Tailoring Water Sensitivity of Soy Protein Bioplastic Films via Plasticizer Formulation and Compression Molding

To develop sustainable alternatives to petroleum-based plastics, this study investigated the solvent-free thermomechanical processing of soy protein isolate (SPI) plasticized with glycerol and water. The study compounded SPI/glycerol (70/30, 60/40, 50/50 wt%) and SPI/glycerol/water (40/30/30 wt%) blends in an internal mixer at 90 °C and then compression-molded them at 120, 150, and 180 °C. Torque curves indicated that higher glycerol contents decreased melt viscosity and mitigated viscous dissipation heating. Water acted as an effective co-plasticizer, promoting melt fluidization and lowering the Specific Mechanical Energy (SME) relative to formulation 50GLI. Compression molding at 150 °C ensured optimal film cohesion and chain unfolding, whereas 120 °C restricted melt flow and 180 °C caused severe Maillard browning. Moisture sorption at 75% relative humidity increased at higher processing temperatures and was highest for 30H2O. Conversely, water solubility after 24 h was primarily driven by plasticizer content via glycerol leaching (30-52 wt%), while thermally crosslinked networks preserved film integrity. Overall, thermomechanical processing of SPI offers a promising, scalable route for disposable biodegradable packaging.