The aim of this study was to determine the Weibull and slow crack growth (SCG) parameters of bulk-fill resin based composites. The strength degradation over time of the materials was also assessed by strength-probability-time (SPT) analysis.
Three bulk-fill [Tetric EvoCeram Bulk Fill (TBF); X-tra fil (XTR); Filtek Bulk-fill flowable (BFL)] and a conventional one [Filtek Z250 (Z250)] were studied. Seventy five disk-shaped specimens (12 mm in diameter and 1 mm thick) were prepared by inserting the uncured composites in a stainless steel split mold followed by photoactivation (1200 mW/cm 2 /20 s) and storage in distilled water (37 °C/24 h). Degree of conversion was evaluated in five specimens by analysis of FT-IR spectra obtained in the mid-IR region. The SCG parameters n (stress corrosion susceptibility coefficient) and σ f0 (scaling parameter) were obtained by testing ten specimens in each of the five stress rates: 10 −2 , 10 −1 , 10 0 , 10 1 and 10 2 MPa/s using a piston-on-three-balls device. Weibull parameter m (Weibull modulus) and σ f0 (characteristic strength) were obtained by testing additional 20 specimens at 1 MPa/s. Strength–probability–time (SPT) diagrams were constructed by merging SCG and Weibull parameters.
BFL and TBF presented higher n values, respectively (40.1 and 25.5). Z250 showed the highest (157.02 MPa) and TBF the lowest (110.90 MPa) σ f0 value. Weibull analysis showed m (Weibull modulus) of 9.7, 8.6, 9.7 and 8.9 for TBF, BFL, XTR and Z250, respectively. SPT diagram for 5% probability of failure showed strength decrease of 18% for BFL, 25% for TBF, 32% for XTR and 36% for Z250, respectively, after 5 years as compared to 1 year.
The reliability and decadence of strength over time for bulk-fill resin composites studied are, at least, comparable to conventional composites. BFL shows the highest fatigue resistance under all simulations followed by TBF, while XTR was at par with Z250.
Bulk-fill resin composites were introduced with claims to save restorative procedure time by enabling up to 4-mm thick placements that can be photopolymerized in one step. As increment size variability greatly depends on the skill level of clinicians , these materials can possibly overcome problems related to traditional layering techniques such as the inclusion of voids or contamination between layers .
The physical, chemical and mechanical properties of bulk-fill resin composites have been extensively studied. These materials may present degree of conversion (DOC) as high as high as 76–86% at 1 mm reaching up to 64% at 4 mm depth similar to conventional resin composites in the 55–60% range at 1 mm . The marginal quality to enamel and dentin as well as internal dentin adaptation of bulk-fill resin composites has been found to be similar to conventional composites . Some low-viscosity bulk-fill resin composites however, present less gap-free marginal interface and compromised internal adaptation to dentin . Moreover, bulk fill resin composites present nanoindentation and bulk compressive creep comparable to conventional resin composites . Conversely, the elastic modulus and flexural strength of bulk-fill resin composites vary as a function of the resin formulation and filler type and loading, making it impossible to generalize for the entire class of material. The elastic modulus of bulk-fill resin composites, can vary from 3.3 to 9.4 GPa . Higher elastic modulus were reported for some bulk-fill composites (∼15 GPa) but the values obtained were lower than the ones measured for conventional resin composites (up to 20 GPa) .
Though some studies have not found discrepancy in the flexural strength while comparing only bulk-fill resin composites , others have reported significant discrepancies when comparing various bulk-fill resin composites and conventional ones . Fracture strength alone cannot predict structural failure as it provides only insight into the stresses that the material will withstand for a given flaw size distribution . Alternatively, Weibull distribution, however, takes into consideration the scatter in strength measurements to describe the reliability of materials (i.e. stress required to fracture a given percentage of specimens) as they are scaled-up in size (larger volume or surface under stress) . The Weibull modulus ( m ) is a dimensionless material-specific parameter that describes the variability of strength of brittle materials. Since it is inversely related to the standard deviation in a normal distribution, high Weibull modulus relates to higher reliability of materials . The second parameter, characteristic strength ( σ 0 ), is a location parameter that corresponds to the stress level for a 63.2% probability of failure . Since it is related to the fracture strength of a material, it may vary with specimen geometry and test set-up . The m values for some bulk fill composites can fluctuate from 10.4 up to 26.7 .
Characterization of a material’s fracture resistance is important for screening, however, because that property is generally determined under static or quasi-static loading and it may not be representative of the material’s strength when in function . This is especially true for dental applications since, in clinical situations, direct and indirect restorative materials are subjected to cyclic masticatory forces under a corrosive environment that ultimately may lead to strength degradation . In fact, resin composites do experience fracture strength degradation while defying fatigue scenarios . The flexural strength of composites can decrease up to 27% after being stressed under rotating fatigue . Likewise, dramatic strength degradations ranging from 32% up to 64% were observed for resin composite subjected to a flexural fatigue regimen of 10 4 cycles . Similar strength degradation (37% up to 67%) were reported by Lohbauer et al . for resin composites after a fatigue challenge of 10 5 cycles .
The strength degradation over time is related to the material’s susceptibility to slow crack growth (SCG) . The SCG parameters n (stress corrosion coefficient) and σ f0 (scaling parameter) can be obtained by the dynamic fatigue test. This methodology relies on mathematical relationships among fracture resistance of specimens tested at different stress rates . Usually, n values ranging from 5 to 30 indicate a high susceptibility to strength degradation under corrosive environment over time. For resin composites, n values ranging from 7 to 34.7 have been reported .
The objective of this study was to determine the strength ( m and σ 0 ) and SCG ( n and σ f0 ) parameters of bulk-fill composite materials. In addition, the reliability degradation of the materials over time was assessed by the analysis of a strength–probability–time (SPT) diagram. The null hypothesis to be tested is that bulk-fill resin composites present similar Weibull and SCG parameters to the conventional resin composite tested.
Materials and methods
The three bulk-fills and one conventional resin composite used in this study are listed in Table 1 . Disk-shaped specimens (12 mm in diameter and 1.0 ± 0.1-mm thick) were prepared by inserting the uncured composites in a stainless steel split mold. The top surface was covered with a Mylar strip. A glass slide was placed over the mold and manual pressure was applied to extrude excess material. The glass slide was removed and the material was photoactivated for 20 s at 1200 mW/cm 2 (Elitedent Q-4, Elitedent Enterprise Inc, USA). The tip of the curing light was kept 2 mm from the composite by a spacer to standardize curing distance. The specimens were subsequently removed from the mold and stored in distilled water at 37 °C for 24 h prior to testing.
|Resin based composites||Group||Resin matrix||Filler||Filler (wt%/vol%)|
|Tetric EvoCeram Bulk fill nanohybrid (Vivadent, USA, batch S14902)||TBF||Bis-GMA, UDMA, Bis-EMA||Ba–Al–Si glass, prepolymer filler (monomer, glass filler and ytterbium fluoride), spherical mixed oxide||81/61|
|x-tra fil hybrid (VOCO, USA, batch 1325395)||XTR||Bis-GMA, UDMA, TEGDMA||N/A||86/70|
|Filtek Bulk fill Flowable (3M ESPE, USA, batch N504062)||BFL||Bis-GMA, UDMA,Bis-EMA, Procrylat resins||Zirconia/silica, ytterbium trifluoride||64/42|
|Filtek Z250 (3M ESPE, USA, batch) 1370A3||Z250||Bis-GMA, UDMA, Bis-EMA||Zirconia/silica||82/60|
The degree of conversion (DOC) was assessed to confirm the efficacy of the polymerization method. Five specimens were dry stored for 24 h at 37 °C and the FT-IR spectrum was measured at the bottom of the specimens in mid-IR region (IFS66 v/S, Bruker, USA) equipped with universal ATR sampling accessory (MIRacle, PIKE, USA) under the following conditions: 4 cm −1 resolution and 138 scans per spectrum. DOC was obtained by measuring the difference in the ratio of the absorbance strength of the vinyl peak at 1638 and a reference peak at 1608 cm −1 corresponding to aromatic absorption before and after photoactivation according to previously reported .
To obtain the SCG and Weibull parameters, specimens were tested under distilled water (37 °C) with a piston-on-three balls device using a universal testing machine (Model 5948 MicroTester, Instron, USA) .
The SCG parameters n (subcritical crack growth susceptibility coefficient) and σ f0 (scaling parameter) were obtained by the dynamic fatigue method which relies on mathematical relationships among fracture resistance values obtained at different constant non-zero stress rates . The higher the n value calculated, the lower susceptibility to SCG. Ten specimens were tested in each of the stress rates (10 −2 , 10 −1 , 1, 10 1 and 10 2 MPa/s) with the exception of 1 MPa/s, for which thirty specimens were tested to perform Weibull statistics . The biaxial flexural strength ( σ f ) was obtained according to ISO 6872 as previously described by Ornaghi et al. :