Chemistry Reference
In-Depth Information
9.6.3 Epoxidized Vegetable Oils as Toughening Agents
Epoxy thermoset polymers may suffer low toughness or brittleness due to
stiff structures with high cross-link densities. Various methods, include the
addition of a either rigid or soft secondary phase, the chemical modification
with a flexible backbone, or a lowering of the cross-link density of the
polymer, have been attempted to improve epoxy toughness.
144
The addition
of rubbery compounds to form phase-separated inclusions has been proved
to be one of the most effective methods for toughening epoxy to avoid major
deterioration of thermal and mechanical properties. The toughening
mechanism is generally thought due to increased shear yielding of the
rubber phases at low strain rate and cavitation at high strain rates.
145
EVOs ability to form heterogeneous phases with petroleum-based epoxies
has been found beneficial as reactive toughening agents in epoxy or other
engineering plastics.
146,147
As mentioned above, for EVO-DGEBA polymer
blends the mechanical and physical properties of an EVO toughened epoxy
are closely related to the network structure in terms of the EVO com-
positions, phase morphology, cross-link density and the chains flexibility.
Most polymerized EVO are of rubbery state but also depend on the curing
system. A cross-linked EVO structure can eciently absorb, transform and
dissipate fracture energy through deformation of molecular networks
analogous to common rubbery compounds. Researchers have shown that
EVO polymers possessed better toughness than those of stiff, neat epoxies
and that the incorporation of EVOs into epoxy can improve impact
strength.
135,139,140,148,149
Shabeer et al.
150
found the fracture toughness of anhydride-cured DGEBA
polymer was greatly improved, by more than 200%, with substitution of
75 wt% DGEBA with epoxidized allyl soyate (EAS). Increase in fracture
toughness of the blend was attributed to a lesser degree of cross-linking.
Ductile fracture behavior at a high concentration of EAS resin was observed.
However, this improvement was also associated with a greatly reduced
storage modulus, T
g
, and cross-link density, e.g., the T
g
of 75 wt% EAS blend
was only 40.5 1C compared to 90 1C for the neat DGEBA polymer. Anhydride-
cured ELO-DGEBF showed a single-phase structure and no apparent im-
provement in toughness with up to 50 wt% ELO. A further increase in ELO
content even resulted in a decrease of fracture toughness and Izod impact
strength
137
while 30 wt% ESO showed improvement in toughness due to
phase separation of rubbery ESO particles within the rigid DGEBF matrix.
151
Tan and Chow
152
indicated the plasticizing effect of EPO, which is rich in
saturated components, improved the fracture toughness of DGEBA by en-
hancing flexibility through cavities occupied by unreacted EPO that increase
resistance to deformation, crack initiation and propagation. The cross-link
density and water absorption capability of the EPO-DGEBA polymer de-
creased with the increase in loading of EPO, but other thermal and mech-
anical properties were not disclosed. Under thermally latent catalysis, Jin
and Park
153
showed the Izod impact strength of a 60 wt% ESO blend was
