DE68925537T2 - Using a lubricant for compressors that use a chlorine-free fluorocarbon refrigerant. - Google Patents

Description

The invention relates to the use of a lubricant for compressors containing a chlorine-free fluorocarbon coolant, e.g. HFC-134a (1,1,1,2- tetrafluoroethane)

Until now, compounds containing fluorine and chlorine as a component, such as R-11 (trichloromonofluoromethane), R-12 (dichlorodifluoromethane) as chlorofluorocarbon (CFC), R-22 (monochlorodifluoromethane) as chlorofluorocarbon (HCFC), for example, have been used as coolants for freezers, air conditioners and refrigerators, for example. In connection with the current problems about the depletion of the ozone layer, new coolants that do not contain chlorine, such as HFC-134a and the like, have been proposed as possible replacements for R-12 that do not cause depletion of the ozone layer.

Many mineral-based oils or synthetic oils are known as lubricants for refrigeration units. However, it has been confirmed that these oils have very poor compatibility with HFC-134a and cannot be used together with it. Therefore, it is important to take countermeasures for this problem. Furthermore, for example, the lubricating effect, electrical Insulating properties, energy saving properties, anti-abrasion properties, sealability, thermal stability and prevention of sludge formation, for example, are required as requirements for lubricants for refrigeration units, so that these must be taken into account when developing the aforementioned countermeasures.

Incidentally, there are already synthetic lubricants that use known polyethers as synthetic oil and are described in Journal of the Oil Chemistry, Volume 29, No. 9, pages 336-343 (1980) and Journal of the Petroleum Technology, Volume 8, No. 6, pages 562-566 (1985). Furthermore, Japanese Laid-Open Patent Publication No. 61-281199 discloses a mixture of polyglycol represented by the general formula R₁[O-(R₂O)m-R₃]n, an alkylbenzene and the like, and Japanese Laid-Open Patent Publication No. 57-63395 discloses an oil obtained by mixing a polyether such as a high molecular weight polyoxypropylene monobutyl ether with an epoxycycloalkyl compound, and Japanese Laid-Open Patent Publication No. 59-117590 discloses a high viscosity mixed oil of a polyether compound and a paraffinic or naphthenic mineral oil.

However, the above-mentioned conventional synthetic lubricants cannot be used as a lubricant for a refrigeration unit using HFC-134a as a refrigerant due to compatibility and the like.

In US-PS 4,755,316, polyoxyalkylene glycol (hereinafter abbreviated as PAG) having hydroxyl groups (-OH) at both end groups is described as a lubricant for a cooling system using HFC-134a. It is further stated that PAG in HFC-134a can be used in a wide temperature range in Compared to general PAG containing a hydroxyl group and an alkyl group at the terminal position, thereby improving the circulation of lubricant in a compressor in the refrigeration system and preventing seizure when the compressor is operated at high temperatures. Furthermore, the temperature range for compatibility with HFC-134a is described as between -40ºC and +50ºC.

On the other hand, HFC-134a is a replacement refrigerant for R-12, and the main application is expected to be for air conditioners in automobiles, refrigerators and the like. In the case of a refrigerator, it is required that there be good compatibility between the lubricant and the refrigerant, and further, the lubricant itself must necessarily have electrical insulating properties because the engine is an essential part of the cooling system. However, the conventional compounds investigated as lubricants for HFC-134a refrigerants, including PAG as disclosed in U.S. Patent No. 4,755,316, have remarkably poor electrical insulating properties compared with the conventional mineral oil used for refrigeration, and they are very hygroscopic.

It is therefore an object of the invention to provide a lubricant for refrigeration units with an excellent compatibility with new non-chlorine containing coolants, such as HFC-134a, in a wide temperature range, a high electrical insulating capacity and a low hygroscopicity.

Currently, some of the commercially available esters are used in systems using, for example, R-12, R-22 refrigerants, but they are classified as HFC-134a new refrigerant, or the compatibility range is very narrow. In this connection, the inventors aimed to require that the esters must have high electrical insulating properties, low hygroscopicity, good lubricating effect and high stability compared with PAG, and they conducted a series of studies on the molecular structure of the esters having a wide compatibility range with HFC-134a and found that only esters having a considerably limited structure can be used in the HFC-134a refrigeration system, and the present invention is based on this.

The present invention relates to the use of a lubricant for compressions using a fluorocarbon coolant not containing chlorine, comprising as a main component one or more esters obtained by reacting (a) neopentyl glycol with (b) at least one chain-branched monovalent fatty acid having a carbon number of 4 to 18, the ester having a total acid number of not more than 3 mg/KOH/g and a hydroxyl number of not more than 50 mg KOH/g.

According to a preferred embodiment, the fluorocarbon refrigerant is 1,1,1,2-tetrafluoroethane (HFC-134a)

The lubricant for refrigeration equipment using HFC-134a and the like comprises an ester obtained by esterifying neopentyl glycol with at least one branched-chain monovalent fatty acid having a carbon number of 4 to 18, the ester having a total acid value of not more than 3 mg KOH/g and a Hydroxyl number of not more than 50 mg KOH/g as the main component.

The reason why the carbon number of the monovalent fatty acid is limited to 4 to 18 is based on the fact that if the carbon number exceeds 18, the obtained ester becomes turbid or has very poor compatibility with the refrigerant HFC-134a and the like.

As monovalent fatty acid, isobutanoic acid, isopentanoic acid, isoheptanoic acid, 2-ethylhexanoic acid, 3,5,5-trimethylhexanoic acid and isostearic acid can be mentioned as an example.

At least one of these monovalent fatty acids is suitably mixed and esterified with neopentyl glycol to obtain an ester having the desired physical properties required for the various refrigeration units.

In order to obtain sufficient compatibility with the refrigerant HFC-134a and the like, it is preferable to use a mixture of a straight-chain fatty acid having a carbon number of 3 to 11, preferably 5 to 10, and a branched-chain fatty acid having a carbon number of 4 to 14, preferably 7 to 9, as the monovalent fatty acid. In this case, the amount of the straight-chain or branched-chain fatty acid used is preferably not less than 50 mol% per the total amount of the monovalent fatty acid used.

According to the invention, in order to obtain a suitable viscosity in the resulting ester, at least one polybasic acid having a carbon number of 4 to 36 is esterified with neopentyl glycol in an amount of not more than 80 mol% per total fatty acid. Among the polybasic acids, a polybasic acid having a carbon number of 4 to 10 is preferred in consideration of improved compatibility with the refrigerant HFC-134a and the like and the physical properties of the obtained ester.

Concretely, the polybasic acid includes succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, acelic acid, sebacic acid, phthalic acid, maleic acid and trimellitic acid, for example. Furthermore, a polybasic acid having a carbon number of not more than 3 is a special product and very difficult to obtain inexpensively, and it causes poor stability of the ester after synthesis. On the other hand, if the carbon number exceeds 36, the compatibility of the obtained ester with HFC-134a and the like is significantly reduced. In the present invention, the reason why the amount of the polybasic acid added is limited to not more than 80 mol% per total fatty acid is due to the fact that if it exceeds 80 mol%, gelation may occur and it is difficult to obtain the desired physical properties.

The ester compounds used in the present invention can be obtained by an esterification reaction by means of a dehydration reaction between the specific polyhydric alcohol and the specific fatty acid of the above-mentioned type, or by a general esterification reaction by means of an acid anhydride, an acid chloride and the like as a derivative of the fatty acid.

Since the ester according to the invention can be obtained by the above-mentioned method, the remaining acid values and hydroxyl values are not particularly critical. However, if the acid value exceeds 3, an unfavorable phenomenon may occur that a metal soap is formed and precipitates upon reaction with the metal used in the cooling system, and therefore the acid value is not more than 3 mg KOH/g. Furthermore, if the hydroxyl value exceeds 50, an unfavorable phenomenon may occur that the obtained ester becomes turbid, and therefore the hydroxyl value is preferably not more than 50 mg KOH/g.

The esters used in the invention have good compatibility with the refrigerant HFC-134a and the like over a wide range from a low temperature to a high temperature as a lubricant for use in a refrigeration system using HFC-134a as a refrigerant, and thereby the lubrication and the thermal stability of the lubricant in the refrigeration system can be significantly improved. Furthermore, they have high electrical insulation ability and have low hygroscopicity as compared with the conventionally studied PAG as a refrigerant lubricant for HFC-134a. Therefore, the refrigerant lubricants comprising the esters of the present invention as a main component solve the problems of compatibility with HFC-134a and hygroscopicity which have not been solved by the conventional methods, and further improve the electrical insulation properties which are a problem when using HFC-134a in a compressor for a refrigeration device. Furthermore, additives which are conventionally used in lubricants can be used. for example antioxidants, anti-abrasion agents and epoxy compounds, may be suitably added to the coolant-lubricant used in the invention.

The following examples describe the invention without limiting it.

Examples 1 to 8, Comparative Examples 1 to 5

The effectiveness of a coolant lubricant using HFC-134a as a coolant was evaluated using eight esters A-1 to A-8 shown in Table 1 below (all of these esters are not commercially available and were prepared according to the invention). For comparison, the same evaluation as above was carried out using the commercially available PAG (B-1 to B-3 manufactured by Asahi Denka Co., Ltd.) and esters (C-1 to C-2 manufactured by Nippon Oil and Fats Co., Ltd.) as a coolant lubricant, and this is shown in Table 2 below.

The lubricating effect, compatibility, thermal stability, electrical insulation properties, hygroscopicity as the evaluation bases for the coolant lubricant for compressors are shown in Tables 1 and 2, and the evaluation was carried out under the following conditions.

Lubricating effect

Seizure load (Falex load capacity) was measured according to ASTM D-3233-73 under a controlled atmosphere of HFC-134a blown thereon.

compatibility

0.6 g of the test lubricant and 2.4 g of the coolant (HFC-134a) were sealed in a glass tube, and then cooling at 1ºC/min and heating were carried out, and the temperature at which two phases appeared was measured.

Thermal stability

1 g of the test lubricant, 1 g of the refrigerant (HPC-134a or R-12) and a catalyst (a wire made of iron, copper or aluminum) were sealed in a glass tube and the mixture was heated to 175ºC and the color of the lubricant after 10 days was evaluated using an ASTM color system according to ANSI/ASHRAE 97-1983.

Electrical insulating properties

The dielectric constant at 80ºC was evaluated according to JIS C-2101.

Hygroscopicity

In a 100 ml beaker, 60 g of the test lubricant was placed and kept at 25ºC at a humidity of 70% for three hours, and then the water concentration was measured. The evaluation results are shown in Table 3 below. TABLE 1 Ester Straight Chain Fatty Acid Polybasic Acid Adipic Acid Sebacin Acid Dynamic Viscosity at 40ºC (cST) Color (ASTM) Note) The amount of starting material was given in mol%. TABLE 2 Type Trade name Colour (ASTM) Dynamic viscosity at 40ºC (cST) Adekapol M-30 1) Adekapol M-110 2) Adekapol MH-50 3) Dioctyl sebacate Unistar MB-816 4) 1)Polyoxypropylene glycol monoalkyl ether 2) Polyoxypropylene glycol monoalkyl ether 3) Polyoxyethylenepropylene glycol monoalkyl ether 4) Monoester of 2-ethylhexanol and palmitic acid TABLE 3 Test lubricant Dielectric constant at 80ºC (Ω cm) Temperature at which separation into 2 phases occurs (ºC) Low temp. High temp. Seizure load (Kgf) Thermal stability Refrigerant R-12 Paint (ASTM) Sludge Refrigerant HFC-134a Hygroscopicity (water content ppm) Acceptable example Comparative example Insoluble None Much *: not higher than -50ºC **: not lower than 80ºC

As is clear from the table, a comparison of the esters used in the present invention with the conventional PGA (B-1 to B-3) shows that the electrical insulating property represented by the dielectric constant is 100,000 times or more, and that two-phase separation does not occur at a high temperature. Furthermore, the seizure load is very high and the hygroscopicity is low. The thermal stability is the same in the case of the HFC-134a system, but it is particularly good in the case of the R-12 system. This is very advantageous for practical use because mixing of HFC-134a and R-12 cannot be avoided when replacing the refrigerant R-12 with HFC-134a.

Furthermore, if one compares the esters used in the invention with the commercially available esters (C-1 to C-2), the two-phase separation temperature is extremely different, and these common esters are insoluble in HFC-134a. In this way, the specially developed molecules used in the present invention show great advantages.

From the above, it is clear that the esters used in the invention have excellent performance as coolants, as can be seen from the comparative examples.

HFC-134a has been mentioned as a possible replacement for R-12 and is used for air conditioning systems in cars, refrigerators and the like. Especially in air conditioning systems in cars, the compressor is operated in the summer season, so the compatibility between oil and refrigerant at high temperatures becomes important. If during operation of the compressor causes the oil and refrigerant to separate into two phases, then the refrigerant with the larger specific gravity stays in the lower part of the compressor and causes the compressor to seize. In a refrigerator, the motor is incorporated into the compressor, so that dissipation of electricity can become problematic. In this connection, the esters used in the invention have a dielectric constant of more than 100 times or more than that of the conventional PAG, and they are excellent in electrical insulating properties and are therefore a refrigerant lubricant for refrigerators.

Recently, HFC-134a, which causes substantially no destruction of the ozone layer, is used instead of R-12 widely used in refrigeration systems in order to cope with the world-wide problem of destruction of the ozone layer by chlorofluorocarbons and chlorofluorocarbons, but the compatibility with the conventional cooling lubricants is poor and this has been an obstacle to the development of the replacement system. The cooling lubricants used in the present invention have sufficient compatibility with HFC-134a as a cooling agent and have high electrical insulating property, and are also excellent in their overall performance, so that they can be used in the conventional systems and also with HFC-134a instead of the conventional R-12 and R-22 cooling agents.

Claims (3)

1. Use of a lubricant for compressors using a fluorocarbon refrigerant not containing chlorine, comprising as a main component one or more esters obtained by reacting (a) neopentyl glycol with (b) at least one branched-chain monovalent fatty acid having a carbon number of 4 to 18, wherein the ester has a total acid number of not more than 3 mg/KOH/g and a hydroxyl number of not more than 50 mg KOH/g. 2. Use according to claim 1, wherein the branched chain monovalent fatty acid is at least one selected from the group consisting of isobutanoic acid, isopentanoic acid, isoheptanoic acid, 2-ethylhexanoic acid, 3,5,5- trimethylhexanoic acid and isostearic acid 3. Use of a lubricant according to claim 1, wherein the fluorocarbon coolant is 1,1,1,2-tetrafluoroethane.