The emission range of the LED light source is centered at a wavelength of nm. The full width at the half maximum is 32 nm, while the half width at the half maximum is 16 nm for our LED light source. Therefore, the samples were irradiated through a cuboid, a tube with both sides open, which was covered inside with a highly reflective coating to make the irradiation spot more homogenous. In addition, the spectral irradiance distribution of the light source was detected with a spectroradiometer EKO Instruments, LS, Japan.
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The emission range of the LED light source is centered at a wavelength of nm. The full width at the half maximum is 32 nm, while the half width at the half maximum is 16 nm for our LED light source. Therefore, the samples were irradiated through a cuboid, a tube with both sides open, which was covered inside with a highly reflective coating to make the irradiation spot more homogenous.
In addition, the spectral irradiance distribution of the light source was detected with a spectroradiometer EKO Instruments, LS, Japan. For the swelling studies, three of the samples at different light intensities, 0. The surfaces of the wet disks were cautiously wiped dry and their weights were measured immediately.
After 24 h, the cured samples were extracted with DMF in a Soxhlet-type extractor overnight. As this wavelength is out of the absorption range of the photoinitiator, the laser light did not affect the photopolymerization process. The monomer was irradiated in a polystyrene cuvette and the spectra were taken before and after polymerization Figure 1.
In addition, in situ investigation of the Raman spectra was completed; the spectra were recorded during the illumination every second.
This measurement setting allows calculating and establishing the kinetics at different types of irradiation. With these measurements, the chosen light intensities varied between 0. The laser was focused to the center of the cuvette, so the spectral information was collected from the volume and not from the surface. The polymerization time was chosen as s to allow for the front of the polymerization to reach the Raman source.
Preliminary Vickers Microhardness Measurements The specimens were prepared as described in Section and were stored at room temperature for 24 h before testing. At each light intensity, three samples were prepared and five microhardness indentations were made per specimen.
Vickers microhardness measurements were made with a g load for 20 seconds in a microhardness testing machine Buehler Vickers Microhardness, Micromet , US. In this preliminary investigation, our aim was to choose the optimal light intensity for polymerization of the samples, where the Vickers microhardness data showed the best results.
The subsequent mechanical measurements were performed at the chosen light intensity. Diametral Tensile Strength Measurements The uncured resin was placed in a Teflon mold and the samples were covered with a thin polyester foil. The polymerization of the samples was performed at a light intensity of 1.
The size of the specimens was 3 mm thick and 6 mm in diameter. The span distance was 18 mm. The three-point flexural strength tests were implemented on prismatic specimens. The samples were photopolymerized with the LED light source at 1. In average, 14 specimens were prepared and stored at room temperature for 24 h before testing. Fifteen cylindrical samples were created. The resin was inserted in a single increment into a Teflon mold, and the top surface was flattened by means of a polyester strip.
For the sample preparation, the light activation was performed for 30 s at 1. The size of the specimens was 6 mm in height and 3 mm in diameter.
Results and Discussion 3. Asmussen and Peutzfeldt [ 24 ] examined the influence of these monomers on the mechanical properties of experimental resin composites. They stated that for the designed mechanical properties of dental composite resins it is best to apply monomers in this optimal ratio for that purpose.
The application of more flexible monomers is expected to increase the conversion of polymerization. Figure 2 The chemical structure of the photoinitiator and acrylate monomers used in this study. In addition to the copolymer composition, the photoinitiator also influences the physical properties of the resins.
Sabol et al. This photoinitiator does not require an electron donor to produce free radicals so it is suitable for the substitution of CQ in dental photopolymer systems. Irgacure absorbs photons at the green exposing wavelength of nm. The absorption of light quanta by Irgacure causes reversible isomerization, resulting in an intermediary isomer with different absorption spectra, which can either relax and return to the original state or cause photocleavage, resulting in a stable acryl compound and an unstable titanocene diradical that can react with a reducing agent to form a stable transparent final product.
Another possible method of forming a final stable product is the reaction of the isomer with a reactive component of the resin matrix [ 25 ]. Since we used a green LED light source to initiate the photopolymerization process, knowing the characteristics of the lamp and sensitivity of the initiator in the green region of visible light is important. The transmittance spectrum of the LED lamp and the absorption spectrum of Irgacure , recorded in toluene, are presented in Figure 3. Figure 3 Ultraviolet-visible Uv-vis absorption spectrum of Irgacure photoinitiator in toluene left and spectral irradiance distribution of the green LED light source.
From Figure 3 , the LED light source emits in a relatively narrow range — nm , which can be assigned to the green region of visible light. The peak of emission was found to be at nm, while the full width of the half maximum is 32 nm. Irgacure has high absorption below nm, which drops rapidly above this wavelength and is practically transparent above nm. Kinetic Investigation of the Photopolymerization To gain insight into the photopolymerization process of our dental resin, kinetic measurements were performed.
However, the chain polymerization of acrylate monomers during photopolymerization may involve very complex reactions; therefore, it is only possible to work with simplified models. The reaction rate of a theoretical radical chain polymerization is usually given by where is the degree of conversion, is the intensity of absorbed light in moles of light quanta per liter-second, is the quantum yield of initiation, and and are the rate constants for propagation and termination, respectively.
Equation 1 is only valid if we assume steady-state conditions. However, during propagation, the reaction kinetics are affected by autoacceleration, which is a consequence of altered diffusion.
Since 1 is no longer applicable, the rate of reaction must be rewritten as a function of the radical concentration, :. Unfortunately, 2 does not consider that the glass transition temperature of the restorative material must be higher than the maximum temperature that can be reached in the oral cavity, in order to prevent failures of the restorations due to thermal fatigue. As the photocuring progresses, of the network formed also increases, and the initially viscous liquid monomer mixture becomes a glassy solid.
In this glassy network, the mobility of the monomers and radicals is greatly reduced, and due to this vitrification effect the reaction becomes diffusion controlled and the termination step of the polymerization is governed by the strong decrease in the molecular mobility. Maffezzoli and Terzi [ 26 ] proposed a simple expression, capable of describing the overall kinetic process by modelling the kinetic behavior of acrylates during photocuring conditions, using a simple pseudo-autocatalytic expression.
The measured reaction rates along with the calculated ones, per 3 , were plotted as a function of the light intensity as shown in Figure 5. Figure 5 as a function of light intensity. The solid line represents the fitted curve based on 3. From Figure 5 , the experimental points can be well fitted using 3. The variable has a power of 0. The applicability of the model, describing the overall kinetic behavior, was tested by comparing the theoretical conversion data with the experimental data.
According to Figure 6 , the conversion of photocuring shows a significant light intensity dependence. Figure 6 Comparison between the degree of conversion measured by Raman spectroscopy and that of calculated based on 3 at different light intensities. Using a low value of 1. However, little difference was observed for.
Irgacure 651 Benzil Dimethyl Ketal CAS 24650-42-8
Virn Another possible method of forming a final stable product is the reaction of the isomer with a reactive component of the resin matrix [ 25 ]. DTS was calculated from the maximum compression load at the specimen fracture in a diametric position, with the following equation: In addition, hypothesis tests, including independent sample -tests and the nonparametric Mann—Whitney tests, were run. For practical applicability, swelling experiments were performed in deionized water for seven days at ambient temperature. However, the chain polymerization of acrylate monomers during photopolymerization may involve very complex reactions; therefore, it is only possible to work with simplified models. Irgacure , Irgacure Suppliers and Manufacturers at Photopolymerization of the Samples To exclude oxygen, the photocuring process was performed under laminate conditions. The photopolymerizations were done in a dark room, without any backlights.