EP0281295A2

EP0281295A2 - Process and composition for producing bonded magnet

Info

Publication number: EP0281295A2
Application number: EP88301482A
Authority: EP
Inventors: James Hugh Raistrick; Tatsuya Shimoda; Masaaki Sakata
Original assignee: Imperial Chemical Industries Ltd; Seiko Epson Corp
Current assignee: Imperial Chemical Industries Ltd; Seiko Epson Corp
Priority date: 1987-03-03
Filing date: 1988-02-22
Publication date: 1988-09-07
Also published as: JP3229948B2; AU601815B2; AU617620B2; EP0281295A3; JPH11150012A; AU1213088A; JPS63289807A; GB8804062D0; AU5793390A; JP2867140B2

Abstract

A process for the production of a shaped article having magnetic properties from a composition which comprises a mixture of a solid melt-processable and cross-linkable organic material and a particulate magnetic material, which process comprises the steps of

(1) shaping the composition in a mould at a temperature at which organic material is in a fluid state,
(2) cooling the thus shaped composition so as to solidify the organic material, and
(3) cross-linking the organic material in the thus-shaped composition to produce a cross-linked material.

Also, a composition as described which also contains a polymeric material which is soluble in or dispersible in the organic material when the organic material is in a liquid state.

Description

This invention relates to a process for producing a shaped article having magnetic properties in which particles of magnetic material are bonded together by means of an organic material. The invention thus relates to a process for producing a so-called bonded magnet.
The invention also provides a composition from which a shaped article having magnetic properties may be produced.
Bonded magnets which are produced from a composition comprising an organic material e.g. an organic polymeric material, and a particulate magnetic material, are well-known. Most commonly such magnets are produced commercially from a composition comprising a mixture of a thermoplastic organic polymeric material and a particulate magnetic material. For example, a composition comprising a mixture of a thermoplastic organic polymeric material and particulate magnetic material may be shaped in plastics processing equipment, e.g. in an injection moulder or in an extruder, or the composition may be processed by compression moulding.
The composition is shaped whilst the thermoplastic organic polymeric material is in a fluid state and the composition is then cooled to a solid state. Optionally, whilst the organic polymeric material is in a fluid state, the composition may be subjected to the influence of a magnetic field in order to align the articles of magnetic material to the direction of easy magnetisation and thus enhance the performance of the magnet. The magnetic field is maintained whilst the organic polymeric material is cooled to a solid state, and thereafter the thus shaped composition is removed from the influence of the magnetic field as, when the organic polymeric material is in a solid state, the magnetic field is no longer needed in order to maintain the alignment of the particles of magnetic material. The thus produced shaped article is then removed from the plastics processing equipment.
The organic polymeric material in the composition used in the production of the bonded magnet may be a polyolefin, for example, polyethylene or polypropylene, but a particularly favoured material for use in such a composition is a polyamide, that is one of the nylons. A particularly favoured nylon is nylon-6. For example, Japanese Patent Publication No. 59 094406 describes a composition of a synthetic resin and a powdered magnetic material whose surface has been treated with a coupling agent. The magnetic material may be a ferrite or a rare-earth/cobalt intermetallic compound, and the synthetic resin may be polypropylene, polyvinyl chloride or a polyamide, e.g. nylon -6, nylon -11 or nylon -12. The use of polyamides, e.g. nylon -6 and nylon -6.6, in such compositions is also described in Japanese Patent Publication No. 60 216524 and in Japanese Patent Publication No. 61 059705.
Magnets produced from compositions in which the organic polymeric material is a thermoplastic such as a nylon or a polyolefin do, however, suffer from disadvantages, as do the processes used in production of the magnets. Thus, the glass transition temperatures of polyolefins and of the nylons may be relatively low such that at relatively low temperatures magnets made from compositions comprising polyolefins and the nylons may tend to distort and become misshapen, particularly under the influence of a strong magnetic field or as a result of repulsion between the aligned particles of magnetic material, with possible serious consequences for the equipment in which the magnet is installed. For example, the glass transition temperatures of nylon -6, nylon -11 and nylon -12 are respectively 62.5°C, 46°C and 37°C. Thus, the effective upper limit of operation of such a magnet may be at a relatively low temperature, and in particular it may be at a temperature which is not as high as might be desired. Furthermore, it is necessary to process the composition at a temperature at which the organic polymeric material is in a fluid state, and the latter material may melt at a temperature which is so high that during the processing there is an adverse effect on the properties of the particles of magnetic material, e.g. as a result of oxidation. Also, in order to produce a bonded magnet having a high magnetic performance it is necessary to use a composition containing a high proportion of particles of magnetic material. Such a composition may have a high viscosity when it is subjected to plastics processing, and it may be difficult if not impossible to shape a composition containing the desired high proportion of particulate magnetic material. Excessively high temperatures may also be needed in order that the organic polymeric material shall be in a sufficiently fluid state that the composition can be shaped, with possible adverse effect on the properties of the particles of magnetic material.
Magnets made from compositions which comprise an organic material and which have a reduced tendency to distort at high temperatures and which thus may be operated at higher temperatures may be made from compositions in which the organic material is a cross-linkable or curable organic material, e.g. a thermosetting resin. In the production of magnets from such a composition a composition comprising a cross-linkable organic material, optionally an additive capable of effecting or assisting cross-linking of the material, and a particulate magnetic material, is shaped on plastics processing equipment at a temperature at which the organic material is in a fluid state, the organic material is cross-linked and the bonded magnet comprising particulate magnetic material and a solid cross-linked organic resin is recovered. When the organic material is in a fluid state the composition may be subjected to a magnetic field in order to align the particles of magnetic material to the direction of easy magnetisation and thus enhance the performance of the magnet. In this case the influence of the magnetic field is maintained until sufficient cross-linking has been effected that the composition has solidified at least to the extent that the aligned particles are able to retain their alignment when the magnetic field is removed, if necessary the cross-linking reaction is completed, and the bonded magnet is recovered. If the influence of the magnetic field was not maintained whilst the organic material in the composition was still in a fluid state the particles of magnetic material would become misaligned due to repulsion between adjacent particles in the fluid composition.
Examples of the production of bonded magnets from compositions comprising a cross-linkable organic material are provided by Japanese Patent Publication No. 60 220905 which describes the production of a bonded magnet from a composition which comprises an epoxy resin, a rare-earth magnetic powder, and an aliphatic carboxylic ester as a lubricant, by Japanese Patent Publication No 60 220906 which describes the production of a bonded magnet from a composition which comprises an epoxy resin, a rare earth magnetic powder, and a aliphatic carboxylic amide as a lubricant, by Japanese Patent Publication No. 60 206111 which describes in a specific example, the production of a bonded magnet from a composition which comprises a bisphenol A novolak epoxy resin, optionally a liquid diluent, and a magnetic powder of an intermetallic compound of samarium and cobalt, and by Japanese Patent Publication No. 60 183705 which describes the production of a bonded magnet from a composition of a ferrite powder which has been treated with a surfactant and an unsaturated liquid polyester resin.
The cross-linked resin in the bonded magnet will generally have a high glass transition temperature and a bonded magnet produced from such a composition has the advantageous property that it may generally be safely operated at a substantially higher temperature than that at which a bonded magnet produced from a composition comprising a thermoplastic organic polymeric material may be operated.
However, the production of magnets from such compositions does suffer from disadvantages. Thus, the magnetic materials in such compositions may be expensive, as is the case for example with ferrites and with some intermetallic compounds of rare earth metals and transition metals, and it is particularly desirable that any of the composition in a defective moulding or any of the composition which is normally wasted, e.g. that part of the composition in the sprues and runners of an injection moulding machine or the flash which is squeezed out of a compression mould, should be reprocessable. However, where the organic material in the composition in a defective moulding or in the flash or the like has been cross-linked it cannot be reprocessed on plastics processing equipment and thus the expensive magnetic material in the defective moulding or in the flash or the like is effectively wasted. Furthermore, in the production of a magnet from such a composition the rate of production is determined by the speed of the cross-linking reaction and by the necessity of maintaining the composition in a mould, e.g in the die of an extruder, until the cross-linking reaction has proceeded to the extent that the composition is able to retain its shape on removal from the mould. Also, where a magnetic field is applied during the process in order to align the particles of magnetic material to the direction of easy magnetisation, the composition must be retained under the influence of the magnetic field until the amount of cross-linking which has been effected is sufficient to result in a solidified composition, that is until the composition has solidified the extent that the aligned particles of magnetic material are able to retain their alignment when the magnetic field is removed. The cross-linking reaction takes a finite time, indeed, it may take several minutes for the necessary amount of cross-linking to be effected, and the productivity, of the process is severely limited. It is also inconvenient and economically disadvantageous to be required to maintain the magnetic field for such a period of time.
The present invention provides a process for the production of a shaped article having magnetic properties from a composition which comprises a cross-linkable organic material and a particulate magnetic material in which that part of the composition in a defective moulding or that part which is normally wasted in the production of the shaped article may be reprocessed, in which it is unnecessary to maintain the influence of the magnetic field during the cross-linking reaction in order to maintain the alignment of the particles of magnetic material, in which the productivity of the process is much greater than that of the known processes as described herein, and in which it is possible, for example by extrusion, to produce bonded magnets having a substantial length, e.g. magnets in the form of a long cylinder.
According to the present invention there is provided a process for the production of a shaped article having magnetic properties from a composition which comprises a mixture of a solid melt-processable and cross-linkable organic material and a particulate magnetic material, and optionally an additive which is capable of effecting or assisting cross-linking of the organic material to produce a cross-linked material, which process comprises the steps of

In operating the process of the invention the shaping step(1) is operated at elevated temperature. This shaping step may be effected rapidly and provided the composition is maintained at elevated temperature for a relatively short time in step (1) of the process prior to solidification of the organic material in step (2) of the process the amount of cross-linking of the organic material which takes place in step (1), if any, will only be very small with the result that the composition in a defective moulding or in that part of the composition which is normally wasted, e.g. the composition in the sprues and runners of an injection moulding machine and the flash which is squeezed out of a compression mould, may be re-used in the process. There is little or no wastage of expensive magnetic material in the composition. Furthermore,as it is necessary to retain the composition in the mould only for the short period of time which is needed to shape the composition and to cool the thus shaped composition and not for the much larger period of time required to effect the cross-linking reaction the productivity of the process is very greatly increased.
The aforementioned process results in production of a isotropic magnet in which the particles of magnetic material have not been aligned to the direction of easy magnetisation. In an alternative embodiment of the process an anisotropic magnet in which the particles of magnetic material are so aligned and which is of enhanced magnetic performance may be produced by

(1) shaping the composition in a mould at a temperature at which the organic material is in a fluid state,
(2) subjecting the composition to the influence of a magnetic field when the organic material is in a fluid state,
(3) cooling the thus shaped composition so as to solidify the organic material, and
(4) cross-linking the organic material in the thus shaped composition to produce a cross-linked material.

In this alternative embodiment it is not necessary to maintain the magnetic field whilst the cross-linking reaction takes place. The shaped composition is cooled in order to solidify the organic material and in the cooled shaped composition the solid organic material itself maintains the alignment of the particles without the influence of the magnetic field. It is necessary to maintain the influence of the magnetic field only until the organic material in the shaped composition has been cooled to a solid state.
In step (4) of this alternative embodiment of the process the organic material in the shaped composition is cross-linked. If the organic material in the composition was to be re-converted to a fluid state in this cross-linking step the repulsion between adjacent aligned magnetic particles would result in distortion of the shaped composition and in loss of alignment of the particles and thus loss in magnetic performance of the resultant bonded magnet. In order to prevent such repulsion between adjacent aligned magnetic particles the cross- linking reaction may be effected whilst the organic material is in a solid state. Alternatively, prior to effecting the cross- linking reaction the particles of magnetic material may be demagnetised so that, should the organic material in the composition be converted to a state having some fluidity during the cross-linking reaction, the adjacent aligned demagnetised particles of magnetic material will not repel each other and there will be little or no loss in magnetic performance of the bonded magnet. In this alternative embodiment of the process all that is required is that the shape of the composition be maintained during the cross-linking reaction. After the cross-linking reaction has been effected the aligned demagnetised particles of magnetic material may be remagnetised by subjecting the bonded magnet to the influence of a magnetic field.
The process of the invention provides substantial flexibility in the design of magnets, and magnets of simple shape or of complex shape may be produced. The magnets which are produced are light in weight and may for example have a weight which is only about two thirds of the weight of a metallic magnet of corresponding size. The magnets produced by the process are also less brittle than are ceramic magnets.
The magnets may be used in many application, for example, in motors, TV sets, in printers and in latching devices, e.g. latching devices on doors.
In the composition which is used in the process of the invention, the organic material is a solid material which is melt processable. In general the organic material will be solid at or about ambient temperature of 25°C, and have a melting point and thus be fluid and melt-processable at a higher temperature. The organic material, and the composition, should be melt-processable on plastics processing equipment, for example, in an injection moulder, or in an extruder, or in a compression mould.
The organic material in the composition may be a solid organic monomeric material, or it may be a solid organic polymeric material. The composition may comprise a mixture of two or more organic monomeric materials, or a mixture of two or more organic polymeric materials, or a mixture of one or more organic monomeric materials and one or more organic polymeric materials.
Where the organic material in the composition is a monomeric material it should of course have a molecular weight which is sufficiently high that the organic material is solid, e.g. at or about ambient temperature. The monomeric material desirably contains an ethylenically unsaturated group, and preferably a plurality of such groups as the presence of such groups assists the cross-linking reaction. Examples of suitable organic monomeric materials include 1:3 diallyl urea,
H₂C= CH-CH₂-NH-CO-NH-CH₂-CH=CH₂,
9 vinyl carbazole
penta erythritol tetramethacrylate,
3,9- divinyl -2,4,8,10 tetraoxa spiro (5,5) undecane,
an adduct of 4,4ʹ diphenyl methane diisocyanate and hydroxy ethyl methacrylate,
Examples of organic polymeric materials which are solid but which are melt processable include ethylenically unsaturated polyester resins and epoxy resins.
Examples of suitable epoxy resins include bisphenol A:-
and epoxidised phenol formaldehyde novolak:-
in which
The epoxy resin will contain a suitable hardener which comprises a plurality of hydroxyl groups.
An example of a suitable hardener is a phenol-formaldehyde novolak:-
The composition may contain, and preferably does contain, an additive which is capable of effecting or assisting cross-linking of the organic material in the composition, although cross-linking may be effected in the absence of such an additive. Suitable such cross-linking additives include free-radical generators e.g. peroxides and azo compounds, for example, azo-bis-iso butyronitrile, especially where the organic material contains ethylenically unsaturated groups, for example, where the organic material is a polyester resin or where it is an acrylic material. Where the organic material is an epoxy resin it may contain an additive which catalyses reaction between the epoxy resin and hardener.
Where the composition contains an additive capable of effecting or assisting cross-linking care should be exercised in choosing the combination of organic material and additive. Thus, in step (1) of the process of the invention the composition is processed at a temperature at which the organic material is in a fluid state. Whilst the organic material is in this fluid state at an elevated temperature the amount of cross-linking, if any, which is effected is desirably kept to a minimum, or at least is not an amount such as to prevent subsequent reprocessing of the composition, and where the composition contains such a cross-linking additive an additive should be chosen whose activity is such that it does not effect such an undesirable amount of cross-linking at the temperature at which the processing is effected. The additive which is chosen will of course depend on the organic material in the composition, and in particular on the temperature at which this latter material is to be processed in the process of the invention. Indeed, an additive which is suitable for use in a composition with one organic material may be quite unsuitable for use in a composition with a different organic material which is melt processable only at a higher temperature as, at the higher temperature, an unacceptably high proportion of cross-linking of the latter organic material may take place during the melt-processing. Suitable combinations of organic material and additive may be selected from a knowledge of the melt processing characteristics of the organic material and of the thermal decomposition characteristics of the additive. However, by way of example, suitable combinations of additives and organic materials include 3,9-divinyl -2,4,8,10- tetraoxaspiro (5,5) undecane which melts at 42°C and azo-bis-isobutylonitrile which dissociates to form radicals at a temperature in excess of about 70°C, and 9-vinyl carbazole which melts at 65°C and azo-bis-dicyclohexane carbonitrile which dissociates to form radicals at a temperature in excess of about 90°C.
By "magnetic material" we mean a material which is magnetic or which is capable of being magnetised. Thus, the magnetic material may not itself be magnetic but it may be magnetized under the influence of the magnetic field when the composition is processed.
Whilst there is no particular limit on the umaximum size of the particles of magnetic material the particles suitably have a size in the range of 0.5 micron to 200 microns.
Examples of suitable magnetic materials include ferrite materials, eg barium hexaferrite (Ba₀.₆Fe₂O₃) and strontium hexaferrite (Sr₀.₆Fe₂O₃). Other magnetic materials which may be used in the process of the invention and from which bonded magnets having high magnetic performance may be produced include intermetallic compounds formed from at least one rare earth metal and at least one transition metal. Rare earth metals from which such a magnetic material may be formed include Sm, Ce, La, Y, Nd, Pr and Gd, and suitable transition metals include Fe, Co, Ni, Zr, Hf, Cu and Ti. The intermetallic compound may, for example, have an empirical formula which may be generally referred to as RCo₅ or RCO₁₇, where R is at least one rare earth metal. An example of a rare earth metal from which the intermetallic compound may be produced is Sm, for example as in the intermetallic compounds which are generally referred to by the empirical formulae SmCo₅ and Sm₂Co₁₇. These latter empirical formulae are not intended to represent exact chemical formulae for the intermetallic rare earth - transition metal compounds as elements other than Sm and Co maybe present in the intermetallic compounds. By way of example, Japanese Patent Publication No 60 227408 refers to a rare earth-transition metal intermetallic compound having the formula Sm (CO₀.₆₇₂ Cu₀.₀₆ Fe₀.₂₂ Zr₀.₀₂₈)₅.₃ and Japanese Patent Publication No 60 220905 to rare earth-transition metal compounds having formulae Sm (Co₀.₆₇₂ CU₀.₀₆ Fe₀.₂₂ Zr₀.₀₂₈)₈.₃₅, Sm₀.₇₅ Y₀.₂₅ (Co₀.₆₅ Cu₀.₀₅ Fe₀.₂₈ Zr₀.₀₂)₇.₈, and Sm₀.₈₁ Ce₀.₁₉ (Co₀.₆₁ Cu₀.₀₆ Fe₀.₃₁ Zr₀.₀₂)₇.₆. Other examples of magnetic materials which are intermetallic compounds of at least one rare earth metal and at least one transition metal include those based as Nd - Fe - B, for example, Nd (Fe₀.₉₀₅ B₀.₀₉₅)₅.₆₇ which is also described in Japanese Patent Publication No. 60 220905. Other examples of such intermetallic compound magnetic materials include
Sm(Co₀.₆₇Cu₀.₀₈Fe₀.₂₂Zr₀.₀₃)₇.₆, Sm(Co₀.₀₇₄CU₀.₁₀ Fe₀.₁₅Ti₀.₀₁)₇.₂, Sm(Co₀.₆₉Cu₀.₁₀Fe₀.₂₀Hf₀.₀₁)₇.₀, Sm₀.₅Pr₀.₅Co₅, Ce(Co₀.₆₉Cu₀.₁₂Fe₀.₁₈Zr₀.₀₁)₆.₀, Sm₀.₅Nd₀.₄C e₀.₁(Co₀.₆₇₂CU₀.₀₈Fe₀.₂₂Zr₀.₀₃)₈.₃₅, and Nd₁₄Fe₈₁B₅.
The composition may of course contain more than one organic material, more than one particulate magnetic material, and/or more than one additive capable of effecting or assisting cross-linking of the organic material.
In the composition the proportions of organic material, of additive capable of effecting or assisting cross-linking, if present, and of particulate magnetic material, may be varied between wide limits. In general, the proportion of magnetic material will be as high as possible, consistent with the composition being melt-processable on plastics processing equipment, in order that the magnetic performance of the bonded magnet which is produced may be as high as possible. In general, the proportion of magnetic material in the composition will be at least 50% by weight of the composition, preferably at least 80% by weight of the composition. A suitable range for the proportion of the magnetic material is 80 to 95% by weight of the composition.
The amount of organic material in the composition should be such as to result in a composition which is is melt-processable on plastics processing equipment, and in general the composition will contain at least 5% of organic material by weight of the composition. A suitable proportion of organic material is in the range 5 to 20% by weight of the composition.
The amount of additive which is capable of effecting or assisting cross-linking will depend to some extent at least on the nature of the additive and on the nature of the organic material but an amount of additive in the range of 0.01% to 5% by weight of the composition will generally suffice. Where the organic material contains ethylenically unsaturated groups, as in a polyester resin or in an acrylic material, and the additive is a free-radical generator, an amount of additive in the range 0.01% to 2% by weight of the composition will generally suffice. Where the organic material is an epoxy resin the amount of additive will generally also be in the range 0.01% to 2% by weight of the composition.
The greater is the amount of such additive in the composition the faster will be the cross-linking of the organic material.
The components of the composition may be mixed by any convenient means. For example, the components when in particulate form may be mixed in any suitable equipment for blending particulate material. A preferred manner of forming a particularly homogenous composition of the organic material and the particulate magnetic material is to mix the composition under conditions of high shear, for example, on a twin roll mill at an elevated temperature at which the organic material is heat-softened. The mixture may be passed repeatedly throughout the nip between the rolls of the mill, and finally, and if desired, the additive which is capable of effecting or assisting cross-linking may be added to the mixture on the mill. This is a particularly convenient means of mixing the components of the composition when the additive itself is liquid. The mixing of the additive should be effected relatively rapidly so that little if any cross-linking of the organic material is effected during the mixing, and for this reason the additive is preferably added at the end of the mixing process.
In an alternative method the components of the composition may be mixed in the presence of a liquid diluent which is subsequently removed from the composition. The liquid diluent assists in producing a homogenous mixture of the components of the composition and it may be removed from the composition, for example by evaporation, particularly when the diluent is a low boiling liquid.
Where the organic material in the composition is a monomeric material, and even where it is a polymeric material, mixing of the components of the composition under conditions of high shear and in particular the formation of a homogenous mixture, and the subsequent melt-processing of the composition, may be assisted by including in the composition a proportion of, and generally a small proportion of, a polymeric material which is soluble in or dispersible in the organic material when the organic material is in a fluid, or liquid, state. The presence of a small proportion of such a polymeric material also assists in the formation of a composition which contains a high proportion of particulate magnetic material and which is also melt-processable on plastic processing equipment, and in a further embodiment of the invention there is provided a composition which comprises a mixture of:

(1) a solid melt-processable and cross-linkable organic material,
(2) a particulate magnetic material, and
(3) a polymeric material which is soluble in or dispersible in the organic material when the organic material is in a liquid state, and optionally
(4) an additive which is capable of effecting or assisting cross-linking of the organic material to produce a cross-linked material.

The polymeric material will generally be a co-polymer containing some functional groups which have an affinity for the magnetic particles. The polymeric material may promote the wetting of the particles by the organic material. Suitable polymeric materials include polyvinyl butyral/polyvinyl alcohol co-polymer, polyvinyl chloride/polyvinyl acetate/polyvinyl alcohol co-polymer, polyvinyl acetate/polycrotonic acid co-polymer, and polyvinylidene chloride/polyacrylonitrile co-polymer. The composition suitably contains from 0.5 to 5% by weight of such polymeric material. The composition may contain more than one such polymeric material.
The composition may be melt-processed and shaped on suitable plastic processing equipment, for example in an extruder, in an injection moulder, or by compression moulding.
In effecting the shaping step of the process of the invention in an extruder the composition is charged to the extruder, the composition is heated in order to convert the organic material to a fluid state, the composition is extruded though a suitable die, the composition is cooled near the exit from the die in order to solidify the organic material, and a shaped composition is removed from the die. Where it is desired to produce an anisotropic magnet the composition in the die of the extruder is subjected to the influence of a magnetic field whilst the organic material is in a fluid state and the particles of magnetic material are aligned to the direction of easy magnetisation. The magnetic field may be an electromagnet positioned adjacent to the die of the extruder. In effecting the shaping step the temperature should not be excessively high, and the time for which the organic material is in a fluid state should be relatively short in order that little if any cross-linking of the organic material takes place at this stage so that defective mouldings, if any, and waste composition can be reprocessed.
The shaping step of the process, and the optional alignment of the particles of magnetic material, may similarly be effected in an injection moulder having a suitable shaped mould into which the composition is injected when the organic material in the composition is in a fluid state. If desired a magnetic field may be positioned adjacent to the mould in order to align the particles of magnetic material to the direction of easy magnetisation. The shaped composition is cooled and removed from the mould.
The composition may also be shaped by compression moulding by charging the composition to a suitably shaped mould, heating the composition in the mould to convert the organic material to a fluid state and compressing the composition in the mould, and finally cooling the composition in the mould. In this case also a suitable magnetic field may be positioned adjacent to the mould in order to align the particles of magnetic material.
Where the shaping step is carried out under the influence of a magnetic field the field is applied so that the particles of magnetic material are aligned to the direction of easy magnetisation and it is necessary to cool the shaped composition under the influence of the magnetic field and to maintain the magnetic field until the composition has cooled and the organic material has solidified at least to the extent that the influence of the magnetic field is no longer necessary in order to maintain the alignment of the particles of magnetic material.
The organic material in the shaped composition is then cross-linked to produce a cross-linked resin.
The cross-linking of the organic material may be effected by heating the shaped composition. However, on heating of the composition the organic material may be converted to a fluid state and it will be necessary to maintain the shape of the composition. The shape may be maintained by placing the composition in a suitably shaped mould. Alternatively, the cross-linking may be effected whilst the organic material is in a solid state, and in this way the shape of the composition is retained. Suitable means of effecting this solid state cross-linking include irradiating the shaped composition with an electron beam, in which case cross-linking of the organic material in the composition may be effected in the presence of or in the absence of an additive of the type hereinbefore described. Cross-linking may be effected by irradiating the shaped composition with γ-rays, for example with γ-rays provided by a Co⁶⁰ source, similarly in the presence of or in the absence of an additive in the composition. The cross-linking may be effected after the shaped composition has been removed from the plastics processing equipment. Indeed this is preferred in order to maximise the utilisation of the plastics processing equipment.
The shape of the composition may be maintained by effecting a part only of the cross-linking whilst the organic material in the composition is in a solid state. For example, an outer layer of the organic material in the shaped composition may be cross-linked when the organic material is in a solid state, eg by use of electron beam irradiation or by use of γ-ray irradiation such that, when the composition is heated at elevated temperature to complete the cross-linking, this outer layer remains solid and serves to maintain the shape of the composition.
Where the particles of magnetic material in the shaped composition have been aligned to the direction of easy magnetisation it is particularly convenient to cross-link the organic material of the shaped composition when this material is in a solid state. If the organic material was converted to a fluid state prior to or during the cross-linking, for example, by heating the composition, repulsion between adjacent magnetic particles would lead to loss of alignment of particles and decreased magnetic performance in the resultant bonded magnet.
Even when cross-linking of the organic material in the shaped composition is effected when material is in a solid state some of the cross-linking may be effected, particularly the later stages of the cross-linking, by heating the shaped composition at elevated temperature provided that sufficient cross-linking has been effected when the organic material is in a solid state that the shaped composition retains its shape at elevated temperature and does not soften to such an extent that the particles of magnetic material do not maintain their alignment.
Prior to cross-linking the organic material of the shaped composition the particles of magnetic material which have been aligned to the direction of easy magnetisation may be demagnetised by any suitable means, for example, by taking the shaped composition around a series of decreasing hysterisis loops. It may not be necessary to demagnetise the particles of magnetic material completely, but it is preferred that they be demagnetised at least to the extent that in the cross-linking step of the process there is at most only a small amount of repulsion between aligned particles when the shaped composition is in a relatively fluid state. Even when cross-linking is effected at elevated temperature at which the organic material in the composition becomes relatively fluid it is a surprising feature of the process that despite that fluidity the particles of magnetic material remain in alignment as the particles are not magnetised and thus there is no repulsion between particles.
After cross-linking of the organic material has been effected the particles of magnetic material which have been demagnetised may be remagnetised.
The invention is illustrated by the following examples in which all parts are expressed as parts by weight.

Example 1

A compostion of
Magnetic particles:
Sm(Co₀.₆₇₂ Fe₀.₂₂ Cu₀.₀₈ Zr₀.₀₂₈)₈.₃₅ powder 91.47 parts
Organic material:
Oligomerised and epoxidised bisphenol A powder 4.13 parts
Phenol-formaldehye novolak powder 2.29 parts
Epoxidised phenol-formaldehyde novolak powder 0.33 parts
Polymeric material:
A powder of a copolymer containing units of vinyl butyral and vinyl alcohol - Pioloform BN 18, Wacker Chemie GmbH 1.26 parts
Silica powder (Aerosil OX 50) 0.2 parts
Calcium stearate 0.17 parts
Bleached Monton wax 0.17 parts
Diuron 0.05 parts

was mixed by hand to form a reasonably homogenous mixture of the powders and the mixture was then charged to a twin-roll mill, the rolls of which were at temperature of 90°C, and the composition was passed repeatedly through the nip between the rolls of the mill to form a plastic sheet. The presence of the organic polymer in the composition assisted in the production of a sheet. The sheet was then callendered on the twin-roll mill at 80°C to a thickness of 0.7 mm and the sheet was placed in a mould at 110°C and pressed to reduce the thickness of the sheet to 0.5 mm. The sheet was then divided into six equal-sized smaller sheets.
One of the smaller sheets was placed in a mould positioned between the poles of a 23.5 kG electromagnet and the sheet was heated rapidly to 140°C and thereafter immediately cooled to ambient temperature. At 140°C the organic material in the composition melted and the magnetic particles became aligned under the influence of the magnetic field. It was possible to remelt a part of the sheet thus demonstrating that the extent of cross-linking which had taken place, if any, was not such as to prevent reprocessing of the sheet. The sheet was then placed between the poles of an electromagnet and a series of decreasing alternating magnetic fields were applied in order to demagnetise the magnetic particles. The sheet was then heated in the mould at 170°C for 30 minutes in order to cross-link the organic material, and finally the sheet was subjected to magnetisation between the poles of a 23.5 kG electromagnet and the sheet was found to have a (BH) max of 4.5 MG Oe.

Comparative Example 1

One of the smaller sheets produced as described in Example 1 was treated by a conventional process in order to align the particles of magnetic material to the direction of easy magnetisation and in order to cross-link the organic material.
The sheet was placed in a mould between the poles of a 23.5 kG electromagnet and the sheet was heated at 170°C for 30 minutes at which temperature the organic material in the composition initially melted, thereby permitting the particles of magnetic material to become aligned under the influence of the magnetic field, and at which the organic material was subsequently cross-linked. The sheet was then cooled to ambient temperature. The (BH) max of the sheet was 5.2 MG Oe.
The sheet, in which the organic material was cross-linked, was no longer melt processable indicating that waste material, or a defective moulding, which would have been produced by such a conventional process would not have been able to be re-used and thus would have had to have been discarded. It is also clear that in the conventional process the magnetic field needed to be maintained for an excessive period of time in order to ensure the alignment of the particles of magnetic material.

Comparative Example 2

One of the smaller sheets produced as described in Example 1 was subjected to the magnetic alignment procedure of Example 1 to produce a sheet in which the organic material was essentially uncross-linked but in which the particles of magnetic material were aligned. The sheet was then heated at 170°C for 30 minutes whilst unrestrained by a mould. The sheet softened and expanded to several times its original volume as a result of repulsion between adjacent magnetic particles. After heating, the resultant sheet was found to be porous and fragile and to have very poor magnetic properties. This comparative example indicates that where the particles of magnetic material have been aligned prior to cross-linking of the organic material the alignment will be lost if the organic material is heated to a fluid state prior to cross-linking, unless the magnetic field is maintained or unless the aligned particles are first demagnetised.

Comparative Example 3

The procedure of Comparative Example 2 was repeated except that the sheet was constrained in a mould when heated at 170°C for 30 minutes. The (BH) max of the sheet was 3 MG Oe, indicating that heating a sheet in which the magnetic particles were aligned and magnetised but in which the organic material was uncross-linked allowed some of the alignment of the particles of magnetic material to be lost when the organic material melted as the particles were not held in alignment by the influence of a magnetic field.

Example 2

One of the smaller sheets produced as described in Example 1 was subjected to the magnetic alignment and subsequent demagnetisation procedure of example 1 to produce a demagnetised sheet in which the organic material was essentially uncross-linked but in which the particles of magnetic material were aligned. The sheet was then machined so as to shape it precisely to the shape required. Pieces of sheet cut off during machining were suitable for re-use as the organic material in the pieces was essentially uncross-linked. The sheet was sprayed with a 20 µm layer of varnish which solidified to give a film which remained solid when heated to 170°C. The sheet was then heated at 170°C for 30 minutes to cross-link the organic material in the sheet. The desired shape of the sheet was retained. After magnetisation between the poles of a 23.5 kg electromagnet the sheet was found to have a (BH) max of 4.3 MG Oe.

Comparative Example 4

The procedure of example 2 was repeated except that the machined piece was not coated with varnish. The resultant sheet did not have the required shape because the machined edges became rounded when the sheet was heated to 170°C and the organic material in the sheet became fluid prior to cross-linking.

Example 3

A composition of
Magnetic particles:
as used in Example 1 187 parts
Organic material:
a powder of an adduct of 4:4ʹ diphenyl methane diisocyanate and hydroxy ethyl methacrylate 18.7 parts,
Polymeric material:
a powder of a copolymer containing units of vinyl butyral and vinyl alcohol - Pioloform BS 18, Wacker Chemie GmbH 3.1 parts

was mixed by hand to form a reasonably homogenous mixture of the powders and the mixture was then charged to a twin-roll mill, the rolls of which were at temperature of 80°C, and the composition was passed repeatedly through the nip between the rolls of the mill to form a plastic sheet. The presence of the organic polymer in the composition assisted in the production of a sheet. 0.2 part of 1,1ʹ azo bis (cyclohexane-carbonitrile) free radical generator was then added and milling was continued for 1 minute and the plastic sheet was removed from the mill and cooled to ambient temperature. The sheet was then callendered on the twin-roll mill at 60°C to a thickness of 0.7 mm and the sheet was placed in a mould at 80°C and pressed to reduce the thickness of the sheet to 0.5 mm. The sheet was then divided into five equal sized smaller sheets.
One of the smaller sheets was placed in a mould positioned between the poles of a 23.5 kG electromagnet and the sheet was heated rapidly to 100°C and thereafter immediately cooled to ambient temperature. At 100°C the organic material in the composition melted and the particles of magnetic material became aligned under the influence of the magnetic field. It was possible to remelt a part of the cooled sheet thus indicating that the extent of cross-linking which had taken place, if any, was not such as to prevent reprocessing of the sheet.
The sheet was then irradiated with Co⁶⁰ γ-rays at ambient temperature in order to cross-link the organic material when the latter material was in a solid state. The irradiation was continued for a time sufficient to result in a cross-linked resin having a glass transition temperature of 60°C. The sheet was then heated to 120°C at which temperature the sheet softened slightly but not to an extent which allowed the sheet to distort nor which allowed the particles of magnetic material in the sheet to become misaligned. Heating at 120°C was continued for 5 minutes to effect more cross-linking, and the sheet was then found to have a glass-transition temperature of 100°C. The (BH) max value for the sheet was 5.0 MG Oe.

Example 4

The alignment procedure under the influence of the electromagnet as described in Example 3 was repeated on another of the smaller sheets and the sheet, after cooling to ambient temperature, was placed between the poles of an electromagnet and a series of decreasing alternating magnetic fields were applied in order to demagnetise the particles of magnetic material. The sheet was then irradiated with a 1 M rad dose of electrons accelerated by a 170 kV potential in order to produce a partially cross-linked resin, particularly at the surface of the sheet, and the sheet was then heated at 120°C for 5 minutes in order to effect more cross-linking. The sheet was then subjected to the magnetisation procedure of Example ₁ and the sheet was found to have a (BH) max of 4.5 MG Oe indicating that although heating at 120°C resulted in the organic material becoming somewhat fluid, particularly in the interior of the sheet, the alignment of the particles of magnetic material was not lost as the particles had been demagnetised.

Comparative Example 5

One of the smaller sheets referred to in Example 3 was placed in a mould between the poles of a 23.5 kG electromagnet and the sheet was heated at 120 C for 5 minutes at which temperature the organic material in the composition melted thereby permitting the particles of magnetic material to become aligned under the influence of the magnetic field. The (BH) max of the sheet was 5.2 MG Oe. However, heating of the sheet at 120°C for five minutes resulted in substantial cross-linking to an extent that the composition was no longer melt-processable.

Comparative Example 6

One of the smaller sheets referred to in Example 3 was subjected to the magnetic alignment procedure of Example 3 to produce a sheet in which the organic material was essentially uncross-linked but in which the particles of magnetic material were aligned. The sheet was then heated at 120°C for 5 minutes whilst unrestrained by a mould. The sheet softened and expanded to several times its original volume as a result of repulsion between adjacent magnetic properties. After heating, the resultant sheet was found to be porous and fragile and to have very poor magnetic properties.

Comparative Example 7

The procedure of Comparative Example 6 was repeated except that the sheet was constrained in a mould when heated at 120°C for 5 minutes. The (BH) max of the sheet was 3 MG Oe, indicating that heating a sheet in which the magnetic particles were aligned but in which the organic material was uncross-linked to a temperature at which the latter melted resulted in loss of alignment of the particles as they were not held in alignment by the influence of a magnetic field.

EXAMPLE 5

A composition of
Magnetic particles:
Nd₁₄Fe₈₁B₅ 93.30 parts
Organic material:
Oligomerised and epoxidised bisphenol A powder 4.13 parts
Phenol-formaldehye novolak powder 2.29 parts
Epoxidised phenol-formaldehyde novolak powder 0.33 parts
Polymeric material:
A powder of a copolymer containing units of vinyl butyral and vinyl alcohol - Pioloform BN 18-Wacker Chemie GmbH 1.26 parts
Silica powder (Aerosil OX 50) 0.2 parts
Calcium stearate 0.17 parts
Bleached Monton wax 0.17 parts
Diuron 0.05 parts

was mixed by hand to form a reasonably homogenous mixture of the powders and the mixture was then charged to a twin-roll mill, the rolls of which were at temperature of 90°C, and the composition was passed repeatedly through the nip between the rolls of the mill to form a plastic sheet. The presence of the organic polymer in the composition assisted in the production of a sheet. The sheet was then callendered on the twin-roll mill at 80°C to a thickness of 0.7 mm and the sheet was placed in a mould at 110°C and pressed to reduce the thickness of the sheet to 0.5 mm.
The sheet was placed in a mould and heated rapidly at 140°C and thereafter immediately cooled to ambient temperature. It was possible to remelt a part of the sheet thus demonstrating that the extent of cross-linking which had taken place, if any, was not such as to prevent reprocessing of the sheet. The sheet was then heated in the mould at 170°C for 30 minutes in order to cross-link the organic material. The sheet was found to have a (BH) max of 5.5 MGOe.

Examples 6 to 9

In four separate examples solid compositions (in weight per cent) as shown in Table 1 were mixed by hand to form reasonably homogeneous mixtures of powders and each mixture was then separately charged to a twin-roll mill and passed repeatedly through the nip between the rolls of the mill. The compositions of Examples 6 and 7 were heated at 95°C on the mill and the compositions of Examples 8 and 9 at 100°C. Each of the compositions was formed into a sheet and removed from the mill. The compositions were then pulverised to particles and each composition was charged to a screw extruder and extruded through a cylindrical die. The temperature of the barrel of the extruder was 120°C and that of the die was 130°C, and the extrusion speed was 1 mm sec⁻¹. During the extrusion the die of the extruder was subjected to a radial magnetic field of 15 KOe in order to align the particles of magnetic material in the compositions. The end of the die was at ambient temperature in order to solidify the extruded compositions. The cylindrical extruded compositions had an external diameter 30 mm and an internal diameter of 26 mm. The magnetic particles in each of the cylinders were then demagnetised following the procedure described in Example 1 and each of the cylinders was then heated at 200°C for 30 minutes in order to cross-link the resin in the compositions.
By way of comparison, in three separate comparative examples, Comparative Examples 8, 9, and 10, compositions as shown in Table 2 (in weight per cent) were mixed to form reasonably homogeneous mixtures and charged separately to twin-roll mills and mixed on the mill at a temperature of either 250°C (Comparative examples 8 and 9) or 260°C (Comparative example 10). The mixtures removed from the mill were extruded from a screw extruder through a cylindrical die at a speed of 0.5 mm sec⁻¹ at a barrel temperature of 240°C and a die temperature of 220°C. The die was subjected to a radial magnetic field of 15 KOe and the end of the die was at ambient temperature in order to solidify the extruded compositions. The cylindrical extruded composition had an external diameter of 30 mm and an internal diameter of 26 mm.
(A composition comprising, in weight per cent, magnetic particles 95.6, nylon-12 powder 4.3, zinc stearate 0.1, could not be compounded satisfactorily on the twin-roll mill nor could it be extruded satisfactorily.)
The magnetic performances of the cylindrical magnets produced in Examples 6 to 9 and in Comparative Examples 8 to 10 is shown in Table 3.
Examples 6 to 9 and Comparative Examples 8 to 10 demonstrate that it is possible to extrude a composition of the invention comprising more than 95 weight per cent of magnetic particles whereas this is not possible with a composition containing a conventional thermoplastic resin and comprising more than 95 weight per cent of magnetic particles. Furthermore, the composition of the present invention, when moulded, has a superior magnetic performance indicating better alignment of the magnetic particles in the composition and that the magnetic particles are easier to align in the composition.

Examples 10 to 13

In four separate examples the procedure of Examples 6 to 9 was repeated to produce cylindrical shaped compositions except that the magnetic particles were 1 to 200 microns in diameter and had the composition Nd₁₄ (Fe₀.₉₅ Co₀.₀₅)₈₀.₅ B₅.₅, the temperature of the twin-roll mill was 95°C and the extrusion speed was 2 mm sec⁻¹.
The compositions in weight per cent are as shown in Table 4.
In two comparative examples, Comparative Examples 11 and 12, compositions comprising, respectively, 90.4 and 91.7 weight per cent of magnetic particles as used in Examples 10 to 13, 9.5 and 8.2 weight per cent of nylon-12 powder, and 0.1 and 0.1 weight per cent of zinc stearate were shaped into cylindrical magnets following the procedure of Comparative Examples 8 to 10 except that the temperature of the twin-roll mill was 250°C, the temperature of the barrel of the extruder was 230°C, the temperature of the die of the extruder was 205°C, and the extrusion speed was 1 mm sec⁻¹.
(A composition containing 95 weight per cent or more of magnetic particles could not be milled satisfactorily on the twin-roll mill nor could the composition be extruded.)
The magnetic performances of the cylindrical magnets are shown in Table 5.
It can be seen that by using a composition of the invention which comprises a solid melt-processable and cross-linkable organic material it is possible to obtain high isotropic performance of the resultant magnets. It is believed that the relatively low isotropic performance of a magnet produced from a composition containing a conventional thermoplastic polymer, e.g. nylon-12, is due in part to oxidative deterioration of the magnetic particles at the high processing temperatures which it is necessary to use in the production of the magnets.

Example 14

A composition as used in Example 7 was mixed on a twin roll-mill and extruded following the procedure described in Examples 6 and 7 except that the cylindrical shaped magnet which was produced had an external diameter of 16 mm and an internal diameter of 14 mm, and the cylinder was cut into 15 mm lengths.
In a Comparative Example 13 a composition as used in Comparative Example 9 was charged to a twin-roll mill and mixed on the mill at a temperature of 250°C. The composition was removed from the mill in the form of a sheet and the sheet was pulverised to small particles which were charged to an injection moulding machine. The die of the injection moulding machine was subjected to a radial magnetic field of 6KOe and the composition was injected into the die to form a cylindrical magnet having a length of 15 mm and external and internal diameters of respectively, 16 mm and 14 mm. The magnetic performance of the magnets was as shown in Table 6.
As can be seen from Table 6 the magnet of Comparative Example 13 had substantially isotropic properties caused, it is believed, by the difficulty of subjecting the composition to a sufficient magnetic field for alignment of the particles when the composition is in the die of the injection moulding machine.

Examples 15 to 18

In four separate examples a composition was extruded in a cylindrical shape having an external diameter of 33 mm and an internal diameter of of 32 mm (Examples 15 and 16) or of 31.6 mm (Examples 17 and 18), and the cylinders were cut up into lengths of 8 mm. The compositions of Examples 15 and 16 was the same as that used in Example 6 and the compositions of Examples 17 and 18 was the same as that used Example 10. In Examples 15 and 16 the conditions of twin-roll milling, extrusion, magnetisation and demagnetisation were the same as those used in Example 6, and in Examples 17 and 18 the conditions of twin-roll milling, extrusion, magnetisation and demagnetisation were the same as those used in Example 10.
In four separate comparative examples, Comparative Examples 14 to 17, an attempt was made to form separate compositions into a cylindrical shape having an external diameter of 33 mm a length of 8 mm, and an internal diameter of 32 mm (Comparative Examples 14 and 15) or of 31.6 mm (Comparative Examples 16 and 17). The composition of Comparative Examples 14 and 15 was the same as that used in Comparative Example 8 , and the composition of comparative Examples 16 and 17 was the same as that used in Comparative Example 11. The twin-roll milling conditions used in Comparative Examples 14 and 15 were the same as those used in Comparative Examples 8, and the twin-roll milling conditions used in Comparative Examples 16 and 17 were the same as those used in Comparative Example 11. Each of the compositions removed from the twin-roll mill was pulverised and charged to an injection moulder operating at a temperature of 295°C. The temperature of the mould was 90°C and the mould was subjected to a radial magnetic field of 15 KOe.
The magnetic performance of the resultant cylindrical magnets as shown in Table 7.
The magnetic performance of the magnet of Comparative Example 15 was about the same as that of an isotropic magnet, whereas the extruded magnet produced from the composition of the present invention had a substantially superior magnetic performance.

Example 18

A cylindrical magnet having radially aligned magnetic particles, an outer diameter of 16 mm, and an internal diameter of 14 mm, was produced from a composition as used in Example 7 following the procedure as described in Example 14 except that the extrusion speed was 1.2 mm sec⁻¹ and the cylindrical magnet was cut into 4 mm lengths. The 4 mm long magnets were thus produced at a rate of 18 magnets per minute.
In Comparative Example 18 a composition as described in Comparative Example 13 was injection moulded under the same conditions as described in Comparative Example 13 to produce magnets having a length of 4 mm, an external diameter of 16 mm, and an internal diameter of 14 mm. The minimum moulding cycle time at which it was possible to operate was 20 seconds, and thus from a single mould only 3 magnets per minute could be produced.

Claims

1. A process for the production of a shaped article having magnetic properties from a composition which comprises a mixture of a solid melt-processable and cross-linkable organic material and a particulate magnetic material, which process comprises the steps of

(1) shaping the composition in a mould at a temperature at which the organic material is in a fluid state,

(2) cooling the thus shaped composition so as to solidify the organic material, and

(3) cross-linking the organic material in the thus shaped composition to produce a cross-linked material.

2. A process as claimed in claim 1 for the production of an anisotropic magnet, which process comprises

(2) subjecting the composition to the influence of a magnetic field when the organic material is in a fluid state,

(3) cooling the thus shaped composition so as to solidify the organic material, and

(4) cross-linking the organic material in the thus shaped composition to produce a cross-linked material.

3. A process as claimed in claim 1 or claim 2 in which cross-linking of the organic material in the shaped composition is effected when the organic material is in a solid state.

4. A process as claimed in any one of claims 1 to 3 in which, prior to effecting cross-linking of the organic material, the particles of magnetic material are demagnetised.

5. A process as claimed in claim 4 in which during the cross-linking of the organic material the shape of the composition is maintained.

6. A process as claimed in claim 5 in which after cross-linking of the organic material the particles of magnetic material are remagnetised.

7. A process as claimed in any one of claims 1 to 6 in which in the composition the organic material has a melting point above 25°C.

8. A process as claimed in any one of claims 1 to 7 in which in the composition the organic material comprises a monomeric material.

9. A process as claimed in any one of claims 1 to 7 in which in the composition the organic material comprises an organic polymeric material.

10. A process as claimed in claim 8 in which the monomeric material comprises one or more ethylenically unsaturated groups.

11. A process as claimed in claim 10 in which the monomeric material is an adduct of 4, 4ʹ diphenyl methane diisocyanate and hydroxethyl methacrylate.

12. A process as claimed in claim 9 in which the organic polymeric material compromises an epoxy resin.

13. A process as claimed in claim 12 in which the epoxy resin compromises epoxidised bisphenol A and epoxidised phenol formaldehyde novolak, and phenol formaldehyde novolak as hardener.

14. A process as claimed in any one of claims 1 to 13 in which the composition comprises an additive capable of effecting or assisting cross-linking of the organic material.

15. A process as claimed in claim 14 in which the organic material comprises one or more ethylenically unsaturated groups and in which the additive is a free-radical generator.

16. A process is claimed in any one of claims 1 to 15 in which the magnetic material has a particle size in the range 0.5 to 200 microns.

17. A process as claimed in any one of claims 1 to 16 in which the particulate magnetic material comprises an intermetallic compound of at least one rare earth metal and at least one transition metal.

18. A process as claimed in claim 17 in which the transition metal is or comprises Co.

19. A process as claimed in claim 17 or claim 18 in which the magnetic material has an approximate empirical formula RCo₅ or RCo₁₇ where R is at least one rare earth metal.

20. A process as claimed in any one of claims 17 to 19 in which the rare earth metal is or comprises Sm.

21. A process as claimed in claim 17 in which the particulate magnetic material comprises an intermetallic compound comprising Nd-B-Fe.

22. A process as claimed in any one of claims 1 to 21 in which the composition contains at least 80% by weight of particulate magnetic material.

23. A process as claimed in any one of claims 1 to 22 in which the composition contains at least 5% by weight of organic material.

24. A process as claimed in any one of claims 14 to 23 in which the composition contains from 0.01 to 2% by weight of additive.

25. A process as claimed in any one of claims 1 to 24 in which the composition contains a polymeric material which is soluble in a dispersible in the organic material when the organic material is in a liquid state.

26. A process as claimed in claim 25 in which the composition contains from 0.5 to 5% by weight of polymeric material.

27. A process as claimed in any one of claims 1 to 26 in which the composition is mixed under conditions of high shear.

28. A process as claimed in any one of claims 1 to 26 in which the composition is shaped in an extruder, in an injection mould, or in a compression mould.

29. A process as claimed in any one of claims 3 to 28 in which the organic material is cross-linked in the solid state by irradiating the composition with an electron beam or with γ-rays.

30. A process as claimed in any one of claims 5 to 29 in which the shape of the composition is maintained by initially cross-linking an outer layer of the shaped composition when the organic material is in a solid state.

31. A magnet produced by a process as claimed in any one of claims 1 to 30.

32. A composition, suitable for use in the production of a shaped article having magnetic properties, which composition comprises a mixture of

(1) a solid melt-processable and cross-linkable organic material,

(2) a particulate magnetic material, and

(3) a polymeric material which is soluble in or dispersable in the organic material when the organic material is in a liquid state.

33. A composition as claimed in claim 32 in which the organic material has a melting point above 25°C.

34. A composition as claimed in claim 32 or claim 33 in which the organic material comprises a monomeric material.

35. A composition as claimed in claim 32 or claim 33 in which the organic material comprises an organic polymeric material.

36. A composition as claim 34 in the monomeric material comprises one or more ethylenically unsaturated groups.

37. A composition as claimed in claim 36 in which the monomeric material is an adduct of 4, 4ʹ diphenyl methane diisocyanate and hydroxyethyl methacrylate.

38. A composition as claimed in claim 35 in which the polymeric material comprises an epoxy resin.

39. A composition as claimed in claim 38 in which the epoxy resin comprises epoxidised bisphenol A and epoxidised phenol formaldehyde novolak, and phenol formaldehyde novolak as hardener.

40. A composition as claimed in any one of claims 32 to 39 which comprises an additive capable of effecting or assisting cross-linking of the organic material.

41. A composition as claimed in claim 40 in which the organic material comprises one or more ethylenically unsaturated groups and in which the additive is a free-radical generator.

42. A composition as claimed in any one of claims 32 to 41 in which the magnetic material has a particle size in the range 0.5 to 200 microns.

43. A composition as claimed in any one of claims 32 to 42 in which the particulate magnetic material comprises an intermetallic compound of at least one rare earth metal and at least one transition metal.

44. A process as claimed in claim 43 in which the transition metal is or comprises Co.

45. A composition as claimed in claim 44 in which the magnetic material has an approximate empirical formula RCo₅ or R₂Co₁₇ where R is at least one rare earth metal.

46. A composition as claimed in any one of claims 43 to 45 in which the rare earth metal is or comprises Sm.

47. A composition as claimed in claim 43 in which the particulate magnetic material comprises an intermetallic compound comprising Nd-B-Fe.

48. A composition as claimed in any one of claims 32 to 47 which contains at least 80% by weight of particulate magnetic material.

49. A composition as claimed in any one of claims 32 to 48 in which the composition contains at least 5% by weight of organic material.

50. A composition as claimed in any one of claims 40 to 48 which contains from 0.01 to 2% by weight of additive.

51. A composition as claimed in any one of claims 32 to 50 which contains a polymeric material which is soluble in or dispersible in the organic material when the organic material is in a liquid state.

52. A composition claimed in claim 51 which contains from 0.5 to 5% by weight of polymeric material.