Polímeros: Ciência e Tecnologia (Polimeros) 3rd. issue, vol. 33, 2023

Volume XXXIII - Issue III - October., 2023

Polímeros

Prof. Ailton de Souza Gomez Emeritus Professor, IMA/UFRJ

021 Pol, 2 B C 16 Preto Ouro th

VOLUME XXXIII - Issue III - October., 2023

Granulated recycled plastics used to test the separation method of sink-float with froth flotation São Paulo 994 St. São Carlos, SP, Brazil, 13560-340 Phone: +55 16 3374-3949 Email: abpol@abpol.org.br 2023 2021

Análise Dinâmica de Imagens ao toque de um botão Litesizer DIA 500

Caracterize de maneira fácil e confiável o formato e o tamanho de partículas entre 0,8 μm e 8.000 μm Use o software Kalliope para começar rapidamente e trabalhar em um espaço digital e amigável Beneficie-se da máxima segurança do usuário e do instrumento, graças a um conceito de segurança exclusivo Aproveite a automação para se concentrar em tarefas mais importantes, com treinamento mínimo

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P o l í m e r o s - I ss u e I I I - V o l u m e X X X I I I - 2 0 2 3 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 ” — “ 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 Ailton S. Gomes (UFRJ/IMA), Rio de Janeiro, RJ (in memoriam) Alain Dufresne (Grenoble INP/Pagora) Bluma G. Soares (UFRJ/IMA) César Liberato Petzhold (UFRGS/IQ) Cristina T. Andrade (UFRJ/IQ) 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 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

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“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: October 2023

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Polímeros / Associação Brasileira de Polímeros. vol. 1, nº 1 (1991) -.- São Carlos: ABPol, 1991Quarterly v. 33, nº 3 (October 2023) ISSN 0104-1428 ISSN 1678-5169 (electronic version)

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1. Polímeros. l. Associação Brasileira de Polímeros.

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Editorial Section News....................................................................................................................................................................................................E3 Agenda.................................................................................................................................................................................................E5 Funding Institutions.............................................................................................................................................................................E6

R e vi e w A r t ic l e Analytical approaches to fiber-reinforced polymer composites: a short review Marcia Murakoshi Takematsu and Rita de Cássia Lazzarini Dutra ................................................................................................... e20230031

O r i g in a l A r t ic l e Separation of plastic mixtures by sink-float combined with froth flotation Fernando Pita ..................................................................................................................................................................................... e20230025

Sustainable composites of eco-friendly polyethylene reinforced with eggshells and bio-calcium carbonate Kássia Peçanha Vieira, Alexandra Augusta Reichert, Gabriel Monteiro Cholant, Dielen Marin, Cesar Augusto Gonçalves Beatrice and Amanda Dantas de Oliveira ................................................................................................................................................................ e20230026

Physical properties of Oriental beech impregnated and coated with some chemicals

Hilmi Toker, Çağlar Altay, Ergün Baysal, İlknur Babahan Bircan and Hüseyin Peker ...................................................................... e20230027

Development and characterization of chitosan-collagen films loaded with honey David Servín de la Mora-López, Tomás Jesús Madera-Santana, Jaime López-Cervantes, Dalia Isabel Sánchez-Machado, Jesús Fernando Ayala-Zavala and Herlinda Soto-Valdez ................................................................................................................... e20230028

Pectin-based films with thyme essential oil: production, characterization, antimicrobial activity, and biodegradability Greice Ribeiro Furlan, Wendel Paulo Silvestre and Camila Baldasso ............................................................................................... e20230029

Novel modified blister test to evaluate composites used in repairing cracked pipeline Payman Sahbah Ahmed, Jafar Abdullah Ali and Serwan Sarbast Mohammed Talabani ................................................................... e20230030

Effect of accelerated weathering environment on the carbon fiber/polyamide 6 composites Larissa Stieven Montagna, Guilherme Ferreira de Melo Morgado, Juliano Marini, Thaís Larissa do Amaral Montanheiro, Alessandro Guimarães, Fabio Roberto Passador and Mirabel Cerqueira Rezende ........... e20230032

Mechanical behavior of snake grass fiber with neem gum filler hybrid composite Arumugam Pachiappan and Senthil Kumar Velukkudi Santhanam .................................................................................................... e20230033

Evaluation of potential biomaterials for application in guide bone regeneration from Bacterial Nanocellulose/Hydroxyapatite Elouise Gaulke, Michele Cristina Formolo Garcia, Bruna Segat, Giannini Pasiznick Apati, Andréa Lima dos Santos Schneider, Ana Paula Testa Pezzin, Karina Cesca and Luismar Marques Porto ................................................................................................. e20230034

Incorporation of organic acids in the crosslinking of polyvinyl alcohol hydrogels Dione Pereira de Castro, Vanessa Zimmer Kieffer and Ruth Marlene Campomanes Santana .......................................................... e20230035

Synergistic electrochemical method to prepare graphene oxide/polyaniline nanocomposite Eric Luiz Pereira, Anderson Gama, Maria Elena Leyva González and Adhimar Flávio Oliveira ..................................................... e20230036

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Fighting global warming with tamarind polymers Start-up wins ISC3 Innovation Award 2023 As one of eight finalists in the Innovation Challenge 2023 of the International sustainable chemistry Collaborative Centre (ISC3), Raj Tanna highlights the consequences of increasing plastic production from fossil raw materials with three alarming facts: global warming, the release of harmful substances and poor end-of-life options. With his start-up, Schutzen Chemical Group, the founder is researching and developing biobased and biodegradable solutions with polymers from the fruit seeds of the tamarind tree, which can replace many fossil raw materials such as silicone, acrylates or polyurethane in certain industries. For this contribution to Sustainable Chemistry, the international expert jury of the Innovation Challenge honoured the start-up with the ISC3 Innovation Award 2023, endowed with 15,000 euros.

Founder Raj Tanna, who initially worked in fossil chemistry and later studied textile technology in Manchester, was motivated to work in Sustainable Chemistry by the ISC3 initiative, among other things. The collaborative centre has been supporting Schutzen as part of the ISC3 Global Start-up Service since 2020 – with success: the Indian start-up has developed a functioning technology to optimize the benefits of polymers from the fruits of the tamarind tree. For example, chemicals such as sodium hydrosulphite can be replaced in indigo dyeing for jeans production. Water consumption can be reduced by up to 60 percent as no rewashing is necessary. Another innovative solution is the replacement of silicone in skin and hair care products with more sustainable tamarind-based conditioning agents. “The ISC3 is a catalyst for sustainable change. Schutzen has been directly motivated and inspired by interactions with ISC3 to develop more Sustainable Chemicals, which over the years have set the direction of thinking regarding the world’s needs,” says Raj Tanna.

Tamarind is a fruit tree that is widespread in India and, unlike the guar bean, for example, from which substances with similar properties to those of Schutzen polymers for textile printing are obtained, requires less fertile soil.

Tamarind seeds have a low value in the food industry as only fruits (also known as Indian dates) are used, and the seeds remain a waste product. This makes tamarind seeds an inexpensive raw material. During his studies, Raj Tanna researched the processing of a tamarind polymer to use it as a natural thickening agent in the textile printing process with fiber-reactive dyes.

“Tamarind polymer has been used in the industry for almost 60 years, and many companies in India have used it for polyester printing over the past 20 years,” explains Raj, who is supported by his father Mahendra Tanna and his experience, including in an internationally active German company for textile specialty chemicals in India.

“So it’s not about a new polymer; it’s more about understanding how this polymer can be processed and repur-

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posed sustainably in a wide range of industries and new applications,” says Raj Tanna. Following his research at the university, the Schutzen Chemical Group launched its own Biobased Resin finishing package to achieve wrinkle free effect of fabrics without the use of toxic Dimethyloldihydroxyethyleneurea (DMDHEU) resins, and in that way enabling more sustainable approach to cellulosic fibres such as cotton.

Tamarind is a natural substance with a high molecular weight and nitrogen content. It must be made water-soluble for processing by depolymerization and carboxylation without impairing the nitrogen content. Schutzen has developed a patented process enabling to obtain two basic tamarind polymers: an amphoteric compound that can be used in skin and hair care products, and a compound without basic or acidic groups that retains the nitrogen content and at the same time has good viscosity, among other things, without affecting the charge density.

This process makes fibres shrink and crease-resistant and is essential for use in resin finishing Source: ChemEurope – chemeurope.com/en/

Ferroelectric polymer goes elastic Although polymers are usually flexible, polymer-based ferroelectric materials tend to be rigid. Adding a small amount of crosslinking material can change that, however, and researchers at China’s Ningbo Institute of Materials Technology and Engineering (NIMTE) say that their new “elastic ferroelectrics” are resilient and flexible enough for use in wearable electronics and implantable medical devices.

Ferroelectricity is a material’s ability to change its electrical properties in response to an applied electric field.

It was discovered just over a century ago in certain naturally-occurring crystals and is now exploited in a wide range of technologies, including digital information storage, sensing, optoelectronics and neuromorphic computing.

Conventional ferroelectrics can be made from either ceramics or polymers, but even polymer-based ferroelectrics are not very elastic. This is because they contain crystalline regions that are rigid. Researchers led by Run-Wei Li have now solved this problem by adding a cross-linking chemical, soft-long-chain polyethylene oxide, to the ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene).

“Crosslinking is a general way to endow resilience to plastic polymers in which the crosslinking density range is 1-10% (that is, one to ten repeat units crosslinked in each one hundred repeat units in polymer chains),” explains study team member Ben-Lin Hu. At the higher end of this range, however, Hu adds that the crystallinity of the mixture decreases dramatically, weakening the material’s ferroelectric response. The crosslinking density needed to make elastic ferroelectrics is therefore much lower, leading the researchers to call it “slight crosslinking”.

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When the NIMTE researchers limited the density of the crosslinker to just 1-2%, they found that a betaphase crystalline structure was uniformly dispersed in the crosslinked polymer network. This new crosslinked polymer network can evenly distribute and bear external forces, say the researchers, mitigating damage to the crystalline regions and creating a new ferroelectric material that combines elasticity with relatively high crystallinity. Indeed, the cross-linked film retains its ferroelectricity even under strains of 70% thanks to its improved elasticity.

The new elastic ferroelectric could be used in wearable/implantable electronics, such as sensors and smart healthcare, as well as in information storage and energy transduction, Hu says. Elastic ferroelectrics also have

some exotic properties that might be useful for structures such as elastomers with a giant (>1000) dielectric constant, spin valves with a large magnetoelectric coupling effect and dielectric capacitors that have energy densities on a par with lithium-ion batteries, but with charging and discharging times on the order of just microseconds.

The researchers say they now plan to optimize the properties of their elastic ferroelectrics and will focus mainly on materials with high dielectric and high piezoelectric constants. “These could be used in energy storage and transduction and information sensing and memory,” Hu tells Physics World.

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Website: www.elsevier.com/events/conferences/esistc4conference

December

May

Polymers in Footwear Date: December 5-6, 2023 Location: Nuremberg, Germany Website: www.ami-events.com/event/ecf39069-81fa-414d-b1f800b020542568/summary?RefId=Website_AMI Polymer Engineering for Energy Date: December 5-6, 2023 Location: London, United Kingdom Website: www.ami-events.com/event/ac4c147b-82c7-4540-90ebf2fa9a2d4333/summary?RefId=Website_AMI Polymers in Hydrogen and CCUS Infrastructure Date: December 7, 2023 Location: London, United Kingdom Website: www.ami-events.com/event/6a43b95c-4d3c-48fa-aed85448c37dbccd/summary?RefId=Website_AMI ICPC 2023: 17. International Conference on Polymers and Composites Date: December 11-12, 2023 Location: Rome, Italy Website: waset.org/polymers-and-composites-conference-indecember-2023-in-rome ICAPSC 2023: 17. International Conference on Applications of Polymers in Synthetic Chemistry Date: December 18-19, 2023 Location: Barcelona, Spain Website: waset.org/polymers-and-composites-conference-indecember-2023-in-rome

Polymer Sourcing and Distribution Date: May 14-16, 2024 Location: Brussels, Belgium Website: www.ami-events.com/event/a555bb4d-c26b-4729-80fe05c535294593/summary?RefId=Website_AMI Fire and Polymers Date: May 12-15, 2024 Location: New Orleans, Louisiana, United States of America Website: https://polyacs.net/24fipo Polymers in Flooring Date: May 15-16, 2024 Location: Hamburg, Germany Website: www.ami-events.com/event/4c1e4b8b-4e49-4c29-b2c0db78fce7a924/summary?RefId=Website_AMI 39th International Conference of the Polymer Processing Society - PPS-39 Date: May 19-23, 2024 Location: Cartagena de Indias, Colombia Website: pps39.uniandes.edu.co/ POLY-CHAR 2024 — Polymers for our future Date: May 27-31, 2024 Location: Madrid, Spain Website: congresosalcala.fgua.es/poly-char2024/ Polymers 2024 - Polymers for a Safe and Sustainable Future Date: May 28-31, 2024 Location: Athens, Greece Website: polymers2024.sciforum.net

2024

June

January Compact seminar: Plastics markets essentials Date: January 1, 2024 Location: On-line Website: pieweb.plasteurope.com/default.aspx?pageid=18003&op en=anmeldung ICPAPNM 2024: 18. International Conference on Properties and Applications of Polymer Nanocomposite Materials Date: January 11-12, 2024 Location: Tokyo, Japan Website: waset.org/properties-and-applications-of-polymernanocomposite-materials-conference-in-january-2024-in-tokyo ICPC 2024: 18. International Conference on Polymers and Composites Date: January 15-16, 2024 Location: Rome, Italy Website: waset.org/polymers-and-composites-conference-injanuary-2024-in-rome ICICPMC 2024: 18. International Conference on Industrial Chemistry, Polymers, Metals and Composites Date: January 29-30, 2024 Location: Istanbul, Turkey Website: waset.org/industrial-chemistry-polymers-metals-andcomposites-conference-in-january-2024-in-istanbul

February Polyethylene Films Date: February 12-14, 2024 Location: Tampa, Florida, United State of America Website: www.ami-events.com/event/3605e8c6-3e644ed6-9a13-2c11444ca907/summary?RefId=website_ AMI&rt=ZJWqCFC1sUuPSrZfsYSo5A 38th Australasian Polymer Symposium Date: February 18-21, 2024 Location: Auckland, New Zealand Website: www.auspolymersymposium.org.au/

March Polymers 2024 International Conference Date: March 6-8, 2024 Location: Seville, Spain Website: www.setcor.org/conferences/polymers-2024 9th International Conference on Fracture of Polymers, Composites and Adhesives Date: March 24-27, 2024 Location: Eurotel Victoria, Les Diablerets, Switzerland

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Polymers for sustainable future 2024 Date: June 24-28, 2024 Location: Prague, Czech Republic Website: imc.cas.cz/sympo/85pmm/ MACRO2024 — 50th World Polymer Congress Date: June 30- July 4, 2024 Location: Coventry, United Kingdom Website: iupac.org/event/50th-world-polymer-congress-macro2024/

July PoWER Conference – Polymer Women Empowerment & Research Date: July 11-12, 2024 Location: Northwestern University Evanston, Illinois, United States of America Website: polymerwomenempowermentresearch.com/ Polymer Engineering & Science International 2024 Date: July 21-25, 2024 Location: Tokyo, Japan Website: www.pesi.tw/

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August International Composites, Polyurethane and Engineering Plastics Fair and Congress 2024 Date: August 20-22, 2024 Location: São Paulo, Brazil Website: feiplar.com/Presencial/

September Polymer Markets Outlook Date: September 10-11, 2024 Location: Brussels, Belgium Website: go.ami.international/polymer-markets-outlook/ Plastics Extrusion World Expo Europe Date: September 11-12, 2024 Location: Brussels, Belgium Website: eu.extrusion-expo.com/home Advances in Polyolefins Date: September 29 – October 2, 2024 Location: Rohnert Park, California, United States of America Website: www.polyacs.net/24apo

November Plastics Extrusion World Expo North America Date: November 13-14, 2024 Location: Cleveland, Ohio, United States of America Website: na.extrusion-expo.com/

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ABPol Associates Sponsoring Partners

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Polímeros, 33(3), 2023

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.20230050

Analytical approaches to fiber-reinforced polymer composites: a short review Marcia Murakoshi Takematsu1*  and Rita de Cássia Lazzarini Dutra1  1

Departamento de Química, Instituto Tecnológico da Aeronáutica – ITA, São José dos Campos, SP, Brasil *marciatakematsu@gmail.com

Abstract A variety of fiber-reinforced polymer (FRP) has been described in literature, with a considerable subset of studies focused on fiber surface treatment (sizing), performance enhancement of matrix and fibers both synthetic and natural, and development of more ecologically sustainable composites. The present review discusses the different types of fibers and matrices and their applications, depending on the chemical and mechanical properties of their composites.

In order to evaluate the performance of FRP composites and explore the characteristics of the involved materials, some analytical techniques are considered paramount, such as thermal analysis, microscopy, Fourier transform-infrared spectroscopy (FT-IR), and others. On this basis, this review addresses the state-of-the-art of material characterization methodologies, provides a comprehensive overview of different types of FRP found in literature, as well as links the analytical techniques with the main applications contributing to future studies and research in this area.

Keywords: analytical techniques, composite, fiber, polymer.

How to cite: Takematsu, M. M., & Dutra, R. C. L. (2023). Analytical approaches to fiber-reinforced polymer composites: a short review. Polímeros: Ciência e Tecnologia, 33(3), e20230031. https://doi.org/10.1590/0104-1428.20230050

1. Introduction Fiber-reinforced polymer (FRP) composites are broadly used in technological applications, for example, aerospace, military, automotive, civil, electronic, transport, renewable energy, and biomedical engineering[1-10]. This remarkable material consists of synthetic or natural fibers with specific properties embedded in a polymeric matrix, and the fibers can also have geometry and/or orientation to enhance the performance target of the composite. In aerospace research, Soutis[11] reports that FRP composites have been employed in aircraft structures since 1903 in the Wright Brother’s Flyer 1, and their use was expanded to military aircraft, satellites, and space launchers.

Nowadays, FRP composites are a fast-developing field of research and development, given the advances in materials and applications. In this context, many derived classes of FRP have been reported, such as carbon fiber-reinforced plastics (CFRP), natural fiber-reinforced polymer composites (NFRPCs), synthetic fiber-reinforced polymer composites (SFRPCs), glass fiber-reinforced polymer (GFRP), continuous carbon fiber-reinforced polymer composites (CCFRPs), discontinuous carbon fiber-reinforced polymer composites (DCFRPs), and fiber-reinforced soft composites (FRSCs)[1,11,12].

Previous studies by Raju and Shanmugaraja [13], Kerni et al.[14], and Chaudhary and Ahmad[15] highlight FRP composites as an engineering material with sustainability potential; it employs renewable sources, such as natural fibers, sustainable and biodegradable polymers. In a recent review, Mahesh et al.[16] reports the use of natural fibers in

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combination with different polymeric matrices, focused on mechanical properties. The recent developments in materials science have been focused on alternatives with less impact on the environment and bringing more sustainability to this area of composites[17-21].

Soutis[11] predicted that 50% or more of the structural mass of an aircraft could be made of CFRP composites.

According to Hui et al.[1], the CFRP based on epoxy matrix composites makes up to 50% of the wings and fuselages of the Boeing 787 Dreamliner and the Airbus A350 XWB models, due to the mechanical properties of composites with failure by diffuse damage, presenting a different and more adequate fracture resistance than solid materials, such as stainless steel.

This statement highlights how FRP composites are crucial in this area, as well as in other fields. Hui et al.[1] also report a new class of FRP composites known as FRSCs, where the matrix is very soft and resistant. In addition, developments in this class could result in a polymeric matrix with self-healing properties with potential to replace epoxy matrices, which have fundamental roles in aerospace engineering. The matrices that could perform this role are polyampholyte hydrogel, acrylic tapes forming a double network (DN), and self-healing hydrogels[1,11,22].

Alemour et al.[23] also report the use of glass fiber, carbon fiber (CF), FRP and a combination of these materials on aeronautical application that significantly reduce the weight of an aircraft with added resistance when compared to metal alloys, reducing fuel consumption, improving efficiency and operating costs.

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Takematsu, M. M., & Dutra, R. C. L. Moreover, the aerospace sector is considered one of the most important fields to invest in composites once it is the main responsible for stratospheric pollution, and advances in different fibers or polymers that reduce weight and add resistance to the composite could reflect in energy efficiency, high performance and eco-friendly engineering structures and less climatic footprint[24].

Szabó et al.[25] studied alternatives to synthetic FRP and developed short CF-reinforced polymers derived from cellulosic materials and polyamide 6, monomers that could be obtained from renewable sources, and consider this composite a potential green alternative to FRP composites.

Imre and Pukánszky[26] mention four factors that determine the properties of composites including FRP: component properties, composition, structure, and interaction. Thus, additive manufacturing technologies are being developed to improve the interaction of the mechanical properties of polymeric composites with continuous fibers, with the use of a suitable binder to associate the FRPCs (fiber-reinforced polymer composites) in the interlayer; with the techniques being mainly focused on fiber alignment, significant reduction of porosity, fiber-matrix adhesion and improvement in the bonding of composite layers[27].

Combining a tough hydrogel and a woven fiber fabric, it is possible to provide a synergistic effect that increases the toughness and tensile properties of composites compared to isolated or neat materials. Huang et al.[28] studied the dissipation of energy performed by hydrogel matrix in the final toughness of composites, interfacial bonding, and synergic effects in the mechanical properties of PA-GF (polyampholyte – glass fiber fabric), PAAm-GF (polyacrylamide – glass fiber fabric), hydrogel composites, and PDMS-GF (polydimethylsiloxane – glass fiber fabric) elastomer composite. This study is an example of soft composites with remarkable fracture resistance and provides a good guide to understanding the synergy between hydrogel and fibers. Hydrogels consist of a soft material that could be used in composites that require softness whilst mechanical properties are also required.

Other promising FRSCs applications are 4D printing, biomimetic composites, and embedded sensing/actuation.

Spackman et al.[29] studied 3D printing of FRSCs and reported limitations in this type of printing due to a lack of control over the positioning of fibrous structures. One way to mitigate this limitation is to develop laminated FRFCs, which deliver regular composite structure and improvements on properties of fiber alignment when printing, through a combination of an ultraviolet curable polymer, providing better mechanical properties to the soft material.

Illeperuma et al.[30] report another type of matrix used in fiber-reinforced composites: hybrid hydrogels. It is challenging to develop a matrix based on hydrogel and use strong fiber to reinforce this material, but new techniques to improve the toughness of hydrogel are being sought to combine this property and stretchability through networks with covalent and ionic cross-links.

Recently, Ren et al.[31] point out the importance of fiber-reinforced polymer nowadays in new high-technology fields and the opportunity to metal replacement in important areas like the aeronautics-aerospace industry, new energy, and military field.

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Basalt fibers (BF) consist of fibers derived from salt rock (volcanic stone) with minerals like plagioclase, pyroxene, and olivine. The features of this fiber are very interesting for FRP application on aerospace, automobile, and navy, as this material is considered more mechanically resistant than GF (glass fiber), eco-friendly, non-toxic to humans, chemically resistant, corrosion-resistant, non-combustible, and stable at high temperatures (above 900 °C). The disadvantage of BF and GF is their high electrical resistance that can interfere in electrostatic discharge, electromagnetic interference shielding, and electric heating. However, CF are being used in composites to complement functions in which GF and BF perform poorly: electrical, thermal, and mechanical properties[10,32,33]. Lopes and D’Almeida[34] studied CF- reinforced ABS (acrylonitrile butadiene styrene) and concluded that the inclusion of CF in the mixture improved thermal stability and mechanical properties in the composite.

Regarding NFRPC (natural fiber-reinforced polymer composite) produced from plant matter, Bledzki et al.[35] reported wood fiber as the lignocellulosic natural fiber most used to reinforce plastic materials. Nevertheless, with the advances in natural fiber treatment, other sources are being studied as reinforcement, such as barley husk, coconut shell, banana, jute, cotton, agave, flax, and others[2,12,13,35,36].

According to Yang et al.[37], aramid fibers present low density, high rigidity, high strength, and high specific modulus. Their main drawback consists in poor interfacial adhesion with common industrial resins; although it could be improved through chemical treatment of the fiber surface with acid solutions, fluorinated compounds, Polyvinyl alcohol (PVA), and dopamine auto-oxidative polymerization with grafting to promote effective chemical bonds and increase adhesion. Plasma and gamma irradiation could also be used.

Thomason[38] recognized the technical importance of characterizing the nature of GF used in FRP production in order to improve quality control, develop new materials and study the prediction of processability influence and composite performance. The study highlights the growing relevance of the analytical methods for polymeric GF sizing in industry and research and synthesizes the main contributions to the field. The set of analyses addressed by Thomason included: X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), electrokinetic analysis (EKA), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), contact angle (CA), dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), DMA, nuclear magnetic resonance (NMR), FT-IR, ultraviolet spectroscopy (UVS), gel permeation chromatography (GPC), high-performance liquid chromatography (HPLC).

As described above, there are several types of FRP to be explored according to fiber surface treatment, advances in the polymeric matrix, and so on. Thus, the main purpose of this review is to address the development, advances, and state-of-the-art on characterization methodologies of FRP composites according to the trends in this class of composites in constant evolution to attend needs in key areas of scientific progress. Furthermore, this review also discusses some applications and the diverse materials used to develop FRP and presents complementary information to recent reviews published on FRP composites[5,13-15,32,39,40].

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Analytical approaches to fiber-reinforced polymer composites: a short review In addition, different studies on FRP stability are found in the literature and consider many conditions that could affect the material within a reasonable period, and analytical techniques to evaluate the tests. These conditions include the influence of humidity, alkaline and acid solutions, temperature variation, ultraviolet radiation, freeze-thaw and wet-dry cycles, and combined conditions. Considering this, the importance of analytical methodologies in supporting FRP research is clear, and this review will address some techniques widely applied to understand and evaluate different treatments in aging experiments[40].

We also evaluate the most important analytical techniques to develop FRP composites, and the main results on their characterization based on the literature.

2. SEM and SEM/EDS (Scanning electron microscopy/ energy-dispersive X-ray spectroscopy) Composites with chemical-treated fibers are widely analyzed by SEM in order to visually analyze the interaction of fiber and matrix, indicating the adhesion process, fractures, and presence of gaps between fibers and matrix.

El-Shekeil et al.[41] studied the effects of treatment of kenaf (Hibiscus cannabinus) fiber-reinforced thermoplastic polyurethane composite and the SEM images suggested that NaOH (sodium hydroxide) + pMDI (polymeric methylene diphenyl diisocyanate) chemical treatment in kenaf fibers presented better results on wetting and adhesion in the studied composite.

On composites produced with CF and ABS, Lopes and D’Almeida[34] performed SEM to observe the expected voids previously reported in the literature, and voids between the CF surface and ABS matrix due to the cooling after the extrusion process. SEM analysis also helped to elucidate the adhesion around the fibers and the mechanism of fractures in neat ABS and reinforced composites.

Furthermore, SEM also contributes to confirming the uniformity of reinforced fibers in the hydrogel matrix. Martin and Youssef[42] used 2wt% (weight %) and 3wt% by weight (relative to swelled hydrogel weight) of E-glass fiber to reinforce alginate/PAAm (polyacrylamide) hydrogel and SEM images showed how the fibers were distributed in the top surface of samples ensuring a degree of uniformity on chopped fiber dispersion. This type of hydrogel reinforced with E-glass fiber could present medical applications.

SEM was also applied to obtain microscopy evidence of some deformation and failure mechanisms such as plastic deformation, crazing, crack arrest, crack deflection, and fiber holes[29]. Images supported Wang et al.’s[43] findings on the durability of epoxy-based GF, basalt fibers, and CF-reinforced polymer composite bars during accelerated stability tests, casting light on fracture morphology.

SEM was also applied to understand the effects of seawater aging on different temperatures, saline concentration, and time on CFRP with epoxy resin, in terms of corrosion damage to the matrix, CF morphology, presence of NaCl (sodium chloride), surface morphology, microcracking, and delamination. These phenomena were evident by images of aging samples[44].

Polímeros, 33(3), e20230031, 2023

This microscopy can be associated with energy-dispersive X-ray spectroscopy (EDS) to perform elementary analysis[45].

This coupled analysis is widely used to characterize minerals, metallic materials, composites, microplastics, and micro and nanomaterials based on polymers.

3. DSC DSC is a fundamental thermal analysis applied to evaluate phase transitions (e.g., glass transition, melting, crystallization) and chemical reactions (e.g., curing, oxidation), as a function of temperature, by equipment that consists of a furnace and electronic system able to register the difference in temperature between reference and sample pans according to the heat flow measured in each pan[46,47]. When phase transitions or curing processes are important to be evaluated in FRP composites, DSC analysis is always required.

As some epoxy resins are commercialized in a pre-preg material, where this thermoset matrix is partially cured in the fibers of the composite to facilitate the handling of material, it is needed to evaluate the conditions of the curing process

by DSC to characterize the material[47].

Bio-composites are being studied to replace synthetic composites, mainly to promote more sustainable products.

Yu et al.[48] reported extensive use of bio-based thermosets in FRP during the last several years. Ferdosian et al.[49] studied the performance of bio-based epoxy composites.

In the bio-polymer field, chicken feathers are being used as reinforcement fibers in the matrix, as these natural sources have interesting chemical (presence of ~90% of keratin protein), physical (low electrical conductivity), and morphological properties; besides the environmental benefits that, associated with synthetic resins, result in applications ranging from electrical insulators to biodegradable plastics.

Chicken feathers were studied via DSC to understand the thermal transition temperatures after chemical treatments with sodium dodecyl sulfate and hydrogen peroxide.

The authors suggest that chicken feathers have the potential to be used as reinforced fibers in composites due to their properties, and also their light weight[50].

Wang and ElGawady[51] studied the influence of moisture in concrete-filled epoxy-based GF-reinforced polymer tubes, specifically in epoxy-based GF. It was observed that the glass transition temperature (Tg) decreased after contact with humidity due to the plasticizing effect of water when epoxy resin absorbs it.

Mgbemena et al.[24] applied DSC to comprehend the shift of Tg via the design of stability studies of FRP composite, and to verify the cure of the polymeric resin. Therefore, this thermoanalytical technique is essential to characterize the phases of composites, aging, and plasticizing process of the polymeric matrix.

4. TGA TGA analysis constitutes a well-known thermoanalytical technique that addresses the thermal stability of samples by monitoring the weight of the sample over time and temperature increase, with a controlled flow of gas during the test to ensure the use of inert or oxidizing gas[46,52].

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Takematsu, M. M., & Dutra, R. C. L. This technique is being used to measure the fiber content in FRP and has the benefit of being faster and requiring less material than digestion methods[53].

Moon et al.[54] applied TGA to determine GF and CF contents of epoxy composites. The different conditions were tested to optimize the fiber content measurement.

The advantages of this method relate to being an easy test to carry out, as it is not demanded constant reweighing and requires a small sample to test.

Another contribution of TGA analysis is related to studies of thermal stability of FRP composites. For instance, CF and aramid fibers were studied in a matrix of polybenzoxazine to indicate their function to provide more thermal stability to FRP composites. CF performed better than Kevlar because of higher carbon content, and the presence of graphite in CF structures[55].

Mak and Fan[56] also investigated the influence of wet-dry cycles in NFRP based on flax and epoxy resin by TGA. The resulting peak derivative temperature, representative of cellulose degradation in the samples, was considered evidence of the reduction of thermal stability of flax.

Lopes and D’Almeida[34] studied the thermal stability of CF-reinforced ABS by TGA methodology. Three heating rates (10, 20, and 30 ºC/min) in an inert atmosphere (N2-Nitrogen gas) and different decomposition levels (2.5 to 20% degradation level) in the samples were applied to investigate the decomposition kinetics using Flynn-Wall calculation that determines the activation energy in each decomposition level per sample. From this calculation, it was inferred that the thermal stability of samples increased or decreased according to the changes in the composition of samples.

5. FT-IR FT-IR spectroscopy is a well-known analytical technique used in many fields of science and technology to identify and characterize substances or materials that absorb specific infrared radiation bands related to different molecular vibration levels. Through infrared spectroscopy, it is possible to evaluate surface and interfacial phenomena, and complex mixtures, by interpretation of spectra in three different regions of the infrared spectral range[57]: •

NIR (near-infrared): 14000 – 4000 cm-1;

•

MIR (medium infrared): 4000 – 400 cm-1, where it is found the fingerprint region (the region with main fundamental bands of MIR: 1500 – 400 cm-1);

•

FIR (far infrared): 500 – 50 cm-1.

FT-IR is also used to characterize the materials used to produce FRP and to evaluate the potential degradation of the polymeric matrix during manufacturing. As studied

by Lopes and D’Almeida[34], the CF-reinforced ABS with a variation of fiber concentration and length produced via extrusion, was investigated to understand the degradation process of ABS (180 - 220 °C) and the interaction of this polymer with CF through ATR (attenuated total reflection) mode in the medium infrared region (4000 – 450 cm-1).

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The results indicated degradation over 200 °C, it was observed absorbance in the region related to the stretching of the carbonyl group (C=O) at 1690 and 1800 cm-1. As ABS does not present oxygen in its molecular structure, this result shows that oxidation is occurring during the extrusion.

No influence of the interaction of CF in the matrix in polymer degradation was observed.

Chua et al.[58] applied FT-IR using ATR to observe the surface chemical composition changes along the aging process (37°C for 1, 3, 6, and 12 months) of CFRP for implementable medical devices. CFRP discs with continuous and discontinuous CF, and different matrices based on epoxy resins or vinyl ester. 3D printing fabrication was also tested employing fused filament fabrication technology with a PA (polyamide) thermoplastic matrix.

They observed some functional groups like C=O (at 1730 cm-1) and C-O (1240 cm-1) indicating an oxidative process from months 0 to 12. Another band also evaluated by this study was about 3400 cm-1, related to O-H stretching and the absorption of water during the aging process. The authors correlated the FT-IR results with an EDS performed in tandem with SEM. Although EDS accused in all samples a significant increase of oxygen level from 1 to 3 months, the FT-IR evaluation did not show the same tendency[58].

This difference observed between methodologies could be derived from the contact of the sample with the crystal in FT-IR analysis. The ATR is very dependent on good contact between the sample and crystal, as mentioned by Sanches et al.[59].

Wang and ElGawady [51] studied concrete filled epoxy-based GF-reinforced polymer tubes to understand the moisture effect in the GFRP, as epoxy can absorb up to 7% moisture by weight, according to the authors, and it will reflect directly in the mechanical properties of the final material. GFRP was analyzed by transmission mode using KBr (potassium bromide) pellets with a ratio of 1:10.

The authors considered the carbon-hydrogen bond (-CH) constant in the GFRP and used the OH/CH ratio as an indicator of moisture absorption in the resin. The -OH was assigned a wavenumber of 3421 cm-1, and -CH, 2926 cm-1.

The authors also mention the importance of this analysis to understand the reduction of the Tg of resin, as moisture can plasticize the epoxy resin and, consequently, cause changes in Tg. Thomason[38] in his review of polymeric GF sizing applied diffuse reflectance Fourier transform infrared (DRIFT) mode to analyze silanes and sizing used in the coating of plates and fibers. The author also referred that DRIFT mode was also carried out in a combination of XPS and CA to study the modification of chopped E-glass with long-chain alcohol adsorption.

Magalhães et al.[60] discusses DRIFT mode regarding the sampling depth degree, and that it is not recommended to characterize chemically the surface of VectranTM fibers, as it is not considered a selective mode for this purpose.

According to the authors, other ways of obtaining spectra by reflection, such as ATR or universal attenuated total reflection (UATR), or obtaining spectra by photoacoustic spectroscopy (PAS) detection could be more appropriate to investigate surface treatment in fibers.

Polímeros, 33(3), e20230031, 2023

Analytical approaches to fiber-reinforced polymer composites: a short review Another important type of synthetic fiber used worldwide to reinforce polymer composite is Kevlar, as these fibers are considered chemically inert and present high tenacity[61,62].

However, Kevlar fibers have a smooth surface, and this physical characteristic requires a surface modification, according to the matrix to be used with them. For this reason, Lin[61] studied the grafting of Kevlar fiber surfaces with bromoacetic acid at 50°C/10h, and epichlorohydrin at 25°C/8h. Infrared spectroscopy showed the presence of the carboxyl group (1750 cm-1) when the fiber was treated bay bromoacetic acid and the epoxy group (2990 cm-1) with epichlorohydrin treatment. According to the results presented by the spectra, it seems that the reflection mode was carried out to perform this experiment and this shows the importance of the technique to solve the surface characterization of fibers.

Kondo et al.[63] also applied FT-IR ATR mode to identify the grafting of 3-Acryloxypropyltrimethoxysilane (APTMS) by electron beam irradiation in PET (polyethylene terephthalate) fibers, using C=C in the vinyl group of APTMS as a marker with an absorption peak at 1639 cm-1 to accuse the surface presence of APTMS. This was corroborated by SEM/EDS analysis, which provided a mapping of silicon (Si), fundamental to confirm the uniform coating of siloxane linkage by electron beam irradiation.

Especially in natural fiber composites, moisture or the presence of humidity can negatively influence the mechanical properties of the composite, as it can lead to interface degradation. It happens for the natural fibers have hydrophilic properties, and absorb more water than the resin normally used in this type of natural FRP; this condition reflecting in swelling of fibers, micro-cracking in the composite, and loss of interfacial adhesion due to the stress induced by water absorption. This results in a lack of adhesion[41,55]. Another drawback is less durability due to high moisture and chemical absorption[12].

It is recommended to use the reflection techniques, such as UATR or ATR rather than the transmission technique to evaluate the influence humidity, as reflection techniques do not need to prepare a KBr pellet, which could absorb more humidity and influence the final result.

Wang et al.[43], studying bars of basalt, GF, and CF-reinforced polymer composites candidates as replacements for steel, pointed out an accelerated test of these composites in seawater and sea sand concrete, applying FT-IR to assess the degradation mechanism. They used ATR mode, and the region of hydroxyl stretching (O-H) at 3400 cm-1 was studied to understand the indicative of water absorption during the wet-dry cycles purposed by the article.

Bansal et al.[64] studied NFRPCs based on bamboo, jute, and coir fibers, with an epoxy resin in random orientations, and discussed the characterization of matrix by FT-IR, applied as a method to differentiate the samples according to the mixture of fibers. Bands related to each type of fiber were detected and they can be used for diagnostic differentiation.

Ramachandran et al.[65] studied bamboo, banana, and linen fibers cut into 2-4 mm of length, with epoxy resin in random orientations. In order to characterize the natural fibers, the authors also carried out FT-IR but did not describe the sample preparation and mode of analysis, as well as the previous study[64]. FT-IR studies have particular conditions Polímeros, 33(3), e20230031, 2023

and preparations, with different modes available to obtain a spectrum. It is recommended to describe this information to understand the real conditions and achieve the same quality of spectrum as the authors. Any modification of condition, preparation, or mode could impact the result of the spectrum, thus these descriptions contribute significantly to the information of a scientific article.

Furthermore, NFRPCs have used coupling agents or compatibilizers to improve the interface between the polymer and natural fiber fillers. Maleic anhydride is commonly used in NFRPCs, being a component able to bond hygroscopic cellulose with a hydrophobic polymer, due to the reaction of anhydride and hydroxyl groups of cellulose with ester bonds or secondary interactions of H-bond. Bajwa et al.[66] used FT-IR with a photoacoustic detector to analyze biochar and oakwood flour as fillers of PLA (polylactic acid) and HDPE (high-density polyethylene) matrices. FT-IR photoacoustic spectroscopy, a non-destructive and near-surface technique, is normally used to analyze infrared spectra of dark samples, once this type of spectroscopy is based on a physical process combining acoustic signal generation and radiation energy absorption regardless of the IR transmission intensity to the detector[66-69].

Senophiyah-Mary and Loganath[70] used printed circuit border (PCB) to obtain carbonaceous slag to reinforce PVA, as an alternative for a membrane used to treat domestic or industrial wastewater. This is an example of FRP performed by electro-spinning used to synthesize a nanofiber membrane.

The FT-IR using transmission mode with potassium bromide pellet associated with Raman spectroscopy allowed to demonstrate that carbonization can transform thermoset polymers derived from PCB into useful activated carbon.

In this study, Raman added value to carbonaceous formations as the results of the spectrum showed the presence of bands of carbon at 1336-1604 cm-1.

Ji et al.[71] verified the interference of increasing temperature in the curing of amino silane coupling agent by monitoring the shift of amine functional group N-H (1596 to 1566 cm-1) bands. The absorption in the region of C=O (1660 cm-1) due to the reaction of the amino group with CO2 (carbon dioxide) and H2O (water) was observed. Other important regions in the reaction of silanization on the surface were related to Si-OH (3355 cm-1) and Si-O-Si (1000 - 1100 cm-1).

In addition, FT-IR could be coupled to TGA, and the analysis of volatile pyrolysis gases can be performed using a heated transfer line and appropriate cell to receive the gases.

Perret et al.[72] carried out experiments with this technique to study flame retardants in CF epoxy resins, and used condensed-product analysis at different phases of thermal decomposition.

A short review of VectranTM fiber explored some conditions and modes of FT-IR in the fiber field. This study also brings concepts of different modes of acquisition of infrared spectra and discusses the sample depth degree according to the chosen mode, focused on the surface analysis by FT-IR techniques, such as ATR, UATR, DRIFT, NIR, NIRA (near-infrared reflectance analysis), FT-IR microscopy and PAS[60]. In addition, VectranTM fibers are very important in aerospace and military fields because of their high mechanical performance, and these fibers are applied in FRP composites with epoxy matrix[73].

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Takematsu, M. M., & Dutra, R. C. L. According to Magalhães et al.[60], it is well known that the studies involving NIR region regularly are associated with chemometrics and algorithm based, increasing the complexity of performing such analysis.

However, with the advent of transflectance analysis, such as NIRA, polymer analysis could be performed directly in the equipment without any preparation of the sample, being non-destructive and with the advantage of being considered with high penetration of IR (infrared) beams, and high resolution.

After scrutinizing SEM, SEM/EDS, DSC, TGA, and FT-IR performed in FRP composites, Table 1 presents the principal applications of each instrumental technique addressed in this review.

Given the importance of experimental conditions, different materials performing important roles in the FRP composites, and the array of advanced technologies available to characterize these materials, this review was carried out to study fiber reinforcement interaction with matrix, future trends, and principal characterization techniques involved in FRP studies. Different FRP composite configurations found in the literature were listed in Table 2 with techniques performed in the related composites. All FT-IR analyses were carried out in MIR.

6. Trend According to the studies reviewed, there is an opportunity related to expanding of the use of FT-IR techniques to characterize polymers, further exploring the NIR region and reflectance techniques (such as NIRA). As the spectrum obtained by NIR brings overtone responses

and combination bands, this region is also important to FRP characterization, especially when quantitative studies have to be carried out[60].

According to Table 2, some articles report using AFM (atomic force microscopy) to acquire images of FRP composites, which presents some advantages when coupled with IR spectroscopy. Nguyen-Tri et al.[105] described some principles of AFM-IR (atomic force microscopy-based infrared spectroscopy), as well as the correlation between this technique and the chemical characterization of polymers, including crystallization mechanisms, phase separation, and spherulitic structures.

The use of recycled materials, such as CF, has attracted attention as it repurposes waste materials in the end-of-life phase, as well as reduces energy consumption.

Fernández et al.[102] studied recycled CF as reinforcement material with PP (polypropylene) by injection process, and suggest that composites made with recycled materials have similar mechanical properties of composites with virgin CF. However, further developments in sizing fiber surfaces could bring more benefits to the use of this eco-friendly material. This study also brings the state-of-the-art on fiber orientation and fiber distribution analysis in composites, using a modern technique of X-ray tomography.

Furthermore, this review reveals a trend in natural sources to develop FRP, as the concerns with sustainability and green alternatives are rising. As an alternative to synthetic material, it has already been reported that bio-based composites are considerably environmentfriendly which can reduce the cost, weight of structure, and environmental impacts.

Table 1. Principal instrumental techniques to characterize FRP composites and applications.

Technique

Applications

Ref.

SEM

Morphological assessment; fiber sizing analysis; evaluation of interaction between fibers and matrix; investigation of failures, fractures, adhesion, gaps, corrosion, and deformation.

[29, 34, 41-44, 74-84]

SEM/EDS

Elemental analysis of surfaces after treatment; identification of ratios and chemical composition.

[35, 45, 82, 85-87]

DSC

Determination of phase transition, mainly Tg temperature in the matrix; evaluation of matrix curing process; curing degree.

[24, 36, 46-51, 78-80, 88-90]

TGA

Determination of fiber content; evaluation of thermal stability of composites; assessment of thermal decomposition of materials; characterization of the effects of dehydration and oxidation on material.

[34, 46, 52-56, 78, 80, 84, 91]

FT-IR

Fiber and matrix characterization; assessment of chemical changes, after surface modification of fiber or matrix through specific molecular vibration absorption; relationship between amount of surface treatment of fibers and intensity of infrared absorption ratio in IR spectra; fiber sizing analysis*; degradation studies; aging or stability studies of FRP; water absorption in FRP development; non-destructive analyses: DRIFT, PAS, ATR, microscopy; destructive analysis: KBr pellet analyzed by transmission mode.

[12, 34, 38, 41, 43, 51, 55-73, 82, 84, 92, 93]

* using microscopy features of FT-IR spectrometer coupled with microscopy.

Siglas: SEM: Scanning electron microscopy; SEM/EDS: Scanning electron microscopy/energy-dispersive X-ray spectroscopy; DSC: Differential scanning calorimetry; TGA: Thermogravimetric analysis; FT-IR: Fourier transform-infrared spectroscopy; IR: Infrared; FRP: Fiber-reinforced polymer; DRIFT: Diffuse reflectance Fourier transform infrared; PAS: Photoacoustic spectroscopy; ATR: Attenuated total reflection; KBr: Potassium bromide.

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Analytical approaches to fiber-reinforced polymer composites: a short review Table 2. FRP composites and characterization techniques with instrumental analysis.

Fiber phase

Matrix phase

Analysis

Ref.

Glass/Kevlar fibers Coconut fiber Short GF GF GF and CF GF Cellulose whisker PET fiber GF Natural fibers CF Kenaf fiber CF Natural fibers GF Nylon-6 BF, GF, and CF Natural fibers GF fabric Kevlar fibers and CF GF UHMWPE Lignocellulose fiber GF and flax fibers GF GF MWCNT-coated BF Wood fiber CF CF Short CF Sugarcane fibers CF CF GF and flax fibers Carbonaceus slag from thermoset Natural fiber and GF Flax fiber CF BF Bagasse fiber Recycled CF CF Flax fiber / PLA woven CF Jute fiber

Epoxy resin PP PBT PP/EPDM Epoxy resin Epoxy resin PVA Natural rubber Epoxy and polyester resin Polypropylene Epoxy resin TPU Phenolic resin Epoxy resin Epoxy resin UV curable polymer Epoxy resin Epoxy resin Polyampholyte gel, NaSS, and DMAEA-Q Polybenzoxazine resins PAAm RPU Ethylene-norbornene copolymer Epoxy resin Epoxy resin PEN-BAPh Epoxy resin HDPE and PLA ABS Epoxy resin Cellulose; PA 6 and PP PP Epoxy resin PEEK-Ti laminates Epoxy resin PVA Vinyl Ester Polyester PEEK HDPE Cardanol PP and PP-MAH Epoxy resin Epoxy resin Epoxy resin doped with graphene oxide Bio-based vanillin-derived epoxy

DSC, SEM, TGA SEM DSC, SEM DSC, SEM, TGA, WAXD SEM, TGA AFM, SEM DMA, DSC, SEM FT-IR, SEM/EDS SEM FT-IR, SEM/EDS, TGA, UV-VIS DMA, FT-IR, TGA FT-IR, SEM FT-IR, LRS, SEM, XPS FT-IR DSC, FT-IR, GPC, titration SEM FT-IR, SEM FT-IR SEM FT-IR, SEM, TGA DMA, SEM CA, FT-IR, SEM AFM, DLS, DMA, DSC, FT-IR, TGA DSC, FT-IR, SEM, TGA DSC, FT-IR, SEM/EDS DSC, DRA, FT-IR, SEM, TGA, UV-VIS FT-IR, LRS, SEM, XPS DMA, FT-IR, SEM, TGA FT-IR, SEM, TGA DMA, FT-IR, SEM DSC, SEM, TGA,XPS SEM DSC, FT-IR, Rheology, TGA FT-IR, SEM, XPS DSC, FT-IR, SEM, TGA AFM, FT-IR, LRS, TGA DSC, TGA 1 H NMR, FT-IR, SEM/EDS AFM, FT-IR, SEM/EDS, XPS, WCA FT-IR, DSC, SEM/EDS, TGA FT-IR, SEM, TGA DSC, SEM, TGA, XCT, XPS AFM, FT-IR, SEM/EDS DMA, DSC, SEM, TGA DLS, FT-IR, Raman, SEM FT-IR, NMR, SEM, tensile test, TGA, WCA

[94] [95] [96] [97] [54] [98] [99] [63] [7] [35] [72] [41] [9] [65] [49] [29] [43] [64] [28] [55] [42] [100] [19] [36] [51] [31] [33] [66] [34] [44] [25] [101] [89] [71] [56] [70] [88] [82] [87] [92] [84] [102] [58] [80] [103] [104]

Siglas: DSC: Differential scanning calorimetry; SEM: Scanning electron microscopy; TGA: Thermogravimetric analysis; PP: Polypropylene; GF: Glass fibers; PBT: Poly(butylene terephthalate); EPDM: Ethylene–propylene–diene terpolymer; WAXD: Wide-angle X-ray diffraction; CF: Carbon fibers; AFM: Atomic force microscopy; PVA: Polyvinyl alcohol; DMA: Dynamic mechanical analysis; PET: Polyethylene terephthalate; FT-IR: Fourier transform-infrared spectroscopy; SEM/EDS: Scanning electron microscopy/energy-dispersive X-ray spectroscopy; UV-VIS: Ultraviolet and visible spectroscopy; TPU: Thermoplastic polyurethane; LRS: Laser Raman scattering; XPS: X-ray photoelectron spectroscopy; GPC: Gel permeation chromatography; UV: Ultraviolet; BF: Basalt fibers; NaSS: Copolymerized from sodium p-styrenesulfonate; DMAEA-Q: Dimethylaminoethylacrylate quaternized ammonium; PAAm: Polyacrylamide; UHMWPE: Ultra-high molecular weight polyethylene; RPU: Rigid polyurethane; CA: Contact angle; DLS: Dynamic light scattering; PEN-BAPh: Phthalonitrile containing aromatic ether nitrile linkage; DRA: Dynamic rheological analysis; MWCNT: Multi-walled carbon nanotube; HDPE: High-density polyethylene; PLA: Polylactic acid; ABS: Acrylonitrile butadiene styrene; PA: Polyamide; PEEK-Ti: Polyetheretherketone-titanium; 1H NMR: Proton nuclear magnetic resonance spectroscopy; PEEK: Polyetheretherketone; WCA: Water contact angle; PP-MAH: Maleic anhydride grafted polypropylene; XCT: X-ray computed assisted tomography; NMR: nuclear magnetic resonance.

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7. Conclusion FRP composites are very important materials in many fields of science and technology, with several studies being developed to create more resistant, sustainable, and advanced materials. Studies with natural fibers including plants, and mineral fibers or matrices based on cellulose have been developed to contribute to more ecological alternatives for the near future.

A comprehensive review was conducted in FRP composites investigating the principal characterization techniques performed to study new developments on these composites, including aging or stability testing, sizing on fibers, interfacial properties between matrix and fibers, and material treatment.

SEM, SEM/EDS, DSC, TGA, FT-IR, and other analytical techniques were discussed, and many applications to study FRP composites were proposed. Through the reviewed experimental studies, it is possible to conclude that two or more associated techniques provide more support for composite development.

Finally, this review provides a systematic understanding of FRP composite applications for the development and characterization of these materials, as well as bringing the latest advances in this segment of material science.

8. Author’s Contribution • Conceptualization – Marcia Murakoshi Takematsu.

• Data curation – NA. • Formal analysis – NA. • Funding acquisition - Rita de Cássia Lazzarini Dutra.

• Investigation – Marcia Murakoshi Takematsu.

• Methodology – Marcia Murakoshi Takematsu.

• Project administration – Rita de Cássia Lazzarini Dutra.

• Resources – Rita de Cássia Lazzarini Dutra.

• Software – NA. • Supervision – Rita de Cássia Lazzarini Dutra.

• Validation – NA. • Visualization – Marcia Murakoshi Takematsu.

• Writing – original draft – Marcia Murakoshi Takematsu.

• Writing – review & editing – Rita de Cássia Lazzarini Dutra.

9. Acknowledgements Conselho Nacional de Desenvolvimento Científico e Tecnológico - Finance Code 301626/2022-7 and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001.

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Takematsu, M. M., & Dutra, R. C. L. 99. Roohani, M., Habibi, Y., Belgacem, N. M., Ebrahim, G., Karimi, A. N., & Dufresne, A. (2008). Cellulose whiskers reinforced polyvinyl alcohol copolymers nanocomposites.

European Polymer Journal, 44(8), 2489-2498. http://dx.doi. org/10.1016/j.eurpolymj.2008.05.024.

100. Meng, L., Li, W., Ma, R., Huang, M., Wang, J., Luo, Y., Wang, J., & Xia, K. (2018). Long UHMWPE fibers reinforced rigid polyurethane composites: an investigation in mechanical properties. European Polymer Journal, 105, 55-60. http:// dx.doi.org/10.1016/j.eurpolymj.2018.05.021.

101. Anggono, J., Farkas, Á. E., Bartos, A., Móczó, J., Antoni, Purwaningsih, H., & Pukánszky, B. (2019). Deformation and failure of sugarcane bagasse reinforced PP. European Polymer Journal, 112, 153-160. http://dx.doi.org/10.1016/j. eurpolymj.2018.12.033.

102. Fernández, A., Santangelo-Muro, M., Fernández-Blázquez, J. P., Lopes, C. S., & Molina-Aldareguia, J. M. (2021). Processing and properties of long recycled-carbon-fibre reinforced polypropylene. Composites. Part B, Engineering, 211, 108653. http://dx.doi.org/10.1016/j.compositesb.2021.108653.

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Received: June 04, 2023 Accepted: Sept. 13, 2023

Polímeros, 33(3), e20230031, 2023

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.20220094

Separation of plastic mixtures by sink-float combined with froth flotation Fernando Pita1*  Centro de Geociências – CGEO, Departamento de Ciências da Terra – DCT, Faculdade de Ciências e Tecnologia – FCTUC, Universidade de Coimbra – UC, Coimbra, Portugal

1

*fpita@ci.uc.pt

Abstract The aim of this research was to separate a mixture of six post-consumer plastics (PS, PMMA, PVC-D, PVC-M, PET-D and PET-S) by combination of sink-float separation and froth flotation.. In sink-float method two mediums of separation were used: sodium chloride water solution and ammonium nitrate water solution. Sink-float method allowed complete separation of the less dense plastic (PS) from intermediate density plastics (PMMA and PVC-D) and from high density plastics (PET-S, PET-D and PVC-M); also allowed good separation of intermediate density plastics (PMMA and PVC-D) from high density plastics (PVC-M, PET-D and PET-S) with an efficiency close to 100%. Separation of PVC-M from PET-D and PET-S by sink-float method led to fair results allowing a separation efficiency of about 60%. Since PMMA and PVC-D have similar density, their separation was achieved by froth flotation, using sodium lignosulfonate as selective wetting agent of PVC-D, with a separation efficiency of 85%.

Keywords: density separation, plastic, mixture, particle size.

How to cite: Pita, F. (2023). Separation of plastic mixtures by sink-float combined with froth flotation. Polímeros: Ciência e Tecnologia, 33(3), e20230025. https://doi.org/10.1590/0104-1428.20220094

1. Introduction The management of urban solid waste (MSW) is one of the major environmental concerns worldwide. Its production has increased continuously, reaching in 2020 a world production of 2.24 billion tons, which corresponds to a per capita production of 0.79 kg/person.day. At worldwide, plastics are one of the main constituents of MSW, representing about 12% of its weight[1]. Due to the low price of plastics and their excellent properties that give them multiple applications, the world production of plastics has increased continuously, reaching 367 million tons in 2020, which is a contrasting value with the approximately 5 million tons produced in the 50s of the last century[2].

Despite the constant increase in their consumption, in recent years plastics have acquired a negative reputation, with strong public pressure on their use. Regarding the management of plastic waste, it can be landfilled, incinerated with energy production or recycled. In view of the non-biodegradability of most plastics, they cause the difficulties in the degradation of fermentable materials placed in landfills, and their incineration generates toxic gases. Thus, the most environmentally and economically correct solution is to proceed with recycling, in which plastics should not be seen as a waste, but rather as a resource[3].

Despite the importance of recycling, the vast majority of plastic waste ends up in landfills or the natural environment, or are incinerated, causing serious environmental problems.

In 2019, only about 9% of world plastic waste was recycled, while 19% was incinerated, almost 50% went to sanitary

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landfills, and the remaining 22% was disposed of in uncontrolled dumpsites, burned in open pits or leaked into the environment[4].

Plastic waste management vary by country income level.

In low-income countries the recovery rate of plastic waste is lower, with most of it being mismanaged or uncollected.

In Europe Union, during 2020, 34.6% of plastic waste was recycled, 42% was recovered through energy recovery processes and 23.4% was landfilled[2]. In Africa nearly 60% of plastic waste was mismanaged or littered, around 5% was recycled and the rest was deposited in landfills[4].

Thus, it is urgent to substantially reduce the use of plastics, and reduce the deposition in landfills or in dumps through recycling. However, in order to recycle plastic waste it is necessary to separate the plastic mixtures into individual plastics because different plastics cannot be recycled together due to chemical incompatibilities, different melting points and thermal stabilities, and therefore cannot be mixed in the recycling process[5]. However, separating plastics is not easy because there are many types and most of them have similar properties. In recent years several methods were developed for plastics separation. These methods include automatic separation based on surface properties: X-ray detection[6]; infrared spectroscopy[7] and optical separation based on color[7]. These methods can only be applied when the plastics are clean and of significant size, larger than 30 mm, as the quality of separation significantly deteriorates with decreasing particle size and surface contamination[7,8].

Other methods to separate plastic are as follows:

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Pita, F. -

Electrostatic separation, which is based on the difference in electrical conductivity of plastics[9].

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Froth flotation separation, the most important method of separation in mineral processing, which is based on the different degree of particles hydrophobicity. It is a physicochemical process that is based on the selective adhesion of particles to air bubbles (hydrophobic particles) or to water (hydrophilic particles)[10-15].

-

Selective solvent dissolution methods[16] that usually involve temperature variations because the solubility of a polymer in a given solvent changes with this variable. The toxic organic solvents associated with high costs makes alternative methods more attractive.

-

Gravity separation methods (density separation), which are based on the difference in plastics density, and can be performed in different equipment, such as jigs, shaking table, liquid fluidized bed techniques and cylindrical cyclone media separator[17,18]. Some means have been used to separate plastics, such as water, saturated water solutions with sodium chloride, calcium chloride and ethanol solutions[19,20].

Several combination techniques are also used for the separation of mixed plastic, like air tabling and triboelectric techniques[21]; sink-float separation and flotation[22]; or jigging and flotation[23].

In density separation (sink-float) the plastics with different density are placed in a liquid of intermediate density, where the denser plastics sink and separates out from the less dense floating plastics. The separation efficiency depends on the medium’s density which lies between the different densities of the plastics. A sink-float separation is efficient when plastics have significantly different density. When plastics have a low density difference, separation may be hard or even impossible.

Sometimes air bubbles adhere to the surface of plastics and alter the float and sink features of plastic particles[20].

To obtain water solutions with different density several products have been used, such as: ethanol to obtain solutions with density lower than 1 g/cm3, and sodium chloride which allows obtaining water solutions with density up to 1.2 g/cm3[20]. To obtain water solutions with a higher density have been used sodium polytungstate, zinc chloride calcium chloride, sodium iodine and lithium metatungstate[22,24,25,26]. However, these higher density solutions are toxic to the environment and often very expensive, limiting their use[26]. On the other hand, sodium chloride it is environmentally benign, cheaper and widely available[26].

This work aimed to study the separation of post-consumer plastic mixture by sink-float (density separation) combined with froth flotation separation. Froth flotation was used when sink-float performance was poor. For the density separation two mediums separation are used: sodium chloride water solution and ammonium nitrate water solution. For the separation of plastic mixtures by froth flotation, sodium lignosulfonate was used as a wetting agent. The effect of the solution density and the effect of particle size and particle hydrophobicity on the density separation performance was analysed.

2. Materials and Methods 2.1 Materials This study used six types of granulated plastics from three recycling companies: Polystyrene (PS), Polymethyl methacrylate (PMMA), Polyethylene Terephthalate (PET-D) and Polyvinyl Chloride (PVC-D) from Daniel Morais, Polyethylene Terephthalate (PET-S) from Selenis, and Polyvinyl Chloride (PVC-M) from Micronipol (Figure 1).

The plastics differed on colour and shape, which facilitated separation through manual sorting at the end of each separation test. The density of these plastics was determined in our previous work[18], represents the average of three samples, ranged from 1.047 g/cm3 (PS) to 1.372 g/cm3 (PET-S) (Table 1).

Figure 1. Photographs of the studied plastic samples.

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Separation of plastic mixtures by sink-float combined with froth flotation Table 1. Density and characteristics of the studied plastics.

Types of plastic PS PMMA PET-S PET-D PVC-M PVC-D

Density (g/cm3) 1.047±0.003 1.204±0.003 1.372±0.007 1.364±0.004 1.326±0.005 1.209±0.006

According to the density of plastics it is possible to separate the six plastics into three groups: the first group constituted only by the PS, that has a less density, a second group that includes PMMA and PVC-D that have intermediate density, and the third group that includes PVC-M, PET-S and PET-D that have high density. So, it is expected that the sink-float method is appropriate to individually separate PS from the others five plastics, and also is suitable to separate PMMA and PVC-D from PET-S, PET-D and PVC-M; in addition, it is not suitable to separate PMMA from PVC-D as they have similar density. It is also expected that it will be difficult to separate PVC-M from PET-S and PET-D because of the slight difference in their density.

Particles size has an important role in separation processes. To study the influence of the particles size in the sink-float separation and in the froth flotation separation, two size fractions of the six plastics were used: +1.4-2.0 mm and +2.8-4.0 mm.

2.2 Density separation experiments To obtain dense solutions, sodium chloride and ammonium nitrate are used because they are cheap, widely available and have a low environmental impact than other products that can also be used to obtain dense solutions, such as sodium polytungstate, zinc chloride, sodium iodine, lithium metatungstate[26]. Also other studies have used sodium chloride[20,26]. Sink-float separation of plastics using sodium chloride water solution and ammonium nitrate water solution with various densities was tested. For this, different amounts of sodium chloride and ammonium nitrate were mixed into water on a mechanical stirrer. The maximum density obtained for the sodium chloride solution was 1.203 g/cm3, and the maximum density obtained for the ammonium nitrate solution was 1.380 g/cm3. Sink-float tests were performed in a glass beaker with a capacity of 1 dm3, and in each test 10 g of plastics were used. Prior to the density separation test, the plastics were mixed with tap water in a stirrer during 5 minutes so that the plastics were completely moistened avoiding air bubble formation. Subsequently, the plastics were left in the separation medium. Plastics with a lower density than separation medium floated to the surface while the plastics with a higher density than separation medium sank to the bottom. The separation tests were carried out for 2 minutes (waiting time required for the particles to fall into the solution). After 2 minutes, the floated and sunk products were manually collected and then washed with tap water, dried and weighed to evaluate their recovery based on mass balance.

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Characteristics dark colored, irregular colorless to white, transparent, irregular blue colored, traslucid, lamellar) colorless, transparent, lamellar light green to white, translucid, lamellar gray colored, irregular

First, tests were carried out with one-component plastic samples in solutions with different densities. Then, density separation of binary plastic mixtures was performed using several bi-component mixtures, contributing each plastic with 50% of the weight in solutions with different densities.

2.3 Froth flotation separation experiments Froth flotation tests were performed in a Denver cell with a capacity of 3 dm3. Each test used 40 g of PMMA/PVC-D mixture, in equal proportions, and was conditioned with sodium lignosulfonate for about 5 minutes and later with frother (Methyl isobutyl carbinol - MIBC) for about 2 minutes before the flotation, at the constant concentration of 30x10-3 g/L in all experiments. After conditioning, floated product was collected over 6 minutes.

After the experiments of density separation and froth flotation, the floated and sunk products were dried and weighed. The control of the separation of binary plastic mixtures was carried out using the recovery and grade of each type of plastic in the floated and in the sunken products, after manual sorting and weighing of the two types of plastics.

This was possible due to the different colours and shapes of the plastics particles. The tests were carried out three times under similar operating conditions, and the values represent the mean value of independent experiments. The effectiveness of the plastic separation was quantified by the efficiency of separation, defined by Schulz[27]: η =RP1-RP2 (where η is the separation efficiency, RP1 is the recovery of plastic 1 in the floated and RP2 is the recovery of plastic 2 in the floated).

3. Results and Discussion 3.1 One-component plastic Figure 2 shows the effects of density of sodium chloride water solution on the recovery in the floated of six plastics for two size fractions: +1.4-2.0 mm and +2.8-4.0 mm. For the two size fractions the recovery in the floated of the six plastics increased with the increasing of medium density. Although the two size fractions show similar results, the coarse fraction (+2.8-4.0 mm) led to slightly lower floated recovery.

When only tap water was used (density = 1.0 g/cm3) some PS and some PVC-D floated. Under these conditions the other four plastics sank completely. PS plastic had a greater increase of floatability when increase the medium density in comparison with the other five plastics. It was observed that for a medium density greater than 1.05 g/cm3 all PS floated and for a medium density greater than 1.200 g/cm3 all PMMA floated.

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Pita, F. For PS and PMMA the recovery in the float increases sharply with the variation of medium density, but for PVC-D floated recovery increases gradually with the variation of medium density. For the range of densities evaluated, the floated recovery of the three densest plastics (PET-S, PET-D and PVC-M) is very small because these plastics have a density clearly higher than the maximum density of the medium (1.203 g/cm3).

The use of ammonium nitrate solution allowed obtaining a medium with higher density, reaching a maximum density of 1.380 g/cm3. Thus, this solution allowed a better evaluation of the behavior of denser plastics (PET-S, PET-S and PVC-M) than the sodium chloride solution. For the six plastics and for the two size fractions the floated recovery increased with the increasing of density of ammonium nitrate water solution (Figure 3). For a density of 1.38 g/ cm3, the floated recovery of the six plastics was 100%.

PET-D is the plastic with the lowest floated recovery. In this solution, the behavior of PS, PMMA and PVC-D is similar to that observed for the sodium chloride solution.

However, recovery in floated of these three plastics is slightly lower when using ammonium nitrate solution.

Although the plastics have been previously washed, it is likely that the formation of some air microbubbles attached to the plastics particles when sodium chloride was used are responsible for the highest floated recovery in this solution.

On the two separation medium, for the six plastics, the fine fraction (+1.4-2.0 mm) showed greater floated recovery than the coarse fraction (+2.8-4.0 mm). This means that large particles sink more easily into the medium than fine particles. For example, for PVC-D in sodium chloride solution, when medium density is 1.180 g/cm3, floated recovery was 36.2% for fraction +1.4-2.0 mm and was 25.4% for fraction +2.8-4.0 mm. For PVC-D in ammonium nitrate solution, when medium density is 1.180 g/cm3, floated recovery was 32.2% for fraction +1.4-2.0 mm and was 22.5% for fraction +2.8-4.0 mm. Since the density of plastics is independent of particle size, the higher floated recovery of the fine fraction may result from the possibility that some air microbubbles attached to the particles and make it difficult for them to sink, being this effect more pronounced for the finer particles because they weigh less (being easier to get particle-bubble aggregates with density lower than the density of medium).

Figure 2. Influence of density of sodium chloride solution on floated recovery of six plastics for fractions +1.4-2.0 mm and +2.8-4.0 mm.

Figure 3. Influence of density of ammonium nitrate water solution on floated recovery of six plastics for fractions +1.4-2.0 mm and +2.8-4.0 mm.

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Separation of plastic mixtures by sink-float combined with froth flotation PMMA and PET-D plastics are the ones more homogeneous, with particles of similar texture, similar shape and the same appearance (color). The other plastics have a less homogeneous texture, with particles of different shape and different appearance (color). For example, for PET-S, particles from the body of a bottle and its neck are visible, these having a different texture. This may mean that in each type of plastic, particularly in less homogeneous ones, there are particles with slightly different density, and so the floated recovery variation is more gradual, occurring in a larger range of density of the medium. This behavior is more evident for PVC-D. On the other hand, the drastic variation of floated recovery versus density of medium is fundamentally observed for homogeneous plastics, such as PMAA and PET-D. The density of the six plastics has the following order: PS < PMMA ≈ PVC-D < PVC-M < PET-D ≈ PET-S. In the two dense medium, and for the two size fractions, the floated recovery of the six plastics follows the order: PS > PVC-D > PMMA > PVC-M > PET-S > PET-D. The order of flotation is slightly different from the order of density. Although PET-D is not the densest plastic, it is the one with the least flotation. Also, although PVC-D has a density similar to PMMA, it has greater flotation than that plastic, particularly for medium density less than 1.18 g/cm3.

Although the separation of plastics in dense medium mainly depends on their density, other physical properties of particles such as size, shape, texture and hydrophobicity grade also influence the behavior of plastics in a medium.

Although the plastics have been previously washed in order to avoid the formation of air bubbles, during the separation tests there is the possibility of air microbubbles attach to the plastic particles, which may affect the results.

The possibility of attaching air microbubbles to plastic particles depends on the hydrophobicity grade of the plastics. In the plastics that are more hydrophobic, i.e. with a higher contact angle, the probability of attachment of the air microbubbles to their surface is greater[10]. Contact angle indicates the degree of wetting when a solid and a liquid interact. If the contact angle is very small, then the air bubbles do not attach to the particle surface, while a very large contact angle results in a very strong bubble attachment to the particle. Previous study[14] showed that these six plastics are naturally hydrophobic, and that the contact angle of the six plastics is as follows: PS - 97º, PMMA - 77º, PET-S - 85º, PET-D - 73º, PVC-M - 85º, and PVC-D - 92º[14]. Thus, since PMMA and PVC-D have similar density, the greater floated recovery of PVC-D than PMMA, may be a consequence of its higher contact angle, as the probability of attachment of air microbubbles is greater. Also PET-S and PET-D have similar density and therefore, the greater recovery in sunk of PET-D may be a consequence of its lower contact angle, thus being less likely for air microbubbles to attached to the PET-D particles, sinking them more easily than PET-S. If air microbubbles attach to the plastic particles, they can float in medium with a density lower than the density of plastics. For example, for PMMA, whose density is 1,204 g/cm3, for fraction +1.4-2.0 mm, when density of sodium chloride solution is 1,185 g/cm3, the floated recovery Polímeros, 33(3), e20230025, 2023

was 48.8%. It was expected that the floated recovery would be none, that is, the entire PMMA would sink, as the medium density is lower than plastic density. These behavior is a result of air microbubbles attaching to plastic particles.

The shape of the particles does not seem to influence their behavior in the dense medium. It would be expected that particles with a more lamellar shape might have a greater tendency to float. The plastic with more lamellar shaped particles is PET-D, however it was the one with the highest recovery in the sunken.

3.2 Separation of bi-component mixtures of plastics by sink-float 3.2.1 Mixtures of PS with other plastics According to the floated recovery of plastics, it is possible to separate the six plastics into three groups: the first group constituted only by PS, that has the highest floated recovery (low density); a second group that includes PMMA and PVC-D that have intermediate floatability (intermediate density), and the third group that includes PVC-M, PET-S and PET-D that have lowest floatability (high density).

In face of these results, further separation tests were developed using bi-component plastic mixtures of PS and other plastic, in equal proportions, for two size fractions (+1.4-2.0 mm and +2.8-4.0 mm), with the intention to obtain a sunk without PS and a floated of PS. For the separation of PS/PMMA and PS/PVC-D mixture was used sodium chloride water solution with a density of 1.08 g/cm3, and for the separation of PS/PET-S, PS/PET-D and PS/PVC-M mixtures was used sodium chloride water solution with a density of 1.12 g/cm3. Thus, these densities led to the most efficiente separation of bi-component plastic mixtures of PS and other plastic. For that range of densities, ammonium nitrate solution could also have been used. It was decided to use the sodium chloride solution because it is cheaper and it has a low environmental impact[26].

It can be stated that sink-floatseparation of PS from PMMA, PET-S, PET-D or PVC-M was efficient (Table 2).

For these four mixtures the separation efficiency was perfect (100%), since all the PS floated and all the other plastics sunk. The influence of the particles size in the separation quality of the four mixtures is not evident.

On the other side, PS/PVC-D had the worst separation efficiency, with separation efficiency close to 85%, because about 15% of PVC-D plastic was recovered in the floated.

These results were consistent with the floatability of plastics observed in the mono-component tests (Figures 2 and 3).

The separation of PS/PVC-D mixture presented best results for the coarse size fraction (+2.8-4.0 mm) because PVC-D recovery in the floated decreased with the increase in particle size.

3.2.2 Mixtures of PMMA with other plastics In order to separate PMMA from PET-S, PET-D and PVC-M mixtures into individual polymers, an ammonium nitrate water solution with density of 1.23 g/cm3 was used (Table 3). This was the density that allowed the greatest separation efficiency of those three plastic mixtures. In this separation, sodium chloride solution was not used because it does not allow obtaining a density greater than 1.203 g/cm3.

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Pita, F. For the three mixtures all the PMMA floated because its density is lower than the density of the medium.

The two size fractions of PET-D and coarser fraction of PET-S and PVC-M completely sink. So, for these mixtures, the separation was perfect, with a separation efficiency of 100%, resulting in a pure sunk and a pure floated.

Although the PMMA/PET-S and PMMA/PVC-M mixtures have led to lesser results for the fine fraction (+1.4-2.0 mm), the separation efficiency is high (about 98%).

3.2.3 Mixtures of PVC-D with other plastics To separate PVC-D from PET-S, PET-D and PVC-M mixtures into individual polymers, also an ammonium nitrate water solution with density of 1.23 g/cm3 was used. The separation of the three mixtures had similar results (Table 4).

These mixtures present similar results to the mixtures of PMMA with the other three plastics. For all mixtures the floated recovery of PVC-D was 100%. The two size fractions of PET-D and coarser fraction of PET-S and PVC-M completely sank.

Table 2. Recovery and grade of the floated (concentrated of PS) and sunk products, in the separation of bi-component mixtures of PS with other plastics by sodium chloride water solution.

Floated (overflow)

100 100

PS Recovery (%) 100 100

Sunk (underflow) PMMA PMMA Grade (%) Recovery (%) 100 100 100 100

+1.4-2.0 +2.8-4.0

PS Grade (%) 100 100

PS Recovery (%) 100 100

PET-S Grade (%) 100 100

PET-S Recovery (%) 100 100

+1.4-2.0 +2.8-4.0

PS Grade (%) 100 100

PS Recovery (%) 100 100

PET-D Grade (%) 100 100

PET-D Recovery (%) 100 100

+1.4-2.0 +2.8-4.0

PS Grade (%) 100 100

PS Recovery (%) 100 100

PVC-M Grade (%) 100 100

PVC-M Recovery (%) 100 100

+1.4-2.0 +2.8-4.0

PS Grade (%) 85.8 89.6

PS Recovery (%) 100 100

PVC-D Grade (%) 100 100

PVC-D Recovery (%) 83.5 88.4

Mixture

Size fraction (mm)

PS/PMMA

+1.4-2.0 +2.8-4.0

PS/PET-S

PS/PET-D

PS/PVC-M

PS/PVC-D

Grade (%)

Table 3. Recovery and grade of the floated and sunk products, in the separation of bi-component mixtures of PMMA with PET-S, PET-D and PVC-M by ammonium nitrate water solution.

+1.4-2.0 +2.8-4.0

Floated (overflow) PMMA PMMA Grade (%) Recovery (%) 98.5 100 100 100

Sunk (underflow) PET-S PET-S Grade (%) Recovery (%) 100 98.5 100 100.0

+1.4-2.0 +2.8-4.0

PMMA Grade (%) 100 100

PMMA Recovery (%) 100 100

PET-D Grade (%) 100 100

PET-D Recovery (%) 100 100

+1.4-2.0 +2.8-4

PMMA Grade (%) 97.7 100

PMMA Recovery (%) 100 100

PVC-M Grade (%) 100 100

PVC-M Recovery (%) 97.6 100

Mixture

Size fraction (mm)

PMMA/PET-S

PMMA/PET-D

PMMA/PVC-M

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Separation of plastic mixtures by sink-float combined with froth flotation For these mixtures, the separation was perfect, with a separation efficiency of 100%. PVC-D/PET-S and PVC-D/ PVC-M mixtures have led to worst results for the fine fraction, but the separation efficiency is high (about 98%).

3.2.4 Mixtures of PVC-M with other plastics In order to separate PVC-M from PET-S and PET-D mixtures into individual polymers an ammonium nitrate water solution with density of 1.33 g/cm3 was used (Table 5). This density led to the most efficient separation of bi-component plastic mixtures of PVC-M/PET-S and PVC-M/PET-D. The optimal density of the medium should be only slightly higher than the density of the lighter plastic (PVC-M - 1.326 g/cm3). Separation tests on medium with density of 1.34 and 1.35 g/cm3 led to poorer quality separations. However, Fu et al.[19] observed the opposite, concluding that the optimum value of the medium’s density lies near that of the denser particles.

For the two mixtures the quality of separation improved slightly with the increase of the particles size. While the PVC-M recovery in the floated had decreased slightly with the increase of the particles size, the PET-S and PET-D recovery in the sunk had increased significantly with the increase of the particles size and therefore, the best separations were achieved for coarse fraction (+2.8-4.0 mm). PVC-M/PET-D

mixture with size +2.8-4.0 mm had the greatest separation efficiency (77.4%), having be obtained a floated with a grade of 88.8% in PVC-M and a sunk with a grade of 88.8% in PET-D. On the other side, size fraction of +1.4-2.0 mm of PVC-M/PET-S mixture had the lowest separation efficiency, of about 46%, with a PVC-M grade in the floated of 66.5% and a PET-S grade in the sunk of 87.7%.

In a gravity separation of two materials, the separation efficiency must be larger when the density difference is greater. Thus, based on plastics density (Table 1), it would be expected that the PVC-M/PET-S and PVC-M/PET-D mixtures had similar results because PET-S and PET-D have similar density. Surprisingly, PVC-M/PET-D mixture showed better results than PVC-M/PET-S mixture. Therefore, these results cannot be explained by the density difference. The best results for PVC-M/PET-D mixture could be explained by the lower contact angle of the PET-D, thus being less likely for air microbubbles to attached to the PET-D particles, sinking them more easily than PET-S. 3.2.5 Separation of PMMA/PVC-D mixture by froth flotation Since PMMA and PVC-D have similar density, separation of the mixture of these two plastics was not possible by gravity method. So, to separate PMMA/PVC-D mixture, selective froth flotation tests had been carried out.

Table 4. Recovery and grade of the floated and sunk products, in the separation of bi-component mixtures of PVC-D with PET-S, PET-D and PVC-M by ammonium nitrate water solution.

+1.4-2.0 +2.8-4.0

Floated (overflow) PVC-D PVC-D Grade (%) Recovery (%) 98.3 100 100 100

Sunk (underflow) PET-S PET-S Grade (%) Recovery (%) 100 98.3 100 100

+1.4-2.0 +2.8-4.0

PVC-D Grade (%) 100 100

PVC-D Recovery (%) 100 100

PET-D Grade (%) 100 100

PET-D Recovery (%) 100 100

+1.4-2.0 +2.8-4.0

PVC-D Grade (%) 97.6 100

PVC-D Recovery (%) 100 100

PVC-M Grade (%) 100 100

PVC-M Recovery (%) 97.5 100

Mixture

Size fraction (mm)

PVC-D/PET-S

PVC-D/PET-D

PVC-D/PVC-M

Table 5. Recovery and grade of the floated and sunk products in the separation of bi-component mixtures of PVC-M with PET-S and PET-D by ammonium nitrate water solution.

+1.4-2.0 +2.8-4.0

Floated (overflow) PVC-M PVC-M Grade (%) Recovery (%) 66.5 92.5 75.9 89.3

Sunk (underflow) PET-S PET-S Grade (%) Recovery (%) 87.7 53.3 87.0 71.6

+1.4-2.0 +2.8-4.0

PVC-M Grade (%) 73.3 88.8

PET-D Grade (%) 89.5 88.6

Mixture

Size fraction (mm)

PVC-M/PET-S

PVC-M/PET-D

Polímeros, 33(3), e20230025, 2023

PVC-M Recovery (%) 92.2 88.6

PET-D Recovery (%) 66.4 88.8

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Pita, F. Table 6. Results of the flotation tests on the mixture of PMMA/PVC-D in the presence of sodium lignosulfonate.

Mixture

Size fraction (mm)

PMMA/PVC-D

+1.4-2.0 +2.8-4.0

Floated (overflow) PMMA PMMA Grade (%) Recovery (%) 94.5 96.2 92.8 82.5

Sunk (underflow) PVC-D PVC-D Grade (%) Recovery (%) 96.1 94.4 84.2 93.3

Figure 4. Flowchart of separation of plastic mixture (PS+PMMA+PVC-D+PVC-M+PET+D+PET-S) by sink-float and by froth flotation.

Plastics are naturally floatable in the absence of a wetting agent.

Thus, in order to separate plastic mixtures by froth flotation, several wetting agents had been tested for the selective flotation of plastic mixtures[28-31]. Pita and Castilho[32] verified that floatability of PMMA and PVC-D decreases with an increase of the sodium lignosulfonate concentration. However, they also verified that in presence of this wetting agent, PVC-D has lower floatability than PMMA. So, in the presence of sodium lignosulfonate, flotation tests were developed using bi-component plastic mixtures of PMMA/PVC-D in equal proportions, to obtain a selective separation.

The ideal concentration of sodium lignosulfonate that led to the most efficient separation of PMMA/PVC-D mixture was selected from previous tests. It was verified that the concentration of 1000 mg/L led to the most efficient separation for the fine fraction (+1.4-2.0 mm), and the concentration of 80 mg/L led to the most efficient separation for the coarse fraction (+2.8-4.0 mm). The experimental results of these separation tests are presented in Table 6. The quality of the flotation separation of PMMA/PVC-D mixture was reasonable.

The separation was more efficient for the fine fraction because there was a greater amount of PMMA recovered in the floated. For this fraction, the separation efficiency was 90.6%, having obtained a floated with a grade of 94.5% in PMMA and a sink with a grade of 96.1% in PVC-D. For the coarse fraction the separation efficiency was only 75.8%.

4. Conclusions In this study the separation of mixed post-consumer plastic (PS, PMMA,VC-D, PVC-M, PET-D and PET-S) by combination of sink–float method and froth flotation was investigated. In sink-float method two mediums of separation were used: sodium chloride water solution and ammonium 8/10

nitrate water solution. In froth flotation sodium lignosulfonate was used as selective wetting agent of PVC-D. The success of the sink-float separation of plastic mixtures depends on the density difference of the plastics and on the density of the separation medium. The application of the sink-float method does not require very different densities between the plastics of the mixture; even for plastic mixtures that have a small density difference, the sink-float method can lead to good quality separations. The optimal density of the medium should only be slightly higher than the density of the lighter plastic. The quality of separation also depends on the particles size and their hydrophobicity (contact angle).

In sink-float method floated recovery of the six plastics decrease with the increase of the medium density and decrease with the increase of particle size. In some mixtures, separation improved with increasing particle size. Separation is better when the floating plastic has a greater contact angle than the sinking plastic. The hydrophobicity of the particles influences plastic behavior in the sink-float method. The possibility of attaching air microbubbles to plastic particles during the separation depends on the hydrophobicity grade of the plastics, the attachment of air microbubbles is smaller for less hydrophobic plastics, that is, have a smaller contact angle, such as PET-D, which has the smallest contact angle among the studied plastics.

In face of the experimental results of the separation of plastic mixtures by sink-float and by froth flotation, it can be assumed that it is possible to separate the mixture of the six plastics (PS, PMMA, PVC-D, PVC-M, PET-D,PET-S) using density separation combined with froth flotation, obtaining almost clean products of PS, PMMA, PET and PVC. Figure 4 shows the sequence of separation operations to which the mixture of six plastics must be submitted and the respective products obtained.

Polímeros, 33(3), e20230025, 2023

Separation of plastic mixtures by sink-float combined with froth flotation For mixture of plastics that join a less density plastic (PS) with intermediate density plastics (PMMA and PVC-D) and with high density plastics (PVC-M, PET-D and PET-S), the separation efficiency of the sink-float method was close to 100%. Only some PVC-D floats together with the PS. Sink-float separation also allowed perfect separation of intermediate density plastics (PMMA and PVC-D) from high density plastics (PVC-M, PET-D and PET-S); all PMMA and PVC-D floats. Separation of PVC-M from PET-D and PET-S by sink-float method led to a reasonable quality of separation, since no clean products were obtained.

The quality of separation improved slightly with the increase of the particles size.

Since it was not possible to separate successfully the PMMA/PVC-D mixture by sink-float method, this separation was carried out by froth flotation, which led to a separation efficiency of 90% for size fraction of +1.4.4-2 mm and of 76% for size fraction of +2.8-4 mm.

5. Acknowledgements This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding, with UIDB/00073/2020 project.

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Laboratory methods for the analysis of microplastics in the marine environment: recommendations for quantifying synthetic particles in waters and sediments. Silver Spring: National Oceanic and Atmospheric Administration. Retrieved in 2023, April 4, from http://hdl.handle.net/11329/1076 26. Quinn, B., Murphy, F., & Ewins, C. (2017). Validation of density separation for the rapid recovery of microplastics from sediment. Analytical Methods, 9(9), 1491-1498. http://dx.doi.org/10.1039/C6AY02542K. 27. Schulz, N. F. (1970). Separation efficiency. Transactions of the Society for Mining, Metallurgy, and Exploration, Inc., 247, 81-87.

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Optimization of froth flotation procedure for poly (ethylene terephthalate) recycling industry. Polymer Engineering and Science, 52(1), 157-164. http://dx.doi.org/10.1002/pen.22058.

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32. Pita, F., & Castilho, A. (2019). Plastics floatability: effect of saponin and sodium lignosulfonate as wetting agents. Polímeros: Ciência e Tecnologia, 29(3), e2019035. http://dx.doi.org/10.1590/01041428.01419.

Received: Oct. 06, 2022 Revised: Mar. 13, 2023 Accepted: Apr. 04, 2023

Polímeros, 33(3), e20230025, 2023

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.20220108

Sustainable composites of eco-friendly polyethylene reinforced with eggshells and bio-calcium carbonate Kássia Peçanha Vieira1 , Alexandra Augusta Reichert1 , Gabriel Monteiro Cholant1 , Dielen Marin2 , Cesar Augusto Gonçalves Beatrice3  and Amanda Dantas de Oliveira1*  Laboratório de Materiais Compósitos, Programa de Pós-graduação em Ciência e Engenharia de Materiais, Centro de Desenvolvimento Tecnológico, Universidade Federal de Pelotas, Pelotas, RS, Brasil 2 Laboratório de Desenvolvimento de Processos, Programa de Pós-graduação em Engenharia Química, Departamento de Engenharia Química, Universidade Regional de Blumenau, Blumenau, SC, Brasil 3 Laboratório de Núcleo de Reologia e Processamento de Polímeros, Departamento de Engenharia de Materiais, Universidade Federal de São Carlos, São Carlos, SP, Brasil 1

*adoliveira@ufpel.edu.br

Abstract This work aimed to obtain and analyze composite materials made from green polyethylene and calcium carbonate extracted from chicken eggshells. The shells were collected and prepared for later extraction of calcium carbonate.

With the X-ray diffraction analysis it was possible to confirm that both the in natura reinforcement and the calcined one, have the same polymorph mineral (calcite). With regard to thermal behavior, compounds with in natura reinforcement showed greater mass loss due to the moisture contained in them. The results showed a significant increase in Young’s moduli of the composites compared to the pure polymer. The scanning electron microscopy showed good dispersion and adhesion between the reinforcement materials and matrix. In view of the results, eggshells have the potential to be used as fillers, where greater rigidity is required. Since these materials come from a waste material, with low cost, their use becomes even more viable.

Keywords: biopolymers, calcium carbonate, composites, eggshells, polyethylene.

How to cite: Vieira, K. P., Reichert, A. A., Cholant, G. M., Marin, D., Beatrice, C. A. G., & Oliveira, A. D. (2023). Sustainable composites of eco-friendly polyethylene reinforced with eggshells and bio-calcium carbonate.

Polímeros: Ciência e Tecnologia, 33(3), e20230026. https://doi.org/10.1590/0104-1428.20220108

1. Introduction A Brazilian petrochemical company created the green polyethylene named “I’m green” to develop a sustainable product from renewable raw materials. Unlike other polymers extracted from fossil fuel sources, such as oil and natural gas, low density green polyethylene (LDGPE) is obtained from sugarcane-derived ethanol. In this way, this product reduces the emission of greenhouse gases while keeping the same properties and performance as conventional polyethylene[1].

Brazil has kilometers of land with sugarcane plantations, thus making the use of a product from this renewable source more viable. An interesting fact in contributing to the development of a network with a more sustainable profile, such as green polyethylene, is that in its production chain it contributes to a cleaner atmosphere, since sugarcane plantations carry out photosynthesis, and thus absorb carbon dioxide from the atmosphere.

The absorption of carbon dioxide (CO2) by green polyethylene occurs in sugarcane plantations that photosynthesize and capture CO2 from the atmosphere, making green polyethylene a “carbon sequestrant”. This process can remove around three tons of CO2 from the

Polímeros, 33(3), e20230026, 2023

atmosphere for each ton of green polyethylene (PE) produced, for conventional polyethylene the opposite is true, its production process releases two tons of CO2 into the atmosphere[2].

From an economic point of view, the cost of production and the value of the green product is higher than conventional polyethylene. This difference can reach 30% for each kilogram of green polyethylene[3]. Several reasons explain this difference such as the need for a vast land area for planting sugarcane, adequate weather conditions, increased water consumption, equipment, capital, and direct competition with ethanol production based on sugarcane[2].

According to the Brazilian Association of Animal Protein (ABPA)[4], Brazil consumes many eggs, consumption of eggs by each Brazilian in 2021 was 257 units, this consumption generates a considerable amount of waste[4]. Eggshell is a porous ceramic material that, despite being light, has high strength[5]. Eggshell is rich in mineral salts, and its main constituent is calcium carbonate (CaCO3), around 94%.

The organic part of the egg (6%) includes internal and external membranes composed of collagen types I, V and X and glycosaminoglycans[6,7].

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

Vieira, K. P., Reichert, A. A., Cholant, G. M., Marin, D., Beatrice, C. A. G., & Oliveira, A. D. Due to the fact that eggshells are highly deposited in the environment, there is a need to explore more applications for their use. Therefore, this research encompassed two materials that are not usually combined because the eggshell, when used in composites, acts as a filling filler and not as the main filler of the composite. Thus, one of the differentiating points of this study is the use of eggshells as the main load.

Furthermore, another interesting novelty of this study is the comparison between different types of reinforcement material.

Calcium carbonate has been applied as a filler in some noteworthy composites. It can be prepared as nanoparticles, optimizing the homogenization between the matrix and the dispersed phase. Charde et al.[8] added calcium carbonate (CaCO3) to a polycarbonate matrix and improved the tribological properties of the composite. Hafez et al.[7] produced a biofilm combining nanocellulose and calcium carbonate.

The CaCO3 increased the thermal stability and retarded the thermal degradation of the cellulose at high temperatures.

Currently, there are numerous works considering that eggshell reinforces composite matrices. But, regarding its use and comparison to the calcium carbonate, obtained through eggshell there is a lack of research and information on the subject. In this way, this comparative study developed composites reinforces with eggshells and calcium carbonate extracted from them. Thus, it is possible to understand better the influence of organic matter present in the eggshell and to indicate possible fields of application for both composites.

Following the above and to promote sustainable development, this work aimed to obtain, characterize, and analyze composite materials made from green polyethylene reinforced with chicken eggshell particles (in natura) and biologically derived calcium carbonate (bio-CaCO3) extracted from the eggshells. The extraction of calcium carbonate was done by the method of calcination of egg shells. At the end of the experimental development, the reinforcements and composites were characterized for the investigation of their microstructural, elemental, thermal and mechanical properties.

2. Experimental 2.1 Materials The polymeric matrix used was a low-density green polyethylene (LDGPE) from Braskem (I’m greenTM),

which is commercially known as SLD4004 resin. The chicken eggshells used as reinforcement material were donated by a bakery of the city of Pelotas-RS (Brazil). Finally, polyethylene grafted with maleic anhydride (PE-g-MA) was provided by Crystal Master and used as compatibilizing agent.

2.2 Preparation of in natura eggshell powder First, the eggshells were collected, cleaned and washed in running water to remove dirt and dried at 100°C for 24h in a Quimis oven, model Q317M-52. After drying, the peels were ground in a Marconi knife mill, MA model.

After that, the peels went through a second milling. For this went through a second milling using a IKA A11 Basic analytical mill. Thus, a fine powder was obtained, which was sieved through a Tyler 200 mesh to obtain a better granulometry.

2.3 Extraction of calcium bio-carbonate The extraction of bio-CaCO3 from the egg shells takes place by removing the organic matter from the shell.

For this, the powder obtained by grinding the eggshells underwent a calcination heat treatment for 2h at 600°C in a Quimis muffle, model Q-318. This simple and affordable procedure removes organic matter, because with the temperature used in the calcination process, the organic matter in the eggshell is degraded and the moisture present in it evaporates, thus leaving only CaCO3. Figure 1 shows the powders obtained.

2.4 Production of the composites A Quimis oven, model Q317M-52 was used to dry the polymer matrix and reinforcement materials (in natura and bio-CaCO3) for 24h at 60°C before being processed.

After drying, the pellets and reinforcement materials were manually mixed in a glass container to obtain better mixing homogeneity. Table 1 presents the mixture formulations.

Then, a single screw extruder from Eco Soluções with a L/D of 20 was used to process and mix the material in the molten state. The pure polymer (unfilled) was also processed following the same conditions used for the composites.

The temperature profile used to process the materials was 140°C/145°C/150°C. The die temperature was set at 90°C and the screw speed at 60 rpm.

Figure 1. (a) ground eggshell; (b) in natura powder; and (c) the bio-CaCO3.

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Polímeros, 33(3), e20230026, 2023

Sustainable composites of eco-friendly polyethylene reinforced with eggshells and bio-calcium carbonate An amount of 5% by weight of each reinforcing material was incorporated into the LDGPE. Formulations were made with and without the compatibilizing agent for a better analysis of the results. A 3 wt% concentration by weight of PE-g-MA was used for the formulations that considered it.

The extruded material was granulated to obtain pellets and stored for subsequent injection molding. Table 1 shows the specific formulations.

A mini bench injector, AX Plastics brand, model AXINJET, performed the injection molding. The specimens followed the dimensions presented in ASTM D638 (Type IV) and ASTM D 256A for tensile and impact tests, respectively. The processing conditions used were an injection temperature of 190°C and a mold temperature of 50°C. Eight specimens of each composition were selected for each mechanical characterization. The specimens were measured with a Starret digital caliper, model EC799. In addition, were weighed on a Shimadzu precision scale, model UX8200S.

2.5 Characterization of in natura eggshell powder and bio-CaCO3 particles The powders were characterized by scanning electron microscopy (SEM) using a Tescan VEGA3 to evaluate their morphology. SEM analysis was used to visualize the geometry of the reinforcing particles as well as particle size uniformity.

The powders obtained from eggshells were also characterized

by X-ray diffraction (XRD) to determine the crystal structure and the degree of crystallinity. A Bruker D8 ADVANCE diffractometer was used. Analyses were performed at a wavelength (λ) of 1.541 Å, operating at 40 kV and 40 mA, using a scan speed of 1°/min, for 2θ between 10° and 90°.

The powders were also characterized by Fourier transform infrared (FTIR) spectroscopy to evaluate their functional groups and bonds. A Shimadzu Prestige-21 was used. The analyses were performed in duplicate in the 400 to 4000 cm-1 wavenumber range. An equipment from the brand Shimadzu, model Prestige-21 performed this analysis.

The analysis of the thermal stability of the powders was performed by thermogravimetry using a TA Instruments TG Q500, with a heating rate of 10°C/min, from room temperature to 800°C, under a nitrogen atmosphere.

2.6 Characterization of the polymer matrix and composites The morphology of the composites was analyzed using SEM under the same conditions and equipment described above. SEM images were obtained from the fractured surface of the specimens after tensile testing.

TGA was performed to evaluate the thermal stability of the composites using a TA Instruments TG Q500 with a heating rate of 10°C/min, from room temperature to 800°C, under an argon atmosphere.

Differential scanning calorimetry (DSC) characterized the melting temperature (Tm), crystallization temperature (Tc), and the degree of crystallinity of the pure polymer and the composites. The samples were initially heated from room temperature to 190°C at a heating rate of 10°C/min and kept at this temperature for 5 min. Then, the samples were cooled to 30°C at a rate of 10°C/min and heated again to 190°C at a rate of 10°C/min, following the ASTM D3418-15 standard.

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Crystallization and melting thermograms were recorded from the first cooling and second heating cycles. For this analysis, a TA Instruments Q2000 was used, with nitrogen as the carrier gas, at a constant flow of 50 mL/min.

From the DSC analysis, the degree of crystallinity of the matrix and composites follows the Equation 1:

Xc(%) =

∆ Hf 1 × ×100% ´ ∆H * f W

(1)

Where Xc is the degree of crystallinity (%), ∆Hf is the enthalpy of crystal fusion (J/g), ∆H*f is the enthalpy of fusion of a 100% crystalline sample (the theoretical ∆H*f value used for LDGPE was 293 J/g[9]); and W is the amount of polymer (wt%).

One of the mechanical tests used was an Izod-type impact test. The notched specimens were measured using a Ceast pendulum impact instrument, Impactor model, coupled to a DAS 4000 data acquisition system via software. The injection-molded specimens were notched in a Ceast notching machine, to a depth of 2.54 ± 0.1mm, with a minimum notching speed. According to ASTM D256A standard, the depth of the notch was checked with an appropriate micrometer when performing the analysis.

A uniaxial tensile test was conducted to investigate the mechanical properties of the composites. The tensile tests were carried out following the ASTM D638 Type IV standard in an Instron testing system, model 5569, at 5 mm/min.

A 50 kN load cell was used and the specimens were deformed until rupture. It was not possible to use the strain gauge to measure the elastic strain due to the size of the specimens.

Eight samples of each formulation were analyzed for each mechanical test.

3. Results and Discussion 3.1 In natura eggshell powder and bio-CaCO3 particles Figure 2 shows the micrographs of the in natura and bio-CaCO3 powders, which are irregularly shaped and have smooth surfaces. However, the uniformity of the particle size was not achieved. Ivanović et al.[10] obtained eggshell particles with geometry and morphology similar to those presented in this study.

Table 1. Formulations of pellets obtained by the extrusion process.

Formulation of pellets (wt%) Pure LDGPE 100 95/5 LDGPE/in natura LDGPE/bio-CaCO3 95/5 92/5/3 LDGPE/in natura/PE-g-MA LDGPE/bio-CaCO3/PE-g-MA 92/5/3 Tm is the crystalline melting temperature measured in the second heating cycle; ∆Hm is the enthalpy of crystal melting measured on the second heating; Tc is the crystallization temperature measured in the cooling cycle; ∆Hc is the enthalpy of crystallization measured in the cooling cycle; and Xc is the degree of crystallinity calculated in the second heating cycle.

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Vieira, K. P., Reichert, A. A., Cholant, G. M., Marin, D., Beatrice, C. A. G., & Oliveira, A. D. As expected, bio-CaCO3 had smaller particles than the in natura powder. This reduction in particle size is a result of the calcination process. A smaller particle size gives bio-CaCO3 increased surface area compared to the fresh powder. In this case, large surface area values guarantee greater viability for the reinforcement. Large surface area usually has better adhesion to the polymer matrix[9].

Figure 3 shows the X-ray spectra of the powders.

It can be seen that the peaks are well defined, which can be explained by the ordering of the atoms when forming the crystalline structure of the particles, as Callister and Rethwisch[11] comment. The peak of greatest intensity occurs at approximately 29.5° (2θ). This aspect indicates that the in natura powder and the bio-CaCO3 contain calcite as the main constituent phase, identified from the rhombohedral structure, corroborating Nawar et al.[12] and Kareem and Naji[13].

There is no difference between the diffractograms presented, due to the fact that the two reinforcement materials have the same constituent phase, (calcite), that is, the same crystalline arrangement of the atoms that form the particle.

Figure 4 presents the FTIR spectra of the reinforcement particles produced. In both spectra of Figure 4 it is possible to observe broad band at 1415 cm−1, which is associated with the C-O bond. Other two bands, at 711 and 875 cm−1, also occur due to the C-O bond. The fresh powder and bio-CaCO3 presented similar bands, except for the very weak band at 2360 cm−1. This weak band happened due to the N-H bond and results from the amines and amides present in the organic part of the eggshell. These results agree with previous studies published by Rezk et al.[14] and Hossain et al.[15].

Figure 2. SEM micrographs of in natura and bio-CaCO3 powders (200×, 500×, and 1000× magnification).

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Polímeros, 33(3), e20230026, 2023

Sustainable composites of eco-friendly polyethylene reinforced with eggshells and bio-calcium carbonate Figure 5 presents the TG/DTG (derivative thermogravimetry) curves of the thermal decomposition analysis of in natura and bio-CaCO3 powders. In the TG/DTG curves in Figure 5, it is possible to observe three stages of weight loss in the in natura powder. The first stage between 0 and 100°C, due to the loss of moisture from the material. A more accentuated second stage that occurred between 200 and 400°C. This stage is a result of the decomposition of organic matter.

The third stage is at 750°C. This characteristic temperature range of the thermal decomposition of calcium carbonate (CaCO3), releasing carbon dioxide (CO2) and transforming into calcium oxide (CaO). The bio-CaCO3 particles, as do not contain an organic part and water, did not present the same first stages as the in natura particles. Bio-CaCO3 particles showed only one stage of weight loss (750°C).

This stage is a consequence of the conversion of CaCO3 into CaO. Similar behavior have been reported by Nath et al.[16] and Razali et al.[17].

3.2 Composite materials

Figure 3. X-ray spectra of in natura and bio-CaCO3 particles.

Figure 4. FTIR spectra of the particles of interest (in natura and bio-CaCO3).

The specimens of LDGPE and composites were subjected to an Izod impact test using an 11J pendulum. None of the specimens broke (Figure 6). Therefore, the impact resistance values ​​were not quantified. Although the composites did not allow the collection of numerical impact strength data, from the analysis of SEM images below, one can observe that the reinforcement and matrix particles had a satisfactory adhesion to each other.

With regard to tensile strength, there is little variation in behavior when comparing the matrix to the composites produced. Behera et al.[18] introduced eggshell particles into epoxy resin and, likewise, did not obtain a differential in tensile strength properties. However, the flexural strength properties showed improvement.

The composites should have considerable impact resistance since good adhesion between matrix and reinforcement favors its improvement. The use of eggshells and calcium carbonate as reinforcement might have the same effect, as reported by Oladele et al.[19] where the mechanical properties were improved for the two types of reinforcement used in an epoxy matrix. Through the SEM analysis, the calcined particulate had a smaller particle size when compared to the in natura particulate.

The particle size of the reinforcement materials is another factor that can influence the impact strength of composites.

Sosiati et al.[20] have reported that composites reinforced with smaller particles have better impact resistance since this materials have a high surface area., This surface area allows a better adhesion between the matrix and the reinforcement.

Figure 5. (a) TG; and (b) DTG curves of in natura and bio-CaCO3 powders.

Polímeros, 33(3), e20230026, 2023

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Vieira, K. P., Reichert, A. A., Cholant, G. M., Marin, D., Beatrice, C. A. G., & Oliveira, A. D. Table 2 shows the results obtained by the uniaxial tensile test. Among the properties analyzed, Young’s modulus presented a significative difference when one compares the composite with the pure polymer. The best properties were observed in LDGPE/in natura composite (an increase of approximately 124%). This increase can be explained by the reinforcement particles being well incorporated into the matrix, which is somehow linked to the geometry and size of the particles, facilitating the interaction with the matrix and not forming agglomerates.

Similar results have been reported by Gbadeyan et al.[21], which improved the Young’s modulus with the addition of eggshells at 5 and 10 wt% in an epoxy matrix. Mustapha et al.[22] used eggshells and calcium carbonate extracted from them as reinforcement materials in a polypropylene matrix. As in the study mentioned above, the Young’s modulus of the composite increased by incorporating the reinforcement material.

The elongation at break decreased by incorporating the reinforcing materials into the polymer matrix, as reported by Williams et al.[23]. This indicates that the composites are less ductile than the pure polymer. Composites do not suffer great deformation until reach rupture. Thus, break more easily.

In composites with the compatibilizing agent, there was no significant improvement in the mechanical results compared to composites without it. This might have happened because a small amount of compatibilizing agent was used.

Perhaps, there were insufficient functional groups to improve the mechanical properties.

In the study presented by Brząkalski et al.[24], the results of the mechanical properties of LDPE obtained from extrusion were 19 MPa for tensile strength and 213 MPa for Young’s modulus. In their research, Liew et al.[25] found the value of 15 MPa for the tensile strength of LDPE. The mentioned values are in agreement with the results of mechanical properties found for LDGPE in this study.

Figure 7 shows the SEM micrographs of the composite materials. The images were obtained from the ruptured area of the specimens. From the images, one can visualize that the reinforcement materials presented a well-dispersed distribution in the matrix. Besides, the presence of large agglomerates was not observed. Despite the voids at the interface, polymer–particle interactions are strong, resulting in good adhesion between the phases of the composite material. The composites do not suffer great deformation until they reach to rupture, breaking more easily.

Particle size can influence the adhesion between the matrix and the reinforcing phase. Based on the micrographs of the composites, the smaller particles presented better incorporation into the matrix. Thus, controlling the particle size can be used to improve adhesion. With the use of the

compatibilizing agent, one can observe an improvement in the covering of the reinforcement particles by the matrix.

In other words, the agent partially improved the adhesion between the matrix and the reinforcement particles. Wu et al.[26] obtained particles of uniform size which resulted in achieving good dispersion of the reinforcement in the matrix of the films. The analysis of the properties showed that Young’s modulus and tensile strength increased as the eggshell content increased. Sosiati et al.[20] reports that composites reinforced with smaller particles have better adhesion with the composite matrix. According to the author, this is due to the fact that the size of the particles, as are smaller, have a greater surface area, enabling good adhesion.

Figure 8 shows the TG/DTG curves of the polymeric matrix and the composites. These thermograms have only one stage of weight loss. Thus, the thermal degradation process of LDGPE occurs in a single step. The only stage of weight loss was at approximately 475°C for all samples, in agreement with Hajinezhad et al.[27], Duque et al.[28] and Saber et al.[29] in similar studies using LDPE. However, each formulation had a different amount of weight loss. Pure LDGPE lost around 78% of its mass, as did the composite LDGPE/bio-CaCO3/PE-g-MA. The composites reinforced with in natura particles had very similar weight losses (88–90%). These formulations showed the largest decreases in mass, probably due to organic matter present in them. For this reason, the in natura particles are not very stable at high temperatures. The composite that showed the lowest weight loss (~70 wt%) was LDGPE/bio-CaCO3. This can be explained by the reinforcement particles calcined at high temperatures, which provided greater thermal stability to the composite.

Figure 6. Specimens after impact test (no break) in (a) pure LDGPE; (b) LDGPE/in natura; (c) LDGPE/in natura/PE-g-MA; (d) LDGPE/bio-CaCO3; (e) LDGPE/bio-CaCO3/PE-g-MA.

Table 2. Mechanical properties of pure LDGPE and composites.

Pure LDGPE LDGPE/in natura LDGPE/ in natura/PE-g-MA LDGPE/bio-CaCO3 LDGPE/bio-CaCO3/PE-g-MA

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Young’s modulus (MPa) 242.4 ± 132.1 544.8 ± 150.3 494.3 ± 66.6 463.4 ± 24.4 458.8 ± 23.6

Tensile strength (MPa) 19.7 ± 1.2 20.1 ± 0.9 19.5 ± 1.2 19.1 ± 1.1 18.6 ± 0.6

Elongation at break (%) 33.7 ± 3.5 22.7 ± 2.9 23.7 ± 1.4 25.3 ± 2.2 24.1 ± 1.8

Polímeros, 33(3), e20230026, 2023

Sustainable composites of eco-friendly polyethylene reinforced with eggshells and bio-calcium carbonate

Figure 7. SEM micrographs of the composites at 200×, 500×, 1050×, and 2250× magnification.

Figure 8. (a) TG; and (b) DTG curves of the composites.

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Vieira, K. P., Reichert, A. A., Cholant, G. M., Marin, D., Beatrice, C. A. G., & Oliveira, A. D.

Figure 9. DSC curves obtained for the composites produced in (a) cooling; and (b) second heating.

Table 3. Thermal parameters obtained from the DSC analysis.

Tm (°C) ∆Hm (J/g) Tc (°C) ∆Hc (J/g) Xc (%)

Pure LDGPE

LDGPE/in natura

LDGPE/in natura/PE-g-MA

LDGPE/bio-CaCO3

LDGPE/bio-CaCO3/PE-g-MA

104 114 97 74 39

104 104 98 74 37

104 93 97 57 39

107 8 97 55 39

104 100 97 69 39

Figure 9 shows the DSC curves of the pure LDGPE and polymer composites samples during cooling and second heating. The endothermic melting peak of LDGPE and the composites appears at approximately 112°C (Figure 9B). This aspect is a characteristic value expected for this polymer.

All samples showed similar peak shapes, but with some variation in width and intensity. Similar behavior has been reported by Santos et al.[30].

Table 3 shows the thermal parameters obtained from the DSC analysis for the matrix and the composites. The obtained values show no significant variation in the Tg, Tm, and Tc parameters. However, there was a reduction in the enthalpy variation values due to the reinforcement particles that do not melt during the heating step.

Li et al.[31] and Saikrishnan et al.[32] have reported values of crystallinity for LDGPE and LDPE between 30 and 50%.

In this study, the pure LDGPE had 39% crystallinity.

The presence of main-chain branches explains this low value. These short “branches” did not fit into the crystal lattice, leading to a large disorder. The same happened in the study by Alkaron et al.[33], where he comments that with the addition of calcium carbonate there was a slight influence on the crystallinity of his composites, however with the reinforcement of eggshells the crystallinity suffered a decrease. The composites containing the compatibilizing agent had a slight increase in crystallinity, but the value did not outweigh pure LDGPE. Brząkalski et al.[24] processed the LDPE through a flat die extruder. The DSC results showed that its melting 8/10

temperature peak (Tm) was around 112°C and crystallization temperature peak (Tc) at 98°C. Zhan et al.[34] indicated that Tm can vary from 105-116°C for the LDPE. These values are similar to the ones found for LDGPE, which were 112°C for the endothermic melting peak, 104°C for Tm, and 97°C for Tc, as shown in Figure 9B and Table 3.

The crystallinity value found for LDGPE also corroborates studies in the literature that investigate the crystallinity of conventional LDPE, such as Yu et al.[35] and Li et al.[31] found around 36% and 38% for the crystallinity of conventional LDPE. In general, both in natura and bio-CaCO3 reinforcement particles did not strongly influence the thermal properties of the polymer. This can be explained by the low amount of reinforcement incorporated into the matrix.

4. Conclusion The thermal treatment by calcination proved to be a simple method to obtain calcium carbonate from eggshells (bio-CaCO3), as confirmed by XRD and FTIR analyses.

There was no significant difference when the reinforcement materials (in natura and bio-CaCO3) were compared. The organic matter and water present in the in natura particles did not decrease the properties of the composite. The fresh in natura powder proved to be as effective as the bio-CaCO3 powder. The low-density green polyethylene (LDGPE) showed properties and characteristics similar to conventional low-density polyethylene.

Polímeros, 33(3), e20230026, 2023

Sustainable composites of eco-friendly polyethylene reinforced with eggshells and bio-calcium carbonate The Young’s modulus and impact strength of the composites were the remarkable properties. The composites, when compared to each other, do not present a significant difference. However, when comparing the pure polymer to them, the Young’s modulus was improved with the incorporation of both reinforcements. It is observed that eggshell powders, in natura or calcined, act mainly on the stiffness and energy absorption during impact tests.

Some factors can be improved in this study to improve certain properties, such as tensile strength. Having a better control of the particle size, aiming to obtain particles of smaller and uniform size. It would be interesting to use a source of PE-g-AM that contains a higher maleic anhydride content or to use a different compatibilizing agent. Incorporate a greater amount of reinforcing particles into the polymeric matrix, with the aim of better understanding the properties that remained the same when comparing pure polymers and composites.

Given the excellent properties of stiffness and impact resistance, some applications for the materials purchased can be pointed out, such as parts for vehicles (windshield wipers, rear-view mirror box), electrical devices (sockets, switches), street lighting, police shield etc.

5. Author’s Contribution • • • • • •

• • • • • • • •

Conceptualization – Kássia Peçanha Vieira; Amanda Dantas de Oliveira.

Data curation – Kássia Peçanha Vieira.

Formal analysis – Kássia Peçanha Vieira.

Funding acquisition – Amanda Dantas de Oliveira.

Investigation – Kássia Peçanha Vieira; Alexandra Augusta Reichert.

Methodology – Kássia Peçanha Vieira; Alexandra Augusta Reichert; Dielen Marin; Gabriel Monteiro Cholant; Cesar Augusto Gonçalves Beatrice; Amanda Dantas de Oliviera.

Project administration – Kássia Peçanha Vieira; Amanda Dantas de Oliveira.

Resources – Amanda Dantas de Oliveira.

Software – NA. Supervision – Amanda Dantas de Oliveira.

Validation – Kássia Peçanha Vieira.

Visualization – Kássia Peçanha Vieira.

Writing – original draft – Kássia Peçanha Vieira.

Writing – review & editing – Kássia Peçanha Vieira.

6. Acknowledgements This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. The authors would like to acknowledge the Composites Materials Laboratory (LabCom) of the Federal University of Pelotas (UFPel), the Southern Electron Microscopy Center (CEME-Sul) of Federal University of Rio Grande (FURG), the Biomaterials Development and Control Center (CDC-Bio) also of UFPel, the Department of Materials Engineering (DEMa) of Polímeros, 33(3), e20230026, 2023

Federal University of São Carlos (UFSCar) and Department of Environmental Engineering of the Regional University of Blumenau (FURB).

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Received: Jan. 03, 2023 Revised: Jul. 27, 2023 Accepted: Aug. 03, 2023

Polímeros, 33(3), e20230026, 2023

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.20230033

Physical properties of Oriental beech impregnated and coated with some chemicals Hilmi Toker1 , Çağlar Altay2* , Ergün Baysal1 , İlknur Babahan Bircan3  and Hüseyin Peker4  Department of Wood Science and Technology, Faculty of Technology, Muğla Sıtkı Koçman University, Muğla, Turkey 2 Department of Interior Design, Aydın Vocational School, Aydın Adnan Menderes University, Aydın, Turkey 3 Department of Chemistry, Faculty of Arts and Sciences, Aydın Adnan Menderes University, Aydın, Turkey 4 Department of Forest Industrial Engineering, Faculty of Forestry, Artvin Çoruh University, Artvin, Turkey 1

*caglar.altay@adu.edu.tr

Abstract In this research, oven-dry density, air-dry density, and water absorption levels of Oriental beech treated with flame-resistant chemicals (FRC) and coated with polyurethane/polyure (PU) and epoxy (EP) were evaluated. According to ISO 3129 standard, the experimental specimens were made from Oriental beech wood. Wood specimens were subjected to 3% aqueous solutions of boric acid, borax, a boric acid and borax mixture (1:1; weight: weight), ammonium sulphate, and polyurethane/polyurethane, and epoxy resins before being coated with these substances. Results showed that oven-dry and air-density values of PU coated wood were much higher than EP coated wood. Water absorption (WA) levels of PU coated of wood were lower than EP coated wood. While FRC treated and PU coated wood resulted in lower WA levels than only PU coated wood, FRC treated and EP coated wood resulted in higher WA levels than only EP coated wood.

Keywords: coating, epoxy resin, impregnation, polyurethane/polyure resin, physical properties.

How to cite: Toker, H., Altay, Ç., Baysal, E., Bircan, İ. B., & Peker, H. (2023). Physical properties of Oriental beech impregnated and coated with some chemicals. Polímeros: Ciência e Tecnologia, 33(3), e20230027. https://doi.org/10.1590/0104-1428.20230033

1. Introduction Wood material has some important properties compared to other engineering materials. For example; wood materials have superior properties such as aesthetically superior to different materials, can be easily processed and shaped, can be combined with simple tools, low cost, environmentally friendly, abundant in nature, carbon-retaining and renewable material[1]. In previous studies, it has been indicated that there is an important relationship between these physical properties and density. For example; as wood density increases, the hardness, flexibility, strength of wood increases and it is more resistant to abrasive effects[2].

The expansion and shrinkage of wood with the effect of moisture is called “working of the wood”[3]. For this reason, undesirable situations such as cracking, shrinkage, and expansion can occur in the wooden material. Generally, wood material is used preferentially in exterior cladding, door-window joinery and in the construction of park-garden furniture and decoration works[3]. However, wood material, which no protection measures are taken, is exposed to weathering effects in a short time. Cracks occur in the material due to continuous wetting and drying, and color and mold fungi develop on the surface, and on the other hand, the sun rays destroy the wood layer and substances that can be removed by the effect of rain and wind. Thus, the wood material gains a dirty appearance[4]. To shield wood against these

Polímeros, 33(3), e20230027, 2023

effects, many different techniques are used. Impregnation, or the penetration of chemicals into wood substance, is the most significant of them. The impregnation method protects the wood material from burning, weathering, fungus and insect infestation, water absorption, etc. recommended to protect against the effects. It is known that chemicals such as pentachlorophenol, copper/chromium/arsenic (CCA), copper/chromium /boron (CCB), acid copper chromate, ammonia, copper/arsenic, which are commonly used as preservatives, are harmful to the environment[5,6]. To eliminate these chemicals, the impregnation industry is in search of new environmentally friendly chemicals. One of them is the aqueous solutions of boron compounds and they can penetrate the wood material very well. Boron compounds have gained currency because of their high resistance to biological destruction, their talents to be easily coated to the wood surface by unraveling in water, their good diffusion to wood material, their easy and cheap availability, and their significantly increasing the wood’s resistance to fire[5,6].

Another method used to protect the wood material surface against the effect of moisture is to stop the contact of water with the wood surface by surface treatments (paints, varnishes, water repellents, etc.). Recently, different surface products coated with wood material have been on the market and many studies have been carried out on these products.

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

Toker, H., Altay, Ç., Baysal, E., Bircan, İ. B., & Peker, H. Wood material becomes more resistant to photochemical degradation, dimensional changes, and biological organisms after being impregnated and applied to the surface with various coatings or materials such as varnish/paint[7-9].

Similar studies have been carried out in the literature on this subject. Baysal et al.[10] investigated some physical properties, such as water uptake of paradise wood treated with various boron compounds and monomer water-repellent substances. In the study; borax, boric acid, and borax and boric acid mixture (7:3; weight: weight) were used as borates; styrene, methyl methacrylate, and a mixture of styrene+methylmethacrylate (7:3; vol:vol) and isocyanate were preferred as water repellents. In the study; vacsol, immersol WR, polyethylene glycol-400, Tanalith-CBC and phosphoric acid, which are commercial impregnation materials, were also tested for comparison purposes. According to the results, the water repellents preferred in the study provided an important decrease in the water absorption rate of the specimens, and a similar effect was achieved in the water repellents treatment coated on boron compounds.

Baysal et al. [11] examined the density of wood polymer composites pretreated with a mixture of boric acid and borax. According to their results, it was found that the density of wood polymer composites increased compared to the untreated control specimen. Studies on the physical features of wood that has been coated with various coating materials and impregnated with various chemicals have been published in the literature. There are, however, hardly any investigations on wood coated with polyurea/polyurethane and epoxy resins and impregnated with flame resistant chemicals. This work represents the first attempt in the body of literature to achieve this goal. In this study, a twostep approach was used to made new coating materials that could enhance the physical properties of the wood. Before the coating procedure, an impregnation approach utilizing FRC which are boron compounds and ammonium sulfate, was used. The wood specimens were first primed with

epoxy resin using Sikafloor®-156 (EPR), and after that, Sikalastic®-851 R, a polyurethane/polyure, was applied to coat them. In this study, it was aimed to investigate some physical characteristics such as oven-dry density, air-dry density, and water absorption levels of Oriental beech treated with some flame-resistant chemicals and coated with polyurethane/polyure and epoxy (resins.

2. Materials and Methods 2.1 Preparation of test specimens In accordance with ISO 3129 standard, specimens of wood were prepared in 20 mm × 20 mm x 20 mm (radial, tangential, and longitudinal) sizes for oven dry density, airdry density, and water absorption tests[12]. The images of oven dry density (ODD), air-dry density (ADD), and water absorption (WA) of levels of control group are given in Figure 1. In Table 1, the sizes and numbers of all specimens prepared according to the test standards are given.

2.2 Impregnation procedure Boric acid (BA), borax (BX), and ammonium sulfate (AS) were used as flame-resistant chemicals. Oriental beech specimens were treated with 3% aqueous solution of BA, BX, AS, and a mixture of BA and BX (1:1; weight/weight).

Boric acid is also known as orthoboric acid. It consists of 3 hydrogen, 3 oxygen and 1 boron atom. Due to its chemical properties, it can show very different effects. These properties Table 1. Test specimens dimensions and numbers.

Specimens’ dimensions

Test types

20 mm × 20 mm x 20 mm 20 mm × 20 mm x 20 mm 20 mm × 20 mm x 20 mm

Oven dry density Air-dry density Water absorption

Number of specimens 110 110 110

Figure 1. Image of control specimens (WA: Water absorption; ADD: Air-dry density; ODD: Oven-dry density).

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Toker, H., Altay, Ç., Baysal, E., Bircan, İ. B., & Peker, H. are mild antiseptic, antifungal and antiviral properties. It is known as the conjugate acid of dihydrogen borate [13].

Borax’s solubility is inversely correlated with the rise in water temperature. In cold water, it is only very slightly soluble, but as the temperature rises, it becomes much more soluble. Borax has a mild alkalinity and reacts with water to produce an alkaline solution. Borax is considerably dissolve in ethylene glycol, only marginally dissolve in acetone, and only moderately dissolve in diethylene glycol and methanol.

Borax melts at 743 °C (anhydrous), reaching its boiling point at 1.575 °C (anhydrous). Borax density is 2.4 g / cm3[14].

Ammonium sulphate contains 20% nitrogen and 22% sulfur in its composition. The inorganic ammonium salt of sulfuric acid is crystalline and very soluble in water. It has an acidic pH and can be broken down easily[15].

The specimens were impregnated according to ASTM D1413-07[16] standard. The retention amount of the FRC was measured by the Equation 1.

Retention =

G.C x103 ( Kg / m3 ) V

(1)

In here; G = T2 -T1 T2 = Specimens’ weight after impregnation (g) T1 = Specimens’ weight before impregnation (g) V = Volume of specimen (cm3) C = Concentration of solution (%)

2.3 Coating procedure The impregnated test specimens were coated after receiving an epoxy component primer coat (Sikafloor®-156) and a polyurethane/polyure coating (Sikalastic®-851 R).

According to specimens with the PU label, the coatings were first produced with epoxy and then coated with polyurethane/polyure.With a low viscosity, solvent-free composition, compressive strength of 95 N/mm2, flexural strength of 30 N/mm2, and shore D hardness of 83 (seven days), Sikafloor®-156 is a two-component flooring product.

A/B’s mixing ratio was one-third of the volume ratio[17].

Sikafloor®-156 consists of 1-part A and 1-part B chemicals.

In line with the recommendations of the company, 3 A component and 1 B component were mixed and coated with 2 layers of wood[18]. Sikalastic®-851 R is a two-component, elastic, crack-bridging, modified polyurethane/polyurea hybrid resin with rapid curing. The two components of Sikalastic®-851 R are component A, an isocyanate derivative, and component B, a polyol/amine derivative[19]. Utilizing specialized polyure coating equipment (GAMA G-30 H) with consumption rates of 1.7 kg/m2 to 2.2 kg/m2, 2 layers were done to the floor, with the second layer beginning no later than six hours following the first layer.

2.4 Oven-dry density test The test specimens’ oven-dry densities were calculated with the help of TS ISO 13061-2 standard[20]. This standard required that test specimens be dried at 103 °C±2 until they reached a consistent weight. Following cooling, the Polímeros, 33(3), e20230027, 2023

specimens were weighed on an analytical scale with a 0.01 g sensitivity, their dimensions were evaluated using a precision caliper with an accuracy of 0.01 mm, and their volumes were measured using the stereo metric method.

Then, using Equation 2, determine the oven-dry density (δ0), oven-dry weight (M0), and oven-dry volume (V0) values. δ0 =

M0 ( g / cm3 ) V0

(2)

In here; M0 = Oven-dry weight of specimen (gr)

V0 = Oven-dry volume of specimen (cm3)

2.5 Air-dry density test The test specimens’ air-dry density values were calculated in accordance with TS 2472[20] standard. Wood specimens were kept in the cabinet at 20°C and 65% relative humidity until they reached a consistent weight. It was then weighed using an analytical balance with a sensitivity of 0.01 g, the dimensions were measured using a caliper with a sensitivity of 0.01 mm, the volumes were calculated using the stereometric method, and the air-dry density was measured using the values of the air-dry weight (M12) and volume (V12) in accordance with Equation 3. δ 12 =

M 12 ( g / cm3 ) V 12

(3)

In this equation; M12 = Air-dry weight of specimen (gr)

V12 = Air-dry volume of specimen (cm3)

2.6 Water absorption test Wood specimens were kept in distilled water for 1, 8, 24, 72, 120, 168, and, 336 hours under room conditions.

After each soaking period, specimens were taken out of water, dried with paper and weighted. The distilled water used in this experiment is a liquid that does not reflect light and is permeable. For this reason, it is used in optical experiments and instruments. In addition, due to the heating and evaporation of the water during the distillation process, the distilled water is largely free of microorganisms. In this study, WA of the by each specimen was calculated with the Equation 4.

WA =

Mf − Moi x100 Moi

(4)

In here; WA = Water absorption (%), Mf = Weight of specimen after water absorption (gr),

Moi = Oven dry weight of specimen after impregnation (gr).

2.7 Statistical evaluation The results of the oven dry density, air-dry density, and water absorption tests were statistically analyzed using the SPSS program, which also applied the Duncan test and an 3/7

Toker, H., Altay, Ç., Baysal, E., Bircan, İ. B., & Peker, H. analysis of variance with a 95% confidence level. Different letters denoted statistical significance in homogeneity groups (HG) statistical evaluations.

3. Results and Discussions 3.1 Oven-dry and air-dry density The oven-dry density values of Oriental beech treated with FRC and coated with PU and EP are presented in Table 2.

The oven dry densities of treated with FRC and coated with PU and EP of wood specimens were changed from 0.72 g/cm3 to 1.32 g/cm3. According to our results, the oven dry density values of treated with FRC and coated with PU and EP of wood specimens were much higher than that of control sample. Oven-dry density increased from 0.65 g/cm3 to 1.21 g/cm3 after the wood was coated with PU. The highest oven-dry density values detected with BA+BX treated and PU coated Oriental beech specimens. PU coated Oriental beech specimens gave a higher oven-dry density values than that of EP coated wood specimens. While impregnation with FRC before PU coating caused to increase oven-dry density values of wood specimens, it generally decreased for FRC treated and EP coated wood specimens. For example, while oven dry- density value was 1.21g/cm3 for PU coated wood, it was changed from 1.26 g/cm3 to 1.32 g/cm3 for FRC treated and PU coated wood. However, while oven dry

density value was 0.76 g/cm3 for only EP coated Oriental beech, it was changed from 0.72 g/cm3 to 0.76 g/cm3 for FRC treated and EP coated Oriental beech.

In Table 3, the air-dry density values of wood treated with FRC and coated with PU and EP resins are presented.

While the highest air-dry density value of Oriental beech was 1.34 g/cm3 for treated with BA+BX and coated with PU, the minimum air-dry density value was 0.69 g/cm3 for un-treated and non-coated wood. There was a statistical difference in air-dry density values between treated with FRC and PU coated groups and control groups. According to our results, PU coated Oriental beech specimens gave higher air-dry density values than EP coated Oriental beech specimens. While impregnation with FRC before PU coating caused to increase air-dry density values of Oriental beech specimens, it decreased for FRC treated and EP coated Oriental beech specimens. For example, while air-dry density value was 1.22 g/cm3 for PU coated Oriental beech wood, it was changed from 1.27 g/cm3 to 1.34 g/cm3 for FRC treated and PU coated Oriental beech. However, while air-dry density value was 0.78 g/cm3 for EP coated wood, it was changed from 0.71 g/cm3 to 0.77 g/cm3 for FRC treated and EP coated wood. Baysal et al. [21] studied oven-dry and air-dry density of borates treated and vinyl monomers coated wood. They found that oven-dry and air-dry density values of wood were highly increased statistical levels treated with solely vinyl monomer treatments and secondary vinyl monomer

Table 2. Oven-dry density values of Oriental beech treated with flame-resistant chemicals (FRC) and covered with PU and EP resins.

Chemicals Control PU BA+PU BX+PU AS+PU (BA+BX)+PU EP BA+EP BX+EP AS+ EP (BA+BX)+ EP

Retention (kg/m3) 16.88 15.74 14.67 13.96 16.21 16.07 14.87 16.05

Oven-dry density values (g/cm3) 0.65 1.21 1.30 1.27 1.26 1.32 0.76 0.72 0.76 0.74 0.76

Standard deviation 0.01 0.10 0.13 0.07 0.17 0.07 0.03 0.15 0.04 0.07 0.05

Homogeneity group D B A AB AB A C CD C CD C

PU: Polyurethane/Polyure; EP: Epoxy; BA: Boric acid; BX: Borax; AS: Ammonium sulphate. Each group received ten replicas. At a 95% confidence level, homogeneity was achieved in the group.

Table 3. Air-dry density values of Oriental beech treated with flame-resistant chemicals (FRC) and covered with PU and EP resins.

Chemicals Control PU BA+ PU BX+ PU AS+ PU (BA+BX)+PU EP BA+EP BX+EP AS+EP (BA+BX)+EP

Retention (kg/m3) 16.88 15.74 14.67 13.96 16.21 16.07 14.87 16.05

Air-dry density values (g/cm3) 0.69 1.22 1.32 1.28 1.27 1.34 0.78 0.71 0.77 0.76 0.77

Standard deviation 0.01 0.10 0.13 0.07 0.17 0.07 0.03 0.06 0.03 0.07 0.04

Homogeneity group D B A AB AB A C CD CD CD CD

PU: Polyurethane/Polyure; EP: Epoxy; BA: Boric acid; BX: Borax; AS: Ammonium sulphate. Each group received ten replicas. At a 95% confidence level, homogeneity was achieved in the group.

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Toker, H., Altay, Ç., Baysal, E., Bircan, İ. B., & Peker, H. treatments on borates (P≤0.05). Geçer et al.[22] investigated oven-dry density values of Oriental beech pre-impregnated with boric acid and borax before styrene coated Oriental beech. They found that oven dry density values of Oriental beech were highly increased statistical levels impregnated with solely styrene coated and secondarily styrene coated on boric acid and borax (P≤0.05). In this context, the results obtained from the literature were similar to the results of this study.

Our results showed that boron compounds and ammonium sulphate showed same effects in terms of air-dry and ovendry densities of wood.

3.2 Water absorption levels Water absorption (WA) amounts of specimens treated with FRC [with 3% aqueous solution of BA, BX, and BA+BX and AS] and coated with PU and EP resins are presented in Table 4.

The finding were in line with the earlier finding[23] and showed that the untreated and uncoated control group’s WA levels were significantly greater throughout the early phases of WA, notably within 1 and 8 h. It might be because water absorbed by wood at the beginning of soaking and reduced wood gaps with time[24]. Untreated and non-coated control specimens absorbed 25.53% of their weight in water after 1 hour, whereas PU and EP coated Oriental beech absorbed 0.90% and 3.46% water, respectively after 1 hour. Less water penetrates Oriental beech thanks to the hydrophobic properties of PU and EP shields, which also affect the wood’s surface and any remaining water in the cell wall and lumen[25].

For all WA periods, the untreated and uncoated control group had the highest WA levels. While the WA level of the untreated and uncoated control group was 52.00% after 8 hours, it was 100.59% after 336 hours. The untreated and uncoated control groups thus received more than half of the total water in the 8-hour period. The WA levels of wood treated with BX and PU coated wood were the lowest during all other WA periods, with the exception of 1 h and 8 h. For all WA periods, PU coated wood specimens had lower WA levels than EP coated Oriental beech. The WA

of PU-coated Oriental beech was 19.66% after 336 hours, whereas the WA of EP-coated Oriental beech was 48.36%.

It may be claimed that the polyurethane/polyurea oligomers’ backbones include more hydrophobic groups than epoxy oligomers do, and as a result, they serve to increase the crosslinked polymeric coating’s water resistance while decreasing its water absorption capacity[25].

For all WA periods, there was a statistically significant difference in the WA levels between the control and treated and coated groups (p≤0.05). As for compared AS and boron chemicals usages as FRC, it has been determined that boron chemicals show higher activities when compared with AS in terms of water absorption levels. While, BX displayed lowest WA level with PU coated specimens, BA exhibited lowest WA level with EP coated specimens. Whereas, WA levels of AS treated and PU coated Oriental beech specimens were 18.34%, it was determined 10.96% for BX treated and PU coated Oriental beech specimens after 336 h WA period.

As for EP coated specimens, it has been determined that WA levels of the AS treated Oriental beech specimens were 72.17%, and 52.50% for the BA treated wood specimens after 336 h WA period. Surprisingly, for EP coated Oriental beech, AS showed same effects with BA+BX in terms of WA levels after 336 h WA period. Moreover, they gave the worst results in terms of WA levels of Oriental beech after 336 h WA period. Usage of BX+BA mixture showed negative effect in terms of WA levels as compared with BX usage for EP coated Oriental beech. Geçer et al.[22] discovered that improving the water absorption levels of Oriental beech wood by impregnating it with various waterrepellent compounds like styrene. Our findings agree with those of Geçer et al.[22] in their data.

4. Conclusions Oriental beech treated with FRC [with 3% aqueous solutions of BA, BX, BA+BX, and AS] and coated with PU and EP resins were measured for oven-dry density, air-dry density, and water absorption levels.

Table 4. Water absorption levels of wood treated with flame-resistant chemicals (FRC) and covered with PU and EP resins.

Chemicals

After 1 hour

H.G

After 8 hours

H.G

After 24 hours

H.G

After 72 hours

H.G

After 120 hours

H.G

After 168 hours

H.G

After 336 hours

H.G

Water absorption levels (%)

Control

25.53

A

52.00

A

65.72

A

76.03

A

81.17

A

88.41

A

100.59

A

PU

0.90

B

3.11

C

4.40

D

8.96

E

9.95

E

13.06

F

19.66

F

BA+PU

1.14

B

2.18

C

3.72

D

8.17

E

9.63

E

11.07

F

15.29

FG

BX+PU

0.50

B

1.81

C

2.35

D

5.42

E

5.89

E

6.22

F

10.96

G

AS+PU

0.39

B

1.58

C

3.26

D

6.92

E

8.12

E

12.38

F

18.34

FG

(BA+BX) +PU

0.78

B

1.78

C

3.73

D

6.49

E

8.65

E

11.21

F

15.39

FG

EP

3.46

B

9.34

BC

18.53

C

32.09

D

35.19

D

39.48

E

48.36

E

BA+EP

5.50

B

16.29

B

25.55

BC

39.17

CD

43.17

CD

47.21

D

52.50

DE

BX+EP

4.51

B

11.82

B

31.19

B

47.36

B

49.18

B

56.96

BC

62.68

C

AS+EP

4.27

B

12.37

B

27.42

B

49.55

B

55.56

B

63.95

B

72.17

B

(BA+BX) +EP)

4.92

B

13.45

B

26.38

B

43.09

BC

47.46

BC

52.87

CD

73.07

B

PU: Polyurethane/Polyure; EP: Epoxy; BA: Boric acid; BX: Borax; AS: Ammonium sulphate. Each group received ten replicas. At a 95% confidence level. H.G.: Homogeneity group. Homogeneity was achieved in the group.

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Toker, H., Altay, Ç., Baysal, E., Bircan, İ. B., & Peker, H. Oriental beech coated with PU and EP had statistically greater oven-dry and air-dry density values than the control group. The density might have risen as a result of Oriental beech wood absorbing PU and EP. Compared to EP coated Oriental beech specimens, our findings demonstrated that PU coated specimens gave higher oven-dry density and air-dry density values. While applying FRC before PU coating led to an increase in both density values for Oriental beech, it typically led to a drop in both density values for Oriental beech specimens coated with EP. The WA of Oriental beech coated with EP and PU was found to be much lower than that of the control group. Meyer [26] asserts that a decrease in WA results from an increase in hydrophobicity. The bulking and water repellency properties of the PU and EP have a major role in the reduction of WA. It might be as a result of the capillaries’ ability to diffuse water more slowly as a result of the addition of polymer[27].

For all WA periods, PU coated wood specimens had lower WA than EP coated wood specimens. The WA levels of FRC applied and PU coated Oriental beech specimens were lower than the WA levels of only EP coated Oriental beech specimens.The findings also support the notion that boron compounds (BA and BX) perform better as FRC than AS in terms of water absorption.

As a result, newly developed PU and EP coatings for wood could serve as substitutes for building materials where high physical qualities are required for outdoor use.

The novel PU and EP coatings in this investigation have excellent physical characteristics, including as rising airdry and oven-dry density values and falling wood water absorption levels.

5. Author’s Contribution • Conceptualization – İlknur Babahan Bircan.

• Data curation – Hilmi Toker.

• Formal analysis – Çağlar Altay.

• Funding acquisition – Ergün Baysal.

• Investigation – Hüseyin Peker.

• Methodology – Çağlar Altay.

• Project administration – Çağlar Altay.

• Resources – İlknur Babahan Bircan.

• Software – Hüseyin Peker.

• Supervision – Çağlar Altay.

• Validation – Ergün Baysal.

• Visualization – İlknur Babahan Bircan.

• Writing – original draft – Hilmi Toker.

• Writing – review & editing – Çağlar Altay.

6. Acknowledgements This article is based in part on the findings of Çağlar Altay’s PhD thesis in Wood Science and Technology from the Institute of Science and Technology at Mula Stk Koçman University. This work is supported by the Muğla Sıtkı Koçman University’s project BAP-20/099/01/2.

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12. International Organization for Standardization – ISO. (2019).

ISO 3129:2019 - wood - sampling methods and general requirements for physical and mechanical testing of small clear wood specimen. Geneva: ISO. 13. ETİMADEN. (2023, July 12). Retrieved in 2023, August 11, from https://www.etimaden.gov.tr/storage/pages/March2019/1borik-asit1.pdf 14. MTA. (2023, July 12). Retrieved in 2023, August 11, from https://www.mta.gov.tr/v3.0/bilgi-merkezi/boraks 15. ASKİMYA. (2023, July 12). Retrieved in 2023, August 11, from http://www.askimya.com/urunler/amonyum-sulfat-7. html 16. ASTM International. (2007). ASTM D1413-07 - standard test method for wood preservatives by laboratory soil-block cultures. West Conshohocken: ASTM International. http:// dx.doi.org/10.1520/D1413-07.

17. Abed, M. S., Ahmed, P. S., Oleiwi, J. K., & Fadhil, B. M. (2020). Low velocity impact of Kevlar and ultra high molecular Polímeros, 33(3), e20230027, 2023

Toker, H., Altay, Ç., Baysal, E., Bircan, İ. B., & Peker, H. weight polyethylene (UHMWPE) reinforced epoxy composites.

Multidiscipline Modeling in Materials and Structures, 16(6), 1617-1630. http://dx.doi.org/10.1108/MMMS-09-2019-0164.

18. Babahan, I., Zheng, Y., & Soucek, M. D. (2020). New bio based glycidal epoxides. Progress in Organic Coatings, 142, 105580. http://dx.doi.org/10.1016/j.porgcoat.2020.105580.

19. Wiwa Wilhelm Wagner GmbH & Co.KG. (2015). Twocomponent hybrid coating for roofs: decades of protection. IST International Surface Technology, 8(3), 10-11. http://dx.doi. org/10.1007/s35724-015-0573-z.

20. Turkish Standards Institution. (2021). TS ISO 13061-2 - Physical and mechanical properties of wood - Test methods for small clear wood specimens - Part 2: Determination of density for physical and mechanical tests. Ankara: Turkish Standards Institution.

21. Baysal, E., Peker, H., Çolak, M., & Göktaş, O. (2003). Çeşitli emprenye maddeleriyle muamele edilen Kayin odununun yoğunluğu, eğilme direnci ve elastikiyet modülü. Fırat Üniversitesi Fen ve Mühendislik Bilimleri Dergisi, 15(4), 655-672. Retrieved in 2023, August 11, from https://search. trdizin.gov.tr/en/yayin/detay/30986/ (in Turkish).

22. Geçer, M., Baysal, E., Toker, H., Türkoğlu, T., Vargun, E., & Yüksel, M. (2015). The effect of boron compounds impregnation on physical and mechanical properties of wood polymer composites. Wood Research, 60(5), 723-737. Retrieved in 2023,

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August 11, from http://www.woodresearch.sk/wr/201505/04. pdf 23. Yalınkılıç, M. K., Baysal, E., & Demirci, Z. (1995). Bazı borlu bileşiklerin ve su itici maddelerin Kızılçam odununun higroskopisitesi üzerine etkileri. Pamukkale Üniversitesi Mühendislik Bilimleri Dergisi, , 1(3), 161-168. Retrieved in 2023, August 11, from http://pajes.pau.edu.tr/en/jvi.aspx?pdi r=pajes&plng=eng&un=PAJES-70852 (in Turkish).

24. Richardson, B. (1987). Wood preservation. Lancester: The Construction Press Ltd. 25. Ang, D. T. C., & Gan, S. N. (2012). Novel approach to convert non-self drying palm stearin alkyds into environmental friendly UV curable resins. Progress in Organic Coatings, 73(4), 409414. http://dx.doi.org/10.1016/j.porgcoat.2011.11.013.

26. Meyer, J. A. (1984). Wood-polymer materials. In R. Rowell (Ed.), The chemistry of solid wood (pp. 257-289). Washington: American Chemical Society. Advances in Chemistry, no. 207. http://dx.doi.org/10.1021/ba-1984-0207.ch006.

27. Langwing, J. E., Meyer, J. A., & Davidson, R. W. (1969). New monomers used in making wood plastics. Forest Products Journal, 19(11), 57-61.

Received: May 06, 2023 Revised: Jul. 14, 2023 Accepted: Aug. 11, 2023

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.20230031

Development and characterization of chitosan-collagen films loaded with honey David Servín de la Mora-López1 , Tomás Jesús Madera-Santana1* , Jaime López-Cervantes2* , Dalia Isabel Sánchez-Machado2 , Jesús Fernando Ayala-Zavala1  and Herlinda Soto-Valdez1  Coordinación de Alimentos de Orígen Vegetal, Centro de Investigación en Alimentación y Desarrollo, Hermosillo, Sonora, México 2 Laboratorio de Biotecnología y Ciencias Alimentarias, Instituto Tecnológico de Sonora, Ciudad Obregón, Sonora, México 1

*madera@ciad.mx; jaime.lopez@itson.mx

Abstract Biomaterials developed with biopolymers contribute to the healing process of healthy or diabetic patients. The objective of the present study was to evaluate the effect of honey incorporation (0.3, 0.6, and 1.2 g/100 mL) in chitosan/collagen/ glycerol composite films. The Ch/Coll/1.2H films revealed the greatest percentage of elongation (27.10%) and Young´s modulus (65.58 MPa). The barrier properties (WVTR and WVP) exhibited a significant increase when the honey was incorporated into the films. The absorption capacity, solubility, and enzymatic biodegradability were lower in films containing honey. The chemical interactions between the functional groups of the films were verified by FTIR. The morphology studied by SEM confirmed the mixture’s homogeneity. Finally, all formulations exhibited antibacterial properties against Staphylococcus aureus, Pseudomonas aeruginosa, Listeria monocytogenes, and Salmonella Typhimurium.

The aforementioned properties of formulated dressings are suitable for their potential application in chronic wounds.

Keywords: chitosan, collagen, Apis mellifera, antibacterial films, bioactive films.

How to cite: Servín de la Mora-López, D., Madera-Santana, T. J., López-Cervantes, J., Sánchez-Machado, D. I., Ayala-Zavala, J. F., & Soto-Valdez, H. (2023). Development and characterization of chitosan-collagen films loaded with honey. Polímeros: Ciência e Tecnologia, 33(3), e20230028. https://doi.org/10.1590/0104-1428.20230031

1. Introduction Wounds are skin breaks caused by physical or thermal damage that can alter the physiological and bodily functions of the underlying tissue[1]. They can be classified as acute or chronic[2]. Chronic wounds affect a great part of the world’s population. Their prevalence has increased with the growth of the adult population and comorbidities such as obesity and diabetes[2]. It has been estimated that around 1 to 2% of the world’s population has suffered or will experience chronic injuries[3,4]. The costs of treating chronic wounds have been extremely expensive. In the United States, around $20 billion is invested annually in the health sector, solely for that purpose[4]. Traditionally, several natural materials such as biopolymers, animal fats, vegetable fibers, honey pastes, cotton fabrics, lint, and gauze have been widely used in medicine for wound healing[5]. The ideal material for wound healing should provide an adequate moist environment, promote gas exchange, possess adequate mechanical properties for handling and manipulation, act as an efficient barrier against infectious microorganisms, be non-toxic, and promote efficient wound healing[6,7].

Chitosan is a biopolymer derived from the thermo-alkali deacetylation of chitin, which is abundantly found in the exoskeletons of crustaceans and insects and in fungal cell walls[8]. The chitosan-free amino groups impart relevant

Polímeros, 33(3), e20230028, 2023

properties such as antimicrobial, biodegradable, biocompatible, and non-toxic characteristics. These properties have served as a basis for providing an efficient treatment for several chronic conditions, such as diabetic foot, third-degree burns, decubitus ulcers, pyoderma gangrenosum, venous ulcers, and pressure ulcers[9].

Collagen is the main extracellular matrix (ECM) component that creates highly organized and elastic structures and confers physical tissue support[10]. It shows potential uses in biomedicine due to its ability to stimulate fibroblast and keratinocyte proliferation and promote the synthesis of proteins that make up the ECM[6,10]. Using additional collagen from external media allows for accelerated healing processes, including in wounds where healing occurs more slowly.

Honey has long been employed in traditional medicine as an effective therapy to treat burn injuries and chronic wounds[11]. The presence of flavonoids and phenolic compounds has favored the antioxidant roles of honey, which provide anti-inflammatory responses in wounds[12]. In addition, the antimicrobial nature of honey is directly associated with its supersaturated concentration of sugars, its high osmolarity in conjunction with its acidic nature, and the presence of hydrogen peroxides and flavonoids[13,14]. Honey can be

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

Servín de la Mora-López, D., Madera-Santana, T. J., López-Cervantes, J., Sánchez-Machado, D. I., Ayala-Zavala, J. F., & Soto-Valdez, H. considered a suitable material because it decreases pain and swelling, and promotes autolytic debridement and healing[14].

The present study was focused on preparing chitosan/ collagen/glycerol/honey films. The effect of incorporating honey at different concentrations in chitosan/collagen/ glycerol films was evaluated. The optical, physicochemical, mechanical, barrier, structural, morphological, thermal, biodegradable, and antibacterial properties of the films were investigated to enhance their possible application as wound dressings.

2. Materials and Methods 2.1 Materials Apis mellifera honey was acquired from a local market in Mani, Yuc. Mexico. The shrimp chitosan powder was supplied

by Quitomex, S.A. (Obregon, Mexico). The chitosan has an average molecular weight of 378 KDa and a deacetylation degree of 87%. Hydrolyzed fish collagen was provided by the Food Science and Technology Laboratory at ITSON (Obregon, Mexico). Glycerol was purchased from REASOL (Mexico City, México). Glacial acetic acid was obtained from FAGALAB (Mocorito, Mexico). Lysozyme (40,000 units/mg) and phosphate-buffered saline (PBS) pH 7.4 were supplied

by Sigma-Aldrich (St. Louis, MI, USA).

2.2 Preparation of chitosan-collagen-glycerol-honeybased films Chitosan-collagen-honey composite films were prepared by the solution casting technique[15]. 100 mL of a solution of acetic acid (1% v/v) was used to prepare a composite blend of chitosan (1% w/v) and collagen (0.5% w/v).

Both chitosan and collagen were mixed in the acetic acid solution. This mixture was considered a control group and named Ch/Coll. Based on this formulation, the other four blends were prepared by adding glycerol, and subsequently, honey at different concentrations, as described in Table 1.

The solutions were homogenized by a magnetic stirrer, Cimarec™ Thermo Fisher Scientific (Waltham, MA, USA), at 25 ºC and subsequently filtered through 100 μm of organza fabric (100 mesh). Finally, the films were produced by pouring the corresponding solution into plastic containers (150 mL) and leaving them to dry at 45 ºC for 48 h in a DZF-6050 vacuum oven (Xiamen, China).

y= 0.3319). Measurements were recorded at three specific points on the film, and an average of nine measurements was reported for each formulation. Using the CIELab scale, the values of L* (lightness) and the chromatic parameters a* (red/green) and b* (yellow/blue) were measured. The color difference (ΔE) of each film was compared against the Ch/ Coll formulation and was calculated using Equation 1: ∆E=

( ∆L *)2 + ( ∆a *)2 + ( ∆b *)2

(1)

where ∆L*= L – LO, ∆a* = a – aO, and ∆b* = b – bO. The L*, a*, and b* parameters are the chromatic values of the sample, and LO*, aO*, and bO* represent the control’s chromatic values (Ch/Coll film).

The transparency measurement was performed according to the methodology established by Escárcega-Galaz et al.

[16] . Rectangular samples (3x1 cm) were made and placed inside a spectrophotometric cell. The absorbance readings were determined at 600 nm in a UV-Vis spectrophotometer Varian Cary 50 Bio. Transparency was calculated using the following Equation 2: A T = 600 t

(2)

where A600 is the recorded absorbance value at 600 nm, and t represents the average thickness of the film in mm. The transparency of the films was analyzed, and the average value of two replicates was reported for each formulation.

2.4 Mechanical properties The mechanical characterization of the films was analyzed using a texture analyzer TA-XT plus texture analyzer, Stable Micro Systems (Surrey, UK). The tensile parameters tensile strength (σmax), elongation at break (εb), and Young’s modulus (E) were calculated according to the ASTM D882-02 standard method[17]. Rectangular samples (10x60 mm) were obtained, and the thickness of each film was measured in triplicate. The separation distance, end-toend, of 30 mm was set, and a 10 mm/min crosshead speed was programmed. The tensile parameters were reported based on the average value of six replicates per film.

2.5 Physicochemical properties

2.3 Optical properties of the films

2.5.1 Water holding capacity and solubility

Color determination of the films was performed using a colorimeter Chroma Meter CR-400, Konica Minolta (Osaka, Japan) calibrated to a standard (Y= 94.10, X= 0.3155, and

Water holding capacity (DS) and solubility (WS) tests were performed according to the methodology reported

by Madera-Santana et al.[18]. with some modifications.

Table 1. Composition of films based on chitosan, collagen, glycerol, and honey.

Formulation Ch/Coll Ch/Coll/Gly Ch/Coll/Gly/0.3H Ch/Coll/Gly/0.6H Ch/Coll/Gly/1.2H

Cha (g) 1.0 1.0 1.0 1.0 1.0

Collb (g) 0.5 0.5 0.5 0.5 0.5

Glyc (g) 0 0.6 0.6 0.6 0.6

Hd (g) 0 0 0.3 0.6 1.2

Ch: Chitosan. bColl: Collagen. cGly: Glycerol. dH: Honey. Solute (g/100 mL).

a

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Polímeros, 33(3), e20230028, 2023

Development and characterization of chitosan-collagen films loaded with honey The films were evaluated with PBS pH 7.4 and deionized water. Circular cuts (3 mm in diameter) were performed in triplicate for each film formulation. The sample weight was recorded and transferred to a Petri dish. Subsequently, PBS or deionized water was added (3 mL), and the samples remained in adsorption equilibrium at different time intervals (5, 10, 15, 30, and 60 min). Excess moisture was removed by placing the film surface on filter paper. Finally, the weight of the wet sample was recorded, and the percentage of water retention was calculated using Equation 3: = % DS

We − Wo ×100 Wo

(3)

where We corresponds to the weight of the film at equilibrium adsorption in PBS medium or deionized water, and Wo represents the initial weight of the film.

To determine solubility, wet samples employed for the %DS assay at time intervals of 60 min in both mediums were allowed to dry for 24 h. Solubility was calculated using Equation 4: = %WS

Wo − Wd ×100 Wo

(4)

where Wo is the initial weight of the film and Wd is the weight of the film after the drying process at 60 ºC.

2.6 Barrier properties 2.6.1 Water Vapor Transmission Rate (WVTR) The test to determine WVTR was performed according to the wet cup gravimetric method established by ASTM E96[19]. Water vapor diffusion was determined by the weight loss from a transmission container (w). For this purpose, a plastic container with deionized water (30 mL) was sealed with a lid containing the sample firmly fixed on its top.

The container was stored at 25±1 °C and 30±5% RH in a desiccator with dry silica. The assay was performed in triplicate for each sample, and the container weight was recorded periodically for 30 h. WVTR was calculated from the slope of the straight line where time (h) vs. weight difference (g) was plotted using Equation 5: WVTR =

w t*A

(5)

where w is the weight loss of the container (g), t is the time in hours and A corresponds to the permeation area (2.85x10-4 m2).

2.6.2 Water Vapor Permeability (WVP) The determination of WVP was calculated from the WVTR and using Equation 6: WVP =

WVTR * l ∆p

(6)

where l is the average thickness of the film and ∆p corresponds to the difference in water vapor pressure on the internal Polímeros, 33(3), e20230028, 2023

and external sides of the container where the film sample is located.

2.7 Fourier transform infrared spectroscopy (FTIR) The infrared spectra of the samples were obtained using FTIR-ATR equipment Nicolet iS50 FTIR, Thermo Fisher Scientific (Waltham, MA, USA). The study was carried out in a wavenumber range from 4000 to 600 cm-1 with a spectral resolution of 4 cm-1, and 64 scans were performed for each formulation.

2.8 Morphological and elemental analysis Surface morphology was analyzed using micrographs taken in a field emission scanning electron microscope (FESEM) model JEOL JSM-7600F (Peabody, MA, USA).

The samples were placed on an aluminum stub, and the observation was performed at an angle of 90º to the surface.

2.9 In vitro enzymatic biodegradation The biodegradation study of the films was performed according to the methodology proposed by MartinezIbarra et al.[9] with some modifications. The assay was performed in a PBS solution containing lysozyme (40,000 units/ mg) at a 2 mg/mL concentration. For this purpose, circular dry samples with diameters of 17 mm were weighed and immersed in a PBS solution containing lysozyme (3 mL).

Samples were incubated at 37 °C for 9 days, and weight was determined at 1, 2, 4, 7, and 9 days. To carry out the measurements, the samples were removed from the enzyme solution and carefully washed with distilled water to interrupt the enzymatic process. The excess moisture was removed using filter paper, the films were dried, and their weight was recorded. In vitro enzymatic biodegradation was evaluated by weight loss, which was calculated by the following Equation 7: %Weight = loss

Wo − Wt ×100 Wo

(7)

where Wo is the weight of the dry sample before contact with lysozyme and Wt is the weight of the sample after contact with the enzyme. The results corresponding to biodegradation were calculated based on an average of three replicates per formulation.

2.10 Antibacterial properties The antibacterial properties of the films were evaluated against Staphylococcus aureus ATCC 6538, Pseudomonas aeruginosa ATCC 10154, Listeria monocytogenes ATCC 7644, and Salmonella Typhimurium ATCC 14028 using the method established by Rodríguez-Núñez et al.[15].

Inoculums were prepared 18 h before the study to reach their exponential phases; a loop of bacteria was introduced into tubes with BD Difco™ Mueller-Hinton broth (10 mL) and incubated at 37 °C. Small aliquots of each inoculum were transferred to tubes with saline solution (NaCl 0.9% w/v) until their absorbance was adjusted to 0.100 at 600 nm in UV/vis spectroscopy (equivalent to 108 CFU/mL).

To evaluate the antibacterial properties of the films, 16 mm 3/11

Servín de la Mora-López, D., Madera-Santana, T. J., López-Cervantes, J., Sánchez-Machado, D. I., Ayala-Zavala, J. F., & Soto-Valdez, H. Table 2. Optical properties of films based on chitosan and collagen loaded with honey.

Formulation Ch/Coll Ch/Coll/Gly Ch/Coll/Gly/0.3H Ch/Coll/Gly/0.6H Ch/Coll/Gly/1.2H

L* 81.30 ± 0.47c 82.01 ± 0.30c 57.94 ± 2.75b 34.44 ± 1.47a 35.67 ± 3.05a

Color parameters a* b* -1.79 ± 0.08ª 35.31 ± 1.86c -1.59 ± 0.07ª 34.23 ± 0.81bc 24.92 ± 2.59b 54.40 ± 2.13d 32.33 ± 0.70c 25.34 ± 0.95ª 33.27 ± 0.33c 32.61 ± 2.81b

Transparency

ΔE ND 4.96 ± 0.90a 36.17 ± 3.72b 57.13 ± 2.64c 54.45 ± 2.20c

1.54 ± 0.03b 0.96 ± 0.02ª 2.67 ± 0.21c 3.41 ± 0.21d 3.74 ± 0.15d

Mean values ± standard deviation is reported for each treatment. ND: No determined. Different letters in the same column indicate significant difference (p<0.05).

diameter film samples were placed in tubes containing 10 mL of Mueller-Hinton broth. Then 10 µL of the adjusted inoculum was introduced. An inoculated tube without a film sample was used as a bacterial control for comparison.

The samples were incubated at 37 °C for 24 h, and 200 µL of the broths were transferred to polypropylene microplate wells. Absorbance readings were performed at 600 nm on a UV-Vis SPECTROStar Omega microplate reader (BMG LabTech GmbH, Germany).

Finally, the colony-forming units (CFU/mL) were calculated from the absorbance obtained using the equations established by Gonzáles-Pérez et al.[20] (Supplementary Material).

2.11 Statistical analysis A completely randomized design was used, where the response variables were measured according to the composition of glycerol (0 and 0.6 g/100 mL) and honey (0, 0.3, 0.6, and 1.2 g/100 mL) in the films. The Ch/Coll films were considered control samples. The mean values ± standard deviations of the replicates were reported for each analysis. An analysis of variance (ANOVA) test was performed using the STATGRAPHICS PLUS 5.1 statistical package. Statistically significant differences between the means of each group were estimated below a significance level of (p<0.05).

3. Results and Discussions 3.1 Optical properties The color parameters of the Ch/Coll, Ch/Coll/Gly, and Ch/Coll/Gly/H films are listed in Table 2. The Ch/Coll/Gly films did not generate changes in L* and a* compared to the Ch/Coll films. In qualitative terms, the Ch/Coll films showed a clear yellow, homogeneous, and transparent coloration, while the addition of glycerol conferred a mostly shiny and smooth surface texture. In contrast, brown coloration was observed due to honey addition in the films (Figure SI1, Supplementary Material). The a* values exhibited low blue tinting in the Ch/Coll films, while reddening appeared in the honey composite films. The b* value showed yellowness in all films, with the Ch/Coll/Gly/0.3H formulation exhibiting the strongest yellow tint. The color difference (ΔE) revealed statistical differences among the films (p<0.05). The lowest values corresponded to the Ch/Coll/Gly formulation, while the Ch/Coll/40Gly/0.6H films had the highest color difference according to the control film (Ch/Coll film).

The browning by the honey Maillard reactions generated a 4/11

Table 3. Mechanical properties of films based on chitosan and collagen loaded with honey.

Formulation Ch/Coll Ch/Coll/Gly Ch/Coll/Gly/0.3H Ch/Coll/Gly/0.6H Ch/Coll/Gly/1.2H

σmax

εb

E

[MPa]

[%]

[MPa]

71.78 ± 5.5d 1.66 ± 0.3a 3882.7 ± 1007.2c 24.74 ± 6.9b 8.51 ± 1.9c 722.5 ± 181.9b 19.84 ± 5.1b 2.94 ± 1.2ab 974.9 ± 310.5b 33.06 ± 9.5c 4.30 ± 1.4b 1110.3 ± 192.1b 8.61 ± 1.7a 27.10 ± 2.3d 65.6 ± 12.9a

Mean values ± standard deviation are reported for each treatment.

Different letters in the same column indicate a significant difference (p<0.05).

greater reddening and decreased lightness (L*) that caused an increment in ΔE. Significant changes were obtained by Escárcega-Galaz et al.[16], who reported significant increases in L*, a*, and ΔE by including honey and glycerol compounds in their chitosan films, which showed a trend very similar to those reported here.

Table 2 illustrates the corresponding transparency of the analyzed formulations. Transparency ranged from 0.96 to 3.74, with the Ch/Coll/Gly films having the lowest transparency value, while the Ch/Coll/Gly/1.2H films exhibited the highest values. The inclusion of glycerol significantly decreased (p˂0.05) the transparency values in the Ch/Coll films. This effect was confirmed by Rivero et al.[21], where they evidenced that the presence of glycerol improved transparency in laminated films based on gelatin/chitosan.

3.2 Mechanical properties In this study, the mechanical parameters measured in the films by the tensile test were determined by the stressstrain curves. These parameters, tensile strength (σmax), elongation at break (εb), and Young’s modulus (E), are shown in Table 3. The σmax of the films presented ranges that fluctuated between 8.61 MPa and 71.78 MPa; the εb ranged from 1.66% to 27.10%; and the E showed values between 65.58 MPa and 3882.73 MPa. The Ch/Coll films exhibited the highest values of σmax and E, although they exhibited the lowest value of εb. This film formulation had high strength, but it was not flexible and showed brittle characteristics.

The incorporation of glycerol into Ch/Coll films produced a significant decrease (p<0.05) in σmax and E values.

In contrast, the εb showed a significant increase (p<0.05).

This combination of results was mainly pronounced in formulations that contained honey. The most noticeable Polímeros, 33(3), e20230028, 2023

Development and characterization of chitosan-collagen films loaded with honey changes were observed in the Ch/Coll/Gly/1.2H film, which provided mostly elastic and flexible structures.

These characteristics were attributed to the capability of glycerol and honey to act as plasticizing agents in the films.

Recently, Rocha-Lemus et al.[22] reported the performance of honey as a plasticizer in graphene oxide-agar films. Other authors have reported that incorporating glycerol and honey can promote a larger free volume between the polymeric chains, causing a decrease in the intermolecular interactions among them[15]. It explains the drastic decreases in elastic modulus values and the increase in elongation rates when both compounds were added to Ch/Coll films, particularly in the Ch/Coll/Gly/1.2H formulation. On the other hand, the Ch/Coll/Gly/0.3H and Ch/Coll/Gly/0.6H films revealed lower elongation at break and a higher elastic modulus compared to Ch/Coll/Gly films but were more elastic and flexible when compared to Ch/Coll films.

3.3 Physicochemical properties 3.3.1 Water holding capacity and solubility Figure 1 shows the kinetics of the water-holding capacity performed on each film for 60 min using PBS solution and deionized water as hydrating agents, both at 25 ºC. The study revealed that all formulations reached equilibrium absorption after 15 min of contact with the PBS solution (Figure 2a). However, the instability in the behavior of the Ch/Coll/Gly formulation was observed between 5 and 10 min, while in the Ch/Coll/Gly/0.3H film, there was a fluctuation between 10 and 15 min. The film that exhibited the highest adsorption capacity in PBS was Ch/Coll, with a retention rate of 111.79% at 5 min and 67.96% at 60 min of contact. In contrast, the Ch/Coll/Gly/0.6H formulation revealed the lowest rates, measured at 32.05% and 6.63% at 5 and 60 min, respectively.

Figure 1b revealed the water-holding capacity of films hydrated with deionized water. The kinetics indicated that the films, except for Ch/Coll/Gly, achieved equilibrium adsorption after being in contact for 10 min with the hydrating medium.

The Ch/Coll/Gly films exhibited a considerable increase in

their adsorption with values between 5 and 30 min; however, at these times, they achieved equilibrium. The highest absorption rates corresponded to the Ch/Coll formulation, and the film with the lowest water retention was the Ch/ Coll/Gly/1.2H formulation. Likewise, the study indicated no considerable differences in the values obtained among the films containing honey. The decrease in water-holding capacity in honey composite films is an effect associated with the hydrophilic nature of honey and the viscosity of the polymers used to produce the films. Likewise, honey can restrict mobility and free rotation among polymer chains by forming strong hydrogen-bonding interactions with chitosan, collagen, and glycerol due to the presence of many functional groups among the components. In this sense, strong interactions between polymers could shorten intermolecular distances and create a much more compact network, resulting in a lower adsorption capacity. Figure SI2 (Supplementary Material) shows the solubility of the different formulations exposed for 60 min in PBS solution and deionized water.

3.4 Barrier properties Figure 2 shows the water permeability behavior for each formulation described. The film’s water vapor diffusion was significantly affected by incorporating glycerol and honey.

According to Figure 2, the Ch/Coll/Gly/0.3H films exhibited the highest WVTR and WVP values; in contrast, the Ch/Coll films presented the lowest WVTR and WVP. The Ch/Coll/ Gly films exhibited higher WVTR and WVP in comparison to the Ch/Coll films. Ziani et al.[23] obtained increases in their WVP values from 0.89 to 1.11 g mm kPa-1 h-1 m-2 when plasticizing chitosan films (96% DD) with glycerol. It can be explained because glycerol possesses short linear chains that allow it to incorporate into adjacent polymeric chains, increasing the free volume and weakening the intermolecular forces. This effect causes the polymeric network to be less dense and favors greater water vapor molecules diffusion.

The honey inclusion promoted significant increases in film permeability. However, no significant differences (p>0.05) were found for WVTR in Ch/Coll/Gly/0.3H and

Figure 1. Kinetics of water-holding capacity of films based on chitosan and collagen loaded with honey hydrated with PBS (a) and deionized water (b).

Polímeros, 33(3), e20230028, 2023

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Servín de la Mora-López, D., Madera-Santana, T. J., López-Cervantes, J., Sánchez-Machado, D. I., Ayala-Zavala, J. F., & Soto-Valdez, H.

Figure 2. WVTR (a) and WVP (b) of films based on chitosan and collagen loaded with honey. Different letters in each column indicate a significant difference (p<0.05).

Figure 3. FTIR-ATR spectra of films based on chitosan and collagen loaded with honey at wavelengths 2600 - 3800 cm-1 (a) and 700 – 1900 cm-1 (b).

Ch/Coll/Gly/0.6H films. Likewise, statistical similarities (p>0.05) were found for WVP in Ch/Coll/Gly/0.3H and Ch/Coll/Gly/1.2H films. Slight decreases in permeability behavior with increasing honey concentrations in the films were observed. During the healing process, biomaterials must control water loss in the wound at optimal levels and provide adequate moisture to prevent excessive dehydration and facilitate tissue healing. Considering these criteria, Ch/ Coll/Gly/0.6H films had a lower WVP, which results in the formulations with the best properties to prevent dehydration and maintain suitable wound environments.

3.5 FTIR analysis The effects of the interactions of the chitosan-collagen films and the subsequent incorporation of glycerol and honey were evaluated by FTIR spectroscopy (Figure 3).

The absorption patterns of the Ch/Coll films revealed extensive bands at 3358–3276 cm-1 attributed to N-H bonds stretching vibrations and the existence of O-H groups linked by hydrogen bonds[24]. The signals found at lengths of 2923– 2873 cm-1 were associated with the stretching vibrations of C-H groups contained in chitosan and collagen molecules.

The absorption band related to Amide I, corresponding to C=O 6/11

group stretching vibrations, was located at 1637 cm-1. The Amide II was found at 1541 cm-1 and was attributed to the C-N group stretching vibrations in conjunction with N-H and CH2 bending vibrations[25]. The collagen’s O-H deformation (COOH) was found at 1401 cm-1. An extensive absorption band between 1199 and 1022 cm-1 showed the C-O stretching vibration, which is characteristic of the polysaccharide structure of chitosan.

Specifically, the peak located at 1151 cm-1 corresponded to C-O-C bond stretching vibrations, and the peaks found at 1065 cm-1 and 1022 cm-1 were related to C-O bond vibrations.

The absorption band associated with the pyranose ring stretching of chitosan was found at 899 cm-1[24].

FTIR analysis showed overlaps of the O-H groups over the N-H groups at 3278 cm-1. This effect explains the strong stretching vibration of the O-H groups of chitosan, collagen, glycerol, and honey[26]. Higher absorption intensities were found at 2923-2873 cm-1, corresponding to C-H groups of carboxylic acids and NH3+ groups of free amino acids, indicating the presence of honey in the films. The most notable changes in the spectra occurred with the appearance of a strong Amide I band, whose peak was augmented with increasing honey concentration in the films.

Likewise, the honey incorporation decreased the absorption bands of Amide II, and the peak was found at 1401 cm-1 (O-H Polímeros, 33(3), e20230028, 2023

Development and characterization of chitosan-collagen films loaded with honey

Figure 4. SEM micrographs of films based on chitosan and collagen: Ch/Coll (a), Ch/Coll/Gly (b), Ch/Coll/Gly/0.3H (c), Ch/Coll/ Gly/0.6H (d), and Ch/Coll/Gly/1.2H (e).

deformation). These changes could indicate a possible interaction of hydrogen bonds between the COOH, NH2, C=O, and OH groups found in the film’s components[26,27].

The glycerol addition caused an overlap at 1023 cm-1, and its intensity increased with the addition of honey. The signal at wavenumbers between 700-940 cm-1 is attributed to bending vibrations of the honey carbohydrate ring in anomeric regions, while at wavenumbers from 940 to 1145 cm-1, it is due to the C-C and C-O groups stretching the carbohydrates.

3.6 Morphological analysis The surface morphology of the films was analyzed by SEM micrographs, as shown in Figure 4. The test was performed under a magnification of 500x at a scale of 10 μm. The study revealed films with smooth, homogeneous, and continuous surfaces, indicating that the components were uniformly dispersed in the polymer matrix. However, it was possible to witness some rough regions in the Ch/Coll/Gly and Ch/ Coll/Gly/0.6H films. In addition, all films exhibited good structural integrity with the absence of porosities, cracks, or fractures on their surfaces. Micrographs revealed good miscibility in Ch/Coll and Ch/Coll/Gly films; however, small crystals started to appear with the incorporation of honey into the membranes. Costa et al.[28] mentioned that this phenomenon is strongly related to glucose, which can favor honey crystallization during storage due to its presence at supersaturated concentrations. The low solubility of glucose in water allows these molecules to separate from water and form crystals, as shown in Figures 4c, 4d, and 4e.

3.7 In vitro enzymatic biodegradation The films presented successive weight losses as a consequence of the dynamic degradation of lysozyme, Polímeros, 33(3), e20230028, 2023

Figure 5. Kinetic of enzymatic degradation by lysozyme in films based on chitosan and collagen loaded with honey.

as shown in Figure 5. The largest weight loss in the films occurred during the first day of incubation. After nine days, Ch/Coll and Ch/Coll/Gly films were completely dissolved in the enzyme-containing solution, while formulations of Ch/Coll/Gly/0.3H, Ch/Coll/Gly/0.6H, and Ch/Coll/Gly/1.2H presented breakage to small particles as a consequence of hydrolysis. The Ch/Coll formulation showed the highest biodegradability rate, with weight losses of 62.10%. The Ch/ Coll/Gly films showed very similar behaviors during the first two days of incubation; however, after day four, a lower weight loss was observed compared to the Ch/Coll films.

On the other hand, the honey incorporation in the films had a less pronounced effect. The Ch/Coll/Gly/0.6H formulation 7/11

Servín de la Mora-López, D., Madera-Santana, T. J., López-Cervantes, J., Sánchez-Machado, D. I., Ayala-Zavala, J. F., & Soto-Valdez, H. recorded the lowest weight loss with values of 36.79% at nine days of incubation. Similarly, the Ch/Coll/Gly/0.3H films revealed higher progressive weight losses over the first four days of incubation but no significant differences (p>0.05) between the honey-based formulations. The above results indicated that glycerol and honey favored lower weight loss. Moreover, there was a direct relationship between the biodegradability of the films and their adsorption capacity.

The Ch/Coll and Ch/Coll/Gly formulations presented the highest biodegradability rates because they had a higher capacity to absorb the enzymatic solution, which favored contact with a greater volume of lysozyme in the films.

3.8 Antibacterial properties The antibacterial activity of films based on chitosan and collagen loaded with honey is given in Figure 6. Significant reductions (p˂0.05) were observed in all formulations compared to the control for each microorganism. When the films were evaluated against S. aureus, some significant differences (p˂0.05) were observed among the treatments employed. The Ch/Coll films presented the highest bacterial reduction of all films (1.57x108 CFU/mL), while the Ch/Coll/ Gly/0.3H formulation revealed the highest microbial count (2.87x108 CFU/mL). The effect of honey, according to the increase in its concentration on the films, caused gradual decreases in the microbial count of S. aureus.

On the other hand, films inoculated with P. aeruginosa presented a different behavior in their microbial growth.

The Ch/Coll composite films exhibited the highest counts with values of 1.31x108 CFU/mL, and the lowest count was recorded for the Ch/Coll/Gly/0.3H formulation (8.48x107 CFU/mL). Increasing the honey concentration in the films caused increases in their microbial counts.

For L. monocytogenes and S. Typhimurium, similar trends were observed among the different treatments. The microbial count values gradually decreased as honey was incorporated and concentrations increased. In both study organisms, statistical similarities (p˂0.05) were observed between the

counts of Ch/Coll and Ch/Coll/Gly films. In contrast, the effect of honey showed significant differences (p˂0.05) when compared in both films.

Both chitosan and honey are compounds that contribute synergistically to the antibacterial properties of the films.

The antibacterial properties of honey lie in its supersaturated concentration of sugars, its high osmolarity, its acidic nature, the constant production of hydrogen peroxides, and the existence of lysozymes, phenolic acid, and flavonoids[13,14].

In contrast, chitosan with positively charged amino groups can interact with the negatively charged bacterial cell wall. This process produces strong electrostatic interactions, resulting in cell wall rupture and leakage of intracellular components, causing cell death[28,29]. In addition, many studies have shown that chitosan can block the cell membrane of Gram+ bacteria and prevent the entry of nutrients into the cell[30].

4. Conclusions Honey is a favorable compound to be incorporated into biomaterials. In this work, films of chitosan, collagen, glycerol, and honey were prepared by using the solution casting technique. All the films were transparent and presented a good appearance and color in their structure. Additionally, the interactions between the functional groups of the compounds used in this study allowed the formation of miscible, homogeneous, and uniformly dispersed mixtures. The effect of honey on the films promoted increases in elongation at break and tear strength, generating mostly flexible and elastic films. The enzymatic biodegradation of films by lysozyme action resulted in lower weight losses in the films containing honey. In addition, this research demonstrated that the films had enhanced antibacterial properties against S. aureus, P. aeruginosa, L. monocytogenes, and S. Typhimurium when honey was incorporated into them. These results reveal that honey-prepared films are a promising alternative for chronic wound treatment in biomedicine.

5. Author’s Contribution

Figure 6. Antibacterial activity of films based on chitosan and collagen loaded with honey against S. aureus, P. aeruginosa, L. monocytogenes, and S. Typhimurium. Different letters in each group of columns per bacteria indicate a significant difference (p<0.05).

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• Conceptualization – Tomás Jesús Madera-Santana; Jaime López-Cervantes; Dalia Isabel Sánchez-Machado; Herlinda Soto-Valdez; Jesús Fernando Ayala-Zavala.

• Data curation – David Servín de la Mora-López; Tomás Jesús Madera-Santana.

• Formal analysis – David Servín de la Mora-López.

• Funding acquisition – Tomás Jesús Madera-Santana.

• Investigation – David Servín de la Mora-López.

• Methodology – David Servín de la Mora-López; Tomás Jesús Madera-Santana.

• Project administration – Tomás Jesús Madera-Santana.

• Resources – Tomás Jesús Madera-Santana; Jaime López-Cervantes; Dalia Isabel Sánchez-Machado; Herlinda Soto-Valdez; Jesús Fernando Ayala-Zavala.

• Software – NA • Supervision – Tomás Jesús Madera-Santana; Jaime López-Cervantes; Dalia Isabel Sánchez-Machado; Herlinda Soto-Valdez; Jesús Fernando Ayala-Zavala.

Polímeros, 33(3), e20230028, 2023

Development and characterization of chitosan-collagen films loaded with honey • Validation – NA. • Visualization – Tomás Jesús Madera-Santana; Jaime López-Cervantes.

• Writing – original draft – David Servín de la Mora-López.

• Writing – review & editing – Tomás Jesús MaderaSantana; Jaime López-Cervantes; Dalia Isabel SánchezMachado; Herlinda Soto-Valdez; Jesús Fernando Ayala-Zavala.

6. Acknowledgements David Servín de la Mora López would like to thank to CONACyT for the scholarship granted for his doctoral studies. Part of this research was conducted at the facilities of Laboratorio Nacional CONACYT LANNBIO-Cinvestav Unidad Mérida (PROY. No. 321119).

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(2016). Apósitos de polímeros naturales para regeneración de piel. Revista Mexicana de Ingeniería Biomédica, 37(3), 235-249. http://dx.doi.org/10.17488/rmib.37.3.4.

11. Majtan, J. (2014). Honey: an immunomodulator in wound healing. Wound Repair and Regeneration, 22(2), 187-192. http://dx.doi.org/10.1111/wrr.12117. PMid:24612472.

12. El-Kased, R. F., Amer, R. I., Attia, D., & Elmazar, M. M. (2017). Honey-based hydrogel: in vitro and comparative In vivo evaluation for burn wound healing. Scientific Reports, 7(1), 9692. http://dx.doi.org/10.1038/s41598-017-08771-8.

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13. Shamloo, A., Aghababaie, Z., Afjoul, H., Jami, M., Bidgoli, M. R., Vossoughi, M., Ramazani, A., & Kamyabhesari, K. (2021).

Fabrication and evaluation of chitosan/gelatin/PVA hydrogel incorporating honey for wound healing applications: An in vitro, in vivo study. International Journal of Pharmaceutics, 592, 120068. http://dx.doi.org/10.1016/j.ijpharm.2020.120068.

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14. Sarhan, W. A., & Azzazy, M. H. M. (2015). High concentration honey chitosan electrospun nanofibers: biocompatibility and antibacterial effects. Carbohydrate Polymers, 122, 135-143. http://dx.doi.org/10.1016/j.carbpol.2014.12.051. PMid:25817652.

15. Rodríguez-Núñez, J. R., Madera-Santana, T. J., SánchezMachado, D. I., López-Cervantes, J., & Soto-Valdez, H. (2014).

Chitosan/hydrophilic plasticizer-based films: Preparation, physicochemical and antimicrobial properties. Journal of Polymers and the Environment, 22(1), 41-51. http://dx.doi. org/10.1007/s10924-013-0621-z.

16. Escárcega-Galaz, A. A., Sánchez-Machado, D. I., LópezCervantes, J., Sanches-Silva, A., Madera-Santana, T. J., & Paseiro-Losada, P. (2018). Mechanical, structural and physical aspects of chitosan-based films as antimicrobial dressings.

International Journal of Biological Macromolecules, 116, 472-481. http://dx.doi.org/10.1016/j.ijbiomac.2018.04.149.

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17. American Society for Testing and Materials. (2002). ASTM D882-02: standard test method for tensile properties of thin plastic sheeting. West Conshohocken, PA: ASTM. 18. Madera-Santana, T. J., Freile-Pelegrín, Y., & Azamar-Barrios, J. A. (2014). Physicochemical and morphological properties of plasticized poly(vinyl alcohol)–agar biodegradable films.

International Journal of Biological Macromolecules, 69, 176-184. http://dx.doi.org/10.1016/j.ijbiomac.2014.05.044.

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19. American Society for Testing and Materials. (2010). ASTM E96/ E96M-10: standard test methods for water wapor transmission of materials. Philadelphia, PA: ASTM. 20. González-Pérez, C. J., Tanori-Cordova, J., Aispuro-Hernández, E., Vargas-Arispuro, I., & Martínez-Téllez, M. A. (2019).

Morphometric parameters of foodborne related-pathogens estimated by transmission electron microscopy and their relation to optical density and colony forming units. Journal of Microbiological Methods, 165, 105691. http://dx.doi. org/10.1016/j.mimet.2019.105691. PMid:31437554.

21. Rivero, S., Garcia, M. A., & Pinotti, A. (2009). Composite and bi-layer films based on gelatin and chitosan. Journal of Food Engineering, 90(4), 531-539. http://dx.doi.org/10.1016/j. jfoodeng.2008.07.021.

22. Rocha-Lemus, L. M., Azamar-Barrios, J. A., Ortíz-Vazquez, E., Quintana-Owen, P., Freile-Pelegrín, Y., Gamboa-Perera, F., & Madera-Santana, T. J. (2021). Development and physical characterization of novel bio-nanocomposite films based on reduced graphene oxide, agar and melipona honey. Carbohydrate Polymer Technologies and Applications, 2, 100133. http:// dx.doi.org/10.1016/j.carpta.2021.100133.

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Servín de la Mora-López, D., Madera-Santana, T. J., López-Cervantes, J., Sánchez-Machado, D. I., Ayala-Zavala, J. F., & Soto-Valdez, H. 23. Ziani, K., Oses, J., Coma, V., & Maté, J. I. (2008). Effect of the presence of glycerol and Tween 20 on the chemical and physical properties of films based on chitosan with different degree of deacetylation. Food Science and Technology (Campinas), 41(10), 2159-2165. http://dx.doi.org/10.1016/j. lwt.2007.11.023.

24. Amiri, N., Moradi, A., Tabasi, S. A. S., & Movaffagh, J. (2018).

Modeling and process optimization of electrospinning of chitosan-collagen nanofiber by response surface methodology.

Materials Research Express, 5(4), 045404. http://dx.doi. org/10.1088/2053-1591/aaba1d.

25. Shah, R., Stodulka, P., Skopalova, K., & Saha, P. (2019). Dual crosslinked collagen/chitosan film for potential biomedical applications. Polymers, 11(12), 2094. http://dx.doi.org/10.3390/ polym11122094. PMid:31847318.

26. Samadieh, S., & Sadri, M. (2021). Preparation and Biomedical properties of transparent chitosan/gelatin/honey/aloe vera nanocomposite. Nanomedicine Research Journal, 5(1), 1-12. http://dx.doi.org/10.22034/nmrj.2020.01.001.

27. Campa-Siqueiros, P., Madera-Santana, T. J., Ayala-Zavala, J. F., López-Cervantes, J., Castillo-Ortega, M. M., & HerreraFranco, P. J. (2020). Nanofibers of gelatin and polyvinylalcohol-chitosan for wound dressing application: fabrication

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and characterization. Polímeros: Ciência e Tecnologia, 30(1), e2020006. http://dx.doi.org/10.1590/0104-1428.07919.

28. Costa, L. C. V., Kaspchak, E., Queiroz, M. B., Almeida, M. M., Quast, E., & Quast, L. B. (2015). Influence of temperature and homogenization on honey crystallization. Brazilian Journal of Food Technology, 18(2), 155-161. http://dx.doi. org/10.1590/1981-6723.7314.

29. Raisi, A., Asefnejad, A., Shahali, M., Kazerouni, Z. A. S., Kolooshani, A., Saber-Samandari, S. S., Moghadas, B. K., & Khandan, A. (2020). Preparation, characterization, and antibacterial studies of N, O-carboxymethyl chitosan as a wound dressing for bedsore application. Archives of Trauma Research, 9(4), 181-188. http://dx.doi.org/10.4103/atr.atr_10_20.

30. Radoor, S., Karayil, J., Jayakumar, A., Siengchin, S., & Parameswaranpillai, J. (2021). A low cost and eco-friendly membrane from polyvinyl alcohol, chitosan and honey: synthesis, characterization and antibacterial property. Journal of Polymer Research, 28(3), 82. http://dx.doi.org/10.1007/ s10965-021-02415-2.

Received: May 29, 2023 Revised: Aug. 12, 2023 Accepted: Aug. 17, 2023

Polímeros, 33(3), e20230028, 2023

Development and characterization of chitosan-collagen films loaded with honey

Supplementary Material Supplementary material accompanies this paper.

Supplementary information Figure SI1. Chitosan-collagen films loaded with honey.

Figure SI2. Solubility of films based on chitosan and collagen loaded with honey in PBS solution and deionized water.

Different letters in each column indicate a significant difference (p˂0.05).

This material is available as part of the online article from https://doi.org/10.1590/0104-1428.20230031

Polímeros, 33(3), e20230028, 2023

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.20230053

Pectin-based films with thyme essential oil: production, characterization, antimicrobial activity, and biodegradability Greice Ribeiro Furlan1 , Wendel Paulo Silvestre2*  and Camila Baldasso1,2  Universidade de Caxias do Sul – UCS, Caxias do Sul, RS, Brasil Programa de Pós-graduação em Engenharia de Processos e Tecnologias – PGEPROTEC, Universidade de Caxias do Sul – UCS, Caxias do Sul, RS, Brasil 1

2

*wpsilvestre@ucs.br

Abstract This work aimed to incorporate thyme essential oil into films composed of pectin to provide antimicrobial action to them. The effect of adding essential oil on the films’ mechanical, physical-chemical, and barrier properties and their degradability was evaluated. Essential oil addition was possible by using Tween® 20 as an emulsifier, and it was possible to observe antimicrobial activity in the films containing 1.0 wt.% and 2.0 wt.% essential oil. The films containing thyme essential oil were more elastic and thicker but less resistant, with high permeability to water vapor and more hydrophilic relative to other formulations. Scanning electron microscopy analysis showed the presence of heterogeneities in the formulations with essential oil. The films produced using the optimized formulation (30 wt.% glycerol, 1.0 wt.% thyme essential oil, and 0.5 wt.% Tween® 20 relative to pectin mass) degraded entirely after 24 days of exposure to standard soil.

Keywords: active packaging, biological activity, biopolymer, terpenes.

How to cite: Furlan, G. R., Silvestre, W. P., & Baldasso, C. (2023). Pectin-based films with thyme essential oil: production, characterization, antimicrobial activity, and biodegradability. Polímeros: Ciência e Tecnologia, 33(3), e20230029. https://doi.org/10.1590/0104-1428.20230053

1. Introduction The use of polymers made transporting and storing various products more convenient, securing their protection, and increasing shelf-life. However, the high turnover of plastic products and their large-scale production causes the accumulation of waste in the environment. These degradationresistant materials remain for years in the soil. Researchers, governments, and the productive chain are searching for ways to reduce plastic waste generation. Research is also being conducted to develop materials capable of decomposing faster using biopolymers[1].

Among the materials from renewable sources that can be used to produce polymer films, polysaccharides, lipids, and proteins stand out. These raw materials give a renewable characteristic to the polymer and, generally, its degradability. However, more in-depth studies are essential for improving these materials’ mechanical and barrier properties and production processes so they can compete with synthetic plastics[2-3].

Another property that can be conferred to biodegradable polymers is antioxidant and/or antimicrobial activity by inserting additives into the polymer matrix. The addition of essential oils to biofilms is being researched since some essential oils show antimicrobial activity, and their use complies with the need for safer food[2,4]. Using films with antimicrobial activity expands the field of applications to products prone to quick

Polímeros, 33(3), e20230029, 2023

deterioration due to microbial action. The use of pectin, a byproduct of apple juice extraction, in the production of biofilms can be a potential destination for this waste[5-6].

Recently, materials were developed to be used in fruit coatings (apples and oranges), such as shellac and carnauba wax, being used alone or in combination[7]. The advantages of this application for biodegradable polymers are the reduction of synthetic plastics as packaging materials and the possibility of adding preservatives and other ingredients to the polymer matrix. The latter option is a possible solution to the growing demand for safe and environmentally friendly food[3].

Among the biopolymers reportedly used in the production of biofilms are alginate[8-9], chitosan[10-11], soy protein[12], whey[13-15], and pectin[2,16], among others. Research also focuses on adding antimicrobial and antioxidant agents to film formulations. However, studies using pectin as the main biopolymer in producing this type of film are scarce[17].

Traditionally, pectin is mainly used in the food industry as a jellifying, thickening, or stabilizing agent. Its application is diverse, being able to be present in products based on fruits, dairy, confectionery, bakery, and even in products from the pharmaceutical industry[2,18]. Commercially used pectin may be extracted from citrus fruits or apple residues[19]. Historically, apple was considered the primary pectin source. However, in the last few years, the use of citrus wastes as a feedstock

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

Furlan, G. R., Silvestre, W. P., & Baldasso, C. for pectin obtainment increased substantially. More recently, beet pulp is also being used as a source of pectin. Europe and citrus-producing countries such as Brazil and Mexico compose the main pectin production centers worldwide[18].

Natural antimicrobial agents can be added to pectin-based films, satisfying consumer demand for food free from chemical additives. Usually, compounds with these characteristics that are added to pectin films are antimicrobial peptides, essential oils, and polyphenols[6]. In this context, antimicrobial agents are added to film formulations and food coatings to avoid or delay food deterioration and reduce the risk of pathogen contamination. The most commonly used substances are organic acids, plant extracts, and essential oils[17].

The antimicrobial activity of essential oils is generally attributed to their terpene content. These substances, due to their lipophilicity, interact with the cell membrane. The action of these compounds causes the disorganization of the cell membrane of the microorganisms, increasing their permeability to ions and eventually leading to the rupture of the membrane[17,20-21].

Several essential oils are reported as antimicrobial additives for biofilms, such as oregano[9,13-14], cinnamon[22], rosemary[13], lavender[23], basil[24], garlic[13,25], sage[26], clove[9,23], anise[23], and thyme[22-23], among others. Thyme essential oil has antimicrobial activity, but few works used this essential oil as an antimicrobial additive in biopolymer-based films[23].

So, this study also aimed to evaluate the characteristics of pectin films with thyme essential oil.

Igarashi[27] evaluated the antimicrobial activity of an edible alginate film, testing the addition of thirteen essential oils. The results showed that adding some essential oils made the film capable of inhibiting microorganisms of the genera Salmonella and Pseudomonas and the bacterium Listeria monocytogenes. Among the essential oils investigated, only clove essential oil inhibited all strains of microorganisms tested.

This essential oil also increased the permeability to water vapor, tensile strength, and elongation at break of the film.

Souza[28] reported that cassava-based films with 2.0 wt.% clove essential oil inhibited P. commune and E. amstelodami, fungi commonly found in bakery products.

Ojagh et al.[29] developed a biodegradable film composed of chitosan and cinnamon essential oil, evaluating the produced film’s mechanical, physical, and antibacterial properties.

Essential oil concentrations between 0.4 v.% and 2.0 v.% were tested. It was found that after the addition of essential oil, there was an increase in the antimicrobial activity of the film, as well as a decrease in moisture content, water solubility, and water vapor permeability.

Thus, this work aimed to produce pectin-based films containing thyme essential oil (Thymus vulgaris L.) as an additive to verify the impact of the presence of the essential oil on the mechanical, physicochemical, barrier, degradability, and antimicrobial capacity of the films.

2. Materials and Methods 2.1 Determination of the formulation and film preparation Preliminary tests were conducted to determine the optimal formulation of the films containing pectin, glycerol, and 2/10

thyme essential oil. Pectin concentration in the filmogenic solution was varied in the proportions of 0.5 wt.%, 1.0 wt.%, 2.0 wt.%, and 3.0 wt.%[30-31], using commercial pectin (CAS number 9000-69-5, > 75 wt.% galacturonic acid, Sigma Aldrich, USA). After determining the adequate pectin concentration, glycerol (CAS number 51-86-5, 99% purity, Sigma Aldrich, USA) addition was tested at 10 wt.%, 20 wt.%, and 30 wt.% relative to pectin mass[32].

Aiming to evaluate the emulsifier that promoted the best interaction between the essential oil and the polymer matrix, soy lecithin (CAS number 8002-43-5, Sigma Aldrich, USA), xanthan gum (CAS number 11138-66-2, Sigma Aldrich, USA), Tween® 20 (VWR Life Science, USA), and Emustab® (Mix Ingredientes, Brazil) were tested, at the concentration of 0.5 wt.%. Thyme essential oil was kept at 1.0 wt.% in all tests. The essential oil was obtained by steam distillation, purchased from Tekton company (Viamão, Brazil); the essential oil was of the thymol chemotype, with 33 wt.% thymol and 25 wt.% p-cymene as the major compounds.

For the previous selection of the optimal concentrations of each component, a visual analysis of the films was carried out, considering aspects such as uniformity, presence of cracks, homogeneity, and flexibility. When choosing the emulsifier, visual and tactile aspects of the filmogenic solution and the formed films were evaluated. The homogeneity of the film-forming solution and the uniformity, roughness, and opacity of the films produced were considered.

To evaluate the most adequate essential oil concentration aiming for an antimicrobial effect, films containing thyme essential oil concentrations of 0.5 wt.%, 1.0 wt.%, and 2.0 wt.% (relative to pectin mass) were produced[33-34].

All films were produced by casting, following the procedures described, regardless of the formulation.

The ingredients were weighed and dissolved in heated distilled water (75 °C) under constant stirring with a magnetic stirrer (120 rpm). After the complete dissolution of the ingredients, the solution was left to rest for 20 min to remove bubbles, and the solution was poured into glass Petri dishes with a diameter of 11 cm. The cast solutions were dried for 48 h at 26 °C to form the films.

2.2 Determination of the antimicrobial activity of the films The disk-diffusion method[35] was used to evaluate the antimicrobial activity of the films containing different concentrations of thyme essential oil. The produced films were cut into 1.0 cm x 1.0 cm squares and put into Petri dishes with a diameter of 11 cm containing agar nutrient medium, previously inoculated with Escherichia coli (ATCC 25922). The Petri dishes were incubated in a B.O.D. at 35 °C for 24 h. The antimicrobial activity of the films was determined by the qualitative evaluation of the inhibition halo formed in the medium, considering the samples with more antimicrobial activity and those with higher inhibition halos relative to the control (without essential oil). The lowest essential oil concentration that produced an antimicrobial effect was determined. This concentration, along with pectin and glycerol concentrations of the filmogenic solution, was used to determine the optimal formulation. For the other tests and characterization analyses, three formulations were tested, using the optimal concentrations determined Polímeros, 33(3), e20230029, 2023

Pectin-based films with thyme essential oil: production, characterization, antimicrobial activity, and biodegradability previously, being denominated: Pec – film composed only of pectin; Pec/Gly – film composed of pectin and glycerol, and Pec/Gly/EO – film composed of pectin, glycerol, and thyme essential oil.

2.3 Film characterization Film thickness was measured using a digital micrometer (Mitutoyo, Japan) with a measurement range of 0.001 – 25 mm and a resolution of 1.0 µm at five random points of the film, and calculating the arithmetic mean of the measurements.

The determination of the mechanical properties of the films followed the ASTM D882-00 standard. The tensile tests were performed using an EMIC universal testing machine, model DL 3000, with a spacer clearance speed of 50 mm·min-1 and initial spacing of 5 cm.

The films’ permeability to water vapor (PWV) was determined following the ASTM E96-00 standard. Film solubility in water was measured following the methods described by Bierhalz[36] and Tong et al.[37]. The contact angle of the films with water was measured[38] using the Surftens software to calculate the contact angles.

The microstructure and morphology of the surface of the films were assessed by scanning electron microscopy (SEM). The presence of metals on the film surface was also assessed by energy dispersive spectroscopy (EDS).

The tests were conducted using an SEM-FEG microscope Mira 3 LM (TESCAN, Czech Republic).

2.4 Degradability tests The films’ degradability was evaluated using a standard soil, following the ASTM G160-12 standard[15]. Film samples made using the optimal formulation were packed with inert net packaging to protect the samples against physical damage and help identify them in the soil. The net packages containing the samples were visually evaluated daily until the complete degradation of the film samples.

2.5 Experimental design and statistical analysis The tests followed a completely randomized design, with three replicates for each treatment (film formulation).

The data regarding the optimal formulation underwent analysis of variance (ANOVA), followed by Tukey’s multiple range test at a 5% error probability (p = 0.05). The statistical analyses were conducted using the Statistica 12 software (StatSoft, USA).

3. Results and Discussions 3.1 Determination of the optimized film formulation The first stage of the study consisted of evaluating the pectin concentration. The concentration of 2.0 wt.% in the filmogenic solution was chosen among the other concentrations tested because it generated films more resistant to handling and easier to release from the Petri dish. Film-forming solutions with very high pectin concentrations result in brittle films, and the increase in the viscosity makes the handling and casting process difficult[39].

In the stage evaluating the plasticizer concentration, it was noticed that in all tested concentrations, there was an Polímeros, 33(3), e20230029, 2023

improvement in the flexibility of the formed films. This could be the result of the interaction of this component with the pectin polymer chains. According to Camargo et al.[40], the plasticizer acts on the intermolecular forces between the pectin chains, decreasing their intensity and increasing the free space in the polymer matrix. Such a phenomenon facilitates movement between the polymer chains, consequently increasing film flexibility.

In this context, due to the greater flexibility, the film containing 30 wt.% glycerol relative to pectin mass (0.6 wt.% in the filmogenic solution) was chosen to compose the other formulations, together with the pectin at 2.0 wt.%. Although visually, this glycerol concentration was the most suitable (by improving film malleability), higher concentrations of plasticizer can also decrease film strength and increase the permeability to water vapor due to the increase in free space in the polymer matrix, facilitating the passage of water vapor[40].

Considering that thyme essential oil is immiscible in the filmogenic solution, it was necessary to use an emulsifier to incorporate it into the film. For the choice of emulsifier, the pre-stipulated concentrations of pectin and glycerol were maintained, and a thyme essential oil concentration of 1.0 wt.% (relative to pectin mass) was used. The emulsifiers soy lecithin, xanthan gum, Tween® 20, and Emustab® were tested at 0.5 wt.% (relative to pectin mass).

In the films produced with soy lecithin, phase separation was observed in the filmogenic solution, probably caused by partial emulsification of the oil. Although the mixing was carried out at a low temperature (50 °C), the formulation also showed the formation of white clots, possibly due to the denaturation of lecithin. Regarding the formulation containing xanthan gum, a homogeneous filmogenic solution was observed, and there was no phase separation, although the films were opaque. In addition, this polysaccharide could interfere with the evaluation of films because xanthan gum is a polymer also used to formulate biodegradable films[17].

Meydanju et al.[17] reported the preparation of lemon peel-based films with the addition of xantham gum and TiO2Ag nanoparticles. The added materials greatly improved the films’ physical-chemical and antimicrobial properties, and that xantham gum alone influenced film color and increased the moisture content. The same authors commented that homogeneous films were formed using xantham gum, probably due to the formation of a homogeneous network between the xantham gum and the polymeric matrix.

The films produced with Tween® 20 and Emustab® as emulsifiers had the best visual appearances among the four formulations. However, films containing Tween® 20 as the emulsifier were more uniform when compared to those produced with Emustab®. The visual aspect of the films produced using each of the four tested emulsifiers is shown in Figure 1.

The Tween® 20 emulsifier is widely used in various formulations containing essential oils and also acts as a stabilizer in terpene-containing emulsions and essential oil encapsulation processes, producing microcapsules and nanocapsules. Tween® 20 acts as an emulsifier by stabilizing the dispersion of essential oil throughout the film structure by forming micelles, helping to distribute 3/10

Furlan, G. R., Silvestre, W. P., & Baldasso, C. and incorporate the essential oil components along the film profile. This mechanism also avoids the coalescence of essential oil particles, a phenomenon caused mainly due to density differences between the essential oil and the other components of the film solution[41-42].

Based on the visual aspect, it was decided to use the emulsifier Tween® 20 to compose the formulations containing essential oil since this emulsifier formed more uniform films without opacity or any other undesirable visual appearance.

3.2 Determination of the optimized film formulation The choice of the optimum thyme essential oil concentration in the films was based on a qualitative evaluation of the antimicrobial action caused by each formulation. Film samples underwent the disk-diffusion test in agar (halo test), evaluating the antimicrobial activity of these films against the bacterium Escherichia coli, which can become pathogenic when ingested through food and drink, causing infections in its host[43]. The results of the disk-diffusion test of the produced films containing thyme essential oil are shown in Figure 2.

The film samples that did not contain thyme essential oil (control) and the film containing 0.5 wt.% essential oil did not show inhibiting effects on the growth of E. coli.

However, formulations containing 1.0 wt.% and 2.0 wt.% essential oil showed inhibition halos (Figure 2), indicating the presence of antimicrobial activity. Furthermore, it is

possible to observe that the inhibition of bacterial growth was proportional to the concentration of essential oil, confirming the antimicrobial activity attributed to thyme essential oil[23].

Almasi et al.[44], evaluating the antibacterial activity of pectin films containing marjoram essential oil, reported that the produced films had antimicrobial activity, although weaker than essential oil emulsions at the same concentration.

Although the formulation containing 2.0 wt.% essential oil had a greater antimicrobial effect against E. coli, its films had an oily appearance, indicating partial incorporation of the oil into the polymer matrix and increasing the production costs of the films due to the greater amount of essential oil used.

In this context, a formulation with 1.0 wt.% essential oil was chosen, considering that this was the lowest concentration to have an inhibitory action on the tested bacterium.

3.3 Film characterization The results regarding the mechanical properties of the films produced (pectin, pectin and glycerol, and pectin, glycerol, and thyme essential oil) are shown in Table 1.

Film thickness varied significantly with the alteration of the formulation, in which the addition of components to the films caused an increase in thickness. This same effect was observed by Lorevice[45] when adding the surfactant Tween® 80 to pectin films, which caused an increase in film thickness.

The same author also pointed out that this variation could be associated with an increase in the amount of solids in the

Figure 1. Visual aspect of the films produced with soy lecithin (A); xanthan gum (B); Tween® 20 (C); and Emustab® (D).

Figure 2. Disk-diffusion test in agar nutrient medium of the films with zero (A); 0.5 wt.% (B); 1.0 wt.% (C); and 2.0 wt.% (D) thyme essential oil (relative to pectin mass) against Escherichia coli.

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Pectin-based films with thyme essential oil: production, characterization, antimicrobial activity, and biodegradability matrix. However, films produced with the same formulation showed a uniform thickness between them. Almasi et al.[44] also observed an increase in the thickness of pectin films with the addition of marjoram essential oil, either through nanoemulsion or stabilized essential oil emulsion.

Caetano[46] commented on the influence of adding oregano essential oil in the polymer matrix on film thickness, causing a thickening of the film. This increase in thickness was also observed in the present work by adding thyme essential oil to the films produced, probably caused by the increase in the spacing between the polymer chains.

The tensile test showed information about the mechanical properties of the different formulations. According to Table 1, adding glycerol caused the formation of more flexible films due to the reduction of attractive intermolecular forces between the pectin chains. However, this effect also caused a reduction in mechanical strength, as evidenced by the decrease in the tensile strength and Young modulus of the films relative to the pectin-only film.

This same characteristic associated with the addition of plasticizer is reported by Camargo et al.[40], in which the addition of glycerol, even at concentrations below 15 wt.%, caused a decrease in the mechanical strength of the films.

The same effects observed with the addition of glycerol also occurred with the addition of thyme essential oil, but more markedly. Essential oils also have plasticizing properties when incorporated into biopolymers such as pectin. Thus, glycerol and thyme essential oil may act together as plasticizers[47-48].

Considering the plasticizing effect of the essential oil, obtaining a more malleable film may be interesting for packaging applications. On the contrary, the decrease in Young modulus may render the film too compliant, making it prone to rupture even when strained by smaller forces.

Thus, it is important to consider that, depending on the application envisaged, a reinforcing agent may be necessary to avoid excessive film maleability[48]. Another possible option is reducing plasticizer (glycerol) content, aiming to balance the plasticizer effects of it and the essential oil[45,48].

Table 1. Mechanical properties of the films composed of pectin (Pec), pectin and glycerol (Pec/Gly), and containing pectin, glycerol, and thyme essential oil (Pec/Gly/EO).

Parameter Pec Thickness (µm) 85.2±4.5 c Elongation at break (%) 3.1±1.0 c Tensile strength (MPa) 50.7±7.1 a Young modulus (MPa) 42.7±13.6 a

Pec/Gly Pec/Gly/EO 101.9±6.3 b 146.0±5.8 a 6.0±2.9 b 12.1±3.1 a 26.9±6.0 b 16.9±2.2 c 22.6±7.2 a 13.3±2.2 b

Means in row followed by the same lowercase letter do not differ statistically by Tukey’s multiple range test at a 5% error probability (p = 0.05).

Lorevice[45] mentioned that Tween® 20, added to the filmogenic solution to emulsify the essential oil, may have a plasticizing effect. Thus, the presence of all these components can justify the increase in elongation at break and the decrease in tensile strength and Young modulus, as observed in the Pec/Gly/EO formulation. However, it is important to note that the addition of essential oil may not have a plasticizing effect in all cases, and it is possible to reduce the elongation at break, especially if the interaction between the polymer and the essential oil is impaired by the presence of components with a destabilizing effect[46].

Considering that commercial films, such as poly(vinyl chloride) films, present elongation at break in the range of 120 – 250% and tensile strength in the range of 15 – 21 MPa[49], it can be observed that the films produced were much less flexible, but their strength was comparable to that of commercial films. For the formulation containing only pectin (Pec), the tensile strength was superior to commercial films (42.7±13.6 MPa).

Table 2 shows the physicochemical properties of solubility, permeability to water vapor, and contact angle with water of the films produced. The images of contact angle measurement are shown in Figure 3.

It was not possible to determine the degree of solubility of the films, as they were completely solubilized after remaining in contact with distilled water for approximately 10 min, preventing the test from being carried out. Ngo et al.

[50] also reported complete solubilization of films composed of pectin as a base polymer and about 45% solubility in films composed of 75 wt.% pectin and 25 wt.% chitosan nanoparticles. Since pectin is highly hydrophilic, like most polysaccharides, the films were expected to be poorly water-resistant[51].

The permeability to water vapor (PWV) values observed show that the addition of glycerol and thyme essential oil caused an increase in the permeability of the films. Despite many studies involving biodegradable polymers, the difficulty in producing films with barrier properties similar or superior to those of plastic films of fossil origin is recurrent[36].

The PWV values determined for each film produced aligned with the values reported in the literature. This parameter can be influenced by the type of process used to prepare the films or the composition of the filmogenic solution. Batista[5] determined the PWV values of pectin films, observing average values of 6.80 g∙day-1∙mm-1∙kPa-1, close to the PWV values determined in this study (6.81 g∙day-1∙mm-1∙kPa-1). On the other hand, Ngo et al.[50], observed a decrease in PWV with the addition of chitosan nanoparticles to pectin films (from 1.33 g·mm-1·day-1·kPa-1 in films composed only of pectin to 0.27 g·mm-1·day-1·kPa-1 in the films containing 75 wt.% pectin and 25 wt.% chitosan nanoparticles). Nisar et al.[52]

Table 2. Physical-chemical properties of the films composed of pectin (Pec), pectin and glycerol (Pec/Gly), and containing pectin, glycerol, and thyme essential oil (Pec/Gly/EO).

Parameter Permeability to water vapor (g∙day-1∙mm-1∙kPa-1) Contact angle with water (°)

Pec 6.8±1.0 c 84±1 a

Pec/Gly 8.6±0.2 b 73±1 b

Pec/Gly/EO 11.4±1.0 a 39±1 c

Means in row followed by the same letter do not differ statistically by Tukey’s multiple range test at a 5% error probability (p = 0.05).

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Figure 3. Images of contact angle measurements of the films composed of pectin (Pec), pectin and glycerol (Pec/Gly), and containing pectin, glycerol, and thyme essential oil (Pec/Gly/EO).

reported a decrease in PWV of chitosan films containing increasing proportions of clove essential oil.

Caetano[46], studying the effects caused by the variation of the concentration of glycerol and essential oil of oregano on the PWV of cassava starch films, reported an inverse relationship between the PWV values and the glycerol concentration used in the production of the films. The plasticizing effect potentiated by the combined action of glycerol and essential oil can weaken interactions between polymer chains and would cause an increase in film permeability.

This fact may explain the increase in PWV in films containing only glycerol and even higher PWV values in films with thyme essential oil relative to films containing only pectin, which had the lowest PWV. However, Ezati and Rhim[53] did not observe a statistical difference in the PWV of pectin films with the addition of curcumin and sulfur nanoparticles, indicating that not all additives used would cause an increase in film PWV. Regarding the contact angles of the films with water, it can be observed that the addition of glycerol and thyme essential oil caused a reduction in the contact angle in relation to the film composed only of pectin (Pec) and the formulation containing both glycerol and essential oil (Pec/ Gly/EO) showed the smallest angle (39°) between the three formulations. Sriamornsak et al.[54] observed water contact angles in the range of 60 – 90° for films composed only of pectin with different specifications. With the addition of mucin, the contact angles of the films were reduced to less than 40°, except for only one type of pectin, which showed a reduction in the contact angle from 85° to 80°. On the other hand, Ezati and Rhim[53] observed an increase in the contact angle of pectin films by adding curcumin and sulfur nanoparticles. Ngo et al.[50] reported a water contact angle of 62° for pectin films. The presence of chitosan nanoparticles influenced the contact angle; the contact angles increased with an increasing nanoparticle content.

Considering that all formulations had contact angles below 90°, the films had a hydrophilic character[55], which was accentuated with the addition of glycerol and essential oil. It is also important to note that the PWV of the produced films showed similar behavior to that of the contact angles of the films with distilled water since these two parameters are related to the type of interaction between the film and water, which was attractive (hydrophilic).

Although thyme essential oil and other essential oils have a hydrophobic character, adding this material increased the hydrophilicity of the films. It is important to observe that the dispersion of essential oil (and other hydrophobic materials) throughout a polar polymeric matrix must be stabilized, and 6/10

this occurs by micelle formation[50,53-54]. Thus, considering that the polar part of the micelles is pointed outward, the presence of these polar moieties may enhance the polarity of the film, easing interaction with water and its transport, which reflects in a higher permeability to water vapor and decreased contact angle, even with the presence of dispersed hydrophobic particles in the polymeric matrix.

In addition, this increase in water permeability and reduction in contact angle may be a hindrance when considering using this material as a packaging material for materials or foods with considerable amounts of water. On the other hand, this behavior can make this material feasible to be used as a water absorber in some packages and for agricultural applications, such as seed coating.

3.4 Microstructure and morphology of the produced films SEM analysis was used to evaluate the morphological aspects of the developed films. The Pec and Pec/Gly formulations were regular and without substantial defects or heterogeneities. However, the films containing thyme essential oil (Pec/Gly/EO) showed irregularities on their surface. The SEM images of the surface of the three formulations studied are shown in Figure 4.

The occurrence of irregularities can be associated with the formation of lipid droplets dispersed in the matrix that were not fully incorporated[56]. Siracusa et al.[57] also observed morphological changes in the films containing the oil, which the authors attributed this behavior to an uneven dispersion of the essential oil into the polymer matrix.

According to energy-dispersive X-ray spectroscopy (EDS) analysis, alkaline and alkaline earth metals were observed in all formulations. Most likely, these elements come from the pectin obtainment processes or even contaminants from the raw material from which the pectin was extracted[58].

The structures with cylindrical shapes were noticed in the films containing thyme essential oil. The SEM image at 5,000 x magnification, showing the presence of these structures on the surface of the films produced containing thyme essential oil, is shown in Figure 5.

These particles can be agglomerates of essential oil due to its incomplete incorporation in the polymeric matrix and its crystallization after drying the filmogenic solution and film formation[57]. Almasi et al.[44] did not observe similar particulates but reported the formation of heterogeneous films containing marjoram essential oil, attributing these heterogeneities to low incorporation of the essential oil into the pectin matrix.

The heterogeneities observed may result from poor essential oil incorporation by the polymer matrix or a Polímeros, 33(3), e20230029, 2023

Pectin-based films with thyme essential oil: production, characterization, antimicrobial activity, and biodegradability

Figure 4. SEM images of the surface of the films composed of pectin (Pec), pectin and glycerol (Pec/Gly), and containing pectin, glycerol, and thyme essential oil (Pec/Gly/EO). Magnification of 1,000 x. The scale bar in the images corresponds to 50 µm.

Figure 5. SEM image of the film’s surface containing pectin, glycerol, and thyme essential oil (Pec/Gly/EO formulation), with a magnification of 5,000 x. The scale bar corresponds to 10 µm.

crystalization process of the essential oil, the emulsifier (Tween® 20), and the plasticizer (glycerol). As the cast film dries, the stabilization of the polymeric chains may cause part of the additives to be expelled from the matrix, building up and crystallizing at the film surface[44,57].

These surface heterogeneities may hinder food applications since the interaction between them and the food can cause these crystals to dissolve and interact with the packaged material. However, a post-treatment may remove these crystals, eliminating this issue. In addition, these crystals can also act as a barrier to further water permeation, avoiding rapid film solubilization in the packaging of low-moisture materials.

3.5 Film degradability tests The degradability test showed that the samples containing thyme essential oil degraded entirely after 24 days in contact with the standard soil used in the test. The evolution of film degradation in this period is shown in Figure 6.

Norcino et al.[59] reported a degradation period of 28 days for pectin films containing copaiba oil (zero to 6.0 wt.%) and exposed to soil. Mendes et al.[60], producing thermoplastic starch Polímeros, 33(3), e20230029, 2023

films containing pectin and lemongrass essential oil, observed that the produced films took 20 days to decompose when exposed to soil. Leonardelli et al.[15], evaluating the biodegradability of whey-based films by adding chitosan as an antimicrobial agent, reported a degradation time of 8 days for films when disposed of in soil. Film degradability was similar to those of other biopolymeric films and superior to commercial films, considering that plastic films have degradation times longer than one year, even when discarded in landfills[61].

The rapid degradation observed for the formulation tested makes it an interesting alternative considering the current environmental problems the synthetic polymers pose and their inherent resistance to degradation. Furthermore, pectin, a common agroindustrial material sometimes regarded as waste, makes this material even more interesting from an environmental standpoint. In this sense, pectin and other polysaccharides can be considered viable and potential alternatives to non-biodegradable polymers as packaging materials, being further studies needed to make this type of material more competitive economically and with standardized feedstock parameters and production procedures for large-scale production.

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Figure 6. Evolution of the degradation of the film composed of pectin, glycerol, and thyme essential oil (Pec/Gly/EO formulation) during 24 days of exposure to standard soil.

4. Conclusions

6. References

The films produced from the filmogenic solution containing 2.0 wt.% pectin and 0.6 wt.% glycerol (30 wt.% relative to pectin mass) had better properties among the formulations tested. Adding thyme essential oil in contents above 1.0 wt.% conferred antimicrobial activity to the film against E. coli.

However, the presence of essential oil reduced the barrier properties and made the films more hydrophilic, causing the reduction of the mechanical properties and thickening of the films. All produced films showed a high degradability, indicating the potential use of these materials as sustainable alternatives to the development of more environmentally friendly packaging materials, being further studies necessary to optimize and standardize the production parameters and feedstock specifications.

1. Platt, D. (2006). Biodegradable polymers: market report.

Shawbury: iSmithers Rapra Publishing.

2. Pirsa, S., & Hafezi, K. (2023). Hydrocolloids: structure, preparation method, and application in food industry. Food Chemistry, 399, 133967. http://dx.doi.org/10.1016/j.foodchem.2022.133967.

PMid:35998495.

3. Vargas, M., Pastor, C., Chiralt, A., McClements, D. J., & González-Martínez, C. (2008). Recent advances in edible coatings for fresh and minimally processed fruits. Critical Reviews in Food Science and Nutrition, 48(6), 496-511. http:// dx.doi.org/10.1080/10408390701537344. PMid:18568856.

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Essential oils: a promising eco-friendly food preservative.

Food Chemistry, 330, 127268. http://dx.doi.org/10.1016/j. foodchem.2020.127268. PMid:32540519.

5. Batista, J. A. (2004). Development, characterization and applications of biofilms based on pectin, gelatin and fatty acids in bananas and broccoli seeds (Master’s dissertation).

Universidade Estadual de Campinas, Campinas.

6. Espitia, P. J. P., Du, W.-X., Avena-Bustillos, R. J., Soares, N. F. F., & McHugh, T. H. (2014). Edible films from pectin: physical-mechanical and antimicrobial properties - a review.

Food Hydrocolloids, 35, 287-296. http://dx.doi.org/10.1016/j. foodhyd.2013.06.005.

7. Miranda, M., Sun, X., Ference, C., Plotto, A., Bai, J., Wood, D., Assis, O. B. G., Ferreira, M. D., & Baldwin, E. (2021).

Nano- and micro- carnauba wax emulsions versus shellac protective coatings on postharvest citrus quality. Journal of the American Society for Horticultural Science, 146(1), 40-49. http://dx.doi.org/10.21273/JASHS04972-20.

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Composition factors affecting the water vapor permeability and tensile properties of hydrophilic films. Journal of Agricultural and Food Chemistry, 43(6), 1432-1435. http://dx.doi.org/10.1021/ jf00054a004.

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5. Author’s Contribution • Conceptualization – Greice Ribeiro Furlan; Camila Baldasso.

• Data curation – Greice Ribeiro Furlan.

• Formal analysis – Wendel Paulo Silvestre.

• Funding acquisition –​ NA. • Investigation – Greice Ribeiro Furlan.

• Methodology – Wendel Paulo Silvestre; Camila Baldasso.

• Project administration – Camila Baldasso.

• Resources – Camila Baldasso.

• Software – NA. • Supervision – Camila Baldasso.

• Validation – Wendel Paulo Silvestre.

• Visualization – Wendel Paulo Silvestre.

• Writing – original draft – ​ Greice Ribeiro Furlan; Wendel Paulo Silvestre.

• Writing – review & editing – Camila Baldasso.

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Journal of Food Science, 61(5), 953-956. http://dx.doi. org/10.1111/j.1365-2621.1996.tb10909.x.

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Physical stability and moisture sorption of aqueous chitosanamylose starch films plasticized with polyols. European Journal of Pharmaceutics and Biopharmaceutics, 58(1), 69-76. http:// dx.doi.org/10.1016/j.ejpb.2004.03.015. PMid:15207539.

12. Kokoszka, S., Debeaufort, F., Hambleton, A., Lenart, A., & Voilley, A. (2010). Protein and glycerol contents affect physicochemical properties of soy protein isolate-based edible films.

Innovative Food Science & Emerging Technologies, 11(3), 503-510. http://dx.doi.org/10.1016/j.ifset.2010.01.006.

13. Seydim, A. C., & Sarikus, G. (2006). Antimicrobial activity of whey protein based edible films incorporated with oregano, rosemary and garlic essential oils. Food Research International, 39(5), 639-644. http://dx.doi.org/10.1016/j.foodres.2006.01.013.

14. Zinoviadou, K. G., Koutsoumanis, K. P., & Biliaderis, C. G. (2009). Physico-chemical properties of whey protein isolate films containing oregano oil and their antimicrobial action against spoilage flora of fresh beef. Meat Science, 82(3), 338-345. http://dx.doi.org/10.1016/j.meatsci.2009.02.004.

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15. Leonardelli, C., Silvestre, W. P., & Baldasso, C. (2020). Effect of chitosan addition in whey-based biodegradable films.

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42. Gorjian, H., Mihankhah, P., & Khaligh, N. G. (2022). Influence of tween nature and type on physicochemical properties and stability of spearmint essential oil (Mentha spicata L.) stabilized with basil seed mucilage nanoemulsion. Journal of Molecular Liquids, 359, 119379. http://dx.doi.org/10.1016/j. molliq.2022.119379.

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45. Lorevice, M. V. (2019). Nanoemulsions of essential oils: stability mechanisms and interactions with pectin in bionanocomposites for use as active packaging (Doctoral thesis). Universidade Federal de São Carlos, São Carlos.

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48. Syafiq, R., Sapuan, S. M., Zuhri, M. Y. M., Ilyas, R. A., Nazrin, A., Sherwani, S. F. K., & Khalina, A. (2020). Antimicrobial activities of starch-based biopolymers and biocomposites incorporated with plant essential oils: a review. Polymers, 12(10), 2403. http://dx.doi.org/10.3390/polym12102403.

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60. Mendes, J. F., Norcino, L. B., Martins, H. H. A., Manrich, A., Otoni, C. G., Carvalho, E. E. N., Piccoli, R. H., Oliveira, J. E., Pinheiro, A. C. M., & Mattoso, L. H. C. (2020). Correlating emulsion characteristics with the properties of active starch films loaded with lemongrass essential oil. Food Hydrocolloids, 100, 105428. http://dx.doi.org/10.1016/j.foodhyd.2019.105428.

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Received: Jul. 11, 2023 Revised: Aug. 21, 2023 Accepted: Sept. 01, 2023

Polímeros, 33(3), e20230029, 2023

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.20230066

Novel modified blister test to evaluate composites used in repairing cracked pipeline Payman Sahbah Ahmed1* , Jafar Abdullah Ali2  and Serwan Sarbast Mohammed Talabani1  1

Strength of Materials Laboratory, Manufacturing & Industrial Engineering Department, Faculty of Engineering, Koya University, Koya, Kurdistan Region, Iraq 2 Strength of Materials Laboratory, Petroleum Engineering Department, Faculty of Engineering, Koya University, Koya, Kurdistan Region, Iraq *payman.suhbat@koyauniversity.org

Abstract The traditional method of fixing cracked pipes by welding needs stopping the production. In this study, a composite material is used to repair pipelines without interrupting production, saving both time and money. Hand Layup and vacuum infusion techniques were used to prepare glass strand mat and intra-ply hybrid glass - carbon fibers composites repairs.

To show the composites’ strength, blister, double cantilever beam, and peel tests were conducted. A novel modified blister test was utilized to show effectiveness of laboratory tests on real pipeline. The results indicate that vacuum infused intra-ply hybrid composite exhibited the highest strength. The experimental results of the best composite were compared with the finite element model under blister test, and the results were found to be identical. The modified blister test on the real pipe provides a better indication about the good strength of the composite repair compared to previous researches.

Keywords: cracked pipe repair, hand lay up, intra-ply hybrid composite, modified blister test, vaccum infusion.

How to cite: Ahmed, P. S., Ali, J. A., & Talabani, S. S. M. (2023). Novel modified blister test to evaluate composites used in repairing cracked pipeline. Polímeros: Ciência e Tecnologia, 33(3), e20230030. https://doi.org/10.1590/0104-1428.20230066

1. Introduction Pipelines are one of the safest modes of transporting oil from fields to consumers, operating under significant pressure, any issue must be fixed cautiously to prevent explosions, and potential harm to; people, equipment and the environment.

Among the various problems that can occur, cracks in the pipe pose the biggest concern and can result from corrosion, stress, or vandalism. Several repair methods exist for corroded pipe such as; welding, clamping and replacement of the damaged section with a new pipe[1]. The process of fixing the cracked pipe via welding is not safe in the oil and gas industries and expensive. This issue is an interesting topic for mechanical researchers for a long time, in particular, repair the fractures using safe, fast and economical method. The use of polymerbased composite systems to repair damaged pipes has been widely used in the offshore and onshore units, due to their suitability and cost-effectivness compared to other to other maintenance options[2]. The advantages of composite over metallic repairs or a full replacement of pipe sections included a better corrosion resistance, higher flexibility for the repair of complex structures, no production shut down or hot work and a minimal lead time[3]. The design of composite repairs of corroded oil and gas pipelines must take into account the strength of the interface adhesion between composite and metal[4]. The degradation of adhesion with time is the main problem[5]. The toughness of composite laminates can be improved by the addition of multi-layers[6]. Although, critical water content and relative humidity could decrease the cohesive strength of composite[7], it is possible, under the

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right conditions, that composite materials be used to repair offshore risers[8].

Researchers in the previous studies; de Barros et al.[2] utilized a composite of fiber glass and epoxy resin with different densities to fix a corroded pipe. Abdelouahed et al.[9] investigated the use of three different composites, graphite with epoxy, boron with epoxy and glass with epoxy. Linden et al.[3] used only the blister test for the composite and CFD package for validation. Liland et al.[10] only carried out the blister test in the study. Cao et al.[11] did a blister test for a graphene/photoresist composite film. Linden et al.[4] investigated blister test on thick fiber-reinforced plates bonded on steel disk. Tan et al.

[7] analyzed the effect of humidity on the composite adhesive with blister test. Barros et al.[12] studied the failure pressure of composite repair using only the blister test and hydrostatic test.

Borowski et al.[6] examined the fracture toughness of carbon fiber reinforced polymer (CFRP) using double cantilever beam (DCB) and delaminate tests. Alexander[8] developed a guideline to help the operators and manufacturers in using composite for fixing offshore risers.

As a result of generation composite hybridization, the demand for high-strength and high-performance composites has recently increased due to their exceptional toughness under diverse thermal and chemical circumstances. Composite hybridzation involves combining tow or more typs of fibers with resin, leveraging the strengths of each fiber while addressing thier limitations. The hybrid composite stacking sequence,

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

Ahmed, P. S., Ali, J. A., & Talabani, S. S. M. where the right types and sequence of fibers are employed and combined, has a significant impact on mechanical properties.

The behavior of the composite material can be significantly improved by taking into account the necessary application and its particular needs for certain features. The number of plies that will be utilized, together with their density and thickness, all affect the fiber type that is chosen[13]. The mechanical and impact properties can be improved by employing a hybrid combination of glass fiber with other types. The usage of carbon-reinforced polymer composites is limited by their high cost, low toughness, and strain to failure, despite the fact that these materials have an extraordinarily high strength. Glass fiber can significantly reduce the cost and strain of failure of carbon-reinforced polymer composites[13]. In the literature, four primary types of hybrid composites are discussed: “interply hybrid composites,” which are made up of different fiber types arranged in different sequences; “intraply hybrid composites,” which are made up of two or more different types of fibers used in the same ply; “interply-intraply hybrid composites,” which are made up of interply and intraply laminates stacked in a specific order; as well as “resin hybrid composites,” which can be created by combining two or more resins rather than changing the type of fiber[14]. The objective of the current study is to employ a composite for reparing fractured pipeline.

The study aims to conduct a comperhanesive laboratory test on multi layers composite. The novelties of this work lies in utilizing intra-ply hybridizing and different manufacturing processes to repair cracked composites, along with modifying the original blister test to better simulate realistic conditions in petroleum fields.

matrix can be found in Table 1. Glass strand mat and Intraply carbon+glass fibres (Figure 1) were used as reinforcement, the properties of fibres are listed in Table 2.

Two Manufacturing Processes were used to prepare composite materials repairs: Hand lay-up and vacuum Infusion Technique as listed in Table 3. Figure 2 shows that the layers of the fibers are placed on a piece of glass, followed by adding the epoxy matrix to each layer then another glass piece is placed over the composite layers to push the bubbles outside the composite plate and then fixed tightly by 4 clamps. The samples were cured as a final preparation step at 112 °C for 4 hrs[15]. Vacuum Infusion is used to prepare the carbon and glass hybrid composite, as shown in Figure 3. Four plies are placed together between

2. Experimental 2.1 Materials Epoxy resin (Sikadur-52, Sika Company) is employed as the matrix and consists of two components, namely, low viscosity resin and hardener, where three parts of resin are mixed with one part of hardener. The properties of the epoxy

Table 1. Properties of epoxy matrix (provided by the supplier).

Compression Strength 53 N/mm2

Flexural Strength 50 N/mm2

Tension Strength 25 N/mm2

Modulus of Elasticity 1.06 KN/mm2

Figure 1. (a) Intra-Ply Hybrid Glass+Carbon; (b) Chopped Strand Mat Glass used in this Study.

Table 2. Properties of fibers (provided by the supplier).

Fiber Type Intra-Ply Hybrid Glass+Carbon Chopped Strand Mat Glass

Young’s Modulus (GPa) 151 7.5

Tensile Strength (MPa) 3625 111

Elongation (%) 2.8 3.5

Density (gm/cm3) 2.18 1.6

Table 3. Composites of this study.

Symbol of Composite Material 6GCH 4GCH 4RGH 4GCV

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Type of fiber Intra-Ply hybridized glass and carbon Intra-Ply hybridized glass and carbon chopped strand mat glass plies Intra-Ply hybridized glass and carbon

No. of Layers 6 4 4 4

Type of Manufacturing Process Hand lay-up Hand lay-up Hand lay-up Vacuum infusion technique

Polímeros, 33(3), e20230030, 2023

Novel modified blister test to evaluate composites used in repairing cracked pipeline

Figure 2. Hand lay-up method.

Figure 3. Vacuum Infusion technique.

two nylon sheets to form the vacuum bag. The vacuum bag is connected to a vacuum motor by a hose from one side, and another hose connects the vacuum bag with the epoxy resin-hardener mix from the other side. A resin trapper is placed between the vacuum bag and the vacuum motor to prevent the epoxy from going into the vacuum motor.

A mesh sheet is placed over the fibre layers to guarantee a homogeneous distribution of the resin inside the fibre layers.

The vacuum bag is tightly sealed to ensure no leakage occurs before pumping the resin into the vacuum bag. When all fibre layers are completely covered with the resin, the two houses are closed tightly, and the composite is left to cure at 112°C for 4 hrs. The molds were cut to be ready for the mechanical tests: peel (ASTM D 3330)[16] double cantilever beam (DCB) (ASTM D5528)[17], and blister test[12].

2.2 Tests 2.2.1 Peel test Peel test specimens were manufactured with 140 mm length substrate and 95 mm width, 150 mm length rigid Polímeros, 33(3), e20230030, 2023

substrate and 20 mm width. The rigid substrate is made of composite repair sticked on the substrate which is made from carbon steel by two adhesive epoxy glue from Tepusi and Sikadur 52, as shown in Figure 4. Tests were performed by a universal tensile machine with a capacity of 100kN and the testing speed was 5 mm/min, the test standard is ASTM D3330[16].

2.2.2 Double Cantilever Beam (DCB) test DCB specimens were manufactured and evaluated in accordance with the ASTM D5528[17] standard test technique to ascertain the composite’s Mode I interlaminar fracture toughness. The composite laminate’s thickness, h, and initial delamination length, a0, were created to meet the requirements of the standard. The configuration of the DCB specimen are shown in Figure 5. Two piano hinges were bonded to the delaminated arms of the DCB specimen after the test coupons were cut to the required sizes (163 mm in length*22 mm in width and 63 mm distance of the open end from the edge). This was done to apply the tensile opening load and cause a Mode I delamination fracture. The Instron 3/9

Ahmed, P. S., Ali, J. A., & Talabani, S. S. M. machine used for the DCB tests (ASTM D5528) has a load cell with a 100 kN capacity. Crosshead speed was maintained at 3 mm/min when loading the specimens[18].

2.2.3 Blister test A universal tensile machine with a capacity of 100kN was used to perform the blister test. The specimen was fixed horizontally and the repaired face by the composite was faced down (to produce tensile stresses) and fixed by two holders. The used cross-head velocity was 2 mm/min and the force was exerted on the shaft which was placed on a small disc to make sure that the load will transfer uniformly to the composite repair. Figure 6 shows the blister test arrangements[19].

2.2.4 Modified blister test The modified blister test consists of wrapping the composite repair directly then adding Epoxy by vacuum infusion technique on 10 mm diameter cracked pipeline as can be seen in Figure 7 and test it by a shaft to resemble the fluid pressure on the composite repair.

3. Numerical Analysis of Blister Test

Figure 4. Peel test.

Figure 5. DCB test.

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To determine a material’s effective qualities from knowledge of its basic rules and the spatial distribution of its constituent parts is one of the main objectives of heterogeneous material physics. The techniques utilized for homogenization have advanced significantly in sophistication and efficacy, particularly when it comes to mechanical characteristics and thermal conductivity. There is also numerical homogenization

Figure 6. Blister test.

Polímeros, 33(3), e20230030, 2023

Novel modified blister test to evaluate composites used in repairing cracked pipeline

Figure 8. RVE of Intraply woven fiber composite.

Table 4. Properties of 4GCV calculated by materials designer.

Figure 7. (a) Vacum Infusion of composite repair around the pipe; (b) Modified pipe blister test.

for the macroscopic effective characteristics. They consist of the popular finite element techniques or the quick Fourier transform. The term “homogenization” describes the process of taking into account a Representative Volume Element RVE, a statistically homogenous representation of a heterogeneous material[20]. Glass and carbon woven fiber composites were modeled in three dimensions using FEA utilizing the ANSYS Material Designer program and a homogenization strategy. The effective characteristics of materials or composites having periodic architectures were calculated using homogenization theory. The simulations were run under the presumption that the structure is made up of an endless network of unit cells. The boundary conditions for FEA utilizing homogenization theory are symmetrical in all planes.

Figure 8 depicts the application of the composite material’s 3D model based on homogenization theory. For the purpose of creating a virtual RVE and setting the volume to Matrix epoxy, epoxy composites with intraply glass and carbon textiles were modelled in three dimensions. The epoxy RVE contained the glass and carbon fibers.

The volume occupied by the of 4GCV was 75.5% and it’s calculated by the set of equations mensioned by Abdalla et.al[21].

Polímeros, 33(3), e20230030, 2023

Property E1 E2 E3 G12 G23 G31 nu12 nu13 nu23

Value 6.1077E+08 6.1077E+08 3.7208E+09 1.9271E+10 3.4679E+08 3.4679E+08 0.98815 0.0066025 0.0066025

Unit Pa Pa Pa Pa Pa Pa

Based on the volume fraction used to structure the oriented glass and carbon textiles in the epoxy matrix, the basic modeling criteria were established. SOLID187 elements with a mesh size of 2 mm were the element type utilized for FEA. The implementation of secondary displacement is supported by SOLID187, making it the best tool for building meshes inside structures with odd shapes. The components utilized for the epoxy matrix and glass and carbon fibers were set to the same size in order to lessen the sensitivity of forecasting physical attributes based on mesh size[22].

The calculated properties are listed in Table 4.

The strength of the adhesive bonding between the composite repair and pipeline can be assessed using shaft-loaded blister tests (Figure 5), in which the shaft stimulates the produced load from the petroleum flow and the consequent pressure on the composite repair. The cohesive zone model (CZM) is used to represent the interface between the pipeline and the composite repair through the blister test by building a model using FEM in the ANSYS workbench. A zone where the material can withstand traction loads immediately in front of a fracture point forms the basis of the CZM method.

Surface cohesive behavior can be employed as an alternative to the cohesive element technique to simulate CZM when the surface thickness is zero[19].

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Ahmed, P. S., Ali, J. A., & Talabani, S. S. M.

Figure 9. Composite repair and the steel base (a) front view; and (b) back view.

The CZM parameters employed in this experiment were traction stress of 8 MPa and interface displacement of 0.06 mm. The quad/tri free face sweep meshing method’s mesh and boundary conditions are shown in Figure 9.

The upper face of the disc is given a displacement in the vertical direction to reveal the shaft[13].

4. Results and Discussions 4.1 Peel test Epoxy matrix resins are applied to wrapped composite repairs consists of carbon or glass fiber reinforcements.

Performance and behavior of composite repairs are significantly influenced by interfacial characterization.

The most significant failure mode between layers in the repair/metal substrate interface occurs after the transferred fluid cracks the pipeline wall and forms a blister, which is known as delamination. Peel tests have been developed to measure a thin film’s ability to stick firmly to adjacent surfaces[11].

Its clearly obvious from Figure 10 that Sikadure-52 gives a better strength than that of comercial Epoxy glue (Tepusi).

This result lead to the use of Sikadure-52 to stick the composite repair on the steel base for this study.

Figure 10. Load-Extension curves for the composites under peel test.

4.2 DCB test The force-load point extension is the primary outcome of the DCB tests (Figure 11). The vacuum infusion technique plays a significant effect in strengthening the bonding between the composite layers, which increases the load value needed to open the crack tip, as can be seen from these curves.

The impact of vacuum in eliminating voids and improving the interface between the fiber and epoxy is evident when comparing the load values of 4GCV and 4GCH. Due to the compression force it experiences during the process, vacuum infusion exhibits better mechanical qualities and can prolong the time it takes for the resin to fully cure.

The procedure is also carried out in a vacuum environment to get rid of the gases inside the mold. The ambient pressure will indirectly shrink the voids or empty space inside the sample. The mechanical characteristics can be improved if the sample’s internal voids and space are minimized[23].

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Figure 11. Load-Extension curves for the composites under DCB test.

Additionally, when comparing the 4GCV and 6GCH, it is noticeable that using a 4-layer vacuum-infused composite results in a weight savings advantage over a 6-layer handlaid composite. The advantage of woven fiber over random strand mat is plainly visible when comparing the 4GCV and 4RGH, where the load value increases by 4 times.

The effect of fiber orientation, which plays a crucial role in the mechanical behavior of the composite, may be responsible for this large improvement. Due to insufficient interface strength between the fiber and matrix and poor stress transfer efficiency, which favors crack initiation, random glass composites have worse mechanical properties.

Polímeros, 33(3), e20230030, 2023

Novel modified blister test to evaluate composites used in repairing cracked pipeline In woven laminates, crack leaping and fiber bridging have a significant impact on the load value in DCB test. Crack hopping, in which the delamination crack spreads through a nearby ply or into the other interface, is a common failure mechanism seen in woven composites. As a result, it appears that crack leaping and fiber bridging have been reduced[24].

The complex weave of woven fabrics used in woven fiber composites, where the fiber bundles are aligned in the force direction and can therefore hold more load, can slow or stop fiber debonding. In other words, the weaved fibers resist tensile stress until they break because they do not separate like randomly distributed fibers. Although the forces may cause the woven threads to stretch, they might not be strong enough to completely break the fibers. Additionally, the polymer matrix can be damaged and separated from the woven fibers[23]. In order to integrate the greatest qualities of each reinforcement ingredient, intraply fibers composites are created. “Intraply hybrid composites” are materials made up of two or more types of fibers combined into a single ply. Additionally, some of the unfavorable reinforcements can be removed. For instance, by combining the benefits of carbon and glass fiber textiles’ high modulus and high strain, a novel hybrid composite material can be created[14].

4.3 Blister test Since the delamination in the test only occurs under small strains, the blister test results are more important when studying the adhesion of the composite repair[11].

Figure 12 shows the load-displacement curves of the blister test for the four composites, first the curve is increased until the point of the critical load which indicates that the initial debonding of the repair is taking place. Then the interfacial debonding grew as a second stage, and finally, the tearing of the composite repair occurred. Figure 12 shows that 4GCV improves the behavior of the composite repair and increases the value of the critical load to 368 N which the highest value compared with other composites.

Vacuum infusion has greater mechanical properties and can extend the time it takes for the resin to fully cure because of the compression force it experiences during the process. To remove the gases from the mold, the technique is also carried out in a vacuum atmosphere. The voids or empty space inside the sample will indirectly contract due to ambient pressure. If the sample’s internal voids and space are reduced, the mechanical properties can be enhanced[23].

This significant improvement can be attributable to the effect of fiber orientation, which is important for the composite’s mechanical behavior. Random glass composites have poor mechanical properties when compared to woven fiber due to insufficient interface strength between the fiber and matrix and poor stress transfer efficiency, which promotes fracture initiation.

Fiber debonding can be slowed down or prevented by the intricate weave of woven fabrics used in woven fiber composites, where the fiber bundles are aligned in the force direction and can therefore hold greater load. In other words, because the woven fibers do not separate like randomly distributed fibers, they resist tensile force until they break.

The woven fibers could stretch as a result of the stresses, yet the fibers may not totally break even if they are strong Polímeros, 33(3), e20230030, 2023

enough to do so[23]. As shown in Figure 8, using intraply woven glass and carbon fibers increases the load and improves the blistering behaviour in comparison to using either carbon or glass alone used by previous researchers[13] and[11] with taking into account weaving and stitching pattern. A larger critical load value is the effect of this. Placing glass with carbon within the same layer may produce better results due to the high-strength carbon fiber and the high strain of the glass in the composite, the composite maintains its strength under the load and pressure of the shaft. While in the case of 4RGH, the composite is unable to keep its strength under pressure because the random glass cannot offer the composite the required amount of protection due toits low strength, which results in the composite repair delaminating prematurely from the pipe[13].

Figure 13 shows the identical behavior of FEM and the practical blister Load-Displacement curve in the critical load value. This shows that the CZM zone of blister test can be successfully presented utilizing the RVE method and the bilinear traction separation law. Due to its simplicity and adaptability, the bilinear cohesive traction-separation rule is widely employed in finite element modeling. According to ISO/TS 2481[25], this law is divided into three regions: firstly: an elastic zone up to full strength, secondly: a softening region, and a third region with full nodal pair separation on zero tractions[25]. However, the fact that FEM evaluates the ideal behavior of the materials without taking into consideration issues in the lab work like improper bonding or the occurrence of gaps is what causes the disparity between the experimental and numerical behavior[20] in the second with 7% difference and third region with 10% difference of the curve.

Figure 12. Load-Displacement curves for the composites under blister test.

Figure 13. Expremintal and numerical Load-Extension curves for 4GCV composite under blister test.

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Ahmed, P. S., Ali, J. A., & Talabani, S. S. M. • Data curation – Payman Sahbah Ahmed.

• Formal analysis – Payman Sahbah Ahmed.

• Funding acquisition - NA. • Investigation – Payman Sahbah Ahmed; Jafar Abdullah Ali. • Methodology – Payman Sahbah Ahmed; Jafar Abdullah Ali; Serwan Sarbast Mohammed Talabani.

• Project administration – NA. Figure 14. Load-Extension curve for 4GCV composite under pipe modified blister test.

• Resources – Payman Sahbah Ahmed; Jafar Abdullah Ali. • Software – Payman Sahbah Ahmed; Serwan Sarbast Mohammed Talabani.

• Supervision – NA.

4.4 Modified blister test The modified blister test consists of wrapping the composite repair directly on cracked pipeline and test it by a shaft to resemble the fluid pressure on the composite repair. As can be seen in the below Load-Extension curve (Figure 14). First, the curve is raised till the critical load point, which denotes the beginning of the repair’s debonding.

The interfacial debonding developed as a secondary step after that, and the composite repair eventually tore. But when comparing the load value (4154N) with that of plain blister test (368.177N), it can be concluded that the plain blister test gives an initial indication about the adhesion strength but the modified blister test can give the real data about the adhesion strength.

5. Conclusions The vacuum infusion technique has a significant impact in strengthening the bonding between the composite layers compared to the hand lay up method. 4-layer vacuum-infused composite results in a weight savings advantage over a 6-layer hand-laid composite. The advantage of woven fiber over random strand mat is plainly visible when comparing the 4GCV and 4RGH, where the load value increases by 4 times. The utilization of intraply woven glass and carbon fibers leads to increase the load and improves the blistering behaviour compared to using either carbon or glass alone.

Furthermore, there is a close correspondence between the behavior of the Finite Element Model FEM and the practical blister Load-Displacement curve in the critical load value. This shows that the CZM zone of blister test can be successfully presented utilizing the RVE method and the bilinear traction separation law. Plain blister test gives an initial indication about the adhesion strength but the modified blister test can give the real data about the adhesion strength. Using natural fibers in repairing cracked piplines may be a potential future perspectives to reduce repairing costs and lowering the harmful effect of using synthetic materials.

6. Author’s Contribution • Conceptualization – Payman Sahbah Ahmed; Jafar Abdullah Ali. 8/9

• Validation – Payman Sahbah Ahmed.

• Visualization – Payman Sahbah Ahmed; Jafar Abdullah Ali; Serwan Sarbast Mohammed Talabani.

• Writing – original draft – Payman Sahbah Ahmed; Jafar Abdullah Ali. • Writing – review & editing – Payman Sahbah Ahmed; Jafar Abdullah Ali; Serwan Sarbast Mohammed Talabani.

7. References 1. Saeed, N. (2015). Composite overwrap repair system for pipelines: onshore and offshore application (Doctoral thesis).

The University of Queensland, Austria.

2. Barros, S., Bdhe, S., Banea, M. D., Rohem, N. R. F., Sampaio, E. M., Perrut, V. A., & Lana, L. D. M. (2018). An assessment of composite repair system in offshore platform for corroded circumferential welds in super duplex steel pipe. Frattura ed Integrità Strutturale, 12(44), 151-160. http://dx.doi.org/10.3221/ IGF-ESIS.44.12.

3. Linden, J. M., Köpple, M., Elder, D., & Gibson, A. G. (2012).

Modelling of composite repairs for steel pressure piping. In Proceedings of the 15th European Conference on Composite Materials - ECCM15 (pp. 1- 8). Venice, Italy: University of Padova.

4. Linden, J. M., Kotsikos, G., & Gibson, A. G. (2016). Strain energy release rate in shaft-loaded blister tests for composite repairs on steel. Composites. Part A, Applied Science and Manufacturing, 81, 129-138. http://dx.doi.org/10.1016/j. compositesa.2015.10.026.

5. Skorski, S. A. (1994). Measurement of adhesion using the island blister test (Doctoral thesis). Massachusetts Institute of Technology, Massachusetts.

6. Borowski, E., Soliman, E., Kandil, U. F., & Taha, M. R. (2015). Interlaminar fracture toughness of carbon fiber reinforced polymer laminates incorporating multi-walled carbon nanotubes. Polymers, 7(6), 1020-1045. http://dx.doi. org/10.3390/polym7061020.

7. Tan, K. T., White, C. C., Hunston, D. L., Clerici, C., Steffens, K. L., Goldman, J., & Vogt, B. D. (2008). Fundamentals of adhesion failure for a model adhesive (poly methel metha acrylate/glass) joint in humid environments. The Journal of Adhesion, 84(4), 339-367. http://dx.doi.org/10.1080/00218460802004428.

8. Alexander, C. R. (2007). Development of a composite repair system for reinforcing offshore series (Doctoral thesis). Texas A&M University, Texas.

9. Abdelouahed, E., Benzaama, H., Mokhtari, M., & Aour, B. (2019). Pipeline repair by composite patch under temperature Polímeros, 33(3), e20230030, 2023

Novel modified blister test to evaluate composites used in repairing cracked pipeline and pressure loading. Frattura ed Integrità Strutturale, 13(49), 690-697. http://dx.doi.org/10.3221/IGF-ESIS.49.62.

10. Liland, K. B., Faremo, H., & Furuheim, K. M. (2019). Blister test as method of measuring adhesion of solids on a flat surface.

In Proceedings of the 26th Nordic Insulation Symposium on Materials, Components and Diagnostics, Tampere (NORD-IS 19) (pp. 108–112). Finland: Norwegian University of Science and Technology.. http://dx.doi.org/10.5324/nordis.v0i26.3288.

11. Cao, Z., Wang, P., Gao, W., Tao, L., Suk, J. W., Ruoff, R. S., Akinwande, D., Huang, R., & Liechti, K. M. (2014). A blister test for interfacial adhesion of large-scale transferred graphene. Carbon, 69, 390-400. http://dx.doi.org/10.1016/j. carbon.2013.12.041.

12. Barros, S., Fadhil, B. M., Alila, F., Diop, J., Reis, J. M. L., Casari, P., & Jacquemin, F. (2019). Using blister test to predict the failure pressure in bonded composite repaired pipes.

Composite Structures, 211, 125-133. http://dx.doi.org/10.1016/j. compstruct.2018.12.030.

13. Ahmed, P. S. (2023). Effect of hybridisation and nano reinforcement on repairing cracked pipeline. Polímeros, 33(1), e20230010. http://dx.doi.org/10.1590/0104-1428.20220111.

14. Uzay, Ç., Acer, D., & Geren, N. (2019). Impact strength of interply and intraply hybrid laminates based on carbon-aramid/ epoxy composites. European Mechanical Science, 3(1), 1-5. http://dx.doi.org/10.26701/ems.384440.

15. Liang, J., Liu, L., Qin, Z., Zhao, X., Li, Z., Emmanuel, U., & Feng, J. (2023). Experimental study of curing temperature effect on mechanical performance of carbon fiber composites with application to filament winding pressure vessel design.

Polymers, 15(4), 982. http://dx.doi.org/10.3390/polym15040982.

PMid:36850262.

16. American Society for Testing and Materials – ASTM. (2002).

ASTM D 3330/D 3330M-02e1: standard test method for peel adhesion of pressure-sensitive tape. West Conshohocken: ASTM International.

17. American Society for Testing and Materials – ASTM. (2013).

ASTM D 5528-13: standard test method for mode I interlaminar fracture toughness of unidirectional fiber-reinforced polymer matrix composites. West Conshohocken: ASTM International.

18. Saadati, Y., Chatelain, J.-F., Lebrun, G., Beauchamp, Y., Bocher, P., & Vanderesse, N. (2020). A study of the interlaminar fracture toughness of unidirectional flax/epoxy composites. Journal of Composites Science, 4(2), 66. http://dx.doi.org/10.3390/ jcs4020066.

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19. Ahmed, P. S., Kamal, A. A., Abdulkader, N. J., Fadhil, B. M., & Khoshnaw, F. (2023). Blister test to evaluate the Multiwall Carbon Nanotubes (MWCNT): woven carbon fiber reinforced epoxy used for repairing pipelines. Multidiscipline Modeling in Materials and Structures, 19(5), 953-965. http://dx.doi. org/10.1108/MMMS-11-2022-0266.

20. El Moumen, A., Kanit, T., & Imad, A. (2021). Numerical evaluation of the representative volume element for random composites. European Journal of Mechanics. A, Solids, 86, 104181. http://dx.doi.org/10.1016/j.euromechsol.2020.104181.

21. Abdalla, F. H., Megat, M. H., Hamdan, M. S., Sapuan, M. S., & Sahari, B. B. (2008). Determination of volume fraction values of filament wound glass and carbon fiber reinforced composites. Journal of Engineering and Applied Sciences, 3(4), 7-11. Retrieved in 2023, July 12, from http://www.arpnjournals. com/jeas/research_papers/rp_2008/jeas_0808_109.pdf 22. Yun, J.-H., Jeon, Y.-J., & Kang, M.-S. (2023). Prediction of the elastic properties of ultra high molecular weight polyethylene reinforced polypropylene composites using a numerical homogenisation approach. Applied Sciences (Basel, Switzerland), 13(6), 3638. http://dx.doi.org/10.3390/app13063638.

23. Farooq, M., & Banthi, N. (2018). An innovative FRP fibre for concrete reinforcement: production of fibre, micromechanics, and durability. Construction & Building Materials, 172, 406421. http://dx.doi.org/10.1016/j.conbuildmat.2018.03.198.

24. Kim, H. S., Wang, W.-X., & Takao, Y. (1999). Effects of temperature and fiber orientation on mode I interlaminar fracture toughness of carbon/epoxy composites. In Proceedings of the 12th International Conference on Composite Materials (ICCM-12) (p. 276). Paris: International Committee on Composite Materials. Retrieved in 2023, July 12, from https:// iccm-central.org/Proceedings/ICCM12proceedings/papers/ pap276.pdf 25. International Organization for Standardization – ISO. (2017).

ISO/TS 24817: petroleum, petrochemical and natural gas industries: composite repairs for pipework: qualification and design, installation, testing and inspection. Geneva: ISO. Received: Jul 12, 2023 Revised: Aug. 31, 2023 Accepted: Sept. 04, 2023

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.20230062

Effect of accelerated weathering environment on the carbon fiber/polyamide 6 composites Larissa Stieven Montagna1* , Guilherme Ferreira de Melo Morgado1 , Juliano Marini2 , Thaís Larissa do Amaral Montanheiro3 , Alessandro Guimarães4 , Fabio Roberto Passador1  and Mirabel Cerqueira Rezende1  Laboratório de Tecnologia de Polímeros e Biopolímeros – TecPBio, Universidade Federal de São Paulo – UNIFESP, São José dos Campos, SP, Brasil 2 Departamento de Engenharia de Materiais, Universidade Federal de São Carlos – UFSCar, São Carlos, SP, Brasil 3 Departamento de Engenharia Mecânica, Instituto Tecnológico de Aeronáutica – ITA, São José dos Campos, SP, Brasil 4 Laboratório de Estruturas Leves – LEL, Instituto de Pesquisas Tecnológicas do Estado de São Paulo – IPT, São José dos Campos, SP, Brasil

1

*larissa.s.montagna@gmail.com

Abstract Prolonged exposure to environmental conditions such as ultraviolet radiation, humidity, and temperature, to which carbon fiber-reinforced thermoplastic polymer components are exposed during their service life, can lead to significant changes in mechanical, physical, and chemicals properties, and can often be irreversible, resulting in premature component failure. This study presents the influence of accelerated weathering exposure times (400 h, 800 h, and 1200 h) on the mechanical, thermal, and structural properties of carbon fiber (CF)/polyamide 6 (PA6) laminates. Analyses of composite surfaces were carried out using microscopy and contact angle measurements, which indicated that the factors of exposure to accelerated only affected the surface of the composites, showing signs of the beginning of degradation.

The tensile strength (609 MPa ± 10 MPa) and interlaminar shear strength (27 MPa ± 0.9 MPa) did not present significant changes, showing that the reinforcement, the matrix, and the interface remained stable after exposure to accelerated.

Keywords: accelerated weathering, carbon fiber, composites, polyamide 6, ultraviolet radiation.

How to cite: Montagna, L. S., Morgado, G. F. M., Marini, J., Montanheiro, T. L. A., Guimarães, A., Passador, F. R., & Rezende, M. C. (2023). Effect of accelerated weathering environment on the carbon fiber/polyamide 6 composites.

Polímeros: Ciência e Tecnologia, 33(3), e20230032. https://doi.org/10.1590/0104-1428.20230062

1. Introduction Carbon fiber reinforced thermoplastic polymer (CFRTP) has proven to be an excellent replacement option for traditional carbon fiber composites with thermoset resins due to its good mechanical properties, such as high strength and rigidity, easy and fast processing, in addition to the possibility of being recycled at the end of its useful life[1,2]. CFRTP has been widely used in sectors that demand lightweight structural applications, such as aerospace[3], automotive[4,5], renewable energies[6], and sports goods[7].

During their lifespan, CFRTPs are exposed to the most aggressive environmental factors, such as humidity, temperature gradients, solar radiation, ozone, pollution, mechanical factors load, and some chemical products, that individually or in combination, may affect their thermal, chemical, and mechanical properties, resulting in a decrease in service life[8]. Among these factors, changes in temperature, humidity, and ultraviolet (UV) radiation prevail. According to Sang et al.[9], the variation in “hot/humid” exposure to which CFRTPs are often subjected during the working

Polímeros, 33(3), e20230032, 2023

period is considered the most severe condition in terms of degradation. Aiming at increasing its useful life, therefore, it is crucial to understand the behavior of the diffusion and damage characteristic of CFRTP when subjected to environmental exposure.

UV radiation from sunlight also affects polymer composites, as the energy of UV photons is similar to the covalent bonds of polymeric materials, thus it can affect and alter the chemical structure of polymeric chains[10,11].

The depth of UV radiation penetration is small, in the order of micrometers. The reinforcement of carbon fiber (CF) in composite materials absorbs and limits the penetration of UV radiation, resulting in the superficial degradation of the composite[12].

However, this phenomenon can lead to the formation of small flaws and surface cracks, which facilitate the penetration and diffusion of moisture and pollutants into the composite. These factors can be intensified with the

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

Montagna, L. S., Morgado, G. F. M., Marini, J., Montanheiro, T. L. A., Guimarães, A., Passador, F. R., & Rezende, M. C. increase in temperature, and in this way, they can act as stress concentrators, and lead to the formation of surface tensions, which, when accumulated, generate stress. This stress may propagate to the interior of the thermoplastic composite, resulting in the formation of cracks, in addition to premature failure during the life of the component[13,14].

Among the engineering thermoplastics, polyamide 6 (PA6) has been highlighted due to the ease of processing and the fact that CF/PA6 composite laminates can be prepared using the same equipment used to prepare thermoset resin laminates.

PA6 is a thermoplastic composed of amide monomers connected by peptide bonds, with good resistance to impact, abrasion, and wear, good mechanical stability, and high resistance to chemical and biological attacks, in addition to being extremely rigid and resistant up to 180 ºC[15]. However, PAs can undergo different degradation processes due to their highly hygroscopic nature, and the presence of moisture results in degradation by hydrolysis of the amide, which can be accelerated with the presence of temperature[16]. The influence of UV radiation combined with oxygen and temperature results in photooxidative, and thermo-oxidative degradation, respectively, causing polymer chain scission, micro-cracking, discoloration due to the production of chromophoric chemical species, the opacity of the polymer surface, and consequently a significant loss in the mechanical properties[17,18].

CF is widely used to reinforce polymeric materials, to be applied in different sectors. CF has good chemical stability and high heat resistance, in addition to absorbing little or no moisture, so it has been used as reinforcement in PA6 matrices to minimize or even delay the effects of weathering conditions of the polymeric matrix[1,19-21].

In addition to improving resistance and mechanical, thermal, and chemical stability. Pinpathomrat, Yamada, and Yokoyama[22] studied the effect of ultraviolet-C (UV-C, 20.2 W/m2) irradiation on neat PA6 and CF/PA6 composites, and observed a drastic decrease in tensile strength in neat PA6, while for CF/PA6 composite no major changes were observed, indicating that CF is useful not only as composite reinforcement but also for UV protection of PA6.

The present work aims at determining the influence of accelerated weathering, including rain cycles, temperature, and UV radiation using a weather meter machine, on the mechanical (tensile strength and interlaminar shear strength) and thermal (melting temperature and degree of crystallinity) properties, and morphological characteristics by optical microscopy (MO) and scanning electron microscopy (SEM) of CF/PA6 composites. CF/PA6 composite laminates are used to manufacture lightweight outdoor structures for the automotive and oil and gas industries. These composites are subjected to climate and environmental change that include the presence of humidity and UV radiation. Therefore, the degradation of polymers-based composite components and/ or structures must be taken into account, aiming at the safety of human health and the environment.

2. Materials and Methods

2.2 Accelerated weatherometer The CF/PA6 composite samples were placed in a UV test fluorescent UV/condensation weathering instrument (Atlas, 22007) with a UV test radiometer (Atlas, 24078), using UVA340 lamps with 0.89 W/m2 of irradiation intensity, with a cycle of 8 h of UVA irradiation at a temperature of 60 ºC (± 3 ºC), intercalated with 4 h of condensation at a temperature of 50 ºC (± 3 ºC). These factors were submitted on both sides of the composite samples, to homogenize and standardize the effects. The CF/PA6 composite specimens were subjected to UV radiation and were removed at 400 h, 800 h, and 1200 h (on both sides), then were tested and evaluated.

Text paragraph within a second subsection.

2.3 Characterizations The contact angle measurements were performed in a Goniometer Ramé-Hart (model 500), and the liquid used was distilled water by the sessile drop method (1.0 μL, 25 °C). Nine measurements were made in each sample to evaluate the surface character of the CF/PA6 composites unexposed and after 400 h, 800 h, and 1200 h exposed to accelerated weathering.

Fourier-transformed Infrared Spectroscopy (FT-IR) measurements of CF/PA6 composites before and after 400 h, 800 h, and 1200 h exposed to accelerated weathering were carried out in a PerkinElmer spectrometer (model 2000) using universal attenuated total reflectance (UATR), with an average of 20 scans with 2 cm−1 resolution, in the range from 4000 to 500 cm−1.

DSC analyses of CF/PA6 composites unexposed and after 400 h, 800 h, and 1200 h exposed to accelerated weathering were performed using TA Instruments equipment (Q2000 model) in an inert nitrogen atmosphere (50 mL/min). The samples were heated from 30 °C to 300 °C, with a heating rate of 10 °C/min, with 3 min isotherm, at 300 °C, to eliminate the thermal history of the samples, and after that, they were cooled and heated again from 300 °C to 30 °C at a rate of cooling and heating of 10 ºC/min. The degree of crystallinity (Xc, %) was determined by Equation 1, where ∆Hm is the enthalpy of melting of the semicrystalline PA6 and ∆HCC is the enthalpy of cold crystallization (∆HCC = 0 J/g, in this case), both according to the result of DSC analysis from the 2nd heating. ϴ is the carbon fiber mass fraction in the composite (according to Montagna et al.[23] this CF/PA6 composite has 51% CF), and ∆Hm0 is the enthalpy of melting of the 100% crystalline polymer (∆Hm0 = 230 J/g for PA6[24]).

= Xc

2.1 Materials and specimens cutting The thermoplastic composite used is polyamide 6 reinforced with carbon fiber (CF/PA6) 5 plies, 2x2 twill 2/10

carbon fiber weave, 415 gsm, and 39 wt% matrix content (Cetex® TC910) from Toray® Advanced Composites Co. (England). The material was used as received. The cutting of the standardized specimens for the mechanical test (tensile test and ILSS) was carried out in a cutting machine (Extec Corp., LAbcut 5000) with a cutting speed of 2 mm/s.

(∆H m − ∆H cc )

(1 − θ ) ∆H m0

×100

(1)

Tensile tests were carried out according to ASTM D3039[25] using an Instron (model 5982), with sample dimensions of Polímeros, 33(3), e20230032, 2023

Effect of accelerated weathering environment on the carbon fiber/polyamide 6 composites 250 mm x 25 mm x 2.3 mm (length x width x thickness), and a crosshead speed of 2 mm.min−1 and load cell of 250 kN. Seven specimens of each sample were tested.

The interlaminar shear strength (ILSS) by short beam method was performed to evaluate CF interfacial adhesion mechanisms with the PA6 matrix. The samples (24 mm x 6.35 mm x 4.0 mm) were performed with a 100 kN load cell on an Instron (model 5982) and a testing speed of 1.0 mm/ min, according to ASTM D2344[26]. For this purpose, six specimens were tested for each case studied. The interlaminar shear strength (τmax) was calculated by dividing the peak recorded reaction force (P) by the area of the cross-section (b x h), as shown in Equation 2. τ max = 0.75

P

(b x h )

(2)

Mechanical test results were analyzed using one-way analysis of variance (ANOVA) and Tukey’s multiple comparisons test on GraphPad Prism 6 (GraphPad Prism 6 Software Inc. USA) at 95 and 99.9% levels, respectively.

The fractured surface after the ILSS test was performed through images obtained by optical microscopy (OM) using a benchtop optical microscope (MP-150). Surface degradation and the fractured surfaces of specimens after tensile tests were analyzed by scanning electron microscopy (SEM) with a scanning electron microscope Inspect S50 (FEI Company®) at an accelerating voltage of 20 kV. The CF/ PA6 composite samples were placed in the aluminum stub with carbon tape and coated with a thin layer of gold for 120s[27], by sputtering from Quorum (Q150RS Plus).

3. Results and Discussions The quantification of moisture absorption by the specimens after accelerated weathering did not show a significant increase in the weight gain of the samples.

This behavior is attributed to the intercalated cycles of wet (water at 50 ºC) and dry (UV irradiation at 60 ºC), which maintained the weight of the specimens around the initial value before conditioning. Although the specimens did not show moisture absorption at the end of conditioning, the

possible influence of the accelerated weathering on the composites was investigated by FT-IR and thermal analyses.

Figure 1 shows the structural changes of CF/ PA6 composite surfaces before and after 400 h, 800 h, and 1200 h in accelerated weathering measured by FT-IR. All the spectra are very similar, occurring a reduction of insignificant intensities of the bands or their displacement.

That is, insignificant chemical degradation was observed

by FT-IR on the surface of CF/PA6 after conditioning in an accelerated weathering by small changes in the carbonyl area and hydrolysis.

The photooxidation of the surface of CF/PA6 composites can be monitored mainly by the appearance of a band around 1715 cm-1, corresponding to the stretching of the carbonyl, referring to the possible oxidation of the matrix that occurred after the process of exposure to UV radiation; and by changes in the intensity of the bands at 3000–3500 cm−1 referring to alcohols and carboxylic acids[28]. In none of the spectra shown in Figure 1-A was observed the band around 1715 cm-1. However, an increase in the intensity of the band at 3300 cm-1 was observed in the exposed CF/PA6 composite at 1200 h, which may be due to moisture absorption during the periods of wet conditioning to which it was subjected[29]. The occurrence of degradation by hydrolysis can be indicated through the reduction in the amide band (1633 cm-1 and 1540 cm-1) intensities after the conditioning test suggesting that the amide was degraded[30].

This behavior was observed in the composites after the exposure time of 400 h and 800 h.

To evaluate the degradation of the samples exposed to accelerated weathering, the ratio between interest bands was calculated, since there is a change in their intensities proportionally to the concentration of these groups (Figure 1-B).

Once the degradation can be monitored by the change in intensity of the amide band, the ratios 3300/1633 and 3300/1540 were obtained. Whereas the degradation by hydrolysis is indicated by the reduction in amide band intensity, is expected that, for samples that were degraded, the ratio between 3300/1633 and 3300/1540 bands increases if compared to the unconditioned sample. Figure 1-B shows the ratios between bands for all samples. It can be concluded that after 1200 h of accelerated weathering the sample presented degradation of the amide band. After 400 h and

Figure 1. FT-IR results: spectra (A) and the results of the ratio between interest bands (B) of CF/PA6 composites before and after the exposure times of 400 h, 800 h, and 1200 h in accelerated weathering.

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Montagna, L. S., Morgado, G. F. M., Marini, J., Montanheiro, T. L. A., Guimarães, A., Passador, F. R., & Rezende, M. C. 800 h, the reduction in the ratios may be justified by some water adsorption of samples. To confirm the degradation, the ratio between both amide bands was also calculated; seeing that, a variation in this ratio indicates that the functional groups were somehow affected. The 1633/1540 ratios were nearly the same for unconditioned, 400 h and 800 h samples, but for 1200 h showed variation, confirming the matrix degradation of this sample.

The thermal properties of samples exposed to accelerated weathering were evaluated using DSC and aimed to verify the influence of different exposure times (400 h, 800 h, and 1200 h) on the melting temperature (Tm) and degree of crystallinity (Xc). Figure 2 presents the DSC curves in the first and second heating. It is possible to observe in the DSC curves referring to the first heating that the composites did not present significant changes in the value of the Tm, and all composites presented Tm values at approximately 217 ºC (± 0.5 ºC). However, when checking the curves for the second heating, the presence of a double endothermic peak in the Tm region is noticeable, with Tm1 at approximately 214 ºC ± 0.8 and Tm2 at 218.8 ºC ± 0.5 ºC. The presence of these double peaks in crystalline melting could be due to different crystalline forms, α phase, and γ phase. The α phase is considered the most stable and predominant, and the γ phase is less stable and is formed by rapid cooling.

A similar behavior was observed in the study carried out

by Oulidi et al.[31].

Also, through the DSC analysis, it was possible to verify a slight difference in the composites after the different exposure times to the accelerated weathering chamber.

Increases of 21%, 26%, and 13% in the Xc values of the CF/PA6 composites after 400 h, 800 h, and 1200 h exposure to accelerated weathering, respectively, when compared to the unconditioned composites were observed. This increase may be related to the breaking of intramolecular bonds and the reorganization of the polymeric chains, associated with the crystalline regions that were influenced by the environmental factors to which they were exposed, such as UV radiation, humidity, and temperature. These factors influenced the increase of the crystalline phase. Similar behavior was observed by Mahat et al.[32], who observed that as the exposure time of CF-reinforced thermoplastic composites increased, crystallinity values increased, as the

molecular structure becomes more ordered with increasing crystallinity and consequently restricts the movement of the amorphous region.

Figure 3 shows the results obtained in the mechanical characterization of unconditioned specimens and those exposed to accelerated weathering. Figure 3-A-B shows the results of the ultimate tensile strength (UTS) and elastic modulus (E) of the CF/PA6 composites unconditioned and after 400 h, 800 h, and 1200 h exposed to accelerated weathering. Through the analysis of variance (ANOVA) and Tukey’s multiple comparisons tests, no statistical difference was observed in the results of the analyzed mechanical properties.

In Figure 3-A, it is possible to observe that the periods of 400 h, 800 h, and 1200 h of accelerated weathering exposure did not cause a significant effect on the UTS values, since all values are within the standard deviation range. Furthermore, this property is governed by the fiber and not by the polymeric matrix, which is more susceptible to degradation, preserving the reinforcement. It is worth remembering that the moisture absorption by the specimens at the end of the accelerated weathering conditioning was not significant, due to the alternating cycles with and without humidity, which favored that the moisture absorbed during the wet cycle was lost during the dry cycle. Thus, the tensile strength and elastic modulus were not significantly affected by the accelerated weathering exposure, as related in the literature for CF-reinforced composites[33].

Figure 3-C shows the UTS vs Xc curve of specimens unconditioned and exposed to different times of conditioning.

This Figure 3-C shows the increase of Xc with the time of exposure in the accelerated weathering about the unconditioned specimen but without a consistent correlation with the UTS values, considering that the variation of this property has no statistical difference according to ANOVA and Tukey’s multiple comparisons. The Xc behavior may be due to structural changes caused by increased exposure, which may be related to a possible restructuring of the molecular chains, altering the degree of crystallinity and/or by the adsorbed humidity which makes greater the free volume among the chains. According to the literature[34], the stiffening of the material can be increased with the Xc increase. In the present study, the combined factors of UV radiation, humidity, and

Figure 2. DSC curves from 1st and 2nd heating of CF/PA6 composites unconditioned and after 400 h, 800 h, and 1200 h exposed to accelerated weathering.

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Effect of accelerated weathering environment on the carbon fiber/polyamide 6 composites

Figure 3. CF/PA6 composites unconditioned and after 400 h, 800 h, and 1200 h exposed to accelerated weathering: ultimate tensile strength, UTS (A), elastic modulus, E (B), UTS vs Xc (C), and interlaminar shear stress, ILSS (D).

temperature may have promoted a slight increase in the UTS property, as observed for the composite after 1200 h of exposure, which presented a slight increase of 1.6%, when compared to the unconditioned composites.

As for the elastic modulus values (Figure 3-B), a reduction of approximately 2.7% (± 1%) was observed in the composites after different exposure periods in accelerated weathering, when compared to the unconditioned composite, probably due to the small amount of the absorbed humidity.

Figure 4 shows representative macroscopic images of the UTS fracture of the CF/PA6 composites, unconditioned and after exposure to accelerated weathering. The images of the unconditioned composites (Figure 4-A-B-C) are typical of tensile fracture with total rupture of the specimen, with CF and matrix fracture, and consequently translaminar, intralaminar, and interlaminar fractures and more extensive delaminations. In the images of the composites after 400 h of exposure to accelerated weathering (Figure 4-D-E-F), it is possible to visualize the partial rupture of the specimen (Figure 4-D), with the rupture of CFs and with several intralaminar, interlaminar, and translaminar fractures. The images referring to the composites after 800 h (Figure 4-G-H-I) and 1200 h (Figure 4-J-K-L) after exposure to accelerated weathering show a more severe rupture, with the combination of intralaminar, interlaminar, and translaminar fractures, and fiber failure with aspects similar to those observed in compression failures, plus wedge split fault (highlighted with a green rectangle, in Figure 4-G and K).

Prolonged exposure to UV radiation, humidity, and temperature can contribute to the degradation of the polymeric matrix, affecting the properties that are governed by the matrix, as is the case of ILSS. Therefore, through the analysis of the ILSS (Figure 3-D), it was possible to verify the adhesion between the CF and the matrix, in addition to the analysis of the interfacial failure, which allows evaluating the integrity of the interface that is a prerequisite for a good stress transfer in the laminate. In this way, the Polímeros, 33(3), e20230032, 2023

ILSS test allowed evaluation of the quality and integrity of the interface between the PA6 matrix and the CF of the composites after 400 h, 800 h, and 1200 h exposed to accelerated weathering, and the OM images (Figure 5) assisted in assessing the failure.

The results obtained from the ILSS of the CF/PA6 composites are shown in Figure 3-D. It is observed that the values of the samples before and after exposure to accelerated weathering are close, 27.2 MPa (± 0.9 MPa), without statistical differences. However, the CF/PA6 composites after being exposed to accelerated weathering for 400 h and 1200 h showed a slight increase of 0.4% and 3.1%, respectively, in the mean resistance values of ILSS when compared to the unconditioned composite. This behavior suggests that the exposure time which may have promoted an improvement in the fiber/matrix interface of the composites, but this behavior needs to be further investigated.

After ILSS tests, OM analyses were carried out to analyze the failure mode resulted from the ILSS tests.

The images obtained from the fracture region are shown in Figure 5. The fracture of the tested specimens occurred in the central region of the transversal face of the specimen, as expected for the ILSS tests. First, the analysis of these images shows a homogeneous distribution of matrix and fibers in the laminate. The unconditioned CF/PA6 composite showed numerous intralaminar fractures in the CF cables at 0o and interlaminar fractures (highlighted in yellow) along the analyzed surface. This type of failure generally tends to fracture in the plane of the laminate, oriented between layers, and results mainly in the fracture of the polymeric matrix with a few exceptions that can fracture the fibers.

CF/PA6 composites after 400 h, 800 h, and 1200 h exposed to accelerated weathering showed more interlaminar, intralaminar, and translaminar fractures. The presence more intense of these three types of fractures may be related to a possible fragility of the polymeric matrix, as they are exposed to accelerated weathering, with the presence of UV 5/10

Montagna, L. S., Morgado, G. F. M., Marini, J., Montanheiro, T. L. A., Guimarães, A., Passador, F. R., & Rezende, M. C.

Figure 4. Representative macroscopic images of tensile fracture surface from the CF/PA6 composites unconditioned and after 400 h, 800 h, and 1200 h exposed to accelerated weathering.

radiation, humidity, and temperature. These factors may have influenced the beginning of matrix degradation. The matrix suffered intralaminar fractures, followed by interlaminar fractures, followed by translaminar fractures, which occur transversely to the plane of the laminate, causing the fiber breakage (highlighted in green).

Figure 6 presents SEM images of the surface of the specimens after exposure at different times in accelerated weathering. No change in the color of the composites was observed after this degradative process. Since the depth of penetration of UV radiation is small, resulting in surface degradation mainly by surface grooves (highlighted in yellow), which can lead to embrittlement of the matrix and the formation of microcracks. This behavior was more pronounced in the CF/PA6 composites after 800 h and 1200 h of exposure, and some regions with signs of erosion followed by the presence of some holes and cavitations (highlighted with red arrows).

Furthermore, since UV radiation affects the surface of materials and causes changes in surface morphology, prolonged exposure to UV radiation can deteriorate the surface, and consequently form surface defects where moisture, environmental pollutants, and other factors can migrate to the interior of components. Then, it can start a degradative process of the material and can result in the deterioration 6/10

and decrease of the mechanical resistance, for example, and even the premature failure of some components in use[8,12].

Consequently, with the increase in the exposure time to accelerated weathering the surface degradation is intensified causing an increase in the surface wettability value. This effect can result in high moisture absorption by the composite as shown in the contact angle presented in Figure 7. The contact angle measurements were done using the distilled water drop.

The values < 90º and > 90º correspond to hydrophilic and hydrophobic surfaces, respectively. The unconditioned CF/ PA6 composite presents a contact angle value of 82º, and after 400 h, 800 h, and 1200 h of exposure to the accelerated weathering, these values decreased to 62º, 57º, and 52º, respectively, that is, after conditioning the material became more hydrophilic, favoring the wettability and, consequently, the moisture absorption.

The approximately 30% reduction in contact angle values suggests that the UV radiation, together with other factors present in the accelerated weathering chamber, such as humidity and temperature, strongly affected the composite surface. This behavior was also observed by Mahat et al.[31], who submitted CF/PPS composites to 120, 240, 360, and 480 h of UV radiation, and suggested that the reduction in contact angle values also implied in the surface erosion mechanism, in addition to an increase in Polímeros, 33(3), e20230032, 2023

Effect of accelerated weathering environment on the carbon fiber/polyamide 6 composites

Figure 5. MO of ILSS fracture of CF/PA6 composite unconditioned and after 400 h, 800 h, and 1200 h exposed to accelerated weathering.

Figure 6. SEM images of the degraded surface after exposure to 0h (A), 400 h (B), 800 h (C), and 1200 h (D) in an accelerated weathering environment, 5,000x.

the wettability of the composites, leading to high moisture absorption and, consequently, greater degradation of the material.

Figure 8 shows SEM micrographs of the tensile fracture of the CF/PA6 composites after exposure to 400 h, 800 h, and 1200 h in an accelerated weathering environment.

In general, in all micrographs of the composites after exposure to accelerated weathering, it is observed that the fiber-matrix interface was preserved, maintaining the structural integrity of the specimens. Furthermore, as seen in the surface micrographs of the composites (Figure 6), few signs of surface degradation were observed, indicating that the exposure times studied resulted in early morphological degradation of the composites.

Similar behavior was observed by Pillay, Vaidya, and Janowski[35], who submitted CF/PA6 composites at 100, 200, 300, 420, and 600 h at only UV exposure, without the combination of other factors, such as temperature and humidity. Therefore, the present work used a longer period of exposure in addition to the combination of factors such as UV radiation, humidity, and temperature, which is more aggressive, and even so, we observed behavior similar to that of the authors[35].

Polímeros, 33(3), e20230032, 2023

Figure 7. Contact angle measurements and distilled water drop image: CF/PA6 composites before and after the exposure times of 400 h, 800 h, and 1200 h in accelerated weathering.

In the images of the unconditioned composites (Figure 8-A-B-C) it is possible to observe delamination (Figure 8-A), regions of broken fibers (Figure 8-B), and the plastic deformation of the matrix, as well as the good adhesion of the matrix in the CF (Figure 8-C).

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Montagna, L. S., Morgado, G. F. M., Marini, J., Montanheiro, T. L. A., Guimarães, A., Passador, F. R., & Rezende, M. C.

Figure 8. Fractography aspects of CF/PA6 composites: unconditioned (500x A, 2,500x B, and 5,000x C), after 400 h (500x D, 2,500x E, and 5,000x F), 800h (500x G, 2,500x H, and 5,000x I), and 1200 h (500x J, 2,500x K, and 5,000x L) exposed to accelerated weathering.

In the images of CF/PA6 composites after different exposure times to accelerated weathering, it is possible to visualize the integrity of the matrix and reinforcement, with the presence of delamination, intralaminar fractures and ruptured CF, typical of tensile fractures (Figure 8 D-G-J).

In addition to the presence of plastic deformation by the matrix and by the fibrils, as well as the good adhesion of the matrix to the CF, indicates CF is covered with the PA6 matrix.

However, when comparing the micrographs of the unconditioned composites and after 400 h, 800 h, and 1200 h exposed to accelerated weathering, it is possible to observe that the unconditioned composite shows the CF well adhered and coated by the matrix (Figure 8-C).

In the images of the composites after the accelerated weathering, smoother CF can be observed, with less PA6 matrix adhered to their surfaces, in addition to more pulled fibers due to the detachment of the matrix, forming micro-cracks. This behavior is more evident in the composites after 1200 h of exposure (Figure 8-L), since in this exposure time the deterioration of the matrix may have started, and consequently the detachment of the interface.

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4. Conclusions This work presented the influence of various weathering conditions, such as UV radiation, temperature, and humidity, at different exposure times (400 h, 800 h, and 1200 h) on the thermal, structural, and mechanical properties of CF/ PA6 composites. The conditioning times studied did not significantly influence the thermal stability of the composites, with no changes being observed in the Tm and only a slight variation in the Xc values. The polymeric matrix was preserved, without major signs of degradation, just some cracks and the beginning of erosion points, which were indicated by SEM images and FT-IR spectra. Thus, the resistance to ILSS, which is dominated by the matrix, did not show statistical differences, as well as the tensile strength, which is dominated by reinforcement, also did not show statistical differences.

And so, through micrographs of the tensile fracture, it was possible to confirm the internal preservation of the composites, without evidence of matrix and reinforcement degradation.

These results confirm that thermoplastic matrix composites can be used in lightweight external structural applications, without major losses in their properties and may be an alternative to the use of thermoset matrices.

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Effect of accelerated weathering environment on the carbon fiber/polyamide 6 composites

5. Author’s Contribution • Conceptualization – Larissa Stieven Montagna; Mirabel Cerqueira Rezende.

• Data curation – Larissa Stieven Montagna; Mirabel Cerqueira Rezende.

• Formal analysis – Larissa Stieven Montagna; Guilherme Ferreira de Melo Morgado; Mirabel Cerqueira Rezende.

• Funding acquisition – Mirabel Cerqueira Rezende; Alessandro Guimarães.

• Investigation – Larissa Stieven Montagna; Guilherme Ferreira de Melo Morgado; Mirabel Cerqueira Rezende.

• Methodology – Larissa Stieven Montagna; Mirabel Cerqueira Rezende.

• Project administration – Mirabel Cerqueira Rezende.

• Resources – Fabio Roberto Passador; Juliano Marini; Mirabel Cerqueira Rezende.

• Software – NA. • Supervision – Mirabel Cerqueira Rezende.

• Validation – Fabio Roberto Passador; Mirabel Cerqueira Rezende.

• Visualization – Larissa Stieven Montagna; Mirabel Cerqueira Rezende.

• Writing – original draft – Larissa Stieven Montagna; Guilherme Ferreira de Melo Morgado; Juliano Marini; Thaís Larissa do Amaral Montanheiro; Alessandro Guimarães; Fabio Roberto Passador; Mirabel Cerqueira Rezende.

• Writing – review & editing – Larissa Stieven Montagna; Guilherme Ferreira de Melo Morgado; Juliano Marini; Thaís Larissa do Amaral Montanheiro; Alessandro Guimarães; Fabio Roberto Passador; Mirabel Cerqueira Rezende.

6. Acknowledgements The authors would like to thank the CNPq processes 305123/2018-1 and 307933/2021-0) for the financial support, and the Federal Government Program ‘Rota 2030’ linked to the “Development of Skills for Design and Manufacturing of Tooling for Composite Parts” n° 27194.03.03/2020.01-00 for the financial support; the Lightweight Structures Laboratory from IPT for the coordination, and also the Brazilian Funding Institutions FIPT for the administrative support.

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35. Pillay, S., Vaidya, U. K., & Janowski, G. M. (2009). Effects of moisture and UV exposure on liquid molded carbon fabric reinforced nylon 6 composite laminates. Composites Science and Technology, 69(6), 839-846. http://dx.doi.org/10.1016/j. compscitech.2008.03.021.

Received: July 10, 2023 Revised: Aug. 30, 2023 Accepted: Sept. 04, 2023

Polímeros, 33(3), e20230032, 2023

ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.20220116

Mechanical behavior of snake grass fiber with neem gum filler hybrid composite Arumugam Pachiappan1  and Senthil Kumar Velukkudi Santhanam2*  1

Department of Mechanical Engineering, Rajalakshmi Engineering College, Chennai, Tamil Nadu, India 2 College of Engineering, Guindy, Anna University, Chennai, Tamil Nadu, India *vsskumar@annauniv.edu

Abstract In this study, the utilization of neem gum powder and snake grass fiber, gathered from snake grass plants is discussed.

The fibers are produced in various volume percentages of 5, 10, 15, 20, 25, 30 and 35% and their mechanical characteristics such as tensile strength, flexural strength, impact strength, and critical stress intensity are investigated. The combination of 30% snake grass fiber, 15% neem gum powder and 55% epoxy resin, in terms of volume, contributes towards the attaining of better mechanical properties. The tensile strength, flexural strength, impact strength, and critical stress intensity of this blend are respectively 36.497±0.429 MPa, 65.87±1.85 MPa, 2682.67±1.866 J/m2 and 42.291±2.61 Pa mm-1/2.

The mechanical properties improve with the addition of the fiber. However, as more fiber is added, the adhesion at the interface gets reduced. The automotive and aerospace sectors can use this composite material, which enhances the mechanical characteristics for interior applications.

Keywords: critical stress intensity, flexural strength, hybrid composite, impact strength, neem gum powder filler.

How to cite: Pachiappan, A., & Santhanam, S. K. V. (2023). Mechanical behavior of snake grass fiber with neem gum filler hybrid composite. Polímeros: Ciência e Tecnologia, 33(3), e20230033. https://doi.org/10.1590/0104-1428.20220116

1. Introduction A composite is a mixture of two materials, of which one is the reinforcing phase and the other is the matrix in which it is embedded. Metal, ceramic, and polymer are all acceptable choices for the matrix and reinforcing materials. Unlike metallic alloys, each material added to the composite maintains its original chemical, physical, and mechanical properties.

These composite materials are also widely known for their excellent strength, stiffness, and low density. The primary load-bearing components of composites typically consist of a fiber or particle phase that is stiffer and more powerful than the continuous matrix phase. The matrix serves as the source of the composite as it is more ductile than the fibers.

Materials comprised of two or more physically and chemically distinct phases that are separated by an interface are known as composites[1,2]. The diverse phases are combined to produce composites that perform like the individual components in terms of structural or functional quality[3].

In terms of weight, rust resistance, fatigue strength, and ease of installation, composite materials are superior to non-composite materials[4,5]. Composites are used in the manufacture of transmission towers, electrical gadgets and packing materials, spacecraft, medical equipment, and aircraft structures[6]. Natural fiber and biodegradable polymers are used to create bio-based green composites, which are further classified into hybrid and textile composites[7].

Currently, natural fibers are more often used in the manufacturing of PMC for a range of functions, including structural ones[8]. Natural fibers are produced by plants,

Polímeros, 33(3), e20230033, 2023

animals, and minerals. Due to their accessibility, environmental friendliness, degradability, and renewability, plant fibers have caught the attention of scientists, researchers, and engineers[9]. Abnormally formed fibers may get deboned and start to break apart from one another at lower loads, which cause fibrillation[10]. The fiber treatment is an alternative, for increasing the fiber surface area, chemical bonding, and interface adhesion between matrices of natural fiber[11]. The effect of sequential placing of fiber mats on the improvement of natural fiber (sisal, aloe Vera, and flax) hybrid composite qualities have also been recently studied by Balasubramanian et al.

[12] . The effect of Barium sulfate on the mechanical, DMA, and thermal behavior of woven aloe vera /flax hybrid composites were investigated by Arulmurugan et al.[13]. The impact properties new hybrid composite material made of woven flax and carbon fibers in an epoxy matrix was the subject of an investigation by Al-Hajaj et al.[14]. Many researchers have shown interest in the use of natural fibers as reinforcement in polymer matrix composites, such as kenaf fiber[15]. Characterization of the failure surface, impact, and inter laminar strength has been investigated on hybrid composites[16]. So far, studies have been carried out on how the different fiber loadings affect the flexural and thermal properties of banana leaf hybrid composites[17]. In composites containing 50% fiber, the flexural strength, modulus of elasticity (MOE), and tensile strength have all been increased[18]. The matrix-to-reinforcement ratio had a big impact on the performance of sugar palm composites[19].

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Pachiappan, A., & Santhanam, S. K. V. The best mechanical properties were attained by the composite specimen with a 10 mm fiber length and a 15 percent fiber loading[20]. Changes in fiber orientations will have a significant impact on the storage modulus, loss tangent, and other mechanical parameters that are investigated[21]. It has been shown that jute fibers may be chemically treated to enhance matrix-fiber adhesion by increasing the interfacial bonding with the polymer matrix, which in turn improved the composites’ tensile properties[22]. Natural fiber reinforced polymer composite materials are used to make wind turbine blades[23]. Unsaturated polyester hybrid composites bonded with sugar palm yarn and glass fiber have been developed for use in automobile components[24]. Fiber orientation and loading have an impact on the mechanical and thermal characteristics of composites that are reinforced with sugar palm yarn and an unsaturated polyester resin[25]. Bio composites must possess the desired qualities in order to displace synthetic fiber-reinforced composites for being used in new industrial applications[26]. Natural fibers are used in automobile structures because of their moderate tensile strength, better stiffness, and high damping capability[27].

Natural fiber-reinforced composites are intended for lowering vibration and noise levels, in addition to component weight when used in automotive applications. Additionally, composites have exceptional resistance to corrosion and fatigue[28]. One of the hybrid composites, specimen D, has 20% weight of snake grass fibre, with increased tensile strength. However, when the snake grass fibre content is increased, the tensile strength got abruptly decreased. The findings suggest that the alkali treatment enhances the elastic behavior and boosts the material’s resilience to failure. The elastic region is formed when the strain rate is reduced and above this region, the specimen experiences plastic deformation. This behaviour due to the resin in the area which begins to deform plastically, causing it to produce small cracks[29]. Hybrid composites made by Rangaraj et al.

[30] with 20% weight of snake grass and 10% weight of areca fibre provided the highest tensile, flexural, impact, and hardness values[30]. In the hardness test, fiber-reinforced polyester composite materials that have been calcium carbonate-treated, have the highest hardness values, scoring 27 BHN, outperforming untreated snake grass fiber-reinforced polyester composite materials by more than 50%. The calcium carbonate-treated reinforced composite has a high mean ultimate strength of 45.335 N/mm2 according to the tensile test. A fiber-reinforced composite coated with calcium carbonate has high impact strength of 3.35 J. A fiber-reinforced composite that has been treated with Ca2CO3 has a high ultimate flexural strength of 4.5 N/mm2[31]. The composites demonstrated that at 30% fibre volume, the maximum mechanical characteristics are obtained. Additionally, adding silica nano filler improved the inter laminar structure’s cohesive strength. The material is strengthened by these behavioral changes at the ideal concentration of 30% hybrid fibers and 3% nano silica[32]. The outcomes showed that banana fibre with 20% by weight produced good results, maintaining the mechanical strength values at the desired level[33]. SiO2 and B4C significantly increase the tensile, flexural, and impact strength of snake grass fiber, according to research by Hariprasad et al.[34]. To create a new composite, Kevlar and Napier grass fibres are reinforced with epoxy 2/9

matrix, and it was assessed that sample A had the highest mechanical strength[35]. The tensile, flexural, interlaminar shear, impact, and hardness of the natural fibre reinforced hybrid composite, which combines jute, snake grass, and kenaf fibres as reinforcement with varying fibre quantities, were evaluated in this research effort. Additionally, by adding Annona reticulata (custard apple) seed powder as a filler, the hybrid composites’ wear behaviour was improved.

This analysis showed that the sample, which contains kenaf fibre (without filler) and snake grass in similar amounts (12.5% of each), had outstanding mechanical properties.

The sample with 5 wt% filler exhibits a lower wear rate than other samples in terms of wear behavior[36]. The tensile, compression, and flexural properties of the epoxy hybrid composites reinforced with banana and snake grass fibres were examined for various stacking orders. To achieve greater interfacial strength, the fibres were properly treated with an alkali solution[37]. The investigations showed that, up to a 20% increase in weight of African tefl fibre, the mechanical qualities increased before degrading. Additionally, it has been shown that natural fibres and bio castor seed shell powder had a stronger combined effect on the mechanical qualities[38]. The chair made of snake grass fiber-reinforced polymer composites is manufactured and utilised in place of wood chairs in commercial settings. The mechanical characteristics, including flexural and compressive strength, as well as the water absorption with time, have been studied.

These characteristics are contrasted with those of SAL wood, and the conclusions arrived at are listed[39]. From the study of this literature survey, it is understood that the nature of composite extraction, processing techniques and their change in the chemical and the mechanical properties with different composition of materials is utilized for various applications.

Also, it is inferred that snake grass fiber acts as the best alternative for glass fiber in all its applications. Neem gum powder as a filler is used in the biomedical field for making prosthetics, and dentistry. A Combination of neem gum powder with snake grass fiber will open the way to many more applications in both mechanical and biomedical fields.

2. Materials and Methods 2.1 Materials 2.1.1 Snake grass fiber The southernmost state of India, Tamil Nadu has an abundance of snake grass from which fibers are extracted using a straightforward water retting method. It is succulent and extremely thick with sturdy leaves that hold the water.

The extracted snake grass fibers are used as reinforcement in fabricating composites.

2.1.2 Neem gum (filler) The Neem tree when scratched, naturally yields neem gum.

Neem gum is a non-bitter substance that is soluble in cold water and it is clear and amber in color. It is a biodegradable natural filler substance which is inexpensive and widely accessible. The parameters of the neem gum powder taken for this study are, particle size ranging from 1 to 15 microns, tensile strength of 12.5 MPa, flexural strength of 18.1 MPa, impact energy of 1 joule, and density of 1.08 g/cm3.

Polímeros, 33(3), e20230033, 2023

Mechanical behavior of snake grass fiber with neem gum filler hybrid composite Different volume fractions of Acacia Nilotica bio filler were used to create composite specimens. The effects of filler loading and chemically treated reinforced fibers influenced the mechanical properties of the composites. When the bio filler content in the polyester matrix increased from 15% to 20% of volume fractions, a little decrease in the mechanical properties was noticed. This could be because the bio filler agglomerates in the composite material at higher volume fractions[40]. In the present investigation, neem gum powder has been used as a bio filler with a 15% volume fraction, because it was found after reading numerous texts on the subject that higher composite characteristics could only be attained below this level.

2.1.3 Epoxy resin (matrix) One of the most important roles played by the resins or matrix in a polymer composite is binding the reinforcements together, maintaining the shape of a component, and passing applied loads to the reinforcing fibers. It protects the reinforcing fibers from abrasion and damaging the environment. Despite its widespread use, thermosetting plastics like polyester and epoxy resins help in determining the mechanical properties of composites. Epoxy resin serves as the paper’s matrix material. Epoxy resins outperform polyester resins in terms of strength and cost, whereas Polyester resins are less susceptible to moisture absorption than epoxy resins. Resins adhere to glass and organic fibers quite well. The LY556 epoxy and the HY951 hardener, with density values of 1.15–1.20 g/ cm3 and 0.97–0.99 g/cm3 respectively are combined to create the composite plate. The weight ratio of the hardener to epoxy is 10:1.

2.2 Methods A simple hand lay-up method is utilized to create the composite plate with different volume percentages of fiber, such as 5%, 10%, 15%, 20%, 25%, 30%, and 35%.The fiber volume percentage varies according to the matrix and fiber densities. The male and female steel dies are used to create the composite plates. The male and female portions of the die are initially coated with a releasing agent to facilitate the removal of the specimens after the solidification procedure.

The fibers are laid one by one over the resin on the female die. This is followed by sealing the mold, putting it in a hydraulic press, and applying 5 bars of compressive pressure for 4 hours at 30°C ambient temperature. In the 300 x 300 x 3 mm mold chamber, the matrix is used to reinforce the fibers. Table 1 provides a detailed of the composites’ volume percentage used in this work.

Tensile testing was done on the composite in accordance with ASTM D638 at a test speed of 5 mm/min utilizing TinusOlesan UTM. The specimens (a) The equipment used for testing and Specimen before (b) and Specimen after fracture (c) are shown in Figure 1.

Flexural testing of the composite was carried out using the Tinus Olesan UTM in the three-point bending mode in compliance with the ASTMD790 standard. Specimen Dimensions: Thickness (d) = 4mm, Width (b) =13 mm, Support Span (L) = 52 mm, σf = 3𝐹𝐿/2𝑏𝑑2 where, σf = Flexural Strength F = Break load (value taken from graph.) Figure 2 shows the Flexural testing samples (a), Equipment used for testing and specimen before fracture (b) and specimen after fracture (c)

Table 1. Composition of all volume percentages of the composites.

S. No Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7

Snake grass fiber(%) 5 10 15 20 25 30 35

Neem Gum Powder(%) 15 15 15 15 15 15 15

Epoxy(%) 80 75 70 65 60 55 50

Figure 1. Various composition specimens and test setup for testing of tensile properties of hybrid composites.

Figure 2. Various composition specimens and test setup for testing of flexural properties of hybrid composites.

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Pachiappan, A., & Santhanam, S. K. V. Using impact testing equipment, the ASTM D256 standard calls for Izod mode impact testing of the composite was carried out. Specimen Dimensions: Width (b) = 13.1 mm, Thickness (t) = 3.2 mm, area = 13.1 × 3.2 = 41.92 mm2, Izod Impact Value = 0.6 J, Impact Strength = (0.6/41.92) × 1000 = 14.31 N-mm/mm2. Figure 3 displays the testing samples.

According to ASTM D709 standards, the crack is in the opening mode, which is a tensile stress normal to the crack’s plane. The Formula for calculating the fracture toughness factor for stress intensity K can be used to estimate the fracture toughness.

It relies on (i) the load, (ii) Flow depth and (iii) the Geometry Critical stress intensity factor for mode 1 is K ic =

  A   F    30  B W   W   P

(1)

A 2+    A   W F    = 1.5   W    A 1 − W   

Where F(A/W)- geometry crack length factor, P is the load at which the first crack appears, B is the test specimen’s thickness, w is the specimen’s length, and A is the initial crack size. The test samples are cut into the required dimensions as shown in Figure 4a and 4b displays the testing samples.

3. Results and Discussions Five samples per composition were tested and the average values of mechanical properties with standard deviation are shown in Tables 2, 3, 4 and 5.

Standard Deviation for each composition of five samples for all mechanical properties are calculated using the formula = σ

1 N

N

∑  X − µ ) i

2

(3)

i =1

σ = Population standard deviation N = Number of observations in population X i = ith observation in the population μ = Population mean

(2)

2  0.866 + 4.66  A  − 13.32  A    W W    3 4   A A  +14.72   − 5.6    W W      

Figure 3. Various composition specimens for testing of impact strength of hybrid composites.

Table 2. Tensile test results.

SAMPLE ID

BREAK LOAD (N)

TENSILE STRENGTH (MPa)

TENSILE MODULUS (MPa)

Sample 1

520± 3.033

13.460±0.073

562.820±3.35

Sample 2

614±2.828

16.075± 0.118

591.871± 3.87

Sample 3

755± 2.93

20.063±0.105

695.763±2.83

Sample 4

870+± 2.39

23.038± 0.292

794.488± 2.11

Sample 5

950± 2.40

29.713± 0.517

898.378± 3.66

Sample 6

1010± 2.60

36.497± 0.429

1082.712± 2.58

Sample 7

915±3.60

24.303± 0.320

943.854± 4.12

SAMPLE ID

BREAK LOAD (N)

Flexural Strength(MPa)

Flexural Modulus (MPa)

Sample 1

54.986± 2

20.62± 1.96

1652.820± 3.87

Sample 2

63.317± 3.6

23.75± 2.4

1681.871± 2.01

Sample 3

76.647±2.2

28.75± 2.58

1765.763± 3.17

Sample 4

107.626±2.2

40.37± 1.41

1874.488± 2.86

Sample 5

140.631±1.83

52.75± 2.58

1898.378± 3.162

Sample 6

175.609± 3.13

65.87±1.85

1952.712± 2.46

Sample 7

71.635± 2.64

26.87± 1.96

1743.854± 3.288

Table 3. Flexural test results.

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Mechanical behavior of snake grass fiber with neem gum filler hybrid composite Table 4. Impact test results.

SAMPLE ID Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7

Impact Energy (J) 0.6± 0.1609 0.8± 0.1135 1.5± 0.172 2.4± 0.219 4.11± 0.273 5.5± 0.231 3.3±0.313

Impact Strength (J /m2) 2214.31± 0.265 2319.08± 0.293 2429.27± 0.426 2555.41±1.744 2593.85± 2.338 2682.67± 1.866 2667.16± 1.721

Table 5. Mode I fracture test result.

SAMPLE No Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7

Ult. Stress (MPa) 17.81 ±2.228 25.434±-2.28 30.22±2.115 33.274± 2.5 38.21±2.961 44.232±1.28 30.42 ±1.766

Displacement (mm) 2.1±0.279 2.5± 0.141 2.9± 0.256 3.2± 0.185 3.8± 0.261 4.2±0.287 4.8± 0.311

Peak Load (N) 140±1.853 145± 2.316 185± 2.786 205± 3.349 215± 2.786 235± 2.442 285±3.547

Critical Stress Intensity Factor (KIC) (Pa mm-1/2) 19.125±1.469 20.252±2.059 22.561±1.853 25.005± 1.414 33.213± 2.315 42.291±2.61 50.664± 1.744

Figure 4. Various composition specimens for testing of mode I crack test of composites.

3.1 Tensile property Measurements of the tensile properties of the various fiber volume percentages in the hybrid composites are made.

With every 5% increase in volume of Snake grass fiber in the hybrid composites, the tensile strength gradually increases.

Figure 5 and Table 2 demonstrate that the composition of 30% snake grass fiber, 15% neem gum powder and 55% epoxy resin achieve the maximum tensile strength and tensile modulus of 36.497±0.429 MPa & 1082.712±2.58 MPa respectively, which are higher than the corresponding values of 14.02 MPa & 960.621 MPa obtained by Vimalanathan et al.[41] and 3.56 MPa & 1023 MPa by Palanikumar et al.[42], due to the increased interfacial adhesion between the fiber and the matrix. A further increase in the fiber volume percentage in hybrid composites to 35% causes a decline in the tensile properties due to insufficient bonding between the fiber and the matrix as shown in the SEM image in Figure 6.

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Figure 5. Comparison of tensile strength for various composition specimens of hybrid composite.

3.2 Flexural property The flexural properties of the hybrid composites are evaluated for each fiber volume percentage.

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Figure 6. SEM image of 35% snake grass fiber with matrix poor interfacial adaption.

Figure 7. Comparison of flexural strength for various composition specimens of the hybrid composite.

and 1810 MPa obtained by Vimalanathan et al.[41] and 27.26 MPa and 1873 MPa by Palanikumar et al.[42], due to better interfacial adhesion between the fiber and the matrix.

The best results are obtained by Sample 6 (30% snake grass fiber), as shown in Figure 7 and Table 3. Flexural strength is observed to decrease as the fiber volume percentage is increased in hybrid composites made of 35% snake grass fiber, 15% neem gum, and 50% epoxy resin due to an inadequate bonding between the fiber and the resin.

3.3 Impact property

Figure 8. Comparison of izod impact strength for various composition specimens of the hybrid composite.

The hybrid composite’s impact characteristics are measured for each fiber volume percentage. Every 5% increase in volume of the snake grass fiber in hybrid composites results in a progressive rise in the impact strength. It has been noted that stronger interfacial adhesion between the fiber and the matrix on the composition of 30% snake grass fiber, 15% neem gum powder and 55% epoxy resin attained the maximum impact strength of 2682.67 ±1.866 J/m2 which is higher than the value of 2500 J/m2 obtained by Vimalanathan et al.[41] and 2671 J/m2 by Palanikumar et al.[42]. Sample 6 (30% snake grass fiber) produces the highest results as seen in Figure 8 and Table 4. Due to the poor bonding between the fiber and the resin in the composition of 35% snake grass fiber, 15% neem gum, and 50% epoxy resin, it was noticed that further increasing the fiber volume percentage in hybrid composites causes a drop in the impact strength.

3.4 Mode I fracture Figure 9. Comparison of critical stress intensity factor for various composition specimens of the hybrid composite.

The Flexural strength increases gradually with every 5% increase in the volume of Snake grass fiber in the hybrid composites. The mixture of 30% snake grass fiber, 15% neem gum powder and 55% epoxy resin was found to have the highest flexural strength and flexural modulus of 65.87±1.85 MPa and 1952.712±2.46 MPa respectively, which are higher than the corresponding values of 26.16 MPa 6/9

In the Mode I fracture test, the hybrid composite’ fracture properties were measured for different fiber volume fractions.

With every 5% increase in volume of Snake grass fiber in hybrid composites, the Critical Stress Intensity Factor and the Ultimate Stress increased rapidly. Due to improved interfacial adhesion between the fiber and the matrix, it was seen that the composition of 30% snake grass fiber, 15% neem gum powder, and 55% epoxy resin gave the highest ultimate strength of 44.232 ±1.28 MPa and Critical Stress Intensity Factor of 42.291±2.61 Pa mm-1/2. Figure 9 and Table 5 demonstrate that sample 6 (30% snake grass fiber) produces the best outcome. The ultimate strength drops when the fiber content in hybrid composites is increased further.

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Mechanical behavior of snake grass fiber with neem gum filler hybrid composite

Figure 10. SEM image of 30% snake grass fiber with good matrix interfacial bonding.

Figure 11. SEM image of 35% snake grass fiber breakage and fiber pull out.

Figure 10 shows the existence of good adhesion between the fiber and the matrix. A strong interface permits the composite to withstand the applied load even if multiple fibers break the load, which can be transferred by the integral portion of the fibers, which in turn increases the mechanical strength.

The fiber pullout and fiber breakage of the tensile fracture and impact fracture of the composite specimens have been examined using a scanning electron microscope (SEM).

Fiber pullout and fiber breakage are evidently noticed on 35% snake grass fiber, in the fiber binding zone. Flexural strength and tensile strength were tested for fiber pullout and fiber breakage are evident in Figure 11. With further increase in the fiber volume fraction, the tensile strength and flexural strength start reducing, due to fiber agglomeration at higher volume fraction. Further, clustering of fiber leads to poor wetting of the fiber by the matrix and hence there is a poor bonding between the fibers[19]. This results in reduction in the mechanical properties.

Similar effect was noticed in this case due to poor interfacial bonding also, at higher volume fraction, as shown in Figure 6.

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4. Conclusion Tests were conducted to evaluate the effect of fiber volume percentage and filler on the mechanical properties of the hybrid composite. The volume of 30% snake grass fiber with 15% neem gum powder and 55% epoxy resin contributes in achieving better mechanical properties such as tensile strength, flexural strength, impact strength and critical stress intensity factor (KIC) of 36.497±0.429 MPa, 65.87±1.85 MPa, 2682.67±1.866 J/m2 and 42.291±2.61 Pa mm-1/2 respectively. The results show that adding fiber increased the mechanical properties to some extent, but too much of fiber impaired the adhesion at the interface, which in turn reduced the mechanical properties of the composite material. The SEM images Figures 6 and 11 prove it. Stress transfers within the matrix material are made more efficient by the inclusion of the filler material as secondary reinforcement. The benefits of combining this snake grass fiber with other comparable biopolymers in composite applications is found to be substantial. Hence, this new sustainable biodegradable material can be taken up for future research for interior applications in automotive and aerospace industries.

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5. Author’s Contribution • Conceptualization – Arumugam Pachiappan; Senthil Kumar Velukkudi Santhanam.

• Data curation – NA. • Formal analysis – Arumugam Pachiappan.

• Funding acquisition – NA. • Investigation – Arumugam Pachiappan; Senthil Kumar Velukkudi Santhanam.

• Methodology – Arumugam Pachiappan.

• Project administration – Senthil Kumar Velukkudi Santhanam.

• Resources – Arumugam Pachiappan; Senthil Kumar Velukkudi Santhanam.

• Software – NA. • Supervision – Senthil Kumar Velukkudi Santhanam.

• Validation – Senthil Kumar Velukkudi Santhanam.

• Visualization – NA. • Writing – original draft – Arumugam Pachiappan.

• Writing – review & editing – Senthil Kumar Velukkudi Santhanam.

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Introduction to biofiller reinforced degradable polymer composites.

In R. Jumaidin, S. M. Sapuan & H. Ismail (Eds.), Biofillerreinforced biodegradable polymer composites (pp. 1-23). Boca Raton: CRC Press. http://dx.doi.org/10.1201/9780429322112-1.

2. Sari, N. H., Pruncu, C. I., Sapuan, S. M., Ilyas, R. A., Catur, A. D., Suteja, S., Sutaryono, Y. A., & Pullen, G. (2020). The effect of water immersion and fiber content on properties of corn husk fibers reinforced thermoset polyester composite.

Polymer Testing, 91, 106751. http://dx.doi.org/10.1016/j. polymertesting.2020.106751.

3. Sapuan, S. M., Aulia, H. S., Ilyas, R. A., Atiqah, A., DeleAfolabi, T. T., Nurazzi, M. N., Supian, A. B. M., & Atikah, M. S. N. (2020). Mechanical properties of longitudinal basalt/ woven-glass-fiber-reinforced unsaturated polyester-resin hybrid composites. Polymers, 12(10), 2211. http://dx.doi.org/10.3390/ polym12102211. PMid:32992450.

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13. Arulmurugan, M., Selvakumar, A. S., Prabu, K., & Rajamurugan, G. (2020). Effect of barium sulfate on mechanical, DMA and thermal behavior of woven aloe vera/flax hybridcomposites.

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17. Zin, M. H., Abdan, K., & Norizan, M. N. (2019). The effect of different fiber loading on flexural and thermal properties of banana/pineapple leaf (PALF)/glass hybrid composite.

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18. Richard, B. D., Wahi, A., Nani, R., Lling, E., Osman, S., & Ali, D. S. H. (2019). Effect of fiber loading on the flexural and tensile strength of oil palm frond fiber reinforced polymer composite. International Journal of Integrated Engineering, 11(7), 122-128. http://dx.doi.org/10.30880/ijie.2019.11.07.016.

19. Baihaqi, N. M. Z. N., Khalina, A., Nurazzi, N. M., Aisyah, H. A., Sapuan, S. M., & Ilyas, R. A. (2021). Effect of fiber content and their hybridization on bending and torsional strength of hybrid epoxy composites reinforced with carbon and sugar palm fibers. Polimery, 66(1), 36-43. http://dx.doi. org/10.14314/polimery.2021.1.5.

20. Karthick, R., Adithya, K., Hariharaprasath, C., & Abhishek, V. (2018). Evaluation of mechanical behavior of banana fiber reinforced hybrid epoxy composites. Materials Today: Proceedings, 5(5), 12814-12820. http://dx.doi.org/10.1016/j.matpr.2018.02.265.

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Mechanical behavior of snake grass fiber with neem gum filler hybrid composite 21. Doddi, P. R. V., Chanamala, R., & Dora, S. P. (2020). Effect of fiber orientation on dynamic mechanical properties of PALF hybridized with basalt reinforced epoxy composites.

Materials Research Express, 7(1), 015329. http://dx.doi. org/10.1088/2053-1591/ab6771.

22. Wang, H., Memon, H., Hassan, E. A. M., Miah, M. S., & Ali, M. A. (2019). Effect of jute fiber modification on mechanical properties of jute fiber composite. Materials, 12(8), 1226. http://dx.doi.org/10.3390/ma12081226. PMid:30991643.

23. Kalagi, G. R., Patil, R., & Nayak, N. (2018). Experimental study on mechanical properties of natural fiber reinforced polymer composite materials for wind turbine blades. Materials Today: Proceedings, 5(1), 2588-2596. http://dx.doi.org/10.1016/j. matpr.2017.11.043.

24. Nurazzi, N. M., Khalina, A., Sapuan, S. M., & Rahmah, M. (2018). Development of sugar palm yarn/glass fiber reinforced unsaturated polyester hybrid composites. Materials Research Express, 5(4), 045308. http://dx.doi.org/10.1088/2053-1591/ aabc27.

25. Nurazzi, N. M., Khalina, A., Chandrasekar, M., Aisyah, H. A., Rafiqah, S. A., Ilyas, R. A., & Hanafee, Z. M. (2020).

Effect of fiber orientation and fiber loading on the mechanical and thermal properties of sugar palm yarn fiber reinforced unsaturated polyester resin composites. Polimery, 65(2), 115-124. http://dx.doi.org/10.14314/polimery.2020.2.5.

26. Ayu, R. S., Khalina, A., Harmaen, A. S., Zaman, K., Isma, T., Liu, Q., Ilyas, R. A., & Lee, C. H. (2020). Characterization study of Empty Fruit Bunch (EFB) fibers reinforcementin Poly(Butylene) Succinate (PBS)/starch/glycerol composite sheet. Polymers, 12(7), 1571. http://dx.doi.org/10.3390/ polym12071571. PMid:32679865.

27. Balla, V. K., Kate, K. H., Satyavolu, J., Singh, P., & Tadimeti, J. G. D. (2019). Additive manufacturing of natural fiber reinforced polymer composites: processing and prospects.

Composites. Part B, Engineering, 174, 106956. http://dx.doi. org/10.1016/j.compositesb.2019.106956.

28. Sathish, S., Prabhu, L., Gokulkumar, S., Karthi, N., Balaji, D., & Vigneshkumar, N. (2021). Extraction, treatment and applications of natural fibers for bio-composites - a critical review. International Polymer Processing, 36(2), 114-130. http://dx.doi.org/10.1515/ipp-2020-4004.

29. Sathish, S., Kumaresan, K., Prabhu, L., & Vigneshkumar, N. (2017).

Experimental investigation on volume fraction of mechanical and physical properties of flax and bamboo fibers reinforced hybrid epoxy composites. Polymers & Polymer Composites, 25(3), 229-236. http://dx.doi.org/10.1177/096739111702500309.

30. Rangaraj, R., Sathish, S., Mansadevi, T. L. D., Supriya, R., Surakasi, R., Aravindh, M., Karthick, A., Mohanavel, V., Ravichandran, M., Muhibbullah, M., & Osman, M. (2022).

Investigation of weight fraction and alkaline treatment on Catechu Linnaeus/Hibiscus cannabinus/Sansevieria Ehrenbergii plant fibers-reinforced epoxy hybrid composites. Advances in Materials Science and Engineering, 2022, 4940531. http://dx.doi.org/10.1155/2022/4940531.

31. Jenish, I., Sahayaraj, A. F., Appadurai, M., Raj, E. F. I., Suresh, P., Raja, T., Salmen, S. H., Alfarraj, S., & Manikandan, V. (2021). Fabrication and experimental analysis of treated snake grass fiber reinforced with polyester composite. Advances in Materials Science and Engineering, 2021, 6078155. http://dx.doi.org/10.1155/2021/6078155.

32. Jenish, I., Sahayaraj, A. F., Suresh, V., Mani raj, J., Appadurai, M., Raj, E. F. I., Nasif, O., Alfarraj, S., & Kumaravel, A. K. (2022).

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Analysis of the hybrid of mudar/snake grass fiber-reinforced epoxy with nano-silica filler composite for structural application.

Advances in Materials Science and Engineering, 2022, 7805146. http://dx.doi.org/10.1155/2022/7805146.

33. Nguyen, T. A., & Nguyen, T. H. (2022). Study on mechanical properties of banana fiber-reinforced materials poly (lactic acid) composites. International Journal of Chemical Engineering, 2022, 8485038. http://dx.doi.org/10.1155/2022/8485038.

34. Hariprasad, P., Kannan, M., Ramesh, C., Sahayaraj, A. F., Jenish, I., Hussain, F., Khedher, N. B., Boudjemline, A., & Suresh, V. (2022). Mechanical and morphological studies of Sansevieria trifasciata fiber-reinforced polyester composites with the addition of SiO2 and B4C. Advances in Materials Science and Engineering, 2022, 1634670. http://dx.doi. org/10.1155/2022/1634670.

35. Ganesamoorthy, R., Reddy, R. M., Raja, T., Panda, P. K., Dhoria, S. H., Nasif, O., Alfarraj, S., Manikandan, V., & Jenish, I. (2021). Studies on mechanical properties of Kevlar/ Napier grass fibers reinforced with polymer matrix hybrid composite. Advances in Materials Science and Engineering, 2021, 6907631. http://dx.doi.org/10.1155/2021/6907631.

36. Kumar, R. P., Muthukrishnan, M., & Sahayaraj, A. F. (2022).

Experimental investigation on jute/snake grass/kenaf fiber reinforced novel hybrid composites with annona reticulata seed filler addition. Materials Research Express, 9(9), 095304. http://dx.doi.org/10.1088/2053-1591/ac92ca.

37. Sahoo, M. R., Gopinathan, R., Kumar, K. V. P., Rani, J. J. A., Pradhan, R., & Parida, L. (2022). Study on the influence of stacking pattern on mechanical behaviour of banana/ snake grass fibers hybrid epoxy composite. Materials Today: Proceedings, 69(Part 3), 1164-1168. http://dx.doi.org/10.1016/j. matpr.2022.08.185.

38. Manickaraj, J., Ramamoorthi, R., Sathish, S., & Makeshkumar, M. (2022). Effect of hybridization of novel African teff and snake grass fibers reinforced epoxy composites with bio castor seed shell filler: experimental investigation. Polymers & Polymer Composites, 30, 09673911221102288. http://dx.doi. org/10.1177/09673911221102288.

39. Sathishkumar, T. P. (2016). Development of snake grass fiber-reinforced polymer composite chair. Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications, 230(1), 273-281. http://dx.doi.org/10.1177/1464420715569291.

40. Bhaskar, K. B., Santhanam, V., & Devaraju, A. (2020).

Dielectric strength analysis of acacia nilotica with chemically treated sisal fiber reinforced polyester composite. Digest Journal of Nanomaterials and Biostructures, 15(1), 107-113. http://dx.doi.org/10.15251/DJNB.2020.151.107.

41. Vimalanathan, P., Venkateshwaran, N., & Santhanam, V. (2016).

Mechanical, dynamic mechanical, and thermal analysis of Shorea robusta-dispersed polyester composite. International Journal of Polymer Analysis and Characterization, 21(4), 314-326. http://dx.doi.org/10.1080/1023666X.2016.1155818.

42. Palanikumar, V., Narayanan, V., & Vajjiram, S. (2018).

Experimental investigation of mechanical and viscoelastic properties of Acacia Nilotica filler blended polymer composite.

Polymer Composites, 39(7), 2535-2546. http://dx.doi.org/10.1002/ pc.24238.

Received: Jan. 31, 2023 Revised: Sept. 04, 2023 Accepted: Sept. 14, 2023

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.20220121

Evaluation of potential biomaterials for application in guide bone regeneration from Bacterial Nanocellulose/Hydroxyapatite Elouise Gaulke1, Michele Cristina Formolo Garcia2,3* , Bruna Segat2 , Giannini Pasiznick Apati2 , Andréa Lima dos Santos Schneider2,3 , Ana Paula Testa Pezzin1 , Karina Cesca4  and Luismar Marques Porto4  Laboratório de Materiais, Programa de Pós-graduação em Engenharia de Processos, Universidade da Região de Joinville, Joinville, SC, Brasil 2 Laboratório de Biotecnologia I, Departamento de Engenharia Química, Universidade da Região de Joinville, Joinville, SC, Brasil 3 Laboratório de Biotecnologia I, Programa de Pós-graduação em Saúde e Meio Ambiente, Universidade da Região de Joinville, Joinville, SC, Brasil 4 Laboratório de Engenharia Biológica, Programa de Pós-graduação em Engenharia Química, Universidade Federal de Santa Catarina, Florianópolis, SC, Brasil 1

*micheleformologarcia@gmail.com

Abstract Bacterial nanocellulose (BNC) membranes have interconnected porous nanostructures and excellent biocompatibility.

Functionalizing these with calcium phosphate sources and metal ions confers optimized properties to the biomaterial.

This study aims to synthesize BNC membranes, functionalize them with copper and magnesium apatites, characterize and evaluate their cytotoxicity and antimicrobial potential. Membranes were synthesized for 8 days in Mannitol Medium.

The biocomposite production was by immersion cycles. The biocomposites were characterized by porosity and swelling capacity, Fourier transforms infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), antimicrobial properties and cytotoxicity assays. The FTIR and SEM results showed that phosphate groups were incorporated into the BNC. The TGA analysis also indicated the incorporation of the inorganic phase. The membrane functionalization with Cu promoted the antimicrobial properties of the biomaterial. However, functionalization with Mg had a more positive behavior on cell viability, proving to be more suitable for use as an implantable material.

Keywords: apatites, bacterial nanocellulose, biocomposites, osteogenesis.

How to cite: Gaulke, E., Garcia, M. C. F., Segat, B., Apati, G. P., Schneider, A. L. S., Pezzin, A. P. T., Cesca, K., & Porto, L. M. (2023). Evaluation of potential biomaterials for application in guide bone regeneration from Bacterial nanocellulose/Hydroxyapatite. Polímeros: Ciência e Tecnologia, 33(3), e20230034. https://doi.org/10.1590/01041428.20220121

1. Introduction The healing process of bone fractures is considered a regenerative and biologically complex process[1]. Autologous bone grafts have been pointed out as the primary solution[2].

However, there is still the risk of transmitting infectious diseases and rejection[3]. In this context, guided bone regeneration (GBR) appears as one of the most effective and reliable methods to promote bone restructuring[4].

In this context, scaffolds for bone regeneration emerged to mimic the natural extracellular matrix of bone (ECM), providing a basic structure and microenvironment for the growth of bone tissue with excellent biocompatibility, adaptable biodegradability, osteoconductivity and minimal immunogenic responses[5]. For this reason, different biomaterials have been studied.

Polímeros, 33(3), e20230034, 2023

BNC is an extracellular polysaccharide produced by some bacterial genera, especially the species of Komagataeibacter[6].

Its surface allows the adsorption of metallic ions or metallic nanoparticles[7]. It has low rejection and inflammatory reaction due to its excellent biocompatibility, good permeability, hygroscopicity, and flexibility, allowing its wide use in tissue engineering as a scaffold[8]. However, BNC alone does not have all the properties necessary to act as a device for a bone implant.

In this context, the incorporation of hydroxyapatite (HAp) in the BNC matrix[9] is studied. The chemical similarity of HAp to the naturally occurring bone matrix, biocompatibility, non-toxicity and high osteoconductivity makes it highly attractive for bone regeneration and implantation[10].

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

Gaulke, E., Garcia, M. C. F., Segat, B., Apati, G. P., Schneider, A. L. S., Pezzin, A. P. T., Cesca, K., & Porto, L. M. The biocomposite of BNC/HAp is formed by the porous membrane reinforced with HAp crystals. This structure allows this material to trigger the fixation and proliferation of bone cells at the defect site[9]. In addition, since the HAp particles are physically attached to the BNC fibers, these particles do not migrate to neighboring areas, preventing damage that could be caused to the surrounding soft tissues[4,10].

Furthermore, adding divalent cations to these biomaterials can promote optimized characteristics.

Copper (Cu) is an essential micronutrient involved in the immune system and has bacteriostatic and antibacterial effects, modifying cell permeability and eventually leading to bacterial cell death[11]. In addition, it has the potential to stimulate angiogenic properties, assist in the regulation of bone resorption rate and increase the deposition of collagen fibers, attributing several functionalities when incorporated into HAp[11].

Magnesium (Mg) is also an essential trace element; it acts in bone resorption processes and stimulates the proliferation of osteoblasts[12]. The degradation product (Mg2+) is a standard human body composition with antitumor and antibacterial characteristics, reducing the risk of infections and the need for further surgeries[13]. Combined with natural calcium phosphate, it helps the spontaneous formation of bone bonding in vivo[14], which can alter the mineral metabolism, resulting in the modification of the dissolution rate of the crystals and the biodegradation of the related materials[15].

In this context, this work aimed to synthesize and characterize BNC membranes functionalized with copper and magnesium apatites, seeking to produce a biocomposite for application in GBR and evaluate its antimicrobial activity and cytotoxicity. The study stands out because it aims at combining biomaterials that have the potential to mimic the native bone structure and the extracellular matrix.

Furthermore, including bivalent cations found in bone tissue may increase the osteogenic potential, optimizing the synergistic effects of this biomaterial.

2. Materials and Methods 2.1 Biosynthesis and purification of BNC membranes For this, the bacterium Komagataeibacter hansenii ATCC 23769 was used. The culture medium used in the cell activation step and the production of membranes was the Mannitol Medium (MM) (pH 7), consisting of (g L-1): 20 g mannitol, 5 g peptone, 5 g yeast extract. In the cell activation step, one Eppendorf tube containing the microorganism was added to an Erlenmeyer flask and incubated at 30 °C under static conditions for two days. Then, this pre-inoculum was transferred to the culture medium at a rate of 20% (v/v), containing an optical density between 0.15 and 0.19, measured by absorbance in a spectrophotometer at 600 nm. The culture medium was incubated at 30 °C under static condition for 8 days to form of BNC membranes. The formed membranes were removed from the surface of the liquid culture, washed with water, and treated with a 0.1 M NaOH solution at 80 °C for 60 min. After that, the membranes were washed with distilled water until reaching pH 7, sterilized, and stored in a refrigerator, for later production of biocomposites.

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2.2 Production of biocomposites The biocomposites functionalized with Mg and Cu were produced according to the methodology described by Hutchens et al.[16]. The wet BNC membranes (never dried) were immersed in hybrid solutions of MgCl2 (0.1 M) and CaCl2 (0.1 M) or CuCl2 (0.1 M) and CaCl2 (0.1 M), with a pH between 4 and 5, in different proportions for 24 h under orbital agitation at 26 °C and 85 RPM. The concentrations of the tested solutions were 50% CuCl2 or MgCl2 and 50% CaCl2, 30% CuCl2 or MgCl2 and 70% CaCl2, and 10% CuCl2 or MgCl2 and 90% CaCl2 concerning the Na2HPO4 solution (0.06 M) with pH 8. Subsequently, the membranes were washed with distilled water to remove residues from the previous step and immersed in a Na2HPO4 0.06 M solution for another 24 h. Three immersion cycles were performed in each solution. The biocomposites formed were named BNC/MgHAp and BNC/CuHAp. Then, the biocomposites were lyophilized for 24 h and then characterized.

2.3 Characterization techniques 2.3.1 Determination of porosity and swelling degree The samples were weighed previously wet and after drying by lyophilization for 24 h. The porosity of biocomposites was determined according to the Equation 1[17].

( m1 − m2 ) = ε (%)

ρ water v

× 100

(1)

Where ε is the porosity of the membrane, m1 and m2 are the masses (g) of the wet and dry membrane, respectively, ρwater is the specific mass of water obtained at 20 ºC, which is equivalent to 0.9982 g/cm3, and v is the volume (cm3) calculated based on the membrane form.

For the determination of the swelling capacity of the biomaterials, the samples were immersed in distilled water until the sample mass remained constant (~1h). After immersion, excess water from the samples was removed with absorbent paper, and the samples were weighed. The degree of swelling was calculated according to Equation 2: = %I

( m1 − m2 ) × 100

(2)

m2

The samples were analyzed in triplicate and submitted to analysis of variance (ANOVA) using the OriginPro® 8.5 software.

2.3.2 Fourier Transform Infrared Spectroscopy (FTIR) Thirty-two scans were performed per sample from 4000 to 500 cm-1, resolution of 4 cm- 1, using the attenuated total reflectance module (ATR) in a spectrometer (Perkin-Elmer).

2.3.3 Scanning Electron Microscopy (SEM) The samples were fixed on metallic supports and covered with gold. The equipment used was a SEM (JEOL, model JSM-6390LV). An electron beam bombarded the samples, and the X-rays emitted from the samples were detected by a silicon solid-state detector device for a point elemental analysis (EDS) of the sample.

Polímeros, 33(3), e20230034, 2023

Evaluation of potential biomaterials for application in guide bone regeneration from Bacterial Nanocellulose / Hydroxyapatite 2.3.4 Thermogravimetric Analysis (TGA) The TG and DTG curves were obtained using the TGA-Q50/TA Instruments. The samples were heated from 25 to 600 °C at a 10 °C/min rate under an oxidizing atmosphere.

2.4 Antimicrobial properties assay The antimicrobial properties assay was performed using the disk diffusion technique of Bauer et al.[18], using Müeller Hinton medium (MH) with the microorganisms Escherichia coli (ATCC 8739) and Staphylococcus aureus (ATCC 25923) activated for 24 h at 37 ºC, in Brain Heart Infusion Broth (BHI). The Mac Farland scale was used as a parameter of cell concentration of the evaluated microorganisms, standardizing the absorbance at 0.28 (according to the 0.5 tube of the Mac Farland scale). Then, disks (Ø 6 mm) of the biocomposites produced and a disk of pure BNC (BR), added to each plate as a control, were placed on the inoculated cells and incubated at 37°C for 24 h. The analysis was carried out in duplicate.

2.5 Cytotoxicity analysis Cytotoxicity analysis was evaluated by measuring the metabolic activity of mouse fibroblasts (line - L929) multiplied in Dulbecco’s Modified Eagle’s Medium (DMEM)

supplied with 15% FBS, following ISO 10993-5[19], using the MTS colorimetric method[20]. The test was conducted in triplicate, and submitted to analysis of variance (ANOVA) using the OriginPro® 8.5 software.

3. Results and Discussions 3.1 Synthesis of the BNC membrane The BNC membrane formed after cultivating had a thickness varying between 2 and 3.5 mm (Figure 1A). After purification, the membranes had a gelatinous appearance in the form of a translucent hydrogel (Figure 1B). On the other hand, lyophilization brought a spongy characteristic to the membrane (Figure 1C).

3.2 Production of biocomposites The biocomposites formed showed a change in their color according to the percentage of the incorporated element (Mg or Cu) (Figure 2). The biocomposites obtained were lyophilized to maintain the pores’ integrity and provide an environment conducive to the migration and cell adhesion of this material when implanted. According to Kim et al.

[21] , how bigger the porosity, the better the permeability of the material, which will facilitate vascularization, its gas exchange and the transport of nutrients to the cells, thus increasing the possibility of successful tissue regeneration.

Figure 1. BNC membranes: (A) biosynthesis; (B) after purification; (C) after lyophilization.

Figure 2. Biocomposites with different concentrations: (A) BNC/MgHAp 50%; (B) BNC/MgHAp 30%; (C) BNC/MgHAp 10%; (D) BNC/CuHAp 50%; (E) BNC/CuHAp 30%; (F) BNC/CuHAp 10%.

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Gaulke, E., Garcia, M. C. F., Segat, B., Apati, G. P., Schneider, A. L. S., Pezzin, A. P. T., Cesca, K., & Porto, L. M. Thus, despite the decreased water absorption capacity, the biomaterials did not lose their swelling capacity and were as quick to absorb as pure BNC.

3.3 Characterization techniques 3.3.1 Determination of porosity and swelling degree BNC has a highly porous structure (96%), and incorporating inorganic material can affect this characteristic. According to Jin et al.[22], the pore size of the matrix decreases with increasing hydroxyapatite mass, and eventually, accumulations may occur in the pore structure.

The concentration of metal ions did not significantly influence the porosity values. However, Figures 3A and 3B show an average reduction of 15% in the average porosity percentage of the biocomposites concerning BNC, suggesting the deposition of inorganic material. This result suggests that metallic apatite deposition occurred superficially and inside the BNC. The biocomposites still maintained a high percentage of average porosity (± 80%). This characteristic is essential for implantable biomaterials because it helps anchor cells and distributes nutrients and growth or differentiation factors to cells.

The swelling of nanofibers is a fundamental parameter for biomaterials since they are designed to be used in high humidity conditions[23]. BNC showed significantly greater swelling capacity than functionalized biomaterials (Table 1). However, within the group of functionalized membranes, there was no significant difference in the samples’ water absorption capacity, demonstrating that the partial replacement of Ca2+ ions by Mg2+ or Cu2+ did not result in significant changes in this property in the biomaterials. This result is likely due to the similarities between these ions, such as similar ionic radius (Ca2+ = 99 pm, Mg2+ = 72 pm and Cu2+ = 73 pm)[24] and the same valence (2+).

3.3.2 Fourier Transform Infrared Spectroscopy (FTIR) The FTIR spectrum (Figure 4) characteristic of pure BNC is marked by bands in the region of 3345 cm-1, which according to He et al.[25], is characteristic of the stretching of the hydroxyl groups present in BNC. In addition, the CH stretching and asymmetric stretching at 2897 cm-1, CH2 deformation at 1427 cm-1, OH deformation at 1315 and 1359 cm- 1, as well as the antisymmetric bridge of the C-O-C stretch at 1109 and 1162 cm-1 and the band in the region of 1056 cm-1 related to the vibrations of the C-O stretch are characteristic of BNC[26,27]. Another exciting band, located around 400 to 700 cm-1, is characteristic of the torsion of the OH groups.

However, it is worth mentioning that typical HAp behaviors are also observed in the composites; this can be verified in the band around 1248 cm-1, which, according to Salarian et al.[28], can be attributed to the P–O stretching vibration of PO43-, the band at 1367 cm-1 was also attributed

by Panda et al.[29] to the presence of carbonate ions in a Ca-deficient HAp, indicating cationic substitution in the biocomposites produced in this work. Another point worthy of note is the band at 3345 cm-1, attributed to the BNC hydroxyls, which had its intensity significantly reduced in the biocomposite spectra. According to Minatti[30], this change in intensity suggests that the HAp crystals affected the hydroxyl groups of the BNC, confirming an interaction between the OH group and the HAp. The chemical interaction between BNC and HAp stabilizes the biocomposite to maintain mechanical integrity.

Figure 3. Analysis of the average percentage of porosity: (A) BNC/MgHAp; (B) BNC/CuHAp.

Table 1. Study the degree of water absorption by samples of BNC and functionalized biomaterials.

Samples

BNC

BNC/ MgHAp50%

BNC/ MgHAp30%

BNC/ MgHAp10%

BNC/ CuHAp50%

BNC/ CuHAp30%

BNC/ CuHAp10%

Swelling capacity (%)

629.3 ± 56.44a

304.9 ± 19.82b

251.6 ± 27.59b

218.7 ± 0.45b

251.6 ± 6.17b

266.1 ± 18.92b

263.9 ± 35.41b

Lowercase letters indicate comparisons between the biocomposites and the BNC (control). Equal letters indicate means that did not differ from each other.

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Evaluation of potential biomaterials for application in guide bone regeneration from Bacterial Nanocellulose / Hydroxyapatite The bands at 1030 cm-1 and 962 cm-1, are related to the PO . The band at 1030 cm-1 was observed only for the samples BNC/CuHAp30% and BNC/MgHAp10%, while the bands at 962 cm-1 were seen in all analyzed samples (Figure 4). According to Hutchens et al.[16], these two bands may represent the elongation mode of the phosphate group vibration, which can overlap the BNC membrane groups at wavenumbers between 1000 and 1100 cm-1.

Bands at 1750 and 873 cm-1 were also observed in all functionalized biomaterials, corresponding to the C=O bond and the carbonate ion (CO3-2), indicating that part of the phosphate group incorporated into the BNC was replaced by carbonate. This replacement can be attributed to the CO2 present in the air since the functionalization was produced under agitation. Wan et al.[33] highlighted that the composition and structure of the incorporated carbonate produce an apatite similar to that found in natural bone.

[31,32] 4

3.3.3 Thermogravimetric Analysis (TGA) Figure 5 shows the TG and DTG curves obtained for BNC, Mg, and Cu apatites. Pure BNC presented the first stage of mass loss of 1.77%, (Table S1), which refers to the water loss present in the sample. The second stage occurred at Tpeak3 at 316 ºC, where the highest percentage of sample mass loss was observed, equivalent to 74.38%, related to cellulose degradation. The last stage of degradation for the BNC showed a mass loss of 17.19%, which occurred at Tpeak4 of 494 ºC and is related to the degradation of carbonaceous residues[34].

The BNC/MgHAp50%, BNC/MgHAp30%, BNC/ MgHAp10% and BNC/CuHAp10% samples showed an additional mass loss stage of 6.6%, 7.1%, 1.6% and 2.0%, respectively, with Tpeak2 around 180 ºC. This event can be attributed to adsorbed water in HAp that is reversibly removed from 25 to 200 °C without affecting the network parameters[35].

The Tpeak3 presented by BNC was 316 ºC, while for biocomposites, it was around 290 ºC. According to

Saska et al.[36], this event can be associated with broken hydrogen bonds to form apatites. Another feature that stands out is the high percentage of residue at the end of the test, which presented a value greater than 60% for all biocomposites produced, while for BNC it was 6.6%.

These residues can be attributed to the inorganic material in the sample because the analysis was performed in an oxidizing atmosphere, which confirms the incorporation of the desired phosphate sources on the BNC membrane[36].

In this context, it is interesting to note that the higher the Cu concentration, the higher the residue, while for Mg, the behavior was the opposite, the higher the Mg concentration, the lower the residue.

3.3.4 Scanning Electron Microscopy (SEM) In Figure 6A, it is possible to observe the morphology of the BNC obtained by SEM, with nanofibers randomly arranged in a three-dimensional structure, forming a highly porous structure. This type of 3D structure makes BNC an excellent biomaterial allowing the exchange of fluids containing nutrients and growth factors, as well as the extracellular matrix, stimulating cell growth. In addition, membrane porosity is essential to promote cell adhesion, initiating the process of tissue regeneration[37]. In the BNC/MgHAp biocomposites with 10%, 30% and 50% M (Figure 6B-6D), the phosphate crystals formed were in the form of plates deposited on the BNC nanofibril network but with low interaction with them. The energy scattering X-ray spectroscopy (EDS) result confirmed the presence of calcium, magnesium and phosphorus elements (Figure 6H).

According to Rabelo[38], the formation of HAp is dependent on the diffusion and adsorption of OH-, Ca2+ and PO43- ions. Thus, at more acidic pH, the adsorption of OH- ions is limited, and the most common morphological growth presents the phosphate crystals in the form of plaques. In this work, BNC was immersed in CuCl2, MgCl2 and CaCl2 with pH between 4 and 5 and Na2HPO4 at pH 8, which may have enabled the formation of these structures.

Figure 4. FTIR spectra obtained for BNC membranes compared with: A) BNC/MgHAp; B) BNC/CuHAp biocomposites.

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Gaulke, E., Garcia, M. C. F., Segat, B., Apati, G. P., Schneider, A. L. S., Pezzin, A. P. T., Cesca, K., & Porto, L. M.

Figure 5. TG (A and C) and DTG (B and D) curves for BNC compared with: A,B) BNC/MgHAp; C,D) BNC/CuHAp biocomposites.

Figure 6. Micrographs of the surface of the membrane of BNC (A) and of the biocomposites incorporated with Mg: (B) BNC/MgHAp50%; (C) BNC/MgHAp30%; (D) BNC/MgHAp10%; (E) BNC/CuHAp 50%; (F) BNC/CuHAp 30% and (G) BNC/CuHAp 10%. The EDS of the sample surfaces are also presented in: H) BNC/MgHAp; I) BNC/CuHAp. 6/11

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Evaluation of potential biomaterials for application in guide bone regeneration from Bacterial Nanocellulose / Hydroxyapatite The morphology of the BNC/CuHAp biocomposites can be seen in Figures 6E-6G. The energy scattering X-ray spectroscopy (EDS) result also confirmed the presence of calcium, copper and phosphorus elements (Figure 6I). In Figures 6E and 6F, it is possible to observe the nanofibrils of the BNC but with the deposition of the inorganic material with lamellar crystalline form or rosettes referring to the compositions BNC/CuHAp 30% and 50%. Interestingly, in Figure 6F, the biomaterial with 30% Cu showed a denser crystalline formation, covering the entire surface of the BNC. While in the BNC/CuHAp10% sample (Figure 6G), there was a second flat crystalline form in addition to those already described as lamellar or rosettes.

Hutchens et al.[16] produced different Ca-deficient HAp BNC (CdHAp) compositions by immersion cycles. The authors observed the formation of crystallites with a needle or lamellar morphology that resembles the “rosette” structure, highlighting the similarity with the structures of apatite found in physiological bone. They found that this “rosette” structure implies that apatite was nucleated from a distinct location on the BNC nanofibrils. Moreover, at higher concentrations of CdHAp, the particles appeared to have a larger and rougher texture, indicating that the molecules of CdHAp provided secondary nucleation sites for additional apatite formation.

This characteristic was also observed in Figure 6F of this work (BNC/CuAp30%). The authors observed that the distinct arrangement of the BNC nanofibers appears to have guided the growth of apatite in clusters, which can also be seen in Figures 6E and 6F. Suggesting that the apatite formed in these cases resembles native bone tissue and, through new nucleations, could stimulate bone regeneration.

3.4 Antimicrobial properties Table 2 shows the result of the antimicrobial susceptibility test with the BNC/MgHAp and BNC/CuHAp biocomposites after 24 h of incubation at 30 ºC with the microorganisms Escherichia coli and Staphylococcus aureus. The absence of inhibition halos in the samples containing Mg indicates that these samples did not show an antimicrobial effect.

On the other hand, all compositions of BNC/CuHAp biocomposites showed inhibition halos that varied between 11 and 13 mm in diameter (S2).

Studies reported by Demishtein et al.[39] propose that magnesium affects cell membrane permeabilization, making bacteria more sensitive. The authors present a survey of different effects on different microorganisms, further highlighting the potential to affect the formation of microbial biofilms. However, in this study, no antimicrobial effect of Mg was observed on the microorganisms tested in any of the concentrations used.

On the other hand, Cu showed higher antibacterial activity, as seen in the formation of inhibition halos for E. coli and S. aureus bacteria (Table 2 and Figure S1). Araújo et al.[7] point out that the Cu(II) ion can act on microorganisms by different mechanisms, breaking the plasma membrane, blocking biochemical pathways, forming complexes with proteins and even causing DNA damage.

3.5 Cytotoxicity analysis A preliminary cytotoxicity assay is one of the critical assessments of the biological properties of biomaterials before in vivo assessment[41]. According to ISO 10993-5:2009[19], if the cell viability is greater than 70% compared to the control group, the material is considered non-cytotoxic.

The cell viability results for the BNC/MgHAp and BNC/ CuHAp biocomposites during the 1, 3 and 7-day incubation period are shown in Table 3.

The BNC/MgHAp biomaterials did not show change concerning the control group (BNC) after the first and third days of the test. Biocomposites containing BNC/CuHAp showed a slightly negative effect on the first analysis day, reducing cellular metabolic activity to 90% but remaining above 70%. However, after 3 days of testing, the Cu samples started to show cytotoxic effects, except for the BNC/ CuHAp30% sample, which still maintained cell viability of 77.27%. However, after 7 days, all BNC/CuHAp samples confirmed a negative effect on long-term cell viability, with values around 35%.

Table 2. Antimicrobial activity test of the BNC/MgHAp and BNC/CuHAp biocomposites against the microorganisms Escherichia coli and Staphylococcus aureus.

Samples

BNC

Ø (mm) E. coli Ø (mm) S. aureus

0 0

BNC/ MgHAp50% 0 0

BNC/ MgHAp30% 0 0

BNC/ MgHAp10% 0 0

BNC/ CuHAp50% 11.5 ± 0.5 12.5 ± 0.5

BNC/ CuHAp30% 13.0 ± 1.0 11.0 ± 1.0

BNC/ CuHAp10% 11.0 ± 1.0 11.5 ± 0.5

Table 3. Cell viability resulting from the cytotoxicity analysis of the biomaterials produced.

Sample BNC BNC/MgHAp50% BNC/MgHAp30% BNC/MgHAp10% BNC/Cu HAp50% BNC/CuHAp30% BNC/CuHAp10%

1 day Cell viability (%) 100.00 100.27 100.27 101.47 90.14 90.60 90.40

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Standard deviation 0.02 0.02 0.02 0.01 0.01 0.02 0.02

3 days Cell viability (%) 100.00 101.72 103.25 108.32 50.00 77.27 45.07

Standard deviation 0.02 0.01 0.02 0.01 0.06 0.12 0.19

7 days Cell viability (%) 100.00 55.41 100.56 52.46 35.93 35.53 37.73

Standard deviation 0.18 0.03 0.15 0.06 0.02 0.01 0.01

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Gaulke, E., Garcia, M. C. F., Segat, B., Apati, G. P., Schneider, A. L. S., Pezzin, A. P. T., Cesca, K., & Porto, L. M. The BNC/MgHAp30% sample maintained the same behavior from the beginning to the end of the test, demonstrating that it does not have a long-term cytotoxic effect. This result makes this biomaterial the most suitable implantable material, despite not having an antimicrobial effect. The presence of Mg2+ ions influence mineral metabolism, playing an important role in the process of bone mineralization through the activities of osteoblasts and osteoclasts[14]. Thus, the presence of Mg helps to obtain adequate mineralization in the correct formation of HAp and stimulates the production of new bone tissue.

Thus, although the biomaterial BNC/MgHAp30% does not show antimicrobial activity, the partial replacement of Ca2+

by Mg2+ can bring other benefits for bone regeneration.

Lima et al.[40] evaluated the biocompatibility of HAp partially replaced by several divalent ions in just 1 day of contact. The authors also observed increased total viable cells in the presence of MgHAp, while CuHAp samples reduced this parameter. In the present study, however, all BNC/CuHAp samples after the first day of testing still showed 90% cell viability. Even after 3 days of testing, the sample containing 30% Cu still showed cell activity higher than 70%. However, unfortunately, in the long term, all concentrations harmed cell viability.

4. Conclusions The results demonstrated a promising interaction between BNC and metallic apatites as a biocomposite for application in GBR, offering good porosity, swelling capacity and available hydroxyls to connect with HAp. These characteristics are essential in producing a biomaterial with the potential for application in GBR because they combine the osteogenic properties of HAp with the properties of BNC that make it similar to the extracellular matrix. Furthermore, the partial replacement of Ca by bivalent cations demonstrated that the effect is directly linked to the chemical nature of the ion.

In this study, incorporating Cu in apatites promoted antimicrobial action against E. coli and S. aureus. However, at the tested concentrations, this element was cytotoxic in comparison, biomaterials incorporated with Mg. With particular emphasis on the BNC/MgHAp30% sample, which maintained cell viability values around 100% throughout the entire study, making this composition the most suitable for application as an implantable material and application in GBR, although not promoting antimicrobial activity. In addition, studies with multiple ionic replacements seeking synergistic effects and more remarkable similarity with natural bone composition are important factors to be investigated as future directions.

5. Author’s Contribution • Conceptualization – Michele Cristina Formolo Garcia.

• Data curation – Michele Cristina Formolo Garcia.

• Formal analysis – Michele Cristina Formolo Garcia; Giannini Pasiznick Apati.

• Funding acquisition - Ana Paula Testa Pezzin.

• Investigation – Elouise Gaulke; Michele Cristina Formolo Garcia; Bruna Segat.

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• Methodology – Elouise Gaulke; Michele Cristina Formolo Garcia; Karina Cesca.

• Project administration – Ana Paula Testa Pezzin.

• Resources – Michele Cristina Formolo Garcia; Andréa Lima dos Santos Schneider.

• Software – NA. • Supervision – Karina Cesca; Ana Paula Testa Pezzin; Luismar Marques Porto.

• Validation – Michele Cristina Formolo Garcia; Karina Cesca.

• Visualization – Elouise Gaulke; Michele Cristina Formolo Garcia; Bruna Segat.

• Writing – original draft – Elouise Gaulke; Michele Cristina Formolo Garcia.

• Writing – review & editing – Michele Cristina Formolo Garcia; Bruna Segat; Karina Cesca; Ana Paula Testa Pezzin.

6. Acknowledgements The authors are grateful for the financial support of FAPESC and FAP/UNIVILLE for the project.

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Received: Jan. 10, 2023 Revised: Jul. 18, 2023 Accepted: Aug. 18, 2023

Polímeros, 33(3), e20230034, 2023

Evaluation of potential biomaterials for application in guide bone regeneration from Bacterial Nanocellulose / Hydroxyapatite

Supplementary Material Supplementary material accompanies this paper.

Table S1. Maximum degradation temperature (Tpeak) and percentage of mass loss (M%) data, obtained from the TG and DTG curves of BNC (standard) and BNC/CuHAp and BNC/MgHAp samples.

Figure S1. Antimicrobial susceptibility test of the BNC/MgHAp and BNC/CuHAp biocomposites against the microorganisms Escherichia coli and Staphylococcus aureus.

This material is available as part of the online article from https://doi.org/10.1590/0104-1428.20220121

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.20230075

Incorporation of organic acids in the crosslinking of polyvinyl alcohol hydrogels Dione Pereira de Castro1* , Vanessa Zimmer Kieffer1  and Ruth Marlene Campomanes Santana1  Laboratório de Materiais Poliméricos (LAPOL), Departamento de Materiais, Escola de Engenharia, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS, Brasil

1

*dione.castro@ufrgs.br

Abstract This work studied the incorporation of organic acids as crosslinking agents and reaction time on the properties of poly(alcohol vinyl) (PVOH) hydrogels to act as scaffold systems to compounds incorporated into agriculture systems.

PVOH hydrogels crosslinked with citric and L-malic acids were prepared, and the effects of heat-treatment time, and temperature on their swelling and hygroscopic performances were investigated by FTIR, thermal analysis and swelling.

Both the swelling and rate of water uptake of hydrogels decreased with increasing heat-treatment time. While the swelling decreased with heat-treatment time, the chemical crosslinking shown in FTIR increased. DSC results indicated adsorbed water in the uncrosslinked PVOH and hydrogels, and the absorbed water changed the melting point and glass transition temperature. TGA analysis showed that the incorporation of organic acids brought thermal stability. The results obtained show effective crosslinking hydrogels by L-malic acids and possibilities to use in scaffold systems and controlled release.

Keywords: PVOH hydrogels, chemical crosslinking, citric and malic acids.

How to cite: Castro, D. P., Kieffer, V. Z., & Santana, R. M. C. (2023). Incorporation of organic acids in the crosslinking of polyvinyl alcohol hydrogels. Polímeros: Ciência e Tecnologia, 33(3), e20230035. https://doi.org/10.1590/0104-1428.20230075

1. Introduction Poly(vinyl alcohol) (PVOH) is a synthetic, semi-crystalline polymer obtained by the hydrolysis of polyvinyl acetate. Its final physical properties are defined by its degree of hydrolysis, which changes its molecular mass and differentiates commercial grades. PVOH exhibits many important characteristics, as it is biodegradable, biocompatible, non-toxic, soluble in water, thermostable and has good chemical and adhesive resistance[1-7]. However, the presence of hydroxyls in PVOH makes PVOH soluble, limiting its applicability[2,3,8].

Physicochemical control of PVOH is possibly by crosslinking using various conventional crosslinking agents such as glutaraldehyde, boric acid, glycidyl methacrylate, genipin, microcrystalline cellulose, thermal crosslinking and photo-induced. In many of these cases, toxic solvents are commonly used, which are of concern for specific hydrogel applications[2,5,7-10]. In this context, carboxylic acids are crosslinking agents for PVOH, and these acids are easily available, odourless, non-toxic, biodegradable, and also good chemical crosslinking agents[2,5,9,11,12]. Chemical modification of hydroxyl groups of PVOH through crosslinking agents is simple, cost-effective and easily controlled; thus, opening up new fields of its applications in new materials[2,7,8,13-15].

This molecular chain flexibility, biocompatibility and hydrophilicity is due to its large number of hydroxyl groups is used to form hydrogels with covalent bonds[6].

Typically, the crosslinking of PVOH with organic acids is carried out by heating the PVOH in solution under stirring with a catalyst and elevated temperature[2,11,14,16].

Polímeros, 33(3), e20230035, 2023

The literature search provides an option for preparing PVOH hydrogels with performance that is matched to the user’s needs using an organic acid as a natural crosslinker[2,12].

The most commonly used crosslinkers have problems of toxicity to living organisms. Citric and malic acids are dicarboxylic acids obtained from natural sources that can be used as a crosslinking agent and are non-toxic, easily available and low-cost[2,12,15]. The effects of crosslinking, which is done by heating, on the swelling and hygroscopic performances of the PVOH hydrogels are studied and elucidated.

2. Materials and Methods 2.1 Materials Poly(vinyl alcohol) (PVOH), with a molecular mass of Mw = 105,000 g.mol−1 and 87% hydrolyzed was obtained from Neon® (Brazil). Organic acids used were citric acid obtained from Neon® and malic acid supplied by Êxodo Científica® (Brazil). A hydrochloric acid catalyst (37% w/w) was obtained from Química Moderna® (Brazil). All solvents and reagents were used without further purification.

2.1.1 Hydrogel preparation A solution of distilled water (~494.00 mL) and PVOH (~26.00 g) was solubilized under constant stirring at room temperature (30 min) and then at 85°C for 40 min.

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

Castro, D. P., Kieffer, V. Z., & Santana, R. M. C. For the synthesis of hydrogels by the casting method, the previously solubilized PVOH solution was mixed with ~114.00 g of distilled water and 1% (w/w) of specific organic acids (citric or malic) and hydrochloric acid (HCl) was used (1M, 4mL) as a catalyst[16,17]. This mixture was stirred continuously for 20 min, 1, 2 or 3 h, following a previously published method[11]. Subsequently, hydrogel solution (aliquots) was weighed and placed in Petri dishes and dried in an oven at 65°C for 24 h, removed for washing to remove residual HCl from the synthesis and dried again for 1 h in an oven at 65°C and stored in a desiccator for 6 days for the final curing process following an adapted methodology[18]. The symbols of the previous PVOH (standard) and respective PVOH hydrogels, as prepared are presented in Table 1.

2.2 Hydrogels characterizations 2.2.1 Fourier transform infrared spectroscopy (FTIR) Samples (90 × 35 × 0.4 mm3) were analyzed in an IR-Spectroscopy (Spectrum 1000, Perkin Elmer®, United Kingdom) with HATR system following ASTM E1252. 32 scans were averaged per spectrum over a wavenumber range from 600 to 4000 cm-1 at a resolution of 4 cm-1. Subsequently, the baseline and HATR corrections (Savitzky-Golay, 5 points) were done using Origin® 2018 software. The chemical crosslinking of the PVOH hydrogels after incorporation of organic acids was characterized according to Awad and Khalaf[19] to evaluate the degree of the crosslinking with changes in the carbonyl and hydroxyl groups showed by characteristic bands associated with carbonyl (1710 cm-1), hydroxyl groups (3280 cm-1) and compared with the vinyl standard group (1462 cm-1), were calculated from the mathematically integrated areas (Equations 1 and 2) of the spectra using Origin® 2018 software, following the equations below.

Carbonyl index ( ICO ) =

I (1710) I (1462)

(1)

Hydroxyl index ( IOH ) =

I (3280) I (1462)

(2)

2.2.2 Thermogravimetric analysis (TGA) Thermogravimetric analyses were performed according to ASTM E1131, on a thermobalance (TA Instruments®, Q50 model, United States). The samples were placed in a platinum crucible and heated in a temperature range from 25 and 900°C, under a dynamic nitrogen atmosphere with a flow rate of 90 mL/min and heating rate of 20°C/min.

2.2.3 Differential exploratory calorimetry (DSC) PVOH and PVOH hydrogels were measured by differential scanning calorimetry (DSC) (TA Instruments®, Q20 model, United States) according to ASTM D3418. Samples sealed

in an aluminium pan were heated from 25 to 220°C at a heating rate of 10°C/min rate and were kept at 220°C heating isothermal for 1 min. The degree of crystallinity of the PVOH was calculated from the integration of the endothermic peak of the DSC curves using Equation 3, following a method applied by Yang et al.[10] with the use of the TA Universal Analysis® software.

X c %=

∆H ×100 ∆H c

(3)

Where: ΔHc is the thermodynamic enthalpy of hydrogels and ΔHc is the thermodynamic enthalpy of fusion of 100% crystalline PVOH (136.8 J g−1)[10].

2.2.4 Swelling Swelling values of PVOH hydrogels were obtained following the method described by Sonker and Verma[11].

1.5 × 1.5 × 0.0325 cm3 hydrogel films were immersed in 50 mL of distilled water for 30 minutes, 2, 4, 6, 8 and 24 h at 25°C and weights were measured before and after water uptake.

Swelling (S%) was calculated with the following Equation 4. = S%

(M f - Mi ) ×100 Mi

(4)

2.2.5 Statistical analysis The obtained results average (FTIR data, swelling) were subjected to one-way ANOVA tests and, whenever the null hypothesis was rejected, the averages were compared using Tukey–Kramer tests at a significance level of 5%.

3. Results and Discussions 3.1 Transform infrared spectroscopy (FTIR) Figure 1 shows the spectra of PVOH and respectively of the hydrogels: citric acid (AC), and malic acid (AM), with reaction times of 20 min, 1, 2 and 3 h, presented in the range of 4000-600 cm-1.

The characteristic peaks of PVOH and its interactions with citric acid (AC) are identified and presented as follows: The absorption bands at 3014–3680 cm−1 are attributed to the stretching vibration of the hydroxyl groups (-OH) present in PVOH and free non-crosslinked AC molecules[7,20-22]; characteristic bands at 2931 and 2853 cm−1 are attributed to the asymmetric and symmetrical elongation of the C-H groups, respectively; a band at 1735 cm–1 arising from residual carbonyl groups in PVOH (vinyl acetate)[11] plus ester carbonyls in the crosslinked hydrogel; characteristic bands at 1426 cm−1 are the result of in-plane deformation of the -CH2- bonds; characteristic band at 1000–1180 cm−1 can be attributed to C-O stretching of PVOH bonds; and the band at 842 cm−1 is correlated with the vibrational stretching of C-C bonds[20,22].

Table 1. Nomenclature of the PVOH hydrogels prepared by different crosslinking organic acids.

Organic acid Citric acid Malic acid

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20 min PVOH-AC (-AC) PVOH-AM (-AM)

PVOH Hydrogels crosslinking time 1h 2h PVOH-AC1 (-AC1) PVOH-AC2 (-AC2) PVOH-AM1 (-AM1) PVOH-AM2 (-AM2)

3h PVOH-AC3 (-AC3) PVOH-AM3 (-AM3)

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Incorporation of organic acids in the crosslinking of polyvinyl alcohol hydrogels

Figure 1. FTIR spectra of PVOH sample and PVOH-AC (A); PVOH-AM (B) hydrogels.

Sabzi et al.[22] and Franco et al.[21] state that the esterification reaction between PVOH and AC can be monitored by an absorption band close to 1722 cm−1, attributed to the elongation of carboxylic acid groups (-COOH) due to unreacted AC in the heat treatment time indicative of an increasing percentage of AC in the hydrogel.

This behaviour of the intense bands of carbonyls (C=O) and hydroxyls (-OH) with the reaction time of the hydrogel samples is also described in the literature and studied in carbonyl/hydroxyl indexes.

The main characteristic bands attributed to malic acid (AM) were described from the literature as stretching vibrations –OH at approximately 3400 cm−1, derived from the hydroxyls present[8,23]. As reaction time increased, there was an increase in the hydroxyl and carbonyl index in the samples[8]. This behaviour of increasing the carbonyl index (C=O) is also observed by other authors. Three possibilities for esterification are presented (Figure 2): intermolecular crosslinks (type A), intramolecular crosslinks (type B) and uncrosslinked (type C). The chemical structures of the interaction among the respective organic acids and PVOH are described in the literature[2,8,11,12,20].

Yang et al.[10] studied the incorporation of tannic acid (TA) in PVOH-based hydrogels by FTIR, the results showed a band shift from 3408 cm−1 to 3373 cm−1, which is attributed to the stretching bands of -OH, indicating that the intermolecular interactions between PVOH and TA via hydrogen bonds are formed, being characterized by the intermolecular types presented above and observed in the analysed spectra of -AM and its variations. Also, Zhang et al.[17] when evaluating the behaviour of AM and AT acids, observed that the zeta potential increased slightly at the beginning of the titration and then decreased, while in the AT series, the surface energy decreased continuously, showing a strong influence of the interaction of hydrogen bonding in AM and showing a greater interaction with hydroxyl groups (-OH), presented under the chemical change in the polymeric chains of poly(2-vinyl pyridine).

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3.1.1 Carbonyl (ICO) and hydroxyl (IOH) index The carbonyl indices shown in Figure 3A revealed that the chemical reaction time for crosslink hydrogels is crucial for the increase in carbonyls (C=O) which indicates a higher concentration of organic acids esterified in the polymeric chain of PVOH (especially seen in the hydrogels of PVOH with AC) however, during the chemical reaction, this concentration of organic acids decreased due to a smaller chemical reaction for crosslinking by esterification during the separation of the aliquots over time (especially seen in hydrogels with AM, with the lowest values being with TA).

Yang et al.[10] described that it was also possible to verify that the intensity of bands at 1714 and 755 cm−1 (C=O) increased with increasing AT concentration, demonstrating that the diffusion of AT molecules in hydrogels is dependent on the final concentration or chemical reaction. Sonker and Verma[11] state that PVOH, because it is a hydrolysis derivative of polyvinyl acetate (PVA), has remaining carbonyl bands (C=O) due to the remaining acetate groups. This behaviour can be observed in the PVOH compared to the other samples in FTIR spectrums (Figures 1 and 2) at 1735-1740 cm-1.

The hydroxyl indices of samples shown in Figure 3B revealed that in addition to the esterification promoted by organic acids reducing the hydroxyls present in PVOH, there may also be an increase in the -OH indices due to the presence of hydrogen bonds and free hydroxyls between organic acids and PVOH[21,22]. This behaviour is exhibited with citric acid (AC), which, because of the free hydroxyls of its chemical structure, had the highest average percentage among all samples due to non-total esterification with PVOH, presenting unstable hydrogels and was not fully crosslinked, according to data presented in the topics of swelling, in addition to this AC having 4 hydroxyl groups (3 of them from -COOH groups) in its molecule. The -AM samples showed similar behaviour in terms of hydroxyl indices, which correspond to swelling tests in which the presence of a three-dimensional structure indirectly reduced the hydroxyl indices and increased the carbonyls.

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Figure 2. Esterification possibilities for PVOH-AC (up); PVOH-AM (below) hydrogels.

Figure 3. Carbonyl (A); hydroxyl (B) indexes of PVOH film, PVOH-AC and PVOH-AM hydrogels.

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Incorporation of organic acids in the crosslinking of polyvinyl alcohol hydrogels The statistical analysis from Figure 3 can be seen in Table S1 and Table S2 – Supplementary Material.

3.2 Thermogravimetric analysis (TGA) Thermal analysis of PVOH film and their hydrogels (Figure 4) show significant mass loss in up to three (PVOH) and four stages (hydrogels), being studied from the second stage onwards. First, the weight loss up to 150°C was attributed to the loss of water present in the hydrogels[11].

The first recorded temperature stage ranged from 150 to 320°C was due to the initial thermal decomposition of the chemical structure of PVOH, by low molecular weight compounds (including acetate groups) as well as the possible evaporation of water occluded within the hydrogel networks, while the second degradation process (320 - 400°C) has been associated with decomposition of the polymer backbone (C-C)[24] and a third step between 400 and 500°C which may be associated with the decomposition of unsaturated macromolecules (products of PVOH decomposition) and the crosslinks of hydrogels. In the DTG curves, the temperatures corresponding to the maximum rate of weight loss of the second stage of degradation of PVOH (443.76°C); AC (350°C); -AC1 (366.24°C); -AC2 (364.35°C) and -AC3 (365.30°C) respectively. This behaviour implied lower thermal stability

of the hydrogels with the incorporation of citric acid, which was confirmed with the loss of mass (DTG) in the second stage due to the decomposition of the PVOH side chain[24].

The thermal degradation temperature of the third stage (hydrogels) and the reaction times applied with the citric acid did not significantly change the TGA/DTG curves among the samples because the crosslinking reaction failed to change the structure of the PVOH main structure. Temperature data was used to compare differences in initial (T10% and T50%) lost mass weight properties from Figures 4 and 5 can be seen in Table S3 – Supplementary Material.

The TGA and DSC curves of the PVOH film and malic acid (AM) hydrogels (Figure 5) show a significant mass loss in up to four stages in the hydrogels after the -AM incorporation, and having it studied from the second stage onwards, the mass loss up to 150°C was attributed to the loss of water present in the PVOH and in the hydrogels between 150°C and 200°C the remaining occluded water due to hydrogen bonds with the PVOH matrix[8,24]. In the DTG curves, the temperatures corresponding to the maximum rate of weight loss of the second stage of degradation of PVOH (443.76°C), AM (360.57°C), -AM1 (356.79°C), -AM2 (353.95°C) and -AM3 (356.79°C) respectively. This behaviour implied lower thermal stability of the hydrogels

Figure 4. TGA (A); DTG (B) curves of PVOH film and PVOH-AC hydrogels.

Figure 5. TGA (A); DTG (B) curves of PVOH film and PVOH-AM hydrogels.

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Castro, D. P., Kieffer, V. Z., & Santana, R. M. C. with the incorporation of malic acid, which was confirmed with the loss of mass (DTG) of the hydrogels, in the second stage due to the decomposition of the PVOH side chain at around 440°C, and with a subsequent increase due to the chemical crosslinking process promoted by the degradation of AM together with PVOH, a similar behaviour was studied and evaluated with the incorporation of citric acid[24]. In the third and fourth stages (440 – 500°C) the samples have a thermal change induced by the decomposition of the main polymer chain of PVOH (including acetate groups) and unsaturated bonds and crosslinked thermal among the organic acids used in hydrogels, making a new volatile compound[24].

Above 600°C, the samples with AM in comparison to AC hydrogels incorporated, had greater thermal stability due to chemical crosslinking induced by thermal degradation.

3.3 Differential exploratory calorimetry (DSC) The DSC curves of PVOH, -AC, -AC1, -AC2, -AC3 shown in Figure 6A observed that the PVOH film has a glass transition temperature (Tg) close to 60.1°C and a melting temperature (Tm) at 190.1°C. The incorporation of

the citric acid (AC) the chemical reaction time is observed for the samples, the PVOH-AC and -AC3 have a decrease in Tg, which cannot be observed, while the -AC1 and -AC2 samples exhibit a decrease in Tg to 38.5°C and 28.6°C respectively. This decrease in Tg is due to the addition of citric acid, which enabled partial crosslinking (esterification) of PVOH, through chemical interactions of esters and free hydroxyls (-OH) present in AC and facilitates the adsorbed water. This adsorbed and occluded water acted as a plasticizer for the hydrogel which resulted in a decrease in its Tg. This reaction behaviour is described by Yu et al.[24] in which the incorporation of AC implies a lower thermal stability of the hydrogels, which is confirmed by the loss of mass in the second step due to the decomposition of the PVOH side chain.

For the DSC curves of PVOH, -AM, -AM1, -AM2, -AM3, (Figure 6B) it is observed that with the incorporation of malic acid (AM) the chemical reaction time for all samples, except for PVOH-AM3, has a decrease in Tg, around 30°C, while the -AM3 hydrogel sample showed a smaller decrease in Tg to 55.03°C. This decrease in Tg is due to the plasticization promoted by the adsorbed water and

Figure 6. DSC curves of PVOH sample and PVOH-AC (A); PVOH-AM (B) hydrogels.

Table 2. DSC data of the evaluated PVOH film and hydrogels.

Samples

Tg (°C)a)

Tm (°C)b)

ΔHf (J/g)c)

% Xcd)

PVOH

60.15

190.18

46.34

32.52

PVOH-AC

e)

*

161.52

9.30

6.53

PVOH-AC1

38.52

162.03

13.87

9.73

PVOH-AC2

28.66

164.37

12.95

9.09

PVOH-AC3

e)*

159.72

12.06

8.46

PVOH-AM

30.40

163.06

10.74

7.54

PVOH-AM1

30.58

163.74

12.66

8.88

PVOH-AM2

31.27

161.82

8.88

6.23

PVOH-AM3

55.03

160.55

9.04

6.34

a)Tmax.: Glass transition temperature. b)Tm (°C): Melting temperature. c)ΔHf (J/g): Enthalpy of fusion. d)% Xc: per cent crystallinity. e)*Non-measured.

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Incorporation of organic acids in the crosslinking of polyvinyl alcohol hydrogels incorporation of AM that rearranged the chemical chains of PVOH in a chemical behaviour previously observed with the PVOH/AC hydrogels. Kanmaz et al.[25] described in their study that this Tg alteration is attributed to the incorporation of organic acid that promotes changes in the structural arrangement of PVOH molecules. Gao et al.[8] state that water molecules, whether occluded or frozen, cannot bind directly to the hydrophilic groups of PVOH, thus being shown to be mass change.

To investigate the effect of the concentration of organic acids on the crystallization of hydrogels, in the DSC analysis in addition to the glass transition temperature (Tg), the melting temperature (Tm) the heat of fusion (ΔHf) and the degree of crystallinity (Χc) were obtained from the curves of samples of PVOH hydrogels with citric (AC) and malic (AM) acids and their time variations are shown in Table 2.

The DSC curve for the PVOH film exhibits an endothermic peak at 190.18°C which is co-responding to the melting temperature of the PVOH crystalline phase[10]. For hydrogels composed of PVOH/AC and -AM, as shown in Table 2, the melting point Tm of -AC, -AC1, -AC2, -AC3 and -AM, -AM1, -AM2, -AM3 are 161.52 °C to 164.37 °C and from 160.55°C to 163.74°C respectively, and all Tm points were shifted to a lower temperature as the concentration of organic acids increased, with similar behaviour for the -AC/-AM groups. This behaviour was also observed in a study by Yang et al.[10] with the incorporation of tannic acid. Meanwhile, the Χc of composite hydrogels decreased from 32.52% to -AC (6.53%; 9.73%; 9.09% and 8.46%); -AM (7.54%; 8.88%; 6.23% and 6.34%) with the incorporation of organic acid in 1% by weight, indicating that the introduction of organic acids can chemically inhibit the crystallization of PVOH due to strong interaction of free hydrogen and -OH bonds formed between AC and AM in the polymeric chains of PVOH[2].

3.4 Swelling The water uptake capacity of the hydrogels was observed by measuring the degree of swelling (S%) as a function of

time. The absorption/adsorption of water during swelling is due to a large number of hydroxyl groups (-OH) present in PVOH resulting in high swelling and therefore reducing its strength and final properties[6,11]. Crosslinking not only reduces the active -OH groups but also covalently bonds the PVOH structure, reducing their interaction with water, and resulting in a reduced final swelling. In addition, the crosslinked structure of the hydrogel promoted by the organic acid confers better mechanical resistance in the presence of moisture between the interpenetrating chains, resulting from the formation of a chemical bond of ester between the chains of the matrix with the organic acids[2,6,11]. Figure 7 shows the degree of swelling and its amplification for PVOH hydrogels with citric acid (-AC) and malic acid (-AM) for up to 24 h. The statistical analysis from Figure 7 can be seen in Table S4 – Supplementary Material.

For the swelling test, only PVOH-AC was different from the other samples. This is caused by the higher swelling of the hydrogel in comparison to other samples. This better stability in the time also promoted better crosslinking time properties in 2 h, increasing the time without disintegration.

This thermal treatment in 3 h to -AC2 and -AC3, also promoted equal values for specific swelling, compared with PVOH-AC hydrogel had the most result obtained (485.38%). The AM hydrogels samples don’t have statistical differences except for the PVOH-AM with more swelling in comparative the other samples with 236.15%.

Evaluating the swelling behaviour applied to the PVOH-AC hydrogel, a swelling speed up is observed up to the maximum limit of 680% in 1 hour of the swelling, for the 20 min reaction due to the low chemical interaction of citric acid (CA) with PVOH, the other samples of -AC1, -AC2 and -AC3 obtained longer-stability times during the test, however with relatively lower rates of swelling and with a gradual loss of mass. This behaviour is also the result of a decrease in the final dimensional strength of the hydrogel and its disintegration due to the high interaction with the hydrolysis promoted with water and acids, causing it to lose its shape and final dimension depending on the elaborated reaction kinetics and the time for hydrogel crosslinking[11].

It is reported that the number of residual free carboxylic groups

Figure 7. Swelling graphs of PVOH-AC (A); PVOH-AM (B) hydrogels.

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Castro, D. P., Kieffer, V. Z., & Santana, R. M. C. (-COOH) increases with the CA content in the hydrogel[22].

This theory is based on the concept of chemical synthesis and stability of hydrogels that was developed from the study of the literature[2,6,11] that was addressed and presented in the topic discussion of FTIR [Figure 2] and had its method applied conceptually to the other organic acids in this study.

Figure 8 shows a hydrogel made up of PVOH-AC (20 min) during the swelling tests up to the limit of two hours.

However, the PVOH-AC1, PVOH-AC2 and PVOH-AC3 hydrogels showed lower percentages of swelling due to increased crosslinking applied, but with a longer stability time when compared to PVOH-AC hydrogel; -AC2 and -AC3 hydrogels, on the other hand, had dimensional stability up to 24 h of testing, but with a gradual loss of mass over time due to hydrolyzation promoted by water in the hydrogel. This behaviour is because the longer the thermal reaction takes, the greater the percentage of cross-linking in the samples, promoting lower swellings but longer stability in liquids, respectively, but the crosslinking kinetics with citric acid does not favour chemical stability which is evidenced by the total hydrolysis of the hydrogel, which will be confirmed in the following topic of swelling kinetics. This behaviour has been studied and reported by several researchers[2,22-27].

For the swelling of PVOH-AM, a decrease in the average percentage of swelling of the samples by 200% was observed, in which the PVOH-AM sample reached the maximum swelling value of 240%. Samples -AM1, -AM2 and -AM3 (Figure 9) had a swelling decrease with increasing reaction time, where sample -AM3 had the lowest swelling value of 180%, and all hydrogel samples had dimensional stability without disintegrating if up to 24 h. Gautam et al.[2] describe that AM, because it has an additional hydroxyl group, is more hydrophilic, creating a strong interlocking of polymeric chains in PVOH, restricting its hydrophilicity, thus resulting in the formation of a more three-dimensionally

and stable crosslinked structure, resulting in lower swelling values in PVOH comparison with others. This behaviour is shown by the low swelling characteristic shown above in citric acid and its chemical crosslinking kinetics by esterification.

4. Conclusions The effects of organic acids on hydrogels based on PVOH were studied by physio-chemical analysis, and the organic acids (citric and malic) modified the structure and physio-chemical properties of PVOH polymer with effectiveness. PVOH-AC hydrogels showed the highest swelling percentages but with shorter swelling times due to dimensional instability and their weak chemical interaction.

The -AM samples obtained satisfactory results with crosslinking percentages and greater dimensional stability as compared to AC hydrogels. Consequently, this behaviour is attributed to chemical interactions promoted mainly by the type A (intermolecular) chemical reaction model for the -AM samples described in FTIR. In the thermal analysis, the samples had similar degradation behaviour in all, where the -AC hydrogels were susceptible due to the percentage of absorbed and adsorbed water that promoted an initial decrease in the degradation temperature demonstrated in all samples as compared to PVOH film.

The analysis of DTG curves showed characteristic and defined stages of degradation, in which the hydrogels of -AM followed by -AC presented the best results of resistance to degradation respectively.

It is concluded that organic acids can be used as crosslinking agents for hydrogels, however, the AC hydrogel does not meet the requirements through this applied methodology and the AM hydrogels are more promising for the development of hydrogels.

Figure 8. PVOH-AC hydrogel in swelling for 30 min (A); 2 h (B).

Figure 9. PVOH-AM hydrogels swelling after 24 h; -AM (A); -AM1 (B); -AM2 (C); and -AM3 (D).

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5. Author’s Contribution • Conceptualization – Ruth Marlene Campomanes Santana; Dione Pereira de Castro; Vanessa Zimmer Kieffer.

• Data curation – Ruth Marlene Campomanes Santana; Dione Pereira de Castro.

• Formal analysis – Ruth Marlene Campomanes Santana; Dione Pereira de Castro.

• Funding acquisition – NA. • Investigation – Dione Pereira de Castro.

• Methodology – Ruth Marlene Campomanes Santana; Dione Pereira de Castro; Vanessa Zimmer Kieffer.

• Project administration – Ruth Marlene Campomanes Santana; Dione Pereira de Castro; Vanessa Zimmer Kieffer.

• Resources – Ruth Marlene Campomanes Santana; Dione Pereira de Castro.

• Software – NA. • Supervision – Ruth Marlene Campomanes Santana.

• Validation – Ruth Marlene Campomanes Santana; Dione Pereira de Castro.

• Visualization – Ruth Marlene Campomanes Santana; Dione Pereira de Castro; Vanessa Zimmer Kieffer.

• Writing – original draft – Ruth Marlene Campomanes Santana; Dione Pereira de Castro.

• Writing – review & editing – Ruth Marlene Campomanes Santana; Dione Pereira de Castro.

6. Acknowledgements This work is financed by National Council for Scientific and Technological Development (CNPq) fellows [grant number: 141265/2020-5]. The authors would like to thank LAPOL/UFRGS for their support in this work.

7. References 1. Awada, H., & Daneault, C. (2015). Chemical modification of poly(vinyl alcohol) in water. Applied Sciences, 5(4), 840-850. http://dx.doi.org/10.3390/app5040840.

2. Gautam, L., Warkar, S. G., Ahmad, S. I., Kant, R., & Jain, M. (2022). A review on carboxylic acid cross-linked polyvinyl alcohol: properties and applications. Polymer Engineering and Science, 62(2), 225-246. http://dx.doi.org/10.1002/pen.25849.

3. Nthoiwa, K. K. M., Diaz, C. A., & Chaudhari, Y. (2016). Vinyl alcohol polymers. In O. Olabisi & K. Adewale (Eds.), Handbook of thermoplastics (pp. 53-88). Boca Raton: CRC Press.

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7. Zhao, J., Li, J., Zeng, Q., Wang, H., Yu, J., Ren, K., Dai, Z., Zhang, H., Zheng, J., & Hu, R. (2022). A chewing gum residue-based gel with superior mechanical properties and self-healability for flexible wearable sensor. Macromolecular Rapid Communications, 43(13), e2200234. http://dx.doi. org/10.1002/marc.202270037. PMid:35483003.

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9. Pei, M., Peng, X., Wan, T., Fan, P., Yang, H., Liu, X., Xu, W., Zhou, Y., & Xiao, P. (2021). Double cross-linked poly(vinyl alcohol) microcomposite hydrogels with high strength and cell compatibility. European Polymer Journal, 160, 110786. http://dx.doi.org/10.1016/j.eurpolymj.2021.110786.

10. Yang, M., Guo, W., Liu, S., Zhang, B., Chen, Y., & Wang, Y. (2021). Highly stretchable gamma-irradiated poly (vinyl alcohol)/ tannic acid composite hydrogels with superior transparency and antibacterial activity. Journal of Polymer Research, 28(11), 412. http://dx.doi.org/10.1007/s10965-021-02777-7.

11. Sonker, A. K., & Verma, V. (2018). Influence of crosslinking methods toward poly(vinyl alcohol) properties: microwave irradiation and conventional heating. Journal of Applied Polymer Science, 135(14), 46125. http://dx.doi.org/10.1002/app.46125.

12. Sethi, S., Medha, Thakur, S., Sharma, D., Singh, G., Sharma, N., Kaith, B. S., & Khullar, S. (2022). Malic acid cross-linked chitosan based hydrogel for highly effective removal of chromium (VI) ions from aqueous environment. Reactive & Functional Polymers, 177, 105318. http://dx.doi.org/10.1016/j. reactfunctpolym.2022.105318.

13. Karoyo, A. H., & Wilson, L. D. (2021). Review on the design and hydration properties of natural polymer-based hydrogels.

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14. Altaf, F., Niazi, M. B. K., Jahan, Z., Ahmad, T., Akram, M. A., Safdar, A., Butt, M. S., Noor, T., & Sher, F. (2021). Synthesis and characterization of PVA/starch hydrogel membranes incorporating essential oils aimed to be used in wound dressing applications. Journal of Polymers and the Environment, 29(1), 156-174. http://dx.doi.org/10.1007/s10924-020-01866-w.

15. Valdés, C., Valdés, O., Bustos, D., Abril, D., Cabrera-Barjas, G., Pereira, A., Villaseñor, J., Polo-Cuadrado, E., Carreño, G., Durán-Lara, E. F., & Marican, A. (2021). Use of Poly(vinyl alcohol)-Malic Acid (CLHPMA) hydrogels and Chitosan Coated Calcium Alginate (CCCA) microparticles as potential sorbent phases for the extraction and quantitative determination of pesticides from aqueous solutions. Polymers, 13(22), 3993. http://dx.doi.org/10.3390/polym13223993. PMid:34833292.

16. Dlamini, D. S., Wang, J., Mishra, A. K., Mamba, B. B., & Hoek, E. M. V. (2014). Effect of cross-linking agent chemistry and coating conditions on physical, chemical, and separation properties of PVAPsf composite membranes. Separation Science and Technology, 49(1), 22-29. http://dx.doi.org/10.1080/01496395.2013.813040.

17. Zhang, Q., Huang, X., Zhang, L., & Jin, Z. (2022). Isothermal titration calorimetry directly measures the selective swelling of block copolymer vesicles in the presence of organic acid.

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18. Omidian, H., & Park, K. (2010). Introduction to hydrogels.

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19. Awad, S. A., & Khalaf, E. M. (2020). Evaluation of the photostabilizing efficiency of polyvinyl alcohol-zinc chloride composites. Journal of Thermoplastic Composite Materials, 33(1), 69-84. http://dx.doi.org/10.1177/0892705718804585.

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Castro, D. P., Kieffer, V. Z., & Santana, R. M. C. 20. Huang, S.-M., Liu, S.-M., Tseng, H.-Y., & Chen, W.-C. (2023).

Effect of citric acid on swelling resistance and physicochemical properties of post-crosslinked electrospun polyvinyl alcohol fibrous membrane. Polymers, 15(7), 1738. http://dx.doi. org/10.3390/polym15071738. PMid:37050352.

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Study of the annealing effect of starch/polyvinyl alcohol films crosslinked with glutaraldehyde. Gels, 7(4), 249. http://dx.doi. org/10.3390/gels7040249. PMid:34940309.

22. Sabzi, M., Afshari, M. J., Babaahmadi, M., & Shafagh, N. (2020). pH-dependent swelling and antibiotic release from citric acid crosslinked poly(vinyl alcohol) (PVA)/nano silver hydrogels.

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23. Zhang, Y., Lin, S., Qiao, J., Kołodyńska, D., Ju, Y., Zhang, M., Cai, M., Deng, D., & Dionysiou, D. D. (2018). Malic acid-enhanced chitosan hydrogel beads (mCHBs) for the removal of Cr(VI) and Cu(II) from aqueous solution. Chemical Engineering Journal, 353, 225-236. http://dx.doi.org/10.1016/j.cej.2018.06.143.

24. Yu, D., Feng, Y.-Y., Xu, J.-X., Kong, B.-H., Liu, Q., & Wang, H. (2021). Fabrication, characterization, and antibacterial

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properties of citric acid crosslinked PVA electrospun microfibre mats for active food packaging. Packaging Technology & Science, 34(6), 361-370. http://dx.doi.org/10.1002/pts.2566.

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26. Uyanga, K. A., & Daoud, W. A. (2021). Green and sustainable carboxymethyl cellulose-chitosan composite hydrogels: effect of crosslinker on microstructure. Cellulose, 28(9), 5493-5512. http://dx.doi.org/10.1007/s10570-021-03870-2.

27. Eid, M., Yehia, R., & Amin, A. (2021). Swelling modelling and kinetics investigation of polymer hydrogel composed of Chitosan-g-(AA-AM). Egyptian Journal of Chemistry, 64(10), 5999-6005. http://dx.doi.org/10.21608/ejchem.2021.88530.4260.

Received: Aug. 02, 2023 Revised: Sept. 09, 2023 Accepted: Sept. 14, 2023

Polímeros, 33(3), e20230035, 2023

Incorporation of organic acids in the crosslinking of polyvinyl alcohol hydrogels

Supplementary Material Supplementary material accompanies this paper.

Table S1. Descriptive and inferential statistics averages were used to compare differences in carbonyl index averages for the PVOH film and hydrogels.

Table S2. Descriptive and inferential statistics averages were used to compare differences in hydroxyl index averages for the PVOH film and hydrogels.

Table S3. Temperature data was used to compare differences in initially lost mass weight properties for the PVOH film and hydrogels in 10 and 50%.

Table S4. Descriptive and inferential statistics averages were used to compare differences in swelling properties for the hydrogels.

This material is available as part of the online article from https://doi.org/10.1590/0104-1428.20230075

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ISSN 1678-5169 (Online)

https://doi.org/10.1590/0104-1428.20220102

Synergistic electrochemical method to prepare graphene oxide/polyaniline nanocomposite Eric Luiz Pereira1 , Anderson Gama1 , Maria Elena Leyva González1*  and Adhimar Flávio Oliveira1  Universidade Federal de Itajubá – UNIFEI, Itajubá, MG, Brasil

1

*mariae@unifei.edu.br

Abstract Graphene oxide (GO) was electropolymerized with polyaniline (PANI) during graphite exfoliation in 0.1 M H2SO4 electrolyte and 0.1 M aniline monomer solution. Characterization techniques, including Infrared Absorption Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), UV-vis spectroscopy, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and cyclic voltammetry, were utilized. XRD analysis confirmed GO multilayer exfoliation from the graphite anode, while UV-vis and FTIR techniques confirmed PANI electropolymerization. SEM images revealed PANI distributed between GO multilayers with a nanoneedle morphology. Cyclic voltammetry in 1 M H2SO4 demonstrated that the GO/PANI composite achieved a specific capacitance of 117.440 Fg-1, in contrast to GO’s 1.243 Fg-1, both at a scan rate of 1 mVs-1. This enhancement is attributed to the improved electrical conductivity from PANI and graphene oxide. These results highlight the potential of the GO/PANI composite for high-performance supercapacitors and energy storage systems.

Keywords: graphene oxide, electrochemical exfoliation, polyaniline, electropolymerization, nanocomposites.

How to cite: Pereira, E. L., Gama, A., González, M. E. L., & Oliveira, A. F. (2023). Synergistic electrochemical method to prepare graphene oxide/polyaniline nanocomposite. Polímeros: Ciência e Tecnologia, 33(3), e20230036. https://doi.org/10.1590/0104-1428.20220102

1. Introduction Supercapacitors are promising energy storage devices and have attracted considerable attention in recent years[1-3].

Due to the pursuit of reducing pollution through electric vehicles, or the explosive growth of portable electronic devices, there has been a boost in the development of highperformance supercapacitors[3-5].

In this context, there are currently two main classes of electrochemical capacitors based on their charging and storage mechanism: (a) electrical double-layer capacitors in which the capacitance arises from the separation of charges at the electrode/electrolyte interface[6] and (b) redox where the pseudocapacitance arises from faradic reactions occurring at the electrode/electrolyte interface[7,8]. Large surface area carbons, noble metal oxides, and conductive polymers are the main families of materials used as electrodes in supercapacitors[9]. Conductive polymers have been widely studied, among the main conductive polymeric materials that have been investigated for application as an electrode of a supercapacitor are polyaniline (PANI)[10], polypyrrole (PPY)[11], polythiophene (PTH) and its derivatives[12].

Among these polymers, PANI is considered the most promising material due to its high capacitance, low cost, and ease of synthesis[13]. However, it can undergo expansion and contraction in volume and have low stability and easy collapse in loading and unloading, which restricts its practical applications[14]. To overcome these disadvantages, various carbon materials such as porous carbon, mesoporous carbon,

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and carbon nanotubes were investigated for having good conductivity, stable physicochemical properties, low cost, and long life cycle[15,16].

Another promising candidate for the fabrication of supercapacitor electrode materials is graphene oxide (GO) [17-20] . GO can be obtained from electrochemical exfoliating graphite[21]. The surface of GO contains a large number of functional groups (carboxyl, hydroxyl, epoxy group, etc.)[22]. These unique functional groups make it disperse and hydrophilic in water. In addition, GO also has a large specific surface area, broad chemical potential, excellent chemical stability, and rich[23,24] coverage morphology.

In this way, graphene oxide can improve the conductivity and chemical stability of the polymer. The combination of nanometer-sized and nanostructured GOs with PANI is a promising topic for research for application in charge storage devices. GO and PANI are compatible materials due to both having conjugated π electrons. GO/PANI nanocomposite has excellent electrical properties that are linked to the large surface area of GO will allow these materials to withstand large current densities[25].

Several articles have reported the preparation of GO/ PANI nanocomposite. The one-step electrochemical polymerization of a thin film of PANI/GO was reached by anodic deposition from an electrolytic solution acid of GO and aniline[25]. Other two methods are reported such as chemical polymerization of PANI in an aqueous dispersion

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

Pereira, E. L., Gama, A., González, M. E. L., & Oliveira, A. F. of GO[26-29], and by physical mixing of the GO and PANI powders dispersed in EtOH[30,31]. The present paper shows a new synthesis route to prepare GO/PANI nanocomposite.

The synergistic method consists of the electrochemical polymerization of aniline by the potentiostatic method during the exfoliation of an anode electrode constituted by a graphite bar.

Thereby, this work presents the results of research whose objective was to obtain a nanocomposite of GO and PANI by electrochemical means using a synergistic method. This simple and inexpensive technique avoids polluting reagents.

The procedure allows obtaining the GO/PANI nanocomposite in just a single synthesis step, from the electrosynthesis of polyaniline during the electrochemical exfoliation of graphite.

The materials obtained were analyzed by morphological, physical-chemical characterization techniques, and the electrochemical performance of the materials as an electrode for supercapacitors showed promising results for application in charge storage devices.

2. Materials and Methods 2.1 Preparation of GO and GO/PANI nanocomposites The synergistic method consists of the potentiostatic electropolymerization of PANI at the same electrode where occurs the electrochemical exfoliation of graphite to obtain graphene oxide (GO) in an electrolytic cell. This two-electrode cell has commercially sourced 8B graphite electrodes, such as anode and cathode. The electrodes were immersed in an electrolytic solution of 0.1 M sulfuric acid containing 0.1 M of aniline monomer.

The electrochemical exfoliation of the graphite electrode (anode) was performed by applying a DC voltage of +5 V for 5 min, then increasing +2 V for 5 min until reaching +13 V, using an Instrutherm Model FA-3005 Digital power supply. Simultaneously the anode was electropolymerized the PANI by a modified potentiostatic method in the same potential window from +5 V to +13 V. It was observed the appearance of bubbles and the increase of the applied tension the formation of foam on the surface of the solution. Then the foam (GO/PANI) was extracted from the electrolysis cell filtered and washed with distilled water until neutral pH and dried in an oven at 100ºC for 24h.

Electrochemical exfoliation of graphite to obtain GO was carried out under the same conditions without adding aniline to the solution. Was also noticed the appearance of bubbles and with the increase of the applied tension the formation of foam on the surface of the solution.

The filtered GO and GO/PANI were dispersed in 0.1 M sulfuric acid its solutions were used to carry out the absorbance measurements by UV-vis. The powder obtained was used for characterizations (TGA, FTIR, SEM and XRD). The same resulting powder was dissolved in dimethylsulfoxide, the solution was poured onto ITO glass to form films characterized by cyclic voltammetry.

2.2 Samples characterization The absorbance measurements for the solutions containing dispersions of GO and GO/PANI in 0.1 M sulfuric acid 2/10

was performed between 200 nm and 800 nm, using a Varin spectrometer, model: Cary 50 Bio in a quartz cuvette with a width of 10~mm and a capacity of 3.5 mL. Thermogravimetric analysis (TGA) was performed to verify the thermal behavior of GO and GO/PANI regarding thermal degradation temperatures and mass losses. For this, the TGA-50 Shimadzu equipment was used, with samples of mass 0.2mg, in the temperature range of 25-800ºC with a heating rate of 20ºC/min, in a nitrogen atmosphere at a flow rate of 50 mL/min.

The samples were KBr pellets and then characterized at room temperature by Fourier transform infrared spectroscopy (FTIR) using the Shimadzu spectrometer, model IR Tracer 100, in the region of 600-4000 cm-1 with a resolution of 4 cm-1.

Scanning electron microscopy (SEM) was performed with the Superscan SSX-550 SEM-EDX equipment (Shimadzu Corporation), with an electron beam of 15 kV, and coupled to an energy dispersive spectroscopy (EDS) analyzer. In the equipment, the samples (powder form) were fixed to support by double-sided carbon tape and previously metalized with gold in an IC-50 ion coater equipment (Shimadzu).

The characterization of XRD was performed by the Panalytical X’Pert Pro equipment that uses a beam of wavelength equal to 0.154 nm, provided by an Analytical Expert Diffractometer, The analysis was performed by XRD - X’Expert PRO, using CuKαwith λ= 1.505Å, at 40 kV and with a current of 40 mA. The 2θ range used was between 10-65º, with a step size of 2º/min.

Raman spectroscopy was realized at room temperature in a spectrometer Jobin-Yvon-64000 micro-Raman system, using a green laser (λ= 532 nm) as an excitation source.

For the cyclic voltammetry (CV) analysis, the potentiostat (Metrohm Autolab) was used to control such equipment, using the NOVA 2.0 Software. Cyclic voltammetry curves were performed to verify the performance of the GO/PANI and GO films for their capacitance[32]. The electrochemical cell was assembled with a conventional three-electrode system: an ITO plate modified with the GO and GO/PANI composite as the working electrode, an Ag/AgCl electrode as the reference electrode, and a Pt wire as the auxiliary electrode. The CV measurements were recorded in 0.1 M H2SO4, in the potential range from -0.2 V to 1.0 V, using a different scan rate of 1, 2, 3, 4, and 5 mV/s.

3. Results and Discussions Figure 1 illustrates the potential program used for synergistic electrochemical exfoliation of graphite and electropolymerization of PANI. The increase of voltage between the working electrode (anode) and the against electrode (cathode) over time proved to be suitable for the electrochemical polymerization of PANI[33].

During the electrochemical polymerization of aniline generally, the polymer is deposited on the working electrode.

The accepted mechanism reaction consists of oxidation of the monomer at the anode leading to the formation of aniline cation radicals (rate-determining step). Followed by the formation of soluble oligomers next to the electrode.

Through a nucleation process, these oligomers are deposited Polímeros, 33(3), e20230036, 2023

Synergistic electrochemical method to prepare graphene oxide/polyaniline nanocomposite on an electrode. The next stage of polymerization is the propagation chain, which allows the deposition of PANI on the electrode[33].

Figure 1. Schematic representation of the potential program used for the preparation of the GO/PANI nanocomposite

Figure 2. FTIR transmittance spectrum of GO and GO/PANI. The spectrum shows the characteristic stretching of the vibration bands at 1568 cm-1 (C = C, quinoid rings),1481 cm-1 (C = C, benzenoid rings), 1296 cm-1 (CN), and 1122 cm-1 (CH).

In our work due to higher initial potential (5V) the rate of formation of aniline cation radicals is favored and synthetized oligomers are rapidly stabilized by the nanosheets of GO. Therefore, the nucleation on the working electrode is not possible and the polymerization of PANI continues between the nanosheets of GO. FTIR spectroscopy was used to structurally characterize the product of electrochemical exfoliation of graphite and in situ, electropolymerizations of polyaniline in GO leaves.

Figure 2 displays the spectra obtained from GO and GO/ PANI. In the FTIR spectrum of the GO sheets, the presence of oxygen is observed, containing the functional groups in GO. The high-intensity band at 1076 cm-1 corresponds to the C-O-C stretch vibration. The band at 1381 cm-1 corresponds to the C-OH stretch. The peak at 1458 cm-1 corresponds to the splitting vibration of the O-H bond of the C-OH group. The stretching vibration of the aromatic rings (C=C) corresponds to the absorption band at 1573 cm-1. The lowintensity band at 1720 cm-1 corresponds to the stretching vibration of the C=O bond, in the -COOH group. Note two bands 2850 cm-1 and 2920 cm-1 corresponding to the vibration of stretching C-H bonds. The widest band at 3350 cm-1, corresponding to the O-H stretching vibrations, suggests that the GO sample contains a large amount of adsorbed water molecules. The FTIR spectrum of graphene oxide shows that the product, obtained by electrochemical exfoliation of graphite, is strongly oxidized and consists mainly of -OH and other oxygen-containing functional groups[34].

The FTIR spectrum of the GO/PANI product shows the characteristic vibration bands of the PANI[35] chemical structure. The broadband at 3444 cm-1 corresponds to the stretching vibration of the N-H bond. The spectrum shows the characteristic stretching of the vibration bands at 1568 cm-1 (C = C, quinoid rings), 1481 cm-1 (C = C, benzenoid rings), 1296 cm-1 (CNC), and 1122 cm-1 (C-NH-C).

All these bands indicate the presence of PANI in the GO/ PANI compound.

Figures 3a and 3b show the TGA and DTG results of the GO and GO/PANI samples degraded between 25 to 800ºC to 20ºC/min, under a nitrogen atmosphere. The TGA curves show a similar decomposition profile in both samples, in three degradation steps. The TGA and DTG curve of the GO

Figure 3. TGA and DTG results from (a) GO and (b) GO/PANI samples degraded between 25 to 800ºC to 20ºC/min, under a nitrogen atmosphere.

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Pereira, E. L., Gama, A., González, M. E. L., & Oliveira, A. F. is shown in Figure 3a. It is observed that under a nitrogen atmosphere the GO degrades in three well-defined steps in the TGA curve: • step A, temperatures ≤ 100ºC refer to water loss (1.8%); • Step B, between 100ºC and 414ºC, refers to the loss of oxygen-containing functional groups (10.2%); • Stage C, between 414ºC and 800ºC (slow and soft loss) attributed to the loss of oxygen functional groups with greater thermal stability (5.5%).

According to the literature, in a nitrogen atmosphere, there is no breakage of the C-C bond, related to graphene pyrolysis[36]. Finally, up to 800ºC, the TGA curve shows a residual of 82.5%.

It can be observed in Figure 3a (DTG curve) that steps A and B, described above, have the following singularities: • Step A: two peaks of maximum degradation velocity at a temperature of 62ºC and 90ºC, We can then distinguish two types of water, one adsorbed on the surface and the other located between graphene sheets; • Two peaks of maximum degradation velocity at temperatures of 216ºC and 330ºC, which must be related to the loss of different oxygen functional groups on the surface of graphene.

Therefore, step B which corresponds to the highest mass loss of 10.2% is due to the decomposition of oxygencontaining functional groups (such as C-OH, COC, -COOH) producing CO2, CO, and H2O that are removed from the GO nanosheets. These functional groups, like -COOH and -OH, on the surface of the GO not only increase dispersibility but can also anchor PANI to the surface[37].

In Figure 3b the TGA and DTG curves of GO/PANI are observed. This sample has a slightly higher moisture content (2%) and is similar to stage A of the GO. Degradation step B, between 150ºC and 430ºC, also shows a higher mass loss (11.8%). In this second step, in addition to the loss of the oxygen functional groups attached to the surface of the GO, we have the loss of mass of the PANI dopant (SO42-).

In step C, ≤ 430ºC, large mass loss occurs (12%) due to the decomposition of PANI in the GO/PANI nanocomposite.

In this last step, the loss of stable oxygen functional groups linked to the GO also occurs. Finally, at 800ºC we have a residue of 74.2%, which we attribute to the GO and stable structures of the PANI, formed during the degradation.

To observe the surface of the GO/PANI nanocomposite more distinctly, SEM images were obtained for comparison.

The results are shown in Figure 4.

As shown in Figure 4a, GO leaves were observed.

Figure 4b shows that PANI was well distributed on the surface of GO sheets with high porosity. It is also possible to identify the PANI in nanoneedles structure in some regions of the GO surface, with an average length of approximately 0.032±0.004 µm. In agreement with some reported references[35].

When analyzing the morphology of the GO/PANI nanocomposite (Figure 4b), it is important to consider the electrical transport of the GO/PANI composite is related to the binding mechanism stated between GO and aniline monomer during electrochemical synthesis. The strong affinity between the negatively charged carboxyl group and positively charged amine nitrogen groups makes the PANI fibers bond tightly with the GO nanosheets. The aniline monomer is first adsorbed on the surface of the GO nanosheets, during the formation of PANI nanofiber under the GO nanosheets due to electrostatic attraction.

The electrochemical oxidation of aniline results in the nanostructured product shown in Figure 4b. Therefore, GO nanosheets can be considered as a support material, providing a large number of active sites for PANI growth.

The X-Ray Diffraction Spectrum for GO/PANI nanocomposite is shown in Figure 5. The first peak at 2θ = 6.46º is characteristic of Graphene Oxide and refers to the (001) plane. Compared to the literature (2 θ = 10.2º and spacing between GO sheets, d = 8.65 Å) this detachment is approximately 5.39º[38].

In the diffractogram of Figure 5, it is also possible to notice two peaks at 2 θ = 20.85º and 26.515º. These peaks refer respectively to the (021) and (200) planes, which

Figure 4. Comparative SEM images of (a) GO and (b) GO/PANI nanocomposite. (a) GO leaves can be observed. (b) shows that PANI was well distributed on the surface of GO sheets with high porosity. It is also possible to identify the PANI in a nanoneedles structure in some regions of the GO surface, with an average length of approximately 0.032±0.004 µm.

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Synergistic electrochemical method to prepare graphene oxide/polyaniline nanocomposite are characteristic of PANI and prove the polymerization of polyaniline in the material. This also suggests that the polymer on the GO surface has the same crystal structure as PANI when pure.

Figure 6 shows the UV-vis spectrum measured in 0.1 M sulfuric acid of GO/PANI and GO. An absorption peak at 314 nm is observed in the GO spectrum, which is related to the excitation n – π* transition of the carbonyl group (C=O).

In the GO/PANI spectrum, the presence of three peaks is noted. The peak at 340 nm can be linked to π - π* transitions within the benzoid ring compound. The third widest peak at 540 nm, due to PANI, originates from the cationic charged species, which are known as polarons. Furthermore, the appearance of this peak indicates that the resulting product contains polyaniline.

Through the UV-vis spectrophotometry technique, it was also possible to calculate the value of the optical

Figure 5. GO/PANI XRD spectrum. The first peak at 2 θ = 6.46º is characteristic of Graphene Oxide and refers to the (001) plane. Two peaks at 2 θ = 20.85º and 26.515º are characteristic of PANI and refer to planes (021) and (200), respectively. These peaks confirm the polymerization of polyaniline in the material.

Figure 6. UV-Vis of GO/PANI and GO dispersion in 0.1 Molar sulfuric acid. In the GO/PANI spectrum, the presence of two peaks is noted. The peak is 340 nm due to a π - π* electronic transition within the benzoid ring.

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bandgap (Eg). The estimate of the Eg was determined from the absorption coefficient using the Wood-Tauc equation[39].

Figure 7 shows the estimate of the indirect bandgap ( n = 1/2) for the GO/PANI compound. It is observed that the G/PANI presents an indirect bandgap Eg of 2.06 eV. Figure 7 shows the estimate of the direct bandgap ( n = 2) for GO/NIAP. A direct Eg of 3.01 eV can be observed for GO/PANI. It is possible to observe that GO has a direct gap Eg of 3.53 eV. The indirect gap for GO is shown in Figure 7, where an indirect gap of Eg of 3.29 eV was found.

Figure 8 shows comparatively the Raman spectra of GO and GO/PANI nanocomposite. Raman spectrum of GO displays the typical peaks at 1350 and 1600 cm−1, assigned to the D band (disordered GO structure, carbon atom hybridized sp3 due to chemical functionalization) and G band (organized hexagonal structure, carbon atom hybridized sp2 ), respectively. The intensity ratio between these two bands (ID/IG) is a signal of the degree of disorder of the carbon structure[40]. The ID/IG ratio of 0.8 to GO it is agrees with the value observed in graphene oxide obtained by the electrochemical method[41].

For GO/PANI the ID/IG ratio shows a lower value. Like results were observed in the GO/PANI composite prepared by chemical method[42]. The interaction between GO and PANI can be by π-π stacking, electrostatic interactions, and hydrogen bonding[43]. The Raman spectrum of GO/PANI displays two new bands between D and G bands of graphene oxide, a band at 1408 cm-1 and a shoulder to 1533 cm−1, both are assigned to the PANI backbone[44]. The performance of GO/PANI and GO Films deposited in ITO were analyzed through the cyclic voltammetry technique. These films were used as electrodes and the specific capacitance Ce in (F/g) was calculated from the results obtained using Equation 1.

E2

∫ i ( E ) dE C = e

(1)

E1

2 ( E2 − E1 ) mv '

where i ( E ) is the current (A),

E2

∫ i ( E ) dE and the total E1

voltammetric charge obtained by integrating positive and negative sweeps during cyclic voltammetry, ( E2 − E1 ) is the potential window (V), m is the average mass of the material in grams (g) and v ' is the rate of sweep (V/s). The values of Ce calculated from the cyclic voltammetry curves for the GO and GO/PANI electrodes at different scan rates are shown in Table 1.

The CV curves of the GO/PANI and GO films on ITO were plotted with different scan rates of 1, 2, 3, 4, and 5 mV/s with the potential range from -0.2 to 1.0 V and are shown in Figure 9. All curves show the redox peak appearance and reflect the mixture of double layer and pseudocapacitance.

The redox peaks originate from the faradaic reactions that occur between the surface of the electrodes and electrolytic ions. The cathodic peak at lower potential values (0.20V) can be linked to the transition leucoemeraldine/emeraldine.

The anodic peak at higher potential values (0.6V) was associated with the transition emeraldine/pernigraniline[45].

The highest specific capacitance of GO/PANI is 117.440215 F/g at 1 mV/s. This is the highest Ce recorded 5/10

Pereira, E. L., Gama, A., González, M. E. L., & Oliveira, A. F.

Figure 7. Tauc plot, and the respective linear extrapolation to obtain the bandwidth for the (a) indirect electronic transition of GO/PANI, (b) direct electronic transition of GO/PANI, (c) direct electronic transition of GO, and (d) indirect electronic transition of GO.

Table 1. Specific capacitance in F/g calculated from electrode cyclic voltammetry curves with GO and GO/PANI nanocomposites at different scan rates.

Scan rate (mV/s) 1 2 3 4 5

F/g (GO/PANI) 117.44 52.94 32.33 24.25 19.55

F/g (GO) 1.24 0.56 0.35 0.25 0.13

among the samples. The difference in specific capacitance is due to the highly porous nature of PANI, which provides more surface area available for redox reactions. The SEM images (Figure 4) clearly show that the GO/PANI nanocomposite film provides several microchannels for the diffusion of counter-ions during the redox reaction, which increases the accessibility in the surface area of the film. In addition, graphene oxide sheets act as Chanel in the compound and facilitate the rapid transfer of ions and electrons across the electrode surface.

Therefore, GO/PANI film shows high capacitance, even higher than some compounds. Higher values of specific capacitance of PANI were reported by some authors[46], as well as for PANI with nanoneedles morphology. The cyclic voltammetry curves of the ITO electrode deposited with GO are shown in Figure 9. The highest specific capacitance of GO is 1.24 F/g at 1 mV/s.

In both electrodes, the specific capacitance decreased with increasing scan rates. This is because the ion concentration at the electrode-electrolyte surface is increasing rapidly and 6/10

Figure 8. Raman spectra of GO and GO/PANI.

the diffusion rates of electrolyte ions at the electrode interface are still not sufficient to satisfy electrochemical reactions.

The device underwent a cyclic stability test, where it was subjected to a cyclic current of 0.3 mA for more than Polímeros, 33(3), e20230036, 2023

Synergistic electrochemical method to prepare graphene oxide/polyaniline nanocomposite

Figure 9. Cyclic voltammetry curves of the ITO electrode with (a) GO/PANI film and (b) GO film, in a solution of 1 M of H2SO4, with different scanning rates in the range potential from -0.2 to 1.0 V.

Figure 10. (a) Charging and discharging the device during an interval of more than 5h, (b) insert of Figure (a).

5 hours, with an application interval of 15 seconds followed by a discharge of 10 seconds in a load that consumed -0.3 mA (see Figure 10). To aid visualization, Figure 10b presents an excerpt from Figure 9a. Remarkably, the specific capacitance exhibited a small initial increase and subsequently maintained a constant level. This increase in specific capacitance can be attributed to insufficient contact between the composite material and the aqueous electrolyte solution during the initial stages of the electrochemical process. The initial capacitance increment is probably the result of favorable wetting of the composite materials by the electrolyte, which facilitates a reduction in the electrode’s internal resistance.

In conclusion, the examined samples exhibit characteristics of graphene oxide (GO) sheets, and the GO/polyaniline (PANI) nanocomposite showcases PANI nanoneedles distributed within the GO sheets, resulting in high porosity. Despite being a cost-effective technique, it proved to be efficient, as the formation of GO was confirmed by various analyses, including TGA, FTIR, XRD, and UV-vis. Furthermore, the studied material, based on graphene oxide and polyaniline, Polímeros, 33(3), e20230036, 2023

demonstrates a high specific capacitance and excellent stability overcharge and discharge cycles. Therefore, it stands as an outstanding candidate for use in charge storage devices, such as capacitors and supercapacitors.

4. Conclusions Composites of graphene oxide and polyaniline were successfully synthesized by a synergistic new method based on electrochemical exfoliation of graphite and potentiostatic electropolymerization of polyaniline.

The X-ray diffraction pattern confirmed the polymerization of polyaniline in the material and indicated that the introduction of the polymer in GO increased the space between the GO layers by 57%. FTIR spectroscopy provided chemical structural information on GO and GO/PANI. The FTIR spectrum of GO/PANI clearly showed the characteristic PANI bands at 1568 and 1481 cm-1. The FTIR spectrum confirmed that the surface of the GO was completely oxidized. The UVVis spectrum confirmed the presence of PANI through the 7/10

Pereira, E. L., Gama, A., González, M. E. L., & Oliveira, A. F. polaronic band. SEM image confirmed the presence of GO nanosheets and PANI nanoneedles interspersed between GO with an average length of approximately 0.032±0.004 µm distributed in the surface regions of the GO nanosheets.

Thermal characterization by thermogravimetry (TGA) showed that both the GO and the GO/PANI nanocomposite degrade in three well-defined steps. TGA showed a high content of functional groups present in the GO structure.

From the analysis of cyclic voltammetry, it is evident that the addition of PANI (GO/PANI) to GO significantly enhanced its electrochemical performance. The highest specific capacitance observed for GO/PANI was 117.44 F/g, measured at a sweep rate of 1 mV/s. Notably, this value is approximately 94.44 times greater than the specific capacitance of GO analyzed under identical conditions.

Furthermore, the device demonstrated excellent performance throughout the charge and discharge tests, indicating its reliability and stability as a charge storage device. These results highlight the potential of GO/PANI as an improved material for use in energy storage applications, such as capacitors and supercapacitors. Taking into account all the characterizations performed, the material studied in this research, based on graphene oxide and polyaniline, is an excellent candidate for applications in energy storage devices, such as supercapacitors, as it has a high specific capacitance.

In addition, despite being a low-cost technique, it proved to be efficient, as the formation of the GO was confirmed by both TGA and FTIR, DRX, and UV-vis. Finally, the images performed in SEM showed characteristics of GO sheets and in the GO/PANI composite, the PANI with nanoneedles morphology appears well distributed between GO sheets with high porosity.

5. Author’s Contribution • Conceptualization – Maria Elena Leyva González; Adhimar Flávio Oliveira.

• Data curation – Maria Elena Leyva González; Adhimar Flávio Oliveira.

• Formal analysis – Eric Luiz Pereira; Maria Elena Leyva; Anderson Gama; Adhimar Flávio Oliveira.

• Funding acquisition – Maria Elena Leyva González.

• Investigation – Eric Luiz Pereira; Maria Elena Leyva González; Anderson Gama; Adhimar Flávio Oliveira.

• Methodology – Eric Luiz Pereira; Maria Elena Leyva González; Anderson Gama; Adhimar Flávio Oliveira.

• Project administration – Maria Elena Leyva González.

• Resources – Maria Elena Leyva González.

• Software – NA. • Supervision – Maria Elena Leyva González; Adhimar Flávio Oliveira.

• Validation – Eric Luiz Pereira; Maria Elena Leyva González; Anderson Gama; Adhimar Flávio Oliveira.

• Visualization – Eric Luiz Pereira; Maria Elena Leyva González; Adhimar Flávio Oliveira.

• Writing – original draft – Eric Luiz Pereira; Maria Elena Leyva González; Adhimar Flávio Oliveira.

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• Writing – review & editing – Eric Luiz Pereira; Anderson Gama; Maria Elena Leyva González; Adhimar Flávio Oliveira.

6. Acknowledgements The authors would like to thank the Brazilian agencies CAPES, CNPq, and Fapemig (Finance Code APQ-0267616 and APQ-00010-18) for financial support.

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Received: Dec. 01, 2022 Revised: Sep. 10, 2023 Accepted: Oct. 05, 2023

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