Production method of cordierite ceramic honeycomb structure

Abstract: A production method of a cordierite ceramic honeycomb structure is provided. 0.1 to 2% by weight of a nonionic polyether lubricant, obtained by adding ethylene oxide and propylene oxide to polyhydric alcohol with an ethylene oxide : propylene oxide ratio of 10 : 90 to 100 : 0 by weight, is added to the cordierite material batch for extruding. The honeycomb structure has a thin partition wall, and is suitable for the mass production by improving the formability (in particular, the lubricity and the shape stability) in extruding.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cordierite ceramic honeycomb structure by extruding. 2. Description of Related Art Cordierite ceramic honeycomb structures are used as an exhaust gas purifying catalyst carrier, a filter, or a heat exchanger for automobiles or the industrial use. In order to improve the purifying performance and the heat exchanging efficiency, and reduce the pressure loss, a thinner partition wall is adopted in the cordierite ceramic honeycomb structures. In general, the extruding method is used for the cordierite ceramic honeycomb structure for improving the mass productivity. For the extrusion convenience, a forming auxiliary agent such as a binder and a surfactant is added in the ceramic material batch for extruding. However, in extruding of a thin cordierite ceramic honeycomb structure as conducted recently, a problem is involved in that the unbaked clay can partially aggregate during the extrusion and thus the extrusion property and the shape stability are deteriorated, besides, the pressing area becomes smaller. Further, if the amount of a binder or a surfactant is increased in order to solve the problems, a crack can be generated due to difficulty in the drying process, and a preheating member needs to be wider due to de-ashing in firing. EP-A-680938 describes cordierite honeycomb production in which the batch of material for extrusion includes 0.1 - 10 parts by weight, per 100 parts of raw material for cordierite, of nonionic surface active agent, together with binder, to achieve good extrudable plasticity. US-A-4745092 describes addition of various surfactants to a cordierite raw material batch, such as alkyl ethers of polyethylene glycol and polyoxyethylene esters. US-A-5021373 describes cordierite honeycomb construction in which a liquid mixture was added to achieve desired batch consistency. The liquid comprised 780 ml chilled water, 14 ml triethylene glycol and 14 ml of a PVA/polyethylene glycol (PEG) mixture (which was 100 parts water, 2 parts PVA and 1 part PEG. SUMMARY OF THE INVENTION Accordingly, in order to solve the above-mentioned conventional problems, an object of the present invention is to provide a production method of a cordierite ceramic honeycomb structure, suitable for mass production, capable of coping with a honeycomb structure with a thin partition wall by improving the formability (in particular, the lubricity and the shape stability) in extruding. According to the present invention, there is provided a method of producing a cordierite ceramic honeycomb structure, as set out in claim 1. It is preferable that the polyhydric alcohol used in the present invention is selected from ethylene glycol, glycerol, pentaerythritol, and sorbitol. Further, it is preferable that the nonionic polyether lubricant used in the present invention has 6 or less carbon atoms (functional groups) in the principal chain. It is more preferable that the end group of the nonionic polyether lubricant is a hydroxy ethyl group. It is further preferable that extrusion dies with a 110 µm or less slit width are used in extruding. Any suitable binder agent may be used. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the production method of a cordierite ceramic honeycomb structure according to the present invention, 0.1 to 2% by weight of the nonionic polyether lubricant is added to the cordierite material batch for extruding. Accordingly, a honeycomb structure having a thin partition wall, and suitable for mass production can be obtained by improving the formability (in particular, the lubricity and the shape stability) in extruding. As a main feature of the production method of a cordierite ceramic honeycomb structure according to the present invention, the nonionic polyether lubricant obtained by the condensation polymerization conducted by adding ethylene oxide, or ethylene oxide and propylene oxide to polyhydric alcohol is used. The nonionic polyether lubricant used in the present invention will be explained in further detail. A nonionic polyether lubricant used in the present invention can be obtained by adding ethylene oxide and propylene oxide to polyhydric alcohol with an ethylene oxide : propylene oxide ratio of 10 : 90 to 100 : 0 by weight for the condensation polymerization. That is, oxyethylene groups and oxypropylene groups exist in the nonionic polyether lubricant used in the present invention according to the molar ratio of ethylene oxide and propylene oxide. Accordingly, since a nonionic polyether lubricant having a large number of oxyethylene groups has a sufficient compressive strength (kgf) at the time of extruding, it has an excellent shape stability. Further, since a nonionic polyether lubricant having a large number of oxypropylene groups can restrain the extrusion pressure (kgf) at the time of extruding, it can contribute to the improvement of the lubricity. However, although a nonionic polyether lubricant having too little oxyethylene groups (less than 10% oxyethylene groups) has an excellent lubricity, it is insufficient in terms of the shape stability so that it is important to contain at least 10% of oxyethylene groups in order to compensate the shape stability. The nonionic polyether lubricant is added by 0.1 to 2% by weight as mentioned above because the effect of the lubricant cannot be performed with less than 0.1% by weight so that a complete compact cannot be obtained. On the other hand, concentration difference in the lubricant can be easily generated at the time of drying with more than 2% by weight so that a tear (crack) can be generated at a portion with a high lubricant concentration. As heretofore mentioned, since the formability (in particular, the lubricity and the shape stability) can be appropriately improved by optionally changing the ratio of ethylene oxide and propylene oxide according to the slit width of a honeycomb structure with a thin partition wall, a honeycomb structure with a thin partition wall having a 110 µm or less slit width can be preferably produced as well as the isostatic strength (the hydrostatic pressure fracture strength: [kg/cm 2 ]) of the honeycomb structure after firing can be 10 kg/cm 2 or more. The polyhydric alcohol used in the present invention is not particularly limited, but any one selected from the group consisting of ethylene glycol, glycerol, pentaerythritol, and sorbitol is preferable for improving the lubricity by having a low molecular weight. It is more preferable that the end group of the nonionic polyether lubricant is a hydroxy ethyl group for improving the shape retention property by improving the hydrophilic property. The present invention will be explained in further detail with reference to examples, but the present invention is not limited thereto. (Examples 1 to 8, Comparative Examples 1 to 3) As a cordierite material, which is the main material, one prepared with talc, kaolin (clay mineral), alumina, aluminum hydroxide and silica in the region with 42 to 56% by weight, preferably 47 to 53% by weight of SiO 2 , 30 to 45% by weight, preferably 32 to 38% by weight of Al 2 O 3 , and 12 to 16% by weight, preferably 12.5 to 15% by weight of MgO, centering the cordierite theoretical composition point (2MgO ·2Al 2 O 3 ·5SiO 2 ) conventionally known as a composition for low thermal expansion cordierite ceramics was used. Hydroxypropyl methyl cellulose was added to the obtained cordierite material as a binder by 5% by weight so as to be used as the cordierite material batch. Further, 1% by weight of a nonionic polyether lubricant shown in Table 1 was added to the obtained cordierite material batch. Further, kneading water was added thereto for mixing and kneading so as to obtain clay mixture. The kneading process was conducted such that a 1200 ± 20 g resistance value can be obtained when a round ball with a 5 mm diameter enters into the clay mixture with a 2 cm/min constant velocity. As shown in Table 1, the nonionic polyether lubricant was obtained by adding ethylene oxide (EO) and propylene oxide (PO) with a predetermined ratio to polyhydric alcohol as the starting group for the condensation polymerization so as to have a predetermined average molecular weight. Kind of polyhydric alcohol Composition ratio of oxide (% by weight) Molecular weight Number of carbon atoms in the principal chain (number of functional groups) EO PO Example 1 Ethylene glycol 100 0 400 2 Example 2 Glycerol 70 30 3000 3 Example 3 Ethylene glycol 50 50 3000 2 Example 4 Pentaerythritol 40 60 3300 4 Example 5 Glycerol 30 70 2400 3 Example 6 Glycerol 20 80 3000 3 Example 7 Sorbitol 20 80 6000 6 Example 8 Ethylene glycol 13 87 4500 2 Comparative example 1 Glycerol 10 90 6000 3 Comparative example 2 Ethylene glycol 0 100 3000 2 Comparative example 3 Glycerol 0 100 6000 3 With the obtained clay mixture, a honeycomb compact and a honeycomb sintered body were produced. Then, the extrusion pressure, the compressive strength, and the isostatic strength (hydrostatic pressure fracture strength: [kg/cm 2 ]) were measured with the below-mentioned conditions. Results are shown in Table 2. (Measuring method for the extrusion pressure) The pressure (kgf) necessary for forming a cylindrical honeycomb compact with a 20 mm diameter at a 30 cm/sec extrusion speed with the obtained clay mixture by extrusion dies having a 110 µm extrusion slit and a 60 cell/cm 2 cell density was measured. (Measuring method for the compressive strength) The maximum resistance value (kgf) generated when a 75 × 75 × 60 mm 3 honeycomb compact formed with the obtained clay mixture passed through extrusion dies having a 920 µm extrusion slit and a 4 cell/cm 2 cell density was compressed with a 40 × 60 mm 2 flat plate at a 2 cm/min constant velocity was measured. (Measuring method for the isostatic strength (hydrostatic pressure fracture strength: [kg/cm 2 ]) A honeycomb sintered body was produced by firing a cylindrical honeycomb compact, which was formed by passing through extrusion dies having a 110 µm extrusion slit and a 60 cell/cm 2 cell density, having a 100 mm diameter and a 150 mm height at 1400°C or more after drying. The pressure (kg/cm 2 ) at which the obtained honeycomb sintered body was broken in the hydrostatic pressure was measured. Moldability ISO strength (kg/cm 2 ) Extrusion pressure (kgf) Compressive strength (kgf) Comprehensive judgment Example 1 28.2 4.5 o ○ 20.6 Example 2 26.5 3.8 o ○ 18.2 Example 3 27.5 3.5 ○ 17.2 Example 4 22.2 2.8 ○ 18.3 Example 5 26.6 2.9 ○ 21.4 Example 6 23.7 3.0 ○ 16.2 Example 7 24.5 2.7 ○ 13.8 Example 8 24.9 2.4 ○ 15.7 Comparative example 1 30.2 2.1 Δ 7.3 Comparative example 2 25.3 1.3 Δ 11.3 Comparative example 3 26.1 1.8 Δ 9.6 In Table 2, o ○ denotes Excellent, o denotes Good and Δ denotes Inferior. As heretofore mentioned, according to a production method of a cordierite ceramic honeycomb structure of the present invention, a honeycomb structure having a thin partition wall, and suitable for the mass production can be obtained by improving the formability (in particular, the lubricity and the shape stability) in extruding.

Claims: A method of producing a cordierite ceramic honeycomb structure, which comprises: adding 0.1 to 2% by weight of a nonionic lubricant to a cordierite material batch; and forming the cordierite material batch by extrusion, wherein the nonionic lubricant is a nonionic polyether lubricant having an average molecular weight of 10,000 or less obtained by polymerising ethylene oxide and optionally propylene oxide with polyhydric alcohol, with an ethylene oxide : propylene oxide ratio of 13:87 to 100:0 by weight. The method of producing a cordierite ceramic honeycomb structure according to claim 1, wherein the polyhydric alcohol is selected from ethylene glycol, glycerol, pentaerythritol and sorbitol. The method of producing a cordierite ceramic honeycomb structure according to claim 1 or 2, wherein an extrusion die or dies with a 110 µm or less slit width is/are used in the extruding.

Inventors: KOTANI WATARU, MAKINO KYOKO, KOTANI, WATARU, MAKINO, KYOKO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1998
Main classification: C04B35
IPC codes: C04B35, B01J21, B28B3, B01J35