Aerogel particles for casting use

Abstract

The invention relates to an additive comprising carbon aerogel particles for sand casting, to the use of carbon aerogel particles as an additive for sand casting and to a method for producing same, and to a casting core comprising sand, a binder and carbon aerogel particles as an additive, to a method for producing same and to the use thereof.

Description

[0001] The present invention relates to an additive containing carbon aerogel particles for sand casting, the use of carbon aerogel particles as an additive for sand casting and a method for producing the same, and a casting core comprising sand, a binder and carbon aerogel particles as an additive, a method for producing the same and its use.

[0002] In sand casting, molds (Formen) and cores (Kerne) are mostly made of quartz sand, but in special applications, they can also be made of other sands (aluminum oxide, zirconium oxide, olivine, chromite). The sand grains in these sands are bonded together by a (polymer) binder and form a shape-stable composite structure during the filling of the mold with liquid metal. This composite structure should be able to redissolve as easily as possible after the melt has solidified, especially for cores that form complex-shaped cavities in the casting in the form of a female mold. Mechanical aids (vibration, shaking, knocking), thermal aids or pressurized water can be used to remove the core or dissolve the mold. In the prior art, phenolic resins, as well as polyurethane, urea and furan resins, are mainly used as binders, and these resins are subjected to complex chemical modifications (chemical additives) to meet the requirements of casting. Aerogel binders are also known. The chemical composition of the binder is optimized for the application to meet conflicting requirements, such as high thermal stability at low gas evolution and low binder usage, as well as easy core removal and high surface quality.

[0003] In the prior art, in order to produce high-quality castings (especially to prevent veining), special additives are also used. These additives can be organic and inorganic materials. However, these additives are also accompanied by various disadvantages. For example, they include high emissions (such as benzene, formaldehyde and phenol), and a mold coating process is required to obtain a high-quality surface on the casting. In addition, this ceramic coating must also be dried energy-consumingly, and if not used, it causes increased cleaning work for the cast components.

[0004] EP 2193858 B1 and EP 2204246 B1 disclose casting cores with improved core removal properties, a method for producing the same and their use. These casting cores contain sand, a binder and hydrophobic resorcinol-formaldehyde aerogel particles, which pyrolyze during casting to form a material with uncertain properties. In this case, the presence of organic components leads to gas evolution.

[0005] The aerogels further described in the prior art are mainly based on inorganic aerogels, in particular oxide aerogel particles containing SiO2, TiO2 and / or ZrO2, and organic aerogels based on phenolic resins (such as resorcinol-formaldehyde). In principle, organic aerogels can be pyrolyzed to convert into carbon aerogels. Carbon aerogels can represent a particularly interesting class of materials used as additive for aerogel binders in foundry cores. Using aerogels as binder additives can generally simplify the manufacturing process and reduce emissions while maintaining the same quality. Carbon aerogels have more advantages than other aerogels. Using carbon aerogels to produce cores is particularly simple (self-lubricating), thus reducing core rejects. The mold coating can be reduced or partially eliminated, thus possibly eliminating a process step. Using carbon aerogels can significantly reduce emissions. Research has shown that using carbon aerogels can maintain the required microstructure of the metal or improve the surface layer. However, it has been shown that foundry cores containing carbon aerogels cannot maintain sufficient stability without problems.

[0006] Therefore, the object of the present invention is to provide an additive for sand casting based on carbon aerogels, thereby avoiding the disadvantages of the prior art. Specifically, it aims to provide an additive for sand casting based on carbon aerogels, by using which a stable and easy-to-process foundry core can be obtained, and the generation of harmful gases during the casting process can be reduced.

[0007] In a first embodiment, the object of the present invention is achieved by an additive for sand casting, which comprises carbon aerogel particles having a particle size in the range of 100 to 800 μm, and the external surface area of the carbon aerogel particles is less than 40 m 2 / g. In a preferred embodiment, the additive can consist of carbon aerogel particles.

[0008] Surprisingly, it has been shown that only when the carbon aerogel particles have the aforementioned particle size and external surface area can a stable foundry core based on carbon aerogels be obtained. Beyond these parameter ranges, the corresponding foundry cores are practically unprocessable. By using the additive for sand casting of the present invention, the amount of mold coating used can be reduced, the cleaning work can be minimized, and emissions can be reduced. Reducing the mold coating process can also reduce the material consumption, shorten the production time, and reduce the energy consumption. At the same time, the core manufacturing process remains process-reliable, and the quality of the cast components remains stable.

[0009] Carbon aerogels in the sense of the present invention particularly refer to aerogels that can be prepared by pyrolyzing organic aerogels (such as those based on phenolic resins, especially resorcinol - formaldehyde). The microscopic and macroscopic properties of the organic aerogels change, especially due to shrinkage. Suitable carbon aerogels can be obtained by all known prior - art methods for preparing carbon aerogels. In the sense of the present invention, it is crucial that the carbon aerogels exist in the form of particles with a particle size in the range of 100 to 800 μm and an external surface area of less than 40 m 2 / g.

[0010] For example, the corresponding aerogel particles can be obtained by grinding and sieving the monolithic aerogel. The particle size can be adjusted by grading the oversize. However, the present invention is not limited to grinding and sieving. Alternatively, the corresponding aerogel particles can also be obtained, for example, by emulsion polymerization, jet - cutting processes, spraying processes, or more commonly, by the dripping process or in a tubular reactor.

[0011] For example, carbon aerogel particles can be prepared by grinding and sieving an organic aerogel and then pyrolyzing the resulting particles. In this case, it must be taken into account that the material may shrink due to pyrolysis, so the grading of the organic aerogel must be adjusted accordingly. Alternatively, for example, the monolithic organic aerogel particles can first be converted into carbon aerogel particles by pyrolysis and then ground and sieved. However, it is preferred to first grade the organic aerogel and then pyrolyze the resulting particles. Carbon aerogels are generally more brittle than organic aerogels, so the effect of grinding and sieving to a predetermined particle size is slightly worse.

[0012] In the sense of the present invention, a particle size in the range of 100 to 800 μm means that the particles of all the used aerogel particles basically have dimensions in this range. In particular, the additive is basically free of particles with a size greater than 800 μm or less than 100 μm. If the aerogel particles contain a significant amount of particles with a size greater than 800 μm or less than 100 μm, they are not suitable as additives for sand casting. Accordingly, the mold cores produced using such additives lack the required workability and are prone to fragmentation. In addition, particle sizes less than 100 μm are also more difficult to process because of their dusty consistency.

[0013] However, the aerogel particles can contain a non - significant amount of portions with a particle size greater than 800 μm or less than 100 μm. Such additives are also considered to comply with the present invention. A non - significant amount especially means a content 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 particles can also contain almost no particles greater than 800 μm or less than 100 μm. If the aerogel particles are obtained by sieving, the particle size can be adjusted very precisely, and particle sizes outside the range of 100 to 800 μm can be almost excluded.

[0014] Preferably, the aerogel particles essentially only contain particles with a size greater than 350 μm, particularly preferably greater than 400 μm, and very particularly preferably greater than 450 μm. Preferably, the aerogel particles essentially only contain particles with a size less than 650 μm, particularly preferably less than 600 μm, and very particularly preferably less than 550 μm.

[0015] In particular, by the principle of static light scattering, the particle size of the milled aerogel particles can be directly measured on the powder. The interaction of the incident light beam with the particles produces a characteristic angle-dependent pattern through diffraction, refraction, reflection, or absorption. The scattering angle and intensity depend on the size of the particles involved. Compared with small particles, large particles (>> the wavelength of the incident light) have a greater scattering intensity and a smaller scattering angle. The particle size distribution is determined based on the obtained scattering pattern using the Fraunhofer theory (> 5 μm) or the Mie theory (< 5 μm). The Mie theory assumes that the particles can be described as quasi-translucent spheres. This means that light can penetrate the material and be elastically scattered by the atoms of the particles. Therefore, in order to evaluate, it is necessary to know the optical material constants such as the refractive index.

[0016] The measurement can be carried out as follows: where dry powder from a vibrating feeder (the powder delivery amount can be adjusted by the vibration intensity) is fed into the laser beam using compressed air. The angle and intensity of the light scattered by the particles are measured, and the scattered light data is converted into particle size information through an algorithm.

[0017] The required external surface area is less than 40 m 2 / g, based on the overall carbon aerogel particles, rather than each individual particle. If the external surface area exceeds this value, a processable casting core cannot be obtained using this additive. The external surface area is preferably below 38 m 2 / g, particularly preferably below 35 m 2 / g. The external surface area is preferably at least 1 m 2 / g, particularly preferably at least 17 m 2 / g, very particularly preferably at least 30 m 2 / g.

[0018] By coordinately selecting the synthesis parameters (reactant concentration, pH value, amount and type of catalyst, temperature, stirring, and drying conditions, etc.), the required external surface area can be adjusted.

[0019] The external surface area, together with the specific surface area and pore size distribution, can be determined by nitrogen adsorption. Here, a certain amount of nitrogen gas is introduced into the aerogel to be measured, and the nitrogen will be physically adsorbed on the material surface. After a certain period of time, a dynamic equilibrium is established based on temperature, pressure, and the amount of adsorption. The relationship between the amount of adsorption and the partial pressure of the gas phase at a constant temperature is described by the adsorption isotherm. By recording the gas pressure, the amount of the adsorbed substance adsorbed on the surface can be determined. Since the sample chamber is cooled with liquid nitrogen, this process occurs below the saturated vapor pressure, thus preventing the condensation of the test gas in the sample chamber. The defined metered addition of nitrogen gas is continuously carried out until the equilibrium pressure is reached. Then, the sample chamber is gradually evacuated again to desorb the adsorbed substance from the surface again. Thus, the adsorption-desorption isotherm is obtained and can be evaluated and interpreted by various methods.

[0020] The specific surface area can be determined by the BET method. It is assumed that under monolayer coverage (pressure range of 0.05 < p / p0 < 0.35), the amount of adsorbed nitrogen is proportional to the specific surface area of the sample. The BET equation can be used to calculate the gas volume V of the monolayer. m V a is the adsorption volume of gaseous nitrogen, p0 is the saturation pressure, p is the measured pressure, and C is an empirical constant:

[0021]

[0022] The monolayer volume V obtained in this way m , the specific surface area S can be calculated BET :

[0023]

[0024] A m = the area occupied by nitrogen molecules on the surface at 77K

[0025] N A = Avogadro's constant

[0026] M V = the molar volume of N2

[0027] M = the mass of the sample

[0028] The pore size distribution can be automatically calculated using the BJH method. Therefore, the pore size is determined by the Kelvin pore radius R K and the monolayer thickness t of the pore wall:

[0029] R 孔 = R K + t (3)

[0030] The Kelvin pore radius R K can be determined according to the Kelvin equation (4):

[0031]

[0032] The layer thickness of the monolayer at the pore wall can be determined by Equation (5):

[0033]

[0034] By analyzing using the t-plot method, the adsorption process in each pore type can be distinguished. Thus, the specific surface area of the porous material can be divided into the external surface area of the outer particles (S 外部 ) and the internal surface area of the micropores or mesopores (S 内部 ). The t-Plot method is based on the mathematical formula for multilayer adsorption of the adsorbate on a non-porous material. The statistical thickness t of the adsorbate layer is described as a function of the partial pressure p / p0. If there are no micropores in the material, the surface area calculated in this way should be equal to the specific surface area value obtained by the BET method. If micropores exist, the corresponding micropore surface area can be determined by the difference between the BET specific surface area and the external surface area obtained by the t-plot.

[0035] When preparing for the measurement, the sample can be thermally pre-treated in vacuo, especially at 200 °C for 12 hours, to remove any possibly adsorbed solvent and water residues.

[0036] The carbon aerogel particles preferably have a pore diameter in the range of 0.3 to 2 nanometers. A pore diameter in the range of 0.3 to 1 nanometer is particularly preferred. If the pore diameter is above this range, more binder is required, and the stability decreases when the amount of binder remains constant.

[0037] The carbon aerogel particles preferably have a specific surface area in the range of 500 to 600 m 2 / g. Particularly preferably, the specific surface area is in the range of 525 to 575 m 2 / g, and most particularly preferably 550 m 2 / g.

[0038] The carbon aerogel particles preferably have a packing density in the range of 0.28 to 0.51 g / cm 3 . A packing density in the range of 0.30 to 0.38 g / cm 3 is particularly preferred. The apparent density, also known as the packing density or envelope density (umhüllende Dichte), refers to the density of a porous solid based on its volume (including the pore space).

[0039] The carbon aerogel particles preferably have a skeletal density in the range of 1.89 to 2.34 g / cm 3 . A skeletal density in the range of 1.98 to 2.27 g / cm 3Skeletal density of the range. True density, also known as skeletal density, refers to the density of the material part of an object, that is, the density without considering the volume of cavities it may contain.

[0040] A sand pycnometer can be used to measure bulk density or envelope density, and the volume displacement principle is applied. The solid medium DryFlo, a liquid-like substance composed of closely distributed small and hard spheres with high fluidity, can be used as a displacing agent. DryFlo particles form a dense filling around the sample to be measured, but do not penetrate the pore volume that may exist.

[0041] The sample chamber (e.g., a precision flask with a diameter of 2.54 cm and a maximum length of 4 cm) is initially filled only with DryFlo to determine the zero volume. Through a controlled compression process, the powder is compressed by a piston to a specified force (e.g., 51 N). The stroke of the piston is recorded. Then, the weighed sample is added and the process is repeated. The volume of the sample can be calculated from the change in the piston stroke length during the compression process.

[0042] V P = π * r * 2 * (h0 - h P ) (6)

[0043] V P = sample volume

[0044] r = piston radius

[0045] h0 = piston stroke length at zero volume measurement

[0046] h p = piston stroke length at sample measurement.

[0047] Based on the mass m of the material and the measured volume V P The density ρ of the material to be measured is calculated according to the following formula 堆积 ,

[0048]

[0049] To improve the measurement accuracy and reproducibility, the ratio of the sample to the displacing agent is preferably 75 / 25 vol%, and the zero volume measurement and the sample measurement are each repeated 10 times.

[0050] A helium pycnometer can be used to measure 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 of the solid to be measured. The inert gas helium can be used as a displacing agent because it hardly interacts with the material surface and its molecules are very small (diameter: ) so that it can even penetrate into porous materials. The weighed sample is placed in a chamber of known volume in a sample crucible and sealed. The sample chamber is connected to a reference chamber (with the same known volume) through a valve. Then the sample chamber is filled with helium gas and the pressure is recorded. By opening the valve between the two 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 to calculate the volume of the solid-phase sample.

[0051]

[0052] V P = sample volume

[0053] V K = chamber volume

[0054] V R = volume of the reference chamber

[0055] p i = initial pressure

[0056] p e = final pressure

[0057] From the mass m of the material and the measured volume V P The skeletal density ρskeleton is calculated according to formula (7).

[0058] The crucible with a diameter of 14.44 mm and a height of 9 mm is filled as much as possible to prevent measurement errors. Each measurement is repeated 10 times. The measurement duration is 30 to 40 minutes.

[0059] The carbon aerogel particles preferably have a porosity of 75% to 90%. The porosity is particularly preferably in the range of 78% to 88%, and most particularly preferably in the range of 80% to 88%.

[0060] Porosity is a dimensionless measurement that represents the ratio of the cavity volume to the total volume of a substance or a mixture of substances. It can be used as a classification measure for the actual existing cavities. Porosity has a great influence on the flow resistance of porous solids. The porosity of the measured material can be calculated according to formula (9) by the ratio of the bulk density to the skeletal density:

[0061]

[0062] The primary particles of the aerogel preferably have a diameter of 4 to 10 microns. This parameter describes the size of the primary particles that basically constitute the microstructure of the aerogel. This diameter can be determined graphically by evaluating the scanning electron micrograph of the aerogel.

[0063] In another embodiment, the object of the present invention is achieved by a method for preparing an additive according to the present invention, wherein the following steps are carried out:

[0064] a. A synthetic organic aerogel monolith,

[0065] b. Grinding the aerogel monolith into aerogel particles,

[0066] c. Screening the aerogel particles,

[0067] wherein, the aerogel monolith before step b) or the aerogel particles after step c) are converted into carbon aerogel by pyrolysis, and screening is carried out in step c) such that the obtained carbon aerogel particles have a particle size in the range of 100 to 800 μm.

[0068] Alternatively, corresponding carbon aerogel particles having the above particle size and optionally the above surface area can be directly produced, thereby avoiding the synthesis, grinding and screening of the monolith.

[0069] If pyrolysis is carried out before grinding and screening, the carbon aerogel particles can be directly classified into a particle size of 100 to 800 microns according to the present invention. If pyrolysis is carried out after grinding and screening, a larger particle size must be selected when classifying the organic aerogel because pyrolysis causes a size shrinkage of 10% to 25%.

[0070] In another embodiment, the object of the present invention is achieved by the use of carbon aerogel particles as an additive for sand casting, wherein the carbon aerogel particles have a particle size in the range of 100 to 800 μm, and an external surface area of less than 40 m 2 / g.

[0071] The carbon aerogel particles used according to the present invention can in principle have all the previously described properties of the carbon aerogel particles contained in the additives according to the present invention.

[0072] In another embodiment, the object of the present invention is achieved by a casting core comprising sand, a binder and an additive according to the present invention comprising carbon aerogel particles according to the present invention.

[0073] The amazing feature of the casting core of the present invention is that by using carbon aerogel as a binder additive, the manufacturing process is simplified, emissions are reduced while maintaining the same quality. It is particularly easy to manufacture cores using carbon aerogel, thereby reducing core scrap. The mold coating can be reduced or even partially eliminated, thus possibly eliminating a process step. Using carbon aerogel results in a significant reduction in emissions.

[0074] Any sand suitable for use in casting cores known in the prior art can be used as the sand. The sand preferably includes quartz sand, Al2O3-based sand and / or mullite-based sand. The sand can contain, for example, SiO2, Al2O3 and / or Fe2O3.

[0075] As sand, it is possible to use in particular new quartz sand available on the German market, with the following sources and average grain sizes (in millimeters):

[0076] Dorsten 0.84 mm (grade D020), 0.56 mm (D030), 0.39 mm (D040), 0.13 mm (DO110);

[0077] 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);

[0078] Gambach 0.37 mm (grade G30), 0.29 mm (G31), 0.23 mm (G32), 0.21 mm (G33), 0.19 mm (G34);

[0079] Haltern 0.36 mm (grade H31), 0.32 mm (H32), 0.26 mm (H33), 0.21 mm (H34) and 0.19 mm (H35).

[0080] As an alternative to the above quartz sand, corundum sand with a similar size (0.1 to 0.9 mm) can also be used.

[0081] All of the quartz sands shown above are new sands; in fact, they are only added in small amounts to the "old sand" used in casting. Old sand refers to the sand produced after the casting is poured out of the mold. After appropriate cooling and reprocessing, it is returned to the mold shop. The reprocessing includes two tasks: removing the binder adhering to the quartz particles and removing the dust-like components. During this process, any remaining agglomerates are mechanically crushed, thereby partially removing the binder shell on the quartz particles. During this process, the originally round surface of the sand grains changes. It changes from round to fragmented. This particle shape is important for the bonding process of the mold material; in this way, it is ensured that only a relatively small amount of binder is required.

[0082] The mixture for producing the casting core preferably contains a sand content of 83 to 95 wt.%, where the new sand content is at most 95 or 100 wt.%, or preferably 1 to 20 wt.% new sand and 80 to 99 wt.% recycled sand (reclaimed mold material, i.e., purified and reused sand). The addition of recycled sand can be omitted as required, especially in the case of copper casting, brass casting, and bronze casting. The binder content is preferably 1 to 10 wt.%. The sum of the contents of sand, binder, and aerogel particles (and the contents of any other optional components) reaches 100 wt.% or vol.%.

[0083] The content of the aerogel particles in the cast core is preferably in the range of 0.1 to 2.0% by weight, particularly preferably in the range of 0.4 to 1.2% by weight, especially in the range of 0.5 to 0.75% by weight.

[0084] The binder is preferably an organic binder, in particular a binder or binder mixture comprising at least one representative 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 the binder undergoes carbonization during the casting process, which further promotes core removal and reduces mineralization.

[0085] In another embodiment, the object of the present invention is achieved by a method for preparing a cast core according to the present invention, wherein the following steps are carried out:

[0086] a. Mixing the carbon aerogel particles according to the present invention with sand and a binder,

[0087] b. Introducing the mixture into the female mold of the core, optionally subsequently compressing the mixture,

[0088] c. Curing the binder, and

[0089] d. Removing the core from the female mold.

[0090] Compression can be achieved, for example, by core shooting, vibration, tapping, and / or ramming. When using a cold box binder, curing can be carried out immediately after core shooting at room temperature. It has been confirmed that temperatures of 20 to 300 °C, especially 80 to 250 °C, are particularly suitable for curing hot box binders. The curing time is preferably from a few seconds to a few minutes. Drying of the cast core can be completed after hardening, or by storing the core at room temperature, at a temperature above room temperature up to 300 °C for 1 to 24 hours, or by placing it in a microwave oven.

[0091] In another embodiment, the object of the present invention is achieved by using the cast core according to the present invention for metal casting, especially for non-ferrous metal casting, light metal casting, or iron casting.

[0092] In particular, the core can be removed after the melt has solidified, for example, by heat treatment at an elevated temperature (especially at a temperature ≥ 350 °C) or by mechanical means (vibration, ultrasound, tapping). Removal by heat treatment is very advantageous because no residue remains after the core decomposes. During the casting process, dimensional accuracy errors caused by core expansion when quartz breaks in the case of using quartz sand can be compensated for by the elasticity of the particles used, which is related to the ratio of the particles and the content of the binder.

[0093] Alternatively, the core can also be removed after the melt has solidified, for example by means of a fluid (especially water) that wets it. Removing the core using a wetting fluid is very advantageous because no residue remains after the core decomposes through the fluid that wets it. Fluids with good wettability (such as water) are particularly suitable for this purpose. Wettability refers to the ability of a liquid to spread on a surface; the better the wettability, the smaller the contact angle during the wetting process. If the contact angle with the surface is up to 90°, the surface is also called (incompletely) wettable. The higher the temperature of the wetting fluid, the easier it is to remove the core. Therefore, a fluid with a temperature of 30 to 100 °C is particularly preferred. This utilizes the property that hydrophilic silica aerogels can be easily destroyed by liquids with good wettability (such as boiling water).

[0094] Alternatively, the core can also be destroyed, for example by means of an alcohol-based fluid or a short-chain alcohol, especially a short-chain alcohol with a chain length of at most 6 carbon atoms. To avoid the risk of fire, a non-flammable alcohol mixture, such as an alcohol mixture containing water, should be used.

[0095] Working Examples

[0096] The following general procedure is used to manufacture casting cores with different carbon aerogel particles as additives:

[0097] Synthesis of Resorcinol-Formaldehyde Aerogel Monoliths

[0098] Molar ratio:

[0099] Resorcinol / sodium carbonate = 1500,

[0100] Resorcinol / formaldehyde = 0.74

[0101] Resorcinol / water = 0.044

[0102] Dissolve resorcinol in water and stir for 30 minutes to ensure that the reaction solution is well mixed. Add 23.5 wt.% formaldehyde in batches and stir until a homogeneous reaction mixture is achieved. Add sodium carbonate and stir for another 10 minutes. Then add 2 M nitric acid until the pH value reaches 5.4 to 5.6. After 30 minutes, fill it into an airtight container or a heat-sealable aluminized bag. The maximum filling height is 4 cm. Age it in an oven at 60 °C for 2 days, then open the reaction container and conduct convective drying in an oven at 80 °C until the residual moisture content in the material is < 5%.

[0103] Grinding and Sieving

[0104] Crush the monolith aerogel on a screen mill (rotating speed 120 rpm, screen width 1 mm) and carefully classify it using a drum sieve machine.

[0105] Pyrolysis of RF Aerogel Particles into Carbon Aerogel Particles

[0106] Heat the RF aerogel particles to 1000 °C (heating rate 450 K / h) in a pyrolysis furnace under N2 or argon, hold at 1000 °C for 1 h, and then cool down.

[0107] Preparation of Casting Cores

[0108] Dry-mix the obtained carbon aerogel particles with commercially available quartz sand of suitable quality and particle size. Then obtain a homogeneous mixture after a few minutes, and add a commercially available two-component binder system thereto. This binder system is an organic binder resin based on phenolic resin containing a suitable activator. The addition process is carried out successively and continuous mixing is maintained.

[0109] After a few minutes of mixing time, a homogeneous and slightly moist core sand mixture is formed. Compact it in a permanent mold and follow certain standard process parameters, such as core shooting pressure. In the same permanent mold, activate the hardening of the core by appropriate amine gas treatment. The core hardens immediately and can be removed from the mold. After standing at room temperature for about 24 hours, the core can be further processed.

[0110] Characterization of Casting Cores

[0111] Using the carbon aerogel particles of the present invention and those not of the present invention, various casting cores according to the present invention and not according to the present invention were prepared, and their processing properties, immediate strength and waste gas emissions were tested. The processing properties were qualitatively evaluated, especially considering the metering addition ability, dust generation and adhesion / contamination on the mixer as well as the maintenance / cleaning of the mixer. The given particle size refers to the particle size distribution measured after pyrolysis.

[0112]

[0113] Control: Organic cold box core based on phenolic resin without aerogel additive

[0114] ++: Sufficient processing; --: Insufficient processing

[0115] n.b.: Not measured

[0116] Comparative Examples V1, V2 and V3 do not have the particle size according to the present invention (V1, V2, V3) or do not have the external surface area according to the present invention (V1, V2), and have poor processing properties or poor immediate strength. Examples 1 and 2 according to the present invention have the particle size according to the present invention and the external surface area according to the present invention, showing sufficient processing properties and immediate strength. Compared with the control core, the use of the additive according to the present invention achieves a significant reduction in waste gas emissions.

Claims

1. An additive for sand casting, comprising carbon aerogel particles having a particle size in the range of 100 to 800 μm, and the external surface area of the carbon aerogel particles is less than 40 m 2 / g.

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

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

4. The additive according to any one of claims 1 to 3, wherein the carbon aerogel particles have a bulk density in the range of 0.28 to 0.51 g / cm 3 range.

5. The additive according to any one of claims 1 to 4, wherein the carbon aerogel particles have a skeletal density in the range of 1.89 to 2.34 g / cm 3 range.

6. The additive according to any one of claims 1 to 5, wherein the additive consists of carbon aerogel particles.

7. A method for preparing the additive according to any one of claims 1 to 6, wherein the following steps are carried out: a. Synthesizing an organic aerogel blank, b. Grinding the aerogel blank into aerogel particles, c. Sieving the aerogel particles, Among them, Converting the aerogel blank before step b) or the aerogel particles after step c) into carbon aerogel by pyrolysis, and sieving in step c) such that the resulting carbon aerogel particles have a particle size in the range of 100 to 800 μm.

8. Use of carbon aerogel particles as an additive for sand casting, wherein the carbon aerogel particles have a particle size of 100 to 800 μm and an external surface area of less than 40 m 2 / g.

9. A casting core, comprising sand, a binder, and the additive according to any one of claims 1 to 6.

10. The cast core according to claim 9, wherein, The content of the carbon aerogel particles in the core is in the range of 0.1 to 2.0% by weight, particularly 0.4 to 1.2% by weight.

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

12. The casting core according to any one of claims 9 to 11, wherein the binder is a phenolic resin-based organic binder.

13. A method for manufacturing the casting core according to any one of claims 9 to 12, wherein the following steps are carried out: a. Mixing the carbon aerogel particles according to any one of claims 6 with sand and a binder, b. Introducing the mixture into the female mold of the core, optionally subsequently compacting the mixture, c. Curing the binder, and d. Removing the core from the female mold.

14. Use of the casting core according to any one of claims 9 to 12 in metal casting, particularly in non-ferrous metal casting, light metal casting, or iron casting.

15. The use according to claim 14, wherein the core is removed by heat treatment at an elevated temperature, particularly at a temperature of 300 °C or higher, or by a fluid that wets it, particularly water.

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