GB2176179A - Porous ceramic body formed by extrusion process - Google Patents

Abstract

A porous ceramic body is composed of filamentary ceramic material, formed by extrusion, and regularly or randomly layed into convolutions closely spaced, mutually contacting and bonded together at their contact points to constitute a self-supporting body having a stereo-reticulated porous structure. <IMAGE>

Description

SPECIFICATION Porous ceramic material This invention relates to porous ceramic material which can be used in place of so-called ceramic foam for such purposes as filters for molten metal and catalyst supports in high temperature conditions.

It has previously been proposed to provide a ceramic structure, suitable for use as a liquid metal filter, consisting of plane layers of parallel ceramic extrusions, which are extruded in separate layers, superimposed and extending at 90" to the extrusion in successive layers.

The present invention also employs extruded ceramic material but enables any required shape of a porous body to be produced without the limitations imposed by a structure of superimposed plane layers of extrusions at right angles.

The basic concept of the invention is that a self-supporting body can be built up by laying extruded filamentary ceramic material in convolutions which are closely spaced between mutually contacting points.

More particularly, the invention provides a porous ceramic body composed of filamentary ceramic material regularly or randomly lying in closely spaced and mutually contacting convolutions bonded together attheir contact points and constituting a self-supporting body having a stereo-reticulated porous structure.

The invention also provides a method of making a porous ceramic body by extruding in filamentary form a plastic preparation of ceramic material and randomly or regularly laying the filamentary extrudate into closely spaced and mutually contacting convolutions constituting a self-supporting body having a stereo-reticulated porous structure, drying and firing the body.

In one method of carrying outthe invention, to form a body by random laying of extruded filamentary material, there is provided a hollow receiver, for extrudate, the receiver having an internal shape which is an envelope of the shape of the required body. For example, for a right cylindrical body the receiver is an open-ended cylindrical drum. Similar drum with a central core or an inner peripheral wall, leaving an annular space, may be used as a receiver for making an annular cylindrical body with an annular wall of substantial, self-supporting, thickness. After forming, the body should be removed from the receiver and supported by a paper or other easily deformable mount which permits shrinkage on drying. Alternatively a rigid receiver may have a deformable foam or like core.

The extrudate, as a single extruded filament or multiple filaments, is directed into the mouth of the receiver which is moved relatively to iaythe extrudate in random convolutions progressively to fill the receiverwhich serves as a mould forthe shape of the body required. After extrusion, the receiver content may be stabilized in form by moderate jolting of the receiver or light pressure applied to the accessible surface ofthefilament composing the body. This improves definition of the surface.

After initial setting in the receiver, and drying with or without heating in or out ofthe receiver, the built-up filamentary material body is fired.

Relative movement may be effected by movement of either the receiver and/orthe extruding head.

The shape, size and number offilaments extruded can be selected to give a required porosity ofthe stereo-reticulated structure. The ceramic material will be selected to suit requirements.

In another method of carrying outthe invention, filamentary extrudate is regualrly laid into convolutions by coil-winding of a single or multiple extruded filament in helical convolutions on to a spindle, bobbin or spool.

Known textile thread, wire orstring coil-winding methods and apparatus may be used for this purpose.

The invention is illustrated by way of example on the accompanying drawings, in which: Figure lisa perspective illustration of extruding a filament into random convolutions in a receiver, Figures 2 and 3 are diagrammatic illustrations of coil-winding of an extruding filament.

Figures 4and 5are respectively a side elevation and an end viewofa cylindrical body made by the method illustrated by Figure 1, and Figures 6 and 7 are respectively a side elevation and an end view of an annular body coil-wound by a method such as is illustrated by Figure 2.

As illustrated in Figure 1, an extruder 1 directs an extruded filament 2 down into a cylindrical receiver 3while either or both the extruder and receiver are moved relatively to one another in horizontal directions, as indicated by the arrows. The result is to build up in the receiver 3 a randomly convoluted filamentary accumulation 4 in which the convolutions intermittently contact and are spaced at close intervals. The extrudate is tacky enough forthe convolutions to interadhere attheir points of contact.

When the receiver 3 is full, the contents are stabilized by joggling or lightly compacted, allowed to set and dry, or are dried, until firm enough to be removed from the receiver as a self-supporting cylindrical body 5, as shown by Figures 4 and 6, which is fired after such further drying as may be required. As shown, the stereo-reticulated structure of the body 5 is irregular, without any discrete layer structure.

Figure 2 shows an extruder 1 from which a filament 2 is laid on to a former on a rotating spindle 6 in helical convolutions 7 resulting from lateral movement, as indicated by the arrow, of a thread guide 12 relatively along the spindle 6 to build up a tubular body or annulus 8. By appropriate dwell in the movement ofthe extruder at the ends of its transverse, end flanges 9 can be formed as portions of increased radial thickness at the ends of the body 8.

By making the former 10 offoam or other deformable material, on which the filament 2 is wound, allowance is made for shrinkage during drying. Figure 3 shows an alternative, part-spherical, shape of wound body 8 and a deformable,withdrawable,former 10.

After setting and drying, the body8 is removed from the spindle, with orwithoutthe former, and fired to produce a self-supporting wound filamentary annulus 8, as shown in Figures 6 and 7, in which thefilamentary material extends substantially helically in multiple convoluted layers.

Suitable materials for making a plastic ceramic preparation forfilamentary extrusion areforexample, alumina, cordierite and titania.

Examples of suitable preparations are; in parts by weight:- Composition l-suitable for the method of Figure 7.

A16Alumina 98 Talc 2 Glycerine 10 Plasticizer, in aqueous solution 17 Composition 11-Suitable forthe method of Figure2.

A16 98 Talc 2 Wax 5 Plasticizer, in aqueous solution 24 For both compositions, a suitable plasticizer is a 5% aqueous solution of M 450 Celacol (Trade Mark) which is a carboxy methyl cellulose. The wax is a parafin wax, Okerine wax, of low melting point, about 25"C. The plasticizer and wax serve also as binders.

The amount of water required to produce an extrudable paste is usually provided by the plasticizer.

The physical properties of the ceramic preparation should be: A. Soft and plastic enough to extrude through a fine nozzle, e.g. 0.2-0.6 mm. Larger diameterfilaments may be used. This property can be controlled by adjustment of th e binder content.

B. The extruded filament should have sufficienttensile strength and cohesion,to retain itswaterduring extrusion and withstand laying or winding, whilst remaining soft and plastic enough, to conform to the former, and tacky so asto adhereto contacted filaments. These properties are controllable by the glycerine orwax content. Drying times should of course allowforthese requirements.

Claims (7)

1. A porous ceramic body composed offilamentaryceramic material regularly or randomly lying in closely spaced and mututally contacting convolutions a self-supporting body having a stereo-reticulated porous structure.

2. A porous ceramic body according to Claim 1, in which the filamentary material extends in convolutions multidirectionally so that the stereo-reticulated structure is irregular, without any discrete layer structure.

3. A porous ceramic body according to Claim 1, in which the filamentary material extends as a coil-wound structure.

4. A porous ceramic body according to Claim 3, in which the body is an annulus and the filamentary material extends substantially helically in multiple convoluted layers.

5. A porous ceramic body substantially as described with reference to Figures 4 and 5 or Figures 6 and 7 of the accompanying drawings.

6. A method of making a porous ceramic body by extruding in filamentary form a plastic preparation of ceramic material and randomly or regularly laying the filamentary extrudate into closely spaced and mutually contacting convolutions constituting a self-supporting body having a stereo-reticulated porous structure, drying and firing the body.

7. A method of making a porous ceramic body substantially as described with referenceto Figure 1 or Figure 2 ofthe accompanying drawings.