Patent Application
20090186787
The invention is in the field of lubricants. It relates to lubricant compositions comprising complex esters of elevated viscosity, and to the use of these lubricant compositions as, for example, transmission oil, industrial oil or motor oil.
The commercially available lubricant compositions or else lubricants are produced from a multitude of different natural or synthetic components. To improve the required properties, according to the field of use, additions and/or further additives are added. The base oils often consist of mineral oils, highly refined mineral oils, alkylated mineral oils, poly-α-olefins (PAOs), polyalkylene glycols, phosphate esters, silicone oils, diesters and esters of polyhydric alcohols. Especially mineral oils of the Solvent Neutral class and mineral oils of the XHVI, VHVI, group II and group III classes are used.
The different lubricants, such as motor oil, turbine oil, hydraulic fluid, transmission oil, compressor oil and the like, must satisfy extremely high criteria such as high viscosity index, good lubricant performance, high oxidation sensitivity, good thermal stability or comparable properties.
High-performance lubricant oil formulations which are used as transmission, industrial or motor oils are especially oils with a high performance profile with regard to shear stability, low-temperature viscosity, long life, evaporation loss, fuel efficiency, seal compatibility and wear protection. Such oils are currently being formulated preferentially with PAO (especially PAO 6) or group II or Group III mineral oils as carrier fluids, and with specific polymers (polyisobutylenes=PIBs, olefin copolymers=ethylene/propylene copolymers=OCPs, polyalkyl methacrylates=PMAs) as thickeners or viscosity index improvers in addition to the customary additive components. Together with PAOs, low-viscosity esters, for example DIDA (diisodecyl adipate), DITA (diisotridecyl adipate) or TMTC (trimethylolpropane caprylate), are typically also being used especially as solubilizers for polar additive types and for optimizing seal compatibilities. Disadvantages in the case of use of the PAOs or of the polymers are generally the high costs and the low shear stability, and also the low-temperature viscosity of the lubricants in the case of use of polymers.
Ester-based lubricant oils are known per se and have already been used for some time (see Ullmanns Encyklopädie der technischen Chemie, 3rd Edition, volume 15, 1964, p. 285-294). Common esters are reaction products of dicarboxylic acids with alcohols, for example 2-ethylhexanol, or reaction products of polyols, for example trimethylolpropane, and fatty acids, for example oleic acid or a mixture of n-octanoic acid and n-decanoic acid. When, for example, dicarboxylic acids are used as well as monocarboxylic acids and polyols in the ester preparation, the dicarboxylic acid has crosslinking action, which leads to an increase in molecular weights of the ester and ultimately to higher viscosities and improved thickening actions in lubricant formulations. Such esters are typically referred to as complex esters. Low-temperature viscosities of the formulations produced with esters and hence improved handling at low temperatures have been described especially for esters with branched alkyl chains.
The industrial requirements on lubricant oils are reflected in the common specifications according to classes, for example multiregion oils which satisfy viscosity classes SAE 75-W90 for transmission oils, or O-W20 or O-W30 for motor oils, can be used virtually in all seasons.
There is still a particular demand for additions of polymeric or oligomeric nature which, used as additives, contribute to the satisfaction of requirements for very shear-stable lubricant compositions which can be used within wide ranges. These additives should additionally at least not worsen the viscosity index. Some viscosity index improvers are known which, however, do not exhibit good shear stability, as shown, for example, in U.S. Pat. No. 4,156,673. EP 488432 (=U.S. Pat. No. 5,070,131) discloses polymers with good shear stability, which are prepared from poly(polyalkenyl) couplings.
DE 3544061 (=U.S. Pat. No. 4,822,508) describes high-shear stability transmission oils which comprise viscosity index-improving additives based on esters of acrylic acid and/or methacrylic acid.
U.S. Pat. No. 5,451,630 describes olefin copolymers (OCPs) which have good shear stability. It is additionally stated that the good shear stability decreases with the size of the molecule and hence with an elevated viscosity. This decomposition of the polymers as a result of elevated shear forces leads to a reduction in the viscosity in the lubricant.
An optimal viscosity index improver exhibits a minor contribution to the viscosity of the lubricant at low temperatures, and a major contribution at operating temperatures. Moreover, a high stability should also be present under elevated shear forces.
It was thus an object of the invention to increase the shear stability of the lubricant composition and to achieve a good low-temperature viscosity. Both are generally worsened by polymeric or oligomeric additions, for example thickeners, viscosity index improvers or polymeric dispersants.
EP 1281701 discloses synthetic lubricants prepared from polyneopentylpolyol and a mixture of linear and branched acids, wherein the ester has a viscosity of from 68 to 400 mm2/s at 40° C. These have been developed for use in cooling compressor fluids.
EP 938536 discloses lubricants which comprise synthetic esters which are obtained by reacting polyols with mixtures of monocarboxylic acids and optionally polybasic acids, and which have an elevated thermal and oxidative stability. The viscosity of the esters at 100° C. is not more than approx. 80 mm2/s. No statements regarding the shear stability were made.
It was firstly an object of the invention to provide high-shear stability lubricant compositions comprising novel thickener systems, which at least do not worsen the viscosity index and are usable within wide ranges. Low-temperature viscosities and/or shear stabilities should be improved in comparison to customary thickeners or VI improvers corresponding to the state of the art, and the compatibility of the thickener system with the remaining components of the lubricant formulations, especially at relatively low temperatures, should remain guaranteed. It was a further object of the invention either to reduce or to eliminate the content of common polymeric and/or oligomeric thickeners or VI improvers (e.g. OCPs, PIBs, polyalkyl methacrylates) in the lubricant compositions, and to replace expensive carrier components such as PAOs with group II or III oils. For lubricant oils which are already formulated with group II or group III oils, in contrast, a replacement of these group II and III oils with cheaper group I oils was desirable. In industry, the reduction or elimination of customary polymers should give rise to advantages with regard to shear stability and low-temperature viscosity.
There is a particular problem when the lubricants, as well as elevated oxidation stability and low-temperature viscosity, must have improved compatibility with respect to seal materials. The known lubricants based on linear esters with good oxidation stability are saturated in nature, but lead to the softening of the customary seal materials. Conversely, unsaturated ester types which originate, for example, from oleic acid have better behavior toward seal materials but have significantly reduced oxidation stabilities. Particular problems occur with respect to seal materials such as NBR (nitrile butyl rubber) and hydrogenated variants thereof (HNBR).
There is still a need for improved lubricants with high biodegradability. A further object of the present invention consisted in providing lubricants which, as well as the properties mentioned, have a good compatibility with respect to seal materials.
At the same time, the other properties, especially the lubricity and rheological properties of the lubricant, must not be adversely affected.
It has been found that particular high-viscosity esters solve the problems outlined above in an outstanding manner.
The invention provides a lubricant composition having a good shear stability determined by the loss of kinematic viscosity at 100° C., comprising base oil and a synthetic complex ester, said complex ester having a kinematic viscosity at 40° C. of greater than 400 and up to 50 000 mm2/s and being obtained by reaction of:
For the complex esters mentioned, it has been found that the shear stability of the lubricant composition comprising these esters achieves very good results and decreases the viscosity only slightly. Furthermore, it has been possible to reduce the content of polymers. The loss of kinematic viscosity was determined at 100° C.
In the context of the invention, shear is considered to be permanent shear. Since the viscosity of the base oil decreases as a result of shear only to a very insignificantly minor degree, if at all, the determination of the loss of viscosity after shear is meaningful as a parameter for the complex esters.
Moreover, it has surprisingly been found that oil temperatures in transmission or axle applications are lower when lubricants are formulated with the high-viscosity complex esters. This has been found by means of the industry standard ARKL test (VW PV 1454).
In addition, it has been found that the further utilization of low concentrations of a polar polymer, for example of an alkyl fumarate-α-olefin, of a polyalkyl methacrylate or of an alkyl methacrylate-α-olefin system, in a lubricant composition comprising the relatively high-viscosity ester in many cases acts as a solubilizer for the ester, and can lower low-temperature viscosities of the lubricant composition in a synergistic manner.
It has additionally been found that expensive, high-viscosity PAO types, for example PAO 60 or PAO 100, or customary thickeners such as OCP or PIB, which have been added to the lubricants as thickeners, can alternatively be formulated with the complex esters to be present in accordance with the invention and lead to comparably good or improved properties. Preference is given to the simultaneous addition of polar polymers as a further component, for example those mentioned above.
The kinematic viscosity of the complex ester for use is preferably from 800 to 25 000 mm2/s, especially from 1200 to 10 000 mm2/s, more preferably from 1300 to 5000 mm2/s and most preferably from 1500 to 3000 mm2/s. It has been found that, surprisingly, the use of these esters leads to very low losses in the kinematic viscosity of the lubricant composition after permanent shear. This property makes possible use in lubricants which are exposed to high shear stress.
Preference is given in accordance with the invention to lubricant compositions comprising the complex ester in a concentration of from 3 to 90% by weight based on the total amount of lubricant composition. Especially preferred is a concentration of 7-50% by weight and more preferably of 10-34% by weight.
In a further preferred embodiment, the lubricant compositions are characterized in that the monocarboxylic acids used in the reaction according to a) are branched monocarboxylic acids or mixtures of linear and branched monocarboxylic acids, each of which has a carbon number of from 5 to 40 carbon atoms, where the content of branched monoacid is preferably greater than 90 mol % based on the total content of the acid mixture. The monocarboxylic acids preferably have from 8 to 30 carbon atoms and especially from 10 to 18 carbon atoms. In particular, the monocarboxylic acids are selected from the group formed by the following branched acids: 2,2-dimethylpropanoic acid, neoheptanoic acid, neooctanoic acid, neononanoic acid, isohexanoic acid, neodecanoic acid, 2-ethylhexanoic acid, 3-propylhexanoic acid, 3,5,5-trimethylhexanoic acid, isoheptanoic acid, isooctanoic acid, isononanoic acid, isostearic acid, isopalmitic acid, Guerbet acid C32, Guerbet acid C34 or Guerbet acid C36, and isodecanoic acid. The linear acids are preferably selected from the group formed by valeric acid, caproic acid, heptanoic acid, caprylic acid, pelargonic acid, capric acid, undecanoic acid, lauric acid, tridecanoic acid, tetradecanoic acid, pentadecanoic acid, palmitic acid, margaric acid, stearic acid, nonadecanoic acid, arachic acid, behenic acid, lignoceric acid, myristic acid, cerotic acid, mellissic acid, tricosanoic acid and pentacosanoic acid, 2-ethylhexanoic acid, isotridecanoic acid, myristic acid, palmitoleic acid, oleic acid, elaidic acid, petroselic acid, linoleic acid, linolenic acid, eleostearic acid, gadoleic acid and erucic acid, and the technical-grade mixtures thereof. Preferred branched monocarboxylic acids are isononanoic acid, isostearic acid and 2-ethylhexanoic acid.
Preference is given to lubricant compositions which comprise complex esters which are obtained by reacting polyols with dicarboxylic acids and branched monocarboxylic acids. These preferred esters formed from polyols, dicarboxylic acids and branched monocarboxylic acids preferably have a viscosity from 1300 to 5000 mm2/s and most preferably from 1500 to 3000 mm2/s.
In the context of the invention, the base oil present in the lubricant composition is understood to mean an oil which is selected from the group formed by mineral oils, highly refined mineral oils, alkylated mineral oils, poly-α-olefins, polyalkylene glycols, phosphate esters, silicone oils, diesters and esters of polyhydric alcohols, and also mineral oils of the Solvent Neutral class and mineral oils of the XHVI, VHVI, group II and group III and GTL basestock (gas-to-liquid base oil) classes. The poly-α-olefins may preferably be formed from C6 to C18-α-olefins and mixtures thereof. Especially preferred are poly-α-decenes.
According to the invention, the polyols are branched or linear alcohols of the general formula (I) R1(OH)n in which R1 is an aliphatic or cycloaliphatic group having from 2 to 20 carbon atoms and n is at least 2. The polyols are preferably selected from the group formed by neopentyl glycol, 2,2-dimethylolbutane, trimethylolethane, trimethylolpropane, trimethylol-butane, monopentaerythitol, dipentaerythritol, tripentaerythritol, ethylene glycol, propylene glycol, polyalkylene glycol, 1,4-butanediol, 1,3-propanediol and glycerol. Especially preferred are trimethylolpropane, monopentaerythritol and dipentaerythritol.
In further preferred embodiments, the lubricant compositions are characterized in that, in the reaction according to b), the monoalcohols used are branched or linear alcohols of the general formula (II) (R2OH) in which R2 is an aliphatic or cycloaliphatic group having from 2 to 24 carbon atoms and bears 0 and/or 1, 2 or 3 double bonds. The monoalcohols are preferably selected from the group formed by caproic alcohol, capryl alcohol, 2-ethylhexyl alcohol, capric alcohol, lauryl alcohol, isotridecyl alcohol, myristyl alcohol, cetyl alcohol, palmitoleyl alcohol, stearyl alcohol, isostearyl alcohol, oleyl alcohol, elaidyl alcohol, petroselinyl alcohol, linolyl alcohol, linolenyl alcohol, elaeostearyl alcohol, arachyl alcohol, gadoleyl alcohol, behenyl alcohol, erucyl alcohol and brassidyl alcohol, and technical-grade mixtures thereof.
The dicarboxylic acids used in accordance with the invention to prepare the complex esters are preferably oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, brassylic acid, thapsic acid and phellogenic acid. The anhydrides of the dicarboxylic acids are also suitable in accordance with the invention for the reaction. Especially preferred are azelaic acid or sebacic acid, and anhydrides thereof.
The conversion to the reaction products of the complex esters proceeds in syntheses known per se for preparing esters. The preparation of the esters can also be carried out in accordance with the invention by known processes such that free carboxyl groups and/or free hydroxyl groups are present in a controlled manner, and these products with free carboxyl and/or free hydroxyl groups are used in the lubricant composition. According to the invention, the free carboxyl groups present may be reacted further with amines to give amides, and the resulting compounds may be present in the lubricant composition as complex esters in the context of the invention.
In further preferred embodiments, the inventive lubricant compositions comprise, as a further component, a polar polymer in a concentration of from 0.5 to 30% by weight based on the total amount of lubricant composition. Preference is given to a concentration of from 1 to 18% by weight and more preferably from 2 to 12% by weight.
The polar polymers for use in accordance with the invention are preferably selected from the group formed by alkyl fumarate-α-olefin copolymer, alkyl maleate-α-olefin copolymer, polyalkyl methacrylate, propylene oxide polymer, ethylene oxide-propylene oxide copolymer and alkyl methacrylate-α-olefin copolymer.
In addition to good shear stability, the complex esters for use in accordance with the invention exhibit a high compatibility toward seal materials which typically find use. The test for compatibility toward seal materials can be carried out, for example, according to the standard test ASTM D 471, for example over 168 h at 100° C. According to this test, the complex esters for use in accordance with the invention exhibit, for the seal materials, a volume increase of not more than 20%, preferably not more than 10%, a hardness loss of less than 15%, preferably less than 10%, and a decrease in the elongation at break of less than 50%, preferably less than 30%.
Stability problems of seal materials with respect to lubricant compositions based on esters occur particularly in the case of use of nitrile rubber or acrylonitrile-butadiene rubber or hydrogenated variants thereof. Typically, these seal materials are softened by esters as lubricants, which is manifested by an increase in volume. This softening leads to reduced hardness and reduced breaking strength or elongation at break.
In a preferred embodiment of the invention, the complex esters for use are compatible toward seal materials which are selected from the group formed by NR (natural rubber), NBR (nitrile-butadiene rubber), HNBR (hydrogenated nitrile butyl rubber), FPM (fluorine rubber), ACM (acrylate rubber), PTFE (Teflon), PU (polyurethane), silicone, polyacrylate and neoprene, more preferably toward NBR, HNBR and ACM.
In a preferred embodiment of the inventive use, the stability of the seal materials toward esters with branched alkyl groups is determined by the ASTM D 471 test mentioned, and the criteria specified are met.
The complex esters for use in accordance with the invention exhibit, in addition to the properties already mentioned, also good oxidation stability and thermal stability. This has been determined to DIN EN ISO 4263-3.
In the context of the invention, the terms “lubricant composition”, “lubricant”, “lubricant oil” and “formulation” are used synonymously.
In addition to the further components mentioned, the inventive lubricant composition may comprise further additives which are selected from the group formed by polymer thickeners, viscosity index improvers, antioxidants, corrosion inhibitors, detergents, dispersants, demulsifiers, defoamers, dyes, wear protection additives, EP (extreme pressure) and AW (antiwear) additives and friction modifiers.
The invention further provides for the use of the inventive lubricant composition, especially in the preferred embodiments, as a vehicle transmission oil, axle oil, industrial transmission oil, compressor oil, turbine oil or motor oil. Particular preference is given to use as a vehicle transmission oil, axle oil, clutch oil or industrial transmission oil.
Table 1 shows a compilation of example and comparative example formulations.
It is found clearly that, based on the high-viscosity esters HVE I or HVE II, transmission oils of SAE class 75W-90 were formulable with good low-temperature properties (low dynamic viscosities, all <300,000 mPa·s; measured at −40° C.). What is noticeable is the improved shear stability of the example formulations (apart from E5 and E6, which are aimed exclusively at one inventive effect of improving low-temperature properties and the possibility of utilizing group III mineral oils instead of PAO 6) as compared with the comparative example (CE1). The effect is all the clearer when it is considered that CE1 has been formulated with PIB and OCP systems which are classified as particularly shear-stable. It is noticeable that the utilization of high-viscosity esters makes available formulations with good low-temperature viscosities likewise by means of PAO 8 or a group III mineral oil instead of by means of PAO 6 (see E4, E5, E6). It is found that the utilization of particular polymers in relatively low concentrations has synergistic effects on an improvement of low-temperature viscosities (see E2 in comparison with E3, E2 in comparison with E7, E2 in comparison with E10 and E5 in comparison with E6). This was shown by means of alkyl methacrylate polymers (see E5 and E6), alkyl methacrylate-α-olefin copolymers (see E3), alkyl maleate-α-olefin copolymers (see E7) and by means of alkyl fumarate-α-olefin copolymers (see E10). In the case of utilization of alkyl methacrylate polymers, the shear stability of the formulations was found to be worse (see E5 and E6), which was attributable to the shear of the alkyl methacrylate polymer. It is likewise apparent that formulations based on HVE II bring advantages in the mean end of test temperature in the ARKL test (VW PV 1454) (see CE1 in comparison to E8 and E9). This test reflects operating oil temperatures in transmission and axle applications and is all the more positive the lower the temperatures observed are. It was likewise apparent that friction values had decreased, as had wear as a result of utilization of the inventive oils. This was shown by means of the industry standard SRV test (see CE1 in comparison to E2).
All methods used and the exact names of the feedstocks used are explained in table 1.
Several motor oils were produced with esters according to the present invention (E13-E15) and their properties were tested. For comparison, the test results for comparable prior art motor oils (Eli and E12) are listed as well. The results can be found in table 2 below:
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 027 602.7 | Jun 2006 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP07/04908 | 6/2/2007 | WO | 00 | 12/18/2008 |
