Aerogel granulate for use in foundry technology

Abstract

The present invention relates to an additive for sand casting, containing a carbon aerogel granulate, to the use of a carbon aerogel as an additive for sand casting, to the production thereof, to a foundry core comprising sand, a binder and a carbon aerogel granulate as an additive, and to the production and use of the foundry core.

Description

[0001] Aerogel granulate for use in foundry

[0002] The present invention relates to an additive for sand casting, containing a carbon aerogel granulate, the use of a carbon aerogel granulate as an additive for sand casting and its production, as well as a foundry core comprising sand, a binder and a carbon aerogel granulate as an additive, its production and use.

[0003] In sand casting, molds and cores are usually made from quartz sand, but for special applications also from other sands (aluminum oxide, zirconium oxide, olivine, chromium ore). In this sand casting, the grains are bonded together by (polymer) binders, forming a dimensionally stable bond for the duration of the mold filling with liquid metal. This bond should be able to be dissolved as easily as possible after the melt has solidified, which is particularly important for cores that negatively represent complexly shaped cavities in the casting. Mechanical aids (vibrating, shaking, tapping), thermal aids, or pressurized water can be used to core or dissolve the mold. Phenolic resins, polyurethanes, ureas, and furan resins, which are complexly chemically modified (chemical additives) to meet the requirements of the foundry, are also used as binders. Aerogel binders are also known.The chemical composition of the binders is optimized for the applications in order to meet contradictory requirements, such as high thermal stability with low outgassing and low binder usage, yet easy core removal and high surface quality.

[0004] The current state of the art continues to use special additives to produce high-quality castings (especially to prevent veining). These can be both organic and inorganic materials. However, these additives also have various disadvantages. These include, for example, high emissions (e.g., benzene, formaldehyde, and phenol) and a required coating process to achieve a high-quality surface on the casting. This ceramic coating also requires energy-intensive drying and, if not used, results in increased cleaning effort on the cast component.

[0005] EP 2 193 858 B1 and EP 2 204 246 B1 disclose foundry cores with improved decoring properties, processes for their production, and their use. The foundry cores contain sand, binder, and hydrophobic resorcinol-formaldehyde aerogel granules, which pyrolyze during the casting process to form a material with unknown properties. The presence of organic components will lead to outgassing.

[0006] The aerogels further described in the prior art are essentially based on inorganic aerogels, particularly oxide aerogel granules comprising SiO2, TiO2, and / or ZrO2, as well as organic aerogels based on phenolic resins such as resorcinol-formaldehyde. Organic aerogels can, in principle, be converted into carbon aerogels by pyrolysis. Carbon aerogels could represent a particularly interesting class of materials for use as aerogel binder additives for foundry cores. The use of aerogels as a binder additive generally leads to a simplification of the manufacturing process and lower emissions while maintaining consistent quality. Carbon aerogels offer advantages over other aerogels. Core production with carbon aerogels is particularly simple ("self-lubricating"), resulting in less core waste.Coating can be reduced or partially eliminated, potentially eliminating a process step. The use of carbon aerogels could lead to a significant reduction in emissions. Studies show that the use of carbon aerogels maintains the desired microstructure in the metal or improves the surface layer. However, it has been shown that foundry cores containing carbon aerogels cannot easily be maintained with sufficient stability.

[0007] The present invention is therefore based on the object of providing carbon aerogel-based additives for sand casting that avoid the disadvantages of the prior art. In particular, the aim is to provide carbon aerogel-based additives for sand casting that can produce stable and easily processable foundry cores and reduce the formation of harmful gases during casting.

[0008] In a first embodiment, the object of the invention is achieved by an additive for sand casting, comprising a carbon aerogel granulate with a grain size in a range of 100 to 800 μm and an external surface of less than 40 m 2 / g of the carbon aerogel granules. In a preferred embodiment, the additive can consist of the carbon aerogel granules.

[0009] Surprisingly, it has been shown that a stable foundry core based on a carbon aerogel can only be obtained if the carbon aerogel granules have the aforementioned grain size and external surface area. Outside these parameter ranges, such foundry cores are practically impossible to process. The additives according to the invention for sand casting can reduce the amount of coating required, minimize cleaning effort, and lower emissions. Reducing the coating process also results in less material usage, shorter throughput times, and lower energy requirements. Nevertheless, the core manufacturing process remains reliable, and the quality of the cast component remains stable.

[0010] Carbon aerogel in the sense of the present invention is understood in particular to be an aerogel that can be produced by pyrolysis of an organic aerogel, for example based on phenolic resins, in particular resorcinol-formaldehyde. The microscopic and macroscopic properties of the organic aerogels change in the process, particularly due to shrinkage. A suitable carbon aerogel can be obtained using all processes known in the art for producing carbon aerogels. It is crucial in the sense of the present invention that the carbon aerogel is in the form of granules with a grain size in the range of 100 to 800 pm and an external surface of less than 40 m 2 / g. Such aerogel granulate can be obtained, for example, by grinding and sieving a monolithic aerogel. The grain size can be adjusted by fractionating the sieved material. However, the present invention is not limited to grinding and sieving. Alternatively, such aerogel granulate can also be obtained, for example, by emulsion polymerization, jet cutter processes, spraying processes, or, more generally, by droplet processes, or in tubular reactors.

[0011] Carbon aerogel granules can be produced, for example, by milling and sieving an organic aerogel followed by pyrolysis of the resulting granules. In this case, it is important to note that the material may shrink during pyrolysis, so the fractionation of the organic aerogel must be adjusted accordingly. Alternatively, a monolithic organic aerogel granule can first be converted into carbon aerogel granules by pyrolysis, which are then milled and sieved. However, it is preferable to first fractionate an organic aerogel and then perform pyrolysis on the resulting granules. Carbon aerogels are generally more brittle than organic aerogels, so milling and sieving to a predefined grain size is somewhat less successful.

[0012] A grain size in the range of 100 to 800 μm means, in the context of the present invention, that essentially all of the grains used in the aerogel granulate have a size in this range. In particular, the additive essentially comprises no grains with a size above 800 μm or below 100 μm. If the aerogel granulate contains grains with a size above 800 μm or below 100 μm in a significant proportion, it is not suitable as an additive for sand casting. Corresponding foundry cores produced using such an additive do not have the required processability but disintegrate. Grain sizes smaller than 100 μm have also proven to be more difficult to process because they have a dusty consistency. However, the aerogel granulate may contain insignificant proportions with a grain size above 800 μm or below 100 μm. Such an additive is also to be regarded as being in accordance with the invention.An insignificant proportion is, in particular, a proportion of less than 20 wt.%, preferably less than 10 wt.%, particularly preferably less than 5 wt.%, very particularly preferably less than 1 wt.%, 0.1 wt.%, or 0.01 wt.%. However, the aerogel granules can also be virtually free of grains above 800 μm or below 100 μm. If the aerogel granules are obtained by sieving, the grain size can be adjusted very precisely, and grain sizes outside the range of 100 to 800 μm can be practically eliminated.

[0013] Preferably, the aerogel granulate contains essentially exclusively grains with a size of more than 350 pm, more preferably more than 400 pm, most preferably more than 450 pm. Preferably, the aerogel granulate contains essentially exclusively grains with a size of less than 650 pm, more preferably less than 600 pm, most preferably less than 550 pm.

[0014] The particle size of ground aerogel granules can be determined directly on the powder using the principle of static light scattering. Interactions between the incoming light beam and the particles create characteristic angle-dependent patterns through diffraction, refraction, reflection, or absorption. The scattering angle and intensity depend on the size of the particles involved. Light is scattered more strongly and at smaller angles by large particles (>> wavelength of the incident light) than by small particles. The particle size distribution is determined from the resulting scattering patterns using either the Fraunhofer (> 5 pm) or Mie theory (< 5 pm). Mie theory assumes that the particles can be described as quasi-translucent spheres. This means that light penetrates the material and is elastically scattered by the atoms of the particle.For evaluation, knowledge of optical material constants such as the refractive index is therefore required.

[0015] The measurement can be performed by passing dry powder through the laser beam using compressed air from a vibrating feeder (the vibration intensity of which can be adjusted to determine the amount of powder transported). The angle and intensity of the light scattered by the particles are measured, and an algorithm converts the scattered light data into particle size information.

[0016] The required external surface of less than 40 m 2 / g refers to the carbon aerogel granules as a whole and not to each individual grain. If the external surface area is above this value, no processable foundry core will be obtained using the additive. Preferably, the external surface area is below 38 m 2 / g, particularly preferably below 35 m 2 / g. The external surface is preferably at least 1 m 2 / g, particularly preferably at least 17 m 2 / g, most preferably at least 30 m 2 / G.

[0017] The required external surface area can be adjusted by selecting the synthesis parameters (reactant concentrations, pH value, amount and type of catalyst, temperature, stirring and drying conditions, etc.) in a coordinated manner.

[0018] The external surface area, together with the specific surface area and a pore size distribution, can be determined by nitrogen sorption. Nitrogen is passed over a defined amount of the aerogel to be tested and physisorbs on the surface of the material. After a certain time, a dynamic equilibrium is established, depending on temperature, pressure, and adsorbed amount. An adsorption isotherm describes the dependence of the adsorbed amount on the partial pressure in the gas phase at constant temperature. By recording the gas pressure, the amount of adsorptive adsorbed on the surface can be determined. Since the sample chamber is cooled with liquid nitrogen, the process takes place below the saturation vapor pressure, preventing condensation of the test gas in the sample chamber. The defined addition of nitrogen continues as long as an equilibrium pressure is established.The sample chamber is then gradually evacuated again so that the adsorptive detaches from the surface again (desorption). This results in adsorption-desorption isotherms, which can be evaluated and interpreted using various methods. The specific surface area can be determined using the BET method. It is assumed that, at a monolayer coverage (in the pressure range of 0.05 < p / po < 0.35), the amount of adsorbed nitrogen is proportional to the specific surface area of ​​the sample. The gas volume V can be determined using the BET equation. m of the monolayer. V a is the adsorbed volume of gaseous nitrogen, po is the saturation pressure, p is the measured pressure and C is an empirical constant:

[0019] Using the volume of the monolayer Vm thus obtained, the specific surface area SBET can be calculated:

[0020] Am = area occupied by a nitrogen molecule at 77 K on a surface

[0021] NA = Avogadro number

[0022] Mv = molar volume N2 m = mass of the sample

[0023] The pore size distribution can be calculated automatically using the BJH method. According to this method, the pore size is determined from the Kelvin pore radius RK and the thickness t of the monolayer at the pore walls:

[0024] The Kelvin pore radius RK can be determined from the Kelvin equation (4):

[0025] And with equation (5) the layer thickness of the monolayer at the pore walls:

[0026] By evaluating the t-plot method, the adsorption processes in the individual pore types can be differentiated from one another. This makes it possible to divide the specific surface area of ​​a porous material into the outer, external particle surface area (Sextem) and the inner surface area of ​​the micro- or mesopores (S / ntem). The t-plot method is based on a mathematical formulation for the multilayer adsorption of an adsorptive on a non-porous material. The statistical thickness t of the adsorbate layer is described as a function of the partial pressure p / po. If no micropores are present in the material, the surface area calculated in this way should equal the specific surface area value obtained using the BET method. If micropores are present, the corresponding micropore surface area can be determined by the difference between the BET specific surface area and the external surface area obtained using the t-plot method.

[0027] In preparation for the measurement, the samples can be thermally pretreated in vacuum to remove possible adsorbed solvent and water residues, in particular for 12 h at 200 °C.

[0028] The carbon aerogel granules preferably have a pore size in the range of 0.3 to 2 nm. Pore sizes in the range of 0.3 to 1 nm are particularly preferred. If the pore size is above this range, more binder is required, and stability decreases with a constant amount of binder.

[0029] The carbon aerogel granules preferably have a specific surface area in a range of 500 to 600 m 2 / g. Specific surface areas in the range of 525 to 575 m 2 / g, most preferably 550 m 2 / G.

[0030] The carbon aerogel granules preferably have a bulk density in a range of 0.28 to 0.51 g / cm 3Bulk densities in the range of 0.30 to 0.38 g / cm 3 . The apparent density, also called bulk density or enveloping density, is the density of a porous solid based on its volume including the pore spaces.

[0031] The carbon aerogel granules preferably have a skeletal density in a range of 1.89 to 2.34 g / cm 3 Particularly preferred are skeletal densities in a range of 1.98 to 2.27 g / cm 3. The true density, also called skeletal density, describes the density of the material part of a body, i.e., without taking into account the volume of any voids it may contain. A sand pycnometer can be used to determine the bulk density or the enveloping density, applying the principle of volume displacement. The solid medium DryFlo, a quasi-liquid consisting of a narrow distribution of small, rigid spheres with high fluidity, can serve as the displacement medium. The DryFlo particles achieve a dense packing around the sample under investigation, but do not penetrate into any pore space that may be present.

[0032] The sample chamber, for example, a precision piston with a diameter of 2.54 cm and a maximum length of 4 cm, is initially filled with DryFlo to determine the zero volume. Using a controlled compression process, the powder is compressed by a piston to a defined force (e.g., 51 N). The travel of the piston is recorded. The weighed sample is then added, and the process is repeated. The sample volume can be calculated from the varying travel lengths of the piston during compression.

[0033] VP = volume of the sample r = radius of the piston ho = path length of the piston at zero volume measurement hp = path length of the piston at sample measurement

[0034] The density pßuik of the material under investigation is then calculated from the mass m of the material and the measured volume VP according to

[0035] To increase measurement accuracy and reproducibility, a sample to displacer ratio of 75 / 25 vol% is preferred and both the zero volume measurement and the sample measurement are repeated 10 times each.

[0036] A helium pycnometer can be used to determine both the skeletal density of porous materials and the true density of powders. This is a non-destructive technique based on the principle of volume displacement by the solid being measured. The inert gas helium serves as the displacement agent because it barely interacts with the surfaces of the materials and its molecules are so small (diameter: ~1 Å) that they can penetrate even porous materials. The weighed sample is placed in a sample crucible in a chamber of known volume and sealed. The sample chamber is connected to a reference chamber (also of known volume) via a valve. The sample chamber is then filled with helium, and the pressure is recorded. By opening the valve between the chambers, the gas expands into the reference chamber.The difference between the pressure when filling the sample chamber and the pressure when expanding into the second empty chamber is measured and allows the volume of the solid phase sample to be calculated.

[0037] VP = volume of the sample

[0038] VK = volume of the chamber

[0039] VR = volume of the reference chamber

[0040] Pi = inlet pressure p e = final pressure

[0041] The skeletal density pskeiett is then calculated from the mass m of the material and the measured volume VP according to equation (7).

[0042] The crucible, with a diameter of 14.44 mm and a height of 9 mm, is filled to its maximum capacity whenever possible to prevent measurement inaccuracies. Each measurement is repeated 10 times. The measurement takes 30–40 minutes.

[0043] The carbon aerogel granules preferably have a porosity in the range of 75% to 90%. Porosities in the range of 78% to 88% are particularly preferred, and 80% to 88% are most preferred.

[0044] Porosity is a dimensionless measurement and represents the ratio of void volume to the total volume of a substance or mixture of substances. It serves as a classifying measure for the actual voids present. Porosity has a significant influence on the resistance to flow through a porous solid. The porosity of the material under investigation is calculated from the ratio of bulk density to skeletal density according to equation (9):

[0045] The primary particles of the aerogel preferably have a diameter of 4 to 10 pm. This parameter describes the size of the primary particles that essentially make up the microstructure of the aerogel. The diameter can be determined graphically by evaluating scanning electron images of the aerogels.

[0046] In a further embodiment, the object of the invention is achieved by a process for producing an additive according to the invention, wherein the following steps are carried out: a. Synthesis of an organic aerogel monolith b. Grinding of the aerogel monolith to form aerogel granules c. Sieving of the aerogel granules, wherein the aerogel monolith before step b) or the aerogel granules after step c) are converted into a carbon aerogel by pyrolysis and are sieved in step c) such that the resulting carbon aerogel granules have a grain size in a range of 100 to 800 pm.

[0047] Alternatively, it is possible to directly produce corresponding carbon aerogel granules of the mentioned grain size and, if applicable, the mentioned surface area, thus avoiding the synthesis, grinding and sieving of a monolith.

[0048] If pyrolysis occurs before grinding and sieving, the carbon aerogel granules can be fractionated directly to the inventive particle size of 100 to 800 pm. If pyrolysis occurs after grinding and sieving, a larger particle size must be selected for fractionation of the organic aerogel, as pyrolysis causes shrinkage of the order of 10 to 25%.

[0049] In a further embodiment, the object of the invention is achieved by the use of a carbon aerogel granulate, wherein the carbon aerogel granulate has a grain size in a range of 100 to 800 pm and an external surface of less than 40 m 2 / g, as an additive for sand casting.

[0050] The carbon aerogel granules used according to the invention can in principle have all the previously described properties of the carbon aerogel granules contained in the additive according to the invention.

[0051] In a further embodiment, the object of the invention is achieved by a foundry core containing sand, binder and the additive according to the invention, comprising the carbon aerogel granulate according to the invention.

[0052] The foundry core according to the invention is surprisingly characterized by the fact that the use of carbon aerogel as a binder additive simplifies the manufacturing process and results in lower emissions while maintaining consistent quality. Core production is particularly easy with carbon aerogels, resulting in less core waste. Coating can be reduced or even partially eliminated, potentially eliminating a process step. The use of carbon aerogels leads to a significant reduction in emissions.

[0053] Any sand known in the art suitable for use in foundry cores can be used as the sand. The sand preferably comprises quartz sand, an Al2O3-based sand, and / or a mullite-based sand. The sand may contain, for example, SiC, Al2O3, and / or Fe2C3.

[0054] The sands that can be used include the new quartz sands commercially available in Germany from the following origins with the following average grain size in mm:

[0055] Dorsten 0.84 mm (grade D020), 0.56 mm (D030), 0.39 mm (D040), 0.13 mm (DO110);

[0056] Frechen 0.32 mm (grade F31), 0.23 mm (F32), 0.22 mm (F33), 0.20 mm (F34), 0.18 mm (F35), 0.16 mm (F36); Gambach 0.37 mm (grade G30), 0.29 mm (G31), 0.23 mm (G32), 0.21 mm (G33), 0.19 mm (G34);

[0057] Holders 0.36 mm (grade H31), 0.32 mm (H32), 0.26 mm (H33), 0.21 mm (H34) and 0.19 mm (H35).

[0058] As an alternative to the quartz sands mentioned, corundum sands of a similar size (0.1 to 0.9 mm) can also be used.

[0059] The quartz sands shown above are new sands; in fact, they are only added to the "used sand" in foundries in small quantities. Used sand is the sand that accrues when castings are emptied from the molds. After appropriate cooling and reprocessing, it is returned to the molding shop. Reprocessing has two tasks: cleaning the quartz grain of any adhering binders and removing dust-like components. During this process, any remaining agglomerates are mechanically crushed, thus partially removing the binder shells from the quartz grains. During this process, the originally rounded surface of the sand grain undergoes a change. From round, it becomes splintery. This grain shape is important for the molding material bonding process; it ensures that only a comparatively small amount of binder is required.

[0060] The mixture from which the foundry core is produced preferably contains a sand content of 83 to 95 wt.%, with up to 95 or 100 wt.% new sand, or alternatively 1 to 20 wt.% new sand and 80 to 99 wt.% reclaimed sand (recycled molding material, i.e., purified, reused sand) being preferred. The addition of reclaimed sand may be omitted if necessary, particularly for red, brass, and bronze castings. The binder content is preferably 1 to 10 wt.%. The sand, binder, and aerogel granulate content (and, if applicable, the proportions of other ingredients) add up accordingly to 100 wt.% or vol.%.

[0061] The proportion of aerogel granules in the foundry core is preferably in a range from 0.1 to 2.0 wt. %, particularly preferably from 0.4 to 1.2 wt. %, in particular from 0.5 to 0.75 wt. %. The binder is preferably an organic binder, in particular a binder or binder mixture comprising at least one member selected from phenolic resins, urea resins, furan resins, polyurethane resins, resorcinol-formaldehyde resins, and RF aerogel binders. Organic binders have proven to be preferred because carbonization of the binder occurs during casting, which further facilitates core removal and reduces mineralization.

[0062] In a further embodiment, the object of the invention is achieved by a method for producing a foundry core according to the invention, wherein the following steps are carried out: a. Mixing the carbon aerogel granulate according to the invention with sand and binder, b. Introducing the mixture into a negative mold of the core, optionally followed by compacting the mixture, c. Curing the binder, and d. Removing the core from the negative mold.

[0063] Compaction is achieved, for example, by core shooting, vibrating, tapping, and / or tamping. When using a cold-box binder, curing can occur immediately after shooting at room temperature. Temperatures of 20 to 300 °C, especially 80 to 250 °C, have proven particularly suitable for curing hot-box binders. Curing preferably takes a few seconds to minutes. Drying of the foundry cores is either completed after curing or occurs by storing the cores at room temperature, at temperatures above room temperature up to 300 °C for 1 to 24 hours, or in a microwave.

[0064] In a further embodiment, the object underlying the invention is achieved by the use of the foundry core according to the invention in metal casting, in particular in non-ferrous metal, light metal, or iron casting. In particular, the core can be removed after the melt has solidified, for example by thermal treatment at elevated temperature, in particular a temperature of > 350 °C, or mechanically (vibration, ultrasound, tapping). Removal by thermal treatment is advantageous because the core decomposes without leaving any residue. Errors in dimensional accuracy that arise due to core expansion during quartz cracking when using quartz sand during casting can be compensated for by the elasticity of the granules used, depending on the granule proportion and binder content.

[0065] Alternatively, the core can be removed after the melt has solidified, for example using a wetting fluid, particularly water. Removal with a wetting fluid is advantageous because the core decomposes without leaving any residue. Well-wetting fluids such as water are particularly suitable for this purpose. Wettability describes the ability of liquids to spread over a surface; the better the wettability, the smaller the contact angle that occurs during wetting. Surfaces are also referred to as (incompletely) wettable if the contact angle with the surface is up to 90°. The higher the temperature of the wetting fluid, the easier the cores can be removed. Fluids with a temperature of 30 to 100 °C are therefore particularly preferred.This exploits the fact that hydrophilic silica aerogels can be easily destroyed by well-wetting liquids (e.g. boiling water).

[0066] Alternatively, the core can also be destroyed by alcoholic fluids or short-chain alcohols, especially those with a chain length of up to six carbon atoms. To avoid the risk of fire, non-flammable alcohol mixtures, for example, with water, should be used.

[0067] Implementation examples

[0068] Foundry cores with different carbon aerogel granules as additive were prepared according to the following general procedure: Synthesis of resorcinol-formaldehyde aerogel monoliths

[0069] Molar ratios:

[0070] Resorcinol / sodium carbonate = 1500, resorcinol / formaldehyde = 0.74, resorcinol / water = 0.044

[0071] Resorcinol was dissolved in water and stirred for 30 minutes to ensure thorough mixing of the reaction solution. 23.5 wt% formaldehyde was added in portions and stirred until a homogeneous reaction mixture was achieved. Sodium carbonate was added and stirred for a further 10 minutes. 2M nitric acid was then added until the pH was 5.4 to 5.6. After 30 minutes, the mixture was filled into hermetically sealed containers or weldable coated aluminum bags. The maximum fill height was 4 cm. Aging was carried out at 60 °C for 2 days in an oven. The reaction vessels were then opened and convective dried in an oven at 80 °C until the residual moisture content of the material was < 5%.

[0072] Grinding and sieving

[0073] Monolithic aerogel was crushed on a sieve mill (speed 120 rpm, sieve mesh size 1 mm) and gently fractionated using a tumbler sieve machine.

[0074] Pyrolysis of RF aerosols to carbon aerosols

[0075] The RF aerogel granules were heated to 1000 °C in a pyrolysis furnace under N2 or argon (heating rate 450 K / h), kept at 1000 °C for 1 h and then cooled.

[0076] Production of foundry cores

[0077] The resulting carbon aerogel granules were dry-mixed with commercially available quartz sand of a suitable quality and grain size. After a few minutes of a uniform mixture, a commercially available two-component binder system was added. This consisted of an organic phenolic resin-based binder resin with a suitable activator. The additions were made sequentially and with continuous mixing. After a few minutes of mixing, a homogeneous, slightly moist core sand mixture was formed. This was compacted in a permanent mold, observing standard process parameters such as the firing pressure. Curing of the sand core was activated by suitable amine gassing in the same permanent mold. The core achieved immediate hardness, and it could be removed from the mold. After approximately 24 hours of resting at room temperature, the core was ready for further processing.

[0078] Characterization of foundry cores

[0079] Various foundry cores according to the invention and not according to the invention were produced using carbon aerogel granules according to the invention and not according to the invention and tested for their processability, initial strength, and exhaust emissions. Processability was assessed qualitatively, taking particular account of metering, dust generation, adhesion / contamination on the mixer, and maintenance / cleaning of the mixer. The specified grain size refers to the measured particle size distribution after pyrolysis.

[0080]

[0081] Reference: Organic ColdBox sand core based on phenolic resin without aerogel additive + + : Adequate processing handling; — : Insufficient processing handling nb : Not determined.

[0082] Comparative Examples VI, V2, and V3 do not have the inventive grain size (VI, V2, V3) or the inventive external surface (VI, V2) and exhibit inadequate processing properties and inadequate immediate strength, respectively. Inventive Examples 1 and 2 have the inventive grain size and the inventive external surface, respectively, and exhibit sufficient processability and immediate strength. In contrast to a reference core, significantly lower exhaust emissions are achieved by using the inventive additive.

Claims

Patent claims 1. Additive for sand casting, comprising a carbon aerogel granulate having a grain size in a range of 100 to 800 μm and an external surface area of ​​less than 40 m 2 / g of carbon aerogel granules.

2. Additive according to claim 1, wherein the carbon aerogel granules have a pore size in a range of 0.3 to 2 nm.

3. Additive according to claim 1 or 2, wherein the carbon aerogel granules have a specific surface area in a range of 500 to 600 m 2 / g.

4. Additive according to one of claims 1 to 3, wherein the carbon aerogel granules have a bulk density in a range of 0.28 to 0.51 g / cm 3 has.

5. Additive according to one of claims 1 to 4, wherein the carbon aerogel granules have a skeletal density in a range of 1.89 to 2.34 g / cm 3 has.

6. Additive according to one of claims 1 to 5, wherein the additive consists of the carbon aerogel granules.

7. A process for producing an additive according to any one of claims 1 to 6, comprising the following steps: a. Synthesis of an organic aerogel monolith b. Grinding the aerogel monolith to form aerogel granules c. Sieving the aerogel granules, wherein the aerogel monolith before step b) or the aerogel granules after step c) are converted into a carbon aerogel by pyrolysis and are sieved in step c) such that the resulting carbon aerogel granules have a particle size in a range from 100 to 800 pm.

8. Use of a carbon aerogel granulate, wherein the carbon aerogel granulate has a grain size in a range of 100 to 800 pm and an external surface of less than 40 m 2 / g, as an additive for sand casting.

9. Foundry core containing sand, binder and an additive according to one of claims 1 to 6.

10. Foundry core according to claim 9, wherein the proportion of carbon aerogel granules in the core is in a range of 0.1 to 2.0 wt.%, in particular 0.4 to 1.2 wt.%.

11. Foundry core according to claim 9 or 10, wherein the sand comprises quartz sand, an Al2O3-based sand and / or a mullite-based sand.

12. Foundry core according to one of claims 9 to 11, wherein the binder is an organic phenolic resin-based binder.

13. A method for producing a foundry core according to any one of claims 9 to 12, comprising the following steps: a. Mixing carbon aerogel granules according to any one of claims 6 with sand and binder, b. Introducing the mixture into a negative mold of the core, optionally followed by compacting the mixture, c. Curing the binder, and d. Removing the core from the negative mold.

14. Use of a foundry core according to one of claims 9 to 12 in metal casting, in particular in non-ferrous metal, light metal or iron casting.

15. Use according to claim 14, wherein the core is removed by a thermal treatment at elevated temperature, in particular a temperature of 300°C or more, or by a fluid wetting it, in particular water.

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