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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 L EP s were dispersed in CO. The parameters of L EP  synthesis were varied and the epoxidation index (EPI) of the L EP s was determined. The L EP s were also submitted to thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. Rheological responses of the L EP /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 L EP s 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.

Influence of epoxidation conditions on the rheological properties of gel-like dispersions of epoxidized kraft lignin in castor oil