Abstract
Objectives
The aim of our study is to find an amine free photoinitiating system (PIS) for the polymerization of representative dental methacrylate resins. A photoinitiating system (PIS) based on camphorquinone (CQ)/triphenylgermanium hydride/diphenyl iodonium hexafluorophosphate is proposed and compared to the conventional CQ/amine couple. The polymerization monitoring of thin (∼20 μm) and thick (1.4 mm) samples of a bisphenol A-glycidyl methacrylate (Bis-GMA)/triethyleneglycol dimethacrylate (TEGDMA) blend (70%/30% w/w) and of a urethane dimethacrylate (UDMA) upon exposure to a commercial blue LED centered at 477 nm under air or in laminate is described. Finally, the impact of the photoinitiating system composition on the final polymer color is evaluated in detail.
Methods
FTIR and DSC experiments are used to record the photopolymerization profiles. ESR spectrometry and steady state photolysis are used to detect the produced radicals. Color measurements are carried out to determine the key parameters in the bleaching of the different dental formulations.
Results
The efficiency of the newly proposed PISs for the photopolymerization of BisGMA/TEGDMA and UDMA for thin (20 μm) or for thick (1.4 mm) samples upon exposure to a dental blue LED under air is excellent. It is noticeably higher than that of the CQ/amine reference couple. Excellent bleaching properties are also observed under irradiation in presence of the new PISs. A good correlation is found between the sample bleaching and the amount of Ph 3 GeH in the formulation. The excited state processes could be established. The overall chemical mechanisms for the initiation step were also clarified.
1
Introduction
Light induced free radical photopolymerization (FRP) and cationic photopolymerization (CP) reactions require the use of photoinitiators (PIs) and/or photoinitiating systems (PISs) that contain a PI and one or more additives, among them a co-initiator (see e.g., in Refs. ). In the field of dentistry, the camphorquinone (CQ)/amine (AH) (e.g., ethyldimethylaminobenzoate (EDB)) couple as a PI/co-initiator photoinitiating system has been thoroughly investigated and largely used for the manufacturing of resin-based composites (see e.g., a review in Ref. and in Refs. ); the amine has also been substituted, by e.g., an iodonium salt or a thiobarbituric derivative . The addition of a propanedione derivative (PD) to CQ/ N , N -cyanoethylmethylaniline increases the polymerization efficiency as the result from an increased light absorption. At least, two separated PISs are operating in a broad range of the UV–vis spectrum: CQ/AH and PD/AH. In the radiation curing area, CQ has been extensively studied as a PI and part of PIS for the FRP of high or low viscosity multifunctional acrylates and methacrylates (see e.g., in Refs. and reference therein) and the CP of epoxides . In this field, using appropriate, newly developed CQ-based PISs has recently led to an efficient CP of epoxides under air and FRP of acrylates in laminate as well as promising results for FRP under air (see e.g., in Refs. ); silanes, stannanes and germanes have also been claimed as efficient co-initiators in PISs .
The search for other PIs or suitable CQ/additives for the FRP of methacrylate dental resins remains to be of continuous interest as indicated e.g., by the proposal of other aromatic ketones and other amine co-initiators , bisimidazole based PISs , organoborates or titanocenes , the addition of monoacylphosphine oxides (e.g., TPO) or bisacylphosphine oxides (e.g., BAPO) , the incorporation of benzoyl germanium derivatives BzGe in CQ/AH/TPO systems , the synthesis of polymer tethered CQ , hydrophilic phosphine oxides , CQ-AH linked PI or BAPO-CQ linked PI or the complete change of CQ/AH for TPO or BAPO . The CQ/amine/iodonium salt system was proposed for the cationic photopolymerization of siloxane/oxirane blend or epoxide based dental composites . A totally different chemistry involving thiol-ene formulations as well as the use of cationic and hybrid matrices , bisphenol A free methacrylates or β-allyl sulfone containing dimethacrylates was also proposed. The development of other photosensitizers , the use of LEDs , the problem of the reciprocity law , the correlation of the experimental properties with the achieved performances have been recently carried out. It still appears that CQ based PISs remain the most attracting systems in the dentistry area today, as indicated by the vast number of works that are currently carried out on the use of e.g., CQ/AH, CQ/AH/TPO or CQ/TPO/BzGe (e.g., in Refs. ). When incorporated into a CQ/AH system, the additive effect of TPO or BAPO results from a direct light absorption of the phosphine oxide leading to an additional generation of initiating radicals rather than to a specific CQ/TPO (or BAPO) interaction where CQ would have played the role of a photosensitizer .
In the present paper, we propose a novel CQ based and amine free three-component PIS – CQ/triphenylgermanium hydride (Ph 3 GeH)/diphenyl iodonium hexafluorophosphate (DPI or Ph 2 I + ) – for the photopolymerization of two representative methacrylate resins ( Scheme 1 ): (i) a bisphenol A-glycidyl methacrylate (Bis-GMA)/triethyleneglycol dimethacrylate (TEGDMA) blend (70%/30% w/w) and (ii) an urethane dimethacrylate (UDMA) upon exposure to a commercial blue LED (used in the dentistry area and centered at 477 nm) under air or in laminate. Their polymerization ability will be checked by FTIR and photoDSC experiments. The involved mechanisms will be studied using ESR experiments.
2
Experimental part
2.1
Compounds
Triphenylgermanium hydride (Ph 3 GeH), diphenyl iodonium (DPI also called Ph 2 I + in some chemical reactions) hexafluorophosphate and camphorquinone (CQ) were obtained from Sigma–Aldrich. Ethyldiethylaminobenzoate (EDB—Esacure EDB from Lamberti) was chosen as the reference amine co-initiator. Bisphenol A-glycidyl methacrylate (Bis-GMA), triethyleneglycol dimethacrylate (TEGDMA) and a urethane dimethacrylate (UDMA) were also obtained from Sigma–Aldrich and used with the highest purity available ( Scheme 1 ).
2.2
Irradiation source
A dental blue LED centered at 477 nm (Dentsply SmartLite Focus; ∼80 mW cm −2 at the surface of the irradiated sample) was used for the irradiation of the samples (emission spectrum in Fig. 1 ).
2.3
Photopolymerization experiments
2.3.1
FTIR procedure
The photosensitive formulations were deposited on a BaF 2 pellet under air or in laminate (25 μm thick) for irradiation with the LED light. The evolution of the double bond content of Bis-GMA/TEGDMA blend was continuously followed by real time FTIR spectroscopy (JASCO FTIR 4100) at about 1630 cm −1 . The evolution of the Ge-H content in the Ph 3 GeH based formulations can be also followed at 2030 cm −1 .
For thick samples (1.4 mm), the polymerization was evaluated under air in the near infrared range following the band at 6160 cm −1 and the procedure presented just above.
2.3.2
PhotoDSC procedure
The polymerization enthalpies Δ R H were measured with a photo-calorimeter DSC 7/DPA 7 (PerkinElmer) having a light intensity in the visible portion of the spectrum of 108 mW/cm 2 in an isothermal mode at 37 °C. Each sample was irradiated twice. After the first run a second run was made that was subtracted from the first one. The subtraction of these runs from one another removed the effect of sample heating by illumination. Typically the DSC experiments were carried out at least twice. The photoDSC procedure is well known to record photopolymerization processes and has been already presented in many studies e.g., Refs. .
2.4
ESR spin trapping (ESR-ST) experiment
ESR-ST experiment was carried out using an X-Band spectrometer (MS 400 Magnettech). The radicals were generated at 477 nm under N 2 and trapped by phenyl- N – tert -butylnitrone (PBN) according to a procedure described elsewhere in detail . The ESR spectra simulations were carried out with the WINSIM software.
2.5
Multiple regression analysis
The multiple regression analyses were carried out with the Orgin 7.1 (Microcal TM Origin) software for estimating the relationships among the independent variables (here the concentrations of the compounds in the photoinitiating system) was used to characterize the final performance (i.e., the monomer conversion and the final color).
2
Experimental part
2.1
Compounds
Triphenylgermanium hydride (Ph 3 GeH), diphenyl iodonium (DPI also called Ph 2 I + in some chemical reactions) hexafluorophosphate and camphorquinone (CQ) were obtained from Sigma–Aldrich. Ethyldiethylaminobenzoate (EDB—Esacure EDB from Lamberti) was chosen as the reference amine co-initiator. Bisphenol A-glycidyl methacrylate (Bis-GMA), triethyleneglycol dimethacrylate (TEGDMA) and a urethane dimethacrylate (UDMA) were also obtained from Sigma–Aldrich and used with the highest purity available ( Scheme 1 ).
2.2
Irradiation source
A dental blue LED centered at 477 nm (Dentsply SmartLite Focus; ∼80 mW cm −2 at the surface of the irradiated sample) was used for the irradiation of the samples (emission spectrum in Fig. 1 ).
2.3
Photopolymerization experiments
2.3.1
FTIR procedure
The photosensitive formulations were deposited on a BaF 2 pellet under air or in laminate (25 μm thick) for irradiation with the LED light. The evolution of the double bond content of Bis-GMA/TEGDMA blend was continuously followed by real time FTIR spectroscopy (JASCO FTIR 4100) at about 1630 cm −1 . The evolution of the Ge-H content in the Ph 3 GeH based formulations can be also followed at 2030 cm −1 .
For thick samples (1.4 mm), the polymerization was evaluated under air in the near infrared range following the band at 6160 cm −1 and the procedure presented just above.
2.3.2
PhotoDSC procedure
The polymerization enthalpies Δ R H were measured with a photo-calorimeter DSC 7/DPA 7 (PerkinElmer) having a light intensity in the visible portion of the spectrum of 108 mW/cm 2 in an isothermal mode at 37 °C. Each sample was irradiated twice. After the first run a second run was made that was subtracted from the first one. The subtraction of these runs from one another removed the effect of sample heating by illumination. Typically the DSC experiments were carried out at least twice. The photoDSC procedure is well known to record photopolymerization processes and has been already presented in many studies e.g., Refs. .
2.4
ESR spin trapping (ESR-ST) experiment
ESR-ST experiment was carried out using an X-Band spectrometer (MS 400 Magnettech). The radicals were generated at 477 nm under N 2 and trapped by phenyl- N – tert -butylnitrone (PBN) according to a procedure described elsewhere in detail . The ESR spectra simulations were carried out with the WINSIM software.
2.5
Multiple regression analysis
The multiple regression analyses were carried out with the Orgin 7.1 (Microcal TM Origin) software for estimating the relationships among the independent variables (here the concentrations of the compounds in the photoinitiating system) was used to characterize the final performance (i.e., the monomer conversion and the final color).
3
Results and discussion
3.1
The polymerization initiating ability of the CQ/Ph 3 GeH/DPI system
The CQ/Ph 3 GeH/DPI photoinitiating system very efficiently initiates the polymerization of the Bis-GMA/TEGDMA blend (70%/30%, w/w; 25 μm thick films) upon exposure to the dental LED centered at 477 nm both in laminate and under air ( Fig. 2 ); the CQ/Ph 3 GeH system still exhibits a good efficiency but lower than that of CQ/EDB. Consequently, we can suggest that the presence of DPI enhances the polymerization initiating ability of the system (as shown by the increase in the initial reaction rate). Also, the polymerization profiles of samples with DPI are clearly indicating faster polymerization rates Rp and higher final conversion rates (FC) than those recorded when using the reference systems (CQ/EDB and CQ/EDB/DPI). The methacrylate conversion profile proceeds parallel to that of the Ge-H content ( Fig. 3 ) which suggests that the hydrogen abstraction from Ge-H is a critical step for the efficiency of the PIS. The stability of the CQ/Ph 3 GeH/DPI-based formulation after storage at RT for 1 month was verified by RT-FTIR—similar polymerization profiles were obtained ( Fig. 4 ).
The consumption of the CQ for the CQ/Ph 3 GeH/DPI system in the monomer formulation during the course of the polymerization can be easily followed by UV–vis spectroscopy (i.e., recording the optical density (O.D. at ∼470 nm)) ( Fig. 5 ). Interestingly, the CQ consumption is comparable in the CQ/Ph 3 GeH/DPI and CQ/EDB initiating systems ( Fig. 5 A vs. B). However, the formation of yellow photoproducts characterized by an increase of the polymer absorption in the 380–450 nm range is observed for the CQ/EDB initiating system but not for the Ph 3 GeH based system. The formation of yellow photoproducts for amine based systems is well known (see in Ref. ); remarkably, this behavior is not observed for Ph 3 GeH as co-initiator leading to better bleaching properties for these latter systems.
3.2
The CQ/Ph 3 GeH/DPI system for the polymerization of UDMA
In order to optimize the initiator concentration, a design of experiment (DOE) was planned and carried out. The composition of the initiating system has been optimized for the polymerization of a urethane dimethacrylate UDMA (UDMA allows high final conversions rendering easier a comparison of the different photoinitiating systems). The different contents of CQ/Ph 3 GeH/DPI are shown in Table 1 . The polymerization process was evaluated under air for thin (20 μm) and thick samples (1.4 mm). The oxygen inhibition being stronger for thinner samples, the use of both conditions is useful for a better understanding of the system performances. Examples of polymerization profiles are reported in Fig. 6 .
| Sample | CQ w/w % | Ph 3 GeH w/w % | DPI w/w % | Δ R H (kJ/mol) |
|---|---|---|---|---|
| 1 | 0.504 | 0.165 | 1.452 | −39.4 |
| 2 | 1.000 | 1.223 | 1.430 | −56.0 |
| 3 | 0.504 | 0.165 | 1.452 | −48.8 |
| 4 | 0.096 | 0.956 | 1.828 | −48.3 |
| 5 | 0.492 | 1.211 | 2.884 | −53.9 |
| 6 | 0.536 | 1.731 | 1.549 | −59.5 |
| 7 | 0.070 | 0.883 | 0.179 | −39.3 |
| 8 | 0.068 | 2.112 | 1.909 | −52.0 |
| 9 | 0.671 | 2.136 | 0.176 | −54.5 |
| 10 | 0.996 | 2.070 | 2.893 | −52.9 |
| 11 | 0.492 | 1.211 | 2.884 | −56.1 |
| 12 | 1.016 | 0.124 | 2.951 | −47.3 |
| 13 | 1.045 | 0.128 | 0.179 | −46.0 |
| 14 | 0.068 | 0.125 | 2.979 | −25.7 |
| 15 | 0.341 | 2.084 | 2.912 | −57.8 |
| 16 | 1.002 | 1.838 | 2.568 | −57.2 |
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