Abstract
The main aim of the study was to implement the most reliable method of measuring the degrees of conversion during photopolymerization of dental fillings. Contrary to the methods used so far, the method is based only on comparison with the monomer absorbance spectrum without reference bands. Another aim of the study was to prepare a comparative analysis of the polymerization kinetics of dental resins under various light sources and different environmental conditions (irradiance, light dose, temperature), with estimation of the degrees of conversion (DC) of the resins being the main metric. HRi Universal Enamel (UE2) and HRi Universal Dentine (UD2) were examined under two different types of light sources used in dentistry, LED and halogen. DC was measured by Fourier transform infrared spectroscopy (FTIR) in transmission mode from 5 s up to 7 days. Spectra were recorded from the parallel optical layers of samples that were placed between the KBr crystals. The results are expressed by the changes in the absorbance spectrum during the polymerization and the calculated conversion rates. The results of each experiment were averaged from three separate measurements of three samples, during which the samples were illuminated under identical conditions. The data were analyzed by performing ANOVA test comparisons between sample groups at the significance level α = 0.05. The degree of conversion of the UD2 resin was higher than that of UE2 for each experimental condition, but there was no statistically significant difference between the DC of those materials (p > 0.05). There was statistically significant difference (p < 0.01) in the DC caused by LED and halogen light sources producing the same light doses (38 J/cm2). This was the result of different features of light transmission to the filler in the resin composite. The efficacy of the LED source is twice as high as that of the halogen light source. Maximal DC without any other differences in conditions, such as resin type or light source, reached around 70% for temperatures of 22–37 °C. For 37 °C, this took 24 h, which is a contrast to the 7 days it took for 23 °C. The influences of different conditions and factors on reaction kinetics are only strong in the early and the rapid stage of conversion. The optimal time of irradiance using either light source is 20 s for a monolayer, and its thickness should not exceed 2 mm.
Highlights
IntroductionDental resin-based composites (RBCs) are among the most used biomaterials in dentistry and the objects of a very large amount of research—e.g., [1–6]
The degree of conversion (DC) of a monomer in a dental resin-based composites (RBCs) is a very important parameter, because the physical and mechanical properties of photo-cured resins are directly influenced by DC [7]
It is believed that a limited DC is caused by the mobility of radial chain ends [9]
Summary
Dental resin-based composites (RBCs) are among the most used biomaterials in dentistry and the objects of a very large amount of research—e.g., [1–6]. The degree of conversion (DC) of a monomer in a dental RBC is a very important parameter, because the physical and mechanical properties of photo-cured resins are directly influenced by DC [7]. A low DC might cause increased cytotoxicity [8] and reduced hardness. The cured compositions based on the derivatives of methacrylate exhibit considerable numbers of remaining double bonds. It is believed that a limited DC is caused by the mobility of radial chain ends [9]. RBCs reach a DC range from 50–70% [3–5], or even 94% [10], depending on the exposure time (or dose), composite composition, temperature, type of photo activator and light source
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