Experimental characterization and constitutive modeling of bulk epoxy under thermo-oxidative aging

Abstract

Most research in the area of high-temperature oxidation of polymers focuses on the realm of reaction-limited oxidation, where the effects of oxidation are homogeneous throughout the thickness of the material. While this may be applicable in applications involving coatings, or thin films, most load-bearing structures rely on finite thickness samples (bulk material) or a composite to support the load. These finite-dimension bulk samples typically demonstrate signs of diffusion-limited oxidation, where the chemical reactions with ambient oxygen mostly reside on the surface due to the restricted ability for oxygen to diffuse through the thickness of the material. Hence, a heterogeneous growth of the oxide layer occurs across the thickness of the bulk sample, resulting in localized alteration of the oxidized network mostly near the outer surfaces. As the formation of this oxide layer continues, depending on the aging period, the mechanical response of the bulk sample degrades significantly. In this study, we used various experimental techniques to investigate the correlation between the growth of the oxide layer and the impact of local microstructural changes within this layer on the macroscopic constitutive response of a bulk thermoset (epoxy) as a function of the aging period. We used a controlled environmental chamber to accelerate the high-temperature oxidation of a structural epoxy commonly used in industry. Experimental techniques such as Fourier transform infrared spectroscopy (FTIR), dynamic mechanical analysis (DMA), nano-indentation, and uni-axial tensile testing were used to characterize how heterogeneous microstructural changes due to diffusion-limited oxidation affect the macroscopic performances of these materials. Additionally, a phenomenological constitutive model based on a well-known nonlinear viscoplastic framework was used to describe the uniaxial tensile response of both virgin and oxidized epoxy. This model incorporates the effects of aging-induced degradation by modifying certain material parameters to reflect the microstructural changes in the polymer network. The major findings from the experiments indicated the formation of an oxide layer on the outer surfaces of the epoxy samples, which exhibited a higher indentation modulus. Increased concentrations of carbonyls were detected in this outer layer by the FTIR spectra corroborating oxidative crosslinking events. These local changes in the macromolecular structures resulted in reduced viscoplastic deformation of the aged epoxy at continuum level testing such as uniaxial tensile, and dynamic mechanical response.