The purpose of this study was to assess the degree of conversion (DC) over time, using FTIR spectroscopy for bulk-fill flowable resin composite materials compared to conventional flowable and regular resin composite materials.
Eight resin composites were investigated including flowable bulk-fill materials SureFil SDR (SDR), Venus bulk-fill (VBF), x-tra base (XB), and Filtek Bulk Fill (FBF). Conventional flowable and regular composite materials included: Venus Diamond flow (VDF), Grandioso flow (GRF), Venus Diamond (VD), and Grandioso (GR). Degree of conversion (DC) was assessed by Fourier transform infrared spectroscopy using attenuated total reflectance technique. DC was measured for samples immediately post-cure ( n = 3), and after 24 h storage period at 37 °C ( n = 3). Results were analysed using one-way analysis of variance (ANOVA), Bonferroni post hoc test, and independent-samples t -test at α = 0.05 significance level.
Immediately post-cure, the mean DC values of the different materials were in the following order: GRF > VDF > SDR > VBF > XB > GR > FBF < VD and ranged from 34.7 to 77.1%. 24 h post-cure, DC values were in the following order: GRF > VBF > VD > SDR > VDF > GR > XB < FBF and ranged from 50.9 to 93.1%. GRF showed significantly higher DC values than all other materials at both time intervals while XB and FBF showed significantly lower values at 24 h post-cure.
The 24 h post-cure DC values of the bulk-fill composites SDR and VBF are generally comparable to those of conventional composites studied; however, the 24 h post-cure DC values of XB and FBF were lower compared to the other materials.
The degree of conversion (DC) of a resin composite is crucial in determining the mechanical performance of the material and its biocompatibility. Strength, modulus, hardness and solubility have been shown to be directly related to the degree of monomer conversion . In addition, assessment of changes of DC during polymerization is considered a useful tool in characterizing and understanding polymerization kinetics using different resin composite formulations and curing techniques.
The final DC depends mainly on intrinsic factors such as the chemical structure of the dimethacrylate monomer and photo-initiator concentration and extrinsic factors such as the polymerization conditions . The DC of several Bis-GMA based resin-composites has been evaluated previously using infrared techniques. The reported DC values were in the range of 52–75%, with most of the materials in the 55–60% range . The DC for adequate clinical performance has not yet been established. However, a negative correlation of in vivo abrasive wear depth with DC has been established for DC values in the range of 55–65% . Accordingly, at least for occlusal restorative layers, DC values below 55% are not recommended .
Many studies have investigated the effect of filler load, size, and geometry on DC of the resin-composite . DC was found to progressively decrease linearly with increasing opaque filler content . Differences in filler geometry did not seem to influence DC of experimental composites. However, DC decreased in composites whose filler particles size approached the output wavelength of the curing unit (470 nm). This was explained by the scattering effect of fillers of this size on penetrating light during photoactivation .
The aim of this study was to assess the DC of some bulk-fill composite materials compared to that of conventional flowable and regular composites using FTIR spectroscopy at two time intervals: immediately post-cure, and 24 h post-cure. Two null hypotheses were investigated: (i) there is no difference in the DC values of the low stress bulk-fill composites SDR, VBF, and XB in comparison to those of conventional composite materials, and (ii) there is no difference between DC values immediately post-cure and 24 h post-cure for all materials.
Materials and methods
Device used for assessment of DC
Eight resin composite materials were investigated including three flowable bulk-fill composites ( Table 1 ). The DC was measured using FTIR (Avatar 360, Nicolet Analytical Instruments, UK) equipped with a single reflection horizontal attenuated total reflectance (ATR) accessory (MIRacle ATR, PIKE Technologies, 6125 Cottonwood Drive, Madison). The FTIR spectrometer was operated under the following conditions: 4000–500 cm −1 wavelength, 6 cm −1 resolution, and 32 scans.
|Group||Material||Code||Type||Manufacturer||Lot no./shade||Organic matrix||Filler||Filler loading|
|Bulk fill composites||SureFil SDR||SDR||Flowable bulk fill||DENTSPLY||1000830/universal||Modified UDMA, EBPADMA, TEGDMA||Ba-Al-F-B silicate glass, Sr-A-F silicate glass||68 wt%|
|Venus Bulk Fill||VBF||Flowable bulk fill||Heraeus||010028/universal||UDMA, EBPADMA||Ba-Al-F silicate glass, YbF 3 , SiO 2||65 wt%|
|x-tra base||XB||Flowable bulk fill||VOCO||1208392/universal||Aliphatic di-methacrylate (UDMA), Bis-EMA||–||75 wt%|
|Filtek Bulk Fill||FBF||Flowable bulk fill||3M ESPE||N377465/universal||Bis-GMA, UDMA, Bis-EMA, Procrylat resins||zirconia/silica, ytterbium trifluoride||64.5 wt%|
|Conventional flowable composites||Venous Diamond Flow||VDF||Flowable||Heraeus||010027/A3||UDMA, EBPADMA||Ba-Al-F silicate glass, YbF 3 , SiO 2||65 wt%|
|Grandioso Flow||GRF||Flowable||VOCO||1104372/A2||Bis-GMA, TEGDMA, HEDMA||–||81 wt%|
|Conventional regular composites||Venus Diamond||VD||Low shrinkage nanohybrid||Heraeus||010021/AM||TCD-DI-HEA, UDMA||Ba-Al-F silicate glass, YbF 3 , SiO 2||81 wt%|
|Grandioso||GR||nanohybrid||VOCO||1048014/A3||Bis-GMA, TEGDMA, HEDMA||–||89 wt%|
Measurement of immediate post-cure DC
Uncured composite material was placed on the ATR crystal making sure that the crystal was completely covered by the material ( n = 3), the FTIR spectra of the uncured samples were then collected. Each material sample was then cured at room temperature for 20 s (as recommended by the manufacturer) using a halogen curing light (Optilux 501, Kerr Corporation, USA) with an output irradiance of 600 mW/cm 2 and standard curing mode. The light tip was kept as close as possible to the sample. The FTIR spectra of the cured samples were then collected immediately.
Measurement of 24 h post-cure DC
To assess the DC of the materials after 24 h storage at 37 °C, thin samples ( n = 3) were prepared by applying a small amount of each material on a piece of glossy transparent polyester film set on a glass microscope slide. The material was then covered with another piece of polyester film and pressed with another glass slide until a thin film was created. Each sample was checked to ensure consistent thickness. The sample was then cured with the curing light exit window against the glass slide. The samples were then stored for 24 h at 37 °C in a lightproof oven within a sealed glass container with silica gel (to prevent water adsorption onto the surface of the samples and avoid the potential source of noise in an FTIR). After 24 h, each sample was carefully placed on the ATR crystal plate and pressed with a clamp to maintain good contact between the sample and the ATR crystal. An FTIR spectrum of the sample was then collected.
For all samples, DC was measured by assessing the variation in the ratio of the absorbance intensities of aliphatic C C peak at 1638 cm −1 and that of an internal standard peak of aromatic C C at 1608 cm −1 of the uncured and cured samples. Due to the lack of aromatic C C, internal standard peaks at 1600 cm −1 and 1720 cm −1 were used in the case of SDR and VD respectively. The percentage DC was calculated for each sample using the following equation:
DC % = 1 − ( 1638 cm − 1 / interrnal standard ) peak area after curing ) ( 1638 cm − 1 / interrnal standard ) peak area before curing × 100