Thermal aging and degradation of epoxy coatings at oxidised interfaces

It is generally understood that coating-metal adhesion is a critical controlling factor in the service performance of corrosion protective coatings. Interfacial failure rarely occurs "cleanly" and even in the absence of a visible film, surface analytical methods such as x-ray photoelectron spectroscopy (XPS) can detect chemical remnants characteristic of the original polymer coating. Thus, it is generally accepted that coating failures occur in a cohesive manner and propagate through a structurally distinct, and weak, polymeric 'interphase' region located close to the metal-polymer interface.
The nature and extent of this interphase region has been the subject of much research, and yet even in the case of widely studied adhesives such as epoxy-amine thermosets, its formation and structure are disputed. For epoxies, a variety of formation mechanisms have been proposed. These include restricted polymer motion in the vicinity of a rigid substrate, the selective adsorption of resin components onto the metal surface, local chemical gradients as a result of metal-induced catalysis of the cure or oxidation, and variations in cross-linking density as a result of thermal gradients or the formation and diffusion of organometallic species into the polymer.
This ambiguity around interphase structures stems from a historical dearth of techniques capable of analysing the buried region directly. For thermoplastic polymers, ultrathin films can be examined as a model approximations of the interphase. However, for thermoset polymers this approach has serious limitations, which stem from the increased influence air-polymer interactions on the structure of thin films. An alternative approach is to examine resin-metal cross-sections, or the organic material remaining on the metal substrates after delamination, directly. The technique of choice, vibrational spectroscopy, has heretofore been restricted by the optical diffraction limit of around 2-3 m and by poor sensitivity. Alternative techniques such as analytical transmission electron microscopy (A-TEM) can be used to determine the elemental composition of interphase regions at high (nm) spatial resolution while XPS delivers true atomic surface sensitivity but with a lateral resolution no better than around 100 m.
At the University of Manchester we have pioneered the use of the atomic force microscopy - infra-red (AFM-IR) technique, which couples the lateral resolution of scanning probe microscopy with the chemical sensitivity of infrared spectroscopy, in the examination of metal-polymer interphase regions. Recently we have use AFM-IR to examine polymeric residues remaining after pull-off adhesion tests after thermal aging at 70 C. Preliminary work finds a strong band at 1658 cm1 implying interfacial oxidation of the epoxy is occurring.

We propose to examine the hypothesis that highly local polymeric oxidation in the interphase region adjacent to metallic substrates and pigments influences interfacial adhesion and overall corrosion performance. We will use predominantly the AFM-IR method supported by complementary analytical approaches including nano-Tg and electrochemical determination of interfacial impedance.
We will explore a number of variables including: resin stoichiometry, curing above or below Tg, post-cure and "overcure" at elevated temperature as a device to promote oxidative stress within the polymer. Of particular interest is the temperatures where the metallic interface might also begin to oxidise significantly. Here we can model this process in a stable fashion by using pre-oxidised iron substrates.