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  Use of nanomaterials as effective viscosity modifiers in lubricating fluidsUSPTO Application #: 20070293405Title: Use of nanomaterials as effective viscosity modifiers in lubricating fluids Abstract: Nanomaterials have been used as a supplement or replacement of traditional polymer-based viscosity modifiers for lubricants and other related fluids. Compared with traditional polymer-based viscosity modifiers, nanomaterials possess better viscosity-index modification functions, i.e., more even viscosity increase across the whole temperature range. Meanwhile, a cost-effective way of making nanomaterials have been developed based on commercially available graphite materials, and the resulting nanoparticles of graphite are nanodisks (nanoplates). Furthermore, it provides a viscosity modifier which exhibits temporary shear loss, which can contribute to fuel economy, but no permanent shear loss. (end of abstract) Agent: Carrithers Law Office, PLLC One Paragon Centre - Louisville, KY, US Inventors: Zhiqiang Zhang, Gefei Wu, Frances E. Lockwood, Thomas R. Smith USPTO Applicaton #: 20070293405 - Class: 508113000 (USPTO) Related Patent Categories: Solid Anti-friction Devices, Materials Therefor, Lubricant Or Separant Compositions For Moving Solid Surfaces, And Miscellaneous Mineral Oil Compositions, Lubricants Or Separants For Moving Solid Surfaces And Miscellaneous Mineral Oil Compositions (e.g., Water Containing, Etc.), Graphite, Coal, Or Elemental Carbon The Patent Description & Claims data below is from USPTO Patent Application 20070293405. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This application claims priority from U.S. Provisional Application Ser. No. 60/592,570 filed on Jul. 31, 2004 and Provisional Application Ser. No. 60/254,959 filed on Dec. 12, 2000 and U.S. Pat. No. 6,783,746 which issued on Aug. 31, 2004 from application Ser. No. 10/021,767 filed on Dec. 12, 2001 and application Ser. No. 10/929,636 filed on Aug. 30, 2004 and application Ser. No. 10/730,762 filed on Dec. 8, 2003 which claims priority from PCT/US02/16888 filed on May 30, 2002 and application Ser. No. 10/737,731 filed on Dec. 16, 2003 all of which are incorporated herein in their entirety. TECHNICAL FIELD [0002] The present invention relates to a novel use of nanomaterial as a viscosity modifier for a lubricating oil and a lubricating oil composition. More particularly, the invention relates to a novel viscosity modifier for a lubricating oil capable of producing a lubricating oil composition having excellent viscosity index and lubricating oil compositions containing such a viscosity modifier. BACKGROUND OF THE INVENTION [0003] The viscosity of petroleum products generally varies greatly with temperature, and for lubricating oils for automobiles, the temperature dependence of the viscosity is desired to be small. Therefore, a polymer has been widely used as a viscosity modifier having an effect of improving viscosity index for the purpose of decreasing the temperature dependence of the lubricating oils. [0004] Viscosity index of a fluid is defined as the relationship of viscosity of that fluid to the temperature. It is determined by measuring the kinematic viscosities of the oil at 40 and 100.degree. C. and then calculated by using the tables or formulas included in ASTM D 2270. High viscosity index fluids (e.g., a base oil with the addition of a viscosity modifier) tend to display less change in viscosity with temperature than low viscosity index fluids (e.g., that base oil), and the effect is illustrated in FIG. 1. [0005] Mineral oils, which are very effective lubricants at low temperatures, become less effective lubricants at high temperatures. At high temperatures, their film-forming ability (in the hydrodynamic lubrication regime) diminishes, because of a drop in viscosity. Prior to the use of viscosity modifiers and the introduction of multigrade oils, this problem was partly overcome through seasonal oil changes. The principal function of a viscosity modifier is to minimize viscosity variations with temperature. Viscosity modifiers are typically added to a low-viscosity oil to improve its high-temperature lubricating characteristics. These are organic polymers that minimize viscosity change with a change in temperature. This represents a practical means by which the operating range of mineral oils is extended to high temperature without adversely affecting their low-temperature fluidity. The mechanism is explained as follows. [0006] At low temperature, the polymer molecules occupy a small volume (hydrodynamic volume) and therefore have a minimum association with the bulk oil. The effect should be little viscosity increase. The situation is reversed at high temperature because polymer chains extend or expand as a consequence of added thermal energy. This increases the association of the polymer with bulk oil because of an increase in surface area. The result is an effective increase in viscosity at this high temperature. FIG. 2 illustrates oil thickening by viscosity modifiers. [0007] Olefin copolymers (OCP), polymethacrylates (PMA), hydrogenated styrene-diene (STD), and styrene-polyester (STPE) polymers are the most common types of viscosity modifiers used in modern lubricant formulations. [0008] However, there is always some undesired viscosity increase at low temperature (under the operating temperature range) caused by these viscosity modifiers. That is, the viscosity index improvement by the polymers is limited. Moreover, these polymers contribute to a higher extreme-low-temperature viscosity and wax formulation. [0009] When the surrounding temperature lowers, a wax component in a lubricating oil is crystallized and solidified to make the lubricating oil lose flowability, so a pour point depressant (PPD) is usually added into the lubricating oil to depress the solidification temperature. The pour point depressant functions to inhibit formation of a three-dimensional network attributed to crystallization of the wax component in the lubricating oil and to depress the pour point of the lubricating oil. Of the low-temperature properties of a lubricating oil containing a viscosity modifier, having an effect of improving viscosity index, and a pour point depressant, the viscosity at a high shear rate is determined by compatibility of a lubricating oil base with the viscosity modifier, but on the other hand, the viscosity at a low shear rate is greatly influenced by the pour point depressant. It is known that when an ethylene/.alpha.-olefin copolymer having specific composition is used as a viscosity modifier, the effect of the pour point depressant is markedly reduced because of an interaction between the copolymer and the pour point depressant (e.g., U.S. Pat. No. 3,697,429 and U.S. Pat. No. 3,551,336). Accordingly, the viscosity modifier to be blended with a lubricating oil which is required to have particularly excellent low-temperature properties is desired to exhibit an excellent effect of improving viscosity index and at the same time not to inhibit the function of the pour point depressant. [0010] The thickening effect of particles to a fluid base is well known (P. C. Hiemenz and R. Rajagopalan, Principles of Colloid and Surface Chemistry, 3.sup.rd ed., Marcel Dekker, Inc., 1997, Chapter 4). The initial theory of explaining this was developed by Albert Einstein in 1906, and there have been various modifications and deviations in this theory, and the details of which are obviously out of the scope of the current invention. Nanoparticles have been added to a fluid for the purpose of increasing thermal conductivity (U.S. Pat. No. 6,221,275, U.S. Pat. No. 6,432,320, and U.S. Pat. No. 6,695,974). However, there has been little or no effort in addressing the issue of viscous thickening effect of these nanoparticles. In most of the cases this viscous thickening effect is undesirable, since the increased viscosity will results in more demand for pumping power, more energy loss due to internal fluid friction, and even malfunction or catastrophic failure of the machinery if the viscosity is way off the desired range. [0011] However, in the current invention, with very careful formulation, the viscous thickening effect of the nanoparticles could be turned into an application as a revolutionary viscosity modifier. And because the nanoparticles are usually not polymer based, they are not going to cause compatibility issue with other polymeric additives/components in a lubricating fluid, and they are usually not contributing to wax formation by themselves. [0012] It is common understanding in the lubricant industry that thinner fluid may provide better fuel economy if adequate film thickness is properly maintained. The reason is that the energy loss due to internal friction of fluid itself is less when the viscosity is lower. Therefore, if the viscosity modifier can be sheared down temporarily (but not permanently), fuel economy benefit could be observed. In the event of current invention, the nanodisks (or nanoplates) orient themselves in a laminar flow regime (Literature cited: Y. Yang, E. Grulke, Z. Zhang, G. Wu, Rheological Behavior of Carbon Nanotube and Graphite Dispersions, submitted to Langmuir), which indicates that temporary shear loss will be observed should the fluid be place in a shear field. SUMMARY OF THE INVENTION [0013] In this invention, the use of nanoparticles as an effective viscosity modifier is illustrated. More specifically the use of carbon nanomaterial will be addressed. More specifically, cost-effective graphite materials and the process of making them into nanoparticles will be illustrated. [0014] The use of nanoparticles in a fluid base is well know, as illustrated by the previous US Patents. The use of graphite in fluids such as lubricants is also 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 or larger, typically mean as 500-800 nm. The viscosity modification advantage of the graphite is not mentioned in the sales literature, nor is the product sold or promoted for its viscosity modification property. [0015] 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 viscosity index of the fluid. 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 viscosity index. While graphite-containing automotive engine oil was once commercialized (Arco graphite), the potential to use graphite as a viscosity modifying material in this oil was not realized. The particle size of graphite used was larger (mean greater than one micron) than for the instnt invention as shown in FIG. 3. 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. [0016] 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. [0017] Bulk graphite is available from POCO Graphite as a graphite foam, and is also available from the Carbide/Graphite Group, Inc. Graphite powders can be obtained from UCAR Carbon Company Inc., and from Cytec Carbon Fibers LLC. These bulk or powdery materials must be reduced to a nanometer-sized particles by various methods for use in the instant invention. [0018] In this invention, fluids of enhanced viscosity index are prepared by dispersing nanometer-sized particles, especially carbon nanomaterials, 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. [0019] The present invention provides at a minimum, a fluid of lubricant containing from 0.001% to 50% by weight nanoparticles, and preferably, from 0.01% to 25% by weight, and more preferably, from 0.1% to 20% by weight of nanoparticles. Preferably, however, a minimum of one or more chemical dispersing agents and/or surfactants are 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 "surfactanf" 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. The particle-containing fluid of the instant invention will have a viscosity index higher than the conventional fluid of the same type. The fluid can have any 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, or thermal property booster. 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. [0020] As set forth above, the nanomaterial could be of any commercially available nanoparticles, or any material which can be wet-milled into nanometer-sized particles using the process developed in this invention which will be explained in detail later. One of the preferable nanoparticles are carbon-based materials. A preferred form of carbon nanomaterials is carbon nanotubes. Another preferred form of carbon nanomaterials is graphite. A preferred form of graphite is POCO Foam from POCO Graphite. Another preferred form is graphite powders from UCAR Carbon Company Inc. Still another preferred form of graphite is graphite powders from Cytec Carbon Fibers LLC. Still another preferred form of graphite is bulk graphite from The Carbide/Graphite Group, Inc. Another preferable nanomaterial is aluminum oxide nanoparticles from Sasol. [0021] The nanoparticle 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 additional conventional polymer-based viscosity improvers. Continue reading... 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