Abstract: To develop highly stable fluorine-free foam extinguishing agents, this study used a base system consisting of a nonionic organosilicon surfactant (LS-408L) and a zwitterionic hydrocarbon surfactant, dodecyl dimethyl betaine (BS-12). Nonionic water-soluble polymer guar gum (GG, 0.2%) and hydrophilic fumed silica (SiO₂, 1.5%) nanoparticles were introduced as foam-stabilizing modifiers to construct four fluorine-free foam mixed systems. The surface tension and electrical conductivity of each mixed solution were systematically characterized, foam stability was analyzed, and the evolution of the rheological behavior of both the foam mixed liquids and the foam phase under thermal action from 25 ℃ to 75 ℃ was thoroughly investigated. The results showed that the addition of either GG or SiO₂ alone increased the surface tension and electrical conductivity of the LS-408L/BS-12 system, whereas their combination exhibited a synergistic effect, characterized by reduced surface tension and a continuously increased electrical conductivity. Both GG and SiO₂ delayed the foam drainage process, and their combination achieved the best foam-stabilizing performance. All four foam mixed liquids exhibited typical shear-thinning non-Newtonian fluid behavior. The systems containing SiO₂ displayed viscoelastic solid-like characteristics with a relatively high yield stress, whereas the SiO₂-free system behaved as a viscoelastic fluid. The modified fluorine-free foams showed an elastic response before the crossover point of the viscoelastic moduli and transitioned to a viscous response after the crossover point. Increasing the temperature generally reduced the viscoelastic moduli of the foams; nevertheless, the composite system maintained the best thermal rheological stability and retained good shear resistance even at the maximum temperature of 75 °C. Within the linear viscoelastic region, the complex modulus of system A-3# was 16.60 Pa, the highest among the four systems. This study elucidates the synergistic regulation mechanism of the GG and SiO₂ combination on the thermal rheological behavior of fluorine-free foam, achieving superior foam-stabilizing ability with a relatively low SiO₂ concentration. These findings provide experimental evidence and theoretical support for component optimization and formulation design of highly stable fluorine-free foam extinguishing agents.

Key words: nanoparticles, polymers, surfactants, foam rheology, fluoride-free foams