Flame Retardant Technology, Mechanisms, And Introduction of Flame Retardants for Rubber

Flame Retardant Technology, Mechanisms, And Introduction Of Flame Retardants For Rubber

I. Flame retardant technology for rubber

Except for a few synthetic rubbers, most synthetic rubber products, like natural rubber, are flammable or combustible materials. Currently, the primary methods to improve flame retardancy include adding flame retardants or flame-retardant fillers, as well as blending with flame-retardant materials. Additionally, introducing flame-retardant groups into monomers during polymerization is also an effective technique in flame retardancy. Increasing the crosslink density of rubber products also positively impacts flame retardancy. The flame retardant technology for rubber is briefly introduced as follows:

1. Hydrocarbon Rubbers

Hydrocarbon rubbers include NR (Natural Rubber), SBR (Styrene-Butadiene Rubber), BR (Butadiene Rubber), IIR (Isobutylene-Isoprene Rubber), EPR (Ethylene-Propylene Rubber), EPDM (Ethylene-Propylene-Diene Monomer), etc. Although NBR (Nitrile Rubber) does not belong to hydrocarbon rubbers, its flame retardant technology is very similar to that of hydrocarbon rubbers, so it is grouped with them.

The oxygen index of hydrocarbon rubbers is approximately between 19 and 21, and their thermal decomposition temperature ranges from 200°C to 500°C. Their heat resistance and flame retardancy are generally poor, and the decomposition products during combustion are mostly flammable gases. The commonly used flame retardant techniques for these rubbers are as follows:

Blending with Flame-Retardant Polymers: Blending with polymers such as polyvinyl chloride (PVC), chlorinated polyethylene (CPE), chlorosulfonated polyethylene (CSM), and ethylene-vinyl acetate (EVA) can appropriately improve the flame retardancy of hydrocarbon rubbers. Compatibility and co-crosslinking issues should be considered during blending.

Adding Flame Retardants: This is an important approach to improving the flame retardancy of hydrocarbon rubbers. The synergistic effects of combined flame retardants can further enhance flame retardancy. Commonly used flame retardants are mostly organic halogen-based, such as perchlorocyclopentadecane, decabromodiphenyl ether, and chlorinated paraffin. Inorganic flame retardants like antimony trioxide are often used in combination, along with zinc borate, hydrated alumina, and ammonium chloride. It is important to ensure that halogenated flame retardants do not contain free halogens, as free halogens can corrode equipment and molds during processing and adversely affect the electrical and aging properties of rubber. Additionally, the negative impact of flame retardant dosage on the mechanical properties of rubber should be considered.

Adding Flame-Retardant Inorganic Fillers: Fillers such as calcium carbonate, clay, talc, silica, and aluminum hydroxide can be used to reduce the proportion of combustible organic materials. Calcium carbonate and aluminum hydroxide have endothermic effects during decomposition. However, this method may reduce certain physical and mechanical properties of the rubber, so the filler content should not be too high.

Increasing Crosslink Density: Experiments have shown that increasing the crosslink density of rubber can improve its oxygen index, thereby enhancing flame retardancy. This may be due to the increased thermal decomposition temperature of the rubber. This method has been applied in ethylene-propylene rubber.

2. Halogen-Containing Rubbers

Halogen-containing rubbers contain halogen elements, with oxygen indices generally ranging from 28 to 45. The oxygen index of FPM (Fluorocarbon Rubber) can even exceed 65. Generally, the higher the halogen content in halogen-containing rubbers, the higher their oxygen index. These rubbers inherently have high flame retardancy and are self-extinguishing. Therefore, their flame retardant treatment is easier compared to hydrocarbon rubbers. To further improve the flame retardancy of halogen-containing rubbers, the addition of flame retardants is typically used.

3. Heterochain Rubber

The most representative of this type of rubber is dimethyl silicone rubber, which has an oxygen index of around 25. The practical flame retardant approaches for it include increasing its thermal decomposition temperature, increasing the residue during thermal decomposition, and slowing down the generation rate of combustible gases.

II. Necessity of Rubber Flame Retardancy

With the continuous advancement of technology, rubber products have been widely used in various industries. Rubber products such as wire and cable, rubber ropes, conveyor belts, rubber hoses, air ducts, rubber belts, and those used in the electronics and electrical industry must meet the corresponding national standard requirements in terms of flame retardancy and mechanical properties. The requirements for the flame retardancy of rubber products are becoming increasingly higher, making the development and application of flame - retardant rubber particularly important.

There are many types of rubber, and the combustion properties of various rubbers are different. Most rubbers have a low oxygen index and a relatively low decomposition temperature, making them prone to combustion. Therefore, studying the combustion characteristics of rubber, adding flame retardants, or improving the combustion properties of the rubber itself have become the main ways to prepare flame - retardant rubber.

III. Several Important Ways of Rubber Flame Retardancy

The main ways of flame retardancy are to slow down thermal decomposition and block the combustion process. The specific flame - retardant methods are as follows:

1. Add one or more substances to change the thermal decomposition behavior of rubber, increase the thermal decomposition temperature of the prepared rubber, and reduce the combustible gases generated during decomposition.

2. The added substances can generate non - combustible gases when heated, or produce viscous substances that can isolate oxygen, or absorb heat when heated, so that the three elements of combustion (combustible substances, oxygen, and ignition temperature) cannot be satisfied.

3. Add substances that can capture HO· to interrupt the chain reaction and terminate the flame propagation.

Modify the structure or properties of the rubber molecular chain to improve its thermal decomposition ability or make it inherently flame - retardant.

Since rubber has good compatibility with various additives, adding various types of flame retardants is still an important means of flame - retardant modification of rubber at present.

IV. The Flame-Retardant Effect and Mechanism of Rubber Flame Retardants

• Flame-Retardant Effect of Flame Retardants

The main reason flame retardants exert their flame-retardant effect is that they can prevent or inhibit the physical changes or oxidation reactions of polymers during combustion. Compounds that possess one or more of the following flame-retardant effects can be used as flame retardants.

1. Endothermic Effect

When a compound decomposes upon heating or releases water of crystallization or dehydrates, it absorbs heat, thereby inhibiting the rise in temperature of the material and producing a flame-retardant effect. This is referred to as the endothermic effect. For example, borax, aluminum hydroxide, and calcium carbonate exert flame-retardant effects due to this mechanism.

2. Coverage Effect (Isolation Effect)

At higher temperatures, flame retardants can form a stable covering layer or decompose to generate foamy substances that cover the surface of the polymer. This prevents the flammable gases produced by the thermal decomposition of the polymer material from escaping and provides thermal insulation and air isolation, thereby achieving a flame-retardant effect. Phosphorus ester compounds and fire-retardant foaming coatings are examples of this type.

3. Dilution Effect

The mechanism of this effect involves the generation of a large amount of non-flammable gases upon thermal decomposition, which dilutes the flammable gases produced by the polymer material, preventing them from reaching a combustible concentration. Gases such as CO₂, NH₃, HCl, and H₂O can serve as dilution gases. Ammonium phosphate, ammonium chloride, and ammonium carbonate, for example, release such non-flammable gases when heated.

4. Inhibition Effect

These are inhibitors that can interrupt the free radical chain reactions responsible for ignition and combustion. These substances can repeatedly react with the hydroxyl radicals (·OH) to form water, breaking the free radical reaction chain and inhibiting the oxidation reaction. This prevents the reaction from becoming intense enough to ignite. Even if ignited in a strong heat source, the material will self-extinguish once the external heat source is removed due to insufficient heat to sustain combustion. Commonly used bromine and chlorine-containing organic halogen compounds have this inhibitory effect.

5. Transformation Effect

The role of this effect is to alter the thermal decomposition mode of polymer materials, thereby inhibiting the production of flammable gases. For example, acids or bases can be used to dehydrate cellulose, causing it to decompose into carbon and water instead of flammable gases, thus preventing combustion. Flame retardants such as ammonium chloride and ammonium phosphate belong to this category.

6. Synergistic Effect

This mainly involves the combined use of flame retardants. Some compounds may have no flame-retardant effect or only a weak effect when used alone, but their flame-retardant efficiency can be significantly enhanced when used in combination. For example, the combination of antimony trioxide with halogenated compounds can greatly improve flame-retardant efficiency and reduce the total amount of flame retardant required.

l Main Flame Retardants and Their Mechanisms

Ø Inorganic Flame Retardants

1. Hydrated Metal Oxides
The main varieties include aluminum hydroxide, magnesium hydroxide, and tin hydroxide, among which aluminum hydroxide has the greatest endothermic effect and provides excellent flame retardancy. Their flame retardant action is primarily due to the endothermic effect, and the generated water vapor also acts as a barrier. The greatest advantage of these flame retardants is their non-toxicity. they do not produce harmful gases and can also reduce the generation of CO during combustion, acting as smoke suppressants. The main drawback is their low decomposition temperature, which requires large amounts for application, limiting their use to polymers processed at lower temperatures and with lower requirements for physical and mechanical properties. Additionally, magnesium hydroxide easily absorbs CO₂ from the air to form magnesium carbonate, causing white spots in the products.

2. Boron and Molybdenum Compounds
This category mainly includes boric acid, hydrated zinc borate, zinc molybdate, calcium molybdate, and ammonium molybdate, with hydrated zinc borate being the most effective. These flame retardants melt at relatively low temperatures, releasing water and forming a glassy layer that provides barrier, endothermic, and dilution effects during combustion. Boron-based flame retardants have a synergistic effect with halogen-based flame retardants. Due to their low decomposition temperature, they cannot be used for flame retardancy in polymers processed at high temperatures.

3. Silicon Compounds
These flame retardants can generate a glassy inorganic layer (SiO2) during combustion, which grafts onto the polymer to produce non-flammable carbon-containing compounds, forming an oxygen barrier that inhibits combustion. They also prevent dripping of the polymer upon heating. They do not produce flames, CO, or smoke during combustion and also have reinforcing effects. Therefore, they represent a highly promising class of non-halogen flame retardants for development.

4. Expanded Graphite
This is a newly developed inorganic flame retardant that has been commercialized in the United States. It provides a barrier effect and has a good synergistic effect with red phosphorus, often used together.

5. Antimony Trioxide
Antimony trioxide has little flame retardant effect in non-halogenated polymers and is generally not used alone as a flame retardant. It shows better flame retardant effects in halogenated polymers and has a good synergistic effect when used with halogen-based flame retardants.

Ø Organic Flame Retardants

1. Organic Halogen-Based Flame Retardants
Organic halogen-based flame retardants are currently the most widely used organic flame retardants, primarily bromine and chlorine compounds. Although bromides are toxic, their flame retardant effectiveness is superior to chlorides, requiring less quantity, which makes them popular among users. The flame retardant capability varies among different types of compounds of the same halogen, in the order: aliphatic > alicyclic > aromatic.

Aliphatic compounds have good compatibility with polymers but poor thermal stability; aromatic compounds have good thermal stability but poor compatibility. Aromatic halogen compounds containing ether groups have good compatibility with polymers and high thermal stability, leading to a rapid increase in their usage. The most commonly used brominated flame retardants are decabromodiphenyl ether and tetrabromobisphenol A. Commonly used chlorinated flame retardants include chlorinated paraffins and perchlorocyclodecane. In recent years, a series of high molecular weight halogen flame retardants have been developed, such as tetrabromobisphenol A carbonate oligomers and tetrabromobisphenol A epoxy oligomers, which show promising application prospects.

Halogen-based flame retardants produce non-flammable hydrogen halide gases upon decomposition, providing dilution and coverage effects. More importantly, hydrogen halides can react with ·H radicals generated during combustion, inhibiting the chain reaction of polymer combustion, thus providing an inhibitory effect. This makes these flame retardants highly effective. Brominated flame retardants are more effective than chlorinated ones, mainly because the reaction rate of HCl with ·OH is slower than that of HBr with ·OH.

2. Organic Phosphorus-Based Flame Retardants
Currently, the commercialized ones are mainly phosphate esters, such as triphenyl phosphate (TPP), tricresyl phosphate (TCP), cresyl diphenyl phosphate (CDP), tris(2,3-dibromopropyl) phosphate, and tris(2,3-dichloropropyl) phosphate. Newly developed varieties include quaternary phosphonium salts, phosphazene compounds, and their polyphosphates, which have good high-temperature resistance but are less effective than the former and have not yet been commercialized. The flame retardant mechanism of these retardants can be summarized as follows.

During combustion, phosphorus compounds decompose to form non-flammable liquid films of phosphoric acid, providing a coverage effect. Simultaneously, phosphoric acid further dehydrates to form metaphosphoric acid, which then condenses to form polymetaphosphoric acid, causing the polymer to dehydrate and carbonize, altering the combustion pattern of the polymer and forming a carbon film on its surface to isolate air and prevent the generation of flammable gases, thereby exerting a stronger flame retardant effect. These flame retardants are effective for polymers containing hydroxyl groups, such as cellulose, polyurethane, and polyester, but less effective for oxygen-free polyolefin polymers.

3. Organic Nitrogen-Based Flame Retardants
These flame retardants generate nitric acid upon combustion, which can dehydrate and carbonize polymers, providing a transfer effect. They are mainly used for flame retardancy in oxygen-containing polymers but are not significantly effective for hydrocarbon polymers. Representative products include melamine and its derivatives.

4. Composite Flame Retardants
Organic phosphorus/nitrogen intumescent flame retardants were a hot topic in the development of flame retardants in the 1990s. They are flame retardants that contain both organic phosphorus and organic nitrogen, which can be a single compound (monomeric) or a mixture of two or more compounds (composite), typically mixtures of phosphate esters and their derivatives with nitrogen-containing flame retardants, such as phosphate esters with triazine derivatives, condensates of organic amines, and derivatives of ammonium polyphosphate. Their flame retardant mechanism involves generating a uniform carbonaceous foam layer on the polymer surface during combustion, providing isolation and heat absorption effects. These flame retardants are highly effective, smoke-suppressive, drip-preventing, low-toxicity, and have considerable development prospects.

• Synergistic Use of Flame Retardants

Organic phosphorus-based flame retardants and organic halogen-based flame retardants used together have an excellent synergistic effect. This is because phosphorus-based flame retardants are effective in the liquid and solid phases, while halogen-based retardants are effective in the gas phase. Their combined use can exert a synergistic effect. Additionally, the reaction of phosphorus with halogens to form PX3, PX5, POX3, and other halogen-phosphorus compounds, which are heavier than hydrogen halides, makes them less volatile and more effective in coverage. The synergistic effect of phosphorus and chlorine flame retardants is somewhat lower than that of phosphorus and bromine. Furthermore, the synergistic effect of inorganic antimony trioxide with halogen-based flame retardants is due to the formation of dense antimony halides like SbCl₃ and SbBr₃ during combustion in the presence of halides, which cover the polymer surface providing a coverage effect and can also capture free radicals in the gaseous state, providing an inhibitory effect. Halogen compounds used with silicon powder can also produce a synergistic effect, similar to the use of halogen compounds with phosphorus compounds. The use of phosphorus compounds with nitrogen compounds can accelerate the formation of polyphosphoric acid during combustion, aiding in the formation of the foam layer and preventing the escape of phosphorus compounds with combustion gases, thus providing a synergistic effect. Phosphorus/nitrogen intumescent composite flame retardants are developed based on this principle.

V. Conclusion

Flame retardancy is crucial for rubber products due to their widespread use in various industries and the inherent flammability of most rubber materials. This article provides a comprehensive overview of flame retardant technologies for rubber, including methods such as adding flame retardants or fillers, blending with flame-retardant polymers, and modifying rubber molecular structures. It also discusses the necessity of flame retardancy, key flame retardant mechanisms (e.g., endothermic, coverage, dilution, and synergistic effects), and the main types of flame retardants used, including inorganic and organic compounds.

YINSU Flame Retardant specializes in the development of advanced halogen free flame retardants for rubber applications such as EP-80, XJ-85, XJ-A2 and others. Those products leverage synergistic effects and innovative formulations to enhance flame retardancy while maintaining rubber's mechanical properties. These flame retardants are designed to meet stringent industry standards and provide effective solutions for both hydrocarbon and halogen free rubbers, ensuring safety and performance in diverse applications.