d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1029–1038
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The influence of particle size and fluorine content of aluminosilicate glass on the glass ionomer cement properties T. De Caluwé a,∗ , C.W.J. Vercruysse a , S. Fraeyman b , R.M.H. Verbeeck a a
Biomaterials Group, Department of Basic Medical Sciences, Faculty of Medicine and Health Sciences, Ghent University, De Pintelaan 185, Ghent 9000, Belgium b Department of Pharmacology, Toxicology and Biochemistry, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, Merelbeke 9820, Belgium
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. Glass ionomer cements (GIC) are clinically accepted dental restorative materials
Received 12 December 2013
mainly due to their direct chemical adhesion to both enamel and dentin and their ability
Received in revised form
to release fluoride. However, their mechanical properties are inferior compared to those of
18 April 2014
amalgam and composite. The aim of this study is to investigate if combinations of nano- and
Accepted 5 June 2014
macrogranular glass with different compositions in a glass ionomer cement can improve the mechanical and physical properties. Methods. Glasses with the composition 4.5 SiO2 –3 Al2 O3 –1.5 P2 O5 –(5 − x) CaO − x CaF2 (x = 0
Keywords:
and x = 2) were prepared. Of each type of glass, particles with a median size of about 0.73 m
Dental materials
and 6.02 m were made.
Glass ionomer
Results. The results show that the setting time of GIC decreases when macrogranular glass
Aluminosilicate glass
particles are replaced by nanogranular glass particles, whereas the compressive strength
Nanogranular glass
and Young’s modulus, measured after 24 h setting, increase. The effects are more pro-
Mechanical properties
nounced when the nanogranular glass particles contain fluoride. After thermocycling,
Fluoride release
compressive strength decreases for nearly all formulations, the effect being most pronounced for cements containing nanogranular glass particles. Hence, the strength of the GIC seems mainly determined by the macrogranular glass particles. Cumulative F− -release decreases when the macrogranular glass particles with fluoride are replaced by nanogranular glass particles with(out) fluoride. Significance. The present study thus shows that replacing macro- by nanogranular glass particles with different compositions can lead to cements with approximately the same physical properties (e.g. setting time, consistency), but with different physicochemical (e.g. F− -release, water-uptake) and initial mechanical properties. On the long term, the mechanical properties are mainly determined by the macrogranular glass particles. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author at: Ghent University, De Pintelaan 185, Bdg B, 4th Floor, Ghent 9000, Belgium. Tel.: +32 93322646. E-mail address: [email protected] (T. De Caluwé) .
http://dx.doi.org/10.1016/j.dental.2014.06.003 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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1.
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1029–1038
Introduction
Glass ionomer cements (GIC), also known as polyalkenoate cements, were discovered in the late 70s and are known for their good esthetical properties, their anti-cariogenic potential and their biocompatibility. Moreover, they have the unique ability to bind to wet tooth tissue. This prevents microleakage and increases the durability of a restoration. However, GIC are highly sensitive to moisture during initial hardening and have poor mechanical properties compared to other restorative materials, such as resin-modified GIC and composites [1,2]. For these reasons they are predominantly used as luting and base cements and as filling materials in low load-bearing dental restorations. Conventional GIC set by an acid-base reaction between a polyalkenoic acid and an aluminosilicate glass powder [3]. The aluminosilicate glass powder normally contains Al, Si, and Ca. The ratio of these components determines whether the glass will form a cement. Si4+ and Al3+ are important compounds as they form a network of tetrahedrae with oxygen bridges [3–5]. In this network, Al3+ induces negative charges. These charges are compensated by Ca2+ , which can also make the glass more basic by its network modifying capability [3,4]. When mixed with polyalkenoic acid, the glass degrades, so that Ca2+ and Al3+ -ions are set free. These ions form complexes with the acid, so that a firm gel is formed. In a later stage, the silica ions eluted from the glass condensate and form a silica gel matrix [3,6]. The chemistry and formulation of the basic glass and the polyalkenoic acid both affect the setting reaction and the properties of the GIC. An increase of the molecular weight of the polyalkenoic acid results in improved mechanical properties, but reduces the handling properties [7]. Lyophilized polyalkenoic acid was introduced, as an aqueous solution is unstable and becomes viscous in time [1]. Photopolymerizable resin modified glass ionomer cements (RM-GIC) were developed by adding hydroxyethylmethacrylate (HEMA) and photo-initiators to a modified polyalkenoic acid. These cements are stronger and have better handling characteristics, but are still prone to water-uptake [2,8]. Over the years, many researchers have focused on the optimization of the composition of the glass to improve the physical and mechanical properties of the cement [5,6,9,10]. Griffin and Hill have incorporated fluoride and phosphate in the calcium aluminosilicate glass and investigated the effect on the setting and mechanical properties of the GIC [11,12]. F− was originally added to the glass mixture as a flux to decrease the melting temperature [10]. It was found that fluoride disrupts the glass-network by forming Al–F–Ca(n) and F–Ca(n) species. In glasses with high fluoride content, additional Si–F–Ca(n) species are formed. The incorporation of calcium into these species results in a reduction of the number of available cations to charge balance non bridging oxygens [13]. This makes the glass more reactive and in this way it improves the mechanical properties of the GIC [4,9,10,14]. Furthermore, as the polyalkenoic acid attacks the glass during setting, F− leaches from the glass and a reservoir of F− is formed within the matrix, which leads to a long-term release of F− [15]. F− has been shown to have an anticariogenic effect
due to the inhibition of the formation of bacterial plaques and due to the formation of fluorapatite, which is more resistant to acid dissolution than hydroxyapatite [16]. Phosphor, on the other hand, can integrate in the tetrahedral network, which might create extra places for the acid to degrade the glass. However, phosphor also balances the charge deficit caused by aluminum and leads to a less reactive glass. During setting, cations can also bind to PO4 3− , so that less cations are available to react with the acid groups. This consequently leads to weaker GIC with longer setting times [5]. Apart from the chemical composition of the glass and the polyalkenoic acid, the contact area between these components also controls the setting and mechanical properties of GICs. So, powder/liquid (P/L) ratio as well as particle size distribution are determining parameters for the setting rate and mechanical strengths of the set cement. Increasing the P/L ratio increases the setting rate and mechanical strength of GICs [17]. Smaller particles reduce the setting time [12], and improve wear resistance, surface hardness and compressive strength [18,19]. However, viscosity increases, which impedes the workability [11]. This is a result of the higher surface area that is available for the acid to react with the glass [12]. On the other hand, a combination of small particles with the original glass particles of the same composition results in GIC with better mechanical and handling properties [11,19]. Although several studies have demonstrated that the properties of GICs can be optimized by changing the composition or the particle size and distribution of the glass, the combined effect of such changes has not yet been explored. The purpose of this study is two-fold. First, the effect of the particle size of the glass on the physical and mechanical properties of the GIC is evaluated. Secondly, as the properties of a GIC are also determined by the composition of the glass, the effect of combining nano- and macrogranular particles of glasses with different compositions on the physical and mechanical properties of the GIC is investigated. The hypothesis is that mixing nano- and macrogranular particles of glasses with different compositions can result in GIC with optimal physicochemical and mechanical properties.
2.
Materials and methods
For all solutions, demineralized water was used (Milli-Q system, Millipore, Bedford, MA, USA).
2.1.
Synthesis of the glass powder
Glass with the composition 4.5 SiO2 –3 Al2 O3 –1.5 P2 O5 –(5 − x) CaO − x CaF2 was prepared, as described by Griffin and Hill, by mixing appropriate amounts of SiO2 (Merck 7536), Al2 O3 (Merck 1095), CaCO3 (Merck 2066), P2 O5 (Fluka 79612) and CaF2 (Mallinckrodt (AR) 4168) [6,10]. A total batch size of 100 g was made. Glass without (x = 0) and with (x = 2) fluoride was synthesized. The powder mixture of glass without fluoride and glass with fluoride was heated respectively at 1475 ◦ C and 1450 ◦ C in alumina crucibles in a furnace (HTF17, Carbolite, Hope valley, UK). After homogenization, the glass melt was poured in demineralized water to produce a glass frit [6,10].
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Table 1 – Overview of the glass combinations. Glass formulation
Notation −
Macrogranular glass without F 90% macrogranular glass without F− + 10% nanogranular glass without F− 80% macrogranular glass without F− + 20% nanogranular glass without F− 90% macrogranular glass without F− + 10% nanogranular glass with F− 80% macrogranular glass without F− + 20% nanogranular glass with F−
M0 M0 10N0
M0 20N0
M0 10N2
M0 20N2
The glass frit was ground in a planetary mill (Pulverisette 6, Fritsch GmbH, Idar-Oberstein, Germany) with a zirconia mortar and balls at 300 rpm and sieved (Vibratory Sieve-Shaker, Analysette 3 Fritsch GmbH, Idar-Oberstein, Germany). The fraction with a particle diameter 0.05).
3.2.
Characterization of the cement
Table 4 shows that consistencies are roughly the same and consequently have no effect on the clinical handling of the cement. Setting times of the different GIC are given in Fig. 3A. Adding F− to the formulation decreases the setting time
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significantly (P < 0.001). The setting time decreases with an increasing amount of nanogranular glass particles replacing the macrogranular glass particles (P < 0.030), the effect being most pronounced for the nanogranular glass particles with fluoride (P < 0.002). Relative water uptake after thermocycling is shown in Fig. 3B. The mass of the GIC increases due to water uptake during thermocycling. There is no significant difference in water uptake between M0 and its formulations with nanogranular glass particles and between M2 and its formulations with nanogranular glass particles. However, water uptake differs significantly between the formulations made with M0 and those made with M2 (P < 0.017). The general trend is that GIC on the basis of macrogranular glass with F− take up less water than GIC on the basis of macrogranular glass without F− . Compressive strengths of GIC as a function of the overall surface area of the glass mixture after 24 h and after thermocycling are shown in Fig. 4A and 4B. Both composition (P < 0.001) and thermocycling (P < 0.001) have an effect on the compressive strength. There is also an interaction between the composition and thermocycling (P < 0.001). For GIC formulations on the basis of M0 , compressive strength after 24 h increases with increasing surface area (P < 0.037), the effect being more pronounced when macrogranular glass particles are replaced by nanogranular particles with fluoride (Fig. 4A). For GIC formulations on the basis of M2 , the compressive strength remains the same when the macrogranular glass
Fig. 3 – Setting time (min) of the experimental GIC (A). Relative water uptake of the set GIC after thermocycling (B). Error bars represent standard deviations at the 95% confidence interval.
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Table 4 – Mean diameter (mm) and standard deviation, between brackets, of the GIC paste measured to determine the consistency. GIC Diameter (mm)
M0
M0 10N0
M0 20N0
M0 10N2
M0 20N2
M2
M2 10N0
M2 20N0
M2 10N0
23.70 (0.99)
20.65 (0.35)
21.83 (0.04)
23.88 (0.14)
19.73 (0.21)
18.60 (0.42)
21.59 (0.34)
22.08 (0.39)
22.08 (0.07)
particles are replaced by nanogranular glass particles without F− (P = 1.000), but increases when the macrogranular glass particles are replaced by nanogranular glass particles with F− (P < 0.039) (Fig. 4A). Thermocycling reduces the compressive strength of all GIC (P < 0.004), except for M0 (P = 0.468) for which the compressive strength increases (Fig. 4A–C). After thermocycling, there is no significant difference in compressive strengths for M0 and its combinations with nanogranular glass particles, nor for M2 and its combinations with nanogranular glass particles, except for M2 20N0 (P = 0.018). The relative reduction of the compressive strength caused by thermocycling (% reduction) is given in Fig. 4C as a function of the overall surface area of the glass mixture. The % reduction increases with increasing surface area, being more pronounced for the GIC formulations on the basis of M0 . This is substantiated by
B
M2 and N2
120 Compressive strength (MPa)
Compressive strength (MPa)
M0 and N0 M2 and N0
100
21.18 (0.34)
the slope of the regression lines, being 18.39 and 8.611 respectively for GIC on the basis of M0 and M2 (Fig. 4C). Fig. 5 shows the Young’s modulus (E) of the GIC after 24 h setting and after thermocycling as a function of the overall surface area of the glass mixture. Composition and thermocycling affect E (P = 0.001). Moreover, there is also an interaction between these parameters (P < 0.001). After 24 h setting, formulations on the basis of M0 have lower E compared to those on the basis of M2 (P < 0.001). Except for the case that fluoride containing macrogranular glass particles are replaced by nanogranular glass particles without fluoride, E also increases with increasing surface area (P < 0.032). Thermocycling increases E for macrogranular glass particles (P < 0.015). In line with the compressive strength, there is no significant difference in E after thermocycling for M0 and
A 130 110
M2 20N0
M0 and N2
90 80 70 60 50
90 85
M2 and N2
80
M0 and N0 M2 and N0
75
M0 and N2
70 65 60 55 50 45
40
40 0
0.5
1
1.5 2 Surface area (m2/g)
2.5
3
3.5
0
0.5
1
1.5 2 Surface area (m2/g)
2.5
3
3.5
% reduction in compressive strength after thermocycling
C 50 M2 and N2
40
M0 and N0 M2 and N0
30
M0 and N2
20
y = 8.61x + 7.29 y = 18.4x - 19.9
10 0 0
0.5
1
1.5
2
2.5
3
3.5
-10 -20 Surface area (m2/g)
Fig. 4 – Compressive strength after 24 h setting (37 ◦ C and 80% relative humidity) (A) and after thermocycling (B) for GIC formulations with macro- and nanogranular glass particles with the same compositions and for formulations with different compositions as a function of the surface area of the glass. Reduction in compressive strength (%) by thermocycling (C) as a function of surface area. The surface area increases by the addition of nanogranular glass particles. Error bars represent standard errors at the 95% confidence interval.
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A
10
850
M2 M210N2 M220N2 M210N0 M220N0 M010N2 M020N2
Cumulative concentration F- (µg/cm²)
9
Young's modulus (MPa)
800 750 700 M2 and N0
650
M2 and N0 + TC
600
M0 and N2 M0 and N2 + TC
550 0
B
0.5
1
1.5 2 Surface area (m2/g)
2.5
3
8 7 6 5 4 3 2 1
3.5
0
800
0
5
10
15
20
25
30
Time (days) Young's modulus (MPa)
750
Fig. 6 – Cumulative F− -release (g/cm2 ) as a function of time (days): the points represent the measured values. The lines show the estimated values according to Eq. (1).
700
650
M2 and N0 M2 and N0 + TC
600
M0 and N2 M0 and N2 + TC
550 0
0.5
1
1.5 2 Surface area (m²/g)
2.5
3
3.5
Fig. 5 – Young’s modulus measured after 24 h setting and after thermocycling for GIC formulations with macro- and nanogranular glass particles with the same compositions (A) and for formulations with different compositions (B) as a function of the surface area of the glass. The surface area increases by the addition of nanogranular glass particles. Error bars represent standard errors at the 95% confidence interval.
its combinations with nanogranular glass particles, nor for M2 and its combinations with nanogranular glass particles (P > 0.193), except for M2 20N0 and M2 20N0 (P < 0.031). Fig. 6 shows the cumulative F− -release (F− cum ) as a function of time. A non-linear regression analysis demonstrates that the F− -release profile can be best represented by Eq. (1). −bt F− ) + ct cum = a(1 − e
summarized in Table 5. After 28 days, GIC based on macrogranular glass with F− (M2 ) shows the highest cumulative F− -release (Fig. 6). In GIC formulations where nanogranular glass with F− replaces M2 , the F− -release decreases (Table 5, Fig. 6). Apparently, nanogranular glass particles without F− decrease the F− -release even more, which can be attributed to the decrease of factors a and c in Eq. (1), while t1/2 values are comparable (Table 5). The initial amount of F− -release in the exponential process (described by constant a) and the rate of F− -release in the linear process (described by constant c) show the same trend as the cumulative F− -release, being highest in M2 , and decreasing in the order: M2 M2 20N2 > M2 10N2 > M2 10N0 M2 20N0 > M0 20N2 > M0 10N2
4.
Discussion
4.1.
The glass
(2)
(1)
This equation includes an exponential factor and a linear factor, suggesting that the fluoride release is the result of two processes occurring simultaneously: a short- and a long-term release corresponding respectively to the first and second term of the equation [23]. The two parameters characteristic for the short-term release are the total amount of fluoride to be released in the initial process, described by constant a, and the so-called half-life time (t1/2 ). t1/2 gives the time needed to release half of the total amount of fluoride and is calculated as ln(0.5)/(−b). Parameter c is characteristic for the long-term release process. The parameters of Eq. (1) are
XRD confirms that the glasses do not contain crystalline phases, so that the glasses are fully amorphous. All reagents have completely reacted and no devitrification occurred. The latter guarantees that the reactivity is determined by the chemical composition and not by the physical composition of the glass [14]. Despite the fact that the glasses in the present study were synthesized using the method described by Hill [6,10], the composition of the glasses containing fluoride deviates from the theoretical composition as clearly indicated by the F/Si and Al/Si molar ratios (Table 2). Apparently, 47 mol% of fluoride and 12 mol% of aluminum are lost during melting (Table 2).
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Table 5 – Values and corresponding standard deviation of the parameters of the cumulative fluoride-release according to −bt ) + ct. t Eq. (1): F− 1/2 represents the half-life time of the fluoride release for the short-term fluoride release. cum = a(1 − e GIC formulation M2 M2 10N2 M2 20N2 M2 10N0 M2 20N0 M0 10N2 M0 20N2
a
b
c
t1/2 (days)
3.7 (0.1) 3.0 (0.4) 3.1 (0.1) 2.5 (0.3) 1.7 (0.1) 1.3 (0.5) 1.5 (0.2)
0.665 (0.004) 0.6 (0.1) 0.54 (0.04) 0.62 (0.03) 0.6 (0.2) 0.8 (0.2) 0.70 (0.05)
0.20 (0.02) 0.15 (0.02) 0.17 (0.01) 0.14 (0.01) 0.10 (0.01) 0.04 (0.01) 0.07 (0.01)
1.04 1.10 1.28 1.13 1.17 0.88 1.00
This is most probably due to a loss of AlF3 from the melt as its sublimation temperature is 1276 ◦ C [24]. Incorporation of F− tends to increase the reactivity of the glasses. The increase, however, is not statistically significant (Fig. 2). Fluoride destabilizes the glass network [4,9,10,14], resulting in less bonds that need to be broken to disrupt the glass network [9,10]. Fig. 2 shows a distinct difference in reactivity between macro -and nanogranular glass particles. Since the composition and density of macro -and nanogranular glass particles are the same, the inherent reactivity of the glass with respect to the acid only depends on the specific surface area. As the specific surface area of macrogranular glass particles is about 10× smaller than that of nanogranular glass particles, the reactivity is also expected to be 10× smaller. This is substantiated by the reaction rate, which is about 10× smaller (Table 3). During the setting of the cement, the surface of the glass particles is gradually covered with a silicagel layer that ultimately protects the glass against further degradation [4]. The larger the surface area, the larger is the amount of glass that needs to be degraded to form such a silicagel layer. Consequently, more cations are leached which result in a higher crosslinking of the GIC matrix. As discussed further, the particle size distribution has a distinct effect on the setting time and compressive strength of the GIC.
4.2.
The GIC
In the present study, the consistencies of the different GIC formulations are quite comparable, though small differences were found (Table 4). As consistency is a measure for the workability of a cement, differences in cement properties such as compressive strength can hardly be ascribed to differences in the mixing procedure. Fluoride is known to destabilize the glass network, which facilitates the dissolution of the glasses and increases cation release in the acid [4,9,10,14]. This is corroborated by the increase of the reaction rate (dpH/dt)t=1 for F− -containing glasses (Table 3). The cations that are released more quickly can also result in a faster formation of cross-links, explaining the higher consistencies and decrease in setting times [10]. On the other hand, the reactivity of the glasses mainly depends on the specific surface area (Fig. 2). In fact, during setting, nanogranular glass particles have a greater contact area with the polyacid due to their high specific surface area. The concentration of the ions leached in the solution increases faster, which in turn leads to a faster reaction rate and consequently to a decrease in setting time of the GIC. This is corroborated by Prentice and co-workers, who found that replacing larger
particles by smaller particles (both in m range) resulted in cements with lower setting times. Increasing the proportion of small particles above 20%, however, resulted in high viscous cements with reduced workability [11]. Nanogranular glass particles with fluoride give the shortest setting times, due to the combined effect of nanogranular glass and F− . Fig. 3B shows an increase in mass of the GIC after thermocycling, due to the uptake of water [25]. During maturation, the cements take up water, in order to form a silicagel-layer and aluminumhydrates [21,25]. An excess of unbound water, however, can weaken the cement. The water-uptake of glasses with F− seems lower than that of glasses without F− , but this was not statistically significant. The increase in compressive strength after 24 h with the incorporation of F− in the glass can be attributed to an increased crosslinking of the polysalt matrix. In this respect, fluoride not only increases the reactivity of the glass (Fig. 2), but it can also form strong hydrogen bridges with the aluminum carboxylate complexes in the GIC matrix [3,9,10,12]. This can also explain the lower water-uptake in glasses with F− . Compressive strength after 24 h increases with increasing specific surface area for GIC based on nano- and macrogranular glass particles of the same glass, as the nanogranular glass particles increase the reactivity of the glass mixture and the crosslinking of the GIC matrix. This effect was also seen by Prentice et al. who found a linear relationship between compressive strength and the proportion of small particles in a GIC [11]. According to Kaplan et al., the flexural strength after 24 h is higher for GIC containing only smaller particles when compared to GIC containing only larger particles. This difference was neglected after one week immersion in artificial saliva or water [26]. The crosslinking effect is most pronounced for nanogranular glass particles with fluoride, as a result of their highest reactivity and the ability of fluoride to form strong hydrogen bonds with the carboxylate matrix. For the same reason, nanogranular glass particles with fluoride result in increased compressive strength when mixing nano- and macrogranular glass particles with different compositions. However, compressive strength is unaffected when macrogranular glass particles with fluoride are replaced by nanogranular glass particles without fluoride. Apparently, the crosslinking of the GIC matrix in this case is mainly determined by the macrogranular glass particles with fluoride. The decrease of the compressive strength after thermocycling can be attributed to the free ions that have leached out or to the bound ions that are released again from the polysalt matrix. Also the aging process is fastened by thermocycling. The general trend is that replacing macrogranular glass particles by N0 decreases the compressive strength after
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1029–1038
thermocycling, while the presence of N2 still contributes to the strength after thermocycling. This is probably due to the formation of hydrogen bonds and to a lesser extent to the higher reactivity caused by fluoride. Furthermore, it is seen that the reduction in strength after thermocycling as a function of the overall surface area of the glass mixture increases with increasing surface area, being more pronounced for the GIC formulations on the basis of M0 (Fig. 4A). This can be ascribed to the absence of F− resulting in poorer crosslinking and higher susceptibility to water-uptake. Moreover, the reduction in strength does not depend on the composition of the nanogranular glass particles (Fig. 4C). On the contrary, the reduction rate of the strength is more determined by the composition of the macrogranular glass particles. Apparently, the initial better crosslinking caused by the nanogranular glass particles is eliminated by thermocycling and the final crosslinking arises mainly from the macrogranular glass particles. The higher dissolution degree of the small glass particles on the long term and the cross-linking of the silica-phase could increase the brittleness of the cements. And thus, larger particles may have a greater influence in the cement forming process [26]. E is a measure for the stiffness of a GIC. GIC become stiffer when glass containing F− is added to the formulation, because F− in the glass results in a better crosslinking and in the formation of hydrogen bridges. This is in agreement with previous studies [3,9,10]. In general E also increases when macrogranular glass particles are replaced by nanogranular glass particles due to the better crosslinking. On the other hand, E remains the same when F− -containing macrogranular glass particles are replaced by nanogranular glass particles without F− (Fig. 5). Apparently the high amount of F− in the macrogranular glass particles dominates the crosslinking of the matrix over the nanogranular glass particles. Thermocycling increases E for M0 and M2 because GIC maturate through a further crosslinking of the matrix [10]. This is in agreement with the results of the compressive strength, which is also higher for M0 after thermocycling. Thermocycling has no effect on E for the GIC formulations of M0 combined with nanogranular glass particles, but decreases E of the GIC formulations of M2 with nanogranular glass particles without fluoride. This corroborates the conclusion that only nanogranular glass particles with fluoride contribute substantially to the crosslinking of the GIC matrix after thermocycling. Eq. (1) suggests that the fluoride release proceeds according to two simultaneous occurring processes, a short- and a long-term process. The short-term process is the result of relatively loose bound F− -ions that are released quite easily. The second process is due to diffusion and depends on the matrix formed by the setting of the GIC [22]. The composition of the glass clearly affects the F− -release profile. The F− -release decreases for formulations based on M2 when macrogranualr glass particles are replaced by nanogranular glass particles (Fig. 6, Table 5). More F− is leached in the GIC matrix when nanogranular glass particles with fluoride are mixed with the polyacid and nanogranular glass particles with fluoride thus result in a greater F− -reservoir. Nevertheless, less F− -release is observed, probably due to the greater amount of crosslinking and hydrogen bridges, which impede F− diffusion [12]. However, when comparing the particle size distribution, it becomes
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clear that the amount of fluoride in the reservoir produced by the nanogranular glass particles, dominates the diffusion impediment. Long-term F− -release experiments are needed to confirm whether the F− -reservoirs, built up in the matrix due to the replacement of macrogranular glass particles by nanogranular glass particles, could lead to higher F− -releases on the long term.
5.
Conclusions
Our hypothesis that particle size and composition, both separately and combined, affect the properties of GIC is confirmed: - The setting time shortens and compressive strength and E increase in formulations containing nanogranular glass particles. In addition, setting time shortens even more and compressive strength and E increase even more if the nanogranular glass particles contain F− . After thermocycling, compressive strength decreases for all formulations, except for M0 . There is no difference in compressive strengths between M0 and its formulations with nanogranular glass particles, and between M2 and its formulations with nanogranular glass particles after thermocycling. Hence, the effect of adding nanogranular glass particles is lost. The strength is thus mainly determined by the macrogranular glass particles, though F− in the nanogranular glass particles still contributes to the strength of the GIC. - The F− -release decreases when a fraction of the macrogranular glass is replaced by nanogranular glass. Both the particle size and the composition of the glass have an effect. Also in this case, the amount of crosslinking seems mainly determined by the macrogranular glass particles. Further investigation for the release profile on long term is necessary. The present study thus shows that GIC can be optimized by changing both particle size and composition of the glass. Replacing macrogranular glass particles by nanogranular glass particles with different compositions can thus lead to cements with approximately the same physical properties (e.g. setting time, consistency), but with different physicochemical (e.g. F− release, water-uptake) and initial mechanical properties. On the long term, the mechanical properties are mainly determined by the macrogranular glass particles.
Acknowledgements The authors would like to thank Ms. Valérie Vanhoorne and Prof. Dr. Chris Vervaet from the Laboratory of Pharmaceutical Technology, Faculty of Pharmaceutical Sciences, for the conduction of the particle size analysis.
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[12]
[13]
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glass-ionomer dental cements. J Mater Chem 2011;21:1319–28. Lohbauer U. Dental glass ionomer cements as permanent filling materials? Properties, limitations and future trends. Materials 2010;3:76–96. Nicholson JW. Chemistry of glass-ionomer cements: a review. Biomaterials 1998;19:485–94. De Maeyer EAP, Verbeeck RMH, Vercruysse CWJ. Reactivity of fluoride-containing calcium aluminosilicate glasses used in dental glass-ionomer cements. J Dent Res 1998;77:2005–11. Griffin SG, Hill RG. Influence of glass composition on the properties of glass polyalkenoate cements. Part II: influence of phosphate content. Biomaterials 2000;21:399–403. Griffin SG, Hill RG. Influence of glass composition on the properties of glass polyalkenoate cements. Part I: influence of aluminium to silicon ratio. Biomaterials 1999;20:1579–86. Wilson AD, Hill RG, Warrens CP, Lewis BG. The influence of polyacid molecular-weight on some properties of glass-ionomer cements. J Dent Res 1989;68:89–94. Sidhu SK. Clinical evaluations of resin-modified glass-ionomer restorations. Dent Mater 2010;26:7–12. De Barra E, Hill RG. Influence of glass composition on the properties of glass polyalkenoate cements. Part III: influence of fluorite content. Biomaterials 2000;21:563–9. Griffin SG, Hill RG. Influence of glass composition on the properties of glass polyalkenoate cements. Part IV: influence of fluorine content. Biomaterials 2000;21:693–8. Prentice LH, Tyas MJ, Burrow MF. The effect of particle size distribution on an experimental glass-ionomer cement. Dent Mater 2005;21:505–10. Wren A, Clarkin OM, Laffir FR, Ohtsuki C, Kim IY, Towler MR. The effect of glass synthesis route on mechanical and physical properties of resultant glass ionomer cements. J Mater Sci Mater Med 2009;20:1991–9. Matsuya S, Stamboulis A, Hill RG, Law RV. Structural characterization of ionomer glasses by multinuclear solid state MAS-NMR spectroscopy. J Non-Cryst Solids 2007;353:237–43. Hill RG, Wilson AD. Some structural aspects of glasses used in ionomer cements. Glass Technol 1988;29:150–8.
[15] Dhondt CL, De Maeyer EAP, Verbeeck RMH. Fluoride release from glass ionomer activated with fluoride solutions. J Dent Res 2001;80:1402–6. [16] Mickenautsch S, Mount G, Yengopal V. Therapeutic effect of glass-ionomers: an overview of evidence. Aust Dent J 2011;56:10–5. [17] Mitsuhashi A, Hanaoka K, Teranaka T. Fracture toughness of resin-modified glass ionomer restorative materials: effect of powder/liquid ratio and powder particle size reduction on fracture toughness. Dent Mater 2003;19:747–57. [18] Guggenberger R, May R, Stefan KP. New trends in glass-ionomer chemistry. Biomaterials 1998;19:479–83. [19] Xie D, Brantley WA, Culbertson BM, Wang G. Mechanical properties and microstructures of glass-ionomer cements. Dent Mater 2000;16:129–38. [20] De Maeyer EAP, Verbeeck RMH. Analysis of bioactive fluoride-containing calcium aluminosilicate glasses. Anal Chim Acta 1998;358:79–83. [21] Crisp S, Lewis BG, Wilson AD. Characterization of glass-ionomer cements.6. A study of erosion and water-absorption in both neutral and acidic media. J Dent 1980;8:68–74. [22] Verbeeck RMH, De Maeyer EAP, Marks LAM, De Moor RJG, De Witte AMJC, Trimpeneers LM. Fluoride release process of (resin-modified) glass-ionomer cements versus (polyacid-modified) composite resins. Biomaterials 1998;19:509–19. [23] Demoor RJG, Verbeeck RMH, DeMaeyer EAP. Fluoride release profiles of restorative glass ionomer formulations. Dent Mater 1996;12:88–95. [24] Lide DR. Handbook of chemistry and physics; 2005. p. 4–46. [25] Nicholson JW, Wilson AD. The effect of storage in aqueous solutions on glass-ionomer and zinc polycarboxylate dental cements. J Mater Sci Mater Med 2000;11:357–60. [26] Kaplan AE, Williams J, Billington RW, Braden M, Pearson GJ. Effects of variation in particle size on biaxial flexural strength of two conventional glass-ionomer cements. J Oral Rehabil 2004;31:373–8.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
The influence of particle size and fluorine content of aluminosilicate glass on the glass ionomer cement properties T. De Caluwé a,∗ , C.W.J. Vercruysse a , S. Fraeyman b , R.M.H. Verbeeck a a
Biomaterials Group, Department of Basic Medical Sciences, Faculty of Medicine and Health Sciences, Ghent University, De Pintelaan 185, Ghent 9000, Belgium b Department of Pharmacology, Toxicology and Biochemistry, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, Merelbeke 9820, Belgium
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. Glass ionomer cements (GIC) are clinically accepted dental restorative materials
Received 12 December 2013
mainly due to their direct chemical adhesion to both enamel and dentin and their ability
Received in revised form
to release fluoride. However, their mechanical properties are inferior compared to those of
18 April 2014
amalgam and composite. The aim of this study is to investigate if combinations of nano- and
Accepted 5 June 2014
macrogranular glass with different compositions in a glass ionomer cement can improve the mechanical and physical properties. Methods. Glasses with the composition 4.5 SiO2 –3 Al2 O3 –1.5 P2 O5 –(5 − x) CaO − x CaF2 (x = 0
Keywords:
and x = 2) were prepared. Of each type of glass, particles with a median size of about 0.73 m
Dental materials
and 6.02 m were made.
Glass ionomer
Results. The results show that the setting time of GIC decreases when macrogranular glass
Aluminosilicate glass
particles are replaced by nanogranular glass particles, whereas the compressive strength
Nanogranular glass
and Young’s modulus, measured after 24 h setting, increase. The effects are more pro-
Mechanical properties
nounced when the nanogranular glass particles contain fluoride. After thermocycling,
Fluoride release
compressive strength decreases for nearly all formulations, the effect being most pronounced for cements containing nanogranular glass particles. Hence, the strength of the GIC seems mainly determined by the macrogranular glass particles. Cumulative F− -release decreases when the macrogranular glass particles with fluoride are replaced by nanogranular glass particles with(out) fluoride. Significance. The present study thus shows that replacing macro- by nanogranular glass particles with different compositions can lead to cements with approximately the same physical properties (e.g. setting time, consistency), but with different physicochemical (e.g. F− -release, water-uptake) and initial mechanical properties. On the long term, the mechanical properties are mainly determined by the macrogranular glass particles. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author at: Ghent University, De Pintelaan 185, Bdg B, 4th Floor, Ghent 9000, Belgium. Tel.: +32 93322646. E-mail address: [email protected] (T. De Caluwé) .
http://dx.doi.org/10.1016/j.dental.2014.06.003 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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1.
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Introduction
Glass ionomer cements (GIC), also known as polyalkenoate cements, were discovered in the late 70s and are known for their good esthetical properties, their anti-cariogenic potential and their biocompatibility. Moreover, they have the unique ability to bind to wet tooth tissue. This prevents microleakage and increases the durability of a restoration. However, GIC are highly sensitive to moisture during initial hardening and have poor mechanical properties compared to other restorative materials, such as resin-modified GIC and composites [1,2]. For these reasons they are predominantly used as luting and base cements and as filling materials in low load-bearing dental restorations. Conventional GIC set by an acid-base reaction between a polyalkenoic acid and an aluminosilicate glass powder [3]. The aluminosilicate glass powder normally contains Al, Si, and Ca. The ratio of these components determines whether the glass will form a cement. Si4+ and Al3+ are important compounds as they form a network of tetrahedrae with oxygen bridges [3–5]. In this network, Al3+ induces negative charges. These charges are compensated by Ca2+ , which can also make the glass more basic by its network modifying capability [3,4]. When mixed with polyalkenoic acid, the glass degrades, so that Ca2+ and Al3+ -ions are set free. These ions form complexes with the acid, so that a firm gel is formed. In a later stage, the silica ions eluted from the glass condensate and form a silica gel matrix [3,6]. The chemistry and formulation of the basic glass and the polyalkenoic acid both affect the setting reaction and the properties of the GIC. An increase of the molecular weight of the polyalkenoic acid results in improved mechanical properties, but reduces the handling properties [7]. Lyophilized polyalkenoic acid was introduced, as an aqueous solution is unstable and becomes viscous in time [1]. Photopolymerizable resin modified glass ionomer cements (RM-GIC) were developed by adding hydroxyethylmethacrylate (HEMA) and photo-initiators to a modified polyalkenoic acid. These cements are stronger and have better handling characteristics, but are still prone to water-uptake [2,8]. Over the years, many researchers have focused on the optimization of the composition of the glass to improve the physical and mechanical properties of the cement [5,6,9,10]. Griffin and Hill have incorporated fluoride and phosphate in the calcium aluminosilicate glass and investigated the effect on the setting and mechanical properties of the GIC [11,12]. F− was originally added to the glass mixture as a flux to decrease the melting temperature [10]. It was found that fluoride disrupts the glass-network by forming Al–F–Ca(n) and F–Ca(n) species. In glasses with high fluoride content, additional Si–F–Ca(n) species are formed. The incorporation of calcium into these species results in a reduction of the number of available cations to charge balance non bridging oxygens [13]. This makes the glass more reactive and in this way it improves the mechanical properties of the GIC [4,9,10,14]. Furthermore, as the polyalkenoic acid attacks the glass during setting, F− leaches from the glass and a reservoir of F− is formed within the matrix, which leads to a long-term release of F− [15]. F− has been shown to have an anticariogenic effect
due to the inhibition of the formation of bacterial plaques and due to the formation of fluorapatite, which is more resistant to acid dissolution than hydroxyapatite [16]. Phosphor, on the other hand, can integrate in the tetrahedral network, which might create extra places for the acid to degrade the glass. However, phosphor also balances the charge deficit caused by aluminum and leads to a less reactive glass. During setting, cations can also bind to PO4 3− , so that less cations are available to react with the acid groups. This consequently leads to weaker GIC with longer setting times [5]. Apart from the chemical composition of the glass and the polyalkenoic acid, the contact area between these components also controls the setting and mechanical properties of GICs. So, powder/liquid (P/L) ratio as well as particle size distribution are determining parameters for the setting rate and mechanical strengths of the set cement. Increasing the P/L ratio increases the setting rate and mechanical strength of GICs [17]. Smaller particles reduce the setting time [12], and improve wear resistance, surface hardness and compressive strength [18,19]. However, viscosity increases, which impedes the workability [11]. This is a result of the higher surface area that is available for the acid to react with the glass [12]. On the other hand, a combination of small particles with the original glass particles of the same composition results in GIC with better mechanical and handling properties [11,19]. Although several studies have demonstrated that the properties of GICs can be optimized by changing the composition or the particle size and distribution of the glass, the combined effect of such changes has not yet been explored. The purpose of this study is two-fold. First, the effect of the particle size of the glass on the physical and mechanical properties of the GIC is evaluated. Secondly, as the properties of a GIC are also determined by the composition of the glass, the effect of combining nano- and macrogranular particles of glasses with different compositions on the physical and mechanical properties of the GIC is investigated. The hypothesis is that mixing nano- and macrogranular particles of glasses with different compositions can result in GIC with optimal physicochemical and mechanical properties.
2.
Materials and methods
For all solutions, demineralized water was used (Milli-Q system, Millipore, Bedford, MA, USA).
2.1.
Synthesis of the glass powder
Glass with the composition 4.5 SiO2 –3 Al2 O3 –1.5 P2 O5 –(5 − x) CaO − x CaF2 was prepared, as described by Griffin and Hill, by mixing appropriate amounts of SiO2 (Merck 7536), Al2 O3 (Merck 1095), CaCO3 (Merck 2066), P2 O5 (Fluka 79612) and CaF2 (Mallinckrodt (AR) 4168) [6,10]. A total batch size of 100 g was made. Glass without (x = 0) and with (x = 2) fluoride was synthesized. The powder mixture of glass without fluoride and glass with fluoride was heated respectively at 1475 ◦ C and 1450 ◦ C in alumina crucibles in a furnace (HTF17, Carbolite, Hope valley, UK). After homogenization, the glass melt was poured in demineralized water to produce a glass frit [6,10].
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Table 1 – Overview of the glass combinations. Glass formulation
Notation −
Macrogranular glass without F 90% macrogranular glass without F− + 10% nanogranular glass without F− 80% macrogranular glass without F− + 20% nanogranular glass without F− 90% macrogranular glass without F− + 10% nanogranular glass with F− 80% macrogranular glass without F− + 20% nanogranular glass with F−
M0 M0 10N0
M0 20N0
M0 10N2
M0 20N2
The glass frit was ground in a planetary mill (Pulverisette 6, Fritsch GmbH, Idar-Oberstein, Germany) with a zirconia mortar and balls at 300 rpm and sieved (Vibratory Sieve-Shaker, Analysette 3 Fritsch GmbH, Idar-Oberstein, Germany). The fraction with a particle diameter 0.05).
3.2.
Characterization of the cement
Table 4 shows that consistencies are roughly the same and consequently have no effect on the clinical handling of the cement. Setting times of the different GIC are given in Fig. 3A. Adding F− to the formulation decreases the setting time
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significantly (P < 0.001). The setting time decreases with an increasing amount of nanogranular glass particles replacing the macrogranular glass particles (P < 0.030), the effect being most pronounced for the nanogranular glass particles with fluoride (P < 0.002). Relative water uptake after thermocycling is shown in Fig. 3B. The mass of the GIC increases due to water uptake during thermocycling. There is no significant difference in water uptake between M0 and its formulations with nanogranular glass particles and between M2 and its formulations with nanogranular glass particles. However, water uptake differs significantly between the formulations made with M0 and those made with M2 (P < 0.017). The general trend is that GIC on the basis of macrogranular glass with F− take up less water than GIC on the basis of macrogranular glass without F− . Compressive strengths of GIC as a function of the overall surface area of the glass mixture after 24 h and after thermocycling are shown in Fig. 4A and 4B. Both composition (P < 0.001) and thermocycling (P < 0.001) have an effect on the compressive strength. There is also an interaction between the composition and thermocycling (P < 0.001). For GIC formulations on the basis of M0 , compressive strength after 24 h increases with increasing surface area (P < 0.037), the effect being more pronounced when macrogranular glass particles are replaced by nanogranular particles with fluoride (Fig. 4A). For GIC formulations on the basis of M2 , the compressive strength remains the same when the macrogranular glass
Fig. 3 – Setting time (min) of the experimental GIC (A). Relative water uptake of the set GIC after thermocycling (B). Error bars represent standard deviations at the 95% confidence interval.
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Table 4 – Mean diameter (mm) and standard deviation, between brackets, of the GIC paste measured to determine the consistency. GIC Diameter (mm)
M0
M0 10N0
M0 20N0
M0 10N2
M0 20N2
M2
M2 10N0
M2 20N0
M2 10N0
23.70 (0.99)
20.65 (0.35)
21.83 (0.04)
23.88 (0.14)
19.73 (0.21)
18.60 (0.42)
21.59 (0.34)
22.08 (0.39)
22.08 (0.07)
particles are replaced by nanogranular glass particles without F− (P = 1.000), but increases when the macrogranular glass particles are replaced by nanogranular glass particles with F− (P < 0.039) (Fig. 4A). Thermocycling reduces the compressive strength of all GIC (P < 0.004), except for M0 (P = 0.468) for which the compressive strength increases (Fig. 4A–C). After thermocycling, there is no significant difference in compressive strengths for M0 and its combinations with nanogranular glass particles, nor for M2 and its combinations with nanogranular glass particles, except for M2 20N0 (P = 0.018). The relative reduction of the compressive strength caused by thermocycling (% reduction) is given in Fig. 4C as a function of the overall surface area of the glass mixture. The % reduction increases with increasing surface area, being more pronounced for the GIC formulations on the basis of M0 . This is substantiated by
B
M2 and N2
120 Compressive strength (MPa)
Compressive strength (MPa)
M0 and N0 M2 and N0
100
21.18 (0.34)
the slope of the regression lines, being 18.39 and 8.611 respectively for GIC on the basis of M0 and M2 (Fig. 4C). Fig. 5 shows the Young’s modulus (E) of the GIC after 24 h setting and after thermocycling as a function of the overall surface area of the glass mixture. Composition and thermocycling affect E (P = 0.001). Moreover, there is also an interaction between these parameters (P < 0.001). After 24 h setting, formulations on the basis of M0 have lower E compared to those on the basis of M2 (P < 0.001). Except for the case that fluoride containing macrogranular glass particles are replaced by nanogranular glass particles without fluoride, E also increases with increasing surface area (P < 0.032). Thermocycling increases E for macrogranular glass particles (P < 0.015). In line with the compressive strength, there is no significant difference in E after thermocycling for M0 and
A 130 110
M2 20N0
M0 and N2
90 80 70 60 50
90 85
M2 and N2
80
M0 and N0 M2 and N0
75
M0 and N2
70 65 60 55 50 45
40
40 0
0.5
1
1.5 2 Surface area (m2/g)
2.5
3
3.5
0
0.5
1
1.5 2 Surface area (m2/g)
2.5
3
3.5
% reduction in compressive strength after thermocycling
C 50 M2 and N2
40
M0 and N0 M2 and N0
30
M0 and N2
20
y = 8.61x + 7.29 y = 18.4x - 19.9
10 0 0
0.5
1
1.5
2
2.5
3
3.5
-10 -20 Surface area (m2/g)
Fig. 4 – Compressive strength after 24 h setting (37 ◦ C and 80% relative humidity) (A) and after thermocycling (B) for GIC formulations with macro- and nanogranular glass particles with the same compositions and for formulations with different compositions as a function of the surface area of the glass. Reduction in compressive strength (%) by thermocycling (C) as a function of surface area. The surface area increases by the addition of nanogranular glass particles. Error bars represent standard errors at the 95% confidence interval.
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A
10
850
M2 M210N2 M220N2 M210N0 M220N0 M010N2 M020N2
Cumulative concentration F- (µg/cm²)
9
Young's modulus (MPa)
800 750 700 M2 and N0
650
M2 and N0 + TC
600
M0 and N2 M0 and N2 + TC
550 0
B
0.5
1
1.5 2 Surface area (m2/g)
2.5
3
8 7 6 5 4 3 2 1
3.5
0
800
0
5
10
15
20
25
30
Time (days) Young's modulus (MPa)
750
Fig. 6 – Cumulative F− -release (g/cm2 ) as a function of time (days): the points represent the measured values. The lines show the estimated values according to Eq. (1).
700
650
M2 and N0 M2 and N0 + TC
600
M0 and N2 M0 and N2 + TC
550 0
0.5
1
1.5 2 Surface area (m²/g)
2.5
3
3.5
Fig. 5 – Young’s modulus measured after 24 h setting and after thermocycling for GIC formulations with macro- and nanogranular glass particles with the same compositions (A) and for formulations with different compositions (B) as a function of the surface area of the glass. The surface area increases by the addition of nanogranular glass particles. Error bars represent standard errors at the 95% confidence interval.
its combinations with nanogranular glass particles, nor for M2 and its combinations with nanogranular glass particles (P > 0.193), except for M2 20N0 and M2 20N0 (P < 0.031). Fig. 6 shows the cumulative F− -release (F− cum ) as a function of time. A non-linear regression analysis demonstrates that the F− -release profile can be best represented by Eq. (1). −bt F− ) + ct cum = a(1 − e
summarized in Table 5. After 28 days, GIC based on macrogranular glass with F− (M2 ) shows the highest cumulative F− -release (Fig. 6). In GIC formulations where nanogranular glass with F− replaces M2 , the F− -release decreases (Table 5, Fig. 6). Apparently, nanogranular glass particles without F− decrease the F− -release even more, which can be attributed to the decrease of factors a and c in Eq. (1), while t1/2 values are comparable (Table 5). The initial amount of F− -release in the exponential process (described by constant a) and the rate of F− -release in the linear process (described by constant c) show the same trend as the cumulative F− -release, being highest in M2 , and decreasing in the order: M2 M2 20N2 > M2 10N2 > M2 10N0 M2 20N0 > M0 20N2 > M0 10N2
4.
Discussion
4.1.
The glass
(2)
(1)
This equation includes an exponential factor and a linear factor, suggesting that the fluoride release is the result of two processes occurring simultaneously: a short- and a long-term release corresponding respectively to the first and second term of the equation [23]. The two parameters characteristic for the short-term release are the total amount of fluoride to be released in the initial process, described by constant a, and the so-called half-life time (t1/2 ). t1/2 gives the time needed to release half of the total amount of fluoride and is calculated as ln(0.5)/(−b). Parameter c is characteristic for the long-term release process. The parameters of Eq. (1) are
XRD confirms that the glasses do not contain crystalline phases, so that the glasses are fully amorphous. All reagents have completely reacted and no devitrification occurred. The latter guarantees that the reactivity is determined by the chemical composition and not by the physical composition of the glass [14]. Despite the fact that the glasses in the present study were synthesized using the method described by Hill [6,10], the composition of the glasses containing fluoride deviates from the theoretical composition as clearly indicated by the F/Si and Al/Si molar ratios (Table 2). Apparently, 47 mol% of fluoride and 12 mol% of aluminum are lost during melting (Table 2).
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Table 5 – Values and corresponding standard deviation of the parameters of the cumulative fluoride-release according to −bt ) + ct. t Eq. (1): F− 1/2 represents the half-life time of the fluoride release for the short-term fluoride release. cum = a(1 − e GIC formulation M2 M2 10N2 M2 20N2 M2 10N0 M2 20N0 M0 10N2 M0 20N2
a
b
c
t1/2 (days)
3.7 (0.1) 3.0 (0.4) 3.1 (0.1) 2.5 (0.3) 1.7 (0.1) 1.3 (0.5) 1.5 (0.2)
0.665 (0.004) 0.6 (0.1) 0.54 (0.04) 0.62 (0.03) 0.6 (0.2) 0.8 (0.2) 0.70 (0.05)
0.20 (0.02) 0.15 (0.02) 0.17 (0.01) 0.14 (0.01) 0.10 (0.01) 0.04 (0.01) 0.07 (0.01)
1.04 1.10 1.28 1.13 1.17 0.88 1.00
This is most probably due to a loss of AlF3 from the melt as its sublimation temperature is 1276 ◦ C [24]. Incorporation of F− tends to increase the reactivity of the glasses. The increase, however, is not statistically significant (Fig. 2). Fluoride destabilizes the glass network [4,9,10,14], resulting in less bonds that need to be broken to disrupt the glass network [9,10]. Fig. 2 shows a distinct difference in reactivity between macro -and nanogranular glass particles. Since the composition and density of macro -and nanogranular glass particles are the same, the inherent reactivity of the glass with respect to the acid only depends on the specific surface area. As the specific surface area of macrogranular glass particles is about 10× smaller than that of nanogranular glass particles, the reactivity is also expected to be 10× smaller. This is substantiated by the reaction rate, which is about 10× smaller (Table 3). During the setting of the cement, the surface of the glass particles is gradually covered with a silicagel layer that ultimately protects the glass against further degradation [4]. The larger the surface area, the larger is the amount of glass that needs to be degraded to form such a silicagel layer. Consequently, more cations are leached which result in a higher crosslinking of the GIC matrix. As discussed further, the particle size distribution has a distinct effect on the setting time and compressive strength of the GIC.
4.2.
The GIC
In the present study, the consistencies of the different GIC formulations are quite comparable, though small differences were found (Table 4). As consistency is a measure for the workability of a cement, differences in cement properties such as compressive strength can hardly be ascribed to differences in the mixing procedure. Fluoride is known to destabilize the glass network, which facilitates the dissolution of the glasses and increases cation release in the acid [4,9,10,14]. This is corroborated by the increase of the reaction rate (dpH/dt)t=1 for F− -containing glasses (Table 3). The cations that are released more quickly can also result in a faster formation of cross-links, explaining the higher consistencies and decrease in setting times [10]. On the other hand, the reactivity of the glasses mainly depends on the specific surface area (Fig. 2). In fact, during setting, nanogranular glass particles have a greater contact area with the polyacid due to their high specific surface area. The concentration of the ions leached in the solution increases faster, which in turn leads to a faster reaction rate and consequently to a decrease in setting time of the GIC. This is corroborated by Prentice and co-workers, who found that replacing larger
particles by smaller particles (both in m range) resulted in cements with lower setting times. Increasing the proportion of small particles above 20%, however, resulted in high viscous cements with reduced workability [11]. Nanogranular glass particles with fluoride give the shortest setting times, due to the combined effect of nanogranular glass and F− . Fig. 3B shows an increase in mass of the GIC after thermocycling, due to the uptake of water [25]. During maturation, the cements take up water, in order to form a silicagel-layer and aluminumhydrates [21,25]. An excess of unbound water, however, can weaken the cement. The water-uptake of glasses with F− seems lower than that of glasses without F− , but this was not statistically significant. The increase in compressive strength after 24 h with the incorporation of F− in the glass can be attributed to an increased crosslinking of the polysalt matrix. In this respect, fluoride not only increases the reactivity of the glass (Fig. 2), but it can also form strong hydrogen bridges with the aluminum carboxylate complexes in the GIC matrix [3,9,10,12]. This can also explain the lower water-uptake in glasses with F− . Compressive strength after 24 h increases with increasing specific surface area for GIC based on nano- and macrogranular glass particles of the same glass, as the nanogranular glass particles increase the reactivity of the glass mixture and the crosslinking of the GIC matrix. This effect was also seen by Prentice et al. who found a linear relationship between compressive strength and the proportion of small particles in a GIC [11]. According to Kaplan et al., the flexural strength after 24 h is higher for GIC containing only smaller particles when compared to GIC containing only larger particles. This difference was neglected after one week immersion in artificial saliva or water [26]. The crosslinking effect is most pronounced for nanogranular glass particles with fluoride, as a result of their highest reactivity and the ability of fluoride to form strong hydrogen bonds with the carboxylate matrix. For the same reason, nanogranular glass particles with fluoride result in increased compressive strength when mixing nano- and macrogranular glass particles with different compositions. However, compressive strength is unaffected when macrogranular glass particles with fluoride are replaced by nanogranular glass particles without fluoride. Apparently, the crosslinking of the GIC matrix in this case is mainly determined by the macrogranular glass particles with fluoride. The decrease of the compressive strength after thermocycling can be attributed to the free ions that have leached out or to the bound ions that are released again from the polysalt matrix. Also the aging process is fastened by thermocycling. The general trend is that replacing macrogranular glass particles by N0 decreases the compressive strength after
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1029–1038
thermocycling, while the presence of N2 still contributes to the strength after thermocycling. This is probably due to the formation of hydrogen bonds and to a lesser extent to the higher reactivity caused by fluoride. Furthermore, it is seen that the reduction in strength after thermocycling as a function of the overall surface area of the glass mixture increases with increasing surface area, being more pronounced for the GIC formulations on the basis of M0 (Fig. 4A). This can be ascribed to the absence of F− resulting in poorer crosslinking and higher susceptibility to water-uptake. Moreover, the reduction in strength does not depend on the composition of the nanogranular glass particles (Fig. 4C). On the contrary, the reduction rate of the strength is more determined by the composition of the macrogranular glass particles. Apparently, the initial better crosslinking caused by the nanogranular glass particles is eliminated by thermocycling and the final crosslinking arises mainly from the macrogranular glass particles. The higher dissolution degree of the small glass particles on the long term and the cross-linking of the silica-phase could increase the brittleness of the cements. And thus, larger particles may have a greater influence in the cement forming process [26]. E is a measure for the stiffness of a GIC. GIC become stiffer when glass containing F− is added to the formulation, because F− in the glass results in a better crosslinking and in the formation of hydrogen bridges. This is in agreement with previous studies [3,9,10]. In general E also increases when macrogranular glass particles are replaced by nanogranular glass particles due to the better crosslinking. On the other hand, E remains the same when F− -containing macrogranular glass particles are replaced by nanogranular glass particles without F− (Fig. 5). Apparently the high amount of F− in the macrogranular glass particles dominates the crosslinking of the matrix over the nanogranular glass particles. Thermocycling increases E for M0 and M2 because GIC maturate through a further crosslinking of the matrix [10]. This is in agreement with the results of the compressive strength, which is also higher for M0 after thermocycling. Thermocycling has no effect on E for the GIC formulations of M0 combined with nanogranular glass particles, but decreases E of the GIC formulations of M2 with nanogranular glass particles without fluoride. This corroborates the conclusion that only nanogranular glass particles with fluoride contribute substantially to the crosslinking of the GIC matrix after thermocycling. Eq. (1) suggests that the fluoride release proceeds according to two simultaneous occurring processes, a short- and a long-term process. The short-term process is the result of relatively loose bound F− -ions that are released quite easily. The second process is due to diffusion and depends on the matrix formed by the setting of the GIC [22]. The composition of the glass clearly affects the F− -release profile. The F− -release decreases for formulations based on M2 when macrogranualr glass particles are replaced by nanogranular glass particles (Fig. 6, Table 5). More F− is leached in the GIC matrix when nanogranular glass particles with fluoride are mixed with the polyacid and nanogranular glass particles with fluoride thus result in a greater F− -reservoir. Nevertheless, less F− -release is observed, probably due to the greater amount of crosslinking and hydrogen bridges, which impede F− diffusion [12]. However, when comparing the particle size distribution, it becomes
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clear that the amount of fluoride in the reservoir produced by the nanogranular glass particles, dominates the diffusion impediment. Long-term F− -release experiments are needed to confirm whether the F− -reservoirs, built up in the matrix due to the replacement of macrogranular glass particles by nanogranular glass particles, could lead to higher F− -releases on the long term.
5.
Conclusions
Our hypothesis that particle size and composition, both separately and combined, affect the properties of GIC is confirmed: - The setting time shortens and compressive strength and E increase in formulations containing nanogranular glass particles. In addition, setting time shortens even more and compressive strength and E increase even more if the nanogranular glass particles contain F− . After thermocycling, compressive strength decreases for all formulations, except for M0 . There is no difference in compressive strengths between M0 and its formulations with nanogranular glass particles, and between M2 and its formulations with nanogranular glass particles after thermocycling. Hence, the effect of adding nanogranular glass particles is lost. The strength is thus mainly determined by the macrogranular glass particles, though F− in the nanogranular glass particles still contributes to the strength of the GIC. - The F− -release decreases when a fraction of the macrogranular glass is replaced by nanogranular glass. Both the particle size and the composition of the glass have an effect. Also in this case, the amount of crosslinking seems mainly determined by the macrogranular glass particles. Further investigation for the release profile on long term is necessary. The present study thus shows that GIC can be optimized by changing both particle size and composition of the glass. Replacing macrogranular glass particles by nanogranular glass particles with different compositions can thus lead to cements with approximately the same physical properties (e.g. setting time, consistency), but with different physicochemical (e.g. F− release, water-uptake) and initial mechanical properties. On the long term, the mechanical properties are mainly determined by the macrogranular glass particles.
Acknowledgements The authors would like to thank Ms. Valérie Vanhoorne and Prof. Dr. Chris Vervaet from the Laboratory of Pharmaceutical Technology, Faculty of Pharmaceutical Sciences, for the conduction of the particle size analysis.
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