Structural characterization of fluorine containing glasses by 19F, 27Al, 29Si and 31P MAS–NMR spectroscopy

Fluorine containing glasses are important components for glass ionomer cements. Their main advantage is the long-term fluoride release during application. This paper deals with the MAS–NMR structural characterization of a series of fluorine containing glasses that could possibly be used for glass ionomer cements and they are based on the following composition: (1.5)SiO2 · Al2O3 · (0.5/0.25)P2O5 · (1 − Z − Y)CaO · (0.5)CaF2 · (Z)SrO · (Y)Na2O, where X = 0.5, Y = 0.0 or 0.2 and Z = 0.0, 0.8 and 1.0. In some of the glasses, CaO is replaced with SrO that gives radio-opacity to the final glass ionomer cement. 29Si MAS–NMR spectra gave strong evidence for the presence of Q4(4Al) and Q4(3Al) species. 27Al MAS–NMR spectra showed the presence of largely Al(IV) and a smaller proportion of Al(VI) particularly in glasses with high phosphorus content. There is evidence for the presence of [P–(OAl)n(4 − n)O](4−n)− species. In low sodium containing glasses, the principal P species was Al – O – PO 4 3 - . The presence of sodium and the low phosphorus content favored amorphous phase separation as shown by the 31P MAS–NMR spectra and the presence of orthophosphate, especially in the case of glasses with low phosphorus content. The main species observed by 19F MAS–NMR spectra were F–Ca/Sr(n) and Al–F–Ca/Sr(n), however when Na2O was present a weak peak attributed to Al–F–Na(n) was also present reflecting the low sodium content of the glasses.