Chemistry Reference
In-Depth Information
ultimate performance.
77,120
Lewis acid catalysts are commonly used for the
EVO-amine curing reaction, e.g., stannous octoate catalyst in an ELO and
MDA curing system.
48
The onset and peak temperature of the reaction
exotherm were significantly reduced while the polymer T
g
was increased by
more than 20 1C. Tertiary amines, imidazoles, and quaternary ammonium
salts are commonly used catalysts for polyacid- or anhydride-curing of EVOs.
The use of imidazoles has advantages compared with tertiary amines in
improving the T
g
value,
121
which may due to a reaction of imidazole with the
epoxy.
122
Supanchaiyamat et al.
75
reported that the mechanical and thermal
properties of diacid-cured ELO films were significantly influenced by the
type of amine catalyst selected. Both 1-methylimidazole and 4-dimethyl-
aminopyridine (DMAP) can significantly enhance the mechanical properties
of the resulting films. For DMAP, etherification may occur due to good
nucleophilicity. The curing speed is highly sensitive to the catalyst amount,
where the optimum DMAP catalyst concentration was 1 wt% of the total ELO
and cross-linkers. Further increase of the DMAP concentration decreased the
Young's modulus.
In an EMI-catalyzed ESO-MHHPA curing system, Tan and Chow
123
found
the rate of polyesterification, the degree of conversion, T
g
, storage modulus,
and cross-link density were improved at higher EMI concentrations. However,
a continued increase in the catalyst concentration led to rapid gelling, but
reduced the conversion due to hindered monomer/oligomer diffusion.
79
Tan
and Chow
124
also compared the type and concentration of catalysts on the
fracture mechanics of MHHPA-cured ESO thermoset polymers. The improve-
ment in fracture toughness with catalyst concentration was due to an increase
in the degree of cure, while extreme cross-link densities led to catastrophic
brittle fracture and low fracture toughness. For the EMI catalyst, fracture
toughness increased with an increase in concentration of EMI, whereas a re-
duction of fracture toughness was observed when using tetraethylammonium
bromide as the catalyst and its concentration exceeded 0.5 wt%.
9.6 Polymer Blends of Epoxidized Vegetable Oils
As mentioned above, an EVO can be polymerized with a variety of curing
agents. The cured thermoset polymers, however, generally show low
thermal/mechanical performance and cross-linking density due to their
flexible structure and lower reactivity compared to DGEBA and cycloaliphatic
epoxy. Commercially available epoxies such as DGEBA and cycloaliphatic
epoxies possess stiffer structures, thus EVOs can be blended with these
petroleum-based epoxy monomers to mutually improve their mechanical
and thermal properties. EVOs generally have lower viscosity, so EVOs or their
derivatives can be used as reactive diluents for DGEBA resins, which are
relatively high-viscosity liquids or solids, to decrease the overall cost and
improve the processability. Due to a less homogeneous structure, ESO is less
ecient in reducing the viscosity of epoxy resin compared to many
petroleum-based reactive diluents.
