Hydrothermal treatments of aqueous cellulose nanocrystal suspensions: effects on structure and surface charge content

Cellulose nanocrystals (CNCs) are ideal rheological modifiers for aqueous oil and gas extraction fluids. CNCs are typically produced with sulfuric acid and their aqueous suspensions have uniform and predictable properties under ambient conditions; however, drastic changes occur at elevated temperatures. Herein, the effects of high temperature treatments (ranging from 80 to 180 °C for 1 h to 7 days) on the properties (including uniformity, colloidal stability, and color) of sulfated, phosphated, and carboxylated CNC suspensions were studied. Additionally, cellulose molecular weight, and CNC surface charge content and crystallinity index were quantified before and after heating. CNCs underwent few morphological changes; their molecular weight and crystallinity index were largely unchanged under the conditions tested. Their surface charge content, however, was significantly decreased after heat treatment which resulted in loss of colloidal stability and aggregation of CNCs. The largest change in suspension properties was observed for sulfated CNCs whereas CNCs with a combination of sulfate and phosphate esters, or carboxylate groups, were less affected and maintained colloidal stability at higher temperatures. In fact, desulfation was found to occur rapidly at 80 °C, while many carboxylate groups persisted at temperatures up to 180 °C; calculated rate constants (based on second order kinetics) suggested that desulfation is 20 times faster than decarboxylation but with a similar activation energy. Overall, this study elucidates CNC suspension behavior after heat exposure and demonstrates routes to produce CNCs with improved high temperature performance.