Use of Microfibrillated Cellulose/Lignosulfonate Blends as Carbon Precursors: Impact of Hydrogel Rheology on 3D Printing

The aim of the present study was to investigate the rheological properties of microfibrillated cellulose (MFC)/lignosulfonate (LS) hydrogels and to use them in the manufacture of carbon objects by 3D printing and carbonization. For this purpose, both flow mode and thixotropic mode were used to characterize the hydrogel rheological behavior, which was subsequently used to search for formulation/processability correlations during 3D printing of square cuboids. At a concentration of 2%, MFC displayed excellent printability, i.e., shear-thinning behavior with high yield stress and a viscoelastic response to a step-down shear rate variation. The addition of LS induced a drop in the yield stress, and above an LS mass fraction of 30%, the MFC/LS hydrogels displayed an inelastic thixotropic response with a drop in printability (viz., the printed cuboids underwent a continuous deformation until spreading of the hydrogel was complete). Above 50% LS, the high viscosity slowed the flow of the MFC/LS hydrogels, and the printed cuboids had minor deformation. Freeze and air drying of cuboids printed with LS mass fractions lower than 20% and higher than 50%, respectively, allowed the cuboids to keep their original shape, and their carbonization under inert gas led to the production of highly conducting objects. In line with the high density of the air-dried samples, carbonized samples displayed an irregular structure with pores and crackles generated during drying and carbonization, whereas the freeze-dried samples had the typical lamellar structure of ice-templated materials.