Coating processes – Particles – flakes – or granules coated or encapsulated – Inorganic base
Reexamination Certificate
2000-05-25
2002-11-05
Lovering, Richard D. (Department: 1712)
Coating processes
Particles, flakes, or granules coated or encapsulated
Inorganic base
C106S490000, C252S062000, C423S338000, C428S405000, C516S100000, C516S101000
Reexamination Certificate
active
06475561
ABSTRACT:
The present invention relates to a method of producing silicon tetrachloride-based organically modified aerogels.
Aerogels, particularly those with above 60%/a porosity and densities below 0.6 g/cu.cm, exhibit an extremely low thermal conductivity and are therefore used as heat insulating materials, as is described for example in EP-A-0 171 122.
Aerogels in the wider sense, i.e. in the sense of “gel with air as a dispersing agent” are produced by drying a suitable gel. In this sense, the term “aerogel” embraces aerogels in the narrower sense, xerogels and kryogels. In this respect, a dried gel is termed an aerogel in the narrower sense when the fluid in the gel is eliminated at temperatures above the critical temperature and starting from pressures above critical pressure. If on the other hand the fluid of the gel is eliminated subcritically, for example with formation of a fluid-vapour interphase, then the gel produced is often referred to as a xerogel.
When the term aerogels is used in the present application, this refers to aerogels in the wider sense, i.e. in the sense of “gel with air as a dispersing agent.”
In addition, aerogels can be basically sub-divided into inorganic and organic aerogels according to the nature of the gel structure. Inorganic aerogels have already been known since 1931 (S. S. Kistler, Nature 1931, 127,741). These first aerogels were produced from water glass and an acid as the starting materials. In that case, the water was exchanged for an organic solvent in the wet gels obtained and this lyogel was then super-critically dried. In this way, hydrophilic aerogels were obtained, as disclosed for example in U.S. Pat. No. 2,093,454.
Until now, various inorganic aerogels were produced. For example, SiO
2
—, Al
2
O
3
—, TiO
2
—, ZrO
2
—, SnO
2
—, Li
2
O—, CeO
2
— and V
2
O
5
—aerogels as well as mixtures thereof could be produced (H. D>Gesser, P. C. Goswarni, Chem Rev. 1989, 89, 765 et seq.).
Silicate based inorganic aerogels are normally produced either on a basis of water glass or acids as the raw material.
If the starting materials water then it is possible for instance, with the aid of an ion exchange resin, to produce a silicic acid aerogel which is polycondensed to an SiO
2
gel by the addition of a base. After exchange of the aqueous medium for a suitable organic solvent, then, in a farther step, the gel obtained is reacted with a chlorine-containing silylating medium. By virtue of their reactivity, it is likewise preferred to use methyl chlorosilanes (Me
4−n
SiCl
n
in which n=1 to 3) as the silylating agents. The resulting SiO
2
gel modified on the source with methyl allyl groups can then be similarly dried out of an organic solvent by exposure to the air. The production method based on this technique is described in detail in EP-0-658 513.
In U.S. Pat. No. 3,015,645, the hydrogel is obtained by adding a mineral acid to a water glass solution. After formation of the hydrogel the water in the gel is exchanged for an organic solvent and subsequently allylated and subcritically dried by means of a silylating agent, preferably a chloroalkyl silane.
The use of a chlorine-free silylating agent is described in DE-C-195 02 543. To this end, for example a silicatic lyogel produced by the method described above is presented and reacted with a chlorine-free silylating agent. The silylating agents used in this case are preferably methyl isopropene oxisilanes (Me
4−n
Si(OC(CH
3
)CH
2
)
n
in which n=1 to 3. The resulting SiO
2
gel modified on the surface with methyl silyl groups can then likewise be dried out of an organic solvent by exposure to the air.
By the use of chlorine-free silylating agents, it is indeed possible to resolve the problem of HCl formation but the chlorine-free silylating agents used have a very high cost factor.
WO 95/06617 and DE-A-195 41 279 disclose methods of producing silicic acid aerogels with hydrophobic surface groups.
In WO 95/06617, the silicic acid-aerogels are obtained by reaction of a water glass solution with an acid at a pH value of 7.5 to 11, substantial liberation of the resulting silicic acid hydrogel from ionic constituents by washing with water or dilute aqueous solutions of inorganic bases, the pH value of the hydrogel being maintained in the range from 7.5 to 11, displacement of the aqueous phase contained in the hydrogel by using an alcohol and subsequent supercritical drying of the alcogel obtained.
In DE-A-195 41 279, similarly to the description in WO 95/06617, silicic acid aerogels are produced and then subcritically dried.
In both methods, however, dispensing with chlorine-containing silylating agents only results in an aerogel with hydrophobic surface groups bonded via nitrogen. These can be split off very readily again in a hydrous atmosphere. Consequently, the aerogel described is only briefly hydrophobic.
The use of water glass as a starting material, however, has the disadvantage that by-products occur such as NaCl and the process is generally quite expensive.
If silanes are used as a raw material for silicated aerogels, then by virtue of the very difficult handling of silicon tetrachloride, from the subsequent products, tetralkyl silanes are used as the staring material.
For example, SiO
2
aerogels can be used by acid hydrolysis and condensation of tetra ethyl orthosilicate in ethanol. The result is a gel which by supercritical drying can be dried while retaining its structure. Production methods based on this drying technique are known for example from EP-A-0 396 076, WO 92/03378 and WO 95106617. One alternative to the above drying is a method for the subcritical drying of SiO
2
gels in which these are reacted with a chlorine-containing silylating agent prior to drying. In this case, the SiO
2
gel can be obtained for example by acid hydrolysis of tetra alkoxy silanes, preferably tetra ethoxy silane (TEOS) in a suitable organic solvent, preferably ethanol, by reaction with water. Once the solvent has been exchanged for a suitable organic solvent, in a further step, the gel obtained is reacted with a chlorine-containing silylating agent. In this respect, by virtue of their reactivity, methyl chlorosilanes (Me
4−n
SiCl
n
in which n=1 to 3) are preferably used as silylating agents. The resulting SiO
2
gel modified on the surface with methyl silyl groups can then be air dried out of an organic solvent. Thus, aerogels with densities below 0.4 g/cc and parasites above 60° C. can be achieved. The production method based on this drying technique is described in detail in WO 94/25149.
The above-described gels can, furthermore, prior to drying in alcoholic solution which contains the quart of water needed for reaction, be mixed with tetra alkoxy silanes and aged in order to enhance the gel network strength, as disclosed for example in WO 92120623.
The tetra alkoxy silanes used in the above-described methods as starting materials can however constitute an extremely high cost factor.
Therefore, it will be substantially more favourable to use silicon tetrachloride as a starting material.
Furthermore, common to all the methods of producing aerogels with a density of less than 300 kg/cu.m which are known from the state of the art is that prior to drying, the water is exchanged for an organic solvent or CO
2
. In the case of supercritical drying, the water from the pores of the hydrogel is previously exchanged for an organic solvent so that in the supercritical state, the water does not partially or completely dissolve the network. With regard to production by subcritical drying, prior to the necessary surface modification, there is likewise an exchange of the water in the hydrogel for an organic solvent. This was and is, in the opinion of the experts in the field, needed since for instance in the case of silylating agents, either the silylating agents do not dissolve in water and cannot therefore be incorporated into the gel via the aqueous phase in which the gel particles are normally to be found, or the silylating agents react in water with the same or with themselve
Cabot Corporation
Lovering Richard D.
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