Lubricating Compositions Containing 5,5'-Dithiobis(1,3,4-Thiadiazole-2-Thiol)

The present invention concerns lubricating compositions and more particularly lubricating grease compositions having improved extreme pressure and other beneficial properties.

Many lubricants which are satisfactory under ordinary lubricating conditions do not provide adequate lubrication under the high frictional forces and temperatures that characterize extreme pressure conditions. Under these conditions friction gives rise to localized high temperatures at the metal surfaces. If this situation is not controlled, the metal parts in contact will seize or be scored causing failure of the parts.

Lubricating additives known as extreme pressure agents are employed to increase the load-carrying capacity of lubricants. The extreme pressure agents promote the formation of an antiweld film and thereby prevent welding of the contacting surfaces.

It is known in the art that certain sulfur containing compounds possess extreme pressure properties. However, many impart to the grease composition the undesirable properties of corrosion and staining of the metal surfaces in contact with the composition. Still others do not possess sufficient efficacy. The latter include polymers of 1,3,4-thiadiazole-2,5-dithiol which possess relatively low extreme pressure properties, insufficient for heavy duty applications when used as a sole extreme pressure agent as indicated in U.S. Pat. No. 4,308,182 granted Eckard et al Dec. 29, 1981.

Surprisingly, it has now been discovered that the dimer of 1,3,4-thiadiazole-2,5-dithiol imparts to lubricating grease composition good load carrying ability without causing the composition to corrode or discolor the metal surface to be protected.

The invention provides a lubricating composition comprising a major amount of lubricating grease or lubricating oil and a minor amount of 5,5'-dithiobis(1,3,4-thiadiazole-2-thiol). The minor amount is sufficient to impart extreme pressure properties to the composition.

Another aspect of the invention concerns a process for preparation of a novel lubricating composition by forming a mixture of lubricating base oil, thickener and grease forming additives and characterized by the improvement of adding to the mixture 0.1 to 10 percent 5,5'-dithiobis(1,3,4-thiadiazole-2-thiol), agitating and milling the mixture to a grease consistency. Optionally, the composition may contain other functional lubricating additives.

The 5,5'-dithiobis(1,3,4-thiadiazole-2-thiol) compound may be prepared by known oxidation methods. Exemplary method of oxidizing 2,5-dimercapto-1,3,4-thiadiazole with iodine to yield the dimer is described in U.S. Pat. No. 3,161,575 granted Wells et al Dec. 15, 1964 and incorporated herein by reference.

The dimer compound of the present invention is of low volatility and substantially ashless in character. Both properties are desirable for lubricating applications under high load conditions.

The lubricating compositions contemplated for use herein include lubricating greases and lubricating oil dispersions.

Lubricating greases are well known in the art. The compositions are prepared by thickening a base oil of lubricating viscosity with a thickening agent. The base oil may be selected from oils derived from petroleum hydrocarbon and synthetic sources. The hydrocarbon base oil may be selected from naphthenic and paraffinic mineral oils. The synthetic oils may be selected from, among others, alkylene polymers, polysiloxanes, carboxylic acid esters and polyglycol ethers.

The thickeners may be selected from organic metal salts, metal complex salts, minerals and polymeric compounds. Exemplary thickeners include, among others, fatty soaps, carboxylic acid salts, mixed and complex salts of alkali metals, alkaline earth metals and aluminum. Other thickeners include polyurea, silica gel and modified clay minerals. Particularly preferred are tetrahydrocarbyl substituted quaternary ammonium bentonite and hectorite clays.

The amount of additive required to be effective for imparting extreme pressure characteristics in lubricating compositions may range from about 0.1 to 10 percent of the total lubricating composition. The preferred range is about 0.5 to about 5.0 percent of the extreme pressure agent of the total lubricating composition.

The extreme pressure additive of the invention may be added directly to the grease composition and dispersed therein by agitation and subsequent milling according to known techniques.

Alternately, the extreme pressure agent may be dispersed in base oil and then added to the grease composition. The compositions may include dispersing agents and detergents.

The lubricating oil dispersion containing the dimer possesses extreme pressure properties and may be used for certain lubricating applications without being formulated into a grease.

In addition to the extreme pressure property, the dimer displays antioxidant properties. The compound is compatible with metals. Lubricating compositions containing the dimer cause no corrosion or discoloration of metals in contact with same.

Depending upon the intended use of the lubricant, other functional additives may be added to enhance a particular property of the lubricant. The lubricating grease and oil dispersion may further contain known antioxidants, antiwear agents, metal passivators, rust inhibitors and other extreme pressure agents.

The following examples are given for the purpose of further illustrating the invention. All percentages and parts are based on weight unless otherwise indicated.

The dimer of the invention was formulated with various commercially available base greases and oil.

The extreme pressure characteristics of the formulations were evaluated by measuring the weld point of the grease formulations in a four-ball tester according to the ASTM D-2596-69 procedure and the oil formulation according to the ASTM D-2783-81 procedure. The results compiled in Table I show that all lubricating formulations containing the dimer yielded high weld load points indicating excellent extreme pressure characteristics.

Lithium 12-hydroxystearate grease was formulated with varying amounts of the dimer of the invention. The weld load points were measured by the procedure described in Example 1. The results compiled in Table II indicate that good weld load points were obtained over the entire experimental range. Optimum results were obtained with formulations containing greater than two parts dimer per hundred parts formulation.

Lithium 12-hydroxystearate grease was formulated with 3.0 percent dimer of the invention. The grease formulation 24 was tested for overall functional properties advantageous for general lubricating applications. Formulation 23 containing no extreme pressure agent was used as control.

Extreme pressure performance was evaluated by the Timken test according to ASTM D-2909 procedure.

The antioxidative efficacy was measured by the ASTM D-942 oxidation test. The compatibility of the dimer with respect to metals was demonstrated by the corrosion test at 100.degree. C. according to ASTM D-130.

The test data are compiled in Table III. The grease composition prepared according to the invention showed good overall lubricating properties. Moreover, metals in contact with the grease showed no discoloration or corrosion as a result of addition of the dimer to the composition.

The above embodiments and illustrations have shown various aspects of the present invention. Other variations will be evident to those skilled in the art and such modifications are intended to be within the scope of the invention as defind by the appended claims.