Objectives: The purpose of this study was to characterise commercial glass ionomer/polyalkenoate cement Carbomer designed to promote remineralisation to fluorapatite in the mouth and to follow the setting reaction.
Methods: Magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy probing at 27Al, 29Si, 31P and 19F nuclei was used to study the initial glass and subsequent cement formation. Cements samples matured from 5 minutes up to 10 months were investigated. X-ray powder diffraction was employed to determine the presence of crystalline phases.
Results: X-ray diffraction showed that the cement powders contain apatite crystals and the initial material was amorphous before addition of fluorapatite. The presence of apatite in the cements was confirmed by 31P MAS-NMR. 19F MAS-NMR showed the apatite to be a mixture of hydroxyapatite with a small fraction of fluorapatite. The apatite was consumed during the cement reaction. 27Al MAS-NMR showed the aluminium to be predominantly four coordinate species, Al(IV), and the presence of Al-O-PO33- units in the glass. On cement formation, the proportion of Al(IV) reduced and the proportion of six coordinated aluminium species, Al(VI), increased.
Conclusion: The structural transformation during the setting reaction in glass ionomer cements can be followed by solid-state NMR. The apatite in this cement was largely hydroxyapatite but not fluorapatite. The apatite was partially consumed during the cement reaction. The changes in the ratio of Al(VI):Al(IV) indicate that the cement reaction continues for at least 10 months.