Isothermal cold crystallization kinetics and properties of thermoformed poly(lactic acid) composites: effects of talc, calcium carbonate, cassava starch and silane coupling agents

Abstract

The effects of three different fillers (i.e., talc, calcium carbonate, and cassava starch) and surface functionalization by 3-aminopropyltriethoxysilane (APTES) and vinyltriethoxysilane (VTES) on morphology, thermal and tensile properties of the poly(lactic acid) (PLA) composites were comparatively examined. Dynamic differential scanning calorimetry (DSC) results revealed that the incorporation of filler can facilitate the cold crystallization of PLA, as confirmed by lowered cold crystallization temperature. By fitting DSC data with Avrami model, the highest isothermal cold crystallization rate constant k and the shortest crystallization half time t1/2 were obtained for the PLA/talc composites under isothermal temperature of 100 °C, implying that talc was the most effective nucleating agent for PLA in this study. The average Avrami index n of neat PLA and its composites lay within the same range of 3.0–3.6, which reflected the three-dimensional spherulitic growth of PLA with the mixture of instantaneous nuclei and sporadic nuclei. In comparison with the composite cast films, the thermoformed films had higher degree of crystallinity as well as higher tensile strength and Young’s modulus owing to the chain orientation upon annealing. Furthermore, the addition of silane-treated talc, especially APTES-treated talc, fastened cold crystallization rate and enhanced tensile properties because of the improved interfacial interaction between talc particles and PLA matrix.