Improvement on properties of experimental resin cements containing an iodonium salt cured under challenging polymerization conditions

Highlights

• The addition of DPI can improve the degree of conversion under low irradiances.

• DPI resins had higher polymerization stress, regardless the radiant exposure.

• Better chemical/mechanical properties are observed at DPI resins.

Abstract

Objective

The use of resin cements in clinical practice entails photopolymerization through prosthetic devices, which precludes light penetration. The objective of this study was to modify experimental resin cements (ERCs) with diphenyliodonium hexafluorophosphate (DPI) in an attempt to improve chemical and mechanical properties of materials cured with reduced irradiance and final radiant exposure.

Methods

A co-monomer base containing a 1:1 mass ratio of 2.2-bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane (bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA) was prepared, with 1 mol% of camphorquinone and 2 mol% of ethyl 4-(dimethylamino)benzoate as initiator system. The resin was divided into 4 fractions according to the DPI concentrations (0, 0.5, 1 and 2 mol%). The challenging polymerization condition was simulated performing the light activation (12, 23 and 46 s) through a ceramic block (3 mm thick). The irradiance was assessed with a calibrated spectrometer (1320 mW/cm²), resulting in three levels of radiant exposure (0.58, 1.1 and 2.2 J/cm²). The polymerization kinetics was evaluated in real-time using a spectrometer (Near-IR). Water sorption and solubility was analyzed and the cohesive strength of resins obtained through the microtensile test. Polymerization stress was assessed by Bioman method.

Results

Resins containing DPI had higher degree of conversion and rate of polymerization than the control (without DPI). The use of DPI reduced water sorption and solubility, and led to higher cohesive strength compared to resins without the iodonium salt. However, the stress of polymerization was higher for experimental resins with DPI.

Significance

Even under remarkably reduced irradiance, cements containing a ternary initiating system with an iodonium salt can present an optimal degree of conversion and chemical/mechanical properties.

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.