PREPARATION OF MIXED ESTERS OF STARCH OR USE OF AN EXTERNAL PLASTICIZER - 2 DIFFERENT WAYS TO CHANGE THE PROPERTIES OF STARCH ACETATE FILMS

In view of the brittleness of acetylated starch-based materials, we in vestigated the possibility of obtaining internal plasticization of suc h products by preparing mixed esters of starch with both acetate group s and fatty-acid ester chains. We prepared such compounds and determin ed their compositions by use of quantitative solid-state NMR. The meth od detailed in this work is not usually used in the area of modified p olysaccharides. The presence of fatty-acid esters is shown to decrease the glass transition temperature. External plasticization of starch a cetate by triacetin is also studied. A wide range of polymer-plasticiz er compositions was investigated, and the variation of glass transitio n temperature is found to follow a Couchman law. The mechanical behavi our of mixed esters is also investigated. The introduction of fatty-ac id esters is shown to depress the glass transition temperature and the alpha relaxation associated with the glass transition, corresponding to a plasticizing effect. The introduction of these substituents is sh own to suppress the beta relaxation, corresponding to a local antiplas ticizing effect. However, the strength modulus of mixed esters remains almost unchanged compared to that of starch acetate, and the material s remain brittle in spite of internal plasticization. This is due to a decrease of glass transition temperature, which is not sufficient to produce a soft material. In contrast, external plasticization resulted in an increased flexibility, when sufficient amounts of plasticizer w ere used. (C) 1998 Elsevier Science Limited. All rights reserved.