Structure-property relationships in phase separated borosilicate glasses containing P2O5

The dilatometric and rheological properties of a series of P2O5-containing sodium borosilicate glasses are reported and discussed in relation with the glass structure. Three borosilicate compositions have been chosen as depending on the predominant phosphate species which appear after introduction of P2O5, i.e. isolated sodium ortho- and pyrophosphates, borophosphate groups and the mixing of both isolated phosphates and borophosphates. The variation of the glass transition and the dilatometric softening temperatures, as well as the changes in viscosity as a function of the phosphorus content, are explained according to the structural units found in these glasses and the different phase separation appearing in every case.