Initial Dynamic Viscoelasticity Change of Composites During Light Curing

Objectives: This study investigated the development of the initial dynamic viscoelasticity of dental composites during light curing.  Methods: A custom, oscillatory rotational rheometer was constructed and consisted of three parts: (1) an oscillatory shear strain induction unit, (2) a measuring unit of parallel plates, and (3) a stress measurement unit. Six commercial composites (Z100, Z250, Z350, DenFil, Tetric Ceram, and Clearfil) were examined. For the measurement of the viscoelastic modulus change in the composites during light curing as a function of time, a dynamic oscillatory shear test was accomplished for 10 seconds of light curing at a frequency of 12 Hz. The specimens were cured with a QTH light (Optilux) at 650 mW/cm². The complex shear modulus G*, storage shear modulus G', loss shear modulus G" and loss tangent tan d were obtained from the strain–stress curves.  Results: The time to reach the G* of 1 MPa ranged 1.27s to 3.32s. Z100 was fastest while DenFil was slowest. The G* of DenFil was lowest (1.36 MPa), and that of Z100 was highest (5.13 MPa) at 5s after the beginning of light exposure.

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Composite           Time (s) to reach         Complex Modulus (MPa)            Loss tangent

Materials               G* of 1 MPa                  5s                10s                     5s             10s

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Z100                      1.27 (0.05)               5.13            103.9                 0.59          0.29

Z250                      1.60 (0.11)               4.75             77.3                  0.68          0.50

Z350                      1.59 (0.12)               3.66             21.7                  0.54          0.47

DenFil                    3.32 (0.38)               1.36             11.6                  0.61          0.42

Tetric Ceram          2.88 (0.34)                1.65             10.8                 0.55           0.45

Clearfil                   1.71 (0.19)               4.33             75.5                 0.52           0.44

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The tan d of the composites rapidly decreased with time as the polymerization proceeded.  Conclusion: The custom made rheometer could measure the initial dynamic modulus change of the composites during light curing, and showed significant differences in the development of viscoelasticity among the materials, with different magnitude of modulus and cure speed.