WO2012076285A1 - A lubricant composition - Google Patents

Abstract

The present invention describes a lubricant composition comprising a viscosity index improver comprising at least one ester group containing polymer, a base oil and a naphthenic oil having a low viscosity.

Description

A lubricant composition

The present invention relates to a lubricant composition having improved properties comprising a viscosity index improver and a base oil. Furthermore the present invention describes a use of naphthenic oils to improve the viscosity performance of a lubricant comprising an ester group containing polymer. For more than 50 years, the lubricant industry has sought efficient ways to modify the viscosity of various fluids to improve the overall lubricity of the fluid for applications in crankcase fluids, transmission fluids, gear oils, and hydraulic oils. The viscosity index (VI) of a fluid, refers to the ability for a fluid to maintain viscosity and lubricity over a specified temperature range, most often between 40 °C and 100 °C. Increasing the VI of a fluid not only leads to enhanced lubrication, but also can provide additional benefits and utilities which may distinguish the overall performance of one fluid versus another. Such benefits may include reduced viscosities at colder

temperatures thus improving low temperature performance and improvements in hydraulic pump efficiency for various hydraulic systems, which can ultimately lead to reduced fuel consumption.

The viscosity index of a lubricant formulation may be modified by addition of a viscosity modifier or by altering the composition of the base fluid. Viscosity index of formulated lubricating oil can be improved by the choice of base oil as well as the viscosity modifier. The base oils used are generally selected from a class of mineral base oils (Groups I-III) or synthetic oils such as poly alpha- olefins (Group IV) or ester-based oils (Group V) . The viscosity index of these base fluids generally increase as the fluid changes from a Group I to Group III. Synthetic base fluids (Groups IV-V) are beneficial for their

favorable low temperature properties and their high

viscosity index.

Viscosity modifiers are generally selected from a class of polymers such as polyolefins and polymethacrylates .

Poly ( alkylmethacrylates ) (PAMAs) are conventionally

employed as VI improvers to obtain favorable viscosity profiles in lubricating oils at high and low temperature. Chemical modification of poly ( alkylmethacrylates ) , such as, for example, compositional modifications, molecular

weight/shear stability adjustments and solvent selection may affect performance of the polymer as a VI improver in a lubricant composition. Poly ( alkylmethacrylates ) (PAMA) represent a class of VI improvers that have been used for many years and boast favorable viscosity profiles in lubricating oils at high and low temperature.

Due to ever increasing demands on lubricants, in

particular, hydrocarbon oil based lubricants, for better performance which would contribute to reduced fuel

consumption and reduced frictional wear leading to

increased engine or pump performance lifetime, the industry is continuously exploring new methods and technologies to improve lubricant performance and increase the VI of the lubricant formulation. The need for increasing viscosity index is important for many applications requiring

lubrication, where incremental increases can result in vast improvements in performance and efficiency. JP2007031666 describes methacrylate-based VI improvers prepared in a solvent such as ester-oil synthetic solvent, which increase the VI of ester-based synthetic fluids. The described viscosity index improvers, contain a copolymer (A) comprising alkyl (meth) acrylate ( al ) selected from group consisting of C₁- alkyl and C₁-4 hydroxyalkyl (meth) acrylate ester, Cn-15 alkyl (meth) acrylate ester (a2), and C16-2 alkyl (meth) acrylate ester (a3) . A solvent (D) may be an

aliphatic solvent, aromatic solvent or ester based

synthetic oil.

JP2007031666 provides no indication that the copolymers are useful for improving the VI of hydrocarbon oil-based formulations .

JP 2006077119 reports the use of various ester oils which are used as solvents for synthetic base fluids. These ester-based synthetic fluids have benefits in low

temperature viscosity, gear lubricity, and hydraulic actuation. However, there is no disclosure or suggestion of improvement in viscosity index of the final fluid.

JP 2627725 describes the synthesis of ethylene-alpha- olefin-MA based copolymers which may contain grafted side- chains and VI improvers containing the copolymers. The VI improvers are added to lubricating oils based on mineral oil, synthetics, ester-based synthetics and mixtures thereof . US 6303548 describes a lubrication oil which is a

combination of a mineral basestock, a poly-alpha-olefin and a synthetic ester. A broad range of potential viscosity improvers which are prepared in a solvent is described. Potential viscosity modifiers in this crankcase application include alkyl methacrylate copolymers, olefin copolymers and poly-hydrogenated butadienes. EP 992570 A3 describes a hydraulic lubrication oil

containing one of a mineral oil, poly-alpha-olefin or ester based synthetic as a base fluid. EP 992570 A3 provides no discussion of VI benefit or notable low temperature benefit by addition of ester oil as an additive.

None of the above patents discloses or suggests that improvement in the VI of a hydrocarbon oil based lubricant can be obtained with a combination of copolymer having a polar composition and a naphthenic oil.

The known polymers show a good efficiency as viscosity index improvers. Accordingly, most of these polymers exhibit a satisfactory property profile. However, there is a permanent effort to improve the relationship of

thickening, viscosity index and shear stability in order to achieve a desired viscosity with minimum use of additive in lubricant oils over a wide temperature range without impairing this property through premature degradation of the polymers.

Furthermore, the viscosity index improvement should be achievable in a simple and inexpensive manner, and

especially commercially available components should be used. In this context, they should be producible on the industrial scale without new plants or plants of

complicated construction being required for this purpose. These objects and also further objects which are not stated explicitly but are immediately derivable or discernible from the connections discussed herein by way of

introduction are achieved by a lubricant composition having all features of claim 1. Appropriate modifications to the lubricant composition are protected in the claims referring back to claim 1.

The present invention accordingly provides a lubricant composition comprising a viscosity index improver and a base oil, characterized in that the viscosity index

improver comprises at least one ester group containing polymer and the lubricant composition comprises a

naphthenic oil having a low viscosity.

The present lubricants provide a high efficiency to the viscosity index improvers while retaining high shear stability. At the same time, the inventive lubricants allow a series of further advantages to be achieved. These include:

The inventive lubricants have a particularly high viscosity index-improving effectiveness in hydrocarbon oils. These properties are achieved by low treating rates and high shear stabilities. The lubricants of the present invention can be prepared in a particularly easy and simple manner. Furthermore, a high amount of high efficient viscosity index improving polymers can be used. It is possible to use customary industrial scale plants. Furthermore, the present polymers impart fuel efficiency to vehicles using the inventive lubricants. In addition, hydraulic fluids comprising the present polymers show very low fuel

consumption . Moreover, the present lubricants show an astonishing low temperature performance. The adjusting of the polarity of the viscosity modifier in combination with the addition of a naphthenic oil having a low viscosity results in a greater lubrication benefit over a broader temperature range, without sacrificing shear stability or viscosity modifier solubility. This approach has enabled to expand the available techniques which improve the viscosity index which has been demonstrated for ester group containing polymers across a broad range of compositions, polarity, and molecular weight. The lubricant of the present invention comprises a

lubricant composition including at least one ester group containing polymer.

The present invention uses ester group containing polymers which preferably have a high oil solubility. The term "oil- soluble" means that a mixture of a base oil and a polymer comprising ester groups is preparable without macroscopic phase formation, which has at least 0.1% by weight, preferably at least 0.5% by weight, of the polymers. The polymer may be present in dispersed and/or dissolved form in this mixture. The oil solubility depends especially on the proportion of the lipophilic side chains and on the base oil. This property is known to those skilled in the art and can be adjusted readily for the particular base oil via the proportion of lipophilic monomers.

Of particular interest, among others, are polymers which comprise ester groups and preferably have a weight-average molecular weight M_w in the range from 10000 to 2 000 000 g/mol, especially from 20 000 to 800 000 g/mol, more preferably 40 000 to 500 000 g/mol and most preferably 60 000 to 250 000 g/mol.

The number-average molecular weight M_n may preferably be in the range from 5000 to 1 000 000 g/mol, especially from 10000 to 800 000 g/mol, more preferably 15000 to

500 000 g/mol and most preferably 20 000 to 80 000 g/mol.

Without intending any limitation by the following

description, the polymers which comprise ester groups preferably exhibit a polydispersity, given by the ratio of the weight average molecular weight to the number average molecular weight Mw/Mn, in the range of 1 to 15, more preferably 1.1 to 10, especially preferably 1.2 to 5.

Astonishing improvements can be achieved with ester group containing polymer having preferably a polydispersity in the range of 1.05 to 2.0, especially 1.10 to 1.65, more preferably 1.15 to 1.4. The polydispersity may be

determined by gel permeation chromatography (GPC) .

The polymer comprising ester groups may have a variety of structures. For example, dispersing polymers may be present as a statistical copolymer or a diblock, triblock,

multiblock, comb and/or star copolymer which has

corresponding polar and nonpolar segments. In addition, the polymer may especially be present as a graft copolymer. According to a preferred embodiment of the present

invention, the polymers comprising ester groups may include polyalkyl (meth) acrylates (PAMAs), polyalkyl fumarates and/or polyalkyl maleates. Polymers comprising ester groups are understood in the context of the present invention to mean polymers

obtainable by polymerizing monomer compositions which comprise ethylenically unsaturated compounds having at least one ester group, which are referred to hereinafter as ester monomers. Ester monomers are known per se. They include especially (meth) acrylates , maleates and fumarates, which may have different alcohol radicals. The expression " (meth) acrylates " encompasses methacrylates and acrylates, and mixtures of the two. These monomers are widely known. In this context, the alkyl part may be linear, cyclic or branched. The alkyl part may also have known substituents . Accordingly, these polymers contain ester groups as part of the side chain.

The polymer comprising ester groups can be used singly or as a mixture of polymers having different molecular

weights, different compositions of repeating units and/or different ester group containing monomers, for example.

The polymer comprising ester groups comprises preferably at least 40% by weight, more preferably at least 60% by weight, especially preferably at least 80% by weight and most preferably at least 90% by weight of repeat units derived from ester monomers.

The term "repeating unit" is widely known in the technical field. The present polymers comprising ester groups can preferably be obtained by means of free-radical

polymerization of monomers, the controlled radical process techniques of ATRP, RAFT and NMP, which will be explained later, being counted among the free-radical processes in the context of the invention, without any intention that this should impose a restriction. In these processes, double bonds are opened up to form covalent bonds.

Accordingly, the repeat unit is obtained from the monomers used.

The polymers comprising ester groups preferably contain repeating units derived from ester monomers having 7 to 4000 carbon atoms in the alcohol part. Preferably, the polymer comprises at least 40 % by weight, especially at least 60 % by weight and more preferably at least 80 % by weight of repeating units derived from ester monomers having 7 to 4000 carbon atoms, preferably 7 to 300 carbon atoms and more preferably 7 to 30 carbon atoms in the alcohol part.

According to a preferred embodiment the polymer may

comprise repeating units derived from ester monomers having 16 to 4000 carbon atoms, preferably 16 to 300 carbon atoms and more preferably 16 to 30 carbon atoms in the alcohol part, and repeating units derived from ester monomers having 7 to 15 carbon atoms in the alcohol part.

The polymer comprising ester groups may contain 5 to 100% by weight, especially 20 to 95% by weight and more

preferably 30 to 70% by weight of repeat units derived from ester monomers having 7 to 15 carbon atoms in the alcohol part . In a particular aspect, the polymer comprising ester groups may contain 0 to 90% by weight, preferably 5 to 80% by weight and more preferably 40 to 70% by weight of repeat units derived from ester monomers having 16 to 4000, preferably 16 to 30 carbon atoms in the alcohol part.

Preferably, the polymer may comprise repeating units derived from ester monomers having 23 to 4000 carbon atoms, preferably 23 to 400 carbon atoms and more preferably 23 to 300 carbon atoms in the alcohol part.

In addition, the polymer comprising ester groups may contain 0.1 to 60% by weight, especially 5 to 40% by weight, preferably 10 to 30% by weight and more preferably 15 to 20% by weight, of repeat units derived from ester monomers having 1 to 6 carbon atoms in the alcohol part, preferably 1 to 4 carbon atoms in the alcohol part, more preferably 1 or 2 carbon atoms in the alcohol part.

Preferably, the polymer comprising ester groups may contain 0.1 to 60% by weight, especially 5 to 40% by weight, preferably 10 to 30% by weight and more preferably 15 to 20% by weight, of repeat units derived from ester monomers having 1 carbon atom in the alcohol part, with methyl

(meth) acrylate being preferred.

According to a preferred embodiment the polymer may

comprise repeating units derived from ester monomers having 23 to 4000 carbon atoms, preferably 23 to 400 carbon atoms and more preferably 23 to 300 carbon atoms in the alcohol part, and repeating units derived from ester monomers having 1 to 6 carbon atoms in the alcohol part, preferably 1 to 4 carbon atoms in the alcohol part, more preferably 1 or 2 carbon atoms in the alcohol part. The polymer comprising ester groups comprises preferably at least 40% by weight, more preferably at least 60% by weight, especially preferably at least 80% by weight and very particularly at least 95% by weight of repeat units derived from ester monomers.

Mixtures from which the inventive polymers comprising ester groups are obtainable may contain 0 to 40% by weight, especially 5 to 30% by weight and more preferably 10 to 20% by weight of one or more ethylenically unsaturated ester compounds of the formula (I)

in which R is hydrogen or methyl, R is a linear or

branched alkyl radical having 1 to 6 carbon atoms, R² and R³ are each independently hydrogen or a group of the formula -COOR' in which R' is hydrogen or an alkyl group having 1 to 6 carbon atoms .

Examples of component (I) include

(meth) acrylates , fumarates and maleates which derive from saturated alcohols, such as methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, isopropyl

(meth) acrylate, n-butyl (meth) acrylate, tert-butyl

(meth) acrylate and pentyl (meth) acrylate, hexyl

(meth) acrylate;

cycloalkyl (meth) acrylates , such as cyclopentyl

(meth) acrylate, cyclohexyl (meth) acrylate .

As component (I), methyl methacrylate is preferred. The compositions to be polymerized preferably contain 0 to 100% by weight, particularly 5 to 95% by weight, especially 20 to 90% by weight and more preferably 30 to 60% by weight of one or more ethylenically unsaturated ester compounds of the formula (II)

in which R is hydrogen or methyl, R⁴ is a linear or

branched alkyl radical having 7 to 15 carbon atoms, R⁵ and R⁶ are each independently hydrogen or a group of the formula -COOR' ' in which R" is hydrogen or an alkyl group having 7 to 15 carbon atoms.

Examples of component (II) include:

(meth) acrylates , fumarates and maleates which derive from saturated alcohols, such as 2-ethylhexyl (meth) acrylate, heptyl (meth) acrylate, 2-tert-butylheptyl (meth) acrylate, octyl (meth) acrylate, 3-isopropylheptyl (meth) acrylate, nonyl (meth) acrylate, decyl (meth) acrylate, 2- propylheptyl (meth) acrylate, undecyl (meth) acrylate,

5-methylundecyl (meth) acrylate, dodecyl (meth) acrylate, 2-methyldodecyl (meth) acrylate, tridecyl (meth) acrylate,

5-methyltridecyl (meth) acrylate, tetradecyl (meth) acrylate, pentadecyl (meth) acrylate ;

(meth) acrylates which derive from unsaturated alcohols, for example oleyl (meth) acrylate ;

cycloalkyl (meth) acrylates such as 3-vinylcyclohexyl

(meth) acrylate, bornyl (meth) acrylate ; and the

corresponding fumarates and maleates. In addition, preferred monomer compositions comprise 0 to 100% by weight, particularly 0.1 to 90% by weight,

preferably 5 to 80% by weight and more preferably 40 to 70 by weight of one or more ethylenically unsaturated ester compounds of the formula (III)

in which R is hydrogen or methyl, R is a linear or

branched alkyl radical having 16 to 4000, preferably 16 to 400 and more preferably 16 to 30 carbon atoms, R⁸ and R⁹ are each independently hydrogen or a group of the formula - COOR' ' ' in which R' ' ' is hydrogen or an alkyl group having 16 to 4000, preferably 16 to 400 and more preferably 16 to 30 carbon atoms.

Examples of component (III) include (meth) acrylates which derive from saturated alcohols, such as hexadecyl

(meth) acrylate, 2-methylhexadecyl (meth) acrylate,

heptadecyl (meth) acrylate, 5-isopropylheptadecyl

(meth) acrylate, 4-tert-butyloctadecyl (meth) acrylate, 5-ethyloctadecyl (meth) acrylate, 3-isopropyloctadecyl (meth) acrylate, octadecyl (meth) acrylate, nonadecyl

(meth) acrylate, eicosyl (meth) acrylate, cetyleicosyl

(meth) acrylate, stearyleicosyl (meth) acrylate, docosyl (meth) acrylate and/or eicosyltetratriacontyl

(meth) acrylate;

cycloalkyl (meth) acrylates such as 2 , 4 , 5-tri-t-butyl- 3-vinylcyclohexyl (meth) acrylate, 2 , 3 , 4 , 5-tetra- t-butylcyclohexyl (meth) acrylate,

and the corresponding fumarates and maleates. Furthermore, the monomers according formula (III)

especially include long chain branched (meth) acrylates as disclosed inter alia in US 6,746,993, filed 07.08.2002 with the United States Patent Office (USPTO) having the

application number 10/212,784; and US 2004/077509, filed 01.08.2003 with the United States Patent Office (USPTO) having the application number 10/632,108. The disclosure of these documents, especially the (meth) acrylate monomers having at least 16, preferably at least 23 carbon atoms are enclosed herewith by reference.

In addition thereto, the C16-C 000 alkyl (meth) acrylate monomers, preferably the C16-C 00 alkyl (meth) acrylate monomers include polyolefin-based macromonomers . The polyolefin-based macromonomers comprise at least one group which is derived from polyolefins. Polyolefins are known in the technical field, and can be obtained by polymerizing alkenes and/or alkadienes which consist of the elements carbon and hydrogen, for example C2-Cio-alkenes such as ethylene, propylene, n-butene, isobutene, norbornene, and/or Cj-Cio-alkadienes such as butadiene, isoprene, norbornadiene . The polyolefin-based macromonomers comprise preferably at least 70% by weight and more preferably at least 80% by weight and most preferably at least 90% by weight of groups which are derived from alkenes and/or alkadienes, based on the weight of the polyolefin-based macromonomers. The polyolefinic groups may in particular also be present in hydrogenated form. In addition to the groups which are derived from alkenes and/or alkadienes, the alkyl (meth) acrylate monomers derived from polyolefin- based macromonomers may comprise further groups. These include small proportions of copolymerizable monomers.

These monomers are known per se and include, among other monomers, alkyl (meth) acrylates , styrene monomers,

fumarates, maleates, vinyl esters and/or vinyl ethers. The proportion of these groups based on copolymerizable

monomers is preferably at most 30% by weight, more

preferably at most 15% by weight, based on the weight of the polyolefin-based macromonomers . In addition, the polyolefin-based macromonomers may comprise start groups and/or end groups which serve for functionalization or are caused by the preparation of the polyolefin-based

macromonomers. The proportion of these start groups and/or end groups is preferably at most 30% by weight, more preferably at most 15% by weight, based on the weight of the polyolefin-based macromonomers. The number-average molecular weight of the polyolefin-based macromonomers is preferably in the range from 500 to 50 000 g/mol, more preferably from 700 to 10 000 g/mol, in

particular from 1500 to 8000 g/mol and most preferably from 2000 to 6000 g/mol.

In the case of preparation of the comb polymers via the copolymerization of low molecular weight and macromolecular monomers, these values arise through the properties of the macromolecular monomers. In the case of polymer-analogous reactions, this property arises, for example, from the macroalcohols and/or macroamines used taking account of the converted repeat units of the main chain. In the case of graft copolymerizations, the proportion of polyolefins formed which have not been incorporated into the main chain can be used to conclude the molecular weight distribution of the polyolefin. The polyolefin-based macromonomers preferably have a low melting point, which is measured by means of DSC. The melting point of the polyolefin-based macromonomers is preferably less than or equal to -10°C, especially

preferably less than or equal to 20°C, more preferably less than or equal to -40°C. Most preferably, no DSC melting point can be measured for the repeat units which are derived from the polyolefin-based macromonomers in the ester group containing copolymer.

Polyolefin-based macromonomers are disclosed in the

publications DE 10 2007 032 120 Al, filed 09.07.2007 at the German Patent Office (Deutsches Patentamt) having the application number DE102007032120.3; and DE 10 2007 046 223 Al, filed 26.09.2007 at the German Patent Office (Deutsches Patentamt) having the application number DE 102007046223.0; which documents are enclosed herein by reference.

The ester compounds with a long-chain alcohol part,

especially components (II) and (III), can be obtained, for example, by reacting (meth) acrylates , fumarates, maleates and/or the corresponding acids with long-chain fatty alcohols, which generally gives rise to a mixture of esters, for example (meth) acrylates with different long- chain hydrocarbons in the alcohol parts. These fatty alcohols include Oxo Alcohol® 7911, Oxo Alcohol® 7900, Oxo Alcohol® 1100; Alfol® 610, Alfol® 810, Lial® 125 and Nafol® types (Sasol) ; Alphanol® 79 (ICI); Epal® 610 and Epal® 810 (Afton) ; Linevol® 79, Linevol® 911 and Neodol® 25E (Shell) ; Dehydad®, Hydrenol® and Lorol® types (Cognis); Acropol® 35 and Exxal® 10 (Exxon Chemicals); Kalcol® 2465 (Kao

Chemicals) . Among the ethylenically unsaturated ester compounds, the (meth) acrylates are particularly preferred over the

maleates and fumarates, i.e. R , R , R , R , R and R of the formulae (I), (II) and (III) in particularly preferred embodiments are each hydrogen.

The weight ratio of units derived from ester monomers having 7 to 15 carbon atoms, preferably of the formula (II), to the units derived from ester monomers having 16 to 4000 carbon atoms, preferably of the formula (III), may be within a wide range. The weight ratio of repeat units derived from ester monomers having 7 to 15 carbon atoms in the alcohol part to repeat units derived from ester

monomers having 16 to 4000 carbon atoms in the alcohol part is preferably in the range from 30:1 to 1:30, more

preferably in the range from 5:1 to 1:5, especially

preferably 3:1 to 1.1:1.

The polymer may contain units derived from comonomers as an optional component. These comonomers include

aryl (meth) acrylates like benzyl (meth) acrylate or phenyl (meth) acrylate, where the acryl residue in each case can be unsubstituted or substituted up to four times; (meth) acrylates of halogenated alcohols like 2,3- dibromopropyl (meth) acrylate, 4-bromophenyl (meth) acrylate, 1 , 3-dichloro-2-propyl (meth) acrylate, 2-bromoethyl

(meth) acrylate, 2-iodoethyl (meth) acrylate, chloromethyl (meth) acrylate; nitriles of (meth) acrylic acid and other nitrogen- containing (meth) acrylates like N- (methacryloyloxyethyl ) diisobutylketimine, N- (methacryloyloxyethyl ) dihexadecylketimine,

(meth) acryloylamidoacetonitrile, 2- methacryloyloxyethylmethylcyanamide, cyanomethyl

(meth) acrylate; vinyl halides such as, for example, vinyl chloride, vinyl fluoride, vinylidene chloride and vinylidene fluoride; vinyl esters like vinyl acetate; vinyl monomers containing aromatic groups like styrene, substituted styrenes with an alkyl substituent in the side chain, such as -methylstyrene and -ethylstyrene,

substituted styrenes with an alkyl substituent on the ring such as vinyltoluene and p-methylstyrene, halogenated styrenes such as monochlorostyrenes , dichlorostyrenes , tribromostyrenes and tetrabromostyrenes ; vinyl and isoprenyl ethers; maleic acid and maleic acid derivatives such as mono- and diesters of maleic acid, maleic anhydride, methylmaleic anhydride, maleinimide, methylmaleinimide ; fumaric acid and fumaric acid derivatives such as, for example, mono- and diesters of fumaric acid; methacrylic acid and acrylic acid.

According to a special aspect of the present invention, the ester group containing polymer comprises dispersing

monomers . Dispersing monomers are understood to mean especially monomers with functional groups, for which it can be assumed that polymers with these functional groups can keep particles, especially soot particles, in solution (cf. R.M. Mortier, S.T. Orszulik (eds.) : "Chemistry and Technology of Lubricants", Blackie Academic & Professional, London, 2^nd ed. 1997) . These include especially monomers which have boron-, phosphorus-, silicon-, sulfur-, oxygen- and

nitrogen-containing groups, preference being given to oxygen- and nitrogen-functionalized monomers.

Appropriately, it is possible to use especially

heterocyclic vinyl compounds and/or ethylenically

unsaturated, polar ester compounds of the formula (IV)

in which R is hydrogen or methyl, X is oxygen, sulfur or an amino group of the formula -NH- or -NR^a- in which R^a is an alkyl radical having 1 to 40 and preferably 1 to 4 carbon

1 0

atoms, R is a radical which comprises 2 to 1000,

especially 2 to 100 and preferably 2 to 20 carbon atoms and has at least one heteroatom, preferably at least two heteroatoms, R¹¹ and R¹² are each independently hydrogen or

1 0 '

a group of the formula -COX'R m which X' is oxygen or an amino group of the formula -NH- or -NR^a'- in which R^a' is an alkyl radical having 1 to 40 and preferably 1 to 4 carbon atoms, and R^10' is a radical comprising 1 to 100, preferably 1 to 30 and more preferably 1 to 15 carbon atoms, as dispersing monomers. The expression "radical comprising 2 to 1000 carbon" denotes radicals of organic compounds having 2 to 1000 carbon atoms. Similar definitions apply for corresponding terms. It encompasses aromatic and heteroaromatic groups, and alkyl, cycloalkyl, alkoxy, cycloalkoxy, alkenyl, alkanoyl, alkoxycarbonyl groups, and also heteroaliphatic groups. The groups mentioned may be branched or unbranched. In addition, these groups may have customary substituents . Substituents are, for example, linear and branched alkyl groups having 1 to 6 carbon atoms, for example methyl, ethyl, propyl, butyl, pentyl, 2-methylbutyl or hexyl;

cycloalkyl groups, for example cyclopentyl and cyclohexyl; aromatic groups such as phenyl or naphthyl; amino groups, hydroxyl groups, ether groups, ester groups and halides.

According to the invention, aromatic groups denote radicals of mono- or polycyclic aromatic compounds having preferably 6 to 20 and especially 6 to 12 carbon atoms. Heteroaromatic groups denote aryl radicals in which at least one CH group has been replaced by N and/or at least two adjacent CH groups have been replaced by S, NH or 0, heteroaromatic groups having 3 to 19 carbon atoms.

Aromatic or heteroaromatic groups preferred in accordance with the invention derive from benzene, naphthalene, biphenyl, diphenyl ether, diphenylmethane,

diphenyldimethylmethane, bisphenone, diphenyl sulfone, thiophene, furan, pyrrole, thiazole, oxazole, imidazole, isothiazole, isoxazole, pyrazole, 1, 3, 4-oxadiazole,

2, 5-diphenyl-l, 3, 4-oxadiazole, 1, 3, 4-thiadiazole,

1.3.4-triazole, 2, 5-diphenyl-l, 3, 4-triazole,

1.2.5-triphenyl-l, 3, 4-triazole, 1 , 2 , 4-oxadiazole, 1,2,4- thiadiazole, 1 , 2 , 4-triazole, 1 , 2 , 3-triazole, 1.2.3.4-tetrazole, benzo [b] thiophene, benzo [b] furan, indole, benzo [c] thiophene, benzo [c] furan, isoindole, benzoxazole, benzothiazole, benzimidazole, benzisoxazole, benzisothiazole, benzopyrazole, benzothiadiazole,

benzotriazole, dibenzofuran, dibenzothiophene, carbazole, pyridine, bipyridine, pyrazine, pyrazole, pyrimidine, pyridazine, 1, 3, 5-triazine, 1 , 2 , 4-triazine,

1.2.4.5-triazine, tetrazine, quinoline, isoquinoline, quinoxaline, quinazoline, cinnoline, 1 , 8-naphthyridine, 1 , 5-naphthyridine, 1 , 6-naphthyridine, 1 , 7-naphthyridine, phthalazine, pyridopyrimidine, purine, pteridine or quinolizine, 4H-quinolizine, diphenyl ether, anthracene, benzopyrrole, benzoxathiadiazole, benzoxadiazole, benzo- pyridine, benzopyrazine, benzopyrazidine, benzopyrimidine, benzotriazine, indolizine, pyridopyridine,

imidazopyrimidine, pyrazinopyrimidine, carbazole,

aciridine, phenazine, benzoquinoline, phenoxazine,

phenothiazine, acridizine, benzopteridine, phenanthroline and phenanthrene, each of which may also optionally be substituted.

The preferred alkyl groups include the methyl, ethyl, propyl, isopropyl, 1-butyl, 2-butyl, 2-methylpropyl , tert- butyl radical, pentyl, 2-methylbutyl , 1 , 1-dimethylpropyl , hexyl, heptyl, octyl, 1 , 1 , 3 , 3-tetramethylbutyl , nonyl, 1-decyl, 2-decyl, undecyl, dodecyl, pentadecyl and the eicosyl group.

The preferred cycloalkyl groups include the cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl and the cyclooctyl group, each of which is optionally substituted with branched or unbranched alkyl groups. The preferred alkanoyl groups include the formyl, acetyl, propionyl, 2-methylpropionyl , butyryl, valeroyl, pivaloyl, hexanoyl, decanoyl and the dodecanoyl group. The preferred alkoxycarbonyl groups include the

methoxycarbonyl , ethoxycarbonyl , propoxycarbonyl , butoxy- carbonyl, tert-butoxycarbonyl , hexyloxycarbonyl ,

2-methylhexyloxycarbonyl , decyloxycarbonyl or dodecyl- oxycarbonyl group.

The preferred alkoxy groups include alkoxy groups whose hydrocarbon radical is one of the aforementioned preferred alkyl groups.

The preferred cycloalkoxy groups include cycloalkoxy groups whose hydrocarbon radical is one of the aforementioned preferred cycloalkyl groups.

Ί 0

The preferred heteroatoms which are present m the R radical include oxygen, nitrogen, sulfur, boron, silicon and phosphorus, preference being given to oxygen and nitrogen .

Ί 0

The R radical comprises at least one, preferably at least two, preferentially at least three, heteroatoms.

Ί 0

The R radical m ester compounds of the formula (IV) preferably has at least 2 different heteroatoms. In this

Ί 0

case, the R radical m at least one of the ester

compounds of the formula (IV) may comprise at least one nitrogen atom and at least one oxygen atom. Examples of ethylenically unsaturated, polar ester

compounds of the formula (IV) include aminoalkyl

(meth) acrylates , aminoalkyl (meth) acrylamides , hydroxyalkyl (meth) acrylates , (meth) acrylates of ether alcohols, heterocyclic (meth) acrylates and/or carbonyl-containing

(meth) acrylates .

The hydroxyalkyl (meth) acrylates include

2-hydroxypropyl (meth) acrylate,

3.4-dihydroxybutyl (meth) acrylate,

2-hydroxyethyl (meth) acrylate ,

3-hydroxypropyl (meth) acrylate,

2.5-dimethyl-l , 6-hexanediol (meth) acrylate and

1 , 10-decanediol (meth) acrylate .

(Meth) acrylates of ether alcohols include

tetrahydrofurfuryl (meth) acrylate, methoxyethoxyethyl (meth) acrylate, 1-butoxypropyl (meth) acrylate,

cyclohexyloxyethyl (meth) acrylate, propoxyethoxyethyl

(meth) acrylate, benzyloxyethyl (meth) acrylate, furfuryl (meth) acrylate, 2-butoxyethyl (meth) acrylate, 2-ethoxy-2- ethoxyethyl (meth) acrylate, 2-methoxy-2-ethoxypropyl

(meth) acrylate, ethoxylated (meth) acrylates , 1-ethoxybutyl (meth) acrylate, methoxyethyl (meth) acrylate, 2-ethoxy-2- ethoxy-2-ethoxyethyl (meth) acrylate, esters of

(meth) acrylic acid and methoxy polyethylene glycols.

Appropriate carbonyl-containing (meth) acrylates include, for example,

2-carboxyethyl (meth) acrylate,

carboxymethyl (meth) acrylate,

oxazolidinylethyl (meth) acrylate, N- (methacryloyloxy) formamide,

acetonyl (meth) acrylate,

mono-2- (meth) acryloyloxyethyl succinate,

N- (meth) acryloylmorpholine,

N- (meth) acryloyl-2-pyrrolidinone,

N- (2- (meth) acryloyloxyethyl) -2-pyrrolidinone,

N- (3- (meth) acryloyloxypropyl ) -2-pyrrolidinone,

N- (2- (meth) acryloyloxypentadecyl ) -2-pyrrolidinone, N- (3- (meth) acryloyloxyheptadecyl ) -2-pyrrolidinone and N- (2- (meth) acryloyloxyethyl) ethyleneurea.

2-Acetoacetoxyethyl (meth) acrylate

The heterocyclic (meth) acrylates include

2- ( 1-imidazolyl ) ethyl (meth) acrylate,

2- ( 4-morpholinyl ) ethyl (meth) acrylate and

1- (2- (meth) acryloyloxyethyl) -2-pyrrolidone .

Of particular interest are additionally aminoalkyl

(meth) acrylates and aminoalkyl (meth) acrylatamides , for example

dimethylaminopropyl (meth) acrylate,

dimethylaminodiglykol (meth) acrylate,

dimethylaminoethyl (meth) acrylate,

dimethylaminopropyl (meth) acrylamide,

3-diethylaminopentyl (meth) acrylate and

3-dibutylaminohexadecyl (meth) acrylate.

In addition, it is possible to use phosphorus-, boron- and/or silicon-containing (meth) acrylates as dispersing units, such as

2- (dimethylphosphato) propyl (meth) acrylate,

2- ( ethylenephosphito ) propyl (meth) acrylate,

dimethylphosphinomethyl (meth) acrylate, dimethylphosphonoethyl (meth) acrylate,

diethyl (meth) acryloyl phosphonate,

dipropyl (meth) acryloyl phosphate, 2- (dibutylphosphono) ethyl (meth) acrylate,

2 , 3-butylene (meth) acryloylethyl borate,

methyldiethoxy (meth) acryloylethoxysilane,

diethylphosphatoethyl (meth) acrylate.

The preferred heterocyclic vinyl compounds include

2-vinylpyridine, 3-vinylpyridine, 2-methyl-5-vinylpyridine, 3-ethyl-4-vinylpyridine, 2,3 dimethyl-5-vinylpyridine, vinylpyrimidine, vinylpiperidine, 9-vinylcarbazole,

3-vinylcarbazole, 4-vinylcarbazole, 1-vinylimidazole, N- vinylimidazole, 2-methyl- 1-vinylimidazole,

N-vinylpyrrolidone, 2-vinylpyrrolidone, N-vinylpyrrolidine, 3-vinylpyrrolidine, N-vinylcaprolactarn,

N-vinylbutyrolactam, vinyloxolane, vinylfuran,

vinylthiophene, vinylthiolane, vinylthiazoles and

hydrogenated vinylthiazoles, vinyloxazoles and hydrogenated vinyloxazoles, particular preference being given to using N-vinylimidazole and N-vinylpyrrolidone for

functionalization .

The monomers detailed above can be used individually or as a mixture .

Of particular interest are especially polymers which comprise ester groups and are obtained using 2- hydroxypropyl methacrylate, 2-hydroxyethyl methacrylate, mono-2-methacryloyloxyethyl succinate,

N- ( 2-methacryloyloxyethyl ) ethyleneurea,

2-acetoacetoxyethyl methacrylate, 2- ( 4-morpholinyl ) ethyl methacrylate, dimethylaminodiglycol methacrylate, dimethylaminoethyl methacrylate and/or

dimethylaminopropylmethacrylamide .

Special improvements can be achieved with ester groups comprise polymers being obtained using N-vinyl-2- pyrrolidine and/or N-vinyl-2-pyrrolidone .

The dispersing and non-dispersing monomers can be

statistically distributed within the ester group comprising polymer. The proportion of dispersing repeat units in a statistical polymer, based on the weight of the polymers comprising ester groups, is preferably in the range from 0 % by weight to 20% by weight, more preferably in the range from 1% by weight to 15% by weight and most preferably in the range from 2.5% by weight to 10% by weight.

More preferably, the dispersing repeating unit can be selected from dimethylaminopropylmethacrylamide (DMAPMA) and/or dimethylaminoethylmethacrylate (DMAPMA) and the amount of dispersing repeating based on the weight of the polymers comprising ester groups, is preferably in the range from 0.5 % by weight to 10% by weight, more

preferably in the range from 1.2 % by weight to 5% by weight .

More preferably, the dispersing repeating unit can be selected from 2- ( 4-morpholinyl ) ethylmethacrylate (MOEMA) , 2-hydroxyethyl (meth) acrylate (HEMA) and/or

hydroxypropylmethacrylate (HPMA) and the amount of

dispersing repeating based on the weight of the polymers comprising ester groups, is preferably in the range from 2 % by weight to 20% by weight, more preferably in the range from 5 % by weight to 10% by weight. According to another aspect of the present invention, the ester group containing polymer may comprise only a low amount of dispersing repeating units. According such aspect, the proportion of the dispersing repeat units is preferably at most 5 %, more preferably at most 2 % and most preferably at most 0.5 %, based on the weight of the polymers comprising ester groups. According to a special aspect of the present invention, the lubricant used in the motor may preferably comprise a mixture of polymers and at least one of the polymers comprises a considerable amount of dispersing repeating units and at least one of the polymers comprises a low amount of dispersing repeating units as mentioned above.

According to a preferred embodiment of the present

invention, the ester group containing polymer is a graft copolymer having an non-dispersing alkyl (meth) acrylate polymer as graft base and an dispersing monomer as graft layer. Preferably non-dispersing alkyl (meth) acrylate polymer essentially comprises (meth) acrylate monomer units according formulae (I), (II) and (III) as defined above and below. The proportion of dispersing repeat units in a graft or block copolymer, based on the weight of the polymers comprising ester groups, is preferably in the range from 0 % by weight to 20% by weight, more preferably in the range from 1% by weight to 15% by weight and most preferably in the range from 2.5% by weight to 10% by weight.

The dispersing monomer preferably is a heterocyclic vinyl compound as mentioned above and below. According to a further aspect of the present invention the ester group containing polymer is an alkyl (meth) acrylate polymer having at least one polar block and at least one hydrophobic block.

Preferably, the polar block comprises at least three units derived from monomers of the formula (IV) and/or from heterocyclic vinyl compounds, which are bonded directly to one another.

Preferred polymers comprise at least one hydrophobic block and at least one polar block, said polar block having at least eight repeat units and the proportion by weight of dispersing repeat units in the polar block being at least 30%, based on the weight of the polar block.

The term "block" in this context denotes a section of the polymer. The blocks may have an essentially constant composition composed of one or more monomer units. In addition, the blocks may have a gradient, in which case the concentration of different monomer units (repeat units) varies over the segment length. The polar blocks differ from the hydrophobic block via the proportion of dispersing monomers. The hydrophobic blocks may have at most a small proportion of dispersing repeat units (monomer units), whereas the polar block comprise a high proportion of dispersing repeat units (monomer units) .

The polar block may preferably comprise at least 8, especially preferably at least 12 and most preferably at least 15 repeat units. At the same time, the polar block comprise at least 30% by weight, preferably at least 40% by weight, of dispersing repeat units, based on the weight of the polar block. In addition to the dispersing repeat units, the polar block may also have repeat units which do not have any dispersing effect. The polar block may have a random structure, such that the different repeat units have a random distribution over the segment length. In addition, the polar block may have a block structure or a structure in the form of a gradient, such that the non-dispersing repeat units and the dispersing repeat units within the polar block have an inhomogeneous distribution.

The hydrophobic block may comprise a small proportion of dispersing repeat units, which is preferably less than 20% by weight, more preferably less than 10% by weight and most preferably less than 5% by weight, based on the weight of the hydrophobic block. In a particularly appropriate configuration, the hydrophobic block comprises essentially no dispersing repeat units.

The hydrophobic block of the polymer comprising ester groups may have 5 to 100% by weight, especially 20 to 98% by weight, preferably 30 to 95 and most preferably 70 to 92% by weight of repeat units derived from ester monomers having 7 to 15 carbon atoms in the alcohol radical. In a particular aspect, the hydrophobic block of the polymer comprising ester groups may have 0 to 80% by weight, preferably 0.5 to 60% by weight, more preferably 2 to 50% by weight and most preferably 5 to 20% by weight of repeat units derived from ester monomers having 16 to 4000 carbon atoms in the alcohol radical. In addition, the hydrophobic block of the polymer

comprising ester groups may have 0 to 40% by weight, preferably 0.1 to 30% by weight and more preferably 0.5 to 20% by weight of repeat units derived from ester monomers having 1 to 6 carbon atoms in the alcohol radical.

The hydrophobic block of the polymer comprising ester groups comprises preferably at least 40% by weight, more preferably at least 60% by weight, especially preferably at least 80% by weight and most preferably at least 90% by weight of repeat units derived from ester monomers.

The length of the hydrophobic and hydrophobic blocks may vary within wide ranges. The hydrophobic block preferably possess a weight-average degree of polymerization of at least 10, especially at least 40. The weight-average degree of polymerization of the hydrophobic block is preferably in the range from 20 to 5000, especially from 50 to 2000.

The proportion of dispersing repeat units, based on the weight of the polymers comprising ester groups, is

preferably in the range from 0.5% by weight to 20% by weight, more preferably in the range from 1.5% by weight to 15% by weight and most preferably in the range from 2.5% by weight to 10% by weight. At the same time, these repeat units preferably form a segment-like structure within the polymer comprising ester groups, such that preferably at least 70% by weight, more preferably at least 80% by weight, based on the total weight of the dispersing repeat units, are part of a polar block. Preferably, the weight ratio of said hydrophobic block and said polar block is in the range from 100:1 to 1:1, more preferably in the range from 30:1 to 2:1 and most

preferably in the range from 10:1 to 4:1.

The preparation of the ester group containing polymers from the above-described compositions is known per se. Thus, these polymers can be obtained in particular by free- radical polymerization and related processes, for example ATRP (= Atom Transfer Radical Polymerization) or RAFT (= Reversible Addition Fragmentation Chain Transfer) .

Customary free-radical polymerization is described, inter alia, in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition. In general, a polymerization initiator and a chain transfer agent are used for this purpose. The usable initiators include the azo initiators widely known in the technical field, such as AIBN and 1 , 1-azobiscyclohexane- carbonitrile, and also peroxy compounds such as methyl ethyl ketone peroxide, acetylacetone peroxide, dilauryl peroxide, tert-butyl per-2-ethylhexanoate, ketone peroxide, tert-butyl peroctoate, methyl isobutyl ketone peroxide, cyclohexanone peroxide, dibenzoyl peroxide, tert-butyl peroxybenzoate, tert-butyl peroxyisopropylcarbonate,

2, 5-bis (2-ethylhexanoylperoxy) -2, 5-dimethylhexane, tert- butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-3, 5, 5- trimethylhexanoate, dicumyl peroxide, 1 , 1-bis ( tert- butylperoxy) cyclohexane, 1, 1-bis ( tert-butylperoxy) -3, 3, 5- trimethylcyclohexane, cumyl hydroperoxide, tert-butyl hydroperoxide, bis ( 4-tert-butylcyclohexyl )

peroxydicarbonate, mixtures of two or more of the

aforementioned compounds with one another, and mixtures of the aforementioned compounds with compounds which have not been mentioned but can likewise form free radicals.

Suitable chain transfer agents are in particular oil- soluble mercaptans, for example n-dodecyl mercaptan or 2- mercaptoethanol , or else chain transfer agents from the class of the terpenes, for example terpinolene.

The ATRP process is known per se. It is assumed that it is a "living" free-radical polymerization, without any

intention that the description of the mechanism should impose a restriction. In these processes, a transition metal compound is reacted with a compound which has a transferable atom group. This transfers the transferable atom group to the transition metal compound, which oxidizes the metal. This reaction forms a radical which adds onto ethylenic groups. However, the transfer of the atom group to the transition metal compound is reversible, so that the atom group is transferred back to the growing polymer chain, which forms a controlled polymerization system. The structure of the polymer, the molecular weight and the molecular weight distribution can be controlled

correspondingly .

This reaction is described, for example, by J-S. Wang, et al., J. Am. Chem. Soc, vol. 117, p. 5614-5615 (1995), by Maty aszewski, Macromolecules , vol. 28, p. 7901-7910

(1995) . In addition, the patent applications WO 96/30421, WO 97/47661, WO 97/18247, WO 98/40415 and WO 99/10387 disclose variants of the ATRP explained above. In addition, useful polymers may be obtained, for example, also via RAFT methods. This process is presented in detail, for example, in WO 98/01478 and WO 2004/083169, to which reference is made explicitly for the purposes of

disclosure .

In addition, useful polymers are obtainable by NMP

processes (nitroxide-mediated polymerization) , which are described, inter alia, in US 4581429.

These methods are described comprehensively, in particular with further references, inter alia, in K. Maty aszewski, T.P. Davis, Handbook of Radical Polymerization, Wiley

Interscience, Hoboken 2002, to which reference is made explicitly for the purposes of disclosure.

The polymerization may be carried out at standard pressure, reduced pressure or elevated pressure. The polymerization temperature is generally in the range of -20° - 200°C, preferably 0° - 160°C and more preferably 60° - 140°C.

The polymerization may be carried out with or without solvent. The term solvent is to be understood here in a broad sense.

The polymerization is preferably carried out in a nonpolar solvent. These include hydrocarbon solvents, for example aromatic solvents such as toluene, benzene and xylene, saturated hydrocarbons, for example cyclohexane, heptane, octane, nonane, decane, dodecane, which may also be present in branched form. These solvents may be used individually and as a mixture. Particularly preferred solvents are mineral oils, diesel fuels of mineral origin, natural vegetable and animal oils, biodiesel fuels and synthetic oils (e.g. ester oils such as dinonyl adipate) , and also mixtures thereof. Among these, very particular preference is given to mineral oils and mineral diesel fuels.

According to a special aspect of the present invention, the lubricant of the invention preferably comprises an ester group containing polymer and a olefinic polymer which preferably have a viscosity index-improving or thickening effect. Such polyolefins have long been known and are described in the documents mentioned in the prior art.

These polyolefins include in particular polyolefin

copolymers (OCP) and hydrogenated styrene/diene copolymers (HSD) . The polyolefin copolymers (OCP) to be used according to the invention are known per se. They are primarily polymers synthesized from ethylene-, propylene-, isoprene-,

butylene- [sic] and/or further -olefins [sic] having 5 to 20 C atoms, as are already recommended as VI improvers. Systems which have been grafted with small amounts of oxygen- or nitrogen-containing monomers (e.g. from 0.05 to 5% by weight of maleic anhydride) may also be used. The copolymers which contain diene components are generally hydrogenated in order to reduce the oxidation sensitivity and the crosslinking tendency of the viscosity index improvers .

The molecular weight Mw is in general from 10 000 to

300 000, preferably between 50 000 and 150 000. Such olefin copolymers are described, for example, in the German Laid- Open Applications DE-A 16 44 941, DE-A 17 69 834, DE-A 19 39 037, DE-A 19 63 039 and DE-A 20 59 981. Ethylene/propylene copolymers are particularly useful and terpolymers having the known ternary components, such as ethylidene-norbornene (cf. Macromolecular Reviews, Vol. 10 (1975)) are also possible, but their tendency to crosslink must also be taken into account in the aging process. The distribution may be substantially random, but sequential polymers comprising ethylene blocks can also advantageously be used. The ratio of the monomers ethylene/propylene is variable within certain limits, which can be set to about 75% for ethylene and about 80% for propylene as an upper limit. Owing to its reduced tendency to dissolve in oil, polypropylene is less suitable than ethylene/propylene copolymers. In addition to polymers having a predominantly atactic propylene incorporation, those having a more pronounced isotactic or syndiotactic propylene

incorporation may also be used.

Such products are commercially available, for example under the trade names Dutral^® CO 034, Dutral^® CO 038, Dutral^® CO 043, Dutral^® CO 058, Buna^® EPG 2050 or Buna^® EPG 5050.

The hydrogenated styrene/diene copolymers (HSD) are

likewise known, these polymers being described, for

example, in DE 21 56 122. They are in general hydrogenated isoprene/styrene or butadiene/styrene copolymers. The ratio of diene to styrene is preferably in the range from 2:1 to 1:2, particularly preferably about 55:45. The molecular weight Mw is in general from 10 000 to 300 000, preferably between 50 00 and 150 000. According to a particular aspect of the present invention, the proportion of double bonds after the hydrogenation is not more than 15%, particularly preferably not more than 5%, based on the number of double bonds before the hydrogenation. Hydrogenated styrene/diene copolymers can be commercially obtained under the trade name^®SHELLVIS 50, 150, 200, 250 or 260.

According to a very preferred embodiment of the present invention the ester group containing polymer is a block copolymer comprising a block of ester group containing units and an olefinic block. Preferably, the olefinic block is derived from HSD polymers and/or OCP polymers.

Block copolymer comprising a block of ester group

containing units and an olefinic block are disclosed in the publications DE 33 39 103 Al, filed 28.10.1983 at the

German Patent Office (Deutsches Patentamt) having the application number P 33 39 103.3; and DE 29 05 954 Al, filed 16.02.1979 at the German Patent Office (Deutsches Patentamt) having the application number P 29 05 954.9; which documents are enclosed herein by reference.

In addition to the ester group containing polymer the lubricant of the present invention includes base oil.

Preferred base oils include especially mineral oils, synthetic oils and natural oils.

Mineral oils are known per se and commercially available. They are generally obtained from mineral oil or crude oil by distillation and/or refining and optionally further purification and finishing processes, the term mineral oil including in particular the higher-boiling fractions of crude or mineral oil. In general, the boiling point of mineral oil is higher than 200°C, preferably higher than 300°C, at 5000 Pa. The production by low-temperature carbonization of shale oil, coking of bituminous coal, distillation of brown coal with exclusion of air, and also hydrogenation of bituminous or brown coal is likewise possible. Accordingly, mineral oils have, depending on their origin, different proportions of aromatic, cyclic, branched and linear hydrocarbons.

In general, a distinction is drawn between paraffin-base, naphthenic and aromatic fractions in crude oils or mineral oils, in which the term paraffin-base fraction represents longer-chain or highly branched isoalkanes, and naphthenic fraction represents cycloalkanes . In addition, mineral oils, depending on their origin and finishing, have

different fractions of n-alkanes, isoalkanes having a low degree of branching, known as mono-methyl-branched

paraffins, and compounds having heteroatoms, in particular 0, N and/or S, to which a degree of polar properties are attributed. However, the assignment is difficult, since individual alkane molecules may have both long-chain branched groups and cycloalkane radicals, and aromatic parts. For the purposes of the present invention, the assignment can be effected to DIN 51 378, for example.

Polar fractions can also be determined to ASTM D 2007. The proportion of n-alkanes in preferred mineral oils is less than 3% by weight, the fraction of 0-, N- and/or

S-containing compounds less than 6% by weight. The fraction of the aromatics and of the mono-methyl-branched paraffins is generally in each case in the range from 0 to 40% by weight. In one interesting aspect, mineral oil comprises mainly naphthenic and paraffin-base alkanes which have generally more than 13, preferably more than 18 and most preferably more than 20 carbon atoms. The fraction of these compounds is generally > 60% by weight, preferably > 80% by weight, without any intention that this should impose a restriction. A preferred mineral oil contains 0.5 to 30% by weight of aromatic fractions, 15 to 40% by weight of naphthenic fractions, 35 to 80% by weight of paraffin-base fractions, up to 3% by weight of n-alkanes and 0.05 to 5% by weight of polar compounds, based in each case on the total weight of the mineral oil. An analysis of particularly preferred mineral oils, which was effected by means of conventional processes such as urea separation and liquid chromatography on silica gel, shows, for example, the following constituents, the

percentages relating to the total weight of the particular mineral oil used:

n-alkanes having approx. 18 to 31 carbon atoms:

0.7-1.0%,

slightly branched alkanes having 18 to 31 carbon atoms: 1.0-8.0%,

aromatics having 14 to 32 carbon atoms:

0.4-10.7%,

iso- and cycloalkanes having 20 to 32 carbon atoms:

60.7-82.4%,

polar compounds:

0.1-0.8%,

loss :

6.9-19.4%.

An improved class of mineral oils (reduced sulfur content, reduced nitrogen content, higher viscosity index, lower pour point) results from hydrogen treatment of the mineral oils (hydroisomerization_r hydrocracking, hydrotreatment , hydrofinishing) . In the presence of hydrogen, this

essentially reduces aromatic components and builds up naphthenic components. Valuable information with regard to the analysis of mineral oils and a list of mineral oils which have a different composition can be found, for example, in T. Mang, W.

Dresel (eds.) : "Lubricants and Lubrication", Wiley-VCH, Weinheim 2001; R.M. Mortier, S.T. Orszulik (eds.) :

"Chemistry and Technology of Lubricants", Blackie Academic & Professional, London, 2^nd ed. 1997; or J. Bartz:

"Additive fiir Schmierstoffe" , Expert-Verlag, Renningen- Malmsheim 1994.

Synthetic oils include organic esters, for example diesters and polyesters, polyalkylene glycols, polyethers, synthetic hydrocarbons, especially polyolefins, among which

preference is given to polyalphaolefins (PAOs), silicone oils and perfluoroalkyl ethers. In addition, it is possible to use synthetic base oils originating from gas to liquid (GTL) , coal to liquid (CTL) or biomass to liquid (BTL) processes. They are usually somewhat more expensive than the mineral oils, but have advantages with regard to their performance .

Natural oils are animal or vegetable oils, for example neatsfoot oils or jojoba oils.

Base oils for lubricant oil formulations are divided into groups according to API (American Petroleum Institute) . Mineral oils are divided into group I (non-hydrogen- treated) and, depending on the degree of saturation, sulfur content and viscosity index, into groups II and III (both hydrogen-treated) . PAOs correspond to group IV. All other base oils are encompassed in group V.

According to a special aspect of the present invention group II and group III oils are preferred. That aspect preferably applies with ester group containing polymers having a low content of repeating units being derived from ethylenically unsaturated ester compounds having 1 to 4, more preferably 1 or 2 carbon atoms in the alkyl residue. Preferably, the lubricant may comprise at least 40 % by weight, more preferably at least 60 % and most preferably at least 80 % by weight of a group II and/or group III oil.

According to a further aspect of the present invention group I oils are preferred. That aspect preferably applies with ester group containing polymers having a high content of repeating units being derived from ethylenically

unsaturated ester compounds having 1 to 4, more preferably 1 or 2 carbon atoms in the alkyl residue. Preferably, the lubricant may comprise at least 40 % by weight, more preferably at least 60 % and most preferably at least 80 % by weight of a group I oil.

These lubricant oils may also be used as mixtures and are in many cases commercially available.

For the purposes of the present invention the term

hydrocarbon oils will be understood to describe both mineral oils (Groups I-III) and poly alpha-olefins (Group IV) .

In addition to the base oil, preferably a hydrocarbon base oil, the lubricant composition lubricant comprises a naphthenic oil having a low viscosity. Naphthenic oils are well known in the art. These oils comprise a high content of naphthenic and/or aromatic compounds. Preferably, the naphthenic oil having a low viscosity may have a viscosity, measured at 40°C to ASTM D 445, of at most 15 mm²/s, especially of at most 10 mm²/s, more

preferably of at most 5 mm²/s and most preferably of at most 4 mm²/ s .

Preferably, the naphthenic oil having a low viscosity may have a viscosity, measured at 100°C to ASTM D 445, of at most 3.5 mm²/s, especially of at most 3.0 mm²/s, more preferably of at most 2.5 mm²/s and most preferably of at most 2.0 mm²/ s .

Preferably, the naphthenic oil having a low viscosity may have a content of aromatic carbon atoms of at least 8 %, especially at least 9 % and more preferably at least 10%.

Preferably, the naphthenic oil having a low viscosity may have a content of naphthenic carbon atoms of at least 50%, especially at least 55 % and more preferably at least 60%. Preferably, the naphthenic oil having a low viscosity may have a content of paraffinic carbon atoms of at most 50%, especially of at most 40 % and more preferably of at most 30%. The determination of the content of aromatic carbon atoms, naphthenic carbon atoms and paraffinic carbon atoms can be done via infrared (IR) spectroscopy or¹³C nuclear magnetic resonance (NMR) spectroscopy. Useful information are provided by Nynas Base oil handbook.

Preferably, the naphthenic oil having a low viscosity may have a ratio of naphthenic carbon atoms to aromatic carbon atoms in the range of 20:1 to 2:1, especially in the range of 15:1 to 3:1 and more preferably in the range of 10:1 to 4:1.

Preferably, the naphthenic oil having a low viscosity may have a ratio of paraffinic carbon atoms to aromatic carbon atoms in the range of 10:1 to 1:1, especially in the range of 5:1 to 3:2 and more preferably in the range of 4:1 to 2:1.

Preferably, the naphthenic oil having a low viscosity may have a ratio of naphthenic carbon atoms to paraffinic carbon atoms in the range of 10:1 to 1:1, especially in the range of 5:1 to 3:2 and more preferably in the range of 4:1 to 2:1.

The polymers comprising ester groups can be mixed with the lubricant oil. Furthermore, the polymers can be prepared in the lubricant oils as mentioned above. In addition thereto, the polymers comprising ester groups can be used as a concentrate or as component of an additive package.

The concentration of the ester group containing polymer, preferably the polyalkyl (meth) acrylate polymer in the lubricant oil composition is preferably in the range from

0.5 to 40% by weight, especially in the range from 1 to 25% by weight, more preferably in the range from 2 to 13% by weight, based on the total weight of the composition. The amount of base oil, preferably hydrocarbon oil, more preferably group I, II and/or group III oil in the

lubricant is usually at least 60 % by weight, more

preferably at least 75 % by weight.

The concentration of the naphthenic oil having a low viscosity in the lubricant oil composition is preferably in the range from 1 to 40% by weight, especially in the range from 2 to 20% by weight, more preferably in the range from 4 to 10% by weight, based on the total weight of the composition .

Surprisingly, the inventive effect can be improved by adjusting the polarity of the ester group containing polymer and the amount of the polymer used within the lubricant. This is especially true with regard to non polar hydrocarbon base oil, as group III oils and polyalpha- olefins (PAO) . Preferably the lubricant may comprise a ester group containing polymer having repeating units being derived from ethylenically unsaturated ester

compounds, preferably alkyl (meth) acrylates having 1 to 4 carbon atoms in the alkyl residue and the amount of ester group containing polymer in the lubricant and the amount of repeating units being derived from ethylenically

unsaturated ester compounds, preferably alkyl

(meth) acrylates having 1 to 4 carbon atoms in the alkyl residue in the polymer is selected such that the lubricant preferably comprises 0.1 to 5 %, especially 0.3 to 3.2, more preferably 0.5 to 3 % and most preferably 0.8 to 2.5 % by weight of repeating units being derived from

ethylenically unsaturated ester compounds, preferably alkyl (meth) acrylates having 1 to 4 carbon atoms in the alkyl residue based on the total weight of said lubricant. In addition to the polymers comprising ester groups for use in accordance with the invention, the lubricant oil

compositions detailed here may also comprise further additives. These additives include viscosity index

improvers, pour point improvers and DI additives

(dispersants, detergents, defoamers, corrosion inhibitors, antioxidants, antiwear and extreme pressure additives, friction modifiers) .

The additionally usable VI improvers include

poly ( iso ) butenes (PIB) , fumarate-olefin copolymers,

styrene-maleate copolymers, hydrogenated styrene-diene copolymers (HSD) and olefin copolymers (OCP) .

Compilations of VI improvers and pour point improvers for lubricant oils are also detailed in T. Mang, W. Dresel (eds.) : "Lubricants and Lubrication", Wiley-VCH, Weinheim 2001: R. M. Mortier, S. T. Orszulik (eds.) : "Chemistry and Technology of Lubricants", Blackie Academic & Professional, London, 2nd ed. 1997; or J. Bartz: "Additive fiir

Schmierstoffe" , Expert-Verlag, Renningen-Malmsheim 1994.

Appropriate dispersants include poly ( isobutylene ) deri- vatives, e.g. poly ( isobutylene ) succinimides (PIBSIs);

ethylene-propylene oligomers with N/O functionalities.

The preferred detergents include metal-containing

compounds, for example phenoxides; salicylates; thio- phosphonates , especially thiopyrophosphonates, thio- phosphonates and phosphonates; sulfonates and carbonates. As metals, these compounds may comprise especially calcium, magnesium and barium. These compounds may be used

preferably in neutral or overbased form.

Of particular interest are additionally defoamers, which are in many cases divided into silicone-containing and silicone-free defoamers. The silicone-containing defoamers include linear poly (dimethylsiloxane) and cyclic

poly (dimethylsiloxane) . The silicone-free defoamers which may be used are in many cases polyethers, for example poly ( ethylene glycol) or tributyl phosphate.

In a particular embodiment, the inventive lubricant oil compositions may comprise corrosion inhibitors. These are in many cases divided into antirust additives and metal passivators/deactivators . The antirust additives used may, inter alia, be sulfonates, for example petroleumsulfonates or (in many cases overbased) synthetic

alkylbenzenesulfonates, e.g. dinonylnaphthenesulfonates ; carboxylic acid derivatives, for example lanolin (wool fat) , oxidized paraffins, zinc naphthenates , alkylated succinic acids, 4-nonylphenoxy-acetic acid, amides and imides (N-acylsarcosine, imidazoline derivatives); amine- neutralized mono- and dialkyl phosphates; morpholine, dicyclohexylamine or diethanolamine. The metal

passivators/deactivators include benzotriazole,

tolyltriazole, 2-mercaptobenzothiazole, dialkyl- 2, 5-dimercapto-l , 3, 4-thiadiazole ;

N, N' -disalicylideneethylenediamine, N,N'-disalicyli- denepropylenediamine ; zinc dialkyldithiophosphates and dialkyl dithiocarbamates .

A further preferred group of additives is that of

antioxidants. The antioxidants include, for example, phenols, for example 2 , 6-di-tert-butylphenol (2,6-DTB), butylated hydroxytoluene (BHT) , 2 , 6-di-tert-butyl- 4-methylphenol , 4,4' -methylenebis ( 2 , 6-di-tert-butylphenol ) ; aromatic amines, especially alkylated diphenylamines ,

N-phenyl-l-naphthylamine (PNA) , polymeric

2 , 2 , 4-trimethyldihydroquinone (TMQ) ; compounds containing sulfur and phosphorus, for example metal dithiophosphates , e.g. zinc dithiophosphates (ZnDTP), "OOS triesters" = reaction products of dithiophosphoric acid with activated double bonds from olefins, cyclopentadiene, norbornadiene, a-pinene, polybutene, acrylic esters, maleic esters

(ashless on combustion) ; organosulfur compounds, for example dialkyl sulfides, diaryl sulfides, polysulfides, modified thiols, thiophene derivatives, xanthates,

thioglycols, thioaldehydes , sulfur-containing carboxylic acids; heterocyclic sulfur/nitrogen compounds, especially dialkyldimercaptothiadiazoles , 2-mercaptobenzimidazoles ; zinc and methylene bis (dialkyldithiocarbamate) ; organophos- phorus compounds, for example triaryl and trialkyl

phosphites; organocopper compounds and overbased calcium- and magnesium-based phenolates and salicylates.

The preferred antiwear (AW) and extreme pressure (EP) additives include phosphorus compounds, for example

trialkyl phosphates, triaryl phosphates, e.g. tricresyl phosphate, amine-neutralized mono- and dialkyl phosphates, ethoxylated mono- and dialkyl phosphates, phosphites, phosphonates , phosphines; compounds containing sulfur and phosphorus, for example metal dithiophosphates, e.g. zinc C3-i₂dialkyldithiophosphates (ZnDTPs), ammonium

dialkyldithiophosphates , antimony dialkyldithiophosphates , molybdenum dialkyldithiophosphates, lead dialkyldithiophosphates , "00S triesters" = reaction

products of dithiophosphoric acid with activated double bonds from olefins, cyclopentadiene, norbornadiene,

a-pinene, polybutene, acrylic esters, maleic esters, triphenylphosphorothionate (TPPT) ; compounds containing sulfur and nitrogen, for example zinc bis (amyl

dithiocarbamate ) or methylenebis (di-n-butyl

dithiocarbamate ) ; sulfur compounds containing elemental sulfur and ¾S-sulfurized hydrocarbons (diisobutylene, terpene) ; sulfurized glycerides and fatty acid esters;

overbased sulfonates; chlorine compounds or solids such as graphite or molybdenum disulfide.

More preferably the antiwear additive and/or extreme pressure additive is selected from phosphorous compounds, compounds comprising sulfur and phosphorous, compounds comprising sulfur and nitrogen, sulfur compounds comprising elemental sulfur and ¾S-sulfurized hydrocarbons,

sulfurized glycerides and fatty acid esters, overbased sulfonates, chlorine compounds, graphite or molybdenum disulfide .

A further preferred group of additives is that of friction modifiers. The friction modifiers used may include

mechanically active compounds, for example molybdenum disulfide, graphite (including fluorinated graphite), poly ( trifluoroethylene) , polyamide, polyimide; compounds which form adsorption layers, for example long-chain carboxylic acids, fatty acid esters, ethers, alcohols, amines, amides, imides; compounds which form layers through tribochemical reactions, for example saturated fatty acids, phosphoric acid and thiophosphoric esters, xanthogenates , sulfurized fatty acids; compounds which form polymer-like layers, for example ethoxylated dicarboxylic acid partial esters, dialkyl phthalates, methacrylates , unsaturated fatty acids, sulfurized olefins or organometallic

compounds, for example molybdenum compounds (molybdenum dithiophosphates and molybdenum dithiocarbamates MoDTC) and their combinations with ZnDTPs, copper-containing organic compounds . Some of the additives detailed above may fulfill multiple functions. ZnDTP, for example, is primarily an antiwear additive and extreme pressure additive, but also has the character of an antioxidant and corrosion inhibitor (here: metal passivator/deactivator ) .

The additives detailed above are described in more detail, inter alia, in T. Mang, W. Dresel (eds.) : "Lubricants and Lubrication", Wiley-VCH, Weinheim 2001; J. Bartz: „Additive fiir Schmierstoffe" , Expert-Verlag, Renningen-Malmsheim 1994; R.M. Mortier, S.T. Orszulik (eds.) : "Chemistry and

Technology of Lubricants", Blackie Academic & Professional, London, 2^nd ed. 1997.

Preferred lubricant oil compositions have a viscosity, measured at 40°C to ASTM D 445, in the range of 10 to

160 mm²/s, more preferably in the range of 20 to 120 mm²/s. The kinematic viscosity KVioo measured at 100°C is

preferably at least 3.5 mm²/s, especially at least

4.0 mm²/s, more preferably at least 5.0 mm²/s and most preferably at least 5.4 mm²/s. In a particular aspect of the present invention, preferred lubricant oil compositions have a viscosity index

determined to ASTM D 2270 in the range of 100 to 400, more preferably in the range of 125 to 325 and most preferably in the range of 150 to 250.

Furthermore, lubricant compositions of the present

invention may preferably comprise a High Temperature High Shear (HTHS) viscosity of at least 2.4 mPas, more

preferably at least 2.6 mPas as measured at 150°C according to ASTM D4683. According to a further aspect of the present invention the lubricant may preferably comprise a high temperature high shear of at most 10 mPas, especially at most 7 mPas more preferably at most 5 mPas as measured at 100°C according to ASTM D4683. The difference between the High Temperature High Shear (HTHS) viscosities as measure at 100°C and 150°C HTHSioo - HTHSiso preferably comprises at most 4 mPas, especially at most 3.3 mPas and more

preferably at most 2.5 mPas . The ratio of the High

Temperature High Shear (HTHS) viscosity measured at 100°C (HTHSioo) to the High Temperature High Shear (HTHS)

viscosity measured at 150°C (HTHSiso) HTHSioo/HTHSiso

preferably comprises at most at most 2.0 mPas, especially at most 1.9 mPas . High Temperature High Shear (HTHS) viscosity can be determined according to D4683.

In addition thereto, the lubricant may comprises a high shear stability index (SSI) . According to a useful

embodiment of the present invention, the shear stability index (SSI) as measured according to ASTM D5621 (40 minutes sonic treatment) could preferably amount to 35 or less, especially to 20 or less more preferably to 15 or less. Preferably, lubricants comprising a shear stability index (SSI) as measured according to DIN 51381 (30 cycles Bosch- pump) of at most 5, especially at most 2 and more

preferably at most 1 could be used.

The present lubricant composition can be used, for example, as engine oils (oils used in engines such as an engine for means of transportation and engine for machine tools); gear oils; transmission lube oils, particularly automatic transmission fluid (ATF) , such as stepped automatic

transmission fluid and continuously variable transmission fluid (CVTF) ; and traction oils, shock-absorber oils, power steering oils, hydraulic oils and the like. The lubricant useful for the present invention can

preferably designed to meet the requirements of the SAE classifications as specified in SAE J300. E.g. the

requirements of the viscosity grades 0W, 5W, 10W, 15W, 20W, 25W, 20, 30, 40, 50, and 60 (single-grade) and OW-40, 10W- 30, 10W-60, 15W-40, 20W-20 and 20W-50 (multi-grade) could be adjusted.

According to a special aspect of the present invention, the lubricant composition is a hydraulic fluid having ISO VG 15, VG 22, VG 32, VG 46, VG 68, VG 100, VG 150, VG 1500 and

VG 3200 fluid grades. The viscosity grades as mentioned above can be considered as prescribed ISO viscosity grade.

Preferably, the ISO viscosity grade is in the range of 15 to 3200, more preferably 22 to 150.

According to a further aspect of the invention the

preferred ISO viscosity grade is in the range of 150 to

3200, more preferably 1500 to 3200. The invention will be illustrated in detail hereinafter with reference to examples, without any intention that this should impose a restriction. All amounts are displayed in weight percent unless otherwise stated.

Examples Synthesis Example - Copolymer 1 (25% MMA, ATRP)

A round-bottom flask equipped with a glass stir rod, nitrogen inlet, reflux condenser and thermometer was charged with 85.8 g of Hydrocal 3145 dilution oil, 270.0 g C12-C13 methacrylate, 105.0 g C₁ -C15 methacrylate, 125.0 g Ci methacrylate, 0.83 g CuBr, 2.00 g

pentamethyldiethylenetriamine . The mixture was heated up to 70°C while stirring and nitrogen bubbling for inertion.

Then polymerization was initiated with 2.25 g Ethyl-2- bromoisobutyrate . Reaction temperature was increased to 95 °C and stirred for 6 hours. After the end of the

polymerization the product was diluted with 126.62 g

Hydrocal 3145 supplied by Calumet. Synthesis Example - Copolymer 2 (25% MMA, free radical)

A round-bottom flask equipped with a glass stir rod, nitrogen inlet, reflux condenser and thermometer was charged with 39.0 g dilution oil, 210.6 g C12-C13

methacrylate, 81.9 g C₁ -C15 methacrylate, 97.5 g Ci

methacrylate. The mixture was heated up to 110°C while stirring and nitrogen bubbling for inertion. Then 3-stage feed for 3 hours feed of a mixture consisting of 3.25 g tert-butyl peroctoate (tBPO) and 51.4 g dilution oil was started. After the feed end the mixture was stirred for an addition 30 minutes. After the end of the polymerization the product was diluted with 292.5 g Hydrocal 3145 supplied by Calumet.

Copolymer 3 was prepared by a similar method as Copolymer 1 wherein the components were adjusted as described in Table 1

Copolymers 4 to 7 were prepared by a similar method as Copolymer 2 wherein the components were adjusted as

described in Table 1 Hydrocal 3145 (HC 3145) is a naphthenic oil having low viscosity ( KVi₀₀= 1.309; KV₄₀ = 3.663) .

100N (I) is a group I oil

RMF5 is a group I oil

P1017 (II) is a group II oil

PC 60 (II) is a group II oil

Yubase 3 is a group III oil

Yubase 4 is a group III oil

Nexbase 3020 (III) is a group III oil

PA02 (IV) is polyalphaolefin

Table 1. Composition of VI improvers

Table 1 (continuation)

The efficiency to improve the viscosity index of lubricant oils has been determined. With regard to Copolymers 1 to 4 a constant viscosity has been used. The Shear Stability Index (SSI) was determined according to ASTM D5621 (40 minutes sonic treatment) . Details are provided in Table 2. Table 2. Viscometrics of VI improvers with varying shear stabilities and polarities Blended in Yubase 3 to KVioo = 6.75cSt, and using a napthenic dilution oil, Hydrocal 3145.

These data clearly show that a narrow molecular weight distribution provides an unexpected improvement in

viscosity index. Furthermore, different dilution oils had been used to show the invention for the Copolymers 3 to 5. In order to provide the same concentration of polymer, base oil and dilution oil, constant treat rates has been used. The polymers had been dissolved in different base oils and different dilution oils. Details are provided in Tables 3, 4 and 5, respectively. Table 3. Viscometrics of Copolymer 5 (12 % by weight) blended in Yubase 4 (80 % by weight), and using different dilution oils.

Table 4. Viscometrics of Copolymer 6 (10.6 % by weight) blended in RMF5 (80 % by weight), and using different dilution oils.

Table 5. Viscometrics of Copolymer 7 (10.6 % by weight) blended in RMF5 (80 % by weight), and using different dilution oils.

Dilution oil KV₁₀₀ KV₄₀ VI

HC 3145 14.22 77.81 191

PC 60 (II) 15.18 87.02 185

P1017 (II) 15.67 91.61 183

Yubase 3

(III) 15.11 86.15 186

Claims

Patent claims

1. A lubricant composition comprising a viscosity index improver and a base oil, characterized in that the viscosity index improver comprises at least one ester group containing polymer and the lubricant comprises a naphthenic oil having a low viscosity.

2. The lubricant according to claim 1 characterized in

that the naphthenic oil having a low viscosity has a viscosity, measured at 40°C according to ASTM D 445, of at most 10 mm²/s.

3. The lubricant according to claim 1 or 2 characterized in that the naphthenic oil having a low viscosity has a viscosity, measured at 100°C according to ASTM D 445, of at most 3.0 mm²/s.

4. The lubricant according to at least one of the

preceding claims characterized in that the naphthenic oil having a low viscosity has a content of aromatic carbon atoms of at least 8 %.

5. The lubricant according to at least one of the

preceding claims characterized in that the naphthenic oil having a low viscosity has a content of naphthenic carbon atoms of at least 50%.

6. The lubricant according to at least one of the

preceding claims characterized in that the naphthenic oil having a low viscosity has a ratio of naphthenic carbon atoms to aromatic carbon atoms in the range of 20:1 to 2:1. The lubricant according to at least one of the

preceding claims characterized in that the naphthenic oil having a low viscosity has a ratio of paraffinic carbon atoms to aromatic carbon atoms in the range of 5:1 to 3:2.

The lubricant according to at least one of the

preceding claims characterized in that the naphthenic oil having a low viscosity has a ratio of naphthenic carbon atoms to paraffinic carbon atoms in the range of 5:1 to 3:2.

The lubricant according to at least one of the

preceding claims characterized in that the ester group containing polymer is an alkyl (meth) acrylate polymer.

The lubricant according to at least one of the

preceding claims characterized in that the ester group containing polymer comprises at least 5 % by weight of repeating units being derived from ethylenically unsaturated ester compounds having 1 to 4 carbon atoms in the alcohol residue of the ester. 11. The lubricant according to at least one of the

preceding claims characterized in that the ester group containing polymer comprises at least 5 % by weight of repeating units being derived from ethylenically unsaturated ester compounds having 10 to 15 carbon atoms in the alkyl residue. The lubricant according to at least one of the

preceding claims characterized in that the ester group containing polymer is an alkyl (meth) acrylate polymer having units being derived from (meth) acrylates having 23 to 4000 carbon atoms.

The lubricant according to at least one of the

preceding claims characterized in that the ester group containing polymer is a graft copolymer having an nonpolar alkyl (meth) acrylate polymer as graft base and an dispersing monomer as graft layer.

14. The lubricant according to at least one of the

preceding claims characterized in that the polymer comprising ester groups has a weight-average molecular weight in the range from 20000 to 1000000 g/mol.

15. The lubricant according to at least one of the

preceding claims characterized in that the ester group containing polymer comprises a polydispersity Mw/Mn in the range of 1.05 to 2.0.

The lubricant according to at least one of the

preceding claims characterized in that the ester group containing polymer is obtainable by polymerizing a monomer composition which comprises

a) 0 to 40% by weight, based on the weight of the monomer composition for preparing the polymer, of one or more ethylenically unsaturated ester compounds of the formula (I)

in which R is hydrogen or methyl, R¹ is a linear or branched alkyl radical having 1 to 6 carbon atoms, R² and R³ are each independently hydrogen or a group of the formula -COOR' in which R' is hydrogen or an alkyl group having 1 to 6 carbon atoms,

b) 5 to 100% by weight, based on the weight of the monomer composition for preparing the polymer, of one or more ethylenically unsaturated ester compounds of the formula (II)

in which R is hydrogen or methyl, R⁴ is a linear or branched alkyl radical having 7 to 15 carbon atoms, R⁵ and R⁶ are each independently hydrogen or a group of the formula -COOR' ' in which R' ' is hydrogen or an alkyl group having 7 to 15 carbon atoms,

c) 0 to 80% by weight, based on the weight of the monomer composition for preparing the polymer, of one or more ethylenically unsaturated ester compounds of the formula (III)

in which R is hydrogen or methyl, R⁷ is a linear or branched alkyl radical having 16 to 4000 carbon atoms, R⁸ and R⁹ are each independently hydrogen or a group of the formula -COOR' ' ' in which R' ' ' is hydrogen or an alkyl group having 16 to 4000 carbon atoms,

d) 0 to 50% by weight, based on the weight of the monomer composition for preparing the polymer, of comonomer .

The lubricant according to at least one of the

preceding claims characterized in that the ester group containing polymer is an alkyl (meth) acrylate polymer having at least one polar block and at least one hydrophobic block.

The lubricant according to claim 17 characterized in that the polar block of said polymer the dispersing repeat units are derived from one or more heterocyclic vinyl compounds and/or ethylenically unsaturated polar ester compounds of the formula (IV)

m which R is hydrogen or methyl, X is oxygen, sulfur or an amino group of the formula -NH- or -NR^a- in which R^a is an alkyl radical having 1 to 40 carbon atoms, R¹⁰ is a radical which comprises 2 to 1000 carbon atoms and has at least one heteroatom, R¹¹ and R¹² are each independently hydrogen or a group of the formula - COX'R^10' in which X' is oxygen or an amino group of the formula -NH- or -NR^a'-, in which R^a' is an alkyl radical having 1 to 40 carbon atoms, and R^10' is a radical comprising 1 to 100 carbon atoms, and/or from

heterocyclic vinyl compounds. The lubricant according to claim 17 or 18, characterized in that the weight ratio of said

hydrophobic block and said polar block is in the range from 100:1 to 1:1.

The lubricant according to at least one of the

preceding claims characterized in that said lubricant comprises a polyalkyl (meth) acrylate polymer having repeating units being derived from alkyl

(meth) acrylates having 1 to 4 carbon atoms in the alkyl residue and the amount of polyalkyl (meth) acrylate polymer in the lubricant and the amount of repeating units being derived from alkyl (meth) acrylates having 1 to 4 carbon atoms in the alkyl residue in the

polyalkyl (meth) acrylate polymer is selected such that the lubricant comprises 0.5 to 3 % by weight of repeating units being derived from alkyl

(meth) acrylates having 1 to 4 carbon atoms in the alkyl residue based on the total weight of said lubricant.

The lubricant according to at least one of the

preceding claims characterized in that said lubricant comprises a group I oil.

The lubricant according to claim 21 characterized in that the lubricant comprises at least 40 % by weight a group I oil. 23. A use of a naphthenic oil having a low viscosity to

improve the viscosity index of a lubricant comprising an ester group containing polymer.