Preparation and properties of highly water-resistant thermoplastic starch/polyvinyl acetate/corn straw fiber composites

Abstract

Thermoplastic starch (TPS) is a promising renewable and biodegradable material, but its poor water resistance and mechanical properties severely restrict its practical application in packaging and disposable container fields. The aim of this study is to improve the water resistance and mechanical performance of TPS by incorporating polyvinyl acetate (PVAc) and corn straw fiber (CSF). In the study, TPS/PVAc/CSF composites were prepared via melt blending, in which PVAc and CSF contents ranging from 27 wt% to 57 wt% (mass ratio of CSF to PVAc fixed at 27/10, g/g); meanwhile, a TPS/CSF composite without PVAc (TPS/CSF30) was prepared as a control. Various methods were applied to systematically evaluate the comprehensive properties of the products. Differential scanning calorimetry (DSC) results revealed that PVAc could inhibit the crystallization of TPS and formed a semi-crystalline structure at a TPS/PVAc mass ratio of 4.23/1, while the melting temperature (T_m) of the composites increased with the increase of PVAc and CSF contents, with PVAc reducing the T_m of TPS and CSF exerting the opposite effect. Scanning electron microscopy (SEM) images demonstrated that the fracture surfaces of TPS/PVAc/CSF composites were more rough and glossy than that of TPS/CSF30, and CSF had good compatibility with the matrix except for the sample with 57 wt% PVAc/CSF (TPS/PVAc/CSF57). Tensile test results indicated that the tensile strength of each TPS/PVAc/CSF composite was higher than that of TPS/CSF30, besides, with the increase in PVAc/CSF content, the tensile strength of TPS/PVAc/CSF composite first increased and then decreased, reaching the maximum value at 47 wt% PVAc/CSF, which was 1.5 times that of TPS/CSF30. Water resistance test results showed that samples containing PVAc had excellent water resistance, which improved with the increase of PVAc content. For instance, TPS/PVAc/CSF57 maintained a tensile strength of 14.3 MPa after soaking in water for 5 min, with a retention rate of 74.5%. Water absorption test results revealed that TPS/PVAc/CSF composites exhibited rapid water absorption at the initial stage, followed by a gradual slowdown. Nearly no mass loss was observed, demonstrating their favorable water-soluble substance retention ability. Soil burial biodegradation test results confirmed that TPS/PVAc/CSF composites still possessed biodegradability. It is concluded that TPS/PVAc/CSF composites prepared in this study exhibit high water resistance, improved mechanical properties, and retained biodegradability, making them suitable for short-term disposable containers for high-moisture items and promising green alternatives to traditional petroleum-based materials.