Abstract:
This invention relates to the use of water-soluble 3-mercaptopropionic acid disulphide salts as extreme pressure additives for aqueous functional fluids. 
     These salts may be prepared by the oxidation of 3-mercaptopropionic acid followed by bringing the disulphide obtained into contact with an organic or inorganic base. 
     Functional fluids are used during industrial operations such as, for example, the machining of metals.

Description:
FIELD OF THE INVENTION 
     The present invention relates to extreme pressure additives for aqueous functional fluids as well as to functional fluids and concentrated aqueous compositions containing the additives. 
     BACKGROUND OF THE INVENTION 
     Many industrial operations such as machining of metals, drilling, grinding, turning, milling, rolling, wire drawing or swaying require so-called functional fluids. 
     The role of these fluids is to decrease cutting forces, to cool the workpiece to obtain good dimension characteristics, to remove the chips from the cutting zone, to impart a good surface funish to the workpiece and to extend the life of tools. 
     Water is the best cooling agent because its specific heat, heat of vaporization and thermal conductivity are high. At the same time, it is the most economical and most harmless cooling agent towards the environment. However, the use of aqueous functional fluids is becoming common. These aqueous functional fluids may be true aqueous solutions of different additives in water which are called synthetic fluids, or alternatively, microemulsions which are called semisynthetic fluids. Semisynthetic fluids contain, in addition to water, mineral oils and surfactants. 
     For high pressure machining operations, the use of aqueous functional fluids has not yet found a satisfactory solution. 
     In these operations, the friction between metal surfaces becomes very high; and it is necessary to use extreme pressure additives. The role of these additives consists in forming a protective layer on the metal surfaces. This protective layer film prevents a work-piece from being seized, or even worse, becoming welded to the working tool. 
     The most commonly used among these extreme pressure additives are sulphur-containing additives because they are the most effective. Contact with hot metal surfaces gives rise to the decomposition of sulphur-containing products and to the formation of a protective metal sulphide layer which is continually renewed. 
     The sulphur-containing products used as extreme pressure additives in lubricants based on mineral oils or emulsions are dialkyl polysulphides, sulphur-containing polyisobutenes and sulphur-containing fatty acid esters. All these products are sparingly soluble or insoluble in water. 
     Attempts were made to overcome this disadvantage by using extreme pressure additives which are soluble in water. 
     The article by R.W. MOULD of British Petroleum Co. Ltd. (Journal of the American Society of Lubrication Engineers, 33(6)291-298 (1977)) examines the efficiency of a number of water-soluble halogenated or sulphur-containing products as extreme pressure additives for aqueous fluids. The efficacy of halogenated, generally chlorinated, products is very low. The sulphur-containing products studied, such as sodium salts of thiosalicylic acid, or 2-mercaptopropionic acid, 2,2&#39;-dithiodibenzoic acid, 2,2&#39;-dithiodipropionic acid, disodium-L-cystine and disodium dithiodiglycolate are not very stable and promote development of bacteria and release of hydrogen sulphide. 
     To stabilize them, the formulations of these products require addition of large quantities of bactericidal agents which are commonly used in emulsions, but are generally excluded from functional fluids. 
     U.S. Pat. No. 4,250,046 describes the use of diethanol disulphide as extreme pressure additive. However, this product is incompatible with many additives which are commonly used in the formulation of semisynthetic fluids. 
     We have now found a water-soluble, effective and stable extreme pressure additive. 
     SUMMARY OF THE INVENTION 
     The extreme pressure additive for aqueous functional fluids according to the invention consists of a water-soluble salt of 3-mercaptopropionic acid disulphide or 3,3&#39;-dithiodipropionic acid (SCH 2  CH 2  CO 2  H) 2 . Water-soluble salt means any inorganic or organic salt the solubility of which in water at room temperature is at least 0.01%. The preferred salts are those with a solubility of at least 0.1%. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     3-mercaptopropionic acid disulphide is a known compound which can readily be prepared by oxidation of 3-mercaptopropionic acid by sulphur or conventional oxidizing agent such as hydrogen peroxide. 
     The salts used in accordance with this invention are obtained in a conventional manner by neutralizing the disulphide, in an aqueous medium, with an organic or inorganic base. Thus, it is possible to use alkali metal or alkaline earth metal oxides, hydroxides or carbonates, liquid ammonia or organic nitrogenous bases. As nitrogenous organic bases yielding water-soluble salts, more particular mention can be made of the mono-, di- or trialkylamines and cycloalkylamines whose total number of carbon atoms is not more than 8 (preferably from 1 to 6), as well as the alkylamines having at least one alkyl radical which brings one or more hydrophilic groups such as OH, COOH or poly(oxyethylene and/or propylene). Mono-, di- or triethanolamines are advantageously used. 
     The aqueous solutions of the salts according to the invention are perfectly stable and can easily be stored without the release of hydrogen sulphide, even in a neutral medium having a pH of 7. If desired, the salts from inorganic bases can also be isolated in form of crystals. 
     The salts according to the invention are incorporated into the aqueous fluids at a gravimetric concentration ranging from 0.01% to 20% and preferably from 0.1% to 10%. 
     They may be used alone; however, they are generally used mixed with other common additives for aqueous fluids. Among these additives, there may be mentioned additives having antiwear, antirust and antifoam effects. 
     The aqueous functional fluids are synthetic or semisynthetic in nature. Synthetic fluids are true aqueous solutions of different additives in water. Their lubricating effect may be improved by adding polyglycols such as polyethylene glycols, polypropylene glycols or their copolymers. 
     Semisynthetic fluids are microemulsions containing a mineral or synthetic oil and a surfactant. The oil improves the lubricating properties of the aqueous fluid. 
     Because the salts of 3-mercaptopropionic acid disulphide are perfectly stable in an aqueous medium, it is possible to store them in the form of concentrated mixtures of additives which are diluted when used. These concentrates contain between 1% and 50% by weight, and preferably betweein 15% and 35%, of 3-mercaptopropionic acid disulphide. They also optionally contain other common additives such as antirust, antiwear and antifoam additives, surfactants, polyglycols or mineral or synthetic oils. 
     The efficacy of the additives according to the invention is assessed by testing on a 4-ball machine by the so-called 10-point test (ASTM standard D 2783). 
     A 4-ball machine test comprises 10 successive tests in which one ball, which is fixed in a chuck, rotates for 10 seconds against three balls held in a cradle filled with the extreme pressure fluid to be tested. By a system of weights, the three balls are pressed with an increasing force, from one test to the following, against the rotating ball. The weights progress geometrically. 
     In each test, the diameter of the scars observed on the three stationary balls is determined and a curve A is plotted on a logarithmic scale which gives the diameter of the scars as a function of the load applied. SEIZING (last load before seizing) is the load above which curve A deviates from an ideal line which is called the HERTZ line. It corresponds to the presence of a few weld points in contact between the balls. The scar or the wear diameter increases all of a sudden. WELDING (or weld load) is the load above which the 4 balls are welded to one another preventing the upper ball from rotating against the three others. MHL (maximum HERTZ load) is a dimensionless coefficient based on the determination of scars formed by the upper ball against the three stationary lower balls. The higher this coefficient, which has no real physical meaning, the better the oil tested is considered to be from the extreme pressure point of view. 
     EXAMPLES 
     The following examples illustrate the invention without, however, limiting it. The salts utilized are obtained and used in the form of aqueous solutions prepared as in the following typical example: 
     Typical preparation 
     Into a stirred reactor, 1050 g (5 moles) of 3-mercaptopropionic acid disulphide are dissolved in 1590 ml of water, then 610 g (10 moles) of pure monoethanolamine are added slowly. The solution thus obtained which contains about 50% of di(monoethanolamine) 3,3&#39;-dithiodipropionate can be used such as it is or in diluted form. 
     Example I 
     Diethanolamine salt of 3-mercaptopriopionic 
     acid disulphide (DEA DAM3P) 
     This salt is incorporated at various concentrations into a synthetic fluid (aqueous solution) containing 5% of ELF XT 6720 or into a semisynthetic fluid (microemulsion) containing 5% of ELF TX 6760. 
     ELF XT 6720 and 6760 are commercial additive concentrates which impart good lubricating properties (lubricity, anti-corrosion properties and the like) to water. 
     XT 6720 is a water-soluble concentrate containing polyglycols. 
     XT 6760 is a microemulsifiable concentrate especially containing a surfactant and a mineral oil. 
     
         ______________________________________     % Sulphur 4-Ball machine test     originating               results (so-calledAdditive  from DEA  10 point test)concentrate     DAM3P     SEIZING   WELDING  MHL______________________________________XT 6720 (5%)     0         80 daN    126 daN  34XT 6720 (5%)     0.15      80 daN    160 daN  37.6XT 6720 (5%)     0.5       80 daN    400 daN  74.0XT 6720 (5%)     1         80 daN    400 daN  78.1XT 6760 (5%)     0         80 daN    126 daN  32XT 6760 (5%)     0.15      80 daN    160 daN  38.6XT 6760 (5%)     0.5       80 daN    400 daN  74.7XT 6760 (5%)     1         100 daN   500 daN  77.6______________________________________ 
    
     The addition of DEA DAM3P into a conventional commercial formulation enables the extreme pressure properties of the latter to be improved significantly. (MHL increasing from 32 or 34 to 77-78.) 
     Example 2 
     Example 1 is repeated but with the monoethanolamine salt (MEA DAM3P). 
     
         ______________________________________     % Sulphur     originating               4-Ball machine testAdditive  from MEA  resultsconcentrate     DAM3P     SEIZING   WELDING  MHL______________________________________XT 6720 (5%)     0         80 daN    126 daN  34XT 6720 (5%)     0.15      80 daN    160 daN  38XT 6720 (5%)     0.5       80 daN    400 daN  73.2XT 6720 (5%)     1         100 daN   500 daN  79.0XT 6760 (5%)     0         80 daN    126 daN  32XT 6760 (5%)     0.15      80 daN    160 daN  38.7XT 6760 (5%)     0.5       80 daN    400 daN  74.5XT 6760 (5%)     1         100 daN   500 daN  78.6______________________________________ 
    
     Example 3 
     Example 1 is repeated but with the ammonium salt (NH 4  DAM3P). 
     
         ______________________________________     % Sulphur     originating               4-Ball machine testAdditive  from NH.sub.4               resultsconcentrate     DAM3P     SEIZING   WELDING  MHL______________________________________XT 6720 (5%)     0         80 daN    126 daN  34XT 6720 (5%)     0.15      80 daN    160 daN  37.7XT 6720 (5%)     0.5       80 daN    400 daN  70.9XT 6720 (5%)     1         80 daN    400 daN  75.5XT 6760 (5%)     0         80 daN    126 daN  32XT 6760 (5%)     0.15      80 daN    160 daN  37.9XT 6760 (5%)     0.5       80 daN    400 daN  71XT 6760 (5%)     1         80 daN    400 daN  75.4______________________________________ 
    
     Example 4 
     Example 1 is repeated but with the sodium salt (Na DAM3P). 
     
         ______________________________________     % Sulphur     originating               4-Ball machine testAdditive  from Na   resultsconcentrate     DAM3P     SEIZING   WELDING  MHL______________________________________XT 6720 (5%)     0         80 daN    126 daN  34XT 6720 (5%)     0.15      80 daN    140 daN  37.3XT 6720 (5%)     0.5       80 daN    400 daN  72.7XT 6720 (5%)     1         80 daN    400 daN  75.5XT 6760 (5%)     0         80 daN    126 daN  32XT 6760 (5%)     0.15      80 daN    160 daN  38.1XT 6760 (5%)     0.5       80 daN    400 daN  73.5XT 6760 (5%)     1         80 daN    400 daN  76.0______________________________________ 
    
     Example 5 
     Example 1 is repeated but with the calcium salt (Ca DAM3P). 
     
         ______________________________________     % Sulphur     originating               4-Ball machine testAdditive  from Ca   resultsconcentrate     DAM3P     SEIZING   WELDING  MHL______________________________________XT 6720 (5%)     0         80 daN    126 daN  34XT 6720 (5%)     0.15      80 daN    160 daN  37.7XT 6720 (5%)     0.50      80 daN    400 daN  71.9XT 6760 (5%)     0         80 daN    126 daN  32XT 6760 (5%)     0.15      80 daN    160 daN  38.4XT 6760 (5%)     0.50      80 daN    400 daN  72.2______________________________________ 
    
     Example 6 
     The diethanolamine salt of 3-mercaptopropionic acid disulphide (DEA DAM3P) is used. A polyethylene glycol having an average molar mass of 400 (called PEG 400) is incorporated into the aqueous phase to improve the lubricating properties thereof. 
     
         ______________________________________   % Sulphur   originating           4-Ball machine test   from DEA           resultsAdditives DAM3P     SEIZING   WELDING  MHL______________________________________PEG 400 (12%)     0         200 daN   20 daN   28PEG 400 (12%)     0.5       250 daN   50 daN   45______________________________________ 
    
     Example 7 
     Using the same salt as above (DEA DAM3P), the lubricating properties are improved by adding polypropylene glycol having an average molar mass of 425 (called PPG 425). 
     
         ______________________________________   % Sulphur   originating           4-Ball machine test   from DEA           resultsAdditives DAM3P     SEIZING   WELDING  MHL______________________________________PPG 425 (12%)     0         126 daN   13 daN   14PPG 425 (12%)     0.1       126 daN   32 daN   29PPG 425 (12%)     0.5       200 daN   40 daN   38______________________________________ 
    
     Although the invention has been described in conjunction with specific embodiments, it is evident that many alternatives and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, the invention is intended to embrace all of the alternatives and variations that fall within the spirit and scope of the appended claims.