Keeping Offcuts Cold: Cryogenic Processing of Prepreg Platelets to Produce Short Carbon Fiber-Reinforced Epoxy Composites

The typical fly-to-buy ratio in the aerospace industry is ca. 65%, and the growing demand for carbon fiber-reinforced polymer composites (CFRPs) is expected to increase the CFRP manufacturing waste. A particular interesting waste is prepreg offcuts, which still preserve the high value and high-performance virgin carbon fibers embedded within a non-cross-linked epoxy. Here, we report a simple method using cryogenic temperature (−196 °C) to aid the processing of prepreg platelets into randomly oriented short carbon fiber-reinforced epoxy composites, eliminating the processing bottleneck brought by prepreg tackiness. Loop tack measurement showed that prepreg at cryogenic temperature loses its tackiness completely. This is because at cryogenic temperature, the non-cross-linked epoxy resin does not deform (i.e., no viscous losses). This then allowed us to mix the prepreg platelets more homogeneously prior to consolidation and cross-linking. The resulting short carbon fiber composites achieved a flexural modulus and strength of 27 GPa and 172 MPa, respectively, at a porosity of only 2%. By comparison, composites molded from prepreg thermally aged for 70 min (a common method to remove prepreg tackiness in this context) were found to possess a flexural modulus and strength of only 19 GPa and 121 MPa, respectively, 30% lower than those processed using the cryogenic method. The higher mechanical properties of cryogenically processed prepreg platelets are attributed to the better adhesion between the platelets, higher carbon fiber loading, and lower composite porosity. Cryogenically processing of prepreg offcuts may motivate the composite industry to upcycle manufacturing waste, reducing the environmental footprint of the aerospace industry.