Cellulose nanocrystals (CNCs) are commonly produced using strong acid hydrolysis of cellulosic fibers. The reduction of cellulose degree of polymerization (DP), sulfation of cellulose, and production of CNC under strong acid concentrations occurred abruptly and simultaneously. As a result, process control and optimization are very difficult. For over 60 years, sulfuric acid concentration of approximately 64wt% has been used as the standard condition for producing CNCs with good dispersion properties due to the formation of sulfate groups that imparts electrostatic stability, important for aqueous processing. Low CNC yield of approximately 30-50% has been the main drawback of this standard production acid concentration despite several optimization studies were carried out in the last several decades. Furthermore, the properties of the resultant CNCs are not much affected by varying other hydrolysis conditions at standard acid concentration of 64 wt%. Here we present the first kinetic study of strong acid hydrolysis reaction of cellulosic fibers. We were able to identify a transition acid concentration of 58% below which low CNC yield is due to insufficient cellulose deploymerization, while above which low CNC yield was caused by CNC dissolution by acid. This finding provides a mechanism for controlling CNC yield by simply controlling acid concentration. Laboratory bench scale experiments verified that CNC yield of over 70% can be achieved when a bleach hardwood pulp was used. We also found that acid concentration can be used to produce desired CNC properties, such as CNC morphology, crystal length, crystallinity, and surface charge. The importance of this study is that the production of CNCs can now be tailored to a specific application, for example, CNCs with large aspect ratio (or long crystal length) may be most desirable for polymer reinforcement in composite, while low aspect ratio CNCs with uniform crystal length and high crystallinity may be more suitable for smart windows or LED displays.

Tuning the properties and yield of cellulose nanocrystals in the production space

249th National Meeting (2015)

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<br /> Tuning the properties and yield of cellulose nanocrystals in the production space