Trials have proven that this low-VOC waterborne self-healing coating is as good as its more obnoxious solvent-borne cousins.
Self-healing materials are extremely useful for effective corrosion resistance. Case in point, consider that although they remain highly regulated known carcinogens, chromate conversion coatings and primers are still used for the protection of critical metal assets by the US Department of Defence due to their effectiveness. A key component of the corrosion resistance afforded by coatings containing chromates is the ability to store Cr(VI) within the coating system such that damage to the coating system allows the Cr(VI) to leach out into the site of damage where it is then reduced to form a highly protective chromium oxide film.
Note that this mechanism, which has been referred to as ‘self-healing’, can be initiated by the exposure of the underlying substrate as a result of damage to the coating system. Due to the carcinogenicity of chromates and the regulation of their use, the vast majority of protective coating systems are chrome-free and, as such, do not exhibit such ability to maintain corrosion resistance after damage. When these coatings are damaged, the underlying substrate is exposed to the environment, leading to corrosion. Unless promptly repaired, corrosion on the substrate will propagate at the coating/substrate interface, compromising the coating’s adhesion to the substrate and, consequently, its ability to protect it.
Adding self-healing technology to protective coating systems
Based on the importance of the self-healing functionality in the corrosion resistance offered by protective coating systems containing chromates, it stands to reason that the future of effective corrosion resistance must leverage some self-healing functionality. Autonomic Materials (AMI) has been researching additives that impart self-healing functionality into protective coatings.
These additives are comprised of microencapsulated healing agents that incorporate polymer precursors, adhesion promoters and corrosion inhibitors. When coatings incorporating these additives are damaged, the microcapsules are ruptured, releasing the healing agent into the site of damage where it polymerises sealing off the damage and restoring the coating’s protective function. Using this approach, AMI has designed additives that have demonstrated the ability to facilitate a reduction in corrosion creep in damaged coatings based on a broad range of chemistries.
Finding waterborne protective coating solutions
One game-changing application for AMI’s self-healing technology is the design of high-performance waterborne protective coating systems. Industrial waterborne coatings have traditionally exhibited inferior corrosion resistance relative to solvent-borne versions. However, global regulatory trends limiting the amount of volatile organic compounds (VOCs) in coatings have led to an increase in demand for higher-performing industrial waterborne coatings. AMI has demonstrated an epoxy waterborne coating with corrosion resistance surpassing many solvent-borne coatings while exhibiting a VOC component of less than 50g/L.
To demonstrate the benefits of the self-healing materials in the formulated epoxy waterborne coating, the corrosion performance of a control system excluding the self-healing functionality, but otherwise identical to the self-healing version was compared to a system incorporating the self- healing low VOC waterborne coating formulation.
For both control and self-healing systems, two coats of the waterborne epoxy were applied at 60-70 microns each to blasted steel panels (SSPC-SP10) followed by a waterborne acrylic topcoat, which was also applied at 60-70 microns. After curing at room temperature for a minimum of seven days, panels coated with control and self-healing materials were scribed using 156 micron and 500 micron scribe tools. The samples were then allowed to equilibrate at room temperature for 24 hours followed by exposure to a salt fog (ASTM B1175) for 2,000 hours.
AMI’s self-healing materials with protective coating systems results
After 2,000 hours of salt fog exposure, the extent of creep from the scribes was evaluated as described in ASTM 1654.6.
The results showed that while the control exhibited average creep values of 14.2mm and 10.9mm for the 156 micron and 500 micron scribes respectively, the incorporation of the self-healing additive resulted in a reduction in scribe creep of 77% to a value of 3.3mm and 69% to a value of 3.4mm for the 156 micron and 500 micron scribes respectively. Such minimal creep following 2,000 hours of salt fog exposure is rare among the best solvent-borne coatings and unprecedented for waterborne formulations.
Within the context of transportation coating applications, the self-healing epoxy formulation affords three main benefits. Firstly, there’s no trade-off between performance and environmental friendliness, the self-healing coating formulation exhibited more efficient protection of metal substrates relative to many incumbent solvent-borne coatings while incorporating substantially lower VOCs.
Secondly, as there’s almost no odour and multiple components are not being mixed on site, application is easier.
Finally, the formulation offers multiple avenues for cost savings including longer-lasting protection leading to lower maintenance costs, shorter application time for multiple coats due to rapid-drying kinetics; and the elimination of the need to manage solvent waste streams, all important considerations for the transport industry.