Polymerization kinetics and reactivity of alternative initiators systems for use in light-activated dental resins

Abstract

Objectives

The purpose of this study was to evaluate the reactivity and polymerization kinetics behavior of a model dental adhesive resin with water-soluble initiator systems.

Methods

A monomer blend based on Bis-GMA, TEGDMA and HEMA was used as a model dental adhesive resin, which was polymerized using a thioxanthone type (QTX) as a photoinitiator. Binary and ternary photoinitiator systems were formulated using 1 mol% of each initiator. The co-initiators used in this study were ethyl 4-dimethylaminobenzoate (EDAB), diphenyliodonium hexafluorophosphate (DPIHFP), 1,3-diethyl-2-thiobarbituric acid (BARB), p-toluenesulfinic acid and sodium salt hydrate (SULF). Absorption spectra of the initiators were measured using a UV–Vis spectrophotometer, and the photon absorption energy (PAE) was calculated. The binary system camphorquinone (CQ)/amine was used as a reference group (control). Twelve groups were tested in triplicate. Fourier-transform infrared spectroscopy (FTIR) was used to investigate the polymerization reaction during the photoactivation period to obtain the degree of conversion (DC) and maximum polymerization rate (R_p^max) profile of the model resin.

Results

In the analyzed absorption profiles, the absorption spectrum of QTX is almost entirely localized in the UV region, whereas that of CQ is in the visible range. With respect to binary systems, CQ + EDAB exhibited higher DC and $R_p^{\max }$ values. In formulations that contained ternary initiator systems, the group CQ + QTX + EDAB was the only one of the investigated experimental groups that exhibited an $R_p^{\max }$ value greater than that of CQ + EDAB. The groups QTX + EDAB + DPIHFP and QTX + DPIHFP + SULF exhibited values similar to those of CQ + EDAB with respect to the final DC; however, they also exhibited lower reactivity.

Significance

Water-soluble initiator systems should be considered as alternatives to the widely used CQ/amine system in dentin adhesive formulations.

Introduction

Dental adhesive system formulations fundamentally contain resin monomers, polymerization initiators, inhibitors or stabilizers, solvents and sometimes inorganic fillers [1]. According to their chemical structures, these components may exhibit hydrophobic or hydrophilic behavior [1].

The hydrophilic components, such as 2-hydroxyethylmethacrylate (HEMA), are required to increase monomer infiltration into wet and demineralized dentin [2]. However, the hydrophobic components used in these materials, such as bisphenol A glycidyldimethacrylate (Bis-GMA), are known to be responsible for enhancing the mechanical properties of the formulation and its compatibility with restorative composites or resin cements [2].

Resin monomer infiltration into the spaces around exposed collagen fibrils depends on the results of dentin demineralization before hybrid layer formation [3]. This interface must exhibit bond strength values that are sufficiently high to counteract the stresses generated by polymerization shrinkage and mastication [4] and to maintain these values over time, which is a critical factor for restoration durability. However, in vivo and in vitro studies have shown that the durability of the bond continues to be a problem [5], [6]. The main factors that explain the reduced longevity of bonded interfaces include incomplete impregnation of collagen fibrils by the monomer, high permeability of the bonded interfaces, sub-polymerized polymers, phase separation and activation of collagenolytic enzymes [7].

Phase separation of Bis-GMA-based adhesives has been shown to occur in the presence of water [8], [9]. Consequently, limited conversion from monomers into a rigid polymer network occurs when hydrophobic molecules are surrounded by a hydrophilic matrix, usually HEMA. This important outcome suggests that the traditional photo-initiator molecules either become isolated within the hydrophobic phase or that they are incompatible with hydrophilic HEMA [8].

The main photoinitiator system used in commercially available dental adhesive systems is based on the visible-light photosensitizer camphorquinone (CQ) [10]. In view of the phenomenon of phase separation that occurs in these materials and its consequences in terms of longevity and biocompatibility, the development of a more efficient adhesive photoinitiator system is necessary. A water-soluble photoinitiator system appears to be an interesting alternative to increase monomer/polymer conversion in Bis-GMA/HEMA-based adhesives. Some ionic derivatives of thioxanthone dyes are water miscible and therefore may represent an interesting alternative for the polymerization of dental adhesives [11]. In addition, iodonium salts seem to be efficient water-soluble co-initiator systems that improve the polymerization rate of dental monomers when camphorquinone is used, even in the presence of a solvent [12], [13].

Organic-acid derivatives can also be used to play the role of water-soluble co-initiators for this type of material, and sulfinic acid derivatives [14] and barbituric acid [15] have both already been used for the polymerization of dental materials.

Although recent studies have proved that the polymerization behavior of dental monomers is beneficial when the aforementioned components are used, little information exists about the efficiency of an iodonium salt or organic acid derivative as co-initiators for a thioxanthone derivative, and this question needs clarification. Consequently, the aim of this study was to evaluate the behavior of a model dental adhesive resin, in terms of its reactivity and polymerization kinetics, when a water-soluble initiator system is added to its formulation. Importantly, in addition exhibiting water solubility, the photoinitiator system might exhibit high polymerization reactivity when used in dental adhesives. The hypotheses tested were that: (1) the combination of QTX + an iodonium salt and/or organic acid derivatives is able to promote satisfactory polymerization and (2) the addition of QTX, an iodonium salt or organic acid derivatives may improve the polymerization efficiency of a CQ-based model dental adhesive resin.