Chemistry Reference
In-Depth Information
on polymer properties. EGS had its unreactive saturated fatty acid com-
ponents removed. Upon curing cationically, neat EGS polymer displayed T
g
values well above room temperature. Even higher T
g
values e.g., greater than
100 1C, and improved mechanical properties, compared to other bio-based
systems, were obtained through careful selection of curing agents and
catalysts.
9.4 Curing Reactions of Epoxidized Vegetable Oils
The polymeric materials of EVOs are three-dimensional cross-linked net-
works prepared through the use of curing agents. There are two types of
curing agent, catalytic and co-reactive. The catalytic curing agent initiates
polymerization of EVOs themselves, i.e., through homopolymerization,
whereas the co-reactive curing agent behaves as a co-monomer for EVO. The
curing process is bond formation through a combination of step-growth
and/or chain-growth mechanisms. Due to the polarity of C-O bonds, the
electron-deficient carbon of oxirane constitutes an active site for nucleo-
philic reaction, while the electron-rich oxygen atom affords an electrophilic
reaction site. The rate of curing is dependent on temperature, curing agent
and thus mechanism, as well as the type and number of epoxy groups pre-
sent in the chemical structure.
Despite a large volume of literature on the reactivity of EVOs, some of
which is conflicting, there is consistency in its general conclusions: due to
sterically hindered and electron-donating alkyl substituents, the rate of re-
action of EVOs with nucleophilic curing agents is lower than glycidyl
(terminal) epoxies, while the rate is higher with electrophilic curing agents.
For instance, EVOs react especially sluggishly with common polyamine
curing agents.
48,58
It is not uncommon for some EVOs to show no, or a re-
duced degree, of curing due to their low reactivity and/or low oxirane con-
tent. In many ways, the curing behaviors of EVOs are analogous to
commercial cycloaliphatic epoxies rather than DGEBA. Polyacids and their
derivative anhydrides, plus cationic catalysts, are commonly used curing
agents for EVOs.
9.4.1 Addition with Polyamines
Polyamines are very frequently used curing agents for epoxy resins. The
overall reaction rate of amine with epoxy resin is influenced by the structure
and electronic properties of the amine. The nucleophilic reactivity of amines
generally follows the order: aliphatic4cycloaliphatic4aromatic. Where the
EVO molecules have long, flexible, and aliphatic structures, cycloaliphatic or
aromatic amines can compensate for this shortcoming through their rigid
structures. Cyclic curing agents favor applications with high thermal and
mechanical strength requirements
59-61
but require higher curing tempera-
tures and longer reaction times.
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