1. Field of the Invention
This invention relates to a method of economically manufacturing thin silica flakes and novel silica flakes produced by that method. In another aspect, the invention also relates to chromatographic articles made with the silica flakes as well the use of the silica flakes in protective coatings
2. Background of the Invention
U.S. Pat. No. 3,795,524 (Sowman) discloses a method of making refractory articles (especially fibers, but also films, flakes, and microspheres) from either an aqueous solution of water-soluble aluminum and boron compounds or a colloidal dispersion of silica and water-soluble or dispersible aluminum compounds and boron compounds. Such a solution or dispersion is often called an "organosol" or a "sol-gel". To make flakes, the organosol is coated onto a non-adhering smooth inert substrate and dried to form a film which may tend to crack and form flakes. After being removed from the substrate, the "film can be comminuted to form irregularly shaped particles, flakes, or aggegates, e.g., 1/16" to 1" large, 1 to 25 microns thick, or ground to powders of small size" (col. 11, ls. 39-42) that were converted to refractory flakes by being fired at temperatures up to 1000.degree. C. "In the form of particulate materials, the refractory products can be used as fillers and/or coloring agents or pigments for paints and enamels . . . " (col. 13, ls. 8-11). They can also be used as abrasion-resistant agents for elastomeric materials (col. 13, ls. 34-40). Refractory flakes are formed in Examples 20-23 of the Sowman patent.
U.S. Pat. No. 4,349,456 (Sowman) discloses a method of making refractory ceramic metal oxide microcapsules from an organosol similar to those of the earlier Sowman '524 patent. The microcapsules after being fired have diameters within the range of about 1 to 1000 .mu.m and a uniform wall thickness in the range generally of 0.1 to 100 .mu.m (col. 8, ls. 16-20). The microcapsules are crushable to flakes, as shown in FIG. 2 of Sowman '456, but we fail to find in Sowman '456 any mention of utility for these refractory flakes.
European Pat. Publ. 240952 (Saegusa), published Oct. 14, 1987, discloses a process similar to that of Sowman '524 in that a thin coating of a solution or sol of a metallic compound is applied to a smooth surface, dried, and scraped off as flakes. The flakes can either be used as such or fired at a temperature of from about 200.degree. C. to about 1100.degree. C., preferably from 500.degree. C. to 900.degree. C. The resulting refractory flakes are said to be from about 0.01 to about 10 .mu.m in thickness and used in lengths of from about 1 to 100 .mu.m. Uses for the flakes are said to include lustrous pigments in nail enamels, enamels, leather products, exterior automotive coatings, paints, and pearl-glazed buttons. Metallic compounds used in the examples include oxides of Zr, Ti, Al, Si, Sn.
It is believed that thin refractory flakes produced by any of the above-discussed methods would be too expensive to be put to any widespread use.
U.S. Pat. Nos. 3,794,503, 3,796,777 and 3,960,583 disclose hollow spheres made from alkali metal silicate. In all cases a coreactant such as aqueous ammonium pentaborate, sodium hexametaphosphate, or sodium pentaborate is required in the preparation. In addition, expansion of the silicate particles is required to take place in a spray dryer.
Known in the art are chromatographic processes for separating and analyzing mixtures of solutions by selective adsorption on materials such as nylon, alumina, and silica. The processes are based on differences in the distribution ratios of the components of mixtures between a mutually immiscible mobile and a fixed stationary phase. In particular, there are formed isolated spots or bands which can be separated mechanically and further examined. In thin layer chromatography, it is known to use thin films, such as silica mixed with a binder (e.g., calcium sulfate) adhered to glass for the separating vehicle.
Chromatographic articles are disclosed in patent application U.S. Ser. No. 01/137,811, and are composites of a poly(tetrafluoroethylene) [PTFE] fibril matrix and non-swellable sorptive particles enmeshed in said matrix, the ratio of said particles to PTFE being in the range of 19:1 to 4:1. The non-swellable sorptive particles can be silica particles that have been treated to be surface-active, are marketed as "TLC grade silica" (Aldrich Chemical Co., Milwaukee, Wis.) and are sometimes called "silica-gel particles", and have an apparent size within the range of 1 to 100 .mu.m. The chromatographic articles are marketed as conformable strips or disks that have a chamois-like feel.
Other composite articles comprising PTFE fibril matrices and particulate enmeshed therein are disclosed in U.S. Pat. Nos. 4,565,663 and 4,460,642 (which are a division of a continuation-in-part and a continuation-in-part, respectively, of U.S. Pat. No. 4,373,519), and in U.S. Pat. No. 4,153,661.