24 Industrial Synthetic Silicas i n Powder
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Form Horst K. F e r c h Department of Applied Research and Technical Services, Silicas and Pigments, Degussa AG, Frankfurt on the M a i n , Germany
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.
THE
D E V E L O P M E N T O F S Y N T H E T I C SILICAS I N P O W D E R F O R M took place i n
the 20th century. Nearly all of the manufacturing processes can be used on an industrial basis, although the extent to which they have been applied varies considerably. The ideas originated in North America and Germany (Table I) ( I ) . Mention was made i n 1887 of electric arc silicas (2), and a detailed account by Potter has been available since 1907 (3). Industrial use of this process, which involves electricity costs, became possible only after further developments by the BFGoodrich Company (4). Silica gels were first described in 1914 i n the Gôttingen thesis of Patrick (5), who then developed patents for methods of production in the United States (6) that were first implemented i n 1920 (7). The first silica aerogels were made by Kistler i n 1931 (8), and production was started i n 1942 (9). Stôwener was an important early figure i n silica gel production i n Germany (10). 0065-2393/94/0234-0481$08.72/0 © 1994 American Chemical Society
Bergna; The Colloid Chemistry of Silica Advances in Chemistry; American Chemical Society: Washington, DC, 1994.
482
T H E C O L L O I D CHEMISTRY O F SILICA
Table I. Overview of SAS
Raw Materials or Kind of After-treatment
Process
Inventor
Commercial "Name
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P Y R O G E N I C O R T H E R M A L SILICAS Flame hydrolysis
S1CI4+H2+O2
Electric arc
quartz + coke
Kloepfer,1941
Aerosil, Cab-O-Sil
Potter, 1907
Fransil E L , T K 900
SILICAS F R O M W E T - P R O C E S S Precipitation
water glass + acid
1940
Hi-sil, Ultrasil V N Vulcasil, Zeolex
Spray drying
water glass + acid
approx. 1964
Ketjensil, Sipernat, Zeosil
Gels
acid + water glass
Patrick, 1914; Stôwener, 1924
Gasil, Sorbsil, Syloid
Overcritical conditions
acid + water glass
Kistler, 1931
Hydrothermal process
sand + lime
1957
Santocel H K 125
A F T E R - T R E A T E D SILICAS Physical after-treatment
thermal
Syloid 73
Chemical
coating
O K 412
reaction on the surface
Aerosil R 972, Sipernat D 17
after-treatment Chemical after-treatment
NOTE: The term SAS here does not include any salts or synthetic sodium aluminosilicates (SSAS), which are not discussed in this chapter.
The first reinforcing silica for rubber, which was in the form of a calcium silicate and known as Silene, was introduced to the U.S. market as early as 1939 (11). The first reinforcing fillers for rubber articles available in West Germany after the Second W o r l d War were also calcium and aluminum silicates. The production of Hi-Sil, a silicate with a high silica content, started in 1946 (12). The first " p u r e " precipitated silica was brought onto the European market in 1953 and was called Ultrasil V N 3 (13). Although the patent was not issued until 1942, aerosil was successfully produced for the first time by the original flame hydrolysis process in 1941
Bergna; The Colloid Chemistry of Silica Advances in Chemistry; American Chemical Society: Washington, DC, 1994.
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24.
FERCH
Industrial Synthetic Silicas in Powder Form
483
(14). The inventor, H . Kloepfer, told me that the idea behind this hightemperature hydrolysis had taken shape in 1934. The details of this process were published in 1959 and later (15,16). In 1955, Flemmert (17) succeeded in exchanging the SiCU used in the aerosil process with S1F4. This Fluosil process was used in Sweden for about 15 years, and a recently built factory belonging to Grace commenced production in Belgium with this method in 1990. Synthetic silicas in powder form are used in a wide range of applications. A breakthrough in certain fields was made possible only by the existence of these synthetic silicas, for example, the silicone rubber that was developed in the United States. The existing market in 1990 in the Western hemisphere was estimated to be about 1 million metric tons per annum. Table II gives the approximate distribution of these products in powder form according to the different types. The capacity has doubled since 1974 (1). If the Eastern hemisphere is included, the value of 1 million metric tons per year increases by approximately 20%. Table II. Estimated Distribution of Synthetic Silicas i n Powder F o r m for 1990 i n the Western Hemisphere
Silica Type
Amount (1000 tons)
Precipitated Fumed Gels Arc Total
810 100 90
