On the sintering characteristics of calcium polyphosphates

Porous calcium polyphosphates (CPP) have been shown to hold promise for tissue engineered implant applications. However initial attempts to form porous CPP structures for these applications by sintering of powders have demonstrated problems with reproducible formation of porous forms of desired geometry from run to run, even for powders taken from a single production batch. Batch to batch variability further confounds this problem. In this study, melt-derived CPP glass powders of prescribed size ranges were gravity sintered under various conditions of temperature, time and humidity in order to better elucidate the role of each of these parameters in controlling the sintering of these powders. No discernible variability in thermal characteristics was detected for each of the powder batches studied. XRD confirmed the consistent amorphous nature of the powder and the formation of monoclinic β-CPP with high temperature sintering. Significant sinter neck formation without extensive densification, in addition to insufficient sinter neck development with crystallized powders under similar sintering conditions, indicated that a viscous flow mechanism dominated. Sinter neck formation and disk density also moderately increased with increasing heating rate (approx. 1-10°C/m), though this effect was less obvious with the larger particle size range. The relative insensitivity of disk density to higher sintering temperatures (800-950°C) at a fixed ramp rate emphasized the importance of controlling sintering kinetics in the 500-700°C range where most of the densification occurs and during which humidity and powder size appear to be influential factors. The results of this study suggest that appropriate selection of sintering conditions can result in the reliable development of adequate porous constructs of desired pore size and crystallographic structure (amorphous or crystalline), ultimately allowing a degree of control over porous CPP degradation characteristics.