An integral description of the volume behaviour of composite tooth filling materials
Internal stresses which build up due to polymerisation shrinkage of composite tooth filling materials during the curing process are still the main cause for failure of restorations. A good understanding of the time dependence of the shrinka geprocess is essential. The dynamics of the polymerisation process form a base for the development of material models which can be used in combination with finite element analysis to improve the material properties and the handling of the filling materials. In order to achieve this goal, the polymer shrinkage is investigated experimentally with the buoyancy method which proved to be a very accurate method for a precise direct measurement of the volume change during a time pe riod of at least 4 hours. The effect of different light intensities and exposure times on the shrinkage was studied for several typical composites. These investigations were restricted to single tests for each parameter variation as a basis for the development of the theoretical model. The experimental data were compared to mathematical descriptions of the shrinkage process which were based on models of the polymerisation kinetics during the initiation phase (i.e. the time period when the initial filling material is exposed to light) and the dark phase ( i.e. the time period after the initiation phase when the filling material develops into a cross-linked bulk material). In addition to polymer shrinkage, the effect of thermal expansion due to the heat of the reaction is included into the description of the volume change. The experimental data and the supporting mathematical models can be ultimately used to improve handling procedures and material properties.