Physical property comparison of 11 soft denture lining materials as a function of accelerated aging

Water sorption, solubility, and tensile bond strength of resilient denture lining materials polymerized by different methods after thermal cycling

STATEMENT OF PROBLEM

The clinical properties of resilient denture lining materials may be influenced by the methods used to polymerize them.

PURPOSE

This study evaluated and compared water sorption, solubility, and tensile bond strength of 2 resilient liner materials polymerized by different methods after being thermal cycled.

MATERIAL AND METHODS

Two acrylic resin-based resilient liner materials were evaluated: 1 (Light Liner) polymerized by visible light, and 1 (Ever-Soft) processed by 2 different methods: hot water bath and microwave energy. To evaluate water sorption and solubility, 10 disc-shaped specimens (50 x 0.5 mm) were fabricated for each polymerization method, using acetate matrices. The specimens were dried and thermal cycled (2000 cycles) between baths of 5 degrees C and 55 degrees C. After thermal cycling, the specimens were weighed, dried, and weighed again to calculate the water sorption values and solubility. For tensile bond strength assessment, 12 rectangular specimens measuring 83 mm in total length and with a cross-sectional area of 10 x 10 mm were fabricated using each polymerization method. The specimens were also thermal cycled (2000 cycles) and, afterwards, tested in a universal testing machine at a crosshead speed of 6 mm/min with 100 Kg load cell. The type of failure was determined by use of stereoscopic microscopy. The data (for water sorption and solubility, n = 30, and for tensile bond strength, n = 36) were submitted to 1-way analysis of variance. The means were compared using the Bonferroni test (alpha=.05).

RESULTS

For water sorption the results showed no significant differences among the polymerization methods. Light Liner showed the lowest solubility values (5.3% +/- 0.3%) when compared to Ever-Soft (hot water bath: 7.3% +/- 1.1% and microwave energy: 7.8 +/- 0.9%). For tensile bond strength, Ever-Soft polymerized by microwave energy (0.7 +/- 0.0 MPa) showed the greatest and most significant ( P <.05) value when compared to Ever-Soft polymerized by hot water bath (0.5 +/- 0.1 MPa) and Light Liner polymerized by visible light (0.5 +/- 0.0 MPa). Most Ever-Soft specimens polymerized by microwave energy and Light Liner specimens showed adhesive/cohesive failures. However, Ever-Soft polymerized by hot water bath showed 50% adhesive/cohesive and 50% adhesive failures. The results at the reline material-resin bond interface showed the absence of cohesive failures.

CONCLUSION

Within the limitations of this study, Light Liner showed the lowest solubility values. Ever-Soft should be polymerized by microwave energy to obtain the greatest tensile bond strength values. Materials polymerized by microwave energy and visible light showed predominantly adhesive/cohesive failures.

Effect of storage duration on tensile bond strength of acrylic or silicone-based soft denture liners to a processed denture base polymer

OBJECTIVE

To investigate the effect of storage duration on the tensile bond strength of acrylic and silicone-based denture base materials with liners either heat-cured or auto-cured.

MATERIAL AND METHODS

The denture liners investigated were Vertex soft (acrylic-based, heat-cured), Coe soft (acrylic-based, auto-cured), Molloplast-B (silicone-based, heat-cured), and Mollosil plus (silicone-based, auto-cured). The soft liner specimens were 10 x 10 x 3 mm and were processed between two PMMA blocks. They were tested following immersion in water at 37 degrees C for 1 day, 1 week, and 1, 3, and 6 months. Tensile bond strength was measured using a universal testing machine (Testometric Micro 500) at a crosshead speed of 20 mm/min (n = 10 specimens per experimental group). Multiple ANOVA and Tukey HSD were used to analyse the data at a pre-set alpha of 0.05.

RESULTS

The results indicate that the tensile bond strength of acrylic-based soft liners is greater than that of silicone-based materials. The bond strength of all lining materials decreases with storage duration; the decrease being greatest for the acrylic-based soft liners. The decrease in bond strength of the auto-cured materials is greater than that of the heat-cured products.

CLINICAL SIGNIFICANCE

Comparison of the materials in this study indicates that the silicone-based, heat-cured soft liner is superior, based on the tensile bond strength property. Use of silicone-based, heat-cured soft liners may provide better clinical success over a long period. These laboratory results need to be verified by clinical testing.

Soft liners

The article provides a background for understanding the properties of soft liner materials, describing associated problems, and discussing clinical applications of soft liners in dental practice. Although not a panacea, soft liner materials provide the practitioner with a variable tool in providing excellent clinical care for patients.

Effect of thermocycling on tensile bond strength of six silicone-based, resilient denture liners

STATEMENT OF PROBLEM

Adhesion failure between silicone resilient denture lining materials and denture base resin is commonly encountered in clinical practice. Adhesion failure results in localized unhygienic conditions at the debonded regions and often causes functional failure of the prosthesis.

PURPOSE

The purpose of this study was to investigate the effect of thermocycling on the tensile bond strength of 6 soft lining materials.

MATERIAL AND METHODS

Six commonly used silicone-based resilient denture liners (Ufigel C, Ufigel P, Mollosil, Molloplast B, Permafix, and Permaflex) were chosen for the investigation. The bond strength was determined, in tension, after processing to PMMA. The resilient denture liners for each group (n = 24) were 10 x 10 x 3 mm and were processed between 2 polymethyl methacrylate specimens according to manufacturer's instructions. Two PMMA specimens were prepared by investing brass dies with a 3-mm-thick spacer in a denture flask. Specimens were made by processing the resilient denture liners against the polymerized PMMA block. After polymerization, the brass spacer was removed from the mold, the 2 PMMA resin specimens were trimmed, and the surfaces to be bonded were smoothed. The PMMA block was placed back into the molds and the resilient denture liners were packed into the space made by brass spacer, trial packed, and polymerized according to the manufacturer's instructions. Half of the specimens for each group were stored in water for 24 hours, and the other half were thermocycled (5000 cycles) between baths of 5 degrees and 55 degrees C. All specimens were placed under tension until failure in a universal testing machine at a crosshead speed of 5 mm/min. The maximum tensile stress before failure and mode of failure were recorded. The mode of failure was characterized as cohesive, adhesive, or mixed mode, depending on whether the fracture surface was in the soft liner only, at the denture base-soft liner interface only, or in both. Failure strength was recorded in kg/cm(2). Results were tested by multiple analysis of variance (ANOVA) for mode of failure (adhesive, cohesive, and mix), 2-way ANOVA (storage-products), and 1-way ANOVA (storage-products interaction, before and after thermocycling). Duncan's test was used to determine whether significant changes in the tensile bond properties of the materials occurred during thermocycling.

RESULTS

Varying degrees of bond strengths were found for soft lining materials and were significantly different (P<.05). Results of this study also indicated that the bond strengths of soft lining materials had significantly decreased after thermocycling except Ufigel C and Mollosil.

CONCLUSIONS

Because the adequate adhesive value for soft lining materials is given 4.5 kg/cm(2), all of the materials were acceptable for clinical use.

Changes in surface roughness and colour stability of soft denture lining materials caused by denture cleansers

Soft denture lining materials were immersed into solutions of denture cleansers for 8 h at room temperature, and immersed into distilled water for the remainder of the 24-h period at 37 degrees C. Surface roughness of the soft denture lining materials was measured by contact type surface roughness instrument. For the colour stability test, soft denture lining materials were immersed in the denture cleansers as described above for 180 days. Finally, the colour changes of each material were quantitatively measured by a photometrical instrument to obtain the colour differences between newly processed specimen and immersed specimens (P < 0.01). An autopolymerizing silicone material, Evatouch, exhibited severe changes in surface roughness by all denture cleanser, and the generic material GC Denture Relining showed the minimal changes. Severe colour changes were also observed with some liner and cleanser combinations (P < 0.01). Except for Evatouth, the four silicone soft liners were more stable in surface roughness and in colour change than the two acrylic soft liners. One autopolymerizing silicone (GC denture relining) and one heat curing silicone (Molloplast B) demonstrated the best stability.

Effect of finishing and polishing procedures on the gap width between a denture base resin and two long-term, resilient denture liners

STATEMENT OF PROBLEM

The junction between a long-term, resilient denture liner and the denture base is difficult to finish and polish due to differences in the hardness of the materials. Gaps tend to form during finishing and polishing procedures.

PURPOSE

This study measured the junctional gap between 2 long-term, resilient denture liners and a denture base material after different finishing and polishing procedures were performed. The surface smoothness of the 2 liner materials also was evaluated.

MATERIAL AND METHODS

Molloplast-B and an experimental, heat-polymerized methyl siloxane-resin-based denture liner were processed (according to the manufacturer's instructions) against Lucitone 199 acrylic resin. Control specimens (n = 20) were 2 mm thick and flat. Experimental specimens (n = 64) were fabricated with a raised center section: a 3- x 5- x 15-mm half-cylindrical ridge with a junction at the top of the cylinder between the liner material and acrylic resin. The specimens were finished with 1 of 6 types of burs and polished with different combinations of rubber-impregnated acrylic polishers, pumice, and tin oxide. The finished specimens were examined and photographed with a scanning electron microscope, and the largest gap along the liner/denture base junction on each specimen was measured. Three-way analysis of variance without replication (P<.05) and post-hoc t tests were used to analyze the data and compare groups. RESULTS; Averaged across finishing and polishing techniques, a larger gap was recorded for the experimental liner material (22 microm) than for Molloplast-B (14 microm) (P<.00005). Qualitative evaluation suggested that the experimental liner material polished better than Molloplast-B. The smoothest surfaces were obtained when specimens were finished with fine-tooth cross-cut carbide burs and polished with both pumice and tin oxide. A comparison between polished and unpolished specimens, averaged over materials and finishing techniques, revealed that polishing reduced gap size (P=.015).

CONCLUSION

Within the limitations of this study, the 2 denture liners tested behaved similarly. The results suggest that the size of the gap at the liner/denture base junction may be affected by finishing and polishing techniques and vary among materials.

Microleakage study of various soft denture liners by autoradiography: effect of accelerated aging

STATEMENT OF PROBLEM

The longevity of soft denture liners is a major clinical problem. Debonding of the soft liner from the denture base material is one of the factors that influence their longevity. Debonding of the soft liner can be attributed to microleakage at the interface.

PURPOSE

This study investigated microleakage at the interface of various soft liners and base materials.

MATERIALS AND METHOD

Six soft liners were investigated. Forty specimens of each material in disk form (10 mm in diameter, thickness of approximately 4 mm) were prepared. Twenty specimens of each material were stored in an accelerated weathering tester for 900 hours. For 2 days, all disks were immersed in (45)Ca radioisotope solution, then they were embedded in acrylic resin blocks and sectioned longitudinally. Autoradiographic imaging was used to determine microleakage at the interface of the soft liners and their bases.

RESULTS

Significant differences between nonaged materials were found (P<.05).The difference between Molloplast B and Mucopren (silanized) was not significant (P<.05). Differences among aged materials were significant (P>.05). Differences between Mucopren (nonsilanized), Mucopren (silanized), and Ufigel P-Tokuyama were not significant (P<.05). Significantly decreased microleakage characteristics were determined for Molloplast B, Mucopren (nonsilanized) and Ufigel P liners after aging.

CONCLUSION

Microleakage of Mucopren and Molloplast B lining materials was the lowest. However, the microleakage of Flexor and Simpa was the highest. The aging process did not significantly affect the microleakage characteristics of the Simpa, Flexor, Mucopren (silanized), or Tokuyama materials. Molloplast B, Mucopren (nonsilanized), and Ufigel P materials should significantly decrease microleakage properties after aging.

Finite element analysis of stress relaxation in soft denture liner

To gather knowledge related to establishing criteria for selecting soft denture liners for individual patients, the effects of certain properties of soft denture liners on stress distribution were evaluated by two-dimensional finite element analysis. A partial mandibular edentulous ridge crest was modelled. Six combinations of thickness (mucosa: 1 or 2 mm; soft denture liner: 1, 2, or 3 mm) and 18 combinations of Young's modulus (mucosa: three kinds; soft denture liner: six kinds) were analysed. The ratio of maximum to minimum stress in the mucosa (stress ratio) was calculated to estimate stress concentration. In the case of thin mucosa (1 mm thickness), the lower the Young's modulus of the soft denture liner, the lower the stress ratio. However, if the soft denture liner had a lower Young's modulus than the mucosa, stress concentrated adversely. These results suggest that the elasticity of the soft denture liner should match the elasticity of the mucosa to obtain the optimum cushioning effect.

Viscoelastic properties of some soft lining materials. II--Ageing characteristics

A non-resonant forced vibration, dynamic mechanical analyser was employed to measure the viscoelastic characteristics of soft lining materials at 1 Hz, after storage in distilled water at mouth temperature for periods up to and including one year. The six commercial products included methacrylate, silicone, and phosphazine based polymers and the one experimental material was a methacrylate. Water sorption of the soft liners, recorded by change in sample mass, ranged from -4.39 to +48.57% and their solubilities from 0.13 to 13.58%, after one year. The heat-cured silicone was the most stable polymer in water in contrast to its autopolymerised counterpart. The excessive water uptake of this latter material resulted in a massive reduction in modulus. At the other extreme one methacrylate with a high plasticiser content hardened substantially after ageing (modulus changed from 5.87 to 72.3 MPa). Changes in loss tangent data were relatively small for all the polymers tested, even for materials with high water uptake. Reduced leaching and/or plasticiser content have led to a more stable generation of soft lining materials.

Physical-property comparison of a chairside- or laboratory-polymerized permanent soft-liner during 1 year

PURPOSE

This investigation examined the influence of polymerization mode and water storage duration on the Shore A hardness, water sorption, resin solubility, and glass transition temperature of Permasoft, a commercial soft denture liner that is polymerized either chairside or in the laboratory.

MATERIALS AND METHODS

Specimen disks (31-mm diameter x 10 mm thick) and bars (44 x 8.5 x 1.2 mm) were polymerized following manufacturer's recommendations. The chairside polymerization process was simulated by pouring the mixed components into a mold and processing at 70 degrees C for 15 minutes under 2-psi pressure. Laboratory-polymerized specimens of the same dimensions were fabricated by processing under 500-psi pressure at 100 degrees C for 45 minutes. Specimens were stored in distilled water at 37 degrees C for 1, 7, and 30 days, and 6 and 12 months. Specimens were tested for Shore A hardness, water sorption, resin solubility, and glass transition temperature after the prescribed interval. To determine the effects of polymerization mode and storage time on material properties, a repeated-measures ANOVA (hardness data) and a two-way ANOVA (sorption and solubility data) with appropriate post-hoc tests were used.

RESULTS

Shore A hardness values increased from a low of 9.4 (+/- 0.5) units immediately after fabrication to a maximum of 15.9 (+/- 1.1) units after 1 year. Mode of polymerization did not influence hardness (p = .9851). Water-sorption values ranged from 4.2 (+/- 0.2%) of dry weight to 14.7 (+/- 2.5%) after 1 year. Resin solubility varied from 10.3 (+/- 0.6%) of preimmersion weight to 15.4 (+/- 1.1%), and immersion duration had no effect on solubility. In addition, after 1 year of storage, no difference in resin solubility or water sorption was found with respect to cure mode. The glass transition temperature for chairside-polymerized samples approximated -10 degrees C, while that for labpolymerized samples approximated -15 degrees C.

CONCLUSIONS

With regard to the material properties evaluated in this study, clinically processed Permasoft liner was equivalent to the laboratory-processed material.

Bond strength and failure analysis of lining materials to denture resin

OBJECTIVES

This study was conducted to investigate the bonding properties of five lining materials to a denture base resin. Two hard (chemical-cured resin: Kooliner "Coe Labs, USA" and light-cured VLC resin: Triad "Dentsply, USA") and three soft (chemical-cured resin: Express "Alcos,-USA", heat-temperature vulcanized 'HTV' silicone material: Molloplast-B "Regneri GmbH, Germany" and room-temperature vulcanized 'RTV' silicone material:Ufi Gel-P "Voco, Germany") liners were used.

METHODS

Paladent 20 "Heraeus Kulzer GmbH, Germany", a conventional heat-cured polymethylmethacrylate (PMMA), was used as the denture base resin and as the control material. Bonding strength and adhesion properties of the liners to PMMA were compared by tensile test and scanning electron microscope (SEM) analysis. After curing, an aging process was applied and the samples were immersed and stored in distilled water at 37 +/- 1 degrees C and taken out at certain aging intervals (at 0, 15, 30 and 90 days) for examination. Specimens (168) were processed for tensile tests and other specimens (24) for fracture tests. The mean and standard deviation values were calculated. Changes in the mechanical properties and the SEM findings of the adhered surfaces were evaluated.

RESULTS

Triad (a hard liner) has the closest tensile strength to the control, indicating the strongest bonding between the base and the liner. Also, during the aging process, formation of better adhesion was observed in SEM micrographs. From the SEM analysis it was found that, Molloplast-B (a soft liner) also has a very good filling capacity.

SIGNIFICANCE

Among the hard lining materials, VLC resin to chemical-cured resin; and among the soft lining materials, HTV resin to RTV and chemical-cured ones should be preferred for relining procedures. Molloplast-B and Express as resilient liners were found to have adequate adhesive values for clinical use.

Color stability of heat-polymerized and autopolymerized soft denture liners

STATEMENT OF PROBLEM

The physical and mechanical properties of soft denture liners are influenced by a number of factors. Aging is one factor that has an effect on the characteristics of polymers. The resistance of denture liners to aging can potentially influence the color stability of these liners.

PURPOSE

This study attempted to determine the color stability of soft liners by subjecting them to an in vitro accelerated aging test.

MATERIAL AND METHODS

Two autopolymerizing and 3 heat polymerizing commercially available soft denture lining materials were tested. Seven sample disks were made of each material according to manufacturers' instructions. Color measurements were recorded before and after the accelerated aging process. Color changes (DeltaE) were calculated by measuring tristimulus values at several wavelengths in the visual spectrum with the use of Commission International de l'Eclairge Lab (CIE-LAB) uniform color scale.

RESULTS

Kruskall-Wallis variance analysis and Mann-Whitney U-Wilcoxon rank sum W tests were performed for statistical comparison of the DeltaE values. Ufigel P and Simpa liner materials showed significantly greater color changes than the other tested materials. There were no significant differences in color change among Ufigel L-Molloplast B, Ufigel L-Flexor, and Molloplast B-Flexor liner materials. However, a significant difference in color change was found between Ufigel P and Simpa liners.

CONCLUSION

Heat-polymerized soft liners were more color stable than autopolymerized soft liners. No significant differences were found among heat polymerized materials; however, significant differences in color change were found between the 2 autopolymerized liners.

Setting and stress relaxation behavior of resilient denture liners

STATEMENT OF PROBLEM

Resilient denture liners are widely used for the patients who are not comfortable with correctly made conventional hard-based dentures because of thin and relatively nonresilient mucosa or severe alveolar resorption. There are several materials used for denture liners and the efficacy in their use is influenced by their viscoelastic properties.

PURPOSE

This study evaluated the setting behavior and viscoelastic properties of various types of resilient denture liners and the changes in viscoelasticity with the passage of time.

MATERIAL AND METHODS

Four types of resilient denture liners were used. Setting behavior of 5 autopolymerizing materials was evaluated with an oscillating rheometer. Stress relaxation tests were conducted to evaluate the viscoelastic properties of 9 materials and changes that occurred over time by means of Maxwell model analogies.

RESULTS

Significant differences were found in the setting behavior of the autopolymerizing materials. The acrylic resin and fluoroethylene materials demonstrated viscoelastic properties and the silicone and polyolephin materials were found to be elastic. The acrylic resin materials exhibited the greatest changes in viscoelastic properties over time when compared with silicone, polyolephin, and fluoroethylene materials.

CONCLUSIONS

The results suggest that it is important to select denture liner materials according to clinical situations because of the wide ranges of setting behavior, viscoelastic properties, and durability over time.

Adhesion and tear energy of a long-term soft lining material activated by rapid microwave energy

STATEMENT OF PROBLEM

Construction of dentures with permanent soft linings is time-consuming in the laboratory and extra costs are related to equipment and materials used.

PURPOSE

The purpose of this in vitro study was to determine whether using microwave energy to activate the polymerization of a silicone rubber denture soft lining material affected its properties.

MATERIAL AND METHODS

Tear energy and adhesive properties were measured in a tensile testing machine by using a pants leg tear test and peel specimens. Tear energy was measured for specimens polymerized conventionally (control) and for 3, 5, and 10 minutes in a microwave. Data were analyzed with one-way analysis of variance and a two-sample Student t test.

RESULTS

The multiple comparison test failed to show a significant difference in tear energy between 3 minutes microwave activation and conventional heat curing. However, 3 minutes microwave activation revealed a significantly stronger material when compared with 5 minutes and 10 minutes (p < 0.05). Application of a two-sample Student t test failed to demonstrate a significant difference between microwave energy and conventional heat activation groups in the adhesion test. In adhesion testing, all specimens presented cohesive failure.

CONCLUSIONS

This method of polymerization does not compromise the strength of a soft lining material and its adhesion to polymethyl methacrylate. This study suggests the use of 3 minutes 650 W microwave energy for processing a silicone soft lining material.

Bond strength of permanent soft denture liners bonded to the denture base

The purpose of this study was to characterize denture and soft liner adhesion and to determine the adhesive and/or cohesive strength of different soft tissue liners bonded to the denture base by use of a new technique. Two groups of five permanent soft liners (dry or exposed to water for 6 months) were tested by use of a tensile mode to characterize the failure characteristics of soft liners bonded to denture base resin. The method differed from previous test methods because of the specimen's ability to align axially during the test. The results indicated significant differences in the bonding of liners to the denture base, and light-cure systems exhibited the greatest amount of stress needed for failure. Low bond strength was observed when the adhesion was poor or when the cohesive strength of the soft liner was low and lead to pure adhesive or cohesive failure. When both adhesive and cohesive bonds were strong, failure occurred at high stresses. Combinations of adhesive and cohesive failures (mixed mode) were also observed in intermediate cases.

Dynamic viscoelastic properties of processed soft denture liners: Part II--Effect of aging

The proper functioning of soft denture liners depends to a great extent on their mechanical properties. As with many polymers these materials are affected by aging. Twelve soft denture liners were processed by a laboratory according to the manufacturers' directions. Five specimens of each material were tested without aging. Five additional specimens of each material were subjected to 900 hours of accelerated aging in a Weather-Ometer instrument. These were tested with a dynamic viscoelastometer at three frequencies and two temperatures, and data for 37 degrees C and 1 Hz was obtained. Two of the ethyl methacrylate resins demonstrated the largest increases in storage (E') and loss moduli (E") after aging. These materials also showed the greatest overall E' and E". One denture liner material exhibited 673% and 488% increases in E' and E", and other materials showed smaller increases. The effects of aging on the damping factor (tan delta) were varied and five materials showed increased tan delta. Only two ethyl methacrylate resins developed lower tan delta. All the silicone and polyphosphazine rubbers showed small changes after aging and had the lowest tan delta values. Significance of differences between materials and treatments was tested with ANOVA, Scheffé intervals, and t-tests at a = 0.05. The ethyl methacrylate soft denture liners were affected the most by accelerated aging, and the silicones and polyphosphazine were least affected. The ethyl methacrylate resins also had the greatest values of E', E", and tan delta after aging.