Concentration-dependent effects of Al(OH)₃ nanoparticles on the thermal stability of surfactant-polymer foams

Water-soluble polymers can improve the stability of fluorine-free foams by increasing the viscosity of the liquid phase; however, this stabilizing effect is often weakened under high-temperature thermal radiation. To investigate a strategy for improving the thermal stability of fluorine-free foams, Al(OH)₃ nanoparticles (NPs) were incorporated into a surfactant-polymer composite foam system consisting of the silicone surfactant LS-408L, the gemini hydrocarbon surfactant SM-2B-30, and guar gum (GG). The effects of Al(OH)₃ NP concentration on the basic properties, foaming ability, foam height decay, drainage behavior, temperature distribution, and bubble coarsening behavior of the foam system were systematically investigated. The results showed that Al(OH)₃ NPs exerted a concentration-dependent influence on the foam mixtures. The surface tension slightly increased at low NP loading and then gradually decreased with increasing Al(OH)₃ NP concentration. Electrical conductivity increased after the addition of 1 wt% Al(OH)₃ NPs but decreased at higher NP concentrations. Low NP loading reduced the viscosity, whereas higher Al(OH)₃ NP concentrations increased viscosity. Under thermal radiation from a heat source maintained at 200 °C, the surfactant-only foam without GG or Al(OH)₃ NPs almost completely collapsed after 60 min, leaving a residual foam height of only 0.4 cm. In contrast, the foam containing 0.2 wt% GG and 5 wt% Al(OH)₃ NPs retained a residual height of 6.9 cm and exhibited the lowest drainage height and the best thermal-insulation performance among the present radiant-heating conditions. Temperature measurements at different foam depths showed that higher Al(OH)₃ NP concentrations delayed the rapid temperature increase associated with upper-layer foam rupture and maintained the deeper foam layers at relatively low temperatures. Microscopic observations at 85 °C further showed that the foam containing 0.2 wt% GG and 5 wt% Al(OH)₃ NPs retained identifiable bubble structures after 420 s, whereas the surfactant-only foam almost completely ruptured after approximately 160 s. The improved thermal stability at higher Al(OH)₃ NP concentrations may be associated with enhanced liquid retention, increased viscosity, suppressed bubble coarsening, and the possible accumulation of particle-containing structures in foam films and Plateau borders. This study provides a physicochemical basis for the further development and fire-performance evaluation of heat-resistant fluorine-free foam formulations.