768 768 Journal of the Ceramic Society of Japan 113 [12] 768–773 (2005) Paper mêÉé~ê~íáçå çÑ _áçã~íÉêá~äë `çãéçëáíÉ ïáíÜ jÉÅÜ~åáÅ~ääó `ç~íÉÇ m~êíáÅäÉë S. KANGWANTRAKOOL, Aki TAKENAKA, J. SUWANPRATEEB and Kunio SHINOHARA School of Ceramic Engineering, Institute of Engineering, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Nishi 8, Kita 13jo, Kita-ku, Sapporo-shi 060–8628 National Metal and Materials Technology Center, Ministry of Science, Technology and Environment, Pathumthani 12120, Thailand S. KangwantrakoolJ. Suwanprateeb School of Ceramic Engineering, Institute of Engineering, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand 060–8628 13 8 National Metal and Materials Technology Center, Ministry of Science, Technology and Environment, Pathumthani 12120, Thailand New preparation routes have been investigated for enhancing mechanical properties of a biomaterial composite consisting of with high-density polyethylene HDPE reinforced with hydroxyapatite HA. HA was used as fine particles to coat each coarse particle of HDPE with an elliptical-rotor-type mixer and with a high-speed rota- tional impact blending machine. The effect of particle size of HDPE and mixing conditions were studied on mechanical properties of the composite material, such as rotor speed, total treatment time, number of prepara- tion steps and total volume fraction of HA. In comparison, it was found that the embedment besides uniform coating and dispersion of HA fine particles onto the surface of HDPE core particle was easily achieved by rota- tional impact blending, due to high impact energy. This yielded relatively high properties. However, multi-coat- ing steps or layers were required due to a high percentage of powder loss during operation. In contrast, uniform and tight coating of core particles was performed without particle loss by elliptical-rotor-type mixing. Neverthe- less, due to a slight embedment of fine particles by gentle shear and compressive stress, HA could not disperse uniformly due to aggregates generated by the molten HDPE of core particles escaped through the thick and loose coating layer during material formation. This resulted in a weak bonding among coated particles to yield lower mechanical properties. Received May 16, 2005; Accepted November 17, 2005 Key-words : Coated particle, Rotational impact blending, Elliptical-rotor-type mixer, Composite biomaterials 1. Introduction High density polyethylene HDPE reinforced with hydro- xyapatite HA is one of composite biomaterials, which has been developed since early 1980s as an analogue for bone replacement. Bonfield et al. employed HA particles of 10– 40 by volume dispersed in HDPE matrix by means of a twin-screw extruder as a macroscopic mixing method. 1 It was demonstrated that an optimum combination of mechanical and biological performance was achieved with the composite containing HA of 40 by volume. 2 Such a composite has a modulus value approaching to that of cortical bone, superior toughness and considerably high bioactivity. The close modu- lus matching of the composite is promising to solve the problem of implants produced with conventional materials which have much higher modulus values than the bone. Im- plants made of the HAHDPE composites encouraged bone apposition rather than fibrous encapsulation, which was en- countered with other implant materials. 3 Recent progress in hydrostatic extrusion of HAHDPE has indicated that com- posites with higher modulus Young's as well as flexural modulus and strength within the bounds of cortical bone can be manufactured for major load bearing skeletal implants. 4 Various aspects of HAHDPE composites have been investi- gated since their invention. One particular topic of great in- terest is the mechanical properties of HAHDPE composites. Tensile modulus and strength of HAHDPE composites increased significantly with HA volume fraction, while the fracture strain decreased. 5 Necking was noted only for com- posites with less than 20 of HA during a tensile test. At higher HA volume, composites exhibited considerable ductili- ty. As the HA filler content increased, the recorded stress- strain curves got steeper and the elongation at yield lower. For composites with 45 of HA, the elongation at yield was equal to that at breaking point as the fracture occurred without any yield. At this HA volume fraction, the material fractured with reduced tensile strength. HAHDPE composites exhibited a sensitivity of modulus and strength to the strain rate as well. Some results obtained from HAHDPE composites were already reported. 6–8 Besides the composition, the uniformity of the distribution of HA particles in HDPE matrix should also influence the mechanical properties of the composite materials. In general, it is almost impossible to achieve the uniform mixing of fine particles of a few micrometers in diameter, in other words, the microscopic mixing by conventional powder mixing such as ball milling, commercial mixing and extrusion. One of the approaches to enhance particle mixing is the preparation of an ordered mixture using coated particles. The fine particles are fixed on the large one that acts as a core particle. Thus, the highly uniform dispersion of fine particles as well as the large ones is attained in a particle scale. Undoubtedly, the final microstructure of the formed composite sample should cor- respond to the characteristics of particle mixture. 9 This paper concentrates on the feasibility study of improv- ing mechanical properties of HAHDPE composites by utiliz- ing coated particles due to microscopic mixing. HA is used as fine particles to coat each coarse particle of HDPE with a high-speed elliptical-rotor-type mixer and a high-speed rota-