Role of hygrothermal ageing temperature and time in degrading the fracture toughness of highly cross-linked epoxy

Abstract

The evolution of plane strain fracture toughness in highly cross-linked epoxy was assessed for different hygrothermal aging temperatures (25°C, 50°C, 75°C) and durations (0-75 days). Water absorption increased with hygrothermal aging, but at a diminishing rate, without saturating. Some absorbed water persisted despite prolonged drying. Thermogravimetric analysis (TGA) revealed that the epoxy decomposition temperature decreased with hygrothermal ageing. ¹³C solid-state NMR revealed an increase in the de-shielding of carbon atoms in the vicinity of electronegative elements like oxygen, with increasing hygrothermal ageing severity. indicating the development of intermolecular hydrogen bonding. X-ray photoelectron spectroscopy (XPS) tracked the chemical bonding state of carbon using a narrow scan on the C1s binding energy region. It is observed that with hygrothermal ageing there was a decrease in C-C/C=C groups indicating main chain scission through hydrolysis and an increase in the C-O species indicating formation of alcohols as hydrolysis reaction products. Thus, the retained water was involved in hydrogen bonding and chemical reactions with the epoxy. With increasing hygrothermal ageing severity, the fracture toughness decreased.  This was consistent with the fracture surface images that showed higher distance amongst crack stretch marks (formed on the fracture surface through crazing) in specimens with lower toughness.