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

Abstract

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 smallamplitude 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.

Comparative Study of Aromatic and Cycloaliphatic Isocyanate Effects on Physico-Chemical Properties of Bio-Based Polyurethane Acrylate Coatings

“…This effect generated constraints for the benzoyl radical to attack the carbon double bond (C = C) of the acrylate chain during the propagation state of radical polymerisation. Meanwhile, JPUA-IPDI offered a more flexible, asymmetrical aliphatic structure of IPDI side chain with less steric hindrance [31]. This made JPUA-IPDI more favourable during the UV curing reaction compared to JPUA-TDI.…”

Evaluation of lignin-enriched side-streams from different biomass conversion processes as thickeners in bio-lubricant formulations

“…The preparation of chemical oleogels was carried out following the procedure detailed by Borrero-López et al [16], wherein the NCOfunctionalized lignin samples, at 20% w/w concentration, were dispersed in castor oil at room temperature for 24 h, using a RW 20 IKA stirrer outfitted with an anchor impeller. Subsequently, the oleogels were stored at room temperature and characterized after one month, to ensure the complete curing of the samples [23]. Oleogel samples have been named as GAHL, GSEL, GEKL and GPKL for those prepared with lignins obtained from the autohydrolysis, steam explosion and eucalypt and pine kraft pulping treatments, respectively.…”

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

“…In this sense, Sánchez et al [8] and Núñez et al [9] evaluated the performance of cellulose derivatives-based oleogels and gel-like dispersions of industrial cellulose pulps, respectively. The potential use of chitin, chitosan and some acylated derivatives as biogenic thickening agents to formulate stable dispersions in vegetable oils was explored by Sánchez et al [10]; whereas Gallego et al [11] and Borrero-López et al [12] reported the preparation of oleogels using NCO-functionalized lignocellulosic materials for the same use.…”