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

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 crosslinking 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 powerlaw parameters were found to increase almost linearly with the EPI. The thermo-rheological characterization provides a softening temperature beyond 50 degrees C.