CERAMIC ENGOBES FOR ELECTROSTATIC POWDER APPLICATION AND A PROCESS FOR THEIR PREPARATION

ENGOBES CERAMIQUES POUR L'APPLICATION ELECTROSTATIQUE D'UNE POUDRE; METHODE POUR L'OBTENTION DE CES ENGOBES

English Abstract

CERAMIC ENGOBES FOR ELECTROSTATIC POWDER APPLICATION
AND A PROCESS FOR THEIR PREPARATION
ABSTRACT OF THE DISCLOSURE
Powders useful in applying a ceramic coating to ceramic
substrates by electrostatic powder application are ceramic engobes
in powder form, containing molten, ground ceramic frits, ground
ceramic raw materials, finely divided minerals, powdered glass,
powdered crockery porcelain and inorganic opacifiers for
electrostatic powder application to ceramic substrates, and
which engobes a surface which has been treated with electrically
insulating substances comprising halogen-free polysiloxanes
or mixtures thereof which react by their reactive groups on
the grain surface of the engobe powders. The ceramic engobes
are prepared by treating ceramic materials with from 0.05 to
0.25% by weight of halogen-free polysiloxanes, based on the
ceramic in powder form, which was ground, before or during the
treatment at a temperature from 70 to 100°C or thermally after-
treated at 70 to 300°C following a cold grinding, and the ground
powder mixture having a fineness of from 10 to 120µ, a specific
electric resistance of from 1013 to 1016 Ohm ? cm, a cubic coefficient
of thermal expansion of 120 x 10-7 to 240 x 10-7K-1 determined
at 20 to 300°C, and a fluidity of from 50 to 90 g/30 sec.
BIT 1

Claims

What Is Claimed Is:
1. In improved ceramic engobes in powder form, containing
molten, ground ceramic frits, ground ceramic raw materials,
finely divided minerals, powdered glass, powdered crockery porcelain
and inorganic opacifier for electrostatic powder application on
to ceramic substrates and which engobes have a surface which
has been treated with electrically insulating substances,
the improvement comprises the electrically insulating substances
being halogen-free polysiloxanes or mixtures thereof which react
by their reactive groups on the surface of the engobe powders
and are present in a quantity of from 0.05 to 0.25% by weight,
based on the ceramic engobe, and wherein the powder has a fineness
of from 1 to 120µ, has a specific electric resistance of from
1013 to 1016 Ohm ? cm, a cubic coefficient of thermal expansion
of 120 x 10-7 to 240 x 10-7K-1 determined at 20 to 300°C, and
a fluidity of from 50 to 90 g/30 sec.
2. The improved ceramic engobe powders according
to claim 1 wherein the polysiloxanes are polymethyl hydrogen
siloxanes corresponding to the formula
<IMG>
wherein n is 5 to 50, or mixtures thereof.
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3 A process for the preparation of ceramic engobe powders
according to claim l having a surface which has been treated
with electrically insulating substances which comprises treating
mixtures of molten ceramic frits, ceramic raw materials, minerals,
glasses, crockery porcelain and inorganic opacifier corres-
ponding to the engobe powders with 0.05 to 0.25% by weight,
based on the total quantity, of halogen-free polysiloxanes or
mixtures thereof which react by their reactive groups on the
surfaces of the ceramic engobe powder particles, wherein before
or during said treatment all the materials are ground together
at 70 to 100°C or are cold milled together followed by a thermal
after-treatment at 70 to 300°C, and the ground powder
mixture having a grain size of from 1 to 120µ,
a specific electric resistance of from 1013 to 1016 Ohm ? cm,
a cubic coefficient of thermal expansion of 120 x 10-7 to 240
x 10-7K-1, determined at 20 to 300°C, and a fluidity of from
50 to 90 g/30 sec.
4. A process for the preparation of ceramic engobe
powders according to claim 3 wherein mixtures corresponding
to the engobe powder materials are treated with polymethyl hydrogen
silxoanes corresponding to the formula
<IMG>
wherein n is 5 to 50, or mixtures thereof.
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5. In an improved method of applying an enamel coating
to a metallic substrate by electrostatically applying a ceramic
powder to the substrate, the improvement comprises electrostatically
applying a powder as claimed in claim 1 to the substrate.
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Description

203~28~
Ceramic engobes for elec~os~atic powder application and a process for their
preparation
_
o The present invention lelates to ceramic engobes in powder fo~n for
elec~ostadc powder a~plica~on to ceram~c subs~a~es and to a process for their
preparation.
BACKGROUND OF THE lNVENTION
It is known that enamel powder can be electrically
charged and applied to metallic surfaces in an electric
field with a potential difference of about 60,000 to
lOO,OOO Volt. For this purpose, the surfaces of the
particles of enamel powder are coated with insulating
substances which are added to the enamel frits in a
quantity of O.l to 2.0~ by weight before or during dry
grinding so that a specific electric resistance of 10l2 to
lOl6 Ohm cm is obtained. The insulating substances used
are silanols, isocyanates, silicon nitrogen compounds,
carbodiimides, chlorosilanes and organopolysiloxanes~
The ceramic surfaces are coated either by a process
of wet application by ~ourinq, spraying, sprinkling or
centrifuging or variations of these techniques or by a dry
process in which ground frits or granulates are applied by
sieving, powdering or pouring to a substrate which has
generally been wet engobed so that the granulate or powder
becomes fixed and then flows evenlv when firin~ is
subsequently carried out.
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Ceramic powders may also be applied electrostadcally to ceranuc surfaces
but the adherence of the powders is so weak that it is virtually impossible to handle
the parts before the sintering and f~nng process since the powder drops from the5 parts.
Engobes are mixtures which may contain molten ceranuc frits, ceranuc raw
materials or minerals, powdered glass or powdered crockery china and inorganic
opacifier. These engobes may be applied on to cerarnic substrates as aquoeous
suspensions of ground material for masking the color of the substrate and improving
10 the subsequent applicadon of coatings or layers without at the same dme
deteriorating the adherence or surface quality of the finished product.
The application on to ceramic substrates is carried out by pouring, spraying,
immersion or variadons of these techniques.
Dry processing of the engobes in the form of powders by sieve application or
15 electrostatically is virtually impossible because the a&erence of the powders is so
weak that handling of the parts before the sintering or firing process would be
impracticable and the powder would drop from the ceramic substrates.
The problem therefore arose of providing ceramic engobes in powder form
which could be applied on to cerarnic surfaces and adhere firmly to these substrates.
20 This problem has been solved by means of the ceramic engobes according to the invention.
BRIEF DESC~RIPIlON OF THE INVENTION
Improved ceramic engobes have been discovered which contain ground
ceramic frits and ground ceramic raw materials, finely divided n~inerals, powdered
25 glass and crockery porcelain and inorganic clouding agents for electrostadc powder
application to ceramic substrates, said engobes having a surface which has been
treated with electrically insuladng substances which are halogen-free polysiloxanes
or mixtures thereof and react by their reactive groups on the grain surfaces of the
engobe powders and are present in a quantity of from 0.05 to 0.25% by weight,
30 based on the ceramic engobe.
DETAILED DESCRIPIlON OF THE lNVENTION
The present invention relates to ceramic engobes in powder forrn for
electrostatic powder application on to ceramic substrates having a surface which has
been treated with electrically insulating substances, characterised in that the
35 electricaUy insulating substances are halogen-free polysiloxanes or mixtures thereof
and react by their reactive groups on the grain surfaces of the ceram~c
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engobe powders and are present in a quantity of from 0.05
to 0.25% by weight, based on the ceramic engobe, and in
that the powder, which has been ground dry at a tempera-
ture from 70 to 1009C or has been thermally -treated
at 70 to 3000C after a cold milling has a particle size of
from 1 to 120 ~, a specific electric resistance of 1ol3 to
1016 Ohm cm, a cubic coefficient of thermal expansion of
(120 to 240) 10-7K-l, determined at 2~ to 300~C, and a
fluidity of from 50 to 90 g/30 sec.
It is preferred to use ceramic engobe powders which
have been treated with polymethyl hydrogen siloxanes
corresponding to the following formula
CH3)3 - Si - O ~ Si - 1- Si - (CH3)3
CH3 n
wherein n = 5 to 50, or mixtures thereof.
The invention further relates to a process for the
preparation of the ceramic engobe powders according to the
invention, characterised in that the mixtures of molten
ceramic frits, ceramic raw materials, minerals, glasses
and crockery porcelain and inorganic o~acifiers used
for the engobe powders are treated with 0.05 to 0.25% by
weight, based on the total quantity, of halogen-free
polysiloxanes or mixtures thereof which rea~t by their
reactive groups on the grain surfaces of the ceramic
engobes, this treatment being carried out before or during
the dry ~rinding of all the ~aterials together
at 70 to 100~C or before or during the corresponding
cold grinding followed by the thermal after-treatment at
70 to 300~C, and the ground powder mixture
having a grain size of from 1 to 120 ~, a specific
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electric resistance of from lol3 to 1ol6 Ohm . cm, a cubic
coefficient of thermal expansion of (120 to 240) 10-7K-l
determined at 20 to 3000C, and a fluidity of from 50 to 90
g/30 sec.
In a preferred method of procedure, the engobe
components giving rise to the engobe powders are treated
with polymethyl hydrogen siloxanes corresponding to the
following formula
H
(CH3)3 - Si - - Sl - O - S; - (CH3)3
CH3
wherein n = 5 to 50, or mixtures thereof.
The engobe powders according to the invention are
used for electrostatic powder application to ceramic
substrates.
The electrostatic dry powder coating according to
the invention has considerable advantages over other
techniques. These manifest themselves primarily when
electrostatic serial coating of ceramic substrates is
carried out with powders for white, transparent or colour
application in the firin~ process (Monocottura) because
in these circumstances the electrostatic powder applica-
tion of the engobe is harmoniously adapted to the
operating process and the flow of material.
Thus the powder application of the engobe only
requires the provision of an electrostatic coating cabin
arranged upstream on the application line so that the two
layers, i.e. the engobe + glaze, can be fired together.
Wet grinding of the engobe for wet application, which
requires the use of aqueous suspensions, is dispensed
with, as is also the drying prooess for removing water
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from the layers of slip before ~irina.
The electrostatic coating process with enqobes
affords a particular advantage in that no liquids are
required as medium so that much thinner substrates may be
used since these will not swell in the process of
electrostatic dry coating and deformations are therefore
prevented (about 3 mm instead of 6 to 9 mm).
In addition, the consumption of material is greatly
reduced since the engobe powder according to the invention
is deposited with an efficiency of >98% in electrostatic
application cabins.
Further, in the electrostatic application process,
the powders remain in a closed system and the air emitted
from the cabins is purified by filters. This has ecologi-
cally important advantages, especially when ecologicallyundesirable products are used. Neither the conventional
dry application process nor the wet application process
provides such a closed application system.
The electrostatic application technique for engobe
powders also considerably simplifies the whole coating
process and provides a considerable saving in space since
the application process takes place on a conveyor belt and
the apparatus required is limited to an electrostatic
cabin with controlled powder nozzles.
Electrostatic powder coating with engobes on
ceramic surfaces requires powders whose components
(ground, molten frits, finely divided ceramic raw
materials, ground minerals, glass and porcelain powder and
inorganic opacifier) have approximately the same
specific gravity and have a specific resistance in ~hm/cm
of the same order of magnitude after the treatment with
electrically insulating substances and a heat treatment
during or after grinding, so that the componants are not
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liable to separate, especially when the powders circulate
in the electrostatic coating cabin.
The invention will now be described in more detail
with the aid of examples which, however, are not to be
regarded as limiting.
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Exam~les
Exam~le 1
A 1000 kg mill is charged as follows:
1850 kg of alubit balls having a diameter of 32
to 45 mm
500 kg of a frit in the form of fla~es or
granules suitable as engobe component,
consisting of
52.5% by wt. of SiO2, 9.8% by wt. of
B2O3, 7.1% by wt. of A12O3, 5.1% by wt.
of ZnO, 6.9% by wt. of Cao, 2.0% by wt.
of MgO, 5.2% by wt. of K2O, 1.2% by wt.
of Na2O and 10.2% by wt. of Zro2
and the following additives:
100 kg of zirconium silicate,
100 kg of potassium feldspar,
100 kg of amblygonite,
200 kg of ground crockery porcelain and
1.2 kg of a mixture of polymethyl hydrogen
siloxanes (= 0.12% by wt.) corresponding
to the following formula
(CH3)3 - Si ~ r Si O ~ Si (CH3)3
CH3 n
wherein n = 5 to 50.
Before the ball mill is closed, the interior is
flushed with gaseous nitrogen to remove oxygen.
The material is ground for about 6 hours to reduce
it to the required degree of fineness, i.e. a grain size
distribution of
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100 percent by weight below 120 micron
about 80 " ~ 30 "
about 60 " " 20 "
about 40 " " 15 "
In the course of grinding, the temperature inside
the mill rises to about 80~C.
After grinding, the powder has a specific resis-
tance of 1016 Ohm cm, a flow of 50 - 70 g/30 sec,
determined with an apparatus of SAMES, and a cubic
coefficient of thermal expansion of (195) 10-7X-1.
The fluidity is determined by an apparatus of
SAMES, Grenoble. The method of measurement is described in
detail in a publication by Dr. H. J. Schittenhelm in
Journal des VDEFa, Volume 32, ~1984), Number 10, pages 137
to 148.
The optimum fluidity for electrostatic application
is in the range of 50 to so g/30 sec. Values below 50 g/30
sec and especially below 40 g/30 sec are to be regarded as
critical in that they may give rise to difficulties in
application due to lack of fluidity.
ExamDle 2
Grinding is carried out in a 1 kg mill containing 3
kg of grinding balls of steatite having a diameter of 20
to 40 mm. The grinding unit is charged with 1 kg of a
ceramic engobe having the composition according to
Example 1 and 1.5 g (= 0.15% by weight) of polymethyl
hydrogen siloxane mixture.
Dry grinding to the desired particle size distribu-
tion as described in ~xample 1 takes place within 4 hours.
The temperature of the material being ground is only
slightly above room temperature. The specific resistance
is 3.0 x 1015 Ohm cm and the fluidity is 5 g~30 sec.
The ground material is thus unsuitable for electro-
static application. It is therefore heated to 120QC for 8
hours. After this heat treatment, the fluidity increases
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to 60 g/30 sec and the specific resistance is 1016 Ohm -
cm. The material is now perfectly suitable for electro-
static application.
Instead of the ball mills which operate batchwise,
continuous grinding units such as vibration mills or
Hardinge mills may be used. The former develop a grinding
temperature of about 1009C while the latter have to be
heated from outside to reach the reguired temperature of
70 to 1009C. Both grinding units must be provided uDstream with
a mixing mill charged with only a few heavy grinding balls
to ensure homogenisation of the grinding stock with the
insulating suhstances. When grinding is carried out at low
temperatures, the milled powder must subsequently be
thermally after-treated for 2 to 8 hours, depending on the
temperature (temperature range of from 70 to 300~C),
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