actinic light

[Ross]

I know that working with ester resins in bright sunlight shortens the gel and hardening times to sometimes unmamageable degrees. Does epoxy react in the same manner?

 

[Ross]

I posted this question also on the wooden boat forum and after several attempts was provided with an answer. To wit:
Originally Posted by AstoriaDave
Aagh. The orginal poster's question is unanswered, to wit: Does sunlight have a photochemical effect on epoxy cure rate?

A chemist's answer: No way, Jose! The epoxy components combine using a plain old condensation-style polymerization, perhaps accelerated by general acid catalysis, but unaffected by UV light on a photon basis, except to the extent the sunlight raises the temperature of the resin once laid on the substrate. These reactions are "even electron" processes, and no free radicals are involved.

In contrast, polyester resins cure using a "free radical" process (meaning: chemical bonds split apart into odd-electron species), which is why organic peroxides are used as catalysts. These peroxides operate by generating free radicals in the polymer mix; said free radicals are generated by action of UV light, as well, so that sunlight helps things along. Of course, increased temperatures also accelerate these processes, so that sunlight is a double whaOriginally Posted by AstoriaDave