Chemistry Reference
In-Depth Information
composites, coatings and toughening agents are being continuously de-
veloped, as will be detailed in later sections. Lastly, but most importantly,
new epoxy monomers derived from VOs have been successfully synthesized
and show greater promise than common EVOs in terms of polymerization
reactivity and thermal and mechanical strength. New monomers are a step
towards more advanced applications, such as structural composites, which
are made possible through improved chemical structure, reactivity, com-
patibility with other monomers, and proper choice of formulation con-
ditions. Properties are the avenue by which to provide the opportunity for
bio-based epoxy resins to replace or supplement their petroleum-based
counterparts. Without appropriate properties, there can be no commercial
opportunities.
9.3 Vegetable-oil-derived Epoxy Monomers
Commercial EVOs, as the major epoxy resins derived from VOs, have in-
herent problems that derive from their chemical structure, flexibility and
hindered reactivity. Most thermoset polymers derived from EVO have very
low glass-transition temperatures (T
g
) and are mainly of a rubbery state,
which inevitably limits their applications. As a result, it has been of interest
to synthesize VO-derived epoxies of enhanced polymerization rate and with
stiffer polymer backbones.
Functionalized oils have been prepared from linseed oils and 1,3-buta-
diene, cyclopentadiene or dicyclopentadiene through Diels-Alder reactions
(Scheme 9.4). Epoxynorbornane linseed oils (ENLOs) were prepared using
hydrogen peroxide with a catalyst.
39,40
The produced cycloaliphatic structure
was expected to improve polymer tensile strength, toughness, and T
g
and be
suitable for cationic polymerization. However, the double bond conversion
of linseed oil had to be limited, e.g.,
o
30%, otherwise only high-viscosity
liquids or soft solids were obtained. Reactive diluents were required to re-
duce the viscosity of the formulation, accelerate the rate of cationic poly-
merization, and increase their final conversions.
41
Epoxidized sucrose esters of fatty acids (ESEFAs), highly functional epoxy
compounds with reasonably well-defined structures, have been synthesized
by Webster and co-workers (Scheme 9.5).
42,43
Anhydride-cured ESEFAs
showed better thermal and mechanical strengths than EVOs. ESEFAs still
possess internal epoxy groups, which are less reactive with common an-
hydride and amine curing agents than the terminal epoxy groups analogous
to DGEBA. Internal epoxy groups are more useful for cationic-cured coating
applications.
44
A drawback to the ESEFA approach is the relatively large
viscosity increase compared to EVOs that hampers some applications.
Polyepoxides were derived from poly(vinyl ether of soybean oil fatty acid
esters) (poly-VESFA) through the transesterification of soybean oil with
ethylene glycol vinyl ether (Scheme 9.6). Poly-VESFA has an increased
number of fatty branches per molecule compared to native soybean oil, thus
the epoxidized poly-VESFA showed faster curing kinetics and improved T
g
