Fire-Retardant Coatings (Flame-Retardant Systems)

So far, two types of paints have been developed that delay the spread of fire. These are;

1. Fire-retardant paints and coatings made from non-flammable materials, 2. Materials that serve to isolate or separate a combustible substance from heat and form a foam structure when heated (These types are Intumescent type. Intumescence: The foaming or swelling of plastic or other materials when exposed to high surface temperature or flame).

The areas requiring the application of fire-retardant paints are divided into three general sections. These are;

1. Interior surfaces of structures, 2. Exterior surfaces of flammable materials, 3. Interior spaces covered by non-flammable materials (Painted surfaces of below-deck areas in steel ships). These are compounds mixed into or applied to the surface of combustible materials to reduce or prevent the burning of materials prone to combustion in the face of low-energy flames such as matches or cigarettes. These types of materials have three application methods: 1. Systems applied as coating or surface finish (Durable; easily removed), 2. Systems applied as solution to be distributed within fiber (Semi-durable and balanced), 3. Systems applied as an inherent part of the polymer structure of synthetic fiber (Durable and permanent). This type of application method provides permanent protection and not only allows the material to be extinguished, but also does not leach from the material as a result of washing and cleaning operations. It is permanent. For this type of method; polymers bonded with flame-retardant material or polyester fibers made from polyethylene terephthalate are used. To obtain a sufficient film thickness suitable for the intumescence system, it is recommended to apply at least two thick coats of paint. Intumescence is basically dependent on gas release consisting of the first three ingredients when the paint is heated to sufficient temperature. These water-soluble ingredients are quite resistant to leaching when the paint is formulated appropriately. Combinations of chlorine in borates and chlorine compounds create fused liquid compounds that prevent combustion and block oxygen. Zinc borate is better than lead borate as it is cheaper and has lower density. Antimony trioxide and chlorine create a similar situation. If there is a sufficient amount of antimony trioxide in the paint formulation, chlorinated compounds are not necessary. Besides this, paints containing high amounts of chlorine do not require antimony trioxide. The best result is achieved by using both materials. Brominated compounds have also been found useful as fire-retardant materials. As the amount of pigment in the paint increases, the combustible binder component decreases, so the flammability of the paint also decreases. However, some pigments such as lead white and basic silico lead pigments have fire-retardant effects because they produce non-flammable gases at their decomposition temperatures. Calcium carbonate, chlorine paraffin and antimony trioxide are used together in flame-delay systems. Calcium carbonate is used because it neutralizes any free acidity and the thermal decomposition temperature is sufficiently low for the formation of carbon dioxide, which aids in the fire-retarding function. Natural iron oxide black pigments are also suitable for this purpose in flame-delay paints as they are neither carbon black nor soot black.

Fire-Retardant Paints

Chlorinated rubber resins are used extensively for various purposes in fire-retardant paints and non-flammable paints. These types of paints are gaining increasing importance in both civilian and military sectors. Generally, a distinction is made between non-flammable paints and fire-retardant paints. Fire-retardant paints delay flame spread on combustible surfaces, while non-flammable paints are designed more to provide heat resistance to metal surfaces. When formulating a non-flammable or fire-retardant paint, both the binder and pigment content in the system must be selected with considerable care. Binders containing high amounts of chlorine are particularly effective. Chlorinated rubber resins contain an average of 67% chlorine. It is more appropriate to use antimony oxide as a pigment along with this type of chlorinated rubber resin in fire-retardant paints. Because at the heat of the flame, antimony oxide reacts with chlorinated rubber to form antimony trichloride. The resulting antimony trichloride is considered to act as a flame extinguisher. Generally, a high pigment ratio in this type of paint provides better fire-retardant properties. Furthermore, studies have determined that high chlorine and inorganic pigment content increases the effectiveness of fire-retardant systems.

Flammability Classification

The combustion properties of materials are determined by taking into account ignition time, heat release time, flame spread time, smoke generation rate and time to flaming. These materials are divided into 6 classes according to Euroclasse. The following six classes are compiled from EN 13 501-1: Class (A): Does not contribute to combustion, Class (B): Contributes very limitedly to combustion, Class (C): Contributes limitedly to combustion, Class (D): Contributes to combustion, Class (E): Can be considered combustible, Class (F): No resistance to combustion has been determined.

Intumescent Paints

Intumescence is an interesting phenomenon. The French verb tumere means "to swell". The Latin equivalent tumescere can be translated as "to swell". Therefore, tumid or tumescent means swelling, swollen or puffy, and the process of becoming swollen is intumescence. In terms of flame retardancy, exposure to heat initiates a series of physical and chemical processes extending to the state of swelling. This condition is characterized by fire-resistant insulating foam. The foam functions to isolate heat and oxygen from the fuel source. In order to provide a complete explanation of intumescence, it is necessary to analyze the chemical and physical processes.

Chemical Mechanism of Intumescence

Mechanisms proposed regarding charring have been discussed by experts. Its chemistry is generally explained in terms of simple acid-catalyzed dehydration reactions. Four reactions are shown below. The first two reactions show acid-catalyzed depolymerization. The following two reactions show polymer dehydration in the presence of phosphoric acid. Both reactions essentially lead to the same result. That is, -OCH2 groups at the end of the polymer chain are produced. These groups condense to form carbon-rich char residues. Briefly, the effectiveness of phosphorus compounds is that they are phosphorylated carbonfiers to form polyol phosphates such as pentaerythritol. These polyol phosphates are then decomposed to form char.

Physical Model of Intumescence

Intumescent flame-retardant materials were initially used in paints and coatings. In typical formulations, a phosphorus compound such as ammonium polyphosphate, along with a blowing agent such as melamine, and a polyol such as pentaerythritol to form char are present. Additionally, a binder is needed to hold these ingredients together. With such intumescent coatings, a burning polymer can be visualized as a block consisting of many separate layers. The upper char layer is followed by an intumescent layer in front where foaming reactions take place. Below is an unburned polymer coating layer still containing flame retardant. The bottom layer shows a polymer material protected by an intumescent coating. The char (foam provides a physical barrier to heat) and mass transfer and therefore prevents the combustion process.

Intumescent Paint Ingredients and Functions

Intumescent paints are paints that, when exposed to heat or flame, swell and thicken, creating a carbonaceous foam layer that prevents the surface from contact with air, heat and fire, slowing combustion. They can be applied to wood, plastic or steel surfaces. These paints can swell up to 100 times their thickness with heat, from 1 mm to 10 cm. They activate and begin to swell at approximately 150-200°C. To formulate intumescent paint, 3 types of special materials are required: 1. Carbonifier: Material providing carbon, such as polyols like pentaerythritol or starch. (Carbonifier materials can be found in Perstorp's Charmor products.) 2. Acid-forming material: Examples and decomposition temperatures: • Ammonium polyphosphate (215°C), • Monoammonium phosphate (417°C), • Melamine phosphate (300°C). 3. Spumific: Gas-providing material. Examples and decomposition temperatures: • Urea (130°C), • Dicyandiamide (210°C), • Melamine (300°C).

Operating Mechanism of Intumescent (Swelling) Paints

• Around 200-250°C the binder in the paint begins to melt, thus activating the materials. • Acid-forming material decomposes to form polyphosphoric acid. • The formed polyphosphoric acid reacts with the hydroxyl groups of the polyol carbonifier material to form polyphosphoric acid ester. • These esters decompose to form a carbon matrix and water and phosphoric acid reappears. • The spumific material also decomposes under the effect of heat, generating non-flammable gases, causing this carbon matrix to become foamed. • As a result, a hard insulating carbonaceous barrier has formed adhering to the surface. Intumescent (swelling) paints are normally applied by airless spray to obtain a smooth decorative surface and in some cases may be a complementary design feature. The paints are designed to remain stable at ambient temperatures. The composition of intumescent (swelling) paints is typically based on organic resins such as acrylic rubber or acrylic epoxy. The resins are filled with active ingredients that react in a fire at temperatures of 250°C to form carbonous char or carbonous foam with thermal insulating properties. The resulting char can expand up to 50 times the original coating thickness. This formed char layer reduces the heating rate of steel and thus extends its load-bearing capacity. A basic formulation of an intumescent (swelling) paint contains a different set of ingredients. These are; • An organic binder, • A carbonifier ingredient; Usually penta or dipentaerythritol, • A spumific or blowing additive; melamine or a formaldehyde derivative, • An acid catalyst source; ammonium polyphosphate or boric acid, • A char reinforcing pigment; such as fine glass flakes or microspheres. As temperature increases, the binder resin begins to melt, the blowing additive releases gases that will provide controlled expansion. At the same time, decomposition of the carbon chain and melting and bonding of inorganic reinforcing materials occur. As a result, solidified char is formed.

Main Ingredients of Intumescent Paint

• Carbon donor (example: Charmor), • Acid donor (example: Ammonium Polyphosphate (APP), • Blowing agent (example: Melamine), • Binder (example: Polyvinyl acetate).

Key Characteristics of Intumescent Paint

1. Mostly physically drying, that is, thermoplastic paint systems, 2. High PVC, 3. Has three basic active ingredients: • Acid donor, • Carbon donor, • Blowing agent / Spumific additive. 4. High film thickness (~1000 μm), • Spray gun or brush application, 5. Heat-activated (200-250°C) insulating paint, 6. Has a swelling characteristic of 40-80 times its own thickness, 7. Application: Mostly applied to structural steel.

Possible Improvements in Intumescent Paint Formula

• Active raw material ratios can be changed, • PVC can be changed, • Change between APP types, • Inorganic flame retardants such as ATH (Aluminum trihydrate) can be added, • Halogenated flame retardants can be added, • Inorganic fibers can be added.

Other Important Factors That Can Affect Performance

• Grinding and dispersion must be adequate, • Surface preparation and cleaning are important. Sandblasting can also be done, • Initial primer selection should be appropriate, • Application methods, • Drying. General Assessment of Intumescent Paints The ratios in which different compounds are present in relation to each other are also of great importance. The optimum ratio must be determined experimentally. One or more of these substances can be replaced by others in the same class or group. Further research has shown that more effective intumescent systems are obtained when two or more elements required for intumescence are contained in the same molecular complex. Intumescent flame-retardant substances also work well in cast polymers such as polypropylene (PP). The effectiveness of intumescent type flame retardant stems from the foamed char created on the surface of the combustible material. The resulting char functions as a physical barrier against heat transfer mechanisms that will affect the surface of the combustible material. Charring reduces the rate of increase in temperature of the surface below the char. This is attempted to be explained in the figure below. The char layer also prevents the diffusion of oxygen to the combustion zone. Dripping of melted plastics is reduced with char formation and thus more sources of flame spread are also reduced. Halogenated compounds such as chlorine paraffin are generally used as carbonfiers in intumescent paints. However, intumescent flame retardants are not commonly used in plastics. Nitrogen-based compounds are used due to their generally favorable environmental effects. This also applies to almost all char-forming flame-retardant systems. Nitrogen-based flame retardants have many advantages over other systems because they produce less smoke and less toxic gases. Their smoke is also less corrosive and the resulting polymer waste after use can be more easily rendered inert. The transition to such environmentally friendly flame retardants has attracted intense interest worldwide. The market is working to move away from halogenated flame retardants, but alternative systems are often less effective or more expensive.

Mineral Filler Synergism

It was believed that by adding inorganic fillers to compounds used as flame retardants, their efficiency could be increased. This stems from the layered microstructure of some filler materials and the stabilizing effect similar to the cell structure of the char foam. The addition of filler materials can reduce the amount of intumescent char, but can provide the char with better resistance and cell structure. Following research on commonly and frequently used inorganic fillers at the end of the 1980s, it was proven that they were revealed. These fillers were found to modify the char structure but reduce the effectiveness of the flame-retardant system. The char created by filler addition is harder and more solid compared to unfilled, while reaching a weaker volume. The mechanism used to explain modifications in the physical and chemical properties of the char is based on the likelihood of reaction between the acid phosphorus group and the filler. Recently, careful research comparing the effect of titanium dioxide (Ti02) and tin oxide (Sn02) with the flame-retardant charring effect of ammonium polyphosphate in polypropylene was conducted by Hoechst-Celanese. Additionally, an intumescent made with a nitrogen-containing resin was also used. Titanium dioxide enhanced flame-retardant properties by producing stronger and more compatible high-efficiency char. On the other hand, tin oxide, conversely, produced more porous char in thinner slices and did not increase char yield. Titanium dioxide likely showed a positive effect due to its physical bridging effect in the char, while tin oxide may have shown a negative effect due to some chemical interaction with phosphorus compounds.

Other Intumescent (Swelling) Systems

Recent research has shown that combinations of potassium carbonate with silica gel and pentaerythritol are effective charring flame retardants. Unfortunately, the water solubility of these systems makes them unsuitable for outdoor use. It should be possible to convert these systems to intumescent (swelling) systems. Other new intumescent (swelling) systems can be created by using specially intumescent (swelling), expandable graphite plates within the carrier resin. Under the effect of heat, these plates expand to one hundred times their initial thickness. This type of material is naturally graphite-based and therefore environmentally friendly. On the other hand, the application of nanocomposite clays as flame retardants in polymers is also being investigated.   M. Namık Kayaalp Chemical Engineer Ecelak Boya Kimya Ltd. Şti.     References: 1. Michael and Irene Ash, Formulary of Paints and Other Coatings, George Godwin 1978. 2. Buckman Laboratories International, Inc. 3. Budenheim Ibérica Comercial, S.A. 4. Perstorp com. 5. Pcimag.com 6. Eliokem Inc. Ohio-USA.