MXPA99010766A - Coating powder for the electrostatic coating of glass and ceramic or metallic substrates, the production and use thereof - Google Patents

Abstract

The invention relates to coating powders for the electrostatic coating of glass and ceramic or metallic substrates. According to the invention, the required high-specific electric volume resistance to electrostatically coat glass and ceramic or metallic substrates is achieved in the presence of a carboxylic by acid salt in a volume of 0.01 to 1 wt%. The coating powders preferably contain alkali salts, alkaline-earth salts or earth metal salts of carboxylic acids with 2 to 6 C-atoms. The resistance can be increased by 3 to 5 powers of ten. The inventive coating powders are used for glazing, slip painting and decorating glass and ceramic substrates as well as for enamelling metallic substrates.

Description

COATING POWDER FOR ELECTROSTATIC COATING GLASS AS WELL AS CERAMIC AND METALLIC SUBSTRATES, ITS PRODUCTION AND UTILIZATION DESCRIPTION The invention relates to a coating powder for electrostatically coating glass as well as ceramic and metal substrates; The main component of the coating powder is a powdery material, which forms a glass layer when cooked. A further object of the invention relates to a process for the production of the coating powder. Another object of the invention is the use of coating powder for glazing, plastering and decorating glass and ceramic substrates, as well as for enameling metal substrates. For the production of ceramic coatings, such as glaze, enamel and vitreous enamel on susceptible substrates, meaning glass and ceramic and metallic substrates, aqueous slips are mainly used. After the application of the slip, the substrate coated with it is fired, melting or sintering the material capable of cooking contained in the slip REF .: 31930 to a vitreous layer, also often called a ceramic layer. Due to the disadvantages linked to the use of aqueous slips, such as drainage problems and a greater demand for energy, electrostatic powder coating is gaining relevance. It is known that coating powders that form a vitreous layer when baked, are sprayed electrostatically on glass as well as ceramic and metal surfaces. Coating powders that can be applied electrostatically must have a sufficiently high specific electrical resistance, usually from 109 to 1014 ohm * m. To achieve the necessary strength, the coating powders are coated with insulating substances, for example, silanols and organopolysiloxanes, isocyanates and carbodiimides. According to the European Patent EP-A 0 382 003, by means of a treatment of ceramic powders with polymethylhydrosiloxanes, in addition to the necessary strength, the adhesion capacity of the powder on the ceramic surface can be simultaneously improved. It is disadvantageous that the process for the production of ceramic powders treated in this way is very expensive. WO 94/26679 instructs on a improvement of the adhesive strength and clamping of an electrostatically applied glass powder: the coating powder, in addition to a glass powder, contains adhesives that cook without leaving residues, such as polyolefins or dextrins, which are activated during firing thermally, or, by means of humidity, to fix the particles of the layer with each other and with the substrate. Preferred coating powders contain glass frits coated with polysiloxane, in admixture with 10 to 15% by weight of thermoplastic or 5 to 101 by weight of dextrin. Only under specially optimized conditions, which, however, lead to a high cost, it is possible to obtain glazes that can be used in double-walled porcelain fragments. Under conditions already slightly modified, depending on the substrate, glass defects, considerable and disturbances occur partially before reaching the necessary layer thickness. WO 97/08115 teaches on an improvement of the aforementioned problems: when using a glazing or coating composition with a certain granule distribution, namely a value ds;. from 5 to 25 μm, a d9o value less than 35 μm and a diA value equal to or greater than 2 μm, in a coating powder additionally containing an adhesive, is possible to simplify the production process without having to lose quality in the glaze. In WO 97/08115 it is also shown that in the presence of an organopolysiloxane, the electrical resistance -specific of the coating powder is increased in a desired manner with an increasing amount of adhesive. Above a certain amount of adhesive, however, retro-threading effects occur, so that the necessary layer thickness can no longer be applied, and thus, the cooked layer is too thin and, therefore, has defects. A problem of the electrostatic coating of glass and ceramic substrates is also that the coating powders, depending on their composition and their granule spectrum, have a very different specific resistance of passage. In addition, the coated substrates have a very diverse surface conductivity and are additionally dependent on the climatic conditions in the coating chamber. A perfect electrostatic coating presupposes that the specific flow resistance of the coating powder is adjusted to the substrate to be coated and to the climatic conditions. Whereas, up to now, a specific resistance of the coating powder as high as possible has always been demanded, meanwhile it has been demonstrated that too high resistances, although in the case of reduced layer thicknesses, such as the usual thicknesses of 10 to 80 μm for glass decorations, are suitable, in the case of higher layer thicknesses, such as 100 to 400 μm , as required for tile glazes, if you have too high a resistance, retro-threading effects occur. Therefore, the object of the present invention is to provide a new way of simply adjusting the specific step resistance of a coating powder, so that it can be applied electrostatically without problems and that produces perfect baked coatings. It has been observed that the salts of the carboxylic acids can be used as effective auxiliaries to increase the specific step resistance of the coating powders. From the time of adding very small amounts of a carboxylic acid salt, for example, 0.05% by weight of magnesium acetate, it is possible to raise the specific step resistance of a glass powder by up to 2 to 3 powers of ten. Thus, the object of the invention is a coating powder for electrostatically coating glass as well as ceramic and metal substrates, which includes a pulverulent material that when baking forms a glass bale, characterized in that it contains a salt of a carboxylic acid in an amount of 0.01 to 1% by weight. The carboxylic acid salts are based on an aliphatic or aromatic carboxylic acid with one or more carboxyl groups and, optionally, additionally other functional groups, such as hydroxyl groups. Preferred are aliphatic carboxylic acids having 1 to 6 carbon atoms and 1 to 2 carboxyl groups, and optionally, 1 or 2 additional functional groups. Particularly suitable are the lower aliphatic monocarboxylic acids, especially acetic acid and propionic acid. The cation assortment of the carboxylic acid salt includes metals of the main and secondary groups of the Periodic Table. Preferably, it is an alkali metal, alkaline earth metal or earth metal cation, particularly preferably Li +, NaX K +, Mg: 'X Ca + and Al3X Since a very small amount of carboxylic acid salt is increased. the resistance of the coating powder. Although an amount of less than 0.01% by weight already has an effect, in practice the amount used will be around, and especially over, 0. 01% by weight Conveniently, the amount used is in the range of 0.01 to 1% by weight.An utilized amount of more than 1% by weight, although possible, no longer increases the specific electrical resistance. they employ 0.05 to 0.5% by weight of carboxylic acid salt, especially an alkali metal, alkaline earth or earth metal acetate or propionate.For an optimum resistance increase, the carboxylic acid salt must be homogeneously distributed in the powder. Such a distribution can be easily achieved by intensive dry mixing, especially by using an intensive mixer Alternatively, the powdery material forming the glass layer can be contacted with an aqueous solution or suspension of a salt of carboxylic acid, for example, in the framework of a grinding process, after which the wet product is dried and, if necessary, further separated. It was completely surprising that the salts of carboxylic acids are able to significantly increase, namely by several powers of ten, generally 3 to 5 powers of ten, the specific electrical resistance of the coating powders. In this way resistors are obtained, such as those they require for the electrostatic application. Whereas, for example, untreated glass powders usually have a specific electrical resistance in the range of 106 to 107 ohm 'm, by adding 0.1 to 0.3% by weight of an acetate, resistances are achieved in the range of 1010 to 1012 ohm 'm. Thus, a treatment with organopolysiloxanes is no longer necessary to increase the strength, but it is not excluded if a resistance is required above that which can be obtained with the carboxylic acid salt. By the presence of a carboxylic acid salt in the coating powder, the electrical charges of the powder particles are better maintained, so that, in the electrostatic spraying, they are better oriented in the electric field towards the workpiece. work connected to earth. In this way, not only the powder can be applied electrostatically, but also, the adhesion of the powder to the substrate is increased, so that the substrates coated in this way can be baked directly. To obtain coatings with an even greater adhesion and clamping force, it may be advisable to make additional use of the technical instruction of WO 94/26679 and / or WO 97/08115. According to WO 94/26679, the powder of The coating according to the invention can additionally contain, in effective amounts, a powdery adhesive, which can be activated physically, in particular thermally, namely by melting and hardening, or chemical. In the case of thermally activatable adhesives, these are in particular thermoplastics, such as polyethylene, whose binding action is achieved by melting and subsequent hardening. For the purpose of melting, the substrate to be coated, before and / or after being coated, is heated to a temperature above the melting point of the thermoplastic. Among the chemically activatable adhesives, there are two-component resin systems, such as "epoxy-hardener resin combinations, as well as isocyanate-hardener combinations." The amount of adhesive is generally in the range of 1 to 20% by weight, based on the coating powder, but in particular from 1 to 10% by weight The adhesive is introduced into the coating powder, in a known manner, before, during or after dry addition of the carboxylic acid salt. in this case, it is convenient to mix intensively below the activation temperature of the adhesive, in order to guarantee a homogeneous distribution, and with it, a coating free of defects before and after after cooking According to WO 97/08115, the adhesion strength can also be improved by using a pulverulent material which, when baked, forms a glass layer, with a specific granule spectrum, in particular, from 5 to 25 μm, less than 35 μm and gave equal / greater than 2 μm. Before or after adding the carboxylic acid salt to the pulverulent material, this, if required, should be ground and / or sieved. As indicated above at the point of the adhesives, it is also here, in many cases, advantageous to heat the substrate to be coated before the electrostatic coating. The temperature and duration depend on the substrate. Preheating is particularly convenient in the case of cooked or second-firing fragments, as well as glass. The material that forms a layer of glass when baked is essentially glaze powder, plaster, vitreous enamel and decoration, as well as enamel powder. It can be essentially uniform material, such as glass frits or mixtures thereof, or mixtures of substances: glazing powders of essentially one or more glass frits and, optionally, additional substances of the group consisting of clay, ollastonite, zirconium silicate and white or colored pigments; vitreous enamel powders of essentially one or several glass frits or the so-called fusions and pigments; In addition to glass frits, they contain substances from the group consisting of fine-particle ceramic raw materials, ground minerals, glass and porcelain flour, opacifiers and pigments. In the case of mixture of substances, it can be advantageous to use them in the form of so-called spray granules, in order to avoid demixing and achieve a uniform melting behavior. Preferably, the materials forming a glass layer are already in the coating powder at 30 to 100% by weight, in particular more than 50% by weight, in the fritted state. The electrostatically coated substrates are baked directly or after further decoration of the surface, for example, including a fixation of the layer, using a. polymeric fixative, and a decoration, using a decal or by serigraphy. The temperature and duration of the cooking depend on the substrate and the type of coating powder. It can be, for example, a typical decoration cooking or a quick cooking at one time, simultaneously cooking the uncooked substrate and the applied coating. In addition to the additives mentioned above in the electrostatically applicable coating powders, these may also contain customary fluidizing agents, such as pyrogenic oxides, such as Si02, A1203, Ti02, Zr02, in effective amounts, such as 0.05 to 3% by weight, especially 0.1 to 0.5% by weight, thereby improving the electrostatic spraying of the powder. The coating powder according to the invention can be used for glazing, plastering, enamelling and decorating cooked and uncooked ceramic substrates and glass, as well as for enameling metals. The use includes the electrostatic coating under known conditions and the firing of the substrate. A special use of the coating powders according to the invention refers to the production of glass decorations, especially corrosion imitations or high-gloss color decorations, which are provided with additional decoration, such as an inscription. In the prior art, this type of decorations, for example, imitations of corrosion with additional decoration, were produced using polymer systems, which required two firings. As has also been found, alternatively to the use of the aforementioned systems according to the invention, i.e. the coating powders with a carboxylic acid salt, this type of decorations can also be obtained by employing other electrostatically applicable coating powders, the specific strength of which is high enough, for example, those containing polysiloxanes. For the production of these decorations, an electrostatically applicable coating powder, especially one with a value dso less than 10 μm and a specific electrical resistance of more than 1010 ohm ', preferably 1012 to 1014 ohm "m, is sprayed electrostatically on a pre-heated glass (100 to 250 ° C), in such an amount that the layer thickness after baking is 10 to 50 μm.The coating can be fixed in a known manner by spraying with a polymer dispersion. applying a decoration in a known manner, for example, by means of silk-screen printing or the decal technique, followed by the cooking, simultaneously cooking the basic decoration, for example, the imitation of corrosion, and the additional decoration. To achieve a corrosion effect, the coating powder contains an opacifier, such as Si02, Zr04, Ti0, Zr02 and spinel, or multi-phase glass systems. If necessary, the coating powder contains a carboxylic acid salt and / or an organopolysiloxane and / or an organic adhesive and / or a fluidizing agent, which it can act as an opacifier, in respectively effective amounts. Preferably, the d50 value of the powder is located around / below 5 μm and the value d90 below the value of the cooked layer. So far it is not known to explain the action of increased resistance of the salts of carbsxílico acid. Due to the ionic character of the carboxylic acid salts, a reduction in the strength rather than the increase in accordance with the invention was to be expected. Other salts, such as sulfates, phosphates and chlorides, do not show the action according to the invention (see comparative tests). It was also surprising that the salts of short-chain aliphatic carboxylic acids, to which, compared to salts of long-chain carboxylic acids, are not attributed per se hydrophobicizing action, proved to be especially effective. Another advantage of the salts of short-chain aliphatic carboxylic acids consists, moreover, in their easy obtainment, their good solubility in water, and with it, their good distribution capacity in the coating powder. The coating powders according to the invention can be easily produced and the specific electric pitch resistance can be adjusted reliably, depending on the amount of use. Other advantages are the good adhesion to the substrate, as well as the possibility of renouncing the use of organopolysilanes and / or adhesives or of reducing the amounts of use of these substances. The carboxylic acid salts in the coating powder do not cause any defect in the baked coating. A further advantage of the coating powders according to the invention, which also contain an adhesive with a high specific electrical resistance, consists in that when electrically coating, no demixing due to electrostatic issues occurs which, until now, could produce that the coated substrate contained a greater portion of adhesive than the powder that did not come into contact with the substrate at the time of spraying. It is obvious that demixing is avoided because the specific electrical resistance of the material forming glass is similar to that of the adhesive. Thus, when using the powder according to the invention, a new adjustment of the adhesive content is not necessary. Another advantage of the coating powders according to the invention is that by occupying the active centers of the glass-forming material with a carboxylic acid salt, the flowability is improved. In this way, you can eliminate the addition of other fluidization agents, and the opacities of a coating baked on glass are avoided. The invention is illustrated in more detail with the aid of the following examples and comparative examples. General specifications: a) Production of coating powders: They are ground for 25 minutes in a mill Bloch-Rosetti 500 g of glass frit with x% carboxylic acid salt in 200 ml of water. They are dried at 110 ° C, then milled 5 minutes dry and, if indicated, sieved. b) Production of a coating powder containing an adhesive: A frit without salt of carboxylic acid, respectively, a frit with carboxylic acid (obtained according to part a)), it is mixed intensively with the adhesive, homogenizing the respective raw mixture using an intensive mixing device. Polyethylene wax having a softening temperature of 135 ° C and a granule spectrum of di; of 1.8 μm, ds, -, of 10.5 μm and dg0 of 16.9 μm. c) Determination of the specific electric pass resistance of the coating powder: Method and measuring device according to DIN IEC 93: - cylindrical measuring cell with protective collar electrode; - distance between electrodes 5 mm; high strength meter type Teralog 6202 (STATRON); - selected measuring voltage UM = 1 V up to a specific resistance of 106 ohm m, 10 V up to 107 ohm 'm; UM = 1000 V in powders with a specific resistance > 109 ohm 'm. The measuring voltage was always applied 1 minute after filling the powder sample in the measuring cell (to guarantee constant and reproducible stacking dust densities in the inter-electrode space). The measurement values are read 1 minute after applying the measurement voltage. d) Electrostatic coating: Double-walled porcelain fragments (15 x 15 cm) were coated, without preheating. High voltage gun with flat jet nozzle; 70 kV with 60 μA. e) Determination of the adhesion strength of the coating powder applied electrostatically: The electrostatically coated fragments They are placed on the head (coated side down) and fixed in a holder provided for this purpose. From a defined height (h = 70 cm) a hard rubber sphere (m = 0.005 kg) is dropped to the center of the substrate, an electrostatically charged powder remains attached to the ceramic fragments after the drop test. The substrate after said test is checked in relation to the total application of original powder.The adhesion force of the glass powder is obtained in percent.

Example 1 For the production of a glaze coating powder for tiles a glass frit was used "TGF 2486 A from Cerdee AG, which contains as main components Si02, ZnO, B203, A1203, MgO, CaO, KXD and ZGO (sample 1.1) In a manner analogous to general specification a), but in a proportion 150 times greater, a grinding duration of 75 minutes in a ball drum mill and roller drying, the frit was mixed with 0.21 in weight of magnesium acetate (sample 1.2), sample 1.1 corresponds to the frit treated in the same way in the absence of magnesium acetate Sample 1.2 was sieved (sample 1.3).

Table 1 shows the specific electrical resistance as well as the granule spectrum of the samples.

Table 1 Sample No. Electrical resistance Specific granule spectrum (ohm 'm) d10 (μm) d50 (μ) d90 (μm) 1. 1 5 X 108 2.6 24.0 66.5 Tests show that even 0.2% by weight of magnesium acetate increases the resistance in 4 powers of ten. By screening the powder, an additional increase is possible.

EXAMPLE 2 Coating powders 1.1 and 1.2 of Example 1, as well as coating powders 1.1 H and 1.2 H containing polyethylene wax as adhesive, were applied according to the general specification d) electrostatically onto second-grade porcelain fragments, and then the coated fragments were baked. The production of samples 1.1 H and 1.2 H was carried out according to the general specification b), the amount of adhesive being respectively 6% by weight, based on the coating powder. The adhesion strength was determined in% (average of 5 measurements) according to the 'general e' specification as well as the application amount for 60 seconds (average of 5 measurements), and also, the specific electrical step resistance according to the general specification c), as well as the surface quality of the baked glaze for 60 minutes at 1170 ° C. The results are shown in Table 2.

Table 2 Sample No. Electrical resistance Ad-Qty Strength of Specific Quality (ohm 'm) hesity (%) api. (g / min) glaze 1.1 5 x 108 5.2 2.4 n.d. * lT2 - 5 x 101 81.5 15.5 good ** 1. 1H 7 x 10 ?: 82.9 15.0 good ** 1. 2H 5 x 10: 84.0 15.1 good ** * n.d. = not determined, as it was not cooked due to insufficient powder application. ** good = smooth and soft surface; no stings, no orange peel effect; without defects (bubbles, points or glazes).

The tests show that an addition of 0.2% by weight of magnesium acetate to a glaze powder increases the adhesion strength in the same way as an addition of 6% by weight of polyethylene wax.

Example 3 A glaze frit (TGF 2486 A from Cerdee AG) was mixed according to the general specification a) with different amounts of magnesium acetate. Sample 3.0 corresponds to the frit treated according to a) in the absence of magnesium acetate; 3.0 granule spectrum: gave 1-2 μm, dso 8.6 μm, d90 34.9 μm. As can be seen in Table 3, the specific electric resistance resistance of the coating powder is considerably increased with an increasing amount of magnesium acetate.

Table 3 Sample No. Magnesium acetate Step resistance (% by weight) elec. spec. (ohm 'm) 3. 0 0 < 107 3.1 0.05 1 x 10- 3. 3 0.25 1 x 10 12 Example 4 Analogously to Example 3, according to the general specification a) se. they applied various salts on the same glass frit respectively (290/498 from the company Cerdee AG), namely respectively in an amount of 0.2% by weight. Table 4 shows the specific electric resistance resistance of the different powders, the powder No. 4.0 not containing any salt and "powders Nos. 4.9 to 4.11 in accordance with the invention." Powders Nos. 4.9 to 4.11 contain other salts that salts of carboxylic acid (not according to the invention), which does not produce any increase in the resistance or only a very small increase.

Table 4 Sample No. Salt Resistance of elec. spec. (ohm 'm) 4. 0 < 10 '4.1 Mg stearate 5 x 1011 4.2 Ca acetate 2 x 1011 4.3 Mg acetate 2 x 1011 4.4 Zn acetate 6 x 1011- 4.5 Na acetate 1 x 10 11 4.6 K acetate 5 x 10 10 4. 7 Al Acetate 4 x 1010 4.8 Acetatohydrate (III) of La 3 x 1011 4.9 MgSO4. 7 H20 < 10 '4.10 MgCl2 < 107 4.11 Mg Phosphate 4 x 108 It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Having described the invention as above, property is claimed as contained in the following:

Claims (9)

1. A coating powder free of organopolysiloxane, for electrostatically coating glass as well as ceramic and metal substrates, which includes a powdery material which, when baked, forms a glass layer, characterized in that it contains a salt of a carboxylic acid in an amount of 0.01 to 1. % in weigh.

2. A coating powder according to claim 1, characterized in that it contains an alkaline, alkaline earth or earth metal salt of a monocarboxylic acid having 2 to 6 carbon atoms.

3. A coating powder according to claim 1 or 2 / characterized in that it contains an alkali metal, alkaline earth or earth metal acetate in an amount of 0.05 to 0.5% by weight.

4. A coating powder according to any of claims 1 to 3, characterized in that it was obtained by a method that includes the contact of an aqueous solution of the carboxylic acid salt with the powdery material forming a glass layer at the cook, and drying.

5. A coating powder according to any of claims 1 to 4, characterized in that it additionally contains a chemically or thermally activatable powdery adhesive in an amount of 1 to 10% by weight.

6. A coating powder according to any of claims 1 to 5, characterized in that the powdery material forming a glass layer has a granule distribution with a dgo value of less than 35 μm, a value dso in the range of 5 to 25 μm and a value gave greater than 1 μm.

7. A coating powder according to any of claims 1 to 6, characterized in that the powdery material forming a glass layer has a composition that forms a glaze or a coating and is present in the form of 30 to 100 * by weight of a glass frit.

8. A process for the production of an organopolysiloxane-free coating powder according to any of claims 1 to 7, characterized in that a pulverulent material which when baked forms a glass layer is contacted wet or dry in a intensive mixing device, with 0.01 to 1 * by weight of carboxylic acid salt and, if the coating powder contains an adhesive, a powdery adhesive is mixed in the dry state to the mixture containing the carboxylic acid salt. .

9. The use of the organopolysiloxane-free coating powder according to any of claims 1 to 7 or obtained according to claim 8, for glazing, plastering and decorating glass and metal substrates, including electrostatic spraying of the coating powder on the substrate and the cooking of the coated substrate.