EP1987118A1

EP1987118A1 - Improvement of energy efficiency in hydraulic systems

Info

Publication number: EP1987118A1
Application number: EP06830061A
Authority: EP
Inventors: Douglas Placek; Christian Daniel Georges Neveu; Steven Neill Herzog
Original assignee: Evonik Rohmax Additives GmbH
Current assignee: Evonik Oil Additives GmbH
Priority date: 2006-02-21
Filing date: 2006-11-21
Publication date: 2008-11-05
Also published as: CA2643098A1; KR20080098498A; WO2007096011A1; US20070197410A1; ZA200807193B; CN101336285A; JP5757675B2; BRPI0621364A2; JP2009527601A

Abstract

The present invention describes use of a fluid having a VI of at least 130 to improve the en- ergy efficiency of a hydraulic system. Furthermore, the present invention relates to a hy- draulic system comprising a hydraulic fluid having a VI of at least 130, a unit for creating mechanical power, a unit that converts mechanical power into hydraulic energy, and a unit that converts hydraulic energy into mechanical work. Preferentially, engine speed can be reduced to decrease load and stress while delivering the same amount of hydraulic power.

Description

RohMax Additives GmbH February 17, 2006

64293 Darmstadt R 05/58

Improvement of energy efficiency in hydraulic systems

Description

The present invention relates to the improvement of energy efficiency in hydraulic systems.

Hydraulic systems are designed to transmit energy and apply large forces with a high degree of flexibility and control. It is desirable to build systems that efficiently convert input energy from an engine, electric motor, or other source into usable work. Hydraulic power can be used to create rotary or linear motion, or to store energy for future use in an actuator. Hy- draulic systems provide a significantly more accurate and adjustable means to transmit energy than electrical or mechanical systems. In general, hydraulic systems are reliable, efficient, and cost effective, leading to their wide use in the industrial world. The fluid power industry is constantly improving the cost effectiveness of hydraulic systems by employing new mechanical components and materials of construction.

Water and many other liquids can be utilized to make practical use of Pascal's Law, which states that a fluid compressed in a closed container will transmit the resulting pressure throughout the system undiminished and equal in all directions. While many innovations in hydraulic system hardware and controls have been commercialized in the past 50 years, there have been very few changes in the functionality of hydraulic fluids. There have been advances in base oil composition and additives that have improved fluid life and metals compatibility, however, fluids are not known to offer any advantages for work productivity or fuel economy. The most widely specified and purchased grades of fluids (ISO VG 46 and 68, HM performance grade) are based on Group I mineral oil and are very similar to those recommended over 50 years ago. Hydraulic fluid is not a critical design element of most hydraulic systems, it is typically the last system element selected, as it is assumed that a standard monograde oil will offer sufficient performance. Standard "HM" monograde oil is typically selected as it is the lowest cost option and has a long history of dependable performance with no maintenance issues. Outdoor applications of fluid power that experience wide variations in temperature will make use of lower viscosity grade fluids in the winter and higher viscosity grade fluids in the summer. Some hydraulic fluids are formulated with PAMA additives as viscosity index improvers, in order to achieve good low temperature fluidity properties under cold start-up conditions ("HV" grade oils). PAMA additives are not known to offer any other performance benefits.

E.g., the document WO 2005108531 describes the use of hydraulic fluids comprising PAMA additives in order to reduce the temperature increase of a hydraulic fluid under work load. However, an improvement with regard to the energy efficiency is not indicated or suggested by that document.

Additionally, the document WO 2005014762 discloses a functional fluid having an improved fire resistance. The fluid can be used in hydraulic systems. However, the document is silent with regard to the energy efficiency of the fluid.

The improvement of energy efficiency is a common object in all fields of technology. Usually such objects are achieved by construction improvements of the unit providing mechanical energy of the hydraulic system, e.g. a combustion engine or an electric motor. However, there is still a need for further improvements with regard to that object.

A further common object of the present invention is the improvement of the performance of a hydraulic system. Usually, the performance of a hydraulic system is improved by using a combustion engine or an electric motor having more power. However, such approach is usually connected with higher energy consumption. Taking into consideration the prior art, it is an object of this invention to provide hydraulic systems having an improved energy efficiency and an improved system performance.

Additionally, it is an object of the present invention to improve the life time of the unit providing mechanical power to the hydraulic system.

These as well as other not explicitly mentioned tasks, which, however, can easily be derived or developed from the introductory part, are achieved by the use of a fluid according to pre- sent claim 1. Expedient modifications of the use in accordance with the invention are described in the dependent claims.

The use of a fluid having a VI of at least 130 provides an unexpected improvement of the energy efficiency of a hydraulic system. Furthermore, the system performance of a hydraulic system can be improved in an unforeseeable manner.

At the same time a number of other advantages can be achieved through the hydraulic fluids in accordance with the invention. Among these are:

The hydraulic fluid of the present invention shows an improved low temperature performance and broader temperature operating window.

The hydraulic fluid of the present invention can be sold on a cost favorable basis with fast investment pay-back time.

The hydraulic fluid of the present invention exhibits good resistance to oxidation and is chemically very stable, compared to a standard HM fluid.

The viscosity of the hydraulic fluid of the present invention can be adjusted over a broad range. Furthermore, the hydraulic fluids of the present invention are appropriate for high pressure applications. The hydraulic fluids of the present invention show a minimal change in viscosity due to good shear stability.

Additionally, the improvement of the system performance and the energy efficiency can be achieved without constructional changes of the hydraulic system. Consequently, also the performance and energy efficiency of old hydraulic systems can be improved at very low costs.

The hydraulic fluid used according to the present invention has a viscosity index of at least 130, preferably at least 150, more preferably at least 180 and most preferably at least 200. According to a preferred embodiment of the present invention, the viscosity index is in the range of 150 to 400, more preferably 200 to 300. The viscosity index can be determined ac- cording to ASTM D 2270.

The use according to the present invention provides an improvement of the energy efficiency of a hydraulic system. The expression energy efficiency means a better effectiveness of the energy provided to the hydraulic system in order to achieve a defined result. Particu- larly, the energy consumption of the system may be lowered at least 5%, more preferably at least 10 % and more preferably at least 20%, based upon the energy consumption of a system using a monograde hydraulic fluid having a VI of about 100 and providing the same work or result of the system. The type of energy usually depends on the unit providing mechanical energy to the hydraulic system. Additionally, the energy consumption based upon a defined period of time can be improved.

Furthermore, the system performance of the hydraulic system can be improved. The expression system performance means the work productivity being done by the hydraulic system within a defined period of time. Particularly, the system performance can be improved at least 5%, more preferably at least 10 % and more preferably at least 20%. The type of work depends on the hydraulic system. In preferred systems, the work cycles per hour are improved. The improvement of energy consumption and system performance can be observed at all typical engine or electrical motor operating speeds. Preferentially, the improvement of energy consumption and system performance can be determined at the about 90% of the maximum performance of the unit providing mechanical energy to the hydraulic system, e.g. 90% throttle, if a combustion engine is used.

Preferentially, engine speed can be reduced to decrease load and stress while delivering the same amount of hydraulic power.

Fluids having a viscosity index of at least 130 are well known in the art. Usually, these fluids are used, e.g. in combustion engines and gears as a lubricant oil.

According to the consumer needs, the viscosity of the hydraulic fluid of the present invention can be adapted with in wide range. ISO VG 15, 22, 32, 46, 68, 100, 150 fluid grades can be achieved, e.g.

Preferably the kinematic viscosity 40⁰C according to ASTM D 445 of is the range of 15 mm²/s to 150 mm²/s, preferably 28 mm²/s to 110 mm²/s. For the use according to the present invention, preferred hydraulic fluids are NFPA (National Fluid Power Association) multigrade fluids, e.g. double, triple, quadra and/or penta grade fluids as defined by NFPA T2.13.13 -2002.

Preferred fluids comprise at least a mineral oil and/or a synthetic oil.

Mineral oils are substantially known and commercially available. They are in general obtained from petroleum or crude oil by distillation and/or refining and optionally additional purification and processing methods, especially the higher-boiling fractions of crude oil or petroleum fall under the concept of mineral oil. In general, the boiling point of the mineral oil is higher than 200⁰C, preferably higher than 300⁰C, at 5000 Pa. Preparation by low temperature distillation of shale oil, coking of hard coal, distillation of lignite under exclusion of air as well as hydrogenation of hard coal or lignite is likewise possible. To a small extent mineral oils are also produced from raw materials of plant origin (for example jojoba, rape- seed (canola), sunflower, and soybean oil) or animal origin (for example tallow or neat foot oil). Accordingly, mineral oils exhibit different amounts of aromatic, cyclic, branched and linear hydrocarbons, in each case according to origin.

In general, one distinguishes paraffin-base, naphthenic and aromatic fractions in crude oil or mineral oil, where the term paraffin-base fraction stands for longer-chain or highly branched isoalkanes and naphthenic fraction stands for cycloalkanes. Moreover, mineral oils, in each case according to origin and processing, exhibit different fractions of n-alkanes, isoalkanes with a low degree of branching, so called monomethyl-branched paraffins, and compounds with heteroatoms, especially O, N and/or S, to which polar properties are attributed. How- ever, attribution is difficult, since individual alkane molecules can have both long-chain branched and cycloalkane residues and aromatic components. For purposes of this invention, classification can be done in accordance with DIN 51 378. Polar components can also be determined in accordance with ASTM D 2007. The fraction of n-alkanes in the preferred mineral oils is less than 3 wt%, and the fraction of O, N and/or S-containing compounds is less than 6 wt%. The fraction of aromatic compounds and monomethyl-branched paraffins is in general in each case in the range of 0-40 wt%. In accordance with one interesting aspect, mineral oil comprises mainly naphthenic and paraffin-base alkanes, which in general have more than 13, preferably more than 18 and especially preferably more than 20 carbon atoms. The fraction of these compounds is in general at least 60 wt%, preferably at least 80 wt%, without any limitation intended by this. A preferred mineral oil contains 0.5-30 wt% aromatic components, 15-40 wt% naphthenic components, 35-80 wt% paraffin-base components, up to 3 wt% n-alkanes and 0.05-5 wt% polar components, in each case with respect to the total weight of the mineral oil.

An analysis of especially preferred mineral oils, which was done with traditional methods such as urea dewaxing and liquid chromatography on silica gel, shows, for example, the following components, where the percentages refer to the total weight of the relevant mineral oil: n-alkanes with about 18-31 C atoms: 0.7-1.0%, low-branched alkanes with 18-31 C atoms: 1.0-8.0%, aromatic compounds with 14-32 C atoms: 0.4-10.7%, iso- and cycloalkanes with 20-32 C atoms: 60.7-82.4%, polar compounds: 0.1-0.8%, loss: 6.9-19.4%.

Valuable advice regarding the analysis of mineral oil as well as a list of mineral oils that have other compositions can be found, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition on CD-ROM, 1997, under the entry "lubricants and related products."

Preferably, the hydraulic fluid is based on mineral oil from API Group I, II, or III. According to a preferred embodiment of the present invention, a mineral oil containing at least 90 % by weight saturates and at most about 0.03 % sulfur measured by elemental analysis is used. Especially, API Group II oils are preferred.

Synthetic oils are, among other substances, organic esters like carboxylic esters and phosphate esters; organic ethers like silicone oils and polyalkylene glycol; and synthetic hydrocarbons, especially polyolefins. They are for the most part somewhat more expensive than the mineral oils, but they have advantages with regard to performance. For an explanation reference is made to the 5 API classes of base oil types (API: American Petroleum Institute).

American Petroleum Institute (API) Base Oil Classifications

Synthetic hydrocarbons, especially polyolefins are well known in the art. Especially polyal- phaolefins (PAO) are preferred. These compounds are obtainable by polymerization of al- kenes, especially alkenes having 3 to 12 carbon atoms, like propene, hexene-1, octene-1, and dodecene-1. Preferred PAOs have a number average molecular weight in the range of 200 to 10000 g/mol, more preferably 500 to 5000 g/mol. According to a preferred aspect of the present invention, the hydraulic fluid may comprise an oxygen containing compound selected from the group of carboxylic acid esters, poly- ether polyols and/or organophosphorus compounds. Preferably, the oxygen containing compound is a carboxylic ester containing at least two ester groups, a diester of carboxylic acids containing 4 to 12 carbon atoms and/or a ester of a polyol. By using an oxygen containing compound as a basestock, the fire resistance of the hydraulic fluid can be improved.

Phosphorus ester fluids can be used as a component of the hydraulic fluid such as alkyl aryl phosphate ester; trialkyl phosphates such as tributyl phosphate or tri-2-ethylhexyl phosphate; triaryl phosphates such as mixed isopropylphenyl phosphates, mixed t-butylphenyl phosphates, trixylenyl phosphate, or tricresylphosphate. Additional classes of organophosphorus compounds are phosphonates and phosphinates, which may contain alkyl and/or aryl substituents. Dialkyl phosphonates such as di-2-elhylhexylphosphonate; alkyl phosphinates such as di-2-elhylhexylphosphinate are useful. As the alkyl group herein, linear or branched chain alkyls comprising 1 to 10 carbon atoms are preferred. As the aryl group herein, aryls comprising 6 to 10 carbon atoms that maybe substituted by alkyls are preferred. Especially, the hydraulic fluids may contain 0 to 60 % by weight, preferably 5 to 50% by weight organophosphorus compounds.

As the carboxylic acid esters reaction products of alcohols such as polyhydric alcohol, monohydric alcohol and the like, and fatty acids such as mono carboxylic acid, poly carboxylic acid and the like can be used. Such carboxylic acid esters can of course be a partial ester.

Carboxylic acid esters may have one carboxylic ester group having the formula R-COO-R, wherein R is independently a group comprising 1 to 40 carbon atoms. Preferred ester compounds comprise at least two ester groups. These compounds may be based on poly carboxylic acids having at least two acidic groups and/or polyols having at least two hydroxyl groups. The poly carboxylic acid residue usually has 2 to 40, preferably 4 to 24, especially 4 to 12 carbon atoms. Useful polycarboxylic acids esters are, e.g., esters of adipic, azelaic, sebacic, phthalate and/or dodecanoic acids. The alcohol component of the polycarboxylic acid com- pound preferably comprises 1 to 20, especially 2 to 10 carbon atoms.

Examples of useful alcohols are methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol and octanol. Furthermore, oxoalcohols can be used such as diethylene glycol, triethylene glycol, tetraethylene glycol up to decamethylene glycol.

Especially preferred compounds are esters of polycarboxylic acids with alcohols comprising one hydroxyl group. Examples of these compounds are described in Ullmanns Encyclopadie der Technischen Chemie, third edition, vol. 15, page 287 -292, Urban & Schwarzenber (1964)).

Useful polyols to obtain ester compounds comprising at least two ester groups contain usually 2 to 40, preferably 4 to 22 carbon atoms. Examples are neopentyl glycol, diethylene glycol, dipropylene glycol, 2,2-dimethyl-3-hydroxypropyl-2',2'-dimethyl-3'-hydroxy propionate, glycerol, trimethylolethane, trimethanol propane, trimethylolnonane, ditrimethylol- propane, pentaerythritol, sorbitol, mannitol and dipentaerythritol. The carboxylic acid component of the polyester may contain 1 to 40, preferably 2 to 24 carbon atoms. Examples are linear or branched saturated fatty acids such as formic acid, acetic acid, propionic acid, oc- tanoic acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid, capric acid, unde- canoic acid, lauric acid, tridecanoic acid, myrisric acid, pentadecanoic acid, palmitic acid, heptadecanoic acid, stearic acid, nonadecanoic acid, arachic acid, behenic acid, isomyiristic acid, isopalmitic acid, isostearic acid, 2,2-dimethylbutanoic acid, 2,2-dimethylpentanoic acid, 2,2-dimethyloctanoic acid, 2-ethyl-2.3,3-trimethylbutanoic acid, 2,2,3,4- tetramethylpentanoic acid, 2,5,5-trimethyl-2-t-butylhexanoic acid, 2,3,3-trimethyl-2- ethylbutanoic acid, 2,3-dimethyl-2-isopropylbutanoic acid, 2-ethylhexanoic acid, 3,5,5- trimethylhexanoic, acid; linear or branched unsaturated fatty such as linoleic acid, linolenic acid, 9 octadecenoic acid, undecenoic acid, elaidic acid, cetoleic acid, erucic acid, brassidic acid, and commercial grades of oleic acid from a variety of animal fat or vegetable oil sources. Mixtures of fatty acids such as tall oil fatty acids can be used.

Especially useful compounds comprising at least two ester groups are, e.g., neopentyl glycol tallate, neopentyl glycol dioleate, propylene glycol tallate, propylene glycol dioleate, di- ethylene glycol tallate, and diethylene glycol dioleate.

Many of these compounds are commercially available from Inolex Chemical Co. under the trademark Lexolube 2G-214, from Cognis Corp. under the trademark ProEco 2965, from Uniqema Corp. under the trademarks Priolube 1430 and Priolube 1446 and from Georgia Pacific under the trademarks Xtolube 1301 and Xtolube 1320.

Furthermore, ethers are useful as a component of the hydraulic fluid. Preferably, polyether polyols are used as a component of the hydraulic fluid of the present invention. These compounds are well known. Examples are polyalkylene glycols like, e.g., polyethylene glycols, polypropylene glycols and polybutylene glycols. The polyalkylene glycols can be based on mixtures of alkylene oxides. These compounds preferably comprise 1 to 40 alkylene oxide units, more preferably 5 to 30 alkylene oxide units. Polybutylene glycols are preferred com- pounds for anhydrous fluids. The polyether polyols may comprise further groups, like e.g., alkylene or arylene groups comprising 1 to 40, especially 2 to 22 carbon atoms.

According to another aspect of the present invention, the hydraulic fluid is based on a synthetic basestock comprising polyalphaolefin (PAO), carboxylic esters (diester, or polyol es- ter), a vegetable ester, phosphate ester (trialkyl, triaryl, or alkyl aryl phosphates), and/or polyalkylene glycol (PAG). Preferred synthetic basestocks are API Group IV and/or Group V oils.

Preferably, the hydraulic fluid is obtainable by mixing at least two components. At least one of the components shall be a base oil with a kinematic viscosity at 40⁰C according to ASTM D 445 of 35 mm²/s or less. Preferably, the hydraulic fluid comprises at least 60 % by weight of at least one component having a kinematic viscosity at 40⁰C according to ASTM D 445 of 35 mm²/s or less. Preferably, at least one of the components may have a viscosity index of 120 or less. According to a preferred embodiment, the hydraulic fluid may comprise at least 60 % by weight of at least one component having a viscosity index of 120 or less.

Particularly, a polymeric viscosity index improver can be used as a component of the hydraulic fluid. Viscosity index improvers are well known and, e.g. disclosed in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition on CD-ROM, 1997.

Preferred polymers useful as VI improvers comprise units derived from alkyl esters having at least one ethylenically unsaturated group. These polymers are well known in the art. Preferred polymers are obtainable by polymerizing, in particular, (meth)acrylates, maleates and fumarates. The term (meth)acrylates includes methacrylates and acrylates as well as mix- tures of the two. These monomers are well known in the art. The alkyl residue can be linear, cyclic or branched.

Mixtures to obtain preferred polymers comprising units derived from alkyl esters contain 0 to 100 wt%, preferably 0,5 to 90 wt%, especially 1 to 80 wt%, more preferably 1 to 30 wt%, more preferably 2 to 20 wt% based on the total weight of the monomer mixture of one or more ethylenically unsaturated ester compounds of formula (I)

where R is hydrogen or methyl, R¹ means a linear or branched alkyl residue with 1-6, especially 1 to 5 and preferably 1 to 3 carbon atoms, R² and R³ are independently hydrogen or a group of the formula -COOR, where R means hydrogen or an alkyl group with 1-6 carbon atoms. Examples of component (a) are, among others, (meth)acrylates, fumarates and maleates, which derived 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, pentyl (meth)acrylate and hexyl (meth)acrylate; cycloalkyl (meth)acrylates, like cyclopentyl (meth)acrylate.

Furthermore, the monomer compositions to obtain the polymers comprising units derived from alkyl esters contain 0 - 100 wt%, preferably 10-99 wt%, especially 20-95 wt% and more preferably 30 to 85 wt% based on the total weight of the monomer mixture of one or more ethylenically unsaturated ester compounds of formula (II)

where R is hydrogen or methyl, R⁴ means a linear or branched alkyl residue with 7-40, especially 10 to 30 and preferably 12 to 24 carbon atoms, R⁵ and R⁶ are independently hydrogen or a group of the formula -COOR", where R" means hydrogen or an alkyl group with 7 to 40, especially 10 to 30 and preferably 12 to 24 carbon atoms.

Among these are (meth)acrylates, fumarates and maleates that 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, 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, 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 3-vinylcyclohexyl (meth)acrylate, cyclohexyl (meth)acrylate, bornyl (meth)acrylate, 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.

The ester compounds with a long-chain alcohol residue, especially component (b), can be obtained, for example, by reacting (meth)acrylates, fumarates, maleates and/or the corresponding acids with long chain fatty alcohols, where in general a mixture of esters such as (meth)acrylates with different long chain alcohol residues results. These fatty alcohols include, among others, Oxo Alcohol® 7911 and Oxo Alcohol ® 7900, Oxo Alcohol® 1100 (Monsanto); Alphanol® 79 (ICI); Nafol® 1620, Alfol® 610 and Alfol® 810 (Sasol); Epal® 610 and Epal® 810 (Ethyl Corporation); Linevol® 79, Linevol® 911 and Dobanol® 25L (Shell AG); Lial 125 (Sasol); Dehydad® and Dehydad® and Lorol® (Cognis).

Of the ethylenically unsaturated ester compounds, the (meth)acrylates are particularly preferred over the maleates and furmarates, i.e., R², R³, R⁵, R⁶ of formulas (I) and (II) represent hydrogen in particularly preferred embodiments.

In a particular aspect of the present invention, preference is given to using mixtures of ethylenically unsaturated ester compounds of formula (II), and the mixtures have at least one (meth)acrylate having from 7 to 15 carbon atoms in the alcohol radical and at least one (meth) acrylate having from 16 to 30 carbon atoms in the alcohol radical. The fraction of the (meth)acrylates having from 7 to 15 carbon atoms in the alcohol radical is preferably in the range from 20 to 95% by weight, based on the weight of the monomer composition for the preparation of polymers. The fraction of the (meth)acrylates having from 16 to 30 carbon atoms in the alcohol radical is preferably in the range from 0.5 to 60% by weight based on the weight of the monomer composition for the preparation of the polymers comprising units derived from alkyl esters. The weight ratio of the (meth)acrylate having from 7 to 15 carbon atoms in the alcohol radical and the (meth) acrylate having from 16 to 30 carbon at- oms in the alcohol radical is preferably in the range of 10: 1 to 1:10, more preferably in the range of 5:1 to 1,5:1.

Component (c) comprises in particular ethylenically unsaturated monomers that can co- polymerize with the ethylenically unsaturated ester compounds of formula (I) and/or (II).

Comonomers that correspond to the following formula are especially suitable for polymerization in accordance with the invention:

where Rl* and R2* independently are selected from the group consisting of hydrogen, halogens, CN, linear or branched alkyl groups with 1-20, preferably 1-6 and especially preferably 1-4 carbon atoms, which can be substituted with 1 to (2n+l) halogen atoms, where n is the number of carbon atoms of the alkyl group (for example CF3), α, β-unsaturated linear or branched alkenyl or alkynyl groups with 2-10, preferably 2-6 and especially preferably 2- 4 carbon atoms, which can be substituted with 1 to (2n-l) halogen atoms, preferably chlorine, where n is the number of carbon atoms of the alkyl group, for example CH₂=CCl-, cycloalkyl groups with 3-8 carbon atoms, which can be substituted with 1 to (2n-l) halogen atoms, preferably chlorine, where n is the number of carbon atoms of the cycloalkyl group; C(=Y*)R5*, C(=Y*)NR^6*R^7*, Y*C(=Y*)R^5*, SOR^5*, SO₂R^5*, OSO₂R^5*, NR^8*SO₂R^5*, PR^5* ₂, P(=Y*)R^5* ₂, Y*PR^5* ₂, Y*P(=Y*)R^5* ₂, NR^8* ₂, which can be quaternized with an additional R^8*, aryl, or heterocyclyl group, where Y* can be NR^8*, S or O, preferably O; R^5* is an alkyl group with 1-20 carbon atoms, an alkylthio group with 1-20 carbon atoms, OR¹⁵ (R¹⁵ is hydrogen or an alkali metal), alkoxy with 1-20 carbon atoms, aryloxy or heterocycly- loxy; R^6* and R^7* independently are hydrogen or an alkyl group with one to 20 carbon at- oms, or R^6* and R^7* together can form an alkylene group with 2-7, preferably 2-5 carbon atoms, where they form a 3-8 member, preferably 3-6 member ring, and R^8* is linear or branched alkyl or aryl groups with 1-20 carbon atoms; R3* and R4* independently are chosen from the group consisting of hydrogen, halogen (preferably fluorine or chlorine), alkyl groups with 1-6 carbon atoms and COOR^9*, where R^9* is hydrogen, an alkali metal or an alkyl group with 1-40 carbon atoms, or R^1* and R^3* can together form a group of the formula (CH₂)_n, which can be substituted with l-2n' halogen atoms or C1-C4 alkyl groups, or can form a group of the formula C(=O)-Y*-C(=O), where n' is from 2-6, preferably 3 or 4, and Y* is defined as before; and where at least 2 of the residues R^1*, R^2*, R^3* and R^4* are hydrogen or halogen.

The comonomers include, among others, hydroxyalkyl (meth)acrylates like 3-hydroxypropyl (meth)acrylate, 3,4-dihydroxybutyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2- hydroxypropyl (meth)acrylate, 2,5-dimethyl-l,6-hexanediol (meth)acrylate, 1,10-decanediol (meth)acrylate;

aminoalkyl (meth)acrylates and aminoalkyl (meth)acrylamides like N-(3- dimethylaminopropyl)methacrylamide, 3-diethylaminopentyl (meth)acrylate, 3- dibutylaminohexadecyl (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;

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;

carbonyl-containing (meth)acrylates like 2-carboxyethyl (meth)acrylate, carboxymethyl (meth)acrylate, oxazolidinylethyl (meth)acrylate,

N-methyacryloyloxy)formamide, acetonyl (meth)acrylate, N-methacryloylmorpholine, N- methacryloyl-2-pyrrolidinone, N-(2-methyacryloxyoxyethyl)-2-pyrrolidinone, N-(3- methacryloyloxypropyl)-2-pyrrolidinone, N-(2-methyacryloyloxypentadecyl(-2- pyrrolidinone, N-(3 -methacryloyloxyheptadecyl-2-pyrrolidinone;

(meth)acrylates of ether alcohols like tetrahydrofurfuryl (meth)acrylate, vinyloxyethoxyethyl (meth)acrylate, methoxyethoxyethyl (meth)acrylate, 1-butoxypropyl (meth)acrylate, 1- methyl-(2-vinyloxy)ethyl (meth)acrylate, cyclohexyloxymethyl (meth)acrylate, methoxy- methoxyethyl (meth)acrylate, benzyloxymethyl (meth)acrylate, furfuryl (meth)acrylate, 2- butoxyethyl (meth)acrylate, 2-ethoxyethoxymethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, ethoxylated (meth)acrylates, allyloxymethyl (meth)acrylate, 1-ethoxybutyl (meth)acrylate, methoxymethyl (meth)acrylate, 1-ethoxyethyl (meth)acrylate, ethoxymethyl (meth)acrylate;

(meth)acrylates of halogenated alcohols like 2,3-dibromopropyl (meth)acrylate, A- bromophenyl (meth)acrylate, l,3-dichloro-2-propyl (meth)acrylate, 2-bromoethyl (meth)acrylate, 2-iodoethyl (meth)acrylate, chloromethyl (meth)acrylate;

oxiranyl (meth)acrylate like 2, 3-epoxybutyl (meth)acrylate, 3,4-epoxybutyl (meth)acrylate, 10,11 epoxyundecyl (meth)acrylate, 2,3-epoxycyclohexyl (meth)acrylate, oxiranyl (meth)acrylates such as 10,11-epoxyhexadecyl (meth)acrylate, glycidyl (meth)acrylate;

phosphorus-, boron- and/or silicon-containing (meth)acrylates like 2- (dimethylphosphato)propyl (meth)acrylate, 2-(ethylphosphito)propyl (meth)acrylate, 2- dimethylphosphinomethyl (meth)acrylate, dimethylphosphonoethyl (meth)acrylate, diethyl- methacryloyl phosphonate, dipropylmethacryloyl phosphate, 2-(dibutylphosphono)ethyl (meth)acrylate, 2,3-butylenemethacryloylethyl borate, methyldiethoxymethacryloylethoxysil- iane, diethylphosphatoethyl (meth)acrylate;

sulfur-containing (meth)acrylates like ethylsulfinylethyl (meth)acrylate, 4-thiocyanatobutyl (meth)acrylate, ethylsulfonylethyl (meth)acrylate, thiocyanatomethyl (meth)acrylate, methyl- sulfinylmethyl (meth)acrylate, bis(methacryloyloxyethyl) sulfide; heterocyclic (meth)acrylates like 2-(l-imidazolyl)ethyl (meth)acrylate, 2-(4- morpholinyl)ethyl (meth)acrylate and l-(2-methacryloyloxyethyl)-2-pyrrolidone;

vinyl halides such as, for example, vinyl chloride, vinyl fluoride, vinylidene chloride and vi- nylidene 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 α-methylstyrene and α-ethylstyrene, substituted styrenes with an alkyl substituent on the ring such as vinyltoluene and p-methylstyrene, halo- genated styrenes such as monochlorostyrenes, dichlorostyrenes, tribromostyrenes and tetrabromostyrenes;

heterocyclic vinyl compounds like 2-vinylpyridine, 3-vinylpyridine, 2-methyl-5- vinylpyridine, 3-ethyl-4-vinylpyridine, 2,3-dimethyl-5-vinylpyridine, vinylpyrimidine, vi- nylpiperidine, 9-vinylcarbazole, 3-vinylcarbazole, 4-vinylcarbazole, 1-vinylimidazole, 2- methyl-1-vinylimidazole, N-vinylpyrrolidone, 2-vinylpyrrolidone, N-vinylpyrrolidine, 3- vinylpyrrolidine, N-vinylcaprolactam, N- vinylbutyro lactam, vinyloxolane, vinylfuran, vinyl- thiophene, vinylthiolane, vinylthiazoles and hydrogenated vinylthiazoles, vinyloxazoles and hydrogenated vinyloxazoles;

vinyl and isoprenyl ethers;

maleic acid derivatives such as maleic anhydride, methylmaleic anhydride, maleinimide, me- thylmaleinimide;

fumaric acid and fumaric acid derivatives such as, for example, mono- and diesters of fu- marie acid. Monomers that have dispersing functionality can also be used as comonomers. These monomers are well known in the art and contain usually hetero atoms such as oxygen and/or nitrogen. For example the previously mentioned hydroxyalkyl (meth)acrylates, ami- noalkyl (meth)acrylates and aminoalkyl (meth)acrylamides, (meth)acrylates of ether alcohols, heterocyclic (meth)acrylates and heterocyclic vinyl compounds are considered as dispersing comononers.

Especially preferred mixtures contain methyl methacrylate, lauryl methacrylate and/or stearyl methacrylate.

The components can be used individually or as mixtures.

The hydraulic fluid of the present invention preferably comprises polyalkylmethacrylate polymers. These polymers obtainable by polymerizing compositions comprising alkyl- methacrylate monomers are well known in the art. Preferably, these polyalkylmethacrylate polymers comprise at least 40 % by weight, especially at least 50 % by weight, more preferably at least 60 % by weight and most preferably at least 80 % by weight methacrylate repeating units. Preferably, these polyalkylmethacrylate polymers comprise C₉-C24 methacry- late repeating units and Ci-Cs methacrylate repeating units.

The molecular weight of the polymers derived from alkyl esters is not critical. Usually the polymers derived from alkyl esters have a molecular weight in the range of 300 to 1,000,000 g/mol, preferably in the range of range of 10000 to 200,000 g/mol and more preferably in the range of 25000 to 100,000 g/mol, without any limitation intended by this. These values refer to the weight average molecular weight of the polymers.

Without intending any limitation by this, the alkyl(meth)acrylate polymers exhibit a polydis- persity, 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, preferably 1.1 to 10, especially prefera- bly 1.2 to 5. The polydispersity may be determined by gel permeation chromatography (GPC).

The monomer mixtures described above can be polymerized by any known method. Con- ventional radical initiators can be used to perform a classic radical polymerization. These initiators are well known in the art. Examples for these radical initiators are azo initiators like 2,2'-azodiisobutyronitrile (AIBN), 2,2'-azobis(2-methylbutyronitrile) and 1,1 azo- biscyclohexane carbonitrile; peroxide compounds, e.g. methyl ethyl ketone peroxide, acetyl acetone peroxide, dilauryl peroxide, tert. -butyl per-2-ethyl hexanoate, ketone peroxide, methyl isobutyl ketone peroxide, cyclohexanone peroxide, dibenzoyl peroxide, tert.-butyl perbenzoate, tert.-butyl peroxy isopropyl carbonate, 2,5-bis(2-ethylhexanoyl-peroxy)-2,5- dimethyl hexane, tert.-butyl peroxy 2-ethyl hexanoate, tert.-butyl peroxy- 3,5,5-trimethyl hexanoate, dicumene peroxide, 1,1 bis(tert. butyl peroxy) cyclohexane, 1,1 bis(tert. butyl peroxy) 3,3,5-trimethyl cyclohexane, cumene hydroperoxide and tert.-butyl hydroperoxide.

Low molecular weight poly(meth)acrylates can be obtained by using chain transfer agents. This technology is ubiquitously known and practiced in the polymer industry and is described in Odian, Principles of Polymerization, 1991. Examples of chain transfer agents are sulfur containing compounds such as thiols, e.g. n- and t - dodecanethiol, 2- mercaptoethanol, and mercapto carboxylic acid esters, e.g. methyl-3-mercaptopropionate. Preferred chain transfer agents contain up to 20, especially up to 15 and more preferably up to 12 carbon atoms. Furthermore, chain transfer agents may contain at least 1, especially at least 2 oxygen atoms.

Furthermore, the low molecular weight poly(meth)acrylates can be obtained by using transition metal complexes, such as low spin cobalt complexes. These technologies are well known and for example described in USSR patent 940,487-A and by Heuts, et al, Macro- molecules 1999, pp 2511-2519 and 3907-3912. Furthermore, novel polymerization techniques such as ATRP (Atom Transfer Radical Polymerization) and or RAFT (Reversible Addition Fragmentation Chain Transfer) can be applied to obtain useful polymers derived from alkyl esters. These methods are well known. The ATRP reaction method is described, for example, by J-S. Wang, et al, J. Am. Chem. Soc, Vol. 117, pp. 5614-5615 (1995), and by Matyjaszewski, Macromolecules, Vol. 28, pp. 7901-7910 (1995). Moreover, the patent applications WO 96/30421, WO 97/47661, WO 97/18247, WO 98/40415 and WO 99/10387 disclose variations of the ATRP explained above to which reference is expressly made for purposes of the disclosure. The RAFT method is extensively presented in WO 98/01478, for example, to which reference is ex- pressly made for purposes of the disclosure.

The polymerization can be carried out at normal pressure, reduced pressure or elevated pressure. The polymerization temperature is also not critical. However, in general it lies in the range of -20-200⁰C, preferably 0-130⁰C and especially preferably 60-120⁰C, without any limitation intended by this.

The polymerization can be carried out with or without solvents. The term solvent is to be broadly understood here.

According to a preferred embodiment, the polymer is obtainable by a polymerization in API Group II or Group III mineral oil. These solvents are disclosed above.

Furthermore, polymers obtainable by polymerization in a polyalphaolefin (PAO) are preferred. More preferably, the PAO has a number average molecular weight in the range of 200 to 10000, more preferably 500 to 5000. This solvent is disclosed above.

The hydraulic fluid may comprise 0.5 to 50 % by weight, especially 1 to 30 % by weight, and preferably 5 to 20% by weight, based on the total weight of the fluid, of one or more polymers derived from alkyl esters. According to a preferred embodiment of the present in- vention, the hydraulic fluid comprises at least 10 % by weight of one or more polymers derived from alkyl esters.

According to a preferred aspect of the present invention, the fluid may comprise at least two polymers having a different monomer composition. Preferably, at least one of the polymers is a polyolefin. Preferably, the polyolefin is useful as a viscosity index improver.

These polyolefins include in particular polyolefin copolymers (OCP) and hydrogenated sty- rene/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 and/or further olefins having 5 to 20 carbon atoms. 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 oxida- tion 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 addi- tion 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 iso- prene/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 10000 to 300 000, preferably between 50000 and 150000. According to a particular aspect of the present invention, the proportion of double bonds after the hydro- genation 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.

Preferably, at least one of the polymers of the mixture comprises units derived from monomers selected from acrylate monomers, methacrylate monomers, fumarate monomers and/or maleate monomers. These polymers are described above.

The weight ratio of the polyolefin and the polymer comprises units derived from monomers selected from acrylate monomers, methacrylate monomers, fumarate monomers and/or maleate monomers may be in the range of 1:10 to 10:1, especially 1:5 to 5:1.

The hydraulic fluid may comprise usual additives. These additive include e.g. antioxidants, antiwear agents, corrosion inhibitors and/or defoamers, often purchased as a commercial additive package. Preferably, the hydraulic fluid has a viscosity according to ASTM D 445 at 40°C in the range of 10 to 120 mm²/s, more preferably 22 to 100 mm²/s.

Preferably, the hydraulic system includes the following components:

1. A unit creating mechanical energy, e.g. a combustion engine or an electrical motor.

2. A fluid flow or force-generating unit that converts mechanical energy into hydraulic energy, such as a pump.

3. Piping for transmitting fluid under pressure. 4. A unit that converts the hydraulic energy of the fluid into mechanical work, such as an actuator or fluid motor. There are two types of motors, cylindrical and rotary.

5. A control circuit with valves that regulate flow, pressure, direction of movement, and applied forces.

6. A fluid reservoir that allows for separation of water, foam, entrained air, or debris be- fore the clean fluid is returned to the system through a filter.

7. A liquid with low compressibility capable of operating without degradation under the conditions of the application (temperature, pressure, radiation).

Most complex systems will make use of multiple pumps, rotary motors, cylinders, electroni- cally controlled with valves and regulators.

The system may be operated at high pressures. The improvement of the present invention can be achieved at pressures in the range of 50 to 500 bar, preferably 100 to 350 bar.

Preferably, the fluid is used in military hydraulic systems, in hydraulic launch assist systems, in industrial, marine, mining and/or mobile equipment hydraulic systems.

Furthermore, the present invention provides a hydraulic system comprising a hydraulic fluid having a VI of at least 130, a unit for creating mechanical power, a unit that converts me- chanical power into hydraulic energy, and a unit that converts hydraulic energy into mechanical work.

Preferentially, engine speed can be reduced to decrease load and stress while delivering the same amount of hydraulic power. Preferably, the the mechanical power output of the engine or electrical motor can be operated at less than 98% of its full power capacity to deliver the same amount of hydraulic power as the hydraulic system utilizing an HM grade fluid with a viscosity index less than 120.

The invention is illustrated in more detail below by examples and comparison examples, without intending to limit the invention to these examples.

Examples and comparative examples

A field test was conducted comparing fuel consumption rates in a 2001 Caterpillar model 318CL hydraulic excavator. Hydraulic flow is generated by a dual piston pump feeding 2 rotary motors to drive the tracks, 1 rotary motor to power the swivel, and 3 linear actuators to power the boom, swivel and bucket. A standardized work protocol was developed which involved moving a pile of loose earth 100 feet. The excavator takes a full scoop of earth, the cab rotates 180 degrees, and travels at full speed in a straight line to dump the bucket. After dumping the load, the cab rotates back 180 degrees and returns in the same track back to the starting point. This completes one work cycle. Diesel fuel consumption was measured for a work day that progressed according to the following schedule:

Field Test Procedures:

15 minute warm up

15 minute break to fill fuel tank to top of fill neck

Hour 1- 55 minutes of work following the standard cycle protocol

5 minute operator break Hour 2- 55 minutes of work following the standard cycle protocol 5 minute operator break

Hour 3- 55 minutes of work following the standard cycle protocol 5 minute operator break Break - 1 Hour Fill fuel tank to top of fill neck, record fuel weight addition to +0.1 grams Hour 4- 55 minutes of work following the standard cycle protocol 5 minute operator break

Hour 5- 55 minutes of work following the standard cycle protocol 5 minute operator break Hour 6- 55 minutes of work following the standard cycle protocol 5 minute operator break Fill fuel tank to top of fill neck, record fuel weight addition to ±0.1 grams

Operating baseline measurements were made with the excavator using the Caterpillar brand monograde oil recommended and sold by Caterpillar for their hydraulic excavators (Caterpillar HYDO 1OW, NFPA viscosity grade L46-46). The total number of work cycles and total fuel consumption were recorded. These test runs were considered as comparative examples. Comparative example 1 was performed at full throttle (2200 rpm) and comparative example 2 was performed at 90% throttle (2000 rpm). The results are represented in Table 1.

For the examples, the monograde oil was then exchanged for "HV" multigrade oil following a triple flush procedure. The multigrade oil was an NFPA grade L32-100, which is an appropriate replacement for this monograde oil according to NFPA recommended practice T2.13.13-2002. The same work protocol was then followed at both full throttle and 90% throttle, measuring the total number of work cycles and total fuel consumption. The example 1 was performed at full throttle and example 2 was performed at 90% throttle. The results are shown in Table 1.

The NFPA quadra grade fluid used in this test was formulated from a blend of Group I and Group II mineral oil, plus the Dynavis® additive system (from Degussa — RohMax Oil Addi- tives). The Dynavis*¹ additive system consists of a shear stable polyalkylmethacrylate viscosity index improver (VISCOPLEX® 8-219) manufactured and diluted in Group II mineral oil, The formulation contained Group II PAMA additive at 16 weight percent, and a zinc based antiwear package at 0.8 weight percent, which enables the fluid to meet the major global performance standards. This extremely high treat rate of PAMA is unusual and not found in any commercial hydraulic fluid.

To conclude the test, the NFPA multigrade oil was exchanged for the Caterpillar brand L46-46 monograde oil, and the baseline runs were re-measured at both full and 90% throttle. The second baseline runs were a good match with first baseline runs. Table 1

The data gathered in the field test demonstrate that the "HV" multigrade oil formulated with Group II PAMA was responsible for both increased productivity (+5.8 to 24.3%) and reduced daily fuel consumption (-8.6 to -13.8%). When fuel consumption per work cycle is considered, the "HV" multigrade fluid produced a 18.4% to 26.3% reduction in the total amount of fuel required to achieve an equivalent amount of work. Especially the improvement based on the same amount of work is very astonishing. This improvement is also achieved at 90% throttle. Furthermore, it was not foreseeable, that the productivity at 90% throttle can be improved in the manner shown by the examples.

Claims

1. A use of a fluid having a VI of at least 130 to improve the energy efficiency of a hydraulic system.

2. The use according to claim 1, wherein the energy consumption is lowered at least

5%.

3. The use according to claim 1 or 2, wherein the system performance of the hydraulic system is improved.

4. The use according to claim 3, wherein the system performance is improved at least

5%.

5. The use according to at least one of the preceding claims, wherein the life time of a unit providing mechanical power to the hydraulic system is improved.

6. The use according to claim 5, wherein the engine speed is reduced to decrease load and stress while delivering the same amount of hydraulic power.

7. The use according to at least one of the preceding claims, wherein the fluid has a VI of at least 150.

8. The use according to claim 7, wherein the has a VI of at least 180.

9. The use according to at least one of the preceding claims, wherein the fluid is a NFPA double viscosity grade, triple viscosity grade, quadra viscosity grade, or penta viscosity grade hydraulic fluid.

10. The use according to at least one of the preceding claims, wherein the fluid is obtainable by mixing a base fluid and a polymeric viscosity index improver.

11. The use according to claim 10, wherein the base fluid has a kinematic viscosity at 40⁰C according to ASTM D 445 of 35 mm²/s or less.

12. The use according to claim 11, wherein the fluid comprises at least 60 % by weight of at least one base fluid having a kinematic viscosity at 40⁰C according to ASTM D 445 of 35 IM²Zs or less.

13. The use according to at least one of the claims claim 10 to 12, wherein the base fluid has a viscosity index of 120 or less.

14. The use according to claim 13, wherein the fluid comprises at least 60 % by weight of at least one base fluid having a viscosity index of 120 or less.

15. The use according to at least one of the preceding claims, wherein the fluid comprises a mineral oil and/or a synthetic oil.

16. The use according to claim 15, wherein the fluid comprises a API group I, API group II, API group III oil, a API group IV or API group V oil.

17. The use according to at least one of the preceding claims, wherein the fluid comprises a polyalphaolefin (PAO), a carboxylic ester, a vegetable ester, a phosphate es- ter and/or a polyalkylene glycol (PAG).

18. The use according to at least one of the preceding claims, wherein the fluid comprises at least one polymer.

19. The use according to claim 16, wherein the polymer comprises units derived from monomers selected from acrylate monomers, methacrylate monomers, fumarate monomers and/or maleate monomers.

20. The use according to claim 19, wherein the fluid comprises a polyalkylmethacrylate polymer.

21. The use according to at least one of the claims 18 to 20, wherein the fluid comprises a polymer obtainable by polymerizing a mixture of olefinically unsaturated mono- mers, which consists of a) 0-100 wt% based on the total weight of the ethylenically unsaturated monomers of one or more ethylenically unsaturated ester compounds of formula (I)

where R is hydrogen or methyl, R¹ means a linear or branched alkyl residue with 1-6 carbon atoms, R² and R³ independently represent hydrogen or a group of the for- mula -COOR, where R means hydrogen or a alkyl group with 1-6 carbon atoms,

b) 0-100 wt% based on the total weight of the ethylenically unsaturated monomers of one or more ethylenically unsaturated ester compounds of formula (II)

where R is hydrogen or methyl, R⁴ means a linear or branched alkyl residue with 7- 40 carbon atoms, R⁵ and R⁶ independently are hydrogen or a group of the formula -

COOR", where R" means hydrogen or an alkyl group with 7-40 carbon atoms, c) 0-50 wt% based on the total weight of the ethylenically unsaturated monomers comonomers.

22. The use according to at least one of the claims 18 to 21, wherein the polymer is ob- tainable by a polymerisation in a API group II or group III mineral oil.

23. The use according to at least one of the claims 18 to 22, wherein the polymer is obtainable by a polymerisation in a polyalphaolefin (PAO).

24. The use according to claim 23, wherein the PAO has a molecular weight in the range of 200 to 10000 g/mol.

25. The use according to one of the claims 18 to 24, wherein the polymers are obtainable by polymerizing a mixture comprising dispersant monomers.

26. The use according to one of the claims 18 to 25, wherein the polymers are obtainable by polymerizing a mixture comprising vinyl monomers containing aromatic groups.

27. The use according to at least one of the claims 18 to 26, wherein the polymer has a molecular weight in the range of 10000 to 200000 g/mol, specifically 25000 g/mol to 100000 g/mol.

28. The use according to at least one of the claims 18 to 27, wherein the fluid comprises 0.5 to 40% by weight polymer.

29. The use according to claim 28, wherein the fluid comprises 10 to 30% by weight polymer.

30. The use according to at least one of the claims 18 to 29, wherein the fluid comprises at least two polymers having a different monomer composition.

31. The use according to claim 30, wherein at least one of the polymers is a polyolefin.

32. The use according to claim 31, wherein at least one of the polymers comprises units derived from alkyl ester monomers.

33. The use according to claim 32, wherein the weight ratio of the polyolefin and the polymer comprises units derived from alkyl ester monomers is in the range of 1 : 10 to 10: 1.

34. The use according to at least one of the preceding claims, wherein the fluid comprises an oxygen containing compound selected from the group of carboxylic acid esters, polyether polyols and/or organophosphorus compounds.

35. The use according to claim 34, wherein the oxygen containing compound is a carboxylic ester containing at least two ester groups.

36. The use according to claim 34 or 35, wherein the oxygen containing compound is a diester of carboxylic acids containing 4 to 12 carbon atoms.

37. The use according to claim 36, wherein the oxygen containing compound is a ester of a polyol.

38. The use according to at least one of the preceding claims, wherein the fluid has an ISO viscosity grade in the range of 15 to 150.

39. The use according to at least one of the preceding claims, wherein the fluid is used at a temperature in the range of -40⁰C to 120⁰C.

40. The use according to at least one of the preceding claims, wherein the fluid comprises antioxidants, antiwear agents, corrosion inhibitors and/or defoamers.

41. The use according to at least one of the preceding claims, wherein the fluid is used in military hydraulic systems, in hydraulic launch assist systems, in industrial, marine, mining and/or mobile equipment hydraulic systems.

42. The use according to at least one of the preceding claims, wherein the hydraulic sys- tern comprises at least one unit providing mechanical energy, at least one unit that converts mechanical energy into hydraulic energy, at least one pipe for transmitting fluid under pressure and at least a unit that converts the hydraulic energy of the fluid into mechanical work.

43. The use according to claim 42, wherein the unit providing mechanical energy comprises a combustion engine.

44. The use according to at least one of the preceding claims, wherein the speed or power output of the unit can be controlled and adjusted.

45. A hydraulic system comprising a hydraulic fluid having a VI of at least 130, a unit for creating mechanical power, a unit that converts mechanical power into hydraulic energy, and a unit that converts hydraulic energy into mechanical work.

46. The hydraulic system according to claim 45, wherein the engine speed can be reduced to decrease load and stress while delivering the same amount of hydraulic power.

47. The hydraulic system according to claim 45 or 46 or the use according to claims 42 to 44 wherein the mechanical power output of the engine or electrical motor can be operated at less than 98% of its full power capacity to deliver the same amount of hydraulic power as the hydraulic system utilizing an HM grade fluid with a viscosity index less than 120.