this post was submitted on 26 Jun 2023
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Technology
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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.