PHYSICAL-PROPERTIES OF ACETYLATED STARCH-BASED MATERIALS - RELATION WITH THEIR MOLECULAR CHARACTERISTICS

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