US3360466A - Heat-generating magnetic ink compositions - Google Patents

Description

United States Patent 3,360,466 HEAT-GENERATING MAGNETIC INK COMPOSITIONS Oliver A. Short, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Nov. 18, 1964, Ser. No. 412,253 4 Claims. (Cl. 252-62.S3)

ABSTRACT OF THE DISCLOSURE A heat-generating magnetic lithographic type ink consisting essentially of a finely divided magnetic pigment possessing a high energy loss and a coercivity of 2 to 20 oersteds, and a liquid vehicle having drying properties and-,containing a metal compound drier. Since the ink heats'instantly to a given high temperature when subjected to amagnetic field, it is particularly useful as a source of sudden high heat, e.g., when a print thereof under a heat-scalable coating on paper or the like is subjected to a magnetic field.

Wax and wax-containing coatings have long been applied to paper, paperboard and similar structures to permit the heat-sealing of such structures to each other. The heat for such heat-sealing has usually been supplied by the hot surface of an electric heating element, gas flames, hot air blasts and the. like, the heat-seal being effected by contacting the hot surface of the heating element or the gas flame with one or both sides of the portions of the coated structures to be sealed.

It was found recently that the heating of such heat-sealable coatings can be greatly speeded up by applying a finely divided magnetically actuable substance to the coatings and subjecting the finely divided substance to a magnetic field to quickly heat and melt the coatings whereby to heat-seal the coated structures. Application of such magnetically actuable substance would be most practical if it could be employed in an ink which could be rapidly applied employing usual printing methods and equipment.

It is an object of the invention to provide a new and improved heat-generating magnetic ink. A particular object is to provide a magnetic ink that can be printed at high speeds, for example, in a lithographic or raised letter press, as an undercoat under a heat-scalable coating on paper or similar material, and has the property of instantaneously heating to a given temperature, when subjected to a magnetic field, to cause the heat-scalable over coating to heat-seal. Other objects of the invention Will be apparent from the following description.

The ink of the invention consists essentially of a finely divided magnetic pigment having a low coercivity but a high energy loss and a viscous tacky vehicle therefor comprising a drying liquid component and a metal compound drier dissolved therein. The weight ratio of the above pigment to the vehicle should be between 0.9:1 to 4:1, the preferred ratios ranging from 1:1 to 3.5: 1.

The pigment, which should be in finely divided form, e.g., have an average particle size of 0.5 to microns in diameter, must have a low coercivity. Pigments with a coercivity of from 2 to 20 oersteds are generally suitable. The finely divided metal ferrites containing, on an oxide weight basis, 3 to 30% NiO, 2 to 20% ZnO, 0 to 5% MnO and at least 66% (generally the remainder) Fe O are magnetic pigments meeting the above requirements. Such magnetic materials which are available commercially as sintered shapes which must be crushed and ground to make pigments, are different in kind from those commonly used on tapes, bank checks and the like where a high remnant magnetism is essential and a high coercivity is desirable. This latter type of magnetic pigment is entirely unsuitable for use in formulating the ink of the present invention.

The ink should possess the property of drying when the print thereof on paper, paperboard or the like is exposed to air. It should also be definitely tacky and adhesive and have a viscosity of 50,000 to 100,000 centipoises in order to permit printing at high speeds in a lithographic or raised letterhead press. In order that these requirements be met, the vehicle for the magnetic pigment particles must include a liquid component having drying properties and a metal compound drier, such as the cobalt, manganese, cerium and other metal acetates, borates, oxalates, linoleates, linoleneates, naphthenates, xanthates and Z-ethylhexanates employed as driers for the oxidative drying of linseed oil, eg, in lithographic varnishes, Such driers should be present in amounts, e. g., from 0.2 toabout 3%, sufficient to effect the desired drying of the printed hyde resins, rosin-modified maleic ester resins and alkyd resins having long chain unsaturated groups, such as the styrenated linseed or tung oil alkyds. If a non-drying resin is used, a solution thereof in a drying oil such as linseed oil or linseed stand oil should be used as the liquid drying component of the vehicle. If a resin having drying properties is used, it may also be employed as a solution in such a solvent, although non-drying solvents such as the ali-.

phatic hydrocarbons boiling at from about 240 to 300 C. can be used. Thus, the drying property of the ink may derive from the use of a drying resin or a drying oil, which drying substance should be so selected as to provide an ink which is tacky and has a viscosity within the range stated above. Minor adjustment of the viscosity to bring it within this range may be effected, when necessary, by thinning With a hydrocarbon solvent of the kind indicated above.

The ink of the invention adheres well both to substrates such as paper and paperboard and to heat-sealable wax and wax-containing coatings. When formulated to contain from 0.9 to 4 parts of the magnetic pigment per one part of the vehicle, prints thereof can be rapidly and effectively applied by lithographic or raised letter presses. The above weight ratio of pigment to vehicle is important since if less than 0.9 part of pigment is used per part of vehicle the print will not contain sufiicient pigment to effect the desired heat sealing unless multiple prints are used. On the other hand, if more than 4 parts of pigment are employed per part of vehicle, bonding of the pigment to the paper substrate will be inadequate.

The invention is illustrated by the following examples.

Example 1 A magnetic pigment was prepared by ball milling a crushed ferrite formed by sintering together NiO, ZnO and Fe O in the weight ratio of 10 NiO:5 ZnO: Fe O This pigment had a coercivity of about 2 oersteds. The ball milling was effected in a porcelain mill with porcelain balls in water for about 16 hours, whereby the average particle size was reduced to about 7 microns. The pigment was mixed with a commercial heat-bodied linseed oil based varnish containing a drier at a Weight ratio of one part of the pigment to one part of the varnish. The pigment-varnish mixture was thoroughly blended by passing it through a 3-roll paint or ink mill. The ink thus formed has a viscosity of 50,000 centipoises (Brookfield Vis- Patented Dec. 26, 1967 cometer RVT No. 6 Spindle at r.p.m.). It was printed by means of a lithographic offset press onto a paperboard typical of that used for frozen food cartons, Carton blanks were then cut from this board and coated with an ethylene/ vinyl acetate copolymer-wax blend. When two pieces of this coated board were held together in an 80 mega cycle magnetic field, a satisfactory seal was formed in 150 milliseconds in those areas where the ink print underlay the copolymer-wax blend coating.

Example 2 An ink generally similar to that of Example 1 was formulated using 3 parts by weight of the magnetic pigment of Example 1 to one part by weight of the varnish as in Example 1 and with sufiicient petroleum spirits thinner to give an ink having a viscosity of 80,000 centipoises (Brookfield Viscometer RVT with No. 6 spindle at 10 rpm). This ink was printed on paperboard which was then coated with an ethylene/ vinyl acetate copo-lymer-wax blend as described in Example 1. The paperboard was cut and formed into food cartons and sealing was obtained in about 50 milliseconds when the inked and coated surface was placed in an 80 megacycle magnetic field against another coated surface which had not been inked.

The finely divided ferrite pigments employed in the ink of the invention have the property of heating substantially instantaneously to a temperature of 200 to 600 C. in a magnetic field of 200 oersteds or less and between 30 and 3000 megacycles. Upon removalor discontinuance of the magnetic field, the ferrite particles of the printed ink return to normal temperature, thus permitting the coating to congeal. The upper temperature reached when the magnetic field is applied is self-limiting, since the ferrite particles on reaching their specific Nel temperature, i.e., that temperature at which the magnetic hysteresis effects cease, will heat no further. Thus, the ink of the invention can be employed to effect substantially instantaneous sealing at temperatures which will never be too high. This feature together with the fact that the ink can be rapidly and effectively printed by lithographic or'raised letter presses makes it highly practical for the purposes indicated.

The embodiments of the invention in which an eXclusive property or privilege is claimed are as follows.

I claim: 0

1. A heat-generating magnetic ink consisting essentially, on a weight basis, of 0.9 to 4 parts of a finely divided magnetic pigment possessing a high energy loss and a coercivity of 2 to 20 oersteds per 1 part of a'liquid vehicle having drying properties and containing a metal compound drier dissolved therein, said pigment being a metal ferrite containing, on an oxide basis, 3 to 30% NiO, 2 to 20% 2110, 0 to 5% MnO and at least 66% Fe O 2. A magnetic ink according to claim 1 in which the vehicle consists essentially of heat-bodied linseed oil and a metal compound drier and the ink has a viscosity of 50,000 to 100,000 centipoises.

3. A heat-generating magnetic ink having a viscosity of 50,000 to 100,000 centipoises and consisting essentially, on a weight basis, of (a) a finely divided magnetic ferrite pigment containing, on an oxide Weight basis, 3 to 30% NiO, 2 to 20% ZnO, 0 to 5% MnO and at least 66% Fe O and (b) a liquid vehicle having drying properties and consisting essentially of a heat-bodied linseed oil having dissolved therein a metal compound-drier.

4. A magnetic ink according to claim 3 wherein the ferrite pigment contains about 10% NiO, 5% ZnO and Fe O References Cited UNITED STATES PATENTS 3,082,171 3/1963 Shoemaker et al. 252-625 TOBIAS E. LEVOW, Primary Examiner.

R. D. EDMONDS, Assistant Examiner.

Claims (1)

1. A HEAT-GENERATING MAGNETIC INK CONSISTING ESSENTIALLY, ON A WEIGHT BASIS, OF 0.9 TO 4 PARTS OF A FINELY DIVEDED MAGNETIC PIGMENT POSSESSING A HIGH ENERGY LOSS AND A COERCIVITY OF 2 TO 20 OERSTEDS PER 1 PART OF A LIQUID VEHICLE HAVING DRYING PROPERTIES AND CONTAINING A METAL COMPOUND DRIER DISSOLVED THEREIN, SAID PIGMENT BEING A METAL FERRITE CONTAINING, ON AN OXIDE BASIS, 3 TO 30% NIO, 2 TO 20% ZNO, 0 TO 5% MNO AND AT LEAST 66% FE2O3.