Stiffness increase during the setting of dental composite resins.

Changes in the dynamic Young's modulus under flexure of self-cured and light-cured composite resins during setting were demonstrated to reflect accurately the condition of the curing composite resins. The rate of stiffness increase during setting varied considerably with the product, and the light-cured composite resins generally showed a faster rate of increase of stiffness. In the initial stage, the Young's moduli were very low, especially for the self-cured composite resins. This accounts for the damage (adhesion disruption and cross-linking interruption) that can be inflicted on a freshly placed composite resin filling that is not yet mechanically stabilized. Therefore, such fillings are vulnerable to aggressive distortion for at least 10 to 15 minutes after placement.