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.
Pharmaceutical Applications
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.
