Patent Publication Number: US-2022212246-A1

Title: Coating composition, method for coating a casting mold, use of the coating composition for coating a casting mold, and casting mold

Description:
TECHNICAL FIELD 
     The present invention is directed to a coating composition capable of suppressing veining in iron and heavy metal casting. The present invention is furthermore directed to a method for coating a casting mold, the use of the coating composition for coating a casting mold, and the corresponding casting mold. 
     STATE OF THE ART 
     Most products in the iron, steel and non-ferrous metal industries undergo casting processes for their initial forming. During these processes, molten materials, ferrous metals or non-ferrous metals, are turned into shaped objects with certain workpiece properties. For forming the castings, casting molds, which can be quite intricate, must first be prepared to receive the molten metal. The casting molds are classified as lost molds, which are destroyed after each casting, or permanent molds, which can be used to produce a large number of castings. The lost molds usually consist of a refractory, granular mold base material, which is solidified by means of a curable binder. 
     Molds are negatives; they comprise the cavity into which the material is cast to obtain the casting to be formed. The inner contours of the future casting are formed by cores. In the manufacture of the mold, the cavity is formed in the mold base material using a model of the casting to be produced. Inner contours are shaped by cores which are formed in a separate core box. 
     For the preparation of the molds, both organic and inorganic binders can be used, the curing of which can be carried out by cold or hot processes. A cold process is a process in which the curing is essentially carried out at room temperature without heating the mold base material mixture. The curing is usually carried out by a chemical reaction, which can be triggered, for example, by having a gaseous catalyst pass through the mold base material mixture to be cured, or by adding a liquid catalyst to the mold base material mixture. In hot processes, the mold base material mixture is heated to a sufficiently high temperature after forming, for example to remove the solvent contained in the binder, or to initiate a chemical reaction by which the binder is cured due to crosslinking. 
     The preparation of the molds can proceed such that the mold base material is first mixed with the binder so that the grains of the mold base material are coated with a thin film of the binder. The mold base material mixture obtained from the mold base material and the binder can then be introduced into a corresponding form and optionally compacted in order to obtain a sufficiently stable mold. Subsequently, the mold is cured, for example by heating it or by adding a catalyst that causes a curing reaction. When the mold has reached at least a certain initial strength, it can be removed from the form. 
     As was already mentioned, casting molds for the production of metal bodies often consist of so-called cores and molds. The cores and molds have to meet different requirements. Molds provide a relatively large surface area to release gases that are formed due to the effect of the hot metal during casting. Cores usually only provide a very small area through which the gases can be discharged. Thus, if too much gas is formed, there is a risk that gas from the core enters the liquid metal and leads to the formation of casting defects. The inner cavities are therefore often provided as sand cores which have been solidified by cold box binders, i.e. a binder based on polyurethanes, while the outer contour of the casting is formed by more cost-effective molds, such as a green sand mold, a mold bound by a furan resin or a phenolic resin, or by a steel mold. 
     For larger molds, organic polymers are mostly used as binders for the refractory, granular mold base material. Washed and classified quartz sand is often used as a refractory, granular mold base material, but other mold base materials such as zircon sands, chromite sands, chamotte, olivine sands, feldspathic sands, and andalusite sands can also be used. The mold base material mixture obtained from the mold base material and the binder is preferably present in a free-flowing form. 
     Currently, organic binders such as polyurethane, furan resin or epoxy acrylate binders, are often used for the production of casting molds, in which the curing of the binder is carried out by the addition of a catalyst. 
     The choice of the suitable binder depends on the shape and size of the casting to be produced, the production conditions, and the material used for the casting. For instance, polyurethane binders are often used in the production of small castings, which are produced in large numbers, as they allow fast cycle times and thus also series production. 
     The use of two-component polyurethane binders for the production of cores has gained significant importance in the foundry industry. One component contains a polyol with at least two OH groups per molecule, and the other one a polyisocyanate with at least two NCO groups per molecule. In one type of core production, the so-called cold box process, the two components are first mixed simultaneously or consecutively with a suitable mold base material, e.g. quartz sand. This mixture, which is referred to as mold base material mixture, is then transferred to the reservoir of a core shooter, then transported into a forming tool by means of compressed air and is finally cured therein by passing through a gaseous low-boiling tertiary amine as a catalyst, thus yielding a solid, self-supporting core (U.S. Pat. No. 3,409,579). As further components, the mold base material may also contain additives, as for example described in EP 0 795 366 A1. 
     Quartz, zircon, or chrome ore sand, olivine, chamotte, and bauxite can for example be used as refractory materials. Furthermore, synthetically produced mold base materials can be used as well, such as hollow aluminum silicate balls (so-called microspheres), glass beads, glass granules or the spherical ceramic mold base materials known as “cerabeads” or “carboaccucast”. Mixtures of the said mold base materials are also possible. 
     In order to improve the surface finish of the castings, the cores and molds can be coated with a coating referred to as finish before use. 
     Conventional coating compositions comprise at least one refractory substance as a purposive portion. The purpose of this refractory material is mainly to influence the surface of the casting to be coated in order to comply with the above-mentioned requirements with regard to the avoidance of sand defects which lead to defects and impurity in the casting. 
     For example, in foundry technology, the refractory material can close the sand pores of a core or molded part to prevent a penetration of the casting metal. 
     Examples of refractory substances are pyrophyllite, zirconium silicate, andalusite, chamotte, iron oxide, kyanite, bauxite, olivine, alumina, quartz, talc, calcined kaolins (metakaolin) and/or graphite alone or mixtures thereof. 
     For example, WO 2004/083321 A1 and EP 2 364 795 A1 describe the use of a fine-grained material, such as metakaolin, which can impregnate the sand pores, and a platelet-shaped coarse pyrophyllite, which covers the surface of the sand and thus provides good protection against both veining and penetration. 
     Conventionally, a coating composition comprises a carrier liquid. The solid components of the coating composition can form a suspension with the carrier liquid, whereby the solid components become processible and are applied to the body to be coated by a suitable method, such as e.g. immersion. 
     In general, when coating porous substances, the application behavior of the coating material is not only determined by the rheology of the coating material, but also by the absorption behavior of the porous body as well as the retention capacity of the carrier liquid by the coating substance. With regard to the absorption behavior of porous bodies, it should be noted that substrates with hydraulic binders such as clay, cement and water glass usually absorb the carrier liquid to a particularly high degree. 
     In the case of coating materials based on an aqueous system, the use of suspending agents, such as natural slimes or cellulose derivatives, is known. Although they result in a high water retention capacity of the coating substance material, the rheology of the system is negatively impacted in that the coating materials exhibit unfavorable, lower intrinsically viscous properties, and that they will flow off more viscously. This can lead to undesirable application features such as drop formation and sagging, as well as uneven layer thicknesses. In particular in immersion coating, the optimization of the flow behavior of the coating material for achieving contour formation, a uniform layer thickness and a low drop formation is important. 
     Basically, any coating material should be kept in a homogeneous state during processing in order to avoid precipitation of the solids in the suspension. In combination with the required application behavior, the rheological character as well as the degree of thixotropy of the complex suspensions should meet the desired requirements. 
     For example, swellable activated layered silicates are used in numerous technical fields as thickeners for aqueous systems. By using shear forces, the layered silicates are dispersed in the system in a finely distributed form, wherein the individual layered platelets largely or completely detach from each other and form a colloidal dispersion or suspension in the system, which leads to a gel structure. 
     A method for the preparation of casting molds and cores from resin-bound molding sand comprises, for example, the manufacture of a basic mold or a base core from the molding sand and the application of a coating containing refractory inorganic components at least on those surfaces of the basic mold/base core that come into contact with the cast metal. On the one hand, the purpose of the mold coatings is to influence the surface of the molded part, to improve the appearance of the casting, to influence the casting metallurgically and/or to prevent casting defects. Furthermore, these coats or coatings serve to chemically insulate the mold from the liquid metal during casting, thereby preventing any adhesion and allowing the subsequent separation of mold and casting. In addition, the coating provides a thermal barrier between the mold and the casting. The heat transfer can be used specifically to influence the cooling of the casting. In the production of cold box cores, problems with expansion errors of the sand occur time and again in practice, which lead to stresses in the core surface due to the so-called quartz inversion from α-quartz to β-quartz, and a length expansion of about 1% at approx. 580° C. This results in so-called veining (cracked cores with penetrated metal) or scabbing (loosened sand layer), which lead to a raised casting surface and, in other areas, to sand inclusions. 
     Another issue is the increasing demand of automotive casting manufacturers for ever thinner-walled castings from 3 to 5 mm with a high dimensional accuracy of 0.2 to 0.3 mm. Since commercially available coatings are typically applied with a dry layer thickness of 0.2 to 0.4 mm in automotive casting to avoid casting defects, the high layer thickness constitutes a limitation for improving dimensional tolerance. So-called fat edge formation or sagging during the application of the coating also lead to problems with the dimensional accuracy and impermeability of thin-walled castings. 
     WO 2011/110798 describes a foundry coating composition comprising a liquid carrier, a binder, and a particulate refractory filler, wherein the particulate refractory filler comprises a first relatively coarse fraction having a particle size of d&gt;38 μm and a second relatively fine fraction having a particle size of d&lt;38 μm, wherein no more than 10% of the total particulate refractory filler has a particle size of 38 μm&lt;d&lt;53 μm and 0 to 50% of the second relatively fine fraction consists of calcined kaolin. 
     DE 10 2009 032 668 A1 describes a ready-to-use wash for producing mold coatings on lost molds or on cores for iron and steel casting, wherein the wash contains a proportion by weight of 0.001% or more, and 1% or less of inorganic hollow bodies, characterized in that the inorganic hollow bodies consist partly or completely of crystalline material. 
     WO 2006/063696 A1 describes a sizing composition for casting molds, comprising a solvent component and a solid component, characterized in that the solid component comprises a mixture of metakaolinite and pyrophyllite as a main component. 
     EP 2 176 018 A1 describes a method for casting vermicular and spheroidal graphite cast iron in a SO 2  epoxy bound sand, wherein carbonates prevent the sulfur from causing a graphite degeneration at the edge of the metal. Inter alia, an earth alkali carbonate is mentioned. 
     DE 10 2016 211 930 A1 describes the use of carbonates and/or phosphates in combination with refractory fillers, preferably on acid-bound sand molds to avoid the so-called white film. 
     WO 2011/075220 A1 describes the use of carbonates as an additive against veining formation in the sand mixture; while there is no indication of the particle size distribution, usually, grain sizes of more than 50 pm are used in order to reduce the loss of strength. 
     It was an object of the present invention to provide a coating composition which allows the production of thin-walled castings with a high dimensional tolerance and which ensures a good protection against veining and penetration. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1 : Schematic side view of a step core 
         FIG. 2 : Schematic top view of a step core 
         FIG. 3 : Schematic side view of a dome core 
     
    
    
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to the following points.
         1. A coating composition, comprising:
           a solids component which comprises a first solid that is able to cleave CO 2  in a temperature range from about 150 to about 1000° C., and that has a D50 value of at most about 10 μm.   
           2. The coating composition according to point 1, wherein the first solid has a D99 value of at most about 30 μm and a D90 value of at most about 20 μm.   3. The coating composition according to point 1 or 2, wherein the first solid is selected from carbonates of the elements of the 2 nd , 7 th , 8 th , 9 th , 10 th , 11 th  and 12 th  groups of the periodic table of elements.   4. The coating composition according to point 3, wherein the first solid is selected from calcium carbonate, magnesium carbonate, dolomite or iron carbonate, or mixtures thereof.   5. The coating composition according to point 1 or 2, wherein the first solid is selected from starch, in particular rice or oat starch.   6. The coating composition according to any of points 1 to 5, wherein the coating composition further comprises a carrier liquid, and wherein the carrier liquid preferably comprises water.   7. The coating composition according to point 6, wherein the solubility product of the first component in the carrier liquid, in terms of pK L , at 25° C., is at least about 4.   8. A method for coating a casting mold, wherein the method comprises the steps of:
           (a) providing a casting mold, optionally equipped with one or more casting cores, comprising a surface which defines a casting cavity,   (b) providing a coating composition according to one of points 1 to 7; and   (c) applying the coating composition to at least one part of the surface which defines the casting cavity.   
           9. Method according to point 8, wherein the coating composition further comprises a carrier liquid and the coating composition is dried after step (c).   10. Method according to point 8 or 9, wherein the coating penetrates at least 2 mm into the coated surface, and the layer thickness of the coating after drying the coating is at most about 100 μm.   11. Use of a coating composition according to any of points 1 to 7 for coating a casting mold, wherein the casting mold, optionally equipped with one or more casting cores, comprises a surface which defines a casting cavity, and the coating composition is applied to at least one part of the surface which defines the casting cavity.   12. Use according to point 11, wherein the coating penetrates at least 2 mm into the coated surface, and the layer thickness of the coating after drying the coating is at most about 100 μm.   13. A coated casting mold, optionally equipped with one or more casting cores, obtainable by the method according to one of points 8 to 10.       

     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to a coating composition, comprising: 
     a solids component which comprises a first solid that is able to cleave CO 2  in a temperature range from about 150 to about 1000° C., and that has a D50 value of at most about 10 μm. 
     Solids Component 
     Solids components are all components that are present as solids after the drying of the ready-to-use coating composition. In this context, the drying temperature can, for example, be 120° C. 
     First Solid 
     The particle size distribution of the individual components of the coating composition, and in particular of the first solid, can be determined on the basis of the passage proportions D99, D90, D50 and D10. They are measures for the particle size distribution. Accordingly, the passage proportions D99, D90, D50 and D10 denote the proportions in 99%, 90%, 50% or 10% of the particles which pass through a sieve with a mesh width corresponding to the designated grain size fraction. For example, at a D50 value of 10 pm, 50% of the particles have a size of less than 10 pm. The grain size and the passage proportions D99, D90, D50 and D10 can be determined by laser diffraction granulometry according to ISO13320. The passage proportions are given on a volume basis. In the case of non-spherical particles, a hypothetical spherical grain size is calculated and used as a basis. 
     In a preferred embodiment, the D10 value of the first solid is no more than about 3 μm, more preferably from about 0.1 μm to about 3 μm, more preferably from about 0.1 μm to about 2 μm, especially preferred from about 0.1 μm to about 1 μm. 
     The D50 value of the first solid is no more than about 10 μm, preferably from about 0.5 μm to about 10 μm, more preferably from about 0.5 μm to about 7 μm, especially preferred from about 0.5 μm to about 6 μm. 
     In a preferred embodiment, the D90 value of the first solid is no more than about 20 μm, more preferably from about 5 μm to about 20 μm, more preferably from about 5 μm to about 15 μm, especially preferred from about 5 μm to about 10 μm. 
     In a preferred embodiment, the D99 value of the first solid is no more than about 30 μm, more preferably from about 5 μm to about 30 μm, more preferably from about 5 μm to about 20 μm, especially preferred from about 5 μm to about 15 μm. 
     In a preferred embodiment, one or more of the above-mentioned D10, D50, D99, and D90 values are fulfilled simultaneously. In a particularly preferred embodiment, the above-mentioned D50, D99, D90 values, and optionally the D10 value, are fulfilled simultaneously. 
     The desired grain size can be adjusted by crushing the first solid until the desired grain size is obtained. Alternatively, or additionally, the desired grain size can be provided by screening. In the case of synthetic first solids, it is also possible to adjust the process parameters during production in such a way that the desired grain size is achieved. The first solid cleaves CO 2  in the temperature range from about 150 to about 1000° C., preferably about 300 to about 1000° C. This can be determined by heating the first solid in a tube furnace under a nitrogen atmosphere, and then, by absorption in a wash bottle with milk of lime and subsequent filtration, drying and gravimetric determination, measuring whether CO 2  is emitted at at least one temperature in the mentioned range. The CO 2  partial pressure should be less than 0.01 bar to avoid errors due to an influence on the chemical equilibrium. Thus, the cleaving of CO 2  according to the present invention only encompasses the decomposition of the first solid, wherein CO 2  is produced, and not a combustion of the same. 
     The chemical composition of the first solid is not particularly limited, provided that the first solid is able to cleave CO 2  as required. 
     In one embodiment, the first solid is selected from carbonates of the elements of the 2 nd , 7 th  8 th , 9 th , 10 th , 11 th , and 12 th  groups of the periodic table. Preferably, the first solid is selected from carbonates of Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu, and Zn, preferably Mg, Ca, Mn, Fe, Cu, and Zn, especially preferred Mg, Ca, Mn, and Fe, even more preferred Mg, Ca, and Fe. 
     Mixed carbonates of the above elements, such as calcium-magnesium carbonate, can also be used as carbonates. Mixtures of the carbonates mentioned above are also possible. 
     Furthermore, it goes without saying that the carbonates can be used not only in their pure form but also in the form of natural minerals. In the case of natural minerals, the content of carbonates of the elements of 2 nd , 7 th , 8 th , 9 th , 10 th , 11 th , and 12 th  groups of the periodic table is preferably more than about 50 wt.-%, more preferably more than about 70 wt.-% and especially preferred more than 90 wt.-%. 
     Calcium Carbonate 
     Calcium carbonate CaCO 3  can be used in its pure form or in the form of a natural mineral such as calcite (calcspar, double spar), aragonite and vaterite. Known natural minerals containing calcite, aragonite or vaterite are chalk, limestone and marble. 
     Calcium carbonate can be produced synthetically according to the methods known in the art. Among other methods, precipitation with carbon dioxide, the lime-soda process, and the Solvay process, in which calcium carbonate is a by-product of ammonia production, are known. 
     Calcium carbonate occurs in several anhydrous forms as well as in two hydrate modifications and other amorphous forms. Starting at a temperature of about 600° C., they decompose into calcium oxide and carbon dioxide: 
       CaCO 3 →CaO+CO 2  
 
     According to the present invention, the type of calcium carbonate is not particularly limited. Any known calcium carbonate can be used. The calcium carbonate can preferably be selected from calcite and aragonite, more preferably, the calcium carbonate is calcite. 
     Magnesium Carbonate 
     Magnesium carbonate MgCO 3  can be used in its pure form or in the form of a natural mineral such as magnesite (bitter spar), barringtonite, nesquehonite and lansfordite. 
     Magnesium carbonate can be produced synthetically according to the methods known in the art. Among other methods, a precipitation with carbon dioxide is known. 
     According to the present invention, the type of magnesium carbonate is not particularly limited. Any known magnesium carbonate can be used. The magnesium carbonate can preferably be selected from magnesite, magnesia alba, and upsalite; magnesite is especially preferred. 
     Calcium-Magnesium Carbonate 
     Calcium carbonates and magnesium carbonate can be used as mixed carbonates. A known calcium-magnesium carbonate is dolomite CaMg(CO 3 ) 2 , which can also be found as dolomite spar, diamond spar and pearl spar. 
     According to the present invention, the type of calcium-magnesium carbonate is not particularly limited; preferably, dolomite can be used. 
     Iron Carbonate 
     Iron carbonate FeCO 3  can be used in its pure form or in the form of a natural mineral such as siderite, iron lime, iron spar, spade iron stone, chalybeate, and steel stone; preferably siderite is used. Alternatively, iron carbonate can be produced synthetically according to the methods known in the art. 
     Manganese Carbonate 
     Manganese carbonate MnCO 3  can be used in its pure form or in the form of a natural mineral such as rhodochrosite. Alternatively, manganese carbonate can be produced synthetically according to the methods known in the art. 
     Zinc Carbonate 
     Zinc carbonate ZnCO 3  can be used in its pure form or in the form of a natural mineral such as smithsonite. Alternatively, zinc carbonate can be produced synthetically according to the methods known in the art. 
     Copper Carbonate 
     Copper carbonate CuCO 3  can be used in the form of basic compounds or in the form of a natural mineral such as malachite and azurite. Alternatively, copper carbonate can be produced synthetically according to the methods known in the art. 
     The following table provides an overview of the properties of preferred carbonate compounds. 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                   
                 CO 2  Cleaving 
                 Solubility Product 
               
               
                   
                 Compound 
                 [° C.] 
                 pK L   
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Calcite 
                 898 
                 8.48 
               
               
                   
                 Magnesite 
                 650 
                 8.03 
               
               
                   
                 Dolomite 
                 800 
                 17.09 
               
               
                   
                 Siderite 
                 200 
                 10.89 
               
               
                   
                 Zinc carbonate 
                 180 
                 10.00 
               
               
                   
                 Copper carbonate 
                 280 
                 9.63 
               
               
                   
                 Malachite* 
                 350-384 
                 33.16 
               
               
                   
                 Rhodochrosite 
                 350 
                 10.39 
               
               
                   
                   
               
               
                   
                 *as CuCO 3  Cu(OH) 2   
               
            
           
         
       
     
     In a preferred embodiment, the first solid is selected from calcium carbonate, magnesium carbonate, dolomite, or iron carbonate, or mixtures thereof. 
     In a second embodiment, the first solid is selected from starch, in particular rice or oat starch. Preferably, the starch should be insoluble in the carrier liquid as described below. 
     In its ready-to-use form, the coating composition preferably comprises a carrier liquid. It is preferred that the first solid be insoluble in the carrier liquid. In the context of the present invention, the term “insoluble in the carrier liquid” means that the first solid has a solubility product, expressed as pK L , of at least about 4, preferably at least about 6, at 25° C. The solubility product can be measured by mixing a certain amount of substance to be measured (e.g. 100 g) into a certain amount of solvent (e.g. 1 liter) at a temperature of 25° C., filtering the liquid, and determining the dissolved content either by evaporation of the solvent or by chemical analysis of the dissolved substance. 
     The amount of the first solid in the solids component is not particularly limited and can be from about 65 to about 99 wt.-%, preferably about 80 to about 99 wt.-%, more preferably about 90 to about 99 wt.-%. The upper limit of the amount of the first solid in the solids component can be 90 wt.-%, preferably 95 wt.-%. Natural minerals often contain mixtures of various chemical compounds. If a natural mineral is used, the amount of the first solid specified above refers to the amount of chemical compound(s) which can cleave CO 2  in the temperature range of about 150 to about 1000° C. In the case of dolomite rock consisting, for example, of 90 wt.-% calcium-magnesium carbonate and 10 wt.-% of other components that cannot cleave CO 2 , only the 90 wt.-% of calcium-magnesion carbonate would be taken into account for the calculation of the above-mentioned amount. The 10 wt.-% of other components which cannot cleave CO 2  would thus be part of the solids component, provided that they remain as a solid residue during the defined drying. 
     Second Solid 
     In addition to the first solid, the solids component may also contain a second solid. The second solid is understood to be any solid that is unable to cleave CO 2  in the temperature range of about 150 to about 1000° C. 
     Examples of the second solid include graphite, mica, non-swellable aluminium silicates and swellable layered silicates. 
     Graphite can be present in an amount from 0 to about 20 wt.-%, preferably from 0 to about 10 wt.-%, based on the solids component. 
     One or more types of mica can be used. Examples include muscuvite or phlogopite mica. Mica can be present in an amount from 0 to about 10 wt. %, preferably from 0 to about 5 wt.-%, more preferably from 0 to about 2 wt.-%, based on the solids component. 
     The second solid can contain one or more non-swellable aluminium silicates. Examples include pyrophyllite, metakaolinite, mullite, kyanite or sillimanite. Pyrophyllite and metakaolinite are preferred. Non-swellable aluminum silicates can be present in an amount from 0 to about 10 wt.-%, preferably from 0 to about 5 wt.-%, based on the solids component. 
     In a preferred embodiment, the D10 value of the second solid (except the swellable layered silicates) is no more than about 15 μm, preferably from about 0.1 μm to about 10 μm, more preferably from about 0.1 μm to about 8 μm, especially preferred about 0.1 μm to about 7 μm. 
     In a preferred embodiment, the D50 value of the second solid (except the swellable layered silicates) is no more than about 50 μm, more preferably from about 0.5 μm to about 30 μm, more preferably from about 0.5 μm to about 25 μm, even more preferably from about 0.5 μm to about 20 μm. 
     In a preferred embodiment, the D90 value of the second solid (except the swellable layered silicates) is no more than about 100 μm, more preferably from about 5 μm to about 90 μm, more preferably from about 5 μm to about 80 μm, especially preferred from about 5 μm to about 75 μm. 
     In a preferred embodiment, the D99 value of the second solid (except the swellable layered silicates) is no more than about 250 μm, more preferably from about 5 μm to about 200 μm, more preferably from about 5 μm to about 150 μm, especially preferred from about 5 μm to about 100 μm. 
     In a preferred embodiment, one or more of the above-mentioned D50, D99, and D90 values are fulfilled simultaneously. In a particularly preferred embodiment, the above-mentioned D10, D50, D99, and D90 values are fulfilled simultaneously. 
     The term swellability refers to the property of solids dispersed in solvents to incorporate the solvent and thus become more voluminous. This usually greatly increases viscosity. 
     In one embodiment, the second solid comprises one or more swellable layered silicates to reduce the settling behavior of the coating composition and to control the rheology. The swellable layered silicates are not particularly limited. Any swellable layered silicate known to the person skilled in the art which is capable of storing water between its layers can be used. Preferably, the swellable layer silicate can be selected from attapulgite (palygorskite), ball clay, serpentines, kaolins, smectites (such as saponite, montmorillonite, beidellite and nontronite), vermiculite, illite, sepiolite, synthetic lithium-magnesium layered silicate LAPONITE RD and mixtures thereof, especially preferred from attapulgit (palygorskite), serpentine, smectites (such as saponite, beidellite and nontronite), vermiculite, illite, sepiolite, synthetic lithium-magnesium layered silicate LAPONITE RD and mixtures thereof, and in particular it is preferred that the swellable layered silicate be attapulgite. Attapulgite is preferred because of the increase in the flow limit. 
     Furthermore, the grain size of the swellable layered silicates is not particularly limited, any usual grain size can be used. The D10, D50, D90, and D99 values given for the second solid also apply to the swellable layered silicates. 
     In a preferred embodiment, the D10 value of the swellable layered silicates is no more than about 5 μm, preferably from about 0.1 μm to about 5 μm, more preferably from about 0.1 μm to about 4 μm, especially preferred about 0.1 μm to about 3 μm. 
     In a preferred embodiment, the D50 value of the swellable layered silicates is no more than about 30 μm, preferably from about 0.5 μm to about 25 μm, more preferably from about 0.5 μm to about 20 μm, especially preferred from about 0.5 μm to about 15 μm. 
     Preferably, the D90 value of the swellable layered silicates can be no more than about 50 μm, more preferably from about 5 μm to about 40 μm, especially preferred from about 5 μm to about 30 μm, and most preferred from about 5 μm to about 25 μm. 
     In a preferred embodiment, the D99 value of the swellable layered silicates is no more than about 100 μm, preferably from about 5 μm to about 90 μm, more preferably from about 5 μm to about 80 μm, especially preferred from about 5 μm to about 75 μm. 
     In a preferred embodiment, one or more of the above-mentioned D50, D99, and D90 values are fulfilled simultaneously. In a particularly preferred embodiment, the above-mentioned D50, D99, and D90 values are fulfilled simultaneously. 
     The amount of the swellable layered silicates (in particular attapulgite) in the coating composition is not particularly limited; it can preferably be from about 0 to about 5 parts by weight, more preferably from about 0.1 to about 5 parts by weight, especially preferred about 0.1 to about 4 parts by weight, and most preferred from about 0.1 to about 2 parts by weight, based on the solids component. 
     Carrier Liquid 
     The coating composition may contain a carrier liquid to facilitate application. A carrier liquid is any component which evaporates when the ready-to-use coating composition is dried, and which is not present in the dried coating. 
     The coating composition can be provided as a dry powder, as a concentrate which contains a portion of the required amount of carrier liquid and has to be diluted with additional carrier liquid before use, or as a ready-to-use coating composition which already contains the desired amount of carrier liquid. 
     The carrier liquid can be selected by a person skilled in the art according to the intended application. Preferably, the carrier liquid may be selected from water, alcohols such as aliphatic C 1 -C 5  alcohols, or mixtures thereof. In a preferred embodiment, the carrier liquid is water, methanol, ethanol, n-propanol, isopropanol, n-butanol, or mixtures thereof, more preferably water, ethanol, isopropanol or mixtures thereof, and especially preferred water. 
     Coating compositions whose carrier liquid consists mainly of water are usually called water coatings. Coating compositions whose carrier liquid consists mainly of alcohol or alcohol mixtures are called alcohol coatings. In one embodiment of the present invention, the carrier liquid comprises about 0 to about 100 wt.-%, preferably approximately about 30 to about 100 wt.-%, more preferably about 60 to about 100 wt.-%, water, and as a further component, about 100 to about 0 wt.-%, preferably about 70 to about 0 wt.-%, more preferably about 40 to about 0 wt.-%, one or more alcohols as defined above, based on the carrier liquid. According to the present invention, pure water coatings as well as pure alcohol coatings as well as water/alcohol mixtures can be used. In a particularly preferred embodiment, water is the sole carrier liquid. Alternatively, it is also possible to produce coating compositions whose solvent component consists of alcohol or, in the case of so-called hybrid coatings, initially consists only of water. By diluting with an alcohol or an alcohol mixture, these coatings can be used as alcohol coatings. Preferably, ethanol, propanol, isopropanol and mixtures thereof are used. 
     If necessary, other organic solvents can be used. Examples include acetic acid alkyl esters such as acetic acid ethyl ester and acetic acid butyl ester, and ketones such as acetone and methyl ethyl ketone. The amount of the other organic solvents is not particularly limited, preferably, they are present in an amount of from about 0 to about 10 wt.-%, more preferably from about 0 to about 5 wt.-%, and especially preferred from about 0 to about 1 wt.-%, based on the carrier liquid. 
     The amount of the carrier liquid in the coating composition is not particularly limited, preferably, it is present in an amount of 80 wt.-% or less, more preferably 75 wt.-% or less, and especially preferred 70 wt.-% or less. 
     Accordingly, the amount of the solids component in the coating composition is preferably about 20 wt.-% or more, more preferably about 25 wt.-% or more, and especially preferred about 30 wt.-% or more. 
     In the ready-to-use coating composition, carrier liquid is preferably present in an amount of from about 40 to about 85 wt.-%, more preferably from about 45 to about 85 wt.-%, and especially preferred from about 50 to about 85 wt.-%. 
     Accordingly, the amount of the solids component in the ready-to-use coating composition is preferably from about 15 to about 60 wt.-%, more preferably from about 15 to about 55 wt.-%, and especially preferred from about 15 to about 50 wt.-%. 
     Optional Additives 
     In addition to the above components, the coating composition may contain common additives such as binders, wetting agents, defoamers, pigments, dyes, and biocidal active ingredients. 
     Binder 
     The function of a binder is primarily to bind the solids component. Preferably, the binder is characterized by an irreversible bond and thus results in an abrasion-resistant coating on the mold. Abrasion resistance is of great importance for the finished coating, as the coating can be damaged if it lacks abrasion resistance. In particular, the binder should not soften due to humidity. According to the present invention, all binders can be used which are conventionally used in, for example, aqueous and/or water-alcohol systems. As a binder, water-soluble starches or dextrins whose D50 value is more than about 10 μm (preferably at least about 15 μm) and which are soluble in the carrier liquid, peptides, polyvinyl alcohol, polyvinyl acetate copolymers, polyacrylic acid, polystyrene, polyvinyl acetate polyacrylate dispersions and mixtures thereof can for example be used. In a preferred embodiment of the present invention, the binder comprises a dispersion of an alkyd resin, which is soluble both in water and in low (e.g. C 1-4 ) alcohols, such as ethanol, propanol and isopropanol. Examples of alkyd resins include unmodified water-dispersible alkyd resins, based on a natural oil or the fatty acids thereof with polyalcohols, as described for example in U.S. Pat. No. 3,442,835, or isocyanate-modified alkyd resins, as described for example in U.S. Pat. No. 3,639,315—which are preferred—or epoxy urethane-modified alkyd resins according to DE 43 08 188. For example, products from the NECOWEL series from ASK Chemicals GmbH, 40721 Hilden, Germany, can be used. Other preferred binders are polyvinyl alcohols and polyvinyl acetate copolymers, in particular polyvinyl alcohols. 
     The term “binder” refers to the effective binding component which can also be present as a solution or dispersion in diluted form. 
     The binders are preferably used in an amount of about 0.1 to about 5 parts by weight, more preferably about 0.2 to about 2 parts by weight, based on all components of the coating composition. 
     Wetting Agents 
     Anionic and non-anionic surfactants of medium and high polarity (HLB value of 7 and higher) known to the person skilled in the art can preferably be used as wetting agents. Examples of wetting agents that can be used in the present invention include disodium dioctylsulfosuccinate, ethoxylated 2,5,8,11-tetramethyl-6-dodecyn-5,8-diol, ethoxylated 2,4,7,9-tetramethyl-5-decyn-4,7-diol or combinations thereof; more preferably, ethoxylated 2,4,7,9-tetramethyl-5-decyn-4,7-diol can be used. 
     The wetting agents are preferably used in an amount of from about 0.01 to about 1 parts by weight, more preferably from about 0.05 to about 0.3 parts by weight, and even more preferably from about 0.1 to about 0.3 parts by weight, based on all the components of the coating composition. 
     Defoamers 
     Defoamers or antifoaming agents are used to prevent foaming during the preparation of the coating composition according to the present invention and during its application. Foaming during the application of the coating composition can lead to an uneven layer thickness and to holes in the coating. Silicone or mineral oil can for example be used as defoamers. 
     Preferably, the defoamer can be selected from the FINASOL product line, commercially available from Total Deutschland GmbH. 
     In the coating composition according to the present invention, defoamers are used in an amount of from about 0.01 to about 1 parts by weight, more preferably from about 0.05 to about 0.3 parts by weight, and especially preferred from about 0.1 to about 0.2 parts by weight, based on all the components of the coating composition. 
     Pigments and Dyes 
     In the coating composition according to the present invention, common pigments and dyes can optionally be used. They are added if necessary, in order to achieve a visible contrast, e.g. between different layers, or to create a stronger separation between the coating and the casting. Examples of pigments include red and yellow iron oxide. Examples of dyes are commercially available dyes such as the LUCONYL products from BASF SE. 
     The dyes and pigments are usually used in an amount of from about 0.01 to about 10 parts by weight, preferably from about 0.05 to about 5 parts by weight, based on all the components of the coating composition. 
     Biocidal Active Ingredients 
     The coating composition can optionally contain one or more biocidal active ingredients (especially if the carrier liquid includes water) in order to prevent a bacterial infestation and thus avoid a negative influence on the rheology and the bonding strength of the binding agents. The biocidal active ingredients are not particularly limited; preferably they can be selected from the group consisting of formaldehyde, glutaraldehyde, tetramethylol acetylene diurea, 2-methyl-4-isothiazoline-3-one (MIT), 5-chloro-2-methyl-4-isothiazoline-3-one (CIT), 1,2-benzisothiazolin-3-one (BIT), or mixtures thereof, more preferably from 2-methyl-4-isothiazoline-3-one (MIT), 1,2-benzisothiazoline-3-on (BIT), or mixtures thereof. 
     The amount of biocidal active ingredients in the coating composition is not particularly limited and depends on the selected biocidal active ingredient. The amount can, for example, range from about 0.001 to about 1.0 parts by weight, preferably from about 0.005 to about 1.0 parts by weight, more preferably from about 0.007 to about 1.0 parts by weight, even more preferably from about 0.007 to about 0.9 parts by weight, and especially preferred from about 0.007 to about 0.8 parts by weight, based on all the components of the coating composition. 
     In a preferred embodiment, a coating composition according to the invention comprises about 10 to about 60 parts by weight calcium carbonate, and about 0.1 to about 2 parts by weight attapulgit, based on the solids component. Furthermore, the coating composition comprises about 0.2 to about 2 parts by weight binder, about 0.1 to about 0.3 parts by weight wetting agents, about 0.1 to about 0.2 parts by weight defoamers, about 0.005 to about 0.3 parts by weight biocidal active ingredient, as well as carrier liquid, preferably water, to make up the difference to 100 parts by weight. 
     Preparation of the coating composition The coating compositions according to the present invention are prepared using common methods. For example, a coating composition according to the present invention is prepared by taking a large part of the carrier liquid (preferably the entire amount of the carrier liquid) and adding swellable layered silicates (if they are used) using a high shear mixer (e.g. about 400 to about 2000 rpm) (premixture B in the examples). Then, additional solids components are stirred in, for example the first solid as well as pigments and dyes (if they are used) until a homogenous mixture is obtained. The order in which the components are added is of little or no relevance and can easily be determined by the person skilled in the art. Finally, the wetting agents, defoamers, biocidal active ingredients, and binders are stirred in, if they are used. The coating composition is for example prepared at a temperature of preferably about 5 to about 50° C., more preferably about 10 to about 30° C., with a stirrer speed of preferably about 400 to about 2000 rpm, more preferably about 1000 to about 1500 rpm, and with a tooth disc with preferably d/D=about 0.3 to about 0.7, more preferably d/D=about 0.4 to about 0.6 (d is the diameter of the tooth disc of the mixer, D is the diameter of the mixing vessel). 
     The properties of the coating composition are preferably adjusted to an impregnating coating so that the first solid can penetrate into the surface of the casting mold. In particular, it is preferred that the impregnating depth of the coating composition be at least about 2 mm, preferably from about 2 mm to about 20 mm, more preferably from about 2 mm to about 6 mm. The impregnation depth can be measured by cutting. 
     The dry layer thickness of the dried coating resulting from the coating composition described above is the thickness of the layer of the dried coating composition (“coating”), which is determined by drying the coating composition by substantially completely removing the carrier liquid. Preferably, the dry layer thickness of the coating can be no more than about 100 μm, more preferably from about 10 μm to about 100 μm, and still more preferably from about 10 μm to about 50 μm. The dry layer thickness of the coating is determined either preferably by measuring 3-point bending bars before and after finishing (dried) with a micrometer screw or by measuring with a wet film thickness comb gauge. For example, the dry layer thickness can be determined with the comb gauge by scraping away the coating on the end marks of the comb until the substrate appears. The dry layer thickness can then be read from the markings of the teeth. Or, it is also possible to measure the wet layer thickness in the matted state (hereinafter referred to as the matte layer thickness) according to DIN EN ISO 2808, wherein the dry layer thickness amounts to 70 to 80% of the thickness of the matted layer. A “matted” layer is a layer that is no longer flowable, in which the solvent content is reduced so much that the surface no longer has a gloss. 
     Preferably, the impregnating depth of the coating composition is at least about 2 mm and the dry layer thickness is no more than about 100 μm. More preferably, the impregnating depth of the coating composition is from about 2 mm to about 20 mm (more preferably from about 2 mm to about 6 mm) and the dry layer thickness from about 10 μm to about 50 μm. 
     The viscosity of the ready-to-use coating composition can be adjusted from about 10 sec to about 16 sec, preferably from about 10 sec to about 13 sec, determined according to DIN 53211; flow cup 4 mm, Ford Cup. 
     The density of the ready-to-use finishing composition can, for example, be from about 20 °Be to about 50 °Bé, preferably from about 25 °Bé to about 35 °Bé, determined according to the Baume buoyancy method; DIN 12791. 
     Use of the Coating Composition 
     The coating composition according to the present invention can be used for coating a casting mold. One possible method comprises the steps of:
         (a) providing a casting mold comprising a surface which defines a casting cavity,   (b) providing a coating composition according to the present invention;   (c) applying the coating composition to at least one part of the surface which defines the casting cavity; and   (d) drying the coating composition.       

     All types of bodies that are necessary for the production of molds are referred to as casting molds. The casting molds are not particularly limited, all casting molds commonly used in the iron, steel and non-metal industries can be used, for example cores, molds or dies. The casting molds can consist of a refractory, granular mold base material, which is solidified by means of a curable binder. The refractory, granular mold base material is not particularly limited, any common mold base material can be used. Preferably, the refractory, granular mold base material can comprise quartz sand, zircon sand, chromite sand, chamotte, bauxite, olivine sand, feldspathic sand, andalusite sand, aluminum silicate hollow balls (also referred to as “microspheres”), glass beads, glass granules, the ceramic spherical mold base material known under the designation “cerabeads” or “carboaccucast”, or mixtures thereof. In particular, the inventive coating composition is used for casting molds wherein quartz sand or proportions of quartz sand were used as a mold base material. 
     The grain size of the sand should be from about 100 to about 600 μm, preferably from about 100 to about 500 μm and most preferably from about 200 to about 400 μm. 
     The curable binder is not particularly limited. Any curable binder known to the person skilled in the art can be used. Preferably, organic binders such as polyurethane, furan resin or epoxy acrylate binders, inorganic binders, such as water glass, or mixtures thereof can be used; it is especially preferred that the binders are based on PUR cold box, water glass CO 2 , resol methyl formate (MF), resol CO 2 , furan resin, phenolic resin, or water glass ester binders. Polyurethane binders are particularly preferred. The amount of curable binder in the casting mold is not particularly limited; the binder may be present in any common amount. Preferably, the binder is present in an amount of about 0.2 to about 5 parts by weight, further preferably of about 0.3 to about 4 parts by weight, more preferably of about 0.4 to about 3 parts by weight, based on 100 parts by weight of the refractory, granular mold base material. The coating compositions are suitable for all conceivable applications where a casting mold is to be coated with a coating. Examples of casting molds, i.e. cores and molds used in foundry applications, include sand cores which are bonded with PUR cold box, water glass CO 2 , resol MF, resol CO 2 , furan resin, phenolic resin or water glass esters. Other examples of preferred casting molds, which can be coated with the coating compositions according to the present invention, are for example described in “Formstoffe und Formverfahren” [Engl. molding materials and molding methods], Eckart Flemming and Werner Tilch, Wiley-VCH, 1993, ISBN 3-527-30920-9. 
     The surfaces of the casting mold, optionally equipped with one or more casting cores, define a casting cavity into which the liquid metal is introduced. The surfaces of a casting mold can be the surfaces of a core or of a hollow mold. According to the present invention, the casting molds and cores can be completely or partially coated. Preferably, the surfaces of the casting mold and cores that come into contact with the casting metal are coated. 
     The coating composition is initially provided in a ready-to-use form. If it is present as a dry powder or as a concentrate, carrier liquid is added to achieve the consistency necessary for application. 
     In one embodiment, for example, the coating composition may be provided in the form of a kit (multi-component pack containing two or more containers for different components). The solids component and the carrier liquid may be present side by side in separate containers. All components of the solids component may be present as a powdered solids mixture in one container. The components of the solids component can alternatively be provided as several separate components. Where applicable, other components to be used, such as binders, wetting agents, defoamers, pigments, dyes, and biocidal active ingredients, may be present in this kit together with the above-mentioned components or in one or more separate containers. The carrier liquid may either include the optional components to be used if necessary, for example in the same container, or it may be present separately from other optional components in a separate container. In order to prepare a ready-to-use coating composition, the appropriate quantities of the components are mixed together. 
     Typically, the coating composition can be present as a water coating. Alternatively, a ready-to-use alcohol coating can be provided from this water coating by adding an alcohol. 
     The application and drying of at least one layer of a coating composition to at least one part of the surface defining the casting cavity is not particularly limited. All conventional application methods described in the art can be used for this purpose. Examples of preferred application methods are immersion coating, flow coating, spray coating, and painting. Conventional application methods are for example discussed in “Formstoffe und Formverfahren” [Engl. molding materials and molding methods], Eckart Flemming and Werner Tilch, Wiley-VCH, 1993, ISBN 3-527-30920-9. 
     During an immersion coating process, the casting mold is immersed, for example, in a container with a ready-to-use coating composition for about 1 second to about 2 minutes. The time it takes for the excess coating composition to flow off after immersion depends on the flow behavior of the coating composition used. After a sufficient flow off period, the coated casting mold is subjected to drying. 
     As a drying process, all conventional drying methods known in the art can be used, such as air drying, drying in dehumidified air, drying with microwave or infrared radiation, drying in a convection furnace, and the like. In a preferred embodiment of the present invention, the coated casting mold is dried at about 100 to about 250° C., more preferably at about 120 to about 180° C., in a convection furnace. When alcohol coatings are used, the coating composition is preferably dried by burning off the alcohol or alcohol mixture. In that case, the coated casting mold is additionally heated by the combustion heat. In a further preferred embodiment, the coated casting mold is air-dried without any further treatment. 
     The coating composition according to the present invention can be used as a base layer. The dry layer thickness of the base layer is, for example, at most about 100 μm, more preferably from about 10 μm to about 80 μm, and still more preferably from about 10 μm to 50 μm. 
     Casting molds coated according to the present invention are preferably used for the production of metal bodies. They are particularly suitable for the manufacture of engine and engine components, brake discs, turbochargers, exhaust manifolds, and general machine components. 
     In a casting process, a casting mold coated according to the invention is provided, liquid metal is filled into the mold, and after the metal has hardened, the casting mold is removed. 
     The invention will be explained in the following examples; however, they shall not restrict the invention in any way. 
     Examples 
     The following components were used: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 Attapulgite 
                 ATTAGEL 40 from BASF, 67063 Ludwigshafen, Germany 
               
               
                   
                 D99 appr. 50 μm, D90 appr. 20 μm, D50 appr. 9 μm, D10 appr. 
               
               
                   
                 2 μm 
               
               
                 Biocide 
                 ACTICIDE F (N) (70 wt.-% tetramethylol acetylene diurea) from 
               
               
                   
                 Thor GmbH, 67346 Speyer, Germany 
               
               
                 Defoamer 
                 FINASOL, Total Deutschland GmbH, 10117 Berlin, Germany 
               
               
                 Wetting agent 
                 SURFYNOL 440, Evonik Corporation, Allentown, PA 18195, 
               
               
                   
                 USA 
               
               
                 Binder 
                 POLYVIOL (25 wt.-% polyvinyl alcohol), Wacker AG, 81737 
               
               
                   
                 Munich, Germany 
               
               
                 Amorphous graphite 
                 Georg H. Luh, 65396 Walluf, Deutschland 
               
               
                   
                 D 99 appr. 90 μm, D90 appr. 60 μm, D50 appr. 18 μm, D10 
               
               
                   
                 appr. 7 μm 
               
               
                 Glossy powdered graphite 
                 Fa. Georg H. Luh GmbH, 65396 Walluf, Deutschland; C content 
               
               
                   
                 at least 85 wt.-%, ash at most 15 wt.-% 
               
               
                   
                 Particle size distribution determined with laser diffraction 
               
               
                   
                 granulometry: 
               
               
                   
                 D10 15 to 30 μm, D50 80 to 120 μm, D90 190 to 250 μm 
               
               
                 Clay 
                 Kärlicher Blauton yellow-burning T7001 KTS, Kärlicher Ton und 
               
               
                   
                 Schamottewerke Mannheim GmbH &amp; Co. KG, 56218 Mülheim- 
               
               
                   
                 Kärlich, Germany; chemical analysis, in wt.-% annealed: SiO 2   
               
               
                   
                 53.66, Al 2 O 3  37, TiO 2  3.75, Fe 2 O 3  2.98, CaO 0.73, MgO 0.63, 
               
               
                   
                 K 2 O 0.75, Na 2 O 0.07; sedimentation analysis by means of 
               
               
                   
                 sedigraph measurement in mass-%: &lt;2.0 μm 95.7, mineral 
               
               
                   
                 analysis in mass-%: kaolinite 70 to 75, illite 7.0, montmorillonite 
               
               
                   
                 15 to 20, quartz 2.0, Fe—Ti minerals 3.0 
               
               
                 Mica 
                 Fa. Georg H. Luh GmbH, 65396 Walluf, Deutschland; chemical 
               
               
                   
                 analysis in wt.-%: SiO 2  43 to 46, Al 2 O 3  33 to 37, Fe 2 O 3  2 to 5, 
               
               
                   
                 K 2 O 9 to 11; screen analysis, passage through 60 μm screen: 
               
               
                   
                 25 to 64 wt.-% 
               
               
                 Calcined kaolin 
                 Satintone W, BASF Corporation, Charlotte, NC, USA 
               
               
                   
                 D99 appr. 65.5 μm, D90 8.53 μm, D50 2.4 μm, D10 appr. 0.4 
               
               
                   
                 μm 
               
               
                   
               
            
           
         
       
     
     The following components were examined as first solid: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 Calcium carbonate 
                 ETIQUETTE VIOLETTE from Omya GmbH, 50679 Cologne, 
               
               
                   
                 Germany 
               
               
                   
                 D99 appr. 15 μm, D90 appr. 8 μm, D50 appr. 3 μm, D10 appr. 1 
               
               
                   
                 μm 
               
               
                 Dolomite 
                 HELADOL10, Helawit GmbH, D-23829 Wittenborn, Germany 
               
               
                   
                 D99 appr. 15 μm, D90 appr. 8 μm, D50 appr. 3 μm, D10 appr. 1 
               
               
                   
                 μm 
               
               
                 Iron carbonate 
                 Cofermin Chemicals GmbH &amp; Co. KG, D-45130 Essen, 
               
               
                   
                 Germany 
               
               
                   
                 as obtained 100 mesh = 150 μm 
               
               
                   
                 micronized by grinding to D99 about 5 μm, D90 about 1.4 μm, 
               
               
                   
                 D50 about 0.5 μm, D10 about 0.2 μm 
               
               
                 Rice starch 
                 Hermann Kröner GmbH, D-49479 Ibbenbüren, Germany 
               
               
                   
                 D50 = about 5 μm 
               
               
                   
               
            
           
         
       
     
     A coating composition for coating a casting mold was obtained as follows: 
     The first solid was combined with premixtures A and B, and the coating composition was adjusted with additional water to a flow viscosity of about 13 sec, determined with an immersion flow cup according to DIN 53211 with a 4 mm nozzle. 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Wt.-% 
                 Premixture A 
                 Premixture B 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Water 
                 99.4 
                 85.1 
               
               
                   
                 Wetting agent 
                 0.6 
               
               
                   
                 Biocide 
                   
                 1.0 
               
               
                   
                 Binder 
                   
                 9.0 
               
               
                   
                 Attapulgite 
                   
                 4.0 
               
               
                   
                 Defoamer 
                   
                 0.9 
               
               
                   
                   
               
            
           
         
       
     
     All values are given in wt.-%. 
     Dome core molded bodies with the dimensions diameter 50 mm and height 50 mm with an upper radius of 25 mm, and step cores with the dimensions R 1 =63 mm/H 1 =25 mm, R 2 =54 mm/H 2 =25 mm, R 3 =45 mm/H 3 =25 mm, R 4 =37 mm/H 4 =25 mm, R 5 =28 mm/H 5 =25 mm, R 6 =21 mm/H 6 =25 mm, and R 7 =12 mm/H 7 =43 mm (wherein R; is the radius and H i  is the height of the i-th stage) were prepared from Sand H32 from the Quarzwerke Group with 0.8 wt.-% ASKOCURE 388 (available from ASK Chemicals, Hilden, Germany) and 0.8 wt.-% ASKOCURE 666, each based on the sand, using a polyurethane cold box process by amine gassing with dimethylethylamine. The level 0 of the dome core shown in  FIG. 1  is glued in and therefore does not come into contact with the metal. 
     Subsequently, the dome cores or step cores were manually immersed in the stirred coating composition and then dried in the drying furnace at 120° C. 
     The dried cores were freed from the coating by rubbing at the core marks and the mirror surface, so that no compressive stress is generated, and glued into the casting mold of sand H32 hardened by water glass/ester by means of ASKOBOND RAPID A (available from ASK Chemicals, Hilden, Germany). After closing and clamping the mold package by means of screw clamps, the castings were performed at 1420° C. with gray cast iron GJL 200. 
     After cooling and unpacking the castings, the cavity of the core was sandblasted (2 bar pressure) and evaluated. 
     The veining was evaluated as follows: 
     Step core: The table below shows the number of veins on the respective steps 
     Dome core: In the table below, the veining was rated with school grades 1 to 6. 
     Examples According to the Present Invention: Dome Core 
       
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                   
                 Iron 
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 Calcium 
                   
                 carbonate 
                   
                   
                 DIN 4 mm 
                 Matte layer 
                 Impregnation 
               
               
                   
                 Premixture A 
                 carbonate 
                 Dolomite 
                 micronized 
                 Rice starch 
                 Premixture B 
                 flow time 
                 thickness 
                 depth 
                 Evaluation 
               
               
                 Example 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (s) 
                 (μm) 
                 (mm) 
                 veining* 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 34 
                 46 
                   
                   
                   
                 20 
                 13.0 
                 50 
                 4 
                 1 
               
               
                 2 
                 34 
                   
                 46 
                   
                   
                 20 
                 12.5 
                 75 
                 4 
                 1 
               
               
                 3 
                 34 
                   
                   
                 46 
                   
                 20 
                 11.5 
                 25 
                 4 
                 2 
               
               
                 4 
                 34 
                   
                   
                   
                 46 
                 20 
                 14.0 
                 100 
                 4 
                 2-3 
               
               
                   
               
               
                 *Grades: 1 = very good; 2 = good; 3 = satisfactory; 4 = adequate; 5 = poor; 6 = insufficient 
               
            
           
         
       
     
     Examples According to the Present Invention: Step Core 
       
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
               
             
            
               
                   
                   
                 Calcium 
                   
                 Graphite 
                   
                 DIN 4 mm 
               
               
                   
                 Premixture A 
                 carbonate 
                 Dolomite 
                 amorphous 
                 Premixture B 
                 flow time 
               
               
                 Examples 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (parts by wt.) 
                 (s) 
               
               
                   
               
               
                 5 
                 39 
                 39 
                   
                   
                 22 
                 12.4 
               
               
                 6 
                 39 
                 35 
                   
                 4 
                 22 
                 12.2 
               
               
                 7 
                 39 
                   
                 39 
                   
                 22 
                 11.5 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Matte layer 
                 Impregnation 
                 Number of 
                 Number of 
                 Number of 
                 Number of 
               
               
                   
                   
                 thickness 
                 depth 
                 veins 
                 veins 
                 veins 
                 veins 
               
               
                   
                 Examples 
                 (μm) 
                 (mm) 
                 Step 2 
                 Step 3 
                 Step 4 
                 Step 5 
               
               
                   
                   
               
               
                   
                 5 
                 75 
                 4 
                 0 
                 0 
                 0 
                 2 
               
               
                   
                 6 
                 50 
                 5 
                 0 
                 0 
                 0 
                 3 
               
               
                   
                 7 
                 50 
                 4 
                 0 
                 0 
                 0 
                 3 
               
               
                   
                   
               
            
           
         
       
     
     When using the step core, which is glued into a cylindrical sand mold with a cavity having a diameter of 150 mm and a height of 200 mm, so that the “foot” is at the top, the volume ratio of metal to sand core gradually increases from the 1st step (the “foot”, R=63 mm). The casting mold is filled from the bottom. The “number of veins step n” indicates the number of veins that are present on the step core casting at the nth level. 
     Furthermore, as a comparative example, a dome core and a step core were coated as described in the patent application DE 10 2018 004 234.1 according to manufacturer&#39;s specifications. 
     Non-Inventive Example: Dome Core 
       
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                   
                 Glossy 
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                   
                   
                 powdered 
                 Calc. 
                   
                   
                 DIN 4 mm 
                 Matte layer 
                 Impregnation 
               
               
                   
                 Premixture A 
                 Mica 
                 Clay 
                 graphite 
                 kaolin 
                 Premixture B 
                 Water 
                 flow time 
                 thickness 
                 depth 
                 Evaluation 
               
               
                 Example 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (s) 
                 (μm) 
                 (mm) 
                 veining* 
               
               
                   
               
             
            
               
                 8 
                 39 
                 20 
                 9 
                 5 
                 5 
                 22 
                 30 
                 12.8 
                 450 
                 1 
                 4 
               
               
                   
               
            
           
         
       
     
     Non-Inventive Example: Step Core 
       
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
               
             
            
               
                   
                   
                   
                   
                 Glossy 
                   
                   
                   
                   
               
               
                   
                   
                   
                   
                 powdered 
                 Calc. 
                   
                   
                 DIN 4 mm 
               
               
                   
                 Premixture A 
                 Mica 
                 Clay 
                 graphite 
                 kaolin 
                 Premixture B 
                 Water 
                 flow time 
               
               
                 Example 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (wt.-%) 
                 (s) 
               
               
                   
               
               
                 8 
                 39 
                 20 
                 9 
                 5 
                 5 
                 22 
                 30.0 
                 12.8 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Matte layer 
                 Impregnation 
                 Number of 
                 Number of 
                 Number of 
                 Number of 
               
               
                   
                   
                 thickness 
                 depth 
                 veins 
                 veins 
                 veins 
                 veins 
               
               
                   
                 Example 
                 (μm) 
                 (mm) 
                 Step 2 
                 Step 3 
                 Step 4 
                 Step 5 
               
               
                   
                   
               
               
                   
                 8 
                 450 
                 1 
                 1 
                 1 
                 1 
                 3 
               
               
                   
                   
               
            
           
         
       
     
     The results show that at a very low matte layer thickness of only 25 to 100 μm, the coating composition according to the present invention shows a sufficiently high protection against veining formation both in the dome core test and in the step core test. For the evaluation, steps 1 to 5 were evaluated during the step test, as this corresponds to the most common and demanding applications in foundries for automotive parts in terms of the wall thickness ratio of metal to sand. In this respect, the effect of the coating composition according to the present invention exceeds the effect of conventional coating compositions against veining defects, which are applied with high layer thicknesses of 300 to 500 μm. 
     It is apparent that the coating composition according to the present invention saves the process step of adding a sand additive and provides a much higher active ingredient concentration in the zone where this concentration is needed. In particular, the gas formation provides a very good protection against so-called veining, which is often found in quartz sand due to the thermal expansion of the quartz sand (quartz inversion) and the insufficient thermal strength, observed especially in polyurethane cold box cores.