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Published on: July 21, 2014
Effect of thickness and irradiance on the polymerization delay at the bottom surface of light-cured resin composites
Richard B Price1, Alexandre P Gareau1, Brett D MacNeil2
1Department of Dental Clinical Sciences, Dalhousie University, 5981 University Avenue, Halifax, Nova Scotia B3H 4R2, Canada.
Objective:
To examine the effects of the resin-based composite (RBC) thickness and incident irradiance on the time delay after the light turns on before polymerization starts at the bottom surface of the RBC.
Methods:
The degree of conversion (DC) was determined using Fourier-transform infrared spectroscopy (FT-IR) at a rate of 13 measurements per second. Six commercial RBCs were tested at three thicknesses (0.2, 2 and 4 mm) and four incident irradiance values (472-3985 mW/cm2).
Results:
There was a significant time delay before polymerization began at the bottom of each RBC. At 942 mW/cm2, representative of many current LCUs, this delay ranged from 0.12 to 1.11 s at the bottom of 2 mm of RBC. When the thickness was increased to 4 mm, even with an irradiance of 1969 mW/cm2, which represents high-power LCUs, the delay ranged from 0.12 to 5.17 s. The highest irradiance of 3985 mW/cm2 did not fully overcome the inherent lag caused by the thickness of the RBC. At 3985 mW/cm2, PowerFlow and PowerFill had the shortest delay times of 0.09 s and 0.30 s, respectively, at the bottom of a 4 mm thick RBC.
Significance:
The presence of a time delay before photo-cured RBCs start to polymerize at the bottom surfaces must now be addressed clinically and when calculating reaction rates.
