Materials Science and Engineering C 54 (2015) 69–75
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Effects of temperature change and beverage on mechanical and tribological properties of dental restorative composites M.R. Ayatollahi a,⁎, Mohd Yazid Yahya b, A. Karimzadeh a, M. Nikkhooyifar a, Amran Ayob b a Fatigue and Fracture Laboratory, Center of Excellence in Experimental Solid Mechanics and Dynamics, School of Mechanical Engineering, Iran University of Science and Technology, Narmak, Tehran 16846, Iran b Center for Composite, Institute for Vehicle System and Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Malaysia
a r t i c l e
i n f o
Article history: Received 3 September 2014 Received in revised form 14 March 2015 Accepted 2 May 2015 Available online 5 May 2015 Keywords: Dental restorative materials Nanoindentation experiment Nanoscratch experiment Thermocycling effect Beverage effect
a b s t r a c t The aim of this study was to investigate the effects of temperature change and immersion in two common beverages on the mechanical and tribological properties for three different types of dental restorative materials. Thermocycling procedure was performed for simulating temperature changes in oral conditions. Black tea and soft drink were considered for beverages. Universal composite, universal nanohybrid composite and universal nanofilled composite, were used as dental materials. The nanoindentation and nanoscratch experiments were utilized to determine the elastic modulus, hardness, plasticity index and wear resistance of the test specimens. The results showed that thermocycling and immersion in each beverage had different effects on the tested dental materials. The mechanical and tribological properties of nanohybrid composite and nanocomposite were less sensitive to temperature change and to immersion in beverages in comparison with those of the conventional dental composite. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Dental composite materials have different applications in dentistry such as filling the tooth cavities, veneering to mask discoloration, and correcting contour and alignment deficiencies. These materials are also used for making dental implants and bonding orthodontic brackets [1,2]. It is often desirable to find new methods for improving the mechanical properties of dental restorative composites [3–5]. In the past few years, one of the most important advancements in dental materials is related to the application of nanotechnology to dental restorative composites. For example, nano-fillers with dimensions of 5 to 100 nm have been added to the composite resins for producing dental nanocomposite. Nano-hybrid composites are also a category of dental restorative materials where in addition to nanometer particles, particles of 0.2 to 1 μm in size are added to the composite resins. It has been shown in the past that dental nanocomposites and nanohybrid composites can be used as dental restorative materials, instead of the conventional dental composites [6–10]. The general aims for producing these new dental materials were to achieve better tensile and compressive strengths, improved fracture toughness and wear resistance, firm bonding with dental enamel surface and desired aesthetic performance [7,11–13]. A good knowledge of the mechanical and tribological properties of new dental materials in various oral conditions would help clinicians in comparing
⁎ Corresponding author. E-mail address: [email protected] (M.R. Ayatollahi).
http://dx.doi.org/10.1016/j.msec.2015.05.004 0928-4931/© 2015 Elsevier B.V. All rights reserved.
the behavior of different dental materials and selecting the appropriate one. It could also be useful for manufacturers of dental materials for improving their products. The variation of temperature in oral environment can affect the mechanical properties of dental restorative materials. Eating hot or cold foods and drinking beverages are the causes of most extreme temperature variation in the oral cavity. Typical minimum and maximum temperatures of tooth surface during the consumption of food stuffs are 1 °C and 50 °C [14–16]. Although the mechanical and tribological properties of commercial dental restorative materials are often measured at room or body temperature [12,14,15,17,18], the mechanical properties of polymeric materials like dental composites are often sensitive to temperature variations. Therefore, it is important to evaluate the effects of oral conditions on the mechanical and tribological properties of dental restorative composites. Thermocycling is an in-vitro process which simulates temperature variations of the oral cavity that occur through eating, drinking, and breathing [19–22]. In addition to the temperature variations in oral conditions, water or other liquids such as saliva, beverages or food components may influence their physical and mechanical properties. Global statistics show that the consumption of carbonated soft drinks in the past 50 years has increased dramatically. Consumption of acidic drinks can degrade both the teeth and the restorative materials. For example, Lussi and Hellwig [23] reported a reduction in enamel hardness after 10 and 20 min of immersion in a commercial orange juice. Aliping-McKenzie et al. [24] immersed dental restorative materials in Coca-Cola and fruit juices and found significant reduction in their surface hardness.
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In this paper, the effects of thermocycling and immersion in two types of beverages on the mechanical and tribological properties of several dental restorative composites are studied. The null hypothesis was that new dental restorative materials preserve their mechanical and tribological properties in different oral conditions compared with the conventional dental composites. Nanoindentation and nanoscratch tests have been found to be powerful methods for measuring the mechanical properties such as modulus of elasticity, hardness and wear resistance of various materials in very small sample sizes such as polyamide-12/layered silicate nanocomposites, clay nanocomposites, bone cement and tooth enamel [25–31]. Therefore, in the current study the nanoindentation and nanoscratch experiments are employed to determine the mechanical and tribological properties of dental restorative composites.
2.3. Immersion in beverages For investigating the effect of beverage alone and also simultaneous effect of thermocycling and beverage on the dental restorative materials, a non-thermocycled sample and a thermocycled sample for each type of material were immersed in carbonated soft drink (Coca-Cola) and another pair was immersed in black tea for 48 h. Each specimen was placed in a separate container. The temperature was kept at 55 ± 5 °C for tea and at 5 ± 2 °C for soft drink to reach more realistic evaluation of beverage effects. All in all, five samples were prepared for each type of the three investigated dental materials: (1) control or intact sample (non-thermocycled and non-immersed in any beverage), (2) non-thermocycled and immersed in tea, (3) non-thermocycled and immersed in soft drink, (4) thermocycled and immersed in tea, and (5) thermocycled and immersed in soft drink.
2. Materials and methods 2.4. Nanoindentation experiment
2.1. Sample preparation Three types of dental restorative materials, i.e. composite, nanohybrid composite and nanocomposite were used to prepare the samples. The characteristics of these materials are presented in Table 1 according to the manufacturer's information. Five specimens from each material were prepared in a disc-shaped mold with a diameter of 10 mm and a thickness of 4 mm. Due to the thickness of specimens, each consists of two light cure increment. According to the manufacturer's instructions of the restorative materials, each increment was light cured for 20 s using a LED light with intensity of 400 mW/cm2. During the light exposure, the light guide tip was held as close to the specimen surface as possible. Since nanoindentation and nanoscratch tests need a very smooth surface, all the specimens were smoothed by sandpapers with 400 to 2500 grits and then polished by diamond pastes with mesh sizes of 1, 0.5 and 0.25 μm.
2.2. Thermocycling procedure In order to study the effect of cyclic thermal stresses that occur in the mouth during the service life of the dental restorative materials, two samples for each type of material were thermocycled. The thermocycling apparatus consists of two stainless steel water baths filled with distilled water and a mechanical arm that transports the specimens from one bath to another. The ISO TR 11450 standard indicates that a thermocycling procedure of 500 cycles in water between +5 °C and +55 °C is an appropriate artificial aging test [32]. However, long-term aging process could be selected for long-lasting restorative materials such as materials used in this study [33–37]. Therefore, two samples from each material were subjected to 2000 cycles between the temperatures of + 5 and +55 °C, with a dwell time of 30 s in each bath per cycle and a transport over time of 15 s. A total number of 2000 cycles of thermocycling in this study can be considered to be equal to maintaining the dental materials for 200 days in the mouth conditions.
The basic mechanical properties of the test specimens, i.e. hardness, elastic modulus and plasticity index were determined by a Triboscope nanoindentation test system (Hysitron Inc., USA) based on ISO 14577 [38]. A Berkovich indenter was used in all experiments. This type of indenter is usually used for bulk samples and for thin films thicker than 100 nm. The test setup and the Berkovich indenter tip are shown in Fig. 1. Oliver and Pharr method [39] was used to calibrate the device and also to analyze the test results. An indentation load of 1950 μN was applied to the surface of each sample in 30 s with a constant loading rate. To take account of likely creep effects, the indenter was kept on the sample at the maximum load for 10 s [40]. Finally, the unloading stage of the test was performed by removing the tip from the sample at the same rate. For each sample, the nano-indentation tests were repeated at least 5 times in different randomly selected sites, and the curves of force versus indenter displacement were recorded through the test instruments. According to the simple power and sample analysis, when the experiment repeats 5 times in each of the 5 groups, the 0.05 level of difference in means will be detected by 90% power, assuming that the common standard deviation is 0.500 [41]. Before and after the experiments, atomic force microscopy (AFM) images were taken from the sample surfaces to analyze the indentation hole. In the AFM process, the same indenter tip as the nano-indentation test, scans the surface of specimen without applying any load. 2.5. Nanoscratch experiment Each nano-scratch test was performed in three main stages. First, a pre-scan of each sample was carried out and AFM images were taken to investigate the roughness and tilt angle of the sample surface. Then, the nano-scratch test was performed on the sample by using the same indenter as that of the nano-indentation test. The normal load was 1950 μN and the scratch length was 4 μm. The indenter penetrated the
Table 1 Dental restorative materials used for experiments. Type
Material
Type of resins
Filler particles
Filler content
Shade
Manufacturer
Composite Nanohybrid composite Nanocomposite
Filtek Z250 Filtek Z250 XT Filtek Z350 XT
Bis-GMA1, UDMA2, Bis-EMA3 Bis-GMA, UDMA,PEGDMA4, TEGDMA5 Bis-GMA, UDMA, TEGDMA, PEGDMA, Bis-EMA
Zirconia/silica Silica, Zirconia/silica Zirconia, silica
60% 67.8% 63.3%
B1 B1 WE
3M ESPE, USA 3M ESPE, USA 3M ESPE, USA
1 2 3 4 5
Bis-GMA: bisphenol A glycidyl methacrylate. UDMA: diurethane dimethacrylate. Bis-EMA: ethoxylatedbisphenol A dimethacrylate. PEGDMA: polyethylene glycol dimethacrylate. TEGDMA: triethylene glycol dimethacrylate.
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where Pmax is the maximum normal load and A is the projected contact surface between the sample and the indenter at the maximum load. The average values of elastic modulus and hardness measured for all specimens are illustrated in Table 2. In the control samples, the elastic modulus and hardness of the nanocomposite and the nanohybrid composite are higher than the corresponding values for the conventional dental composite materials. For all three dental materials, the elastic modulus and hardness values of the “non-thermocycled and immersed” specimens were decreased in comparison to the relevant control sample. A comparison between the elastic modulus and hardness values of the samples immersed in tea and soft drink with those of the corresponding control samples demonstrates that no significant difference exists between the effects of tea and soft drink on these properties. Moreover, the reduction in the properties of conventional dental composite is higher than other two restorative materials. 3.2. Elastic–plastic behavior The plasticity index (ψ) is a tool for characterizing the elastic–plastic response of a material under external loads. In the nanoindentation test, the plasticity index can be calculated from [43]:
Fig. 1. a) Triboscope nanoindentation test system, b) Berkovich indenter tip.
sample surface and was drawn with a constant scratching speed of 0.13 μm · s− 1. Finally, a post-scan was performed by taking an AFM image from the scratch site on the sample. The wear resistance of the material could be studied by comparing the pre-scan AFM images with the post-scan ones. 3. Results 3.1. Elastic modulus and hardness The elastic modulus (E) and hardness (H) of each sample can be obtained from a complete cycle of loading and unloading during the nanoindentation test. In this study, the Oliver and Pharr method [39] was applied to determine these properties from the experimental data. The following equations were used for calculating the elastic modulus and hardness from the results of nanoindentation experiment.
ψ¼
hm −he hm
ð4Þ
ð1Þ
where, Er is the reduced modulus of the material obtained from the nanoindentation test, ν is the Poisson's ratio of the test sample, and Ei and νi are the elastic modulus and Poisson's ratio of the indenter, respectively. Based on the technical data available for the related Triboscope system, Ei and νi are equal to 1140 GPa and 0.07. Considering ν = 0.31 [42], one can determine the elastic modulus of each sample by using Eq. (1). The material hardness (H) could also be calculated from: H¼
ð3Þ
where A1 is the area under the loading segment of load–displacement curve which is equal to the total energy expended during the indentation procedure and A2 is the area under the unloading segment which is equal to the energy released during the indentation due to viscoelastic behavior. These areas are depicted in Fig. 2. Indeed, subtraction of A1 and A2 is equal to the irreversible plastic work during the nanoindentation test. For materials with viscoelastic–plastic behavior such as polymers, the range of this index is from zero which indicates perfectly elastic behavior, to one which represents fully-plastic behavior. Since in this study a Berkovich indenter was used for all experiments, the plasticity index can be calculated by Eq. (4) [30]:
ψ¼ 1 1−ν2 1−νi 2 þ ¼ E Ei Er
A1 −A2 A1
Pmax A
ð2Þ
where hm is the maximum penetration depth of the indenter and he is its elastic reversible depth. The difference between these two depths indicates the residual penetration depth which is measured from the load–displacement curve. Table 3 shows the values of plasticity index calculated by Eq. (4) for all tested materials. Based on Table 3, the values of plasticity index for the nonthermocycled samples don't change significantly after immersion in the tea compared to that of the control group. However, this index increases after immersion in the soft drink for all tested materials. Results show that the plasticity index decreases after thermocycling and immersion in the beverages.
Table 2 Elastic modulus and hardness of samples obtained from the nanoindentation test. Material type:
Control sample Non-thermocycled and immersed in tea Non-thermocycled and immersed in soft drink Thermocycled and immersed in tea Thermocycled and immersed in soft drink
Conventional composite
Nanohybrid composite
Nanocomposite
Elastic modulus (GPa)
Hardness (GPa)
Elastic modulus (GPa)
Hardness (GPa)
Elastic modulus (GPa)
Hardness (GPa)
11.46 ± 0.46 10.17 ± 0.38 9.68 ± 0.43 12.74 ± 0.38 11.96 ± 0.31
0.46 ± 0.05 0.34 ± 0.02 0.31 ± 0.03 0.53 ± 0.03 0.49 ± 0.02
13.77 ± 0.46 13.12 ± 0.34 12.93 ± 0.40 13.81 ± 0.44 13.53 ± 0.36
0.72 ± 0.07 0.65 ± 0.03 0.63 ± 0.04 0.77 ± 0.02 0.73 ± 0.03
12.48 ± 0.42 11.64 ± 0.04 11.31 ± 0.59 12.76 ± 0.54 12.34 ± 0.39
0.71 ± 0.08 0.62 ± 0.03 0.59 ± 0.05 0.74 ± 0.01 0.69 ± 0.03
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have almost the same destructive effects on the wear resistance of all tested materials. 4. Discussion
Fig. 2. A schematic load–displacement curve in nanoindentation tests.
3.3. Wear resistance Due to the considerable roughness of thermocycled and immersed samples the nanoscratch experiment was performed only on the control group and on the “non-thermocycled and immersed in beverage” specimens. In fact, after the thermocycling and immersion procedures the surfaces of samples couldn't be smoothed by sandpapers or be polished. The residual depth of the indenter on the surface of sample is often used to investigate the material deformation after the nanoscratch test. Fig. 3 shows the AFM images and longitudinal scratch sections of control and immersed samples for the nanohybrid composite. Erosion of the sample surface caused by immersing in the beverages is clearly visible in the AFM images of immersed samples. The erosion has increased the surface roughness of the immersed samples compared to the control sample. The AFM images of scratches created by the nanoscratch test on the sample surfaces illustrate that the residual scratch depths on the surfaces of immersed samples are more than that of the control sample. Thus, after immersing in beverage, less force is needed to penetrate and slide the indenter on the sample surface indicating higher softness of the material. This implies that drinking too much tea or soft drink decreases the wear resistance of the dental restorative materials. The ratio of the lateral force to the normal force (LF/NF) during the scratch test was also calculated for each test specimen. Table 4 shows the values determined for this ratio for different samples. It is seen from this table that the ratio of LF/LN reduces when the dental materials are immersed in tea or soft drink. In other words, the immersed samples require less lateral force than the control sample for sliding the indenter on the specimen surface. The reduction of LF/NF for the composite specimens is more than those for the nanohybrid composite ones. The soft drink and the tea
As mentioned earlier, the main objective of this study was to explore whether two new types of dental restorative materials preserve their mechanical and tribological properties in different oral conditions better than conventional dental composites. The results obtained from the nanoindentation and nanoscratch experiments indicated that the nanocomposite and nanohybrid composite samples could preserve their properties better than the dental composite when the samples were thermocycled or immersed in tea or soft drink. Therefore, from a clinical point of view, one may prefer to use these two types of dental materials for dental treatments or in orthodontic applications as their mechanical properties are less sensitive to oral conditions. It is worth mentioning that the main purpose of this paper was to provide a general study on the effects of various oral conditions on the mechanical properties of three different types of dental materials. It is essential to extend this study by performing a statistical analysis on the experimental data. However, for a reliable statistical analysis, a larger number of specimens per group is required. The increase in the elastic modulus and hardness of the nanocomposite and the nanohybrid composite in the control group in comparison with the corresponding values for the conventional composite may be due to the amount of filler loading in these materials, as shown in Table 1. The nanohybrid composite has the highest amount of filler loading and the largest elastic modulus value, while the composite sample has the lowest amount of filler loading and the lowest elastic modulus. Accordingly, a direct relationship is seen between the elastic modulus or hardness of dental restorative materials and their filler loading. Masouras et al. [44] have also found a similar behavior for bulk moduli of resincomposites. Based on the results presented in Table 2, the elastic modulus and hardness values of specimens were decreased significantly when immersed in tea and soft drink. Moreover, the decrease in the properties of conventional composite is higher than those of other two restorative materials. For instance, the hardness value of the composite decreased about 26% after immersion in tea, while for nanohybrid composite and nanocomposite the reductions were about 10% and 12%, respectively. Therefore, it is expected that nanohybrid composite and nanocomposite would be better option than the composite materials specially for long lasting dental applications. This could be attributed to the difference in the filler loadings and the types of resins in these materials. The chemical agents existing in beverages and food can affect polymer composites by their chemical degradation [45]. Cross linking density, fillers, porosity and solvents may influence water sorption on the surface of polymerbased composite materials [46]. The degradation of composites by water molecules occurs by two mechanisms; (i) diffusion into the polymer network and (ii) hydrolysis reaction. In the first mechanism, water molecules diffuse into the polymer network and fill the empty spaces between the polymer chains and the micro voids. This phenomenon leads to the plasticization and swelling of polymer matrix. Thus, the polymer chain scission takes place and results in the monomer elution
Table 3 The value of plasticity index obtained for the tested materials. Plasticity index ( ) Non-thermocycled specimens
Thermocycled specimens
Material
Control
In tea
In soft drink
In tea
In soft drink
Composite Nanohybrid composite Nanocomposite
0.51 ± 0.05 0.47 ± 0.03 0.45 ± 0.03
0.56 ± 0.03 0.50 ± 0.02 0.49 ± 0.03
0.58 ± 0.05 0.52 ± 0.04 0.51 ± 0.03
0.48 ± 0.02 0.44 ± 0.02 0.43 ± 0.02
0.49 ± 0.02 0.46 ± 0.02 0.45 ± 0.01
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Fig. 3. AFM image and scratch section of nanohybrid composite a) Control sample, b) Immersed in tea and c) Immersed in soft drink.
[46–48]. In the second mechanism, hydrolysis reaction degrades the siloxane bonds (i.e. the bond between the silane coupling agent and silanol groups of the silica surface) and this leads to debonding of filler [47]. In general, all of these factors and events cause the degradation and softening of composite resins and consequently decrease some of the physical and mechanical properties of the immersed materials such as elastic modulus and hardness. In addition, the acids in beverages could accelerate and enhance the effects of these mechanisms. Asmussen [49] reported that in the presence of acids, erosion and plasticization of polymer matrix become more pronounced. A reduction in the surface hardness of dental restorative materials has been also reported in [24] after their immersion in the Coca-Cola and fruit juices. The presence of citric acid in these beverages could make chemical activities which increase the surface erosion [50]. The surface erosion leads to a decrease in the mechanical properties of dental restorative materials. Therefore, in the case of immersion in soft drink, reductions in the values of elastic modulus and hardness of the materials are reasonable. According to Table 2, the hardness and elastic modulus of all thermocycled and “immersed in tea” samples are higher than those of the control samples. However, for thermocycled and “immersed in soft drink” samples various trends are observed for different materials. These dissimilar variations seem to be due to different effects on material properties from the thermocycling process and from immersion in the beverages. Thermocycling process can affect the samples in two separate ways: 1) Thermal stresses (caused by temperature changes in the oral conditions) are generated between the structural components of the materials. 2) Moisture affects the structure of dental restorative materials used in dental treatments. Thermal stresses can improve the bond strength between material components and increase the hardness and elastic modulus. The effect of moisture was described earlier in the part dealing with the immersion Table 4 Ratio of lateral force to normal force (LF/NF). LF/NF Material
Control
In tea
In soft drink
Composite Nanohybrid composite Nanocomposite
0.25 ± 0.01 0.28 ± 0.01 0.27 ± 0.02
0.21 ± 0.01 0.26 ± 0.02 0.24 ± 0.02
0.20 ± 0.01 0.25 ± 0.01 0.23 ± 0.01
conditions of samples in beverages. Meanwhile, because of thermal stresses generated in the thermocycling process, the bond strength between the composite components is improved and the empty space between the polymer chains decreases. Thus, in the thermocycled samples, the diffusion of water molecules into the polymer network reduces in comparison with the non-thermocycled ones. Diffusion, on the other hand, leads to the plasticization and swelling of polymer matrix. This would result in scission of polymer chains and elution of monomers [46, 47]. Various effects of humid environment on the mechanical properties of dental restorative composites were observed in previous studies. Some researchers [51,52] have found that the storage of samples in water increases the elastic modulus while some others e.g. [53] have reported a reduction in the elastic modulus. Some papers [54,55] have also reported that it has no influence on the elastic modulus. Therefore, the effects of humid environment on the elastic modulus can be suggested to be dependent on the material characteristics and the storage conditions such as temperature and storage time. In general, the thermocycling process would improve and the water sorption and chemical agents would degrade the elastic modulus and hardness of dental restorative materials. As shown in Table 3, for both non-thermocycled and thermocycled specimens, the plasticity index is more sensitive to immersion in the soft drink than immersion in the tea. The enhancement of the plasticity index observed in non-thermocycled and immersed samples, indicates the reduction of their elastic recovery compared to the control group and demonstrates that the properties of the immersed samples are changed from brittle to ductile behavior. Meanwhile, an opposite behavior is observed for the thermocycled and immersed specimens. According to Eq. (4), the value of residual depth on the sample surface is proportional to the value of plasticity index. For example, AFM images of composite specimens and their residual depth after nanoindentation experiment are shown in Fig. 4. As illustrated in this figure, the control sample possesses the highest value of plasticity index and has the maximum residual depth in comparison with other samples. Water sorption causes softening and plasticization of the sample surfaces. Two factors are of substantial importance in the water sorption and surface erosion of materials. The first factor is filler loading which affects the microvoids and available free volume for water absorption. The higher amount of filler loading generates less microvoids and reduces the available free volume for water uptake [56]. Therefore, in the dental materials with higher filler loading, the erosion and softening may decrease after immersion in the beverages. Monomer type is another important factor in the water sorption procedure of the dental restorative materials. Different hydrophilicities
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Fig. 4. The AFM images and indentation sections of samples made from composite dental material after different simulated oral conditions.
exist between the monomers which form the resins of composites. Higher hydrophilicity is indicative of higher water sorption in the monomers. For example, the hydrophilicity of TEGDMA monomer is higher than both Bis-GMA and UDMA [57]. Thus, the absence of TEGDMA decreases the water sorption in the composite. As a result, the plasticity index values for all states of the conventional composite samples are more than the nanohybrid composite and nanofilled composite.
The results obtained from the nanoscratch test show that the wear resistance of all specimens decreases after immersion in tea and in soft drink. Recalling the reduction in hardness of non-thermocycled and immersed samples, a direct relation exists between the hardness and wear resistance of specimens. Similar results have been reported in the previous studies for different types of dental restorative materials, in which different test procedures were also used for investigating the wear properties [58,59]. By comparing the nanoscratch test results
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with those of the conventional wear experiments, they concluded that as a consequence of factors such as the accuracy of load control and scan size, and also due to the absence of electrical and chemical fields, the nanoscratch test can be a good alternative for investigating the tribological properties of materials. Wear resistance is an important property for dental restorative materials specially when being used as a restoration of large occlusal areas in the posterior teeth [60]. According to Table 4, more lateral force is required for scratching the nanohybrid composite compared to the composite specimens implying that the nanohybrid composite is more resistant to wear in comparison with the conventional composite restorative materials. Thus, the nanohybrid composite is more suitable than the dental composite for restoring the teeth cavities where it is being exposed to high wear situations. 5. Conclusion The results indicated that the mechanical and tribological properties of the dental nanocomposite and the nanohybrid composite were less sensitive to temperature changes and to immersion in beverages than the conventional restorative composite. Thus, the use of nanohybrid composite and nanocomposites can be considered to be more beneficial than the conventional composite materials specially for long lasting clinical applications. For the cases in which wear resistance is an important factor, one may prefer the use of nanohybrid composite instead of the conventional composite or nanocomposite dental materials due to the favorite wear resistance it showed in the simulated oral conditions. References [1] S. Ramakrishna, J. Mayer, E. Wintermantel, K.W. Leong, Compos. Sci. Technol. 61 (2001) 1189–1224. [2] L. Breschi, A. Mazzoni, A. Ruggeri, M. Cadenaro, R. Di Lenarda, E. De Stefano Dorigo, Dent. Mater. 24 (2008) 90–101. [3] N. Moszner, U. Salz, Prog. Polym. Sci. 26 (2001) 535–576. [4] J. Park, Q. Ye, E.M. Topp, A. Misra, S.L. Kieweg, P. Spencer, J. Biomed. Mater. Res. A 93A (2010) 1245–1251. [5] J.G. Leprince, W.M. Palin, M.A. Hadis, J. Devaux, G. Leloup, Dent. Mater. 29 (2013) 139–156. [6] M.-H. Chen, J. Dent. Res. 89 (2010) 549–560. [7] S. Mitra, D. Wu, B. Holmes, J. Am. Dent. Assoc. 134 (2003) 1382–1390. [8] S. Mahmoud, A. El-Embaby, A. AbdAllah, H. Hamama, J. Adhes. Dent. 10 (2008) 315–322. [9] N. Krämer, C. Reinelt, G. Richter, A. Petschelt, R. Frankenberger, Dent. Mater. 25 (2009) 750–759. [10] J.W.V. van Dijken, U. Pallesen, J. Dent. 39 (2011) 16–25. [11] S. Beun, T. Glorieux, J. Devaux, J. Vreven, G. Leloup, Dent. Mater. 23 (2007) 51–59. [12] J.L. Ferracane, Dent. Mater. 27 (2011) 29–38. [13] J. Lin, M. Sun, Z. Zheng, A. Shinya, J. Han, H. Lin, G. Zheng, A. Shinya, Dent. Mater. J. 32 (2013) 476–483. [14] L. Musanje, B. Darvell, Dent. Mater. 20 (2004) 750–765. [15] M. Gale, B. Darvell, J. Dent. 27 (1999) 89–99. [16] D.A. Stewardson, A.C. Shortall, P.M. Marquis, J. Dent. 38 (2010) 437–442. [17] O. Feuerstein, K. Zeichner, C. Imbari, Z. Ormianer, N. Samet, E.I. Weiss, Clin. Oral Implants Res. 19 (2008) 629–633.
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Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Effects of temperature change and beverage on mechanical and tribological properties of dental restorative composites M.R. Ayatollahi a,⁎, Mohd Yazid Yahya b, A. Karimzadeh a, M. Nikkhooyifar a, Amran Ayob b a Fatigue and Fracture Laboratory, Center of Excellence in Experimental Solid Mechanics and Dynamics, School of Mechanical Engineering, Iran University of Science and Technology, Narmak, Tehran 16846, Iran b Center for Composite, Institute for Vehicle System and Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Malaysia
a r t i c l e
i n f o
Article history: Received 3 September 2014 Received in revised form 14 March 2015 Accepted 2 May 2015 Available online 5 May 2015 Keywords: Dental restorative materials Nanoindentation experiment Nanoscratch experiment Thermocycling effect Beverage effect
a b s t r a c t The aim of this study was to investigate the effects of temperature change and immersion in two common beverages on the mechanical and tribological properties for three different types of dental restorative materials. Thermocycling procedure was performed for simulating temperature changes in oral conditions. Black tea and soft drink were considered for beverages. Universal composite, universal nanohybrid composite and universal nanofilled composite, were used as dental materials. The nanoindentation and nanoscratch experiments were utilized to determine the elastic modulus, hardness, plasticity index and wear resistance of the test specimens. The results showed that thermocycling and immersion in each beverage had different effects on the tested dental materials. The mechanical and tribological properties of nanohybrid composite and nanocomposite were less sensitive to temperature change and to immersion in beverages in comparison with those of the conventional dental composite. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Dental composite materials have different applications in dentistry such as filling the tooth cavities, veneering to mask discoloration, and correcting contour and alignment deficiencies. These materials are also used for making dental implants and bonding orthodontic brackets [1,2]. It is often desirable to find new methods for improving the mechanical properties of dental restorative composites [3–5]. In the past few years, one of the most important advancements in dental materials is related to the application of nanotechnology to dental restorative composites. For example, nano-fillers with dimensions of 5 to 100 nm have been added to the composite resins for producing dental nanocomposite. Nano-hybrid composites are also a category of dental restorative materials where in addition to nanometer particles, particles of 0.2 to 1 μm in size are added to the composite resins. It has been shown in the past that dental nanocomposites and nanohybrid composites can be used as dental restorative materials, instead of the conventional dental composites [6–10]. The general aims for producing these new dental materials were to achieve better tensile and compressive strengths, improved fracture toughness and wear resistance, firm bonding with dental enamel surface and desired aesthetic performance [7,11–13]. A good knowledge of the mechanical and tribological properties of new dental materials in various oral conditions would help clinicians in comparing
⁎ Corresponding author. E-mail address: [email protected] (M.R. Ayatollahi).
http://dx.doi.org/10.1016/j.msec.2015.05.004 0928-4931/© 2015 Elsevier B.V. All rights reserved.
the behavior of different dental materials and selecting the appropriate one. It could also be useful for manufacturers of dental materials for improving their products. The variation of temperature in oral environment can affect the mechanical properties of dental restorative materials. Eating hot or cold foods and drinking beverages are the causes of most extreme temperature variation in the oral cavity. Typical minimum and maximum temperatures of tooth surface during the consumption of food stuffs are 1 °C and 50 °C [14–16]. Although the mechanical and tribological properties of commercial dental restorative materials are often measured at room or body temperature [12,14,15,17,18], the mechanical properties of polymeric materials like dental composites are often sensitive to temperature variations. Therefore, it is important to evaluate the effects of oral conditions on the mechanical and tribological properties of dental restorative composites. Thermocycling is an in-vitro process which simulates temperature variations of the oral cavity that occur through eating, drinking, and breathing [19–22]. In addition to the temperature variations in oral conditions, water or other liquids such as saliva, beverages or food components may influence their physical and mechanical properties. Global statistics show that the consumption of carbonated soft drinks in the past 50 years has increased dramatically. Consumption of acidic drinks can degrade both the teeth and the restorative materials. For example, Lussi and Hellwig [23] reported a reduction in enamel hardness after 10 and 20 min of immersion in a commercial orange juice. Aliping-McKenzie et al. [24] immersed dental restorative materials in Coca-Cola and fruit juices and found significant reduction in their surface hardness.
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In this paper, the effects of thermocycling and immersion in two types of beverages on the mechanical and tribological properties of several dental restorative composites are studied. The null hypothesis was that new dental restorative materials preserve their mechanical and tribological properties in different oral conditions compared with the conventional dental composites. Nanoindentation and nanoscratch tests have been found to be powerful methods for measuring the mechanical properties such as modulus of elasticity, hardness and wear resistance of various materials in very small sample sizes such as polyamide-12/layered silicate nanocomposites, clay nanocomposites, bone cement and tooth enamel [25–31]. Therefore, in the current study the nanoindentation and nanoscratch experiments are employed to determine the mechanical and tribological properties of dental restorative composites.
2.3. Immersion in beverages For investigating the effect of beverage alone and also simultaneous effect of thermocycling and beverage on the dental restorative materials, a non-thermocycled sample and a thermocycled sample for each type of material were immersed in carbonated soft drink (Coca-Cola) and another pair was immersed in black tea for 48 h. Each specimen was placed in a separate container. The temperature was kept at 55 ± 5 °C for tea and at 5 ± 2 °C for soft drink to reach more realistic evaluation of beverage effects. All in all, five samples were prepared for each type of the three investigated dental materials: (1) control or intact sample (non-thermocycled and non-immersed in any beverage), (2) non-thermocycled and immersed in tea, (3) non-thermocycled and immersed in soft drink, (4) thermocycled and immersed in tea, and (5) thermocycled and immersed in soft drink.
2. Materials and methods 2.4. Nanoindentation experiment
2.1. Sample preparation Three types of dental restorative materials, i.e. composite, nanohybrid composite and nanocomposite were used to prepare the samples. The characteristics of these materials are presented in Table 1 according to the manufacturer's information. Five specimens from each material were prepared in a disc-shaped mold with a diameter of 10 mm and a thickness of 4 mm. Due to the thickness of specimens, each consists of two light cure increment. According to the manufacturer's instructions of the restorative materials, each increment was light cured for 20 s using a LED light with intensity of 400 mW/cm2. During the light exposure, the light guide tip was held as close to the specimen surface as possible. Since nanoindentation and nanoscratch tests need a very smooth surface, all the specimens were smoothed by sandpapers with 400 to 2500 grits and then polished by diamond pastes with mesh sizes of 1, 0.5 and 0.25 μm.
2.2. Thermocycling procedure In order to study the effect of cyclic thermal stresses that occur in the mouth during the service life of the dental restorative materials, two samples for each type of material were thermocycled. The thermocycling apparatus consists of two stainless steel water baths filled with distilled water and a mechanical arm that transports the specimens from one bath to another. The ISO TR 11450 standard indicates that a thermocycling procedure of 500 cycles in water between +5 °C and +55 °C is an appropriate artificial aging test [32]. However, long-term aging process could be selected for long-lasting restorative materials such as materials used in this study [33–37]. Therefore, two samples from each material were subjected to 2000 cycles between the temperatures of + 5 and +55 °C, with a dwell time of 30 s in each bath per cycle and a transport over time of 15 s. A total number of 2000 cycles of thermocycling in this study can be considered to be equal to maintaining the dental materials for 200 days in the mouth conditions.
The basic mechanical properties of the test specimens, i.e. hardness, elastic modulus and plasticity index were determined by a Triboscope nanoindentation test system (Hysitron Inc., USA) based on ISO 14577 [38]. A Berkovich indenter was used in all experiments. This type of indenter is usually used for bulk samples and for thin films thicker than 100 nm. The test setup and the Berkovich indenter tip are shown in Fig. 1. Oliver and Pharr method [39] was used to calibrate the device and also to analyze the test results. An indentation load of 1950 μN was applied to the surface of each sample in 30 s with a constant loading rate. To take account of likely creep effects, the indenter was kept on the sample at the maximum load for 10 s [40]. Finally, the unloading stage of the test was performed by removing the tip from the sample at the same rate. For each sample, the nano-indentation tests were repeated at least 5 times in different randomly selected sites, and the curves of force versus indenter displacement were recorded through the test instruments. According to the simple power and sample analysis, when the experiment repeats 5 times in each of the 5 groups, the 0.05 level of difference in means will be detected by 90% power, assuming that the common standard deviation is 0.500 [41]. Before and after the experiments, atomic force microscopy (AFM) images were taken from the sample surfaces to analyze the indentation hole. In the AFM process, the same indenter tip as the nano-indentation test, scans the surface of specimen without applying any load. 2.5. Nanoscratch experiment Each nano-scratch test was performed in three main stages. First, a pre-scan of each sample was carried out and AFM images were taken to investigate the roughness and tilt angle of the sample surface. Then, the nano-scratch test was performed on the sample by using the same indenter as that of the nano-indentation test. The normal load was 1950 μN and the scratch length was 4 μm. The indenter penetrated the
Table 1 Dental restorative materials used for experiments. Type
Material
Type of resins
Filler particles
Filler content
Shade
Manufacturer
Composite Nanohybrid composite Nanocomposite
Filtek Z250 Filtek Z250 XT Filtek Z350 XT
Bis-GMA1, UDMA2, Bis-EMA3 Bis-GMA, UDMA,PEGDMA4, TEGDMA5 Bis-GMA, UDMA, TEGDMA, PEGDMA, Bis-EMA
Zirconia/silica Silica, Zirconia/silica Zirconia, silica
60% 67.8% 63.3%
B1 B1 WE
3M ESPE, USA 3M ESPE, USA 3M ESPE, USA
1 2 3 4 5
Bis-GMA: bisphenol A glycidyl methacrylate. UDMA: diurethane dimethacrylate. Bis-EMA: ethoxylatedbisphenol A dimethacrylate. PEGDMA: polyethylene glycol dimethacrylate. TEGDMA: triethylene glycol dimethacrylate.
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where Pmax is the maximum normal load and A is the projected contact surface between the sample and the indenter at the maximum load. The average values of elastic modulus and hardness measured for all specimens are illustrated in Table 2. In the control samples, the elastic modulus and hardness of the nanocomposite and the nanohybrid composite are higher than the corresponding values for the conventional dental composite materials. For all three dental materials, the elastic modulus and hardness values of the “non-thermocycled and immersed” specimens were decreased in comparison to the relevant control sample. A comparison between the elastic modulus and hardness values of the samples immersed in tea and soft drink with those of the corresponding control samples demonstrates that no significant difference exists between the effects of tea and soft drink on these properties. Moreover, the reduction in the properties of conventional dental composite is higher than other two restorative materials. 3.2. Elastic–plastic behavior The plasticity index (ψ) is a tool for characterizing the elastic–plastic response of a material under external loads. In the nanoindentation test, the plasticity index can be calculated from [43]:
Fig. 1. a) Triboscope nanoindentation test system, b) Berkovich indenter tip.
sample surface and was drawn with a constant scratching speed of 0.13 μm · s− 1. Finally, a post-scan was performed by taking an AFM image from the scratch site on the sample. The wear resistance of the material could be studied by comparing the pre-scan AFM images with the post-scan ones. 3. Results 3.1. Elastic modulus and hardness The elastic modulus (E) and hardness (H) of each sample can be obtained from a complete cycle of loading and unloading during the nanoindentation test. In this study, the Oliver and Pharr method [39] was applied to determine these properties from the experimental data. The following equations were used for calculating the elastic modulus and hardness from the results of nanoindentation experiment.
ψ¼
hm −he hm
ð4Þ
ð1Þ
where, Er is the reduced modulus of the material obtained from the nanoindentation test, ν is the Poisson's ratio of the test sample, and Ei and νi are the elastic modulus and Poisson's ratio of the indenter, respectively. Based on the technical data available for the related Triboscope system, Ei and νi are equal to 1140 GPa and 0.07. Considering ν = 0.31 [42], one can determine the elastic modulus of each sample by using Eq. (1). The material hardness (H) could also be calculated from: H¼
ð3Þ
where A1 is the area under the loading segment of load–displacement curve which is equal to the total energy expended during the indentation procedure and A2 is the area under the unloading segment which is equal to the energy released during the indentation due to viscoelastic behavior. These areas are depicted in Fig. 2. Indeed, subtraction of A1 and A2 is equal to the irreversible plastic work during the nanoindentation test. For materials with viscoelastic–plastic behavior such as polymers, the range of this index is from zero which indicates perfectly elastic behavior, to one which represents fully-plastic behavior. Since in this study a Berkovich indenter was used for all experiments, the plasticity index can be calculated by Eq. (4) [30]:
ψ¼ 1 1−ν2 1−νi 2 þ ¼ E Ei Er
A1 −A2 A1
Pmax A
ð2Þ
where hm is the maximum penetration depth of the indenter and he is its elastic reversible depth. The difference between these two depths indicates the residual penetration depth which is measured from the load–displacement curve. Table 3 shows the values of plasticity index calculated by Eq. (4) for all tested materials. Based on Table 3, the values of plasticity index for the nonthermocycled samples don't change significantly after immersion in the tea compared to that of the control group. However, this index increases after immersion in the soft drink for all tested materials. Results show that the plasticity index decreases after thermocycling and immersion in the beverages.
Table 2 Elastic modulus and hardness of samples obtained from the nanoindentation test. Material type:
Control sample Non-thermocycled and immersed in tea Non-thermocycled and immersed in soft drink Thermocycled and immersed in tea Thermocycled and immersed in soft drink
Conventional composite
Nanohybrid composite
Nanocomposite
Elastic modulus (GPa)
Hardness (GPa)
Elastic modulus (GPa)
Hardness (GPa)
Elastic modulus (GPa)
Hardness (GPa)
11.46 ± 0.46 10.17 ± 0.38 9.68 ± 0.43 12.74 ± 0.38 11.96 ± 0.31
0.46 ± 0.05 0.34 ± 0.02 0.31 ± 0.03 0.53 ± 0.03 0.49 ± 0.02
13.77 ± 0.46 13.12 ± 0.34 12.93 ± 0.40 13.81 ± 0.44 13.53 ± 0.36
0.72 ± 0.07 0.65 ± 0.03 0.63 ± 0.04 0.77 ± 0.02 0.73 ± 0.03
12.48 ± 0.42 11.64 ± 0.04 11.31 ± 0.59 12.76 ± 0.54 12.34 ± 0.39
0.71 ± 0.08 0.62 ± 0.03 0.59 ± 0.05 0.74 ± 0.01 0.69 ± 0.03
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have almost the same destructive effects on the wear resistance of all tested materials. 4. Discussion
Fig. 2. A schematic load–displacement curve in nanoindentation tests.
3.3. Wear resistance Due to the considerable roughness of thermocycled and immersed samples the nanoscratch experiment was performed only on the control group and on the “non-thermocycled and immersed in beverage” specimens. In fact, after the thermocycling and immersion procedures the surfaces of samples couldn't be smoothed by sandpapers or be polished. The residual depth of the indenter on the surface of sample is often used to investigate the material deformation after the nanoscratch test. Fig. 3 shows the AFM images and longitudinal scratch sections of control and immersed samples for the nanohybrid composite. Erosion of the sample surface caused by immersing in the beverages is clearly visible in the AFM images of immersed samples. The erosion has increased the surface roughness of the immersed samples compared to the control sample. The AFM images of scratches created by the nanoscratch test on the sample surfaces illustrate that the residual scratch depths on the surfaces of immersed samples are more than that of the control sample. Thus, after immersing in beverage, less force is needed to penetrate and slide the indenter on the sample surface indicating higher softness of the material. This implies that drinking too much tea or soft drink decreases the wear resistance of the dental restorative materials. The ratio of the lateral force to the normal force (LF/NF) during the scratch test was also calculated for each test specimen. Table 4 shows the values determined for this ratio for different samples. It is seen from this table that the ratio of LF/LN reduces when the dental materials are immersed in tea or soft drink. In other words, the immersed samples require less lateral force than the control sample for sliding the indenter on the specimen surface. The reduction of LF/NF for the composite specimens is more than those for the nanohybrid composite ones. The soft drink and the tea
As mentioned earlier, the main objective of this study was to explore whether two new types of dental restorative materials preserve their mechanical and tribological properties in different oral conditions better than conventional dental composites. The results obtained from the nanoindentation and nanoscratch experiments indicated that the nanocomposite and nanohybrid composite samples could preserve their properties better than the dental composite when the samples were thermocycled or immersed in tea or soft drink. Therefore, from a clinical point of view, one may prefer to use these two types of dental materials for dental treatments or in orthodontic applications as their mechanical properties are less sensitive to oral conditions. It is worth mentioning that the main purpose of this paper was to provide a general study on the effects of various oral conditions on the mechanical properties of three different types of dental materials. It is essential to extend this study by performing a statistical analysis on the experimental data. However, for a reliable statistical analysis, a larger number of specimens per group is required. The increase in the elastic modulus and hardness of the nanocomposite and the nanohybrid composite in the control group in comparison with the corresponding values for the conventional composite may be due to the amount of filler loading in these materials, as shown in Table 1. The nanohybrid composite has the highest amount of filler loading and the largest elastic modulus value, while the composite sample has the lowest amount of filler loading and the lowest elastic modulus. Accordingly, a direct relationship is seen between the elastic modulus or hardness of dental restorative materials and their filler loading. Masouras et al. [44] have also found a similar behavior for bulk moduli of resincomposites. Based on the results presented in Table 2, the elastic modulus and hardness values of specimens were decreased significantly when immersed in tea and soft drink. Moreover, the decrease in the properties of conventional composite is higher than those of other two restorative materials. For instance, the hardness value of the composite decreased about 26% after immersion in tea, while for nanohybrid composite and nanocomposite the reductions were about 10% and 12%, respectively. Therefore, it is expected that nanohybrid composite and nanocomposite would be better option than the composite materials specially for long lasting dental applications. This could be attributed to the difference in the filler loadings and the types of resins in these materials. The chemical agents existing in beverages and food can affect polymer composites by their chemical degradation [45]. Cross linking density, fillers, porosity and solvents may influence water sorption on the surface of polymerbased composite materials [46]. The degradation of composites by water molecules occurs by two mechanisms; (i) diffusion into the polymer network and (ii) hydrolysis reaction. In the first mechanism, water molecules diffuse into the polymer network and fill the empty spaces between the polymer chains and the micro voids. This phenomenon leads to the plasticization and swelling of polymer matrix. Thus, the polymer chain scission takes place and results in the monomer elution
Table 3 The value of plasticity index obtained for the tested materials. Plasticity index ( ) Non-thermocycled specimens
Thermocycled specimens
Material
Control
In tea
In soft drink
In tea
In soft drink
Composite Nanohybrid composite Nanocomposite
0.51 ± 0.05 0.47 ± 0.03 0.45 ± 0.03
0.56 ± 0.03 0.50 ± 0.02 0.49 ± 0.03
0.58 ± 0.05 0.52 ± 0.04 0.51 ± 0.03
0.48 ± 0.02 0.44 ± 0.02 0.43 ± 0.02
0.49 ± 0.02 0.46 ± 0.02 0.45 ± 0.01
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Fig. 3. AFM image and scratch section of nanohybrid composite a) Control sample, b) Immersed in tea and c) Immersed in soft drink.
[46–48]. In the second mechanism, hydrolysis reaction degrades the siloxane bonds (i.e. the bond between the silane coupling agent and silanol groups of the silica surface) and this leads to debonding of filler [47]. In general, all of these factors and events cause the degradation and softening of composite resins and consequently decrease some of the physical and mechanical properties of the immersed materials such as elastic modulus and hardness. In addition, the acids in beverages could accelerate and enhance the effects of these mechanisms. Asmussen [49] reported that in the presence of acids, erosion and plasticization of polymer matrix become more pronounced. A reduction in the surface hardness of dental restorative materials has been also reported in [24] after their immersion in the Coca-Cola and fruit juices. The presence of citric acid in these beverages could make chemical activities which increase the surface erosion [50]. The surface erosion leads to a decrease in the mechanical properties of dental restorative materials. Therefore, in the case of immersion in soft drink, reductions in the values of elastic modulus and hardness of the materials are reasonable. According to Table 2, the hardness and elastic modulus of all thermocycled and “immersed in tea” samples are higher than those of the control samples. However, for thermocycled and “immersed in soft drink” samples various trends are observed for different materials. These dissimilar variations seem to be due to different effects on material properties from the thermocycling process and from immersion in the beverages. Thermocycling process can affect the samples in two separate ways: 1) Thermal stresses (caused by temperature changes in the oral conditions) are generated between the structural components of the materials. 2) Moisture affects the structure of dental restorative materials used in dental treatments. Thermal stresses can improve the bond strength between material components and increase the hardness and elastic modulus. The effect of moisture was described earlier in the part dealing with the immersion Table 4 Ratio of lateral force to normal force (LF/NF). LF/NF Material
Control
In tea
In soft drink
Composite Nanohybrid composite Nanocomposite
0.25 ± 0.01 0.28 ± 0.01 0.27 ± 0.02
0.21 ± 0.01 0.26 ± 0.02 0.24 ± 0.02
0.20 ± 0.01 0.25 ± 0.01 0.23 ± 0.01
conditions of samples in beverages. Meanwhile, because of thermal stresses generated in the thermocycling process, the bond strength between the composite components is improved and the empty space between the polymer chains decreases. Thus, in the thermocycled samples, the diffusion of water molecules into the polymer network reduces in comparison with the non-thermocycled ones. Diffusion, on the other hand, leads to the plasticization and swelling of polymer matrix. This would result in scission of polymer chains and elution of monomers [46, 47]. Various effects of humid environment on the mechanical properties of dental restorative composites were observed in previous studies. Some researchers [51,52] have found that the storage of samples in water increases the elastic modulus while some others e.g. [53] have reported a reduction in the elastic modulus. Some papers [54,55] have also reported that it has no influence on the elastic modulus. Therefore, the effects of humid environment on the elastic modulus can be suggested to be dependent on the material characteristics and the storage conditions such as temperature and storage time. In general, the thermocycling process would improve and the water sorption and chemical agents would degrade the elastic modulus and hardness of dental restorative materials. As shown in Table 3, for both non-thermocycled and thermocycled specimens, the plasticity index is more sensitive to immersion in the soft drink than immersion in the tea. The enhancement of the plasticity index observed in non-thermocycled and immersed samples, indicates the reduction of their elastic recovery compared to the control group and demonstrates that the properties of the immersed samples are changed from brittle to ductile behavior. Meanwhile, an opposite behavior is observed for the thermocycled and immersed specimens. According to Eq. (4), the value of residual depth on the sample surface is proportional to the value of plasticity index. For example, AFM images of composite specimens and their residual depth after nanoindentation experiment are shown in Fig. 4. As illustrated in this figure, the control sample possesses the highest value of plasticity index and has the maximum residual depth in comparison with other samples. Water sorption causes softening and plasticization of the sample surfaces. Two factors are of substantial importance in the water sorption and surface erosion of materials. The first factor is filler loading which affects the microvoids and available free volume for water absorption. The higher amount of filler loading generates less microvoids and reduces the available free volume for water uptake [56]. Therefore, in the dental materials with higher filler loading, the erosion and softening may decrease after immersion in the beverages. Monomer type is another important factor in the water sorption procedure of the dental restorative materials. Different hydrophilicities
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Fig. 4. The AFM images and indentation sections of samples made from composite dental material after different simulated oral conditions.
exist between the monomers which form the resins of composites. Higher hydrophilicity is indicative of higher water sorption in the monomers. For example, the hydrophilicity of TEGDMA monomer is higher than both Bis-GMA and UDMA [57]. Thus, the absence of TEGDMA decreases the water sorption in the composite. As a result, the plasticity index values for all states of the conventional composite samples are more than the nanohybrid composite and nanofilled composite.
The results obtained from the nanoscratch test show that the wear resistance of all specimens decreases after immersion in tea and in soft drink. Recalling the reduction in hardness of non-thermocycled and immersed samples, a direct relation exists between the hardness and wear resistance of specimens. Similar results have been reported in the previous studies for different types of dental restorative materials, in which different test procedures were also used for investigating the wear properties [58,59]. By comparing the nanoscratch test results
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with those of the conventional wear experiments, they concluded that as a consequence of factors such as the accuracy of load control and scan size, and also due to the absence of electrical and chemical fields, the nanoscratch test can be a good alternative for investigating the tribological properties of materials. Wear resistance is an important property for dental restorative materials specially when being used as a restoration of large occlusal areas in the posterior teeth [60]. According to Table 4, more lateral force is required for scratching the nanohybrid composite compared to the composite specimens implying that the nanohybrid composite is more resistant to wear in comparison with the conventional composite restorative materials. Thus, the nanohybrid composite is more suitable than the dental composite for restoring the teeth cavities where it is being exposed to high wear situations. 5. Conclusion The results indicated that the mechanical and tribological properties of the dental nanocomposite and the nanohybrid composite were less sensitive to temperature changes and to immersion in beverages than the conventional restorative composite. Thus, the use of nanohybrid composite and nanocomposites can be considered to be more beneficial than the conventional composite materials specially for long lasting clinical applications. For the cases in which wear resistance is an important factor, one may prefer the use of nanohybrid composite instead of the conventional composite or nanocomposite dental materials due to the favorite wear resistance it showed in the simulated oral conditions. References [1] S. Ramakrishna, J. Mayer, E. Wintermantel, K.W. Leong, Compos. Sci. Technol. 61 (2001) 1189–1224. [2] L. Breschi, A. Mazzoni, A. Ruggeri, M. Cadenaro, R. Di Lenarda, E. De Stefano Dorigo, Dent. Mater. 24 (2008) 90–101. [3] N. Moszner, U. Salz, Prog. Polym. Sci. 26 (2001) 535–576. [4] J. Park, Q. Ye, E.M. Topp, A. Misra, S.L. Kieweg, P. Spencer, J. Biomed. Mater. Res. A 93A (2010) 1245–1251. [5] J.G. Leprince, W.M. Palin, M.A. Hadis, J. Devaux, G. Leloup, Dent. Mater. 29 (2013) 139–156. [6] M.-H. Chen, J. Dent. Res. 89 (2010) 549–560. [7] S. Mitra, D. Wu, B. Holmes, J. Am. Dent. Assoc. 134 (2003) 1382–1390. [8] S. Mahmoud, A. El-Embaby, A. AbdAllah, H. Hamama, J. Adhes. Dent. 10 (2008) 315–322. [9] N. Krämer, C. Reinelt, G. Richter, A. Petschelt, R. Frankenberger, Dent. Mater. 25 (2009) 750–759. [10] J.W.V. van Dijken, U. Pallesen, J. Dent. 39 (2011) 16–25. [11] S. Beun, T. Glorieux, J. Devaux, J. Vreven, G. Leloup, Dent. Mater. 23 (2007) 51–59. [12] J.L. Ferracane, Dent. Mater. 27 (2011) 29–38. [13] J. Lin, M. Sun, Z. Zheng, A. Shinya, J. Han, H. Lin, G. Zheng, A. Shinya, Dent. Mater. J. 32 (2013) 476–483. [14] L. Musanje, B. Darvell, Dent. Mater. 20 (2004) 750–765. [15] M. Gale, B. Darvell, J. Dent. 27 (1999) 89–99. [16] D.A. Stewardson, A.C. Shortall, P.M. Marquis, J. Dent. 38 (2010) 437–442. [17] O. Feuerstein, K. Zeichner, C. Imbari, Z. Ormianer, N. Samet, E.I. Weiss, Clin. Oral Implants Res. 19 (2008) 629–633.
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