GICs employ ion-leachable calcium aluminosilicate glass characterised by high Al : Si ratio (~1:2) and high fluorine content up to 23% [45].
1.5.1.1 Synthesis
Mixture of SiO2 and Al2O3 in a fluorite flux (CaF2) is fused at pre-determined proportions in a sillimanite crucible at temperature 1100 - 1500 °C for 45 - 120 min.
Other additives, e.g. AlPO4, Na3AlF6 and AlF3, are also used. The melt is poured onto a metal tray to cool to dull red heat, then plunged into a well of water and ground in a vibratory disc mill to a fine powder. The degree of fineness of the powder is controlled by the time of milling [112]. The powder is usually sifted through a sieve of 45 µm.
1.5.1.2 Composition
Typical glass compositions are shown in Table 3 and the fraction of each element is shown in Table 4. G338 was discovered after G200 and is the glass powder commonly used in commercial products [96].
Table 3. Composition of glass G200 and G338, as wt. % prior to firing.
Component G200 G338
Al2O3 16.6 14.3
SiO2 29 24.9
CaF2 34.2 12.8
AlF3 5.3 4.6
AlPO4 9.9 24.2
Na3AlF6 5 19.2
Only certain compositions (Figure 1.15) in the range have the necessary combination of properties. There must be sufficient alumina for the glass to be basic
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enough to be susceptible to acid attack. However, too much of it results in the separation of Al2O3 as a separate phase (corundum) and this makes the glass opaque.
The advantages of using the fluorite flux include [112]:
It reduces the fusion temperature;
Makes a workable cement;
Increases cement strength;
It is the source of fluoride release which gives GICs anticariogenic characteristic;
The presence of CaF2-rich separated phase assists in creating the tooth-like optical properties of the set cement.
Table 4. Composition of typical glasses for glass ionomer cements, as at. %.
Element G200 G338
Si 13.9 11.8
Al 13.4 16.9
Ca 17.6 6.6
F 20.1 19.7
Na 2.0 6.3
O 30.6 32.5
P 2.5 6.2
However, too much of fluorite makes the material opaque. A certain amount is required to give the opal appearance.
Figure 1.15 Region of Al2O3-SiO2-CaF2 glass that forms cement with polyacrylic acid on the phase diagram. The clear glasses are not satisfactory. Reproduced from [101].
32 1.5.1.3 Effect of particle size and distribution
Reduction in the median glass particle size improves compressive strength [113,114]
and hardness [82]. It also assists the reduction in maturation time because the larger surface area for reactions will result in rapid set and less long-term degradation. Larger glass particle sizes and a more integrated microstructure contributed to a higher wear resistance [82]. And large particles produce cements of increased fracture toughness by deflecting cracks upon failure [115].
Optimising particle size distribution has been recognised as a route to improved mechanical properties [84] and clinical handling characteristics [116] which may enhance the longevity of the restoration. It was proved that powders with a bimodal particle size distribution, i.e. wide distribution of particle sizes with fine particles distributed among large particles, ensured a high packing density of GICs, leading to relatively high compressive strength [117].
1.5.1.4 Effect of alumina : silica ratio
The glass powder‟s reactivity depends on the ratio of alumina (basic) to silica (acidic):
the basicity in the fusion mixture. An increase in the basicity of the glass will increase the setting reaction rate and shorten the setting time.
On the structural aspect, the glass can be considered as a silica network consisting of SiO4 tetrahedral in which four-coordinated aluminium has partly replaced silicon forming Al-O-Si bonds (Figure 1.16). The aluminium acts as network-former. This replacement introduces more basicity and negative charge into the glass structure due to valence difference which is balanced by network-dwellers, i.e. Na+ and Ca2+. When these are not available, Al3+ may go into network-dwelling sites. When the Al2O3:SiO2
ratio approaches unity, the glass structure acquires enough negative charges to become susceptible to acid attack at the Al3+ sites. This explains the decreased setting time when the Al2O3:SiO2 ratio was increased up to 1. When the Al2O3:SiO2 is further increased over 1, the excess Al3+ ions will go into network-dwelling sites increasing the resistance to acid attack while the network charge is not altered [112]. F may also break up the glass network by replacing bridging-oxygen and forming non-bridging fluorines [118].
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Figure 1.16 Qualitative representation of the microstructure of the glass component. Al has partially replaced silicon forming Al-O-Si bonds with Ca and Na charge balancing the network.
P may also replace Al forming Al-O-P bonds. F may also break up the glass network by replacing bridging-oxygen and forming non-bridging fluorines.
1.5.1.5 Effect of sodium
Increase of sodium content increases setting rate but reduces the hydrolytic stability of the set cements. The sodium content should be minimal in order to produce cements with adequate hydrolytic stability [96].
1.5.1.6 Effect of fluorine
Glasses used in glass iononer cements are high in fluorine [108,119]. The fluorine‟s two major roles are:
1. For appearance: it lowers the glass‟ refractive index and enables the match to the polyacylate salts so translucent cements can be produced;
2. For caries inhibitory: it forms fluorine complexes in the cement matrix that lead to fluoride release.
It also breaks up the glass network by replacing bridging-oxygen and forming non-bridging fluorines [118] and facilitates acid attack and fast cement formation [108]. The strengths of the set cements first increase with an increase of fluorine content initially, and then reach a maximum before decrease with further increase of fluorine.
Experimental cements with lower fluorine had increased fracture toughness [104]. For applications where appearance is not important, lower fluorine content may be preferable [119].
34 1.5.1.7 Effect of cation substitution
A study on the influence of certain cation substitutions on an equimolecular basis on the properties of the cements prepared from the glasses revealed that the replacement of CaF2 by MgF2 made the cement paste difficult to mix; replacement of CaF2 by LaF3
produced a rubbery cement mix but it had improved compressive strength; partial replacement of Al2O3 by TiO2 slowed down the setting rate; replacement of NaF by LiF gave a glass that yielded a slow-setting cement with very poor resistance to aqueous attack [96]. None of the above substituting cations have produced as satisfying cement as the present composition.