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The effect of a leucite-containing ceramic filler on the abrasive wear of dental composites
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1181–1187
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
The effect of a leucite-containing ceramic filler on the abrasive wear of dental composites Mohammad Atai a,∗ , Esmaeil Yassini b , Maryam Amini b , David C. Watts c a
Iran Polymer and Petrochemical Institute (IPPI), P.O. Box 14965/115, Tehran, Iran Department of Restorative Dentistry, Dental School, Tehran University of Medical Sciences, Tehran, Iran c Biomaterials Science Research Group, School of Dentistry and Photon Science Institute, The University of Manchester, Manchester M15 6FH, UK b
a r t i c l e
i n f o
Article history:
a b s t r a c t Objectives. The aim of this study was to evaluate abrasive wear of a dental composite based on
Received 10 March 2006
a leucite-containing (KAlSi2 O2 ) ceramic filler, and to compare it with the wear of a composite
Received in revised form
based on commonly used aluminum barium silicate glass filler.
20 March 2007
Methods. IPS Empress (Ivoclar-Vivadent) ingots were ball milled, passed through an 800 mesh
Accepted 23 March 2007
(ASTM) sieve, and used as the leucite ceramic filler. Experimental composites were prepared by mixing the silane-treated fillers with the resin monomers. The resin consisted of 70 wt% Bis-GMA and 30 wt% TEGDMA containing camphorquinone and DMAEMA as the photoini-
Keywords:
tiator system. Glass-based composites were also prepared using silane-treated aluminum
Dental composite
barium silicate glass fillers and the same resin system. TetricCeram® , a commercially avail-
Filler
able dental composite, was used as control. Spherical specimens of the composites were
Abrasive wear
then prepared and kept in water for 2 weeks to reach equilibrium with water. An abrasive
Leucite-containing ceramic
wear test was performed using a device designed in our laboratory and weight loss of the specimens was measured as an abrasion parameter after each 50 h. SEMs were taken from worn and fractured surfaces. Degree-of-conversion of the composites was measured using FTIR spectroscopy. Vickers surface microhardness, flexural strength, and flexural modulus of the composites were also measured. The data were analyzed and compared using ANOVA and Tukey HSD tests (significance level = 0.05). Results. The results showed that there were significant differences among the abrasive wear of the composites (p < 0.05). The ranking from least to most was as: leucite-based composite < TetricCeram® < glass-based composite. The higher wear resistance of leucite-based composite could be related to its higher surface hardness. Significance. Using leucite-containing glass as an alternative for aluminum barium silicate glass fillers in dental composites generated a significant increase in the wear resistance of the resin composites which should be beneficial in the development of dental materials. © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. Tel.: +98 21 44580085; fax: +98 21 44580023. E-mail address: [email protected] (M. Atai). 0109-5641/$ – see front matter © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2007.03.006
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1.
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1181–1187
Introduction
Table 1 – Composition of the experimental composites Composition
Wear is the loss of material through different processes: abrasion, adhesion, fatigue and corrosive effects which act in various combinations depending upon the properties of materials. Abrasion and attrition have been identified as the main clinical wear mechanisms for dental composites [1]. High resistance to wear is one of the desirable properties for restorative materials. Reducing the wear of the materials has been the subject of numerous studies. Most of the studies have centered on the reinforcing phase of the composites considering the filler particle size and surface treatment [2–6], the use of porous fillers [7], silica-fused whiskers [8] and fibers [9] to improve the wear resistance of dental composites. The particulate fillers which are commonly used in composites are silica and/or silicates. The crystalline form of silica (quartz) is stronger and harder, but used alone results in composites that are radiolucent and difficult to finish and polish; therefore most composites are now produced using silicate glasses [10]. IPS Empress ceramic contains leucite crystals (KAlSi2 O6 ) only a few microns in size that are produced by means of controlled crystallization with special glass-containing nucleating agents. The leucite crystals are the crystalline component (35–55%) of the feldspathic rock used in dental porcelains [11–13]. There is no history of using the leucite-containing ceramic, IPS Empress, as fillers in dental composites. In this study the abrasive wear of a dental composite based on fillers prepared from the ceramic was investigated and compared with the abrasion of a composite based on commonly used aluminum barium silicate glass fillers and a commercially available composite, TetricCeram® . Because of the lack of an internationally acceptable method, different methods have been used for assessing the wear of dental materials which can be categorized in two distinct classes: clinical [14,15] and laboratory test methods [16–23]. The clinical testing is complex and time consuming (up to 6 years). Laboratory methods can simplify the oral condition and give some reasonable results to compare different dental materials. An additional goal of this work was to propose a simple method for measuring the abrasive wear of composite resins or other dental materials.
2.
Materials and methods
2.1.
Materials
Glass fillers with the average particle size of 2–5 m (SP345; aluminum fluoride: 5–10%, barium fluoride: 5–10%, calcium fluoride: 5–10%, silicon dioxide: 40–70%, zinc oxide: 5–10%) were obtained from Specialty Glass (USA). TetricCeram® (Lot: F61695) in an A-3 shade and IPS Empress ingots (Lot: D50014) were purchased from Ivoclar Vivadent (Leichtenstein). Bis-GMA and triethylene glycol dimethacrylate ¨ (TEGDMA) were kindly supplied by Rohm (Degussa Group, Germany). -Methacryloxy propyl trimethoxy silane (-
Filler (wt%) Bis-GMA/TEGDMA (70/30 by weight) CQ (wt%) DMAEMA (wt%)
Leucite-based
Glass-based
75 24 0.5 0.5
75 24 0.5 0.5
MPS), camphorquinone (CQ) and N-N -dimethyl aminoethyl methacrylate (DMAEMA) were obtained from Fluka (Germany).
2.2.
Specimen preparation
IPS Empress ingots were ball milled, passed through a 800 mesh (ASTM) sieve using a wet technique, resulting in a powder filler with an average particle size of 3.6 m (75%), measured by a particle-size analyzer (Analysette 22, Fritsch, Germany). Leucite-containing and aluminium–barium–glass fillers were separately surface treated with 1 wt% -MPS. MPS was prehydrolyzed for 1 h in an aqueous solution of 70 wt% ethanol and 30 wt% double-distilled water (pH was adjusted to 3–4 by adding a few droplets of acetic acid). The treated fillers were dried for over 20 days at room temperature. Leucite and glass fillers were then hand mixed with the monomer phase and photoinitiator system according to the composition shown in Table 1. The pastes were then inserted into spherical Plexy Glass molds (diameter = 8 mm; Fig. 1) and light cured on both sides for 40 s using a light source with an irradiance circa 700 mW/cm2 (Optilux 501, Kerr, USA). As the diameter of the specimens were greater than the depth of light penetration, the specimens were then post cured in an oven (120 ◦ C, 2 h) to reach the maximum degree of conversion. TetricCeram® was tested as control. As the water uptake of the specimens could interfere with the weight loss during the abrasion test, the specimens were stored in water for 2 weeks to approach equilibrium with water before starting the test. 5 specimens were tested in each group. The specimens were numbered making small holes on their surfaces using dental drills (diameter <0.5 mm, depth: ca. 2 mm); the holes were then filled with dental waxes to reduce water diffusion which might interfere with the weight loss of the specimens.
Fig. 1 – Abrasive wear testing device.
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2.3.
Degree of conversion
Table 2 – Degree of conversion of the composites before and after post curing
To measure the degree of conversion, the uncured paste of each composite was placed between two polyethylene films, pressed to form a very thin film and the absorbance peaks obtained by transmission mode of FTIR (EQUINOX 55, Bruker, Germany). The specimens were then light cured for 40 s (Optilux 501, 700 mW/cm2 ) and the absorbance peaks were collected for the cured specimens. Specimens were then placed between two glass slides and heat cured in an oven (at 120 ◦ C for 2 h) and the absorbance peaks were also collected after post curing. Degree of conversion (DC%) was determined from the ratio of absorbance intensities of aliphatic C C (peak at 1638 cm−1 ) against internal standard aromatic C· · ·C (peak at 1608 cm−1 ) before and after curing of the specimen. The degree of conversion was then calculated as follows [24]:
DC% =
1−
(1638 cm−1 /1608 cm−1 )peak area after curing (1638 cm−1 /1608 cm−1 )peak area before curing
×100
2.4.
Abrasive wear test
A simple device was designed to measure the abrasive wear of the specimens (Fig. 1). In this device, spherical specimens in an abrasive medium (pumice powder with 100 m average particle size dispersed in water) were placed in a plastic cylindrical container. The container rotated at 50 rpm causing the specimens to slide over each other in the abrasive media resulting in abrasion. The specimens were prepared spherically because the impact fracture of sharp edges of non-spherical shapes could be eliminated. The masses of the spherical specimens in each group (n = 5) were measured by a balance (10−4 g) after each 50 h. The percent weight loss after each cycle was determined as abrasion as follows: Abrasion(%) = weight loss(%) =
W0 − Wt × 100 W0
where W0 was the initial weight and Wt was the specimen weight after each 50 h of wear.
2.5.
Flexural strength and flexural modulus
The flexural strength was measured on the bar specimens (2 mm × 2 mm × 25 mm) by a three-point bending test with a span of 20 mm using a universal testing machine (Instron 6025, UK) at a cross-head speed of 0.5 mm/min. The cured specimens were stored in distilled water for 24 h before test. The flexural strength was calculated as: Flexural strength = 3PL/2bd2
Light cure (%) (S.D.)
Glass-based Leucite-based TetricCeram®
Post cure (%) (S.D.)
71.3 (2.0) 69.0 (1.7) 65.6 (1.5)
83.9 (2.0) 83.0 (1.0) 79.5 (2.2)
where P stands for load at fracture, L for span length, b for width, and d for thickness. Flexural modulus was then determined from the slope of the elastic region of the stress–strain curve. The data were analyzed and compared using one-way ANOVA followed by Tukey HSD tests.
3.
Results
Table 2 shows the degree-of-conversion after light curing and after post curing. The specimens’ degree-of-conversion increased significantly after post-curing (p < 0.05), but there was no significant difference between the groups after postcuring. Table 3 shows weight-loss, for different composite groups which was significantly different (p < 0.05) and the ranking from least to most was: Leucite-based composite < TetricCeram® < glass-based composite. Table 4 presents the flexural strength and flexural moduli of the composites. The glass-based composite showed the lowest flexural strength and modulus. Fig. 2 plots the composite weight-losses with time. These gave linear correlation between weight loss (%) and time of
Table 3 – Abrasion (weight loss %) of the composites vs. time Time (h)
Composite
Weight loss % (S.D.)
50
Gglass-based Leucite-based TetricCeram®
3.54 (0.01) 2.50 (0.07) 2.80 (0.05)
100
Glass-based Leucite-based TetricCeram®
8.50 (0.03) 5.60 (0.11) 6.15 (0.36)
150
Glass-based Leucite-based TetricCeram®
13.59 (0.04) 9.05 (0.16) 10.77 (0.14)
200
Glass-based Leucite-based TetricCeram®
18.83 (0.05) 12.89 (0.25) 14.96 (0.18)
250
Glass-based Leucite-based TetricCeram®
24.17 (0.06) 16.54 (0.33) 19.35 (0.25)
300
Glass-based Leucite-based TetricCeram®
28.47 (0.05) 19.20 (0.40) 22.97 (0.31)
Vickers microhardness
Vickers microhardness on the surface (five points) of the cured specimens was measured using a microhardness tester (Duramin 20, Denmark) applying a load of 1.961 N for 20 s.
2.6.
Composite
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Table 4 – Flexural strength and flexural modulus of the composites Composites Glass-based Leucite-based TetricCeram®
Flexural strength (MPa) (S.D.) 43.8 (3.5) 55.4 (5.6)a 56.8 (9.5)a
Flexural modulus (GPa) (S.D.) 9.0 (1.1) 10.5 (1.0)b 11.2 (1.5)b
Superscript letters (a and b) indicate that the differences are not significant (p > 0.05).
Fig. 2 – Weight loss of the composites vs. time.
test in all the groups (r2 > 0.99). Fig. 3 shows the abrasion rate (%/h) of the composites versus their surface microhardness, where the abrasion rate was seen to increase with decreasing microhardness. Fig. 4 illustrates SEM images of worn surfaces after 300 h abrasion. The glass composite exhibited a coarser surface as compared to the lecuite and TetricCeram® surfaces. Fig. 5 shows the SEM images of fractured surfaces of the composites. No separation of fillers from resin matrices was seen.
Fig. 4 – SEM images of worn surfaces after 300 h (×3000). (a) Leucite-based composite. (b) Glass-based composite. (c) TetricCeram® .
4.
Fig. 3 – Abrasion rate vs. microhardness of the composites.
Discussion
Wear is the loss of material that occurs through contact of two or more surfaces. Abrasive wear occurs when a soft surface comes into contact with a harder one. In dental composites abrasion involves the process of resin matrix loss between filler particles and subsequent dislodgement of the filler. Filler loading, size, inter-particle spacing, degree of polymerization, filler/matrix cracking caused by water sorption and hydrolytic
d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1181–1187
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Although the FTIR measurement method used in our study shows the degree-of-conversion of a thin film of the composites which is not necessarily equal to the cure of the interior of the spherical specimens, it is sufficient for the study as the abrasion occurs at the specimen surface layers. From Table 2, degree-of-conversion significantly increased (p < 0.05) with post curing, but there was no significant difference between the degrees-of -conversion of the composites after post-cure. Therefore, filler type was the only important variable that determined the wear resistance of the experimental composites. Table 3 shows that abrasion of the composite containing amorphous glass filler was always higher than the composite with partially crystalline ceramic filler (leucite). Abrasion correlated linearly with time (Fig. 2) and the rate of abrasive wear, as presented in Fig. 3, was higher for the glass-based composite. To eliminate the effect of water-uptake during the abrasion test, the specimens were previously stored in water for 2 weeks. Water-sorption in a material is a time- and thicknessdependent phenomenon which can be described by the Fick’s second law: ∂c ∂2 c =D 2 ∂t ∂x
Fig. 5 – SEM images of fractured surfaces (×3000). (a) Leucite-based composite. (b) Glass-based composite. (c) TetricCeram® .
degradation of the filler surface are important factors affecting wear resistance of resin composites [25–28]. In this study for both experimental groups (leucite-base and glass-base composites) the filler load (75 wt %) and average filler-particle size (2–5 m) were the same; therefore the effect of these two important factors on wear was equalized. The degree of conversion influences the abrasion resistance and other mechanical properties [29], therefore the specimens were post-cured to reach the same level of conversion and eliminate the effect of conversion on the properties.
where c is the concentration of the diffusing water, x the distance through the thickness of the specimen, t the time, and D the diffusion coefficient in the direction of absorption. Although 2-week storage in water may not have been sufficient for water-diffusion to the center of the spherical specimens, it was sufficient to fully wet the outer layers which are abraded during the test. Dental ceramics are composed of two phases: a glassy phase surrounding a crystalline phase [13]. The size, shape, or quantity of the crystal phase might be factors which influence the wear of ceramics [30]. The hardness of composites has been considered to provide an indication of their wear resistance properties. However, the complexity of the wear process of composites has caused conflicting reports regarding the correlation between the hardness of a material and its wear resistance [31–33]. It has been suggested that a general relationship exists [31]. A positive correlation has been established between the hardness and inorganic filler content of resin composites. Composites with harder filler particles and higher filler load exhibit higher surface hardness [4,34]. Since the process of abrasive wear involves the cutting away of a soft material by a hard abrasive, it seems reasonable that adding hard inorganic filler particles to a softer resin matrix should enhance the overall abrasion resistance of the composite. Hardness of Empress has been reported to be 6.94 GPa [35]. Most glasses, such as barium glass and zinc glass, have hardness valves of about 3–4 GPa [36]. Therefore, it can be concluded that leucite-containing ceramic particles with higher hardness increased the abrasion resistance of the composite containing this crystalline filler. The microhardness of the composites (Fig. 3) also confirms the finding. The composite containing leucite ceramic exhibited higher Vickers microhardness in comparison with the other two composites.
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The abrasion test was also conducted to provide a comparison between the experimental and a commercially available composite. TetricCeram® is a hybrid composite with the fillerparticle size ranging from 0.04 to 3 m and a filler load of 79 wt%. In spite of finer filler and higher filler load, wear of TetricCeram® was greater than the leucite-based composite and this also confirms the importance of the hardness and crystallinity of the filler. As the composition and ingredients of the commercial TetricCeram® are not completely disclosed, detailed conclusions cannot be drawn about this composite. The flexural properties of the composites, as apparent in the Table 4, were also affected by the filler type. The strength and modulus of the ceramic-filler composite were higher than for the glass-based material (p < 0.05). There was no significant difference between the flexural properties of the leucite-based composite and TetricCeram® . A good bond between the matrix-resin and filler surface is a key factor in achieving desirable physical and mechanical properties. SEM micrographs (Fig. 5) of the fractured composite cross-sections show no debonding of fillers from the matrices. This indicates good bonding between silane-treated fillers and each matrix. The results also suggest that the proposed abrasion test method is suitable and can be used to study the effect of different parameters on the wear of dental materials. Although it cannot simulate the exact oral condition, the method has some advantages. It is easy to use, the obtained results are not widely scattered (very small S.D.s), a large number of specimens can be tested each time, the abrasive media can be chosen according to the experimental conditions including different abrasive materials, the pH and viscosity of the media can be changed and controlled and, by selecting the total volume (or mass) of the media, the applied load on the specimens can be controlled.
5.
Conclusion
The effect of filler type on the abrasive wear of experimental dental composites was investigated. The results showed that the ceramic filler (the glass-containing leucite) with higher hardness produced a composite with lower wear, confirming the positive effect of the filler hardness on the wear resistance of the composites. The proposed abrasive wear testing method was shown to be a suitable procedure for assessing the wear of dental materials.
references
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available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
The effect of a leucite-containing ceramic filler on the abrasive wear of dental composites Mohammad Atai a,∗ , Esmaeil Yassini b , Maryam Amini b , David C. Watts c a
Iran Polymer and Petrochemical Institute (IPPI), P.O. Box 14965/115, Tehran, Iran Department of Restorative Dentistry, Dental School, Tehran University of Medical Sciences, Tehran, Iran c Biomaterials Science Research Group, School of Dentistry and Photon Science Institute, The University of Manchester, Manchester M15 6FH, UK b
a r t i c l e
i n f o
Article history:
a b s t r a c t Objectives. The aim of this study was to evaluate abrasive wear of a dental composite based on
Received 10 March 2006
a leucite-containing (KAlSi2 O2 ) ceramic filler, and to compare it with the wear of a composite
Received in revised form
based on commonly used aluminum barium silicate glass filler.
20 March 2007
Methods. IPS Empress (Ivoclar-Vivadent) ingots were ball milled, passed through an 800 mesh
Accepted 23 March 2007
(ASTM) sieve, and used as the leucite ceramic filler. Experimental composites were prepared by mixing the silane-treated fillers with the resin monomers. The resin consisted of 70 wt% Bis-GMA and 30 wt% TEGDMA containing camphorquinone and DMAEMA as the photoini-
Keywords:
tiator system. Glass-based composites were also prepared using silane-treated aluminum
Dental composite
barium silicate glass fillers and the same resin system. TetricCeram® , a commercially avail-
Filler
able dental composite, was used as control. Spherical specimens of the composites were
Abrasive wear
then prepared and kept in water for 2 weeks to reach equilibrium with water. An abrasive
Leucite-containing ceramic
wear test was performed using a device designed in our laboratory and weight loss of the specimens was measured as an abrasion parameter after each 50 h. SEMs were taken from worn and fractured surfaces. Degree-of-conversion of the composites was measured using FTIR spectroscopy. Vickers surface microhardness, flexural strength, and flexural modulus of the composites were also measured. The data were analyzed and compared using ANOVA and Tukey HSD tests (significance level = 0.05). Results. The results showed that there were significant differences among the abrasive wear of the composites (p < 0.05). The ranking from least to most was as: leucite-based composite < TetricCeram® < glass-based composite. The higher wear resistance of leucite-based composite could be related to its higher surface hardness. Significance. Using leucite-containing glass as an alternative for aluminum barium silicate glass fillers in dental composites generated a significant increase in the wear resistance of the resin composites which should be beneficial in the development of dental materials. © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. Tel.: +98 21 44580085; fax: +98 21 44580023. E-mail address: [email protected] (M. Atai). 0109-5641/$ – see front matter © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2007.03.006
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d e n t a l m a t e r i a l s 2 3 ( 2 0 0 7 ) 1181–1187
Introduction
Table 1 – Composition of the experimental composites Composition
Wear is the loss of material through different processes: abrasion, adhesion, fatigue and corrosive effects which act in various combinations depending upon the properties of materials. Abrasion and attrition have been identified as the main clinical wear mechanisms for dental composites [1]. High resistance to wear is one of the desirable properties for restorative materials. Reducing the wear of the materials has been the subject of numerous studies. Most of the studies have centered on the reinforcing phase of the composites considering the filler particle size and surface treatment [2–6], the use of porous fillers [7], silica-fused whiskers [8] and fibers [9] to improve the wear resistance of dental composites. The particulate fillers which are commonly used in composites are silica and/or silicates. The crystalline form of silica (quartz) is stronger and harder, but used alone results in composites that are radiolucent and difficult to finish and polish; therefore most composites are now produced using silicate glasses [10]. IPS Empress ceramic contains leucite crystals (KAlSi2 O6 ) only a few microns in size that are produced by means of controlled crystallization with special glass-containing nucleating agents. The leucite crystals are the crystalline component (35–55%) of the feldspathic rock used in dental porcelains [11–13]. There is no history of using the leucite-containing ceramic, IPS Empress, as fillers in dental composites. In this study the abrasive wear of a dental composite based on fillers prepared from the ceramic was investigated and compared with the abrasion of a composite based on commonly used aluminum barium silicate glass fillers and a commercially available composite, TetricCeram® . Because of the lack of an internationally acceptable method, different methods have been used for assessing the wear of dental materials which can be categorized in two distinct classes: clinical [14,15] and laboratory test methods [16–23]. The clinical testing is complex and time consuming (up to 6 years). Laboratory methods can simplify the oral condition and give some reasonable results to compare different dental materials. An additional goal of this work was to propose a simple method for measuring the abrasive wear of composite resins or other dental materials.
2.
Materials and methods
2.1.
Materials
Glass fillers with the average particle size of 2–5 m (SP345; aluminum fluoride: 5–10%, barium fluoride: 5–10%, calcium fluoride: 5–10%, silicon dioxide: 40–70%, zinc oxide: 5–10%) were obtained from Specialty Glass (USA). TetricCeram® (Lot: F61695) in an A-3 shade and IPS Empress ingots (Lot: D50014) were purchased from Ivoclar Vivadent (Leichtenstein). Bis-GMA and triethylene glycol dimethacrylate ¨ (TEGDMA) were kindly supplied by Rohm (Degussa Group, Germany). -Methacryloxy propyl trimethoxy silane (-
Filler (wt%) Bis-GMA/TEGDMA (70/30 by weight) CQ (wt%) DMAEMA (wt%)
Leucite-based
Glass-based
75 24 0.5 0.5
75 24 0.5 0.5
MPS), camphorquinone (CQ) and N-N -dimethyl aminoethyl methacrylate (DMAEMA) were obtained from Fluka (Germany).
2.2.
Specimen preparation
IPS Empress ingots were ball milled, passed through a 800 mesh (ASTM) sieve using a wet technique, resulting in a powder filler with an average particle size of 3.6 m (75%), measured by a particle-size analyzer (Analysette 22, Fritsch, Germany). Leucite-containing and aluminium–barium–glass fillers were separately surface treated with 1 wt% -MPS. MPS was prehydrolyzed for 1 h in an aqueous solution of 70 wt% ethanol and 30 wt% double-distilled water (pH was adjusted to 3–4 by adding a few droplets of acetic acid). The treated fillers were dried for over 20 days at room temperature. Leucite and glass fillers were then hand mixed with the monomer phase and photoinitiator system according to the composition shown in Table 1. The pastes were then inserted into spherical Plexy Glass molds (diameter = 8 mm; Fig. 1) and light cured on both sides for 40 s using a light source with an irradiance circa 700 mW/cm2 (Optilux 501, Kerr, USA). As the diameter of the specimens were greater than the depth of light penetration, the specimens were then post cured in an oven (120 ◦ C, 2 h) to reach the maximum degree of conversion. TetricCeram® was tested as control. As the water uptake of the specimens could interfere with the weight loss during the abrasion test, the specimens were stored in water for 2 weeks to approach equilibrium with water before starting the test. 5 specimens were tested in each group. The specimens were numbered making small holes on their surfaces using dental drills (diameter <0.5 mm, depth: ca. 2 mm); the holes were then filled with dental waxes to reduce water diffusion which might interfere with the weight loss of the specimens.
Fig. 1 – Abrasive wear testing device.
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2.3.
Degree of conversion
Table 2 – Degree of conversion of the composites before and after post curing
To measure the degree of conversion, the uncured paste of each composite was placed between two polyethylene films, pressed to form a very thin film and the absorbance peaks obtained by transmission mode of FTIR (EQUINOX 55, Bruker, Germany). The specimens were then light cured for 40 s (Optilux 501, 700 mW/cm2 ) and the absorbance peaks were collected for the cured specimens. Specimens were then placed between two glass slides and heat cured in an oven (at 120 ◦ C for 2 h) and the absorbance peaks were also collected after post curing. Degree of conversion (DC%) was determined from the ratio of absorbance intensities of aliphatic C C (peak at 1638 cm−1 ) against internal standard aromatic C· · ·C (peak at 1608 cm−1 ) before and after curing of the specimen. The degree of conversion was then calculated as follows [24]:
DC% =
1−
(1638 cm−1 /1608 cm−1 )peak area after curing (1638 cm−1 /1608 cm−1 )peak area before curing
×100
2.4.
Abrasive wear test
A simple device was designed to measure the abrasive wear of the specimens (Fig. 1). In this device, spherical specimens in an abrasive medium (pumice powder with 100 m average particle size dispersed in water) were placed in a plastic cylindrical container. The container rotated at 50 rpm causing the specimens to slide over each other in the abrasive media resulting in abrasion. The specimens were prepared spherically because the impact fracture of sharp edges of non-spherical shapes could be eliminated. The masses of the spherical specimens in each group (n = 5) were measured by a balance (10−4 g) after each 50 h. The percent weight loss after each cycle was determined as abrasion as follows: Abrasion(%) = weight loss(%) =
W0 − Wt × 100 W0
where W0 was the initial weight and Wt was the specimen weight after each 50 h of wear.
2.5.
Flexural strength and flexural modulus
The flexural strength was measured on the bar specimens (2 mm × 2 mm × 25 mm) by a three-point bending test with a span of 20 mm using a universal testing machine (Instron 6025, UK) at a cross-head speed of 0.5 mm/min. The cured specimens were stored in distilled water for 24 h before test. The flexural strength was calculated as: Flexural strength = 3PL/2bd2
Light cure (%) (S.D.)
Glass-based Leucite-based TetricCeram®
Post cure (%) (S.D.)
71.3 (2.0) 69.0 (1.7) 65.6 (1.5)
83.9 (2.0) 83.0 (1.0) 79.5 (2.2)
where P stands for load at fracture, L for span length, b for width, and d for thickness. Flexural modulus was then determined from the slope of the elastic region of the stress–strain curve. The data were analyzed and compared using one-way ANOVA followed by Tukey HSD tests.
3.
Results
Table 2 shows the degree-of-conversion after light curing and after post curing. The specimens’ degree-of-conversion increased significantly after post-curing (p < 0.05), but there was no significant difference between the groups after postcuring. Table 3 shows weight-loss, for different composite groups which was significantly different (p < 0.05) and the ranking from least to most was: Leucite-based composite < TetricCeram® < glass-based composite. Table 4 presents the flexural strength and flexural moduli of the composites. The glass-based composite showed the lowest flexural strength and modulus. Fig. 2 plots the composite weight-losses with time. These gave linear correlation between weight loss (%) and time of
Table 3 – Abrasion (weight loss %) of the composites vs. time Time (h)
Composite
Weight loss % (S.D.)
50
Gglass-based Leucite-based TetricCeram®
3.54 (0.01) 2.50 (0.07) 2.80 (0.05)
100
Glass-based Leucite-based TetricCeram®
8.50 (0.03) 5.60 (0.11) 6.15 (0.36)
150
Glass-based Leucite-based TetricCeram®
13.59 (0.04) 9.05 (0.16) 10.77 (0.14)
200
Glass-based Leucite-based TetricCeram®
18.83 (0.05) 12.89 (0.25) 14.96 (0.18)
250
Glass-based Leucite-based TetricCeram®
24.17 (0.06) 16.54 (0.33) 19.35 (0.25)
300
Glass-based Leucite-based TetricCeram®
28.47 (0.05) 19.20 (0.40) 22.97 (0.31)
Vickers microhardness
Vickers microhardness on the surface (five points) of the cured specimens was measured using a microhardness tester (Duramin 20, Denmark) applying a load of 1.961 N for 20 s.
2.6.
Composite
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Table 4 – Flexural strength and flexural modulus of the composites Composites Glass-based Leucite-based TetricCeram®
Flexural strength (MPa) (S.D.) 43.8 (3.5) 55.4 (5.6)a 56.8 (9.5)a
Flexural modulus (GPa) (S.D.) 9.0 (1.1) 10.5 (1.0)b 11.2 (1.5)b
Superscript letters (a and b) indicate that the differences are not significant (p > 0.05).
Fig. 2 – Weight loss of the composites vs. time.
test in all the groups (r2 > 0.99). Fig. 3 shows the abrasion rate (%/h) of the composites versus their surface microhardness, where the abrasion rate was seen to increase with decreasing microhardness. Fig. 4 illustrates SEM images of worn surfaces after 300 h abrasion. The glass composite exhibited a coarser surface as compared to the lecuite and TetricCeram® surfaces. Fig. 5 shows the SEM images of fractured surfaces of the composites. No separation of fillers from resin matrices was seen.
Fig. 4 – SEM images of worn surfaces after 300 h (×3000). (a) Leucite-based composite. (b) Glass-based composite. (c) TetricCeram® .
4.
Fig. 3 – Abrasion rate vs. microhardness of the composites.
Discussion
Wear is the loss of material that occurs through contact of two or more surfaces. Abrasive wear occurs when a soft surface comes into contact with a harder one. In dental composites abrasion involves the process of resin matrix loss between filler particles and subsequent dislodgement of the filler. Filler loading, size, inter-particle spacing, degree of polymerization, filler/matrix cracking caused by water sorption and hydrolytic
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Although the FTIR measurement method used in our study shows the degree-of-conversion of a thin film of the composites which is not necessarily equal to the cure of the interior of the spherical specimens, it is sufficient for the study as the abrasion occurs at the specimen surface layers. From Table 2, degree-of-conversion significantly increased (p < 0.05) with post curing, but there was no significant difference between the degrees-of -conversion of the composites after post-cure. Therefore, filler type was the only important variable that determined the wear resistance of the experimental composites. Table 3 shows that abrasion of the composite containing amorphous glass filler was always higher than the composite with partially crystalline ceramic filler (leucite). Abrasion correlated linearly with time (Fig. 2) and the rate of abrasive wear, as presented in Fig. 3, was higher for the glass-based composite. To eliminate the effect of water-uptake during the abrasion test, the specimens were previously stored in water for 2 weeks. Water-sorption in a material is a time- and thicknessdependent phenomenon which can be described by the Fick’s second law: ∂c ∂2 c =D 2 ∂t ∂x
Fig. 5 – SEM images of fractured surfaces (×3000). (a) Leucite-based composite. (b) Glass-based composite. (c) TetricCeram® .
degradation of the filler surface are important factors affecting wear resistance of resin composites [25–28]. In this study for both experimental groups (leucite-base and glass-base composites) the filler load (75 wt %) and average filler-particle size (2–5 m) were the same; therefore the effect of these two important factors on wear was equalized. The degree of conversion influences the abrasion resistance and other mechanical properties [29], therefore the specimens were post-cured to reach the same level of conversion and eliminate the effect of conversion on the properties.
where c is the concentration of the diffusing water, x the distance through the thickness of the specimen, t the time, and D the diffusion coefficient in the direction of absorption. Although 2-week storage in water may not have been sufficient for water-diffusion to the center of the spherical specimens, it was sufficient to fully wet the outer layers which are abraded during the test. Dental ceramics are composed of two phases: a glassy phase surrounding a crystalline phase [13]. The size, shape, or quantity of the crystal phase might be factors which influence the wear of ceramics [30]. The hardness of composites has been considered to provide an indication of their wear resistance properties. However, the complexity of the wear process of composites has caused conflicting reports regarding the correlation between the hardness of a material and its wear resistance [31–33]. It has been suggested that a general relationship exists [31]. A positive correlation has been established between the hardness and inorganic filler content of resin composites. Composites with harder filler particles and higher filler load exhibit higher surface hardness [4,34]. Since the process of abrasive wear involves the cutting away of a soft material by a hard abrasive, it seems reasonable that adding hard inorganic filler particles to a softer resin matrix should enhance the overall abrasion resistance of the composite. Hardness of Empress has been reported to be 6.94 GPa [35]. Most glasses, such as barium glass and zinc glass, have hardness valves of about 3–4 GPa [36]. Therefore, it can be concluded that leucite-containing ceramic particles with higher hardness increased the abrasion resistance of the composite containing this crystalline filler. The microhardness of the composites (Fig. 3) also confirms the finding. The composite containing leucite ceramic exhibited higher Vickers microhardness in comparison with the other two composites.
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The abrasion test was also conducted to provide a comparison between the experimental and a commercially available composite. TetricCeram® is a hybrid composite with the fillerparticle size ranging from 0.04 to 3 m and a filler load of 79 wt%. In spite of finer filler and higher filler load, wear of TetricCeram® was greater than the leucite-based composite and this also confirms the importance of the hardness and crystallinity of the filler. As the composition and ingredients of the commercial TetricCeram® are not completely disclosed, detailed conclusions cannot be drawn about this composite. The flexural properties of the composites, as apparent in the Table 4, were also affected by the filler type. The strength and modulus of the ceramic-filler composite were higher than for the glass-based material (p < 0.05). There was no significant difference between the flexural properties of the leucite-based composite and TetricCeram® . A good bond between the matrix-resin and filler surface is a key factor in achieving desirable physical and mechanical properties. SEM micrographs (Fig. 5) of the fractured composite cross-sections show no debonding of fillers from the matrices. This indicates good bonding between silane-treated fillers and each matrix. The results also suggest that the proposed abrasion test method is suitable and can be used to study the effect of different parameters on the wear of dental materials. Although it cannot simulate the exact oral condition, the method has some advantages. It is easy to use, the obtained results are not widely scattered (very small S.D.s), a large number of specimens can be tested each time, the abrasive media can be chosen according to the experimental conditions including different abrasive materials, the pH and viscosity of the media can be changed and controlled and, by selecting the total volume (or mass) of the media, the applied load on the specimens can be controlled.
5.
Conclusion
The effect of filler type on the abrasive wear of experimental dental composites was investigated. The results showed that the ceramic filler (the glass-containing leucite) with higher hardness produced a composite with lower wear, confirming the positive effect of the filler hardness on the wear resistance of the composites. The proposed abrasive wear testing method was shown to be a suitable procedure for assessing the wear of dental materials.
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