Current coating systems for offshore installations and bridges are in most cases based on epoxy resins in the primer and intermediate layers and polyurethane resins as UV-resistant topcoats. The established lifetime is around 30 years with regular maintenance at exposed areas such as welding seams and joints. The breakdown mechanisms are manifold, but the most important ones are connected to embrittlement due to long-term ageing under thermal cyclic conditions, combined with humidity, UV light and salt. At some point, micro cracks, water ingress and chlorine diffusion to the interface cause corrosion of the underlying steel substrate.

SEM cross-section of coating layer

SEM cross-section of coating layer

Hempel is frequently faced with customer demands requiring coating systems with 50-80 years lifetime expectancy. The choice and selection of the exact coating systems are in these cases based on experience since little is known on the exact chemical and physical changes of the polymeric coating film in the final part of their lifetime. In the literature, the term “endothermic relaxation” is used to cover ageing effects but the underlying mechanism is still unknown.

The aim of this R&D track is to understand the underlying degradation mechanism by using x-ray scattering techniques as well as mechanical testing on severely aged samples. Building upon this, new paint systems will be developed. UV resistance can be increased significantly by using fluorinated binders or including thermoplastic polyvinylidene fluoride (PVDF) dispersed in another resin as topcoat. The properties of the primer and intermediate coats in the final part of the service life will be stabilized by a biphasic binder-system with micro sized droplets of soft resins that can stretch to avoid embrittlement of the main binder phase. Furthermore, functionalized fillers with appending moieties with high affinity to the binder phase such as phenyls will be developed and tested.