WO2009042400A1

WO2009042400A1 - Gear oil compositions, methods of making and using thereof

Info

Publication number: WO2009042400A1
Application number: PCT/US2008/075803
Authority: WO
Inventors: Allan G. Hee; Trevor Miller
Original assignee: Chevron U.S.A. Inc.
Priority date: 2007-09-27
Filing date: 2008-09-10
Publication date: 2009-04-02
Also published as: US20090088355A1; CA2700631A1; JP2010540718A; CN101855323A; MX2010003300A; DE112008002574T5; BRPI0817479A2

Abstract

A gear oil composition is provided. The composition comprises a synergistic blend of mineral oil base stock and polyalphaolefin (PAO) base stock for the oil composition to have a traction coefficient at a slide to roll ratio of 40 percent at 15 mm2/s. of 0.030 or less and a pressure viscosity coefficient of at least 16.0 at 80 °C, 20 Newton load, and 1.1 m/s rolling speed. In one embodiment, the synergistic amount of PAO base stock ranges from 5 to 48 wt. % based on the total weight of the gear oil composition.

Description

Gear Oil Compositions, Methods of Making and Using Thereof

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of US Patent Application Serial No. 11/863,068 filed September 27, 2007, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

[001] The invention relates generally to compositions suitable for use as lubricants, more particularly for use as gear oils.

BACKGROUND

[002] Gear oil is used in industrial applications as well moving equipment such as automobiles, tractors, and the like (collectively referred to as "equipment'). When in use in some applications, the gear oil is present as an oil film between the moving parts, e.g., traction drives. In traction drive applications, power is transmitted via the gear oil film. In some applications, e.g., a hypoid gear of final reduction gear, it is very desirable to form / retain a thick oil film between gears. Increased oil film thickness to a sufficient level can protect a friction surface from damages, greatly improving gear and/or bearing fatigue life and load resistance characteristics. [003] Traction coefficient is the force required to move a load, divided by the load. The coefficient number expresses the ease with which the lubricant film is sheared. It is desirable for gear oils to have a low traction coefficient as the lower the traction coefficient, the less energy is dissipated due to lubricant shearing.

[004] Besides having a low traction coefficient, it is important for a gear oil to have a high pressure-viscosity coefficient. The pressure-viscosity coefficient ("PVC") refers to the relationship between the load placed on the oil film (pressure) at the dynamic load zone and the thickness of the oil film (viscosity) at that load, when all other factors (material, temperature, geometry, speed, load) are constant. The pressure-viscosity coefficient of a gear oil is a fixed value for an oil film thickness in a given set of conditions (elastohydrodynamic regime, also known as an EHL or EHD regime) based on a mathematical estimation as noted in the American Gear Manufacturers Association (AGMA) Information Sheet AGMA 925-A03. It is desirable for gear oils to have a high PVC value.

[005] US Patent Publication No. 2007/0027042 discloses a gear oil composition comprising two mineral base oil and / or hydrocarbon-based synthetic oils of different kinematic viscosities, one of 3.5 to 7 mm²/s at 100 °C and one of 20- 52 mni²/s at 100 ⁰C. US Patent Publication No. 2007/0078070 discloses a gear oil composition comprising at least one Group II base stock and at least one low volatility low viscosity polyalphaolefm base stock.

[006] There is still a need for gear oil compositions having a low traction coefficient, a high pressure- viscosity coefficient, and optimal film thickness properties.

SUMMARY OF THE INVENTION

[007] In one aspect, the invention relates to a gear oil composition comprising: a) a base oil containing a synergistic mixture of at least a polyalphaolefm base stock with a mineral oil base stock having a kinematic viscosity of 3 to 120 mm²/s at 100 ⁰C. and a viscosity index of at least 60; b) 0.001 to 30 wt % at least an additive selected from traction reducers, dispersants, viscosity modifiers, pour point depressants, antifoaming agents, antioxidants, rust inhibitors, metal passivators, extreme pressure agents, friction modifiers, and mixtures thereof; wherein the polyalphaolefm is present in a synergistic amount for the gear oil composition to have a traction coefficient at 15 mm²/s of 0.030 at a slide to roll ratio of 40 percent or less and a pressure viscosity coefficient of at least 16.0 GPa^"1 at 80 °C, 20 Newton load, and 1.1 m/s rolling speed. [008] In another aspect, the invention relates to a method for improving the traction coefficient property of a gear oil, the method comprises adding to a base oil typically used for preparing the gear oil a synergistic amount of at least a polyalphaolefm for the gear oil to have a traction coefficient at 15 mm²/s. of 0.030 or less. In one embodiment, the sufficient amount of the polyalphaolefm to be added to the base oil matrix ranges from 5 to 48 wt. % based on the total weight of the gear oil composition. DESCRIPTION OF THE DRAWINGS

[009] Figure 1 is a graph comparing the pressure-viscosity coefficients of the gear compositions of Examples 1-5 at different temperatures. [010] Figure 2 is a graph comparing the film thickness of the gear compositions of Examples 1-5 at different temperatures.

DETAILED DESCRIPTION

[01 1] The following terms will be used throughout the specification and will have the following meanings unless otherwise indicated.

[012] "Kinematic viscosity" is a measurement in mm²/s of the resistance to flow of a fluid under gravity, determined by ASTM D445-06.

[013] "Viscosity index" (VI) is an empirical, unit-less number indicating the effect of temperature change on the kinematic viscosity of the oil. The higher the VI of an oil, the lower its tendency to change viscosity with temperature. Viscosity index is measured according to ASTM D 2270-04.

[014] Cold-cranking simulator apparent viscosity (CCS VIS) is a measurement in millipascal seconds, mPa.s to measure the viscometric properties of lubricating base oils under low temperature and high shear. CCS VIS is determined by ASTM D 5293-04.

[015] The boiling range distribution of base oil, by wt%, is determined by simulated distillation (SIMDIS) according to ASTM D 6352-04, "Boiling Range Distribution of Petroleum Distillates in Boiling Range from 174 to 700 °C by Gas Chromatography." [016] "Noack volatility" is defined as the mass of oil, expressed in weight %, which is lost when the oil is heated at 250 °C. with a constant flow of air drawn through it for 60 min., measured according to ASTM D5800-05, Procedure B.

[017] Brookfϊeld viscosity is used to determine the internal fluid-friction of a lubricant during cold temperature operation, which can be measured by ASTM D 2983-04. [018] "Pour point" is a measurement of the temperature at which a sample of base oil will begin to flow under certain carefully controlled conditions, which can be determined as described in ASTM D 5950-02.

[019] "Auto ignition temperature" is the temperature at which a fluid will ignite spontaneously in contact with air, which can be determined according to ASTM 659-78.

[020] "Traction coefficient" is an indicator of intrinsic lubricant properties, expressed as the dimensionless ratio of the friction force F and the normal force N, where friction is the mechanical force which resists movement or hinders movement between sliding or rolling surfaces. Traction coefficient can be measured with an

MTM Traction Measurement System from PCS Instruments, Ltd. , configured with a polished 19 mm diameter ball (SAE AISI 52100 steel) angled at 22° to a flat 46 mm diameter polished disk (SAE AISI 52100 steel). The steel ball and disk are independently measured at an average rolling speed of 3 meters per second, a slide to roll ratio of 40 percent, and a load of 20 Newtons. The roll ratio is defined as the difference in sliding speed between the ball and disk divided by the mean speed of the ball and disk, i.e. roll ratio = (Speedl-Speed2)/((Speedl+Speed2) 12).

[021] Molecular weights are determined by ASTM D2503-92(Reapproved 2002). The method uses thermoelectric measurement of vapour pressure (VPO). In circumstances where there is insufficient sample volume, an alternative method of ASTM D2502-04 may be used; and where this has been used it is indicated.

[022] Density is determined by ASTM D4052-96 (Reapproved 2002). The sample is introduced into an oscillating sample tube and the change in oscillating frequency caused by the change in the mass of the tube is used in conjunction with calibration data to determine the density of the sample.

[023] Component A - Group V Polyalphaolefins ("P AOs'"): Component A of the base oil matrix is a Group IV base oil or a mixture of different Group IV base oils. Group IV base stocks consist of polyalphaolefins ("PAOs"), offering superior volatility, thermal stability, oxidative stability and pour point characteristics compared to those of the Group II and III oils with less reliance on additives. [024] PAOs comprise a class of hydrocarbons manufactured by the catalytic oligomerization (polymerization to low-molecular-weight products) of linear α-olefins typically ranging from 1-octene to 1-dodecene, although polymers of lower olefins such as ethylene and propylene can also be used, including copolymers of ethylene with higher olefins. High viscosity PAOs may be conveniently made by the polymerization of an α-olefin in the presence of a polymerization catalyst such as the Friedel-Crafts catalysts including, for example, aluminum trichloride, boron trifluoride or complexes of boron trifluoride with water, alcohols such as ethanol, propanol or butanol, carboxylic acids or esters such as ethyl acetate or ethyl propionate.

[025] In one embodiment, the PAO used is predominantly α-olefin, that is, linear terminal olefin. By predominantly is meant that the PAO contains over about 50 mole percent of α-olefϊns. In another embodiment, the PAO is a high viscosity PAO, comprising hydrogenated polymers or oligomers of α-olefins. The α-olefins include, but are not limited to, C₂ to about C₃₂ α-olefins, e.g., 1-octene, 1-decene, 1- dodecene and the like. In one example, the PAO is a α-olefins selected from the group of poly- 1-octene, poly- 1-decene, and poly- 1-dodecene.

[026] The PAO products for use in the composition can have a wide range of viscosities, varying from highly mobile fluids of low- viscosity, about 2 mm²/s., at 100 °C to higher molecular weight, viscous materials which have viscosities exceeding 1000 mm²/s (cSt.) at 100 °C. In one embodiment, the PAO products have a viscosity ranging from 40 to 500 mm²/s (cSt.) at 40 ⁰C. In one embodiment, the PAO for use as component A has a viscosity of greater than or equal to about 80 mm²/s at 40 ⁰C and less than or equal to about 20 mm²/s at 100 ⁰C. In another embodiment, the PAO base stock has a kinematic viscosity @40 °C. in the range of 80-110 mm²/s. and a kinematic viscosity @100 °C. of 10-16 mm /s. and a viscosity index of 140-160. In yet another embodiment, the PAO base stock is a blend of different PAOs, one having a viscosity of ranging from 30 - 60 mm²/s at 40 °C. and the other having a viscosity of 300 - 600 mm²/s at 40 ⁰C, for a PAO blend having a viscosity of 100 mm²/s at 40 °C. [027] Component B - Mineral Oil: Component B is a mineral oil or mixtures of mineral oils. The mineral oil can be any of paraffinic and naphthenic oils, or mixtures thereof. Mineral oils can be obtained by subjecting a lubricating oil fraction produced by atmospheric- or vacuum-distilling a crude oil, to one or more refining processes such as solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing, hydrorefining, sulfuric acid treating, and clay treatment.

[028] In one embodiment, the mineral oil used as Component B may contain an amount of synthetic oils such as poly-α-olefϊns, ethylene-α-olefins copolymer, and ester-based synthetic oils, in an amount of 50 wt. % or less of the total weight of the gear oil composition.

[029] In one embodiment, Component B is a mineral oil (or blends of mineral oils and/or hydrocarbon-based synthetic oils) having a kinematic viscosity of 3 to 120 mm²/s at 100 °C. and a viscosity index of at least 60. In another embodiment, Component B is a mineral oil having a kinematic viscosity of 2.3 to 3.4 mm²/s at 100 ⁰C. and a % Cp defined by ASTM D 3238 (R2000) is 70 or higher, ASTM D 3238 is a standard test method for calculation of Carbon distribution and structural group analysis of petroleum oils by the ndM method. In yet another embodiment, Component B is a base oil matrix having a kinematic viscosity of less than 80 mm²/s at 40 °C, comprising a mixture of: a "low viscosity" mineral or and / or a synthetic oil having and a kinematic viscosity of 3.5 to 7 mm²/s at 100 ⁰C; and a "high viscosity" mineral-based oil and/or hydrocarbon-based synthetic oil having a kinematic viscosity of 20 to 52 mm²/s at 100 ⁰C.

[030] In one embodiment, the base oil matrix contains sufficient amounts of mineral and PAO oils for the base oil matrix to have a kinematic viscosity at 100 ⁰C. between 10 mm²/s and 15 mm²/s; a kinematic viscosity at 40 °C. between 95 mm²/s and 110 mm /s; and a viscosity index between 95 and 175.

[031] Additional Optional Components: The incorporation of synergistic amounts of mineral and PAO oils allows the composition to have a low traction coefficient without the need for traction reducers in the prior art. However, in one embodiment, small amounts of traction reducers, e.g., from 0.5 to 10 wt. %, can be incorporated in the gear oil composition. Examples of traction reducers include ExxonMobil's Norpar™ fluids (comprising normal paraffins), Isopar™ fluids (comprising isoparaffins), Exxsol™ fluids (comprising dearomatized hydrocarbon fluids), Varsol™ fluids (comprising aliphatic hydrocarbon fluids), and mixtures thereof.

[032] In one embodiment, the gear oil composition comprises 0.01 to 30 wt. % of one or more additives selected from dispersants, viscosity index improvers, pour point depressants, antifoaming agents, antioxidants, rust inhibitors, metal passivators, extreme pressure agents, friction modifiers, etc., in order to satisfy diversified characteristics, e.g., those related to friction, oxidation stability, cleanness and defoaming, etc.

[033] Examples of dispersants include those based on polybutenyl succinic acid imide, polybutenyl succinic acid amide, benzylamine, succinic acid ester, succinic acid ester-amide and a boron derivative thereof. When used, ashless dispersants are typically employed in an amount of 0.05 to 7 wt. %. In one embodiment, the dispersant are selected from the products of reaction of a polyethylene polyamine, e.g. triethylene tetraamine pentaamine, with a hydrocarbon- substituted anhydride made by the reaction of a polyolefin, having a molecular weight of about 700-1400 with an unsaturated polycarboxylic acid or anhydride, e.g. maleic anhydride.

[034] Examples of metallic detergent include those containing a sulfonate, phenate, salicylate of calcium, magnesium, barium or the like. Metallic detergents when used, are typically incorporated in an amount of 0.05 to 5 wt. %.

[035] Examples of antioxidants include but are not limited to amine-based ones, e.g., alkylated diphenylamine, phenyl-α-naphtylamine and alkylated phenyl-x- naphtylamine; phenol-based ones, e.g., 2,6-di-t-butyl phenol, 4,4'-methylenebis-(2,6- di-t-butyl phenol) and isooctyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate; sulfur- based ones, e.g., dilauryl-3,3'-thiodipropionate; and zinc dithiophosphate. When used, antioxidants are incorporated in an amount from 0.05 to 5wt.%. [036] Defoaming agents can be optionally incorporated in an amount of 10-

100 ppm. Examples of defoaming agents include but are not limited to dimethyl polysiloxane, polyacrylate and a fluorine derivative thereof, and poerfluoropolyether. Rust inhibitors can be used in an amount from 0 to 30 wt. %. Examples include a fatty acid, alkenylsuccinic acid half ester, fatty acid soap, alkylsulfonate, polyhydric alcohol/fatty acid ester, fatty acid amine, oxidized paraffin and alkylpolyoxyethylene ether.

[037] Friction modifiers can be incorporated in an amount from 0.05 to 5 wt. %. Examples include but are not limited to organomolybdenum-based compounds, fatty acids, higher alcohols, fatty acid esters, sulfided esters, phosphoric acid ester, acid phosphoric acid esters, acid phosphorous acid esters and amine salt of phosphoric acid ester.

[038] Anti-wear and / or extreme pressure agents can be incorporated in an amount from 0.1 to 10 wt. %. Examples of anti-wear and / or extreme pressure agents include metal-free sulfur containing species including sulfurized olefins, dialkyl polysulfides, diarylpolysulfides, sulfurized fats and oils, sulfurized fatty acid esters, trithiones, sulfurized oligomers of C2-C8 monoolefins, thiophosphoric acid compounds, sulfurized terpenes, thiocarbamate compounds, thiocarbonate compounds, sulfoxides, thiol sulfinates, and the like. Other examples include metal- free phosphorus-containing antiwear and/or extreme pressure additives such as esters of phosphorus acids, amine salts of phosphorus acids and phosphorus acid-esters, and partial and total thio analogs of the foregoing. In one embodiment, the composition comprises an acid phosphate as an anti-wear agent, with the agent having the formula RiO(R₂O)P(O)OH, where R₁ is hydrogen or hydrocarbyl and R₂ is hydrocarbyl.

[039] Pour point depressant can be incorporated in an amount ranging from 0.05 to 10 wt. %. Examples include but are not limited to ethylene/vinyl acetate copolymer, condensate of chlorinated paraffin and naphthalene, condensate of chlorinated paraffin and phenol, polymethacrylate, polyalkyl styrene, chlorinated wax- naphthalene condensate, vinyl acetate-fumarate ester copolymer, and the like.

[040] In one embodiment, the composition further comprises at least one of a polyoxyalkylene glycol, polyoxyalkylene glycol ether, and an ester as a solubilizing agent in an amount from 10 to 25 wt. %. Examples include esters of a dibasic acid (e.g., phthalic, succinic, alkylsuccinic, alkenylsuccinic, maleic, azelaic, suberic, sebacic, fumaric or adipic acid, or linolic acid dimmer) and alcohol (e.g., butyl, hexyl, 2-ethylhexyl, dodecyl alcohol, ethylene glycol, diethylene glycol monoether or propylene glycol); and esters of a monocarboxylic acid of 5 to 18 carbon atoms and polyol (e.g., neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol or tripentaerythritol); polyoxyalkylene glycol ester; and phosphate ester.

[041] In one embodiment, the composition further comprises at least a metal passivator, and sometimes specifically a copper passivator. Examples include thiazoles, triazoles, and thiadizoles. Specific examples of the thiazoles and thiadiazoles include 2-mercapto-l,3,4-thiadiazole, 2-mercapto-5-hydrocarbylthio- 1,3,4-thiadiazoles, 2-mercapto-5-hydrocarbyldithio-l,3,4-thiadiazoles, 2,5-bis- (hydrocarbylthio)-l,3,4-thiadiazoles, and 2, 5-bis-(hydrocarbyldithio)- 1,3,4- thiadiazoles. Other suitable inhibitors of copper corrosion include imidazolines, described above, and the like.

[042] In one embodiment, the composition further comprises at least a viscosity modifier in an amount of 0.50 to 10 wt. %. Examples of viscosity modifiers include but are not limited to the group of polymethacrylate type polymers, ethylene-propylene copolymers, styrene-isoprene copolymers, hydrated styrene-isoprene copolymers, polyisobutylene, and mixtures thereof. In one embodiment, the viscosity modifier is a blend of a polymethacryalte having a weight average molecular weight of 25,000 to 150,000 and a shear stability index less than 5 and a polymethacryate having a weight average molecular weight of 500,000 to 1,000,000 and a shear stability index of 25 to 60.

[043] The gear oil composition of the invention is characterized has having a synergistic amount of mineral and PAO base stock for the composition to have a low traction coefficient, a high pressure-viscosity coefficient, and optimal film thickness properties. In one embodiment, this synergistic amount of PAO base stock ranges from 5 to 48% (based on the total weight of the gear oil composition). In a second embodiment, the synergistic amount of PAO base stock ranges from 15 to 40 wt. %. In a third embodiment, the synergistic amount of PAO base stock ranges from 25 - 30 wt. %. In a fourth embodiment, the synergistic amount of PAO base stock is at least 40 wt. %. In a fifth embodiment, the synergistic amount of PAO base stock ranges from 10 to 35 wt. %.

[044] In one embodiment, the gear oil comprises a blend of 5 to 48 wt. % (based on the total weight of the gear oil composition) of a PAO base stock having a kinematic viscosity at 40⁰C. of 70-120 mm²/s., a kinematic viscosity at 100⁰C. of 12 to 18 mm²/s., and a viscosity index of 130-160; and 25-75 wt. % of a group II neutral base oil having a kinematic viscosity at 40⁰C. of 40-120 mm²/s., a kinematic viscosity at 100°C. of 8 to 14 mm²/s., and a viscosity index of 80-120.

[045] Properties: In one embodiment, the gear oil composition having a synergistic combination of mineral and isomerized base oils has a traction coefficient at 15 mm²/s. of 0.030 or less, a pressure viscosity coefficient of greater than 15.0 GPa^" ¹ at 80 ⁰C, 20 Newton load, and 1.1 m/s rolling speed., and a film thickness of greater than 175 nm at 80 ⁰C. In another embodiment, the gear oil composition has a film thickness of at least 160 nm at 90 ⁰C. or 130 nm at 100 ⁰C. In a third embodiment, the gear oil composition has a pressure viscosity coefficient of at least 15.0 GPa^"1 at a temperature in the range of 70-100 ⁰C, 20 Newton load, and 1.1 m/s rolling speed. In a fourth embodiment, the gear oil composition has a traction coefficient at 15 mm²/s. of 0.030 or less, at a slide to roll ratio of 40 percent.

[046] In one embodiment for use as an automotive gear oil, the composition meets SAE J306 specifications for the designated viscosity grades. For example, under the specifications of SAE J- 306, the measured viscosity at 100 ⁰C (212 ⁰F) of an SAE 90 gear oil must exceed 13.5 mm²/s after 20 hours of testing.

[047] In yet another embodiment, the composition meets at least one of industry specifications SAE J2360, API GL-5 and API MT-I, and military specification MIL-PRF-2105E quality level.

[048] Method for Making: Additives used in formulating the gear oil composition can be blended into base oil blends individually or in various sub- combinations. In one embodiment, all of the components are blended concurrently using an additive concentrate (i.e., additives plus a diluent, such as a hydrocarbon solvent). The use of an additive concentrate takes advantage of the mutual compatibility afforded by the combination of ingredients when in the form of an additive concentrate.

[049] In another embodiment, the composition is prepared by mixing the base oil and the additive(s) at an appropriate temperature, e.g., 60 °C, until homogeneous. [050] Applications: The composition is useful in any system that include elements or parts containing gears of any kind and rolling element bearings. In one embodiment, the composition is used as a gear oil for lubricating industrial gears, e.g., spur and bevel, helical and spiral bevel, hypoid, worm, and the like. In another embodiment, the composition is used in automotive / mobile equipment applications and parts, including aircraft propulsion systems, aircraft transmissions, wind turbine gears, automotive drive trains, transmissions, transfer cases, and differentials in automobiles, trucks, and other machinery. In yet another embodiment, the composition is used in wind turbines, plastic extruder gear boxes, and highly loaded gearboxes used in electricity generating systems, or paper, steel, oil, textile, lumber, cement industries, and the like.

[051] EXAMPLES. The following Examples are given as non-limitative illustrations of aspects of the invention. Unless specified otherwise, the components in the examples are as follows (and expressed in wt. % in Table 1):

[052] RLOP is Chevron™ 600R group II heavy neutral oil from Chevron Corporation.

[053] PAO 8 is a highly branched iso-paraffinic polyalphaolefin commercially available from various sources, including Chevron Phillips as Synfluid™ PAO 8 cSt, with a kinematic viscosity at 100 ⁰C of about 7.8, a kinematic viscosity at 40 ⁰C of 46.6, a viscosity index of 138, and a pour point of -57 ⁰C. [054] PAO 40 is a highly branched iso-paraffinic polyalphaolefin commercially available from various sources, including Chevron Phillips as Synfluid™ PAO 40 cSt, with a kinematic viscosity at 100 ⁰C of about 40, a kinematic viscosity at 4O⁰C of 410, a viscosity index of 145, and a pour point of -34 °C.

[055] Additive X is an industrial gear sulfurphosphorus containing extreme pressure additive commercially available from various sources. [056] The kinematic viscosity, refractive index, and density are properties of the base oil matrix blends, measured using methods known in the art. The traction coefficients of the gear oils in the Examples are measured / calculated using methods and devices known in the art, e.g., a traction coefficient measurement device disclosed in US Patent No. 6691551 , or a Twin-Disc machine designed by Santotrac, for measuring in the elastohydrodynamic (EHD) regime under high pressure of at least 300,000 psi.

[057] The EHL film thickness is calculated using methods known in the art, e.g., the American Gear Manufacturers Association (AGMA) Information Sheet AGMA 925 equation 65, wherein the EHL film thickness is established by the operating temperature of the components. An oil film thickness is determined by the oil's response to the shape, temperature and velocity of the surfaces at the contact inlet. The thickness depends strongly on entraining velocity and oil viscosity. The pressure-viscosity coefficient ("PVC") quantifies the EHL film-generating capability of a gear oil, which can be measured by known methods. The PVC can be measured either directly by assessing viscosity as a function of pressure using high-pressure apparatus, or indirectly by measuring film thickness in an optical interferometer. PVC is the slope of the graphs plotting the log of viscosity vs. pressure.

[058] Results of the experiments establish that the synergistic addition of at least a PAO base stock into the mineral oil helps improve the traction coefficient of the gear oil composition, lowering the traction coefficient of at least 10% to 0.030 or less at 15 ⁰C, with the values of 0.028 or below for compositions containing 25 to 75 wt.% of at least a PAO base stock. The data establishes that the incorporation of a synergistic amount of PAO base stock into a base oil matrix of gear oil compositions in the prior art, e.g., a base oil matrix containing mineral oil(s), provides a gear oil composition having desired optimal properties of low traction coefficient (e.g., 0.030 or less) and high pressure viscosity coefficients or PVC (e.g., greater than 15.0 at a temperature of 65 ⁰C. or higher ~ typical temperatures of gear components). In one embodiment, the addition of excessive amounts of PAO may afford decreased PVC values, with compositions containing 50-75% PAO, for PVC values less than comparable pure PAO or pure mineral compositions. Hence, an antagonistic effect was observed in these cases.

[059] Figures 1 and 2 are graphs comparing the film thicknesses (refractive index corrected) and the pressure-viscosity coefficients of the gear oil examples as a function of temperature. As shown in Figure 1, a gear oil composition consisting essentially of a Group II neutral oil in the prior art shows a relatively moderate PVC profile that exhibits a downward trend toward about 14.0 GPa^"1 or less at 100 °C. A gear oil composition consisting essentially of at least a PAO base stock exhibits lower PVC values than the group II-based oil in the range of 60 - 70 °C; its PVC value is about 15 GPa^"1 or less at less than 70 ⁰C, with a PVC value of 14.4 GPa^"1 at about 60 0C. As shown in the examples, combining the prior art base oil with synergistic amounts of PAO base stock (e.g. 75% RLOP 600R and 25% PAO) results in significantly improved PVC values in the broad 60-100 ⁰C range, with a value of greater than 17.6 GPa^"1 at about 80 °C. Also as shown, this composition shows excellent synergy with the PVC values measured at 80°C and 100 ⁰C being greater than the corresponding values of either the PAO base oil-only or group II-only gear oils. Also as shown, combinations containing excessive amounts of PAO exhibit the opposite effect. Compositions containing 50% or greater PAO show lower PVC values at 80 ⁰C. than the corresponding PVC values for either the PAO base oil-only or prior art group II only gear oils. The most antagonism is observed for a composition containing a small amount of Group II neutral base oil and a large amount of PAO base stock (i.e. 25% RLOP 600R and 75% PAO). The composition exhibited antagonistically decreased PVC values in the 60-80 ⁰C range. Table 1

[060] For the purpose of this specification and appended claims, unless otherwise indicated, all numbers expressing quantities, percentages or proportions, and other numerical values used in the specification and claims, are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained and / or the precision of an instrument for measuring the value, thus including the standard deviation of error for the device or method being employed to determine the value. The use of the term "or" in the claims is used to mean "and/or" unless explicitly indicated to refer to alternatives only or the alternative are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and "and/or." The use of the word "a" or "an" when used in conjunction with the term "comprising" in the claims and/or the specification may mean "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one." Furthermore, all ranges disclosed herein are inclusive of the endpoints and are independently combinable. In general, unless otherwise indicated, singular elements may be in the plural and vice versa with no loss of generality. As used herein, the term "include" and its grammatical variants are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that can be substituted or added to the listed items.

Claims

1. A gear oil composition, comprising: a) a base oil comprising a mixture of at least a polyalphaolefin (PAO) base stock and a mineral base stock having a kinematic viscosity of 3 to 120 mm²/s at 100 °C. and a viscosity index of at least 60; b) 0.001 to 30 wt % at least an additive selected from traction reducers, dispersants, viscosity modifiers, pour point depressants, antifoaming agents, antioxidants, rust inhibitors, metal passivators, extreme pressure agents, friction modifiers, and mixtures thereof; wherein the polyalphaolefin base stock is present in a synergistic amount for the gear oil composition to have a traction coefficient at 15 mm²/s. of 0.030 or less at a slide to roll ratio of 40 percent and a pressure viscosity coefficient of at least 15.0 GPa^"1 at 80 ⁰C, 20 Newton load, and 1.1 m/s rolling speed.

2. The composition of claim 1 , wherein the PAO base stock is present in an amount ranging from 5 to 48 wt. % based on the total weight of the gear oil composition, and wherein the gear oil has a pressure- viscosity coefficient of at least 15.5 GPa^'1 in a temperature range of 70- 100 ⁰C, 20 Newton load, and 1.1 m/s rolling speed.

3. The composition of any of claims 1 - 2, wherein the PAO base stock is present in an amount ranging from 15 to 40 wt. % based on the total weight of the gear oil composition.

4. The composition of any of claims 1 - 3, wherein the PAO base stock is present in an amount of 25 to 35 wt. % based on the total weight of the gear oil composition.

5. The composition of any of claims 1 - 4, wherein the gear oil composition has a pressure viscosity coefficient of at least 16.0 GPa^'1 in a temperature range of 70-100 °C, 20 Newton load, and 1.1 m/s rolling speed.

6. The composition of any of claims 1 - 5, wherein the gear oil composition has a traction coefficient at a slide to roll ratio of 40 percent at 15 mm²/s. of less than 0.028.

7. The composition of any of claims 1 - 6, wherein the gear oil composition has a traction coefficient at a slide to roll ratio of 40 percent at 15 mm /s. of less than 0.026.

8. The composition of any of claims 1 - 7, wherein the gear oil composition has a film thickness of at least 175 nm at 80 °C.

9. The composition of any of claims 1 - 8, wherein the gear oil composition has a film thickness of at least 130 nm at 100 ⁰C.

10. The composition of any of claims 1 - 9, wherein the PAO base stock has a kinematic viscosity at 40 ⁰C. ranging from 40-500 mm²/s.

11. The composition of any of claims 1 - 10, wherein the PAO base stock has a kinematic viscosity at 100 ⁰C. of 10-16 mm²/s. and a viscosity index of 140-160.

12. The composition of any of claims 1 — 1 1, wherein the mineral oil has a kinematic viscosity of 2.3 to 3.4 mm²/s at 100 °C. and a % Cp defined by ASTM D 3238 (R2000) of 70 or higher.

13. The composition of any of claims 1 - 12, wherein the mineral oil is a group II neutral base oil having a kinematic viscosity at 40 °C. of 80-120 mm²/s., a kinematic viscosity at 100 ⁰C. of 10 to 14 mm²/s., and a viscosity index of 80-120.

14. A method for improving the traction properties of a gear oil composition, the method comprises adding a synergistic amount of at least a polyalphaolefm (PAO) base stock to a base oil matrix comprising at least a mineral oil having a kinematic viscosity of 3 to 120 mm²/s at 100 °C. and a viscosity index of at least 60, for the gear oil composition to have a traction coefficient at a slide to roll ratio of 40 percent at 15 mm²/s. of 0.030 or less, a pressure viscosity coefficient of 15.7 or higher at 80 °C, 20 Newton load, and 1.1 m/s rolling speed, and a film thickness of greater than 175 nm at 80 ⁰C, wherein the PAO base stock has a kinematic viscosity at 40 °C. in the range of 80-110 mm²/s. and a kinematic viscosity at 100 °C. of 10-16 mm²/s. and a viscosity index of 140-160.

15. The method of claim 14, wherein the synergistic amount of at least a polyalphaolefin (PAO) base stock ranges from 5 to 48 wt. % based on the total weight of the gear oil composition.