Synergetic Effect of Dual Compatibilizers on in Situ Formed Poly(Lactic Acid)/Soy Protein Composites

In this work, biobased poly(lactic acid) (PLA)/soy protein concentrate (SPC) composites were prepared by twin screw extrusion and injection molding. Poly(2-ethyl-2-oxazoline) (PEOX) and polymeric methylene diphenyl diisocyanate (pMDI) were used sequentially as compatibilization agents to improve phase morphology and interfacial bonding. Properties of the PLA/SPC composites were significantly improved by the application of PEOX and pMDI. The SPC phase was refined and stretched into fine threads during processing under the compatibilization effect of PEOX. With only 0.5 part pMDI, the tensile strength of PLA/SPC composite increased significantly to approach that of pure PLA. With 1 part pMDI, the tensile strength was 6% higher than that of the neat PLA. Scanning electron micrographs evidenced enhanced interfacial adhesion between the two phases. Dynamic mechanical analysis tests revealed that the presence of pMDI enhanced the storage modulus of the composite, especially at temperatures above the glass-transition temperature of PLA, due to the strong interactions between the PLA and SPC phases after pMDI compatibilization. The compatibilized PLA/SPC blends also exhibited significantly reduced water uptakes. Fourier Transform Infrared Spectroscopy confirmed the occurrence of PLA grafting onto SPC molecules through pMDI compatibilization.