Viscoelastic properties, creep behavior and degree of conversion of bulk fill composite resins

Highlights

  • The viscoelastic properties of bulk fill composites are tested.

  • The materials are tested dry and wet under different temperatures.

  • Degree of conversion is calculated at the directly irradiated surface and at 4 mm.

Abstract

Objectives

The aim of this study was to investigate the viscoelastic properties and creep behavior of bulk fill composites under different conditions and evaluate their degree of conversion.

Methods

Seven bulk fill composites were examined: everX Posterior (EV), SDR (SD), SonicFill (SF), Tetric EvoCeram Bulk Fill (TE), Venus Bulk Fill (VE), x-tra base (XB) and x-tra fil (XF). Each material was tested at 21 °C, 37 °C and 50 °C under dry and wet conditions by applying a constant torque for static and creep testing and dynamic torsional loading for dynamic testing. Degree of conversion (%DC) was measured on the top and bottom surfaces of composites with ATR-FTIR spectroscopy. Statistical analysis was performed with two-way ANOVA, Bonferroni’s post hoc test and Pearson’s correlation coefficient.

Results

Shear modulus G ranged from 2.17 GPa (VE) to 8.03 GPa (XF) and flexural modulus E from 6.16 GPa (VE) to 23 GPa (XF) when the materials were tested dry at 21 °C. The increase of temperature and the presence of water lead to a decline of these properties. Flowable materials used as base composites in restorations showed significantly lower values ( p < 0.05) than non-base composites, while being more prone to creep deformation. %DC ranged from 47.25% (XF) to 66.67% (SD) at the top material surface and 36.06% (XF) to 63.20% (SD) at the bottom.

Significance

Bulk fill composites exhibited significant differences between them with base flowable materials showing in most cases inferior mechanical properties and higher degree of conversion than restorative bulk fill materials.

Introduction

The most common method of composite resin placement, considered as standard for large cavities, is the incremental layer technique in which the restorative material is placed in subsequent increments of about 2 mm of thickness. Its purpose is to achieve an optimal degree of conversion and depth of cure by curing thin layers of composite resin with a light curing device that is capable of producing adequate output for the necessary time period . The use of thoroughly cured consecutive layers is considered to be advantageous compared to single thicker layers as a high degree of conversion may result in improved mechanical and chemical properties, decrease of possible elution of monomers and color stability. Moreover, it is suggested that the incremental layer technique results in lower setting shrinkage stresses as the individual layers contribute to a lower C-factor , the number of bonded surfaces to the number of un-bonded surfaces.

However, despite the theoretical advantages of incremental layer, there is no general consensus in literature concerning its benefits, as there have been contradictory reports of increased polymerization stresses after using the incremental technique and cuspal deflection of a larger magnitude than that of bulk filling . This may be attributed to the fact that testing regimens differ due to a lack of standardization and also because the simulation of the resin flow transitions during polymerization is difficult .

Recently, new composite resins with the ability to be cured in bulk were introduced. Some of them can be used to fill the cavity from bottom to top, while others are used as base materials, usually flowable dentin replacement composites, requiring a layer of a conventional composite resin of 1.5–2 mm to finish the restoration. The manufacturers claim that these materials can be thoroughly cured, while problems as the implementations of voids between consecutive layers and possible contamination between them are avoided. Moreover, the use of a bulk filling protocol lessens the amount of time required for the clinician to complete the restoration, which is beneficial both for practitioner and patient.

Dental composite resins are polymer materials of time-dependent mechanical properties. The study of their viscoelastic properties is necessary in order to characterize them mechanically and can be useful in defining microstructural aspects like degree of cure, cross-linking and the bonding between the inorganic fillers and the organic matrix creep of conventional. The high loading forces during chewing may result in failure of composite restorations, so viscoelastic properties, like creep-strain, are essential as in clinical situation there is strain recovery during the unloading phases . According to Watts , the understanding of both elastic moduli and visco-elasticity is valuable in the appraisal of clinical performance of biomaterials. It is therefore important to examine the viscoelastic and creep behavior of these novel materials , as they are used in stress-bearing restorations

The aim of this study was to investigate the viscoelastic mechanical properties of various bulk fill composite resins under different testing conditions and also to determine their degree of conversion. The null hypothesis was that there would be no differences among the materials’ mechanical properties and degree of conversion and that their properties would not be affected by the different testing conditions.

Materials and methods

In the current study seven composite resins were used and are shown in Table 1 along with their composition provided by the manufacturers. Among these materials SD, VE and XB are flowable and EV non-flowable base composites requiring another thin layer of composite on top of them. SF, TE and XF can be used in bulk and are restorative composites that may fill cavities without the need of another composite.

Table 1
The bulk fill composites resins used in the study and their composition.
everX Posterior
GC
Tokyo, Japan
(EV)
SDR
Dentsply De Trey
Konstanz, Germany
(SD)
SonicFill
Kerr
Orange, CA, USA
(SF)
Tetric EvoCeram Bulk Fill
Ivoclar Vivadent
Schaan, Lichtenstein
(TE)
Venus Bulk Fill
Heraeus Kulzer
Hanau, Germany
(VE)
x-tra base
VOCO
Cuxhaven, Germany
(XB)
x-tra fil VOCO
Cuxhaven, Germany
(XF)
Base composite (fiber-reinforced) Base flowable composite Restorative composite Restorative composite Base flowable composite Base flowable composite Restorative composite
Monomers: Bis-GMA, TEGDMA
Fillers: 74.2% wt., 53.6% vol. E-glass fibers 1–2 mm length, PMMA, Ba–B–Si glass filler
Monomers: Modified UDMA, TEGDMA, EBPADMA
Fillers: 68% wt., 44% vol. Ba–B–Si glass
Monomers: Bis-GMA, TEGDMA, Bis-EMA, SIMA
Fillers: 83.5% wt., 66% vol.
Monomers: Bis-GMA, UDMA
Fillers: 79–81% wt., 60–61% vol. Ba–Al–Si glass, prepolymer filler (monomer, glass filler and YbF 3 ), spherical mixed oxide
Monomers: UDMA, EBPADMA
Fillers: 65% wt., 38% vol. Ba–Si glass
Monomers: Bis-EMA, MMA
Fillers: 75% wt., 58% vol. Si glass
Monomers: Bis-GMA, UDMA, TEGDMA
Fillers: 86% wt., 70% vol. Ba–B–Al–Si glass

Mechanical properties

The materials were tested under the following conditions:

  • (i)

    Dry at 21 °C, after 24 h of storage in room temperature of 21 °C.

  • (ii)

    Wet at 21 °C, after 24 h of storage in distilled water at 21 °C.

  • (iii)

    Wet at 37 °C, after 24 h of storage in distilled water at 37 °C.

  • (iv)

    Wet at 50 °C, after 24 h of storage in distilled water at 50 °C.

The different conditions were chosen in order to first evaluate the materials without any other external influence (21 °C dry), under the presence of water (21 °C wet) and then to investigate the behavior when closer to the conditions in the oral cavity (37 °C wet) and lastly the behavior under an excessive increase of temperature (50 °C wet) that may happen in the mouth for brief time intervals.

The experimental method used is capable of studying viscoelastic behavior and creep, by administrating constant load rate, subresonant dynamic and resonant dynamic experiments in bending and torsion. The apparatus ( Fig. 1 ) consists of a permanent high intensity Sm-Co magnet ( M = 1.12 × 10 −2 Nm/A) attached to the end of each specimen and placed at the center of a Helmholtz coil. The magnet is able to produce torque that can be controlled by the current in the coil. A laser beam from a low power He–Ne laser is reflected by a thin mirror ( d = 1.5 mm) cemented on the magnet to a calibrated chart at a distance D = 944 cm. The rotation angle of the mirror φ is given by φ = 2 X / D where X is the displacement of the laser beam on the chart.

Fig. 1
The apparatus used in the study.

The specimens ( n = 4) were fabricated by inserting the uncured composites into capillary tubes ( d = 1 mm, l = 18 mm) and then light-cured according to each manufacturer’s instructions for 30 s (1200 mW/cm 2 , G2 Bluephase, Ivoclar Vivadent). Due to the specimens’ length, each one was light-cured in consecutive sections along its axis in order to achieve thorough polymerization. The cured specimens were subsequently mounted on a Plexiglas disc (0.5 mm thick) by using a jig for centering.

Under static testing a constant torque was applied to each specimen for 10 s and the angular displacement was recorded, while the torque was instantly released. The shear modulus is the ratio of shear stress to shear strain ( G = σ / γ ) and was calculated from Eq. (1) :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='G=2MLπR4φ’>G=2MLπR4φG=2MLπR4φ
G = 2 M L π R 4 φ

with L being the length of the specimen and R its radius. The shear stress and shear modulus at 10 s show the short-time viscoelastic response of the material. Compliance J is the reciprocal of the shear modulus in the elastic case.

By repeating the experiment after the coil was rotated for 90° in order to achieve bending, Young’s modulus of elasticity E was obtained. Again, a constant torque was applied to the specimen for 10 s and then “instantaneously” released. The distribution of flexural strain ɛ , in a circular cylinder in bending is: ɛ = rφ / L. Young’s modulus is the ratio of flexural stress to flexural strain ( E = σ / ɛ ) and was calculated from Eq. (2) :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='E=64MLπd4φ’>E=64MLπd4φE=64MLπd4φ
E = 64 M L π d 4 φ

Dynamic experiments

In this test, a steady-state dynamic vibration was applied by frequencies with a range from 1 to 150 Hz. A function generator is connected to the Helmholtz coil creating sinusoidal torque. The displacement or amplitude n was recorded for every frequency. In dynamic experiments performed within linear viscoelastic behavior, stress and strain vary sinusoidally. Storage modulus ( G 1 ) which is the real part of the complex modulus G * is in phase with strain, while the loss modulus G 2 (the imaginary part of the complex modulus G *, related to the dissipation of energy) is 90° out of phase with strain. In stiff solids the complex modulus G * is approximately equal in magnitude to the storage modulus G 1 because G 2 is small compared to G 1 . The ratio of the imaginary part to the real part ( G 2 / G 1 ) of the complex modulus G * is called the loss tangent (tan δ ) and represents the phase angle between stress and strain sinusoids. Loss tangent is proportional to the energy loss per cycle while in the framework of linear viscoelasticity and was calculated by Eq. (3) :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='13Δνν0,’>(13)Δνν0,13Δνν0,
1 3 Δ ν ν 0 ,

while shear storage modulus G 1 was calculated from Eq. (4) :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='ν0=12πG1πr2LI’>ν0=(12π)G1πr2LIν0=12πG1πr2LI
ν 0 = 1 2 π G 1 π r 2 L I

where r is the specimen’s radius, L is its length and I is the moment of inertia of the magnet which was measured at 5.5 × 10 −8 kg m 2 . Dynamic viscosity was calculated from Eq. (5) :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax style="POSITION: relative" data-mathml='n*=1ω0(G12+G22),’>n*=(1ω0)(G12+G22),n*=1ω0(G12+G22),
n * = 1 ω 0 ( G 1 2 + G 2 2 ) ,

where ω 0 = 2 πν 0 . The quality factor, Q , indicates the shape of the resonance curve. A high value of Q correlates with a peaked resonance curve and little damping. Q was calculated from Eq. (6)

<SPAN role=presentation tabIndex=0 id=MathJax-Element-6-Frame class=MathJax style="POSITION: relative" data-mathml='Q=ν0Δν.’>Q=ν0Δν.Q=ν0Δν.
Q = ν 0 Δ ν .

Creep experiments

The materials were also tested under torsional creep by using the same apparatus. The test was performed 24 h after the fabrication of each specimen, with the specimens being stored dry at a temperature of 37 °C. A constant torque was applied and the angular displacement was recorded for up to 3 h. The stress was then released and the material was let to recover for 50 h. Two torques of 2.12 × 10 −4 N m and 4.24 × 10 −4 N m were applied resulting into initial stresses within the range of linear viscoelasticity (1 MPa and 2 MPa) and below the proportional limit of the materials.

For the flowable base bulk-fill materials (SD, VE, XB), additional specimens consisting of the base material ( l = 12 mm) combined with a conventional composite top layer ( l = 6 mm) were fabricated. For SD and VE the packable composite Prodigy (PR, Kerr) was chosen as a top layer, whereas XF was used for XB. These specimens were made in order to simulate the way the base materials will be used in clinical conditions and to find out if their creep performance is improved and is on the same level with the rest of the non-base bulk fill composites. The proportions of the two composites in the specimens were the same with the ones the manufacturers suggest using in cavities (4 mm and 2 mm layers for base and top composite).

For the statistical analysis in static and dynamic testing, two-way ANOVA with material and testing condition as independent variables and Bonferroni post-tests ( α = 0.05) were used. In the creep experiments material and torque used were the independent variables. Pearson’s correlation coefficient was used to investigate possible interactions between properties and %vol. filler content.

Degree of conversion

Specimens were prepared ( d = 5 mm, h = 4 mm, n = 5 per product) by packing the material pastes in opaque plastic molds pressed between two glass slides (0.5 mm thick) covered with polystyrene matrix strips to remove material excess and avoid oxygen inhibition upon setting. The diameter of the specimens was chosen to allow for a single-phase uniform irradiation, whereas the height to comply with the maximum depth of cure claimed by most manufacturers.

Light-curing was performed for 30 s employing a LED curing unit (G2 Bluephase, Ivoclar Vivadent, 1200 mW/cm 2 ), with the tip of the fiber-optic in contact with the top glass slide. All specimens were stored dry in dark conditions at 37 °C for 48 h. The degree of conversion (%DC) was measured at the top and bottom specimen surfaces by micro-attenuated total reflectance Fourier transform infrared spectroscopy (micro-ATR FTIR). A micro-ATR cell (Golden Gate, Specac, Smyrna, GA, USA) equipped with a 2 mm type IIIa diamond ATR element and ZnSe focusing lenses was attached to an FTIR spectrometer (Spectrum GX, Perkin-Elmer, Bacon, UK). The specimens were demounted, the top or bottom surfaces were pressed against the ATR crystal by the flat stainless steel anvil of the cell and spectra were acquired under the following conditions: 2000–650 cm −1 wavenumber range, 4 cm −1 resolution and 40 scans co addition. The depth of analysis was estimated as to ∼2 μm at 1000 cm −1 .

The quantitative determination of %DC was based on the two-band technique. The peak of the aliphatic C C bond stretching vibrations (1636 cm −1 ), which are consumed during polymerization, was chosen as the analytical absorption band, whereas the peak of the aromatic C⋯C bond stretching vibrations (1608 cm −1 ), which are not affected by the curing process, was selected as the reference absorption. The %DC was calculated by the following Eq. (7) :

<SPAN role=presentation tabIndex=0 id=MathJax-Element-7-Frame class=MathJax style="POSITION: relative" data-mathml='%DC=100×1−Ap(cUnknown node type: glyphc)×Am(c⋯c)Am(cUnknown node type: glyphc)×Ap(c⋯c)’>%DC=100×(1(Ap(cUnknown node type: glyphc)×Am(cc)Am(cUnknown node type: glyphc)×Ap(cc)))%DC=100×1−Ap(cUnknown node type: glyphc)×Am(c⋯c)Am(cUnknown node type: glyphc)×Ap(c⋯c)
% DC = 100 × 1 − A p ( c c ) × A m ( c ⋯ c ) A m ( c c ) × A p ( c ⋯ c )
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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Viscoelastic properties, creep behavior and degree of conversion of bulk fill composite resins
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