US20080026968A1

US20080026968A1 - Lubricant compositions, their preparation and use

Info

Publication number: US20080026968A1
Application number: US11/880,194
Authority: US
Inventors: Douglas E. Deckman; David J. Baillargeon; Andrew G. Horodysky
Original assignee: Individual
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 2006-07-28
Filing date: 2007-07-20
Publication date: 2008-01-31
Also published as: JP2009544804A; CA2658817A1; WO2008013754A2; WO2008013754A3; CA2658817C; EP2049635A2

Abstract

A lubricating composition that has improved air release characteristics is based on a lubricating base oil comprising an oil or mixture of oils derived from waxy hydrocarbons produced in an F-T synthesis process. The composition is substantially free of a viscoelastic fluid having a shear stress greater than 11 kPa and a viscosity greater than 30 cSt at 100° C. It is further characterized as entraining less than 1.7% air in 2 minutes and having an air release rate greater than 0.3%/min. when measured at 50° C. by ASTM D 3427.

Description

This application claims priority of Provisional Application 60/833,871 filed Jul. 28, 2006.

FIELD OF THE INVENTION

The present invention relates to lubricant compositions with good air release characteristics, their preparation and use.

BACKGROUND OF THE INVENTION

Lubricating oils, including hydraulic oils and crankcase oils, often are used in environments in which the oil is subject to mechanical agitation in the presence of air. As a consequence, the air becomes entrained in the oil and also forms a foam.
Foam appears on the surface of an oil as air bubbles greater than 1 mm in diameter. Air entrainment refers to the dispersion within the oil of air bubbles less than 1 mm in diameter.
Air entrainment and foaming in lubricating compositions are undesirable phenomena. For example, air entrainment reduces the bulk modules of the fluid resulting in spongy operation and poor control of a hydraulic system's response. It can result in reduced viscosity of a lubricating composition. Both air entrainment and foaming can contribute to fluid deterioration due to enhanced oil oxidation.
Air entrainment, however, is more problematic than foaming. Foaming is typically depressed in lubricating compositions by the use of antifoamant additives. These additives expedite the breakup of a foam, but they do not inhibit air entrainment. Indeed, some antifoamants, such as silicone oils typically used in diesel and automotive crankcase oils, are known to retard air release.
Air release and air entrainment are referred to herein as the air release characteristics of a lubricating composition and are determined in accordance with the method of ASTM D 3427.
U.S. Pat. No. 6,090,758 discloses that foaming in a lubricant comprising a slack wax isomerate is effectively reduced by use of an antifoamant exhibiting a spreading coefficient of about 2 mN/m without increasing the air release time. While the specified antifoamant does not degrade the air release time, further improvements in enhancing air release characteristics are desirable. Indeed, many modern gasoline and diesel engines are designed to use the crankcase oil to function as a hydraulic fluid to operate fuel injectors, valvetrain controls and the like. For these functions, low air entrainment and rapid air release are indicative of high performance lubricants.
U.S. Pat. No. 6,713,438 discloses a lubricating oil composition that exhibits improved air release characteristics. The composition comprises a basestock, typically a polyalphaolefin (PAO), and two polymers of different molecular weight. One of the polymers is a viscoelastic fluid having a shear stress greater than 11 kPa such as a high VI PAO, and the other preferably is a block copolymer. Synthetic basestocks are relatively expensive, and it would be desirable to provide lubricants having good air release characteristics without their use.

SUMMARY OF THE INVENTION

Is has now been found that the air release characteristics in lubricating compositions can be enhanced by formulating the lubricating composition with a base oil derived from a waxy hydrocarbon produced in a Fischer-Tropsch (F-T) synthesis process.
Thus, one aspect of the invention comprises a method for improving the air release characteristics of a lubricating composition comprising a major amount of a lubricating base oil and a minor amount of at least one lubricant additive, the method comprising using as the base oil an effective amount of one or more oils derived from a waxy hydrocarbon produced in a F-T synthesis process.
Another aspect of the invention is a lubricating composition comprising a major amount of a lubricating base oil comprising an oil or mixture of oils derived from a waxy hydrocarbon produced in an F-T synthesis process and a minor amount of at lease one lubricant additive, the composition being substantially free of a viscoelastic fluid having both a shear stress greater than 11 kPa and a viscosity greater than 30 cSt at 100° C.; further characterized as entraining less than 1.7% air in 2 min. and an air release rate greater than 0.3%/min. when measured at 50° C. by ASTM Test Method D 3427.
In another aspect, lubricating oils formulated according to the invention are particularly useful as crankcase lubricants in engines wherein the lubricant provides a lubricating and hydraulic function.
The foregoing summary and the following detailed description are exemplary of the various aspects and embodiments of the claimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are graphical representations showing the improvement in air release characteristics achieved by the invention.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the specification the specific properties referred to have been determined by the following method:

(1) Air release characteristics—ASTM D 3428

(2) TBN or total base number—ASTM D2896

(3) Kinematic viscosity—ASTM D 445

(4) Viscosity index—ASTM D 2270

(5) Shear stress—As per SAE Paper No. 872043.

For convenience, the invention will be described by reference to engine oils especially internal combustion engine oils; however, it should be appreciated that in some aspects the invention is also applicable to other types of lubricants, such as hydraulic fluids, industrial oils and the like.
A key advantage of the present invention is that it provides a method to control the air release characteristics of a lubricating composition by formulating the composition with a base oil derived from a waxy hydrocarbon produced in an F-T synthesis process.
As is known to those skilled in the art, in an F-T synthesis process, a synthesis gas comprising a mixture of H₂and CO is catalytically converted into hydrocarbons and preferably liquid hydrocarbons. The mole ratio of the hydrogen to the carbon monoxide may broadly range from about 0.5 to 4, but which is more typically within the range of from about 0.7 to 2.75 and preferably from about 0.7 to 2.5.
As is well known, F-T synthesis processes include processes in which the catalyst is in the form of a fixed bed, a fluidized bed or as a slurry of catalyst particles in a hydrocarbon slurry liquid. The stoichiometric mole ratio for an F-T synthesis reaction is 2.0, but there are many reasons for using other than a stoichiometric ratio as those skilled in the art know. In cobalt slurry hydrocarbon synthesis process the feed mole ratio of the H₂to CO is typically about 2.1/1.
The synthesis gas comprising a mixture of H₂and CO is bubbled up into the bottom of the slurry and reacts in the presence of the particulate F-T synthesis catalyst in the slurry liquid at conditions effective to form hydrocarbons, a portion of which are liquid at the reaction conditions and which comprise the hydrocarbon slurry liquid. The synthesized hydrocarbon liquid is separated from the catalyst particles as filtrate by means such as filtration, although other separation means such as centrifugation can be used. Some of the synthesized hydrocarbons pass out the top of the hydrocarbon synthesis reactor as vapor, along with unreacted synthesis gas and other gaseous reaction products. Some of these overhead hydrocarbon vapors are typically condensed to liquid and combined with the hydrocarbon liquid filtrate. Thus, the initial boiling point of the filtrate may vary depending on whether or not some of the condensed hydrocarbon vapors have been combined with it.
Slurry hydrocarbon synthesis process conditions vary somewhat depending on the catalyst and desired products. Typical conditions effective to form hydrocarbons comprising mostly C₅₊ paraffins, (e.g., C₅₊-C₂₀₀) and preferably C₁₀₊ paraffins, in a slurry hydrocarbon synthesis process employing a catalyst comprising a supported cobalt component include, for example, temperatures, pressures and hourly gas space velocities in the range of from about 320-850° F., 80-600 psi and 100-40,000 V/hr/V, expressed as standard volumes of the gaseous CO and H₂mixture (0° C., 1 atm) per hour per volume of catalyst, respectively. The term “C₅₊” is used herein to refer to hydrocarbons with a carbon number of greater than 4, but does not imply that material with carbon number 5 has to be present.
Similarly other ranges quoted for carbon number do not imply that hydrocarbons having the limit values of the carbon number range have to be present, or that every carbon number in the quoted range is present. It is preferred that the hydrocarbon synthesis reaction be conducted under conditions in which limited or no water gas shift reaction occurs and more preferably with no water gas shift reaction occurring during the hydrocarbon synthesis. It is also preferred to conduct the reaction under conditions to achieve an alpha of at least 0.85, preferably at least 0.9 and more preferably at least 0.92, so as to synthesize more of the more desirable higher molecular weight hydrocarbons. This has been achieved in a slurry process using a catalyst containing a catalytic cobalt component. Those skilled in the art know that by alpha is meant the Schultz-Flory kinetic alpha. While suitable F-T reaction types of catalyst comprise, for example, one or more Group VIII catalytic metals such as Fe, Ni, Co, Ru and Re, it is preferred that the catalyst comprise a cobalt catalytic component. In one embodiment the catalyst comprises catalytically effective amounts of Co and one or more of Re, Ru, Fe, Ni, Th, Zr, Hf, U, Mg and La on a suitable inorganic support material, preferably one which comprises one or more refractory metal oxides. Preferred supports for Co containing catalysts comprise titania. Particularly useful catalysts and their preparation are known and illustrative, but nonlimiting examples may be found, for example, in U.S. Pat. Nos. 4,568,663; 4,663,305; 4,542,122; 4,621,072 and 5,545,674.
The waxy hydrocarbon produced in the F-T synthesis process, i.e., the F-T wax, preferably has an initial boiling point in the range of from 650° F. to 750° F. and preferably boils up to an end point of at least 1050° F.
When a boiling range is quoted herein it defines the lower and/or upper distillation temperature used to separate the fraction. Unless specifically stated (for example, by specifying that the fraction boils continuously or constitutes the entire range) the specification of a boiling range does not require any material at the specified limit has to be present, rather it excludes material boiling outside that range.
The waxy feed preferably comprises the entire 650-750° F.+ fraction formed by the hydrocarbon synthesis process, having an initial cut point between 650° F. and 750° F. determined by the practitioner and an end point, preferably above 1050° F., determined by the catalyst and process variables employed by the practitioner for the synthesis. Such fractions are referred to herein as “650-750° F.+ fractions”.
By contrast, “650-750° F.− fractions” refers to a fraction with an unspecified initial cut point and an end point somewhere between 650° F. and 750° F. Waxy feeds may be processed as the entire fraction or as subsets of the entire fraction prepared by distillation or other separation techniques. The waxy feed also typically comprises more than 90%, generally more than 95% and preferably more than 98 wt % paraffinic hydrocarbons, most of which are normal paraffins. It has negligible amounts of sulfur and nitrogen compounds (e.g., less than 1 wppm of each), with less than 2,000 wppm, preferably less than 1,000 wppm and more preferably less than 500 wppm of oxygen, in the form of oxygenates. Waxy feeds having these properties and useful in the process of the invention have been made using a slurry F-T process with a catalyst having a catalytic cobalt component, as previously indicated.
The process of making the lubricating base oil from the F-T wax may be characterized as a hydrodewaxing process. This process may be operated in the presence of hydrogen, and hydrogen partial pressures range from about 600 to 6000 kPa. The ratio of hydrogen to the hydrocarbon feedstock (hydrogen circulation rate) typically range from about 10 to 3500 n.l.l.⁻¹(56 to 19,660 SCF/bbl) and the space velocity of the feedstock typically ranges from about 0.1 to 20 LHSV, preferably 0.1 to 10 LHSV.
Hydrodewaxing catalysts useful in the conversion of the n-paraffin waxy feedstocks disclosed herein to form the isoparaffinic hydrocarbon base oil are zeolite catalysts, such as ZSM-5, ZSM-11, ZSM-23, ZSM-35, ZSM-12, ZSM-38, ZSM-48, offretite, ferrierite, zeolite beta, zeolite theta, and zeolite alpha, as disclosed in U.S. Pat. No. 4,906,350. These catalysts are used in combination with Group VIII metals, in particular palladium or platinum. The Group VIII metals may be incorporated into the zeolite catalysts by conventional techniques, such as ion exchange.
In one embodiment, conversion of the waxy feedstock may be conducted over a combination of Pt/zeolite beta and Pt/ZSM-23 catalysts in the presence of hydrogen. In another embodiment, the process of producing the lubricant oil base stocks comprises hydroisomerization and dewaxing over a single catalyst, such as Pt/ZSM-35. In yet another embodiment, the waxy feed can be fed over Group VIII metal loaded ZSM-48, preferably Group VIII noble metal loaded ZSM-48, more preferably Pt/ZSM-48 in either one stage or two stages. In any case, useful hydrocarbon base oil products may be obtained. Catalyst ZSM-48 is described in U.S. Pat. No. 5,075,269. The use of the Group VIII metal loaded ZSM-48 family of catalysts, preferably platinum on ZSM-48, in the hydroisomerization of the waxy feedstock eliminates the need for any subsequent, separate dewaxing step, and is preferred.
A dewaxing step, when needed, may be accomplished using either well known solvent or catalytic dewaxing processes and either the entire hydroisomerate or the 650-750° F.+ fraction may be dewaxed, depending on the intended use of the 650-750° F.− material present, if it has not been separated from the higher boiling material prior to the dewaxing. In solvent dewaxing, the hydroisomerate may be contacted with chilled solvents such as acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), mixtures of MEK/MIBK, or mixtures of MEK/toluene and the like, and further chilled to precipitate out the higher pour point material as a waxy solid which is then separated from the solvent-containing lube oil fraction which is the raffinate. The raffinate is typically further chilled in scraped surface chillers to remove more wax solids.
Low molecular weight hydrocarbons, such as propane, are also used for dewaxing, in which the hydroisomerate is mixed with liquid propane, a least a portion of which is flashed off to chill down the hydroisomerate to precipitate out the wax. The wax is separated from the raffinate by filtration, membrane separation or centrifugation. The solvent is then stripped out of the raffinate, which is then fractionated to produce the preferred base stocks useful in the present invention. Also well known is catalytic dewaxing, in which the hydroisomerate is reacted with hydrogen in the presence of a suitable dewaxing catalyst at conditions effective to lower the pour point of the hydroisomerate. Catalytic dewaxing also converts a portion of the hydroisomerate to lower boiling materials, in the boiling range, for example, 650-750° F.−, which are separated from the heavier 650-750° F.+ base stock fraction and the base stock fraction fractionated into two or more base stocks. Separation of the lower boiling material may be accomplished either prior to or during fractionation of the 650-750° F.+ material into the desired base stocks.
Any dewaxing catalyst which will reduce the pour point of the hydroisomerate and preferably those which provide a large yield of lube oil base stock from the hydroisomerate may be used. These include shape selective molecular sieves which, when combined with at least one catalytic metal component, have been demonstrated as useful for dewaxing petroleum oil fractions and include, for example, ferrierite, mordenite, ZSM-5, ZSM-11, ZSM-23, ZSM-35, ZSM-22 also known as theta one or TON, and the silicoaluminophosphates known as SAPO's. A dewaxing catalyst which has been found to be unexpectedly particularly effective comprises a noble metal, preferably Pt, composited with H-mordenite. The dewaxing may be accomplished with the catalyst in a fixed, fluid or slurry bed. Typical dewaxing conditions include a temperature in the range of from about 400-600° F., a pressure of 500-900 psig, H₂treat rate of 1500-3500 SCF/B for flow-through reactors and LHSV of 0.1-10, preferably 0.2-2.0. The dewaxing is typically conducted to convert no more than 40 wt % and preferably no more than 30 wt % of the hydroisomerate having an initial boiling point in the range of 650-750° F. to material boiling below its initial boiling point.
The base oil suitable for use in the invention will have a kinematic viscosity in the range of about 2 to 50 mm²/s at 100° C. and preferably in the range of about 3.5 to 30 mm²/s at 100° C. and a VI greater than about 130, preferably greater than 135 and more preferably 140 or greater.
The base oil of the invention is further characterized as having a pour point of −5° C. or lower, preferably about −10° C. or lower and under some conditions advantageously having pour points of about −25° C. to about −40° C.
A preferred base oil is one comprising paraffinic hydrocarbon components in which the extent of branching, as measured by the percentage of methyl hydrogens (BI), and the proximity of branching, as measured by the percentage of recurring methylene carbons which are four or more carbons removed from an end group or branch (CH₂≧4), are such that: (a) BI−0.5(CH₂≧4)>15; and (b) BI+0.85(CH₂≧4)<45 as measured over said liquid hydrocarbon composition as a whole.
The preferred base oil can be further characterized, if necessary, as having less than 0.1 wt % aromatic hydrocarbons, less than 20 wppm nitrogen containing compounds, less than 20 wppm sulfur containing compounds, a pour point of less than −18° C., preferably less than −30° C., a preferred BI≧25.4 and (CH₂≧4)≦22.5. They have a nominal boiling point of 370° C.⁺, on average they average fewer than 10 hexyl or longer branches per 100 carbon atoms and on average have more than 16 methyl branches per 100 carbon atoms. They also can be characterized by a combination of dynamic viscosity, as measured by CCS at −40° C., and kinematic viscosity, as measured at 100° C. represented by the formula: DV (at −40° C.)<2900 (KV @ 100° C.)-7000.
The preferred base oil is also characterized as comprising a mixture of branched paraffins characterized in that the lubricant base oil contains at least 90% of a mixture of branched paraffins, wherein said branched paraffins are paraffins having a carbon chain length of about C₂₀to about C₄₀, a molecular weight of about 280 to about 562, a boiling range of about 650° F. to about 1050° F., and wherein said branched paraffins contain up to four alkyl branches and wherein the free carbon index of said branched paraffins is at least about 3.
In the above the Branching Index (BI), Branching Proximity (CH₂≧4), and Free Carbon Index (FCI) are determined as follows:

Branching Index

A 359.88 MHz 1H solution NMR spectrum is obtained on a Bruker 360 MHz AMX spectrometer using 10% solutions in CDCl₃. TMS is the internal chemical shift reference. CDCl₃solvent gives a peak located at 7.28. All spectra are obtained under quantitative conditions using 90 degree pulse (10.9 μs), a pulse delay time of 30 s, which is at least five times the longest hydrogen spin-lattice relaxation time (T₁), and 120 scans to ensure good signal-to-noise ratios.
H atom types are defined according to the following regions:

- 9.2-6.2 ppm hydrogens on aromatic rings;
- 6.2-4.0 ppm hydrogens on olefinic carbon atoms;
- 4.0-2.1 ppm benzylic hydrogens at the a-position to aromatic rings;
- 2.1-1.4 ppm paraffinic CH methine hydrogens;
- 1.4-1.05 ppm paraffinic CH₂methylene hydrogens;
- 1.05-0.5 ppm paraffinic CH₃methyl hydrogens.

The branching index (BI) is calculated as the ratio in percent of non-benzylic methyl hydrogens in the range of 0.5 to 1.05 ppm, to the total non-benzylic aliphatic hydrogens in the range of 0.5 to 2.1 ppm.

Branching Proximity (CH₂≧4)

A 90.5 MHz³CMR single pulse and 135 Distortionless Enhancement by Polarization Transfer (DEPT) NMR spectra are obtained on a Brucker 360 MHzAMX spectrometer using 10% solutions in CDCL₃. TMS is the internal chemical shift reference. CDCL₃solvent gives a triplet located at 77.23 ppm in the ¹³C spectrum. All single pulse spectra are obtained under quantitative conditions using 45 degree pulses (6.3 μs), a pulse delay time of 60 s, which is at least five times the longest carbon spin-lattice relaxation time (T₁), to ensure complete relaxation of the sample, 200 scans to ensure good signal-to-noise ratios, and WALTZ-16 proton decoupling.
The C atom types CH₃, CH₂, and CH are identified from the 135 DEPT ¹³C NMR experiment. A major CH₂resonance in all ¹³C NMR spectra at ^{{tilde over ( )}}29.8 ppm is due to equivalent recurring methylene carbons which are four or more removed from an end group or branch (CH2>4). The types of branches are determined based primarily on the ¹³C chemical shifts for the methyl carbon at the end of the branch or the methylene carbon one removed from the methyl on the branch.
Free Carbon Index (FCI). The FCI is expressed in units of carbons, and is a measure of the number of carbons in an isoparaffin that are located at least 5 carbons from a terminal carbon and 4 carbons way from a side chain. Counting the terminal methyl or branch carbon as “one” the carbons in the FCI are the fifth or greater carbons from either a straight chain terminal methyl or from a branch methane carbon. These carbons appear between 29.9 ppm and 29.6 ppm in the carbon-13 spectrum. They are measured as follows:
a. calculate the average carbon number of the molecules in the sample which is accomplished with sufficient accuracy for lubricating oil materials by simply dividing the molecular weight of the sample oil by 14 (the formula weight of CH₂);
b. divide the total carbon-13 integral area (chart divisions or area counts) by the average carbon number from step a. to obtain the integral area per carbon in the sample;
c. measure the area between 29.9 ppm and 29.6 ppm in the sample; and
d. divide by the integral area per carbon from step b. to obtain FCI.
Branching measurements can be performed using any Fourier Transform NMR spectrometer. Preferably, the measurements are performed using a spectrometer having a magnet of 7.0 T or greater. In all cases, after verification by Mass Spectrometry, UV or an NMR survey that aromatic carbons were absent, the spectral width was limited to the saturated carbon region, about 0-80 ppm vs. TMS (tetramethylsilane). Solutions of 15-25 percent by weight in chloroform-dl were excited by 45 degrees pulses followed by a 0.8 sec acquisition time. In order to minimize non-uniform intensity data, the proton decoupler was gated off during a 10 sec delay prior to the excitation pulse and on during acquisition. Total experiment times ranged from 11-80 minutes. The DEPT and APT sequences were carried out according to literature descriptions with minor deviations described in the Varian or Bruker operating manuals.
DEPT is Distortionless Enhancement by Polarization Transfer. DEPT does not show quaternaries. The DEPT 45 sequence gives a signal for all carbons bonded to protons. DEPT 90 shows CH carbons only. DEPT 135 shows CH and CH₃up and CH₂180 degrees out of phase (down). APT is Attached Proton Test. It allows all carbons to be seen, but if CH and CH₃are up, then quaternaries and CH₂are down. The sequences are useful in that every branch methyl should have a corresponding CH. And the methyls are clearly identified by chemical shift and phase. The branching properties of each sample are determined by C-13 NMR using the assumption in the calculations that the entire sample is isoparaffinic. Corrections are not made for n-paraffins or cycloparaffins, which may be present in the oil samples in varying amounts. The cycloparaffins content is measured using Field Ionization Mass Spectroscopy (FIMS).
A particularly preferred lubricating composition of the invention comprises a major amount of a base oil comprising an oil or mixture of oils derived from waxy hydrocarbons produced in an F-T process and a minor amount of at least one lubricant additive. By major amount is meant greater than 50 wt %, preferably between 65 wt % to 80 wt % and conveniently between 75 wt % to 90 wt %.
The base oil suitable for the composition is that described in detail above.
In one aspect, the suitable base oil may comprise a blend of an effective amount of an oil or mixture of oils derived from waxy hydrocarbons produced in an F-T process with conventional lubricating oils. By effective amount is meant that the ratio of the F-T derived oil to the conventional oil is sufficient to provide an improvement in the air release characteristics of the mixture over that of the conventional oil alone.
Among suitable lubricant additives are alkaline earth metal detergents, such as metal salicylates, phenates and sulfonates. The preferred alkaline earth metal detergents for the composition of the invention are calcium, magnesium and barium salicylates and preferably calcium salicylates. As commonly used in the art, the term “salicylate” refers to salts of hydrocarbyl-substituted salicylic acid. Typically, the salicylate will be a mono- or di-substituted salicylic acid having from about 8 to about 30 or more carbon atoms in the hydrocarbyl substituent. The detergent may be neutral, overbased, or a mixture thereof. Borated detergents may also be used. In a particularly preferred embodiment, the metal salicylate detergent is a calcium salicylate and present in 0.5 wt % to about 4 wt % based on the total weight of the lubricating composition.
Another component of the composition of the invention may be a dispersant or mixture of dispersants. Suitable dispersants include succinimide depressants, ester dispersants, ester-amide dispersants, Mannich dispersants, polyether dispersants, and the like. Preferably, the dispersant is a succinimide dispersant, especially a polybutenyl succinimide. The molecular weight of the polybutenyl group may range from about 800 to about 4,000 or more and preferably from about 1300 to about 2500. The dispersant may be head capped or borated or both.
Another component of the composition may be an antiwear agent. Commonly used crankcase antiwear agents include zinc dialkyldithiophosphates (ZDDP), sulfurized olefins, polysulfides of thiophosphorous acids or amine salts thereof and the phosphorous acid esters, esters of glycerol and the like. In the ZDDP the alkyl groups typically will have from 3 to about 18 carbon atoms with 3 to 10 carbon atoms being preferred. The ZDDP is typically used in amounts of from about 0.4 to 1.4 wt % of the total lubricating composition, although for a preferred lubricating composition having less than about 0.08% phosphorous, the amount of ZDDP used will be in the range of about 0.01 wt % to about 0.1 wt % of the total lubricating composition.
Another class of suitable additives for the composition of the invention includes antioxidants such as aminic and phenolic antioxidants exemplified by secondary aromatic amines and hindered phenols. Typical phenolic antioxidants include derivatives of dihydroxy aryl compounds in which the hydroxyl groups are in the o- or p-position to each other and which contain alkyl substituents and bis-phenolic antioxidants. Typical aminic antioxidants include alkylated aromatic amines especially those in which the alkyl group contains no more than 14 carbon atoms. Mixtures of phenolic and aminic antioxidants also may be used. Such additives may be used in an amount of about 0.01 to 5 wt %, and preferably about 0.1 wt % to about 2 wt %.
Other additives often used in lubrication compositions include VI improvers such as linear or radial styrene-isoprene VI improvers olefin copolymers, polymethacrylates and the like, metal deactivators such as benzotriazole, thiadiozoles and their derivatives, and pour point depressants such as alkylnaphthalenes, polymethacrylates, fumarates and the like.
The composition will typically comprise various lubricant additives in amounts, on an active ingredient basis, from about 0.5 wt % to about 25 wt % and preferably from about 2 wt % to about 10 wt % based on the total weight of the composition.
The composition of the invention is substantially free of viscoelastic fluids having both a shear stress greater than 11 kPa and a viscosity greater than 30 cSt at 100° C. Any amount of such material that does not affect the air release characteristics of the composition may be present; however, it is preferred that the composition of the invention be totally free of such materials. The composition of the invention is further characterized as entraining less than 1.7% air, and preferably less than 1.6% air, in 2 minutes and having an air release rate greater than 0.3%/min., preferably greater than 0.35%/min. when measured at 50° C. by ASTM D 3427. In another aspect of the invention, the lubricating composition typically has a TBN less than 10, a phosphorous content less than 0.08% and a sulfur content less than 0.3% based on the total composition, and a sulfated ash of 1% or less.

EXAMPLES

In the examples, the Group I and Group II base oils were solvent extracted and dewaxed paraffinic hydrocarbon distillates derived from petroleum crude oil. The Group III base oil was an isomerate of an oil containing about 40% of slack wax. The GTL base oils were oils boiling in the lube oil ranges that were derived from an F-T wax. The designation of Group I, II and III oils refers to the categories of base oil slacks defined by the American Petroleum Institute (API Publication 1509; www.API.org). The additive package used in the example contained a polybutenyl-succinimide dispersant, a calcium salicylate detergent, a silicone defoamant, an ashless antioxidant, a ZDDP antiwear agent and VI improving components.

Example 1

A series of engine oils were formulated from the base oils described above to have the same kinematic viscosity (5.2 cSt at 100° C.). Each of the oils contained the same additive package also described above. The formulated oils had a TBN of less than 10, a phosphorous content of less than 0.08 wt %, calcium less than 0.3 wt % and less than about 1.0 wt % sulfated ash.
The composition of the formulated oils is given in Table 1.

TABLE 1

Formulation, wt %	Group I	Group II	Group III	GTL

Additive Package	18.22	18.22	18.22	18.22
150 N Group I	81.78
4.5 cSt Group II		40.89
6.0 cSt Group II		40.89
5.2 cSt Group III			81.78
3.6 cSt GTL				24.53
6.0 cSt GTL				57.25

Each of the formulations were evaluated for air entrainment and air release at 50° C. using the test method ASTM D 3427.
FIG. 1 shows the amount of air entrained over time for each of the base oils. The results clearly show the beneficial effect of the GTL base oil on air entrainment.
FIG. 2 shows the air release properties of each of the formulations. These results also show the beneficial effect of the GTL base oil on air release.
The results clearly show the unexpected improvement in both the air entrainment and air release rates obtained by the base oil derived from an F-T wax even when compared with one derived from a slack wax.

Claims

1. A method for improving the air release characteristics of a lubricating composition comprising a major amount of a lubricating base oil and a minor amount of at least one additive, the method comprising using as the base oil an effective amount of one or more oils derived from a waxy hydrocarbon produced in an F-T process.

2. The method of claim 1 wherein the base oil has a kinematic viscosity in the range of 2 mm²/s to about 50 mm²/s at 100° C.

3. The method of claim 2 wherein the oil has a VI in the range of 130 to 140 or more.

4. A lubricating composition comprising:

a major amount of a lubricating base oil comprising an oil or mixture of oils derived from waxy hydrocarbons produced in an F-T process; and

a minor amount of at least one lubricant additive,

the composition being substantially free of viscoelastic fluids having both a shear stress greater than 11 kPa and a viscosity greater than 30 cSt at 100° C. and further characterized as entraining less than 1.7%/min. air and having an initial air release rate greater than 0.3%/min. when measured at 50° C. by ASTM D 3427.

5. The composition of claim 4 wherein the base oil has a viscosity of 2 to 50 mm²/s at 100° C.

6. The composition of claim 5 wherein the base oil has a VI in the range of 130 to 140 or more.

7. The composition of claim 6 including a plurality of additives.

8. The composition of claim 7 wherein the additives include one or more of alkaline earth metal detergents, ashless dispersants, antioxidants, antiwear agents, pour point depressants and VI improvers.

9. The composition of claim 8 wherein the composition has a TBN less than 10, a phosphorous content less than 0.08 wt % and calcium content less than 0.3 wt %, each based on the total weight of the composition and a sulfated ash of 1% or less.

10. The composition of claim 8 wherein the additives comprise, on an active ingredient basis, from 0.5 wt % to 25 wt % based on the total weight of the composition.

11. The composition of claim 10 wherein the additives comprise from 2 wt % to 10 wt % of the composition.

12. In the lubrication of an engine with a crankcase lubricant wherein the lubricant additionally is used to perform a hydraulic function, the improvement comprising using as the crankcase lubricant a lubricating composition according to claims 4 to 11.

13. A method for formulating an engine oil lubricant having an air entrainment of less than 1.7% in 2 minutes and an air release rate greater than 0.3%/min. when measured at 50° C. by ASTM D 3427 without adding to the formulation a viscoelastic fluid having a shear rate greater than 1 kPa and a viscosity greater than 30 cSt at 100° C., the method comprising blending a major amount of a base oil consisting essentially of an oil or mixture of oils derived from waxy hydrocarbons produced in an F-T process with one or more engine oil additives.

14. The use of one or more oils derived from a waxy hydrocarbon produced in an F-T process to improve the air release characteristics of a lubricating composition.

15. The use according to claim 14, wherein the one or more oils derived from a waxy hydrocarbon produced in an F-T process is used in an amount of at least 50 wt % based on the total weight of the lubricating composition.

16. The use according to any of claims 14 or 15, wherein the one or more oils derived from a waxy hydrocarbon produced in an F-T process have a kinematic viscosity in the range of 2 mm²/s to 50 mm²/s at 100° C.

17. The use according to any of claims 14 to 16, wherein the one or more oils derived from a waxy hydrocarbon produced in an F-T process have a VI in the range of 130 to 140 or more.

18. The use according to any of claims 14 to 17, wherein the lubricating composition has an air entrainment of less than 1.7% in 2 minutes and an air release rate greater than 0.3%/min. when measured at 50° C. by ASTM D 3427 without adding to the formulation a viscoelastic fluid having a shear rate greater than 1 kPa and a viscosity greater than 30 cSt at 100° C.