Synthetic hydrocarbon fluids and their use as traction fluids

Abstract: Highly branched synthetic hydrocarbon fluids produced by anionically oligomerizing selected dienes in the presence of organoalkali compounds and complexing reagents provide fluids having high traction coefficients and excellent elastohydrodynamic lubricating characteristics.

Description: This invention relates to synthetic hydrocarbon fluids and their use as traction fluids; more particularly, this invention relates to highly branched, saturated synthetic hydrocarbon fluids (SHF) produced by non-destructively hydrogenating anionically oligomerized dienes or polydienes; to their use as traction fluids; and to a method of operating traction drives using these synthetic hydrocarbon fluids. A traction drive efficiently transmits torque or traction force from one rotating, drive shaft to another, driven shaft if there is minimal slippage. This is a function of the traction coefficient which is defined as the quotient of the force transmitted and the normal force which keeps the rolling members in contact or as the quotient of the traction due to the transmission of the traction between the driving and driven elements and the normal force (normal load) between the driving and driven elements. Slip may also be defined as the quotient of the difference between the two circumferential speeds of the roller elements and the greater circumferential speed. For maximum efficiency of transmission the peak coefficient of traction is as high as possible in order to achieve maximum power transmission per unit load. This invention seeks to provide traction fluids which enhance the efficiency of transmission. According to one aspect of this invention, there is provided a process for the preparation of a highly branched, saturated synthetic hydrocarbon fluid, which process comprises: anionically oligomerizing isoprene in the presence of an organo-alkali metal compound and a complexing agent therefor, the molar ratio of complexing agent : organoalkali metal compound being greater than 0.5 at autogenous pressure and at a temperature from -15°C to 120°C thereby producing a highly branched liquid oligomer which comprises from 50 to 85% by weight of residues polymerized by 1,2-addition; and non-destructively hydrogenating the resulting oligomer. The oligomerization reactions give near quantitative yields of oligomers. The organo-alkali metal compound may suitably comprise an organo-lithium compound, organo-sodium compound or organo-potassium compound. The organic moiety of the organo-alkali metal compound may suitably comprise a hydrocarbyl group. Examples include a C₁ to C₁₀ alkyl group; a C₆ to C₁₂ aryl group; a C₇ to C₁₂ alkaryl group. Examples of suitable aromatic hydrocarbyl groups include phenyl, biphenyl, naphthyl and diphenylmethyl groups. Preferably, the organic moiety comprises a C₁ to C₆ alkyl group. The organo-alkali metal compound is preferably an organo-lithium compound with alkyllithium compounds being particularly preferred. Examples include methyllithium,ethyllithium,propyllithium,isopropyllithium,n-butyllithium,t-butyllithium, sec-butyllithium, t-octyllithium, n-decyllithium, phenyllithium, naphthyllithium, 4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium, cyclohexyllithium, 4-butylcyclohexyllithium and 4-cyclohexyl-butyllithium. The organo-alkali metal compound is preferably n-butyllithium. Branching, derived from 1,2-addition of the isoprene is controlled primarily by the complexing agent, reaction temperature and reactant ratio. The complexing agent may suitably comprise any organic solvent which has alkali metal complexing capabilities. Such complexing agents include cyclic or straight-chain, mono- or polyethers, amines, sulphides; and pyridines. Preferred complexing agents include ethers and may comprise tetrahydrofuran, methyltetrahydrofuran, dioxane or diethyl ethers. Tetrahydrofuran is the preferred solvent or complexing agent. A reaction temperature from -15°C to 120°C is used. Lower temperatures favour enhanced 1,2-addition of the isoprene although a temperature from 10°C to 50°C or even 70°C, such as up to 30°C, is found to be very suitable. The complexing agent is used in an amount sufficient to achieve from 50 to 85%, preferably from 75 to 80% 1,2-addition of the isoprene; that is, the molar ratio of complexing agent:organo-alkali metal compound is greater than 0.5. It is preferred that the molar ratio of isoprene: organo-alkali metal compound: complexing agent is at least 1:1:1. An excess of complexing agent may be used: for example, a ratio of complexing agent: organo-alkali metal compound from 1.1:1 to 10:1. It is likewise preferred to use an excess of isoprene: for example, a ratio of isoprene: organo-alkali metal compound greater than 5:1. The reaction may be conducted for 10 hours or more, but preferably from 1 to 3.5 hours, at autogenous pressure. Hydrogenation is carried out in any convenient manner known in the art. For example, the oligomer may be contacted, neat or in hydrocarbon solution, such as hexane, with hydrogen in the presence of a transition metal catalyst such as nickel on Kieselguhr in a ratio of catalyst to oligomerized diene of from 0.5:1 to 100:1 at a temperature from 150°C to 180°C or even 250°C and at a pressure from 2000 to 5000 kPa (300 to 700 psi) hydrogen. The hydrogenation is non-destructive; that is, the hydrogen saturates the olefinic bonds while retaining the branching structure produced in the oligomerization and the molecular weight of the oligomer. Complete hydrogenation is accomplished without cracking or rearrangement. This invention further provides a traction fluid which comprises a highly branched, saturated synthetic hydrocarbon fluid prepared by the process as herein described having a viscosity from 5 to 150 cP at 100°C. This invention also provides a traction fluid which comprises a highly branched, saturated synthetic hydrocarbon fluid prepared by the process as herein described having a traction coefficient from 0.09 to 0.12 at 90°C and a 3000 MPa (400 Kpsi) contact pressure. This invention provides a method of transmitting torque or traction force from a tractional driving element to a driven element thorough a traction fluid, which method comprises utilizing, as traction fluid, such a composition. The hydrogenated (poly)diene fluids in accordance with the invention can be conveniently produced in a wide range of viscosities and their structures can be tailored to produce a wide range of traction properties. The molecular structure produced in these fluids influences their traction properties. In general, highly-branched structures have high traction. Elastohydrodynamic film thicknesses generated by these fluids are significantly larger than mineral oils and other synthetic hydrocarbon fluids at equivalent viscosities. Traction coefficients are as high or higher than commercially available synthetic hydrocarbon fluids designed for use as traction fluids. The synthetic fluids in accordance with the present invention generally have traction coefficients ranging from about 0.090 to about 0.120 at 90°C and 3000 MPa (400 Kpsi) contact pressure. Due to the "living" nature of the oligomerization process, molecular weight of the isoprene oligomers and the resulting viscosity of their nondestructively hydrogenated products is easily controlled by the anionic synthesis. Fluids of any desired viscosity can be produced simply by control of the reactant ratios in this stoichiometric reaction. In addition, due to the nature of the oligomerization process, isoprene oligomers of a variety of microstructures can be achieved and retained in the hydrogenation step. The reaction medium has a profound influence on the microstructure of the oligomers. Structures with greater alkyl branching are obtained by oligomerization in the presence of complexing reagents such as tetrahydrofuran (THF) followed by hydrogenation. This results from a dramatic increase in the selectivity towards 3,4-enchainment in the oligomerization. The increased alkyl branching, relative to the ethylene-propylene oligomers (EPO) type structures derived mainly from 1,4-enchainment, at equivalent molecular weight, are a dramatic increase in viscosity and a dramatic decrease in VI. The microstructure of the hydrogenated isoprene oligomers has great influence on: 1. Viscosity index (VI) 2. Elastohydrodynamic (EHD) film thickness as function of viscosity and temperature and 3. Traction properties in EHD lubrication. Lubricants based upon the disclosed hydrogenated polydienes can accordingly be produced with many desirable properties tailored to meet specific lubrication applications. The structure of these fluids can be easily controlled in the synthesis of the polydiene precursors to achieve the desired properties in the hydrogenated products. No other process for producing lubricating fluids with such diversity and versatility has been reported or is known to applicant. There is a multitude of patented designs for continuously variable transmissions (CVTs) of which approximately ten of the "traction drive" type have been commercialized. CVTs can be used to give a wide range of output speeds for a single input speed, which is a useful feature for industrial drives, or to allow automotive engine speed to be selected for maximum efficiency based upon required torque rather than required output speed, which could potentially improve fuel economy by up to 30%. A traction drive may be considered a transmission without gear teeth. The torque is transmitted through the thin elastohydrodynamic lubricant (EHL) film generated between the rolling elements of the drive; and since there are no teeth, the ratio need not be fixed and can be designed to vary continuously. Fluids appropriate for traction drives are those which have high shear strength at the high contact pressures encountered in EHL contacts. The maximum torque that can be transmitted is determined by the fluid's peak traction coefficient (traction force divided by contact load), which is proportional to the fluid's shear strength at the contact pressure and temperature. The contact load required for a given torque can be reduced by using fluids with a high traction coefficient. This in turn contributes to longer roller fatigue life which is inversely proportional to the third power of the load. Suitable fluids, known as traction fluids, must be able to lubricate the rollers in addition to transmitting torque. Traction drives are usually operated at high speed to maximize power (torque x speed) for a given torque and low viscosity fluids are used to reduce churning losses and heating. Traction force is the response of the fluid to the strain or strain rate caused by small speed differences between the bounding surfaces of an EHL contact. At the high pressures encountered, many lubricants behave as elastic-plastic solids rather than viscous liquids and the highest traction force that can be generated is a function of the maximum shear stress the solidified fluid can sustain before plastic strain occurs. From traction force measurements in a point contact traction rig, an average value of the maximum shear stress is determined for a fluid at a range of pressures. Plots of maximum average shear stress against pressure have been found to be linear at high pressures for most fluids studied so far and for the high traction fluids remain linear throughout the pressure range used. Linearity of such a plot is a characteristic of plastic materials sheared under compression. For purposes of comparing the performance of these fluids in traction drives, it is perhaps easier to think in terms of the traction coefficient (traction force divided by contact load). Table 1 gives values of traction coefficient determined at 90°C and two maximum contact pressures of 1720 and 2760 MPa (250 Kpsi and 400 Kpsi) for a range of fluids. The HPI fluids have substantially greater traction coefficients than naphthenes and paraffinic mineral oils and PAO. The HPI fluids also have traction coefficients equivalent to or higher than commercial traction fluids. These HPI fluids have also been found to be able to generate thicker EHL (elastohydrodynamic lubrication) films than other fluids of similar viscosity. To what degree depends upon their molecular structure. The highly branched hydrogenated polyisoprenes (HPI) which result from increased 3,4-enchainment in the oligomerization step, show greater film thickness. This is the result of greater increase in viscosity with pressure (higher pressure-viscosity coefficients) for these fluids as branching increases. The physical benefits are either greater EHL film thickness at equivalent viscosity, giving more protection in rolling bearings and gears, or equivalent EHL film thickness at substantially reduced viscosity leading to increased energy efficiency due to reduced churning losses. Elastohydrodynamic lubrication is the regime of lubrication whereby the elastic deformation of non-conforming, contacting surfaces plays a major role in the generation and maintenance of hydrodynamic, load-carrying film which maintains separation of the surfaces. This is the mode of lubrication of gears, rolling bearings, cams and traction drives. They operate at very high contact pressures (up to 3000 MPa 400,000 psi) and the increase in viscosity with pressure of lubricating fluids is vital to the protective film formation. The sole lubricant contribution to EHL film thickness is summed up as LP, the lubricant parameter. LP is a product of the dynamic viscosity, η, (cP), and the pressure-viscosity coefficient, α, (psi⁻¹), defined by the equation: LP = 14500 ηα For a given component, speed, load and operating temperature, film thickness and protection can be increased by increasing η or α. Increasing η causes increased churning losses and higher operating temperatures. Increasing α is consequently better. This presumably has been ignored in the past because α is a function of η for related types of mineral oils. In practice LP is determined by measurement of EHL film thickness as a function of rolling speed at constant temperature (and thus viscosity), load and contact geometry. By determining LP as a function of temperature, a fluid's EHL behavior can be fully characterized. In addition the equivalent pressure-viscosity coefficients, α, can be calculated. Such analyses result in logarithmic relationships between absolute viscosity and lubricant parameter which are functions of fluid structure. Paraffinic mineral oils, naphthenic mineral oils, polyalphaolefins, as well as other general structural classes of fluids generally have single characteristic relationships between LP and absolute viscosity. Thus it seems likely that SHF of different molecular structure would show variable behavior of LP with viscosity. This would result from variable pressure-viscosity behavior as a function of molecular structure. Increased branching in HPI fluids gives higher pressure-viscosity coefficients and higher lubricant parameter (LP) values with viscosity. EHL film thicknesses at equivalent viscosities can be up to 2.7 times that of PAO fluids depending upon structure. The HPI fluids of the present invention have increasing pressure-viscosity coefficient (α) with increased branching. Higher α values for HPI fluids result in significantly higher LP values and EHL film thickness than for PAO fluids of similar viscosity. The oligomerization may be carried out under any suitable set of reaction conditions which will ensure the particularly desired polymeric product from substantially pure streams of isoprene. The following Examples illustrate the invention. Examples 1 to 10 Oligomers of isoprene of varied microstructure were synthesized by addition of the isoprene to the concentrated n-butyl lithium solutions (1.6-10.1 molar ratio) in hexane under a blanket of dry nitrogen. Tetrahydrofuran (THF) was used as complexing reagent in 0.5-32 molar ratio of THF to n-butyl lithium for branching control in the oligomers. The solutions of the polyisopropenyl lithium were quenched by water. The diene oligomers were isolated by removal of solvent. Yields approached quantitative within the limits of the experimental methods. Table 1 gives reactant proportions and reaction temperatures at atmospheric pressure. The diene oligomers were hydrogenated neat or as up to 50 wt.% solutions in hexane over nickel on Kieselguhr (1g catalyst/100g diene) at 180-250°C at 4140 kPa (600 psi) hydrogen. The synthetic hydrocarbon fluids were isolated by filtration followed by removal of solvent. Yields approached quantitative within the limits of the experimental methods. Traction measurements were made on a ball-on-plate traction apparatus. Peak traction coefficients were determined in the temperature region of 30-120°C at maximum contact pressures (stresses) from 690 to 2930 MPa (100-425 Kpsi). Traction coefficients of the HPI fluids and other common lubricating fluids are shown in Table 2 (90°C, 1720 and 2760 MPa 250 and 400 Kpsi). Lubricant parameter (LP) values for the selected HPI fluids were determined as a function of temperature on an optical EHL viscometer from film thickness variation with rolling speed at various temperatures. Pressure-viscosity (α) and LP values at 100°C are shown in Table 2. Polyisoprene Synthesis Parameters HPI Example Mol Isoprene Mol n-BuLi Mol THF Rx T°C Mol Ratio THF:n-BuLi 1 2.94 0.62 0.72 25 1.16 2 2.94 0.78 1.23 25 1.58 3 2.94 0.78 1.23 40 1.58 4 15.0 4.04 6.15 -10 1.52 5 15.0 3.03 14.8 -15 4.89 6 1.59 0.16 0.62 25 3.88 7 1.47 0.15 0.074 65 0.5 8 1.91 0.18 0.62 25 3.4 9 14.7 2.15 14.8 25 6.88 10 1.91 0.27 1.84 20 6.8 The data disclosed herein clearly show that relative to other common types of lubricating fluids, the HPI fluids in accordance with the present invention have: 1. Equivalent or better lubricant protection. 2. Equivalent or higher traction coefficients. 3. Superior pressure-viscosity properties which will result in substantially larger film thicknesses at any given viscosity. These superior properties of HPI fluids result in: 1. Equivalent lubricant protection at substantially lower viscosity or better protection at equivalent viscosity; 2. Reduced viscous churning losses for equivalent lubricant protection resulting in better energy efficiency, lower operating temperatures and concomitant longer lubricant life. 3. Better performance in traction drive applications.

Claims: A process for the preparation of a highly branched, saturated, synthetic, hydrocarbon traction drive fluid, which process comprises: anionically oligomerizing isoprene in the presence of an organo-alkali metal compound and a complexing agent therefore at autogenous pressure and at a temperature from -15°C to 120°C, the molar ratio of the complexing agent : organoalkali metal compound being greater than 0.5,; and non-destructively hydrogenating the resulting oligomer. A process according to any preceding claim wherein the organoalkali metal compound comprises an alkyllithium. A process according to claim 2 wherein the alkyllithium comprises butyllithium. A process according to any preceding claim wherein the complexing agent comprises an ether. A process according to claim 4 wherein the ether comprises tetrahydrofuran, methyltetrahydrofuran, dioxane or diethyl ethers. A process according to any preceding claim wherein the molar ratio of complexing agent : organoalkali metal compound is from 1.1:1 to 10:1. A process according to any preceding claim wherein the molar ratio of conjugated diene to organo-alkali metal compound is at least 5:1. A process according to any preceding claim wherein the hydrogenation is effected by contacting the oligomer with hydrogen at an elevated temperature and pressure in the presence of a transition metal-containing catalyst. A process according to claim 8 wherein the temperature is from 150°C to 180°C. A process according to claim 8 or 9 wherein the pressure is from 2000 to 5000 kPa (300 to 700 psi). A process according to claim 8, 9 or 10 wherein the molar ratio of catalyst to oligomer is from 0.5:1 to 100:1. A traction fluid which comprises a highly branched, saturated synthetic hydrocarbon fluid prepared by the process of any of the preceding claims.

Inventors: FORBUS THOMAS REGINALD, FORBUS, THOMAS REGINALD
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 1988
Main classification: C07C2
IPC codes: C10M107