- Email: [email protected]
Influence of drinks on resin composite: Evaluation of degree of cure and color change parameters
ARTICLE IN PRESS
POLYMER TESTING Polymer Testing 26 (2007) 438–444 www.elsevier.com/locate/polytest
Material Performance
Influence of drinks on resin composite: Evaluation of degree of cure and color change parameters Betsy K.M. Luiza, Renata D.M.C. Ambonib, Luiz Henrique M. Pratesc, Jose´ Roberto Bertolinod, Alfredo T.N. Piresd, Programa de Po´s-Graduac- a˜o em Cieˆncia e Engenharia de Materiais, Universidade Federal de Santa Catarina, 88040-900, Floriano´polis-SC, Brazil b Departamento de Cieˆncia e Engenharia de Alimentos, Universidade Federal de Santa Catarina, 88040-900, Floriano´polis-SC, Brazil c Departamento de Estomatologia, Universidade Federal de Santa Catarina, 88040-900, Floriano´polis-SC, Brazil d Departamento de Quı´mica, Universidade Federal de Santa Catarina, 88040-900, Floriano´polis-SC, Brazil a
Received 20 October 2006; accepted 8 December 2006
Abstract Resin composites represent a class of materials widely used in restorative dentistry due to patient demands for better aesthetics. In this study, the degree of conversion and color change parameters of commercial resin composite Charisma after immersion in commonly used drinks were investigated. Degree of conversion and color stability are important parameters of modern resin-based dental filling materials. The specimens, after curing, were immersed in distilled water, a sports drink, yogurt, a soft drink based on cola and red wine for 24 and 168 h at 37 1C. The degree of conversion, evaluated by Raman spectroscopy through the correlation of absorption bands related to CQC aromatic and aliphatic stretching and CQO free stretching, was 70% at all specimen depths. The ratios of these absorption bands for specimens after immersion in the drinks were of the same magnitude, indicating that no chemical reaction occurred at the surface. The influence of drinks on color change of filling resin was investigated by measuring the CIE-Lab-values. The color change parameters (L*, a* and b*) were significantly affected when the specimens were immersed in wine or yogurt for 24 or 168 h. r 2007 Elsevier Ltd. All rights reserved. Keywords: Resin composites; Raman spectroscopy; Degree of conversion; Color parameters
1. Introduction Resin composites represent a class of materials widely used in restorative dentistry and have been improved since the 1960s when they were first described. Although light-cured composites are excellent for aesthetics procedures, both physical and chemical properties are directly related to their Corresponding author. Tel.:55 48 33316845.
E-mail address: [email protected] (A.T.N. Pires). 0142-9418/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.12.005
degree of conversion (DC). Adequate polymerization of composite resin restorative materials is fundamental for optimal physical and chemical properties, as well as for a good clinical performance. Raman spectroscopy and Fourier transformed infrared spectroscopy (FT-IR) have been used to determine the DC. Previous reports in the literature give DC values for bis-GMA-based resin composites measured by FT-IR of 36.9–62.2% [1] and 54.9–65.4% [2] for different specimen depths, depending on the curing conditions.
ARTICLE IN PRESS B.K.M. Luiz et al. / Polymer Testing 26 (2007) 438–444
The storage in a water environment can influence the properties of a resin composite, so if the matrix is hydrophobic, there is less water sorption and only a slight impact on the color is seen. However, a hydrophilic matrix induces an increasing water sorption resulting in a white and opaque shade [3–5]. Also, the type of filler (organic, inorganic, glass, pyrogenic silica) may have an influence on discoloration. In contrast to exogenous discolorations caused by the adsorption of dyes or plaque that can be easily removed by polishing, endogenous discolorations are irreversible [4–7]. Therefore, the components of a filling resin may not only influence the curing characteristics and the strength of the materials, but may also have an impact on the color stability. Color change is another important parameter for resin-based filling materials. Several factors influence the color of photocuring materials, such as photoinitiator component, resin matrix composition, lightcuring device and irradiation times. Camphorquinone is the most commonly used photoinitiator in dental restorative resins and although it is used in small amounts, it significantly influences the color of the material. Other photoinitiators used are tertiary aromatic or aliphatic amines, which act as so-called synergists or accelerators. All amines are known to form by-products during photoreaction, which tend to cause yellow to red/brown discolorations under the influence of light or heat [8]. In 1931, the Commission Internationale de L’E´clairage (CIE) defined a standard light source, developed a standard observer and enabled the calculation of tristimulus values, which represent how the human visual system responds to a given color. The CIE defined a color space, CIE-lab, that supports the accepted theory of color perception based on three separate color receptors in the eye (red, green and blue) and is currently one of the most popular color spaces [9]. In this investigation, the FT-Raman spectroscopy technique was used to evaluate the degree of conversion of a photo-cured resin composite using a light emitting diode (LED) and color measurements were used to verify changes on the resin composite surface after immersion in different drinks for 24 and 168 h.
439
source used to light curing the specimens was Elipar free light (3 M ESPE Dental Products, Minnesota, USA). The storage environments were commonly used drinks, and distilled water was used as the control. The drinks were a sports drink (Gatorades—citric fruits), a yogurt obtained from lactobacillus (Yo-mix TM cultures), a soft drink based on cola (Coca-cola Companys) and a red wine (Cordier Le Merlot—2000). 2.2. Specimens preparation Specimens (diameter 10.070.1 mm, thickness: 1.070.1 mm) were prepared at room temperature using a stainless steel mold. The mold was filled with the test material, avoiding gross excess and entrapped air. A 0.05 mm transparent polyethylene film covered each side of the mold and a glass slide (3 mm) was placed on the top of the mold. A light emitting diode (LED) was used to irradiate (370 mW/cm2) the specimens from one side along the whole extension in constant polymerization mode for 40 s. The glass slide thickness standardized the distance from the light source to the resin composite and provided a smooth, non-air-inhibited surface for subsequent testing. The specimens were then ground with a sequence of abrasive papers (240, 400, 600 and 1200 grit). Five specimens were made for each type of drink (in total 25 specimens). The drinks were changed after every 24 h of immersion. The specimens were immersed in a drink or distilled water for 24 and 168 h and stored at 37 1C. 2.3. Raman Spectroscopy Micro-Raman spectroscopy was performed with a Renishaw Raman microscope. Laser light at 514.5 nm from an Ar laser was used for excitation. A power density of 1.1 mW/cm2 was used and calibration was performed with the Si peak at 520.7 cm1. The error in the peak positions was less than 72.0 cm1. The degree of cure and chemical groups of the resin composites were observed before and after drink immersion. 2.4. Color measurements
2. Materials and methods 2.1. Materials The resin composite used was Charisma (Heraeus Kulzer, Hanau, Germany), shade A2, and the light
For color measurements, a spectrophotometer (Konica Minolta colorimeter, Japan) equipped with an integrating sphere with a 10 mm opening was used. The spectrophotometer measurements were
ARTICLE IN PRESS B.K.M. Luiz et al. / Polymer Testing 26 (2007) 438–444
440
made using the L*a*b* system. D65 illumination and a 101 standard observation angle were selected. A white calibration was used: calibration plate no 19433091 (Konica Minolta, Japan). In the three-dimensional color space, the L* value is a measure of the lightness of an object and is quantified on a scale such that a perfect black has an L* value of 100. The a* value is a measure of redness (positive a*) or greenness (negative a*). The b* value is a measure of yellowness (positive b*) or blueness (negative b*). The a* and b* co-ordinates approach zero for neutral colors (white, grays) and increase in magnitude for more saturated or intense colors. The advantage of the CIE-lab system is that color differences can be expressed in units that can be related to visual perception and clinical significance [10]. The calculation of the color variation DE* between two color positions from values obtained before and after immersion, in the threedimensional (3-D) L*a*b* color space, is given by: DE ¼ ½ðL1 L0 Þ2 þ ða1 a0 Þ2 þ ðb1 b0 Þ2 1=2
evaluate the effect of the drinks on color change and to analyze the FT-Raman results for the composite using statistical software (SPSS for Windows, Version 13.0; SPSS Inc., Chicago, Ill). 3. Results The Raman spectrum of a cured specimen of Charisma maintained in distilled water is showed in Fig. 1, with different characteristic peaks that are related to vibrational frequency given in Table 1. The main vibrational lines are assigned to Si2O bending at 806 cm1, SiO stretching at 1200 cm1, aliphatic CO stretching at 1300 cm1, modes of CQCH2 at 1404 cm1, CQC stretching of the aromatic ring at 1446 and 1607 cm1 (skeletal vibration of the benzene ring), methacrylate CQC stretching at 1638 cm1 and CQO stretching at 1714 cm1 [11–15]. Through an evaluation of the peak corresponding to CQC stretching from methacrylate (1638 cm1) and the aromatic ring (1607 cm1), it is possible to estimate the decrease in carbon double bonds due to the cross-linking reaction (CQC from methacrylate) in the same Raman spectrum. The degree of conversion can then be calculated through
(1)
2.5. Statistical analysis One-way analysis of variance (ANOVA) followed by the Scheffe´ post hoc test (a ¼ 0.05) was used to
DCð%Þ ¼ 100 ½1 H cured =H uncured ,
1607
Intensity (a.u.)
1446 1638 1404 1200
806
400
600
800
1000
1714
1300
1200
1400
1600
1800
2000
2200
Raman shift (cm-1) Fig. 1. Raman spectra of the Charisma composite after being kept in distilled water.
(2)
ARTICLE IN PRESS B.K.M. Luiz et al. / Polymer Testing 26 (2007) 438–444 Table 1 Vibrational line assignments Frequency (cm1)
Peak assignments
806 1200 1300 1404 1446 1457, 1513, 1558 1607 1638 1714
Si–O–Si bending Si–O stretching aliphatic C–O stretching C ¼ CH2 bending C ¼ C of aromatic group vibrations of the benzene ring C ¼ C aromatic stretching C ¼ C aliphatic stretching C ¼ O free stretching
where Hcured and Huncured correspond to the peak height ratios at 1638 and 1607 cm1 for cured and uncured resin, respectively. Using these peak height ratios for uncured resin and for specimens after irradiation for 40 s using LED, a degree of conversion of 70% was obtained, in agreement with values reported in the literature for studies using different light sources including LED [1,2, 12,16–18]. For the cured resin, obtaining several successive spectra showed a constant degree of cure, indicating no influence of the laser beam. No evidence of a change in the degree of cure was observed at different specimen depths. Table 2 shows the average of eight measurements for the peak ratio of the spectra, from the surface to the inner part of the specimen, in steps of 5 mm. The peak ratios for spectra did not show a statistically significant difference within the confidence interval (a ¼ 0.05). The same results were obtained for specimens immersed in yogurt and the soft drink. Table 3 shows the color parameters (L*, a* and b*) for specimens before and after immersion in different drinks and water as the control, for 24 and 168 h. Significant changes in the color parameters were observed for the specimens, for example, the wine change a* from 0.28 to 1.78 and 2.14 after 24 and 168 h, respectively. By applying Eq. (1), the DE* value was calculated, as given in Table 4 along with the standard deviation and the dissimilarity of groups obtained through statistical analysis. 4. Discussion Although dental composite resins have been widely used as filling materials, they have a major drawback, which is their incomplete polymeriza-
441
Table 2 Band ratios related to C ¼ C aromatic stretching (1446 and 1607 cm1) and C ¼ C aliphatic stretching C ¼ C (1638 cm1) for Charisma composite specimens maintained in distilled water, yogurt or soft drink for 168 h Specimens
Peak ratio h1607 /h1638
h1607 /h1446
Control Yogurt Soft drink
Average
SD
Average
SD
1.29 1.38 1.29
0.04 0.05 0.05
2.50 2.33 2.59
0.10 0.03 0.02
tion. This can be detected using several methods, including the measurement of the degree of conversion (DC), which relates to the percentage of double bonds reduced during the polymerization process. Incomplete polymerization results in unreacted monomers, which leach from the material in a wet environment [13,19]. Problems associated with inadequate polymerization include poor physical properties, increased solubility in the oral environment, increased microleakage and consequent color changes. Vibration methods allow a precise assessment of the depth of polymerization and DC of methacrylate composites resins [14,17]. The use of Raman techniques to determine the percent conversion requires that the amount of double bonds, which are present after curing, can be quantified. In bisGMA/TEGDMA this measurement is carried out on a relative basis by comparing the height (or intensity) of the vibrational CQC band at 1638 cm1 to the CQC aromatic ring stretching at 1609 cm1, whose height (or intensity) does not depend on the polymerization process [15–18]. The 70% DC evaluated by Raman spectroscopy, with LED irradiation for 40 s, is of the same order of magnitude as similar resins described in the literature. The cure degree of dimethacrylate-type and related resins were in the range of 50%–60% under a halogen beam [16–18, 20]. The same DC being obtained in the inner part of the specimen, and the high DC using LED as the cure process initiator, indicates the better efficiency of this light source. The hydrophilic nature of a polymer is in large part a function of the chemistry of its monomers and its polymerization linkages. The structure of polymers used in dental composites reveals the presence of hydrolytically susceptible ester groups. The presence of hydroxyl, carboxyl
ARTICLE IN PRESS B.K.M. Luiz et al. / Polymer Testing 26 (2007) 438–444
442
Table 3 Color parameters for Charisma resin stored in distilled water, soft drink, sports drink, wine or yogurt: L*, a* and b* Color parameters
Immersion time (h)
Control
L*
0 24 168 0 24 168 0 24 168
74.7 74.7 74.7 0.28 0.28 0.28 14.5 14.5 14.5
a*
b*
Table 4 Color changes (DE*) for Charisma resin stored in distilled water, soft drink, sports drink, wine or yogurt Immersion time (h)
24 168
Color change (DE*) Control
Soft drink
Sports drink
Wine
0 (0) a 0 (0) a
1.6 (0.1) b 0.8 (0.2) a 4 (0.1) c 1.4 (0.3) b 0.6 (0.3) a 10.6 (0.5) c
Yogurt
1 (0.2) b 1.4 (0.9) b
* Standard deviations are in parentheses. ** Different letters indicate dissimilarity of groups (Po0.05).
and phosphate groups in monomers and their resultant polymers make them more hydrophilic and, supposedly, more prone to water sorption [21]. The lack of change in the height intensity ratio (h1607/h1638) of the vibration line corresponding to CQC stretching from methacrylate (1638 cm1) and the aromatic ring (1607 cm1) for the specimens after immersion in the drinks, indicates that a chemical reaction did not occur between the drink and composite components. There was no significant statistical difference between the ratios of these absorption bands for specimens after immersion in the drinks, compared with the control group. However, since a change in the color properties did occur, it could be that there was some physical adsorption of specific drink components on the composite surface. Due to this hydrophilic behavior of the polymeric matrix, the penetration of drink components into the composite was facilitated by the action of water. Initially, the presence of water softens the polymer by swelling the network and reducing the frictional forces between the polymer chains [22]. After the relaxation process, unreacted monomers trapped in the polymer network are released to the
Soft drink
Sports drink
Wine
Yogurt
74.8 75.0
75.2 75.2
72.1 66.6
75.6 74.8
0.23 0.50
0.03 0.3
1.78 2.14
0.10 0.07
15.8 16.3
15.0 15.1
14.8 20.8
15.0 15.0
surrounding environment at a rate that is controlled by the swelling and relaxation capacities of the polymer. More hydrophilic polymer networks, which have a superior relaxation capacity, permit a faster release of unreacted monomers through nanovoids in the material, showing a decrease in mass within a short time of water immersion. This is somewhat dependent on the DC and the quantity of pendant molecules existing within the network [23–25]. It is extremely difficult to measure whether the appearance of restorative materials mimic natural teeth. In the ideal case, the resin-based composite has to show the same reflectance, translucence and transparency as the tooth structure. The samples were stored for 24 and 168 h in drinks and distilled water at 37 1C to simulate the mouth environment. The composite color changes occurred mainly in specimens maintained in wine, the soft drink and yogurt. Although the citric fruit flavored sports drink has an orange color, this drink did not change the composite color, probably because the components were not adsorbed on the specimen surface. The resistance to staining of the resin composite materials is related to polymerization type, filler particles of the composite materials, and type of staining agent. The considerations relating to these statements are discussed below. Discoloration can be evaluated with various instruments. Since instrumental measurements eliminate the subjective interpretation of visual color comparison, spectrophotometers and colorimeters have been used to measure color change in dental materials. The CIELab system for measuring chromaticity was chosen to record color differences because it is well suited for the determination of small color differences [26–28].
ARTICLE IN PRESS B.K.M. Luiz et al. / Polymer Testing 26 (2007) 438–444
Extrinsic factors for discoloration include staining by adsorption or absorption of colorants as a result of contamination from exogenous sources. Extrinsic factors for discoloration are known to cause staining of oral tissues and restorations, especially in combination with dietary factors, including coffee, tea, and nicotine [29]. In a previous study [30], red wine, tea, coffee, and mouth rinses, were used as staining agents to evaluate the stain resistance of composite materials, and it was found that red wine caused the most severe discoloration. In another study [31], the highest color difference for all the composite materials investigated was also observed for red wine. Based on changes in color (DE) it is possible to compare the values before and after storage in drinks. Because the ability of the human eye to appreciate differences in color differs from individual to individual (as it is a combination of eye characteristics and skill of the operator), three different intervals were used for distinguishing color differences. Values of DEo1 were regarded as not appreciable by the human eye. Values 1oDEo3.3 were considered appreciable by skilled operators, but considered clinically acceptable, whilst values of DE43.3 were considered appreciable also by nonskilled persons and for this reason clinically not acceptable [29,32,33]. DE values for the light-polymerized composite Charisma were significantly higher after immersion in red wine (DE4 3.3), and for yogurt and soft drink they would be considered appreciable by skilled operators, but considered clinically acceptable (1oDEo3.3). For composite specimens immersed in the sports drink, DE values indicated that the color changes were not appreciable by the human eye (Eo1). The discoloration might be due to both absorption and surface adsorption of colorants. Fine colorant particles may be deposited in pitted areas of the light-polymerized material. Large filler particles present on the surface will produce high surface roughness values [34]. Through statistical analysis of variance, followed by the Scheffe´ post hoc test, the average DE values obtained for specimens maintained in wine for 24 and 168 h differ significantly from results obtained for the other drinks. 5. Conclusions From the Raman spectroscopy data it could be determined that the degree of conversion was 70%
443
at the surface and in the inner part of specimens, indicating the efficiency of LED as a cure process initiator. No chemical reaction between the drinks and composite components was observed, indicating that the change in color parameters may be due to surface adsorption and/or absorption of drink components. DE values for light-polymerized composite Charisma were significantly higher after immersion in red wine (DE43.3), and for yogurt and the soft drink were considered observable by skilled operators, but considered clinically acceptable (1oDEo3.3). For composite specimens immersed in the sports drink, DE values were not noticeable to the human eye (Eo1). Acknowledgment This study was supported by CNPq (Conselho Nacional de Desenvolvimento Cientı´ fico e Tecnolo´gico). References [1] T.H. Yoon, Y.K. Lee, B.S. Lim, C.W. Kim, Degree of polymerization of resin composites by different light sources, J. Oral Rehabil. 29 (2002) 1165–1173. [2] Z. Tarle, A. Meninga, A. Knezevick, J. Sutalo, M. Ristic, G. Pichler, Composite conversion and temperature rise using a convencional, plasma arc and an experimental blue LED curing unit, J. Oral Rehabil. 29 (2002) 662–667. [3] D. Dietschi, G. Campanille, J. Holz, J.M. Meyer, Comparison of the color stability of ten new-generation composites: an in vitro study, Dental Mater. 10 (1994) 353–362. [4] W. Buchalla, T. Attin, R. Hilgers, E. Hellwig, The effect of water storage and light exposure on the color and translucency of a hybrid a microfilled composite, J. Prosthet. Dent. 87 (2002) 264–270. [5] C. Ameye, P. Lambrechts, G. Vanherle, Conventional and microfilled composites resins: color stability and marginal adaptation, J. Prosthet. Dent. 46 (1981) 623–630. [6] J.J. Ten Bosch, J.C. Coops, Tooth color and reflectance as related to light scattering and enamel hardness, J. Dental Res. 74 (1995) 374–380. [7] A. Watts, M. Addy, Tooth discolouration and staining: a review of the literature, Br. Dent. J. 190 (2001) 309–316. [8] R. Janta, J.F. Roulet, M. Kaminsky, G. Steffin, M. Latta, Color stability of resin matrix restorative materials as function of the method of light activation, Eur. J. Oral Sci. 112 (2004) 280–285. [9] D.B. MacDogall, Colour measurement of food: principles and practice, in: Colour in Food, Woodhead Publishing Limited, Cambridge, 2002. [10] W.J. O’brien, H. Hemmendinger, K.M. Boenke, J.B. Linger, C.L. Groh, Color distribution of three regions of extracted human teeth, Dent. Mater. 13 (1997) 179–185. [11] S. Kammer, K. Albinsky, B. Sandner, S. Wartewig, Polymerization of hydroxyalkyl methacrylates characterized
ARTICLE IN PRESS 444
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19] [20]
[21]
[22]
B.K.M. Luiz et al. / Polymer Testing 26 (2007) 438–444 by combination of FT-Raman and step-scan FT-I.R. photoacoustic spectroscopy,, Polymer 40 (1999) 1131–1137. J. Xu, I.S. Butler, D.F.R. Gibson, I. Sangel I, High-pressure infrared and FT-Raman investigation of a dental composite, Biomaterials 18 (24) (1997) 1653–1657. R.L. Ore´fice, J.A.C. Discacciati, A.D. Neves, H.S. Mansur, W.C. Jansen, In situ evaluation of the polymerization kinetics and corresponding evolution of the mechanical properties of dental composites, Polym. Test. 22 (2003) 77–81. J.W. Stansbury, S.H. Dickens, Determination of double bond conversion in dental resins by near infrared spectroscopy, Dent. Mater. 17 (2001) 71–79. B. Sandner, S. Kammer, S. Wartewig, Crosslinking copolymerization of epoxy methacrylates as studied by Fourier transform Raman spectroscopy, Polymer 37 (21) (1996) 4705–4712. G. Leloup, P.E. Holvoet, S. Bebelman, J. Devaux, Raman scattering determination of the depth of cure of lightactivated composites: influence of different clinically relevant parameters, J. Oral Rehabil. 29 (6) (2002) 510–515. C. Pianelli, J. Devaux, S. Bebelman, G. Leloup, The microRaman spectroscopy, a useful tool to determine the degree of conversion of light-activated composite resins, J. Biomed. Mater. Res. 48 (1999) 675–681. N. Silikas, G. Eliades, D.C. Watts, Light intensity effects on resin-composite degree of conversion and shrinkage strain, Dent. Mater. 16 (4) (2000) 292–296. A.U.J. Yap, H.K. Lee, R. Sabapathy, Release of methacrylic acid from dental composite, Dent. Mater. 16 (2000) 172–179. W.S. Shin, X.F. Li, B. Schwartz, S.L. Wunder, G.R. Baran, Determination of the degree of cure of dental resins using Raman and FT-Raman spectroscopy, Dent. Mater. 9 (5) (1993) 317–324. J.P. Santerre, L. Shajii, B.W. Leung, Relation of dental composite formulations to their degradation and the release of hydrolyzed polymeric-resin-derived products, Crit. Rev. Oral Biol. Med. 12 (2001) 136–151. J.L. Ferracane, X.H. Berge, J.R. Condor, In vitro aging of dental composites in water-effect of degree of conversion,
[23]
[24]
[25]
[26]
[27]
[28]
[29] [30]
[31]
[32]
[33] [34]
filler volume, and filler/matrix coupling, J. Biomed. Mater. Res. 42 (1998) 465–472. C.S. Brazel, N.A. Peppas, Mechanisms of solute and drug transport in relaxing, awellable hydrophilic glassy polymers, Polymer 40 (1999) 3383–3398. C.S. Brazel, N.A. Peppas, Dimensionless analysis of swelling of hydrophilic glassy polymers with subsequent drug release from relaxing structures, Biomaterials 20 (1999) 721–732. I. Sideridou, V. Tserki, G. Papanastasiou, Study of water sorption, solubility and modulus of elasticity of light-cured dimethacrylate-based dental resins, Biomaterials 24 (2003) 655–665. N. Satou, A.M. Khan, I. Matsumae, J. Satou, H. Shintani, In vitro color change of composite-based resins, Dent. Mater. 5 (1989) 384–387. R.R. Seghi, M.D. Gritz, J. Kim, Colorimetric changes in composites resulting from visible-light-initiated polymerization, Dent. Mater. 6 (1990) 133–137. S.R. Okubo, A. Kanawati, M.W. Richards, S. Childress, Evaluation of visual and instrument shade matching, J. .Prosthet. Dent. 80 (1998) 642–648. C.M. Um, I.E. Ruyter, Staining of resin-based veneering materials with cofee and tea, Quint. Int. 22 (1991) 377–386. T. Stober, H. Gilde, P. Lenz, Color stability of highly filled composite resin materials for facing, Dent. Mater. 17 (2001) 87–94. A.U. Guler, F. Yilmaz, T. Kulunk, E. Guler, S. Kurt, Effects of different drinks on stainability of resin composite provisional restorative materials, J. .Prosthet. Dent. 94 (2) (2005) 118–124. S. Inokoshi, M.F. Burrow, M. Kataumi, T. Yamada, T. Takatsu, Opacity and color changes of tooth-colored restorative materials, Oper. Dent. (1996) 73–80. H.S. Kim, C.M. Um, Color differences between resin composites and shade guides, Quint. Int. 27 (1996) 559–567. A. Maalhagh-Fard, W.C. Wagner, F.E. Pink, A.M. Neme, Evaluation of surface finish and polish of eight provisional restorative materials using acrylic bur and abrasive disc with and without pumice, Oper. Dent. 28 (2003) 734–739.
POLYMER TESTING Polymer Testing 26 (2007) 438–444 www.elsevier.com/locate/polytest
Material Performance
Influence of drinks on resin composite: Evaluation of degree of cure and color change parameters Betsy K.M. Luiza, Renata D.M.C. Ambonib, Luiz Henrique M. Pratesc, Jose´ Roberto Bertolinod, Alfredo T.N. Piresd, Programa de Po´s-Graduac- a˜o em Cieˆncia e Engenharia de Materiais, Universidade Federal de Santa Catarina, 88040-900, Floriano´polis-SC, Brazil b Departamento de Cieˆncia e Engenharia de Alimentos, Universidade Federal de Santa Catarina, 88040-900, Floriano´polis-SC, Brazil c Departamento de Estomatologia, Universidade Federal de Santa Catarina, 88040-900, Floriano´polis-SC, Brazil d Departamento de Quı´mica, Universidade Federal de Santa Catarina, 88040-900, Floriano´polis-SC, Brazil a
Received 20 October 2006; accepted 8 December 2006
Abstract Resin composites represent a class of materials widely used in restorative dentistry due to patient demands for better aesthetics. In this study, the degree of conversion and color change parameters of commercial resin composite Charisma after immersion in commonly used drinks were investigated. Degree of conversion and color stability are important parameters of modern resin-based dental filling materials. The specimens, after curing, were immersed in distilled water, a sports drink, yogurt, a soft drink based on cola and red wine for 24 and 168 h at 37 1C. The degree of conversion, evaluated by Raman spectroscopy through the correlation of absorption bands related to CQC aromatic and aliphatic stretching and CQO free stretching, was 70% at all specimen depths. The ratios of these absorption bands for specimens after immersion in the drinks were of the same magnitude, indicating that no chemical reaction occurred at the surface. The influence of drinks on color change of filling resin was investigated by measuring the CIE-Lab-values. The color change parameters (L*, a* and b*) were significantly affected when the specimens were immersed in wine or yogurt for 24 or 168 h. r 2007 Elsevier Ltd. All rights reserved. Keywords: Resin composites; Raman spectroscopy; Degree of conversion; Color parameters
1. Introduction Resin composites represent a class of materials widely used in restorative dentistry and have been improved since the 1960s when they were first described. Although light-cured composites are excellent for aesthetics procedures, both physical and chemical properties are directly related to their Corresponding author. Tel.:55 48 33316845.
E-mail address: [email protected] (A.T.N. Pires). 0142-9418/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2006.12.005
degree of conversion (DC). Adequate polymerization of composite resin restorative materials is fundamental for optimal physical and chemical properties, as well as for a good clinical performance. Raman spectroscopy and Fourier transformed infrared spectroscopy (FT-IR) have been used to determine the DC. Previous reports in the literature give DC values for bis-GMA-based resin composites measured by FT-IR of 36.9–62.2% [1] and 54.9–65.4% [2] for different specimen depths, depending on the curing conditions.
ARTICLE IN PRESS B.K.M. Luiz et al. / Polymer Testing 26 (2007) 438–444
The storage in a water environment can influence the properties of a resin composite, so if the matrix is hydrophobic, there is less water sorption and only a slight impact on the color is seen. However, a hydrophilic matrix induces an increasing water sorption resulting in a white and opaque shade [3–5]. Also, the type of filler (organic, inorganic, glass, pyrogenic silica) may have an influence on discoloration. In contrast to exogenous discolorations caused by the adsorption of dyes or plaque that can be easily removed by polishing, endogenous discolorations are irreversible [4–7]. Therefore, the components of a filling resin may not only influence the curing characteristics and the strength of the materials, but may also have an impact on the color stability. Color change is another important parameter for resin-based filling materials. Several factors influence the color of photocuring materials, such as photoinitiator component, resin matrix composition, lightcuring device and irradiation times. Camphorquinone is the most commonly used photoinitiator in dental restorative resins and although it is used in small amounts, it significantly influences the color of the material. Other photoinitiators used are tertiary aromatic or aliphatic amines, which act as so-called synergists or accelerators. All amines are known to form by-products during photoreaction, which tend to cause yellow to red/brown discolorations under the influence of light or heat [8]. In 1931, the Commission Internationale de L’E´clairage (CIE) defined a standard light source, developed a standard observer and enabled the calculation of tristimulus values, which represent how the human visual system responds to a given color. The CIE defined a color space, CIE-lab, that supports the accepted theory of color perception based on three separate color receptors in the eye (red, green and blue) and is currently one of the most popular color spaces [9]. In this investigation, the FT-Raman spectroscopy technique was used to evaluate the degree of conversion of a photo-cured resin composite using a light emitting diode (LED) and color measurements were used to verify changes on the resin composite surface after immersion in different drinks for 24 and 168 h.
439
source used to light curing the specimens was Elipar free light (3 M ESPE Dental Products, Minnesota, USA). The storage environments were commonly used drinks, and distilled water was used as the control. The drinks were a sports drink (Gatorades—citric fruits), a yogurt obtained from lactobacillus (Yo-mix TM cultures), a soft drink based on cola (Coca-cola Companys) and a red wine (Cordier Le Merlot—2000). 2.2. Specimens preparation Specimens (diameter 10.070.1 mm, thickness: 1.070.1 mm) were prepared at room temperature using a stainless steel mold. The mold was filled with the test material, avoiding gross excess and entrapped air. A 0.05 mm transparent polyethylene film covered each side of the mold and a glass slide (3 mm) was placed on the top of the mold. A light emitting diode (LED) was used to irradiate (370 mW/cm2) the specimens from one side along the whole extension in constant polymerization mode for 40 s. The glass slide thickness standardized the distance from the light source to the resin composite and provided a smooth, non-air-inhibited surface for subsequent testing. The specimens were then ground with a sequence of abrasive papers (240, 400, 600 and 1200 grit). Five specimens were made for each type of drink (in total 25 specimens). The drinks were changed after every 24 h of immersion. The specimens were immersed in a drink or distilled water for 24 and 168 h and stored at 37 1C. 2.3. Raman Spectroscopy Micro-Raman spectroscopy was performed with a Renishaw Raman microscope. Laser light at 514.5 nm from an Ar laser was used for excitation. A power density of 1.1 mW/cm2 was used and calibration was performed with the Si peak at 520.7 cm1. The error in the peak positions was less than 72.0 cm1. The degree of cure and chemical groups of the resin composites were observed before and after drink immersion. 2.4. Color measurements
2. Materials and methods 2.1. Materials The resin composite used was Charisma (Heraeus Kulzer, Hanau, Germany), shade A2, and the light
For color measurements, a spectrophotometer (Konica Minolta colorimeter, Japan) equipped with an integrating sphere with a 10 mm opening was used. The spectrophotometer measurements were
ARTICLE IN PRESS B.K.M. Luiz et al. / Polymer Testing 26 (2007) 438–444
440
made using the L*a*b* system. D65 illumination and a 101 standard observation angle were selected. A white calibration was used: calibration plate no 19433091 (Konica Minolta, Japan). In the three-dimensional color space, the L* value is a measure of the lightness of an object and is quantified on a scale such that a perfect black has an L* value of 100. The a* value is a measure of redness (positive a*) or greenness (negative a*). The b* value is a measure of yellowness (positive b*) or blueness (negative b*). The a* and b* co-ordinates approach zero for neutral colors (white, grays) and increase in magnitude for more saturated or intense colors. The advantage of the CIE-lab system is that color differences can be expressed in units that can be related to visual perception and clinical significance [10]. The calculation of the color variation DE* between two color positions from values obtained before and after immersion, in the threedimensional (3-D) L*a*b* color space, is given by: DE ¼ ½ðL1 L0 Þ2 þ ða1 a0 Þ2 þ ðb1 b0 Þ2 1=2
evaluate the effect of the drinks on color change and to analyze the FT-Raman results for the composite using statistical software (SPSS for Windows, Version 13.0; SPSS Inc., Chicago, Ill). 3. Results The Raman spectrum of a cured specimen of Charisma maintained in distilled water is showed in Fig. 1, with different characteristic peaks that are related to vibrational frequency given in Table 1. The main vibrational lines are assigned to Si2O bending at 806 cm1, SiO stretching at 1200 cm1, aliphatic CO stretching at 1300 cm1, modes of CQCH2 at 1404 cm1, CQC stretching of the aromatic ring at 1446 and 1607 cm1 (skeletal vibration of the benzene ring), methacrylate CQC stretching at 1638 cm1 and CQO stretching at 1714 cm1 [11–15]. Through an evaluation of the peak corresponding to CQC stretching from methacrylate (1638 cm1) and the aromatic ring (1607 cm1), it is possible to estimate the decrease in carbon double bonds due to the cross-linking reaction (CQC from methacrylate) in the same Raman spectrum. The degree of conversion can then be calculated through
(1)
2.5. Statistical analysis One-way analysis of variance (ANOVA) followed by the Scheffe´ post hoc test (a ¼ 0.05) was used to
DCð%Þ ¼ 100 ½1 H cured =H uncured ,
1607
Intensity (a.u.)
1446 1638 1404 1200
806
400
600
800
1000
1714
1300
1200
1400
1600
1800
2000
2200
Raman shift (cm-1) Fig. 1. Raman spectra of the Charisma composite after being kept in distilled water.
(2)
ARTICLE IN PRESS B.K.M. Luiz et al. / Polymer Testing 26 (2007) 438–444 Table 1 Vibrational line assignments Frequency (cm1)
Peak assignments
806 1200 1300 1404 1446 1457, 1513, 1558 1607 1638 1714
Si–O–Si bending Si–O stretching aliphatic C–O stretching C ¼ CH2 bending C ¼ C of aromatic group vibrations of the benzene ring C ¼ C aromatic stretching C ¼ C aliphatic stretching C ¼ O free stretching
where Hcured and Huncured correspond to the peak height ratios at 1638 and 1607 cm1 for cured and uncured resin, respectively. Using these peak height ratios for uncured resin and for specimens after irradiation for 40 s using LED, a degree of conversion of 70% was obtained, in agreement with values reported in the literature for studies using different light sources including LED [1,2, 12,16–18]. For the cured resin, obtaining several successive spectra showed a constant degree of cure, indicating no influence of the laser beam. No evidence of a change in the degree of cure was observed at different specimen depths. Table 2 shows the average of eight measurements for the peak ratio of the spectra, from the surface to the inner part of the specimen, in steps of 5 mm. The peak ratios for spectra did not show a statistically significant difference within the confidence interval (a ¼ 0.05). The same results were obtained for specimens immersed in yogurt and the soft drink. Table 3 shows the color parameters (L*, a* and b*) for specimens before and after immersion in different drinks and water as the control, for 24 and 168 h. Significant changes in the color parameters were observed for the specimens, for example, the wine change a* from 0.28 to 1.78 and 2.14 after 24 and 168 h, respectively. By applying Eq. (1), the DE* value was calculated, as given in Table 4 along with the standard deviation and the dissimilarity of groups obtained through statistical analysis. 4. Discussion Although dental composite resins have been widely used as filling materials, they have a major drawback, which is their incomplete polymeriza-
441
Table 2 Band ratios related to C ¼ C aromatic stretching (1446 and 1607 cm1) and C ¼ C aliphatic stretching C ¼ C (1638 cm1) for Charisma composite specimens maintained in distilled water, yogurt or soft drink for 168 h Specimens
Peak ratio h1607 /h1638
h1607 /h1446
Control Yogurt Soft drink
Average
SD
Average
SD
1.29 1.38 1.29
0.04 0.05 0.05
2.50 2.33 2.59
0.10 0.03 0.02
tion. This can be detected using several methods, including the measurement of the degree of conversion (DC), which relates to the percentage of double bonds reduced during the polymerization process. Incomplete polymerization results in unreacted monomers, which leach from the material in a wet environment [13,19]. Problems associated with inadequate polymerization include poor physical properties, increased solubility in the oral environment, increased microleakage and consequent color changes. Vibration methods allow a precise assessment of the depth of polymerization and DC of methacrylate composites resins [14,17]. The use of Raman techniques to determine the percent conversion requires that the amount of double bonds, which are present after curing, can be quantified. In bisGMA/TEGDMA this measurement is carried out on a relative basis by comparing the height (or intensity) of the vibrational CQC band at 1638 cm1 to the CQC aromatic ring stretching at 1609 cm1, whose height (or intensity) does not depend on the polymerization process [15–18]. The 70% DC evaluated by Raman spectroscopy, with LED irradiation for 40 s, is of the same order of magnitude as similar resins described in the literature. The cure degree of dimethacrylate-type and related resins were in the range of 50%–60% under a halogen beam [16–18, 20]. The same DC being obtained in the inner part of the specimen, and the high DC using LED as the cure process initiator, indicates the better efficiency of this light source. The hydrophilic nature of a polymer is in large part a function of the chemistry of its monomers and its polymerization linkages. The structure of polymers used in dental composites reveals the presence of hydrolytically susceptible ester groups. The presence of hydroxyl, carboxyl
ARTICLE IN PRESS B.K.M. Luiz et al. / Polymer Testing 26 (2007) 438–444
442
Table 3 Color parameters for Charisma resin stored in distilled water, soft drink, sports drink, wine or yogurt: L*, a* and b* Color parameters
Immersion time (h)
Control
L*
0 24 168 0 24 168 0 24 168
74.7 74.7 74.7 0.28 0.28 0.28 14.5 14.5 14.5
a*
b*
Table 4 Color changes (DE*) for Charisma resin stored in distilled water, soft drink, sports drink, wine or yogurt Immersion time (h)
24 168
Color change (DE*) Control
Soft drink
Sports drink
Wine
0 (0) a 0 (0) a
1.6 (0.1) b 0.8 (0.2) a 4 (0.1) c 1.4 (0.3) b 0.6 (0.3) a 10.6 (0.5) c
Yogurt
1 (0.2) b 1.4 (0.9) b
* Standard deviations are in parentheses. ** Different letters indicate dissimilarity of groups (Po0.05).
and phosphate groups in monomers and their resultant polymers make them more hydrophilic and, supposedly, more prone to water sorption [21]. The lack of change in the height intensity ratio (h1607/h1638) of the vibration line corresponding to CQC stretching from methacrylate (1638 cm1) and the aromatic ring (1607 cm1) for the specimens after immersion in the drinks, indicates that a chemical reaction did not occur between the drink and composite components. There was no significant statistical difference between the ratios of these absorption bands for specimens after immersion in the drinks, compared with the control group. However, since a change in the color properties did occur, it could be that there was some physical adsorption of specific drink components on the composite surface. Due to this hydrophilic behavior of the polymeric matrix, the penetration of drink components into the composite was facilitated by the action of water. Initially, the presence of water softens the polymer by swelling the network and reducing the frictional forces between the polymer chains [22]. After the relaxation process, unreacted monomers trapped in the polymer network are released to the
Soft drink
Sports drink
Wine
Yogurt
74.8 75.0
75.2 75.2
72.1 66.6
75.6 74.8
0.23 0.50
0.03 0.3
1.78 2.14
0.10 0.07
15.8 16.3
15.0 15.1
14.8 20.8
15.0 15.0
surrounding environment at a rate that is controlled by the swelling and relaxation capacities of the polymer. More hydrophilic polymer networks, which have a superior relaxation capacity, permit a faster release of unreacted monomers through nanovoids in the material, showing a decrease in mass within a short time of water immersion. This is somewhat dependent on the DC and the quantity of pendant molecules existing within the network [23–25]. It is extremely difficult to measure whether the appearance of restorative materials mimic natural teeth. In the ideal case, the resin-based composite has to show the same reflectance, translucence and transparency as the tooth structure. The samples were stored for 24 and 168 h in drinks and distilled water at 37 1C to simulate the mouth environment. The composite color changes occurred mainly in specimens maintained in wine, the soft drink and yogurt. Although the citric fruit flavored sports drink has an orange color, this drink did not change the composite color, probably because the components were not adsorbed on the specimen surface. The resistance to staining of the resin composite materials is related to polymerization type, filler particles of the composite materials, and type of staining agent. The considerations relating to these statements are discussed below. Discoloration can be evaluated with various instruments. Since instrumental measurements eliminate the subjective interpretation of visual color comparison, spectrophotometers and colorimeters have been used to measure color change in dental materials. The CIELab system for measuring chromaticity was chosen to record color differences because it is well suited for the determination of small color differences [26–28].
ARTICLE IN PRESS B.K.M. Luiz et al. / Polymer Testing 26 (2007) 438–444
Extrinsic factors for discoloration include staining by adsorption or absorption of colorants as a result of contamination from exogenous sources. Extrinsic factors for discoloration are known to cause staining of oral tissues and restorations, especially in combination with dietary factors, including coffee, tea, and nicotine [29]. In a previous study [30], red wine, tea, coffee, and mouth rinses, were used as staining agents to evaluate the stain resistance of composite materials, and it was found that red wine caused the most severe discoloration. In another study [31], the highest color difference for all the composite materials investigated was also observed for red wine. Based on changes in color (DE) it is possible to compare the values before and after storage in drinks. Because the ability of the human eye to appreciate differences in color differs from individual to individual (as it is a combination of eye characteristics and skill of the operator), three different intervals were used for distinguishing color differences. Values of DEo1 were regarded as not appreciable by the human eye. Values 1oDEo3.3 were considered appreciable by skilled operators, but considered clinically acceptable, whilst values of DE43.3 were considered appreciable also by nonskilled persons and for this reason clinically not acceptable [29,32,33]. DE values for the light-polymerized composite Charisma were significantly higher after immersion in red wine (DE4 3.3), and for yogurt and soft drink they would be considered appreciable by skilled operators, but considered clinically acceptable (1oDEo3.3). For composite specimens immersed in the sports drink, DE values indicated that the color changes were not appreciable by the human eye (Eo1). The discoloration might be due to both absorption and surface adsorption of colorants. Fine colorant particles may be deposited in pitted areas of the light-polymerized material. Large filler particles present on the surface will produce high surface roughness values [34]. Through statistical analysis of variance, followed by the Scheffe´ post hoc test, the average DE values obtained for specimens maintained in wine for 24 and 168 h differ significantly from results obtained for the other drinks. 5. Conclusions From the Raman spectroscopy data it could be determined that the degree of conversion was 70%
443
at the surface and in the inner part of specimens, indicating the efficiency of LED as a cure process initiator. No chemical reaction between the drinks and composite components was observed, indicating that the change in color parameters may be due to surface adsorption and/or absorption of drink components. DE values for light-polymerized composite Charisma were significantly higher after immersion in red wine (DE43.3), and for yogurt and the soft drink were considered observable by skilled operators, but considered clinically acceptable (1oDEo3.3). For composite specimens immersed in the sports drink, DE values were not noticeable to the human eye (Eo1). Acknowledgment This study was supported by CNPq (Conselho Nacional de Desenvolvimento Cientı´ fico e Tecnolo´gico). References [1] T.H. Yoon, Y.K. Lee, B.S. Lim, C.W. Kim, Degree of polymerization of resin composites by different light sources, J. Oral Rehabil. 29 (2002) 1165–1173. [2] Z. Tarle, A. Meninga, A. Knezevick, J. Sutalo, M. Ristic, G. Pichler, Composite conversion and temperature rise using a convencional, plasma arc and an experimental blue LED curing unit, J. Oral Rehabil. 29 (2002) 662–667. [3] D. Dietschi, G. Campanille, J. Holz, J.M. Meyer, Comparison of the color stability of ten new-generation composites: an in vitro study, Dental Mater. 10 (1994) 353–362. [4] W. Buchalla, T. Attin, R. Hilgers, E. Hellwig, The effect of water storage and light exposure on the color and translucency of a hybrid a microfilled composite, J. Prosthet. Dent. 87 (2002) 264–270. [5] C. Ameye, P. Lambrechts, G. Vanherle, Conventional and microfilled composites resins: color stability and marginal adaptation, J. Prosthet. Dent. 46 (1981) 623–630. [6] J.J. Ten Bosch, J.C. Coops, Tooth color and reflectance as related to light scattering and enamel hardness, J. Dental Res. 74 (1995) 374–380. [7] A. Watts, M. Addy, Tooth discolouration and staining: a review of the literature, Br. Dent. J. 190 (2001) 309–316. [8] R. Janta, J.F. Roulet, M. Kaminsky, G. Steffin, M. Latta, Color stability of resin matrix restorative materials as function of the method of light activation, Eur. J. Oral Sci. 112 (2004) 280–285. [9] D.B. MacDogall, Colour measurement of food: principles and practice, in: Colour in Food, Woodhead Publishing Limited, Cambridge, 2002. [10] W.J. O’brien, H. Hemmendinger, K.M. Boenke, J.B. Linger, C.L. Groh, Color distribution of three regions of extracted human teeth, Dent. Mater. 13 (1997) 179–185. [11] S. Kammer, K. Albinsky, B. Sandner, S. Wartewig, Polymerization of hydroxyalkyl methacrylates characterized
ARTICLE IN PRESS 444
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19] [20]
[21]
[22]
B.K.M. Luiz et al. / Polymer Testing 26 (2007) 438–444 by combination of FT-Raman and step-scan FT-I.R. photoacoustic spectroscopy,, Polymer 40 (1999) 1131–1137. J. Xu, I.S. Butler, D.F.R. Gibson, I. Sangel I, High-pressure infrared and FT-Raman investigation of a dental composite, Biomaterials 18 (24) (1997) 1653–1657. R.L. Ore´fice, J.A.C. Discacciati, A.D. Neves, H.S. Mansur, W.C. Jansen, In situ evaluation of the polymerization kinetics and corresponding evolution of the mechanical properties of dental composites, Polym. Test. 22 (2003) 77–81. J.W. Stansbury, S.H. Dickens, Determination of double bond conversion in dental resins by near infrared spectroscopy, Dent. Mater. 17 (2001) 71–79. B. Sandner, S. Kammer, S. Wartewig, Crosslinking copolymerization of epoxy methacrylates as studied by Fourier transform Raman spectroscopy, Polymer 37 (21) (1996) 4705–4712. G. Leloup, P.E. Holvoet, S. Bebelman, J. Devaux, Raman scattering determination of the depth of cure of lightactivated composites: influence of different clinically relevant parameters, J. Oral Rehabil. 29 (6) (2002) 510–515. C. Pianelli, J. Devaux, S. Bebelman, G. Leloup, The microRaman spectroscopy, a useful tool to determine the degree of conversion of light-activated composite resins, J. Biomed. Mater. Res. 48 (1999) 675–681. N. Silikas, G. Eliades, D.C. Watts, Light intensity effects on resin-composite degree of conversion and shrinkage strain, Dent. Mater. 16 (4) (2000) 292–296. A.U.J. Yap, H.K. Lee, R. Sabapathy, Release of methacrylic acid from dental composite, Dent. Mater. 16 (2000) 172–179. W.S. Shin, X.F. Li, B. Schwartz, S.L. Wunder, G.R. Baran, Determination of the degree of cure of dental resins using Raman and FT-Raman spectroscopy, Dent. Mater. 9 (5) (1993) 317–324. J.P. Santerre, L. Shajii, B.W. Leung, Relation of dental composite formulations to their degradation and the release of hydrolyzed polymeric-resin-derived products, Crit. Rev. Oral Biol. Med. 12 (2001) 136–151. J.L. Ferracane, X.H. Berge, J.R. Condor, In vitro aging of dental composites in water-effect of degree of conversion,
[23]
[24]
[25]
[26]
[27]
[28]
[29] [30]
[31]
[32]
[33] [34]
filler volume, and filler/matrix coupling, J. Biomed. Mater. Res. 42 (1998) 465–472. C.S. Brazel, N.A. Peppas, Mechanisms of solute and drug transport in relaxing, awellable hydrophilic glassy polymers, Polymer 40 (1999) 3383–3398. C.S. Brazel, N.A. Peppas, Dimensionless analysis of swelling of hydrophilic glassy polymers with subsequent drug release from relaxing structures, Biomaterials 20 (1999) 721–732. I. Sideridou, V. Tserki, G. Papanastasiou, Study of water sorption, solubility and modulus of elasticity of light-cured dimethacrylate-based dental resins, Biomaterials 24 (2003) 655–665. N. Satou, A.M. Khan, I. Matsumae, J. Satou, H. Shintani, In vitro color change of composite-based resins, Dent. Mater. 5 (1989) 384–387. R.R. Seghi, M.D. Gritz, J. Kim, Colorimetric changes in composites resulting from visible-light-initiated polymerization, Dent. Mater. 6 (1990) 133–137. S.R. Okubo, A. Kanawati, M.W. Richards, S. Childress, Evaluation of visual and instrument shade matching, J. .Prosthet. Dent. 80 (1998) 642–648. C.M. Um, I.E. Ruyter, Staining of resin-based veneering materials with cofee and tea, Quint. Int. 22 (1991) 377–386. T. Stober, H. Gilde, P. Lenz, Color stability of highly filled composite resin materials for facing, Dent. Mater. 17 (2001) 87–94. A.U. Guler, F. Yilmaz, T. Kulunk, E. Guler, S. Kurt, Effects of different drinks on stainability of resin composite provisional restorative materials, J. .Prosthet. Dent. 94 (2) (2005) 118–124. S. Inokoshi, M.F. Burrow, M. Kataumi, T. Yamada, T. Takatsu, Opacity and color changes of tooth-colored restorative materials, Oper. Dent. (1996) 73–80. H.S. Kim, C.M. Um, Color differences between resin composites and shade guides, Quint. Int. 27 (1996) 559–567. A. Maalhagh-Fard, W.C. Wagner, F.E. Pink, A.M. Neme, Evaluation of surface finish and polish of eight provisional restorative materials using acrylic bur and abrasive disc with and without pumice, Oper. Dent. 28 (2003) 734–739.