Resizing and improving properties of Recycled Carbon Fibers as reinforcement for Composites

Recycling carbon fibers (CF) is becoming increasingly essential due to the high energy cost of virgin CF production and the growing volume of composite waste. In this study, carbon fibers recovered by solvolysis (rCFs) were used as feedstock, and their surface properties were restored through a pilot-scale sizing process. Commercial sizing (HP-206), with and without amine-functionalized carbon nanotubes (CNTs), was applied to improve fiber–matrix interfacial performance. The effectiveness of these treatments was assessed by mechanical testing, short-beam test, wettability, and nanoscale interphase characterization. The rCFs exhibited reduced tensile strength (-21.8%) and interlaminar shear strength (-41.5%) compared to reference fibers (Ref.CF), reflecting surface damage and removal of the original sizing during recycling. Application of surface treatments effectively mitigated these losses: both sized_CF and sized-CNT_CF achieved ILSS values exceeding those of Ref.CF, indicating improved fiber–matrix adhesion. The sized_CF group exhibited the highest tensile strength (2.40 GPa), outperforming both rCF and Ref.CF. Although CNT addition did not further increase tensile strength, it enhanced wettability and maintained ILSS performance. SEM confirmed clean fiber surfaces and uniform CNT dispersion without agglomeration. Nanoindentation mapping further revealed that sizing restored interphase width and hardness, while CNT incorporation delivered the largest and stiffest interphase, consistent with the macroscopic improvements in ILSS. Overall, the results confirm that optimized, scalable sizing treatments can restore and even surpass the performance of virgin fibers, supporting the circular reuse of recycled carbon fibers in high-performance structural composites. • Pilot-scale resizing restores properties of solvolysis recycled carbon fibres. • Resized rCFs surpass virgin fibres in tensile strength and ILSS. • CNT-modified sizing improves wettability and interphase properties. • Nanoindentation shows wider and stiffer interphase with CNT sizing. • Scalable resizing enables high-performance reuse of recycled carbon fibres.