US 3451934 A
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June 24, 1969 H. c. HUBBARD PROCESS OF MAKING MOLDED MAGNETIC MATERIAL Filed Feb. 9, 1968 ADD EQUAL PARTS ACETONE AND BAKELITE POW DER 30 SECONDS ADD EQUAL PART IRON POWDER FIG STIR 2 MINUTES ADD MORE ACETONE s'rnz UNTIL COLOR IS UNIFORM (2 MINUTES) DRY AND POWDER MOLD TO DESIRED SHAPE FIG- 3 FIG. 2
IN'VENTOR. HAROLD C. HUBBARD ATTO RN E Y5 United States Patent U.S. Cl. 25262.54 1 Claim ABSTRACT OF THE DISCLOSURE A process for making magnetic molding material for use in fabricating cores for magnetic devices, such as the friction electromagnet in an electrical brake, wherein finely divided metallic magnetic particles containing iron are highly dispersed in and individually coated with an electrically insulating binder material. The binder material, such as phenol formaldehyde resin in powder form, is first added to a solvent such as acetone which is a solvent for said binder but a non-solvent for said particles. Then, While said binder is dissolving in said solvent, the magnetic particles are added to the binder-solvent mixture. The iron particles are broken up and highly dispersed in said mixture by the dissolving action of the solvent on the binder. The resultant dispersion is then dried and powdered to render it conveniently moldable to the desired shape of the finished article.
This application is a continuation-in-part of my copending application Ser. No. 480,869, filed Aug. 19, 1965, now abandoned, and entitled, Process of Making Molding Material.
This invention relates and in particular to a ing material.
Magnetic molding material has been used in fabricating cores for magnetic devices such as transformers, inductors, tuners and remote control devices. Typically, the molding material consists of finely divided metallic particles, usually iron or iron oxide, in an insulating binder which is usually a resin. Bakelite has been used as the insulating binder, but other materials are suitable.
It has been suggested that the metallic particles should be well dispersed in the material, and that each particle should be individually coated with insulation. Some of the reasons for this are explained in United States Patent No. 1,982,689 to W. J. Polydorotf; i.e., to reduce losses due to eddy currents and hysteresis in the electrical operation of the magnetic device in which the material is used. A material in which the particles are well dispersed and individually insulated from each other is characterized by low hysteresis losses, low eddy current losses, high resistivity and good mechanical strength. Material having these characteristics is desirable for many applications and its utility is not limited to the high frequency field. For example, cores of such magnetic molding material may be used advantageously in remotely controlled electrical brakes for trailers and other vehicles.
Known processes for making magnetic molding material have been deficient because of a tendency of the metallic particles to agglomerate when they are added to the resin binder. This tendency cannot be overcome in a practical manner by stirring or otherwise agitating the mixture.
Accordingly, it is an object of this invention to provide an improved process for making molding material containing metallic particles wherein the particles are disgenerally to molding processes, process for making magnetic mold- 3,451,934 Patented June 24, 1969 persed in a binder more effectively than in prior art processes.
Another object of the invention is to provide a process of the above character in which metallic particles are highly dispersed in resin by taking advantage of tensions which exist in a mixture of resin and solvent in which the resin is dissolving.
A further object is to provide a process wherein by proper timing and sequence of steps the unevenly distributed surface and interfacial tensions produced while resin is dissolving in a solvent are caused to act on metallic particles so as to disperse them in the resin.
In the drawings:
FIG. 1 is a block diagram showing the steps of one embodiment of the process of the invention.
FIG. 2 is a greatly enlarged drawing: made from a photomicrograph showing how metallic particles tend to agglomerate when they are added to resin.
FIG. 3 is a greatly enlarged drawing made from a photornicrograph of a magnetic molding material made by the process of the invention, illustrating the particles well dispersed in the resin and individually coated'with resin.
In accordance with the process of the invention, solid resin, preferably a resin powder, is first added to a solvent for the resin. When the resin begins to dissolve in the solvent, unevenly distributed surface tensions and interfacial tensions exist in the resin-solvent mixture because at that time there are unequal concentrations of resin in solution at different portions of the mixture. Magnetic metallic particles (preferably iron or iron oxide) are added to the resin-solvent mixture while the resin is dissolving and While these unevenly distributed tensions are present. The unevenly distributed tensions act rather violently on the particles and quickly disperse them throughout the mixture. The mixture may be stirred, blended, or otherwise agitated after the particles are added to insure that all the particles are exposed to the action of surface tensions. Nevertheless this does not negate the fact that a definite improvement in dispersion is achieved by the action of uneven surface tensions and interfacial tensions on the particles beyond the degree of dispersion which can be achieved by such agitation alone.
The mixture (a slurry at this stage) is dried; i.e., the solvent is evaporated, either by simply allowing the mixture to dry in room air or by other suitable drying procedures. The mixture dries into pieces of molding material, and these pieces may be powdered and molded at low temperatures and pressures into variously shaped objects as desired. The resulting molded product is composed of hard plastic material having magnetic metallic particles dispersed uniformly throughout, and is characterized by high resistivity, low hysteresis and low eddy currents. It is strong and durable to the extent that it may be used as a friction element in a brake.
When iron particles are dispersed in resin binder according to the above-described process, each particle of iron is coated with sufficient resin binder so that when the material is molded a matrix is formed of iron particles and resin binder that is substantially homogeneous, having no large domains either of iron particles or resin binder. [When a magnetic core is molded from the material, the iron particles are disposed in close proximity to each other, yet a great number of them are electrically insulated from one another. A typical ohmmeter reading of such a magnetic core of 3000 ohms between two points A! apart. While this measurement shows enough metallic contact for establishing a magnetic path through the material, the resistivity is also high enough to prevent high residual magnetism or eddy current losses in any magnetic field use.
Although iron and iron oxide particles have been mentioned as preferred examples of magnetic metallic particles, it is sometimes desirable to use either iron or iron oxide in combination with "another material. For example, iron metal may be alloyed with some other metal to obtain metallic particles having magnetic properties desired for some applications. Examples of such alloys are silicon-iron and nickel-iron. Iron powder may also be mixed with other powders such as silicon powder or quartz powder. Mixed oxides are also useful; for example, iron oxide sintered with copper oxide, nickel oxide, magnesium oxide, manganese oxide, cadmium oxide and/or silicon oxide. Such mixed oxides are known as ferrites, and many different compositions are available.
The preferred resin binder is phenol formaldehyde resin, and suitable phenol formaldehyde resin is sold under the trademark Bakelite. Powdered Bakelite resin is preferred. Other resinous materials which are suitable binders for use in the process of the invention are shellac, amber, polystyrene resin and polyvinyl resin.
The solvent is selected from a wide variety of available solvents depending on which resin and metallic particles are used. It should be one in which the resin is soluble and in which the metallic particles are substantially insoluble. When iron particles and Bakelite resin are used, acetone is the preferred solvent but methyl alcohol and ethyl alcohol are also satisfactory.
The steps of a preferred process are illustrated in the block diagram of FIG. 1. \First, equal parts by volume of acetone and powdered Bakelite resin are added to each other in a receptacle. The amount of resin added to the solvent is in excess of the amount required to produce a saturated solution of resin and solvent at the existing conditions of temperature and pressure. The process may be carried out at ordinary room temperature and pressure. The powdered Bakelite resin starts to dissolve in the acetone, and for a short time, which can be determined by observation for a given set-up, there are unequal concentrations of dissolved resin at different places in the acetone. These unequal concentrations produce unevenly distributed surface tensions and also interfacial tensions between regions of high and low concentration.
Iron powder, obtained by thermal decomposition of iron carbonyl, is added to the resin-acetone mixture while the tensions just referred to are active; i.e., while the resin is dissolving and before dissolved resin has diffused enough to minimize concentration gradients.
By way of example, a typical small quantity mix consists of the following:
Parts Iron powder (obtained from iron carbonyl) 8 Black, powdered Bakelite resin 4 Acetone 6 Four parts of acetone are added to four parts of Bakelite resin, mixed, and within 30 seconds the iron powder is added. After stirring for two minutes the remaining two parts of acetone are added and stirring action is continued until the mixture is all the same color, which occurs in approximately 4 minutes. All of these steps are carried out at room temperature and pressure.
In the above example, a part of the acetone dissolves some of the Bakelite resin, thus producing a different density of liquid as compared to some other portion of the acetone. The rate of diffusion increases with increased concentration. This produces active movements of surface tension. This surface tension exerts a tension upon any adjacent objects and adjacent portions of liquid This tension occurs not only on the free surface but also at the interfaces between the portions of the acetone which are not yet concentrated with dissolved resin and other portions of the acetone which are a concentrated solution. Under a microscope, some of the very small particles (iron balls that compose the powder) seem to perform Brownian movements. In a domain of iron balls each possesses some potential energy and the domain breaks up in a violent manner with a repulsive effect. This insures that each ball of iron is coated. If there is more Bakelite resin than required to obtain such uniform coating, it is surplus and does not mean that some portion of iron powder was left uncoated as would result from ordinary mixing. The connecting chain structure of the binder (Bakelite resin) is more dense and homogeneous than if small groups or domains consisting of uncoated particles were present throughout the structure. However, with the right proportions, there is just enough acetone and Bakelite resin so that when the activity ceases, each particle of iron is coated with no large domains or grains of resin present.
Referring to FIG. 2, if the mixture is viewed through a microscope as the iron powder is first added, it may be seen that some of the iron particles are bunched together in groups such as are shown at '10, 11 and 12 in FIG. 2. However, the tensions existing in the mixture quickly break up these groups of particles in a violent manner and disperse the particles throughout the mixture as shown in FIG. 3. The movement of particles due to the tensions is over in a moment, leaving a slurry of iron particles and resin which may be stirred briefly, say for two minutes as indicated in FIG. 1, to increase dispersion of the particles. In the process of FIG. 1, more acetone is added and the slurry is stirred for two more minutes until its color is uniform. The resin does not all dissolve in the acetone.
The slurry is then allowed to dry into hard pieces, and these pieces are powdered to put the material in a convenient form for molding.
The molding material just referred to may be pressed into objects of desired shape using low pressures of from 1000 to 7000 p.s.i. Preferably, the pressure is increased gradually while heating the material. The pressure will fall off when the binder reaches its softening temperature, which for Bakelite resin is around 270 F., and the molding operation is completed at this point since the resin has cured sufficiently.
A wide variety of articles may be molded using the magnetic material obtained from the process. As previously mentioned, friction elements for brakes may be molded from the material. The magnetic material may be combined with non-magnetic molding material in molding special products. For example, an article may be molded in one step for electrical relays wherein magnetic material forms the core path for the relay and non-magnetic material, such as pure Bakelite resin, forms a frame. It is also possible to mold the magnetic material about an insulated coil, and if desired this combination may be encased in pure plastic.
Preferably in the aforementioned small quantity mix, Bakelite powder is employed having a powder particle size of from minus 20 Tyler screen mesh to minus Tyler screen mesh, and a mixture of fairly coarse to very fine particles Within this range is preferred. In the example as shown in FIG. 1, the Bakelite powder particles employed were a random mix having a particle size of minus 20 to minus 80 Tyler screen mesh. In the aforementioned small quantity mix example, the iron powder particles should range in size from minus 60 Tyler screen mesh to minus Tyler screen mesh, and in the specific example of FIG. 1 the iron powder particles were a random mix having a particle size of minus 60 to minus 80 Tyler screen mesh. For the range of particle sizes described above, the ratios specified in the small quantity mix example and in the steps of FIG. 1 are by volume. However, generally speaking, the ratios, to be independent of particle size, are by weight.
The Bakelite powder particles are preferably a random mix because the molecular action generated is promoted by the small particles dissolving first, then the next larger particles and so on, and this helps keep the surface,
tension activity reacting for the time period necessary to complete the operation.
1. A process for making molded magnetic material in which iron powder particles are individually coated with resin, comprising the steps of (1) adding together about equal parts by volume of phenol formaldehyde resin in powder form and liquid acetone to start dissolving said resin in said acetone,
(2) adding about an equal part by volume of said iron powder particles to the mixture of resin and acetone produced in step (1) while said resin is dissolving and at a predetermined time within about thirty seconds after said resin starts dissolving in step (1) and While sufficient concentration gradients exist such that unevenly distributed tensions in said dissolving mixture act on said particles to disperse said particles in said mixture,
(3) agitating said resin-acetone-particle mixture produced in step (2) while said unevenly distributed tensions are still active in causing said dispersion,
(4) adding more acetone after about two minutes of said agitation in an amount less than that required to produce a saturated solution of said resin in said acetone at the existing conditions of temperature and pressure,
(5) continuing said agitation until said resin-acetoneparticle mixture is a uniform color,
(6) drying the mixture of resin, acetone and said particles before all of the resin is dissolved,
(7) powdering said dried mixture, and
5 (8) molding said powdered material into the desired shape under pressures of from about 1000 to 7000 pounds per square inch while heating the material approximately to its softening temperature.
10 References Cited UNITED STATES PATENTS 2,748,099 5/1956 Bruner 260-37 2,078,808 3/1937 Reardon 260-37 3,240,621 3/1966 Flower 26037 2,989,415 6/1961 Horton 25262.5 2,914,480 11/1959 Hagopian 252-62.5 3,003,965 10/1961 Troelstra 252-62.5 2,508,705 5/1950 Beller 252-62.5 1,783,561 12/1930 Eisenman 252-62.5
TOBIAS E. LEVOW, Primary Examiner. W. T. SCOTT, Assistant Examiner.
US. Cl. X.R. 260--38
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