Abstract
Considerable efforts are currently being made by the academic community and industry, aiming to develop environmentally friendly lubricants with suitable technical features for their performance. In this context, lignin could be considered a promising candidate to be used as a bio-sourced thickening agent to formulate eco-friendly lubricating greases. In this work, alkali lignin (AL) was chemically modified with poly(ethylene glycol) diglycidyl ether (PEGDE). Afterwards, the epoxidized lignin was properly dispersed in castor oil (CO) in order to obtain an oleogel for lubricant applications. The epoxidized lignins were characterized by means of epoxy index determination, thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy. The epoxide-functionalized lignin-based oleogels were analyzed from both rheological and tribological points of view. It was found that the viscosity, consistency and viscoelastic functions of these oleogels clearly increased with the epoxy index of the epoxide-modified lignin compound. Thermo-rheological characterization of these oleogels revealed a slight thermal dependence of the viscoelastic moduli below 100 °C, but a significant softening above that critical temperature. In general, these oleogels showed low values of the friction coefficient under the mixed lubrication regime as compared to the neat castor oil.
Highlights
Several million tonnes per year of used lubricants in Europe are poured into the environment as a result of leaks, spillages or other problems [1]
The epoxy index decreased with reaction time, probably due to crosslinking reactions, this effect may depend on both epoxy/lignin ratio and temperature [25]
Suitable thickeners for castor oil were obtained by means of chemical modification of alkali lignin (AL) with poly(ethylene glycol) diglycidyl ether (PEGDE) with the aim of developing environmentally friendly lubricating greases
Summary
Several million tonnes per year of used lubricants in Europe are poured into the environment as a result of leaks, spillages or other problems [1]. In the particular case of lubricating greases, the most feasible measure to be implemented by manufacturers in order to produce environmentally friendly formulations is the replacement of the mineral or synthetic lubricating oil, which comprises 70–90 wt % of grease components, as for instance established in the Blue Angel eco-label in 1978 [2] For this purpose, it is well known that vegetable oils are gaining more importance in environmentally friendly industrial applications due to their inherent biodegradability, renewability, low ecotoxicity and low toxicity towards humans [3,4]. The toxicity of epichlorohydrin and the limited rheological properties imparted to the gel-like dispersion require another pathway for lignin epoxidation to be explored Bearing in mind these assumptions, this work focused on finding a distinctive alternative for lignin valorization by using it as an effective thickening agent in castor oil-based lubricants through the study of the epoxidization of an alkali lignin with poly(ethylene glycol) diglycidyl ether (PEGDE). The influence of lignin/PEGDE ratio used in the epoxidation reaction and concentration of epoxide-functionalized lignin on the performance properties of the resulting oleogels, i.e., rheological, tribological and thermal characteristics, were studied
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