This has been a topic of conversation on thechipboard on several past occassions. (You might want to do a search).

The problem is two-fold; Either you are using plastic flips or pages that contain PVC (polyvinylchloride) a chemical that makes plastics softer but causes a chemical reaction that interacts with coins, tokens, medals, chips, etc. Soulution is to buy flips and/or pages that do not contain PVC.