this post was submitted on 26 Jun 2023
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From my limited experience with laying up carbon fiber, I know there's the raw carbon fiber cloth and there's liquid resin that you spread into the cloth. It's also very common to see carbon fiber cloth that is "pre-impregnated" - the resin is already applied to the cloth. Everyone calls this "pre-preg".
So I've seen a lot of folks online scratching their heads about "how can carbon expire?" or "my carbon fiber (bike/boat/etc.) is N-years old, is it expired?" but I think the most likely thing to expire is the resin. Once the resin is cured it is much more stable.
Any materials folks or structures engineers who want step in and correct me, please do.
I'm seeing a lot of misconceptions in the replies. You have it mainly right from a very high level.
The reason why prepreg "expires" is simply that the resin system is mixed before being impregnated into the fibers, so it starts the curing reaction immediately. These resin systems are usually designed to cure properly at high temperatures, typically 250-400F depending on end-use, but they'll still slowly react at lower temperatures. To further slow the reaction, prepreg is kept frozen. Prepreg also has two types of expirations: "shelf life" and "out life". Shelf life is how long it can last frozen. Out life is how long it can last at room temp.
Theres a few issues that can happen when using expired prepreg. It can be harder to laminate since it will be too stiff and not as sticky. It won't cure correctly causing failures in the resin.
Expired prepreg can be recertified by testing the material for those types of failures. Check if the prepreg can fold over a certain radius and stick to a certain angle without sliding off. Cure a sample and test it to see if cured properly.