Structure Development and Interfacial Interactions in High-Density Polyethylene/Hydroxyapatite (HDPE/HA) Composites Molded with Preferred Orientation Rui A. Sousa, 1,2 * Rui L. Reis, Anto ´ nio M. Cunha, 1 Michael J. Bevis 2 1 Department of Polymer Engineering, University of Minho, 4800-058 Guimara ˜es, Portugal 2 Wolfson Centre for Materials Processing, Brunel University, Uxbridge, Middlesex, UB8 3PH, United Kingdom Received 24 January 2002; revised 22 March 2002; accepted Published online 25 September 2002 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/app.11301 ABSTRACT: Composites of high-density polyethylene (HDPE) filled with sintered and nonsintered hydroxyapatite (HA) powders, designated as HAs and HAns, respectively, were compounded by twin screw extrusion. Compounds with neoalkoxy titanate or zirconate coupling agents were also produced to improve interfacial interaction and filler dispersion in the composites. The composites were molded into tensile test bars using (i) conventional injection molding and (ii) shear-controlled orientation in injection molding (SCORIM). This latter molding technique was used to delib- erately induce a strong anisotropic character to the compos- ites. The mechanical characterization included tensile testing and microhardness measurements. The morphology of the moldings was studied by both polarized light microscopy and scanning electron microscopy, and the structure devel- oped was assessed by wide-angle X-ray diffraction. The reinforcing effect of HA particles was found to depend on the molding technique employed. The higher mechanical performance of SCORIM processed composites results from the much higher orientation of the matrix and, to a lesser extent, from the superior degree of filler dispersion com- pared with conventional moldings. The strong anisotropy of the SCORIM moldings is associated with a clear laminated morphology developed during shear application stage. The titanate and the zirconate coupling agents caused significant variations in the tensile test behavior, but their influence was strongly dependent on the molding technique employed. The application of shear associated with the use of coupling agents promotes the disruption of the HA agglomerates and improves mechanical performance. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2873–2886, 2002 Key words: high-density polyethylene (HDPE); hydroxyap- atite; composites; biomaterials; injection molding; injection molding; shear-controlled orientation in injection molding (SCORIM) INTRODUCTION In an engineering perspective, bone can be considered as a complex composite material, comprised of a poly- mer matrix (collagen fibrils) and an inorganic stiff phase [hydroxyapatite crystals (HA)]. 1–3 The combina- tion of low density and high mechanical performance (featuring high stiffness and strength, strong anisot- ropy, and pronounced viscoelastic behavior) arises from its composition and structure, as well as from the arrangement of the bone constituent elements at dif- ferent scale levels. 3–9 As a result, the mechanical be- havior of human bone varies considerably with its morphology, depending on a large range of features, such as the type, its location, and the personal char- acteristics of the patient. 10 –12 Nevertheless, it is possi- ble to characterize bone with values of tensile modulus, in the longitudinal direction, in the range 7–25 GPa. 10 –12 When developing bone substitute materials, the me- chanical behavior is a crucial aspect because the stiff- ness of the implant determines the amount of load carried by the healing/surrounding tissue. 13,14 It is known that bone remodeling strongly depends on an adequate loading of the healing bone that strictly re- lies on the stiffness of the implant. 13,14 Thus, the re- placement of hard tissues in load-bearing applications demands mechanically biocompatible materials with properties similar to those of the bone. Polymer-based composites are a class of materials that may, in principle, combine adequately high stiff- ness and strength together with a clear anisotropic and viscoelastic character. Bonfield et al. 15–32 introduced the bone-analogue concept by proposing composites composed of a ductile polymer matrix [polyethylene (PE)] and a stiff ceramic phase (HA). The idea was to mimic bone by using a semicrystalline matrix that can develop a considerable anisotropic character through adequate orientation techniques reinforced with a bone-like ceramic that also assures the mechanical reinforcement and the bioactive character of the com- posite. 17,18,24 Attempts to develop a bone-matching mechanical performance have relied on the use of Correspondence to: R. A. Sousa (rui.sousa@set.pt). *Current address: IBEROMOLDES, S.A., Rua Augusto Costa, Picassinos P.O. Box 33, 2431-956 Marinha Grande, Portugal Journal of Applied Polymer Science, Vol. 86, 2873–2886 (2002) © 2002 Wiley Periodicals, Inc.