This invention relates to lubricants and more particularly to their use in heat transfer devices.

Heat transfer devices of the mechanical recompression

5 type, including refrigerators, heat pumps and air conditioning systems, are well known. In such devices, a working fluid of a suitable boiling point evaporates at a low pressure taking heat from the surrounding zone. The resulting vapour is then compressed and passed to a condenser where it condenses and gives

10 off heat to a second zone. The condensate is then returned through an expansion valve to the evaporator, so completing the cycle. The mechanical energy required for compressing the vapour and pumping the fluid is provided by, for example, an electric motor or an internal combustion engine.

15 The working fluids used in these heat transfer devices include dichlorodifluoromethane (R-12) the production of which is likely to be severely limited by international agreement in order to protect the stratospheric ozone layer. As is the case with other mechanical equipπumt, it is necessary for the moving parts

20 of the heat transfer devices to be adequately lubricated and the devices are generally designed to use lubricants which are miscible with the working fluids, mineral oil being used in conjunction with dichlorodifluoromethane.

Conventionally, in the refrigeration industry and in

25 particular with mobile air-conditioning (MAC) industry, for example the automobile air-conditioning industry, it is desirable to use a lubricant which is as miscible as possible with the working fluid. The limit of miscibility is often defined as the maximum temperature (known as the upper miscibility temperature)

-^ at which a mixture comprising a defined ratio of a lubricant and a working fluid remains as a single phase, with the more miscible lubricants showing a higher maximum temperature. ^' Where the t working fluid is a mixture of components, the miscibility of the lubricant is often at a level intermediate to that of the ⁴ - ^~ miscibility of the lubricant in the individual components.

Furthermore, certain refrigeration systems, e.g. automobile air conditioning systems, are often designed so as to require the lubricant to have an intrinsic viscosity at 100°C within a specific range of viscosities, e.g. 9 to 10 cSt, or 14 to 16 cSt, or 20 to 22 cSt. A lubricant having an intrinsic viscosity in the range 9 to lOcSt is generally not suitable for use in a system requiring a 14 to 16 cSt lubricant.

Unfortunately some of the compounds, for example 1,1,1,2-tetrafluoroethane (R-134a) which have been proposed as working fluids to replace dichlorodifluoromethane are insufficiently soluble in mineral oils to allow the latter to be used as lubricants.

Polyalkoxylates, such as polyalkylene glycols (PAGs) having -OH and other terminal groups have been proposed as alternatives. Such materials have been prepared which have miscibility temperatures in excess of 70°C and also intrinsic viscosities of between 9 to 10 cSt. However, materials which have hitherto been prepared having satisfactory intrinsic viscosities in the range 14 to 16 cSt have miscibility temperatures significantly lower than 70°C.

It has been found that certain polyalkoxylates as defined below are excellent lubricants when used with a working fluid such as 1,1,1,2-tetrafluoroethane, related hydro- fluorocarbons such as 1,1,2,2-tetra fluoroethane, and hydrochlorofluorocarbons such as monochlorotetrafluoroethane. Such polyalkoxylates have in combination an intrinsic viscosity of at least 14 cSt at 100°C and an improved miscibility with the working fluid compared with known polyalkoxylate lubricants.

Accordingly, the invention provides a composition for use in a compression type heat transfer device comprising a working fluid and sufficient to provide lubrication of a polyalkoxylate, wherein the working fluid comprises at least one hydrofluoroalkane or fluoroalkane and the polyalkoxylate has an intrinsic viscosity at 100°C of at least 14 cSt, has an upper miscibility temperature of at least 55°C at a weight ratio of 1:9

in the working fluid and comprises a nitrogen compound residue initiating a polyoxyalkylene chain which is terminated by an end capping group.

The invention further provides a method of lubricating a compression type heat transfer device wherein the surfaces of the relatively moveable components of the device are contacted with a composition as defined above.

The working fluid may be one or more refrigerants selected from the group containing R-23, R-32, R-134a, R-152a, R-134, and R-143, and optionally one or more refrigerants selected from the group containing R-125, R-22, R-124, R-124a, R-142a, R-133, R-123, and R-123a. Compositions of the present invention preferably contain a working fluid comprising R-134a, e.g. R-134a in combination with R32 and/or R-125. By suitable selection of the initiating nitrogen compound, polyoxyalkylene chain and end capping group a wide range of materials may be prepared. As stated above, such materials have an intrinsic viscosity at 100°C of at least 14 cSt, and preferably in the range 14 to 16 cSt, in combination with an upper miscibility temperature (at a weight ratio of 1:9 with the working fluid, i.e. 10Z w/w) of at least 55°C, preferably at least 58°C, and most suitably in the range 55 to 75°C.

The initiating nitrogen compound residue is preferably the residue formed by the removal of at least one active hydrogen atom from ammonia, or an organic amine or amide. Suitably the nitrogen compound may be a mono or polyhydric amine having primary and/or secondary amine groups. The nitrogen compound may thus be selected from alkyl, cycloalkyl or aromatic amines and amides having 1 to 50 carbon atoms, preferably 1 to 30 carbon atoms and more preferably 1 to 15 carbon atoms and preferably 1 to 100, more preferably 1 to 10 and most preferably 1 to 6 amine or amide groups. Specifically suitable nitrogen compounds include alkylamines, alkylene diamines and in particular piperazine. The polyoxyalkylene chain may comprise 4 to 170, preferably 5 to 140, and particularly 5 to 120 alkylene oxide

residues, each having 2 to 4 carbon atoms. The polyoxyalkylene chain may contain one or more species of alkylene oxide. Thus, where more than one species of alkylene oxide is present, for example where both propylene oxide and ethylene oxide residues are

5 present, the polyoxyalkylene chain may have the structure of a random or block polymer. Preferred polyoxyalkylene chains are those which contain 0 to 70 mol Z, particularly 0 to 60 mol Z, and especially 0 to 50 mol Z of ethylene oxide residues with the complementary percentage of propylene oxide residues.

1 The end capping group may be provided by an alkyl, or aryl, or aralkyl group. In particular the end capping group is one or more methyl, ethyl, propyl, isopropyl, or butyl groups.

Suitably the above defined polyalkoxylate has the following structural formula

15 R^PR^n wherein R-~ represents the nitrogen compound residue having n active sites, P represents the polyoxyalkylene chain and R^ represents the end capping group or groups.

The polyalkoxylate used in the present invention may be

20 prepared by reacting a nitrogen compound with the appropriate alkylene oxide moieties and thereafter end capping the terminal alkylene oxide residues.

The reaction of the nitrogen compound and alkylene oxide moieties may be conducted in one or more stages, and is usually

25 catalysed by an acid or an alkali such as sodium or potassium hydroxide. The temperature of the reaction is preferably controlled to be in the range 100 to 180°C, preferably 150 to 160°C for ethoxylation, 105 to 130°C, e.g. 125°C, for propoxylation or butoxylation, or 105 to 130°C, e.g. 105°C for

- rt mixed alkoxy reaction. The reaction is preferably conducted at a controlled elevated pressure, in particular at a pressure of 1.5 to 8 bars absolute.

The subsequent endcapping of the terminal alkyle-ie oxide residues is usually achieved by reaction with a suitable derivative of the end capping group, for example where it is

desired to end cap using a methyl group then methyl chloride is usually used in conjunction with a metal alkoxide, e.g. sodium methoxide. The use of an acidic passivating agent, e.g. phosphoric acid, in order to protect any exposed metal of the reactor should be avoided. Failure to omit such passivating agents leads to quaternisation of the polyalkoxylate, thereby giving a final product having inferior viscosity characteristics.

The invention is illustrated but not limited by the following examples. Preparation of Methyl End Capped Polyoxypropylene

Piperazine Propoxylation

An aqueous solution containing 860 g of potassium hydroxide and 546 g of water was added to 18 kg of piperazine. The resulting mixture was then dehydrated by heating to a temperature between 120 and 125°C, under a vacuum of between 5 to 15 mm Hg for about 0.5 hours in an autoclave. Nitrogen was bubbled through the dehydrated mixture thereby increasing the pressure in the autoclave to 2 bars absolute. The propylene oxide was then added in two stages. In the first stage 20.6 kg propylene oxide was introduced into the autoclave over a period of 1.5 hours, whilst maintaining the temperature at 125°C. The pressure within the autoclave increased from 2.4 to 2.8 bars absolute during this period. After the propylene oxide had been added the reaction was continued for 4 hours, whereupon the product was vacuum stripped of unreacted propylene oxide at a temperature of 110°C, and a pressure of between 5 to 15 mm Hg, for 1.5 hours.

In the second stage a further 220 kg of propylene oxide was added to the vacuum stripped product over a period of 11.5 hours, whilst maintaining the temperature at 115°C. During this period the pressure within the autoclave increased from 2 to 5.3 bars absolute. After the propylene oxide had been added the reaction was continued for 4 hours, whereupon the product was again vacuum stripped as described above to produce a piperazine

propoxylate. This material had a viscosity at 40°C of 154.4 cSt. Methylation

The material as produced above was then endcapped with methyl groups in the following manner. 70 kg of the piperazine propoxylate was mixed with 7 kg of sodium methoxide and stirred for 0.5 hours at ambient temperature. The resulting mixture was then heated to 120°C under a vacuum of 27 mbar, whilst continuing to be stirred for a period of 5 hours. The pressure was then increased to 1 bar absolute using nitrogen at which point 7.2 kg of methyl chloride was introduced into the reactor over a period of 2.5 hours, whilst maintaining the temperature of the reactor at between 120 to 130°C. The reaction was then continued at a temperature of 120°C for 1 hour. The product was then vacuum stripped for a period of 0.75 hour as described above whereupon the pressure in the reactor was increased to 1 bar absolute using nitrogen.

After allowing the reactor to cool to 80°C, 38.5 kg of water was added. The resulting mixture was then stirred for 0.5 hours, then decanted over a period of 6 hours to produce a a substantially water insoluble product in a supernatant layer and the wash water as a natant layer. The wash water was then decanted from the bottom of the reactor. The water content of the product was then reduced by heating to a temperature of 105°C for a period of 3 hours whilst bubbling nitrogen through.

The dehydrated product was then demineralised using 1 kg of "Ambosol" (magnesium silicate), and 1 kg of "Dicalite" (filter aid). The resulting mixture was heated to 110°C for 1.5 hours, then filtered at a temperature of 110°C using a Gauthier filter to produce a final product.

The yield of the final product was 60 kg.

The final product was identified as a substantially (approximately 95Z) methyl end capped piperazine propoxylate, having a molecular weight of about 1328. The viscosity of the product at 40°C was 76.3 cSt, at 100°C was 14.1 cSt, thereby giving a viscosity index (VI) of 193. A mixture containing 5Z w/w

of the product and 95Z w/w of refrigerant R-134a had an upper miscibility temperature of 60°C, and a mixture containing 10Z w/w of the product and 90Z w/w of the refrigerant had an upper miscibility temperature of 58°C. For comparison a number of known propoxylate lubricants were prepared and tested. Comparative examples Cl to C6 were initiated on methanol, C7 on butanol and C8 to C9 on glycerol. The polyalkylene chain in all cases contained propylene oxide repeat units, and the end capping groups in C2, C4 and C6 to C9 were methyl. The properties of these known propoxylate lubricantss are displayed in Table 1.

Comparative Examples Cl C2 C3 C4 C5 C6 C7 C8 C9 Molecular Weight 951 1136 1313 1327 1558 1581 1548 1903 1979 Z Endcapping 0 99 0 98 0 97 96 98 96

Viscosity (40°C) 45 43 74 56 90 73 68 71 73 cSt (100°C) 9 10 14 12 16 15 14 14 15

Viscosity Index 186 219 192 219 193 215 215 204 214

Upper Miscibility 71 67 52 59 44 48 43 54 51 Temperature (°C)

(102 w/w lubricant in R-134a)

TABLE 1 It can thus be seen, by comparing C3 and C4, and C5 and C6, that end capping of a methanol initiated propoxylate increases the upper miscibility temperature but decreases the intrinsic viscosity at 100°C of that material. Also, the trends in the series Cl, C3 and C5 and the series C2, C4 and C6 show that reducing the molecular weight of the material increases the upper miscibility temperature but also decreases the intrinsic viscosity at 100°C.

Propoxylates C8 and C9 show that increasing the number of polyoxypropylene chains, which was achieved by using an initiator (glycerol) having more than one hydroxy group, slightly improves the upper miscibility temperature.

Thus, the known propoxylates in Cl to C9 do not possess the combination of desired intrinsic viscosity and upper miscibility temperature. Furthermore, from a knowledge of the structures of the known propoxylates, it is not possible to predict how to achieve such a desirable combination.