Permanent magnets

Description

[0001] Flexible permanent magnets have been used extensively in gasket assemblies for sealing the space between the door and cabinet of refrigerators, food freezers and like structures. Such magnets also have found extensive use in a variety of other applications, including use in advertising sheet materials that can be removably attached to an appropriate metal surface, as an attaching component in storm window framing, in motors, in sheet backing for carpeting to anchor the carpeting to steel flooring, as toy components, etc.

[0002] As described in US-A-2,959,832, such magnets may be made by incorporating a magnetic powder into an elastomer binder and shaping the composition into the desired configuration, followed by orientation of the particles of magnetic powder within the binder in order to enhance the potential magnetic properties of the composition. The resulting composition is subjected to a magnetizing field to magnetize the particles of magnetic powder within the composition.

[0003] US-A-4,022,701 discloses the production of an anisotropic plastics magnet composed of a mixture of one or more ferromagnetic powders having a large magnetic anisotropy constant and a synthetic resin which is characterized in that one or more of a metal crosslinked resin from a copolymer of a-olefin and α,β unsaturated mono- or dicarboxylic acid or a saponified resin of a copolymer of ethylene and vinyl acetate is used as the synthetic resin and the molding is done while the magnetic field is applied under a heated condition.

[0004] DE-A-1 464 613 discloses a flexible permanent magnet composition containing at least 67 Vol.-% barium ferrite with a particle size smaller than 10 pm and an elastomeric binder characterized in that at least one component of the binder is selected from the group consisting of chlorosulfonated polyethylene with a high molecular weight, polyisobutylene and chlorinated polyethylene. If a high amount of the ferrite powder is used in the mix per 100 parts by weight of binder the mixes are difficult to process.

[0005] The mixing operation can be carried out in any convenient manner, such as by mixing the components of the magnetic composition together in a Banbury mixer, on a roll mill or in an extruder. The method or methods which can be used for shaping the composition will depend to a significant degree on the configuration into which the composition is to be formed. For example, if thin sheets of the magnetic composition are desired, the sheets can be formed by a conventional calendering operation. Strip magnets of the type used in refrigerator gasket assemblies normally are formed as a continuous strip by extruding the magnetic composition through an appropriate-shaped extrusion die. Magnetization of the particles of magnetic material within the composition can be accomplished by subjecting the shaped product to a magnetic field of sufficient strength.

[0006] The amount of powdered magnetic material present in the magnetic composition will influence the strength of a magnet that can be formed from the composition. Usually, the larger the quantity of powered magnetic material. in the composition, the greater will be the strength of a magnet which can be formed from the composition (provided that the degree of orientation of the magnetic particles is equal). However, as the quantity of powdered magnetic material in the composition is increased, the stiffness of the composition also increases and ultimately the composition may become friable and unable to be "worked" further. Also, the energy required for mixing or the pressure required for extrusion may become objectionably high as greater quantities of the magnetic powder are added to the composition.

[0007] Therefore it is desirable to find elastomeric binders, so that a high loading of ferrite powder can be used in the unit and the unit can still be processed.

[0008] According to the present invention there is provided a flexible permanent magnet composition comprising finely-divided particles of a ferrite having the formula MO . nFe₂0₃ in which M is barium, lead or strontium and n is an integer, in an elastomeric binder, in an amount up to 1200 parts by weight or 100 parts by weight of said elastomer characterized in that the elastomeric is a polyarylic elastomer having a Mooney viscosity between 20 and 70 when measured in accordance with the procedure of ASTM Designation 1646 with a 4 minute running time at 100°C using the large rotor and with a 1 minute warm-up period, said polyacrylic elastomer being a polymer comprised of from 95 to 100 percent by weight of a backbone component having the structure

derived from an alkyl acrylate, where x is an integer and n is from 2 to 8, or having the structure

derived from an alkyoxy acrylate, where x in an integer, n is 2 and m is 1 or 2, and up to 5% by weight of a copolymerisable component having reactive functional groups.

[0009] The present invention provides a magnetic composition that exhibits outstanding processing characteristics that enable the composition to be processed easily using less energy input and to be extruded at extrusion pressures lower than required with current commercial compositions containing comparable loadings of magnetic material. The composition is able to accommodate higher loadings of the powdered magnetic material without losing its capability of being "worked" and shaped and, as a result of the possible higher loadings, can be formed into magnets of higher strengths.

[0010] Magnets formed from the composition can be made into magnets that have a greater degree of flexibility than previously realized and will withstand higher temperatures without exhibiting objectionable degradation.

[0011] The magnetic material can be combined with the polyacrylic elastomer component of the composition in any convenient manner, such as by mixing the components of the composition together in a Banbury mixer or on a roll mill. The mixed composition, usually is cubed or granulated to put it in a form that can be handled easily for further processing. The composition then is shaped into the desired configuration. If a flat thin sheet is desired, shaping can be accomplished on a calendering mill. If a magnet in the form of a strip is required, the cubed or granulated magnetic composition can be fed into a screw-type extruder and forced through an appropriately-shaped extrusion die to form a continuous strip of the magnetic composition of the desired cross-sectional configuration. The composition also can be shaped by injection molding or by shaping in conventional sectional molds. The shaped composition thereafter is exposed to a magnetizing field to magnetize the magnetic particles with the composition.

[0012] Among the reactive functional groups are reactive halogen groups having the structure

where X is chlorine or bromine (such as results from the copolymerization of the alkyl acrylate or alkoxy acrylate with 2-chlorethyl vinyl ether), or reactive epoxy groups having the structure

(such as results from the copolymerization of the alkyl acrylate or alkoxy acrylate with an alkyl glycidyl ether), or reactive carboxyl groups having the structure

(such as results from the copolymerization of the alkyl acrylate or alkoxy acrylate with acrylic acid or methacrylic acid) or reactive hydroxyl groups having the structure

(such as results from the copolymerization of the alkyl acrylate or alkoxy acrylate with an hydroxalkyl acrylate).

[0013] The magnetic component is a ferrite of barium, lead, or strontium, or a mixture thereof in a finely-divided state so that the individual particles desirably approach the size of the magnetic domains of the material. As used herein, a ferrite is defined as a material having the formula MO . nFe₂0₃ wherein M is barium, lead or strontium and n is an integer. Of the commercial ferrites, barium ferrite (BaFe,₂0,₉) is preferred. The particle size of the magnetic component desirably is within the size range of 0.5 to 10 pm with an average particle size preferably being 1 to 1.5 pm. Although commercial flexible permanent magnets currently being sold can contain as an upper limit only about 1,000 parts by weight of the magnetic component per 100 parts by weight of the binder component the composition of the present invention can accommodate up to about 1200 parts by weight of the magnetic material per 100 parts by weight of the polyacrylic binder component. As a result of the increased amount of magnetic material that can be incorporated into the composition, magnets of greater magnetic strength can be formed.

[0014] If desired, a small quantity of a processing aid for the polyacrylic binder component can be added to the composition. The processing aid should have a softening point below the temperature at which the magnetic composition is mixed and shaped to produce optimum results. Polyethylene homopolymers and copolymers of ethylene with an acrylic acid or vinyl acetate which have softening points when measured in accordance with the procedure described in ASTM Designation No. E-28 of between 60 to 120°C. are preferred processing aids. Desirably, from 1 to 15 parts by weight of such processing aid is used per 100 parts by weight of the polyacrylic elastomer in the magnetic composition.

[0015] The invention will be further understood by reference to the following examples.

Examples I-V

[0016] Magnetic compositions were formed having the following compositions:

[0017] The ingredients were mixed together on a roll mill, sheeted off the mill and granulated. The granules were fed to a conventional extruder fitted with a heated die which produced a strip-type extrusion rectangular in cross-section. As the strip of material was discharged from the extrusion die, it was advanced through a magnetic field to magnetize the particles of ferrite in the composition. Various operating conditions employed in the manufacture of the strip materials and selected physical properties of the strip magnet formed are listed in Table I:

[0018] A typical commercial magnetic composition having the following composition served as a "control" was formed into a strip magnet as described in Examples I-V:

[0019] The "control" strip magnet had the following physical properties:

[0020] The invention is further illustrated by the following examples.

Examples VI-XIII

[0021] Magnetic compositions were formed using the following formulation:

[0022] The polymer binders used in the examples were:

[0023] The compositions were mixed on an open mill. Mixing time for the polyacrylic elastomer binder compositions (Examples VI-X) was ten (10) minutes and mixing time for the chlorosulfonated polyethylene/polyisobutylene binder compositions was 20 minutes. Each composition was run through a capillary extrusion die in a constant load rheometer at the following conditions:

[0024] The data obtained is tabulated in Table II.

Claims

1. A flexible permanent magnet composition comprising finely-divided particles of a ferrite having the formula MO . nFe₂0₃ in which M is barium, lead or strontium and n is an integer, in an elastomeric binder in an amount up to 1200 parts by weight per 100 parts by weight of said elastomer, characterized in that the elastomeric is a polyacrylic elastomer having a Mooney viscosity between 20 and 70 when measured in accordance with the procedure of ASTM Designation 1646 with a 4 minute running time at 100°C. using the large rotor and with a 1 minute warm-up period, said polyacrylic elastomer being a polymer comprised of from 95 to 100 percent by weight of a backbone component having the structure

derived from an alkyl acrylate, where x is an integer and n is from 2 to 8, or having the structure

derived from an alkyoxy acrylate, where x in an integer, n is 2 and m is 1 or 2, and up to 5% by weight of a copolymerisable component having reactive functional groups.

2. The composition of claim 1 characterized in that the reactive functional groups are reactive halogen groups having the structure

where X is chlorine or bromine.

3. The composition of claim 1 characterized in that the reactive functional groups are reactive epoxy groups having the structure

4. The composition of claim 1 characterized in that the reactive functional groups are reactive carboxyl groups having the structure

5. The composition of claim 1 characterized in that the reactive functional groups are reactive hydroxyl groups having the structure

6. The composition of claim 1 characterized in that the polyacrylic elastomer has a Mooney viscosity between 25 to 60 when measured in accordance with the procedure of ASTM Designation 1646 with a 4 minute running time at 100°C. using the large rotor and with a 1 minute warm-up period.

7. The composition of claim 1 characterized in that the finely-divided particles of ferrite have a particle size within the range of from 0.5 to 10 11m and an average particle size from 1 to 1.5 µm.

8. The composition of claims 1, 2, 3, 4, 5, 6 or 7, characterized in that the ferrite is barium ferrite (BaFe₁₂O₁₉).

9. The composition of claims 1, 2, 3, 4, 5 or 6, characterized in that the composition contains from 1 to 15 parts by weight of a polyethylene homopolymer or a copolymer of ethylene with an acrylic acid or vinyl acetate per 100 parts by weight of said polyacrylic elastomer.

Ansprüche

1. Flexible Permanentmagnet-Zusammensetzung, enthaltend feine Teilchen eines Ferrits der Formel MO. nFe₂0₃, in der M Barium, Blei oder Strontium ist und n eine ganze Zahl ist, in einem elastomeren Bindemittel in einer Menge von bis zu 1200 Gew.-Teilen auf 100 Gew.-Teile des Elastomers, dadurch gekennzeichnet, daß das Elastomer ein Polyacryl-Elastomer mit einer Mooney-Viskosität zwischen 20 und 70, gemessen nach der Arbeitsweise der ASTM-Vorschrift 1646 mit einer Laufzeit von 4 min bei 100°C unter Verwendung des großen Rotors und mit einer Aufwärmperiode von 1 min, wobei das Polyacryl-Elastomer ein Polymer ist, das zusammengesetzt ist aus 95 bis 100 Gew.-% einer Skelett-Komponente mit der von einem Alkylacrylat abgeleiteten Struktur

in der x eine ganze Zahl ist und n 2 bis 8 ist, oder mit der von einem Alkyloxyacrylat abgeleiteten Struktur

in der x eine ganze Zahl ist, n 2 ist und m 1 oder 2 ist, und bis zu 5 Gew.-% einer copolymerisierbaren Komponente mit reaktionsfähigen funktionellen Gruppen.

2. Zusammensetzung nach Anspruch 1, dadurch gekennzeichnet, daß die reaktionsfähigen funktionellen Gruppen reaktionsfähige Halogen-Gruppen mit der Struktur

sind, in der X Chlor oder Brom ist.

3. Zusammensetzung nach Anspruch 1, dadurch gekennzeichnet, daß die reaktionsfähigen funktionellen Gruppen reaktionsfähige Epoxy-Gruppen mit der Struktur

₅ sind.

4. Zusammensetzung nach Anspruch 1, dadurch gekennzeichnet, daß die reaktionsfähigen funktionellen Gruppen reaktionsfähige Carboxyl-Gruppen mit der Struktur

sind.

5. Zusammensetzung nach Anspruch 1, dadurch gekennzeichnet, daß die reaktionsfähigen funktionellen Gruppen reaktionsfähige Hydroxyl-Gruppen mit der Struktur

sind.

6. Zusammensetzung nach Anspruch 1, dadurch gekennzeichnet, daß das Polyacryl-Elastomer eine Mooney-Viskosität zwischen 25 und 60, gemessen nach der Arbeitsweise der ASTM-Vorschrift 1646 mit einer Laufzeit von 4 min bei 100°C unter Verwendung des großen Rotors und mit einer Aufwärmperiode von 1 min besitzt.

7. Zusammensetzung nach Anspruch 1, dadurch gekennzeichnet, daß die feinen Teilchen des Ferrits eine Teilchengröße innerhalb des Bereichs von 0,5 bis 10 um und eine mittlere Teilchengröße von 1 bis 1,5 pm besitzen.

8. Zusammensetzung nach Anspruch 1, 2, 3, 4, 5, 6 oder 7, dadurch gekennzeichnet, daß der Ferrit Bariumferrit (BaFe₁₂O₁₉) ist.

9. Zusammensetzung nach Anspruch 1, 2, 3, 4, 5 oder 6, dadurch gekennzeichnet, daß die Zusammensetzung 1 bis 15 Gew.-Teile eines Polyethylen-Homopolymers oder eines Copolymers aus Ethylen mit einer Acrylsäure oder Vinylacetat auf 100 Gew.-Teile des Polyacryl-Elastomers enthält.

Revendications

1. Composition pour aimants permanents flexibles, incluant des particules finement divisées d'un ferrite ayant pour formule MO . nFe₂0₃, dans laquelle M est du baryum, du plomb ou du strontium et n est un nombre entier, dans un liant élastomère, et ce en une quantité allant jusqu'à 1200 parties en poids pour 100 parties en poids dudit élastomère, caractérisée en ce que l'élastomère est un élastomère polyacrylique possédant une viscosité Mooney comprise entre 20 et 70, telle qu'elle est mesurée conformément au mode opératoire selon ASTM Designation 1646 avec une durée de cycle de fonctionnement de 4 minutes à 100°C en utilisant le gros rotor et une période d'échauffement de 1 minute, ledit élastomère polyacrylique étant un polymère constitué entre 95 et 100 pour cent en poids d'on composant formant tronc et possédant la structure:

dérivée d'un acrylate d'alkyle, x étant un nombre entier et n étant compris entre 2 et 8, ou possédant la structure

dérivée d'un acrylate d'alcoxy, x étant un nombre entier, n étant égal à 2 et m étant égal à 1 ou à 2, et par une quantité allant jusqu'à 5 pour cent en poids d'un composant copolymérisable possédant des groupes fonctionnels réactifs.

2. Composition selon la revendication 1, caractérisée en ce que les groupes fonctionnels réactifs sont des groupes réactifs halogène possédant la structure

X étant du chlore ou du brome.

3. Composition selon la revendication 1, caractérisée en ce que les groupes fonctionnels réactifs sont des groupes réactifs époxy possédant la structure

4. Composition selon la revendication 1, caractérisée en ce que les groupes fonctionnels réactifs sont des groupes réactifs carboxyle possédant la structure

5. Composition selon la revendication 1, caractérisée en ce que les groupes fonctionnels réactifs sont des groupes réactifs hydroxyle possédant la structure

6. Composition selon la revendication 1, caractérisée en ce que l'élastomère polyacrylique possède une viscosité Mooney comprise entre 20 et 60 lorsqu'on la mesure conformément au mode opératoire selon ASTM Designation 1646 avec une durée de cycle de fonctionnement de 4 minutes à 100°C en utilisant le gros rotor et avec une période d'échauffement de 1 minute.

7. Composition selon la revendication 1, caractérisée en ce que les particules finement divisées de ferrite possèdent une taille de particule comprise dans la gamme allant de 0,5 à 10 µm et une taille de particule moyenne comprise de 1 à 1,5 pm.

8. Composition selon les revendications 1, 2, 3, 4, 5, 6 ou 7, caractérisée en ce que le ferrite est un ferrite de baryum (BaFe₁₂O₁₉).

9. Composition selon les revendications 1, 2, 3, 4, 5 ou 6, caractérisée en ce que la composition contient de 1 à 15 parties en poids d'un homopolymère du polyéthylène ou d'un copolymère de l'éthylène avec un acide acrylique ou un acétate de vinyle pour 100 parties en poids dudit élastomère polyacrylique.