Because of their excellent mechanical properties, light weight and biodegradability, cellulose nanofibers (CNF) are attractive materials for next-generation reinforced biomaterials and bioplastics. However, the method for separating cellulose to make CNF, known as fibrillation, is energy-intensive, which limits applications for CNF. Now, in a collaborative project, scientists from the Oak Ridge National Laboratory (ORNL; Oak Ridge, Tenn.; www.ornl.gov), along with teams from the University of Tennessee (Knoxville; www.utk.edu) and the University of Maine (Orono; www.umaine.edu), have developed a method for lowering the energy required to produce CNF from cellulose.
First, the researchers used atomistic molecular dynamics simulations to explore candidate solvents that would “facilitate the reduction of interactions between cellulose fibers, thereby reducing energy consumption in fibrillation,” they write. According to the simulation studies, the most suitable medium for fibrillation of cellulose is an aqueous solution of sodium hydroxide and urea in a particular ratio (0.007:0.012 wt.%).
Next, the scientists conducted pilot studies using the NaOH/urea solution, and found that fibrillation energy was reduced by about 21% on average, compared to water alone. The research team explains: The NaOH and urea “act synergistically on CNFs to aid fibrillation, but at different length scales. NaOH deprotonates hydroxyl groups, leading to mesoscale electrostatic repulsion between fibrils, whereas urea forms hydrogen bonds with protonated hydroxyl groups, thus disrupting inter-fibril hydrogen bonds.”
The researchers say the method significantly lowers the production cost of CNF — an ideal biomaterial to use as a composite for 3D-printing structures, such as sustainable housing and vehicle assemblies. With the “winning” solvent, the team estimates the electricity savings potential could amount to about 777 kilowatt hours per metric ton CNF produced.
Testing of the CNF derived from this new method found similar mechanical strength and other desirable characteristics compared with conventionally produced CNF.
The research was published in a recent issue of the Proceedings of the National Academy of Sciences. ■