Synthetic lubricating oil

This application is a U.S. national phase application under 35 U.S.C. §371 of International Application No. PCT/JP2006/322477 filed Nov. 10, 2006 which claims priority of Japanese Patent Application No. 2005-328695 filed Nov. 14, 2005 and Japanese Patent Application No. 2006-240058 filed Sep. 5, 2006.

The present invention relates to a synthetic lubricating oil comprising an ionic liquid selected from an organic cation and a bis(fluorosulfonyl)imide anion, and to a synthetic lubricating oil comprising an ionic liquid composition which is a mixture of two or more ionic liquids containing a 1,3-substituted imidazolium cation.

As a lubricating oil used in a machinery installment, a power transmission device, a metallurgical processing oil, grease and the like, base oil selected from a poly α-olefin, a diester, a polyol ester, silicone and the like as the best ingredient has so far been used under optionally mixing with a suitable additive. However, those lubricating oils are accompanied with a danger of flash ignition or evaporation under specific conditions such as high temperature and highly vacuum pressure, and thus more suitable lubricating oil has been desired. Further, in accordance with high functionality and high efficacy of a device, a lubricating oil having more excellent anti-oxidation property, anti-evaporation property and long-released excellent lubricating property has been desired.

As means for solving the above problems, it is reported in R. A. Reich et al., Journal of the Society of Tribologists and Lubrication Engineers, July 2003, p. 16 to 21, for instance, that a compound comprising a combination of an organic cation and an inorganic anion (ionic liquid, melted salt at normal temperature) can be adopted as lubricating oil, and it has recently been known that the ionic liquid is possibly usable as a material of a lubricating oil because the ionic liquid is excellent in non-volatility, stability in a wide temperature range and fire-resistance, and also it has a high viscosity index and sufficient properties of a coefficient of friction and an abrasion trace diameter which are required for lubricating oil.

However, many of the ionic liquids have generally show high viscosity, and thus it is necessary for practical use as a lubricating oil to find out an ionic liquid having low viscosity, and under the situation, further development has been continued.

And, in a Japanese Patent Publication No. 2005-89667, an ionic liquid containing bis(trifluoromethanesulfonyl)imide anion as an anion is mentioned as an ionic liquid having low viscosity. However, it has been said that the ionic liquid comprising this combination of an anion is not sufficient in reduction of viscosity.

Under the situation, the object of the present invention is to provide a synthetic lubricating oil having a stable lubricating property in a wide temperature range.

Thus, the present inventors have made extensive study under considering the above situation to find surprisingly that an ionic liquid having remarkably low viscosity and excellent viscosity stability can be obtained by using a bis(fluorosulfonyl)imide anion as an anion seed in an ionic liquid using an organic cation selected from an imidazolium cation, a pyridinium cation, a quaternary ammonium cation and a quaternary phosphonium cation and this ionic liquid can suitably be used as a synthetic lubricating oil.

Further, it is found that an ionic liquid composition having a low viscosity and excellent viscosity stability and moreover having a lower melting point compared with a single use can be obtained by mixing an ionic liquid with an ionic liquid containing an imidazolium cation having a specific alkyl group of relatively short alkyl chain and at the same time by incorporating therein two or more kind of compounds having an eutectic point, and consequently the present invention has been completed.

Namely, the present invention relates to a synthetic lubricating oil comprising an ionic liquid composition which comprises an ionic liquid (A) containing an organic cation selected from the group consisting of an imidazolium cation, a pyridinium cation, a quaternary ammonium cation and a quaternary phosphonium cation and a bis(fluorosulfonyl)imide anion and an ionic liquid (B1) containing a 1-ethyl-3-methylimidazolium cation, and an ionic liquid (B2) containing a 1-methyl-3-propylimidazolium cation and/or a 1-methyl-3-isopropylimidazolium cation.

The ionic liquid and the ionic liquid composition used in the present invention are useful as lubricating oil which is excellent in anti-abrasion property and further has stable flowability, and viscosity characteristics required for lubricating oil, and still further has stable lubricating properties in a wide temperature range and a wide using condition.

In the following, the present invention is explained in details.

In the following, the ionic liquid of the present invention means an ionic substance which is in melted state at normal temperature (25° C.).

The present invention relates to a synthetic lubricating oil comprising an ionic liquid composition which comprises an organic cation selected from the group consisting of an imidazolium cation, a pyridinium cation, a quaternary ammonium cation and a quaternary phosphonium cation and a bis(fluorosulfonyl)imide anion and to a synthetic lubricating oil comprising an ionic liquid (A) containing a 1-ethyl-3-methylimidazolium cation, and an ionic liquid (B1) containing a 1-methyl-3-propylimidazolium cation and/or an ionic liquid (B2) containing a 1-methyl-3-isopropylimidazolium cation.