Peel strength and interfacial characterization of maxillofacial silicone elastomers bonded to titanium

d e n t a l m a t e r i a l s 3 2 ( 2 0 1 6 ) e137–e147

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Peel strength and interfacial characterization of maxillofacial silicone elastomers bonded to titanium Ioli Ioanna Artopoulou a,∗ , Mark S. Chambers b , Spiros Zinelis c , George Eliades c a

Department of Prosthodontics, National and Kapodistrian University of Athens, School of Dentistry, Athens, Greece Section of Maxillofacial Prosthodontics and Oncologic Dentistry, Department of Head and Neck Surgery, Division of Surgery, The University of Texas MD Anderson Cancer Center, Houston, USA c Department of Biomaterials, National and Kapodistrian University of Athens, School of Dentistry, 2 Thivon Str., 11 527 Goudi, Athens, Greece b

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. To investigate the effect of three adhesive primers on the morphology, chemistry

Received 1 October 2015

and peel bond strength of two maxillofacial silicone elastomers with commercially pure

Received in revised form

titanium (cpTi).

5 January 2016

Methods. The effect of three primers (PR2:A-304 Primer/A-320 Bonding Enhancer, PR3:Super

Accepted 22 March 2016

Bond, and PR4:Super Glue) on cpTi morphology and chemistry were studied by reflected light polarized microscopy (RPOLM) and reflection Fourier-transform infrared microspectroscopy (RFTIRM). For testing the bond strength between two elastomers (EL1:MDX4-4210, EL2:A-

Keywords:

2006) and primed cpTi surfaces, a 90◦ T peel test was performed (PBS), using as reference

Maxillofacial silicone elastomer

EL1, EL2 specimens bonded to heat-cured poly(methyl methacrylate) resin (PMMA) primed

Bond strength

with A-330G primer (PR1). Failure modes were analyzed under a stereomicroscope, and the

cpTi

percentage of remaining silicone (RS%) on cpTi and PMMA were calculated by image anal-

FTIR

ysis. Scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDX) was

SEM/EDX

used to investigate representative failure patterns on cpTi. Data were analyzed with Weibull analysis, ANOVA plus post hoc tests, and Pearson correlation coefficient (a = 0.05). Results. Thick-irregular (PR2), thin-smooth (PR3), and uniform-porous (PR4) films were identified on cpTi by RPOLM. RFTIRM revealed: a strong peak of Si–O–Si with a distribution following the outline of the image (PR2); COO-M groups developed, but unevenly distributed (PR3); and reduction in C C groups due to in situ polymerization (PR4). Following PBS, the ranking of the statistical significant differences in Weibull scale parameter ( 0 ) of the EL1 group was PMMA PR1 > cpTi PR2,cpTi PR3 > cpTi PR4, whereas for the EL2 group cpTi PR2 > PMMA PR1 > cpTi PR4,cpTi PR3. For RS%, the ranking in the EL1 group was: PMMA PR1 > cpTi PR2 > cpTi PR3 > cpTi PR4, and in the EL2 cpTi PR2 > cpTi PR3 > cpTi PR4,PMMA PR1. There was no statistically significant correlation

∗ Corresponding author at: National and Kapodistrian University of Athens, School of Dentistry, Department of Prosthodontics, 2 Thivon Str., 11 527 Goudi, Athens, Greece. Tel.: +30 210 746 1246; fax: +30 210 746 1240. E-mail address: [email protected] (I.I. Artopoulou). http://dx.doi.org/10.1016/j.dental.2016.03.024 0109-5641/© 2016 The Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

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between PBS and RS%, with the exception of EL1 PMMA PR1. In all groups mixed failure modes were found by SEM/EDX. Significance. Although there is evidence of bonding with cpTi, there are important differences among the primer/elastomer combination that may affect the clinical performance of these materials. © 2016 The Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

A variety of materials is currently used in maxillofacial prostheses including poly(methyl methacrylate) or urethane-backed, medical grade silicone. Room temperaturevulcanizing (RTV) addition silicones are probably the most widely used materials for facial restoration, with MDX4-4210 (Dow Corning, Midland, MI, USA) shown to be the most popular among clinicians [1,2]. These prostheses are retained with adhesives, tissue undercuts, magnets or in some cases extraoral osseointegrated implants [3–5]. The success of the prostheses is strongly related to their retention. The fabrication of effective bone-anchored extraoral prostheses, made possible by the introduction of magnets and osseointgrated implants, greatly improved the treatment outcome. Several attachment systems have been used to retain facial prostheses, such as bar-clips, commercially pure titanium (cpTi) encapsulated magnets, and O-ring types. In case of bar-clip systems and O-ring type attachments, a clear heatcured poly(methyl methacrylate) resin (PMMA) substructure is housing the retentive elements [6–8]. The substructure should extend into the body of the silicone and possess sufficient surface area for efficient bonding [4,9]. In case of magnetic retention, magnetic attachments are positioned and secured in the body of the silicone prosthesis, especially in small defects where space is limited [10]. Encapsulation of samarium-cobalt (Co5 Sm) magnets with tin or cpTi, improved their biocompatibility, minimizing corrosion and cytotoxicity [11]. A strong bond between the silicone and the PMMA substructure of the prosthesis or the cpTi encapsulated magnets is important for sufficient retention and stability, so that the junction will not fail during insertion or removal of the prosthesis. The bonding capacity of facial elastomers to acrylic resin substructures has been the subject of many studies. Several surface treatments have been proposed to improve bonding of PMMA and various facial elastomers [12–17]. However, there is only a single report on the bond strength between silicone materials and cpTi, as mediated by different primers [18]. The purpose of this study was to investigate the effect of three primers on the adhesion of two addition silicone maxillofacial elastomers to cpTi. The experimental methodology included assessment of the primers contribution to the morphological and chemical alterations induced on cpTi surfaces and to the interfacial strength of silicones bonded to cpTi. The null hypothesis was that there are no differences among the primer induced effects on cpTi surfaces including morphological, chemical and bond strength issues.

2.

Materials and methods

The products used in the present study are listed in Table 1.

2.1. Effect of primers on cpTi surface morphology and chemistry Specimens made of cpTi (length = 120 mm, width = 5 mm, thickness = 3 mm, n = 5) were ground with 600-grit size silicon carbide papers and polished with a colloidal silica suspension (OP-S, Struers, Ballerup, Denmark) containing 30% H2 O2 with a 0.4 ␮m polishing cloth (MD-Chem, Struers) in a grinding/polishing machine (DAP-V, Struers) and then cleaned in an ethanol ultrasound bath for 10 min. On these surfaces, the priming treatments (PR2, PR3, PR4) were performed following the manufacturers’ instructions. The specimens were then stored at 37 ◦ C for the maximum setting times given in Table 1, air dried and then studied by reflected light polarized microscopy (RPOLM) and reflection Fourier-transform infrared microspectroscopy (RFTIRM). For RPOLM a microscope (ME 600 Eclipse, Nikon Kogaku, Tokyo, Japan) was used in bright-field mode and 40× magnification. RFTIRM analysis was performed by an FTIR microscope (AutoImage, Perkin-Elmer, Beaconsfield, Bacon, UK) attached to an FTIR spectrometer (Spectrum GX, Perkin-Elmer) operated under the following conditions: Liquid N2 -cooled mercury–cadmium telluride (MCT) detector, 4000–650 cm−1 wave number range, 4 cm−1 resolution, 100 ␮m × 100 ␮m aperture, 400 ␮m × 300 ␮m scan size for mapping and 100 scans co-addition per site. All spectra were subjected to Kramers–Kroning and baseline corrections.

2.2.

Effect of primers on bond strength

For testing the bond strength between silicone elastomers and cpTi, a peel bond strength test (PBS) was performed based on the procedure described in ASTM D3167 standard (90◦ T peel test) [19]. Specimens of cpTi (length = 120 mm, width = 5 mm, thickness = 3 mm, n = 36) were prepared as previously described, whereas PMMA specimens (same dimensions, n = 12) were prepared by conventional flasking heat-cured procedures and polished according to the manufacturer’s recommendations. On each of the specimens prepared as above, rectangular wax base plate patterns with dimensions (length = 120 mm, width = 5 mm, thickness = 3 mm) were attached. Half the wax pattern length was in contact with the metal or polymer surfaces and half in contact with a thin layer of a tin foil separating medium. All specimens, were invested in dental stone, preheated to remove wax, and the areas without the separating medium

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Table 1 – The products used in the study. Product (Lot)

Code

Manufacturer

Composition

Application procedure

EL1

Dow Corning Midland, MI, USA

Mix 1 part of catalyst with 9 parts of base. Setting time at room temperature (RT): 3 days

EL2

Factor II Lakeside, AZ, USA

Base: dimethyl vinyl siloxane, silica, platinum complex Catalyst: dimethyl vinyl siloxane, Dimethyl hydrogen siloxane trimethyl siloxy-terminated, inhibitor Part A: vinyl polysiloxane, silica, platinum complex. Part B: dimethyl, methyl-hydrogen siloxane copolymer, silica

cpTi

I. Moatsos, Athens, GR

–

PMMA

Dentsply Caulk, York, PA, USA

Wt%: C < 0.08, N < 0.03, Fe < 0.30, H < 0.015, O < 0.25, Ti > 99. Liquid: methyl methacrylate, ethylene glycol dimethacrylate Powder: poly methyl methacrylate, dibenzoyl peroxide

Primers A-330G Primer L4707836

PR1

Factor II Lakeside, AZ, USA

Solution of modified polyacrylates in methyl ethyl ketone and dichloromethane

A-304 Primer (L30332)

PR2

Factor II Lakeside, AZ, USA

Wt%: Naphtha (85), tetra-n-propyl silicate (5), tetrabutyltitanate (5), tetra (2methoxyethoxy)silane (5)

Thoroughly clean and degrease the surface to be primed with a chlorinated solvent trichloroethane, acetone, or methyl ethyl ketone. After drying, apply a uniform thin coat of A-330G primer. Setting time (RT): 30–120 min Thoroughly clean and degrease the surface to be primed with a chlorinated solvent trichloroethane, acetone, or methyl ethyl ketone. After drying, apply a uniform thin coat of A-320 bonding enhancer. Allow to dry and apply a uniform thin coat of A304 primer. Setting time (RT): 30–120 min

Silicone elastomers MDX4-4210 medical grade elastomer (DT031806)

A-2006 platinum silicone elastomer (R2375612)

Core materials Commercially pure grade II Titanium (8100-I) Lucitone 199 (061208)

A-320 Bonding Enhancer (R032106) Super-Bond C&B Monomer (LR1)

Super Glue (BI2626)

PR3

Sun Medical, Moriyama, JPN

PR4

Bison Int. Goes, NL

Trichloroethane (85), amorphous treated silica (5) Methyl methacrylate (MMA), 4methacryloxyethyltrimelitic acid anhydride (4-META)

Ethyl-cyanoacrylate

Mix equal parts of A and B. Setting time (RT): 16 h

Powder/liquid ratio: 32 ml/10 ml. Mix for 15 s, cover mixing jar and allow material to reach packing consistency (9 min at RT). Curing time: 9 h in water bath (73 ◦ C ± 1 ◦ C) plus 30 min in boiling water

Apply a thin layer of the monomer and allow to dry Setting time (37 ◦ C): 5–6 min (with T-BBO catalyst) Apply a thin layer. Setting time (RT): 60 s

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were treated as follows: (a) For cpTi, the surfaces were thoroughly cleaned with acetone and primed with PR2, PR3 and PR4 (n = 12 each) respectively, and (b) for PMMA (control group, n = 12), the surfaces were treated with PR1. Then, half the specimens of the primed groups were bonded with the EL1 silicone elastomer, whereas the rest with the EL2. The silicone elastomers were mixed, packed and polymerized following the manufacturers’ instructions. All the specimens were stored in a humidor (100% RH, 37 ◦ C, 24 h). Then, the unbonded ends of the flexible set silicone adherent parts were inserted at 90◦ into the peel test fixture grips, whereas the cpTi framework was attached to a horizontal device with free floating rollers. The specimens were peeled off at 100 mm/min crosshead speed in a universal testing machine (Tensometer 10, Monsanto, Swidon, UK). Failure loads (PBS) were recorded in force per unit width (N/mm or kN/m). Failure modes were analyzed under a reflected light stereomicroscope (M80, Leica, Wetzlar, Germany) at 20× magnification and the percentage remaining silicone (RS%) on cpTi and PMMA surfaces was calculated for each condition. Five photographs were obtained from each cpTi or PMMA specimen and the surface area of the remaining silicone was calculated with an image processing software (SigmaPro, Jandel, S. Rafael, CA, USA). Scanning electron microscopy and energy dispersive X-ray spectrometry (SEM/EDX) were used to investigate in detail the elastomer failure patterns on representative cpTi specimens. Compositional backscattered electron images (BEI) of selective regions were acquired in a SEM (Quanta 200, FEI, Hilsboro, OR, USA) operating at 15 kV accelerating voltage and 90 ␮A beam current in low vacuum mode (1 Pa) under 40× and 300× magnification. The specimens were further subjected to elemental analysis by an EDX system (Sapphire CDU, EDAX Int, Mahwah, NJ, USA), employing a liquid N2 -cooled Si (Li) detector with a super ultra-thin Be window. Spectra were collected from each region of interest in an area scan mode (15 kV accelerating voltage and 110 ␮A beam current) and multielement mapping for C, O, Si and Ti was performed using area scans at the same magnifications.

2.3.

Statistical analysis

A Weibull analysis was employed to analyze the PBS data. The shape or modulus parameter (m; a measure of the variability of the results, expressing the size distribution of the flaws) and the scale or B63.2 parameter ( 0 ; indicates the value of PBS for which the 63.2% of the sample size will be debonded) of the Weibull distributions, as well as the 95% confidence intervals for  0 and m respectively, were calculated by WinSMITH Weibull & Visual 2.0M software (Fulton Findings, Torrance, CA, USA). Survival probability at any peel stress [Ps = exp{−(/ 0 )m }], and fracture probability [Pf = 1 − exp{−(/ 0 )m }], were also calculated. For the statistical analysis of the RS% on the cpTi and PMMA surfaces bonded with the EL1 elastomer, one-way ANOVA, followed by Holm–Sidak post hoc test were used to assess statistically significant differences between the control and the cpTi groups treated with PR2, PR3, and PR4 primers. For the EL2 elastomer, Kruskal–Wallis one-way ANOVA on Ranks and Dunn’s multiple comparison tests were used. For RS% comparisons between the two silicone elastomers (EL1 and EL2)

and among the three cpTi primers (PR2, PR3, and PR4), the data were subjected to a rank transformation function and a two-way ANOVA was performed, using the types of elastomers and primers as independent variables, followed by Holm–Sidak multiple comparisons test. Pearson correlation was employed to determine the relationship between PBS and RS%. All these analyses were performed at a 95% confidence interval (a = 0.05) employing the StatView software (SAS Institute; Cary, NC, USA).

3.

Results

3.1. Effect of primers on cpTi surface morphology and chemistry Representative RPOLM images of the cpTi surfaces following the primer treatments are illustrated in Fig. 1. PR2 treatment created a thick, irregular superficial film component imposed onto a uniform, thin one (Fig. 1a). The film was set after the instructed period of room storage, as confirmed by scratching with a tactile probe (sharp explorer). The almost parallel fibrial arrangement observed at the margins seemed to follow the directions of the brush strokes, used for primer application. The film formed after PR3 application was substantially thinner, creating birefringence patterns due to local submicron variations in thickness (Fig. 1b). Most of the topography of the cpTi surface was still visible through the film, which showed no resistance to tactile probing. PR4 treatment established a uniform film masking the cpTi topography (Fig. 1c). The film appeared thick, with film smears at the margins, demonstrated porosity, was set and showed evidence of marginal debonding. The results of the RFTIRM analysis are presented in Fig. 2a–c. The spectrum of the PR2 film on cpTi (Fig. 2a) demonstrated absorption peaks of –OH s (H-bonded, 3600–3200 cm−1 ), –OH b (1640 cm−1 ), CH3 /CH2 b (1467–1423 cm−1 ), Si–CH3 (1266 and 870 cm−1 ), Si–O–Si cage (∼1150 cm−1 ), Si–O– (asymmetric vibrations 1080 cm−1 for Si–O–Si and 1020 cm−1 for Si–O–C), Si–OH (944 cm−1 ) and Si–O– (symmetric vibrations 805–790 cm−1 ), where s: stretching, b: bending and w: wagging vibrations respectively. On cpTi, the Si–O–Si peak at 1080 cm−1 was increased in intensity, in comparison with the reference PR2 spectrum, indicating the formation of Si–O–Si bonds. Some residual Si–OH groups were identified on cpTi after the setting time (120 min). In the molecular mapping, the distribution of the Si–O–Si group (1080 cm−1 ) followed the outline of the image, with highest absorption located at the thickest film regions. The film following PR3 treatment (Fig. 2b) showed the following absorption peaks (in cm−1 ): –OH s (H-bonded, 3540–3370), –COOH s (3200–3000 and 2630–2520), CH3 /CH2 s (2960–2900), –(C O)–O–(C O)– s (1900), C O s (1725), C C s (1636), aromatic (Ar) C···C s (1609), –COO–M s (1550 asym, 1440 sym, where M: metal), CH3 /CH2 b (1450–1380), C–O s (1300), Ar–O– (1179), C–O–C (1178, 1124), C–OH (1075, 1037), CH b of C C out of plane (950) and Ar–CH b out of plane (760). The peaks associated with the –COO–M were missing from the reference PR3 spectrum. Also, the peak assigned to the COOH group of the acid derivative of 4-META (1740 cm−1 ) found in the reference spectrum, was missing

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Fig. 1 – Reflected light microscopic images of the primers on cpTi. (a) PR2, (b) PR3, and (c) PR4 (40× magnification, bar 1 mm).

from the PR3 spectrum on cpTi. These findings confirm a reaction of PR3 with the substrate. Molecular mapping of the C O (1730 cm−1 ) and COO–M (1550 cm−1 ) components revealed a different distribution, since the first corresponds to both MMA and 4-META, whereas the second only to the reacted fraction of 4-META. The film area was rich in methacrylate C C bonds. The spectrum of the film region imaged after PR4 treatment of cpTi (Fig. 2c), revealed the following peaks (in cm−1 ): –OH s (H-bonded, 3490), CH3 /CH2 s (3050–2890), C N s (2240), C O s (1740), CH3 /CH2 b (1450–1380), C–H b (1290), C–O–C s (1240), C–O s (1150–1015), C–C (857), –N–C– (798) and CH (743). Important differences were identified in comparison with the unset reference PR4 spectrum. The original C C peak (1614 cm−1 ) and the C C–COO peak (1290 cm−1 ) disappeared, and a new peak of CH3 –COO (1250 cm−1 ) appeared on cpTi. Since ethyl isocyanates are single-component, catalyst and solvent-free adhesives, no mapping was performed.

3.2.

Effect of primers on peel strength

Table 2 summarizes the results of the Weibull analysis for all the groups tested. The logarithmic Weibull fracture probability plots are presented in Fig. 3a and b. Within the EL1 group, there were no statistically significant differences in the Weibull modulus (m) values among the cpTi priming treatments or the control (EL1 PMMA PR1). Similar were the results for the EL2 group, with the exception of the statistically significant difference (p < 0.05) of PR4 treatment on cpTi from the corresponding control (EL2 PMMA PR1). For the same primer treatments, the only statistically significant difference between EL2 and EL1 elastomers was found on cpTi after PR3

treatment, with EL2 providing the highest m value. Regarding the characteristic strength ( 0 ) within the EL1 elastomer group, the ranking of the statistical significant differences was EL1 PMMA PR1 > EL1 cpTi PR4, EL1 cpTi PR2 > EL1 cpTi PR3 (p < 0.05), whereas for the EL2 elastomer group the ranking was EL2 cpTi PR3 EL2 cpTi PR2 > EL2 PMMA PR1 > EL2 cpTi PR4, (p < 0.05). For the same primer treatments, EL2 showed significantly higher  0 values than EL1 on cpTi after PR2 and PR3 treatments, while EL1 showed higher  0 values in the PMMA (control) group. Representative reflected light stereomicroscopic images of PMMA and cpTi surfaces after elastomer debonding are presented in Fig. 4. The results of the percentage remaining silicone (RS%) for the EL1 groups are summarized in Table 3. The ranking of the statistically significant differences was EL1 PMMA PR1 > EL1 cpTi PR2 > EL1 cpTi PR3 > EL1 cpTi PR4 (p < 0.05). The results of the RS% for the EL2 groups are presented in Table 4. The corresponding statistical ranking was EL2 cpTi PR2 > EL2 cpTi PR3 > EL2 cpTi PR4, EL2 PMMA PR1 (p < 0.05). The results (transformed-ranked data) of the RS%, for the comparisons between the two silicone elastomers and among the three priming treatment when applied on cpTi surfaces, are summarized in Table 5. Two-way ANOVA showed statistically significant differences in the primer parameter (p < 0.001), but not in the silicone elastomer parameter (p = 0.732), with a significant interaction encountered between the two parameters (p = 0.012). For both elastomers, the most efficient cpTi primer was PR2, followed by PR3 and PR4, with statistically significant differences among all these treatments (p < 0.05). Between the two elastomers, the only statistically significant difference was found when cpTi was primed with PR2 in

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favor of EL1. Pearson correlation coefficient (r) showed no statistically significant correlations between the PBS values (m,  0 ) and RS%, with the exception of EL1 PMMA PR1 (r = 0.856, p < 0.05).

Representative SEM/EDX analyses of selected debonded specimens are illustrated in Fig. 5a and b. Fig. 5a shows the backscattered electron image (BEI) of a region considered as adhesive failure from EL1 cpTi PR2 group. At

Fig. 2 – Reflected light images, FTIR absorbance spectra, absorbance molecular maps for the specified groups and reference spectra (ref) of the primer treatments on cpTi. (a) PR2: Arrows show increased absorbance of Si–O–Si peak on cpTi in comparison with the reference spectrum. (b) PR3: Arrows show COOM peaks on cpTi and COOH peak in reference spectrum. (c) PR4: Arrows show reduction in the absorbance of C C peak on cpTi and increased absorbance in reference spectrum.

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Fig. 2 – (Continued )

300× magnification islands with low mean atomic number material were identified, which corresponded to domains rich in C and O, as revealed in the multielement X-ray mapping. These domains demonstrated a complementary distribution to Ti. The Si map followed the location of these domains, but at a much lower intensity, which cannot be considered as a well-defined elemental distribution. Similar where the observations for Si, when the same primer was combined with EL2. A typical mixed failure mode was observed, with a great part of elastomer retained on cpTi surface (Fig. 5b).

4.

Discussion

The results of this study support the partial rejection of the null hypothesis, since there where statistically significant differences among the effects of primer treatments on (a) cpTi surface morphology and chemistry and (b) in the bond strength between the silicone elastomers and primed cpTi versus the PMMA control. Bonding of the maxillofacial silicone elastomers to acrylic frameworks and cpTi elements may increase the longevity of

Table 2 – Results of the Weibull analysis for the peel bond strength. Silicone elastomer

Core material

PMMA EL1 cpTi

PMMA EL2 cpTi

Primer

Weibull modulus (m, N/mm or kN/m)

95% Confidence intervals for m

Characteristic strength ( 0 , N/mm or kN/m)

95% Confidence intervals for 0

PR1 PR2 PR3 PR4

7.17a,* 4.44a,1 3.02a,1 2.93a,1

3.90–13.18 2.66–7.41 1.84–4.94 1.71–5.02

7.15a,* 1.05b,1 0.41c,1 1.52b,1

6.47–7.90 0.90–1.24 0.32–0.52 1.19–1.93

PR1 PR2 PR3 PR4

11.871,* 3.79a,b,1 9.79a,b,2 3.55b,1

6.44–21.86 2.14–6.71 5.69–16.85 1.99–6.32

2.12b,** 6.14a,2 1.31c,2 1.56c,1

2.0–2.26 5.09–7.43 1.22–1.41 1.27–1.91

Same superscripts indicate values with no statistical significance (p > 0.05) per silicone group (letters), between silicone groups per primer (numbers), and between acrylic controls (asterisks). The higher the Weibull modulus (m), the more reliable the treatment, whereas the higher the Weibull characteristic strength ( 0 ), the higher the peel bond strength.

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Fig. 3 – Weibull plots (log/log) for (a) EL1 treated cpTi and PMMA surfaces, and (b) EL2 treated cpTi and PMMA surfaces. The slope and x-axis intercept of the fitting curve correspond to the Weibull shape or modulus parameter (m), and the scale or B63.2 parameter ( 0 ) respectively.

Fig. 4 – Representative stereomicroscopic images of the core surfaces after elastomer debonding. (a) PMMA after PR1, (b) cpTi after PR2, (c) cpTi after PR3, (d) cpTi after PR4 (reflected light, 20× magnification, bar: 2 mm).

the prostheses. The silicone elastomers (EL1, EL2) and the PR1 and PR2 priming treatments included in the present study, are commonly used by maxillofacial prosthodontists and anaplastologists. The selection of PR3 and PR4, as additional priming treatments, was based on the fact that both contain C C bonds co-polymerizable with the vinyl groups of the addition type silicone elastomers, while they can also bond with cpTi, through ionized carboxyl groups (hydrolyzed and ionized 4-META) or with surface adsorbed hydroxyl groups (cyanoacrylates). Testing of the interfaces of maxillofacial elastomers with acrylics and alloys is a challenging task, due to their soft,

Table 3 – The results of the percentage remaining silicone (RS%) for the groups bonded with EL1 elastomer. Group EL1 PMMA PR1 (Control) EL1 cpTi PR2 EL1 cpTi PR3 EL1 cpTi PR4

RS% (means and standard deviations) 90 (1)a 72 (4)a 42 (6)c 16 (4)d

Same superscript letters imply mean values with no statistically significant difference (p > 0.05).

elastic nature and the inherent problems regarding specimen preparation, retention and loading. Since peeling off the prosthesis from a retentive matrix or skin is a common detachment method used by patients, a 90◦ T shape peel test was employed in order to examine the interfacial strength of these prostheses. This was considered as a more reliable procedure, than the typical tensile tests, for the specific applications. Moreover, the 90◦ angle peel design is a milder testing procedure than the 180◦ peel design for low tensile strength materials, like the maxillofacial silicones tested (5 MPa for MDX4-4210 and 3 MPa for A 2006 as stated by the manufacturers), since the U-shape silicone design may create defects in the material at the start of 180◦ peel testing. Adhesion of vinyl functionalized addition silicone elastomers to cpTi, as mediated by silane coupling agents, involves two reactions with the adherent materials: First, hydrogen bonding with surface adsorbed –OH groups of cpTi and subsequent condensation of silanol groups to form a –Si–O–Si– network, which stabilizes the cpTi–silane interface [20], and second, copolymerization of the silane vinyl functional groups with the vinyl elastomer groups, as catalyzed by the platinum complex of the latter. In PR2, though, the main component is tetra-(ethoxymethoxy)silane [Si(OCH2 CH2 OCH3 )4 ], a silane containing four hydrolysable methoxy groups (–OCH3 ), but

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Table 4 – The results of the percentage remaining silicone (RS%) for the groups bonded with EL2. Group

RS% Median

EL2 EL2 EL2 EL2

PMMA PR1 (Control) cpTi PR2 cpTi PR3 cpTi PR4

4a 80b 40c 13a

25% percentile 2 76 31 9

75% percentile 8 82 44 15

Same superscript letters imply mean values with no statistically significant difference (p > 0.05).

no vinyl groups to act as coupling agent between cpTi and vinyl siloxane elastomers. This non-functional silane is activated to silanol by tetrabutyl titanate [Ti(OCH2 CH2 CH2 CH3 )4 ]. To assist intra- and intermolecular –Si–O–Si– network formation, the bond enhancer used to clean the surface prior to silane application is enriched with amorphous silica. Finally, the tetra-n-propyl silicate [Si(OCH2 CH2 CH3 )4 ], a crosslinking agent for silicone rubber, is used to catalyze the condensation reaction of silanol groups. This crosslinker is preferred over tetra-ethyl silicate, a commonly used agent in condensation type silicone impression materials, since the greater n-propyl group, retards the hydrolysis of the crosslinker and

the silanol condensation reaction [21]. The characteristic microscopic image of the PR2 film on cpTi may imply a degree of phase separation of the activated multicomponent primer. Such non-functional silanes could interact with vinyl elastomers only through a dehydrogenative coupling between residual silanol groups of the primer (Si–OH) and the hydride functionalized siloxanes (Si–H), used for hydrosilylation of vinyl siloxanes, by forming –Si–O–Si– and H2 . However, the FTIR analysis confirmed the presence of a strong –Si–O–Si– peak dominating the primed region, with minimal Si–OH groups, as a result of silanol condensation. Therefore, following PR2 treatment, a hydrophobic cpTi surface has been

Fig. 5 – Backscattered electron images of cpTi surfaces after debonding and the corresponding X-ray maps of carbon, oxygen, silicon, and titanium. (a) cpTi surface treated with PR2 and EL1 (300×, bar = 50 ␮m). (b) cpTi surface treated with PR2 and EL2 (300×, bar = 50 ␮m).

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Table 5 – The transformed (ranked) results of the percentage remaining silicone (RS%) for the two silicone elastomers (EL1, EL2) and the three primers (PR2, PR3, PR4) applied on cpTi surfaces. Primer

RS% (means and standard deviations) EL1

PR2 PR3 PR4

EL2 a,A

28.2 (2.3) 19.5 (3.2)b,A 8.4 (4.0)c,C

32.8 (3.2)a,B 17.3 (4.4)b,A 4.8 (2.8)c,C

Same superscript letters indicate no statistically significant differences (p > 0.05) among primer treatments within the same elastomer group (lower case) and between elastomer groups for the same primer treatment (upper case).

established, compatible with the hydrophobic addition silicone elastomers, but not prone to copolymerization. This may explain the low characteristic strength ( 0 ) of PR2, especially with the slow-set elastomer EL1. Nevertheless, the same primer, when used with EL2, resulted in significantly greater  0 and RS%. Variations in the silicone chemistry (structure and filler content) may explain these findings. Based on the manufacturers’ information, EL2 has lower Shore-A hardness (12 vs 30) and tensile strength (3 vs 5 MPa), but higher percentage of elongation (626% vs 470%) than EL1. FTIR analysis of cpTi surface following PR3 treatment, documented the presence of –COO–M groups, which implies interaction with the superficial TiO2 film. It has been proposed that such bonds are established through electrostatic interaction between the acids of the monomers and the –OH groups of the superficial metal oxide layer, with the basic character, hydrolytic stability and surface concentration of the metal oxides considered as determinant factors for the durability of the bonds [22]. Under these conditions, cpTi surfaces are modified to hydrophobic, rich in methacrylate groups, which are oriented upwards, a configuration that sterically favors crosslinking with the vinyl–silicone elastomers [20]. The primer was not separately cured to allow for copolymerization with the vinyl groups of the elastomers. This applies for most metal primers, even for these used for low-reactive noble alloys [23]. Nevertheless, the  0 values were the lowest recorded for both the elastomers. Again, EL2 showed greater  0 and m values from EL1 (p < 0.05), with no statistically significant difference in RS%. The lowest  0 and RS% of PR3 treatments in comparison with PR2, may indicate problems in copolymerization of PR3 primer with the vinyl groups of the elastomers. This incompatibility has been observed in a previous study, as well [18], although it was not clear if the MMA/4-META monomer had been activated by the partially oxidized tri-n-butyl borane catalyst of the system. Another possible explanation for the inferior  0 and RS% values, is the absence of a condensation mechanism similar to silanes, in order to stabilize the carboxyl groups reacted with the TiO2 surface by a crosslinked network. The setting reaction of PR4 agent is typical of ethylcyanoacrylates, which is based on anionic polymerization initiated by –OH groups [24]. Polymerization was confirmed by the strong reduction in the absorbance of C C bonds in the FTIR spectra. The adhesive film formed was rich in pores due to ethyl-cyanoacrylate degassing (or fuming), since the reaction

starts from the outer surface exposed to the environmental humidity and then proceeds to the bulk material. Such blisters create structure discontinuities and reduce primer strength. Due to the rapid setting reaction, any bonding condition of PR4 is expected to be established with the unset elastomer mass upon packing onto the primed cpTi surface. Although not statistically significant differences were found in m,  0 and RS% values between EL1 and EL2 after PR4 treatment, the  0 values of EL1 were significantly higher than PR2. Nevertheless, the low RS% found after PR4 treatment in both the elastomers indicates an inadequate interfacial bonding. The SEM/EDX analysis of the debonded cpTi specimens confirmed the presence of mixed failures even in cases considered as adhesive failures, based on stereomicroscopy. In these cases, small residual islands of elastomer were identified at higher magnification, rich in C and O. Interestingly, Si mapping failed to follow elastomer distributions, apparently due to the small differences in the atomic numbers among C (6), O (8) and Si (14), and the low Si content, in both the elastomers tested, in comparison with C and O. The control PMMA groups primed with PR1 demonstrated high m values, indicating reliable treatments with a predictable behavior. However, only EL1 resulted in high  0 and RS% values. Therefore, although Weibull statistics minimize the effects of variables associated with the specimen characteristics (preparation, handling, storage, etc.) [25,26], survival analysis should be always considered in accordance with the characteristic strength. The results of the present study may suggest some areas for further developments in the field. Vinyl functionalized silane coupling agents (i.e. allyltrimethoxysilane [CH2 CHCH2 Si(OCH3 )3 ]), currently used as adhesion promoter for vinyl silicones [27], may enhance bonding to cpTi, in comparison with the non-functional silane used in the PR2 primer. Condensation polymerization of the silanol groups formed upon silane hydrolysis should be considered mandatory, prior to vinyl silicone elastomer application, because a stable interface is established with proper molecular orientation of the pendant C C groups of the vinyl silane, favoring thus copolymerization with the elastomer. An alternative solution to silanes would be the use of vinyl functionalized phosphate coupling agents, as those used for base metal primers and titanium in prosthodontic applications [28]. Vinyl functionalized primers with rapid in situ polymerization capacity should be avoided, because the lack of residual C C bonds may hamper primer copolymerization with the slow-setting vinyl silicone elastomers. Compounds, like the universal ethylcyanoacrylate instant-setting adhesives, may be used for the rapid on-site repair of prostheses failures that do not require reprocessing. In such cases, though, it is important to preserve the elastic properties of silicone elastomers, by using a small amount of accurately placed adhesive, to avoid the increase in elastomer stiffness and the possible resultant rupture, associated with this treatment. In the present study no means of aging, other than storage in a 100% RH (37 ◦ C, 24 h), were used. Therefore, the PBS values recorded may be further reduced in service, due to hydrolytic or environmental degradation. Regarding hydrolytic degradation, silane primers are expected to offer superior performance due to integration of

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the reversible hydrolytic bond mechanism. This mechanism involves reversible breaking and reforming of the stressed bonds between the coupling agent and the substrate, with stress relaxation capacity but without rapture of adhesion [29]. An additional finding that may affect PBS, is the porosity of the elastomer specimens. The cross-sections of the fractured elastomer surfaces showed porosity despite that the manufacturers’ information was precisely followed during specimen preparation (slow mixing, de-airing under vacuum, etc.). The introduction of automatic cartridge delivery systems, as in dental impression materials, may substantially diminish porosity and therefore improve the bulk elastomer strength.

Acknowledgements This study was funded by the National and Kapodistrian University of Athens Special Account for Research grants (ELKE), and the University of Texas MD Anderson Cancer Center.

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