Degradation behavior and ageing mechanism of E-glass fiber reinforced epoxy resin composite pipes under accelerated thermal ageing conditions

Abstract

In this work, accelerated ageing experiments with circulating air and simulated oilfield water were carried out at 95 °C on aromatic amine-cured GFRP pipes used in oilfield environments, with effective ageing times of 5000 h and 3000 h respectively. The differences in ageing behavior and ageing mechanisms of the GFRP tubes at different times under the two conditions were then analyzed and discussed. The changes in the apparent morphology and microstructure of the samples after thermal ageing were first observed using optical microscopy and scanning electron microscopy, and the results demonstrated that the thermal oxygen environment triggered more drastic color changes and obvious oxidation layers. The resin matrix manifests a large number of nano-scale micropores and modest matrix shrinkage pores in the micron size range, this is accompanied by damage to the fiber cross-section. Hydrothermal ageing causes further dissolution of the surface resin and dissolution of the exposed fibers. ATR-FTIR analysis verifies the increase in CO bonding of the ester group during thermal oxidation and the decreasing strength of the C–O–C and C–H bonds, while the behavior of ageing water absorption and glass fiber hydrolysis is illustrated by changes in the intensity of the –OH and Si–O characteristic bands. NMR hydrogen spectroscopy verified the formation of amides, methyl ethers and carboxylic acids in the thermal oxidation products of epoxy resins. Furthermore, the decay pattern of the circumferential tensile strength indicates that the hydrothermal environment has a more significant effect on the mechanical properties of GFRP pipes, which is attributed to the physical dissolution of the surface resin, significant hydrolytic damage to the surface exposed fibers, and the combined damage to the resin-fiber interfacial bonding by chloride ions in simulated oilfield water.