Reactive nitrogen–boron polyols for flame-retardant and low-smoke rigid polyurethane foams

Rigid polyurethane foam (RPUF) is extensively employed in construction and transportation for its superior insulation and structural integrity. However, its inherent high flammability restricts its utility across various applications. The excessive use of traditional flame retardants can lead to a reduction in the physical properties of materials and generate toxic by-products during combustion. To address this challenge, we developed an innovative flame-retardant system for RPUF, using nitrogen-containing polyols synthesized from formaldehyde and melamine, and replacing traditional polyols with 2-methyl-1,3-propanediol (MPD), alongside boron-containing polyols derived from glycerol and boric acid. The incorporation of nitrogen- and boron-containing polyols at a 1:1 ratio has markedly raised the limiting oxygen index of RPUF from 19.1 % (pure RPUF) to 26.8 % while maintaining its mechanical properties, with the compressive strength ranging from 0.48 MPa to 0.50 MPa. Compared to traditional polyether polyols, MPD offers an environmentally friendly alternative by reducing toxic byproduct emissions during combustion, providing a safer profile in fire scenarios. Additionally, compared to pure RPUF, the total heat release and total smoke production were significantly reduced by 68.8 % and 52.4 %, respectively. Thermogravimetric analysis demonstrated enhanced thermal stability and char-forming ability, further confirmed by scanning electron microscopy. These findings suggest that the newly developed system has great potential for enhancing RPUF's flame retardancy and smoke reduction while maintaining its mechanical properties at the same time. • Novel reactive flame-retardant polyols were synthesized and applied in RPUF. • Structural integration of FRP N and FRP B improves mechanical properties. • 2-methyl-1,3-propanediol is used instead of traditional polyether polyols. • Nitrogen and boron-based polyols effectively mitigate fire risk in RPUF.