DE29914502U1 - Polyether as a lubricant for CO2 chillers - Google Patents

Description

The invention relates to operating fluid compositions containing lubricants based on polyethers with -(-CH ₂ -CH ₂ -CH ₂ -CH ₂ -O-)-monomer units, which are suitable for the lubrication of refrigeration machines, heat pumps and related systems which are operated with carbon dioxide as an operating fluid.

Carbon dioxide was already used as a refrigeration machine operating fluid at the beginning of modern refrigeration technology. In 1881, the Linde company built the first compression refrigeration machine using carbon dioxide as a refrigerant. Until the middle of this century, carbon dioxide was mainly used in ship refrigeration systems, usually below its critical temperature (critical point T _k = 31.4°C / p _k = 72.9 bar). Glycerine was used as a lubricant and as a sealing fluid. Later, with the introduction of CFC refrigerants,

Carbon dioxide is hardly used anymore.

The phasing out of environmentally harmful chlorofluorocarbons (CFCs) as refrigeration equipment has led to carbon dioxide being considered again as a possible refrigeration equipment fuel. Carbon dioxide is readily and cheaply available and has no damaging effect on the ozone layer. Carbon dioxide continues to be characterised as a refrigeration equipment fuel by favourable thermodynamic properties, such as a relatively high volumetric refrigeration capacity and a low pressure ratio (p _c /p _s index) as well as a

25 resulting good system efficiency.

Refrigeration machine prototypes have demonstrated the technical possibilities of a transcritical carbon dioxide cycle with pressures of over 100 bar. In a transcritical cycle, conditions are passed through during the compression cycle in which the carbon dioxide as an operating medium is in both the subcritical and supercritical state. At the same time, work is also being done on further developing the conventional process using carbon dioxide in the subcritical range.

Compared to conventional refrigerants such as fluoro/chlorine or fluorocarbons, carbon dioxide as a refrigeration machine operating fluid requires higher operating pressures and temperatures, which place high demands on sealing materials, moving parts and lubricants. This applies in particular to refrigeration machines that are operated in a transcritical cycle.

the.

Polyalkylene glycols have already been proposed as lubricants for refrigeration machines using carbon dioxide as an operating medium, e.g. in J. Fahl, Ki Luft- und Kältetechnik 8/1998, pp. 375-379.

JP 10-46169 also relates to operating fluid compositions containing carbon dioxide and polyalkylene glycols. The polyalkylene glycols have C2 to C10 alkylene units, with ethylene oxide (EO) and/or propylene oxide (PO) units being preferred. Polyalkylene glycols produced by alkoxylation of polyfunctional alcohols are also disclosed.

In the German patent DE 197 19 430-Cl, operating fluid compositions containing carbon dioxide and poly(C2- to C6-alkylene) glycols are proposed as lubricants, wherein the polyethers contain at least 40% propylene oxide monomer units.

DE 197 19 430-Cl also discloses that the good solubility of polyol esters can lead to a dramatic drop in viscosity in the area of the bearings of the refrigerant compressor. Under these conditions, less soluble lubricants, such as mineral oils, polyolefins or polyalkylene glycols, do not show the drop in viscosity mentioned. Due to the poorer solubility, however, problems can arise with polyalkylene glycols as operating fluids with regard to oil return transport, particularly in the expansion valve and evaporator components and the suction line, especially at low flow rates. On the one hand, the requirement in the compressor, i.e. to achieve a correspondingly high mixture viscosity in the lubrication gap, must be met, and on the other hand, solubility at low temperatures in the area of the evaporator and suction line components must be guaranteed in order to achieve good

To guarantee heat transfer and oil return. A so-called partial miscibility, i.e. a miscibility gap that exists in a certain temperature range for certain mixing ratios, is of interest here.

The polyethers according to the invention meet the above-mentioned requirements and are soluble in CO 2 for comparatively higher mass fractions (eg greater than 70 wt.%) of lubricant over the entire temperature range down to -55 ⁰ C. At lower lubricant fractions, the polyethers according to the invention are no longer soluble in CO ₂ over the entire temperature range, as is usual in refrigeration machines.

10 Soluble at temperatures down to about -55°C.

The invention is based on the problem of finding polyethers which, in addition to the above-mentioned requirements,

- very good hydrolytic and thermal stability 15 - good viscosity-temperature behavior

- excellent chemical stability

- high compatibility with sealing materials and

but in particular have particularly good lubricating properties.

Surprisingly, the following have proven to be suitable for solving these problems: operating fluid compositions for refrigeration machines, heat pumps and related systems, such as air conditioning systems, containing carbon dioxide as operating fluid and polyethers or mixtures thereof, wherein the polyethers

(a) 15% to 100%, preferably 30 to 60%, of the monomer units consist of monomer units of the type -(-CH ₂ -CH ₂ -CH ₂ -CH ₂ -O-)- and

(b) the remaining residue consists essentially of monomer units of the type -(-R-O-)-, where R is a linear or branched, saturated alkyl group having 2 to 26, preferably 2 to 6 carbon atoms, optionally carrying 1 to 5, preferably 1 to 2, further oxy groups (-O-),

30 and can be different for each monomer unit.

The monomer units of type (b) are preferably ethylene oxide and/or propylene oxide units.

The polyethers or their mixtures can have the following end groups: 35 (i) hydrogen and/or hydroxy end groups and/or

(ii) alkyl, aryl, alkylaryl, hydroxy, alkoxy, aryloxy, carbonate esters,

Ester and/or alkylaryloxy end groups, wherein the

Hydrocarbon radicals 1 to 32, preferably 1 to 12, particularly preferably 1 to 6, carbon atoms and 0 to 8, preferably 0 to 4

5 oxy groups (-O-).

Preferably, the polyethers have one or no free hydroxy group.

The polyethers according to the invention are prepared by ring-opening polymerization of tetrahydrofuran (THF). Acyl and alkyl halides in the presence of Friedel-Crafts-type catalysts such as BF ₃ , AICI3, SnCU, SnF4 etc., lead to polymers with the characteristic group
-(CH2)4-O- . Furthermore, copolymers can be obtained by copolymerization of THF in the presence of ethylene oxide and/or propylene oxide and the catalysts SOCI2 / FeCk. The chlorine end groups of the polymers thus obtained can be converted into methoxyl groups using sodium methylate. Compounds containing active hydrogen, such as alcohols, e.g. n-butanol, propylene glycol, ethylene glycol, phenol, cresol, polyamines, sorbitol or other sugars and neopentyl glycols such as pentaerythritol, amines such as ethylenediamine, aminoethanolamine, triethylenetetramine or hydroquinone, can be used as regulators for the chain length.

20 can be used.

Furthermore, the polyethers according to the invention are also accessible by reacting long-chain 1,2-epoxyalkanes, and optionally additionally C2 to C4 alkylene oxides, and THF in the presence of an alcohol, polyalcohols and/or water by a process according to US 4,481,123. Instead of the long-chain 1,2-epoxyalkanes, glycidyl ethers according to EP 0 382 931 or US 5,180,856 can also be used. The side groups of the polyethers then have additional ether bonds.

According to US 4,228,272, the polyethers according to the invention are accessible by reacting THF and C2 to C3 alkylene oxides in the presence of montmorillonites. However, heteropolyacids of tungsten, vanadium or molybdenum have also been proposed as catalysts.

Polyethers according to the invention having the following structural units -CH ₂ -CH(CH ₃ )-CH ₂ -O- and -CH ₂ -CH ₂ -CH ₂ -CH ₂ -O- are obtainable, for example, by the process disclosed in DE 33 26 178 by reacting 3-methyloxethane and THF in the presence of an acid catalyst.

Particularly preferred with regard to stability, hygroscopicity and solubility in carbon dioxide are polyethers according to the invention which no longer have any free hydroxyl groups.

The polyethers according to the invention preferably have an average molecular weight (number average) of 400 to 3000 g/mol, particularly preferably 600 to 1800 g/mol. The kinematic viscosity of the polyethers is preferably 10 to 400 mm ² /s (cSt) at 40 ⁰ C measured according to DIN 51562.

The operating fluid composition generally contains between 1 and 15% by weight of lubricant - however, depending on the type of refrigeration machine, this value may also be outside the specified range, with preferably at least 20% by weight, particularly preferably at least 50% by weight, of the additives to the operating fluid being polyethers according to the invention, based on all ingredients of the

20 operating resources.

In addition, the operating fluid composition may contain common additives such as wear improvers, anti-foaming agents, antioxidants such as alkylated phenolic or amine antioxidants, viscosity index improvers, corrosion inhibitors or high-pressure additives such as phosphoric acid esters (in particular

Tricresyl phosphate).

Furthermore, the polyethers according to the invention can be used as lubricants together with neopentyl polyol esters, preferably those from DE 197 19 132, polyethers, preferably those from DE 197 19 430, polyvinyl ethers and/or compounds with an olefinic main chain and ester side groups according to DE 198 35 226.

The neopentyl polyol esters are preferably made from:

an acid component, wherein the acid component consists of linear or

branched C5 to C12 monocarboxylic acids, and

an alcohol component with 4 to 8 hydroxy groups, one, two or three quaternary carbon atoms (in the 2-position to the hydroxy group), 5 to 21 carbon atoms and 0 to 4 ether bonds,

wherein the acid component is preferably a C5 to ClO acid component and consists of at least 90 mol% branched carboxylic acids based on the total carboxylic acid group concentration.

The operating agents are used in refrigeration machines, heat pumps and related systems, such as air conditioning systems, preferably in refrigeration machines for vehicles, particularly preferably in refrigeration machines with an internal heat exchanger. The operating agent according to the invention can be used in a transcritical cycle or exclusively in the subcritical range, e.g. in

15 cascade systems.

Lubrication properties under carbon dioxide atmosphere:

To determine the tribological properties of refrigeration machine oils, the seizure load or breaking load is often determined using the Falex test method (ASTM D 3233). This method has proven itself in the development of lubricants for refrigeration machine applications. The pure lubricant is tested, i.e. the presence of the operating fluid or refrigerant is omitted.

Other test methods such as the Almen-Wieland test under a refrigerant atmosphere up to 1 bar (Lubrication Technology 22 (1991) pp. 153-155), four-ball apparatus test (DIN 51350-3) and a modification of the Falex method, ie a test in the presence of the operating fluid at ambient pressure, were carried out to determine the influence of the refrigerant on the tribological properties. The results available so far from test runs of the first prototype compressors for mobile CO ₂ vehicle air conditioning systems in transcritical operation or as tests with subcritically operated cascade systems showed surprisingly good results.

Practical measurements have shown that some lubricants with comparatively high Falex breaking load values, such as polyol esters (approx. 1000 lbs), contrary to expectations, only have fairly mediocre lubricating properties under a CO2 atmosphere. Polyalkylene glycols according to DE 197 19 430-Cl with good breaking loads of approx. 800 lbs gave adequate results.

In contrast, THF polyethers according to the invention with breaking loads of approximately 1000 lbs showed excellent wear characteristics.

Furthermore, a special rolling bearing test bench was constructed, which makes it possible to carry out tribological tests on load-bearing capacity and wear behavior under a CO ₂ atmosphere at pressures of up to 100 bar. In contrast to the tribological test methods used to date, it is possible to test the lubricating properties under a CO2 atmosphere under realistic pressure and temperature conditions. It has proven to be advantageous to set the maximum transferable axial bearing loads from 300 N to

5500 N at 90 ⁰ C and 50 bar CO ₂ .

Polyol esters only allowed a bearing load of up to approx. 300 N, whereas polyalkylene glycols according to DE 197 19 430-Cl allowed a bearing load of around 1700 N. The THF polyethers according to the invention achieved values of over 4600 N. The results of the various measurements

are summarized in Table 1.

Solubility behavior:

The influence of the operating medium CO2 on the lubrication of the compressor or on the lubricant is extremely high under the corresponding pressure conditions compared to other refrigerants such as R 134a. The advantage of the THF polymers or THF copolymers used according to the invention also lies in the poorer solubility in CO2 compared to polyalkylene glycols based on ethylene oxide / propylene oxide (EO/PO) and polyol esters (POE). This results in favorable

Mixture viscosities and associated good tribological properties.

The solubility behavior with CO2 was investigated in the pressure range from 1 to 200 bar. The THF polyethers used according to the invention show advantageous solubility behavior with CO2, especially in the supercritical range. This results in

a viscosity-temperature behavior favorable for lubrication in conjunction with the operating fluid.

Stability:

The stability of the THF polyethers according to the invention was determined using thermogravimetry (TGA) and differential scanning calorimetry (DSC) under a CO2 atmosphere. The stability of the THF polyethers according to the invention under a CO2 atmosphere can be improved by alkylating the hydroxy groups, e.g. methylation (final etherification). The _CO2 solubility increases

thereby slightly.

Table 1: Measured values

product Chemical structure Molars density Cinemat. viscosity Vis.- Pour- Dielectricity Thermal TGA max.- feeding load transferable axial force t
< Dimensions index Point constant stability °C,5% No. M Vl PP DSC _CO2 Falex Bearing test. EO/PO/THF - g/mol kg/ ^{m3 mm2} /s - ^0C - ^0C 280 lbs N <
I Relationship 15 ⁰ C 40 ⁰ C 100°C 2O ⁰ C _CO2 255 fracture 90 ⁰ C / 50 bar CO ₂ POE1 DPE-i-C9 1150 974 170.0 17.2 108 -30 4.33 389 248 750 150 PAGE 1 B0/100/0 2600 1002 223.0 38.0 223 -31 5.18 368 850 1700 PAGE 2 B 0/100/0 M 1200 996 73.5 13.7 193 -48 5.34 349 PAGE 3 B0/100/0 1100 993 58.9 11.4 191 -45 5.36 800 PAGE 4 B 0/100/0 M 1115 995 57.5 12.2 215 -39 4.98 341 198 PAGE 5 M 0/100/0 M 992 42.3 9.2 212 -36 5.16 191 800 THF1 B37 / 20 / 43 1457 1008 194.1 33.5 220 -51 6.01 285 225 1000 3875 THF 1 M B 37 / 20 / 43 M 1464 1007 136.5 26.9 236 -54 6.24 379 THF2 1307 1006 170.8 30.8 224 -54 5.93 379 1000 4650 THF 2 E i-C9 esterified (E) 1445 95.6 18.5 215 -54 THF3 1004 91.5 17,19 206 5.98 THF 3 M 121, partly M 988 66.3 15.04 240 5.58 ► · ·
* · «
* * · THF4 747 999 54.2 10.7 194 -57 *
»·· · THF4X other end group 720 997 57.8 11.4 195 -57 &bull;
»· ·· ·· ·
I · ·
* · · &bull; 9 ·
* a
&bull; * &ngr; * ■ ·
· * · * ·

THF: THF polyether (HomO'/copolymer), POE: polyol ester, PAG: polyalkylene glycol made from ethylene oxide (EO) and/or propylene oxide (PO), M = methylated (or methanol started), B = butanol started, DPE dipentaerythritol ester, i-C9: isononanoic acid group

Claims (10)

1. Operating fluid compositions for refrigeration machines containing carbon dioxide as refrigerant and polyether, wherein the polyether a) 15% to 100%, preferably 30 to 60%, of the monomer units consist of monomer units of the type -(-CH ₂ -CH ₂ -CH ₂ -CH ₂ -O-)- and b) the remaining residue consists essentially of monomer units of the type -(-RO-)-, where R is a linear or branched, saturated alkyl group having 2 to 26, preferably 2 to 6 carbon atoms, optionally carrying 1 to 5 further oxy groups (-O-), and can be different for each monomer unit. 2. Operating agent composition according to claim 1, characterized in that the operating agent composition contains polyethers or mixtures thereof which have end groups which a) hydrogen and/or hydroxy end groups and/or b) alkyl, aryl, alkylaryl, hydroxy, alkoxy, aryloxy, carbonate ester, ester and/or alkylaryloxy end groups, where the hydrocarbon radicals can have 1 to 32 carbon atoms and 0 to 8 oxy groups (-O-). 3. Operating fluid composition according to one of the preceding claims, characterized in that the operating fluid composition contains polyethers or mixtures thereof whose monomer units of type (b) are ethylene oxide and/or propylene oxide units or were prepared therefrom. 4. Operating fluid composition according to one of the preceding claims, characterized in that the operating fluid composition contains polyethers or mixtures thereof which have one or no free hydroxyl group per molecule. 5. Operating fluid composition according to one of the preceding claims, characterized in that the operating fluid composition contains polyethers or mixtures thereof which have an average molecular weight of 400 to 3000 g/mol, particularly preferably of 600 to 1800 g/mol. 6. Operating fluid composition according to one of the preceding claims, characterized in that the operating fluid composition further contains esters or an ester mixture, wherein the esters are neopentyl polyol esters. 7. Operating fluid composition according to claim 6, characterized in that the neopentyl polyol esters consist of 1. an acid component, wherein the acid component consists of linear or branched C5 to C12 monocarboxylic acids, and 2. an alcohol component with 4 to 8 hydroxy groups, one, two or three quaternary carbon atoms, 5 to 21 carbon atoms and 0 to 4 ether bonds. 8. Operating fluid composition according to claim 6, characterized in that the neopentyl polyol esters consist of 1. a C5 to C10 acid component, wherein the acid component consists of at least 90 mol% branched carboxylic acids based on the total carboxylic acid group concentration, and 2. an alcohol component with 4 to 8 hydroxy groups, one, two or three quaternary carbon atoms, 5 to 21 carbon atoms and 0 to 4 ether bonds. 9. Operating fluid composition according to one of the preceding claims, characterized in that the operating fluid composition further contains polyvinyl ether compounds. 10. Operating fluid composition according to one of the preceding claims, characterized in that the operating fluid composition contains, in addition to carbon dioxide and conventional additives, exclusively polyethers according to one of claims 1 to 5, polyether/ester mixtures according to one of claims 6 to 8, polyether/polyvinyl ether mixtures according to claim 9 or polyether/ester/polyvinyl ether mixtures according to one of the preceding claims as lubricants.