Correlation between light intensity and exposure time on the hardness of composite resin

This study evaluated the correlation between light intensity emitted by the curing units and exposure time on the of degree polymerization of dental c omposites by measured Knoop hardness. Specimens with 5 mm diameter and 2 mm height were prepared in a copper mold, covered with a mylar strip and poly merized for 30 sec by curing units with 50, 130, 180, 220, 280 and 520 mW/c m(2), respectively. The output for all units was measured using a Curing Ra diometer (Demetron). Other specimens were polymerized for 45, 60, 75, 90, 1 05, 120, 135, 150, 165 and 180 sec utilizing units of 130, 220 and 280 mW/c m(2). Knoop hardness values of the top and bottom surfaces were measured af ter 24 h. The results indicate that the curing units with higher light inte nsity presented improved values of Knoop hardness on the top and bottom reg ions. As expected, the top surface always presented improved Knoop hardness values in relation to the bottom, after exposure for 30 sec. The values ob tained on the surface were statistically superior compared to the values on the bottom (p < 0.05) for the intensity of 130, 220 and 280 mW/cm(2) after 30, 45 and 60 sec of exposure. (C) 2000 Kluwer Academic Publishers.