The main chain structure of natural and synthetic rubber is mainly composed of C-C bonds, while the main chain structure of silicone rubber is mainly composed of Si-O bonds. There are various types of side groups such as methyl, methyl vinyl, and methyl phenyl vinyl, which belong to semi inorganic, main chain saturated, amorphous, and non-polar elastic rubber materials. There are no double bonds in the main chain, the silicon oxygen chain has a spiral structure, and the high Si-O bond can make silicone rubber exhibit excellent characteristics in thermal stability, radiation resistance, weather resistance, high and low temperature resistance, elasticity, physiological inertness, aging, etc. It is widely used in fields such as wires and cables. With the continuous development of the national economy, industries such as conveyor belts, air ducts, seals, household appliances, ships, and automobiles have also put forward certain requirements for flame retardancy of silicone rubber.

Due to its low oxygen index and poor self extinguishing performance, silicone rubber is highly prone to combustion and fire when exposed to high temperatures, and the toxic and harmful gases released pose a serious threat to human health. In response to this phenomenon, reducing the flammability, flame propagation speed, smoke generation, and toxic and harmful gas emissions of silicone rubber has become a continuous research and exploration direction in the field of flame retardancy in recent years.

1. Aluminum magnesium inorganic flame retardant

Common flame retardants of this type include aluminum hydroxide and magnesium hydroxide, which have the advantages of low cost, environmental protection, and low smoke. This type of compound usually has a moisture content of over 30% and releases water through heating to lower the temperature of the combustion system. Its flame retardant effect is reflected in both solid and gas phase flame retardant mechanisms. The higher decomposition temperature also gives it a certain smoke suppression effect. During the combustion process, it can form a protective layer of alumina and magnesium oxide, which also has a certain catalytic effect on carbonization when preventing heat transfer. Due to the poor compatibility between such flame retardants and silicone rubber, a large amount of addition can lead to a decrease in the physical and mechanical properties of silicone rubber. Therefore, amino silane, vinyl silane, and hexamethyldisilazane are commonly used as bridging agents to surface treat aluminum magnesium flame retardants and improve compatibility with silicone.

2. Phosphorus containing flame retardants

Phosphorus containing flame retardants are mainly divided into inorganic flame retardants mainly composed of red phosphorus, microencapsulated red phosphorus, and ammonium polyphosphate, as well as organic flame retardants mainly composed of phosphate esters, phosphophenanthrene, phosphoronitrile, reactive phosphate esters, organic hypophosphates, and organic hypophosphates. This type of flame retardant generates small molecule substances such as PO •, PO2 • free radicals when heated, which react with H •, HO •, and O • free radicals generated during the combustion process, interrupting the combustion chain reaction. The phosphoric acid, metaphosphate, polyphosphate, and pyrophosphate formed after heating help to dehydrate the flame retardant into carbon. The phosphorus rich substances formed by self combustion will adsorb on the surface of silicone rubber, isolating the contact between combustible materials and oxygen, and can also reduce the heat feedback intensity of flame combustion on internal combustible materials. When used in combination with nitrogen and halogenated flame retardants, the flame retardant effect is even better.

3. Nitrogen containing flame retardants

Nitrogen containing flame retardants mainly include melamine and its salts, commonly including melamine and melamine cyanurate. This type of flame retardant has a high thermal decomposition temperature, and the non combustible gas it decomposes after being heated is harmless to the environment. It can significantly reduce the temperature of the combustion system and the oxygen concentration around combustibles, and has a good flame retardant and smoke suppression effect on silicone rubber. It is currently a new hot research direction in various sectors of society.

4. Boron and antimony flame retardants

The common boron based flame retardants include boric acid, boron oxide, and borate salts. Boron based flame retardants (boric acid, boron oxide, borate) will undergo melting phenomenon after being heated, and the crystal water released by itself can absorb heat and reduce the temperature of the combustion system. At the same time, it can also isolate a certain amount of heat and oxygen through the formed glass cover layer, which has good stability and low toxicity.

Antimony trioxide is one of the most commonly used antimony based flame retardants, but similar to zinc borate, it has a poor effect when used alone. Only when used in combination can it exhibit good flame retardant synergistic effects.

5. Silicon based and platinum based flame retardants

Silicon based flame retardants can be divided into inorganic silicon flame retardants mainly composed of inorganic silica gel, silicate (montmorillonite), and talc powder, as well as organic silicon flame retardants mainly composed of organic silica gel, polysiloxane, and silicone resin.

Platinum based flame retardants can not only enhance the crosslinking degree of silicone rubber, but also form an isolation layer to suppress or delay combustion during combustion. They often appear in the form of halogenated platinum acid, halogenated platinum, and platinum atom centered complexes. When used alone, they have little flame retardant effect and are often used in combination with other types of flame retardants. When such compounds encounter elements such as N, O, P, S, and their compounds, they will become toxic and fail, causing certain effects on the vulcanization rate and degree.

6. Intumescent flame retardants

Intumescent flame retardants can be divided into organic intumescent flame retardants (CIFR) and inorganic intumescent flame retardants (PIFR) mainly composed of expandable graphite EG. They are a type of flame retardant that is heat-insulating, oxygen resistant, halogen-free, environmentally friendly, drip free, and self extinguishing. Organic flame retardants mainly consist of acid sources, carbon sources, and gas sources. Common acid sources mainly include phosphoric acid, boric acid, phosphate, ammonium polyphosphate, etc., carbon sources mainly include pentaerythritol, and gas sources mainly include urea and melamine. The incombustible gas generated by the incombustible gas expands the carbon layer formed by melting, forms a foam carbon protective layer with certain strength on the surface of silicone rubber, delays or suppresses the decomposition rate of combustibles in the combustion system, thus inhibiting the further spread of the flame.