Synthesis of Odorless Sulfur-Based Polymers and Their High Load-Bearing Capacity as Extreme Pressure Lubricant Additives

Sulfurized isobutylene (SIB) has been widely utilized as an extreme pressure (EP) additive in lubricating base oils. However, it has an unbearably bad odor during preparation and use. In this work, a series of sulfur-based polymers, <b>poly(S-<i>r</i>-UnOH-x)</b> (<b>x</b> = <b>LA</b>, <b>SA</b>, <b>OA</b>, LA: lauric acid, SA: stearic acid, OA: oleic acid), have been synthesized via inverse vulcanization with the industrial byproduct of elemental sulfur (S₈) and sequential esterification reactions with long-chain fatty acids. The resulting sulfur-containing polymers are oil-soluble and odorless. The tribological properties of the sulfur-based polymers as an oil additive have been investigated by an SRV-IV oscillating reciprocating tribological machine and a four-ball tribological wear tester. Compared to the base polyalphaolefin (PAO-10), the oil solutions demonstrated substantial reductions in the coefficient of friction and wear volume by up to 59.8 and 93.2%, respectively. Moreover, the sulfur-based polymers demonstrate a high load-bearing capacity of up to 1500 N under SRV-IV oscillating reciprocating test conditions, and the four-ball tribological tests yielded a maximum nonseizure load (<i>P</i>_B) of 1069 N and a weld load (<i>P</i>_D) of 2453 N. In comparison with a representative commercial SIB additive (T321), the sulfur-based polymers exhibit good tribological performance while not suffering from the odor issues typically associated with conventional sulfurized EP additives. Collectively, these odorless sulfur-based polymers represent a class of green extreme pressure additives derived from S₈, providing further insight into the green utilization of the industrial byproduct in lubrication applications.