Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2003-03-11
2004-03-02
Koslow, C. Melissa (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C106S429000, C516S033000
Reexamination Certificate
active
06699316
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for the preparation of nano-zinc oxide dispersions stabilized by hydroxyl group-containing inorganic polymers. The established preparation routes for zinc oxide nanoparticles proceed either via calcination processes of suitable precursors or via wet-chemical precipitation methods. An overview of the preparation of zinc oxide nanoparticles and the dispersibility in water and water- and/or halogen-containing organic solvents, optionally with the addition of stabilizers such as betaines, polyols or aminoalcohols, is given, for example, in DE-A 199 07 704.
EP-A 1 146 069 A2 describes inorganic coating systems based on nano-ZnO particles for the UV-protective coating of various substrates. The preparation of the nano-ZnO particles or corresponding sols used is carried out in accordance with DE-A 199 07 704. EP-A 1 146 069 likewise describes that the nano-ZnO particles can be surface-modified with alkoxysilanes. The dispersions which can be prepared in this way have a high degree of dispersion of the ZnO particles, but exhibit very limited storage stability (<48 hours) and have high contents of halogenated hydrocarbons (>5% by weight).
According to the prior art, it is known that anhydrous nano-zinc oxide dispersions in which the particles are present in the form of a primary particle dispersion can only be prepared in organic solvents if the dispersion medium additionally contains halogen-containing compounds such as chlorinated hydrocarbons. However, halogenated components are toxicologically unacceptable and are therefore unsuitable for the production of coatings. It is also known that nano-zinc oxide dispersions are not obtainable by simply exchanging the solvent, e.g. by adding a higher-boiling, nonhalogenated solvent and distilling off the halogen-containing constituents. This always results in irreversible agglomeration of the nano-zinc oxide particles, and subsequent production of transparent coatings becomes impossible.
An object of the present invention was therefore to provide a process for the preparation of nano-zinc oxide dispersions with improved storage stability. Within a shelf life (pot life) of these dispersions which is sufficiently long for a large number of applications, no clouding and/or flocculation of the nano-zinc oxide dispersions arises as a result of particle aggregation or agglomeration. Neither does gelling or solidification of the system of nanoparticles and dispersion medium (consisting of one or more solvents and optionally a stabilizer) arise. Furthermore, they have a markedly reduced content of halogenated, constituents.
DESCRIPTION OF THE INVENTION
The present invention therefore provides a process for the preparation of nano-zinc oxide dispersions, where the storage stability of the resulting dispersions between −50 and +250° C., preferably between 0 and 80° C., in particular between 10 and 40° C., is greater than 48 hours, and the dispersions have a content of halogenated constituents of less than 5% by weight, comprising:
A) dispersing zinc oxide nanoparticles in a halogen-containing medium,
B) adding the dispersion obtained in step A), optionally with stirring, to a solution of hydroxyl group-containing inorganic polymers, and
C) removing the halogen-containing constituents by distillation at atmospheric pressure or by condensation under reduced pressure.
Reduced pressure is understood as meaning pressures between 0 and 1000 mbar, preferably 10 and 300 mbar.
In the process according to the invention, it is possible to prepare nanozinc oxide dispersions whose storage stability between −50 and +250° C., preferably between 0 and 80° C., in particular between 10 and 40° C., is greater than 48 hours (two days). Over the course of this period, no gellations and/or aggregations or agglomerations of the nano-zinc oxide can be identified (e.g. by transmission electron micrographs, TEM).
According to the invention, storage stability is understood as meaning the pot life, shelf life or gel time. Over the course of this period the dispersion is stable.
The storage stability of the dispersions prepared according to the invention is at least 48 hours (two days). They are sedimentation-stable and storable over days, preferably weeks, in particular over months, without any separation, precipitation, gelling or hardening resulting.
The zinc oxide nanoparticles used in the process according to the invention have-a primary particle size of from 1 to 500 nm, preferably from 1 to 100 nm, and particularly preferably from 1 to 30 nm. The agglomerate size is less than 10 &mgr;m, preferably less than 1 &mgr;m, and particularly preferably less than 0.1 &mgr;m. The zinc oxide nanoparticles used may be crystalline, partially crystalline or amorphous. Preference is given to using X-ray-crystalline zinc oxide nanoparticles.
The particle sizes of the ZnO particles stated refer to results from ultracentrifugation measurements (H. G. Müller, Colloid. Polym. Sci., 267, 1113-1116 (1989)).
The dispersible zinc oxide nanoparticles used in the process according to the invention are prepared, for example, in accordance with the process described in DE-A 199 07 704 A1. It is, however, also possible to use ZnO nanoparticles which have been prepared by other processes.
The hydroxyl group-containing inorganic polymers B) used in the process according to the invention contain at least one element from the 3rd and 4th main group of the Mendeleev Periodic Table of the Elements which contains at least one hydroxyl group and, in addition to the hydroxyl groups, optionally one or more further nonhydrolysable groups. Preference is given to polymers containing the element silicon, germanium, boron, aluminium and/or indium, particularly preferably silicon, boron and/or aluminium.
Inorganic polymers containing the element silicon preferably additionally contain nonhydrolysable groups, such as optionally substituted alkyland/or aryl radicals, particularly preferably C
1
-C
4
-alkyl and/or C
6
-phenyl radicals, very particularly preferably C
1
-C
3
-alkyl radicals.
The inorganic polymers described can be obtained, for example, by hydrolysis and condensation of monomeric and/or oligomeric alkoxysilanes or organoalkoxysilanes (sol-gel process). The hydrolysis and condensation of appropriate alkoxysilanes is part of the prior art and is described in detail, for example, in “Sol-Gel Science”, J. Brinker, 1990.
To prepare the inorganic polymers used in the process according to the invention, alkoxysilanes of the formula (I), for example, can be used,
(Y)
a
(Y′)
b
Si(OR)
c
(I)
where
Y, Y′ are each hydrogen, an optionally substituted C
1
-C
20
-alkyl or C
6
-aryl radical,
R is a C
1
-C
8
-alkyl radical or a phenyl radical,
a, b and c, independently of one another, may be 0, 1, 2, 3 or 4 and
the sum (a+b+c)=4.
Preference is given to using alkoxysilanes of the formula (II)
(Y)
a
(Y′)
b
Si(OR)
c
(II)
where
Y, Y′ are each an H, a C
1
-C
8
-alkyl or C
6
-aryl radical,
R is a C
1
-C
4
-alkyl radical or a phenyl radical,
a, b and c, independently of one another, may be 0, 1, 2 or 3 and
the sum (a+b+c)=4.
Specifically, mention may be made of the following alkoxysilanes and organoalkoxysilanes:
a.) Si(OCH
3
)
4
, Si(OC
2
H
5
)
4
;
b.) CH
3
Si(OCH
3
)
3
, C
2
H
5
—Si(OCH
3
)
3
, phenyl-Si(OCH
3
)
3
, CH
3
—Si(OC
2
H
5
)
3
, C
2
H
5
—Si(OC
2
H
5
)
3
, phenyl-Si(OC
2
H
5
)
3
, 3-glycidoxypropyl-Si(OCH
3
)
3
, 3-acetoxypropyl-Si(OCH
3
)
3
, 3-methacryloxypropyl-Si(OCH
3
)
3
, 3-mercaptopropyl-Si(OCH
3
)
3
, 3-cyanopropyl-Si(OCH
3
)
3
, isocyanatopropyl-Si(OCH
3
)
3
, 3-aminopropyl-Si(OCH
3
)
3
;
c.) (CH
3
)
2
Si(OCH
3
)
2
, (CH
3
)
2
Si(OC
2
H
5
)
2
, (C
2
H
5
)
2
Si(OCH
3
)
2
, (C
2
H
5
)
2
Si(OC
2
H
5
)
2
, (n-butyl)
2
Si(OC
2
H
5
)
2
, (n-butyl)
2
Si(OCH
3
)
2
, (i-propyl)
2
Si(OC
2
H
5
)
2
, (i-propyl)
2
Si(OCH
3
)
2
, (CH
3
)(phenyl)Si(OC
2
H
5
)
2
, (CH
3
)(phenyl)Si(OCH
3
)
2
, (CH
3
)(H)Si(OCH
3
)
2
, (CH
3
)(H)Si(OC
2
H
5
)
2
, (CH
Hofacker Steffen
Mager Michael
Marx Thiemo
Wege Volker
Bayer Atiengesellschaft
Gil Joseph C.
Koslow C. Melissa
Manlove Shalie
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