Abstract
Objectives
The purpose of this in vitro study was to evaluate the bond strength and durability of adhesive bonding systems to amorphous and crystalline PEKK and fiber-reinforced PEEK using five types of surface conditioning methods.
Methods
One hundred and fifty specimens of each material were conditioned mechanically and chemically, bonded with Multilink Automix to Plexiglas tubes, filled with Multicore Flow, and stored in water at 37 °C for 3, 30 and 150 days. The long-term storage series were thermal cycled between 5 and 55 °C for 10,000 times (30 days) or for 37,500 times (150 days) prior to tensile bond strength test (TBS). Statistical analysis was performed using Kruskal–Wallis and Wilcoxon tests with a Bonferroni–Holm correction for multiple testing ( α = 0.05).
Results
Fiber-reinforced PEEK exhibited higher bond strengths in all five conditioning groups and at all three storage times than crystalline and amorphous PEKK, which showed lowest TBS. Highest TBS was achieved after conditioning with silica coating and priming (Rocatec Soft, Monobond Plus, Luxatemp Glaze & Bond; TBS up to 23.6 MPa).
Significance
The conditioning method has a significant influence to the bond strength of the bonding to the amorphous and crystalline PEKKs and fiber-reinforced PEEKs.
1
Introduction
Polyaryletherketones (PAEKs) are a group of high-performance semicrystalline thermoplastic resins, which family members differ according to their ratio of keto- and ether-groups ( Fig. 1 ). With a higher ratio and sequence of keto groups, the rigidity of the polymer chain and the glass, as well as melting temperature are increasing . Different PAEKs have similar high-quality characteristics such as good dimensional stability at high temperatures (melting temperature is over 300 °C), high chemical and mechanical resistance against wear, and high tensile, fatigue and flexural strengths ( , see Table 1 ). According to the manufacturer (Cendres+Métaux, Switzerland) they are compatible with reinforcing materials such as glass and carbon fibers and can be sterilized with the current methods like the gamma- and steam-sterilization . Their characteristics make PAEKs highly attractive for industrial usage. Therefore, their field of application extends from food, aircraft and automobile industry to medical products.
| Physical and mechanical properties | PEKK (amorphous) | PEKK (crystalline) | PEEK (fiber) |
|---|---|---|---|
| Flexural strength (MPa) | 140 | 200 | 312 |
| Tensile strength (MPa) | 89 | 117 | 215 |
| Glass temperature (°C) | 160 | 157 | 143 |
| Softening temperature range or melting temperature (°C) | 305–325 | 364 | 341 |
In medicine, PEEK, the best-known PAEK member, mainly serves as implantation material due to its mentioned features and good biocompatibility. It has been proven as an adequate alternative for the long-term proven titanium in orthopedic applications . In dentistry, the usage of PEEKs is increasing mainly as temporary implant abutments . First results are available as well for its use as dental clasps and frameworks for partial removable dental prostheses .
Due to its chemical and physical properties, the high-performance thermoplastic PEKK outclasses all the other PAEK materials . The manufacturer (Cendres+Métaux) reports about an up to 80% higher compressive strength of PEKK as compared to the unreinforced PAEK material PEEK. A wider processing window of parameters makes PEKK also especially interesting for the fabrication of crowns and fixed dental prostheses (FDPs). The possibility of producing different rigidity also makes PEKK useful for different applications, e.g. crystalline PEKK for crowns and FDPs, amorphous PEKK for removable prosthesis. However, a clinically adequate bonding to PEKK is a prerequisite for intraoral usage of bonded PEKK restorations.
Up to date, only rare data on bonding to the class of PAEK materials exists. When testing the tensile bond strength (TBS) of differently conditioned PEEK no adequate bonding could be achieved using dental universal composite resin cement in contrast to using an adhesive composite system . Another study showed that the conditioning of PEEK with air-abrasion or silica-coating improved the adhesive properties of PEEK, because the micro-roughness enhances the contact surface with its functional groups between the PEEK and the adhesive. Another recent study tested different primers on PEEK after artificial aging. Only when using a multifunctional methacrylates containing primer (Luxatemp Glaze & Bond), a durable resin bonding was achieved, which therefore was recommended for clinical use. For bonding to PEKK no data could be found in the dental literature.
In view of the limited data available on bonding to PAEK materials, the purpose of the current study was to evaluate different methods for bonding to amorphous and crystalline PEKKs and to fiber-reinforced PEEK. In addition the durability of the achieved bonding should be tested. The null hypothesis was that the bonding method of surface conditioning does not influence the bonding durability and the used methods.
2
Materials and methods
In this study, the tensile bond strength and the durability of adhesive bonding of amorphous and crystalline PEKK and fiber-reinforced PEEK discs (150 specimens each) was tested using a luting resin (Multilink Automix, Ivoclar Vivadent AG, Schaan, Liechtenstein). The bonding surfaces of in total 450 discs, with a diameter of 8 mm and a thickness of at least 3 mm each, were polished with rotating silicon carbide paper (SiC Grinding Paper, Grit P600, Bühler GmbH, Düsseldorf, Germany) under water rinsing.
Specimens of each material (amorphous, crystalline, fiber) were divided into the following five groups with different surface conditioning with 30 specimens each.
| Pre | For surface cleaning and activation air-abrasion with alumina particles (Rocatec Pre, 3M Espe, Seefeld, Germany) at 0.25 MPa for 15 s was used, then cleaning with compressed air for 15 s. Bonding with Multilink Automix without any adhesion promoter served as negative control group to reveal whether the application of adhesion promoters is effective |
| PreLu | Air-abrasion with Rocatec Pre at 0.25 MPa for 15 s was used, then cleaning with compressed air for 15 s. Application of the adhesive primer Luxatemp Glaze & Bond (DMG, Hamburg, Germany) for 20 s, light curing (Elipar™ 2500 Halogen Curing Light, 3M ESPE, Seefeld, Germany) for 20 s. Bonding with Multilink Automix. The use of the adhesive primer on the air-abraded surface served as positive control as it has been shown to be effective on PEEK in a previous study |
| PreLu5 | Air-abrasion with Rocatec Pre at 0.25 MPa for 15 s was used, then cleaning with compressed air for 15 s. Application of the adhesive primer Luxatemp Glaze & Bond with storage under a lightproof box for 5 min, light curing for 20 s. Bonding with Multilink Automix. It was assumed that with a longer residence time of the adhesive primer a better penetration into the surface of the substrates might occur resulting in improved bonding |
| SoMB | Tribochemical silica-coating through air-abrasion with Rocatec Soft (3M Espe, Seefeld, Germany) at 0.25 MPa for 15 s was used, then cleaning with compressed air for 15 s. Application of the silane containing universal primer Monobond Plus (Ivoclar Vivadent AG, Schaan, Liechtenstein) with a residence time of 5 min. Bonding with Multilink Automix. It was assumed that the combination of silica coating and silane application might be an alternative bonding approach especially in combination with a multifunctional primer such as Monobond Plus which might be effective on both silica-coated and silica-free areas assuming that the silica coverage is not complete |
| SoMBLu | Tribochemical silica-coating through air-abrasion with Rocatec Soft at 0.25 MPa for 15 s was used, then cleaning with compressed air for 15 s. Application of the silane containing universal primer Monobond Plus with a residence time of 5 min, then application of the adhesive primer Luxatemp Glaze & Bond, light curing for 20 s. Bonding with Multilink Automix. It was assumed that the additional use of the adhesive primer Luxatemp Glaze & Bond might enhance bonding especially in silica-free areas due to better surface penetration |
Before chemical conditioning, all discs were cleaned ultrasonically for 3 min in isopropanol (99%, Otto Fischar GmbH & Co. KG, Saarbrücken, Germany) and dried with oil- and water-free compressed air. The materials utilized are listed in Table 2 .
| System | Main composition a | Manufacturer | Batch no. |
|---|---|---|---|
| PEKK (amorph) | Polyetherketoneketone, titanium dioxide pigments | Cendres+Métaux, Biel/Bienne, Switzerland | modified PEKK Grade A1 – 09.2009 |
| PEKK (crystalline) | Polyetherketoneketone, titanium dioxide pigments | Cendres+Métaux | Pekkton ivory – 03.2011 |
| PEEK (fiber-reinforced) | Polyaryletherketon, glass fibers, pigments | Cendres+Métaux | PEEK Grade C3 – 02.2009 |
| Multicore flow | Dimethacrylates, inorganic fillers, ytterbium trifluoride, initiators, stabilizers, pigments | Ivoclar Vivadent AG, Schaan, Liechtenstein | N09293 N26726 M43997 |
| Multilink Automix | Hydroxyethyl-, dimethacrylates, inorganic fillers, ytterbium trifluoride, initiators, stabilizers, pigments | Ivoclar Vivadent AG | M55053 N47349 |
| Luxatemp Glaze & Bond | Methacrylates, stabilizer, catalyzer, additives | DMG, Hamburg, Germany | 608356 |
| Monobond Plus | Ethanol, silane, silane-, phosphoric-, sulfidemethacrylates | Ivoclar Vivadent AG | M37209 |
| Liquid strip | Glycerine, silicon dioxide, aluminum oxide | Ivoclar Vivadent AG | M32317 |
2.1
Tensile bond strength testing
Plexiglas tubes with an inner diameter of 3.2 mm were filled with the self-curing luting composite Multicore Flow (Ivoclar Vivadent AG, Schaan, Liechtenstein). After 7 min curing time, tubes were bonded to the conditioned amorphous and crystalline PEKK and fiber-reinforced PEEK discs with the dual-curing luting composite Multilink Automix. Before curing, excess material at the bonding margin was removed with foam pellets. A glycerine gel (Liquid strip, Ivoclar Vivadent AG, Schaan, Liechtenstein) was applied to seal the bonding area in order to prevent an oxygenic inhibition layer. The bonded specimens were first light cured with a dental curing light from two opposite sides and then in a curing unit (Dentacolor XS, Heraeus Kulzer GmBH, Wehrheim, Germany) for additional 90 s for definite curing. To ensure a permanent level of contact pressure for all specimens while bonding, this step was completed with an alignment apparatus under a load of 750 g . Subgroups of ten specimens each were stored in water at 37 °C for 3, 30 and 150 days. In addition, the longer-term storage subgroups were thermal cycled between 5 and 55 °C for 10,000 times (30 days) or for 37,500 times (150 days).
TBS testing was done using a universal testing machine (Zwick Z010/TN2A, Ulm, Germany) with a crosshead speed of 2 mm/min. A collet held the tube while an alignment jig allowed self-centering of the specimen. The jig was attached to the load cell and crosshead by upper and lower chains, allowing the whole system to be self-aligning and providing a moment-free axial force application. The tensile bond strength (N/mm 2 = MPa) was a calculated using the force in N, which was needed to debond the specimen, and the bonding area in mm 2 , which was standardized at 8.04 mm 2 .
2.2
Morphological examination
To calculate the debonded fractured interfaces of the amorphous and crystalline PEKK and fiber-reinforced PEEK discs for adhesive and cohesive failure modes, the debonded area was inspected with a light stereoscope (Wild Makroskop M420, Heerbrugg, Switzerland) at 25× magnification. After sputtering a gold-alloy conductive layer of about 15 nm, representative samples were examined using a scanning electron microscope (SEM, XL 30 CP, Eindhoven, The Netherlands) with a working distance of 10 mm and an acceleration voltage of 15 keV. To measure the surface roughness representative specimens without any priming were examined in the SEM and analyzed with the 3d-scanning microscope (VK-X100K/X200K, Keyence, Germany).
2.3
Statistical analysis
For statistical analysis the Kruskal–Wallis test followed by pair-wise comparison of groups and subgroups with the Wilcoxon test was used. Significance level α was determined at 5% and corrected using the Holm–Bonferroni method.
2
Materials and methods
In this study, the tensile bond strength and the durability of adhesive bonding of amorphous and crystalline PEKK and fiber-reinforced PEEK discs (150 specimens each) was tested using a luting resin (Multilink Automix, Ivoclar Vivadent AG, Schaan, Liechtenstein). The bonding surfaces of in total 450 discs, with a diameter of 8 mm and a thickness of at least 3 mm each, were polished with rotating silicon carbide paper (SiC Grinding Paper, Grit P600, Bühler GmbH, Düsseldorf, Germany) under water rinsing.
Specimens of each material (amorphous, crystalline, fiber) were divided into the following five groups with different surface conditioning with 30 specimens each.
| Pre | For surface cleaning and activation air-abrasion with alumina particles (Rocatec Pre, 3M Espe, Seefeld, Germany) at 0.25 MPa for 15 s was used, then cleaning with compressed air for 15 s. Bonding with Multilink Automix without any adhesion promoter served as negative control group to reveal whether the application of adhesion promoters is effective |
| PreLu | Air-abrasion with Rocatec Pre at 0.25 MPa for 15 s was used, then cleaning with compressed air for 15 s. Application of the adhesive primer Luxatemp Glaze & Bond (DMG, Hamburg, Germany) for 20 s, light curing (Elipar™ 2500 Halogen Curing Light, 3M ESPE, Seefeld, Germany) for 20 s. Bonding with Multilink Automix. The use of the adhesive primer on the air-abraded surface served as positive control as it has been shown to be effective on PEEK in a previous study |
| PreLu5 | Air-abrasion with Rocatec Pre at 0.25 MPa for 15 s was used, then cleaning with compressed air for 15 s. Application of the adhesive primer Luxatemp Glaze & Bond with storage under a lightproof box for 5 min, light curing for 20 s. Bonding with Multilink Automix. It was assumed that with a longer residence time of the adhesive primer a better penetration into the surface of the substrates might occur resulting in improved bonding |
| SoMB | Tribochemical silica-coating through air-abrasion with Rocatec Soft (3M Espe, Seefeld, Germany) at 0.25 MPa for 15 s was used, then cleaning with compressed air for 15 s. Application of the silane containing universal primer Monobond Plus (Ivoclar Vivadent AG, Schaan, Liechtenstein) with a residence time of 5 min. Bonding with Multilink Automix. It was assumed that the combination of silica coating and silane application might be an alternative bonding approach especially in combination with a multifunctional primer such as Monobond Plus which might be effective on both silica-coated and silica-free areas assuming that the silica coverage is not complete |
| SoMBLu | Tribochemical silica-coating through air-abrasion with Rocatec Soft at 0.25 MPa for 15 s was used, then cleaning with compressed air for 15 s. Application of the silane containing universal primer Monobond Plus with a residence time of 5 min, then application of the adhesive primer Luxatemp Glaze & Bond, light curing for 20 s. Bonding with Multilink Automix. It was assumed that the additional use of the adhesive primer Luxatemp Glaze & Bond might enhance bonding especially in silica-free areas due to better surface penetration |
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