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Sulfinates and sulfonates as high performance co-initiators in CQ based systems: Towards aromatic amine-free systems for dental restorative materials
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Sulfinates and sulfonates as high performance co-initiators in CQ based systems: Towards aromatic amine-free systems for dental restorative materials Julie Kirschner a , Florian Szillat b , Mariem Bouzrati-Zerelli a , Jean-Michel Becht a , Joachim E. Klee b , Jacques Lalevée a,∗ a
Institut de Science des Matériaux de Mulhouse IS2M, UMR CNRS 7361, UHA, 15 Rue Jean Starcky, 68057 Mulhouse Cedex, France b Dentsply Sirona, De-Trey-Strae 1, Konstanz, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. The aim of our study is to develop amine-free photoinitiating systems (PISs) for
Received 7 June 2019
the polymerization of representative dental methacrylate resins under blue light irradiation.
Received in revised form
PISs based on camphorquinone (CQ)/sulfinate and CQ/sulfonate, eventually in combination
27 October 2019
with an iodonium salt, are proposed and compared to the well-established CQ/amine sys-
Accepted 15 November 2019
tem. The polymerization performances of thick (1.4 mm) samples of different methacrylate
Available online xxx
blends upon exposure to a commercial blue LED centered at 477 nm under air are described.
Keywords:
application.
Sulfinates
Methods. FTIR is used to monitor the photopolymerization profiles. ESR spectroscopy and
Finally, the performances of the new developed PISs are evaluated for dental composites
Sulfonates
electrochemical experiments are used to identify the radicals generated. Mechanical prop-
Photoinitiator
erties measurements and color stability measurements are carried out to determine the key
Camphorquinone
properties of the dental composites prepared.
Methacrylates
Results and Significiance. The performances of the new proposed PISs for the photopolymer-
Dental composites
ization of thick (1.4 mm) samples of methacrylate upon exposure to a blue dental LED under air are excellent. Similar or better performances and bleaching properties are obtained with the new proposed amine-free systems compared to those reached with the CQ/amine reference system. Dental composites with excellent mechanical properties and exceptional color stability are obtained. The involved chemical mechanisms for the initiation step were also established. © 2019 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved.
1.
∗
Introduction
Free radical polymerization has been widely studied in the past decades and has many applications such as 3D printing, stereolithography, inks, coatings and dentistry. Two types of photoinitiators are mainly used in free radical processes
Corresponding author. E-mail address: [email protected] (J. Lalevée). https://doi.org/10.1016/j.dental.2019.11.020 0109-5641/© 2019 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved.
Please cite this article in press as: J. Kirschner, F. Szillat, M. Bouzrati-Zerelli et al.. Sulfinates and sulfonates as high performance co-initiators in CQ based systems: Towards aromatic amine-free systems for dental restorative materials. Dent Mater (2019), https://doi.org/10.1016/j.dental.2019.11.020
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and are classified as Type I and Type II photoinitiators [1]. Type I photoinitiators generate radicals by an homolytic cleavage whereas Type II photoinitiators require the presence of a co-initiator [1,2]. Among them, tertiary aromatic amines are particularly efficient co-initiators and are commonly used as hydrogen donors [3–5]. In dental restorative materials, the camphorquinone (CQ)/amine photoinitiating system (PIS) is clearly themost well-established reference system for the free radical photopolymerization of methacrylates due to its visible light absorption properties in the 400–500 nm spectral region. Ethyl-4-(dimethylamino)benzoate (EDB) is a very efficient hydrogen donor and is largely used in combination with CQ in dental materials [6]. However, the presence of tertiary aromatic amines in (meth-)acrylate containing compositions can cause yellowing (discoloration) of the resulting photocured material. Furthermore, the use of tertiary aromatic amines gives more and more rise to toxicological concerns [7–11]. Therefore, the development of amine-free PISs is of great interest for dental manufacturers. Some alternatives to amines have been proposed in the last years for example: 2-thiobarbituric acid [12], piperonyl alcohol [13], tris(trimethylsilyl)silane [6] and PH3 GeH [14]. These co-initiators present an enhanced biocompatibility and color stability but can remain less efficient than the traditionally used EDB [15]. In previous studies, sulfinates were used in combination with tertiary amines to enhance the adhesion to tooth substrate (dentin, enamel) [16] or to achieve better mechanical properties [17]. However to the best of our knowledge, the use of sulfinates and sulfonates as co-initiator was never investigated in CQ based photoinitiating system and was mainly found by coincidence. In the present paper, sulfinates and sulfonates are studied as efficient co-initiators for the replacement of amines in CQ/amine based PISs. Also a new synthesized iodonium salt (diphenyliodonium ptoluenesulfinate) including a sulfinate as counter anion is reported in this study. The involved chemical mechanisms are studied by electro spin resonance (ESR) and electrochemistry experiments. The performances of the new co-initiators are evaluated in combination with CQ for the free radical polymerization of methacrylates and followed by real-time FTIR spectroscopy. The performances of the proposed PISs are compared with a CQ/amine reference PIS. The bleaching properties playing an important role for the esthetic aspect of a respective material e.g. the color of the material directly after curing and even more important, the color stability of the aged material. Latter are also compared to those achieved with the reference system.
2.
Fig. 1 – The emission spectrum of a blue LED centered at 477 nm (SmartLite Focus from Dentsply Sirona, Germany).
obtained from Sigma Aldrich (Scheme 1). Diphenyliodonium p-toluenesulfinate (DPIpTS) was prepared by ion exchange using sodium p-toluenesulfinate and diphenyliodonium chloride both obtained from Sigma Aldrich (Scheme 1). The synthetic procedure used for the preparation of diphenyliodonium p-toluenesulfinate is described in the Supporting information. Camphorquinone was obtained from Sigma Aldrich and used as a representative Type II PI (Scheme 2). Ethyldimethylaminobenzoate (EDB) was used as additive in multicomponent systems and obtained from Sigma Aldrich (Scheme 2). Speedcure938 (SC938) was obtained from Lambson Ltd (Scheme 2). Bisphenol A-glycidyl methacrylate (BisGMA), triethylene glycol dimethacrylate (TEGDMA) and methacrylic acid (MA) were obtained from Sigma Aldrich and used with the highest purity available (Scheme 3). The BisGMA/TEGDMA (70%/30% w/w) blend was used as a benchmark matrix for methacrylates. 2-Hydroxyethyl methacrylate (HEMA) was obtained from TCI Chemicals (Scheme 3).Spectrum® TPH® 3 resin (mixture of modified Bis-GMA, TEGDMA and other methacrylate monomers) from Dentsply Sirona was used for the preparation of dental composites.
2.2.
A blue LED@477 nm representative for dental materials usage (SmartLite Focus from Dentsply Sirona ∼300 mW cm−2 in the selected conditions) was used for the irradiation of the photocurable samples (the emission spectrum is given in Fig. 1).
Experimental section 2.3.
2.1.
Irradiation sources
Photopolymerization experiments
Compounds
Sodium p-toluenesulfinate (NapTS), sodium 1-methyl 3sulfinopropanoate (NaMeSP), zinc benzylsulfinate (ZnBnS), sodium 4-(acetylamino)benzenesulfinate (NaAcABS), zinc isopropylsulfinate (ZniPrS), Sodium butylnaphtalenesulfinate (NaBuNS) and sodium p-toluenesulfonate (NapTSo) were
The photosensitive formulations were deposited on a BaF2 pellet under air for irradiation with the LED light (SmartLite Focus, 300 mW/cm2 ). The evolution of the methacrylate function conversions of BisGMA, TEGDMA and HEMA was continuously followed by real time FTIR spectroscopy (JASCO FTIR 6600) at about 6160 cm−1 for thick (1.4 mm) samples [18].
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Scheme 1 – Structures of the co-initiators used.
2.4.
ESR spin trapping experiments
ESR experiments were carried out using a Bruker EMXplus spectrometer (X-band). The radicals are generated at room temperature upon the blue LED exposure (SmartLite
Focus from Dentsply Sirona, 300 mW/cm2 ) under N2 . The radicals were trapped by phenyl-N-tert-butylnitrone (PBN, Scheme 2) according to a procedure already described [19]. The ESR spectra simulations were carried out using WINSIM software.
Scheme 2 – Structures of the photoinitiator (CQ) and additives used. Please cite this article in press as: J. Kirschner, F. Szillat, M. Bouzrati-Zerelli et al.. Sulfinates and sulfonates as high performance co-initiators in CQ based systems: Towards aromatic amine-free systems for dental restorative materials. Dent Mater (2019), https://doi.org/10.1016/j.dental.2019.11.020
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Scheme 3 – Monomers used in this work.
2.5. Redox potentials measurements and free energy change calculations The oxidation potential of Sulfinate (NapTS) was determined by cyclic voltammetry by a procedure already presented [20] (Radiometer PST006). The free energy change G for the electron transfer reaction between the sulfinate and CQ was calculated from the classical free energy change equation: G = Eox − Ered − ET + C
(1)
where Eox , Ered , ET , and C are the oxidation potential of the electron donor, the reduction potential of electron acceptor, the excited triplet state energy of the CQ, and the electrostatic interaction energy for the initially formed ion pair, generally considered as negligible in polar solvents.
2.6.
Mechanical properties measurements
Flexural strength (FS) measurements of the experimental composite formulations were determined using three-point bending flexural test according to ISO 4049:2009 (material class II).
2.7.
Fig. 2 – ESR spectra of the radicals generated in CQ/sulfinate (NapTS) and trapped by PBN in methacrylic acid/tert-butylbenzene (∼20:80) upon exposure to a LED@477 nm (SmartLite Focus): N2 saturated medium 䊉 SO R/PBN hyperfine coupling constants: a = 13.3 G, N 2 aH = 1.5 G and oxidized PBN (PBNox) aN = 7.9 G indicated by stars *; (a) experimental and (b) simulated spectra.
Color stability
The color stability of composites was investigated according to a slightly adapted procedure based on the method given in the ISO 4049:2009/ISO 7491 guideline. The procedure is described in the following:three disc specimens of each formula were prepared using a Liculite light oven (irradiation time 90 s/each side). Afterwards, the initial L*a*b* values of each specimen were measured using a Datacolor 800. One specimen was then stored in the dark and dry in the oven at 37 ± 2 ◦ C for 7 days (Scenario 1). One specimen was stored in the dark in the oven in water at 37 ± 2 ◦ C for 7 days (Scenario 2). The last specimen was first stored dark and dry in the oven at 37 ± 2 ◦ C for 24 ± 2 h. After this time, latter specimen was removed from the oven and blanked off half of it with aluminium foil (uncovered side − Scenario 3a/covered side − Scenario 3b). The specimen was then placed in a radiation chamber immersed in water (37 ± 2 ◦ C) and exposed to the radiation for 24 h at 150 000 ± 15 000 Lux. It was ensured that the water level
was 10 ± 3 mm above the specimen. After exposure, the aluminium foil was removed and the specimen was transferred back to the oven at 37 ± 2 ◦ C and stored in the dark and dry for 5 days. The change in color (E) was measured for each specimen after the conducted ageing scenario using a Datacolor 800. The E was calculated using following Eq. (2): E = deltaE =
L∗2 + a∗2 + b∗2
3.
Results and discussion
3.1.
Photochemical mechanisms
3.1.1.
From ESR: spin trapping experiments
(2)
In presence of a spin trap agent (PBN) for the photolysis of CQ/sulfinate e.g. CQ/NapTS under N2 upon exposure to the blue LED centered at 477 nm, the RSO2 •/PBN radical adduct
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Fig. 3 – Cyclic voltamogram of Sulfinate (NapTS) in water/buffer (pH = 3) under N2 for NapTS + buffer (red curve) and buffer alone (black curve). (For interpretation of the references to colour in the figure legend, the reader is referred to the web version of this article.)
is clearly observed: characterized by hyperfine coupling constants (hfcs) aN = 13.3 G, aH = 1.5 G in agreement with hfcs known in the literature [21] (Fig. 2). Therefore, ESR spin trapping experiments show the formation of benzene sulfonyl centered radicals generated from the CQ/sulfinate interaction upon light irradiation.
3.1.2.
From redox potentials
The oxidation potential (Eox ) of NapTS (as measured by cyclic voltammetry, Fig. 3) is 0.7 V (error bar < 0.05 V). The calculated electron transfer free energy change G from the triplet state of CQ is rather favorable (G = −0.1 V) using Ered for CQ = −1.4 V, ET = 2.2 V [1] for CQ (Eq. (1)). The ESR and redox potentials experiments revealed the formation of sulfonyl centered radicals according to reaction 1 (r1) through a favorable electron transfer between the sulfinate and camphorquinone. The sulfonyl radicals can initiate the polymerization of (meth)acrylate monomers in agreement with the good initiating ability of CQ/sulfinate vs. CQ alone found in polymerization experiments (see below, Fig. 4). A similar mechanism is expected between sulfonate and CQ to explain the polymerization initiating ability of CQ/sulfonate initiating systems (see polymerization experiments in Table 1). ∗CQ + RSO2 − → CQ 䊉− + RSO2 䊉
3.2.
(r1)
CQ/sulfinate and CQ/sulfonate initiating ability
3.2.1. Free radical polymerization initiating ability of CQ/NapTS for methacrylate monomers (MA/BisGMA/TEGDMA blend) The polymerization profiles of thick films (1.4 mm) of a MA/BisGMA/TEGDMA(10/63/27% w/w) blend upon exposure to the blue LED at 477 nm (SmartLite Focus) using different photoinitiating systems are depicted in Fig. 4. Under air, for a 300 mW cm−2 light intensity, the system CQ/NapTS exhibits very good polymerization initiating ability in thick samples. High polymerization rate Rp and high final methacrylate
Fig. 4 – Photopolymerization profiles (methacrylate function conversion vs irradiation time) for a MA/BisGMA/TEGDMA blend (10/63/27% w/w; 1.4 mm thick films) upon exposure to the LED@477 nm (I0 = 300 mW cm−2 ) using different photoinitiating systems. (1) CQ (0.5% w/w); (2) CQ/sulfinate (NapTS) (0.5/1% w/w). The irradiation starts for t = 5 s (see the dot line).
function conversion (FC) up to 80% are obtained for the CQ/sulfinate system contrary to CQ alone for which no polymerization occurred. For sulfinate alone, no polymerization is observed. These first experiments clearly highlight that sulfinate behaves as an efficient co-initiator in synergy with CQ. Remarkably, this system can efficiently initiate the free radical polymerization of methacrylates without additional amine.
3.2.2. Free radical polymerization initiating ability of CQ/NapTS/Iod for methacrylate monomers (MA/BisGMA/TEGDMA blend) Furthermore, the three-component system CQ/NapTS/Iod exhibits exceptional performances for the free radical polymerization of a MA/BisGMA/TEGDMA (10/63/27% w/w) blend upon exposure to the blue LED at 477 nm (SmartLite Focus).
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Table 1 – New proposed photoinitiating systems for free radical polymerization of different methacrylate blends upon visible light (under air; thickness = 1.4 mm; blue LED@477 nm, SmartLite Focus, 300 mW cm−2 ); monomer conversions reached after a SmartLite Focus exposure of 120 s (error bar ∼2%).
NaMeSP ZnBuS NaAcABS ZniPrS NaBuNS NapTSo
Two-component system
Three-component system
CQ/co-initiator (0.5/1% w/w)
CQ/co-initiator/Iod (0.5/1/1% w/w)
Resin 1
Resin 2
Resin 3
Resin 1
Resin 2
Resin 3
50% 50% 20% 70% 2% <5%
70% – 30% 70% – <5%
20% 20% 20% 75% – <5%
80% 82% 75% 80% 80% 70%
80% – 80% 70% – 80%
80% 88% 80% 80% – 80%
Resin 1: BisGMA/TEGDMA (70/30% w/w). Resin 2: MA/BisGMA/TEGDMA (10/63/27% w/w). Resin 3: HEMA/BisGMA/TEGDMA (10/63/27% w/w).
CQ 䊉+ + RSO2 − → CQ + RSO2 䊉
(r3)
CQ 䊉− + Ar2 I+ → CQ + Ar䊉 + ArI
(r4)
3.2.3. Free radical polymerization initiating ability of CQ/sulfinate or CQ/sulfonate and CQ/sulfinate/iod or CQ/sulfonate/iod systems for different methacrylate monomer blends
Fig. 5 – Photopolymerization profiles (methacrylate function conversion vs irradiation time) for a MA/BisGMA/TEGDMA blend (10/63/27% w/w; 1.4 mm thick films) upon exposure to the LED@477 nm (I0 = 300 mW cm−2 ) using different photoinitiating systems. (1) CQ/NapTS (1/1% w/w); (2) CQ/EDB (1/1% w/w); (3) CQ/NapTS/Iod (1/1/1% w/w). The irradiation starts for t = 5 s (see the dot line).
The photopolymerization profiles of thick films (1.4 mm) using CQ/sulfinate/iod as photoinitiating system is presented in Fig. 5. Very high Rp and a FC of ∼85% are reached after only 10 s of irradiation with the blue LED using a 300 mW cm−2 light intensity. Remarkably, the amine-free system CQ/sulfinate/iod presents enhanced polymerization performances (higher Rp and FC) compared to the well-established CQ/EDB reference system. The better ability of CQ/sulfinate/iodonium vs. CQ/sulfinate can be ascribed to r2–r4 generating additional initiating radicals. Remarkably the redox potentials of CQ (Ered = −1.4 V and Eox > 1 V) are in agreement with r3–r4 using Ered = −0.7 V for iodonium [1] and Eox = 0.7 V for sulfinate (this work). Remarkably, CQ can be regenerated in r3–r4 in agreement with the very high performance of the three-component system. ∗CQ + Ar2 I+ → CQ 䊉+ + Ar䊉 + ArI
(r2)
The photopolymerization performances of all the sulfinates and sulfonate studied (Scheme 1) were investigated in CQ/sulfinate (or CQ/sulfonate) and CQ/sulfinate/iod (or CQ/sulfonate/iod) systems for the polymerization of thick samples (1.4 mm) of different methacrylate blends upon exposure to the LED at 477 nm, under air, using a 300 mW cm−2 light intensity. The FCs obtained after 120 s of irradiation for the different PISs and monomers investigated are presented in Table 1. For all the three-component systems investigated using sulfinate or sulfonate as co-initiator, very high FCs (>70%) and high Rps are obtained for the different methacrylate blends used. The sulfinates and sulfonate studied are excellent candidate to replace tertiary aromatic amines in combination with CQ. Remarkably, excellent polymerization performances are also obtained for some two-component CQ/Sulfinate systems e.g. in presence zinc isopropylsulfinate (ZniPrS).
3.3. Diphenyliodonium p-toluenesulfinate (DPIpTS) as a new co-initiator and additive in CQ based system The previous results show that sulfinates are particularly efficient co-initiators for CQ in presence of an iodonium salt. An attempt to combine iodonium and sulfinate coinitiators in the same structure has been carried out. Indeed, a novel one component system involving an iodonium salt with a sulfinate as counter anion was synthesized by ion exchange between diphenyliodonium chloride and sodium ptoluenesulfinate. To the best of our knowledge, this structure was never investigated in photopolymerization experiments. The performances of this new co-initiator were studied for the photopolymerization of thick samples (1.4 mm) of methacrylates (BisGMA/TEGDMA) under irradiation with the
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higher final conversions are reached for the CQ/EDB/Iod in presence of additional sulfinate (Fig. 8 curve 1 vs. curve 2).
3.5. CQ/sulfinate system for dental composites: towards excellent mechanical properties, exceptional bleaching and enhanced color stability
Fig. 6 – Photopolymerization profiles (methacrylate function conversion vs.irradiation time) for a BisGMA/TEGDMA blend (70/30% w/w; 1.4 mm thick films) under air upon exposure to the LED@477 nm (I0 = 300 mW cm−2 ) using different photoinitiating systems. (1) CQ/DPIpTS (0.5/1% w/w); (2) CQ/EDB (0.5/1% w/w). The irradiation starts for t = 5 s.
blue LED. Remarkably, similar performances (i.e. very high final methacrylate function conversion (∼80%) and polymerization rate) are obtained with the PIS based on CQ/DPIpTS compared to the reference system CQ/EDB (Fig. 6 curve 1 vs. curve 2). Furthermore, exceptional bleaching properties are obtained with the system based on CQ/DPIpTS after 120 s of irradiation with the blue dental LED. The obtained bleaching properties are similar or better than those achieved with the CQ/EDB based system (Fig. 7).
3.4. Sulfinates and sulfonates as efficient additives for CQ/amine based systems Interestingly, sulfinates and sulfonates can be used as efficient additive in CQ/amine based systems and provide very high polymerization performances as shown in Fig. 8. Indeed
The performances of the new developed CQ/sulfinate PIS were investigated for dental composites applications. The given amounts of the respective formulation (Table 2, wt% based on resin part) were given to 5 g Spectrum® TPH® 3 resin and 15 g Spectrum® TPH® 3 glass filler. Afterwards, the mixture was processed to a paste by using a SpeedMixer (DAC 600-2 VAC-P, Hauschild & Co. KG). Six different formulations (A-1/–F-1/2) with various amounts of the PIS were prepared and the mechanical properties of the resulting composites were measured. For all the different compositions investigated, similar mechanical properties in terms of flexural strength and Emodulus were obtained for the system CQ/NapTS compared to the reference system CQ/EDB (e.g. for formula A-1/2:FS: 136 MPa vs. 142 MPa and E-Modulus: 9042 MPa vs. 9240 MPa). Therefore, the replacement of EDB by the sulfinate in the CQ based system enables the preparation of dental composites without any decrease of the mechanical properties. Three disc specimens of formula A-1 (CQ/NapTS system) and A-2 (CQ/EDB system) were aged according to the color stability procedure described previously. In this test, only unpigmented experimental composite formulations were evaluated. Furthermore, no UV-stabilizers (e.g. 2-hydroxy-4methoxybenzophenone) were used in order to evaluate the real discoloration behavior of the PIS compounds (NapTS vs. EDB).The color stability of the samples was determined by calculating the E value for each sample and each ageing scenario (Table 3). The photos of the samples after ageing are presented in Fig. 9. According to the procedure, the upper side of the composites aged following Scenario 3 (above the black line, Scenario 3a) was exposed to the radiation whereas the lower side (below the black line, Scenario 3b) was protected from the radiation exposure. Remarkably, the difference of color stability of these two sides for the two different composites prepared (CQ/NapTS vs. CQ/EDB) is visible with naked
Fig. 7 – Photos of the samples before and after polymerization (under air; thickness = 1.4 mm; SmartLite Focus (300 mW cm−2 ); 115 s irradiation): (1) CQ/DPIpTS (0.5/1% w/w); (2) CQ/EDB (0.5/1% w/w) in BisGMA/TEGDMA (70/30% w/w). Please cite this article in press as: J. Kirschner, F. Szillat, M. Bouzrati-Zerelli et al.. Sulfinates and sulfonates as high performance co-initiators in CQ based systems: Towards aromatic amine-free systems for dental restorative materials. Dent Mater (2019), https://doi.org/10.1016/j.dental.2019.11.020
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Fig. 8 – Photopolymerization profiles (methacrylate function conversion vs. irradiation time) for a MA/BisGMA/TEGDMA (10/63/27% w/w) or Bis-GMA/TEGDMA blend (70/30% w/w; 1.4 mm thick films) under air upon exposure to the LED@477 nm (I0 = 300 mW cm−2 ) using different photoinitiating systems. (A) (1) CQ/EDB/NaMeSP/SC938 (0.2/0.5/1/1% w/w); (2) CQ/EDB/SC938 (0.2/0.5/1% w/w); (3) CQ/EDB (0.2/0.5% w/w); (B) (1) CQ/EDB/NapTSo/SC938 (0.2/0.5/1/1% w/w); (2) CQ/EDB/SC938 (0.2/0.5/1% w/w); (3) CQ/EDB (0.2/0.5% w/w). The irradiation starts for t = 5 s.
Table 2 – Flexural strength measurements of formula 1–6 for different amounts of CQ/NapTS and CQ/EDB. Examples with NapTS Formula
CQ [wt%]
NapTS [wt%]
BHT [wt%]
Flexural strength [MPa]
E-Modulus [MPa]
A-1 B-1 C-1 D-1 E-1 F-1
0.541 0.375 0.259 0.7 0.463 0.7
0.355 0.127 0.450 0.127 0.697 1.267
0.05 0.05 0.05 0.05 0.05 0.05
136 132 123 143 136 123
9042 8860 8730 9750 9100 8800
Comparative examples with EDB Formula
CQ [wt%]
EDB [wt%]
BHT [wt%]
Flexural strength [MPa]
E-modulus [MPa]
A-2 B-2 C-2 D-2 E-2 F-2
0.541 0.375 0.259 0.7 0.463 0.7
0.355 0.127 0.450 0.127 0.697 1.267
0.05 0.05 0.05 0.05 0.05 0.05
142 132 125 129 143 123
9240 8950 9250 9230 9720 9350
Table 3 – Results of E measurements (Eq. (2)) of the formula A for CQ/NapTS and the comparative example CQ/EDB. Formula A
E
Example with NapTS (Formula A-1)
Comparative Example with EDB (Formula A-2)
Scenario 1
Scenario 2
Scenario 3 a/b
Scenario 1
Scenario 2
Scenario 3 a/b
2.2
2.5
5.9/1.6
4.0
4.2
20.7/2.5
eye. For the composite prepared with CQ/NapTS, only a slight color difference is visible between the covered and uncovered side whereas for CQ/EDB, the uncovered side presents clearly a yellow coloration compared to the reference (covered) side.This observation wasconfirmed by E measurements. Indeed higher E values are obtained for all the composites prepared with the CQ/EDB system compared to those obtained with CQ/NapTS. For Scenario 3, this difference is even higher for the uncovered sides 3a (E: 20.7 for CQ/EDB
vs. 5.9 for CQ/NapTS) and confirms the better color stability of CQ/sulfinate based composites.
4.
Conclusion
In the present paper, sulfinates and sulfonates are introduced as new high performance co-initiators in CQ based systems for the polymerization of methacrylates upon blue light irra-
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Fig. 9 – Photos of the specimens after ageing(for Scenario 3: uncovered side above the black line, covered side below the black line) using CQ/EDB (0.541/0.355% w/w) or CQ/NapTS (0.541/0.355% w/w) as photoinitiating system in Spectrum® TPH® 3 resin (75% w/w of Spectrum® TPH® 3 fillers). (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
diation. Remarkably, sulfinates and sulfonates represent an interesting alternative for the replacement of the amine traditionally used in combination with CQ especially in the dental field. Exceptional bleaching properties and color stability of the final polymers are noted as well as excellent mechanical properties for the composites. The chemical mechanisms were established. The use of these amine-free systems for other fields (3D printing, composites, coatings, inks, etc.) when the amine replacement is important will be examined in forthcoming papers.
Acknowledgments IS2M authors thank Dentsply Sirona and the Grand Est region for the funding of this work. The authors also thank Valérie Monnier (Spectropole, Aix Marseille University) for the elementar analysis experiments and MS experiments.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/ j.dental.2019.11.020.
references
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Sulfinates and sulfonates as high performance co-initiators in CQ based systems: Towards aromatic amine-free systems for dental restorative materials Julie Kirschner a , Florian Szillat b , Mariem Bouzrati-Zerelli a , Jean-Michel Becht a , Joachim E. Klee b , Jacques Lalevée a,∗ a
Institut de Science des Matériaux de Mulhouse IS2M, UMR CNRS 7361, UHA, 15 Rue Jean Starcky, 68057 Mulhouse Cedex, France b Dentsply Sirona, De-Trey-Strae 1, Konstanz, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. The aim of our study is to develop amine-free photoinitiating systems (PISs) for
Received 7 June 2019
the polymerization of representative dental methacrylate resins under blue light irradiation.
Received in revised form
PISs based on camphorquinone (CQ)/sulfinate and CQ/sulfonate, eventually in combination
27 October 2019
with an iodonium salt, are proposed and compared to the well-established CQ/amine sys-
Accepted 15 November 2019
tem. The polymerization performances of thick (1.4 mm) samples of different methacrylate
Available online xxx
blends upon exposure to a commercial blue LED centered at 477 nm under air are described.
Keywords:
application.
Sulfinates
Methods. FTIR is used to monitor the photopolymerization profiles. ESR spectroscopy and
Finally, the performances of the new developed PISs are evaluated for dental composites
Sulfonates
electrochemical experiments are used to identify the radicals generated. Mechanical prop-
Photoinitiator
erties measurements and color stability measurements are carried out to determine the key
Camphorquinone
properties of the dental composites prepared.
Methacrylates
Results and Significiance. The performances of the new proposed PISs for the photopolymer-
Dental composites
ization of thick (1.4 mm) samples of methacrylate upon exposure to a blue dental LED under air are excellent. Similar or better performances and bleaching properties are obtained with the new proposed amine-free systems compared to those reached with the CQ/amine reference system. Dental composites with excellent mechanical properties and exceptional color stability are obtained. The involved chemical mechanisms for the initiation step were also established. © 2019 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved.
1.
∗
Introduction
Free radical polymerization has been widely studied in the past decades and has many applications such as 3D printing, stereolithography, inks, coatings and dentistry. Two types of photoinitiators are mainly used in free radical processes
Corresponding author. E-mail address: [email protected] (J. Lalevée). https://doi.org/10.1016/j.dental.2019.11.020 0109-5641/© 2019 The Academy of Dental Materials. Published by Elsevier Inc. All rights reserved.
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and are classified as Type I and Type II photoinitiators [1]. Type I photoinitiators generate radicals by an homolytic cleavage whereas Type II photoinitiators require the presence of a co-initiator [1,2]. Among them, tertiary aromatic amines are particularly efficient co-initiators and are commonly used as hydrogen donors [3–5]. In dental restorative materials, the camphorquinone (CQ)/amine photoinitiating system (PIS) is clearly themost well-established reference system for the free radical photopolymerization of methacrylates due to its visible light absorption properties in the 400–500 nm spectral region. Ethyl-4-(dimethylamino)benzoate (EDB) is a very efficient hydrogen donor and is largely used in combination with CQ in dental materials [6]. However, the presence of tertiary aromatic amines in (meth-)acrylate containing compositions can cause yellowing (discoloration) of the resulting photocured material. Furthermore, the use of tertiary aromatic amines gives more and more rise to toxicological concerns [7–11]. Therefore, the development of amine-free PISs is of great interest for dental manufacturers. Some alternatives to amines have been proposed in the last years for example: 2-thiobarbituric acid [12], piperonyl alcohol [13], tris(trimethylsilyl)silane [6] and PH3 GeH [14]. These co-initiators present an enhanced biocompatibility and color stability but can remain less efficient than the traditionally used EDB [15]. In previous studies, sulfinates were used in combination with tertiary amines to enhance the adhesion to tooth substrate (dentin, enamel) [16] or to achieve better mechanical properties [17]. However to the best of our knowledge, the use of sulfinates and sulfonates as co-initiator was never investigated in CQ based photoinitiating system and was mainly found by coincidence. In the present paper, sulfinates and sulfonates are studied as efficient co-initiators for the replacement of amines in CQ/amine based PISs. Also a new synthesized iodonium salt (diphenyliodonium ptoluenesulfinate) including a sulfinate as counter anion is reported in this study. The involved chemical mechanisms are studied by electro spin resonance (ESR) and electrochemistry experiments. The performances of the new co-initiators are evaluated in combination with CQ for the free radical polymerization of methacrylates and followed by real-time FTIR spectroscopy. The performances of the proposed PISs are compared with a CQ/amine reference PIS. The bleaching properties playing an important role for the esthetic aspect of a respective material e.g. the color of the material directly after curing and even more important, the color stability of the aged material. Latter are also compared to those achieved with the reference system.
2.
Fig. 1 – The emission spectrum of a blue LED centered at 477 nm (SmartLite Focus from Dentsply Sirona, Germany).
obtained from Sigma Aldrich (Scheme 1). Diphenyliodonium p-toluenesulfinate (DPIpTS) was prepared by ion exchange using sodium p-toluenesulfinate and diphenyliodonium chloride both obtained from Sigma Aldrich (Scheme 1). The synthetic procedure used for the preparation of diphenyliodonium p-toluenesulfinate is described in the Supporting information. Camphorquinone was obtained from Sigma Aldrich and used as a representative Type II PI (Scheme 2). Ethyldimethylaminobenzoate (EDB) was used as additive in multicomponent systems and obtained from Sigma Aldrich (Scheme 2). Speedcure938 (SC938) was obtained from Lambson Ltd (Scheme 2). Bisphenol A-glycidyl methacrylate (BisGMA), triethylene glycol dimethacrylate (TEGDMA) and methacrylic acid (MA) were obtained from Sigma Aldrich and used with the highest purity available (Scheme 3). The BisGMA/TEGDMA (70%/30% w/w) blend was used as a benchmark matrix for methacrylates. 2-Hydroxyethyl methacrylate (HEMA) was obtained from TCI Chemicals (Scheme 3).Spectrum® TPH® 3 resin (mixture of modified Bis-GMA, TEGDMA and other methacrylate monomers) from Dentsply Sirona was used for the preparation of dental composites.
2.2.
A blue LED@477 nm representative for dental materials usage (SmartLite Focus from Dentsply Sirona ∼300 mW cm−2 in the selected conditions) was used for the irradiation of the photocurable samples (the emission spectrum is given in Fig. 1).
Experimental section 2.3.
2.1.
Irradiation sources
Photopolymerization experiments
Compounds
Sodium p-toluenesulfinate (NapTS), sodium 1-methyl 3sulfinopropanoate (NaMeSP), zinc benzylsulfinate (ZnBnS), sodium 4-(acetylamino)benzenesulfinate (NaAcABS), zinc isopropylsulfinate (ZniPrS), Sodium butylnaphtalenesulfinate (NaBuNS) and sodium p-toluenesulfonate (NapTSo) were
The photosensitive formulations were deposited on a BaF2 pellet under air for irradiation with the LED light (SmartLite Focus, 300 mW/cm2 ). The evolution of the methacrylate function conversions of BisGMA, TEGDMA and HEMA was continuously followed by real time FTIR spectroscopy (JASCO FTIR 6600) at about 6160 cm−1 for thick (1.4 mm) samples [18].
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Scheme 1 – Structures of the co-initiators used.
2.4.
ESR spin trapping experiments
ESR experiments were carried out using a Bruker EMXplus spectrometer (X-band). The radicals are generated at room temperature upon the blue LED exposure (SmartLite
Focus from Dentsply Sirona, 300 mW/cm2 ) under N2 . The radicals were trapped by phenyl-N-tert-butylnitrone (PBN, Scheme 2) according to a procedure already described [19]. The ESR spectra simulations were carried out using WINSIM software.
Scheme 2 – Structures of the photoinitiator (CQ) and additives used. Please cite this article in press as: J. Kirschner, F. Szillat, M. Bouzrati-Zerelli et al.. Sulfinates and sulfonates as high performance co-initiators in CQ based systems: Towards aromatic amine-free systems for dental restorative materials. Dent Mater (2019), https://doi.org/10.1016/j.dental.2019.11.020
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Scheme 3 – Monomers used in this work.
2.5. Redox potentials measurements and free energy change calculations The oxidation potential of Sulfinate (NapTS) was determined by cyclic voltammetry by a procedure already presented [20] (Radiometer PST006). The free energy change G for the electron transfer reaction between the sulfinate and CQ was calculated from the classical free energy change equation: G = Eox − Ered − ET + C
(1)
where Eox , Ered , ET , and C are the oxidation potential of the electron donor, the reduction potential of electron acceptor, the excited triplet state energy of the CQ, and the electrostatic interaction energy for the initially formed ion pair, generally considered as negligible in polar solvents.
2.6.
Mechanical properties measurements
Flexural strength (FS) measurements of the experimental composite formulations were determined using three-point bending flexural test according to ISO 4049:2009 (material class II).
2.7.
Fig. 2 – ESR spectra of the radicals generated in CQ/sulfinate (NapTS) and trapped by PBN in methacrylic acid/tert-butylbenzene (∼20:80) upon exposure to a LED@477 nm (SmartLite Focus): N2 saturated medium 䊉 SO R/PBN hyperfine coupling constants: a = 13.3 G, N 2 aH = 1.5 G and oxidized PBN (PBNox) aN = 7.9 G indicated by stars *; (a) experimental and (b) simulated spectra.
Color stability
The color stability of composites was investigated according to a slightly adapted procedure based on the method given in the ISO 4049:2009/ISO 7491 guideline. The procedure is described in the following:three disc specimens of each formula were prepared using a Liculite light oven (irradiation time 90 s/each side). Afterwards, the initial L*a*b* values of each specimen were measured using a Datacolor 800. One specimen was then stored in the dark and dry in the oven at 37 ± 2 ◦ C for 7 days (Scenario 1). One specimen was stored in the dark in the oven in water at 37 ± 2 ◦ C for 7 days (Scenario 2). The last specimen was first stored dark and dry in the oven at 37 ± 2 ◦ C for 24 ± 2 h. After this time, latter specimen was removed from the oven and blanked off half of it with aluminium foil (uncovered side − Scenario 3a/covered side − Scenario 3b). The specimen was then placed in a radiation chamber immersed in water (37 ± 2 ◦ C) and exposed to the radiation for 24 h at 150 000 ± 15 000 Lux. It was ensured that the water level
was 10 ± 3 mm above the specimen. After exposure, the aluminium foil was removed and the specimen was transferred back to the oven at 37 ± 2 ◦ C and stored in the dark and dry for 5 days. The change in color (E) was measured for each specimen after the conducted ageing scenario using a Datacolor 800. The E was calculated using following Eq. (2): E = deltaE =
L∗2 + a∗2 + b∗2
3.
Results and discussion
3.1.
Photochemical mechanisms
3.1.1.
From ESR: spin trapping experiments
(2)
In presence of a spin trap agent (PBN) for the photolysis of CQ/sulfinate e.g. CQ/NapTS under N2 upon exposure to the blue LED centered at 477 nm, the RSO2 •/PBN radical adduct
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Fig. 3 – Cyclic voltamogram of Sulfinate (NapTS) in water/buffer (pH = 3) under N2 for NapTS + buffer (red curve) and buffer alone (black curve). (For interpretation of the references to colour in the figure legend, the reader is referred to the web version of this article.)
is clearly observed: characterized by hyperfine coupling constants (hfcs) aN = 13.3 G, aH = 1.5 G in agreement with hfcs known in the literature [21] (Fig. 2). Therefore, ESR spin trapping experiments show the formation of benzene sulfonyl centered radicals generated from the CQ/sulfinate interaction upon light irradiation.
3.1.2.
From redox potentials
The oxidation potential (Eox ) of NapTS (as measured by cyclic voltammetry, Fig. 3) is 0.7 V (error bar < 0.05 V). The calculated electron transfer free energy change G from the triplet state of CQ is rather favorable (G = −0.1 V) using Ered for CQ = −1.4 V, ET = 2.2 V [1] for CQ (Eq. (1)). The ESR and redox potentials experiments revealed the formation of sulfonyl centered radicals according to reaction 1 (r1) through a favorable electron transfer between the sulfinate and camphorquinone. The sulfonyl radicals can initiate the polymerization of (meth)acrylate monomers in agreement with the good initiating ability of CQ/sulfinate vs. CQ alone found in polymerization experiments (see below, Fig. 4). A similar mechanism is expected between sulfonate and CQ to explain the polymerization initiating ability of CQ/sulfonate initiating systems (see polymerization experiments in Table 1). ∗CQ + RSO2 − → CQ 䊉− + RSO2 䊉
3.2.
(r1)
CQ/sulfinate and CQ/sulfonate initiating ability
3.2.1. Free radical polymerization initiating ability of CQ/NapTS for methacrylate monomers (MA/BisGMA/TEGDMA blend) The polymerization profiles of thick films (1.4 mm) of a MA/BisGMA/TEGDMA(10/63/27% w/w) blend upon exposure to the blue LED at 477 nm (SmartLite Focus) using different photoinitiating systems are depicted in Fig. 4. Under air, for a 300 mW cm−2 light intensity, the system CQ/NapTS exhibits very good polymerization initiating ability in thick samples. High polymerization rate Rp and high final methacrylate
Fig. 4 – Photopolymerization profiles (methacrylate function conversion vs irradiation time) for a MA/BisGMA/TEGDMA blend (10/63/27% w/w; 1.4 mm thick films) upon exposure to the LED@477 nm (I0 = 300 mW cm−2 ) using different photoinitiating systems. (1) CQ (0.5% w/w); (2) CQ/sulfinate (NapTS) (0.5/1% w/w). The irradiation starts for t = 5 s (see the dot line).
function conversion (FC) up to 80% are obtained for the CQ/sulfinate system contrary to CQ alone for which no polymerization occurred. For sulfinate alone, no polymerization is observed. These first experiments clearly highlight that sulfinate behaves as an efficient co-initiator in synergy with CQ. Remarkably, this system can efficiently initiate the free radical polymerization of methacrylates without additional amine.
3.2.2. Free radical polymerization initiating ability of CQ/NapTS/Iod for methacrylate monomers (MA/BisGMA/TEGDMA blend) Furthermore, the three-component system CQ/NapTS/Iod exhibits exceptional performances for the free radical polymerization of a MA/BisGMA/TEGDMA (10/63/27% w/w) blend upon exposure to the blue LED at 477 nm (SmartLite Focus).
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Table 1 – New proposed photoinitiating systems for free radical polymerization of different methacrylate blends upon visible light (under air; thickness = 1.4 mm; blue LED@477 nm, SmartLite Focus, 300 mW cm−2 ); monomer conversions reached after a SmartLite Focus exposure of 120 s (error bar ∼2%).
NaMeSP ZnBuS NaAcABS ZniPrS NaBuNS NapTSo
Two-component system
Three-component system
CQ/co-initiator (0.5/1% w/w)
CQ/co-initiator/Iod (0.5/1/1% w/w)
Resin 1
Resin 2
Resin 3
Resin 1
Resin 2
Resin 3
50% 50% 20% 70% 2% <5%
70% – 30% 70% – <5%
20% 20% 20% 75% – <5%
80% 82% 75% 80% 80% 70%
80% – 80% 70% – 80%
80% 88% 80% 80% – 80%
Resin 1: BisGMA/TEGDMA (70/30% w/w). Resin 2: MA/BisGMA/TEGDMA (10/63/27% w/w). Resin 3: HEMA/BisGMA/TEGDMA (10/63/27% w/w).
CQ 䊉+ + RSO2 − → CQ + RSO2 䊉
(r3)
CQ 䊉− + Ar2 I+ → CQ + Ar䊉 + ArI
(r4)
3.2.3. Free radical polymerization initiating ability of CQ/sulfinate or CQ/sulfonate and CQ/sulfinate/iod or CQ/sulfonate/iod systems for different methacrylate monomer blends
Fig. 5 – Photopolymerization profiles (methacrylate function conversion vs irradiation time) for a MA/BisGMA/TEGDMA blend (10/63/27% w/w; 1.4 mm thick films) upon exposure to the LED@477 nm (I0 = 300 mW cm−2 ) using different photoinitiating systems. (1) CQ/NapTS (1/1% w/w); (2) CQ/EDB (1/1% w/w); (3) CQ/NapTS/Iod (1/1/1% w/w). The irradiation starts for t = 5 s (see the dot line).
The photopolymerization profiles of thick films (1.4 mm) using CQ/sulfinate/iod as photoinitiating system is presented in Fig. 5. Very high Rp and a FC of ∼85% are reached after only 10 s of irradiation with the blue LED using a 300 mW cm−2 light intensity. Remarkably, the amine-free system CQ/sulfinate/iod presents enhanced polymerization performances (higher Rp and FC) compared to the well-established CQ/EDB reference system. The better ability of CQ/sulfinate/iodonium vs. CQ/sulfinate can be ascribed to r2–r4 generating additional initiating radicals. Remarkably the redox potentials of CQ (Ered = −1.4 V and Eox > 1 V) are in agreement with r3–r4 using Ered = −0.7 V for iodonium [1] and Eox = 0.7 V for sulfinate (this work). Remarkably, CQ can be regenerated in r3–r4 in agreement with the very high performance of the three-component system. ∗CQ + Ar2 I+ → CQ 䊉+ + Ar䊉 + ArI
(r2)
The photopolymerization performances of all the sulfinates and sulfonate studied (Scheme 1) were investigated in CQ/sulfinate (or CQ/sulfonate) and CQ/sulfinate/iod (or CQ/sulfonate/iod) systems for the polymerization of thick samples (1.4 mm) of different methacrylate blends upon exposure to the LED at 477 nm, under air, using a 300 mW cm−2 light intensity. The FCs obtained after 120 s of irradiation for the different PISs and monomers investigated are presented in Table 1. For all the three-component systems investigated using sulfinate or sulfonate as co-initiator, very high FCs (>70%) and high Rps are obtained for the different methacrylate blends used. The sulfinates and sulfonate studied are excellent candidate to replace tertiary aromatic amines in combination with CQ. Remarkably, excellent polymerization performances are also obtained for some two-component CQ/Sulfinate systems e.g. in presence zinc isopropylsulfinate (ZniPrS).
3.3. Diphenyliodonium p-toluenesulfinate (DPIpTS) as a new co-initiator and additive in CQ based system The previous results show that sulfinates are particularly efficient co-initiators for CQ in presence of an iodonium salt. An attempt to combine iodonium and sulfinate coinitiators in the same structure has been carried out. Indeed, a novel one component system involving an iodonium salt with a sulfinate as counter anion was synthesized by ion exchange between diphenyliodonium chloride and sodium ptoluenesulfinate. To the best of our knowledge, this structure was never investigated in photopolymerization experiments. The performances of this new co-initiator were studied for the photopolymerization of thick samples (1.4 mm) of methacrylates (BisGMA/TEGDMA) under irradiation with the
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higher final conversions are reached for the CQ/EDB/Iod in presence of additional sulfinate (Fig. 8 curve 1 vs. curve 2).
3.5. CQ/sulfinate system for dental composites: towards excellent mechanical properties, exceptional bleaching and enhanced color stability
Fig. 6 – Photopolymerization profiles (methacrylate function conversion vs.irradiation time) for a BisGMA/TEGDMA blend (70/30% w/w; 1.4 mm thick films) under air upon exposure to the LED@477 nm (I0 = 300 mW cm−2 ) using different photoinitiating systems. (1) CQ/DPIpTS (0.5/1% w/w); (2) CQ/EDB (0.5/1% w/w). The irradiation starts for t = 5 s.
blue LED. Remarkably, similar performances (i.e. very high final methacrylate function conversion (∼80%) and polymerization rate) are obtained with the PIS based on CQ/DPIpTS compared to the reference system CQ/EDB (Fig. 6 curve 1 vs. curve 2). Furthermore, exceptional bleaching properties are obtained with the system based on CQ/DPIpTS after 120 s of irradiation with the blue dental LED. The obtained bleaching properties are similar or better than those achieved with the CQ/EDB based system (Fig. 7).
3.4. Sulfinates and sulfonates as efficient additives for CQ/amine based systems Interestingly, sulfinates and sulfonates can be used as efficient additive in CQ/amine based systems and provide very high polymerization performances as shown in Fig. 8. Indeed
The performances of the new developed CQ/sulfinate PIS were investigated for dental composites applications. The given amounts of the respective formulation (Table 2, wt% based on resin part) were given to 5 g Spectrum® TPH® 3 resin and 15 g Spectrum® TPH® 3 glass filler. Afterwards, the mixture was processed to a paste by using a SpeedMixer (DAC 600-2 VAC-P, Hauschild & Co. KG). Six different formulations (A-1/–F-1/2) with various amounts of the PIS were prepared and the mechanical properties of the resulting composites were measured. For all the different compositions investigated, similar mechanical properties in terms of flexural strength and Emodulus were obtained for the system CQ/NapTS compared to the reference system CQ/EDB (e.g. for formula A-1/2:FS: 136 MPa vs. 142 MPa and E-Modulus: 9042 MPa vs. 9240 MPa). Therefore, the replacement of EDB by the sulfinate in the CQ based system enables the preparation of dental composites without any decrease of the mechanical properties. Three disc specimens of formula A-1 (CQ/NapTS system) and A-2 (CQ/EDB system) were aged according to the color stability procedure described previously. In this test, only unpigmented experimental composite formulations were evaluated. Furthermore, no UV-stabilizers (e.g. 2-hydroxy-4methoxybenzophenone) were used in order to evaluate the real discoloration behavior of the PIS compounds (NapTS vs. EDB).The color stability of the samples was determined by calculating the E value for each sample and each ageing scenario (Table 3). The photos of the samples after ageing are presented in Fig. 9. According to the procedure, the upper side of the composites aged following Scenario 3 (above the black line, Scenario 3a) was exposed to the radiation whereas the lower side (below the black line, Scenario 3b) was protected from the radiation exposure. Remarkably, the difference of color stability of these two sides for the two different composites prepared (CQ/NapTS vs. CQ/EDB) is visible with naked
Fig. 7 – Photos of the samples before and after polymerization (under air; thickness = 1.4 mm; SmartLite Focus (300 mW cm−2 ); 115 s irradiation): (1) CQ/DPIpTS (0.5/1% w/w); (2) CQ/EDB (0.5/1% w/w) in BisGMA/TEGDMA (70/30% w/w). Please cite this article in press as: J. Kirschner, F. Szillat, M. Bouzrati-Zerelli et al.. Sulfinates and sulfonates as high performance co-initiators in CQ based systems: Towards aromatic amine-free systems for dental restorative materials. Dent Mater (2019), https://doi.org/10.1016/j.dental.2019.11.020
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Fig. 8 – Photopolymerization profiles (methacrylate function conversion vs. irradiation time) for a MA/BisGMA/TEGDMA (10/63/27% w/w) or Bis-GMA/TEGDMA blend (70/30% w/w; 1.4 mm thick films) under air upon exposure to the LED@477 nm (I0 = 300 mW cm−2 ) using different photoinitiating systems. (A) (1) CQ/EDB/NaMeSP/SC938 (0.2/0.5/1/1% w/w); (2) CQ/EDB/SC938 (0.2/0.5/1% w/w); (3) CQ/EDB (0.2/0.5% w/w); (B) (1) CQ/EDB/NapTSo/SC938 (0.2/0.5/1/1% w/w); (2) CQ/EDB/SC938 (0.2/0.5/1% w/w); (3) CQ/EDB (0.2/0.5% w/w). The irradiation starts for t = 5 s.
Table 2 – Flexural strength measurements of formula 1–6 for different amounts of CQ/NapTS and CQ/EDB. Examples with NapTS Formula
CQ [wt%]
NapTS [wt%]
BHT [wt%]
Flexural strength [MPa]
E-Modulus [MPa]
A-1 B-1 C-1 D-1 E-1 F-1
0.541 0.375 0.259 0.7 0.463 0.7
0.355 0.127 0.450 0.127 0.697 1.267
0.05 0.05 0.05 0.05 0.05 0.05
136 132 123 143 136 123
9042 8860 8730 9750 9100 8800
Comparative examples with EDB Formula
CQ [wt%]
EDB [wt%]
BHT [wt%]
Flexural strength [MPa]
E-modulus [MPa]
A-2 B-2 C-2 D-2 E-2 F-2
0.541 0.375 0.259 0.7 0.463 0.7
0.355 0.127 0.450 0.127 0.697 1.267
0.05 0.05 0.05 0.05 0.05 0.05
142 132 125 129 143 123
9240 8950 9250 9230 9720 9350
Table 3 – Results of E measurements (Eq. (2)) of the formula A for CQ/NapTS and the comparative example CQ/EDB. Formula A
E
Example with NapTS (Formula A-1)
Comparative Example with EDB (Formula A-2)
Scenario 1
Scenario 2
Scenario 3 a/b
Scenario 1
Scenario 2
Scenario 3 a/b
2.2
2.5
5.9/1.6
4.0
4.2
20.7/2.5
eye. For the composite prepared with CQ/NapTS, only a slight color difference is visible between the covered and uncovered side whereas for CQ/EDB, the uncovered side presents clearly a yellow coloration compared to the reference (covered) side.This observation wasconfirmed by E measurements. Indeed higher E values are obtained for all the composites prepared with the CQ/EDB system compared to those obtained with CQ/NapTS. For Scenario 3, this difference is even higher for the uncovered sides 3a (E: 20.7 for CQ/EDB
vs. 5.9 for CQ/NapTS) and confirms the better color stability of CQ/sulfinate based composites.
4.
Conclusion
In the present paper, sulfinates and sulfonates are introduced as new high performance co-initiators in CQ based systems for the polymerization of methacrylates upon blue light irra-
Please cite this article in press as: J. Kirschner, F. Szillat, M. Bouzrati-Zerelli et al.. Sulfinates and sulfonates as high performance co-initiators in CQ based systems: Towards aromatic amine-free systems for dental restorative materials. Dent Mater (2019), https://doi.org/10.1016/j.dental.2019.11.020
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Fig. 9 – Photos of the specimens after ageing(for Scenario 3: uncovered side above the black line, covered side below the black line) using CQ/EDB (0.541/0.355% w/w) or CQ/NapTS (0.541/0.355% w/w) as photoinitiating system in Spectrum® TPH® 3 resin (75% w/w of Spectrum® TPH® 3 fillers). (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
diation. Remarkably, sulfinates and sulfonates represent an interesting alternative for the replacement of the amine traditionally used in combination with CQ especially in the dental field. Exceptional bleaching properties and color stability of the final polymers are noted as well as excellent mechanical properties for the composites. The chemical mechanisms were established. The use of these amine-free systems for other fields (3D printing, composites, coatings, inks, etc.) when the amine replacement is important will be examined in forthcoming papers.
Acknowledgments IS2M authors thank Dentsply Sirona and the Grand Est region for the funding of this work. The authors also thank Valérie Monnier (Spectropole, Aix Marseille University) for the elementar analysis experiments and MS experiments.
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/ j.dental.2019.11.020.
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Please cite this article in press as: J. Kirschner, F. Szillat, M. Bouzrati-Zerelli et al.. Sulfinates and sulfonates as high performance co-initiators in CQ based systems: Towards aromatic amine-free systems for dental restorative materials. Dent Mater (2019), https://doi.org/10.1016/j.dental.2019.11.020