Effect of P2O5 content in two series of soda lime phosphosilicate glasses on structure and properties – Part II: Physical properties

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Abstract

The effect of the variation in phosphate (P2O5) content on the properties of two series of bioactive glasses in the quaternary system SiO2–Na2O–CaO–P2O5 was studied. The first series (I) was a simple substitution of P2O5 for SiO2 keeping the Na2O:CaO ratio fixed (1:0.87). The second series (II) was designed to ensure charge neutrality in the orthophosphate (PO43-), therefore as P2O5 was added the Na2O and CaO content was varied to provide sufficient Na+ and Ca2+ cations to charge balance the orthophosphate present. Network connectivity’s of the glasses were calculated, and densities and thermal expansion coefficients predicted using the Appen and Doweidar models, respectively. Theoretical densities were measured using the Archimedes principle. Characteristic temperatures, namely the glass transition temperature, Tg, and crystallization temperatures, Tx, were obtained using differential analysis (DTA). Two crystallization exotherms were observed for both glass series (Txi and Txii). Both Tg and Tx decreased with P2O5 addition for both series. The working range of the glasses, TxTg was shown to increase to a maximum at around 4 mol% P2O5 then decrease at higher P2O5 contents for both series. Thermal expansion coefficients were measured using dilatometry increasing with P2O5 addition and showed good agreement with the Appen values. Dilatometric softening points, Ts, were also measured, which increased with P2O5 addition. X-ray diffraction (XRD) was performed on the glasses to confirm their amorphous nature. The glass containing 9.25 mol% P2O5 from series I exhibited well-defined peaks on the XRD trace, indicating the presence of a crystalline phase.

Introduction

There is some controversy as to the role of P2O5 in soda lime phosphosilicate glass structure, bioactivity and bone mineralization. The Hench model [1], [2] predicts that the presence of phosphate is required in the implant material. However, this model does not predict the bioactivity of some glass compositions which do not contain phosphate [3] and neglect the network connectivity of the glass [4], [5], [6]. The Hench model also assumes congruent dissolution, which is not possible as these glasses phase separate into a phosphate rich and silicate rich phase as shown in part I. The phosphate phase will degrade in the presence of water at a much higher rate than the silicate phase. This phase separation results in orthophosphate groups (PO43-) in the phosphate phase which are charge balanced by alkali and alkaline earth cations. This has the effect of reducing the number of non-bridging oxygens in the silicate phase resulting in higher connectivity.

Part I of this study examined the structure of two series of bioglasses with varying phosphate contents using solid state MAS-NMR spectroscopy. The purpose of the second part of the study is to use more conventional characterization techniques to validate the NMR findings and assess properties important for processing promising compositions. Glass densities were measured to compare experimental values to calculated values from glass network connectivities based on glass composition (Doweidar’s model [7], [8]) and using network connectivities obtained from fitting of the 29Si MAS-NMR data. Thermal expansion coefficients were obtained from dilatometry and values compared to calculations using the Appen model [9]. Thermal expansion coefficients are important to characterize, particularly for applications which may involve coatings. If bioactive glass compositions can be identified with thermal expansion coefficients matched to metals typically used in prosthetic implants (or preferably slightly higher than the metal so the glass is in compression), this could provide an important component to the next generation of biomedical devices. Characteristic temperatures were observed using differential thermal analysis (DTA) to examine the effect of phosphate addition on the glass transition temperature and devitrification behavior. The working range, TxTg, is an important parameter in various applications such as viscous flow sintering (coatings) and fiber drawing. Finally, X-ray diffraction, on amorphous and again on subsequently heat treated samples, was used to investigate if any crystallites were present, and if so how this might relate to the structure of the devitrified material to the parent glass.

Section snippets

Glass melting

Details of glass melting can be found in part I of this paper (REF). Glasses for thermal expansion coefficient (TEC) measurements and density were remelted from the frit described above for 30 min and cast into preheated (≈Tg–10 °C) graphite moulds and annealed in an electric furnace overnight. The rod shaped samples formed were cut into sections of 6 mm in diameter and 25 mm in length using a slow speed diamond saw. The ends of these rods were ground flat using SiC abrasive paper. It should be

Density

Fig. 1, Fig. 2 show the density measurements for the glasses from series I and II, respectively.

The plots also show the density values calculated using Doweidar’s model [7], [8] which assumes a binary distribution of silicate Qn species. All glass compositions were normalized to exclude the phosphate content, and all fell in the 33  xt  50 mol% region (where xt is the total modifier mol%), therefore the Qn distribution was Q3 and Q2 in the binary distribution model. The relative proportions of the

Conclusions

The modified Doweidar model, taking into account the effect of the P2O5 not entering the glass network, seems to provide a closer fit to experimental density values. The densities of series I increased and series II decreased with P2O5 addition. Theoretical thermal expansion coefficients (Appen) also showed a relatively good match to experimental values. This can be explained by the increase in the amount of the separate phosphate phase with P2O5 addition which contains weaker P–O bonds

Acknowledgements

MDO and SW would like to thanks EPSRC (Grant No. EP/C549309/1) and Imperial College London for funding this work. MDO would like to thank Dr. Steve Skinner for useful discussions on Rietveld refinement.

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