Structure and Properties of Soy Protein Plastics with ε-Caprolactone/Glycerol as Binary Plasticizers

We successfully prepared a series of soy protein isolate (SPI) plastics with epsilon-caprolactone (CL)/glycerol binary plasticizers via extrusion and compression-molding. The chemical reactions among SPI, CL, and glycerol as well as the influence of CL/glycerol content on the microstructure, thermal degradation, and mechanical properties have been investigated using Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), thermogravimetric analysis-Fourier transform infrared spectroscopy (TGA-FTIR), and mechanical tests. The results of FTIR, SEM, DSC, and DMTA revealed that CL reacted with protein and glycerol molecules under high-temperature, high-shear, and high-pressure conditions. When the CL content was low (less than 25 wt %), the CL added to the protein matrix was dispersed mainly in the glycerol-rich domains and reacted with glycerol. However, at a higher concentration, the CL predominated in the protein-rich domains and reacted with protein molecules. The chemical reactions led to a significant increase in glass transition and mechanical relaxation temperatures of the glycerol-rich and protein-rich domains. Accordingly, the chemical reactions retarded the volatilization of glycerol and the release of NH3 and CO2 and elevated the tensile strength, Young's modulus, and the water resistance of the soy protein plastic sheets.