| Abstract|| |
Purpose: To examine, evaluate, and compare the tensile bond strength of two silicone-based liners; one autopolymerizing and one heat cured, when treated with different chemical etchants to improve their adhesion with denture base resin.
Materials and Methods: Hundred and sixty test specimens of heat-cured polymethyl methacrylate (PMMA) were fabricated; out of which 80 specimens were tested for tensile bond strength after bonding it to autopolymerizing resilient liner (Ufigel P) and rest 80 to heat-cured resilient liner (Molloplast B). Each main group was further divided into four subgroups of 20 specimens each, one to act as a control and three were subjected to surface treatment with different chemical etchants namely dichloromethane, MMA monomer, and chloroform. The two silicone-based denture liners were processed between 2 PMMA specimens (10 mm × 10 mm × 40 mm) in the space provided by a spacer of 3 mm, thermocycled (5-55°C) for 500 cycles, and then their tensile strength measurements were done in the universal testing machine.
Results: One-way ANOVA technique showed a highly significant difference in the mean tensile bond strength values for all the groups. The Student's t-test computed values of statistics for the compared groups were greater than the critical values both at 5% and at 1% levels.
Conclusion: Surface treatment of denture base resin with chemical etchants prior to the application of silicone-based liner (Ufigel P and Molloplast-B) increased the tensile bond strength. The increase was the highest with specimens subjected to 180 s of MMA surface treatment and the lowest with control group specimens.
Keywords: Adhesion, denture liners, polymethyl methacrylate, tensile strength
|How to cite this article:|
Kaur H, Datta K. Comparative evaluation of tensile bond strength of silicone-based denture liners after thermocycling and surface treatment
. Indian J Dent Res 2015;26:514-9
The most expedient, however, the most effective method for treating abused basal tissues is the use of soft denture base liners. They readily adapt to the tissue surface of the denture to form an intervening cushion between denture bearing mucosa and basal denture surface and function by restoring the fit and stability of a denture base. 
|How to cite this URL:|
Kaur H, Datta K. Comparative evaluation of tensile bond strength of silicone-based denture liners after thermocycling and surface treatment
. Indian J Dent Res [serial online] 2015 [cited 2019 Dec 13];26:514-9. Available from: http://www.ijdr.in/text.asp?2015/26/5/514/172076
One of the first synthetic resins developed in 1945 as a soft liner was plasticized polyvinyl resin followed by the introduction of silicones in 1958.  At present, soft lining materials include silicone elastomers and plasticized acrylic resins, former being better in properties such as tensile strength and low absorption.  They also retain their resiliency and cushioning effect for longer periods as compared to plasticized acrylic-based soft denture liners. 
Silicone-based denture liners have a different molecular structure as compared to polymethyl methacrylate (PMMA) denture base resin and cannot be chemically bonded with the latter. Bonding between them completely relies on an interfacial adhesion. There are several problems associated with the use of resilient denture liners, including bond failure between the liner and the denture base, colonization by Candida albicans, porosity, poor tear strength, and loss of softness.  Different types of chemical etchants (such as dichloromethane, MMA monomer, chloroform, acetone, and methylene chloride) can be used for the surface treatment of the denture base resin prior to the application of silicone-based denture liner to improve the bond strength of the liner with the denture base resin, as they help in better penetration of the adhesive. 
Properties such as tensile and shear bond strength have been shown to be dependent on the chemical composition of both reline materials and denture base polymers. A weak bond could harbor bacteria, promote staining and delamination of the lining material. Furthermore, it is suggested that the bond strength between the dentures reline and denture base resins could affect the mechanical strength of the reline denture base.
The literature present on a comparison of auto and heat polymerizing denture liners after surface treatment with various chemicals and thermocycling are scanty. Hence, this study was undertaken to investigate the effect of different chemical etchants on tensile bond strength of denture base resin and silicone-based resilient liners.
The research hypothesis was that the duration of application or the type of chemical etchants would have different effects on the bond strength of the silicone-based resilient liner to denture base resin.
| Materials and methods|| |
The materials used in this study were [Table 1]:
The chemicals used for the surface treatment of specimens were:
A brass die was used to prepare specimens of PMMA of dimensions 10 mm × 10 mm × 40 mm each with 3 mm thick removable brass spacer for measuring tensile bond strength [Figure 1]. Two groups (Group I and Group II) of 80 specimens each of heat-cured PMMA denture base resin were prepared for tensile bond strength. Each group was further divided into four subgroups (A, B, C, and D) of 20 specimens each.
- MMA monomer
Specimens of Group I A and Group II A served as control.
Specimens of Group I B and Group II B were subjected to 30 s of chloroform treatment.
Specimens of Group I C and Group II C were subjected to 180 s of MMA monomer treatment.
Specimens of Group I D and Group II D were subjected to 15 s of dichloromethane treatment.
Group I specimens
Denture base resin (Pyrax) was mixed according to manufacturer's instructions and packed into the brass die with the brass spacer in place. The die was pressed in a clamp, and processing was done in a water bath at 74°C for approximately 2 h, and then temperature of the water bath was increased to 100°C, and processing for further 1 h was carried out. After curing, the die was bench cooled and the blocks were removed and finished by trimming off the excessive flash. According to the group, the bonding surfaces were given surface treatments with different chemicals used in this study. Adhesive was applied and left to dry for 1 min. Then, the spacer of the die was removed and blocks were placed back into the die. The base and the catalyst pastes of Ufigel P were then mixed in the recommended ratio of 1:1 and placed in the die at the place of the spacer. The die was closed and bench pressed for 10 min. The specimen was removed from the flask, and excess material was removed with the help of scalpel [Figure 2].
Group II specimens
Denture base resin (Pyrax) was mixed according to manufacturer's instructions and placed in the dough stage in the die with the spacer in place. The flask was left under bench press for approximately 2 h. This was done to allow the dough stage to reach a firm state that would resist distortion when packing the soft lining material. After 2 h, the flask was opened and the spacer was removed. The bonding surfaces were given surface treatments with different chemicals used in this study according to their group. Molloplast-B was taken with the help of clean spatula and packed into the space created by the spacer in the die against the resin acrylic dough. The flask was then closed and bench pressed for 15 min. Polymerization was done by placing the flask in the cold water slowly heating up to 100°C for approximately 2 h. The flask was cooled down slowly. The specimen was removed and trimmed off for excess flash [Figure 3].
For tensile bond strength measurement (Groups I and II)
All the specimens were thermocycled at 5°C and 55°C in two water baths for 500 cycles with a dwell period of 30 s in each bath to simulate the oral conditions. All specimens were then deformed in Llyod's universal testing machine at the rate of 5 mm/min to determine the maximum tensile load before failure [Figure 4].
|Figure 4: Llyod's Universal Testing Machine for evaluation of tensile bond strength|
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Bond strength was calculated as follows:
Maximum load is the force required before failure of specimens in kilogram force (Kgf).Cross sectional area of sample = Width (mm) × Height (mm)=10 mm × 10 mm =100 mm 2 = 1 cm 2
The readings then obtained were subjected to statistical analysis. The bond strength of each resilient liner material was determined statistically using a two-way ANOVA and the Student's t-test. All statistical testing was performed at a preset alpha level of 0.05.
| Results|| |
Descriptive statistical measures, such as mean, range between maximum and minimum values of tensile bond strength, standard deviation, and coefficient of variation, were computed for all the study groups. These statistical computations are presented in [Table 2] and [Table 3].
A perusal of [Table 2] and [Table 3] indicated that the mean tensile bond strength was maximum for Group I C (9.86 kgf/cm 2 ) and II C (19.28 kgf/cm 2 ) (specimens treated with MMA monomer); and minimum for Group I A (4.17 kgf/cm 2 ) and II A (10.71 kgf/cm 2 ) (control). In order to compare the means of study groups, one-way ANOVA (analysis of variance) test was used [Table 4] and [Graph 1] and [Graph 2].
Analysis of tensile bond strength values for all the study groups by one-way ANOVA technique returned an F value of 205.84 at the degree of freedom = 159. The computed F value was greater than critical F value of 2.07 and 2.76 at 5% and 1% level of significance, respectively, thus indicating a highly significant difference in the mean tensile bond strength values for all the groups (0.01).
In order to make all valid comparisons and to test the difference of strength of any two study groups, the Student's t-test was applied. The computations for tensile bond strength have been presented in [Table 5] and [Table 6].
The Student's t-test to determine the significance of difference for tensile bond strength among group means indicated a highly significant difference between any two groups means that were compared, which indicates that the probability of a significant result due to chance or when no true significance exists is <1% except between Group II B (specimens treated with chloroform) and Group II D (specimens treated with dichloromethane). The computed values of t-statistics for the compared groups were greater than the critical values, both at 5% and at 1% levels, except for the comparison between Group II B and Group II D for which the computed t values was less than critical "t" value even at 5% significant level.
| Discussion|| |
The result of this study supports the hypothesis that says that the duration of application or the type of chemical etchants would have different effects on the bond strength of the silicone-based resilient liner to denture base resin. Bonding between the denture base resin and the silicone based denture reliner relies completely on an interfacial adhesive. Effective bonding is important for longevity of interfacial adhesive; and to achieve this, many techniques have been put forward such as mechanical surface preparation that is roughening of the denture base, airborne particle abrasion or laser treatment of the denture base, application of various chemicals as surface modifiers of denture base, and effect of polymerization stage at which resilient liner is packed with acrylic resin.  This in vitro study was carried out by focusing on thermocycling as water may percolate directly into the bond site, leading to swelling and consequent increase in stress at the liner-denture base interface and reduce the bond strength causing plasticizers from the body of the liner to leach out. This leads to reduction in elasticity of the resilient.
al-Athel et al. enumerated different tests to evaluate the adhesive bond strength of materials which include peel, tensile, shear, fatigue, creep, impact, and cleavage tests.  The tensile bond test is an effective method of investigating the adhesive bond strength. ,, The bonding interface between soft denture liners and denture base materials is mainly subjected to shear and tear stresses during the clinical use. Shear stress is generated at the periphery of the bonding interface during tensile testing; as the bonding area remains the same while the soft denture liner stretches. Therefore, the tensile test used in this study was an acceptable method to evaluate the adhesion between both materials. Although the tensile test does not simulate clinically exposed forces of lining material, it has been considered as good method. 
In the present study, surface treatment of denture base resin with various chemical etchants increased the bond strength of silicone-based lining materials, Ufigel P, and Molloplast-B. The mean measured tensile bond strength of the autopolymerizing resilient liner (Ufigel P) in descending order according to the type of chemical etchant applied was as follows: MMA monomer for 180 s (9.86 kg/cm 2 ), chloroform for 30 s (7.35 kg/cm 2 ), dichloromethane for 15 s (5.84 kg/cm 2 ), and the untreated group which served as control (4.71 kg/cm 2 ). Tensile bond strength of heat polymerizing resilient liner (Molloplast-B) was also measured and depending upon the chemical etchants results were as follows: MMA monomer for 180 s (19.28 kg/cm 2 ), chloroform for 30 s (13.75 kg/cm 2 ), dichloromethane for 15 s (13.30 kg/cm 2 ), and the untreated group which served as control (10.70 kg/cm 2 ). The Student's t-test to determine the significance of difference for tensile bond strength among group means indicated a highly significant difference between any two groups means that were compared, which indicates that the probability of a significant result due to chance or when no true significance exists is <1%. The significance of the difference is nonsignificant in the specimens treated with chloroform and dichloromethane when bonded with Molloplast-B for which the computed t value was less than critical t value even at 5% significant level. Monomer treatment of a polymerized surface can help to prepare that surface for the addition of a new resin. This is possible because the monomer causes the outer surface of the polymer to swell enabling the adhesive resin added later to penetrate between polymer chains and become entangled while the added monomer or solvent can be evaporated. This deeper penetration of the adhesive resin will produce a more intimate and stronger bond. The use of MMA may effectively increase the dissolution of the denture base resin (PMMA) surface prior to soft liner application.
Sarac et al.  suggested the improvement in bond strength of resilient liners to denture base resin after surface pretreatments with chemical etchants such as acetone for 30 or 45 s; methyl methacrylate monomer for 180 s; methyl chloride for 5, 15, and 30 s; preceding the silicone-based resilient liner (Mollosil) application. MMA monomer surface treatment for 180 s was found to be the most effective in improving the bond strength which was confirmed by the studies carried out by Takahashi and Chai  in which the bond strength of denture base resin and three denture lining materials (Kooliner, GC reline, Triad VLC) was examined. Leles et al.  treated the denture base resin before the application of autopolymerizing lining material with MMA monomer, isobutyl methacrylate monomer, experimental adhesives, chloroform, and acetone and confirm the increase in bond strength with MMA monomer. Wieckiewicz et al.  evaluated and compared tensile and shear bond strengths values of three modern autopolymerized silicone relining materials bonded to acrylic plates and found that all tested materials have acceptable adhesion values to acrylic resin. Lau et al.  conducted a study on soft and hard reliners and concluded that the tensile and shear bond strength values of denture soft reliners were significantly lower than denture hard reliners.
Hardness, weight change, tensile strength, tear strength, and color stability are all properties of soft denture liners. A selection of a particular liner cannot be based on any single property. Material selection is influenced not only by the properties available but also by the particular situation being treated. This study was entirely laboratory based. Because of the most appropriate testing environment is the mouth, the long-term clinical studies of these materials are required.
| Conclusions|| |
Within the limitations of the current in-vitro investigation, the following conclusions can be drawn:
Financial support and sponsorship
- Surface treatment of denture base resin with chemical etchants prior to the application of silicone-based liner (Ufigel P and Molloplast-B) increased the tensile bond strength of silicone-based liner to denture base resin
- The increase in tensile bond strength value was the highest with specimens subjected to 180 s of MMA surface treatment and the lowest with control group specimens.
Conflicts of interest
There are no conflicts of interest.
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Department of Prosthodontics, Maulana Azad Institute of Dental Sciences, New Delhi
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]