First demonstrated in polymerized resins via a ring opening metathesis polymerization (ROMP)-based system by White et al.,1 self-healing functionality utilizing microcapsules has been demonstrated to improve the performance of a range of protective coatings.2-4 In these coatings applications, a healing agent is released from microcapsules embedded in the coating film after damage. Once in the site of damage, the generally hydrophobic healing agent renders the damage site more hydrophobic, thereby limiting further moisture ingress, and upon curing, seals the edges of the damage, thus maintaining the coating’s adhesion to the substrate and limiting undercutting. The effect of designing microcapsule-based self-healing functionality into protective coatings is that the lifetime of traditional coatings and their underlying substrates (Figure 1) can be extended in instances where mechanical failure of the coating is the dominant failure mechanism. In these cases, damage to the coating is arrested by the healing functionality, thereby decreasing the rate of onset of the level of failure that might require coating maintenance, repaint or asset replacement (Figure 2). For owners and operators maintaining assets in the most corrosive environments, the life extension afforded by self-healing functionality presents a compelling value proposition due to the potential for longer coating maintenance cycles and less-frequent downtime. Together, these improvements could contribute to a significant decrease in operational costs over the life of the asset.
Since zinc-rich primers are used extensively in the most corrosive environments, they represent a natural target for improvement via self-healing functionality. Zinc-rich coatings are typically used for the protection of steel substrates via sacrificial oxidation. Depending on the specifications of the coating, various sizes and shapes of zinc particles have been used as electrochemically active pigments, and organic binders have included epoxies, polyurethanes and alkyd resins, while inorganic binders are typically alkyl silicates capable of polycondensation in the presence of moisture.6 The pigment volume concentration (PVC) of zinc-rich coatings is generally greater than the critical pigment volume concentration; a characteristic that is essential in ensuring a high degree of electrical connectivity between zinc particles and between the zinc particles and the underlying substrate.