Investigation on moisture absorption behavior on GFRP and neat epoxy systems in hygrothermal salt fog aging

Abstract

This work investigates the moisture absorption behavior in epoxy matrix composites reinforced with glass fibers, used in repairs of offshore topside metal pipelines exposed to aging in saline environment. The materials used were a bicomponent DGEBA epoxy and a woven bidirectional E-glass fabric. In order to simulate the harsh environment, the composites were exposed to accelerated hygrothermal aging tests in three independent salt spray chambers at temperatures of 35, 55 and 70 °C and the materials had their mass gain monitored. The moisture absorption of the neat epoxy resin was also investigated and correlated with that of the composites. Thermal analyses by Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Thermal Analysis (DMTA) were performed to evaluate the curing state of the polymeric materials tested and the effects of temperature on the material's post-curing. Fourier-Transform Infrared (FTIR) analyses were performed to investigate the occurrence of post-curing, hydrolysis and thermo-oxidation processes during aging. As expected, it was seen that temperature increased diffusivity for both the composite and the unreinforced epoxy matrix. The neat epoxy resin presented a decrease in moisture absorbed with temperature, evidencing the effect of post-curing on moisture absorption. Furthermore, from the thermal analyses, it was observed the presence of an interphase between matrix and fibers with a Tg higher than the bulk matrix. Non-Fickian models confirmed structural changes caused by temperature and moisture in the materials. A modification of the Berens-Hopfenberg (BH) non-Fickian model was proposed to account for post-curing effects on moisture absorption.