Silicone Elastomers
The elastomers from high molecular weight silicone polymers must be cross-linked to obtain rubber- like properties. One way this is accomplished is through hydrogen abstraction by free-radicals that are generated by decomposition of added peroxides. 2,4-Dichlorobenzoyl peroxide is often used for this purpose. It is decomposed between 110 and 150°C. The reaction can be shown as follows:
The same can be accomplished by replacing a small quantity of the methyl groups with vinyl ones. This can be done by including a small amount of vinyl methyldichlorosilane into the monomer mix (about 0.1%). The product with a small quantity of pendant vinyl groups cross- links readily by free-radical mechanism. When portions of the methyl groups on the polysiloxane backbones are replaced with phenyl structures, the elastomers exhibit particularly good low temperature properties [159].
Room temperature cross-linkable polysiloxane elastomers (commonly called RTV-s) are prepared by two techniques. In the first one, chloro siloxanes with functionality larger than two are added to hydroxyl-terminated prepolymers. The products are subsequently cross-linked by a second addition of polyfunctional compounds like tetraalkoxysilane in the presence of tin catalysts, like stannous octoate. Cross-linking occurs at room temperature. This reaction may vary from 10 min to 24 h:
In the second one, polysiloxanes terminated by hydroxyl groups are either:
(1) acetylated, (2) converted to ketoximes , or (3) etherified.The cross-linking is activated by reaction with atmospheric moisture:
The acetate-capped RTV-s exhibit good adhesion to substrates, but the released acetic acid can be corrosive.
