Patent ID: 8299004

Claim:
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.