The effect of resin matrix composition on the polymerization shrinkage and rheological properties of experimental dental composites

Objectives. This study was undertaken to evaluate the effect of the resin matrix composition of experimental composites on their polymerization shrinkage and rheological properties. Methods. Six experimental composites consisting of varying ratios of Bis-GMA, TEGDMA, and UDMA were made. All composites had the same amount of filler (barium-aluminum-silicate glass, 76.5 wt.%) and initiator concentrations (camphorquinone, 1.7 wt.%). To investigate the effects of different resin matrices on the polymerization shrinkage, a newly developed measurement method was used. Using a rotational rheometer, a dynamic oscillatory shear test was undertaken to evaluate the rheological properties, including the storage shear modulus (G`), loss shear modulus (G"), loss tangent (tan 6), phase angle (8), and complex viscosity of the experimental composites as a function of frequency (0.1-10Hz). Results. The polymerization shrinkage and complex viscosity of the experimental composites ranged from 2.61 to 3.88 vol.% and from 3.8 to 181.4 Pas, respectively. The experimental composite composed of 17.5% Bis-GMA and 4.4% TEGDMA showed the lowest shrinkage and highest viscosity. The composite composed of 8.7% Bis-GMA and 13.1% TEGDMA showed the highest shrinkage and lowest viscosity. With increasing TEGDMA content, the polymerization shrinkage increased but the viscosity decreased. The substitution of UDMA for TEGDMA reduced the shrinkage level but increased viscosity There was an inverse relationship between the polymerization shrinkage and complex viscosity. All experimental composites exhibited pseudoplasticity. Significance. Within the limitations of this study, resin matrix composition significantly affected the volumetric shrinkage and rheological properties of the experimental composites. (C) 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.1

Slumping resistance and viscoelasticity prior to setting of dental composites

Objectives: The aim of this study was to develop a method for measuring the slumping resistance of resin composites and to evaluate the efficacy using rheological methodology. Methods. Two commercial hybrid composites (Z100 and Z250) and a nanofill composite (Z350) were used to make disc-shaped specimens of 2 mm thickness. Three kinds of aluminum molds with triangular, circular, and square shaped cutting surfaces were pressed onto the resin discs to make standardized imprints. The imprints were light-cured either immediately (non-slumped) or after waiting for 2 min at 25 degrees C (slumped). White stone replicas were made and then scanned for topography using a laser 3-D profilometer. Slumping resistance index (SRI) was defined as the ratio of the groove depth of the slumped specimen to that of the non-slumped specimen. The pre-cure viscoelasticity of each composite was evaluated by an oscillatory shear test and normal stress was measured by a squeeze test using a rheometer. Correlation analysis was performed to investigate the relationship between the viscoelastic properties and SRI. Results. SRI varied between the three materials (Z100 < Z250 < Z350) and the imprint shapes did not affect the slumping tendency The SRI was strongly correlated with the loss shear modulus G `` but not with the loss tangent. Also, slumping resistance was more closely related to the resistance to shear flow than to the normal stress. Significance. Slumping tendency could be quantified using the imprint method and SRI. The index may be applicable to evaluate the clinical handling characteristics of composites. (C) 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Effect of shrinkage strain, modulus, and instrument compliance on polymerization shrinkage stress of light-cured composites during the initial curing stage

OBJECTIVES: The aim of this study was to investigate the influence of shrinkage strain, modulus, and instrument compliance on the polymerization shrinkage stress measurement of light-cured composites, and to determine whether the silorane-based low-shrinkage composite shows a low-polymerization shrinkage stress. METHODS: A universal hybrid; Z250 (Z2), a flowable; Z350 (Z3), and a silorane-based; P90 (P9) composite was examined. A modified "bonded disc method" was used to measure the axial shrinkage strain of the composite. For the measurement of the initial modulus development of composites during light curing, a dynamic oscillatory shear test was undertaken using a custom-made oscillation rheometer. A frequency of 6Hz and strain amplitude of 0.0091rad for 20s was employed and the complex shear modulus (G(*)) was determined. A newly designed stress-strain analyzer was made to measure the shrinkage stress of the composites with two modes: (1) high compliance, or (2) low compliance. The shift between the two modes was controlled by an On-Off switch of a negative feedback circuit. Theoretical shrinkage stress was calculated from the shrinkage strain and modulus measured above, and compared with experimentally measured stress. Data were analyzed with one-way ANOVA and Tukey`s post hoc test (alpha=0.05), and correlation analysis was done to investigate the relationship between measured stress and shrinkage strain, modulus, and theoretical stress. RESULTS: The shrinkage strain of Z3 (4.12%) at 10min was the highest, followed by Z2 (2.31%) and P9 (0.77%). At 10s after light curing, Z2 showed the highest modulus (466.2MPa), Z3 (154.1MPa), and P9 the lowest (130.7MPa). The measured stresses with low compliance were much higher than those with high compliance. With high compliance, the contraction stress of Z3 was the highest (2.75MPa), followed by Z2 (1.54MPa) and P9 (0.48MPa). In low-compliance mode, the stresses of Z3 (7.93MPa) and Z2 (7.48MPa) were similar (p=0.323) while the stress of P9 (3.23MPa) was much lower. A strong correlation was observed between the theoretical stress and the measured stress with low compliance (R=0.996). In high-compliance mode, the shrinkage strain also showed a near-linear relationship with the stress measured (R=0.937), but the modulus showed a low correlation with the measured stress (R=0.398). SIGNIFICANCE: Depending on the instrument compliance, polymerization shrinkage stress showed significant differences for each material. In high-compliance shrinkage strain played a greater role, while in low-compliance shrinkage strain and elastic modulus contributed comparably in determining the shrinkage stress. The low-shrinkage silorane-based composite demonstrated considerable reduction in shrinkage strain and stress. Copyright (C) 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Influence of instrument compliance and specimen thickness on the polymerization shrinkage stress measurement of light-cured composites

objectives. The aim of our study was to investigate the effects of instrument compliance and specimen geometry on the polymerization shrinkage stress measurements of composites. Methods. A custom designed stress-strain analyzer was made using a linear variable differential transformer probe and a cantilever load cell. A sandblasted glass rod was fixed at the free end of the load cell, and another glass rod was located on a base plate. A composite was placed between the two rods and light cured. The end displacement of the load cell during polymerization was recorded for 10 min. A flowable (Filtek Flow), a universal hybrid (Z100), and a packable (P60) composite were studied. To investigate the effect of specimen geometry and instrument compliance, specimen thickness was varied between 0.5, 1.0, and 2.0 mm, and three load cells with maximum capacities of 20, 100, and 500 kgf were used. Ten maxillary premolars were prepared with two sizes of MOD cavities; the bucco-lingual widths and depths of the cavities were 1.5 mm x 2 mm and 3 mm x 2 mm, and the cusp compliance and deflection were measured before and during composite polymerization. Results. The measured polymerization stress decreased in the order of Filtek Flow, Z100, and P60 for all measurement conditions. As the specimen thickness was increased, the shrinkage stress per unit thickness (mm) decreased. The measured stress decreased with increasing instrument compliance. The cusp compliance (3.32 mu m/N) of the 3 mm x 2 mm cavities was similar to that of instrument with a 20 kgf load cell (3.34 mu m/N). Significance. For determining the most clinically relevant values for shrinkage stress of dental composites, the instrument compliance should closely match that of the tooth walls in the cavity geometry to be tested. (C) 2006 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.1

Rheologic properties of flowable, conventional hybrid, and condensable composite resins

Objectives. This research was undertaken to investigate the viscoelastic properties related to handling characteristics of five commercial flowable, two conventional hybrid and two condensable composite resins and to investigate the effect on the viscosity of filler volume fraction of composites. Methods. A dynamic oscillatory shear test was used to evaluate the storage shear modulus (G′), loss shear modulus (G″), loss tangent (tan δ) and complex viscosity (η*) of the composite resins as a function of frequency (ω)—dynamic frequency sweep test from 0.01 to 100 rad/s at 25 °C—using an Advanced Rheometric Expansion System. To investigate the effect on the viscosity of the composites of the filler volume fraction, the filler weight% and filler volume% were measured by the Archimedes' principle using a pyknometer. Results. The complex viscosity η* of flowable composites was lower than that of the hybrid composites and significant differences were observed between brands. The complex viscosity η* of condensable composites was higher than that of hybrid composites. The order of complex viscosity η* at ω=10 rad/s in order of decreasing viscosity was as follows, Synergy compact, P-60, Z-250, Z-100, Aeliteflo, Tetric flow, Compoglass flow, Flow it and Revolution. The complex viscosity of flowable composites, normalized with respect to Z-100, was 0.04–0.56 but Synergy compact was 2.158 times higher than that of Z-100. The patterns of the change of loss tangent (tan δ) of the composite resins with increasing frequency were significantly different between brands. Phase angles δ ranged from 30.9 to 78.1° at ω=10 rad/s. All composite resins exhibit pseudoplastic behavior with increasing shear rate. The relationships between the complex shear modulus G*, the phase angle δ, and the shear rate ω were represented by the frequency domain phasor form, G*(ω)=G* eiδ=G*δ. Only a weak relationship was found between filler volume% and the viscosity of the composite resins. Significance. This investigation shows that the viscoelasticity of composites in the same class is significantly different between brands. This rheologic property of composite resins influences the handling characteristics of the materials. The locus of frequency domain phasor plots in a complex plane is a valuable method of representing the viscoelastic properties of composite resins.This investigation was supported in part by research grant from the Korea Health 21 R&D Project, Ministry of Health and Welfare, Republic of Korea, HMP-99-E-10- 0003

How should composite be layered to reduce shrinkage stress: Incremental or bulk filling?

Objectives. The purpose of this study was to determine the effect of different layering techniques on cuspal deflection in direct composite restorations. Methods. Aluminum blocks were used to prepare MOD cavities divided into three groups. Each cavity was restored with composite using three different filling techniques. Group 1 was filled in bulk, group 2 was restored by a horizontal increment technique, and group 3 by an oblique increment technique. Cuspal deflection was measured with LVDT probes and compared among groups using ANOVA and Scheffe`s post hoc test (alpha = 0.05). Results. The cuspal deflections in groups 1-3 were 21.6 +/- 0.90 mu m, 19.3 +/- 0.73 mu m and 18.4 +/- 0.63 mu m, respectively. The bulk filling technique yielded significantly more cuspal deflection than the incremental filling techniques, while there was no significant difference between the horizontal and oblique increment methods. Significance. Cuspal deflection resulting from polymerization shrinkage can be reduced by incremental filling techniques to obtain optimal outcomes in clinical situations. (C) 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.1

Influence of ceramic thickness and curing mode on the polymerization shrinkage kinetics of dual-cured resin cements

Objectives. The purpose of this study was to assess how ceramic disc thickness and curing mode (light or chemical) affects the polymerization shrinkage of dual-cured resin cements and to evaluate the effect of the ceramic discs on the curing speed of the cements during light exposure. Methods. Six commercial resin cements, RelyX ARC, Bistite II, Duolink, Panavia F, Variolink II and Choice were used. Filler weight contents were determined by the ash method. Four ceramic discs with thicknesses of 0.5, 1, 2 and 4 mm, respectively, were made. The attenuation of light intensity due to the ceramic discs was measured using a radiometer. The polymerization shrinkage kinetics of the resin cements by chemical or light cure through the different ceramic discs was measured using a bonded-disc method. Results. There were differences in filler content among brands of resin cement. The polymerization shrinkage without ceramic disc was 2.61-4.59% by chemical cure and 2.93-4.66% by light cure. The polymerization shrinkage of RelyX ARC and Panavia F by chemical cure was statistically lower than by light cure (p < 0.05). Polymerization shrinkage and filler weight were inversely related (R = -0.965). Both the transmitted light intensity and polymerization shrinkage decreased with increasing thickness of ceramic discs (p < 0.05). The time to reach the maximum shrinkage rate of the resin cements increased with increasing ceramic thickness. The cure speed by light cure was 15-322 times faster than by chemical cure. Significance. The polymerization shrinkage kinetics of dual-cured resin cements significantly differed between brands under various curing conditions. Clinicians should be aware of the setting characteristics of the cements, so they can choose the optimal materials for different clinical situations. (c) 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.1

Slumping tendency and rheological properties of flowable composites

Objectives. The aim of this study was to develop a method for measuring the slumping tendency of flowable resin composites and to correlate the results with those obtained from standard rheological methods. Methods. Five commercial flowable composites (Aeliteflo: AF, Filtek flow: FF, DenFil flow: DF, Tetric flow: TF and Revolution: RV) were used. A fixed volume of each composite was extruded from a syringe onto a glass slide using a custom-made loading device. The composite was allowed to slump for 10 s at 25 degrees C and light cured. The aspect ratio (height/diameter) of the cone or dome shaped specimen was measured to estimate the slumping tendency of the composites. In order to investigate the relationship between the slumping tendency and the rheological properties of the composites, the complex viscosity eta* of each composite was measured by a dynamic oscillatory shear test over a range of angular frequency omega = 0.1-100 rad/s using a rheometer. The aspect ratios of the composites were compared by one-way ANOVA and Tukey`s post hoc test at the 5% significance level. Regression analysis was performed to investigate the relationship between the aspect ratio and the complex viscosity. Results. Slumping tendency based on the aspect ratio varied among the five materials (AF < FF < DF < TF < RV). Flowable composites exhibited pseudoplasticity in which the complex viscosity decreased with increasing frequency. Slumping resistance increased with increase in the complex modulus. Significance. The slumping tendency could be quantified by measuring the aspect ratio of slumped flowable composites. This method may be applicable to evaluate the clinical handling characteristics of these materials. (C) 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Initial dynamic viscoelasticity change of composites during light curing

Objectives. The aim of this study was to measure the initial dynamic viscoelastic modulus change of dental composites during light curing using a custom made oscillation rheometer. Methods. Six commercial universal hybrid resin composites: Z100 (Z1), Z250 (Z2), Z350 (Z3), DenFil (DF), Tetric Ceram (TC), and Clearfil AP-X (CF) were examined. A custom designed oscillation rheometer was made, which consisted of three parts: (1) an oscillatory shear strain induction unit, (2) a measuring unit of parallel plates made of glass rods, and (3) a stress measurement unit. For the measurement of the initial viscoelastic modulus change of composites during light curing as a function of time, a dynamic oscillatory shear test was undertaken at a frequency of 6 Hz and strain amplitude of 0.00579 rad for 10 s. From the strain-stress curves, the complex shear modulus (G*), storage shear modulus (G`), loss shear modulus (G ``) and loss tangent (tan delta), and the time to reach a G* of 10 MPa were determined. The data were analyzed with one-way ANOVA and Tukey`s post hoc test (alpha = 0.05). Results. There were great differences in the development of viscoelasticity among materials. At 10 s, the complex modulus G* of Z1 was the highest (563.7 MPa), followed by CF, Z2, Z3, TC, and finally DF being the lowest (150.3 MPa). The time to reach the G* of 10 MPa ranged from 2.55 s (Z1) to 4.06 s (DF). The tan delta of the composites decreased with time as the polymerization reaction proceeded. Significance. The initial modulus development of composites during curing plays a very important role in determining the polymerization shrinkage stress. The custom made oscillation rheometer was able to measure the very initial dynamic viscoelasticity change of several commercial composites during light curing; therefore, the instrument can be used to study the initial curing kinetics of newly developed composites. (C) 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.