Title of Invention

PROCESS FOR THE PREPARATION OF FERROFLUIDS

Abstract

Process for the preparation of ferrofluid comprising precipitationof the superparamagnetic particles such as ferrites and substituted ferrites like Mn-, Co- ferrites, MxFe2O4 (M=Mn, Zn etc) in the presence of surface modifying agents at pH 8 - 10.5, further processed to obtain superparamagnetic particles capable of being dispersed in carrier liquid to produce in stable ferrofluids of tailored magnetic properties

Full Text

FORM 2 THE PATENTS ACT, 1970 (39 of 1970) COMPLETE SPECIFICATION (See section 10; rule 13) 1. Title of the invention, - Process for the preparation of Ferrofiuids 2. Applicant (a) Indian Institute of Technology (b) Powai, Mumbai 400076, Maharashtra, India (c) An autonomous educational institution and established in India under the Indian institute of Technology Act 1965. The following specification particularly describes the nature of this invention and the manner in which it is to be performed Original Field of the Invention This invention relates to the preparation of ferrofiuids wherein the precipitation of the superparamagnetic particles (SP) are carried out in the presence of surface modifying agents to obtain narrow particle size distribution (3 to 15 nm) that are amenable to controlled dispersion in carrier liquid to produce ferrofiuids of tailored properties. Background of the invention: Preparation of stable ferofluids requires superparamagnetic particles in specific particle size distribution that can be dispersed in a carrier liquid to form a stable colloid without being coalesced or precipitated under the influence of the magnetic force, gravity, centrifugal force etc. To form stable colloidal suspension of magnetic particle, the particles size should be in The characteristics of ferrofluid depend on the particle properties, dispersed phase content, stability of dispersing particles in the external magnetic field. The strength of magnetization of ferrofluid is determined by the volume fraction of magnetic materials i.e. concentration of magnetic fine particles contained in the ferrofiuids. For different applications it is necessary to tailor characteristics of the magnetic fluid. Magnetic fluid particularly with higher concentration is difficult to synthesize since with increasing concentration generally leads to coalescence. It may be noted that transportation of the ferrofluid is hazardous as the hydrocarbon/ oil base ferrofiuids are very sensitive to fire. Further ferrofiuids are bulky thereby increasing the cost of transportation. Two methods commonly used are milling and chemical precipitation. 2 The conventional method of preparation of ferrofluid which is described in Japanese Unexamined Patent Publication No. 44579/[1967] as well as in many other patents including US Patent 3843540 and scientific papers [Journal of colloid and Interface Sc. 155, 152 (1993), Journal of Magnetism and Magnetic Materials, 149, 64, 1995, Journal of Magnetism and Magnetic Materials, 139, 249 (1995), Journal of Magnetism and Magnetic Materials, 123, 262 (1993). IEEE Tran. Mag. Mag-16, No2, 184, (1998). Phy. Rev. B. 55 5585 (1997), Phy. Rev. 54, 309, (1938), IEEE Tran. Mag. Mag-16, No2, 172, 1998] involve the precipitation of the magnetic particles and then subjecting them to surface treatment using surfactants etc followed by subsequent process steps to disperse the magnetic particles in a dispersion medium to obtain the ferrofluid. The steps involved in conventional method are depicted in figure 1. US Patent No; RE32.576 discloses process in which fine particles of the magnetic materials such as magnetite, ferrite, iron, cobalt alloy, etc. are dispersed stably and uniformly in a dispersing medium selected out of an oil group, an ester group or an ether group, whereby a ferrofluid composition having a high magnetizing capacity is produced effectively. In one of the intermediate steps, the dried powder is obtained. However this process consists of several process and intermediates steps, which is complicated. EP0880149 discloses a ferrofluid manufacturing process wherein an antiferromagnetic a-Fe203 was ball milled to render it magnetic and suitable for use in a ferrofluid. The material is placed, together with a solvent and a surfactant, in a commercial attrition mill where the mill action converts the non¬magnetic (a-Fe203) particles to magnetic particles. In order to eliminate a solvent replacement step, which is necessary with oil carrier liquids, water is used as the grinding solvent and as the ferrofluid carrier liquid. The resulting water-based ferrofluid has a high saturation magnetization, low viscosity and good colloidal stability. Using the inventive method, a large volume of fluid can inexpensively be synthesized in a short time. A particular modification made in US3764540 wherein ferrofluids, comprising a stable, colloidal suspension of magnetite, elemental iron, are produced by 3 comminuting a antiferromagnetic precursor (i.e. a-Fe2O3) compound to colloidal size, dispersing the precursor in a carrier liquid and thereafter converting the precursor to superparamagnetic forms while in stable suspension. US Patent 3917538 discloses a process for preparing irreversibly flocked magnetic particles that 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. US Patent 5240626 discloses a water base ferrofluid prepared by coating of the magnetite particles with a single carboxyl functional polymer surfactant. Similarly US Patent 4, 09804 suggests the use of two separate surfactants for water base ferrofluids. EP1224670 which corresponds to US6277298 discloses a composition and process for producing a chemically stable magnetic fluid comprising a plurality of magnetic particles covered first with a small molecular weight surface modifier, which is a nondispersant and acts as a surfactant-accepting layer, and then with at least one surfactant. The surface modifier/surfactant coated magnetic particles are then suspended in a silicone oil-based, hydrocarbon oil-based or an ester oil-based carrier liquid US Patent 62777298, describes a method wherein the final volume of ferrofluid and the magnetization are monitored using the following formula; Vf = (Mh x Vh)/Mf Vf = Volume of final ferrofluid Mf= Saturation magnetization desired for final ferrofluid Vh= Volume of heptanes based ferrofluid Mh= Saturation magnetization of the heptane based ferrofluid 4 The method disclosed in the patent involves the preparation of ferrofluid wherein the magnetite is prepared by conventional precipitation technique followed by addition of silicon oil and a surfactant in heptane. In the final stage of the preparation, heptane is evaporated to obtain a silicone oil base ferrofluid. The saturation magnetization (Ms) of final ferofluid is adjusted by adding silicone oil using above formula. The problem, with these methods is that dilution leads to agglomeration/ precipitation of particles. Further the process of preparation involves heating to remove the solvent, which is undesirable in such systems. The prior art has several drawbacks. In the conventional methods used in the precipitation of the SP particles it is difficult to control the particle sizes especially to prevent the formation of large sized particles (> 15 nm), which with increased particle concentration in the ferrofluid tend to precipitate. The processes involving grinding to achieve uniform particle sizes suffer from the shortcoming that some of the particles may have their coating dislodged and therefore produce unstable ferrofluids when dispersed in an organic medium. In most of the processes the ferrofluids are taken to the final stages of the preparation after the precipitation of the SP particles. The ferrofluids become bulky and their transportation becomes a cumbersome, hazardous due to the organic solvents used in the dispersion and expensive due to their bulk. There is a longstanding; need in the industry to provide process for the preparation of ferrofluids wherein the SP particles of narrow size distribution are produced and that they are amenable to controlled dispersion in carrier liquid to produce stable ferrofluids with tailored magnetization as and when required. Summary of the invention The main object of the invention is to provide a process for preparation of ferrofluids to obtain SP particlesin a narrow size distribution (3-11 nm), avoiding any process steps that lead to destabilization of the particles in a ferrofluid, and also to prepare superparamagnetic particles that are amenable to storage and 5 controlled dispersion in a liquid medium to produce stable ferrofluid with desired magnetisation characteristics.. Another object of the invention is to provide a process for the production of SP particles that avoids any steps of grinding and elaborate process steps. It is yet another object of the invention to provide process in which the surface modified particles are separated by novel methods without steps of pH adjustment as is required in the prior art. It is yet another object of the invention to provide a process in which the product is intermediate stabilized coated SP dried particles that can be easily dispersed in a carrier liquid as per the application requirement thereby avoiding the transportation of ferroliquids as is normally done in the prior art. It is yet another object of the invention to prepare stable ferrofluids of tailored magnetisation by dispersing the required amount of superparamagnetic particles in a carrier liquid carrier without the steps of dilution as is done in the prior art to adjust the magnetization.. Thus in accordance of this invention, the ferrofluid preparation is done in two stages. In the first stage precipitation of the SP particles is carried out in the presence of surface modifying agents and further processed to obtain surface modified dried SP particles that can be stored and transported and in the second stage a calculated amount of the dried magnetic particles are dispersed in a known volume of carrier liquid to obtain tailored ferrofluids without the need to carry out any steps of dilution etc that may destabilize the ferrofluids. Detailed Description Of The Invention: The invention is described with a preferred embodiment involving the precipitation of SP and its further dispersion in an appropriate medium to obtain a ferrofluid. The process of the present invention is illustrated in figure 2. 6 Stage I: Precipitation of the superparamagnetic particles Precipitation of the superparamagnetic particles such as ferrites and substituted ferrites like Mn, Co-ferrites, MxFe1-xFe2O4 (M= Mn, Zn etc) is carried out as follows: Metal sulphate and chlorides are taken in 1:2 mole ratio. > The metal ions are dissolved separately in distilled water and mixed. > A surface modifying agent such as oleic acid, is mixed with acetone. > The precipitation reaction is carried with stirring in air or nitrogen, preferably in nitrogen atmosphere. > The solutions of the metal salts, and solution of the surface modifying agent are stirred and 25 % NH3 is added with vigorous stirring, such that the resultant solution pH is between 8 -10.5 preferably in the rage of 9 -10. > On completion of the precipitation, the reaction mixture is heated to 75°C -95 °C, preferably about 85°C - 90 °C for 5 min - 30 min preferably 10 min - 20 min. > The surface modified SP are separated and collected under the influence of a magnetic field. > Optionally, acetone is added to the reaction mixture and then kept in a magnetic field for separation. The supernatant is decanted. The SP particles are washed 2-3 times with water and finally 5-6 times with acetone and dried. Stage II: Preparation of ferrofluid In this stage a ferrofluid is tailored to any desired magnetization by > dispersing a calculated amount of the SP particles in a known volume of carrier liquid. > The weighted amount of surface modified magnetite particles is mixed with stirring in 1 to 30 volume % preferably 1 - 20 volume % of the base carrier liquid such as kerosene or cyclohexane. The resultant ferrofluid is 7 centrifuged at 12,000 rpm for 15 min to separate the larger particles thereby resulting in a stable ferrofluid. In one of the embodiments of the invention, the back-calculated excess powder (after step 4a in fig. 2) is added to the carrier liquid so "that after centrifugation it gives the required magnetization ferrofluid. In another embodiment the intermediate cyclohexane base ferrofluid is prepared and after evaporation of solvent the powder is obtained (step 7, in figure 2). The calculated amount of the obtained powder is dispersed in carrier liquid to get required magnetization. In other embodiments of the invention surface modified agents are selected from organic acids having 12 to 18 carbon atoms such as Laurie, Stearic, Palmitic, Pentadecanoic acid, etc and their mixtures.Using the methods of this invention, it is possible to prepare ferrofluids involving ferrites and substituted ferrites like Mn-, Co-ferrites, MxFe1-xFe204 (M= Mn, Zn etc) etc. Carrier liquid may be selected from kerosene, hydrocarbon, CHCI3, hexane, heptane, octane pentane, aromatic hydrocarbons such as cyclohexane, toluene etc, ISOPAR-M, paraffin Oil etc. Examples The invention is now illustrated by non limiting examples. Example 1. Magnetite particles are precipitated by controlled co-precipitation method. 19.20 g of FeS04 is dissolved in 75 - 80 cc of distilled water. 22.40 g of FeCI3 is dissolved in 75 - 80 cc of distilled water. The two solutions are mixed and a solution of 10 cc - 20 cc oleic acid dissolved in 20 cc of acetone is added with stirring to the solution. 85 - 90 cc of 25 % of ammonia is added with stirring to a pH of 9.5 and further stirred for 30 min followed by heating at about 90 °C for 15 min with stirring. The resulting slurry is cooled to room temperature and poured in 8 separate beaker and placed on a magnetic field for 5 min. The liquid is decanted and 250 - 300 cc of distilled water at about 40 °C is added. This is further subjected to the magnetic field followed by decantation and addition of distilled water. This process is repeated a few times followed by washing with 100 - 150 cc acetone for 4-5 times. The resulting precipitate is dried and stored in air/inert gas atmosphere for magnetization measurement and ferrofluid preparation. The magnetization measured at room temperature is 49 emu/g at 20,000 Oe (figure 4). To prepare a tailored ferrofluid of desired magnetization, theoretically calculated amount of magnetic particles is dissolved in a measured volume of kerosene (shown in tablel). The mixture is stirred for 30 min and the ferrofluids obtained. The ferrofluid before and after centrifugation, will be referred in the table 1 as FF1 and FF1C respectively. The magnetization data of these ferrofluids are given in this table. The ferrofluid is centrifuged at 12,000 rpm for 15 min and magnetization is measured again. The XRD pattern shows the formation of cubic Fe304 (Matched with JCPDS). The broadness of the peak is attributed to nano-crystallinity. The crystallite size of heated samples is measured from XRD line broading using Scherrer"s formula given below, D = 0.9λ/β CosӨ, where D is average crystalline size, X the X-ray wave length used, (3 the angular line width of half maximum intensity and 6 the Bragg"s angle in degree. The crystalline sizes are estimated to be 8 nm - 9 nm prepared at pH=9 -10.4 respectively. The TEM micrograph (figure 3a) shows that the particles are narrowly distributed in size, varying between 3 to 11 nm. The particle size distribution for one typical sample is given in figure 3(b). It is in agreement with the crystalline size, calculated from XRD peak broadening. The reliability of the synthesis improved significantly with the initial presence of surfactant during precipitation process. Example: 2 Solutions of FeS04 (9.60 g dissolved in 35 to 40 cc of distilled water) and FeCI3. (11.20 g dissolved in 35 - 40 cc of distilled water) are mixed. 5 to 10 cc of oleic acid dissolved in 10 cc of acetone is added to the metal ion mixture with 9 stirring. 45 -50 cc of 25 % of ammonia is added to the metal ion mixture with vigorous stirring. The pH of the resulting reaction mixture is 9.3. The resultant slurry is stirred for 30 min and heated with stirring at around 90 °C for 20 min. The resulting slurry is cooled to room temperature and poured in a beaker and placed on a permanent magnet for 5 min. The liquid is decanted and 150 to 175 cc of distilled water at around 40 °C is added. The steps of subjecting the slurry to the magnetic field, decantation followed by addition of distilled water are repeated 4 times. The resulting precipitate is separated and washed 4-5 times with acetone, air dried and stored in air/inert gas atmosphere for magnetization measurement and ferrofluid preparation. The magnetization is 49.00 emu/g at 20,000 Oe. To prepare the ferrofluid with tailored magnetization, theoretically calculated amount of magnetic particles is mixed with stirring in measured volume of kerosene for 30 min. The resultant ferrofluid will be reffered as FF2 in table 1. Magnetization of the resulting fluid is measured (figure 5). Example 3. The magnetite particles prepared in Example 2 is used to prepare the dispersion in cyclohexane. The dried magnetite particles is added with stirring in 250 cc of cyclohexane for 30 min. The resulting suspension is centrifuged at 10000 rpm for 15 min. and decanted after placing on a strong magnet. The solvent of the suspension is removed using rotary evaporator and the dried powder is characterized for magnetization measurement (47 emu/gm, at 20000 Oe) and ferrofluid is prepared which will be referred as FF3 in Table 1. 10 Table: 1 Magnetization data of different ferrofluid with field measured at room temperature We claim 1. A process for the preparation of ferrofiuid comprising steps a) precipitating superparamagnetic particles such as ferrites and substituted ferrites like Mn-, Co-ferrites, MnxFe1-xFe204 (M=Mn, Zn, etc) in the presence of surface modifying agents at pH 8-10.5, b) processing to obtain dried superpramagenting particles, c) optionally dispersing the superparamagentic particles in a carried liquid to produce stable ferrofiuid of tailored magnetic properties. 2. A process for preparation of ferrofiuid as claimed in claim 1, wherein superparamagentic particles are prepared by reacting a solution of the metal ions such as iron, manganese, zinc, and their like with ammonia solution at pH 8 - 10.5 preferably in the range of pH 9 - 10 at 75 °C - 95 °C, preferably 85 °C - 90 °C in the presence of surface modifying agent(s) dissolved in organic solvents, subjecting the reaction mixture to magnetic filed of ~0 2T, collecting the particles, followed by washing with aqueous and organic solvents and. drying. 3. A process for preparation of ferrofiuid as claimed in ciairms1/ wherein the superparamagnetic particies are dispersed in organic mediam, preferably separating the particies > 15 nm, drying and dispersing calculated amount of the dried superparamagnetic particles below 15 nm in predetermined quantity of carried liquid to obtain a ferrofiuid of tailored magnetization or optionally dispersing known quantity of the dried superparamagnetic particles in predetermined quantity of carried liquid, separating the particies > 15 nm to obtain ferrofluids. 4. A process for preparation of ferrofiuid as claimed in claim 1 wherein the surface modified superparamagnetic particles are separated from the reaction mixture by application of magnetic filed. 5. A process for preparation of ferrofiuid as claimed in claim 1 wherein the surface modified superparamagnetic particies are separated by addition of acetone to the reaction mixture without pH adjustment. 6. A process for preparation of ferrofiuid as claimed in claim 1 wherein ths water-soluble organic medium for dissolution of the surface agent is preferably acetone. 7. A process for preparation of ferrofiuid as claimed in claim 1 wherein the, surface modifying agents are selected from organic acids having 12 to 18 carbon atoms such as lauric, stearic, palmitic, pentadecanoic acids and their mixtures. 8. A process for preparation of ferrofluid as claimed in claim 1 wherein the orgasnic solvent used for washing of the precipitated superparamagnetic particles is preferably acetone. A process for preparation of ferrofluid as claimed in claim 1 wherein the carried liquid for the dispersion of the superparamagnetic particles is selected from kerosene, hydrocarbons, CHCI3, hexane, octane, pentane, aromatic hydrocarbons such as cyclohexane, toluene and their like, ISOPAR-M, paraffin oil. Dated. 21.04.2004 Agent on behalf of Applicant Dr. Prabuddha Ganguli VISION-IPR

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