Creep in injection molded starch/synthetic polymer blends

Creep in uniaxial extension was measured in starch/synthetic polymer blends. In the first experiment, the starch content was kept constant at 70% and the applied stress and temperatures were varied. In a second experiment, the starch content was varied from 0 to 70% at a constant applied stress and varying temperature. The synthetic polymers in the blend were either polybutylene succinate (PBS) or polycaprolactone (PCL), both semi-crystalline biodegradable polyesters. Maleic anhydride-functionalized polyesters (5% by weight) were used to reactively compatibilize starch and the synthetic polymer. The compliance of these blends was found to increase with increasing applied stress and temperature and decrease with increasing starch content. Addition of small amounts of starch was found to significantly inhibit creep. Time-temperature superposition was attempted for blends containing 30, 50 and 70% starch. The compliance master curve was successfully obtained for the blends containing 70 and 50% starch, but empirically estimated shift factors did not obey the WLF equation. WLF constants could be calculated based on the empirical shift factors and were in the range obtained for other polymers. The compliance data could be fitted to several empirical models. Excellent fits were obtained for compliance at different stress levels and temperatures.