The oral cavity contains a diversity of chemical components from food and saliva including alkalis, acids, salts, and alcohol that may be absorbed by composite resin resulting in their degradation [
To evaluate the softening effect of the oral environment on composite resin, the solubility parameter is of extreme importance. It provides a numerical value through which the level of interaction between materials can be expected [
Both ethanol and methyl ethyl ketone FSOS have solubility parameters closely matching that of Bis-GMA (the most commonly used monomer in composite resin) [
Bulk fill composite resin is widely spreading amongst dentists due to its simple application technique. It displayed adequate light-curing to about 5 mm depth by measuring degree of conversion, compressive strength, and top/bottom hardness [
Recent studies have proved that MEK has stronger softening effect on bulk fill composite resin than E [
Two bulk fill composite resins were used in this investigation (Table
Technical specifications of the tested materials according to their manufacturers.
Composite resin | SonicFill |
SureFill |
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Manufacturer | Kerr Corporation, CA, USA/Kavo Germany | Dentsply/Caulk (USA) |
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Increment thickness (mm) | 5 mm | 5 mm |
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Monomer | Bis-GMA |
Bis-GMA, |
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Photoinitiator/coinitiators | Camphorquinone/tertiary amine | Camphorquinone/tertiary amine |
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Filler type | SiO2, |
Barium fluoro alumino borosilicate glass blend of fumed silica |
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Filler loading (wt%) | 83.5 | 82 |
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Filler loading (vol%) | Not declared | 66 |
Twenty specimens were prepared from each composite resin; 10 were kept in E and the other 10 in MEK. The bulk fill composite resin was packed into a cylindrical Teflon mold of 2.5 mm inner diameter and 5 mm height placed on a glass slab. The mold was packed with composite resin as a single layer. SonicFill was applied by sonic activated hand-piece, as recommended by the manufacturer. Light-irradiation was performed for 20 s on the top of the specimens using a light emitting diode curing unit (1200 mW/cm2, Bluephase 20i, Ivoclar Vivadent, Schaan, Liechtenstein). The top surfaces of all specimens were marked. Then the specimens were kept dry for 24 h at room temperature (
The evaluation of crosslink density was determined indirectly by calculating the difference between the top Vickers hardness numbers of the specimens stored dry for 24 h and after their storage in FSOS [
Three indentations (load: 50 g; dwell time: 15 seconds) were performed on the top surface of each specimen using Vickers’s hardness tester (Micromet 6049, Buehler, Illinois, USA). The mean of these three individual measurements was considered as the Vickers hardness number of the overall surface.
The specimens were stored individually in 10 mL of either 75% ethanol or 100% methyl ethyl ketone at
Statistical data were analyzed by using SPSS-20.0 (IBM product-USA). Mean and standard deviation were calculated for the Vickers hardness number of all subgroups. Two way ANOVA was used to assess the overall effect of material and solvent on the crosslink density and the estimated means were compared using Bonferroni adjustment.
The mean Vickers hardness numbers of the tested bulk fill composite resins stored dry for 24 h and stored in E and MEK as well as the calculated crosslink densities are presented in Table
Mean hardness number and crosslink density of the tested bulk fill composite resins after dry storage, storage in 75% ethanol, and storage in 100% methyl ethyl ketone.
Bulk fill composite resin | Dry storage | Storage in 75% E | Storage in 100% MEK |
|
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SureFill | ||||
Vickers hardness number (SD) | 82.4 (2.4) | 76.6 (2.1) | 74.6 (1.7) | |
Crosslink density (SD) | — | 7.0 (1.4) | 9.4 (2.5) |
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SonicFill | ||||
Vickers hardness number (SD) | 99.0 (1.3) | 88.5 (1.6) | 87.2 (1.8) | |
Crosslink density (SD) | — | 10.5 (1.8) | 11.9 (1.3) | 0.06 |
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<0.000 |
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SD: standard deviation.
Crosslink density is an important indicator of the chemical stability of composite resin [
The hardness assessment of the two bulk fill composite resins was conducted to estimate the crosslink density resultant from storage in E and MEK. The results revealed that both solvents produced reduction in hardness of the investigated composite resins. Such reduction can be explained by the solvent-polymer interaction detailed earlier in this context. These results were also consistent with other studies that investigated the effects of organic solvents on composite resins [
SureFill showed significant higher crosslink density than SonicFill where the former showed less decrease in Vickers hardness number after being stored in FSOS. It can be speculated that the polymeric network of SureFill consists of more crosslinked chains rather than the linear polymeric structure present in SonicFill. Such linear polymeric structure can encourage the diffusion of solvent inside as stated by Ferracane 2006 [
The higher resistance of SureFill to softening could be attributed to the presence of UDMA. The latter is an aliphatic flexible oligomer owing to the presence of flexible urethane linkage instead of the stiff iso-propylidene-di-phenoxy center core present in both Bis-GMA and its analog Bis-EMA. This flexibility allowed free initial mobility of UDMA ensuring the close proximity of the radical species, thus creating multiple centers of polymer growth (crosslink centers). In addition, the opportunity of chain transfer reactions through the imino group should be responsible for more rapid rate of polymerization in SureFill, evident in network formation prior to diffusion-controlled propagation stage [
Goņalves et al. 2009 [
Moreover, one should emphasize that the higher crosslink density of SureFill has overshadowed the relatively higher hydrophilic nature of its UDMA oligomer due to the presence of urethane group (-NHCOO-) when compared to BIS-EMA with its ether group (-O-) of lower hydrophilicity. This could be supported by the study of Pfeifer et al. 2009, who concluded that experimental composite resins composed of Bis-GMA : TEGDMA : UDMA offered the best compromise between degree of conversion from one side and flexural properties, fracture toughness, and susceptibility to ethanol degradation from the other side.
In addition to the impact of the monomers, the particulate filler also provides an essential part in defining the resistance to the plasticizing effect of organic solvents. An increase in the filler loading is most probably associated with higher resistance of the tested composite resins to degradation. Surprisingly, SonicFill displayed higher deterioration after conditioning in the organic solvents in spite of its relatively higher filler loading, Table
Within the limitation of this investigation, the following can be concluded: The crosslink density of dimethacrylate-based bulk fill composite resins can be evaluated using either ethanol or methyl ethyl ketone food simulating organic solvents with expected higher values for the latter. SureFill has higher crosslink density than SonicFill composite resin in both ethanol and methyl ethyl ketone food simulating organic solvents.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors thank Professor Dr. Maha Tantawy for her efforts in the statistical analysis of this study.