Polymerization shrinkage and shrinkage stress development in ultra-rapid photo-polymerized bulk fill resin composites

To determine the polymerization shrinkage (%) and shrinkage stress (MPa) characteristics of ultra-rapid photo-polymerized bulk fill resin composites.

Two ultra-rapid photo-polymerized bulk fill (URPBF) materials: PFill and PFlow were studied, along with their comparators ECeram and EFlow. PFill contains an addition fragmentation chain transfer (AFCT) agent. The URPBR materials were irradiated using two different 3 s high irradiance protocols (3000 and 3200 mW/cm² based on Bluephase PowerCure and VALO LCUs, respectively) and one 10 s standard protocol (1200 mW/cm² based on a Bluephase PowerCure LCU). Bonded disk and Bioman II instruments were used to measure Polymerization shrinkage % and shrinkage stress MPa, respectively, for 60 min at 23 ± 1 °C (n = 5). Maximum shrinkage-rate and maximum shrinkage stress-rate were also calculated for 15 s via numerical differentiation. The data were analyzed via multiple One-way ANOVA and Tukey post-hoc tests (α = 0.05).

PFill groups, regardless of their irradiance protocol, showed significantly lower PS than the comparator, ECeram (p < 0.05). However, PFlow irradiated via different protocols, was comparable to EFlow and ECeram (p > 0.05). PFill consistently produced stress results which were significantly lower than ECeram (p < 0.05) and were comparable for both high irradiance protocols (p > 0.05). PFlow only exhibited significantly higher shrinkage stress when polymerized with the 3 sVALO protocol (p < 0.05). The maximum shrinkage strain-rate (%/s) was significantly lower in PFill-10s and PFill-3s groups (using PowerCure LCU) compared to ECeram. However, no differences were seen between PFlow and EFlow (p > 0.05). The maximum shrinkage stress-rate of PFill and PFlow was comparable between different irradiation protocols, as well as to their comparator ECeram (p > 0.05).

High irradiation protocols over ultra-short periods led to slightly lower shrinkage strain but slightly higher stress, possibly due to reduced network mobility. The AFCT agent incorporated in PFill composite seemed to reduce shrinkage stress development, even with high irradiance protocols.