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

This work studied the properties of glasses with the molar compn. 63.8SiO₂-​(11.6-​x)​Na₂O-​(0.7 + x)​B₂O₃-​19.2CaO-​3MgO-​1.5Al₂O₃-​0.2P₂O_5,​ 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, DTA, and heat treatments in the range of 800°-​1000 °C.  The glass crystn. behavior was detd. based on calcd. values of the activation energy of crystn. and the Johnson-​Mehl-​Avrami exponent.  The structural units in the glass network were identified using IR spectroscopy.  Finally, in vitro dissoln. was tested in SBF soln. The addn. of B₂O₃ increased the glass transition temp. and reduced the working temp.  When heat treated at 900 °C, the glass with the smallest amt. of B₂O₃ formed two cryst. phases: magnesium silicate MgSiO₃ and wollastonite CaSiO₃.  In the other compns., only CaSiO₃ was obsd. after heat treatment at 950 °C.  All the glasses crystd. preferentially from the surface.  Changes in the liquidus and crystn. temps. were related to changes in the glass structure.  The formation of [BO₃] units led to glasses with improved resistance to crystn. and decreased liquidus temp.  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 crystn.  All the glasses dissolved slowly in simulated body fluid.