R/Si ratio

Difficulties arise in characterising commercial branched and network structures in this way because of their heterogeneity. In these cases the R/Si ratio (or specifically the CH3/Si ratio in methylsilicones) is a useful parameter. On this basis the R/Si ratios of four types are given in Figure 29.1. 

On the commercial scale silicone resins are prepared batchwise by hydrolysis of a blend of chlorosilanes. In order that the final product shall be cross-linked, a quantity of trichlorosilanes must be incorporated into the blend. A measure of the functionality of the blend is given by the R/Si ratio (see Section 29.3). Whereas a linear polymer will have an R/Si ratio of just over 2 1, the ratio when using trichlorosilane alone will be 1 1. Since these latter materials are brittle, ratios in the range 1.2 to 1.6 1 are used in commercial practice. Since chlorophenylsilanes are also often used, the CH3/CgH5 ratio is a further convenient parameter of use in classifying the resins. 

If the functional groups are used completely, the products are mostly cross-linked, nonmeltable and insoluble however, with the same R Si ratio special techniques for the treatment of original organochlorosilanes can create ladder structures and help to prepare flexible high-melting or non-mentable, but soluble, products. 

Thus, as the R Si ratio increases and the number of cross links decreases, polymers can range from glasslike to rubbery. Most cyclolinear and branched polymers are obtained when the R Si ratio is from 1 1 to 1.6 1. 

In these reactions the composition of the final product is controlled by suitable proportionihg of the intermediate organosilicon halides, the R/Si ratio in the mixture of intermediates being substantially the same as that in the product. 

It has been pointed out that the alkyl silicones of high R/Si ratio are oily liquids or weak gels when polymerized by heat alone, and that in order to get resinous and adhesive properties the R/Si ratio must be brought down to about 1.5 for a methyl silicone and about 1.0 for a butyl or benzyl silicone. 

At the same time, the aryl silicones of high R/Si ratio are brittle vitreous soluble fusible solids, which actually are supercooled liquids. By themselves they are too brittle and weak for coating or impregnation. 

As for the other micas, Rammelsberg (1895) confirmed the half-silicate nature also of paragonite, (H,Na)2AlSi203 (O is an obvious misprint of O ) that has Na H = 1 2. For some muscovites, he reported a R Si ratio to 3 1, and hence a mixture of normal and half silicate (these are the present phengites). Other micas, e.g., the iron-magnesium micas, have compositions which arise from a mixture of several half silicates x R SiO + y R SiO + z R Si Qi [x, y, z being three variable coefficients]. 

Silicone resins are manufactured batchwise by hydrolysis of a blend of chlorosilanes. For the final product to be cross-linked, a certain amount of trichlorosilane must be incorporated into the blend. (In commercial practice, R/Si ratios are typically in the range of 1.2 1-1.6 1) The cross-linking of the resin is, of course, not carried out until it is in situ in the finished product. The cross-linking takes place by heating the resin at elevated temperatures with a catalyst, several of which are described in the literature (e.g., triethanolamine and metal octoates). 

Table 4.1. Structure, siUcon/oxygen ratio, R/Si ratio of the various alkyl-alkoxysilane compounds...

Branched polysiloxanes are more useful as binders for coatings. The branching is introduced by the use of T units in the composition. The organic substitution may be methyl, phenyl or a mixture of both (Figure 2.70). Properties of the final resins are largely governed by the R Si ratio (composition of M, D and T units), and methyl and phenyl content. 

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