The aim of this study is to investigate depth dependent changes of polymerization process and kinetics of visible light-curing (VLC) dental composites in real-time. The measured quantity – “ion viscosity” determined by dielectric analysis (DEA) – provides the depth dependent reaction rate which is correlated to the light intensity available in the corresponding depths derived from light transmission measurements.
The ion viscosity curves of two composites (VOCO Arabesk Top and Grandio) were determined during irradiation of 40 s with a light-curing unit (LCU) in specimen depths of 0.5/0.75/1.0/1.25/1.5/1.75 and 2.0 mm using a dielectric cure analyzer (NETZSCH DEA 231 with Mini IDEX sensors). The thickness dependent light transmission was measured by irradiation composite specimens of various thicknesses on top of a radiometer setup.
The shape of the ion viscosity curves depends strongly on the specimen thickness above the sensor. All curves exhibit a range of linear time dependency of the ion viscosity after a certain initiation time. The determined initiation times, the slopes of the linear part of the curves, and the ion viscosities at the end of the irradiation differ significantly with depth within the specimen. The slopes of the ion viscosity curves as well as the light intensity values decrease with depth and fit to the Lambert–Beer law. The corresponding attenuation coefficients are determined for Arabesk Top OA2 to 1.39 mm −1 and 1.48 mm −1 , respectively, and for Grandio OA2 with 1.17 and 1.39 mm −1 , respectively. For thicknesses exceeding 1.5 mm a change in polymerization behavior is observed as the ion viscosity increases subsequent to the linear range indicating some kind of reaction acceleration.
The two VLC composites and different specimen thicknesses discriminate significantly in their ion viscosity evolution allowing for a precise characterization of the curing process even with respect to the polymerization mechanism.
An important issue in visible light-curing (VLC) composites development and research as well as in photo-chemistry in general is the question: How fast do composites cure, and how deep are they properly cured ? Since the development of VLC dental restoratives numerous studies have investigated the curing kinetics , the degree of cure (DC) and the depth of cure (DoC) . In order to get a better understanding of the curing process and its dependency on curing parameters such as curing time, light intensity, resin formulation, initiator system etc., various test methods have been applied in real time curing investigations. The most common techniques determine the curing-enthalpy via Differential Scanning Calorimetry (DSC) , the DC via Fourier Transformed Infrared Spectroscopy (FTIR) , the shrinkage strain via dilatometric devices (e.g. bonded-disk method) , the shrinkage stress via stress-analyzer and light transmission via radiometer . This time-dependent data is the base for kinetic modeling of the polymerization and curing processes.
Another method to monitor the curing behavior of resins is the dielectric analysis (DEA) which is widely used in composites manufacturing in the aircraft industry , but hardly established in dental science. In some cases it has been applied for the frequency dependent evaluation of dielectric properties in certain states of cure but without tracking the light-curing process itself in a suitable time-resolution .
In previous studies the authors have introduced a DEA setup allowing for analyzing the light-curing process of dental composites at high time resolution . It has been shown that the ion viscosity is suitable to characterize the influence of resin compositions as well as curing conditions on the polymerization process of VLC composites. For many VLC composites the ion viscosity data shows a linear time dependency, and thus it is feasible to derive kinetic parameters .
Thus, in the present study the DEA is employed to investigate the curing behavior of a micro-hybrid and a nano-hybrid composite with respect to the depth within the samples. The hypothesis of this study is that in a VLC dental composite the polymerization and curing process changes significantly with increasing depth in the specimen and as a consequence the corresponding resin structure.