Thickness Dependent Dielectric Monitoring of Light-Curing Composite Samples

Objectives:

The curing behaviour of photo-curing composites is significantly affected by the locally available light intensity, which especially depends on the employed light curing unit and the light absorption of composites. The aim of this study was to investigate the effects of light intensity on the curing behaviour of a dental composite in different specimen depths using three different curing units.  

Methods:

An examplatory dental composite (VOCO, Arabesk Top shade OA2, Germany) was investigated using a dielectric cure analyser (NETZSCH , DEA 231, Germany) with a frequency of 1kHz. The surface of Mini-IDEX-sensors was covered with composite layers of 0.5-2mm. Three curing units – Polofil Lux halogen (VOCO), LED Celalux (VOCO) and LED Bluephase 20i in turbo-modus (IVOCLAR VIVADENT, Liechtenstein) – were used for curing with a polymerisation time of 80s respectively 20s with Bluephase turbo. The DEA-curves were evaluated with respect to reaction-time-constant Tau and maximal slope (Fig.1).

Results:

For all curing units the maximum slope of the ion-viscosity depended significantly on the thickness of the specimens. When exceeding the reaction-time-constant Tau all curves decreased significantly. Reaction-time-constant and maximal slope of the ion-viscosities µ versus sample thickness showed an exponential dependency, Fig.2.

Conclusion:

The DEA provided intensity-dependent data of the curing behaviour. A transition from the primary-curing to post-curing was found when exceeding the reaction-time-constant. This may indicate a glass transition change of the polymer resin at curing temperature. Because the reaction-time depends on the radicalised initiator concentration, the curing rate slowed down in increasing sample depths. The slopes of ion-viscosity during the post-curing allow for the assumption of higher final values meaning a higher degree of conversion up to 1-2mm depth.

Acknowledgement:

Financial support by the Ministry of Education and Research grant no. 17081X10.