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Structure of silicone rubber

Structure of silicone rubber

 

Silicone rubber chain

Polysiloxanes differ from other polymers in that their backbones consist of Si-O-Si units unlike many other polymers that contain carbon backbones. Polysiloxane is very flexible due to large bond angles and bond lengths when compared to those found in more basic polymers such as polyethylene. For example, a C-C backbone unit has a bond length of 1.54 Å and a bond angle of 112˚, whereas the siloxane backbone unit Si-O has a bond length of 1.63 Å and a bond angle of 130˚.

 

 

Repeat unit of silicone rubber

The siloxane backbone differs greatly from the basic polyethylene backbone, yielding a much more flexible polymer. Because the bond lengths are longer, they can move farther and change conformation easily, making for a flexible material. Polysiloxanes also tend to be chemically inert, due to the strength of the silicon-oxygen bond. Despite silicon being a congener of carbon, silicon analogues of carbonaceous compounds generally exhibit different properties, due to the differences in electronic structure and electronegativity between the two elements; the silicon-oxygen bond in polysiloxanes is significantly more stable than the carbon-oxygen bond in polyoxymethylene (a structurally similar polymer) due to its higher bond energy (also because polyoxymethylene decomposes formaldehyde, which is volatile and escapes driving decomposition forward, but Si-containing decomposition products of silicone are less volatile).

 

Mechanical properties    

Hardness, shore A:1090

Tensile strength:      11 N/mm²

Elongation at break :  1001100%

Maximum temperature:         +300 °C

Minimum temperature:          -120 °C

 

Special grades

There are many special grades and forms of silicone rubber, including: steam resistant, metal detectable, high tear strength, extreme high temperature, extreme low temperature, electrically conductive, chemical/oil/acid/gas resistant, low smoke emitting, and flame-retardant. A variety of fillers can be used in silicone rubber, although most are non-reinforcing and lower the tensile strength.

 

Silicone rubber is available in a range or hardness levels, expressed as Shore A or IRHD between 10 and 100, the higher number being the harder compound. It is also available in virtually any colour, and can be colour matched.

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