Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to oxide-based hard
particles, in particular of corundum, which are coated with
ceramic pigments, as well as a method for producing same.
Furthermore, the invention concerns the use of these
particles as colouring, wear-retardiny and roughening
components, in particular in ceramic glazes, enamels,
paints and coating compounds.
To improve the mechanical properties of surfaces
subjected to high loads, in particular the wear resistance
and slip resistance, hard particles are often incorporated
into the surface material. Examples of such surfaces are,
among others, ceramic floor tiles or their glazes, in
particular for floor surfaces in wet rooms, such as public
or private baths, or outdoors, or floor coverings made of
other materials such as plastics or artificial stones which
are exposed to similar loads. For this purpose, corundum
grains have primarily been used as hard particles which, in
contrast to non-oxidic hard materials such as hard metals
or silicon carbide, can be worked into glazes and enamels
without troublesom~ chemical reactions and have
satisfactory hardness and wear resistance at favourable
cost.
However, a disadvantage of conventional corundum
grains lies in the fact that only limited colouring and
surface structures are possible, which results in severe
restrictions, in particular in residential areas and in
puhlic buildings. The reason for this due to the natural
colours of these corundum grains which are white to brown
or grey to blue or, in special corunda, also pink to ruby
and which can, in addition, change during the baking
process when used in ceramic glazes. Satisfactory results
are, therefore, only achieved when the glaze has almost the
same colour, as a result of which the range of colours is
very restricte~d and also all intensive shades are excluded.
~n increase in the amount of colouring matter in the glaze
above the usual level does not afford any relief, because
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the corundum particles must be on the surface of the ylaze
in keeping with its purpose and, moreover, this leads to a
substantial increase in price. Any desired surface
effects, such as sprinklings with different colours between
glaze and hard material, are also only possible to a very
limited degree.
Thus, it is an object of the invention to provide
oxide-based hard particles which can be worked into any
coloured glaze and other surfaces desired without
influencing the colour effect and which permit an expansion
of the range of possible colours, whereby to render
possible homogeneous dyeing and special surface effects,
such as sprinklings, mattings, mottlings, marblings and
other patterns.
Accordingly the invention provides oxide-based
hard particles, formed with a coating which contains at
least one ceramic pigment and at least one binding agent.
In this way, the desirable mechanical properties
of the hard particles can be retained without the need to
tolerate any restrictions in colouring or an adverse affect
on the number of possible structures of those items in
which the particles are used.
The term "ceramic pigment" is intended to include
inorganic compounds (e.g. rutiles, spinels, chromites,
aluminates, ferrites etc.), produced ~rom (inorganic)
oxides in an annealing process, which are used as colouring
components in ceramic glazes and are characterized by
insolubility, or only limited solubility, in ceramic
glazes. In addition, depending on the requirements of khe
specific application in each case, inorganic pigmenks, such
as single-phase or multiphase metallic oxidesl metal
powders etc. as well as coloured frits, can be used as the
ceramic pigment.
An overall summary is given by B. Rompp, Chemie-
Lexikon [Chemical Encyclopedia], 9th Edition, Vol. 4, 1991,G. Thieme Verlag Stuttgart, and By W. Heinz, Silicat-
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Lexikon [Silicate Encyclopedia], 1st Edition 1985,Akademie-Verlag Berlin, p. 226.
In general, all ceramic pigmenks according to the
above definition can be used without difficulty for
surfaces that are not exposed to temperatures above
approximately 300CI such as painted surfaces, plastic
surfaces and concrete.
When using coloured glasses especially in glazes
for tiles, a melting mixture of the coloured glasses with
the glaze takes place around the hard material core, which
can lead, on the one hand, to a desired mottling effect.
When using inorganic pigments in glazes, testing
should be affected in each case ko ascertain the extent to
which there is an inert behaviour of the inorganic pigments
duriny the glaze baking.
Aluminum oxide, zirconium oxide, silicon dioxide,
spinels or mixtures thereof are preferably used as basic
material for the oxide-based coated hard particles of the
invention, optionally with the usual contaminations and/or
additives, such as for example stabilizers or sintered
adjuvants.
An especially preferred basic material is
corundum, in particular melted or sintered corundum.
The particle size of the coated hard particles of
the invention is advantageously from 0.03 to 3 mm,
preferably from 0.06 to 0.5 mm. Especially preferred are
particles with narrow grain bands as described in FEPA
Standard 42-D 1984 or 43-D-1984.
Due to their size, the coated hard particles are
easy to process, since they are free-flowing and dust-free
and do not agglomerate. Thus, they can be used both in a
dry glazing process and in the more common wet Iglazing
process and can be easily conveyed and dosed by automatic
devices.
The ceramic pigment is advantageously used in an
amount of from 2 to 20% by weight, preferably from 5 to 10~
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by weight, based on the quantity of particles prior to
coating. The amount to be appli~d thereby depends on the
grain size of the pigment, the desired colouring strength,
and the depth of colour as well as the grain size of the
hard particles.
When using finer grains (e.g. F 220 according to
FEPA), ceramic glazes can be coloured in such a way that a
homogeneous visual color effect results, as is obtained
with conventional glaze colouring by direct pigment
addition. However, it has been shown that, when using the
coated hard particles of the invention, a substantially
smal].er amount of pigment is sufficient to obtain the same
color saturation as with the conventional glaze colouring.
Thus, for example, by adding 7% by weight of colour-coated
corundum particles with a maximum pigment content of 10% by
weight, that is 0.7% by weight maximum piyment, based on
the entire dry glaze mixture, the same color saturation is
attained as with a direct addition of 5% by weiyht pigment
to the same ylaze.
As a result, it is possible to lower
significantly the heavy metal component of the glazes
without the need to switch to other pigment compositions.
Basically, any binding agent is suitable for use
as binding agent for the coating, which adequately wets
hoth the hard material and the pigment, forms a
sufficiently firmly adhering layer and does not exhibit any
troublesome effects during the proposed use of the coated
particles, for example during glaze baking of tiles.
Preferred binding agents include silicic acids,
silicates, each with or without addition of colourless
glass frits, aluminum monophosphate or mixtures of these
substances. Especially preferred are alkali silicates
("sodium silicates") such as soda waterglass.
The coated hard particles of the invention can be
produced in a simple and cost-effective manner. To this
end, the hard particles to be coated are, for example,
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first wetted with a solution or suspension of the binding
agent. This can be done, for example, by simple mixing in
a common mixer or by spraying onto the particles in motion.
A solution or a suspension with water or another easily
vaporizable liquid is preferably used as the solution or
suspension. Advantageously, not much more solution or
suspension binder is used than is necessary for the uniform
wetting of the particles and the binding of the proposed
pigment quantity. The necessary amount depends on the
specific surface or size of the particles and the viscosity
of the solution or suspension and the pigments.
The particles thus wetted are then thoroughly
mixed with the necessary amount of the finely powdered
pigment, so that a uniform coating is formed. PrefPrably,
the quantitative ratio between binding agent and pigment is
selected in such a way that this coating does not have any
substantial excess binding agent on the outside, because
the particles would otherwise easily stick together and
form agglomerates. After applying the pigment, the
particles are first dried. Further thermal treatment
depends mainly on the desired use and covers a temperature
range of:
- at least 300C for use in plastic surfaces and
concrete, as well as specially when using
corundum coated with coloured glass in tiles to
produce a mottled effect,
to
- 1200C maximum for use in tiles, in the event
an absolutely inert behaviour of the coated
corundum is necessary.
Finally, if necessary, the dried coated particles
may be deagglomerated and/or sifted, to destroy~ and/or
remove resultant agglomerates.
A preferred use of the coated hard particles of
the invention is as an addition to ceramic glazes or
enamels. The particles are primarily used there to
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increase the wear resistance and to roughen the surface and
thus reduce the danger of slipping. To this end, the
particles are preferably embedded in the surface of the
glaze or enamel layer. This can take place by sprinkling
on the glaze or enamel mass already applied as dross or in
powder orm prior to baking or during the baking process.
Moreover, the addition of the particles can also be used to
affect the appearance of the finished glaze or enamel
layer, for example, to obtain a modified surface structure
in the manner of a mat effect, or to produce sprinklings or
similar color effects when different shades and/or colour
intensities are selected for the glaze or enamel base mass
and the particle coating.
In this case, it can also be advantageous,
depending on the degree of transparency of the glaze or
enamel, to uniformly distribute the particles in the glaze
mass or, similar to the so-called underglaze colours, to
first apply th~ particles to the substrate and cover with
the glaze.
Especially preferred is the use of the coated
hard particles of the invention in the glaze for ceramic
floor tiles for use in outside areas, in wet rooms and for
heavily wear-loaded floors, in particular in public
buildings and in industry.
A further preferred use of the coated hard
particles of the invention is their addition of plastics
and coating substances, for example for floor and wall
coverings. In this case also, their advantages, namely the
combination of wear-resistance and colouring, apply.
A further preferred use of the coated hard
particles of the invention is their addition to paints for
mechanically loaded coatings. Moreover, the coated hard
particles of the invention can, for example, be used as an
addition to artificial stone materials, concretes or
plastering.
The following Examples illustrate the invention.
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Example 1
25 ml of a mixture of 60% by volume of soda
waterglass solution (34.2% by weight solids content, 8% by
weight Na2O, 26.2% by weight SiO2, viscosity ca. 70 mPa s)
and 40% by volume of water were added to 1 kg corundum
(precious corundum, grain F-120 (d50 ~ 110 ~m) and mixed for
about 20 minutes, in a paddle mixerO 100 g of pigment
(grain size 100% < 63 ~m), e.g. of the spinel type, was
added to the corundum uniformly wetted in this way and the
mixing process continued for a further 20 minutes. The
coated corundum particles thus obtained were dried at 150C
to a residual moisture content of 0.1% maximum (in a drum
drier for 4 hours with circulating air). After the drying,
a control sifting took place at 212 ~m. The dried
particles, with the exception of the oversize pieces, were
then baked in a rotary kiln at 1000C in an oxidizing
atmosphere (air~. The average duration of baking was 2
hours.
After cooling, the particles were disagglomerated
on a swing sieve using hard rubber ~alls and simultaneously
sifted at 212 ~m in order to separate remaining
agglomerates. The product is a flowable grain which
visually conveys a homogeneous colour effect.
Example 2
In a similar example to that described in Example
1, corundum grains were each coated with 10% by weight of
a ceramic pigment. Pigments of the following colours and
compositions (in % by weight) were usedO
Green: Cr2O3 95%
Al2O3 3%
sio2 2%
I
Brown: ZnO 50%
Fe2O3 25%
Cr2O3 25%
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Blue: Co3O4 20%
Al203 80%
Yellow~ Pb304 40~
5Sb203 40%
~no 10%
CaO 10%
The following Table shows the possible reduction
(vis-a-vis conventional glaze colouring) in the content of
environmentally relavant compounds required in the glaze
(in % by weight) by using the hard particles of the
invention.
TABLE
_ Environmentally During direct When
Colour relevant colouring colouring l
compound with 5% by with 7% by I
weight of weight of
pigment particles of
(% by the invention
weight)* (% by weight)
_
Green Cr203 ~_ 0.67
Brown ZnO 2.50 0.35
_ _ 1.25 0.18
Blue Co304 1.00 0.14
_ Pb304 2.00 0.28
Yellow Sb203 2.00 0.28
- 0.50 0.07
*) The differences to thP indicated 5% are accounted for
by components of the pigment which themsPlves are not
environmentally relevant (e.g. Al203, Sio2, Fe203....)
