When this column debuted in January 2015, the first article discussed mixing organic peroxides into composite materials and focused on the supplier’s point of view. This column is intended to provide insight into the selection and use of peroxides from a user’s perspective.

A Look at Chemistry

Composites are made with a wide variety of organic polymers broadly categorized into thermoset and thermoplastic. In most applications, resins – essentially monomers or copolymers – are cross linked to form a cured resin. Polyesters and vinyl esters are widely used because of ease of processing and comparatively lower cost. These resins are cross linked into a solid by free radical reactions. This cross linking is achieved by introducing an initiator, which are chemical compounds that decompose into free radicals when promoted by reactive metal salts. This free radical generation is perpetuated until all the initiator is decomposed. Typical chemical reaction steps involved are as follows:

The generated free radical bonds with reactive sites in the resin, which then creates another reactive free radical that continues the cross-linking process.

Typically, organic peroxides are used as initiators with polyester, vinyl ester, acrylic and some liquid thermoplastic resins. For more in-depth information on the chemistry aspect, consult with your peroxide supplier to assess suitability for different resin types.

What is My Process?

Applications most often associated with polyester and vinyl ester resins are pultrusion, resin transfer molding, filament winding and spray-up. A variety of peroxides are available for use with polyester and vinyl ester resins, and selection or suitability of these is dictated by the type of applications or processing. These applications or processing parameters also dictate the pot life requirements (the period the two reactive chemicals remain usable when mixed) and, in turn, selection of resin and peroxide combination.

Because peroxides are very reactive (and sometimes even shock-sensitive), special consideration may be needed to process them safely. To mitigate shock sensitivity and process them safely, peroxides are sometimes supplied as suspensions. Some suspensions are water-based. If your resin or process is moisture-sensitive, be mindful to avoid water-based suspensions.

What is My Pot Life?

For processes like resin infusion, hand lay-up and spray-up, where shorter pot life is acceptable, room temperature activated peroxides are suitable. An example is ketone peroxide like methyl ethyl ketone peroxide (MEKP).

For applications like pultrusion and filament winding that require long resin pot life, peroxides that are heat activated are suitable. An example is peroxyester like tert-butylperoxybenzoate.