Valorization of Kraft Lignin as Thickener in Castor Oil for Lubricant Applications

It is known that large amounts of residual lignin are generated in the pulp and paper industry. A new alternative for Kraft lignin valorization, which consists of first a chemical modification using a diisocyanate and then the efficient dispersion in castor oil to achieve stable gel-like systems, is proposed in this work. Rheological properties and microstructure of these materials were determined by means of small amplitude oscillatory shear tests and viscous flow measurements and atomic force microscopy observations, respectively. Moreover, both standardized penetration tests and tribological assays, usually performed in the lubricant industry, were carried out to evaluate the performance characteristics as lubricating greases. Linear viscoelasticity functions are affected by the lignin/diisocyanate ratio and thickener concentration. The thermo-rheological response evidenced a softening temperature of around 105 degrees C. The microstructure of these gel-like dispersions is composed of interconnected thin fibers, homogeneously distributed in castor oil. Moreover, the NCO-functionalized lignin gel-like dispersions studied show lower friction coefficients than traditional lubricating greases.