silicone rubber

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Silicone rubber is a polymer that has a "backbone" of silicon-oxygen linkages, the same bond that is found in quartz, glass and sand. Normally, heat is required to vulcanise (set) the silicone rubber; this is normally carried out in a two stage process at the point of manufacture into the desired shape, and then in a prolonged post-cure process. It can also be injection molded.

Properties

Silicone rubber offers excellent resistance to extreme temperatures, being able to operate normally from -55°C to +300°C. In such conditions the tensile strength, elongation, tear strength and compression set can be far superior to conventional rubbers.

Silicone rubber has excellent high temperature properties. Organic rubber has a carbon to carbon backbone. This can leave them susceptible to ozone, UV, heat and other aging factors that silicone rubber can withstand well. This makes it the material of choice in many extreme environments.

Compared to other organic rubbers, Silicone rubber has a lower tensile strength. Silicone rubber is a highly inert material and does not react with most chemicals. Due to its inertness, it is used in many medical applications and in medical implants.

Silicone rubber also offers excellent electrical/insulation properties.

Silicone rubber, more scientifically known as polysiloxane, is one of the most useful polymers in the modern world. Polysiloxanes differ from other polymers in that their backbones consist of Si-O-Si units unlike many other polymers that contain carbon backbones. Due to its properties silicone is used for a large number of applications and is extensively produced. One interesting characteristic is an extremely low glass transition temperature of about -127˚C (Stevens 1999:428). 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-C backbone unit has a bond length of 1.54 Å and a bond angle of 112˚, where as the siloxane backbone unit Si-O-Si has a bond length of 1.63 Å and a bond angle of 130˚ (Stevens 1999:429; Robert Martin Sok, PhD Thesis: Si-O-Si angle=144º, O-Si-O angle=109.5). The siloxane backbone differs extensively from the basic polyethylene backbone, yielding a much more flexible polymer. Because the bond lengths are longer, they can move further and change conformation easily, making for a flexible material. Another advantage of Polysiloxanes is in their stability. Silicon is in the same group (IV) on the periodic table as carbon, but the properties of these elements are quite different. Silicon has the same oxidation state as carbon, but has the ability to use 3d orbitals for bonding by expanding its valence shell. Si-Si bonds have far less energy than C-C bonds and so are more stable, though in practice Si-Si-bonds are very hard to create. Therefore, the bonding ocuring in silicon polymers is more stable and less reactive(Fitzpatrick 1999:427).

(Polymax 2005)

Specialist grades

There are many specialist grades of silicone rubber with these qualities: Steam resistance, metal detectable, glow in the dark, electrically conductive, chemical/oil/acid/gas resistance, low smoke emission and flame retardant.

Applications

Once milled and coloured, silicone rubber can be extruded into tubes, strips, solid cord or custom profiles within the size restrictions of the manufacturer. Cord can be joined to make "O" Rings and extruded profiles can be joined to make seals. Silicone rubber can be moulded into custom shapes and designs.

An excellent use of silicone rubber is for automotive sunroof seals which have to endure harsh temperatures and other environmental conditions such as ozone, UV light, and pollution, not to mention common automotive cleaners, waxes, etc. One frequent problem encountered with such seals is movement between the seal and the painted surface of the car roof. This movement can cause audible sounds that decrease the enjoyment of driving the vehicle as well as cause abrasion to the paint which could cause premature rusting. One way of alleviating these problems is to coat the seal with a low coefficient of friction coating. Due to the inert nature of silicone rubber it is very difficult to adhere coatings to it, however there are some commercially available coatings designed to be used on silicone rubber.

References

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