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Bonding of maxillofacial silicone elastomers to an acrylic substrate
d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 387–395
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Bonding of maxillofacial silicone elastomers to an acrylic substrate Muhanad M. Hatamleh ∗ , David C. Watts Biomaterials Research Group, School of Dentistry, University of Manchester, Higher Cambridge Street, Manchester, M14 5FH, UK
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To investigate the effect of three different primers on shear and peel bond strengths
Received 18 September 2009
between three maxillofacial silicone elastomers and an acrylic resin after 360 h of acceler-
Received in revised form
ated daylight-aging.
23 December 2009
Materials and methods. Peel and shear-bond strengths of three maxillofacial silicone elas-
Accepted 5 January 2010
tomers (TechSil S25, Cosmesil M511, Cosmesil Z004) to acrylic denture resin bases using three adhesive primers (611, A304, A330-G) were assessed at baseline and after 360 h of accelerated artificial light-aging. Data were collected and statistically analyzed by two-way
Keywords:
ANOVA, one-way ANOVA, and Bonferroni post hoc tests (˛ = 0.05). Independent t-test was
Maxillofacial silicone elastomer
used to investigate the effect of light-aging on bond strengths (˛ = 0.05). Modes of failure
Adhesive primers
were visually analyzed and categorized as adhesive, cohesive, or mixed.
Peel bond test
Results. In the peel bond test, at both baseline and after aging, there was a significant influ-
Shear-bond test
ence of primers and silicones on bond strength (p < 0.001) and a strong interaction was also
Light-aging
found between primers and silicones (p < 0.05). Peel bond strengths ranged from 0.85 to 5.31 and 0.76 to 8.22 N/mm at baseline and after aging, respectively. The Z004 and 611 and Z004 and A330-G combinations showed the highest peel bond strength (5.31 and 8.22 N/mm, respectively) (p < 0.05), as baseline and after aging. In the shear-bond test, there was only a significant influence of silicones on shear-bond strength (p < 0.001), whereas primers did not affect it (p > 0.05), and no interaction between primers and silicones was found (p > 0.05). Shear-bond strengths ranged from 0.42 to 0.66 and 0.48 to 1.00 MPa at baseline and after aging, respectively. The combinations of Z004 and 611, Z004 and A304, Z004 and A330-G, M511 and A304, M511 and A330-G exhibited the highest bond strength (0.59–0.65 MPa) at baseline, and the Z004 with any of the primers (611, A304, and A330-G) showed greater bond strengths (0.89–1.00 MPa) (p < 0.05) after aging. All the silicone elastomers at baseline, regardless of the adhesive primers, failed predominantly by cohesive debonding under peel and shear forces (68.9% and 100% respectively). However, after light-aging, peel and shear forces predominantly exhibited adhesive (79.5%) and cohesive (84.4%) failures, respectively. Conclusions. Shear and peel test-regimes were both relevant and suitable for studying bonding and debonding characteristics of maxillofacial silicone elastomers bonded to an autopolymerising acrylic resin. The silicone/acrylic bond strengths were different for shear versus peel tests: 0.42–1.00 MPa for shear and 0.51–8.22 N/mm for peel. Cohesive failures were predominant with shear-tests, whereas peel-tests showed predominant cohesive failures at baseline but adhesive failures after light-aging. The optimum bonding achieved (best bonding at baseline that increased or was unaffected after light-aging) varied between shear and
∗
Corresponding author. Tel.: +44 0 161 275 6749; fax: +44 0 161 275 6748. E-mail address: [email protected] (M.M. Hatamleh). 0109-5641/$ – see front matter © 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2010.01.001
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peel. For shear, it was achieved using Cosmesil Z004, with any primer, and M511 (but only with A304, and 330-G primers). For peel, it was achieved using both Cosmesil Z004 and TechSil S25 bonded using A330-G primer. Consequently, Cosmesil Z004 along with primer A330-G was the optimum silicone/primer combination to select on the basis of bond strengths. A wide variety of new maxillofacial silicone elastomers and primers used in this study gave serviceable bond strengths. However, Cosmesil Z004 along with primer A330-G gave the optimum silicone/primer combination to select on the basis of bond strengths. © 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Maxillofacial prosthetics is defined as “the art and science of anatomical, functional or cosmetic reconstruction by artificial substitutes of those regions in the maxilla, mandible, and face that are missing or defective because of surgical intervention, trauma, pathology, or developmental or congenital malformation” [1]. The introduction of the osseointegration concept for craniofacial implants in maxillofacial prosthetic rehabilitation has minimized some of the disadvantages associated with traditional retention methods (i.e. medical-grade skin adhesives, eyeglasses, and tissue undercuts) and provided patients with predictable aesthetics and durability, improved prosthesis retention and stability [2–5]. Retentive attachment selection of the implant is made with regard to advantages and disadvantages of bar-clip and magnetic retention [6]. Extra-oral facial prostheses used in conjunction with implants require a retentive matrix to hold the bar clips or magnets. The retentive matrix is commonly made from acrylic resin (i.e. heat-polymerizing, auto-polymerizing, or light-cured materials) to which the facial silicone elastomer is attached. Hence, sufficient bond strength is vital to ensure a serviceable and functional prosthesis. During service, maxillofacial silicone prostheses suffer bond failures between the silicone and denture base, color deterioration and loss of mechanical properties (i.e. tear and tensile strengths) [7,8]. As several potential solutions have been introduced to overcome problems associated with silicone elastomer; delaminating of silicone away from the retentive matrix is still a persisting problem. It was faced by suggesting new design techniques for replacing the retentive acrylic plate with retentive glass-fiber framework [9,10], or the use of bond primers [11–13]. Maxillofacial silicone elastomers are dimethyl siloxane polymers, and have different chemical structure to PMMA denture base resin. Thus an adhesive is supplied to aid their bonding to the denture base [14]. It is likely that adhesive primers have an organic solvent and an adhesive agent that reacts with both silicone and resin materials [15]. They activate the surfaces via etching or promoting hydrogen bonding and covalent coupling, increasing the wettability of the substrate and by impregnating the surface layer with the polymeric ingredients [16]. Bonding of maxillofacial silicone elastomers to either polyurethane or acrylic resin substrates has been studied. Peel-bond strength of maxillofacial silicone elastomer to polyurethane substrate varied with the silicone elastomer, primer, and conditioning performed. It was
enhanced using a combination of MDX 4-4210 silicone and either S-2260 or A-4040 primers [17]. It was greater with primer 1205 than with S-2260 primer regardless of the polymerization method or primer reaction time [18]. Another study reported it as 6.06 N/mm, and showed it decreased after soaking in hot (3.93 N/mm) or room-temperature (2.49 N/mm) soapy water [19]. It was reported to be 1.32, 1.25, and 0.91 N/mm, using SofrelinerMSprimer, Sofreliner, and A-330-G primers, respectively [20]. Bonding of Silastic 891 facial material to acrylic resin was enhanced after using three different primers (Dow corning 4040, S-2260, and 1200), in comparison to the control group, with 4040 showing the greatest increase [11]. The tensile bond strength of silicone elastomers to acrylic denture resin was not affected by the curing method (microwave irradiation and dry heat), but with the type of silicone elastomer. Mollomed showed the highest bond strength (0.53 MPa) in comparison to Silskin II (0.28 MPa), and MDX4-4210 (0.11 MPa), and A-2186 (0.12 MPa) silicone elastomers [21]. The bond strength between different silicone elastomers and acrylic resins was in the range 0.03–0.23 MPa whereas Cosmesil condensationcured silicone elastomer showed the highest bond strength to the acrylic resins in comparison to the addition-cured Ideal silicone elastomer [12]. Frangou et al. reported that the bond strength between silicone elastomers (Cosmesil and Ideal) and acrylic resin using different primers and primer mixtures (Cosmesil, Cosmesil/Z-6020, and Cosmeil/A174) was in the range 0.026–0.22 MPa [13]. They stated that the compatibility and affinity of primer composition with the selected silicone elastomer is important for efficient bonding. New maxillofacial silicone elastomers and primers have been developed to enhance silicone–acrylic bonding, but controlled testing of these materials has been minimal. Commonly used maxillofacial prosthetic silicone elastomers include TechSil S25 (Technovent, Leeds, UK), Cosmesil M511 and Cosmesil Z004 (Principality medical, Newport, UK) [22,23]. A review of the literature revealed a deficiency of studies in comparing the effect of primers on maxillofacial silicone–acrylic bond strengths using different bond tests, or in bond serviceability of primers after conditioning. Accordingly, the aim of this study was to investigate the effects of three different primers on the shear and peel bond strengths between three maxillofacial silicone elastomers and acrylic resin after 360 h of accelerated daylight-aging. The null hypothesis stated that bond strength of maxillofacial silicone elastomers is not affected by the type of silicone elastomer, adhesive primer, aging condition, or bond test type.
389
Factor II, Inc., Lakeside, AZ, USA Lot l4707836 Platinum Primer (A-330-G) [Gold]
Manufacturer
Factor II, Inc., Lakeside, AZ, USA Lot L42587 Platinum Primer (A 304)
Silicone Primers
08/02 08/01 08/02 Lot 07/03 Silicone elastomers
TechSil (S25) Cosmesil Series Materials (M511) Cosmesil Series Materials (Z004) Ancillary Materials Platinum Primer (G611)
Clear repair acrylic liquid (MMA) and powder [1:1] Heat cured for 2 h at 100 ◦ C [9:1] Heat cured for 1 h at 100 ◦ C [10:1] Heat cured for 1 h at 100 ◦ C [1:1] Organic solvent based primer include components of propan-2-ol and various vinyl silanes A mixture of naptha (85%), tetra-n-propyl silicate (5%), tetrabutyltitnate (5%), and tetra (2-methoxyethoxy) silane (5%) A solution of modified ployacrylates in ethylmethylketone and dichloromethane
Composition/processing [mixing ratios] Batch number
2011/07
Brand name Skillbond Acrylic resin
Fig. 1 – Peel bond specimen preparation (dimensions in mm), A = 75 ± 1, B = 50 ± 1 (free silicone), C = 25 ± 1 (bonded silicone). Width = 10 ± 1, and thickness of each silicone and acrylic strips = 3 ± 0.2 mm.
Materials
Materials tested and their mixing ratios and processing parameters are listed in Table 1. For shear-bond strength, specimens’ fabrication method was previously described [23,24]. Autopolymerizing clear acrylic resin was mixed and packed inside hollow brass cylinders (external diameter = 18 mm, internal diameter = 14.4 mm, depth = 25 mm). Their surfaces were prepared for bonding (after 24 h of fabrication) by lapping with a 60grit silicone carbide waterproof abrasive paper. Application of adhesive bond primers was conducted according to manufacturers’ instructions. The acrylic surfaces were cleaned with acetone and left to air dry. Then, a uniform layer of the adhesive primer was applied using a brush over the surfaces, and left for 30 min at 23 ± 1 ◦ C room temperature and 50 ± 5% relative humidity. Teflon disks (PTFE) (external diameter = 18 mm, internal diameter = 8 mm, thickness = 3 mm) were used to define the area over which the silicone elastomers were attached to the acrylic surfaces. Then silicone elastomers were mixed, packed inside the disks, and cured according to manufacturers’ instructions. After curing, specimens were incubated at 37 ± 1 ◦ C for 24 h and then shear-bond tests were performed as described in previous study. A total of 90 specimens were fabricated (Table 2). For the peel test, specimen fabrication followed the principle of the 180◦ peel test. Preparation procedure was conducted at two stages; fabricating the acrylic resin blanks, and then bonding to maxillofacial silicone elastomer. Moulds were fabricated by investing hard wax blanks (75 mm × 65 mm × 3 mm) (Associated Dental products Ltd., Swinton, UK) in dental stone (Class 1, Dentsply, Surrey, UK), and then removed. Then auto-polymerizing clear acrylic denture base material was poured into the moulds and cured using the conventional flasking technique to produce acrylic blanks. The surfaces of the acrylic blanks were prepared for bonding as described earlier. Then, they were cleaned with water, and adhesive tape was used to define the area over which the silicone elastomer was attached to the acrylic substrate. The tape covered an area of (50 mm × 65 mm × 3 mm) leaving an area of (25 mm × 65 mm) of silicone elastomer bonded to the acrylic substrate (Fig. 1). The free silicone was gripped in performing the peel test. Another set of wax blanks of greater thickness (75 mm × 65 mm × 6 mm) were used to make stone moulds as described earlier. The acrylic blanks were fixed inside the moulds and the silicone elastomer was mixed and packed over the acrylic blanks. The silicone elastomer was cured according to manufacturers’ instructions, and moulds were left to bench cool. Prior to packing the silicone elastomers,
Skillbond Direct Ltd., Bucks, UK
Materials and methods
Table 1 – Materials used in the study.
2.
Technovent Ltd., Leeds, UK Principality Medical Ltd., Newport, UK Principality Medical Ltd., Newport, UK Principality Medical, Newport, UK
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Table 2 – Groups of the study. Nine groups were present for each peel and shear-bond tests. Groups (n = 10)a
Silicone and primer
1
Z004 and 611
2 3 4 5 6 7 8 9
Z004 and A304 Z004 and A330-G M511 and 611 M511 and A304 M511 and A330-G S25 and 611 S25 and A304 S25 and A330-G
a
Conditioning For each silicone and primer combination control specimens (n = 5) were tested after 24 h. Remaining specimens (n = 5) were light aged for 360 h, and then tested.
Within each group, half of the specimens acted as control (n = 5).
acrylic surfaces were treated with one of three different primers (according to their manufacturers’ instructions). The treatment process is the same as described previously. After removing the cured specimens from the moulds they were dry stored for 24 h at 23 ± 1 ◦ C room temperature and then cut using a fine saw (Model CBS 355, Clarke Int., UK) into 6 strips of width 10 ± 0.4 mm (the actual width of each strip was measured before testing). The specimen thickness was 6 mm (3 mm of acrylic blank and 3 mm of silicone elastomer). The silicone strip was bonded to the acrylic denture base at one end (25 mm × 10 mm × 3 mm) and free at the other (50 mm × 10 mm × 3 mm). Each free strip was turned back at 180◦ such that the hard base and soft strip could be gripped in a tensile direction. A total of 90 specimens were fabricated (Table 2). Within each silicone and primer combination, half of the specimens (n = 5) acted as control specimens and tested after 24 h of fabrication. The remaining specimens (n = 5) were light aged for 360 h in an aging machine (Suntest Chamber CPS, Heraeus Instruments, Hanau, Germany). Accelerated artificial daylight was generated using filtered Xenon light of 150 klx. A complete weathering cycle lasted for 120 min, including 18 min of wet weathering by controlled-flow of distilled water (29 ± 2 ◦ C), followed by 102 min of dry weathering (36 ± 2 ◦ C). The relative humidity inside the aging chamber was approximately 70%, and air pressure was 700–1060 hPa. The Xenon light was applied for the whole duration of aging (360 h). A universal testing machine (Zwick Roell Z020) was used to peel the maxillofacial silicone elastomers at an angle of 180◦ , and at 10 mm/min crosshead speed (Fig. 2). Each specimen was pulled in tension to peel the silicone elastomer from the denture base resin. The force needed to cause failure and the modes of failure were recorded. Peel bond strength (PS) (N/mm) was determined according to Eq. (1) [25]. PS =
F W
1 + 2
Fig. 2 – Peel test bond failures categories; adhesive (peel), and cohesive (snap or tear).
rial, or cohesive (tear or snap of the silicone elastomer) [26] (Fig. 3). Within each bond test, Univariate two-way ANOVA (release 16, SPSS Inc., Chicago, IL) (p < 0.05) was used to analyze significant differences in bond strengths (dependent variable) between different silicone and primer (independent variables) combinations at baseline, and after 360 h of light-aging. All data were subjected to Levene’s test of homogeneity of variance (␣ = 0.05), following the assumption of equal variances.
+1
(1)
where F is the maximum force recorded (N), W is the width of the specimens (mm), and is the extension ratio of the silicone elastomer (the ratio of stretched to unstretched length). The denture base interface was visually analyzed, and failure modes were characterized as either adhesive (peel), indicating peeling of the silicone elastomer from the denture base mate-
Fig. 3 – Test specimen undergoing 180◦ peel test.
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Table 3 – Mean (SD) values of peel bond strength (N/mm). Group
Silicone/primer combination (n = 5)
1 2 3 4 5 6 7 8 9
Z004 and 611 Z004 and A304 Z004 and A330-G M511 and 611 M511 and A304 M511 and A330-G S25 and 611 S25 and A304 S25 and A330-G
Equal variances were assumed (p > 0.05) and Bonferroni post hoc test was used to analyze significant differences between paired groups. One-way ANOVA and Bonferroni post hoc test were used to detect statistical significances in bond strength between the silicone and primer combinations among peel and shearbond tests. Silicone and primer combination that exhibits the greatest (p < 0.05) bond strengths were reported. Independent t-test was used to investigate the effect of aging on the bond strengths (p < 0.05).
3.
Results
For peel bond strength (Table 3), and at baseline, there was a significant influence of primers and silicones on bond strength (p < 0.001) and a strong interaction was also found between primers and silicones (p < 0.05). For silicone elastomers, Cosmesil Z004 was statistically different from Cosmesil M511 (p = 0.00) and TechSil S25 (p = 0.00). For primers, Bonferroni post hoc test showed that 304 was statistically different from 611 (p = 0.00) and 330 (p = 0.00). After 360 h of light-aging, there was still significant influence of the primers and silicones on bond strength (p < 0.001) and a strong interaction was also found between primers and silicones (p < 0.05). For silicone elastomers, Cosmesil Z004 was statistically different from Cosmesil M511 (p = 0.00) and TechSil S25 (p = 0.042). Cosmesil M511 was statistically different from TechSil S25 (p = 0.004). For primers, Bonferroni post hoc test showed that 330 was statistically different from 611 (p = 0.00) and 304 (p = 0.00).
Conditioning Control
360 h light-aging
5.31 (0.89) 1.39 (0.41) 2.68 (0.66) 1.30 (0.67) 1.78 (0.47) 2.36 (0.66) 1.41 (0.34) 0.85 (0.12) 3.68 (0.72)
1.64 (0.36) 0.94 (0.15) 8.22 (2.24) 0.51 (0.19) 2.12 (0.76) 2.30 (0.35) 1.49 (0.25) 0.76 (0.16) 6.00 (1.02)
For shear-bond strength (Table 4), and at baseline, there was only significant influence of silicones on bond strength (p < 0.001), whereas primers did not affect it (p = 0.572), and no significant interaction between primers and silicones existed (p = 0.138). Bonferroni post hoc test showed no significant differences between all primers (p > 0.05). However, for silicone elastomers, Cosmesil Z004 was statistically different from Cosmesil M511 (p = 0.01) and TechSil S25 (p = 0.00). Also Cosmesil M511 was statistically different from TechSil S25 (p = 0.003). After 360 h of light-aging, there was no difference. There was only significant influence of silicones on bond strength (p < 0.001), while primers did not affect the bond strength (p = 0.184). And, there was no statistically significant interaction between primers and silicones (p = 0.107). Bonferroni post hoc test showed no significant differences between all primers (p > 0.05). However, for silicone elastomers, Cosmesil Z004 was statistically different from Cosmesil M511 (p = 0.00) and TechSil S25 (p = 0.00). Also Cosmesil M511 was statistically different from TechSil S25 (p = 0.011). One-way ANOVA was used to detect statistical significances in bond strengths between the silicone and primer combinations among peel and shear-bond tests. Silicone and primer combination that exhibits the greatest bond strength values (p < 0.05) varied with bond test performed and aging. At baseline, the peel bond strengths range was 0.85–5.31 N/mm (Table 3). The Z004 and 611 combination showed the greatest peel bond strength (5.31 N/mm) (p < 0.05) among other combinations. The S25 and A304 showed weaker bond strength in comparison to Z004 and 611, Z004 and A330-G, M511 and A330-G, S25 and A330-G (p < 0.05). After light-aging, peel bond
Table 4 – Mean (SD) values of shear-bond strengths (MPa). Group
1 2 3 4 5 6 7 8 9 a
Silicone/primer combination (n = 5) Z004 and 611a Z004 and A304 Z004 and A330-G M511 and 611a M511 and A304 M511 and A330-G S25 and 611a S25 and A304 S25 and A330-G
Results of control groups were adopted from previous study [23].
Conditioning Control
360 h light-aging
0.65 (0.10) 0.64 (0.10) 0.66 (0.06) 0.54 (0.07) 0.59 (0.03) 0.59 (0.06) 0.51 (0.03) 0.52 (0.08) 0.42 (0.04)
1.00 (0.10) 0.90 (0.08) 0.89 (0.10) 0.59 (0.02) 0.66 (0.06) 0.64 (0.07) 0.56 (0.04) 0.54 (0.04) 0.48 (0.09)
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Table 5 – Modes of failures of both bond tests exhibited by control and light-aged specimens. Failure mode
Frequency number (%) Peel bond strength (N/mm)a Control (24 h)
Cohesive Adhesive Mixed Total a b
31 (68.9%) 14 (31.1%) 0 45
After light-aging (360 h) 9 (20.5%) 35 (79.5%) 0 44b
Control (24 h) 45 0 0 45
After light-aging (360 h) 38 (84.4%) 4 (8.9%) 3 (6.7%) 45
For Peel bond test modes of failures, peel is considered adhesive, while snap or tear is cohesive. One specimen failed during testing.
strengths range was (0.76–8.22 N/mm). The Z004 and A330G combination exhibited the strongest peel bond strength (8.22 N/mm) (p < 0.05) among other combinations. The S25 and A330-G had higher peel bond strength value (6.00 N/mm) than the remaining combinations, which exhibited the same bond strength values (p > 0.05) which were in the range (0.51–2.30 N/mm). For shear-bond test specimens, and at baseline, bond strengths range was (0.42–0.66 MPa) (Table 4). The combinations of (Z004 and 611, Z004 and A304, Z004 and A330-G, M511 and A304, M511 and A330-G) exhibited greater bond strength (0.59–0.65 MPa) than S25 and A330-G combination (p < 0.05). After light-aging, shear-bond strengths range was (0.48–1.00 MPa). The Z004 with any of the primers (611, A304, and A330-G) showed greater bond strength (0.89–1.00 MPa) than the other silicone and primer combinations (0.48–0.59 MPa) (p < 0.05). Effect of aging on peel bond strength varied (p < 0.05). It increased for Z004 and A330-G and S25 and A330-G combinations; and decreased for Z004 and 611, Z004 and A304, and M511 and 611. Aging for 360 h increased shear-bond strength (p < 0.05) for Cosmesil Z044 silicone elastomer with any of the three primers (611, A304, A330). However, for the other silicone and primer combinations, aging had no effect (p > 0.05). Regarding modes of failures (Table 5), peel bond test specimens exhibited mainly cohesive (68.9%), and adhesive (31.1%) failures at baseline. While after 360 h of light-aging, modes of failures were mainly adhesive (79.5%), and cohesive (20.5%) failures. Shear-bond test specimens exhibited entirely cohesive failures at baseline (100%), while after aging, modes of failures were still predominantly cohesive (84.4%). Other failures were adhesive (8.9%), and mixed (6.7%) failures.
4.
Shear-bond strength (MPa)
Discussion
Bonding of maxillofacial silicone elastomers to retentive acrylic substrates is a crucial factor that enhances the serviceability of maxillofacial silicone prostheses. The bonding was investigated by studying the sole effects of silicones, and following recognizable peel, and shear-tests. This current study showed that primers, silicones, and bond tests affected bond strength between maxillofacial silicone elastomers and acrylic denture resin. Thus, the null hypothesis was rejected. The maxillofacial silicone elastomers and bond primers chosen for this study are commonly used in practice [22], and
their mechanical properties (tensile, tear, and hardness) have been investigated [23]. Patients generally remove implant-retained prostheses by grabbing a part of the prosthesis (i.e. silicone body) or rotating, or peeling, it away from the skin. The test methods followed in the study provided an insight and understanding into the bonding and debonding characteristics of maxillofacial silicone elastomers bonded to a rigid acrylic base. The peel test can simulate the horizontal component of the detaching forces that are generated while patient is holding the silicone to pull the prosthesis out of the defect site. This dislodging action may cause stripping of the silicone elastomer at the prosthesis peripheries. Additionally, the forces that the silicone material is exposed to during service can be related to shear and tear tests. Bond strengths of silicone to rigid acrylic resin base were affected by the chemical affinity between silicones and primers, bond test type and presence of aging. At baseline, combination of Z004 and 611 exhibited the greatest peel strength of 5.31 N/mm, while combinations of (Z004 and 611, Z004 and A304, Z004 and A330-G, M511 and A304, M511 and A330-G) exhibited greater shear-bond strengths of 0.59–0.65 MPa. However, after exposing specimens to accelerated artificial daylight-aging for 360 h, which resembles a prosthesis being in service for 12 months; different combinations, proved to be serviceable. The Z004 and A330-G showed greater peel strength of 8.22 N/mm and Z004 with any of the primers (611, A304, and A330-G) showed greater shear strengths of 0.89–1.00 MPa. The differences in bond strengths within each test type are due to variations in silicone and primer compositions and chemical affinity between them both. The silicone elastomers are mainly composed of short and long chains of polydimethylsiloxane rubber, with varying amounts of surface-treated silica fillers. For TechSil S25, Cosmesil Z004, and Cosmesil M511 silicones respectively, tensile strengths ranged 4.85, 3.86, and 1.86 MPa, tear strengths were 6.55, 7.04, and 6.52 MPa, and Shore A hardness was 25.42, 36.44, and 12.64 [23]. The primers used were also different in chemical formulations (Table 1). The adhesive primers have an organic solvent and adhesive agent that react with both silicone and resin materials [15], serving as a chemical intermediate between the silicones and the acrylic substrate, as the hydrophilic and hydrophobic groups on the primers reactive sites react with the functional groups of silicone [20]. At the same time primers activate the substrate surfaces via etching or promoting hydrogen bond-
d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 387–395
ing and covalent coupling, increasing the wettability of the substrate, and impregnating the surface layer with the polymeric ingredients [16]. Apart from this, it can be noted that the Cosmesil Z004 silicone elastomer, in comparison to other silicone elastomers, exhibited high bond strengths among different bond tests and adhesive primers. It is likely due to its high hardness (more stiffer) and tear strengths, as stress generated at the soft/rigid interface during the application of stress is greater for stiffer materials [26]. Whereas in peel-testing, only one combination achieved the best affinity, in shear-testing the affinity was more silicone dependent (p < 0.05) and was greater with Cosmesil silicones (Z004 and M511) with any of the bond primers investigated. This indicates that shear-testing in more likely to depend on maxillofacial silicone elastomer properties (i.e. hardness) [26]. Additionally, bond strength values were greater under peel forces than shearing forces. It can be due to the direction and nature of testing forces, and bond strength calculations among each test. In peel-testing, the debonding extends through a line, whereas all interfacial area is stressed in shear-testing, with tear resistance of the silicone playing a major role in resisting failure. However, the shear-bond test is claimed to concentrate stresses at the edges, it is affected by the deformation rate chosen [25], nature of materials tested and their testing arrangements [27]. Regarding bond strength calculations, the shear-strength is the measure of maximum shearing force divided by the interfacial cross-sectional bonding area. Peel bond strengths were calculated considering both the elastic deformation () of the silicones, and their adhesive bonding. This formula considers the absorbed energy required to deform the silicone and the energy to cause peeling. Whereas the first one is affected by the compliance (or hardness) and dimensions of the specimen (width and thickness), the second one is affected by the interfacial bond-energy and the area of bonding. Thus, if peeling occurs with a minimal strain, the elastic energy stored in the unattached tab can only be neglected [28]. Other studies calculated the peel-bond work (the maximum peel force recorded per unit of width), where they did not account for the extension ratio [26,29]. Thus their results should be cautiously interpreted. Light-aging enhanced properties and the resistance of silicones to rupture as silicone elastomers continue polymerization by the heating and lighting inside the aging machine [30,31]. Aging effect on bond strengths was different and bond test dependent. Peel-bond strengths increased for Z004 and A330-G and S25 and A330-G combinations; and decreased for Z004 and 611, Z004 and A304, and M511 and 611 combinations. Shear-bond strength either increased (p < 0.05) for Cosmesil Z044 silicone elastomer with any of the three primers (611, A304, A330), or was not affected for the other silicone and primer combinations (p > 0.05). As peeling and shearing forces depend on the silicone properties, thus unstretched silicone part absorbed more energy in peeling, reducing the peel bond strength. Also, silicones were harder and resisted shear deformation exhibiting higher shear strengths. This was proved by the predominant adhesive failures exhibited with peeling forces, and cohesive failures under shearing forces. Nevertheless, reduced bond strengths are also dependent on the
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adhesive primers and their ability to stay chemically reactive after such severe conditioning regime. As aging enhanced silicone resistance to rupture, water storage and continue lighting weakened the chemical intermediate formed by the primer. Additionally, residual stresses that arise when the specimens are cooled to room temperature and due to different materials’ thermal expansion coefficients may also play a role in decreasing bond strength [1]. Type of bond failure was assessed visually and was reported as adhesive, cohesive, or mixed. At baseline, all the silicone elastomers, and regardless of the adhesive primers, failed by predominant cohesive debonding under peel and shear forces (68.9% and 100%, respectively). However, after light-aging, peel and shear forces exhibited predominant adhesive (79.5%) and cohesive (84.4%) failures respectively. For cohesive failures, the peel-bond strength between the silicones and the denture base was stronger than the strength of the silicone material. Although the peel test has the advantage of being the only method in which failure proceeds at a controlled rate and the peel force is a direct measure of the work of detachment, cohesive peel-bond test failures should be interpreted with caution. For extra-oral maxillofacial prostheses, there is a lack of minimum bond strength required to confirm the prosthesis as serviceable. For intra-oral prostheses, it was reported that bond strength of 0.44 MPa is sufficient for bonding soft silicone liners to acrylic denture resin [24,32]. Biting forces in the molar region are in the range of 597 and 847 N for young women and men respectively [33], and normal masticatory forces were reported to be 40% of the biting force [34]. Extraoral forces are much lesser than intra-oral forces, thus all bond strengths exhibited by the silicone and primer combinations should be sufficient to maintain the serviceability of facial prostheses. For clinical relevance, practice requires stable and serviceable bonding between silicones and acrylic. Thus, the best silicone and primer combination are selected based on the highest bond strength achieved and maintained after aging, within each bond test. As shear-bond strengths were not affected by the primer since shear-bond strengths either increased or remained the same after light-aging, it can be stated that the silicone elastomers tested should be bonded well to acrylic resin using any of primers (611, A304 and A330-G). However, higher bond strengths were achieved using Cosmesil silicones (Z004) with any primer and M511 (only with A304, and 330-G primers). Within peel bond tests, primers and silicones had significant effect on bond strengths (p < 0.05). Thus, within each silicone elastomer, the optimum bonding was achieved using primer A-330-G. Although for silicone Z004, primer 611 exhibited higher bond strength at baseline, it was decreased after aging. Hence, and considering bond strengths after light-aging, both silicones (Z004 and S25) bonded using A330-G primer exhibited higher and serviceable bond strengths after light-aging. All in all, Cosmesil silicone elastomer (Z004) along with primer A330G should produce a serviceable bond to auto-polymerizing acrylic resin under daylight conditioning, and peel and shear forces. The current study investigated bond strength using peel and shear-bond tests. Tensile test was not conducted as it
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is a sensitive method and signifies the tensile strength of the materials rather than their bonding to the substrate [35]. However, all bond tests have been subjected to constructive critique [23]. Experimental investigations of bond strengths lack the actual representation of forces acting on the facial prostheses during use. Bond tests apply only one force in one direction, while silicone prostheses are exposed to various different forces in different directions during service. Furthermore, the bonding phenomena are complex and single specimens do not represent the actual silicone prostheses formulation. Despite the difficulty in interpreting results of such tests, they are still useful in comparing and ranking bonding of maxillofacial silicone elastomers to acrylic substrates using different surface primers and treatments.
5.
Conclusions
Within the limitations of this in vitro study, it can be concluded that: 1. Shear and peel test-regimes were both relevant and suitable for studying bonding and debonding characteristics of maxillofacial silicone elastomers bonded to an autopolymerising acrylic resin. 2. The silicone/acrylic bond strengths were different for shear versus peel tests: 0.42–1.00 MPa for shear and 0.51–8.22 N/mm for peel. 3. Cohesive failures were predominant with shear-tests, whereas peel-tests showed predominant cohesive failures at baseline but adhesive failures after light-aging. 4. The optimum bonding achieved (best bonding at baseline that increased or was unaffected after light-aging) varied between shear and peel. For shear, it was achieved using Cosmesil Z004, with any primer, and M511 (but only with A304, and 330-G primers). For peel, it was achieved using both Cosmesil Z004 and TechSil S25 bonded using A330-G primer. 5. Consequently, Cosmesil Z004 along with primer A330-G was the optimum silicone/primer combination to select on the basis of bond strengths. 6. Although the study was useful in ranking best elastomer primer combination, in practice any of the elastomer/primer combinations should be adequate to maintain the serviceability of facial prostheses.
references
[1] Chalian VA, Phillips RW. Materials in maxillofacial prosthetics. J Biomed Mater Res 1974;8:349–63. [2] Parel SM, Branemark PI, Tjellstrom A, Gion G. Osseointegration in maxillofacial prosthetics. Part II: extraoral applications. J Prosthet Dent 1986;55:600–6. [3] Wolfaardt J, Gehl G, Farmand M, Wilkes G. Indications and methods of care for aspects of extraoral osseointegration. Int J Oral Maxillofac Surg 2003;32:124–31. [4] Schoen PJ, Raghoebar GM, van Oort RP, Reintsema H, van der Laan BF, Burlage FR, et al. Treatment outcome of bone-anchored craniofacial prostheses after tumor surgery. Cancer 2001;92:3045–50.
[5] Watson RM, Coward TJ, Forman GH. Results of treatment of 20 patients with implant-retained auricular prostheses. Int J Oral Maxillofac Implants 1995;10:445–9. [6] Karakoca S, Aydin C, Yilmaz H, Korkmaz T. An impression technique for implantretained orbital prostheses. J Prosthet Dent 2008;100:52–5. [7] Jebreil K. Acceptability of orbital prostheses. J Prosthet Dent 1980;43:82–5. [8] Hooper SM, Westcott T, Evans PL, Bocca AP, Jagger DC. Implant-supported facial prostheses provided by a maxillofacial unit in a U.K. regional hospital: longevity and patient opinions. J Prosthodont 2005;14:32–8. [9] 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; in press. [10] Kurunmaki H, Kantola R, Hatamleh MM, Watts DC, Vallittu PK. A fiber-reinforced composite prosthesis restoring a lateral midfacial defect: a clinical report. J Prosthet Dent 2008;100:348–52. [11] McMordie R, King GE. Evaluation of primers used for bonding silicone to denture base material. J Prosthet Dent 1989;61:636–9. [12] Polyzois GL, Frangou MJ. Bonding of silicone prosthetic elastomers to three different denture resins. Int J Prosthodont 2002;15:535–8. [13] Frangou MJ, Polyzois GL, Tarantili PA, Andreopoulos AG. Bonding of silicone extra-oral elastomers to acrylic resin: the effect of primer composition. Eur J Prosthodont Restor Dent 2003;11:115–8. [14] Qudah S, Harrison A, Huggett R. Soft lining materials in prosthetic dentistry: a review. Int J Prosthodont 1990;3:477–83. [15] Minami H, Suzuki S, Ohashi H, Kurashige H, Tanaka T. Effect of surface treatment on the bonding of an autopolymerizing soft denture liner to a denture base resin. Int J Prosthodont 2004;17:297–301. [16] Mutluay MM, Ruyter IE. Evaluation of bond strength of soft relining materials to denture base polymers. Dent Mater 2007;23:1373–81. [17] Singer MT, Mitchell DL, Pelleu Jr GB. Effect of primers on the bond strength of silicone elastomers and polyurethane. J Prosthet Dent 1988;60:602–5. [18] Wang R, Collard SM, Lemon J. Adhesion of silicone to polyurethane in maxillofacial prostheses. Int J Prosthodont 1994;7:43–9. [19] Deng HY, Zwetchkenbaum S, Noone AM. Bond strength of silicone to polyurethane following immersion of silicone in cleaning solutions. J Prosthet Dent 2004;91:582–5. [20] Chang PP, Hansen NA, Phoenix RD, Schneid TR. The effects of primers and surface bonding characteristics on the adhesion of polyurethane to two commonly used silicone elastomers. J Prosthodont 2009;18:23–31. [21] 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;5:101–4. [22] Hatamleh MM, Haylock C, Watson J, Watts DC. Survey about attitudes, opinions, and experience of maxillofacial prosthetists and technologists in the UK toward maxillofacial silicone prostheses. J Maxillofac Prosthet Technol; 2010; (ahead of publication). [23] Hatamleh MM, Watts DC. Mechanical properties and bonding of maxillofacial silicone elastomers. Dental Mater 2010;26(2):79–85. [24] Hatamleh MM, Watts DC. Fibre reinforcement enhances bonding of soft lining to acrylic dental and maxillofacial prostheses. Eur J Prosthodont Restor Dent 2008;16: 116–21.
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[25] Al-Athel MS, Jagger RG. Effect of test method on the bond strength of a silicone resilient denture lining material. J Prosthet Dent 1996;76:535–40. [26] McCabe JF, Carrick TE, Kamohara H. Adhesive bond strength and compliance for denture soft lining materials. Biomaterials 2002;23:1347–52. [27] Della Bona A, van Noort R. Shear vs. tensile bond strength of resin composite bonded to ceramic. J Dent Res 1995;74:1591–6. [28] Kutay O. Comparison of tensile and peel bond strengths of resilient liners. J Prosthet Dent 1994;71:525–31. [29] Tanimoto Y, Saeki H, Kimoto S, Nishiwaki T, Nishiyama N. Evaluation of adhesive properties of three resilient denture liners by the modified peel test method. Acta Biomater 2009;5:764–9. [30] Polyzois GL, Tarantili PA, Frangou MJ, Andreopoulos AG. Physical properties of a silicone prosthetic elastomer
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stored in simulated skin secretions. J Prosthet Dent 2000;83:572–7. Dootz ER, Koran 3rd A, Craig RG. Physical properties of three maxillofacial materials as a function of accelerated aging. J Prosthet Dent 1994;71:379–83. American Society of Testing and Materials. Specification D-903, standard method of test for peel or stripping strength of adhesive bonds. Washington, DC; 1972. Craig RG, Gibbons P. Properties of resilient denture liners. J Am Dent Assoc 1961;63:382–90. Waltimo A, Kononen M. A novel bite force recorder and maximal isometric bite force values for healthy young adults. Scand J Dent Res 1993;101:171–5. Gibbs CH, Mahan PE, Lundeen HC, Brehnan K, Walsh EK, Holbrook WB. Occlusal forces during chewing and swallowing as measured by sound transmission. J Prosthet Dent 1981;46:443–9.
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Bonding of maxillofacial silicone elastomers to an acrylic substrate Muhanad M. Hatamleh ∗ , David C. Watts Biomaterials Research Group, School of Dentistry, University of Manchester, Higher Cambridge Street, Manchester, M14 5FH, UK
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To investigate the effect of three different primers on shear and peel bond strengths
Received 18 September 2009
between three maxillofacial silicone elastomers and an acrylic resin after 360 h of acceler-
Received in revised form
ated daylight-aging.
23 December 2009
Materials and methods. Peel and shear-bond strengths of three maxillofacial silicone elas-
Accepted 5 January 2010
tomers (TechSil S25, Cosmesil M511, Cosmesil Z004) to acrylic denture resin bases using three adhesive primers (611, A304, A330-G) were assessed at baseline and after 360 h of accelerated artificial light-aging. Data were collected and statistically analyzed by two-way
Keywords:
ANOVA, one-way ANOVA, and Bonferroni post hoc tests (˛ = 0.05). Independent t-test was
Maxillofacial silicone elastomer
used to investigate the effect of light-aging on bond strengths (˛ = 0.05). Modes of failure
Adhesive primers
were visually analyzed and categorized as adhesive, cohesive, or mixed.
Peel bond test
Results. In the peel bond test, at both baseline and after aging, there was a significant influ-
Shear-bond test
ence of primers and silicones on bond strength (p < 0.001) and a strong interaction was also
Light-aging
found between primers and silicones (p < 0.05). Peel bond strengths ranged from 0.85 to 5.31 and 0.76 to 8.22 N/mm at baseline and after aging, respectively. The Z004 and 611 and Z004 and A330-G combinations showed the highest peel bond strength (5.31 and 8.22 N/mm, respectively) (p < 0.05), as baseline and after aging. In the shear-bond test, there was only a significant influence of silicones on shear-bond strength (p < 0.001), whereas primers did not affect it (p > 0.05), and no interaction between primers and silicones was found (p > 0.05). Shear-bond strengths ranged from 0.42 to 0.66 and 0.48 to 1.00 MPa at baseline and after aging, respectively. The combinations of Z004 and 611, Z004 and A304, Z004 and A330-G, M511 and A304, M511 and A330-G exhibited the highest bond strength (0.59–0.65 MPa) at baseline, and the Z004 with any of the primers (611, A304, and A330-G) showed greater bond strengths (0.89–1.00 MPa) (p < 0.05) after aging. All the silicone elastomers at baseline, regardless of the adhesive primers, failed predominantly by cohesive debonding under peel and shear forces (68.9% and 100% respectively). However, after light-aging, peel and shear forces predominantly exhibited adhesive (79.5%) and cohesive (84.4%) failures, respectively. Conclusions. Shear and peel test-regimes were both relevant and suitable for studying bonding and debonding characteristics of maxillofacial silicone elastomers bonded to an autopolymerising acrylic resin. The silicone/acrylic bond strengths were different for shear versus peel tests: 0.42–1.00 MPa for shear and 0.51–8.22 N/mm for peel. Cohesive failures were predominant with shear-tests, whereas peel-tests showed predominant cohesive failures at baseline but adhesive failures after light-aging. The optimum bonding achieved (best bonding at baseline that increased or was unaffected after light-aging) varied between shear and
∗
Corresponding author. Tel.: +44 0 161 275 6749; fax: +44 0 161 275 6748. E-mail address: [email protected] (M.M. Hatamleh). 0109-5641/$ – see front matter © 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2010.01.001
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peel. For shear, it was achieved using Cosmesil Z004, with any primer, and M511 (but only with A304, and 330-G primers). For peel, it was achieved using both Cosmesil Z004 and TechSil S25 bonded using A330-G primer. Consequently, Cosmesil Z004 along with primer A330-G was the optimum silicone/primer combination to select on the basis of bond strengths. A wide variety of new maxillofacial silicone elastomers and primers used in this study gave serviceable bond strengths. However, Cosmesil Z004 along with primer A330-G gave the optimum silicone/primer combination to select on the basis of bond strengths. © 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Maxillofacial prosthetics is defined as “the art and science of anatomical, functional or cosmetic reconstruction by artificial substitutes of those regions in the maxilla, mandible, and face that are missing or defective because of surgical intervention, trauma, pathology, or developmental or congenital malformation” [1]. The introduction of the osseointegration concept for craniofacial implants in maxillofacial prosthetic rehabilitation has minimized some of the disadvantages associated with traditional retention methods (i.e. medical-grade skin adhesives, eyeglasses, and tissue undercuts) and provided patients with predictable aesthetics and durability, improved prosthesis retention and stability [2–5]. Retentive attachment selection of the implant is made with regard to advantages and disadvantages of bar-clip and magnetic retention [6]. Extra-oral facial prostheses used in conjunction with implants require a retentive matrix to hold the bar clips or magnets. The retentive matrix is commonly made from acrylic resin (i.e. heat-polymerizing, auto-polymerizing, or light-cured materials) to which the facial silicone elastomer is attached. Hence, sufficient bond strength is vital to ensure a serviceable and functional prosthesis. During service, maxillofacial silicone prostheses suffer bond failures between the silicone and denture base, color deterioration and loss of mechanical properties (i.e. tear and tensile strengths) [7,8]. As several potential solutions have been introduced to overcome problems associated with silicone elastomer; delaminating of silicone away from the retentive matrix is still a persisting problem. It was faced by suggesting new design techniques for replacing the retentive acrylic plate with retentive glass-fiber framework [9,10], or the use of bond primers [11–13]. Maxillofacial silicone elastomers are dimethyl siloxane polymers, and have different chemical structure to PMMA denture base resin. Thus an adhesive is supplied to aid their bonding to the denture base [14]. It is likely that adhesive primers have an organic solvent and an adhesive agent that reacts with both silicone and resin materials [15]. They activate the surfaces via etching or promoting hydrogen bonding and covalent coupling, increasing the wettability of the substrate and by impregnating the surface layer with the polymeric ingredients [16]. Bonding of maxillofacial silicone elastomers to either polyurethane or acrylic resin substrates has been studied. Peel-bond strength of maxillofacial silicone elastomer to polyurethane substrate varied with the silicone elastomer, primer, and conditioning performed. It was
enhanced using a combination of MDX 4-4210 silicone and either S-2260 or A-4040 primers [17]. It was greater with primer 1205 than with S-2260 primer regardless of the polymerization method or primer reaction time [18]. Another study reported it as 6.06 N/mm, and showed it decreased after soaking in hot (3.93 N/mm) or room-temperature (2.49 N/mm) soapy water [19]. It was reported to be 1.32, 1.25, and 0.91 N/mm, using SofrelinerMSprimer, Sofreliner, and A-330-G primers, respectively [20]. Bonding of Silastic 891 facial material to acrylic resin was enhanced after using three different primers (Dow corning 4040, S-2260, and 1200), in comparison to the control group, with 4040 showing the greatest increase [11]. The tensile bond strength of silicone elastomers to acrylic denture resin was not affected by the curing method (microwave irradiation and dry heat), but with the type of silicone elastomer. Mollomed showed the highest bond strength (0.53 MPa) in comparison to Silskin II (0.28 MPa), and MDX4-4210 (0.11 MPa), and A-2186 (0.12 MPa) silicone elastomers [21]. The bond strength between different silicone elastomers and acrylic resins was in the range 0.03–0.23 MPa whereas Cosmesil condensationcured silicone elastomer showed the highest bond strength to the acrylic resins in comparison to the addition-cured Ideal silicone elastomer [12]. Frangou et al. reported that the bond strength between silicone elastomers (Cosmesil and Ideal) and acrylic resin using different primers and primer mixtures (Cosmesil, Cosmesil/Z-6020, and Cosmeil/A174) was in the range 0.026–0.22 MPa [13]. They stated that the compatibility and affinity of primer composition with the selected silicone elastomer is important for efficient bonding. New maxillofacial silicone elastomers and primers have been developed to enhance silicone–acrylic bonding, but controlled testing of these materials has been minimal. Commonly used maxillofacial prosthetic silicone elastomers include TechSil S25 (Technovent, Leeds, UK), Cosmesil M511 and Cosmesil Z004 (Principality medical, Newport, UK) [22,23]. A review of the literature revealed a deficiency of studies in comparing the effect of primers on maxillofacial silicone–acrylic bond strengths using different bond tests, or in bond serviceability of primers after conditioning. Accordingly, the aim of this study was to investigate the effects of three different primers on the shear and peel bond strengths between three maxillofacial silicone elastomers and acrylic resin after 360 h of accelerated daylight-aging. The null hypothesis stated that bond strength of maxillofacial silicone elastomers is not affected by the type of silicone elastomer, adhesive primer, aging condition, or bond test type.
389
Factor II, Inc., Lakeside, AZ, USA Lot l4707836 Platinum Primer (A-330-G) [Gold]
Manufacturer
Factor II, Inc., Lakeside, AZ, USA Lot L42587 Platinum Primer (A 304)
Silicone Primers
08/02 08/01 08/02 Lot 07/03 Silicone elastomers
TechSil (S25) Cosmesil Series Materials (M511) Cosmesil Series Materials (Z004) Ancillary Materials Platinum Primer (G611)
Clear repair acrylic liquid (MMA) and powder [1:1] Heat cured for 2 h at 100 ◦ C [9:1] Heat cured for 1 h at 100 ◦ C [10:1] Heat cured for 1 h at 100 ◦ C [1:1] Organic solvent based primer include components of propan-2-ol and various vinyl silanes A mixture of naptha (85%), tetra-n-propyl silicate (5%), tetrabutyltitnate (5%), and tetra (2-methoxyethoxy) silane (5%) A solution of modified ployacrylates in ethylmethylketone and dichloromethane
Composition/processing [mixing ratios] Batch number
2011/07
Brand name Skillbond Acrylic resin
Fig. 1 – Peel bond specimen preparation (dimensions in mm), A = 75 ± 1, B = 50 ± 1 (free silicone), C = 25 ± 1 (bonded silicone). Width = 10 ± 1, and thickness of each silicone and acrylic strips = 3 ± 0.2 mm.
Materials
Materials tested and their mixing ratios and processing parameters are listed in Table 1. For shear-bond strength, specimens’ fabrication method was previously described [23,24]. Autopolymerizing clear acrylic resin was mixed and packed inside hollow brass cylinders (external diameter = 18 mm, internal diameter = 14.4 mm, depth = 25 mm). Their surfaces were prepared for bonding (after 24 h of fabrication) by lapping with a 60grit silicone carbide waterproof abrasive paper. Application of adhesive bond primers was conducted according to manufacturers’ instructions. The acrylic surfaces were cleaned with acetone and left to air dry. Then, a uniform layer of the adhesive primer was applied using a brush over the surfaces, and left for 30 min at 23 ± 1 ◦ C room temperature and 50 ± 5% relative humidity. Teflon disks (PTFE) (external diameter = 18 mm, internal diameter = 8 mm, thickness = 3 mm) were used to define the area over which the silicone elastomers were attached to the acrylic surfaces. Then silicone elastomers were mixed, packed inside the disks, and cured according to manufacturers’ instructions. After curing, specimens were incubated at 37 ± 1 ◦ C for 24 h and then shear-bond tests were performed as described in previous study. A total of 90 specimens were fabricated (Table 2). For the peel test, specimen fabrication followed the principle of the 180◦ peel test. Preparation procedure was conducted at two stages; fabricating the acrylic resin blanks, and then bonding to maxillofacial silicone elastomer. Moulds were fabricated by investing hard wax blanks (75 mm × 65 mm × 3 mm) (Associated Dental products Ltd., Swinton, UK) in dental stone (Class 1, Dentsply, Surrey, UK), and then removed. Then auto-polymerizing clear acrylic denture base material was poured into the moulds and cured using the conventional flasking technique to produce acrylic blanks. The surfaces of the acrylic blanks were prepared for bonding as described earlier. Then, they were cleaned with water, and adhesive tape was used to define the area over which the silicone elastomer was attached to the acrylic substrate. The tape covered an area of (50 mm × 65 mm × 3 mm) leaving an area of (25 mm × 65 mm) of silicone elastomer bonded to the acrylic substrate (Fig. 1). The free silicone was gripped in performing the peel test. Another set of wax blanks of greater thickness (75 mm × 65 mm × 6 mm) were used to make stone moulds as described earlier. The acrylic blanks were fixed inside the moulds and the silicone elastomer was mixed and packed over the acrylic blanks. The silicone elastomer was cured according to manufacturers’ instructions, and moulds were left to bench cool. Prior to packing the silicone elastomers,
Skillbond Direct Ltd., Bucks, UK
Materials and methods
Table 1 – Materials used in the study.
2.
Technovent Ltd., Leeds, UK Principality Medical Ltd., Newport, UK Principality Medical Ltd., Newport, UK Principality Medical, Newport, UK
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Table 2 – Groups of the study. Nine groups were present for each peel and shear-bond tests. Groups (n = 10)a
Silicone and primer
1
Z004 and 611
2 3 4 5 6 7 8 9
Z004 and A304 Z004 and A330-G M511 and 611 M511 and A304 M511 and A330-G S25 and 611 S25 and A304 S25 and A330-G
a
Conditioning For each silicone and primer combination control specimens (n = 5) were tested after 24 h. Remaining specimens (n = 5) were light aged for 360 h, and then tested.
Within each group, half of the specimens acted as control (n = 5).
acrylic surfaces were treated with one of three different primers (according to their manufacturers’ instructions). The treatment process is the same as described previously. After removing the cured specimens from the moulds they were dry stored for 24 h at 23 ± 1 ◦ C room temperature and then cut using a fine saw (Model CBS 355, Clarke Int., UK) into 6 strips of width 10 ± 0.4 mm (the actual width of each strip was measured before testing). The specimen thickness was 6 mm (3 mm of acrylic blank and 3 mm of silicone elastomer). The silicone strip was bonded to the acrylic denture base at one end (25 mm × 10 mm × 3 mm) and free at the other (50 mm × 10 mm × 3 mm). Each free strip was turned back at 180◦ such that the hard base and soft strip could be gripped in a tensile direction. A total of 90 specimens were fabricated (Table 2). Within each silicone and primer combination, half of the specimens (n = 5) acted as control specimens and tested after 24 h of fabrication. The remaining specimens (n = 5) were light aged for 360 h in an aging machine (Suntest Chamber CPS, Heraeus Instruments, Hanau, Germany). Accelerated artificial daylight was generated using filtered Xenon light of 150 klx. A complete weathering cycle lasted for 120 min, including 18 min of wet weathering by controlled-flow of distilled water (29 ± 2 ◦ C), followed by 102 min of dry weathering (36 ± 2 ◦ C). The relative humidity inside the aging chamber was approximately 70%, and air pressure was 700–1060 hPa. The Xenon light was applied for the whole duration of aging (360 h). A universal testing machine (Zwick Roell Z020) was used to peel the maxillofacial silicone elastomers at an angle of 180◦ , and at 10 mm/min crosshead speed (Fig. 2). Each specimen was pulled in tension to peel the silicone elastomer from the denture base resin. The force needed to cause failure and the modes of failure were recorded. Peel bond strength (PS) (N/mm) was determined according to Eq. (1) [25]. PS =
F W
1 + 2
Fig. 2 – Peel test bond failures categories; adhesive (peel), and cohesive (snap or tear).
rial, or cohesive (tear or snap of the silicone elastomer) [26] (Fig. 3). Within each bond test, Univariate two-way ANOVA (release 16, SPSS Inc., Chicago, IL) (p < 0.05) was used to analyze significant differences in bond strengths (dependent variable) between different silicone and primer (independent variables) combinations at baseline, and after 360 h of light-aging. All data were subjected to Levene’s test of homogeneity of variance (␣ = 0.05), following the assumption of equal variances.
+1
(1)
where F is the maximum force recorded (N), W is the width of the specimens (mm), and is the extension ratio of the silicone elastomer (the ratio of stretched to unstretched length). The denture base interface was visually analyzed, and failure modes were characterized as either adhesive (peel), indicating peeling of the silicone elastomer from the denture base mate-
Fig. 3 – Test specimen undergoing 180◦ peel test.
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Table 3 – Mean (SD) values of peel bond strength (N/mm). Group
Silicone/primer combination (n = 5)
1 2 3 4 5 6 7 8 9
Z004 and 611 Z004 and A304 Z004 and A330-G M511 and 611 M511 and A304 M511 and A330-G S25 and 611 S25 and A304 S25 and A330-G
Equal variances were assumed (p > 0.05) and Bonferroni post hoc test was used to analyze significant differences between paired groups. One-way ANOVA and Bonferroni post hoc test were used to detect statistical significances in bond strength between the silicone and primer combinations among peel and shearbond tests. Silicone and primer combination that exhibits the greatest (p < 0.05) bond strengths were reported. Independent t-test was used to investigate the effect of aging on the bond strengths (p < 0.05).
3.
Results
For peel bond strength (Table 3), and at baseline, there was a significant influence of primers and silicones on bond strength (p < 0.001) and a strong interaction was also found between primers and silicones (p < 0.05). For silicone elastomers, Cosmesil Z004 was statistically different from Cosmesil M511 (p = 0.00) and TechSil S25 (p = 0.00). For primers, Bonferroni post hoc test showed that 304 was statistically different from 611 (p = 0.00) and 330 (p = 0.00). After 360 h of light-aging, there was still significant influence of the primers and silicones on bond strength (p < 0.001) and a strong interaction was also found between primers and silicones (p < 0.05). For silicone elastomers, Cosmesil Z004 was statistically different from Cosmesil M511 (p = 0.00) and TechSil S25 (p = 0.042). Cosmesil M511 was statistically different from TechSil S25 (p = 0.004). For primers, Bonferroni post hoc test showed that 330 was statistically different from 611 (p = 0.00) and 304 (p = 0.00).
Conditioning Control
360 h light-aging
5.31 (0.89) 1.39 (0.41) 2.68 (0.66) 1.30 (0.67) 1.78 (0.47) 2.36 (0.66) 1.41 (0.34) 0.85 (0.12) 3.68 (0.72)
1.64 (0.36) 0.94 (0.15) 8.22 (2.24) 0.51 (0.19) 2.12 (0.76) 2.30 (0.35) 1.49 (0.25) 0.76 (0.16) 6.00 (1.02)
For shear-bond strength (Table 4), and at baseline, there was only significant influence of silicones on bond strength (p < 0.001), whereas primers did not affect it (p = 0.572), and no significant interaction between primers and silicones existed (p = 0.138). Bonferroni post hoc test showed no significant differences between all primers (p > 0.05). However, for silicone elastomers, Cosmesil Z004 was statistically different from Cosmesil M511 (p = 0.01) and TechSil S25 (p = 0.00). Also Cosmesil M511 was statistically different from TechSil S25 (p = 0.003). After 360 h of light-aging, there was no difference. There was only significant influence of silicones on bond strength (p < 0.001), while primers did not affect the bond strength (p = 0.184). And, there was no statistically significant interaction between primers and silicones (p = 0.107). Bonferroni post hoc test showed no significant differences between all primers (p > 0.05). However, for silicone elastomers, Cosmesil Z004 was statistically different from Cosmesil M511 (p = 0.00) and TechSil S25 (p = 0.00). Also Cosmesil M511 was statistically different from TechSil S25 (p = 0.011). One-way ANOVA was used to detect statistical significances in bond strengths between the silicone and primer combinations among peel and shear-bond tests. Silicone and primer combination that exhibits the greatest bond strength values (p < 0.05) varied with bond test performed and aging. At baseline, the peel bond strengths range was 0.85–5.31 N/mm (Table 3). The Z004 and 611 combination showed the greatest peel bond strength (5.31 N/mm) (p < 0.05) among other combinations. The S25 and A304 showed weaker bond strength in comparison to Z004 and 611, Z004 and A330-G, M511 and A330-G, S25 and A330-G (p < 0.05). After light-aging, peel bond
Table 4 – Mean (SD) values of shear-bond strengths (MPa). Group
1 2 3 4 5 6 7 8 9 a
Silicone/primer combination (n = 5) Z004 and 611a Z004 and A304 Z004 and A330-G M511 and 611a M511 and A304 M511 and A330-G S25 and 611a S25 and A304 S25 and A330-G
Results of control groups were adopted from previous study [23].
Conditioning Control
360 h light-aging
0.65 (0.10) 0.64 (0.10) 0.66 (0.06) 0.54 (0.07) 0.59 (0.03) 0.59 (0.06) 0.51 (0.03) 0.52 (0.08) 0.42 (0.04)
1.00 (0.10) 0.90 (0.08) 0.89 (0.10) 0.59 (0.02) 0.66 (0.06) 0.64 (0.07) 0.56 (0.04) 0.54 (0.04) 0.48 (0.09)
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Table 5 – Modes of failures of both bond tests exhibited by control and light-aged specimens. Failure mode
Frequency number (%) Peel bond strength (N/mm)a Control (24 h)
Cohesive Adhesive Mixed Total a b
31 (68.9%) 14 (31.1%) 0 45
After light-aging (360 h) 9 (20.5%) 35 (79.5%) 0 44b
Control (24 h) 45 0 0 45
After light-aging (360 h) 38 (84.4%) 4 (8.9%) 3 (6.7%) 45
For Peel bond test modes of failures, peel is considered adhesive, while snap or tear is cohesive. One specimen failed during testing.
strengths range was (0.76–8.22 N/mm). The Z004 and A330G combination exhibited the strongest peel bond strength (8.22 N/mm) (p < 0.05) among other combinations. The S25 and A330-G had higher peel bond strength value (6.00 N/mm) than the remaining combinations, which exhibited the same bond strength values (p > 0.05) which were in the range (0.51–2.30 N/mm). For shear-bond test specimens, and at baseline, bond strengths range was (0.42–0.66 MPa) (Table 4). The combinations of (Z004 and 611, Z004 and A304, Z004 and A330-G, M511 and A304, M511 and A330-G) exhibited greater bond strength (0.59–0.65 MPa) than S25 and A330-G combination (p < 0.05). After light-aging, shear-bond strengths range was (0.48–1.00 MPa). The Z004 with any of the primers (611, A304, and A330-G) showed greater bond strength (0.89–1.00 MPa) than the other silicone and primer combinations (0.48–0.59 MPa) (p < 0.05). Effect of aging on peel bond strength varied (p < 0.05). It increased for Z004 and A330-G and S25 and A330-G combinations; and decreased for Z004 and 611, Z004 and A304, and M511 and 611. Aging for 360 h increased shear-bond strength (p < 0.05) for Cosmesil Z044 silicone elastomer with any of the three primers (611, A304, A330). However, for the other silicone and primer combinations, aging had no effect (p > 0.05). Regarding modes of failures (Table 5), peel bond test specimens exhibited mainly cohesive (68.9%), and adhesive (31.1%) failures at baseline. While after 360 h of light-aging, modes of failures were mainly adhesive (79.5%), and cohesive (20.5%) failures. Shear-bond test specimens exhibited entirely cohesive failures at baseline (100%), while after aging, modes of failures were still predominantly cohesive (84.4%). Other failures were adhesive (8.9%), and mixed (6.7%) failures.
4.
Shear-bond strength (MPa)
Discussion
Bonding of maxillofacial silicone elastomers to retentive acrylic substrates is a crucial factor that enhances the serviceability of maxillofacial silicone prostheses. The bonding was investigated by studying the sole effects of silicones, and following recognizable peel, and shear-tests. This current study showed that primers, silicones, and bond tests affected bond strength between maxillofacial silicone elastomers and acrylic denture resin. Thus, the null hypothesis was rejected. The maxillofacial silicone elastomers and bond primers chosen for this study are commonly used in practice [22], and
their mechanical properties (tensile, tear, and hardness) have been investigated [23]. Patients generally remove implant-retained prostheses by grabbing a part of the prosthesis (i.e. silicone body) or rotating, or peeling, it away from the skin. The test methods followed in the study provided an insight and understanding into the bonding and debonding characteristics of maxillofacial silicone elastomers bonded to a rigid acrylic base. The peel test can simulate the horizontal component of the detaching forces that are generated while patient is holding the silicone to pull the prosthesis out of the defect site. This dislodging action may cause stripping of the silicone elastomer at the prosthesis peripheries. Additionally, the forces that the silicone material is exposed to during service can be related to shear and tear tests. Bond strengths of silicone to rigid acrylic resin base were affected by the chemical affinity between silicones and primers, bond test type and presence of aging. At baseline, combination of Z004 and 611 exhibited the greatest peel strength of 5.31 N/mm, while combinations of (Z004 and 611, Z004 and A304, Z004 and A330-G, M511 and A304, M511 and A330-G) exhibited greater shear-bond strengths of 0.59–0.65 MPa. However, after exposing specimens to accelerated artificial daylight-aging for 360 h, which resembles a prosthesis being in service for 12 months; different combinations, proved to be serviceable. The Z004 and A330-G showed greater peel strength of 8.22 N/mm and Z004 with any of the primers (611, A304, and A330-G) showed greater shear strengths of 0.89–1.00 MPa. The differences in bond strengths within each test type are due to variations in silicone and primer compositions and chemical affinity between them both. The silicone elastomers are mainly composed of short and long chains of polydimethylsiloxane rubber, with varying amounts of surface-treated silica fillers. For TechSil S25, Cosmesil Z004, and Cosmesil M511 silicones respectively, tensile strengths ranged 4.85, 3.86, and 1.86 MPa, tear strengths were 6.55, 7.04, and 6.52 MPa, and Shore A hardness was 25.42, 36.44, and 12.64 [23]. The primers used were also different in chemical formulations (Table 1). The adhesive primers have an organic solvent and adhesive agent that react with both silicone and resin materials [15], serving as a chemical intermediate between the silicones and the acrylic substrate, as the hydrophilic and hydrophobic groups on the primers reactive sites react with the functional groups of silicone [20]. At the same time primers activate the substrate surfaces via etching or promoting hydrogen bond-
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ing and covalent coupling, increasing the wettability of the substrate, and impregnating the surface layer with the polymeric ingredients [16]. Apart from this, it can be noted that the Cosmesil Z004 silicone elastomer, in comparison to other silicone elastomers, exhibited high bond strengths among different bond tests and adhesive primers. It is likely due to its high hardness (more stiffer) and tear strengths, as stress generated at the soft/rigid interface during the application of stress is greater for stiffer materials [26]. Whereas in peel-testing, only one combination achieved the best affinity, in shear-testing the affinity was more silicone dependent (p < 0.05) and was greater with Cosmesil silicones (Z004 and M511) with any of the bond primers investigated. This indicates that shear-testing in more likely to depend on maxillofacial silicone elastomer properties (i.e. hardness) [26]. Additionally, bond strength values were greater under peel forces than shearing forces. It can be due to the direction and nature of testing forces, and bond strength calculations among each test. In peel-testing, the debonding extends through a line, whereas all interfacial area is stressed in shear-testing, with tear resistance of the silicone playing a major role in resisting failure. However, the shear-bond test is claimed to concentrate stresses at the edges, it is affected by the deformation rate chosen [25], nature of materials tested and their testing arrangements [27]. Regarding bond strength calculations, the shear-strength is the measure of maximum shearing force divided by the interfacial cross-sectional bonding area. Peel bond strengths were calculated considering both the elastic deformation () of the silicones, and their adhesive bonding. This formula considers the absorbed energy required to deform the silicone and the energy to cause peeling. Whereas the first one is affected by the compliance (or hardness) and dimensions of the specimen (width and thickness), the second one is affected by the interfacial bond-energy and the area of bonding. Thus, if peeling occurs with a minimal strain, the elastic energy stored in the unattached tab can only be neglected [28]. Other studies calculated the peel-bond work (the maximum peel force recorded per unit of width), where they did not account for the extension ratio [26,29]. Thus their results should be cautiously interpreted. Light-aging enhanced properties and the resistance of silicones to rupture as silicone elastomers continue polymerization by the heating and lighting inside the aging machine [30,31]. Aging effect on bond strengths was different and bond test dependent. Peel-bond strengths increased for Z004 and A330-G and S25 and A330-G combinations; and decreased for Z004 and 611, Z004 and A304, and M511 and 611 combinations. Shear-bond strength either increased (p < 0.05) for Cosmesil Z044 silicone elastomer with any of the three primers (611, A304, A330), or was not affected for the other silicone and primer combinations (p > 0.05). As peeling and shearing forces depend on the silicone properties, thus unstretched silicone part absorbed more energy in peeling, reducing the peel bond strength. Also, silicones were harder and resisted shear deformation exhibiting higher shear strengths. This was proved by the predominant adhesive failures exhibited with peeling forces, and cohesive failures under shearing forces. Nevertheless, reduced bond strengths are also dependent on the
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adhesive primers and their ability to stay chemically reactive after such severe conditioning regime. As aging enhanced silicone resistance to rupture, water storage and continue lighting weakened the chemical intermediate formed by the primer. Additionally, residual stresses that arise when the specimens are cooled to room temperature and due to different materials’ thermal expansion coefficients may also play a role in decreasing bond strength [1]. Type of bond failure was assessed visually and was reported as adhesive, cohesive, or mixed. At baseline, all the silicone elastomers, and regardless of the adhesive primers, failed by predominant cohesive debonding under peel and shear forces (68.9% and 100%, respectively). However, after light-aging, peel and shear forces exhibited predominant adhesive (79.5%) and cohesive (84.4%) failures respectively. For cohesive failures, the peel-bond strength between the silicones and the denture base was stronger than the strength of the silicone material. Although the peel test has the advantage of being the only method in which failure proceeds at a controlled rate and the peel force is a direct measure of the work of detachment, cohesive peel-bond test failures should be interpreted with caution. For extra-oral maxillofacial prostheses, there is a lack of minimum bond strength required to confirm the prosthesis as serviceable. For intra-oral prostheses, it was reported that bond strength of 0.44 MPa is sufficient for bonding soft silicone liners to acrylic denture resin [24,32]. Biting forces in the molar region are in the range of 597 and 847 N for young women and men respectively [33], and normal masticatory forces were reported to be 40% of the biting force [34]. Extraoral forces are much lesser than intra-oral forces, thus all bond strengths exhibited by the silicone and primer combinations should be sufficient to maintain the serviceability of facial prostheses. For clinical relevance, practice requires stable and serviceable bonding between silicones and acrylic. Thus, the best silicone and primer combination are selected based on the highest bond strength achieved and maintained after aging, within each bond test. As shear-bond strengths were not affected by the primer since shear-bond strengths either increased or remained the same after light-aging, it can be stated that the silicone elastomers tested should be bonded well to acrylic resin using any of primers (611, A304 and A330-G). However, higher bond strengths were achieved using Cosmesil silicones (Z004) with any primer and M511 (only with A304, and 330-G primers). Within peel bond tests, primers and silicones had significant effect on bond strengths (p < 0.05). Thus, within each silicone elastomer, the optimum bonding was achieved using primer A-330-G. Although for silicone Z004, primer 611 exhibited higher bond strength at baseline, it was decreased after aging. Hence, and considering bond strengths after light-aging, both silicones (Z004 and S25) bonded using A330-G primer exhibited higher and serviceable bond strengths after light-aging. All in all, Cosmesil silicone elastomer (Z004) along with primer A330G should produce a serviceable bond to auto-polymerizing acrylic resin under daylight conditioning, and peel and shear forces. The current study investigated bond strength using peel and shear-bond tests. Tensile test was not conducted as it
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is a sensitive method and signifies the tensile strength of the materials rather than their bonding to the substrate [35]. However, all bond tests have been subjected to constructive critique [23]. Experimental investigations of bond strengths lack the actual representation of forces acting on the facial prostheses during use. Bond tests apply only one force in one direction, while silicone prostheses are exposed to various different forces in different directions during service. Furthermore, the bonding phenomena are complex and single specimens do not represent the actual silicone prostheses formulation. Despite the difficulty in interpreting results of such tests, they are still useful in comparing and ranking bonding of maxillofacial silicone elastomers to acrylic substrates using different surface primers and treatments.
5.
Conclusions
Within the limitations of this in vitro study, it can be concluded that: 1. Shear and peel test-regimes were both relevant and suitable for studying bonding and debonding characteristics of maxillofacial silicone elastomers bonded to an autopolymerising acrylic resin. 2. The silicone/acrylic bond strengths were different for shear versus peel tests: 0.42–1.00 MPa for shear and 0.51–8.22 N/mm for peel. 3. Cohesive failures were predominant with shear-tests, whereas peel-tests showed predominant cohesive failures at baseline but adhesive failures after light-aging. 4. The optimum bonding achieved (best bonding at baseline that increased or was unaffected after light-aging) varied between shear and peel. For shear, it was achieved using Cosmesil Z004, with any primer, and M511 (but only with A304, and 330-G primers). For peel, it was achieved using both Cosmesil Z004 and TechSil S25 bonded using A330-G primer. 5. Consequently, Cosmesil Z004 along with primer A330-G was the optimum silicone/primer combination to select on the basis of bond strengths. 6. Although the study was useful in ranking best elastomer primer combination, in practice any of the elastomer/primer combinations should be adequate to maintain the serviceability of facial prostheses.
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