EP2274122B1 - Coating composition which adsorbs odourous and harmful substances and is intended for the box casting of metals - Google Patents

Description

The invention relates to a casting mold for metal casting, a method for producing a casting mold and the use of the casting mold.

Most products of the iron and steel industry and the non-ferrous metal industry undergo casting processes for the first shaping. The molten materials, ferrous metals or non-ferrous metals, are converted into geometrically determined objects with specific workpiece properties. To shape the castings, very complicated casting molds for receiving the melt must first be produced. Molds for the production of metal bodies are composed of so-called cores and molds. The casting mold essentially represents a negative mold of the casting to be produced, wherein cores serve to form cavities in the interior of the casting, while the molds form the outer boundary. Different requirements are placed on the cores and molds. In molds, a relatively large surface area is available to dissipate gases that form during casting by the action of the hot metal. In cores usually only a very small area is available, through which the gases can be derived. If there is too much gas, there is a risk that gas will pass from the core into the liquid metal and lead to the formation of casting defects. Often, the internal cavities are therefore imaged by cores solidified by cold-box binders, a polyurethane-based binder, while the outer contour of the casting is represented by lower cost forms, such as a green sand mold, a furan resin mold. or a phenol resin bound form or by a steel mold.

Casting molds are made of a refractory material, such as quartz sand, whose grains are connected after molding of the mold by a suitable binder to ensure sufficient mechanical strength of the mold. For the production of molds so you use a refractory molding material, which is mixed with a suitable binder. The molding material mixture obtained from molding material and binder is preferably present in a free-flowing form, so that it can be filled into a suitable mold and compacted there. The binder produces a firm cohesion between the particles of the molding material, so that the casting mold obtains the required mechanical stability.

For the production of casting molds both organic and inorganic binders can be used, the curing of which can be effected by cold or hot processes. Cold processes are processes which essentially be carried out at room temperature without heating the molding material mixture. The curing is usually carried out by a chemical reaction, which can be triggered, for example, by passing a gaseous catalyst through the molding material mixture to be cured, or by adding a liquid catalyst to the molding material mixture. In hot processes, the molding material mixture is heated to a sufficiently high temperature after molding to drive off, for example, the solvent contained in the binder, or to initiate a chemical reaction by which the binder is cured by crosslinking.

At present, for the production of casting molds, organic binders, e.g. Polyurethane, furan resin or epoxy-acrylate binders used, wherein the curing of the binder is carried out by adding a catalyst.

The choice of suitable binder depends on the shape and size of the casting to be produced, the conditions of production and the material used for the casting. For example, in the production of small castings that are produced in large numbers, polyurethane binders are often used because they allow fast cycle times and thus also a series production.

Processes in which the curing of the molding material mixture by heat or by subsequent addition of a catalyst, have the advantage that the processing of the molding material mixture is not subject to any special time restrictions. The molding material mixture can first be produced in larger quantities, which are then processed within a longer period of time, usually several hours. The curing of the molding material mixture takes place only after molding, with a rapid reaction is sought. The mold can be removed immediately after curing from the mold, so that short cycle times can be realized. However, in order to obtain a good strength of the mold, the curing of the molding material mixture must be uniform within the mold. If the curing of the molding material mixture by subsequent addition of a catalyst, the mold is gassed after molding with the catalyst. For this purpose, the gaseous catalyst is passed through the casting mold. The molding material mixture cures directly after contact with the catalyst and can therefore be removed very quickly from the mold. As the size of the mold increases, it becomes more difficult to provide an amount of catalyst sufficient to cure the molding material in all sections of the mold. The gassing times are prolonged, but can still arise sections in the mold, which are achieved very poorly or not at all by the gaseous catalyst. The amount of catalyst therefore increases sharply with increasing size of the mold.

Similar difficulties occur in hot curing processes. Here, the mold must be heated in all sections to a sufficiently high temperature. As the size of the mold increases, on the one hand, the times for which the mold must be heated to a certain temperature for curing become longer. Only then can it be ensured that the casting mold also has the required strength in its interior. On the other hand, the hardening with increasing size of the mold on the apparatus side is very complex.

In the production of molds for large castings, such as engine blocks of marine diesel or large machine parts, such as hubs of rotors for wind power plants, therefore, no-bake binders are used mostly. In the no-bake process, the refractory molding material is first coated with a catalyst. Then the binder is added and mixed evenly distributed on the already coated with the catalyst grains of the refractory molding material. The molding material mixture can then be shaped into a shaped body. Since binder and catalyst are evenly distributed in the molding material mixture, the curing is largely uniform even with large moldings.

During casting, the cured binder is said to decompose under the influence of the heat of the liquid metal and the reducing atmosphere generated during casting, so that the casting mold loses its strength. The mold can then be easily removed from the casting. It is particularly important that the cores used in the mold lose their strength, so that the sand, which was used for the production of the cores, can easily pour out of the cavities of the mold. The decomposition of the binder releases a number of gaseous pollutants which, for example, must be collected and removed by suitably designed suction devices. The harmful substances are formed on the one hand by the decomposition of the resin and on the other hand by the decomposition of components which have been added to the binder for curing or for modifying its properties. Thus, for example, the aromatic sulfonic acids used as catalyst in the no-bake process, in particular p-toluenesulfonic acid, benzenesulfonic acid and xylenesulfonic acid, decompose and, besides sulfur dioxide, release aromatic pollutants such as benzene, toluene or xylene (BTX). Some of these decomposition products also remain in the used sand and can be released during reprocessing.

Especially for aromatic pollutants, because of their carcinogenic effects, very low MAK values (MAK = maximum workplace concentration) apply. For benzene, the MAK value is only 3.2 mg / m ³ , for toluene and xylene corresponding to 190 mg / m ³ and 440 mg / m ³ This has become a problem in foundries, as compliance with these limits requires very expensive extraction systems and filters.

The composition of the gaseous mixture produced during the casting is very complex and comprises a large number of compounds which can have very different chemical properties. In addition to the aromatic substances already mentioned, it is also possible, for example, for acidic components, such as sulfur compounds, or basic components, for example amines, to be present in the exhaust gases. In addition to the gaseous components, the exhaust gas produced during casting also contains dusts that are carried along by the released gas. These dusts are usually very fine and therefore can also be harmful to health.

In addition to their health-endangering effect released during casting gaseous substances also because of their strong odor is a problem. The human sense of smell is very sensitive to some compounds, so even low concentrations are sufficient to produce a perceived as unpleasant odor. It is usually not technically possible to completely absorb the exhaust gases released during casting by means of an exhaust system, so that odor nuisance in the corresponding workshop is unavoidable.

The CN 86106521 A and the CN 86106485 A discloses sizing for the inner surfaces of a mold, inter alia, to minimize the adhesion of sand to the surface of castings or to make it more easily removable from the casting. Molds thus treated result in castings having low roughness casting surfaces.

The US 1717820 A discloses release agents in particulate form for the inner surfaces of a mold.

The DE 1212250 B relates to a mold dressing for steel casting, which consists of finely divided carbon slurried in water as the main constituent and an organic binder. It is stated that it is possible with the sizing to compensate for roughness and unevenness of the mold and thus to produce castings with a particularly smooth surface.

The WO 03/015956 A2 discloses a method for reducing pollutant emissions, in particular those which are released in the form of pyrolysis products from metal casting boxes by means of an added combustible substance, which - if it is not gaseous - when heated in the gaseous state passes and on exit from the metal casting boxes together with the Burns pollutants. The additional combustible substance is preferably a wax or an oil.

The invention therefore an object of the invention to provide a mold available that releases a smaller amount of interfering gaseous substances during casting.

This object is achieved with a mold having the features of patent claims 1 and 11. Advantageous embodiments are the subject of the dependent claims.

In the casting mold according to the invention, at least in sections, a layer of a gas discharge surfaces of the outer surface of the mold at least Pollutant absorbing material arranged. The layer of the pollutant-absorbing material is also referred to below as the "absorber layer".

Gas outlet surfaces are understood to be the surfaces of the casting mold through which gaseous components can escape from the casting mold during the casting. The gas exit surface may correspond to the entire outer surface of the mold. But it is also possible that only a part of the outer surface of the mold is used for the discharge of gaseous components. Thus, in case-bound metal casting a box is used for the construction of the mold, which covers the bottom and the side surfaces of the mold. These surfaces are then available for a levy gaseous components or only very limited available. In this case, essentially only the upper side of the casting mold is available for dispensing gaseous components.

An outer surface of the casting mold is understood to mean the surfaces through which exhaust gases produced during the casting can leave the casting mold. This outer surface is visible when viewing the mold from the outside and does not come in contact with the liquid metal in contact. In contrast, an inner surface is understood as meaning, for example, the surface of the mold cavity surrounded by the mold.

Preferably, at least the upper side of the casting mold is at least partially coated with the layer of a material that absorbs pollutants. As the top of the mold, the side of the mold is called, which is located at the top casting. During casting, the majority of the gases released leaves the mold above its top. Since in the casting mold according to the invention there is arranged a layer of material that absorbs pollutants, the gas passes through this layer. This pollutants are contained in the gas are absorbed in the absorber layer and thus removes a significant portion of the pollutants from the gas stream. As such, it would also be possible to provide sections of the outer surface of the casting mold with a gas-tight coating, in order, for example, to selectively expel the exhaust gases from side faces of the casting mold.

By absorbing a pollutant is meant both a binding of the pollutant in the absorber layer and a conversion of the pollutant into innocuous compounds, the innocuous compounds need not necessarily be bound in the absorber layer but also be discharged back into the exhaust stream and leave the mold. By absorbing a pollutant is thus generally understood a removal of the pollutant from the gas stream, which leaves the casting mold during casting.

The pollutants considered are all substances contained in the gas released during casting and which have an environmental or health-damaging effect or which smell strongly. In particular, pollutants are considered to be substances for which limit values apply at the workplace. In particular, such substances are regarded as pollutants whose MAK is less than 1 g / m ³ , preferably less than 500 mg / m ³ .

Preferably, the layer of contaminant-absorbing material covers the entire top of the mold. As such, the side walls of the mold may also be covered with the layer of contaminant-absorbing material. The mold can be provided by the skilled person according to their shape in appropriate sections with the layer of the pollutant absorbing material. If, for example, box-shaped casting molds are used, it is usually not necessary, on the side surfaces of the casting mold, the layer of the pollutants according to the invention provide absorbent material, since the side surfaces are sealed by the box.

The casting mold is initially constructed in the same way as already known casting molds, but at least on a portion of the gas outlet surfaces, ie the outer surface, in particular preferably on the top of the mold additionally a layer of a material absorbing harmful substances is arranged, said absorber layer the top of the mold partially or completely covered.

The mold consists in a conventional manner of a granular refractory molding material which is solidified with a binder. The mold may be composed of molds and cores and includes a mold cavity that substantially conforms to the shape of the casting.

The binders which can be used per se are all binders customary for the production of such casting molds, it being possible to use both inorganic and organic binders. An exemplary inorganic binder is water glass. For example, polyurethane, furan resin or epoxy-acrylate binders can be used as the organic binder, in which the curing of the binder takes place by addition of a catalyst. However, it is also possible to use organic binders which are cured by other methods, for example by heating.

Most preferably, the mold is solidified with one having a furfuryl alcohol-urea resin, a phenol-furfuryl alcohol resin or a phenolic resin.

As refractory standard refractory materials can be used. Exemplary refractories are quartz sand, zircon sand, Olivine sand, aluminum silicate sand and chrome ore sand or mixtures thereof.

The mold may have been pretreated in a conventional manner, for example by covering the surfaces of the mold cavity which come into contact with the liquid metal with a size. In this case, customary sizes can be used.

On at least a portion of the gas outlet surface, in particular the top of the mold, a layer of the pollutant absorbing material is arranged. The layer may initially show an arbitrary structure. Thus, the layer can be constructed homogeneously. But it is also possible that the layer is composed of a layer stack, wherein individual layers of the layer stack may also have a different composition.

The absorber layer should be gas-permeable, ie porous. The porosity should be so high that the released gases can pass through the absorber layer largely unhindered, so in the mold no overpressure arises, which can lead to gas inclusions in the casting. The gas permeability Gd is preferably greater than 50 and is preferably greater than 100, more preferably greater than 200.

The gas permeability Gd indicates how many cm ^{3 of} air at an overpressure of 1 cm water column (WS) in 1 min. pass through a test specimen with a base of 1 cm ² and 1 cm in height average. The measurement is carried out with a permeability tester type PDU from Georg Fischer AG, Schaffhausen, Switzerland.

wobei gilt:

• Q _: durchströmendes Luftvolumen (2000 cm³)
• h: Höhe des Prüfkörpers
• F: Querschnittsfläche des Prüfkörpers (19,63 cm²)
• p: Druck in WS
• t: Durchströmzeit für 2000 cm³ Luft in min.

The gas permeability is determined by the following relationship: $$\mathit{Gd}=\frac{Q\cdot H}{F\cdot p\cdot t}$$

where:

• Q _: air volume flowing through (2000 cm ³ )
• h: height of the test piece
• F: cross-sectional area of the test piece (19.63 cm ² )
• p: pressure in WS
• t: flow-through time for 2000 cm ^{3 of} air in min.

The absorber layer may be constructed of a granular material loosely applied to the top of the mold. But it is also possible that the absorber layer is bonded, that forms a solid, continuous layer on top of the mold.

The absorber layer may comprise a single component which acts as the contaminant absorbing material. However, it is also possible for the layer to comprise a plurality of components, some or all of the components acting as the pollutant absorbing material. In addition to acting as a pollutant absorbing material component, the layer may for example comprise a binder or framework materials, which improve the gas permeability of the absorber layer.

The layer of the pollutant-absorbing material preferably has a different composition than the casting mold, so that a clear separation between the casting mold and the layer of the pollutant-absorbing material is detectable.

The thickness of the layer of contaminant-absorbing material is dependent on the amount of gas released during casting and on the type and amount of pollutants contained in the released gas. For small molds can even a comparatively thin layer of the material absorbing harmful substances may be sufficient, while in the case of casting molds for very large cast pieces, the thickness of the layer of the material absorbing harmful substances can be significantly higher and can be up to several centimeters.

According to a preferred embodiment, the layer of the pollutant-absorbing material has a thickness of at least 2.5 mm. According to a further embodiment, the layer thickness is at least 0.5 cm. It is usually sufficient to clean the gases released during casting if the thickness of the layer is less than 5 cm. However, it is also possible to use even greater layer thicknesses.

The absorption of the pollutants can be done in any way. The pollutants can be physically bound by the material absorbing the pollutants. But it is also possible that the pollutants are bound by means of a chemical reaction of the material absorbing the pollutants, which are for example converted into a non-volatile compound. Finally, it is also possible that the pollutants are decomposed in the pollutant absorbing layer into harmless compounds, such as carbon dioxide or water, which can then be completely or partially released from the layer of the pollutant absorbing material.

According to a first embodiment, it is provided that the layer of the pollutant-absorbing material comprises at least one physical adsorber material which can physically adsorb pollutants. This embodiment is particularly suitable for the removal of relatively non-polar pollutants, such as aromatic hydrocarbons from the released during casting gas.

Preference is given to using compounds which have a high specific surface area as the physical adsorber material. Preference is given to using compounds which have a specific surface area of more than 800 m ² / g, preferably more than 1000 m ² / g, particularly preferably more than 1100 m ² / g. The specific surface area is determined by the BET method according to DIN 66 131.

Such physical adsorber materials preferably have a relatively low bulk density, which is preferably selected in the range of 10 to 2000 g / l. A method for determining the bulk density is given in the examples.

The physical adsorber material preferably has a iodine absorption capacity of at least 300 mg / g, preferably at least 500 mg / g, more preferably more than 800 mg / g. The uptake capacity of the physical adsorbent material for iodine is determined by the method described in standard ASTM D 1510.

In order to achieve a sufficient gas permeability of the layer of the pollutant-absorbing material, the average particle size (D ₅₀ ) of the physical adsorber material is preferably selected to be greater than 100 μm, preferably greater than 150 μm. In order to achieve a uniform structure of the layer, it is preferred that the physical adsorber material has a mean grain size (D ₅₀ ) of less than 500 microns. The particle size distribution can be determined, for example, by laser granulometry.

Preferably, the physical adsorbent material is selected from the group consisting of activated carbon, finely divided silica, acidified clays, ashes and celluloses such as linters, viscose rayon, viscose or similar materials. Preferably, the physical adsorbent material is contained in a proportion of 5 to 50% by weight in the layer of the pollutant absorbing material.

According to a further embodiment, the layer of the pollutant-absorbing material contains at least one chemical absorber material which can bind pollutants by a chemical reaction. The nature of the chemical reaction, which results in removal of pollutants from the gas stream released during casting, is not limited in itself. The chemical reaction may, for example, be a neutralization with which an acidic pollutant, for example an acid sulfur compound, is neutralized or converted into a salt and bound by the chemical absorber material. But it is also possible that the chemical reaction is a redox reaction in which a pollutant, for example, oxidized and converted, for example, into harmless compounds. For this purpose, for example, an oxidation agent or a reducing agent may be contained in the layer of the material absorbing the pollutants, or else a catalyst which catalyzes the oxidation or reduction of the pollutant. But it is also possible to provide compounds in the layer of the pollutant absorbing material, which bind the pollutants coordinately. Suitable compounds are, for example, cyclodextrins, which can store harmful compounds.

The chemical absorber material is preferably contained in granular form in the layer of the pollutant absorbing material. Since the chemical absorber material changes by the reaction with pollutants, the mean grain size of the chemical absorber material is preferably chosen smaller than the mean grain size of the physical adsorber material. Preferably, the chemical absorber material has an average Grain size (D ₅₀ ) of more than 10 microns, preferably more than 20 microns on. According to one embodiment of the invention, the mean grain size of the chemical absorber material is chosen to be smaller than 100 μm, preferably smaller than 50 μm.

According to a preferred embodiment it is provided that the chemical absorber material is a basic material. A basic material is understood as meaning a material or a compound which, upon contact with water, leads to an alkaline reaction. The pH of the water upon contact with the basic material increases to more than 8, preferably more than 9. The measurement of the pH can be carried out, for example, with a glass electrode on a sample containing 10 g of the basic material per liter of water contains. This embodiment is suitable for removing acidic pollutants from the gas released during the casting. Such acidic pollutants arise, for example, when the binder of the mold contains sulfur-containing compounds. Such sulfur-containing compounds are, for example, sulfonic acids, such as those used in the furan or phenolic resin no-bake process.

Preferably, the basic material is selected from oxides, hydroxides and carbonates of the alkali metals and alkaline earth metals. These basic materials are easy and inexpensive to access and can be processed without much difficulty. Both the carbonates and the bicarbonates can be used. Particular preference is given to using calcium carbonate and / or calcium oxide or calcium hydroxide as the basic material.

The at least one chemical absorber material may be present alone in the layer of the pollutant absorbing material or also adjacent to the physical adsorber material. Preferably, a combination of chemical absorber material and physical adsorber material is used.

Preferably, the chemical absorber material is contained in a proportion of 10 to 20 wt .-% in the layer of the pollutant absorbing material.

In addition to the physical or the chemical absorber material, further substances may be contained in the layer of the pollutant-absorbing material. In this case, preferably substances are used, as they are commonly used in coatings for metal casting.

According to one embodiment, at least one refractory material which has an average particle size (D ₅₀ ) of at least 50 μm is contained in the layer of the pollutant-absorbing material.

As refractory material, conventional refractory materials can be used in metal casting. Examples of suitable refractory materials are quartz, alumina, aluminum silicates such as pyropyllite, kyanite, andalusite or chamotte, zircon sands, olivine, talc, mica, graphite, coke, feldspar. The refractory material is provided in powder form. The grain size is chosen so that in the absorber layer, a stable structure is formed and that the layer receives a sufficiently high porosity, so that the resulting gases during casting can pass through the layer without the formation of excessive backpressure. Suitably, the refractory material has an average particle size in the range from 100 to 500 μm, particularly preferably in the range from 120 to 200 μm.

The proportion of the refractory to the layer of the pollutant absorbing material is preferably selected in the range of 30 to 60% by weight, preferably in the range of 40 to 50% by weight.

In addition, in the layer, for example, still contain a binder. As binders conventional binders can be used be, such as clays, especially bentonite. However, other binders may also be present, for example silica sol. All binders which are used in sizing can be contained per se. In this case, both inorganic and organic binders can be used.

The layer of the pollutant-absorbing material preferably still has a residual moisture. As a result, for example, polar pollutants, such as amines, can be retained in the layer. Further, the gases released during casting are cooled as they pass through the layer of contaminant-absorbing material, so that a portion of the pollutants in the layer is deposited. Preferably, the layer of the pollutant-absorbing material has a water content in the range of 0 to 60 wt .-%, preferably 5 to 30 wt .-%, particularly preferably 10 to 20 wt .-% to. The water content refers to the composition of the layer of pollutant absorbing material prior to casting.

According to a further embodiment, the layer of the pollutant absorbing material comprises a porous support framework. On the porous support framework then the absorber materials and the other components of the absorber layer are applied. When calculating the proportions of these components, the weight of the porous support framework is not included.

As a porous support framework, any material can be used which provides a sufficiently strong framework for the recordings of the other components of the absorber layer and which provides a sufficiently high porosity that the gas produced during the casting can pass through the layer. As a porous carrier framework, for example, an open-pore solid foam can be used, or preferably a woven or nonwoven fabric. Suitable materials from which such a fabric or non-woven can be made, for example, mineral wool, glass wool, or mats made of synthetic fibers, such as fibers of perfluorocarbons.

The porous carrier framework is preferably arranged in the form of mats on top of the casting mold, wherein the thickness of the mats are preferably selected in the range of 0.5 to 5 cm, preferably in the range of 1 to 4.5 cm.

The amount of the absorber material and the other components of the layer of the pollutant absorbing material with which the porous carrier material is coated is preferably selected in the range of 0 to 10 g / cm ³ , preferably 0.01 to 1.0 g / cm ³ , calculated as dry matter and based on the weight of the layer of the pollutant absorbing material, including the porous support framework.

The casting mold according to the invention is characterized by an absorber layer, in which pollutants are absorbed or adsorbed, which arise during the casting and exit from the casting mold together with other gaseous and solid components. Another object of the invention is a pollutant-absorbing coating composition, with which such an absorber layer can be produced.

A pollutant-absorbing coating composition for the coating of casting molds for casting metal according to the invention contains at least one pollutant absorbing material.

The components of such a pollutant-absorbing coating composition, in particular the physical adsorber materials and the chemical absorber materials, have been explained in detail in the description of the casting mold according to the invention. The corresponding passages are referred to. The coating composition is similar in composition to a size as it is already used in the production of molds, but additionally containing at least one pollutant-absorbing material.

The coating composition preferably comprises a carrier liquid in which the further constituents of the coating composition can be suspended or dissolved. This carrier liquid is suitably selected so that it can be completely evaporated at the conditions customary in metal casting. The carrier liquid should therefore preferably at normal pressure have a boiling point of less than about 130 ° C, preferably less than 110 ° C. The carrier liquid used is preferably water. However, it is also possible to use alcohols as the carrier liquid, for example ethanol or isopropanol, or else mixtures of these carrier liquids.

The coating composition is preferably provided in the form of a suspension or a paste. The solids content of the coating composition is therefore preferably selected in the range from 20 to 60% by weight, preferably in the range from 30 to 50% by weight. The coating composition can then be applied to the surface of the casting mold by conventional methods, such as brushing or spraying.

If, according to a preferred embodiment, the above-described physical adsorber materials are contained in the coating composition, their proportion is preferably selected in the range from 2.5 to 25% by weight, preferably 4 to 15% by weight, based on the ready-to-use coating composition.

If according to a further embodiment, the above-described chemical absorber materials are contained in the coating composition, their proportion is preferably in the range of 3 chosen to 15 wt .-%, preferably 5 to 10 wt .-%, based on the ready-to-use coating composition.

If, according to a further embodiment, the above-described refractory materials are contained in the coating composition, their proportion is preferably selected in the range from 10 to 30% by weight, preferably from 10 to 20% by weight, based on the ready-to-use coating composition.

In order to prevent a decrease in the solid constituents of the coating composition and at the same time to achieve a uniform application on the casting mold, the viscosity of the coating composition is preferably selected in the range of 1000 to 3000 mPas, particularly preferably 1200 to 2000 mPas.

In the carrier liquid preferably at least one powdered refractory material is suspended. As refractory material, the already mentioned refractories can be used. Examples of suitable refractory materials are quartz, alumina, aluminum silicates such as pyropyllite, kyanite, andalusite or chamotte, zircon sands, olivine, talc, mica, graphite, coke, feldspar. The refractory material is provided in powder form. The grain size is chosen so that in the absorber layer, a stable structure is formed and that the coating composition can be distributed for example with a spray easily on the gas outlet surfaces, preferably the top of the mold. Suitably, the refractory material has an average particle size in the range from 50 to 600 .mu.m, particularly preferably in the range from 100 to 500 .mu.m. As refractory materials are particularly suitable which have a melting point of more than 1200 ° C.

According to a preferred embodiment, the coating composition comprises as further constituent at least one binder. The binder allows for better fixation the coating on the surface, in particular on the top of the mold. In addition, the mechanical stability of the coating is increased by the binder, so that less erosion is observed under mechanical stress or under the action of the gas flowing through the layer. The binders customary binders can be used, such as clays, in particular bentonite. Other exemplary binders are starch, dextrin, peptides, polyvinyl alcohol, polyacrylic acid, polystyrene and / or polyvinyl acetate-polyacrylate dispersions. In general, binder systems are preferably used which can be used in aqueous systems and which do not retract after curing under the action of atmospheric moisture.

According to a further embodiment, the coating composition contains silica sol as a binder. The proportion of the binder is preferably selected in the range of 0.1 to 20 wt .-%, particularly preferably 0.5 to 5 Ges .-%, based on the weight of the coating composition. The silica sol is preferably prepared by neutralizing water glass. The resulting amorphous silica preferably has a specific surface area in the range from 10 to 1000 m ² / g, particularly preferably in the range from 30 to 300 m ² / g.

The coating composition may further comprise at least one adjusting agent. The adjusting agent causes an increase in the viscosity of the coating composition, so that the solid components of the coating material in the suspension do not or only to a small extent decrease. To increase the viscosity, both organic and inorganic materials or mixtures of these materials can be used. Suitable inorganic adjusting agents are, for example, strong swellable clays. As a highly swellable layered silicate, both two-layer silicates and three-layer silicates can be used, such as eg attapulgite, serpentine, kaolins, smectites, such as saponite, montmorillonite, beidellite and nontronite, vermiculite, illite, hectorite and mica. Hectorite also gives the coating composition thixotropic properties, which facilitates the formation of the absorber layer on the casting mold, since the coating composition no longer flows after application.

Suitable organic solvents are, for example, swellable polymers, such as carboxymethyl, methyl, ethyl, hydroxyethyl and hydroxypropylcellulose, mucilages, polyvinyl alcohols, polyvinylpyrrolidone, pectin, gelatin, agar agar, polypeptides and alginates.

Furthermore, the coating composition may contain further constituents which are customary in sizes, for example preservatives, defoamers, wetting agents and dispersants.

For example, cellulose ethers, alginates, mucilages and / or pectins can be used as suspending agents. Examples of suitable wetting and dispersing agents are ionic and nonionic, preferably nonionic surfactants.

The proportion of these further constituents in the ready-to-use coating composition is preferably chosen to be less than 1% by weight.

According to a preferred embodiment, the coating composition is provided in a form in which it is applied to a porous support framework. Suitable support materials have already been described above. The coating composition applied to the porous support framework can be provided in such a way that corresponding mats are already provided which already contain the coating composition. These can then according to the dimensions of the gas outlet surfaces of the mold to be covered, for example, the top of the Cutter cut and placed on this mold. In this embodiment, the coating composition is preferably provided while still wet. The water content of the coating composition is preferably selected in the range from 5 to 30% by weight, preferably in the range from 10 to 20% by weight, based on the coating composition.

Another object of the invention relates to a method for producing a casting mold according to claim 11.

• eine Formstoffmischung bereitgestellt, welche zumindest einen feuerfesten Formstoff und zumindest ein Bindemittel umfasst;
• die Formstoffmischung zu einer Gießform geformt, und
• die Gasaustrittsflächen der Gießform zumindest abschnittsweise mit einer Schicht aus einem Schadstoffe absorbierenden Material bedeckt.

In the method according to the invention is

• providing a molding material mixture comprising at least one refractory molding material and at least one binder;
• shaped the molding material mixture into a casting mold, and
• the gas outlet surfaces of the mold at least partially covered with a layer of a pollutant absorbing material.

First, a casting mold is produced in a manner known per se from a molding material mixture. To produce the molding material mixture, a refractory molding material is mixed with a binder and then molded into a casting mold or a portion of a casting mold.

As a refractory molding material, all refractory materials that are customary for the production of moldings for the foundry industry can be used per se. Examples of suitable refractory molding materials are quartz sand, zircon sand, olivine sand, aluminum silicate sand and chrome ore sand or mixtures thereof. Preferably, quartz sand is used. The refractory molding material should have a sufficient particle size, so that the molded article produced from the molding material mixture has a sufficiently high porosity to allow escape of volatile compounds during the casting process. Preferably, at least 70 wt .-%, particularly preferably at least 80 wt .-% of the refractory molding material has a particle size ≤ 290 microns. The average particle size of the refractory molding material should preferably be between 100 and 350 μm. The particle size can be determined, for example, by sieve analysis. The refractory molding material should be in free-flowing form, so that a binder or a liquid catalyst can be applied well, for example in a mixer to the grains of the refractory molding material.

According to one embodiment, regenerated used sands may be used as the refractory molding material. From the used sand larger aggregates are removed and the used sand is separated into individual grains. After a mechanical or thermal treatment, the old sands are dedusted and can then be reused. Before reuse, the acid balance of the regenerated used sand is preferably tested. In particular, during a thermal regeneration by-products contained in the sand, such as carbonates, can be converted into the corresponding oxides, which then react alkaline. If binders are used which are cured by catalysis by an acid, in this case the acid added as a catalyst can be neutralized by the alkaline components of the regenerated used sand. Likewise, for example, in a mechanical regeneration of a used sand, acid remain in the used sand, which must be considered in the preparation of the binder, as otherwise, for example, the processing time of the molding material mixture can be shortened.

The refractory molding material should be dry. Preferably, the refractory molding material contains less than 1 wt .-% water. To premature curing of the binder by the action to prevent heat, the refractory molding material should not be too warm. Preferably, the refractory molding material should have a temperature in the range of 20 to 35 ° C. Possibly. the refractory molding material can be cooled or heated.

As binders, all binders can be used per se, as are customary for the production of casting molds for metal casting. Both inorganic and organic binders can be used. As the inorganic binder, for example, water glass can be used, which can be cured thermally or by introducing carbon dioxide. Exemplary organic binders are polyurethane no-bake and cold-box binders, binders based on furan resins or phenolic resins, or also epoxy-acrylate binders.

Polyurethanes based on polyurethanes are generally composed of two components, a first component containing a phenolic resin and a second component containing a polyisocyanate. These two components are mixed with the refractory molding material and the molding mixture is brought into a mold by ramming, blowing, shooting or other method, compacted and then cured. Depending on the method in which the catalyst is introduced into the molding material mixture, a distinction is made between the "polyurethane no-bake process" and the "polyurethane cold-box process".

In the no-bake process, a liquid catalyst, generally a liquid tertiary amine, is introduced into the molding material mixture before it is shaped and cured. For the preparation of the molding material mixture phenolic resin, polyisocyanate and curing catalyst are mixed with the refractory molding material. In this case, for example, proceed in such a way that the refractory molding material is first coated with a component of the binder, and then the other component is added. The curing catalyst is added to one of the components. The ready-made molding material mixture must have a sufficiently long processing time, so that the molding material mixture can be plastically deformed for a sufficient time and processed into a molding. The polymerization must be correspondingly slow so that not already in the storage tanks or supply lines hardening of the molding material mixture. On the other hand, the curing should not be too slow to achieve a sufficiently high throughput in the production of molds. The processing time can be influenced for example by adding retarders, which slow down the curing of the molding material mixture. A suitable retarder is, for example, phosphorus oxychloride.

In the cold-box process, the molding material mixture is first brought into a mold without catalyst. Through the molding material mixture, a gaseous tertiary amine is then passed, which may optionally be mixed with an inert carrier gas. On contact with the gaseous catalyst, the binder binds very quickly, so that a high throughput in the production of molds is achieved.

The binder systems based on polyurethanes contain a polyol component and a polyisocyanate component, in which case known components can be used.

The polyisocyanate component of the binder system may comprise an aliphatic, cycloaliphatic or aromatic isocyanate. The polyisocyanate preferably contains at least 2 isocyanate groups, preferably 2 to 5 isocyanate groups per molecule. Depending on the desired properties, it is also possible to use mixtures of isocyanates. The isocyanates used can be mixtures of monomers, oligomers and polymers exist and are therefore referred to below as polyisocyanates.

The polyisocyanate component per se can be any polyisocyanate which is customary in polyurethane binders for molding mixtures for the foundry industry. Suitable polyisocyanates include aliphatic polyisocyanates, e.g. Hexamethylene diisocyanate, alicyclic polyisocyanates, e.g. 4,4'-dicyclohexylmethane diisocyanate, and dimethyl derivatives thereof. Examples of suitable aromatic polyisocyanates are toluene-2,4-diisocyanate, toluene-2,6-diisocyanate, 1,5-naphthalene diisocyanate, xylylene diisocyanate and methyl derivatives thereof, diphenylmethane-4,4'-diisocyanate and polymethylene-polyphenyl-polyisocyanate.

Although, in principle, all conventional polyisocyanates react with the phenolic resin to form a crosslinked polymer structure, it is preferred to use aromatic polyisocyanates, more preferably polymethylene-polyphenyl polyisocyanate, e.g. commercially available mixtures of diphenylmethane-4,4'-diisocyanate, its isomers and higher homologs.

The polyisocyanates can be used both in substance and dissolved in an inert or reactive solvent. A reactive solvent is understood to mean a solvent which has a reactive group, so that it is incorporated into the framework of the binder when the binder is set. The polyisocyanates are preferably used in dilute form in order to be able to coat the grains of the refractory molding material better with a thin film of the binder because of the lower viscosity of the solution.

The polyisocyanates or their solutions in organic solvents are used in sufficient concentration to accomplish the curing of the polyol component, usually in a range of 10 to 500% by weight based on the weight of the polyol component. From 20 to 300% by weight, based on the same base, are preferably used. Liquid polyisocyanates can be used in undiluted form while solid or viscous polyisocyanates are dissolved in organic solvents. Up to 80 wt .-%, preferably up to 60 wt .-%, particularly preferably up to 40 wt .-% of the isocyanate component may consist of solvents.

Preferably, the polyisocyanate is used in an amount such that the number of isocyanate groups is 80 to 120%, based on the number of free hydroxyl groups of the polyol component.

As the polyol component, all polyols used in polyurethane binders can be used per se. The polyol component contains at least 2 hydroxyl groups, which can react with the isocyanate groups of the polyisocyanate component in order to achieve cross-linking of the binder during curing, and thereby better strength of the cured molding.

The polyols used are preferably phenolic resins which are obtained by condensation of phenols with aldehydes, preferably formaldehyde, in the liquid phase at temperatures up to about 180 ° C. in the presence of catalytic amounts of metal. The processes for the preparation of such phenolic resins are known per se.

The polyol component is preferably used liquid or dissolved in organic solvents in order to allow a homogeneous distribution of the binder on the refractory molding material. The polyol component is preferably used in anhydrous form because the reaction of the isocyanate component with water is an undesirable side reaction. Non-aqueous or anhydrous in this context should have a water content of Polyol component of preferably less than 5 wt .-%, preferably less than 2 wt .-% mean.

By "phenolic resin" is meant the reaction product of phenol, phenol derivatives, bisphenols, and higher phenol condensation products with an aldehyde. The composition of the phenolic resin depends on the specific starting materials selected, the ratio of the starting materials and the reaction conditions. For example, play the type of catalyst, the time and the reaction temperature, as well as the presence of solvents and other substances.

The phenolic resin is typically present as a mixture of various compounds and can contain addition products, condensation products and unreacted starting compounds, such as phenols, bisphenol and / or aldehyde, in very different ratios.

By "addition product" is meant reaction products in which an organic component substitutes at least one hydrogen on a previously unsubstituted phenol or condensation product. By "condensation product" is meant reaction products having two or more phenolic rings.

• Novolake sind lösliche, schmelzbare, nicht selbsthärtende und lagerstabile Oligomere mit einem Molekülgewicht im Bereich von ungefähr 500 bis 5.000 g/Mol. Sie fallen bei der Kondensation von Aldehyden und Phenolen im Molverhältnis von 1 : >1 in Gegenwart saurer Katalysatoren an. Novolake sind methylolgruppenfreie Phenolharze, in denen die Phenylkerne über Methylenbrücken verknüpft sind. Sie können nach Zusatz von Härtungsmitteln, wie Formaldehyd spendenden Mitteln, bevorzugt Hexamethylentetramin, bei erhöhter Temperatur unter Vernetzung gehärtet werden.

The condensation reactions of phenols with aldehydes result in phenolic resins, which are subdivided into two product classes, the novolacs and resoles, depending on the proportions of the reactants, the reaction conditions and the catalysts used:

• Novolacs are soluble, meltable, non-self-curing, and shelf-stable oligomers having a molecular weight in the range of about 500 to 5,000 g / mole. They are obtained in the condensation of aldehydes and phenols in a molar ratio of 1:> 1 in the presence of acidic catalysts. Novolacs are methylol group-free Phenolic resins in which the phenyl nuclei are linked via methylene bridges. They may be cured at elevated temperature with crosslinking after addition of curing agents, such as formaldehyde donating agents, preferably hexamethylenetetramine.

Resoles are mixtures of hydroxymethylphenols which are linked via methylene and methylene ether bridges and can be obtained by reaction of aldehydes and phenols in a molar ratio of 1: <1, if appropriate in the presence of a catalyst, for example a basic catalyst. They have a molecular weight M _w of - <10,000 g / mol.

The phenol resins which are particularly suitable as polyol component are known by the name "o-o" or "high-ortho" novolaks or benzyl ether resins. These are obtainable by condensation of phenols with aldehydes in weakly acidic medium using suitable catalysts.

Suitable catalysts for the preparation of benzylic ether resins are salts of divalent ions of metals such as Mn, Zn, Cd, Mg, Co, Ni, Fe, Pb, Ca and Ba. Preferably, zinc acetate is used. The amount used is not critical. Typical amounts of metal catalyst are 0.02 to 0.3 wt .-%, preferably 0.02 to 0.15 wt .-%, based on the total amount of phenol and aldehyde.

For the preparation of phenolic resins, all conventionally used phenols are suitable. In addition to unsubstituted phenols, substituted phenols or mixtures thereof can be used. The phenolic compounds are unsubstituted either in both ortho positions or in an ortho and in the para position to allow polymerization. The remaining ring carbon atoms may be substituted. The choice of the substituent is not particularly limited, unless the substituent is the polymerization of the phenol or the aldehyde adversely affected. Examples of substituted phenols are alkyl-substituted phenols, alkoxy-substituted phenols and aryloxy-substituted phenols.

The abovementioned substituents have, for example, 1 to 26, preferably 1 to 15, carbon atoms. Examples of suitable phenols are o-cresol, m-cresol, p-cresol, 3,5-xylene, 3,4-xylene, 3,4,5-trimethylphenol, 3-ethylphenol, 3,5-diethylphenol, p-butylphenol, 3,5-dibutylphenol, p-amylphenol, cyclohexylphenol, p-octylphenol, p-nonylphenol, 3,5-dicyclohexylphenol, p-crotylphenol, p-phenylphenol, 3,5-dimethoxyphenol and p-phenoxyphenol.

Particularly preferred is phenol itself. Also higher condensed phenols, such as bisphenol A, are suitable. In addition, polyhydric phenols having more than one phenolic hydroxyl group are also suitable. Preferred polyhydric phenols have 2 to 4 phenolic hydroxyl groups. Specific examples of suitable polyhydric phenols are pyrocatechol, resorcinol, hydroquinone, pyrogallol, fluoroglycine, 2,5-dimethylresorcinol, 4,5-dimethylresorcinol, 5-methylresorcinol or 5-ethylresorcinol.

Mixtures of various mono- and polyhydric and / or substituted and / or condensed phenolic components can also be used for the preparation of the polyol component.

In one embodiment, phenols of general formula I:

for the preparation of the phenolic resin component, wherein A, B and C independently of one another from a hydrogen atom, a branched or unbranched alkyl radical, which may have, for example 1 to 26, preferably 1 to 15 carbon atoms, a branched or unbranched alkoxy radical, for example 1 to 26, preferably having from 1 to 15 carbon atoms, a branched or unbranched alkenoxy radical which may, for example, have 1 to 26, preferably 1 to 15, carbon atoms, an aryl or alkylaryl radical, such as, for example, bisphenyls.

Suitable aldehydes for the production of the phenolic resin component are aldehydes of the formula:

R-CHO,

wherein R is a hydrogen atom or a carbon atom radical having preferably 1 to 8, particularly preferably 1 to 3 carbon atoms. Specific examples are formaldehyde, acetaldehyde, propionaldehyde, furfuraldehyde and benzaldehyde. It is particularly preferred to use formaldehyde, either in its aqueous form, as para-formaldehyde or trioxane.

In order to obtain the phenolic resins, an at least equivalent number of moles of aldehyde, based on the number of moles of the phenol component, should be used. The molar ratio is preferably Aldehyde to phenol 1: 1.0 to 2.5: 1, more preferably 1.1: 1 to 2.2: 1, particularly preferably 1.2: 1 to 2.0: 1.

The production of the phenolic resin component takes place by methods known to the person skilled in the art. The phenol and the aldehyde are reacted under substantially anhydrous conditions in the presence of a divalent metal ion at temperatures of preferably less than 130 ° C. The resulting water is distilled off. For this purpose, the reaction mixture, a suitable entraining agent may be added, for example toluene or xylene, or the distillation is carried out at reduced pressure.

For the binder of the molding material mixture, the phenol component is reacted with an aldehyde, preferably benzyl ether resins. The reaction with a primary or secondary aliphatic alcohol to an alkoxy-modified phenolic resin in the one-stage or two-stage process ( EP-B-0 177 871 and EP 1 137 500 ) is also possible. In the one-step process, the phenol, aldehyde and alcohol are reacted in the presence of a suitable catalyst. In the two-stage process, an unmodified resin is first prepared, which is subsequently reacted with an alcohol. When using alkoxy-modified phenolic resins, there is no limitation on the molar ratio per se, but the alcohol component is preferably used in a molar ratio of alcohol: phenol of less than 0.25 so that less than 25% of the hydroxymethyl groups are etherified. Suitable alcohols are primary and secondary aliphatic alcohols having one hydroxy group and 1 to 10 carbon atoms. Suitable primary and secondary alcohols are, for example, methanol, ethanol, propanol, n-butanol and n-hexanol. Particularly preferred are methanol and n-butanol.

The phenolic resin is preferably selected so that crosslinking with the polyisocyanate component is possible. For the construction of a network, phenolic resins comprising molecules having at least two hydroxyl groups in the molecule are particularly suitable. The phenolic resin component or the isocyanate component of the binder system is preferably used as a solution in an organic solvent or a combination of organic solvents. Solvents may be required to keep the components of the binder in a sufficiently low viscosity state. This is u.a. required to obtain a uniform crosslinking of the refractory molding material and its flowability.

As solvents for the polyisocyanate or the polyol component of the binder system based on polyurethanes, all solvents which are conventionally used in such binder systems for foundry technology can be used per se. Suitable solvents are, for example, oxygen-rich, polar, organic solvents. Particularly suitable are dicarboxylic acid esters, glycol ether esters, glycol diesters, glycol diethers, cyclic ketones, cyclic esters or cyclic carbonates. Dicarboxylic acid esters, cyclic ketones and cyclic carbonates are preferably used. Dicarboxylic acid esters have the formula R ^a OOC-R ^b -COOR ^a , wherein the radicals R ^{a are} each independently an alkyl group having 1 to 12, preferably 1 to 6 carbon atoms and R ^{b is} an alkylene group, ie a divalent alkyl group, with 1 to 12, preferably 1 to 6 carbon atoms. R ^b may also include one or more carbon-carbon double bonds. Examples are dimethyl esters of carboxylic acids having 4 to 10 carbon atoms, which are available, for example, under the name "dibasic ester" (DBE) from Invista International S.à.rl, Geneva, CH. Glycol ether esters are compounds of the formula R ^c -OR ^d -OOCR ^e , where R ^{c is} an alkyl group having 1 to 4 carbon atoms, R ^{d is} an ethylene group, a propylene group or an oligomeric ethylene oxide or propylene oxide and R ^{e is} an alkyl group having 1 to 3 carbon atoms. Preference is given to glycol ether acetates, for example butylglycol acetate. Glycol diesters accordingly have the general formula R ^e COO-R ^d OOCR ^e , where R ^d and R ^{e are} as defined above and the radicals R ^{e are} each independently selected. Preferred are glycol diacetates such as propylene glycol diacetate. Glycol diethers can be characterized by the formula R ^c -OR ^d -OR ^c , where R ^c and R ^{d are} as defined above and the radicals R ^{c are} each independently selected. A suitable glycol diether is, for example, dipropylene glycol dimethyl ether. Cyclic ketones, cyclic esters and cyclic carbonates of 4 to 5 carbon atoms are also suitable. A suitable cyclic carbonate is, for example, propylene carbonate. The alkyl and alkylene groups may each be branched or unbranched.

The proportion of the solvent in the binder system is preferably not chosen too high, since the solvent evaporates during the production and application of the molded article produced from the molding compound and thus, for example, can lead to unpleasant odors or leads to smoke during the casting. Preferably, the proportion of the solvent in the binder system is less than 50 wt .-%, more preferably less than 40 wt .-%, more preferably less than 35 wt .-%, selected.

To produce the shaped article, the binder is first mixed with the refractory molding material as described above to form a molding material mixture. If the molding is to be produced by the PU-No-Bake process, a suitable catalyst can also already be added to the molding material mixture. For this purpose, liquid amines are preferably added to the molding material mixture. These amines preferably have a pK _b value of 4 to 11. Examples of suitable catalysts are 4-alkylpyridines wherein the alkyl group comprises 1 to 4 carbon atoms, isoquinoline, arylpyridines such as phenylpyridine, pyridine, acryline, 2-methoxypyridine, pyridazine, 3-chloropyridine, quinoline, n-methylimidazole, 4,4'-dipyridine , Phenylpropylpyridine, 1-methylbenzimidazole, 1,4-thiazine, N, N-dimethylbenzylamine, triethylamine, tribenzylamine, N, N-dimethyl-1,3-propanediamine, N, N-dimethylethanolamine and triethanolamine. The catalyst may optionally be diluted with an inert solvent, for example, 2,2,4-trimethyl-1,3-pentanediol diisobutyrate, or a fatty acid ester. The amount of catalyst added is selected in the range of 0.1 to 15% by weight, based on the weight of the polyol component.

The molding material mixture is then introduced by conventional means into a mold and compacted there. The molding material mixture is then cured to a shaped body. When curing, the shaped body should preferably retain its outer shape.

If the curing is to take place by the PU cold box process, a gaseous catalyst is passed through the molded molding material mixture. As catalyst, the usual catalysts in the field of cold-box process can be used. Particular preference is given to using amines as catalysts, in particular preferably dimethylethylamine, dimethyl-n-propylamine, dimethylisopropylamine, dimethyl-n-butylamine, triethylamine and trimethylamine in their gaseous form or as aerosol.

According to a further preferred embodiment, a furan resin or a phenolic resin is used as the binder, wherein the molding material mixture is cured according to the "furan no-bake" method with catalysis by a strong acid.

Furan and phenolic resins show very good disintegration properties during casting. Under the action of heat of the liquid metal, the furan or phenolic resin decomposes and the strength of the mold is lost. After casting, therefore, cores, possibly after prior shaking of the casting, pour out very well from cavities.

The reactive furan resins contained as the first component in "furan no-bake binders" comprise furfuryl alcohol as an essential component. Furfuryl alcohol can react with itself under acid catalysis and form a polymer. For the production of furan no-bake binders is generally not pure furfuryl alcohol used but added to the furfuryl alcohol further compounds which are polymerized into the resin. Examples of such compounds are aldehydes, such as formaldehyde or furfural, ketones, such as acetone, phenols, urea or polyols, such as sugar alcohols or ethylene glycol. The resins may be added with other components that affect the properties of the resin, such as its elasticity. Melamine can be added, for example, to bind free formaldehyde.

Furan no-bake binders are most often prepared by first producing furfuryl-containing precondensates from, for example, urea, formaldehyde, and furfuryl alcohol under acidic conditions. The reaction conditions are chosen so that only a slight polymerization of furfuryl alcohol occurs. These precondensates are then diluted with furfuryl alcohol. Resoles can also be used to prepare furan no-bake binders. Resoles are prepared by polymerization of mixtures of phenol and formaldehyde. These resoles are then diluted with furfuryl alcohol.

The second component of the furan no-bake binder forms an acid. This acid neutralizes alkaline components, which are contained in the refractory molding material and catalyzed on the other hand, the crosslinking of the reactive furan resin.

As acids mostly aromatic sulfonic acids and in some special cases also phosphoric acid or sulfuric acid are used. Phosphoric acid is used in concentrated form, i. used at concentrations greater than 75%. However, it is only suitable for the catalytic curing of furan resins with a relatively high proportion of urea. The nitrogen content of such resins is more than 2.0% by weight. Sulfuric acid can be added as a relatively strong acid starter for the curing of furan resins to weaker acids. During casting, however, a smell typical of sulfur compounds develops. There is also the danger that the casting material absorbs sulfur and influences its properties. Mostly aromatic sulfonic acids are used as catalysts. Because of their good availability and their high acidity especially toluene sulfonic acid, xylylene sulfonic acid and benzenesulfonic acid are used.

Phenolic resins as the second large group of acid-catalyzed curable no-bake binders contain resoles as reactive resin components, ie phenolic resins which have been prepared with an excess of formaldehyde. Phenol resins show a significantly lower reactivity compared to furan resins and require strong sulfonic acids as catalysts. Phenolic resins show a relatively high viscosity, which increases with prolonged storage of the resin. Especially at temperatures below 20 ° C, the viscosity increases sharply, so that the sand must be heated in order to apply the binder evenly on the surface of the grains of sand can. After the phenol no-bake binder has been applied to the refractory molding material, the molding material mixture should be processed as soon as possible, so as not to impair the quality of the molding compound by premature Curing to accept, which can lead to a deterioration in the strength of the molds produced from the molding material mixture. When using phenol no-bake binders, the flowability of the molding material mixture is usually poor. In the production of the mold, the molding material mixture must therefore be carefully compacted in order to achieve a high strength of the mold can.

The preparation and processing of the molding material mixture should be carried out at temperatures in the range of 15 to 35 ° C. If the temperature is too low, the molding material mixture is difficult to process because of the high viscosity of the phenol no-bake resin. At temperatures of more than 35 ° C, the processing time is shortened by premature curing of the binder.

After casting, molding mixtures based on phenol no-bake binders can also be worked up again, in which case mechanical or thermal or combined mechanical / thermal processes can also be used.

An acid is then applied to the free-flowing refractory to obtain an acid-coated refractory molding material. The acid is applied by conventional methods on the refractory molding material, for example by the acid is sprayed onto the refractory molding material. The amount of acid is preferably selected in the range of 5 to 45 wt .-%, particularly preferably in the range of 20 to 30 wt .-%, based on the weight of the binder and calculated as the pure acid, ie without taking into account any used solvent. If the acid is not already in liquid form and has a sufficiently low viscosity to be distributed in the form of a thin film on the grains of the refractory molding material, the acid is dissolved in a suitable solvent. Exemplary solvents are water or alcohols or mixtures of water and alcohol. Especially when using however, the solution is prepared as concentrated as possible in order to minimize the amount of water entrained into the binder or the molding material mixture. For uniform distribution of the acid on the grains, the mixture of refractory molding material and acid is well homogenized.

An acid-curable binder is then applied to the acid-coated refractory molding material. The amount of the binder is preferably selected in the range of 0.25 to 5 wt .-%, particularly preferably in the range of 1 to 3 wt .-%, based on the refractory molding material and calculated as the resin component. As the acid-curable binder, it is possible to use, as such, all acid-curable binders, especially those acid-curable binders which are already customary for the production of molding compounds for the foundry industry. In addition to a crosslinkable resin, the binder may also contain other customary components, for example solvents for adjusting the viscosity or extenders which replace part of the crosslinkable resin.

The binder is applied to the acid-coated refractory molding material and distributed by moving the mixture on the grains of the refractory molding material in the form of a thin film.

The amounts of binder and acid are chosen so that on the one hand sufficient strength of the casting mold and on the other hand a sufficient processing time of the molding material mixture is achieved. For example, a processing time in the range of 5 to 45 minutes is suitable.

The coated with the binder refractory molding material is then formed by conventional methods to a shaped body. For this purpose, the molding material mixture can be introduced into a suitable mold and be condensed there. The resulting molded body is then allowed to cure.

As furan no-bake binder, all furan resins can be used per se, as they are already used in furan no-bake binder systems.

The furan resins used in technical furan no-bake binders are usually precondensates or mixtures of furfuryl alcohol with other monomers or precondensates. The precondensates contained in furan no-bake binders are prepared in a manner known per se.

According to a preferred embodiment, furfuryl alcohol is used in combination with urea and / or formaldehyde or urea / formaldehyde precondensates. Formaldehyde can be used both in monomeric form, for example in the form of a formalin solution, as well as in the form of its polymers, such as trioxane or paraformaldehyde. In addition to or instead of formaldehyde, other aldehydes or ketones can be used. Suitable aldehydes are, for example, acetaldehyde, propionaldehyde, butyraldehyde, acrolein, crotonaldehyde, benzaldehyde, salicylaldehyde, cinnamaldehyde, glyoxal and mixtures of these aldehydes. Formaldehyde is preferred, this being preferably used in the form of paraformaldehyde.

As ketone component, all ketones can be used which have a sufficiently high reactivity. Exemplary ketones are methyl ethyl ketone, methyl propyl ketone and acetone, with acetone being preferred.

The said aldehydes and ketones can be used as a single compound but also in admixture with each other.

The molar ratio of aldehyde, in particular formaldehyde, or ketone to furfuryl alcohol can be selected within wide ranges. In the preparation of the furan resins, 0.4 to 4 moles of furfuryl alcohol, preferably 0.5 to 2 moles of furfuryl alcohol, may be used per mole of aldehyde.

To produce the precondensates, furfuryl alcohol, formaldehyde and urea can be heated to boiling, for example, after the pH has been adjusted to more than 4.5, water being continuously distilled off from the reaction mixture. The reaction time can be several hours, for example 2 hours. Under these reaction conditions occurs almost no polymerization of furfuryl alcohol. However, the furfuryl alcohol is condensed into a resin together with the formaldehyde and the urea.

According to an alternative method furfuryl alcohol, formaldehyde and urea are reacted at a pH of well below 4.5, for example at a pH of 2.0, in the heat, wherein the water formed in the condensation are distilled off under reduced pressure can. The reaction product has a relatively high viscosity and is diluted with furfuryl alcohol to produce the binder until the desired viscosity is achieved.

Mixed forms of these production methods can also be used.

It is also possible to introduce phenol into the precondensate. For this purpose, the phenol can be reacted under alkaline conditions, first with formaldehyde to a resole resin. This resol can then be reacted or mixed with furfuryl alcohol or a furan group-containing resin. Such furan group-containing resins can be obtained, for example, by the methods described above. For the production of the precondensate It is also possible to use higher phenols, for example resorcinol, cresols or also bisphenol A. The proportion of phenol or higher phenols in the binder is preferably in the range of up to 45% by weight, preferably up to 20% by weight, especially preferably chosen up to 10 wt .-%. According to one embodiment, the proportion of phenol or higher phenols can be greater than 2 wt .-%, according to a further embodiment greater than 4 wt .-% can be selected.

It is also possible to use condensates of aldehydes and ketones, which are then mixed with furfuryl alcohol to produce the binder. Such condensates can be prepared by reacting aldehydes and ketones under alkaline conditions. The aldehyde used is preferably formaldehyde, in particular in the form of paraformaldehyde. The ketone used is preferably acetone. However, other aldehydes or ketones can also be used. The relative molar ratio of aldehyde to ketone is preferably selected in the range of 7: 1 to 1: 1, preferably 1.2: 1 to 3.0: 1. The condensation is preferably carried out under alkaline conditions at pH values in the range of 8 to 11.5, preferably 9 to 11. A suitable base is, for example, sodium carbonate.

The amount of furfuryl alcohol, which is contained in the furan no-bake binder, on the one hand determined by the endeavor to keep the proportion as low as possible for cost reasons. On the other hand, an improvement in the strength of the casting mold is achieved by a high proportion of furfuryl alcohol. However, with a very high proportion of furfuryl alcohol in the binder, very brittle casting molds are obtained which are difficult to process. Preferably, the proportion of furfuryl alcohol in the binder in the range of 30 to 95 wt .-%, preferably 50 to 90 wt .-%, particularly preferably 60 to 85 wt .-% is selected. The proportion of urea and / or formaldehyde on the binder is preferably in the range of 2 to 70 wt .-%, preferably 5 to 45 wt .-%, particularly preferably 15 to 30 wt .-% selected. The proportions include both the unbound portions of these compounds contained in the binder and those bound in the resin.

Other additives can be added to the furan resins, such as ethylene glycol or similar aliphatic polyols, for example, sugar alcohols, such as sorbitol, which serve as an extender and replace part of the furfuryl alcohol. Too high an addition of such extenders can lead to a reduction in the strength of the mold and lowering the reactivity in the worst case. The proportion of these extenders in the binder is therefore preferably less than 25% by weight, preferably less than 15% by weight and more preferably less than 10% by weight. In order to achieve a cost saving without having to put an excessive influence on the strength of the mold, the proportion of extenders is chosen according to an embodiment greater than 5 wt .-%.

The furan no-bake binders may further contain water. However, since water slows the curing of the molding material mixture and water is formed as a reaction product during curing, the proportion of water is preferably chosen as low as possible. The proportion of water in the binder is preferably less than 20% by weight, preferably less than 15% by weight. From an economic point of view, an amount of water of more than 5% by weight in the binder can be tolerated.

As phenolic resins resoles are used in the process according to the invention. Resoles are mixtures of hydroxymethylphenols which are linked via methylene and methylene ether bridges and by reaction of aldehydes and phenols in a molar ratio of 1: <1, if appropriate in the presence of a catalyst, for example a basic Catalyst, are available. They have a molecular weight M _w of ≦ 10,000 g / mol.

For the preparation of the phenolic resins, all conventionally used phenols are suitable, with phenol being particularly preferred. The aldehyde component used is preferably formaldehyde, in particular in the form of paraformaldehyde. Alternative phenols and aldehydes have already been explained in connection with the polyurethane binders. The corresponding passages are referred to.

The binders may contain other customary additives, for example silanes as adhesion promoters. Suitable silanes are, for example, aminosilanes, epoxysilanes, mercaptosilanes, hydroxysilanes and ureidosilanes, such as γ-hydroxypropyltrimethoxysilane, γ-aminopropyltrimethoxysilane, 3-ureidopropyltriethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, β- (3,4-epoxycyclohexyl) trimethoxysilane, N-β- (aminoethyl) -γ-aminopropyltrimethoxysilane.

If such a silane is used, it is added to the binder in a proportion of 0.1 to 3% by weight, preferably 0.1 to 1% by weight.

The binders may also contain other conventional components, such as activators or plasticizers.

The molding material mixture, in addition to the refractory molding material, the binder and optionally the catalyst may contain other conventional ingredients. Exemplary further constituents are iron oxide, ground flax fibers, wood flour granules, ground coal or clay.

The molding material mixture is then formed by conventional methods into a casting mold or a part of a casting mold and optionally cured. After the casting mold has optionally been assembled and the mold cavity provided in the casting mold has possibly been coated with a size, a layer of a substance absorbing harmful substances is applied to the gas outlet surfaces, particularly preferably the top of the casting mold, at least in sections. In this case, all conventional methods for applying such coating compositions can be used per se. The coating can be applied with a brush on the top of the mold or be sprayed by means of a suitable device. Likewise, the coating composition can be poured onto the top of the casting mold and, if necessary, excess coating compound can be drained off.

The coating composition can then be dried if necessary. However, the layer preferably still has a water content in the range from 0 to 60% by weight, preferably 5 to 30% by weight, particularly preferably 10 to 20% by weight.

According to a preferred embodiment, a layer of a porous carrier framework is first applied to the upper side of the casting mold. As already explained, such a carrier framework can be formed, for example, by a solid foam, a woven fabric or a fleece. Suitable materials have already been described above. The thickness of the porous support framework is preferably selected in the range of 0.5 to 5 cm. The porous carrier framework placed on top of the casting mold is then given a coating composition as described above, so that the porous carrier framework is impregnated with the coating composition. Subsequently, the layer can be dried if necessary.

Alternatively, the porous carrier material may also be first coated with the coating composition and then the coated porous carrier material may be attached at least in sections to gas outlet surfaces of the casting mold.

Another object of the invention relates to the use of the above-described mold, which according to the invention comprises an absorber layer, as described above, for metal casting, in particular iron and steel casting.

The invention will be explained in more detail with reference to examples.

Determination of bulk density

A graduated cylinder cut off at the 1000 ml mark is weighed. Then, the sample to be examined is filled by means of a Pulvertrichters so in a train in the measuring cylinder that forms above the end of the measuring cylinder, a pour cone. The pour cone is removed by means of a ruler, which is led over the opening of the measuring cylinder, and the filled measuring cylinder is weighed again. The difference corresponds to the bulk density.

Example 1:

100 parts by weight of quartz sand H 32 (Quarzwerke Frechen, DE) were mixed in a mixer with 0.4 part by weight of hardener. For even distribution of the hardener, the mixture was agitated for one minute. Subsequently, 1.0 part by weight of furan resin was added and the mixture was agitated for another minute. From the obtained molding material mixture was prepared as a specimen an upwardly open tubular mold with a bottom. The mold had an inner diameter of 5 cm, a wall thickness of 5 cm and a height of 30 cm. The composition of the investigated molding material mixture is summarized in Table 1. Table 1: Composition of the molding material mixture Molding mixture Quartz sand H 32 100 GT Methanesulfonic acid (70%) / lactic acid (80%) = 50:50 0.4 GT Furfuryl alcohol-urea resin ^a 1.0 GT ^a : Askuran EP 3576, Ashland-Südchemie-Kernfest GmbH, Hilden, DE

Further, from the components listed in Table 2, a coating composition was prepared by first charging the water and then adding the clay and digested for 15 minutes using a high shear stirrer. Subsequently, the absorbing components, pigments and dyes were stirred for an additional 15 minutes until a homogeneous mixture was obtained. Table 2: Composition of the coating composition component Proportion (% by weight) calcium carbonate 15.00 Aluminum silicate (coarse) 30.00 activated carbon 5.00 clay mineral 3.00 lime 7.00 defoamers 0.20 biocide 0.20 water 39, 60

The side surfaces of the sample body were coated with the coating composition by means of a brush, whereby a layer having a thickness of 2.5 mm was obtained. The top surface of the sample body was ^® with a gas-impermeable size (KERATOP V 107G, ASK Chemicals, Hilden DE). The specimen was then dried for a maximum of 30 minutes at room temperature.

Subsequently, 4.3 kg of liquid iron was poured into the sample body (casting temperature 1400 ° C). The weight ratio of sample and iron was about 1: 1.

The sulfur content of the coating composition before and after casting was determined by infrared spectroscopy.

The benzene content of the coating material before and after the casting was determined quantitatively and qualitatively by gas chromatography in accordance with DIN EN 14662-2. The values determined are summarized in Table 3. Table 3: Sulfur and benzene content of the coating composition component salary before the casting after the casting Sulfur (wt%) 0.028 2.26 Benzene (mg / kg) 0, 035 0.28

The coating composition had a significantly increased content of sulfur and benzene after casting.

Example 2

Comparable measurements were also made in an iron foundry under field conditions. For this purpose, a casting with a weight of about 250 kg (casting temperature about 1400 ° C) was produced. The weight ratio of molding material mixture to iron was about 4: 1. The composition of the for the preparation The molding material mixture used in the casting mold is summarized in Table 4. Table 4: Composition of the molding material mixture Molding mixture Quartz sand H31 100 GT Methanesulfonic acid (70%) / lactic acid (80%) = 50:50 0.35 GT Furfuryl alcohol-urea resin ^a 0.80 GT

One cast was produced with an uncoated casting mold (system 1) and a casting with a casting mold, the top of which was coated with a 2.5 mm thick layer of the coating composition prepared as described above (system 2).

The exhaust gases produced during casting were captured via a suction hood. From the exhaust gas stream, a defined partial stream was sucked off via a sampling probe and the substances contained in the partial stream were adsorbed on activated carbon in accordance with the method according to DIN EN 14662-2. The qualitative and quantitative analysis of the adsorbed substances (benzene, toluene and xylene) was carried out by gas chromatography.

For the determination of the sulfur dioxide content, a partial flow was removed from the exhaust gas and sucked with a vacuum device into a PE bag. The concentration of sulfur dioxide was determined by mass spectrometry.

Furthermore, the gas samples were examined by trained subjects on their odor intensity.

The values determined in the exhaust gases are summarized in Table 5. Table 5: Pollutant levels in the exhaust gases released during casting Sulfur (wt%) System 1 (not according to the invention) System 1 (according to the invention) Odor units (GE / m ³ ) 34700 27000 Benzene (mg / kg) 24.2 6.2 Toluene (mg / kg) 22.0 10.3 Xylene (mg / kg) 0, 6 <0.5 SO ₂ (ppm Vol) 24.5 16.9

By the coating composition, a significant reduction of the odor nuisance and the pollutants in the exhaust gas can be achieved.

Claims (15)

1. A casting mould for the casting of metals, wherein a layer permeable to gas of a material which absorbs harmful substances is arranged on at least sections of gas outlet surfaces of the outer surface of the casting mould, wherein harmful substances are defined to be substances contained in the gas which is released during casting process and are environmentally harmful or harmful to health or strong smelling.
2. The casting mould according to claim 1, wherein the layer of the material which absorbs harmful substances has a thickness of at least 2.5 mm, preferably of at least 5 mm.
3. The casting mould according to claim 1 or 2, wherein the layer of the material which absorbs harmful substances comprises at least one physical adsorber material which can physically adsorb harmful substances.
4. The casting mould according to claim 3, wherein the physical adsorber material has a specific surface area of at least 800 m²/g.
5. The casting mould according to claim 3 or 4, wherein the physical adsorber material is selected from activated charcoal, silica gel, clays digested by an acid, ashes, cellulose, rayon staple.
6. The casting mould according to any one of the preceding claims, wherein the layer of the material which absorbs harmful substances contains at least one chemical absorber material, which can bind or decompose harmful materials by a chemical reaction.
7. The casting mould according to claim 6, wherein the chemical absorber material is a basic material.
8. The casting mould according to claim 7, wherein the basic material is selected from oxides, hydroxides and carbonates of the alkali metals and alkaline-earth metals.
9. The casting mould according to any one of the preceding claims, wherein the layer of the material which absorbs harmful substances contains at least one refractory material which has an average grain size of at least 100 µm.
10. The casting mould according to any one of the preceding claims, wherein the layer of the material which absorbs harmful substances comprises a porous supporting framework.
11. A process for producing a casting mould according to any one of claims 1 to 10, wherein
    a moulding material mixture comprising at least one refractory moulding material and at least one binder is prepared;
    the moulding material mixture is moulded into a casting mould and
    gas outlet surfaces of the casting mould are covered at least in sections with a layer of a material which absorbs harmful substances, wherein harmful substances are defined to be substances contained in the gas which is released during casting process and are environmentally harmful or harmful to health or strong smelling.
12. The process according to claim 11, wherein the layer of the material which absorbs harmful substances is provided by applying a porous supporting framework to at least sections of the gas outlet surfaces of the casting mould and the porous supporting framework is coated with a coating composition for absorbing harmful substances for the coating of metal casting containing at least one material which absorbs harmful substances.
13. The process according to claim 11 or 12, wherein the coating composition comprises a carrier liquid and has a solid content of 20 to 80 % by weight, preferably of 30 to 50 % by weight.
14. The process according any one of claims 11 or 12, wherein the at least one binder is an organic binder.
15. Use of a casting mould according to any one of claims 1 to 10 for the casting of metals.