Rheology of lignin-based chemical oleogels prepared using diisocyanate crosslinkers: Effect of the diisocyanate and curing kinetics

In this work, alkali lignin together with different diisocyanates (hexamethylene diisocyanate (HDI), isophorone diisocyanate (IDI), toluene diisocyanate (TDI) and 4,4′-methylenebis (phenyl isocyanate) (MDI)) have been tested as gelling agents in a castor oil medium. A two-step process comprising first lignin functionalization with a diisocyanate and then the formation of a bio-based polyurethane with gel-like characteristics by combining the functionalized lignin with castor was followed. FTIR and thermogravimetry analysis were carried out on both the gelling agents and resulting oleogels. Moreover, oleogel rheological properties were evaluated by means of small-amplitude oscillatory shear (SAOS) tests and viscous flow measurements. The influences of time-temperature processing conditions during oleogel formation, lignin/diisocyanate ratio and functionalized lignin concentration on the rheological properties of oleogels were analyzed using HDI as crosslinker. 30% (w/w) thickener concentration and room temperature processing were selected to prepare oleogels with the rest of diisocyanates considered. Under the same conditions, HDI-functionalized lignin-based oleogels showed the strongest gel-like behavior whereas TDI-, IDI- and especially MDI-functionalized lignin-based oleogels displayed weak gel-like, or even a liquid-like, behaviors as a consequence of the respective chemical structures, which guide to higher steric hindrance, diminishing the formation of urethane linkages and/or Van der Waals forces. In general, oleogels exhibited an internal curing process due to the progressive formation of urethane linkages, which is closely related to the evolving rheological properties. The kinetics of this curing process was studied and an empirical model has been proposed to predict the evolution of the rheological properties with time.
This work is a part of two research projects (CTQ2014-56038-C3-1R and TEP-1499) sponsored by the MINECO-FEDER and Junta de Andalucía programmes, respectively.