Dental Research

Staining of resin-based veneering materials with coffee and teaChung Moon Um* / 1 , Eystein Ruyter**

Two light-activated, and three heat-polymerized, resin-based veneering materials wereexposed to boiled coffee, filtered coffee, or tea al 50 "C and evaluated for color stabili-ty. Specimens immersed in distilled water in the dark at 37 °C for 4 months were alsoassessed. One of the light-activated, resin-based veneering materials underwent intrinsicdiscoloration during the long-term immersion both in distilled water and in the stainingsolutions. The discoloration of the other materials by tea was mainly due to surfaceadsorption ofthe colorants. Discoloration by coffee was due to adsorption, and also toabsorption of colorants by two of the materials investigated. This absorption and pene-tration of colorants into the organic phase of the veneering materials were probablydue to compatibility of the polymer phase with the yellow colorants of coffee.(Quintessence Int 1991:22:377-386.)

Introduction

Tooth-colored, resin-based composite resins have beenused as dental restorafive materials to meet estheticrequirements. Light-polymerized, resin-based mate-rials have been introduced as veneering materials forcrown and bridgework as alternatives to porcelain.Tooth-colored composite resins and resin-based ve-tieering materials have been unable to retain the colorthey possess at the time of insertion: They lack colorstability.' Leinfelder et aP reported on the clinical eval-uation of composite resin restorative materials. Bothclinical evaluations and laboratory studies of colorstability have been completed,'

Discolorafion of tooth-colored, resin-based mate-rials may be caused by intrinsic and extrinsic factors.The intrinsic factors involve the discoloration of theresin material itself, such as the alteration of the resin

• Professor, Department of Conservative Dentistry, Seoui Na-tionai University, College of Dentistry, 28-2 Yunkun-dong,Chongro-ku, Seoul tiO-744, Korea,

" NIOM—Scandinavian Institute of Dental Materials, Kirkeveien71B, N-1344 Haslum, Norway,

Address all correspondence to I, Eystein Ruyter,

matrix and of the interface of matrix and fillers. Thecause of chemical discoloration has been attributed toa change or oxidation of the amine accelerator,''"' ox-idation in the structure of the polymer matrix," andoxidation of unreaeted pendant methacrylate groups,'The color of composite resins may change after ex-posure to various energy sources'" and immersion inwater for a long period,"'^

Extrinsic factors for discoloration include stainingby adsorption or absorption of colorants as a resultof contamination from exogenous sources. The stain-ing of polymeric materials hy colored solutions,'^ cof-fee and tea,' nicotine,"' and beverages''''^ has beenreported. The extent of discoloration in the oral cavitymay be associated with dietary habits.

The purpose of the present study was to evaluatethe in vitro color stability of several filled and unfilledresin-based veneering materials after exposure toboiled coffee, filtered coffee, and tea in an acceleratedtest. Water absorption and chemical characterizationof the veneering materials, as well as characterizationof the staining solutions, were also assessed.

Method and materials

The two light-activated, resin-based materials and thethree heat-polymerized materials used in this study are

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Table i Heat- and ligbt-polymerized crown and bridge materials investigated

Material

Vi tapanMonopast

Isosit

Shade and batch No.

Enamel material Al1763

Incisa! material S3831074

Biodent Enamel material S13K -f- B Plus Powder: 34/1;

Liquid: 172

Visio-gem

Dentacolor

Color system V, E70P238

Incisai material SA 1046

Code

VI

IS

BI

VG

DC

Manufacturer

Vita Zahnfabrik

Ivoclar AG

DeTrey Dentspiy

ESPE GmbH

Kulzer Inc

Polymerizationmode

Heat, glycerol(lOO^C, 6bar)

Heat, water(120 °C, 6 bar)

Heat, water(95 °C, 6 bar)

Light

Light

Light source

-

-

Espe Visio AlfaEspe Visio Betawith vacuumpump

Dentacolor XS

Curingtime (sec)

900

420

1200

78900

360

Table 2 Coffee and tea brand used

Tea

Filteredcoffee

Coffee forboiling

Earl Grey

DollarFiltermalt

DollarKokmalt

Batch No.

6199X

240290

110190

Distributor

Tower

Samas ArnetAamodt A/S

Samas ArnetAamodt A/S

presented in Table 1, and the Fdter coffee, coffee forboiling, and tea used for immersion solutions areshown in Table 2.

Color stability

Specimen preparation. Nine sample disks of each ma-terial, with a diameter of 35 mm and a thickness ofapproximately 1.4 mm, were prepared in a metal mold.The materials were handled according to each man-ufacturer's instructions. Care was taken to avoid po-rosities by entrapment of air bubbles. The three heat-polymerized materials were cured in Ivomat IP3 (Ivo-clar AG) under the conditions given in Table 1. Thecuring temperatures were controlled with copper-con-

stantan thermocouples with reference junctions at0 ^C (ice water). Visio-gem was first polymerized hyexposure to the light of a Visio-alfa unit, with thespecimen kept directly in the beam at the distance of5 mm, and then polymerized in a Visio-beta unit con-taining a vacuum pump. To minimize porosities, spec-imens of Dentacolor were prepared by a special tech-nique using a wooden roller for filling the mold. Po-lymerization was carried out in a Dentacolor XS unit.All specimens were ground with silicon carbide paperon both sides to a thickness of 1.30 + 0.02 mm.

Preparation of staining solutions. To prepare theboiled coffee solution, 60 g of coffee was poured into1 L of boiling distilled water and simmered for 10minutes. This solution was flltered through a Filterpaper. To prepare the filtered coffee solution, 60 g ofcoffee was put on the fllter paper and 1 L of boileddistilled water passed through the coffee. This solutionwas flltered again through two of the same filter pa-pers and distilled water was added to make 1 L. Toprepare the tea solution, 10 g of tea was poured into1 L of boiled distilled water and brewed for 5 minutes.This solution was flltered through a fllter paper.

Methods of staining. The test specimens were storedin the coffee and tea solutions, which were kept in theincubator at 50 ± 1 "C for 0.5, 1, 2, 3, 6, 9 12 18 24,36, 48, and 1,000 hours.

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Following removal from the staining solutions, thesamples were dipped in distilled water and moved upand down ten times. This procedure was repeated us-inga fresh portion of distilled water. The samples werethen dipped in a soap solution, consisting of 10 mLsoap (Mucaderma, Merz + Co) and 700 mL distilledwater, moved up and down ten times, and subsequent-ly thoroughly rinsed in distilled water. After colormeasurements at the time intervals indicated, the spec-imens were reimmersed in the coffee and tea solutions.The 1,000-hour specimens received an additional treat-ment before color measurement.

Methods of cleansing. The following steps were per-formed to remove the discoloration on nine 48-hoursamples in a separate series.

Method 1: soap wash treatment. The samples wereimmersed in soap solution for 5 minutes and bothsides of the samples were then rubbed with tissue pa-per for 1 minute and subsequently rinsed in distilledwater

Method 2: brush and soap treatment. Both sides ofthe samples were brushed 40 strokes, in two directionsat a 90-degree angle, with a toothbrush (CompactSoft, Jordan A/S; stiffness according to ISO 8627)"moistened with the soap solution, and rinsed with dis-tilled water.

Method 3: brush and toothpaste treatment. Both sidesof the samples were brushed with a 20-mm length oftoothpaste (Colgate Fluor, Colgate-Palmolive) usingthe same method as described for the brush and soaptreatment. Afterward the specimens were rinsed withdistilled water.

l.QOO-hour specimens. Three test specimens of eachbrand were stored in filtered and boiled coffee solu-tions, as well as in a tea solution, at 50 ± 1 °C for1,000-hours, After removal from the .staining solu-tions, the specimens were rinsed in distilled water andcleansed as described earher by soap washing, brushand soap treatment, or brush and toothpaste treat-ment- Both sides of the circular specimens wereground with silicon carbide paper for 60 seconds eachusing finger tip pressure. The thickness was deter-mined with a micrometer (Digitmaster, 0-25 mm,0,002 mm. Tesa S-A,) before and after the grinding.Color measurements were made after the cleansingprocedure and also after the grinding- The color char-acteristics were compared with those of the freshlyprepared specimens.

Color measurements. Color measurements weremade using refiectance spectrophotometry'"" afterthe specimens exposed to tea and the coffee solutions

had been rinsed with distilled water, after the cleansingprocedures, and after grinding. The color character-istics (tristimulus values X, Y, Z, and CIELAB) ofthree specimens of each brand of veneering materiaiswere evaluated.

The color characteristics and the changes in the col-or characteristics (L*, a*, b* and AE*ji,) that re.sultedfrom exposure to the staining solutions were measuredby a computer-controlled spectrometer designed fordiffuse/0" reflectance. This system is based on an ul-tra viol et-visible light single-beam spectrophotometer(Model PMQ3, Carl Zeiss) with one integrating sphererefiectance attachment with gloss trap and with aviewing port of 30-mm diameter (RA3, Carl Zeiss),The spectrophotometer system was connected to acomputer system (Apple II, Apple Computer Inc).

Refiectance values versus wavelengths were ob-tained for each specimen at 10-nm intervals between380 and 720 nm. Each specimen was evaluated in thesample port backed by a white standard.

The CIELAB measurements make it possible toevaluate the amount of perceptible color change ineach sample. The CIELAB is an approximately uni-form color space with coordinates for lightness; ie,white-black (L*), redness-greeness (a*), and yellow-ness-blueness (b*)-''

The color systems with rectangular coordinates arequantitative systems, and they have a meaningful re-lation to human visual perception of color differences.Total color differences are expressed by the formula

A E%b = [(AL*) + (Aa*) + (Ab*)^]''-where âL*. Aa'^, and Âb* are differences in the re-spective L*, a*, and b* values.

Chemical characterization

Fdler content. The content of inorganic fillers was de-termined by burning the sample disks at 575 + 25 °Cfor 30 minutes, in accordance with the internationalstandard for resin-based filling materials, ISO 4049. 'Total filler content, ie, the inorganic part as well asthe polymeric organic material insoluble in tetrahy-drofuran (THF), was determined by removing the sol-uble organic fraction. - The unpolymerized paste ofVI was suspended in THF for 2 hours followed bycentrifugation at 5,000 g for 20 minutes (RC-5 Au-tomatic Superspeed Refrigerated Centrifuge, Dupont/Sorvall) and decanted. The insoluble filler particleswere washed three times with THF and subsequentlydried at 60 °C for 20 hours. The total filler content

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Table 3 Reverse-phase HPLC, Chromatographieconditions

Column 250 mm x 4.6 mm ID, SupelcosilLC-18

Mobile phase Pump A: H2OPump B: CH3OHIsocratic elution: 10% CHjOH

Flow rate 0.8 mL/min

Detector UV, 440 nm

Table 4 Adsorption HPLC, Chromatographie con-ditions

Column 250 mm x 4.6 mm ID, SupelcosilLC-Si

Mobile phase Pump A: CH^Clj; 0.2% HjOPump B: CH^Clj/CHjOH 94/6;0.2 % HiOGradient elution: 50% B, 5-minisocratic50% to 99% B, 5-min hneargradient

Flow rate 0.8 mL/min

Detector UV, 225 nm

was then determined by weight (Model AE240, Mett-ler Instruments AG).

Water sorption. Five sample disk.s of each brandwere prepared as previously described. The disks wereconditioned to constant weight in a desiccator untilthe mass loss of each specimen was less than 0.2 mgin any 24-hour period "* and subsequently immersed indistilled water at 37 °C in the dark. The water waschanged once a day for the first 7 days. Thereafter,the water was changed once a week. The weight in-crease was measured until constant weight, ie, increaseless than 0.2 mg. was obtained. The weight determi-nations were made after 1, 2, 3, 4, 7, 14, 28, 60, 90,120, and 150 days. After constant weight was obtained,the sample disks were reconditioned to constantweigh! in a desiccator.

The color characteristics of the specimens were de-termined before water storage and again after 120days' storage in water.

High-performance liquid chromatography analysis.The coffee and tea solutions were characterized by

reverse-phase high-performance hquid chromatogra-phy (HPLC) analysis. To correspond with the HPLCsolvent system, methanol was added with 10% by vol-ume to the coffee and tea solutions. The insolubleparticles were removed by cen tri fuga tion at 5,000 gfor 20 minutes with the refrigerated centrifuge. Thehquid chromatography system used comprised twopumps and a controller (Model 2150 and 2152, LKB-Produkter AB). The system was equipped with a sy-ringe-loading sample injector (Model 7125, RheodyneInc) with a 20-|aL sample loop. For detection purposes,a variable wavelength spectrophotometer (ModeiSF770 Spectroflow, Schoeffel Instrument Corp) witha tungsten lamp for the visible light region was ap-plied. The Chromatographie conditions are outhned inTable 3.

The monomer system of the material VI was char-acterized by adsorption HPLC using a diode arraydetection system (Model 2140 Rapid Spectral Detec-tor, LKB-Produkter AB) (Table 4). Reverse-phaseHPLC with 80% methanol and 20% water with iso-cratic conditions and the UV detector at 202 nm wasused for quantitative determination of l,6-bis(tne-thacryloyloxy-2-ethoxycarbonylamino)-2,4,4-trime-thyl hexane (UEDMA).-'

Results

Color stability

After 48 hours' immersion, tea produced the greatestcolor changes of all solutions, and filtered coffee pro-duccd more marked color changes than did boiledcoffee (Figs 1 to 3). In filtered and boiled coffee, thespecimens of IS and BI showed better color stabilitytban did the other materials, whereas VI and VGshowed poor color stability. Figures 1 to 3 present thetotal color differences (AE*ab)- As the immersion timeswere prolonged, the AE* ^ value increased graduallyin filtered coffee and boiled coffee and, during the first24 hours, more rapidly in tea. In tea, the specimensof ail brands displayed total color difïcrence values(AE*ji,) of more than 22 after 48 hours' immersion.The difference between the highest (VI) and the lowest(VG) AE*ab value was approximately 8. After exposureto filtered coffee and boiled coffee, the discoloredspecimens appeai cd yellowish, whereas the specimensexposed to tea appeared yellow-brown.

The corresponding values after washing with soap,brushing with soap solution, and brushing with tooth-paste to remove discoloration on the surfaces of the

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Fig 1 Total color diflerence {AE',;,) as a function of timeof immersion in a tea solution at 50 ± 1 °Cfor the veneeringmaterials VI, VG, DC, IS, and Bl. Ttie uniform standarddeviation of A E ' „ averaged ± 5% tor all measurements.

!.' Rg3 Total color difference ( A E ' J as a function of time' of immersion in a boiled coffee solution at 50 ± 1 °C for'- the veneering materials VI, VG, DC, IS, and Bl. The uniformn standard deviation of AE*,» averaged ± 7% for all meas-,- urements.

Fig 2 Total color difference (AE*^,) as a function of limeof immersion in a filtered ccffee solution at 50 ± 1 °C forthe veneering materials VI, VG, DC, IS, and Bl. The uniformstandard deviation of AE",^ averaged ± 8% for all meas-urements.

Fig 4 The total color differences (AE",,,,) and standard de-viations (verticai iines) of veneering materials exposed tofiltered coffee at 50 i 1 °C for 48 hours after cleansingtreatments {1} soap wash, (2) soap and brushing, and (3)toothpaste and brushing.

sample disks are presented in Figs 4 fo 6, Discolora-tion of specimens from contact with tea was moreeasily removed by brush and toothpaste than was dis-coloration from filtered and boiled coffee. The effectof the cleansing procedures was seen as a reductionof the AE*3|, value. There was a gradual reduction ofthe AE*ab values for the specimens treated with filteredcoffee and boiled coffee with progressing cleansingttïatments (Figs 4 and 5). The decrease of the AE*,bvalues was more pronounced for the specimens thathad been exposed to the tea solution (Fig 6), After

cleansing, the specimens of VI treated with the filteredcoffee showed the highest AE*ab values, followed byVG, DC, Bl, and IS. The boiled coffee-treated spec-imens had after-cleansing results similar to those offiltered coffee-treated specimens.

After the cleansing procedures, the samples of Bl,DC, and IS showed discoloration with AE*,h valuesless than 3.0, To allow possible penetration of color-ants, the specimens were stored for 1,000 hours in thestaining solutions. After storage in the staining solu-tions for 1,000 hours at 50 ± 1 °C, the crown and

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Fig 5 The mean color differences (AE'^) and standarddeviations of veneering materials exposed to boiled coffeeat 50 ± 1 'C for 48 hours affer cleansing treatments (1)soap wash, (2) soap and brushing, and (3) toothpaste andbrushing.

Fig 6 The mean color differences (AE\i,) and standarddeviations of veneering materials exposed to tea at 50 ±t °C for 48 hours affer cieansmg treatments (1) soap wash,(2) soap and brushing, and (3) toothpaste and brushing.

Fig 7 The mean coior differences (¿E' ,) and standarddeviations after cleansing treatments (1) soap wash, (2)soap and brushing, and (3) toothpaste and brushing, ofveneering materials exposed for 1,000 hours at a temper-ature of 50 it 1 C to the staining solutions (A) boiled coflee,(B) filtered coffee, and (C) tea.

bridge materials showed different degrees of discol-oration after the cleansing and brushing procedures(Fig 7), After the cleansing procedures that includedbrushing with a dentrifice, only one material (BI)showed an acceptable discoloration. All other mate-rials showed unacceptable discoloraUon at differentlevels. The discoloration varied for the different ma-terials and was also dependent on the staining solu-tion, except for the material DC.

After the surfaces of the specimens were groundwith silicon carbide papers, the thickness of the spec-imens was reduced (Table 5); ie, surface layers withthicknesses of 15 to 35 jam were removed. Despite re-

movai of these surface layers, specimens of VG, DC,and VI that had been stored in both types of coffeeat 50 + 1 °C, continued to show more than a slightchange in color (Fig 8). Only DC continued to showa substantial change in color after storage in tea andremoval ofthe surface layer. The discoloration of thesematerials appeared as an increase in yellowness (Table6).

Chemical characterization

Eiller content. The quantity of inorganic fillers is pre-sented in Table 7. The crown and bridge material BI

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Table 5 Thickness reduction (jam) after grinding ofspecimens stored for 1,000 hours in the staining so-lutions

Boiled colTee Filtered coffee TeaMaterial (mean + SD) (mean + SD) (mean + SD)

VGDCVIISBI

4 1 + 235+ 651 + 1340+ 171+26

49 + 1257+ 138+ 636+ 341+ 1

60+ 641 + 1129 + 1434+1541 + 19

Table 6 Increase in yellowness (Ab* units) aflergrinding of specimens stored for 1,000 honrs in thestaining solutions

Boiled coffee Filtered coffee TeaMaterial (mean+SD) (mean + SD) (mean + SD)

8.20 + 0.90 0.08 + 0.047.81 + 0.17 6.39 + 0.626.02+1.30 2.20 + 0.20

VGDCVI

9.96+1.107.93 + 0.434.79 + 0.14

Fig 8 The mean color differences (AE'^J and standarddeviations after cleansing treatments (1) soap wash, (2)soap and brushing, and (3) toothpaste and brushing, andgrinding of veneering materials exposed for 1,000 hours tothe staining solutions (A) boiled coffee, (B) filtered eoffee,and (C)\ea.

Table 7 Inorganic filler content and organic phase(wt%) in the materials investigated

Material

VIISBIVGDC

Filler content

19.1630.880.18

28.4851.84

Organic phase

80.8469.1299.8371.5248.16

contained only minor quantities of inorganic particlesas pigments, whereas the other materials investigatedhad different quantities of inorganic filler particles.The material VI, which had not been previously char-acterized,'^ contained 44.62 + 0.20 wt% prepoiymer-ized particles with silica microllllers.Water absorption. The water absorption (A) and thewater solubility (S) after 120 days for VI, IS, and VGas well as after 150 days for BI and DC were calculatedby the equations

Tables Water absorption and solubihty (jig/mm^)after 120 days (VI, IS, and VG) and 150 days (BI andDC) in water at 37 + I °C

and

Material

VIISBIVGDC

Water absorption

53.24 + 0.3017.17 + 0.2827.60 + 0.8020.18 + 0.2616.78 + 0.22

Water solubility

0.69 + 0.13<LQ<LQ

0.75±0.13<LQ

<LQ = below limit of quantification.

where ma is the conditioned mass prior to immersionin water; m, is the mass of the specimen after 120 or150 days; nu is the reconditioned mass; and V is thevolume of the spec i m en.' •-

The results for water absorption and water solubil-ity are presented in Table 8. Low solubilities for allflve materials were observed.

The variation in water uptake with time is presented

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t

ao

60

40

CR

EA

SW

EIO

HT

IN

20

10

i

25 SO

I *

76 100TIME, d

— ^ •

1

12S

l/l

• DO/B(VGIS

150

Fig 9 Weight increase due to water uptake with time perunit mass ot organic phase lor the materials VI, DC, Bl, VG,and IS, Uniform standard deviations averaged ± 5.9% be-low 7 days and ± 0,5% above 7 days tor VI; d- 7,3% below14 days and ± 1.3% above 14 days tor DC; i. t,9% forthe whcle period for Bl; ± 2,4% below 3 days and ± 1.2%above 3 days for VG, and ± 3,0% below 3 days and ±t.2% above 3 days for IS,

—H0

A

Ji

V4 6Time ,

à ibTlin

Fig 10 Reverse-phase HPLC chromatograms ol the solu-ble components with absorption at 440 nm of (A) boiletjcoffee and (B) tea.

in Fig 9, The weight increase is calculated per unitmass of organic matrix by the equation

where my is the conditioned mass prior to immersionin water; m, is mass of specimen at each measurementduring water uptake; and /«[ is the mass of inorganicfillers calculated from Table 7,

The variation in water uptake is characterized by arelatively rapid weight increase within the first 14 days.The water uptake then leveled off until equilibriumwas reached. The materials attained equilibrium after4 to 5 months. After storage in distilled water at 37+ 1 "'C for 120 days, the specimens of all materialsshowed only a slight change in color, except for DC,which showed a total color difference value of AE*ai,= 4.89 + 0,64, This change in color was an increasein yellowness. Ab* = 4-86 ± 0,64,

HPLC analysis. The chromatograms of coffee andtea showed longer retention times for the componentsin coffee with absorption in the blue region (440 nm)than for the components in tea with absorption at 440nm (Fig 10), The reverse-phase HPLC analyses andUV-spectra also showed that the filtered coffee andthe boiled coffee solutions contained similar compo-nents.

The quantitative reverse-phase HPLC analysis of thematerial VI showed that the organic phase of the pastecontained 79 + 1 wt% UEDMA, The part of the VIpaste soltible in methanol and dichloromethane alsocontained oligoethyleneglycol dimethacrylate mono-mers, according to the HPLC analysis (Fig 11),

Discussion

Discoloration can be evaluated visually and by in-strumental techniques. Since instrumental measure-ments eliminate the subjective interpretation of visualcolor comparison, spec tro photometers were used in-stead of the Munsell color-order system for visual eval-uation.

In dentistry, a discoloration that is more than per-ceptible (4E*ai, > 1,0} ^ will be referred to as accept-able up to the value AE*ab = 3,3, which is consideredto be the upper limit of acceptability in subjectivevisual evaluations.'^ Discoloration above this level willbe referred to as unacceptable (Fig 12),

The soluble components of tea and coffee with blueabsorption appeared as yellow colorants, because yel-low is the complementary color to blue. The differentreverse-phase liquid Chromatographie retention timesfor these components of tea and coffee revealed thatthe yellow colorants have different polarities. In re-

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Fig 11 An adsorption HPLC c tiro mato g ram ot ttie solubleorganic part of tbe resin-based veneering material VI show-ing (1) UEDrwiA and (2-9) different oligoethyteneglycoi di-methacryiates.

Perceptible

3.3

0 1 2 3 4 5

Acceptable Unacceptable

Fig 12 Selected levels ot perceptibility and clinical ac-ceptability limits expressed on a AE'^^ scale.

verse-phase HPLC, the stationary phase is relativelynonpolar, while the mobile phase is polar; ie, the polarcomponents are eluted first, and less polar solutes areeluted later. ' Accordingly, the yellow colorants of cof-fee were less polar, and thereby less hydrophihc, thanthe yellow colorants of tea (see Fig 10).

The specimens that had been immersed in tea for48 hours were more discolored than those stored infiltered coffee and boiled coffee for the same lengthof time. After the specimens had been cleansed andbrushed, however, all specimens that had been im-mersed in tea showed acceptable discolorations (VI,DC, and BI) or even imperceptible discolorations (VGand IS), compared with their appearance before im-mersion in the staining solutions. During the cleansingprocedures, the specimens stored in the coffee solu-tions showed a gradual decrease in discoloration. VIand VG specimens stored in both types of coffeeshowed higher AE* ^ values after the cleansing pro-cedures (including brushing with a dentrifice) than didthose specimens of the same materials that had beenstored in tea and then cleansed in the same way

After the surface layers were ground away, the spec-imens of three materials, VG, DC, and VI, treatedwith coffee solutions for i,000 hours all showed anunacceptable discoloration (see Fig 8). Of the mate-rials treated with tea at a temperature of 50 °C, onlyDC showed an unacceptable yellow discoloration.This material was also the only material that had anunacceptable yellowing in distilled water at 37 °C. It

has been reported that the rate of discoloration in purewater increases with increasing temperature." Thiswas probably the reason that the discoloration valuesfor the ground DC specimens treated with tea werehigher than for those treated with water. Accordingly,the observed yellowing of tbe composite resin DC wasprobably due to intrinsic discoloration.

The discoloration caused by tea was easily removed,except for in DC. This removable discoloration wasprobably due to adsorption of the polar colorantsfrom tea at the surface of these materials. The dis-coloration from coffee was due both to surface ad-sorption and absorption of colorants. The less polarcolorants from coffee had penetrated deeper into thematerials, probably because the colorants were com-patible with the polymer matrices of the compositeresin materials VG and VI.

All materials investigated had different monomerformulations, according to the findings of this andprevious investigations.^' Of the two materials thatretained exogenous discoloration from coffee afterboth cleansing and grinding, VG contains diacrylatesof tricyclodecanc compounds and 1,6-hexanediol. Thematerial VI contains a mixture of ohgoethyleneglycoldimethacrylates and UEDMA. These two formula-tions are different from the formulation of the materialBI, which underwent the least discoloration in the cof-fee solutions. The material BI is based on the mon-omethacrylate monomers methyl methacrylate and te-trahydrofurfuryl methacrylate, in addition to triethy-

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leneglycol dimethacrylate.-^ The use of diacrylates in-stead of dimethacrylatcs in VG could impart differentproperties with respect to compatibility with colorantsfrom coffee. The use of oligoethyleneglycol dimethac-rylates in VI could make this material more polar thanare the materials Bl, DC, and IS.

The material VI had a high value for water sorptionand relatively high discoloration values with all threestaining solutions. The material VG, which also wasmarkedly discolored hy coffee solutions, had lowwater absorption values. It was observed that VG wasless affected by tea solution.'; than were the other ve-neering materials, but it was heavily discolored by cof-fee solutions. It has been stated that there is a rela-tionship between staining of resin-based materials andwater sorption as well as hydrophohicity/hydrophil-icity of these materials.'" Hydrophobie materials arestained by hydrophobic solutions in oil, and hydro-philic materials with high water absorption are stainedby hydrophilic colorants in aqueous solutions. Thefindings of the present investigation agree to a certainextent with this statement. Further investigations arenecessary to clarify this problem in detail.

Summary

Resin-based veneering materials were stained by coffeeand tea. Dentacolor exhibited intrinsic discolorationin water. Staining after 48 hours" immersion in tea wasremoved by toothbrushing. Staining of the materialswithout perceptible intrinsic discoloration after 1,000hours' immersion in tea was removed by grinding thesurface layer. Staining after 48 hours' immersion incoffee was easily removed from Isosit, Biodent, andDentacolor, whereas Vitapan and Visio-gem had anunacceptable discoloration after toothbrushing. Co-lorants penetrated beneath the surface of Vitapan andVisio-gem, discolored by coffee during long-tenn im-mersion, which indicates compatibihty of the resinmatrices with the yellow colorants in coffee.

The results of the present investigation indieatedthat, for prevention or reduction of discoloration ofresin-based veneering materials, good oral hygienewith a toothbrush and dentifrice is important.

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386 Quintessence Internationai Volume 22, Number 5/1991