Why does the plastic seem to get more brittle over time?

I had a question at the Baltimore coin show this weekend that we encounter from time to time from long time collectors. Over the years, the plastic coin flips and coin pages that I keep my coins in seems to get more brittle over time. With coin flips, they actually seem to crack. Why does this happen?

I know what you mean about the brittle nature of the plastic. This is the inherent problem with plasticizer and softener-free polyester. Over time, they get even harder. Without the softeners in the plastic, they are absolutely getting more brittle. But believe it or not, its a good sign for your coins.

The alternative is putting softeners in the plastic which would harm the coins. Cheaper coin album pages and coin holders/flips are available from other manufacturers that do this, because it may "seem nicer" to non-professional collector. Not realizing that those softening agents cause problems over time to their collection. So if your pages or flips are cracking, rest assured that your coins are safe. If the look is bothering you, perhaps you should switch to a nice coin case with archival materials?