Low-viscosity lubricants are promising solution for improving fuel efficiency and reducing carbon dioxide emission associated with the automobile industry. Friction modifiers have been incorporated in such lubricant formulations to meet the desirable engine protection and durability requirements. Among the available friction modifiers, organic friction modifiers composed of polar groups and hydrocarbon chains have been used for synthesis of high-performance low-viscosity lubricants. However, weak adsorption forces between the metal surface and organic friction modifiers result in poor friction reduction performance thus limiting its applications. Recent research reviews have established that introduction of multiple active sites improves the chelation and adsorption of the organic friction modifiers.
Inspired by the above assertions as well as the coordination performance of oxygen and nitrogen in amino-PEG2-amines with metals, Dr. Wenjing Hu and her Doctoral supervisor Prof. Jiusheng Li from Chinese Academy of Sciences investigated the influence of alkylation on the tribological properties of amino-PEG2-amine-based organic friction modifiers. Specifically, friction reduction and anti-wear properties of three diamines: amino-PEG2-amine (PA), N, N’-disubstituted (SA), and tetrasubstituted products (TA) were explored at elevated temperatures. The friction experiments comprised of universal mechanical tester (UMT) reciprocation and four-ball frictions tests and various characterization methods including X-ray diffraction for analyzing the wear scar surface. The work is currently published in Industrial & Engineering Chemistry Research journal.
Results from the diamine-based friction modifiers exhibited varying tribological properties that depended on the degree of alkylation attributed to the multiple oxygen and nitrogen binding sites. Amino-PEG2-amine and tetrasubstituted products respectively showed superior wear resistance and friction reduction properties. According to the authors, the improved wear performance was due to the difficulty in shearing the protective layer due to the increase in the friction coefficient at stable period. Furthermore, substituting hydrogens on amino acid groups with n-octyl chains improved the friction properties and decreased wear resistance with varying alkylation degrees.
Based on surface topography and composition analysis, lubrication films on the friction surface were found to comprise of iron oxide and organic nitrogen compounds which led to the formation of adsorption film by additives present on the worn surface. The adsorption film was credited with improving the wear resistance and friction reduction properties by preventing the direct contact of the sliding surfaces. Consequently, the direct influence of alkylation degree on steric hindrance and electron density significantly contributed to enhanced surface adsorption and diverse tribological performance.
In a nutshell, the authors performed friction tests analysis on the worn surface to explore the correlation between alkylation degree and tribological performance of amino-PEG2-amine-based organic friction modifiers. The steric hindrance and electron density of atoms as well as the adsorption strength were all observed to be affected by the degree of alkylation. In a statement to Advances in Engineering, Professor Jiusheng Li, the corresponding author, expressed his confidence in organic friction modifiers in developing robust additive technology. There is a huge demand for friction modifiers under the trends of Low-viscosity engine oils, indeed the results of study will play a key role in advancing molecular and structural design for the synthesis of high-performance low-viscosity lubricants.
Hu, W., Zhang, Z., Zeng, X., & Li, J. (2019). Correlation between the Degree of Alkylation and Tribological Properties of Amino-PEG2-amine-Based Organic Friction Modifiers. Industrial & Engineering Chemistry Research, 59(1), 215-225.