Preparation and Uses of Silica Gels and Precipitated Silicas

32 Preparation and Uses of Silica Gels and Precipitated Silicas

Downloaded by UNIV OF MINNESOTA on June 3, 2013 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch032

Robert E. Patterson The PQ Corporation, Research and Development Center, 280 Cedar Grove Road, Conshohocken, PA 19428

An overview of the nomenclature, manufacture, and uses of synthetic silica gels and precipitated silicas that are of significant commercial importance is presented. Typical manufacturing processes are reviewed, and differences in the structure of silica gels and precipitated silicas that come about as a result of their methods of production are discussed. Applications covered include uses as a reinforcing agent, carrier, anticaking and free-flow agent, thickener, adsorbent, defoamer, substrate, abrasive-polishing agent, antiblock agent, and flatting agent.

ILER DEVOTED ONE OF THE SEVEN CHAPTERS of The

Chemistry of Silica (I) entirely to the manufacture, characterization, and uses of "silica gels and powders". In the 160 pages of Chapter 5, lier covered so many products, processes, and applications that it is difficult for anyone not already i n the field to distinguish between what is of commercial importance and what is of theoretical interest. The aim of this chapter is more limited; the emphasis is to provide an overview of the uses of synthetic silica gels and precipitated silicas that are of significant commercial importance. Excluded from this discussion are naturally occurring silicas, including products such as diatomaceous earth and so-called "amorphous silica" minerals (which are actually microcrystalline). Also excluded are fumed and arc silicas, forms of synthetic silica made at high temperature (in contrast to silica gels and precipitated silicas, which are generally made i n aqueous solution and consequently have surface chemical properties quite different from high-temperature silicas) 0065-2393/94/0234-0617$08.00/0 © 1994 American Chemical Society

In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

618

T H E C O L L O I D CHEMISTRY O F SILICA

and products that are not properly classified as silicas (such as insoluble metal silicates). Chapter 24, by Ferch, covers fumed silicas as well as silica gels and precipitated silicas from a somewhat different perspective.


Nomenclature and Manufacture In The Chemistry of Silica (1), Her used the term silica powders as a broad category encompassing silica gels, precipitated silicas, and fumed or pyrogenic silicas. In this chapter, only gels and precipitates are covered, because silica gels and precipitated silicas both being produced by wet processes, possess physical and chemical properties that are similar enough to provide for substantial overlap in applications. In contrast, fumed silicas, which are produced by thermal processes, possess unique properties that confer performance characteristics that cannot generally be matched by wet-process silicas i n specific applications. Silica G e l s . Silica gels and precipitated silicas can usually be distinguished on the basis of pore structure. Silica gels give a Type I V nitrogen adsorption isotherm, whereas precipitated silicas give a Type II isotherm (2). The general shapes of these isotherms are shown in Figure 1. Mercury intrusion-extrusion isotherms exhibit a hysteresis effect i n gels because the stronger pore structure remains relatively intact, whereas in precipitates only the intrusion curve can be measured because the high pressure during intrusion breaks down the pore structure. However, i n practice these terms refer to the method by which the powder was manufactured. A l l silica gels and precipitated silicas of major commercial importance are derived from sodium silicate, with the reaction conditions adjusted to yield one type of silica or the other.

V

Ρ (a) Type II

Po

Ρ

Po

(b) Type IV

Figure 1. Typical Brunauer adsorption isotherms for (left) precipitated silica (Type II) and (right) silica gel (Type IV) (V, volume adsorbed; P, pressure; and Po, saturation pressure).

In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

32.

PATTERSON

Silica Geh and Precipitated Silicas

619


Figure 2 shows a typical manufacturing process for silica gels. Silica gels are formed by the acidification of sodium silicate solution under conditions that form a three-dimensional network of silica polymers that entirely enclose the liquid phase. The liquid that forms just after the mixing of the acid and silicate is referred to as a hydrosol; in time the hydrosol sets into a rigid gel. If setting is rapid, gel beads are possible. W i t h slower setting, a large gel mass is formed that must be crushed before further processing.

- Sodium Silicate Mill

Acid

Gei Beit

Preparation and Uses of Silica Gels and Precipitated Silicas

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