Additive Manufacturing and Mechanical Properties of Cellulose Nanofibril Materials

Cellulose nanofibrils (CNFs) are a nanomaterial derived from plants that have high specific stiffness and strength, can be made into optically transparent sheets, and are biodegradable. These properties make CNFs an attractive building block for bulk structural materials. However, CNFs are typically produced in aqueous suspension at low CNF weight fractions (\u3c 1 wt.%), which makes manufacturing bulk CNF materials challenging due to long processing times and the development of significant residual stresses during drying. As a result, applications of CNFs as structural materials are currently limited to thin sheets and their use as low concentration reinforcement in composite materials. The objective of this dissertation is to overcome current limitations in building neat CNF materials by using additive manufacturing approaches to print sheets from aqueous CNF solutions with controlled fiber orientation and to build bulk structures with mm-scale thicknesses and enhanced mechanical properties. This dissertation reports the use of two additive manufacturing techniques, direct ink writing and laminated object manufacturing, to fabricate neat CNF thin sheets with controlled orientation and materials with millimeter-scale dimensions, respectively. The orientation of the CNFs in the printed sheets and the mechanical properties of the sheets and laminated CNF materials were experimentally characterized. Orientation in the printed CNF sheets was found to be controlled by the drying mechanics, and a correlation between fiber orientation and stiffness was observed. Multi-ply CNF sheets and laminated bulk beams with thicknesses of up to 0.6 mm were fabricated and found to have comparable stiffness and increased toughness compared to single-layer CNF sheets. Key contributions of this dissertation include the development of a printing process to reduce the time to fabricate CNF sheets, a demonstration and a mechanics-based understanding of the control of fiber orientation in printed CNF materials, and a new process to realize bulk neat CNF materials with increased thickness and enhanced toughness