Apparent kinetics of nonisothermal high temperature oxidative degradation of ethylene homopolymers: effects of residual catalyst surface chemistry and structure

Abstract

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 Ea, for n (overall degradation order) = 1 for both polymers. Neither n nor Ea 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 Ea 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 Ea, 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.