Polymerization and shrinkage kinetics and fracture toughness of bulk-fill resin-composites

To determine degree of conversion (DC), maximum polymerization rate (RP_max), polymerization shrinkage (PS), maximum shrinkage rate (PS R_max) and fracture toughness (K_IC) of different types of bulk-fill (BF) composites plus the effect of viscosity reduction techniques.

BF specimens were created in 2 mm deep molds: SonicFill 3 (SF3), Viscalor (VC), One Bulk Fill (OBF) and Beautifil Bulk (BBR). SF3 was applied via sonic insertion using a SonicFill handpiece (Kerr Corp. USA). Viscalor was pre-heated in a Caps Warmer in T3 mode (at 68 °C) for 30 s (T3-30 s) and 3 min (T3-3 min), respectively. Specimens were irradiated at zero distance from the upper surface with an Elipar S10 LED unit (3 M ESPE, USA) of mean irradiance 1200 mW/cm² for 40 s. Real-time polymerization kinetics and DC at 5 min and 24 h post-irradiation (DC_5 min and DC_24 h) were measured using ATR-FTIR (n = 3). PS was measured up to 1 h on 1 mm thick discs via the bonded-disk technique (n = 3) and PS R_max obtained by numerical differentiation (n = 3). For fracture toughness, single-edge-notched specimens (32 × 6 ×3 mm) of each BF composite were prepared and measured by three-point bending after 7 d water storage (n = 5). Data were analysed using One-way ANOVA, independent T-tests and Tukey post-hoc tests (p < 0.05).

SF3 showed the significantly highest DC_5 min, DC_24 h and RP_max (p < 0.05), followed by OBF (p < 0.05). Regardless of pre-heating, VC showed comparable conversion kinetics to BBR (p > 0.05). There was no significant difference in PS of these BF composites, except OBF had the highest PS (p > 0.05). However, PS Rmax significantly varied among materials (p = 0.047) and SF3 had the highest PS R_max. Regarding fracture toughness, BBR had the lowest K_IC (p < 0.05), whereas other composites showed similar K_IC (p > 0.05). Strong correlations of filler content (wt%)-PS/K_IC were found. Different pre-heating times had no significant influences on DC %, RP_max, PS, PS R_max and K_IC of VC (p > 0.05).

Different types of bulk-fill composites showed comparable shrinkage. A highly filled BF giomer composite (BBR) had the lowest fracture toughness, whereas others had similar K_IC. Pre-heating had no adverse effects on Viscalor properties. Sonication and pre-heating are beneficial techniques to enhance composite flowability without either increasing shrinkage or reducing fracture toughness.