Introduction
Glass-ionomer cements, or polyalkenoate cements, contain glass powder and set by ion-exchange reactions. They bond chemically to the tooth and release fluoride, which makes them the most biologically active dental materials.
Setting reaction
Mixing starts an acid-base reaction. The acids dissolve the surface of the glass and free calcium, aluminum and fluoride ions.
- Calcium reacts first, linking the acid chains into calcium polyacrylate. This gives the initial set, but it is soluble in water.
- Aluminum then forms aluminum polyacrylate chains, which are stronger and insoluble.
- Maturation: cross-linking goes on for weeks, and silica gel forms around the glass particles, which protects them from acid.
Fluoride ions take no part in the setting, so they stay free in the matrix.
Water balance
The reaction needs water, which makes up about 20% of the cement. In the first 24 hours the cement is very sensitive. Moisture washes out reacting ions and soluble calcium chains, while drying stops the ion exchange. Both leave it porous, weak and unattractive, so the cement is sealed with a varnish, either a volatile resin solution or a light-cured one.