INFLUENCE OF THE HYDROXYAPATITE POWDER PROPERTIES ON ITS PROPERTIES RHEOLOGY BEHAVIOR

Y.M.Z. Ahmedb, S.M. El-Sheikha * and Z.I. Zakib

aNano-structured materials Division, bCeramic and Refractory Materials Division. Central Metallurgical Research and Development Institute, CMRDI, P.O. Box: 87 Helwan, 11421, Helwan, Egypt, selsheikh2001@gmail.com

ABSTRACT:

One of the key issues involved in successfully achieving HA ceramic body without defects using colloidal processing technique, is the preparation of homogeneous and high solid loading HA ceramic suspension with suitable rheological properties. This could be achieved not only via adjusting the type and amount of the dispersing agent, but also via controlling the properties of the HA powder. The maximum solid loading suspension that could be prepared from the as received powder was found to be 14 vol%. This value is too low to be successfully applied in casting processes. While, the treatment of the as received powder via calcination process prior to suspension preparation was found to play a very versatile role. A solid loading of 57 and 59 vol% would prepare from HA powder calcined at 1000 and 1100°C, respectively with applying the optimum amount of sodium polyacrylate dispersant. The suspensions produced from the calcined powder was found to have acceptable rheological properties for casting processes in terms of low particle interaction, lower number of floc and floc network and higher fluidity.

INTRODUCTION

Nowadays, bone tissue engineering is regarded as the most promising approach in treating diseased or damaged bone tissue. The challenge in this approach is to develop suitable bone scaffold possesses suitable porosity as well as reasonable mechanical properties.1 Among the various materials tested in this concern, hydroxyapatite (HA), which shows excellent biocompatibility with hard tissue and also with skin and muscle tissues with the fact that it do not exhibit any cytotoxic effects is recognized as the most appropriate material for bone tissue engineering.2, 3 However, the main drawback in the application of HA ceramics is its poor mechanical properties and the difficulties in manufacturing parts of complex shapes.4 In recent years a large effort has been made to overcome these problems via the trials in developing different processing techniques for achieving more reliable bioceramic bodies. Colloidal processing technique could be the most promising way for achieving this objective.5 The colloidal processing offers the potential to fabricate bulk bioceramic body having a complex shape similar to the human bone through careful controlling the initial suspensions.6, 7, 8 This technique involves a colloidal dispersion of fine powder particles in a liquid medium and their consolidation into a homogeneous dense green body with minimum defects (both in number and size).9, 10 Consequently, the particle dispersion is the limiting factor in this process which affecting both rheology and homogeneity of the final produced suspension. The HA suspension should have as high as possible solid loading and in the mean time not possesses too low viscosity to avoid critical segregation phenomena as well as not too high to avoid air entrapping and density gradient defects in the final sintered product. Accordingly, in order to produce well dispersed suspension the inter-particle forces should be highly controlled through modification of the reactivity at the solid liquid interface.11, 12, 13 This could be achieved through promoting inter-particle repulsion by any or both of the following methods: 1- through controlling the particle surface charge either by adjusting the media pH or adsorption of dispersant on the particle surface (electrostatic stabilization) and 2- through steric separation of individual particle by adsorption of neutral or charged large chain polymers onto the particle surface (steric or electrosteric stabilization).14, 15, 16, 17 Acrylic based polymers which are dissociated to form negatively ionized polyelectrolyte, are commonly used as dispersant for HA suspension providing enhanced stability via electrosteric force.18, 19 It worthy mentioned that all manuscripts dealt with colloidal processing of HA ceramic or studying the flow behavior of slurry prepared from it, a heat treatment step for the as received HA powder should be firstly performed. This heat treatment step modifies the physical properties of the powder in order to successfully produce high concentrated suspensions from it. This means that the type and amount of dispersant as well as the pH of the media in not the only controlling factors in developing homogeneous and highly concentrated HA suspension. The properties of HA powder in terms of surface area, pore volume, and particle size is a highly important factor. Accordingly, in order to prepare homogeneous and high solid loading HA suspension both dispersant (type and amount) and powder properties should be optimized. To the best of our knowledge, there is no manuscript thoroughly investigate in details the effect of changing the powder properties on the rheological properties of HA suspension. Herein a trial was made in order to understand how much the variation in the properties of HA powder affecting the flow behavior of the final produced suspensions. An extensive study on the rheological properties of suspensions produced from both as received powder and powder calcined at various temperatures were carried out.

3.1. EXPERIMENTAL

2.1. Materials

Hydroxyapatite powder used in this investigation was delivered from Riedel-de Haen Co. (Seelze, Germany). A stock of HA powder was firstly calcined at both 1000 and 1100°C. The various properties of both as received and calcined powders were shown in Table. 1.

Table 1 Properties of both as received and calcined HA powder

The dispersant applied in this investigation is Acumer 9400 was supplied from The Dow Chemical Company, Midland, Michigan, USA. It is a water-soluble sodium salt polymer used to disperse and stabilize high-solids mineral slurries. It is anionic dispersant of sodium polyacrylate polymer (SPA) of solid content 41-43 wt% with a molecular weight of 3000-4000 g/mole.

2.2. Processing

Suspensions produced from both calcined and as received HA powders are prepared by adding the predetermined amount of powder gradually (for obtaining various solid loading suspensions). Both dispersant amount and suspension pH are kept constant at 3.6wt% and 11, respectively for all produced suspensions. To avoid it's contributing in affecting the rheological properties of the finally produced suspensions. The suspensions are then ball milled in a planetary mill for 24 hr using zirconia ball as milling media. The chosen of 24 hr milling time was based on the fact that 24 hour is the last time to promote deagglomeration by ball milling.20

On the other hand to avoid grinding of powder during milling the volume of suspension was maintained at twice the total volume of the balls.21 After preparation of all suspensions at various solid loadings (as received and calcined) it is subject to rheology measurements.

2.2. Characterizations

The particle size distribution and mean particle size (d50) were determined by the laser diffraction method (FRITSCH Model ANALYSETTE 22, Idar-Oberstein, Germany). One gram of the sample was suspended with 10 ml bi-distilled water, then the produced suspension (produced with hand shaking) was employed for measuring size analysis. The specific surface area (SBET) as well as the pore volume of the powders was determined by BET method using a surface area analyzer (Autosorb-1, Quantachrome Instruments, USA).

A laser Zetameter 'Malvern Instruments Model Zetasizer 2000' was used for zeta potential measurements. A 0.1 g of sample was placed in 50 ml Bi-distilled water with pH modifiers; having the ionic strength of 2x10-2M NaCl. The suspension was conditioned for 30 h during which the pH was adjusted. After shaking, the equilibrium pH was recorded. Then 10 ml of the suspension was transferred into a standard cuvette for zeta potential measurement. Suspension temperature was maintained at 25°C. Zeta potential was measured as a function of pH (HNO3 1M and NaOH 1M were used to adjust the pH). Five measurements were taken and the average was reported as the measured zeta potential. The isoelectric point (IEP) was identified at the pH axis crossing point. The rheological behavior of HA suspensions was determined using a Bohlin CVOR controlled stress rheometer (Bohlin Instruments Limited, Gloucester, England) with a truncated cone and plate geometry (a cone spindle with a smooth surface that has a diameter of 60 mm, a cone angle of 4o and a minimum gab width of 0.15 mm have been used for performing these measurements). The apparent viscosity and shear stress versus shear...