Effect of delignification technique on the ease of fibrillation of cellulose II nanofibers from wood

We report on an efficient method for extracting cellulose nanofibers with cellulose II crystal structure that presents a relatively high yield (approximately 82%) after mechanical treatment. Delignification technique plays an important role in the conversion of crystals from cellulose I to cellulose II during the mercerization process and in the subsequent fibrillation of cellulose II nanofibers. Delignified wood pulps (with half of the lignin removed) were treated with 17.5 wt% sodium hydroxide solution to mercerize the cellulose, and then the remaining lignin was further removed to purify the pulps. The resulting pulps were fibrillated by using only one pass through a grinder. X-ray diffraction patterns revealed that the above mercerized pulps were successfully converted into the cellulose II crystal structure. Morphological observation showed that cellulose II nanofibers with a width of approximately 15–90 nm were successfully obtained using the above method. This may have occurred because the remaining half of the lignin in wood pulps partly prevented the interdigitation and aggregation of the cellulose microfibrils during the mercerization process, thus facilitating the subsequent nanofibrillation. However, for the delignified wood pulps (with more than two-thirds of the lignin removed), the microfibrils in the cell wall bound more easily to each other by aggregation during the mercerization process, which may have caused difficulties in the subsequent nanofibrillation.