Effect of Temperature on the Degree of Conversion and Working Time of Dual-Cured Resin Cements Exposed to Different Curing Conditions

Abstract

A high degree of conversion can be achieved when dual-cured resin cements are used with increased temperature even when the curing light is compromised by the presence of ceramic restorations. However, caution is recommended before the clinician decides to warm up the resin cement, as this procedure may compromise the working time, depending on the temperature and product. SUMMARYObjectives: This study evaluated the degree of conversion (DC) and working time (WT) of two commercial, dual-cured resin cements polymerized at varying temperatures and under different curing-light accessible conditions, using Fourier transformed infrared analysis (FTIR).Materials and Methods: Calibra (Cal; Dentsply Caulk) and Variolink II (Ivoclar Vivadent) were tested at 258C or preheated to 378C or 508C and applied to a similar-temperature surface of a horizontal attenuated-total-reflectance unit (ATR) attached to an infrared spectrometer. The products were polymerized using one of four conditions: direct light expo- ) through a glass slide or through a 1.5-or 3.0-mm-thick ceramic disc (A2 shade, IPS e.max, Ivoclar Vivadent) or allowed to self-cure in the absence of light curing. FTIR spectra were recorded for 20 min (1 spectrum/s, 16 scans/spectrum, resolution 4 cm À1 ) immediately after application to the ATR. DC was calculated using standard techniques of observing changes in aliphatic-to-aromatic peak ratios precuring and 20-min postcuring as well as during each 1-second interval. Time-based monomer conversion analysis was used to determine WT at each temperature. DC and WT data (n=6) were analyzed by two-way analysis of variance and Tukey post hoc test (p=0.05).Results: Higher temperatures increased DC regardless of curing mode and product. For Calibra, only the 3-mm-thick ceramic group showed lower DC than the other groups at 258C (p=0.01830), while no significant difference was observed among groups at 378C and 508C. For Variolink, the 3-mm-thick ceramic group showed lower DC than the 1-mm-thick group only at 258C, while the self-cure group showed lower DC than the others at all temperatures (p=0.00001). WT decreased with increasing temperature: at 378C near 70% reduction and at 508C near 90% for both products, with WT reduction reaching clinically inappropriate times in some cases (p=0.00001).Conclusion: Elevated temperature during polymerization of dual-cured cements increased DC. WT was reduced with elevated temperature, but the extent of reduction might not be clinically acceptable.

Correlation between clinical performance and degree of conversion of resin cements: a literature review

“…When a dual cement is self-cured (no activation by light), mechanical properties such as flexural strength, modulus and hardness are reduced by 68.9%, 59.2% and 91.1%, respectively, in comparison to original values presented by dual-cured samples 34 . There are different factors that may affect the DC of self-cured luting systems, such as the relatively high concentration of polymerization inhibitors used to extend the material’s shelf life and to provide a clinically viable working time, ranging from 2 to 5 minutes, which adversely inhibits polymerization during the luting procedure 61 ; the slow rate of polymerization activation and subsequent propagation of radicals in comparison to a directly light-activated material 6 , 49 , 51 , 61 ; and the low concentration of benzoyl peroxide incorporated into those materials 6 , 49 . Furthermore, the hand-mixing of the two pastes incorporates air bubbles that further inhibit polymerization due to the presence of oxygen 79 and may act as stress concentrators that potentially result in cracking throughout the cement layer 56 .…”

“…Under ideal circumstances, light-activated resin cements show higher DC than chemically cured resin cements, irrespective of brand names 49 , 57 . However, the DC of dual-cured cements is material-related, which means that it is more associated with the brand name than with the material classification per se and some systems are significantly more dependent on light activation than others 1 , 10 , 11 , 15 , 16 , 31 , 51 , 73 , 79 . Just as an illustration, the DC of a given dual-cured cement (RelyX ARC, 3M Espe, St. Paul, MN, USA) may vary from 81% to 61% when cured under light as opposed to total absence of light respectively, and from 56% to 26% when another dual-cured cement (RelyX Unicem, 3M Espe, St. Paul, MN, USA) is cured under the same conditions 43 .…”

“…This difference may be explained by the difference in composition between both materials. For instance, some resin-based cements present twice as much benzoyl peroxide than others 51 . The lower DC may affect some critical properties of the resin-based cements 64 .…”

“…Another way to improve the polymerization kinetics of resin-based luting systems is to increase the temperature of the material 30 , 51 . High viscosity cements have significantly lower degree of conversion than low viscosity cements 24 , probably due to the reduced mobility of the monomers in viscous materials.…”

Delayed photo-activation and addition of thio-urethane: Impact on polymerization kinetics and stress of dual-cured resin cements

“…For example, those cements still offer some control of working time, and ensure that the margins of the restoration can be polymerized to relatively high degrees of conversion right after cementation, to avoid premature dissolution – those all being features one can harness from the photo-activated components [1–4]. However, photo-activation cannot guarantee high conversion values in areas that are inaccessible to light, either due to reflection, absorption or scattering phenomena through thick and opaque indirect restorative materials [5,6].…”