|Year : 2015 | Volume
| Issue : 1 | Page : 11-16
The effect of curing time and immersion solutions on discoloration of hybrid composites and nanocomposites
Gülay Uzun1, Filiz Keyf2, Defne Burduroglu2
1 Vocational School of Health Services, Department of Dental Prosthetics Technology, Hacettepe University, Ankara, Turkey
2 Department of Prosthodontics, Faculty of Dentistry, Hacettepe University, Ankara, Turkey
|Date of Web Publication||19-Jan-2015|
Department of Prosthodontics, Hacettepe University, Faculty of Dentistry, 06100 Ankara
Source of Support: None, Conflict of Interest: None
Objectives: The purpose of this study was to evaluate the color change of six esthetic dental composite materials immersed in various solutions for 7 days, and determine the effect of curing time on color change. Materials and Methods: Disk-shaped samples of six types of light curing composite were prepared, and all samples were divided into two groups that were light cured for 20 s and 40 s (n = 7). After 24 h, color measurements were performed with a digital spectrophotometer. Samples were immersed in a solution (coffee, tea, Coke; , cherry juice and distilled water) for 7 days. The discoloration of each specimen was measured and statistically analyzed with a statistics software SPSS Version 17.0 (Released 2008. SPSS Statistics for Windows, Chicago: SPSS Inc.). Results: The discoloration of Filtek™ and Ceram.X was significant. The highest color change values were in Filtek™ group. The lowest color change values were in Herculite classic group. Coffee (ΔE = 12.27) showed the highest influence on color change. There was no effect of curing time on the discoloration of the composites. Significance: The color differences (ΔE) ranged from 3.23 to 12.48. There was no significant difference in color change between two curing times, and coffee was the immersion medium that promoted the highest color change on the tested composite resin.
Keywords: Color change, color stability, composite, curing time
|How to cite this article:|
Uzun G, Keyf F, Burduroglu D. The effect of curing time and immersion solutions on discoloration of hybrid composites and nanocomposites. SRM J Res Dent Sci 2015;6:11-6
|How to cite this URL:|
Uzun G, Keyf F, Burduroglu D. The effect of curing time and immersion solutions on discoloration of hybrid composites and nanocomposites. SRM J Res Dent Sci [serial online] 2015 [cited 2022 May 18];6:11-6. Available from: https://www.srmjrds.in/text.asp?2015/6/1/11/149562
| Introduction|| |
Nanocomposite resins are currently among the most popular esthetic restorative materials in dental clinical practice. The use of these materials has been on the rise for the past several years, and their popularity will increase as manufacturers introduce stronger composite materials. Discoloration of resin composites is an important drawback, which may result in replacement of restorations. 
Structural changes caused by aging, formation of colored degradation products, changes in surface morphology and extrinsic staining may be the reason for discoloration of resin composite restorations.  Consequently, internal and external factors may both change the color of the esthetic restorative material. Discoloration of esthetic materials may be caused by intrinsic factors such as changes in the filler, matrix or silane coating, or extrinsic factors such as absorption of stains, incomplete polymerization, chemical reactivity, diet, oral hygiene and surface smoothness of the restoration.  When the materials are aged under several physical and chemical conditions like ultraviolet exposure, thermal changes and humidity, the intrinsic color of esthetic materials may change. Therefore, many factors were involved in etiology of discoloration in esthetic dental materials. , Hence, this study investigated the different curing times on color of an esthetic restorative material regarding its color stability after immersion in different media.
| Materials and methods|| |
In this study, five different resin composite (shade A2) were used. Name, manufacturer, composition, filler size and filler information of the composites are presented in [Table 1]. A total of 420 disk-shaped specimens (2 mm thickness, 10 mm diameter) was prepared with a specially constructed mold. Uncured resin composite were packed between two glass plates with a transparent polyester strip to minimize the oxygen inhibition layer and to obtain the smoothest surface possible. Each mold was then compressed between two glass plates to remove excess material, and a flat surface was obtained. The surfaces of 210 samples were polymerized for 20 s and the other 210 samples were polymerized for 40 s with a light cure unit (Hilux Ultraplus Dental Curing Light System, Benlioglu Inc., Turkey). The cured specimens were removed from the mold. The glass plates and the polyester strip provided adequate smoothness of the surfaces; therefore, polishing was omitted. The preparation method produced approximately uniform surfaces on all samples. The disk thicknesses were measured using a digital caliper (Longirele Electric, China). All procedures were carried out at room temperature (23°C) by the same operator. All sample disks were immersed in distilled water for 24 h and then the color values were recorded using a digital spectrophotometer (Konica Minolta DM-3600d Sensing Americans Inc., USA). The color was evaluated according to the color system CIEL*a*b* in which L indicates color luminosity (ranging from 0 to 100, that means black to white); a* determines the amount of red (positive values) and green (negative values); b* determines the amount of yellow (positive values) and blue (negative values).
After initial color measurements, 14 sample disks from each composite group (7 samples 20 s and 7 samples 40 s light cured) were immersed in one of the solutions for 7 days, while 14 sample disks were left in distilled water as the control group. The specimens were immersed in tea, coffee, coke and cherry juice solutions. To prepare the tea solution, one tea bag (163 g) was added to 150 mL of water at 100°C and steeped for 5 min. Coffee solution was prepared by dissolving 2.0 g of instant coffee in 150 mL of boiling water (equivalent to two teaspoons/cup). Specimens were then immersed in tea, coffee, coke and cherry juice. The control group was placed in distilled water for the entire test period.
The temperature of the solutions was maintained at room temperature during the immersion period. After 7 days of immersion in the solutions, the specimens were rinsed with distilled water for 2 min, and then dried using blotting paper before color measurements. The second measurements were made with the same digital spectrophotometer. Measurements were taken with the active point of the spectrophotometer in the center of each specimen. The spectrophotometer was calibrated according to the manufacturer's recommendations before each series of measurements. The measured data obtained from the spectrophotometer was an average of three consecutive readings which is calculated by the instrument.
Color change was obtained through Hunter equation ΔE ab * = [(ΔL*) 2 + (Δa*) 2 + (Δb*) 2 ] 1/2 , while luminosity values (ΔL*) were reached using ΔL* = L* (tx) − L* (t0), where (tx) represents immersion time and (t0) represents baseline. In the literature, there is no consensus regarding color perception of the human eye that differs from individual to individual.  The adopted classification of ΔE values was determined by the National Bureau of Standards that considers: 0.0-0.5 values: Extremely slight change; 0.5-1.5: Slight change; 1.5-3.0: Perceivable change; 3.0-6.0: Marked change; 6.0-12.0: Extremely marked change; 12.0 or more: Change to another color. 
The arithmetic means and standard deviations of each material were calculated and were compared to each other statistically by using the Wilcoxon signed ranks test (P ≤ 0.05 significant). Regarding the curing time, each subgroup was analyzed separately according to the immersion solution to study discoloration.
| Results|| |
For statistical analysis, six materials and two curing times were evaluated. The interaction between curing times and materials were performed in the statistical analysis. The information for these materials is presented in [Table 1]. [Table 2] shows mean color changes (ΔE) for the six composite groups in different solutions.
|Table 2: Mean values of color change (ÄE) of all groups after immersion of specimens|
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Tea, coffee, coke and cherry juice solutions were found to be effective in changing the color of composite materials, but the results were statistically insignificant. The Wilcoxon test showed statistically insignificant differences between the means of the first and the second measurements of test specimens with 20 s and with 40 s (P ≤ 0.05).
The mean ΔE values of the six different composite groups with 20 s and 40 s curing time after immersion for 7 days in coffee are shown in [Figure 1]. The highest color change occurred in Filtek™ Ultimate while Spectrum demonstrated the lowest color change. ΔE ranged from 8.144 (Filtek™ Ultimate) to 5.132 (Spectrum). [Figure 2] shows the mean ΔE values regarding color change of the six composite groups with 20 s and 40 s curing time after 7 days of immersion in distilled water. Significant color changes occurred in Filtek™ Ultimate while smaller color changes were observed in Herculite classic. Mean ΔE ranged from 8.906 (Filtek™ Ultimate) to 6.014 (Herculite classic).
|Figure 1: The mean ÄE values of the six different composite groups with 20 s and 40 s curing time after immersion for 7 days in coffee|
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|Figure 2: The mean ÄE values of color change of the six composite groups with 20 s and 40 s curing time after 7 days of immersion in water|
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ΔE were <9.0 for all groups (20 s and 40 s curing time) after 7 days of immersion in coloring solutions. Larger color changes occurred in 40 s groups, while 20 s groups had smaller color changes. After 7 days, ΔE for Filtek™ Ultimate was increased from 8.93 to 11.71 for tea, 5.42-6.60 for a coke and 8.60-12.48 for cherry juice. No significant differences were observed between all groups after immersion in coloring solutions.
| Discussion|| |
Color stability is an important consideration in esthetic restorations and the selection of an appropriate material that duplicates the appearance of natural tooth structure is very important in restorative dentistry. Patients often confront dentists with questions about the staining potential of foods.
There are numerous studies in the literature addressing discoloration of dental composites. It is reported that the amount of discoloration depends on the material, test method, curing time, curing device and aging conditions.  Six resin composite materials were investigated in terms of color change in this study. Shade A2 was selected because it is a light shade and susceptible to greater color changes.
To determine discoloration, both visual and instrumental techniques can be used. Visual technique is not considered to be reliable due to inconsistencies inherent in color perception and description between observers.  Colorimetry, spectrophotometry, and digital image analysis are instrumental techniques used in dental material studies. The most reliable instrumental technique is the spectrophotometry. 
Color perception is influenced by several factors:
- Surface properties (material, surface profile, transparency, and substrate),
- Type of illumination.
- The observer. In this study, sample preparation method allowed production of uniform surfaces. The angle of illumination was fixed (unchangeable, constant).
The observer parameter did not exist, color measurements were done digitally. A consistent specimen position was maintained for each recording with the orientation nubs. Thus, individual color differences due to positional changes were minimized.
Tea, coffee, coke and cherry juice were chosen as the test agents because they have been shown to have greater staining ability on composite resins and natural tooth structure.  In this study, the curing time was an insignificant factor on the color change of these materials. Statistically, an insignificant difference was found between materials tested with different solutions. Also, statistically insignificant differences were found between the control and the tested specimens. Coffee showed much more discoloration than the others. Results of this study found that, there were visually significant color changes after 7 days.
Coffee caused the biggest color change for esthetic restorative materials between 1 st (baseline) and 2 nd measurements with 20 s and 40 s. Coke caused the smallest color change between 1 st and 2 nd measurements with 20 s and 40 s. Tea and coffee affected the specimens more than cherry juice.
Filtek™ Ultimate material indicated color values between 5.42 (coke) and 12.27 (coffee) at 20 s, 5.20 (water) and 12.48 (cherry juice) at 40 s. Grandio indicated color values between 4.61 (tea) and 6.58 (cherry juice) at 20 s, 5.52 (water) and 9.47 (coffee) at 40 s. Herculite classic indicated color values between 4.42 (coffee) and 6.12 (cherry juice) at 20 s, 3.23 (coke) and 8.34 (tea) at 40 s. Tetrik N-Collection indicated color values between 4.36 (tea) and 7.91(coffee) at 20 s, 6.22 (tea) and 7.67 (coke) at 40 s. Ceram.X indicated color values between 7.04 (water) and 9.71 (coffee) at 20 s, 6.64 (tea) and 10.34 (coffee) at 40 s. Spectrum indicated color values between 3.61 (coke) and 7.43 (coffee) at 20 s, 4.38 (coke) and 11.79 (coffee) at 40 s. These findings show that the color values of 40 s were much greater than of 20 s. Filtek™ Ultimate was affected by staining agents much more than other composites.
The progressive increase in the degree of color change was found for materials over the remaining 1-7 day test period. 1 and 7 days were selected as a test period because we wanted to show immediate possible discoloration. After this test period, all the solutions caused a color change.
Discoloration of resin materials may be a result of inefficient polymerization. When the degree of conversion is high, there is less residual monomer which eventually forms colored degraded products.  Therefore, completely polymerized resin would be more stain resistant.
The specimens immersed in distilled water as a control also exhibited color change. The difference between the tested specimens and the control was visually seen but statistically insignificant (P ≤ 0.05).
Smooth surfaces of resin composites are more comfortable for the patient and increased surface roughness leads to staining in in vitro studies.  For these reasons, the faces of the samples were prepared using glass plates. In order to create smooth, uniform and easily cleaned surfaces, composite restorations should be finished and polished. Extrinsic staining and loss of gloss is closely related to polishing and rough surfaces caused by wear and chemical degradation effects discoloration.  Surface smoothness and extrinsic staining are affected by the structure of the resin composite and characteristics of the particles. 
This study compared the effect of five different beverages on the color change stability of five different types of nanocomposites and determined which nanocomposite material has the best color stability. In the present study, compared to other composites, Spectrum and Herculite classic showed less color change after 7 days.
Composites interact differently with certain stains due to structural diversity of the resin and chemical properties of the staining substances. , Discoloration in composite restorations is caused by structural changes in the material due to aging, formation of colored degradation products, changes in surface morphology and extrinsic staining. Intrinsic factors, such as changes in the matrix, filler composition, filler content, minor pigment addition, initiation components and filler coupling agents affect the color of esthetic materials. The color stability is related to the dimension of the filler particles, depth of polymerization and coloring agents. The interactions of each of these components may have a role in the color stability of the material. ,
According to the study of Ferracane and Condon, specimens were immersed in distilled water for 24 h after curing in order to separate unreacted components from the composite and allow for post irradiation and post setting polymerization to occur.  In our study, the same technique was used.
Comparing the results of this work with those of Patel, et al., the values of ΔE found for the specimens immersed in coffee were higher, however, those subjected to Coke, were similar.  Ertas, et al., comparing the immersion of the composite resin Filtek™ Supreme in various media (coffee, tea, red wine, Coke and water), found that both water and coke showed a mild color change, while tea, coffee and wine had greater influence on the staining behavior of this material.  However, Fontes, et al. (2009) observed that after 1-week of immersion in coffee, yerba mate or water, specimens did not show significant differences for ΔE values from the baseline, but the group immersed in grape juice presented significant color change on the nanofill composite resin studied.  In addition, Tunc et al. (2009) observed that among various children's drinks in contact with microhybrid composites, Coke was able to promote the highest color change compared with grape juice, chocolate milk and distilled water. 
According to this study, water also promoted a slight color change in the specimens (ΔE = 1.73) classified as slightly perceptible. The same result was obtained by Omata et al. when comparing the color change of composite resin specimens immersed in distilled water and artificial saliva. Omata et al. observed that the distilled water group did not undergo any color change, while the artificial saliva did.  In our study, water showed similar values with Mutlu's study. 
In the present study, ΔE values >3.0 were considered clinically perceptible based on previous reports.  ΔE values <1 were regarded as not detectable by the human eye. Color differences of 3.0> ΔE >1 may be detectable by a skilled operator but were considered clinically acceptable. On the other hand, values of ΔE >3.0 would be detectable by nonskilled people and were, therefore, considered clinically unacceptable.
Sorption and solubility may cause some chemical and physical changes in the resin structure that effects the material's biocompatibility.  These changes include swelling, plasticization and softening, oxidation and hydrolysis.  As a consequence, there is a decrease in color stability, promoting higher susceptibility to staining.
Mutlu-Sagesen et al. found that a submicron hybrid composite resin (Spectrum) had the least discoloration among other tested composite materials.  Surface smoothness and susceptibility to discoloration are related to the structure of the composite resin and the characteristics of its particles.  The studied resin Filtek™ Ultimate contains silica particles 20 nm in size and silica-zirconia nano agglomerates with size ranging from 4 nm to 11 nm. The resinous matrix is composed of bisphenol-A-diglycidyl methacrylate (Bis-GMA), urethane dimethacrylate (UDMA), Bis-EMA, triethylene glycol dimethacrylate (TEGDMA) and poly (ethylene glycol) dimethacrylate. The combination of large silica particles and nanoparticles in Filtek™ Ultimate seems to exhibit more tendencies to discoloration than the micrometric particles of silica-zirconia found in microhybrid resins. This feature may cause high water absorption thus more discoloration of the resin material.  Due to its low sorption and water solubility characteristics, UDMA is more resistant to discoloration than Bis-GMA. Main components in a microhybrid resin system are Bis-GMA, UDMA, and Bis-EMA. Supplementary components in a composite resin are TEGDMA, hydrophilic monomer mixed with UDMA and Bis-EMA, which promote a hydrophobicity characteristic. According to Ertas, et al., larger amounts of TEGDMA causes higher water absorption and greater susceptibility to staining.  According to this previous study, the hypothesis can be raised that the high staining of Filtek™ Ultimate may be caused by this feature. With the development of composite resins containing nanoparticles, the physical and optical properties are enhanced, but further studies are needed to assess their performance.
| Conclusion|| |
This study has shown that 20 s curing time of the composites tested showed less discoloration than 40 s curing time after 7 days of immersion. We conclude that under our study conditions, the color of all materials was affected. The color changes were within the clinically unacceptable range after 7 days. For these reasons, the dental restorative materials tested in this study are not color stable.
Based on the employed methodology and the obtained results, it may be concluded that:
The use of a spectrophotometer was effective in quantitatively evaluating the color stability of materials.
There was no significant difference between curing times on discoloration of the composite resins; coffee was the tested immersion medium that had the most influence on color stability of the studied composite resin, followed by tea and cherry juice; Filtek™ Ultimate demonstrated much more color change than the other materials. Spectrum demonstrated much less color change.
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[Figure 1], [Figure 2]
[Table 1], [Table 2]