Condensation cross-linking silicone rubber

Silicone polymers can be cross-linked (silicone rubber) by different mechanism (e.g. at high temperatures by peroxides, at room temperatures by condensation reaction). 

Silicone rubber has a three-dimensional network structure caused by cross-linking of polydimethyl siloxane chains. Three reaction types are predominantly employed for the formation of silicone networks (155) peroxide-induced free-radical processes, hydrosilylation addition cure, and condensation cure. Silicones have also been cross-linked using radiation to produce free radicals or to induce photoinitiated reactions. 

Silicone rubber, developed by Dow Corning company, is one of the few polymers developed for medical use. The repeating unit is dimethyl sUoxane which is polymerized by a condensation polymerization. Low molecular weight polymers have low viscosity and can be cross-linked to make a higher molecular weight, rubber-like material. Medical grade silicone rubbers contain stannous octate as a catalyst and can be mixed with a base polymer at the time of implant fabrication. 

Model Networks. Construction of model networks allows development of quantitative structure property relationships and provides the ability to test the accuracy of the theories of rubber elasticity (340-343). By definition, model networks have controlled molecular weight between cross-links, controlled cross-link fimctionality, and controlled molecular weight distribution of cross-linked chains. Silicones cross-linked by either condensation or addition reactions are ideally suited for these studies because all of the above parameters can be controlled. A typical condensation-cured model network consists of an a,[Pg.7595]

On the other hand, liquid silicone rubbers are liquid or paste-type polysiloxanes prepared by using the raw siloxanes having functional groups which are liable to cross-linking at the terminal position. Liquid silicon rubbers are prepared by the cross-linking of these siloxanes. There are two cross-linking types of condensation and addition as follows. 

The chains can be cross-linked to form rubbers if suitable reactive groups are incorporated in the molecule (Moretto et ah, 2002). There are two major classes of room-temperature vulcanizing (RTV) rubbers (Thomas, 1993). The majority (both two-part and one-part) set by a condensation reaction (Figure 13.4, Table 13.2). Two-part RTV-2 rubbers are formulated of the silicone polymer (MW 10,000) with -OH at either end mixed with a filler (usually finely divided silica), a small amount of water and frequently some unreac-tive silicone oil to rednce viscosity of the mix and increase flexibility of the rubber the cross-Unking agent (commonly TEOS) and a dialkyl tin catalyst. 

TABLE 13.2 Some Typical Cross-Linking Agents, CH3-Si(-Y)3, of Condensation-Type Silicone Rubbers... 

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