Development of flax/rPET thermoplastic tapes: Interfacial phenomena, impregnation behaviour and mechanical performance

The development of sustainable thermoplastic composites requires impregnation strategies capable of overcoming the high viscosity of molten polymers while preserving natural-fibre reinforcements. In this study, continuous flax/rPET thermoplastic tapes were manufactured through a capillary-driven solvent-based impregnation process in which post-consumer PET was dissolved in hexafluoroisopropanol (HFIP) and precipitated within flax reinforcements. The interfacial and processing mechanisms were investigated through thermogravimetric analysis, surface-energy measurements, surface-tension characterization, capillary wicking tests and tensile testing. The two flax reinforcements exhibited different apparent surface energies, with values of 63.6 ± 6 and 55.9 ± 13 mN/m for the 500 TEX and 600 TEX reinforcements, respectively. Capillary-rise experiments yielded mean apparent advancing contact angles of 59.8 ± 14.3 ° and 69.4 ± 6.6 ° , respectively, indicating a tendency towards more favourable wetting for the 500 TEX reinforcement, although the difference was not statistically significant. Thermogravimetric analysis showed that the flax-associated degradation peak remained essentially unchanged under nitrogen after tape manufacturing, while under oxygen it shifted from approximately 335.0 ° C for pristine flax to 376.4 ° C in the flax/rPET tapes. After PET incorporation, the apparent tensile responses of the two tapes became comparable, with RaidTEX values of 6.67 ± 0.46 and 5.59 ± 0.58 cN/TEX for the 500 TEX and 600 TEX tapes, respectively. These results demonstrate the feasibility of manufacturing flax/rPET thermoplastic tapes by solvent-assisted capillary impregnation and highlight the combined influence of fibre surface properties, reinforcement architecture and impregnation behaviour on the resulting tape performance.