Silicones
Silicones are a general category of synthetic polymers whose basic structure is made up of polyorganosiloxanes, where silicon atoms are linked to oxygen to create the siloxane bond. The remaining valences of silicon are linked to organic groups, mainly methyl groups.

Silicones are the most stable polymer offer superior reliability, excellent service life and reduced production costs. In addition, silicones significantly resist acids, bases, solvents, chemicals, oils,
and water.

TMG-218
TMG-218 is a one of the organic tin catalysts in our product portfolio. It is a liquid at room temperature and used as a catalyst and stabilizer. Good for transparent rigid products.

Main applications of TMG-218

• Polyurethane catalyst
• RTV-Silicone curing catalyst
• Esterification catalyst
• PVC-Heat stabilizer
• Paint drier

Condensation curing mechanism of TMG-218 in RTV Silicones
In general, room temperature vulcanization (RTV) would typically comprise silicone linear polymers, reinforcing mineral fillers or silicone resins, a crosslinker, a catalyst, and an inhibitor to control the pot life of the mixture, so that it can be processed. The crosslinking is initiated by mixing the two components and occurs by condensation, using an organic-tin catalyst (TMG-218) that generates alcohol as a by-product.

Even though this RTV reaction has been known since the 1950s, its mechanism is poorly understood and is still a matter of controversy. The very small number of reactive sites and the phase change inherent to the cross-linking reaction makes it difficult to assess either by chemical or spectroscopic means. The “accepted” mechanism, as postulated by van der Weij in the 1980s, is shown below.

The reaction starts with the hydrolysis of the organic esters at the tin atom, which diffuse away from the catalytic center. A nucleophilic attack at the alkoxysilane groups generates a transient Sn-O-Si bond, which in turn promotes the attack of a silanol to the activated silicon atom, resulting in the formation of a new siloxane
linkage.

• Polyurethane catalyst
• RTV-Silicone curing catalyst
• Esterification catalyst
• PVC-Heat stabilizer
• Paint drier

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