Properties of a plasticised starch blend - Part 2: Influence of strain rate, temperature and moisture on the tensile yield behaviour

The tensile yield behaviour of a commercial plasticised starch/high molecular weight polyol blend has been studied. The effects of moisture content, strain rate and temperature were investigated and related to dynamic mechanical measurements in order to seek some molecular understanding of the yield behaviour. Dynamic mechanical thermal analysis revealed the presence of two relaxations associated with the glass transition of each phase of the blend. The effect of moisture could be explained by a 'time-moisture' superposition demonstrating that the effect of the water content was analogous to the effect of temperature. The variation in the yield stress with strain rate and temperature was described accurately by a two-process Eyring's model. The process, which dominates the deformation behaviour at low strain rates and/or high temperatures correlated well with the relaxation of the plasticised starch-rich phase, while the second process involved at higher strain rates and/or lower temperatures could be associated with the relaxation of the polyol-rich phase.