Silicone fluids general properties

The frictional and wear properties of the polymers used for gears can be greatly enhanced by the addition of internal lubricants. The most commonly used additives are polytetrafluroethylene, PTFE (Teflon, Fluon) silicone fluids graphite molybdenum sulphide, M0S2. PTFE, with its exceptionally low coefficient of friction is very effective as an internal lubricant. Initial shear causes it to form a high-lubricity film over the surface. It is, of course, expensive, and will generally reduce the mechanical properties of the material. 

Two types of silicone are available. Each is formulated to conform as the generic name implies by following the circuit contours without leaving sharp solder points or other obtrusions uncovered. Their general properties are also shown in Table 3.3. The first of these (4) is a solventless, viscous fluid elastomer which can be applied directly to a circuit board by flow coating, but is usually diluted by solvent to reduce its viscosity. Application may then be by dipping, spraying or flow... 

Diffusional transfers of potassium and silicon between sandstones and shales may be sufficient to accomplish feldspar dissolution, illitization, and quartz cementation (Thyne, 2001 Thyne et al, 2001). Losses of the magnitude observed for detrital carbonates in shales exceed the capacity of diffusion-mediated transfer. Large-scale advection seems required, although our understanding of shale permeabilities seems to preclude this (Bjprlykke, 1989, 1993 and Lynch, 1997). The possibility of convection driven by salinity heterogeneity within thick shale sequences has been demonstrated by Sharp et al (2001), who note that more information for rock properties and fluid compositions within deep basinal shales is needed before the generality of their results can be assessed. 

The silicones which are of commercial interest may be classified into three groups, namely fluids, elastomers and resins. The manufacture of these materials is based on the general reactions described in the preceding section. In the case of elastomers, the linear polymer which is produced initially is subjected to cross-linking reactions in order to develop elastic properties. Similarly, the... 

The most important characteristic of silicone elastomer vulcanizates is their maintenance of physical properties over a large temperature range. General purpose material is serviceable over the approximate range —50 to 200°C but both ends of the range may be extended by the use of special purpose compounds. When elastomers are exposed to air at temperatures above 200°C, cross-linking and embrittlement slowly occur in the absence of air chain scission and softening predominate (cf., fluids). 

Silicone elastomers are based on linear polymers which are analogous to the fluids but which have higher molecular weight. As with other elastomeric materials, it is necessary to cross-link the linear polymers in order to obtain characteristic elastic properties. General purpose elastomers are based on polydimethylsiloxanes but special purpose ihaterials which contain a small proportion of groups other than methyl are also available. These various products are described below. 

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