[0001] 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.
[0002] The invention also provides a composition from which a shaped article having magnetic
properties may be produced.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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
(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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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,

[0022] Examples of organic polymeric materials which are solid but which are melt processable
include ethylenically unsaturated polyester resins and epoxy resins.
[0023] Examples of suitable epoxy resins include bisphenol A:-

and epoxidised phenol formaldehyde novolak:-

in which

[0024] The epoxy resin will contain a suitable hardener which comprises a plurality of hydroxyl
groups.
[0025] An example of a suitable hardener is a phenol-formaldehyde novolak:-

[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] The greater is the amount of such additive in the composition the faster will be
the cross-linking of the organic material.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] The organic material in the shaped composition is then cross-linked to produce a
cross-linked resin.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] After cross-linking of the organic material has been effected the particles of magnetic
material which have been demagnetised may be remagnetised.
[0052] The invention is illustrated by the following examples in which all parts are expressed
as parts by weight.
Example 1
[0053] 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.
[0054] 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
[0055] 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.
[0056] 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.
[0057] 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
[0058] 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
[0059] 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
[0060] 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
[0061] 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
[0062] 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.
[0063] 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.
[0064] 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
[0065] 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
[0066] 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
[0067] 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
[0068] 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
[0069] 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.
[0070] 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
[0071] 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.

[0072] 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
[0073] 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.
[0074] The compositions in weight per cent are as shown in Table 4.

[0075] 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.)
[0076] The magnetic performances of the cylindrical magnets are shown in Table 5.

[0077] 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
[0078] 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.
[0079] 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.

[0080] 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
[0081] 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.
[0082] 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.
[0083] The magnetic performance of the resultant cylindrical magnets as shown in Table 7.

[0084] 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
[0085] 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.
[0086] 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.
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
(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.
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.