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.