Surface modification of feldspar porcelain by corona discharge and its effect on bonding to resin cement with silane coupling agent

journal of the mechanical behavior of biomedical materials 105 (2020) 103708

Contents lists available at ScienceDirect

Journal of the Mechanical Behavior of Biomedical Materials journal homepage: http://www.elsevier.com/locate/jmbbm

Surface modification of feldspar porcelain by corona discharge and its effect on bonding to resin cement with silane coupling agent Yuya Komagata a, Hiroshi Ikeda a, *, Yuki Fujio b, Yuki Nagamatsu a, Hiroshi Shimizu a a b

Division of Biomaterials, Department of Oral Functions, Kyushu Dental University, Fukuoka, 803-8580, Japan Advanced Manufacturing Research Institute (AMRI), National Institute of Advanced Industrial Science and Technology (AIST), Saga, 841-0052, Japan

A R T I C L E I N F O

A B S T R A C T

Keywords: Dental ceramic Plasma Corona Hydrophilicity Adhesion Silane Silanol

This study aims to develop a corona discharge process for a surface treating a glass-ceramic, feldspar porcelain, to improve its bonding to a resin cement with a silane-coupling agent. Corona discharge, a type of plasma process, was performed using a custom-made device on a porcelain surface at temperatures ranging from 25 to 300 � C, for specific treatment times in air. The porcelain was then subjected to a post-heat-treatment at 600 � C to condition the surface state. The resulting surface was primed with a silane-coupling-agent followed by cementing using a resin cement to measure the shear bond strength (SBS). To investigate the effect of surface modifications by the corona discharge treatment, the porcelain was characterized by surface roughness, contact angle, and an X-ray photoelectron spectroscopy analyses. The SBS for the corona-discharge-treated porcelain increased with an in­ crease in treatment-temperature and -time, and reached the maximum value at 200 � C and 5 min. The post-heattreatment improved the bond durability after thermocycling. The SBS for the corona-discharge-treated porcelain was then compared to that of a conventional hydrofluoric-acid-treated one, which showed that the SBSs were comparable. The results of the surface characterizations indicated that the corona discharge treatment generated silanol groups on the porcelain surface giving hydrophilic properties without roughening the surface. It was found that the corona discharge treatment generates silanol groups on the porcelain surface, resulting in an increased SBS. This study is the first to demonstrate that corona discharge treatment is effective for improving bond strength through the modification of the surface of glass-ceramics.

1. Introduction Glass-ceramics have been widely used in dental crown restorations owing to their high mechanical strength, chemical and physical dura­ bility, excellent aesthetic properties, and biosafety (Denry and Hollo­ way, 2010; Li et al., 2014; Zarone et al., 2016). In recent years, various glass-ceramics, based mostly on inorganic amorphous matrices with dispersed crystalline phases, have been developed for practical clinical use. Popular dental glass-ceramics are feldspar porcelain, which is composed of alkali-alumino-silicate glass containing leucite crystal; leucite-reinforced feldspar porcelain, which contains a high amount of leucite crystals; and lithium disilicate glass, which is composed of a lithium-silicate glass matrix with many interlocking plate-like crystals. The standard protocol for the use of glass-ceramics for dental restoration is to first etch the surface of the material with hydrofluoric acid (HF), followed by the application of a silane-primer containing a silane-coupling-agent (Lung and Matinlinna, 2012; Tian et al., 2014).

These surface treatments, which are performed before bonding, are indispensable in the improvement of long-term bond durability in an oral environment (Brentel et al., 2007; Tian et al., 2014; Van den Breemer et al., 2015). The HF treatment roughens the surface of the glass-ceramic due to the etching of glass phases, thereby resulting in an increased surface area and triggering the mechanical interlocking of the glass-ceramic and adhesive (Tian et al., 2014). However, the HF reagent is toxic and hazardous, and hence, a safer method is required for the surface treatment of glass-ceramic. Considerable effort has been devoted to developing alternative pro­ cesses for surface treatment (Lung and Matinlinna, 2012; Matinlinna et al., 2018). For instance, alumina air-abrasion (Burrow et al., 2004; Lu et al., 2001), tribochemical silica coating via blasting (Matinlinna and Vallittu, 2007; Ozcan et al., 2001; Shimakura et al., 2007), laser irra­ diation (Akyil et al., 2011; Ersu et al., 2009), and plasma treatment (Cha and Park, 2014; Ito et al., 2016; Kim et al., 2014; Liebermann et al., 2013; Liu et al., 2016; Nishigawa et al., 2003; Ranjan et al., 2017;

* Corresponding author. E-mail address: [email protected] (H. Ikeda). https://doi.org/10.1016/j.jmbbm.2020.103708 Received 31 December 2019; Received in revised form 14 February 2020; Accepted 17 February 2020 Available online 18 February 2020 1751-6161/© 2020 Elsevier Ltd. All rights reserved.

Y. Komagata et al.

Journal of the Mechanical Behavior of Biomedical Materials 105 (2020) 103708

Valverde et al., 2013) have been extensively studied to improve the bond strength between the ceramics and adhesives. In particular, plasma treatments have been receiving significant attention due to their unique surface modification effects on various materials. Recently, multiple studies that involve the use of various thermal and non-thermal plasmas at low-pressure and atmospheric-pressure and with different gases have €keliler been conducted to improve the bonding not only of ceramics (Ço et al., 2008; Dos Santos et al., 2016; Elias et al., 2019; Hallmann et al., 2016; Ito et al., 2016; Liu et al., 2018; Marcos Daniel Septímio Lanza et al., 2018; Park et al., 2017; Shimizu et al., 2018; Valverde et al., 2013; Vechiato Filho et al., 2014; Vilas Boas Fernandes Junior et al., 2018), but also of teeth (Ritts et al., 2010), polymethyl methacrylate (PMMA) (Liebermann et al., 2013; Nishigawa et al., 2003, 2004), and cobalt-chromium alloy (Maruo et al., 2004). In previous studies, con­ flicting results have been reported regarding whether plasma treatments are effective in improving the bonding of ceramics to adhesives. For instance, argon plasma improved the bond strength between zirconia and resin cement (Elias et al., 2019; Vilas Boas Fernandes Junior et al., 2018), and oxygen plasma increased the bonding of porcelain to resin €keliler et al., 2008). In contrast, argon plasma did not cement (Ço improve the bond strength between zirconia and resin cement (Hall­ mann et al., 2016), and oxygen plasma adversely affected the bond strength of porcelain and resin cement (Liu et al., 2018). Thus, it is considered that the effect of plasma treatments on dental ceramics de­ pends on various factors such as the type of material, plasma species, ambient conditions, among others. However, this is still an open research problem. In order to understand the effects of plasma treat­ ments, it is necessary to examine, in detail, experiments focusing on specific materials and plasmas. Based on the theory of adhesion (Mat­ inlinna et al., 2018), the glass-ceramic can be bonded to a resin cement through a silane-coupling-agent by reacting silanol groups on the glass-ceramic surface and in the silane-coupling-agent. Therefore, con­ trol over the surface silanol groups is considered indispensable for improving the bond strength. In order to achieve desirable control over the surface silanol groups on the glass-ceramics, the present study considers corona discharge, which can efficiently increase the number of silanol groups on the sur­ face of glass (Ikeda et al., 2013). Corona discharge, a type of plasma process, is a non-disruptive electrical discharge created by applying a high DC voltage between needle and plate electrodes (Adamiak and Atten, 2004; Chang et al., 1991). Owing to their high potential for chemical alternation, corona discharge processes have been widely used in industry for the surface treatment of ceramics and polymer materials (Giacometti et al., 1995; Jung et al., 2007; Kawaguchi et al., 2015; Pruneri et al., 1999; Sakai et al., 2007; Smith et al., 2012). In an earlier study focusing on the surface treatment of alkali-silicate glass, corona discharge was found to be effective in increasing the number of silanol (Si–OH) groups on the surface layer of glass (Ikeda et al., 2013; Kawa­ guchi et al., 2014, 2015). From these findings, it can be speculated that the silanol groups generated by the corona discharge increase the bond strength between the glass-ceramic and resin cement through the silane-coupling-agent. Corona discharge is therefore considered to be suitable for the surface treatment of glass-ceramics containing large quantities of silica. Herein, we used feldspar porcelain as a glass-ceramic sample as it has a relatively large amount of silica, which contains silanol groups. This study aims to develop a corona discharge process that improves the bonding between glass-ceramic and resin cement, and further elu­ cidates the surface modification effects. A pretreatment protocol was first established for the corona discharge process using a custom-made device. In this preliminary step, the influence of corona discharge on the surface modification of porcelain and its effects on the bonding to a resin cement with silane-primer were examined. Furthermore, a postheat-treatment was conducted after the corona discharge process to further improve the shear bond strength (SBS) because heat-treatment is a technique used to condition the silanol groups on the silica surface

(Zhuravlev, 2000). The combination of the corona discharge and post-heat-treatment was expected to be a suitable process for condi­ tioning the surface silanol groups on the porcelain. The SBS between the corona-discharge-treated porcelain and the resin cement was then compared with that of conventional HF to demonstrate the effectiveness of corona discharge in the pretreatment of glass-ceramics. The surface modification effects of both the corona discharge process and post-heat-treatment on the porcelain surface were evaluated and discussed. 2. Materials and methods 2.1. Materials Table 1 lists the materials used in the present experiments. A CAD/ CAM block of the feldspar porcelain (Vitabloc Mark II) was cut into plates of 1 � 0.1 mm thickness using a diamond wheel saw. The cutplates were polished by SiC emery papers up to # 1000 and cleaned by ultrasonication for 5 min in distilled water, and were subsequently dried by blowing air. The resultant plates were used for the following experiments. 2.2. Surface treatments 2.2.1. Corona discharge treatment The corona discharge treatment was performed using the custommade device, and the device configuration is illustrated in Fig. 1. This device was developed by partially modifying the device reported in our previous study (Ikeda et al., 2013). A stainless-steel plate and a tungsten needle were used for cathode- and anode-electrodes, respectively. The cathode-plate was fixed on a ceramic-heater and connected to the ground. The anode-needle was connected to a direct-current power supply (HJPQ-10P3, Matsusada precision Inc.). The distance between the cathode-plate and the tip of the anode-needle was set to 5 mm. A data logger was used to monitor the applied voltage and current. The corona discharge treatment was performed on the porcelain sample using the following procedure. The porcelain was put on the cathode-plate and the temperature of the plate was risen to the given temperatures of 25 � C, 100 � C, 200 � C, and 300 � C in air atmosphere Table 1 List of materials used. Product name

Type

Manufacturer

Composition

Vitablocs MarkII

Glass-ceramic (feldspar porcelain)

VITA Zahnfabrik H. Rauter GmhH & Co.KG, Germany

Porcelain Primer

Silane-couplingagent (silane primer) Hydrofluoric acid Dual-cured resin cement

Shofu Inc., Japan

Silicon dioxide 56–64%, Aluminum oxide 20–23%, Sodium oxide 9–11%, Potassium oxide 6–8%, Calcium oxide 0.3–0.6%, Titanium dioxide 0.0–0.1%a Ethanol, γ-MPTS, etc.

Porcelain Etchant ResiCem PASTE

Bisco Inc., U.S.A. Shofu Inc., Japan

9.5% buffered hydrofluoric acid gel Paste A: UDMA, TEGDMA, Fluoroaluminosilicate-glass, Initiator, etc. Paste B: UDMA, TEGDMA, Fluoro-aluminosilicateglass, 4-AET, HEMA, Initiator, etc.

γ-MPTS: 3-methacryloxypropyl trimethoxy silane, UDMA: Urethane dimetha­ crylate, TEGDMA: Triethylene glycol dimethacrylate, 4-AET: 4-acryloxyethyl­ trimellitic acid, HEMA: Hydroxyethyl methacrylate. a Refer to the study (Hu et al., 2016). 2

Y. Komagata et al.

Journal of the Mechanical Behavior of Biomedical Materials 105 (2020) 103708

other hand, for examining the bond durability, the resulting sample was subjected to thermocycling by alternatively immersing in water baths between 5 � C and 55 � C for 20,000 cycles with a 60-s dwell time per each temperature. After thermocycling, the SBS test for the sample was per­ formed using a blade-shape loading device and a mechanical testing machine (AGS-H, Shimadzu Corp.) with a crosshead speed of 1.0 mm/min. The resulting de-bonded surface of the sample was then observed by an optical microscope with 50�magnification, and was classified into the following three categories: A - adhesive failure at the cement–porcelain interface; C - cohesive failure within the porcelain; M - mixed failure of A and C.

Fig. 1. Schematic illustration of the corona discharge device configured for treating porcelain.

using the ceramic heater. After reaching the steady temperature, the applied voltage was gradually increased until the current in the circuit reached a value of 10 μA, which signifies that the corona discharge is occurring around the anode-needle (Ikeda et al., 2013). The voltage was kept constant for 1 min, 5 min, and 9 min to perform a stable corona discharge treatment. The voltage was immediately set to zero after the treatment. The resultant corona-discharge-treated porcelain was used for the following SBS tests and surface characterizations. To enhance the effect of the corona discharge process, the coronadischarge-treated porcelain was further subjected to the post-heattreatment at 600 � C for 10 min using an electric furnace. Both the heat-treated and corona-discharge-treated porcelains were then used for the following experiments.

2.4. Surface characterizations The surface wettability of all treated porcelains was evaluated using a contact angle meter (DMe-211, Kyowa Interface Science Co., Ltd.). The contact angle of distilled water on the porcelain surface at room tem­ perature was obtained by capturing an image of the droplet 5 s after it had fallen on the surface (n ¼ 5). Surface topology of all treated porcelains was measured using a confocal laser scanning microscope (CLSM; VKX-100, Keyence Corp.). Surface roughness (Ra) of each porcelain was calculated from the measured data (n ¼ 10, using ten samples). Chemical states of the porcelain surface were examined using X-ray photoelectron spectroscopy (XPS; ESCA-3400, Shimadzu Corp., Kyoto, Japan) using a monochromatic Mg Kα source for excitation with 10 kV, 20 mA. The analysis area, depth of analysis, instrument resolution, and pass energies used were 6 mm circle, 1–10 nm, 0.9 eV, and 75 eV, respectively. The obtained spectra were deconvoluted by means of a curve background and fitted with a Gaussian function.

2.2.2. Hydrofluoric acid (HF) treatment A conventional HF-treatment was performed on the porcelain sur­ face for the comparative sample. A commercial etchant gel (Porcelain Ethcant, Bisco Inc.) was applied on the polished porcelain using a brushing technique and kept at room temperature for 60 s. The treated sample was subsequently rinsed with running water and cleaned by ultrasonication for 5 min in distilled water, and then dried by blowing air.

2.5. Statistical analysis The obtained data was statistically analyzed using the statistical software, EZR (Saitama Medical Center, Jichi Medical University). Stu­ dent’s t-test was applied to analyze the difference between the results of the SBSs of the two groups before and after thermocycling. For multiple comparisons, one-way analysis of variance (ANOVA) was conducted to compare the SBS, the surface roughness (Ra), and contact angle of all treated porcelains, respectively. Tukey’s post-hoc test was performed in the groups if the results of the one-way ANOVA were significant. The significance level was set at 0.05 for all analyses.

2.3. Shear bond strength (SBS) tests Table 2 summarizes the surface preparation processes for the por­ celain. The bond strength between each treated porcelain and resin cement was measured using an SBS test via the following procedure from a study (Shimizu et al., 2006). Each porcelain was fixed in an acrylic ring using self-cured resin. A Teflon tube, with an internal diameter of 5 mm, was positioned on the porcelain surface using a fixed tape, to regulate each bonding area. The silane-primer (Porcelain primer, Shofu Inc.), which contains 3-methacryloxypropyl trimethoxy silane (γ-MPTS) as the silane-coupling-agent, was applied onto the porcelain surface, and was subsequently dried with a dryer. The resin cement (Resicem PASTE, Shofu Inc.) was loaded on the silanized surface through the Teflon tube to form a 3-mm-thick rod. The cement was cured for 5 min using a light irradiator (α LIGHT II N, J. Morita Corp.) and stored for 6 days in order to cure the resin cement sufficiently. The Teflon tube was then removed from the porcelain surface and the cemented sample was stored in distilled water at 37 � C for 1 days. The resulting sample was then subjected to the SBS tests (n ¼ 5 or 10). On the

3. Results 3.1. Optimization of the corona discharge treatment conditions The optimal condition (treatment-time and -temperature) of the corona discharge process was first determined by evaluating the con­ ditions where the SBS was maximum. Fig. 2 shows the dependence of the treatment-time and treatment-temperature on the SBS for the corona discharge process. As shown in Fig. 2(a), the SBSs in the 5-min and 9-min treatments were significantly higher than that of the 1-min treatment, while there was no significant difference between the SBSs in the 5-min and 9-min treatments. As shown in Fig. 2(b), the SBS tends to increase with an increase in the treatment temperature. The SBS in the 200 � Ctreatment is significantly higher than that of 25 � C-treatment. In the 300 � C-treatment, however, the corona discharge process was not stable because the high temperature caused an uncontrollable discharge. The optimal treatment-temperature and -time were therefore considered to be 200 � C and 5 min, respectively. Based on these results, the following corona discharge treatments were performed at 200 � C for 5 min. The influence of corona discharge on the bond durability was examined by conducting the SBS tests after the thermocycling. Fig. 3

Table 2 Surface treatments for the feldspar porcelain. Abbreviation

Surface treatment

No-treatment HF-treatment Corona-discharge treatment Corona-post-heating

Nothing. Etching by the HF application. Treating using corona discharge under given conditions. Corona discharge treatment followed by post-heating at 600 � C

3

Y. Komagata et al.

Journal of the Mechanical Behavior of Biomedical Materials 105 (2020) 103708

Fig. 2. Dependence of (a) treatment-time and (b) treatment-temperature for the corona-discharge treatment of porcelain. Temperature and time were fixed at 200 � C (a) and 5 min (b), respectively. The different letters indicate a significant difference between the groups (p < 0.05), (Tukey test, n ¼ 5).

shows the SBSs for the corona-discharge-treated samples before and after thermocycling. The results showed that the SBS significantly decreased because of thermocycling. To improve the bond durability of the corona-discharge-treated porcelain, the corona discharge treatment was performed followed by the post-heat-treatment. Fig. 4 shows the SBSs before and after thermocycling for the corona-post-heated sample. No significant difference was found between the groups, which indicates that post-heat-treatment improved the bond durability of the coronadischarge-treated sample. 3.2. Comparison of the SBSs of the samples from each treatment The corona-post-heating gave better bond durability to the porce­ lain. Thus, we employed the corona-post-heating to compare the SBS with those of the conventional treatments. Fig. 5 shows the SBSs of the samples that underwent the corona-post-heating and the conventional treatments. The results show that the SBS of the corona-post-heated sample was significantly higher than that of the non-treated sample, and comparable to that of the HF-treated sample. The failure modes for each sample after the SBS tests are summarized in Table 3. A large number of cohesive failures were observed in both the HF-treated and corona-post-heated samples than in the non-treated sample.

Fig. 4. SBSs between the corona-post-heated porcelain and the resin cement, (a) before and (b) after thermocycling. Corona discharge treatment on the porcelain was performed at optimal condition (at 200 � C for 5 min), and sub­ sequently post-heated at 600 � C for 10 min. There is no significant difference between the groups (p < 0.05, Student t-test, n ¼ 10).

3.3. Surface characterizations of the treated porcelain Fig. 6 shows the surface roughness of each treated porcelain. Both

Fig. 5. SBSs between each treated porcelain and the resin cement after ther­ mocycling; (a) the non-treated porcelain, (b) the HF treated porcelain, (c) the corona-post-heated porcelain (corona discharge at 200 � C for 5 min, followed by heat-treatment at 600 � C for 10 min). Different asterisks (* and **) present significant differences between the groups (p < 0.05, Tukey test, n ¼ 10). Fig. 3. SBSs between the corona-discharge-treated porcelain and the resin cement, (a) before and (b) after thermocycling. Corona discharge treatment on the porcelain was performed at optimal condition (at 200 � C for 5 min). Different asterisks (* and **) represent significant differences between the groups (p < 0.05, Student t-test, n ¼ 10).

the surface roughness of the corona-discharge-treated porcelain and the corona-post-heated porcelain did not change from that of the nontreated sample. Meanwhile the surface roughness of the HF-treated porcelain is significantly higher than that of the other treated porcelains. 4

Y. Komagata et al.

Journal of the Mechanical Behavior of Biomedical Materials 105 (2020) 103708

A around 532 eV is due to the oxygen bond in siloxane (-Si-O-Si-) groups, the peak B around 536 eV is attributed to the oxygen bond in silanol (Si–OH) groups, the peak C around 540 eV is attributed to the oxygen bond in H2O in the surface. The percentages of each peak area for O 1s are listed in Table 4. The areas for peak A and B are in the following order; peak A: (a) > (d) > (b) > (c), and peak B: (c) > (b) > (d) > (a). The area percentage of peak B for the corona-discharge-treated porcelain (Fig. 8(c) and Table 4(c)) is larger than that of the non-treated one (Fig. 8 (a) and Table 4(a)), thereby suggesting that the silanol groups are generated on the porcelain surface during the corona discharge. The area percentage of peak B for the corona-post-heated porcelain (Fig. 8(d) and Table 4(d)) is smaller than that of the corona-discharge-treated one (Fig. 8(c) and Table 4(c)), thereby suggesting that the silanol groups on the corona-discharge-treated porcelain decrease because of post-heating. Fig. 9 shows the XPS results of Si 2p spectra for each treated porce­ lain. The results indicate that there are two peaks in each spectrum; the peaks D (around 102 eV) and E (around 106 eV) are attributed to the siloxane (-Si-O-Si-) groups and silanol (Si–OH) groups, respectively (Gross et al., 1992; Hooshmand et al., 2001; Paparazzo et al., 1992). The percentages of each peak area for Si 2p are listed in Table 5. The areas for peaks D and E are in the following order; peak D: (a) > (d) > (b) > (c), and peak E: (c) > (b) > (d) > (a). The area percentage of peak E for the corona-discharge-treated porcelain (Fig. 9(c) and Table 5(c)) is higher than that of the non-treated porcelain (Fig. 9(a) and Table 5(a)), while the peak area for the corona-post-heated porcelain (Fig. 9(d) and Table 5(d)) is lower than that of the corona-discharge-treated one (Fig. 9 (c) and Table 5(c)), thereby suggesting that the number of silanol groups increased through corona discharge and then decreased through post-heating. The changes in the O 1s and Si 2p spectra were observed to be similar, suggesting that the silanol groups were generated on the porcelain surface during the corona discharge treatment, and the generated silanol groups were removed by post-heat-treatment.

Table 3 Failure modes of the thermocycled samples after the SBS tests, as shown in Fig. 5. Sample (a): the non-treated sample (b): the HF treated sample (c): the corona discharge sample treated at 200 � C for 5 min followed by heat-treatment at 600 � C for 10 min. Sample (a) No-treatment (b) HF-treatment (c) Corona-post-heating

Failure mode (n ¼ 10) A

M

C

6 2 2

3 5 4

1 3 4

A - adhesive failure at the cement–porcelain interface; C - cohesive failure within the porcelain; M - mixed failure of A and C.

Fig. 6. Surface roughness (Ra) of each treated porcelain measured by CLSM; (a) the non-treated porcelain, (b) the HF-treated porcelain, (c) the coronadischarge-treated porcelain, (d) the corona-discharge-treated porcelain, and subsequently post-heating. Different asterisks (* and **) present significant differences between the groups (p < 0.05, Tukey test, n ¼ 10).

4. Discussion The changes in the surface state of the porcelain caused by the corona discharge process were discussed. Fig. 6 shows that corona discharge does not affect the surface roughness of the porcelain. Meanwhile, the results of contact angle, as shown in Fig. 7, indicate that the corona discharge treatment hydrophilized the porcelain surface by forming silanol groups, which was confirmed in the XPS results, shown in Figs. 8 and 9. From these results, it can be determined that corona discharge generated numerous silanol groups on the porcelain without roughening the surface. The same phenomenon was reported for alkali silicate glass (Ikeda et al., 2013), which has a similar glass composition to the por­ celain. The study highlighted that corona discharge treatment formed the saturated amount of silanol groups on the surface layer of the glass, which was reproduced on the porcelain surface in this study. Influence of post-heating on the corona-discharge-treated porcelain was further discussed. From the results of the surface roughness shown in Fig. 6, the corona-post-heating did not affect the surface roughness. Additionally, the wettability of the corona-post-heated porcelain was significantly higher than that of the non-treated as well as the coronadischarged one. XPS analysis showed that the types of the silanol groups formed on the surfaces of the corona-discharge-treated and the corona-post-heated porcelains were different, suggesting that the corona-post-heated porcelain had a larger number of silanol groups when compared to the non-treated porcelain, but less than that of the corona-discharge-treated one. This suggests that post-heating had a trimming effect, which removed the excess silanol groups. Based on the results of the present and the earlier studies on the surface silanol groups (Zhuravlev, 2000), the surface states of the corona-discharge-treated porcelain can be speculated and described, as shown in Fig. 10. Because it is difficult to determine the types of surface silanol groups from the XPS analysis, we speculate on the surface states

Fig. 7 shows the contact angle for each treated porcelain. The contact angles for both the corona-discharge-treated and the corona-post-heated porcelains were significantly lower than that of the non-treated porce­ lain, and comparable to that of the HF-treated one. Fig. 8 show the XPS spectra of O 1s for each treated porcelain. The results show that the spectra have three peaks (Gross et al., 1992; Hooshmand et al., 2001; Mohai et al., 1990; Nasu et al., 1988); the peak

Fig. 7. Contact angles of distilled water on each treated porcelain; (a) the nontreated porcelain, (b) the HF-treated porcelain, (c) the corona-discharge-treated porcelain, (d) the corona-discharge-treated porcelain, and subsequently postheating. Different asterisks (* and **) present significant differences between the groups (p < 0.05, Tukey test, n ¼ 5). 5

Y. Komagata et al.

Journal of the Mechanical Behavior of Biomedical Materials 105 (2020) 103708

Fig. 8. XPS spectra of O 1s for each treated porcelain surface after deconvolution; (a) the non-treated porcelain, (b) the HF-treated porcelain, (c) the coronadischarge-treated porcelain, (d) the corona-discharge-treated porcelain, and subsequently post-heating. The peaks A, B and C are attributed to siloxane (-Si-O-Si-) groups, silanol (Si–OH) groups, and H2O in the surface, respectively.

germinal silanol groups, and many isolated silanol groups, as described in Fig. 10(d). For silanol groups on the HF-treated porcelain surface, it is assumed that the state of the silanol groups is intermediate between Fig. 10(c) and (d) because the percentage of XPS peak area due to silanol groups of the HF-treated porcelain is between that of the corona-discharge-treated porcelain and corona-post-heated porcelain (see Tables 4 and 5). Based on the results of the surface characterizations, the influence of the corona discharge treatment on the SBS was discussed. The optimal treatment-temperature and -time for the corona discharge treatment are found to be at 200 � C for 5 min, as shown in Fig. 2. The previous study suggested that the surface silanol groups were saturated within a few minutes during the corona discharge process (Ikeda et al., 2013). In the present study, the silanol groups were therefore saturated by performing the corona discharge treatment at 200 � C for 5 min. No further effect on the number of silanol groups was observed even after performing the treatment at temperatures higher than 200 � C for over 5 min. Figs. 3 and 4 indicate that the bond durability improved because of the corona-post-heating treatment. This positive effect was considered to be related to the types of the surface silanol groups present on the porce­ €derholm and Shang, 1993), iso­ lain. According to a previous study (So lated silanol groups exhibited a higher reactive nature in the presence of a silane-coupling-agent when compared to the geminal or vicinal groups. In this experiment, as highlighted earlier, the corona-discharge-treated porcelain surface had many geminal and vicinal silanol groups, while the corona-post-heated porcelain surface had a relatively large number of isolated silanol groups. It can be pre­ sumed that the bond durability was improved by corona-post-heating because of the optimization of the types of surface silanol groups. Fig. 5 shows that the SBS of the corona-post-heated sample was comparable to that of the conventional HF-treated sample. In addition, the failure modes of the corona-post-heated and HF-treated samples

Table 4 Percentage of each peak from the XPS analysis of O 1s spectra as shown in Fig. 8. (a) The non-treated porcelain, (b) the HF-treated porcelain, (c) the coronadischarge-treated porcelain, (d) the corona-discharge-treated and subsequently post-heated porcelain. Sample (a) (b) (c) (d)

Fitting χ2 value

Percentage of peak area (%) Peak A

Peak B

Peak C

74 53 32 72

11 47 68 28

15 – – –

0.994 1.789 1.042 1.259

from references; an earlier study (Zhuravlev, 2000) reported that a silica surface consists of either a siloxane or three types of silanol groups, namely isolated silanol, vicinal silanol, and geminal silanol groups, as shown in Fig. 10(a). These functional groups can be tailored by per­ forming heat-treatment at different temperatures. The non-treated por­ celain in the present study, as shown in Fig. 10(b), only had a small number of silanol groups on the surface. This surface would be covered with siloxane groups and has a small number of isolated, vicinal and/or geminal silanol groups. On the other hand, through the corona discharge treatment, the silanol groups on the surface were saturated, as shown in Fig. 10(c). The results showed the presence of vicinal and/or geminal silanol groups without any isolated silanol groups. Furthermore, because of the corona-post-heating, the silanol groups were partially removed as shown in Fig. 10(d). According to the earlier study (Zhur­ avlev, 2000), the geminal or vicinal silanol groups undergo condensa­ tion reactions with each other because of the heat-treatment performed at 600 � C, leading to the formation of isolated silanol groups. These isolated silanol groups are stable up to 900 � C. Thus, the porcelain surface treated by the post-heating at 600 � C has fewer vicinal and 6

Y. Komagata et al.

Journal of the Mechanical Behavior of Biomedical Materials 105 (2020) 103708

Fig. 9. XPS spectra of Si 2p for each treated porcelain surface after deconvolution; (a) the non-treated porcelain, (b) the HF-treated porcelain, (c) the coronadischarge-treated porcelain, (d) the corona-discharge-treated porcelain, and subsequently post-heating. The peaks D and E are attributed to siloxane (-Si-O-Si-) and silanol (Si–OH) groups, respectively.

durability of the bond. Furthermore, the present study points out that isolated silanol groups are especially effective in improving the bond strength. In the future, further research should be conducted to disclose the influence of the types of silanol groups on adhesive bonding.

Table 5 Percentage of each peak from the XPS analysis of Si 2p spectra as shown in Fig. 9. (a) The non-treated porcelain, (b) the HF-treated porcelain, (c) the coronadischarge-treated porcelain, (d) the corona-discharge-treated and subsequently post-heated porcelain. Sample (a) (b) (c) (d)

Percentage of peak area (%) Peak D

Peak E

78 65 49 67

22 35 51 33

5. Conclusion

Fitting χ2 value

A corona discharge treatment was developed for the surface treating feldspar porcelain to improve its bond strength with the resin cement. In the present experimental setup, the corona discharge process was opti­ mized by treating at 200 � C for 5 min, followed by heating at 600 � C. The treatment process generated large amount of silanol groups on the porcelain surface while post-heating removed the excess silanol groups, resulting in an increase in the SBS. This demonstrated that corona discharge is an attractive alternative to the conventional HF-treatment for glass-ceramics.

1.144 1.033 0.817 0.995

were similar. These results suggest that corona discharge treatment had a positive effect on the bonding between the porcelain and resin cement. From the results of the surface characterizations, corona-post-heating increased the surface silanol groups without increasing the surface roughness. The improvement of the SBS by the corona discharge treat­ ment is therefore considered to be because of the chemical alternation on the porcelain surface. Previous studies have demonstrated that some plasma treatments can improve the bonding between ceramics and resin cements, as mentioned in the introduction section. Although some studies revealed that the conventional plasma treatment can modify the surface of the ceramics to hydrolyze, no research has been conducted to elucidate the relationship between the surface silanol groups and the bond strength. The present study is the first to demonstrate that the silanol groups formed by the corona discharge treatment can improve the SBS between the porcelain and the resin cement. The bond strength between the ce­ ramics and resin cement with the silane-coupling-agent are normally reduced by water storage and aging due to the hydrolysis of the poly­ siloxane network at the interface (Heikkinen et al., 2013). The corona discharge treatment moderates the hydrolysis and improves the

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. CRediT authorship contribution statement Yuya Komagata: Conceptualization, Methodology, Validation, Formal analysis, Writing - original draft, Visualization. Hiroshi Ikeda: Conceptualization, Methodology, Validation, Resources, Data curation, Writing - review & editing, Visualization, Supervision. Yuki Fujio: Investigation, Writing - review & editing. Yuki Nagamatsu: Investiga­ tion. Hiroshi Shimizu: Conceptualization, Resources, Data curation, Writing - review & editing, Project administration, Funding acquisition.

7

Y. Komagata et al.

Journal of the Mechanical Behavior of Biomedical Materials 105 (2020) 103708

Fig. 10. Schematic illustrations of (a) the types of silanol groups and siloxane on the surface, and the surface state of porcelain with focus on silica species; (b) nontreated surface, (c) corona-discharge-treated surface, and (d) corona-post-heated surface.

Appendix A. Supplementary data

Hu, M., Weiger, R., Fischer, J., 2016. Comparison of two test designs for evaluating the shear bond strength of resin composite cements. Dent. Mater. 32, 223–232. Ikeda, H., Sakai, D., Funatsu, S., Yamamoto, K., Suzuki, T., Harada, K., Nishii, J., 2013. Generation of alkali-free and high-proton concentration layer in a soda lime glass using non-contact corona discharge. J. Appl. Phys. 114. Ito, Y., Okawa, T., Fukumoto, T., Tsurumi, A., Tatsuta, M., Fujii, T., Tanaka, J., Tanaka, M., 2016. Influence of atmospheric pressure low-temperature plasma treatment on the shear bond strength between zirconia and resin cement. J. Prosthodont. Res. 60, 289–293. Jung, H.J., Park, Y.J., Choi, S.H., Hong, J.M., Huh, J., Cho, J.H., Kim, J.H., Park, C., 2007. Thin film fabrication of PMMA/MEH-PPV immiscible blends by corona discharge coating and its application to polymer light emitting diodes. Langmuir 23, 2184–2190. Kawaguchi, K., Ikeda, H., Sakai, D., Funatsu, S., Uraji, K., Yamamoto, K., Suzuki, T., Harada, K., Nishii, J., 2014. Accelerated formation of sodium depletion layer on soda lime glass surface by corona discharge treatment in hydrogen atmosphere. Appl. Surf. Sci. 300, 149–153. Kawaguchi, K., Suzuki, T., Ikeda, H., Sakai, D., Funatsu, S., Uraji, K., Yamamoto, K., Harada, K., Nishii, J., 2015. Alkali ion migration between stacked glass plates by corona discharge treatment. Appl. Surf. Sci. 338, 120–125. Kim, J.H., Lee, M.A., Han, G.J., Cho, B.H., 2014. Plasma in dentistry: a review of basic concepts and applications in dentistry. Acta Odontol. Scand. 72, 1–12. Li, R.W., Chow, T.W., Matinlinna, J.P., 2014. Ceramic dental biomaterials and CAD/CAM technology: state of the art. J. Prosthodont. Res. 58, 208–216. Liebermann, A., Keul, C., Bahr, N., Edelhoff, D., Eichberger, M., Roos, M., Stawarczyk, B., 2013. Impact of plasma treatment of PMMA-based CAD/CAM blanks on surface properties as well as on adhesion to self-adhesive resin composite cements. Dent. Mater. 29, 935–944. Liu, Y., Liu, Q., Yu, Q.S., Wang, Y., 2016. Nonthermal atmospheric plasmas in dental restoration. J. Dent. Res. 95, 496–505. Liu, Y.C., Hsieh, J.P., Chen, Y.C., Kang, L.L., Hwang, C.S., Chuang, S.F., 2018. Promoting porcelain-zirconia bonding using different atmospheric pressure gas plasmas. Dent. Mater. 34, 1188–1198. Lu, Y.C., Tseng, H., Shih, Y.H., Lee, S.Y., 2001. Effects of surface treatments on bond strength of glass-infiltrated ceramic. J. Oral Rehabil. 28, 805–813. Lung, C.Y., Matinlinna, J.P., 2012. Aspects of silane coupling agents and surface conditioning in dentistry: an overview. Dent. Mater. 28, 467–477. Lanza, Marcos Daniel Septímio, Lanza, Flavia Juliani Souza Rodrigues, Manso, Adriana Pigozzo, Matinlinna, Jukka Pekka, Carvalho, Ricardo Marins, 2018. Innovative surface treatments for improved ceramic bonding: lithium disilicate glass ceramic. Int. J. Adhesion Adhes. 82, 60–66. Maruo, Y., Nishigawa, G., Oka, M., Minagi, S., Suzuki, K., Irie, M., 2004. Does plasma irradiation improve shear bond strength of acrylic resin to cobalt-chromium alloy? Dent. Mater. 20, 509–512. Matinlinna, J.P., Lung, C.Y.K., Tsoi, J.K.H., 2018. Silane adhesion mechanism in dental applications and surface treatments: a review. Dent. Mater. 34, 13–28. Matinlinna, J.P., Vallittu, P.K., 2007. Bonding of resin composites to etchable ceramic surfaces - an insight review of the chemical aspects on surface conditioning. J. Oral Rehabil. 34, 622–630. Mohai, M., Bert� oti, I., R� ev�esz, M., 1990. XPS study of the state of oxygen on a chemically treated glass surface. Surf. Interface Anal. 15, 364–368. Nasu, H., Heo, J., Mackenzie, J.D., 1988. XPS study of non-bridging oxygens in Na2OSiO2 gels. J. Non-Cryst. Solids 99, 140–150.

Supplementary data to this article can be found online at https://doi. org/10.1016/j.jmbbm.2020.103708. References Adamiak, K., Atten, P., 2004. Simulation of corona discharge in point–plane configuration. J. Electrost. 61, 85–98. Akyil, M.S., Yilmaz, A., Bayindir, F., Duymus, Z.Y., 2011. Microtensile bond strength of resin cement to a feldspathic ceramic. Photomed. Laser Surg. 29, 197–203. Brentel, A.S., Ozcan, M., Valandro, L.F., Alarca, L.G., Amaral, R., Bottino, M.A., 2007. Microtensile bond strength of a resin cement to feldpathic ceramic after different etching and silanization regimens in dry and aged conditions. Dent. Mater. 23, 1323–1331. Burrow, M.F., Thomas, D., Swain, M.V., Tyas, M.J., 2004. Analysis of tensile bond strengths using Weibull statistics. Biomaterials 25, 5031–5035. Cha, S., Park, Y.S., 2014. Plasma in dentistry. Clin. Plasma Med. 2, 4–10. Chang, J., Lawless, P., Yamamoto, T., 1991. Corona discharge processes. IEEE Trans. Plasma Sci. 19, 1152–1166. € Ç€ okeliler, D., Erkut, S., Shard, A.G., Akdoğan, E., Ozden, N., I_mirzalıoğlu, P., Mutlu, M., 2008. A novel approach for improvement of the interfacial binding of ceramics for dental materials: chemical treatment and oxygen plasma etching. J. Appl. Polym. Sci. 110, 2656–2664. Denry, I., Holloway, J.A., 2010. Ceramics for dental applications: a review. Materials 3, 351–368. Dos Santos, D.M., da Silva, E.V., Vechiato-Filho, A.J., Cesar, P.F., Rangel, E.C., da Cruz, N.C., Goiato, M.C., 2016. Aging effect of atmospheric air on lithium disilicate ceramic after nonthermal plasma treatment. J. Prosthet. Dent 115, 780–787. Elias, A.B., Simao, R.A., Prado, M., Cesar, P.F., Botelho Dos Santos, G., Moreira da Silva, E., 2019. Effect of different times of nonthermal argon plasma treatment on the microtensile bond strength of self-adhesive resin cement to yttria-stabilized tetragonal zirconia polycrystal ceramic. J. Prosthet. Dent 121, 485–491. Ersu, B., Yuzugullu, B., Ruya Yazici, A., Canay, S., 2009. Surface roughness and bond strengths of glass-infiltrated alumina-ceramics prepared using various surface treatments. J. Dent. 37, 848–856. Giacometti, J.A., Ribeiro, P.A., Raposo, M., Maratmendes, J.N., Campos, J.S.C., Dereggi, A.S., 1995. Study of poling behavior of biaxially stretched poly(vinylidene fluoride) films using the constant-current corona triode. J. Appl. Phys. 78, 5597–5603. Gross, T., Ramm, M., Sonntag, H., Unger, W., Weijers, H.M., Adem, E.H., 1992. An XPS analysis of different SiO2 modifications employing a C 1s as well as an Au 4f7/2 static charge reference. Surf. Interface Anal. 18, 59–64. Hallmann, L., Ulmer, P., Lehmann, F., Wille, S., Polonskyi, O., Johannes, M., Kobel, S., Trottenberg, T., Bornholdt, S., Haase, F., Kersten, H., Kern, M., 2016. Effect of surface modifications on the bond strength of zirconia ceramic with resin cement resin. Dent. Mater. 32, 631–639. Heikkinen, T.T., Matinlinna, J.P., Vallittu, P.K., Lassila, L.V., 2013. Long term water storage deteriorates bonding of composite resin to alumina and zirconia short communication. Open Dent. J. 7, 123–125. Hooshmand, T., Daw, R., van Noort, R., Short, R.D., 2001. XPS analysis of the surface of leucite-reinforced feldspathic ceramics. Dent. Mater. 17, 1–6.

8

Y. Komagata et al.

Journal of the Mechanical Behavior of Biomedical Materials 105 (2020) 103708 Shimizu, H., Inokoshi, M., Takagaki, T., Uo, M., Minakuchi, S., 2018. Bonding efficacy of 4-META/MMA-TBB resin to surface-treated highly translucent dental zirconia. J. Adhesive Dent. 20, 453–459. Shimizu, H., Kurtz, K.S., Tachii, Y., Takahashi, Y., 2006. Use of metal conditioners to improve bond strengths of autopolymerizing denture base resin to cast Ti-6Al-7Nb and Co-Cr. J. Dent. 34, 117–122. Smith, N.J., Lanagan, M.T., Pantano, C.G., 2012. Thermal poling of alkaline earth boroaluminosilicate glasses with intrinsically high dielectric breakdown strength. J. Appl. Phys. 111, 083519. S€ oderholm, K.J., Shang, S.W., 1993. Molecular orientation of silane at the surface of colloidal silica. J. Dent. Res. 72, 1050–1054. Tian, T., Tsoi, J.K., Matinlinna, J.P., Burrow, M.F., 2014. Aspects of bonding between resin luting cements and glass ceramic materials. Dent. Mater. 30, e147–162. Valverde, G.B., Coelho, P.G., Janal, M.N., Lorenzoni, F.C., Carvalho, R.M., Thompson, V. P., Weltemann, K.D., Silva, N.R., 2013. Surface characterisation and bonding of YTZP following non-thermal plasma treatment. J. Dent. 41, 51–59. Van den Breemer, C.R., Gresnigt, M.M., Cune, M.S., 2015. Cementation of glass-ceramic posterior restorations: a systematic review. BioMed Res. Int. 2015, 148954. Vechiato Filho, A.J., dos Santos, D.M., Goiato, M.C., de Medeiros, R.A., Moreno, A., Bonatto Lda, R., Rangel, E.C., 2014. Surface characterization of lithium disilicate ceramic after nonthermal plasma treatment. J. Prosthet. Dent 112, 1156–1163. Vilas Boas Fernandes Junior, V., Barbosa Dantas, D.C., Bresciani, E., Rocha Lima Huhtala, M.F., 2018. Evaluation of the bond strength and characteristics of zirconia after different surface treatments. J. Prosthet. Dent 120, 955–959. Zarone, F., Ferrari, M., Mangano, F.G., Leone, R., Sorrentino, R., 2016. “Digitally oriented materials”: focus on lithium disilicate ceramics. Int. J. Dent., 9840594 Zhuravlev, L., 2000. The surface chemistry of amorphous silica. Zhuravlev model. Colloids Surf., A 173, 1–38.

Nishigawa, G., Maruo, Y., Oka, M., Okamoto, M., Minagi, S., Irie, M., Suzuki, K., 2004. Effect of plasma treatment on adhesion of self-curing repair resin to acrylic denture base. Dent. Mater. J. 23, 545–549. Nishigawa, G., Maruo, Y., Oka, M., Oki, K., Minagi, S., Okamoto, M., 2003. Plasma treatment increased shear bond strength between heat cured acrylic resin and selfcuring acrylic resin. J. Oral Rehabil. 30, 1081–1084. Ozcan, M., Alkumru, H.N., Gemalmaz, D., 2001. The effect of surface treatment on the shear bond strength of luting cement to a glass-infiltrated alumina ceramic. Int. J. Prosthodont. (IJP) 14, 335–339. Paparazzo, E., Fanfoni, M., Severini, E., Priori, S., 1992. Evidence of Si–OH species at the surface of aged silica. J. Vac. Sci. Technol.: Vacuum Surf. Films 10, 2892–2896. Park, C., Yoo, S.H., Park, S.W., Yun, K.D., Ji, M.K., Shin, J.H., Lim, H.P., 2017. The effect of plasma on shear bond strength between resin cement and colored zirconia. J. Adv. Prosthodont. 9, 118–123. Pruneri, V., Samoggia, F., Bonfrate, G., Kazansky, P.G., Yang, G.M., 1999. Thermal poling of silica in air and under vacuum: the influence of charge transport on second harmonic generation. Appl. Phys. Lett. 74, 2423–2425. Ranjan, R., Krishnamraju, P.V., Shankar, T., Gowd, S., 2017. Nonthermal plasma in dentistry: an update. J. Int. Soc. Prev. Community Dent. 7, 71–75. Ritts, A.C., Li, H., Yu, Q., Xu, C., Yao, X., Hong, L., Wang, Y., 2010. Dentin surface treatment using a non-thermal argon plasma brush for interfacial bonding improvement in composite restoration. Eur. J. Oral Sci. 118, 510–516. Sakai, D., Harada, K., Kamemaru, S., Fukuda, T., 2007. Hologram replication technique in glass plates using corona charging. Appl. Phys. Lett. 90, 061102. Shimakura, Y., Hotta, Y., Fujishima, A., Kunii, J., Miyazaki, T., Kawawa, T., 2007. Bonding strength of resin cement to silicate glass ceramics for dental CAD/CAM systems is enhanced by combination treatment of the bonding surface. Dent. Mater. J. 26, 713–721.

9