The monsoons are here, and they are notorious for creating gelling problems and gel-time issues. It is natural to blame the supplier, but the truth is a little knowledge can help you get to the root of the problem yourself. If you are aware of the curing mechanism and approach the gel-time problem equipped with the correct knowledge you won’t have any of these seasonal problems at all. So read on.

Did you know that the rate of cure of a resin is based on several factors? It depends on the amount of the resin, the type and amount of peroxide catalyst & accelerators based on the resin & curing conditions.

Peroxide catalysts: The catalysts used with polyesters and vinyl ester resins are called peroxides. Their function is to crosslink the resin, at first causing a gel and then a complete cure.
Precautions with Peroxide Catalysts
• Because peroxides and accelerators react explosively, they should never be mixed together directly. Hence resins for use at room temperature are generally preferred pre-accelerated.
• If additional accelerator is needed, it should be mixed thoroughly into the resin before adding the catalyst.
• Strictly observe safety instructions that come in the data sheets.

Let’s focus on MEKP and accelerators.

Methyl ethyl ketone peroxide (MEKP); is made from methyl ethyl ketone and hydrogen peroxide. Some of the factors governing peroxide catalyst usage are:

• Amount—Resins and peroxide catalysts are formulated so that from 0.75 to three percent catalyst solution is enough to generate the free radicals needed. If too much catalyst is used, too many polymer chains start  growing; resulting in a weak cured resin with poor physical properties. If too little peroxide catalyst is used, the gel time will be very long. The resin may never cure properly even if post cured and tend to be physically weak and possibly rubbery.

• Heat—Enough heat must be supplied to properly cure the resin. It can come from an external source or from the exotherm of the resin itself. Exotherm is the heat given off as the resin cures. If the exotherm is more concentrated the part will get hotter and cure faster. If the part is a thin laminate it will cure slowly.
• Shop conditions are very important. If temperatures are below 25ºC, cure will be greatly extended. However, if the temperature is in the 40’s, gel and cure will be faster.
• Cutting back on the peroxide catalyst may result in enough working time, but there may not be enough free radicals to properly cure the resin.

Accelerators: Resins formulated for cure at room temperature contain accelerators or promoters. They increase the rate at which peroxide catalysts breakdown into free radicals. The amount of both the accelerator and peroxide catalyst should be such that the fabricator has enough working time to form the part, and at the same time, enough speed of cure to make the process economically viable.
Accelerators used in most products are generally metal salts (metal soaps) and amines. They include cobalt, calcium, copper and potassium salts and amines such as dimethyl aniline and diethyl aniline. Some of these accelerators are described as follows:

• Cobalt—Solutions of cobalt impart a pink to red color to the resin. Cobalt acts on most peroxide catalysts to form free radicals.
• Amines—Amines generally color polyester resins yellow to brown depending on the amount present. They also can cause accelerated yellowing of cured parts.
• DMA and DEA act directly with benzoyl peroxide so that this catalyst can be used at room temperature. Without an amine present, BPO is too slow in generating free radicals at ambient temperatures for use in polyester or vinyl ester resins.
Source: From the Composites Applications Guide of CCP

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