Thermal degradation behaviors of polyethylene and polypropylene. Part I: Pyrolysis kinetics and mechanisms A. Aboulkas a,b, * , K. El harfi a,b , A. El Bouadili b a Laboratoire de Recherche sur la Réactivités des Matériaux et l’Optimisation des Procédés «REMATOP», Département de chimie, Faculté des Sciences Semlalia, Université Cadi Ayyad, BP 2390, 40001 Marrakech, Morocco b Laboratoire Interdisciplinaire de Recherche en Sciences et Techniques, Faculté polydisciplinaire de Béni-Mellal, Université Sultan Moulay Slimane, BP 592, 23000 Béni-Mellal, Morocco article info Article history: Received 24 July 2008 Received in revised form 18 May 2009 Accepted 13 December 2009 Available online 20 January 2010 Keywords: HDPE LDPE PP Thermal analysis Isoconversional methods Master-plot method abstract Study of the decomposition kinetics is an important tool for the development of polymer recycling in industrial scale. In this work, the activation energy and the reaction model of the pyrolysis of high den- sity polyethylene (HDPE), low density polyethylene (LDPE) and polypropylene (PP) have been estimated from non-isothermal kinetic results. Firstly, the activation energy values obtained by Friedman, Kissing- er–Akahira–Sunose and Flynn–Wall–Ozawa isoconversional methods, are 238–247 kJ/mol for HDPE, 215–221 kJ/mol for LDPE and 179–188 kJ/mol for PP. Secondly, the appropriate conversion model of the process was determined by Coats–Redfern and Criado methods. The pyrolysis reaction models of HDPE and LDPE are accounted for by ‘‘Contracting Sphere” model, whereas that of PP by ‘‘Contracting Cylinder” model. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Polyethylene and polypropylene are a major component of plas- tic waste from domestic refuse. Its efficient reutilization has a growing importance these years due to the increased demand for resource recycling and environmental protection. In general, plas- tic waste has been mainly disposed of by landfill or incineration, but these processes are not fully acceptable under current interna- tional policy, which focuses on efficient recovery of raw material and energy. Pyrolysis and gasification processes are promising routes for optimal upgrading from waste. Moreover, pyrolysis of plastic, based on the decomposition of polymers at different tem- peratures, allows the treatment of polymers with simultaneous decomposition and separation [1,2]. These processes allow the obtainment of combustible, gases and/or energy, with the reduc- tion of landfilling as an added advantage [3,4]. The first step for a suitable design of any pyrolysis reactor is knowledge of the kinetics. Thermal degradation of polymers has great interest as an alter- native source of energy or chemical raw materials, as well as it contributes to the solution of environmental problems [5]. The determination of the parameters of the thermal decomposition process by means of thermogravimetric techniques allows the development of the recycling process of these materials in an industrial scale. Thermal behavior of plastics can be improved by knowing ther- mal degradation kinetics. Many studies on pyrolysis kinetics of plastic wastes have been carried out, and most of these studies have been developed on the assumption that the reaction can be described by a nth order reaction model [3,6–10]. The assumption of a nth order reaction model would result in the Arrhenius param- eters deviating from the real ones. Few attempts, however, have been made to identify the reaction model of the polyethylene and polypropylene pyrolysis [11,12]. Isothermal reduced-time plots have been applied to determine the reaction model of the thermal decomposition of solids [13–17]. Kim et al. [11,12,15– 17] confirmed the applicability of the isothermal RTP to identify the pyrolysis reaction models of polyethylene [12,15,16] and poly- propylene [11,17]. However, any one have been attempted to iden- tify reaction models using Coats–Redfern and Criado methods under non-isothermal conditions that were new trials. A thermogravimetric analysis (TGA) technique is an excellent way for studying the kinetics of thermal degradation [18–20]. It provides information on activation energy and kinetic model. Two materials in significant quantities in municipal plastic wastes 0196-8904/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2009.12.017 * Corresponding author. Address: Laboratoire Interdisciplinaire de Recherche en Sciences et Techniques, Faculté polydisciplinaire de Béni-Mellal, Université Sultan Moulay Slimane, BP 592, 23000 Béni-Mellal, Morocco. Tel.: +212 23 485112/22/82; fax: +212 23 485201. E-mail address: a.aboulkas@yahoo.fr (A. Aboulkas). Energy Conversion and Management 51 (2010) 1363–1369 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman