CA1252624A - Article having magnetic properties and production thereof - Google Patents
Article having magnetic properties and production thereofInfo
- Publication number
- CA1252624A CA1252624A CA000451913A CA451913A CA1252624A CA 1252624 A CA1252624 A CA 1252624A CA 000451913 A CA000451913 A CA 000451913A CA 451913 A CA451913 A CA 451913A CA 1252624 A CA1252624 A CA 1252624A
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- composition
- water
- polymeric material
- organic polymeric
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
- H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
- H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
- H01F1/032—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials
- H01F1/10—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials non-metallic substances, e.g. ferrites, e.g. [(Ba,Sr)O(Fe2O3)6] ferrites with hexagonal structure
- H01F1/11—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials non-metallic substances, e.g. ferrites, e.g. [(Ba,Sr)O(Fe2O3)6] ferrites with hexagonal structure in the form of particles
- H01F1/113—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of hard-magnetic materials non-metallic substances, e.g. ferrites, e.g. [(Ba,Sr)O(Fe2O3)6] ferrites with hexagonal structure in the form of particles in a bonding agent
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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Compositions Of Macromolecular Compounds (AREA)
- Soft Magnetic Materials (AREA)
Abstract
Abstract Article Having Magnetic Properties And Production Thereof A shapeable composition comprising a homogeneous mixture of (a) at least one particulate ferrite material having magnetic properties, (b) at least one water-soluble or water-dispersible organic polymeric material, and (c) water, in composition the components (a), (b) and (c) being present in a proportion by volume of the composition of respectively 40 to 90%, 2 to 25%, and not more than 60%, a product produced therefrom, and a process for producing said product by removing water from the composition. The composition optionally contains an additive capable of insolubilising the organic polymeric material with respect to water. The product, that is the shaped article, produced from the composition possesses magnetic properties and may contain a high volume proportion of particulate ferrite material, and in the product it is not necessary to sinter the particles of ferrite material.
Description
-1- l.ZS~ 4 ARTICLE HAVI~G MAGNETIC PROPERTIES AND PRODUCTIOM
THEREOF
This invention relates to a shaped article having magnetic properties, particularly to a shaped article comprising particulate ferrite material having magnetic properties, to a process for the production of said article, and to a composition for use in such production.
Within the scope of the terms shaped article having magnetic properties, and particulate material having magnetic properties, there are included articles and materials whose properties may be described as ferromagnetic or ferrimagnetic. Both these latter terms include articles and materials which once magnetised remain magnetised when removed from the influence of a magnetic field, and also articles and materials which are capable of being magnetised but which do not remain magnetised when removed from the influence of a magnetic field. Such articles and materials may be considered to be, respectively, permanently and temporarily magnetisable, and are frequently referred to as "hard" and "soft" respectively.
The shaped article of the present invention is made of a particulate ferrite material. The term ferrite is a term well known in the art. Ferrites are magnetic oxides containing iron as a major metallic component and in addition another metal component, e.g.
manganese, zinc, lead, strontium, barium, lithium or nickel. The term can include spinels, perovskites, ~¢
~S;~ 4 magnetoplumbites, and garnets. Ferrite materials which are ferrimagnetic include Ba Fe6 19' which is permanently magnetisable, and (Mn, Zn) Fe204, which is temporarily magnetisable.
S Shaped articles of ferrites may be made, for example, by a powder compaction process in which the powdered ferrite is compacted under pressure in a suitably shaped mould and the powder is subsequently sintered by heating at a high temperature which may be in excess of 1200~C. Controlled heating and cooling rates may be required, as may be heating for some hours at the peak temperature. The presence of air or air enriched with oxygen may also be required.
Shaped articles of ferrites may be made from a composition comprising, ferrite powder and a solution of a polymer binder. The article may be shaped, for example, by extrusion, injection moulding, or compression moulding, pyrolysing the polymer binder and finally sintering the ferrite powder.
Shaped articles of ferrites may also be made by filling of plastics materials, although in this case the article generally contains a relatively low volume proportion of ferrite and thus possesses relatively poor magnetic properties.
Permanently magnetisable ferrite materials are used in a wide variety of applications, for example, in ferrite motors ,and in loud speakers, Temporarily magnetisable ferrite materials are used in transformers, in antennae rod , in recording heads and in memory cores. In addition, certain temporarily magnetisable materials, e.g. soft ferrites, are able to absorb certain frequencies of electro-magnetic radiation.
For example ferrites may be used in conjunction with ~Z~ 4 microwave transmitters in order to assist in defining the size and direction of the micro-wave beams; in microwave heating devices, e.g. domestic ovens, in order to absorb potentially harmful microwave radiation which may otherwise escape from the device;
and in the cladding of structures, e.g buildings, ships and aircra~ft, in order to absorb and prevent reflection of radar waves. The frequency at which the absorption is greatest depends on a number of factors, e.g. on the chemical composition and crystal structure of the ferrite, on the thickness of the absorbing structure, and on the distribution of the ferrite within the absorbing structure. An absorber which is manufactured in the manner described later may be most effective at a frequency which differs from that at which a sintered ferrite material is most effective. Furthermore, certain ferrites possess the unusual feature of being able to damp vibrations. Articles made in the manner described later may have a good combination of high modulus and good vibration damping.
The present invention relates to shaped articles of particulate ferrite materials having magnetic properties, and to the production of such articles by a process which does not involve a lengthy and expensive sintering step, and which contain a high volume proportion of the particulate material.
According to the present invention there is provided a process for the production of a shaped article of a particulate ferrite material having magnetic properties which method comprises shaping a homogeneous mixture of ~a) at least one particulate ferrite material having magnetic properties, (b) at least one water-soluble or water-dispersible organic polymeric material, and (c) water, and removing water fxom the thus formed shaped mixture, in the composition the components (a), (b), and (c) being present in a proportion by volume of the composition of respectively, 40 to 90%, 2 to 25%, and not more than 60%.
In a further embodiment of the present invention thare is provided a shapeable composition comprising a homogeneous mix~ure of (a) at least one particulate ferrits material having magnetic properties~
(b) at least one water-soluble or water-dispersible organic polymeric material, and (c) water in the composition the components (a), (b) and (c) being present in proportions by volume of the composition of respectively 40 to 90%, 2 to 25%, and not more than 60%.
There is also provided a shaped article of a particulate ferrite material having magnetic properties produced by removing water from the composition hereinbefore described.
The particulate ferrite material having magnetic properties will generally be referred to hereinafter as the particulate material.
In producing the shapeable composition the components thereof should be thoroughly mixed ~o as to form a homogeneous mixture.
For example, the components o~ the composition are preferably mixed under conditions of high shear, for example in a bladed high shear mixer. I desired, ~2S~4 and where the composition has a suitable consistency, a composition 60 formed may be further mixed under conditions of high shear by passing the composition repeatedly through the nip between a pair of rollers which may be rotating at the same or at different peripheral speeds.
The mixing may be effected at elevated temperature in order, for example, to reduce the viscosity of the composition and thus aid the mixing.
~- 10 However, the elevated temperature, should not be such as to result in premature drying of the composition by loss of water nor in excessive loss of water from the composition by evaporation.
The homogeneous composition of the invention may be shaped by a variety of techniques, depending on the consistency of the composition. Thus, where the composition comprises a relatively large proportion of water in the range up to 60~ by volume of the composition the composition may be sufficiently fluid as to be capable of being cast in a suitably shaped mould.
The composition of the invention may contain a proportion of water, e.g. up to 30% by volume, such that the composition has a dough-like consistency, and the composition may be shaped by techniques known in the plastics or rubber processing art. For example, where the composition has a dough-like consistency it may be shaped by extrusion, e.g. into a rod or tube shape or by injection moulding into a desired shape, or it may be calendered to produce a ~heet-like form. The composition may also be shaped by compression moulding of the composition in a suitably shaped mould.
.. . . . . . .. . . . .. . . .
-6- i2~
Compositions having a dough-like consistency are preferred as it i5 generally possible to use plastics or rubber processing equipment with such compositions, such compositions generally contain a relatively low proportion of water and thus there is a reduced amount of water to remove from the composition, and the shaped articles produced from such compositions are generally of higher flexural strength.
The temperature at which shaping of the composition may be effected may depend on the nature of the components of the composition and their relative proportions. Where the composition is to be shaped under relatively high pressure the composition may be shaped at or near ambient temperature. However, we have found that, particularly where the composition has the characteristics of a thermoplastic, it may be desirable, or even necessary, in order to readily effect the shaping process, to use an elevated temperature in order effectively to shape the composition. A suitably elevated temperature may be chosen by means of simple experiment.
In the final step of the process for the production of the ~haped article water is removed from the shaped composition, that is it is dried. Drying may be effected merely by allowing the water to evaporate.
However, in order to speed up the drying process it is preferred to dry th~ shaped composition at elevated temperature, for example at a temperature ~reater than 50C. A temperature of 100C or greater may be used.
However, the elevated temperature, and the length of time at the elevated temperature, should not be such as to result in sub6tantial reduction in the strength of the shaped article, which may be brought about, for example, by degradation of the polymeric material at elevated temperature.
The shaped article of the invention may have a high flexural strength, for example, a flexural strength in excess of 40 MPa. The shaped article may have a flexural strength in excess of 100 MPa.
As the shaped article comprises an organic polymeric material which i5 water-soluble or water-dispersible the article will be sensitive to water.
Indeed, it may lose dimensional stability when contacted with water, particularly when soaked in water, and in a preferred embodiment of the invention the composition also comprises at least one additive which is capable of reacting with the polymeric material to insolubilise the material with respect to water. Use of such an additive substantially increases the dimensional stability of the shaped article when the article is contacted with water.
Where the composition contains such an additive the final step of the process for the production of the shaped article comprises drying of the shaped composition to remove the water from the composition and reacting the additive with the organic polymeric material in order to insolubilise the latter material with respect to water. In this case~this final step is referred to a~ setting.
The cond~tion~ under which setting of the shaped composition may be effected will depend on the nature of the components of the composition, and in particular on the nature of the organic polymeric material and on the nature of the additive reactive therewi~h. Suitable conditions for use in effecting the setting reaction will be disclosed hereafter in respect of compositions 8~
containing specific organic polymeric materials and additives reactive therewith. Setting of the shaped composition may be effected at or near ambient temperature or it may be effected at elevated temperature, for example at a temperature in excess of 50C. A temperature of up to 100C or even greater may be used. Elevated temperatures may be desirable in order to initiate reaction of the additive with the organic polymeric material, or at least to increa~e the rate of this reaction. The elevated temperature, and the length of time at such elevated temperature should not be such as to result in substantial reduction in the strength of the product.
Where particularly high flexural strength shaped articles are to be produced it is preferred that the components of the composition of the invention are selected such that a test composition comprising 63~ by volume of particulate material, 7% by volume of water-soluble or water-dispersible organic polymeric material, and 30% by volume of water, when extruded in a capillary rheometer at an extrusion pressure up to a maximum of 500 atmospheres under~oes an increase of at least 25%, and preferably at least 50%, in shear stress when a ten-fold increase in the shear rate of the test composition is effected when the shear rates as ~easured are within the range 0.1 to 5 second~l .
A capillary rheometer in which the test composition is extruded comprises a piston in a cylindrical barrel and a capillary orifice through which the test composition may be extruded.
9~ 4 The shear stress in kN cm~2 is defined by F d 1~L D2 and the shear rate in second~l by 2 v D2 15 d3 where D is the diameter of the barrel of the rheometer in cm, v is the rate of travel of the piston in the barrel of the rheometer in cm min~1, d is the diameter of the capillary of the rheometer in cm, L is the length of the capillary of the rheometer in cm, and F is the force in kN applied to the piston of the rheometer. In general, D will be in the range 1 to 3 cm, d in the range 0.2 to 0.5 cm, and L in the range 5d to 20d.
The particulate material in the test composition should not be of a size so great nor of such a shape that the particulate material itself inhibits pa~sage of the composition through the capillary of the rheometer. For use in the capillary rheometer test particulate material having a size which results in a readily extrudable compo ition will be chosen, and a size in the range up to 100 microns will generally be suitable. It may be necessary to choose a particular size, or a combination of sizes, for the particulate material in this range in order to produce a test composition which satisfies the criteria of the capillary rheom~ter test. The composition, and shaped article, of the invention are not limited to particulate material having a size in this range, A shaped article produced from the composition of the invention will be of higher flexural strength where the particulate material and organic polymeric material together are selected so that the test composition satisfies the aforementioned criteria of -10- l~S;~
the capillary rheometer test than is the case where the particulate material and the organic polymeric material selected are such the the test composition does not satisfy the aformentioned criteria.
For example, where the organic polymeric material and the particulate material are selected so that the test composition satisfies the aforementioned criteria a shaped article produced from a composition containing these materials will have a flexural strength higher than that of (1) a shaped article produced from a composition containing the same organic polymeric material and a different particulate material which in combination do not satisfy the criteria of the capillary rheometer test, and ~2) a shaped article produced from a composition containing the same particulate material and a different organic polymeric material which in combination do not satisfy the criteria of the capillary rheometer test, Suitable combinations of particulate material and organic polymeric material which in the test composition satisfy the aforementioned capillary rheometer test will be disclosed hereinafter.
In general, the greater lS the change in shear stress observed,when the shear rate is increased ten-fold the greater will be the flexural strength of the shaped article produced from the composition of the invention, and for this reason it is preferred that the test composition undergoes an increase of at least 75%
in shear stress when a ten-fold increase in shear rate of the test composition is effected.
The test composition for use in the capillary rheometer test should of course be thoroughly mixed and .
be sufficiently fluid that the composition itself i8 capable of being extruded in the capillary rheometer.
In order that the test composition should have sufficient fluidity that shear rates in the range of 0.1 to 5 second~l are obtained it may be necessary to carry out the test at elevated temperature, for example at a temperature greater than 50~C, e.g. at about 80C.
On the other hand, it may be necessary, particularly where the test composition is of high fluidity, to carry out the capillary rheometer test at a temperature below ambient temperature. In effecting the extrusion the composition should not separate into its component parts, for example, water should not tend to separate from the composition.
In order to produce an extrudable composition it may be necessary to select a suitable molecular weight of organic polymeric material or use in the test composition. The composition of the invention is not of course limited to use of a material of the selected molecular weight. m e molecular weight is merely selected for the purposes of the test.
For particularly high flexural strength shaped articles it is preferred that not more than 2%, and more preferably not more than 0.5~, of the total volume of the article comprises pores having a maximum dimension exceeding 100 microns, preferably S0 microns, and more preferably 15 microns, as measured by the method of quantitative microscopy. ~hese pore size criteria do not include pores which may be present in the particulate material, for example, where the particulate material comprises hollow particles.
The production of such a prefexred shaped article is assisted by application of high shear during mixing of the composition, which may be effected in the -12- 125~4 substantial absence of air, for example, under vacuum and/or by application of at least a moderate pressure, e.g. an applied pressure of l to 5 MPa in the shaping step, particularly with a dough-like composition.
Quantitative microscopy is a technique well known in the art. A surface of a sample of the shaped article is polished to produce a plane surface on the sample, the sample is washed to remove the polishing debris from the surface, and the surface is illuminated to ensure that the holes in the surface are contrasted with the plane parts of t~e surface, and the surface is viewed by means of an optical microscope, typically at a magnification of xlO0, and the holes exceeding lO0 microns, or 50 microns or 15 microns in size, are determined, as described in "Quantitative Microscopy"
by De ~off and Rhines, McGraw Hill 1968. Sufficient area of the surface of the sample should be viewed to reduce the statistical error, and usually, lO00 holes are counted. The sample is then subjected to further polishing in order to expose anot~er surface and the optical examination is repeated. In general ten ~uch surfaces are examin0d.
It is also preferred,~for additional improvements in flexural strength, that the total volume of pores in the shaped article e~pressed as a proportion of the apparent volume~of the article, including the pores, does not exceed 20~. Porosities not exceeding 15%, and even porosities not exceeding 10% are more preferred. The poro ity may even be less than 2%. These porosity criteria sxclude pores which may be present in the particulate material, for example, where the particulate material comprises hollow particles.
-13~
Low porosity is a eature of ~haped articles produced from compositions in which the organic polymeric material and the particulate material are selected so as to satisfy the criteria of the capillary rheometer test.
In the composition of the invention the particulate material is insoluble in water and is substantially unreactive with water, although we do not exclude use of particulate material which may be very slightly reactive with water.
The dimen~sions of the particles of the particulate material may vary over a broad range.
Where the particulate material has a small size, however, undesirably large proportions of water may be required in order to produce a composition which is readily shapeable, and for this reason it is preferred, although not essential, that the median particle size is greater than 0.3 micron, more preferably greater than 3 microns.
The particulate material may comprise a plurality of particle sizes. For example, the particulate material may comprise a first fraction and a second fraction of size less than that of the first fraction.
The use of such a pIurality of particle sizes results in good packing of particles in the product and also may lead to a reduction in the proportion of organic polymeric material which otherwise may be required.
Mixtures of different particulate ferrite materials having magnetic properties may be used.
Shaped articles made of ferrites have a wide variety of applications.
~Z~i~t~
Ferrites are magnetic oxides containing iron as a major metallic component and in addition another metal component. The other metal component may be, for example, manganese, zinc, lead, strontium, barium, lithium or nickel. Examples of ferrites include (Mn~ Zn) Fe24~ BaFel201~, MnFe204 and (Ni, Zn) Fe204.
Many other examples of ferrites are described in the art.
The composition, and the shaped article produced therefrom, may comprise particulate material other than a particulate ferrite material having magnetic properties.
The composition, and the shaped article produced therefrom, may include ~ibrous material. Although the fibrous material may be in the form of random, chopped fibre, difficulty may be e~perienced in incorporating such fibrous material into the composition. For this reason the fibrous material is preferably in the form of a mat, which may be woven or non-woven. The mat may be pressed into the composition of the invention, or it may be formed in situ, e.g. by filament winding.
The particulate ferrite material may be present in the composition of the invention in a proportion of 40 to 90~ by volume. It is preferred to use a relatively high proportion of particulate ferrite material, for example a proportion in the range 60 to 90~ by volume.
Such preferred compositions may contain a relatively low proportion of organic polymeric material, which material will generally be flammable, and it is thus of advantage that the shaped article of the invention contains a relatively low proportion of such material. Also, compositions containing a high proportion o~ particulate material will generally contain a relatively low proportion of water. This is of advantage as there is then a lower proportion of water to remove from the composition during production of the shaped article.
The organic polymeric material in the composition of the invention should be water-soluble or water-dispersible. The function of the organic polymeric material is to aid in the processing of the composition, e.g. to aid in the production of a composition which is readily shaped, e.g. a composition of dough-like consistency, and to provide shape-retaining properties to the shaped article of the invention.
It is preferred that the organic polymeric material is soluble in water, rather than water-dispersible, and that the polymeric material is film-forming and contains groups, for example, hydroxyl or carboxylic acid groups, which have an affinity for the particulate material.
Examples of organic polymeric materials include hydroxy propyl methyl cellulose, polyethylene oxide, polyethylene glycol, polyacrylamide, and polyacrylic acid. A particularly preferred organic polymeric material which, with a number of different particulate materials ha~ing magnetic propertie~ in the form of a test composition satisfies the criteria of the afore-mentioned capillary rheometer test, is a hydrolysed polymer or copolymer of a vinyl ester, e.g. a hydrolysed vinyl acetate polymer or copolymer. The polymer may be a copolymer of vinyl acetate and a monomer copolymerisable therewith, but it is preferably a hydrolysed poly(vinyl acetate).
The degree of hydrolysis of the vinyl acetate (co)polymer has a bearing on whether or not the -16- 12~2~
(co)polymer in combination with a particulate material in the test composition satisfies the aforementioned criteria of the capillary rheometer test. In order that in the capillary rheometer test an increase of at least 25~ in shear stress should be produced by the ten-fold increase in shear rate, it is preferred that the degree of hydrolysis of the vinyl acetate (co)polymer be at least 50~ but not more than 97%, and more preferably in the range 70 to 90%, that is, it is preferred that at least 50% but not more than 97~, and more preferably 70% to 90% of t~e vinyl acetate units in he polymer or copolymer, are hydrolysed to the alcohol form.
For a given proportion of hydrolysed vinyl acetate ~co)polymer in the composition of the invention the properties of the shaped article produced therefrom are relatively insensitive to variakions in the molecular weight of the hydrolysed vinyl acetate (co)polymer.
In general, h~wever, the molecular weight of the hydrolysed vinyl acetate ~co)polymer will be at least 3000, e.g. in the range 5000 to 125,000. Such (co)polymers are readily available. The (co)polymer may have a higher molecular weight.
In the compo ition of the invention there is present 2 to 25% of organic polymeric material by volume of the composition. The ea~e of shaping of the composition generally improves with increase in the proportion of polymeric material in the composition, and a proportion of at least 7% by volume is preferred.
On the other hana as the polymeric material is generally capable of burning a proportion of not more than 20~ by volume of polymeric material is preferred.
The proportion of water in the composition has an effect on the properties of the shaped article produced from the composition. In order to produce an -17- l~S~6~
article of particularly high flexural strength the composition shou]d contain no more than 30~ by volume of water. It is preferred to use as low a proportion of water as possible consistent with producing a composition which is shapeable. We prefer to use less than 20~ by volume of water. In general it will be found necessary to use at least 5% by volume of water.
However, a proportion of water may be used in the composition which is greater than that which would result in production of a very high strength article and ome strength may be sacrificed in order to produce a composition which is more readily shaped.
Where the high green strength is desired in the moulded composition of the invention, that is before setting of the composition, the composition may suitably comprise a gelling agent for the organic polymeric material, that is a compound which forms labile bonds ~ith the organic polymeric material.
An alternative way of achieving high green strength in the compo ition is to include in the composition a proportion of an organic polymeric material which is ~oluble in the water of the composition at elevated temperature but which forms a gel at low temperature, e.g. at or near ambient temperature. For example the composition may also comprise a proportion of a substantially fully hydrolysed poly(vinyl acetate) which is soluble in the wat0r of the composition at elevated temperature but which forms a gel at ambient temperature.
It is a preferred feature of the invention that the composition comprises an additive capable of reacting with the organic polymeric material to insolubilise the material with respect to water.
-18- ~25~Z4 The nature of this additive will depend on the particular organic polymeric material in the composition.
Where the organic polymeric material comprises a plurality of reactive functional groups the additive may be a material reactive with the functional groups under the conditions used in forming the shaped article of the invention from the composition. In this case the insolubilisation of the organic polymeric material with respect to water~ may be achieved by cross-linking of the material. For example, where the polymeric material comprises a plurality of hydroxyl groups, e.g. as in a hydrolysed vinyl ester polymer or copolymer such as hydrolysed poly(vinyl acetate), the additive may be a compound of a polyvalent metal capable of reacting with the hydroxyl groups. Particular examples of suitable compounds of a polyvalent metal include compounds of aluminium, A12(0H)5~03,~and A12(0H)5 halide, for example, A12~OH)5Cl. Other examples of compounds of a polyvalent metal include Zr (OH)2 Cl~, (NH4)2Cr207 and ~r(OH)108(~3)1.2-Selection of suitable combinations of water-soluble or water-dispersible organic polymeric materials and insolubilising additives may be made by reacting mixtures of such materials and additives and testing the product of reaction for water insolubility.
In effecting setting of the composition comprising such an addi~ive the additive in~the composition is reacted with the polymeric material to insolubilise the material and water is removed from the composition. Where the additive is a polyvalent metal compound reaction is suitably effected at elevated temperature. For e~ample, -19 ~2S~ 4 the temperature may be greater than 100C, which temperature serves to remove the water in the composition. A temperature of, for example, up to 250C
may be used.
Where the polymeric material comprises a plurality of hydroxyl groups the additive capable of reacting with the polymeric material to insolubilise the material with respect to water may itself be an organic compound reactive with the hydroxyl groups, for example, a dialdehyde, e~g. glyoxal.
In this case a suitable reaction temperature is ambient temperature. However, eleva*ed temperatures are suitably used, e.g. up to about 100C, in order to remove the water from the composition and to accelerate the reaction.
In the composition of the invention the proportion of additive capable of reacting with the polymeric material will depend on the particular organic polymeric material and the particular addi~ive in the composition.
In general the composition will contain a proportion of additive in the range 5 to 100% by volume of the organic polymeric material in the composition e.g. 10 to 50% by volume.
It is preferred to 6elect a proportion of additive which is sufficient not merely to insolublise the organic po~ymeric material with respect to water but which reacts with the polymeric material to produce - a polymeric product which swells at most only to a limited extPnt in water, for example, which takes up not more than 50% by weight of water when the product of reaction of the organic polymeric material and the in olubilising additive is soaked in water. Suitable ;2~
proportions may be selected by test on mixtures of organic polymeric material and insolubilising additive.
In a particularly preferred embodiment of the invention the composition of the invention also comprises an additive capable of effecting coupling between the polymeric material and the surface of the particulate material having magnetic properties in the composition.
Although shaped articles having high flexural strength may be produced from compositions which do not contain such an additive capable of effecting coupling it has been found that such articles may suffer a substantial loss in flexural modulus when contacted with water. Where the composition from which the shaped article is produced contains such an additive capable of effecting coupling the 105s of flexural modulus of the article when the article is contacted with water, if any, is very much reduced.
The coupling additive which may suitably be used in a composition will depend on the nature of the particulate material and the organic polymeric material in the composition.
It is preferred that the additive capable of insolubilising the organic polymeric material be the same as the additive capable of effecting coupling between the polymeric material and the particulate material.
For example, where the additive capable of reacting with the organic polymeric material to insolubilise the latter with respect to water is a polyvalent metal compound certain of the latter compounds are also capable of effecting coupling -21- ~z5~Z~
betwee4n ferrite particulate materials and the organic polymeric material. Suitable additive to fulfil both these functions include A12(OH)5Cl, (NH4)2 Cr2O7, Cr (oH)l~8(~o3)l~2 and A12 (OH)s NO3.
In general the additive capable of eff~cting coupling, when different from the additive capable of reacting with the organic polymeric material to insolublise the latter material with respect to water will be present in the composition in a relatively low proportion, although the proportion required may depend on the particle size of the particulate material. For example, the additive may be present in a proportion of 0.01 to 3% by volume of the particulate material in the composition.
The invention is illustrated by the following examples in which all parts are parts by volume, unless otherwise stated.
Example 1 128 parts of a particulate ferrite, BaFel2Olg, having a particle size of 10 microns, and 22.8 parts of hydrolysed poly(vinyl acetate) (Gohsenol GH175 Nippon Gohsei, degree of hydrolysi6 88% degree of polymerisation 2000) were thoroughly mixed in a bladed mixer. 4 parts of resorcinol in 15 parts of water were mixed with 40 parts of an aqueous solution containing 30 parts of water and 10 parts of aluminium hydroxy chloride the so~ution containing 12.1% w/w Al, 8.75 %
w/w Cl, the latter solution having a viscosity of 18 cps, and the resultant solution added to the mixed solids in the bladed mixer to form a crumble.
The crumble was then charged to a twin-roll mill the rollers of which were heated to a temperature of 70C and the crumble was formed into sheet on the mill, -22- ~2S~Z4 the sheet being passed repeatedly through the nip between the rolls. The milling was contlnued for 5 minutes during which time some of the water evaporated, and the resultant sheet was removed from the mill.
The sheet contained 128 parts of particulate ferrite, 22.8 parts of hydrolysed poly(vinyl acetate), 10 parts of aluminium hydroxy chloride, 4 parts of resorcinol, and 25 parts of water.
The sheet was then placed between two sheets of polyethylene terephthalate the faces of which were coated with mould release agent and the sheet was pressed in a hydraulic press at a temperature of 80C
and a pressure of 10 MPa for 10 minutes.
The platens of the press were then cooled by flowing cold water through the platens, the sheet was removed from the press, and the sheets of polyethylene terephthalate were removed from the sheet.
Setting of the sheet was completed by placing the sheet between two flat pieces of wood, the æheet was allowed to stand for 1 day at 20C, it was then heated at 80C for 1 day, and finally it was heated at 180C for 1 hour.
The sheet had a flexural ~trength of 112.6 MPa and a flexural modulus of 48.3 GPa, and contained 78%
by volume of ferrite and 22~ by volume of cross-linked polymer.
The sheet had the following magnetic properties.
Remenance (Br) 1430 Gauss Coercivity (Hc) 750 Oersteds BHmaX product 0.30 x 106 gauss Oersteds saturation magnetisation 2720 gauss.
~L~2S~i24 Exampl _ The mixing, shaping, and setting procedure of Example 1 was repeated on a composition comprising (Mn, Zn) ferrite 150 micron mean size 669.6 parts (Mn, Zn) ferrite 1 micron mean size 224.1 parts Hydrolysed poly(vinyl acetate) Gohsenol GH 17S 115.8 parts Polyviol V03-140 (WacXer-Chemie) 21.0 parts degree of hydrolysis 86-89%, degree of polymerisation 300 Aluminium hydroxy chloride solution ~03.3 parts (as used in Example 1) Water 140 parts The sheet, which contained 83~ by volume of ferrite and 17~ by volume of cross-linked polymer, had a flexural strength of 106 MPa and a flexural modulus of 44.7 GPa. After soaXing in water for 1 day the flexural strength of the sheet was 56 MPa and the flexural modulus was 20 GPa.
The sheet had the following magnetic properties Remenance 355 gauss Coercivity (Hc) 9.85 Oersteds Initial permeability 19.8 Maximum permeability 26.0 Saturation magnetisation 4480 gauss A ring was cut from the sheet and the low field permeability of the ring was measured by comparing the inductance of a coil wound on the ring with the theoretical inductance for an air-cored coil. The low field permea~ility was 19.1.
Example_3 The mixing, shaping, and setting procedure of Example 1 was repeated on a composition comprising (Mn, Zn) ferrite 150 micron mean size 54.2 parts (Mn, Zn) ferrite 1 micron mean si~e 18.1 parts Hydrolysed poly(vinyl acetate) Gohsenol GH 17S 29.3 parts Polyviol V03-140 5.4 parts Aluminium hydroxy chloride solution (as used in Example 1) 51.4 parts Water 14.3 parts The sheet which was produced contained 61~ by volume of ferrite and 39% by volume of cross-linked polymer, and had a low field permeability, measured as described in Example 2, o 7.2.
Example ~
-The mixing, shaping and setting procedure of Example 1 was repeated on a composition comprising (Mn, Zn) Ferrite 150 micron mean size 60.8 parts (Mn, Zn) Ferrit0 1 micron mean size 20.2 parts Hydrolysed PVA
GH17S 21.1 parts V03/140 3.8 parts A12(0H)sCl solution36.8 parts Water 25 parts The sheet which was produced contained ~0% by volume of ferrite and 30% by volume of cross-linked polymer and had a low field permeability measured as described in Example 2, of 13.6.
THEREOF
This invention relates to a shaped article having magnetic properties, particularly to a shaped article comprising particulate ferrite material having magnetic properties, to a process for the production of said article, and to a composition for use in such production.
Within the scope of the terms shaped article having magnetic properties, and particulate material having magnetic properties, there are included articles and materials whose properties may be described as ferromagnetic or ferrimagnetic. Both these latter terms include articles and materials which once magnetised remain magnetised when removed from the influence of a magnetic field, and also articles and materials which are capable of being magnetised but which do not remain magnetised when removed from the influence of a magnetic field. Such articles and materials may be considered to be, respectively, permanently and temporarily magnetisable, and are frequently referred to as "hard" and "soft" respectively.
The shaped article of the present invention is made of a particulate ferrite material. The term ferrite is a term well known in the art. Ferrites are magnetic oxides containing iron as a major metallic component and in addition another metal component, e.g.
manganese, zinc, lead, strontium, barium, lithium or nickel. The term can include spinels, perovskites, ~¢
~S;~ 4 magnetoplumbites, and garnets. Ferrite materials which are ferrimagnetic include Ba Fe6 19' which is permanently magnetisable, and (Mn, Zn) Fe204, which is temporarily magnetisable.
S Shaped articles of ferrites may be made, for example, by a powder compaction process in which the powdered ferrite is compacted under pressure in a suitably shaped mould and the powder is subsequently sintered by heating at a high temperature which may be in excess of 1200~C. Controlled heating and cooling rates may be required, as may be heating for some hours at the peak temperature. The presence of air or air enriched with oxygen may also be required.
Shaped articles of ferrites may be made from a composition comprising, ferrite powder and a solution of a polymer binder. The article may be shaped, for example, by extrusion, injection moulding, or compression moulding, pyrolysing the polymer binder and finally sintering the ferrite powder.
Shaped articles of ferrites may also be made by filling of plastics materials, although in this case the article generally contains a relatively low volume proportion of ferrite and thus possesses relatively poor magnetic properties.
Permanently magnetisable ferrite materials are used in a wide variety of applications, for example, in ferrite motors ,and in loud speakers, Temporarily magnetisable ferrite materials are used in transformers, in antennae rod , in recording heads and in memory cores. In addition, certain temporarily magnetisable materials, e.g. soft ferrites, are able to absorb certain frequencies of electro-magnetic radiation.
For example ferrites may be used in conjunction with ~Z~ 4 microwave transmitters in order to assist in defining the size and direction of the micro-wave beams; in microwave heating devices, e.g. domestic ovens, in order to absorb potentially harmful microwave radiation which may otherwise escape from the device;
and in the cladding of structures, e.g buildings, ships and aircra~ft, in order to absorb and prevent reflection of radar waves. The frequency at which the absorption is greatest depends on a number of factors, e.g. on the chemical composition and crystal structure of the ferrite, on the thickness of the absorbing structure, and on the distribution of the ferrite within the absorbing structure. An absorber which is manufactured in the manner described later may be most effective at a frequency which differs from that at which a sintered ferrite material is most effective. Furthermore, certain ferrites possess the unusual feature of being able to damp vibrations. Articles made in the manner described later may have a good combination of high modulus and good vibration damping.
The present invention relates to shaped articles of particulate ferrite materials having magnetic properties, and to the production of such articles by a process which does not involve a lengthy and expensive sintering step, and which contain a high volume proportion of the particulate material.
According to the present invention there is provided a process for the production of a shaped article of a particulate ferrite material having magnetic properties which method comprises shaping a homogeneous mixture of ~a) at least one particulate ferrite material having magnetic properties, (b) at least one water-soluble or water-dispersible organic polymeric material, and (c) water, and removing water fxom the thus formed shaped mixture, in the composition the components (a), (b), and (c) being present in a proportion by volume of the composition of respectively, 40 to 90%, 2 to 25%, and not more than 60%.
In a further embodiment of the present invention thare is provided a shapeable composition comprising a homogeneous mix~ure of (a) at least one particulate ferrits material having magnetic properties~
(b) at least one water-soluble or water-dispersible organic polymeric material, and (c) water in the composition the components (a), (b) and (c) being present in proportions by volume of the composition of respectively 40 to 90%, 2 to 25%, and not more than 60%.
There is also provided a shaped article of a particulate ferrite material having magnetic properties produced by removing water from the composition hereinbefore described.
The particulate ferrite material having magnetic properties will generally be referred to hereinafter as the particulate material.
In producing the shapeable composition the components thereof should be thoroughly mixed ~o as to form a homogeneous mixture.
For example, the components o~ the composition are preferably mixed under conditions of high shear, for example in a bladed high shear mixer. I desired, ~2S~4 and where the composition has a suitable consistency, a composition 60 formed may be further mixed under conditions of high shear by passing the composition repeatedly through the nip between a pair of rollers which may be rotating at the same or at different peripheral speeds.
The mixing may be effected at elevated temperature in order, for example, to reduce the viscosity of the composition and thus aid the mixing.
~- 10 However, the elevated temperature, should not be such as to result in premature drying of the composition by loss of water nor in excessive loss of water from the composition by evaporation.
The homogeneous composition of the invention may be shaped by a variety of techniques, depending on the consistency of the composition. Thus, where the composition comprises a relatively large proportion of water in the range up to 60~ by volume of the composition the composition may be sufficiently fluid as to be capable of being cast in a suitably shaped mould.
The composition of the invention may contain a proportion of water, e.g. up to 30% by volume, such that the composition has a dough-like consistency, and the composition may be shaped by techniques known in the plastics or rubber processing art. For example, where the composition has a dough-like consistency it may be shaped by extrusion, e.g. into a rod or tube shape or by injection moulding into a desired shape, or it may be calendered to produce a ~heet-like form. The composition may also be shaped by compression moulding of the composition in a suitably shaped mould.
.. . . . . . .. . . . .. . . .
-6- i2~
Compositions having a dough-like consistency are preferred as it i5 generally possible to use plastics or rubber processing equipment with such compositions, such compositions generally contain a relatively low proportion of water and thus there is a reduced amount of water to remove from the composition, and the shaped articles produced from such compositions are generally of higher flexural strength.
The temperature at which shaping of the composition may be effected may depend on the nature of the components of the composition and their relative proportions. Where the composition is to be shaped under relatively high pressure the composition may be shaped at or near ambient temperature. However, we have found that, particularly where the composition has the characteristics of a thermoplastic, it may be desirable, or even necessary, in order to readily effect the shaping process, to use an elevated temperature in order effectively to shape the composition. A suitably elevated temperature may be chosen by means of simple experiment.
In the final step of the process for the production of the ~haped article water is removed from the shaped composition, that is it is dried. Drying may be effected merely by allowing the water to evaporate.
However, in order to speed up the drying process it is preferred to dry th~ shaped composition at elevated temperature, for example at a temperature ~reater than 50C. A temperature of 100C or greater may be used.
However, the elevated temperature, and the length of time at the elevated temperature, should not be such as to result in sub6tantial reduction in the strength of the shaped article, which may be brought about, for example, by degradation of the polymeric material at elevated temperature.
The shaped article of the invention may have a high flexural strength, for example, a flexural strength in excess of 40 MPa. The shaped article may have a flexural strength in excess of 100 MPa.
As the shaped article comprises an organic polymeric material which i5 water-soluble or water-dispersible the article will be sensitive to water.
Indeed, it may lose dimensional stability when contacted with water, particularly when soaked in water, and in a preferred embodiment of the invention the composition also comprises at least one additive which is capable of reacting with the polymeric material to insolubilise the material with respect to water. Use of such an additive substantially increases the dimensional stability of the shaped article when the article is contacted with water.
Where the composition contains such an additive the final step of the process for the production of the shaped article comprises drying of the shaped composition to remove the water from the composition and reacting the additive with the organic polymeric material in order to insolubilise the latter material with respect to water. In this case~this final step is referred to a~ setting.
The cond~tion~ under which setting of the shaped composition may be effected will depend on the nature of the components of the composition, and in particular on the nature of the organic polymeric material and on the nature of the additive reactive therewi~h. Suitable conditions for use in effecting the setting reaction will be disclosed hereafter in respect of compositions 8~
containing specific organic polymeric materials and additives reactive therewith. Setting of the shaped composition may be effected at or near ambient temperature or it may be effected at elevated temperature, for example at a temperature in excess of 50C. A temperature of up to 100C or even greater may be used. Elevated temperatures may be desirable in order to initiate reaction of the additive with the organic polymeric material, or at least to increa~e the rate of this reaction. The elevated temperature, and the length of time at such elevated temperature should not be such as to result in substantial reduction in the strength of the product.
Where particularly high flexural strength shaped articles are to be produced it is preferred that the components of the composition of the invention are selected such that a test composition comprising 63~ by volume of particulate material, 7% by volume of water-soluble or water-dispersible organic polymeric material, and 30% by volume of water, when extruded in a capillary rheometer at an extrusion pressure up to a maximum of 500 atmospheres under~oes an increase of at least 25%, and preferably at least 50%, in shear stress when a ten-fold increase in the shear rate of the test composition is effected when the shear rates as ~easured are within the range 0.1 to 5 second~l .
A capillary rheometer in which the test composition is extruded comprises a piston in a cylindrical barrel and a capillary orifice through which the test composition may be extruded.
9~ 4 The shear stress in kN cm~2 is defined by F d 1~L D2 and the shear rate in second~l by 2 v D2 15 d3 where D is the diameter of the barrel of the rheometer in cm, v is the rate of travel of the piston in the barrel of the rheometer in cm min~1, d is the diameter of the capillary of the rheometer in cm, L is the length of the capillary of the rheometer in cm, and F is the force in kN applied to the piston of the rheometer. In general, D will be in the range 1 to 3 cm, d in the range 0.2 to 0.5 cm, and L in the range 5d to 20d.
The particulate material in the test composition should not be of a size so great nor of such a shape that the particulate material itself inhibits pa~sage of the composition through the capillary of the rheometer. For use in the capillary rheometer test particulate material having a size which results in a readily extrudable compo ition will be chosen, and a size in the range up to 100 microns will generally be suitable. It may be necessary to choose a particular size, or a combination of sizes, for the particulate material in this range in order to produce a test composition which satisfies the criteria of the capillary rheom~ter test. The composition, and shaped article, of the invention are not limited to particulate material having a size in this range, A shaped article produced from the composition of the invention will be of higher flexural strength where the particulate material and organic polymeric material together are selected so that the test composition satisfies the aforementioned criteria of -10- l~S;~
the capillary rheometer test than is the case where the particulate material and the organic polymeric material selected are such the the test composition does not satisfy the aformentioned criteria.
For example, where the organic polymeric material and the particulate material are selected so that the test composition satisfies the aforementioned criteria a shaped article produced from a composition containing these materials will have a flexural strength higher than that of (1) a shaped article produced from a composition containing the same organic polymeric material and a different particulate material which in combination do not satisfy the criteria of the capillary rheometer test, and ~2) a shaped article produced from a composition containing the same particulate material and a different organic polymeric material which in combination do not satisfy the criteria of the capillary rheometer test, Suitable combinations of particulate material and organic polymeric material which in the test composition satisfy the aforementioned capillary rheometer test will be disclosed hereinafter.
In general, the greater lS the change in shear stress observed,when the shear rate is increased ten-fold the greater will be the flexural strength of the shaped article produced from the composition of the invention, and for this reason it is preferred that the test composition undergoes an increase of at least 75%
in shear stress when a ten-fold increase in shear rate of the test composition is effected.
The test composition for use in the capillary rheometer test should of course be thoroughly mixed and .
be sufficiently fluid that the composition itself i8 capable of being extruded in the capillary rheometer.
In order that the test composition should have sufficient fluidity that shear rates in the range of 0.1 to 5 second~l are obtained it may be necessary to carry out the test at elevated temperature, for example at a temperature greater than 50~C, e.g. at about 80C.
On the other hand, it may be necessary, particularly where the test composition is of high fluidity, to carry out the capillary rheometer test at a temperature below ambient temperature. In effecting the extrusion the composition should not separate into its component parts, for example, water should not tend to separate from the composition.
In order to produce an extrudable composition it may be necessary to select a suitable molecular weight of organic polymeric material or use in the test composition. The composition of the invention is not of course limited to use of a material of the selected molecular weight. m e molecular weight is merely selected for the purposes of the test.
For particularly high flexural strength shaped articles it is preferred that not more than 2%, and more preferably not more than 0.5~, of the total volume of the article comprises pores having a maximum dimension exceeding 100 microns, preferably S0 microns, and more preferably 15 microns, as measured by the method of quantitative microscopy. ~hese pore size criteria do not include pores which may be present in the particulate material, for example, where the particulate material comprises hollow particles.
The production of such a prefexred shaped article is assisted by application of high shear during mixing of the composition, which may be effected in the -12- 125~4 substantial absence of air, for example, under vacuum and/or by application of at least a moderate pressure, e.g. an applied pressure of l to 5 MPa in the shaping step, particularly with a dough-like composition.
Quantitative microscopy is a technique well known in the art. A surface of a sample of the shaped article is polished to produce a plane surface on the sample, the sample is washed to remove the polishing debris from the surface, and the surface is illuminated to ensure that the holes in the surface are contrasted with the plane parts of t~e surface, and the surface is viewed by means of an optical microscope, typically at a magnification of xlO0, and the holes exceeding lO0 microns, or 50 microns or 15 microns in size, are determined, as described in "Quantitative Microscopy"
by De ~off and Rhines, McGraw Hill 1968. Sufficient area of the surface of the sample should be viewed to reduce the statistical error, and usually, lO00 holes are counted. The sample is then subjected to further polishing in order to expose anot~er surface and the optical examination is repeated. In general ten ~uch surfaces are examin0d.
It is also preferred,~for additional improvements in flexural strength, that the total volume of pores in the shaped article e~pressed as a proportion of the apparent volume~of the article, including the pores, does not exceed 20~. Porosities not exceeding 15%, and even porosities not exceeding 10% are more preferred. The poro ity may even be less than 2%. These porosity criteria sxclude pores which may be present in the particulate material, for example, where the particulate material comprises hollow particles.
-13~
Low porosity is a eature of ~haped articles produced from compositions in which the organic polymeric material and the particulate material are selected so as to satisfy the criteria of the capillary rheometer test.
In the composition of the invention the particulate material is insoluble in water and is substantially unreactive with water, although we do not exclude use of particulate material which may be very slightly reactive with water.
The dimen~sions of the particles of the particulate material may vary over a broad range.
Where the particulate material has a small size, however, undesirably large proportions of water may be required in order to produce a composition which is readily shapeable, and for this reason it is preferred, although not essential, that the median particle size is greater than 0.3 micron, more preferably greater than 3 microns.
The particulate material may comprise a plurality of particle sizes. For example, the particulate material may comprise a first fraction and a second fraction of size less than that of the first fraction.
The use of such a pIurality of particle sizes results in good packing of particles in the product and also may lead to a reduction in the proportion of organic polymeric material which otherwise may be required.
Mixtures of different particulate ferrite materials having magnetic properties may be used.
Shaped articles made of ferrites have a wide variety of applications.
~Z~i~t~
Ferrites are magnetic oxides containing iron as a major metallic component and in addition another metal component. The other metal component may be, for example, manganese, zinc, lead, strontium, barium, lithium or nickel. Examples of ferrites include (Mn~ Zn) Fe24~ BaFel201~, MnFe204 and (Ni, Zn) Fe204.
Many other examples of ferrites are described in the art.
The composition, and the shaped article produced therefrom, may comprise particulate material other than a particulate ferrite material having magnetic properties.
The composition, and the shaped article produced therefrom, may include ~ibrous material. Although the fibrous material may be in the form of random, chopped fibre, difficulty may be e~perienced in incorporating such fibrous material into the composition. For this reason the fibrous material is preferably in the form of a mat, which may be woven or non-woven. The mat may be pressed into the composition of the invention, or it may be formed in situ, e.g. by filament winding.
The particulate ferrite material may be present in the composition of the invention in a proportion of 40 to 90~ by volume. It is preferred to use a relatively high proportion of particulate ferrite material, for example a proportion in the range 60 to 90~ by volume.
Such preferred compositions may contain a relatively low proportion of organic polymeric material, which material will generally be flammable, and it is thus of advantage that the shaped article of the invention contains a relatively low proportion of such material. Also, compositions containing a high proportion o~ particulate material will generally contain a relatively low proportion of water. This is of advantage as there is then a lower proportion of water to remove from the composition during production of the shaped article.
The organic polymeric material in the composition of the invention should be water-soluble or water-dispersible. The function of the organic polymeric material is to aid in the processing of the composition, e.g. to aid in the production of a composition which is readily shaped, e.g. a composition of dough-like consistency, and to provide shape-retaining properties to the shaped article of the invention.
It is preferred that the organic polymeric material is soluble in water, rather than water-dispersible, and that the polymeric material is film-forming and contains groups, for example, hydroxyl or carboxylic acid groups, which have an affinity for the particulate material.
Examples of organic polymeric materials include hydroxy propyl methyl cellulose, polyethylene oxide, polyethylene glycol, polyacrylamide, and polyacrylic acid. A particularly preferred organic polymeric material which, with a number of different particulate materials ha~ing magnetic propertie~ in the form of a test composition satisfies the criteria of the afore-mentioned capillary rheometer test, is a hydrolysed polymer or copolymer of a vinyl ester, e.g. a hydrolysed vinyl acetate polymer or copolymer. The polymer may be a copolymer of vinyl acetate and a monomer copolymerisable therewith, but it is preferably a hydrolysed poly(vinyl acetate).
The degree of hydrolysis of the vinyl acetate (co)polymer has a bearing on whether or not the -16- 12~2~
(co)polymer in combination with a particulate material in the test composition satisfies the aforementioned criteria of the capillary rheometer test. In order that in the capillary rheometer test an increase of at least 25~ in shear stress should be produced by the ten-fold increase in shear rate, it is preferred that the degree of hydrolysis of the vinyl acetate (co)polymer be at least 50~ but not more than 97%, and more preferably in the range 70 to 90%, that is, it is preferred that at least 50% but not more than 97~, and more preferably 70% to 90% of t~e vinyl acetate units in he polymer or copolymer, are hydrolysed to the alcohol form.
For a given proportion of hydrolysed vinyl acetate ~co)polymer in the composition of the invention the properties of the shaped article produced therefrom are relatively insensitive to variakions in the molecular weight of the hydrolysed vinyl acetate (co)polymer.
In general, h~wever, the molecular weight of the hydrolysed vinyl acetate ~co)polymer will be at least 3000, e.g. in the range 5000 to 125,000. Such (co)polymers are readily available. The (co)polymer may have a higher molecular weight.
In the compo ition of the invention there is present 2 to 25% of organic polymeric material by volume of the composition. The ea~e of shaping of the composition generally improves with increase in the proportion of polymeric material in the composition, and a proportion of at least 7% by volume is preferred.
On the other hana as the polymeric material is generally capable of burning a proportion of not more than 20~ by volume of polymeric material is preferred.
The proportion of water in the composition has an effect on the properties of the shaped article produced from the composition. In order to produce an -17- l~S~6~
article of particularly high flexural strength the composition shou]d contain no more than 30~ by volume of water. It is preferred to use as low a proportion of water as possible consistent with producing a composition which is shapeable. We prefer to use less than 20~ by volume of water. In general it will be found necessary to use at least 5% by volume of water.
However, a proportion of water may be used in the composition which is greater than that which would result in production of a very high strength article and ome strength may be sacrificed in order to produce a composition which is more readily shaped.
Where the high green strength is desired in the moulded composition of the invention, that is before setting of the composition, the composition may suitably comprise a gelling agent for the organic polymeric material, that is a compound which forms labile bonds ~ith the organic polymeric material.
An alternative way of achieving high green strength in the compo ition is to include in the composition a proportion of an organic polymeric material which is ~oluble in the water of the composition at elevated temperature but which forms a gel at low temperature, e.g. at or near ambient temperature. For example the composition may also comprise a proportion of a substantially fully hydrolysed poly(vinyl acetate) which is soluble in the wat0r of the composition at elevated temperature but which forms a gel at ambient temperature.
It is a preferred feature of the invention that the composition comprises an additive capable of reacting with the organic polymeric material to insolubilise the material with respect to water.
-18- ~25~Z4 The nature of this additive will depend on the particular organic polymeric material in the composition.
Where the organic polymeric material comprises a plurality of reactive functional groups the additive may be a material reactive with the functional groups under the conditions used in forming the shaped article of the invention from the composition. In this case the insolubilisation of the organic polymeric material with respect to water~ may be achieved by cross-linking of the material. For example, where the polymeric material comprises a plurality of hydroxyl groups, e.g. as in a hydrolysed vinyl ester polymer or copolymer such as hydrolysed poly(vinyl acetate), the additive may be a compound of a polyvalent metal capable of reacting with the hydroxyl groups. Particular examples of suitable compounds of a polyvalent metal include compounds of aluminium, A12(0H)5~03,~and A12(0H)5 halide, for example, A12~OH)5Cl. Other examples of compounds of a polyvalent metal include Zr (OH)2 Cl~, (NH4)2Cr207 and ~r(OH)108(~3)1.2-Selection of suitable combinations of water-soluble or water-dispersible organic polymeric materials and insolubilising additives may be made by reacting mixtures of such materials and additives and testing the product of reaction for water insolubility.
In effecting setting of the composition comprising such an addi~ive the additive in~the composition is reacted with the polymeric material to insolubilise the material and water is removed from the composition. Where the additive is a polyvalent metal compound reaction is suitably effected at elevated temperature. For e~ample, -19 ~2S~ 4 the temperature may be greater than 100C, which temperature serves to remove the water in the composition. A temperature of, for example, up to 250C
may be used.
Where the polymeric material comprises a plurality of hydroxyl groups the additive capable of reacting with the polymeric material to insolubilise the material with respect to water may itself be an organic compound reactive with the hydroxyl groups, for example, a dialdehyde, e~g. glyoxal.
In this case a suitable reaction temperature is ambient temperature. However, eleva*ed temperatures are suitably used, e.g. up to about 100C, in order to remove the water from the composition and to accelerate the reaction.
In the composition of the invention the proportion of additive capable of reacting with the polymeric material will depend on the particular organic polymeric material and the particular addi~ive in the composition.
In general the composition will contain a proportion of additive in the range 5 to 100% by volume of the organic polymeric material in the composition e.g. 10 to 50% by volume.
It is preferred to 6elect a proportion of additive which is sufficient not merely to insolublise the organic po~ymeric material with respect to water but which reacts with the polymeric material to produce - a polymeric product which swells at most only to a limited extPnt in water, for example, which takes up not more than 50% by weight of water when the product of reaction of the organic polymeric material and the in olubilising additive is soaked in water. Suitable ;2~
proportions may be selected by test on mixtures of organic polymeric material and insolubilising additive.
In a particularly preferred embodiment of the invention the composition of the invention also comprises an additive capable of effecting coupling between the polymeric material and the surface of the particulate material having magnetic properties in the composition.
Although shaped articles having high flexural strength may be produced from compositions which do not contain such an additive capable of effecting coupling it has been found that such articles may suffer a substantial loss in flexural modulus when contacted with water. Where the composition from which the shaped article is produced contains such an additive capable of effecting coupling the 105s of flexural modulus of the article when the article is contacted with water, if any, is very much reduced.
The coupling additive which may suitably be used in a composition will depend on the nature of the particulate material and the organic polymeric material in the composition.
It is preferred that the additive capable of insolubilising the organic polymeric material be the same as the additive capable of effecting coupling between the polymeric material and the particulate material.
For example, where the additive capable of reacting with the organic polymeric material to insolubilise the latter with respect to water is a polyvalent metal compound certain of the latter compounds are also capable of effecting coupling -21- ~z5~Z~
betwee4n ferrite particulate materials and the organic polymeric material. Suitable additive to fulfil both these functions include A12(OH)5Cl, (NH4)2 Cr2O7, Cr (oH)l~8(~o3)l~2 and A12 (OH)s NO3.
In general the additive capable of eff~cting coupling, when different from the additive capable of reacting with the organic polymeric material to insolublise the latter material with respect to water will be present in the composition in a relatively low proportion, although the proportion required may depend on the particle size of the particulate material. For example, the additive may be present in a proportion of 0.01 to 3% by volume of the particulate material in the composition.
The invention is illustrated by the following examples in which all parts are parts by volume, unless otherwise stated.
Example 1 128 parts of a particulate ferrite, BaFel2Olg, having a particle size of 10 microns, and 22.8 parts of hydrolysed poly(vinyl acetate) (Gohsenol GH175 Nippon Gohsei, degree of hydrolysi6 88% degree of polymerisation 2000) were thoroughly mixed in a bladed mixer. 4 parts of resorcinol in 15 parts of water were mixed with 40 parts of an aqueous solution containing 30 parts of water and 10 parts of aluminium hydroxy chloride the so~ution containing 12.1% w/w Al, 8.75 %
w/w Cl, the latter solution having a viscosity of 18 cps, and the resultant solution added to the mixed solids in the bladed mixer to form a crumble.
The crumble was then charged to a twin-roll mill the rollers of which were heated to a temperature of 70C and the crumble was formed into sheet on the mill, -22- ~2S~Z4 the sheet being passed repeatedly through the nip between the rolls. The milling was contlnued for 5 minutes during which time some of the water evaporated, and the resultant sheet was removed from the mill.
The sheet contained 128 parts of particulate ferrite, 22.8 parts of hydrolysed poly(vinyl acetate), 10 parts of aluminium hydroxy chloride, 4 parts of resorcinol, and 25 parts of water.
The sheet was then placed between two sheets of polyethylene terephthalate the faces of which were coated with mould release agent and the sheet was pressed in a hydraulic press at a temperature of 80C
and a pressure of 10 MPa for 10 minutes.
The platens of the press were then cooled by flowing cold water through the platens, the sheet was removed from the press, and the sheets of polyethylene terephthalate were removed from the sheet.
Setting of the sheet was completed by placing the sheet between two flat pieces of wood, the æheet was allowed to stand for 1 day at 20C, it was then heated at 80C for 1 day, and finally it was heated at 180C for 1 hour.
The sheet had a flexural ~trength of 112.6 MPa and a flexural modulus of 48.3 GPa, and contained 78%
by volume of ferrite and 22~ by volume of cross-linked polymer.
The sheet had the following magnetic properties.
Remenance (Br) 1430 Gauss Coercivity (Hc) 750 Oersteds BHmaX product 0.30 x 106 gauss Oersteds saturation magnetisation 2720 gauss.
~L~2S~i24 Exampl _ The mixing, shaping, and setting procedure of Example 1 was repeated on a composition comprising (Mn, Zn) ferrite 150 micron mean size 669.6 parts (Mn, Zn) ferrite 1 micron mean size 224.1 parts Hydrolysed poly(vinyl acetate) Gohsenol GH 17S 115.8 parts Polyviol V03-140 (WacXer-Chemie) 21.0 parts degree of hydrolysis 86-89%, degree of polymerisation 300 Aluminium hydroxy chloride solution ~03.3 parts (as used in Example 1) Water 140 parts The sheet, which contained 83~ by volume of ferrite and 17~ by volume of cross-linked polymer, had a flexural strength of 106 MPa and a flexural modulus of 44.7 GPa. After soaXing in water for 1 day the flexural strength of the sheet was 56 MPa and the flexural modulus was 20 GPa.
The sheet had the following magnetic properties Remenance 355 gauss Coercivity (Hc) 9.85 Oersteds Initial permeability 19.8 Maximum permeability 26.0 Saturation magnetisation 4480 gauss A ring was cut from the sheet and the low field permeability of the ring was measured by comparing the inductance of a coil wound on the ring with the theoretical inductance for an air-cored coil. The low field permea~ility was 19.1.
Example_3 The mixing, shaping, and setting procedure of Example 1 was repeated on a composition comprising (Mn, Zn) ferrite 150 micron mean size 54.2 parts (Mn, Zn) ferrite 1 micron mean si~e 18.1 parts Hydrolysed poly(vinyl acetate) Gohsenol GH 17S 29.3 parts Polyviol V03-140 5.4 parts Aluminium hydroxy chloride solution (as used in Example 1) 51.4 parts Water 14.3 parts The sheet which was produced contained 61~ by volume of ferrite and 39% by volume of cross-linked polymer, and had a low field permeability, measured as described in Example 2, o 7.2.
Example ~
-The mixing, shaping and setting procedure of Example 1 was repeated on a composition comprising (Mn, Zn) Ferrite 150 micron mean size 60.8 parts (Mn, Zn) Ferrit0 1 micron mean size 20.2 parts Hydrolysed PVA
GH17S 21.1 parts V03/140 3.8 parts A12(0H)sCl solution36.8 parts Water 25 parts The sheet which was produced contained ~0% by volume of ferrite and 30% by volume of cross-linked polymer and had a low field permeability measured as described in Example 2, of 13.6.
Claims (23)
1. A shapeable composition comprising a homogeneous mixture of (a) at least one particulate ferrite material having magnetic properties, (b) at least one water-soluble or water-dispersible organic polymeric material, and (c) water, in the composition the components (a), (b) and (c) being present in a proportion by volume of the composition of respectively, 40 to 90%, 2 to 25%, and not more than 30%.
2. A shapeable composition as claimed in Claim 1 characterised in that the particulate ferrite material having magnetic properties has a median particle size of greater than 0.3 micron.
3. A shapeable composition as claimed in Claim 1 or Claim 2 characterised in that the particulate ferrite material having magnetic properties comprises a plurality of particle sizes.
4. A shapeable composition as claimed in Claim 1 characterised in that the composition comprises particulate ferrite material having magnetic properties in a proportion of 60% to 90% by volume.
5. A shapeable composition as claimed in Claim 1 characterised in that the components of the composition are selected such that a test composition comprising 63% by volume of particulate ferrite material having magnetic properties, 7%
by volume of water-soluble or water dispersible organic polymeric material, and 30% by volume of water, when extruded in a capillary rheometer at an extrusion pressure up to a maximum of 500 atomspheres undergoes an increase of at least 25% in shear stress when a tenfold increase in the shear rate of the test composition is effected when the shear rates as measured are within the range 0.1 to 5 second -1.
by volume of water-soluble or water dispersible organic polymeric material, and 30% by volume of water, when extruded in a capillary rheometer at an extrusion pressure up to a maximum of 500 atomspheres undergoes an increase of at least 25% in shear stress when a tenfold increase in the shear rate of the test composition is effected when the shear rates as measured are within the range 0.1 to 5 second -1.
6. A shapeable composition as claimed in Claim 1 characterised in that the organic polymeric material comprises a hydrolysed polymer or copolymer of a vinyl ester.
7. A shapeable composition as claimed in Claim 6 characterised in that the organic polymeric material comprises hydrolysed poly(vinyl acetate).
8. A shapeable composition as claimed in Claim 1 characterised in that the organic polymeric material is present in a proportion of 7% to 20% by volume.
9. A shapeable composition as claimed in Claim 8 characterised in that the composition contains 5% to 20% by volume of water.
10. A shapeable composition as claimed in Claim 1 characterised in that the composition comprises an additive capable of reacting with the organic polymeric material to insolubilise the material with respect to water.
11. A shapeable composition as claimed in Claim 10 characterised in that the additive is aluminium hydroxy chloride.
12. A shapeable composition as claimed in Claim 10 or Claim 11 characterised in that the additive is present in a proportion of 5% to 100% by volume of the organic polymeric material in the composition.
13. A shapeable composition as claimed in Claim 1 characterised in that the composition comprises an additive capable of effecting coupling between the organic polymeric material and the surface of the particulate ferrite material having magnetic properties.
14. A shapeable composition as claimed in Claim 13 characterised in that the additive capable of insolubilising the organic polymeric material is the same as the additive capable of effecting coupling.
15. A shaped article having magnetic properties characterised in that said article is produced by removing water from a shaped composition comprising a homogeneous mixture of (a) at least one particulate ferrite material having magnetic properties, (b) at least one water-soluble or water-dispersible organic polymeric material, and (c) water, in the composition the components (a), (b) and (c) being present in a proportion by volume of the composition of respectively, 40 to 90%, 2 to 25%, and not more than 30%.
16. A shaped article as claimed in Claim 15 characterised in that there is present an additive capable of insolubilising the organic polymeric material with respect to water, and in that said additive is reacted with said material.
17. A shaped article as claimed in Claim 15 or Claim 16 characterised in that not more than 2% of the total volume of said article comprises pores having a maximum dimension exceeding 100 microns.
18. A shaped article as claimed in Claim 15 or Claim 16 characterised in that not more than 0.5% of the total volume of said article comprises pores having a maximum dimension exceeding 15 microns.
19. A shaped article as claimed in any one of Claim 15 or Claim 16 characterised in that the total volume of pores in the shaped article does not exceed 20%.
20. A process for the production of a shaped article having magnetic properties characterised in that the process comprises shaping a composition comprising a homogeneous mixture of (a) at least one particulate ferrite material having magnetic properties, (b) at least one water-soluble or water-dispersible organic polymeric material, and (c) water, in the composition the components (a), (b) and (c) being present in a proportion by volume of the composition of respectively, 40 to 90%, 2 to 25%, and not more than 30%, and removing water from the composition.
21. A process as claimed in Claim 20 characterised in that the composition is shaped by calendering, injection moulding, compression moulding, or by extrusion.
22. A process as claimed in Claim 20 or Claim 21 characterised in that water is removed from the shaped composition by heating at a temperature of 100°C or greater.
23. A process as claimed in Claim 20 characterised in that the composition contains an additive capable of insolubilising the organic polymeric material with respect to water and in that said additive is reacted with said material by heating the shaped composition at a temperature of 100°C or geater.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8310997 | 1983-04-22 | ||
| GB8310997 | 1983-04-22 | ||
| GB8405302 | 1984-02-28 | ||
| GB8405302 | 1984-02-28 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1252624A true CA1252624A (en) | 1989-04-18 |
Family
ID=26285916
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000451913A Expired CA1252624A (en) | 1983-04-22 | 1984-04-13 | Article having magnetic properties and production thereof |
Country Status (8)
| Country | Link |
|---|---|
| AU (1) | AU577285B2 (en) |
| CA (1) | CA1252624A (en) |
| DE (1) | DE3415243A1 (en) |
| FR (1) | FR2712729A1 (en) |
| GB (1) | GB2188641B (en) |
| IT (1) | IT8420567A0 (en) |
| NL (1) | NL8401183A (en) |
| SE (1) | SE461882B (en) |
Families Citing this family (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS6245660A (en) * | 1985-08-26 | 1987-02-27 | Polyplastics Co | Composite material composition |
| GB8727852D0 (en) * | 1987-11-27 | 1987-12-31 | Ici Plc | Compositions for production of magnets and magnets produced therefrom |
| US4904836A (en) * | 1988-05-23 | 1990-02-27 | The Pillsbury Co. | Microwave heater and method of manufacture |
| DE4140944A1 (en) * | 1991-12-12 | 1993-06-17 | Deutsche Aerospace | ABSORBER FOR ELECTROMAGNETIC RADIATION |
| DE10146805B4 (en) * | 2001-09-22 | 2015-03-12 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. | Method for producing a film for lining casings |
| DE10334141A1 (en) * | 2003-07-24 | 2005-03-03 | Siemens Ag | Inductive system for train security has trackside sending and train side receiving coils on antenna monolithic cores of soft magnetic plastic |
Family Cites Families (15)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1065864A (en) * | 1963-04-11 | 1967-04-19 | Tsukamoto Kenkichi | A disintegrable permanent magnet for use in the soil |
| US4001363A (en) * | 1970-03-19 | 1977-01-04 | U.S. Philips Corporation | Method of manufacturing a ceramic ferromagnetic object |
| DE2231591C3 (en) * | 1972-06-28 | 1979-08-09 | Max 5060 Bergisch Gladbach Baermann | Annular disk-shaped permanent magnet for a magnetic bearing, preferably for electricity meters, and a permanent magnetic bearing constructed therefrom |
| US3901816A (en) * | 1974-06-17 | 1975-08-26 | Dow Chemical Co | Magnetic tape coating |
| GB1497658A (en) * | 1975-03-10 | 1978-01-12 | Ici Ltd | Process for coating films |
| GB1531587A (en) * | 1976-11-11 | 1978-11-08 | Muanyagipari Kutato Intezet | Flat-armature for motors and electro magnets |
| NL7614182A (en) * | 1976-12-21 | 1978-06-23 | Philips Nv | MAGNETIC REGISTRATION ELEMENT IN WHICH A SALT OF AN AMINE AND A PHOSPHORIC ACID ESTER IS USED AS A DISPERSANT. |
| BR8000887A (en) * | 1979-02-23 | 1980-10-21 | Minnesota Mining & Mfg | DISPERSION OF INORGANIC PARTICLES, STABLE, COVERABLE, IN A WATER BASED LATEX, PROCESS TO MAKE A COATING OF A MAGNETIC REGISTRATION MEDIA AND MAGNETIC REGISTRATION MEDIA |
| DE2914659A1 (en) * | 1979-04-11 | 1980-10-23 | Agfa Gevaert Ag | MAGNETIC STORAGE MEDIUM AND METHOD FOR THE PRODUCTION THEREOF |
| DE2920334A1 (en) * | 1979-05-19 | 1980-12-04 | Basf Ag | MAGNETIC RECORDING CARRIERS |
| US4263188A (en) * | 1979-05-23 | 1981-04-21 | Verbatim Corporation | Aqueous coating composition and method |
| NL8006065A (en) * | 1980-04-24 | 1981-11-16 | Indiana General Corp | ELECTROPHOTOGRAPHIC COMPOSITE CARRIER WITH SELF-CLEANING OPERATION DURING USE IN A COPIER. |
| JPS5815573A (en) * | 1981-07-22 | 1983-01-28 | Toyo Ink Mfg Co Ltd | Radiation-curing magnetic paint and magnetic recording medium obtained therefrom |
| DE3375387D1 (en) * | 1982-06-09 | 1988-02-25 | Ici Plc | Mouldable composition and shaped product produced therefrom |
| WO1984002445A2 (en) * | 1984-04-02 | 1984-07-05 | Imperial Chemical Industries Plc | Article having magnetic properties and production thereof |
-
1984
- 1984-03-19 GB GB08407105A patent/GB2188641B/en not_active Expired
- 1984-04-09 AU AU28375/84A patent/AU577285B2/en not_active Ceased
- 1984-04-13 CA CA000451913A patent/CA1252624A/en not_active Expired
- 1984-04-13 NL NL8401183A patent/NL8401183A/en not_active Application Discontinuation
- 1984-04-17 IT IT8420567A patent/IT8420567A0/en unknown
- 1984-04-18 SE SE8402172A patent/SE461882B/en not_active IP Right Cessation
- 1984-04-24 FR FR8406406A patent/FR2712729A1/en not_active Withdrawn
- 1984-04-24 DE DE3415243A patent/DE3415243A1/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| SE461882B (en) | 1990-04-02 |
| IT8420567A0 (en) | 1984-04-17 |
| GB2188641A (en) | 1987-10-07 |
| FR2712729A1 (en) | 1995-05-24 |
| AU2837584A (en) | 1987-08-20 |
| SE8402172L (en) | 1987-11-21 |
| GB8407105D0 (en) | 1987-07-29 |
| SE8402172D0 (en) | 1984-04-18 |
| GB2188641B (en) | 1988-02-10 |
| AU577285B2 (en) | 1988-09-22 |
| NL8401183A (en) | 1987-11-02 |
| DE3415243A1 (en) | 1988-02-18 |
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