DE2050225A1 - Process for producing oxidation-resistant, ferromagnetic metal particles by means of polymer coating - Google Patents

Description

WESTERN ELECTRIC COMPMY INCORPORATED 195, Broadway, New York, N.Y. 10007 7 USA

Process for producing oxidative-resistant ferromagnetic Metal particles by means of polymer coating

The invention relates to polymer-coated ferromagnetic metal particles, a method for making such Particles and products made from them.

Magnetic metal and magnetic alloy particles have always been of interest, particularly because of their utility in permanent magnets and magnetic tapes. A major difficulty in using these materials is their tendency to oxidize in air. Iron, made by reducing fine particles of Fe ₂ O ₃ at low temperatures, starts to burn when exposed to air. Fine iron particles which are produced in other ways, for example by reduction from the solution or from electrolytic solution, oxidize more slowly, but are nevertheless so poorly stable that they are practically unusable. Fine particles of the MnBi intermetallic compound are excellent

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Properties as permanent magnets at temperatures around and above room temperature, but have not been used commercially to a large extent because of their instability. Certain intermetallic compounds made from transition metals from the 3rd period and rare earths, such as Co ₅ Sm, have recently aroused lively interest because of their extremely high coercive forces in finely divided form. However, their instability in this form is so great that a sample in air at room temperature can be completely oxidized within 24 hours.

Various methods have been developed to attempt to slow the oxidation of such particles or to prevent it if possible. For example, immersing the particles in acetone, or benzene grants other organic solvents for a limited time resistance to oxidation. In the USA patent 3.206.338 a process for the production of non-self-igniting ferromagnetic particles has been described, after an aqueous medium an iron salt and an alkali metal borohydride were added to a ferromagnetic precipitate of essentially iron and small fc to produce amounts of boron and oxygen. Such particles are not self-igniting, but it is advisable to to apply a metal coating in order to achieve long-term durability. Metal coatings grant long-term durability, but are in some cases unusable, e.g. if it is necessary to partially or to be completely eliminated when the particles are deformed into a compact body, thereby making a larger one Maintain percentage of magnetic material in the body. In addition, metallic coatings are useless if the Use of the particles for magnetic tape recording tasks is intended because a significant mechanical

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Wear between the coated particles and the playback head occurs.

Another method of stabilizing fine iron particles has been described in U.S. Patent 3,228,381, after which, in a complicated procedure, an organic iron compound is thermally broken down in a polymer solution will. It is believed that the polymer is preferentially absorbed onto the particle surface and thereby agglomeration avoided and a stable suspension of the particles is achieved. However, it is difficult to get out of suspension to obtain a useful magnetic material. That will by U.S. Patent 3,290,252 which is one Describes method of removing the polymer from the suspension, making the trapped ferromagnetic Material is concentrated. According to this method, resistant particles can be obtained over long / conductive spaces, however, the process is too complicated to be used for commercial purposes. Further this method is useless if the use for magnetic tape technology is intended, namely because the needle-shaped particles required for strip material cannot be produced in this way.

In the context of the invention it has been found that ferromagnetic Metal particles can be made resistant to oxidation for an unlimited period of time by a process, after which the metal particles are contacted with a solution of a monomer under conditions under which the particles remain practically non-oxidized, whereupon the resulting monomer-coated particles from the excess monomer solution can be removed and thereafter the monom:! - coatings of particles are polymerized, in that et./a the ioriomer-coated particles in a, ~ e-

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a suitable dispersant which is practically immiscible with the monomer is added and the mixture is heated or is irradiated.

Ferromagnetic particles that work in an advantageous manner have been coated in accordance with the invention include any ferromagnetic metal particles formed in accordance with Requirement to have been made resistant to oxidation.

According to a preferred embodiment of the invention, polymer-coated iron particles can be produced by acicular ^ * - Fe20g in hydrogen under such conditions is reduced so that the particles, when they are reduced to metal, about their size and needle shape maintained that thereupon the reduced particles directly from the hydrogen atmosphere into the monomer solution are transferred and that the procedure indicated above is then followed.

The resulting, flowable polymerized individual Particles have an unlimited resistance to oxidation under normal storage conditions and can come in various Wise used, for example for powder cores, permanent magnets and recording media, such as magnetic tapes and the like ..

Fig. 1 shows a graph of the field strength (H) in kilooersted versus the magnetic moment (»g) in E.M.E, per gram for four different types of ferromagnetic particles according to the invention, which are in different Starch are coated with polymerized tetraethylene glycol dimethacrylate;

Fig. 2 represents an electron photomicrograph one produced by the process according to the invention

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typical acicular ferromagnetic particle;

Fig. 3 is a prospective view of a magnetic recording tape used for magnetic sound recording and reproduction suitable is; and

FIG. 4 is a graph similar to FIG. 1 for a ferromagnetic particle material according to the invention, which has been coated with polymerized divinylbenzene.

The magnetic material of the end product can consist of any ferromagnetic metal that is required in Way has been made resistant to oxidation. Among the particularly interesting ferromagnetic metallic ones Substances include iron, cobalt and nickel and all those numerous ferromagnetic alloys that are based on build up at least one of these substances. Typical non-magnetic alloy components are known to include Aluminum, manganese and carbon. The inter-metal connections MnBi and the large one are also important Group of ferromagnetic substances based on intermetallic compounds of transition metals of the 3rd period, such as Fe, Co and Mn, and rare earths Sc, Y, La, Ce, Pr, Wd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Cp. Typical compounds of the latter group can be represented by A RE, where A is a transition metal or more transition metals, RE one or more rare earths and χ the value 2 or a value between 5 and 8.9 has.

The method of making the metal particles is not critically, for example a massive part of the substance can be ground, the metal ions can be in solution either chemically reduced, for example by the action of a

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reducing agent (e.g. hydrazine) or electrolytically, for example by depositing metal particles on a cathode. A method for separating iron particles in a Qiecksilberkathode is described in US Patent 2,974 · 10 ¹ I. Preferably, however, because of the ease of implementation, particles of metal compounds are reduced in a reducing atmosphere, for example in an atmosphere containing hydrogen or carbon monoxide. The oxides, sulfates and other compounds which produce the oxides on heating are preferred. Typical reduction

P conditions for iron oxide are temperatures between 200 and 500 ^° C. for 2 to 48 hours. Below 200 ^° C., the reduction rate is so low that it becomes economically uninteresting, while above 500 ^° C. a noticeable sintering together of the particles can occur. If the agglomeration of particles is to be avoided by even a slight sintering, a temperature of more than 400 ^° C. must not be exceeded for about 48 hours. Acicular iron particles are easily obtained by reducing acicular iron 'd-Fe20 ~ particles under conditions that do not deform into rounded particles. There are also temperatures

Fe up to 500 ⁰ C for a maximum of 24 hours. When the particles have reached the metal state, they are preferably transferred directly from the reducing atmosphere into the monomer-containing solution because of their tendency to self-ignite. If, on the other hand, the metal particles are produced by reduction from the solution or by electrolytic reduction, they are not self-igniting and, because of their less pronounced instability, can therefore be quickly transferred from an oxidizing atmosphere into the monomer solution. However, in order to eliminate any possibility of oxidation, it is more beneficial to remove the particles from the solution (e.g. by filtering) in a drying room or some other non-

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To bring oxidizing atmosphere and to transfer it directly from the non-oxidizing atmosphere into the monomer solution.

The thickness of the ionomer coating can be controlled by varying it diluting the monomer in a suitable solvent such as benzene, toluene or ethylene dichloride. The thickness of the coating is important as some of the magnetic properties of the end product of the weight fraction (in%) of those present in the product ferromagnetic. *! Depend on particles. For example this is magnetic moment directly proportional to the weight percent of the ferromagnetic material present and can can be increased by decreasing the layer thickness, e.g. by increasing the dilution of the monomer in the solution will. Of course, the coating must remain strong enough to allow the particles to resist oxidation is achieved, and therefore the monomer: solvent ratio should generally not exceed the value 1: 15 go out. However, if the metal particles are particularly sensitive to oxidation, as is the case with rare earths containing particles there? general composition AgRE If this is the case, a monomer: solvent ratio of 1: 6 should generally not be exceeded. The solvent should be removed prior to polymerization in order to ensure the porosity of the finished polymer coating to avoid. This can be achieved in a satisfactory manner if, for example, the coated particles 2 Can be placed under vacuum for up to 10 hours.

Monomers which can be used according to the invention are all difunctional or polyfunctional monomers, which on the one hand protect the particles sufficiently against oxidation before the polymerization protect and, on the other hand, easily pass through polymerization into a state in which they practically remove the particles.

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Make the table resistant to oxidation for an unlimited period of time.

Difunctional or polyfunctional monomers, such as tetraethylene glycol dimethyl acrylate, Ethylene glycol diacrylate and divinylbenzene are generally preferred because they characteristically with or without additional catalysts solely by heating or electron irradiation can be polymerized and good thermal stability during polymerization because of their crosslinkability own. The degree of polymerization required for adequate particle persistence is generally depends on the thickness of the monomer coating and on the extent of crosslinking of the resultant Polymerization product from and can be easily determined. When flowable ferromagnetic individual particles are to be obtained, the polymerization is preferably carried out in a dispersant that contains is practically immiscible with the monomer and is stable when heated at the polymerization temperatures is. Dispersants that can be used include mineral oil, silicone oil, and chlorinated hydrocarbons as well V / ater. Divinylbenzene as a monomer is preferably dispersed in ethylene glycol or a similar material, because there is a certain miscibility with the other dispersants mentioned.

Such flowable individual particles can then be deformed into compact structures, for example, at elevated temperature or pressure or under application increased pressure and temperature are pressed in a mold. The conditions can be chosen in this way be that during compression molding the polymer coating burns off partially or completely, depending on what is being pressed! 1 intended use. Generally will

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however, the aim is to keep the particles against each other to keep isolated, for example in the manufacture of magnetic recording media, permanent magnets and soft iron inductive cores with low eddy current losses.

Components made from polymer-coated particles can also be made by dispersing the particles into a deformable non-magnetic Matrix, such as a curable resin or plastic. A preferred method for Making a body of polymer coated particles for magnetic sound recording consists in making the particles with an elastomeric polymer such as a chlorobutadiene (e.g. neoprene) or a polyether urethane (e.g. Adipren) and mixed with suitable hardeners, lubricants and plasticizers by pouring or otherwise brought into the correct shape and hardened into a solid body.

If no flowable individual particles are required, the process step, the monomer-coated particles in a dispersant prior to the deforming step or prior to embedding the particles in a component polymerize, be left out. In favorable cases, the particles can first enter the desired system installed and then polymerized their coatings in situ will. For example, in the manufacture of thin magnetic tapes, a monomer-coated particle provided tape can be guided through an irradiation zone in which the polymerization takes place.

Example I.

Iron sulfate powder was heated for five hours at 70 ° C

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- ίο -

anneals to produce ck _{-Fe 2} O _3. The mean particle diameter of the Fe ₂ O ₃ was about 0.1 micron (1000 Å). The oxide was reduced to metallic iron by heating in a hydrogen-filled reaction tube at 350 ° C. for a period of about three hours. The iron powder was allowed to cool to room temperature in the hydrogen atmosphere and was then transferred from the hydrogen atmosphere into a beaker with tetraethylene glycol dimethyl acrylate, which was located in the cold end of the reaction tube. This procedure was repeated using solutions of the monomer in benzene in a monomer: benzene ratio of 1: 1, 1: 6 and 1:15, respectively. After removal from the reaction tube, the monomerbeschichteten samples were kept for several hours at 6O ⁰ C in a vacuum oven, to remove the benzene. The samples were then placed in beakers containing mineral oil and stirred vigorously to prevent clumping between the particles. Subsequently, the Hineralölaufschwemmungen were heated to 110 ⁰ C and maintained at this temperature for four hours to conduct polymerization. After the suspensions had cooled, the particles were separated off by filtration and any remaining mineral oil was removed through vials with hexane.

X-ray diffraction measurements indicated that apparently no other metal phase than iron was present. Of the Influence of the monomer dilution on the magnetic moment is shown in FIG. 1, in which the magnetic moment in E.M.E./g a compact component of the invention obtained material is plotted against the field strength (H) in Oersted of the measuring field. You can see that with increasing Monomer dilution and thus decreasing strength and decreasing weight fraction of the polymer coating magnetic moment increases, and realizes that there is a

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-u-

clear control of the thickness of the polymer coating and the magnetic moment of the finished product Change the monomer dilution in the solvent leaves. The pressed parts can be made according to the invention Material provided coercive forces of approximately 250 oersteds.

Example II

The procedure was the same as in Example I, however was ^ -Fe ^ O., with average dimensions of 0.1 χ 0.5 micron used as output. Fig. 2, a Drawing after an electron microscope photograph, shows a typical ferromagnetic particle 10, the is coated with polymer 11; you can see that the dobby shape maintained unci no significant change in the Particle sprout has occurred. Such needle-shaped Particles are very useful in making magnetic tape for sound recording; they are used for this purpose generally incorporated into a non-magnetic support matrix. For the resulting from Examples I and II Materials were measured for resistance to oxidation and the results in Table I. summarized. Minor occurrences of oxidation were seen in values of the percent change in magnetic Moments (<& £) expressed.

Table I.

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Table I.

From initial material 2 ° 3

Ratio of monomer undiluted 1: 1 1: 6 1:15 to solvent monomer

Investigation conditions

97 ^U C, 67 hours in air

SO

C, 40 hours in air

MO

MO

180 C, 18 hours in air

Immersion in 2N HCl

0 VSO

VSO VSO

VSO

Legend:

S - resistant 0 - oxidized

VSO - very slightly oxidized MO - moderately oxidized SO - easily oxidized

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- 13 -

It is found that the polymer-coated particles are practically completely oxidation resistant at temperatures up to about 140 ⁰ C after 40 hr. Exposure in air. Very slight oxidation occurs after 67 hours in air at 97 ° C only with a 1:15 ratio between monomer and benzene.

Example III

A sound recording tape was made by mixing the particles from Example II with polyether urethane, a curable Elastomer, produced in connection with minor additions of hardening and shaping aids; the Mixture was poured and cured. The tape was wrapped around a cylinder so that a continuously revolving A recording surface was created in the manner of the illustration in FIG. 3, according to which the tape according to the invention 12 is stretched over the cylinder 13. The human voice is recorded and reproduced through use the top of the belt 12.

Example IV

The procedure of Example II was followed, but the iron powder was poured into a beaker with undiluted divinylbenzene. The coated particles were suspended in ethylene glycol, the whole was then heated to 125 ° C. and kept at this temperature for four hours to carry out the polymerization, then washed with water and dried. The. Oxidation resistance was obtained by heating the particles, which had been kept at room temperature for 65 hours, to 110 ° C. for 65 days. and to 130 ⁰ C for a further 65 hours.

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checks. The results are shown in Fig. 4, in which the magnetic moment is plotted against the field strength. The comparison of the record with that for the undiluted monomer according to FIG. 1 shows no noticeable decrease in the magnetic moment, which is an indication represents for undetectable oxidation.

Example V

CorSm chunks were first ground in benzene in a ball mill for 18 hours, after which an equal volume of the monomer tetraethylene glycol dimethyl acrylate was added and the sample was ground in the ball mill for a further half an hour. The mixture was then filtered and the benzene was stripped off in vacuo. Thereafter, the monomerbeschichteten particles were dispersed in mineral oil and the polymerization dutch five hours of heating to 110 ⁰ C. Magnetic measurements on this coated material showed a coercive force of 10,000 Oersted and a magnetic moment of about 70 EME / g. Thereafter, the coated material was kept in air for 6 5 hours at room temperature, then in air for 48 hours at 110 ^° C. The measurement of the magnetic moment was repeated and it was found that it kept the same value within the experimental error so that a noticeable oxidation had not occurred.

Example VI

Cobalt hydroxide (Co / Oh ₂ ·)) was reduced to cobalt by heating in hydrogen at 350 ° C. for six hours. The reduced particles were dissolved in a solution of one part of tetraethylene glycol dimethacrylate in six parts of benzene

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given. The benzene was removed and the coating polymerized as described in Example I. By electron microscopic methods, the mean size of the reduced particles was determined to be 0.05 microns. The magnetic moment of the coated particles was found to be 93 EME / g, the intrinsic coercive force was 1,800 Oersted. After heating the coated particles in air at 97 ° C. for 67 hours, the magnetic moment was 91 EME / g, ie no noticeable oxidation had occurred. After heating to 140 ^° C. for a further HO hours, a certain degree of oxidation was found to occur as the magnetic moment fell to 58 EMU / g.

Example VIl

Powdered alloys of the composition Coq hF ^e ng were prepared as follows: an aqueous solution of iron sulfate and cobalt sulfate was added to saturated oxalic acid solution, whereby iron and cobalt oxalate were precipitated side by side. The precipitate was converted into the oxides by heating in oxygen (1 hour at 300 ^{° C.).} Reduction and polymer coating were then carried out in the manner described in Example IV. The mean particle size was measured to be 0.1 microns. The Eigenkoerzxtivkraft was 900 Oersted and the magnetic moment 93 EME / g. After 67 hours of heating to 97 ° C. in air and subsequent heating to 140 ° C. for 15 3 hours and finally to 166 ° C. for 24 hours, the magnetic moment was 88 EME / g, ie the oxidation was negligible.

The following examples illustrate a technique by which powder-form materials suitable for a coating

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alloys can be produced easily.

Example VIII.

Two solutions as indicated below were mixed and the resulting mixture was ^{heated to 80 to 85 °} C. for two hours.

Solution 1 solution 2

CoCl ₂ .6H ₂ O, 25 g H ₂ O 750 cm ³

e ₂ O ₃ 100 g

The resulting solid material was filtered, washed and dried ^{at 60 ° C. in vacuo.} After reducing the material by heating it to 350 ° C. (cf. Example I), a powdery alloy with the approximate composition Fe _{n fl} , Co _{n 17} was obtained.

Example IX

The procedure given in Example VIII was followed, but 2 5g Ni / NOg) .6H ₂ O were added to the solution. The resulting alloy powder had an approximate composition of Fe _{Q 86} Co _{0 w} Ni _Q

NH ₁₊ OH 250 cm NH ₁₊ Cl 3 , 5 g H ₂ O 150 cm

The invention has been described in terms of a limited number of embodiments. As part of the Invention, however, there are a number of modifications which are readily apparent to the person skilled in the art. To the

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For example, it may prove advantageous to coat the metal particles by the monomer or to promote the monomer solution by adding at least one wetting agent.

(Patent claims:

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Claims (1)

Patent claims : / lJ Process for the production of oxidation-resistant ferromagnetic Metal particles by coating the ferromagnetic metal particles with a polymer, characterized in that for coating the metal particles with a liquid solution containing a difunctional or polyfunctional monomer, be coated under conditions that are practical do not cause any oxidation of the metal particles, that the liquid-covered particles of the Excess liquid solution are separated and that the monomer in the particle coating is polymerized. 2. The method according to claim 1, characterized in that the metal particles by reducing metal ions can be generated in an aqueous solution. 3. The method according to claim 2, characterized in that the reduction is carried out electrolytically. Method according to claim 1, characterized in that the metal particles are produced by heating particles, which contain at least one metal compound are generated in a non-oxidizing atmosphere, the at least hydrogen or carbon monoxide as a reducing gas for reducing the particles 109826/148 ^ contains, and that the reduced particles directly from the non-oxidizing atmosphere into the monomer be convicted. A method according to claim k, characterized in that the metal particles are produced by heating an iron oxide in an atmosphere consisting essentially of hydrogen for 2 to 18 hours at a temperature of 200 to 500 ° C. 6. The method according to claim 5, characterized in that needle-shaped χ - ^Fe 2 ° 3 is heated for ^{up to 2} ^ hours. 7. The method according to any one of the preceding claims, characterized in that the liquid solution is monomer in a ratio of up to 1 part of monomer to 15 parts of solvent. 8. The method according to any one of the preceding claims 1 to 7, characterized in that before the polymerization process virtually all of the solvent is removed from the particle coatings. 9. The method according to claim 8, characterized in that the solvent is removed by heating in a vacuum will. 109826 / H8A 10. The method according to any one of the preceding claims, characterized in that the monomer by introducing the liquid-coated particles in a dispersant and heating up to a temperature, take place during the polymerization, is polymerized. 11. The method according to claim 10, characterized in that that the monomer tetraethylene glycol dimethyl acrylate or ψ ethylene glycol diacrylate and as a dispersant Mineral oil, silicone oil or a chlorinated hydrocarbon is selected. 12. The method according to claim 10, characterized in that the monomer divinylbenzene and the dispersant Ethylene glycol is used. 13. Product manufactured according to the method specified in one of claims 1 to 12. 14. The method according to any one of the preceding claims 1 to 12, characterized in that the particles according to polymerizing the monomer coatings surrounding the particles to form pressed parts. 15. The method according to claim 14, characterized in that the pressed part is heated. 109826 / U8A 16. The method according to claim 15, characterized in that the heating is so intense that the polymer coatings at least partially removed. 17. According to the method specified in claims 14, 15 or 16 produced pressed part, 18. The method according to any one of the preceding claims 1 to 12, characterized in that the monomer coatings of the particles are polymerized by dispersing of the particles in a deformable non-magnetic Matrix is carried out. 19. The method according to claim 18, characterized in that needle-shaped particles in chlorobutadiene or polyether urethane be dispersed as an elastomeric polymer. ι 20. According to the method specified in claims 18 or 19 manufactured product. 109826 / U84 Blank page