ility J. L. Shotwell,

A. Koran,


School of Dentistry,

of long-term

DDS, MS,a M. E. Razzoog, MS,b University

of Michigan,

soft denture DDS,




Ann Arbor, Mich.

Tbe use of resilient denture liners in complete denture construction has become increasingly popular for providing comfort for denture wearers. The primary disadvantage of these materials is that the physical and mechanical properties change rapidly with time in a service environment. The purpose of this study is to evaluate the color stability of five commercially available soft denture liners as a function of accelerated aging. Color measurements were made before aging with a calorimeter and data processor. The samples were then weathered for 100 hours in an accelerated aging chamber in the presence of a xenon ultraviolet visible-light source, an intermittent water spray at 110” F, and 90% humidity. After aging, color measurements were made again and color differences (AE) were calculated. Results were statistically tested with analysis of variance and Scheffe intervals were calculated at 0.96. It ‘was concluded that accelerated aging can be used to evaluate color stability of soft denture liners. (J PROSTHET DENT 1992;68:836-8.)

he development and use of resilient denture lining materials has provided the dental profession with an alternative for patient care when there is a history of chronic tissue irritation and s0reness.r From the patient’s perspective, resilient materials offer comfort and in many situations relief from previous symptoms.2b3Unfortunately none of the available soft denture liners have demonstrated an extended service life of more than a few years nor have they satisfactorily met all of the criteria for resilient denture liners proposed by Craig and Gibbons4 in 1961. These criteria include (1) permanent resilience or resilience over an extended period of time, (2) capability of forming a strong bond with acrylic or other rigid denture base materials, (3) dimensional stability, (4) adequate tear strength, and (5) permanence including color stability, shelf life, and freedom from absorption of flavors, odors, or bacterial growth. One of these criteria, color stability, may provide important information on the serviceability of these materials.5, 6 The purpose of the present study was to determine the color stability of various resilient denture liners by subjecting them to an in vitro accelerated aging test.



The resilient liners selected for the study were divided into three groups: silicones, plasticized acrylic resins, and

Supported by National Institute of Dental Research Center grant No. DE09296. Presented at the International Association for Dental Research meeting, Acapulco, Mexico. aAssociate Professor, Department of Prosthodontics. bProfessor, Department of Prosthodontics. 1011l39547




Materials used in the study*










Coe Super-Soft

P101089A L060189A 161-8013


Young Dental

Maryland Heights, MO 63043

Astron LC

Hygenic Corp. Akron, OH 44310 Buffalo Dental Manufacturing Co., Inc. Syosset, NY 11791 Coe Company Chicago, IL 60658-159’7 Astron Dental Corp. Wheeling, IL 60090

*Prolastic and Molloplast-B represent silicones, Coe Super-Soft and Astron LC represent plasticized acrylics, and Novus represents a polyphosphazine.

polyphosphazine (Table I). Five sample disks were made for each material with a Teflon (Du Pont Co., Wilmington, Del.) mold, 28 mm in diameter and 4 mm thick. All materials were processed according to the manufacturer’s instructions. Before testing the samples were stored in distilled water for 24 hours. At that time the color of all samples was measured with a calorimeter (Minolta Chroma Meter II with data processor DP-100, Minolta Camera Co., Ltd., Osaka, Japan). After the initial color measurements, the samples were placed in an accelerated aging chamber (Weather-Ometer, model 25 WR, Atlas Electric Devices, Chicago, Ill.) and exposed to a xenon ultraviolet visible-light source. The weathering chamber was maintained at 110“ F (a black panel temperature of 145’ F) and 90% relative humidity, and intermittent distilled water spray was used every 102 minutes for a period of 18 minutes. The xenon light source











Scheffe Interval

= 1.1

1. Results of accelerated aging of soft liner materials tested. X axis shows the different materials tested; Y axis shows units of color change described as AE.


is similar to the carbon arc weathering apparatus referred to in the American Society for Testing Materials test D750-68 for artificially weathering rubber compounds.7 The samples were maintained in the weathering chamber for 100 hours. At that time there were visible color changes in some of the samples. After 100 hours the color of the samples was measured again with the calorimeter. Color changes (AE) were calculated by measuring tristimulus values at several wavelengths in the visual spectrum with the use of the Commission International de 1’Eclairge L*a*b* (CIE-LAB) uniform color scale.8 This system represents a three-dimensional color spacehaving components of lightness (L*), red-green (a*), and yellow-blue (b*). An important aspect of the CIELAB system is that color differences between specimens can be given a single parameter, AE*ab.8, g The color difference between two samples, each given in terms of L*, a*, and b*, is calculated as follows: AE*ab = ((AL*)2 + (Aa*)

+ (Ab*)2}1/2.

Mean AE values were calculated for each material and compared statistically with a one-way analysis of variance and calculating Scheffe intervals at 0.95.10,l1

The results of accelerated aging on the samples are seen in Fig 1. The Scheffe interval was 1.1. The color of Prolastic silicone material with a AE of 0.8, was least affected by accelerated aging and was statistically different from all other materials. Astron LC plasticized acrylic with a AE of 3.0 and Molloplast-B silicone material with a AE of 3.6 demonstrated slightly greater changes but were not statis-





Table II Critical remarks of color difference

Trace Slight Noticeable Appreciable Much Very much

Textile terms (NBS units) 0.0 0.5 1.5 3.0 6.0 >12.0

- 0.5 - 1.5 - 3.0 - 6.0 - 12.0

tically different from one another. Coe Super-Soft plasticized acrylic with a AE of 8.9, demonstrated a greater change and was statistically different from all other materials tested. Novus polyphosphazine with a AE of 34.2 had the greatest change and was also statistically different from all other materials. The color changes associated with the Coe Super-Soft and Novus materials were easily detected visually. The critical remark of color change (AE) has been quantified by the National Bureau of Standards (NBS) with NBS units of color differenceI (Table II). NBS units are expressed by the following formula: NBS unit = AE*ab x 0.92. When the mean AE values of the weathered soft denture liner samples were converted to NBS units (NBS unit = AE*ab X 0.92), the critical remarks of color difference were as follows: Prolastic/NBS unit = 0.8 X 0.92 = 0.7-Slight Astron LC/NBS unit = 3.0 X 0.92 = 2.7-Noticeable Molloplast/NBS unit = 3.6 X 0.92 = 3.3--Appreciable



Coe Supersoft/NBS unit = 8.9 x 0.92 = &Z-Much Novus/NBS unit = 34.2 X 0.92 = 31.4-Very much

DISCUSSION The NBS parameter is important for color comparison and quality control functions, because only the allowable AE*ab values need to be specified instead of the individual L”, a*, and b* values. The accelerated aging chamber and the calorimeter were effective for evaluating the color stability of soft denture liners. This same technology has also been used to measure the color stability of dental composites and maxillofacial materials.i3, I4 In this study particular attention was given to the preparation of samples, because several required either a stone backing on the Teflon mold or a glass plate to allow for complete polymerization. Accelerated aging did affect the color stability of the materials tested. Prolastic silicone, which is an unpigmented material, demonstrated the least color change. The color changes seen with the other materials could be attributed to changes in the colorants used, a change in color of the elastomer, or both. In addition, some coIorants or elastomers may be affected by the high humidity or warmer temperatures in the aging chamber. If the color change is associated with the colorants, the manufacturers could select pigments that are more color stable. The results of this study should prove valuable to clinicians when they are choosing soft liners for their denture patients, if they are concerned about color stability. In addition, research is currently being done to evaluate other parameters associated with color changes in soft denture liners, and the data from this study will serve as a basis for comparison.

CONCLUSIONS 1. The combination of accelerated aging in a weathering chamber and measurement of color changes with a colorimeter was effective in evaluating the color stability of long-term soft denture liners.





2. The color of all soft denture liners evaluated in this study changed after 100 hours of accelerated aging. There was wide variation in color stability of the materials tested. The color change ranged from slight (Prolastic, NBS = 0.7) to very much (Novus, NBS = 31.4). REFERENCES 1. Mack PJ. Denture soft linings: materials available. Aust Dent J 1989;34:517-21. 2. Makila E. Soft lining to relieve soreness beneath dentures. J Oral Rehabil 1976;3:145-50. 3. Schmidt Jr WF, Smith DE. A six-year retrospective study of MolloplastB-lined dentures. Part I: patient response. J PROSTHET DENT 1983; 50:308-13. 4. Craig RG, Gibbons P. Properties of resilient denture liners. J Am Dent Assoc 1961;63:382-90. 5. Bunch J, Johnson GH, Brudvik JS. Evaluation of hard direct reline resins. J PROSTHET DENT 1987;57:512-9. 6. Makila E, Honka 0. Clinical study of a heat-cured silicone soft lining material. J Oral Rehabil 1979;6:199-204. I. 1968 Book of ASTM Standards, part 28. Philadelphia: American Society for Testing Materials, 1968:1202. 8. Wyszecki G, Stiles WS. Color science: Concepts and methods, quantitative data and formulae. 2nd ed. New York: John Wiley & Sons, 1985:950. 9. Bouma PJ. Physical aspects of colour. 2nd ed. New York: St. Martins Press, Inc., 19’71;280. 10. University of Michigan Statistical Research Laboratory. A manual of elementary statistics using MIDAS. Ann Arbor: University of Michigan Press, 1975:301. 11. Guenther WC. Analysis of variance. Englewood Cliffs, NJ: PrenticeHall, 1964:199. 12. Nimeroff I. Calorimetry. National Bureau of Standards Monograph 104, 1968:47. 13. Powers JM, Dennison JB, Koran A. Color stability of restorative resins under accelerated aging. J Dent Res 1978;57:964-70. 14. Craig RG, Koran A, Yu R, et al. Color stability of elastomers for maxillofacial appliances. J Dent Res 1978;57:866-71. Reprint requests to: DR. JEFFREY SHOTWELL 1395 DENTAL BUILDING 1011 N. UNIVERSITY ST. ANN ARBOR. MI 48109-1078







Color stability of long-term soft denture liners.

The use of resilient denture liners in complete denture construction has become increasingly popular for providing comfort for denture wearers. The pr...
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