Tribology International 40 (2007) 1246–1252 The tribological behavior of diester-containing polysulfides as additives in mineral oil Wu Hua, Li Jing, Yi Hongling, Zeng Xiangqiong, Lv Lingbo, Ren Tianhui à School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China Received 6 October 2005; received in revised form 14 September 2006; accepted 16 January 2007 Available online 3 April 2007 Abstract The diester-containing polysulfides were synthesized and their anti-wear (AW) and friction-reducing behaviors as additives in HVIW H150 base oil were evaluated using a four-ball tester. The copper corrosion test was conducted to identify their anti-corrosion property. The nature of the film formed in the rubbed surface was investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Based on the experimental results, diester-containing polysulfides show good anti-corrosion properties and excellent AW abilities similar to that of ZDDP. The results of XPS analyses illustrate that an S-containing inorganic layer, generated by the reaction of diester-containing polysulfides with the metallic surfaces, mainly contains sulfate, sulfite and little sulfide, and there exists a polymer film protective layer over the inorganic layer surface. r 2007 Elsevier Ltd. All rights reserved. Keywords: Diester-containing polysulfide; Wear and friction; Anti-corrosion; XPS 1. Introduction Additives are necessary to enhance aspects of oil performance and replace compounds in an oil which may have been lost in the refining. Additive packages are typically present at around 10% w/w of the final blend, although this can vary depending on the final application. ZDDP have been utilized widely for several decades as multifunctional lubricant additives mainly for their anti- oxidant properties and their ability to prevent wear. However, ZDDP give rise to ash, which contributes to particulate matter in automotive exhaust emissions. Reg- ulatory agencies are seeking to reduce emissions of zinc into the environment. In addition, the phosphorus in these compounds is also suspected to limit the service of catalytic converters that are used on cars to reduce pollution. It is important to limit the particulate matter and pollution formed during engine use for toxicological and environ- mental reasons, but it is also important to maintain the anti-wear properties of the lubricating oil [1]. Organosulfur compounds have been as additives in lubricants for many years, manly in the form of sulfurised sperm oil, sulfurised terpenes, sulfurised olefins and load- carrying additives, such as diphenyl disulfide, di-n-tutyl disulfide, etc. Many basic studies have been engaged in the anti-wear mechanisms and properties of this type of additives [2–5]. However, previous research has shown that the higher sulfur content the compounds have, the better EP performance they have, but the corrosion ability of the sulfur-containing EP additives also increases. So, the balance must be found between excellent load-carrying capacity and effect of corrosion. Many reports have proved that the organo-polysulfides have excellent load-carrying capacity and low corrosion [2,5]. If other active groups are introduced into organo-polysulfide, the anti-wear property and anti-corrosion ability would be further improved. Attention has also been paid to the study of oxygen- containing lubrication additives because of their non- toxicity and biodegradability [6,7]. However, fatty acids, fatty alcohols and esters as additives in lubricants usually exhibit relatively poorer extreme pressure and anti-wear properties as compared with the traditional commercial additives. So, to open a new door in the development of ARTICLE IN PRESS www.elsevier.com/locate/triboint 0301-679X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2007.01.024 à Corresponding author. Fax: +86 21 54741297. E-mail address: thren@sjtu.edu.cn (R. Tianhui).