The use of a composition in an hydraulic fluid for power steering

Abstract: A hydraulic fluid composition for power steering is disclosed, which comprises (a) at least one phosphorus containing compound selected from the group of alkyl, or alkyl substituted or unsubstituted phenyl phosphorus acid compounds, alkyl, or alkyl substituted or unsubstituted phenyl phosphorus thioacid compounds, alkyl, or alkyl substituted or unsubstituted phenyl phosphorus dithioacid compounds, and (b) one or more thiadiazole derivatives represented by the formula: where R₁ and R₂ independently denote a linear or branched alkyl group having 1 to 12 carbons. The composition enables hydraulic systems to be used for longer times without accelerating deterioration of rubber materials employed in the hydraulic system.

Description: FIELD OF THE INVENTION The present invention relates to a working fluid composition for power steering, more particularly to a hydraulic fluid composition for a hydraulic power steering system which multiplies steering power by an oil pump driven by engine and a control valve, an actuator, etc. actuated by a driver's steering. BACKGROUND OF THE INVENTION Power steering systems, which were employed in approximately 20% of all passenger cars about five years ago, have recently become so popular that approximately 80% of all passenger cars employ a power steering system. Accordingly, the type of the hydraulic fluid for power steering is changing from the double-purpose type which also employs automatic transmission fluid (AFT) to the single-purpose type which is an oil for power steering. In use, a hydraulic fluid for power steering is circulated by means of a vane pump or a gear pump at a pressure of from about 80 to 105 kg/cm²G (7.8 to 10.3 MPa) at a temperature of about 60 to about 120°C. Moreover, the hydraulic fluid is normally not replaced before the car is scrapped. Moreover, the hydraulic fluid is required to have low temperature starting characteristics and to provide smooth cylinder movement in cold areas. Therefore, the hysraulic fluid should prevent abrasion, should show low temperature fluidability, have good thermal oxidation stability, have low friction characteristics and be inert to sealing materials used in the power steering system. To satisfy these requirements, various additives are incorporated in a selected base oil to provide a hydraylic fluid composition for power steering. DESCRIPTION OF THE PRIOR ARTS Heretofore, most hydraulic fluids for power steering contained zinc dithiophosphate which serves as an abrasion preventing and oxidation preventing agent for the purpose of satisfying the above mentioned requirements. Such a fluid may deteriorate rubber parts in the power steering system on prolonged use at normal service temperature of 60 to 100°C to form crack, such rubber parts, thus causing leakage of the fluid or malfunction of the power systems. The inventors of the present invention assumed that the combination of the rubber parts in the system caused dissolution of copper or plated copper in the system into the hydraulic fluid and the copper reacted with the nitrile group of nitrile rubber parts to form a complex, resulting in the deterioration (hardening) of rubber (nitrile rubber) in the hydraulic system. The dissolution of the copper is assumed to be first due to the elution of copper caused by the added zinc dithiophosphate. The elution of copper is decreased to some extent in systems which do not contain zinc dithiophosphate, but cannot completely be prevented, so that the deterioration of the rubber material cannot be avoided. For this reason, an additive to replace zinc dithiophosphate as well as an additive capable of effectively suppressing the dissolution of copper has been desired. US-A-3 879 306 relates to an automatic transmission fluid composition comprising succinamic acid derivatives as major components and may additionally contain bis(octyl)-dithiothiadiazole. As a further additive zinc dialkylphenyldithiophosphate may be used. SUMMARY OF THE INVENTION The inventors of the present invention, after extensive research to solve the above problems, found that the dissolution of copper into the hydraulic fluid is inhibited by adding a phosphorus acid compound in combination with a thiadiazole derivative, and with such combination it is not necessary to use zinc dithiophosphate, and thus completed the present invention. An object of the present invention is to suppress the dissolution of copper in a hydraulic fluid composition for power steering systems, thereby enabling stable use of hydraulic systems for long terms without accelerating the deterioration of rubber materials employed in the hydraulic system. The present invention is accomplished by the use of a composition consisting essentially of: a) at least one phosphorus containing compound selected from the group consisting of a1) pyrophosphate esters represented by the formula: a2) acid phosphate esters represented by the formula: or a3) neutral amine salts or partially neutralized amine salts of acid phosphate esters represented by the formula: or a4) acid thiophosphate esters represented by the formula: a5) neutral amine salts or partially neutralized amine salts of acid thiophosphate esters represented by the formula: or wherein said phosphorus containing compound is contained in an amount ranging from 0.005 % to 0.5 % by weight in terms of phosphorus content based on the base oil, wherein R₁ to R₁₇ represent alkyl or alkyl-substituted or unsubstituted phenyl groups, said alkyl groups being straight or branched alkyl having 1 to 18 carbon atoms; b) one or more thiadiazole derivatives represented by the formula: wherein R₁₈ and R₁₉ independently denote a linear or branched alkyl group having 1 to 12 carbons; wherein said thiadiazole derivative is contained in an amount ranging from 0.007 % to 0.33 % by weight in terms of sulfur content based on the base oil; and c) a base oil. for suppressing the dissolution of copper in a hydraulic fluid for power steering in which copper or plated copper is used inside the hydraulic piping line. These phosphorus compounds may be employed singly or as a combination of two or more thereof. The phosphorus compounds are added singly or as a combination of two or more thereof in an amount ranging from about 0.005% to about 0.5% by weight in terms of phosphorus content based on the base oil, preferably from about 0.02% to 0.07% by weight. The addition of too much thereof does not give a corresponding effect while cost increases, while insufficient addition does not give the intended effect. One or more of the thiadiazole derivatives represented by the formula below of the present invention: where R₁₈ and R₁₉ independently denote straight or branched alkyl groups having 1 to 12 carbons, are added in an amount ranging from about 0.007% to about 0.33% by weight in terms of sulfur content, preferably from about 0.018% to about 0.18% by weight based on the base oil. The thiadiazole derivatives can be prepared, for example, according to the method disclosed in U.S. Patents 2,719,125, and 2,719,126. The thiadiazole derivatives have a straight or branched alkyl group of 1 to 12 carbons, more preferably 1 to 8 carbons, as R₁₈ and R₁₉ in formula (I), respectively. Particularly preferable is 2,5-bis(tert-octyldithio)-1,3,4-thiadiazole. Specific examples of R₁₈ and R₁₉ groups are methyl, ethyl, propyl, butyl, hexyl, and octyl. The base oil employed in the present invention may be a mineral oil purified by a solvent treatment or a hydrogenation treatment, or a synthetic oil as mentioned below, having an appropriate viscosity. Examples of synthetic oils are poly-α-olefins, polybutenes, diesters, polypropylenes, polyglycols, hindered esters, etc. However, poly-α-olefins, polybutenes, and polypropylenes which are analoguous to mineral oil are preferable in consideration of the solubility of additives therein. The hydraulic fluid composition of a power steering system for the use according to the present invention may additionally contain a known additives in conventional amounts such as an anticorrosion agent, e.g., an amine; an antioxidizing agent, e.g., of the phenol type; a viscosity index improver, e.g., a polymethacrylate; a detergent dispersant, e.g., a sulfonate; and an antifoaming agent. More particularly, the anticorrosion agents include amine type anticorrosion agents, alkenylsuccinic imides, alkenyl succinic esters, etc. The antioxidation agents include those of the amine type, the phenol type, etc. The viscosity index improvers include polymethacrylates, olefin copolymers, etc. Useful detergents of the metal type include alkaline earth metal sulfonate, alkaline earth metal phenates, etc. Useful ashless type dispersants include alkenylsuccinic imides, alkenylsuccinic esters, amides of a long-chain fatty acid with a polyamine (amino-amido type), etc. Useful friction controlling agents such as a fatty acid and an organic molybdenum compound may be used. Useful antifoaming agents include silicone compounds, esters, etc. Zinc dithiophosphate may be present in an amount that does not affect elution of rubber additives: namely, less than about 0.01% by weight based on the total composition. The total amount of the additives in the composition of the present invention is preferably from 2.0% to 20% by weight, more preferably 3.0% to 15% by weight. Generally, hydraulic fluids for power steering have a viscosity of approximately 5 to 9 centistokes (mm²/s), preferably approximately 7 to 8 centistokes (mm²/s), at 100°C, and a viscosity of 50,000 centipoise (mPa.s) or less, preferably 5,000 centipoise (mPa.s) or less, at -20°C. Therefore, a base oil is preferably used which has a viscosity of approximately 3.0 to 6.0 centistokes (mm²/s), preferably approximately 3.0 to 4.5 centistokes (mm²/s) at 100°C, to which a polymethacrylate type polymer, or a combination of polypropylene or polybutene with a polymethacrylate type polymer are added in order to increase the viscosity and to lower the pour point. Generally, the polymethacrylate type polymer used in the present invention has a weight average molecular weight (Mw) of about 50,000 to 400,000 and a number average molecular weight (Mn) of about 20,000 to 150,000, and it is added in the range of about 2wt% to 10wt% based on the base oil. Generally, the polypropylene used in the present invention has a weight average molecular weight of about 40,000 to 250,000 and the polybutene used in the present invention has a weight average molecular weight of about 50,000 to 300,000, and they are added in the range of about 2wt% to 15wt% based on the base oil. The hydraulic fluid for power steering of the present invention, which contains the phosphorus compound and the thiadiazole derivative, is capable of preventing damage to piston sealing materials of a power cylinder caused by corrosion, thus preventing leakage of the hydraulic fluid, and providing long term, stable power steering operation, which could not be achieved by the prior art, without impairing other performance levels of conventional power steering hydraulic fluids. The present invention is now illustrated by Examples and Comparative Examples. In the Examples and the Comparative Examples, the compositions were evaluated as below. Metal corrosion test: This test comprises two test stages: a pre-treatment of extracting rubber compounding ingredients (extraction test), and a metal corrosion test employing the above extraction liquid. The procedure of the extraction test is as follows: (1) A Teflon stirrer is put in a 1000 ml glass beaker and a stainless metal gauze is set in the bottom of the beaker in such a manner that the metal gauze is not prevent the turning of the stirrer. (2) A rectangular rubber component is placed on the stainless metal gauze. (3) 800 ml of test oil is poured into the beaker, and the test is conducted according to the following test condition. The procedure of metal corresion test is as follows: (1) With respect to the metal catalyst, a steel plate, a cast iron plate and an aluminum plate are installed on a copper plate in almost similar intervals using a stainless bolt and a Teflon washer. Then this copper plate is changed into a pipe shape and inserted into a 400 ml of glass beaker. (2) Next, the oil obtained after the extraction test is poured into the glass beaker and the test is conducted according to the following test condition. The metal content and the change in the weight and appearance of the metal catalyst are evaluated after the corrosion test. Extraction test conditions Temperature: 100°C Method of stirring: Stirrer, about 200 r.p.m. Time: 96 hours Rubber parts: A rubber parts (butadiene/acrylonitrile copolymer; hardness(Hs) 74 point (determined by the spring type hardness tester specified in JIS K6301), tensile strength 150kgf/cm² (14.7 MPa), extension 270%) used in a hydraulic system is peeled off and cut into rectangular pieces of 5 cm in length, 2 cm in width, and 0.2 cm in thickness for the test. Metal corrosion test conditions Tester: Indiana stirring oxidation stabilization tester (JIS K2514 3.1) Test oil: 300 ml (the oil used for extraction) Temperature: 100°C Rotation speed: 1300 r.p.m. Time: 144 hours Metal catalyst: Copper plate (75x180x0.8 mm), steel plate, cast iron plate, and aluminum plate (respectively 12x80x0.8 mm) Rubber material deterioration test: The procedure of the rubber material deterioration test is as follows: (1) 150 ml of the oil which is obtained after the metal corrision test is poured into a 200 ml beaker. (2) A sealing material (U packing having outer diameter of 34mm and inner diameter of 22mm) is suspended on a stainless wire (diameter 1mm) and the sealing material is dipped into the oil. (3) The sealing material is allowed to stand according to the following test condition. (4) After the test, the sealing material is taken off from the beaker and washed with n-hexane. The groove of the sealing material is observed with a light microscope (100 magnifications) to see if a foreign matter is formed. Test conditions Test oil: 150 ml (oil after metal corrosion test) Temperature: 100°C Time: 144 hours Sealing material: NBR (hardness(Hs) 75 point, tensile strength 190 kgf/cm²) (18.6 MPa), acrylic rubber (hardness(Hs) 70 point, tensile strength 104kgf/cm² (10.2 MPa), extension 200%) Actual Driving Test: The test oil is charged to a test car (commercially available 1800cc gasoline engine car having a rack-and-pinion type power steering system), and is tested under normal driving conditions for an extended period. The hydraulic system is then disassembled to observe the state of the rubber therein and to determine the quantity of copper in the oil. Examples 1 to 4, and Comparative Examples 1 to 4 The compositions employed in the Examples and Comparative Examples are shown in Table 1. The tricresyl phosphate used in Example 1 had a phosphorus content of 8.4% by weight and a total acid value of 0.05 mgKOH/g. The trilauryl phosphate used in Examples 2 and 3 and Comparative Example 4 had a phosphorus content of 5.1% by weight and a total acid value of 0.05 mgKOH/g. The trialkyl thiophosphate (where the alkyl was C₁₂/C₁₃=50/50 by mol) used in Example 4 had a phosphorus content of 4.8% by weight and a sulfur content of 5.4% by weight. The 2,5-bis(tert-octyldithio)-1,3,4-thiadiazole used in Examples 1 to 4 and Comparative Example 1 had a sulfur content of 35.8% by weight, and a nitrogen content of 6.0% by weight. Other additives used in Examples and Comparative Examples were as below. The succinic imide dispersant was made by KARONITE CHEMICAL CO., LTD. with the trade name "OLOA-1200" (nitrogen content of 2.1% by weight). The polymethacrylate viscosity index improver was made by Sanyo Chemical Industries, Ltd. with the trade name of "Aclube 516". The Ca sulfonate had a calcium content of 11.5% by weight and a total base number of 300 mgKOH/g. The magnesium sulfonate had a magnesium content of 9.5% and a total base number of 400 mgKOH/g. The alkyldiphenylamine had a nitrogen content of 3.4% by weight; it was made by R.T. Vanderbilt Co., Inc. with the trade name of "VANLUBE". The zinc di(2-ethyl hexyl)dithiophosphate had a zinc content of 8.8% by weight. The 1,2,3-benzotriazole had a nitrogen content of 22% by weight. The silicone type defoaming agent was made by Shin-Etsu Chemical Co., Ltd. with the trade name of "KF-96"(10,000 centistokes (mm²/s) at 25°C). As shown in Table 2, in the case of fluids containing a phosphate ester or a phosphite ester, and 2,5-bis(tert-otyldithio)-1,3,4-thiadiazole (Examples 1 to 4), the elution of copper was inhibited, rubber was not affected, and no abnormality was observed at in the actual driving test. In the case of fluids containing zinc dithiophosphate (Comparative Examples 1 to 3), copper elution was significant and the rubber material was deteriorated, even with 2,5-bis(tert-octyldithio)-1,3,4-thiadiazole added to the fluid (Comparative Example 1). Moreover, 1,2,3-benzotriazole, which is considered to be usually effective in copper elution inhibition, was not effective (Comparative Examples 2 and 4). As shown in Table 3, the hydraulic fluid composition of the present invention had appropriate properties such as a suitable viscosity for use as a hydraulic fluid for a hydraulic system.

Claims: Use of a composition consisting essentially of: a) at least one phosphorus containing compound selected from the group consisting of a1) pyrophosphate esters represented by the formula: a2) acid phosphate esters represented by the formula: or a3) neutral amine salts or partially neutralized amine salts of acid phosphate esters represented by the formula: or a4) acid thiophosphate esters represented by the formula: a5) neutral amine salts or partially neutralized amine salts of acid thiophosphate esters represented by the formula: or wherein said phosphorus containing compound is contained in an amount ranging from 0.005 % to 0.5 % by weight in terms of phosphorus content based on the base oil, wherein R₁ to R₁₇ represent alkyl or alkyl-substituted or unsubstituted phenyl groups, said alkyl groups being straight or branched alkyl having 1 to 18 carbon atoms; b) one or more thiadiazole derivatives represented by the formula: wherein R₁₈ and R₁₉ independently denote a linear or branched alkyl group having 1 to 12 carbons; wherein said thiadiazole derivative is contained in an amount ranging from 0.007 % to 0.33 % by weight in terms of sulfur content based on the base oil; and c) a base oil. for suppressing the dissolution of copper in a hydraulic fluid for power steering in which copper or plated copper is used inside the hydraulic piping line. The use of a composition according to claim 1, wherein said thiadiazole derivative is 2,5-bis(tert-octyldithio)-1,3,4-thiadiazole. The use of a composition according to claim 1, wherein said phosphorus containing compound is contained in an amount ranging from about 0.02 to about 0.07 % by weight in terms of phosphorus content based on the base oil. The use of a composition according to claim 1, wherein said thiadiazole derivative is contained in an amount ranging from about 0.018 to about 0.18 % by weight in terms of sulfur content based on the base oil. The use of a composition according to claim 1, wherein the composition has a viscosity ranging from about 5 to 9 cSt (mm²/s) at 100°C, and from about 200 to about 50.000 cp (mPa.s) at -20°C. The use of a composition according to claim 1, wherein the composition has a viscosity ranging from about 7 to 8 cSt (mm²/s) at 100°C, and from about 500 to about 5.000 cp (mPa.s) at -20°C.

Inventors: HIROSE YASUNORI, KIKUCHI HIROTO, TOCHIGI HIROSHI, HIROSE, YASUNORI, KIKUCHI, HIROTO, TOCHIGI, HIROSHI
Applicants: COSMO OIL CO LTD, NISSAN MOTOR, COSMO OIL COMPANY, LTD, NISSAN MOTOR CO., LTD.
Application year: 1990
Main classification: C10M141
IPC codes: C10N40, C10M141, C10M163, C10N30