Organofunctional silicones

The concept of Reactive Silicones comprises a great number of different silicon compounds containing various functional groups. Examples of such organofunctional silicones include hydroxy-, amino-, epoxy-, carboxy-, acryloxyalkyl- or vinyl-functionalized polydimethylsiloxanes. 

In a similar way, ethylene, for example, can also be added. The reaction is interesting as a route to organofunctional silicones. 

Organofunctional silicones possess Si—CH2—type groups, since those of the Si—type are too reactive. Here, Y is a reactive group. Such organofunctional silicones can be obtained, for example, by the direct chlorination of methyl groups. 

Common to all of the composites discussed is a large interfacial area between the organic polymer and the fiber and/or filler. To optimize both the ease of fabrication and the resulting composite properties, it is necessary that as a minimum the organic polymer wet the fiber or filler surface and ideally that it bond to it. This chapter is concerned with the use of organofunctional silicon compounds to promote the wetting of the fiber/filler surface, establish bonds between the polymer and the fiber/filler, and retain these bonds under a hostile environment. 

Although there are several possible methods for the synthesis of organofunctionally terminated disiloxanes, hydrosilation reactions have become the most popular and practical process. Hydrosilation is the term used for the addition of silicon hydrides... 

The synthesis of organosilicones and organosilicone surfactants has been well described elsewhere [36-39] and hence only a brief review is given here. Industrially the manufacture of silicones is performed stepwise via the alkylchlorosilanes, produced through the reaction of elemental silicon with methyl chloride (the Muller—Rochow Process) [40,41]. Inclusion of HC1 and/or H2(g) into the reaction mixture, as in Eq. (1.2), yields CH3HSiCl2, the precursor to the organofunctional silanes, and therefore the silicone surfactants ... 

Aliphatic organofunctional silanes recommended as coupling agents for glass-reinforced polyesters, epoxies, or phenolics have heat stabilities of typical aliphatic organic chemicals and are not at all comparable to the methyl or phenyl silicones found in silicone polymers and resins. 

In this review, we will briefly discuss selected literature on hydrolysis and condensation reactions at silicon which are relevant to organofunctional silanes. Some studies will involve simpler (lower functionality) models some may involve related chemical reaction classes illustrating a mechanistic point. We will then discuss selected references focused on silanes of the types used as coupling agents and crosslinkers. Finally, some reactions at the interface will be mentioned. 

A critical concern in the use of organofunctional silanes is the effects that the organofunctional groups and the silane ester groups have on the hydrolysis reactions occurring at silicon. 

Demethylation by sulfuric acid can be successfully extended to certain organofunctional methyldisilanes. For instance, (chloromethyl)penta-methyldisilane easily undergoes demethylation under similar conditions to give l-(chloromethyl)-2-chloro(or fluoro)tetramethyldisilane upon treatment with an ammonium halide, in good yield (120). Moreover, this method is still satisfactory for synthesis of l,3-dichloro(or difluoro-)hexamethyl-trisilane (III), which is obtainable in 40% (or 55%) yield from octamethyl-trisilane (in this case, demethylation occurs concurrently at both the 1- and 3-silicon atoms, unlike the demethylation of hexamethyldisilane) (102,106, 107). 

Y = organofunctional group OX = silicon-functional group n = typically 0 or 3... 

The organofunctional group Y links with the polymer. This group must be chosen carefully to ensure maximum compatibility with the resin. The silicon-functional groups OX, usually alkoxy groups, must be hydrolyzed to the silanol (Si-OH) first before they can bond to the substrate or crosslink. 

In the technical literature the ligands are listed in the order organo-functional-, non-functional- and silicon-functional-ligands. The term organofunctional ligand means a unit containing a functional group linked to a carbon atom (e.g. -Cl, -NH2, -C=C-). 

Alkoxysilanes, including tetraalkoxysilanes, are utilized as crosslinking components in silicone rubbers, in the manufacture of silicone resins, as adhesion promoters with special organofunctional groups e.g. glass fiber layers, in the formulation of corrosion-resistant paints and in mold construction. 

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