Effect of alumina air-abrasion on mechanical bonding between an acrylic resin and casting alloys.

Abstract

This study examined the effect of alumina air-abrasion with different pressure on bonding between an acrylic resin and casting alloys. Disk specimens (8 and 10 mm in diameter) were cast from a silver-palladium-copper-gold (Ag-Pd-Cu-Au, Castwell M.C.12) alloy and a titanium-aluminum-niobium alloy (Ti-6Al-7Nb, T-Alloy Tough). The disks were air-abraded with alumina particles (50-70 microm) under different air-pressures (0 unabraded, 0.1, and 0.6 MPa). The disk pairs were bonded together with a tri-n-butylborane (TBB)-initiated acrylic resin, and shear bond strengths were determined both before and after thermocycling. Bond strength varied from a maximum of 37.1 MPa to a minimum of 3.6 MPa for the Ag-Pd-Cu-Au alloy, whereas bond strength to Ti-6Al-7Nb alloy ranged from 34.7 MPa to 0.1 MPa. Specimens abraded with 0.6 MPa pressure recorded the greatest post-thermocycling bond strength (21.7 MPa and 17.9 MPa), and unabraded specimens showed the lowest strength (3.6 MPa and 0.1 MPa) for both alloys. Post-thermocycling bond strength to the Ag-Pd-Cu-Au alloy was higher than that to the Ti-6Al-7Nb alloy under identical air-abrading conditions. It can be concluded that alumina air-abrasion with an air-pressure of 0.6 MPa is effective in enhancing retentive characteristics of the TBB-initiated resin joined to the alloys.

Full text

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Abstract:This study examined the effect of alumina
air-abrasion with different pressure on bonding between
an acrylic resin and casting alloys. Disk specimens (8
and 10 mm in diameter) were cast from a silver-
palladium-copper-gold (Ag-Pd-Cu-Au, Castwell
M.C.12) alloy and a titanium-aluminum-niobium alloy
(Ti-6Al-7Nb, T-Alloy Tough). The disks were air-
abraded with alumina particles (50-70 µm) under
different air-pressures (0 unabraded, 0.1, and 0.6 MPa).
The disk pairs were bonded together with a tri-n-
butylborane (TBB)-initiated acrylic resin, and shear
bond strengths were determined both before and after
thermocycling. Bond strength varied from a maximum
of 37.1 MPa to a minimum of 3.6 MPa for the Ag-Pd-
Cu-Au alloy, whereas bond strength to Ti-6Al-7Nb
alloy ranged from 34.7 MPa to 0.1 MPa. Specimens
abraded with 0.6 MPa pressure recorded the greatest
post-thermocycling bond strength (21.7 MPa and 17.9
MPa), and unabraded specimens showed the lowest
strength (3.6 MPa and 0.1 MPa) for both alloys. Post-
thermocycling bond strength to the Ag-Pd-Cu-Au alloy
was higher than that to the Ti-6Al-7Nb alloy under
identical air-abrading conditions. It can be concluded
that alumina air-abrasion with an air-pressure of 0.6
MPa is effective in enhancing retentive characteristics
of the TBB-initiated resin joined to the alloys. (J Oral
Sci 51, 161-166, 2009)
Keywords: abrasion; Ag-Pd-Cu-Au alloy; alumina;
bonding; Ti-6Al-7Nb alloy; resin.
Introduction
The development of laboratory and adhesive techniques
has led to the application of silver-palladium-copper-gold
(Ag-Pd-Cu-Au) alloy, titanium-aluminum-niobium (Ti-6Al-
7Nb) alloy, and other alloys for resin-bonded fixed partial
dentures (RBFPDs) (1-6). The bonding between polymeric
materials and Ag-Pd-Cu-Au alloys has improved by use
of thiol or thione monomers (7,8), whereas bonding to Ti-
6Al-7Nb has improved through the use of primers with
phosphate (9,10).
Before the bonding procedure, the priming systems
require air-abrasion with alumina to mechanically clean
the surfaces and to increase the surface bonding area. It
is therefore necessary to determine the appropriate
conditions for alumina air-abrasion. Although the effects
of alumina abrasion on bond strength have been reported
for cobalt-chromium, nickel-chromium, and gold alloys
as well as titanium (11-15), only limited information is
available about Ag-Pd-Cu-Au and Ti-6Al-7Nb alloys (16).
The purpose of the current study was to evaluate the
influence of air-abrasion with alumina on mechanical
bonding between an acrylic resin and two casting alloys.
Journal of Oral Science, Vol. 51, No. 2, 161-166, 2009
Original
Correspondence to Dr. Hiroyasu Koizumi, Department of Fixed
Prosthodontics, Nihon University School of Dentistry, 1-8-13
Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8310, Japan
Tel: +81-3-3219-8145
Fax: +81-3-3219-8351
E-mail: koizumi@dent.nihon-u.ac.jp
Effect of alumina air-abrasion on mechanical bonding
between an acrylic resin and casting alloys
Takaya Ishii1), Hiroyasu Koizumi2,3), Naomi Tanoue4), Koji Naito1),
Miyuki Yamashita1)and Hideo Matsumura2,3)
1)Division of Applied Oral Sciences, Nihon University Graduate School of Dentistry, Tokyo, Japan
2)Department of Fixed Prosthodontics, Nihon University School of Dentistry, Tokyo, Japan
3)Division of Advanced Dental Treatment, Dental Research Center, Nihon University School of Dentistry,
Tokyo, Japan
4)Nagasaki University Hospital of Medicine and Dentistry, Nagasaki, Japan
(Received 24 December 2008 and accepted 15 January 2009)

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Materials and Methods
An Ag-Pd-Cu-Au alloy (Castwell M.C.12, GC Corp.,
Tokyo, Japan) and Ti-6Al-7Nb alloy (T-Alloy Tough, GC
Corp.) designed for cast restorations and partial denture
frameworks were selected as the substrate materials.
Alumina powder (50 – 70 µm grain size; Hi-Aluminas,
Shofu, Inc., Kyoto, Japan) was used for air-abrasion. An
unfilled acrylic resin material initiated with tri-n-
butylborane derivative (TBB) was employed as the luting
material. The details of the materials used in the experiment
are summarized in Table 1.
A total of 66 paired disk specimens (8 and 10 mm in
diameter; 2.5 mm in thickness) were prepared from the two
alloys, respectively. The Ag-Pd-Cu-Au alloy was cast in
a cristobalite investment material (Cristoquick 20, GC
Corp.) using a high-frequency argon-arc casting machine
(Argoncaster, Shofu, Inc.), whereas the Ti-6Al-7Nb alloy
was cast in a magnesia-based investment material (Selevest
CB, Selec, Osaka, Japan) using a spin-cast centrifugal
apparatus (Ticast Super R, Selec). All disks were sanded
with 800-grit silicon-carbide abrasive paper.
Sixty-six disk pairs were divided into three sets of 22
disk pairs each. Of these, the first set (22 pairs for each
alloy) was not air-abraded with alumina, and considered
as the control group. The second set (22 pairs for each alloy)
was air-abraded with alumina by means of an air-borne
particle abrader (Jet Blast II, J. Morita Corp., Suita, Japan)
for 10 s with 0.1 MPa air-pressure. The remaining set was
also air-abraded with 0.6 MPa air-pressure. The distance
of the orifice from the disk surface was 10 mm. After surface
preparation, a piece of tape with a circular hole, 5 mm in
diameter, was positioned on each 10-mm disk specimen
to define the bond area. The identically treated disk pairs
were bonded together with the TBB-initiated non-adhesive
resin using the brush-dip technique. Twenty-two specimens
were prepared for each surface preparation group. Thirty
minutes after preparation, the specimens were immersed
in 37°C water for 24 h before testing for bond strength.
One-half of the specimens (11 specimens; two alloys;
three surfaces) were subsequently thermocycled in water
between 5°C and 55°C for 10,000 cycles with 1-minute
dwell time per bath (Thermal Shock Tester TTS-1 LM,
Thomas Kagaku Co., Ltd., Tokyo, Japan).
Each specimen was embedded in an acrylic resin mould
and seated in a shear-testing jig. Shear bond strengths
were then determined with a mechanical testing machine
(Type 5567, Instron Corp., Canton, MA, USA) at a
crosshead speed of 0.5 mm/minute. Debonded surfaces
were observed through an optical microscope (8x; SZX9,
Olympus Corp., Tokyo, Japan), and the failure modes
were classified into the following three categories: adhesive
failure, cohesive failure within the luting material, and
combination of adhesive and cohesive failures. Typical
specimens after surface preparation, and before bonding,
were observed with a scanning electron microscope (S-
4300, Hitachi High-Technologies Corp., Tokyo, Japan) with
an accelerating voltage of 15 kV.
The bond strength results were analyzed with software
for statistical analysis (SPSS version 15.0, SPSS, Inc.,
Chicago, IL, USA). Equality of variance of the bond
strengths was primarily analyzed by Levene test and F test.
When the Levene test and F test did not show equality of
variances, Steel-Dwass comparison (Kyplot 4.0, KyensLab,
Tokyo, Japan) was performed to evaluate the effect of
alumina air-abrasion. Mann-Whitney U-test was used to
evaluate the difference in bond strengths between pre-
Table 1 Materials assessed

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and post-thermocycling groups, or the difference in bond
strengths between the two alloys under identical surface
preparation. Significance level was set at 0.05 for all
analyses.
Results
Levene test and F test run on the bond strength results
revealed that several groups did not show equality of
variance. Steel-Dwass test was therefore applied to evaluate
the influence of alumina air-abrasion on bond strength. In
addition, Mann-Whitney U-test was used for comparison
of bond strengths between two different conditions.
Table 2 shows the results of shear bond testing and
statistically categorized groupings. Bond strength varied
from a maximum of 37.1 MPa to a minimum of 3.6 MPa
for the Ag-Pd-Cu-Au alloy. Bond strength to Ti-6Al-7Nb
alloy ranged from 34.7 MPa to 0.1 MPa. Unabraded
specimens showed lower bond strength (categories a, f, c,
and h) than the air-abraded specimens for both alloys.
Among the three abrasion conditions, the specimens
abraded with 0.6 MPa pressure recorded the greatest post-
thermocycling bond strength (21.7 MPa for the Ag-Pd-Au-
Cu alloy and 17.9 MPa for the Ti-6Al-7Nb alloy), and
unabraded specimens showed the lowest strength (3.6
MPa for the Ag-Pd-Au-Cu alloy and 0.1 MPa for the Ti-
6Al-7Nb alloy). Post-thermocycling bond strength to the
Ag-Pd-Cu-Au alloy was significantly higher than that to
the Ti-6Al-7Nb alloy under an identical air-abrading
condition (3.6>0.1, 15.9>10.3, and 21.7>17.9). Bond
strength of all six groups was reduced by thermocycling.
Table 3 shows the mode of failures after shear bond
testing. All of the unabraded specimens showed adhesive
Table 2 Results of shear bond strength in MPa
Table 3 Failure modes after shear bond testing

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failure, whereas many alumina-blasted specimens showed
a combination of adhesive and cohesive failures. Complete
cohesive failure was not observed in any of the specimens
in the current study.
Figure 1 shows the ground surfaces of two alloys.
Scratches generated by the abrasive paper could be seen
in both specimens. Figures 2 and 3 show the alloy surfaces
after air-abrasion. The surfaces of the specimens became
rougher in accordance with air-pressure. The Ag-Pd-Cu-
Au alloy surface abraded with 0.6 MPa pressure was
rougher than the Ti-6Al-7Nb alloy surface abraded with
the same pressure.
Fig. 2 Scanning electron micrographs of alumina-abraded Ag-Pd-Cu-Au alloy with 0.1 MPa (a)
and 0.6 MPa (b) air-pressures (original magnification, ×500).
Fig. 1 Scanning electron micrographs of (a) Ag-Pd-Cu-Au alloy and (b) Ti-6Al-7Nb alloy ground
with #800 SiC abrasive paper (original magnification, ×500).
Fig. 3 Scanning electron micrographs of alumina-abraded Ti-6Al-7Nb alloy with 0.1 MPa (a) and
0.6 MPa (b) air-pressures (original magnification, ×500).

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Discussion
This study compared the influence of different alumina-
abrasion pressures on bonding between an acrylic resin and
two casting alloys. The authors did not use adhesive
functional monomers in order to evaluate the single effect
of alumina air-abrasion.
As shown in Table 1, alumina air-abrasion effectively
enhanced bond strength of the resin material (categories
a and b; f and g). The difference between the two air-
pressure conditions, however, was not significant (categories
b or g) at the pre-thermocycling state. This is probably due
to cohesion of resin material to the roughened alloys
before penetration of water into the adhesive interface. Also,
the result was slightly different from that of a previous paper
reporting effects of alumina air-abrasion on bond strength
to Ti-6Al-7Nb alloy (16). The authors speculate that the
disparity in results could be attributed to the differences
in the bonded material, the distance between the surface
to be abraded and aperture of the air-borne particle abrader
(10 mm in this paper and 20 mm in reference 16), and the
abrasion time period (10 s in this paper and 15 s in reference
16).
After thermocycling, significant differences in bond
strength were found among the three surface conditions.
Specifically, specimens abraded with 0.6 MPa pressure
recorded the highest bond strength (categories e and j),
while unabraded specimens resulted in the lowest bond
strength (categories c and h). The results can be explained
by the difference in surface roughness of the specimens
shown in Figs. 1 – 3. The rougher surface of the Ag-Pd-
Cu-Au alloy as compared with the Ti-6Al-7Nb alloy may
be due to the difference in hardness between the two alloys
(146 VHN for Ag-Pd-Cu-Au alloy and 320 VHN for Ti-
6Al-7Nb alloy). Although the rougher surface might be
more retentive than the smoother surface for the non-
adhesive acrylic resin, clinicians should note that
mechanical properties of Ag-Pd-Cu-Au alloy are inferior
to those of Ti-6Al-7Nb alloy (17), even if Ag-Pd-Cu-Au
alloy has been age-hardened.
Statistical analyses also revealed that post-thermocycling
bond strength of the TBB resin to the Ag-Pd-Cu-Au alloy
was significantly higher than that to the Ti-6Al-7Nb alloy
for all three surfaces (c>h, d>i, and e>j). Since the resin
material did not contain any functional monomer in the
composition, this could be explained by the difference in
surface texture of the alloys. Among the three surface
preparations, Ag-Pd-Cu-Au alloy appeared to be rougher
than Ti-6Al-7Nb alloy when the surface was abraded with
0.6 MPa air-pressure (Figs. 2b and 3b). The difference in
surface roughness between the two alloys is consistent with
the difference in post-thermocycling bond strength reported
here.
Overall, shear bond testing results demonstrated the
effectiveness of alumina air-abrasion for bonding to both
the casting alloys. The question of the bonding durability
of mechano-chemical retention systems remains, and
needs to be evaluated. On the basis of the current study,
the application of alumina air-abrasion with 0.6 MPa
pressure is recommended for bonding the TBB-initiated
acrylic resin to the two casting alloys.
Acknowledgments
This work was supported in part by Grant-in-Aid for
Scientific Research C 20592302 (2008) from the Japan
Society for the Promotion of Science (JSPS), Special
Research Grant for the Development of Distinctive
Education from the Promotion and Mutual Aid Corporation
for Private School of Japan (2008), a grant from Dental
Research Center A (2008), and Sato Fund (2008), Nihon
University School of Dentistry.
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