Lubricant including an ester of pyromellitic acid for refrigeration systems

Abstract

A working fluid comprising a lubricant and a refrigerant, the lubricant comprising a tetra-alkyl ester of pyromellitic acid, wherein each of the four alkyl groups of the tetra-alkyl ester of pyromellitic acid contains 1 to 4 carbon atoms.

Description

4822-01LUBRICANT INCLUDING AN ESTER OF PYROMELLITIC ACID FOR REFRIGERATION SYSTEMSBACKGROUND

[0001] The exemplary embodiment relates to a working fluid for a compressor of a cooling system and finds particular application in connection with a lubricant for a working fluid which includes a difluoromethane-based refrigerant.

[0002] Cooling systems are widely used for cooling air in domestic and commercial refrigerators, automobiles, refrigerated transport vehicles, heat pumps, and air conditioners. Such systems generally include a compressor for pressurizing the gaseous refrigerant before it enters a condenser. The compressor is lubricated by a lubricant, which should be compatible with the refrigerant being used. In the industry, there is a desire to shift to refrigerants that have a lower global warming potential (GWP) as well as less of an environmental impact. One of those refrigerants is difluoromethane (R-32).

[0003] Conventional lubricants for compressors, such as polyolester (POE)-based lubricants, tend not to provide the miscibility / solubility properties needed to enable these new refrigerants to perform satisfactorily and meet the system performance requirements set forth by the hardware manufacturers. Aromatic esters have been considered as potential components of a lubricant that may be used with hydrofluorocarbon-based refrigerants. For example, U.S. Pub. No. 20200318023A1 , published October 8, 2020, entitled AROMATIC ESTER LUBRICANT FOR USE WITH LOW GLOBAL WARMING POTENTIAL REFRIGERANTS, by Bujouves, et al., describes a working fluid for a refrigeration system including a refrigerant, and a lubricant that includes at least one aromatic ester and a polyolester.

[0004] Another aromatic ester based on gallic acid is described in U.S. Pub. No. 20150307762A1 , published October 29, 2015, entitled REFRIGERATING MACHINE OIL AND WORKING FLUID COMPOSITION FOR REFRIGERATING MACHINE, by Saito, et al.

[0005] JP5546726B2, published July 9, 2014, entitled REFRIGERATOR OILAND WORKING FLUID COMPOSITION FOR REFRIGERATOR, describes a working fluid composition for a refrigerator which contains an ester of anaromatic carboxylic acid selected from phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, and pyromellitic acid, an aliphatic dihydric alcohol having 2 to 15 carbon atoms, and a 2,3,3,3-tetrafluoropropene.

[0006] GB2216541A, published October 11 , 1989, entitled WORKINGFLUID / LUBRICANT COMBINATION, describes a working fluid / lubricant combination for use in a mechanical vapor recompression type heat transfer device. The working fluid includes a hydrofluorocarbon, hydrochlorofluorocarbon, or chlorofluorocarbon and the lubricant includes an ester having a molecular weight greater than 250, such as tetrabutyl pyromellitate.

[0007] WO200174977A2, published October 11 , 2001 , entitled LUBRICANTAND FLUSHING COMPOSITIONS, describes a lubricant composition including an ester derivable from the reaction of an aromatic monocarboxylic acid with a monovalent aliphatic alcohol having 1 to 15 carbon atoms.

[0008] EP0461435, published entitled APPLICATION OF AROMATICCARBOXYLIC ESTERS AS LUBRICANT IN REFRIGERANT COMPRESSORS, describes esters produced from aromatic carboxylic acids and monohydric alcohols as lubricants for refrigerant compressors which are operated with chlorine- free, partially fluorinated hydrocarbons as refrigerants. The esters can be derived from aromatic carboxylic acids, such as trimellitic acid and pyromellitic acid and straight-chain or branched, primary monohydric alcohols having 4 to 20 carbon atoms.BRIEF DESCRIPTION

[0009] In accordance with one aspect of the exemplary embodiment, a working fluid includes a lubricant and a refrigerant, the lubricant including a tetra-alkyl ester of pyromellitic acid wherein each alkyl group in the tetra-alkyl ester of pyromellitic acid is selected from alkyl groups containing 1 to 4 carbon atoms, or 3 to 4 carbon atoms, or 4 carbon atoms.

[0010] In the working fluid, the tetra-alkyl ester of pyromellitic acid may be at least 50 wt.%, at least 60 wt%, at least 70 wt.%, 80 wt. %, at least 90 wt. %, or at least 95 wt.%, or at least 98 wt. % of the lubricant. The balance of the lubricant may include tetra-alkyl ester of pyromellitic acid and a tri-alkyl ester of trimellitic acid, wherein the alkyl groups of the tetra-alkyl ester of pyromellitic acid or tri-alkyl ester of trimellitic acid each independently contain 4 to 16carbon atoms with the proviso that the refrigerant is miscible in the final lubricant blend.

[0011] The alkyls in the tetra-alkyl ester of pyromellitic acid may be selected from linear and branched alkyl groups comprising 1 to 4 carbon atoms, or 2 to 3 carbon atoms, or 4 carbon atoms.

[0012] The lubricant may include a total of no more than 10 wt. %, or no more than 1 wt. %, or no more than 0.5 wt. %, or at least 0.005 wt. %, or at least 0.01 wt. %, or at least 0. 1 wt. % of at least one additive selected from the group consisting of corrosion inhibitors, foam inhibitors, lubricity additives, surfactants, and combinations thereof.

[0013] The refrigerant in the working fluid of any preceding claim may include at least one difluoromethane, wherein the difluoromethane comprises 10 wt% to 100 wt% of the refrigerant.

[0014] The working fluid as described in any of the embodiments herein may find use in a refrigeration system comprising a compressor and an evaporator.

[0015] In another aspect of the exemplary embodiment, a method of lubricating a compressor of a refrigeration system includes forming a working fluid in the refrigeration system, the working fluid including a lubricant and a refrigerant, the lubricant comprising a tetra-alkyl ester of pyromellitic acid, the refrigerant comprising a difluoromethane.

[0016] In the method, the forming of the working fluid comprises supplying the lubricant to a compressor of the refrigeration system, the lubricant mixing with the refrigerant in the compressor to form the working fluid during normal operation of the compressor.

[0017] In the method in the compressor, the lubricant may reach a maximum concentration in the working fluid of at least 1 wt. %, or at least 5, wt. %, or at least 10 wt. %, or at least 15 wt. %, or at least 20 wt. %, or up to 25 wt. %.

[0018] In this method the four alkyls of the tetra-alkyl ester of pyromellitic acid may be selected from linear and branched alkyl groups comprising 1 to 4 carbon and mixtures thereof, for example, each alkyl group may independently be 1 , 2, 3, or 4 carbon atoms.

[0019] In another aspect of the exemplary embodiment, a method of improving solubility of difluoromethane refrigerant in a working fluid comprisingsupplying a lubricant to the working fluid, the lubricant comprising a tetra-alkyl ester of pyromellitic acid as described herein.BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIGURE 1 is a block diagram of a refrigeration system in accordance with one aspect of the exemplary embodiment,DETAILED DESCRIPTION

[0021] Aspects of the exemplary embodiment relate to a lubricant which includes a tetra-alkyl ester of pyromellitic acid (1 ,2,4,5-benzenetetracarboxylic acid) (or its dianhydride) and to a working fluid which includes a refrigerant and the exemplary lubricant, and to methods of lubricating a refrigeration system.

[0022] The lubricants described herein has improved miscibility with difluoromethane (R-32) refrigerant

[0023] As used herein, the term “refrigeration system” refers generally to any system, or any part or portion of such a system, which employs a refrigerant to provide cooling and / or heating. Such refrigeration systems include, for example, air conditioners, electric refrigerators, chillers, heat pumps, organic Rankine cycle systems, and the like. The exemplary lubricant finds particular use in a compression refrigeration system, such as an air conditioning system, heat pump, or an organic Rankine cycle system, in which a refrigerant is circulated. The refrigerant is a fluid which is used in the refrigeration cycle of the refrigeration system. It generally undergoes a repeated phase transition from a liquid to a gas and back again. The lubricant combines with the refrigerant to form a working fluid. A ratio by weight of the lubricant to the refrigerant in the working fluid typically varies throughout the refrigeration cycle, with a maximum weight ratio of lubricant: refrigerant of at least 1 :99, or at least 5:95, or at least 10:90, or up to 40:60, or up to 30:70, apart from in the compressor itself, where the lubricant: refrigerant ratio may be higher, such as up to 99:1 , or higher.The Lubricant

[0024] The tetra-alkyl ester of pyromellitic acid (which may also be formed from pyromellitic anhydride) used in the lubricant herein may the general formula (I):independently an alkyl group, e.g., a C1-C4 alkyl group, which can be branched or unbranched;R5is a C1-C5 hydrocarbyl group; and n is from 0 to 2.

[0025] R1, R2, R3, and R4can be the same or different. In one embodiment, they are the same. In one embodiment, all of R1, R2, R3, and R4contain 4 carbon atoms.

[0026] Example hydrocarbyl groups suited to use as R5include C1-C5 alkyl groups and C2-C5 alkenyl groups, which can be linear or branched. In some embodiments, the hydrocarbyl groups may include heteroatoms and heteroatom substituents which, in the context of this invention, do not alter the predominantly hydrocarbon nature of the substituent (e.g., halo (especially chloro and fluoro), hydroxy, alkoxy, mercapto, alkylmercapto, nitro, nitroso, and sulfoxy). Representative alkyl and alkenyl groups include methyl, ethyl, propyl, butyl, butyl, butyl, and pentyl groups, unsaturated equivalents thereof, and the like.

[0027] In one embodiment, n is 0, i.e. , R5is absent.

[0028] In one embodiment, the tetra-alkyl ester of pyromellitic acid is reacted with a small amount of diol to form a lubricant comprsing a mixture of the tetra-alkyl ester of pyromellitic acid as well as dimers, trimers, or higher oligomers. Suitable diols include 1 ,3-propanediol, 1 ,4-butanediol, 1 ,3- pentanediol, 1 ,4-pentanediol, 1 ,5-pentanediol, 1 ,3-hexanediol, 1 ,4-hexanediol, 1 ,6-hexanediol, and combinations thereof. In one embodiment, the diol is 1 ,4-butanediol. The diol may be present in a reaction mixture with the pyromellitate and the diol in an amount of 0.25 to 10 wt%, or 0.5 to 5 wt%, or 0.75 to 2 wt%, or 0.8 to 1 .5 wt% .

[0029] In one exemplary embodiment, wherein the tetra-alkyl ester of pyromellitic acid a tetra-butyl ester of pyromellitic acid coupled with 1 ,4- butanediol , a variety of dimer and trimer species are formed, for example:O 0 Formula V.

[0030] In one embodiment, the species of Fomula II, Formula III, and Formula IV, will comprise greater than 80 wt% of the total of the coupled product. In one embodiment, the lubricant comprises at least 50 wt% of the species of Formula II, 15 wt% to 30 wt% of the species of Formula III, 5 wt%to 10 wt% of the species of Formula IV, and 2 wt% to 5 wt% of the species of Formula V, wherein the remainder of the lubricant comprieses other higher oligomers or dibutyl ester products. In another embodiment, the lubricant comprises greater than 50 wt% of the tetra-butyl ester of pyromellitic acid and further comprises 10 wt% to 40 wt% or 15 wt% to 30 wt% of dimers, and less than 20 wt% or less than 15 wt% or less than 12 wt% trimers and other higher oligomers.

[0031] The tetra-alkyl ester of pyromellitic acid may the only component of the lubricant (other than impurities). In other embodiments, the lubricant may further include one or more other lubricant oils and / or one or more additives. However, care should be used when adding other lubricant oils and / or aditives so that the desirable properties of the tetra-alkyl ester relative to its use with the refrigerant is not unduly diminished.

[0032] Examples of other lubricant oils which may be used in the lubricant include a polyol ester, a polyol ether, a polyalkylene glycol a hydrocarbon oil, or a mixture thereof. Specific examples of polyol esters and polyol ethers which may be employed in the lubricant include (i) an aromatic ester comprising the reaction product of an aromatic hydrocarbon having at least one carboxylic functional group and a (mono)alkylalcohol and / or a glycol ether; (ii) a polyolester oil, wherein the polyolester oil comprises a polyol esterified with at least one (mono)carboxylic acid that has at least 5 carbon atoms; (iii) a polyol esterified with a m ixture of (mono)carboxylic acids or their anhydrides, wherein the (mono)carboxylic acids or anhydrides, individually, have 5 to 13 carbon atoms; and mixtures thereof. Suitable polyols include trimethylolpropane, dipentaerythritol, neopentylglycol, monopentaerythritol, polypentaerythritol, and combinations thereof. In some embodiments, the polyol ester may comprise esters and / or complex esters of aromatic polycarboxylic acids or their anhydrides. The complex ester may be composed of polyol oligomeric units (such as trimethylolpropane, dipentaerythritol, neopentylglycol, monopentaerythritol, and / or polypentaerythritol), and a polyacid or acid anhydride (such as succinic, glutaric, adipic, citric, trimellitic, and / or pyromellitic). The complex ester may be fully or partially capped with functional (mono)carboxylic acids or (mono)alkylalcohols or singly-capped glycol ethers, or a mixture thereof.

[0033] In one embodiment, the lubricant contains, in total, no more than 10 wt. %, or no more than 5 wt. %, or no more than 2 wt. %, or no more than 1 wt. %, or no more than 0.1 wt. % of polyol ester(s) and polyol ether(s). In one embodiment, the lubricant is free or substantially free of polyol ester oils, wherein the polyol ester oil comprises a polyol esterified with at least one (mono)carboxylic acid that has at least 5 carbon atoms. In one embodiment, the lubricant contains no more than 5 wt. %, or no more than 1 wt. %, or no more than 0.1 wt. % of polyol ester oils, or no polyol ester oil.

[0034] The polyol ester and / or polyol ether, where present, may have a neat viscosity of at least 4 cSt or up to 400 cSt, measured at 40 °C according to ASTM D445-21. In other embodiments, the neat viscosity may be at least 5 cSt, or at least 10 cSt, or at least 30 cSt, or at least 100 cSt, or at least 170 cSt, or at least 200 cSt, or up to 350 cSt, or up to 200 cSt, or up to 170 cSt, measured at 40 °C according to ASTM D445-21 . Example ranges include 200 to 400 cSt, 200 to 350 cSt, 170 to 200 cSt, 100 to 170 cSt, 32 to 120 cSt, 46 to 68 cSt, or 5 to 30 cSt, measured at 40 °C according to ASTM D445-21 .

[0035] Example hydrocarbon oils include C9-C16 alkanes and mixtures theeof, e.g., petrolleum distillates, such as mineral oil, vegetable oils, and mixtures thereof. Where present, hydrocarbon oils, in total, may be no more than 10 wt. %, or no more than 5 wt. %, or no more than 2 wt. %, or no more than 1 wt. %, or no more than 0.1 wt. % of of the lubricant.

[0036] In one embodiment, the lubricant contains, in total, no more than 10 wt. %, or no more than 5 wt. %, or no more than 2 wt. %, or no more than 1 wt. %, or no more than 0.1 wt. % of lubricant oils other than the tetra-alkyl ester(s) of pyromellitic acid or anhydride.

[0037] The lubricant (and / or the refrigerant) may further include one or more additional additives selected from antioxidants, corrosion inhibitors, anti-wear agents, friction modifiers, foam inhibitors, viscosity modifiers, lubricity additives, and combinations thereof. In one embodiment the additional additive(s), in total, are at least 0.005 wt. % or up to 0.5 wt. % of the lubricant (and / or the refrigerant).

[0038] Suitable antioxidants include butylated hydroxytoluene (BHT), butylatedhydroxyanisole (BHA), phenyl-a-naphthyl amine (PANA), octylated / butylated diphenyl amine, high molecular weight phenolicantioxidants, hindered bis-phenolic antioxidant, di-alpha-tocopherol, di-tertiary butyl phenol, and mixtures thereof.

[0039] In some embodiments, the antioxidant includes one or more of: (i) hexamethylene bis(3,5-di-tert-butyl 4-hydroxyhydrocinnamate), CAS registration number 35074-77-2, available commercially from BASF; (ii) N- phenylbenzenamine, reaction products with 2,4,4-trimethylpentene, CAS registration number 68411 -46-1 , available commercially from BASF; (iii) phenyl-a-naphthylamine and / or phenyl-b-naphthylamine, for example N- phenyl-ar-(1 , 1 ,3,3-tetramethylbutyl)-1 -naphthalenamine, available commercially from BASF; (iv) tetrakis [methylene(3,5-di-tert-butyl-4- hydroxyhydrocinnamate)] methane, CAS registration number 6683-19-8; (v) thiodiethylenebis (3,5-di-tert-butyl-4-hydroxyhydrocinnamate), CAS registration number 41484-35-9, which is also listed as thiodiethylenebis (3,5- di-tert-butyl-4-hydroxy-hydro-cinnamate) (vi) butylated hydroxytoluene (BHT); (vii) butylated hydroxyanisole (BHA), (viii) bis(4-(1 ,1 ,3,3- tetramethylbutyl)phenyl)amine, available commercially from BASF; (ix) benzenepropanoic acid, 3,5-bis(1 ,1 -dimethylethyl)-4-hydroxy-, thiodi-2,1 - ethanediyl ester, available commercially from BASF.

[0040] The antioxidant(s) may be present in the lubricant in an amount of from 0.02 wt.% to 1 wt. %.

[0041] Suitable corrosion inhibitors include (i) triazoles or substituted triazoles, such as tolyltriazole (5-methyl-1 H-benzotriazole); / V, / V-bis(2- ethylhexyl)-ar-methyl-1 H-benzotriazole-1 -methanamine, CAS registration number 94270-86-70, sold commercially by BASF under the trade name Irgamet™ 39; (ii) fatty acids derived from animal and / or vegetable sources, and / or the hydrogenated forms of such fatty acids, for example Neo-Fat™ which is commercially available from Akzo Nobel Chemicals, Ltd.; (iii) / V- methyl- / V-(1 -oxo-9-octadecenyl)glycine, CAS registration number 110-25-8; (iv) dodecanoic acid; (v) Triphenyl phosphorothionate, CAS registration number 597-82-0; and

[0070] (vi) phosphoric acid, mono- and dihexyl esters, compounds with tetramethylnonylamines and C11-14 alkylamines.

[0042] The corrosion inhibitors, where used, may be present in the lubricant at a concentration of at least 0.01 wt. %, or at least 0.02 wt. %, or to 6.0 wt.%, or up to 0.1 wt.%, or up to 0.05 wt. %.

[0043] To inhibit wear on the metal surfaces of the compressor, the lubricant may include one or more of an anti-wear agent. In some cases, a compound or composition may provide two or more of these functions. Anti-wear agents are polar additives that attach to frictional metal surfaces. They react chemically with the metal surfaces when metal-to-metal contact occurs in conditions of mixed and boundary lubrication and are activated by the heat of contact to form a film that minimizes wear. Friction modifiers are typically used to alter the friction between moving parts and can operate at lower loads that are not activated by contact temperatures. Examples of anti-wear agents may include triaryl phosphates such as tricresyl phosphate (TCP), which is available from Chemtura as Kronitex™ TCP; and t-butylphenyl phosphate, which is available from ICL Industrial Products as Syn-O-Ad™ 8478.

[0044] The anti-wear agent, EP additive, and friction modifiers may be from 0.1 wt. % to 4 wt. % of the lubricant and may be used separately or in combination.

[0045] In some embodiments, the lubricant includes a viscosity modifier. Example viscosity modifiers include ethylene vinyl acetate, polybutenes, polyisobutylenes, polymethacrylates, olefin copolymers, esters of styrene maleic anhydride copolymers, hydrogenated styrene-diene copolymers, hydrogenated radial polyisoprene, alkylated polystyrene, and complex ester. Example tackifiers include natural rubber solubilized in oil. The addition of a viscosity modifier improves the viscosity and viscosity index of the lubricant. When used, the viscosity modifier, and / or tackifier may be at least 0.01 wt. %, or at least 0.05 wt. % or at least 0.1 wt. % of the lubricant, or up to 10 wt. %, or up to 5 wt. % of the lubricant. Example viscosity modifiers include a complex ester available from Index Chemical Co. Philadelphia, Pa, as CG 5000™, which can also serve as a pour point depressant.The Working Fluid

[0046] The working fluid includes the tetra-alkyl ester of pyromellitic acid or pyromellitic dianhydride, a refrigerant, and optionally other components of the lubricating composition. A ratio by weight of the at least one the tetra-alkyl ester of pyromellitic acid and the tetra-alkyl ester of pyromellitic anhydride to the refrigerant in the working fluid may be from 1 :99 to 99:1 , or at least 5:95,or at least 10:90, at least within the compressor of a refrigerant system during normal operation.

[0047] The refrigerant is a compound or compounds capable of maintaining a fluid state at temperatures over a range of at least -30 to 170 °C or up to 120 °C or 80 °C and which is able to undergo a repeated phase transition from a liquid to a gas and back again. The refrigerant is one which is sufficiently miscible with the tetra-alkyl ester of pyromellitic acid and / or dianhydride.

[0048] In the present invention, the working fluid contains one or more hydrofluorocarbon (HFC) refrigerants. The one or more hydrofluorocarbon compounds are selected from difluoromethane (R-32), pentafluoroethane (R- 125), 1 ,1 ,1 ,2-tetrafluoroethane (R-134a), 1 ,1 -difluoroethane (HFC-152a),1 .1 .2.2-tetrafluoroethane (HFC-134), and 1 ,1 ,1 ,2,3,3, 3-heptafluoropropane (HFC-227ea), or any mixture thereof. In one embodiment, the refrigerant comprises or consists of difluoromethane (R-32).

[0049] In some embodiments, the refrigerant may contain a blend of a HFC and a hydrofluoroolefin (HFO). HFO refrigerants may be selected from 1 , 1 ,1 ,4,4,4-hexafluorobut-2-ene (mixture of isomers) (HFO-1336mzz or R- 1336mzz); (Z)-1 ,1 ,1 ,4,4,4-hexafluorobut-2-ene (HFO-1336mzz-(Z) or R- 1336mzz(Z)); (E)-1 ,1 ,1 ,4,4,4-hexafluorobut-2-ene (HFO-1336mzz-(E) or R- 1336mzz(E)); (E)-1 ,2,3,3, 3-pentafluoropropene (HFO-1225ye-(E) or R- 1225ye(E)) 2,3,3,3-tetrafluoropropene (HFO-1234yf or R-1234yf); 1 , 3,3,3- tetrafluoropropene (mixture of isomers) (HFO-1234ze or R-1234ze); (Z)-1 .3.3.3-tetrafluoropropene (HFO-1234ze-(Z) or R-1234ze(Z)); (E)-1 , 3,3,3- tetrafluoropropene (HFO-1234ze-(E) or R-1234ze(E)); trifluoroethylene (HFO- 1123 or R-1123); 3,3,3-trifluoropropene (HFO-1243zf or R-1243zf); or any mixture thereof.

[0050] Blended refrigerants of HFO and / or HFC may be selected from R- 407A (blend containing R-32, R-125 and R-134a at 20 / 20 / 40 weight percent, respectively); R-407C (refrigerant blend containing R-32, R-125 and R-134a at 23 / 25 / 52 weight percent, respectively); R-410A (refrigerant blend containing R-32 and R-125 at 50 / 50 weight percent, respectively); R-454A (refrigerant blend containing R-32 and HFO-1234yf at 35 / 65 weight percent, respectively); R-454B (refrigerant blend containing R-32 and HFO-1234yf at 68.9 / 31.1 weight percent, respectively); R-454C (refrigerant blend containing R-32 andR-HFO-1234yf at 21.5 / 78.5 weight percent, respectively); and R-449A (refrigerant blend containing R-32, R-125, R-134a and HFO-1234yf at 24.3 / 24.7 / 25.7 / 25.3 weight percent, respectively).

[0051] The refrigerant may further include one or more saturated halogenated carbon compounds (halocarbons), for example hydrofluorocarbons and / or hydrochlorocarbons. Exemplary saturated halocarbons trifluoromethane (R-23), difluoromethane (R-32), pentafluoroethane (R-125), 1 ,1 ,1 ,2-tetrafluoroethane (R-134a), 1 ,1 ,1 - trifluoroethane (R-143a), and 1 ,1 -difluoroethane (R-152a).

[0052] The refrigerant may further include one or more halogen-free organic refrigerants (organic compounds that include no halogen atoms) such as propane, butane, or ammonia.

[0053] Where present, the halogen-free organic compound(s) may be at least 0.01 wt. %, or at least 0.05 wt. %, or at least 0.1 wt. %, or at least 1 wt. %, or at least 2 wt. %, or at least 3 wt. % of the refrigerant, or up to 90 wt. %, or up to 50 wt. %, or up to 20 wt. %, or up to 10 wt. %, or up to 5 wt. % of the refrigerant, in total.

[0054] In some embodiments, the refrigerant may comprise carbon dioxide. In some embodiments, the refrigerant may include a difluoromethane and carbon dioxide.

[0055] The working fluid may further include one or more additives which do not fall within the categories described above. Example additives include nanoparticles, stabilizers, surfactants, tracing agents, fluorescent agents, odorants and solubilizers.

[0056] The exemplary lubricant is miscible with the selected difluoromethane refrigerant or refrigerant blend over the operational temperatures of the compression refrigeration system. Miscibility of the lubricant with the refrigerant over operational temperatures ensures that the lubricant that enters into the refrigeration system out of the compressor can be carried through the evaporation orifice and heat transfer equipment back to the compressor where it functions as a lubricant and that a non-miscible lubricant portion is not present as blockage in the system restricting refrigerant movement through the system. It also assures that minimal lubricant oil films exist on the heat transfer equipment where it might interfere with the efficiencyof heat transfer by acting as a thermal insulating film. While the necessary extent of miscibility of the lubricant and the refrigerant may vary depending upon application in embodiment, a desired range is from -20 °C to 80 °C, or from -10°C to 70° C weight ratios of lubricant: refrigerant of 5:95, or 10:90, and / or 20:80.

[0057] The amount of each chemical component described is presented exclusive of any solvent or diluent oil, which may be customarily present in the commercial material, that is, on an active chemical basis, unless otherwise indicated. However, unless otherwise indicated, each chemical or composition referred to herein should be interpreted as being a commercial grade material which may contain the isomers, by-products, derivatives, and other such materials which are normally understood to be present in the commercial grade.

[0058] It is known that some of the materials described above may interact in the final formulation, so that the components of the final formulation may be different from those that are initially added. For instance, metal ions (of, e.g., a detergent) can migrate to other acidic or anionic sites of other molecules. The products formed thereby, including the products formed upon employing the lubricant / working fluid in its intended use, may not be susceptible of easy description. Nevertheless, all such modifications and reaction products are included within the scope of the present invention; the present invention encompasses the composition prepared by admixing the components described above.Method of Preparing the Lubricant

[0059] In one embodiment, a method of preparing the lubricant includes (i) reacting pyromellitic acid and / or pyromellitic dianhydride with a branched or unbranched monohydric alkyl alcohol having from 1 to 4 carbon atoms, in a sufficient amount to form the tetra-alkyl ester, and optionally (ii) combining the tetra-alkyl ester of pyromellitic acid and / or pyromellitic dianhydride with one or more lubricant oils (i.e. , other than the reaction product of step (i)), and / or one or more additives as described above. In one embodiment, a small amount, for example, up to 6 equivalents, for example, 0.1 to 6 equivalents of a shortchain diol (e.g. 1 ,4-butanediol) may be added as a coupling agent for the ester.

[0060] The esterification can be performed by heating pyromellitic acid with the alkyl alcohol at a temperature of about 200 to 250 °C, under reflux. The alky alcohol may be in excess over the stoichiometric amount, such as an excess of about 10%, or more. An esterification catalyst, such as sulfuric acid, may be employed, if desired. Water generated in the reaction and any residual alcohol are removed.

[0061] Pyromellitic acid and pyromellitic anhydride are widely available commercially.Refrigeration System

[0062] As illustrated in FIGURE 1 , in an exemplary refrigeration system 10, a refrigerant is compressed by a compressor 12 and directed, in a compressed gaseous state, from an outlet 14 of the compressor, through a first fluid line 16, to a condenser 18, where the compressed refrigerant gas is liquified, by cooling it. The liquified refrigerant passes through a second fluid line 20 to an expansion device 22, such as a valve, where the pressure of the refrigerant is reduced, lowering its temperature. The refrigerant then enters an evaporator 24, where the liquified refrigerant serves to remove heat from the surrounding atmosphere and, as a result, returns to a gaseous state. A third fluid line 26 carries the gaseous refrigerant back to the compressor, via a compressor inlet 28. A lubricant inlet 30 supplies the lubricant from a reservoir 32 to the moving parts of the compressor 12. The refrigerant picks up some of the lubricant while in the compressor, forming the working fluid, which carries some of the lubricant towards the condenser, a process referred to as entrapment. Generally, at least some of the lubricant is separated out from the compressed refrigerant. For example, a receptacle 34 collects the lubricant from the first fluid line 16 between the compressor and condenser. The collected lubricant may be returned to the lubricant inlet 30 of the compressor via a return line 36. Accordingly, other than within the compressor itself, the concentration of the lubricant in the working fluid is at its highest at a location 38 in the first fluid line 16 adjacent the outlet 14. At this location, the ratio of lubricant : refrigerant may be at least 1 :99, or at least 5:95, or at least 10:90, or up to 40:60, or up to 30:70.

[0063] In another embodiment, a method of lubricating a compressor is disclosed. The method may include supplying to the compressor a working fluid including (a) a lubricant comprising (i) at least one tetra-ester of pyromellitic acid or pyromellitic dianhydride, and (b) a refrigerant.

[0064] In one embodiment, the components (a) and (b) are introduced separately to the compressor, for example, by introducing component (a) to the compressor while the refrigerant is passing through the compressor.

[0065] Methods of improving the working viscosity of a refrigerant for a refrigeration system are also disclosed. The method may comprise adding a lubricant, as described herein, to a refrigerant.

[0066] The present methods, systems and compositions are adaptable for use in connection with a wide variety of heat transfer systems in general and refrigeration systems in particular, such as air-conditioning (including both stationary and mobile air conditioning systems), refrigeration, heat-pump systems, and the like.

[0067] The tetra-alkyl esters derived from pyromellitic acid or anhydride described herein, have improved miscibility with HFC refrigerants and may exhibit improved wear and foaming properties.

[0068] Without intending to limit the scope of the exemplary embodiments, the following examples demonstrate the advantages of tetra-alkyl esters derived from pyromellitic acid or its anhydride.EXAMPLES

[0069] Example 1 : An exemplary coupled tetra-alkyl ester of pyromellitic acid a tetra-butyl ester of pyromellitic acid coupled may be prepared by the following process: Pyromellitic dianhydride (e.g. 1.0 equivalent) and butan-1 - ol (e.g. 3.0 equivalents) are charged to a reaction flask having a Dean-Stark added (butan-1 -ol). The mixture is stirred and heated (e.g. up to 80°C). Once stabilized at 80°C, a Bronsted acid (e.g. sulfuric acid, phosphorous acid, or phosphoric acid) (e.g. 0.03 equivalents) is charged to the reaction. The reaction is heated (e.g. up to 120 °C) for up to 16 hours. The reaction is cooled (e.g. to at least 90 °C), followed by charges of butan-1 -ol (0.5-2.9 equivalents), 1 ,4-butanediol (0.1 -2.5 equivalents) and a Bronsted acid (0.03 equivalents) to the reaction. The reaction is heated (e.g. up to 120 °C) for up to 48 hoursfollowed by another heating of up to 160 °C for up to 24 hours. Excess alcohol is stripped off under vacuum. The product is then washed with 1 M NaOH (equivalent volume) and washes with 80% MeOH in H2O (equivalent volume). The product can be filtered and stripped under vacuum to provide a yellow oil. The product is a coupled butyl ester of pyromellitic acid, which may have a kinematic viscosity at 40°C of about 23 5 cSt.

[0070] Multiple pyromellitic acid tetra-ester lubricants were evaluated for high temperature miscibility with a variety of refrigerants. Lubricants were characterized by measuring kinematic viscosity at 40 C (ASTM D445). Samples are summarized below (Table 1 ).Table 1’ Kinematic viscosity at 40 °C (cSt)

[0071] Miscibility of lubricant and refrigerant is carried out according to ANSI / ASHRAE Standard 218-2019. This procedure is designed to determine the critical solution locus of miscible properties of a lubricant and refrigerant mixture. The lubricants are combined with a series of fluorinated refrigerants, including those containing R-32 (difluoromethane) and evaluated for miscibility at various concentrations (Table 2).Table 21. Treat rate of lubricant in refrigerant (weight percent)2. Mixture of R-1234yf (56%) and R-134a (44%)3. Mixture of R-32 (68.9%) and R1234yf (31. 1%)

[0072] The coupled lubricant of Example 1 can also be evaluated using ANSI / ASHRAE Standard 218-2019.

[0073] As will be appreciated under the operating conditions of a compressor in a refrigeration system, the results may differ.

[0074] Each of the documents referred to above is incorporated herein by reference. Except in the Examples, or where otherwise explicitly indicated, all numerical quantities in this description specifying amounts of materials, reaction conditions, molecular weights, number of carbon atoms, and the like, are to be understood as modified by the word “about.” Unless otherwise indicated, each chemical or composition referred to herein should be interpreted as being a commercial grade material which may contain the isomers, by-products, derivatives, and other such materials which are normally understood to be present in the commercial grade. However, the amount of each chemical component is presented exclusive of any solvent or diluent oil, which may be customarily present in the commercial material, unless otherwise indicated. It is to be understood that the upper and lower amount, range, and ratio limits set forth herein may be independently combined. Similarly, the ranges and amounts for each element of the invention may be used together with ranges or amounts for any of the other elements.

[0075] It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Claims

WE CLAIM:

1. A working fluid comprising a lubricant and a refrigerant, the lubricant comprising a tetra-alkyl ester of pyromellitic acid wherein each alkyl group in the tetra-alkyl ester of pyromellitic acid is selected from alkyl groups containing 1 to 4 carbon atoms, or 3 to 4 carbon atoms, or 4 carbon atoms and wherein the refrigerant comprises a hydrofluorocarbon or a hydroflouroolefin.

2. The working fluid of any preceding claim, wherein the lubricant further comprises a total of no more than 10 wt. %, or no more than 1 wt. %, or no more than 0.5 wt. %, or at least 0.005 wt. %, or at least 0.01 wt. %, or at least 0. 1 wt. %, of at least one additive selected from the group consisting of corrosion inhibitors, foam inhibitors, lubricity additives, surfactants, and combinations thereof.

3. The working fluid of any preceding claim, wherein the refrigerant comprises difluoromethane.

4. The working fluid of claim 3, wherein the refrigerant comprises at least 10 wt%, at least 20 wt%, at least 30 wt%, at least 40 wt%, at least 50 wt%, at least 60 wt%, at least 70 wt%, at least 80 wt%, at least 90 wt%, at least 95 wt%, or 100 wt% difluoromethane.

5. The working fluid of any preceding claim, wherein the tetra-alkyl ester of pyromellitic acid comprises one or more dimers of the tetra-alkyl ester, wherein the dimer of the tetra-alkyl ester is formed by coupling the tetraalkyl ester with a diol.

6. The working fluid of claim 5, wherein the diol is selected from 1 ,3- propanediol, 1 ,4-butanediol, 1 ,3-pentanediol, 1 ,4-pentanediol, 1 ,5- pentanediol, 1 ,3-hexanediol, 1 ,4-hexanediol, 1 ,6-hexanediol.

7. The working fluid of claim 5 or 6, wherein the diol comprises 1 ,4-butanediol and the lubricant comprises at least 50 wt% of a tetra-butyl ester of pyromellitic acid, 10 wt% to 40 wt% or 15 wt% to 30 wt% dimers, and less than 20 wt% or less than 15 wt% or less than 12 wt% trimers and other higher oligomers.

8. A working fluid comprising a lubricant and a refrigerant, wherein the refrigerant comprises a hydrofluorocarbon or a hydroflouroolefin and the lubricant comprises a tetrabutyl ester of Formula I:

9. The working fluid of claim 8, wherein the lubricant further comprises a dimer of Formula II:

10. The working fluid of claim 8 or 9, wherein the lubricant further comprises a dimer of Formula III:

11. The working fluid of any of claims 8 to 10, wherein the lubricant further comprises a trimer of Formula IV:

12. The working fluid of any of claims 8 to 11 , wherein the lubricant further comprises a total of no more than 10 wt. %, or no more than 1 wt. %, or no more than 0.5 wt. %, or at least 0.005 wt. %, or at least 0.01 wt. %, or at least 0. 1 wt. %, of at least one additive selected from the group consisting of corrosion inhibitors, foam inhibitors, lubricity additives, surfactants, and combinations thereof.

13. The working fluid of claim 8 to 12, wherein the refrigerant comprises difluoromethane.

14. The working fluid of claim 13, wherein the refrigerant comprises at least 10 wt%, at least 20 wt%, at least 30 wt%, at least 40 wt%, at least 50 wt%, at least 60 wt%, at least 70 wt%, at least 80 wt%, at least 90 wt%, at least 95 wt%, or 100 wt% difluoromethane.

15. The use of a working fluid of any preceding claims in a refrigeration system comprising a compressor and an evaporator.

16. A method of lubricating a compressor of a refrigeration system comprising forming a working fluid of any of claims 1 to 14 in the refrigeration system.

17. A method of improving solubility of a difluoromethane refrigerant in a working fluid comprising supplying a lubricant to the working fluid wherein the lubricant comprises a tetra-alkyl ester of pyromellitic acid wherein each alkyl group in the tetra-alkyl ester of pyromellitic acid is selected from alkyl groups containing 1 to 4 carbon atoms, or 3 to 4 carbon atoms, or 4 carbon atoms.

18. The method of claim 17, wherein the lubricant comprises a tetrabutyl ester of Formula I:

19. The method of claim 17 or 18, wherein the lubricant further comprises a dimer of Formula II:

20. The method of any of claims 17 to 19, wherein the lubricant further comprises a dimer of Formula III:

1. The method of any of claims 17 to 20, wherein the lubricant further comprises a trimer of Formula IV:

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