Degradation mechanisms of amine-cured epoxy novolac and bisphenol F resins under conditions of high pressures and high temperatures

Projections of continued growth in the global hydrocarbon demand and fast depleting resources push the oil and gas industry to explore and produce in geological formations with abnormal high pressures and temperatures, so-called HPHT conditions. In the present study, the largely unexplored degradation mechanisms for amine-cured epoxy novolac (EN) and bisphenol F (BPF) epoxy resins at HPHT are investigated at lower limits of HPHT.

Using a batch-like reactor encompassing the three relevant phases (a gas mixture of nitrogen and carbon dioxide, a hydrocarbon phase of aromatic para-xylene, and an artificial seawater phase), the conditions of high pressures and high temperatures were simulated. The EN and BPF coated steel panels were placed inside the batch reactor.

In the gas phase-exposed zone, both EN and BPF remained essentially intact with no major defects. However, due to para-xylene uptake that resulted in a free volume increase (i.e. lowering of the glass transition temperature), the hydrocarbon-exposed zones of EN and BPF were partly covered by an oxide of iron, the origin of which was found to be diffusion of anodically-dissolved iron from the steel-coating interface.

The enhanced resin chain mobility at the hydrocarbon-seawater interphase allowed higher rates of diffusion of seawater ions to the steel-coating interface with clear signs of coating degradation. Finally, the seawater phase induced small blisters in the EN coating, whereas for BPF, a complete loss of adhesion between the coating and the substrate was observed.

Simulation of Rapid Gas Decompression (RGD), uncovered the role of RGD in the iron oxide formation process for both EN and BPF coatings. In summary, when compared to BPF, the EN network showed superior performance under conditions of HPHT.