Lets look a little bit closer at the chemistry of curing a polyester/vinylester resin. There is lots of chemistry involved but we will try to simplify it here to reduce complication.

The initiator is the correct technical term for what some call the catalyst or hardener. This initiator is the additive that begins or speeds up the chemical reaction and becomes part of the crosslinked polymer.

Free radical polymerization is the process that occurs during the curing cycle of a polyester resin. The initiator (catalyst) decomposes into free radical molecules, which work to crosslink the polyester and styrene molecules in the resin. The rate of cure can be increased with the addition of more initiator.

Selecting the appropriate initiator is important to the control of the chemical reaction. Styrene-based resins use several types of initiators. These include: ketone peroxides, cumine hydroperoxides, acetylacetone peroxides, and benzoyl peroxides. These can also be blended and the initiator package will be recommended by the resin supplier.

Methyl Ethyl Ketone Peroxide(MEKP) is the most common and widely used initiator, and is most cost effective and easy to use. It comes in clear or red tinted and can be used as a fine-tuning for resin cure time with the adjustment of its percentage from 1.25% to 3.0%.

Cumene Hydroperoxides (CHP) achieve cure using lower exotherm temperatures which reduces resin shrinkage. The choice for use with vinyl esters, they also work well on thick laminates to control the mass exotherm. These can be blended with MEKP for specialized purposes.

Benzoyl Peroxide (BPO) is the next most common initiator, and is available in several forms. It is commonly available as a paste and is safer for handling and health hazards.

Room temperature curing resins must have the addition of a promoter to help rapidly decompose the initiator and ensure an appropriate curing time.

Room temperature curing resins must have the addition of a promoter to help rapidly decompose the initiator and ensure an appropriate curing time. These chemicals are metal salts or amine compounds. The most common promoter used with MEKP initiator is cobalt napthenate (CoNap) or cobalt octoate. For BPO initiator, amine promoters are used such as dimethylaniline (DMA), diethyaniline (DEA) and dimethyacetoacetamide (DMAA). These amine promoters may be used in concert with cobalt promoters to produce a rapid cure folloowing gpelation.

Styrene and polyester are mixed at the factory and would polymerize without an initiator (MEKP). This is controlled with the addition of an inhibitor. When initiator is added to the system, it first reacts with the inhibitor free radicals before it moves to crosslinking the styrene and polyester resin. Inhibitors include hydroquinone(HQ), tertiary butyl catechol(TBC) and toluhydroquinone(THQ).

There are many chemicals involved in the composition of the resin system. They all have a purpose, they all are important, and none of them should be taken lightly.