Impact of refrigeration on adhesive resins resins back into the refrigerator for 24 h. The degree of conversion (DC) for each scan was calculated as previously described,17 con- sidering the intensity of C=C stretching vibra- tion (peak height) at 1635 cm-1 and using, as an internal standard, symmetric ring stretching at 1608 cm-1 from polymerized and unpolymerized samples. DC (%) was determined by subtracting the percentage of remaining aliphatic C=C from 100%. Final DC values were submitted to two-way ANOVA (material vs. post-refrigeration time) fol- lowed by Tukey’s test (P<.05). Average conversion vs. time data were plotted and Hill’s 4-parameter non-linear regressions were used for curve fitting. As the coefficient of determination was greater than 0.99 for all curves, the rate of polymerization (RP) was calculated using these data-fitted plots, and the maximum rate of polymerization (Rpmax) was recorded. rEsuLts The results for temperature and viscosity as a function of the post-refrigeration time are shown in Figure 1. Clearfil presented a faster increase in temperature (increase is prominent in the first 5 min) after exposure to room temperature than Scotchbond. The results for the viscosity analysis are also shown in Table 2. The factors ‘material’ and ‘post-refrigeration time’ were both significant (P<.001), and so was their interaction (P<.001). In general, all post-refrigeration times presented significant differences in viscosity compared to each other. A continuous decrease in viscosity with increased post-refrigeration time was ob- served for both bonding resins. For Scotchbond, the decrease was more evident up to 10 min after refrigeration, while for Clearfil the decrease was more evident after 10 min. Comparing the control and 20 min samples, slight differences in viscosity were observed for both systems. At the immediate post-refrigeration time, Scotchbond was signifi- cantly more viscous than Clearfil (P<.001), while Clearfil was significantly more viscous for the times 5 and 10 min (P<.001). Similar viscosities for these two materials were detected after 15 min at room temperature (P≥.45). The results for final DC values are shown in Table 2. The factors ‘material’ (P<.001) and ‘post- refrigeration time’ (P=.018) were both significant, whereas the interaction between the two factors was not significant (P=.223). The profiles of po- lymerization kinetics are shown in Figures 2 and 3. For Scotchbond, a significantly higher final DC value was detected for the control compared with the immediate and 5 min post-refrigeration groups (P<.001). On the other hand, similar DC was observed for the times 10, 15, and 20 min in comparison with the control sample (P≥.558). For Clearfil, the control sample showed significantly higher final DC than all post-refrigeration groups (P≤.003), which were similar among them (P≥.858). Comparing the different materials, Clearfil always showed significantly higher DC than Scotchbond (P≤.012), regardless of the post-refrigeration time. Polymerization rate profiles are shown in Figure 4. For Scotchbond, although the control group showed the highest Rpmax value, similar Rp Figure 1. Results for temperature and viscosity as a function of the post-refrigeration time. The 3D scatter plot shows that a continuous decrease in viscosity and increase in temperature with increased post-refrigeration time was observed for both bonding resins. Table 1. Dental adhesive resins used in the study. Material Scotchbond Dual-Cure Clearfil SE Bond Manufacturer 3M ESPE Kuraray Constituents* Bis-GMA, TEGDMA Bis-GMA, MDP, HEMA, colloidal silica *As provided by the manufacturers. The primer solutions were not tested. Bis-GMA: bisphenol-A glycidyl dimethacrylate; TEGDMA: triethylene glycol dimethacrylate; MDP: methacry- loxydecyl dihydrogen phosphate; HEMA: hydroxyethyl methacrylate. 152 European Journal of Dentistry