Roles of hygrothermal aging temperature and time in degradation of the fracture toughness of highly cross‐linked epoxy

Abstract

AbstractThe evolution of plane‐strain fracture toughness in highly cross‐linked epoxies was assessed for different hygrothermal aging temperatures (25, 50, and 75°C) and durations (0–75 days). The absorption of water increased with hygrothermal aging, but at a diminishing rate and without saturating. A fraction of the absorbed water persisted despite prolonged drying. Thermogravimetric analysis revealed that the epoxy‐decomposition temperature decreased with hygrothermal aging. 13C solid‐state nuclear magnetic resonance spectroscopy indicated an increase in the de‐shielding of carbon atoms in the vicinity of electronegative elements like oxygen with hygrothermal aging severity, indicating the development of intermolecular hydrogen bonding. X‐ray photoelectron spectroscopy tracked the chemical bonding state of carbon by using a narrow scan on the C1s binding energy region. Upon hygrothermal aging, there was a decrease in CC/CC groups, indicating main chain scission through hydrolysis. A simultaneous increase in the CO species indicates the formation of alcohols as hydrolysis reaction products. In this manner, retained water was involved in hydrogen bonding and chemical reactions with the epoxy. With increasing severity of hygrothermal aging, the fracture toughness decreased. This was consistent with fracture surface micrographs that showed a greater distance amongst crack stretch marks in specimens with less toughness.