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Table of Contents   
ORIGINAL RESEARCH  
Year : 2013  |  Volume : 24  |  Issue : 5  |  Page : 605-609
Color change of composite resins subjected to accelerated artificial aging


1 Department of Dental Materials and Prosthodontics, Faculty of Dentistry of Ribeirão Preto, São Carlos, Brazil
2 Department of Materials Engineering, Federal University of São Carlos, São Carlos, Brazil

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Date of Submission26-Nov-2012
Date of Decision25-Dec-2012
Date of Acceptance10-Sep-2013
Date of Web Publication21-Dec-2013
 

   Abstract 

Aim: The aim of this study was to evaluate the influence of accelerated artificial aging (AAA) on the color change of composite resins used in dentistry.
Materials and Methods: Three composite resins were evaluated: Two microhybrids and one hybrid of higher viscosity, with different amounts and sizes of filler particles, shades C2 and B2. A total of 54 specimens were obtained (18 for each composite resin), made of a Teflon matrix (15 mm in diameter and 2 mm in height). The color measurements were obtained with a Spectrophotometer, (PCB 6807 BYK Gardner) before and after AAA. Data were submitted to the Kolmogorov-Smirnov test (α >0.05), ANOVA and Tukey test (α <0.05). After statistical analysis, the color difference among composite resins with the same shades was analyzed.
Results: All composite resins showed unacceptable color changes after AAA (ΔE > 3). Considering the variable ∆E, it was observed that the color tone C2 was already statistically different for the microhybrid composite resin prior to AAA (P < 0.05) and in shade B2 for hybrid of higher viscosity and microhybrid with barium glass fluoride aluminum and silica dioxide (P < 0.01). After this process, a statistically significant difference was observed only for shade B2 between microhybrid composite resins (P < 0.01) and for hybrid of higher viscosity and microhybrid with barium glass fluoride aluminum and silica dioxide (P < 0.05). Regarding the color difference within a same composite resin group, before aging the composite resin hybrid of higher viscosity B2 showed the highest color variation rate and microhybrid with zirconium/silica C2 showed the lowest.
Conclusions: All composite resins presented unacceptable color changes after 382 h of aging and different composite resins with same hue, presented different colors before being subjected to the aging process (B2 and C2) and after (B2). It was also observed color difference within a group of the same composite resin and same hue.

Keywords: Aging, color, composite resins

How to cite this article:
Tornavoi DC, Agnelli JM, Panzeri H, Reis AD. Color change of composite resins subjected to accelerated artificial aging. Indian J Dent Res 2013;24:605-9

How to cite this URL:
Tornavoi DC, Agnelli JM, Panzeri H, Reis AD. Color change of composite resins subjected to accelerated artificial aging. Indian J Dent Res [serial online] 2013 [cited 2019 Mar 25];24:605-9. Available from: http://www.ijdr.in/text.asp?2013/24/5/605/123390
Cosmetic dentistry has currently become feasible due to the materials available, but it may raise questions when it comes to selecting restorative materials given the uncertainty about the effects of time on their chemical compounds, hence leading to defects and decrease in durability.

The demand for restorative materials with good color stability has increased. [1] Number of laboratory studies have evaluated the mechanical and aesthetic properties of the composite resins available to minimize failures related to mechanical performance and color change. [2],[3],[4],[5]

These studies are relevant to the development of those composite resins and contribute to the development of new materials and application techniques. [6],[7],[8],[9] This has led to a major scientific, industrial and technological development, which shows an increased use of such materials that go beyond their initial indicated use, which was for anterior teeth restoration. [10]

Major structural changes occur in these materials over time such as superficial degradation, superficial and marginal pigmentation among others. [11],[12] Moreover, there is evidence suggesting a relationship between their color stability and particle composition. The interface between the organic matrix and the filler particles has been considered a critical area for water absorption and consequently to promote more color instability of composite resins. Furthermore, the type of particle and its size load directly influences the surface finish of the material and consequently in the susceptibility of color change. [13],[14]

Studies report that composite resins with larger particles tend to suffer greater degradation being more susceptible to water absorption and color change. [13],[15] Another factor to be considered when it comes to color instability is the shade as evidently clearer composite resins tend to absorb more pigments when compared to darker. [13]

Of all the changes undergone by the composite resins, color change is undoubtedly one of the most complex, resulting in immediate and intolerable patient dissatisfaction, casting doubts on the need for their total or partial replacement. [11],[16]

Several intrinsic and extrinsic reasons may cause composite resins to discolor over time. Thus, artificial aging could cause such a change through the action of ultraviolet light, humidity and temperature changes, since it simulates the greatly complex oral environment. [17]

Due to the need to know more about the effects of time on color change of photopolymerizable composite resins, in this study, the specimens of these materials were subjected to accelerated artificial aging (AAA) process in order to assess this property. The hypothesis is that these dental composite resins undergo different levels of color change since their composition differs in terms of type and/or inorganic content percentage.


   Materials and Methods Top


Three composite resins with different amounts and sizes of filler particles of C2 and B2 shades were used in this study as shown in [Table 1] below.

The Teflon matrix was developed with a perfectly smooth inner coating, meeting the requirements to obtain specimens with dimensions of 15 mm in diameter and 2 mm in height, which resulted in samples with opacity and diameter compatible for the color readings using the spectrophotometer calorimeter.
Table 1: Description of the composite resins used


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Before the preparation of each specimen, the light curing unit (Ultralux Electronic, Dabi Atlante) was measured by a digital radiometer Dabi Atlante, which was set at 400 mW/cm 2 . To prepare the specimens, the incremental technique was used with the curing time specified by the manufacturer. After this time, the specimens were removed from the matrix and underwent a surface treatment with the extra fine finishing and polishing kit Sof-Lex Pop-On 3 M, in abrasive sequence applied intermittently and interspersed with surface wetting to avoid overheating and surface changes.

A total of 18 specimens were prepared for each composite resin, a total of n = 54. The initial color readings were performed with PCB 6807 Spectrophotometer, BYK (Gardner, Gerestried, Germany), which consisted of 30 light emitting diode lights of 10 different colors, using the standard observer CIE L*a*b*. This system uses three parameters to define color, light, shade and saturation. [18] Luminosity represents the light and dark degree of the object, represented by the value of L* (L* =100 for white and L* =0 for black). The parameters a* and b* (shade), represent the red if + a* and green if -a*, yellow if + b* and blue if -b*. [18]

When the spectrophotometer is activated, the different colors arranged in a circular manner, reflect light beams on the material surface at a 45° angle using the primary standard D65, which simulates the spectrum of natural daylight. This beam is reflected back to the unit at 0°. This then captures and records the values of L, a and b of each sample.

After the first reading, the samples were placed in the AAA chamber under ultra violet (UV) radiation and condensation in different repeated cycles (successively and automatically). In this equipment, the UV-B source was fluorescent light bulbs that emitted concentrated ultraviolet light. Condensation was produced by exposing one surface of a specimen to a heated, saturated mixture of air and water vapor while the reverse side of the specimen was adhered to metal plates with silicone, specifically indicated, under the action of the condensation process at a distance of 50 mn from the light source.

The system was programmed to operate for 4 h of exposure to UV-B at 50°C and 4 h of condensation at 50°C, totaling 382 h of aging, which is equivalent to 10 years of aging. [19]

After aging, the specimens were subjected to another color reading process by the spectrophotometer. Thus, we obtained measures of color change (∆E) before and after artificial accelerated aging, which was automatically calculated by the formula: [20],[21]



Values of ∆ L*, ∆a*, ∆b*, correspond to the difference of the values L*, a*, b*, respectively, compared to the first color reading (initial). ∆E values ≥ 3.3 are considered clinically unacceptable. [22]

The data obtained were submitted to statistical analysis using the normality test (Kolmogorov-Smirnov -α = 0.05), two-way ANOVA (materials and Δ-axis) and the test for multiple comparisons (Tukey-α =0.05).


   Results Top


The colorimetric results of the samples obtained before and after the artificial accelerated aging are shown in [Table 2] and [Table 3], respectively.

Before the AAA, only composite resin Charisma B2 showed clinically unacceptable color change (ΔE = 3.5).
Table 2: Mean colorimetry results of non-aged composite resins


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Table 3: Mean colorimetry results of aged composite resins


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It was found that all the composite resins showed unacceptable color changes after EAA (ΔE > 3), influenced mainly by the strong tendency of these resins presented blueness (negative Δb).

The composite resin Charisma B2, microhybrid with less inorganic particle (60 Vol%) showed the greatest color change after the EAA (ΔE = 13.20) and microhybrid composite resin of higher viscosity, P60 B2 also presented unacceptable color change, but at a lower proportion compared the others [Table 4].

In relation to the axis of luminosity/opacity stands out negatively composite resin Z100 B2 because was that it became darker, unlike all other resins analyzed (P < 0.05).
Table 4: Comparison of samples before and after aging


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The data analysis showed that different composite resins, with the same hue exhibited distinct colors. Before AAA there was statistically significant difference in the shade C2 between the microhybrid composite resins (P < 0.05) and in shade B2 between hybrid of higher viscosity and microhybrid composite resin with barium glass fluoride aluminum and silica dioxide (P < 0.01). After this process, statistically significant difference was observed only for shade B2 between microhybrid composite resins (P < 0.01) and hybrid of higher viscosity and microhybrid with barium glass fluoride aluminum and silica dioxide (P < 0.05).

Another analysis performed to identify, which composite resin had the least variation among the specimens was the determination of the coefficient of variation. The composite resin hybrid of higher viscosity B2 showed the highest color variation before the aging process and microhybrid with zirconium/silica C2 showed the lowest, after.

These results show that when the composite resins are aged there is a tendency to stabilize the color difference [Table 5].
Table 5: Coefficient of variation in color of composite resin in groups, B and A artificial accelerated aging


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   Discussion Top


This study evaluated the influence of accelerated aging on the color change of three composite resins, shades C2 and B2. In a matter of a few days or weeks, C-UV can cause damages that would otherwise occur in months or years. [19]

Although, the composite resins in light colors tend to have higher color change when compared darkest as they absorb a greater amount of pigment, the results demonstrated that all composite resins showed unacceptable change in color after the procedure, influenced mainly by the strong tendency of these resins submit bluish color (all exhibited negative Δb). This can be explained by the possibility of a large degradation of the organic matrix resins, pronouncing the tones of the filler particles in the final measurements of color. The hypothesis is that aging promotes an erosion aspect on the surface of the restorative materials, exposing the load components, which contributes to the increase in the staining of composite resins, since the color changes are related to the porosity of the surface and breaking of the constituents. [23]

In relation to the axis of luminosity/opacity, there is negatively composite resin Z 100 B2 as was that it became darker, unlike all other analyzed (P < 0.05).

Before the AAA, only composite resin Charisma B2 showed clinically unacceptable color change (ΔE = 3.5), this color change was due in large part to the tendency of this composite resin present bluish color (sharp deviation in the blue-yellow axis in sense of the color blue-very negative Δb).

Different composite resins, with the same hue exhibited distinct colors. Different composite resins, with the same hue exhibited distinct colors. Before the AAA, the tone C2 was different between the microhybrid composite resins (P < 0.05) and in shade B2 between hybrid of higher viscosity and microhybrid composite resin with barium glass fluoride aluminum and silica dioxide (P < 0.01). After this process, statistically significant difference was observed only for shade B2 between microhybrid composite resins (P < 0.01) and hybrid of higher viscosity and microhybrid with barium glass fluoride aluminum and silica dioxide (P < 0.05). This demonstrates that the color varies depending on the composition and trademark.

Besides the color difference found in various composite resins with the same hue within a group and the same composite resin with the same hue was also found color difference. Before the aging composite resin P60B2 showed the highest growth rate and the Z 100 C2, lowest. However, we note that there is a tendency to reduce this difference to the process of aging, which may result from complete polymerization. According to Condon and Ferracane, [24] there is a correlation between the polymerization and properties of composite resins and the degree of polymer conversion may have influence on the stability of these materials.

As to type of monomer used for each material, studies claim that the presence of urethane dimethacrylate (UDMA) can result in a better conversion and therefore a greater color stability of the material while the combination of bisphenol a glycidyl methacrylate (Bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA), would lower color stability due to the higher propensity to water absorption. [17] In this study, considering the color change undergone after aging, the composite resin p60B2, which contains UDMA showed unacceptable color change but at a lower proportion when compared the other consisting of Bis-GMA and TEGDMA (hydrophilic monomer), but the same composite resin in hue C2, showed intermediate values.

According to some authors, the color stability of these materials is directly related to the size, type and volume of charged particles, type of matrix and monomer used, depth of polymerization, degree of adherence to the composite resin matrix and colorants. [25],[26],[27],[28] Schulze (2003) [18] found that the composite resins submitted to AAA that had smaller number of inorganic particles underwent major color changes, showing the relationship of this property with the composition of these materials, which was against the results of present study, where despite all the composite resins having presented unacceptable color change after the EAA, the composite resin Charisma B2 (60% by volume) showed the most color change (ΔE = 13.20).

Kawaguchi [15] states that composite resins with large particles are more susceptible to water absorption and color change. Thus, these results contradict the data of the present study since the higher viscosity composite resin hybrid and consequent low flow, not presented the highest values of color change when compared composite resins with reduced particle size.

The substantial contribution of this study is to evaluate the degradation of the composite resins through the C-UV aging method, since it uses heating and cooling intervals as well as light and humidity that are the closest to those of nature, according to the ASTM standard. [29] The fact of the artificial aging method employed in the study did not simulate the action of colorants and acids from foods is a limitation of the study as these factors in clinical practice can cause major color change in composite resin restorations. As a suggestion for future study can cite a study that could reduce clinical longitudinal this and any other limitation comes from in vitro studies.

The data allow us to say that, although the present composites have undergone a great development and constant change in their formulations these materials still present a major problem in the long term, their instability cor, [1],[30],[31],[32],[33] and this a major cause of failures and a major reason for replacing restorations.


   Conclusion Top


The following conclusions can be drawn from the present study:

  • All the composite resins presented unacceptable color changes after 382 h of aging.
  • Different composite resins with same hue, presented different colors before being subjected to the aging process (B2 and C2) and after (B2).
  • Within a group of the same composite resin and the same hue was also difference in color while the composite resin with higher inorganic particle with the lowest color variation.


 
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Correspondence Address:
Andréa Cândido Dos Reis
Department of Dental Materials and Prosthodontics, Faculty of Dentistry of Ribeirão Preto, São Carlos
Brazil
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.123390

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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]

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