Polymer Testing 22 (2003) 647–649 www.elsevier.com/locate/polytest Material behaviour Gel fraction measurements in gamma-irradiated ultra high molecular weight polyethylene Amal Elzubair a , Joa ˜o Carlos Miguez Suarez a,* , Claudia Maria Chagas Bonelli a , Eloisa Biasotto Mano b a Departamento de Engenharia Meca ˆnica e de Materiais, Instituto Militar de Engenharia, Prac ¸a General Tibu ´rcio 80, 22290-270 Rio de Janeiro, Brazil b Instituto de Macromole ´culas Professora Eloisa Mano, Universidade Federal do Rio de Janeiro, PO Box 68525, 21945-970 Rio de Janeiro, Brazil Received 4 October 2002; accepted 4 December 2002 Abstract Ultra high molecular weight polyethylene (UHMWPE) was irradiated at room temperature in air using an industrial 60 Co γ-source. Samples were exposed to integrated doses of 50, 250, 500, 750, 1000, 1500 and 1750 kGy and the soluble part of each irradiated specimen was extracted by exposure to refluxing boiling xylene solvent for 10 h using the Soxhlet method. From such extractions, the swelling ratio and the gel fractions were determined. It was found out that an increase in radiation dose leads to an increase in the gel fraction and a simultaneous decrease in the swelling ratio. These tendencies are attributed to the formation of a crosslinked network.  2003 Elsevier Science Ltd. All rights reserved. Keywords: Ultra high molecular weight polyethylene, UHMWPE; Gamma irradiation; Swelling ratio; Gel fraction; Crosslinking 1. Introduction Polymeric materials when irradiated with ionizing radiation may undergo an increase in molecular weight due to crosslinking and/or a decrease in molecular weight due to chain scission. In the event that the poly- mer is radiation-resistant, no significant changes in mol- ecular weight are expected [1]. Since several high-tech- nology industries require polymers that exhibit a specific, economically beneficial response upon exposure to radi- ation, it is not surprising that such reaction of polymers to ionizing radiation is an area of rapidly increasing interest [2–4]. In that respect, ultra high molecular weight polyethylene (UHMWPE) has an exceptional combination of physical and mechanical properties * Corresponding author. Tel.: +55-21-25467248; fax: +55- 21-25467049. E-mail address: jmiguez@epq.ime.eb.br (J.C.M. Suarez). 0142-9418/03/$ - see front matter  2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0142-9418(02)00170-8 allowing it to perform well under the most demanding conditions. Such properties as toughness, high impact strength, low coefficient of friction and low moisture adsorption make UHMWPE adequate for impact, wear, and sliding applications. Additional applications include lining, articulating surfaces of artificial knees, total hip replacements, underwater bearings, gear, textiles, pulleys and slides. The standard method used in the sterilization of UHMWPE medical devices is gamma radiation exposure. The objective of this work is to estimate the degree of crosslinking in a gamma-irradiated UHMWPE (using different integrated doses) via determination of the gel fraction and swelling ratio. 2. Theoretical background [5] In air irradiated UHMWPE, alkyl radicals are pro- duced by the scission of carbon–hydrogen bonds. The