Exploring the effect of the pulp bleaching on the thermo-rheological behavior of sustainable cellulose nanofiber-based oleogels

Abstract

Taking advantage of the high thickening capacity of cellulose nanofibers in castor oil, sustainable lubricating oleogels were obtained. Methanol was used as mediator to transfer the nanofibers, originally present as a hydrogel, into the vegetable oil. The effect of the pulp bleaching on the oleogels’ thermo-rheological behavior was analyzed. Viscous flow curves and small/large amplitude dynamic shear tests were carried out on both bleached and unbleached samples of the same elm kraft pulp. Substantial differences were observed, above all when the oleogels were subjected to stress values beyond the onset of their linear viscoelastic regimes. The oleogel prepared with nanofibers from the unbleached pulp was found to endure large shear deformations better than that from the bleached pulp, and was less sensitive to temperature. Hence, the complex shear modulus (|G*|) corresponding to the oleogel based on 1.4 wt% bleached nanofiber decreased by 55.7% upon a non-linear stress of 200 Pa was applied for 30 min. However, a less severe structural breakdown, i.e., a decay in |G* | of 28.9% (half of the previous one), was monitored when the oleogel based on the unbleached nanofiber was submitted to the same testing conditions. It should also be underlined that such an oleogel showed an extraordinary capacity to recovery its structure after the large shear deformation process.