Origin and development of internal stresses during polymerization of composite resins
This study is focused on the development of polymerization stresses. These are non functional stresses and arise especially when the adhesive composite materials are cured in a cavity, in which the polymerization contraction is restrained by the cavity walls. During masticatory functioning these residual polymerization stresses elevate the overall stress levels. They exercise a cumulative effect, and therefore it is essential to evaluate the internal prestressed state of the tooth-composite complex. To study the stress conditions in these complexes, the right tools have to be employed. These consist of dynamic material properties during polymerization, and an appropriate equivalent stress expression for the strength differential effect. The origin of polymerization stresses is a combination of volumetric contraction and development of stiffness during the curing process. Two stages can be distinguished: pre-gel and post-gel. Because stiffness develops after the gel-point, only the post-gel phase plays a role in the development of polymerization stresses. An enhanced strain gage method is developed to determine the dynamic post-gel linear contraction. The development of elastic modulus cannot be measured directly, but has to be assessed indirectly through a polymerization stress evaluation. In an attempt to derive the modulus development, strain and stress data are combined. It turns out that the rate of strain and stress development is greatly dependent on the curing rate, and therefore on the test setup. Two temperature effects(exothermic heating and radiation of the curing light), which play an important role in the volumetric phenomena during the polymerization, are also investigated. Polymerization stress concentrations as a result of different filling techniques are analyzed in a finite element calculation. The numerical analysis suggests that an incremental filling technique raises stresses and cusp displacements. Furthermore, it shows that it is not necessary to model the timedependent behaviour of the development of mechanical parameters if the worsed case is sought.