CA2036281A1

CA2036281A1 - Ceramic engobes for electrostatic powder application and a process for their preparation

Info

Publication number: CA2036281A1
Application number: CA002036281A
Authority: CA
Inventors: Paul Zybell; Giovanni Broggi
Original assignee: Bayer Italia SpA
Current assignee: Bayer SpA
Priority date: 1990-02-15
Filing date: 1991-02-13
Publication date: 1991-08-16
Also published as: IT9019379A0; IT1241063B; IT9019379A1; EP0442109A1

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.

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.

Bit I - I -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 Bit 1 - 2 -20362~1 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 203~281 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 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 2~36281 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 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.

20362~1 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 203~281 100 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 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),

Claims

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 wherein n is 5 to 50, or mixtures thereof.

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 wherein n is 5 to 50, or mixtures thereof.

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.