Color stability of denture liners values of all the tested materials’ (except UH and violet light have a direct effect on the properties of MP) increased (to white) after immersion in DW, liners and cause them to swell.19 The changes that while these values (except DL and TR) decreased may occur in polymers as a result of this process (to black) after aging. b* values of the materials’ are; scission of the polymer chains by UV light, (except DL after immersion in DW) increased (to oxygen cross-linking, leaching of plasticizers and yellow) both after immersion in DW and aging. absorption of water.37 The National Bureau of Standards (NBS) es- Previous studies have shown the effectiveness tablished a rating system to describe color dif- of accelerated aging in evaluating the color stabil- ferences by NBS units.20 ∆E* values of this in vi- ity of soft denture liners.17,22 Similarly, in the cur- tro study were converted to NBS units to facilitate rent study, significant color changes were seen in comparison with clinical studies. According to all of the tested materials after aging. the NBS units; DL showed change to other color The clinical color stability behavior or perfor- (∆E*DL = 16.30), while UP and MP revealed marked mance of acrylic and silicone based lining ma- change (∆E*UP = 3.46, ∆E*MP = 4.78) after aging. As terials would be different due to their different a result, UP showed the highest color stability af- structures.38 Water sorption and solubility can ter immersion in DW and aging. This property may dramatically affect stain resistance, dimensional be of advantage to its long-term serviceability in stability, physical and mechanical properties.6 severe oral cavity environment. When immersed in saliva during clinical use and An accelerated aging process has been used in may be soaked in water or cleansing agents when this study in order to simulate the oral environ- not in use; plasticizers and other soluble may ment. Even if the oral environment is more com- leach out over extended periods while water is ab- plex, this aging treatment is still useful for the sorbed until equilibrium is reached.16 At 1 week, comparison of different lining materials as the water sorption value should not be more than 0.8 closest simulation of oral environment.17 It has mg/cm2 and the solubility should not be more than been reported that water spray and visible ultra- 0.04 mg/cm2 for different liners.27 Water sorption Table 4. Color changes of the liner materials. Material Treatment Mean ∆E* SD DL Immersion in DW 0.50 0.18 Aging 16.30 3.25 TR Immersion in DW 1.19 0.70 Aging 13.01 1.37 UH Immersion in DW 7.13 0.62 Aging 8.68 2.12 MP Immersion in DW 1.75 1.07 Aging 4.78 1.05 UP Immersion in DW 0.41 0.11 Aging 3.46 1.23 ∆E*: Color Change Value SD: Standard Deviation NBS UNIT: National Bureau of Standards Unit Table 5. Kruskal-Wallis non-parametric test results of the liner materials. UP DL TR Immersion in DW Mean ∆E* 0.41 0.50 1.19 Mean rank 5.33 8.33 17.00 Aging Mean ∆E* Mean rank Kruskal-Wallis H=25.455 UP MP 3.46 4.78 4.67 8.50 Kruskal-Wallis H=25.974 UH 8.68 15.50 P<.001 P<.001 MP 1.75 19.33 TR 13.01 22.17 NBS unit 0.46 14.99 1.10 11.96 6.56 7.99 1.61 4.39 0.38 3.18 UH 7.13 27.50 DL 16.30 26.67 148 Lines connect values that were not significantly different at the P<.001 level. European Journal of Dentistry