Method for producing light ceramic materials

Abstract

The present invention relates to a novel process for producing ceramic materials, in particular refractory materials having a reduced relative density. In particular, the invention relates to a process for producing light, refractory materials having non-contiguous pores based on shaped and unshaped materials. These materials can be used as working lining in high-temperature applications. The process is based on the production of spherical, closed and isolated pores in the microstructure of the material. The pores having a pore diameter which can be set in a targeted manner are generated by use of polymer particles, in particular polymethacrylates, in particular polymers or copolymers prepared by means of suspension polymerization, as pore formers which can be burnt out. The polymers or copolymers are present in the form of small spheres having a defined diameter. The introduction of isolated spherical pores allows the production of ceramic materials having a sometimes significantly reduced relative density and improved corrosion resistance and better mechanical strength compared to the prior art. The specific, closed pore system at the same time contributes to reducing the thermal conductivity of the ceramic materials. In addition, the novel process has the advantage that there is no risk of formation of undesirable black cores, even in the production of thick-walled ceramic products.

Claims

1. A process for producing a ceramic material, comprising admixing a ceramic raw composition with from 0.5 to 90% by weight of spherical polymer particles having a diameter in the range from 5 m to 3 mm, based on the sum of ceramic raw composition and spherical polymer particles, to obtain a mixture, optionally drying the mixture, and optionally thermally treating the mixture, wherein said ceramic raw composition comprises at least one member selected from the group consisting of aluminum oxide (Al.sub.2O.sub.3), silicon oxide (SiO.sub.2), chrome oxide (Cr.sub.2O.sub.3), zirconium dioxide (ZrO.sub.2), titanium(IV) oxide (TiO.sub.2), magnesium oxide (MgO), tin oxide (SnO), mullite (3Al.sub.2O.sub.3.2SiO.sub.2), spinel (MgO.Al.sub.2O.sub.3), zirconium silicate (ZrO.sub.2.SiO.sub.2), a first calcium aluminate (6Al.sub.2O.sub.3.CaO), a second calcium aluminate (CaO.Al.sub.2O.sub.3), forsterite (2MgO.SiO.sub.2), calcium silicate (2CaO.SiO.sub.2), calcium zirconate (2CaO.ZrO.sub.2), cordierite (2MgO.2Al.sub.2O.sub.3.5SiO.sub.2), aluminum titanate (Al.sub.2O.sub.3.TiO.sub.2).

2. A process according to claim 1, further comprising processing the ceramic raw composition into a ceramic material, wherein the ceramic raw composition comprises less than 10% by weight of ceramic particles which are larger than 0.6 mm, and the process further comprises subsequently drying the mixture, and, optionally, conditioning and thermally treating the mixture at a temperature of greater than 1,000 C.

3. A process according to claim 1, wherein the spherical polymer particles have a monomodal particle size distribution.

4. A process according to claim 1, wherein the spherical polymer particles comprise a polymer having a ceiling temperature of less than 280 C., and the process further comprises baking the ceramic raw composition at a temperature that is at least 100 C. above the ceiling temperature.

5. A process according to claim 1, wherein the polymer is a polymethacrylate, polyoxymethylene or poly--methylstyrene and wherein the particles have a diameter in the range from 10 m and 200 m.

6. A process according to claim 5, wherein the polymethacrylate has an oxygen content of at least 25% by weight.

7. A process according to claim 6, wherein the polymer is a polymethacrylate having a methyl methacrylate content of at least 80% by weight.

8. A process for producing a ceramic material, comprising admixing a ceramic raw composition with from 0.5 to 90% by weight of spherical polymer particles having a diameter in the range from 5 m to 3 mm, based on the sum of ceramic raw composition and spherical polymer particles, to obtain a mixture, optionally drying the mixture, and optionally thermally treating the mixture, wherein said ceramic raw composition comprises at least one member selected from the group consisting of aluminum oxide (Al.sub.2O.sub.3), silicon oxide (SiO.sub.2), chrome oxide (Cr.sub.2O.sub.3), zirconium dioxide (ZrO.sub.2), titanium(IV) oxide (TiO.sub.2), magnesium oxide (MgO), tin oxide (SnO), mullite (3Al.sub.2O.sub.3.2SiO.sub.2), spinel (MgO.Al.sub.2O.sub.3), zirconium silicate (ZrO.sub.2.SiO.sub.2), a first calcium aluminate (6Al.sub.2O.sub.3.CaO), a second calcium aluminate (CaO.Al.sub.2O.sub.3), forsterite (2MgO.SiO.sub.2), calcium silicate (2CaO.SiO.sub.2), calcium zirconate (2CaO.ZrO.sub.2), cordierite (2MgO.2Al.sub.2O.sub.3.5SiO.sub.2), aluminum titanate (Al.sub.2O.sub.3.TiO.sub.2), wherein the proportion of the spherical polymer particles added to the ceramic raw composition is in the range from 40 to 70% by weight.

9. A process according to claim 1, wherein the polymer of the spherical polymer particles is a thermoplastic suspension polymer.

10. A process according to claim 1, wherein the spherical polymer particles have a particle size distribution in the range from 0.5 to 2.0.

11. A process according to claim 2, wherein the spherical polymer particles are present in the form of a suspension in liquid, synthetic resin, or alcohol.

12. A process according to claim 2, wherein the spherical polymer particles are present in the form of a suspension in water, synthetic resin, or alcohol.

13. A process according to claim 1, which comprises drying the mixture, and optionally thermally treating the mixture.

14. A process according to claim 1, which comprises drying the mixture and thermally treating the mixture.

Description

EXAMPLES

(1) Measurement Methods

(2) Particle size distribution of the polymer particles: the measurement is carried out using a Coulter instrument LS 200. Sample preparation: 2 spatulas of test substance are slurried in about 20 ml of deionized water in a 50 ml glass beaker. One drop of Triton X-100 solution is subsequently added and the sample is de-aerated for 1 minute in an external ultrasonic bath.

(3) Measurement procedure: the measurement is carried out at a concentration of from 9% to 11%. The course of the measurement is computer-controlled. Three individual measurements are carried out. The results reported are the average d.sub.V50 of these.

(4) Thermogravimetric (TGA) tests in an air atmosphere using a heating rate of 5 K/min to a maximum temperature of 1000 C. This temperature is maintained until the weight is constant.

(5) Weight of sample: pure polymer particles: about 2 g

(6) Polymer particles embedded in a refractory matrix: about 20 g

(7) The determination of the basic properties of the materials examined was carried out by methods based on the following EN DIN standards:

(8) Open porosity (OP) and overall density (OD): as per EN DIN 993-1

(9) Cold compressive strength (CCS): as per EN DIN 993-5

(10) Shrinkage (S): as per EN DIN 993-10

(11) The polymers of the Degacryl grade (available from Evonik Rhm GmbH) used are suspension polymers of pure PMMA. In detail, the products used have the following characteristics. The weight average molecular weight was determined by means of gel permeation chromatography (GPC).

(12) DEGACRYL M449: PMMA with M.sub.w: 400 000-500 000 and d.sub.y50: 90-110 m

(13) DEGACRYL M527: PMMA with M.sub.w: 450 000-560 000 and d.sub.y50: 33-41 m

(14) DEGACRYL M546: PMMA with M.sub.w: 400 000-500 000 and d.sub.y50: 55-70 m

A) Light Thermally Insulating Ceramics

Examples 1-4: Light Cast Refractory Materials

(15) A fine-grained refractory concrete as mixture of calcined alumina (CT) and a polymer body was examined as ceramic raw composition. To study the influence of a coarser aluminium oxide, mixtures comprising sintered alumina (T60, particle size less than 45 m) were also examined. As binder, use was made of 4 parts by weight of calcium aluminate refractory cement: the concrete composition was produced using 12% by weight (per 100 parts of dry mass) of water (referred to as make-up water). DEGACRYL M449 (M449) in various added amounts was used as polymer body. The polymer particles were firstly mixed with the fully mixed concrete raw composition by stirring. Cylindrical test specimens (diameter and height 46 mm) were cast from the ceramic raw composition containing the polymer particles. This was effected by pouring into a plastic mould. The test specimens were subsequently dried at 110 C. for four hours. After drying, the ceramic was fired at 1500 C. in an air atmosphere for 4 hours. The composition of the mixtures examined and the properties of the fired test specimens are shown in Table 1.

(16) TABLE-US-00001 TABLE 1 Light cast refractory materials Cold Overall density compressive Composition (%) (fired) strength CT T60 M449 (g/cm.sup.3) relative (%) (N/mm.sup.2) Shrinkage (%) Example 1 50.0 50.0 0.76 19.5 0.8 2.8 Example 2 31.3 31.3 37.5 0.95 24.4 3.1 1.3 Example 3 65.0 35.0 1.13 29.0 1.7 4.7 Example 4 31.3 31.3 37.5 0.85 21.8 0.6 1.7
Results The cast light refractory materials according to the invention display very low overall densities. The overall density is in the range from about 16 to 30% of the theoretical value. This corresponds to a porosity in the range from 70 to 84%. The densification of the microstructure can be controlled by the magnitude of the proportion of polymer particles. The materials have a low cold compressive strength which is characteristic of the type of material due to the high porosity. The CCS value can be additionally influenced in a positive way by targeted measures. These include, inter alia, the partial replacement of the calcined alumina by fine sintered alumina. It is clear from the TGA curves appended to the drawings that the polymer particles used according to the invention can be removed from the ceramic without leaving a residue at temperatures of less than 500 C.

Example 5: Light Cast Alumina Materials (with Comparative Example 1)

(17) As ceramic raw composition, a slip was produced from 90% by weight of calcined alumina and 10% by weight of a calcium aluminate as binder. The concrete composition was admixed with 14.5% by weight (per 100 parts of dry mass) of make-up water. 30% by weight of DEGACRYL M527 (M527) (based on 100% by weight of slip) was used as polymer particles. The polymer particles were firstly mixed with the fully mixed ceramic raw composition by stirring. Cylindrical test specimens (diameter and height 46 mm) were subsequently cast from the ceramic raw composition containing the polymer particles. This was effected by pouring into a plastic mould. The test specimens were subsequently dried at 110 C. for four hours. After drying, the ceramic was fired at 1500 C. in an air atmosphere for 4 hours.

(18) For comparison, mixtures with 20% by weight of a conventional aggregate which can be burnt out (shavings) were also examined. To be able to incorporate this amount of aggregate into the ceramic composition, the amount of water added had to be increased to about 28% by weight. The other production and test conditions were identical. The results achieved are shown in Table 2.

(19) TABLE-US-00002 TABLE 2 light cast alumina materials Aggregate Dried Fired Amount OD CCS OD relative CCS Shrinkage Designation (% by weight) g/cm.sup.3 N/mm.sup.2 g/cm.sup.3 (%) N/mm.sup.2 % Example 5 DEGACRYL 30 1.90 13.3 1.57 39.3 43.0 5.2 M527 Comparative Conventional 20 1.58 0.8 1.55 38.8 24.7 5.1 Example 1
Results The ceramic composition with an addition of DEGACRYL requires significantly less (about 50%) make-up water compared to the conventional product despite a higher amount of aggregate. The strength of the dried DEGACRYL composition is very high compared to the conventional aggregate At an approximately equal overall density, the fired ceramic compositions containing proportions of polymer display a very high strength. This is almost 75% higher than that of the materials containing the conventional other aggregate.

Examples 6-9: Light Cast Mullite Materials (with Comparative Example 2)

(20) A homogeneous mixture of a mullite raw mixture and Degacryl M449 was produced by means of stirring. The mullite raw mixture was a raw materials mixture used industrially for producing sintered mullite. The moisture content of the mixture was about 16% by weight. The proportion of Degacryl was, dependent on the example, in the range from 10 to 70% by weight (see Table 3). The make-up water requirement was, as a function of the proportion of Degacryl, in the range from 33 to 45% by weight. The ceramic raw materials were firstly mixed with the make-up water. Degacryl M449 was introduced at the end of the mixing process and homogeneously dispersed. The test specimens (diameter and height in each case 46 mm) were produced without binder by casting into a plastic mould. The mouldings were dried at 110 C., for 24 hours. The test specimens were subsequently subjected to a two-stage firing process in an air atmosphere, using the following parameters:

(21) Stage I. firing temperature 1000 C., heating rate 1 K/min, without hold time

(22) Stage II. firing temperature 1600 C., heating rate 5 K/min, hold time 4 h

(23) The overall density (OD) was determined on the dried test specimens. The overall density (OD), the open porosity (OP), the cold compressive strength (CCS) and the shrinkage (S) were determined on the fired specimens.

(24) TABLE-US-00003 TABLE 3 110 C. 1000 C. 1600 C. Degacryl H.sub.2O OD OD OD CCS M449 % requirement % g/cm.sup.3 g/cm.sup.3 OP % S % g/cm.sup.3 OP % S % N/mm.sup.2 CE 2 0 33.5 1.32 1.27 59.0 0.9 1.77 42.7 11.7 32.5 Ex. 6 10 34.0 1.27 1.10 64.7 1.0 1.58 49.2 12.7 27.4 Ex. 7 30 34.0 1.21 0.86 72.4 1.0 1.24 60.1 12.7 10.5 Ex. 8 50 42.5 1.14 0.70 77.3 1.9 1.02 67.1 13.5 7.8 Ex. 9 70 42.5 1.10 0.59 80.9 1.9 0.88 71.6 14.6 4.1 The H.sub.2O additions are based on the mass without Degacryl M449.

(25) The results show that light, highly porous mullite materials having good strength values can be produced with the addition of polymer particles.

Examples 10-13: Light Cast Al2O3-CA6 Materials (with Comparative Example 3)

(26) In these examples, refractory ceramics corresponding to commercially available ceramics containing calcium hexaaluminate (CaO*6Al.sub.2O.sub.3) as light microporous aggregate were produced according to the invention. The said product contains about 8.5% by weight of CaO and about 91% by weight of Al.sub.2O.sub.3. It has an open porosity of about 75% by volume. The product is used as particulate material for producing light thermally insulating refractory products. A disadvantage of the material is the relatively high CaO content which impairs the corrosion resistance and the thermomechanical properties of the refractory materials.

(27) In these examples, ceramics according to the invention having a lower CaO content but thermal insulation properties comparable to the conventional product were produced. Degacryl M 527 was used as pore former.

(28) The ceramic composition used as slip was composed of 90% by weight of calcined alumina NO 645, 10% by weight of calcium aluminate cement SECAR 71, 1% by weight of plasticizer (ADS, ADW) and various amounts of make-up water (see Table 4). The calculated chemical composition after setting of the mixture used is 3% by weight of CaO and 97% by weight of Al.sub.2O.sub.3. This corresponds to a calcium hexaaluminate content of about 34% by weight. The remainder is formed by -alumina (-Al.sub.2O.sub.3). The Degacryl content was, depending on the example, in the range from 10 to 70% by weight (see Table 4). The ceramic raw materials were firstly mixed with the make-up water. The make-up water requirement was, depending on the Degacryl content, in the range from 12 to 30% by weight (see Table 4). Degacryl M527 was added at the end of the mixing process and homogeneously dispersed.

(29) The test specimens (diameter and height in each case 46 mm) were produced by casting into a plastic mould. After setting, the mouldings were dried at 110 C. for 24 hours. The test specimens were subsequently subjected to a two-stage firing process in an air atmosphere using the following parameters:

(30) Stage I. firing temperature 1000 C., heating rate 1 K/min, without hold time

(31) Stage II. firing temperature 1600 C., heating rate 5 K/min, hold time 4 hours

(32) The overall density (OD) was determined on the dried test specimens. The overall density (OD), the open porosity (OP), the cold compressive strength (CCS) and the shrinkage (S) were determined on the fired specimens.

(33) TABLE-US-00004 TABLE 4 110 C. 1000 C. 1600 C. Degacryl H.sub.2O OD OD OD CCS M527 % requirement % g/cm.sup.3 g/cm.sup.3 OP % S % g/cm.sup.3 OP % S % N/mm.sup.2 CE 3 0 12.5 2.67 2.58 33.8 0.1 3.07 21.4 6.0 261 Ex. 10 10 12.5 2.37 2.13 45.5 0.4 2.48 36.5 5.6 217 Ex. 11 30 14.5 1.95 1.45 62.9 0.2 1.68 56.9 5.0 38 Ex. 12 50 20.0 1.64 1.07 72.6 0.1 1.30 66.6 6.7 21 Ex. 13 70 30.0 1.40 0.83 78.7 0.9 1.03 73.5 8.2 9.8 The H.sub.2O additions are based on the mass without Degacryl M527.

(34) The additive Degacryl M527 brings about an increase in the open porosity. At an addition of over 30% by weight, an increase to more than 55% by volume can even be achieved. Even greater added amounts result in light, highly porous materials having an open porosity of greater than 70% by volume. These materials in particular display a very good ratio of strength to overall density.

Examples 14-17: Light Cast Alumina Materials Having a Proportion of Coarse Particles (with Comparative Example 4)

(35) These examples show that light ceramic materials according to the invention can be produced even with addition of coarsely particulate components. On the basis of the results achieved in the preliminary tests, a ceramic composition was used as slip composed of 47.5% by weight of sintered alumina T60, 47.5% by weight of calcined alumina No 645, 5% by weight of calcium aluminate cement SECAR 71, 1% by weight of plasticizer (ADS, ADW) and various amounts of make-up water (see Table 5) for the experiments.

(36) Degacryl M546 was used as polymer body. The Degacryl content was in the range from 10 to 70% by weight (for amounts, see Table 5). The ceramic raw materials were firstly mixed with the make-up water. The make-up water requirement was, depending on the Degacryl content, in the range from 12 to 30% by weight (see Table 5). Degacryl M546 was added at the end of the mixing process and homogenously dispersed by means of stirring. The test specimens (diameter and height in each case 46 mm) were produced by casting into a plastic mould. After setting, the mouldings were dried at 110 C. for 24 hours. The test specimens were subsequently subjected to a two-stage firing process in an air atmosphere using the following parameters:

(37) Stage I. firing temperature 1000 C., heating rate 1 K/min, without hold time

(38) Stage II. firing temperature 1600 C., heating rate 5 K/min, hold time 4 h

(39) The overall density (OD) was determined on the dried test specimens. The overall density (OD), the open porosity (OP), the cold compressive strength (CCS) and the shrinkage (S) were determined on the fired samples.

(40) TABLE-US-00005 TABLE 5 110 C. 1000 C. 1600 C. Degacryl H.sub.2O OD OD OD CCS M546 % requirement % g/cm.sup.3 g/cm.sup.3 OP % S % g/cm.sup.3 OP % S % N/mm.sup.2 CE 4 0 12.5 2.64 2.58 33.8 0.1 2.90 25.6 4.1 227 Ex. 14 10 12.5 2.33 2.07 46.9 0.2 2.34 40.1 3.9 140 Ex. 15 30 14.5 1.86 1.40 64.2 0.2 1.56 60.0 3.8 23 Ex. 16 50 20.0 1.53 1.00 74.2 0.3 1.14 70.9 4.5 12 Ex. 17 70 30.0 1.33 0.75 80.9 0.2 0.87 77.6 5.3 3.8 The H.sub.2O additions are based on the mass without Degacryl M546.

(41) Even after addition of a coarse particle fraction, a refractory ceramic according to the invention can be produced. In addition, the firing shrinkage of the materials can even be reduced here by addition of Degacryl. On the other hand, the coarse particle fraction effects a reduction in the strength values.

(42) The formation of spherical and isolated pores in the materials from Examples 15 and 17 and the absence of such pores in the case of Comparative Example CE 4 can readily be seen from FIGS. 5-7.

Example 18: Comparison with Conventional Aggregates which can be Burnt Out (with Comparative Example 5)

(43) A ceramic composition comprising 90% by weight of calcined alumina NO 645 and 10% by weight of calcium aluminate cement SECAR 71 was used as slip. 1% by weight of plasticizer (ADS+ADW) and 14.5 percent by weight of make-up water were additionally added to this. This composition was divided into two equal portions. 30% by weight of Degacryl M527 was subsequently added to the first portion for Example 18 and 20% by weight of shavings was added to the second portion for Comparative Example 5. Both amounts of the aggregates had the same volume.

(44) The polymer particles are firstly mixed with the fully mixed ceramic composition by stirring in Example 18. Cylindrical test specimens (diameter and height in each case 46 mm) were produced from the slip containing the polymer particles by casting into a plastic mould. The test specimens were subsequently dried at 110 C. for 4 hours. After drying the ceramic is fired at 1500 C. in an air atmosphere for 4 hours.

(45) In the case of Comparative Example 5, the corresponding procedure was followed with addition of 20% by weight of a conventional aggregate which can be burnt out (shavings). In order to be able to incorporate this amount of aggregate into the slip of ceramic composition, the addition of water had to be increased to about 28% by weight. The other production and test conditions were the same in both cases. The results achieved are shown in Table 5

(46) TABLE-US-00006 TABLE 5 Light cast Al.sub.2O.sub.3 materials Aggregate Dried Fired OD CCS OD CCS Designation Amount % g/cm.sup.3 N/mm.sup.2 g/cm.sup.3 relative (%) N/mm.sup.2 Shrinkage % Ex. 18 DEGACRYL 30 1.90 13.3 1.57 39.3 43.0 5.2 M527 CE 5 Shavings 20 1.58 0.8 1.55 38.8 24.7 5.1

(47) The ceramic composition with addition of Degacryl required significantly less (by about 50%) make-up water compared to the product according to the prior art despite a larger amount of aggregate. The strength of the dried Degacryl composition is very high compared to the aggregate according to the prior art. At approximately the same overall density, the fired ceramic compositions containing proportions of polymer are characterized by a very high strength. It is virtually 75% higher than that of the materials containing the conventional aggregate.

Example 19 and Comparative Example 6: Dry-Pressed Light Materials

(48) A finely particulate mullite material, max. particle size 100 m, served as model. DEGACRYL M449 was employed as the aggregate which could be burnt out. The proportion of the polymer was 30% by weight. The mullite raw material was dry mixed with the DEGACRYL. 10% by weight of sulphite waste liquor was added as binder. Cylindrical standard test specimens 5050 mm were produced from the homogeneously mixed mix by uniaxial pressing in a steel mould. The pressing pressure was 50 MPa. The test specimens were dried at 110 C. for 24 hours and then fired at 1500 C. for 2 hours. The cold compressive strength and overall density of the fired test specimens were examined. For comparison, mullite materials produced using 30% by weight of wood sawdust were also examined. Production and testing were identical. The results achieved are shown in Table 6.

(49) TABLE-US-00007 TABLE 6 Overall density Aggregate which can be relative to CCS burnt out (g/cm.sup.3) TD (%) (N/mm.sup.2) Example 10 DEGACRYL M449 1.03 32.9 0.9 CE 6 Wood sawdust 1.03 32.9 0.4 TD = Theoretical density of the material (pure density)

(50) The strength of the light mullite materials produced using DEGACRYL is a factor of 2.2 higher than that obtained using conventional aggregate.

Example 20 and Comparative Example 7: Plastically Moulded Refractory Light Materials

(51) This example specifically relates to a plastically moulded schamotte material. A refractory clay served as basic raw material. DEGACRYL M527 was employed as the aggregate which could be burnt out. The plastic ceramic composition was produced from 82% by weight of clay and 12% by weight of water. The composition was then homogeneously mixed with the polymer in a ratio of 30% by weight of DEGACRYL M527 per 100% by weight of ceramic composition. Cubic test specimens having an edge length of 30 mm were produced from the plastic composition. The test specimens were dried at 110 C. for 24 hours and then fired at 1000 C. for 2 hours. The cold compressive strength and the overall density were determined on the fired test specimens. For comparison, schamotte materials produced using wood sawdust were also examined (see Comparative Example 4). Owing to difficulties with production of the composition containing large amounts of wood sawdust, the proportion of this aggregate was reduced to 20% by weight. The other production and test conditions remain unchanged. The results achieved are shown in Table 7.

(52) TABLE-US-00008 TABLE 7 Properties of plastic light schamotte materials Amount added (parts by weight per 100 parts by Overall Aggregate which weight of ceramic density CCS can be burnt out composition) (g/cm.sup.3) (Nmm.sup.2) Example DEGACRYL M527 30 0.83 4.5 20 CE 7 Wood sawdust 20 1.01 4.7

(53) The strength of the light schamotte produced using DEGACRYL is, at a virtually 20% lower overall density, approximately as strong as the product produced using traditional aggregates.

B) Dense Refractory Ceramics

Examples 21-25: Pressed Alumina Materials with Comparative Example 8

(54) The objective of this series of experiments was to compare various Degacryl grades: DEGACRYL M449, DEGACRYL M527, DEGACRYL M546.

(55) The amount of polymer particles added is:

(56) Comparative Example 8: 0% by weight

(57) Example 21: 1% by weight of DEGACRYL M449

(58) Example 22: 5% by weight of DEGACRYL M449

(59) Example 23: 10% by weight of DEGACRYL M449

(60) Example 24: 5% by weight of DEGACRYL M527

(61) Example 25: 5% by weight of DEGACRYL M546

(62) The experiments were carried out on a pressed alumina material having the following particle structure:

(63) Sintered alumina: 1-2 mm50% by weight

(64) Sintered alumina: 0.2-0.6 mm10% by weight

(65) Sintered alumina: <0.1 mm40% by weight

(66) Sulphite liquor (4% by weight) is used as temporary binder. The polymer particles (amounts: see below) are firstly mixed dry with the ceramic raw composition by stirring. Test specimens having edge lengths of 36 mm are pressed from the ceramic raw composition containing the polymer particles. This is effected by uniaxial pressing in a steel mould under a pressing pressure of 100 MPa. The test specimens are subsequently dried at 110 C. for 5 hours. After drying, the ceramic is fired at 1500 C. in an air atmosphere for 4 hours.

(67) The results are shown in Table 8.

(68) TABLE-US-00009 TABLE 8 Cold Overall density Overall density compressive (dried) (fired) strength (g/cm.sup.3) relative (g/cm.sup.3) relative (N/mm.sup.2) relative Shrinkage (%) CE 8 2.84 100.0 2.82 100.0 24.1 100.0 0.3 Ex. 21 2.79 98.2 2.74 97.1 18.4 76.4 0.3 Ex. 22 2.62 92.0 2.49 88.2 6.8 28.1 0.4 Ex. 23 2.42 85.1 2.21 78.5 2.4 10.1 0.4 Ex. 24 2.57 90.4 2.45 87.0 5.0 20.9 0.4 Ex. 25 2.60 91.3 2.47 87.7 6.3 26.2 0.4
Results Addition of DEGACRYL to a pressed alumina material brings about a significant decrease in its overall density In a direct comparison, the M449 and M527 products perform better than M546.

Example 26: Pressed Alumina Material Using a Reactive Binder with Comparative Example 9

(69) The objective of the study was to test whether the decrease in strength caused by DEGACRYL could be reduced by use of a reactive binder. The experiments were carried out on a pressed alumina material having the particle structure as in Examples 21-25. Degacryl M527 served as polymer particles. The product was introduced dry and mixed with other components. The amount added was 2% by weight. The test specimens (diameter=height=36 mm) were produced by uniaxial pressing in a steel mould using a pressing pressure of 100 MPa. SDX gel (4%) is used as reactive binder. The dried (110 C., 10 h) test specimens are fired at 1500 C., in an air atmosphere for 4 hours. The results are shown in Table 9.

(70) Comparative Example 9 was carried out analogously without addition of the polymer particles.

(71) TABLE-US-00010 TABLE 9 Cold compressive Overall M527 content strength density Shrinkage (% by weight) (N/mm.sup.2) (g/cm.sup.3) (%) Comparative 0 48.4 2.92 0 Example 9 Example 26 2 34.6 2.78 0
Results The addition of 2% by weight of polymer particles results in a reduction in the overall density of about 5%. The decrease in strength of the pressed alumina materials bound by means of SDX gel is about 23%. Addition of a comparable amount of M449 to the alumina material with a conventional binder brings about a reduction in strength of about 40%. It follows from this that the weakening of the microstructure of the materials produced using DEGACRYL can be significantly reduced by use of a reactive binder.

Examples 27-31: Finely Particulate Alumina Concrete with Comparative Example 10

(72) A finely particulate alumina concrete having the following particle structure: sintered alumina <0.045 mm50% by weight, calcined alumina 50% by weight served as experimental material. The DEGACRYL product M527 was used as pore former. It was introduced dry and mixed with other components. The added amount is: 0, 1, 2, 5, 7 and 10% by weight. The test specimens (diameter=height=46 mm) were produced by casting into a plastic mould. A calcium aluminate refractory cement (4%) is used as binder. The set and dried (110 C., 10 h) test specimens were fired at 1600 C. in an air atmosphere for 4 hours. The following properties are determined on the fired test specimens as a function of the amount of M527 added: overall density (OD), open porosity (OP), cold compressive strength (CCS), linear shrinkage (S). The results obtained are shown in Table 10.

(73) TABLE-US-00011 TABLE 10 Proportion of M527 110 C. 1600 C. (% by OD CCS OD weight) (g/cm.sup.3) S (%) (N/mm.sup.2) OP (%) (g/cm3) CE 10 0 2.63 4.9 274.4 22.7 3.05 Example 27 1 2.59 5.4 270.9 23.7 3.01 Example 28 2 2.57 5.0 266.8 24.4 2.96 Example 29 5 2.50 5.3 232.4 29.0 2.77 Example 30 7 2.42 5.4 221.7 32.4 2.65 Example 31 10 2.37 5.2 179.1 35.6 2.53
Result The overall density of the finely particulate alumina concretes can be reduced by up to 5% by addition of M527 without any appreciable impairment of the other materials parameters. The added amount of M527 required for this is about 2-3%.

Examples 32-33: Coarsely Particulate Alumina Concrete with Comparative Example 11

(74) An industrial alumina concrete served as experimental material. The DEGACRYL product M527 was used as pore former. It was introduced dry and mixed with other components. The amount added is: 0, 2, 5% by weight. The test specimens (diameter=height=46 mm) are produced by casting into a plastic mould. A calcium aluminate refractory cement (4%) is used as binder. The set and dried (110 C., 10 h) test specimens were fired at 1600 C. in an air atmosphere for 4 hours. The following properties are determined on fired test specimens as a function of the amount of M527 added: overall density (OD), open porosity (OP), cold compressive strength (CCS), linear shrinkage (S). The results obtained are shown in Table 11.

(75) TABLE-US-00012 TABLE 11 110 C. 1600 C. Proportion of OD S CCS OP OD M527 (%) (g/cm.sup.3) (%) (N/mm.sup.2) (%) (g/cm3) Comparative 0 3.21 0.80 247.14 11.97 3.25 Example 11 Example 32 2 3.10 0.71 222.16 18.28 3.08 Example 33 5 2.96 0.52 77.79 26.19 2.84
Result

(76) In a coarsely particulate industrial alumina concrete, the justifiable added amount of M527 is 2-3%. The resulting reduction in the weight of the industrial concretes is 5-6%.

DRAWINGS

(77) FIG. 1: Schematic depiction of the microstructure of a light refractory ceramic with pore formation according to the prior art with (1) matrix of the ceramics; (4) pore which is not according to the invention; the coarse particles of a pressed ceramic which are optionally present in a manner analogous to FIG. 2 are not shown in the interests of clarity

(78) FIG. 2: Schematic depiction of the microstructure of a light, pressed refractory ceramic with pore formation as per the process of the invention with (1) matrix of the ceramic; (2) pore; (3) coarse particle. A cast ceramic would have no coarse particles.

(79) FIG. 3: TGA of the polymer particle DEGACRYL M449

(80) FIG. 4: TGA of DEGACRYL M449 in a refractory alumina concrete as per Example 3; weight normalized to polymer content

(81) FIG. 5: TGA of DEGACRYL M449 in a refractory alumina concrete as per Example 16; weight normalized to polymer content

(82) FIG. 6: Optical micrograph of a cross section of the fired ceramic from Comparative Example CE 8

(83) FIG. 7: Optical micrograph of a cross section of the fired ceramic from Example 25 (with 30% by weight of Degacryl M546)

(84) FIG. 8: Optical micrograph of a cross section of the fired ceramic from Example 27 (with 70% by weight of Degacryl M546)