Cellulose fiber and nanofibril characteristics in a continuous sono-assisted process for production of TEMPO-oxidized nanofibrillated cellulose

A hardwood dissolving pulp and bleached softwood Kraft pulp were subjected to continuous ultrasonic cavitation assisted TEMPO-mediated oxidation. The effects of such processing on the yield of carboxylic groups, morphological changes of the fibers as well as the effects on the final nanocellulose dispersion were studied. Ultrasonic cavitation in the TEMPO-mediated oxidation phase enhanced the yield of carboxylic groups on both pulps while having a negligible effect on materials losses due to fiber fines formation. The effect of ultrasonic cavitation was purely mechanical and acted as an additional high-shear mixer in the pre-treatment phase. As a result, the morphological changes on the fibers were enhanced, with additional swelling and fiber straightening being observed. Furthermore, the ultrasonic cavitation also influenced the properties of the nanocellulose dispersion obtained from subsequent microfluidization. The sonicated samples exhibited higher optical clarity, higher elasticity in gels while also having somewhat lower viscosities. On nanoscale, ultrasonic cavitation helped the subsequent microfluidization in releasing better individualized nanofibrils as they had smaller diameters than in non-sonicated samples. Sonication also had no effect on the crystallinity properties of the nanocellulose, the observed slight reduction was a result of intense microfluidization that was used to produce the nanocellulose dispersion. Ultrasonic cavitation in the TEMPO-oxidized pre-treatment phase was shown to be a method that can increase the throughput in lab-scale by mechanical activation of pulps and enabling shorter processing times for TEMPO-mediated oxidation. Graphical abstract