Objectives: The aim of this study was to investigate the effect of a metal etchant on resin bonding durability to a titanium framework. Methods: The etchant consisted of 1% ammonium hydrogen fluoride (AHF) and 40% phosphoric acid (PA). Three control groups without AHF and/or PA were also prepared. Disk-shaped specimens cast with a commercially pure titanium (Titan Ingot JS2, Selec), a phosphate primer (Epricord Opaque Primer, Kuraray), and a self-curing adhesive resin (Super-Bond C&B, SB, Sun Medical) were used. The titanium surfaces were sanded with silicon-carbide paper, air-abraded with alumina, etched for 30 s, rinsed with water, and then primed. An acrylic rod was bonded to the delineated area (ø5 mm) of each specimen with the SB resin. Shear bond strengths were determined after both 24-hours of water storage (thermocycle 0) and 10,000 thermocycles (4 and 60˚C). The bond strength data was compared by ANOVA and a multiple comparison test (p<0.05). Results: All of the pre-thermocycling groups showed the same level of bond strength. No significant difference was found with respect to 1%AHF-No PA or No AHF-40%PA when compared to No AHF-No PA. After thermocycling, the maximum bond strength of 31.2 (2.8) MPa was obtained for 1%AHF-40%PA, the value of which was statistically the same as that before thermocycling. Conclusion: The present findings suggest the cooperative effect of AHF and PA on resin bonding durability to titanium. Supported in part by a grant from JSPS (B) 14771096.

Table. Mean and standard deviation of shear bond strength (MPa)

Thermocycle	Concentration of AHF and PA in metal etchant
	No AHF-No PA	1%AHF-No PA	No AHF-40%PA	1%AHF-40%PA
0	31.7 (3.8) a	37.7 (2.1) a	36.3 (6.6) a	34.7 (3.9) a
10,000	17.5 (3.7) b	16.0 (8.4) b	18.0 (5.5) b	31.2 (2.8) a

Identical letters indicate no significant difference (p≥0.05) among groups.