Stock material or miscellaneous articles – Structurally defined web or sheet – Honeycomb-like
Reexamination Certificate
2000-04-28
2004-01-20
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Honeycomb-like
C428S034400, C428S188000, C264S631000, C264S630000, C264S638000, C264S669000, C264S670000, C055S523000, C055S524000, C422S180000, C422S222000
Reexamination Certificate
active
06680101
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an extrusion-molded honeycomb material used, for example, as a catalyst carrier for automobile exhaust gas purification, as well as to a process for producing the honeycomb material.
In recent years, the regulation of exhaust gases has become stricter for environmental protection. In this connection, a catalyst for exhaust gas purification having a higher purification ability is required. Meanwhile, an engine of lower fuel consumption and higher output is required strongly. To respond to such a situation, the catalyst for exhaust gas purification is required to show lower pressure loss in addition to the higher purification ability.
To satisfy these requirements, it is vigorously desired to allow the honeycomb structure used in the catalyst for exhaust gas purification to have a thinner partition wall so that the honeycomb structure can enable easier gas flow and lower pressure loss and the catalyst can have a lighter weight and lower heat capacity and can have a higher purification ability during engine warm-up. The partition wall thickness of honeycomb structures has heretofore been 150 &mgr;m (6 mil) mainly, but it is shifting to 50 &mgr;m (2 mil) mainly. Incidentally, “honeycomb structure” refers to a structure in which a large number of cells are separated by partition walls.
A honeycomb structure is ordinarily produced by mixing a raw material powder (e.g. a ceramic powder or a metal powder) with a binder or the like, subjecting the resulting mixture to extrusion molding through a die having lattice-like slits, and drying and firing the resulting extrudate. As the above binder, there has been used a water-soluble thermosetting methyl cellulose type binder.
As the partition wall of honeycomb structure becomes thinner, the width of the slits of the die needs to be smaller. Therefore, in molding a honeycomb structure having a thin partition wall, it is required to use a binder of high fluidity which can quickly flow into a die. Further, as the partition walls of honeycomb structures become thinner, the fresh extrudate from the die has a lower strength and tends to deform due to its own weight. Therefore, it is necessary to use a binder of high shape retainability which can solidify quickly after leaving the die.
Hence, it has been conducted to mold a honeycomb structure by using a molding material of high hardness and higher shape retainability, or a molding material of low hardness and high fluidity. Such molding materials, however, have had the following problems. A molding material of high hardness is inferior in fluidity; therefore, (1) it does not flow into a die easily, resulting in low productivity and (2) it requires a high molding pressure and repeated molding brings about deformation or wear of the die.
A molding material of low hardness must be hardened by thermal gelation of the binder contained in the molding material, by way of dielectric drying, to allow the extrudate from the die to have a desired strength. In this case, the transfer of the extrudate to a dielectric dryer is conducted with a gas flow applied to the extrudate from below the extrudate to prevent the deformation of the extrudate caused by its own weight. As a result, the extrudate comes to have cracks caused by drying at the portion to which the gas flow is applied.
SUMMARY OF THE INVENTION
In view of the above situation, the present invention aims at providing a molded honeycomb material having a thin partition wall and a process for mass-producing such a honeycomb structure without impairing the honeycomb quality.
According to the present invention, molded honeycomb material is obtained by subjecting a mixture of a raw material powder and a binder to extrusion molding. No open pores are present in the honeycomb material, and the binder comprises a thermoplastic material which is molten at the molding temperature.
In the molded honeycomb structure of the present invention, the binder is preferably water-insoluble and is preferably a wax, a thermoplastic resin or a mixture thereof.
When the binder is a mixture of a wax and a thermoplastic resin, the mixing ratio of the thermoplastic resin in the binder is preferably 35 to 80% by weight, more preferably 40 to 70% by weight, further preferably 45 to 60% by weight. The raw material powder can be a ceramic powder (e.g. cordierite) or a metal powder.
The molded honeycomb material of the present invention can be used as a carrier for the catalyst for removing harmful substances and dust from an automobile exhaust gas.
According to the present invention, a process for producing the above-mentioned molded honeycomb material is also provided, comprising the steps of heating a mixture of a raw material powder and a binder to a molding temperature to melt the binder, subjecting the mixture to extrusion molding, and cooling and solidifying the extrudate.
DETAILED DESCRIPTION OF THE INVENTION
The molded honeycomb material of the present invention can be produced by subjecting a mixture of a raw material and a binder to extrusion molding. As the binder, a thermoplastic material is used which is molten at the molding temperature.
The thermoplastic material can be melted by heat and can have different viscosities at different temperatures. Therefore, it can have the desired fluidity by appropriately selecting the temperature which enables efficient mass production of a molded honeycomb material.
The molten thermoplastic material solidifies when cooled. Therefore, the binder can be easily solidified by rapidly cooling the extrudate with cold water, cold air or the like, before the extrudate deforms due to its own weight, whereby the extrudate can retain its shape.
In the present invention, a water-insoluble binder is used, which makes drying of the molded material unnecessary. In extrusion molding of a mixture of a raw material powder and a water-soluble binder (a mixture of a raw material and a binder is hereinafter called “raw material mixture”), foam in the raw material mixture must be removed by vacuum defoaming, and local drying taking place during vacuum defoaming hardens the dried portion and causes plugging of the die. In contrast, in extrusion molding of a mixture of a raw material powder and a water-insoluble binder, no local drying takes place, no plugging of the die takes place, and productivity is high. Incidentally, no vacuum defoaming is necessary in extrusion molding using a water-insoluble binder.
In the present invention, the specific water-insoluble binder is preferred to be a wax or a thermoplastic resin. As the wax, paraffin wax, microcrystalline wax, etc are preferred. As the thermoplastic resin, ordinary thermoplastic resins such as EVA, polyethylene, polystyrene, liquid crystal polymer, engineering plastics and the like are preferred. In the present invention, these binders can be used singly or in combinations of two or more kinds. An auxiliary agent such as a coupling agent, lubricant, dispersing agent or the like may be added to the binder.
In the present invention, when a mixture of a wax and a thermoplastic resin is used as the binder, the mixing ratio of the thermoplastic resin in the binder is preferably 35 to 80% by weight, more preferably 40 to 70% by weight, further preferably 45 to 60% by weight.
The reason why the above mixing ratio of the thermoplastic resin is preferred is that the amount of the thermoplastic resin in the binder influences the shape retainability and molding pressure during molding, as well as the amount of expansion, amount of cracks and adhesion to the setter during dewaxing and firing.
As the amount of the thermoplastic resin in the binder becomes larger, the shape retainability during molding is better, the molding pressure required is higher, and the amount of expansion and number of defects during dewaxing and firing are lower.
For the above reasons, the upper limit of the mixing ratio of the thermoplastic resin in the binder is set preferably at 80% by weight, more preferably at 70% by weight, further preferably at 60% by weight. Thereby,
Boss Wendy
Burr & Brown
Jones Deborah
NGK Insulators Ltd.
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