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Effect of immersion time of restorative glass ionomer cements and immersion duration in calcium chloride solution on surface hardness Maho Shiozawa a , Hidekazu Takahashi b,∗ , Naohiko Iwasaki b , Takahiro Wada c , Motohiro Uo c a
Removable Partial Prosthodontics, Department of Masticatory Function Rehabilitation, Division of Oral Health Sciences, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan b Oral Biomaterials Engineering, Course of Oral Health Engineering, School of Oral Health Care Sciences, Faculty of Dentistry, Tokyo Medical and Dental University, Tokyo, Japan c Advanced Biomaterials, Department of Restorative Sciences, Division of Oral Health Sciences, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The objective of this study was to evaluate the effect of immersion time of restor-
Received 22 October 2013
ative glass ionomer cements (GICs) and immersion duration in calcium chloride (CaCl2 )
Received in revised form
solution on the surface hardness.
25 March 2014
Methods. Two high-viscosity GICs, Fuji IX GP and GlasIonomer FX-II, were selected. Forty-
Accepted 8 August 2014
eight specimens were randomly divided into two groups. Sixty minutes after being mixed,
Available online xxx
half of them were immersed in a 42.7 wt% CaCl2 solution for 10, 30, or 60 min (Group 1); the remaining specimens were immersed after an additional 1-week of storage (Group 2).
Keywords:
The surface hardness of the specimens was measured and analyzed with two-way ANOVA
Glass ionomer cement
and the Tukey HSD test (˛ = 0.05). The surface compositions were examined using energy-
Calcium chloride
dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy.
Surface hardness
Results. The surface hardness of Group 1 significantly increased as the immersion dura-
Elemental analysis
tion in CaCl2 increased; that of Group 2 significantly increased only after 60-minute CaCl2 immersion. After CaCl2 immersion, the amounts of Ca increased as the immersion duration increased. The surface hardness after CaCl2 immersion significantly correlated with the amount of Ca in Group 1, but not in Group 2. The binding energy of the Ca2p peak was similar to that of calcium polyalkenoate. These findings indicated that the Ca ions from the CaCl2 solution created chemical bonds with the carboxylic acid groups in the cement matrix. Significance. Immersion of GICs in CaCl2 solution at the early stage of setting was considered to enhance the formation of the polyacid salt matrix; as a result, the surface hardness increased. © 2014 Published by Elsevier Ltd on behalf of Academy of Dental Materials. All rights reserved.
∗ Corresponding author at: Oral Biomaterials Engineering, Course of Oral Health Engineering, School of Oral Health Care Sciences, Faculty of Dentistry, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo, 113-8549, Japan. Tel.: +81 3 5803 5379; fax: +81 3 5803 5379. E-mail address: [email protected] (H. Takahashi) .
http://dx.doi.org/10.1016/j.dental.2014.08.366 0109-5641/© 2014 Published by Elsevier Ltd on behalf of Academy of Dental Materials. All rights reserved.
Please cite this article in press as: Shiozawa M, et al. Effect of immersion time of restorative glass ionomer cements and immersion duration in calcium chloride solution on surface hardness. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.08.366
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1.
Introduction
Glass ionomer cements (GICs) are widely used as a dental restorative material due to their fluoride release and chemical adhesion to tooth structure without pretreatment. However, the mechanical properties of conventional GICs are insufficient for occlusal restoration [1–3]. Recently, high-viscosity GICs have been developed with improved mechanical properties due to an increase in the powder/liquid ratio and the modified particle size distribution of the glass powder [2,4–6]. Because high-viscosity GICs have greater compressive strength, surface hardness, and wear resistance compared to conventional GICs and some resin-modified GICs [6–8], highviscosity GICs are the material of choice for the Atraumatic Restorative Treatment (ART) technique [3,9]. The setting of the GICs is caused by cross-linking of the carboxylic acid groups in the aqueous solution of polyacrylic acids with Al and Ca ions released from fluoroaluminosilicate glass powder [10,11]. When the aqueous solution and the glass powder are combined, the hydrogen ions of the polyacid attack the glass particles, and then Al and Ca ions are released from the glass. These metal ions combine with the carboxylic acid groups of the polyacid to form the polyacid salts matrix, and glass surface is changed to a silica hydrogel. The initial increase in the mechanical strength of GICs is mainly caused by polycarboxylate gel formation [12]. The mechanical properties of GICs have been reported to increase with storage time due to the continuing maturation of the acid–base reaction [4]. However, the increase of mechanical properties of GICs occurs more slowly than do those of resin-modified GICs and composite resins [13]. The mechanical properties of high-viscosity GICs are still insufficient at an early stage of the setting. There are several reports describing efforts to overcome this drawback, such as using ultrasonic excitation to accelerate the initial setting reaction [14] and the application of a surface coating with light-cured resin to protect the surface from ambient moisture during setting [15]. The surface of the cement is directly affected by environmental conditions; therefore, the enhancement of the GIC surface at early setting stages is considered to be valuable for the stability of the GICs’ mechanical properties. Immersion in a higher concentration of calcium chloride (CaCl2 ) solution for 1 day and 1 week was reported to cause a greater increase in the surface hardness of high-viscosity GICs [16]. After immersion in the CaCl2 solution, Ca was detected in the cement matrix, suggesting that the Ca ions of the CaCl2 solution reacted with the unreacted carboxylic acid groups remaining in the cement matrix. These reactions enhanced the reaction by forming a polyacid salts matrix. Immersing GICs in CaCl2 solution is quite simple, and it is not necessary to change the GIC compositions and restoration procedure. If short-term immersion of GICs in CaCl2 solution can improve their surface hardness at an early stage of setting, this method is practical in the clinical situation. However, the adequate setting stage and duration of CaCl2 solution immersion to obtain an effective increase in the surface hardness have not been clearly elucidated. The objective of the present study was to evaluate the effect of immersion time of restorative GICs and immersion
duration in CaCl2 solution on the surface hardness by measuring the Vickers hardness and analyzing the surface composition. The null hypothesis was that the immersion time of GICs and immersion duration in CaCl2 solution had no influence on the surface hardness of GICs.
2.
Materials and methods
2.1.
Materials and storage medium
The two high-viscosity GICs evaluated in the present study are listed in Table 1. These GICs were mixed according to each manufacturer’s instructions at 23 ± 2 ◦ C. Calcium chloride (CaCl2 ; Calcium Chloride, Anhydrous, JIS Special Grade, Wako Pure Chemical Industries, Osaka, Japan) was dissolved in deionized water to make a saturated concentration of 42.7 wt% Ca at 20 ◦ C. This solution was used as the immersion media for the following experiments.
2.2.
Surface hardness measurement
Freshly mixed cement was placed in an acrylic cavity mold (5 mm inner diameter; 2 mm high), then slightly overfilled and compressed with a polyethylene sheet and a metal plate. The mold was stored in a 37.0 ◦ C incubator (NIB-10, AGC Techno Glass Co., Ltd., Shizuoka, Japan) for 1 h before removal of the polyethylene sheet from the specimen surface. The forty-eight specimens were randomly divided into two groups. Half of the specimens were not stored in water (Group 1), and the remainder was used after an additional 1-week of storage in deionized water (Group 2). The surface hardness of each specimen was measured using a micro-hardness testing machine (MVK-H2, Akashi, Kawasaki, Japan) and a Vickers indenter with a load of 50 gf and a dwell time of 15 s. The Vickers hardness of each specimen before immersion in CaCl2 solution was determined using the average of five indentations at least 1 mm apart; these values were registered as references. Six specimens of each group were immersed in 10-mL CaCl2 solution for 10, 30, or 60 min or in 10-mL deionized water for 60 min in a plastic container in the 37.0 ◦ C incubator. The Vickers hardness of each specimen was then determined.
2.3.
Energy-dispersive X-ray spectroscopy
Twenty-seven specimens of each GIC were prepared and stored using the above-mentioned method. Three specimens of each group were immersed in 10-mL CaCl2 solution for 10, 30, or 60 min or in 10-mL deionized water for 60 min in a plastic container in the 37.0 ◦ C incubator. Three specimens that were not immersed in either CaCl2 solution or water were used as references. Subsequently, the specimens in the mold were kept in a desiccator containing silica gel for at least 1 day. The specimens were removed from the mold and mounted on aluminum stubs, then coated with carbon. The elemental composition of the specimen surface was analyzed using a scanning electron microscope (SEM; S-4500, Hitachi High-Technologies Corp., Tokyo, Japan) and an energydispersive X-ray spectroscope (EDS; EMAX-7000, Horiba, Kyoto,
Please cite this article in press as: Shiozawa M, et al. Effect of immersion time of restorative glass ionomer cements and immersion duration in calcium chloride solution on surface hardness. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.08.366
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Table 1 – Restorative glass ionomer cements investigated. Material
Code
Manufacturer
P/L
Shade
Fuji IX GP
FIX
GC Corp. (Tokyo, Japan)
3.6
A3
GlasIonomer FX-II
GFX
Shofu Inc. (Kyoto, Japan)
2.6
A2
Japan). EDS analysis was performed on each specimen at three randomly selected areas using the acceleration voltage of 15 kV for 100 s.
2.4.
Fourier transform infrared (FTIR) spectroscopy
Freshly mixed cement was placed in an acrylic mold (3 mm inner diameter; 0.5 mm thick), then slightly overfilled and compressed with polyethylene sheets and glass plates. The molds were stored in a 37.0 ◦ C incubator for 1 h, and then the specimens were removed from the mold and immersed in 10-mL CaCl2 solution or deionized water for 60 min. The surfaces of the specimens were observed using a FTIR spectrometer (FTIR-8300, Shimadzu, Kyoto, Japan) equipped with an attenuated total reflectance accessory (DuraSampIIR, ASI Technologies, Danbury, CT, USA). A total of 64 scans ranging from 4000 to 600 cm−1 were recorded with a resolution of 4 cm−1 . A non-immersed specimen and liquid of each GIC were observed as references.
2.5.
X-ray photoelectron spectroscopy
Freshly mixed cement was placed in an acrylic mold (2 mm inner diameter; 0.5 mm thick), then slightly overfilled and compressed with polyethylene sheets and glass plates. The molds were stored in a 37.0 ◦ C incubator for 1 h, and then the specimens were removed from the mold and immersed in 10-mL CaCl2 solution for 10, 30, or 60 min. The chemical structures of the specimen mount on a measuring holder were observed using an X-ray photoelectron spectrometer (JPC-9010MC, JEOL Ltd., Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) analysis was performed under a base pressure of less than 10−7 Pa. The Al-K␣ X-ray was obtained using a 10-kV accelerating voltage and 10-mA filament current. A non-immersed specimen of each GIC and powder of CaCl2 were observed as a reference.
2.6.
Statistical analysis
The surface hardness values after immersion were analyzed by one-way analysis of variance (ANOVA) to assess the effect of the immersion condition. The Tukey HSD test was used to compare the individual data. The relationship between the Ca amount on the specimen surface and the surface hardness was evaluated using linear regression analysis. The level of significance was set at ˛ = 0.05.
Components P: fluoroaluminosilicate glass, polyacrylic acid L: polyacrylic acid, distilled water, polybasic carboxylic acid P: fluoroaluminosilicate glass, pigments, fluorescent material L: acrylic acid-tricarboxylic acid co-polymer, tartaric acid, distilled water
3.
Lot No. P: 1108171 L: 1108171 P: 061049 L: 061034
Results
The surface hardness values of FIX before the CaCl2 immersion of Group 1 (without water storage) and Group 2 (after 1-week water storage) were 53.0 ± 2.9 and 74.6 ± 3.0, respectively; those of GFX before the CaCl2 immersion of Group 1 and Group 2 were 43.8 ± 2.2 and 61.4 ± 3.1, respectively. The hardness of both FIX and GFX in Groups 1 and 2 did not change after immersion in deionized water for 60 min. Fig. 1 shows the means and standard deviations of the hardness of FIX and GFX after immersion in CaCl2 solution or in deionized water. The hardness changes of FIX and GFX showed the same tendency. Regarding Group 1, the one-way ANOVA suggested that the immersion condition (immersion solution and duration) was significant. The hardness after CaCl2 immersion was significantly greater than that after water immersion. The hardness increased as the immersion duration increased. For Group 2, the one-way ANOVA suggested that the immersion condition was significant. The hardness after 60-minute CaCl2 immersion was significantly greater than that after water immersion. The elemental compositions of FIX and GFX surfaces before and after immersion in CaCl2 solution or in deionized water are summarized in Table 2. Not more than 0.1% of Ca and Cl were detected on the surface before immersion, but obvious amounts of Ca and Cl were detected on the surface after CaCl2 immersion. The amounts of Ca and Cl increased as the immersion duration increased. The relative amounts of Ca and Cl after the CaCl2 immersion of Group 2 (after 1-week water storage) were greater than those of Group 1 (without water storage). The relative amounts of Sr, F, and Na (FIX only) of Group 2 decreased after immersion. The elemental compositions of FIX and GFX showed a similar tendency, but the amounts of Ca and Cl after the immersion of GFX in CaCl2 were greater than those of FIX. Fig. 2 indicates the relationship between the amount of Ca on the specimen surface and the surface hardness of FIX and GFX. The hardness of both FIX and GFX in Group 1 (without water storage) significantly increased with an increase of the Ca amount (p = 0.02 and 0.01). The hardness of both FIX and GFX in Group 2 (after 1-week water storage) showed the same tendency, but was not significant (p = 0.08 and 0.05). Representative FTIR spectra of FIX and GFX are shown in Fig. 3. Regarding the liquid of GICs, the peaks at 1700 cm−1 and 1640 cm−1 were observed and assigned to the carboxylic acid groups and water, respectively. Regarding the set GICs, new peaks appeared around 1590 cm−1 and 1550 cm−1 , which were assigned to aluminum polyacrylate and calcium polyacrylate, respectively [10]. The absorbance of the latter peak
Please cite this article in press as: Shiozawa M, et al. Effect of immersion time of restorative glass ionomer cements and immersion duration in calcium chloride solution on surface hardness. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.08.366
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Table 2 – Elemental compositions (at%) of FIX and GFX surfaces before and after immersion. FIX Compositions (at%)
Immersion time Si
Al
P
Sr
Na
F
Ca
Cl
31.8 (0.4)
29.3 (0.2)
6.9 (0.3)
14.7 (0.2)
2.7 (0.1)
14.5 (0.7)
0.1 (0.1)
0.0 (0.0)
Water 60 min CaCl2 10 min CaCl2 30 min CaCl2 60 min
33.6 (0.4)
30.2 (0.2)
6.9 (0.5)
14.3 (0.2)
2.3 (0.1)
12.7 (0.4)
0.0 (0.1)
0.0 (0.0)
32.3 (0.4)
29.3 (0.3)
6.7 (0.4)
13.9 (0.5)
2.6 (0.1)
14.0 (0.8)
1.2 (0.2)
0.2 (0.1)
31.8 (1.0)
29.1 (0.4)
6.6 (0.2)
13.6 (0.4)
2.4 (0.3)
14.1 (0.2)
1.6 (0.4)
0.7 (0.5)
30.9 (0.3)
28.6 (0.3)
6.6 (0.3)
13.2 (0.7)
2.4 (0.5)
14.6 (0.6)
2.9 (1.4)
0.9 (0.8)
Water 60 min CaCl2 10 min CaCl2 30 min CaCl2 60 min
36.5 (0.5)
35.9 (0.3)
8.4 (0.4)
11.1 (0.2)
1.0 (0.1)
7.0 (0.1)
0.0 (0.0)
0.0 (0.0)
34.5 (0.5)
35.4 (0.6)
8.2 (0.6)
10.6 (1.0)
1.2 (0.1)
7.1 (0.2)
1.7 (0.6)
1.3 (0.7)
34.2 (0.1)
34.9 (0.8)
7.9 (0.0)
9.9 (0.5)
0.9 (0.2)
7.0 (0.1)
3.0 (0.2)
2.2 (0.9)
32.8 (0.4)
33.7 (0.4)
7.5 (0.2)
9.5 (0.4)
0.9 (0.0)
7.1 (0.2)
4.4 (0.2)
4.1 (0.0)
Before immersion Without water storage (Group 1)
1-week water storage (Group 2)
GFX Compositions (at%)
Immersion time Si
Al
P
Sr
F
Ca
Cl
32.0 (0.3)
31.0 (0.3)
12.2 (0.2)
11.8 (0.4)
12.8 (0.2)
0.1 (0.1)
0.0 (0.0)
Water 60 min CaCl2 10 min CaCl2 30 min CaCl2 60 min
33.3 (0.9)
31.6 (0.4)
12.5 (0.2)
11.4 (0.2)
11.2 (0.6)
0.0 (0.0)
0.0 (0.0)
30.6 (0.7)
29.9 (0.4)
11.9 (0.3)
10.1 (0.5)
12.9 (0.4)
3.1 (0.5)
1.5 (0.7)
30.3 (0.7)
29.5 (0.4)
11.5 (0.3)
10.0 (0.8)
13.0 (0.2)
3.3 (0.6)
2.4 (0.9)
29.9 (0.5)
28.8 (0.2)
11.2 (0.3)
9.3 (0.9)
12.5 (0.1)
4.8 (0.3)
3.4 (0.5)
Water 60 min CaCl2 10 min CaCl2 30 min CaCl2 60 min
33.0 (0.1)
37.0 (0.1)
14.1 (0.6)
8.3 (0.4)
7.5 (0.4)
0.1 (0.1)
0.0 (0.0)
31.2 (0.7)
34.8 (0.5)
12.4 (0.9)
6.6 (0.1)
7.1 (0.5)
4.0 (0.6)
3.8 (0.6)
29.6 (0.8)
33.3 (0.4)
11.4 (0.8)
5.9 (0.2)
6.7 (0.2)
6.3 (0.7)
6.8 (0.4)
28.8 (0.7)
33.0 (0.1)
11.4 (0.6)
6.1 (0.1)
6.8 (0.4)
6.8 (0.7)
7.2 (0.5)
Before immersion Without water storage (Group 1)
1-week water storage (Group 2)
Standard deviations in parentheses.
increased after immersion. The absorbance around 1550 cm−1 after CaCl2 solution immersion was greater than that after water immersion. The XPS wide-scan spectra of FIX and GFX showed a similar pattern. Before immersion, the F1s, O1s, C1s, Si2s, P2p, Sr3d, Si2p, and Al2p peaks were assigned; after CaCl2 immersion, the Ca peak was also assigned. The intensity of the Ca2p peak increased as the immersion duration increased. The XPS narrow-scan spectra of the Ca2p region of FIX and GFX (Fig. 4) showed that Ca2p3/2 and Ca2p1/2 peaks were recognized at binding energies of 347.5 eV and 351.1 eV, respectively. The Ca2p3/2 and Ca2p1/2 peaks of CaCl2 powder was detected at a binding energy of 348.3 eV and 351.8 eV, respectively.
4.
Discussion
Several concentrations of CaCl2 solutions were used as an immersion solution in a previous study [16]. Immersing GICs in 42.7 wt% CaCl2 solution produced the greatest increase in surface hardness regardless of the immersion duration and the product. These results suggested that the 42.7 wt% CaCl2 solution was the most effective concentration to increase GIC surface hardness. For Group 1 (without water storage), the surface hardness after CaCl2 solution immersion significantly increased compared to that after water immersion, and the hardness increased with an increase of the immersion duration. For Group 2 (after 1-week water storage), only the hardness after
Please cite this article in press as: Shiozawa M, et al. Effect of immersion time of restorative glass ionomer cements and immersion duration in calcium chloride solution on surface hardness. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.08.366
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Fig. 1 – Surface hardness of FIX and GFX after immersion. Bars with the same letter were not significantly different (p > 0.05).
Fig. 2 – Relationship between the amount of Ca on the specimen surface and the surface hardness of FIX and GFX. The coefficient of determination (r2 ) with the asterisk was significant (p < 0.05).
Fig. 3 – FTIR spectra changes of FIX and GFX after immersion in CaCl2 solution and water. The peak after CaCl2 solution immersion around 1550 cm−1 (↓) relatively increased compared to that around 1590 cm−1 (↑). Please cite this article in press as: Shiozawa M, et al. Effect of immersion time of restorative glass ionomer cements and immersion duration in calcium chloride solution on surface hardness. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.08.366
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Fig. 4 – XPS narrow-scan spectra of the Ca2p region of FIX and GFX.
60-minute CaCl2 solution immersion significantly increased compared to that after water immersion. These results revealed that the effect of CaCl2 immersion on the Group 1 surface hardness was greater than on the Group 2 surface hardness. The null hypothesis that the immersion time of GICs and immersion duration in CaCl2 solution had no influence on the surface hardness of GICs was rejected. Thus, the setting stage was one of the important factors for increasing GICs surface hardness. The immersion duration was also a factor for increasing the surface hardness of Group 1. Calcium was detected on the surface in both Groups 1 and 2 after CaCl2 immersion. The amounts of Ca increased with the immersion duration. After the Ca-containing GICs were mixed [10,17,18], the hydrogen in the carboxylic acid groups was progressively replaced by Ca released from the glass particles, and calcium polyacrylate preferentially formed in the early stages of the cement-forming reaction. Calcium can adopt six-fold coordination and force the creation of sites in the structure where additional oxygen atoms are present. These reactions cause the alteration in the structure of the glass [19]. After the storage of GICs in a Ca-containing solution [16,20], Ca was observed in the cement matrix but not in the glass core. Unreacted carboxylic acid groups still remained in the cement matrix after the GIC set [10,11]. These reports indicated that the Ca ions from the solution would create chemical bonds with the unreacted carboxylic acid groups remaining in the matrix. According to the FTIR observation, the peak around 1550 cm−1 was assigned to calcium polyacrylate, which participated in the cross-linking. The absorbance of this peak increased after 60-minute immersion in the CaCl2 solution comparing in water. This result indicated that the amount of calcium polyacrylate increased after CaCl2 solution immersion. The increase of calcium polyacrylate has been reported due to enhancement of setting reaction [10,12]. According to the XPS analysis, the Ca2p3/2 peak of CaCl2 powder was recognized at a binding energy of 348.3 eV, which coincided with that of previous reports, 347.8–348.7 eV [21–24]. The binding energy of the Ca2p3/2 peak of the specimen after CaCl2 immersion (347.5 eV) was similar to the binding energy
of calcium polyalkenoate (347.23 eV) [25], but was less than that of CaCl2 . This finding supported the assumption that the Ca ions from the CaCl2 solution would behave as the network modifier, thus enhancing the formation of the polyacid salt matrix. The surface hardness after the CaCl2 immersion of Group 1 significantly correlated with the amount of Ca on the specimen surface; that of Group 2 showed a similar tendency but it was not significant. The surface hardness increase after the CaCl2 immersion of Group 2 was smaller than that of Group 1, while the amount of Ca on the Group 2 surface after CaCl2 immersion was greater than that on the Group 1 surface. Before the CaCl2 solution immersion of Group 2, the surface hardness was greater than that of Group 1 due to the maturation of the acid–base reaction. The surface hardness of the GICs was reported to drastically increase during the first week and remained stable up to 6 months [26]. These findings suggested that the setting reaction of the Group 2 specimens greatly progressed during the 1-week water storage, and the unreacted carboxylic acid groups in the cement matrix decreased. Therefore, the amount of newly created chemical bonds between the Ca ions and the carboxylic acid groups might be limited to the Group 2 specimens. As a result, the hardness increase of Group 2 was less than that of Group 1. However, a higher amount of Ca was detected on the surface after the immersion of Group 2 compared to amounts on the Group 1 specimens. High amounts of cement chemical components, such as Sr, F, and Na, were released during the 1-week water storage of the Group 2 specimens. It could be supposed that the matrixes of the Group 2 specimens had much more space than those of the Group 1 specimens due to the greater component release, so a higher amount of Ca penetrated the matrixes in Group 2. The immersion of the GICs in CaCl2 solution at the early stage of the setting was considered to enhance forming the polyacid salt matrix. The method of improving the GIC surface hardness in the present study was quite simple, and it was not necessary to change the GIC compositions and restoration procedure. However, a method of maintaining the CaCl2 solution on the GIC surface should be developed for the
Please cite this article in press as: Shiozawa M, et al. Effect of immersion time of restorative glass ionomer cements and immersion duration in calcium chloride solution on surface hardness. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.08.366
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clinical situation. For example, a surface coating might be suitable for preserving the CaCl2 solution, which will be evaluated in a future study.
5.
Conclusions
Within the limitations of the experimental methods employed in the present study, the following conclusions can be drawn:
1. The surface hardness of GICs at an early stage of setting significantly increased as the CaCl2 immersion duration increased. 2. The surface hardness of GICs 1 week after the start of mixing significantly increased due to only a 60-minute CaCl2 immersion. 3. Regardless of the setting stage, the amounts of Ca after CaCl2 immersion increased as the immersion duration increased. 4. After CaCl2 immersion, the surface hardness at early setting stages significantly correlated with the Ca amount on the specimen surface. Specimens immersed 1 week after the start of mixing showed a similar tendency but was not significant. 5. By FTIR observation, the peak around 1550 cm−1 assigned to calcium polyacrylate was detected on the GIC surfaces. The absorbance of this peak after the CaCl2 solution immersion was greater than that after water immersion. 6. The XPS narrow-scan spectra suggested that the binding energy of the Ca2p3/2 peak was similar to the binding energy of calcium polyalkenoate. This finding indicated that the Ca ions from the CaCl2 solution created chemical bonds with the carboxylic acid groups in the cement matrix.
Acknowledgement The authors wish to express their gratitude to Ms. Jeanne Santa Cruz for her proofreading of this paper.
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Effect of immersion time of restorative glass ionomer cements and immersion duration in calcium chloride solution on surface hardness Maho Shiozawa a , Hidekazu Takahashi b,∗ , Naohiko Iwasaki b , Takahiro Wada c , Motohiro Uo c a
Removable Partial Prosthodontics, Department of Masticatory Function Rehabilitation, Division of Oral Health Sciences, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan b Oral Biomaterials Engineering, Course of Oral Health Engineering, School of Oral Health Care Sciences, Faculty of Dentistry, Tokyo Medical and Dental University, Tokyo, Japan c Advanced Biomaterials, Department of Restorative Sciences, Division of Oral Health Sciences, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The objective of this study was to evaluate the effect of immersion time of restor-
Received 22 October 2013
ative glass ionomer cements (GICs) and immersion duration in calcium chloride (CaCl2 )
Received in revised form
solution on the surface hardness.
25 March 2014
Methods. Two high-viscosity GICs, Fuji IX GP and GlasIonomer FX-II, were selected. Forty-
Accepted 8 August 2014
eight specimens were randomly divided into two groups. Sixty minutes after being mixed,
Available online xxx
half of them were immersed in a 42.7 wt% CaCl2 solution for 10, 30, or 60 min (Group 1); the remaining specimens were immersed after an additional 1-week of storage (Group 2).
Keywords:
The surface hardness of the specimens was measured and analyzed with two-way ANOVA
Glass ionomer cement
and the Tukey HSD test (˛ = 0.05). The surface compositions were examined using energy-
Calcium chloride
dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy.
Surface hardness
Results. The surface hardness of Group 1 significantly increased as the immersion dura-
Elemental analysis
tion in CaCl2 increased; that of Group 2 significantly increased only after 60-minute CaCl2 immersion. After CaCl2 immersion, the amounts of Ca increased as the immersion duration increased. The surface hardness after CaCl2 immersion significantly correlated with the amount of Ca in Group 1, but not in Group 2. The binding energy of the Ca2p peak was similar to that of calcium polyalkenoate. These findings indicated that the Ca ions from the CaCl2 solution created chemical bonds with the carboxylic acid groups in the cement matrix. Significance. Immersion of GICs in CaCl2 solution at the early stage of setting was considered to enhance the formation of the polyacid salt matrix; as a result, the surface hardness increased. © 2014 Published by Elsevier Ltd on behalf of Academy of Dental Materials. All rights reserved.
∗ Corresponding author at: Oral Biomaterials Engineering, Course of Oral Health Engineering, School of Oral Health Care Sciences, Faculty of Dentistry, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo, 113-8549, Japan. Tel.: +81 3 5803 5379; fax: +81 3 5803 5379. E-mail address: [email protected] (H. Takahashi) .
http://dx.doi.org/10.1016/j.dental.2014.08.366 0109-5641/© 2014 Published by Elsevier Ltd on behalf of Academy of Dental Materials. All rights reserved.
Please cite this article in press as: Shiozawa M, et al. Effect of immersion time of restorative glass ionomer cements and immersion duration in calcium chloride solution on surface hardness. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.08.366
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1.
Introduction
Glass ionomer cements (GICs) are widely used as a dental restorative material due to their fluoride release and chemical adhesion to tooth structure without pretreatment. However, the mechanical properties of conventional GICs are insufficient for occlusal restoration [1–3]. Recently, high-viscosity GICs have been developed with improved mechanical properties due to an increase in the powder/liquid ratio and the modified particle size distribution of the glass powder [2,4–6]. Because high-viscosity GICs have greater compressive strength, surface hardness, and wear resistance compared to conventional GICs and some resin-modified GICs [6–8], highviscosity GICs are the material of choice for the Atraumatic Restorative Treatment (ART) technique [3,9]. The setting of the GICs is caused by cross-linking of the carboxylic acid groups in the aqueous solution of polyacrylic acids with Al and Ca ions released from fluoroaluminosilicate glass powder [10,11]. When the aqueous solution and the glass powder are combined, the hydrogen ions of the polyacid attack the glass particles, and then Al and Ca ions are released from the glass. These metal ions combine with the carboxylic acid groups of the polyacid to form the polyacid salts matrix, and glass surface is changed to a silica hydrogel. The initial increase in the mechanical strength of GICs is mainly caused by polycarboxylate gel formation [12]. The mechanical properties of GICs have been reported to increase with storage time due to the continuing maturation of the acid–base reaction [4]. However, the increase of mechanical properties of GICs occurs more slowly than do those of resin-modified GICs and composite resins [13]. The mechanical properties of high-viscosity GICs are still insufficient at an early stage of the setting. There are several reports describing efforts to overcome this drawback, such as using ultrasonic excitation to accelerate the initial setting reaction [14] and the application of a surface coating with light-cured resin to protect the surface from ambient moisture during setting [15]. The surface of the cement is directly affected by environmental conditions; therefore, the enhancement of the GIC surface at early setting stages is considered to be valuable for the stability of the GICs’ mechanical properties. Immersion in a higher concentration of calcium chloride (CaCl2 ) solution for 1 day and 1 week was reported to cause a greater increase in the surface hardness of high-viscosity GICs [16]. After immersion in the CaCl2 solution, Ca was detected in the cement matrix, suggesting that the Ca ions of the CaCl2 solution reacted with the unreacted carboxylic acid groups remaining in the cement matrix. These reactions enhanced the reaction by forming a polyacid salts matrix. Immersing GICs in CaCl2 solution is quite simple, and it is not necessary to change the GIC compositions and restoration procedure. If short-term immersion of GICs in CaCl2 solution can improve their surface hardness at an early stage of setting, this method is practical in the clinical situation. However, the adequate setting stage and duration of CaCl2 solution immersion to obtain an effective increase in the surface hardness have not been clearly elucidated. The objective of the present study was to evaluate the effect of immersion time of restorative GICs and immersion
duration in CaCl2 solution on the surface hardness by measuring the Vickers hardness and analyzing the surface composition. The null hypothesis was that the immersion time of GICs and immersion duration in CaCl2 solution had no influence on the surface hardness of GICs.
2.
Materials and methods
2.1.
Materials and storage medium
The two high-viscosity GICs evaluated in the present study are listed in Table 1. These GICs were mixed according to each manufacturer’s instructions at 23 ± 2 ◦ C. Calcium chloride (CaCl2 ; Calcium Chloride, Anhydrous, JIS Special Grade, Wako Pure Chemical Industries, Osaka, Japan) was dissolved in deionized water to make a saturated concentration of 42.7 wt% Ca at 20 ◦ C. This solution was used as the immersion media for the following experiments.
2.2.
Surface hardness measurement
Freshly mixed cement was placed in an acrylic cavity mold (5 mm inner diameter; 2 mm high), then slightly overfilled and compressed with a polyethylene sheet and a metal plate. The mold was stored in a 37.0 ◦ C incubator (NIB-10, AGC Techno Glass Co., Ltd., Shizuoka, Japan) for 1 h before removal of the polyethylene sheet from the specimen surface. The forty-eight specimens were randomly divided into two groups. Half of the specimens were not stored in water (Group 1), and the remainder was used after an additional 1-week of storage in deionized water (Group 2). The surface hardness of each specimen was measured using a micro-hardness testing machine (MVK-H2, Akashi, Kawasaki, Japan) and a Vickers indenter with a load of 50 gf and a dwell time of 15 s. The Vickers hardness of each specimen before immersion in CaCl2 solution was determined using the average of five indentations at least 1 mm apart; these values were registered as references. Six specimens of each group were immersed in 10-mL CaCl2 solution for 10, 30, or 60 min or in 10-mL deionized water for 60 min in a plastic container in the 37.0 ◦ C incubator. The Vickers hardness of each specimen was then determined.
2.3.
Energy-dispersive X-ray spectroscopy
Twenty-seven specimens of each GIC were prepared and stored using the above-mentioned method. Three specimens of each group were immersed in 10-mL CaCl2 solution for 10, 30, or 60 min or in 10-mL deionized water for 60 min in a plastic container in the 37.0 ◦ C incubator. Three specimens that were not immersed in either CaCl2 solution or water were used as references. Subsequently, the specimens in the mold were kept in a desiccator containing silica gel for at least 1 day. The specimens were removed from the mold and mounted on aluminum stubs, then coated with carbon. The elemental composition of the specimen surface was analyzed using a scanning electron microscope (SEM; S-4500, Hitachi High-Technologies Corp., Tokyo, Japan) and an energydispersive X-ray spectroscope (EDS; EMAX-7000, Horiba, Kyoto,
Please cite this article in press as: Shiozawa M, et al. Effect of immersion time of restorative glass ionomer cements and immersion duration in calcium chloride solution on surface hardness. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.08.366
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Table 1 – Restorative glass ionomer cements investigated. Material
Code
Manufacturer
P/L
Shade
Fuji IX GP
FIX
GC Corp. (Tokyo, Japan)
3.6
A3
GlasIonomer FX-II
GFX
Shofu Inc. (Kyoto, Japan)
2.6
A2
Japan). EDS analysis was performed on each specimen at three randomly selected areas using the acceleration voltage of 15 kV for 100 s.
2.4.
Fourier transform infrared (FTIR) spectroscopy
Freshly mixed cement was placed in an acrylic mold (3 mm inner diameter; 0.5 mm thick), then slightly overfilled and compressed with polyethylene sheets and glass plates. The molds were stored in a 37.0 ◦ C incubator for 1 h, and then the specimens were removed from the mold and immersed in 10-mL CaCl2 solution or deionized water for 60 min. The surfaces of the specimens were observed using a FTIR spectrometer (FTIR-8300, Shimadzu, Kyoto, Japan) equipped with an attenuated total reflectance accessory (DuraSampIIR, ASI Technologies, Danbury, CT, USA). A total of 64 scans ranging from 4000 to 600 cm−1 were recorded with a resolution of 4 cm−1 . A non-immersed specimen and liquid of each GIC were observed as references.
2.5.
X-ray photoelectron spectroscopy
Freshly mixed cement was placed in an acrylic mold (2 mm inner diameter; 0.5 mm thick), then slightly overfilled and compressed with polyethylene sheets and glass plates. The molds were stored in a 37.0 ◦ C incubator for 1 h, and then the specimens were removed from the mold and immersed in 10-mL CaCl2 solution for 10, 30, or 60 min. The chemical structures of the specimen mount on a measuring holder were observed using an X-ray photoelectron spectrometer (JPC-9010MC, JEOL Ltd., Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) analysis was performed under a base pressure of less than 10−7 Pa. The Al-K␣ X-ray was obtained using a 10-kV accelerating voltage and 10-mA filament current. A non-immersed specimen of each GIC and powder of CaCl2 were observed as a reference.
2.6.
Statistical analysis
The surface hardness values after immersion were analyzed by one-way analysis of variance (ANOVA) to assess the effect of the immersion condition. The Tukey HSD test was used to compare the individual data. The relationship between the Ca amount on the specimen surface and the surface hardness was evaluated using linear regression analysis. The level of significance was set at ˛ = 0.05.
Components P: fluoroaluminosilicate glass, polyacrylic acid L: polyacrylic acid, distilled water, polybasic carboxylic acid P: fluoroaluminosilicate glass, pigments, fluorescent material L: acrylic acid-tricarboxylic acid co-polymer, tartaric acid, distilled water
3.
Lot No. P: 1108171 L: 1108171 P: 061049 L: 061034
Results
The surface hardness values of FIX before the CaCl2 immersion of Group 1 (without water storage) and Group 2 (after 1-week water storage) were 53.0 ± 2.9 and 74.6 ± 3.0, respectively; those of GFX before the CaCl2 immersion of Group 1 and Group 2 were 43.8 ± 2.2 and 61.4 ± 3.1, respectively. The hardness of both FIX and GFX in Groups 1 and 2 did not change after immersion in deionized water for 60 min. Fig. 1 shows the means and standard deviations of the hardness of FIX and GFX after immersion in CaCl2 solution or in deionized water. The hardness changes of FIX and GFX showed the same tendency. Regarding Group 1, the one-way ANOVA suggested that the immersion condition (immersion solution and duration) was significant. The hardness after CaCl2 immersion was significantly greater than that after water immersion. The hardness increased as the immersion duration increased. For Group 2, the one-way ANOVA suggested that the immersion condition was significant. The hardness after 60-minute CaCl2 immersion was significantly greater than that after water immersion. The elemental compositions of FIX and GFX surfaces before and after immersion in CaCl2 solution or in deionized water are summarized in Table 2. Not more than 0.1% of Ca and Cl were detected on the surface before immersion, but obvious amounts of Ca and Cl were detected on the surface after CaCl2 immersion. The amounts of Ca and Cl increased as the immersion duration increased. The relative amounts of Ca and Cl after the CaCl2 immersion of Group 2 (after 1-week water storage) were greater than those of Group 1 (without water storage). The relative amounts of Sr, F, and Na (FIX only) of Group 2 decreased after immersion. The elemental compositions of FIX and GFX showed a similar tendency, but the amounts of Ca and Cl after the immersion of GFX in CaCl2 were greater than those of FIX. Fig. 2 indicates the relationship between the amount of Ca on the specimen surface and the surface hardness of FIX and GFX. The hardness of both FIX and GFX in Group 1 (without water storage) significantly increased with an increase of the Ca amount (p = 0.02 and 0.01). The hardness of both FIX and GFX in Group 2 (after 1-week water storage) showed the same tendency, but was not significant (p = 0.08 and 0.05). Representative FTIR spectra of FIX and GFX are shown in Fig. 3. Regarding the liquid of GICs, the peaks at 1700 cm−1 and 1640 cm−1 were observed and assigned to the carboxylic acid groups and water, respectively. Regarding the set GICs, new peaks appeared around 1590 cm−1 and 1550 cm−1 , which were assigned to aluminum polyacrylate and calcium polyacrylate, respectively [10]. The absorbance of the latter peak
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Table 2 – Elemental compositions (at%) of FIX and GFX surfaces before and after immersion. FIX Compositions (at%)
Immersion time Si
Al
P
Sr
Na
F
Ca
Cl
31.8 (0.4)
29.3 (0.2)
6.9 (0.3)
14.7 (0.2)
2.7 (0.1)
14.5 (0.7)
0.1 (0.1)
0.0 (0.0)
Water 60 min CaCl2 10 min CaCl2 30 min CaCl2 60 min
33.6 (0.4)
30.2 (0.2)
6.9 (0.5)
14.3 (0.2)
2.3 (0.1)
12.7 (0.4)
0.0 (0.1)
0.0 (0.0)
32.3 (0.4)
29.3 (0.3)
6.7 (0.4)
13.9 (0.5)
2.6 (0.1)
14.0 (0.8)
1.2 (0.2)
0.2 (0.1)
31.8 (1.0)
29.1 (0.4)
6.6 (0.2)
13.6 (0.4)
2.4 (0.3)
14.1 (0.2)
1.6 (0.4)
0.7 (0.5)
30.9 (0.3)
28.6 (0.3)
6.6 (0.3)
13.2 (0.7)
2.4 (0.5)
14.6 (0.6)
2.9 (1.4)
0.9 (0.8)
Water 60 min CaCl2 10 min CaCl2 30 min CaCl2 60 min
36.5 (0.5)
35.9 (0.3)
8.4 (0.4)
11.1 (0.2)
1.0 (0.1)
7.0 (0.1)
0.0 (0.0)
0.0 (0.0)
34.5 (0.5)
35.4 (0.6)
8.2 (0.6)
10.6 (1.0)
1.2 (0.1)
7.1 (0.2)
1.7 (0.6)
1.3 (0.7)
34.2 (0.1)
34.9 (0.8)
7.9 (0.0)
9.9 (0.5)
0.9 (0.2)
7.0 (0.1)
3.0 (0.2)
2.2 (0.9)
32.8 (0.4)
33.7 (0.4)
7.5 (0.2)
9.5 (0.4)
0.9 (0.0)
7.1 (0.2)
4.4 (0.2)
4.1 (0.0)
Before immersion Without water storage (Group 1)
1-week water storage (Group 2)
GFX Compositions (at%)
Immersion time Si
Al
P
Sr
F
Ca
Cl
32.0 (0.3)
31.0 (0.3)
12.2 (0.2)
11.8 (0.4)
12.8 (0.2)
0.1 (0.1)
0.0 (0.0)
Water 60 min CaCl2 10 min CaCl2 30 min CaCl2 60 min
33.3 (0.9)
31.6 (0.4)
12.5 (0.2)
11.4 (0.2)
11.2 (0.6)
0.0 (0.0)
0.0 (0.0)
30.6 (0.7)
29.9 (0.4)
11.9 (0.3)
10.1 (0.5)
12.9 (0.4)
3.1 (0.5)
1.5 (0.7)
30.3 (0.7)
29.5 (0.4)
11.5 (0.3)
10.0 (0.8)
13.0 (0.2)
3.3 (0.6)
2.4 (0.9)
29.9 (0.5)
28.8 (0.2)
11.2 (0.3)
9.3 (0.9)
12.5 (0.1)
4.8 (0.3)
3.4 (0.5)
Water 60 min CaCl2 10 min CaCl2 30 min CaCl2 60 min
33.0 (0.1)
37.0 (0.1)
14.1 (0.6)
8.3 (0.4)
7.5 (0.4)
0.1 (0.1)
0.0 (0.0)
31.2 (0.7)
34.8 (0.5)
12.4 (0.9)
6.6 (0.1)
7.1 (0.5)
4.0 (0.6)
3.8 (0.6)
29.6 (0.8)
33.3 (0.4)
11.4 (0.8)
5.9 (0.2)
6.7 (0.2)
6.3 (0.7)
6.8 (0.4)
28.8 (0.7)
33.0 (0.1)
11.4 (0.6)
6.1 (0.1)
6.8 (0.4)
6.8 (0.7)
7.2 (0.5)
Before immersion Without water storage (Group 1)
1-week water storage (Group 2)
Standard deviations in parentheses.
increased after immersion. The absorbance around 1550 cm−1 after CaCl2 solution immersion was greater than that after water immersion. The XPS wide-scan spectra of FIX and GFX showed a similar pattern. Before immersion, the F1s, O1s, C1s, Si2s, P2p, Sr3d, Si2p, and Al2p peaks were assigned; after CaCl2 immersion, the Ca peak was also assigned. The intensity of the Ca2p peak increased as the immersion duration increased. The XPS narrow-scan spectra of the Ca2p region of FIX and GFX (Fig. 4) showed that Ca2p3/2 and Ca2p1/2 peaks were recognized at binding energies of 347.5 eV and 351.1 eV, respectively. The Ca2p3/2 and Ca2p1/2 peaks of CaCl2 powder was detected at a binding energy of 348.3 eV and 351.8 eV, respectively.
4.
Discussion
Several concentrations of CaCl2 solutions were used as an immersion solution in a previous study [16]. Immersing GICs in 42.7 wt% CaCl2 solution produced the greatest increase in surface hardness regardless of the immersion duration and the product. These results suggested that the 42.7 wt% CaCl2 solution was the most effective concentration to increase GIC surface hardness. For Group 1 (without water storage), the surface hardness after CaCl2 solution immersion significantly increased compared to that after water immersion, and the hardness increased with an increase of the immersion duration. For Group 2 (after 1-week water storage), only the hardness after
Please cite this article in press as: Shiozawa M, et al. Effect of immersion time of restorative glass ionomer cements and immersion duration in calcium chloride solution on surface hardness. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.08.366
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Fig. 1 – Surface hardness of FIX and GFX after immersion. Bars with the same letter were not significantly different (p > 0.05).
Fig. 2 – Relationship between the amount of Ca on the specimen surface and the surface hardness of FIX and GFX. The coefficient of determination (r2 ) with the asterisk was significant (p < 0.05).
Fig. 3 – FTIR spectra changes of FIX and GFX after immersion in CaCl2 solution and water. The peak after CaCl2 solution immersion around 1550 cm−1 (↓) relatively increased compared to that around 1590 cm−1 (↑). Please cite this article in press as: Shiozawa M, et al. Effect of immersion time of restorative glass ionomer cements and immersion duration in calcium chloride solution on surface hardness. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.08.366
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Fig. 4 – XPS narrow-scan spectra of the Ca2p region of FIX and GFX.
60-minute CaCl2 solution immersion significantly increased compared to that after water immersion. These results revealed that the effect of CaCl2 immersion on the Group 1 surface hardness was greater than on the Group 2 surface hardness. The null hypothesis that the immersion time of GICs and immersion duration in CaCl2 solution had no influence on the surface hardness of GICs was rejected. Thus, the setting stage was one of the important factors for increasing GICs surface hardness. The immersion duration was also a factor for increasing the surface hardness of Group 1. Calcium was detected on the surface in both Groups 1 and 2 after CaCl2 immersion. The amounts of Ca increased with the immersion duration. After the Ca-containing GICs were mixed [10,17,18], the hydrogen in the carboxylic acid groups was progressively replaced by Ca released from the glass particles, and calcium polyacrylate preferentially formed in the early stages of the cement-forming reaction. Calcium can adopt six-fold coordination and force the creation of sites in the structure where additional oxygen atoms are present. These reactions cause the alteration in the structure of the glass [19]. After the storage of GICs in a Ca-containing solution [16,20], Ca was observed in the cement matrix but not in the glass core. Unreacted carboxylic acid groups still remained in the cement matrix after the GIC set [10,11]. These reports indicated that the Ca ions from the solution would create chemical bonds with the unreacted carboxylic acid groups remaining in the matrix. According to the FTIR observation, the peak around 1550 cm−1 was assigned to calcium polyacrylate, which participated in the cross-linking. The absorbance of this peak increased after 60-minute immersion in the CaCl2 solution comparing in water. This result indicated that the amount of calcium polyacrylate increased after CaCl2 solution immersion. The increase of calcium polyacrylate has been reported due to enhancement of setting reaction [10,12]. According to the XPS analysis, the Ca2p3/2 peak of CaCl2 powder was recognized at a binding energy of 348.3 eV, which coincided with that of previous reports, 347.8–348.7 eV [21–24]. The binding energy of the Ca2p3/2 peak of the specimen after CaCl2 immersion (347.5 eV) was similar to the binding energy
of calcium polyalkenoate (347.23 eV) [25], but was less than that of CaCl2 . This finding supported the assumption that the Ca ions from the CaCl2 solution would behave as the network modifier, thus enhancing the formation of the polyacid salt matrix. The surface hardness after the CaCl2 immersion of Group 1 significantly correlated with the amount of Ca on the specimen surface; that of Group 2 showed a similar tendency but it was not significant. The surface hardness increase after the CaCl2 immersion of Group 2 was smaller than that of Group 1, while the amount of Ca on the Group 2 surface after CaCl2 immersion was greater than that on the Group 1 surface. Before the CaCl2 solution immersion of Group 2, the surface hardness was greater than that of Group 1 due to the maturation of the acid–base reaction. The surface hardness of the GICs was reported to drastically increase during the first week and remained stable up to 6 months [26]. These findings suggested that the setting reaction of the Group 2 specimens greatly progressed during the 1-week water storage, and the unreacted carboxylic acid groups in the cement matrix decreased. Therefore, the amount of newly created chemical bonds between the Ca ions and the carboxylic acid groups might be limited to the Group 2 specimens. As a result, the hardness increase of Group 2 was less than that of Group 1. However, a higher amount of Ca was detected on the surface after the immersion of Group 2 compared to amounts on the Group 1 specimens. High amounts of cement chemical components, such as Sr, F, and Na, were released during the 1-week water storage of the Group 2 specimens. It could be supposed that the matrixes of the Group 2 specimens had much more space than those of the Group 1 specimens due to the greater component release, so a higher amount of Ca penetrated the matrixes in Group 2. The immersion of the GICs in CaCl2 solution at the early stage of the setting was considered to enhance forming the polyacid salt matrix. The method of improving the GIC surface hardness in the present study was quite simple, and it was not necessary to change the GIC compositions and restoration procedure. However, a method of maintaining the CaCl2 solution on the GIC surface should be developed for the
Please cite this article in press as: Shiozawa M, et al. Effect of immersion time of restorative glass ionomer cements and immersion duration in calcium chloride solution on surface hardness. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.08.366
DENTAL-2428; No. of Pages 7
ARTICLE IN PRESS d e n t a l m a t e r i a l s x x x ( 2 0 1 4 ) xxx.e1–xxx.e7
clinical situation. For example, a surface coating might be suitable for preserving the CaCl2 solution, which will be evaluated in a future study.
5.
Conclusions
Within the limitations of the experimental methods employed in the present study, the following conclusions can be drawn:
1. The surface hardness of GICs at an early stage of setting significantly increased as the CaCl2 immersion duration increased. 2. The surface hardness of GICs 1 week after the start of mixing significantly increased due to only a 60-minute CaCl2 immersion. 3. Regardless of the setting stage, the amounts of Ca after CaCl2 immersion increased as the immersion duration increased. 4. After CaCl2 immersion, the surface hardness at early setting stages significantly correlated with the Ca amount on the specimen surface. Specimens immersed 1 week after the start of mixing showed a similar tendency but was not significant. 5. By FTIR observation, the peak around 1550 cm−1 assigned to calcium polyacrylate was detected on the GIC surfaces. The absorbance of this peak after the CaCl2 solution immersion was greater than that after water immersion. 6. The XPS narrow-scan spectra suggested that the binding energy of the Ca2p3/2 peak was similar to the binding energy of calcium polyalkenoate. This finding indicated that the Ca ions from the CaCl2 solution created chemical bonds with the carboxylic acid groups in the cement matrix.
Acknowledgement The authors wish to express their gratitude to Ms. Jeanne Santa Cruz for her proofreading of this paper.
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Please cite this article in press as: Shiozawa M, et al. Effect of immersion time of restorative glass ionomer cements and immersion duration in calcium chloride solution on surface hardness. Dent Mater (2014), http://dx.doi.org/10.1016/j.dental.2014.08.366
