Most silicone products including fluids, RTV's, and rubber are derived from the same chemical starting materials and are later differentiated by viscosity or degree of polymerization. The process begins with the reduction of silica (sand) to elemental silicon metal which is then mechanically ground and reacted with methyl chloride at 300ºC in the presence of a copper catalyst.

This results in the formation of reactive methylchloro silanes which are fractionally distilled and separated into their mono, di, and tri counterparts. Note that the dichloro species is most important for forming long linear polymer chains since its bifunctionality allows it to “grow” chemically in two dimensions. The trichloro species forms three dimensional crosslink networks and, although important for rigid, nonelastomeric molding resins, must be separated from the di stream to avoid gelling the polymer.

After distillation, the dimethyldechlorosilanes are hydrolyzed to form silanols which rapidly condense to cyclic siloxanes and low molecular weight linear siloxanes. The latter are reacted with caustic to produce cyclic siloxanes, specifically dimethyl tetramer or D₄ which is the primary input for all dimethyl silicone rubber polymer and which is a clear, low viscosity liquid. Ring opening polymerization of the cyclic D₄ is then accomplished via strong base resulting in linear polymer whose molecular weight (viscosity) is controlled by the addition of monofunctional siloxanes which function as chain stoppers. This process is diagrammed in Figure 3:

Figure 3

Silicone Polymer Manufacture

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