Time-dependant degradation of highly cross-linked epoxy due to hygrothermal ageing at three different temperatures

Abstract

Laminating grade, highly cross-linked, epoxy specimens were hygrothermally aged by immersing in de-ionized water baths maintained at 75°C, 50°C and room temperature respectively for a period of 75 days. An increase in mass which exacerbated with temperature of ageing was observed for all the aged specimens and the rate of increase became sluggish as ageing progressed without attaining a saturation in mass. The diffusion coefficient (D) increased with hygrothermal ageing temperature. A fraction of the absorbed water (RF) was retained by the epoxy and it increased with ageing temperature and time. This absorption led to hydrolysis reaction in the epoxy matrix as inferred from the Fourier-transform infrared spectroscopy (FTIR) spectrum where we observe an increase in the absorbence peak of C = O (carboxylic), -OH (bending carboxylic) and C-O (stretching alcohol), which are the reaction products of hydrolysis. Differential scanning calorimetry (DSC) indicated a decrease in the glass transition temperature (TG) with severity of ageing till 50°C but showed only a slight decrease in TG for the specimens aged at 75°C possibly due to the formation of a secondary cross-link network by the retained water molecules which are predominantly Type II bonded water. Uniaxial tensile strength decreased with ageing time and temperature accompanied by a decrease in ductility. Tensile fracture surface showed a crack initiation region usually at an edge, a crack stretch region followed by a catastrophic failure region which encompassed the balance region of the fracture surface. The spacing of the crack stretch marks, formed due to crazing, increased with severity of ageing due to strain localization. However, uniaxial compression tests showed an increase in strength with hygrothermal ageing.-