Influence of light energy density on effectiveness of composite cure

This study investigated the influence of light energy density (intensity x time) on the effectiveness of composite cure in view of the curing profiles of new light-polymerization units. This investigation used a digital microhardness tester to evaluate the hardness of the top/bottom surfaces and hardness ratio of 2 mm thick composite specimens after exposure to different light energy densities. Parameters included five light intensities (200, 300, 400, 500 and 600 mW/cm2) and nine irradiation times (10, 20, 30, 40, 60, 80, 100, 120 and 180 seconds). Six samples were evaluated for each light energy density. KHN values and the hardness ratio obtained with 40 seconds cure at 400 mW/cm2 was used as control. Results were analyzed with one-way ANOVA and Scheffe's post-hoc test at significance level (0.05). Correlation between curing time and hardness values and ratio was done using Pearson's correlation at significance level 0.01. Results showed that the adequate hardness for surface finishing could be obtained with 20 seconds irradiation at lower intensities of 200 or 300 mW/cm2. Optimal cure of the bottom surfaces could not be achieved with 200 mW/cm2, but was attained with 300 mW/cm2 only after 120 seconds of irradiation. Optimal cure of the bottom surfaces was possible with 30 and 20 seconds irradiation at 500 and 600 mW/cm2, respectively. Effective cure was not achieved with low light intensities (200 to 300 mW/cm2) but could be achieved with high intensities (500 and 600 mW/cm2) after 30 seconds of irradiation.