Adhesion problems don’t have a preference when it comes to the kind of damage they cause. Whether the end result is structural or cosmetic, adhesion-related failure headaches for manufacturers abound. For aircraft OEMs, one of the most crucial aesthetic operations is the topcoat of paint, although there are also very real structural risks when the topcoat does not properly adhere to the primer below it.
The visual story that a well-painted plane or jet tells is instrumental to brand image and customer perception. The top coat is also the last line of defense against all manner of fluids, corrosive chemicals, and environmental perils such as weather, acidic air quality, and so on. As a decorative and protective outer layer, the top coat needs to be pristine and resilient.
Changes to regulations of coating content and higher demands for longer-lasting paint jobs have forced all aircraft manufacturers to re-examine their topcoat processes. Using new cleaning products, new paints, and application processes comes with several challenges in preparing the surface, applying the paint, and achieving a finished appearance that exceeds the former techniques.
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The cost of redoing the painting of an aircraft is astronomical. Most adhesion issues require a complete removal of the finish and a total re-cleaning and refinishing of the affected area. With the proper controls in place to ensure that cleaning is performed so that the primer is fully prepared for the topcoat will alleviate a great many of the worries of aircraft manufacturers and operators.
New Requirements Mean New Processes
In the late 1990s, the US Environmental Protection Agency issued the National Emissions Standards for Hazardous Air Pollutants (NESHAP), which put restrictions on the amount of volatile organic compound (VOC) content that primers and topcoats can contain.
This meant that new primers and new paints needed to be ushered into the painting hangar. New solvents and cleaning methods were also employed to be in compliance with the NESHAP.
It was found that the new high-solids primers and paints (these have a higher resin content and lower VOC content) have an intense sensitivity to contamination. Conventional paints are more tolerant of contamination because the large quantities of solvents they contain help dissolve and distribute the contamination. With these inherent solvents removed for environmental reasons, the new paints are more susceptible to contaminants causing adhesion failures.
Defects caused by this sensitivity include dewetting and wrinkling. Wrinkling in the topcoat paint occurs when the paint layer forms a cured outer film while the inner layer stays liquid. Strong solvents reacting with the paint before application cause it to not cure evenly all the way through. Dewetting occurs because the primer is uneven beneath where the topcoat is applied. Bare metal often appears where the primer pulls back, and the topcoat doesn’t adhere uniformly. Dewetting is caused by how the new primers interact with the various and complex materials and chemicals used on the airplane.
When new materials are introduced to any coating or adhesion process, it is critical to understand how they interact with current materials and to ensure that the cleaning operations compensate for these new relationships. Methods of testing these materials and processes need to have a scope that includes new contamination possibilities and chemical interactions.
Testing Topcoat Application Needs to Translate From the Lab to the Line
With all these new methods and materials, a tremendous amount of testing has had to be done. Most of this testing was used to investigate the mechanical and chemical resistance properties of the new paints. These properties range from color stability to hydraulic-fluid resistance to adhesion reliability. A major hurdle to these tests, as with all laboratory-based performance tests, is that perfectly replicating the environments of planes while in service within the lab is impossible, and the instrumentation and tests utilized in the lab do not translate seamlessly to the line where the primers and paints are being applied.
When it comes to adhesion, the lab tests can provide recommendations for cleaning techniques, but if there is not a quantifiable and easy way to test that this cleaning has been done per the recommendations, then these instructions are moot.
For instance, lab tests are performed to identify potential appearance defects such as solvent pop and micro-blistering that occur because of either solvent breaking through the paint or carbon dioxide outgassing. Accounting for these possible defects during the actual coating process requires tests that are sensitive to these contaminants.
The new primers and paints have necessitated new cleaning methods that eschew the traditional aggressive abrasion techniques. Often an acid etch will be done, and then a water rinse is used to clean a surface before the primer is applied. Typically a light scrub of the primer is done before the topcoat application, and in both these cases, the tests to ensure proper cleaning has taken place is either a visual test or a water break test. A water break test involves rinsing the surface, observing how the water runs off, and if there are breaks in the water, then it indicates there is contamination. Additional cleaning might be done until the water sheets off completely, without any breaks. This thoroughly subjective test gives no sense of just how chemically clean the surface actually is and offers no quantitative evaluation of the surface's cleanliness.
What’s needed is a test like a contact angle measurement, which is sensitive to the top few molecular layers of a surface (where the adhesion-averse contaminants reside) and can be done identically in the lab and in factory production conditions. Contact angle measurements can detect the kind of chemical contaminants, like silicone - which is extremely common in manufacturing environments, that cause all the defects already mentioned, as well as “orange peel,” fisheyes, pinholes, and slug tracks. All of these are due to invisible contaminants that might only exist in a small area of the aircraft and will be undetectable if a visual or water break test is all that is used.
Topcoats cover the entirety of the exterior of planes, which includes rivets, fasteners, and joints that have tiny crevices that are very difficult to clean or inspect. Contact angle measurements are able to inspect these areas that are the most susceptible to adhesion failure and, ultimately, to corrosion. Additionally, these measurements are extremely accurate when it comes to verifying the uniformity of a coating. If a coating is not being applied uniformly over the material surface, it could indicate that contaminants are not being fully handled in the surface preparation process.
Contaminants can come from a variety of non-obvious places, including the equipment used to apply the coatings. Ensuring clean nozzles and spray guns can help reduce unwanted chemical species mixing with your paints and primers. Using contact angle measurements as a process control method throughout the entire adhesion process will allow operators to track when contaminants are being introduced and then eradicate the source.
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Control Cleanliness to Create Predictable Topcoat Adhesion
It’s important to remember that changes to one aspect of a manufacturing process have implications for how operations upstream and down need to compensate. When these changes are made, for any reason, testing needs to look closely at the chemical relationships between the materials. Furthermore, cleanliness testing needs to be sensitive to these chemical interactions and be able to be performed at the exact spot the topcoat is applied.
For more information about creating a process you can be confident will be free from adhesion problems, download our eBook: Predictable Adhesion in Manufacturing Through Process Verification