The effect of irradiation time on the physical properties of light-cured resins

The physical properties of polymers whose initiation is by visible light are extremely dependent upon the intensity of that light, which may not be ideal in dental clinical conditions. This investigation determined the flexural strength, flexural modulus and indentation hardness of a range of dental composite resins. The curing was initiated by irradiation for 20, 40 and 60 s and the effects of conditioning were measured. Further simulation of clinical conditions was done by holding the light 5 mm away from the materials. The results showed that the materials were adversely affected by short irradiation times and that their conversion was poor immediately after irradiation. Highly significant increases in strength were caused by conditioning at 37 degrees C for 24 h. There was good correlation between the hardness and flexural properties. Flexural strength proved to be a good predictor of conversion. It is suggested that restorations may be very vulnerable to loading in their early stages.