Dental Materials Journal 2020; 39(4): 657–663
Young’s modulus and Poisson ratio of composite materials: Influence of wet
and dry storage
Ana Lúcia LOURENÇO1, Niek De JAGER1, Catina PROCHNOW2, Danilo Antonio MILBRANDT DUTRA2
and Cornelis J. KLEVERLAAN1
1
Department of Dental Materials Science, Academic Center for Dentistry Amsterdam (ACTA), University of Amsterdam and Vrije Universiteit
Amsterdam, Amsterdam, The Netherlands
2
Oral Science (Prosthodontics Units), Faculty of Dentistry, Federal University of Santa Maria (UFSM), Santa Maria, Rio Grande do Sul State, Brazil
Corresponding author, Niek De JAGER; E-mail: n.de.jager@acta.nl
In the oral environment dental materials are subject to a wet condition what might in time change their elastic properties. In
this article, we evaluated the influence of the storage condition (dry versus wet) on the Young’s modulus and the Poisson ratio in
compression of three composite materials. The data of the Young’s modulus and Poisson ratio published of dental composite materials
are not always comparable, due to different test methods and sample dimensions influencing the results. Therefore, we established
the degree of exactness of the results out of the test set-up used. Since the present study depicted differences of the properties after
dry and wet storage, the elastic properties should be measured after wet storage. The bonding between the matrix and the filler
particles showed to have an influence on the elastic properties and on the influence of a wet environment.
Keywords: Dental materials, Storage conditions, Elastic properties, Test methods
INTRODUCTION
Nowadays, direct composite restorations are extensively
used to restore posterior teeth due to their low cost,
less time-consuming technique, minimal intervention
approach and good performance comparing to indirect
restorations1,2). Follow-ups of 10 up to 30 years showed
relatively low annual failure rates (1.1 to 2.2%) for direct
composite restorations3-8). These rates are similar to the
annual failure rates of indirect ceramic and composite
restorations, allowing to conclude that indirect
restorations do not have a better longevity than direct
composite restorations2).
In the oral environment, restorations are subjected
to stresses from mastication action. These forces act on
teeth and/or restoration materials leading to deformation,
which can compromise their durability over time9,10).
In this sense, the elastic properties of the composites
applied in the posterior region (high stress-bearing area)
are important to consider; the more elastic the material
is, the higher the deformation under masticatory loads
will be. This deformation can lead to fracture of the
restoration and surrounding tooth structures, increase
the risk of micro leakage by marginal separation,
secondary decay and/or filling dislodgement11). Therefore,
since the restorative materials are in contact with the
dental tissues (enamel or dentin), the elastic properties
of composite materials should be matched to those of the
teeth, resulting in a more uniform load transmission
across the restoration-tooth interfaces11,12). This is also in
accordance with the biomimetic approach, aiming not to
use the strongest restoration material but rather create
Color figures can be viewed in the online issue, which is available at J-STAGE.
Received Jun 11, 2019: Accepted Sep 9, 2019
doi:10.4012/dmj.2019-165 JOI JST.JSTAGE/dmj/2019-165
a restoration compatible with the mechanical, biologic
and optical properties of underlying dental tissues13).
In a homogeneous isotropic material, within the
elastic range, the Young’s modulus (E ) represents the
stiffness of a material, the higher the value, the higher
the stiffness.
The Poisson ratio (v) is the relation between lateral
to axial strain during axial loading, and it is also a
measure of the relative resistance to dilatation and
shearing. Both properties can be determined from a
stress-strain curve10,14).
Determination of the elastic constants of
homogeneous materials has been well established and
it is not difficult compared with the determination of
these properties of anisotropic, multi-layer or coating
materials9). There are several techniques to determine E
and v; the most used techniques can be grouped in static
and dynamic methods. Static methods measure the
deformation of a specimen by a known static compressive,
tensile or flexural load9,14-16). Dynamic methods consist
often of ultrasounds that generate waves inside the
specimen without destruction of the sample, in which E
is obtained from the ultrasound waves’ velocity11,17,18).
The data published of dental composite materials are
not always comparable, due to the different test methods
and sample dimensions used to determine the Young’s
modulus and Poisson’s ratio, influencing the results19).
For this reason, the geometry of the test setup was
simulated in FEA in order to establish to what degree
the test results represent the material properties.
Also, the storage condition might influence the
results since in a wet environment composite materials
absorb water eventually changing their mechanical
properties11). In the oral environment, the materials are
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subject to a wet condition what might in time change
their properties. Therefore, it is relevant to establish the
differences of the elastic properties between dry and wet
storage.
Taking into consideration the aforementioned
concepts, this in vitro study evaluated the Young’s
modulus and Poisson Ratio of three composite materials
used for posterior restorations after one month of storage
(dry and wet conditions) by using a static compressive
test.
The assumed hypotheses were; (1) The storage
condition will influence the elastic properties of the
composite materials, and (2) the properties determined
with the static compressive test set-up used will
represent well the elastic properties.
MATERIALS AND METHODS
The materials used and their technical specifications
are presented in Table 1. These materials were selected
because they are commonly used, represent materials
with a wide range of filler content, with different types
of filler; zirconia versus barium, and silanized versus not
silanized particles.
Specimens preparation
Thirty-six cylinders of three different dental composite
materials (Filtek Supreme, APX, and ELS) were
fabricated with a POM mold (internal diameter=3.1 mm;
length=5 mm; Mparts Mechanical Solutions, Lisse, the
Netherlands). The composite was placed inside the mold
and pressed between two glass plates. The cylinders
were light-cured (LED Curing Light Elipar™ S10, 3M
ESPE, St Paul, MN, USA) for 40 s from each surface.
After removal of the cylinders from the molds, additional
light-curing was performed for 90 s in an unit Dentacolor
XS light (Heraeus Kulzer, Hanau, Germany), in order to
Table 1
obtain as full polymerization as possible.
The specimens were stored for one month prior to
testing, and divided into two storage conditions (n=6),
as follow: dry (storage in the dark in a sealed vessel at
37°C), and wet (storage in the dark in a sealed vessel
with distilled water at 37°C).
Static compressive tests
Before testing, the length of each cylinder was measured
with a digital caliper. The compressive tests (0.05 mm/
min) were performed in a universal testing machine
(Instron 6022, Instron, Canton, MA, USA) with a loading
cell of 10 kN.
In order to register the elastic deformation during
the tests, a micrometer (Millitron, model 1202D, Mahr,
Deterco, Houston, TX, USA) connected to the testing
machine registered the decrease in length.
The enlargement of the diameter was measured
at the central part of the specimens with a laser scan
micrometer (LSM-6000, Mitutoyo, Kawasaki, Japan).
Differences in length and diameter were registered
continuously, starting at 100 N and after every load step
of 200 N until the test reach a total of 2,100 N.
The Young’s modulus was calculated using the
following equation, as previously described by Craig
(2012)10):
E=σ/ε
E=(F/A)/(∆L/L)
where E is the Young’s modulus (MPa), σ is the stress
(MPa), ε is the longitudinal or axial strain, F is the load
(N), A is the area (mm2), ∆L is the change in length (mm),
and L is the original length (mm).
The Poisson ratio was calculated using the following
formula10) :
Materials used in the present study with their respectively composition and technical specifications according to
the manufacturers
Filler size
Filler
content
% by
volume
Non-agglomerated/non- aggregated
silica filler, non-agglomerated/nonaggregated zirconia filler, aggregated
zirconia/silica cluster filler
20 nm silica
particles and 4
to 11 nm zirconia
particles
63.3%
3M ESPE,
St Paul,
MN, USA
BIS-GMA,
TEGDMA
Silanized barium glass fillers, silanized
silica fillers, silanized colloidal silica
0.02 to 17 μm
71%
Kuraray Medical,
Okayama, Japan
Bis-GMA,
Bis-EMA
Barium glass and silica
4–3,000 nm
49%
Saremco,
Rohnacker,
Switzerland
Material and
color
Resin
Filtek
Supreme XTE
(FS) A2b
Bis-GMA,
Bis-EMA,
UDMA,
TEGDMA
APX A2
ELS A2
Filler
Manufacturer
Bis-GMA: bisphenol A-glycidyl methacrylate; Bis-EMA: ethoxylated bis-phenol-A-dimethacrylate; UDMA: urethane
dimethacrylate; TEGDMA: triethyleneglycol dimethacrylate.
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v=εt/εl
v=(∆D/D)/(∆L/L)
where εt is the transverse strain, εl is the longitudinal
or axial strain, ∆D is the change in diameter (mm), D is
the original diameter (mm), ∆L is the change in length
(mm), and L is the original length (mm).
Finite element analyses (FEA)
A FEA model was made of the test set-up. The model
consisted of the composite cylinder (diameter=3.15
mm; length=5.1 mm) between the two steel supports. A
friction coefficient of 0.45 was assumed for the contact
surface between the steel and the composite. Since the
model is symmetric, only half of the model was made to
facilitate the boundary conditions.
The finite element modeling was carried out using
FEMAP software (FEMAP 10.1.1, Siemens PLM
software, Plano, TX, USA), while the analysis was
performed using the NX Nastran software (NX Nastran,
Siemens PLM Software).
The material properties used are based on the
results of the tests. Since the materials are linear
elastic, the Young’s modulus used is only the average
Young’s modulus found of the three composite materials
tested. The Poisson ratio’s cover the range of the Poisson
ratio’s found of FS and APX. The models were composed
of 21,104 parabolic tetrahedron solid elements. A load
of 1,000 N was applied on the top surface of the steel
support of the models. The nodes in the centric plane of
the half samples were allowed for sliding in the surface
only. The nodes at the top of the steel support were fixed
in the horizontal plane, allowing movement only in the
vertical direction. The nodes in the center of the model
were also fixed in the horizontal plane with no rotation
allowed around the vertical axis. The displacement
in vertical direction and the increase of the diameter
were calculated with the 1,000 N force as input. In
post processing, the contour option “average elemental”
without use of the “corner data” was used for visualizing
the results.
Table 2
Statistical analyses
The statistical analyses were performed with SPSS
(IBM SPSS Statistics Version 24, IBM, Armonk, NY,
USA). A parametric distribution (Levene’s test) of the
Young’s modulus and Poisson ratio data was assumed.
One-way ANOVA and post hoc Tukey HSD (α=0.05) were
performed to compare different composite materials for
each storage condition. To compare the different storage
conditions for each composite material, independent t
tests (α=0.05) were performed.
RESULTS
The results of the Young’s modulus and Poisson ratio are
presented in Table 2. Figure 1 shows the stress-strain
curves and Fig. 2 the transverse strain-longitudinal or
axial strain curves for all composite materials tested in
the present study.
For the Young’s modulus, all the materials differed
from each other (APX>FS>ELS). Comparing only the
storage condition, FS and ELS wet presented statistically
significant lower Young’s moduli than the dry stored
samples. APX showed no statistical difference between
Fig. 1
Stress-strain curves to establish the Young’s
modulus of the three composite materials tested
after wet and dry storage.
Young’s modulus (in MPa) and Poisson ratio data (mean and standard deviation) after the static compressive test
in both storage conditions (wet and dry)
Storage condition*
Test
Composite material
Dry
Wet
Young’s modulus
(MPa)
FS
APX
ELS
7,502 (736)Ba
12,499 (1993)Aa
2,970 (119)Ca
5,694 (147)Bb
12,063 (661)Aa
2,459 (45)Cb
Poisson ratio
FS
APX
ELS
0.43 (0.05)Ba
0.24 (0.03)Cb
0.64 (0.05)Aa
0.41 (0.02)Ba
0.30 (0.03)Ca
0.51 (0.02)Ab
*Different uppercase letters indicate statistical difference among the materials in each test condition (One-way ANOVA,
Tukey HSD; α=0.05); different lowercase letters indicate statistical difference between dry and wet condition for each
composite material (Independent t-tests; α=0.05).
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dry and wet storage.
Statistical differences were found among the
Poisson ratio of the tested groups (ELS>FS>APX). In
Fig. 2
Transverse strain versus longitudinal strain curves
to establish the Poisson ratio of the three composite
materials tested after wet and dry storage.
Fig. 3
The stress-strain curve of the APX samples
showing the two different Young’s moduli E=σ/ϵ for
and after a load of 150 MPa and the results of the
same sample retested.
Table 3
the individual comparisons (dry and wet), there was
statistical difference for ELS and APX, being ELS dry
higher than ELS wet and APX dry lower than APX wet.
The APX samples showed a different Young’s
modulus at lower loads than after approximately 150
MPa, see Fig. 3. In order to understand this behavior, the
samples were retested after testing, the samples in the
first test were named APX 1 and the retested samples
were named APX 2. When retesting the same samples,
APX showed a Young’s modulus equal to the one in the
first test after the 150 MPa load (Table 3, Fig. 3).
The FEA model is presented in Fig. 4, where it is
possible to observe that the increase of the diameter is
constant over the length of the sample, except close to
the steel supports. Table 4 shows the materials used for
the FEA (Young’s modulus and Poisson ratio) and the
results calculated, showing a difference of less than 5%
Fig. 4
Deformation of the APX wet composite sample.
Young’s modulus (in MPa) and Poisson ratio data (mean and standard deviation) between the two tests of APX:
APX 1 (first time tested); APX 2 (second time tested)
Young’s modulus (MPa)
Dry
Poisson ratio
Wet
Dry
Wet
APX1
12,499 (1993)
12,063 (661)
0.24 (0.03)
0.30 (0.03)
APX2
8,542 (728)
7,510 (192)
0.48 (0.04)
0.49 (0.01)
Table 4 Young’s modulus and Poisson ratio used for the FEA analysis and the respective results after calculation
Material prproperti
Results
Young’s modulus (MPa)
Poisson ratio
Calculated Young’s modulus (MPa)
Calculated Poisson ratio
7,198
0.24
7,131
0.24
7,198
0.43
7,446
0.44
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Dent Mater J 2020; 39(4): 657–663
between the values used as input and those obtained by
the FEA analysis.
DISCUSSION
The present study evaluated the Young’s modulus and
Poisson ratio of three composite materials used in the
posterior region after aging in two different storage
conditions (dry and wet), by a static compressive test.
Since not all the groups presented differences between
dry and wet condition for both elastic properties (Table
2), the first hypothesis that the storage condition would
influence the elastic properties was partially denied.
Also, our study purposed to establish to what degree
the test results represent the material properties; our
results showed a difference of less than 5% for the data
used as input, being the material properties and the
results obtained by the FEA analysis in the geometry of
the test set-up (Table 4). The small difference is caused
by the exception of the constant increase of the diameter
over the length of the sample in the area close to the
steel supports, being the second hypothesis accepted.
The knowledge of the material properties is
essential to support the correct indication of the
composite materials and expecting herewith a better
performance12). In this sense, in the oral environment,
the composite dental materials are exposed to a wet
environment. Several studies have searched the elastic
properties of composite materials to predict their
behavior over time17,20-22). However, most of the studies
did not compare dry and wet storage conditions, and also
the test set-up is not standardized, making it difficult to
compare data (Table 5).
In the present study, the storage period of one
month was chosen to allow for the decline of all
leachable, unreacted components and post-cure of the
composite20). Incomplete polymerization may influence
Table 5
The Young’s modulus and Poisson ratio values found in the present study compared with the data found by other
authors.
Author
FS
the results10) due to a heterogeneity of the specimen (i.e.,
porosities), compromising the stress distribution during
the tests16,23). Also, the storage condition (i.e., dry or wet)
of the samples can influence the results. The composite
materials tested had the elastic properties partly affected
by the storage condition (Table 2, Figs. 1 and 2).
This is in accordance with the findings of
Papadogiannis et al.20) that the wet storage at higher
temperatures (37 and 50°C) adversely affected the
properties of the tested materials in a range from 40 to
60 % compared to storage at room temperature (21°C)
and dry condition.
The stiffness (i.e., Young’s modulus) was different for
the composite dental materials tested (APX>FS>ELS),
meaning that APX needs higher loads to deform than FS
and ELS. Comparing only the storage condition, FS and
ELS showed significant lower Young’s modulus after
wet than after dry storage. Therefore, the degradation
of the composites during the wet storage turned those
materials less stiff, in line with the results after wet
storage reported by Papadogiannis et al.20). However,
APX did not present a reduction in the stiffness after
water storage. According to Boaro et al.24) composite
materials with higher filler content present lower water
sorption, explaining the similar elastic modulus found
for APX (approximately 70% of filler content by volume)
in both storage conditions.
The Poisson ratio of several dental composites was
found in the literature to be between 0.23 and 0.44 [10,
11, 14, 18]. Our results are in agreement with that for
APX and FS (0.24 up to 0.43), however ELS presented
values higher than 0.50. Theoretically, values above
0.50 are not possible for homogeneous materials, but the
tested materials are composites.
Partly lack of bond between the matrix and filler
particles, making the matrix an object with holes, might
explain the higher than 0.50 Poisson ratio. Further
Type of test
Young’s modulus (MPa)
Poisson ratio
This study
Static compressive
1 month storage dry/wet
7,502/5,694
0.43/0.41
Masouras et al.17)
Static compressive
24 h of dry storage
5,760
0.45
Papadogiannis et al.15)
Dynamic
1 month storage dry/wet
12,400/9,780
0.37/0.38
This study
Static compressive
1 month storage dry/wet
12,499/12,063
0.24/0.30
Thomsen and Peutzfeldt21)
Three-point bending
24 h of wet storage
15,300
This study
Static compressive
1 month storage dry/wet
2,970/2,459
Bacchi et al.22)
Static compressive
24 h of dry storage
3,100
APX
ELS
—
0.64/0.51
—
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Dent Mater J 2020; 39(4): 657–663
study will be necessary to prove this. FS did not present
statistical difference between wet and dry conditions,
which means that the water storage was not able to
change the resistance to dilatation (i.e., Poisson ratio).
On the other hand, APX presented higher values of
Poisson ratio after wet storage.
APX material showed for both storage conditions a
different behavior after a load of approximately of 150
MPa, being the Young’s modulus lower after a load of
approximately 150 MPa (Fig. 3). Locally stresses due to
occlusal forces will occur in the posterior region, which
exceeds these stresses as has been demonstrated by
Yang et al.25). The found stresses were >250 MPa for a
composite with a Young’s modulus of 9.5 GPa, comparable
with APX after a load of 150 MPa, before this load the
stresses will be still higher due to the higher Young’s
modulus. However, this stresses occurred only in the
points of occlusion. This means that only very locally the
properties of APX will deteriorate with deformation of
the restoration at the occlusal points of contact.
This lower Young’s modulus is permanent; after
retesting the tested samples APX1, the retested samples
APX2 show only one Young’s modulus, which is equal
to the one in the first test after the load of 150 MPa.
After this load, APX behaves like the other materials,
having a lower Young’s modulus after wet storage (Table
3, Fig. 3). This phenomenon can be explained by that
the bond between filler particles and matrix for APX is
partly broken after a certain load (approximately 150
MPa). After the filler particles become partly loose from
the matrix, they do contribute less to the resistance to
deform, and as a consequence, the composite become
less stiff. This is in line with the findings of Sahu and
Broutman26). After the filler particles become partly
loose of the matrix, the elastic properties of APX are also
influenced by the wet storage, showing the influence of
the bond between the filler particles and the matrix on
the resistance to the change of the elastic properties.
Although, in the clinical situation the compressive
strength is intuitively important, most restorations are
likely to fail in tension or shear. Still, the restorations
are loaded in compression during mastication making
the mechanical properties as the Young’s modulus and
Poisson ratio in compression important. A material with a
low Young’s modulus will deform more under masticatory
forces. Such deformations could have catastrophic effects
on the restoration and the surrounding tooth structures.
Deformations could further increase the risk of micro
leakage. Therefore, it seems reasonable to conclude that
the usage of composites with low elastic modulus should
be avoided in posterior regions. The most appropriate
modulus for a composite would be one comparable to
that of the for dentin12). Clinicians must be aware of
this aspect to make a proper material selection based
on specific properties and indications of each material
relevant to a particular clinical situation.
These properties should be measured after wet
storage and with the indication if it was tested in tension
or in compression. Wet storage is comparable with the
exposure and consequently possible degradation in
the clinical situation; therefore, the Young’s modulus
and Poisson ratio of the composite materials should be
measured after wet storage of the samples.
CONCLUSIONS
Wet storage is comparable with the exposure and
consequently possible degradation in the clinical
situation. The present study depicted differences of
the properties after dry and wet storage, therefore, the
Young’s modulus and Poisson ratio of the composite
materials should be measured after wet storage. The
bonding between the matrix and the filler particles
showed to have an influence on the elastic properties and
on the influence of the exposure to a wet environment on
these properties. In order to prevent degradation in the
clinical situation, composites with a good bond between
filler particles and matrix are the preferred composites.
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