BPI can be used in ternary activation systems substituting DPI salt.
Use of iodonium salts increase the polymerization shrinkage of resins.
Ternary systems containing BPI improve the flexural strength and modulus of resins.
To evaluate the influence of different co-initiators (diphenyliodonium hexafluorophosphate – DPI – and bis(4-methyl phenyl)iodonium hexafluorophosphate – BPI) on chemical and mechanical properties of resins.
Nine experimental resins (50% Bis-GMA and 50% TEGDMA, w/w) with 60 wt% filler particles were formulated. The initiating system used was camphorquinone (CQ-1 mol%) and ethyl dimethylaminobenzoate (EDAB-2 mol%). Experimental groups were established according to DPI and BPI quantities (0.25, 0.5, 0.75, and 1 mol%). The control group was a resin containing only CQ-EDAB. Light transmission through the resin during polymerisation was analysed with a UV–vis spectrophotometer. Real-time polymerisation of the systems was evaluated using an FTIR spectrometer. Real-time polymerisation shrinkage strain was evaluated, and the flexural strength and modulus of materials were obtained by 3-point bending. Experimental groups were statistically analysed by Analysis of Variance and Tukey’s test ( α = 0.05). Dunnett’s test was applied to compare experimental groups with control.
Light transmission rapidly increased initially for resins containing DPI or BPI. After 30 s cure, the irradiance on the lower surface of resin specimens was similar for all groups. After 10 s of light irradiation, groups containing DPI and BPI had higher conversion than the control. However, conversion after 120 s post-irradiation was similar for all groups. The rate of polymerisation, shrinkage strain, and the maximum strain rate were higher for groups containing DPI/BPI. The use of iodonium salts increased the flexural strength and flexural moduli of resins.
DPI and BPI increased resin reactivity similarly. Increased rate of polymerization influenced light transmission through the resin in the first seconds of polymerisation and increased resin shrinkage and rate of shrinkage, as well as flexural strength and moduli.
Resin-based materials play an essential role in Dentistry, with different purposes, from adhesive systems to resin-composites. Camphorquinone (CQ) is the most used photoinitiator for such materials. To generate free radicals, CQ is combined with a co-initiator, which is usually a tertiary amine (4-ethyl dimethyl aminobenzoate-EDAB or dimethylaminoethyl acrylate-DMAEMA). Among the used amines, EDAB stands out in comparison to dimethylaminoethyl acrylate DMAEMA as it delivers better reaction, improving the degree of conversion when combined with CQ [ ].
Alternative photoinitiators to CQ [ ] such as monoacylphosphine oxide (MAPO) and bis-acyl phosphine oxide (BAPO), have been successfully evaluated [ , , , ]. These are Type I photoinitiators, which undergo cleavage after exposure to light of specific wavelength with no need for a co-initiator [ , ]. Such photoinitiators provide a significantly greater rate and degree of polymerisation compared with CQ/amine systems, especially when DMAEMA is used with the type II initiator cited [ , , ]. However, these alternative agents require the use of a dual emission (polywave) light source due to the absorption peak of the molecules, which can be considered a disadvantage since polywave light sources are usually more expensive than traditional single-peak LEDs. In addition, violet light has reduced transmission through the resin composite [ ], which can compromise the depth of cure of resins containing alternative initiators as MAPO and BAPO.
Alternatively, a ternary activation system (CQ/amine/iodonium salt) may be used with the addition of an electron acceptor that is compatible with CQ/amine. This may improve system reactivity without the need for light source adjustments, as CQ remains the main photoinitiator [ , , ]. Diphenyl iodonium hexafluorophosphate (DPI) is a molecule widely described in literature as an hydrogen atom acceptor, providing an extra free radical, hindering CQ molecules reverting to the stable state and accelerating polymerisation [ , ]. This agent reacts along with CQ/amine as a co-initiator, forming a CQ-amine-DPI ternary system.
Bis(4-methyl phenyl)iodonium hexafluorophosphate (BPI) is another iodonium salt that may also act as an hydrogen acceptor, as previously described for DPI. This salt results from addition of a methyl group to the aromatic ring of DPI. It is unknown if this molecular change may influence the reactivity of the system, by modulating the polymerisation reaction and, consequently, the properties of the resultant resin.
Therefore, the objective of this study was to evaluate the effects of BPI on light transmission through resin, the degree of conversion, the rate of polymerisation, shrinkage strain, flexural strength and flexural modulus, compared with experimental composites containing DPI. A binary initiated composite system containing CQ and EDAB was used as control. The hypotheses were (1): resins containing DPI and BPI present similar rates of polymerisation and degrees of conversion to resins with a binary initiator system; and (2): resins with DPI/BPI would not have different properties from the control resin (iodonium-salt free).
Materials and methods
Resins composites were prepared with 50wt% of 2.2-bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane (Bis-GMA) and 50wt% of triethyleneglycol dimethacrylate (TEGDMA) (Esstech Inc., Essington, PA, USA). For all composites, camphorquinone (CQ-1 mol%) and ethyl dimethylamino benzoate (EDAB-2 mol%) was used at the initiator systems. The mixture was blended and homogenized for 1 h at room temperature (25 °C) with a magnetic stirrer (2000 rpm). The monomer mixture was aliquoted into 9 fractions and experimental groups were formed according to the type of co-initiator (DPI or BPI) and concentration of each co-initiator (0.25; 0.5; 0.75 or 1 mol%) added to the CQ/EDAB system. Resin composite using a binary initiator system (CQ/EDAB), without the iodonium salts evaluated, was used as control. Each formulation was loaded with 60 wt% of silanated barium borosilicate glass fillers (Esstech Inc, 0.7-μm average size). The experimental composites were mixed and homogenized using a dual asymmetric centrifuge at 3500 rpm (SpeedMixer, Flack-Tek, USA). All chemicals were used without further purification. For all light curing procedures a light-emitting diode (LED Bluephase G2, Ivoclar Vivadent AG, Schaan, Liechtenstein) was used, with radiant emittance of 1200 mW/cm 2 , and exposure time of 40 s.
Silicon molds (8 mm diameter, 1 mm thickness) were positioned over the sensors of the spectrophotometer (MARC™; Bluelight Analytics Inc., Halifax, Canada). A baseline measurement of irradiance was performed before the measurement of the irradiance beneath the resin. Resin composite specimens were inserted in the mold, and a glass slide (0.1 mm thick) placed over the upper surface. The LED tip was positioned in contact with the glass slide, and the curing light transmission through the resin composites were analysed in real time (n = 3), during polymerisation for 30 s. The transmitted irradiance was analysed and recorded via the system software (MARC-RC, Halifax, Canada).
Real-time degree of polymerisation
Using a Fourier transform infrared attenuated total reflectance method (FTIR-ATR, Nicolet 6700, Thermo Scientific, Pittsburgh, PA, USA), the degree of conversion (DC) was measured. A silicon mold (8 mm diameter, 1 mm thick) was fixed onto the ATR plate to ensure concentric alignment and reproducible specimen dimensions. The silicon mold was filled with each resin composite and the upper surface covered with a glass slide (0.1 mm thick). Real time DC was measured using aliphatic (1638 cm −1 ) and aromatic (1608 cm −1 ) C C absorption at the lower surface (3 scans averaged per spectrum, 16 cm −1 resolution). The specimens (n = 3) were light cured for 40 s and absorption measured for a total of 120 s. Conversion measurements were based on peak height ratios between the C C (aliphatic) stretching mode (1638 cm −1 ) and isosbestic point (aromatic: typically 1608 cm −1 ) according to Eq. (1) :