Silicones
Silicones are synthetic polymerised organo–silicon. Polymers containing
linkages along with
linkages present as the side chains.
linear silicone
Preparation:
Silicones are generally prepared by the hydrolysis of alkyl or aryl substituted silicon chloride like R2SiCl2 and RSiCl3 etc., followed by the subsequent condensation of the resulting unstable silicols (which are called silanols) through intermolecular elimination of H2O molecules.
R2SiCl2 on hydrolysis and subsequent condensation gives rise to a linear and cyclic silicones .Preparation of silicones from R2SiCl2 hydrolysis
Cyclic silicones
Cross–linked silicones
RSiCl3 on hydrolysis gives a cross–linked silicone. Its formation takes place through the following steps:
It may be noted that R3SiCl on hydrolysis produce only a dimer, R3Si–O–SiR3
Where R = – CH3, – C2H5 or – C6H5 groups.
Uses of silicones
Silicone oil
Used in Lubrication and cosmetics
Properties of Silicones
Lower silicones are oily liquids, but higher members are waxy and rubbery solids. Silicones are quite stable towards heat and chemical reagents. These are water–repellent. Temperature does not affect the viscosity of silicone oils.
Uses of Silicones
- Silicon oils are used for high temperature oil baths.
- Silicon greases are used as lubricants, over a wide range of temperatures, e.g. –4O°C to 200°C.
- Used for water–proofing cloth, and paper.
- Silicon rubbers are used as insulating material.
- Used in paints and enamels to make them resistant to the effects of high temperature, sunlight, chemicals and damp.
- Silicones do not become too viscous on cooling and are, therefore, used for low–temperature lubrication.