Technical Field of the Invention

The present invention relates to lubricating oil compositions for use in wet brakes having excellent wet brake friction characteristics and wear resistance as well as oxidative stability and so being suitable as a general-purpose lubricating oil for agricultural tractors or construction machinery vehicles. Background of the Invention

The wet brake systems of agricultural tractors require a braking capability that corresponds to the applied pedal force, but at the same time there is a requirement for preventing abnormal noise or abnormal vibrations due to sliding of parts under friction. Specifically, it is preferable if the static friction coefficient is higher for braking performance, but squealing and abnormal vibrations due to the anti-stick- slip phenomenon are apt to occur just before stopping. Therefore, if we assume friction characteristics such that the dynamic friction coefficient is raised at the point where the sliding velocity is high, as when braking is first applied, and the static friction coefficient is low at the point where the velocity is low, as when the brake is stopping, excellent brake stopping capability and anti-slip-stick properties can both be achieved together. If the friction characteristics are categorised like this, in making excellent brake stopping capability compatible with antistick-slip properties, if the dynamic friction coefficient (μd) is high and the static friction coefficient (μs) is low, the value of μs/ μd becomes small. In contrast, if the brake stopping capability and the antistick-slip properties deteriorate, the dynamic friction coefficient is low and the static friction coefficient becomes high, so that the μs/μd value becomes large, and so it is possible to encompass the characteristics by obtaining the size of this ratio. In order to impart the friction characteristics necessary for a lubricating oil as used in wet clutches and wet brakes, it is usual to blend in a friction modifier (FM) , Fatty acids, ester-based compounds, alcohol-based compounds, amine-based compounds, amide- based compounds, phosphorus-based compounds, sulphur- based compounds and solid lubricants are used for such friction modifiers. Friction modifiers form adsorption films by adsorbing onto the surfaces of the brake material and clutch material. The friction resistance and peel strength of these films become the frictional force, but if the adsorption force is large, this frictional force becomes large. On the other hand, if the adsorption force is small, peeling will occur readily and as a result the frictional force will become small. How much this adsorption force varies in the case where the sliding velocity is large and the case where it is small determines the wet friction characteristics, and that comes back to the relationship between the dynamic friction coefficient and the static friction coefficient mentioned above.

Fatty acids have excellent adsorptive properties, but are inferior as regards thermal and oxidative stability, and also have poor water separation. Blends of ester-based compounds, alcohol-based compounds and amide- based compounds instead have different lipophilic properties and adsorption strengths because of the structures of the individual compounds. In other words, they each have their good points and bad points. With ester-based compounds hydrolysis at times of moisture ingress is a problem, and with amine-based compounds lacquer formation is apt to occur at high temperatures. Alcohol-based compounds have weak lipophilic properties as an FM.

As examples relating to lubricating oil compositions for use in wet brakes, mention may be made of Japanese

Laid-open Patent H6-200269 (1994} and Japanese Laid-open Patent 2004-204002. The former discloses acidic phosphate ester amine salts, oleic acids, oleic acid diethanolamides, oleic acid triglycerides or stearyl alcohols as friction modifiers, and the latter discloses ethers of higher alcohols with 8 or more carbons and glycerol, and phosphate ester compounds. Also, Japanese Patent Specification 4132280 discloses combinations of diols and phosphates as friction modifiers. The aim of these inventions is to offer lubricating oil compositions which have friction characteristics such that, as mentioned above, the dynamic friction coefficient is raised at points when the sliding velocity is high, as when the brake is first applied, and the static friction coefficient is low at points when the velocity is low, as when the brake is stopping, and which allows both effective brake stopping capability and antistick-slip properties together in, for example, the wet brake systems of large vehicles such as tractors used in agriculture.

Summary of the Invention

According to the present invention there is provided a lubricating oil composition for use in wet brakes comprising a base oil selected from mineral oils and synthetic oils, and mixtures thereof, and from 0.01 to 0.5% by mass, relative to the total amount of the lubricating oil composition, of a diol compound (A) represented by Formula 1

C _n H _2n+2 O ₂ or C _n H _2n O ₂ (Formula 1} wherein the number of carbons n is from 10 to 24.

According to the second aspect of the present invention there is provided a lubricating oil composition for use in wet brakes comprising a base oil selected from mineral oils and synthetic oils, and mixtures thereof, and from 0.01 to 0.3% by mass, relative to the total amount of the lubricating oil composition of a diol compound (A) represented by Formula 1

C _n H _2n+2 O ₂ or C _n H _2n O ₂ (Formula 1) wherein the number of carbons n is from 10 to 24; and from 0.05 to 5% by mass, relative to the total amount of the lubricating oil composition, of a phosphorus compound (B) represented by Formula 2

(RO) ₂ PH - 0 (Formula 2) wherein R is an alkyl group or an alkenyl group with from 14 to 18 carbons; the μs/μd (μ3/μ38) ratio at 40 ⁰ C in a microclutch test being not more than 0.8.

According to a third aspect of the present invention there is provided a composition wherein a lubricating oil composition for use in wet brakes in accordance with the present invention is further blended with at least one of a metallic detergent-dispersant, ashless dispersant, zinc dialkyl dithiophosphate, hindered phenol-based antioxidant, amine-based anti-oxidant, sarcosinic acid-based corrosion inhibitor, polymethacrylate-based viscosity index improver or silicone-based defoamer. The present invention offers lubricating oil compositions which improve the brake stopping capability as well as the antistick-slip properties and so are effective as a lubricant for use in the wet brake systems used, for example, in large vehicles such as tractors used in agriculture.

The present invention involves an improvement to the prior art as described above. By dint of intensive and repeated investigations to overcome the defects of alcohol-based compounds as friction modifiers, we have discovered that, by combining friction modifiers which are alcohol-based compounds having a special structure preferably together with a specified phosphate ester, and preferably limiting the ratio of the static friction coefficient at 3 mm/s and the dynamic friction coefficient at 38 mm/s in a microclutch test, that is the μs/μd ratio at 40 ⁰ C, to not more than 0.8 and more preferably 0.5 to 0.8, but even more preferably 0.6 to 0.8, it not only has braking capability but also excellent antistick-slip properties. We have thus arrived at the discovery of lubricating oil compositions for use in wet brakes that take advantage of this. Detailed Description of the Invention

Specifying the compositions of these inventions in more detail, one preferred form of the present invention is to take a lubricating oil base oil selected from mineral-type lubricating oils or refined products thereof (referred to below as "mineral oil") preferably having a viscosity at 100°C of approximately 1 to 50 cSt and synthetic lubricating oil bases preferably having a viscosity at 100 ⁰ C of approximately 1 to 50 cSt, and to blend 0.01 to 0.5% by mass, but preferably approximately 0.05 to 0.3% by mass, of an alcohol component (A) represented by Formula (1} in the mineral oil or hydrocarbon-based synthetic oil. If the proportion is less than this, the basic effect will not be displayed. On the other hand, if it is more than this, problems may arise with the solubility in oil. Also, it is possible to increase the amount of additive further by adding a small amount of an ester in order to increase the solubility. Further, in applications such as greases, it is possible also to increase the amount of additive because there is no need to dissolve it completely in the base oil.

C _n H ₂ n _÷ 2θ ₂ or C _n H _2n O2 Formula 1 In Formula I the number of carbon atoms n is in the range of from 10 to 24.

The aforementioned aliphatic alcohol component, in particular, is a diol in which the number of carbon atoms is in the range of from 8 to 18, though more preferably in the range of from 12 to 16, and which has two OH groups. More preferably, the OH groups make an aliphatic 1,2-diol, that is they are bonded at the first and second positions of the aliphatic carbon. It is immaterial whether the aliphatic portion is an alkyl group or an alkenyl group.

The following compounds may be mentioned as representative compounds .

As examples with OH groups bonded in the first and second positions mention may be made of 1, 2-decanediol, 1,2-undecanediol, 1, 2-dodecanediol, 1, 2-tridecanediol, 1,2-tetradecanediol, 1, 2-pentadecanediol, 1,2- hexadecanediol, 1, 2-heptadecanediol, 1, 2™octadecanediol, 1, 2-~nonadecanediol, 1, 2-eicosanediol, 1,2- heneicosanediol, 1,2-docosanediol, 1, 2-tricosanediol and 1, 2-tetracosanediol . The phosphorus compound (B) represented by Formula 2 is preferably included in an amount of from 0.05 to 5% by mass, relative to the total amount of the composition.

(RO) ₂ PH = 0 (Formula 2)

In Formula 2, R is an alkyl group or an alkenyl group with from 12 to 18 carbon atoms.

Especially preferred are dialkyl hydrogen phosphites or dialkenyl hydrogen phosphites. A characteristic of these hydrogen phosphites is that, because they have no acidity, compared to acidic phosphate esters they do not impair the oxidative stability of the lubricating oil. Preferred examples are diesters such as di-n-dodecyl hydrogen phosphite (dilauryl hydrogen phosphite) , di-n™ tetradecyl hydrogen phosphite (dimyristyl hydrogen phosphite) , di-n-hexadecyl hydrogen phosphite (dipalmityl hydrogen phosphite) , di-n-octadecyl hydrogen phosphite (distearyl hydrogen phosphite) , di^9-~ octadecenyl hydrogen phosphite (dioleyl hydrogen phosphite) and di-n-eicosenyl hydrogen phosphite, or n- dodecyl hydrogen phosphite (lauryl hydrogen phosphite) , n-hexadecyl hydrogen phosphite (palmityl hydrogen phosphite) , n-octadecyl hydrogen phosphite (stearyl hydrogen phosphite) , 9-octadecenyl hydrogen phosphite (oleyl hydrogen phosphite) and n-eicosenyl hydrogen phosphite. Dioleyl hydrogen phosphite is most preferred. If the amount of either of the aforementioned components (A) and (B) in the blend is too small, the additive effect will appear inadequate. If it is too large, the improvement in effect may not be as expected yet there will be the drawback of increased costs. It has been discovered that performance is improved synergistically by using these components in combination rather than alone. Apart from the aforementioned components (A) and (B) , known additives in common use, such as metallic detergent-dispersants, ashless dispersants, seizure and wear inhibitors, anti-oxidants, rust inhibitors, viscosity index improvers, pour-point depressants and defoarners, are added to the lubricating oil compositions of these inventions as required by purpose.

Typical examples of the mineral oil-type base oils of these inventions are refined oils produced by one or more treatments such as solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing or hydrorefining of a lubricating oil fraction obtained by vacuum distillation of an atmospheric residuum obtained by atmospheric distillation of a crude oil, or wax isomerised mineral oils and lubricating oil base oils produced by a method in which a GTL wax (gas-to-liquid wax) prepared by a Fischer-Tropsch process is isomerised.

Also, as examples of synthetic oils, mention may be made of poly-α-olefins (PAO) , α-olefin copolymers, polybutenes, alkylbenzenes, polyol esters, dibasic acid esters, polyoxyalkylene glycols, polyoxyalkylene glycol esters, polyoxyalkylene glycol ethers, hindered esters and silicone oils. As mentioned above, these synthetic oils may each be used alone or in mixtures of two or more, and in addition mineral oils and synthetic oils may be used in combination.

As examples, for metallic detergent-dispersants mention may be made of alkaline earth metal sulphonates and alkaline earth metal phenates, for ashless dispersants mention may be made of alkenyl succinimides, alkenyl succinate esters and amides of long-chain fatty acids and amines (aminomides) , for seizure and wear inhibitors mention may be made of sulphurised oils and fats, sulphurised olefins, sulphides, phosphate esters, phosphite esters and thiophosphate esters, for antioxidants mention may be made of amine-based and phenol- based anti-oxidants, for rust inhibitors mention may be made of benzotriazoles and alkenyl succinate esters, for viscosity index improvers mention may be made of polymethacrylates and olefin copolymers, for pour-point depressants mention may be made of polymethacrylates, and for defoamers mention may be made of silicone compounds and ester-based defoamers. Examples _^

As a means of evaluating the wet friction characteristics, measurements were carried out on the basis of the Test Method for Friction Characteristics (JCMAS P 047 4) of the Hydraulic Fluids for Construction Machinery (JCMAS P 047:2004) of the Japan Construction Machinery Association.

The details of the method for the microclutch tests are shown below. Test specimen material:

Clutch disc facing material: D-0512 (JASO M349 material)

Plate material: SS400

Test conditions: Temperatures: 40 and 80 ⁰ C

Surface pressure: 1 Mpa Sliding velocities: 3.0 and 38.0 mm/s Friction duration: 5 min

The present invention is explained below by means of examples and comparative examples under the aforementioned microclutch test conditions, but these are only representative examples and the Invention is in no way limited by them.

Examples 1 to 7 and Comparative Examples 1 to 5

For the examples and comparative examples, to the commercial tractor oil of Comparative Example 1 (additives package containing overbased calcium sulphonate, zinc di-2-ethylhexyldithiophosphate, tripheπyl phosphite, polybutenyl succinimide and succinate ester) was added 0.2% by mass of alcohol compounds 1, 2 and 3 respectively for Examples 1, 2 and 3, the phosphorus compound being further combined with Example 2 to give Example 4.

To Comparative Example 1 was added 0.2% by mass of alcohol compounds 4, 5, 6 and 7 respectively for Comparative Examples 2, 3, 4 and 5.

To Comparative Example 1 was added 0.1% by mass and 0.3% by mass of alcohol compound 2 respectively to give Examples 5 and 6, 0.2% by mass of the phosphorus compound being further combined with Example 6 to give Example 7. The constituent components and characteristics of

Comparative Example 1 are given in Table 1, and the blends of the examples and comparative examples are given in Tables 2 and 3.

Alcohol compounds (A) and phosphorus compound (B) used in the Examples and Comparative Examples:

Alcohol compound 1 : 1, 2-tetra/hexadecanediol mixture

Alcohol compound 2 1 , 2-hexadecanediol Alcohol compound 3 1, 2-dodecanediol Alcohol compound 4 1-hexadecanol

Alcohol compound 5 1-dodecanol Alcohol compound 6 1-octanol Alcohol compound 7 1, 2-octanediol Phosphorus compound : Dioleyl hydrogen phosphite

The characteristics and constituent components of the commercial tractor oil of Comparative Example 1 are shown in Table 1 below. Table 1

The commercial tractor oil of Comparative Example 1 is an oil prepared by adding 6% by mass of a package of additives containing overbased calcium sulphonate, zinc di-2-ethylhexyldithiophosphate, triphenyl phosphite, polybutenyl succinimide and a succinate ester, and 6% by mass of a polymethacrylate-based viscosity index improver to a base oil (mineral oil based, kinetic viscosity 5.95 mmVs @ 100°C and 37.5 mτn ² /s @ 40 ⁰ C) . Also added were, as a percentage on top, 0.1% by mass of a polymethacrylate- based pour-point depressant, and 10 ppm of a silicone- based defoamer as a foaming inhibitor.

The compositions of Examples 1 to 7 are shown in Table 2 below. Table 2

The compositions of Comparative Examples 1 to 5 are shown in Table 3 below. Table 3

The friction coefficients were measured in the micrσclutch tests described above for the Examples and Comparative Examples at sliding velocities of 3 mm/s and 38 mm/s at oil temperatures of 40 ⁰ C and 80 ⁰ C. The results are shown in Tables 4 and 5. Table 4

Table 5

Table 6

Looking at the results of the tests at 4O ⁰ C, Comparative Example 1 was the commercial tractor oil, and the friction coefficient μs (3 πvm/s) was 0.108 whereas μd (38 miti/s) was 0.117, so that the μs/μd ratio was 0.923. But when the C14 and C16 diol mixture of Example 1 was added, μs was 0.082 and μd was 0.113, so that the μs/μd ratio was low at 0.726. This shows that the static friction is lower and anti stick-slip in a wet clutch is constrained. Also, in the case of the C16 1,2- hexadecanediol, the μs/μd ratio was 0.741 and in the case of the C12 dodecanediol it was 0.739, low in both cases and imparting excellent friction characteristics, but in the case of the monoalcohol C16 1-hexadecanediol as in Comparative Example 2 the μs/μd ratio was 0.862, in the case of 1-dodecanol it was 0.802 and in the case of 1,2- octanediol it was 0.808, μs/μd ratios lower than the commercial tractor oil of Comparative Example 1, but the μs/μd ratios were high compared with the examples according to the invention. The superior friction characteristics of the diols of the examples according to the invention were thus shown. Similarly, examining the results of the tests at

80 ⁰ C, the μs/μd ratios of Examples 1, 2 and 3 were, respectively, 0.694, 0.692 and 0.692, whereas the μs/μd ratios of Comparative Examples 1, 2, 3, 4 and 5 were 0.731, 0.764, 0.736, 0.716 and 0.704. As in the case of 40 ⁰ C, the examples showed clearly superior friction characteristics .

As demonstrated by Example 4, where the μs/μd ratio at 80 ⁰ C was 0.659, the friction characteristics are further improved by combining a diol with dioleyl hydrogen phosphite.

In addition, in the case of Examples 5 and 6, the friction behaviour was checked by adding 0.1 and 0.3% by mass of additives to the 1, 2-hexadecanediol . The friction characteristics were further improved in the case of Example 7 where 0.2% by mass of dioleyl hydrogen phosphite was added at the same time to Example 6.