And then there are disposal issues. The chromate in traditional strippers is a problem for the sewage companies as they have certain kinds of bacteria that can clean up the sewage. And if you put the chromated material in the waste stream it’ll cause a problem. If you can eliminate the chromate it’s much easier to dispose of this residue, Schoeder says.
As of right now the selectively strippable system hasn’t been approved for composites, according to Schoeder. “For the most part the strip and repaint process is being used on aluminum surfaces. The reason that Boeing and Airbus are so enthusiastic about this system while designing and testing their new aircraft is because of the larger amounts of composites. They want to be able to apply the stripper and have the intermediate coat on the whole aircraft including the composite parts.”
Right now airlines mask off areas that are composites that have the selectively strippable coatings on it. Composite areas like the vertical fin and body fairing don’t have to be stripped but they need to be sanded down. The airlines are waiting to get permission from the aircraft manufacturers to strip the whole aircraft including the composite areas. “The issue with composites is you’re not allowed to have strong chemicals like strippers come in contact with them as it can change the tensile strength, all the things that make it work structurally,” Schoeder says.
The selectively strippable system uses a weak stripper, one that has a weak activation agent. It could be acid, amine, or hydrogen peroxide. The normal levels shouldn’t come in contact with composites but if the level is reduced far enough it would allow the paint to be stripped off the composite surfaces.
“The theory is if you can take a container, say a quart can, that has the stripper in it and take some of the composite material and leave it in there for a certain amount of time, take it out, test it, and make sure it wasn’t affected by the stripper then you would know that even if there was a break in the paint film it wouldn’t ruin the composite if the stripper came in contact with it,” Schoeder says.
Manufacturers such as Boeing and Airbus want to go with a paint system that can be taken off with a mild chemical stripper on all parts of the aircraft including the composite parts, Schoeder says. And the airlines want a chromate primer so manufacturers can apply it in a very controlled environment and the airline would never have to respray the chromate primer. All they have to do is reapply the intermediate coat and topcoat.
“Airbus is committed to making information available in early 2006 for airlines to safely selectively strip composite parts,” Schoeder says. “The idea is you can strip the paint off composite parts if it has an intermediate coating; however, you have to use these particular strippers from the major stripping companies.” The aircraft manufacturers are working closely with coatings suppliers to find the best solution.
As the amount of composites used on aircraft increase with the emphasis on improved performance and fuel efficiency, the market is ready for a product that will fit the new structural requirements. The time and money saved by not having to sand and being able to use a weaker stripper in the process offer advantages to manufacturers and maintenance departments. The number of aircraft per manufacturer using the system, according to Schoeder, includes 200 for British Airways, more than 50 for Boeing, and more than 100 for Airbus. The topcoat alone has been applied to more than 10,000 aircraft worldwide.
British Airways is the recipient of the 500th new aircraft painted with the Desothane(R) selectively strippable aircraft exterior coatings system.