CA1105759A - Method for forming a spray dried toner of superparamagnetic particles dispersed in polymer - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A superparamagnetic developer comprising particles of superparamagnetic material dispersed in a solid polymer.
The developer is formed by adding a ferrofluid comprising a colloidal dispersion of the superparamagnetic in a carrier liquid, which is either a solvent for a polymer or miscible with a solvent for a polymer, to a solution of the polymer in the solvent; and spray drying the resulting colloidal dispersion.

Description

BACKGROUND OF THE INVENTION
This invention relates to magnetic developers;
and, more particularly, to single component superparamagnetic dry developer.
There has recently been introduced a magnetic imaging system which employs a latent magnetic image on a magnetizable recording medium which can then be utilized for purposes such ; as electronic transmission or in a duplicating process by repetitive toning or transfex of the developed image, Such latent magnetic image is provided by any suitable magnetization procedure whereby a magnetized layer of marking material is magnetized and such magnetism transferred imagewise to the magnetic substrate. Such a process is more fully described in U.S. Patent 3,804,511 to Rait et al.
The latent magnetic image, by way of analogy to xerographic imaging, is developed with a magnetic developer to render the latent magnetic image visible. The developed, visible, magnetic image is then typically transferred to a receiver such as, for example, a sheet of paper to produce ~;~20 a final copy or print. This final copy or print is typically referred to as hard copy.
Concurrently with the growth of interest in magnetic imaging, there has been a growth of interest in magnetic ;developers to render the latent magnetic images visible.
For example, U.S. Patent 3,221,315 to Brown et al.is directed to the utilization of encapsulated ferrofluids in a magnetic recording medium wherein the ferrofluid orientation in the , ~
~; presence of a magnetic field exhibits a variable light-responsive characteristic. In this case, the magnetic recording medium is self-developing in the sense that magnetic marking material need not be employed to present a visible image. In other

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situations, latent magnetic images are rendered visibl~ by magnetic marking material, For example, U.S. Patent 3,627,~82 to ~all et al,is directed to binary toners for developing latent magnetic images. These binary toners include a particulate hard magnetic material and a particulate soft magnetic material in each toner particle. The toner particle includes these two materials and a binder material. For dry development, the toner particles are mechanically mixed with polystyrene beads to form a two component developer~
U,S Patenti 2,826,63~ to Atkinson et al,discloses the use of iron or iron oxide particles, either alone or en-capsulated in low-melting resins or binders, for developing latent magnetic images. Low optical density and relative unresponsiveness to weak magnetic fields are exhibited by relatively large iron or iron oxide-based magnetic particles.
Other patents 0v.idencing the continuing interest in improved magnetic developers include U,S. Patent 3,520,811 to Swoboda and U,S, Patent 3,905,841 to Simonetti. ~he Swoboda patent is based on the discovery that magnetic particles of chromium dio~ide appear to catalyze a surface polymarization of organic air-dryi~g film-forming vehicles, such as those employed in oil-based paints, so that a coating of polymerized vehicle is formed about the particle. The Simonetti patent is directed to the prevention of agglomeration and the formation of homo-geneous dispersion of cobalt-phosphorous particles into an organic resin binder by treatment with a solution containing sulfuric acid.
In new and growing areas of technology, it is often desirable to provide components~with improved functions that are prepared by relatively simple procedures.

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, Thus, in accordance with the present invention, there is provided a method for forming a toner comprising superparamagnetic particles disbursed in a solid polymer. A homogeneous liquid col-loidal dispersion is formed by adding a ferrofluid comprising superparamagnetic particles suspended in a carrier liquid to a ~ solution of polymar dissolved in a solvent, the carrier liquid and ; the solvent being miscible. The liquid colloidal dispersion so formed is spray dried.
More specifically, and in accordance with this inver,tion, a homogeneous liquid colloidal dispersion may be formed by adding ; a errofluid comprising ultra-fine, superparamagnetic particles suspended in a solvent for a polymer to a solution of said polymer in the same solvent; and, spray drying the liquid colloidal dis-persion. The resulting superparamaynetic developer provides:
higher permeabilities compared to relatively larger particles of the same magnetic materials, zero remanent magnetization, no hysteresis, and the option of lower magnetic material loadings for equivalent initial magnetization. Hard copy of good visual quality is obtainable with developers having relatively low loading of the magnetic component.
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DETAILED DESCRIPTION OF ~HE PREFE _ D EMBODIMENTS
"Ferrofluids" as used herein, means stable colloidal dispersions of *erri- or ferromagnetic particles in a liquid medium. These particles do not flocculate ox settle out under the influance of either gravitational or magnetic fields.
Ferrofluids behave much like their liquid vehicle in the absence of a magnetic field; however, when a field is applied, 35~
the colloidal particles carry their surfactant coating in a substantial solvation sheath along the field gradient.
Typically, -the surfactant employed is a long-chain organic acid such as oleic acid, linoleic acid,' succinic acid derivatives, ECA 3852 manufactured by Exxon Corporation.
The particles are less than a critical size to be colloidally stable and in order that the energy of domain wall formation be greater than the energy for rotation of all the spins in the particle.
For a description of ferrofluids, see "Magnetic Fluids", by S.E. Khalafalla in Chem. Tech., page 540, September, 1975; and "Some Applications of Ferrofluid~-Magnetic Colloids", by Kaiser and Miskolczy, IEEE Transactions on Maqnetics, page 694, Vol. M~G-6, No. 3, Septemher, 1970.
Generally speaking, magnetic material can exhibit three modes of magnetic behavior depending on particle size.
Superparamagnetic behavior, one of the three modes, is exhibited by different materials at particle sizes which vary with the identity of the material. A simpli~ied rela~ionship which allows an approximation for determining t~le maximum particle diameter at which a given material will exhiblt superpara-magnetism is given by the equation "critical volume = 25 kT/K"
where k is Bo1tzmann'slcbnstant;~ 3~ x 10~16 erg/--), T~is-the Kelvin tamperature, and K is the magnetic anisotropy in erg/cm3.
This equation is an approximation for spherical uniaxial particles of equal size. The anisotropy values for K are relaked tv the measureable anisotropy constants Kl and K2 by:
K = Kl/4 when Kl is greater than 0; and K = Kl/12 + K2/27 when Kl is less khan 0. For example, at T = 298 Kelvin, the maximum diametars for superparamagnetic behavior in spherical ; particles of iron, co~alt, and magnetite are raspectively:
250~, 1202 and 600~. For a more thorough discussion of super-7 ~ ~

paramagnetism, see "Superparamagnetism" by C.P. Bean and J.D. Livings~on, J. Appl. Phys., Supplement t~ Vol. 30, ~o. 4, page 1205 (1959).
l'ypical suitable ferrofluids for use in accordance with the practice of the present invention may be prepared by ball-milling the magnetic particles for periods of about 1000 hours in the presence of a surfactant as disclosed in "Magnetic Properties of Stable Dispersions of Sub-Domain Magnetite Particles", J. Appl. Phys., Vol. 41, page 1064, R. Kaiser and G. Miskolczy (1970). Residual coarse material must be centrifuged or allowed to settle out in order to produce a stable ferrofluid by this mechanical milling method. Other methods of preparing ferrofluids include chemical precipitation methods. See, for example, "Preparing Magnetic Fluids by a Paptizing Method", U.S. Bureau of Mines Technical Progress Report 59, G,W. ~eimers and S.E. Khalafalla, Septe~ber, 1972, and U.S. Patent 3,228,881 to Thomas directed to a method of preparing a dispersion of discrete particles of ferromagnetic metals.
"Ultra-fine'' as used herein means the particle size or sizes at which magnetic materials are both single domain and exhibit superparamagnetism. See, "Introduction to Magnetic Ma~erials", B.D. Cullity, Addison-Wesley Publishing Co., Reading, Massachusetts, 1972, page 387. The maximum diameter for an assumed spherical uniaxial particle at which superparamagnetism is exhibited by any particular magnetic material is given by the equation previously discussed. Accordingly, for any given magnetic material, particle diameters at and below this maximum diameter for that material come wi~hin the meaning of "ultra-fine".
Commercially available ferrofluids, such as, for exampls, 3~ ferrofluids exhibiting superparamagnetic behavior can be employed in the practice of the present invention. Such ferrofluids are commercially available from Ferrofluidics Corporation of Burlington, Mass.

Once the superparamegnetic ferrofluid has been prepared, by whatever method, or obtained commercially, it can be added to polymers in solution and the resulting dis-persion may be spray dried to give a solid polymer particle containing dispersed superparamagnetic material. As previously stated, the carrier or liquid portion of the ferrofluid can be either a solvent for the polymer to be employed or a liquid which is miscible with the polymer solvent. A solution of the polymer and solvent is separately prepared in order that the ferrofluid and polymer solution are brought together to form a homogeneous liquid colloidal dispersion. If the ferrofluid prepared or obtained contains a carrier liquid of which is not a solvent for the polymer desired to be used, the superparamagnetic particles of the ferrofluid are separated from the carrier liquid by drying or other suitable techniques and redispersing the colloidal particles in the appropriate solven~ for the desired polymer with the aid of dispersing agents. Refere~ce may be had to U.S. Patents 3,917,538 and 3,928,220 which teach suitable carrier liquid exchange techniques.
Further, the ultra fine particles in an a~ueous carrier can be extracted or flushed into a hydrocarbon carrier by means of transfer ~gents or utilization of flushing technigues such as those outlined by H.N. Steuber in American Ink Maker, Vol, 30, page 6, 1952. Methods of preparing ferrofluids, transferring and ~lushing from aqueous to hydrocaxbon carriers of the magnetic particles in ferrofluids, and the identification of superpara~
magnetic commercially available ferrofluids are known in the art and need not be repeated herein.
Similarly, methods of spray drying to produce electro-statographic ~xerographic) and magnetic toners and developers are known in the art and any of the conventional spray drying ~methods can ~e utilized, whether the Rayleigh mode or disc ~praying modo or other modes are desired to be used, It will also be appreciated that several modifications to the developer provided by the present invention can be employed without departing from the spirit of the present inven~ion. For example, to reduce triboelectrification of the developer, the polyrner utilized can be compounded with antistatic agents, electroconductive polymers, or conductive fillers. These additives can be dispersed throughout each developer particla by putting the additives into the homo-geneous liquid colloidal dispersion before spray drying.
Alternatively, these additives can be added only to the developer particle surface.
Latent magnetic images developed with the super-paramagnetic solid polymer developer of the present invention are very nearly black in color but do have a slight brown-red hue, This hue may be altered by the addition of either dyes or pigments as is conventional and well known in the graphic arts. For example, the addition of a cyan colorant would increase scattering and/or absorption in the red region of the spectrum and thereore render the developer guite black in color.
Alternatively, carbon black can be added to the dispersion prior to spray drying and at a lev~ of about 2% or less by weight, Such a small amount of carbon black would have only a minor influence on the developer melt flow.
The solid polymer can comprise any suitable polymer ~5 conventional to xerographic toners or a semicrystalline polymer with a low melting temperature; i.e,~ below the scorch temperature of paper. Preferably, the solid polymer is fusible and therefore is preferably a thermoplastic with a low glass transition temper-ature and a low rnelt viscosity. Typical xerographic toner polymers include styrene-n-butylmethacrylate, polystyrene and low melting polyesters, Other suitablc thermoplastic materials include Staybelite Este ~10, a partially hydrogenated rosin ester from Hercules Corporation~
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more particularly described in U.S, Patent 3,307,941 to Gundlach; Eloral Ester, a hydrogenated rosin triester, and Neolyne 23, an alkyd resin, all from Hercules Corporation;
SR type silicone resins available from General Electric Corp.;
Sucrose Benzoate, from Eastman Chemicals; Velsicol X-37, a polystyrene-olefin copolymer from Velsicol Chemical Corp.;
hydrogenated Piccopale 100, Piccopale H-2, a highly branched olefin; Piccotex 100, a styrene-vinyl toluene copolymer, Picco-lastic A-75, 100 and 125, all polystyrenes; Piccodiene 2215, a polyst~rene-olefin copolymer, all from Hercules Incorporated;
Araldite 6060 and 6071, epoxy resins from Ciba; R50~1A, a phenylmethyl silicone resin, from Dow Corning; Epon 1001, a bisphenyl A-epichlorohydrin epoxy resin, from Shell Chemical -Corp.; and PS-2, PS-3, both polystyrenes, and ET-693, a phenol formaldehyde resin, from Dow Chemical; custom synthesized copolymers of styrene and hexylmethacrylate, a custom synthesized polydiphenylsiloxane; a custom synthesized polyadipate; acrylic resins available under the trademark Acryloyd from Rohm and Haas Company and available under the trademark ~ucite from the E,I. DuPont de ~emours and Co.; thermoplastic resins available under the trademark Pliolite from the Goodyear Tire and Rubber Company; a chlorinated hydrocarbon available under the trademark Aroclor from Monsanto Chemical Co.; thermoplastic polyvinyl resins available under the trademark Vinylite from Union Carbide CoO; other thermoplastics disclosed in Gunther et al U.S. Patent 3,196,011.
; Typical conductive softenable materials, when these are desired to be incorporated in the blend to overcomc tribo-electriEi~ation, include Ethocel, an ethylcellulose material from Dow Chemical Co., polyhexamethylene sebacate; polyvinyl alcohol; polyvinylbenzyltrimethyl ammonium chloride and others. As previously indicated, preferred materials would _.9_ . .

have a low glass transition ter,lperature or melting temperatur2, By "low" we mean materials having a transition temperature below about 60C.
_ The solid polymers can be dissolved in any suitable solvent Typical suitable solvents include toluene, tri-chloroethylene, xylene, methylethyl ketone, cyclohexanone, kerosene, carbon tetrachloride, petroleum ether, silicon oils such as dimethylpolysiloxanes, long chain aliphatic hydrocarbons oils, chlorobenzene, benzene, hexane, acetone, vegetable oils, fluorinated halocarbons, Sohio Odorless Solvent 3440, a kerosene fraction available from Standard Oil of Ohio, etc~ It will be ; appreciated that solvents selection is made on the basis of the selected solid polymer and requirements of volatility for spray drying.
The invention will now be described in more detail by way of exa~ples. All parts and percentages are by weight unless otherwise indicated These examples are intended to be illustrative of preferred embodiments of the present invention and the invention is not to be corstrued as limited thereto, EXAMæLE I
~ polymer solution is prepared by adding about 1.22 grams of polystyrene commercially available from Dow Chemical Co, under the trademark Styron, No. 666U, to 113 milliliters of toluene. The ferrofluid employed comprises ultra-fine magnetite (Fe3O4) particles dispersed in toluene, commercially ; available from Ferrofluidics Corporation, F436B. Upon adding 1.56 milliliters of the ferrofluid to the polymer solution previously prepared, a homogeneous non~separating liquid colloidal dispersion, of dark brown color, is formed.
I~e homogeneous liquid colloidal dispersion is spray dried in a Rayleigh mode sprayer to form nearly sphcrical .~ .

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particles of solid polystyrene from about 5 to about 15 mic~-meters in diameter containing the ultra-fine magnetite particles dispersed in the polymer. The average size of the nearly spherical polymer particles is about 11 micrometers as determined 5 with the use of a Coulter Counter.
Electron micrographs were taken on about 500~ thick sections cut by ultramicrotome from an epoxy embedment of 41~q/~ nearly spherical polymer partic~es. The micrographs resolve individual magne-tite particles of about 150A in diameter and reveal that the magnetite particles are dispersed as aggregates of particles within the polystyrene, The magnetite particle aggregates range from about 3~ to about 2100~ in diameter.
The nearly spherical polymer particles are discovered to have, in addition to magnetite particles dispersed therein, a crust of magnetite particles which partially, or sometimes completely surrounds the particles; with the crust ranging from about 2402 to about 2800A in thickness.
The nearly spherical polymer particles are very nearly black in color and have the characteristics o~ mobile powders. They are readily attracted by a bar magnet. T~le magnetic pr~perties were investigated by the use o~ a vibrating i sample magnetometer. With~n the estimated uncertainties of the measurements, the polymer particles are determined to have zero coercivity and zero remanent magnetization. They ar~, therefore, superparamagnetic as was tha precursor ferrofluid.
Magnetization M in units of emu/gram as a function of both increasing and decreasing applied magnetic ~ield H in units o~
Gauss were obtained by use o~ the vibrating sample magnetometer.
These values are tabulated in Table I, below.

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TABLE I*
Increasinq Field ecreasinq Field H (Gauss) M (emu/qram) H ~Gauss) M (emu/qram) o.o 0.00 -4,5 -0,22 55.7 0.29 -2.8 -0.13 10.0 0.50 -1.6 -0.07 1.19 -0.~ 0.00 2.22 0.8 0.06 3.16 25 1.24 10100 4.05 50 2.28 150 5.56 100 4.09 200 6.83 150 5.58 250 8.02 300 ~,96 300 9.02 15400 10.59 500 11.91 750 14.23 1000 15.731000 15,80 `1500 17.551500 17.62 202000 1~,742000 18.81 3000 20.19 4000 21.00 5000 21,69 7500 22.70 2510000 23,26 12000 23.64 14000 23.89 * Estimated uncertainties are: H + 1 Gauss or ~ 1%
above 10 kilogauss; M + 2%.

EXAMPLE II
Example I is repeated exc~pt;that the polymer - solution comprises 1 part by weight styrene-n-butylmethacrylate dissolved in 113 milliliters of toluene. Upon addition of 1.6 , milliliters o~ ferroflu.id,,,, a homogeneous colloid resulted ; 35 without any evidence of phase,separa~ion. ~S~ray~rying o~
this homogeneous colloidal dispersion was achieved by Rayleigh mode spraying to provide nearly spherical particles having a diameter of from about 5Ito`about 15 micrometers. The resulting spheres are dark brown in color.
With the use of a vibrating sample magnetometer, ~he magnetization M as a function of applied increasing and decreasing magnetic ~ield H was determined and is reported in Table II, below:

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TABLE II*

Increasinq Field Decreasinq Field H (Gauss) M (emu/gram) H (Gauss) M (emu/qram) 0 0 0 0,04 0.23 5 0.28 0.46 10 0.48 1.08 25 1.13 2.05 50 2.08 2.90 75 2,92 10100 3,67 100 3.69 150 5.03 150 5 04 200 6.19 200 6 22 250 7,21 250 7.22 300 8,02 300 8.10 15400 g.53 400 9.53 ~00 10.73 500 10.73 750 12.8~ 750 12.80 1000 14.15 1000 14.15 .: 1500 15.82 ~02000 16.86 2000 16.86 3000 18.13 3000 18.13 4000 18.93 4000 18.93 5000 19,49 5000 19 49 7500 20.36 7500 20 36 2510000 20.92 10000 20.92 2000 21.~4 12000 21.24 14000 21.4~

. * Same estimated uncertainties as in Table I.
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EXAMPLE III
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Th~ magnetic developer comprising ultra-fine ~`~ magnetite particles dispersed in a solid polymer produced by Example II is used to develop a latent magnetic image. The superparamagnetic developer produced by Example II comprises about 41 weight percent (12 volume percent) magnetité--dispersed in the styrene-n-butylmethacrylate (melt index 50). The magnetic labent-.~mage ~si:~r~pared by;sexposi~g a~~pre-r~corded~magnetic tape commercially available from Dupont under the trademark Crolyn to the exposure o~ a Xe~on gaseous discharge lamp through a mask so that light impinges on~t~e~:magnetic.tape in imagewise configuration at an intensity sufficient to raise the tape above its Curie point (about 130C) in imagewise configuration.
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. -13-:, - ., The exposure is a flash exposure and is conducted in the absence of a magnetic field; the tape being allowed to cool in the absence of the magnetic field thereby thermo-remanently creating a latent magnetic image. The superpara-magnetic polymer developer was dusted onto the magnetictape and the tape tilted back and forth to move the polymer particles over the tape surface. This rendered the latent magnetic image on the tape visible as the polymer particles were magnetically attracted to the image area. Excess developer was removed by tilting the tape and by a gentle blow-off with Freon from a pressure can.
Transfer was accomplished by placing the developed tape on an 8 1/2 x 11" paper, commercially available from Xerox Corporation as Xerox 1024 Paper, and holding the tape in position on the paper with transparent tape. Another sheet of the same paper was placed on top of the image and this sandwiched structure was passed between motor driven steel rollers, Developer transferred from the magnetic tape surface to the other sheet of paper. The transfer process resulted in a partially fixed image. Fixing was completed conventionally in a Model D processor oven, Tha resulting hard copy is of high visual quality with acceptable background and excellent solid area coverage, having a maximum density of about 1.1. Using the Air Force Resolution Test Pattern on the hard copy, the resolution measured was abo~t 18 line pairs per millimeter which is of the same order as the resolution of the magnetic image on the magnetic tape, While this invention has been described with respect to particularly prcferred embodiments, it will be appreciated by those skilled in the art that the invention is not limited , :

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thereto. For example, any magnetic material whether con-ventionally designated "hard" or "soft" can be employed in the practice of the present invention provided the particles are sufficiently small to exhibit superparamagnetism. Typical such particle sizes include, but are not limited to, particles from about 50A to about 600A in size.
Further, the carrier liquid or solvent of the precursor ferrofluid need not be the same solvent in which the solid polymer is dissolved; it being understood that the object sought is a homogeneous colloidal dispersion upon addition of the polymer solution and the ferrofluid. Accordingly, the carrier liquid or solvent in the ferrofluid need only be miscible with the solvent in the polymer solution.
Further, it will be understood that when pressure alone is desired for fixing,low molecular weight polymers having approximate molecular weight at or helow about-25,000 are.preferr~d, , ';

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Claims (8)

WHAT IS CLAIMED IS:

1. A method for forming a supernaramagnetic toner comprising superparamagnetic particles dispersed in a solid polymer, comprising the steps of:
(a) forming a homogeneous liquid colloidal dispersion by adding a ferrofluid including a surfactant comprising super-paramagnetic particles suspended in carrier liquid to a solution of an organic polymer dissolved in a solvent, said carrier liquid and solvent being miscible; and (b) spray drying the liquid colloidal dispersion.

2. The method as claimed in Claim 1 wherein said superparamagnetic particles are within the particle size range of from about 50.ANG. to about 600.ANG..

3. The method according to Claim 1 wherein said superparamagnetic particles comprise magnetite.

4. The method according to Claim 3 wherein said magnetite comprises particles of about 150.ANG. in diameter.

5. The method according to Claim 3 wherein said polymer comprises polystyrene.

6. The method according to Claim 3 wherein said polymer comprises styrene-n-butylmethacrylate.

7. The method of Claim 5 wherein said carrier liquid and said solvent comprise toluene.

8. The method of Claim 6 wherein said carrier liquid and said solvent comprise toluene.