| 摘要 |
PROBLEM TO BE SOLVED: To obtain a hydraulic fluid composition excellent in oxidation stability, lubricity and effect on preventing sludge formation by compounding a base oil comprising a mineral oil and/or a synthetic oil with a phenolic antioxidant, an aminic antioxidant, a phosphorus compound and a dispersion-type viscosity index improver and substantially preventing zinc dithiophosphate from being present in the composition. SOLUTION: A base oil comprising a mineral oil and/or a synthetic oil and having kinematic viscosity (40 deg.C) of 10-10,000 mm2/s, a viscosity index of 80-500 and a flow point of -5 deg.C or lower is compounded with 0.01-3 mass %, preferably 0.1-2 mass %, phenolic antioxidant, 0.01-3 mass %, preferably 0.1-2 mass %, aminic antioxidant, 0.01-10 mass %, preferably 0.05-3 mass %, phosphorus compound and 0.01-3 mass %, preferably 0.05-2 mass %, dispersion-type viscosity index improver having a number average mol.wt. of 1,000-1,500,000, pref. 10,000-200,000. When present in an amount of 0.0001 mass % or higher in the composition, zinc dithiophosphate itself is highly liable to become sludge. |
