole is a consequence of the oxidative polymerization of pyrrole monomers: positive charges are created at fixed intervals of three to five monomers along the main chain of polypyrrole[36,37]. This relevant cationic behavior confers an important antibacterial activity for the resulting polymeric chain, described as follows.

1.1 Mechanisms of antibacterial activity of polypyrrolebased systems The resulting positive net charge of chains of polypyrrole electrostatically interacts with the overall surface charge of bacterial wall cell that is negatively chaged[36]. Based on this aspect, the bioactivity of polypyrrole has been attributed to

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Silva Júnior, F. A. G., Vieira, S. A., Botton, S. A., Costa, M. M., &, Oliveira, H. P. the resulting positive charged species along the synthesized chains[38]. These species are stabilized by the introduction of anions that act as counter-ions such as Cl- and SO4-. The overall process of antibacterial can be described by the following steps: The initial electrostatic interaction between conducting polymer and bacteria results in the adhesion of microorganisms to the polymer surface. The physical interaction step is followed by the diffusion of nanoparticles and active counter-ions particles in the direction of the cytoplasmic membrane. This process is established by the permeation of the species into the cell, which provokes the death of bacteria[36]. The general scheme for the overall process of attachment of bacterial cells on the polypyrrole film surface is drawn in Figure 1 in which is indicated that interaction with the charged surface and the diffusion of reactive into the cell wall results in the death (leakage of vital components from the cells).

The direct measurement of the zeta potential of the resulting material (polypyrrole-based composite) denotes important information about the overall surface charge signal (positive or negative) and consequently the overall potential for use as an antibacterial. Bin-Jumah et al.[39] reported the use of ocular chitosan nanoparticles in which the zeta potential is explored to determine the presence of positively charged chitosan groups on the external surface of particles and to infer the level of bioadhesion. To reach the desired condition in the zeta potential response (stability and positively charged surface), it worth mentioning that antibacterial activity of these materials is pH-dependent, being possible to reach the condition of protonation or deprotonation of polypyrrole (from positively charged to neutral) with direct consequences on bioadhesion: negative

zeta potential is observed for both Gram-positive bacteria (in the response of prevailing polysaccharides) or Gram-negative bacteria (teichoic acids bonded to the peptidoglycan layer)[40].

Despite this relevant intrinsic property of polypyrrole, the morphology (aggregation level) of polymeric chains represents a critical drawback that needs to be circumvented by the use of specific formulations, being considered the possibility of production of polypyrrole-based nanoparticles[41].

The incorporation of polypyrrole as a filler or primary matrix for chemical modification is explored from the production of composites with carbon derivatives (such as carbon nanotubes and graphene oxide), metal nanoparticles (silver and palladium), natural materials (chitosan-based matrix), and from interaction with other polymers (such as polyaniline) – the different possibilities for the production of effective antibacterial composites by incorporation of additives/ fillers are summarized in Figure 2.

Besides, the interaction of components in the composite opens the possibility of the synergistic interaction towards more effective antibacterial devices. In the following section, it is discussed about the most relevant strategies for the optimization of polypyrrole-based composites as antibacterial agents.

2. Polypyrrole-based nanocomposites The production of polypyrrole/ metal nanoparticlebased composites represents an important step towards the development of superior multifunctional materials based on the synergistic interaction of components to reach potential performance against bacterial growth, proliferation, and the following cell death. With this aim, the polypyrrole has been

Figure 1. Scheme of electrostatic processes involved in the general mechanism of antibacterial activity of polypyrrole and the following step of cell death – leakage of DNA and vital components.

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Antibacterial activity of polypyrrole-based nanocomposites: a mini-review

Figure 2. Interaction of polypyrrole and chemical compounds for the development of effective antibacterial nanocomposites.

combined with different elements such as iron oxide[42], zinc oxide[43], palladium[44], and silver nanoparticles[45-47]. The advantage related to the production of multifunctional metal/ polymer nanocomposites with antibacterial activity regards the incorporation of intrinsic properties of inorganic phases, such as the magnetic separation of Fe3O4-based composites, the photocatalytic activity of ZnO-based samples, and the plasmonic properties of silver-based composites.

Despite all of the efforts to understand the general mechanism involved in bacterial cell death induced by these experimental prototypes, the overall process is not completely identified. The most accepted hypothesis is attributed to the strong diffusive activity of metal nanoparticles (palladium, silver, or iron oxide) that migrate in direction to bacteria and permeate into the cell wall to provokes the leakage of the cytoplasmic components which is facilitated by strong “adhesive” properties of conducting polymer layer that electrostatically attracts the oppositely charged bacterial cells.

Hasantabar et al.[42] reported the development of an interesting core-shell-shell structure in which iron oxide nanoparticles are coated by a first thin layer of polyxanthone triazole for the following deposition of polypyrrole layer. The authors attributed the superior performance in antibacterial activity to the mutual response of pyrrolinium, triazole ring, xanthone, and Fe3O4 nanoparticles that damage the cell wall, provoking the leakage of vital components and the death of the cells.

Moreover, the combination of metal oxide nanoparticles into polypyrrole-based composites, such as zinc oxide into polypyrrole/ chitosan composition can be considered as a promising strategy not only for improved antibacterial assays but also for anticancer performance[43]. The reinforcement provided by ZnO nanoparticles has been attributed to the Polímeros, 30(4), e2020048, 2020

oxidative stress provoked by nanoparticles after permeation through the cell barrier, which restricts bacterial growth.

The most common metal/ polypyrrole nanocomposites (polypyrrole/ palladium and polypyrrole/ silver nanoparticles) are described in the following sections.

2.1 Polypyrrole/palladium (PPy-Pd) Nanocomposites of polypyrrole and palladium have been intensively explored in the literature for different applications such as catalysts[48,49], electrocatalysts[50], antibacterial[44], and antibiofilm activity[51]. These compounds can incorporate magnetic properties from ternary composites of reusable ironconducting polymer-noble metal[48], different disposition of components such as sandwich structured Pd-PPy-Pd[50], and more elaborate experimental systems as scaffolds (association of PPy, Pd and reduced graphene oxide) with antibiofilm properties applied in implants[51]. Despite these relevant applications, the typical procedure for nanocomposites synthesis can be considered in a simple step process: the micro emulsion-based polymerization procedure mediated

by Fecl3 induces the polymerization of polypyrrole on Pd nanostructures, as reported in Ref[44].

The interaction of polypyrrole and Pd in composites is justified by the strong antibacterial activity of palladium nanoparticles. As described in the literature, the antibacterial activity of palladium is size-dependent[52] with interesting results for inhibition of bacteria due to the interaction of Pd nanoparticles with bacterial cell wall – in this case, the interaction of Pd nanoparticles and phosphor/ sulfur moieties causes the death of the cells. As a resistance mechanism from specific classes of bacteria (such as E. coli), it is reported the possibility of the use of efflux complexes that pump biocidal nanoparticles from cells[52] and can be explored as 3/9

Silva Júnior, F. A. G., Vieira, S. A., Botton, S. A., Costa, M. M., &, Oliveira, H. P. a source for superior performance of these nanoparticles against S. aureus cells.

Superior antibacterial properties were observed for PPy-Pd composites as a consequence of the incorporation of Pd nanoparticles into the polymeric matrix, making use of advantages such as the electrostatic interaction between polymer surface and bacterial cell wall for the following step of nanoparticles release[44]. The overall process favors not only the antibacterial activity but also favors the antibiofilm activity since the process tends to be established in the polymer surface, due to the attraction of negatively charged cells of bacteria.

2.2 Polypyrrole-silver nanoparticles (PPy-AgNPs) The antibacterial activity of silver has been extensively reported in the literature[53-55]. As previously reported for Pdbased systems, the effective action of silver against bacteria is a size-dependent process in which particles with a size in the order of 7 nm reaches the nuclear content and release Ag+ ions, being favored by the high available surface area of nanoparticles[53]. Based on this condition, the incorporation of silver nanoparticles on the polymeric matrix depends on the aggregation level of structures that can be conveniently guided from the growth of polymeric structures[56].

To reach adequate dispersion of silver nanoparticles on different morphology of supports, different strategies of synthesis have been established, as follows: The production of cylindrical polymeric templates decorated with silver chloride (AgCl)[57] makes use of the selfaggregation of methyl orange to form cylinders in microscale which are explored as templates for the polymeric growth.

With the use of ammonium persulfate as the oxidizing agent in the presence of silver nitrate and monomers of pyrrole, the polymerization takes place and the polypyrrole chains grown on tubular supports, acquiring the morphology of hollow tubes decorated with AgCl nanoparticles.

Another alternative is reported by J. Upadhyay et al.[58] that produced silver nanoparticles-decorated polypyrrole nanocomposites (PPy-AgNPs) from in situ reductions of silver nitrate. The authors identified a direct relationship between the amounts of silver in the composite with the overall antibacterial activity of the resulting material. The electrostatic interaction between the conductive layer of polypyrrole and negatively charged bacteria and the release of Ag+ ions into the cells are general mechanisms for this antibacterial system.

Another interesting property for the decorated nanostructures with silver nanoparticles is reported by Saad et al.[59]. For this process, silica nanoparticles were impregnated with pyrrole monomers while silver nitrate was explored as a photosensitizer. The polymerization was initiated by ultraviolet irradiation and silica@PPy composites in which silver nanoparticles were synthesized as a result of the polymerization.

2.3 Polypyrrole/chitosan (PPy-chitosan) Chitosan is a hydrophilic polysaccharide derived from chitin and extensively reported as a natural material with antibacterial activity[60,61]. The mechanism of bacterial 4/9

inhibition is similar to the polypyrrole due to the high-density of cationized amines on the chitosan surface, which results in a positively charged surface that affects the attached bacteria inducing osmotic imbalances and provoking the hydrolysis of the peptidoglycan layer with the following leakage of intracellular electrolytes[62]. As observed for previously reported systems, the aggregation represents a strong limiting factor due to the inhibition of active sites for the adhesion of microorganisms. The development of composites based on the interaction of chitosan and conducting polymer represents an important strategy to improve the density of active sites for bacterial adhesion[63-65].

Soleimani et al.[66] reported the preparation of polypyrrole/ chitosan nanocomposites, exploring the chemical polymerization of polypyrrole that uses the chitosan as a substrate. The synergistic interaction of components (with a higher density of electrostatic active sites) results in superior performance for the composite, that follows the order (in terms of antibacterial activity): PPy-chitosan> pure PPy> chitosan. The presence of chitosan creates the effect of a phospholipid sponge, in which negatively charged phospholipid cells attached to cell membrane migrated in direction to the porous structure rich in charged polymeric chains and amine groups.

2.4 Polypyrrole/ carbon nanotubes (PPy-CNT) Carbon nanotubes have been successfully applied against groups of microorganisms (such as bacteria, protozoa, and viruses)[67,68] in planktonic and biofilm forms with a primary mechanism based on the physical elements that penetrate the membrane and lead to the leakage of components such as protein and nucleic acid leakage[69]. However, the poor solubility degree of CNT in different solvents inhibits its potential antibacterial activity (characteristic of isolated nanotubes). To circumvent this limitation, a promising strategy refers to the incorporation of CNT into the polymer as a filler component[67]. Thus, the adequate disposition of CNT into the polymeric matrix avoids further bundle formation steps and favors the effective physical disruption of the bacterial membrane. The development of composites based on PPy and CNT makes use of superior properties of both materials, such as high conductivity, potential antibacterial activity, and strong absorption of light in the near-infrared region.

Tondro et al.[69] combined these properties and incorporated the components in a phototherapy treatment based on IR light irradiation. The direct incidence of irradiation combines the effect of physical rupture of cells (from CNT), the electrostatic attraction of polypyrrole to the bacterial cells, and the improved generation rate of reactive oxygen species (ROS) induced by laser irradiation. Therefore, it is observed a reduction in the viability of the cells, as a result of protein and nucleic acid leakage and ROS production inhibit vital processes in the bacteria[69].

2.5 Polypyrrole-based ternary composites As observed for polypyrrole-based systems, the intrinsic antibacterial activity of PANI has been observed for emeraldine salt form in the response of high doping level of this structure, which offers a high density of sites for electrostatic interaction with bacterial cells and favors Polímeros, 30(4), e2020048, 2020

Antibacterial activity of polypyrrole-based nanocomposites: a mini-review the production of hydrogen peroxidase that participate in the reactive oxygen species activity (cell damage and cell death)[70-72]. On the other hand, the high conductivity of PANI perturbs the flow of electrons in bacterial cells, establishing an alternative mechanism for bacterial control.

The development of ternary nanocomposites (structures with three active antibacterial agents) can be explored as an important source for antibacterial activity and also as systems with two- and three-level of interaction between components[73].

There are at least two important possibilities of combination: a complete electrostatic sponge-like structure (a combination of conducting polymers and chitosan) or a polymeric template with nanoparticles and physical agents (polypyrrole, silver nanoparticles, and carbon nanotubes), described as follows.

Kumar et al.[74] reported the synthesis of composites of polypyrrole containing polyaniline that were functionalized with chitosan to act as ternary antibacterial agents. The comparison of the performance of ternary with binary and isolated compounds follows the order: chitosan+PA NI+PPy>PANI+PPy>PPy>PANI>chitosan, confirming the potential of isolated polypyrrole and a promising synergistic interaction with chitosan and polyaniline in a better antibacterial experimental system.

The interaction of carbon nanotubes, nanoparticles, and polymeric surface[75] was reported from oxidative polymerization (in situ) of polypyrrole induced by reaction with silver nitrate in a matrix loaded with different amounts of carbon nanotubes. The resulting composites (CNT0-60/ PPy/ AgNPs) demonstrated a good synergistic interaction between polypyrrole and CNT that is favored by the available surface area of carbon nanotubes in an electroactive matrix of polypyrrole decorated with silver nanoparticles. The performance of recent state-of-art in the area for corresponding systems is compared in Table 1, which described the type of synthesis and the resulting inhibition halo for each composition.

As reported in the literature, the biocidal activity of polypyrrole is derived from its intrinsic positive charge. Thus, the attachment with cells disrupts the negatively charged cytoplasmic membrane of bacteria and causes the leakage of internal components, leading to the death of bacteria[78].

However, Gram-negative bacteria such as E. coli present an extracellular membrane that reduces the total negative

charge per cell. This process makes these organisms less prone to be electrostatically adsorbed on positive charges at the surface of composites[79]. The complexity of the outer membrane of Gram-negative bacteria, by selective proteins and efflux systems, is also a prohibitive barrier for inward diffusion of drugs, acting as an inducing factor for resistance to antibacterial agents. In consequence, concentrationdependent processes and the synergistic interaction with nanosized-scale structures represent important strategies to circumvent this limitation against Gram-negative species.

These antibacterial new barriers can be categorized as dependent on different processes, summarized as follows: • Incorporation of diffusive fillers; • Interaction with physical processes; • Incorporation of fillers for ROS production; • Development of porous supports that minimize the aggregation level of antibacterial components.

The doping level of conducting polymers is a key role in the overall process since it confers relevant properties not only in terms of electronic properties but also to increase the diffusive counter ions concentration. These species are relevant in the following step of attachment of organisms on conducting polymer surface, being responsible by the penetration and the subsequent step of inhibition of vital processes in the organisms. Besides that, the interaction of conducting polymers and carbon nanotubes introduces an additional advantage related to the physical rupture of the cytoplasmic layer and the following leakage of internal components that are additional advantages of the toxic activity of carbon nanotubes. It refers to the incorporation of some external excitation conditions that can improve the performance of these devices.

2.6 The effect of light and electric field on the antibacterial activity of polypyrrole-based composites The introduction of an external excitation can be conveniently addressed to optimize the overall antibacterial activity of the composites. The interaction with light, in the infrared region, makes use of strong absorbance of conducting polymer-based systems, promoting the additional ROS generation[69]. Another important aspect that can be explored in high conductivity-based polymeric antimicrobial agent refers to the association of thermal effects and ROS-induced

Table 1. Different systems containing polypyrrole (PPy), type of synthesis, and amount of each composite are used to inhibit different bacterial organisms.

Material

Synthesis

PPy-Pd

Chemical polymerization

PPy-AgCl PPy- NTs: Ag-NPs PPy-chitosan PPy-silica-AgNPs CNT0-60/ PPy/ Ag

Reactive Self-degradation In situ reduction Chemical Polymerization Oxidative Photopolymerization Chemical Polymerization

Polímeros, 30(4), e2020048, 2020

Mass (mg) 100

100 100 200-300 200

Bacterium Bacillus spp.

S. aureus E. coli K.pneumoniae E. coli S. aureus E. coli E. coli E. coli

CFU 108

108 108 108 107 108

Inhibition halo (mm) 23 10 16 14 23 23 20 10 -

Ref. [2]

[76] [77] [18] [24] [58]

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Silva Júnior, F. A. G., Vieira, S. A., Botton, S. A., Costa, M. M., &, Oliveira, H. P. generation by the external electric field. Da Silva et al.[19] reported the electrochemical modulation of cationic species by an external low electrical field. Similar to this phenomenon in photodynamic phototherapy, polypyrrole-based systems can act as electrical heating sources with the advantages of metal-free electrodes.

These strategies are favored by increasing surface area for resulting devices. With this aim, the development of substrates for polymer deposition with high surface area circumvents the limiting aspect related to the progressive adhesion of bacteria on the polymer surface. The deposition of conducting polymers on porous polyurethane (as an example) offers a bulky structure to the impregnation with microorganisms and treatment. The combination of these strategies favors the increase in the intrinsic and stronger antimicrobial activity of conducting polymers that can be considered as promising candidates for alternative strategies against increasing antibiotic-resistant organisms.

3. Conclusions The development of alternative antibacterial agents based on conducting polymers represents a promising strategy to circumvent the increasing resistance to antibiotics, which can be enriched by the incorporation of fillers and physical methods such as phototherapy and electrical excitation to avoid limitations related to the progressive attachment of microorganisms in the polymer surface – strategies of interest in different areas such as medical, food and textile industries. Promising options to reach adequate activity involve the development of substrates with high porosity degree, surface area, and flexibility to be applied as prototypes for wound dressing devices activated by different external excitation for use in the topical treatment of microbial infections. Based on these aspects, the interaction of carbon derivatives, metal nanoparticles and polypyrrole immersed in porous substrate represents an important prototype for antibacterial applications, due to the high surface area, high conductivity, intrinsic activity of polypyrrole (for electrostatic interaction) and carbon nanotubes (for physical rupture of the membrane) in composites that can be optimized in terms of heat treatment and ROS generation for effective rupture and leakage of components from microorganisms.

The polycationic behavior of the resulting material can be favored by both components, as observed for PPy-chitosan composites that present good performance in the response of positive zeta potential of the arrangement, favoring the electrostatic interaction with oppositely charged species (Gram-positive and Gram-negative bacteria).

4. Acknowledgements This work was partially supported by Brazilian funding agency Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq.

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Received: Aug. 26, 2020 Revised: Nov. 21, 2020 Accepted: Dec. 08, 2020

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