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Water Film Forming Fire Extinguishing Foam for Fuel Tanks and Gas Stations [9]

1.1.2. Water film forming foaming agent

Aqueous film forming foam (AFFF) was researched and developed since 1960 as a highly effective fire extinguishing agent, used for fires involving flammable liquids that are insoluble in water such as gasoline, oil, etc. [24]. The application of aqueous film forming fire extinguishing foam in some fires is shown in Figure 1.2.



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Figure 1.2 : Fire-fighting foam creates a water film to extinguish fires in oil tanks and gas stations [9]

AFFF is used in civil applications worldwide, in concentrations of 1%, 3% or 6%. The foam concentrate is mixed with water in a certain ratio, carried out in a mixing system to form a water foam solution as follows:

- Concentration 6%: 6 parts foam in 94 parts water;

- Concentration 3%: 3 parts foam in 97 parts water;

- Concentration 1%: 1 part foam in 99 parts water.

1% firefighting foam is essentially six times more concentrated than 6% foam, and 3% foam is twice as dense as 6%. The firefighting properties of foam solutions made from 1%, 3%, and 6% foam concentrates from a single manufacturer are virtually identical.

Figure 1.3 : Foam solution created from 6% concentrated foaming agent – ​​94% water

Lowering the foam concentrate percentage is necessary to produce highly effective firefighting foam. Using 3% foam concentrate instead of 6% foam can halve the storage space, thus reducing weight and transportation costs, while providing the same firefighting performance.

The foam solution after being sprayed will form small bubbles, these bubbles will spread rapidly to form a floating water film on the surface of most hydrocarbon fuels. This spreading property is due to the fluorinated surfactant component in AFFF due to the property of reducing the surface tension of the solution to a low value (15-20 mN/m). As a result, the water solution spreads rapidly on the surface of the hydrocarbon liquid. This formed film includes both the foam layer and the water film. It has the effect of preventing fuel vapor from rising and separating oxygen from the fuel vapor, cooling and finally extinguishing the flame as shown in Figure 1.4. The film-forming property shows that even when the foam bubbles have disappeared, the water film formed from the foam solution still exists covering the surface of the hydrocarbon liquid [25]. Therefore, the search for surfactants focuses on the important factors of the ability to reduce surface tension and form a water film on a certain type of fuel.

Figure 1.4 : Image showing how to create a foam film that spreads on a liquid hydrocarbon fire [26]

AFFF aqueous film-forming foam concentrates are mainly composed of surfactants and foam stability enhancing components such as glycol ether, ethylene or propylene glycol. The surface tension is reduced to low values, the foam is stabilized and more durable when using a combination of fluorinated surfactants and hydrocarbon surfactants. Fluorinated surfactants are used in all current AFFF formulations.

1.1.3. Alcohol-resistant water-based foaming agent

Although AFFF is commonly used for flammable liquid fires, it is not effective for fires of water-soluble flammable fuels such as alcohols, ketones with low hydrocarbon chains and esters because the foam is easily dissolved and decomposed by the fuel [27]. To solve this problem, scientists have researched and manufactured a fire-fighting foam product called alcohol-resistant water-based fire-fighting foam (symbol AR-AFFF) [28].

Typical alcohol-resistant water-based foam concentrates also have the same main components as water-based foam concentrates, including: one or more fluorinated surfactants; one or more hydrocarbon surfactants; solvents such as glycol or glycol ether and other additives such as complexing agents, pH buffers, anti-corrosion agents, etc. [29-31]. In addition, it also has an important component that helps the foam concentrate not dissolve in polar solvents, which is a water-soluble polymer. Fire extinguishing mechanism of foam

Alcohol-resistant water-based fire extinguishing foam is similar to water-based fire extinguishing foam, shown in Figure 1.5. However, the film formed on the surface of the burning fuel, in addition to the surfactants, also contains polymers. These polymers precipitate when in contact with water-soluble fuel, creating a protective film between the fuel and the foam.


Figure 1.5: Fire extinguishing mechanism of fire-fighting foam that creates a water-resistant alcohol film on fuel [29]

High molecular weight water-soluble polymers commonly used in AR-AFFF are polysaccharides, such as xanthan gum. AR-AFFF foams are effective on both hydrocarbon fuels and water-soluble fuels. To form an effective AR-AFFF foam concentrate, a high concentration of xanthan gum is required. However, xanthan gum is a highly viscous substance, which makes it difficult to spray the foam through the nozzle and reduces the foaming ability of the solution. Therefore, the use of some fluoropolymer surfactants that have the same effect as xanthan gum, but with a lower viscosity increase, has also been investigated.

1.2. Components of fire-fighting foam

Through research of reference documents [32, 33], fire-fighting foam concentrates usually consist of hydrocarbon surfactants; fluorinated surfactants; surface-active auxiliaries and additives. The detailed composition ratios of AFFF and AR-AFFF fire-fighting foam concentrates are presented in Tables 1.1 and 1.2.

Table 1.1: Probable formula of water film forming foaming agent [34]


Ingredient

Chemical

Ratio


Foaming

Nonionic surfactants, amphoteric surfactants, anionic surfactants


10-30%


Create membrane

Non-ionic fluorinated surfactants, amphoteric fluorinated surfactants


2-20%

Increase foam stability

Hydroxyethyl cellulose, Gelatin, CMC

0.1-5%

Antioxidant

Buthyl cellosol, Butyl diglycol

2-10%

Antifreeze

Glycerin, Ethylene diamine tetraacetate, triethylamine…

5-10%

Anti-settling agent

Cocamidopropyl betaine

2-8%

Viscosity modifier

Urea

2-8%

Preservatives

Potassium sorbate, Sodium benzoate

0.5-2%

Anti-corrosion agent

Methylchloroisothiazolinone

0-2%

Table 1.2: The formula of the foaming agent that creates a stable water film on alcohol [34]


Ingredient

Chemical

Ratio


Foaming agent

Nonionic surfactants, amphoteric surfactants, anionic surfactants


5-25%

Film forming agent

Amphoteric fluorinated surfactants

5-15%

OSB auxiliaries

Partially fluorinated acrylic copolymer

1-5%

Film forming polymer

Diutan gum, Pectin, Xanthan gum, Guar gum

0.1-2%

Increase foam stability

Hydroxyethyl cellulose, Gelatin, CMC

0.1-2%

Antioxidant

Buthyl cellosol, Butyl diglycol

1-8%

Antifreeze

Glycerin, Ethylene diamine tetraacetate, triethylamine…

1-8%

Urea

0 - 10%

Anti-settling agent

PEG-400, Cocamidopropyl betaine

1-8%

Preservatives

Potassium sorbate, Sodium benzoate

0.1-2%

Anti-corrosion agent

Methylchloroisothiazolinone

0.1-2%

Viscosity modifier


1.2.1. Surfactants

Surfactants are the main and most important components of fire-fighting foams. They have the task of separating oxygen from burning fuel and releasing water to reduce the heat of the fire. The choice of surfactants used in fire-fighting foams depends on the research objectives:

+ Ambient temperature

+ Type of fire

+ Foaming level

+ Product form (1%, 3%...) and other factors

The surfactant used in fire-fighting foam must meet the following requirements:

- Reduce surface tension at low concentrations and maintain this value for the required period of time;

-Environmentally friendly or has little impact on the environment.

The key parameters to determine the effectiveness of a surfactant are the reduction in surface tension and the critical micelle concentration (CMC). In firefighting applications, properties such as foaming ability, foam stability, and spreading are related to the reduction in surface tension. The use of surfactants at CMCs will result in better foaming performance [22, 35].

1.2.1.1. Hydrocarbon surfactants

Hydrocarbon surfactants are substances that can be adsorbed or concentrated at the liquid-liquid interface at dilute concentrations in solution and reduce surface tension. The structure of a surfactant consists of a non-polar hydrocarbon part called the hydrophobic part and a polar part called the hydrophilic part. A typical surfactant has a low molecular weight, the polar part is called the “head” of the molecule and the non-polar hydrocarbon part is called the “tail”. Some hydrophobic tails and hydrophilic heads commonly found in surfactant formulas are shown in Table 1.3. Surfactants are classified into four types according to the chemical structure of the hydrophilic group including anionic, nonionic, cationic and amphoteric. All of these types of surfactants can be used to make foam.

Fire fighting depends on the requirements and functions of each type of foam.

Table 1.3: Some important hydrophobic tails and hydrophilic heads of surfactants [36]


Hydrophobic tail

Hydrophilic head


C n H 2n+1 C n H 2n-1 C n H 2n+1 C 6 H 4 C n F 2n+1

COO K + , SO 3 Na +

PO 3 2 2Na +

OSO 3Na +

OPO 3 2 2Na +

(OCH 2 CH 2 ) n OSO 3 NH 4 +

(OCH 2 CH 2 ) n OH


N(CH 3 ) 2 O, NR 3 + Cl

PR 3 + Cl , SR 2 + Cl

CON[(OCH 2 CH 2 ) n OH] 2

N+(CH 3 ) 2 CH 2 COO

The important factors affecting the foaming ability of fire-fighting foam concentrates are the solubility of surfactants and the CMC concentration. The CMC value is a basic property and the saturation point of surfactants on the air/water surface. The use of hydrocarbon surfactants helps maintain a stable foam structure on the surface, reducing surface tension. Using surfactants below CMC in fire-fighting foams is ineffective because the solution is not saturated, so the maximum surface tension reduction has not been achieved. Using surfactants higher than CMC is wasteful, because when CMC is reached, adding surfactants will not reduce surface tension. Therefore, when using hydrocarbon surfactants, it is necessary to pay attention to its CMC and develop a suitable concentrated foam concentrate formula.

Surfactant molecules exchange inside and outside the micelles, they attach to each other and detach from the air/water interface. When the air/water interface is formed or when foam is formed or the foam cover is disturbed, the micelles act as reservoirs of water and provide surfactant to form a new interface. If the micelles are stable, the foam is also stable. In addition to the above effect, studies have shown that the micelle structure in the foam helps to increase the stability of the foam by protecting the formed foam film [37-39]. Surfactants with high water solubility are often used.

Surfactants are commonly used in firefighting foams [40-43]

Sulfonate surfactants

R- SO 3M

In which: R: alkyl group (straight chain or branched chain) M: Na + , NH 4+

Sulfonate surfactants are less affected by environmental pH, the C-S bond has a fairly high resistance to hydrolysis reactions, they interact poorly with Ca 2+ and Mg 2+ ions (in hard water) so they form less precipitates than carboxylate surfactants. However, sulfonate surfactants are weaker than some other surfactants such as alcohol sulfate and nonion. This is a family of heat-stable surfactants (decomposition temperature > 350°C), including the following types:

- Linear alkyl benzene sulfonate (LAS)


R: alkyl (straight chain or branched chain C10 ÷ C18) in ortho, meta or para position relative to the –SO 3 M group. LAS can reduce the surface tension to low values ​​depending on the alkyl chain structure.

- Olefin sulfonate (AOS)


Surfactants have straight hydrocarbon chains, are heat-stable, soluble in water, can withstand higher water hardness than LAS and reduce surface tension to low values. AOS has foaming ability due to its straight hydrocarbon chains and is more biodegradable than substances with branched chains or aromatic rings.

- Ethoxylate sulfonate

Is a family of surfactants suitable for use under high temperature conditions.

Sulfate surfactants

R – O – SO 3 M

Sulfate surfactants are more soluble but less thermally stable than sulfonate surfactants because the sulfate group is more soluble in water than the sulfonate group (there is more than one oxygen atom in the molecule) and the COS bond in sulfate is more susceptible to hydrolysis than the CS bond in sulfonate. The general advantage of alkyl ethoxylate or propoxylate surfactants with anionic groups such as sulfate and sulfonate is the combination of many hydrophobic and hydrophilic groups that work effectively when used.

Ethoxylated alcohol surfactant

Nonionic surfactants are highly soluble in water. Their hydrophilicity is determined by the hydrogen bonding of water to the ethoxylate group. The more atoms

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