JP7645059B2 - Lubricating oil composition for transmission - Google Patents

Description

The present invention relates to a lubricating oil composition suitable for use in transmissions, primarily automobile transmissions, and particularly manual transmissions.

Conventionally, when considering a lubricating oil composition for an automobile transmission, it has been extremely difficult to satisfy all of the requirements for that application, namely improved fuel economy, load resistance, and good low-temperature viscosity characteristics.
In view of the above, the applicant has discovered that a GTL low-viscosity base oil containing a predetermined amount of Group 1 high-viscosity base oil, preferably containing no viscosity index improver, has the desired physical properties as a transmission oil, and has provided this (Patent Document 1).

The viscosity index of the above-mentioned lubricating oil composition depends largely on the base oil to be blended. In particular, in environments where the oil temperature is kept relatively low, such as when driving in urban areas with repeated starts and stops as seen in recent years, the viscosity becomes high, and further improvement in fuel economy is also desired.
If a viscosity index improver is simply added to remedy this, it can cause a decrease in oxidation stability against heat and a decrease in viscosity due to shear, which can lead to early deterioration of the lubricating oil and trouble with the transmission.

Patent No. 6284865

The present invention is an improvement over the conventional lubricating oils described above, and aims to obtain a lubricating oil that has a low viscosity, a high viscosity index, excellent viscosity characteristics at low temperatures, and high stability at high temperatures, and can be used suitably over a wide temperature range as a lubricating oil composition for transmissions, has load resistance, shear stability, and oxidation stability, is excellent in high-temperature detergency, and can also improve fuel economy.

The present invention relates to a lubricating oil composition for transmissions which contains a base oil and a poly(meth)acrylate and an olefin copolymer as a viscosity modifier, the ratio of the poly(meth)acrylate and the olefin copolymer in the viscosity modifier being 100:0 to 20:80 in mass %, and which has a kinetic viscosity at 100°C of 5.5 ^mm2 /s or less, a viscosity index of 160 or more, a decrease in the 100°C kinetic viscosity after a KRL shear stability test (60°C, 20 hr) of 2% or less, and a deposit amount in a panel coking test of 120 mg or less.

As the base oil, a low-viscosity base oil having a 100° C. kinetic viscosity of 1 to 5 mm ² /s or less can be used, and as such a low-viscosity base oil, a GTL (gas-to-liquid) base oil can be suitably used.
The poly(meth)acrylate in the viscosity modifier may be a dispersion type poly(meth)acrylate or a non-dispersion type 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. It also has a low evaporation rate at high temperatures, and can be made into a lubricating oil composition with good oxidation stability while maintaining friction characteristics. Even during high-temperature oxidation, the variation range of the kinetic viscosity and viscosity index is small, and it has a variety of well-balanced functions such as a power transmission medium, lubrication of gears, a heat transfer medium, and maintenance of constant friction characteristics. Therefore, it can be suitably used as a lubricating oil composition for transmissions, as it has excellent fuel efficiency and excellent durability that allows it to be used for a long time under the same conditions at any time.

Furthermore, when the oil is used as a lubricant for a manual transmission, it can effectively prevent deposits from becoming caught in the synchronizer ring of the manual transmission, thereby preventing poor synchronization.
Furthermore, this lubricant composition can be widely and effectively used as an industrial lubricant oil, such as automotive gear oil, AT oil, MT oil, CVT oil and other transmission oils, industrial gear oil, hydraulic oil, compressor oil and other industrial lubricants.

As the base oil, mineral oil, synthetic oil, etc. are used. One such base oil that is preferably used is GTL (Gas to Liquid) base oil, which is synthesized by the Fischer-Tropsch process, a technology for converting natural gas into liquid fuel. Compared to mineral oil base oils refined from crude oil, this GTL base oil has an extremely low sulfur and aromatic content and an extremely high paraffin composition ratio, so it has excellent oxidation stability, a high flash point, and very little evaporation loss, making it suitable for use as the base oil of the present invention.

Such base oils preferably have a kinetic viscosity of 5 mm ² /s or less, and preferably 1 mm ² /s or more, at 100° C. If the kinetic viscosity is less than 1 mm ² /s, evaporation becomes significant, and there is a concern that a sufficient oil film cannot be secured, whereas if it exceeds 5 mm ² /s, the viscosity at low temperatures becomes high, and there is a possibility that stirring resistance increases.

The above GTL base oils typically have a total sulfur content of less than 1 ppm and a total nitrogen content of less than 1 ppm. In addition, the aniline point of these base oils is 90°C or higher and 110°C or lower, more preferably 95°C or higher and 107°C or lower, and the refractive index is 1.42 or higher and 1.46 or lower, more preferably 1.43 or higher and 1.45 or lower. One example of such a GTL low-viscosity base oil is Shell GTL.

The above GTL base oil should be used in an amount of 75-85% by mass, preferably 80-84% by mass. If it is used in an amount of less than 75% by mass, problems may occur with properties such as low-temperature fluidity, and the desired effects may not be obtained.

A viscosity modifier is used with the base oil to adjust its viscosity.
Viscosity modifiers include poly(meth)acrylates (PMA) and olefin copolymers (OCP).
The viscosity modifier has a number average molecular weight of 5,000 to 30,000, preferably 5,000 to 20,000, and more preferably 5,000 to 12,000.

The viscosity modifier may be blended in an amount of about 2 to 20% by mass, preferably 5 to 15% by mass, based on the total amount of the composition.
If the blending amount is too small, the high temperature viscosity of the composition may decrease, and wear of mechanical parts may increase when used in a transmission, whereas if the blending amount is too large, the viscosity of the lubricating oil composition may increase, resulting in an inconvenience of increased friction loss.

The poly(meth)acrylates mentioned above include non-dispersed poly(meth)acrylates and dispersed poly(meth)acrylates, either of which can be used. In some cases, both types can be used in combination.

As the viscosity modifier, it is preferable to basically use poly(meth)acrylate (PMA) and olefin copolymer (OCP) in combination.
The ratio of poly(meth)acrylate to olefin copolymer (OCP) in the viscosity modifier is in the range of 100:0 to 20:80, preferably 100:0 to 25:75, and more preferably 100:0 to 30:70, by mass.
This provides an appropriate viscosity index and also provides high temperature detergency.

Such a lubricating oil composition has a kinematic viscosity at 100° C. of 5.5 mm ² /s or less, preferably 5.2 mm ² /s or less, more preferably 5.1 mm ² /s or less.
If the viscosity is higher than this, the stirring resistance increases, which affects fuel economy.
Also, the kinetic viscosity at 40° C. is set to 25 mm ² /s or less, preferably 22 mm ² /s or less.

The viscosity index must be at least 160, preferably at least 165, and more preferably at least 170. If it is lower than this, the viscosity will be high at low temperatures, increasing the resistance to stirring, and at high temperatures it will be difficult to maintain the oil film, increasing the possibility of increased wear.

Furthermore, in a KRL shear stability test measured under conditions of 60°C and 20 hours (hr), the rate of decrease in kinetic viscosity at 100°C after the test must be 2.0% or less, preferably 1.5% or less, and more preferably 1.0% or less. If this shear stability is poor, the viscosity of the composition will decrease significantly, which will affect oil film retention at high temperatures.

In addition, the amount of deposits in the panel coking test described below is also set to 120 mg or less.
If the amount of deposits increases, this indicates that the cleaning performance at high temperatures is deteriorating.

The transmission lubricating oil composition of the present invention may contain, as necessary, known additives, such as extreme pressure agents, dispersants, metal detergents, friction modifiers, antioxidants, corrosion inhibitors, rust inhibitors, demulsifiers, metal deactivators, pour point depressants, seal swelling agents, antifoaming agents, colorants, and the like, either alone or in combination.
In such cases, it is often sufficient to use commercially available transmission additive packages, either alone or in combination.

EXAMPLES The lubricating oil composition for transmissions of the present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited thereto in any way.
In order to prepare the examples and comparative examples, the following materials were prepared.

(1) Base oil (Base oil A-1): GTL (Gas-to-Liquid) base oil (Properties: Kinematic viscosity at 40°C is 9.7 ^mm2 /s, and kinematic viscosity at 100°C is 2.7 ^mm2 /s)
(Base oil A-2): GTL (Gas Liquid) base oil (Properties: Kinematic viscosity at 40°C is 5.4 ^mm2 /s, and kinematic viscosity at 100°C is 1.8 ^mm2 /s)

(2) Additives (Additive B-1): Viscosity modifier, non-dispersed poly(meth)acrylate (number average molecular weight 8400)
(Additive B-2): Viscosity modifier, dispersant type poly(meth)acrylate (number average molecular weight 12,000)
(Additive B-3): Viscosity modifier, olefin copolymer (number average molecular weight 8700)

(Additive C): Additive package (Zn-based GL-4 additive package for commercial manual transmissions)
(Additive D): Pour point depressant (Additive E-1): Antifoaming agent (commercially available silicone-based antifoaming agent)
(Additive E-2): Defoamer (commercially available silicone-based defoamer)

Example 1
To 80.64 mass% of a GTL base oil (base oil A-1), 11.5 mass% of a dispersant-type poly(meth)acrylate (additive B-2), 7.8 mass% of an additive package (additive C), and 0.06 mass% of an antifoaming agent (additive E-2) were added and thoroughly mixed to obtain the transmission lubricant composition of Example 1.
Furthermore, the ratio (mass %) of Additive B-3 in Additive B in the above composition was determined and shown in Tables 1 and 2. In Example 1, it was 0 mass %.

(Examples 2 to 6)
Using the compositions shown in Table 1 and following the procedure of Example 1, transmission lubricating oil compositions of Examples 2 to 6 were obtained.

(Comparative Examples 1 to 3)
Using the compositions shown in Table 2 and otherwise following the procedure of Example 1, lubricating oil compositions of Comparative Examples 1 to 3 were obtained.

〔test〕
In order to examine the properties and performance of the above-mentioned Examples and Comparative Examples, the following tests were carried out as appropriate.
(Kinematic Viscosity at 40°C)
The 40° C. kinematic viscosity (mm ² /s) was measured based on JIS K2283.
(Kinematic Viscosity at 100°C)
The 100° C. kinematic viscosity (mm ² /s) was measured based on JIS K2283.
(Viscosity Index)
Calculation was based on JIS K2283.
Evaluation criteria: 160 or above: Good (○)
Less than 160: Defective (x)

(KRL Shear Stability Test)
According to CEC-L-45-A-99, the material was treated at 60° C. for 20 hours, and the kinetic viscosity at 100° C. after the treatment was measured, and the decrease rate (%) of the kinetic viscosity at 100° C. after the treatment compared to before the treatment was calculated.
Evaluation criteria: The decrease in kinetic viscosity at 100°C is 2.0% or less...Good (○)
The decrease in kinetic viscosity at 100°C exceeds 2.0%: poor (x).

(Panel caulking test)
In accordance with the US Federal Test Method Standard 791-3462, a test was conducted in which a test oil with an oil temperature of 90°C was splashed by a rotating splasher onto an aluminum panel heated and set to the specified temperature of 290°C for two hours at intervals of 15 seconds rotation time and 45 seconds rest time, and the test oil's ability to inhibit deposit formation, i.e., cleanliness, was evaluated based on the increase in weight of the panel before and after the test.
The test results are shown in terms of the weight increase (mg) of the aluminum panel, and the indices for evaluating high-temperature cleanliness are as follows:
Evaluation criteria: 0 mg to 80.0 mg: Excellent (◎)
Over 80.0 mg to 120.0 mg or less: Good (○)
More than 120.0 mg ・・・Not possible (×)

(comprehensive evaluation)
The results of the above tests were taken together and an overall evaluation was made of the transmission lubricant oil composition according to the following criteria.
Particularly suitable as a lubricating oil composition for transmissions: Excellent (◎)
Suitable as a lubricating oil composition for transmissions: Good (◯)
Unsuitable as a transmission lubricant composition: Not acceptable (X)

(result)
The results of the above tests are shown in Tables 1 and 2. In the tables, the blanks for the KRL shear test results indicate that the test was omitted based on the results of the other tests.

(Discussion)
Example 1 uses A-1 GTL low viscosity base oil as the base oil and Additive B-2 as Additive B (the ratio of Additive B-3 is 0% by mass), and has appropriate values for 40°C kinematic viscosity and 100°C kinematic viscosity, and also passes the viscosity index with a score of 165. In addition, the amount of deposits in the panel coking test was small at 31.1 mg, and the overall evaluation was excellent (◎).
In Example 2, the amount of additive B-2 used was increased compared to Example 1. The viscosity index was improved to 174, and the 100°C kinematic viscosity reduction rate of the KRL shear stability was very small at 0.9%, but the amount of deposits in the panel coking test was slightly high at 109.2 mg, and the overall evaluation was slightly inferior to Example 1, being good (◯).

In Example 3, the amount of additive B was approximately the same as in Example 1, and additives B-2 and B-3 were used in combination, with the ratio of B-3 being 21 mass %.The viscosity index and KRL shear stability were good, and the amount of deposits in the panel coking test was small at 72.2 mg, so the overall evaluation was excellent (◎), just like in Example 1.
In Example 4, additive B-2 and B-3 were used in combination, with the ratio of B-3 being 51% by mass. The viscosity index and KRL shear stability were both good, and the amount of deposits in the panel coking test was slightly high at 103.2 mg, resulting in an overall evaluation of good (◯).

In Example 5, additive B-1 was used instead of additive B-2 in Example 4, and additive B-3 was used in the same ratio of 51 mass %, and the viscosity index and the values in the panel caulking test were almost the same, and the overall evaluation was good (◯). Thus, it can be seen that almost the same results can be obtained even if the dispersion type poly(meth)acrylate of B-2 used in combination with additive B-3 is replaced with the non-dispersion type poly(meth)acrylate of B-1.
In Example 6, the amount of additive B-2 used was reduced and the amount of B-3 used was increased, making the ratio of B-3 65% by mass. However, the results of the viscosity index and panel coking test were almost the same as those of Example 4 above, and the overall evaluation was also good (O).

On the other hand, Comparative Example 1 uses additives B-2 and B-3 in combination, but with a high ratio of additive B-3 at 81% by mass. The 40°C kinematic viscosity, 100°C kinematic viscosity, and viscosity index are similar to those of Example 6, but the result in the panel caulking test is a large value of 141.1 mg, and the overall evaluation results are poor (×).

In Comparative Example 2, additive B was used so that the ratio of additive B-3 was 100 mass%, and the defoamer was changed from E-2 to E-1. The viscosity index was almost the same as in each Example, and better results were obtained in the KRL shear stability than in each Example. However, the result in the panel coking test was 143 mg, which was a large value like in Comparative Example 1, and the overall evaluation was poor (×).
In addition, in Comparative Example 3, A-2 was used as the base oil, the amount of additive B-3 was increased from that in Comparative Example 2 to 100 mass%, and a small amount of additive D, a pour point depressant, and additive E-1, an antifoaming agent, were used. Although the viscosity index was significantly improved to 207, the result in the panel coking test was deteriorated to a large value of 171.8 mg, and the overall evaluation was unacceptable (×).

Claims (2)

The oil composition comprises a base oil and at least one of a poly(meth)acrylate and an olefin copolymer as a viscosity modifier, the ratio of the poly(meth)acrylate:olefin copolymer in the viscosity modifier being 100:0 to 20:80 by mass%, the oil composition has a kinetic viscosity at 100°C of 5.5 ^mm2 /s or less, a viscosity index of 171 or more, a decrease in the kinetic viscosity at 100°C after a KRL shear stability test (60°C, 20 hr) of 2% or less, and a deposit amount in a panel coking test of 120 mg or less,
The base oil is a low-viscosity base oil having a kinematic viscosity at 100°C of 1 to 5 ^mm2 /s, and contains a GTL (gas-to-liquid) base oil;
The content of the GTL base oil is 80.4 to 84% by mass,
The content of the viscosity modifier in the lubricating oil composition for transmissions is 2 to 20 mass % based on the total amount of the composition . 2. The transmission lubricating oil composition according to claim 1, wherein the poly(meth)acrylate in the viscosity modifier is a dispersant type poly(meth)acrylate and/or a non-dispersant type poly(meth)acrylate.