Emerging trends in flame retardancy of rigid polyurethane foam and its composites: A review

Owing to the superior thermal insulating attributes of rigid polyurethane foam (RPUF)compared to other insulating materials (expanded and extruded polystyrene, mineralwool), it remains the most dominant insulating material and most studied polymer foam.Like other polyurethane foam, RPUF is highlyflammable, necessitating the incorporationofflame retardants (FR) during production to lower combustibility, promoting itscontinuous use as insulation material in construction, transportation, and others. Thepopular approaches for correcting the highflammability of RPUF are copolymerizationand blending (with FR). The second method has proven to be most effective as there arelimited trade-offs in RPUF properties. Meanwhile, the highflammability of RPUF is still asignificant hindrance in emerging applications (sensors, space travel, and others), and thishas continuously inspired research in theflame retardancy of RPUF. In this study,properties, and preparation methods of RPUF are described, factors responsible for thehighflammability of PUF are discussed, andflame retardancy of RPUF is thoroughlyreviewed. Notably, most FR for RPUF are inorganic nanoparticles, lignin, intumescent FR systems of expandable graphite (EG), ammonium polyphosphate (APP), and hybridizedAPP or EG with other FR. These could be due to their ease of processing, low cost, andbeing environmentally benign. Elaborate discussion on RPUF FR mechanisms were alsohighlighted. Lastly, a summary and future perspectives infireproofing RPUF are provided,which could inspire the design of new FR for RPUF.