The main chain of silicone material contains "silicon-oxygen-silicon" structure, and the side chain contains a small amount of organic groups. Therefore, it has the dual advantages of both organic and inorganic materials, such as high and low temperature resistance, radiation resistance and weathering resistance (no double bond, not easy to be decomposed by ultraviolet light and ozone), hydrophobicity, physiological inertia, oil resistance, solvent resistance and so on, which other materials can not be compared with.

Silicone rubber foaming material (also known as silica gel foamed cotton/silica gel) is a porous, low density, compressible high molecular elastomer material, which is prepared from raw rubber, filler, vulcanizing agent and foaming agent of silicone rubber, after mixing uniformly and foaming vulcanization at high temperature. Nowadays, silicone rubber foam is widely used because of its excellent properties of silicone rubber and foam material. It has high elasticity of rubber, and has the properties of sound absorption, shock absorption and so on. It is mainly used for cushioning pad, sealing pad, sound insulation material, insulating material, aviation thermal insulation material and other high performance materials.

1. General properties of silicone rubber

Silicone rubber has many excellent properties:

(1) High and low temperature resistance

Silicone rubber has excellent mechanical properties at high temperature (above 200 C), and still has certain toughness and elasticity. The glass transition temperature (Tg) can be as low as - 70 C. Generally speaking, the working temperature of silicone rubber is - 100 C to 350 C. It can withstand thousands of degrees of instantaneous high temperature, so it is often used to make thermal protection coatings, such as aviation.

(2) Electrical insulation

Silicone rubber has good insulation. After combustion, silicon dioxide is still an insulating material. In addition, silicone rubber is not easy to scorch when discharged by arc, because the number of carbon atoms in its structure is small, and its reinforcing filler silica is suitable for high voltage applications.

(3) Physiological inertia

The surface of silicone rubber is hydrophobic, so the physical properties of silicone rubber decrease little in wet environment. Poor cohesion can play a role in isolation. It is non-toxic, tasteless, harmless to human health, and can only react slightly with the body, which has created its excellent physiological inertia, at the same time, it is also very superior in the physiological aging performance.

2. silicone rubber foam (foaming silica gel) characteristics

Silicone rubber foam is widely used. Because of its excellent properties of silicone rubber and foam material, it has high resilience and aging resistance, and has the properties of sound absorption, damping and other foam. It is widely used in waterproof, shock absorbing, sealing gaskets (sealing grade IP68), noise insulation, insulation, aviation insulation and other performance requirements. According to the bubble structure, silicone rubber foam is divided into three types: closed, open and mixed.

Closed cell foaming silicone foam materials have better shockproof cushioning, sound insulation, heat insulation and flame retardant and explosion-proof properties. They are mainly used in automotive field for insulation foam tube for automotive air conditioning, automotive shock absorption, foaming silicone seal gaskets for new energy vehicle batteries, etc.

At present, most automotive interior materials, such as flooring, ceiling, steering wheel, car seat and so on, are polyurethane foam materials. This is because the technology of polyurethane foam is relatively mature, and its performance meets the needs of use. On the other hand, the price of polyurethane foam is low. However, polyurethane foam has poor weatherability and flammability and releases a lot of toxic gases harmful to human body during combustion. Therefore, with the popularization of foamed silica gel materials and the improvement of people's understanding of them, the future foamed silica gel materials are expected to replace traditional polyurethane foaming materials.

Density Selection of 2.1 Foamed Silica Gel

The density of silicone rubber matrix is 1.17g/cm3. However, through foaming treatment, the density of silicone rubber foam material produced in the current mature process can be 0.16-0.20 g/cm3, which can be used for car seat and headrest. While conventional silicone rubber foam material (density of 0.45 g/cm3) is widely used in sealing, shock absorption and other parts of the gap filling.

Flame Retardant Properties of 2.2 Foamed Silica Gel

Vertical combustion method is often used to characterize the flame retardancy of polymers. Its combustion mode is very close to the actual situation of object combustion, so it can well reflect the actual flame retardant effect of silicone rubber. The effect of flame retardant dosage on the vertical combustion performance of silicone rubber is shown in Table 2. From Table 2, it can be seen that silicone rubber without flame retardant burns completely; silicone rubber with 40 phr of flame retardant reaches FV-1 grade and has certain self-extinguishing ability; silicone rubber with 80 phr of flame retardant reaches FV-0 grade, showing good self-extinguishing ability.

Electrical Insulation Properties of 2.3 Foamed Silica Gel

With the increase of the amount of physical filler, the volume resistivity and surface resistivity of silicone rubber decrease, while the dielectric constant and dielectric loss factor increase. It can be seen that the electrical insulation properties of silicone rubber are damaged to a certain extent by adding physical fillers.

3. Application of Foamed Silica Gel in New Energy Vehicles

The key technology of new energy electric vehicle lies in its charging and discharging technology and safety performance of high energy density lithium battery. Lithium batteries must maintain excellent waterproof and dust-proof effect in the use process, and easy to heat and spontaneous combustion is the first problem affecting their safe use. Under extreme conditions such as rainstorm, shoal and haze, Tesla and other American automobile companies took the lead in applying foamed silica gel as a minority material to power batteries in order to meet the requirements of sealing and cushioning protection of power batteries during driving.

For example, in order to reduce module weight and improve safety, PACK pack of Tesla Model 3 battery uses a lot of silicone foam encapsulation materials to protect a single core, which can prevent the heat transfer from the upper part of the battery pack to the core in a limited time, leading to thermal runaway. Due to Tesla's benchmarking role in power battery pack technology, the application and popularization of silicone gel foam material in PACK package of power battery are greatly accelerated.

At present, there are mature liquid organosilicon foamed silicone sheets with similar properties in China.

At the same time, foamed silica gel is also widely used in the anti-leakage, dust-proof, shock absorption and air tightness treatment around the LED, as well as the anti-noise and filling the gap treatment around the loudspeaker and sound.