DE3784732T2 - - Google Patents

Description

This invention relates to a traction fluid. More particularly, the present invention relates to a traction fluid comprising a diester having two cyclohexyl rings or a derivative thereof and a branched poly-α-olefin as a base oil.

Traction drive power transmissions, which transmit power to a driven part through a traction drive mechanism, have attracted attention in the field of automobiles and commercial machinery, and research and development in this field has progressed in recent years. The traction drive mechanism is a power transmitting mechanism that uses rolling friction. Unlike conventional drive mechanisms, it does not use a gear, which enables reduction in vibration and noise, as well as smooth speed change in high-speed rotation. An important goal in the automotive industry is to improve the fuel economy of automobiles. It has been proposed that when the traction drive is applied to the transmission of automobiles to convert the transmission into a continuously variable transmission, fuel consumption can be reduced by 20% or more compared with conventional transmission systems because the drive can always be performed at the optimum speed ratio. Previous research has led to the development of materials having high fatigue resistance and to the theoretical analysis of traction mechanisms. With respect to the traction fluid, the correlation of traction coefficients is gradually being understood at a level of the molecular structure of the components. The term "traction coefficient" as used herein is defined as the ratio of the traction force caused by slip at the contact points between rotators in contact with each other in a rolling friction type power transmission to the normal load.

It is required that the driving fluid be made of a lubricating oil having a high traction coefficient. It is assured that a driving fluid having a molecular structure having a naphthene ring exhibits high performance. "Santotrack" manufactured by Monsanto Chemical Company is generally known as a commercially available driving fluid. Japanese Patent Publication No. 35763/1972 discloses di(cyclohexyl)alkane and dicyclohexane as driving fluids having a naphthene ring. This patent publication discloses that a fluid obtained by mixing the above-mentioned alkane compound with a perhydrogenated (α-methyl)styrene polymer, hydrindane compound or the like has a high traction coefficient. Furthermore, Japanese Patent Laid-Open No. 35763/1972 discloses di(cyclohexyl)alkane and dicyclohexane as driving fluids having a naphthene ring. 191797/1984 a drive fluid containing an ester compound having a naphthene ring. It teaches that an ester obtained by hydrogenation of the aromatic nucleus of dicyclohexyl cyclohexanedicarboxylate or dicyclohexyl phthalate is preferable as a drive fluid.

As mentioned above, progress has been made in recent years in the development of continuously variable transmissions. The higher the traction coefficient of the drive fluid, the greater the transmission force in the device. This contributes to a reduction in the size of the device with a corresponding reduction in exhaust gas and thus environmental pollution. Therefore, there is a need for a fluid having a traction coefficient as high as possible. However, even the use of a drive fluid which exhibits the highest performance of all currently commercially available fluids in such a traction drive device provides insufficient performance in terms of traction coefficient and economy. The drive fluid proposed in Japanese Patent Publication No. 35763/1971 contains Santotrack or the like as a component that is unsatisfactory in terms of performance and economy.

The inventors have made extensive and intensive studies with a view to developing a traction fluid which not only exhibits a high traction coefficient but is also inexpensive. As a result, they have found that incorporation of a certain amount of a branched poly-α-olefin into a diester in which two cyclohexyl rings are connected by a linear hydrocarbon chain or a derivative thereof can provide an economical high-performance base oil fluid, and have achieved the present invention.

The present invention relates to a drive fluid which is characterized in that it comprises a mixture of a diester having the following general formula

wherein each of the groups A', which may be the same or different, is an ester bond of -COO- or -OOC-, n is an integer from 1 to 10, each of the groups R₁, which may be the same or different, is independently selected from hydrogen and alkyl groups having 1 to 8 carbon atoms, and each of the groups R₂, which may be the same or different, is independently selected from hydrogen and alkyl groups having 1 to 3 carbon atoms) or a derivative thereof with 0.1 to 95 wt.% of a branched poly-α-olefin.

The first object of the present invention is to provide a high performance drive fluid having a high traction coefficient The second object of the present invention is to provide a drive fluid which is not only economical but also readily available and easily applicable to transmissions.

The drive fluid of the present invention comprises a base oil consisting of two components, i.e. component A consisting of a diester or a derivative thereof, and a certain amount of a component B consisting of a branched poly-α-olefin.

In the present invention, component A is a diester having two cyclohexyl rings and represented by the above-mentioned structural formula, or a derivative thereof. A' of the ester bond is -COO- or -OOC-, and the number, n, of carbon atoms in the hydrocarbon skeleton is 1 to 10, preferably 1 to 4. When n is 0, the traction coefficient is low, while when n is 11 or higher, the viscosity is unfavorably high. This diester or the derivative thereof can be prepared by the methods given below and has a viscosity of 5 to 50 mm²/s (cst), preferably 7 to 30 mm²/s (cst) at 40°C and 1 to 10 mm²/s (cst), preferably 2 to 6 mm²/s (cst) at 100°C. The derivative includes an amine compound, an ether compound, etc.

The component A can be prepared by any of the following methods. The first method comprises an esterification reaction of a dihydric alcohol with a cyclohexanecarboxylic acid compound. The dihydric alcohol is selected from the group consisting of those having 1 to 10 carbon atoms, preferably 1 to 4 carbon atoms. Specific examples of the dihydric alcohol include ethylene glycol, 1,3-propanediol, 1,3-butanediol and 1,4-butanediol. Examples of the cyclohexanecarboxylic acid compound include, besides cyclohexanecarboxylic acid, those having an alkyl group having 1 to 8 carbon atoms, for example, methylcyclohexanecarboxylic acid, Ethylcyclohexanecarboxylic acid, etc. Cyclohexanecarboxylic acid is particularly preferred. The esterification reaction is carried out at a molar ratio of alcohol to acid of 1:2 or in the presence of an excess amount of acid. The former method requires the use of a catalyst and has the additional disadvantage of producing a monoalcohol as a by-product. Therefore, it is preferred that the esterification reaction is carried out in the presence of an excess amount of acid. Specifically, 1 mole of the dihydric alcohol is reacted with the acid in a 2 to 5-fold excess (particularly preferred is a 2.5 to 4-fold molar excess). The reaction temperature is about 150 to 250°C, preferably 170 to 230°C, and the reaction time is 10 to 40 hours, preferably 15 to 25 hours. Although the esterification reaction can be carried out under either elevated or reduced pressures, it is preferred from the standpoint of ease of operating the reaction that the reaction be carried out under atmospheric pressure. Under these conditions, the excess acid serves as a catalyst. An alkylbenzene such as xylene or toluene may be added in an appropriate amount as a solvent. The addition of the solvent enables the reaction and the temperature to be easily monitored. As the reaction proceeds, the water formed during the reaction evaporates. The reaction is complete when the amount of water reaches twice that of the alcohol on a molar basis. The excess acid is neutralized with an aqueous alkaline solution and removed by washing with water. When an acid which is difficult to extract with an alkaline wash is used, the reaction is carried out using the acid in a 2 to 3.5-fold molar excess over the alcohol in the presence of a catalyst. Examples of the catalyst include phosphoric acid, p-toluenesulfonic acid and sulfuric acid. The most preferred catalyst is phosphoric acid because it increases the reaction rate and the yield of ester. The reaction product is finally distilled under reduced pressure to water and the solvent, thereby obtaining the diester compound of the invention.

The second method for producing the component A of the present invention comprises esterifying a cyclohexanol compound with a dicarboxylic acid having 3 to 12 carbon atoms. Examples of the cyclohexanol compound include, in addition to cyclohexanol, those having an alkyl group having 1 to 8 carbon atoms, e.g., methylcyclohexanol and tert-butylcyclohexanol. Cyclohexanol is particularly preferred. The dicarboxylic acid includes one having 3 to 12 carbon atoms in its main chain, preferably one having 3 to 6 carbon atoms in its main chain. Examples of the dicarboxylic acid include malonic acid, succinic acid and glutaric acid.

The esterification reaction is carried out at a molar ratio of alcohol to acid of 2:1 or in the presence of an excess amount of alcohol. In the former process, there is a possibility of forming a monocarboxylic acid as a by-product. Therefore, it is preferred that the esterification reaction is carried out in the presence of an excess amount of alcohol. Specifically, 1 mole of the dicarboxylic acid is reacted with the alcohol in a 2.5 to 5-fold molar excess. The reaction temperature is about 150 to 250°C, preferably 170 to 230°C, and the reaction time is 10 to 40 hours, preferably 15 to 25 hours. Although the esterification reaction can be carried out under either elevated or reduced pressures, it is preferred from the standpoint of ease of operating the reaction that the reaction be carried out under atmospheric pressure. An alkylbenzene such as xylene or toluene may be added in an appropriate amount as a solvent. The addition of the solvent allows the reaction temperature to be easily monitored. As the reaction progresses, the water that forms during the reaction evaporates. The reaction is complete when the amount of water on a molar basis reaches twice the amount of alcohol. Phosphoric acid, p-toluenesulfonic acid and sulfuric acid can be used as a catalyst. The most preferred catalyst is phosphoric acid because it increases the reaction rate and the yield of ester. The reaction product is finally distilled under reduced pressure to remove water, the solvent and excess alcohol, thereby obtaining the diester compound of the present invention.

The poly-α-olefin component B has either a quaternary carbon atom or a tertiary carbon atom in its main chain and is a polymer of an α-olefin having 3 to 5 carbon atoms or the hydrogenation product thereof. Examples of the poly-α-olefins include polypropylene, polybutene, polyisobutylene and polypentene and the hydrogenation products thereof. Polybutene, polyisobutylene and the hydrogenation products thereof are particularly preferred. The polyisobutylene is represented by the following structural formula:

The hydrogenation product of polyisobutylene is represented by the following structural formula:

In the above mentioned formulas, the degree of polymerization, n, is 6 to 200.

Although the polybutene and polyisobutylene used may be commercially available, they may also have been prepared by conventional and well-known polymerization techniques. The hydrogenation product thereof is prepared by reacting of polyisobutylene or the like in the presence of hydrogen. The molecular weight of the poly-α-olefin is preferably in the range of 500 to 10,000, particularly in the range of 900 to 5,000. The molecular weight can be adjusted by suitable methods such as decomposing high molecular weight poly-α-olefins and mixing poly-α-olefins having different molecular weights. Although an α-olefin copolymer (OCP) is one type of poly-α-olefin, it is not suitable for use as component B in the present invention. This is because OCP is obtained by polymerizing two or more α-olefins and has such a structure that these α-olefins are bonded in an irregular manner, unlike the polybutene, etc. of the present invention which have a regular geminal dialkyl structure.

Component A of the invention, e.g. a diester of succinic acid with cyclohexanol, exhibits a traction coefficient of 0.102 to 0.106, while component B, e.g. polybutene, exhibits a traction coefficient of 0.075 to 0.085.

Since component A of the present invention exhibits a high traction coefficient, use of components A alone in a traction drive device results in high performance. However, a further improved traction fluid can be obtained by blending component A with 0.1 to 95% by weight, particularly 10 to 70% by weight of the poly-α-olefin of component B. Although component B has a lower traction coefficient than component A, particularly the geminal dialkyl group in component B cooperates with the cyclohexyl ring in component A to exhibit a synergistic effect (in terms of improving the traction coefficient). In addition, since component B is inexpensive and exhibits excellent viscosity properties, a traction fluid can be obtained economically by blending the Component A with 0.1 to 95 wt.% of component B without reducing the traction coefficient.

Various additives may also be added to the drive fluid of the present invention depending on its application. In particular, when the drive device is subjected to high temperature and large load, at least one additive selected from an antioxidant, an anti-wear agent and an anti-corrosive agent may be added in an amount of 0.01 to 5% by weight. Similarly, when a high viscosity index is required, a known viscosity index improver is suitably added in an amount of 1 to 10% by weight. However, since the use of polymethacrylates or olefin copolymers lowers the traction coefficient, it is preferred that they, if present, be used in an amount of 4% by weight or less.

The term "driving fluid" as used in the present invention means a fluid for use in devices that transmit torque by point contact or line contact or for use in transmissions having a similar structure. The driving fluid of the present invention exhibits a higher traction coefficient than conventional known fluids, i.e., exhibits a traction coefficient that is 5 to 15% higher than those of the conventional fluids, although the value varies depending on the viscosity. Accordingly, the driving fluids of the present invention can be advantageously used for relatively low-power drive transmissions including internal combustion engines of small passenger cars, spinning machines and food-producing machines, as well as high-power drive transmissions such as industrial machines.

The traction coefficient of the drive fluid of the present invention is remarkably superior to that of The reason why the drive fluid of the present invention exhibits a high traction coefficient is not fully known. However, it is believed that the reason lies mainly in the unique molecular structure of the drive fluid of the present invention.

Component A of the traction fluid of the present invention comprises a diester having two cyclohexyl rings in its molecule. The two ester bonds create an interdipolar force between the molecules. It is believed that the interdipolar force serves to convert the fluid into a stable, glass-like state under high load conditions, thereby increasing the shear strength. Furthermore, component B in the traction fluid of the present invention has a geminal dialkyl type quaternary carbon atom. Therefore, when the traction device is under high load conditions, the cyclohexyl rings near the geminal dialkyl portions of the quaternary carbon in component B are engaged with each other like gears, while when the device is released from the load, this engagement is quickly released, thereby causing liquefaction.

Examples 1 to 13

Diester A of the present invention was synthesized by the following procedure.

First, 250 g of cyclohexanol and 104 g of malonic acid (i.e., 0.4 mole per mole of cyclohexanol) were charged into a reactor, and phosphoric acid was added in an amount of 1 wt.% based on the total weight of the reactants. The reactor was heated to 180°C. The contents of this reactor were allowed to react at a temperature ranging from 180°C to 210°C under atmospheric pressure. The reaction was stopped at a point when the water evolved during the reaction was twice the molar amount of the amount of malonic acid. The reaction mixture was washed with an alkaline solution to remove unreacted compounds such as unreacted cyclohexanol and To remove phosphoric acid from the mixture of reaction products, i.e. a diester of cyclohexanol with malonic acid, followed by vacuum distillation, whereby a pure diester (A₁) was isolated.

Using the same procedure as described above, diesters A₂ and A₁ of the invention were prepared using the following raw materials:

A₂ Ethylene glycol and cyclohexanecarboxylic acid (acid in excess)

A₃ Succinic acid and cyclohexanol

The diesters thus prepared were each mixed with polybutene having an average molecular weight of 900 to 2,350, followed by measurement of the traction coefficient. The measurement conditions of the traction coefficient were as follows: Measuring device Soda-type four-roll traction testing machine Test conditions Liquid temperature 20ºC Roll temperature 30ºC Mean Hertzian pressure 1.2 GPa Roll speed 3.6 m/s Slip ratio 3.0%

It was found that the drive fluids of the invention were remarkably superior to the conventional fluids, as shown in Table 1.

Comparative examples 1 to 9

The traction coefficients of the following drive fluids were measured under the same conditions as used in the above examples:

a drive fluid consisting of 100% by weight of component B ;

Drive fluids obtained by mixing components A₁ to A₃ with 5 to 30 wt.% OCP or PMA and a commercially available drive fluid (Santotrack ).

The results are shown in Table 1. As can be seen from Table 1, all comparative samples show traction coefficients 5 to 15% lower than those of the drive fluids of the present invention. Incidentally, OCP is an olefin copolymer. Specifically, an ethylene/propylene copolymer having an average molecular weight of 150,000 to 300,000 was used as OCP. PMA is polymethacrylate. Specifically, a polymer having an average molecular weight of 50,000 to 300,000 was used as PMA. Table 1 Viscosity mm²/s (Cst) Content, % Molecular weight Viscosity index Traction coefficient Reference Example Comparative example Santotrack values were obtained by calculation using an equation with reference to a kinetic viscosity of 17 to 43 cst.

Commercial applicability

The present invention relates to a traction fluid comprising a component A consisting of a diester or the like having two cyclohexyl rings and linear-chain hydrocarbons as a skeleton and a certain amount of a component B consisting of a branched poly-α-olefin, which not only has an extremely high traction coefficient but also is inexpensive and exhibits excellent viscosity properties.

Therefore, the use of this drive fluid in a power transmission device, especially a traction device, results in a significant increase in the shear force under heavy load while enabling the reduction in the size of the device and reduced cost of the device.

Claims (10)

1. Drive fluid containing as base oil a mixture of a diester with the formula wherein each of the groups A, which may be the same or different, is an ester bond of -COO- or -OOC-, n is an integer from 1 to 10, each of the groups R₁, which may be the same or different, is independently selected from hydrogen and alkyl groups having 1 to 8 carbon atoms, and each of the groups R₂, which may be the same or different, is independently selected from hydrogen and alkyl groups having 1 to 3 carbon atoms, or a derivative thereof with 0.1 to 95% by weight of a branched poly-α-olefin. 2. A drive fluid according to claim 1, wherein the poly-α-olefin is polybutene. 3. A drive fluid according to claim 1 or 2, which comprises 10 to 70 wt.% of the poly-α-olefin. 4. A drive fluid according to any one of claims 1 to 3, wherein the poly-α-olefin has an average molecular weight of 500 to 10,000. 5. A drive fluid according to any one of claims 1 to 4, wherein each of the groups R₁ is selected from hydrogen and alkyl groups having 1 to 4 carbon atoms. 6. Drive fluid according to one of claims 1 to 5, wherein n is an integer from 1 to 4. 7. A drive fluid according to any one of claims 1 to 6, wherein each of the groups R₂ is independently selected from hydrogen or a methyl group. 8. Drive fluid according to one of claims 1 to 7, which further comprises at least one additive selected from the group consisting of an antioxidant, a wear inhibitor and a corrosion inhibitor in an amount of 0.1 to 5% by weight. 9. A drive fluid according to any one of claims 1 to 8, further comprising a viscosity index improver in an amount of 1 to 10% by weight. 10. Use of a diester according to claim 1 together with a branched poly-α-olefin as a base oil for a drive fluid.