The mixing of organic peroxides (OPs) into unsaturated polyester resin (UPR) can make a critical difference between a good part and an increasing scrap rate. The key is in the order of mixing and reducing the possibility of reacting the OP before it gets into the UPR. As a rule of thumb, it’s best to add the OP last and mix it thoroughly. If only it were this simple.

Organic peroxides are added to UPRs with the intent of co-polymerizing the UPR with the reactive diluent (i.e. styrene) to make the UPR a liquid solution at ambient temperature. It is important to know that all OPs can be reacted in and out of a composite resin, and there are safety and health risks to using OP before and during a mixing operation.

There are two conditions for the use of OP in UPR – ambient and elevated temperatures. At the molecular level, at ambient cure temperatures the most common UPR contains a reactive metal complex, typically a cobalt-based system. The metal causes the decomposition of the organic peroxide to a free radical.


The free radical of the peroxide, represented as RO•, is highly energetic and attacks sites in the UPR that are rich with electrons, the double bonds. In one scenario, the free radical can attach to one end of the double bond and create another free radical on the other end capable of continuing in a chain reaction to form the finished polymer.


At elevated temperatures, different types of OPs that react due to heating are used. The end result is the same: Free radicals create the conditions for polymerization.

The key to the manufacture of a thermoset composite is then having a good mix, which places the decomposing organic peroxide near the double bonds of the reactive monomer and UPR to have an efficient polymerization.

Safe mixing can be demonstrated using the example of an auto-cast application. The prepromoted UPR (containing materials ready to produce decomposition) is first mixed with reinforcements through the action of an auger or a static mixer to reach a level of thoroughly wet out reinforcement. This UPR matrix then continues to where the OP is introduced and also thoroughly augered in. The OP is but a small percentage of the final product (1 to 2.5 percent), and therefore this mixing step is critical to give the end composite the best physical properties. The same order and attention to mixing seen in this “closed” system should be followed in an open-air process. The closed system is very safe, but in open application of peroxide there must be other precautions taken.