METHOD FOR PRODUCING A CERAMIC MOULDED BODY

Abstract

The invention relates to a method for producing a ceramic moulded body, comprising the following steps: a) producing a green body containing ceramic material, binding agents and an organic pore forming agent; b) heating the green body to a temperature equal to or higher than the sublimation temperature of the pore forming agent; c) burning the green body to form a ceramic moulded body. According to the invention that the organic pore forming agent is selected from the group consisting of dicarboxylic acids and mixtures of dicarboxylic acids, the sublimation temperature being at least 80 k lower than the decomposition temperature.

Claims

1-14. (canceled)

15. A method for producing a porous ceramic molding comprising the steps of: a) producing a green body from a mixture comprising ceramic material, a ceramic binder, and fumaric acid particles; b) removing the fumaric acid particles from the green body by sublimation, wherein said removing comprises heating the green body to a temperature at or above 180° C. and below 350° C., wherein the fumaric acid particles are sublimated to produce a porous green body; c) firing the porous green body to form the porous ceramic molding.

16. The method of claim 15, wherein the fumaric acid particles comprise one or more of fumaric acid powder and fumaric acid granules.

17. The method of claim 15, wherein removing the fumaric acid particles comprises heating the green body to a temperature from 180 to 240° C.

18. The method of claim 15, wherein removing the fumaric acid particles comprises heating the green body to a temperature from 180 to 220° C.

19. The method of claim 15, wherein removing the fumaric acid particles comprises heating the green body at a heating rate of 2 to 80° C./h.

20. The method of claim 15, wherein removing the fumaric acid particles comprises heating the green body at a heating rate of 20 to 60° C./h.

21. The method of claim 15, wherein the fumaric acid particles comprise at least two different particle size fractions comprising a fine particle size fraction and a large particle size fraction.

22. The method of claim 21, wherein the fine particle size fraction has a particle size from 1-100 μm.

23. The method of claim 21, wherein the large particle size fraction has a particle size of 1 mm.

24. The method of claim 15, wherein the proportion of fumaric acid particles of the total weight of the green body in step a) is between 2 and 60% by weight.

25. The method of claim 24, wherein the proportion of fumaric acid particles of the total weight of the green body in step a) is 2 and 50% by weight.

26. The method of claim 24, wherein the proportion of fumaric acid particles of the total weight of the green body in step a) is 10 and 50% by weight.

27. The method of claim 24, wherein the proportion of fumaric acid particles of the total weight of the green body in step a) is 10 and 30% by weight.

28. The method of claim 24, wherein the proportion of fumaric acid particles of the total weight of the green body in step a) is 15 and 20% by weight.

29. The method of claim 15, wherein prior to step b), the green body is heated to a temperature of between 30 and 50° C. and maintained at that temperature for a period of 4 to 48 h.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 shows a hardness curve for PEG-fumaric acid-wetting

[0024] FIG. 2 shows grinding forces of the abrasive bodies produced with fumaric acid or naphthalene as pore former.

[0025] FIG. 3 shows G ratio of the abrasive bodies produced with fumaric acid or naphthalene as pore former

[0026] FIG. 4 shows roughness R.sub.z of the abrasive bodies produced with fumaric acid or naphthalene as pore former.

DETAILED DESCRIPTION

[0027] The invention relates to a method for producing a ceramic molding comprising the steps of:

[0028] a) producing a green body comprising ceramic material, binders and an organic pore former;

[0029] b) heating the green body to a temperature equal to or above the sublimation temperature of the pore former;

[0030] c) firing the green body to form a ceramic molding.

[0031] In accordance with the invention, it is provided that the organic pore former is selected from the group consisting of dicarboxylic acids, of which the sublimation temperature is at least 80K below the decomposition temperature. In addition, some terms used in the context of the invention are further explained.

[0032] The terms “ceramic molding”, “tool composed of bonded abrasive”, “abrasive”, “binder” and “green body” are used in such a way in the present application as they are familiar to those skilled in the art from the prior art.

[0033] Ceramic material is an inorganic material which remains largely unchanged as such in the sintering process. An example is abrasive grit.

[0034] The term binder includes both ceramic binders for producing the ceramic binding during the sintering process and temporary binders. Temporary binders serve firstly for the production and protection of the dimensional stability in the course of the production, storage and movement of the green body and secondly for protecting the structural integrity in the course of heating for the removal of pore formers.

[0035] The core of the invention lies in the use of defined pore formers according to the invention which allows the production of a defined porosity without impairment of the structural integrity of the green body.

[0036] The dicarboxylic acids used in accordance with the invention have a sublimation temperature which is at least 80K, preferably 100K, more preferably 120K below the decomposition temperature.

[0037] This means that, on heating the green body to remove the pore former, this sublimes without decomposition. This has a series of advantages compared to the prior art, which uses oxalic acid for example.

[0038] Oxalic acid starts to sublime at about 100° C. and decomposition occurs from about 150 to 160° C. If it is desired to remove oxalic acid as pore former practically completely by sublimation without decomposition, on one hand the temperature of about 100° C. has to be exceeded in the entire volume in the green body while on the other hand it must at no point exceed the range from about 150° C. This means that only a very slow heating must take place and a range from about 120 to 130° C. has to be maintained over a relatively long period, for example, of about 24 h.

[0039] If the heating is effected more rapidly, it may happen, due to the geometrical shape of the oven or of the molding or due to other non-uniformities in the heating process, that the decomposition temperature of about 150 to 160° C. is already exceeded locally and, instead of sublimation, decomposition of the oxalic acid results, forming high volumes of gas.

[0040] Even independently of the rate of the heating process, there may be local temperature inhomogeneities in an oven which result in the decomposition temperature already being exceeded in some regions of the green body. On escape of the considerable volume during decomposition, this can result in undesirable crack formation in the green body.

[0041] In the case of sublimation, substantially lower volumes of gas escape under conditions that are more easily controlled, such that crack formation of this kind can be avoided. This allows the production of a green body with defined porosity without impairment of the structural integrity. The use of a pore former provided in accordance with the invention having a sublimation temperature at least 80K below the decomposition temperature ensures that, even during relatively rapid heating or in the case of temperature inhomogeneities in the oven, undesirable decomposition forming high volumes of gas can be avoided. In the context of the invention it is therefore frequently possible to avoid a separate upstream step for removal of the pore former and to effect this removal of the pore former in the course of the heating in preparation for the sintering process.

[0042] A pore former escaping by sublimation may be captured and reused. No decomposition products are formed which may be toxic or aggressive.

[0043] The sublimation temperature of the pore former is preferably between 160 and 240° C., preferably 180 and 220° C. The gaseous pore former escaping without decomposition can therefore be removed before reaching the actual sintering temperature of the green body.

[0044] The pore former is in the solid state, preferably plastically deformable and has no or only very low springback. In this manner, it is avoided that the green body is damaged by springback after compression and volume enlargement of the pore former linked thereto.

[0045] Generally, pores should be distributed as homogeneously as possible in the tool. For this purpose, it is necessary to mix the pore former equally homogeneously with the remaining mixture constituents of the green body. In order to largely avoid an undesirable demixing in this case, it is preferable that the density of the pore former is similar to the density of the remaining constituents of the green body. Preferably, the density of the pore former can be between 1.3 and 2 g/cm.sup.3, preferably 1.4 and 1.8 g/cm.sup.3.

[0046] In accordance with the invention, particular preference is given to fumaric acid as pore former. Fumaric acid has a sublimation temperature of about 200° C. and only decomposes above 350° C. It can therefore sublime on heating the green body and escape undecomposed with formation of comparatively low volumes of gas.

[0047] Fumaric acid is storage-stable and non-hygroscopic. It therefore does not accumulate any water of hydration as pore former. This is of particular advantage since water of hydration evaporates with formation of large volumes on heating a green body even at temperatures from 50° C. and can result in crack formation in the green body. Oxalic acid used in the prior art for example is highly hygroscopic and therefore when used as pore former regularly results in large amounts of water of hydration being incorporated.

[0048] Fumaric acid is non-toxic and is approved as a food additive. When used and processed therefore, no corresponding precautions have to be met. This is a major advantage compared to other subliming pore formers used in the prior art such as naphthalene for example.

[0049] The ignition temperature of fumaric acid is more than 150K above the sublimation temperature. The green body can therefore be heated to remove the fumaric acid without hazard.

[0050] The proportion of pore former of the total weight of the green body according to the invention can preferably be between 2 and 60% by weight, more preferably 2 and 50% by weight, more preferably 10 and 50% by weight, more preferably 10 and 30% by weight, more preferably 15 and 20% by weight. The use of subliming pore formers according to the invention renders the use of high proportions of a pore former possible, for example in the range of 50% by weight or more, without resulting in damage to the green body due to high gas volumes on escape of the pore former. Accordingly, moldings having high porosity can be produced.

[0051] In order to release the gas volumes which escaped during sublimation of the pore former in a controlled manner and without adversely affecting the green body it may be preferable to carry out the heating at or above the sublimation temperature of the pore former using a defined temperature regime. Preference is given here to a heating rate of 2 to 80° C./h, more preferably 20 to 60° C./h. A particular advantage of the invention is that even comparatively rapid heating is possible without adversely affecting the structural integrity of the green body.

[0052] In accordance with the invention, it can be provided that, prior to heating at or above the sublimation temperature of the pore former, additionally, heating to a temperature below the sublimation temperature of the pore former, preferably 40 to 90° C., more preferably 30 to 50° C., is carried out and the green body is maintained at this temperature for a time period, which in particular allows the evaporation of volatile constituents such as water or solvent. This time period is preferably between 4 and 48 h. When using a non-hygroscopic pore former such as fumaric acid, this heating below the sublimation temperature can be brief or omitted completely.

[0053] The ceramic molding produced in accordance with the invention can be in particular a tool composed of bonded abrasive. Likewise conceivable is a formation as ceramic molding for other purposes, in particular commercial or industrial.

[0054] Preference is given to using additionally a binder during production of the green body.

[0055] This binder is a temporary binder and serves to preserve the structural integrity and true shape during production and handling of the green body, until this is ensured by the actual sintering to give the ceramic molding.

[0056] In the prior art, dextrin/water systems for example are used as temporary binders. This binder system is not a risk to health and can be readily removed (burned off) in the course of the firing process, but alters the strength of a green body associated thereto depending on the humidity/water content, such that the storage of a green body produced using this binder system at undefined humidity may lead, for example, to defects (dry cracks).

[0057] Wax is also known as temporary binder in the prior art. A major disadvantage of wax is that ignitable mixtures can form on heating/burning off such that complex protective measures should be taken.

[0058] In accordance with a particularly advantageous aspect of the invention, it is therefore provided that the binder comprises polyglycols, particularly polyethylene glycols (PEG). Advantageous molecular weight ranges for the polyethylene glycols are 100 to 20 000, more preferably 200 to 10 000, more preferably 250 to 8000. PEGs in the molecular weight range up to 600 are typically liquid. A preferred range of such liquid PEGs is 300 to 600.

[0059] The invention has identified that, surprisingly, polyethylene glycols with the dicarboxylic acids used as pore formers form an advantageous temporary binder system.

[0060] An attempt at an explanation for this surprising advantageous behavior that does not limit the invention is that the terminal OH groups of the polyethylene glycols react with the carboxyl groups forming esters and in this manner form a temporary binder system. The bonding (esterification) of acid and polyethylene glycols takes place preferably at a temperature which is below the sublimation temperature of the pore former used. For example, fumaric acid esterifies with polyethylene glycols at about 165°, which is significantly below the sublimation temperature of 180 to 200° C. A sufficient esterification of fumaric acid can be achieved in accordance with the invention even at lower temperatures, for example about 90° C. However, this requires the temperature to be maintained longer.

[0061] An esterification of this kind takes place particularly with low molecular weight liquid PEG, for example in the molecular weight range of 300 to 600. The temporary binder system thus formed from about 165° C. serves particularly to stabilize the green body in the course of the subsequent removal of the pore former. To be distinguished therefrom is a temporary binder system stage upstream, which already serves during the production, storage and movement of the green body to ensure the desired dimensional stability and handling prior to the first heating. For these initial binders, temporary binder systems known from the prior art can be used which also comprise, for example, PEG. In this case, preference is given to using higher molecular weight solid PEGs such as PEG 6000 for example.

[0062] In a particularly preferred embodiment of the invention, therefore, the pore former can perform a double function. In the first step, it becomes a constituent of a temporary binder by means of reaction (esterification) with polyethylene glycols. In the second step (at higher temperature), the remainder of the pore former sublimes and is thus removed from the green body. At a still higher temperature, prior to sintering, the temporary binder system is then burned off The invention has identified that this binder system can be burned off without leaving residues and without forming ignitable gas mixtures. So that the pore former can perform this double function, it must be added in sufficient stoichiometric excess relative to the polyethylene glycols. The proportion that did not react with the polyethylene glycols in the first step then remains as pore former in the green body.

[0063] In the context of the invention, a pore former, such as fumaric acid in particular, can be used in two different particle size fractions. A fraction with small particle sizes of, for example, 100 μm or less, preferably 1-100 μm, more preferably 1-30 μm and more preferably 1-20 μm is intended to be available as far as possible in finely distributed form in the green body as reactant for the esterification with PEG.

[0064] A larger granulated fraction, having an average particle size of 1 mm for example, serves primarily for pore formation.

[0065] The invention is elucidated below by means of working examples.

[0066] Example 1:

[0067] 2 identical abrasive bodies of dimensions 300×20×127 were produced from the following formulations. Formulation 1 is a comparative example and formulation 2 is inventive. The ceramic binder used in this and all further examples is a mixture of 50% by weight frit 90158 (Ferro), 25 percent by weight clay and 25% by weight petalite.

TABLE-US-00001 1. Sintered corundum F100 20.8% Special fused alumina F100 62.5% Ceramic binder 16.7% Naphthalene 13.0% (powder ca. 200 μm particle size) Dextrin powder  2.0% Water  2.8% Density after firing: 1.78 g/cm.sup.3 Modulus of elasticity: 29.0 GPa 2. Sintered corundum F100 20.8% Special fused alumina F100 62.5% Ceramic binder 16.7% Fumaric acid 16.1% (powder ca. 200 μm particle size) PEG 300  3.0% PEG 6000  1.0% Density after firing: 1.77 g/cm.sup.3 Modulus of elasticity: 29.0 GPa

[0068] The abrasive body composition according to formulation 1 with naphthalene as pore former was compressed and subsequently dried at 80° C. in a drying oven equipped with thermal afterburning system and the naphthalene was debinded. The abrasive bodies were then fired at a maximum temperature of 950° C. in a ceramic kiln (Energo oven).

[0069] The abrasive body composition according to formulation 2 with fumaric acid as pore former was compressed and subsequently hardened/esterified at a maximum temperature of 165° C. for 24 h in a hardening oven (Reinhardt oven). The following hardness curve was applied (FIG. 1).

[0070] The abrasive bodies were subsequently fired at a maximum temperature of 950° C. in a ceramic kiln (Energo oven). In the heating phase, the heating rate was ca. 30-50° C./h. In the heating phase the fumaric acid was debinded.

[0071] The abrasive bodies produced with the fumaric acid and naphthalene pore formers were compared by sanding on a grinding test stand (Blohm flat grinding machine). The grinding force (FIG. 2), the G ratio (FIG. 3), and the roughness (FIG. 4) were measured in each case relative to the machined workpiece volumes.

[0072] The grinding forces caused by these abrasive bodies are almost identical.

[0073] The G ratio of the abrasive body produced with fumaric acid is somewhat greater and the grinding force somewhat lower.

[0074] The roughness of the abrasive body is also to be assessed as about the same.

[0075] Overall, it can be shown that the grinding performance of both abrasive bodies can be rated as equivalent.

[0076] Example 2:

[0077] Production of a highly porous test specimen (diameter 202, height 100 mm).

[0078] Formulation:

TABLE-US-00002 Special fused alumina F80 88.6% Ceramic binding 11.4% PEG 300 4.0% PEG 6000 3.0% Fumaric acid 40.0% (Granules 500-800 μm) Density after firing: 1.39 g/cm.sup.3

[0079] The formulation components were homogeneously mixed and subsequently compressed. The disk was then debinded in a drying oven equipped with thermal afterburning system. The debinding curve included the heating at 50° C./h up to 200° C., maintaining the maximum temperature of 200° C. over 48 h and the natural cooling of the oven to room temperature. The strength was then fully sufficient to assemble the disk on the kiln car for ceramic firing. The abrasive bodies were then fired at a maximum temperature of 950° C. in a ceramic kiln (Energo oven).