Evaluation of the Peel Bond Strength of a Room Temperature Vulcanizing Maxillofacial Silicone when bonded to Three Different High-impact Heat-cured Acrylic Resins: An In Vitro Comparative Study
Corresponding Author: Aishwarya Shastry, Department of Prosthodontics, Government Dental College and Research Institute, Bengaluru, Karnataka, India, e-mail: firstname.lastname@example.org
Received on: 28 April 2023; Accepted on: 24 May 2023; Published on: 28 June 2023
Purpose: The purpose of the study was to evaluate and compare the peel bond strength of a room temperature vulcanizing (RTV) maxillofacial silicone bonded to three different high-impact heat-cure acrylic resins.
Materials and methods: A total of 80 standardized rectangular specimens of dimensions 75 × 10 × 6 mm were fabricated. The specimens were categorized into four groups (n = 20 each) as group I—conventional heat-cure acrylic resin (Trevalon) and three brands of high-impact heat-cure acrylic resins (group II: Lucitone 199, group III: Acralyn H, and group IV: Acryl Hi). Each standardized specimen consisted of a rectangular acrylic component to which silicone was bonded using a primer. Peel bond strength between the acrylic and silicone was measured using a universal testing machine. Kruskal–Wallis H test was performed to compare the peel bond strength among all groups, followed by Dunn post hoc test to find the significant difference for pairwise comparison.
Results: Standardized test specimens belonging to group II (Lucitone 199), bonded to RTV maxillofacial silicone, showed significantly (p = 0.013) greater peel bond strength (0.0372 ± 0.013 MPa) among all the groups. The test specimens of group III (Acralyn H, 0.0213 ± 0.007 MPa), group I (Trevalon, 0.0185 ± 0.009 MPa), and group IV (Acryl Hi, 0.0148 ± 0.007 MPa) showed peel bond strength to RTV silicones in decreasing order.
Conclusion: The high-impact heat-cure acrylic resin (Lucitone 199) had significantly higher bond strength to maxillofacial silicones with the use of a primer than other acrylic resins used in the study. The other two brands of high-impact heat-cure acrylic resins showed comparable peel bond strength to that of conventional heat-cure acrylic resins. Therefore, high-impact heat-cure acrylic resins can be used as alternatives to conventional heat-cure acrylic resins in conjunction with RTV silicones in the fabrication of maxillofacial prostheses.
How to cite this article: Shastry A, Balehonnur P, Krishnappa VS, et al. Evaluation of the Peel Bond Strength of a Room Temperature Vulcanizing Maxillofacial Silicone when bonded to Three Different High-impact Heat-cured Acrylic Resins: An In Vitro Comparative Study. Int J Prosthodont Restor Dent 2023;13(2):76-80.
Source of support: Nil
Conflict of interest: None
Keywords: Heat-cure acrylic resin, High-impact acrylic resin, Lucitone-199, Maxillofacial silicone, Peel bond strength, Primer A330G
Face contributes to the distinctiveness of a person. The form, function, and esthetics of the maxillofacial region can be compromised due to congenital or acquired defects. These defects become a cause of shame, anguish, and suffering that has a deep psychological impact on the patients.1,2 Surgical or prosthetic restoration of the defective tissues can be done in order to improve the admissibility on a personal and social level.3 An important consideration in prosthetic rehabilitation is the chosen material. There has been a quest in search of an ideal material.4
Silicone is one of the most sought-after materials for prostheses, as it offers advantages such as chemical inertness, thermal and dimensional stability, elasticity, flexibility, and tissue-like texture. In addition, color incorporation is easier, giving a natural appearance overall.5 However, acrylic resins are also used in conjunction with silicone to provide a rigid substructure or house retentive components. Auto polymerizing, heat-polymerizing, or visible light-curing denture resin may be used for this purpose, which should be enveloped in silicone.6,7
The bond between the two materials must be strong enough to overcome tensile forces during the handling of prostheses by the patient and also during reservicing.7 Studies6,8 have shown that silicone and acrylic being chemically unlike, fail to bond with each other without surface treatments such as the use of primers. Adhesive primers consist of organic solvents and adhesive agents. These components can bring about the adhesion between acrylic resins and silicone elastomers, forming a strong bond between the two, thereby improving the longevity of the prosthesis.7,9 Apart from poor bond strength, another concern with maxillofacial prostheses is a fracture of the acrylic component.9 This may occur especially when handling the prosthesis while cleaning and also during usage. A good remedial measure would be the use of high-impact acrylic resins instead of conventional acrylic resins. These high-impact acrylic resins have been developed by incorporating a rubber phase in the polymer particles, which is known to improve the impact strength. They differ from conventional resins in their physical and mechanical properties.10
Studies11,12 have shown that high-impact heat-cure acrylic resins are good alternatives to conventional heat-cure resins in complete dentures. However, there is a dearth of literature indicating the usage of high-impact heat-cure resins as an alternative to conventional heat-cure acrylic resins in conjunction with silicones in maxillofacial prostheses.
The purpose of this study was to evaluate and compare the peel bond strength of a RTV maxillofacial silicone bonded to three different high-impact heat-cure acrylic resins using a primer. The null hypothesis was that there would be no difference in the peel bond strength of the RTV maxillofacial silicone to acrylic resins when high-impact heat-cure acrylic resins are used instead of conventional heat-cure acrylic resins.
MATERIALS AND METHODS
This in vitro study was done in the Department of Prosthodontics, Government Dental College and Research Institute, Bengaluru, Karnataka, India. The ethical clearance was obtained from the Ethical Committee and Review Board of the institute with No: GDCRI/IEC-ACM2/12/2022-23. The testing of the specimen was done at the Indian Institute of Science, Bengaluru, Karnataka, India.
The sample size was established using G*power version 18.104.22.168. A sample size of 76 was obtained and was rounded off to 80. Test specimens of dimensions 75 × 10 × 6 mm were fabricated as per American Society for Testing and Materials specification no—D1876,13 consisting of RTV silicone bonded to acrylic resins using the primer A-330 G (Factor II, Inc. Lakeside, Arizona, United States of America).
The specimens were categorized into four groups (n = 20 each) based on the type of acrylic resins used, which included conventional heat-cure acrylic resin (group I, Trevalon, Dentsply India Pvt. Ltd, Gurgaon, India) and three brands of high-impact heat-cure acrylic resins (group II, Lucitone 199, Dentsply Sirona, New York, United States of America; group III, Acralyn-H, Super Hi-impact Asian acrylates, Mumbai, India; group IV, Acryl-Hi Pyrax Polymars, Roorkee, India) (Fig. 1).
Wax blocks of dimensions 75 × 10 × 3 mm were flasked to create mold spaces for the acrylic components. Processing of the heat-cure acrylic resins was carried out as per the manufacturer’s instructions. The acrylic blocks were subjected to primer application.6 Adhesive tapes covered the acrylic blocks such that an area of 25 × 10 mm alone was available for bonding with silicone. Primer A-330 G (Factor II, Inc. Lakeside, Arizona, United States of America) was applied over the exposed area as per the manufacturer’s instructions. The acrylic blocks were then housed in the cassette (Fig. 2). The part A and part B components of S-25 TechSil 25 maxillofacial silicone (Technovent series material, Principality Medical Ltd, South Wales, United Kingdom) was mixed in the ratio 9:1 by weight. The manipulated silicone was compacted in increments over the acrylic test specimens, which were housed in the metallic cassette. Adding in increments prevented air entrapment. The cassette was then closed and secured with nuts and bolts. The closed cassette was left undisturbed for 24 hours to allow for room-temperature polymerization. The test specimens were retrieved, and the flash of excess silicone was carefully trimmed using straight scissors and dry stored. Therefore, each specimen had two rectangular components, one acrylic component of thickness 3 mm, and one maxillofacial silicone of thickness 3 mm, bonded by one primer (Fig. 3).6,14
The standardized test specimens were evaluated for the 180° peel strength between maxillofacial silicone elastomer and four brands of heat-cure acrylic resin. The test was performed as per the American Society for Testing and Materials D903 specifications using a universal testing machine (Mecmesin, Multitest 10i, West Sussex, United Kingdom), with crosshead speed at 10 mm/minute, where the free end of silicone was peeled away from resin at 180, and point of failure was noted (Fig. 4). The peel strength was then calculated using the formula–
The statistical analysis was done with the Statistical Package for the Social Sciences (SPSS) software package (IBM Corp. Released 2013. IBM SPSS Statistics for Windows, Version 22.0. Armonk, New York, United States of America: IBM Corp.). Statistical significance was considered at p < 0.05 (at a confidence interval of 95%). Mean and standard deviation (SD) was calculated for the data. The normality of data was analyzed by using Shapiro–Wilk test. As the data were skewed and not normally distributed, nonparametric tests were applied. Kruskal–Wallis H test was performed to compare the peel bond strength among all groups, followed by Dunn post hoc test to find the significant difference for pairwise comparison.
The mean bond strength is presented in Table 1 and Figure 5. Group II (Lucitone 199), bonded to RTV maxillofacial silicone, showed the greatest peel bond strength (0.0372 ± 0.013 MPa) among all the groups. The test specimens of group III (Acralyn H, 0.0213 ± 0.007 MPa), group I (Trevalon, 0.0185 ± 0.009 MPa), and group IV (Acryl Hi, 0.0148 ± 0.007 MPa) showed peel bond strength to RTV silicones in decreasing order. There was a significant difference (p = 0.013) in the peel bond strength of RTV maxillofacial silicone bonded to three different high-impact heat-cure acrylic resins (Table 2).
|Groups||N||Mean ± SD (MPa)||Standard error||Minimum||Maximum|
|Group I||20||0.0185 ± 0.009||0.0022||0.007||0.0372|
|Group II||20||0.0372 ± 0.013||0.0029||0.0124||0.058|
|Group III||20||0.0213 ± 0.007||0.00171||0.0057||0.034|
|Group IV||20||0.0148 ± 0.007||0.00161||0.0048||0.036|
*Indicates p < 0.05 as statistically significant
Dunn’s post hoc test was carried out to find the significant difference for pairwise comparison. The results showed a significant difference in peel bond strength between group II and each of the other groups (Table 3).
|Group||Group I||Group II||Group III||Group IV|
*Indicates p < 0.05 as statistically significant
The combination of acrylic and silicone is interesting, as several complex prostheses require an acrylic substructure which in turn is enveloped in silicone. This ensures the necessary rigidity. A critical factor in such combinations is the silicone-acrylic interface. Studies6,7 have reported a tendency of debonding of silicone from acrylic due to the difference in their chemical structures. However, the introduction of adhesive primers into maxillofacial prosthetics has minimized this tendency to a great extent, as they have an organic solvent and an adhesive agent that help the silicone to adhere to acrylic. They bring about the superficial surface activation of the substrate by mechanisms such as etching, hydrogen bonding, or covalent coupling, which in turn increases the wettability of the substrate.7 Several primers have been used over the years. However, as per literature,14-16 one of the most clinically efficient primers is A-330 G, which is a modified polyacrylate in methyl ethyl ketone and dichloromethane.
According to Hatamleh and Watts,14 Cosmesil Z004 with primer A-330 G gave optimum results. Similar results were shown in a study by Shetty and Guttal.7 Tanveer et al.15 found that primer A-330 G among three primers provided the best bonding between plexiglass acrylic and silicone A-2186. However, a study by Sanokhan et al. showed that Sofreliner, which is a primer used with intraoral relining materials, yields better results in comparison with A-330G.16 Similarly, Farooqui et al. reported better results with Sofreliner in comparison with A-330 G when M-511 silicone was used.17 While there have been several studies15-18 evaluating the bond strength between conventional acrylic resin and silicone, there is a dearth of literature discussing the bond strength between maxillofacial silicone and high-impact resins, which could be potential alternatives to conventional resins in order to reduce the chances of fracture of prostheses during use and service.
This study aimed to evaluate and compare the peel bond strength of an RTV maxillofacial silicone when bonded to three different high-impact heat-cure acrylic resins with the use of a primer. The results of this study show that the bond strength of maxillofacial silicone, bonded to the treated surface of the acrylic resin with a primer, was improved appreciably when high-impact heat-cure acrylic resins were used instead of conventional heat-cure acrylic resins. Thus, the null hypothesis was rejected as there was a significant difference in the peel bond strength between the control and test specimen groups.
The results obtained were comparable to those obtained in a study by Patel et al.,18 in which peel bond strength was evaluated between conventional heat-cure acrylic resins and maxillofacial silicone. The study reported an increase in peel bond strength when Trevalon was used instead of Dental Product of India (DPI) heat-cure acrylic resin. The mean value for the Trevalon group was 4.826 N/mm, and for the DPI group was 4.351 N/mm.
In the current study on the comparison of the control group with groups II and III, an increase in bond strength was observed when high-impact resins were used instead of conventional heat-cure resins. This increase was significant between groups I and II. The increase was in accordance with a study conducted by Mittal et al.,19 in which bond strength was greater when high-impact resins were bonded to intraoral silicone soft liners than conventional heat-cure resins. The highest mean value with high-impact resins was 1.028 MPa, while the lowest value with conventional heat-cure acrylic resins was 0.289 MPa. However, the results were in contrast with a study by Chauhan et al.,20 which showed that in comparison with high-impact resins, conventional acrylic resins showed higher bond strength with intraoral silicone soft liners. The bond strength of group II with high-impact resin Lucitone 199 was significantly higher than all other groups. This was in accordance with results obtained by Mittal et al.19 As per a study conducted by Artopoulou et al.,21 high bond strength was seen between silicone and Lucitone-199 when A-330 G (7.15 N/mm) and A-304 primers (2.12 N/mm) were used versus the comparison groups.
This was in line with the results obtained in the current study. This was also in line with a study published by Al-Athel et al.,22 which stated that the bond strength of intraoral soft-liner material Novus was dependent on the acrylic resins used and was greater with Lucitone 199 with a mean value of 2.50 N/mm. A study by Wemken et al.,23 showed in their study that the bond strength of a soft chairside relining material Ufi Gel SC which is a polysiloxane, to denture base resins remained unaffected by the denture base material. This is in contrast with the results of the current study.
Studies10,12 have shown significant differences in the physical properties of high-impact heat-cure and conventional heat-cure acrylic resins. As per O’Brien,24 high-impact acrylic resins are created by the incorporation of a rubber phase into the polymer beads. The rubber component remains dissolved in the monomer until sufficient polymer is added during manipulation, during which the rubber precipitates out. The growing chains of polymethylmethacrylate (PMMA) may graft into the butadiene rubber. Thus, islands of rubber may be dispersed in the material. Lucitone 199 shows this type of high-impact rubber graft.22 This obvious difference in the structure may be the reason for the significant increase in bond strength when high-impact resins are used. Bond failure was always adhesive in all four groups of the current study. This indicated that cohesive strength exceeded bond strength in all the specimens.17 This was in line with a study published by Farooqui et al.17 in which specimens containing PMMA resins showed predominantly adhesive failures, while specimens fabricated with fiber-reinforced composite resins showed predominantly cohesive failures when they were bonded to silicone with the use of primers.
Limitations of the current study were that clinical conditions were not adequately replicated as it was an in vitro study. In the current study, only 180° peel bond stresses were simulated, whereas, in function, the prostheses are subjected to forces that are far more complex. This study did not evaluate the effect of silicone coloring pigments, biological solutions such as sweat, cleaning agents, and aging on bond strength. Hence further studies need to be carried out to address the above-mentioned lacunae.
Within the limitations of this study it can be concluded that, Lucitone 199 had greater bond strength to maxillofacial silicone than any other acrylic resin used in the group. Acryl Hi showed the least bond strength to maxillofacial silicone; however, results obtained with Acralyn-H and Acryl Hi were comparable to the results obtained in the control group. This indicated that high-impact heat-cure acrylic resin can be used as an alternative to conventional heat-cure acrylic resin in maxillofacial prostheses and has adequate bond strength to the maxillofacial silicone in the presence of the primer.
Aishwarya Shastry https://orcid.org/0009-0009-7567-277X
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