Composition adapted for inductive heating and method for using same

Patented Jan. 22,1946

2,393,541

UNITED STATES PATENT OFFICE

2,393,541

COMPOSITION ADAPTED FOR INDUCTIVE HEATING AND METHOD FOR USING SAME

Fred Kohler, New York, N. Y., assignor to Induction Heating Corp., New York, N. Y., a corporation of New York

No Drawing. Application May 21, 1943, Serial No. 487,984

Claims.

This invention relates to compositions and materials adapted to be heated by high frequency magnetic induction and to methods for inductively heating non-conductive material. Among other possible uses, the invention is particularly adapted for the inductive heating of adhesives, plastics and other non-conductive materials, which in the course of manufacturing processes, are required to be heat-treated within a predetermined limited temperature range to effect polymerization, curing or softening for various molding or adhesive purposes.

Methods utilizing electrostatic fields have recently been perfected for heating assemblies of dielectric material which are to be secured together by thermosetting phenolic resin glues and the like. A layer or film of the glue is, for example, interposed between two pieces of wood or other dielectric material, and the assembly then subjected to an electrostatic field of extremely high frequency, the assembly being held together under pressure until the glue is heated and thereby hardened. This process requires considerable time, in that not only the glue but also the objects to be secured together are necessarily heated dielectrically up to the desired thermosetting temperature for the glue. Also there is a considerable waste of power in heating the whole assembly, and in some cases, as where the objects are of wood for example, the heating thereof may cause excessive drying or other undesirable modification.

Since objects such as wood and the like, and the adhesives available for securing same together, are non-conductive and also non-magnetic, they cannot be satisfactorily heated through high frequency magnetic induction by any method heretofore available so far as I am aware. I have discovered, however, that upon mixing with the glue, adhesive, plastic or other non-conductive material, finely divided magnetic particles of the ferromagnetic class, it is possible to very quickly heat the mixture by magnetic induction, i. e., by subjecting the mixture to a high frequency electromagnetic field, which quickly heats the magnetic particles because of the hysteresis effect therein, and consequently the hot particles in turn quickly heat the associated non-magnetic material to the desired temperature. Iron filings, for example, might be used as the magnetic particles, but since iron particles become quickly heated to red10 (Cl. 20—0.5) ness in a strong high frequency magnetic field, it is undesirable to use iron for the purpose, unless the nbn-magnetic material to be heated is capable of withstanding the temperatures of red hot iron particles without injury. Since it is too difficult in practice to uniformly control and limit the intensity of high frequency magnetic fields as applied to all of the particles, it is not practical to use iron filings for the purpose with the usual glues and adhesives.

I have discovered, however, that by properly selecting other kinds of finely divided metal particles and alloys having ferromagnetic properties, the heating temperature in the presence of a high frequency magnetic field may be readily limited and controlled to the particular temperature or temperature range necessary for the heat-treating of materials such as glue, adhesives or plastics. As is well-known, when any of the various metals or alloys of the ferromagnetic class become heated to a particular temperature, known as the Curie point therefor, then the ferromagnetic qualities cease. As a result, any further application of a high frequency field is substantially ineffective to cause further heating if the particles are small. Masses of metal of substantial size may be heated by magnetic induction due to the setting up of eddy currents, as well as because of hysteresis effects in the case of metals of the ferromagnetic class. In accordance with my invention, however, the ferromagnetic particles being finely divided and effectively insulated from each other by the adhesive or other dielectric material mixed therewith, there is no substantial heating above the Curie point due to eddy currents. Accordingly the heating effect is substantially confined to that resulting from the effects of hysteresis, and consequently is limited to the Curie temperatures for the particular magnetic particles used. Thus a layer, coating, or film of adhesive, or a mass of plastic material when mixed or associated with particles having a properly selected Curie point, may be heated by magnetic induction to the exact temperature or temperature range necessary for curing, molding or softening the non-conductive material without danger of overheating or burning the same. In fact, particles may be chosen such that the heating effect is discontinued upon reaching the Curie point for the particles, so that the non-conductive material is protected against

9,393,641 be varied from several thousand to several hundred thousand. The work colls and associated equipment by which the high frequency field is applied to the assembly, may for example be of 5 types such as disclosed in U. S. patents to Goodridge, No. 2,308,240 and to Dravneek, No. 2,321,189. The type and shape of work coil used will of course depend upon the external shape of the assembly being glued together and in general for 10 most rapid and economical results, the turn or turns of the coil should be so shaped as to come as close as possible to the layer of glue mixture being heated.

The relative proportions of the magnetic par15 tides and plastic or adhesive material, are not critical and may vary considerably depending upon the characteristics of the adhesive or plastic and the amount of high frequency power available, and the desired.time of treatment. In gen20 era! it has been found desirable that the mixture contain in the neighborhood of about 20 to 30% of magnetic particles or alloy.

While the degree of fineness of the particles is not critical, it is believed that for most purposes, 25 the best results may be obtained by using particles or powder of very fine mesh, Tor example 200-300 mesh or less, which will assure the maintenance of good suspension in the mixture.

It will be apparent that the various features 30 of the invention are applicable for use with adhesives, resins and plastics other than the particular examples above given and with materials which are thermoplastic, as well as those which are thermosetting, and with both organic and in35 organic adhesives which are adapted to be rendered adhesive by heating. The invention may also be used in ways other than by mixing the magnetic particles with liquid or semi-liquid nonmagnetic material. For example, films of ther40 moplastic or thermosetting compositions may be used with the magnetic particles sprinkled over the surface thereof or impressed into the surface, or the particles may be interposed between two or more such films. Plastics or resins within 45 or upon which the particles may be used according to the invention, include formaldehyde urea, “Glyptal,” vinyl resins, cellulose acetate or nitrate resins, and methacrylate resins.

Films of plastic material have also been se50 cured together in accordance with one phase of the invention, by interposing therebetween an area of wire screening, and then subjecting to a high frequency magnetic field. That is, the metal of which the wire is formed was selected from 55 those herein disclosed as having a suitably low Curie point. If the wire is of small diameter, heating due to eddy currents is minimized.

In some manufacturing operations, such as in the assembly and gluing together of wooden 60 objects, it may be found desirable to use the above types of alloy resin mixtures with high frequency magnetic induction applied only at intervals along the joint. In this way, what may be termed “spot gluing” may be quickly 65 accomplished so that the securing clamps may then be taken off for prompt reuse. Thereafter, if desired, the remaining areas of glue may be heated either dielectrically or by high frequency magnetic induction, or otherwise cured if de70 sired over a longer time interval.

The invention affords a convenient means for securing objects together in such a way that they may be again readily and quickly separated by heating the adhesive layer without substanT5 tially heating the objects. For this purpose, variinjury if it is of a character which might be injured merely by more prolonged heating without a further temperature rise.

Various other features, objects and advantages of the invention will be apparent from the follow- 5 ing more detailed description. The invention resides in the novel compositions, methods and combinations of method .steps hereinafter described by way of examples, and as will be more particularly pointed out in the appended claims. 10

The invention has been successfully carried out for gluing together articles of wood with phenolic formaldehyde resins of the thermosetting types heretofore used in connection with dielectric heating. That is, the proper normal thermosetting 15 temperature range for such resin glues is first determined, and then the glue is mixed with a substantial amount of finely divided metal particles of the ferromagnetic class, and selected as having a Curie point within or not excessively 20 above siich normal thermosetting range. This mixture is then interposed in the usual way between the articles which are to be secured together. The assembly may be clamped or otherwise held together under pressure, as hereto- 25 fore, and then subjected to a high frequency electromagnetic field. Because of hysteresis effects, this will cause the magnetic particles to be quickly brought up to the Curie temperature. Consequently the resin or adhesive material associated 30 or mixed therewith will also be promptly heated to the desired curing temperature, and no higher. All danger of burning or destructive decomposition is thus avoided, even though the high frequency field is continued indefinitely. The ar- 35 tides of wood or other dielectric material which are to be secured together, moreover will not be subjected to any heating effect except temporarily at the surfaces immediately adjacent the magnetic particles. Thus the heating effect is con- 40 fined to the seam or joint, and heating of the entire assembly, as occurs with the dielectric heating method, is avoided with a substantial saving of power and with avoidance of drying out or otherwise injuring the articles being secured to- 45 gether. With a given amount of available power, there is also a substantial reduction in the time required for the heating step.

In carrying out one example of the invention, well-known phenolic formaldehyde resin glues 50 were used in a semi-liquid state, available on the market under the trade name Catabond” No. 400 or No. 590. These were mixed with finely divided nickel-copper alloy (20% copper and 80% nickel) in a proportion of approximately 30% 55 of the metal powder to 70% of the glue. Wooden articles to be secured together were coated with this resin alloy mixture and then while being pressed together, were subjected to a high frequency magnetic field sufficient to heat the 60 metal alloy particles up to their Curie temperature, which in this case is approximately 170° C. This caused the glue to polymerize and harden in about 10-12 minutes or less, without heating the adjacent bodies of wood appreciably, the heat 65 being generated directly in the glue mixture and substantially confined thereto. The high frequency induction heating equipment was of a type now commonly used for induction heating purposes such as brazing and heat-treating of 70 metal, the generator having a capacity of 20 kilowatts and a frequency of in the neighborhood of 375,000 cycles. For assemblies of considerable size however, a 2-kiIowatt generator will be found sufficient. The frequency is not critical and may

3,888,041 ous well-known thermoplastic materials may be used either in the form of liquids mixed with the magnetic particles or in the form of films associated with the particles and interposed between the objects to be secured together. That g is, the objects may be clamped together with the interposed thermoplastic adhesive, containing or associated with the magnetic particles, and then subjected to a high frequency magnetic field, and thereafter allowed to cool. Then if it be- 10 comes necessary to later separate the objects again, the assembly is again subjected to a high frequency magnetic field and the objects are separated while the interposed thermoplastic is heated to its softening temperature. IS

The invention may be used with the various types of inorganic material such as sodium or calcium silicate adhesives of known types, mixed with finely divided particles having a Curie point at the temperature customarily used for the ap- 20 plication of such adhesives under heat.

As is well known, certain alloys of the ferromagnetic class, such as various alloys of manganese, when once heated up to the Curie temperature, do not reacquire their ferromagnetic 28 properties until after they have been cooled to a temperature very substantially below the Curie point. For example, in the case of certain manganese alloys, the ferromagnetic properties are not reacquired until the alloy is cooled far below 30 room temperature. The use of metal particles of alloys of this type is advantageous with the present invention in conjunction with adhesives or other non-magnetic material which might be injured by any prolonged heating even at tern- ³⁵peratures below the Curie point or by repeated heating up to the Curie point of the metal particles. The same property may also be utilized to insure against any injury to the particular adheslve used in case a finished assembly should accidentally be repeatedly subjected to a high frequency magnetic field. That is, the initial application of the high frequency field for heating the adhesive up to the Curie point of the _4gmetal particles will terminate the generation of heat in the assembly so that it cannot be inductively heated any longer, nor heated again inductively, by high frequency, unless the assembly is first cooled to a very low temperature for re- go establishing the ferromagnetic properties of the metal particles. By using such particles the power output or load of the high frequency generator discontinues automatically upon the initial heating to the Curie point. 55

The principles of the invention may be used in connection with molding or shaping operations, by mixing the metal particles with plastics adapted to be polymerized, hardened or otherwise cured or shaped upon heating in a mold. In this case ⁶⁰the mold may be formed of a non-conductive material whereby the mixture in the mold may be subjected to a high frequency field by placing a high frequency work coil adjacent the exterior of the mold.

Listed below are numerous examples of metals and alloys having various relatively low Curie points and thus particularly adapted for use as the magnetic particles in accordance with the in- γθ vention. Those having the higher Curie points, of course, are not adapted for use with organic adhesives. The invention is not intended to be confined to the use of these particular illustrative examples. 75

Nickel-chromium alloys

<td> Per cent chromium (remainder nickel)</td><td> Curie point, degree· C.</td>

<td> 0......................................</td><td> 360</td>

<td> 2.5............._................._ .</td><td> 190</td>

<td> 5....................................</td><td rowspan="2"> 20</td>

<td></td>

Heusler alloys (manganese-aluminum-copper)

<td> Manganese, per cent</td><td> Aluminum percent</td><td> Copper percent</td><td> Curie point, degrees C.</td>

<td> 13.9</td><td> 7.2</td><td> 78.9</td><td> 205</td>

<td> 6</td><td> 13.2</td><td> 80.8</td><td> 305</td>

<td> 26.5</td><td> 16.3</td><td> 57.2</td><td> 525</td>

<td> 11.6</td><td> 25.0</td><td> 63.4</td><td> 5-10</td>

Nickel-copper alloys

<td> Per cent copper (remainder nickel)</td><td> Curie point, degrees C.</td>

<td> o..........................................</td><td rowspan="8"> 360 320 270 220 170 120 70</td>

<td> 5........................................</td>

<td> 10......................................</td>

<td> 15.............................</td>

<td> 20..................................</td>

<td> 25........................................</td>

<td> 30.....................................</td>

<td></td>

<td colspan="2"> Nickel-manganese alloys</td>

<td> Per cent manganese (remainder nickel)</td><td> Curie point, degrees C.</td>

<td> 0......................... ..</td><td rowspan="5"> 360 270 170 70</td>

<td> 5...............................</td>

<td> 10.........................</td>

<td> 15....................</td>

<td></td>

<td colspan="2"> Nickel-aluminum alloys</td>

<td> Per cent aluminum (remainder nickel)</td><td> Curie point, degrees C.</td>

<td> 0-..........................</td><td rowspan="4"> 360 230 70</td>

<td> 2.5........................ ......</td>

<td> 5......................</td>

<td></td>

From the above it will be apparent that a wide variety of alloys may be selected having the proper Curie temperature for heat-treating any of the commonly used adhesives, plastics, etc. If higher temperature ranges are desired, it is of course possible to use particles of iron, cobalt or alloys thereof. Iron has a Curie point of 770° C., and cobalt 1125° C. Particles of these metals are thus adapted for incorporation in asbestos, mica or fire clay mixtures for example, to form stove plates or the like, capable of being heated to limited predetermined temperatures by high frequency magnetic induction.

While the invention has been described in detail with respect to certain preferred examples, it will be understood by those skilled in the art after understanding the invention that various changes and modifications may be made without departing from the spirit and scope of the invention, and it is intended therefore, in the appended claims to cover all such changes and modifications.