Thermal and Acoustic Properties of Flax and Hemp Epoxy Bio-Composites Fabricated Using Vacuum-Assisted Resin Infusion Moulding

Synthetic fibre composites deliver high mechanical performance at a substantial energy and carbon cost, motivating a shift to renewable reinforcements. Flax and hemp are credible candidates, but their uptake is constrained by thermal stability and inherent combustibility, and their performance depends strongly on how completely the laminate is consolidated during manufacture. Vacuum-Assisted Resin Infusion Moulding (VARIM) yields low-void, well-consolidated laminates, yet the thermal and acoustic behaviour of VARIM-processed flax and hemp composites has not been compared directly. This study evaluates unidirectional flax/epoxy and hemp/epoxy laminates, each consisting of five plies produced under identical VARIM conditions, using TGA, DSC, limiting oxygen index (LOI) and UL 94 HB testing, SEM, and four-microphone impedance tube transmission loss (TL) measurements (ASTM E2611). Both laminates remained thermally stable to approximately 200–220 °C. Hemp/epoxy recorded a higher 5 wt.% degradation temperature (222 versus 207 °C), char residue (10.22 versus 9.29 wt.%) and average TL (13.22 versus 11.56 dB, 250–2000 Hz), while both laminates returned an identical LOI of 21.54% and a UL 94 HB rating. The fibre-governed properties therefore differed between the two laminates whereas the flammability response, governed by the shared epoxy matrix, did not. The glass transition temperature also differed (71.9 versus 65.3 °C) but is attributed to a small difference in degree of cure rather than to the reinforcement. The results provide baseline data for selecting VARIM-processed bio-composites for thermal management and passive noise control. The findings support Sustainable Development Goals (SDGs) 9, 12, and 13 through the development of sustainable, low-carbon bio-composite materials.