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

Abstract

Objectives

To investigate the effect of three adhesive primers on the morphology, chemistry and peel bond strength of two maxillofacial silicone elastomers with commercially pure titanium (cpTi).

Methods

The effect of three primers (PR2:A-304 Primer/A-320 Bonding Enhancer, PR3:Super 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-2006) and primed cpTi surfaces, a 90° T peel test was performed (PBS), using as reference EL1, EL2 specimens bonded to heat-cured poly(methyl methacrylate) resin (PMMA) primed with A-330G primer (PR1). Failure modes were analyzed under a stereomicroscope, and the percentage of remaining silicone (RS%) on cpTi and PMMA were calculated by image analysis. Scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDX) was 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 ( σ 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 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.

Introduction

A variety of materials is currently used in maxillofacial prostheses including poly(methyl methacrylate) or urethane-backed, medical grade silicone. Room temperature-vulcanizing (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 . These prostheses are retained with adhesives, tissue undercuts, magnets or in some cases extraoral osseointegrated implants . 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 heat-cured poly(methyl methacrylate) resin (PMMA) substructure is housing the retentive elements . The substructure should extend into the body of the silicone and possess sufficient surface area for efficient bonding . 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 . Encapsulation of samarium-cobalt (Co 5 Sm) magnets with tin or cpTi, improved their biocompatibility, minimizing corrosion and cytotoxicity .

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 . However, there is only a single report on the bond strength between silicone materials and cpTi, as mediated by different primers .

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.

Materials and methods

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

Table 1
The products used in the study.
Product (Lot) Code Manufacturer Composition Application procedure
Silicone elastomers
MDX4-4210 medical grade elastomer (DT031806) EL1 Dow Corning
Midland, MI, USA
Base: dimethyl vinyl siloxane, silica, platinum complex
Catalyst: dimethyl vinyl siloxane, Dimethyl hydrogen siloxane trimethyl siloxy-terminated, inhibitor
Mix 1 part of catalyst with 9 parts of base. Setting time at room temperature (RT): 3 days
A-2006 platinum silicone elastomer (R2375612) EL2 Factor II
Lakeside, AZ, USA
Part A: vinyl polysiloxane, silica, platinum complex.
Part B: dimethyl, methyl-hydrogen siloxane copolymer, silica
Mix equal parts of A and B. Setting time (RT): 16 h
Core materials
Commercially pure grade II Titanium (8100-I) cpTi I. Moatsos, Athens, GR Wt%: C < 0.08, N < 0.03, Fe < 0.30, H < 0.015, O < 0.25, Ti > 99.
Lucitone 199 (061208) PMMA Dentsply Caulk, York, PA, USA Liquid: methyl methacrylate, ethylene glycol dimethacrylate
Powder: poly methyl methacrylate, dibenzoyl peroxide
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
Primers
A-330G Primer
L4707836
PR1 Factor II
Lakeside, AZ, USA
Solution of modified polyacrylates in methyl ethyl ketone and dichloromethane 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
A-304 Primer
(L30332)
PR2 Factor II
Lakeside, AZ, USA
Wt%: Naphtha (85), tetra- n -propyl silicate (5), tetrabutyltitanate (5), tetra (2-methoxyethoxy)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-320 bonding enhancer. Allow to dry and apply a uniform thin coat of A304 primer. Setting time (RT): 30–120 min
A-320 Bonding Enhancer
(R032106)
Trichloroethane (85), amorphous treated silica (5)
Super-Bond C&B Monomer
(LR1)
PR3 Sun Medical, Moriyama, JPN Methyl methacrylate (MMA), 4-methacryloxyethyltrimelitic acid anhydride (4-META) Apply a thin layer of the monomer and allow to dry
Setting time (37 °C): 5–6 min (with T-BBO catalyst)
Super Glue
(BI2626)
PR4 Bison Int. Goes, NL Ethyl-cyanoacrylate Apply a thin layer. Setting time (RT): 60 s

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% H 2 O 2 with a 0.4 μm 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 N 2 -cooled mercury–cadmium telluride (MCT) detector, 4000–650 cm −1 wave number range, 4 cm −1 resolution, 100 μm × 100 μm aperture, 400 μm × 300 μm scan size for mapping and 100 scans co-addition per site. All spectra were subjected to Kramers–Kroning and baseline corrections.

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) . 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 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 μA 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 N 2 -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 μA beam current) and multielement mapping for C, O, Si and Ti was performed using area scans at the same magnifications.

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 ( σ 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 σ 0 and m respectively, were calculated by WinSMITH Weibull & Visual 2.0M software (Fulton Findings, Torrance, CA, USA). Survival probability at any peel stress [ P s = exp{−( σ / σ 0 ) m }], and fracture probability [ P f = 1 − exp{−( σ / σ 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).

Materials and methods

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

Table 1
The products used in the study.
Product (Lot) Code Manufacturer Composition Application procedure
Silicone elastomers
MDX4-4210 medical grade elastomer (DT031806) EL1 Dow Corning
Midland, MI, USA
Base: dimethyl vinyl siloxane, silica, platinum complex
Catalyst: dimethyl vinyl siloxane, Dimethyl hydrogen siloxane trimethyl siloxy-terminated, inhibitor
Mix 1 part of catalyst with 9 parts of base. Setting time at room temperature (RT): 3 days
A-2006 platinum silicone elastomer (R2375612) EL2 Factor II
Lakeside, AZ, USA
Part A: vinyl polysiloxane, silica, platinum complex.
Part B: dimethyl, methyl-hydrogen siloxane copolymer, silica
Mix equal parts of A and B. Setting time (RT): 16 h
Core materials
Commercially pure grade II Titanium (8100-I) cpTi I. Moatsos, Athens, GR Wt%: C < 0.08, N < 0.03, Fe < 0.30, H < 0.015, O < 0.25, Ti > 99.
Lucitone 199 (061208) PMMA Dentsply Caulk, York, PA, USA Liquid: methyl methacrylate, ethylene glycol dimethacrylate
Powder: poly methyl methacrylate, dibenzoyl peroxide
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
Primers
A-330G Primer
L4707836
PR1 Factor II
Lakeside, AZ, USA
Solution of modified polyacrylates in methyl ethyl ketone and dichloromethane 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
A-304 Primer
(L30332)
PR2 Factor II
Lakeside, AZ, USA
Wt%: Naphtha (85), tetra- n -propyl silicate (5), tetrabutyltitanate (5), tetra (2-methoxyethoxy)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-320 bonding enhancer. Allow to dry and apply a uniform thin coat of A304 primer. Setting time (RT): 30–120 min
A-320 Bonding Enhancer
(R032106)
Trichloroethane (85), amorphous treated silica (5)
Super-Bond C&B Monomer
(LR1)
PR3 Sun Medical, Moriyama, JPN Methyl methacrylate (MMA), 4-methacryloxyethyltrimelitic acid anhydride (4-META) Apply a thin layer of the monomer and allow to dry
Setting time (37 °C): 5–6 min (with T-BBO catalyst)
Super Glue
(BI2626)
PR4 Bison Int. Goes, NL Ethyl-cyanoacrylate Apply a thin layer. Setting time (RT): 60 s

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% H 2 O 2 with a 0.4 μm 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 N 2 -cooled mercury–cadmium telluride (MCT) detector, 4000–650 cm −1 wave number range, 4 cm −1 resolution, 100 μm × 100 μm aperture, 400 μm × 300 μm scan size for mapping and 100 scans co-addition per site. All spectra were subjected to Kramers–Kroning and baseline corrections.

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) . 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 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 μA 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 N 2 -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 μA beam current) and multielement mapping for C, O, Si and Ti was performed using area scans at the same magnifications.

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 ( σ 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 σ 0 and m respectively, were calculated by WinSMITH Weibull & Visual 2.0M software (Fulton Findings, Torrance, CA, USA). Survival probability at any peel stress [ P s = exp{−( σ / σ 0 ) m }], and fracture probability [ P f = 1 − exp{−( σ / σ 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).

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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Peel strength and interfacial characterization of maxillofacial silicone elastomers bonded to titanium
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