Ashless extreme pressure additive offered for greases

By Thomas J. Karol, Ph.D. and Ronald J. Tepper, Ph. D. (Senior Chemist), Steven G. Donnelly (Manager of Technical Services)

The R.T. Vanderbilt Company is commercially offering a new ashless extreme pressure (EP) additive (VANLUBE^® 972) for greases and certain glycol fluids.  The material is a proprietary additive with patent pending. Vanlube^® 972 is aimed at being environmentally friendly with cost effectiveness equivalent to the heavy metal EP additives in use today.   The new additive is a "readily" biodegradable liquid material that affords high Timken pass values and four ball weld results with 1.5 to 2% additive treatment. 

Theory of Thiadiazoles as EP additives

Thiadiazole dimer technology (VANLUBE 829) has been used successfully in grease, which requires high load carrying properties as measured by the Four-Ball EP Test (ASTM 2596). The weld point reported in this test is the lowest applied load at which the sliding steel-on-steel surfaces seize and then weld. Most greases weld at between 126 kgf to 160 kgf while grease treated with 2.0 mass percent VANLUBE 829 will reach 250 kgf and sometimes 315 kgf; at 3.0 mass percent a 620 kgf weld point is possible. On occasion, we have been able to obtain the maximum 800 kgf loading without seizure.

We believe a possible explanation of why this additive can sustain such high loads and prevent steel-on-steel seizure is the ability to complex to the metal surface. Thiadiazole monomer technology (VANLUBE 871) is an effective antiwear additive in motor oils but doesn’t demonstrate the EP properties of the dimer technology. This suggests that the dimer technology affords a "bidentate coordination" (meaning "two teeth") on the metal surface. The fact that the mono thiadiazoles afford only wear protection, supports the "bidentate coordination" as a reasonable explanation of the improved metal coordination that would be necessary for high load (or EP) protection. We believe that the pseudo aromatic nature of the ring allows p -complexation with the metal surface. The following structure depicts our visualization of how the ring could orientate planar to the metal and affect a double complexation or bidentate effect. In this light either ring’s decomplexation with the metal surface, does not eliminate the material from the metal surface.



Next Generation Thiadiazole Technology (thiadiazole dimer bridging)

The R.T. VANDERBILT research group set about to improve the ability of the thiadiazole technology to coordinate with metal surface thereby improving EP properties:

 

1. Obtain a liquid product
    
2. Increase the Timken OK load (versus VANLUBE 829)
    
3. Maintain good 4 ball EP test performance
    
4. Obtain an environmentally friendly product (ashless, biodegradable)
    
5. Cost effective versus metal technology
    

Our new grease technology (Vanlube^® 972) consists of a complex between the 2,5-dimercapto-1,3,4-thiadiazole (DMTD) disulfide dimer and butoxytriglycol and obtains all of these above goals.

Vanlube^® 972: Thiadiazole / Glycol Complex

The pure disulfide dimer of DMTD (VANLUBE 829) shows excellent four ball weld properties. However, the product is a solid material with relatively low Timken values. We have recently found that the complex between the solid material and butoxytriglycol is an easily handled liquid with excellent Timken and 4-Ball Wear values and has been found to be biodegradable.

It is believed that the improved extreme pressure properties are due to coordination between the thiadiazole bis dimer and butoxytriglycol which affords a bridging capable of many thiadiazole rings interacting with the surface but still linked by the glycol interaction between dimers. This affords a polydentate (versus a bidentate of the simple dimer) surface interaction. The butoxytriglycol bis DMTD derivative multiple association has a maximized surface interaction.



Physical Evidence of Bridging dimer Thiadiazole Technology

Performance is the most compelling evidence. The liquid complex Vanlube^® 972 can obtain 80-pound Timken with 400 kgf 4-ball EP at very low levels (1-2%). To obtain this with metal technology took about 5%. If you were to evaluate the individual components of the dimer thiadiazole-glycol complex, the glycol has essentially no EP performance while the dimer thiadiazole performs exceptionally only in the 4-ball-EP test (not the Timken). The complex at the minimized treatment level (1-2%) which still obtains the maximum OK Timken Load result (80 pound Timken possible) produces a 4-ball EP test result of 315-400 kgf (typical). It is clear that this concentration level (thiadiazole is about 40% of VANLUBE^® 972) of dimer thiadiazole (without the glycol bridge) would produce lower 4-ball-EP test results (VANLUBE 829 @ 0.5% in a Lithium grease gave 215 kgf 4-ball EP test) and a very low Timken result (~ 20-30 pound).

The maximum load carrying performance of VANLUBE^® 972 is at least an 80 pound Timken (maximum attempted due to variability in test) and a 400 kgf 4-ball EP test (although reached at 1-2%, evaluated to 5%).

Major shifts in IR absorption peaks are further evidence for the complex formation.

 

Additional evidence of true complexation of glycols to dimer thiadiazoles is shown by the interaction with the glycol as measured by simple solubility. Dimer thiadiazoles have little interaction with most solvents as seen by their saturation point in that solvent. Higher interactions would be seen by more solvation, which would be evident by higher levels dissolving (again higher interaction). The solvolysis/complexation is evidence of this higher interaction. We believe that the glycol must have the ability to coordinate with two dimer molecules to function a chain link in the complex and afford the EP properties.

 

Formation of the complex frequently requires heating to approximately 130^oC. Stirring indefinitely at room temperature will not dissolve the solid thiadiazole derivative in the glycol.

Copper to Gold color changes (1a- 2e) occur on copper strip. This is the "typical thiadiazole coating" with the vast majority of 1,3,4-thiadiazole analogs. Air exposed copper (versus "freshly polished") demonstrates a 1a color. Thiadiazole exposed copper demonstrates a gold color metal would be rated 2e. Test were conducted to determine if this color change was a protective coat or corrosion of the copper metal.

 

Results indicate that Vanlube^® 972 does not corrode copper but coats the metal with a film. Most of the film can be removed with acetone and a paper towel. Acetone is highly polar and coordinates well with the metal sites and still some thiadiazole coordination remains.

We wished to test if this coating was "protective" to corrosion by treating the copper with lubricant with copper corrosive additive alone and compare that to the corrosive additive with the Vanlube^® 972 thiadiazole present.

 

Results indicate that Vanlube^® 972 protects against copper corrosion. Molybdenum dithiophosphate shows a weight loss with a high degree of darkening (4a). Molybdenum dithiophosphate shows no corrosion with VANLUBE^® 972 as seen by a weight gain and the characteristic gold color (2e) formed.

Improving Wear Scar

Most EP additives do have some effect on the wear scar of the lubricant. Since they have a high metal interaction, one might expect some degree of "corrosive" effect on the metal. This could explain the higher wear scar seen in a grease treated with Vanlube^® 972 (versus the base grease) in the Shell 4-ball wear test.

In a second aluminum complex grease, it was found that the Timken properties, 4-Ball Wear properties, and the 4-Ball Weld properties are improved in the presence of the rust inhibitor, OD-9316 (USP 5,599,779; material available from R.T. VANDERBILT Co).

Synergism between Vanlube^® 972 and Corrosion Inhibitors

 

Biodegradation Study of Vanlube^® 972

One standard test method for determining aerobic biodegradation of lubricants and their components is the Gledhill Shake flask ASTM method D-6139-97. This test method was used for evaluating the biodegradation of Vanlube^® 972. Doses of Vanlube^® 972 were subjected to the conditions of the test, and there appeared to be a dose-related response. The percent degradation was measured by the amount of carbon dioxide produced versus the theoretical amount. The amount of CO₂ that formed was determined by reaction of carbon dioxide with barium hydroxide, which forms barium carbonate as the product:
 

It appears that the higher levels of glycol diminish the bacterial activity. This would be expected from the biodegradation of the glycol itself where it is stated "biodegradable in low levels". The biodegradation of Vanlube^® 972 was compared with the percent biodegradation of sodium benzoate and canola oil, which are known to have very high biodegradation and are typically used as standards. We were very delighted that Vanlube^® 972 (at low levels) is "readily" biodegradable.

 

Conclusion

 

Grease formulators should now consider the possible benefits of this new cost effective, environmentally friendly, ashless antiwear additive. Commerical samples of Vanlube^® 972 are readily available for evaluation, and may be received by request at Petro@RTVanderbilt.Com.

 

*Some softening of the grease