Hydraulic fluid and hydraulic system

1. A hydraulic fluid comprising an ester as a base oil, the ester having two or more ring structures, wherein the two or more ring structures are at least one selected from the group consisting of an aromatic ring and a saturated naphthenic ring, and wherein the ester is at least one compound selected from the group consisting of a compound comprising an aromatic ester skeleton of formula (1)

wherein n is 0 or 1, m is 0, p and q each are an integer of 0 to 3, and X and Y each represent an alkyl group that may include a cycloalkyl group or an aromatic group having 1 to 30 carbon atoms, a cycloalkyl group or an aromatic group having 5 to 12 carbon atoms, an alkyloxycarbonyl group that may include a cycloalkyl group or an aromatic group having 2 to 30 carbon atoms, or an alkylcarbonyloxy group that may include a cycloalkyl group or an aromatic group having 2 to 30 carbon atoms;

a compound comprising an aromatic ester skeleton of formula (3)

wherein n and m each are 0 or 1, p and q each are an integer of 0 to 3, R 1 and R 2 each represent hydrogen or an alkyl group having 1 to 10 carbon atoms, and A represents an alkylene group having 2 to 18 carbon atoms that may contain oxygen in a main chain and may contain a side chain; and

a compound comprising an aromatic ester skeleton of formula (4)

wherein j and k each are 0 or 1, n and m each are 1 or 2, p and q each are an integer of 0 to 3, R 1 and R 2 represent hydrogen or an alkyl group having 1 to 10 carbon atoms, and Z represents an alkylene group having 1 to 18 carbon atoms that may include a side chain.

2. The hydraulic fluid according to claim 1 , wherein the ester is the compound comprising an aromatic ester skeleton of formula (1) and said compound comprising an aromatic ester skeleton of formula (1) is a compound comprising a phenyl benzoate skeleton structure of formula (2)

wherein p and q each are an integer of 0 to 3; and X and Y represent an alkyl group that may include a cycloalkyl group or an aromatic group having 1 to 30 carbon atoms, a cycloalkyl group or an aromatic group having 5 to 12 carbon atoms, an alkyloxycarbonyl group that may include a cycloalkyl group or an aromatic group having 2 to 30 carbon atoms, or an alkylcarbonyloxy group that may include a cycloalkyl group or an aromatic group having 2 to 30 carbon atoms.

3. The hydraulic fluid according to claim 1 , wherein the hydraulic fluid contains 10 mass % or more of the ester as the base oil.

4. The hydraulic fluid according to claim 1 , wherein the base oil has:

(a) a kinematic viscosity (40 degrees C.) of from 15 to 100 mm 2 /s;

(b) a pour point of −10 degrees C. or less;

(c) a density (15 degrees C.) of 1.0 g/ml or more;

(d) a tangential bulk modulus (K value) at 40 degrees C. and 50 MPa of 1.65 GPa or more; and

(e) a flash point of 200 degrees C. or more.

5. A hydraulic system comprising the hydraulic fluid according to claim 1 .

6. The hydraulic fluid according to claim 1 , wherein the ester is the compound comprising an aromatic ester skeleton of formula (1).

7. The hydraulic fluid according to claim 1 , wherein the ester is the compound comprising an aromatic ester skeleton of formula (3).

8. The hydraulic fluid according to claim 1 , wherein the ester is the compound comprising an aromatic ester skeleton of formula (4).

9. The hydraulic fluid according to claim 1 , further comprising one or more additives selected from the group consisting of a viscosity index improver, an antioxidant, a detergent dispersant, a friction modifier, a metal deactivator, a pour point depressant, an antiwear agent, an antifoaming agent, and an extreme pressure agent.

10. The hydraulic fluid according to claim 3 , wherein the hydraulic fluid comprises up to 90 mass % of one or more base oils other than said ester.