Influence of equilibrium relative humidity and plasticizer concentration on the water content and glass transition of starch materials

Sorption behaviour and calorimetric glass transition were measured on cast starch films plasticized with varying concentrations of different components (glycerol, sorbitol, lactic acid sodium, urea, ethylene glycol, diethylene glycol, PEG 200, glycerol diacetate). Precision analysis showed that the water level in samples conditioned at 57% relative humidity reached a minimum for a plasticizer content of 10-20% (dry basis). Starting from 14.8% of water (dry basis) as measured in the amorphous starch-water system, a minimum of 12.7 to 14.4% can be attained according to the type of plasticizer added. Glass transition, as determined on samples conditioned at a constant relative humidity (57%), depended on the type of plasticizer used, although general behaviour was broadly similar for all components except glycerol diacetate. Casting trials performed with this substance revealed an evident phase separation and T-g of starch did not appear really modified by its presence. For glycerol, sorbitol and lactic acid sodium, glass transition was measured with respect to plasticizer and water content. Couchman's relation was applied, which describes the T-g variation of the monophasic polymer-diluant system. The likelihood of phase separation for a high plasticizer level is considered. (C) 1997 Elsevier Science Ltd.