To investigate the effect of three different primers on shear and peel bond strengths between three maxillofacial silicone elastomers and an acrylic resin after 360 h of accelerated daylight-aging.
Materials and methods
Peel and shear-bond strengths of three maxillofacial silicone elastomers (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 ANOVA, one-way ANOVA, and Bonferroni post hoc tests ( α = 0.05). Independent t -test was used to investigate the effect of light-aging on bond strengths ( α = 0.05). Modes of failure were visually analyzed and categorized as adhesive, cohesive, or mixed.
In the peel bond test, at both baseline and after aging, 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). 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.
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 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.
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” .
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 . Retentive attachment selection of the implant is made with regard to advantages and disadvantages of bar-clip and magnetic retention .
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) . 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 , or the use of bond primers .
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 . It is likely that adhesive primers have an organic solvent and an adhesive agent that reacts with both silicone and resin materials . 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 . 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 . It was greater with primer 1205 than with S-2260 primer regardless of the polymerization method or primer reaction time . 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 . It was reported to be 1.32, 1.25, and 0.91 N/mm, using SofrelinerMSprimer, Sofreliner, and A-330-G primers, respectively .
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 . 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 . The bond strength between different silicone elastomers and acrylic resins was in the range 0.03–0.23 MPa whereas Cosmesil condensation-cured silicone elastomer showed the highest bond strength to the acrylic resins in comparison to the addition-cured Ideal silicone elastomer . 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/A-174) was in the range 0.026–0.22 MPa . 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) .
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.
Materials and methods
Materials tested and their mixing ratios and processing parameters are listed in Table 1 . For shear-bond strength, specimens’ fabrication method was previously described . 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 ).
|Materials||Brand name||Batch number||Composition/processing [mixing ratios]||Manufacturer|
|Acrylic resin||Skillbond||2011/07||Clear repair acrylic liquid (MMA) and powder [1:1]||Skillbond Direct Ltd., Bucks, UK|
|Silicone elastomers||TechSil (S25)||08/02||Heat cured for 2 h at 100 °C [9:1]||Technovent Ltd., Leeds, UK|
|Cosmesil Series Materials (M511)||08/01||Heat cured for 1 h at 100 °C [10:1]||Principality Medical Ltd., Newport, UK|
|Cosmesil Series Materials (Z004)||08/02||Heat cured for 1 h at 100 °C [1:1]||Principality Medical Ltd., Newport, UK|
|Silicone Primers||Ancillary Materials Platinum Primer (G611)||Lot 07/03||Organic solvent based primer include components of propan-2-ol and various vinyl silanes||Principality Medical, Newport, UK|
|Platinum Primer (A 304)||Lot L42587||A mixture of naptha (85%), tetra-n-propyl silicate (5%), tetrabutyltitnate (5%), and tetra (2-methoxyethoxy) silane (5%)||Factor II, Inc., Lakeside, AZ, USA|
|Platinum Primer (A-330-G) [Gold]||Lot l4707836||A solution of modified ployacrylates in ethylmethylketone and dichloromethane||Factor II, Inc., Lakeside, AZ, USA|
|Groups ( n = 10) a||Silicone and primer||Conditioning|
|1||Z004 and 611||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.|
|2||Z004 and A304|
|3||Z004 and A330-G|
|4||M511 and 611|
|5||M511 and A304|
|6||M511 and A330-G|
|7||S25 and 611|
|8||S25 and A304|
|9||S25 and A330-G|
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, 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) .
P S = F W 1 + λ 2 + 1
where F is the maximum force recorded (N), W is the width of the specimens (mm), and λ 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 material, or cohesive (tear or snap of the silicone elastomer) ( Fig. 3 ).