Effect of thickness on the degree of conversion of two bulk-fill and one conventional posterior resin-based composites at high irradiance and high temporal resolution

Objectives: This study: 1) measures the effect of sample thickness and high irradiance on the depth-dependent time delay before photopolymerization reaction onset; 2) determines if exposure reciprocity exists; 3) measures the conversion rate at four irradiance levels; 4) determines the time, t₀, at which the maximum DC rate is reached for two bulk-fill and one conventional posterior resin-based composites (RBCs). Methods: Tetric PowerFill IVA shade (Ivoclar Vivadent) and Aura bulk-fill ultra universal restorative (SDI), and one conventional posterior resin-based composite (RBC), Heliomolar A3 (Ivoclar Vivadent), that were either 0.2 mm, 2 mm, or 4 mm thick were photocured using a modified Bluephase G4 (Ivoclar Vivadent) light-curing unit (LCU) that delivered a single emission band (wavelength centered at 449 nm). The same radiant exposure of 24 J/cm² was delivered at irradiances ranging from 0.5 to 3 W/cm² by adjusting the exposure time. PowerFill was also photocured for 3 s or 6 s using a Bluephase PowerCure LCU (Ivoclar Vivadent) on the 3 s mode setting. The degree of conversion (DC) was measured in real-time at a high temporal resolution at 30 °C using Attenuated Total Reflection (ATR) FTIR spectroscopy with a sampling rate of 13 DC data points per second. The DC data were analyzed using a phenomenological autocatalytic model. The RBC viscosity was measured at 21 °C and 30 °C. Light transmission through the RBC samples at 22 °C was monitored with time to calculate the extinction coefficients of the RBCs. Results: The time delay before photopolymerization started increased as the RBC thickness increased and the irradiance decreased. An autocatalytic model described the DC data. The time t₀ was less than 77 ms for the 0.2 mm thick samples of PowerFill irradiated using the highest irradiance of 3 W/cm². Among the three RBCs for each sample thickness and irradiance level, the PowerFill had the smallest time t₀. There was a time delay of 0.59 s and 1.25 s before the DC started to increase at the bottom of 4 mm thick samples for the PowerFill and Aura, respectively, when an irradiance of 1 W/cm² was delivered. The time delay increased to 3.65 s for the Aura when an irradiance of 0.5 W/cm² was delivered. The extinction coefficients near 449 nm were 0.78 mm⁻¹, 0.76 mm⁻¹, and 1.55 mm⁻¹ during the first 2 s after the start of photocuring of PowerFill, Aura, and Heliomolar, respectively. Only PowerFill followed exposure reciprocity. At T = 30 °C, the viscosity was 3400, 17000, and 5200 Paˑs for PowerFill, Aura, and Heliomolar, respectively. Significance: The time delay between when photopolymerization starts at the top and bottom of 2- or 4-mm thick RBC restorations may affect the structural integrity of the bond between the tooth and the bottom of the restoration. Only PowerFill followed exposure reciprocity between irradiance levels of 0.5 to 3 W/cm². Exposure reciprocity did not occur for Aura or Heliomolar, neither of which are optimized for short light exposure or high irradiance conditions.