DE4105956A1 - Lubricant used in refrigerators and air conditioners - Google Patents

Abstract

A lubricant (I) comprises an ester or ester mixt. produced by reacting 4-10C straight-chain, satd. monocarboxylic acids with alcohols having 3-6 OH gps. and one or two quaternary C atoms. Pref. ester (mixt. contains 5-9C monocarboxylic acids and pentaerythritol and/or dipentaerythritol. All OH gps. of the alcohol component are esterified with the acid component. 30-100, pref. 60-80, mol. wt. % monocarboxylic acid, pref. pentanoic acid, is present, or up to 30 mol. wt. %, pref. 3-10, mol. wt. % branched monocarboxylic acid, pref. 2-ethyl hexanoic acid. Mol. wt. of the ester is 360-850 (400-700) pentaerythritol ester and 560-1300 (700-850) dipentaerythritol ester.

Description

The invention relates to lubricants based on synthetic Esters, which are used for the lubrication of refrigeration machines, Heat pumps and related systems such as air conditioners, are suitable, in particular for the lubrication of with Fluorocarbon refrigerants such as tetrafluoroethane, Di fluoroethane and similar compounds, operated cold machinery.

Various compounds are known as refrigerants. The refrigerant HFC 134a, 1,1,1,2-tetrafluoroethane of the chemical formula CFH ₂ -CF ₃ , has been developed as a replacement product primarily for the refrigerant CFC 12, dichlorodifluoromethane of the chemical formula CCl ₂ F ₂ , which is known to coexist others such as CFC 11, trichlorofluoromethane of the chemical formula CCl ₃ F is involved to a large extent in the depletion of the ozone layer. Because of the environmental hazard, chlorofluorocarbons - CFCs for short - of the CFC 12 or CFC 11 type will no longer be used in the future. According to the Montreal Protocol of 1987 - "Montreal Protocol on Substances that Depletes the Ozone Layer" - as well as other international agreements, they must be replaced by those that do not cause this kind of damage to the environment. This also includes the refrigerant HFC 152a, 1,1-difluoroethane. Because of the easy flammability of this fluorocarbon, it will probably only be used in mixtures with other refrigerants.

Lubricants for refrigeration machines still exist today mainly from highly refined mineral oils, which are used for ver  improvement of the flow properties in the cold over that for Lubricating oils are standard dewaxed. In be significant quantities are also synthetic or partial synthetic refrigeration oils based on alkylbenzenes used, such as in DE-PS 35 22 165 described. To a limited extent, poly alpha olefins used.

The refrigeration oils used so far, be it based of mineral oils or based on synthetic liquids, are all in combination with the previously known cold have been used.

For the proper functioning of a refrigeration compressor the compatibility of the refrigerant with the cold machine oil vital. The thermo dynamic properties as well as the flow behavior of the Working fluid, consisting of the refrigerant and the Lubricant, their miscibility and chemical Stability are factors when choosing a lubricant must be taken into account.

If refrigerating machine oils based on hydrocarbons such as mineral oil or synthetic oil are now to be used together with HFC 134a, there is the difficulty that the latter cannot be solved. The function of the refrigeration compressor is impaired. The reason for this is the higher polarity compared to CFC 12, due to the asymmetry of the CFH ₂ -CF ₃ molecule. The same applies to the fluorohydrocarbon difluoroethane (CHF ₂ -CH ₃ ).

As synthetic liquids that ver these disadvantages polar polyalkylene glycols should be avoided are known to be excellent lubricants. tries  however, suggest that the use of poly alkylene glycols is not entirely unproblematic. It proved proved difficult, for example, due to the high Polarity, the humidity within tolerable limits too hold. However, water in refrigeration systems can not only disrupt the refrigeration cycle, but also ver causes. In addition, the free hydroxyl groups the polyalkylene glycols also certain aluminum attack alloys that are increasingly common in cold weather compressors and corresponding systems are used. Another disadvantage is the incompatibility with some commonly used in refrigeration compressors Elastomers. The strain of polyalkylene glycols over a long period at high temperatures - over 150 ° C - can also lead to the formation of degradation products, which usually cause corrosion as acids.

The object of the present invention is therefore as Lubricants for chillers ready such substances places that do not have the disadvantages mentioned, probably but are compatible with the refrigerant used.

According to the invention, this object is achieved by Provision of lubricants for chillers that are used in essentially from an ester or ester mixture of the um settlement of predominantly straight-chain, saturated mono carboxylic acids with 4 to 10 carbon atoms as acid component and alcohols with 3 to 6 hydroxyl groups and one or two quaternary carbon atoms as alcohol component exist.

Esters are compounds in an equilibrium reaction from an alcohol and an acid, for example one Carboxylic acid, with the elimination of water.  

The esterification of monocarboxylic acids with alcohols takes place today mostly in the presence of acidic catalysts Temperatures between 100 and 200 ° C. The di various cleaning procedures to ensure an anhydrous and to get largely neutral product.

The alcohols used for the esterification must according to the invention one or two quaternary carbon contain atoms, such as trimethylolethane and Trimethylolpropane, each having three hydroxyl groups exhibit. The one preferably used according to the invention Alcohol is pentaerythritol. It will on a technical scale from a reaction of Formaldehyde obtained with acetaldehyde and calcium hydroxide:

8 HCHO + 2 CH₃CHO + Ca (OH) ₂ ⇄ 2 C (CH₂OH) ₄ + Ca (HCOO) ₂

In the production of pentaerythritol, dipentaerythritol of the chemical formula (CH ₂ OH) ₃ CCH ₂ -O-CH ₂ C (CH ₂ OH) ₃ is also obtained in different amounts. It is contained in the so-called technical pentaerythritol in a range of 5 to 15%. Esters according to the invention have been prepared both from technical pentaerythritol (TPE), pure pentaerythritol (PE) and dipentaerythritol (DPE) and from mixtures of these alcohols.

In the course of the development of lubricants according to the invention it was recognized that particularly good results, in particular in terms of solubility when achieved as a major Component n-pentanoic acid was used for the esterification.

The preferred n-pentanoic acid is used technically Oxidation of the corresponding oxo aldehydes with oxygen posed. The higher molecular weight monocarboxylic acids come in nature as esters (fats) and are made from them Fat splitting won, e.g. B. from coconut oil. The branched Monocarboxylic acids are either by oxidation of  Oxoaldehydes or by carbonylation of olefins or Alcohols in the presence of acidic catalysts after Koch's carboxylic acid synthesis.

As a lubricant for the combination with the refrigerants HFC 134a and HFC 152a and similar refrigerants in cold machines are particularly suitable for the esters according to the lying invention. The high polarity enables one for the function of the esters as lubricants good solubility especially in the refrigerants HFC 134a and HFC 152a.

The polarity of an ester can be specified as "Non Polarity Index "- use the following formula:

According to this formula, the NPI should act as a lubricant suitable esters for use in chillers if possible lie low. Have correspondingly low indices the esters according to the present invention.

In the case of pentaerythritol esters, the Molecular weights in the range from 360 to 850, preferably between 400 and 700, for dipentaerythritol esters in the range from 560 to 1300, preferably between 700 and 850.  

For example, esters based on due to their low NPI together with refrigerants like HFC 134a or similar can be used:

As technical products and especially when used ver of various monocarboxylic acids, the esters fall only partially into the listed - idealized - formulations. In the Usually there are mixtures of different esters, the number of Weighing of raw materials and process engineering depends.

Since the NPI is proportional to the molecular weight of the ester, should already be the molecular weight of the carbon used  acids be low. Become different monocarbon acids used so that mixtures of different esters ent should stand, the acid component based on the total carboxylic acid use from 30-100 mol%, preferably from 60-80 mol% by weight of n-pentanoic acid exist. In case ver branched carboxylic acids it can from up to 30 mol.%, preferably from 3 to 10 mol% of 2-ethylhexanoic acid stand, also based on the total carboxylic acid commitment.

The esters or ester mixtures according to the present Er The invention has been studied in detail. The tests he stretched from identifying physicochemical Data until it is tested under practical conditions in Refrigeration compressors.

Table 1

The second table gives an overview of the solubility the ester in HFC 134a depending on polarity and molecular weight. Your influence on the solubility of the Ester emerges from the comparison. The examples 1 to 6 show solubility values that these esters call Lubricant for refrigeration compressors, where as cold medium HFC 152a and HFC 134a are used as be award particularly suitable. Here is the decreasing Solubility tendency from Example 1 to Example 6 increases detect.

Table 2

The hygroscopic properties of the ester, i. H. admission of water, for example through moist air, are special important for use as a refrigerating machine oil. A too high a water content can not only function properly negatively affect a refrigeration system, but also promotes the tendency to corrosion in the system. Refrigeration oils must therefore be as dry as possible in refrigeration compressors be filled in. Drying of the products is essential inevitable. As substances of a polar character, prerequisite for their solubility in polar refrigerants like HFC 134a, esters are necessarily more hygroscopic than substances polar in nature. In contrast, polyalkylene glycols are considerable more hygroscopic than ester.  

To determine the hygroscopic properties, the products dried to a certain water content Amounts of 10 ml each were placed in test tubes, which then unlocked at a relative humidity of 75% and stored at a temperature of 20 ° C (average values) were. The water was determined according to the known one Karl Fischer's method. The results are in the table 3 listed. Examples 1 to 6 relate to esters according to the present invention according to Table 1. Bei Examples 7 and 8 are polyalkylene glycols with the disadvantages described above are offered as refrigeration oils.

Table 3

For the detection of thermal, chemical and hydrolytic The stability of the esters was based on that of ASHRAE (ANSI / ASHRAE 97-1989) standardized stability test subjected.  

The thermal stability of pentaerythriol esters according to the Invention is due to its molecular structure - in There is no water in the beta position of the alcohol group fabric - excellent. It is known that esters of this type are stable at least up to 260 ° C. In refrigeration compressors these temperatures are generally not reached. It However, use cases are conceivable in which the Temperature is correspondingly high, for example at very high high pressure to achieve low temperatures so that the Esters must be stabilized with the help of additives. It It has been shown that the decomposition of the esters forming radicals very well with so-called "radical scavengers" be converted into a form that is harmless to the system can. Products like radicals are among radical scavengers for example to understand triphenylmethane, their effects is generally known (Fieser, Organic Chemistry, 2. Edition, pages 409-415).

If there is a radical cleavage of esters, it proceeds according to the following mechanism:

The free radicals are present in the presence of a radical scavenger like triphenylmethane.

The tests with dried and moist samples were carried out according to the following procedure: Prepared accordingly - cleaned and acidified - glass tubes of 200 mm length and an inner diameter of 5 to 7 mm were on one Welded end and at the other end in a length of about  Tapered 5 cm to 1.5 mm. In glasses so prepared a copper wire (length 30 mm / cross section 1 mm), a steel strips (50 × 3 mm) and an aluminum strip (30 × 5 mm) introduced, weighed 1.5 g of lubricant and in one special device under vacuum and freezing 0.7 g of the HFC 134a refrigerant added. With stick The glasses were then fabric after the addition of the refrigerant welded. The glasses then came into a dryer cabinet that is tested in a given period temperature of 175 ° C was to be heated. The test duration was 2 weeks. In the course of the test, the samples were colored Changes and deposits visually checked. The Metals have also been shown to change the surface observed.

The different degrees of moisture of the oils examined were set as follows: The dried oils (moisture grade 1) vessels were removed, in which they previously over had been stored for a period of 2 months. These were checked before filling the oils into the test tubes open for 15 minutes. The wet samples were the same (opened) containers after storage at 95% relative humidity after one hour (humidity grade 2) and removed after 24 hours (moisture level 3) and filled into the test tubes.

Table 4 below shows the moisture levels for the Refrigeration oils of Examples 4, 5, 6, and for a poly alkylene glycol listed in Comparative Example 7.

Table 4

The test results are summarized in Table 5. How these results show subject to the esters themselves in Presence of relatively high proportions of water thermal or hydrolytic decay. The tendency to Moreover, the purer the hydrolyzing, the less Esters are present after the manufacturing process. A note for their purity is the neutralization number, measured according to DIN 51 558. It should be used for esters according to the present Invention do not exceed the value of 0.2 mgKOH / g.

Table 5

The function of a refrigeration compressor depends largely on the perfect lubrication of all moving parts. Esters, especially polyol esters, have been used for many years excellent lubricants for a wide variety Areas of application known. Because of their polarity they have a high affinity for metal surfaces and protect them thus in the area of boundary lubrication for metal / metal loading stirring better against wear than, for example, the be knew lubricants based on pure hydrocarbons such as mineral oil or polyalphaolefin.

The test was carried out according to the Falex method, which is carried out according to ASTM-D 2670 is standardized. Following this procedure, two prismatic test specimens with continuously increasing force pressed against a rotating shaft in a test oil bath running. Depending on the load capacity of the lubricant to be tested occurs at a certain load on the sliding surfaces to seizure wear. By doing this by leaps and bounds increasing friction force breaks the shaft. The amount of he Ranged load - in "pounds" - is an assessment benchmark for the lubricity of the lubricant and is commonly referred to as "feeding load".

The comparative tests with the Falex tester show clearly the superiority of the esters, especially over Mineral oil and polyalphaolefin. Results of the investigations are shown in Table 6.  

Table 6

Lubrication properties of the esters compared to poly alpha olefins, mineral oil and polyalkylene glycol

Experiments with esters according to the present invention the conditions mentioned, but in the presence of HFC 134a, show a 10 to 15% higher resilience.

Should be under certain, difficult to lubricate prevailing use cases additives to minimize the Wear may be required, provided that they are for such applications are suitable to be added. Numerous additives are used in lubrication technology Prevention or minimization of wear known. Most of time their mode of action is based on chemical reactions with the metal surface to be lubricated to form a front Wear protective layer. Are known for example wise so-called sulfur carriers such as sulfurized olefins, their Sulfur with the iron of the steel surface with formation of iron sulfide, a wear-reducing layer responds: Fe + S = FeS. Wear-reducing layers also form organic compounds of chlorine, phosphorus  and nitrogen alone or in combinations, some fat acids, etc. Organometallic compounds of lead, Molybdenum and soaps are also well-known anti wear additives.

The compatibility of the lubricant / refrigerant mixture with sealing materials (elastomers) is another advantage Exposure to the functionality of a cold compressor. In the industry are different Applications different types of elastomers included puts. With esters according to the present invention have been described in Combination with HFC 134a and in comparative tests with the conventional refrigerant CFC 12 following in the industry commonly used elastomers for compatibility looking for: chloroprene A, chloroprene B and hydrogenated nitrobutyl rubber.

The experiments were carried out using the refrigerator oil of the example 6 and for comparison with a commercially available conventional refrigeration oil based on mineral oil in the presence of the refrigerants HFC 134a or CFC 12 leads.

The samples were prepared in such a way that because a mixture of 15% refrigerant / 85% cold machine oil was filled into a glass tube that contained the elastomer to be tested. Subsequently the glass tubes were welded and one 165 hours Exposed to temperature of 120 ° C. After this Test time, the tubes were opened and the elastomers in  Based on those standardized according to DIN ISO 1817 Methods examined. The result is shown in Table 7 summarized. It should be noted that between the Samples containing the HFC 134a refrigerant and the no significant differences with the CFC 12 refrigerant occur. Such elastomers have been investigated which are widely used in refrigeration technology. For the This test gives users the indication that at Change of refrigerant from CFC 12 to HFC 134a none There is a need to use different sealing material turn.

Table 7

Suitability of the esters according to the present invention could also directly, namely in practical tests in the cold compressors can be detected. There were compressors of the Danfoss and Bitzer companies used for these tests.

The tests ran under certain performance requirements a period of 2000 hours. After the end of the tests were both the compressors and the lubricants medium ester used in an in-depth review pulled.

In no case did a compressor fail. All parts were in perfect condition. The Lubricants themselves were unchanged in all data.

The tests ran under the (averaged) conditions of Table 8.

Table 8

Claims (10)

1. Lubricant for refrigeration machines, essentially be standing from an ester or mixture of esters Implementation of predominantly straight-chain, saturated Monocarboxylic acids with 4 to 10 carbon atoms as Acid component and alcohols with 3 to 6 hydroxyl groups and one or two quaternary carbon atoms as an alcohol component. 2. Lubricant according to claim 1, consisting essentially from an ester or ester mixture of monocarboxylic acids with 5 to 9 carbon atoms and the polyvalent ones Alcohols. 3. Lubricant according to claim 1 or claim 2, in consisting essentially of an ester or Ester mixture, the alcohol component Pentaery is thritol and / or dipentaerythritol. 4. Lubricant according to claim 3, consisting essentially from an ester or mixture of esters, all of which OH groups of the alcohol component with the acid component are esterified. 5. Lubricant according to one of the preceding claims, consisting essentially of an ester or Ester mixture, the acid component based on the total carboxylic acid input from 30 to 100 mol%, preferably 60 to 80 mol%, monocarboxylic acid consists and preferably of n-pentanoic acid, the The rest are other monocarboxylic acids.   6. Lubricant according to one of the preceding claims, consisting essentially of an ester or Ester mixture, the acid component based on the Total carboxylic acid use up to 30 mol%, preferably from 3 to 10 mol%, from branched Monocarboxylic acids, in particular from 2-ethylhexanoic acid, consists. 7. Lubricant according to one of the preceding claims, characterized in that the molecular weights of the esters are each in the following ranges:
Pentaerythritol esters from 360 to 850,
preferably from 400 to 700,
Dipentaerythritol esters from 560 to 1300,
preferably from 700 to 850. 8. Lubricant according to one of the preceding claims, characterized in that the polarity of the esters according to the invention, determined according to the equation: for a pentaerythritol ester in the range from 20 to 65, preferably 25 to 50, and for a dipentaerythritol ester in the range from 30 to 100, preferably 45 to 70. 9. Lubricant according to one of the preceding claims, that the lubricant agents to improve its chemical stability and / or improvement wear behavior and / or to prevent Foaming and, if necessary, known radical scavengers contains.   10. Lubricant according to claim 9, characterized in that to improve the chemical stability of the Lubricant 0.01 to 1.0% by weight, preferably 0.05 up to 0.2% by weight, based on the total lubricant, Triphenylmethane is included.