Abstract:
The invention relates to a composition and a process for producing a chemically stable magnetic fluid comprising finely divided magnetic particles covered with surfactants. A surface modifier is also employed which is added to cover thoroughly the free oxidizable exterior surface of the outer layer of the particles to assure better chemical stability of the colloid under different environmental conditions.

Description:
The present invention is a continuation-in-part of U.S. patent application Ser. No. 08/622,315 filed Mar. 26, 1996, now U.S. Pat. No. 5,676,877. 
    
    
     BACKGROUND OF THE INVENTION 
     Magnetic fluids used in technical applications, commonly referred to as ferrofluids, are a dispersion of finely divided magnetic or magnetizable particles ranging between thirty (30) and one hundred fifty (150) angstroms in size and dispersed in a liquid carrier. 
     The magnetic particles are typically covered with surfactants or a dispersing agent. The majority of industrial applications using magnetic fluids incorporate iron oxides as magnetic particles. The most suitable iron oxides, for magnetic fluid applications, are ferrites such as magnetite (Fe 3  O 4 ) or ferric oxides (Fe 2  O 3 ) such as gamma. Ferrites and ferric oxides offer a number of physical and chemical properties to the magnetic fluid, the most important of these being saturation magnetization, viscosity, magnetic stability, and chemical stability of the whole system. The amount of magnetic particles in the magnetic fluid composition can range up to 40% by volume. 
     The surfactants have two major functions. The first is to assure a permanent distance between the magnetic particles to overcome the forces of attraction caused by Van der Waal&#39;s force and magnetic interaction, and the second is to provide a chemical composition on the outer layer of the covered particle which is compatible with the liquid carrier and the chemicals in the surrounding environment. Most of tne magnetic fluids employed today have one (1) to three (3) types of surfactants arranged in one (1), two (2) or three (3) layers around the magnetic particles. The surfactants, for magnetic fluids, are long chain molecules having a chain length of at least sixteen (16) atoms such as carbon, or a chain of carbon and oxygen, and a functional end group at one end. The functional end group can be of a cationic, an anionic or a nonionic nature. The functional end group is attached to the outer layer of oxides (magnetic particles) by either chemical bonding or physical force or a combination of both, and the chain or tail of the surfactant provides a permanent distance between the particles and compatibility with the liquid carrier. For all practical purposes, the amount of surfactant in the magnetic fluid composition can range up to thirty (30)% by volume. 
     The carrier is generally an organic molecule, either polar or non polar, of various chemical composition such as hydrocarbon (polyalpha olefins, aromatic chain structure molecules), esters (polyol esters), silicone, or fluorinated and other exotic molecules with a molecular weight range up to five thousand (5,000). 
     There are several physical and chemical properties of the magnetic fluid related to the type of carrier such as viscosity, evaporation rate, resistance and compatibility with the surrounding environment. 
     There are many patents related to the preparation of magnetic fluids and the most relevant of which for this invention are: 
     U.S. Pat. No. 3,531,413 describes a process where magnetic particles are initially dispersed in a non-polar solvent, and then flocculated with a polar solvent whereby the particles are separated from the initial solvent and resuspended in a different solvent. 
     U.S. Pat. No. 3,917,538 describes a process which consists of grinding coarse magnetic particles in an aqueous carrier using a dispersing agent. The aqueous ferrofluid obtained from the grinding process is flocculated and the magnetic particles are separated out of the aqueous solution. The particles are then washed, dried and resuspended in an organic carrier using a second dispersant. 
     U.S. Pat. No. 3,700,595 describes using a carboxylic acid having at least a twelve (12) carbon chain as a surfactant which is oil soluble and water insoluble, or a high molecular weight polyisobutene carboxylic acid surfactant. 
     U.S. Pat. No. 4,280,918 describes a process for preparation of a magnetic dispersion for use in magnetic coating. The magnetic particles are coated with a uniform material, preferably colloidal silica. The coating prevents aggregation of magnetic particles. The pH of the slurry is adjusted to between three (3) and six (6), by an acid, to produce a positive electrostatic charge on the magnetic particles and to mix a colloidal silica having a negative electrostatic charge. The two oppositely charged particles are attracted to and the silica particles are irreversibly bonded to the magnetic particles. 
     U.S. Pat. No. 4,315,827 describes a method of preparing a stable ferrofluid composition by dispersing magnetic particles in polyphenyl ether using surfactants with one functional polar group reactive with the surface of the particles, and a tail group containing phenyl, benzyl or phenoxy groups soluble in the liquid carrier. 
     U.S. Pat. No. 4,356,098 describes a method of preparing a stable silicone oil ferrofluid composition which comprises a colloidal dispersion of finely divided magnetic particles in a liquid silicone oil carrier, a dispersing amount of silicone oil surfactants containing a functional group which forms a chemical bond with the surface of magnetic particles, and a tail group which is soluble in the silicone oil carrier to provide a stable magnetic composition. The tail group of the surfactant has a number of atoms of silicon and oxygen chains, or siloxane, in order to be soluble in the silicone oil. 
     U.S. Pat. No. 4,430,239 describes a stable ferrofluid composition comprising a colloidal dispersion of finely divided magnetic particles in a liquid carrier, and a dispersing amount of a dispersing agent, which agent comprises an acid phosphoric acid ester of a long chain alcohol, the long chain alcohol being compatible with the polar liquid carrier. 
     U.S. Pat. No. 4,576,725 describes a method of preparing a magnetic fluid by dispersing metallic magnetic particles, having an average diameter of several hundreds of angstroms, in a base liquid. The particles are obtained by condensation of metallic vapor in the liquid carrier. The metal magnetic particles in the ferrofluid are oxidized very rapidly. The oxidation process of the metallic particles will dramatically change the initial property of the ferrofluid. 
     U.S. Pat. No. 4,599,184 describes an attempt to improve the oxidation and magnetic stability of the magnetic metal particles obtained from metallic vapor condensation by coating the particles with a surface active agent or surfactant. In order to obtain a stable magnetic fluid, the particles have to be covered with a surfactant as in any other process, to obtain a stable magnetic fluid. 
     U.S. Pat. Nos. 4,604,229 and 4,687,596 describe methods for producing stable electrically conductible magnetic fluids using cationic high molecular weight surfactants and polar carriers. 
     U.S. Pat. No. 4,608,186 describes a magnetic fluid comprising fine metal particles of cobalt, and a surface active agent selected from a group consisting of polyglycerine fatty acid esters, sorbitan fatty acid esters and a mixture thereof. The liquid carrier is a hydrocarbon. The composition contains tocopherol as an antioxidant additive. 
     U.S. Pat. No. 4,624,797 describes a magnetic fluid comprising fine particles of cobalt, and a surface active agent selected from the group consisting of oil soluble anionic sulfosuccinate and nonionic polyglycerine fatty acid ester or the group consisting of polyethyleneglycol alkyl ether and a low volatility solvent medium. 
     Metallic magnetic particles of a diameter less than two hundred (200) angstroms and evenly coated with a surfactant are highly unstable and oxidized very rapidly. Today, there are no commercial applications of such fluid using magnetizable metal particles. The major drawback of this process is the oxidation of the magnetic particles. 
     U.S. Pat. No. 4,938,886 describes a super paramagnetic fluid comprising magnetic particles; a dispersing agent of a formula A-X-B anchored to the magnetic particles, wherein A is derived from a nonionic surface active agent precursor having a terminal OH group, the precursor being selected from a group consisting of ethoxylated or propoxylated alcohols and other ethoxylated compounds, B is a carboxylic acid group which anchors the dispersing agent to the magnetic particles and X is a connecting group between A and B; and a carrier liquid which is a thermodynamically good solvent for A. 
     U.S. Pat. No. 5,013,471 describes a magnetic fluid where the particles are covered with a chlorosilane surfactant having a chain with ten (10) to twenty-five (25) atoms of carbon. Fluorine atoms are substituted for the hydrogen atoms of the hydrocarbon chain of the chlorosilane surfactant and fluorinated oil is used as a carrier. There is no other surfactant used in this process. According to this reference, the surfactant chlorosilane has to be large enough to disperse the particles and to assure the colloidal stability of the magnetic fluid by providing sufficient distance between the particles. 
     One object of the present invention is to use a silane surface modifier of very low molecular weight, e.g. one (1) to ten (10) carbon atoms, in the tail chain to be able to penetrate between the existing surfactant to cover the free (exposed) surface which is not covered by the large molecular weight surfactant. According to the present invention the silane can not be used to disperse the magnetic particle alone. 
     U.S. Pat. No. 5,064,550 describes a super paramagnetic fluid which is a stable colloid comprising a non-polar hydrocarbon carrier, and the magnetic particles are coated with at least one acid selected from the group consisting of an organic acid containing only carbon and hydrogen atoms in the chain connected to the carboxyl group where the chain contains at least nineteen (19) carbon atoms and an amino acid acylated with the fatty acid, wherein the organic and amino acids are branched, unsaturated or both, and an ashless polymer is provided to increase the viscosity of the super paramagnetic fluid. 
     U.S. Pat. No. 5,085,789 describes a ferrofluid composition consisting essentially of fine particles of ferromagnetic particles with alkylnaphtalene being used as the carrier and a surfactant with the hydrophobic portion consisting of alkylnaphtalene structure. 
     U.S. Pat. No. 5,124,060 describes a ferrofluid composition consisting essentially of an organic solvent carrier, ferromagnetic particles coated with oleophilic groups exhibiting an affinity for said organic solvent, and a fluorocarbon surface active material. 
     U.S. Pat. No. 5,143,637 describes a magnetic fluid consisting of ferromagnetic particles dispersed in an organic solvent, a low molecular weight dispersing agent, and an additive with a carbon number between twenty-five (25) and fifteen hundred (1,500). The low molecular weight dispersing agent is used to disperse the particles in an organic carrier. In the summary of this reference there is a discussion about using a coupling agent, such as silane, as a dispersant. However, the coupling agent has to have a large enough molecular weight to perform as a dispersant. It should be mentioned that, in U.S. Pat. No. 5,143,637, there is no particular disclosure claim directed to using silane as an additive or even as a dispersant. The thermal stability of the fluid is increased by adding a high molecular weight additive, e.g. up to twenty thousand (20,000), such as polystyrene, polypropylene, polybutene, or polybutadiene polymers. 
     U.S. Pat. No. 5,147,573 describes a method of preparing a colloidal dispersion of electrically conductive magnetic particles consisting essentially of superparamagnetic particles, an electrically conductive organo metallic compound, a dispersing agent comprising a nonionic, an anionic or a cationic surfactant, and a hydrocarbon organic solvent. 
     U.S. Pat. No. 4,554,088 employs polymeric silane as a coupling agent. The coupling agents are a special type of surface active chemicals which have functional groups at both ends of the long chain molecules. One end of the molecule is attached to the outer oxide layer of the magnetic particles and the other end of the molecule is attached to a specific compound of interest in those applications, such as drugs, antibody, enzymes, etc. 
     U.S. Pat. No. 5,240,628 describes a process for producing a magnetic fluid, which comprises adding a solution of N-polyalkylenepolyamine-substituted alkenylsuccinimide in a water-insoluble or water-sparingly-soluble organic solvent to an aqueous suspension of fine particles of ferrites and stirring the resulting mixture, thereby forming an emulsion and absorbing the N-polyalkylenepolyamine-substituted alkenylsuccinimide onto the fine particles of ferrites, then distilling off water and the organic solvent therefrom and dispersing the fine particles of N-polyalkylenepolyamine-substituted alkenylsuccinimide-absorbed ferrites in a base oil of low vapor pressure having a vapor pressure of not more than 0.1 mm Hg at 25° C. 
     In none of the above discussed patents is there an attempt to cover the surface area of the magnetic particles which is not already covered by the large size surfactants. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a chemically stable magnetic fluid composition and a process of preparing such a composition. 
     A magnetic fluid has to exhibit stability in two areas in order to be used in current industrial applications. The first is to have magnetic stability under a very high magnetic field gradient since the magnetic particles tend to agglomerate and aggregate under high magnetic field gradients and separate out from the rest of the colloid. The second is to have chemical stability relating to oxidation of the surfactant and the organic oil carrier. All the organic oils undergo a slow or rapid oxidation process, over the course of time, which increases with temperature and the concentration of the oxygen in the surrounding environment in contact with the oil. This oxidation process results in an increased viscosity of the oil to the point where the oil becomes a gel or solid. There is also a different mechanism where the molecules break down and evaporate out of the system more quickly. This is the most important condition to assure a chemically stable colloid which is exposed to oxygen and high temperature. 
     The present invention relates to a chemically stable magnetic fluid composition comprising: 0.1 to 40 parts by volume of magnetic particles; 0.1 to about 30 parts by volume of at least one surfactant, the surfactant preferably having a molecular weight of at least 150; 10 to about 99 parts by volume of an organic carrier fluid, the carrier fluid is preferably an organic molecule compatible with the surfactant and comprises a concentration of between ten percent to ninety-five percent by volume of the composition, and preferably 10 to 95 parts per volume; and 0.01 to 30 parts per volume of at least one of a surface modifier and an additive to improve the chemical oxidation of said composition. 
     The present invention also relates to a process for preparing an improved chemically stable magnetic fluid comprising a plurality of magnetic particles, at least one surfactant, an organic carrier fluid, and a surface modifier or an additive to improve the chemical oxidation of said composition, said process comprising the steps of: preparing a solvent base magnetic fluid containing at least one of a cationic, an anionic or a nonionic surfactant, whereby said surfactant is attached to the outer surface of the magnetic particles of the fluid, in order to disperse the particles in a compatible solvent base; adding a low molecular weight surface modifier or additives to cover exposed area of the outer layer of the magnetic particle previously uncovered by the surfactant, and said surface modifier or additive comprises one of an acid base additive, an amine base additive, a silane base surface modifier, and a titanium coupling agent; adding a high molecular weight organic carrier; and increasing the temperature of the mixture to evaporate the solvent carrier and to disperse the magnetic particles in the carrier fluid. Structure diagrams of certain above mentioned modifier and additive compounds are detailed in Table 6. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example, with reference to the accompanying drawing in which: 
     FIG. 1 shows a magnetic particle with a long tail surfactant attached thereto. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A long tail surfactant (S) will have the arrangements on the magnetic particles (MP) as can be seen in FIG. 1. A long tail surfactant, however, can not completely cover the entire outer oxidizable surface of the magnetic particles. 
     Repeated experiments show that an organic oil undergoes a faster oxidation in contact with a solid surface, especially oxides. The life of the oil is significantly reduced by mixing the oil with very small size magnetic particles. A simple calculation shows that a cubic centimeter of magnetic fluid of two hundred (200) gauss saturation magnetization has around ten (10) to power sixteen (16) number of magnetic particles of one hundred (100) angstrom diameter. This number of particles will provide approximately thirty (30) square meters of oxidizable outer area surface per cubic centimeter of magnetic fluid or per approximately 0.7 cubic centimeter volume of oil (about 0.55 grams). The area could be much larger considering that the surface of the outer oxidizable area is not uniform but has a topography of &#34;mountains and valleys&#34;. Theoretically, because of stearic repulsion and geometry, the surfactant will cover at best eighty percent (80%) to ninety percent (90%) of the outer oxidizable area of the particles. There is about three (3) to six (6) square meters of uncovered outer oxidizable area in contact with a very small amount of oil (0.55 grams). This simple calculation shows that the major oxidation effect of the oil and surfactant is due to the immense surface of oxide from the uncovered surface area of the magnetic particles. 
     The present invention uses a surface modifier to cover the area not covered by the surfactant used in the preparation of the magnetic fluid. The present invention requires the surface modifier to have a very low molecular weight and not to be a dispersant so it can penetrate through the tails of the existing surfactant to cover the free area of the particles uncovered by the existing surfactant. 
     The surface modifier has to be of a very small molecular weight and size in order to be able to penetrate the uncovered oxidizable surface of the magnetic particles through the tail of the surfactants already connected to that surface, to attach and cover the surface, and to protect the surface against oxidation. 
     The surface modifier may consists of one (1) to three (3) similar functional groups, at one end of the molecule, and a very short tail of one (1) to ten (10) atoms. The modifier can be represented by the formula 
     
         R.sup.1.sub.n Si R.sup.2.sub.4-n 
    
     wherein the group R 1  denotes a hydrolyzable radical chosen from the group consisting of alkoxides of one to three carbon atoms; R 2  denotes an alkyl radical having one (1) to ten (10) carbon atoms; and n is 1, 2 or 3 on the average. In particular, isobutyltrimethoxy silane has been found to be a particularly useful surface treatment agent employable in the present invention and is represented by the above formula where R 1  denotes a methoxy radical, R 2  denotes the isobutyl radical and n is three. The mechanism of coupling to the free oxidizable surface by the silane is thought to be: the alkoxy part of the surface modifier reacts with the proton from the inorganic hydroxyl group to form alcohol as a byproduct, and the silicon connects to the oxygen from the former hydroxyl group present on the outer layer of the magnetic particles. 
     During the reaction with the surface, the surface modifier becomes even smaller because approximately one third (1/3) of the molecule is eliminated as a byproduct of this reaction. 
     There are several other ways to improve the chemical stability of the magnetic fluid such as adding a proper amount of antioxidant, choosing a good combination of a surfactant(s) and an oil carrier(s), having a substantially uniform particle size closer to one hundred (100) angstroms, etc. After all these options have been carefully considered, further improvement to chemical oxidation of the magnetic fluid can be achieved by adding isobutyltrimethoxysilane or other small molecules with the same capability to cover the magnetic particles. 
     EXAMPLE 1 
     13.0 g of ferrous sulfate heptahydrate and 24.0 g of ferric chloride hexahydrate were dissolved in water and the total amount of the solution was adjusted to be 70 cc with water. 30 cc of 28% ammonia solution was added to the iron salt solution and Fe 3  O 4  particles were precipitated. 
     Oleic soap that consisted of 2.1 g of oleic acid and 27 cc of 3% ammonia solution was also prepared. The oleic soap was then added to the Fe 3  O 4  particle slurry and the particles were covered with an oleic ion. 30 cc of heptane was poured into the oleic covered particle slurry, and the entire slurry was stirred and left to set. The oleic coated particles were peptized in heptane and the heptane base magnetic fluid was siphoned into a 200 cc beaker. 
     The oleic covered magnetite particles were flocculated with 50 cc of acetone and the supernatant was removed. The particles were washed four (4) times with 50 cc of acetone. 75 cc of water and 15 cc of a 28% ammonia solution were added into the beaker and the particles were suspended by gentle agitation, e.g. about 60 rpm. 
     The slurry was heated up to 70° C, and 11 cc of isobutyl trimethoxysilane was added, and the slurry temperature was maintained at about 75±5° C. for 30 minutes. 
     After cooling the slurry, the supernatant was removed and the particles were washed five (5) times with 50 cc of acetone. 
     Then the washed particles were dispersed in heptane, and 20 cc of 2 cSt at 100° C. of polyalphaolefin oil was added to the heptane base magnetic fluid, the heptane was removed by heating it, and the saturation magnetization of the oil base magnetic fluid was adjusted to be 200 gauss by adding oil. 
     Magnetic fluid, sample #1-1, that was 200 gauss and 2 cSt oil base was obtained. Another magnetic fluid, sample #1-2, that was 200 gauss and 2 cSt oil base was prepared in the same manner as the sample #1-1 except that isobutyl trimethoxysilane was not applied to the particles during the process. 
     The magnetic fluids samples #1-1 and #1-2, respectively, were placed in a glass dish having an inside diameter of 12.9 mm, an outside diameter of 15.0 mm, and a length of 10 mm. The thickness of the magnetic fluid in the glass dish was 3 mm. The glass dishes were placed in a hole drilled in an aluminum plate (110 mm×110 mm×10 mm), the hole being sized such that the glass dish would fit snugly. The aluminum plate was then placed on an aluminum block (220 mm×220 mm×20 mm) in an oven at a controlled temperature. A test was carried out at 80° C. and the result is shown in table 1. 
     
                       TABLE 1______________________________________Gel time test result for the samples #1-1 and #1-2Type of magnetic fluid          Gel time at 80° C. (hours)______________________________________Sample #1-1    82-91Sample #1-2    42-51______________________________________ 
    
     EXAMPLE 2 
     The oleic covered and isobutyl trimethoxysilane treated heptane base magnetic fluid was prepared in the same manner as described in Example 1. 
     7 cc of polyisobutenylsuccinimide and 13 cc of 6 cSt at 100° C. oil of polyalphaolefin was added into the heptane base magnetic fluid, the heptane was removed by heating it, and the saturation magnetization of the oil base fluid was adjusted to be 200 gauss by adding the oil. 
     A magnetic fluid, sample #2-1, that was 200 gauss and 6 cSt oil base was obtained. 
     Another magnetic fluid, sample #2-2, that was 200 gauss and 6 cSt oil base was prepared in the same manner as the sample #2-1, except that isobutyl trimethoxysilane was not applied to the particles during the process. 
     A gel time test was carried out in the same manner as described in Example 1 for the samples #2-1 and #2-2, but the test temperature was raised to 150° C. Table 2 shows the test results. 
     
                       TABLE 2______________________________________Gel time test result for the samples #2-1 and #2-2Type of magnetic fluid          Gel time at 150° C. (hours)______________________________________Sample #2-1    101-130Sample #2-2     94-101______________________________________ 
    
     EXAMPLE 3 
     52 g of ferrous sulfate heptahydrate was diluted with water to about 200 cc and stirred until dissolved. To this was added 85 cc of 42 Baume ferric chloride and stirred until the mixture was homogeneous. About 125 cc of about 26% ammonium hydroxide in about 70 cc of water was added to this mixture and stirred until homogeneous. The mixture reached a temperature of 60-70° C. About 50 cc of di-12-hydroxystearic acid isostearate, dissolved in about 450 cc of heptane, was heated to about 70° C. and added to the stirring warm magnetite mixture. This mixture was then stirred for about 5 minutes. To this was added about 350 cc of acetone and the mixture was stirred for about 5 minutes. The mixture was then allowed to separate for one hour. 
     The fluid, which rose to the top, was then siphoned off and the volume was reduced to about 150 cc by heating to remove some of the heptane solvent. This fluid was then cooled to room temperature. The fluid was flocced with about 350 cc of acetone by stirring and allowing settlement over a large Alnico V magnate. The supernatant was decanted and the remaining fluid was then flocced a final time and washed with 2, 50 cc portions of acetone. It was then dried two times with about 150 cc acetone and drained well. The remaining magnetite was dispersed in about 450 cc heptane and the remaining water and acetone were evaporated. The fluid was then filtered through a Whatman #4 filter and a trimellitate ester carrier liquid was added. The solvent was evaporated and the magnetization was adjusted to about 250 gauss by adding enough trimellitate ester. 
     Samples of about 0.50 g were weighed into small glass dishes (about 1 cm diameter×0.5 cm height). The dishes were set into drilled wells of a thick aluminum plate and the time of gellation at about 140° C. was obtained. 
     EXAMPLE 4 
     52 g of ferrous sulfate heptahydrate was diluted with water to about 200 cc and stirred until dissolved. To this was added 85 cc of 42 Baume ferric chloride and stirred until the mixture was homogeneous. About 125 cc of about 26% ammonium hydroxide in about 70 cc of water was added to this mixture and stirred until homogeneous. The mixture reached a temperature of 60-70° C. About 50 cc of di-12-hydroxystearic acid isostearate, dissolved in about 450 cc of heptane, was heated to about 70° C. and added to the stirring warm magnetite mixture. This mixture was then stirred for about 5 minutes. To this was added about 350 cc of acetone and the mixture was stirred for about 5 minutes. The mixture was then allowed to separate for one hour. 
     The fluid, which rose to the top, was then siphoned off and the volume was reduced to about 150 cc by heating to remove some of the heptane solvent. This fluid was then cooled to room temperature. The fluid was flocced with about 350 cc of acetone by stirring and allowing settlement over a large Alnico V magnate. The supernatant was decanted and the remaining magnetite was re-suspended to about 150 cc in heptane. This procedure was repeated four times. The fluid was then flocced a final time and washed with 2, 50 cc portions of acetone. It was then washed three times with about 300 cc of a 70:30 acetone to water mixture. The magnetite was then placed in about 600 cc of cold water and stirred vigorously. The pH was adjusted to 9-11, with 26% ammonium hydroxide, and subsequently about 6 g of acetic acid was added. This mixture was stirred for about 30 minutes. 
     The magnetite was collected over an Alnico V magnet and drained well. It was then washed two times with about 300 cc portions of acetone, three times with about 300 cc portions of the acetone/water (70:30) mixture and finally dried two times with about 300 cc portions of acetone and drained well. The remaining magnetite was dispersed in about 450 cc of heptane and the remaining water and acetone were evaporated. The fluid was then filtered through a Whatman #4 filter and a trimellitate ester carrier liquid was added. The solvent was evaporated and the magnetization was adjusted to about 250 gauss by adding additional trimellitate ester carrier liquid. 
     Samples of about 0.50 g were weighed into small glass dishes (about 1 cm diameter×0.5 cm height). The dishes were set into drilled wells of a thick aluminum plate and the time of gellation of about 140° C was obtained. 
     EXAMPLE 5 
     This example followed the same procedure as Example 4, except about 15.5 g of allantoin were added instead of acetic acid. 
     EXAMPLE 6 
     This example followed the same procedure as Example 4, except that about 13.5 g of tripropyl amine were added instead of acetic acid. 
     EXAMPLE 7 
     52 g of ferrous sulfate heptahydrate was diluted with water to about 200 cc and stirred until dissolved. To this was added 85 cc of 42 Baume ferric chloride and stirred until the mixture was homogeneous. About 125 cc of about 26% ammonium hydroxide, in about 70 cc of water, was added to the above and stirred until the mixture was homogeneous. The mixture reached a temperature of 60-70° C. About 50 cc of di-12-hydroxystearic acid isostearate dissolved, in about 450 cc of heptane, was heated to about 70° C. and then added to the stirring warm magnetite mixture. This was then stirred for about 5 minutes. To this was added about 350 cc of acetone and the mixture was stirred for about 5 minutes. The mixture was then allowed to separate for one hour. 
     The fluid, which rose to the top, was then siphoned off and the volume was reduced to about 150 cc by heating to remove some of the heptane solvent. This was then cooled to room temperature. The fluid was flocced with about 350 cc of acetone by stirring and allowing settlement over a large Alnico V magnet. The supernatant was decanted and the remaining fluid was then flocced a final time and washed with 2, 50 cc portions of acetone. It was then washed three times with about 300 cc of a 70:30 acetone to water mixture. This was then dried two times with about 150 cc acetone and drained well. The remaining magnetite was dispersed in about 450 cc heptane and the remaining water and acetone were evaporated. The fluid was then filtered through a Whatman #4 filter and a trimellitate ester carrier liquid, with 6% of an alkyl diphenylamine antioxidant, was added. The solvent was evaporated and the magnetization was adjusted to about 250 gauss by adding enough trimellitate ester/antioxidant mixture. 
     Samples of about 0.50 g were weighed into small glass dishes (about 1 cm diameter×0.5 cm height). The dishes were set into drilled wells of a thick aluminum plate and the time of gellation at about 160° C. was obtained. 
     EXAMPLE 8 
     52 g of ferrous sulfate heptahydrate was diluted with water to about 200 cc and stirred until dissolved. To this was added 85 cc of 42 Baume ferric chloride and stirred until the mixture was homogeneous. About 125 cc of about 26% ammonium hydroxide, in about 70 cc of water, was added to the above and stirred until the mixture was homogeneous. The mixture reached a temperature of 60-70° C. About 50 cc of di-12-hydroxystearic acid isostearate, dissolved in about 450 cc of heptane, was heated to about 70° C. and then added to the stirring warm magnetite mixture. This mixture was then stirred for about 5 minutes. To this was added about 350 cc of acetone and the mixture was stirred for about 5 minutes. The mixture was then allowed to separate for one hour. 
     The fluid, which rose to the top, was then siphoned off and the volume was reduced to about 150 cc by heating to remove some of the heptane solvent. This mixture was then cooled to room temperature. The fluid was flocced with about 350 cc acetone by stirring and allowing settlement over a large Alnico V magnet. The supernatant was decanted and the remaining magnetite was re-suspended in heptane to about 150 cc. This flocced procedure was repeated four times. The fluid was then flocced a final time and washed with 2, 50 cc portions of acetone. It was then washed three times with about 300 cc of a 70:30 acetone to water mixture. The magnetite was then placed in about 600 cc of cold water and stirred vigorously. The pH was adjusted to between about 9-11 with 26% ammonium hydroxide. Subsequently, about 6.0 g of acetic acid was added and stirred for about 30 minutes. 
     The magnetite was collected over an Alnico V magnet and drained well. It was then washed two times with about a 300 cc portion of acetone, three times with about a 300 cc portion of an acetone/water (70:30) mixture and finally dried two times with about a 300 cc portion of acetone and drained well. The remaining magnetite was dispersed in about 450 cc of heptane and the remaining water and acetone were evaporated. The fluid was then filtered through a Whatman #4 filter. A trimellitate ester carrier liquid, with 6% of an alkyl diphenylamine antioxidant, was added. The solvent was evaporated and the magnetization was adjusted to about 250 gauss by adding enough trimellitate ester/antioxidant mixture. 
     Samples of about 0.50 g were weighed into small glass dishes (about 1 cm diameter×0.5 cm height). The dishes were set into drilled wells of a thick aluminum plate and the time of gellation at about 160° C. was obtained. 
     EXAMPLE 9 
     This example followed the same procedure as Example 8, except that about 13.5 g of tripropyl amine was added instead of acetic acid. 
     EXAMPLE 10 
     This example followed the same procedure as Example 8, except that about 15.0 g of benzoic acid was added instead of acetic acid. 
     EXAMPLE 11 
     200 g of ferrous sulfate heptahydrate was dissolved in water and the total amount of the solution was adjusted to be 840 cc by adding water. An amount of 370 cc of 42 Baume ferric chloride was dissolved in the above ferrous sulfate solution. An ammonia solution was prepared by mixing 530 cc of 26% ammonia solution with 300 cc of water and this ammonia solution was carefully added to the iron mixture solution and mixed for about three minutes. The iron mixture solution was preheated to 60*C prior to adding the ammonia solution to it. A 2000 cc slurry of Fe 3  O 4  magnetic particles precipitate was obtained. 
     The above slurry was mixed with 46 cc of 26% ammonia solution. 37 g of an oleic acid solution was dissolved in 500 cc of heptane and added to the slurry and stirred for three minutes. The entire magnetic particle slurry was left to set for ten hours. 
     The oleic acid coated particles were peptized in heptane and the heptane base magnetic fluids were heated to 100° C. and adjusted to be 250 gauss. 
     60 cc of acetone was added to a 40 cc heptane base fluid, the coated magnetic particles were flocculated and the supernatant fluid was removed. The particles were washed three times with 60 cc of acetone. 
     The washed particles were peptized in 40 cc of heptane, heated to 100° C. and 40 cc of Emery 3004 oil was added. The heptane was evaporated and the particles were suspended in the oil. 
     EXAMPLE 12 
     200 g of ferrous sulfate heptahydrate was dissolved in water and the total amount of the solution was adjusted to be 840 cc by adding water. An amount of 370 cc of 42 Baume ferric chloride was dissolved in the above ferrous sulfate solution. An ammonia solution was prepared by mixing 530 cc of 26% ammonia solution with 300 cc of water and this ammonia solution was carefully added to the iron mixture solution and mixed for about three minutes. The iron mixture solution was preheated to 60° C. prior to adding the ammonia solution to it. A 2000 cc slurry of Fe 3  O 4  magnetic particles precipitated was obtained. 
     The above slurry were mixed with 46 cc of 26% ammonia solution. 37 g of an oleic acid solution was dissolved in 500 cc of heptane and added to the slurry and stirred for three minutes. The entire magnetic particle slurry was left to set for ten hours. 
     The oleic acid coated particles were peptized in heptane and the heptane base magnetic fluids were heated to 100° C. and adjusted to be 250 gauss. 
     A solution of 13.5 g of titanium triisostearoyl isopropoxide, dissolved in 50 cc of heptane, was then added to the heptane base fluid prior to mixing it with the Emery 3004 oil. 
     60 cc of acetone was added to a 40 cc heptane base fluid, the coated magnetic particles were flocculated and the supernatant fluid was removed. The particles were washed three times with 60 cc of acetone. 
     The washed particles were peptized in 40 cc of heptane, heated to 100° C. and 40 cc of Emery 3004 oil was added. The heptane was evaporated and the particles were suspended in the oil. 
     EXAMPLE 13 
     This example followed the same procedure as Example 12, except that a solution 3.8 g of Methacryloxypropyl trimethoxysilane, dissolved in 50 cc of heptane, was added to the heptane base fluid, instead of the titanium triisostearoyl isopropoxide, prior to mixing it with the Emery 3004 oil. 
     TEST DATA 
     The thermal stability experiments consist of all samples being respectively placed in a glass dish having an inside diameter of 13 mm, an outside diameter of 15 mm, and a length of 10 mm. The thickness of magnetic fluid in the glass dish was about 2 mm. The glass dishes were placed on an aluminum plate (250 mm×250 mm×10 mm). The aluminum plate was then placed in an oven at a controlled temperature. A test was carried out at 180° C., and the result is shown below. 
     The gel time data is presented in Tables 3, 4 and 5 and the chemical composition of the additives are presented in Table 1. 
     
                       TABLE 3______________________________________Hydroxystearic acid isostearate base magnetic fluidgel time dataExample # Additives.sup.1                 Gel Time in Hours @ 140° C.______________________________________3         No additives                 6904         Acetic Acid 7865         Allantoin   7866         Tripropyl Amine                 786______________________________________ .sup.1 See Table 6. 
    
     
                       TABLE 4______________________________________Hydroxystearic acid isostearate base magnetic fluidget time dataExample #    Additives.sup.1                   Gel Time in Hours @ 160° C.______________________________________7        ADA.sup.2  only                   4998        ADA.sup.2  + Acetic                   785    Acid9        ADA.sup.2  + Tripropyl                   618    Amine10       ADA.sup.2  + Benzoic                   785    Acid______________________________________ .sup.1 See Table 6. ADA = alkyl diphenylamine antioxidant. 
    
     
                       TABLE 5______________________________________Oleic acid base magnetic fluid gel time dataExample # Additive Name                 Gel Time in Hours @ 180° C.______________________________________11        Magnetic fluid                 3     with no     additives12        Titanium    14.0     Triisostearoyl     isopropoxide13        Methacryloxy                 14.0     propyl     trimethoxysilane______________________________________ 
    
     
                       TABLE 6______________________________________1. Acetic Acid ##STR1##2. Allantoin ##STR2##3. Tripropyl Amine ##STR3##4. Benzoic Acid ##STR4##5. Titanium Triisostearoyl - isopropoxide ##STR5##6. Methacryloxypropyltrimethoxysilane ##STR6##______________________________________