Improvement on properties of experimental resin cements containing an iodonium salt cured under challenging polymerization conditions
Introduction
The polymerization is one of the most critical steps on clinical procedures for restorative dental treatments. Materials with an optimal degree of conversion present better mechanical properties, such as flexural strength and modulus, low water sorption and solubility, reduced lixiviation of unreacted monomers, consequently presenting lower cytotoxicity [[1], [2], [3], [4]]. Together, these factors are crucial to increase longevity of the restorative procedures. However, high conversion in general translates into stiffer materials (high flexural modulus), which is a factor in increasing the stress of polymerization on the cement line [5]. The stress has been correlated to gap formation at the margin, which can facilitate bacteria recolonization, and accelerate degradation of the materials exposed to the oral environment [[5], [6], [7]], reducing the long-term stability of the procedure. In summary, ideal materials would be able to combine high conversion with low stress generation.
Clinically, there are challenging situations that can compromise the photopolymerization due to the reduced light that can reach the resin material. Restoration of deep cavities, cementation of fiber posts, as well as of indirect restorations, are examples of those situations [[8], [9], [10]]. Regarding full crowns, the proximal area of posterior restorations or endocrowns (thicker than 4 mm) represents situations of challenging light activation of the resin cements. In those situations, the use of dual-cure resin cements is indicated. The rationale for the dual-cure mode is that the redox polymerization reaction compensates for the deficient light penetration in certain areas [11,12], but this often translates into reduced working time, which is considered a disadvantage of these materials [13]. This is because the removal of resin cement flash on the margin, and the correct positioning of the restoration can be compromised.
One possible way to increase the conversion in areas of low light exposure is to use iodonium salts as part of the initiator system, since it has been shown that it improves the rate and degree of polymerization of the materials, as well as the mechanical properties [[14], [15], [16], [17], [18]]. Iodonium salts works in combination with camphorquinone (CQ) and amine, increasing the number of radicals formed per molecule of CQ (3 instead 1 of the conventional CQ/amine system), improving the polymerization of the system [14,16,[15], [16], [17], [18]]. Also, iodonium salts inhibit the back electron transfer, meaning that all CQ activated by the blue light can form a free radical, not returning to the stable state [14,16,[15], [16], [17], [18]]. Therefore, it is possible to imagine that cementation materials could be designed with improved conversion and properties that are not as reliant on the redox polymerization mechanism.
However, to our knowledge, there is no information regarding the effect of iodonium salts on polymerization of dental materials under extremely reduced irradiances and resulting radiant exposures, which would be relevant to understand the behavior of the resin materials on specific challenging situations. Therefore, the present study aimed to evaluate the influence of diphenyliodonium hexafluorophosphate (DPI) under simulated clinical challenging conditions. For this purpose, three concentrations of DPI (0.5, 1 and 2 mol%) were tested, and the resin cements cured through extremely reduced radiant exposure (0.58, 1.1 and 2.2 J/cm2). The influence of experimental conditions were evaluated on different resin properties (rate of polymerization, degree of conversion, water sorption and solubility, cohesive strength and stress of polymerization). The tested hypothesis was that DPI would significantly improve the chemical and mechanical properties of ERCs compared to the one containing a binary system, even at lower radiant exposure.
Section snippets
Experimental material composition
For the study, experimental resin cements (ERCs) were formulated composed of bis-phenol A diglycidyl dimethacrylate (Bis-GMA; Esstech, Essington, PA, USA), and tri-ethylene glycol dimethacrylate (TEGDMA; Esstech) at a 50:50 mass ratio. Camphorquinone (CQ-1 mol%) and ethyl dimethylamino benzoate (EDAB-2 mol%) were used at the initiator system for all resins. To this base co-monomer mixture, different concentration of diphenyl iodonium hexafluorophosphate (DPI — 0, 0.5, 1 and 2 mol%) were added.
Photopolymerization reaction kinetics and degree of conversion
For the rate of polymerization, the interaction of the factors was not statistically relevant (p = 0.1407), but the two factors were (p < 0.0001 for concentration and p = 0.0046 for radiant exposure). The rate of polymerization increased significantly with the addition of DPI to the ERCs, with results 3 or 4 times higher than the control resins without DPI (Table 1, Fig. 1A). The rate was significantly reduced for the resins containing 2 mol% of DPI.
For DC, the interaction between the factors was
Discussion
The influence of iodonium salts has been extensively studied, demonstrating the positive effect of the agents on the properties of dental resin materials [[14], [15], [16], [17]]. Despite this, the efficacy of DPI on ERCs facing challenging polymerization conditions, with extremely reduced irradiance and radiant exposure was not studied. This study demonstrates that even with lower radiant exposure obtained through reduced irradiance, resins containing DPI can significantly improve chemical and
Conclusions
The use of DPI on ERCs was able to increase conversion in relation to the control even under challenging polymerization conditions, such as reduced radiant exposure (1.1 and 2.2 J/cm2), under reduced irradiance (48 mW/cm2) caused by the interposition of a 3 mm thick ceramic disc. The rate of polymerization was 3–4 times higher than the control resins. Also, significantly reduced water sorption and solubility, and high cohesive strength were verified in resins with the ternary initiator system. The
Acknowledgment
This study was partially supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP grant # 2015/12324-0).
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