The potential of recycling carbon fiber reinforced polymers (CFRP) as a sustainable solution for waste management is yet to be fully understood. This study reports on the evolution of mechanical, and chemical properties of reclaimed carbon fibers when recycled multiple times via pyrolysis and partial oxidation. The performed work aims at filling the knowledge gap related to repetitive recycling when moving towards a circular flow of resources. A recycling process existing at industrial scale is used to ensure the relevance and usefulness of the results in the current industry scene. Two sets of three identical model composites are recycled using distinct recycling parameters, and their properties are characterized at the end of each recycling cycle. Results show that recycling can lead to an increase in stiffness but can have a negative impact on strength of recovered fibers. Mechanical behaviour shows recovered fibers suitable for secondary applications with medium performance requirements after two recycling cycles. The findings highlight the importance of understanding the material properties evolution during recycling processes. This research contributes to the development of sustainable waste management strategies and a more environmentally friendly future.

Carbon fiber, despite its exceptional properties, remains underutilized due to monetary and environmental concerns. Concurrently, the imminent challenge associated with rising quantities of End-of-Life CFRP (carbon fiber reinforced polymer) demands the further development of recycling strategies. This study focuses on optimizing the recycling process parameters of pyrolysis and oxidation thermal treatment to maximize the retention of mechanical properties in the recycled fibers in the shortest process time. To assess the result of the pyrolysis, single fiber tensile tests were executed to measure strength and stiffness. Additionally, microscopy and spectroscopy studies were carried out to evaluate fiber geometry as well as surface quality. At the laboratory scale, experiments demonstrated that the combination of pyrolysis and oxidation yields clean, reusable fibers with mechanical properties suitable for secondary applications. The influence of various treatment parameters on the strength and stiffness of the recycled fibers was explored, establishing a clear correlation. The outcome is a set of optimized parameters that contribute to mechanical property retention, including a novel recycling method that allows for reduced processing times, as short as 10 min. This work paves the way for a more eco-friendly and cost-effective approach to harnessing the potential of carbon fiber in a wide range of applications while mitigating environmental concerns associated with landfill disposal.

The research field of composite materials is particularly fascinating due to the design freedom they offer and the infinite number of constituent combinations, including those that are already explored, and many more that are yet to be tried. One composite material that holds great potential contains carbon in its fiber shape. Carbon fibers possess unique properties that excel in mechanical aspects, as well as interesting electrical and thermal properties that are yet to be fully explored. These fibers are readily available on the market and can be introduced as reinforcement in various lengths and orientations, yielding diverse results depending on the intended effect. Although carbon fiber reinforced polymer composites (CFRP) are present on the market for quite some time, specifically in high-performance applications, they are predominantly used when their performance outweighs their cost. Meanwhile, carbon fiber composite waste is starting to cumulate in noticeable amounts. This waste originates from both, production scrap and end-of-life scenarios, as components introduced in service life in the past 30 years are being decommissioned and discarded. Unfortunately, the prevalent solution for handling this waste is landfilling, due to its ease, affordability, and accessibility. Consequently, substantial amounts of composite waste are accumulating worldwide. Furthermore, it has finally come to our attention that our planet's resources are finite. Our exploitation of these resources has been largely devoid of consideration for the needs of future generations. As a result, recently, sustainability has emerged as a key enabler for a circular economy, driven by increasing environmental concerns and demands from customers and users for market transformation. The implementation of sustainable practices is now underway, albeit at a gradual pace.

In summary, we find ourselves facing a trifold predicament: a splendid material being underutilized due to production costs, the cumulative generation of CFRP waste resulting from a lack of foresight and suitable alternatives, and the urgent need to transition towards a circular economy due to resource depletion. This research work aims to address all three challenges by developing an integrated solution.

The current work demonstrates that it is possible to recycle carbon fiber model composites through a two-step pyrolysis treatment, a fully mature recycling technology. The study has been done in two stages which are presented in two journal papers included in the thesis. The primary objective of the first paper is to identify and optimize process parameters that maximize the retention of mechanical properties in the recovered fibers. The overall results achieved show good retention value; with over 90% retention on stiffness and 90% on strength. Encouraging results from initial experimental work, have spurred the research focus towards further investigation. Thus, the second paper reports on repetitive manufacturing and recycling cycles of two sets of identical model composites by using the two most effective recycling treatments identified through the parameter optimization. The mechanical performance and structural changes of the recycled fibers are characterized and analyzed. Although further analysis is required, current mechanical behavior shows recovered fibers suitable for secondary applications after two recycling cycles, with an abrupt decay in fiber properties after the third cycle.

With the waste challenge under control, through successful recycling of composite waste, it is time to find concrete applications for this research. Having recycled carbon fibers (rCF) with comparable performance to virgin carbon fibers (vCF) opens up opportunities for rCF mats and other intermediate products to compete in previously inaccessible markets.

Carbon fiber, despite its exceptional properties, remains underutilized due to monetary and environmental concerns. Concurrently, the imminent challenge associated with rising quantities of End-of-Life CFRP (carbon fiber reinforced composites) demands the further development of recycling strategies. This study focuses on optimizing the recycling process parameters of pyrolysis and oxidation thermal treatment to maximize the retention of mechanical properties in the recycled fibers. To assess the result of the pyrolysis, single fiber tensile tests were executed to measure strength and stiffness. Additionally, microscopy and spectroscopy studies were carried out to evaluate fiber geometry as well as surface quality. At the laboratory scale, experiments demonstrated that the combination of pyrolysis and oxidation yields clean, reusable fibers with mechanical properties suitable for secondary applications. The influence of various treatment parameters on the strength and stiffness of the recycled fibers was explored, establishing a clear correlation. The outcome is a set of optimized parameters that contribute to mechanical property retention and lead to reduced processing times as short as 10 minutes, enhancing the overall sustainability of recycling CFRP. This work paves the way for a more eco-friendly and cost-effective approach to harnessing the potential of carbon fiber in a wide range of applications while mitigating environmental concerns associated with landfill disposal.