Direct Ink Writing and Photocrosslinking of Hydroxypropyl Cellulose into Stable 3D Parts Using Methacrylation and Blending

Two 50% solid content solutions of methacrylated hydroxypropyl cellulose (MAHPC) with respective substitution degrees of 1.85 ± 0.04 (L_MAHPC) and 2.64 ± 0.04 (H_MAHPC) were screened for rheological properties, photocrosslinking kinetics and printability in relevance to direct ink writing (DIW). Photo-rheological and printability studies reveal that the rheological properties of both MAHPC inks are better suited for DIW than those of hydroxypropyl cellulose (HPC) inks. Namely, methacrylate grafting improves shear dynamic moduli at low strain but also shear thinning and shear recovery. Both inks completely cure within 30 s upon shining UV light. Photocrosslinking is found to follow the phenomenological autocatalytic Sestak-Berggren kinetic model. However, prolonged exposure to UV light past full cure upon DIW leads to part fracture. The narrow UV-cure time window consequently precludes the production of multilayer parts using UV-assisted DIW for these neat MAHPC inks. In contrast, when blending MAHPC with HPC, an optimal balance between curing kinetics and DIW conditions is achieved, and stable, high-fidelity 150-layered parts are produced. Altogether this research highlights the need to design the content of photocrosslinkable moieties of cellulose derivatives to photoprint high fidelity and stable 3D parts from HPC inks.