Impact of refrigeration on adhesive resins a faster increase in temperature after exposure also a significant linear correlation between % DC to room temperature, the viscosity of Clearfil de- and increase in temperature. creased more slowly than Scotchbond. This might Despite the increased effect of refrigeration be a result of the greater hydrogen bonding po- on Clearfil, this bonding resin presented signifi- tential of HEMA compared with TEGDMA, slowing cantly higher DC than Scotchbond, regardless of down the viscosity recovery after refrigeration. the post-refrigeration time. This result is most The presence of fillers in Clearfil may also explain probably related to the type of diluent used in this result, since the differences in the thermal each system. The dimethacrylate TEGDMA poten- conductivity of the glass particles compared with tially renders the polymer network more densely the resin matrix may have influenced heat transfer cross-linked11,19 but also reduces the limiting DC through the material, maintaining the viscosity of for Scotchbond. For Clearfil, the presence of the the system at higher levels for a longer period of monomethacrylate HEMA reduces the amount time. As no fillers are present in Scotchbond, the of cross-linking reactions but increases the final system increases its fluidity faster than Clearfil. conversion. It is difficult to forecast whether the Regarding the polymerization process, refrig- differences in conversion observed here might eration significantly decreased the DC measured predict a distinct clinical performance for the immediately and after 5 min for both adhesives. bonding resins. Although higher DC is usually After 10 min, Scotchbond presented a similar linked with improved mechanical properties,20 the conversion to the control sample. Therefore, the cross-link density also strongly affects the prop- hypothesis for conversion was also confirmed, as erties of the polymer,19 especially by reducing refrigeration presented a time-dependent influ- the access of water into the network, which is re- ence on the DC. Nonetheless, the impact of re- sponsible for degrading the bonding layer over the frigeration on DC was also material-dependent; course of time.4 Clearfil showed significantly lower DC for all The present outcomes showed a time- and refrigerated samples, even after a 20 min post- material-dependent effect of refrigeration on refrigeration time. This might be linked to the both the viscosity and polymerization of bonding aforementioned longer time required for Clearfil resins. Therefore, it seems adequate to indicate to increase fluidity, which may interfere with the that adhesive systems should be removed from mobility of the monomers in the system.12 Re- refrigerator and exposed to room temperature for stricted mobility may decrease the polymeriza- at least 20 min before they are used. In addition, tion rate and the conversion of double bonds; all it can be suggested that the post-refrigeration refrigerated groups showed lower Rpmax than the period should be longer for materials containing control sample for Clearfil. In corroboration, Lo- fillers. According to Lohbauer et al,18 the recom- hbauer et al,18 testing resin composites, observed mendation for resin restoratives is similar; the au- lower monomer conversion for specimens at the thors indicated that composites should be used at temperature 10±2oC compared with 23±2oC, and room or physiological temperatures. However, the effect of refrigeration is likely to be even greater for composites; thus, earlier removal from the refrigerator is advisable. The present results do not take into account other effects that might af- fect the performance of refrigerated materials in a clinical situation, such as differences in tem- perature and humidity in the oral environment. Further investigations are necessary to define the time required for proper performance of adhesive systems after refrigeration. Figure 4. Profiles of polymerization rate. For Scotchbond, al- though the control group showed the highest R max value, simi- lar Rp profiles were detected for the 10, 15, anpd 20 min post- refrigeration times; the immediate and 5 min times showed lower R max compared with all other groups. In contrast, for p Clearfil, all post-refrigeration times showed lower Rpmax in comparison with the control group. Comparing the bonding resins, the R was slightly slower, and higher Rpmax values were p usually detected for Clearfil. 154 European Journal of Dentistry concLusIons Refrigeration at 4oC presented a significant time- and material-dependent effect on the vis- cosity and polymerization kinetics of the two den-