Effect of air-drying and solvent evaporation on the strength of HEMA-rich versus HEMA-free one-step adhesives

Objectives The objectives of this study were (1) to clarify the relationship between the duration of air-drying of one-step self-etch adhesives (1-SEAs) and the evaporation degree (ED) of solvents, and (2) to evaluate the effect of ED on the ultimate micro-tensile strength (μTS) of the adhesives. Methods The ED of one HEMA-rich 1-SEA, Clearfil S3 Bond (Kuraray), and two HEMA-free 1-SEAs, iBond (Heraeus-Kulzer) and G-Bond (GC), was determined without air-drying and after, respectively, 5- and 10-s air-drying using a gravimetric method. Next, the μTS of the adhesives at ED equivalent to 0-, 5- and 10-s air-drying was measured. Results The ED increased with extension of air-drying time. Among the adhesives tested, iBond showed the largest ED, followed by G-Bond and Clearfil S3 Bond in this order. A longer air-drying time for 10-s resulted in a statistically significantly higher μTS for the HEMA-rich Clearfil S3 Bond. The μTS of the latter was higher than that of the other two HEMA-free adhesives for each air-drying time. Significance Air-drying of 1-SEAs had a significant effect on the degree of solvent evaporation (ED) and also on the mechanical properties (μTS) of the 1-SEAs upon setting. It is therefore beneficial to remove solvents of the 1-SEAs as much as possible by thorough, strong air-drying in order to achieve a strong adhesive layer at the interface.