Synergistic effects of chain extenders and natural rubber on PLA thermal, rheological, mechanical and barrier properties

Abstract

Poly (lactic acid) (PLA) has high potential to replace conventional petroleum-based plastics in a wide range of applications. However, the high brittles, thermal degradation, low melt strength and low viscosities of PLA pose challenges for its processing and performance satisfied for industrial use. Previously, we have shown that natural rubber (NR) incorporated into PLA improved the melt strength and viscosity and thus the processability of biopolyesters. Chain extension is a practical and cost-effective method of improving PLA processability by enhancing the molecular weight and re-bonding degraded chains. We hypothesized that addition of epoxy-based chain extenders will work synergistically with natural rubber to improve the elongational viscosity and strain hardening of PLA. The thermal transition, thermal degradation, mechanical and moisture barrier properties were studied to provide a comprehensive view of the chain-extended and NR-modified PLA. The results shows that chain extender with higher functionality (Joncryl 4368) had a more significant effect on chain extending, branching and restoring of PLA. Although the chain extender or the NR alone reduced both the dynamic and steady shear viscosities and minimally affected the elongational viscosity of PLA, they together resulted in shear-thickening and strain-hardening of PLA, suggesting a synergistic effect of the chain extender and NR on improving PLA rheological performance. The improved elongational viscosity of PLA was comparable to film grade low density polyethylene (LDPE). Additionally, NR addition significantly reduced the water vapor permeability of PLA. The PLA with improved melt viscosity and moisture barrier are promising for film and flexible packaging applications.