Use of polyesters as viscosity index improvers for aircraft hydraulic fluids

1. An aircraft hydraulic fluid composition comprising from 5.0 wt% to 20 wt%, based on the weight of the aircraft hydraulic fluid composition, of a polyester and not less than 50 wt.%, based on the weight of the aircraft hydraulic fluid composition, of a phosphate ester base stock,

wherein

the polyester has a weight average molecular weight Mw from 10,000 g/mol to 40,000 g/mol and consists of 50 mol%, based on the total number of repeating units constituting the polyester, of repeating units of Formula I

and

50 mol%, based on the total number of repeating units constituting the polyester, of repeating units of Formula II

wherein R 1 is a methyl group,

R 2 is a methyl group; and

wherein the phosphate ester base stock substantially consists of tributylphosphate, triisobutylphosphate or a mixture thereof,

and wherein the hydraulic fluid composition contains less than 5 wt % of compounds selected from the group consisting of triaryl phosphates, diarylalkyl phosphates, and aryldialkyl phosphates, based on the total weight of the aircraft hydraulic fluid composition,

wherein the aircraft hydraulic fluid composition has a pour point of below −30° C.

2. The aircraft hydraulic fluid composition according to claim 1 , wherein the aircraft hydraulic fluid has a pour point of below −40° C.

3. The aircraft hydraulic fluid composition according to claim 1 , wherein the weight average molecular weight Mw of the polyester is from 10,000 g/mol to 30,000 g/mol determined according to DIN 55672-1.

4. The aircraft hydraulic fluid composition according to claim 1 , wherein the aircraft hydraulic fluid composition has a pour point of below −50° C.

5. The aircraft hydraulic fluid composition according to claim 1 , wherein the polyester has an acid value from 0.01 to 3.0 mg KOH/g and a hydroxyl value from 5.0 to 20.0 mg KOH/g.

6. The aircraft hydraulic fluid composition according to claim 1 , wherein the polyester has a polydispersity index Mw/Mn from 2.0 to 5.0.

7. The aircraft hydraulic fluid composition according to claim 1 , wherein the polyester comprises end groups and more than 80 mol% based on the total number of end groups in the polyester, are hydroxyl groups.

8. The aircraft hydraulic fluid composition according to claim 1 , wherein the polyester is an amorphous polymer and has a glass transition temperature Tg below −35 ° C.

9. The aircraft hydraulic fluid composition according to claim 1 , wherein the aircraft hydraulic fluid composition substantially consists of the polyester and the phosphate ester base stock.

10. The aircraft hydraulic fluid composition according to claim 1 , wherein the aircraft hydraulic liquid composition substantially consists of the polyester, the phosphate ester base stock and at least one further additive selected from corrosion inhibitors, antifoam agents, acid scavengers and dyes.

11. The aircraft hydraulic fluid composition according to claim 1 , wherein the aircraft hydraulic fluid composition has a kinematic viscosity at 100° C. (KV100) of from 2.0 to 5.0 cSt and a kinematic viscosity at −54° C. (KV-54) of not higher than 2,000 cSt.

12. The aircraft hydraulic fluid composition of the composition according to claim 1 , wherein the aircraft hydraulic fluid composition has a viscosity index (VI) from 200 to 400.

13. The aircraft hydraulic fluid composition according to claim 1 , wherein the polyester has a polydispersity index Mw/Mn from 2.5 to 4.5.

14. The aircraft hydraulic fluid composition according to claim 1 , wherein the polyester comprises end groups and more than 90 mol% based on the total number of end groups in the polyester, are hydroxyl groups.

15. The aircraft hydraulic fluid composition according to claim 1 , wherein the weight average molecular weight Mw of the polyester is from 20,000 g/mol to 30,000 g/mol determined according to DIN 55672-1.