A lot goes into painting an aircraft. This massive, multi-stage process begins long before that final, shiny topcoat you see on the wings and tail is applied. The components are molded, then they’re prepared (which is often a multi-step process in-and-of itself), then they’re assembled, then there are likely several layers of coatings and primers applied to the surface to make sure the finished aircraft is completely ready to fly.
So how do you know if all of these steps are truly moving towards a reliable and beautifully painted aircraft? And in particular, how do you know that the primer coating is truly going to stick to those metal or composite components?
Manufacturers in the aerospace industry know they need to have very specific kinds of primers, paints and coatings, but do they know the other half of the bonding equation--the chemical cleanliness of the surface being painted or coated? Only controlling 50% of material system leaves a huge gap and major vulnerabilities in the process; things that need to be avoided at all costs when we’re talking about flying vehicles that move humans around.
In our experience, many manufacturers do not have a handle on their surface preparation processes to ensure predictable and flawless adhesion for coatings. This isn’t due to a lack of trying either. Until now, there hasn’t been a reliable way to verify that the surface preparation is actually going to result in a strong bond between the material the aircraft is built out of and the paint or coating being applied to it.
The multi-faceted process of constructing aircraft spans several disciplines and, usually, multiple locations. And all of these parts of the process have implications for the adhesion of the final product.
For this article, we’re just going to focus in on the primer coating and what leads to it properly adhering to the body of the plane or jet. It’s helpful to break out this one aspect of the process because the complexity of getting it right is indicative of the manufacturing process at a macro-level. Getting primer coating to stick to an airplane is an entire adhesion process all on its own, and it’s only one step!
Aircraft components are typically extruded aluminum or, more and more, molded composite. These parts need to be cleaned of mold release and debris that come from the fabrication process. Usually this is done with a solvent bath and perhaps some wiping down of the parts by hand as well.
We have written before about what can be overlooked in the process of washing parts in industrial baths. Check out those articles here. The most important thing to remember is to measure the surface quality before and after these cleanings to get an accurate look at how the surface is changing (hopefully for the best).
Additional pre-primer steps might include chemical etching and plasma treatment to prepare the components for adhesive assembly. Once the aircraft is constructed, further surface treatment needs to be done before applying the primer. This has been made much more feasible with automated atmospheric plasma treatment. These systems allow treatment directly on the line and can even get into difficult to reach places such as countersunk rivets that, if not properly coated, can be a starting place for corrosion that compromises the entire top-coat paint.
By the time the aircraft is at the priming stage the material surfaces have been through a lot and have had numerous opportunities for the chemical composition of the surfaces to be compromised along the way. Preserving these surfaces and validating their chemical cleanliness is imperative to maintaining the consistency of the primer performance.
Traditionally the closest thing to a surface quality test manufacturers use is the water break test. This is generally unreliable due to its subjectivity and lack of sensitivity to the chemical make-up of the surface. The fact that it has to be done on samples and cannot be adequately employed directly in the production setting makes it particularly disruptive and unhelpful in the aerospace industry. The water break test needs to be done on large, flat surfaces and, as we’ve already pointed out, the places that need to be inspected the most have tiny geometries and are not uniform.
Here’s an article that goes over all the ways that the water break test is an out-moded and frustratingly deficient method for understanding surfaces.
As opposed to leaving the understanding of surfaces up to guesswork, you can actually get an accurate and quantifiable metric of how ready your surface is to have the primer stick to it.
Water contact angle measurements are a perfect way to assess the chemical state of surfaces prior to treatment and prior to coating. Measuring the wettability (i.e. how the drop of water interacts with the surface) of the material surface gives you a definable and objective picture of the preparedness of the surface. Measuring first and measuring often also allows you to know precisely where contamination is being introduced or where a surface preparation step isn’t changing the surface as expected.
Looking at how water contact angle measurements correlate to crosshatch tape tests shows just how sensitive and reliable these measurements are. This correlation is especially helpful in the application of painting and priming aircraft because the crosshatch tape test is often used in laboratories to assay adhesion in these . Download the white paper linked to above to learn about the relationship between water contact angle measurements and the tape test.
For a brief summary of the relationship take a look at these charts:
Being able to measure real parts in a real production setting is crucial to saving every resource. Water contact angles can be measured directly on the parts being cleaned and coated and can even be used to inspect the small and complex geometries that are impossible for other test methods. Most surface preparation inspections and tests are done in a laboratory or are done on a part that needs to be scrapped once the part is tested upon. Water contact angle measurements eliminate this problem as they are non-destructive and can be collected right on the part that is actually going to be coated.
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