Prediction of stress induced by heterogeneous oxidation: Case of epoxy/amine networks

Abstract

A methodology to predict the formation of superficial stress during the diffusion-limited oxidation of thick epoxy/amine samples is proposed. This quantitative methodology is based on the understanding of mechanisms responsible for this stress lead to superficial micro-cracks. The first step consists of simulating the homogeneous oxidation on the surface of thick samples. The extent of oxidation is assessed through the concentration of oxidation products as amide groups measured using Fourier-transform infrared spectroscopy (FTIR). These experimental results are compared to simulations obtained from a kinetic model based on a close loop mechanistic scheme in chain oxidation. Shrinkage and tensile behavior changes are correlated with an oxidation tracer, being amide groups in our case. The second step considers diffusion-limited oxidation in thick samples where the gradients of oxidation products characterized through thickness by micro-FTIR are simulated by coupling the previous kinetic model with oxygen diffusion. Finally, the gradient of strain and stress induced by oxidation of a thick epoxy/amine sample is simulated by inserting the mechanical behavior modifications and shrinkage gradient associated with the amide gradient in finite element code (Abaqus®). As a result, it is shown that it is possible to predict the superficial stress level as a function time/temperature of exposure and sample geometry.