RESEARCH
Degree of Conversion, Depth of Cure, and Color Stability of Experimental Dental Composite Formulated with Camphorquinone and Phenanthrenequinone Photoinitiators PEDRO PAULO A. C. ALBUQUERQUE, DDS*, MARCUSV. L. BERTOLO, DDS†, LARISSA M. A. CAVALCANTE, DDS, MSc, PhD‡, CARMEM PFEIFER, DDS, MSc, PhD§, LUIS F. S. SCHNEIDER, DDS, MSc, PhD**¶
ABSTRACT Purpose: This study evaluated the applicability of 9,10-phenanthrenequinone (PQ) in experimental dental composites. Materials: Camphorquinone (CQ), PQ, ethyl 4-N,N-dimethylaminobenzoate (EDMAB) and diphenyliodonium salt (DPI) were employed. A mixture of 2,2-bis(4-[2-hydroxy-3-methacryloxypropoxy]phenyl)-propane/triethylene glycol dimethacrylate (60:40%) and silanated glass filler at 60% were used. A two-peak-based light-emitting diode (LED) was used. Methods: The photoinitiator absorption and the light emission spectra were determined by a Ultraviolet–visible spectroscopy and a spectroradiometer, respectively. Relative photon absorption (RPabs) was calculated. Fouriertransformed infrared spectroscopy analysis was used to determine the degree of conversion (DC). The optical properties were determined with a spectrophotometer. Depth of cure was assessed from adapted International Organization for Standardization (ISO) 4049. Results were analyzed with descriptive analysis, analysis of variance, and Tukey’s test (α = 5%). Results: PQ showed higher RPabs than CQ. Regarding the DC, CQ + EDMAB (control), CQ + EDMAB + DPI, PQ + DPI, and PQ + EDMAB + DPI produced statistically similar results. Groups formulated with CQ presented higher depth of cure. Only the group formulated with CQ + EDMAB presented satisfactory color stability (ΔE < 3.3). Conclusion: PQ presented higher RPabs than CQ and it was able to produce DC similar to CQ + EDMAB, when used with DPI. However, groups formulated with PQ produced lower depth of cure, greater yellowing, and less color stability than the traditional combination CQ and amine.
CLINICAL SIGNIFICANCE Although research with novel photoinitiator systems should be encouraged, the traditional camphorquinone and amine pair remains as a reliable combination for the formulation of dental composites. (J Esthet Restor Dent ••:••–••, 2015)
*MSc Student, Department of Biomaterials and Oral Biology, School of Dentistry, University of São Paulo—USP, São Paulo, Brazil † MSc Student, Dental Material Area, Piracicaba Dental School, State University of Campinas—UNICAMP, Piracicaba, SP, Brazil ‡ Adjunct Professor, Department of Restorative Dentistry, School of Dentistry, Federal Fluminense University—UFF, Niteroi, Brazil) and (Salgado de Oliveira University—UNIVERSO, Niterói, Brazil) § Assistant Professor, Department of Restorative Dentistry, Biomaterials and Biomechanics, School of Dentistry, Oregon Health and Science Unviersity—OHSU, Portland, OR, USA **Adjunct Professor, Department of Restorative Dentistry, School of Dentistry, Federal Fluminense University—UFF, Niteroi, Brazil) ¶ Dental Biomaterials Research Lab, School of Dentistry, Veiga de Almeida University—UVA, Rio de Janeiro, Brazil
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INTRODUCTION In the last 40 years, dental composites have been used as the material of choice for direct restorative procedures. Since the introduction of the visible-light-activated dental composites, the combination of camphorquinone (CQ) with an amine remains the most traditional photoinitiator/co-initiator system used by dental materials manufacturers.1,2 Unfortunately, this interesting binary system presents some drawbacks, and studies involving alternative photoinitiators are encouraged. Initially, dental composites were formulated with benzoin-methyl-ether and activated by ultraviolet (UV)-emitting sources. However, UV light might cause harmful effects for patients and operators as well as produce inferior material properties.2 Therefore, the combination CQ/amine was added in the formulation of dental composites due to the fact that CQ absorbs wavelengths in the blue region of the spectrum.2,3 However, due to its intense yellow hue4 and the need of a co-initiator, studies have shown that composites formulated with CQ tend to undergo significant color changes with time, which might be resultant from the oxidation of the amine-deviated co-initiator and/or due to the incomplete reaction of the CQ after material photoactivation.4,5 Therefore, modifications in the material photoinitiator/co-initiator systems have been encouraged. Phenylpropanedione (PPD) was proposed as an alternative photoinitiator system in 1999. In theory, this component would be able to reduce the yellowing rate and to present a synergistic effect with CQ on the generation of free radicals.6 However, specific results for optical properties concluded that the degree of yellowing of PPD-based materials might be higher than those based on CQ, and this fact was associated with the reduced efficiency of dissociation of the molecule of PPD to connect with co-initiator amine.4,5 Other studies have tested photoinitiators derived from phosphine oxides, and it has been observed that these components are able to produce materials with greater color stability than the combination CQ/amine over time.7,8 Phosphine oxide-derived components are classified as
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Norrish type I photoinitiators, meaning that they do not require a co-initiator during the polymerization process.9 After light is absorbed, the molecules undergo direct cleavage that generates a higher number of reactive free radicals.10 Unfortunately, such components tend to produce materials with lower depth of cure than those formulated with CQ, which in turn might create restorations with low mechanical properties and/or higher toxicity.11 In effect, the alternative photoinitiators tested have also presented serious disadvantages when compared with the binary CQ/amine system, and new formulations are still desired. 9,10-Phenanthrenequinone (PQ) is an aromatic diketone that appears as an orange solid. Theoretically, this photoinitiator could present lower yellowing than CQ due to a lower absorption peak (λmax 420 nm). This initiator has been used for holographic data storage in the last few years,12,13 but there is no research confirming the benefits of using PQ as well as no data reporting the need for co-initiators in dental materials applications. Therefore, the main objective of this study was to determine the applicability and effectiveness of using PQ as a photoinitiator in formulations of experimental dental composites. The specific objectives were to determine: (1) the relative photon absorption (RPabs), (2) the degree of conversion (DC), (3) the depth of cure, and (4) the color stability of composites formulated with different photoinitiator/co-initiator systems. The following research hypotheses were tested: The RPabs would be higher for CQ than PQ Composites formulated with CQ would present higher DC than those with PQ regardless of the addition of a co-initiator Composites formulated with CQ would present higher depth of cure than those with PQ Composites formulated with PQ would present improved color stability than those with CQ
MATERIALS AND METHODS A 2,2-bis(4-[2-hydroxy-3-methacryloxypropoxy]phenyl)propane and triethylene glycol dimethacrylate (Esstech
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Inc., Essington, PA, USA) blend was obtained at a molar ratio of 60:40%, respectively. CQ and PQ were added as photoinitiators. Ethyl 4-N,N-dimethylaminobenzoate (EDMAB; Sigma-Aldrich, Chemie, Steinheim, Germany) and diphenyliodonium salt (DPI, Sigma-Aldrich) were tested as potential co-initiators. The components were added according to the following experimental groups: Group 1: CQ (0.5% mol) + EDMAB− (1% mol) − control group Group 2: CQ (0.5% mol) + EDMAB (1% mol) + DPI (0.5% mol) Group 3: PQ (0.5% mol) Group 4: PQ (0.5% mol) + EDMAB (1% mol) Group 5: PQ (0.5% mol) + DPI (1% mol) Group 6: PQ (0.5% mol) + EDMAB (1% mol) + DPI (0.5% mol)
of 350 to 520 nm. Calibration was performed by pure absolute ethanol without the addition of photoinitiator. The molar extinction coefficients (mm/mol/L) were calculated from the absorbance values using the Beer–Lambert law:
A (λ ) = ε (λ ) [c ] l where A(λ) is the spectrophotometer absorbance at each wavelength, ε(λ) is the molar extinction coefficient, [c] is the molar concentration of the photoinitiator, and l is the optical pathlength through the cuvette. Therefore, the molar extinction coefficient is:
ε (λ ) = A (λ ) [ c ] l .
Silane-treated fillers were added at a 60 total wt% (75 wt% of barium–aluminium–silicate with 2 μm average size; 15 wt% barium–aluminium–silicate with 0.7 μm average size; and 10 wt% silicon dioxide with 16 nm average size; R972, Evonik Degussa, Essen, Germany).
RPabs was used to quantify the possible relations between the light emission spectrum and the CQ and PQ absorption spectra. This was calculated as:
Light Emission and Photoinitiator Absorption Spectra Readings, Molar Extinction Coefficient, and RPabs Calculation
in which E(λ) is the spectral irradiance of the LCU in (mW/cm2)/nm emitted from the LED LCU and abs(λ) is the photoinitiator absorption at that given wavelength. The E(λ) values used for the Absorbed power density (PDabs) calculation were those obtained when the light guide was positioned close to the power meter.
The total light-curing unit (LCU) power output (mW) of a light-emitting diode (LED) LCU (Bluephase G2; Ivoclar Vivadent, Liechtenstein, Liechtenstein) was measured with a power meter (Powermax 5200; Molectron, Portland, OR, USA). Irradiance (E), in mW/cm2, was determined by dividing the power output by the area of the light guide. The LCU emission spectrum was determined in the 350 to 550 nm range by using an integrating sphere connected to a spectrophotometer (USB2000; Ocean Optics, Dunedin, FL, USA). Photoinitiators (0.5 mol%) were diluted in 20 mL of absolute ethanol (Sigma-Aldrich). Absorption spectra of the photoinitiators were measured using a Ultraviolet– visible diode array spectrophotometer (DU 800; Beckman Coulter, Indianapolis, IN, USA) over the range
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RPabs = ∫ E (λ ) abs (λ )
Degree of C = C Conversion The DC was evaluated using Fourier-transformed infrared spectroscopy (FTIR, Alpha; Bruker Optik GmbH, Ettlingen, Germany), with an attenuated total reflectance device. A preliminary reading of the unpolymerized composite was taken in the absorbance mode (24 scans, 4 cm resolution). After the monomer reading, the composite was immediately photoactivated for 40 seconds. A support was coupled to the spectrometer in order to hold the curing unit and standardize a 0.5 mm distance between the fiber tip and the material. Five minutes after the photoactivation procedure, the DC reading was repeated. Calculation
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was carried out using baseline techniques, considering the intensity of C = C stretching vibration (peak height) at 1,638 cm, and as an internal standard, using the symmetric ring stretching at 1,608 cm. Five specimens per material were tested under controlled temperature (25 ± 1°C) and humidity (45 ± 5%) conditions. DC was calculated with the following formula:
{ (
%DC = 100 × 1 − ⎡⎣ C = C cured Aromatic cured C = C uncured Aromatic uncured ⎤⎦
(
)}
)
Depth of Cure The depth of cure was based on the method described in ISO 4049. Disc specimens were prepared by placing composites in split cylindrical molds (10 mm height × 2 mm diameter). After the photoactivation procedure (40 seconds), the mold was opened and the non-polymerized material was removed with a plastic instrument by the scratching method. The remaining material height was measured with a digital caliper (N = 5).
at room temperature (25 ± 1°C) in the dark. A spectrophotometer (CM2600d; Konica Minolta, Ramsey, NJ, USA) was used to evaluate the CIEL*a*b* coordinates of the specimens. The Commission Internationale de l’Eclairage (CIELAB) system is composed of three axes or coordinates: L* (lightness, from 0 = black to 100 = white), a* (from −a = green to +a = red), and b (from −b = blue to +b = yellow). The measurements considered D65 illuminant and a 10° observer. Color readings were taken immediately after the photoactivation procedure; they were repeated 24 hours after photoactivation (baseline) and repeated after the specimens were stored in distilled water at room temperature for 2 months (storage medium changed weekly). The color stability was reported as the color change (ΔE) after storage by using the following equation:14
ΔE = ([ ΔL]2 + [ Δa ]2 + [ Δb]2 ) . 12
Statistical Analyses Data were analyzed with descriptive analysis, analysis of variance, and Tukey’s test (p = 0.05).
Optical Properties Analyses The composites were photoactivated for 40 seconds inside a stainless steel mold (8.7 mm inner diameter, 1 mm thickness) placed between two clear matrix strips (N = 5). The photoactivated specimens were dry-stored
RESULTS Figure 1 shows spectral irradiance of the LCU used. The third-generation LED-based LCU emits a
FIGURE 1. Spectral irradiance of the light-curing unit (measured irradiance = 1,200 mW/cm2).
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“two-peak” spectrum, with the highest one centered in the blue region and a lower second peak on the violet region of the spectrum. Figure 2 shows the absorption profiles as a function of the wavelength for the different photoinitiators tested. Although CQ has a maximum absorption at 468 nm, the absorption peak for PQ occurred at 413 nm. The molar extinction coefficients were 47 mm/mol/L for CQ and 554 mm/mol/L for PQ. Figure 3 details the plots obtained as a result from the product of the LCU emission spectrum with each of the photoinitiators absorption spectra. The higher relationship between PQ and LCU is reflected on the
results of RPabs, in which PQ presented a higher value (335) than found for CQ (53). Table 1 shows the mean value results of DC, depth of cure, and ΔE after 2 months of storage. Data obtained from the FTIR analyses indicate that the results of DC for PQ + DPI, PQ + EDMAB + DPI, CQ + EDMAB + DPI, and CQ + EDMAB did not differ statistically. On the other hand, these groups produced higher values than those obtained for PQ + EDMAB and PQ alone. The depth of cure was higher for materials formulated with CQ than those with PQ.
FIGURE 2. Light absorption spectra of the different photoinitiator systems.
FIGURE 3. Plots obtained as a result from the product of the light-curing unit (LCU) emission spectrum with each of the photoinitiators absorption spectra.
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The color change observed for the control group (CQ + EDMAB) was statistically lower than those obtained for the other groups, which did not differ statistically among themselves. Figure 4 shows the L* axis values obtained for each material according to the storage time and condition. All groups presented a very narrow darkening effect when comparing the values obtained after 24 hours with those taken immediately after photoactivation. Groups formulated with PQ present lower values of lightness after 2 months in wet conditions. Figure 5 shows the a* axis values obtained for each material according to the storage time and condition. After 2 TABLE 1. Mean values and standard deviations for degree of conversion (DC), depth of cure, and color stability (ΔE) Group
DC in %
Depth of cure in mm
ΔE
CQ + EDMAB
59.3 (1.5) A
4.2 (0.3) A
2.7 (0.3) B
CQ + EDMAB + DPI
61.3 (2.1) A
3.9 (0.2) A
5.0 (0.3) A
PQ
51.3 (0.5) B
2.7 (0.1) B
6.1 (1.3) A
PQ + EDMAB
54.1 (2.0) B
2.4 (0.1) B
5.8 (1.0) A
PQ + DPI
63.0 (2.0) A
2.6 (0.1) B
6.0 (1.1) A
PQ + EDMAB + DPI
62.3 (1.5) A
2.6 (0.1) B
6.3 (1.3) A
Values followed by different upper case letter in the same column are statistically different (p < 0.05). For DC, depth of cure and ΔE, p < 0.001.
months in water, it can be observed that groups formulated with CQ presented negative values of a*, which translates into a greenish coloration. On the other hand, groups formulated with PQ showed a reddish coloration, observed by the positive values of a*. Figure 6 shows the b * axis values obtained for each material according to the storage time and condition. For all groups, there was an intense increase in the +b axis values after 24 hours dry storage, meaning that samples became yellowish. After storage in water, groups formulated with CQ became less yellow with time. Differently, those groups formulated with DPI became more yellow within the next 2 weeks of water storage and, subsequently, stabilized.
DISCUSSION In order to formulate new dental composites to overcome the drawbacks of materials containing the CQ/amine system, alternative photoinitiators have been tested in experimental formulations. Therefore, the present study verified the applicability of PQ in the formulations of dental composites. From the results shown in the emission spectrum profile by the LCU (Figure 1) and the absorption spectra obtained from the photoinitiators (Figure 2), it can be inferred that the light source and initiators would be compatible. This relationship was quantified by calculating RPabs, with
FIGURE 4. L-values according to photoinitiator system used and the storage time/condition.
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FIGURE 5. a-values according to photoinitiator system used and the storage time/condition.
FIGURE 6. b-values according to photoinitiator system used and the storage time/condition.
the conclusion that PQ showed higher value than CQ and, therefore, the first hypothesis was rejected. The second hypothesis tested—that groups formulated with CQ would present higher values of DC than groups formulated with PQ regardless of the addition of a co-initiator—was also rejected since PQ + DPI and PQ + EDMAB + DPI were able to produce similar result as those from groups formulated with CQ. Although PQ alone produced statistically lower conversion than the control group, the material formulated with this component was able to generate 50% of conversion. This finding might relate to high molar extinction coefficient and RPabs, and may be
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useful if other applications are considered, as, for example, to overcome the problems associated with inactivation of the tertiary amine in acidic medium.15 Since PQ and PQ + EDMAB presented lower conversion than PQ + DPI and PQ + EDMAB + DPI—that produced statistically similar conversion as CQ + EDMAB and CQ + EDMAB + DPI—it might be speculated that the co-initiator used plays fundamental role for those groups formulated with PQ. Previous studies indicate that DPI is able to optimize the monomer conversion in two ways: through the reactivation of inactive free radicals and also regenerating the original photosensitizer (e.g., CQ) to start the polymerization process.16,17 Those prior studies involve CQ-based initi-
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ating systems and the results here demonstrate that the benefits of DPI use extend to the PQ initiator as well. The third hypothesis proposed—that composites formulated with CQ would present higher depth of cure than those with PQ—was accepted since all materials formulated with CQ developed higher depth of cure than those based on PQ and the poor depth of polymerization of PQ could be related with the absorption range of this component. PQ presents the absorption peak near the UV region and presents a curve extended to the visible region of the spectrum. The high absorption in the UV region decreases the light irradiance at shorter wavelengths and, thus, reduces the possibility of light penetration through the body of the restoration.11 One previous study based on photoinitiator systems derived from phosphine oxides showed similar trends to those obtained in the present study.7 According to the color analysis, the fourth hypothesis—that composites formulated with PQ would present lower values of color change than those with CQ—was rejected. It has been considered that satisfactory values of color change are those lower than 3.3. In Table 1, it is possible to observe that only CQ + EDMAB was able to produce “low” color change (ΔE = 2.7) after 2 months of storage in water. Conversely, groups formulated with PQ showed high values of color change at the end of the analysis and several mechanisms might explain this behavior. Analyzing the axis L*, of the CIELab parameter, it can be seen that all groups showed a “darkening effect” after 24 hours. It is well known that the polymerization process continues after the light exposure ends (“dark-cure”)18 and also that the refractive index mismatch between fillers and monomers become less prominent as the polymerization process proceeds. Therefore, it is possible that after 24 hours, the material achieves higher DC and becomes more translucent and, consequently, darker. For those groups formulated with DPI (CQ + EDMAB + DPI, PQ + DPI, PQ + EDMAB + DPI), there was a trend of a very narrow increase of L* values after the samples were exposed to water; this can be associated with the
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leaching of these components. However, additional studies should be conducted to clarify this behavior. Considering the results for a* and b* axes, it seems that both chromatic coordinates follow the same trend and are probably related to photoinitiators’ ability to generate free radicals and/or leach after immersion in water. In Figure 6, it is possible to verify that all groups formulated with PQ showed higher values of yellowing (+b) than CQ, in special after 2 months of storage in distilled water. Although DC analysis in the FTIR did not differ from the groups formulated with CQ + EDMAB, CQ + EDMAB + DPI, PQ + DPI, and PQ + EDMAB + DPI, the depth of cure test reveals that those formulated with CQ improved the polymerization efficiency throughout the samples if compared with PQ. Considering only those groups formulated with PQ, the addition of DPI increases DC and reduces yellowing, since the amount of unreacted PQ decreases, which also brings the yellow values down. For those groups formulated with CQ, the b * axis values decrease with the constant water changes and this might be related to the leaching process of the non-reactive components on the polymerization process.7
CONCLUSIONS The materials formulated with CQ presented similar DC as those formulated with PQ when this alternative photoinitiator is tested in conjunction with DPI and/or EDMAB + DPI. However, composites containing CQ presented higher depth of cure and color stability than the proposed systems for PQ. Considering the overall results from the current study, it is possible to state that the traditional combination of CQ and amine remains as a reliable combination for the formulation of dental composites.
DISCLOSURE AND ACKNOWLEDGEMENTS This work was supported by FAPERJ (Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro), grant E-26/112.027/2012. The authors do not have any financial interest in any of the companies
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whose products are included in this article. This work was supported by FAPERJ (Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro), grant E-26/112.027/2012.
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Reprint requests: Pedro Paulo A. C. Albuquerque, DDS, MSc Student, Biomaterials and Oral Biology, University of São Paulo, Carlos de Vasconcelos Street, nº45/4, 20521-050, Rio de Janeiro, RJ, Brazil; email: [email protected]
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Degree of Conversion, Depth of Cure, and Color Stability of Experimental Dental Composite Formulated with Camphorquinone and Phenanthrenequinone Photoinitiators PEDRO PAULO A. C. ALBUQUERQUE, DDS*, MARCUSV. L. BERTOLO, DDS†, LARISSA M. A. CAVALCANTE, DDS, MSc, PhD‡, CARMEM PFEIFER, DDS, MSc, PhD§, LUIS F. S. SCHNEIDER, DDS, MSc, PhD**¶
ABSTRACT Purpose: This study evaluated the applicability of 9,10-phenanthrenequinone (PQ) in experimental dental composites. Materials: Camphorquinone (CQ), PQ, ethyl 4-N,N-dimethylaminobenzoate (EDMAB) and diphenyliodonium salt (DPI) were employed. A mixture of 2,2-bis(4-[2-hydroxy-3-methacryloxypropoxy]phenyl)-propane/triethylene glycol dimethacrylate (60:40%) and silanated glass filler at 60% were used. A two-peak-based light-emitting diode (LED) was used. Methods: The photoinitiator absorption and the light emission spectra were determined by a Ultraviolet–visible spectroscopy and a spectroradiometer, respectively. Relative photon absorption (RPabs) was calculated. Fouriertransformed infrared spectroscopy analysis was used to determine the degree of conversion (DC). The optical properties were determined with a spectrophotometer. Depth of cure was assessed from adapted International Organization for Standardization (ISO) 4049. Results were analyzed with descriptive analysis, analysis of variance, and Tukey’s test (α = 5%). Results: PQ showed higher RPabs than CQ. Regarding the DC, CQ + EDMAB (control), CQ + EDMAB + DPI, PQ + DPI, and PQ + EDMAB + DPI produced statistically similar results. Groups formulated with CQ presented higher depth of cure. Only the group formulated with CQ + EDMAB presented satisfactory color stability (ΔE < 3.3). Conclusion: PQ presented higher RPabs than CQ and it was able to produce DC similar to CQ + EDMAB, when used with DPI. However, groups formulated with PQ produced lower depth of cure, greater yellowing, and less color stability than the traditional combination CQ and amine.
CLINICAL SIGNIFICANCE Although research with novel photoinitiator systems should be encouraged, the traditional camphorquinone and amine pair remains as a reliable combination for the formulation of dental composites. (J Esthet Restor Dent ••:••–••, 2015)
*MSc Student, Department of Biomaterials and Oral Biology, School of Dentistry, University of São Paulo—USP, São Paulo, Brazil † MSc Student, Dental Material Area, Piracicaba Dental School, State University of Campinas—UNICAMP, Piracicaba, SP, Brazil ‡ Adjunct Professor, Department of Restorative Dentistry, School of Dentistry, Federal Fluminense University—UFF, Niteroi, Brazil) and (Salgado de Oliveira University—UNIVERSO, Niterói, Brazil) § Assistant Professor, Department of Restorative Dentistry, Biomaterials and Biomechanics, School of Dentistry, Oregon Health and Science Unviersity—OHSU, Portland, OR, USA **Adjunct Professor, Department of Restorative Dentistry, School of Dentistry, Federal Fluminense University—UFF, Niteroi, Brazil) ¶ Dental Biomaterials Research Lab, School of Dentistry, Veiga de Almeida University—UVA, Rio de Janeiro, Brazil
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INTRODUCTION In the last 40 years, dental composites have been used as the material of choice for direct restorative procedures. Since the introduction of the visible-light-activated dental composites, the combination of camphorquinone (CQ) with an amine remains the most traditional photoinitiator/co-initiator system used by dental materials manufacturers.1,2 Unfortunately, this interesting binary system presents some drawbacks, and studies involving alternative photoinitiators are encouraged. Initially, dental composites were formulated with benzoin-methyl-ether and activated by ultraviolet (UV)-emitting sources. However, UV light might cause harmful effects for patients and operators as well as produce inferior material properties.2 Therefore, the combination CQ/amine was added in the formulation of dental composites due to the fact that CQ absorbs wavelengths in the blue region of the spectrum.2,3 However, due to its intense yellow hue4 and the need of a co-initiator, studies have shown that composites formulated with CQ tend to undergo significant color changes with time, which might be resultant from the oxidation of the amine-deviated co-initiator and/or due to the incomplete reaction of the CQ after material photoactivation.4,5 Therefore, modifications in the material photoinitiator/co-initiator systems have been encouraged. Phenylpropanedione (PPD) was proposed as an alternative photoinitiator system in 1999. In theory, this component would be able to reduce the yellowing rate and to present a synergistic effect with CQ on the generation of free radicals.6 However, specific results for optical properties concluded that the degree of yellowing of PPD-based materials might be higher than those based on CQ, and this fact was associated with the reduced efficiency of dissociation of the molecule of PPD to connect with co-initiator amine.4,5 Other studies have tested photoinitiators derived from phosphine oxides, and it has been observed that these components are able to produce materials with greater color stability than the combination CQ/amine over time.7,8 Phosphine oxide-derived components are classified as
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Norrish type I photoinitiators, meaning that they do not require a co-initiator during the polymerization process.9 After light is absorbed, the molecules undergo direct cleavage that generates a higher number of reactive free radicals.10 Unfortunately, such components tend to produce materials with lower depth of cure than those formulated with CQ, which in turn might create restorations with low mechanical properties and/or higher toxicity.11 In effect, the alternative photoinitiators tested have also presented serious disadvantages when compared with the binary CQ/amine system, and new formulations are still desired. 9,10-Phenanthrenequinone (PQ) is an aromatic diketone that appears as an orange solid. Theoretically, this photoinitiator could present lower yellowing than CQ due to a lower absorption peak (λmax 420 nm). This initiator has been used for holographic data storage in the last few years,12,13 but there is no research confirming the benefits of using PQ as well as no data reporting the need for co-initiators in dental materials applications. Therefore, the main objective of this study was to determine the applicability and effectiveness of using PQ as a photoinitiator in formulations of experimental dental composites. The specific objectives were to determine: (1) the relative photon absorption (RPabs), (2) the degree of conversion (DC), (3) the depth of cure, and (4) the color stability of composites formulated with different photoinitiator/co-initiator systems. The following research hypotheses were tested: The RPabs would be higher for CQ than PQ Composites formulated with CQ would present higher DC than those with PQ regardless of the addition of a co-initiator Composites formulated with CQ would present higher depth of cure than those with PQ Composites formulated with PQ would present improved color stability than those with CQ
MATERIALS AND METHODS A 2,2-bis(4-[2-hydroxy-3-methacryloxypropoxy]phenyl)propane and triethylene glycol dimethacrylate (Esstech
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PHOTOINITIATOR SYSTEMS AND PROPERTIES OF COMPOSITE Albuquerque et al.
Inc., Essington, PA, USA) blend was obtained at a molar ratio of 60:40%, respectively. CQ and PQ were added as photoinitiators. Ethyl 4-N,N-dimethylaminobenzoate (EDMAB; Sigma-Aldrich, Chemie, Steinheim, Germany) and diphenyliodonium salt (DPI, Sigma-Aldrich) were tested as potential co-initiators. The components were added according to the following experimental groups: Group 1: CQ (0.5% mol) + EDMAB− (1% mol) − control group Group 2: CQ (0.5% mol) + EDMAB (1% mol) + DPI (0.5% mol) Group 3: PQ (0.5% mol) Group 4: PQ (0.5% mol) + EDMAB (1% mol) Group 5: PQ (0.5% mol) + DPI (1% mol) Group 6: PQ (0.5% mol) + EDMAB (1% mol) + DPI (0.5% mol)
of 350 to 520 nm. Calibration was performed by pure absolute ethanol without the addition of photoinitiator. The molar extinction coefficients (mm/mol/L) were calculated from the absorbance values using the Beer–Lambert law:
A (λ ) = ε (λ ) [c ] l where A(λ) is the spectrophotometer absorbance at each wavelength, ε(λ) is the molar extinction coefficient, [c] is the molar concentration of the photoinitiator, and l is the optical pathlength through the cuvette. Therefore, the molar extinction coefficient is:
ε (λ ) = A (λ ) [ c ] l .
Silane-treated fillers were added at a 60 total wt% (75 wt% of barium–aluminium–silicate with 2 μm average size; 15 wt% barium–aluminium–silicate with 0.7 μm average size; and 10 wt% silicon dioxide with 16 nm average size; R972, Evonik Degussa, Essen, Germany).
RPabs was used to quantify the possible relations between the light emission spectrum and the CQ and PQ absorption spectra. This was calculated as:
Light Emission and Photoinitiator Absorption Spectra Readings, Molar Extinction Coefficient, and RPabs Calculation
in which E(λ) is the spectral irradiance of the LCU in (mW/cm2)/nm emitted from the LED LCU and abs(λ) is the photoinitiator absorption at that given wavelength. The E(λ) values used for the Absorbed power density (PDabs) calculation were those obtained when the light guide was positioned close to the power meter.
The total light-curing unit (LCU) power output (mW) of a light-emitting diode (LED) LCU (Bluephase G2; Ivoclar Vivadent, Liechtenstein, Liechtenstein) was measured with a power meter (Powermax 5200; Molectron, Portland, OR, USA). Irradiance (E), in mW/cm2, was determined by dividing the power output by the area of the light guide. The LCU emission spectrum was determined in the 350 to 550 nm range by using an integrating sphere connected to a spectrophotometer (USB2000; Ocean Optics, Dunedin, FL, USA). Photoinitiators (0.5 mol%) were diluted in 20 mL of absolute ethanol (Sigma-Aldrich). Absorption spectra of the photoinitiators were measured using a Ultraviolet– visible diode array spectrophotometer (DU 800; Beckman Coulter, Indianapolis, IN, USA) over the range
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RPabs = ∫ E (λ ) abs (λ )
Degree of C = C Conversion The DC was evaluated using Fourier-transformed infrared spectroscopy (FTIR, Alpha; Bruker Optik GmbH, Ettlingen, Germany), with an attenuated total reflectance device. A preliminary reading of the unpolymerized composite was taken in the absorbance mode (24 scans, 4 cm resolution). After the monomer reading, the composite was immediately photoactivated for 40 seconds. A support was coupled to the spectrometer in order to hold the curing unit and standardize a 0.5 mm distance between the fiber tip and the material. Five minutes after the photoactivation procedure, the DC reading was repeated. Calculation
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was carried out using baseline techniques, considering the intensity of C = C stretching vibration (peak height) at 1,638 cm, and as an internal standard, using the symmetric ring stretching at 1,608 cm. Five specimens per material were tested under controlled temperature (25 ± 1°C) and humidity (45 ± 5%) conditions. DC was calculated with the following formula:
{ (
%DC = 100 × 1 − ⎡⎣ C = C cured Aromatic cured C = C uncured Aromatic uncured ⎤⎦
(
)}
)
Depth of Cure The depth of cure was based on the method described in ISO 4049. Disc specimens were prepared by placing composites in split cylindrical molds (10 mm height × 2 mm diameter). After the photoactivation procedure (40 seconds), the mold was opened and the non-polymerized material was removed with a plastic instrument by the scratching method. The remaining material height was measured with a digital caliper (N = 5).
at room temperature (25 ± 1°C) in the dark. A spectrophotometer (CM2600d; Konica Minolta, Ramsey, NJ, USA) was used to evaluate the CIEL*a*b* coordinates of the specimens. The Commission Internationale de l’Eclairage (CIELAB) system is composed of three axes or coordinates: L* (lightness, from 0 = black to 100 = white), a* (from −a = green to +a = red), and b (from −b = blue to +b = yellow). The measurements considered D65 illuminant and a 10° observer. Color readings were taken immediately after the photoactivation procedure; they were repeated 24 hours after photoactivation (baseline) and repeated after the specimens were stored in distilled water at room temperature for 2 months (storage medium changed weekly). The color stability was reported as the color change (ΔE) after storage by using the following equation:14
ΔE = ([ ΔL]2 + [ Δa ]2 + [ Δb]2 ) . 12
Statistical Analyses Data were analyzed with descriptive analysis, analysis of variance, and Tukey’s test (p = 0.05).
Optical Properties Analyses The composites were photoactivated for 40 seconds inside a stainless steel mold (8.7 mm inner diameter, 1 mm thickness) placed between two clear matrix strips (N = 5). The photoactivated specimens were dry-stored
RESULTS Figure 1 shows spectral irradiance of the LCU used. The third-generation LED-based LCU emits a
FIGURE 1. Spectral irradiance of the light-curing unit (measured irradiance = 1,200 mW/cm2).
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“two-peak” spectrum, with the highest one centered in the blue region and a lower second peak on the violet region of the spectrum. Figure 2 shows the absorption profiles as a function of the wavelength for the different photoinitiators tested. Although CQ has a maximum absorption at 468 nm, the absorption peak for PQ occurred at 413 nm. The molar extinction coefficients were 47 mm/mol/L for CQ and 554 mm/mol/L for PQ. Figure 3 details the plots obtained as a result from the product of the LCU emission spectrum with each of the photoinitiators absorption spectra. The higher relationship between PQ and LCU is reflected on the
results of RPabs, in which PQ presented a higher value (335) than found for CQ (53). Table 1 shows the mean value results of DC, depth of cure, and ΔE after 2 months of storage. Data obtained from the FTIR analyses indicate that the results of DC for PQ + DPI, PQ + EDMAB + DPI, CQ + EDMAB + DPI, and CQ + EDMAB did not differ statistically. On the other hand, these groups produced higher values than those obtained for PQ + EDMAB and PQ alone. The depth of cure was higher for materials formulated with CQ than those with PQ.
FIGURE 2. Light absorption spectra of the different photoinitiator systems.
FIGURE 3. Plots obtained as a result from the product of the light-curing unit (LCU) emission spectrum with each of the photoinitiators absorption spectra.
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The color change observed for the control group (CQ + EDMAB) was statistically lower than those obtained for the other groups, which did not differ statistically among themselves. Figure 4 shows the L* axis values obtained for each material according to the storage time and condition. All groups presented a very narrow darkening effect when comparing the values obtained after 24 hours with those taken immediately after photoactivation. Groups formulated with PQ present lower values of lightness after 2 months in wet conditions. Figure 5 shows the a* axis values obtained for each material according to the storage time and condition. After 2 TABLE 1. Mean values and standard deviations for degree of conversion (DC), depth of cure, and color stability (ΔE) Group
DC in %
Depth of cure in mm
ΔE
CQ + EDMAB
59.3 (1.5) A
4.2 (0.3) A
2.7 (0.3) B
CQ + EDMAB + DPI
61.3 (2.1) A
3.9 (0.2) A
5.0 (0.3) A
PQ
51.3 (0.5) B
2.7 (0.1) B
6.1 (1.3) A
PQ + EDMAB
54.1 (2.0) B
2.4 (0.1) B
5.8 (1.0) A
PQ + DPI
63.0 (2.0) A
2.6 (0.1) B
6.0 (1.1) A
PQ + EDMAB + DPI
62.3 (1.5) A
2.6 (0.1) B
6.3 (1.3) A
Values followed by different upper case letter in the same column are statistically different (p < 0.05). For DC, depth of cure and ΔE, p < 0.001.
months in water, it can be observed that groups formulated with CQ presented negative values of a*, which translates into a greenish coloration. On the other hand, groups formulated with PQ showed a reddish coloration, observed by the positive values of a*. Figure 6 shows the b * axis values obtained for each material according to the storage time and condition. For all groups, there was an intense increase in the +b axis values after 24 hours dry storage, meaning that samples became yellowish. After storage in water, groups formulated with CQ became less yellow with time. Differently, those groups formulated with DPI became more yellow within the next 2 weeks of water storage and, subsequently, stabilized.
DISCUSSION In order to formulate new dental composites to overcome the drawbacks of materials containing the CQ/amine system, alternative photoinitiators have been tested in experimental formulations. Therefore, the present study verified the applicability of PQ in the formulations of dental composites. From the results shown in the emission spectrum profile by the LCU (Figure 1) and the absorption spectra obtained from the photoinitiators (Figure 2), it can be inferred that the light source and initiators would be compatible. This relationship was quantified by calculating RPabs, with
FIGURE 4. L-values according to photoinitiator system used and the storage time/condition.
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FIGURE 5. a-values according to photoinitiator system used and the storage time/condition.
FIGURE 6. b-values according to photoinitiator system used and the storage time/condition.
the conclusion that PQ showed higher value than CQ and, therefore, the first hypothesis was rejected. The second hypothesis tested—that groups formulated with CQ would present higher values of DC than groups formulated with PQ regardless of the addition of a co-initiator—was also rejected since PQ + DPI and PQ + EDMAB + DPI were able to produce similar result as those from groups formulated with CQ. Although PQ alone produced statistically lower conversion than the control group, the material formulated with this component was able to generate 50% of conversion. This finding might relate to high molar extinction coefficient and RPabs, and may be
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useful if other applications are considered, as, for example, to overcome the problems associated with inactivation of the tertiary amine in acidic medium.15 Since PQ and PQ + EDMAB presented lower conversion than PQ + DPI and PQ + EDMAB + DPI—that produced statistically similar conversion as CQ + EDMAB and CQ + EDMAB + DPI—it might be speculated that the co-initiator used plays fundamental role for those groups formulated with PQ. Previous studies indicate that DPI is able to optimize the monomer conversion in two ways: through the reactivation of inactive free radicals and also regenerating the original photosensitizer (e.g., CQ) to start the polymerization process.16,17 Those prior studies involve CQ-based initi-
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ating systems and the results here demonstrate that the benefits of DPI use extend to the PQ initiator as well. The third hypothesis proposed—that composites formulated with CQ would present higher depth of cure than those with PQ—was accepted since all materials formulated with CQ developed higher depth of cure than those based on PQ and the poor depth of polymerization of PQ could be related with the absorption range of this component. PQ presents the absorption peak near the UV region and presents a curve extended to the visible region of the spectrum. The high absorption in the UV region decreases the light irradiance at shorter wavelengths and, thus, reduces the possibility of light penetration through the body of the restoration.11 One previous study based on photoinitiator systems derived from phosphine oxides showed similar trends to those obtained in the present study.7 According to the color analysis, the fourth hypothesis—that composites formulated with PQ would present lower values of color change than those with CQ—was rejected. It has been considered that satisfactory values of color change are those lower than 3.3. In Table 1, it is possible to observe that only CQ + EDMAB was able to produce “low” color change (ΔE = 2.7) after 2 months of storage in water. Conversely, groups formulated with PQ showed high values of color change at the end of the analysis and several mechanisms might explain this behavior. Analyzing the axis L*, of the CIELab parameter, it can be seen that all groups showed a “darkening effect” after 24 hours. It is well known that the polymerization process continues after the light exposure ends (“dark-cure”)18 and also that the refractive index mismatch between fillers and monomers become less prominent as the polymerization process proceeds. Therefore, it is possible that after 24 hours, the material achieves higher DC and becomes more translucent and, consequently, darker. For those groups formulated with DPI (CQ + EDMAB + DPI, PQ + DPI, PQ + EDMAB + DPI), there was a trend of a very narrow increase of L* values after the samples were exposed to water; this can be associated with the
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leaching of these components. However, additional studies should be conducted to clarify this behavior. Considering the results for a* and b* axes, it seems that both chromatic coordinates follow the same trend and are probably related to photoinitiators’ ability to generate free radicals and/or leach after immersion in water. In Figure 6, it is possible to verify that all groups formulated with PQ showed higher values of yellowing (+b) than CQ, in special after 2 months of storage in distilled water. Although DC analysis in the FTIR did not differ from the groups formulated with CQ + EDMAB, CQ + EDMAB + DPI, PQ + DPI, and PQ + EDMAB + DPI, the depth of cure test reveals that those formulated with CQ improved the polymerization efficiency throughout the samples if compared with PQ. Considering only those groups formulated with PQ, the addition of DPI increases DC and reduces yellowing, since the amount of unreacted PQ decreases, which also brings the yellow values down. For those groups formulated with CQ, the b * axis values decrease with the constant water changes and this might be related to the leaching process of the non-reactive components on the polymerization process.7
CONCLUSIONS The materials formulated with CQ presented similar DC as those formulated with PQ when this alternative photoinitiator is tested in conjunction with DPI and/or EDMAB + DPI. However, composites containing CQ presented higher depth of cure and color stability than the proposed systems for PQ. Considering the overall results from the current study, it is possible to state that the traditional combination of CQ and amine remains as a reliable combination for the formulation of dental composites.
DISCLOSURE AND ACKNOWLEDGEMENTS This work was supported by FAPERJ (Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro), grant E-26/112.027/2012. The authors do not have any financial interest in any of the companies
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whose products are included in this article. This work was supported by FAPERJ (Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro), grant E-26/112.027/2012.
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Reprint requests: Pedro Paulo A. C. Albuquerque, DDS, MSc Student, Biomaterials and Oral Biology, University of São Paulo, Carlos de Vasconcelos Street, nº45/4, 20521-050, Rio de Janeiro, RJ, Brazil; email: [email protected]
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