Setting reactions

After mixing powder and liquid, the acid etches the gllas which reslts in a release of calcium, aluminium, sodium and fluoride ions into solution. This is an acid-base reaction where the water serves as the medium for the reaction. The metal ions react with the carboxyl (COO) groups to form a polyacid salt, which becomes the cement matrix, and the surface of the glass becomes a silica hydrogel. The unreacted cores of the glass particles remain as a filler [79, 84].

Although the clinical set is completed within a few minutes, a continuing 'maturation' phase occurs over subsequent months. This is predominantly due to the slow reaction of the aluminium ions [45] and is the cause of the set material's sensitivity to water balance. The set material needs to be protected from salivary contamination for several hours, otherwise the surface becomes weak and opaque, and from water loss for several months, otherwise the material shrinks and cracks and may debond [45, 79].

The RM-GICs also undergo an acid:base reaction (which is a pre-requisite for any material to be described as a glass-ionomer cement). However, there is an additional resin polymerization phase. Depending on the product, the resin polymerization may be self-cure, light-cure or both. On mixing powder and liquid, the acid:base reaction, and if present, the self-cure resin polymerization reaction, begin and setting commences. Restorative RM-GICs (in contrast to luting RM-GICs) undergo photopolymerization on exposure to light, resulting in clinical set. However, the acid:base reaction continues, albeit much more slowly. Although the set material can be contoured and polished under water spray immediately following polymerization, delayed polishing has been recommended [88]. However, dehydration remains a potential problem. All GICs show an increase in translucency at seven days compared to that at placement, resulting in an aesthetic improvement [45, 79].

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