Effect of prolonged application times on resin-dentin bond strengths.

Purpose: To evaluate the cumulative loss of mass of two-step solvent-based adhesives, examine the effect of prolonged application time on the resin-dentin microtensile bond strength (microTBS), and correlate the microTBS with the evaporation rate of adhesive systems.
Materials and Methods: An ethanol/water based (Single Bond: SB) and an acetone-based system (One Step: OS) were employed. Approximately 16 microl of each product was transferred to a small glass container and placed in an analytical balance. The cumulative loss of mass (CLM) was determined in every minute for 5 min to the nearest 0.0001 mg and again after 20 min (n = 4). For the microTBS test, a flat superficial dentin surface was exposed in 24 human third molars by wet abrasion. The adhesives were applied to a delimited area of 52 mm2 on wet surfaces, for 40, 90, and 150 s. Four teeth were assigned to each experimental group. Composite crowns were constructed incrementally. After storage in water at 37 degrees C for 24 h, teeth were sectioned to obtain sticks with cross-sectional areas of 0.8 mm2 to be tested in tension (0.5 mm/min). The CLM (%) for both adhesives was compared using a Student-t test at each minute (alpha = 0.05). The microTBS values were analyzed by two-way ANOVA and Tukey's test (alpha = 0.05).
Results: The CLM for OS was higher than for SB during the first 5 min, but these values were only statistically different at minutes 3, 4, and 5 (p < 0.05). A CLM of approximately 50% was observed for both adhesive systems after 20 min. The factor application duration and the interaction between application duration and adhesive were significant (p < 0.05). The increase in the application duration led to an increase in microTBS for both systems; however, the differences were only statistically significant for OS over the 150 s period (p < 0.05). A strong correlation was found between the microTBS and the cumulative loss of mass of adhesives.
Conclusion: Prolonged application times can increase the resin-dentin microTBS of two-step adhesive systems, but the application duration is dependent on the solvent present in the bonding system.