Overview of DLO modeling and approaches to predict heterogeneous oxidative polymer degradation

Abstract

Accelerated polymer aging under thermal oxidative conditions often involves heterogeneous degradation processes due to diffusion limited oxidation (DLO). DLO depends on geometrical shape and size, intrinsic oxidation rate and oxygen permeability of the material and occurs through the competition of simultaneous diffusional transport and reaction of oxygen. Definitions of the many parameters that describe polymer aging under DLO conditions are summarized for several applicable situations, which include the average oxidation rate, the oxidizing material fraction (i.e. fraction of the material oxidizing at a reference oxidation rate), and the depth of a specific flux reduction (i.e. generalized total oxidized layer (TOL)). Analytical and numerical solutions are derived for the prediction of spatially dependent oxidation behavior within a systematic framework for 1D to 3D geometries, multi-material situations and for a range of boundary conditions. Traditional mathematical approaches are expanded to embrace an arbitrary reaction domain through the finite element method (FEM). Concepts are also developed that address time dependent DLO profiles when the underlying oxidation rate or permeability may in itself depend on the evolving oxidation level. Understanding the occurrence of DLO and having the ability to predict spatially resolved oxidation reactions in polymers is critically important to determine when DLO (i.e. non uniform degradation) will interfere as part of accelerated aging studies, or to predict the aging behavior of multi-material polymer components of any geometry. This overview wishes to guide the polymer aging community towards embracing DLO phenomena and share newly developed computational approaches that facilitate comprehensive predictive modeling capabilities.