Effect of acidic primers on bonding between stainless steel and auto-polymerizing methacrylic resins

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Journal of Dentistry, Vol. 25, Nos 34, pp. 285-290, 1997 Copyright 0 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0300&5712/97$17.00+0.00

SO300-5712(96)00023-l

ELSEVIER

Effect of acidic primers on bonding between stainless steel and auto-polymerizing methacrylic resins H. Matsumura, Department

T. Tanaka and M. Atsuta

of Fixed Prosthodontics,

Nagasaki

University School of Dentistry,

1-7-1 Sakamoto,

Nagasaki,

852, Japan

ABSTRACT Objectives: The purpose of this study was to evaluate the effect of acidic primers on bonding between

methacrylic resins and SUS 316 stainless steel. Methods: The primers were single liquid metal conditioners containing either a phosphate monomer (Ceseadopaque primer, CO; Metal primer, MP) or a carboxylic monomer (Super-Bond liquid, SB; Acryl bond, AB; MR bond, MR). Disk metal specimenswere air-abraded with alumina followed by priming. The disks were bonded with a methacrylic resin using a brush-dip technique (Super-Bond C & B, CB or Repairsin, RE). Specimenswere thermocycled in water and bond strengths were determined. Results: Shear bond strengths after the thermocycling were 11.9 MPa for CO-CB, 7.6 MPa for CO-RE, 4.9 MPa for SB-RE, 3.9 MPa for MP-RE, 3.3 MPa for AB-RE, 2.5 MPa for MR-RE, 1.9 MPa for None-CB, and 0 MPa for None-RE. The two systems primed with CO primer showed greater bond strengths than the other groups (P
Adhesive,

J. Dent 1997; 25: 285-290

Bonding, Magnet, Stainless steel (Received

14 November

1995; accepted 23 February

INTRODUCTION Stainless steel is being used for the cap, yoke and keeper components of dental magnetic attachments’. The keeper is usually cast-bonded, brazed, or cemented to the root post in the laboratory, then seated into the root canal as a root cap. The magnetic attachment is embedded into the denture base with auto-polymerizing adhesive resin. It is necessary for the denture that the attachment and the denture base be strongly bonded to avoid detachment of the magnetic component from the denture base. Since the magnetic attachment is covered with stainless steel, adhesive bonding of this attachment Correspondence should be addressed to: Dr H. Matsumura, Department of Fixed Prosthodontics, Nagasaki University School of Dentistry, 1-7-1 Sakamoto, Nagasaki 852, Japan. Tel.: +81-958-497688. Fax: +81-958-49-7689.

1996)

with polymeric material is essentially the same clinical situation as bonding of stainless steel with adhesive resin. Various cases and experiments of adhesive bonding of stainless steel have been reported. Magi’ used a selfcuring adhesive resin containing 4-methacryloyloxyethyl trimellitate anhydride (4-META) to bond stainless steel orthodontic brackets to enamel. Various types of 4-META resins were subsequently used as bonding promoters for SUS 304 stainless steel mesh veneer plates3, stainless steel denture bases4, composite resin veneered primary stainless steel crowns5’6, steel pin for core build-up7, and steel attachment sleeves’. Matsumura et aL9 reported strong bonding of SUS 304 stainless steel using a composite luting agent that contained a phosphate-methacrylate monomer (Panavia opaque, Kuraray Co., Ltd, Osaka, Japan). Panavia

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J, Dent 1997; 25: Nos 3-4

Table /. Primers and auto-polymerizing Trade name Primer Acryl Bond Cesead opaque primer Metal primer MR Bond Super-Bond liquid Resin material Repairsin Super-Bond

C & B

resins investigated

Abbreviation

Manufacturer/trader

Lot No.

Functional

A6 co MP MR SB

Shofu, Inc., Kyoto, Japan Kuraray Co., Ltd, Osaka, Japan G-C International, Chicago, USA Tokuyama America Inc., San Mateo, USA Sun Medical Co., Ltd, Moriyama, Japan

019213 00260 101132 002 21002

4-AET MDP MEPS MAC-10 4-META

RE

G-C International

CB

Sun Medical Co., Ltd

280532 090541 30801 50402 411042

Powder Liquid Powder Liquid Initiator

monomer

4-META

4-AET, 4-acryloyloxyethyl trimellitate; MDP, 1 0-methacryloyloxydecyl dihydrogen phosphate; MEPS, methacryloyloxyalkyl thiophosphate derivative; MAC-l 0, 11 -methacryloyloxyundecan-1 ,l -dicarboxylic acid; 4-META, 4-methacryloyloxyethyl trimellitate anhydride.

material was also used to bond stainless steel6including pins7, attachment sleeve8, primary stainless steel crownslo and orthodontic brackets”. Although various adhesive systemsare being reported to bond stainless steel, limited information is available concerning comparison of bonding agents, especially as related to chemical ingredients or functional monomer?. The purpose of this study was to evaluate the effect of acidic primers on the bond strength and durability of two auto-polymerizing methacrylic resins joined to SUS 316 stainless steel.

MATERIALS

AND METHODS

SUS 3 16 stainless steel (Wago Industrial Ltd, Nagasaki, Japan) was used as the adherend substrate material. According to the manufacturer, the alloy contains 18.0% chromium, 12.0% nickel and 3.0% molybdenum. The SUS 316 steel is the same material as the cap component of a magnetic attachment (Ma&t 600, G-C Corp., Tokyo, Japan)13. Five primers designed for conditioning base metal alloys were investigated, all of which contained either a carboxylic or a phosphoric acid derivative monomer. Two auto-polymerizing methacrylic resins (Super-Bond C & B, Sun Medical Co., Ltd, Moriyama, Japan, and Repairsin, G-C Corp., Tokyo, Japan) were selected as the resin materials. Super-Bond resin consisted of three components: initiator, monomer liquid and powder. The initiator was partially oxidized tri-n-butylborane (TBB). The monomer liquid was 5% 4-META in methyl methacrylate (MMA). The powder was a finely pulverized poly (methyl methacrylate) (PMMA). This material, abbreviated as 4-META/MMA-TBB resin, is usually applied by a brush-dip technique. Repairsin material was a benzoyl peroxide-amine redox initiated MMA-PMMA material designed for repair of acrylic denture bases. Information on the materials is summarized in Table I. One hundred and twenty-eight stainless steel disk specimens 10 mm in diameter by 2.5 mm thick were

Table II. Combinations

of primer and resin material assessed

Group

Primer

Resin material

Abbreviation

1 2 3 4 5 6 7 8

None Acryl Bond Cesead opaque primer Metal primer MR Bond Super-Bond liquid None Cesead opaque primer

Repairsin Repairsin Repairsin Repairsin Repairsin Repairsin Super-Bond Super-Bond

None-RE AB-RE CO-RE MP-RE MR-RE SB-RE None-CB CO-CB

machined from a rod using a low speed cutting instrument with water coolant. All disks were sanded with No. 600 silicon-carbide abrasive paper followed by air-abrading with 50 urn alumina for 10 s (Micro Blaster MB102, Comco Inc., Burbank, USA). The air pressure was 0.5 MPa and the distance of the nozzle from the metal surface was 5 mm. A piece of double coated tape with a 5 mm circular hole was positioned on the surface of each disk to control the area of the bond. The disks were divided into eight combinations of five primers and two resins (Table IZ). In two of these eight combinations no primer was used. Since 4-META/MMA-TBB resin contained a carboxylic monomer (4-META) in the liquid component, carboxylit primers were not used together with the 4-META resin. Sixteen specimens for each combination were prepared. Each primer was applied to the 5 mm diameter area with sponge pellet. A brass ring (6 mm inside diameter by 2 mm deep and with a 1 mm thick wall) was next placed surrounding the opening area. The ring was filled with either Super-Bond material or Repairsin material with a brush-dip technique. Thirty minutes later, all the specimenswere immersed in 37°C water for 24 h. This state was defined as thermocycle 0. Half of the specimens(eight sets of eight specimens)were tested for 24-h shear bond strength at thermocycle 0. The remaining eight setsof eight specimenswere then placed in a thermocycling apparatus (Thermocycling machine,

Matsumura

Shearing load i

Primed area Stainless steel disk

Fig. 1. Assembly

Auto-polymerized resin

used for determination

of shear bond strength

Rika-Kogyo, Hachioji, Japan) and cycled between 4 and 60°C water with a 1 min dwell time per bath for 20,000 cycles. Each specimen was embedded in a resin mould and seated in an ISO/TR 11405 shear testing jig (Fig. I). Shear bond strengths were then determined on a universal testing device (DCS-500, Shimadzu Corp., Kyoto, Japan) at a crosshead speedof 0.5 mm/min. For each set of samples, the mean and standard deviation (S.D.) of eight specimenswere calculated. The values of each group were compared by factorial analysis of variance (ANOVA). When the F-tests were significant, the Duncan new multiple range interval was further performed with the value of statistical significance set at the 0.05 level. Debonded surfaces were observed by means of an optical microscope (SMZ 100, Nikon Corp., Tokyo, Japan) and the location of each failure was recorded as a cohesive failure within the luting agent (C), an adhesive failure at the resin-metal interface (A), or a combination of cohesive and adhesive failures (CA). Some of the specimens after shear testing were dried, sputtercoated with gold (IB-3, Eiko Engineering Co., Ltd, Tokyo, Japan), and observed with a scanning electron microscope (H-520, Hitachi Ltd, Tokyo, Japan) operated at 20 kV.

RESULTS Results of shear testing are summarized in Fig. 2. The three-factor ANOVA of bond strengths showed that thermocycling (F=4937.2, P=O.OOOl)was the variable with the highest F value, followed by type of primer (F=94.2, P=O.OOOl),and then type of resin material (Fy4.9, PzO.0292). All interactions were significant (P
et a/.: Bonding of stainless steel

287

standard deviations, and Duncan groupings are presented in Tables III and 1Y for the pre-thermocycled and thermocycled groups, respectively. Bond strengths of the two resins joined to the stainless steel varied from 21.0 to 46.0 MPa before thermocycling, whereas the values ranged from 0 to 11.9 MPa after 20,000 thermocycles. Bond strengths of all groups were significantly reduced by the application of thermal stress (PcO.05). The difference between primed and unprimed groups indicates that the five primers were effective in elevating the bond strengths of Repairsin material (Table III). The bond strengths of primed and Repairsin bonded groups were divided into four groups. Cesead opaque (CO) primer was effective in bonding both resins joined to the stainless steel. When the metal was unprimed, Super-Bond resin showed greater bond strength as compared with Repairsin material. Table IV summarizes the Duncan grouping of thermocycled specimens. The two groups primed with CO material demonstrated greater bond strength than other groups (Table Iv?. The differences among groups 2, 4 and 6, or among groups 2, 4, 5 and 7 were not significant. Failure modes after shear testing are presented in Table I? The unprimed Repairsin bonded group showed adhesive failure for all specimens. All other groups demonstrated an increased area of adhesive failure due to the application of thermocycling. It was notable that eight specimens conditioned with CO primer exhibited cohesive-adhesive failure after 20,000 thermocycles. Figures 3 and 4 illustrate the debonded surface of a group 8 specimen after thermocycling. A typical combined cohesive-adhesive failure, consisting of a cohesively failed central area surrounded by an adhesively failed area, can be seen in Fig. 3, with a close-up of the interface of the cohesively and adhesively failed area in Fig. 4.

DISCUSSION Stainless steel is categorized as a basemetal alloy. Many types of chemical bonding systemsfor base metal alloys are being introduced and the majority of these systems contain an acidic monomer capable of bonding both enamel and casting alloys. This study evaluated the effect of primers on the bond strength and the durability of two resins joined to stainless steel in relation to the acidic functional monomers contained in the primers. Shear testing results of pre-thermocycling groups indicate that the bond strength of Repairsin material was considerably elevated by the use of primers, whereas improvement of the bond strength of SuperBond material was limited. Although both resins are based on MMA-PMMA auto-polymerizing material, the latter contains 4-META functional monomer in the liquid component. The difference in the effect of

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1997; 25: Nos 3-4

CO-CB None-CB SB-RE MR-RE MP-RE CO-RE AB-RE None-RE 0

5

10

15 20 25 30 35 40 Shear bond strength (MPa)

45

5

Fig. 2. Shear bond strengths of two resins joined to stainless steel. Tab/e 111.Statistical Group

analysis results for shear bond strengths of pre-thermocycling

groups

Primer

Resin material

Mean

S. D. (MPa)

Cesead opaque primer Acryl Bond Metal primer MR Bond Super-Bond liquid Cesead opaque primer None None

Repairsin Repairsin Repairsin Repairsin Repairsin Super-Bond Super-Bond Repairsin

46.0 43.6

3.7 0.9

38.4 36.3 36.0

2.3 1.7 1.4

I I

34.2 31.5 21.0

1.2 1.4 4.3

I

Duncan grouping I I

I I

Vertical lines in the same column indicate that the values are not statistically 0.05 significance level.

different at the

Tab/e IV. Statistical

groups

analysis results for shear bond strengths of thermocycled

Group

Primer

Resin material

Mean

S. D. (MPa)

8 3 6 4 2 5 7 1

Cesead opaque primer Cesead opaque primer Super-Bond liquid Metal primer Acryl Bond MR Bond None None

Super-Bond Repairsin Repairsin Repairsin Repairsin Repairsin Super-Bond Repairsin

11.9 7.6

3.6 3.9

3.9 3.3 4.9 2.5 1.9

1.0 1.4 1.1 0.6 0.6

Vertical lines in the same column indicate that the values are not statistically 0.05 significance level.

priming at pre-thermocycling can therefore be explained according to whether or not the resins contain adhesive monomer. Among the primers used together with Repairsin material, CO primer exhibited the greatest bond strength. The result suggests the effectiveness of a phosphate-methacrylate functional monomer (MDP) contained in the CO primer for conditioning stainless steel. Among the thermocycled groups, two groups conditioned with CO primer exhibited greater bond strengths

Duncan grouping

I I //

different at the

as compared with other groups. Taira et all4 reported on bond strength of titanium using CO primer and Super-Bond resin. Their results showed that the bond strength of Super-Bond resin joined to pure titanium was improved by the application of CO primer. The same result was generated in the present investigation using stainless steel. These findings suggest that bonding between the dihydrogen phosphate group in MDP and the surface of base metal alloys is more stable against hydrolysis or thermal stress as compared with

Matsumura

et a/.: Bonding of stainless steel

289

Table V. Failure modes after shear testing

Group

Primer

‘I 2 3 4 5 6 7 8

None Acryl Bond Cesead opaque primer Metal primer MR Band Super-Bond liquid None Cesead opaque primer

Resin material Repairsin Repairsin Repairsin Repairsin Repairsin Repairsin Super-Bond Super-Bond

Thermocycle C CA

0 A

20000 Cycles C CA A

0 053008 0 0 5 1 0 3

0

8

0

0

8

8 4 3 3 5 5

0 4 0 4 3 0

0 0 0 0 0 0

4 0 0 0 0 8

4 8 8 8 8 0

C, Cohesive failure within the resin material; A, adhesive failure at the metal-resin CA, complex of C and A

interface:

Fig. 3. Scanning electron micrograph of debonded surface of a group 8 (CO-CB) specimen after thermocycling. Cohesively failed area at the center as well as surrounding adhesively failed area can be seen. Arrowheads at the right corner indicate the margin of the resin material.

Fig. 4. Higher magnification of Fig. 3 specimen. Fracture within Super-Bond (CB) resin material can be seen on the left side of photograph, whereas adhesrvely failed resin which can be seen the right side of the photograph represents the impression of alumina-blasted metal surface.

the the on the

bonding between other functional groups and base metals. Thermocycling revealed that the reduction in bond strength after ageing was statistically significant (P~0.05) for all specimens, and the rate of reduction varied from 65.2% (group 8) to 100% (group 1). The reduction rate for metal-to-resin bond strength (group 7, stainless steel-Super-Bond) was 94.0%. The authors determined metal-to-metal bond strength using sandblasted SUS 316 stainless steel and Super-Bond resin to compare durability of the bond of metal-to-resin

bonded specimens and metal-to-metal bonded specimens. Shear bond strengths recorded were 42.0 MPa before thermocycling and 32.6 MPa after thermocycling, hence the reduction rate was 22.4%. These results demonstrate that the reduction rate is obviously greater for metal-to-resin bonded specimens than for metal-to-metal bonded specimens. The results may be attributed to the difference in assembly of specimen. Figure 5 illustrates the metal-toresin bonded specimen used in the present study as well as the metal-to-metal bonded specimen used in the

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Nos 3-4

results also suggest that an appropriate combination of priming agent and methacrylic resin should be selected according to the type of adherend metal alloy used. References

L

1

1. Okuno 0, Ishikawa S, Iimuro FT. et al. Development of sealed cup yoke type dental magnetic attachment. Dent Muter J 1991; 10: 172-184. 2. Mogi M. Study on the application of 4-METAIMMATBB resin to orthodontics. Adhesion to metal. J JOB Ovthodont Sot 1982;41:272-282.

Metal

3.

4,

Metal

Metal

Fig. 5. Comparison of metal-to-resin bonded specimen used in this study and metal-to-metal bonded specimen used in other studies. Arrows indicate the difference in coefficient of thermal expansion between resin and metal.

previously reported study14,15. The former is a model for a magnetic attachment directly bonded to a denture base resin, whereas the latter constitutes a model for a magnetic attachment bonded to the metal framework of denture base. It is easily understandable that the metalto-resin bonded specimen is more severely affected by thermal stress due to the seven to eight-fold difference in the coefficients of thermal expansion between the methacrylic resin (81 x 10e6/“C) and the stainless steel (11 x 10e6/“C)16. Moreover, unlike metal-to-metal bonded specimens, the possibility of water penetration from the resin side will increase for metal-to-resin bonded specimens during the long-term ageing test. Figures 3 and 4 show the scanning electron micrographs of a group 8 specimen after 20,000 thermocycles. The micrographs suggest the debonding of the resin from the metal surface began at the border of the bonded area, then proceeded to the center. It is likely that the stronger the bonding ability of the functional monomer contained in the primer, the slower the propagation of the border of cohesive and adhesive failure. Shear testing results and the findings of scanning electron microscopy suggest that metal-to-resin bonded specimens are affected more severely by thermal stress than metal-to-metal bonded specimens. Hence magnetic attachments should be embedded into metal framework skeletons using adhesive resin, rather than embedded directly into denture base resin, in order to prevent detachment of magnets from the denture base. The

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Salonga JP, Matsumura H, Yasuda K, Yamabe Y. Bond strength of adhesive resin to three nickel-chromium alloys with varying chromium content. J Prosthet Dent 1994; 72: 582-584. S. Illustrated Dictionary of Dentistry. 16. Jablonski Philadelphia: W. B. Saunders Co., 1982; 478 pp.

15.