Plastic performance of soybean protein components

Soybean proteins recently have been considered as petroleum polymer alternatives in the manufacture of adhesives, plastics, and various binders. The objective of this work was to characterize the plastic performance of soybean protein components during molding processes. Two major soybean protein fractions, 7S-rich globulin (7S-RG) and 11S-rich globulin (11S-RG) were separated from defatted soybean flour, and their purity was examined by sodium dodecyl sulfate-polyacrylamide gel eletrophoresis and high-performance liquid chromatography. The thermal transition properties of the two fractions at 10% moisture content were 137.6°C for 7S and 163°C for 11S, as analyzed using differential scanning calorimetry (DSC). Plastics were prepared using a hot press at various molding temperatures that were selected based on the proteins’ thermal transition temperatures obtained by DSC. The plastics were evaluated for mechanical properties, water absorption, and microstructure. The plastics prepared with temperatures at or close to the thermal transition temperature showed a smooth, uniform, and complex structure. Results showed that the plastics made from 11S-RG at its thermal transition temperature were stronger (35 MPa) and had lower water absorption than those made from 7S-RG at 145°C (26 MPa). The plastics made from the 7S- and 11S-RG mixture had the highest tensile strength (39 MPa) and medium water absorption compared to those made from 7S- and 11S-RG alone. These mechanical properties and water absorption behaviors were significantly affected by molding temperatures. The results obtained from this research indicated that interaction between 7S- and 11S-RG could occur during molding and that thermal transition temperature played an important role in thermal processing of soybean proteins.