A “layer by layer plus UV curing” strategy for low-smoke, flame retardant polyurethane foams

An efficient flame retardant coating was developed for rigid polyurethane foam (RPUF) through the integration of layer-by-layer (LBL) self-assembly and UV curing. The RPUF surface was first activated by UV ozone to enable covalent grafting of branched polyethyleneimine (BPEI) as the initial layer. Subsequently, L-serine intercalated Kaolinite (K*aol), diethylene triamine penta (methylene phosphonic acid) (DTPMPA), and BPEI were alternately assembled to form two bilayers, followed by the application of a UV curing phosphorus/nitrogen-containing topcoat to construct a dense protective network. The optimized sample (R-K* 5 D 30 -P 8 N 3 ) exhibited a limiting oxygen index (LOI) of 36.9 % and achieved a V-0 rating in UL-94, accompanied by a 31.8 % reduction in peak heat release rate (pHRR) and a 30 % decrease in total smoke production compared to neat RPUF. Even after 12 h of water leaching, it maintained a LOI of 34.0 % and displayed a 300 % increase in time to ignition and 18.6 % reduction in pHRR compared with neat RPUF during cone calorimetry. The rapid formation of continuous physical barriers accounted for the enhanced flame retardancy, while UV curing immobilized the coating layers, imparting long-term water resistance. This facile “LBL plus UV curing” strategy provides an effective pathway toward robust, low-smoke, and flame retardant coatings for polymeric foams.