MODIFICATION OF STARCH-POLY(METHYL ACRYLATE) GRAFT-COPOLYMERS BY STEAM JET COOKING

Starch-g-poly(methyl acrylate) containing 12.3, 31.9, 51.7, and 58.3% PMA, by weight, were prepared by eerie ammonium nitrate-initiated poly merization of methyl acrylate onto granular cornstarch. The granular s tructures of these graft copolymers were not disrupted by steam jet co oking at 140 degrees C. At most, only 13% of the polymer was dissolved , and this soluble fraction was comprised largely of starch. The proba bility of crosslinking within these graft copolymer granules was consi dered. Physical properties of extruded ribbons depended upon whether o r not granular graft copolymers were jet cooked prior to extrusion. Al though tensile strengths were not greatly affected by steam jet cookin g, cooked samples showed significant increases in both percent elongat ion and tear resistance. The effects of jet cooking upon the propertie s of extruded ribbons can be explained by gelatinization of starch wit hin the grafted starch granules. Although jet-cooked granules still re main intact, gelatinization of the starch moiety causes these granules to be less rigid, more deformable, and more easily plasticized by sma ll amounts of water. Loss of starch crystallinity after steam jet cook ing was proved by both differential scanning calorimetry and X-ray dif fraction. (C) 1997 John Wiley & Sons, Inc.