This paper describes recent work to develop 2-component aqueous topcoat formulations, based on PVDF-acrylic hybrid dispersions crosslinked with polyisocyanates, which meet the requirements of the new SSPC Paint 471 standard for highly weatherable fluoropolymer topcoats, and when combined with conventional primer and midcoat technology, also meet the requirements of IEEE(1) standards C57.12.282 and C57.12.293 for pad-mounted transformer enclosures. Enhanced weatherability is primarily conferred by having relatively high levels of fluoropolymer in the coating binder. Chemical resistance is obtained by a combination of high fluoropolymer level plus crosslink density. Barrier properties contributing to corrosion resistance depend on both these compositional factors, as well as on other formulation details.
Typical service lifetimes for protective coating systems range from 10-50 years, depending on how extreme the service environment is, as well as on all the details of substrate preparation, coating composition, and coating application.4 While the primary consideration for specifiers of protective coating systems is the service life related to corrosion protection, there is often also a requirement of durability of decorative properties, e.g. color and gloss. This is true not only for monumental steel structures, but also for instance for industrial and offshore structures where "safety colors" are used. This segment therefore has many similarities to the "architectural coatings" segment (coatings for monumental buildings), where the substrates may be different, but where a multi-layer system approach is still used, and owners expect the durability of both the protective and decorative functions.
Unlike the protective coating service life, which is a system property, the decorative service life often depends almost exclusively only on the system topcoat or finish coat. As a result, in the protective coatings standards world, it is often the case that standards for individual coating system layers do NOT have include corrosion-related tests (corrosion resistance being a system property), while decorative durability requirements are most commonly seen in finish coat requirements. The SSPC(2) protective coatings standards, widely used in North America, illustrate this point. The SSPC Paint 36 standard5 for 2-k urethane topcoats has only three main performance requirements: weathering, solvent (methyl ethyl ketone) resistance, and adhesion (to the primer over which it is specified).