The Effect of Composition, Temperature and Post-Irradiation Curing of Bulk Fill Resin Composites on Polymerization Efficiency

Materials Research. 2016; 19(2): 466-473 DOI:http://dx.doi.org/10.1590/1980-5373-MR-2015-0614 © 2016 The Effect of Composition, Temperature and Post-Irradiation Curing of Bulk Fill Resin Composites on Polymerization Efficiency Dimitrios Dionysopoulosa*, Kosmas Tolidisa, Paris Gerasimoua Department of Operative Dentistry, School of Dentistry, Aristotle University of Thessaloniki, Greece a Received: October 10, 2015; Revised: January 25, 2016; Accepted: February 9, 2016 Objectives: To evaluate the polymerization efficiency of bulk fill resin-based composites (RBCs) and how their composition, temperature and post-irradiation polymerization influence the results. Methods and Materials: Eight bulk fill RBCs were investigated. Five specimens for each material were prepared for Vickers hardness measurements. The measurements were performed in three depths from the top of the surface of the specimens (0, 2 or 4 mm) immediately and 24 h after polymerization and for the highly viscous bulk fill RBCs after preheating at 54oC. Statistical analysis was performed using one-way ANOVA and Tukey post hoc tests at a=0.05. Results: As the depth increases microhardness values reduce. The Vickers hardness measurements 24 h after photo-polymerization revealed higher values compared to those obtained immediately after photo-polymerization (p<0.05). There was an increase in microhardness of the RBC materials when preheated at 54oC in comparison with the room temperature specimens (p<0.05).Conclusions: Polymerization efficiency of bulk fill RBCs is affected by their composition and increases with temperature and post-irradiation polymerization. Keywords: Bulk fill resin composites, Resin composite preheating, post-irradiation polymerization, Vickers hardness Introduction Resin-based composites (RBCs) are the most popular restorative materials providing very good esthetics and adequate longevity. When restoring cavities, resin composites should be applied only to increments up to 2 mm thick to ensure adequate light transmittance and full polymerization of the material1. Sufficient polymerization of resin composites is needed to achieve appropriate physical and mechanical properties2 and biocompatibility3. In order to overcome and simplify the time-consuming incremental technique of RBC restorations, a new resin composite category has been recently developed the so‑called “bulk fill” RBCs. The bulk fill RBCs are claimed to be curable to a thickness of 4-5 mm4, create a lower polymerization shrinkage stress5 and exhibit higher light transmission properties due to reduction of light scattering at the filler–matrix interface by either decreasing the filler amount6 or increasing the filler size7. The bulk fill RBCs are further classified into highly viscous and flowable (low-viscosity) resin composites. Highly viscous resin composites contain a greater amount of inorganic fillers compared to flowable resin composites, which adapt better on the cavity walls but exhibit greater polymerization shrinkage and inferior mechanical properties, as a result of their lower filler content. Due to lower mechanical properties of flowable bulk fill resin composites, restorations is recommended to be finished with a 2-mm capping layer of a high-viscosity resin composite when restoring areas which are submitted to occlusal stresses8. *e-mail: The evaluation of surface microhardness of a resin composite is an effective method to indirectly determine the degree of monomer conversion9. In the present study, Vickers hardness measurements were performed in order to evaluate the curing efficiency of various bulk fill RBCs. To achieve the acceptable curing efficiency bulk fill materials have to meet the requirement of ≥80% bottom/top percentage microhardness. Flury et al.10, who evaluate if depth of cure determined by the ISO 4049 method is accurately reflected with bulk fill materials when compared to depth of cure determined by Vickers microhardness profiles, concluded that the ISO 4049 scraping method overestimated depth of cure compared to depth of cure determined by Vickers hardness profiles. For this reason we used microhardness method for the present study to evaluate curing efficiency. It has been demonstrated that preheated resin composites exhibit reduced viscosity11 and increased polymerization efficiency12. Heating the resin composites prior to placement in the cavity and immediately light-curing increases monomer conversion rate and thus the duration of the irradiation period may be reduced13. With increased paste temperature, free radicals and developing polymer chains become more fluid as a consequence of decreased paste viscosity and they react to a greater extent, resulting in a more complete polymerization reaction and greater cross-linking14. The increase in the degree of polymerization of RBCs may lead to better internal adaptation to cavity walls15, improved mechanical properties and increased wear resistance13. Taubock et al.,16 reported that RBC pre-heating significantly reduces shrinkage force formation of high-viscosity bulk-fill and conventional resin The Effect of Composition, Temperature and Post-Irradiation Curing of Bulk Fill Resin Composites on Polymerization Efficiency 2016; 19(2) composites, while maintaining or increasing the degree of monomer conversion, dependent upon the specific composite material used. Surface hardness will develop over time after lightcuring period which is mainly attributed to post-irradiation polymerization17. There are discrepancies in outcomes of previous reports concerning the polymerization kinetics after light-curing of RBCs. Hansen18 found that the most of the polymerization reaction takes place during the first minutes after irradiation or one hour after removal of the irradiation source, while Leung et al.,19 reported that a significant portion of reaction may occur during the first 24 h. Moreover, in a recent study it has been reported a further increase in microhardness of 50-80% of the immediate readings for some RBC materials in room temperature after 168 h20. The extent of post-irradiation polymerization may be influenced by various intrinsic factors such as monomer composition, availability of free radicals and initial degree of conversion. However, it is not fully understood how these factors affect this phenomenon21. Therefore, the aim of this in vitro study was to evaluate the polymerization efficiency of four highly viscous and four flowable bulk fill resin composites at three depths (0, 2 and 4 mm) from the surface and to determine if they 467 achieve the curing efficiency requirement for microhardness of ≥80%VHN of the top surface (0 mm). Moreover, we investigated how the preheating at 54oC of the high-viscosity resin composites and the post-irradiation polymerization after 24 h may influence the results. The first null hypothesis of the study (Ho1) was that there were no differences in microhardness among the RBC materials of the (...truncated)