Can CD jewel cases melt in a library drop box?

Q: Some libraries claim that their CD cases melt due to high temperature if they were left in a drop box for couple of days. Is that true? What is the melting temperature of a CD case?

A: Traditional CD cases are made of polystyrene, which has a melting temperature of around 374-500° F (190-260° C). Temperature can be an issue for polystyrene, but not because of its melting temperature; it's because of something called "glass transition temperature." This basically means the temperature at which a polymer turns from something that's hard and brittle to something that's soft and bendy.



Polystyrene is used at temperatures that are BELOW its transition temperature. That means we use polystyrene when it's hard and brittle. Its glass transition temperature is pretty low, though, around 181-212° F (82.78-100° C).


That's pretty close to the temperature at which water boils and, I think it's pretty reasonable to assume that a metal library box could reach those temperatures on a hot, summer day. In my previous career life, I sold chocolate and, if I learned one thing during that time, it was that outside temperature is only part of the equation. The temperature inside an enclosed space — such as inside a library drop box, a mail box or even a delivery truck — can be much, much higher than the temperature outside.


So, while a polystyrene case won't "melt" per se, it can warp enough to make it unusable.

Personally, I think it's a bad idea to put polystyrene into a drop box anyway, because polystyrene is naturally brittle. Thinking of that poor CD case sitting at the bottom of a drop box, with piles of heavy books being dropped onto it throughout a long holiday weekend… it's enough to make me shudder.

A better choice for libraries would be a polypropylene case. Polypropylene is chemically non-aggressive, which makes it a good choice for archiving and long-term storage. It is also more flexible and resilient to breaking, so it can withstand heavy use. It also stands up to heat a little better than polystyrene because, although it has a lower melting point, it also has a lower glass transition temperature. We use polypropylene while it's in the soft, bendy phase. So, when the temperature goes up, it gets "bendier" – but it was bendy to begin with. As long as the temperature stays above 14° F (-10° C), it will remain soft and bendy instead of getting brittle. That means if there's a chance it will drop below freezing where you store your polypropylene cases, there's a chance your polypropylene cases could crack.

Here's a couple of polypropylene packaging options that would be a good for libraries:

Poly jewel box This has the same dimensions as a traditional jewel box, so you can use the same inserts and other printed materials with it.

Clamshell CD Jwel Cases The ultra-compact design of the Clamshell jewel case leaves no room for an insert card, so your disc art will get to take center stage.

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