Lubricants with enhanced thermal conductivity containing nanomaterial for automatic transmission fluids, power transmission fluids and hydraulic steering applications
A lubricant composition having an enhanced thermal conductivity, up to 80% greater than its conventional analogues, and methods of preparation for these fluids are identified. One preferred composition contains a base oil, nanomaterial, and a dispersing agent or surfactant for the purpose of stabilizing the nanomaterial. One preferred nanomaterial is a high thermal conductivity graphite, exceeding 80 W/m in thermal conductivity. The graphite is ground, milled, or naturally prepared to obtain a mean particle size less than 500 nm in diameter, and preferably less than 100 nm, and most preferably less than 50 nm. The graphite is dispersed in the fluid by one or more of various methods, including ultrasonication, milling, and chemical dispersion. Carbon nanostructures such as nanotubes, nanofibrils, and nanoparticles are another type of graphitic structure useful in the present invention. Other high thermal conductivity carbon materials are also acceptable. To confer long-term stability, the use of one or more chemical dispersants or surfactants is useful. The thermal conductivity enhancement, compared to the fluid without graphite, is proportional to the amount of nanomaterials added. The graphite nanomaterials contribute to the overall fluid viscosity, partly or completely eliminating the need for viscosity index improvers and providing a very high viscosity index. Particle size and dispersing chemistry is controlled to get the desired combination of viscosity and thermal conductivity increase from the base oil while controlling the amount of temporary viscosity loss in shear fields. The resulting fluids have unique properties due to the high thermal conductivity and high viscosity index of the suspended particles, as well as their small size.
This application is a Continuation-In-Part of copending U.S. application Ser. No. 10/737,731 filed on Dec. 16, 2003 which claims priority from U.S. application Ser. No. 10/021,767 filed on Dec. 12, 2001 which claims priority from U.S. Provisional Application Ser. No. 60/254,959 filed on Dec. 12, 2000; PCT Application S.N. PCT/US02/16888 filed on May 30, 2002; U.S. nonprovisional application Ser. No. ______ filed on Dec. 8, 2003; and claims priority from U.S. Provisional Application Ser. No. 60/433,798 filed on Dec. 16, 2002 all of which are incorporated by reference in their entirety.
This application is part of a government project, Contract No. W031-109-ENG-38 by the Department of Energy. The Government has certain rights in this invention.
BACKGROUND OF THE INVENTION1. Field of the Invention
The invention relates to the field of providing lubricants and functional fluids containing nanomaterial dispersed within automatic transmission fluids, power transmission fluids, and hydraulic steering fluids which exhibit enhanced thermal conductivity as compared to conventional fluids without the nanomaterial dispersions.
2. Description of the Prior Art
Lubricants and coolants of various types are used in equipment and in manufacturing processes to remove waste heat, among other functions. Traditionally, water is most preferred for heat removal, however, to expand it's working temperature range, freezing point depressants such as ethylene glycol and/or propylene glycol are added, typically at levels above 10% concentration by volume. For example, automotive coolant is typically a mixture of 50-70% ethylene glycol, and the remainder is water. The thermal conductivity of the freezing point depressed fluid is then about ⅔ as good as water alone, as illustrated in Table 1. In many processes and applications, water can not be used for various reasons, such as corrosion, temperature restriction, etc., and then a type of oil, e.g., mineral oil, polyalphaolefin, ester synthetic oil, ethylene oxide/propylene oxide synthetic oil, polyalkylene glycol synthetic oil, etc. is used. The thermal conductivity of these oils, is typically 0.12 to 0.16 W/m at room temperature, and thus they are inferior as heat transfer agents to water, since water has a much higher thermal conductivity, 0.61 W/m as set forth in Table 1. Usually these oils have many other important functions, and they are carefully formulated to perform to exacting specifications for example for friction and wear performance, low temperature performance, fuel efficiency performance etc. Often designers will desire a fluid with higher thermal conductivity than the conventional oil, but are restricted to oil due to the many other parameters the fluid must meet.
The use of graphite in fluids such as lubricants is well known. The graphite is added as a friction reducing agent, which also carries some of the load imposed on the working fluid, and therefore helps to reduce surface damage to working parts. In order to be low friction, it is well known that the graphite layered structure must contain some water or other material to create the interlayer spacing and thereby lamellar structure. There are various commercially available graphite suspensions, e.g., from Acheson Colloid Co., which are specifically intended for use in lubricants. The size of the particles is varied for different dispersions, but the minimum average size for commercially available products is in the submicron range, typically mean as 500-800 nm (nanometers). The thermal advantage of the graphite is nor mentioned in the sales literature, nor is the product sold or promoted for its thermal conductivity property.
While there have been various patents filed on lubricants containing graphite, e.g. U.S. Pat. No. 6,169,059, there are none which specifically rely on graphite to improve the thermal conductivity of the fluid formulated for specific applications. Furthermore, there are none which teach specifically the use of nanometer-sized graphite with mean particle size much significantly less than 1000 nm in order to increase thermal conductivity and that reducing particle size improves thermal conductivity. While graphite-containing automotive engine oil was once commercialized (Arco graphite), the potential to use graphite as a heat transfer improving material in this oil was not realized. The particle size of graphite used was larger (mean greater than one micron) than for the instant invention. As a result, the graphite had some settling tendency in the fluid. Graphite of this size also significantly affects the friction and wear properties of the fluid, and heretofore has been used to reduce friction and improve wear performance of the fluid, e.g. in metalworking fluids. On the other hand, the use of graphite in lubricants for recirculating systems was made unpopular, partly due to evidence that micron size graphite could “pile up” in restricted flow areas in concentrated contacts, thereby leading to lubricant starvation. No recognition of effect of graphite particle size on this phenomena was made.
Previously, naturally formed “nano-graphites” have not been available in the marketplace at all. Recently, Hyperion Catalysis International, Inc. commercialized carbon nanotubes or so-called carbon fibrils, which have a graphitic content, e.g., U.S. Pat. No. 5,165,909. Carbon nanotubes are typically hollow graphite-like tubules having a diameter of generally several to several tens nanometers. They exist in the form either as discrete fibers or aggregate particles of nanofibers. The thermal conductivity of the Hyperion Catalysis International, Inc. material is not stated in their product literature. However, the potential of carbon nanotubes to convey thermal conductivity in a material is mentioned in their patents, U.S. Pat. No. 5,165,909. Actual measurement of the thermal conductivity of the carbon fibrils they produced was not given in the patent, so the inference of thermal conductivity is general and somewhat speculative, based on graphitic structure.
Bulk graphite with high thermal conductivity is available from Poco Graphite as a graphite foam, with thermal conductivity higher than 100 W/m, and is also available from the Carbide/Graphite Group, Inc. Graphite powders can be obtained from UCAR Carbon Company Inc., with thermal conductivity 10-500 W/m, and typically >80 W/m, and from Cytec Carbon Fibers LLC, with thermal conductivity 400-700 W/m. These bulk materials must be reduced to a nanometer-sized powder by various methods for use in the instant invention.
Utilization of these inexpensive sources of nanomaterials have not been released in lubrication formulations before and a point of novelty in the instant invention is the ability to reduce the graphite to produce an inexpensive nanomaterial having a particle size suitable for long term dispersion in lubricating compositions and the method of dispersing same.
Automatic transmission fluids, (“ATF”), power transmission fluids and hydraulic steering fluids have stringent requirements for viscosity, stability to oxidation, temperature and shear, low temperature fluidity, and static and dynamic coefficient of friction and their relative levels over many shift cycles. Additionally, the heat transfer requirements in transmissions and pumps are significant. It is generally necessary to use some form of cooling for the transmission fluid, and some designs of transmissions are prevented due to insufficient capability to eliminate waste heat. Because of the friction control requirements, and the relatively large particle size of conventional graphite dispersions, the use of graphite in these fluids is not known.
While the present invention is applicable in automatic transmission fluids, (ATF), power transmission fluids and hydraulic steering fluids, the examples and further discussion will focus on automatic transmission fluids; however, the claims are applicable to the power transmission fluids, hydraulic steering fluids, and other types of oil based noncompressible fluids as well.
SUMMARY OF THE INVENTIONIn this invention, automatic transmission fluids of enhanced thermal conductivity are prepared by dispersing nanometer-sized carbon nanomaterials of thermal conductivity higher than 80 W/m into the fluid The term carbon nanomaterials used in this invention refers to graphite nanoparticles, carbon nanotubes or fibrils, and other nanoparticles of carbon with graphitic structure. Stable dispersion is achieved by physical and chemical treatments.
For example, the instant invention provides a method for making a composition for an automatic transmission fluids that have enhanced thermal conductivity, up to 80% greater than their conventional analogues. One preferred composition contains an effective amount of at least one base oil such as mineral oil, hydrocracked mineral oil with high viscosity index, vegetable derived oils, polyalphaolefins, poly-internal-olefins, polyalkylglycols, polycyclopentadienes, propylene oxide or ethylene oxide based synthetics, silicone oils, phosphate esters or other synthetic esters, or any suitable base oil; an effective amount of at least one type of nanomaterial, preferably graphite nanoparticle or carbon nanotubes, and an effective amount of at least one dispersing agents or surfactants for the purpose of stabilizing the nanoparticles.
One preferred nanomaterial is a high thermal conductivity graphite, exceeding 80 W/m in thermal conductivity, and ground, milled, or naturally prepared with mean particle size less than 500 nm in diameter, and preferably less than 100 nm, and most preferably less than 50 nm. The graphite is dispersed in the fluid by one or more of various methods, including ultrasonication, milling, and chemical dispersion. It is contemplated that nanoparticles can be selected from any metal from the Group IV elements, such as carbon materials (carbon nanotubes, fullerenes, graphite, amorphous carbon, carbon particles, carbon fibrils and combinations thereof, etc.), silicone carbide, and clay materials, metal (including transition metals) particles (such as silver, copper, aluminum, etc.), metal oxides, alloy particles, and combinations thereof may be applicable to the instant invention.
Carbon nanotubes with a graphitic structure are another preferred type of nanomaterial or particles. Other high thermal conductivity carbon materials are also acceptable as long as they meet the thermal conductivity and size criteria set forth heretofore.
To confer long-term stability, an effective amount of one or more chemical dispersants or surfactants is preferred, although a special grinding procedure in base oil will also confer long term stability. The thermal conductivity enhancement, compared to the fluid without graphite, is proportional to the amount of nanomaterials added. The graphite nanoparticles or nanotubes contribute to the overall fluid viscosity, partly or completely eliminating the need for viscosity index improvers and providing a very high viscosity index. Particle size and dispersing chemistry is controlled to get the desired combination of viscosity and thermal conductivity increase from the base oil while controlling the amount of temporary viscosity loss in shear fields. The resulting fluids have unique properties due to the high thermal conductivity and high viscosity index of the suspended particles, as well as their small size.
The present invention provides at a minimum, a fluid of lubricant containing less than 10% by weight graphite nanoparticles. Preferably, however, a minimum of one or more chemical dispersing agents and/or surfactants is also added to achieve long term stability.
The term dispersant in the instant invention refers to a surfactant added to a medium to promote uniform suspension of extremely fine solid particles, often of colloidal size. In the lubricant industry the term dispersant is generally accepted to describe the long chain oil soluble or dispersible compounds which function to disperse the “cold sludge” formed in engines. The term surfactant in the instant invention refers to any chemical compound that reduces surface tension of a liquid when dissolved into it, or reduces interfacial tension between two liquids or between a liquid and a solid. It is usually, but not exclusively, a long chain molecule comprised of two moieties; a hydrophilic moiety and a lipophilic moiety. The hydrophilic and lipophilic moieties refer to the segment in the molecule with affinity for water, and that with affinity for oil, respectively. These two terms, dispersant and surfactant, are mostly used interchangeably in the instant invention for often a surfactant has dispersing characteristics and many dispersants have the ability to reduce interfacial tensions.
The particle-containing fluid of the instant invention will have a thermal conductivity higher than the neat fluid, wherein the term ‘neat’ is defined as the fluid before the particles are added.
The fluid can have other chemical agents or other type particles added to it as well to impart other desired properties, e.g. friction reducing agents, antiwear or anticorrosion agents, detergents, antioxidants, dispersants to define a lubricant composition suitable for use in vehicle applications or the like. Furthermore, the term fluid in the instant invention is broadly defined to include pastes, gels, greases, and liquid crystalline phases in either organic or aqueous media, emulsions and microemulsions.
For instance, U.S. Pat. No. 4,029,587 by Koch teaches the use of a variety additives for functional fluids applicable to the present invention and is hereby incorporated by reference in its entirety. Moreover, U.S. Pat. No. 4,116,877 by Outten et al. teaches the use of a variety additives for hydraulic fluids such as automatic transmission fluids and power steering fluids applicable to the present invention and is hereby incorporated by reference in its entirety.
As set forth above, the preferred carbon nanomaterials are selected from graphitic carbon structures with bulk thermal conductivity exceeding 80 W/m. A preferred form of carbon nanomaterials is carbon nanotubes. Another preferred form of carbon nanomaterials is high thermal conductivity graphite. A preferred form of the high thermal conductivity graphite is Poco Foam from Poco Graphite. Another preferred form of high thermal conductivity graphite is graphite powders from UCAR Carbon Company Inc. Still another preferred form of high thermal conductivity graphite is graphite powders from Cytec Carbon Fibers LLC. Still another preferred form of graphite is bulk graphite from The Carbide/Graphite Group, Inc.
Of course, one of the major drawbacks concerning commercial use of the carbon nanotubes and other prepared carbon structures is the cost of preparation and availability of same for commercial applications. The instant invention has resulted in the development of a method of reducing very inexpensive graphite to a nanomaterial comprising particles, fabrils and flakes suitable for use and long term dispersion in lubricant compositions.
The carbon nanomaterial containing dispersion may also contain a large amount of one or more other chemical compounds, such as polymers, antiwear agents, friction reducing agents, anti-corrosion agents, detergents, metal passivating agents, antioxidants, antifoaming agents, corrosion inhibitors, pour point depressants, and viscosity index improvers that are not for the purpose of dispersing, but to achieve thickening or other desired fluid characteristics.
Furthermore, the carbon nanomaterial dispersion can be pre-sheared, in a turbulent flow, such as a nozzle, or high pressure fuel injector, ultrasonic device, or mill in order to achieve a stable viscosity. This may be especially desirable in the case where carbon nanotubes with high aspect ratio are used as the graphite source, since they, even more than spherical particles, will thicken the fluid but loose viscosity when exposed in turbulent flows such as the flow regime in engines. Pre-shearing, e.g. by milling, sonicating, or passing through a small orifice, such as in a fuel injector, is a particularly effective way to disperse the particles and to bring them to a stable size so that their viscosity increasing effect will not change upon further use.
The milling process itself, or other pre-shearing process, can have a rather dramatic effect on the long term dispersion stability.
A novel method has been developed whereby graphite particles are milled to form a thick pasty liquid of particles with mean size less than 500 nanometers in diameter. The pasty liquid is then used as concentrate to prepare lubricants of various viscosity grades, and can be easily diluted to make a fluid with suitable viscosity for an automatic transmission fluid. A very effective paste can be made by mixing particles in a viscous base fluid in a loading of 5 to 20% by weight and milling for a period of several hours. The base fluid preferably contains from 20% up to 100% of the dispersant/surfactant mixture with the remainder being natural, synthetic, or mineral base oil. Once the thermally conductive concentrate prepared by milling is diluted to liquid consistency with base oil and other transmission fluid components, the entire fluid can (optionally) be passed through a small orifice to further increase the uniformity and decrease the size of dispersed particles.
An important aspect of this invention is that the final ATF composition should be prepared to give an acceptable lubricant film thickness at the maximum shear rate and temperature of use in the target transmission. The maximum concentration of particles in the final (diluted) automatic transmission fluid is limited by the relationship between viscosity increase of the fluid caused by the particles, and the temporary loss of viscosity (associated with the particles) at maximum temperature and shear rate of fluid use. In general, the heat transfer improvement with the ATF of the instant invention will be greater at room temperature than at the highest temperature of use due to the excellent viscosity index of the particle-containing fluids, depending on the particle size and their thickening effect. Viscosity index is defined as the relationship of viscosity to the temperature of a fluid. It is determined by measuring the kinematic viscosities of the oil at 40° C. and 100° C. and using the tables or formulas included in ASTM D2270. It is important to note that the smaller particles give the best thermal conductivity increase, and higher viscosity index of fluid, but also contribute to higher temporary viscosity loss in shear fields. A fluid made with heat transfer improvement of 20% at 100° C. may have an improvement of 60% or more when compared to a conventional fluid at 40° C. Therefore the heat transfer improvement due to the particles may be twofold, due to the higher thermal conductivity of the particles, and also due to the exceptional viscosity index of the particle-containing fluid.
It is an object of the present invention to provide a method of preparing a lubricant as a stable dispersion of the carbon nanomaterials in a liquid medium with the combined use of dispersants/surfactants and physical agitation.
It is an object of the present invention to provide a in which the carbon nanomaterials are made from cost-effective high-thermal-conductivity graphite (with thermal conductivity higher than 80 W/m).
It is an object of the present invention to provide a method of developing a method of forming carbon nanomaterials from inexpensive bulk graphite.
It is an object of the present invention to provide a method of utilizing carbon nanotube, graphite flakes, carbon fibrils, carbon particles and combinations thereof.
It is an object of the present invention to provide a method of using carbon nanotubes which are either single-walled, or multi-walled, with typical aspect ratio of 500-5000.
It is an object of the present invention whereby the carbon nanomaterial can optionally be surface treated to be hydrophilic at surface for ease of dispersing into the aqueous medium.
It is an object of the present invention to provide a method wherein the said dispersants/surfactants are soluble or highly dispersible in the said liquid medium.
It is an object of the present invention to provide a process for preparing a lubricant composition containing nanomaterial by
a) dissolving the said dispersants/surfactants or dispersant additive package into the base fluid; b) adding a high concentration (5-20% by weight) of the said carbon nanomaterials into the above mixture while being strongly agitated by high impact milling, and/or ultrasonication, to form a pasty liquid; and c) the pasty liquid obtained in b) is further diluted with base oil and additives as needed to make the final lubricant.
It is an object of the present invention to provide a method of using a liquid medium selected from a natural oil (vegetable or animal oil), or a synthetic oil, or a mineral oil or a combination thereof.
It is an object of the present invention to provide a method of defining an appropriate dispersants/surfactants for a liquid medium of the type used in the lubricant industry, whereby it is a surfactant or a mixture of surfactants with low HLB value (<8), preferably nonionic or mixture of nonionic and ionic surfactants.
It is an object of the present invention to provide that the dispersants can be the ashless polymeric dispersants used in the lubricant industry.
It is an object of the present invention to provide a uniform dispersion in the form of a gel or paste with designed viscosity of carbon nanomaterials in base oil medium.
It is an object of the present invention to provide a uniform dispersion in a form as a gel or paste of high thermal conductivity graphite nanoparticle in petroleum, natural, or synthetic liquid medium.
It is an object of the present invention to provide a uniform dispersion in its final form as an automatic transmission fluid of relatively low viscosity (kinematic viscosity less than 10 centistokes at 100° C.).
It is an object of the present invention to provide a uniform and stable dispersion in a form containing dissolved non-dispersing, other functional compounds in the liquid medium.
It is an object of the present invention to provide a uniform and stable dispersion in a form without polymeric viscosity index improvers, where all viscosity index improvement comes from the carbon nanomaterials.
It is an object of the present invention to provide a uniform and stable dispersion where due to the absence of polymeric materials the dispersion exhibits no permanent, only temporary loss in viscosity due to shear fields and turbulence.
It is an object of the present invention to provide a uniform and stable dispersion where the carbon nanomaterials are used to convey an extremely high viscosity index, >200, and even >300.
It is an object of the present invention to provide a uniform and stable dispersion where the thermal conductivity and heat transfer capability of the fluid is at least more than 20% improved compared to conventional mineral oil based automatic transmission fluids.
Other objects, features, and advantages of the invention will be apparent with the following detailed description taken in conjunction with the accompanying drawings showing a preferred embodiment of the invention.
A better understanding of the present invention will be had upon reference to the following description in conjunction with the accompanying drawings in which like numerals refer to like parts throughout the several views and wherein:
The present invention provides a graphitic dispersion in fluid medium that gives a high thermal conductivity and improved heat transfer capability compared to conventional fluids of the same medium. In the present invention the fluid medium is targeted in its viscosity, friction, and antioxidant characteristics to perform in modern automatic transmissions.
One preferred type of graphitic particles are carbon nanotubes, the nanotubes can be either single-walled, or multi-walled, having a typical nanoscale diameter of 1-200 nanometers. More typically the diameter is around 10-30 nanometers. The length of the tube can be in submicron and micron scale, usually from 500 nanometers to 500 microns. More typical length is 1 micron to 100 microns. The aspect ratio of the tube can be from hundreds to thousands, more typical 500 to 5000. The surface of the nanotube can be treated chemically to achieve certain level of hydrophilicity, or left as is from the production. Unfortunately, the commercial availability of the prepared nanotubes is limited making them too expensive for incorporation into commodity type lubricants at this time.
Therefor, a novel method has been developed to form nanomaterials suitable for use with commodity type lubricants at a low cost and capable of being produced in a large quantity using readily available equipment. Other acceptable form of graphite is a high-thermal-conductivity graphite commercially available, e.g. POCO FOAM, available from Poco Graphite, Inc., and graphite powders available from UCAR Carbon Company Inc. POCO FOAM is a high thermal conductivity foamed graphite, thermal conductivity typically in the range 100 to 150 W/m. A readily commercially available graphite is graphite powders from UCAR Carbon Company Inc., thermal conductivity of 10 to 500 W/m, and typically >80 W/m. Still another acceptable form of graphite is the high-thermal-conductivity graphite, Part#875G, from The Carbide/Graphite Group, Inc.
These graphite are prepared for the instant invention by pulverizing to a fine powder, dispersing chemically and physically in a fluid of choice, and then ball milled or otherwise size reduced until particle, flake, fibril or combinations thereof produce nanomaterial of a size of less than 500 nm mean size is attained. The preferred method is to disperse the graphite by ball milling in a viscous fluid of most additives (detergents, dispersants, etc.) and then diluting the obtained concentrate with base oil and other additives as needed to attain the final viscosity and performance characteristics. The finer the particle size attained upon milling, the better the thermal conductivity increase but also the more viscosity thickening effect of the pasty concentrate on the final blend. These effects must be balanced to attain a suitable lubricating film thickness at the maximum shear rate and temperature of fluid use. In general, any high thermal conductivity graphite can be used, provided that pulverization, milling and other described chemical and physical methods can be used to reduce the size of the final graphite dispersion to below a mean particle size of 500 nm or less.
In the process of making lubricating fluid such as the automatic transmission fluid with the nanomaterial, the mechanical process and sequence of adding the components are important in order to fully take advantage of the high viscosity index of the nanoparticles an to make the final fluid product with exceptionally high viscosity index. High impact mixing is necessary to achieve a homogeneous dispersion. Ball mill is one of the examples of a high impact mixer. In the instant invention, an Eiger Mini Mill (Model: M250-VSE-EXP) is used as the high impact ball mill. It utilizes high wear resistant zirconia beads as the grinding media and circulates the dispersion constantly during milling.
To achieve the best milling effect and therefore the best viscosity index improvement, the proper milling procedure has been developed. Firstly a 5% to 20% by weight of graphite powders, and more preferably 10% by weight of graphite powders, in base oil dispersion is milled into a paste state. Usually this step takes about 3 to 4 hours. Then add the appropriate effective amount of at least one dispersing agent(s), usually 1 to 2 times of the weight of graphite, into the mill. With the addition of dispersing agent(s) the paste changes from paste into liquid almost instantly, and extended milling becomes possible. For most cases the extended milling period is 4 hours. It should be pointed out that if the mixture in the mill turns into a paste, the recirculation of it becomes very difficult and thus a poor milling is resulted. It is also found that if the dispersing agent(s) is(are) added into the mill at the very beginning, the viscosity index of the final nanofluids made from the milling process is not as high.
Graphite nanomaterials are obtained by pulverizing big graphite chunks weight several pounds or kilograms obtained from The Carbide/Graphite Group. The resulting pulverized material is size-selected through a mesh filter to be less than 75 Thirty (30) grams of the above pulverized graphite particles and 270 grams of a base oil, DURASYN 162 (a commercial 2 centistokes polyalphaolefin) were added into the Eiger Mini Mill (Model: M250-VSE-EXP). The milling speed was gradually increased to 4000 rpm. In about 4 hours the above mixture turned into thick paste. About 60 grams of this paste was discharged and labeled Paste ‘A’. Forty-eight (48) grams of a dispersant package from Lubrizol, LUBRIZOL 9677MX, was added to the rest of the mixture in the mill. The paste became very thin, and successful recirculation was restored. Stopped the mill after another 4 hours of milling and labeled the discharged paste as Paste B. Paste C was obtained by milling a mixture of 30 grams of graphite with diameter less than 7560 grams of LUBRIZOL 9677MX, and 270 grams of DURASYN 162 at 4000 rpm for 8 hours Note here the dispersing agent LUBRIZOL 9677MX was added into the mill at the very beginning. Then three automatic transmission fluids were formulated, A through C, using the above three pastes as concentrates respectively. The final compositions were exactly the same by weight and ingredients except for the graphite material: 2% graphite, 4% LUBRIZOL 9677 MX, 18% DURASYN 162, 76% DURASYN 166 (a commercial 6 centistokes polyalphaolefin base oil) (all percentage by weight). Example 1 illustrates the 100° C. viscosity and viscosity index (VI) of the fluids. It was also found that the graphite particle size before milling was an important variable to control the viscosity modification effect as well. For example, starting with graphite smaller than 10 (obtained as graphite powder from UCAR Carbon Company Inc.) and following the same procedure as Paste B, a thin Paste D was obtained. An automatic transmission fluid D was formulation with the same composition as ATF A and result is list in Example 1 as well. The particle size is measured by atomic force microscopy (AFM), and
The petroleum liquid medium can be any petroleum distillates or synthetic petroleum oils, greases, gels, or oil-soluble polymer composition. More typically, it is the mineral basestocks or synthetic basestocks used in the lube industry, e.g., Group I (solvent refined mineral oils). Group II (hydrocracked mineral oils), Group III (severely hydrocracked oils, sometimes described as synthetic or semi-synthetic oils), Group IV (polyalphaolefins), and Group V (esters, naphthenes, and others). One preferred group includes the polyalphaolefins, synthetic esters, and polyalkylglycols.
Synthetic lubricating oils include hydrocarbon oils and halo-substituted hydrocarbon oils such as polymerized and interpolymerized olefins (e.g., polybutylenes, polypropylenes, propylene-isobutylene copolymers, chlorinated polybutylenes, poly(l-octenes), poly(l-decenes), etc., and mixtures thereof; alkylbenzenes (e.g., dodecylbenzenes, tetradecylbenzenes, dinonylbenzenes, di-(2-ethylhexyl)benzenes, etc.); polyphenyls (e.g., biphenyls, terphenyls, alkylated polyphenyls, etc.), alkylated diphenyl, ethers and alkylated diphenyl sulfides and the derivatives, analogs and homologs thereof and the like. Alkylene oxide polymers and interpolymers and derivatives thereof where the terminal hydroxyl groups have been modified by esterification, etherification, etc. constitute another class of known synthetic oils.
Another suitable class of synthetic oils comprises the esters of dicarboxylic acids (e.g., phtalic acid, succinic acid, alkyl succinic acids and alkenyl succinic acids, maleic acid, azelaic acid, suberic acid, sebacic acid, fumaric acid, adipic acid, alkenyl malonic acids, etc.) with a variety of alcohols (e.g., butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol diethylene glycol monoether, propylene glycol, etc.). Specific examples of these esters include dibutyl adipate, di(2-ethylhexyl) sebacate, di-hexyl fumarate, dioctyl sebacate, diisooctyl azelate, diisodecyl azealate, dioctyl phthalate, didecyl phthalate, dicicosyl sebacate, the 2-ethylhexyl diester of linoleic acid dimer, the complex ester formed by reacting one mole of sebacic acid with two moles of tetraethylene glycol and two moles of 2-ethylhexanoic acid, and the like.
Esters useful as synthetic oils also include those made from C C12 monocarboxylic acids and polyols and polyol ethers such as neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol, tripentaerythritol, etc. Other synthetic oils include liquid esters of phosphorus-containing acids (e.g., tricresyl phosphate, trioctyl phosphate, diethyl ester of decylphosphonic acid, etc.), polymeric tetrahydrofurans and the like.
Preferred polyalphaolefins (PAO), include those sold by Mobil Chemical Company as SHF fluids, and those sold by Ethyl Corporation under the name ETHYLFLO, or ALBERMARLE. PAO include the ETHYL-FLOW series by Ethyl Corporation, “Albermarle Corporation,” including ETHYL-FLOW 162, 164, 166, 168, and 174, having varying viscosity from about 2 to about 460 centistokes.
Mobil SHF-42 from Mobil Chemical Company, EMERY 3004 and 3006, and Quantum Chemical Company provide additional polyalphaolefins basestocks. For instance, EMERY 3004 polyalphaolefin has a viscosity of 3.86 centistokes at (100° C.) and 16.75 centistokes at (40° C.). It has a viscosity index of 125 and a pour point of −98° C. Moreover, EMERY 3006 polyalphaolefin has a viscosity of 5.88 centistokes at 212° C. and 31.22 centistokes at 104° C. It has a viscosity index of 135 and a pour point of −87° C.
Additional satisfactory polyalphaolefins are those sold by Uniroyal Inc. under the brand SYNTON PAO-40, which is a 40 centistokes polyalphaolefin. Also useful are the Oronite brand polyalphaolefins manufactured by Chevron Chemical Company.
It is contemplated that Gulf SYNFLUID 4 centistokes PAO; commercially available from Gulf Oil Chemicals Company, a subsidiary of Chevron Corporation, which is similar in many respects to EMERY 3004 may also be utilized herein. MOBIL SHF-41 PAO, commercially available from Mobil Chemical Corporation, is also similar in many respects to EMERY 3004.
Preferably the polyalphaolefins will have a viscosity in the range of about 2-100 centistokes at 100° C., with viscosity of 4 and 10 centistokes being particularly preferred.
The most preferred synthetic base oil ester additives are polyolesters and diesters such as di-aliphatic diesters of alkyl carboxylic acids such as di-2-ethylhexylazelate, di-isodecyladipate, and di-tridecyladipate, commercially available under the brand name EMERY 2960 by Emery Chemicals, described in U.S. Pat. No. 4,859,352 to Waynick. Other suitable polyolesters are manufactured by Mobil Oil. Mobil polyolesters P-43, M Ø45 containing two alcohols, and Hatco Corp. 2939 are particularly preferred.
Diesters and other synthetic oils have been used as replacements of mineral oil in fluid lubricants. Diesters have outstanding extreme low temperature flow properties and good resistance to oxidative breakdown.
The diester oil may include an aliphatic diester of a dicarboxylic acid, or the diester oil can comprise a dialkyl aliphatic diester of an alkyl dicarboxylic acid, such as di-2-ethyl hexyl azelate, di-isodecyl azelate, di-tridecyl azelate, di-isodecyl adipate, di-tridecyl adipate. For instance, Di-2-ethylhexyl azelate is commercially available under the brand name of EMERY 2958 by Emery Chemicals.
Also useful are polyol esters such as EMERY 2935, 2936, and 2939 from Emery Group of Henkel Corporation and Hatco 2352, 2962, 2925, 2938, 2939, 2970, 3178, and 4322 polyol esters from Hatco Corporation, described in U.S. Pat. No. 5,344,579 to Ohtani et al., and Mobil ester P 24 from Mobil Chemical Company. Mobil esters such as made by reacting dicarboxylic acids, glycols, and either monobasic acids or monohydric alcohols like EMERY 2936 synthetic-lubricant basestocks from Quantum Chemical Corporation and Mobil P 24 from Mobil Chemical Company can be used. Polyol esters have good oxidation and hydrolytic stability. The polyol ester for use herein preferably has a pour point of about −100° C. or lower to −40° C. and a viscosity of about 2-460 centistokes at 100° C.
Group III oils are often referred to as hydrogenated oil to be used as the sole base oil component of the instant invention providing superior performance to conventional ATFs with no other synthetic oil base or mineral oil base.
A hydrogenated oil is a mineral oil subjected to hydrogenation or hydrocracking under special conditions to remove undesirable chemical compositions and impurities resulting in a mineral oil based oil having synthetic oil components and properties. Typically the hydrogenated oil is defined as a Group III petroleum based stock with a sulfur level less than 0.03, severely hydrotreated and isodewaxed with saturates greater than or equal to 90 and a viscosity index of greater than or equal to 120, and may optionally be utilized in amounts up to 90 percent by volume, more preferably from 5.0 to 50 percent by volume and more preferably from 20 to 40 percent by volume when used in combination with a synthetic or mineral oil.
The hydrogenated oil my be used as the sole base oil component of the instant invention providing superior performance to conventional motor oils with no other synthetic oil base or mineral oil base. When used in combination with another conventional synthetic oil such as those containing polyalphaolefins or esters, or when used in combination with a mineral oil, the hydrogenated oil may be present in an amount of up to 95 percent by volume, more preferably from about 10 to 80 percent by volume, more preferably from 20 to 60 percent by volume and most preferably from 10 to 30 percent by volume of the base oil composition.
A Group I or II mineral oil basestock may be incorporated in the present invention as a portion of the concentrate or a basestock to which the concentrate may be added. Preferred as mineral oil basestocks are the Marathon Ashland Petroleum (MAP) 325 Neutral defined as a solvent refined neutral having a Sabolt Universal viscosity of 325 SUS at 100° C. and MAP 100 Neutral defined as a solvent refined neutral having a Sabolt Universal viscosity of 100 SUS at 100° C., both manufactured by the Marathon Ashland Petroleum.
Other acceptable petroleum-base fluid compositions include white mineral, paraffinic and MVI naphthenic oils having the viscosity range of about 20-400 centistokes. Preferred white mineral oils include those available from Witco Corporation, Arco Chemical Company, PSI and Penreco. Preferred paraffinic oils include solvent neutral oils available from Exxon Chemical Company, HVI neutral oils available from Shell Chemical Company, and solvent treated neutral oils available from Arco Chemical Company. Preferred MVI naphthenic oils include solvent extracted coastal pale oils available from Exxon Chemical Company, MVI extracted/acid treated oils available from Shell Chemical Company, and naphthenic oils sold under the names HYDROCAL and CALSOL by Calumet, and described in U.S. Pat. No. 5,348,668 to Oldiges.
Finally, vegetable oils may also be utilizes as the liquid medium in the instant invention. Soybean or rapeseed oil, particularly of the high oleic or mid oleic genetically engineered type, commercially available from Archer Daniels Midland Company, are good examples of these oils. Soybean oil is of interest because it has a high thermal conductivity itself.
Dispersants Used in Lubricant IndustryDispersants used in the lubricant industry are typically used to disperse the “cold sludge” formed in gasoline and diesel engines, which can be either “ashless dispersants”, or containing metal atoms. They can be used in the instant invention since they are found to be an excellent dispersing agent for nanoparticles with graphitic structure of this invention. They are also needed to disperse wear debris and products of lubricant degradation within the transmission.
The ashless dispersants commonly used in the automotive industry contain an lipophilic hydrocarbon group and a polar functional hydrophilic group. The polar functional group can be of the class of carboxylate, ester, amine, amide, imine, imide, hydroxyl, ether, epoxide, phosphorus, ester carboxyl, anhydride, or nitrile. The lipophilic group can be oligomeric or polymeric in nature, usually from 70 to 200 carbon atoms to ensure oil solubility. Hydrocarbon polymers treated with various reagents to introduce polar functions include products prepared by treating polyolefins such as polyisobutene first with maleic anhydride, or phosphorus sulfide or chloride, or by thermal treatment, and then with reagents such as polyamine, amine, ethylene oxide, etc.
Of these ashless dispersants the ones typically used in the petroleum industry include N-substituted polyisobutenyl succinimides and succinates, alkyl methacrylate-vinyl pyrrolidinone copolymers, alkyl methacrylate-dialkylaminoethyl methacrylate copolymers, alkylmethacrylate-polyethylene glycol methacrylate copolymers, and polystearamides. Preferred oil-based dispersants that are most important in the instant application include dispersants from the chemical classes of alkylsuccinimide, succinate esters, high molecular weight amines, Mannich base and phosphoric acid derivatives. Some specific examples are polyisobutenyl succinimide-polyethylenepolyamine, polyisobutenyl succinic ester, polyisobutenyl hydroxybenzyl-polyethylenepolyamine, bis-hydroxypropyl phosphorate. Commercial dispersants suitable for transmission fluid are for example, Lubrizol 890 (an ashless PIB succinimide), Lubrizol 6420 (a high molecular weight PIB succinimide), ETHYL HITEC 646 (a non-boronated PIB succinimide). The dispersant may be combined with other additives used in the lubricant industry to form a ispersant-detergent (DI) additive package for transmission fluids, e.g., LUBRIZOL 9677MX, and the whole DI package can be used as dispersing agent for the nanoparticle dispersions.
Other Types of DispersantsAlternatively a surfactant or a mixture of surfactants with low HLB value (typically less than or equal to 8), preferably nonionic, or a mixture of nonionics and ionics, may be used in the instant invention.
The dispersants selected should be soluble or dispersible in the liquid medium. The dispersant can be in a range of up from 0.01 to 30 percent, more preferably in a range of from between 0.5 percent to 20 percent, more preferably in a range of from between 1 to 15 percent, and most preferably in a range of from between 2 to 13 percent. The carbon nanomaterials can be of any desired weight percentage in a range of from 0.001 up to 10 percent. For practical application it is usually in a range of from between 0.01 percent to 10 percent, and most preferably in a range of from between 0.1 percent to 5 percent. The remainder of the formula is the selected medium.
It is believed that in the instant invention the dispersant functions by adsorbing onto the surface of the carbon nanomaterials.
Other Chemical CompoundsThis dispersion may also contain a large amount of one or more other chemical compounds, preferably polymers, not for the purpose of dispersing, but to achieve thickening or other desired fluid characteristics. These can be added but reduce the amount of particulate that can be used without excessive thickening.
The viscosity improvers used in the lubricant industry can be used in the instant invention for the oil medium, which include olefin copolymers (OCP), polymethacrylates (PMA), hydrogenated styrene-diene (STD), and styrene-polyester (STPE) polymers. Olefin copolymers are rubber-like materials prepared from ethylene and propylene mixtures through vanadium-based Ziegler-Natta catalysis. Styrene-diene polymers are produced by anionic polymerization of styrene and butadiene or isoprene. Polymethacrylates are produced by free radical polymerization of alkyl methacrylates. Styrene-polyester polymers are prepared by first co-polymerizing styrene and maleic anhydride and then esterifying the intermediate using a mixture of alcohols.
Other compounds which can be used in the instant invention in the oil medium include: acrylic polymers such as polyacrylic acid and sodium polyacrylate, high-molecular-weight polymers of ethylene oxide such as Polyox WSR from Union Carbide, cellulose compounds such as carboxymethylcellulose, polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), xanthan gums and guar gums, polysaccharides, alkanolamides, amine salts of polyamide such as DISPARLON AQ series from King Industries, hydrophobically modified ethylene oxide urethane (e.g., ACRYSOL series from Rohmax), silicates, and fillers such as mica, silicas, cellulose, wood flour, clays (including organoclays) and nanoclays, and resin polymers such as polyvinyl butyral resins, polyurethane resins, acrylic resins and epoxy resins.
Chemical compounds such as seal swell agents or plasticizers can also be used in the instant invention and may be selected from the group including phthalate, Adipate, sebacate esters, and more particularly: glyceryl tri(acetoxystearate), epoxidized soybean oil, epoxidized linseed oil, N,n-butyl benzene sulfonamide, aliphatic polyurethane, epoxidized soy oil, polyester glutarate, polyester glutarate, triethylene glycol caprate/caprylate, long chain alkyl ether, dialkyl diester glutarate, monomeric, polymer, and epoxy plasticizers, polyester based on adipic acid, hydrogenated dimer acid, distilled dimer acid, polymerized fatty acid trimer, ethyl ester of hydrolyzed collagen, isostearic acid and sorbian oleate and cocoyl hydrolyzed keratin, PPG-12/PEG-65 lanolin oil, dialkyl adipate, alkylaryl phosphate, alkyl diaryl phosphate, modified triaryl phosphate, triaryl phosphate, butyl benzyl phthalate, octyl benzyl phthalate. alkyl benzyl phthalate, dibutoxy ethoxy ethyl adipate, 2-ethylhexyldiphenyl phosphate, dibutoxy ethoxy ethyl formyl, diisopropyl adipate, diisopropyl sebacate, isodecyl oieate, neopentyl glycol dicaprate, neopenty giycol diotanoate, isohexyl neopentanoate, ethoxylated lanolins, polyoxyethylene cholesterol, propoxylated (2 moles) lanolin alcohols, propoxylated lanoline alcohols, acetylated polyoxyethylene derivatives of lanoline, and dimethylpolysiloxane. Other plasticizers which may be substituted for and/or used with the above plasticizers including glycerine, polyethylene glycol, dibutyl phthalate, and 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, and diisononyl phthalate all of which are soluble in a solvent carrier. Other seal swelling agents such as Lubrizol 730 can also be used.
Antioxidants are an important part of transmission fluids. General classes include zinc
dialkyldithiophosphates, alkyl and aryl phenols, alkyl and aryl amines, and sulfurized olefins. Commercial examples are Ciba L57 (phenyl amine) and Etnyl Hitec 1656.
Pour point depressants, either of polymethyl methacrylate or ethylene propylene olefin co-polymer type are useful to decrease the low temperature Brookfield viscosity of the ATF. Examples include ROHMAX 3008, ROHMAX 1-333, LUBRIZOL 6662A.
Friction Modifiers are used to control friction and torque characteristics of the fluid. Commercial examples include LUBRIZOL 8650 and HITEC 3191.
Physical AgitationThe physical mixing includes high shear mixing, such as with a high speed mixer, homogenizers, microfluidizers, a Kady mill, a colloid mill, etc., high impact mixing, such as attritor, ball and pebble mill, etc., and ultrasonication methods or passing through a small orifice such as a fuel injector. Turbulent flows of any type will assist mixing.
Ball milling is the most preferred physical method in the instant invention since it is effective at rapidly reducing the graphite particles to very small size while simultaneously dispersing them into a concentrated paste as previously described. The concentrate can then be diluted with base oil and other additives to hit a final target viscosity, depending on the maximum temperature and shear conditions anticipated in the target transmission. For further size reduction and reducing particle maximum size the diluted oil can be passed through a small orifice such as a fuel injector. The raw material mixture may be pulverized by any suitable known dry or wet grinding method. One grinding method includes pulverizing the raw material mixture in the fluid mixture of the instant invention to obtain the concentrate, and the pulverized product may then be dispersed further in a liquid medium with the aid of the dispersants described above. However, pulverization or milling reduces the carbon nanotube average aspect ratio. A detailed description has been given in an earlier section of the instant invention.
Ultrasonication is another physical method in the instant invention since it is less destructive to the carbon nanomaterial structure than the other methods described. Ultrasonication can be done either in the bath-type ultrasonicator, or by the horn-type ultrasonicator. More typically, horn-type ultrasonication is applied for higher energy output. Sonication at the medium-high instrumental intensity for up to 30 minutes, and usually in a range of from 10 to 20 minutes is desired to achieve better homogeneity.
The instant method of forming a stable dispersion of carbon nanomaterials in a solution consist of three steps. First select the appropriate concentrate of dispersant or mixture of dispersing and other additives for the carbon nanomaterials, and the medium, and dissolve the dispersant into the liquid medium to form a concentrate solution (keeping in mind the final additive concentrations desired following dilution); secondly add a high concentration of the carbon nanomaterials into the dispersant-containing solution, initiate strongly agitating, ball milling, or ultrasonicating, or any combination of physical methods named; following an agitation time of several hours, the resulting paste will be extremely stable and easily dilutable into more base oil and additives to give the final desired concentrations of additives and the desired final viscosity.
EXAMPLESThe following examples describe preferred embodiments of the invention. Other embodiments within the scope of the claims herein will be apparent to one skilled in the art from consideration of the specification or practice of the invention as disclosed herein. It is intended that the specification, together with the examples, be considered exemplary only, with the scope and spirit of the invention being indicated by the claims which follow the examples. In the examples all percentages are given on a weight basis unless otherwise indicated. Reference to documents made in the specification is intended to result in such patents or literature cited are expressly incorporated herein by reference, including any patents or other literature references cited within such documents as if fully set forth in this specification.
Example 1 Automatic Transmission Fluids and Viscosity Data
Thermal conductivity is measured by a transient hot-wire rig constructed by the inventors in-house according to Nagasaka et al. (Y. Nagasaka and A. Nagashima, Absolute measurement of the thermal conductivity of electrically conducting liquids by the transient
hot-wire method, Journal of Physics E: Sci. Instrum. 1981, 14, 1435-1440). A diagram of the rig is shown in
The foregoing detailed description is given primarily for clearness of understanding and no unnecessary limitations are to be understood therefrom, for modification will become obvious to those skilled in the art upon reading this disclosure and may be made upon departing from the spirit of the invention and scope of the appended claims. Accordingly, this invention is not intended to be limited by the specific exemplification presented herein above. Rather, what is intended to be covered is within the spirit and scope of the appended claims.
Claims
1. A method of preparing a lubricant as a stable dispersion of the carbon nanomaterials in a liquid medium with the combined use of dispersants/surfactants and physical agitation, comprising the steps of:
- a) dissolving an effective amount of at least one of the group consisting of a dispersant, a surfactant, a dispersant additive package, and combinations thereof into an effective amount of at least one base oil forming a first mixture;
- b) adding a high concentration of up to 20 percent by weight of a carbon nanomaterial into said first mixture while being strongly agitated by high impact milling, and/or ultrasonication, to form a pasty liquid; and
- c) diluting said pasty liquid with an effective amount of a base oil.
Type: Application
Filed: Oct 31, 2007
Publication Date: Nov 20, 2008
Inventors: Zhiqiang Zhang (Lexington, KY), Gefei Wu (Lexington, KY), Frances E. Lockwood (Georgetown, KY)
Application Number: 11/981,720
International Classification: C10M 169/04 (20060101);
