Comparison of vacuum assisted resin infusion and resin transfer moulding on thickness uniformity, surface quality and mechanical properties of flax fibre bio-epoxy composites

Composites composed of plant fibres and bio-based resin systems expand the range of sustainable materials for lightweight applications. However, natural fibres require special handling during manufacturing due to their high hydrophilic nature in order to achieve acceptable quality, and the end properties strongly depend on the applied manufacturing process. In this study composites made of unidirectional and woven flax fibres and a bio-based epoxy resin (Sicomin GreenPoxy® 56) were investigated. The fibres were pre-dried under vacuum, also using nitrogen gas in order to reduce void formation within the final composite. Further, two different manufacturing techniques, vacuum-assisted resin infusion (VARI) and resin transfer moulding (RTM) were considered. The novelty of this work lies in the development of a cost-effective, lab-scale RTM method that achieves high consolidation without a hydraulic press, and the introduction of a nitrogen-assisted (N 2 ) vacuum drying strategy to effectively mitigate fibre moisture and void formation. Laminates fabricated using RTM exhibited reduced thickness variation (standard deviation for woven flax fibres reduced from 0.15 mm to 0.05 mm, i.e. by about 66.7%), improved surface quality and a superior fibre volume content of 48% (unidirectional fabric) compared to those manufactured by VARI (38%). Tensile and flexural tests were conducted to compare the mechanical performance of both manufacturing approaches. Unidirectional flax-fibre composites exhibited a tensile modulus of 27 GPa and a tensile strength of 293 MPa, tested in the 0° direction. In contrast, woven fabric reinforced composites tested in the 0° direction reached a tensile modulus and strength of 13 GPa and 122 MPa, respectively. In flexural testing the overall highest modulus and strength were recorded for unidirectional RTM samples at 26 GPa and 309 MPa, while woven samples reached 13 GPa and 174 MPa. In general, the reproducibility of results in case of RTM was higher than that achieved with VARI.