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.