This study has investigated the influence of BisGMA, BisEMA, BisEMA 30, and two UDMA-based monomers (UDMA and Fit 852), with TEGDMA as co-monomer, on the degree of conversion, water sorption, water solubility, and optical properties of experimental dental composites.
Materials were formulated at 70/30 molar rations using BisGMA, BisEMA, BisEMA 30, UDMA or FIT 852, as base monomers, combined with TEGDMA. 60 wt% of silanated-glass particles was added. Degree of conversion (DC) and polymerization kinetics were monitored using Fourier-transformed infrared spectroscopy in the near-IR range. Water sorption (Wsp) and solubility (Wsl) were assessed using mass variation after 60 days water storage. Color was evaluated using a digital spectrophotometer, applying the CIELab parameters 24 h after dry storage and 60 days after water immersion to calculate ΔE values. All data were analyzed using ANOVA and Tukey’s test (pre-set alpha = 0.05).
The BisGMA-based co-monomer mixture presented the lowest DC (62 ± 1%), whereas BisEMA 30 had the highest DC value (95 ± 2%). The highest Wsp was observed for BisEMA 30 (12.2 ± 0.8%), and the lowest for BisEMA (0.4 ± 0.1%). BisEMA has shown the lowest Wsl (0.03 ± 0.01%) and BisEMA 30 the highest one (0.97 ± 0.1%). The ΔE values showed that BisEMA 30 (7.3 color units) and Fit 852 (3.8 color units) altered the color stability providing ΔE > 3.3, which is considered clinically unacceptable.
The chemical composition and structure of the base monomer influenced the degree of conversion, water sorption, water solubility, and color stability. Considering the overall results, it is possible to state that the base monomer BisEMA mixed with the co-monomer TEGDMA presented the best performance in terms of all the parameters tested.
The resin matrix composition might influence physical property degradation processes and color stability of dental resin composites. Formulations based on BisEMA seem most promising for materials’ development.
The association of aesthetic characteristics, mechanical properties, ease of handling and affordable cost makes dental composites the materials of choice for direct restorations . However, the stability and longevity of composite resin are a major concern, since many factors in the oral environment affect these parameters . It has been stated that the main reasons for failure of posterior composite restorations are secondary caries and fractures , whereas for anterior restorations, the reasons of replacement seem to be associated to aesthetic aspects of the restoration . Maintenance of restoration structural and color characteristics over time is an important aspect to determine the success or need of replacement . Composite formulation plays a large role in determining material properties and degradation resistance .
Most commercial composite products are based on methacrylates, such as bisphenol A- glycerolate dimethacrylate (BisGMA), a high viscosity monomer with a stiff aromatic central core, which can form a strong polymer network when associated with diluent co-monomers, usually triethylene glycol dimethacrylate (TEGDMA) . Among other base monomers, dimethacrylate monomers UDMA (urethane dimethacrylate) and BisEMA (ethoxylated bisphenol A dimethacrylate) may be used both as diluent co-monomers and also as base monomers themselves .
In addition to conventional BisEMA, a different version of this monomer, having a longer spacer (BisEMA 30), can also be used. This monomer is currently being tested as a possible replacement for HEMA in adhesive formulations . BisEMA 30 may also be considered for use in resin composite formulations. Due to its hydrophilicity, which would facilitate the interaction with the dentin, as well as lesser tendency to undergo hydrolysis in comparison to HEMA, this monomer can be envisioned for use in self-adhesive composite formulations’ . Other UDMA-based monomers, such as FIT 852 (FIT; Esstech, Essington, P’A, USA), have also been tested for dental composite due to low shrinkage associated to high conversion rate, in homopolymer resin and in resin composites . FIT 852 is a non-linear bifunctional methacrylate with molecular weigh between 1100 and 1200, with no aromatic group . A recent study by Manojlovic et al. reported a higher degree of conversion (DC) and lower shrinkage, but it has also pointed lower mechanical properties in relation to BisGMA/TEGDMA model composites, which suggest that FIT 852 would be used as a shrinkage reducing factor in composite matrix formulations .
Although there is vast literature on the influence of traditional monomers on the degree of conversion, the structure of polymeric networks, and their effect on physical–chemical properties and mechanical properties only few studies have evaluated the influence of monomer composition on the stability of the optical properties of dental composites subjected to artificial aging . This is also important due to the fact that variations in monomeric compositions have been applied to develop bulk-fill composites . Therefore, the aim of this study was to determine the effect of the replacement of BisGMA by BisEMA, BisEMA 30, UDMA and Fit 852 (UDMA-modified) with TEGDMA diluent co-monomer, on the polymerization kinetics, degree of conversion, water sorption and solubility and resultant color stability of experimental composites. The research hypotheses were that replacement of the conventional base monomer BisGMA using select monomers BisEMA, UDMA and alternative monomers BisEMA 30 and FIT 852 would significantly:
improve the polymerization kinetics and degree of conversion,
reduce water sorption and solubility, and
improve color stability.
Materials and methods
Preparation of experimental composites
The monomers BisGMA, UDMA, BisEMA, BisEMA 30, and FIT 852 were used as base monomers. TEGDMA was used as a diluent co-monomer. All materials were provided by Esstech (Essington, PA, USA) and used as received without further purification. Five experimental co-monomer mixtures with TEGDMA were obtained at a molar ratio of 70:30 (base to co-monomer ratio): BisGMA; UDMA; BisEMA; BisEMA 30; and Fit 852. The photo-initiator system was composed of 0.5 wt% camphorquinone, 1.0 wt% ethyl 4-dimethylamino benzoate (EDAB), while 0.05 wt% butyl hydroxytoluene (BHT) was added as a polymerization inhibitor. Fillers were incorporated into the organic matrix by hand-mixed procedure, silanated inorganic fillers were added at a total of 60% by weight; this was the maximum wt% achieved by hand-mixed incorporation for blend BisGMA/TEGDMA in pilot test. Thus, in order to keep standardization, the same filler amount (60%) was incorporated in the other blends tested. Of this amount, 20% were 2-μm barium–aluminium–silicate (Esstech), 20% were 0.7-μm barium–aluminium–silicate (Esstech), and 60% were 14-nm silicon dioxide (Evonik Industries AG, Hanau-Wolfgang, Germany). All the components were homogeneously mixed (DAC 150 Speed Mixer, Flacktek, Landrum, SC, USA) for 1 min at 1300 rpm.
Photoactivation procedures were performed using a blue-only light emitting diode (LED) source (Radii-call, SDI, Australia). The light irradiance was 1150 mW/cm 2 , which was derived from measurements with a power meter in mW (Ophir Optronics Inc., Danvers, USA) and the light source tip area, determined in cm 2 . A total of 15 specimens of each model composite was made for water sorption and solubility analyses (n = 10) and for color stability (n = 5). The specimens were randomly allocated in test groups.
Degree of conversion and polymerization kinetics
Conversion and polymerization kinetics were monitored using spectra obtained from an infrared spectrometer (Nicolet 6700 FTIR, Thermo Scientific, Pittsburgh, PA, USA) in the near-IR range, equipped with an extended KBr beam splitter and an InGaAs detector. Specimens were placed in silicone rubber molds (n = 3; Ø = 6.5 mm; t = 0.8 mm) sandwiched between glass slides, and then photoactivated for 40 s. The photoactivation was carried out with the tip of the light guide at a 45° angle from the top of the specimen. The irradiance reaching the specimen was measured with the same angle using a laboratory grade spectrometer. The IR beam passed through the middle of the specimen. The average irradiance reaching the center of the specimen was kept consistent for all specimens.
FTIR spectra (2 scans averaged per spectrum, 4 cm −1 resolution) were collected during 5 min. The peak area used corresponded to vinyl stretching (6165 cm −1 ). The near IR technique used here does not benefit from the use of internal reference peaks, because the monomer and subsequent polymer spectra are collected from the same specimen, with the same cross-sectional area, and the absence of adjacent, overlapping peaks. Therefore, degree of conversion (DC) was calculated from the ratio of peak area in monomeric and polymeric states , according to the following equation: (DC = (1-(area polymer/area monomer)) × 100).
D C = ( 1 − ( a r e a p o l y m e r a r e a m o n o m e r ) ) × 100