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ORIGINAL ARTICLE
Year : 2017  |  Volume : 9  |  Issue : 1  |  Page : 48-51

Tensile bond strength of facial silicone and acrylic resin using different primers


1 Department of Prosthetic Dentistry, Faculty of Dentistry, Prince of Songkla University, Hat Yai, Songkhla, Thailand
2 Department of Conservative Dentistry and Dental Materials Research Unit, Faculty of Dentistry, Prince of Songkla University, Hat Yai, Songkhla, Thailand

Date of Web Publication14-Jun-2017

Correspondence Address:
Sasiwimol Sanohkan
Department of Prosthetic Dentistry, Faculty of Dentistry, Prince of Songkla University, Hat Yai, Songkhla
Thailand
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0975-8844.207944

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  Abstract 


Aim: To evaluate the tensile bond strength between Silastic MDX 4-4210 silicone and autopolymerizing acrylic resin (Orthojet) with two facial silicone primers (A306 and A330-G) and three primers used for silicone-based intraoral reliner materials [Sofreliner tough primer (ST); Mucosoft bond liner primer (ML); and Mucopren adhesive (MA)]. Materials and Methods: Sixty specimens were divided into six groups according to the primers used to attach the facial silicone to the acrylic resin. All specimens were loaded in tension mode in a universal testing machine with a crosshead speed of 10 mm/min until bonding failure occurred. Results: The highest bond strength values were found in the ST group (1.42 ± 0.24 MPa) followed by the MA group (1.39 ± 0.20 MPa) and the ML group (1.32 ± 0.24 MPa), which were significantly different from the A330-G group (1.12 ± 0.10 MPa), A306 group (0.69 ± 0.0.11 MPa), and the control group (0.18 ± 0.08 MPa). The mode of failure for all specimens was found to be adhesive failure at the facial silicone and bonding agent interface. Conclusion: This study suggests that silicone-based intraoral reliner materials can be used as facial silicone primers.

Keywords: Acrylic resin, facial silicone, primer, tensile bond strength


How to cite this article:
Sanohkan S, Kukiattrakoon B, Peampring C. Tensile bond strength of facial silicone and acrylic resin using different primers. J Orofac Sci 2017;9:48-51

How to cite this URL:
Sanohkan S, Kukiattrakoon B, Peampring C. Tensile bond strength of facial silicone and acrylic resin using different primers. J Orofac Sci [serial online] 2017 [cited 2020 Feb 26];9:48-51. Available from: http://www.jofs.in/text.asp?2017/9/1/48/207944




  Introduction Top


Maxillofacial prosthetics rehabilitation is required in patients who suffer from craniofacial deformity resulting from trauma, burns, surgical treatment of neoplasm, or congenital malformations.[1] A facial prosthesis may be made of silicone, acrylic resin, or a combination of the two. There are two types of silicone used for facial prosthesis, which are classified as per their polymerization, heat temperature vulcanizing and room temperature vulcanizing (RTV) silicone. Silastic MDX4-4210 is brandname of commercial silicone is a medical-grade RTV silicone that is commonly used in practice because of its acceptable properties.

In the past, facial prostheses have been retained by skin adhesives. For better retention of a prosthesis in large defects such as an auricular prosthesis, nasal prosthesis, and an orbital prosthesis, an implant-retained facial prosthesis is preferred.[2] In implants retained for such facial prosthesis, housing with autopolymerizing acrylic resins is preferred because of their good handling: property. The rigidity of autopolymerizing acrylic resin is enough to secure the attachment in place and create a mechanical lock to prevent dislodging of the attachment. However, the different chemical structure of acrylic resins and facial silicone causes there to be a low bond strength between them and leads to clinical problems. Therefore, adhesive primers are used to enhance the bond between acrylic resins and facial silicone.[3] In dental treatment, gradual changes of supporting tissue are occurring. Thus, complete or partial dentures must be relined to fill the space between the denture base and the tissue; thereafter, their adaptation to supporting tissues will be improved.

A resilient or soft liner is indicated in dentures immediately after extraction or in sensitive mucosa. The weak bonding between a soft liner and an acrylic denture base may result from swelling and stress build-up at the bond interface, and when the viscoelastic property is reduced, leaching out of plasticizers occur.[4],[5] The results of mechanical surface treatment studies of acrylic resin are controversial. Abrasion pretreatment with Al2O3 weakened the tensile bond strength between the acrylic resin and soft liner.[6],[7],[8] Several studies have shown that chemical treatment with primers are more effective in the improvement of bond strength between acrylic resin and silicone reliner.[9] The primer is used for enhancing the bond strength between the silicone soft liner and acrylic resin. Therefore, using dental silicone soft liner primer to enhance bond strength between facial silicone and acrylic resin is the optimal way to get desirable clinical outcome, with reduced cost. This study then proposed to evaluate the tensile bond strengths between autopolymerizing acrylic resins and Silastic MDX 4-4210 silicone using facial silicone primer, and comparing it with dental silicone soft liner primer.


  Materials and Methods Top


Materials used in this study are listed in [Table 1]. The specimens were divided into six groups according to their surface treatment, which contained 10 samples per group. Autopolymerizing acrylic resin specimens were fabricated in polyethylene tubes, which were of 30 mm diameter and 5 mm thickness. The autopolymerized acrylic resin was manipulated in a ratio of 2:1 by weight according to the manufacturer’s recommendation. On reaching the dough stage, the mixture was kneaded and placed into the polyethylene tube. All specimen surfaces were polished with silicon carbide paper no. 240–600. The acrylic resin specimens were kept in a desiccator for 24 h before bonding to the silicone. Acetone was applied onto the acrylic resin for 30 s to clean the surfaces. Thereafter, the surfaces of the acrylic resin were treated with the following primers: A306, A330-G, Sofreliner tough primer (ST), Mucosoft bond liner primer (ML), and Mucopren adhesive (MA). There was no surface treatment in the control group. A thin layer of each primer was evenly applied onto both surfaces of the acrylic resin according to the manufacturer’s recommendation and allowed to dry. Then, a metal collar, 10 mm in diameter and 3 mm thickness, was placed in the middle of each acrylic resin plate. Silastic MDX 4-4210 silicone base and catalyst were mixed according to the manufacturer’s recommendation in a ratio of 10:1 of base-to-catalyst by weight. One drop of thixotropic agent (A-300-1) was added to reduce air bubbles in the silicone. The mixed silicones were then put into the metal collar, and the primed acrylic resin disc was placed onto the filled silicone metal collar. The two acrylic resins were clamped to secure the specimens in position. The excess of silicone was removed with a small brush. The specimens were placed into a vacuum pot to eliminate air bubbles for 10 min. The specimens were then polymerized at 75°C in a hot air oven (Memmert, BE 500, Memmert GmbH Co., Ltd, Germany) for 30 min and allowed to cool at room temperature. A metal rod was adhered to the middle of the specimen using cyanoacrylate glue to secure specimens with the machine. As the crosshead speed of the testing machine moved up, the lower acrylic plate was fixed in place. The prepared specimens were then kept at room temperature for 3 days before testing.
Table 1: Materials examined in this study

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The tensile bond strength between the silicone and acrylic resin was tested according to McCabe et al.[10] using a universal testing machine (Loyd Instruments, Model LRX-Plus, AMETEK Lloyd Instruments, Ltd., Hamphshire, UK) [Figure 1]. A 250 N load cell was applied to the specimen at a crosshead speed of 10 mm/min until the specimens were separated. The maximum tensile force was recorded for each specimen. Tensile bond strength was calculated as maximum force at failure (N) divided by the bonding area (78.57 mm2).
Figure 1: Tensile bond strength testing

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The bond failure surfaces were then examined under a stereoscope (Eclipse G400 POL, Nikon, Tokyo, Japan) at ×20 magnification. The mode of failure was classified as “adhesive” when the remaining material on the silicone surface was less than 25%, “cohesive” when it was more than 75%, and “combination” when it ranged between 25 and 75%).[11] Bond strength was analyzed statistically using one-way analysis of variance technique. A multiple comparison test, Tukey’s HSD (Honestly Significant Difference), was used to determine the pair differences. Ethic approval for this study was provided by the Human Research Ethics Committee of Faculty of Dentistry, Prince of Songkla University, on 6 March 2017. This study met the criteria of the Exemption Determination (EC6003-09-L-LR).


  Results Top


The mean and standard deviation of the tensile bond strength for the groups studied are shown in [Table 2]. The highest tensile bond strength was found in the ST group (1.42 ± 0.12 MPa), followed by the MA group (1.39 ± 0.20 MPa) and the ML group (1.32 ± 0.24 MPa), which was significantly different from A330-G (1.12 ± 0.10 MPa), A306 (0.69 ± 0.0.11 MPa), and the control groups (0.18 ± 0.08 MPa), respectively (P < 0.05). The mode of failure for all specimens, evaluated under a stereoscope, was found to be adhesive failure at the facial silicone and bonding agent interface.
Table 2: Tensile bond strength between autopolymerizing acrylic resins and Silastic MDX 4-4210 using commercial primers

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  Discussion Top


In general, the average life span for a silicone craniofacial prostheses is 1.5–2 years. The main reasons for remaking a craniofacial prosthesis include discoloration of the prosthesis, attachment problems with the acrylic resin housing to the silicone, rupture of the silicone, and poor fitting of the prosthesis. The clinical problem for an implant-retained maxillofacial prosthesis is that there is no chemical bonding between acrylic resin and facial silicone. The loss of bonding between the acrylic resin baseplate and the silicone results from their different chemical structure. Facial prostheses are affected by various types of stresses and directions of forces during removing and cleaning. The bonding between facial silicone and acrylic resin substrate is the key for a successful facial prosthesis. The bond strength test for maxillofacial silicones to an acrylic resin has been tested using different testing methods such as the peel test,[3],[12] the shear bond test,[3],[12],[13] and the tensile bond test.[14] The tensile bond strength test in this study was performed in the same way as a previous study, in which force was applied to the bonding area of both materials (facial silicone and acrylic resin).[10] To ensure even stress distribution at the adhesive interfaces, the crosshead speed was maintained at 10 mm/min. However, the results of the tensile bond strength values were different from previous studies because of the use of different types of substrates, the bonding testing machine, the crosshead speed, and the fixation method; therefore, careful interpretation is needed. Al-Athel and Jagger[15] showed that the bond strength depends critically upon the nature, particularly the direction, of the debonding force and the thickness of the layer of soft material. A metal collar of 10 mm diameter and 3 mm thickness was used in this study to control the amount of silicone. The known volume of material over the controlled bonding area was constrained within the metal collar. However, there was no adhesion between this collar and the test material. The collar did not interfere with the debonding process. The direction of tensile force was also correctly controlled in this study. The metal jig was fabricated to control the center of the bonding area to the center of the test piece in the universal testing machine, and the alignment of the adherent surface was perpendicular to the loading axis.

Differences in tensile bond strengths were observed in each group because of variations in the primer composition. The nature of the type of primer solvent is a factor in enhancing bond strength. McCabe et al.[10] concluded that a primer based on ethyl acetate performed better than one based on toluene. Ethyl acetate is a good solvent for acrylic resins and is able to soften the surface of the hard base more readily, encouraging the deposition of a more tenacious layer of the silane-bonding agent. ML, ST, and MA primers are used regularly to condition and enhance the bonding between acrylic resin and silicone base relining material. The results of this study suggested that all dental silicone liner groups (ST, ML, and MA groups) could be used as primers for bonding autopolymerizing acrylic resin and facial silicone (A306 and A330-G groups). The ST primer showed the significantly highest tensile bond strength value. Because ST primer contains ethyl acetate as a solvent, and the active ingredients are Poly(methyl methacrylate) (PMMA) and polyorganisiloxane, this primer exhibited a high bond strength. The bonding mechanism can assume that PMMA in a primer bond to acrylic resin, and polyorganisiloxane will bond to a facial silicone substrate. ML contains methacrylate (MMA) in a dichloromethane solvent, whereas MA contains MMA in ethyl acetate. A previous study found that the surface treatment with MMA and ethyl acetate improved the bond strength of a silicone-based denture liner to PMMA.[16] MMA has good swelling properties and the ability to enhance the MMA molecules and adhesive monomer diffused into acrylic resin surface. When the depth of a swollen layer increased, the bond strength improved.[17],[18] Two commercial facial silicone primers (A306 and A330-G) were also used in this study. A-306 contains a mixture of 5% silane in naptha (85%). It enhances the bond between platinum (addition cure) silicone elastomers to various substrates. The A330-G primer contains modified polyacrylates in methyl ethyl ketone and dichloromethane. It chemically enhances the bond of silicone elastomer to acrylic resins, plastics, and metal.

The type of bond failure was assessed as adhesive, cohesive, or mixed. At baseline, all the facial silicone, regardless of the adhesive primers, failed by predominant adhesive debonding under tensile forces. In clinical practice, adhesive failure between facial silicone and acrylic resin is more favorable than cohesive failure in silicone, because it is easier to repair. However, patients should be instructed to remove the prosthesis by holding the thickest portion of the prosthesis to prevent tearing of the facial silicone. According to the study of Goiato et al.,[19],[20] the removing force for an O-ring, bar clip, and magnet were 20.57 N, 29.22 N, and 13.75 N, respectively. For clinical success, the minimal bond strength value between facial silicone and acrylic resin had an average 0.07 MPa.[19],[20],[21] The results obtained from this study found that all experimental primers show higher tensile bond strength than minimal bond strength (0.07 MPa), which was considered as the minimally acceptable value for clinical use.


  Conclusion Top


Within the limitations of the study, the results suggested that the ST, ML, and MA could promote optimal bonding between Silastic MDX 4-4210 facial silicone and autopolymerizing acrylic resin. The monomer or the solvents used were compatible with acrylic resin and facial silicone.

Financial support and sponsorship

Prosthodontics and occlusion rehabilitation research unit, Faculty of Dentistry Research Fund, Prince of Songkla University.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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Seimenis I, Sarafianou A, Papadopoulou H, Papadopoulos T. Shear bond strength of three veneering resins to a Ni-Cr alloy using two bonding procedure. J Oral Rehabil 2006;33:600-8.  Back to cited text no. 11
    
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