Roles of Soft Segment Length in Structure and Property of Soy Protein Isolate/Waterborne Polyurethane Blend Films

Waterborne polyurethanes (WPUs) with varying lengths of soft segment were synthesized and used to physically modify soy protein isolate (SPI), a protein with good film-forming ability and oxygen barrier characteristic but poor toughness and water resistance, via a simple blending technique without any compatibilizer. The effects of soft segment structure of WPU on the structure and property of resultant blend films were investigated. The tensile strength, elongation at break, and water resistance of the SPI-based films were improved and modulated by adding WPUs with different lengths of soft segment. Compared with neat SPI film, the equilibrium water uptake was decreased by 138% at most for a blend film, and the tensile strength and elongation at break were improved by 96% and 82%, respectively. All the blend films possessed uniform cross-section structures due to the strong interactions between SPI and WPU. Moreover, the blend films exhibited a satisfied water vapor barrier property. It was proved that the modification of SPI with WPU might be an effective way to fabricate SPI-based films for practical applications.