Lubricants For Refrigeration Systems

Abstract

A lubricating composition which comprises: (A) a polyalkyleneglycol of the general formula (I): RO(R.sup.aO).sub.x(R.sup.bO).sub.y(R.sup.cO).sub.zR.sup.d (I) wherein: R is a C.sub.4 to C.sub.6 substituent comprising a heterocyclic ring, in which the heteroatom(s) in said ring(s) is (are) oxygen and/or sulphur, R.sup.a is a C.sub.2 alkylene group, R.sup.b is a C.sub.3 alkylene group, R.sup.c is a C.sub.4 alkylene group, R.sup.d is the same as R, or is a hydrogen atom or a C.sub.1-C.sub.20 alkyl or C.sub.1-C.sub.20 acyl group, x, y, z are each independently 0 or an integer in the range of from 1 to 100, and x+y+z=4 to 100; and (B) an ester of a alcohol containing two or more hydroxyl groups with a mono- or dicarboxylic acid, or an ester of an dicarboxylic acid with an alcohol containing one hydroxyl group.

Description

RELATED APPLICATIONS

[0001] This application claims priority from U.S. Provisional Application No. 60/693,779, filed Jun. 27, 2005.

FIELD OF THE INVENTION

[0002] The present invention relates to lubricating compositions for use in refrigeration apparatus, particularly compression-type refrigerators.

BACKGROUND INFORMATION

[0003] In a compression-type refrigerator, a mixture of a refrigerant and a lubricating oil is circulated through a number of units including a compressor, which exerts pressure on the gaseous refrigerant to convert it into a liquid. Such units comprise moving parts, and the lubricating oil is necessary to reduce friction between these parts. In addition to such lubricating properties, to ensure maximum system efficiency it is necessary for the lubricant to have appropriate miscibility characteristics with the refrigerant across the system's temperature operating range (commonly within the region of -50 to +80.degree. C.).

[0004] Polyalkylene glycols ("PAGs") and esters are well known as lubricants, and lubricants containing mixtures of PAGs with esters are known. For example, U.S. Pat. No. 4,302,343 discloses a defined mix of PAGs with esters for use in rotary screw compressors, while U.S. Pat. No. 4,751,012 discloses mixtures for use in reciprocating air compressors. EP 913457 discloses a liquid refrigeration composition comprising a defined fluorine-containing refrigerant and a lubricant comprising a defined ester of a carboxylic acid and a polyhydroxy compound together with a defined alkoxylated alcohol or an alkoxylated glycol. U.S. Pat. No. 4,851,144 discloses lubricant compositions miscible in hydrofluorocarbon and hydrochlorofluorocarbon refrigerants in the range from -20.degree. C. to greater than 65.degree. C. and having a viscosity greater than 75 centistokes at 38.degree. C., comprising about 5 to 95% of a defined polyether polyol together with about 95 to 5% of an ester made from a polyhydric alcohols with an alkanoic acid, or an ester made from an alkanedioic acid with an alkanol. EP 980416 discloses the use of particular PAGs in carbon dioxide refrigeration systems, and states that the lubricant composition may also contain neopentylpolyol esters.

SUMMARY OF THE INVENTION

[0005] Briefly described, according to one aspect of the invention, a lubricating composition which includes (A) a polyalkyleneglycol of the general formula (I): RO(R.sup.aO).sub.x(R.sup.bO).sub.y(R.sup.cO).sub.zR.sup.d (I)

[0006] in which R is a C.sub.4 to C.sub.6 substituent including a heterocyclic ring, in which the heteroatom(s) in said ring(s) is (are) oxygen and/or sulphur, R.sup.a is a C.sub.2 alkylene group, R.sup.b is a C.sub.3 alkylene group, R.sup.c is a C.sub.4 alkylene group, R.sup.d is the same as R, or is a hydrogen atom or a C.sub.1-C.sub.20 alkyl or C.sub.1-C.sub.20 acyl group, x, y, z are each independently 0 or an integer in the range of from 1 to 100, and X+y+z=4 to 100; and (B) an ester of a alcohol containing two or more hydroxyl groups with a mono- or dicarboxylic acid, or an ester of an dicarboxylic acid with an alcohol containing one hydroxyl group is provided.

[0007] According to another aspect of the invention, a refrigerant composition for refrigeration apparatus which includes a lubricant composition including: (A) a polyalkyleneglycol of the general formula (I): RO(R.sup.aO).sub.x(R.sup.bO).sub.y(R.sup.cO).sub.zR.sup.d (I)

[0008] in which R is a C.sub.4 to C.sub.6 substituent comprising a heterocyclic ring, in which the heteroatom(s) in said ring(s) is (are) oxygen and/or sulphur, R.sup.a is a C.sub.2 alkylene group, R.sup.b is a C.sub.3 alkylene group, R.sup.c is a C.sub.4 alkylene group, R.sup.d is the same as R, or is a hydrogen atom or a C.sub.1-C.sub.20 alkyl or C.sub.1-C.sub.20 acyl group, x, y, z are each independently 0 or an integer in the range of from 1 to 100, and X+y+z=4 to 100; and (B) an ester of a alcohol containing two or more hydroxyl groups with a mono- or dicarboxylic acid, or an ester of an dicarboxylic acid with an alcohol 10 containing one hydroxyl group; together with a refrigerant is provided.

[0009] According to another embodiment of the invention, a refrigeration apparatus which includes a refrigerant composition which includes: (A) a polyalkyleneglycol of the general formula (I): RO(R.sup.aO).sub.x(R.sup.bO).sub.y(R.sup.cO).sub.zR.sup.d (I)

[0010] in which R is a C.sub.4 to C.sub.6 substituent comprising a heterocyclic ring, in which the heteroatom(s) in said ring(s) is (are) oxygen and/or sulphur, R.sup.a is a C.sub.2 alkylene group, R.sup.b is a C.sub.3 alkylene group, R.sup.c is a C.sub.4 alkylene group, R.sup.d is the same as R, or is a hydrogen atom or a C.sub.1-C.sub.20 alkyl or C.sub.1-C.sub.20 acyl group, x, y, z are each independently 0 or an integer in the range of from 1 to 100, and X+y+z=4 to 100; and (B) an ester of a alcohol containing two or more hydroxyl groups with a mono- or dicarboxylic acid, or an ester of an dicarboxylic acid with an alcohol containing one hydroxyl group; together with a refrigerant is provided.

DETAILED DESCRIPTION OF THE INVENTION

[0011] We have now found that selection of a particular class of PAG in a mixture of PAG with ester, gives a lubricant oil for refrigeration apparatus with particularly useful properties.

[0012] Accordingly, the present invention provides a lubricating composition which comprises:

[0013] (A) a polyalkyleneglycol of the general formula (I): RO(R.sup.aO).sub.x(R.sup.bO).sub.y(R.sup.cO).sub.zR.sup.d (I)

[0014] wherein:

[0015] R is a C.sub.4 to C.sub.6 substituent comprising a heterocyclic ring, in which the heteroatom(s) in said ring(s) is (are) oxygen and/or sulphur,

[0016] R.sup.a is a C.sub.2 alkylene group,

[0017] R.sup.b is a C.sub.3 alkylene group,

[0018] R.sup.c is a C.sub.4 alkylene group,

[0019] R.sup.d is the same as R, or is a hydrogen atom or a C.sub.1-C.sub.20 alkyl or C.sub.1-C.sub.20 acyl group,

[0020] x, y, z are each independently 0 or an integer in the range of from 1 to 100, and

[0021] x+y+z=4 to 100; and

[0022] (B) an ester of a alcohol containing two or more hydroxyl groups with a mono- or dicarboxylic acid, or an ester of an dicarboxylic acid with an alcohol containing one hydroxyl group.

[0023] In the compound of the general formula I, R is a substituent comprising a C.sub.4 to C.sub.6 heterocyclic species, in which the heteroatom (s) in the heterocyclic ring is oxygen and/or sulphur. Preferably, the heterocyclic ring comprises oxygen or sulphur. The heterocyclic ring may be saturated or unsaturated. For example, R may comprise a saturated cyclic ether or saturated cyclic thioether. Such cyclic compounds may or may not be substituted. When substituted, the heterocycle may be linked to the rest of the molecule via the ring or via a substituent, which in such a case may be a hydrocarbyl linkage, e.g. --CH.sub.2--, --C.sub.2H.sub.4-- or --C.sub.3H.sub.6--. R preferably comprises a C.sub.4 to C.sub.6 heterocyclic moiety which is attached to the rest of the molecule either directly or via a hydrocarbyl linkage. For example, the heterocyclic moiety may be a furan or a thiophene ring. The heterocyclic moiety may alternatively be furfuryl, or a furfuryl derivative such as tetrahydrofurfuryl, attached to the rest of the molecule either directly or via a hydrocarbyl linkage. Examples of compounds from which R may be derived include tetrahydrofuran, methyltetrahydrofuran, tetrahydrothiophene or methyltetrahydrothiophene substituents. In a preferred embodiment of the invention, R represents a 5-membered oxygen-containing heterocycle linked to the rest of the molecule either directly or via a CH.sub.2 group. An especially preferred example of R is derivable from 2-hydroxymethyltetrahydrofuran, which may be regarded as having the formula R--OH according to the above definition, such that R is a tetrahydrofuranmethyl (tetrahydrofurfuryl) group.

[0024] Each of R.sup.a, R.sup.b and R.sup.c may be represented by the general formula II: [--C(R.sup.e)(R.sup.f)-C(R.sup.g)R.sup.h)--] (II)

[0025] In the case of R.sup.a, each of R.sup.e, R.sup.f, R.sup.g, and R.sup.h is hydrogen. In the case of R.sup.b, one of R.sup.e, R.sup.f, R.sup.g, and R.sup.h is methyl and the others are hydrogen. In the case of R.sup.c, either one of R.sup.e, R.sup.f, R.sup.g, and R.sup.h is ethyl or two of R.sup.a, R.sup.f, R.sup.g, and R.sup.h are methyl, the remainder being hydrogen. For example, R.sup.c may be a butylene group or an iso-butylene group.

[0026] Preferably, only one or two of R.sup.a, R.sup.b, and R.sup.c are present in the compound of Formula I. In other words, in preferred embodiments of the invention one or two of x, y and z is 0. For example, x may be zero, and/or z may be 0. Preferably, either z is 0 or z and x are both 0, i.e. the compound of Formula I is a polyalkylene glycol based upon propylene oxide, or upon a mixture of ethylene oxide and propylene oxide. If more than one of x, y and z is present, the compound of Formula I may be either a block copolymer or a random copolymer. The use of random copolymers is preferred. Preferably, the sum of x, y and z is equal to a number in the range of from 5 to 80, more preferably from 7 to 65.

[0027] R.sup.d may be the same as R, or may be a hydrogen atom or a C.sub.1-C.sub.20 alkyl or C.sub.1-C.sub.20 acyl group. Where R.sup.d is a C.sub.1-C.sub.20 alkyl or a C.sub.1-C.sub.20 acyl group, it preferably has 1 to 15 carbon atoms, more preferably 1 to 10 carbon atoms, and most preferably, 1 to 6 carbon atoms. When R.sup.d is a C.sub.1-C.sub.20 alkyl group, the alkyl group may be a straight chain, branched chain, or cyclic alkyl group. Suitable alkyl groups include methyl, ethyl, propyl, butyl, pentyl, and hexyl groups. When R.sup.d is a C.sub.1-C.sub.20 acyl group, the acyl group may be straight chain, branched chain or cyclic. The acyl group may have one or more further substituents, for example alkyl substituents, which may be a straight chain, branched chain or cyclic alkyl substituent. Such alkyl substituents include methyl, ethyl, propyl, butyl, pentyl, and hexyl groups. Preferably R.sup.d represents a hydrogen atom or, especially, a C.sub.1-4 alkyl group, preferably a methyl group.

[0028] In an especially preferred embodiment of the invention, the compound of the formula I has the following formula Ia: R.sup.1O--(R.sup.2O).sub.m--R.sup.3 (Ia)

[0029] in which R.sup.1 is a tetrahydrofurfuryl group; R.sup.2is a mix of C.sub.2 and C.sub.3 alkylene groups, the number ratio of C.sub.2 to C.sub.3 groups preferably being in the range of from 27:75 to 75:25, especially 40:60 to 60:40, or R.sup.2 is a C.sub.3 alkylene group; m is from 5 to 80, especially from 7 to 65; and R.sup.3 is a hydrogen atom or, preferably, a C.sub.1-4 alkyl group, especially a methyl group.

[0030] The compounds of the formula (I) are disclosed in EP 1257623. They may be prepared by any suitable method known in the art, including the methods described or referenced in EP 1257623.

[0031] Preferably the acid from which the ester component (B) is derived is an alkanoic or alkandioic acid, and preferably the alcohol is a mono-, di- or poly-hydroxyl alkanol. The ester may be a mixed ester derived from one or more alcohols and/or one or more acids. Preferably the ester is an ester of neopentyl glycol, monopentaerythritol, dipentaerythritol and/or trimethylolpropane with a C.sub.2-18 mono or di-alkanoic acid, more preferably an ester of neopentyl glycol and/or monopentaerythritol with a C.sub.2-10 mono alkanoic acid or a C.sub.6 dialkanoic acid, or a dipentaerythritol ester of a C.sub.2-10 mono alkanoic acid or a C.sub.8-9 dialkanoic acid, most preferably an ester of monopentaerythritol with a C.sub.5-9 mono alkanoic or a C.sub.6 dialkanoic acid. Esters of mixed acids are commonly used.

[0032] Suitable esters for use in the invention are well known, and many are commercially available. They may be prepared by methods known in the art.

[0033] In addition to the compound of the formula I and the ester, the composition according to the invention may also contain one or more additives, such as extreme pressure, antiwear, anticorrosion, antioxidants, and viscosity improver additives. Other additives include acidity regulators, reactive water-eliminating additives, antifoams, and demulsifiers. Typical additive packages include those available under the Trade Marks Irgalube 349, TBPP (tributyl phenyl phosphate), TCP (tricresyl phosphate) and Cardura E-10 (glycidyl neodecanoate).

[0034] Preferably the composition contains from 5 to 95% wt ester and from 5 to 95% of the compound of the general formula I.

[0035] Preferably the composition is formulated to have a viscosity in the range of from 2 to 220, more preferably 5 to 100, especially 5 to 68, cSt at 40.degree. C.

[0036] The lubricant composition according to the invention may be used with a wide range of refrigerants. Accordingly, the invention also provides a refrigerant composition for refrigeration apparatus which comprises a lubricant composition according to the invention together with a refrigerant. Preferably the refrigerant is 30 selected from fluorocarbons, hydrofluorocarbons (for example 1,1,1,2-tetrafluoroethane (R134a), difluoromethane (R32, pentafluoroethane (R125) and 1,1,1-trifluoroethane (R143a)), hydrocarbons (for example iso-butane), ammonia, and carbon dioxide. Preferably the refrigerant is a hydrofluorocarbon, for example 1,1,1,2-tetrafluoroethane (R134a), or carbon dioxide. Mixtures of different refrigerants may be used.

[0037] Compositions of the invention find particular application in compressors used in refrigeration and air-conditioning systems, for example domestic and industrial refrigeration and air-conditioning systems, and also automotive air-conditioning systems. Throughout this specification and claims, references to refrigeration systems and apparatus should be understood to include all refrigeration and air-conditioning systems, and also heat-pump systems and apparatus: these are refrigeration systems with a reverse heat flow compared with cooling systems. The invention further provides a refrigeration apparatus which includes a refrigerant composition according to the invention.

[0038] The lubricant compositions of the present invention provide a number of advantages. In particular, they provide advantageous lubricity, hygroscopicity and electrical properties.

[0039] The following Examples illustrate the invention.

EXAMPLES 1 to 5

[0040] The following lubricant compositions were tested: [0041] Example 1 (comparative). BREOX RFL (1250 g/mol mol.wt). [0042] Example 2 (comparative). MPE (monopentaerythritol) ester of iso-C.sub.8 (2-ethyhexanoic) (46.8 wt. %) and iso-C.sub.9 (iso-nonanoic acid) (53.2 wt %). [0043] Example 3. BREOX RFL (1250 g/mol) (50 wt %) blended with MPE (monopentaerythritol) ester (50 wt %) of iso-C.sub.8 (2-ethyhexanoic) (46.8 wt %) and iso-C.sub.9 (iso-nonanoic acid) (53.2 wt %). [0044] Example 4. BREOX RFL (1750 g/mol) (70 wt %) blended with MPE ester (30 wt %) of iso-C.sub.5 (57 wt %) and iso-C.sub.9 acid (43 wt %).

[0045] Example 5 (comparative). BREOX MAC (1550 g/mol) (67 wt %) blended with 33% MPE ester (33 wt %) of iso-C.sub.5 (isopentanoic) acid (57 wt %) and iso-C.sub.9 (iso-nonanoic) acid (43 wt %).

[0046] The BREOX RFL products are commercial refrigerator lubricants available from Cognis, and contain a compound of the formula I in which R is a tetrahydrofurfuryl group, x and z are both 0, and R.sup.d is a methyl group. The BREOX MAC product is a commercial refrigerator lubricant available from Cognis, and contains a polypropylene glycol initiated with an n-butyl group and terminated with a hydroxy group.

[0047] Experiments were carried out and data collected using a Falex Block on Ring test machine as follows. The procedure was in accordance with ASTM D3704, with some modification to the procedure to allow for the correct refrigerant atmosphere. The chamber containing the test equipment was evacuated of air and recharged with R410a HFC refrigerant, typically to a pressure of 180-280 psi. The equipment was computer software controlled to generate the coefficient of friction and wear data. Tests were run using steel blocks and rings of compressor grade steel. Tests were run in periods of 5 minutes at 3001b break-in load followed by 3501b load for 45 minutes, at 300 rpm. Operating temperature was 120.degree. C. Surface finish measurements were obtained using a commercially available standard profilometer. Results are shown in the following Table.

[0048] Examples 1 and 2 were carried out using a compound of formula I alone, and using an ester alone. Examples 3 and 4 were carried out using two different mixtures, and the results show that the blends in accordance with the invention are better than the separate components in coefficient of friction and surface finish (relates to wear) on the test pieces.

[0049] Example 5 is a comparative experiment using an ester of the same type as in Example 4 with a polyalkylene glycol which does not have the formula I. The results show a significant reduction in performance, both with respect to friction and to wear, compared with the mixtures using a compound of formula I. TABLE-US-00001 TABLE Coefficient of Wear Surface Finish Example No. Friction (inches) (Delta Ra) 1 0.0868 0.0006 3.2425 (comparative) 2 0.1153 0.0008 6.1171 (comparative) 3 0.0719 0.0006 1.5353 4 0.0765 0.0006 1.7695 5 0.0874 0.0010 4.1156 (comparative)

EXAMPLES 6 to 9

[0050] Further testing was carried using the following materials:

[0051] (A) BREOX RFL (Trade Mark) (1750 g/mol mol.wt)

[0052] (B) ProExo 32M (Trade Mark): MPE ester of ISO-C.sub.5 (57 wt %) and ISO-C.sub.9 acid (43 wt %)

[0053] (C) ProEco 100 C: adipate ester of ISO-C.sub.5 (57 wt %) and iso-C.sub.9 acid (43 wt %)

[0054] (D) A blend of 30% (B) and 70% (C) (which is a blend conforming to ISO 68).

[0055] All materials were formulated together with an Extreme Pressure/antiwear t-butylphenyl diphenyl phosphate type, SYN-O-AD 8478 (Trade mark).

[0056] The various materials were tested by a variety of standard tests. In all cases, the composition according to the invention gave superior results to comparison compositions.

EXAMPLE 6

[0057] Materials were tested by the Falex 4-ball wear test under the test procedure of ASTM D4172-94, using steel balls, under the following conditions: load: 20 kg; speed: 1200 rpm; temp: 107.degree. C.; time: 1 hour. The wear scar was measured, and the results are given in the following Table: TABLE-US-00002 Test Material Wear Scar (mm) B (comparison) 0.75 C (comparison) 0.67 A (comparison) 0.63 D (comparison) 0.68 B + A (30:70) 0.32

EXAMPLE 7

[0058] Materials were tested using the block-on-ring test using M2 vane steel blocks and cast iron rings, using the test procedure according to ASTM D-2714-94. Conditions were as follows: load: 300 lbs (break-in) and 250 lbs (test); speed: 300 rpm; temp: 120 C; time: 5 mins (break-in) and 45 mins (test). The results are given in the following Table: TABLE-US-00003 Test Material Ring Weight (mg) A (comparison) 3.00 D (comparison) 5.40 A + B 70:30 1.50

EXAMPLE 8

[0059] Materials were tested using the Pin and VBlock test according to ASTM D-2670-95. Conditions were as follows: aluminium pins, steel blocks; load: 350 lbs (break-in) and 400 lbs (test); time: 5 mins (break-in) and 30 mins (test). Pin weight was recorded. The results are given in the following Table. TABLE-US-00004 Materials Tested Pin Weight (mg) A (comparison) 13.8 D (comparison) 10.8 A + B 70:30 33.0

EXAMPLE 9

[0060] Materials were tested in a boundary friction test according to ASTM D-6079 with temperature modification. The equipment was a high frequency reciprocating rig, and the test conditions were as follows: stroke length: 1.+-.0.02 mm; frequency: 50.+-.1 Hz; temp: 50.degree. C., 100.degree. C., 120.degree. C.; duration: 75.+-.0.1 min; load: 200.+-.1 g. The results (average film and average friction at each temperature) are given in the following Table. TABLE-US-00005 Friction Film Friction Film Friction Test Materials Film (50.degree. C.) (50.degree. C.) (100.degree. C.) (100.degree. C.) (120.degree. C.) (120.degree. C.) A + B 70:30 90.8 0.079 68.0 0.106 85.9 0.120 A (comparison) 97.6 0.094 96.2 0.126 97.4 0.126

Claims

1. A lubricating composition which comprises: (A) a polyalkyleneglycol of the general formula (I): RO(R.sup.aO),(R.sup.bO).sub.y(R.sup.cO).sub.zR.sup.d (I) wherein: R is a C.sub.4 to C.sub.6 substituent comprising a heterocyclic ring, in which the heteroatom(s) in said ring(s) is (are) oxygen and/or sulphur, R.sup.a is a C.sub.2 alkylene group, R.sup.b is a C.sub.3 alkylene group, R.sup.c is a C.sub.4 alkylene group, R.sup.d is the same as R, or is a hydrogen atom or a C.sub.1-C.sub.20 alkyl or C.sub.1-C.sub.20 acyl group, x, y, z are each independently 0 or an integer in the range of from 1 to 100, and X+y+z=4 to 100; and (B) an ester of a alcohol containing two or more hydroxyl groups with a mono- or dicarboxylic acid, or an ester of an dicarboxylic acid with an alcohol containing one hydroxyl group.

2. A composition as claimed in claim 1, in which R represents a 5-membered oxygen-containing heterocycle linked to the rest of the molecule either directly or via a CH.sub.2 group.

3. A composition as claimed in claim 2, in which R represents a tetrahydrofurfuryl group.

4. A composition as claimed in claim 1, in which either z is 0 or z and x are both 0.

5. A composition as claimed in claim 1, in which the sum of x, y and z is equal to a number in the range of from 5 to 80.

6. A composition as claimed in claim 1, in which R.sup.d represents a hydrogen atom or a C.sub.1-4 alkyl group.

7. A composition as claimed in claim 6, in which R.sup.d represents a methyl group.

8. A composition as claimed in claim 1, in which the compound of the formula I has the formula Ia: R.sup.1O--(R.sup.2O).sub.m--R.sup.3 (Ia) in which R.sup.1 is a tetrahydrofurfuryl group; R.sup.2 is a mix of C.sub.2 and C.sub.3 alkylene groups, the number ratio of C.sub.2 to C.sub.3 groups preferably being in the range of from 27:75 to 75:25, or R.sup.2 is a C.sub.3 alkylene group; m is from 5 to 80; and R.sup.3 is a hydrogen atom or a C.sub.1-4 alkyl group.

9. A composition as claimed in claim 1, in which the acid from which the ester component (B) is derived is an alkanoic or alkandioic acid, and the alcohol is a mono-, di- or poly-hydroxyl alkanol.

10. A composition as claimed in claim 9, in which the ester is an ester of neopentyl glycol, monopentaerythritol, dipentaerythritol and/or trimethylolpropane with a C.sub.2-18 mono- or di-alkanoic acid.

11. A composition as claimed in claim 10, in which the ester is an ester of neopentyl glycol and/or monopentaerythritol with a C.sub.2-10 mono alkanoic acid or a C.sub.6 dialkanoic acid, or a dipentaerythritol ester of a C.sub.2-10 mono alkanoic acid or a C.sub.8-9 dialkanoic acid.

12. A composition as claimed in claim 11, in which the ester is an ester of monopentaerythritol with a C.sub.5-9 mono alkanoic acid or a C.sub.6 dialkanoic acid.

13. A refrigerant composition for refrigeration apparatus which comprises a lubricant composition comprising: (A) a polyalkyleneglycol of the general formula (I): RO(R.sup.aO).sub.x(R.sup.bO).sub.y(R.sup.cO).sub.zR.sup.d (I) wherein: R is a C.sub.4 to C.sub.6 substituent comprising a heterocyclic ring, in which the heteroatom(s) in said ring(s) is (are) oxygen and/or sulphur, R.sup.a is a C.sub.2 alkylene group, R.sup.b is a C.sub.3 alkylene group, R.sup.c is a C.sub.4 alkylene group, R.sup.d is the same as R, or is a hydrogen atom or a C.sub.1-C.sub.20 alkyl or C.sub.1-C.sub.20 acyl group, x, y, z are each independently 0 or an integer in the range of from 1 to 100, and X+y+z=4 to 100; and (B) an ester of a alcohol containing two or more hydroxyl groups with a mono- or dicarboxylic acid, or an ester of an dicarboxylic acid with an alcohol containing one hydroxyl group; together with a refrigerant.

14. A refrigerant composition as claimed in claim 13, in which the refrigerant is selected from fluorocarbons, hydrofluorocarbons, hydrocarbons, ammonia, and carbon dioxide.

15. A refrigerant composition as claimed in claim 14, in which the refrigerant is selected from 1,1,1,2-tetrafluoroethane or carbon dioxide.

16. Refrigeration apparatus which includes a refrigerant composition which comprises: (A) a polyalkyleneglycol of the general formula (I): RO(R.sup.aO).sub.x(R.sup.bO).sub.y(R.sup.cO).sub.zR.sup.d (I) wherein: R is a C.sub.4 to C.sub.6 substituent comprising a heterocyclic ring, in which the heteroatom(s) in said ring(s) is (are) oxygen and/or sulphur, R.sup.a is a C.sub.2 alkylene group, R.sup.b is a C.sub.3 alkylene group, R.sup.c is a C.sub.4 alkylene group, R.sup.d is the same as R, or is a hydrogen atom or a C.sub.1-C.sub.20 alkyl or C.sub.1-C.sub.20 acyl group, x, y, z are each independently 0 or an integer in the range of from 1 to 100, and X+y+z=4 to 100; and (B) an ester of a alcohol containing two or more hydroxyl groups with a mono- or dicarboxylic acid, or an ester of an dicarboxylic acid with an alcohol containing one hydroxyl group; together with a refrigerant.

17. A composition as claimed in claim 8, in which the acid from which the ester component (B) is derived is an alkanoic or alkandioic acid, and the alcohol is a mono-, di- or poly-hydroxyl alkanol.

18. A composition as claimed in claim 17, in which the ester is an ester of neopentyl glycol, monopentaerythritol, dipentaerythritol and/or trimethylolpropane with a C.sub.2-18 mono- or di-alkanoic acid.

19. A composition as claimed in claim 18, in which the ester is an ester of neopentyl glycol and/or monopentaerythritol with a C.sub.2-10 mono alkanoic acid or a C.sub.6 dialkanoic acid, or a dipentaerythritol ester of a C.sub.2-10 mono alkanoic acid or a C.sub.8-9 dialkanoic acid.

20. A composition as claimed in claim 19, in which the ester is an ester of monopentaerythritol with a C.sub.5-9 mono alkanoic acid or a C.sub.6 dialkanoic acid.