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Recycling High-Performance Carbon Fiber Reinfor...

Nondestructive retrieval of expensive carbon fibres (CFs) from CF-reinforced thermosetting advanced composites widely applied in high-tech fields has remained inaccessible as the harsh conditions required to recycle high-performance resin matrices unavoidably damage the structure and properties of CFs. Degradable thermosetting resins with stable covalent structures offer a potential solution to this conflict. Here we design a new synthesis scheme and prepare a recyclable CF-reinforced poly(hexahydrotriazine) resin matrix advanced composite. The multiple recycling experiments and characterization data establish that this composite demonstrates performance comparable to those of its commercial counterparts, and more importantly, it realizes multiple intact recoveries of CFs and near-total recycling of the principal raw materials through gentle depolymerization in certain dilute acid solution. To our best knowledge, this study demonstrates for the first time a feasible and environment-friendly preparation-recycle-regeneration strategy for multiple CF-recycling from CF-reinforced advanced composites.

Recycling High-Performance Carbon Fiber Reinfor...

We take the carbon fiber reclaimed from scrapped parts and apply it to create advanced materials. These materials are used to manufacture new, high-performance parts. Carbon fiber is prized for the durability, strength, stiffness and light-weighting qualities it brings to products. It is especially useful in sports equipment, electronics, automotive and aerospace applications.

Carbon Conversions is the worldwide technology leader in the reclamation of carbon fiber for utilization in advanced materials to manufacture high-performance components. Our primary mission is to profitably create and develop a new ecosystem based on recycled carbon fiber.

Asahi Kasei's method of recycling carbon fiber enables CFRP and CFRTP waste to be decomposed efficiently and inexpensively so that it yields seamless, continuous strands of carbon fiber that retain the same strength and other properties of the original material. (Photo: Business Wire)

The growing use of carbon and glass fibres has increased awareness about their waste disposal methods. Tonnes of composite waste containing valuable carbon fibres and glass fibres have been cumulating every year from various applications. These composite wastes must be cost-effectively recycled without causing negative environmental impact. This review article presents an overview of the existing methods to recycle the cumulating composite wastes containing carbon fibre and glass fibre, with emphasis on fibre recovery and understanding their retained properties. Carbon and glass fibres are assessed via focused topics, each related to a specific treatment method: mechanical recycling; thermal recycling, including fluidised bed and pyrolysis; chemical recycling and solvolysis using critical conditions. Additionally, a brief analysis of their environmental and economic aspects are discussed, prioritising the methods based on sustainable values. Finally, research gaps are identified to highlight the factors of circular economy and its significant role in closing the life-cycle loop of these valuable fibres into re-manufactured composites.

In recent decades, various studies have assessed market requirements for new composites and the amount of cumulating wastes to avoid the inevitable negative consequences. By 2020, The US market for fibre-reinforced composites (FRC) will reach an estimated value of $12 billion, with an annual growth rate of 6.6% [15]. Similarly, by the same year, the annual global demand for carbon fibres (CF) is expected to increase from 72,000 tonnes to 140,000 tonnes, and the CFRP global revenue expected to increase from $28.2 billion to $48.7 billion [16]. To keep up with such a drastic demand for virgin carbon fibre (vCF), the cumulating CFRP waste should be recycled efficiently to reduce environmental impacts and satisfy the need [17]. Indeed, recycling CFRP into a valuable resource is a challenging issue affecting the future of the fibre-based recycling industry [18].

Even though the mechanical recycling process is capable of recycling both CFRP and GFRP, most of the research focuses on GFRP [2, 33, 37]. Discontinuous recyclates and their re-incorporation with low-value applications like fillers or reinforcements can provide the main reasons for such research variation [4]. Besides, CFs are expensive compared to GF. Disrupting their physical integrity by mechanical recycling can lead to economic and fibre property loss. Since the early development of the process, serious drawbacks have been involved, even though studies like Mou et al. [38] showed the improved flexural strength of concrete after the addition of GF recyclates as filler materials. Studies like Pickering [33], however, noted that the GF recyclates used as fillers are not commercially feasible due to the availability of alternative cheap virgin fillers such as calcium carbonate or silica.

Like any other recycling process, pyrolysis also suffers from certain limitations, with the possibility of char formation on the resulting fibre surface considered the most challenging of all [4]. A significant percentage decrease in the mechanical properties can be observed in the recovered fibres due to the char. Methods such as chemical treatment [26, 58] and post-heating the fibres result in reducing the char formation, but only to a certain extent [1]. Recent studies have used carbon dioxide (CO2) and water vapours to remove the char formation from CFRP [59]. Also, oxidising the fibre surface will result in the formation of an oxygen-rich surface, improving the adhesive nature of the fibre with resins [60].

It is easy to achieve a supercritical state with alcohols as opposed to water [116]. Supercritical alcohols possess good recycling capabilities when used with waste polymer composites. Among all the supercritical alcohols, propanol is better than ethanol and methanol. When comparing methanol, ethanol, acetone, and 1-propanol, research showed that methanol has a low mass-transfer rate under subcritical conditions. On the other hand, 1-propanols three atoms of carbon and high solvation capacity performs better than methanol and ethanol [115, 117].

Contradicting the studies with higher resin decomposition ratio, Bai et al. [131] research concluded that the decomposition ratio increase above 96.5 wt% in recycling CF would decrease the recycled fibre strength, indicating the damage to the fibre surface after complete resin removal. In that study, the authors managed to obtain an 85 wt% decomposition ratio by using supercritical water and adding oxygen as a catalyst. In recycling CF using water at CC, catalysts play an important role in boosting the resin decomposition ratio [126]. To analyse, this phenomenon, Okajima et al. [132] investigated the resin degradation with a catalyst (2.5 wt% potassium carbonate) and without a catalyst using subcritical water as a solvent. The catalyst process had a better outcome with only a 15% tensile strength decrease of rCF compared to vCF. However, recent studies have avoided the use of a catalyst, increasing temperature and pressure to maintain higher process efficiency instead [130, 133]. The studies based on the catalyst are summarised in Table 5.

In contrast to Lee et al. [85], a Khalil [149] 2018 study used GaBi LCA software and showed opposite results, in which conventional thermolysis via pyrolysis and solvolysis using supercritical water (SCW) was compared. The study showed that the solvolysis process had a 78 times greater human health impact, 76 times greater ecotoxicity, 17 times greater carbon footprint (global warming) and 3 times greater ozone depletion when compared to pyrolysis. Also, the quantitative evidence proves that CFRP recycling via pyrolysis had the advantage of positive environmental and human health value compared to solvolysis using SCW.

Asahi Kasei, a diversified Japanese multinational company, has developed a new technology for recycling carbon fiber plastic compounds together with the National Institute of Technology, Kitakyushu College and Tokyo University of Science.

Carbon fiber reinforced plastics (CFRP) are highly attractive for various industries in demanding application fields due to their unique balance of rigidity, mechanical strength and light weight. However, CFRPs are expensive and challenging from a recycling perspective, as it is difficult to extract the carbon fibers from the resin after usage.

Together with its project partners at the National Institute of Technology at Kitakyushu College and the Tokyo University of Science, Asahi Kasei has developed a recycling method that allows carbon fibers to be extracted from CFRP or carbon fiber reinforced thermoplastics (CFRTP) used in automobiles. This results in high-quality, inexpensive continuous carbon fiber that can be recycled perpetually, contributing to circular economy.

The conventional technologies for recycling carbon fibers by chopping and re-applying them results in a product with lower quality and less durability, insufficient for high-performance applications. To address this issue, Asahi Kasei developed an electrolyzed sulfuric acid solution method that allows the carbon fiber to retain its original strength and continuous nature while fully decomposing the resin in which the carbon fiber is embedded.

This allows for its continued use in high-performance applications and presents an inexpensive, circular solution to the end-of-life dilemma of carbon fiber plastic compounds. Thus, these carbon fiber compounds present in vehicles for weight reduction. It can be easily and inexpensively be broken down at end-of-vehicle-life and reapplied to new vehicles in the future.

Toho Tenax Europe GmbH (TTE) has developed a new compound, Tenax-E Compound rPEEK CF30, based on recycled thermoplastic carbon fiber material and PEEK (polyetheretherketone) thermoplastic. A serial aircraft wing access panel was manufactured as a demonstrator part. 041b061a72


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