Influence of Oxygen Inhibition on Surface Free-energy of Self-etch Adhesives

Methods: The systems used were: Bond Force (Tokuyama Dental), Clearfil tri-S Bond (Kuraray Medical), and G-Bond Plus (GC). Bovine mandibular incisors were mounted in self-curing resin and labial dentin was wet ground with #600-grit SiC paper. The adhesives were applied according to each manufacturer’s instructions and light irradiated The oxygen-inhibited layer was either retained or removed with ethanol. Contact angle measurement was conducted using a sessile drop and θ/2 (half angle) method by means of DropMaster (DM500, Kyowa Interface Science). The surface free energies and their components (γ^LW, γ⁺, γ^–) were determined by measuring the contact angles of three test liquids placed on the cured adhesives. The dentin bond strengths of specimens with and without the oxygen-inhibited layer were measured. ANOVA and Tukey HSD tests at a level of 0.05 were performed. SEM observations of the treated dentin surface were also conducted.

Results: For all surfaces, the value of the estimated surface tension component γ^LW was relatively constant (35.5~39.8 mN/m). The value of the γ⁺ component increased slightly when the oxygen-inhibited layer was removed, whereas that of the γ^–component decreased significantly (17.5~25.0 mN/m) when compared to those with the layer (34.2~56.6 mN/m). The dentin bond strengths of the self-etch adhesives were significantly lower in the specimens without the layer (13.2~13.6 MPa) when compared to those with the oxygen-inhibited layer (15.6~18.4 MPa). The oxygen-inhibited layer, in which the Lewis-base component remained, acted as an electron donor, which accelerated the polymerization reaction at the adhesive–resin composite interface.

Conclusions: The current study suggests that the presence of an oxygen-inhibited layer in single-step self-etch adhesives promoted higher dentin bond strength.