ABSTRACT
Somewhere between 11% and 24% of the set cement is water, so that a glass-ionomer can be accurately described as a water-based material. The water content has been somewhat arbitrarily divided into ‘loosely bound’ water, which is easily removed by dehydration, and ‘tightly bound’ water, which cannot be removed, and remains an important part of the setting reaction as well as of the finished set cement. It is essential to be aware that, in the early stages following mixing,
and prior to the final set, the calcium polyacrylate chains remain highly soluble in water, and this means that further water can be taken up into the immature cement. Conversely, the loosely bound water can be lost by evaporation if the cement is exposed to air. This problem of water loss or water uptake, that is, the water balance, is probably the most important and least understood problem with this group of materials (Figures 1.1 and 1.2). However, achievement of rapid resistance to water uptake can be gained; but only with the sacrifice of some degree of translucency. A rapid-set material can be developed, during the manufacturing process, by stripping excess calcium ions from the surface of the glass particles in an acid bath. The aluminium ion exchange will then commence earlier in the process of setting, and rapid water uptake will be
less of a problem. Ultimate physical properties will be enhanced, but translucency will be reduced or lost, and it will still be possible to lose water from dehydration if a restoration is left exposed for any length of time.
