The increasing demand for high-performance materials must be considered within the context of finite resources and growing sustainability challenges. As the availability of raw materials becomes increasingly constrained, it is necessary to consider not only what is consumed, but also how existing materials can be used more efficiently through advances in processing, recycling, and manufacturing. In this context, resource efficiency is no longer limited to reducing consumption; it also involves extending the useful life of materials already extracted from the environment.

One way to achieve this is by recovering and reusing materials at the end of their service life. Rather than being treated as waste, materials can be reintroduced into the value chain through routes that maximize their retained value while minimizing environmental impact. Recycling therefore becomes more than a waste-management strategy; it becomes a means of improving resource efficiency and supporting the transition toward more circular material systems.

Within this framework, recycled carbon fiber (rCF) emerges as a material of growing interest. Recovered from end-of-life composites and manufacturing waste, rCF offers the possibility of retaining much of the value embedded in carbon fibers while avoiding the environmental burden associated with producing virgin carbon fiber (vCF). However, recycling does not reproduce the original material exactly. The recycling process modifies the fibers through thermal exposure, oxidation, and repeated handling, resulting in changes to their structure, surface condition, and performance. Consequently, rCF should not be considered a direct replacement for vCF, but rather a material with its own characteristics, capabilities, and limitations.

The work presented in this thesis demonstrates that carbon fibers can be recovered with high quality through thermal recycling when pyrolysis and oxidation are carefully controlled. The results show that stiffness can be largely preserved, whereas strength is more sensitive to oxidation damage and the accumulation of surface defects. Studies on repeated recycling further reveal that the properties of rCF evolve with recycling history, highlighting the importance of considering recycled fibers as a distinct material system with its own performance window rather than as a degraded version of virgin fibers.

A thorough characterization of recycled fibers is therefore essential. While mechanical properties remain central for structural applications, the performance of rCF cannot be understood solely through stiffness and strength. Surface chemistry, wettability, and fiber–matrix interactions also play a critical role in determining the quality of the composites manufactured from recovered fibers. The findings of this thesis show that recycling affects the fiber surface more significantly than the bulk structure and that these surface modifications strongly influence interfacial behavior. Although re-sizing treatments can improve wettability, they cannot fully compensate for damage introduced during recycling, emphasizing the importance of carefully controlling recycling conditions.

The practical value of rCF ultimately depends on how it is used in new products. Because recycled fibers are typically discontinuous, their most promising applications are found in short-fiber composite systems where the limitations imposed by fiber length are less critical. The manufacturing studies presented in this thesis demonstrate that wet-laid processing provides an effective route for producing homogeneous mats and reproducible laminates from virgin, recycled, and hybrid fiber systems. The resulting composites show that recycled fibers can retain significant stiffness, while hybrid architectures combining virgin and recycled fibers can recover a substantial portion of the strength losses associated with recycling and provide a balance between performance and sustainability.

This thesis lies at the intersection of recycling, characterization, remanufacturing, and environmental assessment. Its overarching objective is to improve the understanding of how recycled carbon fibers can be recovered, characterized, processed, and evaluated to support their integration into more circular and resource-efficient composite systems. The results demonstrate that rCF should not be viewed simply as a lower-performance alternative to virgin carbon fiber, but rather as an emerging material system with its own technical, manufacturing, and sustainability profile. By preserving much of the intrinsic value of carbon fibers while reducing the environmental burden associated with new material production, rCF creates opportunities for new material and product concepts that balance performance, resource efficiency, and sustainability. Ultimately, the findings show that the successful use of recycled carbon fibers depends not only on how closely they replicate virgin materials, but also on understanding and exploiting their own unique capabilities, limitations, and potential contributions to future composite design.