Abstract:
Mineral oil or synthetic hydrocarbon base hydraulic fluids containing a zinc bis(dialkyldithiophosphate) as an antiwear agent are stabilized against degradation at elevated operating temperatures by the incorporation in the hydraulic fluid of an appropriate Group I or Group II metal phosphonate. For example, a hydraulic fluid containing a zinc bis(dialkyldithiophosphate) is stabilized by the presence of a minor amount of a sodium phosphonate.

Description:
SUMMARY OF THE INVENTION 
     This invention relates to mineral oil base or synthetic hydrocarbon base hydraulic fluids which contain a zinc bis(dialkyldithiophosphate) as an antiwear agent, and more particularly, it relates to the use of a minor amount of an alkali metal or alkaline earth metal phosphonate, such as sodium phosphonate in hydraulic fluids containing a zinc bis(dialkyldithiophosphate) in order to retard the thermal decomposition of the zinc bis(dialkyldithiophosphate) and to minimize sludge formation and metal corrosion resulting from the thermal decomposition products. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hydraulic systems are apparatus for transmitting force over a distance through the agency of a fluid--the hydraulic fluid. This hydraulic fluid not only functions in power transmission but it also must lubricate the moving parts and must seal the closely fitting parts. Additionally, it should resist chemical breakdown, it should not cause rust or corrosion and it should resist foaming. The hydraulic fluid being the heart and most vital part of the system, is the primary recipient of the excessive and variable demands on the system such as shock, overload and high temperatures. As a result the great preponderance of hydraulic system failures directly relate to the hydraulic fluid. And in recent years with expanding uses and more rigorous applications, there is an ever increasing potential for fluid failure. 
     Mineral oil base hydraulic fluids fortified with appropriate additives have been most commonly used in hydraulic systems. The additives serve to better adapt the oil to this use and to extend its useful life in the hydraulic system. One additive in general use which functions well as an antiwear and antirust agent is a zinc bis(dialkyldithiophosphate). However, these zinc bis(dialkyldithiophosphate)s tend to break down in the more rigorous applications. The higher pumping pressures required by more demanding uses cause a temperature buildup in the fluid particularly at the pump and valves and at other critical points which become the center of hot spots in the system. 
     It has been determined that the zinc bis(dialkyldithiophosphate) additive begins to exhibit significant decomposition when the fluid temperature reaches a level of about 200° F. (93.3° C.). This decomposition results in the formation of insoluble sludge sediments and deposits in the hydraulic fluid which can build up to a substantial volume and lead to excessive wear and plugging of filters and constriction of orifices. The decomposition also results in the formation of acidic decomposition products in the sludge which actively attack the metals in the system, particularly the copper in the bearing alloys, seals and other parts. The resulting corrosion will eventually lead to the failure of the hydraulic system. 
     I have discovered that a minor amount of an appropriate alkali metal or alkaline earth metal phosphonate will stabilize the hydraulic fluid and the zinc bis(dialkyldithiophosphate) antiwear agent at temperatures in the hydraulic fluid up to about 300° F. (148.9° C.), and preferably up to about 275° F. (135° C.). Since significant decomposition begins at about 200° F. (93.3° C.), the use of this alkali metal or alkaline earth metal phosphonate is particularly desirable when fluid operating temperatures of at least about 175°-200° F. (79.4°-93.3° C.) are anticipated. 
     The stabilizer composition comprises a metal phosphonate having the following general formula: ##STR1## in which M is the alkali metal or alkaline earth metal, n is the valence of the metal, R is lower alkyl having one to about four carbon atoms and R&#39; is higher alkyl having from about 10 to about 30 carbon atoms, preferably about 16 to about 20 carbon atoms. Lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium and barium can be used as the metal, but I prefer to use sodium as the metal in the stabilizer composition. The metal phosphonate exhibits a stabilizing effect in the hydraulic fluid when it is used in an amount of between about 0.01 and about one volume percent, and preferably between about 0.05 and about 0.5 volume percent. 
     In general, the zinc bis(dialkyldithiophosphate) antiwear agent is used in the hydraulic fluid in an amount between about 0.1 to about 2.0 volume percent, and preferably between about 0.2 and about 1.0 volume percent. The alkyl groups in this compound will generally have between about four and about twelve carbon atoms, and preferably they will have between about seven and about nine carbon atoms. 
     A mineral oil is generally used as the base fluid in hydraulic fluids in an amount comprising from about 90 to 99.9 percent of the total hydraulic fluid. These oils are preferably highly refined to remove any non-hydrocarbon components which could lead to corrosion, deposits, and the like. The 100° F. (37.8° C.) viscosity of the base oil useful in hydraulic fluids will range between about 100 SUS (20.6 cs.) (2.06×10 -5  m 2  /s) and about 1,000 SUS (215 cs.) (2.15×10 -4  m 2  /s). 
     A suitable synthetic hydrocarbon oil can also be used as the base fluid, such as, for example, an alpha-olefin oligomer. These oligomers are currently being produced primarily for use as lubricants in automotive engines and in jet aircraft engines. These alpha-olefin oligomers are generally prepared from 1-decene but any alpha-olefin or mixture of alpha-olefins from 1-butene to 1-dodecene can be used. 
     The hydraulic fluid can also contain other additives such as antioxidants, antifoamers, V.I. improvers, vapor phase inhibitors, pour point depressants, demulsibility improvers, and the like. Although the zinc bis(dialkyldithiophosphate) provides some antioxidation protection in addition to its antiwear and antirust properties, it may be desirable to add an additional anti-oxidant such as di-t.butyl-p-cresol to the fluid. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following heat stability tests, the testing procedure developed by the Cincinnati Milacron Company, Cincinnati, Ohio was used. This test procedure utilizes two clean weighed rods of 0.25 inch diameter and three inches long, one of 99.9 percent copper and the other, one percent carbon steel. The rods are submerged in 200 cc. of the test oil in contact with each other, and the oil and test rods are heated to 135° C. After 168 hours (seven days) at 135° C., the rods are removed from the oil and loose sludge is squeegeed back into the oil with a plastic policeman. At this point the copper rod is visually evaluated and rated as to stain and discoloration by ASTM D130. 
     The copper rod is washed with acetone to remove oil before being weighed to determine the total weight of the rod plus sludge deposit. It is then subjected to a ten percent solution of potassium cyanide for one minute to strip the sludge deposit from the rod and is then sequentially washed in distilled water and acetone before being weighed again. The difference in the weight of this cleansed rod and the initial rod weight is the copper loss. The difference in the weight of this cleansed rod and the weight obtained prior to cleansing is the weight of the sludge deposit. 
     The oil is filtered through a filter paper and the residue on the filter paper is washed with naphtha to free it of oil. The dried weight of this residue is the filter paper sludge. A portion of the oil filtrate is filtered through an eight micron millipore filter pad and this residue is also washed free of oil with naphtha. The dried weight of this residue is the millipore filter sludge. The total sludge in milligrams per 100 milliliters of oil is determined from the weight of the sludge deposit, the filter paper sludge and the millipore filter sludge, each adjusted to mg. per 100 ml. of oil. 
     The stain and discoloration evaluation under ASTM D130 is the result of a visual comparison with 12 preprepared strips of increasing stain and discoloration which are available as standards for making the comparison. Group 1 represents slight tarnish, group 2 represents moderate tarnish, group 3 represents dark tarnish and group 4 represents corrosion (black). Increasing discoloration within each group, indicated by color changes, is represented by the letters A, B, etc. Therefore, a matching with the first strip gives a 1A rating, a matching with the second strip gives a 1B rating and a matching with the twelfth strip gives a 4C rating, which is the most severe rating under this procedure. 
     The base oil that was used in the test was a solvent refined neutral mineral oil having a 100° F. (37.8° C.) viscosity of 200 SUS (43.2 cs.) (4.32×10 -5  m 2  /s). It contained 0.30 volume percent of a commercial pour point depressant (Hitec E672, Edwin Cooper Co., St. Louis, Mo.), 0.20 weight percent added of di-t.butyl-p-cresol and one ppm. of a polymerized dimethylsiloxane as an antifoam agent. Three different zinc bis(dialkyldithiophosphate) antiwear agents were tested without stabilizer and then one of the zinc bis(dialkyldithiophosphate)s was separately tested with a sodium phosphonate and a barium phosphonate stabilizer in which R in the above general formula was methyl and R&#39; was a mixture of polyisobutyl groups having between about 10 and 30 carbon atoms. The following table identifies the alkyl groups in the antiwear agents and the amount of the antiwear agents that were used as well as the metal in the stabilizers and the amount of the stabilizers that were used. 
     
         ______________________________________                   Sludge   CuAntiwear agent        Stabilizer mg./100  loss  ASTMalkyl    Vol.%   metal  Vol.% ml.    mg.   D130______________________________________hexyl    0.75    --     --    404.4  10.6  4Cisooctyl 0.75    --     --    466.2  8.69  4C2-ethylhexyl    0.50    --     --    253.0  12.4  4C2-ethylhexyl    0.50    Na     0.30  7.7    nil   1B2-ethylhexyl    0.50    Na     0.20  3.2    nil   1B2-ethylhexyl    0.50    Na     0.10  3.6    nil   1B2-ethylhexyl    0.50    Ba     0.30  11.7   nil   4C2-ethylhexyl    0.50    Ba     0.20  19.1   nil   4C______________________________________ 
    
     It is noted from this data that the sodium and barium phosphonates effect a significant decrease in the sludge formation and copper loss. It is further noted that the improvement in stain and discoloration is very substantial with the sodium phosphonate stabilizer. 
     It is to be understood that the above disclosure is by way of specific example and that numerous modifications and variations are available to those of ordinary skill in the art without departing from the true spirit and scope of the invention.