Structural and mechanical properties of a giomer-based bulk fill restorative in different curing conditions

ABSTRACT Objective The main goal of this study was to compare the polymerization degree of bulk-fill giomer resin cured with three different light-curing units (LCUs): a polywave third-generation (Valo); a monowave (DemiUltra: DU); and a second-generation LED (Optima 10: Opt) LCUs by using structural and mechanical properties. Material and methods Giomer samples of 2 and 4 mm cured with three LCUs were employed in vitro analysis. The degree of curing (DC%) was determined with Fourier-Transform Infrared Spectroscopy (FTIR). Microstructural features were observed with scanning electron microscopy (SEM). Flexural strength (FS), compression strength (CS), elastic modulus and fracturing strain were determined for mechanical properties. Surface microhardness (SMH) values were also measured. Oneway ANOVA, two-way analysis of variance and Tukey multiple comparison tests were used for statistically analyzing the FS and SMH. Results DC% values were 58.2, 47.6, and 39.7 for the 2 mm samples cured with DU, Opt., and Valo LCUs, respectively. DC% values of the 4 mm samples were 50.4, 44.6, and 38.2 for DU, Opt, and Valo, respectively. SMH values were Valo, Opt<DU at top of the samples; Valo<DU, Opt at 2 mm, and DU, Valo<Opt at 4 mm depth. Giomer samples cured with Opt and DU exhibited higher FS values than Valo. CS values were similar but compressive modulus and fracturing strain (%) varied depending on the curing protocol. Conclusions Based on the results, it can be concluded that curing device and protocol strongly affect crosslinking reactions and thus DC%, SMH, compressive modulus and strain at break values. Consequently, it can be deduced that curing protocol is possibly the most important parameter for microstructure formation of highly-filled composite restoratives because it may bring some structural defects and physical frailties on restorations due to lower degree of polymerization.


Introduction
Bulk-fill resins have been developed to speed up the emplacement of restorative material. These new restorative composites can be used to fill cavities by single or multiple increments 6 . Bulk-fill restoratives generally include small-sized or lower amount of fillers to decrease light scattering. This structural feature allows the material to be applied up to 4 mm increments at a time. Innovation in bulk fills introduced various new photoinitiators and fillers 16 .
Giomer is an alternative novel hybrid dental restorative material containing pre-reacted glassionomer filler particles in a resin matrix that provides some advantages for fluoride releasing and recharging, in addition to enhanced mechanical, esthetical and handling properties 5 . Recently a bulk-fill giomer in low viscosity and high viscosity forms was introduced, claiming a combination of anticarious properties, esthetic, durability and fast-treatment comfort 13 .
Light-cured resin materials allow controlling curing time but also require incremental polymerization, which has been suggested to be restricted to 2 mm until recently for the majority of composites with most of the light-curing unit (LCU) in the market 7,8 .
In addition to the compositional improvements in resin phase and polymerization issues, some LCU manufacturers have claimed to decrease curing time and thus treatment period by increasing the irradiance outputs (mW/cm 2 ) of their equipment, following the assumption that radiant exposure has a simple reciprocal relationship: if the irradiance is increased, the light-curing time can be decreased 18,26 .
Most recent advancements in curing technology have appeared in light-emitting diode (LED) LCUs. These devices have become popular due to their several operational advantages such as shorter exposure times, longer service time, lower weight and thermal effects compared to halogen lights and ultraviolet (UV) predecessors 11,19,24 . First-and second-generation LED-LCUs were able to polymerize 2 mm thick resin samples in 20-40 s and emitted a narrow monowave light spectrum (450-470 nm), which corresponds to the spectral peak absorbance of camphorquinone.
However, some resin manufacturers have started to use alternative photoinitiators, which necessitated suitable LCU. Recently, third-generation LED-LCUs were developed, which can emit multi-wave light to activate multi-component photo-initiator systems with high irradiance outputs and provide sufficient polymerization with shorter curing 11,19,24 .
Polymerization of dental restoratives can be determined indirectly by scraping methods, depth of cure and surface microhardness (SMH) tests or directly with Fourier-Transform Infrared Spectroscopy (FTIR).
FTIR is a spectroscopic technique used to analyze the chemical bonds of polymers by comparing the peaks of C=C bands. Although FTIR has been reported to be a superior measure for quantifying polymerization, it is a complex, high-cost and time-consuming method 23 .
SMH is a very common and simpler method, which uses the measurements of its specific indenter to test polymerization and has been reported as a good indicator of polymerization 7,8,11 .      where A is the intensity of a particular absorption peak, ac and bc represent conditions of "after curing" and "before curing", respectively.
The morphological properties of a representative sample (G-DU) were investigated in a field emission scanning electron (FE-SEM, FEI Quanta FEG 450) microscope. In the SEM analysis, fractured surfaces of the sample after the compression test were directly imaged in the instrument after a proper sample preparation route sputter-coated with gold.

Microstructure and morphology
The intensity of the characteristic absorption peak of unsaturated aliphatic C=C double bond originated from the methacrylate group at 1636 cm -1 and that of aromatic C=C double bond at 1604 cm -1 can be used to quantify the DC%, as previously described 21 .
Another characteristic -COO-NH-stretching peak originated from UDMA is clearly seen at 1716  since these peaks can be influenced by the interfacial interaction or the adhesion between the resin phase and filler particles. Ilie and Fleming 15 (2015) compared physical properties of a bulk-fill giomer and two resinbased bulk fill composites cured by a single LED device.
They reported that giomer exhibited lower DC values than resin-based bulk fill composites due to its higher filler content.
In this study, SEM image of polymerized resin ( Figure 5) implied that the resin phase successfully covered all filler surfaces and no holes or microvoids at the interfacial region appeared. This result also represents that the possible cracking mechanism mentioned before is originated from the cracking of the crosslinked resin phase rather than the particle or interfacial failure.

Surface hardness
It is a well-known fact that when light is applied to a resin material, the irradiance decreases as it is It was found that all the samples considered in our study met the FS requirements, which also implied that they could be safely used in load-bearing areas.