Mechanical viscoelastic behavior of dental adhesives

Abstract

Objectives

The purpose of the study was to evaluate the mechanical properties of dental adhesive materials at different testing temperatures after dry and wet storage.

Methods

Specimens ( d = 1 mm, l = 18 mm) from six materials were tested: Silorane Adhesive System (SL), Heliobond (HE), One-Step Plus (OS), Optibond Solo Plus (OP), cmf Adhesive System (CF) and Protobond (PR). Static and creep testing was performed by applying a constant torque below the proportional limit of the materials, while dynamic testing consisted of dynamic torsional loading. Experiments were performed after 24 h of dry and wet storage under temperatures from 21 °C to 50 °C and various viscoelastic parameters were calculated.

Results

Shear modulus ranged from 0.19 to 1.99 GPa, while flexural modulus from 0.67 to 5.69 GPa. Most of the materials were affected by the presence of water and increase of temperature. OP showed the highest recovery after creep, while SL exhibited the highest permanent deformation.

Significance

Contact with water after polymerization and increase of temperature resulted in a decline of the mechanical properties, especially for the HEMA-containing adhesives.

Introduction

The need for restorative materials that could replace amalgam led to the introduction of composite resins that are now widely used by dentists. One of the most important steps that consequently led to the popularity of these materials was the breakthrough in adhesive technology. The finding that acid etching enamel led to higher bond strength between resin and enamel resulted in further research and the understanding of hybrid layer and dentin etching. Dental adhesives have evolved since then and many commercial products are available with different compositions and different approach to the way they deal with tooth tissue . Their aim is to achieve strong bonding between dental tissues and restorative materials which will provide clinical longevity to the restoration.

Despite the advancements in bonding, the bonded interface of composite restorations is still the weakest area of the restoration and the main reason for failures such as marginal discoloration and poor marginal adaptation which may later lead to loss of retention . This is apparent in the fact that various strategies and types of adhesives are used in order to achieve the most satisfactory bonding performance. Without regard to the steps required, the approaches contemporary adhesives use are two: etch-and-rinse and self-etch. In the former approach the tooth substrate is first etched and rinsed (conditioning stage), followed by a priming stage and the application of the bonding resin . On the other hand, self-etch approach does not require a separate etch-and-rinse stage and uses acidic monomers that simultaneously condition and prime dentin and enamel .

The resulting composite restorations are a complex system that consists of different substrates and interfaces. The long-term performance of these restorations is the result of the behavior of the various components and their ability to withstand stress and deformation. Testing separately the various components of the composite restoration can help to identify which is the least stable under different conditions . While the viscoelastic behavior of the restorative composite resins is often studied, adhesive resins are not commonly examined regarding their mechanical behavior. This phenomenon may be attributed to the fact that research is focused mainly on their bonding properties and that some techniques cannot be applied directly to materials that are used in thin layers . The technique being used in the current study was developed in order to test small specimens appropriate for dental materials, as most instruments used for this type of testing are optimized for much larger specimens. It has been previously used for the determination of the mechanical properties of other dental materials as composite resins , resin cements impression materials and fiber-reinforced posts . The advantages of this method is that it can be used both for static and dynamic testing under different conditions that can be controlled by the operator and can provide measurements for various viscoelastic parameters.

The aim of this study was to evaluate the mechanical properties of various commercial dental adhesives under different conditions both under static and dynamic testing. The null hypothesis was that the materials will not present differences in their properties and will not be affected by storage and testing conditions.

Materials and methods

Six commercially available materials were tested and are shown in Table 1 . In the case the material consisted of more than one component only the bonding component was tested. Cylindrical specimens (diameter d = 1 mm, length L = 18 mm) from each material were made with the use of glass capillary tubes. Each material was poured to a plastic funnel to be gently dried and then let to flow into the transparent glass tube. Due to the specimens’ length, each one was light-cured in consecutive sections along its axis in order to achieve thorough polymerization and for a time according to the manufacturers’ instructions (600 mW/cm 2 , Coltulux 4 light, Coltene Whaledent, Altstätten Switzerland).

Table 1
The materials used in the study.
Material Composition Type
Silorane adhesive system (SL)
3M ESPE – Seefeld, Germany
Hydrophobic dimethacrylate, phosphorylated methacrylates, TEGDMA, initiators, stabilizers
Fillers: Silane treated silica
Two-step self-etch: self-etch primer and bond
Heliobond (HE)
Ivoclar Vivadent – Liechtenstein
Bis-GMA, TEGDMA, initiators, stabilizers Unfilled enamel bonding agent
One-Step Plus (OS)
Bisco – Schaumburg, IL, USA
Bis-GMA, HEMA, BPDM, acetone
Fillers: 8.5% wt. glass ionomer
Two-step etch and rinse: etching gel and bond
OptiBond Solo Plus (OP)
Kerr – Orange, CA, USA
Bis-GMA, HEMA, GDM, GPDM, ethanol
Fillers: 15% wt. 0.4 μm barium glass, fumed silica, sodium hexafluorosilicate
Two-step etch-and-rinse: etching gel and bond
cmf adhesive system (CF)
Saremco – St Gallen, Switzerland
Bis-GMA, Bis-EMA
Fillers: silanized barium glass
Three-step etch-and-rinse: etching gel, primer and bond
Protobond (PR)
Dental Co-operative – Thessaloniki, Greece
Bis-GMA, Bis-EMA, HPMA, initiators, stabilizers Two-step etch-and-rinse: etching gel and bond
Bis-GMA: bisphenol A glycidyl methacrylate, TEGDMA: triethylene glycol dimethacrylate, HEMA: 2-Hydroxyethylmethacrylate, BPDM: biphenyl dimethacrylate, GDM: glycerol dimethacrylate, GPDM: glycerol phosphate dimethacrylate, Bis-EMA: bisphenol A ethoxylate dimethacrylate, HPMA: hydroxypropyl methacrylate.

The materials were tested under four different conditions ( n = 4 for each condition):

  • i)

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

  • ii)

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

  • iii)

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

  • iv)

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

The specimens were mounted using a jig for centering between a Plexiglas disc (0.5 mm thick) and a rod. The experiments were performed using an apparatus ( Fig. 1 ), previously described by Lakes , that is capable of testing cylindrical specimens under torsion or bending and measuring viscoelastic behavior. After the specimen was centered to the disc, a thin plastic tube (16 mm in diameter and 18 mm high) was placed over the specimen and attached to the disc, creating a chamber to hold water during testing. The top of the chamber remained open to receive a heating element and a thermocouple in order to control temperature (accuracy of ±0.5 °C). A permanent Samarium-Cobalt magnet was attached to the end of each specimen and placed in the center of a Helmholtz coil producing a torque (M) of 3.53 × 10 −3 Nm/A controlled by the current in the coil. The weight of the magnet caused only a small constant axial tensile stress and there was no constraint on the specimen for neither torsion, nor extension. The laser beam from a low power He–Ne laser was reflected by a small mirror (diameter of 1.55 mm) attached to the magnet to a calibrated chart at a distance D = 944 cm. The rotation angle of the mirror is given by φ = 2X / D , where X is the displacement of the laser beam to the chart.

Fig. 1
The apparatus used in the study. Torque was generated by the magnet placed at the center of a Helmholtz coil. The spot of a laser beam was reflected from the thin mirror cemented onto the magnet to the calibrated chart at a known distance. The displacement of the laser beam on the chart provided the mirror rotation angle φ during testing, which was used for the measurement of the viscoelastic properties.

Static and creep experiments

The determination of the static shear modulus of the materials was achieved with the rapid application of a constant torque to the specimen for 10 s. The angular displacement was recorded and the torque was then released. The shear modulus is the ratio of shear stress to shear strain ( G = σ / γ ) and was calculated from the equation:

G = 2 M L π R 4 φ

where L is the length of the specimen and R is 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.

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

E = 64 M L π d 4 φ .

Poisson’s ratio ν was calculated from:

E = 2 G ( 1 + v ) ,

using the values of G and E calculated in static measurements.

Creep measurements were also made, with the specimens being tested dry 24 h after fabrication (dry storage at 21 °C and testing under the same temperature). A constant torque was applied to the materials and the angular displacement was recorded for 3 h. The stress was then released and the recovery was recorded for 50 h. Three different torques were used corresponding to stresses of 2.03, 3.38 and 4.73 MPa.

Dynamic experiments – storage modulus

Under dynamic experiments 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 usually 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.

Frequencies ranging from 1 to 150 Hz were applied in the dynamic vibration of the specimens. A function generator connected to the Helmholtz coil creates a sinusoidal torque. The displacement or amplitude was measured on the chart for each frequency. Viscoelastic parameters were calculated from the resonance frequency ν 0 , corresponding to the peak amplitude and the resonance full width Δ ν which is the difference between the two frequencies at which the amplitude is half of the maximum. The loss tangent (tan δ ) is equal to:

1 3 Δ ν ν 0 ,

while shear storage modulus G 1 was calculated from the following equation:

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:

n = 1 ω 0 ( G 1 2 + G 2 2 ) ,

where ω 0 = 2πν 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 the equation Q = v 0 v .

Statistical analysis

The results were analyzed statistically using two-way analysis of variance (ANOVA) and Bonferroni post hoc tests ( α = 0.05). The independent parameters were the materials and the testing conditions for the static and dynamic testing, while for creep testing the independent parameters were the materials and the initial stress.

Materials and methods

Six commercially available materials were tested and are shown in Table 1 . In the case the material consisted of more than one component only the bonding component was tested. Cylindrical specimens (diameter d = 1 mm, length L = 18 mm) from each material were made with the use of glass capillary tubes. Each material was poured to a plastic funnel to be gently dried and then let to flow into the transparent glass tube. Due to the specimens’ length, each one was light-cured in consecutive sections along its axis in order to achieve thorough polymerization and for a time according to the manufacturers’ instructions (600 mW/cm 2 , Coltulux 4 light, Coltene Whaledent, Altstätten Switzerland).

Table 1
The materials used in the study.
Material Composition Type
Silorane adhesive system (SL)
3M ESPE – Seefeld, Germany
Hydrophobic dimethacrylate, phosphorylated methacrylates, TEGDMA, initiators, stabilizers
Fillers: Silane treated silica
Two-step self-etch: self-etch primer and bond
Heliobond (HE)
Ivoclar Vivadent – Liechtenstein
Bis-GMA, TEGDMA, initiators, stabilizers Unfilled enamel bonding agent
One-Step Plus (OS)
Bisco – Schaumburg, IL, USA
Bis-GMA, HEMA, BPDM, acetone
Fillers: 8.5% wt. glass ionomer
Two-step etch and rinse: etching gel and bond
OptiBond Solo Plus (OP)
Kerr – Orange, CA, USA
Bis-GMA, HEMA, GDM, GPDM, ethanol
Fillers: 15% wt. 0.4 μm barium glass, fumed silica, sodium hexafluorosilicate
Two-step etch-and-rinse: etching gel and bond
cmf adhesive system (CF)
Saremco – St Gallen, Switzerland
Bis-GMA, Bis-EMA
Fillers: silanized barium glass
Three-step etch-and-rinse: etching gel, primer and bond
Protobond (PR)
Dental Co-operative – Thessaloniki, Greece
Bis-GMA, Bis-EMA, HPMA, initiators, stabilizers Two-step etch-and-rinse: etching gel and bond
Bis-GMA: bisphenol A glycidyl methacrylate, TEGDMA: triethylene glycol dimethacrylate, HEMA: 2-Hydroxyethylmethacrylate, BPDM: biphenyl dimethacrylate, GDM: glycerol dimethacrylate, GPDM: glycerol phosphate dimethacrylate, Bis-EMA: bisphenol A ethoxylate dimethacrylate, HPMA: hydroxypropyl methacrylate.

The materials were tested under four different conditions ( n = 4 for each condition):

  • i)

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

  • ii)

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

  • iii)

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

  • iv)

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

The specimens were mounted using a jig for centering between a Plexiglas disc (0.5 mm thick) and a rod. The experiments were performed using an apparatus ( Fig. 1 ), previously described by Lakes , that is capable of testing cylindrical specimens under torsion or bending and measuring viscoelastic behavior. After the specimen was centered to the disc, a thin plastic tube (16 mm in diameter and 18 mm high) was placed over the specimen and attached to the disc, creating a chamber to hold water during testing. The top of the chamber remained open to receive a heating element and a thermocouple in order to control temperature (accuracy of ±0.5 °C). A permanent Samarium-Cobalt magnet was attached to the end of each specimen and placed in the center of a Helmholtz coil producing a torque (M) of 3.53 × 10 −3 Nm/A controlled by the current in the coil. The weight of the magnet caused only a small constant axial tensile stress and there was no constraint on the specimen for neither torsion, nor extension. The laser beam from a low power He–Ne laser was reflected by a small mirror (diameter of 1.55 mm) attached to the magnet to a calibrated chart at a distance D = 944 cm. The rotation angle of the mirror is given by φ = 2X / D , where X is the displacement of the laser beam to the chart.

Fig. 1
The apparatus used in the study. Torque was generated by the magnet placed at the center of a Helmholtz coil. The spot of a laser beam was reflected from the thin mirror cemented onto the magnet to the calibrated chart at a known distance. The displacement of the laser beam on the chart provided the mirror rotation angle φ during testing, which was used for the measurement of the viscoelastic properties.

Static and creep experiments

The determination of the static shear modulus of the materials was achieved with the rapid application of a constant torque to the specimen for 10 s. The angular displacement was recorded and the torque was then released. The shear modulus is the ratio of shear stress to shear strain ( G = σ / γ ) and was calculated from the equation:

G = 2 M L π R 4 φ
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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Mechanical viscoelastic behavior of dental adhesives
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