Stability Mechanism And Fire Extinguishing Performance Of Highly Stable Fluorine-Free Protein Foam

Aqueous film-forming foam(AFFF)was considered the gold standard for pool fire suppression.However,the core component of AFFF,perfluorooctactyl sulfonate(PFOS),has been restricted and listed as a persistent organic pollutant(POPs)by the Stockholm Convention and thus AFFF containing PFOS has been phased out globally.The current fluorine-free foams generally have poor stability and high viscosity,and the performance of fluorine-free foams cannot meet the application requirements in large petrochemical enterprises,large buildings and other places.It is urgent to develop a low viscosity,high stability fluorine-free foam formula suitable for extinguishing pool fires.In this paper,a highly stable fluoride-free foam with excellent fire extinguishing performance,as well as low solution viscosity,was prepared using hydrolyzed rice protein(HRP)as the foaming agent and metal ions as the stabilizer.Glycoside surfactant was used to tune the performance of the highly stable foam.The critical conditions for the generation of highly stable foam and the foam evolution dynamic were deeply analyzed,the physical and chemical mechanism of fuel-induced defoaming was revealed,and the optimal formulation and foaming parameters of highly stable foam with excellent fire extinguishing performance were determined.The conclusions of this work are as follows:A low-viscosity,highly stable fluoride-free foam formulation was prepared by combining hydrolyzed rice protein(HRP)and metal ions,and the correlation between the critical bubble size and the critical aggregate size for the formation of highly stable foam was given.It was found that the binary complexes formed by HRP and Fe(Ⅱ)in situ could inhibit the liquid drainage,coarsening and coalescence of foam.It was observed that the bubble surface was completely covered by HRP-Fe(Ⅱ)complexes,forming the solid interfaces,and preventing bubble coarsening and coalescence.The excess metal complexes in the continuous phase slowed down the liquid flow in the foam film and the Plateau borders.Based on Plateau’s law and Kelvin foam theory,the correlation between the critical bubble size and aggregate size of foam at a given foam expansion ratio(liquid fraction)was established.The drainage delay of foam was stem from the blockage of aggregates in the Plateau borders,and the restart of liquid drainage was related to the increase in foam bubble size.The influence mechanism of alkyl glycoside surfactant(APG)on the evolution of highly stable protein foam was revealed,and the power law relationship between the average bubble size and time in the self-similar growth stage of foam was confirmed.The addition of APG could significantly reduce the equilibrium/dynamic surface tension and enhance the foaming ability of the HRP-Fe(Ⅱ)solution.The difference in foaming ability was closely related to the adsorption characteristic time of different fonnulations from dynamic surface tension instead of equilibrium surface tension.Adding a small amount of APG led to the desorption of HRP-Fe(Ⅱ)at the air-liquid interface and promoted the conversion of the rigid film interface to the mobile interface,thus the fluidity of aqueous foam was enhanced and the stability was weakened.The interfacial adsorption of the HRP-Fe(Ⅱ)complexes led to a significant increase in the film thickness and inhibited the growth of the bubble size,while the addition of APG resulted in a decrease in the liquid film thickness and accelerated the bubble coarsening rate.It was confirmed that in the bubble self-similar growth stage,the variation of the average bubble diameter with time followed the power-law scaling of 1/2.The interaction between oil droplets and bubble liquid film was studied,the defoaming mechanism of fuel on the foam was revealed,the acceleration effect of fuel on bubble coarsening was confirmed,and the physical model to predict the liquid drainage time of highly stable protein foam was given.For the fluoride-free surfactant-based foam and HRP foam,the presence of oil could destroy the liquid film of foam and accelerate the degradation of foam.The entry,spreading and bridging coefficient(ESB coefficient)could be used to predict the status of oil on the liquid film surface and the stability of foam in the presence of oil.For the fluorinated foam,oil had little effect on the lifetime of the foam,but the presence of fuel could accelerate the coarsening of bubbles at the fuel foam interface.For the highly stable HRP-Fe(Ⅱ)foam,the thermodynamic ESB coefficient could not predict foam stability.A physical model of foam liquid drainage was proposed by considering the drainage delay effect,which coupled the key parameters including bubble size,foam height and effective diffusion coefficient.The theoretical model and experimental results showed that the inhibition of the HRP-Fe(Ⅱ)adsorption layer on liquid drainage and bubble coarsening was the key to the stability of foam on the fuel surface.Based on the national standard fire extinguishing experiment,the optimal formulation composition and foaming parameters of highly stable HRP-Fe(Ⅱ)foam with excellent fire extinguishing performance were determined.The addition of Fe(Ⅱ)significantly improved the fire-extinguishing performance of HRP foam.After the addition of Fe(Ⅱ),the fire extinguishing time was shortened by 3 times,and the burn-back time was increased by 20 times.When the foam expansion ratio was between 8-12 and pH was between 8-10,HRP-Fe(Ⅱ)foam showed the best fire extinguishing performance.The addition of surfactant APG significantly increased the spreading rate of foam on the fuel surface but significantly reduced the extinguishing performance of highly stable HRP foam.Compared with the properties such as foam fluidity and surface tension,the stability of foam/liquid film in the presence of fuel was the most critical factor determining the fire-extinguishing performance of the foam.