Influence of Plasticizers on Thermal and Mechanical Properties and Morphology of Soy-Based Bioplastics

Bioplastics from soy protein and biodegradable polyester amide with plasticizers including glycerol, d-sorbitol, and blends of these two have been made using extrusion and injection molding. The thermal properties, mechanical properties, and morphologies of the soy-based bioplastics were evaluated with a dynamic mechanical analyzer (DMA), a united testing system (UTS), and an environmental scanning electron microscope (ESEM). The influence of plasticizers on the physical properties of the soy-based bioplastics was determined. It was found that sorbitol-plasticized soy-based bioplastic (SSBP) had a higher tensile modulus and tensile strength than glycerol-plasticized soy-based bioplastic (GSBP). GSBP had the highest impact strength, whereas SSBP had the highest thermal stability. The mixed plasticized soy-based bioplastic (MSBP) that was obtained using a mixture of glycerol and d-sorbitol showed an intermediate range of tensile modulus and tensile strength values in comparison to those of SSBP and GSBP. The ESEM results suggested that SSBP had brittle fracture features, whereas GSBP had local ductile fracture features, which is consistent with the results obtained for the mechanical properties. The glass transition temperatures from DMA measurements of the soy protein and polyester amide in the soy-based bioplastics differed from those of the neat components. Using Fox equation calculations, this was interpreted as compatibility between the soy protein and polyester amide, which causes thermal and mechanical changes in different soy-based bioplastics.