The impact of non-thermal plasma on the adhesion of polyetherketoneketone (PEKK) to a veneering composite system
Introduction
Over the past years, high-performance polymers known as polyaryletherketones (PAEK) were introduced in the field of prosthodontics as an alternative to traditional metal or zirconia frameworks (Oh et al., 2017). Because of their chemical nature, these materials became attractive in dentistry as they provide high dimensional stability, chemical resistance, and superior strength-to-weight ratio (Kurtz, 2019). Therefore different fixed and removable prosthetic frameworks could be effortlessly constructed from these materials in thinner sections without affecting their strength (Arvai and Schmid, 2014; Najeeb et al., 2016; Dawson et al., 2017).
Polyetherketoneketone (PEKK) is the most recent material among all PAEK used in prosthodontics and has a big potential in this field (Fuhrmann et al., 2014). Although, because of its opaque grey shade, it must be veneered with composite resins for application in anterior region. Like all polymers, PEKK possesses inert hydrophobic surface, which must be treated before bonding with veneering resins to avoid chipping and fracture of veneering materials (Najeeb et al., 2016).
Various surface treatment methods were described in the literature to improve the bonding between PEKK and veneering composites. Fuhrmann et al. were the first to investigate the influence of different air abrasion techniques on the bonding behavior of two commercially available PEKKs (Fuhrmann et al., 2014). The effect of chemical etching with sulfuric and vinyl sulfonic acids in different concentrations on the bonding to PEKK materials was also assessed in the following researches (Lee et al., 2017; Sakihara et al., 2018). The inconclusive results of these studies prompt to search for alternative surface treatment methods that could provide consistently better bonding.
Generally, the plasma surface treatment was suggested as an effective method for treating different polymer surfaces (Comyn et al., 1996; Grace and Gerenser, 2003). In dentistry the plasma was initially introduced for cleaning and sterilization of dental instruments, root canal disinfection in endodontic treatment, teeth bleaching (Cha and Park, 2014). A significantly positive impact of air, oxygen, and argon plasma treatment on the bonding strength between polyetheretherketone (PEEK) materials and veneering composites was reported in a row of studies (Stawarczyk et al., 2013, Stawarczyk et al., 2014; Schwitalla et al., 2017; Bötel et al., 2018; Younis et al., 2019). Nevertheless, in contrast to PEEK, an extra ketone group in PEKK makes this material more resistant to the surface treatment and little evidence is provided about its surface treatment methods. The cold plasma bombardment of the PEKK may provide a greater adhesion to the resin cements (Labriaga et al., 2018). To authors’ best knowledge the influence of surface treatment with various plasma types on the bonding between PEKK and veneering composites was not compared yet.
In the literature, shear and tensile tests are used mutually to assess the bond strength between PEKK and veneering composite resins or resin cements (Stawarczyk et al., 2013, Stawarczyk et al., 2014; Fuhrmann et al., 2014; Lee et al., 2017; Sakihara et al., 2018; Fokas et al., 2019). Even though, tensile tests show more uniform and homogenous stress distribution, the specimens preparation technique is more sensitive and time-consuming in comparison to shear bond test (Sirisha et al., 2014).
Therefore, the first aim of the present study was to propose, which test among shear and tensile tests suits better for a bond strength analysis of veneering composites to PEKK. The second aim was to quantify the influence of sandblasting, adhesive application and various gaseous plasma treatment on the bonding of veneering composites to PEKK. It was hypothesized that there would be no significant differences between air, argon, oxygen, nitrogen and acetylene plasma types after thermocycling.
Fig. 1 depicts the workflow of this study composed of 2 tests.
PEKK specimens (n = 240) with the following dimensions 20 mm × 10 mm x 2 mm were designed in Solidworks software (Solidworks Corp, Massachusetts, USA) then exported in STL format and milled of PEKK CAD/CAM blocks (Pekkton ivory, Cendres + Métaux, Biel-Bienne, Switzerland). Afterwards, all milled specimens were polished using abrasive papers P180 followed by P320 (Buehler UK LTD, Coventry, England). Subsequently, all specimens were cleaned in an ultrasonic bath containing 70% ethanol for 20 min using an ultrasonic device (Sonorex, Bandelin, Berlin, Germany) then left to dry in the air prior to surface treatment.
For the surface treatment of the milled specimens with different plasmas, special jigs were designed using Siemens NX 10.0 CAD software (Siemens PLM, Texas, USA) following ISO 10477. Two different jigs were constructed. Seating jigs, where specimens were seated on and bonding jigs with dimensions 20 mm × 10 mm x 2.5 mm. With these jigs the bonding steps were made through a hole in its center with a diameter of 5 mm. The seating jigs were printed with Fused Filament Fabrication (FFF) technology (MakerBot Replicator+, MakerBot Industries, NY, USA), and the bonding jigs were printed using stereolithography (SLA) (Form2, Formlabs, Somerville, Massachusetts, USA).
All materials and machines used for the plasma treatment, sandblasting and bonding are listed in Table 1.
80 specimens were divided into four groups. Two control groups were made for both shear and tensile tests (ntest1_contr_shear = 20, ntest1_contr_tensile = 20), in which the direct bonding with a veneering composite (Sinfony, 3 M ESPE AG, Seefeld, Germany) was performed without any surface treatment. While in the other two groups, the specimens were treated with low-pressure cold argon plasma DENTA PLASPC (Diener electronic GmbH, Ebhausen, Germany) (ntest1_AR_shear = 20, ntest1_Ar_tensile = 20) according to manufacturer instructions with the following settings of surface treatment: 10 min treatment duration at pressure of 0.3 mbar, temperature of 20 °C, frequency of 40 kHz and power output of 100 W.
160 specimens were sorted randomly, comprising the following 8 groups of 20 specimens each: Control (untreated surface) (ntest2_contr = 20), sandblasting (ntest2_sandbl = 20), adhesive (application of adhesive without prior plasma treatment) (ntest2_adh = 20) and 5 different gaseous cold plasma treatment groups including Argon (ntest2_argon = 20), Oxygen (ntest2_oxygen = 20), nitrogen (ntest2_nitrogen = 20), air (ntest2_air = 20) and acetylene (ntest2_acetyelen = 20).
In the sandblasting group the specimens were treated with 110 μm Al2O3 (Rocatec Pre, 3 M Espe, Seefeld, Germany) following by tribochemical coating using 110 μm silica-modified Al2O3 (Rocatec Plus, 3 M Espe, Seefeld, Germany). Both were done at a pressure of 2.8 bar, with vertical irradiation at a beam distance of 1 cm and a beam time of 15 s per 1 cm2 area. Afterwards the silane (Rocatec Sil, 3 M Espe, Seefeld, Germany) was applied with microbrush for conditioning the surface and left dry for 5 min.
In the adhesive group the adhesive (Visiolink, Bredent, Senden, Germany) was applied with a microbrush on the surface of specimens through the hole in the 3D printed jig. The specimens were immediately cured in a light-curing unit (Speed Labolight, HAGER &WERKEN GmbH &Co.KG, Duisburg, Germany) with wavelengths ranging from 320 to 550 nm for 90 s.
In the plasma group the specimens were treated with cold plasma utilizing corresponding feeding gases for 10 min at the pressure of 0.3 mbar, temperature of 20 °C, frequency of 40 kHz and power output of 100 W.
Following the surface treatment, all specimens were put into the printed jigs. With the use of printed jigs, bonding procedures were performed to produce composite button of 2 mm thickness and 5 mm diameter. First, the opaquer was applied in powder-liquid ratio of 1:1 with clean disposable brushes and was light cured for 10 s using a halogen curing unit with 400–515 nm wavelength. Thereafter the first composite layer of 1 mm was applied through the jig directly from the dispenser and cured for 5 s (ELipar Trilight, 3 M Espe, Seefeld, Germany). The second composite layer was applied to fill the entire jig hole and cure again intermediately for another 5 s. Afterwards then specimens were delicately detached from the jigs and undergone the final curing for 1 min of light exposure without vacuum and 14 min light curing with vacuum (Visia Beta Vario, 3 M Espe, Seefeld, Germany) (Fig. 2).
All veneered specimens (ntest1, ntest2) were subjected to artificial aging by using Thermocycler (SD Mechatronik, Feldkirchen-Westerham, Germany). Following the ISO 10477, the specimens were exposed to thermal stresses by immersing them in distilled water for 5000 cycles at two temperatures 5 °C (±1) and 55 °C (±1). The specimens were kept immersed in water in each bath for 30 s with a dwell time of 20 s in between both temperatures.
The specimens were placed in a holding device, which was installed in an universal testing machine. This holding device is designed to ensure axial force application without any momentum. The specimens were pulled apart axially from bonding jigs through an upper chain, while the system is self-centered the whole procedure to purely tensile stresses. The crosshead had a constant speed of 1 mm/min until debonding or fracture of veneering.
The SBS test was performed using the same universal testing machine (Z010 Zwick Ulm, Germany). For shear testing a customized specimen holder was made, through which specimens were firmly fixed throughout the testing procedure. A chisel-shaped rod applied force constantly parallel to the bond surface at a distance of 0,5 ± 0,02 mm from the surface of the PEKK specimen with a crosshead speed of 1 mm/min and starting from a 0-N load, which increased gradually until fracture of the veneering composite. Then, the maximum force at fracture was detected.
The TBS and SBS were calculated according to the following equation:where σ is the TBS or SBS, F is the load applied in Newtons, and A is the bonded area in mm2. Specimens that did not survive the aging test and showed premature debonding of veneering composites during thermocycling were assigned 0 MPa and considered as pre-failures. Fig. 3 represents a schematic illustration of shear and bond testing.
Following shear bond testing, the specimens were examined under microscope Wild M400 photomacroscope (Wild Heerbrugg, Gais-Switzerland) to determine the type of failure as follows:
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adhesive failure which means no resin remnants were left on the PEKK surface.
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cohesive failure where failure is in the bulk layer of the resin.
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mixed where resin remnants partially left on PEKK surface exposed.
The surface roughness of a specimen from each group was measured using Perthometer S6P (Mahr, Göttingen, Germany). This device had a needle with a 2 μm diamond tip, which allowed two-dimensional tracing of a given surface. The stylus traversed the center of each specimens surface at a constant speed of 0.5 mm/s in an area of 3 mm length and width. 121 measurement lines with 25 μm distance between the lines were performed. In the MountainsMap Universal 7.3 software (Digital Surf, Besançon, France) the surface roughness (Ra) was calculated.
The surface topography of one specimen per group was further analyzed by using Zeiss LEO 1430 microscope (Zeiss, Oberkochen, Germany) at ×500 and ×5000 magnification.
All gathered data was analyzed with JMP 13.1 software package (SAS Corp. Heidelberg, Germany). The measurements were tested for normality of distribution by goodness of fit with Kolmogorov test using p < 0.05. The bond strength data was distributed normally, therefore ANOVA and Tukey-Kramer post-hoc test were applied on the alpha level of α = 0.05. As the non-normal distribution was revealed for the surface roughness measurements, the Wilcoxon test was applied to determine the statistical significance. The pearson correlation was calculated between the Ra and SBS values (see Table 3).
Section snippets
Comparison of shear and tensile bond strength tests
In shear bond test, argon plasma treated specimens showed significant higher mean bond strength value of 8.14 ± 1.70 MPa than control group (5.83 ± 1.42 MPa). In tensile bond testing, argon plasma treated specimens showed significant higher mean bond strength value of 1.50 ± 0.51 MPa than control group (0.58 ± 0.50 MPa) (Fig. 4). The shear test showed higher bond strength values for both argon plasma and control groups than tensile test.
Comparison of different surface treatment types on shear bond strength of o composite resins to PEKK
Rocatec group showed the highest mean bond strength value
Discussion
Up to the present time, in-vitro bond strength tests have been used to assess the influence of various surface treatment methods on the adhesion capacities of dental substrate polymers and anticipate their clinical performance (Kantheti Sirisha and Ravikumar, 2014; Keul et al., 2014; Sirisha et al., 2014). Such studies may be helpful to gather data about material properties and to spot the possible reasons for failures, but still can not replace clinical trials simply because of presence of
Conclusion
Within the limitations of this in-vitro study, the plasma surface treatment can be proposed as a valuable surface treatment option to provide sufficient bonding of dental PAEK to veneering composites. Herein, the PEKK surface treatment with acetylene plasma can provide bonding strength on the same level to sandblasting and solely adhesive application and may be for this reason preferable to argon, nitrogen, air, and oxygen feeding gases. Whereas, the oxygen plasma may have no positive impact on
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The study was financed completely from the university's budget.
Acknowledgement
The authors thank Prof. Dr. J. Geis-Gerstorfer for his scientific guidance during this study. Katharina Brenner is acknowledged for her technical consultancy during this study, and Diener electronic GmbH + Co. KG for providing the device.
References (47)
- et al.
The effect of different surface treatments on the shear bond strength of luting cements to titanium
J. Prosthet. Dent.
(2012) - et al.
The effect of ethanol on surface of semi-interpenetrating polymer network (IPN) polymer matrix of glass-fibre reinforced composite
J. Mech. Behav. Biomed. Mater.
(2019) - et al.
Influence of different low-pressure plasma process parameters on shear bond strength between veneering composites and PEEK materials
Dent. Mater.
(2018) - et al.
Plasma in dentistry
Clin. Plasma Med.
(2014) - et al.
Plasma-treatment of polyetheretherketone (PEEK) for adhesive bonding
Int. J. Adhesion Adhes.
(1996) - et al.
The effects of surface treatments on tensile bond strength of polyether-ketone-ketone (PEKK) to veneering resin
J. Mech. Behav. Biomed. Mater.
(2019) - et al.
Resin bonding to three types of polyaryletherketones (PAEKs) - durability and influence of surface conditioning
Dent. Mater.
(2014) - et al.
Influence of surface conditioning on bonding to polyetheretherketon (PEEK)
Dent. Mater.
(2012) - et al.
Aspects of silane coupling agents and surface conditioning in dentistry: an overview
Dent. Mater.
(2012) - et al.
Acetylene plasma polymer treated by atmospheric dielectric barrier discharge
Vacuum
(2014)
Applications of polyetheretherketone (PEEK) in oral implantology and prosthodontics
J. Prosthodont Res.
Effect of composition, viscosity and thickness of the opaquer on the adhesion of resin composite to titanium
Dent. Mater.
Spotlight on bond strength testing - unraveling the complexities
Dent. Mater.
Glycine: a potential coupling agent to bond to helium plasma treated PEEK?
Dent. Mater.
The impact of argon/oxygen low-pressure plasma on shear bond strength between a veneering composite and different PEEK materials
Dent. Mater.
Modification of polymers by plasma treatment and by plasma polymerization
Radiat. Phys. Chem.
Autohesion of plasma treated semi-crystalline PEEK: comparative study of argon, nitrogen and oxygen treatments
Colloid. Surf.
Das neue Hochleistungspolymer Pekkton ® ivory – eine Werkstoffklasse
Quintessenz Zahntech
Polyetherketoneketone (PEKK), a framework material for complete fixed and removable dental prostheses: a clinical report
J. Prosthet. Dent.
Analysis and comparison of different bond strength tests
JSM Dent.
Ultra-thin plasma polymer films as a novel class of adhesion promoters: a critical review
Rev. Adhes. Adhes.
Bonding between CAD/CAM resin and resin composite cements dependent on bonding agents: three different in vitro test methods
Clini. Oral Investig.
Plasma treatment of polymers
J. Dispersion Sci. Technol.
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