Application of Alkyl Amidopropyl Betaine in Fire Fighting Foam Extinguishing Agent

. In order to improve the fire extinguishing performance of foam fire extinguishing agents in non-polar liquid fires, the application of alkyl amidopropyl betaine with different chain lengths in aqueous film-forming foam fire extinguishing agents was studied. The relationship between the structure of alkylamidopropyl betaine and surface tension, foaming property and foam stability was analyzed. On this basis, different foam fire extinguishing agent formulations were formed, and then the surface tension, foam performance and fire extinguishing performance of each formulation were tested. The results show that the alkyl chain of alkylamidopropyl betaine is directly proportional to the foaming property. The shorter the alkyl chain, the less oily the foam is and the better the foam's anti-burning performance. The combination of alkylamidopropyl betaine with different chain lengths is conducive to comprehensive product foam performance and oleophobic performance to achieve the best fire extinguishing effect.

In recent years, accidents of large oil-filled equipment in substations have occurred frequently, resulting in serious economic losses and social impact [1,2]. The core equipment of a substation is an oil-immersed transformer, which contains a large amount of insulating material and transformer oil.Once an oil-immersed transformer has an operating accident, it will easily lead to high-temperature transformer oil leakage and fire hazards. The national standard stipulates that "the main transformer with a single capacity not less than 125MVA should be equipped with a water spray fire extinguishing system, a synthetic foam spray system or other fixed fire extinguishing devices." Foam extinguishing agent is the most effective extinguishing medium for liquid fires, and it is suitable for preventing and controlling fires in large oil-filled equipment in substations. At present, the performance of commercially available foam fire extinguishing agents is uneven, and the fire extinguishing performance requirements of foam fire extinguishing agents used in substations are relatively low. It is stipulated in 3.2 of " Technical standard for foam extinguishing system " [3] that when an aqueous filmforming foam liquid is used for fire extinguishing, its fire resistance level should not be lower than Class C specified in the current national standard " Foam Extinguishing Agent " [4]. When a fire occurs in a substation scene, the transformer oil burns violently. In order to ensure effective fire extinguishing of substation fires, higher fire extinguishing performance requirements should be put forward for foam fire extinguishing agents.
The key factor affecting the performance of foam fire extinguishing agent is foaming agent, which mainly includes amphoteric, nonionic, anionic and cationic surfactants. The commonly used foaming agent in the daily chemical industry is betaine amphoteric surfactant, which has good foaming ability [5] and foam viscosity. At the same time, the betaine amphoteric surfactant has good biodegradability, and the biodegradation rate can reach 81% (C14-15 betaine) and 91% (C12 betaine) in the closed bottle test [6]. Considering its low biological irritation, good resistance to hard water, and strong compounding ability, the application of betaine amphoteric surfactants in fire-fighting foam fire extinguishing agents has great prospects. In order to improve the fire extinguishing efficiency of foam fire extinguishing agent for Class B fire combustibles, this paper studies the influence of different types of alkylamidopropyl betaines on the performance of new foam fire extinguishing agents, laying a theoretical foundation for the development of environmentally friendly and efficient aqueous film-forming foam fire extinguishing agents, but also provides technical support for the safe operation of the substation.

Experimental method
The relative molecular masses of the three alkylamidopropyl betaines were determined by mass spectrometer in positive ion mode. An aqueous solution with a mass fraction of 2% surfactant was prepared, and the surface tension of the solution at 20°C was measured using a fully automatic surface tensiometer. The measurement results were obtained by taking the average value of two experiments. The Foam test of alkylamidopropyl betaines refers to the determination method of surfactant foaming power [7]. Under the condition that the temperature of the aqueous solution is 50°C, the foam volume of 500mL surfactant aqueous solution dropped from a height of 450mm was measured. The foam solution and deionized water were prepared at a volume ratio of 3:97; the surface tension of the foam solution at (20±1)°C was measured; (5-7) mm thick cyclohexane was added to the foam solution, The interfacial tension was measured after waiting 6 min. The measurement results were obtained by taking the average value of two experiments. Calculate the diffusion coefficient between the foam solution and cyclohexane according to formula (1): In the formula: S--diffusion coefficient, mN/m; γ c -the surface tension of cyclohexane, mN/m; γ f -the surface tension of the foam solution, mN/m; γ ithe difference between the foam solution and cyclohexane The foam generating device is built according to the national standard "Foam Fire Extinguishing Agent", as shown in Figure 1. First, the inlet pressure of the foam gun was adjusted to 0.63±0.03MPa, and the foam expansion ratio and 25% draining time of the foam fire extinguishing agent were tested in a 11.4L/min foam generator. Referring to the fire extinguishing performance test method of low-expansion foam fire extinguishing agent, the fire extinguishing performance of different fire extinguishing agent series on rubber industrial solvent oil fires was tested. The area of the oil pan is set to 4.52m 2 , the fuel consumption is set to 144L, and the preburn time is set to 60s. Adjust nozzle to ensure that the jet continues to hit the centre of the backboard, apply foam for (180±2) s.If the fire within the fire tray is extinguished prior to the end of foam application, record the extinction time as the period from the start of foam application until the time all flames are extinguished within the fire tray. After a further (300±10)s, place a burn-back pot, containing (2±0.1)L of fuel in the centre of the tray and ignite. Record the 25 % burn-back time.

Product structure test
The mass spectra of three different alkylamidopropyl betaines are shown in Figure 2.

Surface activity test
Surface activity is usually expressed by critical micelle concentration cmc and critical surface tension γcmc. They respectively represent the ability of the system to form micelles and the ability to reduce the surface tension of the liquid [8]. Surface tension tests of alkylamidopropyl betaines with different alkyl chain lengths are shown in Figure 3. The surface tension of alkylamidopropyl betaine decreased first and then increased with the increase of mass concentration. In contrast, CAB had the lowest critical micelle concentration. When the mass concentration is 0.1%, the surface tension reaches 34.0 mN/m. When the amount of OLAB added was 0.2%, the surface tension reached a minimum of 29.5 mN/m. When the addition of ODAB is 0.5%, the surface tension reaches the minimum of 27.3mN/m. Alkyl chain shortening of surfactants leads to an increase in critical micelle concentration and a decrease in critical surface tension. The surface activity of surfactants is affected by the chain length of the hydrophobic group and the chain length distribution. The cmc value mainly depends on the length of the hydrophobic group of the surfactant. The larger the chain length of the alkyl chain, the smaller the cmc.γcmc is less affected by the chain length of the hydrophobic group of the surfactant, but greatly affected by the chain length distribution of the surfactant molecule in the saturated adsorption layer. The closer the chain length distribution, the smaller the γcmc value.

Foaming ability and foam stability test
The foamability and foam stability of different alkylamidopropyl betaine aqueous solutions were tested. As shown in Figure 4, the increase of the alkyl chain greatly improves the foamability and foam stability of the surfactant. In general, the lower the critical micelle concentration of a surfactant, the stronger the ability to generate foam. The critical micelle concentration of CAB is lower than that of OLAB and ODAB, so CAB shows more excellent foaming ability at the same concentration. The structure of the surfactant also has a certain influence on the stability of the foam. The length of the hydrophobic chain can affect the arrangement of the surfactant in the liquid film and the diffusion rate of the gas through the liquid film, thereby affecting the life of the foam. As shown in Figure 5, the surfactant is adsorbed on the air-liquid interface of the bubbles. The hydrophilic polar chain ends extend into the water, and the hydrophobic hydrocarbon chains extend in the gas phase. The surfactant aligns and forms bimolecular film bubbles. Bubbles can achieve long lifetimes under the protection of bilayer membranes. As the bubbles are continuously generated, they gradually accumulate on the surface of the liquid to form a foam layer. The growth of the hydrophobic chain to a certain extent is conducive to increasing the interaction force between surfactant molecules, making the arrangement of non-polar groups more compact, thereby enhancing the foam stability.

Preparation of foam fire extinguishing agents with different formulas
The existing foam fire extinguishing agent formula is composed of Capstone 1470, APG0810, CAB, ethylene glycol, diethylene glycol butyl ether, urea, xanthan gum and water. Based on the existing product formula, ODAB and OLAB are used to replace CAB in the foam fire extinguishing agent. The main components and the addition amount are shown in Table 1. The formed foam fire extinguishing agent is mixed with water at a volume ratio of 3:97.

Physicochemical performance analysis of formula
The surface tension, interfacial tension, foaming ratio, and 25% draining time of different product formulations were tested. Surface tension, interfacial tension, and diffusion coefficient are the main ways to characterize the spreading properties of foam liquid. As shown in Table 2, the use of fluorosurfactants in combination with hydrocarbon surfactants exhibits lower surface tension (<18mN/m). When the hydrocarbon surfactant is replaced with a shorter alkyl chain betaine surfactant, the formulation has lower surface tension. Capstone 1470 is a perfluoroalkyl betaine with a relatively short fluoroalkyl chain (C6), which has good environmental protection and low toxicity [9]. As the length of the hydrophobic chain of the alkyl betaine surfactant increases, the hydrocarbon chain may bend to cover part of the fluorocarbon chain. This phenomenon causes a relative decrease in the density of the outermost fluorocarbon chains, resulting in an increase in surface tension. It can be seen from Table 2 that the increase of the hydrophobic chain length of alkylamidopropyl betaine significantly improves the foaming ability and 25% draining time of the formula. This is consistent with the foaming and foam stabilizing properties of surfactants. Adding a small amount of CAB on the basis of ODAB/OLAB can also improve the foam characteristics of the product and achieve better foaming ability and foam stability.

2.6
Analysis of formula fire extinguishing performance Different formulations have completed the performance test of strong release fire extinguishing. As shown in Table 3, when CAB is used as the main blowing agent, the ability to control the fire at the edge of the oil pan is low, the sealing ability of the foam and water film is poor, the flame can burn through the foam, and the fire cannot be extinguished within 3 minutes. When the short alkyl chain betaine surfactant is used as the main foaming agent, it has a strong ability to control and extinguish the fire of fuel. When OLAB is used as the main foaming agent, the product has strong fire extinguishing ability. When ODAB is used as the main foaming agent, the foam's antireburning ability is greatly improved. The longer the non-polar alkyl chain of the surfactant, the better the lipophilicity of the foam produced, and the worse the blocking ability of the fuel. Therefore, when CAB is used as the main foaming agent, the fire extinguishing performance of the product is greatly reduced. The shortening of the alkyl chain increases the HLB value, and the hydrophilic ability of the formed foam is enhanced, which has a stronger sealing ability for liquid combustibles, so the fire extinguishing effect is greatly enhanced. When ODAB is used as the main blowing agent, the foam performance of the product is relatively poor, and the fire extinguishing ability is reduced. As shown in formulation 4, the combination of betaine surfactants with different alkyl chains can improve the foam coverage and water film sealing characteristics of the product. On the one hand, a small amount of long alkyl chain betaine surfactant can greatly improve the foaming ability and foam stability of the foam. On the other hand, a large amount of short alkyl chain betaine surfactants can improve the problem of poor foam sealing. Under the double action of foam and water film, the best effect of fire extinguishing and anti-resurgence is achieved.

Conclusion
The article studies the application of different chain lengths of alkylamidopropyl betaines in aqueous filmforming foam fire extinguishing agents, and draws the following conclusions: (1) The longer the hydrophobic chain segment of alkylamidopropyl betaine, the higher the surface activity and the better the foam performance.
(2) The structure of alkylamidopropyl betaine has little influence on the spreading performance of the foam fire extinguishing agent system, but has a greater influence on the foaming performance and foam stability of the product. The hydrophobic chain growth of alkylamidopropyl betaine is beneficial to Improve the foam performance of the product.
(3) The shortening of the hydrophobic chain length of alkylamidopropyl betaine is conducive to the improvement of fire extinguishing performance. The combination of betaine surfactants with different alkyl chains can improve the foam coverage and water film sealing characteristics of the product.