Nanocellulose-Based Emulsion for 3D-Printing: Taking Advantage of Creaming Behavior

Oil-in-water emulsions stabilized by cellulose nanofibrils (CNF) are known to form stable, high-volume creaming layers as a synergistic result of the Pickering mechanism and the formation of percolating networks by CNF. Methyl cellulose (MC) can be used as a co-emulsifier together with CNF to further increase the viscosity of the system, and can moreover be used to tune the droplet size distribution of the formed emulsion. In this work, these highly stable creaming layers are used in in-situ polymerization of styrene-in-water emulsions, with the target to prepare an emulsion-based ink for 3D-printing. We show that the simultaneous creaming and polymerization enable effortless water removal, resulting in a paste-like material consisting of polystyrene beads decorated with CNF and MC. The formed paste was characterized regarding its structure and performance.  Solid-state NMR confirmed the successful polymerization of the droplets with average molecular weight (Mw) of approximately 700 000.  Gas chromatography indicate that leftover styrene was less than 1%. A significant increase of the dry solids content took place after polymerization, rising to 20% compared to 1-2% of the starting mixtures containing CNF and MC. Shrinkage of the paste, a common challenge during drying, was at an acceptable level for this hygroscopic biocomposite. The formed paste, was tentatively demonstrated to print small objects in direct-ink-writing.