Ferrofluid compositions and process of making same

Abstract

A process for preparing irreversibly flocked magnetic particles, which process increases the versatility of the size-reduction process for the preparation of ferrofluids. The process comprises producing a ferrofluid in an aqueous carrier liquid with a dispersing agent by grinding coarse magnetic materials, removing the dispersing agent and attaching a different dispersing agent to the ground magnetic particles, and redispersing the particles in another carrier liquid. The process provides for the preparation of irreversibly flocked magnetic particles, and the preparation of alternate ferrofluids containing such particles.

Description

BACKGROUND OF THE INVENTION

Ferrofluid is a ferromagnetic fluid displaying superparamagnetism, having a magnetic polarizability that is substantially uniform and having the property such that when a gradientmagnetic field is applied to it, a body force is developed within it which can exceed by orders of magnitude the ordinary force of gravity on a unit volume of the material. Typically, ferrofluid comprises a colloidal dispersion of finely divided magnetic particles of subdomain size whose liquid condition is remarkably unaffected by the presence of an applied magnetic field, and which particles resist settling under the influence of gravitational, centrifugal, magnetic or other force fields. Ferrofluid particles typically ranging in size up to about 300 A remain uniformly dispersed throughout the liquid carrier due to thermal agitation.

Ferrofluids are described in my publication "Magnetic Fluids," International Science and Technology, July 1966, pp 48-56; U.S. Pat. No. 3,215,572; in the publication of R. Kaiser and G. Miskolczy, "Magnetic Properties of Stable Dispersions of Subdomain Magnetite Particles," J. Applied Physics, Vol. 41, No. 3, Mar. 1, 1970, pp 1064-1072; in "A Catalog of Magnetic Fluids", Ferrofluidics Corporation, Burlington, Massachusetts 1972 and elsewhere.

In the prior art, a magnetic powder is reduced in size to the colloidal range as by ball-mill grinding in the presence of a liquid carrier and a grinding aid which serves also as a dispersing agent. The dispersing agent is typically a surfactant comprising a polar long-chain molecule whose polar group adsorbs onto the surface of the particle to produce a monomolecular protective coating that prevents particles from attaching to each other.

U.S. Pat. No. 3,531,413, hereby incorporated by reference, describes a method for transferring magnetic-coated particles of a ferrofluid from one carrier liquid to another by a process of flocculation using a foreign solvent, separation of flocculated particles from supernatant liquid, and transfer of the particles into an alternate solvent in which the coated particles are dispersible. The carrier liquids that are employed must be similar in their physical chemical properties. The flocculation process is here termed reversible with the coating of a dispersing agent on the particles as in the prior art remaining attached and unchanged. As a result, the coated, flocked particles are redispersable into the pure original solvent.

SUMMARY OF THE INVENTION

My invention relates to an improved process for the preparation of ferrofluids and to the ferrofluids so produced, and in particular, concerns a process for the flocculation of magnetic particles from a ferrofluid in an irreversible manner, and the treatment of such irreversibly flocked particles to prepare improved ferrofluid compositions.

I have discovered that ferrofluids may be flocculated by the addition of foreign agents or solvents in an irreversible manner such that the dispersant employed; that is, the surfactant that originally stabilized the magnetic particles in the ground ferrofluid, is removed and separated from the ground particles. I have further found that the irreversibly flocked magnetic particles from which the first dispersant agent has been removed may be treated with another dispersant agent so that the dispersant agent is attached to said particles, and that such particles may be employed to prepare alternate ferrofluid compositions. Such alternate ferrofluid compositions prepared by my process and the particles of my process exhibit improved properties over prior art ferrofluid compositions. In particular, the ferrofluid compositions of my invention are well suited for application in magnetic fluid seals, bearings and related devices. My alternate ferrofluid compositions exhibit better; that is, lower, viscosity properties, while the high packing of the volume fractions of the irreversibly flocked particles in the ferrofluid composition permits saturation magnetization of over 300 gauss. My ferrofluid composition provides for the advantages of a wide temperature range of operation, high magnetic intensity, low viscosity, oxidation resistance and colloidal stability in the presence of intense magnetic field gradients.

My process comprises preparing a ferrofluid composition by preparing a dispersion of colloidal-size magnetic particles, and a dispersant in a liquid carrier, such as by grinding through a ball-milling operation coarse magnetic solids in the presence of a long-chain polar molecule as a surfactant and dispersant, with a water carrier. My process then includes removing the dispersant from the ground magnetic particles of the ferrofluid composition, such as by adding a foreign agent or solvent to flocculate the particles, and the wet dispersant-stripped particles removed from the supernatant liquid. The wet dispersant-stripped particles are then washed free of any solvent and any remaining dispersant agents, and heated to drive off moisture and to obtain dried dispersant-stripped irreversibly flocked particles. Such irreversibly flocked particles are then treated with a dispersant, and redispersed in another and different carrier liquid to provide an alternate ferrofluid composition having improved properties. The treatment of the irreversibly flocked, dried magnetic particles with a dispersant attaches the dispersant to the magnetic particles. The second dispersant may be attached to the magnetic particles by grinding the particles in a ball mill, together with the dispersant and the alternate carrier liquid to obtain the alternate ferrofluid composition.

My process permits the preparation of ferrofluids wherein the ratio of magnetic solid-core particle volume relative to the total volume of the dispersant-coated particle is greater than found in ferrofluids prepared by direct grinding processes. This greater volume permits much better packing of the magnetic solid particles in the liquid carrier. For example, a comparison of a 120 Angstrom versus a 100 Angstrom spherical particle, with both having a 20 Angstrom dispersant coating, provides for a 16 percent increase in solid packing density; however, I have found that such increase in solid packing by my process provides for unexpected increases in saturation mgnetization over that expected for the improved packing density.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a, b and c are schematic presentations of the stability of magnetic particles in ferrofluid compositions.

FIG. 2 is a schematic process-flow sheet illustrating the process of preparing the improved alternate ferrofluid compositions of my invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 a, b and c schematically illustrate various states of stability of colloidal magnetic particles of a ferrofluid composition, with FIGS. 1 a and b directed to stability states of the particles of the prior art, while FIG. 1 c is directed to the stability state of the magnetic particles of my invention, which particles have been irreversibly flocculated. As illustrated, the magnetic particles 12 are suspended in a liquid carrier 10, the carrier containing the dispersing agent for dispersant, typically, a long-chain surfactant molecule having a polar group 14, with the polar group illustrated by a circular configuration, with the long-chain portion of the molecule schematically illustrated by a jagged line attached to the polar circular head.

FIG. 1 a illustrates the stabilized magnetic particles of the prior art, the particles showing a stabilized condition, with the surfactant molecules adsorbed on the particle surface. The long-chain-like tails of the surfactant molecules are extended and well solvated with the surrounding liquid carrier 10. The adsorbed monomolecular surfactant layers on the adjacent particles furnish a steric hindrance, preventing the particles' surfaces from approaching close enough to agglomerate the particles under the influence of van der Waal's or magnetic forces of attraction. The stabilized condition of the magnetic particles as illustrated in FIG. 1 a are those prepared in a typical process for preparing ferrofluid compositions.

FIG. 1 b illustrates a reversibly flocculated group of particles wherein the surfactant, while still adsorbed on the surface of the particles, is recoiled to a smaller effective radius by such reversible flocculation. The shielding is incomplete with the particles loosely bound together under the forces of mutual attraction. Ferrofluid compositions containing reversibly flocculated particles as illustrated in FIG. 1 b are typically prepared in a manner set forth in my U.S. Pat. No. 3,531,413.

FIG. 1 c schematically illustrates the irreversibly flocculated particles' condition in a ferrofluid composition of the present invention and process, wherein the surfactant molecules 14 are no longer secured to the particles' surface, but instead, by my process, have been removed and are held in solution by a surrounding liquid carrier. The surfactant-stripped magnetic particles illustrated in FIG. 1 c and prepared by my process are more or less firmly attached to each other as illustrated, and will not redisperse in the flocculant-free carrier liquid, whereas the particles of the prior art in a reversible flocculation as depicted in FIG. 1 b would redisperse. An important feature of my invention is my discovery and recognition of two different types of flocculations in ferrofluid compositions containing magnetic particles, and the incorporation of such recognition into a process which provides particular advantages for the preparation of ferrofluids and for the ferrofluid composition so prepared.

The materials which may be employed in the practice of my process and the preparation of my improved ferrofluid compositions are those materials, such as magnetic particles, dispersants and the carriers which are presently employed for ferrofluid compositions. For example, the finely divided magnetizable particles include the materials usually recognized as being magnetic, such as magnetite, gamma iron oxide, chromium dioxide, ferrites, such as manganese-zinc ferrite, manganese ferrite, nickel ferrite and many similar materials. Such materials include also elements and metallic alloys, such as cobalt, iron, nickel, gadolinium, and samarium-cobalt. The preferred materials for practice of the present invention are magnetite and gamma iron oxide. Typically, such magnetic particles are present in a ferrofluid composition in particle size ranging from about 20 A to 300 A, with the average in particle size being from about 100 A to 120 A. The magnetic particles are usually present up to about 20% by volume of the ferrofluid composition, and more typically, from about 2 to 15% by volume.

The liquid carrier employed in ferrofluid compositions in the practice of my invention initially should be a liquid which is relatively inexpensive, easily evaporated, of low viscosity and noncombustible, with the preferred liquid carrier for the initial stages or the preparation of the initial ferrofluid composition being water. As more fully described hereinafter, the second carrier liquid employed in the preparation of the alternate ferrofluid composition by my process is typically characterized by being of a relatively low viscosity, having a wide temperature range for stability which would include a low pour point and a high flash point, and have a low volatility such that the resulting ferrofluid composition may be usefully employed and sealed in a vacuum condition on rotary bearings or similar applications. A wide variety of liquid materials may be employed as the liquid carrier in either the first or second stage of my invention, but more typically, the second stage of the invention to prepare the alternate ferrofluid compositions, which materials would include hydrocarbons, both aromatic and aliphatic; for example, toluene, xylene, cyclohexane, heptane, kerosene, mineral oils and the like; halocarbons, such as fluorocarbons which would include the fluorinated and chlorinated ethers, esters and the derivatives of C.sub.2 -C.sub.6 materials, such as perfluorinated polyethers; esters to include polyesters, di and triesters, such as azealates, phthalates, sebaccates, such as, for example, dioctyl phthalates, di-2 ethylhexyl azealates, silicate esters and the like.

A dispersant or dispersing agent which is typically a surfactant which may be employed in my ferrofluid process and composition includes a wide variety of materials which would aid in the dispersion of the magnetic particles. Such dispersants are characterized as surfactants or surface-active agents, and would include, for example, succinates, sulfonates, phosphated alcohols, amine long-chain acid reaction products, phosphate esters, polyether alcohols to include alkylphenoxypolyethoxyethanols, polyether acids and similar materials which are characterized by suppressing the surface tension of water, and which include a polar group and a long-chain tail; for example, C.sub.6 -C.sub.20. The surfactant is typically present in my ferrofluid compositions in a ratio of surfactant to magnetic particles of about 1:2 to 10:1 by volume; for example, 5:2 by volume, of the solid magnetic particles.

FIG. 2 illustrates a process-flow block diagram in my process for preparing irreversibly flocked particles and alternate ferrofluid compositions. In my process, coarse magnetic solids, typically having a size of 1 to 2 microns (10,000 to 20,000 Angstroms in size), were reduced by grinding a ball mill in the presence of a surfactant-water solution to produce a stable, colloidal, magnetizable ferrofluid, as presently known in the prior art and as illustrated in FIG. 1 a. The nature and behavior of ferrofluids are quantitatively described in the insertion "Ferrohydrodynamics" in the Encyclopedia Dictionary of Physics, Supplement 4, Pergamon Press. The surfactant employed is chosen to produce a ferrofluid composition which may be irreversibly flocculated.

The ferrofluid composition so prepared is then irreversibly flocculated by the addition of a solvent-flocculating agent, typically a polar solvent, such as a ketone, ester or alcohol; for example, acetone or methylethyl ketone. On the addition of the solvent-flocculated agent, the colloidal magnetic particles of the ferrofluid composition are then precipitated free of the liquidwater carrier, and also free of the initial surfactant employed. The initial liquid carrier; that is, water, and the surfactant are removed in the supernatant liquid after precipitation of the particles, and the wet dispersant-stripped particles so precipitated recovered.

The wet dispersant-stripped particles are then water-washed several times to remove any traces of residual solvent or surfactant. The washed magnetic particles are then carefully heated to drive off the moisture so as to provide dry, dispersant-stripped magnetic particles. The dry, dispersant-stripped particles are then treated with another and typically a different surfactant, such as by coating, or as shown, grinding in a ball mill, in the presence of an alternate carrier liquid to form an alternate ferrofluid composition of my invention, and having enhanced characteristics and properties. The alternate ferrofluid composition so prepared may be of the reversible or irreversible type. As illustrated, the alternate ferrofluid composition is prepared by again grinding in a ball mill the dry, dispersant-stripped particles in the presence of the alternate dispersant, and the alternate carrier liquid. By my process, alternate ferrofluid compositions having improved magnetic and viscous properties have been produced, lengthy processing time eliminated, and more versatile ferrofluid compositions produced which have alternate and improved chemical compositions.

In the preferred embodiment of my invention, the initial carrier liquid is water, while the alternate carrier liquid to prepare the alternate ferrofluid composition may be water, but is preferably an organic material. Further, while my tests indicate that the original surfactant employed for the preparation of the first ferrofluid compositions may be employed as the alternate dispersant, the use of such surfactant has not been found as effective as a new and different surfactant as the alternate dispersant in preparing the alternate ferrofluid compositions. Where the same or similar carrier liquid is employed, as employed in the first ferrofluid composition, the alternate ferrofluid compositions prepared by my process exhibit improved properties over the first ferrofluid compositions using such liquid carrier.

In the process of my invention, the term "irreversibly flocked" refers to that condition in which the magnetic particles will not disperse in any liquid carrier in that condition. To obtain the benefits of my process, the flocculating agent originally employed must be removed, and a new surfactant employed. I have found, for example, that prior art ferrofluid compositions of magnetic particles and oil typically have an average size of 100 Angstroms, while my alternate ferrofluid compositions prepared by my process using the same materials have an average size of 120 Angstroms, and because of their condition of larger particle size, better packing is accomplished, and, therefore, better magnetic susceptibility and lower viscosity is obtained. Thus, in my process, in its preferred embodiment, the original liquid carrier employed is water, while the alternate dispersant employed should be different from the dispersant originally employed. The advantages and techniques of my process and alternate ferrofluid compositions will be illustrated by the following examples wherein Tables I and II provide details of the materials employed in the examples.

TABLE I __________________________________________________________________________ CARRIER LIQUIDS Mfr's Viscosity Density Liquid Source Designation ctsk. (.degree.F) g/cc (.degree.F) __________________________________________________________________________ White Mineral Oil Humble Oil & Isopar M 2.31 (100) 0.77 (77) Refining Co. Silicate ester Chevron Oil Co. Oronite M-2 V 16.6 (100) 0.94 (68) Fluorinated ether DuPont Co. E-3 1.3 (77) 1.72 (77) Dibasic acid ester 11.0 (100) 0.91 (77) (Di-2-ethyl hexyl azelate) __________________________________________________________________________

TABLE II __________________________________________________________________________ STABILIZING AGENTS USED TO REDISPERSE COLLOIDAL MAGNETITE PARTICLES Stabilizing Agent Source Description __________________________________________________________________________ Aerosol 22 American Cyanamid Co. Tetrasodium N-(1,2 dicar- boxyethyl)N-octadecylsul- fosuccinamate Alkanol BG DuPont Co. Sodium alkylnapthalene sulfonate Alkanol 189S DuPont Co. Sodium hydrocarbon sulfonate Pyronate 50 Witco Chemical Corp. Petroleum sulfonate (M.W. 340/360) Victawet 58B Stauffer Chemical Co. Phosphorated higher alcohol (Capryl).sub.5 -NA.sub.5 (P.sub.3 O.sub.10).sub.2 3 Polyisobutene Enjay Chemical Co. M.W. 1000 (approx.) Reaction succinic acid product with polyethylene derivative (PIBSA) amine Di-2-ethyl hexyl Pfaltz and Bauer phosphate Inc. Acto 500 Humble Oil & Refining Alkylaryl sodium sulfonate Co. (M.W. 460 Avg) Antar LM400 GAF Corp., Chem. Div. Free acid of a complex organic phosphate ester Bryton HY Bryton Chemical Co. Synthetic sodium petroleum sulfonate Bryton Calcium 45 Bryton Chemical Co. Synthetic calcium petroleum sulfonate Teric N8 I.C.I.A.N.Z. Ltd. Nonylphenol + 8 moles ethylene oxide Surfactant 157 DuPont Co. Perfluoro-polyether car- boxylic acid __________________________________________________________________________

EXAMPLE 1

A ferrofluid was formed by ball milling finely divided magnetite, carrier liquid and dispersing agent (surfactant) to obtain a ferrofluid composition as follows:

Saturation magnetixation 200 gauss Carrier liquid Distilled water (83% vol.) Magnetic particle Magnetite (Fe.sub.3 O.sub.4) (4% vol.) Dispersing Agent Aerosol C61 (13% vol.) American Cyanamid Co. An ethanolated alkylguani- dine amine complex

To the ferrofluid so prepared was added an equal volume of acetone as a flocculating agent, and the solution stirred. The colloidal particles immediately flocculated and began to settle under gravity. A source of magnetic field or other means may be used to accelerate the settling process. The supernatant liquid was decanted and discarded.

Water was added to the precipitated particles with stirring, and it was found that the particles did not stably redisperse; i.e., the flocculation was irreversible. This water was discarded as a supernatant and served to remove excess acetone and any dispersant that may be present. The wash was repeated three times, then the solids were dried. Drying was carried out under infrared lamps or in an oven at 200.degree.F until a dry black granular solid powder was obtained. Care should be taken not to heat excessively, since overheating leads to formulation of a dry brownish solid that is not suited for further processing.

The powder comprised magnetic particles of the desired colloidal size which were free from any dispersant. These particles were loosely bonded together, but were well suited to be redispersed in an alternate carrier liquid by my process.

The dried dispersant-free powder was then combined with a surfactant and carrier liquid, and redispersed in a ball mill in an alternate ferrofluid composition as follows:

Alternate Carrier Liquid Toluene (90% vol.) Magnetic solids Fe.sub.3 O.sub.4 (4% vol.) Alternate Stabilizing Agent PIBSA (see Table II) (6% vol.)

Ball milling was carried out for a period of one to three weeks. I found that virtually all the magnetic solid became colloidally dispersed in the new liquid carrier.

Of course, it is possible to ball mill directly the magnetite with toluene as the liquid carrier and PIBSA as the dispersant to produce a colloidal ferrofluid. However, the produce is greatly inferior in its ability to be concentrated due to its higher viscosity at a given level of magnetic saturation.

The data of Table III compares a ferrofluid of the prior art with the alternate ferrofluid of my invention. The direct grind ferrofluid was prepared with a dibasic acid ester (see Table I), while the alternate ferrofluid employed water as the direct grind carrier with the same dibasic acid ester employed as the carrier after irreversible flocculation as described.

The comparative viscosities and ability to concentrate are typified in Table III below.

TABLE III ______________________________________ COMPARISON OF VISCOSITY VS MAGNETIZATION MAGNETIZATION (gauss) VISCOSITY (.eta., cp at 30.degree.C) Alternate Saturation Magnetization Direct Grind Ferrofluid Ferrofluid (Invention) ______________________________________ 100 150 20 200 400 40 300 2000 120 400 -- 325 500 -- 1760 ______________________________________

Ferrofluid employing PIBSA as produced in the prior art by direct grind techniques cannot be concentrated beyond about 300 gauss, while my invention permits concentration of over 300 gauss and beyond 500 gauss. My improved ferrofluids may have viscosities of 100 to 2000 cps at gauss levels of 300 to 800. In fact, ferrofluid compositions with saturation magnetization up to and in excess of 800 gauss have been prepared by my process.

Although not wishing to be bound by any particular theory, it is believed that the improved properties of lower viscosity at a given level of gauss for my alternate ferrofluids are based on the better concentrating ability; i.e., ability of particles to pack as in FIG. 1 c, due to the larger particle size present in preparations carried out according to my process. Electron micrograph study shows that the average particle size in Water/C61 dispersant ferrofluid is larger (120 Angstroms) than for particles prepared by direct grinding in the system Toluene/PIBSA dispersant (100 Angstroms). Thus, at a given level of saturation gauss, a system of coated large particles by irreversible flocculation takes up less bulk than a system of small particles flocculated reversibly having the same thickness of coating, and, accordingly, the viscosity is lower.

EXAMPLE 2

Stripped dispersant-free magnetite particles obtained as described in Example 1 may be coated with many other dispersing agents. A useful screening test is to add a small quantity of the stripped particles that have been pulverized to a small quantity; e.g., 5 c.c., of carrier liquid/surfactant solution and stir. Suitable combinations of materials produce an instantaneous peptization of at least a portion of the magnetic particles.

My process and the above testing technique are very valuable in eliminating the time and expense associated with the filling, running and clean-up of balls and ball mills associated with experimental searching for successful ferrofluid grind combinations in the past. My screening test requires but a few minutes and provides information that previously required one to several weeks to obtain.

Table IV identifies a series of surfactant/solvent combinations that have been identified and successfully produced in this fashion. Details of the surfactants and solvents are provided in Tables I and II.

TABLE IV

The combination of Surfactant 157/Dupont E-3 is especially useful, since a higher gaussage, higher initial permeability fluorocarbon base ferrofluid results as compared to direct grinds made from the same ingredients. The coated particles obtained here may be dispersed in any member of the family of perfluorinated polyether liquids, such as KRYTOX (perfluorinated polyether supplied by DuPont Co.) to produce ferrofluids of widely varying viscosity, volatility, inertness and other properties.

EXAMPLE 3

For the purposes of illustration only of the range and nature of my invention, redispersion of stripped dispersant-free magnetic particles from the initial aqueous ferrofluid composition of Example 1 back into a water carrier using an alternate and different dispersing agent has been demonstrated, employing the following dispersants: Aerosol 22, Alkanol BG, Alkanol 189S, Pyronate 50 and Victawet 58B.

TABLE IV ______________________________________ ALTERNATE FERROFLUIDS OF NONAQUEOUS BASE PRODUCED BY CONVERSION OF THE WATER BASE FERROFLUID Surfactant Solvent ______________________________________ 1. Acto 500 Chevron M2V 2. Antara LM400 Chevron M2V 3. Bryton HY Chevron M2V 4. Bryton Calcium 45 Chevron M2V 5. Teric N8 Chevron M2V 6. Di-2 Ethyl hexyl White Mineral Oil Phosphate 7. Surfactant 157 DuPont E-3 ______________________________________

the aqueous alternate ferrofluid employing as a dispersant Aerosol 22 is particularly useful in display devvices, such as those described in my U.S. Pat. No. 3,648,269. This ferrofluid composition combines the properties of immiscibility with white mineral oil, colloidal stability against freeze/thaw, permits polystyrene surfaces to be preferentially wetted by white mineral oil, and freedom from phase separation by emulsification of the ferrofluid into an adjacent liquid phase of white mineral oil. By comparison, the original direct grind C61 stabilized ferrofluid emulsifies into the white mineral oil over a period of time, creating a clouding appearance that makes it totally unsuitable for most consumer or industrial applications.

I have found that ferrofluid is not produced at all when surfactant A22 is direct ground in the ball mill with coarse magnetite in water. Evidently, the processes of size reduction and particle stabilization are independent in this case, while many surfactants utilized to produce ferrofluids appear effective in promoting both my and the prior art processes. The A22 dispersant is effective only in the role of steric protection.

A further general advantage provided by the process of my invention is the time saving in producing lots of specialty ferrofluid compositions. Thus by my process, a master batch of C61/water base ferrofluids may be processed in a series of operations that require from three to six or more months time to complete to provide a dry, stripped powder of the particles. The powder may then be relatively rapidly converted to the desired specialty ferrofluid in typically 1 to 3 weeks. In fact, if complete conversion is not desired, a useful product can be often obtained on an overnight basis in many instances. Thus, as little as one-day processing produces a ferrofluid that previously typically required several months, or was not producible at all.

Claims

What I claim is:

1. A process for preparing an improved ferrofluid composition, which process comprises in combination:

a. preparing a first ferrofluid composition comprising a dispersion of colloidal-size magnetic particles in a dispersant in a polar liquid carrier;

b. adding a flocculating agent to the first ferrofluid composition to precipitate the magnetic particles from the first composition separating the precipitate from the supernatant liquid to obtain flocked precipitated magnetic particles which are characterized in that such flocked particles will not redisperse in the polar liquid carrier without the addition of a dispersant;

c. recovering essentially dispersant-free magnetic precipitated particles;

d. coating the surface of the flocculated precipitated magnetic particles with a second dispersant; and

e. redispersing said coated particles in a second carrier liquid to provide a second ferrofluid composition of improved properties and characteristics.

2. The process of claim 1 which includes drying the recovered magnetic precipitated particles.

3. The process of claim 1 wherein the second dispersant employed to obtain the second ferrofluid composition is different from the dispersant in the first ferrofluid composition.

4. The process of claim 1 wherein the second liquid carrier in the second ferrofluid composition is different from the liquid carrier in the first ferrofluid composition.

5. The process of claim 1 wherein both the second dispersant and the second carrier liquid are different from the first dispersant and the first carrier liquid of the first ferrofluid composition.

6. The process of claim 1 wherein the first and second carrier liquids comprise water.

7. The process of claim 1 wherein the first carrier liquid of the first ferrofluid composition comprises water, and the second carrier liquid of the second ferrofluid composition comprises a nonaqueous liquid.

8. The process of claim 1 wherein the step of coating the flocked particles and redispersing such particles in the second carrier liquid is accomplished by grinding the recovered flocked particles in a ball mill in the presence of the second dispersant and the second carrier liquid.

9. The process of claim 1 which includes the step of washing the dispersant-free particles to remove residual traces of the dispersant from the surface of the particles, and, thereafter, drying the magnetic particles.

10. The process of claim 1 wherein the first carrier liquid is water, and wherein the flocculating agent added to the first ferrofluid composition comprises a polar solvent.

11. The process of claim 1 wherein the second carrier liquid comprises a liquid fluorocarbon.

12. The process of claim 11 wherein the fluorocarbon comprises a perfluorinated polyether liquid.

13. The process of claim 1 wherein the second carrier liquid comprises a diester liquid.

14. The process of claim 1 wherein the magnetic particles of the first ferrofluid composition have an average particle size of from about 20 to 300 Angstroms.

15. The process of claim 1 wherein the magnetic particles comprise magnetite or gamma iron oxide.

16. The process of claim 1 wherein the second disperssant comprises, in a ratio of dispersant-to-magnetic particles, from about 1:2 to 10:1 by volume.

17. The process of claim 1 wherein the second ferrofluid composition comprises from about 2 to 15% by volume of magnetic particles.

18. The process of claim 1 wherein the first liquid carrier is water, and the second flocculating agent is a ketone, ester or alcohol.

19. The process of claim 1 wherein the magnetic particles of the second ferrofluid composition have an average particle size of about 120 Angstroms.

20. The process of claim 1 which includes:

a. drying the essentially dispersant-free magnetic precipitated particles; and

b. ball milling the dry magnetic particles with the second carrier liquid and second dispersant to provide the second ferrofluid composition.

21. The process of claim 1 wherein the first carrier liquid is water, and the second carrier liquid is a mineral oil.

22. A ferrofluid composition which comprises a solution of a low-volatility liquid diester carrier and a dispersant, and colloidal-size magnetic particles dispersed in said solution, the composition having a saturation magnetization of between about 300 to 800 gauss, and having a viscosity of from about 100 to 2000 cps, the magnetic particles comprising magnetite or gamma iron oxide, the dispersant comprising, in a ratio of dispersant-to-magnetic particles, from about 1:2 to 10:1 by volume, and the composition comprising from about 2 to 15% by volume of magnetic particles, the magnetic particles having a particle size of about 120 Angstroms.

23. The flocked dispersant-free magnetic particles having a particle size of from about 20 to 300 Angstroms produced by the process of claim 1, the particles essentially free of adsorbed surfactant and incapable of being redispersed in the first carrier liquid without the addition of a dispersant.

24. A process of preparing an improved ferrofluid composition of low viscosity and high magnetization saturation, which process comprises in combination:

a. grinding in a ball mill coarse magnetic solids, a water-soluble dispersant, and a water carrier liquid to provide a first ferrofluid composition of colloidal-size stable magnetic particles;

b. adding a polar solvent as a flocculating agent to the first ferrofluid composition to precipitate the magnetic particles from the first ferrofluid composition separating the precipitate from the supernatant liquid to obtain flocked, dispersant-free magnetic particles which are characterized in that such flocked particles will not redisperse in the polar liquid carrier without the addition of a dispersant;

c. washing the dispersant-free precipitated particles with water to remove the flocculating polar solvent and any residual traces of the dispersant;

d. heating the washed precipitated particles to a temperature not in excess of about 200.degree.F to remove moisture, and to produce dry, flocculated dispersant-free, magnetic particles; and

e. grinding in a ball mill the dry flocked particles in the presence of an alternate and different dispersing agent and a second carrier liquid to provide a second ferrofluid composition.

25. A ferrofluid composition which comprises a solution of a diester liquid carrier, a dispersant, and colloidal-size magnetic particles dispersed in said solution, the composition having a saturation magnetization of over 300 gauss, and having a viscosity of from about 100 to 2000 cps.

26. The composition of claim 25 wherein the liquid carrier comprises a nonaqueous carrier, and the average particle size is about 120 Angstroms.

27. The composition of claim 25 wherein the dispersant is a polyisobutene succinic acid derivative.

28. The composition of claim 25 wherein the composition comprises a ratio of dispersant-to-magnetic particles of about 1:2 to 10:1 by volume.

29. The composition of claim 25 wherein the magnetic particles comprise magnetite or gamma iron oxide.

30. The composition of claim 25 wherein the magnetic particles of the ferrofluid composition have an average particle size of from about 20 to 300 Angstroms.

31. The composition of claim 25 wherein the ferrofluid composition comprises from about 2 to 15% by volume of magnetic particles.

32. The composition of claim 25 wherein the composition has a magnetic saturation of between about 300 to 800 gauss.