Softening patterns of post-cure heat-treated dental composites

The softening of post-cure, heat-treated dental composites in various solutions was evaluated by Knoop hardness measurements. The samples were heated at 120 degrees C for 7 min immediately after curing and immersed in various solutions, water, ethanol, heptane and 0.1 N NaOH. Significant increases in hardness were observed for heated samples when compared to unheated samples. The various solutions softened both heated and unheated composites but heated samples were softened to a lesser degree. Plots of hardness of heated and unheated samples were nearly parallel, indicating that mechanisms such as penetration of the resin matrix, degradation of the silane coupling agent and fillers may be involved in the softening of both heated and unheated composites. Post-cure heat treatment improved the resistance to softening of dental composites.