Novel in-line plasma compounding of flax fibre-reinforced PA6 composites

Plasma surface treatment of flax (linum usitatissimum) reinforcement has been shown to enhance performance of polymeric composites. However, natural fibre plasma treatment processing methodology has been limited to batch processing, which inherently faces scalability issues. To introduce alternatives to plasma batch modification, three continuous treatments were integrated into established short fibre composite processing steps to enhance flax fibre (FF)-reinforced polyamide 6 (PA6) performance: atmospheric pressure plasma treating the (i) fibre yarn, (ii) matrix, and (iii) both in a novel reactive compounding process. Flax degradation was investigated by thermogravimetric analysis and infrared thermography. The tensile, flexural, and rheological properties of the short fibre-reinforced composites were evaluated. Continuous surface treatment of the flax yarn did not degrade the monofilament tensile strength. Visual inspection and thermogravimetric analysis show that the treatment removed smooth and loose surface features that comprised a thin film of residual waxes and oils. This led to a cohesive composite failure. Composites produced with plasma surface treated FF had the topmost tensile and flexural strength enhancement of 28.1 and 31.7%, respectively, compared to untreated composites. Reactive compounding of FF and PA6 during melt-blending was also effective, yielding a tensile and flexural strength enhancement of 15.3 and 33.5%, respectively. The presented work introduces two viable composite processes. The topmost performance is attained by pretreating the continuous FF yarn because wax is efficiently removed from the monofilament surface, facilitating bonding between the matrix and the bare surface. However, reactive compounding offers a promising method that minimises process steps while sufficiently enhancing composite properties. • Introduction of two continuous fibre treatments to functionalise and remove wax. • Plasma compounding is a promising alternative to ex-situ fibre surface treatment. • Flexural strength enhanced up to 33.5% relative to baseline. • Plasma compounding the matrix alone is not sufficient for performance enhancement.