Vyazovkin model-free approach is applied to study non-isothennal decomposition of waste PE sample using various temperature integral approximations such as Coats and Redfern, Gorbachev, Agrawal & Siva Subramanian and a new approximation [J. Cai, F. Yao, W. Yi, F. He, AIChE J. 52 (2006) 1554] and a new method for direct integration. E-alpha dependency on alpha is compared with several literature data. Initial strong and increasing E-alpha dependency on alpha in case of non-isothermal decomposition is associated with initiation at the weak links. Weak but increasing E-alpha dependency on alpha in the later stages is possibly due to dependence of distribution of size of volatile products on heating rate and random scission. Weak and decreasing followed by weak but increasing function of alpha in case of isothermal decomposition is possibly due to existence of two opposite effects, nucleation, and distribution of size of volatile products. The sample used follows neither perfectly the LDPE trend nor the HDPE trend. Thus, it becomes mandatory to study the decomposition behaviour of any waste material before design of an incinerator and/or pyrolysis unit without using blindly the information available for standard samples. Optimum E-alpha dependencies from non-isothermal data at 15 K min(-1) heating rate best predicted the isothermal data. (C) 2006 Elsevier B.V. All rights reserved.