Distribution of transient properties during polymerization of a light-initiated restorative composite

To assess residual shrinkage stress, transient properties must be defined that describe the composite transformation during polymerization. The purpose of this study was to determine the development and distribution of properties that affect the creation of residual stresses in a light-initiated restorative composite.

Microhardness and shrinkage strain were experimentally measured during and/or after light-initiated polymerization. The data was acquired for different combinations of light intensities and light exposure times. Light attenuation experiments were used to derive local light intensities inside the composite samples.

For the microhardness, a nonlinear correlation was found with the administered light energy, defined as the product of light intensity and exposure time. However, shrinkage strain depended on the initiation intensity rather than the light energy. Higher initiation intensities resulted in higher shrinkage strain rates and values. Microhardness and strain values continued to increase after the light initiation.

Similar microhardness values, and hence degree of cure and mechanical properties, can be achieved by application of comparable light energy. Therefore, microhardness as a function of light energy can be used to describe transient elastic properties during polymerization. Shrinkage strain, and therefore post-gel shrinkage and residual stress, depends primarily on initiation light intensity. Although mechanical properties achieved at a certain light energy level may be similar, residual stresses may differ depending on initiation intensity.