Biaxial orientation of polylactide/thermoplastic starch blends

The biaxial stretchability and film properties of polylactide/ thermoplastic starch blends were investigated. Polylactide (PLA) and thermoplastic starch (TPS) were blended in various proportions. Blends containing 27, 42 and 60 wt.% TPS were prepared via a twin-screw extrusion process. Interfacial modification was performed by grafting the PLA with maleic anhydride. These blends were subsequently cast into sheets and biaxially drawn using a laboratory biaxial stretcher The morphology of extruded strands and cast sheets was investigated using scanning electron microscopy. The compatibilized blends exhibited a much finer morphology as well as preferential minor phase size orientation along the machine direction. The addition of starch did not affect significantly the biaxial stretchability of the pure PLA. Even at high starch content (60 wt. %), the biaxial draw ratio at which the sample breaks is very similar to that of the pure PLA. At higher temperatures, all blends could be stretched at significantly much higher biaxial draw ratio and very thin films could be obtained. The starch content and processing variables affected the tensile properties.