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Publication numberUS3531413 A
Publication typeGrant
Publication date29 Sep 1970
Filing date22 Sep 1967
Priority date22 Sep 1967
Publication numberUS 3531413 A, US 3531413A, US-A-3531413, US3531413 A, US3531413A
InventorsRosensweig Ronald E
Original AssigneeAvco Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Method of substituting one ferrofluid solvent for another
US 3531413 A
Abstract  available in
Images(2)
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Claims  available in
Description  (OCR text may contain errors)

sePf- 29, 1970 R. E. ROSENSWEIG 3,531,413

METHOD OF SUBSTITUTING ONE FERROFLUID SOLVENT FOR ANOTHER Filed sept. 22, 1967 2 sheets-sheet 1 G ENERGY ff E a PARTncLE SEPARATION PARTCLE ATTRACTING ENERGY REPELLIN (VAN DER WAAL) ATTRACTING ENERGY REPELLING ENERGY VAN DER WAAL) INVENTOR.

RONALD E. ROSENSWEIG sept 29, 1970 l R. E. RosENswElcsA 3,531,413

METHOD OF SUBSTITUTING ONE FERROFLUID SOLVENT FOR ANOTHER Filed sept. 22, 1967 2 sheets-sheet 2 INVE R. RONALD E. ROSE WEIG United States Patent @ii 3,53lAl3 Patented Sept. 29, 1970 3,531,4ll3 METHD F SUBSTH'EUTENG @NE lFlERlRUFLlUlD SULVENT FR ANG'I'HER Ronald lE. Rosensweig, Lexington, Mass., assigner to Aveo Corporation, Cincinnati, Unio, a corporation of Delaware Filed Sept. 22, 1967, Ser. No. 669,938 int. Cl. Bllld 2]/0] US. Cl. 252-62.62 6 Claims ABSTRACT @E THE DISCLOSURE This invention covers a method of substituting one ferroiluid solvent for another. A typical class of ferrofluid contains magnetic particles with a polar surfactant adsorbed on the surface thereof suspended in a non-polar solvent. To substitute one solvent for another in accordance with the invention, a polar ilocculating solvent is introduced into the ferrofluid causing the particles with the adsorbed surfactant to tlocculate, and to settle out of solution.

The particles and solvent are separated. The particles are then redistributed within another solvent which may be the same as the original solvent or a different composition.

BACKGROUND OF THE INVENTION The invention relates to ferrouid and more particularly to methods of making ferrouids. Related information may be found in Pat. No. 3,281,403.

In general, ferrofluids contain submicron particles of magnetic material such as magnetite and ferrites. A surfactant or dispersing agent is adsorbed on the surface of these particles and acts as a coupling agent `between the particle and a solvent in which the particles are dispersed. Additional information on the structure of typical ferrolluids are found in an article entitled Magnetic Fluids by R. E. Rosensweig in the July 1966 issue of International Science and Technology.

Ferrotluids are generally made by grinding magnetic particles together with a surfactant and solvent over a prolonged period of time.

Not all surfactants or solvents are suitable grinding media. At times they are totally unsuitable and other times ineilicient.

Examples of magnetic material which are suitable for this purpose are magnetite (.FE3O4), manganese-zinc ferrite, v-FezOa, and any solid magnetic material that can be `formed in the proper particle size. Surfactants of the following general nature have been used to make ferrolluids. Carboxylic acids and their metallic salts, amines, succinic acid derivatives, condensation product of an amino ester of a fatty acid. The chain length should usually be eight carbon atoms or greater. Examples of solvents are aliphatic hydrocarbons such as heptane, octane, decane, mineral oil, kerosene etc., halogenated hydrocarbons such as carbon tetrachloride or trichlorethylene and aromatic solvents such as benzene, toluene and their non-polar derivatives. All of these solvents are suitable for grinding and are interchangeable.

Pentane is a suitable solvent but not useful as a grinding aid because it is very volatile.

Flocculation is used herein to denote the aggregation of individual particles into larger masses. These aggregations eventually grow large enough to deposit out of suspension.

It is an object of the invention to provide a method of substituting one solvent of a ferrofluid by another.

It is an object of the invention to provide a method of substituting one solvent of a ferroiluid by another by inducing particle flocculation to cause the magnetic particle to deposit out of solution.

It is another object of the invention to provide a means for formulating ferrolluids containing solvents that are not suitable as grinding or manufacturing aids.

It is yet another object of the invention to provide means for altering the particle concentration in a ferroiiuid particularly Where a ferrofluid surfactant is more volatile than the solvent.

It is still another object of the invention to induce flocculation for the purpose of substituting solvents by reducing the repelling forces overriding the Van der Waal forces of attraction.

In accordance with the invention a method of substituting one ferrofluid solvent for another ferrolluid comprises the steps of introducing a flocculation agent into the ferrofluid thereby causing the particles with adsorbed surfactant to come out of suspension. The particles are separated from the solvent and this is followed by resuspending them in another solvent.

The novel features that are considered characteristic of the invention are set forth in the appended claims; the invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specic embodiment when read in conjunction with the accompanying drawings, in which:

FIG. 1 shows a curve useful in explaining the forces acting on ferrofluid particles.

FIG. 2 is a schematic representation of a ferrouid structure.

FIG. 3 shows a second curve useful in explaining the forces acting on ferrofluid particles.

FIG. 4 depicts the relationship of the various components of a ferrofluid in which a flocculation solvent has been added.

FIG. 5 illustrates an alternate method of causing particles to flocculate.

Characteristically, magnetic particles in a ferrouid remain in suspension without changing the characteristics of the ferrofluid as a homogeneous medium under the influence of applied magnetic fields and magnetic field gradients. There are two contributing factors that maintain the magnetic particles in suspension. The rst relates to the size of the particles which typically are in the submicron region so that particle motion is maintained by thermal agitation. Secondly, the surfactant 0r dispersing agent acts to maintain the particles sufliciently remote from one another to overcome the force of attraction caused by Van der Waals forces.

The origin of the Van der Waal force is the attraction of a fluctuating electric dipole for a neighboring induced dipole. So long as the surfaces of adjacent particles are about one radius apart, the particles will not be strongly attracted toward each other and flocculation can be avoided. The function of the adsorbed coating or surfactant on the particles is to provide at least the minimum distance required for separation.

In FIG. 1 curve 21 represents the magnitude of Van der Waal energy as a function of distance separating two particles. Curve 20 represents the repelling energy generated by coatings of adsorbed surfactant on the surface of two adjacent particles. Curve 22 represents the algebraic sum of the attracting and repelling energies.

The repulsive force equals the negative rate of change of energy with distance dE (Prin (assuming the convention that positive force represents repulsion.) Clearly that repulsion occurs to the right of the apex 25 of curve 23 since the slope of the curve 23dE/dX or the rate of change of energy as a function of distance is negative.

Additionally, if the magnitude of energy at the apex exceeds the thermal energy per particle, the thermal fluctuations of the particles will now throw the particles close enough together to come within the attraction region, to the left of the apex.

Referring to FIG. 2 of the drawings, there is a schematic representation of two adjacent particles and 11 suspended within a non-polar solvent 12. Absorbed on each surface of the particles 10 and 11 are molecules 13 of a polar surfactant. It will Ibe noted that the molecules 13 contain a polar head 14 and a non-polar tail 15. The surfactant molecules 13 coat the surface of the particles, and in effect, form as an elastic boundary between the particles. When two particles such as 10 and 11 approach each other the coating is` compressed and provides an elastic repulsion greater than the attractive forces that would otherwise cause particles to come into contact until flocculation occurs and the solid material settles out.

Referring to FIG. 4 of the drawings, the particles 10 and 11 are depicted in substantially the same environment as shown for FIG. 2, except for the fact that a occulating solvent depicted by molecules 16 has been added to the ferrofluid. The flocculating solvent is a polar solvent. Its effect on the ferroiiuid is to make the surfactant molecules less compatible with its fluid environment comprising the combination of ferrofluid solvent and lilocculating solvent. The tail sections of the surfactant molecules are repelled by the combination of solvents and tend to fold back on themselves toward the surface of the particles. Thus, the depth of coating is materially reduced.

The curves in FIG. 3 denote, as before, Van der Waal attraction, surfactant repulsion and the algebraic sum. Energy curve 23 in FIG. 3 reflects the shortened surfactant thickness as shown in FIG. 4. At no point does it go above the abscissa and therefore nowhere does the repulsion energy exceed the Van der Waal attraction energy.

Particles in their random movement through the combined solvent no longer encounter a repelling force in excess of the Van lder Waal attraction force. The particles adhere to each other, flocculate and deposit out of the combined solvent.

From this point it is a relatively simple matter to separate the particles from the combined solvent. A little of the flocculating and ferrofluid solvent remains on the particles. These solvents appear as a very small impurity when the particles are resuspended in the new solvent. No problems are encountered by these impurities as they are compatible with the new ferrofluid in these small quantities.

In the event the purpose in occulating the particles was to increase the concentration, it is now clearly obvious that the one need only add a reduced amount of solvent to the particles. On the other hand, if the purpose in occulating the particles was to substitute one solvent for another, this step is now clearly possible.

Typical formulations used in practicing the invention are provided in Table No. 1 which follows. The ferrofluid sCl/ents and flocculation solvents are fully interchangea e.

Another means for causing flocculation, called polymeric flocculation, is shown in FIG. 5. Flocculation is caused by linking of two-or more-particles 10, 11, 17

4 by a polymer molecule 18 as shown in FIG. 5. The particles that settle out of solution may be redispersed by supplying excess solvent to the particles to dissolve some of the entangling species.

The amount of flocculating agent that is required differs with each application. It is, however, determinable through routine procedures.

Observed iiocculating agents are polyisobutylene which works with aliphatic and chlorinated aliphatic solvents; polystyrene which may be used with aromatic solvents including toluene; and dimethylene siloxane polymers with the aliphatics.

Two important criteria of polymer occulation agents are: (i) they be miscible in the ferrouid solvent, and (ii) the molecule length shall be several times in the diameter of a particle since a linking of particles is an important consideration.

CONCLUSION In its broadest aspect, the invention covers the substitution of one ferrofluid solvent for another by introducing into the ferrofluid a flocculating agent. The agent can reduce the forces acting in opposition to the Van der Waal forces to the extent thatA the particles can be attracted to one another and deposit out of solution. Alternatively, the occulating agent can act to link particles causing occulation and ultimately the separation of particles and solvent. V

Secondarily, with respect to the former approach, the invention covers the concept of shrinking the thickness of the surfactant coating thus enabling the Van der Waal forces to bring and maintain adjacent particles together. Another aspect of the invention causes the coating to shrink and reduce its effective thickness by introducing into a ferrouid containing a polar surfactant and a nonpolar solvent, a polar flocculating agent.

The various features and advantages of the invention are thought to be clear from the foregoing description. Various other features and advantages not specifically enumerated will undoubtedly occur to those versed in the art, as likewise Will many variations and modifications of the preferred embodiment illustrated, all of which may be achieved without departing from the spirit and scope of the invention as deiined by the following claims.

What is claimed is:

1. A method of substituting one ferrouid solvent for another ferrouid solvent in a ferrofluid consisting essentially of suspended magnetic particles, a polar surfactant and a non-polar ferrofluid solvent comprising the steps of z (a) introducing a polar flocculation agent into the ferrofluid causing theiparticles with adsorbed surfactant to occulate and settle out of suspension;

(b) separating the original ferrofluid solvent and flocculation agent from the particles with adsorbed surfactant; and

(c) dispersing said occulated particles in another nonpolar ferrofluid solvent.

2. A method as defined in claim 1 in which the surfactant is a long chain molecule having a chain length of at least eight carbon atoms having a polar end group and a non-polar tail, said ferrofluid solvent is a non-polar organic fluid and said flocculating agent is a polar organic fluid soluble in the ferrofluid solvent.

3. A method as defined in claim 1 in which the surfactant is a member of the class consisting of carboxylic acids and their metallic salts, amines, succinic acid derivatives, condensation product of an amino ester of a fatty acid, said ferrouid solvent is a member of the class consisting of aliphatic hydrocarbons, halogenated aliphatic hydrocarbons and aromatic hydrocarbons, and said flocculating agent is a member of the class consisting of acetone, ethyl alcohol, dioxane, ethyl acetate and acetone.

4. A method of substituting one ferrouid solvent for another ferrofluid solvent in a ferroiluid consisting essentially of magnetic particles a surfactant adsorbed on said 6 particles and a non-polar ferrofluid solvent comprising the fluid solvent is an aromatic hydrocarbon and the occusteps of: lating agent is polystyrene.

(a) introducing a flocculating agent which is a miscible polymer particle linking agent having a 4molecular References Cited chain length at least twice the diameter of the mag- 5 UNITED STATES PATENTS netlc particles mto the ferrouid and occulatlng sald 2 590 997 4/ 1952 Mitchell 252-326 partlcles causmg said partlcles to settle out of suspension; 3,290,252 12/ 1966 Larsen 252-6254 (b) separating the ferrofluid solvent and polymer link- FOREIGN PATENTS ing agent from the particles with adsorbed surfactant; 10 974 627 11/1964 Great Britain and (c) dispersing said flocculated particles in another non- TOBIAS E LEVOW, Primary Examiner polar ferrouid solvent. 5. A method as defined in claim 4 in which the ferro- I' COOPER Assistant Exammer fluid solvent is an aliphatic hydrocarbon and the tioc- 15 U S C1 XR culating agent is selected from the group consisting of polyisobutylene and dimethyl siloxane. 252-6251, 62,56, 309, 327

6, A method as dened in claim 4 in which the ferro- Patent No. 3 531: 413 Dated September Z9, 1970 Inventor(s) Ronald E. Rosenweg It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column l, line 48, for "(FE O read{Fe O4) Column Z, line l5, after "ferroflud'read"solven Column 3, line 8, for "now" read not; Column 3, line 39, after "thickness" omitas; Column 3, line 57, after "that" omit--the.

SIGNED AND (l5-LED DEC im tSEAL) Attest:

M Flethg Ir- Eo y l :'i p nl L! OEE @omissionsor PatentaJ

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Classifications
U.S. Classification252/62.62, 516/140, 516/33, 252/62.51R, 252/62.56
International ClassificationH01F1/44, B03D1/001
Cooperative ClassificationH01F1/44, B03D1/001
European ClassificationH01F1/44, B03D1/001