Coating processes – Particles – flakes – or granules coated or encapsulated – Inorganic base
Reexamination Certificate
1999-01-13
2002-02-05
Metzmaier, Daniel S. (Department: 1712)
Coating processes
Particles, flakes, or granules coated or encapsulated
Inorganic base
C427S352000, C516S100000, C106S490000, C428S405000
Reexamination Certificate
active
06344240
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention generally relates to methods for the surface modification of silica.
BACKGROUND OF THE INVENTION
Silica, found in ordinary sand, is an abundant inorganic material having silicon dioxide (SiO
2
) as a basic structural unit. Due in part to its natural abundance and ready accessibility, a significant amount of research and development has been undertaken regarding methods for refining silica and for its use. Some of this research has resulted in the discovery of different types of silica particles, which in turn can be used in a number of commercial applications. Exemplary of these particles are fumed silica and precipitated silica.
Fumed silica is a material having a relatively small particle size, about 2-20 nm. Although it can be formed using a variety of processes, it is most commonly prepared in a vapor phase. For example, raw silica (typically sand) is vaporized at about 2000° C., and cooled to form fumed silica. Alternatively, raw silica can be sublimed at about 1500° C. in the presence of a reducing agent (e.g., coke) to form SiO, which is then oxidized to form fumed silica.
Precipitated silica is a particulate that can have an average diameter of from about 2 nm to greater than about 1 &mgr;m. This silica is typically prepared by coagulation of silica particles from an aqueous solution.
Another common form of silica is colloidal silica. This silica is characterized by a stable dispersion of amorphous silica particles in water. Colloidal silica, however, also can be provided in non-aqueous liquids.
When placed into solution, silica can exhibit polymerization behavior. In an aqueous medium, silica dissolves (and/or depolymerizes) to form Si(OH)
4
, which in turn undergoes polymerization to form discrete particles with internal Si—O—Si bonds and external (on the particle surface) Si—OH bonds. Generally, under neutral or alkaline conditions (pH 7 or greater), the silica particles will tend to agglomerate, first forming chains, and eventually three dimensional structures. This agglomeration may be enhanced if salts are present, the salts tending to neutralize the charge produced on the particle surface.
The formation of a stable dispersion of discrete, colloid-sized silica particles in a solution is referred to as a sol. Under appropriate storage conditions, sols do not gel or settle even after several years of storage, and can contain up to about 50 wt. % silica and have particle sizes up to about 300 nm. As alluded to previously, a sol can be formed, for example, by agglomerating silica particles to the desired size in a weakly alkaline solution, or by the addition of dilute acid to a solution of sodium silicate (e.g., NaSiO
3
) with rapid mixing until the pH is about 8-10, followed by removal of Na
+.
Silica sols can be used to form gels. A gel is a coherent, rigid, continuous three-dimensional structure of silica particles. The preparation of a gel is typically provided by contacting the silica with an acid in the absence of neutralizing salts. The creation of a gel, however, also may depend upon other factors, including the silica particle size and nature of the silica particle surface.
Various types of gels can be formed, including hydrogels, alcogels, and organogels. As its name implies, a hydrogel is a gel in which the pores (the spaces within the gel microstructure) are filled with water. Similarly, an alcogel is a gel in which the pores are filled with an alcohol, while an organogel is a gel in which pores are filled with an organic liquid. When a gel is dried by a means which collapses the coherent gel microstructure (e.g., by evaporation), a relatively high density powder, commonly referred to as a xerogel, is formed. In contrast, when a gel is dried so that the gel structure is preserved (e.g., by supercritical drying, such as that described in U.S. Pat. No. 3,652,214), a low density powder, commonly referred to as an aerogel, is formed.
Aerogels have very unusual, yet highly desirable, properties. They can be hydrophilic or converted to a hydrophobic species by the introduction of hydrophobic functional groups to their surface. They also are substantially optically transparent and possess extremely low density and a low thermal conductivity. They are used in a wide variety of applications, including in thermal insulators and as reinforcing fillers in elastomers.
Although raw material costs are very low, the costs of processing the various silica particles, including imparting hydrophobicity to the particles, has been high. Further, it is desirable for the hydrophobic groups to be introduced onto the particles in a dense and uniform manner. This has traditionally required resort to relatively high cost difunctional hydrophobic reagents, such as hexamethyldisiloxane (HMDS) and hexamethyldisilazane (HMDZ). While some progress has been made on the economic front, the development of processes that provide high quality aerogels while offering cost savings over traditional processes has lagged.
U.S. Pat. Nos. 2,739,075 and 2,786,042 are relatively early disclosures of surface-modified silicas and general methods for their preparation. U.S. Pat. Nos. 3,015,645 and 3,122,520 disclose methods for providing improved reinforcing silica fillers, e.g., aerogels, for silicone rubber. The method set forth in the '520 patent comprises heating a silica hydrosol under strong acid conditions (e.g., HCl at a pH of 1 or less) prior to reacting the silica hydrosol with a hydrophobing agent. The resulting silica is a hydrophobic aerogel having a surface area of from 100 to 650 m
2
/g as determined by the Brunauer theory (BET measurement).
Despite the knowledge provided by the foregoing, there remains a need for a method which can provide acid-treated silica particles in a manner that is more economical than those presently utilized, while not having any undue deleterious effect on the degree to which the silica can be hydrophobicized, the uniformity thereof, or the physical properties of any gel used in preparing or produced from those hydrophobic surface-treated particles, such as density, surface area, and the like. The present invention provides such a method.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a method of treating silica, wherein dry silica is contacted with a reaction medium consisting essentially of concentrated aqueous acid and a hydrophobing agent selected from the group consisting of organosiloxanes and organochlorosilanes. The silica is then reacted with the hydrophobing agent in the reaction medium for about 90 minutes or less at a temperature from about 10° C. to about 40° C. to provide a hydrophobic treated silica. The hydrophobic treated silica then is recovered.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides an improved method for the treatment of silica, particularly the preparation of hydrophobic silica. The present inventive method generally involves contacting a dry silica with a reaction medium consisting essentially of concentrated aqueous acid and a hydrophobing agent selected from the group consisting of organosiloxanes and organochlorosilanes. The silica then is reacted with the hydrophobing agent in the reaction medium for about 90 minutes or less at a temperature from about 10° C. to about 40° C. to provide a hydrophobic treated silica. The hydrophobic treated silica then is recovered.
While not intending to be bound by any particular theory, it is believed that the absence of excess water in the composition allows the concentrated acid to remain at a relatively high concentration during the reaction process, as opposed to being diluted by the introduction of secondary water sources, including, for example, by adding water itself to the system or via significantly wet silica. This dilution is believed to result in an undesirably lengthy hydrophobic-treatment processing time, which lowers the efficiency of the overall silica surface-treatment process.
The dry silicas that can be utilized in the present inventive method are preferably s
Koehlert Kenneth C.
Menon Vinayan C.
Smith Douglas M.
Cabot Corporation
Metzmaier Daniel S.
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