COOLING-EQUIPMENT WORKING-FLUID
COMPOSITION Technical Field
[0001] The present invention relates to a working fluid composition for a refrigerating machine. It is noted that the term "refrigerating machine" herein includes a vehicle air conditioner, a dehumidifier, a refrigerator, a refrigerated warehouse, a vending machine, a showcase, a cooling apparatus used in a chemical plant or the like, a household air conditioner, a package air conditioner, a heat pump for hot water supply, and the like. Background Art
[0002] In the field of refrigeration and air conditioning, 1,1,1,2-tetrafluoroethane (R134a) that is a hydrofluorocarbon (HFC) and R410A that is a mixed refrigerant of difluoromethane (R32) and pentafluoroethane (R125) in a mass ratio of 1/1 are currently widely used as refrigerants for a refrigerator, a vehicle air conditioner, a room air conditioner, industiial refrigerating machine, and the like. Although such HFC refrigerants have an ozone depletion potential (ODP) of zero, they have a high global warming potential (GWP) of 1000 or more, and therefore, their usage is limited in accordance with what is called the F-gas regulations aiming at the global environmental protection.
[0003] As a substitute for a refrigerant having a high GWP, single use of difluoromethane (R32) is under examination as a candidate owing to its theiTnodynamic characteristic. Although the GWP of R32 is 675

and rather high, it is under examination as a potential candidate because its gas pressure is high and it is a highly efficient refrigerant. [0004] In using a refrigerant increasing a pressure, such as R32 or a mixed refrigerant containing R32, since the discharge temperature in a compressor becomes high, an oil film of a refrigerating machine oil for attaining lubrication in the compressor becomes thin to cause a severe lubrication condition, and therefore, a refrigerating machine oil having good lubricity is necessary.
[0005] Patent Literature 1 proposes, as a refrigerating machine oil for an R32 refrigerant, an ester synthesized from trimethylolpropane and/or neopentyl glycol, a specific dibasic acid, and a monohydric alcohol or a monocarboxylic acid, and Patent Literature 2 proposes, as one for an R32 single refrigerant or a mixed refrigerant containing 50% or more of R32, an ester-based refiigerating machine oil that contains 20 to 60% by mass of a monobasic fatty acid having 5 or less carbon atoms, uses a branched fatty acid as a fatty acid having 8 or more carbon atoms, and has a viscosity at 40°C of 32 to 100 est. With respect to a base oil for a general industrial lubricating oil, Patent Literature 3 proposes a lubricating oil base oil containing a synthetic ester obtained by reacting an alcohol component containing 90% by mass or more of trimethylolpropane with a carboxylic acid component that contains a monocarboxylic acid having 8 to 12 carbon atoms and adipic acid, and further contains, in a total amount of QOYo by mass or more, caprylic acid and/or a monocarboxylic acid having 8 to 12 carbon atoms containing 90%) by mass or more of capiylic acid in total, and adipic acid.

Citation List Patent Literature
[0006] Patent Literature 1: Japanese Patent Application Laid-Open No. 2011-184536
Patent Literature 2: Japanese Patent Application Laid-Open No. 2002-129179
Patent Literature 3: Japanese Patent Application Laid-Open No. 2012-102235 Summary of Invention Technical Problem
[0007] There is a trend, however, that a load condition in a sliding portion of a refrigerating machine becomes more severe in the future, and therefore, there is a demand for a working fluid showing an excellent antiwear property in a case where an R32 refrigerant coexists and is dissolved in a refiigerating machine oil.
[0008] The present invention was accomplished in consideration of the above-described problem, and an object is to provide a working flnid composition for a refrigerating inachine that can retain a thick oil film and show a high antiwear effect and is good in long-term reliability even under a severe lubrication condition caused when a refiigerant coexists and is dissolved in a refiigerating machine oil. Solution to Problem
[0009] The present inventoi^s have discovered that a working fluid for refiigeration/air conditioning, which comprises a refrigerating machine oil using, as a base oil, an ester containing a complex ester obtainable by synthesis fi"om specific polyhydric alcohol, polybasic acid,

monohydric alcohol or monocarboxylic acid, and a polyol ester obtainable by synthesis from specific polyhydric alcohol and monocarboxylic acid; and difiuoromethane used as a refrigerant, and has a high refrigerant dissolved viscosity under a specific condition, foims a thick oil film to show high antiwear property, resulting in accomplishing the present invention.
[0010] Specifically, the present invention provides a working fluid composition for a refrigerating machine according to the following [1] to [6];
[1] A working fluid composition for a refrigerating machine, comprising: a refrigerating machine oil comprising, as a base oil, a mixed ester of (A) a complex ester and (B) a polyol ester, wherein (A) the complex ester is obtainable by synthesis from at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane and pentaerythiitol, a polybasic acid having 6 to 12 carbon atoms, and a monohydric alcohol having 4 to 18 carbon atoms or a monocarboxylic acid having 4 to 18 carbon atoms, wherein (B) the polyol ester is obtainable by synthesis from at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane, pentaerj^hiitol and dipentaeiythritol, and a monocarboxylic acid having 4 to 18 carbon atoms, and wherein a mass ratio of (A) the complex ester/(B) the polyol ester is 5/95 to 95/5; and difiuoromethane used as a refrigerant, and the working fluid composition having a refrigerant dissolved viscosity of 5.5 mm /s or more, at a temperature of 80°C and an absolute pressure of 2.1 MPa. [2] The working fluid composition for a refrigerating machine

according to [1] above, wherein the polyhydric alcohol constituting (A)
the complex ester is neopentyl glycol and/or trimethylolpropane.
[3] The working fluid composition for a refrigerating machine
according to [1] or [2] above, wherein the polybasic acid constituting
(A) the complex ester is adipic acid and/or sebacic acid.
[4] The working fluid composition for a refrigerating machine
according to any one of [1] to [3] above, wherein the monohydric
alcohol constituting (A) the coinplex ester is an alcohol having 8 to 10
carbon atoms.
[5] The working fluid composition for a refrigerating machine
according to any one of [1] to [4], wherein (B) the polyol ester is an
ester obtainable by synthesis fr'om neopentyl glycol and/or
pentaeiythritol and a monocarboxylic acid having 4 to 9 carbon atoms.
[6] The working fluid composition for a refrigerating machine
according to any one of [1] to [5] above, wherein (B) the polyol ester is
an ester obtainable by synthesis from pentaerythritol and a mixed acid
of a branched fatty acid having 4 carbon atoms and
3,5,5-U'imethyihexanoic acid.
Advantageous Effects of Invention
[0011] A working fluid composition for a refrigerating machine of the
present invention has, even under a severe lubrication condition in
which the viscosity of a refrigerating machine oil is lowered because a
difluoromethane refiigerant is dissolved therein, high antiwear property
and excellent stability, and exhibits a remarkable effect that an apparatus
can be used stably over a long period of time.
Description of Embodiments

[0012] A preferred embodiment of the present invention will now be described in detail.
[0013] A working fluid composition for a reMgerating machine according to the embodiment of the present invention comprises: a refrigerating machine oil containing, as a base oil, a mixed ester of (A) a complex ester and (B) a poiyol ester, in which (A) the complex ester is obtainable by synthesis from at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane and pentaerythiitol, a polybasic acid having 6 to 12 carbon atoms, and a monohydiic alcohol having 4 to 18 carbon atoms or a monocarboxylic acid having 4 to 18 carbon atoms, (B) the poiyol ester is obtainable by synthesis from at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane, pentaeiythritol and dipentaeiythritol, and a monocarboxylic acid having 4 to 18 carbon atoms, and a mass ratio of (A) the complex ester/(B) the poiyol ester is 5/95 to 95/5; and difluoromethane used as a refrigerant, in which a refrigerant dissolved viscosity of the working fluid composition, at a temperature of 80°C and an absolute pressure of 2.1 MPa, is 5.5 mm Is or more. [0014] The aforementioned complex ester has a lower compatibility with a refrigerant because the refrigerant is difficult to dissolve therein as compared with a conventional refrigerating machine oil, but has a characteristic that it can retain an oil film thick. Besides, the aforementioned poiyol ester is good in the compatibility with a refrigerant. In the present embodiment, since the complex ester and the poiyol ester thus having different characteristics (of, particularly, a refrigerant solubility) are blended, the oil film can be made thick in

coexistence of a refrigerant, and accordingly the antiwear property of a resultant working fluid can be improved.
[0015] Incidentally a complex ester is difficult to be compatible with a refrigerant because it has a high molecular weight and hence is an ester with a high viscosity, and it is not suitably used singly as a base oil of a refrigerating machine oil requiring compatibility with a refrigerant from the viewpoint of oil return to a compressor. As one characteristic of the present embodiment, it is possible to make the characteristics balanced by mixing the complex ester with an oil having a good compatibility with a refrigerant, such as the above-described polyol ester.
[0016] A preferable kinematic viscosity of the complex ester is 20 to 500 min /s at 40°C, a more preferable kinematic viscosity is 40 to 400 mm /s, a frirther preferable kinematic viscosity is 50 to 200 mm /s, and a particularly preferable kinematic viscosity is 50 to 90 mmVs. Besides, the viscosity index is preferably 100 or more, and particularly preferably 110 to 160.
[0017] Examples of a synthesizing method for the complex ester include (A) a method in which a molar ratio between a polyhydric alcohol and a polybasic acid is adjusted for obtaining an ester intermediate having a carboxyl group of the polybasic acid remaining therein, and the carboxyl group is esterified by a monohydric alcohol, and (B) a method in which a molar ratio between a polyhydric alcohol and a polybasic acid is adjusted for obtaining an ester intennediate having a hydroxyl group of the polyhydric alcohol remaining therein, and the hydroxyl group is esterifred by a monocarboxylic acid. A

complex ester obtained by the method of (B) above is rather inferior in stability to a complex ester obtained by the method of (A) above because a comparatively strong acid is produced if the former is hydrolyzed when used as a refrigerating machine oil. The complex ester of the present embodiment is preferably a complex ester with higher stability obtained by the method of (A) above. [0018] The polyhydric alcohol constituting the complex ester is preferably neopentyl glycol or trimethylolpropane for attaining a suitable viscosity as a base oil. Incidentally, if tetravalent pentaeiythritol is used, as compared with a case of using neopentyl glycol or trimethylolpropane, the viscosity of a resultant complex ester tends to be higher and a low temperature characteristic also tends to be poorer. Besides, neopentyl glycol whose viscosity can be widely adjusted is moreprefeiTcd.
[0019] Besides, the polyhydric alcohol constituting the complex ester
preferably further contains, in addition to at least one selected from
neopentyl glycol, trimethylolpropane and pentaeiythritol, a dihydric
alcohol having 2 to 10 carbon atoms except neopentyl glycol because
thus the lubricity can be improved. Examples of the dihydric alcohol
having 2 to 10 carbon atoms except neopentyl glycol include ethylene
glycol, propanediol, butanediol, pentanediol, hexanediol,
2-methyl-l ,3-propanediol, 3-methyl-1,5-pentanediol and
2,2-diethyl-1,3 -pentanediol, and butanediol that can attain good characteristic balance of a resultant synthesized base oil is prefeired, and examples of the butanediol include 1,2-butanediol, 1,3-butanediol, 1,4-butanedioi and 2,3-butanediol, among which 1,3-butanediol and

1,4-butandiol are more prefeiTcd from the viewpoint of characteristics. The dihydric alcohol having 2 to 10 carbon atoms except neopentyl glycol is used in an amount of preferably 1.2 mol or less, paiticularly preferably 0.8 mol or less, and further preferably 0.4 mol or less based on 1 mol of the polyhydric alcohol selected from neopentyl glycol, trimethylolpropane and pentaeiythritol.
[0020] The polybasic acid constituting the complex ester is a polybasic acid having 6 to 12 carbon atoms. Examples of such a polybasic acid include adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid and trimellitic acid, among which adipic acid and sebacic acid that are good in the characteristic balance of a resultant synthesized ester and in availability are prefeiTed, and in particular, adipic acid is more preferred. The polybasic acid is used in an amount of preferably 0.4 to 4 mol, particularly preferably 0.5 mol to 3 mol, and further preferably 0.6 mol to 2.5 mol based on 1 mol of the polyhydric alcohol selected fi'om neopentyl glycol, trimethylolpropane and pentaerythritol. [0021] If a carboxyl group remains in a complex ester intermediate produced through the reaction between the polyhydric alcohol and the polybasic acid, the carboxyl group is esterified by a monohydric alcohol having 4 to 18 carbon atoms. Examples of the monohydric alcohol having 4 to 18 carbon atoms include straight or branched butanol, straight or branched pentanol, straight or branched hexanol, straight or branched heptanol, straight or branched octanol, straight or branched nonanol, straight or branched decanol, straight or branched dodecanol, and an aliphatic alcohol such as oleyl alcohol. From the viewpoint of the characteristic balance, monohydric alcohols having 6 to 10 carbon

atoms, and 8 to 10 carbon atoms in particular are preferred, among which 2-ethylhexanoi and 3,5,5-trimethylhexanol are preferred from the viewpoint of a good low temperature characteristic of the synthesized complex ester.
[0022] Alternatively, if a hydroxyl group remains in the complex ester intermediate produced through the reaction between the polyhydric alcohol and the polybasic acid, the hydroxyl group is esterified by a monocarboxylic acid having 4 to 18 carbon atoms. Examples of the monocarboxylic acid having 4 to 18 carbon atoms include straight or branched butanoic acid, straight or branched pentanoic acid, straight or branched hexanoic acid, straight or branched heptanoic acid, straight or branched octanoic acid, straight or branched nonanoic acid, straight or branched decanoic acid, straight or branched dodecanoic acid, and oleic acid. Preferably, monocarboxylic acids having 8 to 10 carbon atoms are used, among which 2-ethylhexanoic acid and 3,5,5-trimethylhexanoic acid are preferred from the viewpoint of the low temperature characteristic.
[0023] On the other hand, the polyol ester of the present embodiment is a polyol ester obtainable by synthesis from at least one polyhydiic alcohol selected from neopentyl glycol, trimethylolpropane, pentaer3^hritol and dipentaeiythiitol, and a monocarboxylic acid having 4 to 18 carbon atoms.
[0024] As the polyhydric alcohol constituting the polyol ester, trimethylolpropane and pentaerythritol are prefeiTed, and pentaerythritol is more preferred from the viewpoint of the characteristic balance. [0025] Examples of the monocarboxylic acid having 4 to 18 carbon

atoms constituting the polyol ester include straight or branched butanoic acid, straight or branched pentanoic acid, straight or branched hexanoic acid, straight or branched heptanoic acid, straight or branched octanoic acid, straight or branched nonanoic acid, straight or branched decanoic acid, straight or branched dodecanoic acid, and oleic acid. From the viewpoint of the low temperature characteristic, a monocarboxylic acid having 4 to 9 carbon atoms is preferred, and branched butanoic acid, branched pentanoic acid, branched hexanoic acid; branched heptanoic acid, 2-ethylhexanoic acid and 3,5,5-trimethylhexanoic acid are more prefeiTed.
[0026] From the viewpoint of the compatibility with a refiigerant, a polyol ester obtainable by synthesis from a pentaei-ythritol used as the polyhydric alcohol and a mixed acid of branched fatty acid having 4 to 9 carbon atoms used as the monocarboxylic acid is the most preferred. [0027] In the working fluid composition for a refrigerating machine of the present embodiment, assuming that the content of the complex ester is (A) and the content of the polyol ester is (B), the ratio (A)/(B) in a mass ratio is 5/95 to 95/5, and pvofeiably 20/80 to 80/20, and more preferably 30/70 to 70/30 in order to take more advantages of the characteristics of the respective esters.
[0028] The kinematic viscosity at 40°C of the refrigerating machine oil of the present embodiment is preferably 3 to 500 mmVs, more preferably 8 to 150 mm/s, and further more preferably 20 to 100 mm^/s. Besides, the viscosity index of the refrigerating machine oil is preferably 50 or more and particularly preferably 80 to 120. [0029] The pour point of the refrigerating machine oil of the present

embodiment is preferably -10°C or less, and more preferably -20°C or
less.
[0030] The acid value of the refrigerating machine oil of the present
embodiment can be preferably 0.1 mgKOH/g or less, and more
preferably 0.05 mgKOH/g or less in order to prevent corrosion of a
metal used in refrigerating machine or piping and to suppress
degradation of the refrigerating machine oil itself It is noted that the
acid value herein means an acid value measured in accordance with JIS
K2501 "determination method of acid value".
[0031] The flash point of the refrigerating machine oil of the present
embodiment is preferably 120°C or more, and more preferably 200°C or
more.
[0032] The moistuie content of the refrigerating machine oil of the
present embodiment is preferably 200 ppm or less, more preferably 100
ppm or less, and most preferably 50 ppm or less. In particular, if it is
used in hermetic refrigerating machine, the moisture content is required
to be small from the viewpoint of the stability and electric insulation of
the refrigerating machine oil.
[0033] Besides, the refrigerating machine oil of the present embodiment
may fuither contain, in addition to the complex ester and the polyol
ester described above, another base oil such as a mineral base oil or a
synthetic base oil. The total content of the above complex ester and
the polyol ester is preferably 80% by mass or more, and particularly
preferably 95% by mass or more of the refrigerating machine oil.
[0034] With respect to refrigerating machine, there is a trend that
conventionally used HFC refrigerants having a high GWP are shifted to

refrigerants having a low GWP from the viewpoint of the prevention of global warming as described above, and therefore, refrigerating machine oils suitable for such refrigerants are necessaiy, and a suitable working fluid of a mixture of a refrigerant and a refiigerating machine oil is demanded. According to the present invention, difluoromethane is contained as the refrigerant, and the content of the difluoromethane in the refiigerant is preferably 60 to 100% by mass, and particularly preferably 80 to 100% by mass.
[0035] The mixing ratio between the refrigerating machine oil composition and the refrigerant in the working fluid composition for a refrigerating machine of the present embodiment is not especially limited, and the amount of the refrigerating machine oil composition is preferably 1 to 500 parts by mass, and more preferably 2 to 400 parts by mass based on 100 parts by mass of the refrigerant.
The refiigerant dissolved viscosity, at a temperature of 80°C and an absolute pressure of 2.1 MPa, of the working fluid composition for a refrigerating machine of the present embodiment is 5.5 mm /s or more, and preferably 6.0 mm^/s or more. Besides, the refrigerant dissolved viscosity, at a temperature of 80'^C and an absolute pressure of 2.1 MPa, of the working fluid composition for a refiigerating machine of the present embodiment can be 10.0 mmVs or less.
[0036] The working fluid composition of the present embodiment can further contain various additives for further improving the antiwear property. A preferable example of the additives includes a phosphate, and particularly preferable compounds are triphenyl phosphate (TPP) and tricresyl phosphate (TCP).

[0037] Besides, suitable examples of a sulfrir :additive include sulfides, and there are a large number of sulfide compounds, among which a monosulfide compound is prefeiTed. This is because, for example, a highly active sulfiir compound such as a disulfide compound degrades the stability of a refrigerating machine oil and changes the quality of copper ofl:en used within refrigerating machine.
[0038] The working fluid composition for a refrigerating machine of the present embodiment can contain, in addition to the aforementioned additives, additives conventionally used in a lubricating oil, such as an antioxidant, a fiiction modifier, an antiwear agent, an extreme pressure agent, a mst inhibitoi; a metal deactivator and an antifoaming agent, in a range not impairing the object of the present invention, for fiaither improving the peiformance.
[0039] As the antioxidant, a phenol-based compound such as di-tert-butyl-p-cresol, an amine-based compound such as alkyldiphenylamine, or the like can be contained. In particular, it is preferable to contain a phenol-based compound antioxidant in an amount of 0.02 to 0.5% by mass based on the total amount of the refrigerating machine oil.
[0040] Examples of the friction modifier include aliphatic amines, aliphatic amides, aliphatic imides, alcohols, esters, phosphate amine salts and phosphite amine salts, an example of the antiwear agent includes zinc dialkyldithiophosphate, examples of the extreme pressure agent include olefin sulfide and suliurized fats and oils, examples of the mst inhibitor include alkenyl succinic esters or partial esters, examples of the metal deactivator include benzotriazole and benzotriazole

derivatives, and examples of the antifoaming agent include silicone
compounds and polyester compounds.
Examples
[0041] The present invention will now be described more specifically
on the basis of Examples and Comparative Examples, and it is noted
that the present invention is not limited to the following Examples at all.
[0042] [Examples 1 to 12 and Comparative Examples 1 to 5]
In Examples 1 to 12 and Comparative Examples 1 to 5, base oils having compositions shown in Tables 1 to 3 were first prepared by using the following base materials. [A] Complex Esters
(A-1) An ester (having a kinematic viscosity at 40''C of 68.8 mm /s and a viscosity index of 120) obtained by causing an ester intermediate resulting from a reaction between trimethylolpropane (1 mol) and adipic acid (2.4 mol) to further react with 2-ethylhexanol (2.0 mol) and distilling off a remaining unreacted substance.
(A-2) An ester (having a kinematic viscosity at 40°C of 71.5 mm/s and a viscosity index of 114) obtained by causing an ester intermediate resulting from a reaction between neopentyl glycol (1 mol) and adipic acid (0.8 mol) to further react with 3,5,5-trimethylhexanoic acid (0.5 mol) and distilling off a remaining unreacted substance.
(A-3) An ester (having a kinematic viscosity at 40°C of 77.3 mm /s and a viscosity index of 148) obtained by causing an ester intermediate resulting from a reaction of trimethylolpropane (1 mol) and 1,3-butanediol (0.2 mol) with sebacic acid (2.4 mol) to further react with normal heptanol (1.6 mol) and distilling off a remaining um'eacted

substance.
(A-4) An ester (having a kinematic viscosity at 40°C of 68.2 mm /s and a viscosity index of 144) obtained by causing an ester intermediate resulting from a reaction of neopentyl glycol (1 mol) and 1,4-butanediol (0.3 mol) with adipic acid (2.4 mol) to ftirther react with 2-ethylhexanol (2.4 mol) and distilling off a remaining unreacted substance.
(A-5) An ester (having a kinematic viscosity at 40*^0 of 32.2 minVs and a viscosity index of 161) obtained by causing an ester intermediate resulting from a reaction of neopentyl glycol (1 mol) and 1,4-butanediol (0.4 mol) with adipic acid (3.1 mol) to iurther react with 3,5,5-trimethylhexanol (3.5 mol) and distilling off a remaining unreacted substance.
(A-6) An ester (having a kinematic viscosity at 40°C of 67.8 mm /s and a viscosity index of 145) obtained by causing an ester intermediate resulting from a reaction of neopentyl glycol (1 mol) and 1,4-butanediol (0.3 mol) with adipic acid (2.4 mol) to further react with 3,5,5-trimethylhexano] (2,5 mol) and distilling off a remaining unreacted substance.
(A-7) An ester (having a kinematic viscosity at 40'^C of 150.0 mm/s and a viscosity index of 138) obtained by causing an ester intermediate resulting fi'om a reaction of neopentyl glycol (1 mol) and 1,4-butanediol (0.2 mol) with adipic acid (1.9 mol) to fiirther react with 3,5,5-tiimethylhexanol (1.0 mol) and distilling off a remaining unreacted substance. [B] Polyol Esters

(B-1) An ester (having a kinematic viscosity at 40°C.of 8.3 mm /s and a viscosity index of 56) of neopentyl glycol and 2-ethylhexanoic acid.
(B-2) An ester (having a kinematic viscosity at 40°C of 69.4 mm^/s and a viscosity index of 95) of pentaeiythritol and a mixed acid of 2-methyl propanoic acid and 3,5,5-trimethylhexanoic acid in a molar ratio of 35:65.
(B-3) An ester (having a kinematic viscosity at 40°C of 46.0 mm^/s and a viscosity index of 78) of pentaeiythritol and a mixed acid of 2-methyl propanoic acid and 3,5,5-trimethylhexanoic acid in a molar ratio of 60:40.
(B-4) An ester (having a kinematic viscosity at 40°C of 70.1 mm^/s and a viscosity index of 90) of pentaerythritol and a mixed acid of 2-ethylhexanoic acid and 3,5,5-trimethylhexanoic acid in a molar ratio of 45:55.
(B-5) An ester (having a kinematic viscosity at 40''C of 23.4 mm /s and a viscosity index of 118) of pentaeiythritol and a mixed acid of normal pentanoic acid and 3,5,5-trimethylhexanoic acid in a molar ratio of 80:20.
(B-6) An ester (having a kinematic viscosity at 40°C of 51.0 mm /s and a viscosity index of 105) of pentaeiythritol and a mixed acid of nonnal pentanoic acid and 3,5,5-trimethylhexanoic acid in a molar ratio of 40:60.
In these base oils, synthesis reactions of the esters of [A] and [B] are performed without using a catalyst and a solvent, and a slight amount of impurities was removed by an adsoiption treatment (a clay

treatment) in the final process. Incidentally, the kinematic viscosity
and the viscosity index were measured and calculated in accordance
withJISK2283.
[0043] Next, a refrigerating machine oil was prepared by mixing
di-tert-butyl-p-cresol (DBPC) serving as an antioxidant in an amount of
0.1% by mass with each of the base oils of Examples 1 to 12 and
Comparative Examples 1 to 5.
[0044] The refrigerating machine oils of Examples 1 to 12 and
Comparative Examples 1 to 5 were subjected to measurement of
dissolved viscosity and a lubricity test as follows.
[0045] (Measurement of Refiigerant Dissolved Viscosity)
A 200 ml pressure resistant vessel in which a vibration type viscometer was put was charged with 100 g of each of the refngerating machine oils, and after vacuum degassing the vessel, a working fluid composition was prepared by adding an R32 refi'igerant thereto, and the viscosity was measured with the pressure of the refrigerant and the temperature of the pressure resistant vessel adjusted to conditions of a temperature of 80^C and an absolute pressure of 2.1 MPa. The obtained results are shown in Tables 1 to 3. [0046] (Lubricity Test)
For the lubricity test, a high pressure ambience friction and wear tester (employing a rotating/sliding method using a rotating vane member and a fixed disk member) manufactured by Shinko Engineering Co., Ltd. capable of attaining a refrigerant atmosphere similar to that of an actual compressor was used. The test conditions were: oil amount: 600 ml, test temperature: 110°C, rotation speed: 500 rpm, applied load:

80 kgf, test thne: 1 hour, and SKH-51 and FC250 were used respectively as the vane member and the disk member. As the test conditions, R32 was used as the refiigerant, and the inside pressure of a . test vessel was set to 3.1 MPa. The evaluation ofthe antiwear property was made on the basis of a wear depth of the vane member because the wear amount of the disk member is extremely small. The obtained results are shown in Tables 1 to 3. [0047] [Table 1]

[0050] It is undei-stood that the working fluid compositions of Examples 1 to 12 have high refrigerant dissolved viscosities and are good in the antiwear property. Industrial Applicability
[0051] A working fluid composition for a refrigerating machine of the present invention has a high viscosity when a difluoromethane refrigerant is dissolved therein, and is good in antiwear property even under a severe lubrication condition, and therefore exhibits a remarkable effect of greatly improving lubricity. Accordingly, it can be suitably

used in a refrigeration/air conditioning system with high cooling efficiency including a compressor, a condenser, a throttle device, an evaporator and the like for circulating a refrigerant among these, in particular, in a system including a rotaiy type, swing type or scroll type compressor, and hence is useful in the fields of room air conditioners, package air conditions, refrigerators, vehicle air conditioners, industrial refiigerating machine and the like.

CLAIMS
1. A working fluid composition for a refrigerating machine,
comprising:
a refrigerating machine oil comprising, as a base oil, a mixed ester of (A) a complex ester and (B) a polyol ester, wherein (A) the complex ester is obtainable by synthesis of at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane and pentaerythritol, a polybasic acid having 6 to 12 carbon atoms, and a monohydric alcohol having 4 to 18 carbon atoms or a monocarboxylic acid having 4 to 18 carbon atoms, wherein (B) the polyol ester is obtainable by synthesis of at least one polyhydric alcohol selected from neopentyl glycol, trimethylolpropane, pentaeiythritol and dipentaeiythritol, and a monocarboxylic acid having 4 to 18 carbon atoms, and wherein a mass ratio of (A) the complex ester/(B) the polyol ester is 5/95 to 95/5; and
difluoromethane used as a refrigerant, and
the working fluid composition having a refrigerant dissolved Viscosity of 5.5 mm /s or more, at a temperature of 80°C and an absolutepressureof 2.1 MPa.
2. The working fluid composition for a refiigerating. machine according to claim 1, wherein the polyhydric alcohol constituting (A) the complex ester is neopentyl glycol and/or trimethylolpropane.
3. The working fluid composition for a refiigerating machine according to claim 1 or 2, wherein the polybasic acid constituting (A) the complex ester is adipic acid and/or sebacic acid.
4. The working fluid composition for a refiigerating machine

according to any one of claims 1 to 3, wherein the monohydric alcohol constituting (A) the complex ester is an alcohol having 8 to 10 carbon atoms.
5. The working fluid composition for a refrigoraiing machine according to any one of claims 1 to 4, wherein (B) the polyol ester is an ester obtainable by synthesis from neopentyl glycol and/or pentaeiythiitol and a monocarboxylic acid having 4 to 9 carbon atoms.
6. The working fluid composition for a refrigerating machine according to any one of claims 1 to 5, wherein (B) the polyol ester is an ester obtainable by synthesis from pentaerythritol and a mixed acid of a branched fatty acid having 4 carbon atoms and 3,5,5-trimethylhexanoic acid.