Polymerization kinetics and impact of post polymerization on the Degree of Conversion of bulk-fill resin-composite at clinically relevant depth

ObjectiveSince bulk-fill (BF) resin composites should cure efficiently to a depth up to 4 mm, the aim of the study was to determine the time-dependence of degree of conversion (DC) at that depth during 24 h post-irradiation.MethodsEight representative BF resin composites were studied [x-tra base (XTB), Venus Bulk Fill (VBF), Tetric EvoCeram Bulk Fill (TECBF), Sonic Fill (SF), Filtek Bulk Fill (FBF), everX Posterior (eXP), Beautifil-Bulk Flowable (BBF), Beautifil-Bulk Restorative (BBR)]. Specimens were fabricated in white Delrin moulds of 4 mm height and 5 mm internal diameter directly on an attenuated total reflectance (ATR) accessory attachment of an (FTIR) spectrometer (Nicolet iS50, Thermo Fisher, Madison, USA). Upper specimen surfaces were irradiated in situ for 20 s with an LED curing unit (Elipar S10 with average tip irradiance of 1200 mW/cm2). Spectra from the lower surface were recorded continuously in real-time for 5 min and then at 30 and 60 min and 24 h post irradiation.ResultsMean ranges of DC4mm (%) of the materials at 4 mm depth were 39–67; 48–75; 45–74; and 50–72 at 5, 30 and 60 min and 24 h respectively. DCs for XTB, VBF, TECBF, FBF, BBR increased significantly 30 min after irradiation (p < 0.05) and were not affected by subsequent time up to 24 h (p > 0.05). DC for SF was not affected by subsequent time after 5 min (p > 0.05). For eXP and BBF, DC increased 24 h after irradiation (p < 0.05). The data were described by the superposition of two exponential functions characterising the gel phase (described by parameters a, b) and the glass phase (described by parameters c and d).SignificancePost polymerization impact of bulk-fill composites is material dependent. Five materials exhibited their maximum DC4mm already 30 min after starting the irradiation while DC4mm for two materials optimized after 24 h. BF materials were found to exhibit after 24 h a DC between 50 and 72% at 4 mm depth under the stated irradiation conditions.