Plasticization of extruded thermoplastic starch via xylitol–glycerol low transition temperature mixtures: synergistic effects compared to conventional polyol systems

Thermoplastic starch (TPS) blends are promising bioplastics as an alternative to conventional thermoplastics, due to their compostability, biodegradability, low cost and biocompatibility. However, TPS requires bio-based, abundant, cost-effective and efficient plasticizers to achieve adequate processability, low plasticizer migration and water absorption. This study investigates the behaviour of starch films, processed by extrusion, plasticized by a previously formed low transition temperature mixture of xylitol and glycerol, compared to the effect of the same polyols just directly mixed with starch. Two different molar ratios, one equimolar (1:1) and another glycerol-rich (1:3) were studied, finding that, at equimolar conditions, the formation of the low transition temperature mixture prevents xylitol recrystallization after extrusion, yielding better transparency and mechanical performance (61.7 % increase in terms of tensile strength). In addition to this, at glycerol-rich mixtures, the formation of the liquid phase low transition mixture yielded better improvements in terms of tensile strength (42.9 %) and modulus (283 %), a decrease in water absorption by 19.4 % and a decrease of plasticizer migration by 34.8 %. These results confirm that greater interaction with starch and limited polyol mobility can be achieved when the formation of a low transition temperature mixture is considered.