Conversion kinetics of rapid photo-polymerized resin composites

To measure the degrees of conversion (DC), conversion kinetics, and the effect of post-irradiation time on rapid photo-polymerized bulk-fill resin composites under conditions equivalent to clinical depths of 1 and 4mm.

36 specimens (n=3), based on two resin composites incorporating PowerCure rapid-polymerization technology in two consistencies (PFill; PFlow) and two comparators with matching consistencies (Eceram; EFlow), were investigated from the same manufacturer (Ivoclar AG, Liechtenstein). Specimens were prepared within 4mm diameter cylindrical molds, of either 1mm or 4mm depths respectively, to simulate near-surface and deep locations in a bulk-fill restoration. The independent variables in this study were: materials, thickness and time. Two high irradiance polymerization protocols were utilized for PowerCure materials: 2000 and 3050mW/cm² for 5 and 3s, respectively. A standard (1200mW/cm²) polymerization protocol was used with control materials. FTIR was utilized to measure DC in real-time for 24h post-irradiation. The data were analyzed using Welch's-ANOVA, Games-Howell post-hoc test, kinetic dual-exponential sum function and independent sample t-tests (p=0.05).

The DC of the materials ranged between 44.7-59.0 % after 5min, which increased after 24h reaching 55.7-71.0 % (p<0.05). Specimen thickness did not influence the overall DC. At 5min, the highest DC was shown in EFlow. But PFlow, irradiated for 3s and 5s exhibited comparable results (p>0.05). PFill composite irradiated with the 3s and 5s protocols did not differ from ECeram (p>0.05). Specimen thickness and material viscosity affected polymerization kinetics and rate of polymerization (RP_max). Faster polymerization occurred in 1mm specimens (except PFill-5s and ECeram). PFill and PFlow exhibited faster conversion than the controls. RP_max varied across the specimen groups between 4.3-8.8 %/s with corresponding DC _RPmax between 22.2-45.3 %.

Polymerization kinetics and RP_max were influenced by specimen thickness and material viscosity. PFill and PFlow materials produced an overall comparable conversion at 5min and 24h post-irradiation, despite the ultra-short irradiation times, throughout the 4mm specimen thickness.