Encapsulated Particulate Binary Magnetic Toners For Developing Images

Abstract

Described and claimed are flowable, particulate, binary toners for developing magnetic images comprising a particulate hard magnetic material, e.g., Fe.sub.3 O.sub.4 or CrO.sub.2, and a particulate soft magnetic material, e.g., Fe, each type of material being present in substantially each toner particle.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to, and has as its principal object provision of, improved encapsulated particulate magnetic pigments of developers for developing magnetic images, which particulate substances contain a hard magnetic material and a soft magnetic material essentially within each particle.

Magnetic developers are frequently called "toners" in the art. The term "toner" will accordingly be used in the following discussion to refer to a particulate material capable of developing or making visible a magnetic image. The novel toners of this invention are particularly useful in the readout step of the thermomagnetic copying processes disclosed in the Nacci Belgian Pat. No. 672,017 and thermomagnetic imaging processes disclosed in the Nacci Belgian Pat. No. 672,018. See also the copending, coassigned Nacci U.S. application Ser. No. 682,234, filed Nov. 13, 1967, now abandoned.

2. Description of the Prior Art

As shown, for example, by Atkinson et al. U.S. Pat. No. 2,826,634, the use of iron or iron oxide particles, either alone or encapsulated in low-melting resins or binders, for developing magnetic images, is well known to the art. These toners have been successfully used to develop magnetic images recorded on magnetic tapes, films, drums and printing plates. The encapsulating resin or binder may aid in transferring the decorated magnetic image (or the developing pigment) to paper and can further be heated, pressed or vapor softened or subjected to combinations of these treatments to attach physically or fix the magnetic particles to the surface fibers of the copy paper. In general, images on copy paper using either iron- or iron oxide-based magnetic particles have been of low-optical density or unpleasing appearance due to the difficulty of hiding the natural metallic luster of iron or due to the feeble pickup of iron oxide particles resulting from their tendency to form agglomerates that are in an internally magnetically satisfied state, relatively unresponsive to the weak fields emanating from the surface of magnetic images recorded on films, tapes, printing plates or other magnetic storage media.

The present invention provides improvements in toners. In particular, it provides toners with desirable printing characteristics For example, slurries made from these toners, although settling rapidly, are easily reslurried and are nonreactive chemically with the dispersing medium (such as, the rusting of iron by water) so that they print equally well even after long periods of storage. The dispersions have less tendency to agglomerate due to magnetic forces than dispersions containing either hard or soft magnetic materials used alone. Also the optical reflectance density of the ultimate copy and its appearance in terms of image crispness and sharpness are generally improved with these toners that both pack more tightly on the magnetic image (because of lack of magnetic agglomeration) to give superior areawise coverage on the ultimate copy surface and are more strongly attracted to the magnetic image.

THE DRAWING

The achievement of the above-noted and other objects of the invention will be evident from the remainder of the specification and from the drawing (essentially FIG. 6 of the above-mentioned application of Nacci, Ser. No. 682,234) wherein:

The figure shows a schematic view of apparatus for the thermographic repoduction of documents in which the magnetic toners of this invention are particularly useful. In the figure, a magnetized thermomagnetic copying member in the form of a film 35 is stretched over the surface of a transparent drum 20 which is driven in the direction indicated in the arrow. The document which is to be copied, 21, is fed through the machine in stationary relationship with the copying member by friction applied by the flexible belt 22 which holds the document in contact with the copying member and moves synchronously therewith over the idling rollers 23 and 24. At the center of drum 20 is positioned a Xenon lamp 25 which emits flashes of light at high intensity with a duration of the order of a millisecond over the surface of the member in contact with the document as defined by the stationary mask or shield 36. Each flash forms a magnetic image of the document on the copying member. The copying member returns to its initial thermal state in about 0.5 second, and the flashes are spaced in time at somewhat longer intervals. The speed of document feed and drum rotation is maintained so that each portion of the document is exposed to the radiation at least once while in contact with the copying member.

The magnetic image can be developed by padding on a toner of this invention by the padding roll 26 which dips in a bath 27 containing the toner in suspension. Surplus toner is removed by wiping means 28. The image is then transferred to paper 38 which is fed from a roll 29, passing over the idling roll 30 and thence in contact with the recording member by pressure roll 31, when the image is transferred. The toner particles are then fused to the copying paper by a band of heaters 32, and the paper is removed on the roll 33.

Once the document has passed through the machine forming the magnetic image, a large number of copies can be made by continued rotation of drum 20, since the image is substantially permanent. The image can be destroyed and the magnetic recording member returned to its uniformly magnetized state ready to copy further documents by operation of the magnetic head 34 .

DESCRIPTION OF THE INVENTION

In accordance with the present invention, it has now been found that a surprising improvement in the quality of images printed on copy paper, transparent plastic sheets and the like can be effected through the use of resin-encapsulated magnetic pigments that contain binary mixtures of at least one each of a magnetically hard and a magnetically soft powder material that may optionally contain opacity control agents, release agents, and the like. The magnetically soft powder material may be iron or another high-permeability, low-remanence material, such as certain of the ferrites, e.g., (Zn, Mn)Fe.sub.2 O.sub.4, or permalloys, while the magnetically hard material may be an iron oxide, preferably Fe.sub.3 O.sub.4, .nu.-Fe.sub.2 O.sub.3, other of the ferrites, e.g., BaFe.sub.12 O.sub.19, or chromium dioxide. The ratio of the hard to soft components may vary considerably but is preferably in the range of 1:6 to 4:1 by weight. The encapsulating resin is generally present in the order of 20-40 percent, preferably 20-30 percent, by weight of the final composition. Small amounts of the order of 1-5 percent of additives, such as carbon black or black or colored dyes, for a blacker or colored copy, and stearamide or silicone derivatives for easier transfer to paper may be added if desired. For reasons given later, particle diameter in the range of 3-20 microns is preferred.

The improved binary magnetic toners of this invention are prepared by a process in which a hard and soft magnetic material are mixed together in desired proportions with an encapsulating resin in a solvent for the resin, ball-milled and spray dried. More specifically, the respective hard and soft magnetic powders (cf. seq. for meaning of "hard" and "soft") in ratio of 1:6 to 4:1, but preferably approximately equal quantities by weight, are intimately mixed and dispersed, preferably by ball-milling for a length of time extending up to 17 hours or more at about 60 percent by weight nonvolatiles content, in a multicomponent dispersions system consisting of:

1. An organic liquid selected on the basis of its chemical inertness toward the magnetic materials, its volatility characteristics, and its ability to dissolve the encapsulating polymer;

2. A readily fusible, organosoluble organic binder or encapsulating resin; and

3. Optionally (a) additives such as carbon black, pigments or dyes to control color, (b) agents such as stearamide or silicones to promote easy release during magnetic image transfer, and (c) agents such as conductive carbons or electron donors and acceptors to control the electrostatic properties of the toner particles.

The dispersion resulting from the steps above is separated from the ceramic balls, sand, or other grinding means, diluted, and spray dried at a nonvolatiles content of about 20 percent by weight. The diluent is a compatible organic liquid, usually the same solvent employed during preparation of the dispersion. Spray drying is accomplished by conventional means, e.g., by dropping the diluted dispersion onto a disk rotating at high speed or by using a conventional spray drying nozzle. The droplets are dried in a chamber through which a heated gas is flowing, obtained, for example, by combustion of natural gas in air. Gas flow and temperature are adjusted in known manner to remove solvent quickly, leaving discrete, free-flowing, approximately spherical toner particles, preferably about 3-20 microns in size.

Spray-dried toners tend to have the magnetic particles buried in the resin spheres. Occasionally it is advantageous to have these particles at the surface of the toner spheres. This may be accomplished by the additional step of abrading the spheres by fluid-energy milling (jet pulverizing) to expose the magnetic particles.

The toners prepared as above may be applied to imaged magnetic films either from a dispersion, that is, such as an ink, or in the dry state. In the former case, a nonsolvent dispersion medium such as water, etc., is generally employed. In the latter case, the toner is conveniently mixed with an inert solid, particulate material such as polystyrene beads. In either case, the toners can be transferred from the imaged magnetic film to an ultimate copy which can be further treated as by heat if desired.

The "magnetically hard" and "magnetically soft" materials which form the basis of the present toners are substances which are, respectively, permanently magnetizable and substantially nonpermanently magnetizable under similar conditions below the Curie point of the materials (cf. British Pat. No. 1,108,102). A magnetically hard material, as the term is used here, has a coercivity of at least 40 Oe and exhibits a significant remanence. The latter is 20 percent or more of the saturation magnetization and the material can be used to fashion a permanent magnet. Magnetically soft material has low coercivity, e.g., of the order of an oested or less when in bulk form, and high permeability, permitting saturation to be obtained with a small applied field. More importantly, the soft material exhibits a remanence of less than 5 percent of the saturation magnetization. The ideal magnetic properties for soft materials are found in high permeability, low-loss compositions used in transformers, but such properties are seldom realized in small-diameter particles. Nevertheless, as long as the remanence is low, these particles serve very well even though they are not, strictly speaking, high-permeability materials.

"Soft" magnetic materials are discussed widely in the literature, e.g., by E. W. Lee and R. L. Lynch, Advances in Physics, supplement to Philosophical Magazine 8 (July 1959), pp. 292-348. This high permeability implies a narrow hysteresis loop, a low-energy product, (BH).sub.max, and low-hysterisis loss. Such materials are used in transformers and motors. Examples are soft iron, silicon-iron alloys, and the permalloys, i.e., magnetic alloys of nickel and iron. Certain ferrites (such as Mn.sub.0.5 Zn.sub.0.5 Fe.sub.2 O.sub.4) can also be used but their low magnetizations generally give an inferior toner. Preferred as soft magnetic materials for use in this invention are iron-based pigments such as carbonyl iron, iron flakes and iron alloys.

Many magnetic materials usually designated as soft may become hard and show high-coercive force when prepared as fine particles. Geometric factors, including size and shape of the particle, are important. For example, iron is normally considered a "soft" magnetic material with a coercivity of a fraction of an oersted. However, small iron particles composed of single magnetic domains with lengths great compared to their diameters can be expected to show coercivities of the order of 10.sup.3 -10.sup. 4 Oe. In this case, high coercivity is due to shape anisotropy. For some other materials, such as manganese bismuthide or cobalt, high coercivity for single domain particles may be the result of magnetocrystalline anisotropy arising from an easy direction of magnetization along a particular crystalline direction. Even fine nickel particles should show a high coercivity under uniaxial stress. Many normally "soft" magnetic materials not in single domain form can be made to exhibit a high coercivity after being subjected to cold work or other similar treatments designed to introduce defects or internal strains which serve to pin or block movement of domain walls. Further discussion of "Hard Magnetic Materials" can be found in the article by that title by E. P. Wohlforth, Advances in Physics, supplement to Philosophical Magazine 8 (Apr. 1959), pp. 87-224 and in the book by R. M. Bozorth on "Ferromagnetism," D. Van Nostrand and Company, Princeton, New Jersey (1951), particularly the section on fine particles, pp. 828-834.

"Hard" magnetic materials are usually characterized by having a high-intrinsic coercivity, iH.sub.c, ranging from a few tens of oersteds, e.g., 40 Oe, to several hundred or even several thousand oersteds and a relatively high remanence, B.sub.r, when the materials are removed from a magnetic field. Accordingly, these "hard" materials will in general have a high-energy product (BH).sub.max, ranging as high as several tens of thousands of joules/m.sup.3. Such materials are of relatively low permeability and require high fields for magnetic saturation.

Examples of hard magnetic materials include the permanent magnetic materials, such as the "Alnicos," the "Lodexes" (acicular iron-cobalt alloys encased in lead or plastic, manufactured by the General Electric Company), the "Indox" barium ferrite compositions, and materials used in tape recording, magnetic discs, and magnetic printing inks. These latter materials include .nu.-iron oxide (Fe.sub.2 O.sub.3), magnetite (black Fe.sub.3 O.sub.4), chi-iron carbide and chromium dioxide (CrO.sub.2).

Preferred Fe.sub.3 O.sub.4 particles include the commercially available types "3000" and "4000" (Wright Industries), the much less expensive naturally occurring and modified magnetites, and acicular forms of Fe.sub.3 O.sub.4, such as the IRN 100 sold by C. K. Williams and Company. Other hard magnetic iron oxides can be used such as .nu.-Fe.sub.2 O.sub.3, especially when reddish or brownish toner particles are desired. The recently available black, magnetically hard pigment CrO.sub.2 is especially preferred because of its magnetic properties.

The ratio of the hard to soft component may vary considerably but is preferably in the range 1:6 to 4:1 by weight, a ratio of about 1:1 being more preferred. It is preferred that the soft magnetic material be less than 5 microns in maximum dimension and that the hard magnetic material have a maximum dimension not exceeding about 1 micron.

A variety of organic solvents may be used as solvents for binders and dispersion media for the hard and soft magnetic materials during toner preparation. Use of a 1:1 mixture of n-propylalcohol:toluene and n-propyl alcohol:xylene is described in the examples. The particular organic solvent selected depends primarily upon its ability to dissolve the binder.

It is important, moreover, that the solid toner particles of this invention be free-flowing with a sticking temperature above that encountered in shipment and storage. Stick temperature is related to thermoplasticity, i.e., the melting or fusion temperature of the binder, and if too low, the toner particles may become cemented or sintered together, thereby interfering with their use in image development systems. In wet systems, binders of somewhat lower sticking temperature may sometimes be used provided the particles are protected by a film of the dispersing medium. It will be appreciated that free-flowing characteristics are particularly important for toners used in dry imaging methods, e.g., the cascade method described in table I and in examples 5 and 6 below. Particles precoated with or containing surfactants (see below) for use in liquid development systems should also be free-flowing to facilitate complete dispersion and to prevent clogging of replenishment devices.

Binders are preferably of the thermoplastic type in order to permit fixing to the paper by melting or fusion. Certain waxy polymers are also useful and can be caused to flow into paper by pressure. Preferred binders include the low molecular weight polyamides and ethylene/vinyl acetate copolymers. Of these, for ease of spray drying, ease in image development from aqueous or nonaqueous dispersions and ease of fixing to paper by fusion, the low-molecular weight polyamides are especially preferred. The binder (encapsulating resin) is generally present in the order of 20 -40 percent, preferably 20 -30 percent, by weight of the final composition.

Binders and solvents other than those described in the examples may be used. Preferred binders readily melt to fluid liquids, but have a sticking temperature above about 60.degree. C. In some instances, the solutions must be kept warm to prevent gelation prior to spray drying. The binders are readily soluble in organic liquids that are sufficiently volatile for spray drying, and they are chemically resistant toward the hard and soft magnetic materials present in the toners. The binder serves to fix the developed image to paper and hence is a functional part of the toner. Binders and solvents in addition to those described in the examples include:

1. Copolymers with functional groups, e.g., styrene/dimethylaminomethacrylate copolymer dissolved in xylene or toluene; ehtylene-methacrylic acid copolymer dissolved in 7:2 parts by weight toluene: Triclene trichloroethylene; and styrene/acrylonitrile copolymer dissolved in toluene.

2. Copolymers containing vinyl acetate, e.g., ethylene/vinyl acetate copolymer dissolved in xylene; and mixtures of ethylene/vinyl acetate copolymer and waxes dissolved in toluene.

3. Hydrocarbon types, e.g., "Paraflint," or "Tervan" 2,865 hydrocarbon waxes dissolved in toluene or xylene; Piccolastic D125 styrene resin dissolved in toluene; and styrene/indene copolymer dissolved in toluene or xylene.

4. Fluorine-containing copolymers, e.g., tetrafluoroethylene/vinylidene fluoride/vinyl butyrate copolymer dissolved in methyl ethyl ketone, and tetrafluoroethylene/vinyl acetate copolymer dissolved in cyclohexanone.

Additives to enhance the functional behavior of the toners include color and opacity control agents, surface modifiers, release agents, materials to increase affinity for the copy paper, and the like.

Since toners are normally used to give black copy on white paper, carbon black or a black dye such as "Nigrosine" SSB may be added to give a more pleasing appearance, enhance reflectance optical density, and hide the metallic gray of particulate iron in the toners and exert control over electrostatic properties of the toners. Other dyes and pigments may be added to give a range of colored toners; for many of these .nu.-Fe.sub.2 O.sub.3 (yellow-red) is used in preference to Fe.sub.3 O.sub.4 (black) as the hard magnetic species along with dyes or pigments. Conductive carbons such as acetylene blacks or graphite and certain electron donors or acceptors may be used to control electrostatic properties of the toner particles. Stearamide or silicones may be added to promote easy release during the magnetic image transfer to paper. Other modifications to the surface of the toner particles to enhance these properties are well within the state of the art. Small amounts of the order of 1-5 percent of additives, such as carbon black or colored dyes, for a blacker or colored copy, and stearamide or silicon derivatives for easier transfer to paper are usually adequate.

Other additives of particular importance where the toners are to be applied to a magnetic image from a liquid dispersion are surfactants. These materials aid greatly in dispersing the binary toners in water. A preferred surfactant is "Lakeseal" (see below).

The surfactant or dispersing agent can be added to the toner before it is spray dried, in which case it is distributed throughout the particle, or the particles can be coated with the agent after spray drying, in which case it is carried only on the surface of the particle. Note example 7, below. Both types of toner particles, i.e., those carrying up to about 2 percent by weight of surfactant distributed throughout the particles or those carrying it held on the surface, constitute additional aspects of the invention.

As noted above, in the case of dry development of the toner particles, other additives may be desirable. Thus, the buildup of static charges between toner particles and the image-bearing magnetic film can lead to high-toner pickup in background or nonimage areas. This can be controlled by use of conductive recording media and conductive toner particles or carriers; conductive carbon particles, for example, added to the toner help to dissipate the electrical charges. Carries, e.g., polystyrene or glass beads, which give a preferred charge orientation, are also useful in preventing agglomeration as a result of static in dry development systems.

The liquid used for preparing toner dispersions in "wet" development systems plays an important role. Preferably, it should be of high density, low viscosity and low-surface tension, nonflammable or with a high-flash point to minimize fire hazards, nontoxic, easily volatile with a low heat of vaporization, cheap and readily available, nonreactive with and exerting no undesired solvent action on the image-bearing magnetic storage medium or with the toner binder or the toner pigments, and polar to aid in dissipating static charges induced by the cyclical processes in a copier or duplicator. Compromises, of course, must be made in selecting a liquid to meet as many of these conflicting requirements as possible. Liquids that have been used successfully include Freon 113 (1,1,2-trichlorotrifluoroethane, b.p. 47.6.degree. C.), certain alcohols (methanol), saturated hydrocarbons such as hexane or higher boiling petroleum fractions, e.g., "Isopar-G" (Humble Oil Co.) or Du Pont No. 711 odorless paint thinner and water. Especially preferred among these liquids are water or high-boiling hydrocarbon fractions containing minor amounts of dissolved species such as methanol or antistatic agents.

Water-based toner slurries, which contain nearly 50 weight percent solids, were used for developing magnetic images in the majority of the examples below. In such systems, the role of the dispersing agent used with the toner is of great importance. Investigation of a number of commercially available dispersants including representative soaps, anionic, cationic, and nonionic surfactants, both pure and in so-called "built" formulations showed that most of these were operable, but most presented difficulty after aging for ca. 10 days in that the toner tended to stick to the image-carrying magnetic film, making cleaning difficult. An especially preferred dispersing agent for aqueous systems is the commercially available built detergent laboratory glass cleaner sold under the name of "Lakeseal" discussed below. "Lakeseal" dispersions of the preferred toners have been found to function satisfactorily after long periods of use and storage times in excess of 6 months.

The type of toner used will vary depending on the nature of the ultimate copy sheet, i.e., where the image is to reside finally. Thus, as described in detail in the examples, if the final residual copy sheet contains an, or is, adhesive, the image can be successfully transferred from the imaged magnetic surface by simple pressure after the image is developed using suitable magnetic toner powder. On the other hand, if it is desired to transfer the image to a nonadhesive copy support e.g., conventional white bond paper or a clear film for projection, a different type of toner particle may be used. In such a permutation, the magnetic particles, whatever their nature, will have been previously coated with a thermoplastic material, such as a relatively low-melting polymer or copolymer which will be inert to magnetic fields. Thus in this permutation, the developing toner powder will be imagewise attracted to the imagewise recorded magnetic record and then can be transferred by pressure, fusion, or a combined pressure/fusion step to white bond paper or clear plastic film, or the like, thereby resulting in a fixed or permanent image, the fixing being due to the thermoplasticity of the coating on the developing toner particles on heating.

Image resolution, uniformity and quality are functions of the magnetic and electrostatic properties of the toner and also of its particle size and size distribution. For high resolution the toner particles should be small (3--5 microns in longest dimension), but there should be, and normally will be, larger particles present also. The response of the toner to the magnetic fields of the image will be controlled in part by proper adjustment of the range of coercivity and saturation magnetization of the toner particles as well as the electrostatic properties which may be used as a bias to aid in development and in the final toner transfer step.

If toner particles are smaller than about 1 micron in diameter, they are attracted to surfaces with or without magnetic images and adhere tenaciously by Van der Waal's forces or electrostatic attraction. Particles much larger than 20 microns in size are too easily removed by fluid drag forces or gravity. Further, magnetic forces drop off approximately as the cube of the separation, and appreciable magnetic signal fields from the image areas extend no more than 25-40 microns from the surface of the film. Furthermore, in the case of large particles, much of the magnetic material on the side of the particle farthest removed from the magnetic image is beyond the range of effective magnetic attraction. Accordingly, the optimum particle size for image development should be in the range 3-20 microns.

Particle size distribution as reported herein is based on a 200 particle count of toners dispersed in a viscous liquid, using an optical microscope at 400X. The average size reported is the arithmetic mean based on this count.

Microscopic examination of a number of spray-dried toners has shown them to consist of nearly spherical particles, occasionally with a somewhat roughened or wrinkled surface. Photomicrographs of particle cross sections have shown a major number of particles to contain one to several iron particles surrounded by a relatively uniform oxide/binder matrix.

Throughout this specification, values in the more conveniently measured electromagnetic units/gram (emu/g.) are reported rather than B values in gauss. Thus, .sigma..sub.4,400 Oe or .sigma..sub.s is used to denote magnetization of the toner particles in a 4,400 Oe field, corresponding to magnetization in a saturating field, often designated as B.sub.s. Remanence values, .sigma..sub.r, correspond to remanence magnetization values designated as B.sub.r in induction units.

The sigma values employed herein are defined on pp. 5-8 of Bozorth's "Ferromagnetism," D. Van Nostrand Co., New York (1951). These sigma values are determined in fields of 4,400 oersteds on apparatus similar to that described by T. R. Bardell on pp. 226-228 of "Magnetic Materials In The Electrical Industry," Philosophical Library, New York (1955). The definition of intrinsic coercive force is given in Special Technical Publication No. 85 of the American Society for Testing Materials entitled "Symposium on Magnetic Testing" (1948), pp. 191-198. The values for the intrinsic coercive force given herein are determined on a DC ballistic-type apparatus which is a modified form of the apparatus described by Davis and Hartenhiem in the Review of Scientific Instruments, 7, 147 (1936).

As noted above, one advantage of the present invention is the improvement in optical reflectance obtained when the toners are developed. Reflection optical density of toner images transferred to paper gives a measure of the darkness or blackness of an image. Reflection optical density is the logarithmn of the reciprocal of the fraction of incident light reflected from a given area. For example, for a reflection optical density of 1.0, one-tenth of the incident light is reflected:

Typical printing in books, magazines, etc. will have a reflection optical density of 1.0-1.4. A very black image that reflected only one-thousandth of the incident light would have a reflection optical density of 3:

The reflectance optical density of white paper is usually in the range 0.1-0.14 and the images produced by many commercial copying processes are in the range of 0.5-0.8.

EMBODIMENTS OF THE INVENTION

There follow some nonlimiting examples illustrative of the invention in detail. In these examples, spray-drying operations were carried out below the lower explosive limit of the mixtures involved. Caution, of course, is necessary when mixtures of metals and/or oxides, especially when finely divided, and organic materials are exposed to high temperatures.

The designation, source, identity, and properties of iron, iron oxide, binders, and other materials used in the examples are given immediately hereinafter.

The term "carbonyl iron" refers to essentially pure iron powder produced commercially by the General Aniline and Film Corporation by pyrolysis of iron carbonyl:

Aver. Particle .sigma..sub.s .sigma..sub.r Desig- Size in Microns nation (Based on Wgt.) (emu/g.) (emu/g.) __________________________________________________________________________ GS- 6 5 199 1.0 SF 3 199 0.5 L 20 196 3.3 __________________________________________________________________________

black magnetic oxide of iron (ferroso-ferric oxide, Fe.sub.3 O.sub.4) employed had a modified cubic crystal structure and the properties: ##SPC1##

Carbon blacks used in the examples were:

1. Raven 30, a product of the Columbian Carbon Company, is an all purpose carbon black of high-tinting strength and low-vehicle demand. It has an arithmetic mean particle diameter of 25 millimicrons, a surface area of 82 square meters per gram, an oil absorption by the Venuto method of 90 gallons/100lbs., a pH of 8, a fixed carbon content of 99 percent, and a covering power of 102 (tinting strength index).

2. Statex R, also produced commercially by the Columbian Carbon Company, is a high-abrasion furnace black with an arithmetic mean particle diameter of 26 millimicrons, a surface area of 100 square meters per gram, and an oil absorption of 14 gallons/100 pounds.

3. Darco carbon black refers to Grade G-60, produced commercially by Atlas Chemical Industries, Inc., as a premium grade of powdered activated carbon used for decolorizing, purifying and refining. It is made by activating lignite with heat and steam.

4. "Permanent Black," made by the General Aniline and Film Corporation is a finely divided carbon black pigment.

A description of other previously undescribed mater materials used in the examples follows immediately hereinafter.

Versamid 930 is a low-molecular weight polyamide resin with molecular weight of about 3,100, an inherent viscosity of 0.24, and a softening ("stick") temperature of 105-115.degree. C. available from the Chemical Division of General Mills. Polyamide resins of this type are described in U.S. Pat. No. 2,450,940, J. C. Cowan, L. B. Falkenburg, H. M. Teeter, and P. S. Skell to the U.S.A., in the Handbook of Material Trade Names by Zimmerman & Levine, p. 257, Supp. I, in "Polyamide Resins," D. E. Floyd, Reinhold Plastics Application Series (1958), and in General Mills Bulletin No. 11--D-3. They are prepared by condensing polyamides such as ethylene diamine with polymeric fatty acids, e.g., dilinoleic acid, derived by polymerization of natural, oleaginous materials of animal and vegetable origin.

Tween 20 made by Atlas Chemical Industries, Inc. is a polyoxyethylene sorbitan monolaurate, a nonionic surfactant with approximately 20 polyoxyethylene units in the chain and a hydrophilic-lipophilic balance of 16.7 ["Emulsions: Theory and Practice," Amer. Chem. Soc. Monograph, p. 238 (1966)].

"450 H" is a coumarone/indene resin with a stick temperature of 90.degree.-100.degree. C. marketed by the Pennsylvania Industrial Chemicals Corporation.

"Lakeseal," sold by Peck's Products Company of St. Louis, Missouri for use as a laboratory glass cleaner, is an especially preferred agent for dispersing toners in aqueous systems. "Lakeseal" is a "built" detergent consisting of sodium phosphates, sodium carbonates, and biodegradable anionic and nonionic surfactants; the detergent contains appreciable quantities of inert materials resulting from its method of manufacture.

EXAMPLE 1

A. preparation of Pigment/Binder Dispersions

A preferred magnetic toner was made up from 40 weight percent of carbonyl iron (particle size approximately 2-5 microns), 39 weight percent of Fe.sub.3 O.sub.4 (particle size 0.03-0.06 micron), 1 weight percent of carbon black and 20 weight percent of a polyamide resin with molecular weight of ca. 3,100. Carbonyl iron powder, type GS-6 or type SF, was used as the soft magnetic material. The black Fe.sub.3 O.sub.4 pigment was grade "4,000". The carbon black was Raven 30 carbon black, and the polyamide resin was Versamid 930.

The low-molecular weight polyamide resin was dissolved in a 1:1 mixture by weight of n-propyl alcohol and toluene to give a moderately viscous (100-200 centipoise) solution. The specified ingredients were mixed in proper quantity to give the weight ratio of nonvolatiles shown in table I. A ceramic ball-mill was selected of such a size that when the ball-mill was about one-half to two-thirds full of high-density stone balls, the above ingredients including the solvent just covered the balls, and the mixture was ball-milled at 60 percent nonvolatiles to break up agglomerates. Ball-milling was carried out for 17 hours. After discharging the ball-mill and diluting with more solvent to reduce the total solids (pigments plus resin) to approximately 20 weight percent, the dispersion was ready for spray drying.

B. preparation of Magnetic Toner Particles by Spray Drying

Spray-drying apparatus manufactured by Bowen Engineering, Inc. of North Branch, New Jersey, was used. Precautions were taken to stir the pigment/binder solvent dispersions and maintain a uniform feed composition. The procedure consisted in atomizing a dispersion (prepared as described under Part A) by dropping it onto a disc rotating at 30,000 r.p.m. into a chamber through which heated air was swirling at a high velocity. [Bifluid nozzle atomization in which a stream of slurry containing the pigments, binders, and solvents is atomized by a second stream of air as it leaves the nozzle, or other well-known means may also be used for atomization.] The exact temperature and air velocity depend mainly on the stick point of the resin and boiling point of the solvent. In the following toner preparation involving use of a rotating disc, the inlet gas temperature was 350.degree.-363.degree. F., obtained by combustion of natural gas in air. Total gas volume was 250 standard cu. ft./minute. Under appropriate conditions, the solvent was quickly removed from the dispersed droplets, leaving discrete toner particles of pigmented resin. The particles were classified to some extent by a cyclone collection system. Toner adhering to the sides of the chamber and that from the first or bottom cyclone separator were removed by brushing into a bottle, combined, air dried a few days to remove residual solvent, designated as main fraction, and evaluated.

The main fraction of nearly spherical spray-dried particles had an average particle size of approximately 10 microns with a range of from 5-20 microns. The magnetic properties of the final encapsulated pigment were as follows: .sub.i H.sub.c, 85 Oe; saturation magnetization, .sigma..sub.s, measured at 4,400 Oe, 110 emu/g.; and remanent magnetization, .sigma..sub.r, 9 emu/g.

Magnetic toners prepared by the above procedure and with the specified composition are designated by the letter "A" in table I. For comparison, samples of the same carbonyl iron were dispersed in the same polyamide resin and spray dried by the same procedure, these samples are designated by the letter "B". In addition, an iron oxide toner was prepared in a similar fashion except that coumarone/indene resin of stick temperature 90.degree.-100.degree. C., commercially available as "450 H" from Pennsylvania Industrial Chemical Corporation, was used as binder; the toner is designated by the letter "C" in table I.

C. preparation of Thermomagnetic Recorded Images

Toner evaluation comparisons were made on chromium dioxide magnetic tapes prepared as described in Cox, U.S. Pat. No. 3,278,263. A 1,500-cycles/second sine wave signal was recorded on this 1/4 inch wide tape at 7-1/2 inch per second using a full width recording head. This is equivalent to 1,500/7.5 or 200 cycles/inch or 400 flux reversals or bits/inch of tape. The flux reversals were recorded at various levels from near saturation to a small fraction of saturation. The signal level was determined in arbitrary units by playing back the recorded tape at 7-1/2inch/second on a tape recorder and amplifying the signal from the playback head; the signal level is given in volts as a subheading in table I immediately above optical density data discussed hereinafter.

The signalled tapes were then heated to 85.degree. C. and exposed through a photographic transparency bearing printed text to a xenon flash lamp discharge as taught by Belgian Pat. No. 672,018. The flux reversals recorded on the CrO.sub.2 tape were erased by the flash exposure in areas corresponding to the clear areas of the transparency but remained as recorded where protected by those opaque areas of the transparency bearing the text. The magnetic images thus created by thermomagnetic recording were developed by the magnetic toner particles as discussed below.

D. preparation of Magnetic Toner Particle Dispersions

The toner particles were dispersed in a liquid medium for most of these tests. The toner dispersions were made from 0.75 g. of the magnetic toner particles in 150 ml. of a nonsolvent liquid, that is, hexane, methanol or water containing a small amount of Tween 20 surfactant. Ultrasonic agitation using a 10-minute exposure on a laboratory unit (General Ultrasonic Company, Model 400 ) operated at 22-52 kilohertz tuned to give maximum coupling was used in all cases. The resulting dispersions could be maintained by gentle stirring.

E. development of Magnetic Images

The chromium dioxide tapes bearing the thermomagnetically recorded images were mounted on slides and immersed for 6 seconds in the stirred dispersion from above. This 6-second time was found to be sufficient to permit the magnetic image to attract an equilibrium amount of toner. The slides were then carefully removed from the toner slurry and allowed to drain and dry. The dry toner particles were then stripped from the chromium dioxide tape by using an adhesive-coated, transparent tape that was subsequently transferred to ordinary bond paper. The reflection optical density of the transferred image was then determined using a standard device for measuring optical density, i.e., a Welch "Densichron."

For dry development, 1 g. of the magnetic toner was mechanically mixed with 5 g. of commercially available polystyrene beads (100-150 mesh, Koppers Company) containing 0.01 g. of Darco carbon black to improve powder flow characteristics. In this case the thermomagnetically recorded image was developed by pouring the toner mixture containing the polystyrene beads over the chromium dioxide tape mounted in a trough at a 35.degree. angle to the horizontal.

F. interpretation of Table I

Comparison of optical densities at corresponding tape output voltages shows a definite advantage, particularly at intermediate output levels, for the A-1 or A-2 toner formulations containing both iron and iron oxide over the corresponding B-1 or B-3 formulations containing only iron as the magnetic pigment at the same resin concentration, whether dry or in a liquid system. Higher iron concentration (B-5) or lower iron concentration (B-2, B-4, B-6) were also inferior. The carbonyl iron used made comparatively little difference. ##SPC2##

EXAMPLE 2

A cured, filled, magnetized CrO.sub.2 line pattern film embossed in the surface of polyurethane coated on 5-mil polyester film and prepared according to the copending coassigned application of Nacci Ser. No. 636,955, filed May 8, 1967, now abandoned was reflex-imaged to an original containing representative line text, including both type and graph forms. The image was developed using a machine simplified as described in example 4. The toner employed was a plastic-coated particulate magnetic composition (average size 10 microns) composed, by weight, of 40 percent of a commercially available carbonyl iron (GS-6), 39 percent of a commercially available iron oxide (Fe.sub.3 O.sub.4 -Mapico Black), 20 percent of Versamid 930 and 1 percent of a commercially available Raven 30 carbon black, which formulation was prepared in the desired particulate form by spray drying from a 50:50 blend by volume of xylene and n-propanol. The spray-dried toner had an arithmetic mean particle size of 9.2 microns, coercivity .sub.i H.sub.c of 85 Oe, .sigma..sub.s measured at 4,400 Oe of 107 emu/g., and .sigma..sub.r of 5.8 emu/g.

The slurry or dispersion used in the printing machine was prepared by mixing 170 parts of the above toner, 8 parts of a commercially available laboratory detergent ("Lakeseal" Laboratory Glass Cleaner), and 400 parts of warm (50.degree. C.) water, which mixture was finally dispersed by 10 minutes of ultrasonic agitation with stirring. Two such dispersions were combined and allowed to settle, and 520 parts of toner-free supernatant liquid was decanted therefrom for later use. The developer tank of the printing machine was charged with the remainder of the above slurry (dispersion) after agitation, and the reflex film described in the initial paragraph of this example was used as the printing master. Five thousand copies were run off at the rate of 12 per minute, at a transfer pressure of 44 lbs./linear inch using apparatus similar to that depicted in the drawing.

During the printing run the operating level of the developer slurry was maintained by readding the 520 parts of supernatant liquid previously decanted from the original dispersion preparation, 290 ml. of an aqueous solution containing 1 percent "Lakeseal" and 0.5 percent aryl alkyl sulfonate (G3,300 of Atlas Chemical Industries, Inc.) dispersing agent and 100 ml. of water. The following tabulation shows the reflection optical density of the printed images as a function of the number of copies printed versus that of the first:

Start

During the run, 159 g. of toner was consumed in printing an image recorded on an area 9-1/4.times.2-1/4 inch.

The decrease in copy density during the run was the result of slight loss of chromium dioxide form the film. This was demonstrated by checking the development and transfer with a separate but similar imaged film at the start and end of the 5,000 copy run; the reflection optical density of the printing from this film was 0.85 at both times showing substantially no loss in printing properties of the toner slurry.

EXAMPLE 3

A 480-line per inch chromium dioxide pattern incised in the surface of a5-mil thick polycarbonate (General Electric's "Lexan" ) film in the manner described in the above-mentioned copending application of Nacci, Ser. No. 636,955, was magnetized by passing it over the pole pieces of a bar magnet of approximately 1,500 gauss average field strength. The film was next imagewise demagnetized by reflex imaging against a printed text using multiple flashes about 0.72 second apart of a xenon lamp operating at 1,790 volts and 128 microfarads while both the original and the magnetized copying member were passing in intimate contact over a 5-inch diameter polymethyl methacrylate driven drum with the xenon lamp in a reflector inside with intimate pressure being maintained by an external polyurethane foam belt as in the drawing.

The resultant magnetic image was developed with an aqueous slurry containing a magnetic pigment mixture of average particle size 10 microns composed of 25 percent of a commercially available low-melting polyamide (Versamid 930 ), 43.8 percent of a commercially available Fe.sub.3 O.sub.4 ("3,000"), 29.2 percent of a commercially available carbonyl iron (GS-6), 2 percent of Raven 30 carbon black and 0.4 percent by total weight of a commercially available stearamide, which formulation was prepared by spray drying the above ingredients from a 50:50 by weight blend of xylene and n-propanol. The thus developed sheet was washed gently in water to remove toner from the background and air dried.

In another case the toner was applied to the CrO.sub.2 film, mounted on a rotary drum, by a fountain and excess toner from the background was removed by wiping means. These toner images on the CrO.sub.2 films were transferred to and fused by pressure and heat (150.degree. C.) onto the surface of a standard imaging paper normally used in A.B. Dick duplicators. The imaging papers having the fused toner on them were placed on an A.B. Dick Litho-Offset duplicator, washed with the prescribed etchant to remove the protective coating and to make the background hydrophilic. Printing was with regular litho ink, offset-blanket, and water roll. The to-be-printed areas were wet with the oil-based ink and the background, kept moist by the hydrophilic surface, was free of ink. The imaging obtained on a good commercial grade of lithopaper gave printings of the text of the original positives showing good resolution and fidelity for all the letters.

EXAMPLE 4

Toners were prepared by ball-milling the ingredients and spray drying the dispersion as described in example 1. The compositions of the final toner particles were as shown in table II. These toners were prepared under closely similar conditions, and were dispersed in water using "Lakeseal" dispersing agent and ultrasonic agitation as described in example 2. In testing the toner dispersions a simplified apparatus similar to that of the drawing was used. The simplified apparatus had no provision for magnetization and exposure and did not have a transparent drum. Instead, the CrO.sub.2 -containing reflex film was magnetized by passing it over the pole pieces of a permanent magnet, exposed using a Xenon flash lamp through an image-bearing photographic transparency, and affixed to the film drum using a double-sided adhesive tape.

The magnetic image on the CrO.sub.2 film was developed by padding on toner by means of the flock-covered padding roll 26 of the figure. The particles were carried by the flock covering and elevated to the nip with the film drum where they were squeezed between the surface of the flock-covered roll and the film drum. Both the roll and the film drum were driven at the same surface speed. Excess toner was removed and the developed image was transferred to copy.

The toners in table II were tested as nearly as possible under identical conditions. Some minor variations were made in operating conditions to optimize performance for each individual toner. The compositions, magnetic properties, particle sizes, and reflectance optical density of the copies are tabulated. ##SPC3##

EXAMPLE 5

Using a reflex film and apparatus similar to that of example 4, but without the flock-covered padding roll, a magnetic image was developed by pouring or cascading toner onto the film drum at 2 o'clock position with the machine running at 1.5 inches per second. Excess toner fell off at the 6 o'clock position. The toner was that coded A-2 in table I to which, however, 0.5 percent by weight Cab-O-Sil, a finely divided silica of about 15-20 millimicrons particle size sold by the Cabot Corporation, had been added to improve its flow properties. After transfer to paper the developed image had an average reflectance optical density of 0.65.

EXAMPLE 6

A magnetic toner was prepared by spray drying a mixture containing 22.5 percent of carnauba wax, 33.75 percent Mapico Black Fe.sub.3 O.sub.4, 33.75percent GS-6 carbonyl iron and 10 percent of the cationic modifier Aliquat 207 dimethyldistearylammonium chloride, (percentage by weight). The spray-dried product was sieved through a 400 -mesh screen after adding 0.5 Cab-O-Sil silica to render the toner free flowing. The particles were predominantly 5-10 micron spheres. This toner was termed JCS-1. Another toner termed JCS-2 was prepared, which was identical in preparation and composition to JCS-1, except that the magnetic component consisted entirely of Mapico Black, which comprised 67.5 percent of the toner.

Toner properties of the two compositions were compared by cascading each against the magnetically imaged surface of a thermomagnetic film exposed in the manner described in the first paragraph of example 4. The developed images were printed by pressure contact with paper. The toner designated as JCS-1 was clearly superior, giving a reflectance optical density of 0.86 in the image area as compared to 0.50 for the toner JCS-2.

EXAMPLE 7

Spray-dried toner (120 g. ), prepared as described in example 1 from 40 percent GS-6 carbonyl iron, 39 percent Mapico Black Fe.sub.3 O.sub.4 1 percent "Permanent Black", and 20 percent Versamid 930, was dispersed in 400 ml. of 2 percent "Lakeseal" in water, evaporated to dryness in an oven at 50.degree. C., and gently crushed to pass a 40-mesh screen. The resulting surfactant-coated particles dispersed easily in water and served both to prepare dispersions for use in printing machines as in example 2 and to replenish toner removed from a dispersion by image development. By additions of the powder and deionized water, a single dispersion was used to print about 55,000 copies.

Other surface active agents may also be employed, usually at a concentration of 2 percent or less by weight based on toner, to provide self-dispersing properties. These include Atlas "G B3300" , an anionic, general purpose, branched-chain alkyl aryl sulfonate surfactant, DuPont "Merpol" SE, a general purpose, nonionic, ethylene oxide condensate, and DuPont "Product BCO", a C-cetyl betaine amphoteric surfactant

EXAMPLE 8

Spray-dried toner consisting of 40 percent GS-6 carbonyl iron, 39 percent Mapico Black Fe.sub.3 O.sub.4, 1percent "Permanent Black", and 20 binder (percentage by weight) was prepared as described in example 1. The "binder" consisted of a mixture of 20 parts of Elvax polyethylene/vinyl acetate, 10 parts of carnauba wax, and 1 part of an alkyl aryl sulfonate surfactant. The toner dispersed in water upon stirring without further addition of dispersant, and the resulting dispersion was successfully used in the manner described in example 4 in image development and transfer operations.

Many of the disadvantages apparent for the prior art toners do not exist for the binary toners of this invention. First of all, not only does the hard magnetic material contribute to the magnetic properties of the product, but also it is the primary color agent of toner; e.g., toners and their decorations containing only approximately one-fourth by weight of Fe.sub.3 O.sub.4 or CrO.sub.2, the rest being Fe and resin, are very black. During the resin encapsulation process the finely divided hard magnetic material, which is close to single domain in size, has sufficient magnetic moment to magnetically interlock all of the magnetic particles including the soft magnetic material into a loose resin-impregnated three dimensional network. This network effectively resists any tendency for segregation of resin from the magnetic material. Any slight tendency of the large particles of soft magnetic material to be less than uniformly distributed among the final toner particles is of small consequence because all particles uniformly contain the vary finely divided hard magnetic material and so no nonmagnetic toner particles exist.

The mixed toners possess a desirable balance of magnetic properties. Their magnetic moments in the fields close to a magnetically imaged surface are somewhat less than for soft magnetic toners but are significantly higher than the remanent moments of hard magnetic toners. The remanent moments of the mixed toners are modest. They are large enough to result in an attractive force on the toner at some distance from an imaged surface; yet, they are not large enough to make magnetic flocculation of the toner a problem. Also considerable flexibility in tailoring the magnetic properties to the needs of an application exists via adjustment of the ratios of the two magnetic materials. The superior performance of the mixed toners in a practical application, which results from their desirable combination of properties, is clearly evident in the data of tables I and II. The optical densities of visual images formed magnetically using these toners are consistently the highest obtained.

Since obvious modifications and equivalents will be evident to those skilled in the art, we propose to be bound solely by the appended claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A flowable particulate magnetic toner substantially each particle of which comprises at least one finely divided magnetically hard material having a remanence of at least about 20 percent of the saturation magnetization, and at least one finely divided magnetically soft material having a remanence of less than about 5 percent of the saturation magnetization, said hard and soft materials being substantially uniformly dispersed in a compatible nonmagnetic thermoplastic resin present at about 20 percent to 40 percent by weight of the entire composition, the weight ratio of magnetically hard to magnetically soft material being in the range 1:6 to 4:1.

2. The magnetic toner of claim 1 wherein the particles are substantially spherical.

3. The magnetic toner of claim 1 wherein the weight ratio of magnetically hard to magnetically soft material is about 1:1.

4. The magnetic toner of claim 1 wherein the diameter of the flowable particles ranges from about 3 to about 20 microns.

5. The magnetic toner of claim 1 wherein the resin is a polyamide.

6. The magnetic toner of claim 1 containing additionally from about 1 to 5 percent of a color control agent selected from the group consisting of carbon black, pigments and dyes.

7. The magnetic toner of claim 6 wherein the color control agent is black.

8. The magnetic toner of claim 1 wherein the magnetically hard material is a magnetic iron oxide.

9. The magnetic toner of claim 1 wherein the magnetically hard material is magnetic chromium oxide.

10. The magnetic toner of claim 1 wherein the magnetically soft material is iron.

11. The magnetic toner of claim 1 wherein the magnetically hard material is Fe.sub.3 O.sub.4 and the magnetically soft material is iron.

12. The magnetic toner of claim 1 wherein the magnetically hard material is magnetic chromium oxide and the magnetically soft material is iron.

13. The magnetic toner of claim 1 carrying up to about 2 percent by weight of a dispersing agent.

14. The magnetic toner of claim 1 wherein the dispersing agent is adhered to the outside of the particle.

15. The magnetic toner of claim 1 wherein the dispersing agent is distributed throughout the particles.

16. A dispersion of the magnetic toner of claim 1 in an inert, liquid dispersions medium.

17. The dispersion of claim 16 wherein the dispersions medium is a polar solvent.

18. The dispersion of claim 16 wherein the dispersions medium is water.

19. The dispersion of claim 16 wherein the dispersions medium is hydrocarbon.

20. The dispersion of claim 16 wherein the dispersions medium is a halogenated hydrocarbon.

21. A dry mixture of the magnetic toner of claim 1 with an inert solid, particulate carrier.