Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


3.2 DMTD derivatives and molybdenum alkyl dithiophosphate synergistic combination in polyurea grease MoDDP, DMTD dimer and DMTD/PAG complex were added to the polyurea base grease at the indicated treat levels. Four-ball wear and extreme pressure tests were performed on the treated grease, results shown in Table 5.


Table 5. Additive performance on friction, wear and extreme pressure properties in treated polyurea base grease.


Table 5 Additive performance on friction, wear and extreme pressure properties in treated polyurea base grease


As can be seen from Table 5, polyurea grease treated with MoDDP does not show any improvement in extreme pressure performance. Its friction reducing and antiwear properties are also not obvious. But this does not mean that all organic molybdenum additives are not effective in polyurea greases. As a matter of fact, molybdenum dibutyl dithiocarbamate, i.e. a MoDTC [10] with molybdenum content of approximately 28% and sulfur content of about 24.5% is very effective in friction reducing and antiwear performance in the same polyurea used in this study. Adding 1.0% and 2.0% of MoDTC in the polyurea grease can effectively reduce friction coefficient from 0.089 to 0.071 and 0.058, respectively. It can also reduce 4-ball wear scar diameter from 0.423 mm to 0.359 mm and 0.349 mm, respectively. At the same time, the weld point can be improved from 250 kgf to 315 kgf. Therefore, MoDTC might be more effective than MoDDP in polyurea greases (data for MoDTC treated grease is not shown in Table 5).


It can also be seen from Table 5, polyurea grease treated with DMTD extreme pressure agents DMTD dimer and DMTD/ PAG complex, the weld point of the grease can be effectively increased, but there is no improvement in four-ball wear scar diameter, indicating that these additives are poor in antiwear performance. But unexpectedly, 2.0% DMTD/PAG complex can reduce friction coefficient from 0.089 to 0.077 in the treated grease, showing a strong friction reducing performance. When combination of MoDDP and DMTD dimer or DMTD/PAG complex was used in this grease, the weld point of the treated grease can remain at high values (though LNSL is lower), and four-ball wear scar diameter is smaller than when only DMTD extreme pressure agent was used, although still higher than the value for the base grease. This indicates that MoDDP can be used to improve the antiwear performance when DMTD containing extreme pressure agents are used. When combination of 2.0% MoDDP and 2.0% DMTD/PAG complex was used, the overall performance of the


36 LUBE MAGAZINE NO.126 APRIL 2015


Table 6. Additive performance on friction, wear and extreme pressure properties in treated calcium sulfonate complex base grease.


As it can be seen from Table 6, molybdenum dithiophosphate MoDDP can provide some extreme pressure performance in calcium sulfonate complex grease, and friction reducing effect is also relatively obvious, but there is almost no antiwear improvement. This is mainly due to the calcium sulfonate complex base grease having very good antiwear performance, making MoDDP less effective. Overall, calcium sulfonate complex grease treated with 2.0% MoDDP performed pretty well. The weld point was increased from 315 kgf to 400 kgf, and last nonseizure load was increased from 100 kgf to 152 kgf , friction coefficient was lowered from 0.098 to 0.072, and wear scar diameter at 0.373 mm is still considered very good.


It can also can be seen from Table 6, adding an extreme pressure agent such as DMTD dimer or DMTD/PAG complex to calcium sulfonate complex grease, can effectively improve the weld point of the grease, especially when DMTD dimer was added. When 2.0% DMTD dimer was added to the grease, the weld point can reach 620 kgf. In fact, when 3.0% DMTD dimer was added to the grease, the weld point will reach 800 kgf (data not shown in Table 6). But, in both cases when DMTD dimer or DMTD/PAG complex was added to the grease, the grease will have an increased four-ball wear scar diameter, indicating that antiwear performance of the grease actually deteriorated. When combination of MoDDP and DMTD/PAG complex was used, the overall performance of the grease did not show any major improvement, except a slight increase of the weld point to 500 kgf. On the other hand, when a combination of 2.0% MoDDP and 2.0% DMTD dimer was used, the treated grease gave a very high weld point (620 kgf) , small wear scar diameter (0.426 mm) and low coefficient of friction (0.078).


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polyurea grease is well balanced with a high weld point, a low wear scar diameter and a small coefficient of friction.


3.3 DMTD derivatives and molybdenum alkyl dithiophosphate synergistic combination in calcium sulfonate complex grease MoDDP, DMTD dimer and DMTD/PAG complex were added to the calcium sulfonate complex base grease at the indicated treat levels. Four-ball wear and extreme pressure tests were performed on the treated grease, results shown in Table 6.


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