Compressor oils with high viscosity index

Abstract

Polyalkyl (meth)acrylates are useful in compressor oils. A method of increasing the energy efficiency of a compressor involves operating the compressor with a compressor oil containing a polyalkyl (meth)acrylate-based viscosity index improver.

Claims

1. A method of increasing the energy efficiency of an air compressor, the method comprising: operating the air compressor with a compressor oil, wherein the compressor oil comprises: (i) 1 wt. % to 30 wt. % of a polyalkyl methacrylate-based viscosity index improver consisting of: (a) 0 wt. % to 25 wt. % of methyl methacrylate; (b) 75 wt. % to 100 wt. % of at least one straight-chained or branched C10-18 alkyl (meth)acrylate; and (c) 0 wt. % to 2 wt. % of at least one straight-chained or branched C5-9 alkyl (meth)acrylate or at least one straight-chained or branched C20-24 alkyl (meth)acrylate, wherein a weight average molecular weight (Mw) of the polyalkyl (meth)acrylate-based viscosity index improver is in a range of 5,000 g/mol to 400,000 g/mol; (ii) 70 wt. % to 99 wt. % of a base oil, wherein the base oil is an oil of American Petroleum Institute (API) group II, III, IV, or V, or a mixture thereof; and (iii) optionally, up to 2.5 wt. % of a performance package comprising one or more further additives, wherein the compressor oil has a viscosity index of at least 140.

2. The method according to claim 1, wherein the polyalkyl methacrylate-based viscosity index improver consists of: (a) 0.2 wt. % to 25 wt. % of the methyl methacrylate; (b) 75 wt. % to 99.8 wt. % of the at least one C10-18 alkyl (meth)acrylate; and (c) 0 wt. % to 2 wt. % of the at least one straight-chained or branched C5-9 alkyl (meth)acrylate or the at least one straight-chained or branched C20-24 alkyl (meth)acrylate.

3. The method according to claim 1, wherein the weight average molecular weight (Mw) of the polyalkyl (meth)acrylate-based viscosity index improver is in a range of 5,000 g/mol to 200,000 g/mol.

4. The method according to claim 1, wherein the performance package (iii) comprises at least an antiwear agent, an anticorrosion agent, and an antioxidant.

5. The method according to claim 1, wherein the base oil (ii) is an API group II, III, or IV oil, or a mixture thereof, and the compressor oil has a kinematic viscosity at 40 C. in a range of 28.8 and 74.8 cSt.

6. The method according to claim 1, comprising: operating the air compressor with the compressor oil, wherein the compressor oil comprises: (i) 1 wt. % to 20 wt. % of the polyalkyl methacrylate-based viscosity index improver consisting of: (a) 0.2 wt. % to 25 wt. % of the methyl methacrylate; (b) 75 wt. % to 99.8 wt. % of the at least one C10-18 alkyl (meth)acrylate; and (c) 0 wt % to 2 wt. % of the at least one straight-chained or branched C5-9 alkyl (meth)acrylate or the at least one straight-chained or branched C20-24 alkyl (meth)acrylate, wherein the weight average molecular weight (Mw) of the polyalkyl (meth)acrylate-based viscosity index improver is in the range of 5,000 to 400,000 g/mol; (ii) 80 wt. % to 99 wt. % of an API group II, III, or IV base oil, or a mixture thereof; and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent, and an antioxidant, wherein the compressor oil has a kinematic viscosity at 40 C. in the range of 28.8 and 74.8 cSt and a viscosity index of at least 140.

7. The method according to claim 5, wherein the compressor oil has a pour point of 33 C. or lower.

8. The method according to claim 1, wherein the compressor oil has a viscosity index of at least 180.

9. The method according to claim 2, wherein the polyalkyl methacrylate-based viscosity index improver consists of: (a) 4 wt. % to 16 wt. % of the methyl methacrylate; (b) 84 wt. % to 96 wt. % of the at least one C10-18 alkyl methacrylate; and (c) 0 wt. % to 2 wt. % of the at least one straight-chained or branched C5-9 alkyl (meth)acrylate or the at least one straight-chained or branched C20-24 alkyl (meth)acrylate.

10. The method according to claim 4, wherein the performance package (iii) is zinc-free.

11. The method according to claim 4, wherein the performance package (iii) is ashless.

12. The method according to claim 1, wherein the compressor oil comprises: (i) 1 wt. % to 20 wt of the polyalkyl methacrylate-based viscosity index improver; (ii) 80 wt. % to 99 wt. % of the base oil; and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent and an antioxidant.

13. The method according to claim 1, wherein the compressor oil comprises: (i) 1 wt. % to 15 wt. % of the polyalkyl methacrylate-based viscosity index improver; (ii) 85 wt. % to 99 wt. % of the base oil; and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent and an antioxidant.

14. The method according to claim 1, wherein the compressor oil comprises: (i) 1 wt. % to 10 wt. % of the polyalkyl methacrylate-based viscosity index improver; (ii) 90 wt. % to 99 wt. % of the base oil; and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent and an antioxidant.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The FIGURE shows the test settings used to determine the effects on energy consumption in an air compressor.

DETAILED DESCRIPTION OF THE INVENTION

(2) An object of the present invention is directed to a method of increasing the energy efficiency of a compressor, comprising operating a compressor with a compressor oil, characterized in that the compressor oil comprises: (i) 1 wt. % to 30 wt. % of a polyalkyl methacrylate-based viscosity index improver comprising: (a) 0 wt. % to 25 wt. % of methyl methacrylate; (b) 75 wt. % to 100 wt. % of straight-chained or branched C10-18 alkyl (meth)acrylates; and (c) 0 wt. % to 2 wt. % of straight-chained or branched C5-9 alkyl (meth)acrylates or straight-chained or branched C20-24 alkyl (meth)acrylates, wherein the weight average molecular weight (M.sub.w) of the polyalkyl (meth)acrylate-based viscosity index improver is in the range of 5,000 to 400,000 g/mol; (ii) 70 wt. % to 99 wt. % of a base oil selected from API group II, III, IV and V and mixtures thereof, and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent and an antioxidant,
wherein the compressor oil has a viscosity index of at least 140, preferably at least 160, more preferably at least 180.

(3) In a further object, the compressor oil comprises: (i) 1 wt. % to 20 wt. %, preferably 1 wt. % to 15 wt. %, preferably 1 wt. % to 10 wt. %, of a polyalkyl methacrylate-based viscosity index improver as outlined further above; (ii) 80 wt. % to 99 wt. %, preferably 85 wt. % to 99 wt. %, preferably 90 wt. % to 99 wt. %, of a base oil selected from API group II, III, IV and V and mixtures thereof; and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent and an antioxidant.

(4) In a further object, the polyalkyl methacrylate-based viscosity index improver comprises: (a) 0.2 wt. % to 25 wt. %, preferably 4 wt. % to 16 wt. %, of methyl methacrylate; (b) 75 wt. % to 99.8 wt. %, preferably 84 wt. % to 96 wt. % of straight-chained or branched C10-18 alkyl methacrylates; and (c) 0 wt. % to 2 wt. % of straight-chained or branched C5-9 alkyl (meth)acrylates or straight-chained or branched C20-24 alkyl (meth)acrylates.

(5) The content of each component (i), (ii) and (iii) is based on the total composition of the compressor oil. In a particular embodiment, the proportions of components (i), (ii) and (iii) add up to 100% by weight.

(6) The content of each component (a), (b) and (c) is based on the total composition of the polyalkyl (meth)acrylate-based viscosity index improver. The proportions of components (a), (b) and (c) add up to 100% by weight.

(7) The weight-average molecular weight M.sub.w of the polyalkyl acrylate polymers according to the present invention is preferably at least 5,000 g/mol or 8,000 g/mol or 10,000 g/mol or 30,000 g/mol and preferably at most 400,000 g/mol or 200,000 g/mol or 100,000 g/mol or 80,000 g/mol; for example in the range of 5,000 g/mol to 400,000 g/mol, preferably in the range of 5,000 g/mol to 200,000 g/mol or 5,000 g/mol to 100,000 g/mol or 8,000 g/mol to 100,000 g/mol or 10,000 g/mol to 200,000 g/mol or 30,000 g/mol to 100,000 g/mol or 10,000 g/mol to 80,000 g/mol.

(8) M.sub.w is determined by size exclusion chromatography (SEC) using commercially available polymethylmethacrylate standards. The determination is affected by gel permeation chromatography with THF as eluent.

(9) The term (meth)acrylate refers to both, esters of acrylic acid and esters of methacrylic acid. In accordance with the present invention, methacrylates are preferred.

(10) The C.sub.5-9-alkyl (meth)acrylates for use in accordance with the invention are esters of (meth)acrylic acid and straight-chained or branched alcohols having 5 to 9 carbon atoms. The term C.sub.5-9-alkyl (meth)acrylates encompasses individual (meth)acrylic esters with an alcohol of a particular length, and likewise mixtures of methacrylic esters with alcohols of different lengths.

(11) Suitable C.sub.5-9-alkyl (meth)acrylates include, for example, pentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, 2-ethylhexyl (meth)acrylate and nonyl (meth)acrylate.

(12) The C.sub.10-18 alkyl (meth)acrylates for use in accordance with the invention are esters of (meth)acrylic acid and straight chain or branched alcohols having 10 to 18 carbon atoms. The term C.sub.10-18 alkyl (meth)acrylates encompasses individual (meth)acrylic esters with an alcohol of a particular length, and likewise mixtures of (meth)acrylic esters with alcohols of different lengths.

(13) Suitable C.sub.10-18 alkyl (meth)acrylates include, for example, 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, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate and octadecyl (meth)acrylate.

(14) The C.sub.20-24 alkyl (meth)acrylates for use in accordance with the invention are esters of (meth)acrylic acid and straight-chained alcohols having 20 to 24 carbon atoms. The term C.sub.20-24 alkyl (meth)acrylates encompasses individual (meth)acrylic esters with an alcohol of a particular length, and likewise mixtures of (meth)acrylic esters with alcohols of different lengths.

(15) Suitable straight-chained C.sub.20-24 alkyl (meth)acrylates include, for example, eicosyl (meth)acrylate and docosyl (meth)acrylate.

(16) The dispersant monomers for use in accordance with the invention are selected from the group consisting of hydroxyethyl methacrylate, N,N-dimethylaminoethyl methacrylate (DMAEMA), N-(3-(dimethylamino)propyl)methacrylamide (DMAPMAm) and N-vinylpyrrolidinone (NVP).

(17) For the synthesis of the polyalkyl(meth)acrylate-based viscosity index improver (i), the monomer mixtures described above can be polymerized by any known method. Conventional 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-azodiisobutyronitile (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.-butylper-2-ethyl hexanoate, ketone peroxide, methyl isobutyl ketone peroxide, cyclohexanone peroxide, dibenzoyl peroxide, tert.-butylper-benzoate, tert.-butylperoxy isopropyl carbonate, 2,5-bis(2-ethylhexanoyl-peroxy)-2,5-dimethyl hexane, tert.-butylperoxy 2-ethyl hexanoate, tert.-butylperoxy-3,5,5-trimethyl hexanoate, dicumene peroxide, 1,1 bis(tert.-butylperoxy) cyclohexane, 1,1 bis(tert.-butylperoxy) 3,3,5-trimethyl cyclohexane, cumene hydroperoxide and tert.-butyl hydroperoxide.

(18) Poly(meth)acrylates with a lower molecular weight can be obtained by using chain transfer agents. This technology is ubiquitously known and practiced in the polymer industry and is de-scribed in Odian, Principles of Polymerization, 1991.

(19) 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 expressly made for purposes of the disclosure.

(20) The polymerization can be carried out at normal pressure, reduced pressure or elevated pressure. The polymerization temperature is in the range of 20 to 200 C., preferably 60 to 120 C., 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 I, II or III mineral oil or in API group IV synthetic oil.

(21) The base oil to be used in the compressor oil comprises an oil of lubricating viscosity. Such oils include natural and synthetic oils, oils derived from hydrocracking, hydrogenation, and hydro-finishing, unrefined, refined, re-refined oils or mixtures thereof.

(22) The base oil may also be defined as specified by the American Petroleum Institute (API) (see April 2008 version of Appendix E-API Base Oil Interchangeability Guidelines for Passenger Car Motor Oils and Diesel Engine Oils, section 1.3 Subheading 1.3. Base Stock Categories).

(23) The API currently defines five groups of lubricant base stocks (API 1509, Annex EAPI Base Oil Interchangeability Guidelines for Passenger Car Motor Oils and Diesel Engine Oils, September 2011). Groups I, II and III are mineral oils which are classified by the amount of saturates and sulphur they contain and by their viscosity indices; Group IV are polyalphaolefins; and Group V are all others, including e.g. ester oils. The table below illustrates these API classifications.

(24) TABLE-US-00001 Group Saturates Sulphur content Viscosity Index (VI) I <90% >0.03% 80-120 II at least 90% not more than 0.03% 80-120 II at least 90% not more than 0.03% at least 120 IV Polyalphaolefins (PAOs) V All others not included in Groups I, II, III or IV (e.g. ester oils)

(25) The kinematic viscosity at 100 C. (KV.sub.100) of appropriate apolar base oils used to prepare a compressor oil in accordance with the present invention is preferably in the range of 1 mm/s to 20 mm/s, more preferably in the range of 2 mm/s to 10 mm/s, determined to ASTM D445.

(26) Particularly preferred compressor oils of the present invention comprise at least one base oil selected from the group consisting of API Group II oils, API Group III oils, polyalphaolefins (PAO) and mixtures thereof.

(27) Further base oils which can be used in accordance with the present invention are Group II-III Fischer-Tropsch derived base oils.

(28) Fischer-Tropsch derived base oils are known in the art. By the term Fischer-Tropsch derived is meant that a base oil is, or is derived from, a synthesis product of a Fischer-Tropsch process. A Fischer-Tropsch derived base oil may also be referred to as a GTL (Gas-To-Liquids) base oil. Suitable Fischer-Tropsch derived base oils that may be conveniently used as the base oil in the compressor oil of the present invention are those as for example disclosed in EP 0 776 959, EP 0 668 342, WO 97/21788, WO 00/15736, WO 00/14188, WO 00/14187, WO 00/14183, WO 00/14179, WO 00/08115, WO 99/41332, EP 1 029 029, WO 01/18156, WO 01/57166 and WO 2013/189951.

(29) The compressor oil used according to the present invention may also contain one or more further additives selected from the group consisting of pour point depressants, dispersants, defoamers, detergents, demulsifiers, antioxidants, antiwear additives, extreme pressure additives, friction modifiers, anticorrosion additives, metal deactivators and metal passivators and mixtures thereof; preferably antiwear additives, anticorrosion additives and antioxidants.

(30) The compressor oil used according to the present invention may preferably comprise up to 2.5% by weight, preferably 0.5% to 1.5% by weight, of a performance package containing at least an antiwear agent, an anticorrosion agent and an antioxidant.

(31) The performance package is preferably a zinc-free performance package, more preferably fully ashless.

(32) Preferred pour point depressants are, for example, selected from the group consisting of alkylated naphthalene and phenolic polymers, polyalkyl methacrylates, maleate copolymer esters and fumarate copolymer esters, which may conveniently be used as effective pour point depressants. The compressor oil may contain 0.1% by weight to 0.5% by weight of a pour point depressant. Preferably, not more than 0.3% by weight of a pour point depressant is used.

(33) Appropriate dispersants include poly(isobutylene) derivatives, for example poly(isobutylene)succinimides (PIBSIs), including borated PIBSIs; and ethylene-propylene oligomers having N/O functionalities. The compressor oil may contain 0.05% to 5% by weight of at least one dispersant, based on the total weight of the compressor oil.

(34) Suitable defoaming agents include, for example, silicone oils, fluorosilicone oils, and fluoroalkyl ethers. The compressor oil may contain 0.01% to 0.02% by weight of at least one defoaming agent, based on the total weight of the compressor oil.

(35) The detergents include metal-containing compounds, for example phenoxides; salicylates; thiophosphonates, especially thiopyrophosphonates, thiophosphonates and phosphonates; sulfonates and carbonates. As metal, these compounds may contain especially calcium, magnesium and barium. These compounds may preferably be used in neutral or overbased form.

(36) Preferred demulsifiers include alkyleneoxide copolymers and (meth)acrylates including polar functions.

(37) The suitable antioxidants include, for example, phenols, for example 2,6-di-tert-butylphenol (2,6-DTB), 2,6-di-tert-butyl-4-ethylphenol, 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-1-naphthylamine (PNA), N,N-di-phenyl-p-phenylenediamine, polymeric 2,2,4-trimethyldihydroquinone (TMQ); OOS triesters=reaction products of dithiophosphoric acid with activated double bonds from olefins, cyclopentadiene, norbornadiene, -pinene, polybutene, acrylic esters, maleic esters (ashless on combustion); organophosphorus compounds, for example triaryl and trialkyl phosphites; organocopper compounds and overbased calcium- and magnesium-based phenoxides and salicylates. The compressor oil may contain 0.05% to 5% by weight of at least one antioxidant, based on the total weight of the compressor oil.

(38) The preferred antiwear and extreme pressure 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 or phosphines. The compressor oil may contain 0.05% to 3% by weight of at least one antiwear and extreme pressure additive, based on the total weight of the compressor oil.

(39) Examples of the metal deactivators include triazoles, thiadiazoles and salicylidenes, like e.g. N,N-disalicyliden-1,2-diaminopropane.

(40) Rust inhibitors are widely used. Common chemistries are carboxylates like succinic acid half esters, sulfonates, alkyl amines and phosphates, e.g. amine neutralized phosphate esters.

(41) Friction modifiers used may include mechanically active compounds, for example molybdenum disulfide, graphite (including fluorinated graphite), poly(trifluoroethylene), polyamide, polyimide; compounds that 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 that form polymer-like layers, for example ethoxylated dicarboxylic partial esters, dialkyl phthalates, methacrylates, unsaturated fatty acids, and sulfurized olefins.

(42) All components being part of the formulation need to show acceptable compatibility with the refrigerant over a wide range of operating temperatures.

(43) The above-detailed additives are described in detail, inter alia, 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.

(44) The total concentration of the one or more additives in a compressor oil is up to 5% by weight, preferably 0.1% to 4% by weight, more preferably 0.5% to 3% by weight, based on the total weight of the compressor oil.

(45) A further object of the present invention is directed to the method of increasing the energy efficiency of a compressor as outlined further above, wherein the compressor is selected from the group consisting of household or domestic refrigeration units, air compressors and CO.sub.2 compressors.

(46) A further object of the present invention is directed to the method of increasing the energy efficiency of a compressor as outlined further above, wherein the compressor is part of a household or domestic refrigeration unit, the base oil (ii) is selected from API group IV or V oils and mixtures thereof and the compressor oil has a kinematic viscosity at 40 C. in the range of 2.88 and 7.48 cSt.

(47) This range encompasses the ISO viscosity grades 3 to 7.

(48) The refrigerant used in the household or domestic refrigeration unit may be isobutane or propane, preferably isobutane.

(49) A further object of the present invention is directed to the method of increasing the energy efficiency of a household or domestic refrigeration unit using isobutane or propane, preferably isobutane, as refrigerant, comprising operating the refrigeration unit with a compressor oil, wherein the compressor oil comprises: (i) 1 wt. % to 10 wt. % of a polyalkyl methacrylate-based viscosity index improver comprising: (a) 0.2 wt. % to 25 wt. %, preferably 4 wt. % to 16 wt. %, of methyl methacrylate; and (b) 75 wt. % to 99.8 wt. %, preferably 84 wt. % to 96 wt. %, of C10-18 alkyl (meth)acrylates, wherein the weight average molecular weight (M.sub.w) of the polyalkyl (meth)acrylate-based viscosity index improver is in the range of 5,000 g/mol to 200,000 g/mol, preferably 10,000 g/mol to 200,000 g/mol; (ii) 90 wt. % to 99 wt. % of the API group IV or V base oils and mixtures thereof; and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent and an antioxidant,
wherein the compressor oil has a kinematic viscosity at 40 C. in the range of 2.88 and 7.48 cSt and a viscosity index of at least 140, preferably at least 160, more preferably at least 180.

(50) In a further preferred object, the base oil (ii) is selected from naphthenic oils of API Group V and mixtures thereof being characterized by a C.sub.N value of at least 40%.

(51) The content of each component (i), (ii) and (iii) is based on the total composition of the compressor oil. In a particular embodiment, the proportions of components (i), (ii) and (iii) add up to 100% by weight.

(52) The content of each component (a) and (b) is based on the total composition of the polyalkyl (meth)acrylate-based viscosity index improver. The proportions of components (a) and (b) add up to 100% by weight.

(53) A further object of the present invention is directed to the method of increasing the energy efficiency of a household or domestic refrigeration unit as outlined further above, wherein the compressor oil has a pour point of 60 C. or lower.

(54) A further object of the present invention is directed to the method of increasing the energy efficiency of a compressor as outlined further above, wherein the compressor is an air compressor, the base oil (ii) is selected from API group II, III and IV or mixtures thereof and the compressor oil has a kinematic viscosity at 40 C. in the range of 28.8 and 74.8 cSt.

(55) This range encompasses the ISO viscosity grades 32 to 68.

(56) A further object of the present invention is directed to the method of increasing the energy efficiency of an air compressor, comprising operating the air compressor with a compressor oil, wherein the compressor oil comprises: (i) 1 wt. % to 20 wt. % of a polyalkyl methacrylate-based viscosity index improver comprising: (a) 0.2 wt. % to 25 wt. % of methyl methacrylate; (b) 75 wt. % to 99.8 wt. % of C10-18 alkyl (meth)acrylates; and (c) 0 wt. % to 2 wt. % of straight-chained or branched C5-9 alkyl (meth)acrylates or straight-chained or branched C20-24 alkyl (meth)acrylates, wherein the weight average molecular weight (M.sub.w) of the polyalkyl (meth)acrylate-based viscosity index improver is in the range of 5,000 g/mol to 400,000 g/mol, preferably in the range of 5,000 g/mol to 200,000 g/mol and more preferably in the range of 10,000 g/mol to 80,000 g/mol; (ii) 80 wt. % to 99 wt. % of API group II, III or IV base oils or mixtures thereof; and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent and an antioxidant,
wherein the compressor oil has a kinematic viscosity at 40 C. in the range of 28.8 and 74.8 cSt and a viscosity index of at least 140, preferably at least 160, more preferably at least 180.

(57) The content of each component (i), (ii) and (iii) is based on the total composition of the compressor oil. In a particular embodiment, the proportions of components (i), (ii) and (iii) add up to 100% by weight.

(58) The content of each component (a), (b) and (c) is based on the total composition of the polyalkyl (meth)acrylate-based viscosity index improver. The proportions of components (a), (b) and (c) add up to 100% by weight.

(59) A further object of the present invention is directed to the method of increasing the energy efficiency of an air compressor as outlined further above, wherein the polyalkylmethacrylate based VI improver further comprises (c) up to 5 wt. % of a dispersant monomer selected from the group consisting of hydroxyethyl methacrylate, N,N-dimethylaminoethyl methacrylate (DMAEMA), N-(3-(dimethylamino)propyl)methacrylamide (DMAPMAm) and N-vinylpyrrolidone (NVP).

(60) Typical compressed air systems work at pressures of at least 5 bar or at higher pressures when high forces are required. Some blow molding applications are even operated at air pressures of 40 bar.

(61) The effect of the inventive compressor oil on compressor performance is stronger at high gas pressures.

(62) Preferably, the air compressor is run at an air pressure of at least 5 bar, more preferably at least 7 bar, and more preferably at least 9 bar.

(63) A further object of the present invention is directed to the method of increasing the energy efficiency of a compressor as outlined further above, wherein the compressor is a carbon dioxide compressor, the base oil (i) is selected from API group III, IV or V and mixtures thereof and the compressor oil has a kinematic viscosity at 40 C. in the rage of 41.4 and 110 cSt.

(64) This range encompasses the ISO viscosity grades 46 to 100.

(65) A further object of the present invention is directed to the method of increasing the energy efficiency of a carbon dioxide compressor, comprising operating the carbon dioxide compressor with a compressor oil, wherein the compressor oil comprises: (i) 1 wt. % to 20 wt. % of a polyalkyl methacrylate-based viscosity index improver comprising: (a) 0.2 wt. % to 25 wt. %, preferably 4 wt. % to 16 wt. %, of methyl methacrylate; and (b) 75 wt. % to 99.8 wt. %, preferably 84 wt. % to 96 wt. %, of C10-18 alkyl (meth)acrylates, wherein the weight average molecular weight (M.sub.w) of the polyalkyl (meth)acrylate-based viscosity index improver is in the range of 5,000 g/mol to 100,000 g/mol, preferably 30,000 g/mol to 100,000 g/mol; (ii) 80 wt. % to 95 wt. % of a polyolester base oil or mixtures of different polyester base oils; and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent and an antioxidant, wherein the compressor oil has a kinematic viscosity at 40 C. in the range of 41.4 and 110 cSt and a viscosity index of at least 140, preferably at least 160, more preferably at least 180.

(66) The content of each component (i), (ii) and (iii) is based on the total composition of the compressor oil. In a particular embodiment, the proportions of components (i), (ii) and (iii) add up to 100% by weight.

(67) The content of each component (a) and (b) is based on the total composition of the polyalkyl (meth)acrylate-based viscosity index improver. The proportions of components (a) and (b) add up to 100% by weight.

(68) The compressor oils commonly used in the carbon dioxide compressors is typically based on polyolester with a viscosity of 68 cSt at 40 C.

(69) Commercially available Fuchs Reniso C oils based on polyolesters are available with KV.sub.40 of 55, 80 and 178 cSt. Viscosity indices are always well below 150.

(70) A further object of the present invention is directed to the method of increasing the energy efficiency of a carbon dioxide compressor, comprising operating the carbon dioxide compressor with a compressor oil, wherein the compressor oil comprises: (i) 1 wt. % to 30 wt. % of a polyalkyl methacrylate-based viscosity index improver comprising: (a) 0.2 wt. % to 25 wt. %, preferably 4 wt. % to 16 wt. %, of methyl methacrylate; and (b) 75 wt. % to 99.8 wt. %, preferably 84 wt. % to 96 wt. %, of C10-18 alkyl (meth)acrylates, wherein the weight average molecular weight (M.sub.w) of the polyalkyl (meth)acrylate-based viscosity index improver is in the range of 5,000 g/mol to 100,000 g/mol, preferably 10,000 g/mol to 80,000 g/mol; (ii) 80 wt. % to 99 wt. % of a polyalphaolefin base oil or mixtures of different polyalphaolefin base oils; and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent and an antioxidant,
wherein the compressor oil has a kinematic viscosity at 40 C. in the range of 41.4 and 110 cSt and a viscosity index of at least 140, preferably at least 160, more preferably at least 180.

(71) The content of each component (i), (ii) and (iii) is based on the total composition of the compressor oil. In a particular embodiment, the proportions of components (i), (ii) and (iii) add up to 100% by weight.

(72) The content of each component (a) and (b) is based on the total composition of the polyalkyl (meth)acrylate-based viscosity index improver. The proportions of components (a) and (b) add up to 100% by weight.

(73) A further object of the present invention is directed to the method of increasing the energy efficiency of a carbon dioxide compressor, comprising operating the carbon dioxide compressor with a compressor oil, wherein the compressor oil comprises: (i) 1 wt. % to 30 wt. % of a polyalkyl methacrylate-based viscosity index improver comprising: (a) 0 wt. % to 25 wt. % of methyl methacrylate; (b) 60 wt. % to 99.8 wt. % of C10-18 alkyl (meth)acrylates; and (c) 0 wt. % to 40 wt. % of C8-12 alpha-olefins, wherein the weight average molecular weight (M.sub.w) of the polyalkyl (meth)acrylate-based viscosity index improver is in the range of 5,000 to 100,000 g/mol; (ii) 70 wt. % to 99 wt. % of a polyalphaolefin base oil or mixtures of different polyalphaolefin base oils; and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent and an antioxidant,
wherein the compressor oil has a kinematic viscosity at 40 C. in the range of 41.4 and 110 cSt and a viscosity index of at least 140, preferably at least 150, more preferably at least 160.

(74) The content of each component (i), (ii) and (iii) is based on the total composition of the compressor oil. In a particular embodiment, the proportions of components (i), (ii) and (iii) add up to 100% by weight.

(75) The content of each component (a), (b) and (c) is based on the total composition of the polyalkyl (meth)acrylate-based viscosity index improver. The proportions of components (a), (b) and (c) add up to 100% by weight.

(76) The compressor oils commonly used in air compressors is typically based on API group I, II or III oil with a viscosity of 46 cSt at 40 C. and a viscosity index below 140. Oils are available from all major oil and compressor OEM's, e.g. Kaeser Sigma Fluid MOL with a KV.sub.40 of 46 cSt and a VI of 106.

(77) The pour point of that fluid is at 30 C.

(78) A further object of the present invention is directed to the method of increasing the energy efficiency of an air compressor as outlined further above, wherein the compressor oil has a pour point of 33 C. or lower.

(79) The FIGURE illustrates the test settings used to determine the effects on energy consumption in an air compressor.

(80) The invention is further illustrated by the following non-limiting examples and comparative example (reference oil). The examples below serve for further explanation of preferred embodiments according to the present invention but are not intended to restrict the invention.

EXPERIMENTAL PART

Abbreviations

(81) Synesstic5 alkylated naphthalene base oil from ExxonMobil with a KV.sub.40 of 29 cSt Berylane 230 naphthenic base oil from Total with a KV.sub.40 of 2.3 cSt and a CN value of about 45% KV kinematic viscosity measured according to ASTM D445 KV.sub.40 kinematic viscosity measured @40 C. to ASTM D445 KV.sub.100 kinematic viscosity measured @100 C. to ASTM D445 M.sub.n number-average molecular weight M.sub.w weight-average molecular weight NS3 naphthenic base oil from Nynas with a KV.sub.40 of 2.9 cSt and a C.sub.N value of about 57% PAO6 Group IV base oil with a KV.sub.100 of 6 cSt PAO8 Group IV base oil with a KV.sub.100 of 8 cSt PDI polydispersity index PP pour point T3 naphthenic base oil from Nynas with a KV.sub.40 of 3.6 cSt and a C.sub.N value of about 52% T9 naphthenic base oil from Nynas with a KV.sub.40 of 9.1 cSt and a C.sub.N value of about 45% VI viscosity index
Test Methods

(82) The polyalkyl methacrylate-based polymers according to the present invention were characterized with respect to their weight-average molecular weight.

(83) The compressor oils including the polyalkyl methacrylate-based polymers according to the present invention and the comparative examples were characterized with respect to their kinematic viscosity at 40 C. (KV.sub.40) and 100 C. (KV.sub.100) to ASTM D445, their viscosity index (VI) to ASTM D2270, their pour point to ASTM D5950, their flash point ASTM D92 and their viscosity shear loss.

(84) Determination of effects on energy consumption in a household or domestic refrigeration unit A standardized performance test rig measured the power consumption of the compressor at specified rating conditions. It allowed to ensure the same operating conditions for a number of tests. Furthermore, the performance test comprised the calculation of the coefficient of performance (COP; corresponding to the ratio of cooling power to electric drive power) at the specified rating conditions and the volumetric efficiency, the ratio of real volume flow to geometrically possible volume flow. The latter indicated the sealing properties of the working chamber of the compressor.

(85) The test-rig setup was designed for performance tests of small capacity refrigerant-compressors in accordance with the ASHRAE standard 23.1 (2010), resp. DIN EN 13771-1 (2017). Based on a standard vapor compression cycle, the test bench included a calorimeter evaporator and a flow meter to determine the refrigerant mass flow rate. Besides the main components like the compressor, the condenser, and an electronic expansion device, the cycle was additionally equipped with an oil separator, a filter dryer, sight glasses, and an accumulator.

(86) The compressor was a hermetic reciprocating piston compressor of type Embraco VEMX 7C, refrigerant was R600a (isobutane). The compressor was operated at three speeds: 50 Hz, 100 Hz and 150 Hz. CECOMAF (Comit europen des constructeurs de matriel frigorifique) conditions were applied: gas temperature on suction side=32 C., dew point on suction side=25 C., dew point on pressure side=+55 C., ambient temperature=35+2 C.

(87) The general processing of the acquired data for this experimental investigation followed the European standard on compressor rating (DIN EN 13771-1, 2017).

(88) TABLE-US-00002 TABLE 1 Formulations and results retrieved with inventive and comparative refrigeration compressor oils. Composition CE 1 Ex 1 Ex 2 Ex 3 CE 2 Polymer 1 3.40 2.60 6.20 [wt. %] Performance 0.8 0.8 0.8 0.8 package*.sup.) [wt. %] Nynas T3 96.60 93.00 32.40 [wt. %] Nynas NS3 95.80 [wt. %] Nynas T9 66.80 [wt. %] Genuine oil, 100 Alkyl benzene based [wt. %] Total [%] 100 100 100 100 100 KV.sub.40 [mm.sup.2/s] 4.90 4.12 4.94 7.08 7.08 ISO VG 5 4**.sup.) 5 7 7 KV.sub.100 [mm.sup.2/s 1.59 1.63 1.79 2.55 1.99 VI 64 240 164 238 58 PP [ C.] 57 87 81 78 69 Shear loss 7.3 6.2 11 ASTM D5621 [%] Efficiency (COP) 1.26 1.39 at 50 Hz [%] Efficiency (COP) 1.42 1.47 at 100 Hz [%] Efficiency (COP) 1.33 1.37 at 150 Hz [%] Volumetric 62 65 Efficiency at 50 Hz Volumetric 61 64 Efficiency at 100 Hz Volumetric 59.8 61.3 Efficiency at 150 Hz *.sup.)As performance package, a commercially available zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent and an antioxidant was used to protect the compressor. **.sup.)The value of KV 40 = 4.12 mm.sup.2/s is slightly below the defined range for ISO VG 5; an ISO VG 4 is not defined.

(89) Polymer 1 consists of 13 wt. % of methyl methacrylate, 86.5 wt. % of C10-16 alkyl methacrylates and 0.5 wt. % of C11-18 alkyl methacrylates (Mw=77,000 g/mol, 80% solids dissolved in highly refined mineral oil).

(90) As Comparative Example 1 (CE 1) was used a commercially available alkyl benzene based oil having a KV.sub.40 of 4.90 mm.sup.2/s (corresponding to ISO VG 5). Comparative Example 2 (CE 2) is a mixture of different naphthenic base oils, having a KV.sub.40 of about 7 (corresponding to ISO VG 7). The comparative examples do not contain any polyalkyl (meth)acrylate.

(91) Working examples 1-3 (Ex 1-3) are also based on naphthenic base oils and Polymer 1 as a polyalkyl (meth)acrylate. Ex 1-3 are formulated to a KV.sub.40 of 4 mm/s (Ex 1), 5 mm/s (Ex 2) and 7 mm/s (Ex 3), corresponding to ISO VG 4, VG 5 and VG 7, respectively.

Conclusions

(92) The inventive oil has shown an improvement of the volumetric efficiency and the coefficient of performance at all driving speeds (50/100/150 Hz). The compressor oils with high VI show good compatibility (no detrimental separation and accumulation was observed) with the refrigerant and allow an improvement of equipment performance.

(93) Determination of Effects on Energy Efficiency in Air Compressors

(94) Another aspect of the invention was the improvement of air compressor efficiency.

(95) Compressor oils with VI 140 and higher were tested in a Kaeser SX4 screw compressor and were compared with the commercially used mineral oil-based monograde fluid of Kaeser having a VI of 106.

(96) A second air compressor of larger size was used to determine energy efficiency benefits, Atlas Copco GA75VSD.

(97) The test settings used are described in the FIGURE.

(98) Characterization of air compressors as used in relevant test procedures:

(99) TABLE-US-00003 (1) KAESER SX4 Date of Manufacture: 2019 September Manufacturer: Kaeser Compression Medium: Air Reference Frequency: 50 Hz Maximum Air Volume Flow Rate: 0.36 m.sup.3/min Presurre Stages: 1 Maximum discharge pressure: 11 bar Motor Capacity: 3.0 kW (2) Atlas Copco GA75VSD P A 13 MK5 Date of Manufacture: 2019 January Manufacturer: Atlas Copco Compression Medium: Air Reference Frequency/ 73/20 Hz Lower limit Frequency: Maximum Air Volume Flow Rate: 14.76 m.sup.3/min Presurre Stages: 1 Maximum discharge pressure: 13 bar Motor Capacity: 75 kW

(100) The following parameters were measured: oil sump temperature, air temperature at the suction and discharge side, ambient air temperature, pressure and humidity; air pressure on suction and discharge side, air flow rate, and the power demand of the equipment. On the discharge side, a condensation air dryer was used to maintain dry air with less than 0.1% water in the compressed air.

(101) Stationary operating conditions with two different oil temperatures and four different air pressures were adjusted. Air flow rates and power demand resulted in specific power demand values in W/(bar*L/min).

(102) The following Table 2 shows the formulations and results retrieved with inventive and comparative air compressor oils.

(103) TABLE-US-00004 TABLE 2 Formulations and results retrieved with inventive and comparative air compressor oils (AirEx and AirCE). Composition AirCE 1 AirEx 1 AirEx 2 AirEx 3 AirEx 4 AirEx 5 AirEx 6 Polymer 2 [wt. %] 0 0 0 9.5 13.6 0 11.8 Polymer 3 [wt. %] 0 0 0 0 0 5.0 0 Polymer 4 [wt. %] 0 1.0 10.5 0 0 0 0 Performance 0.8 1.5 0.8 0.8 0.8 0.8 package*.sup.) [wt. %] PAO6 [wt. %] 10.0 PAO8 [wt. %] 89.0 Kaeser genuine 100 fluid [wt. %] Group III oil with 29.3 54.2 **) KV.sub.40 of about 4 mm.sup.2/s [wt. %] Group III oil with 88.0 60.4 31.4 94.2 **) KV.sub.40 of about 6 mm.sup.2/s [wt. %] Synesstic5 [%] 6.0 Total [%] 100 100 100 100 100 100 100 KV.sub.40 [mm.sup.2/s] 46.0 45.99 46.27 45.96 46.81 46.34 55.0 ISO VG 46 46 46 46 46 46 KV.sub.100 [wt. %] 6.92 7.83 8.17 9.0 9.73 9.66 10.3 VI 106 140 151 181 200 200 180 PP [ C.] 30 54 45 45 45 45 42 Shear loss at <1 <1 <1 4.8 6.4 >20 5.8 100 C., ASTM D5621 [%] *.sup.)As performance package, a commercially available zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent and an antioxidant was used to protect the compressor. **) mixture of Group III oils adding up to 81.4% by weight

(104) Polymer 2 consists of 13 wt. % of methyl methacrylate and 87 wt. % of C10-16 alkyl methacrylates (M.sub.w=56,000 g/mol, 74% solids dissolved in highly refined mineral oil).

(105) Polymer 3 consists of 11.3 wt. % of methyl methacrylate, 88.3 wt. % of C10-18 alkyl methacrylates and 0.4 wt. % of C20-22 alkyl methacrylates (M.sub.w=375,000 g/mol; 42% solids dissolved in highly refined mineral oil).

(106) Polymer 4 consists of 0.2 wt. % of methyl methacrylate and 99.8 wt. % of iso C12-15 alkyl methacrylates (M.sub.w=13,800 g/mol).

(107) As comparative example 1 (AirCE 1) was used a genuine fluid (commercially available from Kaeser) having a KV.sub.40 of 48 mm.sup.2/s (corresponding to ISO VG 46). It does not contain any polyalkyl (meth)acrylate.

(108) Working examples 1-6 (AirEx 1-6) arm based on different Group III base oils and contain a polyalkyl (meth)acrylate. AirEx 1-5 were formulated to a KV.sub.40 of about 48 mm.sup.2/s, corresponding to ISO VG 48; AirEx 6 was formulated to a KV.sub.40 of about 55 mm.sup.2/s.

(109) The effects on energy consumption in an air compressor were received by using the compressor oils according to the present invention are summarized in the following Tables 3a, 3b and 3c.

(110) TABLE-US-00005 TABLE 3a Effects on energy consumption and efficiency in an air compressor by using compressor oils according to the present invention at an air pressure p.sub.air in the range of 8.39 to 9.43 bar. T.sub.Air Air flow Efficiency p.sub.Air T.sub.Oil P.sub.Total outlet rate P.sub.specific Power ratio improvement Ex # [bar] [ C.] [W] [ C.] [L/min] ([W*min)/L] [(W*min)/(bar*L)] [%] AirCE 1 8.39 91.9 3111 70 294.5 10.56 1.26 9.11 73.4 3242 59 310.5 10.44 1.15 AirEx 4 8.85 93.7 3226 70 307.8 10.48 1.18 4.3 9.43 74.9 3321 60 320.5 10.36 1.10 2.9 AirEx 5 8.85 92.7 3215 70 298.6 10.77 1.22 1.5 9.39 75.0 3310 60 314.6 10.52 1.12 1.0

(111) TABLE-US-00006 TABLE 3b Effects on energy consumption and efficiency in an air compressor by using compressor oils according to the present invention at an air pressure p.sub.air in the range of 7.06 to 7.67 bar. T.sub.Air Air flow Efficiency p.sub.Air T.sub.Oil P.sub.Total outlet rate P.sub.specific Power ratio improvement Ex # [bar] [ C.] [M] [ C.] [L/min] [(W*min)/L] [(W*min/(bar*L)] [%] AirCE 1 7.06 87.9 2872 67 304.8 9.42 1.34 7.44 70.7 2949 57 318.4 9.26 1.25 AirEx 4 7.27 89.0 2948 68 318.2 9.26 1.27 3.9 7.67 70.7 3013 58 329.7 9.14 1.19 3.4 AirEx 5 7.22 87.8 2926 67 309.6 9.45 1.31 1.3 7.63 70.7 3003 57 325.1 9.24 1.21 2.0

(112) TABLE-US-00007 TABLE 3c Effects on energy consumption and efficiency in an air compressor by using compressor oils according to the present invention at an air pressure p.sub.air in the range of of 4.89 to 5.15 bar. T.sub.Air Air flow Efficiency p.sub.Air T.sub.Oil P.sub.Total outlet rate P.sub.specific Power ratio improvement Ex # [bar] [ C.] [W] [ C.] [L/min] [(W*min)/L] [(W*min)/(bar*L)] [%] AirCE 1 4.89 81.2 2539 63 318.1 7.98 1.63 5.04 68.1 2585 55 328.6 7.87 1.56 AirEx 4 5.00 81.1 2596 63 331.3 7.84 1.57 3.4 5.15 68.3 2613 55 337.1 7.75 1.51 3.0 AirEx 5 4.92 80.6 2559 63 322.5 7.93 1.61 0.8 5.15 68.4 2613 55 334.2 7.82 1.52 2.2 p.sub.Air: air pressure at air discharge T.sub.Oil: Compressor oil temperature P.sub.total: total power demand of compressor Air flow rate: air flow at air discharge side (dry air at pair) P.sub.specific: power demand of compressor unit divided by air flow rate Power ratio: power demand of compressor unit divided by (air flow rate air discharge pressure)

(113) The efficiency improvement was calculated from P.sub.specific, suction pressures and the individual compression ratios at test conditions vs reference conditions (correction factor):

(114) ${\mathrm{Efficiency}}_{\mathrm{relative}}=\frac{{}_2}{{}_1}-1=\frac{P_{\mathrm{specific},1}}{P_{\mathrm{specific},2}}\frac{p_{\mathrm{suction},2}}{p_{\mathrm{suction},1}}\mathrm{correction}-1$

(115) Additional tests were run on Atlas Copco GA75VSD. The oil temperature was controlled to 90 C. Three different discharge air pressures were investigated at 8 bar, 10 bar and 12.5 bar.

(116) The following Table 4 shows the results retrieved with using Atlas Copco GA75VSD.

(117) TABLE-US-00008 TABLE 4 results retrieved with using Atlas Copco GA75VSD rel. efficiency KV90 T.sub.Oil P.sub.air, out P.sub.specific improvement Fluid VG VI [cSt] [ C.] [bar] [W*min/L] [%] mineral-based 46 105 8.69 90 8 7.11 VG46 - 90 10 7.92 Reference 90 12.5 9.11 AirEx3 46 180 11.05 90 8 6.99 1.7 90 10 7.77 1.9 90 12.5 8.91 2.2 AirEx6 55 180 12.55 90 8 7.00 1.6 90 10 7.75 2.2 90 12.5 8.87 2.7

(118) TABLE-US-00009 TABLE 5 Shear loss of oils during test procedure after 1 day testing at various conditions: KV40 KV100 KV40 KV100 AVI AKV40 VI (cSt) (cSt) VI (cSt) (cSt) (%) (%) Fluid Fresh oil After test AirCE1 106 46.1 6.9 106 46.2 6.9 0 +0.2 AirEx3 181 45.9 9.0 181 45.9 9.0 0 0 AirEx4 200 46.8 9.7 198 46.7 9.7 1 0.2 AirEx5 200 46.3 9.7 177 40.6 8.1 11.5 12.4
Conclusions:

(119) The electric power demand was measured for at least 15 minutes after stationary operating conditions were achieved at various discharge pressures and oil temperatures.

(120) The power ratio was defined by the ratio of the measured electric power demand and the output power, measured in air volume flow rate in liter per minute multiplied by the pressure at the compressor air discharge side. Constant and repeatable ambient conditions were achieved by operating the equipment in a controlled air-conditioned room.

(121) The investigations on the air compressor test rigs have clearly shown an efficiency advantage of compressor oils with a VI of at least 140 and high shear stability. The efficiency was significantly improved at all investigated operating conditions. At oil temperatures of about 75 C., a reduction of the power ratio from 1.15 (W*min)/(bar*L) to 1.10 (W*min)/(bar*L) was achieved with changing the compressor oil from AirCE1 to AirEx4, the fluid comprising Polymer 2 and having a VI of 200. At an oil temperature of 92 to 94 C., an even stronger improvement from 1.26 (W*min)/(bar*L) of AirCE1 to 1.18 (W*min)/(bar*L) of AirEx4 was observed. The corresponding efficiency improvement was calculated to 4.3%. The fluid AirEx5 comprising Polymer 3 and having a VI of 200 also allowed to increase the efficiency. The improvement at oil temperatures above 90 C. and an air discharge pressure of about 9 bar was about 1.5%. The molecular weight of the polymer used in AirEx5 was higher and the shear stability of the oil was lower compared to compressor oil AirEx4. A higher shear stability is advantageous for the efficiency improvement and for the lifetime of the oil. The inventive fluids had a maximum KV.sub.100 shear loss of 40% in the 40 minutes sonic shear test method according to ASTM D5621. Preferred is a lower shear loss of maximum 20% and more preferred a shear loss of less than 10% according to ASTM D5621.

(122) Table 5 shows the viscosities of oils before and after the testing on the compressor test rigs. Viscosities of AirEx3 and AirEx4 have not changed over time of the test duration, however, the viscosity of oil AirEx5 with Polymer 3 dropped down by more than 10% under real life conditions. The molecular weight of polymer 3 is relatively high and shear stability is not good enough for a long-term efficiency improvement of air compressors.

(123) The pour point of the compressor fluids according to the present invention were 33 C. or lower. High VI, low pour point and high shear stability were achieved by blending Group II, Group III or PAO base oils with the polyalkyl methacrylate-based viscosity index improvers according to the present invention having a defined composition and a maximum molecular weight of 400,000 g/mol, preferably below 200,000 g/mol and more preferably below 100,000 g/mol. It was recognized that the equipment can be operated at lower temperatures with higher VI and more shear stable lubricants. When using more efficient fluids it became necessary to block the cooling units to achieve higher oil operating temperature levels of 90 C. as requested for the test runs. The investigations have shown that overheating can be avoided by using compressor oils according to the present invention, as a more efficient air compressor has the tendency to run at lower temperatures.