How serious is the issue of corrosion?
According to the U.S. Government Accountability Office’s GAO-490-R, published in April 2011, the Department of Defense (DoD) reported the cost of damage to military assets caused by corrosion in 2010 exceeded $22.9 billion dollars annually and was continuing to trend upward.(1)
The report noted that the damage resulting from this deterioration affects not only military assets such as infrastructure and field equipment but also military readiness by removing critical systems from available use.That, in turn, creates safety hazards.
The need for affordable, sustainable and effective coatings for military applications has never been greater. As technologies continue to evolve, coatings manufacturers must find a balance between assisting with the military’s goal of enhanced performance through reduced corrosion while maintaining environmental compliance and ensuring cost effective solutions.The need to meet this challenge has led manufacturers to increasingly recommend the use of powder coatings.
Perhaps the best known initiative in the military application of powder coatings is the development of Chemical Agent Resistant Coating (CARC)Powder Topcoats for use on the Army’s equipment. But opportunities as presented through new or revised specifications from both the Army and Navy have brought forth numerous opportunities and will continue to do so.
In direct alignment with DoD environmental initiatives, powder coatings require no solvents and release minimal VOCs during the cure stage. With careful management, up to 95 percent of powder coating spray that does not adhere to parts can be reclaimed and reused, making them cost effective. Overspray that cannot be recycled can be easily disposed of in an environmentally friendly manner and is not classified as hazardous waste. When applied by properly trained personnel using the right equipment, powder coating results in less overall waste than liquid coatings. Additionally, the higher film builds that can be achieved support the DoD’s fight against corrosion.And the use of a powder topcoat will potentially eliminate thousands of pounds of HAPs and VOC emissions annually.
Evolution of Powder Coatings for Military Use
Over the past 25 years, powder coatings have been used sporadically for military applications, with no clear direction or emphasis placed on the technology until recently.
The renewed emphasis followed a more non-traditional route, with Army approvals coming under an Experimental Product Program (EPP)and Navy approvals granted through placement on the Advanced Coatings and Cleanings Method (ADM). In both cases, First Article Testing was performed through the intended application process to ensure that the formulations were acceptable for use.
MIL-PRF-24712A was issued for use of powder coatings on naval assets in 1989, but no technology was successful at meeting its requirements. Naval Sea System (NAVSEA)recognized that this powder specification encompassed too many qualifications and made progressions that eased the introduction of powder to naval applications:
- In June 2007, a separate Naval specification for coating aluminum, TT-P-28H, was updated to include a Type II,defined as a functional aluminum pigmented, heat-resistant powder coating that withstands elevated temperatures to a maximum of 372°C. Coatings that meet this specification are eligible for qualification in the Qualified Products Database (QPD)(2), the DoD’s official repository of eligible products for specifiers to reference (including subcontractors performing DoD work).
- MIL-PRF-23236, a specification characterized for regulated ship and structure coatings, was amended in September2009 as MIL-PRF-23236D. This added Types VIII(non-metallic functional powder coating system for severe marine service having a maximum VOC of 75 grams of solvent per 4.54 kilograms [0.17 pounds of solvent per 10 pounds] of powder) and VIIIa (functional powder coating system with two coats which may contain zinc for severe marine service with a maximum VOC of 75grams of solvent per 4.54 kilograms [0.17 pounds of solvent per 10 pounds] of powder) to the specification for a20-year immersion service powder. Both are eligible for approval to be listed in the QPD.
The true emergence for powder coating chemistries in military applications began in 2009, when DoD entities issued approval for powder coatings to be used in place of or in addition to the electrocoat (E-Coat) layer found in traditional layered coating systems.In traditional systems, the E-Coat layer serves as the first defense against corrosion. In theory, both powder and electro-coating are organic coatings that are applied to metal surfaces via electrodeposition and require baking for cross-link cure. The main difference between the two is that the powder particles are charged in the powder coat, whereas the liquid component is charged in electrocoat, and the film is deposited by a reaction. Although both technologies have advantages and disadvantages, powder coatings can be applied at heavier films, creating a thicker barrier to corrosion. Both powder and liquid coatings are available in epoxy and urethane chemistries and are dependent upon the substrate, pretreatment, and/or primer combinations used.
Then, in November 2010, the Army Research Laboratory (ARL) issued the first military specification—MILPRF-32348 (see table 1)—to illustrate formal approval of the use of powders on all military tactical equipment, including ground, aviation, and related support assets. This was the first specification that required products be validated for use on military assets to be included in QPD.
MIL-PRF-32348 outlines four specified types of powder technologies for primers and topcoat. The specification has stringent requirements for chemical resistance, corrosion, flexibility, and weathering. There are multiple technologies currently available that are qualified to Types I, II, and IV. However, the true test comes in the development of coatings to meet Type III—those that may be used as camouflage and are chemical agent resistant (CARC).
CARC technologies must demonstrate:
- The critical properties of a specified IR signature for prevention of detection by enemy forces and
- The ability to be decontaminated from chemical and biological warfare agents, while maintaining a maximum 60 degree gloss level as low as 0.6, color dependent.
For successful agent resistance at the low gloss levels required by the specification, attributes such as crosslink density and surface porosity must be well understood. A initiative funded through the Strategic Environmental Research and Development Program (SERDP) was conducted to perform basic and applied research for CARC powder technologies. Prior to this, there was no active research to understand this correlation. As a result of the project, the technology has been successfully developed, trialed, and qualified.
In support of the corrosion initiative, CARC powders offer superior surface preparation and pretreatment prior to topcoat application. From a technology perspective, it is now understood the relationship that must exist between the structural formations of the coatings and the effect that each partition has on final film formation. The polymer construction is specifically synthesized in the formulation to maximize light scattering efficiency and cross link density, which results in the highly durable chemical agent resistant coating in a low gloss.
An important aspect of CARC powder technologies deals with system development rather than independent layer relationships. The initial release of the specification required compatibility with only liquid primers but has since been amended to include powder primers. One surprising note is the fact that the limited technologies developed in powder CARC to date indicate superior weathering compared to liquid coatings of the same technology after only two years of equivalent exposure (see table 2).
In naval applications, the use of powder coatings is also trending higher. The Navy’s newest vessel, the USS Gerald R. Ford (CVN-78)—the lead ship of a class of U.S. Navy supercarriers that is set for release in 2015—will have more powder coating than any other vessel in the fleet. With approximately 70 percent of the naval fleet already built, powders will primarily be used in repair and recoat activities.
And most recently, in February 2014, NAVSEA issued the performance specification MIL-PRF-24712B (see table 3). This specification is intended for non-immersion use as both single coat and combined primer-topcoat systems and is inclusive of four technology variations and four classes. NAVSEA determined that topside products that were approved by trade name in 1994 and remain in use today will be qualified. From an application perspective, ongoing improvements are in process for the use of powder coating louvers on naval vessels. Research testing indicates that a zinc rich powder and a triglicydyl isocyanurate (TGIC) powder may be installed in one heating step, extending the service life of the parts.
Opportunities and Trends
While the military evolves in its adoption of powder coatings, there are multiple opportunities for improvement and technology advances. Consider the topic of pretreatments; one of the potential opportunities for successful powder growth. The most common pretreatment technology in use today at military powder coating sites, specifically the job shops and depots, is zinc phosphate. As very few of these application sites have ferrous substrates, the optimal pretreatment is not being used, which can eliminate the opportunity to utilize powder or fail to deliver optimal performance. An enhanced pretreatment program can open the door for further powder usage.
Development of an enhanced touch-up system for powder coatings is another opportunity. There are multiple touch-up choices for liquid coatings, such as aerosols, cartridges or roll applications. The only option for touch-up on a powder-cured surface is with a liquid system. This combination, which can be acceptable depending on the technology choice, may also pose the risk for poor color match or performance loss.
While not yet covered under military specifications, newer technologies are being discussed or developed. The industry has enjoyed success with metal-rich epoxy primers in both liquid and powder primers. Zinc is the metal commonly used; as with liquid, the industry has an identified need for a metal-rich powder primer that will include choices outside of the traditional zinc. To this end, a military specification is in the planning stage to identify types for both liquid and powder technologies.
Use of high temperature powders is growing in the metal markets, and it is expected to potentially converge into some military applications. The silicone-based powders may see extended applications on areas such as engines and exhaust systems. Improved weathering characteristics are also a defined need; the powder industry is researching alternate resins to improve weatherability.
In terms of overall finishing trends, any effect that will shorten lead times and increase throughput while delivering specified asset protection is of interest in any military application. Specific to powder coating trends, temperature and film build ranges are focus areas. Lower temperature curing powders, ranging around 200° to 215°F, are desirable for metal and plastic substrates. Lower bakes not only save time and energy but reduce performance issues such as recoat and adhesion failures. Likewise, as previously identified, powder applications allow for higher film builds, which translates to improved corrosion protection. However, this also translates to higher costs for materials and bake; the industry is working to identify formulation processes and resin adaptations which will allow for lower films builds and smoother appearances.
As its role continues to become more defined and focused through specification and new product development, powder coating technology is key to military applications in terms of asset protection, cost control and environmental compliance while delivering perhaps its most important contribution— protecting the lives of the military men and women who serve the United States.
(1) Defense Management: The Department of Defense’s Fiscal Year 2012 Corrosion Prevention and Control Budget Request (http://www.gao.gov/products/GAO-11-490R)
(2) Department of Defense Qualified Products Database(http://qpldocs.dla.mil/)
Dr. Beth Ann Pearson is Global Products Manager, metal and plastics, Sherwin Williams Product Finishes. She can be reached at 216-566-3293 or via email at email@example.com