Predicting glass transition and crystallization temperatures of silicate bioglasses using mixture designs

Mixture designs have been applied on bioglasses in the range 42–55%SiO2, 13.5–48%CaO, 10–35%Na2O, 0–5%P2O5, and 0–13.5%CaF2 (mol%). This study focused on glass transition temperature (Tg), crystallization temperature (Tc) and working range (temperature gap between Tg and Tc). The designs, elaborated from data obtained by Differential Thermal Analysis (DTA), consist of equations connecting the properties with the glass molar composition. Using this powerful mathematical method, Tg, Tc and the working range of bioglasses can be precisely predicted and optimized. We found that a Na, P or F addition decreases Tg. Crystallization occurs at higher temperature when phosphorus is added in small quantity, due to network polymerization, although further addition induces a decrease of Tc related to a decrease of the overall strength of the glass network. Fluoride affects crystallization of both manners, depending on the calcium and sodium contents. Last, as a network modifier, Na lowers Tc. The widest working ranges are obtained for glasses with a large quantity of SiO2, CaO, and P2O5 and a medium quantity of CaF2, allowing to reach a difference between Tc and Tg of up to 260 °C.