Room temperature aging of Narmco 5208 carbon‐epoxy prepreg. Part I: Physicochemical characterization

Abstract Samples of Narmco Rigidite 5208/WC3000 carbon‐epoxy prepreg were exposed to ambient temperature (22°C) and 50% relative humidity for different periods up to 66 days. They were analyzed in depth using various techniques to determine the extent of the chemical changes occurring. The physicochemical techniques used were Fourier transform infrared spectroscopy (transmission, attenuated total reflection, diffuse reflection), liquid chromatography (reverse‐phase, high‐speed reverse‐phase, and high‐performance size‐exclusion), thermal analysis (differential scanning calorimetry and thermogravimetric analysis), and pyrolysis‐gas chromatography. All showed evidence of significant changes, the most sensitive being Fourier transform infrared spectroscopy (FT‐IR) and reversephase liquid chromatography (RPLC). FT‐IR showed that the number of unreacted epoxy groups decreases steadily at a rate of 0.34% per day, based on the initial amount. At the same time, the number of free amine‐hardener molecules, as monitored by RPLC, drops at a rate of 1.05% per day. RPLC also showed that the amount of initial epoxy‐amine reaction product increases significantly over the first 30 days, but then declines as it undergoes further reaction to give higher‐molecular‐weight products. The heat of polymerization of the resin, measured by thermal analysis, decreased by 0.26% per day from its initial value of 561 J/g.