The effects of two supported residual catalysts - one Ziegler-Natta and another metallocene - on the nonisothermal thermooxidative degradation of the resulting ethylene homopolymers were investigated using TGA experiments and kinetic modeling. The rigorous constitutive kinetic model (developed in this study), unlike the analytical Horowitz and Metzger model, fitted very well to the entire TGA curve, without distribution of activation energy E a, for n (overall degradation order) = 1 for both polymers. Neither n nor E a varied as a function of fractional weight loss of the polymer. Hence, the proposed unified molecular level concept of surface chemistry and structure of the residual catalysts held all through the degradation process. The above feature of n and E a also indicates the suitability of the model formulation and the effectiveness of the parameter-estimation algorithm. Random polymer chain scission, with the cleavage of the -C-C- and the -O-O- (hydroperoxide) bonds, prevailed. The types of residual catalyst surface chemistry and structure varied the bond cleavage process. The metallocene Zr residual catalyst caused more thermooxidative degradation in MetCat HomoPE than what the Ti one did in Z-N HomoPE. The rigorous constitutive model-predicted apparent kinetic energy E a, and frequency factor Z also support this finding. The proposed degradation mechanism suggests that the Zr residual catalyst more (i) decreased the activation energy required to decompose the -C-C- and the -O-O- bonds, and (ii) eliminated β-hydrogen (by the carbonyl functionalities) from the polymer chains. These findings were attributed to the differences in surface chemistry and structure of the residual catalysts. Therefore, the current study presents a rigorous constitutive kinetic model that duly illustrates the influence of the characteristic surface chemistry and structure of the residual catalysts on the high temperature oxidative degradation of polyethylenes.
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Acknowledgments The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) via the Science & Technology Unit at KFUPM through Project Number 08-PET90-4 as part of the National Science and Technology Innovation Plan, as well as that, by the Center of Research Excellence in Petroleum Refining and Petrochemicals (CoRE-PRP), established at King Fahd University of Petroleum & Minerals (KFUPM), Dhahran, Saudi Arabia by the Saudi Arabian Ministry of Higher Education (MOHE). The technical assistance from the Center of Refining & Petrochemicals (CRP) of KFUPM Research Institute; Department of Chemistry; and Dr. Pilar del Hierro of Polymer Char, Spain are gratefully acknowledged. The authors also thank PQ Corporation, Philadelphia, USA for donating the silica.
- Ethylene homopolymers
- Kinetic modeling
- Residual catalyst
- Surface chemistry and structure
- Thermooxidative degradation and mechanism
ASJC Scopus subject areas
- Polymers and Plastics
- Organic Chemistry
- Materials Chemistry