Recent advances in fire-retardant rigid polyurethane foam

Driven by global environmental concerns, many efforts have been made to develop halogen-free flame retardants for rigid polyurethane foam (RPUF). These environmentally benign flame retardants are mainly divided into (i) reactive, (ii) additive, and (iii) coating types. The last decade has witnessed great progress of these three strategies, which enhance the fire safety of RPUF and maintain even improve the thermal insulation properties. This comprehensive review focuses on the up-to-date design of the reactive, additive, and coating flame retardants, and their effects on flame retardancy and thermal conductivity of RPUF. Moreover, the practical applications of the as-prepared flame-retardant RPUFs are highlighted. Finally, key challenges associated with these three kinds of flame retardants are discussed and future research opportunities are also proposed.