Material properties and glass transition temperatures of different thermoplastic starches after extrusion processing

Four different starch sources, namely waxy maize, wheat, potato and pea starch were extruded with the plasticizer glycerol, the latter in concentrations of 15, 20 and 25% (w/w). The glass transition temperatures of the resulting thermoplastic products were measured by Dynamic Mechanical Thermal Analysis (DMTA). Beside mechanical and structural properties also the transition temperatures of the materials were evaluated during tensile and impact tests. Above certain glycerol contents, dependent on the starch source, a lower glass transition temperature T-g resulted in decreased modulus and tensile strengths and increased elongations. Lowering the T-g at different glycerol contents did not influence the impact strength. When the amylose/amylopectin ratio increased a decrease in T-g was found. For pea, wheat, potato and waxy maize starch the T-g was 75degreesC, 143degreesC, 152degreesC and 158degreesC, respectively. Therefore products with higher percentages of amylose are more flexible. The shrinkage of the specimens made by injection molding was considerable compared to the specimens made by pressing.