Tribological characterization of green lubricating greases formulated with castor oil and different biogenic thickener agents: a comparative experimental study

摘要

Purpose - The aim of this paper is to characterize some selected formulations based on castor oil and a variety of biogenic thickeners from a tribological point of view and compare them with some traditional lithium greases. Design/methodology/approach - The evolution of the friction coefficient in several tribological tests performed using several ball-on-disc configurations and coupling materials was monitored for the different oleogels proposed as biodegradable lubricating greases. Both a rotational ballon- disc classical tribometer designed at MuT laboratory and a nanotribometer were used in rotational and oscillatory modes. Findings - Generally, the use of castor oil-based formulations potentially applicable as biodegradable lubricating greases provides similar or lower values of the friction coefficient than traditional lithium greases, depending on the nature of thickener agent employed and tribological contact. In all cases, biodegradable formulations provide significantly lower values of the friction coefficient in tribological tests performed in the oscillatory mode. Weak oleogels like those thickened with glyceryl and sorbitan monostearates or acylated chitosan, provide the lowest values of the friction coefficient in every type of configuration or frictional test analysed. Biogrease formulations containing cellulose or chitin derivatives as thickener agents generally yield higher values of the friction coefficient, which may be comparable to those obtained with the reference lithium greases depending on the thickener and tribological configuration. In frictional tests performed in the rotational mode, the inclusion of ethyl cellulose in the formulation yields high values of the friction coefficient, which was attributed to the castor oil viscosity modification exerted by this additive. Wear results depend on the balance between the frictional behaviour, especially in the initial transient regime, and oleogel mechanical stability. Practical implications - This investigation proposes different new alternatives to replace the traditional thickener agent in lubricating greases with others based on renewable resources in order to obtain a completely biodegradable formulation for different industrial applications. Originality/value - This paper provides a resource of new practical friction coefficient data as well as a comparative analysis of the tribological response of these new formulations based on biogenic thickeners and other traditional lithium greases.