Chemistry Reference
In-Depth Information
The epoxidized fatty acids can capture acid groups through catalytic
degradation of their epoxide groups, thus the final product is stabilized.
The tensile strength was reduced from 21 to 10 MPa when the amount of
plasticizer increased from 23.1 to 44.4%, while the elongation-at-break
increased from about 150 to 240%.
5.2.3.2 Toughening Agents
Soybean-oil-based polymers can also be used as toughening agents for dif-
ferent polymeric systems with poor toughness. Epoxy resins have poor
toughness and low impact strength, and are brittle after curing. By adding a
certain amount of flexibility while maintaining the strength of the epoxy
resin, the toughness of the cured product increases. Chen et al.
72
used ESO
as the flexible segment for the toughening of epoxy resins. Anhydride was
used as the curing agent, opening both the epoxy group and ESO, allowing
cross-linking with each other to form a network structure. The combination
of commercial epoxy resin with 20 wt% ESO resulted in a bio-resin with the
optimum properties, a high T
g
of 130.5 1C, good thermal stability, a high
tensile strength of 74.89 MPa and an impact resistance of 48.86 kJ m
2
. PLA
is another brittle polymer with low impact strength, strain-at-break, and
tensile toughness. Many approaches have been taken to improving these
properties; the effect of the polymer stereochemistry, processing history and
the addition of plasticizers have all been studied. In addition, PLA has been
blended with many materials such as polyethylene, PCL, poly(hydroxyalk-
anoates), PBS and others. The challenge remains to find a completely
renewable and bio-degradable toughening agent that enhances PLA's prop-
erties as effectively as existing non-renewable and non-bio-degradable
blending partners. Robertson et al.
73
melt-blended PLA with polymerized
soybean oil (PSO) in order to increase the toughness of PLA in an all-
renewable blend. A critical interparticle distance of around 1 mm was found,
below which increases in the tensile toughness were observed. As a result,
the PLA-PSO blends had a tensile toughness and a strain-at-break four and
six times greater than those of unmodified PLA, respectively.
In addition, bio-composites synthesized from PLA and starch also need a
toughening agent, not only for enhancing their mechanical properties but
also for increasing compatibility between these two components. Xiong
et al.
74
melt-blended PLA-starch composites with ESO as a toughening agent
as well as a reactive compatibilizer. The starch granules were grafted with
MA to enhance their reactivity with ESO. The ready reactions between the
epoxy groups on ESO and the MA groups on MA-grafted starch and the end
carboxylic acid group of PLA brought blending components together and
formed a compatible compound. The elongation-at-break was 140%
for the blend compared with 5% for neat PLA, and the impact strength
was 42 kJ m
2
which is much larger than for pure PLA (only 2.4 kJ m
2
).
This worked showed that ESO can be used as a bio-based reactive plasticizer
for PLA and starch compounds, and the effect was enhanced by chemically
