The flame retardancy of polyamide 6—prepared by in situ polymerisation of ε-caprolactam—For T-RTM applications

The complexity of recycling crosslinked polymer composites has promoted the use of thermoplastic in situ polymerisable matrices, including ε-caprolactam-based polyamide 6 (PA6) in long-fibre reinforced composites. Their uptake by the automotive industry has created a need for adequate flame retardancy, which poses several challenges: flame retardants may hinder the in situ polymerisation reaction; moreover, the fibre reinforcement can filter out the additives not soluble in ε-caprolactam during liquid moulding composite processing, such as thermoplastic resin transfer moulding (T-RTM). We discussed the feasibility of the flame retardancy of ε-caprolactam-based PA6 and its composites, including the solubility of the additives in ε-caprolactam and their effect on the polymerisation reaction and fire performance. We evaluated the correlations between the flame retardant structure and the inhibition of polymerisation. Finally, we briefly summarised the composite processing and coating technologies suitable for preparing flame retarded ε-caprolactam-based PA6 matrix and composite systems, as well as the most often used tests for their characterisation.