Originally Posted by AndrewD
I wonder if the mystery reaction is somehow causing the exposed side of the material to expand or contract?
No, I think the plasticizers in the plastic are being depleted, leaving the plastic brittle. There are lots of different formulations used as plastisizers; this is what gives all plastics their flexibility. Plasticisers are not water soluble, so beer isn't going to accelerate their migration from the plastic. But they are subject to oxidative degradation, and this is what I suspect is causing the problem. Oxyclean is an oxidative cleaner. Probably heat exacerbates the effect.
I replaced my cracked and brittle sight glasses about 2 years ago (see this
). I used to pump oxyclean through my BK overnight. Sometimes it would sit longer than that. I stopped doing that with my new sight glasses (Bobby's) and I haven't had any issue with them (they aren't as clean as I'd like, meh).