Physical properties of acetylated starch-based materials: Relation with their molecular characteristics

In order to obtain thermoplastic starch-based materials with a reinforced hydrophobicity, amylose and amylopectin were acetylated following the pyridine-acetic anhydride procedure. An amylose-rich starch triacetate was saponified under controlled conditions to obtain products with different acetyl contents. The adsorption of water by these polymers equilibrated in a 98% r.h. atmosphere was studied using thermogravimetric analysis and differential scanning calorimetry. A slight increase of non-freezing water content is observed when the acetyl content increases up to DS approximate to 1.3-1.4. For higher acetyl contents, the hydrophobicity of the material is efficiently enhanced and no more freezing water is adsorbed. Tensile strength measurements were performed on films with different amylose acetate-amylopectin acetate ratios: no variation of the tensile modulus (2.5 +/- 0.5 GPa) was observed when composition changed, but the elongation at break was strongly decreased when the amylopectin acetate content increased. The influence of acetyl content on thermal properties was also studied. The position of glass transition (similar to 150 degrees C in the dried state) was unchanged when DS ranged from 1.7 to 3, and moved towards higher temperatures when the acetyl content decreased. This thermal characteristic when DS > 1.7 will allow easy processing of these materials. Copyright (C) 1996 Elsevier Science Ltd.