Preparation of a magnetic layer

A process is disclosed for the preparation of semi-soft or soft magnetic layers. The process involves chemical reduction in aqueous medium of one or more types of metal ions including nickel ions, washing the resulting dispersion of metal particles, and coating this dispersion onto a support. The resulting magnetic layers can be used in an anti-theft system.

FIELD OF THE INVENTION
 The present invention relates to the preparation of particular types of
 magnetic layers and to their use in an anti-theft system.
 BACKGROUND OF THE INVENTION
 The lagging of magnetization behind the magnetizing force as the magnetic
 condition of a ferromagnetic material is changed, e.g. when applying an
 alternating external field, is called magnetic hysteresis. When a
 ferromagnetic sample that is initially demagnetized is subjected to an
 increasing external magnetic field H it reaches a particular flux density
 B.sub.sat at the maximum value of H. When the value of H is decreased
 again the decreasing flux density does not follow the path of increase but
 decreases at a rate less than that at which it rose. When H has reached
 zero again the value of B is not reduced to zero but to a value called the
 retentivity or remanence. The sample has retained a permanent
 magnetization. The value of B may be reduced to zero by reversing th
 magnetic field to negative and increasing its value to the so-called
 coercive force or coercivity. By further increasing H to negative values
 and then again reversing its direction a hysteresis loop as represented in
 FIG. 1 is completed.
 BRIEF AND DETAILED DESCRIPTION OF THE DRAWINGS
 We define in the curve of FIG. 1:
 the saturation magnetization B.sub.sat which is proportional to the amount
 of material;
 the coercive force C.sub.M which is dependent on the chemical composition,
 the particle size, the temperature, etc.;
 the magnetic permeability (susceptibility or permitivity) P.sub.M which is
 dependent on the chemical composition, the degree of deformation of the
 material, etc.
 A so-called soft ferromagnetic material shows a rather low coercive force;
 a so-called semi-soft ferromagnetic material shows a rather high coercive
 force. This properties are used in a special type of anti-theft labels,
 e.g. for preventing the theft from clothes out of shops, called EM-EAS
 labels (Electro Magnetic Electronic Article Surveillance). The principle
 works as follows. A label carrier is covered on one side with a soft
 magnetic layer having a coercive force of about 0.5 Oe, and on the other
 side with a semi-soft magnetic layer having a coercive force of about 100
 Oe. The detection zone consists of a transmitter which transmits an
 alternating magnetic field with a force Z.sub.M positioned between 0.5 and
 100 Oe, and of a receiver.
 Under normal conditions the soft magnetic material will follow the
 alternating magnetic field. This is the case when the semi-soft layer is
 not magnetized (active situation). When one walks with this label through
 the detection zone the reversing of the magnetic dipole due to the high
 permeability (&gt;40,000) will be detected by the receiver and as a
 consequence an alarm will go off. On the contrary, when the semi-soft
 layer is magnetized the soft material will be magnetized as well in the
 opposite sense. The transmitter is in this case not able to influence the
 soft magnetic material since the field strenght of the semi-soft material
 is larger than the strenght of the transmitted alternating field. As a
 consequence nothing is detected. The situations explained are briefly
 summarized in FIG. 2 and FIG. 3.
 For a particular brand of commercially available labels the semi-soft
 magnetic layer consists of a nickel mesh, and the soft magnetic layer
 consists of a complex alloy of Ni.sub.a Fe.sub.b Co.sub.c (MO) .sub.d
 B.sub.e. The problem with these magnetic layers is the fact that they are
 nowadays applied by means of sputtering in vacuo, a cumbersome and
 expensive technique.
 OBJECTS OF THE INVENTION
 It is an object of the present invention to provide a simple and cheap
 method for producing soft and semi-soft magnetic layers.
 It is a further object of the invention to provide a use for such magnetic
 layers in the design of EM-EAS labels.
 SUMMARY OF THE INVENTION
 The above mentioned objects are realised by providing a process for the
 preparation of a magnetic element comprising a support and at least one
 magnetic layer, said process comprising the steps of:
 (1) preparing an aqueous solution containing one or more type of metal ions
 including nickel ions,
 (2) chemically reducing said one or more metal ions by means of a reducing
 agent thus forming an aqueous dispersion of metal particles including
 nickel,
 (3) removing all superfluous ions from said aqueous dispersion by means of
 a washing step, preferably an ultrafiltration and/or diafiltration step,
 or by means of centrifugation,
 (4) coating the resulting aqueous dispersion onto a support.
 DETAILED DESCRIPTION OF THE INVENTION
 The present invention will now be explained on the hand of a preferred
 embodiment whereby the metal ions undergoing reduction are solely
 nickel(II) ions.
 In a first step an aqueous solution of nickel(II) ions is prepared. A most
 suitable salt is Ni(NO.sub.3).sub.2.6H.sub.2 O. The solution is acidified
 with a small amount of nitric acid.
 In a following step the nickel ions in the solution are reduced to highly
 dispersed metallic nickel particles of nanosize by means of the addition
 of a reducing agent. A preferred reducing agent is KBH.sub.4. The reducing
 agent can be added to the original nickel salt solution as a solid powder.
 More preferably, the reducing agent may be dissolved separately in a
 second aqueous medium and added to the nickel salt solution according to a
 single jet or a double jet procedure. Preferably, according to the double
 jet principle, the aqueous medium containing the nickel ions and the
 second solution containing the reducing agent are added together to a
 third aqueous medium.
 The second aqueous solution comprising the reducing agent preferably also
 contains sulphite ions which strongly enhance the chemical stability of
 this solution.
 In order to keep the nickel nanoparticles formed by reduction in colloidal
 dispersion a protective binder is preferably added to one or more of the
 three aqueous solutions involved. Preferably, this protective binder is
 added to the third aqueous medium wherein both others are jetted. A
 particularly preferred protective binder is carboxymethylcellulose (CMC).
 Other possible binders include gelatin, arabic gum, poly(acrylic acid),
 cellulose derivatives and other polysaccharides.
 Preferably also a complexing agent is present in one of the three aqueous
 media described above. A preferred complexant is simply the well-known
 ethylenediaminetetraacetic acid (EDTA) or a homologous compound or a salt
 thereof. Another preferred one is citrate, e.g. triammonium citrate. Other
 suitable complexants include diethylenetriamine-pentaacetic acid (DTPA),
 trans-1, 2-diaminocyclohexane-N,N,N',N'-tetraacetic acid (CDTA),
 ethyleneglycol-O,O'-bis(2-aminoethyl)-N,N,N',N'-tetraacetic acid (EGTA),
 N-(2-hydroxyethyl)ethylenediamine-N,N,N'-triacetic acid (HEDTA), etc. The
 complexing agent is preferably present in the third aqueous medium to
 which the other solutions are added according to the double jet principle.
 In a following step 3 of the present invention the superfluous salts are
 first removed from the aqueous medium by a washing process, preferably
 involving ultrafiltration and/or diafiltration. Additionally or
 alternatively centrifugation can be used.
 In any of the solutions involved in the preparation a so-called dispersing
 aid can be present. In a preferred embodiment this compound is added to
 the diafiltration liquid at the last stage of the preparation. Suitable
 dispersing aids in the case of nickel are phosphates, more particularly a
 hexametaphosphate such as sodium hexametaphosphate. Probably, the
 hexametaphosphate adsorbs to the surface of the alloy particles so that
 they become negatively charged. By electrostatic repulsion they are kept
 in dispersion. Also the phosphate inhibits further oxidation of the
 surface of the formed nanoparticles. In other words, the thin nickel oxide
 shell that will be formed inevitably around the nanoparticles since the
 reducing medium disappears during the washing step will be passivated by
 the hexametaphosphate. So in a preferred embodiment the nickel particles
 are ultrafiltrated e.g. through a Fresenius F60 cartridge and subsequently
 diafiltrated against a solution of sodium hexametaphosphate in
 water/ethanol (98.5/1.5). Apart from the diafiltration liquid the
 hexametaphosphate is also preferably added to the third aqueous solution.
 Preferably after the addition of one or more coating agents the obtained
 final colloidal composition is coated on the substrate by means of a
 conventional coating technique, such as slide hopper, curtain coating and
 air-knife coating.
 Suitable coating agents include non-ionic agents such as saponins, alkylene
 oxides e.g. polyethylene glycol, polyethylene glycol/polypropylen glycol
 condensation products, polyethylene glycol alkyl esters or polyethylene
 glycol alkylaryl esters, polyethylene glycol esters, polyethylene glycol
 sorbitan esters, polyalkylene glycol alkylamines or alkylamides,
 silicone-polyethylene oxide adducts, glycidol derivaties, fatty acid
 esters of polyhydric alcohols and alkyl esters of saccharides; anionic
 agenst comprising an acid group such as a carboxy, sulpho, phospho,
 sulphuric or phosphoric ester group; ampholytic agents such as aminoacids,
 aminoalkyl sulphonic acids, aminoalkyl sulphates or phosphates, alkyl
 betaines, and amine-N-oxides; and cationic agents such as aklylamine
 salts, aliphatic, aromatic, or heterocyclic quaternary ammonium salts,
 aliphatic or heterocyclic ring-containing phosphonium or sulphonium salts.
 Other suitable surfactants include perfluorinated compounds.
 Useful transparent organic resin supports include e.g. cellulose nitrate
 film, cellulose acetate film, polyvinylacetal film, polystyrene film,
 polyethylene terephthalate film, polycarbonate film, polyvinylchloride
 film or poly-.alpha.-olefin films such as polyethylene or polypropylene
 film. The thickness of such organic resin film is preferably comprised
 between 0.05 and 0.35 mm. In a most preferred embodiment of the present
 invention the support is a polyethylene terephthalate layer provided with
 a subbing layer. This subbing layer can be applied before or after
 stretching of the polyester film support. The polyester film support is
 preferably biaxially stretched at an elevated temperature of e.g.
 70-120.degree. C., reducing its thickness by about 1/2 to 1/9 or more and
 increasing its area 2 to 9 times. The stretching may be accomplished in
 two stages, transversal and longitudinal in either order or
 simultaneously. The subbing layer, when present, is preferably applied by
 aqueous coating between the longitudinal and transversal stretch, in a
 thickness of 0.1 to 5 mm. In case of a nickel magnetic recording layer the
 subbing layer preferably contains, as described in EP 0 464 906, a
 homopolymer or copolymer of a monomer comprising covalently bound
 chlorine. Examples of said homopolymers or copolymers suitable for use in
 the subbing layer are e.g. polyvinyl chloride; polyvinylidene chloride; a
 copolymer of vinylidene chloride, an acrylic ester and itaconic acid; a
 copolymer of vinyl chloride and vinylidene chloride; a copolymer of vinyl
 chloride and vinyl acetate; a copolymer of butylacrylate, vinyl acetate
 and vinyl chloride or vinylidene chloride; a copolymer of vinyl chloride,
 vinylidene chloride and itaconic acid; a copolymer of vinyl chloride,
 vinyl acetate and vinyl alcohol, etc. Polymers that are water dispersable
 are preferred since they allow aqueous coating of the subbing layer which
 is ecologically advantageous.
 Alternatively, the support may be opaque, such as a paper support, e.g. a
 plain paper support or a polyolefin coated paper. Furtheron glass, e.g.
 thin glass packed on roll can be used.
 The coated substantially pure nickel layer is ferromagnetic of the
 so-called semi-soft type.
 In a further important embodiment of the present invention the nickel ions
 are not the sole ion type undergoing reduction but they are mixed with one
 or more other types of ions. Preferred types of salts for admixture with
 the nickel salt are iron salts, cobalt salts or molybdene salts, or
 mixtures of those.
 By choosing the appropriate type(s) of ions and the appropriate admixture
 ratio(s) so-called soft magnetic layers can be prepared.
 In a particular embodiment of the present invention a support, preferably a
 paper support, is coated on one side with a soft magnetic layer based on
 substantially pure nickel, and on the opposite side with a soft magnetic
 layer based on an admixture of nickel particles with other metal
 particles. When this material is cut into small dimensions the resulting
 labels can be used in a EM-EAS system (Electro Magnetic Electronic Article
 Surveillance)
 The present invention will now be illustrated by the following examples
 without however being limited thereto.

EXAMPLES
 Example 1
 Preparation of a magnetic material (NiFe).
 The following solutions were prepared:

Solution 1
 Ni(NO.sub.3).sub.2 .multidot. 6H.sub.2 O 41.8 g
 Fe(SO.sub.4).sub.2 .multidot. 6H.sub.2 O 14.3 g
 water to 150 ml
 HNO.sub.3 3 ml
 Solution 2
 Water 150 ml
 NH.sub.3 (26% in water) 0.5 ml
 KBH.sub.4 10 g
 Na.sub.2 SO.sub.3 2 g
 Solution 3
 Na6P6O18 (2% in water/ethanol (85/15) 100 ml
 Triammoniumcitrate (70% in water) 9.1 ml
 Carboxymethylcellulose (3% in water) 25.5 ml
 The Ni-Fe-dispersion was prepared as follows:
 To solution 3, held at room temperature and stirred at 300 rpm, solution 1
 at a flow rate of 12.4 ml/min was simultaneously added with solution 2 at
 12.4 ml/min. After the reduction, the NiFe dispersion was ultrafiltrated
 through a Fresenius F60 cartridge and diafiltrated with a 0.36% solution
 of sodium hexametaphosphate in water/ethanol (98.5/1.5).
 The dispersion was stirred and 10 ml of a 12.5% solution of Saponine
 Quillaya (Schmittmann) in water/ethanol (80/20) was added. This is the
 Ni-Fe dispersion.
 The dispersion was analysed for its particle size distribution (weight
 average d.sub.wa) with the Disc Centrifuge Photosedimentometer BROOKHAVEN
 BI-DCP. A d.sub.wa of 56 nm (s.sub.wa =10) was obtained.
 Subsequently this dispersion was coated on a substrated PET foil so that an
 amount of 0.90 g/m.sup.2 was obtained.
 A Squid magnetometer was used to measure the magnetic properties. A
 coercive field of 110 Oe was measured.