Degree of conversion of 3D printing resins used for splints and orthodontic appliances under different postpolymerization conditions

To measure the degree of conversion (DC) of different 3D printing resins used for splints or orthodontic appliances under different postpolymerization conditions. Five 3D-printed photopolymer resins were studied. Each resin was analyzed in liquid form (n = 15), and then cylindrical specimens (n = 135) were additively manufactured and postcured with Form Cure (Formlabs) at different times (10, 60, and 90 min) and temperatures (20 °C, 60 °C, and 80 °C). The DC of each specimen was measured with Fourier transform infrared spectroscopy (FTIR). The data were statistically analyzed using a 3-way ANOVA followed by Tukey’s post hoc test. The time and temperature of postpolymerization significantly influenced the DC of each resin: when time and/or temperature increased, the DC increased. For all resins tested, the lowest DC was obtained with a postcuring protocol at 10 min and 20 °C, and the highest DC was obtained at 90 min and 80 °C. However, at 80 °C, the samples showed a yellowish color. With the Form Cure device, the time and temperature of postcuring could have an impact on the DC of the 3D printing resins studied. The DC of the 3D printing resins could be optimized by adjusting the postpolymerization protocol. Regardless of the resin used, when using the Form Cure device, postcuring at 60 min and 60 °C would be the minimal time and temperature conditions for achieving proper polymerization. Beyond that, it would be preferable to increase the postcuring time to boost the DC.


Introduction
The recent development of additive manufacturing, also known as 3D printing, in dentistry provides new opportunities [1,2]. The main additive manufacturing technologies used include stereolithography (SLA) and digital light processing (DLP), each offering advantages depending on the type of object that is created [2,3]. These techniques polymerize photosensitive liquid resins to fabricate an object with a laser beam (SLA) or a light beam (DLP). After printing, the object is usually cleaned with 99% isopropylic alcohol (IPA) to remove uncured resins and then postcured in a lightcuring unit (LCU), as recommended by manufacturers to improve mechanical properties and complete the polymerization process [3][4][5][6].
In orthodontics, the fabrication of occlusal splints, retainers, and clear aligners usually involves thermoforming different thermoplastic sheets on models. The latter can be plaster models or currently 3D-printed models [7,8]. Ideally, 3D printing resins should be used to make direct occlusal splints, retainers, and clear aligners to bypass model printing and thermoforming processes, increasing efficiency and decreasing the environmental impact [1,7,[9][10][11].
However, while thermoplastic materials are very biocompatible [8,[12][13][14][15][16], few studies have examined the biocompatibility of 3D printing resins designated for intraoral use [11,12]. These resins are generally photopolymers composed of methacrylate, which is also found in bonding materials used in conventional fixed techniques [3,11,12]. Thus, these resins can release monomers into the oral environment. In addition, unlike brackets bonded to the buccal or lingual surfaces of each tooth where the release of monomers takes place on a tiny area (corresponding to the bonding joint and a small area of excess on the surface of the tooth), an occlusal splint or a clear aligner covers the entire surface of each tooth. The release of monomers could be much more important [17].
A material's degree of conversion (DC) is the primary indicating factor of its biocompatibility [18,19]. It is used to evaluate the conversion rate of monomers into polymers during polymerization and, therefore, the rate of residual monomers likely to be released. DC also plays an important role in mechanical properties or color stability [20]. DC of an object fabricated with the SLA or DLP process depends on several factors, such as print layer thickness and postcuring conditions (time, temperature, intensity of the light source, etc.) [21][22][23][24][25][26][27][28]. One method for measuring the DC of a material is Fourier transform infrared spectroscopy (FTIR), which determines the vibrations of C = C double bonds involved in the polymerization process [18,19]. In contrast with the studies on bonding materials or acrylic resins used for orthodontic appliances [29][30][31][32][33][34], there are no studies on the DC of 3D printing resins used for occlusal splints or orthodontic appliances.
The aim of this study was to measure the DC of five 3D printing resins used for occlusal splints, retainers, and clear aligners under different postpolymerization conditions.

Materials tested
Five resins for the 3D printing of occlusal splints or orthodontic appliances were tested (Table 1) For specimen fabrication, a cylinder 30 mm in diameter and 2 mm in thickness was designed with CAD software Fusion 360® (Autodesk, San Rafael, CA, USA), and data were exported in standard tessellation language (STL) format.

Sample preparation and protocols tested
First, each resin studied was analyzed before printing in a liquid, unpolymerized state (3 measurements of each specimen: 5 × n = 3 × 3). Then, each resin was analyzed in the polymerized state after printing and postprocessing (3 measurements of 3 specimens with 9 postcuring protocols: 5 × 9 × n = 3 × 3). Two printers were used for specimen manufacturing depending on their compatibility with the resins: a DLP printer (Sprintray Pro 95, Sprintray Inc., Los Angeles, CA, USA) for the Ortho Clear, Ortho Rigid, Ortho IBT, and KeySplint Soft resins and an SLA printer (Form 2, Formlabs, Somerville, MA, USA) for the Dental LT resin. For each resin, printing was carried out with a layer thickness of 100 µm, an angulation of 45°, and the addition of supports for better adhesion of pieces on the printer tray.
After printing, specimens were cleaned with isopropylic alcohol (IPA) for 20 min for the Dental LT resin and for 5 min for the other resins according to the manufacturer's recommendations for removing the excess uncured resin (FormWash, Formlabs, Somerville, MA, USA). Then, they were all postcured with the same LCU (Form Cure, Formlabs, Somerville, MA, USA) with a 405-nm wavelength according to different protocols: 3 different times (10, 60, and 90 min) and three different temperatures (20 °C, 60 °C, and 80 °C). This LCU was chosen because it allows the control of temperature, polymerization time, and irradiance ( Table 2).
After postprocessing, the supports were removed with a resin bur, and the specimens were polished with sandpaper (800 grits).

Measurement of the degree of conversion (DC)
DC was measured with Fourier transformed infrared spectroscopy (FTIR) with a Nicolet™ iS10 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) in attenuated total reflectance (ATR) mode. Spectra were recorded ( Fig. 1) with OMNIC software (Thermo Electron Corporation, Waltham, MA, USA). After measuring the background, all measurements were obtained in the spectral region of 500 to 4000 cm −1 under the following parameters: a resolution of 4 cm −1 and 32 internal scans per reading.
For each specimen, spectra were recorded 3 times. The DCs of all the samples in this experiment were measured by the same operator to avoid interoperator variability.
For each spectrum, the height of the absorption band of the aliphatic C = C bond (1638 cm −1 ) and that of the aromatic C = C bond (1608 cm −1 ) were measured, first in the monomer state and then after the polymerization process. The degree of conversion was determined by evaluating the change in the height ratio of the aliphatic C = C peak and the aromatic C = C peak during the polymerization process, according to the following formula: For the Dental LT and KeySplint Soft resins, peaks corresponding to aromatic C = C double bonds at 1608 cm −1 were replaced by the peaks of C = O double bonds at 1720 cm −1 . This peak is also a reference peak that is stable and easy to identify in spectra.

Statistical analysis
DC was expressed in percent by means and standard deviations. A Shapiro-Wilk test confirmed a normal distribution, and the equality of variances was assessed with a Levene test. A 3-way ANOVA followed by Tukey's post hoc test was performed for the factors "resin," "time," and "temperature." The significance level was set for all the tests at p < 0.05. Tests were carried out with XLSTAT (Addinsoft, Paris, France) and GraphPad Prism 9 software (GraphPad Software Inc., San Diego, CA, USA).

Results
The DC values obtained for each 3D printing resin are reported in Table 3 and summarized in Fig. 2.

3
The time and temperature of postpolymerization significantly influenced the DC of each resin: when the time and/ or temperature increased, the DC increased. For all resins tested, the lowest DC was obtained with a postcuring protocol of 10 min and 20 °C (54.6 ± 1.9 for Dental LT, 41.0 ± 2.2 for Ortho Clear, 37.8 ± 2.5 for Ortho Rigid, 40.5 ± 3.1 for Ortho IBT, and 40.3 ± 2.5 for KeySplint Soft), and the highest DC was obtained with a postcuring Fig. 1 a Example of spectra recorded for the Ortho IBT® resin in the unpolymerized state. b Example of spectra recorded for the Ortho IBT® resin in the polymerized state (10 min at 20 °C). For a time of 10 min, the temperature significantly influenced the DC between 20 and 60 °C but also between 60 and 80 °C. For a time higher than 60 min, the temperature significantly influenced the DC between 20 and 60 °C but not between 60 and 80 °C.
For a temperature of 20 °C, the time significantly influenced the DC between 10 and 60 min and between 60 and 90 min. For temperatures higher than 60 °C, the time generally significantly influenced the DC between 10 and 60 min but not between 60 and 80 min.
In addition, even if the DC values of the Dental LT resin were significantly higher than those of other resins studied for some time and temperature, there was no significant influence of the type of resin on DC (p > 0.05).
Finally, at 80 °C, the samples showed a yellowish color.

Effect of time and temperature on DC
The results of this study showed that the DC of 3D printing resins significantly increased with postpolymerization time and temperature. These results are confirmed by the findings of other studies based on 3D printing prosthetic materials [23][24][25][26][27]. Katheng [26]. In addition, the influence of the polymerization time and temperature on DC has already been reported for acrylic resins used for dentures and orthodontic appliances [34]. Indeed, when the temperature increases, the viscosity of the resin decreases; thus, the frequency of collisions between macromolecular chains increases, and the energy required to initiate polymerization reactions decreases, making monomer polymerization easier [23]. In addition, the number of residual monomers decreases with a longer polymerization time [23,25,26]. However, high postcuring temperatures can affect material properties if not properly controlled. Some authors have reported that this can hasten the aging mechanism of the material [22][23][24][25][26][27][28].
The DC values of each resin were higher than the DC values of the adhesive and resin composites (approximately 30-35% after light curing and 75% after 1 week) [29][30][31][32][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 can explain these higher  [3,21,22]. Indeed, it has been reported that a direct resin composite has a lower DC if the filler rate in its matrix is high [29][30][31][32][33]. The implementation technique can also explain the differences in DC values presented. With SLA or DLP, the material is polymerized in thinner layers than a material used in a direct approach [2,4,6]. With 3D printing, the postcuring protocol applied to the material increases its DC [6,[23][24][25][26]. Nevertheless, the DC values of 3D printing resins are not 100%. In the conventional technique with brackets, the surface exposure of the resin composite is limited to the excess and joint. With an occlusal splint or a clear aligner, the developed surface covers the entire surface of each tooth [17]. However, materials for thermoforming remain in a polymer state, without leading to monomer release above the toxicity rate. [12][13][14][15][16]. Therefore, 3D-printed occlusal splints or clear aligners could release more monomers than the materials in the former techniques.

Effect of 3D resin and other parameters on the DC
Our study showed no significant influence of the type of resin on the DC. Each resin differed by its chemical composition (in particular, the photoinitiator concentration) and its polymerization kinetics [3,21]. A study showed that the DC increases with the concentration of photoinitiator when the concentration remains low but decreases at a high concentration of photoinitiator [33]. Moreover, the printing technique used depends on the resins studied. Dental LT resin is a resin printed by SLA, while DLP is used to print the four other resins. Some authors have shown an influence of the printing technique on the DC, mainly through the printing parameters (print layer thickness, deposition time of each layer, etc.) [25][26][27][28]. A study reported that the print layer thickness influences the DC: a layer thickness of 50 µm or 100 µm yields a better DC than 25 µm [25]. Our study used the same print layer thickness for each resin (100 µm).
The type of equipment used for postcuring (light curing unit, LCU) also influences the DC, as reported by Reymus et al. [25], due to various parameters, including the type of light source (UV or LED), light intensity [26], 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 also has an impact on the DC [25,26].
Usually, each manufacturer recommends postcuring their resins with their specific LCU. However, in our study, we postcured all specimens with the same LCU (Form Cure at a wavelength of 405 nm) to study the mechanism of polymerization. That would explain the slightly better DC values observed for the Dental LT resin. Moreover, shorter postcuring times are recommended for the other resins because their specific LCU has a higher irradiance than Form Cure [26].

Comparison with manufacturer recommendations
For the Dental LT resin, the manufacturer defines two postcuring protocols: a classic protocol of 30 min at 60 °C and an optimized protocol (full postcure) of 60 min at 60 °C. However, our results showed a higher DC value for a longer time (83.5%) or a higher temperature (83% at 60 min and 80 °C and 85.4% at 90 min and 80 °C) without significant differences. Therefore, the full postcuring, postpolymerization protocol recommended by the manufacturer allows an optimal DC value.
For the Ortho IBT, Ortho Rigid, and Ortho Clear resins, the manufacturer recommends a 10-min postcuring protocol without specifying any associated temperature. Likewise, for the KeySplint Soft resin, the manufacturer indicates a 25-min postcuring protocol without an associated temperature. (These shorter times are probably related to their specific LCU devices with a higher irradiance than the Form Cure device used in this study [3,6]). For these four resins, using the Form Cure LCU with a time of 10 min, a temperature of 80 °C allows a significant increase in the DC, but without achieving the highest DC. Therefore, an increase in time, such as to 60 min, is necessary. In contrast, at a temperature of 20°, a time of 60 min and even 90 min did not allow or even increase the DC. Consequently, increasing the temperature, such as to 60 °C, is necessary. However, with the specific LCU device of each of these four resins, the temperature control is unknown [3,26].
Thus, using the Form Cure LCU, whatever the resin used, it seems that postcuring at 60 min and 60 °C would be the minimal conditions for achieving proper polymerization. Then, an increase in temperature to 80 °C or in time to 90 min can often significantly improve the DC values. In addition, at 80 °C, the samples showed a yellowish color. A competition between photo-and thermopolymerization processes can explain this. When thermopolymerization is too quick, free radicals of an initiator/coinitiator couple (probably phosphonyl emitted by phosphine oxides, such as BAPO) are unable to polymerize because they are blocked in the resin that is already cured, resulting in this yellow color [3,21]. For this reason, it would be preferable to increase the postcuring time to boost the DC [3,21,26].

Limitations
Only a single LCU (Form Cure) was used in our study; thus, the results obtained are not applicable to other LCUs. In addition, the DC measurements were limited to the specimen surface, and the DC in deeper layers remains to be investigated.

Conclusion
Under the conditions of this study, it was shown that postcuring time and temperature have an impact on the DC of 3D printing resins used in orthodontics for manufacturing occlusal splints, retainers, or clear aligners.
In our study, with the Form Cure LCU, a minimum time (approximately 60 min) combined with a minimum temperature (approximately 60 °C) yielded satisfactory DCs. While a higher temperature increases the DC but causes a yellowish resin coloration, a longer time contributes to improving the DC without color changes. Likewise, assessing if a longer time and/or a higher temperature can affect their mechanical properties would be relevant.
Further studies on the release of monomers are needed to complete the DC data regarding the biocompatibility of these resins.