Influence of various irradiation processes on the mechanical properties and polymerisation kinetics of bulk-fill resin based composites

Abstract

Objectives: To assess the effect of irradiation time and distance of the light tip on the micro-mechanical properties and polymerisation kinetics of two bulk-fill resin-based composites at simulated clinically relevant filling depth.

Methods: Micro-mechanical properties (Vickers hardness (HV), depth of cure (DOC) and indentation modulus (E)) and polymerisation kinetics (real-time increase of degree of cure (DC)) of two bulk-fill resin-based composites (Tetric EvoCeram(®) Bulk Fill, Ivoclar Vivadent and x-tra base, Voco) were assessed at varying depth (0.1-6mm in 100μm steps for E and HV and 0.1, 2, 4 and 6mm for DC), irradiation time (10, 20 or 40s, Elipar Freelight2) and distances from the light tip (0 and 7mm). Curing unit's irradiance was monitored in 1mm steps at distances up to 10mm away from the light tip on a laboratory-grade spectrometer.

Results: Multivariate analysis (α=0.05), Student's t-test and Pearson correlation analysis were considered. The influence of material on the measured mechanical properties was significant (η(2)=0.080 for E and 0.256 for HV), while the parameters irradiation time, distance from the light tip and depth emphasise a stronger influence on Tetric EvoCeram(®) Bulk Fill. The polymerisation kinetics could be described by an exponential sum function, distinguishing between the gel and the glass phase. The above mentioned parameters strongly influenced the start of polymerisation (gel phase), and were of less importance for the glass phase.

Conclusions: Both materials enable at least 4mm thick increments to be cured in one step under clinically relevant curing conditions.

Clinical significance: The susceptibility to variation in irradiance was material dependent, thus properties measured under clinically simulated curing conditions might vary to a different extent from those measured under ideal curing conditions.

Keywords: Bulk-fill resin based composites; Hardness; Irradiance; Modulus of elasticity; Polymerisation kinetic.