Effect of Light Aging on Silicone-Resin Bond Strength in Maxillofacial Prostheses Gregory Polyzois, DDS, Dr Dent, MScD,1 Antonis Pantopoulos, DDS, MS,2 Triantafillos Papadopoulos, DDS, Dr Dent,3 & Muhanad Hatamleh, BSc, MPhil, MSc, Dip (MaxFac), PhD4 1
Associate Professor and Chief of Maxillofacial Prosthetics Unit, Department of Prosthodontics, Dental School, University of Athens, Athens, Greece 2 Private Practice, Korinthos, Greece 3 Associate Professor, Department of Biomaterials, Dental School, University of Athens, Athens, Greece 4 Senior Maxillofacial Prosthetist, Cranio-Maxillofacial Prosthetics Unit, King’s College Hospital NHS Foundation Trust, London, UK
Keywords Bond strength; denture resins; light aging; silicone facial prosthesis. Correspondence Gregory Polyzois, Department of Prosthodontics, Dental School, University of Athens, 2 Thivon Str Goudi, 115 27 Athens, Greece. E-mail: [email protected] The authors deny any conflicts of interest. Accepted March 7, 2014 doi: 10.1111/jopr.12202
Abstract Purpose: The aim of this study was to investigate the effect of accelerated light aging on bond strength of a silicone elastomer to three types of denture resin. Materials and Methods: A total of 60 single lap joint specimens were fabricated with auto-, heat-, and photopolymerized (n = 20) resins. An addition-type silicone elastomer (Episil-E) was bonded to resins treated with the same primer (A330-G). Thirty specimens served as controls and were tested after 24 hours, and the remaining were aged under accelerated exposure to daylight for 546 hours (irradiance 765 W/m2 ). Lap shear joint tests were performed to evaluate bond strength at 50 mm/min crosshead speed. Two-way ANOVA and Tukey’s test were carried out to detect statistical significance (p < 0.05). Results: ANOVA showed that the main effect of light aging was the most important factor determining the shear bond strength. The mean bond strength values ranged from 0.096 to 0.136 MPa. The highest values were recorded for auto- (0.131 MPa) and photopolymerized (0.136 MPa) resins after aging. Conclusions: Accelerated light aging for 546 hours affects the bond strength of an addition-type silicone elastomer to three different denture resins. The bond strength significantly increased after aging for photo- and autopolymerized resins. All the bonds failed adhesively.
Facial defects and disfigurement resulting from tumor surgery, trauma, or congenital disorders can debilitate patients by preventing them from a normal life in our society. Patients with facial and large orofacial defects are rehabilitated using a variety of facial and combined (intra-/extraoral) maxillofacial prostheses. In many cases composite prostheses are fabricated from silicone elastomers, acrylic, visible light-curing (VLC), or fiberreinforced composite resins (FRC), and titanium.1-5 These prostheses consist of a resin or metal framework and a silicone overstructure. Osseointegration and craniofacial implants have led to advances in the management of various facial or combined defects.6 Silicone facial prostheses can be anchored to implants using retentive elements (e.g., magnets, bar-clips, ball attachments, or a combination of them) through a framework or retentive matrix. The framework is commonly fabricated with heat-polymerizing, autopolymerizing, or VLC resin to which the silicone facial elastomer is attached.4 Hence, a sufficient
tenacious bond is vital to ensure serviceability and functionality of prostheses. Nevertheless, the resin/silicone bond is the weakest link in the restoration, and the silicone may separate from the resin during handling of the prosthesis.7 To overcome this problem, several potential solutions have been introduced (e.g., use of bond primers and replacing retentive resin framework with an FRC one).1,8-11 Various studies on silicone facial elastomers have investigated their bonding characteristics by using peel,8,9,12,13 lap shear,14-17 shear,8,9 tensile,11,18,19 and pull-out10,20 testing. The effect of light aging on the bond strength of silicone facial elastomers to dentures or FRC resins has rarely been reported in the relevant literature and remains elusive.8,10,20 Therefore, the aim of this study was to investigate the effect of accelerated light aging on bond strength of a silicone elastomer to three types of denture resin. The null hypothesis tested was that light aging does not affect the bond strength between silicone elastomer and denture resins.
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Table 1 Materials used Trade name Denture resin SR 3/60 NF SR 3/60 NF Triad VLC Silicone elastomer Episil-E Bonding primer A-330-G
Chemical basis
Polymerization/application method
Heat-polymerizing acrylic resin Autopolymerizing acrylic resin Urethane dimethacrylate resin
12 hours at 75°C (dry heat) 20 minutes at room temperature 10 minutes visible light curing
Ivoclar AG, Schaan, Liechtenstein Ivoclar AG, Schaan, Liechtenstein Dentsply, York, PA
Silicone-addition cure (platinum)
24 hours at room temperature
Dreve-Dentamid GmbH, Unna, Germany
Solution of modified polyacrylates in ethyl methyl ketone and dichloromethane
2 hours at room temperature
Factor II, Inc., Lakeside, AZ
Figure 1 Lap joint specimen configuration and dimensions (mm). A = B = 13; C = D = 3 ± 0.2 mm; E = 23; F = G = 48.
Materials and methods The silicone facial elastomer, denture resins, and bond primer used in this study are listed in Table 1. To assess the bond strength of silicone elastomer to denture resins, the single lapshear-joint specimen and testing procedure was used. R Stone molds were made by investing machined Perspex (Lucite Int, Lancashire, UK) patterns of the single lap-joint specimen in denture flasks. The dimensions of the patterns and resultant specimens are shown in Figure 1. The lap-joint specimens were prepared by bonding the silicone elastomer to prefabricated denture resin specimens. The denture resins were prepared according to manufacturers’ instructions and polymerized as shown in Table 1 in the lower section of the flasks. The bonding area of the resin specimens was prepared by lapping with an 80-grit SiC waterproof paper in a polishing machine (Ecomet III; Buehler, Evanston, IL) under water flow. The areas to be primed were treated with acetone twice and left to dry for 15 minutes. Then, a thin coat of A-330-G primer was applied with a brush twice in opposite directions and allowed to dry for 2 hours at room temperature (23 ± 1°C and 50 ± 5% RH). The stone mold surfaces were treated with a tinfoil substitute, and denture resin specimens were placed into the molds with the bond area uppermost. The silicone elas-
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tomer, which comes in twin cartridges, was automixed using a mixing gun with cannula and directly injected without trapping air or creating voids into the upper section of the flasks. The flasks were closed and the silicone vulcanized for 24 hours at room temperature. After careful removal from the flasks, each specimen was trimmed of excess material with a sharp blade and inspected for air bubbles and surface defects. A total of 60 specimens were fabricated, 20 for each denture resin. Within each resin group, half of the specimens (n = 10) acted as controls, dark stored at room temperature and tested after 24 hours of fabrication. The remaining specimens (n = 10) were light aged in an aging machine (Suntest CPS+; Atlas Material Testing Technology GmbH, Linsengericht, Germany). Accelerated artificial daylight was generated by a quartz-filtered Xenon lamp with an illuminance of 160 klux and irradiance (E) of 765 W/m2 within the range of 300 to 800 nm. A complete weathering cycle lasted 120 minutes, including 18 minutes of wet weathering (29 ± 2°C) by distilled water and 102 minutes of dry weathering (36 ± 2°C).8,20 The Xenon light was applied for the whole period of aging, temperature in the chamber was kept to a maximum of 38°C using the integrated cooling unit, and the black standard temperature (BST) was 55°C. The radiant exposure (H = E × 3600 × hours) of the specimens was 1503.7 MJ/m2 for the 546-hour (23-day) aging period. Specimens were stored for 24 hours in a dark place at room temperature and then tested in tension with a universal testing machine (Tensometer 10; Monsanto Ltd., Swindon, UK) using a 500 N loading cell (U4000; Maywood Instruments Ltd., Reading, UK). During placement of the specimens in the grips, care was taken so that the applied load coincided with the long axis of the test specimen. Specimens were pulled at a 50 mm/min crosshead speed, and the maximum force indicated the point of failure by separation. The shear bond strength (MPa) was calculated as the maximum force (N) divided by the bond area (mm2 ). The mode of failure was examined visually and characterized as adhesive, cohesive, or mixed. Adhesive failure was defined as when a complete separation of resin and silicone elastomer occurred. Cohesive failure was when a tear or snap in the elastomer bulk was revealed.
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Table 2 Mean (SD) values of shear bond strength (MPa), (n = 10) Control SR 3/60 NF (heat polymerizing) SR 3/60 NF (autopolymerizing) Triad VLC
a
546-hour light aging
0.108 (0.008)
0.105a (0.010)
0.096a (0.008)
0.131b (0.015)
0.098a (0.008)
0.136b (0.012)
Note: Means with the same superscripts are not statistically different (p > 0.05).
Statistical analysis
Data were subjected to Levene’s test for homogeneity of variances and Kolmogorov-Smirnov test for normality. A two-way ANOVA was then used, followed by post hoc comparisons with Tukey’s test at α = 0.05. All analyses were computed with SPSS for Windows software (SPSS 16.0; SPSS, Inc, Chicago, IL).
Results All the specimens failed adhesively (i.e., the elastomer separated from the denture resin) implying that the tensile strength of the silicone was greater than the bond strength to the different types of resins. Mean shear bond strengths, standard deviations, and results of Tukey’s test are presented in Table 2. Two-way ANOVA revealed significant differences in shear bond strength among denture resins (F = 4.931, p = 0.011) and aging (F = 66.479, p = 0.0001) with a significant interaction between them (F = 24.858, p = 0.0001). In addition, ANOVA showed that the main effect of light aging was the most important factor determining the shear bond strength. The mean bond strength values ranged from 0.096 to 0.136 MPa.
Discussion The null hypothesis of this study was rejected because there was a significant main effect of light aging on the bond strength between silicone elastomers and denture resins. Bonding of silicone elastomers to retentive resin matrices is a crucial factor that enhances the clinical life of facial prostheses. This is particularly relevant with implant-retained facial prostheses and tight-fitting retentive elements exerting considerable forces at the silicone/resin interface, and failure is commonplace. A patient trying to remove a prosthesis usually grips part of it and rotates or peels it away from implants or skin. This action and detaching force have a horizontal component, which is well simulated in a peel type test.8,12 However, clinically and during service, the forces exerted on the silicone/resin interface are more closely related to shear and tear types.8,19 The single lap shear test used in this study has been also employed previously,14-17 and it has been proposed for comparing and selecting adhesives or bonding processes for susceptibility to fatigue and environmental changes.21 It has been reported that the stress distribution in the lap joint across and along the bond length is very complex, resulting in elevated shear and peel stresses at bondline ends.22 In a few studies, the same testing protocol was followed and similar or different materials and variables were investigated.14-17
Polyzois et al15,16 using Triad VLC and Paladon 65 (heatpolymerized) denture resins alongside Silskin silicone elastomer (addition type), different primers, and heat aging (50°C) for 120 days reported bond strengths from 0.053 to 0.160 MPa and from 0.046 to 0.163 MPa, respectively. Our findings are within this range. Polyzois and Frangou17 tested SR 3/60 (heatand autopolymerized) Triad VLC resins with Ideal additiontype silicone and a different primer and reported values from 0.030 to 0.040 MPa. Frangou et al14 tested SR 3/60 NF and Ideal silicone with three different primers and showed bond strengths from 0.026 to 0.057 MPa, similar to the current study, taking into account the different primer and silicones used. The longevity of a facial prosthesis is dependent on several factors such as color fading, tear of silicone, delamination from resin framework, and mechanical failures of retentive elements.6 Facial prostheses during their clinical lifetime and service are exposed to various environmental factors, such as solar radiation, temperature, moisture, pollutants, dust, and wind, that can cause degradation of their chemical, physical, and mechanical properties. The results of this study indicated that accelerated light aging for 546 hours induced changes in the shear bond strength of addition-type silicone elastomer to denture resins. It increased significantly (p < 0.05) for SR 3/60 NF (autopolymerized) and Triad VLC and remained the same (p > 0.05) for SR 3/60 (heat polymerized). Direct comparisons with other studies that conducted bonding of silicone elastomers to different resins after light aging or not is not possible due to various testing methods followed, loading rates, and materials used; however, Hatamleh and Watts8 reported increased peel and shear bond strengths for an addition-type silicone and autopolymerizing acrylic resin primed with A330-G after 360 hours of light aging. Our results corroborate this finding. Primer A330-G is specifically designed by its manufacturer for bonding platinum-cured silicone elastomers to acrylic resin (Table 1). The primer molecules collectively serve as a chemical intermediate of the silicone and resin substrate by swelling the surface and improving wettability of the substrate via organic solvents, promoting hydrogen bonding, covalent coupling, and the formation of an interpenetrating network (IPN) at the boundary interphase.23 The increased bond strengths for SR 3/60 NF (autopolymerized) and Triad VLC noticed in this study suggest further polymerization of silicone elastomer and reactions of pendant C═C bonds from reactive free radicals of denture resins by the heating and lighting inside the aging chamber.24,25 Silicones vulcanized in dental stone molds reported not to be completely polymerized, and accelerated light aging for 546 hours further enhanced polymerization, exhibiting higher bond strengths.26 Additionally, the quantities of available pendant unreacted groups in aged SR 3/60 NF (autopolymerized) and Triad VLC resins, seems to be higher for SR 3/60 NF (heat polymerized) resins, which showed the same bond strength as the unaged group.12,27,28 The light aging period selected simulates a silicone prosthesis in service for 18.2 months. Patients wear prostheses for 8 to 12 hours each day and expect to be exposed to solar radiation for at least 1 hour. Therefore, 1 month of clinical service equals
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Table 3 Extrapolated time (days) in typical climates corresponding to radiance exposure (1503.7 MJ/m2 for 23 days) of aging test Wavelength range, nm
Central Europe
Southern France
Arizona
Florida
2900 (186)
4524 (120)
3712 (146)
2
Average radiance exposure (MJ/m ) per year 300–800 outdoors (days) 2059 (264)
Source: Atlas Material Testing Solutions Bulletin (March 18, 2003) and AWSG (Atlas Weathering Services Group, available at: (http://www.atlaswsg.com/azweath/ weathpdf.asp).
30 hours of light aging. Eighteen months of aging falls within the anticipated lifespan of 14 to 24 months for an implantretained facial prosthesis.29,30 The extrapolated time of aging tests in typical weathering sites and climates corresponding to radiant exposure (1503.7 MJ/m2 in 23 days) is presented in Table 3. It seems that 546 hours aging corresponds to 186 days in Southern European climates and 120 days in Arizona, since the radiant exposure is higher in Arizona. These data could be a useful source of information concerning bonding characteristics of the tested materials for specific time periods. There is currently a lack of international standards/guidelines covering the minimum and clinically acceptable bond strength of facial materials to various substrates and suitable tests for this purpose. It would be very useful if laboratory testing data could give us a prediction of clinical in-service performance. For this purpose, further studies investigating the effect of natural weathering on bonding characteristics of various silicone elastomers to substrates treated with different primers are warranted.
Conclusion Considering the limitations of this laboratory study it may be concluded that accelerated light aging for 546 hours affects the bond strength of an addition-type silicone elastomer to three denture resins. The bond strength significantly increased after aging for photo- and autopolymerized resins. All the bonds failed adhesively.
References 1. Kurunm¨aki H, Kantola R, Hatamleh MM, et al: A fiber-reinforced composite prosthesis restoring a lateral midfacial defect: a clinical report. J Prosthet Dent 2008;100:348-352 2. Marion LR, Rothenberger SL, Minsley GE: A method of fabrication of a facial prosthesis that improves retention and durability: a clinical report. J Prosthet Dent 1997;77:457-460 3. Polyzois G, Frangou M: Combined polymeric-silicone nasal prosthesis enhancing longevity and serviceability: a case report. Acta Stomatol Croat 2012;46:65-70 4. Ciocca L, Maremonti P, Bianchi B, et al: Maxillofacial rehabilitation after rhinectomy using two different treatment options: clinical reports. J Oral Rehabil 2007;34:311-315 5. Wolf BH, Reitemeier BK, Schmidt AE, et al: In vitro testing of the bond between soft materials used for maxillofacial prostheses and cast titanium. J Prosthet Dent 2001;85:401-408 6. Ariani N, Visser A, van Oort RP, et al: Current state of craniofacial prosthetic rehabilitation. Int J Prosthodont 2013;26:57-67 218
7. Karakoca S, Aydin C, Yilmaz H, et al: Retrospective study of treatment outcomes with implant-retained extraoral prostheses: survival rates and prosthetic complications. J Prosthet Dent 2010;103:118-126 8. Hatamleh MM, Watts DC: Bonding of maxillofacial silicone elastomers to an acrylic substrate. Dent Mater 2010;26:387-395 9. Hatamleh MM, Watts DC: Mechanical properties and bonding of maxillofacial silicone elastomers. Dent Mater 2010;26:185-191 10. Hatamleh MM, Watts DC: Effects of bond primers on bending strength and bonding of glass fibers in fiber-embedded maxillofacial silicone prostheses. J Prosthodont 2011;20:113-119 11. Kantola R, Lassila L, Vallittu P: Adhesion of maxillofacial silicone elastomer to a fiber-reinforced composite resin framework. Int J Prosthodont 2011;24:582-588 12. Shetty US, Guttal SS: Evaluation of bonding efficiency between facial silicone and acrylic resin using different bonding agents and surface alterations. J Adv Prosthodont 2012;4:121-126 13. Haddad MF, Goiato MC, Santos DM, et al: Bond strength between acrylic resin and maxillofacial silicone. J Appl Oral Sci 2012;20:649-654 14. Frangou MJ, Polyzois GL, Tarantili PA, et al: Bonding of silicone extra-oral elastomers to acrylic resin: the effect of primer composition. Eur J Prosthodont Restor Dent 2003;11:115-118 15. Polyzois GL, Frangou MJ, Andreopoulos AG: The effect of bonding agents on the bond strengths of facial silicone elastomers to a visible light-activated resin. Int J Prosthodont 1991;4:440-444 16. Polyzois GL, Andreopoulos AG: Effect of silanes on the bond strength of silicone facial elastomers and acrylic resin. Quintessence Dent Technol 1992;15:163-165 17. Polyzois GL, Frangou MJ: Bonding of silicone prosthetic elastomers to three different denture resins. Int J Prosthodont 2002;15:535-538 18. Polyzois GL: Effect of interface roughening on the bond strength of silicone facial elastomers to acrylic resin. Quintessence Dent Technol 1995;18:207-210 19. Polyzois GL: A comparison of microwave and dry-heat curing methods on the bond strength of silicone facial materials applied to acrylic resin. J Prosthodont 1996;5:101-104 20. Hatamleh MM, Watts DC: Effects of accelerated artificial daylight aging on bending strength and bonding of glass fibers in fiber-embedded maxillofacial silicone prostheses. J Prosthodont 2010;19:357-363 21. ASTM D3163 – 01(2008) Standard Test Method for Determining Strength of Adhesively Bonded Rigid Plastic Lap-Shear Joints in Shear by Tension Loading. West Conshohocken, PA, ASTM International, DOI: 10.1520/D3163-01R08 22. Ebnesajjad S: Adhesives Technology Handbook. Norwich, NY, William Andrew Inc, 2008, pp 273-275 23. Witucki GL: A silane primer: chemistry and applications of alkoxy silanes. J Coat Technol 1993;65:57-60 24. Polyzois GL, Kakaboura AI, Eliades GC: Curing efficiency of visible light- and dual-cured denture reliners. Int J Prosthodont 2000;13:520-525
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25. Stathi K, Tarantili PA, Polyzois G: The effect of accelerated ageing on performance properties of addition type silicone biomaterials. J Mater Sci Mater Med 2010;21: 1403-1411 26. Lai JH, Hodges JS: Effects of processing parameters on physical properties of the silicone maxillofacial prosthetic materials. Dent Mater 1999;15:450-455 27. Kopperud HM, Kleven IS, Wellendorf H: Identification and quantification of leachable substances from polymer-based orthodontic base-plate materials. Eur J Orthod 2011;33: 26-31
Light Aging on Silicone-Resin Bond Strength
28. Vallittu PK, Miettinen V, Alakuijala P: Residual monomer content and its release into water from denture base materials. Dent Mater 1995;11:338-342 29. Hatamleh MM, Haylock C, Watson J, et al: Maxillofacial prosthetic rehabilitation in the UK: a survey of maxillofacial prosthetists’ and technologists’ attitudes and opinions. Int J Oral Maxillofac Surg 2010;39:1186-1192 30. Hooper SM, Westcott TP, Evans LL, et al: Implant-supported facial prostheses provided by a maxillofacial unit in a U.K. regional hospital: longevity and patient opinions. J Prosthodont 2005;14:32-38
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Associate Professor and Chief of Maxillofacial Prosthetics Unit, Department of Prosthodontics, Dental School, University of Athens, Athens, Greece 2 Private Practice, Korinthos, Greece 3 Associate Professor, Department of Biomaterials, Dental School, University of Athens, Athens, Greece 4 Senior Maxillofacial Prosthetist, Cranio-Maxillofacial Prosthetics Unit, King’s College Hospital NHS Foundation Trust, London, UK
Keywords Bond strength; denture resins; light aging; silicone facial prosthesis. Correspondence Gregory Polyzois, Department of Prosthodontics, Dental School, University of Athens, 2 Thivon Str Goudi, 115 27 Athens, Greece. E-mail: [email protected] The authors deny any conflicts of interest. Accepted March 7, 2014 doi: 10.1111/jopr.12202
Abstract Purpose: The aim of this study was to investigate the effect of accelerated light aging on bond strength of a silicone elastomer to three types of denture resin. Materials and Methods: A total of 60 single lap joint specimens were fabricated with auto-, heat-, and photopolymerized (n = 20) resins. An addition-type silicone elastomer (Episil-E) was bonded to resins treated with the same primer (A330-G). Thirty specimens served as controls and were tested after 24 hours, and the remaining were aged under accelerated exposure to daylight for 546 hours (irradiance 765 W/m2 ). Lap shear joint tests were performed to evaluate bond strength at 50 mm/min crosshead speed. Two-way ANOVA and Tukey’s test were carried out to detect statistical significance (p < 0.05). Results: ANOVA showed that the main effect of light aging was the most important factor determining the shear bond strength. The mean bond strength values ranged from 0.096 to 0.136 MPa. The highest values were recorded for auto- (0.131 MPa) and photopolymerized (0.136 MPa) resins after aging. Conclusions: Accelerated light aging for 546 hours affects the bond strength of an addition-type silicone elastomer to three different denture resins. The bond strength significantly increased after aging for photo- and autopolymerized resins. All the bonds failed adhesively.
Facial defects and disfigurement resulting from tumor surgery, trauma, or congenital disorders can debilitate patients by preventing them from a normal life in our society. Patients with facial and large orofacial defects are rehabilitated using a variety of facial and combined (intra-/extraoral) maxillofacial prostheses. In many cases composite prostheses are fabricated from silicone elastomers, acrylic, visible light-curing (VLC), or fiberreinforced composite resins (FRC), and titanium.1-5 These prostheses consist of a resin or metal framework and a silicone overstructure. Osseointegration and craniofacial implants have led to advances in the management of various facial or combined defects.6 Silicone facial prostheses can be anchored to implants using retentive elements (e.g., magnets, bar-clips, ball attachments, or a combination of them) through a framework or retentive matrix. The framework is commonly fabricated with heat-polymerizing, autopolymerizing, or VLC resin to which the silicone facial elastomer is attached.4 Hence, a sufficient
tenacious bond is vital to ensure serviceability and functionality of prostheses. Nevertheless, the resin/silicone bond is the weakest link in the restoration, and the silicone may separate from the resin during handling of the prosthesis.7 To overcome this problem, several potential solutions have been introduced (e.g., use of bond primers and replacing retentive resin framework with an FRC one).1,8-11 Various studies on silicone facial elastomers have investigated their bonding characteristics by using peel,8,9,12,13 lap shear,14-17 shear,8,9 tensile,11,18,19 and pull-out10,20 testing. The effect of light aging on the bond strength of silicone facial elastomers to dentures or FRC resins has rarely been reported in the relevant literature and remains elusive.8,10,20 Therefore, the aim of this study was to investigate the effect of accelerated light aging on bond strength of a silicone elastomer to three types of denture resin. The null hypothesis tested was that light aging does not affect the bond strength between silicone elastomer and denture resins.
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Table 1 Materials used Trade name Denture resin SR 3/60 NF SR 3/60 NF Triad VLC Silicone elastomer Episil-E Bonding primer A-330-G
Chemical basis
Polymerization/application method
Heat-polymerizing acrylic resin Autopolymerizing acrylic resin Urethane dimethacrylate resin
12 hours at 75°C (dry heat) 20 minutes at room temperature 10 minutes visible light curing
Ivoclar AG, Schaan, Liechtenstein Ivoclar AG, Schaan, Liechtenstein Dentsply, York, PA
Silicone-addition cure (platinum)
24 hours at room temperature
Dreve-Dentamid GmbH, Unna, Germany
Solution of modified polyacrylates in ethyl methyl ketone and dichloromethane
2 hours at room temperature
Factor II, Inc., Lakeside, AZ
Figure 1 Lap joint specimen configuration and dimensions (mm). A = B = 13; C = D = 3 ± 0.2 mm; E = 23; F = G = 48.
Materials and methods The silicone facial elastomer, denture resins, and bond primer used in this study are listed in Table 1. To assess the bond strength of silicone elastomer to denture resins, the single lapshear-joint specimen and testing procedure was used. R Stone molds were made by investing machined Perspex (Lucite Int, Lancashire, UK) patterns of the single lap-joint specimen in denture flasks. The dimensions of the patterns and resultant specimens are shown in Figure 1. The lap-joint specimens were prepared by bonding the silicone elastomer to prefabricated denture resin specimens. The denture resins were prepared according to manufacturers’ instructions and polymerized as shown in Table 1 in the lower section of the flasks. The bonding area of the resin specimens was prepared by lapping with an 80-grit SiC waterproof paper in a polishing machine (Ecomet III; Buehler, Evanston, IL) under water flow. The areas to be primed were treated with acetone twice and left to dry for 15 minutes. Then, a thin coat of A-330-G primer was applied with a brush twice in opposite directions and allowed to dry for 2 hours at room temperature (23 ± 1°C and 50 ± 5% RH). The stone mold surfaces were treated with a tinfoil substitute, and denture resin specimens were placed into the molds with the bond area uppermost. The silicone elas-
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tomer, which comes in twin cartridges, was automixed using a mixing gun with cannula and directly injected without trapping air or creating voids into the upper section of the flasks. The flasks were closed and the silicone vulcanized for 24 hours at room temperature. After careful removal from the flasks, each specimen was trimmed of excess material with a sharp blade and inspected for air bubbles and surface defects. A total of 60 specimens were fabricated, 20 for each denture resin. Within each resin group, half of the specimens (n = 10) acted as controls, dark stored at room temperature and tested after 24 hours of fabrication. The remaining specimens (n = 10) were light aged in an aging machine (Suntest CPS+; Atlas Material Testing Technology GmbH, Linsengericht, Germany). Accelerated artificial daylight was generated by a quartz-filtered Xenon lamp with an illuminance of 160 klux and irradiance (E) of 765 W/m2 within the range of 300 to 800 nm. A complete weathering cycle lasted 120 minutes, including 18 minutes of wet weathering (29 ± 2°C) by distilled water and 102 minutes of dry weathering (36 ± 2°C).8,20 The Xenon light was applied for the whole period of aging, temperature in the chamber was kept to a maximum of 38°C using the integrated cooling unit, and the black standard temperature (BST) was 55°C. The radiant exposure (H = E × 3600 × hours) of the specimens was 1503.7 MJ/m2 for the 546-hour (23-day) aging period. Specimens were stored for 24 hours in a dark place at room temperature and then tested in tension with a universal testing machine (Tensometer 10; Monsanto Ltd., Swindon, UK) using a 500 N loading cell (U4000; Maywood Instruments Ltd., Reading, UK). During placement of the specimens in the grips, care was taken so that the applied load coincided with the long axis of the test specimen. Specimens were pulled at a 50 mm/min crosshead speed, and the maximum force indicated the point of failure by separation. The shear bond strength (MPa) was calculated as the maximum force (N) divided by the bond area (mm2 ). The mode of failure was examined visually and characterized as adhesive, cohesive, or mixed. Adhesive failure was defined as when a complete separation of resin and silicone elastomer occurred. Cohesive failure was when a tear or snap in the elastomer bulk was revealed.
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Table 2 Mean (SD) values of shear bond strength (MPa), (n = 10) Control SR 3/60 NF (heat polymerizing) SR 3/60 NF (autopolymerizing) Triad VLC
a
546-hour light aging
0.108 (0.008)
0.105a (0.010)
0.096a (0.008)
0.131b (0.015)
0.098a (0.008)
0.136b (0.012)
Note: Means with the same superscripts are not statistically different (p > 0.05).
Statistical analysis
Data were subjected to Levene’s test for homogeneity of variances and Kolmogorov-Smirnov test for normality. A two-way ANOVA was then used, followed by post hoc comparisons with Tukey’s test at α = 0.05. All analyses were computed with SPSS for Windows software (SPSS 16.0; SPSS, Inc, Chicago, IL).
Results All the specimens failed adhesively (i.e., the elastomer separated from the denture resin) implying that the tensile strength of the silicone was greater than the bond strength to the different types of resins. Mean shear bond strengths, standard deviations, and results of Tukey’s test are presented in Table 2. Two-way ANOVA revealed significant differences in shear bond strength among denture resins (F = 4.931, p = 0.011) and aging (F = 66.479, p = 0.0001) with a significant interaction between them (F = 24.858, p = 0.0001). In addition, ANOVA showed that the main effect of light aging was the most important factor determining the shear bond strength. The mean bond strength values ranged from 0.096 to 0.136 MPa.
Discussion The null hypothesis of this study was rejected because there was a significant main effect of light aging on the bond strength between silicone elastomers and denture resins. Bonding of silicone elastomers to retentive resin matrices is a crucial factor that enhances the clinical life of facial prostheses. This is particularly relevant with implant-retained facial prostheses and tight-fitting retentive elements exerting considerable forces at the silicone/resin interface, and failure is commonplace. A patient trying to remove a prosthesis usually grips part of it and rotates or peels it away from implants or skin. This action and detaching force have a horizontal component, which is well simulated in a peel type test.8,12 However, clinically and during service, the forces exerted on the silicone/resin interface are more closely related to shear and tear types.8,19 The single lap shear test used in this study has been also employed previously,14-17 and it has been proposed for comparing and selecting adhesives or bonding processes for susceptibility to fatigue and environmental changes.21 It has been reported that the stress distribution in the lap joint across and along the bond length is very complex, resulting in elevated shear and peel stresses at bondline ends.22 In a few studies, the same testing protocol was followed and similar or different materials and variables were investigated.14-17
Polyzois et al15,16 using Triad VLC and Paladon 65 (heatpolymerized) denture resins alongside Silskin silicone elastomer (addition type), different primers, and heat aging (50°C) for 120 days reported bond strengths from 0.053 to 0.160 MPa and from 0.046 to 0.163 MPa, respectively. Our findings are within this range. Polyzois and Frangou17 tested SR 3/60 (heatand autopolymerized) Triad VLC resins with Ideal additiontype silicone and a different primer and reported values from 0.030 to 0.040 MPa. Frangou et al14 tested SR 3/60 NF and Ideal silicone with three different primers and showed bond strengths from 0.026 to 0.057 MPa, similar to the current study, taking into account the different primer and silicones used. The longevity of a facial prosthesis is dependent on several factors such as color fading, tear of silicone, delamination from resin framework, and mechanical failures of retentive elements.6 Facial prostheses during their clinical lifetime and service are exposed to various environmental factors, such as solar radiation, temperature, moisture, pollutants, dust, and wind, that can cause degradation of their chemical, physical, and mechanical properties. The results of this study indicated that accelerated light aging for 546 hours induced changes in the shear bond strength of addition-type silicone elastomer to denture resins. It increased significantly (p < 0.05) for SR 3/60 NF (autopolymerized) and Triad VLC and remained the same (p > 0.05) for SR 3/60 (heat polymerized). Direct comparisons with other studies that conducted bonding of silicone elastomers to different resins after light aging or not is not possible due to various testing methods followed, loading rates, and materials used; however, Hatamleh and Watts8 reported increased peel and shear bond strengths for an addition-type silicone and autopolymerizing acrylic resin primed with A330-G after 360 hours of light aging. Our results corroborate this finding. Primer A330-G is specifically designed by its manufacturer for bonding platinum-cured silicone elastomers to acrylic resin (Table 1). The primer molecules collectively serve as a chemical intermediate of the silicone and resin substrate by swelling the surface and improving wettability of the substrate via organic solvents, promoting hydrogen bonding, covalent coupling, and the formation of an interpenetrating network (IPN) at the boundary interphase.23 The increased bond strengths for SR 3/60 NF (autopolymerized) and Triad VLC noticed in this study suggest further polymerization of silicone elastomer and reactions of pendant C═C bonds from reactive free radicals of denture resins by the heating and lighting inside the aging chamber.24,25 Silicones vulcanized in dental stone molds reported not to be completely polymerized, and accelerated light aging for 546 hours further enhanced polymerization, exhibiting higher bond strengths.26 Additionally, the quantities of available pendant unreacted groups in aged SR 3/60 NF (autopolymerized) and Triad VLC resins, seems to be higher for SR 3/60 NF (heat polymerized) resins, which showed the same bond strength as the unaged group.12,27,28 The light aging period selected simulates a silicone prosthesis in service for 18.2 months. Patients wear prostheses for 8 to 12 hours each day and expect to be exposed to solar radiation for at least 1 hour. Therefore, 1 month of clinical service equals
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Table 3 Extrapolated time (days) in typical climates corresponding to radiance exposure (1503.7 MJ/m2 for 23 days) of aging test Wavelength range, nm
Central Europe
Southern France
Arizona
Florida
2900 (186)
4524 (120)
3712 (146)
2
Average radiance exposure (MJ/m ) per year 300–800 outdoors (days) 2059 (264)
Source: Atlas Material Testing Solutions Bulletin (March 18, 2003) and AWSG (Atlas Weathering Services Group, available at: (http://www.atlaswsg.com/azweath/ weathpdf.asp).
30 hours of light aging. Eighteen months of aging falls within the anticipated lifespan of 14 to 24 months for an implantretained facial prosthesis.29,30 The extrapolated time of aging tests in typical weathering sites and climates corresponding to radiant exposure (1503.7 MJ/m2 in 23 days) is presented in Table 3. It seems that 546 hours aging corresponds to 186 days in Southern European climates and 120 days in Arizona, since the radiant exposure is higher in Arizona. These data could be a useful source of information concerning bonding characteristics of the tested materials for specific time periods. There is currently a lack of international standards/guidelines covering the minimum and clinically acceptable bond strength of facial materials to various substrates and suitable tests for this purpose. It would be very useful if laboratory testing data could give us a prediction of clinical in-service performance. For this purpose, further studies investigating the effect of natural weathering on bonding characteristics of various silicone elastomers to substrates treated with different primers are warranted.
Conclusion Considering the limitations of this laboratory study it may be concluded that accelerated light aging for 546 hours affects the bond strength of an addition-type silicone elastomer to three denture resins. The bond strength significantly increased after aging for photo- and autopolymerized resins. All the bonds failed adhesively.
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