Monomer selection for natural fibre-reinforced thermoplastic composite manufacture by monomer infusion under flexible tooling (MIFT)

Awareness of the environmental issues has led to increasing interest in using “greener” composite materials to replace synthetic fibre reinforcements and petrochemical polymer matrices. Preventing plastics pollution could become a major constraint on markets for thermoset matrix fibre-reinforced composites in marine applications. Natural fibre bio-based thermoplastic composites could be appropriate alternative materials with the advantages including renewable sources, recyclability and reduced environmental impacts. The selection of the polymer system will significantly affect the cost, manufacturing process, mechanical properties and durability of the chosen composite system. This review considers the available thermoplastic monomers suitable for in-situ polymerisation during resin (monomer) infusion under flexible tooling (RIFT). The criteria for the selection of appropriate monomers are limited by process temperatures (to minimise energy demand and to avoid damage to the natural fibres), and other considerations including monomer viscosity, polymer mechanical properties, and composite recyclability. Given the systems currently available, methyl methacrylate (MMA) seems to be the most suitable monomer for marine composite production. MMA has low process temperatures, a long open window for infusion, and low moisture absorption. However, end-of-life recovery may be an issue. The research literature suggests that bio-based MMA will become available in the near future. Polylactide (PLA) may be an alternative to MMA as the infusible monomer, but the relatively high processing temperature may compromise natural fibre properties, could require expensive consumable materials, and there may be durability issues in the marine environment.