During the curing process of light curing dental composites the mobility of molecules and molecule segments is reduced leading to a significant increase of the viscosity as well as the ion viscosity. Thus, the kinetics of the curing behavior of 6 different composites was derived from dielectric analysis (DEA) using especially redesigned flat sensors with interdigit comb electrodes allowing for irradiation at the top side and measuring the ion viscosity at the bottom side. As the ion viscosities of dental composites change 1–3 orders of magnitude during the curing process, DEA provides a sensitive approach to evaluate their curing behavior, especially in the phase of undisturbed chain growth. In order to determine quantitative kinetic parameters a kinetic model is presented and examined for the evaluation of the ion viscosity curves. From the obtained results it is seen that DEA might be employed in the investigation of the primary curing process, the quality assurance of ingredients as well as the control of processing stability of the light curing dental composites.
Light curing dental resin composites are used as an alternative to amalgam fillings now for over 40 years. However, in spite of their long availability in dentistry and a lot of research work in optimizing their performance and endurance, the primary curing within the initial stages of irradiation, in particular the curing kinetics, was out of focus for a long time.
Some first attempts to investigate the primary curing process of dental resin (composites) were made in the 90s. It is known that most of the curing process happens within the first 10–20 s after irradiation. In order to obtain reliable data a method having a time resolution of minimum five readings per second is required. Furthermore, the experimental conditions and boundaries should be chosen very close to application procedures in dentistry. The methods employed for real time monitoring of the curing process include (i) optical differential scanning calorimetry (DSC) , (ii) shrinkage measurements , (iii) FT-IR spectroscopy in the ATR mode show that in the final cured state remain 20–25% unreacted MA groups , and (iv) dielectric analysis (DEA) using frequencies in the kilohertz range.
DEA has been used to monitor curing reactions of epoxy resins and optimizing RIM processing of polyurethanes . A thorough overview of real-time in situ applications of DEA is given by NAHM . The first applications of DEA to dental resins and resin composites were limited because of the necessity to irradiate through parallel arranged capacitor plates set up to measure the dielectric constant frequency and time dependently. Due to this technical obstacle DEA was considered to be hardly applicable to light curing dental resins and composites and mainly applied to auto-curing resins .
The situation changed with the upcoming of small and flat sensors such as
IDEX-sensors having interdigit comb electrodes with distances of 115 μm, 100 μm and 30 μm and allow for to measure the local curing processes within a layer above the sensor surface having the thickness of the electrode distance and
monotrode sensors measuring an overall dielectric constant of samples having a thickness of a few millimeters .
Both sensor types allow for a dentist-like handling of the light curing dental resin composites with respect to procedure and sample mass. In order to gain information of degree of conversion or curing kinetics one requires only the time dependent change of the dielectric constant–more precisely: the change of the conductivity of the sample is the relevant quantity describing the actual curing state. As the sample is irradiated from the top and the DEA measurement is performed at the bottom, it can be investigated how the curing behavior is affected by the initiator content, different monomers and monomer ratios, or the thickness of sample .
The frequency and time dependent dielectric loss consists of a conductivity part due to the motion of ions in the resin and a dipole part due to the orientation of polar groups of the resin molecules.
ε ″ r ( ω , t ) = σ ion ( ω , t ) ε 0 ω + ε ″ dipole ( ω , t )
with dielectric loss the sample ε″r
ε ″ r
, ion conductivity σ ion , dielectric susceptibility ɛ 0 , frequency ω and dielectric loss of dipoles ε″dipole
ε ″ dipole
. If there are sufficient ions in the resin they dominate the dielectric loss of resins completely. Therefore, the contribution of the dipoles to the dielectric loss is negligible in the phase of primary curing. During curing reactions the dielectric loss changes significantly because the ion conductivity σ ion decreases with the ongoing polymerization and cross-linking. In analogy to the viscosity in rheology one defines the ion viscosity η ion
η ion ( ω , t ) = 1 σ ion ( ω , t ) = 1 ε 0 ω ε ″ r ( ω , t ) 1 − ( ε ″ dipole ( ω , t ) / ε ″ r ( ω , t ) ) ︸ < < 1 in resins ≈ 1 ε 0 ω ε ″ r ( ω , t )