Properties of injection moulded starch/synthetic polymer blends - I. Effect of processing parameters on physical properties

Corn starch was melt-blended in a twin screw extruder with ethylene vinyl acetate (EVAMA) and polyethylene (EMA) each having a maleic anhydride functional group that could interact with the hydroxyl groups in the starch. The percentage of starch in the blend was 70% by weight. These blends were injection moulded at different melt temperatures, screw speeds, injection pressures and back pressures. The effect of these processing parameters on the tensile strength, percentage elongation, break energy, flexural strength and flexural modulus are reported. Results from statistical analysis indicate that temperature and weldline affected some of the properties. Samples with weldline had lower tensile strength, break energy, and percentage elongation for starch/EVAMA blends. Similarly, samples with weldline had lower tensile strength and flexural modulus for starch/EMA blends. Starch/EVAMA blends absorbed between 22% and 37% by weight of water over a three month period, while starch/EMA blends absorbed between 20% and 27% over the same period. The diffusion mechanism varied from Fickian to anomalous, depending on the process conditions. For both blends, the process parameters had similar effects on the equilibrium water up-take and relaxation time. The major thermal degradation products were ethanoic acid and methanoic acid as identified by head-space-gas chromatography. Starch/EVAMA formed four times more ethanoic acid than the starch/EMA blend did during ageing, probably due to the cleavage of the acetate group in the vinyl acetate part of the EVAMA.