Evolution of cellulose pulp microfibrillation during disc grinding and microfluidization indicated by apparent specific surface area

Abstract Fast, reliable, and online-feasible analysis of the degree of microfibrillation is crucial for monitoring the production and specifications of microfibrillated cellulose (MFC). This study applies a previously introduced concept based on the apparent specific surface area (SSA*) derived from optical fiber image processing of MFC to compare the progress of microfibrillation in a disc grinder and a microfluidizer at various consistencies using bleached birch kraft pulp. Regardless of operating consistency, the generation of the new specific surface area (ΔSSA*) correlated linearly with specific energy consumption according to Rittinger’s law in disc grinding, whereas in microfluidization linearity was observed only at higher degrees of microfibrillation. This divergence, supported by measurement of transmittance and fines content, suggests that the primary mechanisms of fiber fragmentation differ between the two methods. Despite their different mechanisms and potentially different size distributions, MFC quality—assessed through the mechanical properties of MFC films—developed similarly as a function of SSA* in both methods at consistencies below a critical threshold consistency (around 1–1.5%). The critical threshold consistency found in disc grinding altered grinding behavior adversely, resulting in inferior mechanical properties due to decreased aspect ratios. The raw material was seen to play an important role in disc grinding. Pulps that appeared to be similar produced MFCs of divergent quality. Overall, SSA* appeared to be a feasible parameter for monitoring the evolution of MFC, thus enabling relevant comparisons between microfibrillation methods at various consistencies.