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

Abstract

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.

Modification of Alkali Lignin with Poly(Ethylene Glycol) Diglycidyl Ether to Be Used as a Thickener in Bio-Lubricant Formulations

“…The dispersion of epoxidized lignin samples in castor oil produces physically stable oleogels, except for the case of the EAL5 sample with the lower epoxy index, which shows phase separation immediately after preparation. This suggests the occurrence of chemical gelation via chemical interactions between castor oil hydroxyl groups and the remaining epoxy groups, as previously proposed for oleogels prepared with epichlorohydrin-functionalized lignins [21]. Figure 3 shows viscous flow curves of stable epoxide-functionalized lignin-based oleogels, as a function of both epoxy index (Figure 3a) and thickener concentration (Figure 3b).…”

“…As previously described [21,26], apart from the initial weight loss of around 5% at 50–100 °C due to moisture, the thermal degradation of this alkali lignin comprises the dehydration of hydroxyl groups at around 150 °C and the cleavage of ether linkages at around 275 °C. At this temperature, volatile gases such as CO, CO 2 , and CH 4 [15,33] are produced, with subsequent pyrolytic degradation, which involves the cleavage of carbon-carbon linkages at a T max around 380 °C.…”

“…Epoxidation of a variety of lignins with epichlorohydrin has been previously investigated [21,25,26,27]. In these studies, the effects of temperature, reaction time, NaOH/lignin ratio and epoxy/lignin ratio on the chemical modification of lignin were analyzed.…”

“…In these studies, the effects of temperature, reaction time, NaOH/lignin ratio and epoxy/lignin ratio on the chemical modification of lignin were analyzed. Cortés-Triviño et al [21] and Malutan et al [25] pointed out that high reaction temperatures had a negative effect on epoxidation due to the occurrence of secondary reactions. Thus, a relatively low temperature (30 °C) was selected in this research.…”

“…In previous work, lignin was epoxidized using epichlorohydrin, and then was dispersed in castor oil thus resulting in gel-like systems stabilized by chemical crosslinking [21]. However, the toxicity of epichlorohydrin and the limited rheological properties imparted to the gel-like dispersion require another pathway for lignin epoxidation to be explored.…”