A model of the viscoelastic behavior of flowable resin composites prior to setting.

Objective: The aim of this study is to develop fractional derivative models for the assessment of viscoelastic properties related to handling characteristics of dental resin composites belonging to two classes: flowable (Revolution Formula 2 and Filtek Ultimate) and posterior "bulk-fill" flowable base (Smart Dentin Replace).

Methods: Rheological measurements on all materials tested in this study were performed using dynamic oscillatory rheometer at temperature of 23°C. A parallel plates module with a diameter of 20mm was used to measure the properties of the resin composites tested. We developed two models to describe the obtained data: the generalized Newton model and the generalized Zener model (the so-called three parameter model). Both models contain fractional derivatives of Riemann-Liouville type. By determining the parameters of the model we were able to fit experimental data with high accuracy.

Results: Our results show that flowable "bulk-fill" resin-composite material (SDR) has distinct properties as compared to other two flowable resin composite materials (Revolution Formula 2 and Filtek Ultimate). Thus, previously found SDR properties as "bulk-fill" flowable base results in the fact that it is described by generalized Zener model (i.e., it has properties of solid like viscoelastic material).

Significance: Our model may be used to predict behavior of tested composites in different flow conditions. The SDR has initially small almost constant complex viscosity showing that it has good self-leveling property.