Traditional Unsaturated Polyesters

Traditional unsaturated 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 1.

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 2.

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.