Dental Materials Journal 27(1)：105－116, 2008
Effect of Bleaching on Color Change and Refractive Index of Dental Composite Resins
İhsan HUBBEZOGLU1, Barış AKAOĞLU2, Arife DOGAN3, Selda KESKİN4, Giray BOLAYIR5,
Süleyman ÖZÇELİK2 and Orhan Murat DOGAN5
1Department of Endodontics, Faculty of Dentistry, Cumhuriyet University, Sivas, Turkey
2Department of Physics, Gazi University, Ankara, Turkey
3Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey
4Department of Chemistry, Middle East Technical University, Ankara, Turkey
5Department of Prosthodontics, Faculty of Dentistry, Cumhuriyet University, Sivas, Turkey
Corresponding author, Arife DOĞAN; E-mail: email@example.com, firstname.lastname@example.org
Received June 16, 2007/Accepted August 28, 2007
This study investigated the effects of three bleaching agents (Whiteness Perfect, Whiteness Super, and Whiteness HP) on
the color change and refractive index of three dental composites (Admira, Durafill VS, and Gradia Direct). Twenty disk-
shaped specimens (10×2 mm) of each composite were prepared and divided into four subgroups (n=5). An unbleached
group was used as a control, while the remaining specimens in the three subgroups were bleached with one of the bleach-
ing agents respectively. Color change was assessed according to CIELAB color system and refractive indices were deter-
mined by phase modulated spectroscopic ellipsometry. Color differences between bleaching and baseline value (ΔE) were
less than 3.3 for all groups. However, bleaching with Whiteness HP led to noticeable color changes for Admira and Durafill
VS. While this agent had no effect on the refractive indices of these composites, the other two agents containing carbamide
peroxide increased their refractive indices. Therefore, results suggested that replacement of such composite restorations
may be required after bleaching.
Keywords: Bleaching, Composites, Color
Bleaching is a relatively non-invasive approach to
lightening teeth stained extrinsically or intrinsically.
Bleaching techniques may be classified by whether
they involve vital or non-vital teeth or whether the
procedure is performed in-office or has an at-home
Bleaching agents usually contain some form of
peroxide (generally carbamide and hydrogen perox-
ide) in gel or liquid form to be in contact with teeth
for several minutes to several hours, depending
on the formulation of material used3-6). It has been
reported that bleaching effect is directly related to
the exposure time and concentration of active bleach-
ing ingredient2,7). The longer the exposure time and
the higher the concentration of whitening mate-
rial, the greater will be the oxidation process and
color change. The associated side effects are namely
porosity, increased surface roughness, and reduction
in surface hardness of the existing composite resto-
rations2,8). Previous investigations reported that the
color change of resin composites was caused by many
factors such as the chemical structure, chemical acti-
vator, resin initiator and inhibitor, activator process,
polymer quality, type and quantity of filler, oxidation
of unreacted C=C bonds, UV illumination, heat, and
Color assessments of teeth and composite mate-
rials after bleaching have been made using value-
oriented shade guides, colorimeters, and digitized
photographs ― each with their own advantages and
disadvantages4,11). The use of a colorimeter gives
more objective results than shade tabs4,7,11), but it is
affected by some factors including translucency of the
Translucency is essential for dental restorative
materials. Most of the organic molecules present in
the matrix phase of dental composites and glasses
(frequently used as fillers) do not effectively absorb
visible light. As a result, scattering of light might
be considered as the main reason for low translu-
cency. Magnitude of light scattered depends on the
dimensions and surface area of the dispersed phase
(fillers), their segregation, microporosity, and surface
roughness. These properties of the microstructure
also affect the overall refractive index of the com-
posite material. It should be noted that in general,
magnitude and direction of scattering depends on
the average magnitude of refractive index fluctuation
in the composite material12). Therefore, individual
refractive indices of the dispersed phase (fillers) and
matrix phase (resin) should be perfectly matched in
order to obtain transluceny close to that of tooth tis-
sue12,13). If this were not so, the tooth would have
poor esthetical properties and reduced cure depth
with visible light14). In this respect, refractive indi-
ces are extensively used for the selection of compos-
Very often in daily dental practice, tooth-colored
restorations exist in the teeth that are planned to be
bleached1). Therefore, unintended application of the
Effect of bleaching on color change106
bleaching products on existing restorations by the
patients cannot be excluded if bleaching is not per-
formed and monitored by the dentist15). The change
of color and loss of shade match of composite resto-
rations with surrounding tooth structure are perhaps
the most frequent reasons for replacement of existing
restorations after bleaching6).
Many studies have evaluated the effect of bleach-
ing agent on composite resin properties. One such
investigation used a colorimeter to show that 10％
carbamide peroxide gels somewhat lightened the
color of composite resins16). In the same vein, analy-
sis of surface reflectance showed significant changes
in microfilled and hybrid resin composites after
application of highly concentrated tooth whiteners
with 30－35％ hydrogen peroxide17).
Due to significant advances in adhesive den-
tistry, resin composite materials have demonstrated
ongoing improvements in strength, wear resistance,
handling properties, and esthetics3,12). Besides, the
introduction of ormocers has also brought on a range
of highly esthetic composites. Ormocer is the term
for organically modified ceramics. This class of
material is characterized by incorporation of novel
organic-inorganic copolymers in the formulation
that allow a modification of the mechanical proper-
ties over a wide range3). Although some studies7,18-20)
have compared the effect of carbamide peroxide
against hydrogen peroxide on resin composites
ranging from microcomposite to polyacrylic resins,
ormocers’ bleaching-related changes in color have
not yet been fully documented. It should be high-
lighted that drastic color changes in existing resto-
rations may compromise esthetics. Therefore, it is
also important to understand the effect of bleach-
ing agents on the color of ormocer-based restorative
This study was conducted to compare color
changes and also refractive indices of a microfill, a
microhybrid, and an ormocer-based resin composite
exposed to bleaching agents of different formulation
MATERIALS AND METHODS
To examine the effects of three bleaching agents on
the color change of resin composites, one product
from each type of contemporary resin-based filling
material was chosen to investigate if the composition
Whiteness PerfectWhiteness SuperWhiteness HP
Composition16% carbamide peroxide,
glycol, distilled water, potassium
nitrate, sodium fluoride
37% carbamide peroxide,
neutralized carbopol, potassium
ions, glycerin, deionized water
35% hydrogen peroxide,
mixture of pigments, glicol,
thickener, deionized water
for 14 days
(20 minutes each)
with a 7 day interval
(15 minutes each)
with a 7 day interval
Table 2 Bleaching agents used
AdmiraDurafill VSGradia Direct
Typeormocer-based resin compositemicrofill resin compositemicrohybrid resin composite
Organic Martrixanorganic-organic copolymers
(ormocers), aliphatic and aromatic
Filler typeBa-Al-B-silicate glass, SiO2
pyrogenic SiO2, splinter polymerFluoro-alumino silicate glass,
silica and pre-polymerized fillers
(mean 0.7 μm)
pyrogenic SiO2: 20-70 nm
splinter polymer: 10-20 μm
Filler volume %564064-65
Data are according to manufacturers’ infomation.
Table 1 Restorative materials used
HUBBEZOGLU et al.107
influenced the results. For all the chosen resin com-
posites, shade A3 was used. Table 1 lists the com-
posite materials and their details. Included were
a microfill resin composite, Durafil VS; a microhy-
brid composite, Gradia Direct; and a ormoser-based
The bleaching agents used were Whiteness Perfect
(16％ carbamide peroxide), Whiteness Super (37％
carbamide peroxide), and Whiteness HP (35％ hydro-
gen peroxide) (Table 2). All the selected bleaching
agents were marketed by the same manufacturer
(FGM Produtos Odontológicos, Joinville, SC, Brasil)
for different applications and claimed not to bleach
restorative materials. Whiteness Perfect was to be
applied daily at home by the patients for 3－4 hours
per day for 14 days consecutively. As for the other
two products, they were recommended for in-office
use by dentists for non-vital and vital teeth. It was
recommended that Whiteness HP be applied for 15
minutes each for two sessions, whereas Whiteness
Super for 20 minutes each for three sessions.
Twenty disk-shaped specimens from each resin com-
posite material (60 specimens in total), 10 mm in
diameter and 2mm in depth, were prepared in Tef-
lon molds. The materials were handled according to
manufacturers’ instructions. The mold was placed
on a transparent polyester film strip (3M Flip-Frame,
3M Visual Systems Division, Austin, TX, USA) and
a glass microscope slide. The composite was packed
into the mold until it was intentionally overfilled.
The material was covered with another polyester
film strip and a glass microscope slide. Excess mate-
rial was extruded by light pressure, and resin com-
posites were polymerized using a blue light-emitting
diode (LED) unit (UltraLight PB-070, Fine Vision
Electronics Co., Sanchung City, Taipei County,
Taiwan). This source emitted light at 440－480 nm,
and had an intensity of 1000 mW/cm2. Curing time
was set at 20 seconds. Distance between the light
source and specimen was standardized by the use of
a 1-mm glass slide. The end of the light guide was
in contact with the cover glass during polymeriza-
tion. After light curing, all specimens were stored
in distilled water for 24 hours at 37℃ to ensure
complete polymerization. The top surfaces of the
specimens were then polished flat using a sequence
of 600-, 800-, and 1200-grit silicon carbide papers
and Sof-Lex (3M ESPE, USA) disks.
Twenty specimens from each composite group were
randomly divided into four subgroups. For control,
five specimens of each composite were immersed in
distilled water. Then, five specimens in each sub-
group were bleached by one of the bleaching agents.
To simulate the bleaching process, the first subgroup
was immersed in Whiteness Perfect (16％ carbamide
peroxide gel) for three hours for 14 consecutive
days; the second subgroup immersed in Whiteness
Super (37％ carbamide peroxide gel) for 20 minutes
for three sessions; the third subgroup immersed
in Whiteness HP (35％ hydrogen peroxide gel) for
15 minutes for two sessions. Whiteness Super and
Whiteness HP were applied in intervals of seven
days. Throughout the experiment, specimens were
stored in a dark environment at room temperature
(23±1℃). During test intervals, the specimens were
rinsed with tap water for one minute to remove the
bleaching agents, blotted dry, and placed in Petri
dishes filled with distilled water for storage. For
each new session, bleaching agents were replenished
Before and after treatment with each of the bleaching
agents, the surface of each specimen was inspected
to determine whether any changes in the color of
the specimen’ s surface were visible to the naked
eye. Before baseline color measurement, specimens
were rinsed under tap water for one minute and blot-
ted dry. A colorimeter (Mercury™ 2000, Datacolor,
Lawrenceville, NJ, USA) was used to record the color
variables according to the CIELAB (Commission
Internationale de I’ Eclairage L*, a*, b*) system.
Aperture size diameter was 5 mm and illuminat-
ing and viewing configurations were CIE diffuse/8°.
The illumination source was provided by a pulsed
xenon lamp filtered to D65, and a white calibration
ceramic was used (CIE L* = 95.93, a* = －0.41, and
b* = 1.56). The specimens were positioned so that
their surfaces were in contact with the aperture head
of the colorimeter. Each specimen was measured
twice by the same person, and the average baseline
values of L*, a*, and b* were calculated. L* repre-
sents the degree of gray and corresponds to a value
of brightness, such that high L* values are obtained
from bright or white specimens. The value a* repre-
sents the red-green axis, and the value b* represents
the blue-yellow axis.
After bleaching, the same procedure was
repeated to determine the chromatical values. Mag-
nitude of total color difference (ΔE*) was calculated
using the following equation21):
ΔE* = [(ΔL*)2 + (Δa*)2 + (Δb*)2]1/2
where ΔL*, Δa*, and Δb* are changes in L*, a*,
and b* after bleaching, respectively. ΔE* values >1
were considered to be visible to the naked eye, and
ΔE* ≥3.3 was considered as clinically unacceptable9).
Effect of bleaching on color change108
Determination of refractive index
Refractive indices of specimens were determined
by phase modulated spectroscopic ellipsometry at a
70° angle of incidence (HORIBA UVISEL, Jobin
Ivon, Chilly Mazarin, France). The measurements
were performed in configuration II where the modu-
lator and analyzer angles were fixed to 0° and
45° respectively. All measurements were performed
with a spot size of 2.9 mm2. The ratio of complex
reflection coefficients of an incident light polarized
parallel (rp) and perpendicular (rs) to the plane of
incidence was expressed in terms of ellipsometric
angles Ψ and Δ as
. The refractive
index n of specimen was determined by assuming a
simple air/material (one interface) model and that
the material was assumed as nonabsorbing22). Within
the frame of this method, refractive index was deter-
mined using the following equation23):
where φ0 is the angle of incidence and
angles Ψ and Δ determine the changes in amplitude
and phase, respectively, of the p (parallel) and s (per-
pendicular) components of a wave upon reflection. ρ
is equivalent to the ratio of the polarization states of
reflected and incident waves.
Scanning microscope analysis
For the evaluation of topography and surface struc-
ture, a total of 12 specimens were examined. One
specimen from each bleaching agent and one con-
trol specimen was selected for each resin compos-
ite. After sputter-coating with 25 to 30 μm of gold
(Hummer VII, Analect, USA), the specimens were
examined at ×2000 magnification with a scanning
electron microscope (SEM) (Jeol JSM 6400, Noran
Instruments, Tokyo, Japan).
After data collection, mean values and standard devi-
ations were calculated by using a SPSS statistical
software program (version 13.0, SPSS Inc., Chicago,
USA). The data were subjected to statistical analysis
using two-way analysis of variance (ANOVA). Where
significant differences were present, Tukey’ s post hoc
test was applied to make pairwise comparison at a
significant level of 0.05.
Table 3 and Figs. 1a－c show the chromatical val-
ues of resin composites before and after bleaching.
For the L* values, it could be seen that Admira and
Durafill VS ― not Gradia Direct ― showed increase
in brightness with the different bleaching agents.
For the a* values, all the bleached specimens of
Admira showed small shifts when compared to the
control. By contrast, Durafill VS and Gradia Direct
resin composite specimens bleached with White-
ness Perfect (16％ carbamide peroxide) revealed a
relatively larger shift in a* value when compared
to the controls and their other own bleached speci-
mens. For the b* values, all resin composites tended
to shown an increase with the use of Whiteness HP
(35％ hydrogen peroxide). This led to a yellow shade
of the specimens that was visible to the naked eye.
A complete evaluation of color changes based on
ΔE* values by two-way ANOVA revealed an interac-
tion between the bleaching agents and resin compos-
ites (F=141.229). The highest score was recorded for
Admira bleached with Whiteness HP, while micro-
hybrid resin composite Gradia Direct bleached with
Whiteness Super (37％ carbamide peroxide) ranked
the lowest among the materials (Table 4, Fig. 1(d)).
To clarify the effect of different bleaching agents
on the same resin composite, Tukey’ s test revealed
that the color of each composite did not change sta-
tistically when bleached with Whiteness Perfect and
Whiteness Super (p>0.05). However, when com-
pared against Whiteness HP for Admira and Durafill
VS specimens, the use of the former two bleaching
agents registered statistically significant differences
(p<0.05) respectively. For Gradia Direct specimens
bleached with Whiteness Super and Whiteness HP,
the color change was found to be significant when
comparing the mean ΔE* values of the specimens
(p<0.05). Conversely, for Gradia Direct specimens
bleached with Whiteness Perfect and Whiteness HP,
no statistically significant differences were noted
Bleaching with 16％ and 37％ of carbamide per-
oxides led to statististically significant differences
among the resin composites (p<0.05). Color change
was found to be statistically significant for all the
resin composites except for Admira and Durafill VS.
On the other hand, the color changes of the three
resin composites tested were found to be statistically
different from each other after bleaching with 35％
hydrogen peroxide (p<0.05).
Figure 2 shows the refractive indices of the
composites before and after bleaching. The refrac-
tive indices of unbleached specimens of Admira and
Durafill VS resin composites were found to be higher
than those of Gradia Direct (1.425, 1.425, and 1.375
respectively). The refractive indices of Admira and
Durafill VS resin composites increased as they were
bleached with both carbamide peroxide gels, being
more so with 37％ carbamide peroxide. As for
Gradia Direct, bleaching with both carbamide per-
HUBBEZOGLU et al.109
n=5 specimens per experimental condition
Groups depicted as WP, WS and WHP are the speciments bleached with Whiteness Perfect, Whiteness Super, and Whiteness HP, respec-
Table 3 ANOVA analysis results of the chromatical values of L*, a*, and b*
AdmiraDurafill VSGradia Direct
n=5 specimens per experimental condition
By two-way ANOVA: F=141.229, p=0.000, p<0.05.
Tukey’ s test indicates statisticl difference (p<0.05) for means followed by the same letters; small letters in the column are for the compari-
son of different bleaching agents within the same material; capital letters in the rows are for the comparison of the composites bleached
with the same agent.
Mean ± SD
0.6956 ± 0.1250a,A
0.7496 ± 0.1255b,C
1.4117 ± 0.1180a,b,E,F
Mean ± SD
0.6893 ± 0.0794c,B
0.8073 ± 0.0606d,D
1.1133 ± 0.0290c,d,F,G
Mean ± SD
0.4962 ± 0.0168A,B
0.4007 ± 0.0288e,C,D
0.5874 ± 0.1400e,E,G
Table 4 Color changes (ΔΕ*) of restorative materials after bleaching
Fig. 1 Chromatical values (a) L*, (b) a*, (c) b* and (d) ΔΕ* of the specimens. I, II, III, and IV show
unbleached and bleached specimens with Whiteness Perfect, Whiteness Super, and Whiteness HP
respectively. Resin composites Admira, Durafill VS, and Gradia Direct are denoted by squares,
triangles, and circles respectively. Magnitude of standard deviation is within the marker size.
Effect of bleaching on color change110
Fig. 3 Representative SEM images of Admira: (a) control group; (b) Whiteness Perfect group;
(c) Whiteness Super group; and (d) Whiteness HP group (original magnification ×2000).
Fig. 2 Refractive indices of specimens at a wavelength of 632.8 nm for samples (I)
unbleached and bleached with (II) Whiteness Perfect, (III) Whiteness Super,
and (IV) Whiteness HP agents. Resin composites Admira, Durafill VS, and
Gradia Direct are denoted by squares, triangles, and circles respectively.
HUBBEZOGLU et al.111
Fig. 4 Representative SEM images of Durafill VS: (a) control group; (b) Whiteness Perfect group;
(c) Whiteness Super group; and (d) Whiteness HP group (original magnification ×2000).
Fig. 5 Representative SEM images of Gradia Direct: (a) control group; (b) Whiteness Perfect
group; (c) Whiteness Super group; and (d) Whiteness HP group (original magnification
Effect of bleaching on color change 112
oxide gels led to increased refractive indices ― but
the differing concentrations of carbamide peroxide
did not result in significant differences between the
two refractive indices. Bleaching with 35％ hydro-
gen peroxide did not affect the refractive indices of
Durafill VS and Admira; however, a considerable
change was observed for Gradia Direct resin
Figures 3－5 show the typical SEM images of the
resin composites. In each figure were included the
surfaces of the composite for unbleached (i.e., control)
(a) and bleached with Whiteness Perfect (b), White-
ness Super (c), and Whiteness HP (d). The surfaces
of all the control specimens were seen as relatively
flat. However, those of Admira and Durafill VS pre-
sented an irregular area with some indentations
which might be a result of the polishing procedures
(Figs. 3a, 4a, and 5a). After bleaching, all the Admira
samples showed shallow pitting on the whole surface
(Figs. 3b－d). Additionally, a river-like pattern was
seen on the surface bleached with Whiteness HP
(Fig. 3d). For Durafill VS samples, there were also
some observable effects of bleaching agents on the
surface morphology. Whiteness Perfect led to etch-
ing-like features such as striations and larger pitting
on the surface (Fig. 4b), whereas the sample bleached
with Whiteness Super showed evidence of some
deeper-splitting features such as river-like patterns
(Fig. 4c). The latter appearance was also seen on
the surface bleached with Whiteness HP, but to a
worse extent (Fig. 4d). Nonetheless, apart from the
aforementioned specific features, the remaining sur-
faces of all Admira and Durafill VS samples were
observed to be relatively smooth. For Gradia Direct,
the sample bleached with Whiteness Super did not
seem to be much affected in surface morphology,
when compared to bleaching by the other two agents.
The sample bleached with Whiteness Super seemed
to be relatively flat (Fig. 5c), whereas the other two
samples exhibited more irregular features ― such as
small pores and scratches ― and a rougher surface
(Figs. 5b and d).
Tooth bleaching has become extremely popular.
Owing to its widespread popularity, the effect of
this procedure on the esthetic appearance of resin
composites ― which may exist in teeth ― needs to be
taken into consideration. In particular, the organic
matrices of resin composites are prone to chemical
alteration induced by the acidic component of bleach-
ing agents. This may then compromise the color
matching of resin composite restorations to adjacent
tooth structure, giving a reason for their replace-
In color assessment, the choice of an appropri-
ate method is important because of the path length
of incident light in the material tested21). It is well
known that instrumental evaluation presents more
objective data versus the subjectivity of visual color
determination4,7,10,11). On this ground, a colorimeter
with white colored plate for the background was used
in the present study. The chromatical values backed
by a white colored plate could be considered as the
color of resin composites applied on the lining mate-
rial in the oral cavity10).
It has been reported that two key factors deter-
mine the overall whitening efficacy of peroxide-con-
taining products: peroxide concentration and applica-
tion duration4). Therefore, to determine the changes
in color and refractive indices of three resin compos-
ites subsequent to bleaching, 16％ and 37％ carb-
amide peroxide and 35％ hydrogen peroxide were
tested as bleaching agents using different application
times (Tables 1 and 2).
Upon examining the chromatical values of the
bleached resin composites, it could be seen that
ormocer-based and microfill resin composites showed
somewhat an increase in brightness ― based on L*
values ― after bleaching with the different agents.
For the a* values, small shifts were observed for all
the bleached specimens of ormocer-based compos-
ite, whereas relatively large shifts were observed
for microhybrid and microfill resin composite speci-
mens after bleaching with 16％ carbamide peroxide.
Although it is difficult to give a clear explanation for
this phenomenon, some amine synergists may create
red/brown by-products to a certain extent9). For the
b* values, a heightened increase was observed for
all the tested composites following the use of 35％
hydrogen peroxide. This led to a yellow shade which
was noticeable to the naked eye.
Several authors have shown that color differ-
ences greater than 1 ΔE* unit were considered to be
visible to the naked eye by 50％ of human observers,
and ΔE* values equal to or greater than 3.3 were
considered as clinically unacceptable25-27). In the pres-
ent study, none of the bleaching regimes resulted in
color changes with ΔE* ≥3.3. However, application
of 35％ hydrogen peroxide gave ΔE* >1 for ormocer-
based and microfill resin composites. The interaction
between this bleaching agent and restoratives could
be of clinical significance, as the color change could
be noticeable to the patient. On the other hand, the
use of 16％ and 37％ carbamide peroxide did not lead
to noticeable color change of the restoratives used
because the amount of color change of the bleached
specimens were lower than 1 ΔE* unit.
Differences in the bleaching effect of the agents
on the same material might be attributed to their dif-
ferent hydrogen peroxide contents. The higher effi-
cacy of 35％ hydrogen peroxide gel could be due to an
excess of active ingredient that readily diffused. It is
HUBBEZOGLU et al.113
noteworthy that carbamide peroxide is a vehicle for
the delivery of low concentrations of hydrogen per-
oxide5). During the process, one-third of carbamide
peroxide decomposes into hydrogen peroxide and the
rest into urea2,4,5,7). Urea further breaks down into
ammonia and carbon dioxide, thereby enabling the
evaluation of intraoral pH3).
Hydrogen peroxide is an aggressive oxidant
capable of degrading the polymer matrix of resin-rich
composite materials3,15,28). It breaks down into water
and oxygen, as well as free radicals which result
in oxidation of the pigments or amine compounds
within the structure2,3,20). In addition to its reactivity,
hydrogen peroxide demonstrates an extensive ability
for diffusion15). Oxidation of the pigments may occur
as a result of direct interaction with hydrogen per-
oxide on the resin surface21). Peroxides might induce
oxidative cleavage of polymer chains. Therefore, any
unreacted double bonds are expected to be the most
vulnerable parts of the polymers. Furthermore, free
radicals induced by peroxides may impact the resin-
filler interface and cause filler-matrix debonding.
Microscopic cracks are formed, resulting in surface
roughness and leading to diffusion of agent15).
In the present study, the resin composites used
were tightly cross-linked with high-molecular-weight
polymer molecules21). This could be a reason why
there was not much color change. This finding was
in good agreement with that of a previous study,
whereby 10％ carbamide peroxide somewhat light-
ened the color of composite resins16). Furthermore,
in another study, color changes following the use of
10％ carbamide peroxide for 312 hours were unde-
tectable to the naked eye20).
Due to their organic matrix, composite resin
materials are rendered more prone to chemical
alterations compared to inert metal or ceramic res-
torations15). Our results indicated that color change
induced by the same bleaching agent might be
dependent upon the monomer structure, volume of
the resin matrix, as well as the filler systems of com-
posite materials tested. The structures of the organic
matrices of all the resin composites used in this
study were different. The organic matrix of microfill
composite contained bisphenol-A dimethacrylate and
urethane dimethacrylate, whereas that of the micro-
hybrid composite was based on urethane dimethacry-
late only, hence it had lower surface hardness than
the other composites tested. On the other hand,
ormocer was based on a resin system in which mul-
tifunctional urethane- and thioether(meth)acrylate
alkoxysilanes as sol-gel precursors have been devel-
oped for the synthesis of inorganic-organic copolymer
ormocer composites as dental restorative materials.
The alkoxysilyl groups of silane allow the formation
of an inorganic Si-O-Si network by hydrolysis and
polycondensation reactions, and the (meth)acrylate
groups are available for photochemically induced
organic polymerization. The ormocer matrix has
been suggested to exhibit significantly less wear than
composite matrices29) and to have high surface hard-
ness values because of a more rigid matrix3). How-
ever, in the current study, this material and microfill
resin composite underwent a significant color change
when bleached by an agent containing a high concen-
tration of hydrogen peroxide. This suggested that
the volume of resin matrix and filler type had a great
influence on the color parameters of dental compos-
ites than the structure of the organic matrix3,29).
It could be seen in Table 1 that the volumes of
the resin matrices of ormocer and microfill compos-
ites were greater than that of microhybrid compos-
ite because of lower volumetric filler content. This
meant that a higher degree of oxidation was induced
by the bleaching agents in their resin polymer matri-
ces3). Hydrogen peroxide is an aggressive oxidant
capable of degrading the organic matrix and in turn
breaks down into free radicals, which eventually
combine to form molecular oxygen and water. Some
aspect of this chemical process may accelerate the
hydrolytic degradation of resin composites, leading to
It has been suggested that barium-containing
glass fillers are more susceptible to water attacks
than both quartz and fairly purified amorphous
SiO2. However, because of a larger total surface
area, the microfill particles have more Si available
for leaking. The attack of water on the fillers might
also influence the esthetic properties of the com-
posites30). In this study, the materials tested con-
tained different type of fillers (Table 1): ormocer had
Ba-containing glass fillers and microfill resin com-
posite contained pyrogenic SiO2, which were likely to
be softer than that of microhybrid composite includ-
ing fluoroalumina silicate glass. Differences in the
structures of the fillers could explain why the ormo-
cer and microfill resin composites revealed greater
color change compared to microhybrid composite
after bleaching with a high concentration of hydro-
For dental materials, the overall refractive index
(i.e., pertaining to both resin matrix and filler par-
ticles) is generally larger than about 1.4512,13,31,32).
Therefore, the resulting refractive indices of the com-
posite materials seem somewhat underestimated31).
The approach applied for the determination of refrac-
tive indices in this study assumed a simple air/mate-
rial model which ignored the surface overlayer of the
composite material. This surface overlayer might
have come into existence due to surface roughness,
surface reconstruction, surface oxide, etc. At this
juncture, it must be reiterated that ellipsometry is
a highly surface-sensitive measurement technique.
The surface overlayer, which is not modelled in the
Effect of bleaching on color change114
ellipsometric analysis, might result in a decreased
density of the microstructure, which in turn leads
to a smaller refractive index33). According to the
Clausius-Mossotti relationship34), the dielectric func-
tion is proportional to the density of polarizable
species. Therefore, a decrease in the density of the
microstructure would cause a decrease in the refrac-
tive index. It should also be noted that this eventual
density deficit is not only restricted to the surface,
but also translated to the bulk of the composite mate-
rial. Within the frame of this approach, the obtained
refractive index value should be referred to as an
apparent or pseudo-refractive index value.
Debonding caused by water will result in gaps
between the resin and fillers, changing the refrac-
tive index of the composite30). It has been shown that
upon exposure to plaque acids, resin-based materials
underwent softening and loss of surface integrity35).
Similarly, although it was not studied in this study,
the deletorius effect of oxidizing agents might impair
the surface integrity, affecting the penetration depth
of bleaching agent. This was a logical suggestion to
propose because SEM images revealed that all the
composites tested underwent surface alterations after
bleaching (Figs. 3－5).
The refractive indices of the composites are
given in Fig. 2. The refractive indices of unbleached
ormocer, microfill, and microhybrid composites were
determined as 1.425, 1.425, and 1.375 respectively.
The surface of the microhybrid resin composite
revealed more irregular features and seemed rougher
(Fig. 5). It could thus be said that a decrease in the
density of its microstructure contributed to the low
refractive index of this composite. In addition, there
was a stronger change in the surface morphology of
this microhybrid composite bleached with 16％ carb-
amide peroxide in comparison to ormocer and micro-
fill composites. This observation was also echoed in
the refractive index values shown in Fig. 2. In light
of these observations, it was confirmed that the sur-
face properties of a material considerably affected its
overall apparent refractive index.
Upon bleaching with carbamide peroxide gels,
the refractive indices of the resin composites exhib-
ited an increasing tendency (Fig. 2). Bleaching with
35％ hydrogen peroxide did not seem to affect the
refractive indices of ormocer and microfill compos-
ites, whereas a considerable change was observed for
microhybrid composite. In addition, the concentra-
tion of carbamide peroxide did not affect the refrac-
tive index of microhybrid resin composite ― a behav-
ior seemingly in agreement with the behaviors of b*
and ΔE* variables in Fig. 1.
The refractive indices of ormocer and microfill
resin composite were found to be higher than those
of microhybrid composite. The average particle size
of the fillers in microhybrid was relatively larger
than those of the other two materials tested, and
such a large particle size might enhance microporos-
ity in the structure. In this connection, it came as
little surprise that smaller refractive index values
were found for this microhybrid composite tested.
On the other hand, this density deficit might only be
partially related to the enhanced surface roughness,
which promotes plaque accumulation36).
With smaller filler particles, the specific surface
area of the particles increases and consequently the
interaction between the matrix and filler increases14).
However, it should be noted that small particles may
agglomerate in the matrix, thereby decreasing the
interaction between the matrix and filler. The micro-
fill composite has been found to show large aggre-
gates made of silica microfillers, embedded in a pre-
polymerized organic matrix32). It had fine particles
(pyrogenic SiO2) which were much smaller than the
large particles of splinter polymers37). According to
the manufacturer’ s information, the ormocer used
consisted of large prepolymerized molecules orga-
nized in a matrix of inorganic-organic copolymers,
and that its surface-modified inorganic fillers were of
spherical shape. Although the average particle size
and filler shape of ormocer and microfill composites
were quite different, their refractive index behav-
iors under different bleaching agents were similar.
This observation suggested agglomerated fillers in
the microfill composite. The similar strong changes
in ΔE* for these two materials also supported this
suggestion. Further, the changes in chromatical val-
ues of ormocer and microfill specimens under differ-
ent bleaching agents in Fig. 1 were very similar to
the refractive index changes given in Fig. 2. At this
juncture, it is noteworthy that color changes (i.e.,
ΔE*) of these materials with larger refractive indi-
ces were found to be stronger.
Based on the results obtained in this study, it
could be concluded that the high concentration of
hydrogen peroxide in a proprietary bleaching gel had
a noticeable color change effect on ormocer-based
and microfill resin composite restorative materials.
Therefore, patients should be informed that replace-
ment of such existing composite restorations may be
required after tooth bleaching.
Within the limitations of this in vitro study, the fol-
lowing conclusions were drawn:
(1) None of the bleaching agents tested made
any significant clinical effects ― with ΔE*
≥3.3 ― on the color of resin composites tested.
(2) Admira and Durafill VS bleached with White-
ness HP containing 35％ hydrogen peroxide
yielded ΔE* >1, a noticeable color change to
the naked eye.
HUBBEZOGLU et al.115
(3) Refractive indices of unbleached Admira and
Durafill VS were found to be larger than that of
(4) Bleaching with carbamide
increased the refractive index values of all the
composites tested. However, the differing con-
centrations of these agents made no impact on
the refractive index of Gradia Direct.
(5) Bleaching with 35％ hydrogen peroxide had no
effect on the refractive indices of Admira and
Durafill VS, whereas it caused a considerable
change to that of Gradia Direct.
(6) Color changes were found to be stronger in
materials with larger refractive indices.
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