Silicone products room temperature vulcanizing

Organotins are used as catalysts in room temperature vulcanization via a condensation reaction. Dibutyltin laurate is the most commonly used organotin catalyst for this application. It is typically used at between 0.01% and 0.1% by weight. Between 50 and 100 tonnes of organotin catalysts were used in the production of silicones in the EU in 2000 (ETICA, 2002). 

Room temperature-vulcanizing (RTV), silicon rubbers make use of the room temperature reaction of certain groups that can be placed on polydimethylsiloxanes, which react with water. When exposed to water, such as that normally present in the atmosphere, cross-links are formed creating an elastomeric product. 

These are silicones in liquid or paste form that can be transformed into cross-linked elastomers at or near room temperature. There are two basic types, one component and two component. The one-component room temperature vulcanizing (RTV) elastomers cure upon exposure to air. A typical material of this type is synthesized from a dihydroxy-terminated siloxane oligomer and methyltriacetoxysilane (Scheme 2). The resulting tetraacetoxy compound will flow as a paste and so can be molded into shape. Hydrolysis of the acetoxy groups takes place over a period of hours. Condensation of the resulting silanols gives a highly cross-linked end product. 

Silicones are available in one- and multicomponent forms. The one-component types are commercially the most important and will be the focus of most of this discussion. These products, which generally cure by reaction with atmospheric moisture, are called RTV (room-temperature vulcanizing) sealants or adhesives. The surface cure rate of these products is a function of the cure system, but the rate of cure in depth depends on the ability to transmit water vapor through the mass of sealant. Silicones are highly permeable to moisture vapor, and generally the one-component types cure at a rate of about 0.3 cm/day. Due to this high vapor permeability, the one-component silicones typically cure faster than do their nonsilicone counterparts. 

The third area for casting is the use of cast elastomer products, particularly for industrial and medical products. Polyurethane and polyester based formulations are generally used, as is the room temperature vulcanizing silicone (RTV) material. These formulations can be cured at room temperature or at low oven temperatures and they... 

Silicones go into many different products (see Figs. 6-22 and 6-23). The so-called room temperature vulcanizing (RTV) system is an example of a specialty silicone compound that is very popular. All kinds of products are made in SI, such as complex molds in artistically designed shapes. The RTVs come in one- and two-component systems and... 

As shown in Scheme 8, condensation can take place by the reaction of terminal OH groups of poly(dimethylsiloxane) with tri-21 or tetraalkoxy-22 silane monomers or with polyalkoxysilanes in the presence of a catalyst such as dibutyltin dilaurate or tin octanoate, which are colorless and pourable. The products from these reactions are white or beige, room temperature-vulcanized two-component system (RTV-2) silicone rubbers. Polyalkoxy silane undergoes similar reaction with poly(dimethylsiloxane) to give RTV-2 silicone rubber. The reaction is isothermal and a volatile alcohol forms from the corresponding alkoxy groups. 

Sealants are often based on butyl rubber, polyisopropylene, polyurethane, or silicone rubber. Reactive sealants may be vulcanized at room temperature (RTV) or at high temperature (HTV). Silicone sealants are chemically-hardening, single-component systems in which cross-linking of the substrate is initiated by moisture from the air. This separates the reaction products. After rapid formation of a skin on the surface, cross-linking continues inside the mixture until it is fully hardened. 

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