In recent years, the fibre-reinforced polymer composite industry has faced strict global targets to address the end-of-life challenges posed by thermoset polymer matrix composites. Thermoplastic composites, despite their well-demonstrated improved recyclability, thermoformability and weldability, are yet to attain the same technological maturity as thermoset composites. In situ polymerisable thermoplastic resins have been identified as attractive emerging solutions for improving the processibility of thermoplastics. This lecture covers two important families of thermoplastic infusible monomer systems for high fibre volume-fraction composites, namely polyamide-6 (PA-6, processed at temperatures above 150°C) and acrylics (processed at room temperature). A brief survey of active research in these fields is presented, together with some emerging industrial applications for the technologies.

Reactive thermoplastic resin transfer moulding has been used to produce stitched-unidirectional glass fibre reinforced PA-6 composites by employing a simple 2-part injection of low viscosity monomer precursors and in-situ polymerisation. The low monomer viscosity resulted in injection pressures of around 10% of that required for a typical thermoset RTM resin, thus negating the need for expensive equipment. A specially-developed novel reactive fibre sizing was used to compare to a standard silane glass fibre sizing. Transverse mechanical properties of the composites reinforced with the novel reactive sizing were 20–28% higher than those with the standard fibre sizings, demonstrating improved fibre-matrix interfacial properties. Average mode I fracture toughness was also measured to be 10–30% higher. Drop-weight impact testing showed that the different fibre sizings had little effect on the post-impact compressive properties.

Acrylic polymers can be used to infuse composites at room temperature, followed by a chemically-activated polymerisation reaction. This offers potentially recyclable materials for large composite structures, for instance in renewable energy, marine and infrastructure applications, where elevated tooling temperatures would be economically prohibitive. The mechanical and thermomechanical characteristics of acrylic matrix composites have been benchmarked against an epoxy composite reference. The acrylic composite exhibited superior tensile (90°), flexural (0°), interlaminar shear and fracture toughness properties. Thermomechanical characterisation also revealed marked superiority in the damping behaviour of the acrylic composite. Acrylics, however, as amorphous thermoplastic polymers, are highly susceptible to solvent attack which makes them unsuitable for application in particular environments. It has been shown that incorporating up to 5% of an engineering thermoplastic, such as poly(phenylene ether) (PPE) into a reactive acrylic resin to produce a hybrid-matrix system is a simple, yet effective strategy towards enhancing solvent resistance of this family of infusible thermoplastic composites.