Rheological characterization of dental resin composites : fundamental study and clinical applications

(eng) While many publications concentrate on the characterization of the mechanical and physical properties of resin composites in order to guide practitioners in their material’s choice according to particular clinical conditions there is only very scarce reference of studies on their rheological properties. The purpose of this work was to study the flow characteristics of resin composites before curing as an attempt to complete their physical and mechanical characterization. A first part of this work presents a characterization study of composites focusing on the comparison of their mechanical properties and the characterization of their inorganic fraction. Such characterization, although giving much useful information for the prediction of composites performance, is insufficient to determine proper clinical indications to restorative materials. The rheological study of resin composites first focuses on the viscoelastic properties of commercially available materials. It was shown that they exhibit complex rheological properties. They are non-Newtonian, viscoelastic materials. Their viscosity dramatically decreases as the frequency or rate of deformation increases, they show solid-like behavior at rest and their rheological properties are time-dependant in such a way that when they are submitted to flow, viscosity drastically decreases and takes many hours to return to its original value. From there, an evaluation of the influence of each component, organic as well as inorganic, on the rheological properties of final materials was performed with the help of model experimental formulations. The complex flow properties that were demonstrated with commercial formulations were confirmed by the experimental ones. It was also shown that the inorganic particles, and in particular sub-micronic particles, are the determinants of the complex rheological behavior since they induce many types of interaction mechanisms with their surrounding environment that lead to time-dependant local rearrangements of the structure of materials. Finally, a third part develops links from rheological properties to possible clinical implications. Different flowable resin composites and pit and fissure sealants were characterized mechanically and rheologically, and attempts of clinical indications based upon both their flow properties and their mechanical behavior were made. On the other hand, since rheology is time-dependent, mechanical properties were also investigated as a function of processing time. It was demonstrated that some materials present a progressive increase of their dynamic elastic modulus as a function of time after deformation. This particular behavior is linked to the type of microfiller and nanofiller they contain but cannot be explained on the basis of their rheological properties alone.