Polysiloxanes

Silicone resins are normally supplied for use as solutions, prepared as described above. The final conversion of the partially polymerized soluble material into a fully cross-linked product is carried out in situ. In the crosslinking process, remaining silanol groups are condensed by heating in the presence of a catalyst, e.g. zinc octo ate, cobalt naphthenate or triethanolamine.

Silicone resins consist of branched polymers, production of which is based on the hydrolysis of trichlorosilanes. If pure trichlorosilanes are hydrolysed, the products are highly cross-linked and intractable and are unsuitable for normal applications. In order to reduce the degree of cross-linking, it is usual to subject a blend of tri- and dichlorosilanes to hydrolysis. A convenient measure of the functionality of a blend is given by the R/Si value, which is the ratio of the numbers of organic groups and silicon atoms. (Thus pure dimethyldichlorosilane and methyltrichlorosilane have R/Si values of 2 and 1 respectively.)For the preparation of commercial resins, R/Si values in the range 1.2-1.6 are usual. Most commercial silicone resins contain both methyland phenyl groups. The introduction of phenyl groups into the methylsiloxane network results in improved heat resistance, flexibility and compatibility with pigments although pure phenyl silicone resins give products which are too weak for most applications. In methyl phenyl resins, the methyl and phenyl groups may be attached to either the same or different silicon atoms.

Silicones are inert synthetic polymers with a wide range of uses including as sealants, cookware, adhesives and medical applications. Silicones contain next to silicon atoms also carbon, hydrogen and oxygen atoms. Silicones (oligo and polysiloxanes) are the most widely used class of silicon-based compounds clinically. Silicones can be found in plastics, lubricants, catheters, implants and a variety of other medically used items. Silicone fluids, such as simethicone, are known for their antifoaming properties.