Plasma-Enhanced Continuous Carbon Fibre-Reinforced Composites Manufactured from Recycled Thermoplastics
Compared to cross-linking thermoset matrix composites, the thermo-reversibility of thermoplastic is an attribute that facilitates the reuse of recovered material to reduce depletion of and reliance on virgin fossil resources. Process development for integrating waste thermoplastics into long-service-life structural materials, such as long-fibre-reinforced composite tape, is especially encouraged. Notwithstanding the numerous tape manufacturing processes described in the original literature, their effectiveness with waste thermoplastic remains unexplored. The research objective of this thesis is to gain an in-depth understanding of continuous carbon fibre (CF) surface functionalisation by atmospheric pressure plasma jet (APPJ) treatments with dry air working gas, then incorporate this understanding to enhance continuous CF-reinforced thermoplastic tape manufactured with novel techniques to facilitate the impregnation with hard-to-recycle waste polymer blends. Two blends, commonly utilised in multilayer film, were investigated: polyamide 6 (PA6)/polypropylene (PP) and PA6/linear low-density polyethylene (LLDPE). A die, consisting of a slot channel with a series of crests, was developed in this thesis to impregnate CF tows with thermoplastic blends. Static and crest pressure in the die facilitated a high degree of impregnation at manufacturing speeds exceeding 1 m.min−1. The melt impregnation and APPJ treatment processes were combined to increase the CF surface O/C ratio by 36% at a pulling speed of 0.83m.min−1. Compression moulded unidirectional CF-reinforced PA6 short-beam strength was increased by 45.2% and 125.4%, relative to the baseline, by plasma surface treating the tow from one side and from two sides, respectively. The surface treatment also increased the mean PA6/PP (50:50 wt.%) matrix composite short-beam strength by 89.8%, compared to the baseline. Performance enhancement by polar CF surface functionalisation was hindered by weak fibre-matrix bonding in blends constituting 80 wt.% non-polar LLDPE. In conclusion, the introduced melt impregnator enables, for the first time, continuous CF-reinforced tape manufacturing from hard-to-recycle thermoplastic blends by elevating the static pressure and manipulating the tow over crests, offering a viable alternative to conventional virgin matrix composites. Moreover, in-line APPJ surface treatment with oxidative species is a promising approach to enhance interfacial strength with polar matrix constituents.
Surface treatment, Fibre-reinforced thermoplastic, Composites processing
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