Effect of starch type on the properties of starch containing films

Abstract

Ultimate tensile strength (UTS), percent elongations at break (%E), tear resistance, and impact strength were investigated for starch-poly(vinyl alcohol) (PVA) cast films. Films contained 41% starch, 41% PVA, 15% glycerol and 3% poly(ethylene- co-acrylic acid) (EAA). Films were made with each of the following starch types: normal cornstarch, waxy cornstarch, high amylose cornstarch (50% amylose and 70% amylose), wheat starch, potato starch, and tapioca starch. Films were aged before testing 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 greatest 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 decrease 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 effected by RH. Films stored at high RH (93%) absorbed so much water that they became weak and had very little tear resistance. Film stored at low RHs (15 and 33%) also had low tear resistance. Films stored at intermediate RHs had high tear resistance. The difference in tear resistance is thought to be due to chain mobility. At low RH the polymer chains in the films have less mobility to absorb the stress associated with tearing. Impact strengths of the films were about the same, except films made with waxy cornstarch, which were significantly lower. Aging the films for 28 days did not significantly effect the impact strength of the films. All films, except films containing high amylose cornstarch, showed a large decrease in %E after aging for 168 days. Films made with waxy cornstarch showed the greatest loss in %E after aging, going from 144% after 7days to 34% after 168days. UTS of the films increased with aging, except films made with waxy cornstarch. Films other than those containing waxy cornstarch showed no %E less than 100% or an UTS of greater than 35MPa, even after aging for 168 days. Films containing high amylose cornstarch had the most consistent properties over the entire range of test conditions.