GB2188641A

GB2188641A - Article having magnetic properties and production thereof

Info

Publication number: GB2188641A
Application number: GB08407105A
Authority: GB
Inventors: James Hugh Raistrick; Stephen John Battersby
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1983-04-22
Filing date: 1984-03-19
Publication date: 1987-10-07
Also published as: SE8402172D0; SE461882B; AU2837584A; IT8420567A0; DE3415243A1; SE8402172L; NL8401183A; GB2188641B; AU577285B2; CA1252624A; GB8407105D0; FR2712729A1

Abstract

A shapeable composition comprising a homogeneous mixture of (a) at least one particulate ferrite material having magnetic properties, (b) at least one water-soluble or water-dispersible organic polymeric material, and (c) water, in the composition the components (a), (b) and (c) being present in a proportion by volume of the composition of respectively, 40 to 90%, 2 to 25%, and not more than 60%, a product produced therefrom, and a process for producing said product by removing water from the composition. The composition optionally contains an additive capable of insolubilising the organic polymeric material with respect to water.

Description

SPECIFICATION Article having magnetic properties and production thereof This invention relates to a shaped article having magnetic properties, particularly to a shaped article comprising particulate ferrite material having magnetic properties, to a process for the production of said article, and to a composition for use in such production.

Within the scope of the terms shaped article having magnetic properties, and particulate material having properties, there are included articles and materials whose properties may be described asferromagneticor ferrimagnetic. Both these latter terms include articles and materials which once magnetised remain magnetised when removed from the influence of a magnetic field, and also articles and materials which are capable of being magnetised but which do not remain magnetised when removed from the influence of a magnetic field. Such articles and materials may be considered to be, respectively, permanently and temporarily magnetisable, and are frequently referred to as "hard" and "soft" respectively.

The shaped article of the present invention is made of a particulate ferrite material. The term ferrite is a term well known in the art. Ferrites are magnetic oxides containing iron as a major metallic component and in addition another metal component, e.g. manganese, zinc, lead, strontium, barium, lithium ornickel.The term can include spinels, perovskites, magnetoplumbites, and garnets. Ferrite materials which are ferrimagnetic include Ba Fe6 019, which is permanently magnetisable, and (Mn ,Zn) Fe204, which is temporarily magnetisable.

Shaped articles of ferrites may be made, for example, by a powder compaction process in which the powdered ferrite is compacted under pressure in a suitable shaped mould and the powder is subsequently sintered by heating at a high temperature which may be in excess of 1200 C. Controlled heating and cooling rates may be required, as may be heating for some hours at the peaktemperature. The presence of air orair enriched with oxygen may also be required.

Shaped articles of ferrites may be made from a composition comprising, ferrite powder and a solution of a polymer binder. The article may be shaped, for example, by extrusion, injection moulding, or compression moulding, pyrolysing the polymer binder and finally sintering the ferrite powder.

Shaped articles of ferrites may also be made by filling of plastics materials, although in this case the article generally contains a relatively low volume proportion of ferrite and thus possesses relatively poor magnetic properties.

Permanently magnetisable ferrite materials are used in a wide variety of applications, for example, in ferrite motors and in loud speakers. Temporarily magnetisableferrite materials are used in transformers, in antennae rods, in recording heads and in memory cores. In addition, certain temporarily magnetisable materials, e.g. soft ferrites, are able to absorb certain frequencies of electro-magnetic radiation. For example ferrites may be used in conjunction with microwave transmitters in order to assist in defining the size and direction of the micro-wave beams; in microwave heating devices, e.g. domestic ovens, in order to absorb potentially harmful microwave radiation which may otherwise escape from the device; and in the cladding of structures, e.g. buildings, ships and aircraft, in order to absorb and prevent reflection of radar waves.The frequency at which the absorption is greatest depends on a number offactors, e.g. on the chemical composition and crystal structure ofthe ferrite, on the thickness of the absorbing structure, and on the distribution ofthe ferrite within the absorbing structure. An absorber which is manufactured in the manner described later may be most effective at a frequency which differs from that at which a sinteredferrite material is most effective. Furthermore, certain ferrites possess the unusual feature of being able to damp vibrations. Articles made in the manner described later may have a good combination of high modulus and good vibration damping.

The present invention relates to shaped articles of particulate ferrite materials having magnetic properties, and to the production of such articles by a process which does not involve a lengthy and expensivesintering step, and which contain a high volume proportion ofthe particulate material.

According to the present invention there is provided a process for the production of a shaped article of a particulate ferrite material having magnetic properties which method comprises shaping a homogeneous mixture of (a) at least one particulate ferrite material having magnetic properties, (b) at least one water-soluble or water-dispersible organic polymeric material, and (c) water, and removing waterfrom the thus formed shaped mixture, in the composition the components (a), (b), and (c) being present in a proportion by volume of the composition of respectively, 40 to 90%, 2 to 25%, and not morethan 60%.

In a further embodiment of the present invention there is provided a shapeable composition comprising a homogeneous mixture of (a) at least one particulate ferrite material having magnetic properties, (b) at least one water-soluble orwater-dispersible organic polymeric material, and (c) water in the composition the components (a), (b) and (c) being present in proportions by volume of the composition of respectively 40 to 90%, 2 to 25%, and not more than 60%.

There is also provided a shaped article of a particulate ferrite material having magnetic properties produced by removing water from the composition hereinbefore described.

The particulate ferrite material having magnetic properties will generally be referred to hereinafter as the particulate material.

In producing the shapeable composition the components thereof should be thoroughly mixed so asto form a homogeneous mixture.

For example, the components of the composition are preferably mixed under conditions of high shear, for example in a bladed high shear mixer. If desired, and where the composition has a suitable consistency, a composition so formed may bye further mixed under conditions of high shear by passing the composition repeatediythrough the nip between a pair of rollers which may be rotating at the same or at different peripheral speeds.

The mixing may be effected at elevated temperature in order, for example, to reduce the viscosity ofthe composition and thus aid the mixing. However, the elevated temperature, should not be such as to result in premature drying of the composition by loss of water nor in excessive loss ofwaterfrom the composition by evaporation.

The homogeneous composition ofthe invention may be shaped by a variety of techniques, depending on the consistency of the composition. Thus, where the composition comprises a relatively large proportion of water in the range up to 60% by volume of the composition the composition may be sufficiently fluid as to be capable of being cast in a suitably shaped mould.

The composition of the invention may contain a proportion of water, e.g. up to 30% by volume, such that the composition has a dough-like consistency, and the composition may be shaped by techniques known in the plastics or rubber processing art. For example, where the composition has a dough-like consistency it may be shaped by extrusion, e.g. into a rod ortube shape or by injection moulding into a desired shape, orit may be calendered to produce a sheet-like form. The composition may also be shaped by compression moulding ofthe composition in a suitably shaped mould.

Compositions having a dough-like consistency are preferred as it is generally possible to use plastics or rubber processing equipment with such compositions, such compositions generally contain a relatively low proportion of water and thus there is a reduced amount of water remove from the composition, and the shaped articles produced from such compositions are generally of higherflexural strength.

The temperature atwhich shaping ofthe composition may be effected may depend on the nature ofthe components of the composition and their relative proportions. Where the composition is to be shaped under relatively high pressure the composition may be shaped at or nearambienttemperature. However, we have found that, particularly where the composition has the characteristics of a thermoplastic, it may be desirable, or even necessary, in orderto readily effectthe shaping process, to use an elevated effect the shaping process, to use an elevated temperature in order effectively to shape the composition. A suitably elevated temperature may be chosen by means of simple experiment.

In the final step ofthe process for the production ofthe shaped article water is removed from the shaped composition, that is it is dried. Drying may be effected merely by allowing the water to evaporate. However, in orderto speed up the drying process it is preferred to drythe shaped composition at elevated temperature, for example-at a temperature greater than 50"C. Atemperature of 1 00"C or greater may be used. However, the elevatedtemperature, and the length of time atthe elevated temperature, should not be such asto resultin substantial reduction in the strength of the shaped article, which may be brought about, for example, by degradation of the polymeric material at elevated temperature.

The shaped article of the invention may have a high flexural strength, for example, a flexural strength in excess of 40 MPa. The shaped article may have a flexural strength in excess of 100 MPa.

As the shaped article comprises an organic polymeric material which iswater-soluble orwater-dispersible the article will be sensitive to water. Indeed, it may lose dimensional stabilitywhen contacted with water, particularly when soaked in water, and in a preferred embodiment of the invention the composition also comprises at least one additive which is capable of reacting with the polymeric material to insolubilisethe material with respectto water. Use of such an additive substantially increases the dimensional stability ofthe shaped article when the article is contacted with water.

Where the composition contains such as additive the final step ofthe process forthe production ofthe shaped article comprises drying ofthe shaped composition to remove the water from the composition and reacting the additive with the organic polymeric material in order to insolubilise the latter material with respect to water. In this case this final step is referred to as setting.

The conditions underwhich setting of the shaped composition may be effected will depend on the nature ofthe components of the composition, and in particular on the nature ofthe organic polymeric material and on the nature of the additive reactive therewith. Suitable conditions for use in effecting the setting reaction will be disclosed hereinafter in respect of compositions containing specific organic polymeric materials and additives reactive therewith. Setting of the shaped composition may be effected at or near ambient temperatue or it may be effected at elevated temperature, for example at a temperature in excess of 50"C. A temperature of up to 1 OO"C o r even greater may be used. Elevated temperatures may be desirable in orderto initiate reaction of the additive with the organic polymeric material, or at leastto increase the rate of this reaction. The elevated temperature, and length of time at such elevated temperature should not be such asto resu It insubstantial reduction in the strength of the product.

Where particularly high flexural strength shaped articles are to be produced it is preferred that the components of the composition of the invention are selected such that a test composition comprising 63% by volume of particulate material, 7% by volume of water-soluble orwater-dispersible organic polymeric material, and 30% by volume of water, when extruded in a capillary rheometer at an extrusion pressure upto a maximum of 500 atmospheres undergoes an increase of at least 25%, and preferably at least 50%, in shear stress when a ten-fold increase in the shear rate of the test composition is effected when the shear rate as measured are within the range 0.1 to 5 second~1 .

A capillary rheometer in which the test composition is extruded comprises a piston in a cylindrical barrel and a capillary orifice through which the test composition may be extruded.

The shear stress in kN cm-2 is defined by Fd TrL D2 and the shear rate in second- by 2vD2 15d3 where D is the diameter of the barrel ofthe rheometer in cm, v is the rate of travel of the piston in the barrel of the rheometer in cm min-,d is the diameter of the capillary of the rheometer in cm, L is the length ofthe capillary of the rheometer in cm, and Fis the force in kN applied to the piston of the rheometer. In general, D will be in the range 1 to 3cm, din the range 0.2 to 0.5 cm,and L in the range 1 to 3cm, din the range 0.2toO.5 cm, and L in the range 5d to 20d.

The particulate material in the test composition should not be of a size so great nor of such a shape thatthe particulate material itself inhibits passage of the composition through the capillary of the rheometer. For use in the capillary rheometertest particulate material having a size which results in a readily extrudable composition will be chosen, and a size in the range upto 100 microns will generally be suitable. It maybe necessary to choose a particular size, or a combination of sizes, for the particulate material in this range in order to produce a test composition which satisfies the criteria of the capillary rheometertest. The composition, and shaped article, of the invention are not limited to particulate material having a size in this range.

A shaped article produced from the composition of the invention will be of higherflexural strength where the particulate material and organic polymeric material together are selected so that the test composition satisfies the aforementioned criteria of the capillary rheometer test than is the casewherethe particulate material and the organic polymeric material selected are such the the test composition does not satisfy the aforementioned criteria.

For example, where the organic polymeric material and the particulate material are selected sothatthetest composition satisfies the aforementioned criteria a shaped article produced from a composition containing these materials will have a flexural strength higher than that of (1) a shaped article produced from a composition containing the same organic polymeric material and a different particulate material which in combination do not satisfy the criteria of the capillary rheometertest, and (2) a shaped article produced from a composition containing the same particulate material and a different organic polymeric material which in combination do not satisfy the criteria ofthe capillary rheometertest.

Suitable combinations of particulate material and organic polymeric material which in the test composition satisfy the aforementioned capillary rheometertestwill be disclosed hereinafter.

In general, the greater is the change in shear stress observed when the shear rate is increased tenfold the greaterwill be the flexural strength of the shaped article produced from the composition of the invention, and for this reason it is preferred that the test composition undergoes an increase of at least 75% in shear stress when a ten-fold increase in shear rate of the test composition is effected.

The test composition for use in the capillary rheometer test should of course be thoroughly mixed and be sufficiently fluid that the composition itself is capable of being extruded in the capillary rheometer. In order that the test composition should have sufficient fluidity that shear rates in the range of 0.1 to 5 second1 are obtained it may be necessaryto carry outthetest at elevated temperature, for example at a temperature greater than 50"C, e.g. at about 80"C. On the other hand, it may be necessary, particularly where the test composition is of high fluidity, to carry outthe capillary rheometertest at a temperature below ambient temperature.In effecting the extrusion the composition should not separate into its component parts,for example, water should not tend to separate from the composition.

In orderto produce an extrudable composition it may be necessary to select a suitable molecularweightof organic polymeric material for use in the test composition. The composition of the invention is not ofcourse limited to use of a material of the selected molecularweight. The molecular weight is merely selected forthe purposes of the test.

For particularly high flexural strength shaped articles it is preferred that not more than 2%, and more preferably not more than 0.5%r ofthetotal volume of the article comprises pores having a maximum dimension exceeding 100 microns, preferably 50 microns, and more preferably 15 microns, as measured by the method of quantitative microscopy. These pore size criteria do not include pores which may be present in the particulate material, for example, where the particulate material comprises hollow particles.

The production of such a preferred shaped article is assisted by application of high shear during mixing of the composition, which may be effected in the substantial absence of air, for example, undervacuum and/or by application of at least a moderate pressure, e.g. an applied pressure of 1 to 5 MPa in the shapig step, particularly with a dough-like composition.

Quantitative microscopy is a technique well known in the art. A surface of a sample of the shaped article is polished to produce a plane surface on the sample, the sample is washed to remove the polishing debris from the surface, and the surface is illuminated to ensure that the holes in the surface are contrasted with the plane parts of the surface, and the surface is viewed by means of an optical microscope, typically at a magnification of xl 00, and the holes exceeding 100 microns, or 50 microns or 15 microns in size, are determined, as described in "Quantitative Microscopy" by De Hoff and Rhines, McGraw Hill 1968. Sufficient area ofthe surface of the sample should be viewed to reduce the statistical error, and usually, 1000 holesare counted.The sample is then subjected to further polishing in order to expose another surface and the optical examination is repeated. In general ten such surfaces are examined.

It is also preferred, for additional improvements in flexural strength, that the total volume of pores in the shaped article expressed as a proportion of the apparent volume of the article, including the pores, does not exceed 20%. Porosities not exceeding 15%, and even porosities not exceeding 10% are more preferred. The porosity may even be less than 2%. These porosity criteria exclude pores which may be present in the particulate material,for example, where the particulate material comprises hollow particles.

Low porosity is a feature of shaped articles produced from compositions in which the organic polymeric material and the particulate material are selected so as to satisfy the criteria of the capillary rheometertest.

In the composition of the invention the particulate material is insoluble in water and is substantially unreactive with water, although we do not exclude use of particulate material which may be very slightly reactive with water.

The dimensions of the particles of the particulate material may vary over a broad range. Where the particulate material has a small size, however, undesirably large proportions of water may be required in order to produce a composition which is readily shapeable, and for this reason it is preferred, although not essential, that the median particle size is greater than 0.3 micron, more preferably greaterthan 3 microns.

The particulate material may comprise a plurality of particle sizes. For example, the particulate material may comprise a firstfraction and a second fraction of size less than that of the firstfraction.

The use of such a plurality of particle sizes results in good packing of particles in the product and also may lead to a reduction in the proportion of organic polymeric material which otherwise may be required.

Mixtures of different particulate ferrite materials having a magnetic properties may be used.

Shaped articles made of ferrites have a wide variety of applications.

Ferrites are magnetic oxides containing iron as a major metallic component and in addition another metal component. The other metal component may be, for example, manganese, zinc, lead, strontium, barium, lithium or nickel. Examples of ferrites include (Mn, Zn) Fe2O4, BaFe12O19, MnFe2O4 and (Ni, Zn) Fe204.

Many other examples of ferrites are described in the art.

The composition, and the shaped article produced therefrom, may comprise particulate material other than a particulate ferrite material having magnetic properties.

The composition, and the shaped article produced therefrom, may include fibrous material. Although the fibrous material may be in the form of random, chopped fibre, difficulty may be experienced in incorporating such fibrous material into the composition. For this reason the fibrous material is preferably in the form of a mat, which may be woven or non-woven. The mat may be pressed into the composition of the invention, or it may be formed in situ, e.g. by filament winding.

The particulate ferrite material may be present in the composition of the invention in a proportion of 40to 90% by volume. It is preferred to use a relatively high proportion of particulate ferrite material, for example a proportion in the range 6Qto 90% by volume.

Such preferred compositions may contain a relatively low proportion of organic polymeric material, which material will generally be flammable, and it is thus of advantage that the shaped article of the invention contains a relatively low proportion of such material. Also, compositions containing a high proportion of particulate material will generally contain a relatively low proportion of water. This is of advantage as there is then a lower porportion of water remove from the composition during production of the shaped article.

The organic polymeric material in the composition ofthe invention should be water-soluble of water-dispersible. The function of the organic polymeric material is to aid in the processing ofthe composition, e.g. to aid in the production of a composition which is readily shaped, e.g. a composition of dough-like consistency, and to provide shape-retaining properties to the shaped article of the invention.

It is preferred that the organic polymeric material is soluble in water, rather than water-dispersible, and that the polymeric material is film-forming and contains groups, for example, hydroxyl or carboxylic acid groups, which have an affinityforthe particulate material.

Examples of organic polymeric materials include hydroxy propyl methyl cellulose, polyethylene oxide, polyethylene glycol, polyacrylamide, and polyacrylic acid. A particularly preferred organic polymeric material which, with a numberof different particulate materials having magnetic properties in the form of a test composition satisfies the criteria of the aforementioned capillary rheometertest, is a hydrolysed polymer or copolymer of a vinyl ester, e.g. a hydrolysed vinyl acetate polymer or copolymer. The polymer may be a copolymer of vinyl acetate and a monomer copolymerisable therewith, but it is preferably a hydrolysed poly (vinyl acetate).

The degree of hydrolysis of the vinyl acetate (co)polymer has a bearing on whether or not the (co)polymer is combination with a particulate material in the test composition satisfies the aforementioned criteria of the capillary rheometertest. In order that in the capillary rheometertest an increase of at least 25% iri shear stress should be produced by the ten-fold increase in shear rate, it is preferred that the degree of hydrolysis ofthe vinyl acetate (co)polymer be at least 50% but not more than 97%, and more preferably in the range 70 to 90%, that is, it is preferred that at least 50% but not more than 97%, and more preferably 70% to 90% ofthevinyl acetate units in the polymer or copolymer, are hydrolysed to the alcohol form.

For a given proportion of hydrolysed vinyl acetate (co)polymer in the composition of the invention the properties of the shaped article produced therefrom are relatively insensitive to variations in the molecular weight of the hydrolysed vinyl acetate (co)polymer.

In general, however, the molecularweightofthe hydrolysed vinyl acetate (co)polymerwill be at least 3000, e.g. in the range 5000 to 125,000. Such (co)polymers are readily available. The (co)polymer may have a higher molecular weight.

In the composition of the invention there is present 2 to 25% of organic polymeric material by volume ofthe composition. The ease of shaping of the composition generally improves with increase in the proportion of polymeric material int he composition, and a proportion of at least 7% by volume is preferred. On the other hand as the polymeric material is generally capable of burning a proportion of not more than 20% byvolume of polymeric material is preferred.

The proportion of water in the composition has an effect on the properties of the shaped article produced from the composition. In order to produce an article of particularly high flexural strength the composition should contain no more than 30% by volume of water. It is preferred to use as low a proportion of water as possible consistent with producing a composition which is shapeable. We prefer to use less than 20% by volume of water. In general it will be found necessary to use at least 5% by volume of water. However, a proportion of water may be used in the composition which is greater than that which would result in production of a very high strength article and some strength may be sacrificed in orderto produce a composition which is more readily shaped.

Where the high green strength is desired in the moulded composition of the invention, that is before setting of the composition, the composition may suitably comprise a gelling agent forthe organic polymeric materiai,that is a compound which forms labile bonds with the organic polymeric material.

An alternative way of achieving high green strength in the composition is to include in the composition a proportion of an organic polymeric material which is soluble in the water of the composition at elevated temperature but which forms a gel at lowtemperature, e.g. at or near ambient temperature. For examplethe composition may also comprise a proportion of a substantiallyfully hydrolysed poly(vinyl acetate) which is soluble in the water of the composition at elevated temperature but which forms a gel art ambient temperature.

It is a preferred feature of the invention that the composition comprises an additive capable of reacting with the organic polymeric material to insolubilise the material with respect to water.

The nature of this additive will depend on the particular organic polymeric material in the composition.

Where the organic polymeric material comprises a plurality of reactive functional groups the additive may be a material reactive with the functional groups underthe conditions used in forming the shaped article of the invention from the composition. In this case the insolubilisation of the organic polymeric material with respect to water may be achieved by cross-linking of the material.For example, where the polymeric material comprises a plurality of hydroxyl groups, e.g. as in a hydrolysed vinyl ester polymer or copolymer such as hydrolysed poly(vinyl acetate), the additive may be a compound of a polyvalent metal capable of reacting with the hydroxyl groups. Particular examples ofsuitable compounds of a polyvalent metal include compounds of aZuminium,AI2(0H5NO3, and A12(OH)S halide, for example, Al2(0H)5CI. Other exampies of compounds of a polyvalent metal include Zr (OH)2 Cl2, (NH4)2Cr207 and Cr(OH)1.8(NO3)1.2.

Selection of suitable combinations or water-soluble or orwater-dispersible organic polymeric materials and insolubilising additives may be made by reacting mixtures of such materials and additives and testing the product of reaction for water insolubility.

In effecting setting of the composition comprising such an additive the additive in the composition is reacted with the polymeric material to insolubilise the material and water is removed from the composition.

Where the additive is a polyvalent metal compound reaction is suitably effected at elevated temperature. For example,thetemperature may be greater than 1 OO"C, which temperature serves two removethe water in the composition. Atemperature of, forexample, up to 250"C may be used.

Where the polymeric material comprises a plurality of hydroxyl groups of the additive capable of reacting with the polymeric material to insolu bilise the material with respectto water may itself be an organic compound reactive with the hydroxyl groups, for example, a dialdehyde, e.g. glyoxal.

In this case a suitable reaction temperature is ambienttemperature. However, elevated temperatures are suitably used, e.g. up to about 100 C, in order to removethe water from the composition and to accelerate the reaction.

In the composition of the invention the proportion of additive capable of reacting with the polymeric material will depend on the particular organic polymeric material and the particular additive in the composition.

In general the composition will contain a proportion of additive in the range 5to 100% by volume ofthe organic polymeric material in the composition e.g. 10 to 50% by volume.

It is preferred to seiect a proportion of additive which is sufficient not merely to insolublise the organic polymeric material with respect to water but which reacts with the polymeric material to produce a polymeric productwhich swells at mostonlyto a limited extent in water, for example, which takes up not mprethan 50% by weight of water when the product of reaction ofthe organic polymeric material and the insolubilising additive is soaked in water. Suitable proportions may be selected by test on mixtures of organic polymeric material and insolubilising additive.

In a particularly preferred embodiment of the invention the composition of the invention also comprises an additive capable of effecting coupling between the polymeric material and the surface of the particulate material having magnetic properties in the composition.

Although shaped articles having high flexural strength may be produced from compositions which do not contain such an additive capable of effecting coupling it has been found that such articles may suffer a substantial loss in flexural modulus when contacted with water. Where the composition from which the shaped article is produced contains such an additive capable of effecting coupling the loss offlexural modulus of the article when the article is contacted with water, if any, is very much reduced.

The coupling additive which may suitably be used in a composition will depend on the nature ofthe particulate material and the organic polymeric material in the composition.

It is preferred that the additive capable of insolubilising the organic polymeric material be the same as the additive capable of effecting coupling between the polymeric material and the particulate material.

For example, where the additive capable of reacting with the organic polymeric material to insolubilisethe latter with respect to water is a polyvalent metal compound certain of the latter compounds are also capable of effecting coupling between ferrite particulate materials and the organic polymeric material. Suitable additiveto fulfil both these functions include Al2(OH)5Cl, (NH4)2 Cur207, Cr (OH)18(NO3) 2 and Al2 (OH)5 NO3.

In general the additive capable of effecting coupling, when differentfrom the additive capable of reacting with the organic polymeric material to insolublisethe latter material with respectto waterwill be present in the composition in a relatively low proportion, although the proportion required may depend on the particle size of the particulate material. For example, the additive may be present in a proportion of 0.01 to 3% by volume of the particulate material in the composition.

The invention is illustrated by the following examples in which all parts are parts by volume, unless otherwise stated.

Example 1 128 parts of a particulate ferrite, BaFe12O19, having a particle size of 10 microns, and 22.8 parts of hydrolysed poly(vinyl acetate) (Gohsenol GH17S Nippon Gohsei, degree of hydrolysis 88% degree of polymerisation 2000) were throughly mixed in a bladed mixer. 4 parts of resorcinol in 15 parts ofwaterwere mixed with 40 parts ofan aqueous solution containing 30 parts ofwarerand 10 parts of aluminium hydroxy chloride the solution containing 12.1% w/wAI,8.75% w/w Cl, the latter solution having a viscosity of 18 cps, and the resultant solution added to the mixed solids in the bladed mixer to form a crumble.

The crumble was then charged to a twin-roll mill the rollers of which were heated to a temperature of 70"C and the crumble was formed into sheet on the mill, the sheet being passed repeatedly through the nip between the roils. The miiling was continued for 5 minutes during which time some of the water evaporated, and the resultant sheet was removed from the mill.

The sheet contained 128 parts of particulate ferrite, 22.8 parts of hydrolysed poly(vinyl acetate), 10 parts of aluminium hydroxy chloride,4 parts of resorcinol, and 25 parts of water.

The sheetwasthen placed between two sheets of polyethylene terephthalate the faces of which were coated with mould release agent and the sheet was pressed in a hydraulic press at a temperature of 80"C a nd a pressure of 10 M Pa for 10 min utes.

The platens ofthe press were then cooled by flowing coldwaterthrough the platens, the sheetwas removed from the press, and the sheet of polyethylene terephthalate were removed from the sheet.

Setting of the sheet was completed by placing the sheet between two flat pieces of wood, the sheetwas allowed to stand for 1 day al 20"C, it was then heated at 80"Cfor 1 day, and finally it was heated at 180"C for 1 hour.

The sheet had aflexural strength of 112.6 MPa and aflexural modulus of48.3 GPa, and contained 78% by volume offerrite and 22% by volume of cross-linked polymer.

The sheet had the following magnetic properties.

Remenance (Br) 1430 Gauss Coercivity (Hc) 750 Oersteds BHmaX product 0.30 x 106 gaussOersteds saturation magnetisation 2720 gauss.

Example 2 The mixing, shaping, and setting procedure of Example 1 was repeated on a composition comprising (Mn, Zn) ferrite 150 micron mean size 669.6 parts (Mn, Zn) ferrite 1 micron mean size 224.1 parts Hydrolysed poly(vinyl acetate) Gohsenol GH 17S 115.8 parts Polyviol V03-140 (Wacker-Chemie) 21.0 parts degree of hydrolysis 86-89%, degree of polymerisation 300 Aluminium hydroxy chloride solution 203.3 parts (as used in Example 1) Water 140 parts The sheet, which contained 83% by volume of ferrite and 17% by volume of cross-linked polymer, had a flexural strength of 106 MPa and a flexural modulus of44.7 GPa. After soaking in waterfor 1 daytheflexural strength ofthe sheet was 56 MPa and the flexural modulus was 20 GPa.

The sheet had the following magnetic properties Remenance 355 gauss Coercivity (Hc) 9.85 Oersteds Initial permeability 19.8 Maximum permeability 26.0 Saturation magnetisation 4480 gauss A ring was cut from the sheet and the lowfield permeability of the ring was measured by comparing the inductance of a coil wound on the ring with the theoretical inductance for an air-cored coil. The lowfield permeabilitywas 19.1.

Example 3 The mixing, shaping, and setting procedure of Example 1 was repeated on a composition comprising (Mn, Zn) ferrite 150 micron mean size 54.2 parts (Mn, Zn)ferrite 1 micron mean size 18.1 parts Hydrolysed poly(vinyl acetate) Gohsenol GH 17S 29.3 parts Polyviol V03-140 5.4 parts Aluminium hydroxy chloride solution (as used in Example 1) 51.4 parts Water 14.3 parts The sheetwhich was produced contained 61% by volume of ferrite and 39% by volume of cross-linked polymer, and had a low field permeability, measured as described in Example 2, of 7.2.

Example 4 The mixing, shaping and setting procedure of Example 1 was repeated on a composition comprising (Mn, Zn) Ferrite 150 micron mean size 60.8 parts (Mn, Zn) Ferrite 1 micron mean size 20.2 parts Hydrolysed PVA GH17S 21.1 parts V03/140 3.8 parts A12(OH)5Cl solution 36.7 parts Water 25 parts The sheet which was produced contained 70% by volume of ferrite and 30% by volume of cross-linked polymer and had a low field permeability measured as described in Example 2, of 13.6.

Claims

1. A shapeable composition comprising a homogeneous mixture of (a) at least one particulate ferrite material having magnetic properties, (b) at ieastonewater-soluble of water-dispersible organic polymeric material, and (c) water, in the composition the components (a), (b)and (c) being presentin a proportion byvolumeofthe composition of respectively, 40 to 90%, 2 to 25%, and not more than 60%.

2. A shapeable composition as claimed Claim 1 characterised in that the particulate ferrite material having magnetic properties has a median particle size of greaterthan 0.3 micron.

3. A shapeable composition as claimed in Claim 1 or Claim 2 characterised in that the particulate ferrite material having magnetic properties comprises a plurality of particle sizes.

4. A shapeable composition as claimed in any one of Claims 1 to 3 characterised in that the composition comprises particulate ferrite material having magnetic properties in a proportion of 60%to 90% byvolume.

5. A shapeable composition as claimed in any one of Claims 1 to 4 characterised in that the components of the composition are selected such that a test composition are selected such that a test composition comprising 63% by volume of particulate ferrite material having magnetic properties, 7% by volume of water-soluble orwater-dispersible organic polymeric material, and 30% by volume of water, when extruded in a capillary rheometer at an extrusion pressure up to a maximum of 500 atmospheres undergoes an increase of at least 25% in shear stress when a tenfold increase in the shear rate of the test composition is effected when the shear rates as measured are within the range 0.1 to 5 second~5.

6. A shapeable composition as claimed in any one of Claims 1 to 5 characterised in that the organic polymeric material comprises a hydrolysed polymer or copolymer of a vinyl ester.

7. A shapeable composition as claimed in Claim 6 characterised in that the organic polymeric material comprises hydrolysis poly(vinyl acetate).

8. A shapeable composition as claimed in any one of Claims 1 to 7 characterised in that the organic polymeric material is present in a proportion of 7% to 20% by volume.

9. A shapeable composition as claimed in any one of claims 1 to 8 characterised in that the composition contains 5% to 20% by volume of water.

9. A shapeable composition as claimed in any one of Claims 1 to 8 characterised in that the composition contains not more than 30% by volume of water.

10. A shapeable composition as claimed in Claim 8 characterised in that the composition contains 5%to 20% by volume of water.

11. A shapeable composition as claimed in any one of Claims 1 to 10 characterised in thatthe composition comprises an additive capable of reacting with the organic polymeric material to insolubilise the material with respect to water.

12. A shapeable composition as claimed in Claim 11 characterised in that the additive is aluminium hydroxy chloride.

13. Ashapeablecomposition as claimed in Claim 11 or Claim 12 characterised inthatthe additive is present in a proportion of 5% to 100% by volume of the organic polymeric material in the composition.

14. Ashapeablecomposition as claimed in anyone of Claims 1 to 13 characterised in thatthe composition comprises an additive capable of effecting coupling between the organic polymeric material and the surface ofthe particulate ferrite material having magnetic properties.

15. Ashapeable composition as claimed in Claim 14 characterised in that the additive capable of insolubilising the organic polymeric material is the same as the additive capable of effecting coupling.

16. A process for the production of a shaped article having magnetic properties which process comprises shaping a composition as claimed in any one of Claims 1 to 15, and removing water from the thus shaped composition.

17. A process as claimed in Claim 16 characterised in that the composition is shaped bycalendering, injection moulding, compression moulding, or by extrusion.

18. A process as claimed in Claim 16 or Claim 17 characterised in that water is removed from the shaped composition by heating at a temperature of 1 00"C or greater.

19. A process as claimed in any one of Claims 16 to 18 characterised in that water is removed from the shaped composition, and the additive capable of insolubilising the organic polymeric material with respect to water, when said additive is present, is reacted with said material by heating at a temperature of 1 OO"C or greater.

20. A shaped article of a particulate material having magnetic properties characterised in that said article is produced by removing water from a shaped composition as claimed in any one of Claims 1 to 15.

21. A shaped article as claimed in Claim 20 characterised in insolubilising the organic polymeric material with respect to water, and in that said additive is reacted with said material.

22. A shaped article as claimed in Claim 20 or Claim 21 characterised in that not more than 2% ofthetotal volume of said article comprises pores having a maximum dimension exceeding 100 microns.

23. A shaped article as claimed in Claim 22 characterised in that not more than 0.5% of the total volume of said article comprises pores having a maximum dimension exceeding 15 microns.

24. Ashaped article as claimed in any one of Claims 20 to 23 characterised in that the total volume of pores in the shaped article does not exceed 20%.

Amendments to the claims have been filed, and have the following effect:- Claims 1,9 and 10 above have been deleted ortextuallyamended.

Newortextuallyamended claims have been filed as follows :- Claims 11-24 above have been re-numbered as 10-23 and their appendancies corrected.

1. A shapeable composition of dough-like consistency, suitable for shaping on plastics or rubber processing equipment and comprising a homogeneous mixture of (a) at least one particulate ferrite material having magnetic properties.

(b) at least one water-soluble orwater-dispersible organic polymeric material, and (c) water, in the composition the components (a), (b) and (c) being present in a proportion by volume ofthe composition of respectively, 40 to 90%, 2 to 25% and not more than 30%.