THE EFFECT OF DIFFERENT CONCENTRATIONS OF GLYCEROL ON PROPERTIES OF BLEND FILMS BASED ON POLYVINYL ALCOHOL-ALLYSUMHOMOLOCARPUM SEED GUM

Abstract

Introduction:Packaging is an important factor in food industry and is dominated by petroleum-derived polymers. Therefore, the amount of research involving the production and characterization of biodegradable films has increased substantially, mainly due to interest in minimizing the ecological impact caused by the use of synthetic packaging materials. Several biopolymers have been exploited to develop eco-friendly food packaging materials. Usually, films based on biopolymers are highly sensitive to environmental conditions and generally present low mechanical resistance. As a result, several researchers have developed films based on mixtures of biopolymers and synthetic polymers. In order to increase the workability and flexibility of biodegradable films, various plasticizers, usually poly-ols, have been widely used, glycerol being one of the most preferred and most studied. Plasticizers reduce intermolecular forces, increase the mobility of the biopolymer chains and thereby improve the mechanical properties of the films. Therefore, the aim of the present study were to investigate the effect of different proportions (20e100% w/w) of plasticizer (glycerol) on physicochemical, mechanical, permeability, surface and thermal properties of biodegradable PVA-AHSG blend films. Polymer blending is one of the most effective methods to have new material with desired properties. Films formed by blending of polymers usually results in modified physical and mechanical properties compared to films made of individual components. Since synthetic polymers are easily obtained and have low production cost, blending of natural and synthetic polymers improves the cost performance ratio of the resulting films. Since Alyssum homolocarpumseed gum (AHSG) is environmentally friendly due to its biodegradability and has good film forming properties, it is considered as a very promising biopolymer. Some synthetic polymers from non-renewable sources are also biodegradable, such as polyvinyl alcohol (PVA). PVA is a synthetic, water soluble polymer with excellent film forming, emulsifying, and adhesive properties. It also imparts good tensile strength (TS) and biodegradability and hence has been used in many biomaterial applications. PVA has also been approved for use in packaging meat and poultry products by the USDA (DeMerlis&Schonek, 2003). AHSG contains free hydroxyl and amine groups, and is therefore miscible with PVA due to the formation of hydrogen bonds. Materials and methods: The aim of this study was to investigate the possibility of producing a novel biodegradable blend film from PVA-AHSG with glycerol as plasticizer in the different concentrations. Films were prepared by the casting method using PVA and AHSG (60:40 ratio). Glycerol was used ac plasticizer because it is compatible with PVA-AHSG blend improving film flexibility, facilitating its handling and preventing cracks. The PVA–AHSG blend film was prepared with different glycerol concentration (20–70%, w/w).The optical properties such as opacity and color were measured. Water vapor permeability, moisture content, water solubility and density of the films were also investigated. Films were evaluated for mechanical and antitoxin properties. The PVA–AHSG blend films were characterized using DSC, FTIR and scanning electron microscopy. Results and Discussion: The results of this study showed that blend of PVA and AHSG could be used as a new film-forming material. However, it was not possible to make PVA-AHSG blend films without addition of glycerol as a plasticizer to the formula. Glycerols in 20-70% (w/w) concentration were used to prepare the blend films. At the level of 20% (W/W) of glycerol, PVA-AHSG blend films had the lowest thickness (0.065 mm), moisture sorption (118.76%), water vapor permeability (WVP) values (4.9 g mm m-2 kPa−1 d-1), elongation at break (EB)(2.1%), moisture content (22.5%) and water solubility (16.6%) and the highest values for tensile strength (TS)(64.6 MPa), young modulus (YM) (892 MPa),density (0.109 g cm-3),opacity (0.069 A/mm) and water contact angle (74.52◦). Increasing of glycerol concentration in PVA-AHSG blend films resulted in increase in water vapor permeability and percent of elongation while, decreased tensile strength and surface hydrophobicity. Increasing the glycerol concentration significantly (p < 0.05) diminished initial water contact angle of films from 74.52◦ to 37.80◦. It has been shown that the addition of plasticizers diminished the films’ water contact angle, which in turn, decreased hydrophobicity of the films. The higher hydro-philicity of the samples is attributable to the hygro-scopicity (water-binding capacity) of the plasticizer. Plasticizer can diminish interactions between biopolymer molecules and increase solubility due to its hydrophilic nature, giving the polymer molecules higher affinity to attract water. The moisture content increased significantly from 22.5% to 40.9% as the plasticizer content increased (p < 0.05). Because of glycerol acts as a water-holding agent, with the higher number of water molecules in glycerol-plasticized films increasing plasticizing activity.WVP increases as plasticizer content of the film increases due to its hydrophilic nature. WVP can be directly related to the quantity of OH group on the molecule. Also, environmental conditions can significantly affect the WVP. Increasing plasticizer concentration decreased the intermolecular forces between polymer chains and increased free volume and segmental motions, allowing water molecules to diffuse more easily and giving a higher WVP. Mechanical strength of films decreases due to plasticizer addition resulting in decreased tensile strength and increased elongation. The measurement of color values showed that by the increasing of the glycerol concentration in polymers blend matrix, the b and L values increased while a value decreased. Furthermore, the addition of glycerol promoted the interactions among PVA, AHSG and glycerol through hydrogen bonding as reflected on the shifting of main peaks of the glycerol-free film to higher wavenumbers as shown by FTIR spectra. Microscopic views indicated smooth and uniform surface morphology without obvious cracks, breaks, or openings on the surfaces after the incorporation of glycerol as a plasticizer. Scanning electron microscopy showed that the microstructure of PVA-AHSG blend films have a critical effect on their physical and mechanical properties that is important in food packaging applications.