Effect of fines percentage on Ultrasonic dewatering of Cellulose Nanofibrils

Abstract

Abstract In this study, we perform a comprehensive examination of ultrasonic dewatering in Cellulose Nanofibril (CNF) suspensions, with particular emphasis on the role of fines content. The production of cellulose nanofibrils (CNF) 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. % CNF 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 CNF, we systematically dewatered both the suspensions until reaching concentrations of 15, 25 and 35 wt.% 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 CNF fines percentages on dewatering duration and suspension stability during ultrasonic dewatering, contributing to improved processing strategies in industrial cellulose applications.