polyesters are polycondensation products based on saturated and
unsaturated dicarboxylic acids such as maleic or fumaric acids and
primary bivalent alcohols. Typically, these systems are dissolved
in styrene which reacts with and cross-links the unsaturated resin
when a dryer system, that is, a cobalt salt and an organic peroxide,
are used. The cobalt salt decomposes the peroxide to form free radicals,
R'·, which initiate the crosslinking of the system. This
reaction mechanism is described in Figure
These types of systems
are two-component coatings with the cobalt in the formulated product
and the peroxide added just prior to use. They have a short pot
life which is typically about 5-10 minutes. They also have a problem
with surface drying; the coatings surface does not cure and will
be sticky because the oxygen in the air inhibits the free radical
mechanism. A mechanism for oxygen termination is given in Figure
To overcome this effect,
a paraffin wax is usually added to the formulation. While the system
is curing, the paraffin wax migrates to the surface. Oxygen from
the atmosphere is excluded, and the chain termination is prevented.
This technique works but requires removal of the wax since the wax
leaves a low-gloss appearance. The surface must then be sanded and
polished to get a high-gloss appearance. This additional sanding
and polishing is very labor intensive.
Work in the mid-1950s
saw the development of non-air-inhibited, unsaturated polyesters
that cured without the aid of waxes. These unsaturated polyesters
would have the same hardness and chemical resistance of traditional
unsaturated polyesters and would give a high gloss without sanding
and polishing. The oxygen inhibition could be prevented through
the introduction of allyl ether groups to the modified fumarates.
Figure 3 shows a
reaction mechanism on the surface. The radical R'· attacks
the allyl ether forming the allyl ether radical (Step A. This allyl
ether radical can then react with a hydrogen atom (Step B), and
in conjunction with another allyl ether radical and oxygen, form
hydroperoxides, which will aid in the further development of new
radicals in the presence of a cobalt drier (Step C).
A reaction path within
the film is shown in Figure
4. The radical R'·, attacks the allyl ether forming the
allyl ether radical (Step A). This radical then attacks the double
bond of a fumaric acid group (Step B), forming a radical that can,
in time, form the three-dimensional network of the polymer.
How these two separate
reactions may take place within the coating is described in Figure
5. On the surface, the oxygen actually aids in the cure due
to the allyl ether groups. Below this surface, the copolymerization
of styrene and fumaric acid groups occurs without being inhibited.
To learn more about unsaturated
polyester resins from Bayer, browse our Roskydal®
unsaturated polyester resin product line.