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Silicone Rubber: Properties and Applications

Prachi Patkar Jan 22, 2019
Silicone is a polymer, comprising silicon and oxygen. Silicone rubber is a synthetic inorganic elastomer belonging to the silicone family. Here's more about the properties and applications of this elastomer.

Heat Resistance

Silicone rubber remains stable at high temperatures, as much as 200ºC (392ºF) for almost 10 hours. Whereas, it remains stable at low temperatures, between -60ºC and -70ºC (-76ºF to -94ºF).
Silicon is one of the most abundant elements in the Earth's crust. It is a non-metallic element, having the atomic number 14. Siloxanes are chemical compounds composed of oxygen and silicon. Silicone basically refers to any of the classes of siloxanes.
Most of us are aware of the fact that the binding energy of C-C bond is about 355 KJ/mol. However, with a value higher than that for C-C bond, silicone are stronger as compared to other polymers. This compound has a linked-chain structure.
Silicone rubber exists in the liquid form. It needs to be converted into a solid in order to use it. In order to solidify this compound, it must be vulcanized, catalyzed, or cured. Here are two processes that can be used for the same.

Platinum-based System

This method uses platinum as a catalyst. The compound used in this process is vinyl and hydride siloxane polymer. This results in the formation of a bridge between the reactant and the catalyst. This system can also be called an addition system.
Its advantages are that, the product is non-deformable, stable, and possesses a good grade of rubber. It also offers high resistance to high temperatures.

Tin-based System

Alkoxy cross-linker on undergoing hydrolysis has a hydroxyl group remaining. This hydroxyl group then reacts with another group (belonging to the polymer) of the same type in a condensation reaction. This process happens at its own pace at room temperature. This method can also be termed as RTV, which is the short form for 'Room Temperature Vulcanizing'.

Structure of Silicone Rubber

Silicone molecules are spiral, with low inter molecular forces. This makes the molecules highly elastic and compressible. The image represents the structure of silicone rubber. The silicon bondage with two hydrocarbons and oxygen is a repeating unit. The diagram also depicts two repeating units.
This structure has a longer bond length and larger bond angles. Since the bond length is longer, the molecules comprising the compound can stretch further, thus increasing the flexibility of the compound. The bond between silicon and oxygen is stronger, which imparts inertness to this compound.

Properties of Silicone Rubber

┗ Silicone rubber is resistant to chemicals, solvents, fluids, and oil.
┗ The binding energy of the -Si-O-Si- bond is 430 KJ/mol.
┗ The value of mechanical property of tensile strength for this compound is 11 N/mm².
┗ This type of rubber is water-repellent.
┗ It is ozone and UV resistant.
┗ The thermal conductivity of this compound is 0.22 W/m.K.
┗ The density of silicone rubber is 1.15 - 1.30 g/cc.
┗ It is known to be highly stable, gaining an edge over most other compounds.
┗ Silicone rubber has a higher binding energy when compared with organic polymers.
┗ It has a high heat resistance value.
┗ They are solids having indefinite molecular weight.
┗ It can withstand extreme low temperatures.
┗ The melting point of silicone rubber is 205ºC - 220ºC (401ºF - 428ºF).
┗ This elastomer is flame-retardant, and resistant to extreme weather conditions.
┗ Its hardness (shore A) is 10-90.
┗ It is highly permeable when it comes to drugs and gases.

Applications of Silicone Rubber

Silicone rubber sealant is used to fill fissures and cracks.
┗ It is usually molded into desired shapes. It is used to make pipes, tubes, strips, etc.
┗ It is used in making footwear and sportswear.
┗ It is used in making phone and keyboard casings.
┗ Silicone rubber, in the non-dyed form, is used in making wiring, which is used for aerospace industrial purposes. This wiring is made by mixing iron oxide with silicone rubber.
┗ It gels well with any substrate, thus, can be used in painting wood and canvas.
┗ It is used in making storage containers, cookware, and baking containers.
┗ It is widely used in the garment industry for making undergarments.
Be it extremely high or low temperatures, silicone rubber is very stable as compared to organic rubbers. Its ability to resist most fluids makes it apt for meeting diverse needs.