Printing and mechanical characterization of cellulose nanofibril materials

Cellulose nanofibrils (CNF) are a promising building block of structural materials because they are biodegradable, can be made into optically transparent bulk materials, and have exceptional specific strength and stiffness compared to common synthetic polymers. The manufacturing of bulk materials from CNFs is a challenge because CNFs form networks in solution at low solids concentration, which can result in long processing times as well as large residual stresses and distortion upon water removal. Here, a method to form materials from CNF suspensions via direct ink writing, a type of additive manufacturing, is demonstrated. Multilayer printing of CNFs provides a route to control drying time by depositing thin layers one at a time. A printing system with a pressure-controlled dispensing system was used to deposit aqueous CNF suspensions onto a temperature-controlled substrate. The geometry, roughness, and mechanical properties of the printed structures were characterized. The shape of the printed line profile is controlled by a combination of the wettability of the substrate, dispense rate, printing speed, and temperature of the substrate. Spatial variation of the elastic modulus of printed CNF structures was assessed with nanoindentation and the average percent difference was found to be small at ± 2.6% of the mean over the area of the printed lines. Through multilayer printing freestanding films with thicknesses greater than 60 μm were achieved. Tensile specimens were printed and characterized; a tensile strength of 72.6 MPa ± 7.4 MPa and a Young’s modulus of 10.2 GPa ± 1.2 GPa were measured.