Ester composition for refrigerator oil and working fluid composition for refrigerator

Abstract

Provided is an ester composition for a refrigerator oil, the ester composition for a refrigerator oil having excellent lubricity and thermal stability and having a lubricity that does not readily deteriorate even at high temperatures and in the presence of air, wherein: contained in the ester composition for a refrigerator oil are an ester (A) that is a triester of oleic acid and a trihydric alcohol having a neopentyl skeleton and an ester (B) that is a diester of oleic acid and a trihydric alcohol having a neopentyl skeleton; the mass ratio ((A):(B)) of the ester (A) to the ester (B) is 99.9:0.1 to 95.0:5.0; and the peroxide value of the ester composition is 0.1 to 10.0 meq / kg.

Description

Ester Composition for Refrigeration Oil and Refrigerant Composition for Refrigeration Equipment

[0001] The present invention relates to an ester composition used as a base oil for refrigeration oil, and more particularly to an ester composition for refrigeration oil used as a working fluid for refrigeration and air-conditioning equipment using a hydrocarbon refrigerant. Furthermore, the present invention relates to a refrigerant composition for refrigeration equipment containing the ester composition for refrigeration oil and a hydrocarbon refrigerant.

[0002] In refrigeration and air-conditioning equipment such as room air conditioners or package air conditioners, low-temperature equipment such as household refrigerators and freezers, industrial refrigerators, and car air conditioners such as hybrid cars or electric vehicles, in consideration of the global environment, the refrigerant used is being converted from a refrigerant with a high global warming potential (GWP) to a refrigerant with a low GWP in order to reduce the greenhouse effect. Against this background, the R32 refrigerant (difluoromethane, GWP: 675) is becoming more popular. On the other hand, for further reduction of the greenhouse effect, conversion to hydrocarbon refrigerants such as the R290 refrigerant (propane, GWP: 3) with an even lower GWP is being considered. As a refrigeration oil having excellent compatibility with these low-GWP refrigerants, for example, Patent Document 1 discloses a refrigeration oil for a hydrocarbon refrigerant containing an ester of a fatty acid and a polyhydric alcohol in which the proportion of linear fatty acids having 10 to 22 carbon atoms is 50 to 100 mol%. Specifically, Patent Document 1 discloses an ester of oleic acid and a polyhydric alcohol such as pentaerythritol as a base oil for refrigeration oil.

[0003] In addition, in the refrigerant circulation system of refrigeration and air-conditioning equipment, moisture and air may be mixed in. For example, Patent Document 2 discloses a base oil for refrigeration oil having excellent stability when moisture is mixed in, containing an ester of a trihydric alcohol and a fatty acid having 14 to 20 carbon atoms, wherein the ester includes a diester and a triester, and the content of the triester is 93 mol% or less based on the total amount of the diester and the triester. Specifically, Patent Document 2 discloses a mixture of diesters and triesters of a plurality of types of fatty acids having oleic acid as a main component and trimethylolpropane as a base oil for refrigeration oil.

[0004] On the other hand, hydrocarbon refrigerants do not contain fluorine, which enhances lubricity, within their hydrocarbon molecules. Therefore, unlike hydrofluorocarbon (HFC) refrigerants, the lubricity-enhancing effect of the refrigerant cannot be expected. For this reason, particularly superior lubricity is required for refrigeration oils used with hydrocarbon refrigerants. Furthermore, because hydrocarbon refrigerants have high compatibility with refrigeration oils, the viscosity of the refrigeration oil tends to decrease in the presence of hydrocarbon refrigerants. As a result, the operating conditions for refrigeration oil in refrigeration and air conditioning equipment using hydrocarbon refrigerants become even more severe. This makes frictional heat more likely to be generated in the sliding parts of the refrigerant compressor, and the refrigeration oil exposed to locally high temperatures may undergo thermal decomposition. The resulting decomposition products may corrode metal components or degrade resin materials. In addition, since air is mixed into the refrigerant circulation system of refrigeration and air conditioning equipment, degradation of the refrigeration oil due to air is also expected. For this reason, there is a need to develop esters for hydrocarbon refrigeration oils that exhibit excellent lubricity even under such harsh conditions and have excellent thermal stability.

[0005] Japanese Patent Publication No. 2010-90284 Japanese Patent Publication No. 2023-149825

[0006] The object of the present invention is to provide an ester composition for refrigeration oil that has excellent lubricity and thermal stability and whose lubricity does not deteriorate easily even at high temperatures and in the presence of air, and further to provide a working fluid composition for refrigeration containing the ester composition for refrigeration oil and a hydrocarbon refrigerant.

[0007] As a result of diligent research to solve the above problems, the present inventors have found that when a specific mixture of triesters and diesters has a specific peroxide value, it exhibits excellent thermal stability and lubricity, and can maintain its lubricity even under harsh conditions such as high temperature and the presence of air, thus completing the present invention.

[0008] In other words, the present invention is as follows: [1] An ester composition for refrigeration oil containing the following ester (A) and the following ester (B), wherein the mass ratio of ester (A):(B) is 99.9:0.1 to 95.0:5.0, and the peroxide value of the ester composition is 0.1 to 10.0 meq / kg. Ester (A): A triester ester of a trihydric alcohol having a neopentyl skeleton and oleic acid. (B): A diester of a trihydric alcohol having a neopentyl skeleton and oleic acid.

[0009] [2] A working fluid composition for a refrigerator, comprising the ester composition for refrigeration oil described in [1] above and a hydrocarbon refrigerant, wherein the content of the ester composition is 1 part by mass or more and 500 parts by mass or less per 100 parts by mass of the hydrocarbon refrigerant.

[0010] The present invention provides an ester composition that has excellent lubricity, is resistant to thermal decomposition, and can maintain excellent lubricity even when exposed to harsh environments such as high temperatures and the presence of air. The present invention further provides a working fluid composition for refrigerators containing the ester composition and a hydrocarbon refrigerant.

[0011] The embodiments of the present invention will be described below, but the present invention is not limited to the embodiments described herein and can be modified in various ways without departing from the spirit of the invention. In the present invention, numerical ranges defined using the symbol "~" include the numerical values ​​at both ends (upper and lower limits) of "~". For example, "2 to 10" represents a range of 2 to 10. In addition, in numerical ranges described herein, the upper or lower limit of the numerical range can be replaced with the value shown in the example or a value uniquely derived from the example. Furthermore, unless otherwise specified, the numerical values ​​described for the purpose of explaining the present invention are numerical values ​​obtained by rounding the digit that was one place smaller than the smallest digit included in the numerical value.

[0012] [Ester Composition for Refrigeration Oil] The ester composition for refrigeration oil of the present invention (which may be simply referred to as "ester composition" in this specification) is an ester composition containing the following ester (A) and the following ester (B), wherein the mass ratio of ester (A):(B) is 99.9:0.1 to 95.0:5.0, and the peroxide value of the ester composition is 0.1 to 10.0 meq / kg. Ester (A): Triester ester of a trihydric alcohol having a neopentyl skeleton and oleic acid. (B): Diester of a trihydric alcohol having a neopentyl skeleton and oleic acid.

[0013] As described above, the ester composition for refrigeration oil of the present invention exhibits excellent lubricity and thermal stability, and is less prone to deterioration even when air is mixed in. Therefore, it can maintain excellent lubricity even when exposed to harsh environments such as high temperatures and the presence of air. Refrigeration oils for hydrocarbon refrigerants require excellent lubricity, thermal stability that resists thermal decomposition, and the ability to maintain lubricity even under harsh conditions. For these reasons, the ester composition for refrigeration oil of the present invention can be suitably used as a base oil for refrigeration oils for hydrocarbon refrigerants.

[0014] Ester (A) used in the present invention is a triester of a trivalent alcohol having a neopentyl skeleton and oleic acid, and ester (B) used in the present invention is a diester of a trivalent alcohol having a neopentyl skeleton and oleic acid. A "trivalent alcohol having a neopentyl skeleton" is an alcohol having a neopentyl skeleton that does not have a hydrogen atom at the β-position carbon relative to the hydroxyl group, and whose alcohol valency is trivalent. In the present invention, a "trivalent alcohol having a neopentyl skeleton" may be referred to as a "trivalent neopentyl polyol." The triester (ester (A)) is a compound in which all three hydroxyl groups of the trivalent neopentyl polyol are esterified with oleic acid, and the diester (ester (B)) is a compound in which two of the hydroxyl groups in the trivalent neopentyl polyol are esterified with oleic acid. The triester (ester (A)) and the diester (ester (B)) can be synthesized by adjusting the amount of oleic acid reacted with the trivalent neopentyl polyol.

[0015] The trivalent neopentyl polyol used in the present invention preferably has 5 to 7 carbon atoms. This further improves the lubricity and thermal stability of the ester composition of the present invention, and further suppresses the deterioration of lubricity under harsh conditions such as high temperature and the presence of air. Examples of preferred trivalent neopentyl polyols include trimethylolethane, trimethylolpropane, and trimethylolbutane, with trimethylolpropane being particularly preferred. These trivalent neopentyl polyols can be used individually or in combination of two or more.

[0016] In the synthesis of ester (A) and ester (B) described above, industrially available oleic acid can be used, and among them, oleic acid with an oleic acid content of about 70% by mass or more (about 70 to 100% by mass) is preferably used.

[0017] The above ester (A) can be produced, for example, by the following method. First, a trivalent neopentyl polyol and an excess amount of oleic acid (preferably 1.1 to 1.3 equivalents) relative to the equivalent amount of trivalent neopentyl polyol are reacted under a nitrogen stream at a temperature of 180 to 250°C until the hydroxyl value becomes 0.1 mg KOH / g or less to obtain a crude ester. The reaction time is preferably 1 to 20 hours. Catalysts such as Lewis acid catalysts or Brønsted acid catalysts may be used in the reaction. The excess acid contained in the obtained crude ester is treated with alkali with potassium hydroxide or the like, followed by washing with water and dehydration. Then, oleic acid triester (ester (A)) is obtained by purification treatment such as adsorbing impurities with an adsorbent such as activated clay or aluminum oxide and filtering.

[0018] The above ester (B) can be produced, for example, by the following method. First, a trivalent neopentyl polyol and 0.5 to 0.8 equivalents of oleic acid relative to the equivalent amount of trivalent neopentyl polyol are reacted under a nitrogen stream at a temperature of 180 to 250°C until the hydroxyl value becomes 85.0 ± 5.0 mg KOH / g to obtain a crude ester. The reaction time and catalyst used to obtain the crude ester are the same as in the production of ester (A) described above. The obtained crude ester is subjected to alkali treatment, washing with water and dehydration, as in the production of ester (A) described above, followed by purification to obtain an oleic acid ester mixture containing a monoester, diester, and triester. By utilizing the difference in boiling points of each ester contained in the obtained oleic acid ester mixture, distillation purification is performed to separate them and obtain only the oleic acid diester (ester (B)).

[0019] In the ester composition of the present invention, the mass ratio of ester (A) to ester (B) ((A):(B)) is 99.9:0.1 to 95.0:5.0. As a result, the ester composition of the present invention has excellent lubricity and thermal stability, and can maintain excellent lubricity even under harsh conditions such as high temperature and the presence of air. It is presumed that the ester composition of the present invention has excellent lubricity because it contains an appropriate amount of ester (B), which is a partial ester, and an appropriate amount of hydroxyl groups on ester (B) are oriented on the surface of metal members, etc. If the content of ester (B) is too high or too low, the ester composition will have insufficient lubricity. Furthermore, if the content of ester (B) is too high, the thermal stability of the ester composition deteriorates and it becomes more susceptible to deterioration by air, which in turn makes it more prone to deterioration of lubricity under high temperature and the presence of air. From this viewpoint, in the ester composition of the present invention, the mass ratio of ester (A) to ester (B) ((A):(B)) is preferably 99.5:0.5 to 96.0:4.0, and more preferably 99.0:1.0 to 97.0:3.0. The mass ratio of ester (A) to ester (B) can be determined from the amount of ester (A) and ester (B) added when manufacturing the ester composition, and from the ester composition after manufacturing, １３ This can be determined by C-NMR analysis. １３ C-NMR Measurement Conditions > Measurement Method: Proton complete decoupling method Magnetic field strength: 400 MHz Solvent: Deuterated chloroform Temperature: Room temperature

[0020] The peroxide value of the ester composition of the present invention is 0.1 to 10.0 meq / kg. This allows the ester composition of the present invention to have excellent thermal stability and maintain excellent lubricity even under harsh conditions such as high temperatures and the presence of air. The peroxide value of the ester composition is an indicator of the amount of primary degradation products (peroxides) of the ester generated in the ester composition. Since peroxides decompose in several stages due to heat and ultimately become acids, the higher the peroxide value of the ester composition, the more likely the acid value of the ester composition is to increase at high temperatures, i.e., the lower its thermal stability. If the peroxide value of the ester composition exceeds 10.0 meq / kg, the ester is more likely to decompose under heat, and the lubricity of the ester composition is likely to deteriorate at high temperatures and the presence of air. From this viewpoint, the peroxide value of the ester composition of the present invention is preferably 8.0 meq / kg or less, more preferably 5.0 meq / kg or less. The lower limit of the peroxide value of the ester composition of the present invention may be adjusted from the viewpoint of ease of manufacture, and is usually 0.1 meq / kg or more, and may be 0.2 meq / kg or more. Furthermore, the longer the contact time between the ester and air during the production of the ester composition, the higher the peroxide value of the resulting ester composition tends to be. Therefore, the peroxide value of the ester composition can be adjusted by the amount of air introduced during production. In this invention, bubbling a gas when mixing ester (A) and ester (B) is effective in reducing the water content in the ester composition. When air is used as the bubbling gas, the ester comes into contact with the air due to bubbling. On the other hand, by using an inert gas such as nitrogen as the bubbling gas, the contact between the ester and air can be reduced. Therefore, one method for obtaining an ester composition with a low peroxide value is to use bubbling with an inert gas such as nitrogen instead of bubbling with air. On the other hand, when using air as the bubbling gas, the peroxide value of the ester composition can be adjusted by adjusting the bubbling time.

[0021] The amount of water contained in the ester composition of the present invention is not particularly limited, but from the viewpoint of improving the stability of the ester composition of the present invention, it is preferably 500 ppm or less, more preferably 100 ppm or less. In this invention, the amount of water contained in the ester composition is measured in accordance with JIS K 2275.

[0022] From the viewpoint of heat resistance, the acid value of the ester composition of the present invention is preferably 0.5 mg KOH / g or less, and more preferably 0.1 mg KOH / g or less. A sufficiently low acid value of the ester composition can further improve its thermal stability.

[0023] The hydroxyl value of the ester composition of the present invention is preferably 0.1 to 5.0 mg KOH / g, more preferably 0.5 to 4.0 mg KOH / g, and even more preferably 0.5 to 3.0 mg KOH / g. The hydroxyl value of the ester composition tends to increase as the amount of ester (B) contained in the ester composition increases. When the hydroxyl value of the ester composition is within the above range, the amount of hydroxyl groups contained in the ester composition becomes appropriate, which tends to improve the lubricity of the ester composition. Furthermore, when the hydroxyl value of the ester composition is below the upper limit, the thermal stability is further improved, and deterioration due to air is further suppressed, thus further suppressing the deterioration of lubricity under harsh conditions.

[0024] To ensure that the effects of the ester composition of the present invention are easily exhibited, the total content of ester (A) and ester (B) in 100% by mass of the ester composition of the present invention is preferably 98% by mass or more, more preferably 99% by mass or more, and even more preferably 99.9% by mass or more.

[0025] The ester composition of the present invention can be preferably used as a base oil for refrigeration oil, and is particularly preferably used as a base oil for refrigeration oil for hydrocarbon refrigerants. Furthermore, the ester composition of the present invention has excellent thermal stability and can maintain excellent lubricity even at high temperatures and in the presence of air, making it suitable for use in compressors of refrigeration and air conditioning equipment where stability under harsh conditions is particularly required. The ester composition of the present invention can be used alone as a base oil for refrigeration oil, or it can be used in combination with other base oils. In addition, the ester composition of the present invention can be used in combination with known additives used in refrigeration oil, as needed. Examples of additives include phenolic antioxidants, metal deactivators such as benzotriazole, thiadiazole, or dithiocarbamate, acid scavengers such as epoxy compounds or carbodiimide, and phosphorus-based extreme pressure agents.

[0026] The method for producing the ester composition of the present invention is not particularly limited. For example, the ester composition of the present invention can be produced by mixing ester (A) and ester (B) in the mass ratio described above. The method for mixing ester (A) and ester (B) is not particularly limited. For example, ester (A) and ester (B) can be prepared separately and mixed by a known method. Ester (A) and ester (B) can be prepared, for example, by the method described above. Furthermore, as described above, when mixing ester (A) and ester (B), it is preferable to blow gas into the mixture and perform bubbling, from the viewpoint of reducing the water content in the ester composition. The bubbling conditions are not particularly limited, and can be adjusted as appropriate so that the water content in the ester composition reaches a desired value. For example, the gas flow rate can be set to 200 to 2000 ml / min and the bubbling time to 1 to 10 hours.

[0027] [Working fluid composition for refrigerators] The working fluid composition for refrigerators of the present invention contains the above-described ester composition for refrigerant oil and a hydrocarbon refrigerant. Because the working fluid composition for refrigerators of the present invention contains the above-described ester composition for refrigerant oil as a base oil, it has excellent lubricity and can maintain excellent lubricity even in harsh environments such as high temperatures and the presence of air. Furthermore, because thermal decomposition products of the base oil are less likely to be generated, deterioration of components inside refrigeration and air conditioning equipment can be suppressed.

[0028] In the working fluid composition for refrigerators of the present invention, the content of the ester composition of the present invention is preferably 1 part by mass or more and 500 parts by mass or less per 100 parts by mass of hydrocarbon refrigerant. The content of the ester composition of the present invention per 100 parts by mass of hydrocarbon refrigerant may be 2 parts by mass or more as a lower limit and 400 parts by mass or less as an upper limit. When the content of the ester composition of the present invention is within the above range, the effects of the ester composition of the present invention are easily exhibited.

[0029] The hydrocarbon refrigerant used is not limited to any known hydrocarbon refrigerant. For example, at least one selected from the group consisting of ethylene, ethane, propane (R290), propylene, cyclopropane, n-butane, isobutane (R600a), cyclobutane, and methylcyclopropane can be used. In particular, from the viewpoint that the effects of the ester composition of the present invention are easily exhibited, hydrocarbons having 2 to 4 carbon atoms are preferred, more preferably at least one selected from the group consisting of propane (R290) and isobutane (R600a), and even more preferably propane (R290). These hydrocarbon refrigerants can be used individually or in combination of two or more.

[0030] The working fluid composition for refrigerators of the present invention may contain refrigerants other than hydrocarbon refrigerants as the refrigerant, but in order to easily exhibit the effects of the ester composition of the present invention, it is preferable that the content of hydrocarbon refrigerants in 100% by mass of the refrigerant be 90% by mass or more, more preferably 95% by mass or more, even more preferably 99% by mass or more, and most preferably that the refrigerant consists only of hydrocarbon refrigerants.

[0031] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples. Unless otherwise specified, parts and percentages are by mass.

[0032] (Synthesis Example 1: Synthesis of Oleic Acid Tryster (Ester (A))) A 5 L four-necked flask equipped with a thermometer, nitrogen inlet tube, stirrer, and Liebig condenser contained 3054.6 parts of oleic acid (manufactured by NOF Corporation, product name: NAA-400) and 445.4 parts of trimethylolpropane (manufactured by Perstop, product name: Trimethylolpropane) to prepare the reaction solution. The equivalent ratio of carboxylic acid to alcohol (carboxylic acid / alcohol) in the reaction solution was 1.1. The reaction solution was then heated to 250°C under a nitrogen atmosphere, and the acid value of the reaction solution was measured every hour. The reaction was continued until the decrease in acid value every hour was 0.5 mg KOH / g or less, and the hydroxyl value was 0.1 mg KOH / g. At the end of the reaction, the reaction solution was cooled to 85°C. Based on the acid value at the end of the reaction, 1.5 equivalents of sodium hydroxide, the amount required to neutralize the reaction solution, were diluted with deionized water to prepare a 10% by mass aqueous sodium hydroxide solution. This solution was added to the reaction solution after the reaction was complete to obtain a mixture, which was then stirred for 1 hour. After stopping the stirring, the mixture was allowed to stand for 30 minutes, and the aqueous layer separated at the bottom was removed from the mixture to obtain the oil layer (reaction solution). Next, an amount of deionized water equivalent to 20% by mass of the obtained reaction solution was added to the reaction solution to obtain a mixture, which was then stirred at 85°C for 10 minutes, allowed to stand for 15 minutes, and the separated aqueous layer was removed from the mixture. This process was repeated 5 times. After that, the obtained oil layer (reaction solution) was dehydrated by stirring at 100°C and 30 Torr for 1 hour. Finally, activated clay in an amount equivalent to 2% by mass relative to the reaction solution was added to the reaction solution to obtain a mixture. This mixture was stirred at 80°C and 30 Torr for 1 hour, and the activated clay was removed from the mixture by filtration to obtain oleic acid triester (ester (A)).

[0033] (Synthesis Example 2: Synthesis of Oleic Acid Diester (Ester (B))) A 5 L four-necked flask equipped with a thermometer, nitrogen inlet tube, stirrer, and Liebig condenser contained 2847.5 parts of oleic acid (manufactured by NOF Corporation, product name: NAA-400) and 652.5 parts of trimethylolpropane (manufactured by Perstop, product name: Trimethylolpropane) to prepare the reaction solution. The equivalent ratio of carboxylic acid to alcohol (carboxylic acid / alcohol) in the reaction solution was 0.7. The reaction solution was then heated to 250°C under a nitrogen atmosphere, and the acid value of the reaction solution was measured every hour. The reaction was continued until the decrease in acid value every hour was 0.5 mg KOH / g or less, and the hydroxyl value was 85 mg KOH / g. At the end of the reaction, the reaction solution was cooled to 85°C. Based on the acid value at the end of the reaction, 1.5 equivalents of sodium hydroxide, the amount required to neutralize the reaction solution, were diluted with deionized water to prepare a 10% by mass aqueous sodium hydroxide solution. This solution was added to the reaction solution to obtain a mixture, which was then stirred for 1 hour. After stopping the stirring, the mixture was allowed to stand for 30 minutes, and the aqueous layer separated at the bottom was removed from the mixture to obtain the oil layer (reaction solution). Next, an amount of deionized water equivalent to 20% by mass of the obtained reaction solution was added to the reaction solution to obtain a mixture, which was then stirred at 85°C for 10 minutes, allowed to stand for 15 minutes, and the separated aqueous layer was removed from the mixture. This process was repeated 5 times. After that, the obtained oil layer (reaction solution) was dehydrated by stirring at 100°C and 30 Torr for 1 hour. Finally, activated clay equivalent to 2% by mass of the reaction solution was added to the reaction solution to obtain a mixture. This mixture was stirred at 80°C and 30 Torr for 1 hour, and the activated clay was removed from the mixture by filtration to obtain a mixture of oleic acid monoester, oleic acid diester, and oleic acid triester. Subsequently, using the difference in boiling points of each ester, only the oleic acid diester (ester (B)) was recovered by distillation.

[0034] [Preparation of Ester Compositions for Refrigerator Oil] Ester (A) obtained in Synthesis Example 1 and ester (B) obtained in Synthesis Example 2 were weighed into a flask to obtain a mixture with the composition shown in Table 1 and totaling 3 kg. The refrigerator oil ester compositions of Examples 1 to 4 and Comparative Examples 1 to 2 were prepared by bubbling the obtained mixture under the conditions shown in Table 1. The amounts of ester (A) (unit: mass%) and ester (B) (unit: mass%) shown in Table 1 are the amounts when the total amount of the refrigerator oil ester composition is taken as 100% by mass.

[0035] The water content in the ester compositions for refrigeration oil of Examples 1 to 4 was measured according to JIS K 2275, and in all cases it was 100 ppm or less.

[0036] Various analyses of the ester composition for refrigeration oil were performed according to the following methods. The hydroxyl value was measured in accordance with JIS K0070. The peroxide value was measured in accordance with JOCS 2.5.2.1. The acid value was measured in accordance with JIS K2501.

[0037] [Lubricity (Vibration Friction and Wear Test (SRV Test))] The lubricity of ester compositions for refrigeration oil was evaluated by performing an SRV test and measuring the diameter of the wear marks. The SRV test was performed using the ball-on-disk method with ball test specimens (material: SUJ2) and disc test specimens (material: SUJ2). The test conditions were a test temperature of 90°C, a load of 100N, an amplitude of 1 mm, and a vibration frequency of 50 Hz, and the diameter of the wear marks was measured after 25 minutes of testing.

[0038] For each example and comparative example, the hydroxyl value, peroxide value, and acid value were measured for the ester compositions for refrigeration oil immediately after preparation. Furthermore, an SRV test was performed to measure the abrasion mark diameter. The measurement results are shown in Table 1 as initial analytical values.

[0039] Based on the initial analysis value of the wear scar diameter measured by the SRV test, the lubricity of the ester composition for refrigeration oil was evaluated according to the following evaluation criteria. In the column of "Lubricity (SRV)" of the initial analysis values in Table 1, the evaluation results of lubricity are shown, and together with this, the value of the wear scar diameter (unit: μm) is shown in parentheses. <Lubricity evaluation criteria> ◎: Wear scar diameter is 470 μm or less. 〇: Wear scar diameter exceeds 470 μm and is less than 500 μm. ×: Wear scar diameter is 500 μm or more

[0040] For the ester composition for refrigeration oil after the following heating test, the acid value was measured, and the SRV test was conducted to measure the wear scar diameter. [Heating test] The ester composition for refrigeration oil was heated in a constant temperature bath at 90°C for 120 hours (5 days) in an air atmosphere.

[0041] Based on the acid value of the ester composition for refrigeration oil after the heating test, the thermal stability was evaluated according to the following evaluation criteria. In the column of "Acid value" of the analysis values after the heating test (90°C, 5 days, in air) in Table 1, the evaluation results of thermal stability are shown, and together with this, the value of the acid value (unit: mgKOH / g) is shown in parentheses. <Thermal stability evaluation criteria> ◎: Acid value is less than 0.50 mgKOH / g. 〇: Acid value is 0.50 mgKOH / g or more and less than 0.60 mgKOH / g. ×: Acid value is 0.60 mgKOH / g or more

[0042] Based on the wear scar diameter measured by conducting the SRV test on the ester composition for refrigeration oil after the heating test, the stability at high temperature and in the presence of air was evaluated according to the following evaluation criteria. In the column of "Lubricity (SRV)" of the analysis values after the heating test (90°C, 5 days, in air) in Table 1, the evaluation results of the stability at high temperature and in the presence of air are shown, and together with this, the value of the wear scar diameter (unit: μm) is shown in parentheses. <Stability evaluation criteria at high temperature and in the presence of air> ◎: Wear scar diameter is less than 480 μm. 〇: Wear scar diameter is 480 μm or more and less than 500 μm. ×: Wear scar diameter is 500 μm or more

[0043]

[0044] The ester compositions for refrigeration oil of Examples 1 to 4 contained the above-mentioned ester (A) and ester (B) in a mass ratio ((A):(B)) of 99.9:0.1 to 95.0:5.0, and had a peroxide value of 0.1 to 10.0 meq / kg, thus representing the ester compositions for refrigeration oil of the present invention. As shown in Table 1, the ester compositions for refrigeration oil of Examples 1 to 3 showed excellent lubricity, as evidenced by the small wear mark diameter measured by the SRV test. Furthermore, the ester compositions for refrigeration oil of Examples 1 to 4 maintained their lubricity even at high temperatures and in the presence of air, as evidenced by the small wear mark diameter even after a heating test (90°C, 5 days, under air), demonstrating excellent stability. In addition, the ester compositions for refrigeration oil of Examples 1 to 4 showed excellent thermal stability, as evidenced by the suppressed increase in acid value after a heating test (90°C, 5 days, under air). On the other hand, the ester composition for refrigeration oil in Comparative Example 1 had a mass ratio of ester (A) to ester (B) outside the range of 99.9:0.1 to 95.0:5.0, resulting in a large wear mark diameter measured by the SRV test, poor lubricity, a large difference in wear mark diameter before and after the heating test, and a tendency for lubricity to deteriorate at high temperatures and in the presence of air. Furthermore, it exhibited a large increase in acid value after the heating test, indicating poor thermal stability. The ester composition for refrigeration oil in Comparative Example 2 had a peroxide value exceeding 10.0 meq / kg, resulting in good lubricity immediately after production, but a large difference in wear mark diameter before and after the heating test, a tendency for lubricity to deteriorate at high temperatures and in the presence of air, and furthermore, a large increase in acid value after the heating test, indicating poor thermal stability.

Claims

1. An ester composition for refrigeration oil containing the following ester (A) and the following ester (B), wherein the mass ratio of ester (A):(B) is 99.9:0.1 to 95.0:5.0, and the peroxide value of the ester composition is 0.1 to 10.0 meq / kg. Ester (A): A triester ester of a trivalent alcohol having a neopentyl skeleton and oleic acid. (B): A diester of a trivalent alcohol having a neopentyl skeleton and oleic acid.

2. A working fluid composition for a refrigerator, comprising the ester composition for refrigeration oil described in claim 1 and a hydrocarbon refrigerant, wherein the content of the ester composition is 1 part by mass or more and 500 parts by mass or less per 100 parts by mass of the hydrocarbon refrigerant.

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