Mechanical properties
3.3.1. Effect of relative humidity
Mechanical properties of starch and starch/PLA foams
(after being conditioned in a specified relative humidity
atmosphere for 7 days at 25 8C) were measured and the
results are shown as a function of relative humidity in Fig. 4.
The values of both the tensile and flexural strength for all
of the foam specimens investigated were found to
increase initially with increasing relative humidity reach a maximum at the relative humidity of 42% RH, and
decrease with further increase in the relative humidity. At
‘low’ relative humidity levels (i.e. less than 42% RH), the
cell structure of starch-based foams was so brittle that
cracks could form and propagate, whereas at ‘high’ relative
humidity levels (i.e. greater than 42% RH), the high amount
of absorbed moisture weakened the cell structure appreciably
(Shogren, Lawton, Doane, & Tiefenbacher, 1998). For
a given value of relative humidity, addition of PLA was
responsible for a significant increase in both the tensile and
flexural strength of the hybrid foams over those of the pure
starch foams. Both percentage of elongation at break and
maximum flexural strain of the foams were found to initially
increase with increasing relative humidity level (up to ca.
42% RH) and then became practically constant at relative
humidity levels greater than ca. 42% RH (with an exception
to the starch/10 wt% PLA foams which were found to
increase slightly)