Additive Manufacturing of Cellulosic Materials with Robust Mechanics and Antimicrobial Functionality

DOI: 10.1002/admt.201600084 mechanical property improvements.[11,12] 3D printing of cellulose has also been achieved after dissolution in ionic liquid or as an aqueous suspension; however, the cellulose concentration is low, and the liquid must be removed following deposition, either through cellulose precipitation or freeze-drying, leading to low-density structures and limited dimensional stability.[9,10] Here we present a technique for cellulose-based AM via extrusion of cellulose acetate (CA) followed by optional conversion of the finished part to cellulose. In CA, typically 80% of the hydroxyl groups on the cellulose molecule are replaced by acetate groups, effectively disrupting the strong hydrogenbonding network (Figure 1b). Like cellulose, the low cost, wide availability (annual production of ≈980 000 t), formability, and glossy appearance of CA have led to its wide commercial use and suggest further uses if it could be processed by AM.[6] For 3D printing of CA, a viscous yet flowable CA feedstock is prepared by dissolution of CA powder in acetone (Figure 2). Acetone is chosen because it is an inexpensive solvent that evaporates quickly due to its high vapor pressure, and could be recycled if condensation schemes are introduced.[6,13] Extrusion of the CA feedstock is performed using a gantry-style desktop 3D printer (see the Experimental Section), where the standard polymer filament extruder is replaced by a capillary nozzle connected to a fluid dispenser that is loaded with the CA solution (Figure S1, Supporting Information). Acetone evaporates from the CA ink upon exit from the nozzle, allowing a solid cellulose acetate part to be built in a layer-by-layer process using a programmed toolpath. To enable and understand this process, the pressure-flow behavior of the CA was characterized (Figure S2, Supporting Information), and the apparent viscosity was calculated assuming a power law fluid model with the Rabinowitsch correction, with an exponentially decaying flow behavior index.[14–17] The viscosity was found to increase exponentially with cellulose acetate concentration, and a CA concentration of 25–35 wt% was found to be most suitable for the remainder of the work, providing a balance between flowability through the nozzle, and shape retention of the extruded line. When a higher molecular weight cellulose was used, the extrusion quality was best at a comparatively lower concentration in the feedstock. Optical imaging of a static filament hanging after exiting the nozzle was used to observe the shrinkage that occurs as acetone evaporates. As shown in Figure 2, shrinkage causes the initially circular cross-section to become ribbon-like and wrinkled as it simultaneously hardens, though this wrinkling is more pronounced when the filament is hanging freely. During the printing process the wet ink uniformly contacts the previous layer and then shrinks as the acetone evaporates. Therefore, when accounted for in the printing parameters, the local shrinkage of the filament due to evaporation does Additive manufacturing (AM), whereby 3D objects are built through the layer-by-layer deposition of material, is poised to have profound effects on many industries via rapid customization, fabrication of complex geometries, and redistribution of supply chains.[1] To fully realize the benefits of AM, we must create AM techniques that can process abundant materials at high rate and low cost, with functional properties that meet or exceed those achieved by conventional manufacturing routes. The potential for local on-demand production using AM, along with the development of renewable feedstocks, can also have significant impact on the sustainability of polymer-based manufacturing. Cellulose, as the primary reinforcement phase of diverse biological organisms, is the most abundant polymer on Earth.[2] It is also mechanically robust, inexpensive, biorenewable, biodegradable, and chemically versatile. Cellulose is therefore ubiquitous in modern life, and finds wide use in pharmaceuticals, construction, packaging, clothing, thermal insulation, and other applications (Figure 1a).[3] The mechanical properties of cellulose are also of significant interest for cost-effective composite materials; for example, regenerated cellulose fibers were measured to have 778 MPa tensile strength and 41 MJ m−3 toughness, which compares favorably to the toughness of Kevlar (33 MJ m−3).[4,5] Further, cellulose is easily chemically modified; many derivatives, such as water-soluble methyl cellulose and carboxymethyl cellulose, are used as thickeners, stabilizers, and binders in areas including building materials, pharmaceuticals, cosmetics, and foods.[2,6] Cellulose and its derivatives would thus be a sustainable alternative to the use of petroleum-derived thermoplastics in AM, and could enable the tailoring of material chemistry and mechanics to a wide variety of applications. However, because of its strong hydrogen bonding, pure cellulose degrades upon heating before becoming sufficiently flowable for extrusion-based AM.[6] As a result, extant methods of AM using cellulose-containing feedstock have used low cellulose concentrations, either as a filler in other materials[7,8] or in solvent/ suspension.[9,10] For example, cellulose nanocrystals have been incorporated into polymers for additive manufacturing,[7,8] but the low cellulose concentration combined with the general difficulties of bonding cellulose to polymers result in only small www.advmattechnol.de