Chemistry Reference
In-Depth Information
Table 7.5 Effect of citrate glyceride (CAG) concentration on the mechanical
properties of PLA-CAG composites compared to neat PLA.
Treatment
a
s
u
/MPa
E/MPa
%El
PLA
60
1.4
607
20
13
0.4
PLA-10CAG
39
0.5
523
35
12
0.5
PLA-20CAG
34
1.3
514
15
9
0.4
PLA-30CAG
22
0.8
445
11
7
0.2
PLA-40CAG
13
0.4
326
6.5
5
0.1
PLA-50CAG
9
0.3
251
3.8
4
0.2
PLA-60CAG
b
6
0.6
194
12
4
0.2
PLA-70CAG
b
3
0.3
110
8
4
0.3
a
Tensile bars were obtained from single-screw extrusions and stamped.
b
Blends that were extruded and then compression molded.
The mechanical properties (i.e., s
u
, E and %El values) of PLA-CAG com-
posites were significantly different to those of neat PLA, see Table 7.5. PLA is
a linear aliphatic polyester thermoplastic derived from lactic acid mono-
mers. CAG is a complex and highly cross-linked polyester thermoset derived
from citric acid and glycerol. The presence of the ester linkages in these two
plastics allows for hydrolytic degradation. Both CAG and PLA are hydrophilic
in nature, however PLA is considerably more hydrophobic compared to
CAG.
27
Therefore the PLA-CAG composite has relatively weak interfacial
bonds between these two dissimilar ingredients. This in turn is reflected in
the mechanical properties of the composite blends, see Table 7.5. Tensile
bars of PLA-CAG showed significantly lower s
u
, E and %El values compared
to tensile bars composed of neat PLA. Increasing the concentration of CAG
in the PLA-CAG composites resulted in progressively lower mechanical
properties compared to PLA-CAG composites containing lower or no CAG.
For example, PLA-10CAG and PLA-50CAG blends exhibited reductions of s
u
,
E and %El of
35,
14, and
8% and
85,
60,
70%, compared to neat
PLA, respectively. These results differ from other PLA composite studies
where employment of a lignocellulosic fillers such as wood flour typically
increases the stiffness (E) while s
u
and %El values only slightly decline.
28
It
is obvious that PLA and CAG are incompatible as evidenced by the mech-
anical tests, see Table 7.5. One positive interpretation on the reduction of
mechanical properties obtained by the blending of CAG with PLA is that co-
polymerized PLA-CAG materials will degrade much more rapidly than neat
PLA. PLA is noted for its slow degradation rate.
68,69
In a subsequent study, PLA-CAG blends containing 10, 25 and 35% CAG were
prepared by extrusion as previous described, stamped into tensile bars and
then exposed to 30% (dry/laboratory room conditions) or 95% humidity (wet
conditions) for 528 h prior to the mechanical testing. This test was conducted
to determine how exposure to moisture influences the mechanical properties
of the blends. Weights were periodically measured over the incubation period
but no weight equilibration time was achieved indicating that the samples
would continue to take up more water if allowed to incubate further. At the
end of the incubation period, the average percentage weight gain for PLA,
PLA-10CAG, PLA-25CAG and PLA-35CAG was 2.2, 2.6, 6.3 and 6.8%,
