FOLYOL ESTER LUBRICANTS FOR REFRIGERANT HEAT TRANSFER

FLUIDS

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to lubricants, lubricant base stocks, refrigerant working fluids including lubricants along with primary heat transfer fluids, and methods for using these materials. The lubricants and lubricant base stocks are particularly suitable for use with substan¬ tially chlorine-free, fluoro-group-containing organic refrigerating heat transfer fluids such as tetrafluoro- ethanes.

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". Other fluoro-substituted ethanes are also desirable working flu¬ ids. U. S. Patent 5,021,179 of June 4, 1991 to Zehler et al. (the same applicants as for this invention) discloses and claims many general classes and specific types of pol- yol esters suitable for refrigerant lubricants, including some but not all of the specific teachings below. The following patents and published patent applica¬ tions also teach many general classes and specific exa -

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pies of polyol esters useful as refrigerant lubricants with chlorine-free fluoro group containing heat transfer fluids, but are not believed to teach any of the specific esters or mixtures of esters in the appended claims here- in:

U. S. Patent 5,096,606 of March 17, 1992 to Hagihara et al.

WO 90/12849 EP 0 406 479 EP 0 430 657

EP 0 435 253

EP 0 445 610 and 0 445 611 UK 2 216 541. 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 stated is usually preferable, however. Summary of the Invention

It has now been found that certain esters and mix- tures of esters of polyols provide excellent lubricants and/or lubricant base stocks for use with fluoro-group- containing refrigerant heat transfer fluids, particularly chlorine-free fluorocarbon heat transfer fluids, most particularly 1,1,1,2-tetrafluoroethane. These esters contain a sufficient fraction of acyl groups with branched chains, i.e., groups containing at least one carbon atom that is bonded to at least three other carbon atoms by single bonds, and/or acyl groups with a total of from one to six, more preferably from one to five, carbon atoms, as described in detail in U. S. Patent 5,021,179, the entire specification of which, except to the extent duplicative of or inconsistent with any explicit statement herein, is

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hereby incorporated herein by reference. Most such esters suitable for use in the invention are known compounds per se, and all may be prepared by known methods.

In making most or all of the preferred esters accord- ing to this invention, the acid(s) reacted will be lower boiling than the alcohol(s) reacted and the product est¬ er(s) . When this condition obtains, it is preferred to remove the bulk of any excess acid remaining at the end of the esterification reaction by distillation, most prefer- ably at a very low pressure such as 0.05 torr.

After such vacuum distillation, the product is often ready for use as a lubricant and/or base stock according to this invention. If further refinement of the product is desired, the content of free acid in the product 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 be¬ hind in the product without harm. If neutralization with alkali is used as the refinement method, subsequent wash- ing 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 preferred before using the product as a lubricant and/or base stock accord¬ ing 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 inhib¬ itors, metal deactivators, lubricity additives, viscosity index improvers, pour and/or floe point depressants,

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detergents, dispersants, antifoaming agents, anti-wear agents, and extreme pressure resistant additives. Many additives are multifunctional. For example, certain additives 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 lub¬ ricant 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 phen- ylnaphthyl-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-dodec- yl)phenyl-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-

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di-(t-butyl) phenol, 4,4 ^I -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-methylbenzamida- zole, 3,5-dimethyl pyrazole, and methylene bis-benzotria- zole. Benzotriazole derivatives are preferred. Other ex¬ amples of more general metal deactivators and/or corro- sion inhibitors include organic acids and their esters, metal salts, and anhydrides, e.g., N-oleyl-sarcosine, sor- bitan monooleate, lead naphthenate, dodecenyl-succinic acid and its partial esters and amides, and 4-nonylphenoxy acetic acid; primary, secondary, and tertiary aliphatic and cycloaliphatic amines and amine salts of organic and inorganic acids, e.g., oil-soluble alkylammonium carboxyl- ates; heterocyclic nitrogen containing compounds, e.g., thiadiazoles, substituted imidazolines, and oxazolines; quinolines, quinones, and anthraquinones; propyl gallate; barium dinonyl naphthalene sulfonate; ester and amide derivatives of alkenyl succinic anhydrides or acids, di- thiocarbamates, dithiophosphates; 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 copolymers.

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 include polybutenylsuccinic acid amides; polybutenyl phos-

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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 products 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 dialkyl dithiocarba ate; chlorinated and/or fluorinated hydrocarbons, 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 by 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

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transfer fluid such as a fluorocarbon and a lubricant ac¬ cording to this invention. Preferably, the two necessary components should have chemical characteristics and be present in such a proportion to each other that the work- ing 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 ade¬ quate if the working fluid remains single phase up to +30° C, although it is increasingly more preferable if the sin¬ gle phase behavior is maintained up to 40, 56, 71, 88, or 100 ° C. Similarly, 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 behavior persists to -10, -20, -30, -40, or - 55 ° C. Single phase mixtures with chlorine free hydro- fluorocarbon refrigerant working fluids can often be ob- tained with the suitable and preferred types of esters described above, with the most preferred esters most likely to give such single phase behavior over a wide temperature range.

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.

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The term "miscible" is used in the refrigeration lubrica¬ tion art and in this description when two phases are formed but are readily capable of being mixed into a uni¬ form dispersion that remains stable as long as it is at least moderately agitated mechanically. Some refrigera¬ tion (and other) compressors are designed to operate sat¬ isfactorily with such miscible mixtures of refrigerant working fluid and lubricant. In contrast, mixtures that lead to coagulation or significant thickening and form two or more phases are unacceptable commercially and are des¬ ignated herein as "immiscible". Any such mixture de¬ scribed below is a comparative example and not an embod¬ iment of the present invention.

The ranges and preferred ranges of viscosity and var- iation of viscosity with temperature for lubricant compo¬ sitions according to this invention are generally the same as established in the art for lubricants to be used in re¬ frigeration systems together with a heat transfer fluid, particularly a fluorocarbon and/or chlorofluorocarbon heat transfer fluid. In general, it is preferred that lubri¬ cants according to this invention have International Or¬ ganization for Standardization ("ISO") viscosity grade numbers between 10 and 220, or more preferably between 10 and 100. The viscosity ranges for the ISO viscosity grade numbers are given in Table 1.

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(ε) 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

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Table 1

ISO Viscosity Grade Number Viscosity Range in Centi- stokes at 40 ° C

2 3 5

7 10 15

22 32 46

68 100 150

220 320 460

680 1,000 1,500

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 number had fallen below 15.0 mg of KOH per gram of mixture for reactions including divalent acid, or below 5.0 mg of KOH per gram of mixture for other reactions, the majority

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of the excess acid was removed by distillation after ap¬ plying the highest vacuum obtainable with the apparatus used, corresponding to a residual pressure of about 0.05 torr, while maintaining the reaction temperature. The re- action mixture was then cooled, and any residual 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.

Results of compatibility testing of several esters and ester mixtures with Refrigerant 134a are shown in Tab- les 2 - 5 following. In these tables, all percentages are by weight unless otherwise stated.

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Notes for Table 2 ISOP = Union Carbide commercial "ISOPENTANOIC ACID" = about 35 % 2-methyl butanoic acid and about 65 % penta¬ noic acid; i-C ₇ = (about 65 % 2-methyl hexanoic acid + about 20 % 2-ethyl pentanoic acid + about 10 % heptanoic acid + a balance of other C _? acids) ; n-C ₇ = > 90 % hepta- noic acid; i-C ₉ = > 90 % 3,5,5-trimethyl hexanoic acid; n- C ₉ = > 90 % nonanoic acid; n-C ₈ = > 90 % octanoic acid; n- C ₁₀ = > 90 % decanoic acid. For all the esters in this table, the alcohol moieties were derived from a mixture of 85 % pentaerythritol and 15 % dipentaerythritol.

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Table 3: LUBRICANTS OR LUBRICANT BASE STOCKS INCLUDING

DIVALENT ACID MOIETIES

Polyol Used Dibasic Acid Used Phase Compatibility

Test Result at -29° C

PE Adipic Soluble

(11 % DPE ^{+ AdiPiC SOlUble}

TMP Adipic . Miscible

PE Azelaic Miscible

Notes for Table 3

Esters were synthesized from a mixture with an equivalents ratio of 1.00 : 0.25 : 0.75 for polyol(s) : dibasic acid : monobasic acids. The monobasic acids in each case were Union Carbide "ISOPENTANOIC ACID", a mixture of about 65 % of pentanoic and 35 % of 2-methyl butanoic acids. The compatibility tests in these instances were not extended below -29° C. PE = pentaerythritol; DPE = dipentaerythri- tol; TMP = 2,2-dimethylol-l-butanol; all alcohols were > 95 % pure.

Notes for Table 4

All alcohols were > 95 % pure. "ISOPENTANOIC ACID" as in Table 3 was the acid used.

Some additional particularly useful esters and mix¬ tures of esters for use with 1,1,1,2-tetrafluoroethane specifically are described in Table 6. All of these es¬ ters and mixtures of esters are at least "miscible" as defined above with, and in most cases fully soluble in, this chlorine-free refrigerant. These esters were made by the same general methods as described above, with neces¬ sary modifications as known to those skilled in the art. For this special purpose, it has been found that dipenta- erythritol, even in the amount of 15 % commonly present in technical grade pentaerythritol, is disadvantageous, and

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Table 5 : EXAMPLES OF SUITABLE LUBRICANTS INCLUDING ADDITIVES

Additive Amount, %

Trade Name Chemical and Functional of Esters Characterization

Irganox L-109 phenolic antioxidant 0.5 Irganox™ L-57 amine antioxidant 0.5 Irganox™ L-115 sulfur-containing phenolic antioxidant 0.5

Van lube™ 7723 dithiocarbamate antioxidant and extreme pressure agent 0.5

Emery™ 9844 sulfurized ester friction 0.5 modifier

Syn O Ad TM 8478 triarylphosphate ester anti- 0.5 wear agent

Irgalube™ 349 amine phosphate anti-wear 0.1 agent and rust inhibitor

Reocor™ 12 alkenyl succinic acid de¬ 0.1 rivative rust inhibitor

Cobratec™ 99 benzotriazole copper cor¬ rosion inhibitor 0.1

Reomet™ 39 triazole derivative copper 0.1 corrosion inhibitor

Notes for Table 5

For all examples in this table, the base stock was a mix¬ ture of esters of (26 % 2-methyl butanoic acid + 49 % pen¬ tanoic acid + 25 % 3,5,5-trimethyl hexanoic acid) with (85 % pentaerythritol + 15 % dipentaerythritol) . All the mix¬ tures shown in this table were fully soluble in the phase compatibility test with Refrigerant 134a at -40° C; in this case, the tests were not extended to lower temper¬ atures.

it is preferred, with increasing preference in the order given, that the PE used to make the ester lubricants con¬ tain no more than 10, 4, 1.8, 0.9, or 0.5 % by weight of dipentaerythritol.

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Table 6

SOME ESTERS AND ESTER MIXTURES ESPECIALLY SUITED FOR USE WITH 1,1,1,2-TETRAFLUOROETHANE (REFRIGERANT 134a)

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Notes for Table 6

The abbreviations for the alcohols and acids used have the same meaning as in the preceding tables, where the same abbreviations are used. In addition "di-C ₆ " = adipic acid; "n-C ₅ " = technical grade pentanoic acid with > 90 pure pentanoic acid, except that when n-C ₅ and i-C ₅ are shown in the same mixture, they mean pentanoic and 2- methylbutanoic acid respectively and were both derived from ISOPENTANOIC ACID of Union Carbide as noted in the preceding tables; "i*-C ₉ " = ECR 1900, a commercial product of Exxon Corp. , reported to be a mixture of many isomeric branched C _g monobasic acids; and "i**-c ₅ " = 3-methylbuta- noic acid. The PE used for the preparation of the esters and mixtures thereof in this table had a purity > 98 %.) The percentages shown for alcohols and acids used to make the esters shown are percentages of the total amount of each chemical type (acid or alcohol) separately consid¬ ered. The amount of acid(s) used in the synthesis was al¬ ways in excess of the stoichiometric amount required for complete esterification of the alcohol(s) used.

The esters described in Table 6 had the following ISO

Viscosity Grades: ISO 10 - Item 6.1; ISO 32 - Items 6.2,

6.3, 6.4, 6.9, and 6.10; ISO 46 - Items 6.5, 6.6, and 6.11; ISO 68 - Item 6.7; ISO 100 - Item 6.8; and ISO about

1000 - Item 6.12.

The invention claimed is:

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