Impact Symmetry on Tribology Performance of 2,5-Bis-Cycloalkylsulfenyl[1,3,4]Thiadiazole Derivatives

Bismuthiol (DMTD) is one of the most important and well-known heterocyclic compounds. Their derivatives have been employed as additives in metalworking fluids to improve their tribology performance. To investigate the symmetry impact on thermal behavior and tribology performance, three metal-free, non-phosphorus, and ashless additives, including two symmetrical derivatives viz 2,5-bis-cyclopentylsulfenyl[1,3,4]thiadiazole {(cC5S)2TD} and 2,5-bis-cyclohexylsulfenyl[1,3,4]thiadiazole {(cC6S)2TD}, and one asymmetrical substituted DMTD, namely 2-(cyclohexylsulfenyl)-5-(cyclopentylsulfenyl)[1,3,4]thiadiazolec (C6S-TD-ScC5), were synthesized, and their tribology performance as additives in highly isoparaffinic Group III base oil, namely Petronas Etro 6, were evaluated using four-ball friction and wear tester. Interestingly, suitable needlelike crystals of 2,5-bis-cyclopentylsulfenyl[1,3,4]thiadiazole could be isolated after tedious work, whereas, the asymmetric derivative was a low viscous liquid. Then, the influence of symmetry on the thermal behavior of three 1,3,4-thiadiazole derivatives was investigated by thermogravimetric analysis/derivative thermogravimetry (TGA/DTG) and differential scanning calorimeter (DSC). Although no significant difference in thermal stability of the three DMTDs was observed, a sharp degradation was displayed and Tdec. of (cC6S)2TD, (cC5S)2TD, and cC6S-TD-ScC5 were 295.14, 290.47, and 276.09 °C, respectively. No residue remained after degradation. The DSC profiles showed a lower melting point for (cC5S)2TD than (cC6S)2TD, whereas, cC6S-TD-ScC5 exhibited a high Tg of 150.03 °C. In addition, the tribology results of three DMTD derivatives compared with those of zinc dialkyldithiophosphate (ZDDP, ADEKA Kiku-Lube Z-112) revealed an excellent load-carrying capacity with a maximum non-seizure load of 150 Kg load and welding load of 180 Kg load and they displayed better anti-wear properties compared to ADEKA Kiku-Lube Z-112. In addition, three DMTDs exhibited good anti-corrosion and excellent extreme pressure properties.