Viscoelastic stability of resin-composites aged in food-simulating solvents

Abstract

Objective

To study time-dependent viscoelastic deformation (creep and recovery) of resin-composites, after conditioning in food-simulating solvents, under a compressive stress at 37 °C.

Methods

Five dimethacrylate-based composites: (Spectrum TPH, Premise Body, Tetric Ceram HB, Filtek P60, X-tra fil), and two Ormocers (Experimental Ormocer V 28407, Admira) were studied. Three groups of cylindrical specimens (4 mm × 6 mm) were prepared and then conditioned in 3 solvents: methyl ethyl ketone (MEK), ethanol, and water for 1 month at 37 °C. The compressive creep-strain under 35 MPa in 37 °C water was recorded continuously for 2 h and then the unloaded recovery-strain was monitored for another 2 h. The data were analyzed by one-way ANOVA and Bonferroni’s test.

Results

The materials all exhibited classic creep and recovery curves, with most parameters being significantly different ( p < 0.0001) for each solvent condition. All materials showed lower creep-strain in water than in ethanol or MEK solvents. Maximum creep-strain and permanent-set gave negative linear-regression ( r 2 > 0.98) with logarithm of the solvent solubility-parameter . The % mean (SD) creep-strain ranged from a minimum of 0.82 (0.01) for the Exp. Ormocer in water to the maximum of 4.19 (0.30) for Admira in MEK. Similar trends were found for permanent-set. The dimethacrylate-based composites behaved as an intermediate group, apart from X-tra fil that had similar stability to the Exp. Ormocer.

Significance

The viscoelastic stability (low creep and permanent-set) of the Exp. Ormocer, compared to many dimethacrylate-based composites, in food-simulating solvents may be due to its diluent-free formulation. This was closely matched by a highly-filled dimethacrylate material (X-tra fil).

Introduction

During the early use of resin composites, restorations lost their original shape and surface character over time . In the oral environment, restorative materials are subjected to various factors that may affect their long-term use, including humidity from saliva, chemicals from food, temperature changes, dynamic load during chewing, and static load during clenching. Restorations may be exposed to chemical agents either intermittently during eating and drinking, or continuously from adherent bacterial debris . Dimensional changes caused by these factors can result in the development of internal stresses and subsequent formation of microcracks, which ultimately lead to bulk fracture of the composite. Viscoelastic stability of composite material is influenced collectively by these conditions, which may cause degradation and subsequent failure of the restoration . A wide range of polymeric dental materials, including resin-composites are viscoelastic and show intermediate behavior between a viscous liquid and elastic solid . Viscoelastic behavior has many forms and is conveniently measured via creep and recovery phenomena.

Among newer types of dental composite is a pure (experimental) Ormocer material that also has several non-dental applications . Ormocers have been used in dentistry as a substitute for Bis-GMA. They incorporate ceramic and polymeric structures linked via covalent bonds at the molecular level . The process of its formation begins by hydrolysis and polycondensation reactions (sol–gel-processing) of functionalized organosilane precursor (alkoxy silane) to form an oligomeric Si O Si nano-structure (polymeric inorganic condensate) ( Fig. 1 ). After the polycondensation reaction, a 3-dimensional inorganic network structure is formed and its mechanical properties vary with the nature of the reactive functionalized organic group attached . Different types of functionalized inorganic fillers, dental monomers and essential additives like a photo-initiator and activator are incorporated to obtain a paste-like light-curable Ormocer-composite .

Fig. 1
Chemical structure of an Ormocer matrix.

New multifunctional urethane- and thioetheroligo(meth)acrylate alkoxysilanes as sol–gel precursors have been developed to alter the property of the formed Ormocer. Two dental Ormocer-based materials, Admira and Definite, were produced within the past 15 years . They are admixed-matrix systems, composed partly from Ormocer and partly from dimethacrylates . However, a pure Ormocer for dental use is still experimental.

Viscoelastic behavior, especially creep of acrylic resin and dental composites, has been studied previously . It is affected by several factors such as filler composition, resin-matrix chemistry and degree of monomer conversion . Contact with food-simulated solvents can decrease strength and modulus of the material and thus may also influence creep . Therefore, the aims were: (i) to determine the viscoelastic stability of some dental posterior composites by creep and recovery measurements, after conditioning in food-simulating solvents; and (ii) to investigate the correlation between the composite’s creep behavior and the solvent solubility parameter ( δ ). The null hypotheses were: (1) the structural type of resin-composite has no effect on creep; (2) food-simulating solvents have no effect on creep of resin-composites; (3) there is no correlation between creep and the solubility parameters of solvents in which composite specimens are stored.

Materials and methods

Materials

The materials used in this study are listed in Table 1 . Five dimethacrylate-based composites: [Spectrum TPH (SP), Premise Body (Prb), Tetric Ceram HB (TCB), Filtek P60 (FP60), X-tra fil (XF)], and two Ormocers [Experimental Ormocer V 28407 (ORM), Admira (ADM)] were investigated.

Table 1
Manufacturers’ composition of the light-cured resin composites.
Material Code Shade Manufacturer Batch no. Resin matrix Filler (wt.%) MPS (μm) Type of filler Material’s Type
Admira ADM A3 VOCO, Cuxhaven, Germany 440690 Bis-GMA
TEGDMA
di-UDMA
BHT
78

0.7

0.04–1.2

3-Dimensionally linked inorganic-organic copolymer
Ba–Al borosilicate glass
Microfiller SiO 2
Admixed Ormocer
Spectrum TPH SP A3 Dentsply DeTrey GmbH., Konstanz Germany 0404002074 Bis-GMA
Bis-EMA
TEGDMA
77 <1.5

0.04

Ba–Al-borosilicate glass
Highly dispersed fumed SiO 2
Submicron hybrid
Premise Body Prb A3 Sds Kerr Corp., Orange, CA, 92867 USA 433538 Ethoxylated bis phenol A dimethacrylate
TEGDMA
81 0.4

0.02
30–50

Barium glass
Non-agglomerated
discrete silica
Silica nanoparticles
pre-polymerized filler
Microhybrid
Tetric ceram
HB
TCB 230
A3.5
Ivoclar Vivadent Schaan, Liechtenstein D63227 Bis-GMA, UDMA
decandiol dimethacrylate
81 0.04–3

0.7

Barium glass
Ba–Al-fluorosilicate glass
Ytterbium trifluoride
Highly dispersed silicon
Hybrid
Filtek P60 FP60 A3 3M ESPE Dental Products, Seefeld, Germany 20030411 Bis-GMA, Bis-EMA, UDMA 83 0.19–3.3 Zirconia/silica (non-silanated) Packable hybrid
X-tra fil XF U VOCO, Cuxhaven, Germany 541384 Bis-GMA,UDMA TEGDMA, BHT 86 Multi-hybrid filler Posterior hybrid
Experimental Ormocer ORM VOCO, Cuxhaven, Germany V 28704 Pure Ormocer Pure Ormocer
Abbreviations : Bis-GMA, bisphenol A diglycidylmethacrylate; Bis-EMA, bisphenol A diglycidyl ether dimethacrylate; TEGDMA, triethylene glycol dimethacrylate; UDMA, urethane dimethacrylate; BHT, butylated hydroxy toluene; MPS, mean particle size.

Specimen preparation and group organization

Cylindrical specimens (4 mm × 6 mm) of each composite material tested were prepared into a split Teflon mold to allow easy removal of the cured specimen. The mold was positioned on a glass slab against which the composite material was packed with a stainless steel condenser to minimize the inclusion of air bubbles into the specimen. The top of the packed material was covered with a transparent Mylar strip on the top of which a microscope glass slide was placed with slight pressure to flatten the surface. The specimen was irradiated with a calibrated quartz-tungsten-halogen light unit (Optilux 501, 2 Demetron, Kerr, USA) of 600 mW/cm 2 irradiance. The irradiation was carried out at a zero mm distance from the glass slide for 40 s from both upper and lower surfaces. After removal of the specimen from the mold, it was cured from diametrically opposite sides for another 40 s each, to ensure optimal conversion of the material.

Nine specimens were fabricated at room temperature (23 ± 1 °C) for each material and then divided into three groups of three specimens. These groups were allocated for storage as follows: (i) distilled water as control, (ii) 99% ethanol, and (iii) methyl ethyl ketone (MEK). Each solvent has a specific Hildebrand solubility parameter ( δ ) with a standard international (SI) unit: δ /MPa 1/2 ( Table 2 ). The specimens were stored in the corresponding solvent contained in dark bottles for one month in an incubator at 37° ± 1 °C.

Table 2
Hildebrand solubility parameter ( δ ).
Materials Solubility parameter ( δ ) or δ /MPa 1/2 Logarithm ( δ )
PMMA 18.6 2.92
MEK 19.3 2.96
Ethanol 26.2 3.27
Water 48.0 3.87

Creep apparatus and experimental procedures

The creep and recovery behavior of the materials was examined with a creep apparatus ( Fig. 2 ), which has been described previously . The creep testing fixture consisted of a rigid stainless steel (SS) base. A rigid cylindrical SS platform rose from the base of a circular water bath where circulated water at 37 °C was maintained during the measurement period. The bath was connected to a heat controller by which the desired temperature was selected and maintained within ±0.5 °C.

Fig. 2
Apparatus for measurement of uniaxial compressive creep at 37 °C.

The platform supported the test specimen that was in axial alignment with the SS loading rod, (10 mm diameter; 95 mm length). The flat lower end of the rod rested vertically on the composite specimen while its frictionless top contacted a loading lever arm. Angular motion of the lever produced linear displacement of the rod. The SS lever arm, (49 cm long), pivoted at one end via a bearing pin providing flexible arm movement. A removable load of 9 kg was hung at the free end of the loading arm. The desired static compressive stress of 35 MPa was thereby exerted symmetrically onto the test specimen and along its axis via the loading rod. Creep-strain was recorded continuously for 2 h and then the unloaded recovery-strain for another 2 h.

The deformation developed in the specimen was monitored in mV units by a Linear Variable Differential Transducer (LVDT) which rested on a small outrigger clamped to the loading rod. Any vertical movement of the rod due to dimensional change in the length of the test-specimen generated corresponding movement on the transducer plunger. The LVDT was connected to a signal conditioner and A/D converter which converted the analog displacement signals to digital signals. The latter were recorded and gathered by a software program (DASYLab, Version 5.02, DATALOG GmbH & Co. KG, Moenchengladbach, Germany) which monitored displacement in mV vs. time (min).

The load was applied lightly onto the specimen to prevent exceeding the required load and care was also exercised to avoid vibration and sudden, impact loading. The applied stress was quickly removed after the 2 h period of compressive loading by raising the lever arm, and thus releasing the hanging weight. Recovery was recorded during the following 2 h.

The LVDT was calibrated periodically throughout the study. The deformation (μm) in the length of the specimen was transformed into nominal strain (%) by the following equation:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='Strain(εn)%=ΔLLo×100′>Strain(εn)%=ΔLLo×100Strain(εn)%=ΔLLo×100
Strain ( ε n ) % = Δ L L o × 100

Δ L is the deformational change occurring in the specimen’s length. L o , the original length of the specimen (6 mm) before loading.

The creep-recovery curve in nominal strain (%) vs. time (min) was generated, giving the three main parameters of the creep-recovery test, i.e. maximum creep strain, maximum recovery strain and residual strain (permanent deformation or set). The data were analyzed and graphically plotted utilizing Sigmaplot software. The parametric data for the three dependent variables were analyzed using two-way ANOVA computed using IBM SPSS Statistics software.

Materials and methods

Materials

The materials used in this study are listed in Table 1 . Five dimethacrylate-based composites: [Spectrum TPH (SP), Premise Body (Prb), Tetric Ceram HB (TCB), Filtek P60 (FP60), X-tra fil (XF)], and two Ormocers [Experimental Ormocer V 28407 (ORM), Admira (ADM)] were investigated.

Table 1
Manufacturers’ composition of the light-cured resin composites.
Material Code Shade Manufacturer Batch no. Resin matrix Filler (wt.%) MPS (μm) Type of filler Material’s Type
Admira ADM A3 VOCO, Cuxhaven, Germany 440690 Bis-GMA
TEGDMA
di-UDMA
BHT
78

0.7

0.04–1.2

3-Dimensionally linked inorganic-organic copolymer
Ba–Al borosilicate glass
Microfiller SiO 2
Admixed Ormocer
Spectrum TPH SP A3 Dentsply DeTrey GmbH., Konstanz Germany 0404002074 Bis-GMA
Bis-EMA
TEGDMA
77 <1.5

0.04

Ba–Al-borosilicate glass
Highly dispersed fumed SiO 2
Submicron hybrid
Premise Body Prb A3 Sds Kerr Corp., Orange, CA, 92867 USA 433538 Ethoxylated bis phenol A dimethacrylate
TEGDMA
81 0.4

0.02
30–50

Barium glass
Non-agglomerated
discrete silica
Silica nanoparticles
pre-polymerized filler
Microhybrid
Tetric ceram
HB
TCB 230
A3.5
Ivoclar Vivadent Schaan, Liechtenstein D63227 Bis-GMA, UDMA
decandiol dimethacrylate
81 0.04–3

0.7

Barium glass
Ba–Al-fluorosilicate glass
Ytterbium trifluoride
Highly dispersed silicon
Hybrid
Filtek P60 FP60 A3 3M ESPE Dental Products, Seefeld, Germany 20030411 Bis-GMA, Bis-EMA, UDMA 83 0.19–3.3 Zirconia/silica (non-silanated) Packable hybrid
X-tra fil XF U VOCO, Cuxhaven, Germany 541384 Bis-GMA,UDMA TEGDMA, BHT 86 Multi-hybrid filler Posterior hybrid
Experimental Ormocer ORM VOCO, Cuxhaven, Germany V 28704 Pure Ormocer Pure Ormocer
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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Viscoelastic stability of resin-composites aged in food-simulating solvents
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