Patent Description:
When a lubricating oil composition is adapted for an automobile transmission, it has been very difficult to improve fuel efficiency, which is required of this application, to provide load resistance, and to satisfy all desirable low temperature viscosity characteristics. In response, the present applicant discovered that a gas-to-liquid (GTL) low-viscosity base oil containing a predetermined amount of a Group <NUM> high-viscosity base oil and preferably containing no viscosity index improver provides the physical properties desired of a transmission oil, and provided such a lubricating oil composition, as disclosed in <CIT>.

The viscosity index of such a lubricating oil composition depends largely on the base oil to be blended with the composition, and the viscosity of the lubricating oil composition increases especially in an environment where the oil temperature remains relatively low, such as driving in an urban area where starting and stopping occur repeatedly, as has become more common in recent years, and further improvement in fuel efficiency is desired. When a viscosity index improver is simply added to improve fuel efficiency, oxidation stability may decline relative to heat, viscosity may decline due to shear, and problems may occur such as early deterioration in the lubricating oil and trouble with the transmission.

<CIT> describes a transmission oil including: as low-viscosity base oils: (i) between <NUM> and <NUM> mass % of a Fischer-Tropsch synthetic low-viscosity base oil with a <NUM>° C. kinematic viscosity of between <NUM> mm2/s and <NUM> mm2/s, and between <NUM> and <NUM> mass % of other than a Fischer-Tropsch synthetic low-viscosity base oil with a <NUM>° C. kinematic viscosity of between <NUM> mm2/s and <NUM> mm2/s, and (ii) between <NUM> and <NUM> mass % of a base oil wherein the <NUM>° C. kinematic viscosity is greater that <NUM> mm2/s and no greater than <NUM> mm2/s; and (iii) between <NUM> and <NUM> mass % of an olefin polymer or co-polymer, as a high-viscosity base oil, wherein the <NUM>° C. kinematic viscosity is between <NUM> and <NUM> mm2/s, wherein: the <NUM>° C. kinematic viscosity of the composition is between <NUM> and <NUM> mm2/s, the viscosity index is no less than <NUM>, the flashpoint is no less than <NUM>° C. , and the reduction ratio in the <NUM>° C. kinematic viscosity after KRL shear stability testing, at <NUM>° C. for <NUM> hours, is no greater than <NUM>%.

An object of the present invention is to provide a lubricating oil that makes further improvements compared to the prior art, that has a low viscosity, a high viscosity index, excellent viscosity characteristics at low temperatures, and high stability at high temperatures, that can be suitably used as a lubricating oil composition in a transmission over a wide temperature range, that provides load resistance, shear stability, and oxidation stability, that has excellent high temperature cleanliness, and that can improve fuel efficiency.

The present invention provides a lubricating oil composition for a transmission, comprising: a gas-to-liquid (GTL) base oil in an amount of <NUM> to <NUM>% by mass and having a kinematic viscosity at <NUM> of <NUM> to <NUM><NUM>/s; and a poly(meth)acrylate and optionally an olefin copolymer as a viscosity modifier, wherein the ratio of the poly(meth)acrylate to the olefin copolymer in the viscosity modifier is <NUM>:<NUM> to <NUM>:<NUM> in terms of the mass percentage, the viscosity modifier has a number average molecular weight of <NUM> or more and <NUM> or less, and the viscosity modifier is blended in an amount of <NUM> to <NUM>% by mass relative to the overall mass of the composition, the kinematic viscosity at <NUM> is <NUM><NUM>/s or less, the viscosity index is <NUM> or more, the rate of decrease in the kinematic viscosity at <NUM> after a KRL shear stability test (<NUM>, <NUM> hr, according to CEC-L-A-<NUM>) is <NUM>% or less, and the amount of deposit in a panel coking test (according to US Federal Test Method Standard <NUM>-<NUM>) is <NUM> or less. The poly(meth)acrylate in the viscosity modifier may be a dispersed poly(meth)acrylate and/or a non-dispersed poly(meth)acrylate.

The lubricating oil composition of the present invention has a low viscosity, a high viscosity index, excellent viscosity characteristics at low temperatures, and good shear stability. In addition, an amount of evaporation at high temperature is low, and a lubricating oil composition with good oxidation stability while maintaining frictional characteristics can be obtained. During high temperature oxidation, the fluctuation in the kinematic viscosity and the viscosity index is low, and the lubricating oil composition provides a good balance of functions such as acting as a power transmission medium, lubricating gears and other components, acting as a heat transfer medium, and keeping friction characteristics constant. As a result, it can be used as a lubricating oil composition for a transmission that provides excellent fuel efficiency and that has durability enabling it to be used in the same state at any time over a long period of time.

When used as a lubricating oil in a manual transmission, it effectively keeps deposits from being caught in the synchronizer rings of the manual transmission and prevents synchronization failure. In addition, this versatile lubricating oil composition can be used effectively as an automobile gear oil, a transmission oil such as an AT oil, MT oil or CVT oil, or as an industrial lubricating oil such as an industrial gear oil, hydraulic fluid, or compressor oil.

The type of oil used as the base oil is a gas-to-liquid (GTL) base oil synthesized by the Fischer-Tropsch process commonly used as a natural gas liquid fuel technology. Compared to a mineral oil base oil refined from crude oil, a GTL base oil has extremely low sulfur content and aromatic content, and has an extremely high paraffin component ratio. As a result, it can be used as a base oil in the present invention having excellent oxidative stability, a high flash point, and very low evaporation loss.

The base oil has a kinematic viscosity at <NUM> of <NUM><NUM>/s or less and <NUM><NUM>/s or more. When the kinematic viscosity is less than <NUM><NUM>/s, significant evaporation occurs and a sufficient oil film may not be secured. When the kinematic viscosity exceeds <NUM><NUM>/s, the viscosity at low temperature may increase and stirring resistance may rise.

A GTL base oil typically has a total sulfur content of less than <NUM> ppm and a total nitrogen content of less than <NUM> ppm. Also, the aniline point is <NUM> or more and <NUM> or less, more preferably <NUM> or more and <NUM> or less, and the refractive index is <NUM> or more and <NUM> or less, more preferably <NUM> or more and <NUM> or less. Shell GTL is one example of a GTL low-viscosity base oil.

The GTL base oil is used in an amount of <NUM> to <NUM>% by mass, preferably <NUM> to <NUM>% by mass. When <NUM>% by mass or less is used, problems may occur due to properties such as low temperature fluidity, and the desired effect may not be obtained.

A viscosity modifier is used with the base oil to adjust its viscosity. Examples of viscosity modifiers include poly(meth)acrylates (PMA) and olefin copolymers (OCP). The viscosity modifier has a number average molecular weight of <NUM>,<NUM> or more and <NUM>,<NUM> or less, preferably <NUM>,<NUM> or more and <NUM>,<NUM> or less, and more preferably <NUM>,<NUM> or more and <NUM>,<NUM> or less.

The viscosity modifier may be blended in a range of about <NUM>% by mass to <NUM>% by mass, and preferably <NUM>% by mass to <NUM>% by mass, relative to total mass of the composition. When the amount is too low, the high-temperature viscosity of the composition decreases, and there is an increased risk of mechanical component wear when used in a transmission. When the amount is too high, the viscosity of the lubricating oil composition may increase, resulting in an increase in friction loss.

The poly(meth)acrylates mentioned above include non-dispersive poly(meth)acrylates and dispersed poly(meth)acrylates, and either type can be used. In some cases, both types may be used together.

A poly(meth)acrylate (PMA) and an olefin copolymer (OCP) may essentially be used together as the viscosity modifier. The ratio of poly(meth)acrylate to olefin copolymer (OCP) in the viscosity modifier is in the range of <NUM>:<NUM> to <NUM>:<NUM>, preferably in the range of <NUM>:<NUM> to <NUM>:<NUM>, and more preferably in the range of <NUM>:<NUM> to <NUM>:<NUM> in terms of mass percentage. When used in this way, a suitable viscosity index can be obtained, and high-temperature cleanliness can also be realized.

The lubricating oil composition should have a kinematic viscosity at <NUM> of <NUM><NUM>/s or less, preferably <NUM><NUM>/s or less, and more preferably <NUM><NUM>/s or less. If the viscosity is any higher, the stirring resistance increases and fuel efficiency is adversely affected. The kinematic viscosity at <NUM> should be <NUM><NUM>/s or less, preferably <NUM><NUM>/s or less.

The viscosity index has to be <NUM> or more, preferably <NUM> or more, and more preferably <NUM> or more. If the viscosity index is any lower, the viscosity at low temperature increases, stirring resistance rises, and it becomes difficult to maintain an oil film and prevent increased wear at high temperatures.

In a KRL shear stability test conducted at <NUM> for <NUM> hours (hr), the rate of decrease in kinematic viscosity at <NUM> after the test is <NUM>% or less, preferably <NUM>% or less, and more preferably <NUM>% or less. When the shear stability is poor, the viscosity of the composition decreases significantly, which adversely affects maintenance of an oil film at high temperatures.

The amount of deposit in the panel coking test described below should be <NUM> or less. A high amount of deposit indicates that the cleanliness at high temperature has deteriorated.

When necessary, any additive common in the art may be added to the lubricating oil composition for a transmission in the present invention. Examples include extreme pressure agents, dispersants, metal cleaners, friction modifiers, antioxidants, corrosion inhibitors, rust inhibitors, anti-emulsifiers, metal deactivators, flow point depressants, seal swelling agents, defoamers, and colorants. These types of additives can be used alone or in combination. In this case, the use of commercially available additive packages for transmissions alone or in combination is preferred.

The lubricating oil composition for a transmission in the present invention will now be described in greater detail with reference to examples and comparative examples. However, the present invention is not limited to these examples. The following materials were prepared in order to create the examples and comparative examples.

First, <NUM>% by mass of the dispersed poly(meth)acrylate in (Additive B-<NUM>), <NUM>% by mass of the additive package in (Additive C), and <NUM>% by mass of the defoamer in (Additive E-<NUM>) were added to <NUM>% by mass of the GTL base oil in (Base Oil A-<NUM>), and the components were mixed together thoroughly to obtain the lubricating oil composition for a transmission in Example <NUM>. The percentages (mass%) of Additive B-<NUM> to Additive B in the examples and in comparative examples were determined and shown in Table <NUM> and Table <NUM>, respectively. In Example <NUM>, it is <NUM>% by mass.

The lubricating oil compositions for transmissions in Examples <NUM> to <NUM> were obtained in the same manner as Example <NUM> using the compositions shown in Table <NUM>.

The lubricating oil compositions for transmissions in Comparative Examples <NUM> to <NUM> were obtained in the same manner as Example <NUM> using the compositions shown in Table <NUM>.

The following tests were conducted to determine the properties and performance of the examples and comparative examples.

The kinematic viscosity (mm<NUM>/s) at <NUM> was measured based on JIS K2283.

The viscosity index was calculated based on JIS K2283. Evaluation Criteria:.

For the KRL Shear Stability Test, processing was performed at <NUM> for <NUM> hours based on CEC-L-<NUM>-A-<NUM>. The kinematic viscosity at <NUM> was measured after the processing, and the rate of decrease (%) in the viscosity after processing at a kinematic viscosity of <NUM> relative to the value before the processing was determined.

For the Panel Coking Test, a rotating splasher splashes test oil at an oil temperature of <NUM> on an aluminum panel heated to a temperature of <NUM> for <NUM> hours at intervals of <NUM> seconds of rotation time followed by <NUM> seconds of stopping in accordance with US Federal Test Method Standard <NUM>-<NUM> to evaluate the suppression of test oil deposits based on the weight increase of the panel before and after the test, which is an indicator of cleanliness. The test results are indicated in terms of the weight increase (mg) in the aluminum panel, and the evaluation criteria indicating high-temperature cleanliness are as follows. Evaluation Criteria:.

The results from each test were combined in order to comprehensively evaluate the suitability of each lubricating oil composition as a lubricating oil composition for a transmission according to the following criteria.

Especially suitable as a lubricating oil composition for a transmission: Excellent (⊚) Suitable as a lubricating oil composition for a transmission: Good (o)
Unsuitable as a lubricating oil composition for a transmission:
: No good (×).

The results of each test are shown in Table <NUM> and Table <NUM>. When the results of the KRL shear test are left blank in the tables, it means the test was omitted due to the results of other tests.

In Example <NUM>, the GTL low-viscosity base oil in Base Oil A-<NUM> was used as the base oil, and Additive B-<NUM> was used as Additive B (the ratio of Additive B-<NUM> was <NUM>% by mass). Appropriate values were obtained for the <NUM> kinematic viscosity and the <NUM> kinematic viscosity, and the viscosity index at <NUM> met the appropriate criteria. In the panel coking test, the amount of deposit was low at <NUM>, and an overall assessment of excellent (⊚) was obtained. In Example <NUM>, the amount of Additive B-<NUM> used was greater than the amount in Example <NUM>. The viscosity index improved to <NUM>, and the rate of decrease in the <NUM> kinematic viscosity after the KRL shear stability test was very low <NUM>%. However, the amount of deposit in the panel coking test was a slightly higher <NUM>, so the overall assessment was somewhat lower than that of Example <NUM> at good (o).

In Example <NUM>, the amount of Additive B used was nearly the same as Example <NUM>, but Additives B-<NUM> and B-<NUM> were used in combination, and the ratio of B-<NUM> was <NUM>% by mass. Because the viscosity index and KRL shear stability were good, and the amount of deposit in the panel coking test was low at <NUM>, an overall assessment of excellent (⊚) was obtained, which is the same as that of Example <NUM>. In Example <NUM>, Additives B-<NUM> and B-<NUM> were used in combination as Additive B, and the ratio of B-<NUM> was <NUM>% by mass. Because the viscosity index and KRL shear stability were good, and the amount of deposit in the panel coking test was somewhat higher at <NUM>, the overall assessment was good (o).

In Example <NUM>, Additive B-<NUM> was used instead of Additive B-<NUM> as in Example <NUM>, and Additive B-<NUM> was used in combination to reach the same percentage of <NUM>% by mass. The viscosity index and results of the KRL shear stability test were about the same, so the overall assessment was good (o). As indicated here, nearly the same result can be obtained by replacing the dispersed poly(meth)acrylate in Additive B-<NUM> used in combination with Additive B-<NUM> with the non-dispersed poly(meth)acrylate in Additive B-<NUM>. In Example <NUM>, the amount of Additive B-<NUM> was reduced and the amount of Additive B-<NUM> was increased to reach a percentage of <NUM>% by mass for Additive B-<NUM>. The viscosity index and results of the KRL shear stability test were about the same as Example <NUM>, so the overall assessment was good (o).

In Comparative Example <NUM>, Additive B-<NUM> and Additive B-<NUM> were used in combination but the percentage of Additive B-<NUM> was higher at <NUM>% by mass. Values similar to those in Example <NUM> were obtained for the <NUM> kinematic viscosity, the <NUM> kinematic viscosity, and the viscosity index. However, the result from the panel coking test was high at <NUM>, so the overall assessment was no good (×).

In Comparative Example <NUM>, the ratio of Additive B-<NUM> was <NUM>% by mass for Additive B, and the defoamer was changed from Additive E-<NUM> to Additive E-<NUM>. The viscosity index was nearly similar to those of the examples of the present invention, and the KRL shear stability was better than those of the examples of the present invention. However, the result from the panel coking test was as high as Comparative Example <NUM> at <NUM>, so the overall assessment was no good (×). In Comparative Example <NUM>, Additive A-<NUM> was used as the base oil, the amount of Additive B-<NUM> was increased compared to Comparative Example <NUM> to a percentage of <NUM>% by mass. Small amounts of the pour point lowering agent in Additive D and the defoaming agent in Additive E-<NUM> were also used. Although the viscosity index improved significantly at <NUM>, the result in the panel coking test rose to <NUM>, which indicates deterioration, so the overall assessment was no good (×).

Claim 1:
A lubricating oil composition for a transmission, comprising:
a gas-to-liquid (GTL) base oil in an amount of <NUM> to <NUM>% by mass and having a kinematic viscosity at <NUM> of <NUM> to <NUM><NUM>/s; and
a poly(meth)acrylate and optionally an olefin copolymer as a viscosity modifier, wherein the ratio of the poly(meth)acrylate to the olefin copolymer in the viscosity modifier is <NUM>:<NUM> to <NUM>:<NUM> in terms of the mass percentage, the viscosity modifier has a number average molecular weight of <NUM> or more and <NUM> or less, and the viscosity modifier is blended in an amount of <NUM> to <NUM>% by mass relative to the overall mass of the composition,
the kinematic viscosity at <NUM> is <NUM><NUM>/s or less, the viscosity index is <NUM> or more, the rate of decrease in the kinematic viscosity at <NUM> after a KRL shear stability test (<NUM>, <NUM> hr, according to CEC-L-A-<NUM>) is <NUM>% or less, and the amount of deposit in a panel coking test (according to US Federal Test Method Standard <NUM>-<NUM>) is <NUM> or less.