A Study on Physical, Morphological and Antibacterial Properties of Bio Polymers Reinforced Polyvinyl Acetate Foams

Nadir Yildirim Ertan Ozen Mehmet Emin Ergun Berk Dalkilic About the authors

Abstract

In this study, foaming-agent free novel polyvinyl acetate (PVAc) foams reinforced with bio polymers were manufactured through freeze-drying technique. The physical, morphological and antibacterial properties of foams which were reinforced with different ratio of zinc borate and water-soluble chitosan were investigated according to relevant standards. The PVAc foams showed low densities (0.12 g/cm3 – 0.21 g/cm3) and high porosity rates (87.50% - 79.05%). The results showed that although the foams have no antibacterial character against Escherichia Coli, they have antibacterial character against Staphylococcus Aureus bacteria. This study mainly focusses on physical and morphological properties of the foams. However, researchers also performed accelerated weathering tests to determine its usability in different industries. The effects of accelerated weathering on the surface of foams were investigated by measuring surface color. The highest color difference was determined 8.09. This foam can be used as a low-density packaging material and/or medical box with its promising physical and morphological properties with hazardous-chemical free structure.

Keywords:
Antibacterial effect; bio polymers; freeze-drying; polyvinyl acetate (PVAc) foam


1. Introduction

Foams consist of solid polymeric matrix and gas phase which are generally obtained with a foaming agents, such as pentane, and hydrochlorofluorocarbon11 Jin F-L, Zhao M, Park M, Park S-J. Recent trends of foaming in polymer processing: a review. Polymers. 2019;11(6):953.. However, these foaming agents are highly flammable and harmful to both human health and the environment22 Wang Y, Li J, Lv D, Jiang N, Zhang Y. Analysis of the dangerous consequences of storing pentane in polyurethane foam enterprises. In: Huang C, Nivolianitou Z, editors. Risk analysis based on data and crisis response beyond knowledge. London: CRC Press; 2019. . These are used for creation of voids in the foam to reduce the density and increase the thermal insulation properties, also produces decrease in the amount of raw material needs to be used to manufacture the foams33 Bledzki AK, Faruk O. Injection moulded microcellular wood fibre–polypropylene composites. Compos, Part A Appl Sci Manuf. 2006;37(9):1358-67.,44 Park CB, Cheung LK. A study of cell nucleation in the extrusion of polypropylene foams. Polym Eng Sci. 1997;37(1):1-10.. Conventional raw materials such as polystyrene, polyurethane, polyvinyl chloride, and polyolefin are extensively utilized as the matrix of foams and they are commonly used in insulation, cushioning, packaging of valuable goods and food packaging fields. But due to the inadequate recyclability and decomposability of these materials, most of the foams are wasted at the end of their product life. This creates increase in pollution55 Zhang X, Teng Z, Huang R, Catchmark J. Biodegradable starch/chitosan foam via microwave assisted preparation: morphology and performance properties. Polymers. 2020;12(11):2612.,66 Yildirim N, Erdonmez FS, Ozen E, Avci E, Yeniocak M, Acar M, et al. Fire-retardant bioproducts for green buildings. In: Pacheco-Torgal F, Ivanov V, Tsang DCW, editors. Bio-based materials and biotechnologies for eco-efficient construction. Cham: Woodhead Publishing; 2020. p. 67-79. (Woodhead Publishing Series in Civil and Structural Engineering)..

Although PVAc is a synthetic polymer, it is consumed by Penicillium and Aspergillus fungi species in laboratory conditions77 Trejo AG. Fungal degradation of polyvinyl acetate. Ecotoxicol Environ Saf. 1988;16(1):25-35.. Based on its versatility of structure and related properties88 Amann M, Minge O. Biodegradability of Poly(vinyl acetate) and Related Polymers. In: Rieger B, Künkel A, Coates GW, Reichardt R, Dinjus E, Zevaco TA, editors. Synthetic biodegradable polymers. Berlin: Springer; 2012. p. 137-172. (Advances in Polymer Science)., PVAc usually utilizes as an adhesive in wood industry. Recently, it has shown potential for use in development of composite materials99 Liu L, Guo H, Zeng X, Lai X. Preparation and properties of PVAc-nano-OMMT/PP-EVA composite. Acta Materiae Compositae Sinica. 2011;28(2):64-9., films1010 Dubininkas M. Production and characterization of 3-methacryloxypropyltrimethoxysilane modified polyvinyl acetate dispersion. J Chem Chem Eng. 2016;65(3-4):119-26. and foams1111 Ergün ME, Özen E, Yildirim N, Dalkiliç B. Manufacture of wood fiber reinforced polyvinyl acetate rigid foams. Ormancılık Araştırma Dergisi. 2020;7(2):104-12.. Biopolymer additives, such as calcium carbonate1212 Cakir R, Öksüz M. Polivinilasetat (pvac) emulsiyon kompozitlerinin mekanik ve morfolojik özellikleri. Gazi Üniv Müh Mim Fak Der. 2013;27(3):481-90., nanocellulose1313 Pracella M, Haque MM, Alvarez DPV. Preparation and characterization of PLA nanocomposites with nanocellulose filled PVAC. In: European Conference on Composite Materials; 2012; Venice, Italy. Proceedings. Padova: University of Padova; 2012. p. 24-8., and sisal cellulose1414 Garcia de Rodriguez NL, Thielemans W, Dufresne A. Sisal cellulose whiskers reinforced polyvinyl acetate nanocomposites. Cellulose. 2006;13(3):261-70., were incorporated into the PVAc matrix to prepare sustainable products with improved properties. The addition of reinforcement of higher strength than the matrix polymer improves the mechanical properties of plastic resins. Carbon nanotubes and carbon fibers are studied to improve the mechanical performance of reinforced polymers, and they are commonly used for polymer reinforcement1515 Hu X, Sun J, Li X, Qian L, Li J. Effect of phosphorus-nitrogen compound on flame retardancy and mechanical properties of polylactic acid. J Appl Polym Sci. 2021;138(7):49829.

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Cellulose is a natural biopolymer that is renewable, biodegradable and non-toxic. It is the most abundant natural biopolymer in the world. Cellulose occupies 40% – 50% of the earth’s total biomass reserves2222 Yi T, Zhao H, Mo Q, Pan D, Liu Y, Huang L, et al. From cellulose to cellulose nanofibrils: a comprehensive review of the preparation and modification of cellulose nanofibrils. Materials. 2020;13(22):5062.. Cellulosic fibers have advantageous compared to synthetic fibers. The natural fibers bend rather than break (fail) during processing and manufacturing2323 Komal UK, Lila MK, Singh I. Processing of PLA/pineapple fiber based next generation composites. Mater Manuf Process. 2021. In press.. In addition, cellulose has a flattened oval cross-section, which enhances stress transfer by presenting a higher aspect ratio2424 Pickering K. Properties and performance of natural-fibre composites. Cambridge: Elsevier; 2008..

The most challenging issue with use of cellulose-based materials for packaging purposes is their vulnerability to microbial growth2525 Peelman N, Ragaert P, De Meulenaer B, Adons D, Peeters R, Cardon L, et al. Application of bioplastics for food packaging. Trends Food Sci Technol. 2013;32(2):128-41.,2626 Petersen K, Væggemose Nielsen P, Bertelsen G, Lawther M, Olsen MB, Nilsson NH, et al. Potential of biobased materials for food packaging. Trends Food Sci Technol. 1999;10(2):52-68.. Polyhexamethylene guanidine hydrochloride2727 Heydarifard S, Pan Y, Xiao H, M. Nazhad M, Shipin O. Water-resistant cellulosic filter containing non-leaching antimicrobial starch for water purification and disinfection. Carbohydr Polym. 2017;163:146-52., copper oxide2828 Ashjari HR, Dorraji MSS, Fakhrzadeh V, Eslami H, Rasoulifard MH, Rastgouy-Houjaghan M, et al. Starch-based polyurethane/CuO nanocomposite foam: antibacterial effects for infection control. Int J Biol Macromol. 2018;111:1076-82., zinc oxide2929 Jain S, Bhanjana G, Heydarifard S, Dilbaghi N, Nazhad MM, Kumar V, et al. Enhanced antibacterial profile of nanoparticle impregnated cellulose foam filter paper for drinking water filtration. Carbohydr Polym. 2018;202:219-26. and silver3030 Jain P, Pradeep T. Potential of silver nanoparticle-coated polyurethane foam as an antibacterial water filter. Biotechnol Bioeng. 2005;90(1):59-63. are used to acquired antibacterial properties to the foam material and they have a great protection against gram positive and gram negative bacterias.The majority of commercial antibacterial products are based on a leaching technique, in which biocides are introduced into the material and leached into the environment to destroy microorganisms. Silver particles are widely utilized for this aim, despite the fact that silver ions are venomous to mammalian cells and can result in silver-resistant bacteria3131 Greulich C, Kittler S, Epple M, Muhr G, Köller M. Studies on the biocompatibility and the interaction of silver nanoparticles with human mesenchymal stem cells (hMSCs). Langenbecks Arch Surg. 2009;394(3):495-502.

32 Kittler S, Greulich C, Diendorf J, Köller M, Epple M. Toxicity of silver nanoparticles increases during storage because of slow dissolution under release of silver ions. Chem Mater. 2010;22(16):4548-54.
-3333 Silver S. Bacterial silver resistance: molecular biology and uses and misuses of silver compounds. FEMS Microbiol Rev. 2003;27(2-3):341-53.. Therefore, the focus has been on alternative antimicrobial polymers that do not harm mammals. Chitosan, which is produced by deacetylation of chitin, is the second most abundant polymer in nature, has widespread use in many areas due to its antibacterial and antifungal properties3434 Ottenhall A, Seppänen T, Ek M. Water-stable cellulose fiber foam with antimicrobial properties for bio based low-density materials. Cellulose. 2018;25(4):2599-613.. This is frequently owing to intense interactions between the positively charged groups of the polymer and the negative charges on the bacterial and fungal cell walls3535 Lichter JA, Rubner MF. Polyelectrolyte multilayers with intrinsic antimicrobial functionality: the importance of mobile polycations. Langmuir. 2009;25(13):7686-94.,3636 Lichter JA, Van Vliet KJ, Rubner MF. Design of antibacterial surfaces and interfaces: polyelectrolyte multilayers as a multifunctional platform. Macromolecules. 2009;42(22):8573-86.. Cell walls are negatively charged on the surface, and cationic polymers bind to anionic components on the cell walls of microorganisms, changing the permeability of the cell wall and leading to cell death3434 Ottenhall A, Seppänen T, Ek M. Water-stable cellulose fiber foam with antimicrobial properties for bio based low-density materials. Cellulose. 2018;25(4):2599-613.,3737 Vaara M. Agents that increase the permeability of the outer membrane. Microbiol Mol Biol Rev. 1992;56(3):395-411..

Many foam manufacturing methods has been studied both commercially and scientifically. The well-known methods can be named specifically, extrusion3838 Rahimi SK, O’Donnell K, Haight B, Machado A, Martin C, Meng F, et al. Supercritical-CO2 foam extrusion of hydroxypropyl methyl cellulose acetate succinate/itraconazole amorphous solid dispersions: processing-structure-property relations. J Pharm Sci. 2021;110(4):1444-56., injection molding3939 Hou J, Zhao G, Wang G. Polypropylene/talc foams with high weight-reduction and improved surface quality fabricated by mold-opening microcellular injection molding. J Mater Res Technol. 2021;12:74-86. and batch process4040 Yeh S-K, Liu Y-C, Wu W-Z, Chang K-C, Guo W-J, Wang S-F. Thermoplastic polyurethane/clay nanocomposite foam made by batch foaming. J Cell Plast. 2013;49(2):119-30., melting the raw materials under high temperature and providing the formation of highly porous final product (foam) using foaming agents4141 Deng Y, Dewil R, Appels L, Ansart R, Baeyens J, Kang Q. Reviewing the thermo-chemical recycling of waste polyurethane foam. J Environ Manage. 2021;278:111527.. Although the production time is very short with these techniques, they do not allow pre-surface modification. Because the chemicals used during surface modification degrade at high temperatures during foam production and have a negative effect on the materials4242 Chakrabarty A, Teramoto Y. Recent advances in nanocellulose composites with polymers: a guide for choosing partners and how to incorporate them. Polymers. 2018;10(5):517.. On the other hand, many studies have been carried out on the freeze-drying technique that enables the production of more qualified products and allowing surface modification4343 Li Y, Wang B, Sui X, Xu H, Zhang L, Zhong Y, et al. Facile synthesis of microfibrillated cellulose/organosilicon/polydopamine composite sponges with flame retardant properties. Cellulose. 2017;24(9):3815-23.

44 Sultana N, Wang M. PHBV/PLLA-based composite scaffolds fabricated using an emulsion freezing/freeze-drying technique for bone tissue engineering: surface modification and in vitro biological evaluation. Biofabrication. 2012;4(1):015003.
-4545 Zhang S, Zhang S, Li W, Pei Y, Liu B, Yang H. PEI-functionized cellulose foam for the visual recognition and removal of Cr(VI). Mater Lett. 2021;293:129685..

The freeze-drying procedure is divided into three stages: freezing, primary drying, and secondary drying4646 Jennings TA. Lyophilization: introduction and basic principles. Boca Raton: CRC Press; 2019.. During the freezing process, the water molecules create ice crystals in the first part. The drying process then begins under particular pressure and temperature settings. As ice crystals sublimate during the drying step, the final pore shape is formed. As a result, the foam structure is directly relevant to the dispersion and size of the frozen system4747 Svagan AJ, Samir MASA, Berglund LA. Biomimetic foams of high mechanical performance based on nanostructured cell walls reinforced by native cellulose nanofibrils. Adv Mater. 2008;20(7):1263-9..

Biopolymer-based foams for different purposes were produced with the freeze-drying technique. For instance; the highly porous, low density, and cross-linked natural cellulose foams which modified with silane based hydrophobic chemical from rice straw was produced. These cellulose foams can be utilized as a worthwhile sorbent for adsorption of oil and organic solvents4848 Dilamian M, Noroozi B. Rice straw agri-waste for water pollutant adsorption: relevant mesoporous super hydrophobic cellulose aerogel. Carbohydr Polym. 2021;251:117016.. Another study, polyvinyl alcohol reinforced cellulose-based foams were produced from waste fibers which have good thermal property. Density and porosity value of foams were 0.019–0.046 g/cm3 and more than 96%, respectively. These foams exhibited good heat insulation with low thermal conductivity (0.039–0.043 W/m K) due to the porous structure inside4949 Do NHN, Tran VT, Tran QBM, Le KA, Thai QB, Nguyen PTT, et al. Recycling of pineapple leaf and cotton waste fibers into heat-insulating and flexible cellulose aerogel composites. J Polym Environ. 2021;29(4):1112-21.. The produced cellulose foams have a low density (0.0554 g/cm3), good thermal stability, and a high porosity (89.32%). Furthermore, directed cellulose foams exhibit good mechanical properties, with a compressive stress of 0.35 MPa at 70% strain. The cellulose foams that have been treated with directed freezing have promising applications in the field of sound absorption5050 Lou C-W, Zhou X, Liao X, Peng H, Ren H, Li T-T, et al. Sustainable cellulose-based aerogels fabricated by directional freeze-drying as excellent sound-absorption materials. J Mater Sci. 2021;56(33):18762-74.. Alumina reinforced polyvinyl alcohol/chitosan-based foam produced and they have the smoke-suppressant, high-strength and fire-retardant. It stated that foams used for potential alternatives to traditional flame-retardant foams. It was stated that the produced foams can be used as potential alternatives to conventional flame retardant foams5151 Yang Z, Li H, Niu G, Wang J, Zhu D. Poly(vinylalcohol)/chitosan-based high-strength, fire-retardant and smoke-suppressant composite aerogels incorporating aluminum species via freeze drying. Compos, Part B Eng. 2021;219:108919..

The novel idea presented in this study is to design and development of PVAc foams with bleached kraft pulp, water-soluble chitosan and zinc borate and investigating their physical and morphological properties.

2. Materials and Methods

2.1. Material

In this work, the polyvinyl acetate (Figure 1a) (PVAc: Mad Wolf brand, made in Akpinar Building Materials Industry and Trade Inc./Istanbul) was bought from a local supplier. The solid content of PVAc was 42.8%. The density of PVAc was 1.2 g/cm3. The brightness value of bleached kraft pulp was 90% (ISO 2470), and the degree of polymerization was 933, was supplied from EUROPAP (İzmir, Turkey). The density of bleached kraft pulp (Figure 1b) which was produced from Eastern Spruce was 0,55 gr/cm3 , and the pulp was bleached elementary chlorine free (ECF). Zinc borate (Figure 1c) (Zn3BO6) (Kimetsan Chemistry) had a purity degree of 85.27% and the density of zinc borate was 2.6 g/cm3. Chitosans were purchased from Sigma-Aldrich in low (50000-190000 Da), medium (200000-300000 Da) and high molecular (310000-375000 Da) weights. Glycidyltrimethylammonium chloride (GTMAC) (Sigma-Aldrich) was used in the synthesis of water-soluble chitosan (Figure 1d).

Figure 1
Materials used in foam production a. PVAc, b. Bleached kraft pulp, c. Zinc borate, d. Water-soluble chitosan.

2.2. Modification of chitosan

Chitosan is soluble in acidic condition. Acidic condition was led to degrade polyvinyl acetate. Therefore, in this study was needed to synthesize water-soluble chitosan. Initially, chitosan (30.00 g) was dispersed in 300 ml water. After that, the glycidylthyrimethylammonium chloride (GTMAC) (112.90 g) was added, and the mixture was stirred at 80 °C for 8 h. The reaction products were filtered, concentrated, precipitated in acetone, and dried at 60 °C for overnight to obtain the water-soluble chitosan.

2.3. Foam preparation

This study was conducted in two sections. The first section, has investigated the effects of zinc borate in different ratios. In the second section, the optimum zinc borate ratio was kept constant, and the effects of molecular weights and ratios of water-soluble chitosan were investigated on foams. Ingredient of the foam were given in Table 1. Mixtures were blended for 15 minutes at 2000 rpm after each reinforcement addition.

Table 1
Ingredient of the PVAc foam reinforced with the different bio polymers.

All samples were poured into the mold, and they were frozen at -25 °C for 24 hours. The frozen samples were freeze-dried in a lyophilizer (ModulyoD, Termo Savant) at a condenser temperature of -50 °C under 0.40 mBar pressures for 48 hours to produce the foams. The preparation method of PVAc foams was shown in Figure 2.

Figure 2
Manufacturing process of PVAc foams reinforced with bio polymers.

2.4. Characterization

2.4.1. Determination of molecular weight

A molecular weight measurement of chitosan and water-soluble chitosan were analyzed with a Ubbelohde viscometer. Viscosity values of chitosan solutions were prepared in a mixture of 0.25 M acetic acid and 0.50 sodium acetate solvent to concentrations of 0.10 to 0.50 g/dl, depending on the sample5252 Hamilton V, Yuan Y, Rigney DA, Puckett AD, Ong JL, Yang Y, et al. Characterization of chitosan films and effects on fibroblast cell attachment and proliferation. J Mater Sci Mater Med. 2006;17(12):1373-81.. The reduced viscosity (η) values were found from the equations of the curves of the graphs drawn with the determined ηspes/c values. Then, the molecular weights of chitosan were determined according to the following Mark-Houwink Equation 1.

η = K . M V α (1)

where [η] is the intrinsic viscosity and M is molecular weight. The constants, α and K , are 0.79 and 1.57 10-4 dl/g, respectively5353 Kasaai MR, Arul J, Charlet G. Intrinsic viscosity-molecular weight relationship for chitosan. J Polym Sci, B, Polym Phys. 2000;38(19):2591-8..

2.4.2. FTIR spectrum

FTIR measurements were used to determine changes in the chemical structure of water-soluble chitosan. For this purpose, FTIR (Thermo Fisher Scientific brand Nicolet™ iS10) device was used and measurements were made in the spectra range of 600-4000 cm-1. The data obtained as a result of the measurement process were visualized with OMNIC™ Specta Software.

2.4.3. Density and porosity determination

Produced PVAc foams (Figure 3) were hold in laboratory conditions for 24 hours for conditioning. The seven (7) samples with 10 mm x 50 mm x 50 mm dimensions from each group were measured according to ASTM C303 (2010) standards.

Figure 3
Produced PVAc foams (a. Z-2, b. C-6).

The porosities of the developed foams were determined by the formulas given below (2).

P t = 100 x ( 1 d b u l k d p a r t i c l e ) (2)

Pt= porosity (%)

dbulk= density of foam (g/cm3)

dparticle= density of particle (g/cm3)

2.4.4. Antibacterial activity test

The disc-diffusion method was used to determine the antibacterial activity. As test organisms, 24-hour cultures of Escherichia Coli and Staphylococcus Aureus bacteria were used. After inoculation of 100 µl with the help of a drigalski spatula, 0.5 McFarland dilutions were prepared from liquid cultures of test microorganisms in petri dishes containing Iso-Sensitest agar (Oxoid CM0471), and the petri dishes were incubated for one hour. Then, specimens prepared with different materials and cut into 10 mm diameter discs were placed in petri dishes. After this process, the petri dishes were examined after 48 hours of incubation at 30°C and the zone of inhibition (ZOI) formed was measured.

2.4.5. Morphological structure

The morphologies of the foams were examined using a JSM-7600F brand scanning electron microscope (SEM). The surfaces of the foams were coated to increase conductivity with gold. During the examination of the microstructure images, the operating voltage of the microscope was chosen as 15 kV.

2.4.6. Accelerated weathering test and color measurement

The accelerated weathering test was carried out in a QUV weathering device (Q - Lab, USA) outfitted with eight UVA 340 lamps in accordance with the ASTM G154 (2006) standard. For a total of 50 hours, foams were exposed to cycles of 8 hours UV-light irradiation followed by 4 hours condensation in the accelerated weathering test divece. At the maximum intensity of 340 nm wavelengths, the average irradiance was 0.89 W/m2 (max = 340 nm). The temperatures during the light irradiation and condensation periods were 60 °C and 50 °C, respectively.

The CIEL*a*b* method was used to calculate the color parameters L*, a*, and b*. The lightness is represented by the L* axis, while the chromaticity coordinates are represented by a* and b*. The parameters +a* and -a* represent red and green, respectively. The +b* parameter represents yellow, while the -b* parameter represents blue. L* can range from 100 (white) to 0 (black). The colorimeter (X-Rite SP Series Spectrophotometer) was used to measure the colors of the specimens before and after accelerated weathering. The color differences of foams (ΔE) were determined according to ASTM D1536–58 T (1964).

2.4.7. Statistical analysis

All results were compared using one-way means/ANOVA to check for significant differences (alpha= 0.01). The Tukey-Kreamer Honest Significant Differences (HSD) test (alpha=0.05) was used to examine whether there were significant differences between the groups.

3. Results and Discussions

3.1. Determination of molecular weight

In this study, water-soluble chitosan was used to prevent the degradation of the PVAc matrix in acidic conditions. As a result of the viscosity measurements, the intrinsic viscosities ([η]), molecular weights and densities of chitosan before and after synthesis were given in Table 2.

Table 2
Intrinsic viscosity, molecular weights and density of chitosans and water-soluble chitosans.

Molecular weights of water-soluble chitosan which were synthesized via glycidyltrimethylammonium chloride (GTMAC) were decreased because intermolecular bonds of chitosan were broken in the alkaline condition5454 Dutta PK, Dutta J, Tripathi VS. Chitin and chitosan: chemistry, properties and applications. J Sci Ind Res. 2004;63(1):20-31..

3.2. FTIR spectrums of chitosan and water-soluble chitosan

The GTMAC was used in the production of water-soluble chitosan. The general chemical structures of chitosan synthesized with GTMAC, and the determination of the changes in the chemical structures after synthesis have been demonstrated by FTIR diagrams as given in Figure 4.

Figure 4
FTIR spectra for chitosan and water soluble chitosan.

The NH2 group, which was the characteristic peak of water-soluble chitosan, was observed at approximately 1646 cm-1. In addition, a new peak was formed at 1479 cm-1. This weak peak was observed due to the binding of ammonium groups to methyl groups. On other hand, the presence of a quaternary amine group, which allowed chitosan to dissolve in water after modification with this peak, was determined5555 Li R, Guo Z, Jiang P. Synthesis, characterization, and antifungal activity of novel quaternary chitosan derivatives. Carbohydr Res. 2010;345(13):1896-900..

3.3. Density and porosity determination

The variation on the porosity and density values depending on the reinforcements were given in Figure 5.The test results showed that the concentration ratio of zinc borate and water-soluble chitosan changed the density and the porosity as expected.

Figure 5
Density and porosity results of foams (A, B, C, D, E, and F letters indicate the significant differences between the groups).

Density values depending on concentration ranged from 0.12 g/cm3 to 0.21 g/cm3. The highest value was obtained from C-9 which high molecular weight water-soluble chitosan at 70 g/L concentration. Z-1 gave the lowest density value with 0.12 g/cm3 and C-9 gave the highest density value with 0.21 g/cm3. Due to fibers clump, attractive forces (intermolecular forces and capillary) and particles and adhere together during solvent removal. This phenomenon led to occur more dense foams. Besides, although particle dimensions can be influenced by the source, fiber-based foams can benefit from the natural morphology of the cellulose source5656 Dufresne A. Nanocellulose: from nature to high performance tailored materials. Berlin: Walter de Gruyter GmbH & Co KG; 2017.. Selecting naturally lightweight fibers (highly porous and/or hollow lumen), such as bleached kraft pulp which was used in our study (0.55 g/cm3), is an obvious way to reduce density. Other low-density fibers are found, for example, in bamboo5757 Savastano H Jr, Fiorelli J, Santos SFD. Sustainable and nonconventional construction materials using inorganic bonded fiber composites. Cambridge: Woodhead Publishing; 2017. (0.26–1.21 g/cm3), flax5858 Fan M, Fu F. Advanced high strength natural fibre composites in construction. London: Woodhead Publishing; 2016. (0.34–0.74 g/cm3), jute5959 Kandemir A, Pozegic TR, Hamerton I, Eichhorn SJ, Longana ML. Characterisation of natural fibres for sustainable discontinuous fibre composite materials. Materials. 2020;13(9):2129. (0.41–0.78 g/cm3) and sugarcane bagasse6060 Monteiro SN, Lopes FPD, Barbosa AP, Bevitori AB, Silva ILAD, Costa LLD. Natural lignocellulosic fibers as engineering materials: an overview. Metall Mater Trans, A Phys Metall Mater Sci. 2011;42(10):2963. (0.10–0.49 g/cm3). On the other hand, shrinkage occurs when materials were freeze-dried, and it was shown that shrinkage can reduce the volume of foams by up to 30%6161 Martoïa F, Cochereau T, Dumont PJJ, Orgéas L, Terrien M, Belgacem MN. Cellulose nanofibril foams: links between ice-templating conditions, microstructures and mechanical properties. Mater Des. 2016;104:376-91.. The porosity values in Figure 5 and voids in Figure 6 provided qualitative information about how the pore ratio diminishing with increasing amount of water-soluble chitosan and zinc borate in suspensions after freeze-drying, occurring a decrease in porosity. Porosity values depending on concentration found between 87.50% to 79.05%. C-9 were given the lowest porosity values with 79.05%. The highest porosity value was obtained from Z-1 with 87.50%. The pore sizes and ratio decrease with an increase in the amount of water-soluble chitosan in the samples. The ice templating theory of suspensions can articulate that case6161 Martoïa F, Cochereau T, Dumont PJJ, Orgéas L, Terrien M, Belgacem MN. Cellulose nanofibril foams: links between ice-templating conditions, microstructures and mechanical properties. Mater Des. 2016;104:376-91.. According to this theory, the higher cellulose and water-soluble chitosan content in the gels prevents broader interconnected ice crystal formation during the ice templating process, resulting in smaller pores ratio and size distribution in the samples after freeze-drying. Also, partly low amount of cellulose and water-soluble chitosan during the ice templating step, because of the lower resistance to ice crystal formation, higher porosity gels with varied pore size and ratio distributions were produced6262 Hossen MR, Talbot MW, Kennard R, Bousfield DW, Mason MD. A comparative study of methods for porosity determination of cellulose based porous materials. Cellulose. 2020;27(12):6849-60.. In additon, during the freezing process of water, ice crystals nucleate, develop, and concentrate cellulose in the interstitial spaces between crystals, lead to the formation of aggregated foam cell walls6363 Wainwright SA, Gosline JM, Biggs WD, Currey JD. Mechanical design in organisms. Princeton: Princeton University Press; 1982.. Additional aggregation is very likely to occur during the sublimation phase. As a result, the porosity of the foams is diminishing6464 Sehaqui H, Zhou Q, Berglund LA. High-porosity aerogels of high specific surface area prepared from nanofibrillated cellulose (NFC). Compos Sci Technol. 2011;71(13):1593-9..

Figure 6
The SEM images of PVAc foam reinforced with different rate of zinc borate (a. Z-1, b. Z-2, c. Z-3) and different rate and molecular weight of water-soluble chitosan (d. C-1, e. C-2, f. C-3, g. C-4, h. C-5, i. C-6, j. C-7, k. C-8, l. C-9) at x50 magnifications.

Cellulose-based foams were produced with different drying techniques before. For example, freeze-drying6565 Pääkkö M, Vapaavuori J, Silvennoinen R, Kosonen H, Ankerfors M, Lindström T, et al. Long and entangled native cellulose I nanofibers allow flexible aerogels and hierarchically porous templates for functionalities. Soft Matter. 2008;4(12):2492-9.

66 Aulin C, Netrval J, Wågberg L, Lindström T. Aerogels from nanofibrillated cellulose with tunable oleophobicity. Soft Matter. 2010;6(14):3298-305.
-6767 Cervin NT, Aulin C, Larsson PT, Wågberg L. Ultra porous nanocellulose aerogels as separation medium for mixtures of oil/water liquids. Cellulose. 2012;19(2):401-10. or ambient drying, supercritical drying6868 Sehaqui H, Zhou Q, Ikkala O, Berglund LA. Strong and tough cellulose nanopaper with high specific surface area and porosity. Biomacromolecules. 2011;12(10):3638-44.. The density of the material produced via supercritical drying (nanopaper) is 0,64 g/cm3 and that of foams made from varibale forms of ambient drying6969 Liu Y, Lu P, Xiao H, Heydarifard S, Wang S. Novel aqueous spongy foams made of three-dimensionally dispersed wood-fiber: entrapment and stabilization with NFC/MFC within capillary foams. Cellulose. 2017;24(1):241-51. varies between 0,033 g/cm3 and 0,066 g/cm3. The foam produced in this study, with a density of 0,12 to 0, 21 g/cm3 is definitely equivalent to these materials, as well as expanded polystyrene foam7070 Tillotson TM, Hrubesh LW. Transparent ultralow-density silica aerogels prepared by a two-step sol-gel process. J Non-Cryst Solids. 1992;145:44-50. (EPS) with a density ranging from 0,02 and 0,64 g/cm3. In our study, the porosity of the foam, which varies from, 87,68 to 79,05%, is comparable with other cellulose based foams7171 Cervin NT, Andersson L, Ng JBS, Olin P, Bergström L, Wågberg L. Lightweight and strong cellulose materials made from aqueous foams stabilized by nanofibrillated cellulose. Biomacromolecules. 2013;14(2):503-11..

In this study, it was observed solid content of foams considerably affected the final density and porosity. In previous studies, the density and porosity of foams produced from chitosan and silica were found between 0.07 g/cm3 to 0.25 g/cm3 and 81.60% to 97.50%, respectively7272 Wang J, Zhou Q, Song D, Qi B, Zhang Y, Shao Y, et al. Chitosan–silica composite aerogels: preparation, characterization and Congo red adsorption. J Sol-Gel Sci Technol. 2015;76(3):501-9.. The density of the samples produced with wood fiber, cellulose nanofiber and cationic polyacrylamide varied between 0.01 g/cm3 and 0.06 g/cm3, and the porosity rate was found over 90%6969 Liu Y, Lu P, Xiao H, Heydarifard S, Wang S. Novel aqueous spongy foams made of three-dimensionally dispersed wood-fiber: entrapment and stabilization with NFC/MFC within capillary foams. Cellulose. 2017;24(1):241-51.. The porosity values ​​of the composite material produced from bacterial cellulose changed between 33.70% to 57.40%7373 Vasconcelos NF, Andrade FK, Vieira LAP, Vieira RS, Vaz JM, Chevallier P, et al. Oxidized bacterial cellulose membrane as support for enzyme immobilization: properties and morphological features. Cellulose. 2020;27(6):3055-83.. The density of foams produced from tapioca starch bleached kraft pulp and chitosan varied between 0.12 g/cm3 and 0.15 g/cm3. In addition, it was determined that the density of the foams increased as chitosan and fiber content increased7474 Kaisangsri N, Kerdchoechuen O, Laohakunjit N. Biodegradable foam tray from cassava starch blended with natural fiber and chitosan. Ind Crops Prod. 2012;37(1):542-6.. Pakornpadungsit et. al. (2020) stated that the porosity value of the materials produced for porous tissue scaffolds from deoxyribonucleic acid sodium salt and chitosan varies between 49% and 62%7575 Pakornpadungsit P, Prasopdee T, Swainson NM, Chworos A, Smitthipong W. DNA:chitosan complex, known as a drug delivery system, can create a porous scaffold. Polym Test. 2020;83:106333.. Chitosan-based aerogels reinforced with microcrystalline cellulose and their density and porosity values ranged 0.03 g/cm3 to 0.11 g/cm3 and 94.73% to 62.30%, respectively1919 Ozen E, Yildirim N, Dalkilic B, Ergun ME. Effects of microcrystalline cellulose on some performance properties of chitosan aerogels. Maderas Cienc Tecnol. 2021;(23):1-10.. As a result, the density and porosity values ​​of the PVAc foams in this study were found to be compatible with the literature.

3.4. Morphological structure

The microstructures of the foams were investigated at x50 magnification using Scanning Electron Microscope as given in Figure 6. The hierarchical irregular pore formations with variable diameters were seen.

The amount of PVAc and bleached kraft pulp were kept constant and the effect of the zinc borate (a, b, c) and different molecular weights of water-soluble chitosan reinforcement (d to l) on the foams were investigated in Figure 6. The increase in zinc borate and water-soluble chitosan amount decreased the inter-pore gap, and the more stringent structures were observed. The freeze-dried foams obviously demonstrate a network interconnected porous structure. Foams exhibits an irregular porous layered structure (Figure 6a, b, c) which is the path left by the ice crystals comprise of same directions after freeze-drying. The structures of the foams change due to the amount of zinc borate and water-soluble chitosan. Water soluble chitosan molecules are taken shape into large blocks after the addition, restricting the mass mobility and affecting their rearrangement at the borders between the growing ice crystals7676 Pojanavaraphan T, Magaraphan R, Chiou B-S, Schiraldi DA. Development of biodegradable foamlike materials based on casein and sodium montmorillonite clay. Biomacromolecules. 2010;11(10):2640-6.. However, increasing the amount of water-soluble chitosan leads in an increase in solution viscosity, which retarding the ice crystal growth when frozen. When the concentration of water-soluble chitosan reaches 70 g/L, the lamellar structure is replaced by a tight architecture (Figure 6f, i, l) due to an increase in solution viscosity and a decrease in mass mobility of precursor solutions7777 Ferreira ES, Rezende CA, Cranston ED. Fundamentals of cellulose lightweight materials: bio-based assemblies with tailored properties. Green Chem. 2021;23(10):3542-68.. Because of the strong hydrogen bonding, when zinc borate is introduced to the precursor solution, the zinc borate is encapsulated by water-soluble chitosan molecules7878 Wang L, Sánchez-Soto M, Abt T, Maspoch ML, Santana OO. Microwave-crosslinked bio-based starch/clay aerogels. Polym Int. 2016;65(8):899-904.,7979 MacKenzie RC. Clay - water relationships. Nature. 1953;171(4355):681-3.. Moreover, some water-soluble struts can be found bridging the gaps between zinc borate particle (Figure 6h, k). It is known that matrices containing hydroxyl groups can form hydrogen bonds with inorganic particles8080 Bakeeva IV, Doktorova AV, Damshkaln LG, Lozinsky VI. A study of cryostructuring of polymer systems. 54. Hybrid organo-inorganic poly(vinyl alcohol) cryogels filled with in situ formed silica. Colloid J. 2021;83(1):49-63..

The amount of bleached kraft pulp was kept constant in this study because our previous study was investigated to effect of bleached kraft pulp amount1111 Ergün ME, Özen E, Yildirim N, Dalkiliç B. Manufacture of wood fiber reinforced polyvinyl acetate rigid foams. Ormancılık Araştırma Dergisi. 2020;7(2):104-12.. The bleached kraft pulp led to decreased the inter-pore gap. Although the morphological structure of nano-fibril cellulose and starch foam material had uniform pore shapes4747 Svagan AJ, Samir MASA, Berglund LA. Biomimetic foams of high mechanical performance based on nanostructured cell walls reinforced by native cellulose nanofibrils. Adv Mater. 2008;20(7):1263-9.,8181 Dash R, Li Y, Ragauskas AJ. Cellulose nanowhisker foams by freeze casting. Carbohydr Polym. 2012;88(2):789-92., morphological structure of PVAc foams was shown to be tight structure and irregular pore shape. The individual particle size of the bleached kraft pulp was in the macro dimension. This macro dimension disturbed interpore gaps and structure. Similar tight and irregular structure were procured with bleached kraft pulp-based foams3434 Ottenhall A, Seppänen T, Ek M. Water-stable cellulose fiber foam with antimicrobial properties for bio based low-density materials. Cellulose. 2018;25(4):2599-613.,6969 Liu Y, Lu P, Xiao H, Heydarifard S, Wang S. Novel aqueous spongy foams made of three-dimensionally dispersed wood-fiber: entrapment and stabilization with NFC/MFC within capillary foams. Cellulose. 2017;24(1):241-51..

Zinc borate particles were aggregated on bleached kraft pulp. When the amount of reinforcement ingredients increased, the density increased, and a tighter structure was formed. In addition, water-soluble chitosan, like PVAc, was found to contribute to the cell wall formation of foams. Also, no foaming agent was used in the foam process. The separation of solvent and solute occur due to most solutes cannot fit within the formed ice crystal structure. So, the final pore morphology was given rise to as ice crystals were sublimated during the drying step8282 Svagan AJ, Azizi Samir MAS, Berglund LA. Biomimetic polysaccharide nanocomposites of high cellulose content and high toughness. Biomacromolecules. 2007;8(8):2556-63.. MatLab software was used to investigate pore size determination. There was a wide range of pore sizes determined. Pore diameters ranged from a few micrometers to several millimeters. The primary cause of this variation is assumed to be related to the manufacturing process, in which the sublimation process could not be regulated uniformly.

In previous studies, the foam - produced with the freeze-drying technique was examined and the morphological structure of foams had a honeycomb appearance4747 Svagan AJ, Samir MASA, Berglund LA. Biomimetic foams of high mechanical performance based on nanostructured cell walls reinforced by native cellulose nanofibrils. Adv Mater. 2008;20(7):1263-9.,8181 Dash R, Li Y, Ragauskas AJ. Cellulose nanowhisker foams by freeze casting. Carbohydr Polym. 2012;88(2):789-92.,8383 Li R, Du J, Zheng Y, Wen Y, Zhang X, Yang W, et al. Ultra-lightweight cellulose foam material: preparation and properties. Cellulose. 2017;24(3):1417-26.. But in this study, it was observed that the morphological structures of the foams were irregular. This situation was related to bleached kraft pulp fibers which were in macro size and these macro-dimensional fibers disrupted the cell structures and spaces. On the other hand, some studies showed that foams produced from natural polymers derivatives such as deoxyribonucleic acid sodium salt, chitosan7575 Pakornpadungsit P, Prasopdee T, Swainson NM, Chworos A, Smitthipong W. DNA:chitosan complex, known as a drug delivery system, can create a porous scaffold. Polym Test. 2020;83:106333., hemicellulose citrate8484 Salam A, Venditti RA, Pawlak JJ, El-Tahlawy K. Crosslinked hemicellulose citrate-chitosan aerogel foams. Carbohydr Polym. 2011;84(4):1221-9., silica7272 Wang J, Zhou Q, Song D, Qi B, Zhang Y, Shao Y, et al. Chitosan–silica composite aerogels: preparation, characterization and Congo red adsorption. J Sol-Gel Sci Technol. 2015;76(3):501-9. had irregular and open-cell pore structures. That results were similar to our study.

3.5. Antibacterial activity test

The antibacterial properties of PVAc foams reinforced with bleached kraft pulp, zinc borate and water-soluble chitosan were researched against gram-negative Escherichia coli and gram-positive Staphylococcus aureus bacteria. PVAc foams were measured inhibition zones around disks against Escherichia coli and Staphylococcus aureus (Table 3).

Table 3
Diameters of inhibition zones against E.coli and S. aureus.

When Table 3 was examined, it was determined that the Z-coded samples did not show antibacterial properties. Although all samples have no antibacterial effect against E. Coli bacteria (Figure 7a), all C coded samples which excepted C-4 and C-7 showed antibacterial effect against S. aureus bacteria (Figure 7b).

Figure 7
a. Antimicrobial activity of foams against E.coli (1. Control, 2. C-5, 3. C-6), b. Antimicrobial activity of foams against S. aureus (1. C-5, 2. C-6).

It was determined that as the molecular weight of water-soluble chitosan decreased, its effectiveness increased on S.aureus bacteria . Also, the antibacterial effect of foams directly proportionates to the amount of water-soluble chitosan. The reason why the samples were not effective against E.coli bacteria in this study; It was thought that the chitosan concentration lower than previous studies8585 Goy RC, Britto D, Assis OBG. A review of the antimicrobial activity of chitosan. Polímeros. 2009;19(3):241-7.,8686 Netanel Liberman G, Ochbaum G, Bitton R. (Malis) Arad S. Antimicrobial hydrogels composed of chitosan and sulfated polysaccharides of red microalgae. Polymer. 2021;215:123353.. On the other hand, it has been reported earlier gram-negative bacteria are more resistant to antibacterial polymers and even show no effect, compared to gram-positive bacteria8787 Özderin S. Determination of some chemical properties of wild pear (Pyrus spinosa Forsk.). BioResources. 2022;17(1):1659-69.,8888 Biswas B, Rogers K, McLaughlin F, Daniels D, Yadav A. Antimicrobial activities of leaf extracts of guava (Psidium guajava L.) on two gram-negative and gram-positive bacteria. Int J Microbiol. 2013;2013:e746165.. Gram-positive bacteria have a mesh-like peptidoglycan coating that allows polymers to penetrate more easily8989 Rameshkumar KB, George V, Shiburaj S. Chemical constituents and antibacterial activity of the leaf oil of Cinnamomum chemungianum Mohan et Henry. J Essent Oil Res. 2007;19(1):98-100.. In previous studies, foams which produced from cellulose and antibacterial thermoplastic starch proved to the effectiveness against E.coli2727 Heydarifard S, Pan Y, Xiao H, M. Nazhad M, Shipin O. Water-resistant cellulosic filter containing non-leaching antimicrobial starch for water purification and disinfection. Carbohydr Polym. 2017;163:146-52.. Polyvinylamine and chitosan were added during the foam production to give the foams water-stability and antibacterial properties. Manufactured foams detected to have a good resistant to Aspergillus brasiliensis, a sporulating mold and Escherichia coli, a common model bacteria3434 Ottenhall A, Seppänen T, Ek M. Water-stable cellulose fiber foam with antimicrobial properties for bio based low-density materials. Cellulose. 2018;25(4):2599-613.. Another study stated that hydrophobically modified chitosan foams had water resistant and very effective for removing bacteria from solution as demonstrated by Escherichia coli and Staphylococcus aureus bacteria. However, produced foams showed a much better bacteria removal performance to Escherichia coli9090 Vo D-T, Lee C-K. Antimicrobial sponge prepared by hydrophobically modified chitosan for bacteria removal. Carbohydr Polym. 2018;187:1-7..

There are various approaches to the mechanism of action of chitosan on bacteria. First approach, electrostatic interaction occurs between the negatively charged bacteria cell walls and the positively charged chitosan, changing the permeability properties of the cell wall membrane. As a result, imbalances occur in osmotic pressure and inhibit the growth of cells. In addition, when chitosan leaks into the cell, it forms complex structures with lipids, fats and proteins. Thus, the cell cannot benefit from lipid, fat and protein. Since this mechanism is based on electrostatic interaction, it is thought that the antimicrobial effect will increase as the number of cationized amines increases8585 Goy RC, Britto D, Assis OBG. A review of the antimicrobial activity of chitosan. Polímeros. 2009;19(3):241-7.. Another approach is thought to be due to the fact that chitosan binds to bacterial DNA and prevents mRNA formation9191 Eikenes M, Alfredsen G, Christensen BE, Militz H, Solheim H. Comparison of chitosans with different molecular weights as possible wood preservatives. J Wood Sci. 2005;51(4):387-94..

Studies with quaternary salts of chitosan were shown that antimicrobial activity against bacteria was greater than chitosan9292 Sadeghi AMM, Amini M, Avadi MR, Siedi F, Rafiee-Tehrani M, Junginger HE. Synthesis, characterization, and antibacterial effects of trimethylated and triethylated 6-NH2-6-deoxy chitosan. J Bioact Compat Polym. 2008;23(3):262-75.. It was stated that the activity of unmodified chitosan against E. coli was 20 fold lower than chitosan modified with N-propyl-N,N-dimethyl , which means that derivatives with cationic charge had especially high activity against bacteria9393 Muxika A, Etxabide A, Uranga J, Guerrero P, de la Caba K. Chitosan as a bioactive polymer: processing, properties and applications. Int J Biol Macromol. 2017;105:1358-68.. At neutral pH, the degree of protonation of NH2 was very low, so the repulsion of NH3+ was weak. Under these conditions, intramolecular and intermolecular hydrogen bonds result in the formation of micro-domains that form hydrophobic and hydrophilic parts in the polymer chain. These results related to the interaction between the bacterial cell wall and water-soluble chitosan8585 Goy RC, Britto D, Assis OBG. A review of the antimicrobial activity of chitosan. Polímeros. 2009;19(3):241-7.,9494 Xie W, Xu P, Wang W, Liu Q. Preparation and antibacterial activity of a water-soluble chitosan derivative. Carbohydr Polym. 2002;50(1):35-40..

3.6. Accelerated weathering test and color measurement

Accelerated weathering test is the most commonly used method for estimating the resistance in the characteristics of the tested material over time. This is a simulated version of natural weathering test in a shorter time frame9595 Liszkowska J, Moraczewski K, Borowicz M, Paciorek-Sadowska J, Czupryński B, Isbrandt M. The effect of accelerated aging conditions on the properties of rigid polyurethane-polyisocyanurate foams modified by cinnamon extract. Appl Sci. 2019;9(13):2663.. When foams are exposed to accelerated weathering and their some properties deteriorate. It gives information about application field where foams can be used. Before and after 50 hours accelerated weathering, foams can be seen in Figure 8.

Figure 8
PVAc foams before (a), after (b) the accelerated weathering test.

The accelerated weathering test was terminated in a short time like 50 hours. Due to the heat, UV light and sprayed water were deteriorated the structure of PVAc which used as the matrix. When PVAc subject to UV light, heat and moisture, the photochemical deacetylation reactions occur such as oxygen derivatives and alkenes chains9696 Bubev E, Georgiev A, Machkova M. ATR-FTIR spectroscopy study of the photodegradation protective properties of BP-4 and 4HBP in polyvinyl acetate thin films. J Mol Struct. 2016;1118:184-93.. Therefore, PVAc foams have a short service life at outdoor applications.

As a result of the accelerated aging test, materials exposed to color change. L*, a*, b* values of PVAc foams before and after accelerated weathering and the changes of ΔE of PVAc foams after 50 hours accelerated weathering were given in Table 4.

Table 4
Total color changes of foams.

When the color values ​​are examined before accelerated weathering in Table 4, L*, a*, b* values ​​of the PVAc foams were ranged from 86.52 to 92.20, -0.30 to -0.80, and 0.76 to 9.44, respectively. It was determined that the a* value and b* values ​​of the samples increased after accelerated weathering. According to the color difference results of the samples, C-2 gave the highest value. The main reason for this change was that the chitosan solution had a yellowish color. As a result, b* values ​​of the foams were increased. As seen in Figure 9, the lowest total color difference of the samples gave Z-3.

Figure 9
Total color differences of bio polymers reinforced foams.

In Figure 9, the difference in total color changes were very insignificant with increasing zinc borate. This result can be explained by the instability of zinc borate under the weathering conditions. Zinc borate was leached or hydrolysed by water or oxidized by UV light9797 Bussiere P-O, Gardette J-L, Rapp G, Masson C, Therias S. New insights into the mechanism of photodegradation of chitosan. Carbohydr Polym. 2021;259:117715.. On the other hand, total color changes of addition of water-soluble chitosan were higher than zinc borate. PVAc foams which included water soluble chitosan led to yellowing. This situation was attributed to the formation of carbonyl groups and the cleavage of glycosidic bonds during thermooxidation and photooxidation9898 Lizárraga-Laborín LL, Quiroz-Castillo JM, Encinas-Encinas JC, Castillo-Ortega MM, Burruel-Ibarra SE, Romero-García J, et al. Accelerated weathering study of extruded polyethylene/poly (lactic acid)/chitosan films. Polym Degrad Stabil. 2018;155:43-51.. Due to the carbonyl functional groups released from the chromophore groups9999 Turku I, Kärki T. Accelerated weathering of fire-retarded wood–polypropylene composites. Compos, Part A Appl Sci Manuf. 2016;81:305-12., the total color change of the water-soluble chitosan added samples increased. In the other study, foams produced from sugar cane fiber, chitosan, starch, and polyvinyl alcohol were increased amount of chitosan, the b* color parameter increased100100 Debiagi F, Mali S, Grossmann MVE, Yamashita F. Biodegradable foams based on starch, polyvinyl alcohol, chitosan and sugarcane fibers obtained by extrusion. Braz Arch Biol Technol. 2011;54(5):1043-52..

4. Conclusions

This study focuses on optimized freeze-drying production process for obtaining highly porous foams and investigation of their physical and morphological properties. The process enables the fabrication of foams with high porosity, up to 87.50% and a low density of 0.12 g/cm3. The optimal reinforcements concentration is found as the compromise between two competing requirements: (i) a concentration as low as possible, to reduce zinc borate and water-soluble chitosan mass and increase porosity; and (ii) a high-water soluble chitosan concentration to avoid foam collapse upon drying. As a result of the accelerated weathering tests, PVAc foam showed low resistance to be used for outdoor application. The color differences of foam which included water-soluble chitosan were so distinctive at end of 50 hours. The foams containing water soluble chitosan had inhibitory activity against S. aureus. It was determined that water-soluble chitosan had more effective antimicrobial activity when both the molecular weight of chitosan was decreased, and the amount of chitosan was increased. On the other hand, more deeply study of some toxicity test will be performed in upcoming study.

These features highlight the high potential of multifunctionality of PVAc foams as porous materials including packaging materials, medical boxes. Other potential indoor and outdoor applications can be provided with hydrophobicity modification of the foams.

5. Acknowledgments

This work was financially supported by Scientific Research Project at Mugla Sitki Kocman University (Project No: 17/246 and 18/012). Authors have a patent pending for this study (TURK PATENT, Application number: 2021/013967). The authors would like to express their sincere thanks to Prof. Dr. Nurettin ŞAHİN who determined the antibacterial activity of the foams from Faculty of Education in Mugla Sitki Kocman University.

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Publication Dates

  • Publication in this collection
    05 Aug 2022
  • Date of issue
    2022

History

  • Received
    14 Nov 2021
  • Reviewed
    12 June 2022
  • Accepted
    19 July 2022
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