Chemistry Reference
In-Depth Information
appears complex. The suggested curing mechanism
1
follows: base acceler-
ators catalyze curing reactions by the generation of carboxyl anions with
anhydride [Scheme 9.11(g)]. The carboxylate ion then acts as a nucleophile in
the ring-opening of the epoxide, resulting in an alkoxide [Scheme 9.11(h)].
The alkoxide anion in turn ring-opens an anhydride group to generate a
carboxylate anion [Scheme 9.11(i)].
77
Continuation of these alternating steps
results in a polyester. Etherification between epoxies and alkoxide anions is
less likely.
78
Boquillon and Fringant
79
modeled the cure kinetics of an ELO-tetra-
hydropthalic anhydride system catalyzed with 2-methylimidazole using dif-
ferential scanning calorimetry (DSC) and an nth-order rate equation. The
curing reaction of their systems followed first-order kinetics at extents of
cure above 0.7. Liang and Chandrashekhara
80
studied the catalyzed soya
epoxy-anhydride curing system where the curing showed autocatalytic be-
havior. The overall reaction order was approximately two, based on Kamal's
autocatalytic model using DSC and rheology results. Using the same model,
Tan et al.
81
studied a methylhexahydrophthalic anhydride (MHHPA) cured
ESO system in the presence of a 2-ethyl-4-methylimidazole (EMI) catalyst.
The EMI content and curing temperature had a significant influence on the
reaction rate constant and reaction order. The overall reaction order ranged
from 1.5 to 3 and the E
a
values decreased inversely with EMI catalyst
concentration.
Kinetic analysis of a similar 1-methyl imidazole catalyzed ELO-methyl
nadic anhydride system by iso-conversion methods found that E
a
increased
at the beginning of the curing and decreased as cross-linking proceeded.
82
The increased E
a
might be due to the slow initiation mechanism by the
catalyst and the decrease in E
a
by gelation and vitrification or autocatalysis.
The curing kinetics of EMO and epoxidized bio-diesel (of sunflower and
linseed oil origin) with cis-1,2-cyclohexanedicarboxylic anhydride catalyzed
by triethylamine were investigated by Nicolau et al.
83
Their results indicated
E
a
was related to the oxirane content and to the locations of the oxiranes in
the fatty acid structure. The oxirane at (C9-C10), which is close to the ester
group, showed a higher E
a
than those at positions C12-C13 or C15-C16. The
difference may be due to steric hindrance.
9.4.3 Cationic Polymerization
Catalytic ring-opening of EVOs by Lewis acids is well known
84
and improves
reactivity compared to either polyamines or anhydrides alone. Boron tri-
halides, super acids, have been widely used for cationic cure of EVOs. Due
to their high reactivity and concomitant diculty in handling, these
catalysts are generally added as latent complexes, which are inert under
normal conditions, such as ambient temperature, but release active species
upon external stimulation, such as with heating or photo-irradiation.
A boron trifluoride ethylamine complex (BF
3
NH
2
C
2
H
5
) is used extensively in
commercial epoxy formulations. Catalytic polymerization of ESO by boron
