|Year : 2009 | Volume
| Issue : 2 | Page : 174-179
|Tensile bond strength of composite luting cements to metal alloys after various surface treatments
Saip Denizoglu1, Cem S Hanyaloglu2, Bunyamin Aksakal3
1 Department of Prosthodontics, Faculty of Dentistry, Yeditepe University, Istanbul, Turkey
2 Department of Mechanical Engineering, Faculty of Engineering, Akdeniz University, Antalya, Turkey
3 Department of Mechanical Education, Faculty of Technical Education, Firat University, Elazig, Turkey
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|Date of Submission||28-Feb-2008|
|Date of Decision||31-Mar-2008|
|Date of Acceptance||21-Jul-2008|
|Date of Web Publication||23-Jun-2009|
| Abstract|| |
Aims: To evaluate the effects of two different surface treatments and bonding agents on tensile bond strength between a Co-Cr and a Ni-Cr cast alloy and two resin-luting cements.
Materials and Methods: Two hundred and forty alloy samples were cast and subjected to surface treatments such as sandblasting, chemical etching, and sandblasting plus chemical etching. Panavia F and CandB cement were used as cementing mediums. The etching qualities were examined by a stereooptic microscope. Failure surfaces were examined throughout scanning electron microscopy. The data were evaluated using statistical methods, namely analysis of variance and multiple comparison test (Tukey HSD).
Results: Significant differences were found in the bonding provided by the various cements (P < 0.001) and also type of surface treatments (P < 0.001). For all groups, sandblasted surfaces showed the highest bond strength values. There was no significant difference between the Cr-Co and the Cr-Ni alloys (P > 0.05).
Conclusions: Panavia F showed higher tensile strength and the sandblasted samples possessed higher tensile strength.
Keywords: Adhesion, metal casting alloys, surface treatment
|How to cite this article:|
Denizoglu S, Hanyaloglu CS, Aksakal B. Tensile bond strength of composite luting cements to metal alloys after various surface treatments. Indian J Dent Res 2009;20:174-9
Resin-bonded castings are commonly used as periodontal splints, post orthodontic splints, fixed partial dentures, extracoronal attachments for removable prostheses, and many other dental applications; however, some problems related to the cementing of metal retainers in clinical applications have been reported elsewhere. ,,,,,
|How to cite this URL:|
Denizoglu S, Hanyaloglu CS, Aksakal B. Tensile bond strength of composite luting cements to metal alloys after various surface treatments. Indian J Dent Res [serial online] 2009 [cited 2013 Dec 10];20:174-9. Available from: http://www.ijdr.in/text.asp?2009/20/2/174/52896
The etched enamel-composite resin bond is a reliable bond.  The surface treatments of enamel by washing away the inorganic matrix from dental structures using orthophosphoric acids was a starting point of development in adhesive dentistry. , The micromorphological, physical, and physiological characteristics of enamel and dentin in combination with the establishment of acid-etch and total-etch techniques have led to major improvements in this field.  Self-etching adhesive systems have produced high tensile bond strengths to human coronal dentin and enamel surfaces. 
Failures that occur at the resin-metal interface may lead to most bonding failures in resin-bonded fixed partial dentures (RBFPD). The factors contributing to this failure can be summarized as the type of adhesive, thickness of the cement, , thickness of retainers,  the nature of the alloy, the treatment of the surfaces, and the stresses undergone by the prosthesis.  Two types of retainer designs are suggested for retention.  First, a macromechanical way with a perforated retainer commonly referred to as the Rochette and, secondly, the micromechanical way, which is derived from the etched cast metal retainer called a Maryland fixed partial denture.  The etching procedure for retaining was first described by Tanaka et al. using an acrylic resin facing.  An electrolytic etching technique for a retentive mechanism, which etches the inner side of the RBFPD, was described. Afterwards, many investigations were carried out for developing etch patterns by changing the etching solution, etching time, and temperature. ,,, However, through recent progresses in material science, various surface treatments with the purpose of increasing tensile bond strength of luting cements to metal surfaces have been introduced, e.g. alumina blasting, tin plating, silicoating. ,,
The purpose of this study was to determine and compare the tensile strength of sandblasted and/or chemically treated alloys bonded with two resin-based luting cements. In order to achieve a good bonding between cement and metal, two different chemical solutions and sandblasting were used to corrode the surface of Cr-Co and Cr-Ni alloys and compared with each other.
| Materials and Methods|| |
Two different beryllium-free metal cast alloys Co-Cr (Wironit, BEGO Bremen, Germany) and Ni-Cr (Remanium CS, Dentaurum, Ispringen, Germany) were used. Sandblasting, chemical etching, and sandblasting plus chemical etching processes were applied to create micromechanic retention cavities on the surfaces of cast metal specimens. Two different types of commercial adhesives were used: Panavia F (resin-based luting cement; Kuraray Medical Inc. Sakazu, Kurashiki, Okayama, Japan) and CandB cement (resin-based luting cement; Bisco, Inc. Schaumburg, IL, USA) for cementing processes. The flow chart of current experimental processes is illustrated in [Figure 1].
Preparation of the holder and the samples
As shown in [Figure 2], a dovetail couple was designed and manufactured as sample holders to be fixed on the tensile testing equipment. The dental base casting model made from bronze alloy was machined to 13 mm diameter and 2 mm thickness. In order to cast 240 specimens, the die cavity was achieved using the silicon-based material (Panasil putty soft and contact plus; Kettenbach Dental, Eschenburg, Germany). Then, the die cavities were filled with melted inlay wax (Cerin; Spofa Dental, Praha, Czech Republic) and 240 standardized items of wax patterns were produced. Finally, the sprues were placed up on those patterns and were invested in phosphate-bonded investment (Deguvest® F Degussa, Wolfgang, Germany). The dies were divided into two main groups: One group for Ni-Cr (60 couples) and one group for Co-Cr (60 couples). Two hundred forty samples (2 × 60 couples) were cast in total using the lost-wax process according to the manufacturer's instruction (Bego, Fornax 35 M, Bremer, Germany).
The cast disc samples were grinded automatically using an automatic grinding and polishing machine (Mecapol, P262; Briι et Angonnes Grenoble, France). In order to get a good polished disc surface, the grinder speed was determined and set to 300 rpm and abrasive silicon carbide papers were used with grid numbers of 80, 320, 400, and 800 in sequence. Each group was also subdivided into groups depending on the surface treatment of the adhesive, as shown in [Figure 1].
Eighty discs (40 Co-Cr and 40 Ni-Cr) were sandblasted using a blasting machine (Bego Minipol, Bremen, Germany). Alumina (Al 2 O 3 ) beads with a mean particle size of 50 µm and 60 psi air pressure were used. The distance between the nozzle and the discs was kept at 5 mm and the blasting time was 120 s. The sandblasted discs were cleaned using detergent under running tap water and then rinsed twice in distilled water and ethanol (99.6%), respectively. The specimen alloys were then finally ultrasonically cleaned in ethanol for 10 min and dried in air.
Etching cast metals by means of chemical etchants is simple, faster than sandblasting, and gives uniformly treated surfaces. Co-Cr dental alloys contain a high volume of Co, thus, possessing good corrosion resistance with suitable mechanical and chemical properties. However, despite difficulties in finding a suitable etchant for such alloys, it was thought that possibly a higher tensile bond strength could be obtained by a combination of surface treatments. Also, it was important to know and show the results and effects of chemical etching for such chosen alloys. Modified Murakami's solution was used for alloy etching. To that end, the chemical etching solution was found and determined by a trial and error fashion and prepared by mixing two solutions with each other. Firstly, solution (A) was prepared, consisting of 12 g of copper ammonia chloride dissolved in 100 ml distilled water. Secondly, solution (B), consisting of 60 g of ferric chloride dissolved in 200 ml hydrochloric acid (HCl) was prepared in our lab. The final etching solution was obtained by pouring the solution (B) into the solution (A) gradually. To determine the optimum temperature-time etching parameter ranging from the temperature of 22 to 70°C for 1-5 min, Co-Cr alloys were tested again by the trial and error method. Finally, the best-etched microstructures were achieved at the temperature of 50°C for an etching time of 2 min.
Although the same etching solution was used for Ni-Cr alloys, the best-etched microstructures were obtained at an ambient temperature and 90 sec of immersion time. The etched discs were rinsed twice in distilled water and ethanol, respectively. Then, the discs were dried in air following ultrasonic treatment in ethanol.
Tensile bond strength testing
Treated surfaces were painted with metal primers. The resin cements were applied uniformly on the forehead of the surface-treated discs in accordance with the specifications of the manufacturers. After applying the cements, a static load of 2 kg was applied for all disc pieces to the upper mounting rod for 5 min to ensure constant pressure distribution. In order to initiate the dual-cure polymerization of Panavia F, a source of halogen ray was used (Translux, Heraeus Kulzer, Hanau, Germany). In the beginning of the polymerization reactions, for the initial curing, a light source was applied on the prepared samples for 40 s from a distance of 2 mm and then was left at room temperature for 24 h for the secondary polymerization phase before the test. The polymerization of self-cure CandB cement was conducted at constant pressure. To provide an adequate bonding reaction, the temperature was kept constant at 35°C throughout 5 min. When Panavia F is used, a coating layer named as Oxyguard II (Kuraray Medical Inc. Sakazu) was applied along open parts of resins to avoid the inhibition during polymerization. In order to provide a complete dual curing of the Panavia F bonding agent, the bonded discs were kept for 24 h in distilled water for CandB cement before tensile tests. To determine the tensile bond strength, 10 specimens from each group were randomly selected and tested using a universal tensile testing equipment (Autograph AG-IS 100 kN; Shimadzu, Kyoto, Japan). The tensile bond strengths were determined by increasing the tensile force gradually with a crosshead speed of 0.5 mm/min until the failure occurred. The data were recorded on a PC, according to the MPa unit system, using the the software (Trapezium, Shimadzu, Kyoto, Japan) provided with the test machine.
Data were analyzed using a statistical package program of SPSS Version 10.0 (SPSS Inc., Chicago IL, USA). Descriptive statistics, including the mean and standard deviation (SD) values were calculated for all variables in each group. Analysis of variance was used to determine if there were significant differences in the measurements between the groups with different adhesion values. When significant differences were present, a Tukey HSD post hoc multiple comparison test was used to determine the differences of the processes.
| Results|| |
The statistical analysis showed that there was a statistically significant interaction between the variables: "adhesive-surface treatment," "alloy-surface treatment," and "adhesive-surface treatment-alloy" [Table 1]. Mean and SDs of data were calculated to the 12 groups indicated in [Figure 3]. Panavia F always showed higher mean tensile bond strength values in comparison with CandB, regardless of the factors "alloys" and "surface treatments." Regarding the effect of surface treatments on tensile bond strengths, sandblasted Co-Cr and Ni-Cr alloys bonded by Panavia F revealed the highest tensile bond strength. As for the Co-Cr alloy bonded by CandB, the sandblasted group indicated lower tensile bond strength compared with the etched and sandblasted + etched groups, although the sandblasted group showed the highest tensile bond strength in the bonding to the Ni-Cr alloy.
The surface treatment procedures such as etching, sandblasting, and sandblasting plus etching were analyzed statistically (Tukey HSD) in terms of tensile bond test results and found to be statistically significant (P < 0.001). The etching and sandblasting-etching processes were statistically significant (P < 0.05) for different bonding stresses, as presented in [Table 2].
[Figure 4] shows a scanning electron microscopy (SEM) profile sample of the failure surface after tensile test for the chemically etched Ni-Cr alloy bonded using Panavia F. It can be clearly observed that the failure occurred in the grey area as marked by the arrow, which is an indication of local failure due to the pulling-out effect of the etched interlayer. As seen from the SEM micrograph, the failure mechanism for the chemically etched surface was observed for both adhesive and cohesive failures. These failures appeared to be uniform on all surfaces due to the bonding agent used and they were described as adhesive failure. According to the results, the sandblasted surfaces for both alloys exhibited higher tensile bond strength.
| Discussion|| |
Adhesive resins generally provide more reliable bonding on etched enamel surfaces than untreated metal surfaces. In the present study, the aim was to find and achieve the best combination of adhesive resin together with surface treatment. Therefore, sandblasting and chemical etching, which are the procedures for increasing contact surface, were conducted to provide a clinically applicable and reliable bonding strength between two metal surfaces.
Even though sandblasting and chemical etching combinations are expected to increase the bond strength, ,,,, in the present study, it was observed that a combination of the two treatments did not provide higher bond strength values. Therefore, overetching might have caused poor bonding. Some more detailed combinations of parameters such as etching type, temperature, and time should also be taken into consideration in future work.
In the present study, the etching solution used was determined in a trial and error fashion. However, the etched alloys revealed lower tensile bond strength in comparison with sandblasted alloys in almost all combinations of adhesive cement and alloys. Therefore, the tensile bond strength might not be decided only by the etching pattern of the alloys. The chemical etching technique might have great advantages, such as high process speed and uniformly treated surfaces. As the highest tensile bond strength was observed in the combination of "CandB"-"Co-Cr"-"sandblasting + etching," a more suitable condition of etching might improve the tensile bond strength in the other combinations. Therefore, more suitable conditions of etching should be decided for each combination of adhesive cements and alloys in the future studies.
The polymerization shrinkage of the luting agents may influence the resin cements in bonding to metal as much as differences of the metal retainers and film thickness of the cement. Beside the ultimate adhesive and cohesive bond strength values of resin composite cement, the polymerization shrinkage and residual stresses are also important factors for bond formation. It was also reported that, except for Panavia EX, the other commercial adhesives showed cohesive failures.  The sandblasting of substrate surfaces increases only micromechanical retention. Watanabe et al.  reported a cohesive failure mode in a significant number of enamel specimens bonded by Panavia EX resin.
In metal-ceramic failures, where the metal becomes exposed, a reliable resin-metal bond is desired for repair. For this, it is necessary to apply surface treatments on the metal and there are numerous reports n the literature regarding this issue. For example, Petridis et al. used Al 2 O 3 blasting with different particle sizes and the highest shear bonding was achieved at 50 microns of particle size.  The micromechanic bonding for all resin systems is achieved easily and quickly by sandblasting.
Neto et al.  also studied the chemical, electrolytic etching, and sandblasting of Ni-Cr alloys and reported that the level of microporosity on the etched alloy surfaces was too high and that non polished surfaces showed deeper etching. The depth of microporosities in electrochemical etching was measured as 3 microns for the chemical etching and 2.5 microns for the sandblasting. As a result, it was pointed out that the etching systems that were used were clinically reliable enough. Although the electrochemical and chemical etching techniques have greatly improved for resin-bonded retainers, several disadvantages have been noted, such as requirement of sensitive procedure and precise estimation of the surface area to be etched as well as controlling voltage and current. In order to provide some possible potential in using chemical etching for future work, the governing processes parameters such as the choice of etchant, etching time, temperature, and pH must be carefully determined and controlled for different alloys.
In the present study, the sandblasted surfaces gave the highest tensile bond strength whereas the remaining combinations of surface treatments surprisingly gave lower tensile bond strength values. This can be explained as weak bonding of the corrosion layer that was formed along the grain boundaries on the substrate surface as a result of the chemical etching. This layer probably caused brittle fracture originating from grain boundaries and fractured easily during tensile tests. Kohli et al.  also reported that sandblasted surfaces of metal alloys presented clinically quite reliable results. In fact, in many papers, it was emphasized that the abraded surfaces had more successful clinical applications. ,,, Although the chemical etching technique has great advantages, such as high process speed and ability to provide uniformly treated surfaces, it requires precise determination and control of the etching solution, temperature, and time and, thus, is a technique-sensitive procedure.
| Conclusion|| |
Within the limitations of the present study, the sandblasting procedure appeared to be more practical in clinical applications. The following conclusions can finally be emphasized:
- A comparison of the surface treatments between sandblasted, etched, and sandblasted + etched surfaces, tensile bond strengths of sandblasted surfaces were higher and statistically significant.
- With the comparison of resin-based luting cements, the bonding performance of Panavia F was higher than CandB.
- Regarding Panavia F, the highest tensile bond strength was obtained by "sandblasting."
- As for CandB, the highest tensile bond strength was obtained by "sandblasting" in the bonding to Ni-Cr alloy. However, the lowest tensile bond strength was observed in the bonding to the Co-Cr alloy by the "sandblasting" procedure.
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Department of Prosthodontics, Faculty of Dentistry, Yeditepe University, Istanbul
[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2]
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