The main chain of silicone material contains "Si-O-Si" structure, and the side chain contains a small number of organic groups, so it has the dual advantages of organic and inorganic materials. High and low-temperature resistance, radiation resistance and weather resistance (no double bond, not easy to be decomposed by ultraviolet light and ozone), hydrophobic, flame retardant, physiologically inert, oil resistance, solvent resistance and other properties, which other materials can’t compare.
Silicone rubber foam material (also known as silicone foam / foamed silica gel) is made of silicone rubber raw rubber, filler, vulcanizing agent, foaming agent, etc. as raw materials. After mixing, it is a porous, low-density, compressible polymer elastomer material prepared by foaming and vulcanization at high temperatures.
Today, silicone rubber foam is increasingly used because it combines the excellent properties of silicone rubber and foam materials. It not only has the high elasticity of rubber, but also has the performance of foam such as sound absorption and shock absorption, and is mostly used in shock absorbing gaskets, sealing gaskets, sound insulation materials, insulation materials, thermally conductive, aviation heat insulation materials and other materials with relatively high-performance requirements.
The generality of silicone rubber
Silicone rubber has many excellent properties:
(1) High and low-temperature resistance
Silicone rubber has excellent mechanical properties under the extreme temperature (above 200℃) environment and still has certain toughness and elasticity. The glass transition temperature (Tg) can be as low as -70℃. In general, the working temperature of silicone rubber is -100℃ to 350℃. It can withstand instantaneous high temperatures of thousands of degrees, so it is often used to make heat-resistant coatings, such as in the aerospace field.
(2) Electrical insulation
Silicone rubber has good insulating properties, and after combustion, the generated silica is still an insulating substance. In addition, silicone rubber is not easy to scorch during arc discharge, because of its small number of carbon atoms in its structure, and its reinforcing filler silica, which is suitable for high-voltage applications.
(3) Physiological inertia
The surface of silicone rubber is hydrophobic, therefore, the physical properties decrease little in the wet environment. Poor adhesion can play a role in isolation. And it itself is non-toxic and tasteless, no harm to human health, and can only have a slight reaction with the body, which also creates its excellent physiological inertia, and at the same time, it is also very superior in terms of physiological aging performance.
Characteristics of silicone rubber foam materials
Silicone rubber foam is increasingly widely used, because it has the excellent performance of both silicone rubber and foam materials, with both the high resilience of rubber, aging resistance, and sound absorption, shock absorption and other properties of foam, mostly used in waterproof, shock absorption, gasket (sealing grade IP68), sound insulation, insulation, aerospace insulation and other performance requirements are relatively high cases.
According to the bubble structure, silicone rubber foam is divided into three types: closed-cell structure, open-cell and hybrid structures.
Figure 2 Microstructure diagram of foamed silicone
Closed-cell silicone sponge has better shock absorption and cushioning sound insulation, heat insulation and heat preservation, as well as flame retardant and explosion-proof characteristics, which is mainly used in the automotive field for heat insulation foam tubing for automotive vehicle air conditioning, automotive shock absorption, and foam silicone sealing gaskets for new energy vehicle batteries.
At present, most of the automobile interior materials, such as floor, roof, steering wheels, car seats, etc. are polyurethane foam materials, because on the one hand, the technology of polyurethane foam materials is relatively mature, and the performance meets the use demand; on the other hand, the price of polyurethane foam materials is low. However, polyurethane foam materials have poor weather resistance, are flammable and release a lot of toxic gases harmful to the human body in the process of combustion. Therefore, with the promotion of silicone foam and the improvement of people's awareness of it, silicone foam is expected to replace traditional polyurethane foam in the future.
| No. | Item | Standard | Indicator |
| 1 | Density | GB/T 6363-2009 | 0.35-0.45 g/cm³ |
| 2 | Water absorption rate | QBT/T 2669-2004 | <5% |
| 3 | Temperature trial range: low temperature toughness | ASTM D 1056 | <55℃ |
| 4 | Temperature trial range: continuous use maximum temperature | SAE J 2236 | >200℃ |
| 5 | Compression set (22hrs, 70℃) | GB/T 6669-2008 | <1% |
| 6 | Compression set (22hrs, 100℃) | GB/T 6669-2008 | <5% |
| 7 | Tensile strength | GB/T 6344-2008 | >250KPa |
| 8 | Elongation at break | GB/T 6344-2008 | >50% |
| 9 | Flame retardant grade | GB/T 10707-2008 | FV-0 |
| 10 | Flame spread index (Ls) | ASTM E 162 | <25 |
| 11 | Smoke density (Ds) | ASTM E 662 |
|
|
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| Tested at 4.0 minutes | <50 |
|
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| Tested at 1.5 minutes | <20 |
| 12 | Toxic gas emission level | SMP-800C | PASS |
Table 1 Main performance parameters of foamed silica gel index
(1) Density selection of silicone foam
The density of the silicone rubber matrix is 1.17 g/cm³, but through the foam treatment, the density of silicone rubber foam prepared in the current mature process can be as low as 0.16-0.20 g/cm³, which can be used for car seats, headrests and other parts; while the conventional silicone rubber foam (density of 0.45 g/cm³) is widely used for sealing, shock absorption and other parts of gap filling.
(2) The flame retardant performance of foamed silicone
Oxygen index (oxygen index: OI) refers to the minimum oxygen concentration required for flaming combustion of the material in the oxygen-nitrogen mixture under the specified conditions, and it is one of the indicators to determine the flame retardant performance of the polymer. Usually: The oxygen index < 22 belongs to flammable materials; 22 < oxygen index < 27 belongs to combustible materials; oxygen index > 27 belongs to refractory materials.
The effect of the amount of flame retardant on the oxygen index of silicone rubber is shown in Figure 3. From Figure 3, it can be seen that the oxygen index of silicone rubber foam increases significantly with the increase of flame retardant dosage. The oxygen index of silicone rubber without flame retardant is 28, and the oxygen index of silicone rubber with 80 parts of flame retardant is as high as 43. It can be seen that flame retardant can obviously improve the flame retardant performance of silicone rubber foams.
Figure 3 Amount of flame retardant WT%
In addition to the oxygen index, the vertical burning properties can also be used to characterize the flame-retardant properties of polymers. Compared with the oxygen index method, the burning pattern of the vertical combustion method is closer to the actual burning of the object, so it can better reflect the actual flame retardant effect of silicone rubber. The effect of the amount of flame retardant on the vertical combustion performance of silicone rubber is shown in Table 2. From Table 2, it can be seen that the silicone rubber without flame retardant burns completely; the vertical burning grade of silicone rubber with 40 parts of flame retardant reaches FV-1, which has a certain self-extinguishing ability; the vertical burning grade of silicone rubber with 80 parts of flame retardant reaches FV-0, which shows good self-extinguishing property.
Table 2 Performance of the amount of flame retardant on the vertical combustion performance of foamed silica gel
| Item | Flame retardant dosage wt% | ||||
| 0 | 20 | 40 | 60 | 80 | |
| The 1st burning time/S | Complete combustion | 18 | 14 | 10 | 4 |
| The 2nd burning time/S |
| Complete combustion | 39 | 21 | 10 |
| Vertical combustion grade |
|
| FV-1 | FV-1 | FV-0 |
(3) Electrical insulation performance of foamed silica gel
As the number of physical filler increases, the volume resistivity and surface resistivity of silicone rubber tend to decrease, and the dielectric constant and dielectric loss factor generally increases. It can be seen that the addition of physical fillers damages the electrical insulation properties of silicone rubber to a certain extent.
Table 3 Effect of Physical Filler on Electrical Insulation Properties of Foamed Silica Gel
| Item | Physical filler dosage wt% | ||||
| 0 | 20 | 40 | 60 | 80 | |
| volume resistivity×10-15（Ω/cm） | 34.6 | 26.4 | 6.9 | 3.5 | 2.9 |
| Surface resistivity×10-15Ω | 10.6 | 8.4 | 3.2 | 2.8 | 2.9 |
| Dielectric constant | 3.21 | 3.46 | 3.58 | 3.49 | 3.77 |
| Dielectric loss factor×10-12 | 2.33 | 2.31 | 2.28 | 2.57 | 2.49 |
Application of foamed silica gel in new energy vehicles
The key technology of new energy electric vehicles lies in the charging and discharging technology and safety performance of its high energy density lithium battery. Lithium batteries must maintain an excellent waterproof and dustproof effect during use, and the easy-to-heat and spontaneous combustion is the number one problem affecting their safe use. Under extreme conditions such as heavy rain, shoals, and smog, in order to meet the requirements for sealing and buffering protection of power batteries during driving, Tesla and other American car companies took the lead in applying foamed silica gel, a niche material, to power batteries. .
For example: In order to reduce weight of the module and improve the safety of the Tesla Model 3 battery pack, a large number of silicone foam potting materials are used to protect a single battery cell. Due to Tesla's benchmarking role in the convenience of power battery pack technology, the application and promotion of silicone foam materials in power battery packs have been greatly accelerated.
At the same time, foamed silica gel can also be custom die-cut parts widely used for light leakage prevention, dust prevention, shock absorption, and air tightness treatment around LEDs; as well as noise prevention and gap-filling treatment around speakers and speakers.
Fig. 4 Foamed silicone sealing washer
Figure5 Tesla Battery Pack
