Industrial Applications of Silicones

The silicones are straight or branched linear polymers in which the difunctional —O—SifRj)—O— or trifunctional —O—Si(R) (O)—O— siloxane elements form a system of extended chains. These chains can be prepared, if necessary, with molecular weights on the order of 1 million. Both ends of the chain are capped with a monofunctional residue RjSiO—, or with reactive groups (H, OH, OR, halogen, etc.) The basic unit of all industrial silicones is dimethylpolysiloxane. The starting material for this is dichlorodimethylsilane, which is hydrolysed to form dimethylsilanediol and dimethylcyclosiloxane (Eq. 4.1, 4.2)  

Dimethylpolysiloxanes are obtained from both of these by subsequent condensation (with removal of water) or polymerisation (with ring splitting). If chemically pure dimethylpolysiloxane oils are desired, the hydrolysis product is mixed with a sufficient amount of hexamethyldisiloxane to cause the polymerisation to stop at the desired molecular weight. For example, if the hydrolysis product of 1350 moles (H3C)2SiCl2 is mixed with 2 moles (H3C)3SiCl, the structure of the polymerisation product is 

4 Applications of Organic Silicon in Industry, Synthesis Operations and Medicine 

A small amount of curing agent is required for the preparation of cross-linked or vulcanised silicone resins and silicone rubber. This has the desired effect either during the polymerisation or later, during the baking or vulcanisation steps. 

In a few special cases, chloroorganosilanes may be directly transmuted into finished silicones, for example in the hydrolysis of a 1 1 mixture of dichlorodimethylsilane and dichlorodiphenylsilane in water/acetone [529]. 

---

Silicones find practical application in different membrane unit operations for treating gaseous and liquid mixtures. This is due to their solubility controlled transport, which allows the selective separation of organics from air or from water. Polymer blending, polymer grafting, addition of different solid fillers or ionic Hquids, are the most effective strategies for improving the stabihty as well as the selective transport of silicones. The industrial applications of silicone-based membrane systems present environmental benefits such as reduced waste and recovered/recycled valuable raw materials that are currently lost to fuel or to the flares. 

---