Characterizing surface viscoelastic integrity of ultra-fast photo-polymerized composites: Methods development. Issue 10 (October 2020)

• Record Type: Journal Article
• Authors: Watts, David C.; Algamaiah, Hamad
• Is Part Of: Dental materials. Volume 36:Issue 10(2020)
• Page Start: 1255
• Page End: 1265
• Publication Date: 2020-10
• Subjects: Photopolymerization -- Dimethacrylate -- AFCT -- RAFT -- Resin-composite -- Indentation-creep -- Stretched-exponential -- Viscoelasticity -- Post-cure; Dentistry -- Periodicals; Dental materials -- Periodicals; 617.695
• DOI: 10.1016/j.dental.2020.07.009
• Languages: English
• ISSNs: 0109-5641

Abstract

Highlights: The rapidly developing surface integrity of ultra-fast (3 s) photo-cure composites can be probed "immediately" by a new macroscopic indentation-creep methodology. PowerCure system and materials demonstrated an acceptable level of polymerization/property performance, despite the ultra-short irradiation times, as judged by this experimental criterion. Abstract: Objective: Resin-Composites are now available designed for polymerization using 3 s of intense light irradiation. The aim was to develop an experimental method to probe their surface viscoelastic integrity immediately following such rapid photo-cure via macroscopic surface indentation under constant stress as a function of time. Methods: Two bulk-fill composites (Ivoclar AG) were studied: Tetric PowerFill (PFill) and PowerFlow (PFlow). Split molds were used to fabricate cylindrical {4 mm (dia) × 4 mm} paste specimens, irradiated at 23 °C at 0 mm from the top surface with a Bluephase PowerCure LED-LCU, with 3 s or 5 s modes, emitting 3 and 2 W/cm 2, respectively. Post-irradiation specimens were immediately transferred to an apparatus equipped with a flat-ended indentor of 1.5 mm diameter. 14 MPa compressive stress at the indentor tip was applied centrally in < 2 min and maintained constant for 2 h. Indentation (I) magnitudes were recorded in real-time (t), with I(t) data re-expressed as % indentation relative to the 4 mm specimen height. After 2 h, the indentor was unloaded and indentation recovery was monitored for a further 2 h. Parallel sets of measurements were made where indentation was delayed for 24 h. Further measurements were made with more conventional composites: EvoCeram Bulk Fill (ECeram) and Tetric EvoFlow Bulk Fill (EFlow). These were irradiated for 20 s at 1.2 W/cm 2 . Kinetic data were curve-fitted to exponential growth functions and key parameters analyzed by ANOVA and post-hoc tests (α = 0.05). Results: I(t) plots looked initially similar to bulk creep/recovery: rapid deformation plus viscoelastic response; then, upon unloading: rapid (elastic) recovery followed by partial viscoelastic recovery. However, unlike multiply irradiated and stored bulk-creep specimens, the present specimens were exposed to only 3 or 5 s "occlusal" irradiation; generating "hard" surfaces. Subsequently, during the 2 h indentation, the polymer matrix network continued to harden and consolidate. Upon initial loading, I(t) reached 2–3% indentation, depending upon the formulation. Upon unloading at 2 h, elastic recovery was only ca. 1 %. Delayed loading for 24 h, generated I(t) plots of significantly reduced magnitude. Most importantly, however, the I(t) plots for ECeram and EFlow, after 20 s irradiation, showed I(t) magnitudes quite comparable to the PFill and PFlow rapid-cure composites. Significance: Macroscopic indentation creep has been shown to be a workable procedure that can be applied to rapid-cure materials to assess their immediate surface integrity and developing viscoelastic characteristics. The applied stress of 14 MPa was relatively severe and the indentation/recovery profiles of PowerFill materials with only 3 or 5 s irradiation demonstrated comparability with their established 20 s cure siblings, evidencing the suitability of the PowerCure system for clinical application.