Injection-molded thermoplastic cassava starch modified with single and mixed polyol plasticizers

Increasing interest in biodegradable and cost-effective materials derived from renewable resources has led to the development of injection-moldable thermoplastic starch (TPS). Plasticizers constitute an indispensable additive for manufacturing TPS, influencing both its thermomechanical processability and performance. The aim of the current work is thus to study the effect of single and mixed polyol plasticizers on the performance of injection-molded TPS. This study was carried out by using three types of polyols - glycerol, xylitol, and sorbitol- as single and equal-weight binary mixed plasticizers to modify cassava starch via extrusion. The resulting TPS extrudates were converted to test specimens via injection molding. Melt flow index, moisture content, water contact angle, and tensile and dynamic mechanical thermal properties of the materials were examined. In addition, Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis were also conducted. The results show that the higher-molecular-weight polyols (xylitol and sorbitol) exhibited lower plasticizing efficiency but formed denser hydrogen-bonding interactions with starch molecules than the smaller-molecular-weight one (glycerol). In both single and mixed plasticizer systems, increasing the molecular weight of the polyol plasticizer led to enhancements of the tensile strength (up to 27 MPa (200 % increment) and 26 MPa (115 % increment) for single and mixed plasticizer systems, respectively), Young's modulus (up to 386 MPa (240 % increment) and 458 MPa (185 % increment) for single and mixed plasticizer systems, respectively), decomposition temperature (2-3 °C), and glass transition temperature (30-50 °C) of TPS and a reduction in its moisture content (60-80 %) and surface stickiness, though disadvantages included poorer thermomechanical processability and higher melt viscosity and brittleness. In conclusion, using a co-plasticizer of glycerol and xylitol is appropriate for injection-molded TPS articles considering both processability and performance. The obtained TPS has potential applications in edible or single-use biodegradable rigid products, such as pet chew toys, chopsticks, cutlery, plant pots, etc.