Compositions: coating or plastic – Coating or plastic compositions – Alkali metal silicate containing
Reexamination Certificate
2002-03-27
2004-11-02
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Alkali metal silicate containing
C106S601000, C106S602000, C501S080000, C501S085000, C501S141000, C501S145000, C501S146000, C501S149000, C264S044000, C423S333000, C423S334000, C252S062000
Reexamination Certificate
active
06811602
ABSTRACT:
The present invention relates to a process for preparing a silicate porous product, which comprises degassing a dispersion of a clay mineral to remove gases dissolved in the dispersion, then freezing the dispersion and drying it in its frozen state under reduced pressure, and firing the obtained dried product. More particularly, it relates to a process for preparing a silicate porous product which is useful particularly for a ceramic filter, a heat insulating material, a carrier for catalysts or the like.
A ceramic porous product is excellent in heat resistance and corrosion resistance and thus is used as a filtration filter to be used at a high temperature where an organic material can not be used or under a strongly acidic or alkaline condition, as a heat insulating material or as a carrier for catalysts.
As a material which is actually used for a ceramic porous product, cordierite, alumina or mullite may, for example, be mentioned. Further, one prepared by applying a film of e.g. silica or zeolite to a ceramic porous product made of such a material, has also been reported. The above-mentioned ceramic porous products are formed of oxides, but a porous body of silicon carbide or silicon nitride has also been developed.
As a method for producing a porous product, for example, a method of mixing a foaming or combustible material to a ceramic powder or its slurry, followed by firing, or a method of depositing ceramics to a foam such as urethane itself, followed by firing, is known. Further, JP-A-5-238848 and JP-A-6-24859 disclose a method wherein a slurry is deposited on a synthetic resin foam having a three dimensional network structure, and an excess slurry is removed, followed by drying and firing to form a non-oxide ceramic.
Further, a method for producing a cordierite or alumina honeycomb porous product by means of an extrusion method, is also known and widely used. Further, a porous product produced from fibrous ceramics, is being used as a filter for diesel particulates.
The above-mentioned ceramic porous products have a wide range of applicability in various fields, for example, for physically removing or separating solids from a gas or liquid as filters for filtration or as carriers for catalysts, or for separating and concentrating a desired specific gas from a gas mixture, or as sound-shielding (sound proofing) materials or heat insulating materials.
Further, JP-A-63-230581 discloses a method wherein a mixture of clay and water is rapidly frozen, and ice crystals are permitted to disappear to form a porous product. Further, JP-A-9-132475 discloses a method wherein a material comprising clay, water glass and fiber, is rapidly frozen, then dried and heated to produce a clay composite porous product.
As mentioned above, a ceramic porous product is excellent in heat resistance and corrosion resistance, and thus has excellent characteristics as a filtration filter, a heat insulating material or a carrier for catalysts, which is used at a high temperature where an organic material can not be used or under a strongly acidic or alkaline condition.
Among the above-mentioned ceramic porous products, one to be used mainly as a filter, has a porosity of from 40% to 50% in many cases, but there may be one having a heavy mass. On the other hand, one to be used as a heat insulating material is, in many cases such that fibrous ceramics are used as the raw material, whereby handling is inefficient. Further, porous products of silicon carbide and silicon nitride, as non-oxides, have disadvantageous such that the raw materials are expensive, the production costs are high, and oxidation resistance at a high temperature is not adequate, and they are used only for limited applications.
Whereas, silicate ceramics represented by clay mineral, are light in weight and relatively inexpensive.
JP-A-63-230581 proposes a method for obtaining a porous product having a very high porosity by rapidly freezing a mixture of clay and water and then letting ice crystals disappear. However, there has been a problem that it is not easy to control the freezing speed while lowering the freezing temperature extremely in order to carry out rapid freezing, and it has been difficult to obtain a thick porous product. Further, JP-A-9-315877 discloses a method for producing a porous product, which comprises heating and concentrating an aqueous dispersion containing clay, followed by freezing and drying. However, there has been a problem that the viscosity of dispersion becomes very high by the concentration, whereby handling and processing of the dispersion are difficult.
Further, JP-A-5-78181, JP-A-8-277174 and JP-A-11-79860 disclose a clay
atural fiber composite porous product, a clay/fumic acid composite porous product and a clay/water-soluble polymer composite porous product, respectively. However, these products contain organic substances and thus have had a problem that they can not be used at a high temperature. Further, JP-A-9-132475 discloses a clay composite porous product comprising clay, water glass and fiber. However, it contains fiber, whereby its use tends to be limited, and it contains water glass, whereby the temperature for its use can not be made sufficiently high.
Here, from the viewpoint of practical use of a ceramic porous product, a porous product having a uniform pore size is easy to use. From such a viewpoint, the above-mentioned JP-A-11-79860 discloses use of a composite sol having air bubbles and a surfactant dispersed in its interior. However, it is difficult in many cases to uniformly disperse air bubbles from the solubility of air bubbles and its temperature dependency. Further, even in a case where a surfactant is used, there is a problem in that its use is limited to a low temperature.
Further, in the use of a ceramic porous product, in addition to control of the pore size, the strength is also an important factor. With a porous product made of a clay mineral as the raw material and not fired, the strength is not sufficient. Further, if pores are not uniformly present, the strength tends to be low.
Further, a clay mineral has a certain degree of heat resistance, but if it is used at a high temperature without firing, heat shrinkage will be remarkable, and due to such shrinkage, it may not be useful as a structural material. Further, as mentioned above, a clay mineral is not easy to form it into a porous product, and it has not been used as a porous product. As mentioned above, it has been desired to develop an inexpensive method while balancing various required properties, a method for producing a ceramic porous product which brings about little environmental pollution, particularly a silicate ceramic porous product using a clay mineral as the starting material and having it fired, and a process for producing it.
Accordingly, the present invention has been made to solve the above problems, and it is an object of the present invention to provide a silicate porous product which is excellent in pore characteristics and strength characteristics and which can be produced inexpensively, and a process for its production.
The present inventors have conducted extensive studies to accomplish the above object and as a result, the present inventors have found it possible to accomplish the above object by presenting a process for preparing a silicate porous product according to the present invention, and the present invention has been accomplished based on such a discovery.
Accordingly, the present invention provides a process for preparing a silicate porous product, which comprises a step of forming a clay mineral into a dispersion by means of a dispersant, a step of removing gases dissolved in the dispersion, a step of freezing the dispersion and drying it in its frozen state under reduced pressure, and a step of firing the dried product obtained by the drying.
The clay mineral to be used in the present invention is one belonging to a clay mineral according to a mineralogical classification, and it preferably is sheet silicate, amorphous silicate or quasi-crystalline silicate, or a mixture the
Beppu Yoshihisa
Kondoh Shinji
Asahi Glass Company Limited
Marcantoni Paul
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