Fabricating highly hydrophobic thermoplastic starch/polyvinyl alcohol composites via twin-screw extrusion using citric acid–chitosan co-plasticizers

Abstract

As one of the most prevalent polymers in nature, starch has the advantages of biodegradability, non-toxicity and low cost. Typically, starch granules can be broken down by gelatinization and plasticization in the presence of water and plasticizers to obtain thermoplastic starch (TPS). In this work, TPS was prepared by optimizing the water content (50 wt%) for corn starch gelatinization. TPS/polyvinyl alcohol (TPS/PVA) composites were successfully fabricated via extrusion and injection molding, using citric acid (CA) and chitosan (Cs) co-plasticizers. Structural analysis with ATR-FT/IR confirmed that CA and Cs underwent esterification and N-acetylation reactions with TPS and PVA, forming a cross-linked structure. Incorporating CA and Cs improved TPS/PVA composite compatibility with inorganic fillers, enhancing surface smoothness and processing stability. However, ATR-FT/IR, viscometer, oscillating shear rheometer and DSC tests demonstrated that when CA addition exceeded 15 wt%, TPS underwent significant acid hydrolysis of glycosidic bonds, resulting in decreased viscosity and even the disappearance of crystallinity. The findings showed that when CA addition was 10 wt% (TC10), the mechanical properties of the TPS/PVA composite improved significantly; accordingly, its tensile strength, elastic modulus, flexural strength, and flexural modulus increased by approximately 200%, 660%, 290%, and 220%, respectively. Furthermore, TC10 exhibits remarkable hydrophobicity, with its contact angle increasing from 75° to 110°. CA–Cs co-plasticization addresses the migration problem of conventional low molecular weight plasticizers under high temperature, significantly enhancing blending uniformity, mechanical properties, and water resistance. TPS/PVA composites are expected to become an ideal choice for environmentally friendly products.