N. LAISSANI et al.: EFFECT OF FOOD-SIMULATING LIQUIDS ON MECHANICAL PROPERTIES AND ...
717–722
EFFECT OF FOOD-SIMULATING LIQUIDS ON MECHANICAL
PROPERTIES AND SURFACE OF A DENTAL RESIN
VPLIV SIMULIRANIH PREHRANSKIH TEKO^IN NA MEHANSKE
LASTNOSTI IN POVR[INO ZOBNEGA POLNILA
Nassira Laissani
1
, Lakhdar Smata
2
, Ferhat Djeddou
3
1,2
Setif1 University, Institut d’Optique et Mécanique de Précision, Physics and Mechanics of Metallic Materials Laboratory, 19000 Algeria
3
Setif1 University, Institut d’Optique et Mécanique de Précision, Applied Mechanics Laboratory, 19000 Algeria
smata_lakhdar@univ-setif.dz
Prejem rokopisa – received: 2018-03-18; sprejem za objavo – accepted for publication: 2018-06-14
doi:10.17222/mit.2018.047
The purpose of this study was to evaluate the effects of food-simulating liquids on the mechanical properties and surface of a
dental resin. The mechanical properties considered are fatigue and flexion. Samples (n = 108) were immersed into three liquids:
distilled water, citric acid and heptane. The storage time was extended to four months. However, the specimens that were not
broken after 10,000 shocks were subjected to flexion. The resulting histograms showed a weak increase in the flexion resistance.
Moreover, a microscopic examination made with AFM indicated a modification of the surface morphology of the samples stored
in distilled water for 90 d. The citric acid presented undulations with peaks, whereas heptane had no effect compared with the
intact resin.
Keywords: acrylic resin, storage, mechanical properties, food-simulating liquids
Namen {tudije je bil ovrednotenje vpliva simuliranih prehranskih teko~in na mehanske lastnosti in povr{ino zobnega polnila.
Mehanske lastnosti polnila so avtorji ~lanka raziskovali z utrujanjem in upogibanjem. Vzorce (n=108) so potapljali v tri
teko~ine: destilirano vodo, citronsko kislino in heptan. ^as shranjevanja vzorcev v njih je bil do {tiri mesece. Vzorce, ki se niso
prelomili po 10.000 udarcih, so izpostavili {e upogibanju. Dobljeni histogrami so pokazali rahlo izbolj{anje odpornosti proti
upogibanju. Mikroskopske preiskave z mikroskopom na atomsko silo (AFM; angl.: Atomic Force Microscopy) so pokazale
spremembe povr{inske morfologije vzorcev, shranjenih v destilirani vodi za 90 dni. Hranjenje v citronski kislini je nakazalo
valovito morfologijo povr{ine z ostrimi vrhovi, medtem ko hranjenje v heptanu ni imelo vpliva na povr{ino v primerjavi z
nepo{kodovanim polnilom.
Klju~ne besede: akrilna smola (zobno polnilo), hramba, mehanske lastnosti, simulacija prehranjevanja
1 INTRODUCTION
One of the most widely used dental materials is
acrylic resin. Since its introduction in dentistry, it was
widely employed for the confection of dental pros-
thesis
1,2
due to its ease of manipulation, low cost, light
weight, good aesthetic characteristics and biocompatibil-
ity.
Restorative materials used in dentistry are required to
have long-term durability in the oral cavity. In different
regions of the mouth, dentures are continuously bathed
in a complex environment, particularly saliva, that con-
tains substances such as water, proteins, amylases, etc.
3
During eating and drinking, materials are also exposed to
food components and beverages that consist of a variety
of substances including proteins, lipids, carbohydrates,
water and citric acids. In vitro, these products are simu-
lated by suitable liquids.
Under clinical conditions, the mechanical strength of
dental materials is normally characterized using static
methods but fatigue tests are particularly well appro-
priated for the response of a material subjected to
repeated loads such as chewing forces.
4
The cyclic masti-
catory loading is an important mechanical load that can
lead to a denture failure and thus decrease its lifetime.
Therefore, the fatigue resistance is an ideal mechanical
property for the study of the clinical lifetime of a den-
ture.
The fracture of a denture is a rather common occur-
rence. After a period of service, it may be easily and
accidentally fractured by biting a hard object or by an
impact received when it is outside the mouth.
5,6
Hence, a
static force such as flexion, after a failure due to fatigue,
reproduces this phenomenon well.
There are several researchers who have studied the
mechanical properties of different dental materials after
immersing them in food-simulating liquids.
7–9
The
majority of these works were mainly oriented on various
dental composites. On the other hand, only few resear-
ches have focused on acrylic resins and apparently no
study has been done on the effect of these liquids on the
fatigue prior flexion. This study was consolidated by ob-
serving the surfaces of different samples with AFM after
three months of storage where no investigation had been
done on the surface states after any type of storage.
However, explanations were given, describing the break-
ing mechanism of the biomaterial.
Materiali in tehnologije / Materials and technology 52 (2018) 6, 717–722 717
UDK 620.1:67.017:616.314-74 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(6)717(2018)

In this context, the intent of this study was to evaluate
the effects of three food-simulating liquids (distilled
water, citric acid and heptane) used with an acrylic resin,
on the mechanical properties, using fatigue and flexion.
The immersion time was extended to four months.
Therefore, the impact of these liquids on the biomaterial
before and after the storage was studied by character-
izing the surface morphologies.
2 EXPERIMENTAL PART
114 rectangular specimens (2×2×2 5 )m mwere
prepared using a stainless steel mold. After hardening,
the specimens were taken out of the mold. The con-
trolled specimens (n = 6) were stored at room tempera-
ture in air. Others were randomly divided into three
groups and then immersed at room temperature as
follows: distilled water, 0.02 N citric acid, and heptane.
The periods of storage time were (15, 30, 45, 60, 90 and
120) d.
The fatigue was determined using a device for cyclic
loading with a shaft guided and ended with a spherical
steel indenter at the center of a 4.0 mm diameter. Fatigue
tests were run at 2 Hz until the completion of the 10
4
cycles or the failure of the material. The samples that
resisted 10
4
shocks were subject to flexion. Flexural
strength was determined using a three-point flexion test,
performed in a universal testing machine (ZWICK
ROWELL) at a crosshead speed of 0.5 mm/min. The
values of the flexion strength were achieved using the
software of the machine. To detect flaws in the material,
cross-sections of the ruptured samples were examined
using an optical microscope (Axiovert 40 MAT ZEISS).
The morphologies of the surfaces after storing the
samples in various media for 90 days were observed with
atomic force microscopy (Pacific Nanotechnology, Santa
Clara, CALIFONIA, USA).
3 RESULTS
The number of fatigue cycles is reported in Figure 1.
The immersion of the samples in the three liquids
provided constant values (10 000 cycles) of the fatigue
up to the 60th day. After the 90th day, the fatigue was
strongly decreased, in water to 7902 cycles and in acid
storage to 7468 cycles, while the heptane storage caused
a failure after 9634 cycles. At the end of the storage (120
d), in distilled water, the fatigue dropped to 7468 cycles,
in acid to 2430 cycles and in heptane to 176 cycles.
The specimens which were not broken after 10 000
shocks were subjected to the three-point flexion. The
measurement of this resistance is reported in Figure 2.
The histograms show a weak increase in the flexion
résistance. The values of the flexion increased from 66.4
MPa to 69.2 MPa for the storage in water and from 66.4
MPa to 72.0 MPa for heptane, while the acid shows a
noticeable growth (from 66.4 MPa to 74.2 MPa).
Images of the cross-sections of the broken samples,
obtained with optical microcopy, are shown in Figure 3.
Cracks that propagate from the porosities through the
resins are visualized in theses figures. Microfissures are
detectable due to the enlargement of the framed zones of
the two micrographs. Other small porosities (A and B)
are also visible in this figure.
On the other hand, a microscopic-examination result,
compared to the resin without storage in Figure 4a,in-
dicates a modification of the surface morphology of the
samples stored in water for 90 d as shown in Figure 4b.
This texture is completely different from the other
pieces. In Figure 4c, the citric acid presents undulations
with peaks, whereas heptane from Figure 4d had no
effect compared with dry resin.
N. LAISSANI et al.: EFFECT OF FOOD-SIMULATING LIQUIDS ON MECHANICAL PROPERTIES AND ...
718 Materiali in tehnologije / Materials and technology 52 (2018) 6, 717–722
Figure 2: Mean flexion resistance versus immersion time
Figure 1: Mean number of cycles versus immersion time

4 DISCUSSION
There is a large variability in oral environments from
one individual to another and at different times of the
same day. In vitro, different conditions can be simulated
by suitable liquids. Due to the diversity of environmental
conditions, the oral and food-simulating liquids used for
storing the specimens from this study were reduced
primarily to distilled water, which substantially simulates
natural saliva, and water as it is found in beverages or
foods. On the other hand, citric acid represented juices
and soft drinks as the product is part the composition of
most of these beverages. Heptane substituted vegetable
oils, butter and fatty meats.
8,10
Restorative materials used in dentistry are required to
have long-term durability in the oral cavity. In addition
to the aqueous environment of the oral cavity, during
eating, restorative materials are also subject to cyclic
loading due to the repeated movements of chewing
forces.
11
Primarily, this cyclic masticatory force indicates
that fatigue is responsible for the failure of dental resto-
ration materials.
12
Therefore, fatigue is one of the most
important mechanical characteristics that needs to be
considered when studying the long-term clinical perfor-
mance of dental materials.
To evaluate the material resistance to fatigue, the
specimens, after storage in the mediums, were mostly
subjected to 10
4
impact-load cycles or until a failure
occurred. The frequency of the cycles was 2 Hz.
However, other test frequencies ranging from 0.3 Hz to 5
Hz
13–16
were also used. These values varied in accord-
ance with the type of fatigue, the materials and environ-
mental conditions used. The number of the practiced
load cycles of 10
4
was set based on the previous
studies.
15–17
It was also estimated that the intraoral stress
received by dental restorations during mastication is
repeated more than3×10
5
times per year.
13
The number
of loading cycles used in this study was low compared to
the total number of load cycles that a restoration must
withstand during its life service. However, some other
works used cycles in the order of 10
3
,10
5
and 10
6
, res-
pectively.
13,14,18
The measuring results of the fatigue
showed two distinct stages: the first, which lasts up to
N. LAISSANI et al.: EFFECT OF FOOD-SIMULATING LIQUIDS ON MECHANICAL PROPERTIES AND ...
Materiali in tehnologije / Materials and technology 52 (2018) 6, 717–722 719
Figure 4: Atomic-force-microscopy images of the surfaces of acrylic-resin samples stored in: a) air, b) distilled water, c) citric acid, d) heptane
Figure 3: Optical-microscopy micrographs of the cross-sections of the
broken resin samples: a) with cracks and small porosities (A and B),
b) magnification of the framed area

60 d, is constant and the second is variable where a
decrease in the fatigue resistance is observed. This be-
havior is characteristic of three storage environments.
For the first period, one possible explanation indicates
that the storage time is insufficient to see any change in
the resistance to fatigue. It assumes that liquids do not
have enough effect on the fatigue of the resin in the 10
4
cycles imposed and that it takes more stress or more
storage time to see a possibility for a material rupture. In
this step, the flexural-strength test was used after the
fatigue test to evaluate the mechanical properties of the
previously "fatigued" materials.
In this study, the flexure resistance of acrylic resin
was found to be increased after the immersion in the
three simulating liquids. This behavior may be justified
by the fact that the polymerization of the resin was
incomplete. This phenomenon was reported in previous
articles
16,19
where some residual monomer remained after
polymerization.
20,21
It is well known that when a polymerization reaction
is activated, the monomer converts into polymer chains
and this reaction finishes when all the monomer is
reacted. When more chains are formed due to the
polymerization, there is less residual monomer and the
material becomes more resistant. It should be noted that
the monomers lower the mechanical properties of dental
resins.
21,22
After the immersion in water, two processes
occur: plasticizer release and water absorption. Mainly,
plasticizers provide the flexibility and softness of
resins,
23
which may contribute to the reduction of the
mechanical properties.
24,25
But the residual monomer
acted as a plasticizer in the polymer structure,
22,26
con-
sequently leaching the residual monomer from the
specimens and contributing to the increase in the flexion
strength after the water storage.
16,27
The water absorption
of resin allows a release of unreacted components, prin-
cipally monomers.
25,28
The amount of the plasticizer that
leached from the resin increased with the increasing
immersion time, improving the strength of this material.
However, the present study also took into account the
effects of the other storage media (other than water) and
the same phenomena applied to the other two liquids. It
should be noted that the flexion-resistance variations are
not important enough for dry resin and stored resins.
Deterioration of the materials used for restorations often
occurs gradually during a process of fatigue. This is
illustrated in this work where the results of the second
phase showed that the fracture strength under dry con-
ditions was always higher than under wet conditions, in
which the fatigue-fracture resistance of the acrylic resin
after the immersion in the three liquids decreased with
the storage time. Before the decrease in the resistance to
fatigue, the samples resisted to the imposed 10
4
cycles
and the effects of the storage liquids appeared only after
60 d of immersion.
The failure due to the fatigue effort starts from a
small structural defect such as a void or microcrack in
the material, from which cracks are nucleated. Internal
defects, such as air bubbles or porosities, can act as
stress concentrators, contributing to the formation of mi-
crocracks under loading, thereby favoring a catastrophic
failure.
6,17
Under the action of cyclic forces, cracks begin
to form in the matrix and propagate, eventually causing a
fast fracture. The latter phase is very short and the cyclic
loading is able to drive the crack propagation, called a
slow crack growth.
14
However, the cracks continue to
propagate and interconnect, eventually resulting in a
catastrophic failure of the resin.
17
An inherent fault or
imperfection able to trigger an initiation of cracks occurs
in composites during the fabrication, as well as
restoration procedures.
29
During the fabrication, air voids
may be introduced when mixing the resin and when
setting the paste in the mold; these pores are trapped
inside the material due to the application of the pressing
force to the upper part of the mold. However, other
porosities resulting from the polymerization process may
also be formed.
30
The presence of these defaults persists
even after the hardening of the materials.
The fissuring mechanism mentioned above is
validated in Figure 3 where cracks initiated from the
pores are observed. It should be noted that the propaga-
tion of cracks is due to the action of external excitations.
However, in the present work, these are the cyclic
loading and the flexion, which cause the breaking of the
material. Figure 3b shows well the beginning of the
formation of two microcracks, propagating from a
defect. On the other hand, the small porosities shown in
Figure 3a and 3b are not wide enough to have sufficient
stress concentrations to create cracks.
Considering the effect of the leaching of monomers,
the number of porosities is not important enough to
undergo this failure mechanism before the 60th day of
the immersion since no failure is produced due to the
fatigue process. During this period, only flexion varies,
as previously described, due to a lower residual mono-
mer inside the material. Another aspect that must be
considered is the fact that the residual monomer will also
cause the formation of more porosities in the polymer
matrix when removed from it.
22
Therefore, when the
level of residual monomer is high, the number of voids
in the resin after leaching off the residual monomer will
be high. This phenomenon is remarkable for long pe-
riods of storage and thus the number of porosities is
more important, resulting in a decreased fatigue resis-
tance after 60 d and 90 d of storage.
From the obtained values of the fatigue, we can say
that time and exposure environment have effects on the
intensification of the internal defects. Thus, with citric
acid, the resin underwent a rapid degradation compared
to those of distilled water and heptane. This shows a
marking effect of citric acid on the resin compared to
distilled water. For heptane, its effect appeared only after
90 d of immersion when a very rapid fall of resistance
N. LAISSANI et al.: EFFECT OF FOOD-SIMULATING LIQUIDS ON MECHANICAL PROPERTIES AND ...
720 Materiali in tehnologije / Materials and technology 52 (2018) 6, 717–722

occurred. This medium was manifested better after 120 d
of immersion when the resin was completely weakened.
In the present study, surface morphologies of the
specimens were evaluated with AFM after 90 d of stor-
age. The AFM micrographs revealed differences in the
surface topography of the acrylic resin. This indicates a
change in the surface texture of the investigated material
caused by the immersion liquids compared with the resin
without storage. This illustrates the effects of these two
liquids on the surface of the biomaterial where a chemi-
cal reaction is produced between the antagonistic
elements. These new factors participated in a sudden
drop in the fatigue strength, in addition to the presence
of the porosities within the samples, as it was previously
described. These weaknesses occurred after 90 d of
storage when the time and nature of the immersion
liquids played a great role in reducing the fatigue of the
biomaterial. This fact can be explained with the cavities
caused by distilled water, which appear to provide an
easy field for crack propagation while the peaks exhibi-
ted by the citric acid can be considered as the stress
concentrators. On the other hand, heptane did not have a
large effect on the surface of the resin where the original
resin structure seems not to be changed. This resulted in
a feeble decrease of the fatigue resistance caused by a
relatively low porosity. However, 120-day storage seems
to be the cause of the biomaterial’s catastrophic failure.
This can be explained with the fact that the immersion
time allowed heptane to seep inside the material and ruin
it.
According to this study, longer periods of food-simu-
lating liquid storage are necessary to determine the dura-
bility of acrylic-resin restoration material. Under the
buccal condition, this component can have a significant
effect on the performance of this material, which may
result in a reduction in the physico-mechanical proper-
ties. Hence, oral hygiene is necessary to increase the
lifetime of this restoration material once it is placed in an
oral environment.
5 CONCLUSIONS
The effects of the food-simulating liquids such as
distilled water, citric acid and heptane on the mechanical
properties and surface morphology of acrylic resin were
studied. For the used liquids, results indicated that the
fatigue resistance remained unchanged during the first
60 d of storage, whereas flexion increased in a progres-
sive manner with the immersion time. After this period, a
fatigue-strength drop was recorded after the cyclic
loading where the resin was completely weakened by
heptane at the end of the fatigue test.
The AFM micrographs indicated differences in the
topography surface caused during the 90-day storage in
distilled water and citric acid, while heptane did not have
a significant effect.
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