Isolation and rheological characterization of cellulose nanofibrils (CNFs) produced by microfluidic homogenization, ball-milling, grinding and refining

The aim of this work is to study and compare the morphology and rheology of cellulose nanofibrils (CNFs) from bleached softwood kraft pulp by several mechanical treatment methods. Three different kinds of CNFs were prepared in this work: (i) ones produced by PFI milling and microfluidic homogenization (RM-CNFs), (ii) ones produced by grinding and microfluidic homogenization (GM-CNFs), and (iii) ones produced by ball milling and ultrasonication (BU-CNFs). RM-CNFs had a high aspect ratio and the highest viscosity, storage modulus, and loss modulus. GM-CNFs had homogeneous size distributions, but low aspect ratio due to severe shearing of narrower orifice of chamber in microfluidic homogenization process. For BU-CNFs, the size distributions were heterogeneous, but they had a high aspect ratio. The three kinds of CNFs all had a lower crystallinity index than BSKP, and the degree of polymerization decreased with the increase of treatment intensity. Results of rheology measurements showed that those CNFs had a high degree of fibrillation and had a wider linear viscoelastic region in oscillatory strain measurements. Also the CNFs with high aspect ratio had a high viscosity and modulus. Additional treatments did not always increase the viscosity and modulus due to too severe treatments, which could cause the decrease of aspect ratio of CNFs. The RM-CNFs had a high viscosity and modulus, which means they are suitable in applications of rheology control.