A Review of Fire-Retardant Additives in Polyurethane: Evolution of Formulation Strategies and Fire Testing Methodologies for Aerospace Applications

Polyurethane (PU) is a highly versatile class of polymer utilised in many industries, including the aerospace sector. In conjunction with its superior mechanical properties, chemical resistance, and durability, it can be highly flammable depending on its form. This poses a risk aboard aircraft, which contain numerous fire hazards and cramped cabin spaces, proving an obstacle for the evacuation of passengers in an emergency. Flame-retardant additives have proven to enhance the thermal properties of polyurethane, but their toxicity and tendency to degrade mechanical performance make them unappealing. This review addresses three main topics: (1) the basic synthesis and structure of PU and modification through additives; (2) types of PU, their properties, and applications in the aerospace industry; and (3) evaluation methodologies for characterising PU performance, studying mechanical properties and thermal degradation. Several key challenges remain, including understanding the long-term durability of modified PU, optimising between fire performance and mechanical properties, improving the sustainability of PU throughout its lifetime, and validating numerical simulation as a viable testing method. This review aims to guide future research on modified PU technology to achieve safer, high-performing, and sustainable solutions for the aerospace industry and beyond.