Moisture sorption behavior of an epoxy-based structural adhesive and its impact on the mode I interlaminar fracture toughness of bonded joints in the oil and gas industry

Structural adhesives are widely used across industry sectors and, specifically in oil and gas, they appear as an alternative for structural joining instead of welding, to mitigate the risks arising from sparks in an environment with highly flammable products. In this sense, this article aims to evaluate the mechanical behavior of adhesive joints using the Double Cantilever Beam (DCB - to determine the GIC, mode I interlaminar fracture toughness) test before and after exposing these joints to severe environmental conditions, such as high humidity and high temperature. Also, three different surface treatments (manual sanding with abrasive sponge, solvent cleaning, and peel-ply application) of the adherend (carbon fiber/epoxy composite) are examined, as well as two different thicknesses of the adhesive layer (0.5 and 1.0 mm) to verify possible influences on the GIC behavior before and after conditioning. The results show that reducing manual surface treatment processes is a better option, to reduce the factors that cause greater dispersion in GIC values. In particular, cleaning the adherend with just isopropyl alcohol (solvent) before applying the adhesive yielded better GIC results among the tested techniques. Regarding thickness, the findings suggest that a greater thickness of the adhesive layer can lead to an increase in the plastic regions, enhancing the dissipation energy and improving the mode I interlaminar fracture toughness. Hygrothermal conditioning revealed the adverse effects of humidity on the adhesive, resulting in a 20–30 % reduction in GIC. Differential Scanning Calorimetry (DSC) analyses demonstrate that conditioning caused a notable reduction in the glass transition temperature (Tg) of the adhesive.