Rheology of Hydroxyapatite Dispersions Davide Gardini and Carmen Galassi w ISTEC – CNR, Via Granarolo 64, I-48018 Faenza (RA), Italy Romano Lapasin DICAMP – University of Trieste, Piazzale Europa 1, I-34127 Trieste, Italy Hydroxyapatite (HA) is an interesting ceramic material for or- thopedic applications, in particular for implant operations and bone regeneration, owing to its bioactivity and biocompatibility with the surrounding tissues. Even if different shaping processes can be used in order to obtain porous ceramic bodies with op- timal final properties, the sponge impregnation method with aqueous HA dispersions is particularly suitable to achieve an appropriate macroporosity to bone regeneration. As for conven- tional slip or tape casting, the rheological behavior of HA sus- pensions must be properly fitted to process conditions and controlled through a satisfactory stabilization of the disperse phase, i.e., through an appropriate dispersant dosage and an accurate homogenization of dispersed powder. In the present work, HA powders with different crystallinity degree were used to prepare aqueous dispersions with different contents of solids and dispersant (ammonium polyacrylate). The specific surface area of HA powders is very high, and then special attention must be dedicated to the dispersant selection and the dispersion proc- ess since the rheology of HA suspensions is strongly influenced by the structural conditions of the disperse phase. Even small differences in dispersant concentration can lead to dramatic changes in the rheological properties also at relatively low values of solids volume concentration. Above a critical concentration, the viscosity drop associated with the apparently plastic behavior is confined within a very narrow stress range, a neat transition is observed in the linear viscoelastic properties, and the time- dependent effects induced by the shear history become quite im- portant and crucial for the experimental characterization. I. Introduction R HEOLOGICAL behavior of concentrated colloidal suspensions mainly depends on the content of solid particles and their degree of agglomeration. At very low shear, the elementary solid particles group themselves in aggregates that may form a 3-D network involving the whole volume of suspension. This net- work imparts elastic properties to suspension and is responsible for apparent yield stress. On increasing the shear rate, the net- work breaks into aggregates whose sizes decrease continuously. The size and spatial arrangement of aggregates depend on the shear rate applied. Such microstructural changes, induced by a shear rate increase, produce a viscosity decrease, i.e., a shear- thinning behavior. At higher shear rate, aggregates break down themselves in primary flocs that constitute the smaller kinetic units flowing at the highest shear condition. Agglomeration and breaking kinetics are not instantaneous, so the rheological prop- erties change with time, giving thixotropic phenomenon. There- fore, most of the macroscopic evidences can be explained, considering the existence of a structure originated from parti- cles’ agglomeration in the suspension and its changes with shear and time. These agglomeration phenomena are governed by force balances involving Brownian, hydrodynamic, and inter- particle (attractive or repulsive) forces. Van der Waals (London) attraction between primary flocs is the major force responsible for flocculation. Its effect can be partially reduced by electro- static repulsive forces arising from ionic atmosphere surround- ing the charged solid particles (electric double layer), but usually this kind of stabilization is not sufficient to avoid agglomeration. Often, a steric stabilization is necessary by the addition of prop- er amount of effective dispersant (a polymer), which is adsorbed on particle surfaces. Such dispersant creates repulsive forces be- tween primary flocs of dispersed systems that prevail over van der Waals attractive interactions. The effects of strong interpar- ticle attractive forces are more considerable for colloidal sus- pensions that are characterized by elementary particles of small size. This is the case of aqueous suspensions of hydroxyapatite (HA) powder studied in this work. Such systems were stabilized introducing an anionic polyelectrolyte dispersant that has the effect of increasing either the electrostatic repulsion or the steric repulsive forces. The aim of this paper is to characterize the rheological be- havior of aqueous suspensions of HA powder used in the sponge impregnation method for manufacturing porous ceramic bodies. This technique substantially consists of two stages: the stage of impregnation of the sponge in the suspension and the stage of homogeneous covering of sponge surfaces. In the first stage, when the pressure on the sponge is removed and the sponge re- covers its original shape, the dispersion is subjected to high shear rates; in this case, the viscosity must be sufficiently low so that the whole suspension can penetrate easily through the pores of the polymeric foam without any filtration effect. In the second stage, after a moderate squeezing applied to eliminate the excess of suspension, a sufficiently high viscosity level is required to avoid dripping. These requirements are met if the suspension exhibits an apparently plastic behavior, with an appropriate dy- namic yield stress value. The flow behavior of HA aqueous suspensions with different contents of solids and dispersant was investigated. Elastic effects appear at sufficiently high solids concentration, so that the vis- coelastic properties of these systems could also be analyzed. Particular attention was paid to individuate the critical strain marking the border of the linear viscoelastic range and the strain associated with incipient flow. II. Materials and Instrumentation HA [Ca 10 (PO 4 ) 6 (OH) 2 ] powders with two different crystallinity degrees, 20% (referred to as HAL) and 35% (referred to as HAC), were considered. Powders with the higher crystallinity degree are characterized by elementary particles with a larger size and lower specific surface area. The specific surface areas, measured by the Brunauer–Emmett–Teller (BET) Single Point 271 J ournal J. Am. Ceram. Soc., 88 [2] 271–276 (2005) DOI: 10.1111/j.1551-2916.2005.00068.x T. B. Troczynski—contributing editor w Author to whom correspondence should be addressed. e-mail: carmen@istec.cnr.it Manuscript No. 10862. Received February 19, 2004; approved August 18, 2004.