MEAM Doctoral Dissertation Defense: “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 materials, 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 (<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 in structural materials are currently limited to thin films 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 films 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 films with controlled orientation and materials with millimeter-scale dimensions, respectively. The orientation of the CNFs in the printed films and the mechanical properties of the films and laminated CNF materials were experimentally characterized. Orientation in the printed CNF films was found to be controlled by the drying mechanics, and a correlation between orientation and stiffness was observed. The multi-ply CNF films and laminated bulk beams with thicknesses of up to 0.6 mm were found to have comparable stiffness and strength and increased toughness compared to single-layer CNF films. Key contributions of this dissertation include the development of a printing process to decrease the time to fabricate CNF films, 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.