‘Coating: Heal thyself’

Dr. Neil Canter, Contributing Editor | TLT Tech Beat June 2009

A self-repairing technology could extend the performance of anticorrosive coatings for longer periods. 

KEY CONCEPTS
• The intent of a self-healing coating is to protect a substrate once the coating has been damaged. Previous approaches negatively affected the properties of the coating.
• A new self-healing technology utilizes microcapsules of monomer and catalyst incorporated into the incumbent coating. The microcapsules open only when the coating is ruptured and do not impact the coating.
• The self-healing technology is flexible and can be tailored to ensure maximum compatibility with the coating used.

Corrosion continues to be a major challenge for those of us in the lubricant industry as end-users seek products that last longer under more severe operating conditions. Protection of metal substrates under conditions such as those encountered with seawater can be very daunting. Just the image of salt water on bare metal leads to immediate thoughts of corrosion.

While there are a number of different corrosion inhibitor technologies available to withstand these conditions, none of these products are effective if the metal substrate becomes damaged and then exposed to the working environment.

The concept of using a self-healing coating has been proposed in an attempt to protect the substrate once the coating has been damaged. After the damage has occurred, a self-healing coating will automatically take care of the problem area and continue to protect the substrate in a similar fashion to a band aid covering a wound.

Dr. Gerald Wilson, technology platform manager for Autonomic Materials, Inc., says, “There have been a number of attempts made to produce a coating that repairs itself during use. But these techniques mainly employ technologies that may have an adverse effect on the properties of the coating. In addition, they may involve the use of some type of external intervention such as heat, pressure or ultraviolet light. For example, a coating can be repaired by a reversible cross-linking reaction that is initiated by heat, but this chemistry requires a synthetic modification of the polymer binder.” 

Other self-repair methods include the utilization of microvascular networks, nanoparticle phase separation and polyionomers. A better approach would be to develop a self-healing mechanism that has no impact on the integrity or properties of the coating.

No technology has been developed using this strategy until now.

MICROENCAPSULATION
Work conducted by researchers at The University of Illinois has led to the development of a self-healing coating that does not affect the integrity or chemistry of the coating and is triggered automatically when the substrate is damaged.

Wilson explains, “Monomer and catalyst needed to prepare a self-healing coating are encapsulated in separate microcapsules which are incorporated when the incumbent coating is produced. While the coating is in use, the microcapsules and their contents remain in the matrix and do not disturb or impact its function. But when the coating is ruptured, some of the microcapsules of the monomer and catalyst open, releasing their contents at the site of the problem. The ensuing reaction creates a self-healing coating that protects the substrate exposed to the environment.” 

In a recent article, an example provided was the use of hydroxyl end-functionalized polydimethylsiloxane (HOPDMS) and polydiethoxysiloxane (PDES) as the monomers with the catalyst being polyurethane microencapsulated; dimethyldineodecanoate tin (DMDNT) catalyst solution (1). Experimental work was conducted with this self-healing coating imbedded in an epoxy vinyl ester matrix.

A razor blade was used to hand scribe through a 100-micron coating into a steel substrate. Samples with the self-healing coating and a control were then immersed in 5% sodium chloride for 24 hours at a temperature of 50 C. The control rapidly showed extensive corrosion within the 24-hour period. But the self-healing samples displayed no evidence of corrosion even when the experiment was extended for 120 hours.

Wilson says, “We have compared the fracture toughness of an epoxy thermoset coating before and after healing. Self-healing efficiency was measured as the ratio of the healed fracture toughness to the virgin fracture toughness. With our approach, we have seen that the toughness of the coating remains virtually unchanged after self-healing.” 

Much of the evaluation work for the self-healing microcapsules was to incorporate them into anticorrosive coatings. Evaluation work was done through use of the salt spray test (ASTM B117). This test method involves the periodic spraying of a 5% sodium chloride solution onto samples coating steel panels in a chamber maintained at 35 C. 

Figure 2 shows on the right a control panel that has extensive rust after exposure of the coated steel for 24 hours in the salt spray test. The left image shows the same steel panel with self-healing microcapsules incorporated into the coating. No rust is detected on that panel.


Figure 2. Coated steel panels that were fractured were evaluated in the salt spray test for 24 hours. The panel on the left contains the self-healing technology and shows no rust, while the panel on the right contains just the coating and contains extensive rust. (Courtesy of Autonomic Materials, Inc.)

Wilson says, “We have found that scribed or intentionally damaged samples prepared with self-healing coatings can be run up to 400 hours without any evidence of corrosion.” 

The coatings used typically have a thickness between 150 and 500 microns. The self-repair microcapsules constitute between 5% and 10% of the total coating. Most of the work has been done using panels made from various steel alloys.

The technology has been licensed to Autonomic Materials, which is exploring commercial applications. Wilson says, “We have developed three different chemistries that are tailored for elastomers, powder coatings and thermosets.” 

The most important feature of this self-healing technology is its flexibility. Wilson adds, “We can tailor the microencapsulation to ensure maximum compatibility with the coating used. This includes ensuring that the microcapsules do not affect the coating. As part of this process, microcapsules can be produced which range in diameter from single digit microns up to 500 microns.” 

This extendable technology has a thermal stability up to 190 C, is sprayable without breaking the microcapsules and can be applied in both solvent and waterborne coating systems. Wilson says, “Most of our work with anticorrosive coatings has been done with solvent-based coatings.” 

This technology can be used to assist coatings that are protecting substrates in nonaccessible locations such as those found in sea water environments. In the past, a catastrophic coating failure would require maintenance engineers to travel out to a difficult location. But the self-repair coating provides the potential for extending the performance of an anticorrosive coating for a longer period.

Other functionalities can also be incorporated into the microcapsules to provide additional performance benefits in coatings. As an example Wilson says, “We have been looking at including pigments to provide color enhancement in case of a problem. In an automotive application, we are exploring the incorporation of dampening agents into the microcapsules to take care of problems associated with noise.

Larry Evans, CEO of Autonomic Materials, says, “We believe this technology allows for the production of true smart coatings that can be used in extreme environments.” 

The self-repair coatings are being evaluated in automotive and marine applications. Commercial products will be available in the first quarter of 2010. Further information can be found at www.autonomicmaterials.com or by contacting Evans at lfe@autonomicmaterials.com. 

REFERENCE
1. Cho, S., White, S. and Braun, P. (2009), “Self-Healing Polymer Coatings,” Advanced Materials, 21 (6), pp 645 – 649.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.