Lubricating oil containing an antiwear antioxidant and frictionreducing additive

Abstract: The addition of an O-alkyl-N-alkoxycarbonylthionocarbamate to a lubricating oil imparts antiwear, antioxidant and/or friction reducing performance to the oil.

Description: This invention relates to a lubricating oil composition having good antiwear, antioxidant and/or friction reducing performance. Engine lubricating oils require the presence of additives to protect the engine from wear. For almost forty years, the principal antiwear additive for engine lubricating oils has been zinc dialkyldithiophosphate (ZDDP). Typically, ZDDP must be used in concentrations of about 1.0 to 1.4 wt.% or greater to be effective in reducing wear. However, phosphate may cause the deactivation of emission control catalysts used in automotive exhaust systems. In addition, ZDDP alone does not provide the enhanced antiwear protection necessary in oils used to lubricate today's small, high performance engines. Furthermore, ZDDP also adds to engine deposits which cause increased oil consumption and increased particulate and regulated gaseous emissions. Accordingly, reducing or eliminating the amount of phosphorus-containing additives (such as ZDDP) in the oil would be desirable. O-alkyl-N-alkoxycarbonylthionocarbamates and their method of preparation are known - see U.S. Patent 4,659,893. However, no mention is made of using O-alkyl-N-alkoxycarbonylthionocarbamates in a lubricating oil. EP-A-0,376,889 discloses a composition comprising (a) a lubricant or a hydraulic fluid and (b) at least one compound of the general formula I in which x=l or 2 and, if x=1, R is as defined for R 1 or R 3 -O- or, if x=2, R is as defined for R 8 , and R 1 and R 3 are, for example, alkyl, cycloalkyl, alkenyl, phenyl, naphthyl, aralkyl or alkaryl, or alkyl, cycloalkyl, alkenyl, phenyl, naphthyl, aralkyl or alkaryl, which are monosubstituted or polysubstituted by groups from the series comprising halogen, cyano, nitro, e.g. -OCH 3 or e.g. -COOCH 3 , or alkyl, cycloalkyl, alkenyl, aralkyl or alkaryl which are interrupted by one or more groups from the series comprising -O-, -S-, -NH-, or alkyl, cycloalkyl, alkenyl, aralkyl or alkaryl which are monosubstituted or polysubstituted by groups from the series comprising halogen, cyano, nitro, e.g. -OCH 3 , or e.g. -COOCH 3 and interrupted by one or more groups from the series comprising -O-, -S-, -NH-, and R 2 is NR 4 R 5 , -OR 6 or -SR 7 , and R 4 and R 5 are identical or different and are, for example, -H, alkyl, phenyl, naphthyl, aralkyl, or alkaryl, or R 4 and R 5 , together with the N atom linking them, form, for example, a piperidine or morpholine radical, and R 6 and R 7 can be alkyl which may be interrupted by one or more groups from the series comprising -O-, -S-, -NH-, or are, for example, phenyl, naphthyl, alkaryl, or aralkyl, and R 8 can be alkylene which may be interrupted by at least one -O- group or alkylidene which may be interrupted by at least one -O- group. The compositions are said to show an improved stability towards oxidative degradation, confer protection under extreme pressures and reduce frictional wear. In one embodiment, this invention provides a lubricating oil composition which comprises (a) a lubricating oil basestock, and (b) an O-alkyl-N-alkoxycarbonylthionocarbamate having the formula where R 1 is a hindered phenol of the formula or an aniline moiety of the formula R 2 is an alkyl group, an aryl group, an alkaryl group, an arylalkyl group, or substituted derivatives thereof, containing from 1 to 20 carbon atoms, R 3 and R 4 are alkyl containing from 1 to 12 carbon atoms, and R 5 is alkyl containing from 2 to 12 carbon atoms. In another embodiment, this invention provides an additive concentrate containing the above-described O-alkyl-N-alkoxycarbonylthionocarbamate that is suitable for blending with a lubricating oil. In a further embodiment, this invention concerns the use of the above-described salt in a lubricating oil composition to provide antiwear, antioxidant and/or friction-reducing properties. In general, the lubricating oil will comprise a major amount of a lubricating oil basestock (or base oil) and a minor amount of an O-alkyl-N-alkoxycarbonylthionocarbamate. The lubricating oil basestock can be derived from natural lubricating oils, synthetic lubricating oils, or mixtures thereof. In general, the lubricating oil basestock will have a kinematic viscosity ranging from about 5 to about 10,000 cSt at 40°C, although typical applications will require an oil having a viscosity ranging from about 10 to about 1,000 cSt at 40°C. Natural lubricating oils include animal oils, vegetable oils ( e . g ., castor oil and lard oil), petroleum oils, mineral oils, and oils derived from coal or shale. Synthetic oils include hydrocarbon oils and halo-substituted hydrocarbon oils such as polymerized and interpolymerized olefins ( e . g . polybutylenes, polypropylenes, propylene-isobutylene copolymers, chlorinated polybutylenes, poly(1-hexenes), poly(1-octenes), poly(1-decenes), etc., and mixtures thereof); alkylbenzenes ( e . g . dodecylbenzenes, tetradecylbenzenes, dinonylbenzenes, di(2-ethylhexyl)-benzene, etc.); polyphenyls ( e . g . biphenyls, terphenyls, alkylated polyphenyls, etc.); alkylated diphenyl ethers, alkylated diphenyl sulfides, as well as their derivatives, analogs, and homologs thereof; and the like. Synthetic lubricating oils also include alkylene oxide polymers, interpolymers, copolymers and derivatives thereof wherein the terminal hydroxyl groups have been modified by esterification, etherification, etc. This class of synthetic oils is exemplified by polyoxyalkylene polymers prepared by polymerization of ethylene oxide or propylene oxide; the alkyl and aryl ethers of these polyoxyalkylene polymers ( e . g ., methyl-polyisopropylene glycol ether having an average molecular weight of 1000, diphenyl ether of polyethylene glycol having a molecular weight of 500-1000, diethyl ether of polypropylene glycol having a molecular weight of 1000-1500); and mono- and polycarboxylic esters thereof ( e . g ., the acetic acid esters, mixed C 3 -C 8 fatty acid esters, and C 13 oxo acid diester of tetraethylene glycol). Another suitable class of synthetic lubricating oils comprises the esters of dicarboxylic acids ( e . g ., phthalic acid, succinic acid, alkyl succinic acids and alkenyl succinic acids, maleic acid, azelaic acid, suberic acid, sebasic acid, fumaric acid, adipic acid, linoleic acid dimer, malonic acid, alkylmalonic acids, alkenyl malonic acids, etc.) with a variety of alcohols ( e . g ., butyl alcohol, hexyl alcohol, dodecyl alcohol, 2-ethylhexyl alcohol, ethylene glycol, diethylene glycol monoether, propylene glycol, etc.). Specific examples of these esters include dibutyl adipate, di(2-ethylhexyl) sebacate, di-n-hexyl fumarate, dioctyl sebacate, diisooctyl azelate, diisodecyl azelate, dioctyl phthalate, didecyl phthalate, dieicosyl sebacate, the 2-ethylhexyl diester of linoleic acid dimer, and the complex ester formed by reacting one mole of sebacic acid with two moles of tetraethylene glycol and two moles of 2-ethylhexanoic acid, and the like. Esters useful as synthetic oils also include those made from C 5 to C 12 monocarboxylic acids and polyols and polyol ethers such as neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol, tripentaerythritol, pentaerythritol monoethylether, and the like. Silicon-based oils (such as the polyakyl-, polyaryl-, polyalkoxy-, or polyaryloxy-siloxane oils and silicone oils) comprise another useful class of synthetic lubricating oils. These oils include tetraethyl silicone, tetraisopropyl silicone, tetra-(2-ethylhexyl) silicone, tetra-(4-methyl-2-ethylhexyl) silicone, tetra(p-tert-butylphenyl) silicone, hexa-(4-methyl-2-pentoxy)-disiloxane, poly(methyl)-siloxanes and poly(methylphenyl) siloxanes, and the like. Other synthetic lubricating oils include liquid esters of phosphorus-containing acids ( e . g ., tricresyl phosphate, trioctyl phosphate, diethyl ester of decylphosphonic acid), polymeric tetrahydrofurans, polyalphaolefins, and the like. The lubricating oil may be derived from unrefined, refined, rerefined oils, or mixtures thereof. Unrefined oils are obtained directly from a natural source or synthetic source ( e . g ., coal, shale, or tar sands bitumen) without further purification or treatment. Examples of unrefined oils include a shale oil obtained directly from a retorting operation, a petroleum oil obtained directly from distillation, or an ester oil obtained directly from an esterification process, each of which is then used without further treatment. Refined oils are similar to the unrefined oils except that refined oils have been treated in one or more purification steps to improve one or more properties. Suitable purification techniques include distillation, hydrotreating, dewaxing, solvent extraction, acid or base extraction, filtration, and percolation, all of which are known to those skilled in the art. Rerefined oils are obtained by treating refined oils in processes similar to those used to obtain the refined oils. These rerefined oils are also known as reclaimed or reprocessed oils and often are additionally processed by techniques for removal of spent additives and oil breakdown products. The O-alkyl-N-alkoxycarbonylthionocarbamates used in this invention are oil soluble and have the general formula: where R 1 is a hindered phenol of the formula or an aniline moiety of the formula R 2 is an alkyl group (straight, branched, or cyclic), an aryl group, an alkaryl group, an arylalkyl group, or substituted derivatives thereof, containing from 1 to 20 carbon atoms, and R 3 and R 4 are alkyl containing from 1 to 12 carbon atoms, R 5 is alkyl containing from 2 to 12 carbon atoms. Preferably R 3 and R 4 are alkyl containing from 3 to 8 carbon atoms, especially t-butyl. Preferably R 5 is alkyl containing from 3 to 10 carbon atoms. Preferably, R 2 is a straight alkyl group containing from 1 to 15 carbon atoms, more preferably from 2 to 8 carbon atoms, and most preferably from 2 to 4 carbon atoms. R 1 and R 2 together should contain a sufficient number of carbon atoms such that the O-alkyl-N-alkoxycarbonylthionocarbamate is soluble in the oil. Examples of suitable substituted groups in R 2 include alkyl, aryl, hydroxy, alkylthio, amido, amino, keto, ether, ester groups, thio, and the like, with hydroxy being a preferred substituted group. The O-alkyl-N-alkoxycarbonylthionocarbamates of the invention are preferably trifunctional, i.e., antiwear, antioxidant and friction reducing properties are combined in a single molecule. This has the advantages of ease of formulation, reduction in additive compatability problems, lower component inventory with fewer components necessary, and reduced mutual antagonisms wherein one component reduces effectiveness of another component. The amount of O-alkyl-N-alkoxycarbonylthionocarbamate used in this invention need be only an amount which is necessary to impart antiwear, antioxidant and/or friction reducing performance to the oil, i . e ., a wear, oxidation and/or friction reducing amount. Typically, however, the concentration of the O-alkyl-N-alkoxycarbonylthionocarbamate in the lubricating oil will range from about 0.1 to about 5 wt.%, preferably from about 0.4 to about 1.5 wt.%, of the oil. If desired, other additives known in the art may be added to the lubricating oil basestock. Such additives include dispersants, other antiwear agents, other antioxidants, corrosion inhibitors, detergents, pour point depressants, extreme pressure additives, viscosity index improvers, other friction modifiers, and the like. These additives are typically disclosed, for example, in "Lubricant Additives" by C. V. Smalhear and R. Kennedy Smith, 1967, pp. 1-11 and in U.S. Patent 4,105,571. O-alkyl-N-alkoxycarbonylthionocarbamates are prepared by methods well known to those skilled in the art. A general method of preparation is illustrated as follows using 2,6-di-t-butyl-4-hydroxy-benzylphenol as the hindered phenol: The resulting product is 0-(3,5-di-t-butyl-4-hydroxybenzyl)-N-ethoxyl-carbonylthionocarbamate, which is a preferred additive. O-alkyl-N-alkoxycarbonylthionocarbamates containing an aniline moiety are prepared by reacting an aniline compound with ethylene oxide followed by reaction with carbonylisothiocyanate. This reaction is illustrated as follows using 4-hexylaniline The resulting product is O-(2-(-4-hexylanilino)ethyl)-N-ethoxycarbonylthionocarbamate, which is a preferred additive. The O-alkyl-N-alkoxycarbonylthioncarbamates can be added directly to the lubricating oil. Often, however they can be made in the form of an additive concentrate to facilitate their handling and introduction into the oil. Typically, the concentrate will contain a suitable organic diluent and from about 10 to about 90 wt.%, preferably from about 30 to about 80 wt.%, of the additives. Suitable organic diluents include mineral oil, naphtha, benzene, toluene, xylene, and the like. The diluent should be compatible ( e . g . soluble) with the oil and, preferably, substantially inert. A lubricating oil containing the O-alkyl-N-alkoxyzarbonylthionocarbamates described above can be used in essentially any application where wear protection, antioxidant protection and/or friction reduction is required. Thus, as used herein, "lubricating oil" (or "lubricating oil composition") is meant to include automotive lubricating oils, industrial oils, gear oils, transmission oils, and the like. In addition, the lubricating oil composition of this invention can be used in the lubrication system of essentially any internal combustion engine, including automobile and truck engines, two-cycle engines, aviation piston engines, marine and railroad engines, and the like. Also contemplated are lubricating oils for gas-fired engines, alcohol ( e . g . methanol) powered engines, stationary powered engines, turbines, and the like. This invention may be further understood by reference to the following examples, which include a preferred embodiment of this invention. The results of Example 3 are described with reference to the accompanying Figure which is a graph showing the increase in viscosity due to oxidation over time for a lubricating oil according to the invention (Oil D) and comparative lubricating oils (Oils A, B and C). Example 1 - Antiwear Performance of O-alkyl-N-alkoxycarbonylthionocarbamates Four Ball Wear tests were performed to determine the effectiveness of various O-alkyl-N-alkoxycarbonylthionocarbamates relative to zinc dialkyldithiophosphate (ZDDP) in reducing wear in various lubricating oils. The Four Ball test used is described in detail in ASTM method D-2266, the disclosure of which is incorporated herein by reference. In this test, three balls are fixed in a lubricating cup and an upper rotating ball is pressed against the lower three balls. The test balls utilized were made of AISI 52100 steel with a hardness of 65 Rockwell C (840 Vickers) and a centerline roughness of 25 nm. Prior to the tests, the test cup, steel balls, and all holders were washed with 1,1,1 trichloroethane. The steel balls subsequently were washed with a laboratory detergent to remove any solvent residue, rinsed with water, and dried under nitrogen. The Four Ball wear tests were performed at 100°C, 60 kg load, and 1200 rpm for 45 minutes duration. After each test, the balls were washed and the Wear Scar Diameter (WSD) on the lower balls measured using an optical microscope. Using the WSD's, the wear volume (WV) was calculated from standard equations (see Wear Control Handbook, edited by M. B. Peterson and W. O. Winer, p. 451, American Society of Mechanical Engineers (1980]). The percent wear reduction (% WR) for each oil tested was then calculated using the following formula: The results of these tests and calculations are shown in Table 1. The data in Table 1 show that O-alkyl-N-alkoxycarbonylthionocarbamates impart comparable antiwear performance to lubricating oils as does ZDDP. Thus, the use of O-alkyl-N-alkoxycarbonylthionocarbamates allows the formulation of a lubricating oil having effective antiwear performance but without the presence (or with a reduced amount) of phosphorus containing compounds such as ZDDP. Example 2 - Friction Reducing Performance of O-alkyl-N-alkoxycarbonylthionocarbamates Ball on Cylinder (BOC) friction tests were performed on several samples of Oil (1) from Example 1 containing some of the O-alkyl-N-alkoxycarbonylthionocarbamates tested in Example 1. The BOC tests were performed using the experimental procedure described by S. Jahanmir and M. Beltzer in ASLE Transactions, 29, No. 3, p. 425 (1985) except that a force of 0.8 Newtons (1 Kg) rather than 4.9 Newtons was applied to a 12.5 mm steel ball in contact with a rotating steel cylinder having a 43.9 mm diameter. The cylinder rotates inside a cup containing a sufficient quantity of lubricating oil to cover 2 mm of the bottom of the cylinder. The cylinder was rotated at 0.25 rpm. The frictional force was continuously monitored by means of a load transducer. In the tests conducted, friction coefficients attained steady state values after 7 to 10 turns of the cylinder. Friction experiments were run at an oil temperature at 100°C. The results of these tests are shown in Table 2 below. Additive Wt.% BOC Friction Coefficient None --- 0.300 I 1.0 0.110 I 2.0 0.110 II 1.0 0.155 II 2.0 0.100 II 3.0 0.095 III 1.0 0.153 VIII 1.0 0.053 The data in Table 2 show that the presence of O-alkyl-N-alkoxycarbonylthionocarbamates improves the friction reducing performance of a lubricating oil. Example 3 - Accelerated Oxidation Test This test is performed by heating oil samples to 172°C and aerating the sample at a rate of 1 liter/min. Test duration is 46 hrs. The stability of the oil is rated by determining viscosity prior to and after oxidation periods. Viscosity is the kinematic viscosity at 40°C. Accelerated oxidation tests were carried out with the following oils: Oil A - Superflo Supreme, a fully formulated commercial passenger car engine oil. Oil B - Oil A without ZDDP and with half the supplementary antioxidant removed. Oil C - Oil B with 2 wt.% of O-isobutyl-N-ethoxycarbonylthionocarbamate, Oil D - Oil B with 2 wt.% of O-(3,5-di-t-butyl-4-hydroxybenzyl)-N-ethoxycarbonylthionocarbamate. The results of the accelerated oxidation tests on the above oils are shown in the Figure as a function of percent viscosity increase as a function of time. With reference to the Figure, Oil A readily passes, incurring a 12% viscosity increase after 46 hrs. Oil B, suffered a 140% viscosity increase after 23 hrs. and was too viscous to measure by the end of the test. Similarly, Oil C's viscosity increased rapidly with an unmeasurable viscosity by the end of the test. By contrast Oil D is very similar to Oil A and passes with little viscosity change as compared to Oil A thereby confirming that Oils A & D have similar antioxidant properties. These results demonstrate that the O-alkyl-N-alkoxycarbonylthionocarbamate of the invention containing a hindered phenol moiety provides antioxidant properties whereas the o-alkyl counterpart exemplified by the isobutyl moiety imparts no antioxidant protection. Example 4 - Comparison of Hindered Phenols Many hindered phenols impart antioxidant properties at room temperature but are not effective in oils at high temperatures. Table 3 shows the behavior of Oil B from Example 3 containing 2 wt.% of various antioxidants in the accelerated oxidation test. Antioxidant in Oil B % Viscosity Increase at Indicated Test Hours 3 19 23 46 α-tocopherol 0 31 44 TVTM Trolox 0 12 20 TVTM Butylated hydroxytoluene 0 3 4 129 Butylated hydroxyanisole 0 5 22 134 none 0 74 101 TVTM phenol 0 85 103 TVTM o-(3,5-di-t-butyl-4-hydroxybenzyl)-N-ethoxycarbonylthionocarbamate 0 0 0 23 The results demonstrate that covalently bonding the hindered phenol as a moiety within the overall carbonylthionocarbamate molecule provides antioxidancy properties at high temperatures in oils. Example 5 Compound VIII in Table 1 of Example 1 was prepared by ethoxylating hexylaniline with ethylene oxide. 2.25 moles of hexylaniline were reacted with 1 mole of ethylene oxide in a Parr bomb at 120°C until the pressure dropped indicating the reaction was over (30 min). The temperature was raised to 150°C for 1 hour to ensure completeness of the reaction. The product was vacuum distilled to remove unreacted hexylaniline, and analysis of the product showed N-(-2-hydroxyethyl)-4-hexylaniline plus a minor amount of di-ethoxylated hexylaniline. The ethoxylated hexylaniline was then reacted with ethoxycarbonylisothiocyanate in a 1:1 mole ratio by refluxing in ether for 6 hours to form compound VIII. Ether was removed by vacuum evaporation and the resulting dark oil containing compound VIII was subjected to oxidative differential scanning calorimetry. Oxidative differential scanning calorimetry (oxidative DSC) is another procedure that assesses the antioxidancy of a lubricating oil. In this DSC test, a sample of oil is heated in air at a programmed rate, e.g., 5°C/minute and the sample temperature rise relative to an inert reference measured. The temperature at which an exothermic reaction (the oxidation onset temperature) is a measure of the oxidative stability of the sample. For comparative purposes, the DSC tests were conducted on a sample of oil B from Table 1 and oil B containing 1 wt.% of compound VIII. The results are tabulated below: Oil DSC Oxidation Onset Temp. °C Oil B 195 Oil B + 1% Compound VIII 235 These results demonstrate that compound VIII provides antioxidancy protection.

Claims: A lubricating oil composition which comprises (a) a lubricating oil basestock, and (b) an O-alkyl-N-alkoxycarbonylthionocarbamate having the formula where R 1 is a hindered phenol of the formula or an aniline moiety of the formula R 2 is an alkyl group, an aryl group, an alkaryl group, an arylalkyl group or derivatives thereof having substituted groups selected from hydroxy, alkylthio, amido, amino, keto, ether, ester and thio groups, containing from 1 to 20 carbon atoms, R 3 and R 4 are each an alkyl group containing from 1 to 12 carbon atoms, and R 5 is an alkyl group containing from 2 to 12 carbon atoms. The composition of claim 1 wherein R 3 and R 4 are each a C 3 to C 8 alkyl group. The composition of claim 2 wherein R 3 and R 4 are tertiary butyl. The composition of any preceding claim wherein R 5 is a C 3 to C 8 alkyl group. The composition of any preceding claim wherein R 2 is a C 1 to C 15 straight chain alkyl group The composition of any preceding claim wherein the concentration of component (b) is from 0.1 to 5 wt.%, based on oil. An additive concentrate suitable for blending with lubricating oils to provide a lubricating composition having antiwear and friction reducing performance which comprises an organic diluent and from about 10 to about 90 wt.% of the O-alkyl-N-alkoxycarbonylthionocarbamate of claim 1. The concentrate of claim 7 wherein the organic diluent is mineral oil, naphtha, benzene, toluene, or xylene. Use of an O-alkyl-N-alkoxycarbonylthionocarbamate as defined in any of claims 1 to 5 in a lubricating oil composition to provide antiwear and/or antioxidant and/or friction reducing properties.

Inventors: BELTZER MORTON, COLLE KARLA SCHALL, HABEEB JACOB JOSEPH, BELTZER, MORTON, COLLE, KARLA SCHALL, HABEEB, JACOB JOSEPH
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1992
Main classification: C10M135
IPC codes: C10M135