WORKING FLUID COMPOSITION

Abstract: A working fluid composition comprising (A) a refrigerant which consists essentially of difluoromethane (R-32) and (B) a lubricant comprising an ester base oil which comprises the reaction product of at least one neopentyl polyol and at least one aliphatic carboxylic acid or an esterifiable derivative thereof. A heat transfer device which contains the working fluid composition is also claimed.

Description: WORKING FLUID COMPOSITION The present invention relates to working fluid compositions for use in heat transfer devices and is particularly concerned with working fluid compositions for use in heat transfer devices which comprise means for controlling the compressor discharge temperature. Heat transfer devices of the mechanical compression type such as refrigerators, freezers, heat pumps and automobile air conditioning systems are well known. In such devices a refrigerant liquid of a suitable boiling point evaporates at low pressure taking heat from a surrounding zone. The resulting vapour is then compressed and passes to a condenser where it condenses and gives off heat to a second zone. The condensate is then returned through an expansion valve to the evaporator so completing the cycle. The mechanical energy required for compressing the vapour and pumping the liquid may be provided by an electric motor or an internal combustion engine. In addition to having a suitable boiling point and a high latent heat of vaporisation, the properties preferred of a refrigerant include low toxicity, non- flammability , non-corrosivity , high stability and freedom from objectionable odour. Hitherto, heat transfer devices have tended to uses chlorofluorocarbon refrigerants such as dichlorodifluoromethane (Refrigerant R-12) , chlorodifluoromethane (Refrigerant R-22) , or the azeotropic mixture of chlorodifluoromethane and chloropenta luoroethane (Refrigerant R-115); the azeotrope being Refrigerant R-502. However, the fully and partially halogenated chlorofluorocarbons have been implicated in the destruction of the earth's protective ozone layer and as a result the use and production thereof is to be severely limited by international agreement. Whilst heat transfer devices of the type to which the present invention relates are essentially closed systems, loss of refrigerant to the atmosphere can occur due to leakage during operation of the equipment or during maintenance procedures. It is important, therefore, to replace fully and partially halogenated chlorofluorocarbon refrigerants by materials having low or zero ozone depletion potentials. In addition to the possibility of ozone depletion, it has been suggested that significant concentrations of chlorofluorocarbon refrigerants in the atmosphere might contribute to global warming (the so-called greenhouse effect). It is desirable, therefore, to use refrigerants which have relatively short atmospheric lifetimes as a result of their ability to react with other atmospheric constituents such as hydroxyl radicals . The use of certain hydrofluoroalkanes , i.e. compounds which contain only carbon, hydrogen and fluorine atoms in their structure, in place of the chlorofluorocarbons has been proposed. The hydrofluoroalkanes of particular interest are those compounds which have comparable boiling points and other thermal properties to the chlorofluorocarbons which they are replacing, but which are also less damaging or benign to the ozone layer. Particular mention may be made of 1 , 1 , 1 , 2-tetrafluoroethane (R-134a ) . In a heat transfer device, the refrigerant forms part of a working fluid composition which also comprises a lubricant. The lubricant circulates around the device along with the refrigerant and provides for continual lubrication of the compressor. In addition to possessing good lubricity in the presence of the refrigerant, the properties desired of a lubricant - include good hydrolytic stability and good thermal stability. Moreover, in order to provide for the return of the lubricant to the compressor, the lubricant should be compatible with the refrigerant, which in practice means that the lubricant and refrigerant -- should possess a degree of mutual solubility, i.e. the lubricant and the refrigerant should be at least partially soluble in one another. Hitherto, heat transfer devices have tended to use mineral oils as lubricants. The good solubility of 5 chlorofluorocarbons with mineral oils allows the mineral oil to circulate around the heat transfer device together with the chlorofluorocarbon, and this in turn ensures effective lubrication of the compressor. Unfortunately, however, the replacement 0 refrigerants tend to have different solubility characteristics to the chlorofluorocarbons presently in use and tend to be insufficiently soluble in mineral oils to allow the latter to be used as lubricants. Consequently, the need to replace the 5 chlorofluorocarbon refrigerants has presented industry with very real difficulties, since not only is there the problem of finding a viable replacement refrigerant which has the required low or zero ozone depletion potential, but also, in many cases, there is also the 0 problem of developing a lubricant which will perform satisf ctorily with the replacement refrigerant. The present applicants have developed a working fluid composition which comprises a refrigerant having a zero ozone depletion potential and a lubricant which - A - is able to provide effective lubrication of the compressor in the presence of the refrigerant. The working fluid composition of the invention can provide a viable replacement for at least some of the working fluid compositions presently in use which comprise a chlorofluorocarbon refrigerant and a mineral oil lubricant . According to the present invention there is provided a working fluid composition which comprises: (A) a refrigerant consisting essentially of difluoromethane (R-32); and (B) sufficient to provide lubrication of a lubricant comprising an ester base oil which comprises the reaction product of at least one neopentyl polyol and at least one aliphatic carboxylic acid or an esterifiable derivative thereof. Difluoromethane, being a small molecule, will tend to result in a high compressor discharge temperature when used for high lift, single stage compression. In consequence, heat transfer devices which use difluoromethane as the refrigerant will preferably comprise means for controlling the compressor discharge temperature in terms of limiting the maximum discharge temperature which is attained. Heat transfer devices which are able to control the compressor discharge temperature are known in the art. Typically, this temperature control is brought about by injecting a flow of lubricant into the compressor and/or by injecting a flow of liquid refrigerant into the compressor. Both these techniques provide for efficient cooling of the compressor and thus control of the compressor discharge temperature. Heat transfer devices which comprise means for controlling the compressor discharge temperature often employ screw compressors, which may be of single or twin screw construction, although heat transfer devices of the reciprocating compressor type which make use of liquid refrigerant injection for cooling purposes are also known in the art. Preferably, the heat transfer device should be able to maintain the compressor discharge temperature below 150°C, for example below 130°C. Accordingly, in a preferred aspect of the present invention there is provided a heat transfer device which comprises means for controlling the compressor discharge temperature and which uses as a working fluid composition a composition comprising: (A) a refrigerant consisting essentially of difluoromethane (R-32); and (B) sufficient to provide lubrication of a lubricant comprising an ester base oil which comprises the reaction product of at least one neopentyl polyol and at least one aliphatic carboxylic acid or an esterifiable derivative thereof. The lubricant component of the working fluid composition of the invention comprises an ester base oil. The ester base oil should be capable of dissolving enough of the difluoromethane refrigerant to allow for the transport of the lubricant around the heat transfer system and the return of the lubricant to the compressor. In general, this means that the ester base oil should be capable of dissolving at least 5.0 Z by weight on the weight of the oil of the difluoromethane refrigerant at a pressure of 40 psi (40 lb/in 2 ) and a temperature of 0°C. Generally, the lubricant component should have a viscosity at 40°C within the range of from 46 to 220 cSt. When the heat transfer device is of a type which uses a liquid refrigerant injection technique to control the compressor discharge temperature, the lubricant component will preferably have a viscosity at 40°C in the range of from 46 to 100 cSt, more preferably in the range of from 46 to 85 cSt and especially in the range of from 46 to 68 cSt. However, when the heat transfer device is of a type which uses the cooling action produced by injecting a flow of lubricant to control the compressor discharge temperature, the lubricant component will preferably have a viscosity at 40°C in the range of from 68 to 220 cSt, more preferably in the range of from 100 to 190 cSt and especially in the range of from 125 to 175 cSt. The viscosity index of the lubricant component, which is a measure of how the viscosity changes with temperature, is preferably in the range of from 50 to 200, more preferably in the range of from 60 to 150 and particularly preferably in the range of from 90 to 130. In addition, preferred lubricants have a pour point of below -20°C, more preferably below -30°C and particularly preferably below -40°C. Suitable neopentyl polyols for the formation of the ester base oil include pentaerythritol , polypentaerythritols such as di- and tripentaerythritol , trimethylol alkanes such as trimethylol ethane and trimethylol propane, and neopentyl glycol. The esters may be formed with linear and/or branched aliphatic carboxylic acids, such as linear and/or branched alkanoic acids. A minor proportion of an aliphatic polycarboxylic acid, e.g. an aliphatic dicarboxylic acid, may also be used in the synthesis of the ester in order to increase the viscosity thereof. However, where such an aliphatic polycarboxylic acid is employed in the synthesis, it will preferably constitute no more than 30 mole Z, more preferably no more than 10 mole Z of the carboxylic acids used in the synthesis. Usually, the amount of the carboxylic acid(s) which is used in the synthesis will be sufficient to esterify all of the hydroxyl groups contained in the polyol, although residual hydroxyl functionality may be acceptable. It will be appreciated that esterifiable derivatives of carboxylic acids may be used in the synthesis if desired. A preferred lubricant for use in the working fluid composition of the invention is one in which the ester base oil comprises the reaction product of at least one neopentyl polyol selected from pentaerythritol, dipentaerythritol ar * ! tripentaerythritol and at least one aliphatic carboxylic acid selected from the straight chain (linear) aliphatic carboxylic acids and the branched aliphatic carboxylic acids . As indicated above, esterifiable derivatives of the carboxylic acids may also be used in the synthesis, such as the acyl halides, anhydrides and lower alkyl esters thereof, and hereinafter whenever reference is made to the use of carboxylic acids in the synthesis of the ester base oil the use of esterifiable derivatives thereof is not to be excluded. Suitable acyl halides are the acyl chlorides and suitable lower alkyl esters are the methyl esters . The base oil in this preferred lubricant comprises one or more ester compounds of formula: R(0 - C - Rl) n wherein R is the hydrocarbon radical remaining after removing the hydroxyl groups from pentaerythritol, dipentaer thritol or tripentaerythritol ; each ΕL -~ is, independently, a straight chain (linear) aliphatic hydrocarbyl group or a branched aliphatic hydrocarbyl group; and n is an integer of 4, 6 or 8. The aliphatic hydrocarbyl groups specified for -- above may be substituted, e.g. by pendant atoms or groups such as chloro, fluoro and bromo , and/or by in chain hetero atoms such as oxygen and nitrogen. Preferably, however, such hydrocarbyl groups are unsubstituted and thus contain only carbon and hydrogen atoms . It will be appreciated that a suitable base oil may comprise a single ester compound of Formula I, i.e. the product which is formed from the reaction of a single pentaerythritol type neopentyl polyol and a single carboxylic acid. Alternatively, a suitable base oil may comprise two or more compounds of Formula I. Such a base oil may be prepared by utilising two or more pentaerythritol type neopentyl polyols and/or two or more carboxylic acids in the synthesis of the ester, or by combining a mixture of different ester compounds each of which is the reaction product of a particular pentaerythritol type neopentyl polyol and a particular carboxylic acid. Furthermore, different mixed ester compositions, each of which has been prepared by utilising two or more pentaerythritol type neopentyl polyols and/or two or more carboxylic acids in their synthesis, may also be blended together to form the ester base oil . A preferred ester base oil comprises one or more compounds of Formula I in which each R 1 is, independently, a linear alkyl group or a branched alkyl group. Preferred linear alkyl groups for R 1 are the C^_ linear alkyl groups, and ester compounds containing such alkyl groups can be prepared by using at least one c 5-10 linear alkanoic acid selected from pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid and decanoic acid in the synthesis of the ester base oil. Particularly preferred linear alkyl groups for R 1 are the C _β linear alkyl groups, especially the linear C4 and linear Cg alkyl groups which are derived from pentanoic and heptanoic acids respectively. The linear C Q alkyl group is an especially preferred linear alkyl group. Preferred branched alkyl groups for R 1 are the C 6 - 9 branched alkyl groups, and ester compounds containing such alkyl groups can be prepared by using at least one C _IQ branched alkanoic acid in the synthesis of the ester base oil, such as 2-methylhexanoic acid, 3-methylhexanoic acid, 4-methylhexanoic acid, 2,2-dimethylpentanoic acid, 2 , -dimethylpentanoic acid, 2-ethylpentanoic acid, 3-ethylpentanoic acid, 2-ethylhexanoic acid, 3 , 5-dimethylhexanoic acid, 2, 2-dimethylhexanoic acid, 2-methylheptanoic acid, 3-methylheptanoic acid, 4-methylheptanoic acid, 2,2-dimethylheptanoic acid, 3 , 5 , 5-trimethylhexanoic acid, 2-methyloctanoic acid, 3-methyloctanoic acid, 2-ethylheptanoic acid and -.odecanoic acid. Particularly preferred branched alkyl groups for R 1 are the C 7 _g branched alkyl groups, especially the branched Cβ alkyl groups which are derived from branched C 9 alkanoic acids. A particularly preferred ester base oil is obtained when a combination of linear and branched alkanoic acids are used in the synthesis thereof so that the oil comprises one or more compounds of Formula I in which at least a proportion of the K-- groups are linear alkyl groups and at least a proportion of the R-- groups are branched alkyl groups. Preferably from 30 to 80 number Z, more preferably from 40 to 80 number Z and particularly preferably from 50 to 70 number Z of the total number of R 1 groups in the one or more ester compounds constituting the base oil are branched alkyl groups, with the remaining R 1 groups being linear alkyl groups . The ester base oil will preferably comprise one or more ester compounds derived from dipentaerythritol, i.e. compounds of Formula I in which R is the hydrocarbon radical remaining after removing the hydroxyl groups from dipentaerythritol. Such ester oils may also comprise one or more ester compounds of Formula I in which R is the hydrocarbon radical remaining after removing the hydroxyl groups from pentaerythritol and/or tripentaerythritol . Preferably at least 25 weight Z, more preferably at least 50 weight Z and particularly preferably at least 75 weight Z of the ester base oil will be comprised of one or more ester compounds derived from dipentaerythritol. Typically, the lubricant will also comprise one or more of the additives which are conventional in the refrigeration lubricants art. Specific mention may be made of antioxidants , deacidifying agents, anti-wear agents and extreme pressure resistance agents. Such additives are well known to those skilled in the art. Where the lubricant comprises such additives, they may be present in the amounts conventional in the art. Preferably, the cumulative weight of all the additives will not be more than 8 Z, e.g. 5 Z, of the total weight of the lubricant. Preferred antioxidants are the compounds containing a hindered phenol in the molecule, preferred deacidifying agents are the compounds containing an epoxy group in the molecule, and the preferred anti-wear and extreme pressure resistance agents are the organophosphorous compounds, particularly the organo phosphates, phosphites and thiophosphates . The following compounds are representative of the preferred antioxidants, deacidifying agents, anti-wear agents and extreme pressure resistance agents. Antioxidants : 2, 6-di-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-ethylphenol, 2, 6-di-t-butyl-4-hydroxyphenol, 2,2' -methylenebis (4-methyl-6-t-butylphenol) , 2,2 ' - methylenebis(4-ethy1-6-t-butylphenol), 2,2'-butylidenebis(4-methyl-6-t-butylphenol), 4,4'-butylidenebis( 3-methyl-6-t-butylphenol ) , 1,1,3-tris ( 2-methyl-4-hydroxy-5-t-butylphenyl ) butane , 1,3, 5-trimethyl-2,4 ,6-tris(3,5-di-t-butyl-4- hydroxybenzyl )benzene and the like. Deacidifying agents: Phenyl glycidyl ether, butylglycidyl ether, bisphenol A epichlorohydrin condensate, vinylcyclohexene dioxide, 2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxycyclohexane)- meta-dioxane, 3,4-epoxycyclohexylmethy1-3,4-epoxy- cyclohexane carboxylate, polypropylene glycol diglycidyl ether, resorcin diglycidyl ether, polyethylene glycol diglycidyl ether and the like. Anti-wear and extreme pressure agents: Tricresyl phosphate, trisnonylphenylphosphite , distearyl phosphate, tributylphosphate , trilaurylphosphate , trilaurylphosphite , mono-di-mixed-lauryl phosphite. mono-di-mixed-tridecyl phosphate, mono-2-ethylhexyl- 2-ethylhexylphosphate , di-2-ethylhexyl phosphate and the like . The working fluid compositions of the invention typically comprise from 30 to 99 Z by weight, e.g from 70 to 99 Z by weight, of R-32 and from 1 to 70 Z by weight, e.g. from 1 to 30 Z by weight, of the lubricant . The working fluid compositions may be used to provide cooling by a method involving condensing the refrigerant and thereafter evaporating it in a heat exchange relationship with a body to be cooled. They may also be used to provide heating by a method involving condensing the refrigerant in a heat exchange relationship with a body to be heated and thereafter evaporating it. The present invention is now illustrated but not limited with reference to the following Example. Example 1 A refrigeration test rig incorporating a screw compressor was filled with "Emkarate" (TM) RL-150S refrigeration lubricant (see below) and difluoromethane (R-32) refrigerant and the performance of the resulting working fluid composition in the test rig examined over a prolonged period of operation. The test rig was kept running for over 1000 hours and no serious operational problems were encountered, showing that the lubricant provided effective lubrication of the compressor in the presence of the difluoromethane refrigerant. "Emkarate" (TM) RL-150S is available from ICI Chemicals & Polymers Ltd. The lubricant comprises an ester base oil which is derived from the reaction of dipentaerythritol, n-heptanoic acid and 3 , 5 , 5-trimethy1 hexanoic acid.

Claims: C l a ims : 1. A working fluid composition comprising: (A) a refrigerant consisting essentially of - difluoromethane (R-32); and (B) sufficient to provide lubrication of a lubricant comprising an ester base oil which comprises the reaction product of at least one neopentyl polyol and at least one aliphatic carboxylic acid or an esterifiable derivative thereof. 2. A working fluid composition as claimed in claim 1 wherein the lubricant component has a viscosity at 40°C in the range of from 46 to 220 cSt, a viscosity index in the range of from 50 to 200 and a pour point of below -20°C. 3. A working fluid composition as claimed in claim 1 or claim 2 wherein the ester base oil comprises the reaction product of at least one neopentyl polyol selected from pentaerythritol, dipentaerythritol, tripentaerythritol , trimethylol ethane, trimethylol propane and neopentyl glycol and at least one aliphatic carboxylic acid selected from the linear and branched aliphatic carboxylic acids, or an esterifiable derivative of such an acid(s). 4. A working fluid composition as claimed in claim 1 or claim 2 wherein the ester base oil comprises one or more ester compounds of formula: R(0 C - Rl) n wherein R is the hydrocarbon radical remaining after removing the hydroxyl groups from pentaerythritol, dipentaerythritol or tripentaer thritol ; each R-l is, independently, a straight chain - (linear) aliphatic hydrocarbyl group or a branched aliphatic hydrocarbyl group; and n is an integer of 4, 6 or 8. 5. A working fluid composition as claimed in claim 4 wherein the ester base oil comprises one or more compounds of Formula I in which each R* is, independently, a linear alkyl group or a branched alkyl group. 6. A working fluid composition as claimed in claim 5 wherein the ester base oil comprises one or more compounds of Formula I in which each R 1 is, independently, a C4_g linear alkyl group or a Cg.g branched alkyl group. 7. A working fluid composition as claimed in claim 6 wherein the ester base oil comprises one or more compounds of Formula I in which each R 1 is, independently, a linear C4 alkyl group, a linear C Q alkyl group or a C 7 _g branched alkyl group. 8. A working fluid composition as claimed in any one of claims 5 to 7 wherein the ester base oil comprises one or more compounds of Formula I in which at least a proportion of the R 1 groups are linear alkyl groups and at least a proportion of the R 1 groups are branched alkyl groups . 9. A working fluid composition as claimed in claim 8 wherein from 30 to 80 number Z of the R 1 groups in the one or more ester compounds of Formula I which constitute the base oil are branched alkyl groups. 10. A working fluid composition as claimed in claim 8 or claim 9 wherein at least a proportion of the branched alkyl groups are branched Cβ alkyl and at least a proportion of the linear alkyl groups are linear C Q alkyl. 11. A working fluid composition as claimed in claim 8 - > or claim 9 wherein the ester base oil comprises one or more compounds of Formula I in which R is the hydrocarbon radical remaining after removing the hydroxyl groups from dipentaerythritol. 12. A working fluid composition as claimed in claim 11 wherein at least 25 weight Z of the ester base oil is composed of one or more ester compounds of Formula I which are derived from dipentaerythritol. 13. A working fluid composition as claimed in claim 12 wherein at least 50 weight Z of the ester base oil is composed of one or more ester compounds of Formula I which are derived from dipentaerythritol. 14. A heat transfer device which comprises means for controlling the compressor discharge temperature and which uses as a working fluid composition a composition as claimed in any one of the preceding claims. 15. A heat transfer device which comprises means for controlling the compressor discharge temperature and which uses as a working fluid composition a composition comprising : (A) a refrigerant consisting essentially of difluoromethane (R-32); and (B) sufficient to provide lubrication of a lubricant comprising an ester base oil which comprises the reaction product of at least one neopentyl polyol and at least one aliphatic carboxylic acid or an esterifiable derivative thereof.

Inventors: CORR STUART, CORR, STUART
Applicants: CORR STUART, ICI PLC, IMPERIAL CHEMICAL INDUSTRIES PLC
Application year: 1993
Main classification: C10M171
IPC codes: C09K5, C10M105, C10M171, C10N20