Effect of fines percentage on ultrasonic dewatering of cellulose nanofibrils

In this study, we perform a comprehensive examination of the ultrasonic dewatering of cellulose nanofibril (CNF) suspensions, with particular emphasis on the role of fines content. The production of CNFs involves mechanical fibrillation which leads to the presence of different percentages of fines (fibrils under 200 μm) in the final product. Although fines have demonstrated mechanical advantages in composite materials, they also increase water retention by the fibrils, leading to increased dewatering time and energy. We selected two distinct CNF samples with 60% and 90% fines, respectively, and subjected them to ultrasonic drying until 100 wt% CNFs is reached. We found that the 90% fines samples displayed 20% longer drying times, indicating a higher water retention capacity than the 60% fines samples due to increased hydrogen bonding sites. Both fines types exhibit a biphasic pattern in water removal, with the second phase, commencing upon the elimination of half the water, displaying similar rates regardless of the fines content. As dewatering and drying processes often induce agglomeration in CNFs, we systematically dewatered both the suspensions until concentrations of 15, 25, and 35% were achieved and then redispersed to 0.01 wt% CNF. To evaluate the stability of redispersed samples, we monitored their settling behavior and conducted UV–vis transmittance analyses. Results showed that while 60% fines samples could be redispersed in 1 min, the 90% fines samples required up to 5 min to reach a similar level of stability to their original suspensions. Notably, UV–vis transmittance values remained consistent across both the 60% and 90% fines samples and their initial suspensions, indicating a lack of significant agglomeration following redispersion. These findings provide critical insights regarding the impact of fines percentages in CNFs on dewatering duration and suspension stability during ultrasonic dewatering, contributing to improved processing strategies in industrial cellulose applications.