1

Introduction

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.

2

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.

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. :

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