MAS-NMR spectroscopy studies in the setting reaction of glass ionomer cements

Abstract 

Objectives

The main objective is the characterisation of the setting reaction in glass ionomer cements based on experimental ionomer glasses with different fluorine content and a commercial glass ionomer cement liquid by using ¹³C CP/MAS-NMR, ²⁹Si, ²⁷Al and ³¹P MAS-NMR spectroscopy in order to receive information specifically about the cross-linking process.

Methods

Different fluorine containing glass compositions based on 4.5SiO₂–3Al₂O₃–1.5P₂O₅–(5−z)CaO–zCaF₂ where z=0–3, were mixed with a commercially available polymer liquid to form glass ionomer cements. The cements were subjected to ²⁷Al, ¹³C CP/MAS, ²⁹Si, and ³¹P MAS-NMR analysis.

Results

: The ²⁷Al spectra showed clearly the formation of six-fold coordinate Al(VI), that may crosslink the carboxyl groups in the poly-acid molecules. A shift towards to more positive values of the carboxyl peak in the ¹³C CP/MAS-NMR spectra showed clearly the proton dissociation of the carboxyl groups. A shift towards more negative values was observed in the ²⁹Si MAS-NMR spectra, suggesting formation of hydrated silica gel and consequently formation of additional Si–O–Si bonds. ³¹P MAS-NMR spectra also reflected changes in the coordination state around a PO₄³⁻ tetrahedron. Increasing the fluorine content of the glasses resulted generally in increased reactivity during setting, due to promoting cross-linking and repolymerisation of the silicate phase, followed by clear changes in the MAS-NMR spectra.

Conclusions

: The cross-linking process during the setting reaction of glass ionomer cements can be followed by MAS-NMR spectroscopy observing the conversion of Al(IV) to Al(VI). The acid base setting reaction is completed in 1 day and no further significant changes in the MAS-NMR spectra can be observed. Further study is required in order to understand the role of phosphorus.

Keywords: Glass ionomer cements, Fluorine containing glasses, Setting reaction, Cross-linking, MAS-NMR