Properties of extrusion cast sheets of blends of natural and aliphatic polyesters

Natural and synthetic polymers of various compositions were blended in a twin-screw extruder. These blends were then sheeted into thin sheets with a coat hanger die attached to a single-screw extruder. The natural content in the blend was varied between 5 and 50 wt%, and the mechanical and morphological properties of the blends were evaluated. At 50 wt% natural content, the tensile strength decreased to a third of that of the synthetic polymer. The use of a compatibilizer doubled the tensile strength for the 50 wt% natural content blend. The sheets displayed equal strengths in the machine and transverse direction. The tear strength decreased as the natural content increased, and the decrease was greater in the anhydride-compatibilized blends than in the uncompatibilized blends. The blends displayed two distinct glass transitions, one for each component, indicating phase separation. The crystallinity of the blends decreased as the starch content increased. This result was confirmed by differential scanning calorimetry (DSC), which showed that the melting endotherm decreased as the starch content increased. Gel permeation chromatography (GPC) results showed that the peak position was at the same location irrespective of blend composition, indicating minimal degradation of starch moieties. The water absorption was diffusion controlled, with a sharp initial burst of water uptake. Scanning electron microscopy (SEM) showed melting of starch granules that formed a co-continuous phase with the synthetic polyester. Increasing the natural content also increased the surface roughness of the sheets.