SYNTHETIC TRACTION FLUID

Abstract: A synthetic traction fluid which is prepared by compounding as a base oil one or more of mono-, di-, and tri-esters represented by general formula (I), (wherein Y represents one, two or more members selected for among formula (II) and (III), A' represents an ester bond of -COO- or -OOC, n represents 1-3, R1 represents one or two (the same or different) members selected from among a hydrogen atom and alkyl groups having 1 to 8 carbon atoms) or their derivatives. This fluid is used for power-transmitting devices, particularly traction driving devices.

Description: This invention relates to a synthetic traction fluid. More particularly, the present invention is concerned with a synthetic traction fluid comprising an ester or its derivative having 1 to 3 cyclohexyl rings as the 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 industrial machinery, and in recent years extensive research and development the thereon has been conducted. The traction drive mechanism is a power transmitting mechanism. Unlike conventional drive mechanisms, it does not use any gears, which enables a reduction in vibration and noise as well as a smooth speed change in high-speed rotation. An important goal in the automobile industry is an improvement in the fuel consumption of automobiles. It has been suggested that if the traction drive is applied to the transmission of automobiles to convert the transmission to the continuous variable-speed transmission the fuel consumption can be reduced by at least 20% compared to conventional transmission systems, since the drive can always be in the optimum fuel consumption region of an engine. Recent studies have resulted in the development of materials having high fatigue resistance as well as a development of the theoretical analysis of traction mechanisms. Regarding the traction fluid, the correlation of traction coefficients is gradually being understood on a level of the molecular structure of the components. The term "traction coefficient" as used herein is defined as the ratio of the tractional force which is caused by slipping at the contact points between rotators which are in contact with each other in a power transmission of the rolling friction type to the normal load. A traction fluid must be comprised of a lubricating oil having a high traction coefficient. It has been confirmed that a traction fluid possessing a molecular structure having a naphthene ring exhibits a high performance. "Santotrack®", manufactured by the Monsanto Chemical Company, is widely known as a commercially available traction fluid. Japanese Patent Publication No. 35763/1972 discloses di(cyclohexyl)alkane and dicyclohexane as traction fluids having a naphthene ring. This patent publication discloses that a fluid obtained by incorporating the above-mentioned alkane compound in a perhydrogenated (α -methyl)styrene polymer, hydrindane compound. or the like, has a high traction coefficient. Further, Japanese Patent Laid-Open No. 191797/1984 discloses a traction fluid containing an ester compound having a naphthene ring. It discloses that dicyclohexyl cyclohexanedicarboxylate or dicyclohexylphthalate is preferred as the traction fluid. US-A-3803037 describes the use for traction fluids of cyclohexyl-based compounds, in particular trimethylolpropane tricyclohexyl carboxylate, possibly in connection with high molecular weight polyolefins as viscosity index improvers. As mentioned above, the development of continuous variable-speed transmissions has advanced in the automobile industry. The higher the traction coefficient of the traction fluid the larger the allowable transmission force in the same device. This contributes to a reduction in the size of the entire device with a corresponding reduction in the emission of polluting exhaust gases. Therefore, there is a strong demand for a fluid having a traction coefficient as high as possible. However, even the use of the traction fluid that exhibits the highest performance of all the currently commercially available fluids in such a traction drive device provides unsatisfactory performance with respect to the traction coefficient, and is costly. The traction fluid which has been proposed in Japanese Patent Publication No. 35763/1971 contains Santotrack® or its analogue as a component and, therefore, is also unsatisfactory with respect to its performance and cost. The present 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 relatively inexpensive. As a result, the present inventors have found that the incorporation of an ester or its derivative having 1 to 3 cyclohexyl rings can provide an economical high-performance base oil fluid. The present invention has been made based on this finding. According to the present invention there is provided a synthetic traction fluid comprising, as a first component, at least one ester or its derivative selected from monoesters or their amination product or ether compound derivatives, diesters or their amination product or ether compound derivatives, and triesters or their amination product or ether compound derivatives, represented by formula (I): wherein each of the groups Y, which may be the same or different, is independently selected from and ―(CH₂)n―OH wherein A' is an ester linkage -COO- or -OOC-, n is an integer of 1 to 3, and R₁ is independently selected from hydrogen and C₁ to C₈ alkyl groups, with the proviso that at least one Y is and as a second component, a poly-α-olefin having a molecular weight to the range of 500 to 3000. A first object of the present invention is to provide a high-performance traction fluid having a high traction coefficient. A second object of the present invention is to provide a traction fluid which is not only economical but also readily available and easily applicable to transmissions. The traction fluid of the present invention contains an ester (hereinafter often referred to as "ester A") having 1 to 3 cyclohexyl rings incorporated therein. The traction fluid of the present invention comprises an ester or its derivative having 1 to 3 cyclohexyl rings and having the above structural formula. A' of the ester linkage is -COO-or -OOC-, and the number, n, of the methylene groups is 1 to 3, particularly preferably 1. Specifically, the ester of the present invention comprises either a single ester or a mixture of two or more esters selected from among monoesters, diesters and triesters each having 1 to 3 cyclohexyl rings. The triesters are particularly preferred. This ester or derivative thereof has a viscosity of 50 to 500 mm²s⁻¹ (cst), particularly preferably 100 to 400 mm²s⁻¹ cst at 40°C, and 1 to 20 mm²s⁻¹ (cst), particularly preferably 2 to 15 mm²s⁻¹ (cst), at 100°C. The derivatives of the esters are specifically their amination products and ether compounds. The esters can be prepared by any of the following methods: The first method comprises an esterification reaction of a trihydric alcohol with a cyclohexanecarboxylic acid compound. The trihydric alcohol to be used has 4 to 13 carbon atoms, particularly preferably 5 to 9 carbon atoms. Specifically, examples of the trihydric alcohols include trimethylolpropane, trimethylolbutane, and trimethylolpentane. Examples of the cyclohexanecarboxylic acid compounds include, besides cyclohexanecarboxylic acid, those acids having an alkyl group with 1 to 8 carbon atoms. e.g., methylcyclohexanecarboxylic acid, ethylcyclohexanecarboxylic acid, etc. Cyclohexanecarboxylic acid is particularly preferred. The esterification reaction is conducted in an alcohol/acid molar ratio of 1:3 or in the presence of an excess amount of the acid. The former method requires the use of a catalyst. Therefore, it is preferred that the esterification reaction be conducted in the presence of an excess amount of the acid. Specifically, 1 mol of the trihydric alcohol is reacted with the acid in 3 to 6-fold mol excess (particularly preferably in 3.5 to 5-fold mol excess). The reaction temperature is about 150 to 250° C, preferably 170 to 230°C, and the reaction time is 10 to 40 hrs., preferably 15 to 25 hrs. Although the esterification reaction may be conducted under either elevated or reduced pressures, it is preferred that the reaction be conducted at atmospheric pressure from the standpoint of ease of reaction operation. Under this condition the excess acid serves as a catalyst. An alkylbenzene such as xylene or toluene can be added in a suitable amount as a solvent. The addition of the solvent enables the reaction temperature to be easily controlled. As the reaction proceeds water formed during the reaction evaporates, The reaction is terminated when the amount of the water reaches a three-fold mol excess of the alcohol. 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 alkali washing is used the reaction is conducted using the acid in an amount of 3 to 3.5-fold mol 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 enhances the reaction rate and increases the yield of the ester. The reaction product is finally distilled under reduced pressure to remove water and the solvent thereby obtaining the ester compound of the present invention. A monoester, diester, or a mixture of these, with a triester, can also be synthesized in the same manner as that described above. In this case, the reaction is terminated when the number of moles of water formed during the reaction reaches the number of the ester linkages of the desired ester. The second method of producing the esters comprises esterification of a cyclohexanol compound with a tricarboxylic acid having 5 to 14 carbon atoms. Examples of the cyclohexanol compounds include, besides cyclohexanol, those having an alkyl group with 1 to 8 carbon atoms, e.g., methylcyclohexanol and tert-butylcyclohexanol. Cyclohexanol is particularly preferred. The tricarboxylic acid includes one having 5 to 14 carbon atoms, preferably one having 5 to 9 carbon atoms. The esterification reaction is conducted in an alcohol/acid molar ratio of 3 : 1 or in the presence of an excess amount of the alcohol. It is preferred that the esterification reaction be conducted in the presence of an excess amount of the alcohol. Specifically, 1 mol of the tricarboxylic acid is reacted with the alcohol in 3.5 to 6-fold mol excess. The reaction temperature is about 150 to 250°C, preferably 170 to 230°C, and the reaction time is 10 to 40 hrs., preferably 15 to 25 hrs. Although the esterification reaction may be conducted under either elevated or reduced pressures, it is preferred that the reaction be conducted at atmospheric pressure from the standpoint of ease of reaction operation. An alkylbenzene such as xylene or toluene can be added in a suitable amount as a solvent. The addition of the solvent enables the reaction temperature to be easily controlled. As the reaction proceeds, water formed during the reaction evaporates. The reaction is terminated when the amount of the water reaches a three-fold mol excess of the tricarboxylic acid. Phosphoric acid, p-toluenesulfonic acid or sulfuric acid is used as a catalyst. The most preferred catalyst is phosphoric acid because it enhances the reaction rate and increases the yield of the ester. Finally, the reaction product is distilled under reduced pressure to remove the water, solvent and excess alcohol, thereby obtaining the ester compound of the present invention. It is noted in this connection that the monoester or diester prepared by this method has a carboxyl group and is therefore unstable. Therefore, it is necessary to convert the ester into its derivative, e.g., a salt. The esters utilized the present invention with the incorporation of a poly-α-olefin as a second component provide an improvement in the traction coefficient. The poly-α-olefin which is used as the second component may have either a quarternary carbon atom or a tertiary carbon atom in its main chain and may be a polymer or 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. Particularly preferred are polybutene and polyisobutylene and the hydrogenation products thereof. The polyisobutylene is represented by the following structural formula: The hydrogenation product of the polyisobutylene is represented by the following structural formula: In the above formulae the degree of polymerization, n, is 5 to 150. Although the polybutene and polyisobutylene may be commercially available, they may also be produced by conventional polymerization methods. The hydrogenation product thereof is produced by reacting polyisobutylene or the like in the presence of hydrogen. The molecular weight of the poly-α-olefin is in the range of 500 to 3,000. The molecular weight can be adjusted by suitable methods such as decomposition of a poly-α-olefin having a high molecular weight and mixing of poly-α-olefins having different molecular weights. Although an α-olefin copolymer (OCP) is a kind of a poly-α-olefin, it is unsuited for use as the second component in the present invention. This is because OCP is obtained by polymerization of two or more α-olefins and has a structure where these α-olefins are irregularly linked, as opposed to the polybutene etc. which have a regular gem-dialkyl structure. The ester component of the present invention, e.g., a triester of trimethylolpropane with cyclohexanol, exhibits a traction coefficient of 0.098 to 0.100; the second component, e.g., polybutene, exhibits a traction coefficient of 0.075 to 0.085. Since the ester (first component) of the present invention exhibits a high traction coefficient the use of this first component alone in a traction drive device results in a high performance. However, a further improved traction fluid is obtained by blending the first component with preferably 0.1 to 95% by weight, particularly 10 to 70% by weight, of the second component comprised of a poly-α-olefin. Specifically, although the traction coefficient of the second component is lower than or equal to that of component A, the gem-dialkyl group or cyclohexyl ring of the second component cooperates with the cyclohexyl ring of the first component to exhibit a synergistic effect in improving the traction coefficient. Furthermore, since the second component is relatively inexpensive and exhibits excellent viscosity characteristics, a traction fluid can be economically obtained by blending the first component with 0.1 to 95% by weight of the second component without lowering the traction coefficient. Various additives may also be added to the traction fluid of the present invention depending upon its application. Specifically, when the traction device is subject to high temperatures and a large load, at least one additive selected from an antioxidant, a wear inhibitor, and a corrosion inhibitor, 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 added in an amount of 1 to 10% by weight. However, the use of polymethacrylate and olefin copolymer unfavorably lowers the traction coefficient. Therefore, it is preferred that if they are present they be added in an amount of 4% by weight or less. The term "traction fluid" as used in the present invention is intended to mean a fluid for use in devices which transmit a rotational torque through spot contact or line contact, or for use in transmissions having a similar structure. The traction fluid of the present invention exhibits a traction coefficient higher than those of conventionally known fluids, i.e., exhibits a traction coefficient 5 to 15% higher than those of conventional fluids. Therefore, the traction fluid of the present invention can be advantageously used for relatively low power drive transmissions such as industrial machines, etc. The synthetic traction fluid of the present invention is remarkably superior in its traction coefficient to the conventional fluids. The reason why the traction fluid of the present invention exhibits a high traction coefficient is not yet fully understood. However, basically, the reason is believed to reside in the unique molecular structure of the traction fluid of the present invention. The traction fluid (first component) of the present invention comprises an ester having 1 to 3 ester linkages in its molecule. The 1 to 3 ester linkages bring about an interdipolar force between the molecules. It is believed that the interdipolar force serves to bring the fluid into a stable glassy state under high load conditions, thereby increasing the shearing force. Furthermore, when the ester component of the present invention is blended with the second component which has a gem-dialkyl quaternary carbon atom or cyclohexyl ring, the cyclohexyl ring of the first component is firmly engaged, like gears, with the gem-dialkyl portion of the quaternary carbon atom or cyclohexyl ring of the second component under high -load conditions of the traction device, while when the device is released from the load this engagement is quickly broken, thereby causing fluidization. EXAMPLES 1-7 Ester A₁ according to the present invention was synthesized by the following method: First, cyclohexanecarboxylic acid and trimethylolpropane (in a molar ratio of 3.3 : 1) and toluene solvent were charged into a reactor. Then the reactor was heated to 170°C, and the contents of the reactor were allowed to react at a temperature in the range of 170°C to 230°C under atmospheric pressure. The heating was stopped at a point when the water generated accompanying the reaction amounted to three times by mole of the cyclohexanecarboxylic acid. The reaction mixture was washed with an alkaline solution to remove unreacted compounds, i.e., cyclohexanecarboxylic acid and toluene, from a mixture of a reaction product, i.e., a triester of cyclohexanecarboxylic acid with trimethylolpropane, and the unreacted compounds, followed by vacuum distillation, thereby isolating a pure ester A₁. A partial ester A₂ according to the present invention was synthesized using the following materials in the same manner as described above, except that heating was stopped at a point when the water generated accompanying the reaction amounted to twice by mole of the acid. A₂...trimethylolpropane and cyclohexanecarboxylic acid (average number of the ester linkages: 2) The ester A₁ or A₂ thus produced was next blended with polybutene B₁ having an average molecular weight of 900 followed by measurement of the traction coefficient. The measurement conditions of the traction coefficient are described below. Measurement conditions: Measuring equipment: Soda-type four roller traction test machine. Test conditions : a fluid temperature of 20°C; a roller temperature of 30°C; a mean Hertzian pressure of 1.2 GPa; a rolling velocity of 3.6 m/s; and a slipping ratio of 3.0%. As can be seen from Table 1, the traction fluid of the present invention was found to be remarkably superior in its traction performance to the conventional traction fluids. COMPARATIVE EXAMPLES 1-4 A traction fluid consisting of polybutene alone (i.e., 100 weight percent) and a commercially available traction fluid (Santotrack) were used as comparative samples. Traction coefficients of these comparative samples were measured under the same conditions as in the above Examples. The results are shown in Table 1. As can be seen from Table 1 all the comparative samples exhibited traction coefficients 5 to 15% smaller than that of the traction fluid of the present invention. The traction fluid of the present invention comprises an ester having 1 to 3 cyclohexyl rings and 1 to 3 ester linkages as the base a oil, and not only exhibits an extremely high traction coefficient, but is also inexpensive and exhibits excellent viscosity characteristics. Therefore, the use of the traction fluid of the present invention in a power transmission, particularly in a traction drive device, leads to a remarkable increase in shearing force under a high load, which enables a reduction in size of the device and allows it to be made inexpensively.

Claims: A synthetic traction fluid comprising, as a first component, at least one ester or its derivative selected from monoesters or their amination product or ether compound derivatives, diesters or their amination product or ether compound derivatives, and triesters or their amination product or ether compound derivatives represented by formula (I) wherein each of the groups Y, which may be the same or different is independently selected from and ―(CH₂) n ―OH wherein A' in an ester linkage -COO- or -OOC-, n is an integer of 1 to 3, and R₁ is independently selected from hydrogen and C₁ to C₈ alkyl groups, with the proviso that at least one Y is and as a second component, a poly-α-olefin having a molecular weight in the range of 500 to 3000. A traction fluid as claimed in claim 1 wherein said poly-α-olefin is selected from polypropylene, polybutene, polyisobutylene, polypentene and the hydrogenation products thereof. A traction fluid as claimed in any one of claims 1 and 2 wherein R₁ is independently selected from hydrogen and C₁ to C₄ alkyl groups. A traction fluid as claimed in any one of claims 1 to 3 wherein n is 1. A traction fluid as claimed in any one of claims 1 to 4 wherein the content of said second component is 0.1% to 95% by weight. A traction fluid as claimed in claim 5 wherein the content of said second component is 1 to 70% by weight. A traction fluid as claimed in any one of claims 1 to 6, which further comprises one or more additives selected from an antioxidant, a wear inhibitor, a corrosion inhibitor, or a viscosity improver in an amount of 1 to 10% by weight.

Inventors: KOMATSU, YASUJI, TOMIZAWA, HIROTAKA, YOSHIMURA, NARIHIKO
Applicants: TONEN CORPORATION
Application year: 1987
Main classification: C10M105
IPC codes: C10M111, C10M105, C10M169, C10N40, C10M171