Mechanical Processing of Cellulose from Spent Coffee Grounds for Extrusion-Based 3D Printing

Spent coffee grounds (SCG) are an abundant lignocellulosic biomass resource; however, their use as structural materials has been limited by their low interfacial reactivity. In this study, we present a solvent-free approach for converting SCG-derived cellulose into a processable material through mechanical pretreatment alone. A key aspect of this work is the development of a water-dispersible cellulose self-matrix that provides structural integrity without the use of synthetic polymer binders. Ball milling was employed to increase the specific surface area and partially disrupt the crystalline domains of cellulose, thereby exposing a greater number of reactive hydroxyl groups. These structural changes improved slurry dispersion stability and enabled precise control of its rheological behavior. X-ray diffraction (XRD) analysis revealed an approximately 29% reduction in crystallinity, confirming effective decrystallization of the cellulose structure. Rheological measurements performed at a shear rate of 10 s⁻¹ showed a significant increase in viscosity together with pronounced shear-thinning behavior, demonstrating excellent printability for extrusion-based additive manufacturing. Field-emission scanning electron microscopy (FE-SEM) further indicated that these rheological characteristics originated from the increased surface roughness of cellulose particles and enhanced physical entanglement between them. The optimized slurries, containing 9–12 wt% cellulose, exhibited excellent extrudability and shape fidelity, enabling the fabrication of uniform three-dimensional architectures without phase separation or nozzle clogging. In addition, urushiol, a natural coating material traditionally used in Korea, was incorporated to reinforce the printed cellulose structures and improve their water resistance. FT-IR spectroscopy confirmed the chemical interactions between cellulose fibers and the urushiol-based polyphenol network. As a result, the fabricated composites exhibited substantially higher Young's modulus and flexural strength than untreated samples. Contact angle measurements also demonstrated an increase in surface hydrophobicity, indicating enhanced resistance to moisture. This study demonstrates a practical strategy for producing high-performance, biodegradable cellulose composites from SCG without the use of chemical binders or organic solvents. The proposed approach highlights the potential of SCG-derived cellulose as a sustainable structural material and a promising alternative to petroleum-based plastics.