What happens to the chlorine in epichlorohydrin when a two-part epoxy is air-cured? Where does the chlorine go? Does it evaporate as a by-product or does it remain in the epoxy? I'm using a two-part resin and hardener and curing them at room temp. The resin is bisphenol-A-epichlorohydrin, and I believe the final cured epoxy does not have chlorine as a part of its main structure. I need to know the whereabouts of Cl because I'm trying to study neutron interaction with materials in my detector, and I suspect there's some chlorine in there but not sure entirely.
 A: Ten or fifteen years ago, I did exactly what you are trying to do.  A colleague had started preparing some useful neutron shielding by mixing lithium-6 carbonate powder into a two-part bisphenol-based epoxy, and then pouring the epoxy into usefully-shaped molds so that it cured into machinable tiles.
We wanted to use the stuff, but for our experiment neutron capture on chlorine produced a background signal that could have easily overwhelmed the effect that we were trying to measure.
I had some conversations with the epoxy manufacturer and they confidently assured me that the cured epoxy contained no chlorine, probably on the same kind of logic in John Rennie's answer. So we bought some of the epoxy and made a bunch of this shielding.
Another collaborator took a sample of this "chlorine-free" shielding to the NIST Nuclear Measurements Group, which performed a Prompt Gamma-ray Activation Analysis. Turns out that our idea of "no chlorine" and the manufacturer's idea of "no chlorine" were different, and there were plenty of other contaminants in the material as well.  The email from the scientist at NIST who analyzed the material included the sentence "I wouldn't use this stuff for neutron shielding."
The moral of the story is: when in doubt, measure.
A: The epichlorohydrin is used in the synthesis of the bisphenol-A-epichlorohydrin. It attaches the epoxy groups to the ends of the molecule and the chlorine is removed (as sodium chloride) during the reaction. So despite the name bisphenol-A-epichlorohydrin does not contain any chlorine except for any chlorine atoms present as impurities.
So in principle your epoxy resin doesn't contain any chlorine, neither before nor after hardening. I have no idea how much chlorine is present as impurities. I think you would have to analyse the resin to find out as I very much doubt the manufacturer would be able to tell you.
