Influence of Etching Protocols on the Bonding Stability of Universal Adhesives to Dentin

This study aimed to assess the effect of etching protocols on the bonding performance of three universal adhesives to dentin under different aging conditions. Specimens were obtained from human molars and randomly allocated to 6 groups (n = 10) according to adhesive type (Clearfil S3 Bond Universal (CBU), Clearfil Universal Bond Quick (CUBQ), and Zipbond Universal (ZBU)) and application strategy (self-etch or etch-and-rinse). Each adhesive was applied according to the manufacturer’s instructions for the respective strategy, followed by composite resin build-up. Resin–dentin beams were prepared and subjected to three aging conditions: storage in distilled water at 37 °C for 24 h, 10,000 thermocycles, or pH cycling. Microtensile bond strength was measured, and the degree of conversion was evaluated using Fourier-transform infrared spectroscopy. Data were evaluated using analysis of variance and Weibull statistics (α = 0.05). Both adhesive type and application protocol significantly influenced bond strength (p < 0.05), whereas aging conditions had no significant effect (p > 0.05). At 24 h, ZBU in the etch-and-rinse strategy showed the highest bond strength and significantly outperformed CUBQ. After thermocycling and pH cycling, no significant differences were found among adhesives. All adhesives demonstrated higher reliability when used in the self-etch mode. Weibull analysis indicated that ZBU in the self-etch mode had the lowest probability of failure, with fractures occurring at higher stress levels. CUBQ showed the lowest degree of conversion.