Interpretation of bond failure through conversion and residual solvent measurements and Weibull analyses of flexural and microtensile bond strengths of bonding agents

Objectives: An inverse correlation between initial acetone content of bonding agents and microtensile bond strength (MTBS) has previously been reported. Here it was investigated whether the inferior MTBS of acetone-rich bonding agents was caused by flaws and low adhesive layer strength, insufficient to resist polymerization shrinkage stresses of the overlying composite. Methods: Bonding solutions with (27–67) mass fraction% acetone were dried for 10 s or 30 min. Specimens for infrared (IR) spectroscopy and flexural strength (FS) were cured for 10 s (cure mode 1) to represent the adhesive layer properties when polymerizing the composite resin or for 10 s plus an additional 60 s through composite resin (cure mode 2) to represent adhesive layer properties when testing the bond strength. Degree of conversion (DC) and residual acetone content were evaluated for films by mid-IR and bulk specimens by near-IR spectroscopy. FS and MTBS data using these bonding solutions were interpreted by Weibull statistics and ANOVA (α=0.05). Results: In bulk specimens, initial and residual acetone content were positively correlated (P=0.003). FS in cure mode 1 was significantly lower than in cure mode 2 and decreased with increasing initial acetone content, while DC remained consistent, suggesting pores from acetone evaporation as flaw initiating sites affecting the mechanical adhesive properties. Significance: From DC and Weibull analyses of MTBS and FS, early low strength of the adhesive layer and shrinkage stresses acting on pre-existing flaws are suggested as critical factors affecting the dentin/adhesive bond strength.