Mechanical Characterization of Cellulose Nanofibril Materials Made by Additive Manufacturing

Abstract

Cellulose nanomaterials have high specific stiffness and strength, are optically transparent, and are biodegradable, making them an attractive building block for bulk materials. The overall dimensions of neat bulk cellulose nanofibril (CNF) materials is significantly limited by the development of residual stresses generated during the drying process, when the source CNF is 1.0 wt.% in water or less, or by agglomeration, when the source CNF is greater than 1.0 wt.%. Here, we overcome these issues by producing CNF films and structures by additive manufacturing (i.e., 3D printing) of a shear thinning aqueous CNF suspension onto hydrophobic substrates under controlled drying conditions. Films of enhanced thicknesses, greater than 80 μm, are achieved as a result of the multistep layer-by-layer manufacturing process. The mechanical properties of the resulting materials are characterized via nanoindentation and tensile testing. Nanoindentation is used primarily to map the mechanical properties and examine variations in properties spatially and through the thickness. Tensile testing, with strain measurement via digital image correlation, is used to characterize the bulk properties. Mechanical characterization is supported by additional characterization via atomic force, optical, and electron microscopy. This study demonstrates the ability to additively manufacture stiff, strong, uniform, and scalable cellulose nanofibril materials.