Study of Rheological Properties of Biodegradable Polyesters

Abstract

Biodegradable polyesters are considered as one of the most cost effective and environmentally friendly solutions to waste-disposal problems associated with traditional thermoplastics. The technologies for converting the resins into useful items require knowledge about the rheological properties of these materials. Adequate rheological models are essential for the design and optimization of the process technologies. Rheological properties of two commercial biodegradable polyesters- poly(lactic acid) (PLA) and aliphatic-aromatic co-polyester (AAC) Ecoflex -have been investigated using parallel plate and capillary rheometers. Results from a study on the extrusion instabilities of biodegradable polymers are reported for the first time. The experimental studies found that the biodegradable polyesters exhibit pseudoplastic (shear-thinning) behaviour and the Cox-Merz rule is obeyed. A Cross model was proposed to describe their shear-thinning behaviour. The viscosity of both PLA grades is more temperature sensitive than the viscosity of Ecoflex. It was observed that the extensional viscosity of Ecoflex is larger than that of PLA and that the extensional viscosity of biodegradable polymers is similar to that of LLDPE. The experimental results indicate that biodegradable polymer melts slip at the die wall. It was observed that with increasing shear rate PLA exhibits sharkskin and gross melt fracture while Ecoflex exhibits only gross melt fracture. With regards to flow instabilities PLA behaves like linear polyolefins, however without exhibiting stick-spurt phenomenon. While Ecoflex behaves like branched LDPE, its gross melt fracture starts at higher values of wall shear stress than LDPE. Both biodegradable materials exhibit small extrudate swell: up to 28% for PLA and up to 34% for Ecoflex, which is comparable to that of rigid PVC. It was observed that biodegradable polymers substantially degrade during extrusion processing. It was also found that blending PLA and Ecoflex produced immiscible blends. Melts of these blends exhibited sharkskin and gross melt fracture at higher shear stresses than the neat resins. This effect was attributed both to degradation during blending and to some sort of lubricating effect.