Non-destructive retrieval of carbon fibres from CF-reinforced thermosetting composites has remained an impossible process for material scientists, as the harsh conditions required to recycle these class of composites damages the matrix and the carbon fibres within it- rendering the carbon fibres unusable. Now, a team of researchers from Guangzhou, China have developed an eco-friendly preparation-recycle-regeneration solution to the battle between high composite performance and non-destructive carbon fibre recovery.
High-performance advanced composites that consist of a thermosetting resin matrix reinforced with carbon fibres are a widely-used material in many high-end applications. Because they are so widely-used, they produce an awful amount of waste and current efforts to recycle the carbon fibres within the matrix are chemically harsh and only end up damaging the fibres, to the point where they can’t be used again. The internal structure of the composites is very stable and can’t be broken down by gentle heating, dissolution or other traditional recycle methods. There is now a consensus to produce effective and non-destructive alternatives to recover the expensive carbon fibres and thermosetting matrix materials.
Many recycling methods have been put forward in an attempt to solve this problem, but even advanced recycling techniques that have been recently reported are too harsh and will destroy the size, ordering or surface morphology of carbon fibres. Some of these methods can produce recycled carbon-fibres that are fine for low-end applications, but could never be recycled back into a similar high-end application that they came from. This puts restrictions on the commercial viability of the recycle process and limits their practical use.
To solve the issue of an ineffective recycle process, researchers have turned to a different approach. There have been some promising discoveries that has led to the development of a novel method by these researchers. Previous research groups have recently changed the composition of many thermosetting composites, to form a new thermosetting material that retains its high performance, but can also be recycled much more easily. This has allowed the design and synthesis of a novel composite to be formed, that not only exhibits the same performance as commercial composites, but also allows for multiple recycling of the carbon fibres within the material.
The Eco-Friendly Method
The formation of the recyclable material itself is a multi-step wet synthetic process that involves the formation of a resin, carbon fibre prepregs and a composite laminar structure using moulds. Both the thermosetting resin matrix and carbon fibres are recyclable in this material and can be implemented as an eco-friendly full-recycle process.
Unlike other composites, the resin can be easily broken down through depolymerisation by a mild acidic solution. The acidity of the pH solution is strong enough to break down the matrix, but gentle enough to leave the carbon fibres intact. The depolymerisation of the matrix fully dissolves the resin allowing for it be collected as a powder (after drying). The depolymerisation process also exposes the carbon fibres, which can be removed and reused in other composite materials. To determine that the structures had not degraded during this process, the researchers performed SEM, XRD, XPS, carbon fibre monofilament tensile testing and photographing analysis.
The researchers have a documented recovery processes greater than 90% from the recycle methods. In addition to a high recoverability, the structural architectures and mechanical properties of the materials have been proven to be unchanged, even after multiple recycles.
This process not only provides a synthetic route to produce a recyclable thermosetting polymer, the recycle process itself is eco-friendly, non-destructive and provides a high recoverability for multiple recycles. The method provides practically relevant preparation-recycle-regeneration solution to the balancing issues between having a high-performance composite and a recyclable composite, where it has the potential to be used commercially to replace current carbon-fibre reinforced composites on the market today.
Yuan Y., Sun Y., Zhao J., Liu S., Zhang M., Zheng X., Jia L., Multiply fully recyclable carbon fibre reinforced heat-resistant covalent thermosetting advanced composites, Nature Communication, 2017, 8, 14657