Control of the thermal properties of slow bioresorbable glasses by boron addition

This work studied the properties of glasses with the molar composition 63.8SiO2-(11.6-x)Na2O-(0.7 + x)B2O3-19.2CaO-3MgO-1.5Al2O3-0.2P2O5, in which x = 0, 1, 2, 3. These glasses are of interest for the development of slowly dissolving fibers to be incorporated in composites for medical applications. The thermal properties were recorded using hot stage microscopy, differential thermal analysis, and heat treatments in the range of 800°-1000 °C. The glass crystallization behavior was determined based on calculated values of the activation energy of crystallization and the Johnson-Mehl-Avrami exponent. The structural units in the glass network were identified using infrared spectroscopy. Finally, in vitro dissolution was tested in SBF solution.The addition of B2O3 increased the glass transition temperature and reduced the working temperature. When heat treated at 900 °C, the glass with the smallest amount of B2O3 formed two crystalline phases: magnesium silicate MgSiO3 and wollastonite CaSiO3. In the other compositions, only CaSiO3 was observed after heat treatment at 950 °C. All the glasses crystallized preferentially from the surface. Changes in the liquidus and crystallization temperatures were related to changes in the glass structure. The formation of [BO3] units led to glasses with improved resistance to crystallization and decreased liquidus temperature. In the glasses with 2.7 and 3.7 mol% B2O3, [BO3] units were transformed into [BO4] units. The formation of [BO4] led to an increase in fragility and a decrease in resistance to crystallization. All the glasses dissolved slowly in simulated body fluid.

► Progressive replacement of Na2O for B2O3 led to increased glass transition temperature and lower working temperature.
► Transformation of [BO3] units into [BO4] units took place via the transformation of metaborate into pentaborate.
► Presence of [BO3] units and decrease in non-bridging oxygen increased the glass crystallization tendency.
► Presence of [BO4] units made the glass structure more floppy, decreasing the glass resistance to crystallization.
► SBF testing suggests slow dissolution, indicating that thin fibers can be used as reinforcing component in composites.