Fluoride-containing bioactive glasses: Fluoride loss during melting and ion release in tris buffer solution

Melt-derived bioactive glasses (SiO2–P2O5–CaO–Na2O–CaF2; CaF2 0 to 17.76 mol%) lost fluoride during melting, but nominal and analysed CaF2 contents in the glass correlated linearly. Analysed CaO contents were increased, showing that fluoride was lost as hydrofluoric acid after reaction with atmospheric water during melting. Weight loss on ignition reduced linearly with increasing CaF2, suggesting that CaF2 impedes absorption of atmospheric water. pH changes in tris buffer solution showed that pH is controlled by the silicate matrix (via ion exchange processes), and fluoride release contributes less to the overall pH. Glasses formed apatite in tris buffer; phosphate concentration of the glass was the limiting factor, resulting in fluorite formation for increasing fluoride content in the glass and calcite formation for the fluoride-free composition. These results allow for tailoring of novel fluoride-containing bioactive glasses to address specific needs, particularly in dentistry and for remineralising toothpastes.