Chemistry Reference
In-Depth Information
9.5.2 Curing Agents
The influence of the curing agent is just as critical to the final properties of
thermoset polymers as the epoxy resin component. Since the curing agent
will become part of the cross-linked network structure, special attention
should be paid to its structure and stoichiometry. Lu
69
found that aromatic
polyamines were unable to react with ESO. Polymers produced with aliphatic
TETA were rubbery with a T
g
of 15 1C. Cycloaliphatic polyamines, such as
PACM, reacting with the same ESO monomer enhanced the T
g
to 58 1C, and
the highest flexural strength was achieved at a ESO/PACM molar ratio of
0.53 : 1. Juangvanich
72
also found that the reaction of diaminodiphenyl
sulfone or MDA with ESO did not occur even at high temperatures. Aromatic
amines, e.g., p-phenylenediamine, reacted to a smaller extent compared to
curing with more nucleophilic aliphatic amines. An imperfect network was
formed when using p-phenylenediamine as a curing agent due to intra-
molecular cross-linking.
Anhydride is one of the most important curing agents for EVOs. Gerbase
et al.
115
investigated the mechanical and thermal behavior of ESO reacted
with various anhydrides in the presence of tertiary amine accelerators.
Thermoset polymers showed higher T
g
, storage modulus, and cross-link
density when the system was cured with the more rigid phthalic, hexa-
hydrophthalic, or maleic anhydride than the more flexible dodecenylsuccinic
or succinic anhydride. Similar results were also reported by R¨sch.
116
Due to
the effects of steric factors and the rigidity of the formed diester segment, ELOs
cured by phthalic anhydride and endo-3,6-methylene-1,2,3,6-tetrahydrophthalic
anhydride in the presence of 2-methylimidazole showed lower cross-linking
densities than those cured with cis-1,2,3,6-tetrahydrophthalic anhydride.
79
In addition to the structure of anhydride, a variation in stoichiometric
ratios of epoxy to anhydride was also found to have significant effect on the
resulting network structure and performance of thermoset polymers. From
the epoxy-anhydride polyesterification curing mechanism,
117-119
the max-
imum cross-linking degree, storage modulus, and T
g
may be achieved at a
stoichiometric ratio R
¼
1.0. However, in practical formulations, less than
stoichiometric ratios are commonly used to achieve balanced properties and
also account for competitive reactions such as epoxy homopolymerization.
The reaction of ELO with cis-1,2,3,6-tetrahydrophthalic anhydride catalyzed
by imidazole indicated complete conversion at the stoichiometric ratio of
R
¼
0.8. The increase in anhydride to R
¼
0.8 caused an increase in T
g
and
stiffness but with a sacrifice in chain mobility. The T
g
was reduced for R41
due to the reduced cross-link density.
79
9.5.3 Catalysts
Due to the low reactivity of EVOs during nucleophilic curing reactions, the
choice of catalyst and the amount used are critically important factors that
strongly influence the cross-link density, network morphology/structure, and
