High temperatures and pressures during cooking hinder the nanofibrillation of purified pulp

Kraft pulp is an abundant raw material for producing cellulose nanofibers. However, our previous study demonstrated that kraft cooking hinders the nanofibrillation of never-dried pulp. Therefore, this study aims to elucidate the specific factors of kraft cooking hindering nanofibrillation, particularly the effects of high temperature and pressure. Undried purified pulp, produced using the Wise method, was subjected to high temperature and pressure conditions, with and without subsequent alkali treatment. After fibrillating the pulps for different durations, the degree of nanofibrillation was evaluated by sedimentation tests. Undried Wise-based pulp, with excellent nanofibrillation ability, becomes less capable of nanofibrillation after cooking at high temperature and pressure. Thus, even in the undried state, harsh cooking is considered to have caused irreversible aggregation of microfibrils or hornification in the pulp cell wall. Moreover, high temperatures and pressure should be avoided when removing the matrix, e.g., lignin and hemicellulose, for producing pulp for cellulose nanofibers.