WO1993024588A1 - Polyol ester lubricants for high efficiency refrigerators - Google Patents

Abstract

A nearly ideal lubricant for high efficiency home refrigerators, especially those using chlorine free hydrofluorocarbon refrigerant working fluids, is the ester of trimethylolpropane with pentanoic acid. Mixtures including substantial quantities of this ester along with some other esters of hindered polyols having no more than four hydroxyl groups and acyl groups with predominantly no more than five carbon atoms are also satisfactory.

Description

POLYOL ESTER LUBRICANTS FOR HIGH EFFICIENCY REFRIGERATORS

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copend- ing International Application No. PCT/US92/04438 designat¬ ing the United States and filed June 3, 1992, the entire disclosure of which, except to the extent contrary to any explicit statement herein, is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention This invention relates to lubricant base stocks, which can also serve as complete lubricants in some cases; compounded lubricants, which include at least one additive for such purposes as improving high pressure resistance, corrosion inhibition, and the like along with the lubri- cant base stocks which contribute the primary lubricity to the compounded lubricants; refrigerant working fluids in¬ cluding lubricants according to the invention along with primary heat transfer fluids, and methods for using these materials. The lubricants and lubricant base stocks are generally suitable for use with most or all halocarbon refrigerants and are particularly suitable for use with substantially chlorine-free, fluoro-group-containing or¬ ganic refrigerating heat transfer fluids such as penta- fluoroethane, 1,1-difluoroethane, 1,1,1-trifluroethane, and tetrafluoroethanes, most particularly 1,1,1,2-tetra- fluoroethane. The lubricants and base stocks, in combina¬ tion with these heat transfer fluids, are particularly suitable for domestic refrigerators with high operating efficiency, where relatively low viscosity lubricants are needed.

Statement of Related Art

Chlorine-free heat transfer fluids are desirable for use in refrigerant systems, because their escape into the atmosphere causes less damage to the environment than the currently most commonly used chlorofluorocarbon heat transfer fluids such as trichlorofluoromethane and di- chlorodifluoromethane. The widespread commercial use of chlorine-free refrigerant heat transfer fluids has been hindered, however, by the lack of commercially adequate lubricants. This is particularly true for one of the most desirable working fluids, 1,1,1,2-tetrafluoroethane, com- monly known in the art as "Refrigerant 134a" or simply "R134a". Other fluoro-substituted ethanes are also desir¬ able working fluids.

The following patents and published patent applica¬ tions also teach many general classes and specific exam- pies of polyol esters useful as refrigerant lubricants with chlorine-free fluoro group containing heat transfer fluids: US 4,851,144; UK 2 216 541; US 5,021,179; US 5,096,606; WO 90/12849 (Lubrizol) ; EP 0 406 479 (Kyodo Oil); EP 0 430 657 (Asahi Denka KK) ; EP 0 435 253 (Nippon Oil) ; EP 0 445 610 and 0 445 611 (Hoechst AG) ; EP 0449 406; EP 0 458 584 (Unichema Chemie BV) ; and EP 0 485 979 (Hitachi) . DESCRIPTION OF THE INVENTION

Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quant¬ ities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the term "about" in defining the broadest scope of the invention. Practice of the invention within the boundaries corresponding to the exact quantities stat¬ ed is usually preferable, however.

Home refrigerators of the normal type generally are required to maintain cool temperatures within a relatively small and well insulated space, and the temperature of their environment, to which waste heat must be discharged, does not normally vary over a wide range, because it usu¬ ally is a temperature in which humans can be reasonably comfortable. Therefore, relatively low power compressors are generally employed for such normal home refrigerators, and lubricants with relatively low viscosities at normal human comfort temperatures are generally satisfactory e- chanically. Because the environmental temperature never rises very high, any drastic decrease in viscosity of the lubricant with temperature is largely avoided. Under such conditions, low viscosity lubricants are preferred for economy of operation, because with all other factors held equal, a low viscosity lubricant reduces power consumption by the lubricated machinery, and, with the rising cost of electric power, consumers are becoming more sensitive to efficiency in their home appliances. On the other hand, a certain minimum viscosity is required to avoid excessive depletion of the lubricant from those surfaces of the re¬ frigerating machinery that need lubrication while running but are not immersed in the refrigerant working fluid, during periods of idleness for the compressor and other mechanically active parts of the refrigeration machinery. More specifically, esters according to this invention should have a viscosity of not more than 17, or with in¬ creasing preference in the order given, not more than 15.0, 14.2, 13.8, 13.0, 12.6, 12.0, 11.7, 11.3, or 11.0, centistokes at 40° C. Independently, esters according to this invention should have a viscosity of at least 6.1, or with increasing preference in the order given, at least 6.5, 6.9, 7.3, 7.7, 8.0, 8.3, 8.5, 8.7, or 8.9, centi¬ stokes at 40° C

It has now been found that selected polyol esters provide high quality lubrication for this kind of service. Specifically effective are esters or mixtures of esters made by reacting (i) a mixture of alcohol molecules in which at least 45 % are molecules of 2,2-dimethylol-l- butanol (also called "trimethylolpropane" and often ab- breviated hereinafter as "TMP") and at least 75 % of the remainder of the molecules are selected from the group consisting of 2,2-dimethylpropane-l,3-diol (also called "neopentyl glycol" and often abbreviated hereinafter as "NPG") , 2,2-dimethylol-l-propanol (also called "trimeth- ylol ethane" and often abbreviated hereinafter as "TME") , and 2,2-dimethylolpropane-l,3-diol (also called "penta- erythritol" and often abbreviated hereinafter as "PE") with (ii) a mixture of acid molecules in which at least 70 % of the molecules are those of pentanoic acid and at least 75 % of the balance of the molecules are selected from the group consisting of butanoic acid, 2-methylpropa- noic acid, 2-methylbutanoic acid, and 3-methylbutanoic acid. (Of course, for this and all the other types of esters described herein as part of the invention, it is possible to obtain the same esters or mixture of esters by reacting acid derivatives such as acid anhydrides, acyl chlorides, and esters of the acids with lower molecular weight alcohols than those desired in the ester products according to this invention, instead of reacting the acids themselves. The acids are generally preferred for economy and are normally specified herein, but it is to be under- stood that the esters defined herein by reaction with ac¬ ids can be equally well obtained by reaction of alcohols with the corresponding acid derivatives, or even by other reactions. The only critical feature is the mixture of acyl groups and alcohol moieties in the final mixture of esters formed.)

It is to be understood that only the desired alcohols and acids are explicitly specified, but some amount of the sort of impurities normally present in commercial or in¬ dustrial grade products can be tolerated in most cases. In general, however, it is preferred, with increasing pref¬ erence in the order given, that not more than 25, 21, 17, 12, 8, 5, 3, 2, 1, 0.5, or 0.2 % of either the hydroxyl groups in the alcohol mixtures specified herein or of the carboxyl groups in the acid mixtures specified herein should be part of any molecules other than those explicit¬ ly specified for each type of lubricant base stock. Per¬ centages of specific chemical molecules or moieties speci- fied herein, such as the percentages of carboxyl and hy¬ droxyl groups stated in the preceding sentence, are to be understood as number percentages, which will be mathemat¬ ically identical to percentages by chemical equivalents, with Avogadro's number of each specified chemical moiety regarded as a single chemical equivalent.

With increasing preference in the order given, at least 70, 78, 85, 89, 93, 96, or 98 % of the alcohol mole¬ cules reacted to make the ester molecules in the lubri- cants according to this invention are TMP. With increas¬ ing preference in the order given, at least 74, 78, 82, 85, 92, 97, or 99 % of the acid molecules reacted to make the esters according to this invention are selected from acids containing five carbon atoms per molecule, and inde- pendently, with increasing preference in the order given, at least 72, 76, 81, 85, 90, 93, 95, or 97 % of the total of the five carbon acid molecules in this acid component consist of pentanoic acid (straight chain) .

These preferences were determined by taking into ac- count a wide variety of factors determined empirically, including the following generalizations: Branched acids give esters with a lower viscosity index, but with usually higher absolute viscosities at normal ambient human com¬ fort temperatures or any lower temperature, than un- branched acids with the same number of carbon atoms. Al¬ cohols with two hydroxyl groups yield esters with lower viscosities than those with three hydroxyl groups, while alcohols with four or more hydroxyl groups yield esters of still higher viscosity. Acids with six or more carbon at- oms yield esters with more viscosity than those with fewer carbon atoms, and acids with four or, especially, fewer than four, carbon atoms are substantially more easily hy- drolyzed during use than those with higher numbers of car¬ bon atoms; such hydrolysis is disadvantageous, because the acid produced by hydrolysis promotes damaging corrosion of at least some of the types of metal surfaces normally be¬ ing lubricated. Also, the corrosiveness of a carboxylic acid to most metals found in refrigerator compressors in¬ creases as the number of carbon atoms in the acid molecule decreases.

As a result of these various factors, molecules of the ester of TMP with pentanoic acid have proved close to ideal for low viscosity lubrication, because they are as low in viscosity as is ever normally desirable for exist¬ ing refrigerating machinery, are miscible in all propor¬ tions with most or all fluorocarbon refrigerants over a temperature range of at least -55° C up to at least the boiling point of the fluorocarbon refrigerant at normal atmospheric pressure and often to still higher tempera¬ tures which can easily be reached in the compression stage of a refrigerator, are capable of solubilizing other high- er viscosity and less soluble hindered polyol esters, and are substantially less likely than esters of acids with fewer than five carbon atoms per molecule to promote cor¬ rosion.

The above descriptions for each of the acid and al- cohol mixtures reacted to produce lubricant esters accord¬ ing to this invention refers only to the mixture of acids or alcohols that actually reacts to form esters and does not necessarily imply that the mixtures of acids or alco¬ hols contacted with each other for the purpose of reaction will have the same composition as the mixture that actu¬ ally reacts. In fact, it has been found that reaction be¬ tween the alcohol(s) and the acid(s) used proceeds more effectively if the quantity of acid charged to the reac¬ tion mixture initially is enough to provide an excess of 10 - 25 % of equivalents of acid over the equivalents of alcohol reacted with the acid. (An equivalent of acid is defined for the purposes of this specification as the amount containing one gram equivalent weight of carboxyl groups, while an equivalent of alcohol is the amount con- taining one gram equivalent weight of hydroxyl groups.) The composition of the mixture of acids that actually re¬ acted can be determined by analysis of the product ester mixture for its acyl group content.

In making most or all of the esters and mixtures of esters preferred according to this invention, the acid(s) reacted will be lower boiling than the alcohol(s) reacted and the product ester(s) . When this condition obtains, it is preferred to remove the bulk of any excess acid remain¬ ing at the end of the esterification reaction by distilla¬ tion, most preferably at a low pressure such as 1 - 5 torr. After such vacuum distillation, the product is often ready for use as a lubricant or lubricant base stock ac¬ cording to this invention. If further refinement of the product is desired, the content of free acid in the prod¬ uct after the first vacuum distillation may be further reduced by treatment with epoxy esters as taught in U. S. Patent 3,485,754 or by neutralization with any suitable alkaline material such as lime, alkali metal hydroxide, or alkali metal carbonates. If treatment with epoxy esters is used, excess epoxy ester may be removed by a second distillation under very low pressure, while the products of reaction between the epoxy ester and residual acid may be left behind in the product without harm. If neutrali¬ zation with alkali is used as the refinement method, sub¬ sequent washing with water, to remove any unreacted excess alkali and the small amount of soap formed from the excess fatty acid neutralized by the alkali, is strongly pre¬ ferred before using the product as a lubricant and/or base stock according to this invention.

Under some conditions of use, the ester(s) as de- scribed herein will function satisfactorily as complete lubricants. It is generally preferable, however, for a complete lubricant to contain other materials generally denoted in the art as additives, such as oxidation resist¬ ance and thermal stability improvers, corrosion inhibi- tors, metal deactivators, lubricity additives, viscosity index improvers, pour and/or floe point depressants, de¬ tergents, dispersants, antifoaming agents, anti-wear agents, and extreme pressure resistant additives. Many additives are multifunctional. For example, certain ad¬ ditives may impart both anti-wear and extreme pressure resistance properties, or function both as a metal de- activator and a corrosion inhibitor. Cumulatively, all additives preferably do not exceed 8 % by weight, or more preferably do not exceed 5 % by weight, of the total com¬ pounded lubricant formulation.

An effective amount of the foregoing additive types is generally in the range from 0.01 to 5 % for the anti- oxidant component, 0.01 to 5 % for the corrosion inhibitor component, from 0.001 to 0.5 % for the metal deactivator component, from 0.5 to 5 % for the lubricity additives, from 0.01 to 2 % for each of the viscosity index improvers and pour and/or floe point depressants, from 0.1 to 5 % for each of the detergents and dispersants, from 0.001 to 0.1 % for anti-foam agents, and from 0.1 - 2 % for each of the anti-wear and extreme pressure resistance components. All these percentages are by weight and are based on the total lubricant composition. It is to be understood that more or less than the stated amounts of additives may be more suitable to particular circumstances, and that a single molecular type or a mixture of types may be used for each type of additive component. Also, the examples listed below are intended to be merely illustrative and not limiting, except as described in the appended claims.

Examples of suitable oxidation resistance and thermal stability improvers are diphenyl-, dinaphthyl-, and phenyl- naphthyl-amines, in which the phenyl and naphthyl groups can be substituted, e.g., N,N'-diphenyl phenylenediamine, p-octyldiphenylamine, p,p-dioctyldiphenylamine, N-phenyl- 1-naphthyl amine, N-pheny1-2-naphthyl amine, N-(p-dodecyl)- pheny1-2-naphthyl amine, di-1-naphthylamine, and di-2- naphthylamine; phenothazines such as N-alkylphenothia- zines; imino(bisbenzyl) ; and hindered phenols such as 6- (t-butyl) phenol, 2,6-di-(t-butyl) phenol, 4-methyl-2,6- di-(t-butyl) phenol, 4,4'-methylenebis(-2,6-di-{t-butyl} phenol) , and the like.

Examples of suitable cuprous metal deactivators are imidazole, benzamidazole, 2-mercaptobenzthiazole, 2,5-di- mercaptothiadiazole, salicylidine-propylenediamine, pyr- azole, benzotriazole, tolutriazole, 2-methylbenzamidazole, 3,5-dimethyl pyrazole, and methylene bis-benzotriazole. Benzotriazole derivatives are preferred. Other examples of more general metal deactivators and/or corrosion inhib¬ itors include organic acids and their esters, metal salts, and anhydrides, e.g., N-oleyl-sarcosine, sorbitan monoole- ate, lead naphthenate, dodecenyl-succinic acid and its par¬ tial esters and amides, and 4-nonylphenoxy acetic acid; primary, secondary, and tertiary aliphatic and cycloali- phatic amines and amine salts of organic and inorganic ac- ids, e.g., oil-soluble alkylam onium carboxylates; hetero- cyclic nitrogen containing compounds, e.g., thiadiazoles, substituted imidazolines, and oxazolines; quinolines, qui- nones, and anthraquinones; propyl gallate; barium dinonyl naphthalene sulfonate; ester and amide derivatives of al- kenyl succinic anhydrides or acids, dithiocarbamates, di- thiophosphates; amine salts of alkyl acid phosphates and their derivatives.

Examples of suitable lubricity additives include long chain derivatives of fatty acids and natural oils, such as esters, amines, amides, imidazolines, and borates.

Examples of suitable viscosity index improvers in¬ clude polymethacrylates, copolymers of vinyl pyrrolidone and methacrylates, polybutenes, and styrene-acrylate co¬ polymers. Examples of suitable pour point and/or floe point de¬ pressants include polymethacrylates such as methacrylate- ethylene-vinyl acetate terpolymers; alkylated naphthalene derivatives; and products of Friedel-Crafts catalyzed con¬ densation of urea with naphthalene or phenols. Examples of suitable detergents and/or dispersants in¬ clude polybutenylsuccinic acid amides; polybutenyl phos- phonic acid derivatives; long chain alkyl substituted aro- matic sulfonic acids and their salts; and metal salts of alkyl sulfides, of alkyl phenols, and of condensation prod¬ ucts of alkyl phenols and aldehydes.

Examples of suitable anti-foam agents include sili- cone polymers and some acrylates.

Examples of suitable anti-wear and extreme pressure resistance agents include sulfurized fatty acids and fatty acid esters, such as sulfurized octyl tallate; sulfurized terpenes; sulfurized olefins; organopolysulfides; organo phosphorus derivatives including amine phosphates, alkyl acid phosphates, dialkyl phosphates, aminedithiophos- phates, trialkyl and triaryl phosphorothionates , trialkyl and triaryl phosphines, and dialkylphosphites, e.g., amine salts of phosphoric acid monohexyl ester, amine salts of dinonylnaphthalene sulfonate, triphenyl phosphate, tri- naphthyl phosphate, diphenyl cresyl and dicresyl phenyl phosphates, naphthyl diphenyl phosphate, triphenylphos- phorothionate; dithiocarbamates, such as an antimony di¬ alkyl dithiocarbamate; chlorinated and/or fluorinated hy- drocarbons, and xanthates.

Under some conditions of operation, it is believed that the presence in lubricants of the types of polyether polyols that have been prominent constituents of most pri¬ or art lubricant base stocks taught as useful with fluoro- carbon refrigerant working fluids are less than optimally stable and/or inadequately compatible with some of the most useful lubricant additives. Thus, in one embodiment of this invention, it is preferred that the lubricant base stocks and lubricants be substantially free of such poly- ether polyols. By "substantially free", it is meant that the compositions contain no more than about 10 % by weight, preferably no more than about 2.6 % by weight, and more preferably no more than about 1.2 % by weight of the materials noted. One major embodiment of the present invention is a refrigerant working fluid comprising both a suitable heat transfer fluid such as a fluorocarbon and a lubricant ac- cording to this invention. Preferably, the refrigerant working fluid and the lubricant should have chemical characteristics and be present in such a proportion to each other that the working fluid remains homogeneous, i.e., free from visually detectable phase separations or turbidity, over the entire range of working temperatures to which the working fluid is exposed during operation of a refrigeration system in which the working fluid is used. This working range may vary from -60° C to as much as +175° C. It is often adequate if the working fluid remains single phase up to +30° C, although it is increasingly more preferable if the single phase behavior is maintained up to 40, 56, 71, 88, or 100 ^β C. Similar¬ ly, it is often adequate if the working fluid compositions remains a single phase when chilled to 0° C, although it is increasingly more preferable if the single phase behav¬ ior persists to -10, -20, -30, -40, or -55 ^β C. Single phase mixtures with chlorine free hydrofluorocarbon re¬ frigerant working fluids are usually obtained with the suitable and preferred types of esters described above.

Inasmuch as it is often difficult to predict exactly how much lubricant will be mixed with the heat transfer fluid to form a working fluid, it is most preferable if the lubricant composition forms a single phase in all proportions with the heat transfer fluid over the temper¬ ature ranges noted above. This however, is a very strin¬ gent requirement, and it is often sufficient if there is single phase behavior over the entire temperature range for a working fluid mixture containing up to 1 % by weight of lubricant according to this invention. Single phase behavior over a temperature range for mixtures containing up to 2, 4, 10, and 15 % by weight of lubricant is suc¬ cessively more preferable.

In some cases, single phase behavior is not required. The term "miscible" is used in the refrigeration lubrica¬ tion art and hereinafter, except when part of the phrase "miscible in all proportions", when two phases are formed but are readily capable of being mixed into a uniform dis¬ persion that remains stable as long as it is at least mod¬ erately agitated mechanically. Some refrigeration (and other) compressors are designed to operate satisfactorily with such miscible mixtures of refrigerant working fluid and lubricant. In contrast, mixtures that lead to coagu¬ lation or significant thickening and form two or more phases are unacceptable commercially and are designated herein as "immiscible". Any such mixture described below is a comparative example and not an embodiment of the present invention.

Another major embodiment of the invention is the use of a lubricant according to the invention, either as total lubricant or lubricant base stock, in a process of operat- ing refrigerating machinery in such a manner that the lub¬ ricant is in contact with the refrigerant working fluid.

The practice of the invention may be further under¬ stood and appreciated by consideration of the following examples and comparative examples. General Ester Synthesis Procedure

The alcohol(s) and acid(s) to be reacted, together with a suitable catalyst such as dibutyltin diacetate, tin oxalate, phosphoric acid, and/or tetrabutyl titanate, were charged into a round bottomed flask equipped with a stir- rer, thermometer, nitrogen sparging means, condenser, and a recycle trap. Acid(s) were charged in about a 15 % mol¬ ar excess over the alcohol(s). The amount of catalyst was from 0.02 to 0.1 % by weight of the weight of the total acid(s) and alcohol(s) reacted. The reaction mixture was heated to a temperature be¬ tween about 220 and 230° C, and water from the resulting reaction was collected in the trap while refluxing acids were returned to the reaction mixture. Partial vacuum was maintained above the reaction mixture as necessary to achieve a reflux rate of between 8 and 12 % of the orig¬ inal reaction mixture volume per hour.

The reaction mixture was sampled occasionally for de- termination of hydroxyl number, and after the hydroxyl num¬ ber had fallen below 5.0 mg of KOH per gram of mixture, the majority of the excess acid was removed by distilla¬ tion after applying the highest vacuum obtainable with the apparatus used, corresponding to a residual pressure of about 0.05 torr, while maintaining the reaction temper¬ ature. The reaction mixture was then cooled, and any re¬ sidual acidity was removed, if desired, by treatment with lime, sodium hydroxide, or epoxy esters. The resulting lubricant or lubricant base stock was dried and filtered before phase compatibility testing.

General Procedure for Phase Compatibility Testing

One milliliter ("ml") of the lubricant to be tested is placed into a thermal shock resistant, volumetrically graduated glass test tube 17 millimeters ("mm") in diam¬ eter and 145 mm long. The test tube is then stoppered and placed into a cooling bath regulated to -29 ± 0.2° C. Af¬ ter the tube and contents have equilibrated in the cooling bath for 5 minutes ("min") , sufficient refrigerant working fluid is added to give a total volume of 10 ml.

At least 15 min after the working fluid has been add¬ ed, during which time the tube and contents have been equilibrating in the cooling bath and the contents may have been agitated if desired, the tube contents are visu- ally examined for evidence of phase separation. If there is any such phase separation, the tube is shaken to deter¬ mine whether the combination can be rated as miscible or is totally unacceptable.

If there is no evidence of phase separation at -29° C, the temperature of the cooling bath is usually lowered at a rate of 0.3° per min until phase separation is ob¬ served. The temperature of first observation of phase separation, if within the range of the cooling equipment used, is then noted as the insolubility onset temperature. Composition of Specific Example

A suitable ester mixture as described above was pre¬ pared by reacting a mixture of alcohol molecules in which 99.9 % were TMP molecules, with most of the remainder be¬ ing DTMP molecules, with a mixture of acid molecules that included 99.6 % pentanoic (= n-valeric) acid, with the re¬ mainder predominantly 2-methylbutanoic acid. This lubri- cant base stock had an ISO grade of 10.

Claims

The invention claimed is: CLAIMS

1. A composition of matter suitable for serving as a lubricant or lubricant base stock, said composition being a liquid with a viscosity of not more than 17 centistokes at 40° C and consisting essentially of a mixture of polyol ester molecules in which (i) at least about 45 % of the alcohol moieties are those of TMP and a total of at least about 75 % of the remainder of the alcohol moieties are selected from those of the group consisting of NPG, TME, and PE and (ii) at least 70 % of the acyl groups are straight chain pentanoyl groups and at least about 75 % of the balance of the acyl groups are those selected from the group consisting of butanoyl, 2-methylpropanoyl, 2-methyl- butanoyl, and 3-methylbutanoyl.

2. A composition according to claim l, wherein (i) at least about 70 % of the alcohol moieties are those of TMP and at least about 75 % of the remainder of the alcohol moieties are selected from those of the group consisting of NPG, TMP, TME, and PE; (ii) at least about 85 % of the acyl groups contain five carbon atoms each and at least about 85 % of these acyl groups containing five carbon atoms each are straight chain pentanoyl groups; and (iii) the viscosity of the composition is not greater than about 15.0 centistokes at 40° C.

3. A composition according to claim 2, wherein (i) at least about 78 % of the alcohol moieties are those of TMP and at least about 75 % of the remainder of the alcohol moieties are selected from those of the group consisting of NPG, TMP, TME, and PE; (ii) at least about 85 % of the acyl groups contain five carbon atoms each and at least about 90 % of these acyl groups containing five carbon atoms each are straight chain pentanoyl groups; and (iii) the viscosity of the composition is not greater than about 14.2 centistokes at 40° C.

4. A composition according to claim 3, wherein (i) at least about 85 % of the alcohol moieties are those of TMP and at least about 75 % of the remainder of the alcohol moieties are selected from those of the group consisting of NPG, TMP, TME, and PE; (ii) at least about 92 % of the acyl groups contain five carbon atoms each and at least about 90 % of these acyl groups containing five carbon atoms each are straight chain pentanoyl groups; and (iii) the viscosity of the composition is not greater than about 13.8 centistokes at 40° C.

5. A composition according to claim 4, wherein (i) at least about 96 % of the alcohol moieties are those of TMP and at least about 75 % of the remainder of the alcohol moieties are selected from those of the group consisting of NPG, TMP, TME, and PE; (ii) at least about 97 % of the acyl groups contain five carbon atoms each and at least about 93 % of these acyl groups containing five carbon atoms each are straight chain pentanoyl groups; and (iii) the viscosity of the composition is not greater than about 11.3 centistokes at 40° C.

6. A compounded lubricant consisting essentially of at least about 95 % by weight of a composition according to claim 5 and a balance of one or more additives selected from the group consisting of oxidation resistance and thermal stability improvers, corrosion inhibitors, metal deactivators, lubricity additives, viscosity index im¬ provers, pour and floe point depressants, detergents, dis- persants, antifoaming agents, anti-wear agents, and ex¬ treme pressure resistant additives.

7. A compounded lubricant consisting essentially of at least about 92 % by weight of a composition according to claim 3 and a balance of one or more additives selected from the group consisting of oxidation resistance and thermal stability improvers, corrosion inhibitors, metal deactivators, lubricity additives, viscosity index im¬ provers, pour and floe point depressants, detergents, dis- persants, antifoa ing agents, anti-wear agents, and ex¬ treme pressure resistant additives.

8. A compounded lubricant consisting essentially of at least about 92 % by weight of a composition according to claim 2 and a balance of one or more additives selected from the group consisting of oxidation resistance and thermal stability improvers, corrosion inhibitors, metal deactivators, lubricity additives, viscosity index im¬ provers, pour and floe point depressants, detergents, dis- persants, antifoaming agents, anti-wear agents, and ex¬ treme pressure resistant additives.

9. A compounded lubricant consisting essentially of at least about 92 % by weight of a composition according to claim 1 and a balance of one or more additives selected from the group consisting of oxidation resistance and thermal stability improvers, corrosion inhibitors, metal deactivators, lubricity additives, viscosity index im¬ provers, pour and floe point depressants, detergents, dis- persants, antifoaming agents, anti-wear agents, and ex- treme pressure resistant additives.

10. A refrigerant working fluid consisting essentially of at least 85 % by weight of a primary heat transfer fluid selected from the group consisting of pentafluoroethane, 1,1-difluoroethane, 1,1,1-trifluroethane, tetrafluoroeth- anes, and mixtures thereof and a balance of a lubricant according to claim 9.

11. A refrigerant working fluid consisting essentially of at least 85 % by weight of a primary heat transfer fluid selected from the group consisting of pentafluoroethane, 1,1-difluoroethane, 1,1,1-trifluroethane, tetrafluoroeth- anes, and mixtures thereof and a balance of a lubricant according to claim 8.

12. A refrigerant working fluid consisting essentially of at least 85 % by weight of a primary heat transfer fluid selected from the group consisting of pentafluoroethane, 1,1-difluoroethane, 1,1,1-trifluroethane, tetrafluoroeth- anes, and mixtures thereof and a balance of a lubricant according to claim 7.

13. A refrigerant working fluid consisting essentially of at least 85 % by weight of 1,1,1,2-tetrafluroethane and a balance of a lubricant according to claim 6.

14. A refrigerant working fluid consisting essentially of at least 85 % by weight of a primary heat transfer fluid selected from the group consisting of pentafluoroethane, 1,1-difluoroethane, 1,1,1-trifluroethane, tetrafluoroeth- anes, and mixtures thereof and a balance of a lubricant according to claim 5.

15. A refrigerant working fluid consisting essentially of at least 85 % by weight of a primary heat transfer fluid selected from the group consisting of pentafluoroethane, 1,1-difluoroethane, 1,1,1-trifluroethane, tetrafluoroeth- anes, and mixtures thereof and a balance of a lubricant according to claim 4.

16. A refrigerant working fluid consisting essentially of at least 85 % by weight of a primary heat transfer fluid selected from the group consisting of pentafluoroethane, 1,1-difluoroethane, 1,1,1-trifluroethane, tetrafluoroeth- anes, and mixtures thereof and a balance of a lubricant according to claim 3.

17. A refrigerant working fluid consisting essentially of at least 85 % by weight of a primary heat transfer fluid selected from the group consisting of pentafluoroethane, 1,1-difluoroethane, 1,1,1-trifluroethane, tetrafluoroeth- anes, and mixtures thereof and a balance of a lubricant according to claim 2.

18. A refrigerant working fluid consisting essentially of at least 85 % by weight of a primary heat transfer fluid selected from the group consisting of pentafluoroethane, 1,1-difluoroethane, l,1,1-trifluroethane, tetrafluoroeth- anes, and mixtures thereof and a balance of a lubricant according to claim 1.

19. A process for operating a refrigerator comprising cyc¬ lic compression, liquefaction, expansion, and evaporation of a heat transfer fluid, said heat transfer fluid consist- ing essentially of at least 85 % by weight of 1,1,1,2- tetrafluoroethane and a balance of a lubricant according to claim 6.

20. A process for operating a refrigerator comprising cyclic compression, liquefaction, expansion, and evapor- ation of a heat transfer fluid, said heat transfer fluid consisting essentially of at least 85 % by weight of a primary heat transfer fluid selected from the group con¬ sisting of pentafluoroethane, 1,1-difluoroethane, 1,1,1- trifluroethane, tetrafluoroethanes, and mixtures thereof and a balance of a lubricant according to claim 1.