EFFECT OF STARCH TYPE ON THE PROPERTIES OF STARCH CONTAINING FILMS

Ultimate tensile strength (UTS), percent elongations at break (%E), le ar resistance, and impact strength were investigated for starch-poly(v inyl alcohol) (PVA) cast films. Films contained 41% starch, 41% PVA, 1 5% glycerol and 3% poly(ethylene-co-acrylic acid) (EAA). Firms were ma de with each of the following starch types: normal cornstarch, waxy co rnstarch, high amylose cornstarch (50% amylose and 70% amylose), wheat starch, potato starch, and tapioca starch. Films were aged before tes ting from 7 to 168 days, or stored at 15 to 93% relative humidity (RH) for 7 days prior to testing. All films showed an increase in %E and a decrease in UTS as RH increased. The high amylose films showed the gr eatest stability over the RH range for %E. The largest decrease in UTS of the films occurred between 15 and 33% RH, followed by a linear dec rease in UTS as RH increased. The large decrease in UTS was thought to be due to the T-g of the films. Tear resistance of the films was effe cted by RH. Films stored al high RH (93%) absorbed so much water that they became weak and had very little tear resistance. Film stored at l ow RHs (15 and 33%) also had low tear resistance. Films stored at inte rmediate RHs had high tear resistance. The difference in tear resistan ce is thought to be due to chain mobility. At low RH the polymer chain s in the films have less mobility to absorb the stress associated with tearing. Impact strengths of the films were about the same, except fi lms made with waxy cornstarch, which were significantly lower. Aging t he firms for 28 days did not significantly effect the impact strength of the films. All films, except films containing high amylose cornstar ch, showed a large decrease in %E after aging for 168 days. Films made with waxy cornstarch showed the greatest loss in %E after aging, goin g from 144% after 7 days to 34% after 168 days. UTS of the films incre ased with aging, except films made with waxy cornstarch. Firms other t han those containing waxy cornstarch showed no %E less than 100% or an UTS of greater than 35 MPa, even after aging for 168 days. Films cont aining high amylose cornstarch had the most consistent properties over the entire range of test conditions. Copyright (C) 1996 Published by Elsevier Science Limited.