title: Initial dynamic viscoelasticity change of composites during light curing.
abstract: OBJECTIVE
  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 6Hz and strain amplitude of 0.00579rad for 10s. From the strain-stress curves, the complex shear modulus (G*), storage shear modulus (G'), loss shear modulus (G'') and loss tangent (tandelta), and the time to reach a G* of 10MPa 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 10s, the complex modulus G* of Z1 was the highest (563.7MPa), followed by CF, Z2, Z3, TC, and finally DF being the lowest (150.3MPa). The time to reach the G* of 10MPa ranged from 2.55s (Z1) to 4.06s (DF). The tandelta of the composites decreased with time as the polymerization reaction proceeded.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Conservative Dentistry and Dental Research Institute, Seoul National University, Yeongeon-Dong, Jongro-Gu, South Korea.
authors.forename: Min-Ho
authors.initials: MH
authors.lastname: Kim
authors.forename: Sun-Hong
authors.initials: SH
authors.lastname: Min
authors.forename: Jack
authors.initials: J
authors.lastname: Ferracane
authors.forename: In-bog
authors.initials: IB
authors.lastname: Lee
mesh_terms: D000704:Analysis of Variance; D003188:Composite Resins / Q000737:chemistry* / Q000528:radiation effects*; D003799:Dental Stress Analysis; D004548:Elasticity; D055115:Light-Curing of Dental Adhesives; D008422:Materials Testing; D009991:Oscillometry; D012212:Rheology / Q000295:instrumentation; D033081:Shear Strength; D018709:Statistics, Nonparametric; D014783:Viscosity
chemical_list: D003188:Composite Resins