dmj_39-4_00_2019-165.inddINTRODUCTION
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
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
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:
[email protected]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
Color figures can be viewed in the online issue, which is avail- able at J-STAGE. Received Jun 11, 2019: Accepted Sep 9, 2019 doi:10.4012/dmj.2019-165 JOI JST.JSTAGE/dmj/2019-165
Dental Materials Journal 2020; : –
Table 1 Materials used in the present study with their respectively composition and technical specifications according to the manufacturers
Material and color
Bis-GMA, Bis-EMA, UDMA, TEGDMA
Non-agglomerated/non- aggregated silica filler, non-agglomerated/non- aggregated 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
APX A2 BIS-GMA, TEGDMA
ELS A2 Bis-GMA, Bis-EMA
Barium glass and silica 4–3,000 nm 49% Saremco, Rohnacker, Switzerland
Bis-GMA: bisphenol A-glycidyl methacrylate; Bis-EMA: ethoxylated bis-phenol-A-dimethacrylate; UDMA: urethane dimethacrylate; TEGDMA: triethyleneglycol dimethacrylate.
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
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) :
2 Dent Mater J 2020; : –
Fig. 1 Stress-strain curves to establish the Young’s modulus of the three composite materials tested after wet and dry storage.
Table 2 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)
Test Composite material Storage condition*
Dry Wet
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).
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.
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
3Dent Mater J 2020; : –
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/ for and after a load of 150 MPa and the results of the same sample retested. Fig. 4 Deformation of the APX wet composite sample.
Table 3 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) Poisson ratio
Dry 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
dry and wet storage. Statistical differences were found among the
Poisson ratio of the tested groups (ELS>FS>APX). In
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%
4 Dent Mater J 2020; : –
Table 5 The Young’s modulus and Poisson ratio values found in the present study compared with the data found by other authors.
Author Type of test Young’s modulus (MPa) Poisson ratio
FS
Static compressive 24 h of dry storage
Dynamic 1 month storage dry/wet
7,502/5,694
5,760
12,400/9,780
0.43/0.41
0.45
0.37/0.38
Three-point bending 24 h of wet storage
12,499/12,063
15,300
0.24/0.30
Static compressive 24 h of dry storage
2,970/2,459
3,100
0.64/0.51
—
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
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
5Dent Mater J 2020; : –
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…