Method of preparing organically modified silica

Details Method of preparing organically modified silica Method of preparing organically modified silica

1999-01-13

2001-01-16

Lovering, Richard D. (Department: 1712)

Colloid systems and wetting agents; subcombinations thereof; pro

Continuous or semicontinuous solid phase

The solid phase contains organic material

C106S490000, C423S338000, C427S220000, C428S405000

Reexamination Certificate

active

06174926

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method of preparing organically modified silica in an aqueous medium.
BACKGROUND OF THE INVENTION
Amorphous silica particles can be used as fluid thickeners, thermal insulation, anti-caking agents, molecular sieve materials, and reinforcing fillers for silicone and rubber. The silica particles used for these purposes have physical properties (e.g., particle size, surface area, density, and surface polarity) that vary according to the particular application.
For example, it is known that when silica particles are to be used as fillers in silicone rubber formulations, they become increasingly reinforcing as their sizes diminish. It is further known that the undesirable phenomenon of “crepe hardening” in filler applications can be avoided if the silica particles have been organically modified to make their surfaces hydrophobic or lipophilic. This can be accomplished by any number of procedures, but generally involves transforming some of the hydrophilic silanol groups into hydrophobic silyl ethers. This process occurs according to the general equation:
Si—OH+R-LG→Si-OR+H-LG,
wherein R is an aromatic or aliphatic hydrocarbon-containing moiety, such as trimethylsilyl, and LG is a leaving group, such as chloride. This procedure is commonly referred to as “hydrophobing”, “treating”, or “capping” the silica, and the organic reagents used for this purpose are called hydrophobing, treating, or capping agents.
Hydrophobing reactions are typically performed in the presence of organic co-solvents at either low or high pH. For example, U.S. Pat. No. 2,786,042 (Iler) discloses the conversion of a silica hydrosol to a lipophilic silica organosol. The method comprises reacting the hydrosol with a hydrophobing agent, such as a chlorosilane, under acidic conditions in the presence of a cosolvent such as tert-butyl alcohol. A brine-immiscible organic solvent is then added while the pH is maintained at approximately 2 to 3 to prepare the organosol, from which the lipophilic silica may be obtained after drying.
In U.S. Pat. No. 3,122,520 (Lentz), a silica hydrosol is again converted to a lipophilic silica organosol, but under more acidic conditions. In particular, a pH of less than about 1 is maintained during the reaction of the hydrosol with a chlorosilane or siloxane, and the subsequent preparation of the organosol is also conducted in the presence of a strong acid catalyst. The hydrophobed silica then can be obtained by boiling off the organic solvent.
The above procedures are indirect in that they use silica hydrosols as starting materials, which are made from silica and water. The silica, in turn, is made from another starting material (e.g., a chlorosilane), and the hydrophobing reaction takes place only after the execution of these preliminary steps. Furthermore, these procedures suffer from the fact that exogenous acids and organic solvents are used in the hydrophobing reactions, which adds cost and complexity to the overall process.
The most common source of silica for such reactions is that obtained via acidification of a solution of a sodium silicate. Alternatively, silica can be formed by burning SiCl
4
in the presence of methane or hydrogen to produce SiO
2
, H
2
O, and HCl. Silica prepared by these methods is completely oxygenated (e.g., SiO
2
) and is extremely hydrophilic. Significant quantities of hydrophobing agent are therefore required in order to satisfactorily increase the lipophilicity of the silica for use in applications such as fillers for silicone rubber.
A direct method for preparing organically modified silica suitable for use as a silicone rubber filler is described in U.S. Pat. No. 4,344,800 (Lutz). This patent teaches the use of an alkyl orthosilicate (e.g., methyl orthosilicate) as a starting material instead of a silica hydrosol. The alkyl orthosilicate is mixed with water, an alcohol, a basic catalyst, and a hydrophobing agent, aged for a period of time, and then filtered and/or oven-dried to provide the filler material. In this process, the silica is prepared in situ from the alkyl orthosilicate and treated in one step. However, it requires the use of an expensive orthosilicate, a basic catalyst, an organic solvent, and a significant quantity of a hydrophobing agent.
A need exists for a simple method of preparing organically modified silica that uses inexpensive processes and starting materials, as well as a reduced quantity of a hydrophobing agent. Furthermore, the method should not require the use of exogenous acids, bases, or organic solvents, but should have the flexibility to provide silica particles in a range of hydrophobicities or lipophilicities, surface areas, densities, etc. The present invention provides such a method.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to a method of preparing organically modified or lipophilic silica in an aqueous medium in the absence of added acids, bases, or organic solvents. In particular, the method comprises contacting a trifunctional silane such as methyltrichlorosilane, with water and a tetrafunctional silane such as silicic acid. One or more hydrophobing agents, such as trimethylchlorosilane and hexamethyldisiloxane, also can be added to the mixture. According to the method of the present invention, using a trifunctional silane as a starting material allows organically modified silica to be produced from inexpensive silica precursors and allows lipophilic silica to be produced using a reduced amount of hydrophobing agent. Moreover, both products (organically modified silica and, the more hydrophobic, lipophilic silica) optionally can be produced in the absence of a organic cosolvents. The tetrafunctional silane imparts improved physical properties (e.g., lower density and larger surface area) to the lipophilic silica. No acids, bases, or external catalysts of any kind are required in the reaction mixture, and the method allows for the production of hydrophobic silica particles in a wide range of surface areas, densities, and hydrophobicities or lipophilicities.
These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention relates to a method of preparing organically modified silica that is performed in an aqueous medium and which can be performed in the absence of added acids, bases, or organic solvents. Organically modified silica prepared under some embodiments of the present invention is sufficiently hydrophobic, such that it is lipophilic. In particular, the present inventive method of preparing organically modified silica comprises (a) providing a mixture consisting essentially of (i) at least one organically modified silica precursor which is a trifunctional silane, (ii) water, and (iii) at least one tetrafunctional silane, (b) allowing the organically modified silica to form in the mixture, and (c) removing the organically modified silica from the mixture.
By a trifunctional silane is meant a substituted silicon compound of the general formula RSiX
3
, wherein R is independently selected from the group consisting of aliphatic hydrocarbon and fluorocarbon radicals of 6 carbon atoms or less (e.g., methyl, trifluoromethyl, ethyl, pentafluoroethyl, propyl, butyl, isopropyl, tert-butyl, amyl, etc.) and phenyl radicals (e.g., phenyl, tolyl, fluorophenyl, chlorophenyl, nitrophenyl, hydroxyphenyl, etc.), and each X is independently selected from the group consisting of halogen radicals (e.g., chloro, bromo, iodo, etc.), and hydroxyl radicals and salts thereof (e.g., OH, O—Li, O—Na, O—K, etc.). The trifunctional silanes of the present invention react with water to provide oxygen-substituted silicon compounds of the general formula RSi(OH)
x
O
1.5−x/2
, wherein x is a number from 0 to 3, inclusive, and R is as previously defined.
According to the method of the present invention, preferred trifunctional silanes include chlorosilanes of the general

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