The present invention refers to a refrigeration lubricant comprising at least one monoester or diester of at least one 1,3-dioxane having at least one hydroxy or hydroxyalkyl group and at least one monocarboxylic acid having 2 to 12 carbon atoms and optionally one or more additives. The present invention further refers to a refrigeration working fluid comprising at least one lubricant according to the invention and at least one halocarbon and/or halo-olefin refrigerant.

Esters and polyesters used as lubricants are well-known in the art and used in a wide range of applications including automotive and aviation oils, metal working fluids, gear oils, turbo oils, hydraulic fluids and refrigeration lubricants. Refrigeration systems like refrigerators, freezers, air conditioners, heat pumps normally include a circulating and heat transferring refrigerant. A refrigeration compression circuit typically comprises a compressor, a condenser, expansion device(s) and an evaporator. The refrigerant and compressor lubricant in most cases circulate throughout the system without phase separation. A potential phase separation may include inadequate lubrication of the compressor and reduced heat exchange efficiency. Lubricants for refrigeration systems must thus be compatible and miscible within a wide range of temperatures with said refrigerant in order for the resulting refrigeration working fluids to work properly and lubricate moving parts in the system.

The properties and performances of halocarbon refrigerants are well-known and those containing chlorine are presently, due to their negative environmental effect, being replaced by chlorine free compounds such as fluorocarbons. Retrofitting of compressor refrigeration systems with newer refrigerants that exhibit no ozone depletion and low global warming potential has become important. The changes made in moving from chlorinated fluorocarbons (CFCs) to hydro fluorinated olefins (HFOs) have impacted the type of lubricant basestock that can be used with the refrigerant. The original lubricants used with CFCs and hydrochlorofluorocarbons (HCFCs) were based on refined naphthenic base oils. For applications that required better stability at higher or lower temperatures, alkylated benzenes were used. However, alkylated benzene-based compressor oils are not compatible with chlorine-free hydroflurocarbon (HFC) refrigerants. Accordingly, polyol esters, which are compatible with these refrigerants, replaced alkylated benzenes. The CFCs and HCFCs used mineral oils. All the new options use synthetic lubricants like polyalkylene glycols and esters. The synthetic oils are more hydrophilic than mineral oils and therefore need to be handled more carefully. Polyalkylene glycols and polyol esters are polar basestocks that have significantly better HFC and HFO miscibility and excellent low-temperature characteristics compared with mineral oils and alkylated benzenes. Polyol esters are the most commonly used lubricants for stationary refrigeration applications. Polyalkylene glycols are used in most automotive air conditioning applications. Alkylated benzenes and polyvinyl ethers are used in some applications.

The missions of a refrigeration lubricant are to lubricate internal parts, remove heat generated during compression, clean the system, act as a fluid seal and reduce energy requirements. Compressor design and operating conditions are important parameters when selecting a lubricant. When a gas is compressed, the lubricant in the compression zone picks up some of the heat of compression, which is subsequently dissipated elsewhere in the system. The lubricant also assists in keeping the refrigeration compressor clean. A filter is usually present and the lubricant helps transport various solid contaminants to the filter for removal. Refrigeration systems also have a variety of 'non-contact' surfaces that require sealing (e.g., cylinder/piston walls). The lubricant seals these surfaces and prevents the refrigerant gas from passing through. Other functions of the lubricant are to control foam and reduce noise generated by moving parts of the compressor.

The lubricant may travel with the refrigerant as it escapes from the compressor, a concept called "entrapment" in the system. Lubricant entrapped by refrigerant can be in quantities up to 5% or more. This entrapment can lead to poor oil return resulting in oil starvation, if not managed properly by making sure of sufficient refrigerant miscibility or by making hardware accommodations (such as an oil separator). This entrapment can also coat the inner surfaces of the system devices responsible for evaporating and condensing the refrigerant. This phenomenon can lead to a decrease in the effectiveness of the heat exchange, which reduces the performance of the refrigeration system.

Small amounts of additives are usually added to lubricants used with HFC refrigerants to boost antiwear protection, metal inactivation, acid scavenging and defoaming. The movement away from chlorine-containing refrigerants has led to a greater need for antiwear additives in specific applications. It is well known that chlorine-containing refrigerants possess some lubricity properties. These refrigerants can react chemically with metals in the refrigeration system to form protective surface films composed of metal chlorides. The antiwear additives used today are based on sulfur and phosphorus chemistries and can boost the performance of the lubricant.

The viscosity and miscibility properties of lubricant esters can be modified by the use of different polyols and by the use of carboxylic acids that have either branched or linear chains. Esters based on linear chain acids in general have good lubricity but often suffer from limited miscibility with hydrofluorocarbon refrigerants. Similarly, esters based on branched chain acids exhibit good miscibility but poorer viscosity. Typically, a mix of acids is used to formulate a polyester blend with specific properties required by the refrigeration system. Polyester lubricants that are based on lower molecular weight alcohols tend to be more miscible than those based on higher molecular weight alcohols. Polyester lubricants based on linear acids are often less miscible when manufactured in higher viscosity grades due to the use of higher molecular weight acids. Thus in order for the lubricant to be compatible with the refrigerant, linear chained acids and low molecular weight are essential.

The present invention refers to a lubricant that has quite unique properties that are highly suitable for refrigeration systems. The problem of obtaining both satisficing viscosity and miscibility with the refrigerant is overcome by using the lubricant of the present invention. Said lubricant comprises an ester that has shown to have a high viscosity, which is requested for good lubricity. The lubricant ester of the present invention has furthermore a low molecular weight and a polar characteristic similar to hydrofluorocarbons and hydro fluorinated olefins, resulting in good mutual solubility. High polarity is often required for newer refrigerants. The fact that the lubricant ester of the present invention is polar gives the lubricant oil some antiwear properties. The polar end can attach to a metal surface and create a film that acts as a barrier to reduce friction and wear by providing a cushioning effect when one coated surface connects with another coated surface. Often are polar molecules, so called antiwear agents, added to lubricants in order to obtain such an antiwear effect. Compared to some other known synthetic lubricant esters, such as esters based on neopentyl glycol and triethylene glycol, the lubricant ester of the present invention has surprisingly high viscosity and low evaporation rate.

Many of the commonly used refrigerants are soluble in the lubricant. This means that the 'fluid' actually doing the lubrication is a combination of both lubricant and refrigerant, often called 'working fluid', and the viscosity of this fluid is less than that of the lubricant alone. This dilution effect of the refrigerant with a corresponding decrease in viscosity is countered by the enhanced viscosity of the lubricant of the present invention. The lubricant ester of the present invention has shown to exhibit surprisingly high viscosity in relation to molecular weight.

Another advantageous property of the lubricant ester of the invention is low volatility. A low volatility contributes to low oil consumption. Most refrigeration systems are hermetic, i.e. closed systems, but there are also semi-hermetic refrigeration systems, in which a low volatility of the lubricant is of essence. When a lubricant with low volatility is used, service intervals can be extended, and then the need to add more surfactant oil to the refrigeration system is decreased. The low weight loss indicates that the lubricant ester of the present invention has a high thermal stability and oxidation resistance. These are properties that help minimise deposit formation and also contributes to extended service intervals. Lower volatility also means a lower fire risk which makes the handling of the lubricant oil a lot safer.

Although the lubricating ester of the present invention primarily is intended to be used in refrigeration system, its polarity and high viscosity in relation to molecular weight also makes it suitable as lubricant in for instance hydraulic oils, transmission fluids, automotive 4-stroke oils and rotary vane compressors.

The present invention accordingly refers to a refrigeration lubricant comprising at least one monoester or diester of at least one 1,3-dioxane having at least one hydroxy or hydroxyalkyl group and at least one monocarboxylic acid having 2 to 12 carbon atoms and optionally one or more additives. Said 1,3-dioxane preferably has at least two hydroxy or hydroxyalkyl groups, which suitably are vicinal hydroxy and/or hydroxyalkyl groups. The 1,3-dioxane is preferably 5,5-dihydroxyalkyl- 1,3-dioxane, where the alkyl is C 1-C3 and most preferably the 1 ,3-dioxane is 5,5-dihydroxymethyl-l ,3-dioxane. Said monocarboxylic acid is valeric acid, isovaleric acid, hexanoic acid isohexanoic acid, 2-methylhexanoic acid, 2-ethylhexanoic acid, 2,2-dimethylhexanoid acid, 3,5,5-trimethylhexanoic acid, heptanoic acid, isoheptanoic acid, octanoic acid, isooctanoic acid, nonanoic acid, isononanoic acid and/or 2-propylheptanoic acid. The refrigeration lubricant of the present invention preferably comprises at least one monoester or diester of 5, 5-dihydroxymethyl- 1,3-dioxane and valeric acid and/or 2- ethylhexanoic acid. The refrigeration lubricant of the present invention may further comprise one or more additives selected from the group consisting of antiwear agents, extreme pressure agents, antioxidants, detergents, polymers improving the viscosity index, pour point improvers, dispersants, antifoaming agents, acidity regulators, corrosion inhibitors, metal inactivators, stabilisers and thickeners. Many additives are multifunctional, for example, certain additives can act both as extreme pressure agents and antiwear agents, or both function as a metal inactivator and a corrosion inhibitor.

The present invention further refers to a refrigeration working fluid comprising at least one lubricant according to the present invention and at least one halocarbon and/or halo-olefin refrigerant, said refrigerant preferably being a fluorocarbon or a hydrofluorocarbon. More preferably said refrigerant is a difluoroethane, a trifluoroethane, a tetrafluoroethane and/or a pentafluoroethane. Most preferably said refrigerant is 1,1 -difluoroethane, 1,1,1- trifluoroethane, 1 , 1 , 1 ,2-tetrafluoroethane, 2,3,3,3-tetrafluoro prop-l-ene and/or trans- 1,3, 3,3 - tetrafluoroprop-l-ene. Mixtures comprising hydrofluorocarbon and hydro fluoro-olefin refrigerant may or may not additionally include carbon dioxide, saturated or unsaturated hydrocarbons of length C ₃ to C ₆ , iodotrifluoromethane, perfluoroketones, hydro fluoroketones, hydrochlororfluoroketones, or hydrochlorofluoroolefins. The refrigeration working fluid of the present invention preferably comprises 1-60% by weight of the lubricant and 40-99% by weight of the refrigerant.

The present invention is further explained with reference to enclosed embodiment examples, which are to be construed as illustrative and not limiting in any way.

Example 1 : Preparation of 5,5-dihydroxymethyl - 1 ,3-dioxane.

Example 2: Preparation of a lubricant ester of 5,5-dihydroxymethyl- 1,3-dioxane and valeric acid.

Example 3: Preparation of a lubricant ester of 5,5-dihydroxymethyl- 1,3-dioxane and 2-ethyl hexanoic acid.

Example 4: Preparation of a comparative lubricant ester of neopenthylglycol and valeric acid.

Example 5: Preparation of a comparative lubricant ester of neopenthylglycol and 2-ethyl hexanoic acid. Example 6: Preparation of a comparative lubricant ester of triethylene glycol and 2-ethyl hexanoic acid.

Example 7: Evaluation of viscosity and volatility of the esters of examples 2-6. Example 1

Preparation of 5,5-dihydroxymethyl-l,3-dioxane

580.6 g (4.2 moles) of 2,2-bishydroxymethyl-l,3-propanediol, 255.2 g (4.2 moles) of formaldehyde, 1062.4 g of water and 1.8 g of sulphuric acid were charged in a reaction vessel equipped with stirrer, thermometer, condenser and electrical heater. The reaction mixture was heated to 99°C under stirring and maintained at this temperature for 8 hours. Obtained product was then neutralised with NaOH, followed by water removal by vacuum distillation. A final distillation was performed at 190-200°C using a column (1 m long DN40 column, inside diameter 43 mm, containing packing LDX type from Sulzer), which distillation resulted in a pure (99 m%) final product of 5,5-dihydroxymethyl-l,3-dioxane.

Example 2

Preparation of a lubricant ester of 5,5-dihydroxymethyl-l ,3-dioxane and valeric acid

200 g (1.4 moles) of the 5,5-dihydroxymethyl-l,3-dioxane (obtained in Example 1), 331 g (3.2 moles) of valeric acid, 4 wt% of xylene and 0.1 wt% of butylstannoic acid - Fascat 4100 were charged in a reaction vessel equipped with stirrer, pressure gauge, nitrogen inlet, thermometer, cooler and water trap (Dean-Stark). The temperature was raised to 165°C, at which temperature esterification water was formed. The temperature was then raised to 200°C and maintained for 17 hours until the esterification was complete. Xylene and unreacted valeric acid were removed by vacuum distillation. Obtained product was then, at 130°C, neutralised with Ca(OH) ₂ , followed by water removal by addition of a filter agent (Celite 545) and finally filtered at 100°C and cooled to room temperature.

Obtained ester exhibited the following properties:

Acid value, mg KOH/g 0,08

Hydroxyl value, mg KOH/g 1 ,3

Viscosity at 23 °C, mPas 28

Volatility (mass loss at Noack time), % 55 Dry content, % >99

The volatility was measured by using the Noack test. The Noack volatility of an oil is defined as the weight loss of the oil when it is held under isothermal conditions at 250 °C for a period of 1 hour under a constant flow of air. Thermos-gravimetric analysis (TGA) was used to determine the Noack evaporation loss. This is also valid for Examples 3-6 below.

Example 3

Preparation of a lubricant ester of 5,5-dihvdroxymethyl-l,3-dioxane and 2-ethylhexanoic acid

200 g (1.4 moles) of 5,5-dihydroxymethyl-l,3-dioxane (obtained in Example 1), 467 g (3.2 moles) of 2-ethylhexanoic acid, 4 wt% of xylene and 0.1 wt% of butylstannoic acid - Fascat 4100 were charged in a reaction vessel equipped with stirrer, pressure gauge, nitrogen inlet, thermometer, cooler and water trap (Dean-Stark). The temperature was raised to 165°C, at which temperature esterification water was formed. The temperature was then raised to 230°C and maintained for 36 hours until the esterification was complete. Xylene and unreacted valeric acid were removed by vacuum distillation. Obtained product was then, at 130°C, neutralised with Ca(OH) ₂ , followed by water removal by addition of a filter agent (Celite 545) and finally filtered at 100°C and cooled to room temperature.

Obtained ester exhibited the following properties:

Acid value, mg KOH/g 0,30

Hydroxyl value, mgKOH/g 7,5

Viscosity at 23 °C, mPas 80

Volatility (mass loss at Noack time), % 27

Dry content, % >99

Example 4 (comparative)

Preparation of a lubricant ester of neopentyl glycol and valeric acid

140 g (1.4 moles) of neopentyl glycol, 331 g (3.2 moles) of valeric acid, 4 wt% of xylene and 0.1 wt% of butylstannoic acid - Fascat 4100 were charged in a reaction vessel equipped with stirrer, pressure gauge, nitrogen inlet, thermometer, cooler and water trap (Dean-Stark). The temperature was raised to 165°C, at which temperature esterification water was formed. The temperature was then raised to 200°C and maintained for 19 hours until the esterification was complete. Xylene and unreacted valeric acid were removed by vacuum distillation. Obtained product was then, at 130°C, neutralised with Ca(OH) ₂ , followed by water removal by addition of a filter agent (Celite 545) and finally filtered at 100°C and cooled to room temperature.

Obtained ester exhibited the following properties:

Acid value, mg KOH/g 0,37

Hydroxyl value, mgKOH/g 0,8

Viscosity at 23 °C, mPas 5

Volatility (mass loss at Noack time), % 100

Dry content, % >99

Example 5 (comparative) 230/35

Preparation of a lubricant ester of neopentyl glycol and 2-ethylhexanoic acid

140 g (1.4 moles) of neopentyl glycol, 467 g (3.2 moles) of 2-ethylhexanoic acid, 4 wt% of xylene and 0.1 wt% of butylstannoic acid - Fascat 4100 were charged in a reaction vessel equipped with stirrer, pressure gauge, nitrogen inlet, thermometer, cooler and water trap (Dean-Stark). The temperature was raised to 160°C, at which temperature esterification water was formed. The temperature was then raised to 230°C and maintained for 35 hours until the esterification was complete. Xylene and unreacted valeric acid were removed by vacuum distillation. Obtained product was then, at 130°C, neutralised with Ca(OH) ₂ , followed by water removal by addition of a filter agent (Celite 545) and finally filtered at 100°C and cooled to room temperature.

Obtained ester exhibited the following properties:

Acid value, mg KOH/g 0,02

Hydroxyl value, mgKOH/g 1 ,4

Viscosity at 23 °C, mPas 13

Volatility (mass loss at Noack time), % 74

Dry content, % >99

Example 6 (comparative)

Preparation of a lubricant ester of triethylene glycol and 2-ethylhexanoic acid 203 g (1.4 moles) of triethylene glycol, 467 g (3.2 moles)of 2-ethylhexanoic acid, 4 wt% of xylene and 0.1 wt% of butylstannoic acid - Fascat 4100 were charged in a reaction vessel equipped with stirrer, pressure gauge, nitrogen inlet, thermometer, cooler and water trap (Dean-Stark). The temperature was raised to 165°C, at which temperature esterification water was formed. The temperature was then raised to 230°C and maintained for 36 hours until the esterification was complete. Xylene and unreacted valeric acid were removed by vacuum distillation. Obtained product was then, at 130°C, neutralised with Ca(OH) ₂ , followed by water removal by addition of a filter agent (Celite 545) and finally filtered at 100°C and cooled to room temperature.

Obtained ester exhibited the following properties:

Acid value, mg KOH/g 0, 1 1

Hydroxyl value, mgKOH/g 0,6

Viscosity at 23 °C, mPas 15

Volatility (mass loss at Noack time), % 46

Dry content, % >99

Example 7

Evaluation of the esters obtained in Example 2-6

The results of the viscosity and volatility measurements are shown in the following two graphs, were CPF is short for cyclic pentaerythritol formal (5,5-dihydroxymethyl-l,3- dioxane), Neo is neopentyl glycol and TEG is triethylene glycol.

aph 1 Viscosity and volatility of valerates

The results show that the lubricant esters according to the present invention, ester of 5,5- dihydroxymethyl-l,3-dioxane and valeric acid (CPF- Valeric acid) and ester of 5,5- dihydroxymethyl-l,3-dioxane and 2-ethylhexanoic acid (CPF-2-EHA), exhibit a higher viscosity and a lower evaporation rate than the comparative lubricant esters.