The modification of castor oil (CO) with lignin was the focus of this research to create a lubricating medium with improved gel-like properties. Namely, an alkali lignin (L) was epoxidized with epichlorohydrin (EP) and the resulting LEPs were dispersed in CO. The parameters of LEP synthesis were varied and the epoxidation index (EPI) of the LEPs was determined. The LEPs were also submitted to thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. Rheological responses of the LEP/CO dispersions were investigated through small-amplitude oscillatory shear (SAOS) tests. Linear viscoelasticity functions are quantitatively affected by the epoxidation parameters, such as temperature, reaction time and L/EP and L/NaOH ratios. In general, lignins with higher EPI show higher values of the SAOS functions, which are indicative of better gel-strength due to a higher cross-linking density between the LEPs and CO. A power-law equation describes well the evolution of the complex modulus, G*, with frequency of gel-like dispersions, where the power-law parameters were found to increase almost linearly with the EPI. The thermo-rheological characterization provides a softening temperature beyond 50°C.
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.
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