Stress relaxation of starch/synthetic polymer blends

Stress relaxation behaviour of injection-moulded starch/synthetic polymer blends were studied. In one experiment, the starch content was kept constant at 70% while the amylose to amylopectin ratio was varied. The synthetic polymers in the blend included high-density (HPDE) and low-density polyethylene (LPDE), and ethylene vinyl acetate (EVA). In the second experiment, the starch content in the blend was varied. A small amount of maleic anhydride-functionalized synthetic polymer (5% by weight) was added to compatibilize reactively the starch and the synthetic polymer. Starch/HDPE and starch/LPDE blends had a ductile behaviour, while starch/EVA blends displayed rubbery characteristics. Blends containing EVA relaxed the fastest while those containing LPDE took the longest time. A double logarithmic plot of stress versus time at constant strain was linear and the slopes of the plots were insensitive to starch type but were affected by the starch content. Residual internal stress which developed during moulding was also estimated from the stress relaxation measurement. The stress - relaxation data fitted several empirical stress-time non-linear models well.