Innovative flame-retardant systems for rigid polyurethane foam: Synergistic effects of nitrogen and phosphorus polyols

Rigid polyurethane foam (RPUF) is widely used for building insulation. However, RPUF has significant safety concerns due to its flammability. It ignites easily, burns rapidly, and releases substantial heat and toxic smoke in a short time. Researchers have explored various additive flame retardants as a solution. However, these additives often weaken the mechanical strength of RPUF, leading to unsatisfactory performance. In this study, we investigated for the first time the application of nitrogen-containing polyols synthesized from the reaction of melamine and formaldehyde, and phosphorus-containing polyols produced from the reaction of trimethyl phosphate and 1,1,1-tris(hydroxymethyl)ethane in flame-retardant RPUF. These polyols were used in a reactive flame-retardant strategy for RPUF. This approach improved the effectiveness of the flame retardants. It also minimized the negative impact on the mechanical properties of RPUF. We combined nitrogen- and phosphorus-containing flame-retardant polyols in a 2:1 ratio. This combination significantly improved flame retardancy and thermal stability. The RPUF composites achieved a limiting oxygen index of 27.2 % and a UL-94 V-0 rating. The composites also showed a significant reduction in total heat release and total smoke production by 55.9 % and 65.8 %, respectively, compared to pure RPUF. These improvements are due to a dense residual carbon layer formed during combustion. This layer shields the RPUF matrix from heat and oxygen. Additionally, the composites reduce the concentration of combustible gases in the gas phase, lowering the overall temperature.