Silicone Rubber: Complete Guide on Highly Durable Elastomer

Silicone rubber is a high-performance elastomer characterized by an unusual combination of properties. These properties range from high-temperature performance to durability, excellent electrical insulation properties as well as its transparency.

But, what makes Silicone rubber a high-performance material in several sectors? What are its key properties and applications? Get detailed technical information on Silicone rubbers here and hence, filter out your possibilities to evaluate a wide array of material properties to find the suitable material meeting demanding performance specifications.

What Makes Silicone Rubber Versatile?

Silicone rubber is a durable & highly-resistant elastomer (rubber-like material) composed of silicone (polymer) containing silicon together with other molecules like carbon, hydrogen and oxygen. Its structure always comprises a siloxane backbone (silicon-oxygen chain) and an organic moiety bound to the silicon.

Hence, the properties of silicone rubber can vary greatly depending on the:

• Organic groups (methyl, vinyl, phenyl, trifluoropropyl or other groups)
• Chemical structure

As compared to organic rubber, silicone rubber has a Si-O bond in its structure, and hence, it has better:

• Heat resistance
• Chemical stability
• Electrical insulation
• Abrasion resistance
• Weatherability as well as Ozone resistance

Silicone rubbers can withstand temperatures ranging from -50°C to 350°C (depending on the duration of exposure). Parts made of silicone rubber when exposed to wind, rain and UV rays for long periods result in virtually no change in physical properties. Unlike most organic rubbers, silicone rubber is not affected by ozone as well.

Silicone Rubber’s special features are hence, originated from its unique molecular structure that they can carry both inorganic and organic properties.

With these unique characteristics, silicone rubber is widely used in industries such as aerospace, automotive, construction, medical, E&E, food processing, etc. Overall silicone rubbers are used in various applications as elastomers, adhesives & sealants, potting, and encapsulating compounds as well as in coatings, lubricants, etc.

Commercial grade Silicone rubbers were first introduced by Dow Corning (now 100% Dow subsidiary) in 1943. Today, silicone rubbers are manufactured by several companies.

Different Types & Methods Used to Synthesize Silicone Rubbers

The organic groups in silicone rubbers may be methyl, vinyl, phenyl or other groups. According to ASTM D1418 standard, which covers a system of general classification or nomenclature for rubber and rubber lattices, silicone rubbers are classified as:

• Methyl Group – Also known as dimethylsilicone elastomer/rubber or simply methyl silicone rubber. It is also referred by MQ.
• Methyl and Phenyl Groups – Also known as methyl-phenylsilicone elastomer/rubber or phenylsilicone rubber. It is referred as PMQ and it has an excellent low-temperature performance.
• Methyl and Vinyl Groups – Also known as methylvinylsilicone elastomer/rubber. It is referred as VMQ as well.
• Methyl, Phenyl and Vinyl Groups – It is referred as PVMQ as well andis known for its excellent low-temperature performance.
• Fluoro, Vinyl and Methyl Groups – Also known as fluorinated rubber or fluorosilicone rubber. It is referred as FVMQ and they are highly resistant to chemical attack (fuel, oil, solvent…).

Apart from its molecular structure, another factor for classifying silicone rubber are viscosity and the method employed for their processing. Silicone rubber is available in three main forms:

• Solid Silicone Rubber or High-Temperature Vulcanized, HTV — Solid silicone rubber contains polymers with a high molecular weight and relatively long polymer chains. They are available in an uncured form and required traditional rubber processing techniques.
• Liquid Silicone Rubber, LSR — Liquid silicone rubber contains polymers of lower molecular weight and hence shorter chains. It has better flow properties. It is processed on specially designed injection molding and extrusion equipment.
• Room Temperature Vulcanized, RTV – RTV silicone rubber is a type of silicone rubber made from one-part (RTV-1) or two-component (RTV-2) systems where their hardness range of very soft to medium. They are available for potting, encapsulations, sealants etc.

Liquid Silicone Rubber maintains mechanical properties over a wide range of temperatures (from -50°C to 250°C). This heat-cured elastomer provides excellent optical clarity, durability and design freedom. This innovative transparent material serves diverse applications such as high-power LED lighting, electronics, automotive lighting and many others.

Method of Synthesis

Overall, silicone rubbers synthesis majorly involves three steps i.e. preparation of chlorosilanes followed by hydrolysis and then polymerization yielding silicone elastomers.

Today, silicones are obtained commercially from chlorosilanes prepared following the direct process of Rochow. The reaction giving chlorosilanes takes place in a fluidized bed of silicon metal powder in which flows a stream of methylchloride, usually at temperatures of 250 to 350°C and at pressures of 1 to 5 bars. A copper-based catalyst is used.

A mixture of different silanes is obtained containing mainly the dimethyldichlorosilane, Me₂SiCl₂.

Mixture of Different Silanes
(Source: Dow Corning)
The dimethyldichlorosilane is separated via distillation and used as the monomer to produce Polydimethylsiloxanes by the hydrolysis of dimethydichlorosilane in the presence of excess water.

Linear & Cyclic Oligomer Synthesis Silicone Rubber
(Source: Dow Corning)
This heterogeneous and exothermic reaction gives formally a disilanol “Me₂Si(OH)₂” [2] which readily condenses, with HCl acting as a catalyst, to give a mixture of linear [3] or cyclic [4] oligomers by inter- or intramolecular condensation.

The linear and cyclic oligomers obtained by hydrolysis of the dimethyldichlorosilane have too short a chain for most applications. They must be condensed (linears) or polymerized (cyclics) and crosslinking to obtain elastomers.