Surface hardness development in light-cured composites

The surface microhardness of 5 visible light-cured dental composite materials for anterior use was measured at a series of time intervals up to one month, commencing from the end of the light irradiation period. The microhardness steadily increased with time and tended towards a maximum, usually reached after one week. The rate of increase was substantial over the first hour, in all cases, and was greater for samples stored at 37°C as compared with 23°C. These results reflect the progressive cross-linking reaction in the resin phase of the materials which continues after photo-activation. The surfaces of samples stored in water were softened compared to dry samples. Differences between materials may be attributed to compositional differences and extent of cure.