Carbon fibre is a very versatile material which can be used for many different applications. The material is light, but compact at the same time making it a very suitable material for passenger cars and even racing cars. Its only drawback is that it is very expensive, energy intensive to produce and also releases a lot of CO2 into the atmosphere. Moreover, until recently no sustainable processes existed to recycle carbon fibre.
Now (2022), scientists have made significant progress in making carbon fibre more environmentally friendly with the help of a bio-based material which would enable recycling carbon fibre at an industrial scale. Scientists at National Renewable Energy Laboratory have discovered that making carbon fibre composites with bio-based epoxies and an anhydride hardener can turn the material into a product which can be fully recycled by integrating linkages that are more easily degraded. Moreover, the recycling process, methanolysis, can be started at room temperature without negatively impacting the quality or orientation of the fibres. This would turn carbon into a circular product which can be used over many cycles and accordingly also make it cheaper.
Carbon fibre is a composite material made up of long threads of pure carbon and a glue-like epoxy coating called "thermoset." During curing, the molecules in the liquid resin bind with each other and around the woven carbon threads to form a strong and rigid lattice. However, the thermoset-nature of the cured epoxy also makes it difficult to break them apart, without severely damaging the carbon threads. This is why spent carbon applications are often simply brought to landfills without attempting to recycle it.
Therefore, the first task of the team of scientists was to find a material or composite to make carbon fibres more sustainable and recyclable. They started out by exploring the chemistry of different biomass materials in order to create a new epoxy which might fulfil the purpose. They replaced the commonly-used epoxy amines resins with epoxy and anhydrides which they had received from the biological and chemical conversion of sugars. Their experiments proved that reformulated resin showed or even exceeded all the properties today's epoxy amine resins had, but could also make carbon fibres easily recyclable at room temperature. With the help of a special catalyst, the NREL team broke down the bio-based resin at room temperature, a process known as "depolymerisation." This allowed them to recover the carbon threads while preserving their quality and alignment.
Scientists have long sought to make the production of carbon fibres more environmentally friendly. In 2018, scientists designed a novel multiscale hydrothermal carbon layer (MHTCL) for carbon fibre (CF) surface modification. The MHTCL was a multiscale high-disorder amorphous carbon coating with abundant functional groups, a large specific surface area, a high surface energy, and good wetting ability. The functional groups were mainly comprised of C–O and CO groups. In the course of the low-concentration glucose hydrothermal treatment with the carbon fibres (CFs), the glucose produced furan derivative intermediates, which could be absorbed by the surface of the CFs and carbonise continuously, and eventually formed the MHTCL on the CFs. New nucleation centres on the CF surface were produced during the fracture and rupture of the MHTCL in the forming process, which resulted in abundant multiscale irregular particles.
Image: SEM images of samples (a) CF1, (b) CF2, (c) CF3, (d) CF4, (e) CF5, (f) CF6, (g) CF7 and (h) CF8. Images (i-l) were selected from sample CF4 to highlight the fractures of the MHTCL.
Source: Xianfeng Xi, Yousi Chen, Jie Wang, Yaoyao Li/ A multiscale hydrothermal carbon layer modified carbon fiber for composite fabrication/ RSC Advances 8(41):23339-23347, June 2018/ DOI:10.1039/C8RA04064H/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported
In 2019, scientists managed to improve the interface between carbon fibres and polyetheretherketone-based chemical grafting of aminated polyetheretherketone (PEEK-NH2) on carbon fibres to design an interfacial layer which worked well with the PEEK matrix. They also analysed the changed chemical composition, surface morphology, surface energy, and interlaminar shear strength. They found that after grafting the interlaminar shear strength (ILSS) was improved by 33.4% owing to the covalent bonds in the interface layer and that the compatibility between the interface modifier and PEEK was improved. Dynamic Mechanical Analysis (DMA) and Scanning Electron Microscopy (SEM) observation also confirmed that the properties of the modified CF/PEEK composites interface were enhanced.
Image: SEM images of (A) [email protected], (B) [email protected], (C) [email protected], and (D) [email protected]
Source: Elwathig. A. M. Hassan, Tienah Elagib, Hafeezullah Memon, Muhuo Yu/ Surface Modification of Carbon Fibers by Grafting PEEK-NH2 for Improving Interfacial Adhesion with Polyetheretherketone/ Materials 12(5):778, March 2019/ DOI:10.3390/ma12050778/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International
There are several benefits to recycling carbon fibres: recycling and extracting carbon fibre will allow the material to be used in large-scale settings for electric vehicle production, as lighter materials also make the vehicle lighter and leave more space for batteries. It would also make the used carbon fibres more sustainable by 20-40%. Moreover, all of this could be achieved without increasing manufacturing costs, as it could be produced for the same price as petroleum-based epoxy-amine resins are today.
Eventually the new carbon fibre material would enable designing advanced materials with performance and environmental advantages. Using bio-based feedstocks instead of petrochemical feedstocks, no extra energy has to be used to fundamentally change their chemistries, which, in turn, allows for manufacture of more precise and advanced materials in an environmentally-friendly manner. .