Journal of Basrah Researches ((Sciences)) Volume 37. Number 3A/ 15 June ((2011)) 36 Water absorption and mechanical properties of high – density polyethylene/ egg shell composite Abdullah A. Hussein 1 , Rusel D. Salim 2 and Abdulwahab A. Sultan 3 1 Department of Material Science, Polymer Research Centre, 2 Department of Physics, College of Education, University of Basrah 3 Basra Technical College ISSN ˰˰1817 ˰˰2695 ((Received 20/3/2011, Accepted 15/5/2011)) ABSTRACT: In this research, the water absorption and mechanical properties of (high-density polyethylene /egg shell) composites as a function of egg shell powder weight content in the grain size (75,125 ȝm) were investigated. Polymer composite was fabricated by mixing (HDPE) with (5, 10, 15, 20, 25) wt.% of egg shell powder to obtain desirable properties. The parameters such as tensile strength, tensile modulus, elongation at break and impact test were carried out on the prepared samples. It was found that the addition of egg shell powder to the polymer leads to decrease the tensile strength, modulus of elasticity, shore-D hardness on other hand it increases the % elongation at break, and for the impact strength. Water absorption of the composites behaviours as function of days has also been investigated, and it increases by increasing exposure time for the same filler content, while the absorbed amount of water increases, by increasing the wt.% of egg shell constant exposure time. Keywords: HDPE, Egg shell, water absorption, Mechanical properties. INTRODUCTION:- A composite is a heterogeneous substance consisting of two or more materials which do not lose the characteristics of each component. This combination of materials brings about new desirable properties [1]. Polymer composites consist of a polymer resin as the matrix, with fibers as the reinforcement medium [2]. Considerable interest has been generated in the manufacture of thermoplastic composites due to their unique properties, including their good mechanical properties, their thermal stability, and a reduced product cost [3]. Due to the combination of more than one material, the properties of composites are influenced by many factors such as filler characteristics, filler content, and interfacial adhesion [4]. This can cause the behaviour of filled polymers to be more complex than their unfilled counterpart [5]. Mechanical properties of polymeric materials are important for nearly all applications in industry, technology, and the household. Particularly, stiffness, strength, and toughness are decisive properties in many uses. Mechanical properties depend strongly on chemical as well as on supermolecular structure of the polymeric material [6]. While the chemical, molecular structure defines some basic properties such as rigidity, thermal softening, and melting behavior, the ultimate mechanical properties are fixed by the supermolecular structures or morphology. The same molecular structure can yield to many varied morphologies dependent on factors such as orientation due to fabrication, different cooling rates, changes in thermal history, and secondary crystallization [7]. High – density polyethylene (HDPE) is an important commercial polymer which is widely used for different engineering applications in modern technology [8]. HDPE is used in a variety of applications and industries where excellent impact resistance, high tensile strength, low moisture absorption, excellent low temperature toughness, relatively high softening temperatures and chemical and corrosion resistance properties are required [9]. In order to reduce the cost or enhance the physical and mechanical properties of HDPE, some additives can be added. Fillers and reinforcement used include talc, calcium carbonate, mica,