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(12) EUROPEAN PATENT APPLICATION
(43) Date of publication: (51) int. CI.6: H01F1/44 25.11.1998 Bulletin 1998/48
(21) Application number: 98102781.6
(22) Date of filing: 18.02.1998
(84) Designated Contracting States: (72) Inventors: AT BE CH DE DK ES Fl FR GB GR IE IT LI LU MC • Raj, Kuldip NL PT SE Merrimack, New Hampshire 03054 (US) Designated Extension States: • Aziz, Lutful M. AL LT LV MK RO SI Nashua, New Hampshire 03060 (US)
(30) Priority: 21.02.1997 US 37416 P (74) Representative: 10.02.1998 US 21228 Kindermann, Manfred Patentanwalt, (71) Applicant: Sperberweg 29 Ferrofluidics Corporation 71032 Boblingen (DE) Nashua New Hampshire 03061 (US)
(54) Method for manufacturing ferrof luid
(57) A ferrofluid manufacturing process applies energy to a nonmagnetic
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Description ids include sealing, damping, heat transfer, noise con- trol, material separation, sensing and parts inspection. FIELD OF THE INVENTION They are used in such diverse products as exclusion seals, loudspeakers and stepper motors. The industries This invention relates to processes for manufactur- s in which ferrofluid-based products are typically used ing ferrofluid in substantially larger volumes and in less are: the semiconductor, computer, aerospace, oil pros- time than is possible with existing techniques. pecting and mining industries. Known manufacturing processes for producing fer- BACKGROUND OF THE INVENTION rofluids characteristically begin by producing suitably- 11 sized particles of a ferrous material, such as Fe304 Ferrofluids are colloidal systems containing mag- (magnetite). Magnetite particles in a subdomain size netic particles, typically having a diameter of the order (e.g., about 10 nm) necessary for use in a ferrofluid, are of 10 nm, suspended in a liquid carrier. The difference not commercially available. Consequently, two methods between ferrofluids and other well-known colloids is that are used to produce suitably sized particles: ball mill ferrofluids specifically utilize particles which possess 15n grinding and chemical precipitation. These methods are magnetic properties, whereas other colloids are com- described in detail in a book entitled prised of non-magnetic particles. Commercial ferroflu- Ferrohydrodynamics by R.E. Rosensweig; Cambridge ids are generally comprised of magnetite or mixed University Press and a book entitled Magnetic Fluid ferrite particles, although it is possible to use particles of Handbook and Applications Handbook, editor B. Berko- other magnetic materials, such as iron, cobalt, nickel, 202t voski, Begell House, Inc., New York (1996). chromium dioxide, iron nitride or magnetic alloys of A typical ball mill grinding process starts with com- these materials. In general, the ferrofluid is colloidally mercially-available magnetic powder, such as magnetite stable and has a relatively low viscosity. powder, in which the particles are of "micron size", e.g. In order to form a stable colloid, the particle size about 0.15 to 0.3 microns (i.e. about 150 nm to 300 nm). should be in the range of 10 nm. In practice, the parti- 252t The commercially-available magnetite particles are cles are generally spherical in shape and are small then ground to reduce their size about 90%, to about 1 0 enough to form a single magnetic domain representing nm. A typical ball milling process is described in the U.S. a tiny permanent magnet with an associated north and Patent No. 3,917,538. In this process, proper amounts a south pole. The particles are further coated with lay- of magnetite powder, surfactant and a solvent are ers of surfactant to prevent agglomeration caused by 303< placed in a stainless steel milling jar about 40% filled magnetic and Van der Waals attractive forces. It is also with grinding media, such as 1/4" carbon steel balls. For possible to form a stable colloid by using either positive grinding to be effective, the viscosity of the solvent or negative electrical charges to keep the particles sep- should be low. In a water-based ferrofluid, the water car- arated; these formulations are referred to as "ionic fer- rier is of low viscosity and consequently, it can be used rofluids". The surfactant used in each type of ferrofluid is 353i as the solvent in the grinding process. In oil-based fer- specific to the carrier because the surfactant must be rofluid, the carrier liquid often has a relatively high vis- chemically compatible with the carrier. The particles cosity. Consequently, a low molecular weigh solvent is may be coated either with a single or double surfactant often added to the oil carrier during the grinding process layers and may be either cationic, anionic or non-ionic in to reduce the viscosity. This solvent is subsequently nature. 404t removed by evaporation to increase the saturation mag- A typical ferrofluid may consist of the following vol- netization in the final ferrofluid. ume fractions: 4% particles, 8% surfactant and 88% liq- The rolling action of the mill causes the media to uid carrier. Ferrofluids are characterized by the liquid impact repeatedly the coarse magnetite breaking it into carrier in which the particles are suspended because it subdomain size particles and coating some of the parti- is the dominant component. For example, a water- 454t cles at the same time with the surfactant. Because the based ferrofluid is a stable suspension of magnetic par- milling media generate a relatively low shear energy, a ticles in water, whereas an oil-based ferrofluid is a sta- conventional ball milling operation takes anywhere from ble suspension of magnetic particles in an oil (such as a two to six weeks to complete and the dispersion quality hydrocarbon, an ester, a fluorocarbon, a silicone oil or is poor. The colloid formed by this process generally polyphenyl ether, etc.) The physical properties of a fer- 50st includes uncoated particles and large aggregates and rofluid are also based on the selection of the liquid car- thus requires a subsequent refinement in which unde- rier since it is the majority component. In addition, as sirable particles and aggregates are removed. mentioned above, the surfactants for water- and oil- Moreover, the finished product often has a high vis- based ferrofluids are different. cosity due to the presence of small particles produced Magnetic colloids can be viewed as "liquid mag- st during the grinding process. These small particles are nets" which can be manipulated and positioned with a further known to degrade the thermal stability of the magnetic field to provide sealing and increase heat fluid. It is also believed that the process may only transfer rates. The commercial applications of ferroflu- reduce large aggregates of useful small particles which
2 3 EP 0 880 149 A1 4 are initially present in commercial magnetite powder. suitable for use in a ferrofluid. The yield is poor, preparation times are long and the In accordance with one embodiment, the nonmag- associated costs are high so that the ball milling method netic starting material is 3 5 EP 0 880 149 A1 6 attrition mill which applies a high level of shear energy processed by mill 114, the entire contents of vessel 122 to the material to convert the non-magnetic red iron are transferred back , via piping 124, to vessel 100 and oxide powder to magnetic magnetite (Fe304). Attrition the grinding process is repeated. mills can be purchased from a number of sources. In the In an attrition mill, the grinding action is much more examples below, two different machines manufactured 5 aggressive than in a ball mill. Consequently, satisfactory by Union Process Company, Akron, Ohio, were used. results can be achieved with an attrition mill in a much The first attrition mill was a model 01-HDDM (1.4 liters shorter time than with a ball mill and the use of an attri- shell capacity) which is a vertical lab attritor for process- tion mill is an important factor in reducing the grinding ing of small amounts of materials. The second attrition time for, and the cost of, producing the ferrofluid. As an mill is a model DM-20 (20 liters shell capacity) which is w illustration, the same water-based ferrofluid was pre- a horizontal attritor for medium size production. The pared using the aforementioned lab attritor and a con- grinding media used in these mills was carbon steel ventional ball mill. The constituents of ferrofluid were balls with diameter of 0.85 or 0.25 mm and the mills can used in the same proportion in both the attrition mill and be operated at various speeds. The attrition mill con- the ball mill. Figure 2 shows the results of this illustra- tains a rotating shaft on which is mounted a series of 75 tion. A stable colloid with acceptable saturation magnet- perforated discs which transmit kinetic shaft energy to ization is formed much more quickly with the attritor the grinding media and the slurry in the mill. Heat gen- than with the ball mill. For example, a ferrofluid with a erated by the process is removed by the flow of agitated saturation magnetization of 60 Gauss was produced in suspension and by outside cooling. These mills are tra- 60 minutes with the attritor, but the ball mill had to be run ditionally used in grinding and dispersion applications, 20 for about 60 hours to produce a ferrofluid with an equiv- for example for preparation of inks, paints and coatings. alent saturation magnetization. When the model 01-HDDM lab attrition mill is used, The model 01-HDDM lab attritor was used for test- the materials used in the grinding process are directly ing of a wide variety of surfactants for producing water- poured into the vessel one by one through an opening. based ferrofluids. Some sunfactants worked better than The shaft is first rotated at a slow speed to mix the 25 others and some surfactants were not suitable. The materials and then it is increased for colloid formation. tests were conducted using the following mixture: 30 When material is processed in the model DM-20 attri- grams 4 7 EP 0 880 149 A1 8 were stirred and mixed to obtain 45 gallons of a homo- of a new surfactant into another medium such an oil car- geneous slurry. rier. Forty-five gallon batches of this slurry were then While this invention has been particularly shown processed using the apparatus in Figure 1 operating in and described with references to preferred embodi- a batch mode. The mixture from vessel 100 was fed to 5 ments thereof, it will be understood by those skilled in the attrition mill 1 14 at a rate of 20 gallons per hour. The the art that various changes in form and detail may be entire output of the mill was coilected in an empty vessel made therein without departing from the spirit and 1 22. This operation constituted "pass #1 ." The total con- scope of the invention as defined by the appended tents of vessel 1 22 was then transferred to vessel 1 00 claims. for a second pass. This process was repeated a total of 10 four passes for each 45 gallon batch. The total time of Claims each pass was approximately two hours. The results for five 45 gallon batches are shown in 1. A method for making a ferrofluid, the method com- Figure 4. The particle size measurement in each batch prising the steps of: was about 90 A. In about 8 hours, 45 gallons of "ready 15 to use" ferrofluid was produced with this process. The (a) combining non-magnetic iron oxide parti- saturation magnetization of the fluid was 165 Gauss. cles, a carrier liquid and a surfactant to form a The use of a larger capacity attritor will allow more slurry; and throughput. There was no waste generated during the (b) subjecting the slurry to mechanical energy preparation. 20 to convert the non-magnetic iron oxide parti- Attempts were also made to synthesize water- cles to magnetic iron oxide particles. based ferrofluids using both the lab attritor and ball mill with the best black magnetite powders (in terms of par- 2. The method as described in claim 1 wherein the ticle size) available in the market place. The same rec- non-magnetic iron oxide is 5 9 EP 0 880 149 A1 9. The method as described in claim 8 wherein the surfactant is a sodium salt of lignosulfonic acid. 10. The method as described in claim 8 wherein the surfactant is Westvaco Reax 88B. s 1 1 . The method as described in claim 8 wherein the 13. The method as described in claim 8 wherein the is water is deionized. 14. The method as described in claim 8 wherein step (b) comprises the step of: 20 (b1) placing steel grinding media in the attritor mill. 15. The method as described in claim 14 wherein the steel grinding media are .25 mm carbon steel balls. 25 16. A method for making a ferrofluid, the method com- prising the steps of: (a) forming a slurry which is, by weight, 19% a- 30 Fe203 powder, 73% water and 8% Westvaco Reax 88B surfactant; (b) placing the slurry in an attrition mill; and (c) operating the attrition mill for a period of time sufficient to convert the non-magnetic iron 35 oxide particles to magnetic iron oxide particles. 17. The method as described in claim 16 wherein the period of time in step (c) is sufficiently long such that the produced ferrofluid attains a predetermined 40 saturation magnetization of at least 1 50 Gauss. 18. The method as described in claim 16 wherein step (b) comprises the step of: 45 (b1) placing carbon steel grinding media in the attrition mill. 50 55 — ' 5r o o o > St I O ra i2 J5 2 x co j"|~0 CO CO CO CO Oil- 0 z LU gS (3 9 j- < < < o o wSS 2 ^ ? 5 3 o ^ g _ -5, 0 ^ >_ C CO 0) E JS 2 E g to ■§ £ to -5 Q- 1 Z 1 g 1 3 ill I ^ 1 1 8 3 ? Iff £ O co < Zi a. co > 5 < uj 1- o o c- q; co « co <2 — -1 — ^ ^ mo) wo «J "g ajw O «i 5i i^S >> CO 5 X 5 X cQh O mi s «E X X 5 CO (0 J- 1- S 56 S 5 O co < 9 EP 0 880 149 A1 DRUM NO. PASS NO. FERROFLUID FERROFLUID SATURATION DENSITY (gm/ml) MAGNETIZATION (GAUSS) 1 1 108 1.22 1 2 155 1.24 1 2 151 1.24 1 3 175 1.25 1 3 177 1.25 2 3 169 1.24 2 3 169 1.24 3 3 160 1.24 3 3 161 1.24 3 4 178 1.25 3 4 177 1.25 4 3 158 1.24 4 3 156 1.24- 4 4 171 1.25 4 4 173 1.25 5 3 158 1.24 5 3 158 1.24 5 4 178 1.25 5 4 175 1.25 FIG. 4 EP 0 880 149 A1 European Patent Number J EUROPEAN SEARCH REPORT Application Office EP 98 10 2781 DOCUMENTS CONSIDERED TO BE RELEVANT Category Citation of document with indication, where appropriate, Relevant CLASSIFICATION OF THE of relevant passages to claim APPLICATION (lnt.CI.6) Y FR 2 498 587 A (AIMANTS UGIMAG SA) 30 July 1,3,5 H01F1/44 1982 A * the whole document * 6-8,14, 15 PATENT ABSTRACTS OF JAPAN 1,3,5 vol. 017, no. 183 (E-1348), 9 April 1993 & JP 04 335502 A (N0K CORP), 24 November 1992, * abstract * TECHNICAL FIELDS SEARCHED (lnt.CI.6) H01F The present search report has been drawn up for all claims Place of search Date ot completion of the search Examiner THE HAGUE 20 May 1998 Decanniere, L CATEGORY OF CITED DOCUMENTS T : theory or principle underlying the invention E : earlier patent document, but published on, or X : particularly relevant if taken alone after the filing date Y : particularly relevant if combined with another D : document cited in the application document of the same category L : document cited for other reasons A : technological background O : non-written disclosure & : member of the same patent family, corresponding P : intermediate document document 11