Method of producing hydrophobic pyrogenically produced oxides

Coating processes – Particles – flakes – or granules coated or encapsulated – Fluidized bed utilized

Reexamination Certificate

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Details

C427S220000, C427S255360, C427S255600

Reexamination Certificate

active

06316050

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to hydrophobic, pyrogenically produced oxides, a process for their production, as well as their use.
BACKGROUND OF THE INVENTION
It is known to treat pyrogenically produced oxides, oxide mixtures or mixed oxides of metals and/or metalloids with hydrophobing agents, for example dimethyldichlorosilane (DE AS 11 63 784).
It is also known to treat finely particulate oxides in a bed with octamethylcyclotetrasiloxane (GB-A 887 257).
It is furthermore known to render finely particulate oxides hydrophobic with octamethylcyclotetrasiloxane in a one-pot process (GB-A 932 753, U.S. Pat. No. 2 803 617).
The known processes have the disadvantage that the octamethylcyclotetrasiloxane is not completely bonded to the surface of the oxides.
SUMMARY OF THE INVENTION
The object of the invention is accordingly to develop a process for the surface treatment (hydrophobing) of highly dispersed oxides, oxide mixtures or mixed oxides of metals and/or metalloids that does not have these disadvantages.
The invention relates to hydrophobic, pyrogenically produced oxides, oxide mixtures or mixed oxides of metals and/or metalloids, which is characterised in that the above compounds have a ratio of dimethylsilyl groups to monomethylsilyl groups of 100:0 to 50:0, preferably 100:0 to 70:30.
The ratio of dimethylsilyl groups to monomethylsilyl groups is determined by means of Si-29-solid body NMR spectroscopy.
The temperature control during the hydrophobing steps is important as regards the adjustment of the ratio of dimethylsilyl groups to monomethylsilyl groups.
In the treatment of pyrogenic silicic acid having a hydrophilic surface of 200 m
2
/g at temperatures of 350° C. to 450° C., products containing 100% dimethylsilyl groups are obtained on the silicic acid surface. The thickening action of the hydrophobic silicic acids obtained is 2500 to 3000 mpas as measured in Araldite. If the temperature is raised to 550° C. to 600° C., the monomethylsilyl fraction rises to about 30%. The ratio of dimethylsilyl groups to monomethylsilyl groups is thus 70:30. The thickening of the resultant hydrophobic silicic acids is between 400 to 1000 mpas. The amount of chemically bound carbon increases from on average 1.2% to 1.6-1.9%. The methanol wettability rises by 5 to 8% and reaches values of on average 45% (maximum 51%).
The possibility of being able to produce hydrophobic silicic acids having a specific dimethylsilyl to monomethylsilyl ratio and consequently having a defined rheology (thickening behaviour) and low chloride content provides advantages for their use in products that should have as small a residual content of corrosive by-products as possible (for example hydrochloric acid or ammonia).
Examples of possible applications of a hydrophobic silicic acid according to the invention having a dimethylsilyl to monomethylsilyl group ratio of 100:0, i.e. a 100% dimethylsilyl fraction with a relatively low methanol wettability as well as a low chloride content, are particularly stable sealants for the building and construction industry, especially if a high transparency is necessary, for example when jointing glass facades or in the sanitary sector.
For a hydrophobic silicic acid according to the invention having a dimethylsilyl to monomethylsilyl group ratio of 70:30, i.e. with a 70% dimethylsilyl fraction and low thickening, high methanol wettability as well as low chloride content, possible applications include self-levelling sealants for optimum joining/bridging of structural parts and joints, silicone compositions having a low viscosity and thixotropy that ensure an optimum casting and shaping accuracy and reproduction of details (for example casting compositions, dental casting materials), highly transparent polymethacrylate compositions, which previously were normally produced by a compression moulding process but which now have a lower viscosity on account of the use of the silicic acid according to the invention and can therefore be produced by a substantially quicker injection moulding process, for example highly transparent shoe soles based on EPDM, and in sealing electronic circuits based on silicone rubber, where a very low chloride content is important for the longevity of the electronic components (corrosion protection).
The invention also provides a process for the surface treatment of highly dispersed oxides, oxide mixtures or mixed oxides of metals and/or metalloids that have been obtained by thermal decomposition of volatile compounds of these metals or metalloids in the vapor state in the presence of gases or vapors having a hydrolyzing and/or oxidizing action, by reacting their free OH groups or released OH groups located on the surface with hydrophobing agents that are capable of reacting with hydroxyl groups, in a fluidized bed, until they have a ratio of dimethylsilyl groups to monomethylsilyl groups of 100:0to 50:50, preferably 100:0 to 70:30 in which connection the still acidic oxides, oxide mixtures or mixed oxides are mixed as homogeneously as possible directly following their preparation, with a previously evaporated hydrophobing agent and are treated, under the exclusion of oxygen, together with small amounts of steam and optionally with an inert gas in continuous operation at temperatures of about 200° to about 800° C., preferably about 400° to about 600° C., in a fluidized bed, and the solid reaction products are if necessary post-deacidified and dried, contact with oxygen before cooling to below about 200° C. preferably being avoided, which process is characterized in that unhalogenated, chain-type siloxanes, monocyclic methyl-dimethylcyclosiloxane mixtures (D3 to D9), preferably octamethylcyclotetrasiloxane (D4) are used as hydrophobing agent.
The polydimethylcyclosiloxanes (D3 to D9) are known compounds of the general empirical formula [(CH
3
)
2
SiO]
n
, where n may be 3 to 9. This nomenclature is given in Ullmanns Encyklopädie der technischen Chemie (1982), vol. 21, p. 515, as well as in W. Noll, Chemie und Technologie der Silicone (Chemistry and Technology of Silicones) (1968), Verlag Chemie, p. 237, the disclosures of which are incorporated herein by reference.
Pyrogenically produced, in particular by flame hydrolysis or high temperature hydrolysis, oxides, oxide mixtures or mixed oxides of metals and/or metalloids and their preparation are known (Ullmanns Encyklopadie der technischen Chemie, 4th Edition, vol. 21, p. 464 ff., which disclosure is incorporated herein by reference).
The Aerosil types Aerosil 200 and Aerosil 300, which have long been commercially available, are likewise disclosed in the same source. Aerosil 200 and Aerosil 300 are furthermore known from “Aerosil-Herstellung, Eigenschaften und Anwendung”, (Aerosil production, properties and uses) Degussa company publication, June 1978 edition, the disclosure of which is incorporated herein by reference.
Pyrogenically produced oxides with a BET surface of 20 to 400 m
2
/g, preferably 50 to 380 m
2
/g, may preferably be used as hydrophilic starting materials.
In an embodiment of the invention, the oxides mixed with the octamethylcyclotetrasiloxane (D4) may be added together with the carrier medium to the fluidized bed and continuously removed from the upper part, preferably after passing through a settling zone known per se.
The treatment may furthermore be carried out in an internally heated fluidized bed known per se.
The required heat may be added in a manner known per se, partly in the form of steam and/or a hot inert gas.
The steam and/or the inert gas may be used in a manner known per se at the same time for the pneumatic conveyance of the reactants.
In the process according to the invention, the ratio of dimethylsilyl groups to monomethylsilyl groups on the hydrophobic, pyrogenically produced oxide may be controlled by the temperature regulation during the hydrophobing.
In a preferred embodiment of the invention, the temperature in one region may be varied from 500° to 600° C., a residence time of 0.5±0.2 hour being maintained.
The addition o

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