Silicone elastomers, or rubbers, are made from linear polymers that bear hydroxyl, vinyl or other reactive side chains. They can be crosslinked in various ways to yield highly elastic, more or less open-pored structures. The constituents of silicone rubbers are long-chain polysiloxanes and various fillers such as pyrogenic (fumed) silica, chalk, quartz, mica and kaolin, which give elastomers good mechanical properties (elasticity, absorption, tear strength). The rubbers are rendered flexible by vulcanisation or curing, a process in which the chains are crosslinked. The different rubbers are classified according to the type of vulcanisation (crosslinking agents, temperature) and base-polymer viscosity employed and are broadly distinguished as room-temperature-vulcanising and high-temperature-vulcanising types.

» High-temperature-vulcanising rubbers

Chemical structure
Heat cure silicone elastomers are otherwise known as HTV or HCE elastomers (High Temperature Vulcanising/Heat Cured Elastomers). They are basically made from reactive silicone gums, or in other words straight chains, with very high molecular weight macromolecules containing various reactive groups.

The most frequently used gums are vinylated gums. Various raw materials are added to give additional properties to the HCR e.g. pyrogenic (fumed) or precipitated silicas with high specific surface area to improve mechanical properties, heat stabilisers and various other additives such as plasticisers or anti-structuring agents. Cross-linking occurs using organic peroxides or a platinum catalyst in polyaddition systems - see Chemical reactions on the "finished" silicone.

Properties
Heat cure silicone elastomers have outstanding properties and are far superior to conventional organic elastomers. They exhibit exceptional mechanical strength at temperature and can be used at temperatures ranging from - 50°C to + 300°C. The ageing resistance of these elastomers highlights their fundamental qualities such as the fact that they are chemically inert, their photo-oxidative stability and the absence of residual reactive groups once cured. A wealth of colourings are possible. By using a sufficiently fine silica filler it is even possible to make transparent articles for use in food and paramedical applications.

Typical applications
Automotive gaskets, boots and hoses, electrical protection, office equipment, safety cables, insulators for electrotechnical applications, spark plug boots, cables, connectors for automotive applications, tubes, profiled sections and food, pharmaceutical and biomedical applications.

» Room-temperature-vulcanising two-component rubbers (RTV-2)

Chemical structure
RTV-2 silicone rubbers are two-component, pourable, spreadable or kneadable compounds that cure highly flexible cured silicone rubbers on addition of the cross-linking agent. Vulcanisation takes place at room temperature (hence "RTV") by one of the mechanisms, condensation or addition. The former utilizes an organo-tin catalyst and generates alcohol as a by-product, whereas the latter involves a platinum catalyst but does not form any by-product.

Properties
Most RTV-2 cured silicone rubbers retain flexibility up to temperatures as high as 200°C, with some even withstanding brief exposure to 300°C. At the other end of the temperature scale, they are still totally flexible at -50°C, special types withstanding -100°C. Generally, their thermal conductivity allows insulation of electrical equipment without heat accumulation. Their electrical characteristics, especially their insulating resistance, dielectric strength and dissipation factor, are highly advantageous.

RTV-2 silicone rubbers are ten times more permeable to gas at room temperature than is natural rubber. Not even years of outdoor weathering will significantly impair their properties. A large number of applications also benefit from the pronounced release effect that cured rubbers have for organic and inorganic materials.

Typical applications
Moulds, electronics, mechanical and plant engineering and healthcare devices.

» Room-temperature-vulcanising one-component rubbers (RTV-1)

Chemical structure
RTV-1 silicone rubbers are one-component, ready-to-use RTV systems. They comprise polydimethylsiloxanes, crosslinking agents, fillers, and auxiliaries. Crosslinking is triggered by contact with atmospheric moisture and proceeds with the elimination of by-products. It therefore starts off at the surface with the formation of a film and gradually extends further and further into the rubber. The by-product, which is formed in small quantities, varies with the type of crosslinking agent employed and may be an amine, acetic acid or a neutral compound, such as alcohol.

Properties
RTV-1 silicone rubbers are the ideal solution to a large number of sealing, bonding and coating problems. They have excellent resistance to weathering and ageing because of their unique chemical properties. Special additives ensure that RTV-1 silicone rubbers can also withstand extremely high and low temperatures.

Typical applications
RTV-1 silicone rubbers are effective in nearly all sealing, bonding and coating applications. As a result, they are used extensively in the automotive, construction, electrical, electronics and textile industries.