effect of an acidulated fluoride etchant on bonding between titanium and two luting materials

Effect of an Acidulated Fluoride Etchant on Bonding betweenTitanium and Two Luting Materials

Lei Yang, Yohsuke Taira, Mitsuru Atsuta

Division of Fixed Prosthodontics and Oral Rehabilitation, Nagasaki University Graduate School of Biomedical Sciences,Nagasaki, Japan

Received 1 May 2005; revised 9 August 2005; accepted 15 August 2005Published online 6 December 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.30468

Abstract: Adhesive bonding between resin and titanium is useful for resin-bonded prosthe-ses. The purpose of this study was to investigate the efficacy of an etchant, consisting ofammonium hydrogen fluoride (AHF) and phosphoric acid (PA), in titanium bonding. Castspecimens of commercially pure titanium were air-abraded with alumina and etched for 30 s,after which a primer (ALP) was applied. An acrylic rod was bonded to the specimen with oneof the two luting agents being examined (Super-Bond QUICK and Panavia F2.0). Shear bondstrengths were determined following 10,000 thermocycles. When Panavia F2.0 was applied,neither the etchant nor the ALP primer showed significant effect on bond strength. Thepostthermocycling bond strength of Super-Bond QUICK was significantly improved with theuse of an etchant and ALP primer. Although microscopic observation revealed that consid-erable numbers of submicron pits were created on the specimens etched using AHF with PA,no significant difference in bond strength was detected in the application of AHF, with orwithout PA. The present findings suggested that the improved bonding durability was due tothe micromechanical retention between the resin and the microscopically roughened titaniumsurface. © 2005 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 78B: 161–166, 2006

Keywords: adhesion; metal; surface modification

INTRODUCTION

Titanium is a useful option for fabricating cast restorations1–3

and denture bases4–6 when a patient is allergic to other metalelements.7 Strong and durable bonding between the metalframework and the luting material is essential for applyingtitanium to a resin-bonded-fixed partial denture.8

Modification of the adherend surface prior to bonding cansometimes improve adhesion. High-energy abrasion,9 silica-coating techniques,10–13 and primers14–18 have been reportedas means to modify the adherend surface. In recent dentalbonding systems, acidic primers containing functional mono-mers are applied to the metal surface, following sand-blast-ing. However, an essential problem of detachment has beenclinically observed in resin-bonded fixed partial denture.19,20

In our previous studies,21,22 a chemical etching with 5wt%ammonium hydrogen fluoride (AHF) significantly improvedthe bond strength of the resin to commercially pure titaniumand a titanium alloy. An etchant containing sodium fluoridewith phosphoric acid (PA) has been evaluated as an alterna-tive to sand-blasting.23 Considering safety and handling, theuse of AHF should be limited, as well as that of hydrogen

fluoride. The concentration of these fluorides is also a matterof great concern, with respect to the etching effect.

The purpose of the present study was to determine whetheran etching agent containing AHF and PA improves bondingdurability to commercially pure titanium through shear bondstrength testing. The authors hypothesized that the addition ofPA to a fluoride agent would facilitate the ionization oftitanium surfaces.

MATERIALS AND METHODS

The titanium metal, etchant, primers, and luting agents usedherein are listed in Table I. The eight etchants used in thepresent experiment were six different concentrations of AHF(0, 0.1, 0.5, 1, 5, and 10wt%) without PA, 1wt% AHF with40wt% PA (1AHF-40PA), and 40wt% PA without AHF(0AHF-40PA). A total of 240 disk-shaped specimens, 10 mmin diameter and 2.5 mm in thickness, were embedded in amagnesia-based investment material (Selevest CB; Selec,Osaka, Japan), and then cast with a centrifugal, argon-arccasting apparatus (Ticast Super R; Selec). All disks wereground on a series of silicon-carbide paper (320, 400, and600-grit) followed by air-abrasion (Jet Blast III; Morita, To-kyo, Japan) with 50-�m alumina (Hi-Aluminas; Shofu,Kyoto, Japan) for 15 s. Air-pressure of 0.5 MPa was supplied,

Correspondence to: Y. Taira (e-mail: [email protected])

© 2005 Wiley Periodicals, Inc.

161

and the orifice was positioned at 10 mm from the specimensurface. Subsequently, 3 �L of the etching agent was appliedto the specimen with a micropipette (Eppendorf AG, Ham-burg, Germany) for 30 s, and the surface was rinsed withwater for 15 s, and then air-dried for 5 s. A 50-�m-thick pieceof masking tape, with a circular hole of 5 mm in diameter,was positioned on the surface of each specimen to delineatethe bonding area. Following priming, under finger pressure,an acrylic rod (8 mm in diameter and 5 mm in height) wasbonded to the specimen with the luting agents, according tothe manufacturer’s instructions. In Super-Bond QUICK (SQ),the luting agent was applied to the surface of the specimen,using the brush-on technique. Sixteen groups, including con-trols, were prepared for each respective combination of thefour etching agents, one primer, and two luting agents.

The bonded specimens were stored in an atmosphericenvironment for 60 min, and were then immersed in water at37°C for 24 h. Half of the specimens (40 sets of 6 specimens)were tested for 24-h shear bond strength, and this state wasdefined as thermocycle 0. The other specimens were thermo-cycled for 10,000 cycles between water held at 4 and 60°C,with a dwelling time of 1 min, by means of a thermocyclingapparatus (Rika Kogyo, Tokyo, Japan). The shear bondstrength was determined on a universal testing machine(AGS-10kNG; Shimadzu, Kyoto, Japan) at a crosshead speedof 0.5 mm/min. The means and standard deviations of sixspecimens were calculated for each group. The data wereanalyzed by two-way analysis of variance (ANOVA), corre-sponding to Figure 1 and by three-way ANOVA, correspond-ing to Table III or IV. In each Figure and Table, the meanvalues were compared by post-hoc Tukey Compromise test

(p � 0.05), following one-way ANOVA. The debondedsurfaces of all specimens were observed through an opticalmicroscope (SMZ-10; Nikon, Tokyo, Japan), with a magni-fication of 20�.

Four additional titanium specimens were prepared for mi-crophotographic evaluation. The surfaces were sputter-coatedwith gold (Ion Coater IB-3; Eiko Engineering Co., Mito,Japan) and then observed using a scanning electron micro-scope (S-3500N; Hitachi Corp., Tokyo, Japan) with a mag-nification of 10,000�.

RESULTS

The relation between the concentration of AHF in the etchantand the bond strength of the SQ resin bonded to titanium isshown in Figure 1. The means and standard deviations of thebond strengths before and after thermocycling for 10,000cycles are listed in Tables III and IV, respectively.

Table II shows ANOVA results for shear bond strengthcorresponding to Figure 1 and Tables III and IV. The bondstrength corresponding to Figure 1 was influenced by ther-mocycling, by etchant, and by their interaction [Table II(a)].With the exception of the primer and the luting agent, all ofthe sources of variation corresponding to Table III showed nosignificant effect [Table II(b)]. The bond strength correspond-ing to Table IV was influenced by the etchant, primer, lutingagent, and by the combinations of etchant, primer, and lutingagent [Table II(c)].

Before thermocycling, all groups with SQ exhibited higherbond strengths than Panavia F2.0 (PF; Table III). The two

TABLE I. Etchants, Primers, and Luting Agents Evaluated in the Present Study

Name Component Manufacturer/Trader Lot No.

Titanium metalTitan Ingot JS3 Ti � 99.315, H � 0.015, O � 0.30,

N � 0.07, Fe � 0.30%Selec Co., Osaka, Japan K-9611

Metal etchantExperimental Ammonium hydrogen fluoride (AHF)

� 0, 0.1, 0.5, 1, 5, or 10wt%Wako Pure Chemical

Ind., Osaka, JapanKSJ4437

Phosphoric acid (PA) � 0 or 40wt% Wako Pure Chemical PEF1027Distilled water

PrimerAlloy Primer (ALP) MDP, VTD Kuraray Medical Co.,

Osaka, Japan00124A

Luting agentSuper-Bond QUICK (SQ) Quick monomer: 4-META, MMA,

hydrophilic multifunctional methacrylateSun Medical Co.,

Moriyama, Japan02051

Polymerization initiator: TBB FW63Clear polymer powder: PMMA KL1

Panavia F2.0 (PF) A paste: MDP, methacrylate monomer, filler,photoinitiator, initiator

Kuraray Medical 00002A

B paste brown: methacryalte monomer, filler,NaF, photoinitiator, initiator

00001A

Oxyguard II: polyethylene glycol, accelerator 00449A

MDP, methacryloyloxydecyl dihydrogen phosphate; VTD, 6-(4-vinylbenzyl-n-propyl)amino-1,3,5-triazine-2,4-dithiol-dithione tautomer; 4-META, 4-methacryloyloxyethyltrimellitate anhydride; MMA, methyl methacrylate; TBB, tri-n-butylborane; PMMA, poly(methyl methacrylate).

Journal of Biomedical Materials Research Part B: Applied BiomaterialsDOI 10.1002/jbmb

162 YANG, TAIRA, AND ATSUTA

groups that produced the greatest postthermocycling bondstrength, higher than 26 MPa, were 1AHF-0PA/ALP/SQ and1AHF-40PA/ALP/SQ (Table IV). After thermocycling, thebond strengths of the groups bonded with PF were equivalent.Although no significant difference was found between thegroups 0AHF-0PA/ALP/SQ and 0AHF-0PA/NoPrimer/SQ,the bond strength of 0AHF-40PA/ALP/SQ was significantlyhigher than that of 0AHF-40PA/NoPrimer/SQ. All specimens

tested in this study were observed to fail in mixed adhesiveand cohesive modes.

Representative titanium specimen surfaces, consisting of(a) an alumina-blasted and nonetched control (0AHF-0PA),(b) an alumina-blasted specimen modified with 1AHF-0PA,(c) an alumina-blasted specimen modified with 0AHF-40PA,and (d) an alumina-blasted specimen modified with 1AHF-40PA, are shown in Figure 2. The surfaces of specimens (b)and (d) were obviously roughened when compared with thoseof specimens (a) and (c). Specimen (d) exhibited a greaternumber of submicron pits than do specimen (b).

DISCUSSION

The experiment, of which data was shown in Figure 1, wasperformed to select the concentration of AHF. A previousstudy revealed that etching with 5wt% AHF for 10 s waseffective for improving the bond strength of the resin totitanium surfaces.21 The present results indicate that etchingwith 0.5–10wt% AHF for 30 s is also effective.

The thermocycling test performed in the present studyshould be considered as an in-vitro experiment, under con-trolled thermal stresses in water. With respect to bondingdurability, therefore, no definitive conclusion can be drawnwithout clinical evaluation.

After thermocycling, groups 1AHF-0PA/ALP/SQ and1AHF-40PA/ALP/SQ exhibited greater bond strengths com-pared with the groups 1AHF-0PA/NoPrimer/SQ and 1AHF-40PA/NoPrimer/SQ (Table IV), suggesting that bonding is

Figure 1. The relation between the AHF concentration in the etchantand the bond strength evaluated before and after the thermocyclingfor 10,000 cycles. The titanium surface was modified successively by(1) alumina-blasting, (2) fluoride etchant without PA, and (3) a phos-phate primer (ALP) and was then bonded with an acrylic rod using SQresin. Bond strength values assigned with the same letters indicate nostatistically significant difference, as determined by a Tukey Compro-mise test (p � 0.05).

TABLE II. Results of Analysis of Variance for Shear Bond Strength

Source of Variation df Sum of Squares Mean Squares F-Value p-Value

(a) ANOVA corresponding to Figure 1Thermocycling 1 4121.5 4121.5 136.1 0.0001Etchant 5 760.4 152.1 6.0 0.0007Thermocycling/etchant 5 1082.1 216.4 7.1 0.0001Residual 60 1817.4 30.3

(b) ANOVA corresponding to TABLE IIIEtchant 3 26.4 8.8 0.4 0.7Primer 1 172.9 172.9 8.5 0.005Luting agent 1 15560.6 15560.6 763.9 0.0001Etchant/primer 3 52.6 17.5 0.9 0.5Etchant/luting agent 3 23.6 7.9 0.4 0.8Primer/luting agent 1 4.7 4.7 0.2 0.6Etchant/primer/luting agent 3 6.9 2.3 0.1 1.0Residual 80 1629.7 20.4

(c) ANOVA corresponding to TABLE IVEtchant 3 891.3 297.1 31.9 0.0001Primer 1 1413.4 1413.4 151.6 0.0001Luting agent 1 480.3 480.3 51.5 0.0001Etchant/primer 3 512.9 171.0 18.3 0.0001Etchant/luting agent 3 414.7 138.2 14.8 0.0001Primer/luting agent 1 1119.1 1119.1 120.1 0.0001Etchant/primer/luting agent 3 253.4 84.5 9.1 0.0001Residual 80 745.6 9.3


163FLUORIDE ETCHANT FOR TITANIUM BONDING

facilitated through the phosphate monomer, methacryloy-loxydecyl dihydrogen phosphate (MDP) rather than throughthe carboxylic monomer, 4-methacryloyloxyethyl trimellitateanhydride (4-META). However, no effect was found with theALP primer as long as the PF resin was applied.

Although the PF resin also contained the MDP monomer,the groups using PF showed relatively low bond strengths.This may be due to the difference in polymerization initiatorsystems between PF and SQ. The authors speculate that even

if the monomer diffused into the submicron pits created onthe surface, no micromechanical retention could be obtainedwithout polymerization of resin at the bonded interface. Thepolymerization of the PF resin is initiated with benzoyl per-oxide (BPO), amine, sulfinic acid derivative, and a photoini-tiator. In comparison to the BPO/amine and photoinitiatorsystems, the tri-n-butylborane (TBB) initiator employed inthe SQ resin has the advantage of creating free radicals in thepresence of water molecule or oxygen.24 Accordingly, water

TABLE III. Means and Standard Deviations of Shear Bond Strengths between the Luting Agents and Commercially Pure Titaniumbefore Thermocycling

Group Name

Conc. in Etchant(wt%)

Primer Luting Agent Mean � SD (MPa)AHF PA

0AHF-0PA/NoPrimer/PF 0 0 None PF 11.2 � 1.7a




0AHF-0PA/ALP/PF 0 0 ALP PF 12.0 � 2.7a




0AHF-0PA/NoPrimer/SQ 0 0 None SQ 36.0 � 3.1b




0AHF-0PA/ALP/SQ 0 0 ALP SQ 39.0 � 6.7b




Bond strength values assigned with the same letters indicate no statistically significant difference, as determined by Tukey Compromise test ( p � 0.05).

TABLE IV. Means and Standard Deviations of Shear Bond Strengths between the Luting Agents and Commercially Pure Titaniumafter Thermocycling for 10,000 Cycles

Group Name

Conc. in Etchant(wt%)

Primer Luting Agent Mean � SD (MPa)AHF PA

0AHF-0PA/NoPrimer/PF 0 0 None PF 7.8 � 2.6abc

0AHF-40PA/NoPrimer/PF 0 40 None PF 5.2 � 0.6ab



0AHF-0PA/ALP/PF 0 0 ALP PF 6.5 � 2.2abc

0AHF-40PA/ALP/PF 0 40 ALP PF 5.5 � 1.3ab

1AHF-0PA/ALP/PF 1 0 ALP PF 10.9 � 1.9bc

1AHF-40PA/ALP/PF 1 40 ALP PF 8.2 � 2.4abc

0AHF-0PA/NoPrimer/SQ 0 0 None SQ 3.4 � 1.3a

0AHF-40PA/NoPrimer/SQ 0 40 None SQ 3.5 � 1.1a

1AHF-0PA/NoPrimer/SQ 1 0 None SQ 6.4 � 2.8abc

1AHF-40PA/NoPrimer/SQ 1 40 None SQ 5.0 � 2.0ab

0AHF-0PA/ALP/SQ 0 0 ALP SQ 9.0 � 3.2abc

0AHF-40PA/ALP/SQ 0 40 ALP SQ 11.6 � 3.7c

1AHF-0PA/ALP/SQ 1 0 ALP SQ 26.5 � 5.3d

1AHF-40PA/ALP/SQ 1 40 ALP SQ 29.2 � 6.8d

Bond strength values assigned with the same letters indicate no statistically significant difference, as determined by Tukey Compromise test ( p � 0.05).



remaining on the etched titanium surface might allow thepolymerization of resin at the interface. Furthermore, theTBB-initiated polymerization has characteristics, such thatboth the decrease in residual methyl methacrylate and theincrease in molecular weight poly(methyl methacrylate) wererelatively fast.25

Alumina-blasted titanium surfaces are not made of puretitanium, but rather a contaminated surface containing tita-nium oxides and aluminum oxides.26–28 The corrosion resis-tance of titanium originates from the titanium oxide layer.However, it is considered that fluorides react with the surfaceof the titanium oxide layer and replace the titanium-boundoxygen to form titanium-fluoride compounds.29,30 When

AHF was applied to the titanium specimen in this experiment,bubbles were formed on the titanium surface. This bubblingwas thought to have been due to the oxygen from the titaniumoxide, which suggests that the titanium oxide layer wasmomentarily broken.

The authors speculated that the role of the PA is such thatonce the AHF has broken down the oxide layer on thetitanium surface, the PA ionizes the underlying titaniumsubstance. This may be the reason for the slight differencesobserved between 1AHF-0PA and 1AHF-40PA in the micro-graphs of relatively high magnification. However, it wasimpossible to distinguish 1AHF-0PA from 1AHF-40PA,based on bond strength data.

Figure 2. Scanning electron micrographs (�10,000 original magnification) of titanium specimensurfaces: (a) Air-abraded with alumina. The specimen was scratched with the alumina particles to formrelatively smooth grooves on the surface; (b) Modified with 1wt% AHF after alumina-blasting. Anumber of submicron pits were observed on the grooves; (c) Modified with 40wt% PA after alumina-blasting. The surface texture was similar to that of specimen (a); (d) Modified with 1wt% AHF and40wt% PA after alumina-blasting. Specimen (d) exhibited a greater number of submicron pits thanspecimen (a), (b), or (c).


165FLUORIDE ETCHANT FOR TITANIUM BONDING

In conclusion, the maximum bond strength after thermo-cycling was obtained using a metal etchant containing 1wt%AHF, irrespective of the addition of 40wt% PA. It would beadvantageous for titanium bonding to use these etchants inconjunction with an ALP primer and a TBB-initiated lutingagent.

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effect of an acidulated fluoride etchant on bonding between titanium and two luting materials

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