Abstract
To reduce harmful sulfur content in lubricant additives, making use of isosterism has been shown to be an effective strategy. When thiobenzothiazole compounds were used as templates, the exchange of sulfur atoms in the thiazole ring with oxygen atoms and NH groups produced twelve isosteres. Similarly, 2-benzothiazole- S-carboxylic acid esters were used as template molecules to produce six isosteres. About 30% of the isosteres exhibited a satisfactory deviation of ±5% relative to the template, ignoring the specific changes in the base oils, the differences in molecular structure, and the friction or wear properties. The template molecules and isosteres in triisodecyl trimellitate exhibited better tribological properties than in trimethylolpropane trioleate or bis(2- ethylhexyl) adipate. Comparative molecular field analysis (CoMFA)- and comparative molecular similarity index analysis (CoMSIA)-quantitative structure tribo-ability relationship (QSTR) models were employed to study the correlation of molecular structures between the base oils and additives. The models indicate that the higher the structural similarities of the base oils and additives are, the more synergetic the molecular force fields of the lubricating system are; the molecular force fields creating synergistic effects will improve tribological performance.
Highlights
The largest user of lubricant additives is the automotive industry
The results of the similarity assessment concerning isostere anti-wear performance revealed that: (1) within a ±1% deviation, there are five compounds (13.89% of the isosteres); (2) within a ±5% deviation, there are 11 compounds (30.56% of the isosteres); (3) within a ±10% deviation, there are 19 compounds (52.78% of the isosteres); (4) there are eight compounds with absolute values of deviation greater than 15% (22.22% of the isosteres); (5) there are 18 compounds (50.00% of the isosteres), with an anti-wear performance that are consistent with or better than that of the template molecules
For the 72 groups of isosteres, in the similarity assessment of the tribological performance, the results indicate that: (1) within a ±1% deviation, there are eight groups (11.11% of the experiments); (2) within a ±5% deviation, there are 28 groups (38.89% of the experiments); (3) within a ±10% deviation, there are 50 groups (69.44% of the experiments); (4) there are nine groups with the absolute values of deviation greater than 15% (12.50% of the experiments); and (5) there are 39 experiment groups (54.17% of the experiments) with a friction-reduction or anti-wear performance that are consistent with or better than that of the template molecules
Summary
The largest user of lubricant additives is the automotive industry. Over the past ten years, the development of lubricant additives has been affected by new laws and regulations. In order to control pollution, European and American countries have enacted stringent new emission standards that require an extended life for exhaust systems, improved fuel efficiency, and the use of less toxic biodegradable lubricants with acceptable environmental compatibility. The new specified limits of sulfated ash, phosphorous, and sulfur in lubricants are lower than previously defined ones [1]. These limits require adjusting and improving engine oil formulations, the gradual introduction of ash-less antioxidants, anti-wear additives, and enhancements in dispersant and viscosity indices. To effectively reduce environmental pollution, lubricants having high-performance friction-reduction and anti-wear additives with less sulfur content need to replace the widely used additives having high sulfur content
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