Stabilization of g-sterilized biomedical polyolefins by synergistic mixtures of oligomeric stabilizers Sameh. A.S. Alariqi a , A. Pratheep Kumar a , B.S.M. Rao b , Amit K. Tevtia a , R.P. Singh a, * a Division of Polymer Science and Engineering, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune, Maharashtra 411008, India b Department of Chemistry, University of Pune, Pune, Maharashtra 411005, India Received 8 February 2006; accepted 17 March 2006 Available online 19 May 2006 Abstract The stabilization of polyolefins against g-sterilization has been studied with single or binary additive system. The present attempt is to utilize synergistic mixtures of stabilizers which are approved for food contact applications. Ethyleneepropylene (EP) copolymer has been melt-mixed with hindered amine stabilizers (HAS), phenolic antioxidants and organo-phosphites (hydroperoxide decomposer). Samples were sterilized with different doses of g-radiation. Stabilization was characterized in terms of changes in the functional groups (oxidation products), tensile prop- erties, yellowing and surface morphology by FTIR spectroscopy, Instron, colorimetry (reflectance) and scanning electron microscopy (SEM), respectively. Results were discussed by comparing the stabilizing efficiency of mixtures with and without phenol system. Among phenol con- taining systems, where we mostly observe discoloration due to the oxidation of hindered phenol and the combination of secondary HAS, tertiary HAS, organo-phosphite and hindered phenol exhibited improved stabilization efficiency than single or binary additive systems. The mixture of secondary HAS and tertiary HAS, has shown antagonistic effect of stabilization whereas their combination with organo-phosphite has exhibited synergistic effect of stabilization even at higher doses of g-sterilization. The effects have been explained through the interaction between the stabilizers. Ó 2006 Elsevier Ltd. All rights reserved. Key words: Ethyleneepropylene copolymer; g-Sterilization; Stabilization; Discoloration; Tensile properties; Antioxidants 1. Introduction The unique properties of ethyleneepropylene homo-/copol- ymers, such as mechanical properties, transparency, low cost, durability, resistance to weathering and photo-degradation as well as biological attack and hydrophobicity have contributed to their skyrocketing utility in different applications. In bio- medical field, they have been the choice of materials for med- ical supplies such as syringes, catheters, vials, blood transfusion bags, dialyzers for blood purification etc., and these medical devices are mostly sterilized by g-radiation. The most commonly validated dose used to sterilize medical devices is 25 kGy [1]. However, sterilization of biomedical polymers by g-radiation is also known to result in physical changes including embrittlement, stiffening, softening, discol- oration, odour generation, and decrease in molecular weight [2e5]. Embrittlement occurs not only during g-irradiation but also during the subsequent post-irradiation storage [6]. Scheme 1 shows the general pathways of degradation and sta- bilization where radical formation is initiating and vital step for polymer degradation. It can be seen that stabilization can be done at different stages by adding very small amount of additives called as ‘‘stabi- lizers’’. They are radical scavengers, antioxidants and hydroper- oxide decomposers and follow different action mechanism. Hindered amine stabilizers (HAS) are widely used radical scavengers having multifunctional capabilities for scavenging radicals. A substituted piperidine was found to give good pro- tection (little yellowing or embrittlement) against g-irradiation as well as post-irradiation storage under accelerated test con- ditions (60  C in air) [7]. Then, antioxidants are also widely * Corresponding author. Tel./fax: þ91 20 25893234. E-mail address: rp.singh@ncl.res.in (R.P. Singh). 0141-3910/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2006.03.010 Polymer Degradation and Stability 91 (2006) 2451e2464 www.elsevier.com/locate/polydegstab