With the increased use of carbon fibre polymer matrix composites comes the important question of their management at the end of their life cycle. Given the high costs associated with carbon fibre production, recycling carbon fibres from composite waste is a desirable source of reinforcing fibres for new applications. However, current recycling methods result in recycled carbon fibres that are short with little to no orientation which can only be used in applications requiring intermediate strength at a fraction of the potential of the continuous, aligned virgin fibres. Thus, a method to recycle fibres with their original length and orientation intact is vital to truly realising a circular economy for carbon fibre polymer composites.

Our research introduces a novel hierarchical composite aimed at preserving the length and orientation of carbon fibres on recycling. Virgin carbon fibres are encapsulated in an insoluble epoxy matrix to form tapes that serve as the primary units of the hierarchical structure. The primary epoxy matrix protects the fibres from chemical and environmental elements while maintaining their permanent orientation. The primary tape units are subsequently embedded in a secondary recyclable matrix polymer to make larger composite structures. Elium, a thermoplastic that dissolves in acetone and has mechanical properties comparable to epoxy, was chosen as the secondary matrix of choice in this study. This approach aims to achieve a composite that is mechanical equivalent to thermoset composites while facilitating easy recycling with minimal impact on the fibres in the primary unit.

Hierarchical composite laminates studied up to 3 recycling cycles and their mechanical properties were investigated through lap shear tensile tests, short beam bending tests, ±45° tensile tests, and single fibre push-out tests. Laminates employing Elium as the secondary matrix achieved 60-90% of the mechanical performance of epoxy. Minimal degradation was observed between recycling rounds of Elium composites, implying successful recycling of these hierarchical composites. The fibre length, orientation and volume fraction were fully preserved. Pushout tests revealed that the fibres in the primary tapes were sufficiently shielded from the recycling process. These findings validate the feasibility of using a recyclable, soluble matrix that can rival traditional non-recyclable epoxy composites. Our research aims to serve as a proof of concept, and as such, has ample opportunities for tuning the properties of their hierarchical composite, either through the shape, size and matrix of the primary units or by using different secondary matrices. Such work is anticipated in the future.