KR20030007038A - Lubricating oil composition for refrigerating machine - Google Patents

Abstract

PURPOSE: To provide a lubricating oil composition for a refrigerating machine, excellent in stability to hydrolysis and oxidation, scarcely causing color development of a lubricating oil nor generating sludge when used for a long period, and suitable for the refrigerating machine for which a refrigerant free from chlorine, such as a hydrofluorocarbon, carbon dioxide or a hydrocarbon, is used. CONSTITUTION: This lubricating oil composition for the refrigerating machine for which the hydrofluorocarbon, the carbon dioxide or the hydrocarbon is used as the refrigerant is given by adding (A) at least one kind of compound which contains an alkyl glycidyl ether, alkyl glycidyl ester or cyclohexane oxide in its molecule in an amount of not less than 0.01 wt.% but less than 1 wt.% and (B) at least one kind of diphenylcarbodiimide and a bis(alkylated phenyl)carbodiimide compound in an amount of not less than 0.01 wt.% but less than 1 wt.% to an ester-based or ether-based synthetic lubricating base oil.

Description

Lubricant composition for refrigerators {LUBRICATING OIL COMPOSITION FOR REFRIGERATING MACHINE}

The present invention relates to a lubricating oil composition for a refrigerator using a refrigerant containing no chlorine such as hydrofluorocarbon, carbon dioxide, and hydrocarbons.

As refrigerators such as refrigerators and air conditioners, lubricating oils are used for preventing friction, abrasion, burning and the like in the compression sliding parts. The characteristics required for the lubricating oil include excellent stability to the refrigerant and excellent solubility with the refrigerant because of contact with the refrigerant. Conventionally, as a refrigerant | coolant, a proton refrigerant containing chlorine, such as chlorofluorocarbons, such as trichloromonofluoromethane (R11) and dichlorodifluoromethane (R12), hydro such as monochlorodifluoromethane (R22), etc. Chlorofluorocarbons are used, and naphthenic mineral oil, paraffinic mineral oil, alkylbenzene synthetic oil, polyαolefin synthetic oil (PAO) and the like have been used as lubricating oils for these refrigerants containing chlorine.

However, because their chlorine-containing pron refrigerants destroy the stratospheric ozone layer, their use is regulated internationally and does not contain chlorine, such as 1,1,1,2-tetrafluoroethane (R134a), as an alternative prolon refrigerant. Hydrofluorocarbons, ammonia, carbon dioxide, hydrocarbons and the like are used as refrigerants. Lubricating oils corresponding to these refrigerants are examined, and polyether-based synthetic oils such as polyoxyalkylene glycol (PAG) or polyolester-based synthetic oils for hydrofluorocarbon refrigerants, and ether-based synthetic oils such as PAG and PAO for carbon dioxide. For synthetic hydrocarbon compounds such as ether, synthetic ethers, naphthenic mineral oils and paraffinic mineral oils have been proposed.

However, in the case of a refrigerant containing no chlorine, the extreme pressure effect (improvement of lubrication) of the chlorine cannot be expected, so it is easy to cause lubrication defects in the bearings, pistons, seals, etc. of the refrigerator, causing energy loss, increased wear, and crushing. It may also cause corrosion by promoting decomposition of refrigerant or lubricant. In particular, a small amount of water and oxygen are present in refrigeration system systems such as compressors of refrigerators, so that when polyol ester lubricants are used, they are hydrolyzed to generate free acids, and in the case of ether lubricants, oxidation deterioration. As a result, the acid value was increased, and none of them had problems such as corrosion of materials in the apparatus, acceleration of deterioration of refrigerant and lubricating oil, and clogged capillary due to generation of refrigerant insoluble components.

In order to improve these methods, a method of adding an epoxy compound (Japanese Patent Laid-Open No. Hei 2-276880, 5-17792) or a method of adding a compound having a carbodiimide group (Japanese Patent Laid-Open No. Hei 7-133487, WO 94/21759) has been proposed. . However, the epoxy compound has a slow reaction with the free acid and a compound having a carbodiimide group alone. In order to achieve a substantial effect, the epoxy compound needs to be added in a large amount, and in particular, the refrigeration system does not have a device or a means for reducing system moisture. In refrigerators such as car air conditioners or air conditioners, the amount of addition must be 2% by weight or more, and in the case of carbodiimide, the coloring of lubricating oil causes a significant decrease in product value. Both the epoxy compound and the carbodiimide compound have a problem of producing sludge with long-term use.

As a result of earnest research to solve the above problems, the inventors of the present invention surprisingly can reduce the amount of addition of these compounds alone compared to the case of adding a single epoxy compound and a carbodiimide compound. It was found that the generation of sludge due to coloring or prolonged use can be suppressed.

The present invention is based on these findings, and an object of the present invention is chlorine, such as hydrofluorocarbon, carbon dioxide, or hydrocarbon, which is excellent in stability against hydrolysis or oxidation and does not generate sludge in coloring of lubricants or prolonged use. It is to provide a lubricating oil composition most suitable for a refrigerator using a refrigerant containing no.

The present invention,

(1) To ester-based or ether-based synthetic lubricant base oils,

(A) 0.01% by weight or more and less than 1% by weight of one or more compounds containing alkylglycidyl ether, alkylglycidyl ester or cyclohexene oxide in a molecule, and

(B) 0.01% by weight or more but less than 1% by weight of at least one kind of diphenylcarbodiimide or bis (alkylated phenyl) carbodiimide compound

Lubricating oil composition for a refrigerator comprising a mixture of hydrofluorocarbons, carbon dioxide or hydrocarbons,

(2) to ester or ether synthetic lubricants,

(A) 0.01% by weight or more but less than 1% by weight of one or more kinds of a compound containing alkylglycidyl ether, alkylglycidyl ester or cyclohexene oxide in a molecule,

(B) 0.01% by weight or more but less than 1% by weight of one or more kinds of diphenylcarbodiimide or bis (alkylated phenyl) carbodiimide compound

(C) 0.01- 1% by weight of di-tert-butyl-p-cresol

Lubricating oil composition for a refrigerator comprising a mixture of hydrofluorocarbons, carbon dioxide or hydrocarbons,

(3) The lubricating oil composition for a refrigerator according to (1) or (2) above, wherein the refrigerator having a hydrofluorocarbon, carbon dioxide, or hydrocarbon as a refrigerant does not have a device or a means for reducing the system moisture of the refrigerator.

(4) the lubricating oil composition for a refrigerator according to the above (1) to (3), wherein the ester synthetic lubricant base oil is a polyol ester;

(5) The lubricating oil composition for a refrigerator according to the above (1) to (3), wherein the ester synthetic lubricant base oil is a complex ester;

(6) the lubricating oil composition for a refrigerator according to the above (4), wherein the polyol ester is an ester synthesized from pentaerythritol, neopentyl glycol, trimetholpropane, or dipentaerythritol and monovalent fatty acid;

(7) The lubricating oil composition for a refrigerator according to the above (4) or (6), wherein the polyol ester is an ester of a pentaerythritol with a mixture of two or more monovalent fatty acids.

It is about.

Epoxy and carbodiimide compounds simultaneously react with corrosive compounds in the refrigerator, such as moisture, fatty acids, and oxidative oxides, to prevent corrosion components from causing deterioration of materials in the refrigerator and deterioration of refrigerants and lubricants. Have

The carbodiimide compound reacts rapidly with water, fatty acids, and oxidative oxides, and reacts with fatty acids, oxidized oxides, or initial moisture in the freezer remaining in the lubricating oil after purification.

Epoxy compounds, on the other hand, are relatively slow in reaction and react with corrosion components such as fatty acids and oxidative degradation products generated from heat loads and sliding parts during operation of the freezer.

When both compounds are used together, the said effect rises and these addition amounts can be reduced.

Furthermore, the addition of di-tert-butyl-p-cresol with both compounds can enhance the effect.

Embodiments of the Invention

The present invention can be applied to a refrigerator using refrigerants such as hydrofluorocarbons, carbon dioxide, and hydrocarbons, and is particularly suitable for a refrigerator having no device or means for reducing system moisture in a refrigeration system in a car air conditioner or an air conditioner. Do. It can also be applied to one or a mixture of two or more of these refrigerants containing no chlorine.

As hydrofluorocarbons, for example, 1,1,1,2-tetrafluoroethane (R134a), 1,1,1-trifluoroethane (R143a), pentafluoroethane (R125), difluoromethane ( Refrigerants such as R32), and mixed refrigerants thereof.

As the hydrocarbon refrigerant, for example, a hydrocarbon compound having 1 to 6 carbon atoms can be used. Specifically, compounds, such as methane, ethane, propane, n-butane, i-butane, n-pentane, i-pentane, neopentane, cyclopropane, cyclobutane, cyclopentane, are mentioned. These compounds may be used alone or in combination of two or more thereof as appropriate.

As a base oil of the lubricating oil for refrigerators in this invention, it is preferable to use 1 or more types of ester compounds and ether compounds.

Examples of the ester compounds include polyol esters obtained by esterification of polyhydric alcohols (polyols) and monocarboxylic acids (monoaliphatic acids), complex esters of polyhydric alcohols with monocarboxylic acids and polyhydric acids, or mixtures thereof. desirable. Especially as polyhydric alcohol, neopentyl polyols, such as neopentyl glycol, a trimethol propane, pentaerythritol, and dipentaerythritol, are preferable. Examples of monocarboxylic acids include n-pentanoic acid, n-hexanoic acid, n-heptanoic acid, n-octanoic acid, n-nonanoic acid, n-decanoic acid, i-pentanoic acid, i-hexanoic acid, i-heptanoic acid, 2 -Ethyl pentanoic acid, 2-methylhexanoic acid, i-octanoic acid, 2-ethylhexanoic acid, i-nonanoic acid, 3,5,5-trimethylhexanoic acid, i-decanoic acid, and the like.

Moreover, as polyhydric acid, dicarboxylic acid, such as malonic acid, a succinic acid, a glutaric acid, adipic acid, a pimeric acid, a sublinic acid, an azeline acid, is preferable.

Various ethers are mentioned as an ether compound. Representative of the following general formula (1)

(Meal,

X: hydrocarbon group in the form of monool or polyol except hydroxyl group,

A: an alkylene group having 2 to 4 carbon atoms,

R ¹ : hydrogen or an alkyl group having 1 to 10 carbon atoms,

m: mantissa of X

n is an integer of 2 or more)

The polyether compound represented by these is mentioned. In addition, (AO) n is one or two or more of oxyethylene, oxypropylene, or oxybutylene, which are n groups as a whole, and the aforementioned oxyalkylene groups can be arranged in a desired order.

As base oil, you may use the said synthetic oil suitably 1 type, or 2 or more types.

The lubricating oil composition of this invention can be manufactured by adding the additive of the (A) and (B) component shown below and the (c) component to said base oil. Furthermore, you may add another additive (D) as needed.

(A) a compound selected from compounds containing alkylglycidyl ether, alkylglycidyl ester or cyclohexene oxide in a molecule

(A1) alkyl glycidyl ether

Alkyl glycidyl ether is represented by following General formula (2).

(In formula (2), R <^2> represents a C5-C18 alkyl group.)

When the carbon number of the alkyl group is less than 5, the stability of the epoxy compound is deteriorated, so that it may decompose before reacting with moisture, fatty acids, or oxidizing oxides, or may cause self-polymerization of the epoxy compounds to polymerize. . On the other hand, if the carbon number exceeds 18, it is not preferable because the solubility with the refrigerant, especially the hydrofluorocarbon refrigerant, may be lost, causing precipitation in the refrigerating device, which may cause unsatisfactory conditions such as poor cooling.

In addition, phenyl, an allyl group, and the like are not preferable because they may lose solubility with the refrigerant and may precipitate in the freezer, resulting in an unsatisfactory condition such as poor cooling.

Specifically, pentyl glycidyl ether, hexyl glycidyl ether, octyl glycidyl ether, 2-ethylhexyl glycidyl ether, nonyl glycidyl ether, decyl glycidyl ether, neodecyl glycidyl ether, dode Silglycidyl ether, neododecyl glycidyl ether, palmityl glycidyl ether, stearyl glycidyl ether and the like.

(A2) alkyl glycidyl esters

Alkyl glycidyl ester is represented by following General formula (3).

(Wherein R ³ represents an alkyl group having 5 to 18 carbon atoms)

When the carbon number of the alkyl group is less than 5, the stability of the epoxy compound is deteriorated, so that it may decompose before reacting with moisture, fatty acids, or oxidizing oxides, or may cause self-polymerization of the epoxy compounds to polymerize. . On the other hand, if the carbon number exceeds 18, it is not preferable because the solubility with the refrigerant, especially the hydrofluorocarbon refrigerant, is lost and may precipitate in the freezer, resulting in an unsatisfactory condition such as poor cooling.

In addition, phenyl, an allyl group, and the like are not preferable because they may lose solubility with the refrigerant and may precipitate in the freezer, resulting in an unsatisfactory condition such as poor cooling.

Specifically, pentyl glycidyl ester, hexyl glycidyl ester, octyl glycidyl ester, 2-ethylhexyl glycidyl ester, nonyl glycidyl ester, decyl glycidyl ester, neodecyl glycidyl ester, dode Silglycidyl ester, neododecyl glycidyl ester, palmityl glycidyl ester, stearyl glycidyl ester and the like.

(A3) A compound containing cyclohexene oxide in a molecule

The compound containing cyclohexene oxide in a molecule | numerator is represented by following General formula (4).

In said formula (4), R <^4> is hydrogen or the C1-C30 alkyl group or alkenyl group, the phenyl group, the alkylated phenyl group, or the oxyalkylene group represented by following General formula (5),

(In Formula (5), A represents a C1-C4 alkylene group and n represents the integer of 1-10.)

Group containing polyester represented by following General formula (6),

(In Formula (6), B is a C1-C9 alkylene group and m shows the integer of 0-10.)

or,

Group containing carbonyl represented by the following general formula (7)

(In formula (7), R <^5> is hydrogen or a C1-C20 alkyl group, p shows the integer of 0-20.)

Which is either.

Specifically, cyclohexene oxide, epoxidized tetrahydrobenzyl alcohol, lactone-modified epoxylated tetrahydrobenzyl alcohol, vinylcyclohexene monooxide, etc. are mentioned, for example. In particular, epoxidized tetrahydrobenzyl alcohol is preferable.

Moreover, the condensate obtained by the at least 1 sort (s) of polycondensation or dehydration condensation of the structure shown by said formula can also be used.

At least one selected from the above-mentioned compounds containing alkylglycidyl ether, alkylglycidyl ester and cyclohexene oxide in the molecule is 0.01% by weight or more and less than 1% by weight with respect to the base oil, preferably 0.05% by weight or more. Less than 1% by weight, more preferably 0.1% by weight or more and less than 1% by weight. If the addition amount is too small, it is not effective. If the addition amount is too large, it is not preferable because sludge is generated by self-polymerization in which epoxy compounds are polymerized in long-term use.

(B) diphenylcarbodiimide or bis (alkylphenyl) carbodiimide

As bis (alkylphenyl) carbodiimide in this invention, a dithryl carbodiimide, bis (isopropylphenyl) carbodiimide, bis (diisopropylphenyl) carbodiimide, and a bis (triisopropylphenyl) knife Bodyimide, bis (butylphenyl) carbodiimide, bis (dibutylphenyl) carbodiimide, bis (nonylphenyl) carbodiimide and the like.

The carbodiimide compound is added in an amount of 0.01% by weight or more and less than 1% by weight with respect to the base oil, preferably 0.05% by weight or more and less than 0.5% by weight. If the amount is too small, it is not effective. If the amount is too large, the lubricating oil is colored yellow to brown, which is not preferable because the product value is lowered and the sludge is formed by long-term use.

(C) di-tert-butyl-p-cresol

The di-tert-butyl-p-cresol is added in an amount of 0.01 to 1% by weight, preferably 0.05 to 0.5% by weight, based on the base oil. Too little added flavor is not effective. Too much added flavor is not preferable because sludge is produced by prolonged use.

(D) other additives

In the present invention, it is possible to appropriately add the following known additives in a range that imparts comprehensive performance and does not impair the characteristics of the lubricating oil composition. Furthermore, you may mix | blend the following additives with the lubricating oil composition of this invention.

Anti-abrasive: Sulfur-based, Phosphorous, Zinc Thiophosphate, etc.

Antioxidant: Phenolic, Amine, Phosphorus, etc.

Metal deactivator: benzotriazole (derivative), thiadiazole, dithiocarbamate, etc.

Acid trapping agent: Epoxy compound other than the above,

Carbodiimide compounds other than the above

Oily agent: higher fatty acids, alcohol, etc.

Antifoam: Silicone Oil

In addition, well-known additives for lubricants

The lubricating oil composition of the present invention is used in refrigerators such as car air conditioners, air conditioners, dehumidifiers, refrigerators, freezers, freezers, vending machines, showcases, chemical plants, etc. having a compressor of a recipe or rotary type, but especially as a refrigeration cycle. A refrigerator comprising at least a compressor, an expansion mechanism, an evaporator, a condenser, or a gas cooler and including no device or means such as a dryer for reducing moisture in the refrigeration cycle system can exhibit the maximum effect of the lubricating oil of the present invention. to be.

EXAMPLES

Hereinafter, although it demonstrates based on an Example and a comparative example, this invention is not limited to an Example. The base oil and the additive which were used for the Example and the comparative example are as follows.

Base oil:

Base oil 1: Polyol ester obtained from pentaerythritol and 2-ethylhexanoic acid / 3,5,5-trimethylhexanoic acid (50 weight% / 50 weight%).

Base oil 2: Polyol ester obtained from pentaerythritol and 2-ethylhexanoic acid.

Base oil 3: Polyol ester obtained from neopentyl glycol and 2-ethylhexanoic acid.

additive :

(1) Epoxy Compound

A1: 2-ethylhexyl glycidyl ether

A2: neodecyl glycidyl ester

A3: oxycyclohexenol oxycyclohexenoic acid ester

(2) Carbodiimide Compounds (CDⅠ):

B1: bis (dibutylphenyl) carbodiimide

(3) di-tert-butyl-p-cresol (DBPC)

Table 1 shows the compositions of the lubricating oil compositions of Examples and Comparative Examples.

Table 1

Base oil Epoxy compound CDI DBPC One 2 3 A1 A2 A3 B1 Example 1 100 0.05 0.05 0.1 Example 2 100 0.25 0.05 Example 3 100 0.25 0.05 0.1 Example 4 100 0.5 0.05 Example 5 100 0.5 0.05 0.1 Example 6 100 0.9 0.01 0.1 Example 7 100 0.25 0.05 0.1 Example 8 100 0.25 0.05 0.1 Example 9 50 50 0.5 0.1 0.1 Example 10 50 50 0.5 0.1 0.1 Example 11 50 50 0.5 0.1 0.1 Comparative Example 1 100 0.5 0.1 Comparative Example 2 100 1.0 0.1 Comparative Example 3 100 2.0 0.1 Comparative Example 4 100 2.0 2.0 0.1 Comparative Example 5 100 2.0 0.1 Comparative Example 6 100 2.0 0.1 Comparative Example 7 100 1.0 0.1 Comparative Example 8 100 2.0 0.1 Comparative Example 9 100 6.0 0.1 Comparative Example 10 100 6.0 0.1 Comparative Example 11 50 50 1.0 0.1 Comparative Example 12 50 50 1.0 0.1

Using the lubricating oil compositions of these examples and comparative examples, the stability against hydrolysis or oxidative degradation was evaluated by a shield tube test under the following conditions.

Shield tube test (stability check):

Test conditions: Conforms to JIS K2211 Annex 2

Water-in-oil: 1000 ppm

Lubricant / R134a = 7ml / 3ml

Catalyst: Fe, Cu, Al

Aging condition: 1) 175 ℃ × 14 days

2) 200 ℃ × 14 days

The color, the acid value, and the presence or absence of precipitation of the sample after aging were measured.

Table 2 shows the test results of the lubricating oil compositions of Examples and Comparative Examples.

Table 2

170 ℃ 14 days 200 ℃ 14 days Color ASTM Acid value mgKOH / g Precipitation Color ASTM Acid value mgKOH / g Precipitation Example 1 L0.5 0.01 none L0.5 0.01 none Example 2 L0.5 0.01 none L0.5 0.02 none Example 3 L0.5 0.01 none L0.5 0.01 none Example 4 L0.5 0.01 none L0.5 0.01 none Example 5 L0.5 0.01 none L0.5 0.01 none Example 6 L0.5 0.01 none L0.5 0.01 none Example 7 L0.5 0.01 none L0.5 0.01 none Example 8 L0.5 0.01 none L0.5 0.01 none Example 9 L0.5 0.01 none L0.5 0.01 none Example 10 L0.5 0.01 none L0.5 0.01 none Example 11 L0.5 0.01 none L0.5 0.01 none Comparative Example 1 L0.5 0.04 none L1.0 0.78 none Comparative Example 2 L0.5 0.03 none L1.0 0.52 none Comparative Example 3 L0.5 0.02 none L1.0 0.31 has exist Comparative Example 4 L1.5 0.01 has exist L2.5 0.01 has exist Comparative Example 5 L1.0 0.03 none L1.0 0.36 has exist Comparative Example 6 L0.5 0.02 none L1.0 0.27 has exist Comparative Example 7 L1.5 0.03 none L2.0 0.39 has exist Comparative Example 8 L2.5 0.02 has exist L3.5 0.23 has exist Comparative Example 9 L1.0 0.02 has exist L1.0 0.26 has exist Comparative Example 10 L3.5 0.01 has exist L5.0 0.16 has exist Comparative Example 11 L0.5 0.03 none L0.5 0.40 none Comparative Example 12 L2.0 0.03 none L2.0 0.36 none

From the above results, it was found that the addition of both the epoxy compound and the carbodiimide compound as in the examples, or the addition of di-tert-butyl-p-cresol to them was excellent in stability.

Moreover, the compressor endurance test which corresponded to a practical test in Examples 2, 7, 8, and Comparative Examples 2, 4, 7 was performed on the following conditions.

Compressor Endurance Test

Exam conditions :

Compressor: 1HP, Rotary Compressor

Water-in-oil: 1000 ppm

Refrigerant: R407C

Discharge Pressure: 31 ㎏ / ㎠

Inlet Pressure: 6 ㎏ / ㎠

Test time: 1000 hours

The refrigerant cycle consisted of a compressor, a condenser, an expansion valve (needle valve), and an evaporator, and was tested in a cycle without a dryer.

Table 3 shows the test results of the lubricating oil compositions of Examples and Comparative Examples.

Table 3

Color: ASTM Computer Value: ㎎KOH / g Precipitation Example 2 L1.0 0.03 none Example 7 L1.0 0.02 none Example 8 L1.0 0.01 none Comparative Example 2 L2.0 0.21 none Comparative Example 4 L5.5 0.04 has exist Comparative Example 7 L6.0 0.05 has exist

From the above results, it was found that the addition of both an epoxy compound and a carbodiimide compound as in the examples, or the addition of di-tert-butyl-p-cresol to these were excellent in stability.

The lubricating oil composition for a refrigerator using the specific epoxy compound and the carbodiimide compound in combination with the present invention is excellent in stability against hydrolysis and oxidation, and does not generate sludge in coloring or long-term use of lubricating oil and further hydrolysis. There is a particular effect of reducing corrosion of materials in the apparatus by products or oxidative deterioration, acceleration of deterioration of refrigerant or lubricating oil, and blockage of capillary caused by the generation of refrigerant non-hazardous components.

Claims (7)

In ester type or ether type synthetic lubricating base oil, (A) 0.01% by weight or more but less than 1% by weight of at least one compound containing an alkylglycidyl ether, an alkylglycidyl ester, or a cyclohexene oxide in a molecule; and (B) 0.01% by weight or more but less than 1% by weight of at least one kind of diphenylcarbodiimide or bis (alkylated phenyl) carbodiimide compound A lubricating oil composition for a refrigerator, comprising a mixture of hydrofluorocarbon, carbon dioxide, or hydrocarbon. In ester or ether synthetic lubricants, (A) 0.01% by weight or more but less than 1% by weight of one or more of the compounds containing alkylglycidyl ether, alkylglycidyl ester, or cyclohexene oxide in a molecule, (B) 0.01% by weight or more but less than 1% by weight of at least one of diphenylcarbodiimide or bis (alkylated phenyl) carbodiimide compound, and (C) 0.01- 1% by weight of di-tert-butyl-p-cresol A lubricating oil composition for a refrigerator, comprising a mixture of hydrofluorocarbon, carbon dioxide, or hydrocarbon. The lubricating oil composition for a refrigerator according to claim 1 or 2, wherein the refrigerator using a hydrofluorocarbon, carbon dioxide, or hydrocarbon as a refrigerant does not have a device or a means for reducing the system moisture of the refrigerator. The lubricating oil composition for a refrigerator according to claim 1, wherein the ester synthetic lubricating base oil is a polyol ester. The lubricating oil composition for a refrigerator according to claim 1, wherein the ester-based lubricating base oil is a complex ester. The lubricating oil composition for a refrigerator according to claim 4, wherein the polyester is an ester synthesized from pentaerythritol, neopentylglycol, trimetholpropane, and / or dipentaerythritol and monovalent fatty acid. The lubricating oil composition for a refrigerator according to claim 4 or 6, wherein the polyester is an ester of a pentaerythritol with a mixture of two or more monovalent fatty acids.