Actually, it was chips that had been stored long term in the more rigid, non pvc flips that had the problem. Not only did I find this problem, so did Chris Colangelo of tiny treasures. The chips turned dark brown and black along the top edge. I have never had a problem with the softer PVC flips.

PVC's react with the metal in coins and tokens which gives them a bad rap, but I think for chips you need to use the softer PVC ones.

Greg