Chemistry: physical processes – Physical processes – Agglomerating
Reexamination Certificate
1995-06-07
2001-10-16
Griffin, Steven P. (Department: 1754)
Chemistry: physical processes
Physical processes
Agglomerating
C423S335000, C423S608000, C423S610000, C423S625000
Reexamination Certificate
active
06302926
ABSTRACT:
The invention relates to preparations of monodisperse spherical oxide particles with which it is particularly easy to disperse the oxide particles homogeneously in polymer systems.
Spherical oxide particles, in particular SiO
2
particles, but also TiO
2
, ZrO
2
and Al
2
O
3
particles, are employed in industrial and scientific applications in diverse ways. It is known that particularly advantageous use properties result if these particles are in monodisperse form, that is to say have the lowest and narrowest possible particle size scatter at a given particle size. An essential industrial use of monodisperse spherical oxide particles is, for example, the use as a filler and modifying agent in polymer materials. In this case, the particles often not only have a substitution and dilution function, but impart to the polymeric matrix material certain properties or functions for the first time. Such particles are thus employed as optical diffusers in embedding compositions for light-emitting diodes or as fillers which reduce thermal stresses in transparent casting compositions for other optical, electro-optical and optoelectronic components. These particles are used in films of plastic, for example for magnetic recording media, to improve the slip properties. They can furthermore be used as opacifying agents in dental compositions. Reference may be made, for example, to the patent documents WO 94/07 948, WO 93/25 611 and EP 0 236 945 A2 with respect to such uses, in which monodisperse spherical oxide particles are particularly advantageous because of their monodispersity.
When used industrially, these oxide particles are in general not incorporated into polymeric binder systems in the form of the native powder. This is because handling in practice and in particular the preparation of homogeneous agglomerate-free and non-sedimenting dispersions of the oxide particle powders with the polymer systems is difficult for the user above all. The oxide particles are therefore preferably provided in a predispersed form specific to the use, whereby later incorporation into the intended polymer systems is said to be facilitated. For example, dispersions in aliphatic alcohols, glycols and oligoglycols are customary. Constituents of the polymeric binder systems into which they are to be incorporated later are occasionally also used as the dispersing medium.
Nevertheless, the oxide particles turn to sediment in such dispersions, especially during prolonged storage. It has been shown that precisely with highly monodisperse spherical oxide particles, sediments with highly ordered structures which approximately resemble the closest spherical packing are formed. Such sediments can be redispersed again only with great difficulty, and it is scarcely possible to achieve the original state of a homogeneous agglomerate-free dispersion again.
Another disadvantage of the use of oxide particles in predispersed form is that at an oxide particle content of as a rule not more than 50% by weight, not inconsiderable amounts of dispersing medium are also incorporated into the polymer system. Attention must be paid to compatibility problems here, and the recipes need to be adjusted accordingly in each case.
The object was therefore to provide monodisperse spherical oxide particles in such a form which is itself stable and with which the particles can be incorporated homogeneously into polymer systems without problems and without thereby also incorporating undesirable amounts of other auxiliaries.
It has now been found that preparations of monodisperse spherical oxide particles having particle diameters of between 10 nm and 10 &mgr;m and an essentially non-volatile binder in the form of a free-flowing powder of agglomerates having diameters of between 1 &mgr;m and 500 &mgr;m, in which 70 to 95% by weight of oxide particles and 30 to 5% by weight of binder are present, meets these requirements in an excellent manner.
Such preparations can be obtained, for example, by mixing the oxide particles homogeneously with the binder in a corresponding ratio together with a solvent which is volatile at temperatures of between 0 and 120° C. under a pressure between 1 and 0.1 bar, the binder and solvent being present in a weight ratio of 1:1 to 1:100, and then removing the solvent from the resulting suspension or dispersion by distillation.
The invention therefore relates to preparations, characterized as above, of monodisperse spherical oxide particles.
The invention furthermore relates to a process for the preparation of such preparations in which 70 to 95% by weight of monodisperse spherical oxide particles having a particle diameter of between 10 nm and 10 &mgr;m are is mixed homogeneously with 30 to 5% by weight of an essentially non-volatile binder together with a solvent which is volatile at elevated temperature and/or under reduced pressure, the binder and solvent being present in a weight ratio of 1:1 to 1:100, and the solvent is then removed from the resulting suspension or dispersion by distillation, a free-flowing powder of agglomerates having diameters between 1 &mgr;m and 500 &mgr;m being obtained.
All monodisperse spherical oxide particles, the particle diameter of which is in the range between 10 nm and 10 &mgr;m, are in principle suitable for the preparations according to the invention. The term monodisperse is to be understood as meaning that the particles of a certain diameter have the lowest possible particle size scatter lying within a very narrow range. Monodisperse spherical oxide particles having a standard deviation of the particle diameter of less than 10%, and in particular less than 5%, are preferred. The particle sizes are preferably in the range between 100 nm and 1 &mgr;m, in particular between 250 nm and 500 nm. All the customary solid oxides or oxidic materials, such as, in particular, metal oxides, are in principle suitable as the base materials of the particles. Oxide particles which comprise SiO
2
, TiO
2
, ZrO
2
, Al
2
O
3
or corresponding mixed oxides are preferred. Particles of SiO
2
and TiO
2
, but in particular of SiO
2
, are particularly preferred. They can be prepared in any desired manner per se, as long as the particles are monodisperse and spherical, and preferably furthermore compact and non-porous.
The monodisperse spherical oxide particles to be used according to the invention are known per se from the prior art. Oxide particles which can be obtained by hydrolytic polycondensation from alcoholate compounds of corresponding elements and are thereby obtained in the form of monodisperse compact spherical particles are particularly suitable. The basic reaction conditions for preparation of SiO
2
particles by hydrolytic polycondensation can be found, for example, in the publications by W. Stöber et al. in J. Colloid and Interface Science 26, 62 (1968) and 30, 5168 (1969) and U.S. Pat. No. 3,634,588. Other particles, such as, for example, TiO
2
or ZrO
2
, can also be prepared by this method. For the preparation of highly monodisperse non-porous spherical SiO
2
particles which have a standard deviation of not more than 5%, reference is made to EP 0 216 278, which discloses a correspondingly targeted preparation process based on hydrolytic polycondensation. The core of the process which is particularly preferred for the preparation of the particles according to the present invention is a two-stage procedure. In this, a sol or a suspension of primary particles is first formed by hydrolytic polycondensation of tetraalkoxysilanes in an aqueous-alcoholic-ammoniacal medium and is then subsequently brought to the desired final size by metered addition of further tetraalkoxysilane. The process according to EP 0 216 278 can be applied without reservation and with the same result to other oxides and also to mixed oxide systems. An appropriate process for the preparation of various metal oxides in the form of spherical particles having a narrow particle size distribution is to be found in EP 0 275 688. A corresponding two-stage process for the preparation of various metal oxides and also mixe
Anselmann Ralf
Griessmann Carsten
Loch Manuela
Marquard Kurt
Griffin Steven P.
Merck Patent Gesellschaft mit beschrankter Haftung
Millen White Zelano & Branigan P.C.
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