Developments in flame retardants for heat and fire resistant textiles—the role of char formation and intumescence

The currently available heat and flame resistant textiles are reviewed. These fall into two groups, each of which is based on the use of non-thermoplastic fibres as the major fibre component. The first group consists of flame retarded cellulosic, wool and certain man-made fibre-containing fabrics which are well established in various markets, have moderate to high heat and flame resistance and are available at reasonable cost. Performance limitations are determined by stability of the chars produced following interaction of the flame retardants and the fibre when heated. The second group relies on the exploitation of the inherent heat and flame resistance of aromatic and carbonized fibres which form chars with superior mechanical stabilities, and hence barrier properties, compared with the first group. However, fibres within this group are expensive, often difficult to process and, from an environmental point of view, difficult to recycle. Thermoplastic fibres such as polyester, polypropylene and polyamide, even when flame retarded using either comonomeric, modifications or additives introduced during polymerisation and/or fibre extrusion stages, melt drip and/or form holes when exposed to flame. They cannot, therefore, be used in applications, such as protective clothing and barrier textiles, where sustained thermal protection via char formation is an essential requirement. The mechanism and role of char formation are discussed and the incorporation of intumescents in textile materials explored. Recent developments have shown that combinations of flame retarded char-forming fibres and intumescents may give rise to a consolidated fibrous char-reinforced intumescent char which exhibits enhanced heat and flame resistance compared to individual chars. These interactive fibre-intumescent combinations offer opportunities for creating high performance barrier textiles based on more conventional, cost effective and environmentally acceptable raw materials.