Improvement in Thermal Conductivity in UV-Curable Polymer Composites via h-BN and Graphite Hybrid Fillers for DLP 3D Printing

UV-curable polymer composites are attractive for fabricating complex components by digital light processing (DLP), but improving thermal transport while preserving printability remains challenging at high filler loadings. In this work, solvent-free UV-curable formulations filled with hexagonal boron nitride (h-BN) and h-BN/graphite hybrids were developed for DLP 3D printing using commercially available equipment. The effects of filler composition on viscosity, printability, microstructure, through-thickness thermal conductivity, electrical conductivity, and tensile behavior were investigated. Viscosity increased markedly with filler loading, yet reliable DLP printing was achieved up to 40 wt% h-BN through composition-dependent adjustment of build parameters. Thermal analysis supported negligible macroscopic sedimentation during printing, while optical and FE-SEM observations revealed generally uniform platelet dispersion, visible 50 μm layer stratification, and limited phase segregation in the hybrid systems. The through-thickness thermal conductivity increased from ~0.25 W/mK for the neat resin to ~1.95 W/mK at 40 wt% h-BN. At a fixed 20 wt% h-BN, graphite addition led to a smaller increase in thermal conductivity, up to ~1.16 W/mK, while increasing electrical conductivity and reducing mechanical performance. A phenomenological percolation-type model captured the thermal-conductivity trend of the h-BN series. Overall, h-BN-rich formulations provided the most effective route to enhance thermal conductivity while preserving electrical insulation.