Effect of time and temperature on DC
Results of this study showed that DC of 3D printing resins significantly increased with post-polymerization time and temperature. These results are confirmed by the findings of other studies [23–27]. Indeed, when temperature increases, viscosity of resin decreases, thus frequency of collisions between macromolecular chains increases and energy required to initiate polymerization reactions decreases, making monomer polymerization easier . In addition, the number of residual monomers decreases with a longer polymerization time [23, 25, 26]. However, the use of high post-curing temperatures could affect material properties if not properly controlled. Some authors have reported this would hasten the aging mechanism of the material [22–28].
DC values for each resin were higher compared to DC values of adhesive and resin composites [29–33]. 3D printing resins are composed of methacrylate like resin composites used for bonding brackets but contain less or no fillers in their matrix, which could explain these higher DC values [3, 21, 22]. Indeed, it has been reported that a direct resin composite has a lower DC if fillers rate in its matrix is high [29–33]. Differences in DC values could also be explained by the implementation technique. With SLA or DLP, material is polymerized in thinner layers than a material used in direct technique [2, 4, 6]. And, with 3D printing, the post-curing protocol applied to the material increases its DC [6, 23–26].
Nevertheless, DC values of 3D printing resins are not of 100%. In conventional technique with brackets, the surface exposed of resin composite is limited to excess and joint and with an occlusal splint or a clear aligner, the surface developed covers the entire surface of each tooth . However, materials for thermoforming stay in polymer state, without leading monomer release above the toxicity rate. [12–16]. So, 3D printed occlusal splints or clear aligners could release more monomers than previous techniques.
Effect of 3D resin and other parameters on DC
Our study showed no significant influence of the type of resin on DC. Each resin differs by its chemical composition (in particular photo-initiators concentration) and by their polymerization kinetics [3, 21]. A study showed that DC increases with the concentration of photo-initiators when concentration remains low but decreases at high concentration of photo-initiators . Moreover, printing technique used depends on resins studied. Dental LT is a resin printed by SLA while the four other resins are printed by DLP. Some authors showed an influence of printing technique on DC particularly on printing parameters (print layer thickness, deposition time of each layer…) [25–28]. A study reported that print layer thickness would influence DC: a layer thickness of 50 µm or 100 µm gives better DC than 25 µm . In our study, we used the same print layer thickness for each resin (100 µm).
Type of equipment used for post-curing (Light curing unit or LCU) would also influence DC, due to different parameters: including type of the light source (UV or LED), light intensity , spectrum of absorbed wavelengths, irradiation type (constant or with repeated flashes) [3, 23, 25, 26], time and temperature. The position of the specimen in the device has also an impact on DC [25, 26]. Normally, each manufacturer recommends post-curing their resins with their specific LCU. However, in our study, we post-cured all specimens with the same LCU (Form Cure at a wavelength of 405 nm) in order to study the mechanism of polymerization. That would explain the slight best DC values for the Dental LT resin. Besides, it shows that all devices can be used for all resins in adapting the LCU parameters. Moreover, shorter post-curing time are recommended for the other resins, because their specific LCU has highest irradiance than Form Cure .
Comparison with manufacturer recommendation
For Dental LT, manufacturer defines two post-curing protocols: a classic protocol of 30 min at 60° and an optimized protocol (full post-cure) of 60 min at 60°. Our results showed a higher DC value for a longer time (83,5%) or a higher temperature (83% at 60 min 80° and 85,4% at 90 min 80°), however without significant differences. So, the full post-cure post-polymerization protocol recommended by the manufactured would allow an optimal DC value.
For Ortho IBT, Ortho Rigid and Ortho Clear, manufacturer recommends a 10 min post-curing protocol without specifying any associated temperature. Likewise, for Keysplint soft, manufacturer indicates a 25 min post-curing protocol without associated temperature. (These shorter times are probably related to their specific LCU devices with higher irradiance than Form Cure used in this study [3, 6]). For these four resins, when a time of 10 min is chosen, a temperature of 80° allows a significant increase of DC, but without reaching the highest DC. Therefore, an increase of time such as 60 min, is necessary. On the contrary, at a temperature of 20°, a time of 60 min and even 90 min did not allow or almost an increase of DC. Consequently, increasing temperature such as 60°, is necessary. However, with the specific LCU device for these four resins, the control of the temperature is unknown [3, 26].
Thus, whatever the resin used it seems that a post-curing at 60 min and 60° would be the minimal conditions to obtain a proper polymerization. Then, an increase of temperature to 80° or time to 90 min can significantly often improve the DC values. Besides, at 80°, samples showed a yellowish color. This could be explained by a competition between photo and thermo-polymerization processes. When thermo-polymerization is too quick, free radicals of an initiator/co-initiator couple (probably phosphonyl emitted by the phosphine oxides such as BAPO) are unable to polymerize because blocked in the resin already cured, giving this yellow color [3, 21]. For this reason, it would be preferable to increase post-curing time to boost the DC [3, 21, 26].
At last, higher post-curing time and/or temperatures values could affect mechanical properties or microscopic deformations [22–28].