|Year : 2016 | Volume
| Issue : 4 | Page : 208-214
Comparative evaluation of water sorption and solubility of two autopolymerizing soft denture liners in distilled water and artificial saliva
Ravudai Singh Jabbal1, Kusum Datta2
1 Department of Prosthodontics, Genesis Institute of Dental Sciences and Research, Firozpur, Punjab, India
2 Department of Prosthodontics, Punjab Government Dental College and Hospital, Amritsar, Punjab, India
|Date of Web Publication||27-Dec-2016|
Ravudai Singh Jabbal
H. No. 202, Diamond Avenue, Majitha Road, Amritsar - 143 001, Punjab
Source of Support: None, Conflict of Interest: None
Statement of the Problem: Sorption and solubility by soft liners is accompanied by volumetric changes, microbial infestation, distortion, hardening, color change, and reduced bond strength between the liner and denture base. Water sorption and solubility of soft liners are, therefore, of obvious importance because of their influence on the long-term stability of a soft liner. Aim: The purpose of this study was to evaluate and compare the water sorption and solubility of two chemically distinct soft liners (acrylic-based soft liner, Viscogel and silicone-based soft liner, Mollosil) at different storage intervals over 3-month period. Methods: Sixty disc-shaped specimens (30 mm × 1 mm) of each liner were fabricated, weighed, and divided into six groups of ten specimens each. The specimens were immersed in distilled water and in artificial saliva, and their weight data following immersion and subsequent drying were collected at 1 week, 1 month, and 3 months. The percentage sorption and solubility were calculated, and the data were submitted to one-way analysis of variance (ANOVA). The means were compared using Tukey's honest significant difference post hoc comparison. Results: One-way ANOVA revealed significant difference in the sorption and solubility between the two materials. Acrylic-based soft liner, Viscogel, demonstrated higher percentage absorption and solubility than its silicone-based counterpart, Mollosil, at all-time intervals in both distilled water and artificial saliva. Conclusion: On the basis of lower water sorption and solubility, silicone-based soft liner, Mollosil, may provide for better clinical success than acrylic-based soft liner, Viscogel.
Keywords: Resilient liners, solubility, tissue conditioners, water sorption
|How to cite this article:|
Jabbal RS, Datta K. Comparative evaluation of water sorption and solubility of two autopolymerizing soft denture liners in distilled water and artificial saliva. Indian J Dent Sci 2016;8:208-14
|How to cite this URL:|
Jabbal RS, Datta K. Comparative evaluation of water sorption and solubility of two autopolymerizing soft denture liners in distilled water and artificial saliva. Indian J Dent Sci [serial online] 2016 [cited 2017 Jan 19];8:208-14. Available from: http://www.ijds.in/text.asp?2016/8/4/208/196811
| Introduction|| |
A resilient liner is a viscoelastic material forming all or part of the impression surface of a denture that acts as a cushion and reduces the masticatory forces transmitted by the prostheses to the underlying tissues. The friability of the supporting mucosa and the significantly higher stiffness of acrylic resin denture bases can result in tissue trauma due to force concentration or misfit of the denture base. Resilient liners help alleviate the possibility of discomfort arising from such denture base force transfer to oral mucosa.
Soft denture liners are valuable when treating patients with ridge atrophy, bony undercuts, xerostomia, bruxing tendencies, congenital or acquired oral defects requiring obturation, and for dentures opposing natural dentition. In addition, soft liners are used to modify transitional prostheses after Stage I and II implant surgery.
Materials which are contenders as soft denture liners include plasticized acrylics, silicone rubbers, plasticized vinyl polymers and copolymers, hydrophilic polymers, polyurethane, fluoroethylene, and polyphosphazene fluoropolymers. Of these, silicones and plasticized acrylics are the most popular contemporary soft liners. Both are available in auto-cured and heat-cured forms.
Acrylic-based soft liners are available in a powder/liquid form – the powder consisting of a higher methacrylate polymer and a liquid consisting of higher methacrylate monomer along with a plasticizer. Temporary soft liners or tissue conditioners are also acrylic based, with the liquid consisting of an alcohol (usually ethyl alcohol) as solvent, and a phthalate as a plasticizer. Silicone rubbers are commonly based on polydimethylsiloxane and fillers, such as finely divided silica.
The desirable properties of resilient denture liners include cushioning effect upon the mucosa, permanent resilience, dimensional stability, minimum fluid sorption and solubility, and inhibitive action on fungal growth. During clinical use, the resilient lining materials are immersed in saliva and when not in use may be soaked in water or other cleansing agents. When immersed in such solutions, plasticizers and other soluble components are leached out and water or saliva is absorbed. Water sorption and solubility, thus, are problems that are commonly encountered when using soft denture liners.,,, These problems are associated with changes in structure and properties of the material that result in swelling, distortion, support of Candida albicans growth, reduced bonding to the denture base, and a hard, less resilient product over time.
This study was undertaken to evaluate and compare the water sorption and solubility of two autopolymerizing soft denture liners: One acrylic based and one silicone based in distilled water and in artificial saliva over a 3-month period.
| Methods|| |
The two commercially available autopolymerizing soft denture liners used were:
- Mollosil (Detax GmbH, Germany) – silicone-based, chairside autopolymerizing soft liner [Figure 1]
- Viscogel (Dentsply De Trey, Germany) – plasticized, acrylic-based chairside autopolymerizing soft liner [Figure 2].
The artificial saliva used was Wet Mouth (ICPA Health Products Ltd., India) that contained sodium carboxymethyl cellulose and glycerin in an aqueous base.
Disc-shaped liner specimens 30 mm in diameter and 1 mm in thickness were fabricated using a brass mold. An analytical balance (METTLER TOLEDO AL 204) was used to weigh the liner specimens. A temperature of 37°C ± 2°C was maintained for all specimens using a hot air oven. A total of 120 specimens were prepared, sixty for each liner material, and divided into two main groups:
- Group I: Sixty specimens of autopolymerizing acrylic-based soft denture liners, Viscogel [Figure 3]
- Group II: Sixty specimens of autopolymerizing silicone-based soft denture liners, Mollosil [Figure 4].
Group I: Autopolymerizing acrylic-based soft liner, Viscogel, was mixed according to the manufacturer's instructions and placed in the brass mold between two glass slabs for polymerization for 20 min. Sixty specimens were processed in this manner. The specimens were dried in a desiccator containing anhydrous calcium chloride at 37°C and weighed using an analytical balance (METTLER TOLEDO AL 204) until a constant weight (±0.0001 g) was obtained. This was recorded as the initial weight of the specimen (W1).
Group II: Autopolymerizing silicone-based soft liner, Mollosil, was mixed according to the manufacturer's instructions and placed in the brass mold between two glass slabs for polymerization for 7 min. Sixty specimens were processed, and then dried and weighed in the same manner as the Group I specimens. The initial weight of the specimen was recorded (W1).
Specimens from Group I and Group II were placed in sealed plastic containers in an oven maintained at 37°C ± 2°C and further divided into six groups of ten specimens each:
- Group I (A) and Group II (A): Ten specimens were immersed in distilled water for 1 week
- Group I (B) and Group II (B): Ten specimens were immersed in distilled water for 1 month
- Group I (C) and Group II (C): Ten specimens were immersed in distilled water for 3 months
- Group I (D) and Group II (D): Ten specimens were immersed in artificial saliva for 1 week
- Group I (E) and Group II (E): Ten specimens were immersed in artificial saliva for 1 month
- Group I (F) and Group II (F): Ten specimens were immersed in artificial saliva for 3 months.
After immersion for 1 week, 1 month, and 3 months, the specimens in corresponding groups were removed from their containers. Excess water was blotted off by means of blotting paper, and all specimens were weighed again using the analytical balance. This was recorded as the specimen weight following absorption or desorption of distilled water or artificial saliva (W2).
After each sorption cycle, the specimens were dried in a desiccator containing anhydrous calcium chloride at 37°C. The specimens were reweighed at regular intervals until a constant weight (±0.0001 g) was reached. This was recorded as the final weight after desiccation (W3).
The percentage absorption and percentage solubility were determined as follows:
W1: The initial weight of the specimen.
W2: The specimen weight after absorption.
W3: The final weight of specimen after desiccation.
| Results|| |
The mean percentage absorption and solubility of both liners were tabulated and compared using descriptive statistical measures [Table 1],[Table 2],[Table 3],[Table 4]. Acrylic-based liner, Viscogel, demonstrated higher percentage absorption and solubility than silicone-based liner, Mollosil, in both distilled water and in artificial saliva. The mean percentage absorption and percentage solubility were maximum for Group I (C) (3.85% for Viscogel after 3 months in distilled water), and Group I (F) (2.03% for Viscogel after 3 months in artificial saliva), respectively. Both the mean percentage absorption and percentage solubility were minimum for Group II (D) (0.26% and 0.19%, respectively, for Mollosil after 1 week in artificial saliva). The sorption and solubility values were found to increase over the 3-month period for both liner materials [Graph 1] and [Graph 2].
|Table 1: Descriptive statistics for Group I (Viscogel) in distilled water|
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|Table 2: Descriptive statistics for Group II (Mollosil) in distilled water|
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|Table 3: Descriptive statistics for Group I (Viscogel) in artificial saliva|
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|Table 4: Descriptive statistics for Group II (Mollosil) in artificial saliva|
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One-way analysis of variance revealed a significant difference (P < 0.05) in the percentage sorption and solubility for the two liners [Table 5] and [Table 6]. To make all possible valid comparisons, Tukey's post hoc test was applied [Table 7] and [Table 8].
| Discussion|| |
In clinical use, the soft lining materials are constantly bathed in saliva, and when out of the mouth, they are usually immersed in water for storage or solutions of denture cleansers for cleansing purpose. In these situations, water or saliva can be absorbed into the lining material, and plasticizers or other soluble components can be leached out. Both processes are likely to affect the physical properties and dimensional stability of soft lining material, which in turn has a direct effect on the performance of the soft liner in the mouth. Therefore, sorption and solubility properties are an important means to evaluate the longevity of a particular liner.
The clinical behavior of acrylic resin-based soft liners is dependent upon the balance between the loss of plasticizers in solution and the absorption of water. The water absorbed by the acrylic lining material is a less effective plasticizer than chemicals meant for the purpose; thus, the loss of plasticizer coupled with the absorption of water reduces the compliance of acrylic lining materials making them hard and brittle with time.
Pure silicone rubber is highly permeable to water and water passes through the silicone rubber approximately ten thousand times as fast as through acrylic, but high permeability does not mean a high absorption and the water absorption of pure silicone rubber is low. The soluble material lost by the silicone has no plasticizing function and would hardly affect the compliance. The small quantity of absorbed water, however, will disturb the molecular integrity of the material and result in an increased compliance.
To estimate the sorption and solubility, the approach by Kazanji and Watkinson  is more representative than the American Dental Association (ADA) standard and was used in the study. This method measures the net difference between the saturated weight and the final desiccated weight (W2–W3), while the ADA Standard measures the relative difference between the saturated weight and the initial weight (W2–W1), and in addition measures the sorption and solubility in terms of the surface area (mg/cm 2). Because the surface is imperfect, the actual surface area is expected to be larger than the calculated area, which adds an additional variable to the calculation.
The mean percentage absorption for acrylic-based liner, Viscogel, in distilled water was 0.98% at 1 week, 1.85% at 1 month, and 3.85% at 3 months. Thus, the absorption was considerably greater than the loss of ethyl alcohol and plasticizer by the acrylic liner. Although the initial loss of ethanol is faster than the absorption of water, the rates of diffusion of ethanol and water become equal at 3.75 h for 1 mm thick liner specimens. Following this, water absorption exceeds the rate of ethanol and plasticizer loss, thus accounting for the increase in weight. The sorption values in this study were comparable to those reported by Dinçkal Yanikoglu and Yesil Duymus  and El-Hadary and Drummond  for plasticized acrylic liners in distilled water.
The mean percentage absorption for Viscogel in artificial saliva was 0.50% at 1 week, 0.58% at 1 month, and 0.84% at 3 months. The lower uptake in artificial saliva is explicable in terms of ionic impurities in the polymer. The osmotic pressure is proportional to the difference in ionic concentration between the polymer and the external liquid, this difference being greater for water than for artificial saliva. This leads to enhanced uptake in distilled water. A similar osmotic mechanism has been described by Parker et al. The values obtained in this study were lower than those reported by Kazanji and Watkinson  who reported a percentage absorption of 1.28% and 1.26% after 1 week and 1 month in artificial saliva, respectively.
Acrylic-based liner, Viscogel, demonstrated significantly higher percentage solubility in artificial saliva than in distilled water. The mean percentage solubility for Viscogel in distilled water was 0.48% at 1 week, 0.72% at 1 month, and 1.44% at 3 months. In artificial saliva, the corresponding percentage solubility was 0.73%, 1.08%, and 2.03%. The solubility may be due to the loss of ethanol and leaching out of plasticizers from the acrylic liners. The greater solubility demonstrated by Viscogel in artificial saliva probably occurred because the plasticizers are more soluble in solutions than in water., Aloul and Shen  also reported that plasticizers leached at higher levels in artificial saliva.
Silicone-based soft liner, Mollosil, demonstrated significantly higher percentage absorption in distilled water than in artificial saliva. The water sorption of the silicones is due to the inorganic filler and probably correlates to the type of filler and the degree to which the filler is bonded to the silicone rubber., The mean percentage absorption in distilled water was 0.56% at 1 week, 0.78% at 1 month, and 0.91% at 3 months. In artificial saliva, the corresponding percentage absorption was 0.26%, 0.44%, and 0.71%. High sorption values for room-temperature-vulcanized (RTV) silicones have been reported by Amin et al. and Waters et al. The lower values obtained in this study could be due to better bonding of the filler to the silicone rubber. Parr and Rueggeberg  also reported low sorption value (approximately 1% after 1 month) for an autopolymerizing silicone soft liner that was comparable to its heat-cured counterpart.
The percentage solubility for Mollosil increased with time, and at 3 months, the mean percentage solubility in artificial saliva (0.86%) was significantly higher than in distilled water (0.69%). Waters et al. reported a similar solubility value of 0.75% after 3 months for an RTV silicone modified with surface-treated fillers. It has been theorized that the water solubility of silicones cross-linked at room temperatures is due to the loss of alcohol formed as a reaction by-product and extraction of the metallic salt catalyst which is left unchanged in the reaction.
Silicone-based liner, Mollosil, demonstrated lower percentage absorption and solubility than acrylic-based liner, Viscogel, in both distilled water and in artificial saliva. The percentage absorption increased for both the liners over the 3-month period. The findings of this study were more or less in line with the sorption and solubility studies of Kazanji and Watkinson, Dinçkal Yanikoglu and Yesil Duymus  and Nair et al. Sun et al. also found that the percentage absorption and solubility for acrylic resin liners was higher than that for the silicones.
The results of this study partly contradict the studies of Kawano et al. and Hekimoglu and Anil  who reported a negative increase in solubility for some of the soft lining materials after aging in water, thus indicating water retention and gain in mass by the liner after desorption. The results also partly contradict the study of Parker et al. who reported a higher solubility for the soft liners in distilled water than in solution. The differences may be a result of the different types of lining materials used, geometry and size of the specimens, duration of immersion, and type of solution used in different studies. In addition, the complex diffusion pattern of soft liners contributes to disagreement about properties of even similar liners.
| Conclusion|| |
- The percentage absorption for both the liners was higher in distilled water than in artificial saliva
- The percentage solubility for both the liners was higher in artificial saliva than in distilled water with few exceptions
- The percentage absorption and solubility values were observed to increase over the 3-month period for both the liners
- The mean percentage absorption and solubility for the silicone-based soft liner, Mollosil, were observed to be lower than that for the acrylic-based soft liner, Viscogel, at all times in either solution and may provide for better clinical success.
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Conflicts of interest
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8]