Over the past few years due to the growing awareness of environmentally sustainable technology, biocomposite or green composite material has been a major concern. The use of petrol-based polymeric materials is caused some environmental effects such as toxicity, ecological problems [1,2]. Biodegradable natural and synthetic polymers offer the solution to these problems. Biocomposites may be obtained by the blending of bio-based materials (lipids, starch, proteins) and synthetic biopolymers (poly (lactic acid), poly (vinyl alcohol)) [2]. Proteins have good film-forming properties and they are used for the food packaging applications. Casein is a milk protein and it has some exceptional properties such as operative barriers to gases and aroma, transparency. However, casein films have relatively low mechanical properties so its mechanical properties should be improved [3]. The plasticizers and polymers are good solutions to provide them improve the mechanical properties.
The aim of this study is to determine the effects of plasticizer on the properties of casein-polymer packaging films. Mechanical properties of the casein films can be increased with the addition of synthetic biopolymers. Poly (vinyl alcohol) (PVA) is one of the most willingly biodegradable and water-soluble vinyl polymers. It is easily accessible, cheap and compatible with proteins. It offers high gas barrier property, tensile strength and flexibility [1,4]. On the other hand, casein-based films have brittle structure due to the hydrogen bonding between protein chains. Plasticizers are small molecular weight molecules which can use to provide flexibility. Selection of plasticizer is based on compatibility between protein and plasticizers. Polyol type hydrophilic plasticizers obstruct the interactions between the protein chains and make the films more flexible. Sorbitol is one of the plasticizers that make biodegradable films more flexible [5].
In our study, casein was plasticized with the different amount of sorbitol and then blended with PVA to enhance the film properties. The biodegradable films were prepared by solution casting method. Casein and sorbitol dissolved in 5% triethanolamine solution at 40 oC to obtain plasticized casein film. Sorbitol loading level was changed as 0, 10, 15, 20, 25 % in the casein. Meanwhile, PVA dissolved in the water at 80 oC. After that plasticized casein and PVA solutions were blended at the ratio of 20%:80% and stirred at 40 oC for 15 min. The total solid amount the all solutions was fixed as 6%. The film-forming plasticized casein/PVA solutions were spread onto a polystyrene petri plate end of the stirring process. Then, they left to dry at room conditions for 2 days. After the drying, films peeled off from the petri plate. The un-plasticized casein/PVA film (0% sorbitol) is used as a control sample and used to compare the properties of plasticized films. The characterization of the films was carried out by using thermogravimetric analysis (TGA), tensile test, differential scanning calorimetry (DSC), water vapor permeability (WVP) and Fourier-transform ýnfrared spectroscopy (FTIR).
The tensile test results showed that tensile strength of un-plasticized casein/PVA film was 35 MPa and this value decreased with the addition of sorbitol. It was seen that sorbitol increased the strain at break value of un-plasticized film. According to TGA, thermal degradation temperature and char residue of un-plasticized film were slightly decreased by incorporation of sorbitol. In the DSC results, a reduction was seen at the melting temperature of un-plasticized film with increasing amount of sorbitol. It was concluded that the optimum sorbitol loading level is 15% at the end of the all characterization tests.
Anahtar Kelimeler: Biodegradable film, Poly (vinyl alcohol), Casein, Plasticization
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