Development of fire retardant epoxy-based gelcoats for carbon fibre reinforced epoxy resin composites

Abstract

This research article focuses on the development of multifunctional, flame retarded epoxy gelcoat formulations based on phosphorus-containing, intumescent flame retardant (FR). We prepared epoxy resin-based gelcoats with 5 %, 10 % and 15 % phosphorus (P) content using ammonium polyphosphate (APP), and compared their thermal stability, glass transition temperature, crosslinking reaction enthalpy and fire performance to the non-flame retarded reference and a commercial FR gelcoat. We coated carbon fibre reinforced pentaerythritol (PER)-based epoxy composites (non-FR, and FR) with these gelcoats in two thicknesses (0.5 mm and 1.0 mm). The APP-based gelcoats outperformed both the commercial reference and FR gelcoat in terms of thermal stability. When the APP concentration in the gelcoat was increased, the thermal stability and the limiting oxygen index (LOI) increased. All FR gelcoats reached the self-extinguishing (V-0) classification during the UL-94 test and showed remarkable char formation during the mass loss calorimetry (MLC) test even at a lower heat flux (25 kW/m2). In terms of heat release rate, the gelcoat with 10 % P from APP behaved similarly, while the formulation with 15 % P from APP outperformed the commercial FR gelcoat. The tendency in the fire performance of the coated composites was similar to the behaviour of the gelcoat matrices: above 10 % P content, the coated sample outperformed the commercial FR gelcoat. The addition of a liquid, organophosphorus flame retardant (resorcinol bis(diphenyl phosphate) (RDP), acting mainly in the gas phase) to the epoxy matrix of the composite resulted in the further decrease of heat release rate. Non-FR gelcoats considerably deteriorated the fire performance of the fibre reinforced composites, even when applied in a thickness of 0.5 mm. If a gelcoat layer is necessary on the surface of the composite part, a substantial increase in heat release must be considered, or an FR multifunctional gelcoat should be used as an alternative. We reached a 67 % reduction in peak heat release rate when we replaced a 1 mm thick non-FR gelcoat with a gelcoat containing 15 % P on the PER reference composite.