Gel-Like Dispersions of HMDI-Cross-Linked Lignocellulosic Materials in Castor Oil: Toward Completely Renewable Lubricating Grease Formulations

In this work, several lignocellulose pulps from different origins and/or submitted to different treatments were cross-linked with hexamethylene diisocyanate (HMDI) and further dispersed in castor oil in order to obtain gel-like formulations based on renewable resources, which can be potentially applicable as semisolid lubricants. The rheological and tribological properties attained as well as physical and mechanical stability were suitable to consider these gel-like dispersions as efficient alternatives to traditional lubricating greases. The rheological behavior was evaluated by means of both small-amplitude oscillatory shear tests (SAOS) and viscous flow measurements, at different temperatures. The HMDI/cellulose pulp weight ratio applied in the cross-linking reaction can be used to modify and modulate the consistency and the values of rheological functions of these gel-like dispersions. However, the rheological behavior is not qualitatively affected by the amount of HMDI used as coupling agent. The thermorheological response evidences a softening temperature of around 100 degrees C. Mechanical stability of HMDI-cross-linked lignocellulose-based oleogels, evaluated as the loss of consistency after submitting the sample to a working test, was significantly improved respecting the non-cross-linked lignocellulose gel-like dispersion. Microstructural morphology was examined using atomic force microscopy (AFM) and consists of long and entangled fibers, extremely similar to that found in lithium greases. Moreover, friction coefficient and resulting wear mark diameters obtained in a ball-on-disc contact lubricated with these eco-friendly formulations were, in most cases, comparable to that obtained with commercial lubricating greases. Therefore, HMDI-cross-linked lignocellulose materials are proposed as effective and promising eco-friendly thickener agents in vegetable oils.