DE1252178B - Method of heating heat-activated material - Google Patents

Description

Method of Heating Heat Activatable Material For many There is a need for a method for heating heat-activatable substances Degree, the exact location, and the speed and duration of the heating and cooling periods to keep exactly under control, e.g. B. when heat welding flexible materials, such as paper, cardboard, glassine, etc., those with heat-activated organic protective adhesive layers are coated to make cardboard boxes and other containers, such as milk containers or Manufacture cartons or packs for other foods. With such procedures the amount of heating is important. Excessive heat destroys or damages the organic coating and / or leads to charring of the base. Likewise can it can happen that the whole layer of the coating melts completely and into the porous base diffuses into it, which destroys the effectiveness of the adhesive will. In the event of insufficient heating, the topping will not activate properly, and it will no firm bonds are formed. The place of heating is also important, because the topping only needs to be heated in the exact places where it is activated must be, z. B. in the surfaces of the overlapping tabs. With some procedures It is of vital importance that the heat-activatable substance is only in the whole precisely defined areas is heated. Also the speed and duration of the The heating and cooling periods are important. When the heating rate too slow and the heating time is too long, the process works too slowly, to allow sufficient heating.

On the other hand, if the cooling takes too long, the individual Workpieces are not immediately stacked or brought into contact with other surfaces , whereby the ejection speed of the process is impaired. However, if the heating time is too short, the topping will not be sufficiently activated. Similar problems arise with other methods of heating other heat-activated ones Fabrics on.

There are several cases in which these problems arise Process for heating heat-activated substances developed. However, there are no fully satisfactory methods known yet. For example, in U.S. Patent 2,393,541 describes a method by which conductive Metal particles can be distributed in a heat-activated substance, whichever is desired is applied to a base. The whole thing then becomes a magnetic field exposed, whereby the metal particles, mainly as a result of hysteresis losses, heat and thereby the heat-activating Activate the substance. This method is suitable for many purposes; the heating of the metal particles lasts too long, however, in that several minutes are required for the heat-activated To heat the fabric sufficiently. This method is therefore not suitable for high-speed mass production.

According to another method, which is described in U.S. Patent 2 457 758, heat-weldable surfaces can be activated, by making an electrically conductive band of metal particles with the heat-sealable Brings surfaces into contact and there is the action of an electromagnetic The field. The electrically conductive tape is produced by induction (eddy current losses) heated, and thereby the heat-sealable surfaces are activated. Even this method is suitable for many purposes. The electrically conductive one If the tape is heavily loaded with the electrically conductive metal particles, heated in a fraction of a second. But it is difficult to control the degree of heating to be kept under control with this method, since with inductive heating the Temperature of the metal particles further up to the Curie point of the same increases. This can lead to the decomposition of the heat-activated material and to charring of the base and lead to other undesirable effects.

The invention relates to a method for heating heat-activatable Material in which a finely divided, electrically non-conductive antiferromagnetic Substance brought into contact with the material and the product then exposed to it exposed to a high-frequency alternating magnetic field of at least 10 megahertz will.

The ones used to describe the magnetic properties of substances Technical terms are not used consistently in the literature.

The definition of antiferromagnetic substances as used in the framework of the invention is to be understood, and the differentiation thereof from other substances is with W a l d r o n, »Ferrites«, Verlag D. Van Nostrand Company, Ltd., London, 1961, p. 31, and in V a n D e r Z iel, "Solid State Physical Electronics", 1957, Pp. 552/553. These antiferromagnetic substances are generally uncompensated, as described in the work of V a n D e r Z i e 1.

These antiferromagnetic substances are sulfides, oxides, and mixtures of oxides of chromium, manganese, iron, cobalt and nickel, either on their own or together with oxides or mixtures of oxides of the alkali metals (i.e. lithium, Sodium, potassium and rubidium), the alkaline earth metals (i.e.

Beryllium, magnesium, calcium, strontium, barium and radium), the Rare earth metals (i.e. lanthanum and the other elements with atomic numbers of 57 to 71) as well as other metals such as copper, zinc, vanadium, titanium and aluminum, wherein the compound has a certain crystal structure, in particular the structure of spinel, garnet, perovskite or pyrrhotite. The preferred antiferromagnetic Substances are ferrites, i. H. the oxides and oxide mixtures of chromium, manganese, iron, Cobalt and nickel, either alone or in combination with other metals, such as described above, with the crystal structure of a spinel. These substances are dem Known to those skilled in the art.

The antiferromagnetic substances are electrical non-conductors, i. that is, they have an electrical resistance of at least 10-2 ohm-cm. In typical These substances have electrical resistances of up to 109 ohm-cm and more.

It is important that the antiferromagnetic substances are finely divided are present. But it is just as important that these particles are multi-district, d. H. that each particle contains at least one and preferably many »Bloch-Wändett, the magnetization areas separate from each other and are referred to as "districts". These domains are thin laminar transition areas in which the direction of magnetization from the direction on one side of the wall outside the same direction to the direction on the other side of the wall outside the same direction changes, with these directions either 180 or 90 degrees from each other differ. The lower limit of the particle size is determined by the requirement that every particle must be multi-district. The exact size of the district varies with different substances. There are already particles on the order of only 0.01 , u known as multidistrict. The particles can also have sizes of have about 511.

Preferably the particle size is in the range of 0.1 to 5. the Particle size is crucial in the context of the invention, primarily to ensure the appropriate heating properties, but also to ensure a suitable one To achieve suspension of the particles in a liquid medium and thus the out coating consisting of these particles has a smooth handle.

It should be noted that these antiferromagnetic substances are used throughout have the property of being extremely friable. Therefore, ordinary grinding devices, such as ball mills, can be used to produce the fine particle sizes required. In contrast, the previously used electrically conductive metal particles much less friable. If these previously used electrically conductive metals are crushed, the particles melt together, or they "smear" if they approach the particle size used in the context of the present invention. It is extremely difficult to make particles of less electrically conductive metal than produce about 75 su. There are a few cumbersome methods of doing this to produce fine particles of electrically conductive metals, such as. B. certain intricate chemical precipitation processes in liquid media or deposition from the vapor phase on a liquid surface. However, these procedures are complete unsuitable for the purposes of use which come into consideration according to the invention.

For the sake of simplicity, these finely divided, multi-district, non-conductive antiferromagnetic particles hereinafter referred to simply as "antiferromagnetic" Called particles.

The Néel temperature of the antiferromagnetic substances determines the highest temperature to which these substances reach by the action of an alternating magnetic field can be heated. Once the material has reached the respective Nel temperature reached, the antiferromagnetic effects cease, and the temperature of the Materials does not rise any further. This effect is similar to that found in ferromagnetic Metals for the Curie temperature is known.

In the case of antiferromagnetic substances, however, takes place at their Néel temperature a much more sudden transition takes place than is the case with ferromagnetic materials is the case at their Curie temperature. Furthermore, the antiferromagnetic Substances generally have a relatively high and uniform permeability the entire temperature range from room temperature to the Neel temperature, so that at all temperatures between the starting temperature and the desired Considerable heating takes place at the end temperature. This makes an extremely possible rapid heating and at the same time fine control of the temperature.

So the degree or extent of heating is determined by the selection an antiferromagnetic substance with a special Neel temperature precisely controlled.

Fabrics with different Neel temperatures are available in stores, and therefore it is possible for those skilled in the art to select suitable antiferromagnetic substances. Normally an antiferromagnetic material must be chosen, its Néel temperature at least equal to the activation temperature of the heat-activatable to be heated Substance is. Upwards the temperature is only due to the decomposition temperature of the heat-activatable substance and / or the decomposition temperature the Underlay or any other body in contact with it.

The alternating magnetic field must have a frequency of at least 10 megahertz, preferably from 40 to 2500 megahertz. Ordinary electrically conductive metal particles such as they were used in the previously known methods, speak magnetic Alternating fields on the order of thousands of kilohertz or at most 1 Megahertz on, d. that is, they heat up under the influence of such alternating magnetic fields. However, the antiferromagnetic particles used according to the invention speak up frequencies that were previously considered "high frequency" are not sufficient Dimensions. Rather, they must be exposed to a field with a frequency of at least 10 megahertz exposed in order to deal with a practical consideration coming speed to heat.

The particles reach their Neel temperature when exposed to them extremely high frequencies within milliseconds, whereas the ordinary ones electrically conductive metal particles for this purpose on the order of several times Minutes. After removal from the magnetic field or after the interruption of the magnetic field cools the particles within Milliseconds down to room temperature.

So far in connection with relatively low frequency magnetic Electrically conductive metals used in radiation can be used within the scope of the invention, according to which extremely high-frequency alternating magnetic fields of at least 10 Megahertz and preferably at least 40 megahertz are applied, do not use. When the electrically conductive metal particles are exposed to such frequencies sparks and, accordingly, streaking occurs Charring of the substrate and uneven heating.

Incidentally, it should be mentioned that the density of the previously used electrical conductive metals is about twice as high as that of the inventively used antiferromagnetic substances. Therefore, it is difficult to use the previously used electrically conductive metal particles in a liquid medium to form a satisfactory To suspend printing ink or the like.

In order to achieve the best efficiency, the magnetic field must have a force flux density of at least 50 gauss; the normal working range is 50 to 500 gauss, and the range of 100 to 300 gauss is preferred.

The invention represents a novel heating method for the most diverse heat-activated substances available. The term "heat-activated material" refers to substances that are used to achieve a certain effect on a certain Temperature. For example, certain solid thermoplastic Substances are melted to give the adhesive properties of so-called hot melt binders to achieve. Volatile liquids can be evaporated by placing them on their The evaporation point. Gases and liquids can affect the temperature are heated, in which they are reactive for certain chemical reactions. This is particularly advantageous in the case of corrosive gases or liquids which heating coils or other heating devices chemically attack. Curable Resins can be heated to the temperature at which chemical crosslinking takes place.

According to the method of the invention, for example with the aid of heat-activated adhesives one metal to another metal, metal to paper, Paper can be bound to paper, leather to paper, etc. Documents, such as paper, Metal, plastic, leather, glass or textile that are glued together are to be at least partially with a heat-activated adhesive mass and coated with a mass containing the antiferromagnetic particles described above contains.

The coating containing the antiferromagnetic particles is im generally from a dispersion of the particles in a binder, such as a natural one or synthetic resin or glue, preferably polyvinyl acetate, in a liquid dispersing medium or solvent for the binder, such as a lower Alcohol, e.g. B. methanol, ethanol, isopropanol, etc. produced. This t coating mass is applied to one or both of the surfaces to be bonded together at those locations applied to be glued together. The top layer is then at least applied to the places where the above-described, the antiferromagnetic Coating containing particles is located; preferably the top covering is at least applied to an entire surface of the base. Thereupon the document folded as desired and passed through an alternating magnetic field in this way, that the surfaces to be bound together are in contact with one another. Through this the particles are heated to their Néel temperature within milliseconds, and they cool down to the same temperature within milliseconds the topping is no longer sticky, resulting in extremely rapid mass production is made possible.

The invention can also be applied to ordinary printing inks when used in high-speed printing presses to dry by making finely divided antiferromagnetic Particles dispersed in the ink. Immediately after the pad has been printed has been, it is guided by an alternating magnetic field, causing the particles heat and the solvent contained in the printing ink to evaporate bring so that the printing ink dries almost instantly and the printed Products can be stacked immediately after printing. Since this also only the ink is heated and there is heating and cooling of the ink inside A split second is going on, the paper itself is undetectable Extent warmed. Therefore, the water contained in the paper does not evaporate, and so does it as a result, there is no shrinkage of the paper.

This method can also be used with inks that have a contain oxidizable liquid carriers, such as lithographic inks on the Base of linseed oil. According to the method of the present invention, the printing ink can be practical instantly heated to the correct oxidation temperature and then cooled, whereby the advantages mentioned above are achieved.

The invention also provides a method for heating gases or liquids to the temperatures suitable for chemical reactions Available by the gases or liquids through a rest pouring or fluidized bed of finely divided, multi-district antiferromagnetic particles which are constantly under the influence of an alternating magnetic field are located. Such a method can be carried out in a very effective manner if the particles in the catalyst or catalyst support used in the process are embedded.

Example A carton of that described in U.S. Patent 2,695,745 Art is made using only the areas intended for overlapping and gluing of the cardboard are coated with a mass, the finely divided, non-conductive antiferromagnetic Contains particles. The mass is made from a commercially available ferrite, the consists essentially of about 100% NiO, 6% ZnO, 1% MnO and 83% Fe2O3 and a Néel temperature of 385 ° C, an initial permeability of 115 and a has a specific volume resistivity of 2.5 10 .ohm-cm at 30 "C. This ferrite is ground in water in a ball mill for 16 hours. The grinding sludge is filtered, dried and crushed to an average particle size of 3 u. Then will the ferrite particles with a 30% solution of polyvinyl acetate (weight average molecular weight 70,000) in a mixture of methyl alcohol and ethyl alcohol mixed to a mass, 67% of the ferrite and 33% of the polyvinyl acetate solution exists and has a viscosity of 700 cP.

This mass is then, as described above, on the overlap applied to certain parts of the cardboard punched pieces. Then the whole Cardboard punch with a mass of a copolymer of ethylene and vinyl acetate and paraffin wax coated. The die cut is folded and placed on a mandrel in placed near an electrode assembly in a press pad. It becomes a magnetic one Alternating field generated with 40 megahertz and 125 Gauss. Here the covering reaches up the overlapping flaps its welding temperature within 100 milliseconds, whereupon the alternating magnetic field is interrupted and they come into contact with each other standing flaps cool to room temperature within 200 milliseconds and touch each other be bound. There is no external cooling of the mandrel or the press pad for this purpose necessary. Solid, liquid-tight bonds are formed that are stronger than are the tear strength of the paper itself. The topping does not melt on any other Place of the cardboard. The result is a durable cardboard box.

When repeating the above example using a force flux density of 200 Gauss, the coating on the tabs reaches its welding temperature within 20 milliseconds, after which the alternating magnetic field is interrupted and each other the contacting tab surfaces to cool to room temperature within 100 milliseconds. In turn, solid, liquid-tight bonds are formed that are stronger than the tear resistance of the paper, the covering does not melt anywhere else on the Cardboard boxes, and you get a durable cardboard box.

Claims (9)

Claims: 1. Method for heating heat-activatable Material, characterized in that it is a finely divided, electrically non-conductive antiferromagnetic substance is brought into contact with the material and that the product of the action of a high-frequency alternating magnetic field of at least 10 megahertz. 2. The method according to claim 1, characterized in that a ferrite used as an anti-ferromagnetic substance. 3. The method according to claim 1 and 2, characterized in that the Particle size of the particles of the antiferromagnetic substance to be set 0.01 to 5 is 9. 4. The method according to claim 1 to 3, characterized in that the Force flux density of the magnetic field is at least 50 Gauss. 5. The method according to claim 1 to 4, characterized in that the The frequency of the magnetic field is 40 to 2500 megahertz. 6. The method according to claim 1 to 5, characterized in that as heat-activatable material, a coating compound, an adhesive or a printing ink is used. 7. The method according to claim 1 to 6, characterized in that the antiferromagnetic particles are heated to their Néel temperature. 8. The method according to claim 1 to 7, characterized in that the activating material of the action of the alternating magnetic field for a period of time is suspended for up to 100 milliseconds. 9. The method according to claim 1 to 8, characterized in that the heat-activated material on a base made of paper, metal, plastic, Leather, glass or fabric.