Dissolution rates of borophosphate glasses in deionized water and in simulated body fluid

Particles of borophosphate glasses with the nominal molar compositions 16Na₂O-(24-y)CaO-ySrO-xB₂O₃-(60-x)P₂O₅ (mol%), where 0 ≤ x ≤ 60 and y = 0, 12, and 24, were reacted in deionized water and in simulated body fluid (SBF) at 37 °C. For the dissolution experiments in water, the pH of the solution at the conclusion of the experiments increased systematically, from 2.1 to 9.5, for y = 0 glasses when ‘x’ increased from 0 to 60. The reaction rates over the first 8–24 h of dissolution in both SBF and deionized water followed linear kinetics, with reaction rates dependent on glass composition. For glass particles in SBF, replacing P₂O₅ with up to 20 mol% B₂O₃ decreased the dissolution rate (fraction dissolved) by two orders of magnitude, from 7.0 × 10⁻³ h⁻¹ for x = 0 to 2.0 × 10⁻⁵ h⁻¹ for x = 20. Further replacement of P₂O₅ by B₂O₃ increased dissolution rates by three orders of magnitude, to 2.3 × 10⁻² h⁻¹ at x = 60. The compositional dependence of the dissolution rates is explained by changes in the glass structure, with the most durable glasses possessing the greatest fraction of tetrahedral borophosphate sites in the glass network. Crystalline brushite was detected on Ca-glasses with 35 and 40 mol% B₂O₃, but the dominant precipitation phase on both the Ca- and Sr-glasses is an x-ray amorphous material constituted from orthophosphate and pyrophosphate anions.