Developing aligned discontinuous flax fibre composites: Sustainable matrix selection and repair performance of vitrimers

Sustainability of fibre reinforced polymer composites has become vital for reaching the global sustainable development goals. Natural fibres, particularly flax, and bioderived matrices are possible sustainable solutions for the composites industry, due to the constituents’ embedded environmental impact reduction. According to the circular economy paradigm, sustainability can also be achieved by delaying the disposal of materials. This work reports the interfacial properties of flax fibres with three potentially sustainable advanced matrices, i.e., a vitrimer that combines the beneficial properties of both thermosets and thermoplastics, an entirely bio-based thermoset, and an advanced thermoplastic resin. Each of the selected matrices offers the potential for either recyclability, repairability, reusability, or the use of renewable sources and a reduction in the emissions of volatile organic compounds. Microbond tests were used to evaluate the interfacial shear strength and critical fibre length. It was found that the vitrimer and the bio-based thermoset matrices had a higher level of adhesion with flax fibres (∼20 and ∼24 MPa, respectively) compared to a traditional epoxy matrix (∼12 MPa); the advanced thermoplastic resin (∼6 MPa) shows the poorest adhesion. The vitrimer matrix was selected as a candidate for a sustainable and repairable discontinuous flax fibre reinforced composite. Mechanical and low-temperature rapid repair performance of an aligned discontinuous flax fibre composite, produced using the HiPerDiF method, were investigated. End-to-end and single patch repair methods were performed: vitrimer matrix composites show the potential for a mechanical strength recovery (∼P-70) that would allow them to be reused over several life cycles, enabling a circular economy.