A sustainable methanol-based solvent exchange method to produce nanocellulose-based ecofriendly lubricants

Abstract

A prospective methodology aimed to develop totally sustainable oleogels with potential application in lubrication is reported. Oleogels were prepared with cellulose nanofibrils from elm pulps in castor oil. Even at a concentration as low as 1.4 wt%, the oleogels showed rheological behaviors similar to a traditional lithium grease used as reference. In that sense, the high thickening power of cellulose nanofibers was remarked. Never-dried bleached and unbleached elm pulps were mechanically treated with PFI mill refiner and microfluidizer. Moreover, as an alternative way of producing cellulose nanofibrils, the never-dried bleached elm pulp was chemically oxidized using 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO). The three types of nanofibers were characterized and used in the production of oleogels. The great challenge was to remove the high amount of water retained by the nanofibrils during the nanofibrillation treatment before it was incorporated into the vegetable oil-based lubricant. Alternative strategies such as liophilization or drying failed because the nanofibers, with very high specific surface and strong capacity to interact among them by hydrogen bonding, underwent severe aggregation. Instead, the proposed methanol-based solvent exchange method enabled the transfer of the cellulose nanofibrils from the origin hydrogel to the vegetable oil with no detriment of both their aspect ratio and thickening capacity. Chemical considerations are provided with regard to such a solvent-mediated method, which yielded homogeneous and storage-stable oleogels. This work may attract the interest of lubricant manufacturers to produce nanocellulose-based eco-lubricating greases for industrial applications.