Polyester lubricant for working fluids comprising difluoromethane

1. A working fluid for a heat transfer device comprising:

i) a hydrofluorocarbon refrigerant comprising difluoromethane (R-32) and

ii) a polyol ester lubricant composition having a kinematic viscosity at 40° C. of from 32 to 100 cSt and comprising a mixture of C 5-10 alkylcarboxy neopentyl polyol esters, said neopentyl polyol esters comprising a mixture of C 5-10 alkylcarboxy esters of pentaerythritol, di-pentaerythritol, tri-pentaerythritol and oligomers comprising 4 or more pentaerythritol monomers,

wherein at least 10 wt % of all neopentyl polyol esters are esters of oligomers containing 4 or more neopentyl polyol monomer units, and

wherein 75 mole % or more of all the C 5-10 alkylcarboxy esters of pentaerythritol, di-pentaerythritol, tri-pentaerythritol and oligomers comprising 4 or more pentaerythritol monomers are linear or branched C 5 alkylcarboxy esters, and 15-40% of all the C 5 alkylcarboxy esters are branched.

2. The working fluid according to claim 1 wherein the polyol ester lubricant composition further comprises C 5-10 alkylcarboxy esters of neopentyl glycol and/or C 5-10 alkylcarboxy esters of trimethylol propane wherein at least 10 wt % of all neopentyl polyol esters are esters of oligomers containing 4 or more neopentyl polyol monomer units, and 50 mole % or more of all the C 5-10 alkylcarboxy neopentyl polyol esters are linear or branched C 5 alkylcarboxy esters, and 15-40% of all the C 5 alkylcarboxy esters are branched.

3. The working fluid according to claim 1 wherein the polyol ester lubricant composition comprises:

a) from about 30 to about 85 wt % C 5-10 alkylcarboxy esters of pentaerythritol,

b) from about 1 to about 20 wt % C 5-10 alkylcarboxy esters of di-pentaerythritol,

c) from about 1 to about 10 wt % C 5-10 alkylcarboxy esters of tri-pentaerythritol, and

d) from about 10 to about 45 wt % C 5-10 alkylcarboxy esters of pentaerythritol oligomers of 4 or more pentaerythritol monomer groups

wherein the wt % is based on all neopentyl polyol esters in the lubricant.

4. The working fluid according to claim 3 wherein the polyol ester lubricant composition comprises:

a) from about 30 to about 55 wt % C 5-10 alkylcarboxy esters of pentaerythritol,

b) from about 1 to about 20 wt % C 5-10 alkylcarboxy esters of di-pentaerythritol,

c) from about 1 to about 10 wt % C 5-10 alkylcarboxy esters of tri pentaerythritol, and

d) from about 25 to about 45 wt % C 5-10 alkylcarboxy esters of pentaerythritol oligomers of 4 or more pentaerythritol monomer groups,

wherein the wt % is based on all neopentyl polyol esters in the lubricant.

5. The working fluid according to claim 3 wherein the polyol ester lubricant composition comprises:

(I) from about 50 to about 95% by weight, based on all neopentyl polyol esters in the lubricant composition, a mixture consisting of

a) from about 30 to about 55 wt % C 5-10 alkylcarboxy esters of pentaerythritol,

b) from about 1 to about 20 wt % C 5-10 alkylcarboxy esters of di-pentaerythritol,

c) from about 1 to about 10 wt % C 5-10 alkylcarboxy esters of tri pentaerythritol, and

d) from about 25 to about 45 wt % C 5-10 alkylcarboxy esters of pentaerythritol oligomers of 4 or more pentaerythritol monomer groups,

wherein the wt % is based on the weight of all esters of pentaerythritol, di-pentaerythritol, tri-pentaerythritol and oligomers comprising 4 or more pentaerythritol monomers,

and

(II) from about 5 to about 50% by weight, based on all neopentyl polyol esters in the lubricant composition, C 5-10 alkylcarboxy esters of neopentyl glycol and/or C 5-10 alkylcarboxy esters of trimethylol propane,

wherein 50 mole % or more of all the C 5-10 alkylcarboxy neopentyl polyol esters are linear or branched C 5 alkylcarboxy esters, and 15-40% of all the C 5 alkylcarboxy esters are branched.

6. The working fluid according to claim 5 wherein the polyol ester lubricant composition comprises

(I) from about 60 to about 90% by weight, based on all neopentyl polyol esters in the lubricant composition, of the mixture consisting of a), b), c), and d), and

(II) from about 10 to about 40% by weight, based on all neopentyl polyol esters in the lubricant composition, C 5-10 alkylcarboxy esters of neopentyl glycol and/or C 5-10 alkylcarboxy esters of trimethylol propane.

7. The working fluid according to claim 5 wherein from 25 to 60 mol % of all the C 5-10 alkylcarboxy neopentyl polyol esters are branched.

8. The working fluid according to claim 3 wherein from 25 to 60 mol % of all the C 5-10 alkylcarboxy neopentyl polyol esters are branched.

9. The working fluid according to claim 1 wherein the polyol ester lubricant has a kinematic viscosity at 40° C. of from 46 to 100 cSt.

10. The working fluid according to claim 1 wherein the polyol ester lubricant has a kinematic viscosity at 40° C. of from 68 to 100 cSt.

11. The working fluid according to claim 1 wherein the hydrofluorocarbon refrigerant comprises R-32 and less than 15% by weight of the refrigerant is other than R-32 and is selected from other HFC refrigerants, CO 2 , ammonia and hydrocarbon refrigerants.

12. The working fluid according to claim 11 wherein the hydrofluorocarbon refrigerant comprises R-32 and less than 10% by weight of the refrigerant is other than R-32.

13. The working fluid according to claim 12 wherein the hydrofluorocarbon refrigerant consists essentially of R-32.

14. The working fluid according to claim 1 further comprising in addition to components i) and ii), one or more of a mineral oil, poly-α-olefin, alkylbenzenes, carboxylic acid ester other than the neopentyl polyol ester of component ii), polyether, polyvinyl ether, perfluoropolyether, phosphoric acid ester or mixture thereof.

15. The working fluid according to claim 1 further comprising one or more antioxidant, extreme-pressure additive, antiwear additive, friction reducing additive, defoaming agent, profoaming agent, metal deactivator, acid scavenger or mixture thereof.