Chemistry Reference
In-Depth Information
compared to ESO due to the higher number of epoxy groups per molecule.
45
The much higher viscosity, a reduced molecular mobility associated with a
polymeric structure, as compared with EVOs, and the presence of only in-
ternal epoxy groups limits the use of epoxidized poly-VESFA in coating
applications.
46
Unlike the internal oxiranes of monomers such as EVOs, terminal epoxies
such as glycidyl show improved reactivities during nucleophilic curing re-
actions. For instance, the terminal epoxy groups of epoxidized triglyceride
esters of undecylenic acid (Scheme 9.7) have been synthesized and suc-
cessfully used in epoxy-amine or epoxy-anhydride curing.
47,48
The prepared
coating compounds also exhibited UV stability due to the predominance of
aliphatic structures.
49
The epoxidation rate of the terminal electron-deficient
alkenes in undecylenic acid by peracid is much lower than for the internal
double bonds of natural fatty acids.
50-52
Undecylenic acid is produced by the pyrolytic cracking of castor oil under
pressure. The non-natural fatty acids then must be reacted with glycerol to
reform triglyceride esters and increase the cross-link density. The maximum
3 oxiranes per epoxidized triglyceride ester of the undecylenic acid molecule
is still lower than that of ESO with about 4.5 oxiranes per triglyceride. Cured
thermoset polymers offer few advantages over their similar, but more readily
available, ESO or ELO counterparts with the exception of minimized pendant
alkyl chain content.
Terminal epoxies of glycidyl esters synthesized from dimer or trimer fatty
acids have been commercially available for some time.
53-55
Recently, both
dicarboxylic acids and a tricarboxylic acids were synthesized by Huang
et al.
56
using Diels-Alder addition onto tung oil (eleostearic) fatty acid with
acrylic acid and fumaric acid, respectively. The corresponding diglycidyl or
triglycidyl esters were prepared using base and EPCH (Scheme 9.8). Both
epoxies showed higher reactivities and improved performance compared to
ESO. The triglycidyl ester version displayed comparable strength, modulus,
and T
g
to a DGEBA control.
Using readily available soybean oil or its free fatty acids, a fatty glycidyl
ester epoxy was synthesized by Wang and Schuman.
57
The versatility of
transesterification and epoxidation reactions (Scheme 9.9) provides several
routes towards the synthesis of the glycidyl esters of epoxidized fatty acids
(EGSs). EGS merits include higher oxirane content and lower viscosity than
commercial ESO, ELO or DGEBA. A structure-property relationship study
measured the effects of oxirane content and presence of saturated fatty acids
Scheme 9.7 The synthesis of the epoxidized triglyceride ester of undecylenic acid.
