Relationship Between Degree of Polymerization and Enamel Bonding Strength with Self-etching Adhesives

Purpose: To investigate the relationship between the degree of conversion of double bonds of all-in-one adhesives and their shear bond strength on ground human enamel. Materials and Methods: Six commercially available systems and one experimental adhesive were tested: Absolute (ABS; Dentsply-Sankin), Clearfil S3 Bond (CSB; Kuraray), G-Bond (GBO; GC), Hybrid Bond (HYB; Sun Medical), iBond (IBO; Heraeus Kulzer), Xeno IV (XEN; Dentsply Caulk), and experimental iBond NG (ING; Heraeus Kulzer). Conventional shear bond strengths (SBS, n = 8) of adhesive-coated enamel specimens bonded to Venus composite (Heraeus Kulzer) and degrees of conversion (DC) (FTIR, n = 5) were determined after 1 and 10 min, 1, 2, and 24 h of storage. Data were statistically analyzed using ANOVA and Duncan's post hoc test (α 0.05). Results: The adhesives' SBS (MPa) and DC (%) by testing time followed logarithmic regression lines established by the least square method. Mean shear bond strengths after 1 min/24 h were: ABS 9.8/14.9; CSB 14.7/23.4; GBO 14.8/22.0; HYB 9.7/17.0; IBO 11.3/22.3; XEN 9.1/17.3; ING 9.7/25.6. The corresponding mean DC values were: ABS 51.3/66.2; CSB 83.1/90.8; GBO 75.8/87.7; HYB 49.6/67.2; IBO 72.6/93.7; XEN 61.6/74.1; ING 64.9/89.1. Linear regressions for the relationship DC vs SBS were significant with coefficients of determination (r2) between 0.72 and 0.97. Conclusion: Despite similar acidity, the adhesives showed different SBSs on enamel. Based on the relationships between DC and SBS, the cohesive failure patterns observed, and the composition-property relations discussed, it is concluded that the percentage DC is the main parameter influencing an adhesive's bonding efficacy to ground enamel. Keywords: self-etching adhesives, enamel bonding, degree of conversion, FTIR