GB2372250A

GB2372250A - Ceramic articles and method of producing said articles

Info

Publication number: GB2372250A
Application number: GB0104028A
Authority: GB
Inventors: John Michael Gallimore; Edward Shaw
Original assignee: DYSON IND Ltd
Current assignee: DYSON IND Ltd
Priority date: 2001-02-19
Filing date: 2001-02-19
Publication date: 2002-08-21
Anticipated expiration: 2021-02-19
Also published as: WO2002066399A1; GB2372250B; GB0104028D0; EP1363862A1

Abstract

A ceramic article is produced by mixing a ceramic material, a binder and a plasticizer under conditions of high shear and vacuum. The mixture is then shaped, dried and fired. Preferably the binder is polyvinyl alcohol, whilst the plasticizer is glycerol, glycerine, polyethylene glycol and/or ethylene glycol.

Description

CERAMIC ARTICLES The invention relates to a ceramic composition and a method for producing ceramic articles using the composition.

In use, a shape is moulded from a ceramic composition and the shape is dried and fired to form the article. Flaws are usually present in ceramic articles after firing of the ceramic composition. These flaws may be holes or cracks formed during removal of the organic ingredients in the composition, or by the presence of aggregations of ceramic particles in the finished shape. In conventional ceramic production processes flaws having a diameter greater than 500 microns are known. These large flaws are a major cause of weakness in conventionally produced ceramic articles.

Viscous polymer processing (VPP) is a process for mixing ceramic compositions using very high shears to input high energy into the mixture. Articles produced using VPP process have smaller and fewer flaws and are stronger than articles produced by conventional ceramic production methods. This process enables thinner and lighter weight articles to be produced.

It has now been found that by mixing the ceramic composition under high shear and vacuum, and by adjusting the proportions of a selected ingredient in the composition, stronger and thinner articles can be produced having smaller flaws.

According to the invention in one aspect there is provided a method of producing a shaped ceramic article from a composition comprising a ceramic material, an organic binder and a plasticiser, the method comprising the steps of : mixing the composition under high shear and under a vacuum; shaping the composition; drying the composition, and firing the composition, wherein the proportion of the plasticiser is selected to reduce the size and frequency of flaws in the finished ceramic article.

The content of the plasticiser should be controlled within a relative narrow range. We prefer to have from about 1.5 parts by weight relative to 100 parts by weight of the dry ingredients to about 2.5, say 1.8 parts.

According to the invention in a second aspect there is provided a composition for use in the method described above, the composition comprising a ceramic material, an organic polymer binder, and a plasticiser, wherein the proportions of the plasticiser are selected to reduce the size and frequency of flaws in the finished ceramic article.

According to the invention in a third aspect there is provided a ceramic article made according to the method described above. Suitable plasticisers include glycerol, glycerine, polyethyleneglycol and ethylenglycol. Preferably the plasticiser is glycerol. It is convenient to add the glycerol as an aqueous solution.

By using high shear mixing of the composition under a vacuum, and by selecting the proportion of the glycerol in the composition we have found that we can produce articles in a normal drying and firing time, and significantly reduce the size and frequency of flaws in the article.

The composition may be mixed using any known method of high shear mixing under a vacuum, for example, using a high shear mixer or by extrusion of the composition through a small diameter die. Preferably the composition is mixed under high shear using suitable blades.

The mixed composition is preferably shaped by extruding the composition to the required shape, or by roll-milling the composition to form a sheet which is then formed into the required article.

The ceramic material may be any solid inorganic material. Preferably the ingredients of a composition may be selected to form a cordierite.

Preferably the proportion of ceramic material present in the composition is at least 50% by weight. Ideally the proportion of ceramic particles present in the composition should be about 60 to about 70%.

The binder is preferably an organic binder preferably a polymer such as polyvinyl alcohol and the like.

The formed ceramic article will typically have a mean flaw size of less than 20 microns.

The mean maximum flaw size of the fired ceramic article is measured by polishing the surface of the article and using an electron microscope to measure the diameter of twenty flaws at the widest point of the defect, and taking the average of these measurements.

In order that the invention may be well understood it will now be described by way of illustration only with reference to the following examples in which all proportions are stated in parts by weight.

Example 1 A dry mixture was prepared by mixing 55 parts of grog, 25 parts of ball clay and 20 parts of powdered magnesium carbonate. 100 parts of ceramic powders prepared as above were mixed with 5 parts of polyvinyl alcohol. 18 parts of a 10% aqueous solution of glycerol was added to the dry mix and the whole was mixed at high shear using a zblade high shear mixer while the apparatus is connected to a vacuum pump to evacuate the system. The resultant mixture having a paste like consistency, was then passed through a twin roll mill to form a sheet. The sheet was then dried to remove water from the composition and then transferred to a kiln and fired. During the firing process the organics, i. e. PVA and glycerol, are burnt off and the particles of ceramic are sintered to form a cordierite article. The selection of the content of the glycerol and the high shear mixing of the composition resulted in the following beneficial results: 1. the composition was very easy to work, for example, it mixed well, was relatively plastic and did not tear during roll milling; 2. the mean maximum flaw diameter was reduced to less than 20 microns; 3. the tensile and flexural strength of the finished article were high.

Claims

CLAIMS 1. A method of producing a shaped ceramic article from a composition comprising a ceramic material, an organic binder and a plasticiser, the method comprising the steps of: 'mixing the composition under high shear and vacuum; * shaping the composition; 'drying the composition, and * firing the composition, wherein the proportion of plasticiser is selected to reduce the size and frequency of flaws in the ceramic article.
2. A method according to Claim 1 wherein the plasticiser is from 1.5 to 2.5 parts by weight where the dry material is 100 parts.
3. A method according to Claim 2 wherein the content of plasticiser is 1.8 parts by weight.
4. A method according to any preceding Claim wherein the plasticiser is one or more of glycerol, glycerine, polyethyleneglycol and ethyleneglycol.
5. A composition comprising a ceramic material, an organic binder and a plasticiser wherein the proportion of plasticiser is selected to reduce the size and frequency of flaws in the finished ceramic article.
6. A composition according to Claim 5 containing 1.5 to 2.5 parts by weight of plasticiser.
7. A composition according to Claim 6 containing 1.8 parts by weight of plasticiser.
8. A composition according to Claim 5 wherein the plasticiser is glycerol.
9. A composition according to any of Claims 5 to 8 wherein the binder is a polymer.
10. A composition according to Claim 9 wherein the binder is polyvinylalcohol.