Abstract: This paper investigates the enhancement of flame retardancy in hyperbranched flame-retardant polyurethane rigid foam (RPUF) through the incorporation of modified montmorillonite and sepiolite. The inorganic fillers, montmorillonite and sepiolite, were first functionalized using hyperbranched poly(hydroxymethyl phosphine oxide) phosphonate (PHPOP) and subsequently integrated into the RPUF matrix. A series of composite foams were then developed by varying the filler content. The findings demonstrate that the thermal stability and flame retardant performance of RPUF were markedly improved by the addition of the modified fillers. Cone calorimetry and thermogravimetric analyses reveal that the modified fillers effectively reduce the peak heat release rate and total heat release rate of RPUF, while also enhancing its thermal stability and char formation. Moreover, the study highlights the synergistic interaction between the modified montmorillonite, sepiolite, and PHPOP, which significantly boosts the material's electrical insulation properties and smoke suppression. These results provide valuable insights and methodologies for the flame-retardant modification of polyurethane rigid foam.

Key words: hyperbranched flame-retardant polyurethane rigid foam, montmorillonite, sepiolite, modification, flame retardant properties