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Working of Organic Peroxides:

Mechanism of action

Unsaturated polyester resins (or UP resin) refer to a large variety of products all containing reactive double bonds in the polymeric chain. The presence of double bonds provide the product with the ability to crosslink in the presence of a monomer, also acting as a diluent, via a free-radical mechanism.

Example of chemical structure

The reactivity, the viscosity and the final properties of UP resins can be adjusted by:

  • chemical composition (aliphatic, aromatic, number of double bonds)
  • resin molecular weight
  • Nature and concentration of the crosslinking monomer (containing at least one double bond)

The general crosslinking mechanism is shown on figure 2 below:


Figure 2: Crosslinking mechanism of UP resins

The crosslinking monomer:


In most cases the crosslinking monomer is Styrene introduced at concentrations between 25 and 45 wt%, but other monomers can also be used.

Curing catalyst:

To initiate the curing reaction a source of radicals is needed. In most of the cases organic peroxides are used.

  • The first step is the decomposition of the peroxide.

  • The second step is chain propagation reaction via styryl radical formation capable to react with a double bond

For cold curing, initiators such as amines or metallic salts are also introduced.


In order to prevent premature curing and to extend pot-life, inhibitors are also introduced into the system. Inhibitors are free radical scavengers like Hydroquinone derivatives, terbutylcatechol, cresol derivatives




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