Urethane-acrylate-based photo-inks for digital light processing of flexible materials

Urethane-acrylate-based photo-inks containing various concentrations (0.1-1.5 wt.%) of two photo-initiators, namely ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate (TPOL) or diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (BPO), for digital light processing (DLP) were developed. According to photo-DSC kinetics investigations, no significant difference was detected between the photo-activity of formulations containing BPO or TPOL at various concentrations. BPO (1.0 wt.%) with a high molar extinction coefficient (500 L/mol•cm at 365 nm) resulted in higher controllability on the layer thickness (100 µm) during the 3D printing process. The surface cracks that appeared during the post-curing process could be avoided by splitting the exposure time (5 min) into short intervals (5 × 1 min) without affecting double bond conversion (DBC). Several flexible objects were successfully 3D printed in good quality and their thermomechanical properties and layer-bylayer morphology were investigated.