Evaluating oxygen shielding effect using glycerin or vacuum with varying temperature on 3D printed photopolymer in post-polymerization

The photosensitive resin used in additive manufacturing is cured by free radical polymerization by UV irradiation. However, undesired reaction with oxygen during polymerization inhibits polymerization and results in an under-cured polymer. Therefore, in this study, the hypothesis that successful oxygen shielding in the post-polymerization step could affect the properties of the final polymer was tested. 3D printed specimens using denture base resin were post polymerized either by immersion in glycerin for oxygen shielding (GL group) or placed in a medium-low vacuum chamber at 5 × 10-2 Torr (VA group). Specimens cured with no additional conditioning served as the control (CON group). To consider the effect of temperature, all groups were additionally compared with 80 °C and without an increase in temperature (room temperature) during post-polymerization. Fourier transform infrared spectroscopy was used to measure the monomer conversion ratios between different groups. In addition, the mechanical properties were quantified by the micro-hardness, flexural strength, and elasticity of the surface, and the water sorption and solubility. Dynamic mechanical analysis (DMA) was conducted to observe the trend in storage and loss modulus between the groups against temperature. Differences in the surface as a function of the post-polymerization conditions were qualitatively observed by scanning electron microscopy (SEM). The result shows that oxygen shielding during post-polymerization showed an increase in the degree of conversion (DC) and hardness of the resin surface. The highest DC was observed for GL group specimens at both room temperature and 80 °C. This result was confirmed by the SEM micrographs of the resin surface, where the interface between the layers of the GL group structure becomes more robust. However, a difference was observed between the samples prepared at room temperature and 80 °C. The flexural modulus was highest in the GL group, followed by the VA group, and lowest in the CON group at 80 °C. No difference in water absorption was observed for any groups, but high water solubility was observed in the GL group at room temperature. Overall, more significant differences in the properties were observed for the samples post-polymerized at 80 °C than at room temperature. The results of DMA analysis to determine the glass transition temperature showed a similar trend in all groups, and the storage modulus and loss rate obtained in the same experiment decreased in the order of GL, CON, and VA. In conclusion, an oxygen shielded post-polymerization environment at elevated temperature effectively improves the mechanical properties of photosensitive resin.