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 δ), 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 (α = 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 δ 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.