Nanostructured forms and layers and method for producing...

Stock material or miscellaneous articles – Composite – Of silicon containing

Reexamination Certificate

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C106S287160, C106S287170, C106S490000, C427S164000, C428S405000, C516S080000, C516S086000, C516S090000, C516S093000

Reexamination Certificate

active

06620514

ABSTRACT:

This application is a 371 of International Application No. PCT/EP99/02396, filed Apr. 8, 1999.
The present invention relates to nanostructured mouldings and layers and to their preparation via stable water-soluble precursors, and in particular to nanostructured mouldings and layers suitable for optical purposes.
In the literature, processes for preparing transparent materials comprising organic/inorganic composites, using water-containing precursors, have already been described for coating purposes.
In particular, JP-A-53-6339 describes the synthesis of a composite starting from a reactively organically modified silane and an inertly organically modified silane and conducting hydrolysis in the presence of aqueous silica sol and phoshoric acid as hydrolysis catalyst. The alcohol formed in the condensation reaction is not removed.
JP-A-63-37168 describes the synthesis of a composite from free-radically crosslinking, acrylate-based monomers dispersed in an aqueous medium and from organically modified silanes, the organic radical of these silanes likewise constituting, a free-radically crosslinking system, in the presence of colloidal silica and nonionic surfactants. Hydrolysis and condensation reactions are conducted in a separate process step. Here again, the alcohol formed in the condensation reaction is not removed.
A similar description is contained in JP-A-63-37167 for a system wherein the silane component possesses cationically crosslinking radicals.
U.S. Pat. No. 5,411,787 describes the synthesis of a composite from polymeric binders dispersed in water, at least one aminosilane component and colloidal particles having a size of less than 20 nm. In this case too, the alcohol formed by the hydrolysis of the silane is not removed.
U.S. Pat No. 4,799,963 describes the preparation of silane-based composites into which, additionally, colloidal silica or nanoscale cerium oxide is incorporated.
The cited literature references contain no indications concerning the mechanism of action and, moreover, little information on the pot life of the systems they describe. Likewise, in the majority of cases, there is a lack of information on residual solvent contents, although a mathematical reworking of the syntheses suggests residual solvent contents of more than 10% by volume.
On the basis of the prior art as described, an investigation was carried out into the extent to which a reduction in the water sensitivity, i.e., in the progress of the hydrolysis and condensation reaction, is achievable through controlled coating of colloidal systems with functional silanes, and into the extent to which such systems may be used to prepare stable systems for the production of mouldings and layers, which are suitable, inter alia, for industrial application.
The object of the present invention, therefore, was to provide a process for preparing nanostructured mouldings and layers, preferably those suitable for optical purposes, via stable water-soluble intermediates.
In accordance with the invention it has been found that aqueous, electrostatically stabilized (and hence extremely concentration-sensitive) colloidal suspensions with reactive monomeric or oligomeric components (silanes or precondensates thereof) may be applied by coating and as a-consequence do not display, the course of concentration, the effect described by Stern (Z. Elektrochem., 508 (1924)) of the aggregation of two particles of like charge as they approach one another, and in particular do not display the chemical reactions, which otherwise proceed spontaneously, between reactive surface groups of the two particles. The concentration and shifting of the reaction equilibrium towards the product side, with formation of the surface condensates, is achieved by means of the removal, performed under reduced pressure, of the alcohol formed in the condensation reaction (generally methanol or ethanol), resulting in a combination of very high storage stability of the condensates (>14 days) with relatively low residual solvent contents (generally not more than 20% by weight and in particular not more than 10% by weight).
By virtue of the reversibility of the surface modifier/particle bonding (e.g. hydrogen bonding or metal-oxygen bonding (—Al—O—Si—, —Ti—O—Si—, etc., see e.g. Chem. Mat. 7 (1995), 1050-52)) the process described above may be reversed when heat is supplied, so that the particles are able to crosslink, accompanied by solidification. Further reaction may also take place by way of appropriately selected organic groups on the surface modifier (e.g. reaction of these groups with one another).
In this way it is possible to react, for example,. aqueous sols, such as boehmite, TiO
2
, ZrO
2
or SiO
2
sols, but also other aqueous sols of compounds of metals of the main groups and transition groups of the Periodic Table, with organically modified alkoxysilanes in such a way that stripping of the solvent and, if desired, subsequent dispersal of the liquid residue in water produces clear solutions which are stable over a relatively long period of time. This stripping of the solvent (alcohol) is necessary in order to take the reaction of the coating of the particles with the organically modified alkoxysilanes to a point where a hydrolysis- and condensation-stable liquid system is produced. Using customary techniques, these systems may be employed, for example, for coating purposes and, depending on the functional group on the organically modified alkoxysilane, may be cured thermally or photochemically with the aid, if desired, of appropriate catalysts. In the case of thermal curing, inorganic networks are formed, and if appropriate organic groups are used organic linkages are formed in parallel thereto as well. The resultant nanocomposites are notable for high transparency. If used as a layer, they exhibit good adhesion to a very large number of substrates, and extremely high scratch resistance.
The present invention accordingly provides a process for preparing a composition for producing nanostructured mouldings and layers which comprises contacting an aqueous and/or alcoholic sol of a compound of an element selected from silicon and metals of the main groups and the transition groups of the Periodic Table with species possessing hydrolysable alkoxy groups and comprising at least one organically modified alkoxysilane or a precondensate derived therefrom, under conditions which lead to (further) hydrolysis of the species, and subsequent removal of the alcohol formed and any alcohol already present originally, and is characterized in that the alcohol is removed in an amount such that the residual alcohol content of the composition is not more than 20% by weight, preferably not more than 15% by weight and, in particular, not more than 10% by weight.
The present invention also provides the compositions obtainable by the above process and for their use for producing nanostructured mouldings and substrates provided with nanostructured layers.
The process of the invention differs from similar processes of the prior art in particular by virtue of the fact that a considerable fraction of the solvent (alcohol) present in the system is removed from the system. This shifts the hydrolysis and condensation equilibrium towards the product side and brings about stabilization of the corresponding liquid system. In general, at least 30% by weight, in particular at least 50% by weight and preferably at least 70% by weight of the theoretical amount of alcohol formed by hydrolysis of alkoxy groups is removed. With particular preference, at least 80% by weight, and more preferably still 90% by weight, of this alcohol is removed. This calculation does not include any alcohol present originally (e.g. from the sol starting material; it is assumed that the corresponding amount of alcohol is removed 100%), but does include the amount of alcohol already formed during the preparation of any precondensates used. As a result, it is generally ensured that 10-80% (preferably 20-50%) of all present condensable (hydrolysed) groups of the silane undergo a condensation react

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