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Huh, I didn't even know carbon fiber had a shelf life, I figured it'd be pretty stable over time.

Here's the reason why, in case anyone's curious (I know I was):

Fabrics and prepregs also degrade gradually over time. In composite laminates it is not the fiberglass, carbon fiber or Kevlar fibers themselves that initially degrade. It is the sizing or fiber primer however that degrades over time. Sizings are placed upon the fibers to make them more compatible with a given resin system during cure. As time goes on fiber sizings diminish, weakening the future link between the fabric and the resin, thus yielding a possibly weaker composite laminate.

The exact storage life of most sized fabrics is not always listed on a technical data sheet. Often one may need to research further into a manufacturer’s technical manuals for an expected shelf life of a fabric sizing. Fiber sizing life can range from a year to beyond five when stored under favorable conditions. For prepreg materials a general shelf life is first a year. After a year or after initial expiration some fabrics and prepregs may be re-tested for performance against its original stated performance specifications and bonding characteristics. In the case of a DIY project possibly using expired fabrics it is important to test a laminated section of the expired fabric to ensure its quality is up to par for its intended use.