TECHNICAL FIELD

The present invention relates to a lubricant composition containing silicone oil and a lubricating agent containing the same.

BACKGROUND ART

Lubricants and lubricant compositions are used in order to reduce friction and wear between movable parts and between movable surfaces of various mechanical devices.

Recently, development and compactness of mechanical devices have been advanced as the environment where transportation apparatuses are used is more expanded and harsher. Due to the expansion and the even more harshness of the environment where transportation apparatuses are used owing to the development and the compactness of mechanical devices, a lubricant having a high viscosity index (VI, i.e., having a small viscosity variation to a temperature change) and a wide usable temperature range has been demanded. Lubricant having a high VI is excellent in the energy saving performance (energy-saving) because of having a low viscosity at a low temperature and becoming small in the energy loss due to viscous resistance of the lubricant itself. Besides, lubricant having a high VI is unlikely to have an excessively low viscosity under a high temperature atmosphere compared with lubricant having a low VI, and can thus secure an oil film required for lubrication on a lubrication surface. Further, since the lubricant can retain an appropriate viscosity, a splatter of the lubricant can be suppressed to thereby prevent the lubricant from contaminating surroundings.

Conventionally, as means of raising the viscosity index of a hydrocarbon-based lubricant, a high molecular compound such as polymethacrylic acid ester and polybutene is generally used as a VI improver (see Patent Literatures 1 and 2).

In recent years, a lubricant composition has been proposed which contains a silicone oil (hereinafter, referred to as “Si oil”) known as lubricant having a high VI as a lubricant base (see Patent Literatures 3 and 4).

However, a lubricant using the conventional VI improver disclosed in Patent Literature 1 has a problem of having a low resistance against a shear force, and of being incapable of maintaining the viscometric property at an initial period of use for a long period of time (i.e., of lowering the viscosity index). Besides, Patent Literature 2 indicates a possibility of increasing the shear stability by use of polymethacrylic acid ester having a specified structure. However, the problem still remains that an increase in the viscous resistance at a low temperature is inevitable due to the use of the high molecular compound, resulting in an inferior energy saving performance when used under a low temperature atmosphere.

On the other hand, the technology disclosed in Patent Literature 3 uses the silicone oil together with a mineral oil-based or an isomerized wax-based base oil aiming at achieving both the high VI and the lubricity. However, since dimethyl silicone having a poor compatibility with hydrocarbon-based lubricants is used as a silicone oil, a silicone oil having a high VI cannot be added in a large amount. Accordingly, it is necessary to use a conventional VI improver such as polymethacrylic acid ester and polybutene together with a silicone oil to secure a high VI. However, the problem still remains that although the additional amount of VI improver can be reduced compared with the conventional hydrocarbon-based lubricant, the viscosity increases at a low temperature, and viscometric property at an initial period of use cannot be maintained for a long period of time (i.e., the viscosity index lowers).

Besides, in the technology disclosed in Patent Literature 4, the high VI is maintained by using a silicone oil containing an aryl group having a high compatibility with the hydrocarbon-based lubricant to increase the additional amount of silicone oil. However, the lubricant composition added with a large amount of silicone oil containing an aryl group has a low lubricity and thus requires to increase the additional amount of ester oil as an opposite component to obtain a high lubricity. Thus, there is the problem that both the VI and the lubricity could not be satisfied.

An object of the present invention is to solve the aforementioned problems. Namely, the present invention is aimed at providing a lubricant composition that has both an excellent lubricity and a high viscosity index (VI), and can be used stably for a long period of time, and in a wide temperature range.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Patent Publication No. 2015-172165

Patent Literature 2 Japanese Patent Publication No. 2017-155193

Patent Literature 3: Japanese Patent Publication No. 2012-207082

Patent Literature 4: Japanese Patent Publication No. 2003-261892

SUMMARY OF INVENTION

The present inventors have made studies extensively to overcome the above-mentioned drawbacks, and as a result of the studies, the inventors have found that the above-mentioned object can be achieved by using a lubricant composition having a structure described below, and have completed the present invention by further making studies based on this finding.

Namely, a lubricant composition according to an aspect of the present invention contains, at least: (A) 50 to 80 mass % of silicone oil represented by a formula (1) below, and having a mass-average molecular weight of 900 to 4000, a ratio (C/Si ratio) of carbon to silicon of 3.03 or higher in the structure, and a viscosity index (VI) of 300 or higher; (B) 10 to 49 mass % of hydrocarbon-based lubricant; and (C) 1 to 10 mass % of antioxidant.


(In the formula (1), R1and R2represent an alkyl group or an aralkyl group with 1 to 12 carbons, and n represents an integer between 2 and 44.)

