MXPA98001366A - Procedure for the preparation of ceramic coatings and recubrimie powder - Google Patents

Abstract

Until now it was the manufacture of ceramic coatings on substrates capable of being baked by an electrostatic process using coating powders, a complicated process, the quality of the coating was not satisfactory. The invention eliminates this problem: the coating powder according to the invention consists of a material capable of baking in powder form with a d90 value of less than 35 microns, a d50 value of 5 to 25 microns, and a heat d10 equal to or greater than 2 microns, and an organic adherent in the form of chemical or physically activatable powder. The coating covers the electrostatic application of coating powder, the physical or chemical activation of the adherent and the baking of the coated substrate. Fail-free coatings are obtained

Description

PROCEDURE FOR THE PREPARATION OF CERAMIC COATINGS AND COATING POWDER DESCRIPTION OF THE INVENTION: The invention is directed to a coating powder applicable electrostatically for the manufacture of ceramic coatings on metal and ceramic substrates, especially non-burnable ceramic substrates and sponge cake. The invention is further directed to a process for the manufacture of coating powder as well as its use for glazing, engobe and decoration of ceramic substrates, where the method of use includes an electrostatic deposition of a coating powder. For the manufacture of ceramic coatings, such as glazes and glazes, on substrates capable of baking, predominantly aqueous agglomerates are used, after the application of the agglomerate the already coated substrate is baked, where the agglomerate including the material capable of baking melts together in a ceramic layer or sinter. Due to the disadvantages associated with aqueous agglomerates, such as the problem of eliminating water and the high energy expenditure, electrostatic production of metallic and ceramic powder coating has gained importance.

REF: 26845 In the electrostatically powder coating of metal substrates, enamel powder is used, which in order to increase the surface resistance is usually covered with organosilicon compounds - see patent document DE -AA 3909 656. The production of such coating enamel powders requires, in addition to intensive mixing of the enamel powder with the components of the coating, a heat treatment, with which the enamel powder becomes more expensive. The powder coating of ceramic substrates such as glass, porcelain, clay, glazed clay with a material capable of baking, still presents greater problems than the coating with metal dust. According to GB-B 1 198 462 an improvement is achieved by pre-treatment of the substrate with an aqueous electrolytic solution. This treatment increases the costs of the coating and gives another problem, namely the insufficient adherence of the powder to the substrate, a defect that can not be removed. According to DE-AS 22 04 982 the adhesion is improved, a substrate heated to 450-550 ° C is immediately electrostatically coated. This procedure is technically complicated and can only be applied, the coating powder melts at the said temperature, which does not occur for many glazes. For the production of glazed mosaic, according to DE-PS 29 41 026, glazing powders having a particle size of less than 10ohohm are applied electrostatically; The specific electrical resistance is achieved as with the aforementioned enamel powders by coating with a polysiloxane. The slip of ceramic substrates - EP-A 0 442 109 is similarly carried out. Finally, the adhesiveness of an electrostatically applied glaze powder is allowed to increase and at the same time the application of a sufficient quantity of the glaze powder is allowed, if the The substrate is first provided with an adherent component, for example a polymer, and a component capable of forming the glass in an adherent layer - DE-A 42 39 541. The requirement of the wet adhesive layer as a preliminary step for the glaze is disadvantageous. One possibility, to improve the adhesion capacity of a baked ceramic material electrostatically placed on a substrate capable of baking, is that the coated substrate without danger of falling coating powder applied before baking proper, can be safely handled, teaches the WO 94/26679: here contains the coating powder in addition to a powder capable of being baked, some of a glaze powder, a burnable adhesive without residue in the baking, which is thermally or moisture activated, so that the particles of the layer are fixed to each other and to the substrate sufficiently. Thermoplastic materials, such as polyolefins, are physically activated when the substrate before electrostatic spraying or later rises to a temperature above the melting point of the polymer: by cooling and with this solidification of the thermoplastic a fixation is obtained, another type of activation , properly a treatment of the coated substrate in a humidification chamber, the use of dextrin as adhesion agent is proposed. The preferred coating powder contains glass co-coated with polysiloxane in a mixture of 10 to 15% by weight thermoplastic or 5 to 10% dextrin. The combined process according to a preferred embodiment of the electrostatic glazing according to WO 94/26679 is very complicated. Firstly, the glazing powders without the polysiloxane layer are considered to be insufficiently electrostatically chargeable, so that the electrostatic coating is not carried out without problems. For the manufacture of a coating powder capable of glazing, firstly the glaze is mixed with a polysiloxane and heat treated, then the glaze powder covered by the polysiloxane is mixed at a temperature above the melting point of the adherent agents such as polyethylene , and then the mixture is ground. A powder containing polyethylene as an adherent, thus manufactured, is not only complicated to manufacture, but also tends to run. The fluidization can be improved in a known manner by the use of pyrogenic oxides, such as Si02, but this has the disadvantage that the amount required in the practice of gloss in the glaze is clearly reduced. In addition, the application of silicon dioxide negatively affects the electrostatic behavior of the coating powder. By means of tests carried out in accordance with the procedure described in WO 94/26679 with a simple preparation of the coating powder - without polysiloxane coating and without thermal treatment of the glass / polyethylene mixture - it was determined as a disadvantage that in the use of the powder, the surface of the glaze is strongly undulated and supersaturated with glaze faults and using a higher amount of the adherent, the applied quantity is limited, so that no coarseness that was sufficient could be obtained from the glaze layer - which should usually be from at least 100 to 250 micras - see comparative examples (VB 1 and 2). The glazing faults are partially explained, because with the investigated coating powders in the impact plates, accumulations appear and in the central electrode of the electrostatic spray gun, which from time to time explode and arrive as dispersions to the substrate that has of coating and remain visible even after baking. Therefore, it is an object of the present invention to create improved coating powders which are comparable in comparison to the powders of WO 94/26679, and which can be prepared in a simple manner. With the use of the powders produced, coatings should be presented without faults and in the thickness of application and quality desired. The objective is achieved by means of an electrostatically applicable coating powder, consisting of a powder-shaped baking material and an agent of the series of glazes, engobes and enamels and a physically or chemically activatable organic adherent, which is characterized in that, the baked material in powder form has a grain distribution of a value d, 0 less than 35 microns, a value d50 in the range of 5 to 25 microns and a value d10 that is equal to or greater than 2 microns. The advantages achievable by the selection of the grain spectrum of the bakeable material in the coating powder have many facets and were not foreseeable. It was determined that the property of commercially available glass powder whose grain spectrum is at most between 1 and 70 microns and its value d9C. It is around 40 micras, it could be improved remarkably for its electrostatic application, if its grain spectrum was narrowed, and precisely by reducing the coarser grain fraction, so that a value of d90 will be less than 35 microns, preferably less than 32 microns. As long as the value d90 was already below the values mentioned according to the invention, the value d10 was less than 2 microns, an improvement could be achieved in reference to the use of electrostatically applicable powders, where the dust fraction was reduce, so that the value d10 is equal to or greater than 2 microns. A grain spectrum with d10 greater than 2 microns, preferably greater than 3 microns, and d 90 less than 32 microns, would be preferred. The d50 value is between 5 and 25 microns, preferably between 10 and 25 microns. The values d1Q and d90 are essential for the invention. The dso value of the commercially available glass powder is often between 10 and 25 microns. As is clear from the examples and comparison examples, this leads to a lowering of the d90 value from 34 to 28 microns with a simultaneous elevation of the d10 value from 1.5 to 2.3 microns for a remarkable improvement in coating powders. The values of index 10, 50, and 90 give the diameter of the grain, in which the passage of 10, 50 and 90% by weight of the powder is worth. The values of the indices 10, 50, and 90 can be obtained in a known manner by the granulometric curves. The determination is made by commercially available apparatuses (such as CILAS HR 850-B granulometer) - see also German Industry Standard 66141. The transfer of materials capable of baking with a granulation not according to the invention by division of the grain and / or narrowing of the grain spectrum by filtering to obtain the desired band of spectrum according to the invention, can be carried out by usual milling means, such as ball mills or sifting mills, and as long as necessary , using one or several stage sifters. For the purpose of the grain spectrum of the adherent agent, a d10 value greater than 1 miera and a d9D value of less than 35 microns, preferably less than 25 microns, and a preferred dso value of 5 to 15 microns are sought. The amount of adherent, in reference to the material capable of baking, is maintained, on the one hand, as low as possible, on the other hand so high, that a final handling of the electrostatic powder coating, such as surface decoration by especially filter pressure or other techniques of pressing and placement in the furnace, without damage of the coating. In general the amount of adherent will be less than 10% by weight and greater than 0.1% by weight, preferably the amount of adherent remains between 2 and 6% by weight, each time in reference to the material capable of baking. According to the invention, the glaze coating powder with from 2 to 6% adherent, in reference to the glaze, is allowed to be applied without any problem similar to those of the powders that are not of the invention, and leads its use to glazes without failure As disclosed, the coating powder according to the invention consists essentially of a material capable of being baked in powder form, and an adherent. The word "adherent" will be understood to mean that the coating powder may additionally contain processing aids, and in an amount of up to 5% by weight, preferably up to 2% by weight, based on the material capable of being baked. The auxiliary materials are to be understood as those with which the fluidization of the coating powder can be improved. In addition, they can be substances, with which the electrostatic parameter of the coating powder can be varied in the desired direction. In the fluidisation auxiliaries, these are especially oxides made of pyrogen, which in turn can be hydrophobic. Suitable fluidization agents are for example silicon dioxide, titanium dioxide and aluminum oxide. While in known powders such fluidizing aids must be present in an amount ranging from 0.5 to 3% by weight, in the coating powders according to the invention, it is the presence of such auxiliary agents, either completely eliminated. or reduced to a nominal value. The coating powder of the preferred invention contains between 0 and 0.3% by weight, especially 0 to 0.2% by weight of fluidizing agent, with reference to the material capable of baking. Under the term "material capable of being baked", it is to be understood, the substance or the mixture of substances that in a ceramic burn are fused together to a ceramic layer or at least sinter together. The softening point of the material capable of baking is therefore usually in the temperature range of 450 to 1450 ° C. Under materials capable of baking are used especially those presenting the composition of glazes, engobes, and enamels. The compositions of the glazes, engobes and enamels are known to the technicians. In the case of glazes, these are mostly glass or glass chip fritters or mixtures thereof with up to 10% by weight of clay minerals, especially kaolin. Essential components of slip are glass frits, fine particulate ceramic materials, ground minerals, glass or porcelain flour as well as inorganic turpentine agents. Essential constituent of enamels are glaze frits. In as much, that the glazes, engolbes, enamels and preparations of decoration contain inorganic pigments, fall, also within the concept of materials able to be baked. The material capable of baking can contain a hydrophobic coating, something like one made of polysiloxane. But it is an advantage of the coating powder according to the invention that such a cover can usually be dispensed with. The powder of the invention contains an organic adherent in the form of physically or chemically activatable powder. It can be a single material or a mixture of materials. The adherent is selected in such a way that, under the conditions of the ceramic firing, essentially it burns completely and does not lead to any failure in the ceramic layer obtained after burning. The adherent must not exercise its adherent effect under the conditions of the coating powder manufacture, but must manifest it after the electronic spray by chemical or physical activation. In the physical activation of the adherent in the coating powder, it will preferably be a heat treatment at a temperature above the melting point of the adherent, where one of the particles of the coating with each other and with the substrate; with the cooling the adherent solidifies and results in a fixedly adhered coating and suitable before the baking itself to other decorating steps. Suitable adhesion agents, which are activatable by a heat treatment, are thermoplastic homopolymers and copolymers with a softening point in the range of 60 to 250 ° C, preferably between 80 and 200 ° C and especially between 80 and 150 ° C. thermoplastic adhesive is preferably polyolefins, such as paraffin waxes and low density polyethylenes (ls-PE) as well as polyvinyl acrylate and methacrylate polymers and copolymers, such as polystyrene, polyvinyl acetate, ethylene / vinyl acetate copolymers, styrene / acrylate copolymers, polyester and copolyester as well as polyamide and copolyamide are also usable. In another class of physically activatable adherents, these are those that can be activated by moisture, such as water vapor. Suitable in this group are the polysaccharides and their derivatives, such as dextrin. In a chemically activatable class of adherents, it is a two-component polymer system, which hardens at a temperature in the range of 60 to 250 ° C, especially between 80 and 200 ° C and especially between 100 to 180 ° C. The two component system contains a polymer or prepolymer and a curing agent in a certain proportion to obtain a crosslinking or sufficient. According to the invention, the coating powder with a two-component system as a hardener is treated after its electrostatic application on the substrate thermally, in order to carry out the crosslinking reaction and with this the fixing of the layer. In preferred two-component systems, these are epoxy resins with an anhydride hardener or at least two carboxyl groups contained in the hardener as well as acidic polyester with hardener containing at least two glycidyl groups. Such two-component systems are obtainable commercially (usually to make powder coatings) or can be obtained by mixing, if necessary by kneading, with subsequent grinding. If necessary, oxides and / or silicates in the form of powder, which in turn will be a constituent part of the baked coating, are added in the production of the adherent of two components, in order to reach an adherent in the form of a powder free of charge. glue. The epoxy resin is usually based on a resin -Abiphenol; as hardeners of the epoxy resin, anhydride hardeners are preferably used; but hardeners containing carboxyl groups can also be used, such as cyclohexanone 2, 2, 6, 6 tetracarboxyethyl, as well as hardeners with application of imide structure. Another preferred two-component system is based on carboxyl polyester resins at saturation and triglycidyl isocyanurate as a hardener. Another alternative of a hardener is based on a mixture of a polymer containing vinyl groups and a crosslinker containing at least two vinyl groups; the polymer and the crosslinker are selected in such a way that the system is able to harden by radiation. Finally, deacylated isocyanate groups which are contained in polyurethane resins as the main constituent part of the adherent come into consideration; such resins are allowed to chemically activate, either with steam or ammonia treatment, or treatment with acid crosslinkers. The coating powders applicable electrostatically according to the invention are obtained by dry-blending a powder-shaped baking material of the series of glazes, engobes and enamels with a grain distribution according to the invention with an activable adhesive. by thermal treatment, treatment with water vapor, ammonia or other active cross-linking or by radiation. The mixing is carried out under conditions, in which an activation of the adherent still does not take place. Intensive mixing devices are preferred, wherein the mixing is carried out at room temperature or at a slightly elevated temperature, such as a plug tear mixer. In this way, since the adherent is not activated during the manufacture of the coating powder, a grinding process after mixing is superfluous, as is usual in the prior art. The co-utilization of fluid adherent in the manufacture of the coating powder, as found in application WO 94/26679, is not precisely excluded, in view of the additional application necessary, but the use of such fluid agents is usually eliminated. The coating powders according to the invention are characterized by an outstandingly good flowability, so that they can be applied electrostatically without problems. The fluidization factor, expressed as the ratio of the maximum height of the layer with respect to the rest protection, is generally between 1.3 and 1.6. The coating powder of the same chemical composition but not of grain distribution according to the invention, of the material capable of baking, presents, on the contrary, an unsatisfactory fluidizing behavior, a factor below 1.3 or by unmeasurable disorders. As the adherent fraction grows, the specific electrical resistance of the coating powder grows. Although the specific electrical resistance with an adherent in the preferred quantity range is generally between 109 to 1013 ohms x meter and this is below the pre-known powders of a material with a polysiloxane cover, they allow such coating powders they are applied without problems, surprisingly, electrostatically. Here, thick layers are allowed, such as those required for glazing of at least 100 to 250 microns and partially to 400 microns. Another object of the invention relates to the use of coating powders for the manufacture of a ceramic coating on a substrate capable of baking. It is understood under a substrate capable of baking, those substrates, capable of ceramic burning, that is a baking in the temperature range of usually between 450 and 1450 ° C. Among the burning conditions used is that the substrate is stable in its form . Metals capable of enamelling as well as ceramic materials are therefore included under these substrates. In the ceramic materials there are glass, fine ceramics, for example porcelain, Bone China and Vitreous China construction ceramics, for example roof and cover mosaic, as well as sanitary ceramics and ornamental ceramics. To the process according to the invention belong, fully cooked or burned ceramic substrates, partially cooked, cooked as biscocho, and unburned materials. Preferably the method of use in the coating is governed to partially or unburned ceramic materials, for the manufacture of the ceramic layer in the substrate the coating powder is applied to the substrate in the dry state with the substrate preheated through sieves, sprayers or agitators, but preferably by means of an electrostatic powder coating. The substrate covered with the powder is then subjected to a ceramic firing. In the important field of application of the glaze, the ceramic glaze is burned usually in the range of 900 and 1250 °. As long as the burning of the ceramic material and the glazing are carried out simultaneously, the burning temperature can also be in the region of up to 1450 ° C. The electrostatic application of the coating powder to the substrate to be treated takes place under usual conditions for the application of the powder.The coating powder - for the glaze is fluidized in a device and using usual electrostatic guns is electrically charged, sprayed and attracted by the substrate grounded. The loading of the coating powder in the coating guns usually takes place in accordance with the principle of crown and super crown, the voltage is usually between 30 and 100 KV, especially between 40 and 80 KV, the intensity of the current is between 50 and 90 microamps. The electrostatic application of the dust is done in cabins built ex profeso. Dust that has not been attracted to the substrate can be recycled without problems and reused. By the use of a bakeable material with the grain distribution according to the invention, the adhesion strength in the substrate can rise surprisingly with respect to the bakeable materials that do not have that grain distribution characteristic of the invention. The investigated glazes is shown in Table 1 of Example 1; the adhesion resistance in the substrate as a function of temperature is shown in Table 2 of that example. It was determined that the adhesion resistance of a glaze powder electrostatically placed on a substrate is allowed to increase markedly, if the substrate is preheated. This was very surprising because the thermal treatment increases the electrical resistance of the ceramic materials in several powers of ten (in the case of cooked in biscuit increases of 1010 allo16). According to a preferred embodiment, the substrate to be covered correspondingly was heated before the electrostatic coating from 60 to 250 ° C, preferably from 80 to 200 ° and especially from 100 to 180 °. As already indicated, the coating powder of the invention contains a physical or chemical adhesion agent. The process for generating a ceramic layer on a substrate capable of baking also comprises an activation step within the aforementioned temperature field. This thermal activation is effective in adherent agents, which melt below the applied temperature, in addition to adherent agents which, at the applied temperature, exert their adherent activity as a consequence of a chemical reaction. A thermal activation may take place when the preheated substrate is heated to the corresponding activation temperature. According to a preferred embodiment of the process used, a subsequent thermal treatment is further carried out within the mentioned temperature range - 60 to 250 °, preferably 80 to 200 °, especially 100 to 180 ° C -. A subsequent thermal process is especially suitable for the purpose pursued if high mechanical strength is expected from the coating. This is necessary if the coating before baking should be decorated by sieve pressure or other coating techniques. While the coating powder contains an adhesion agent activated by moisture or ammonia, activation is allowed to occur, because the coated substrate is brought into a chamber containing moisture, such as a water mist, or water vapor, or ammonia. According to a preferred embodiment, the coating powder contains a moisture-activatable adhesion agent and an organic hydrophobic agent in the form of a powder, especially a polyolefin. The hydrophobic agent, which during the manufacture of the coating powder preferably for the purpose pursued simultaneously with the moisture activated adherent is added dry to the baked-on material, serves to increase the specific electrical resistance of the coating powder, the amount It is used of the hydrophobic agent, in which it can be a thermally activatable hydrophobic adhesive, it is between 0.5 and 10% by weight, preferably 0.5 to 6% by weight, in reference to 1 material capable of being baked. In the use of this coating powder, as long as no adhesion strength is required before activation, the preheating of the substrate can be dispensed with, the activation can take place outside the wetting by means of finely sprayed water or also by means of of water vapor, while the temperature of the water vapor is above the activation temperature of a thermally activatable hydrophobic medium present, the hydrophobic agent may act as an additional adherent In the case of the presence of a radiation-resistant adherent in the coating powder the coated substrate will be subjected to an effective radiation source for activation. In connection with the activation and optionally decoration of the unbaked coating, the burning is carried out under such conditions, as the material capable < the content of the coating is determined, and an even melting or at least sintering of the coating is allowed. The coating powder according to the invention leads to the electrostatic coating of the substrate capable of baking to extraordinary and unexpected advantages. The powder and coating of the invention is essentially better fluidizable than the usual coating powders. The fluent factor is generally above 1.3 m, often in 1.6. When obtaining a good fluidisation capacity, the use of a partial or total fluidization aid can be dispensed with. Due to the good fl uidization of the coating powder, no technical problem arises during spraying. Another advantage of the powder sprayed according to the invention is that there is no cooking of the powder in the impact plate or in the central electrode of the spray gun. Due to the lack of such cooking, a ceramic layer surface is reached without failures, which was unavoidable until the present by the uncontrolled placing of the baked or baked goods and with this precipitation of the same. By the use of a coating powder with a grain distribution according to the invention, the degree of efficiency of the electrostatic separation with respect to coating powders that do not have a grain composition according to the invention can be approximately doubled. material capable of baking. By using a powder according to the invention, the frequent problem that arises can be solved, that a too insignificant layer thickness of the coating is dissolved (frequently below 100 microns). By the invention they are achieved without thick coating problems between 100 and 700 microns. With this the procedure can also be used for glazing, where the thicknesses of the layers are necessarily at least 250 and partially up to 400 microns. The coating according to the invention of a substrate capable of being baked with coating powder according to the invention leads to a high strength of the coating without baking. Such coated substrates are then allowed to decorate before burning. An essential advantage of the use of the powders of the present invention in the manufacture of ceramic layers by means of the electrostatic application of the coating powder remains in the obtaining of a surface of high qualitative value of the coating after burning. the surface is smooth or only slightly wavy, whereas on the contrary when using coating powders not in accordance with the invention, a strong undulation is mostly produced, and an unpleasant effect of hammer blows occurs. Additionally, the ceramic layers produced according to the invention are free of glazing holes and other glaze faults. As already indicated, the coating powders according to the invention are generally indicated by a low content of adherent and because they are allowed to be manufactured more simply than the coating powders known in the past. The invention will be explained in more detail by means of the following examples: EXAMPLE 1 The influence of the preheating was investigated every (30 minutes) of the object to be coated on the adhesion resistance of an electrostatically applied glaze powder (without other additional or adherent material). The adhesion strength was determined using chemically identical glazes under different grain spectra, see Table 1. Table 1 ** GE1 and GE2 have a grain distribution according to the invention, GH and GH3 no distribution according to the invention. * determined by Granulometer HR850 - B CILAS according to DIN 66141 Table 2 also shows the influence of the preheating temperature as well as the grain spectrum of the glaze powder on the mechanical strength of the adhesion. By means of a high mechanical strength of the adhesion of the coated object is handled better before burning. The mechanical strength of adhesion of Table 2 was determined as follows: the electrostatically coated articles were placed on their head (coated side down) and fixed in a retention provided for this purpose. From a defined height (H = 70cm) a hard rubber sphere (m = 0.005 kg) centered on 1 substrate was dropped. An electrostatically charged powder adhered after the drop test to the ceramic article. The powder fixed to the substrate after the drop test was measured in comparison to the amount of powder previously existing. The adhesion strength of the glaze powder was obtained as a percentage.

Table 2 EXAMPLE 2 The coating powders were manufactured by dry mixing the glaze powder, Gh, Gel in reference to Ge2 with 2, 4, 6, 8 and 10% by weight with reference to the glaze, of adhesion. Polyethylene wax (PE) in the powder form applied had a softening temperature of 135 ° C, a value of 1.8 microns, a d50 value of 10.5 microns and a d90 value of 16.9 microns. The two-component adhesive used based on an epoxy resin based on biphenol A and an anhydride hardener (EPP / H) had a value of 1.5 microns, d50 value of 9.9 microns and a value of 90 of 17 microns, the temperature of mixing was approximately room temperature. A fluidization aid was not used. Some properties of the substances important for the electrostatic coating of Ge2, Gel and Gh as well as the powders they contained are shown in Table 3 Table 3 Determination of the fluidisation capacity The fluidisation capacity is important for the judgment about the powder processability in pneumatic impulse installations, as well as the preparation of a powder-air mixture in the reserve container and the pneumatic transport through a hose of impulsion towards the spraying apparatus. The fluidization measuring device consists of a glass cylinder with a floor plate with fine pores. The amount of air flow is increased slowly. In this way the volume of the applied powder increases, until the maximum expansion (amount of gas limit) is achieved. The well fluidized powder shows a smooth surface with no fine fractions, the fluidizing factor F is expressed by the ratio of the maximum height of the hmax layer to the resting discharge ho. The well-fluidizable coating powder has a fluidization factor greater than 1.3 to 1.6. The poorly fluidizable powders are characterized by the formation of vertical channels in the application of dust, which due to the high current speeds that prevail also lead to a rough surface and the separation of fine dust. Determination of the specific electrical resistance Measurement method and device according to DIN IEC 9 • - cylindrical measuring cell with protective annular electrode - electrode distance 5 mm; high ohmic resistance measuring device type Teralog 6202 (STRATRON); - selected measuring voltage Um = 1 V up to a specified resistance = 106 ohm. m, 10 V up to 107ohm. m, Um = 1000 V in a powder with a specific resistance higher than 109ohm. m. The measuring voltage was always switched on 1 minute after filling the dust test in the measuring cell (to guarantee constant and reproducible application layers in the intermediate space of the electrodes) the reading of the measured value was carried out 1 minute after the connection of the measurement voltage From the data given in Table 3 it can be seen that the fluidization and the specific electrical resistance are increased favorably for the coating by the grain spectrum of the invention. EXAMPLE 3 AND COMPARATIVE EXAMPLE 1 Electrostatic glazes were investigated, wherein the coating powder, containing a commercially available glaze (Gh) was used in comparison with that which contained glazes with the grain spectrum according to the invention (Gel) and Ge2). The adherent was a polyethylene wax with the substance data given in Example 2. Spraying conditions: Biscuit articles were sprayed (surface 225 cm 2). The articles were preheated before the electrostatic coating 30 minutes at 180 ° C. Electrostatic spraying was carried out with the use of a high voltage gun (Gema Volstatic signature, CH- St Gallen) Supercorona principle, voltage 70kV, current intensity 60 microamps , impulse air pressure 1 bar, fluidization air pressure 1.2 bar; quantity of air dosed 0.5 m3 / h; amount of spray air 4 m3 / h. After the electrostatic spraying in order to determine the adhesion capacity, 10 minutes were set at 180 °. The burn was performed 60 minutes at 1170 ° C. The results are shown in table 4. The process according to the invention is indicated by the following advantages: Improved fluidization, no accumulation of lumps in the delivery hose, distribution without problem of the desired amount of powder, no cooking in the Impact plate, uniform glazing surface only slightly undulating, no glazing hole, optimal result with a lower amount of adherent than the state of the art.

EXAMPLE 4 AND COMPARATIVE EXAMPLE 2 Analogous to Example 3 and Comparative Example 1 coating powders were manufactured and tested. Instead of the polyethylene wax, a heat hardenable resin (EP / H) itself was used as an adherent, a powder of 84% by weight of epoxy resin based on biphenol A (Araldit GT 6450, Ciba Geigy) and 16% on weight of anhydride hardener (H? T 3380, Ciba Geigy). This adherent formulation had a gel time at 180 ° C of 80 seconds. The powder adherent had a value of 1.5 microns, a d50 value of 9.9 microns and a d90 value of 17.0 microns. The adherent and the glaze were mixed dry for preparation of the coating powder at room temperature. The glaze Gh (comparison example)) and the Gel (example of the invention) were used. The substrate, biscuit article, was preheated before coating 30 minutes at 180 ° and after coating for 10 minutes at 180 ° it was tempered. Burning or baking was 60 minutes at 1170 ° C. By means of the grain-glaze spectrum according to the invention, the degree of efficiency of the powder application (= to the amount of powder applied compared to the amount of powder sprayed) could be surprisingly increased, thereby the economy improved. See Table 5 Table 5 The ab to 6 specifies the specific resistance of the coating powders with the glaze Gh or Gel and the adherent EP / H depending on the amount of adherent. As the amount of adherent increases, the thickness of the applicable layer becomes smaller due to the retro-spraying effect.

Table 6 The results with respect to coating and the properties of the glaze are presented in Table 7. Table 7 B_4 VB 2 Mechanical strength (a): 95 (a): 95% after tempering (b) - (d) : 100 (b) - (d): 100 Fac.de fluidización (c) bien, F = 1.6 (c) bad, F = not measurable Cookings in plate (a) - (d): none (a) and (d): impact and strong electrode central cooking Glazed sup (a) - (c): smooth, (a) and (b): continuous, no many cracks crack, none and stitching seams, no (c) and (d): hole many holes and craters for strong re-ionization Quantity applied (a) - (c): 18-20 (a) - (c): 18-20 (g / plate) (d): 8-10 (d): 8 B: means example of the invention. VB2: means example of comparison.

Claims (15)

1. CLAIMS 1. Electrostatically applicable coating powder consisting of a powder material capable of baking and an organic adhesive in the form of a powder activable by physical or chemical means, characterized in that the material capable of baking in the form of powder of the series of glazes , engobes and enamels and an organic adherent in the form of powder activable by a heat treatment, treatment with water vapor, ammonia or an acid or radiation crosslinker, and the material capable of baking in the form of powder presents a grain distribution with a d90 value less than 25 microns, a d50 value in the range of 5 to 25 microns and a d10 value equal to or greater than 2 microns.
2. 2. Coating powder according to claim 1, characterized in that the value d9Q of material capable of baking is less than 32 microns and the value d50 is between 10 and 25 microns.
3. 3. Coating powder according to claim 1 or 2, characterized in that the adherent in powder form has a grain distribution with a value d90 less than 35 microns and a value d10 greater than 1 miera.
4. 4. Coating powder according to one of claims 1 to 3, characterized in that it basically consists of a material capable of baking and 2 to 6% by weight of adherent as well as 0 to 0.3% by weight of fluidizing agent, each one in reference to the material capable of baking.
5. 5. Coating powder according to one of claims 1 to 4, characterized in that the material capable of baking has a glaze-forming composition or engobe.
6. 6. Coating powder according to one of claims 1 to 5, characterized in that it contains as adherent a thermoplastic polymer having a softening point between 80 and 150 ° C of the series of polyolefins or polyacryl or plimetacril esters.
7. 7. - Coating powder according to one of claims 1 to 5, characterized in that, as adherent, it contains a two-component, heat-curable polymer system made from a series of epoxy resins with an anhydride or a hardener containing at least two carboxyl groups and the acid polyester with a hardener containing at least two glycid groups.
8. 8. Process for the preparation of an electrostatically applicable coating powder according to one of claims 1 to 7, which comprises a dry mixing of a material capable of being baked in the form of powder of the series of glazes, engobes or enamels with an adherent activable by heat treatment, by treatment with water vapor, ammonia or an acid or radiation crosslinker characterized in that, as a material capable of baking, one whose grain distribution has a value d90 less than 35 microns, a value is used dBO of 5 to 25 microns and a value d10 equal to or greater than 2 microns, and the temperature and atmosphere do not induce any activation of the adherent.
9. 9. - Process for the preparation of a ceramic coating on a substrate capable of burning, covering the electrostatic application of a coating powder, activation of an organic adherent contained in the coating powder in order to increase the mechanical strength of adhesion on the substrate and baking of the coated substrate under burning conditions determined by the baking material containing the coating powder, characterized in that one coating powder is used according to claims 1-7.
10. 10. Process according to claim 9, characterized in that, as a substrate capable of baking, a non-metallic ceramic substrate is used that is not capable of being burned, which only partially does so.
11. 11. Method according to claim 9 or 10, characterized in that the substrate to be coated is heated before the electrostatic coating at a temperature in the range of 60 to 250 ° C.
12. 12. - Procedure according to one of claims 9 to 11, characterized in that, the coated substrate at the end of the electrostatic coating is treated with a coating powder containing an adherent at a temperature of 60 to 250 ° C, in order to activate the adherent.
13. 13. Method according to one of claims 9 to 12, characterized in that, after activation of the adherent but before burning in the coated substrate, under usual pressure techniques, especially with the sieve pressure technique, it is applied a decoration and this is baked together with the coating.
14. 14. With a substrate coated with a coating capable of baking, obtained within the framework of the process according to one of the claims 9 to 13, where the adherent is activated, but the coating has not yet been baked.
15. 15. Use of a coating powder according to one of claims 1 to 10 for glazing, engobe or decoration of a ceramic substrate.