1.2.1. Melting and forming
Melt-derived bioactive glasses are melted and formed with methods similar to traditional soda-lime glasses. However, the requirements at the processing must meet the standards for materials used in medical applications. The batches are mixed of high purity analytical- and reagent-grade raw chemicals and thus the content of trace impurities in the glasses is low. Bioactive glasses are produced by melting batch components at an elevated temperature, typically 1350 to 1450 °C, in electrically heated furnaces. The glasses are melted in platinum crucibles to avoid any contamination from oxide crucibles. Usually, no fining agents are added to the batches. The low viscosity of typical bioactive glass compositions at the melting temperature aids in eliminating gaseous inclusions from the melt. Melting times of small batches for laboratory testing varying from 1 to 24 hours have been employed. The glasses are often melted twice in order to increase homogeneity. Volatilization of components with high vapor pressures at high temperatures should also be taken into account. In bioactive glasses alkalis, boron, phosphorus and fluorides may vaporize. The glasses can be melted in covered crucibles to minimize losses. The vaporization in a certain process can also been taken into account by adjusting the batch composition.
Forming and shaping procedures vary depending on the product type; casting into monoliths and drawing into rods or fibres are the main forming processes for bioactive glasses. After forming, the glass is annealed at a temperature corresponding to the viscosity 1013 dPa · s (1013 Poise), to remove residual stresses caused by cooling after forming. Granulates and powdered glass are produced by crushing and sieving the annealed plates into desired particle fractions. Also quenching the melt between stainless steel plates or pouring the melt into deionized water are further steps in the process of granulate fabrication. However, bioactive glasses start to react easily in aqueous solutions, which might affect the composition of the particle surfaces. Crushing and sieving increase the risk of contamination from the equipment used in the particle manufacture. Thus, in all processing of bioactive glasses into specific shapes, care should be taken in order to minimize any contamination.