The effect of light intensity on double bond conversion and flexural strength of a model, unfilled dental resin

Objective. Two visible light sources (tungsten-quartz-halogen and xenon-arc plasma) with vastly different intensities (200 and 1800 mW/cm2) but similar spectral outputs, were used to examine the effects of light intensity on conversion and flexural strength of a model dental resin formulation (75/25 wt% bis-GMA/TEGDMA). Methods. The exact same polymer samples were used to correlate double bond conversion (measured with near-IR spectroscopy) to flexural strength, both immediately after light exposure and after storage. Results. In general, polymers which were irradiated with the high light intensity source exhibited greater double bond conversion. However, increasing the light intensity also increased the maximum temperature reached during polymerization. Therefore, the greater double bond conversion was caused by a combination of both photo and thermal effects. Regardless of the light intensity, a single linear relationship existed between conversion and final flexural strength (measured 4 days after cure) over the conversion range analyzed (50–80%). However, deviations from linearity were noted in several samples that were tested immediately after exposure. Significance. These findings illustrate that light intensity does not affect the final flexural strength of a dental resin as long as the final conversions are similar.