The integration of repurposed and recycled carbon fibers into high-performance composites is essential to promote their use as structural materials due to their low cost, high formability, and reduced environmental impact. The critical issue and challenge in reusing recycled carbon fiber (rCF) were to fully exert its outstanding properties in an efficient and environmental-friendly way. In the present work, recycled carbon fibers, recuperated from thermoplastic carbon fiber scraps, were extracted by a patented procedure developed by ENEA and mixed with a virgin thermoplastic polymer. Composites with different recycled carbon fiber (rCF) content have been manufactured. The thermo-mechanical and viscoelastic behavior has been investigated. The addition of short fibers has been experimentally observed to slow the stress relaxation of viscoelastic polymers, producing a change in the relaxation time constant. The recycling procedure modified the fiber-matrix interface to explain the relaxation constants’ changes in short-fiber composite, recycled carbon fiber, and stress relaxation. © 2023, ASM International.

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