Synthetic silicas, classification

Pure synthetic silicon dioxide in powdered form is discussed. After a brief history, the significance of this product group is shown by the total production quantity in the Western hemisphere. A clear classification of synthetic silicas is given, and the principal differences between thermal and wet-process products are illustrated. After-treated silicas are also discussed. Various applications of synthetic silicas are described in detail. Questions about useful handling methods, registration, approval, and toxicology are addressed. 

Sihcon dioxide exists in several structural forms polymorphic crystalline sihca, synthetic quartz crystals, amorphous silica, and vitreous silica. This classification is not complete as there are other forms of silica synthesized for speciahzed apphcations. Various forms of sihca are mentioned briefly below. 

In addition to the classification of liquid chromatographic enantioseparation methods by technical description, these methods could further be classified according to the chemical structure of the diverse CSPs. The chiral selector moiety varies from large molecules, based on natural or synthetic polymers in which the chirality may be based on chiral subunits (monomers) or intrinsically on the total structure (e.g., helicity or chiral cavity), to low molecular weight molecules which are irreversibly and/or covalently bound to a rigid hard matrix, most often silica gel. 

According to our earlier classification, the stationary phase can be a solid, a liquid, or a bonded phase. In the latter two cases, the phase must be coated on, or bonded to, particles of a porous solid support. Only a few materials have found widespread use as stationary solid supports they are silica, synthetic polymers such as the styrene-divinylbenzene copolymer, diatomaceous earths, and some polysaccharides. The most common types and uses are given in Table 2. 

Until 1985 silicas were usually divided into two groups natural and synthetic. Most of the naturally occurring SiC>2 modifications are crystalline, but this group also includes amorphous or mostly amorphous products, such as diatomaceous earth or kieselguhr. However, when the observations made at the end of the introduction are considered, this classification no longer proves sufficient, as fly ashes and silica fume are also synthetically produced, although not deliberately. These airborne dusts are not harmless (22). 

Synthetic amorphous silica (SAS) became industrially relevant in the second half of the 20th century. From all the developments mainly three different manufacturing processes are now used on an industrial basis. The ideas originated in North America and Germany (see Table 43.1). Ferch in [1] provides an overview and a classification of the different types of silica. 

Classification. Extenders are mainly natural minerals that are converted into a usable form by working natural deposits, separating secondary constituents, and comminution (micronization). Synthetic products such as precipitated carbonates [e.g., calcium carbonicum praecipitatum (ccp), precipitated sulfates (e.g., blanc fixe), precipitated and pyrogenic silica, and silicates are used for optical brightening. Synthetic fibers (generally organic) are used for reinforcement. The classification of extenders used in surface coatings is summarized in Table 4.1. 

Classifications of Fused Silica and Methods of Manufacture. Methods of manufacture that involve gas-oxygen combustion as a heat source allow considerably more water vapor (a product of combustion) to be incorporated within the glass structure as hydroxyl ions than do electric melting methods. Synthetic precursors, as opposed to naturally occurring minerals, allow greater chemical purity. These two factors, heat source and raw material source (natural versus synthetic) have lead to a commonly accepted classification of fused silica types (after Heatherington et al. and Bruckner ) ... 

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