In vitro comparison of polymerisation kinetics and the micro-mechanical properties of low and high viscosity giomers and RBC materials

ObjectivesThe study aims to characterise a low and high viscosity giomer bulk fill resin restorative with established low and high viscosity resin-based composite (RBC) restoratives at simulated clinical relevant specimen depths.MethodsThe irradiance of a light curing unit (Bluephase 20i) was measured on a laboratory-grade spectrometer at distances up to 10 mm from the light tip (in 1 mm increments). Polymerization kinetics (real-time decrease of CC double bond and degree of cure, DC) and micro-mechanical properties (Vickers hardness/HV; Depth of cure/DOC; Indentation modulus/E) were assessed at varying specimen depths (0.1-6 mm in 100 μm steps for E, DOC and HV and 0.1, 2, 4 and 6 mm for DC) for a 20 s irradiation.ResultsOne and two-way ANOVA (α = 0.05) were performed. The parameter material has a significant (p < 0.001) effect on DC (ηP2 = 0.839), HV (ηP2 = 0.683), and E (ηP2 = 0.536), whereas the specimen depth influenced only DC (ηP2 = 0.584) but not HV (p = 0.093) or E (p = 0.133). The polymerisation kinetic could be described by an exponential sum function, distinguishing between the gel and the glass phase. The hardness drop out was lower than 20%, indicated a depth of cure of 6 mm for all materials.ConclusionsThe low and high viscosity giomer bulk-fill restoratives fulfil the requirements of bulk-fill placement, since under the analysed irradiation conditions an adequate depth of cure of >4 mm was identified. Variation of DC with specimen depth was low, with little variation at specimen depths of <4 mm. The micro-mechanical properties of the giomer materials were higher compared with the conventional RBC materials.