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Table of Contents   
ORIGINAL RESEARCH  
Year : 2012  |  Volume : 23  |  Issue : 1  |  Page : 64-68
Evaluation of tensile bond strength of heat cure and autopolymerizing silicone-based resilient denture liners before and after thermocycling


Department of Prosthodontics, Punjab Government Dental College and Hospital, Amritsar, Punjab, India

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Date of Submission06-Mar-2011
Date of Decision23-Jun-2011
Date of Acceptance13-Oct-2011
Date of Web Publication26-Jul-2012
 

   Abstract 

Aim: To assess the effect of simulated mouth conditions reproduced with thermocycling on the tensile bond strength of two silicone based resilient denture liners with acrylic resin bases.
Materials and Methods: Two silicone-based soft denture liners (Mollosil - Chairside autopolymerization and Molloplast B - Heat polymerization) were tested. For each liner, 30 specimens with a cross-sectional area of 10 Χ 10 mm and thickness 3 mm were processed between two acrylic blocks (Trevalon). Specimens were divided into a control group that was stored for 24 hours in water at 37°C and a test group that was thermocycled (2500 cycles) between baths of 5° and 55°C. Tensile bond strength (kg/cm 2 ) was determined in a universal testing machine using crosshead speed of 5 mm/min.
Statistical Analysis Used: The student t-test was used to determine the significance of the difference in bond strength between the two liners.
Results: The mean tensile bond strength for control and thermocycled specimens of the two liners were: Mollosil (6.82 kg/cm 2 and 8.41 kg/cm 2 ) and Molloplast-B (16.30 kg/cm 2 and 13.67 kg/cm 2 ), respectively. Comparison of bond strength of control specimens with thermocycled specimens of the liners indicated a significant difference for both Mollosil (P=0.045) and Molloplast-B (P=0.027). Comparison between control specimens of both liners and thermocycled specimens of both liners indicated a highly significant difference (P<0.001).
Conclusions: Heat polymerized resilient denture liner Molloplast-B had higher tensile bond strength than autopolymerizing liner Mollosil regardless of thermocycling. The bond strength of Mollosil increased after thermocycling while that of Molloplast-B decreased after thermocycling.
Clinical Implications: Although heat-polymerized denture liners require more processing time than autopolymerizing liners, but they display much better adhesion properties to denture base resin and should thus be preferred when soft liner has to be used for a longer duration of time.

Keywords: Soft denture liners, tensile bond strength, thermocycling

How to cite this article:
Madan N, Datta K. Evaluation of tensile bond strength of heat cure and autopolymerizing silicone-based resilient denture liners before and after thermocycling. Indian J Dent Res 2012;23:64-8

How to cite this URL:
Madan N, Datta K. Evaluation of tensile bond strength of heat cure and autopolymerizing silicone-based resilient denture liners before and after thermocycling. Indian J Dent Res [serial online] 2012 [cited 2021 Apr 19];23:64-8. Available from: https://www.ijdr.in/text.asp?2012/23/1/64/99041
Resilient lining materials for dentures are products that are applied to the intaglio surface of dentures for the purpose of achieving a more equal distribution of the load and a reduction of local point pressures. Resilient denture liners were developed to alleviate the discomfort arising from transfer of load from the denture base to oral mucosa by acting as a cushion or a shock absorber between the hard denture base and the underlying tissues. Soft denture liners have been found useful for treating patients with ridge atrophy or resorption, bony undercuts, bruxing tendencies, congenital or acquired oral defects requiring obturation, xerostomia, and dentures opposing natural dentition in the opposing arch. [1]

Currently, commonly used soft liners are either plasticized acrylics or silicone elastomers. Both types are available in autopolymerizing and heat curing forms. Plasticized acrylic liners which consist of a powder (polymethylmethacrylate polymers and co-polymers) and liquid (methacrylate monomers and plasticizers) tend to become hard and lose their resiliency because of gradual leaching out of plasticizers. In silicone elastomers which are basically dimethyl siloxane polymers, no plasticizer is necessary for the softening effect, thus resiliency and cushioning effect is retained for prolonged periods.

One of the most serious problems associated with silicone based resilient denture liners is loss of adhesion to the denture base. Polymethyl methacrylate (PMMA) denture base resin and silicone lining material have different molecular structures and cannot be chemically bonded. The bond is achieved by means of a silicone polymer (such as methyl siloxane) in a volatile solvent or by the use of alkylsilane bonding agents. Adhesion failure between the silicone denture lining materials and the denture base is very common [2],[3],[4] and results in a potential surface for bacterial growth, plaque, and calculus formation at the debonded regions and often causes functional failure of the prosthesis.

Parameters such as absorption of water and saliva from the oral cavity may affect the bond between the resilient lining material and the denture base. Also, cyclic thermal stresses caused by the intake of hot and cold foods provoke repetitive shrinkage and expansion causing shear stress at the bonding interface. This results in a difference of thermal volumetric change between the denture base and the soft denture liner. [5],[6] Also, diffusion of water into the interface and contact with adhesive primers causes hydrolytic degeneration of the bond. McCabe JF et al.[7] reported that the bond strength of soft denture liners to PMMA denture base resins is weak, and when separation takes place, the localized area may become unhygienic and non-functional.

Clinically, the ability of denture lining materials to resist de-bonding and internal fracture is necessary in order to avoid interface failure during the service life of the prosthesis. It is thus extremely important to evaluate the bond between silicone based denture soft lining materials and PMMA denture base resins and study the effect of varying mouth conditions on this bond strength.

The purpose of this study was to assess and compare the effect of simulated mouth conditions reproduced with thermocycling on the tensile bond strength of two silicone-based resilient denture liners to denture base resin.


   Materials and Methods Top


The materials used in this study are listed in [Table 1]. Thirty specimens of both the liners were prepared with cross-sectional area of 10 mm × 10 mm and thickness 3 mm by processing between two PMMA blocks [Figure 1]. To prepare the specimens, a special flask made of brass with three detachable parts was used. A sample mould was cut out in the middle part with a removable 3 mm brass spacer in the center [Figure 2].
Figure 1: Configuration and size of fabricated samples

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Figure 2: Special three part brass flask with removable 3 mm thick spacer in the middle part

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Table 1: Materials used in the study

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Mollosil specimens: Denture base resin (Trevalon) was mixed according to manufacturer's instructions and packed into the special brass flask with the brass spacer in place. The flask was pressed in a clamp and processing was done in a water bath at 74°C for two hours and then increasing the temperature of water bath to 100°C and further processing for one hour. After curing, the flask was bench cooled, the blocks were removed from the flask and any excessive flash was trimmed off. The surfaces of the acrylic resin blocks to be bonded to autopolymerizing liner were cleaned with fine grit sandpaper. Mollosil adhesive No. 03007 was applied on the dried and degreased surfaces of both blocks and left to dry for one minute. The blocks were then placed back in the flask and the spacer was removed. The base and catalyst pastes of Mollosil were taken, mixed in the recommended ratio of 1:1 and the material packed into the space created by the removal of the spacer. The flask was closed and bench-pressed for 15 minutes. The sample was removed from the flask and any excess material was carefully removed with a scalpel.

Molloplast-B specimens: Denture base resin (Trevalon) was mixed according to manufacturer's instructions and packed in the dough stage in the flask with the brass spacer in place. The flask was left at 100 kp for two hours under the press. This was done to allow the dough to reach a firm state that would resist distortion when packing the soft lining material. After two hours, the flask was opened and the spacer removed. Molloplast-B was taken with the help of a clean spatula and packed into the space created by the removal of the spacer against the acrylic resin dough. The flask was then closed and bench-pressed for 15 minutes at 100 kp. Polymerization was done by placing the flask in cold water, slowly heating up to 100°C and then keeping it at 100°C for two hours. The flask was cooled down slowly. The sample was removed and trimmed off any excess flash.

Thirty specimens of each liner were prepared. For both the liners, 15 control specimens were stored for 24 hours in a water bath at 37°C and 15 test specimens were subjected to 2500 thermal cycles (Thermal Shock Chamber, Standard Environmental Inc., USA) between water baths of 5°C and 55°C with a dwell time of 30 seconds in each bath.

All the specimens were then deformed at the rate of 5 mm/minute in a Lloyds Universal Testing Machine linked to an IBM compatible computer to determine the maximum tensile load before failure [Figure 3].
Figure 3: Sample placed in Lloyd's Universal Testing Machine for checking tensile bond strength

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Bond strength was calculated as maximum load before failure (kgf) divided by the cross-sectional area of the specimen (cm 2 ). Mean, standard deviation (SD) and standard error of mean (SE m ) were computed for the four groups [Table 2]. Significance of the statistical differences between two means was determined by use of the student-t test.
Table 2: Basic statistics in respect of tensile strength (kgf/cm2) of the study groups

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   Results Top


Comparisons of mean tensile strength were made between Mollosil and Molloplast-B before and after thermocycling. The difference in mean tensile strengths in respect of the different groups is portrayed in [Figure 4]. The student-t test was used to determine whether the difference was statistically significant. The computations for the test have been presented in [Table 3] (comparison of control with thermocycled specimens) and [Table 4] (comparison of Mollosil with Molloplast-B specimens).
Figure 4: Bar Graph showing difference in mean tensile bond strength between control and thermocycled specimens of Mollosil and Molloplast-B

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Table 3: t-test for comparison of tensile strength of control with thermocycled specimens of the liners

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Table 4: t-test for comparison of tensile strength of Mollosil with Molloplast-B specimens

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Comparison of bond strength of control specimens with thermocycled specimens of the liners indicated a significant difference for both Mollosil (P=0.045) and Molloplast-B (P=0.027).

Comparison of bond strength of control and thermocycled specimens of Mollosil with control and thermocycled specimens of Molloplast-B, respectively, indicated a highly significant difference (P<0.001).


   Discussion Top


Several factors may affect the bond between the resilient lining materials and the denture base, such as aging in water, use of adhesive, and the nature of the denture base material. [8] The effect of water on the adhesive properties of the resilient lining materials is of utmost importance in final clinical success. Water absorbed by the material has both direct and indirect effects on bonding of liners to denture base resin. 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. In the case of acrylic liners, water may indirectly decrease the bond strength by causing plasticizers from the body of the liner to leach out. Diminished plasticizer will increase the stiffness and reduce the cushioning effect of the liner material. Ultimately, this reduction in elasticity will increase the vulnerability of the bond because tensile forces are transmitted directly to the interface instead of being absorbed by the liner.

To simulate these natural conditions in the present study, the specimens were subjected to 2500 thermal cycles. Temperature variations of 5°C and 55°C were chosen since they are similar to the temperature of foods ingested and are well tolerated by the oral mucosa without causing any damage to it. [9]

The bond failure in this study was determined by the tensile mode of testing. Tensile strength provided information on the ultimate strength properties in tension. Bates and Smith [2] and Kawano F [10] considered the tensile test a good method of investigating the bond strength of resilient lining materials, because it gives information on the strength of the bond in comparison to the tensile strength of the material.

It was found that the mean bond strength for the control group of Mollosil (6.82 kg/cm 2 ) was less than the mean bond strength for its thermocycled group (8.41 kg/cm 2 ); whereas the mean bond strength for the control group of Molloplast-B (16.30 kg/cm 2 ) was more than the mean bond strength for its thermocycled group (13.87 kg/cm 2 ). Craig RG [6] reported that the soft denture materials having 10 pounds per inch (4.5 kg/cm 2 ) bond strength were acceptable for clinical use. Taking this criterion into consideration, both the materials tested had satisfactory bond strength to PMMA denture base resin.

The bond strength values of the liners obtained after testing were statistically analyzed using student t-test. After the analysis it was found that the heat-polymerized liner Molloplast-B had significantly higher bond strength than autopolymerizing liner Mollosil, regardless of thermocycling. These findings were in agreement with the studies of Adyin AK et al.[8] and Saber-Sheikh K et al. [11] who confirmed the superiority of the heat-temperature vulcanized (HTV) over the room-temperature vulcanized (RTV) soft liners.

There was considerable change in the mean bond strength values of Mollosil and Molloplast-B liners after thermocycling. The effect of water on the bond strength of resilient materials has been evaluated by several investigators. [2],[6],[12] Dootz et al.[13] and Hekimoglu and Anil [14] have also shown that accelerated aging dramatically affects the physical and mechanical properties of many of the liners. Excessive water absorption in autopolymerizing silicone materials compared to heat-polymerizing silicones has been reported by Saber-Sheikh et al.[11] and Braden M and Wright PS. [12]

The bond strength of Molloplast-B decreased significantly after thermocycling which may be attributed to the high water uptake caused by the filler content. The reduction in bond strength is the result of swelling and stress buildup at the bond interface or of the changed viscoelastic properties of the resilient lining material which renders the material stiffer and transmits the external loads to the bond site. After thermocycling, Mollosil showed a significant increase in bond strength which may indicate that the material became more brittle and less viscoelastic. The strength of the filler-polymer bond also influences the tensile properties.

The findings of this study were more or less in line with the bond strength conclusions of the study drawn by Kulak-Ozkan Yasemin et al.[15] who found that the bond strength of Molloplast-B decreased significantly after thermocycling while that of autopolymerizing liner Mollosil had a significant increase after thermocyling. Pinto JRR et al.[16] reported that thermocycling had a deleterious effect on the bond strength of Molloplast-B liner (13.7 kg/ cm 2 for thermocycled group as compared to 15.1 kg/cm 2 for control group) which is again in accordance with this study (13.87 kg/cm 2 for the thermocycled group compared to 16.30 kg/cm 2 for control group). Polyzois GL [13] also concluded that water storage reduced the bond strength of resilient liners. However, the results of this study contradict those of Craig and Gibbons, [6] Dootz et al.[17] and Emmer TJ et al.[18] who reported that tensile strength of resilient lining materials of different chemical compositions, whether heat-cured or autopolymerized, generally increased after storage in water. These differences may be due to differences in specimen size, acrylic resin type, processing method, and the number of thermal cycles.

It is important to assess the physical properties of soft lining materials in clinical use and the effect of the oral environment on such properties. Because it is not possible to completely simulate clinical conditions and reproduce oral environment in the laboratory, so the final evaluation should be carried out in a clinical situation.


   Conclusion Top


  • The bond strength of heat cure soft denture liner Molloplast-B was greater than the bond strength of autopolymerizing soft denture liner Mollosil.
  • The bond strength of Mollosil, increased after thermocycling while that of Molloplast-B, decreased after thermocycling.
  • The adequate adhesive value of soft lining material for clinical usage is considered to be 10 pounds per inch (4.5 kg/cm 2 ). Since the adhesive bond strength observed in this study was minimum 6.82 kg/cm 2 , both the soft lining materials used are acceptable for clinical usage.
  • Within the limitations of this in vitro study, it was concluded that the heat polymerized resilient denture liner Molloplast-B had higher tensile bond strength than autopolymerizing liner Mollosil regardless of thermocycling.


 
   References Top

1.Winkler S. Essentials of complete denture prosthodontics. In: Gonzalez JB, editors. The use of resilient liners. 2 nd ed. Ishiyaku EuroAmerica Inc. U.S.A. 2000p. 427-8.  Back to cited text no. 1
    
2.Bates JF, Smith DC. Evaluation of indirect resilient liners for denture laboratory and clinical tests. J Am Dent Assoc 1965;70:344-53.  Back to cited text no. 2
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3.Schmidt WF Jr, Smith DE. A six-year retrospective study of Molloplast-B lined dentures. Part II: Liner Serviceability. J Prosthet Dent 1983;50:459-65.  Back to cited text no. 3
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4.Wright PS. The success and failure of denture soft lining materials in clinical use. J Dent 1984;12:319-27.  Back to cited text no. 4
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5.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.  Back to cited text no. 5
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6.Craig RG, Gibbons P. Properties of resilient denture liners. J Am Dent Assoc 1961;63:382-90.  Back to cited text no. 6
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7.McCabe JF, Carrick TE, Kamohara H. Adhesive bond strength and compliance for denture soft lining materials. Biomaterials 2002;23:1347-52.  Back to cited text no. 7
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8.Aydin AK, Terzioglu H, Akinay AE, Ulubayram K, Hasirci N. Bond strength and failure analysis of lining materials to denture resin. Dent Mater 1999;15:211-8.  Back to cited text no. 8
    
9.Ernst CP, Canbek K, Euler T, Willershausen B. In vivo validation of the historical in vitro thermocycling temperature range for dental materials testing. Clin Oral Investig 2004;8:130-8.  Back to cited text no. 9
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10.Kawano F, Dootz ER, Koran A 3 rd , Craig RG. Comparison of bond strength of six soft denture liners to denture base resins. J Prosthet Dent 1992;68:368-71.  Back to cited text no. 10
    
11.Saber-Sheikh K, Clarke RL, Braden M. Viscoelastic properties of some soft lining materials II - Aging characteristics. Biomaterials 1999;20:2055-62.  Back to cited text no. 11
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12.Braden M, Wright PS. Water absorption and water solubility of soft lining materials for acrylic dentures. J Dent Res 1983;62:764-8.  Back to cited text no. 12
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13.Polyzois GL. Adhesion properties of resilient lining materials bonded to light cured denture resins. J Prosthet Dent 1992;68:854-8.  Back to cited text no. 13
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14.Hekimoglu C, Anil N. Sorption and solubility of soft denture liners after accelerated aging. Am J Dent 1999;12:44-6.  Back to cited text no. 14
    
15.Kulak-Ozkan Y, Sertgoz A, Gedik H. Effect of thermocycling on tensile bond strength of 6 silicone based resilient denture liners. J Prosthet Dent 2003;89:303-10.  Back to cited text no. 15
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16.Pinto JR, Mesquita MF, Henriques GE, Nobilo MA. Effect of thermocycling on bond strength and elasticity of 4 long term soft denture liners. J Prosthet Dent 2002;88:516-21.  Back to cited text no. 16
    
17.Dootz ER, Koran A, Craig RG. Physical property comparison of 11 soft denture lining materials as a function of accelerated aging. J Prosthet Dent 1993;69:114-9.  Back to cited text no. 17
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18.Emmer TJ Jr, Emmer TJ Sr, Vaidyanathan J, Vaidyanathan TK. Bond strength of permanent soft denture liners bonded to the denture base. J Prosthet Dent 1995;74:595-601.  Back to cited text no. 18
    

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Correspondence Address:
Nishtha Madan
Department of Prosthodontics, Punjab Government Dental College and Hospital, Amritsar, Punjab
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.99041

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]

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