Silicones or poly siloxanes are organo silicon polymers with general empirical formula (R2SiO). Since their empirical formula is similar to that of ketone (R2CO), they were named “silicones”. These silicones may be linear or cross linked. Because of their very high thermal stability they are called high –temperature polymers.
Generally silicones are prepared by the hydrolysis of dialkyldichlorosilanes (R2SiCl2) or diaryldichlorosilanes Ar2SiCl2, which are prepared by passing vapours of RCl or ArCl over silicon at 570 K with copper as a catalyst.
The hydrolysis of dialkylchloro silanes R2SiCl2 yields to a straight chain polymer which grown from both the sides
The hydrolysis of monoalkylchloro silanes RSiCl3 yields to a very complex cross linked polymer.. Linear silicones can be converted into cyclic or ring silicones when water molecules is removed from the terminal –OH groups.
They are obtained by the hydrolysis and subsequent condensation of dialkyl or diaryl silicon chlorides.
1. Silicone rubbers: These silicones are bridged together by methylene or similar groups
2. Silicone resins: They are obtained by blending silicones with organic resins such as acrylic esters.
These are obtained by the hydrolysis of R2SiCl2.
They are obtained by hydrolysis of RSiCl3
The extent of cross linking and nature of alkyl group determine the nature of polymer. They range from oily liquids to rubber like solids. All silicones are water repellent. This property arises due to the presence of organic side groups that surrounds the silicon which makes the molecule looks like an alkane. They are also thermal and electrical insulators. Chemically they are inert. Lower silicones are oily liquids whereas higher silicones with long chain structure are waxy solids. The viscosity of silicon oil remains constant and doesn’t change with temperature and they don't thicken during winter
· Silicones are used for low temperature lubrication and in vacuum pumps, high temperature oil baths etc...
· They are used for making water proofing clothes
· They are used as insulting material in electrical motor and other appliances
· They are mixed with paints and enamels to make them resistant towards high temperature, sunlight, dampness and chemicals.