In this study, photo-curing kinetics for urethane-acrylate-based photo-inks for 3D printing were evaluated using a photo-differential scanning calorimetry analysis. Initially, the photopolymerization kinetics of di- and monofunctional monomers were separately studied at different temperatures (5–85 °C). Later, the photo-curing kinetics and mechanical properties of photo-inks based on different monomer mixtures (40/60–20/80) were evaluated. The results showed that urethane-dimethacrylate (UrDMA) and urethane-acrylate (UrA) had no light absorption in the region of 280–700 nm, making them a proper crosslinker and a reactive diluent, respectively, for the formulation of 3D-printing photo-inks. The kinetics investigations showed a temperature dependency for the photo-curing of UrDMA, where a higher photopolymerization rate (Rp,max: from 5.25 × 10−2 to 8.42 × 10−2 1/s) and double-bound conversion (DBCtotal: from 63.8% to 92.2%) were observed at elevated temperatures (5–85 °C), while the photo-curing of UrA was independent of the temperature (25–85 °C). Enhancing the UrA content from 60% to 80% in the UrDMA/UrA mixtures initially increased and later decreased the photopolymerization rate and conversion, where the mixtures of 30/70 and 25/75 presented the highest values. Meanwhile, increasing the UrA content led to lower glass transition temperatures (Tg) and mechanical strength for the photo-cured samples, where the mixture of 30/70 presented the highest maximum elongation (εmax: 73%).
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-by-layer morphology were investigated.
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