Heat Resistance of Carbon Fiber and Resin Tg
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Carbon fiber itself handles far more heat than a car or machine will throw at it. The resin holding it together is the limit. Above a certain temperature the resin softens, and the part loses stiffness and strength quickly, especially in compression and shear.
Glass transition temperature
That softening point is the glass transition temperature, or Tg. Below it, cured epoxy is hard and glassy. Through the transition it turns rubbery. Tg isn't a single sharp point, and the number depends on how it's measured, so compare values from the same test method. DMA is the common one.
Typical numbers
- Room temperature cure laminating epoxies: often only around 50 to 60 °C unless they are post-cured. A post-cure at a higher temperature can raise that substantially.
- Prepregs cured around 120 °C: Tg commonly in the 120 to 150 °C range, depending on the system and cure.
- Prepregs cured around 180 °C: Tg often around 180 to 220 °C.
- High temperature systems such as bismaleimides and polyimides go much higher, at a much higher price.
Moisture lowers Tg. Epoxies absorb a little water over time, and the wet Tg can sit noticeably below the dry value on the data sheet.
How much margin to leave
Aerospace practice, from the CMH-17 composites handbook, sets the maximum operating temperature at the wet Tg minus 28 °C (50 °F). It's conservative, but it's a good starting point for any part that carries load. A cosmetic cover can live closer to the limit. A structural bracket shouldn't.
Where it bites
- Engine bays, exhausts and brakes. Parts near hot components need a high-Tg resin, heat shielding or both.
- Sun on dark surfaces. A black part in strong summer sun can get well above 70 °C, already past the Tg of an epoxy that was never post-cured.
- Paint and clear coat ovens. A part that goes through a bake cycle needs a resin that survives it.
- Enclosures around warm electronics, which should be designed with the resin's limit in mind.
What to ask your supplier
- Which resin system, and what are its dry and wet Tg?
- Is the part post-cured, and at what temperature?
- What is the highest temperature the part will see, including sun and nearby heat sources?
Matching the resin to the job is part of our carbon fiber work. For other limits, see carbon fiber vs aluminum.