November 4, 2014
In reality, nothing could be further from the truth. We have found that the new PFOA-free formulations not only don’t have an adhesion problem, but many of them have improved adhesion because these new formulations benefit from the very latest in technology and knowledge.
However, it is true that some of the new PFOA-free PTFE coatings have significantly less tolerance for contamination, coating preparation issues, and surface preparation issues, especially at thin-film thicknesses. In-shop quality practices that were completely sufficient for previous PTFE formulations may need to be reevaluated for the newer formulations. There is a learning curve with some of these new formulations, and it’s obvious that many manufacturers – especially in the medical field – are being affected in a big way by the growing pains.
Fortunately, manufacturers don’t need to switch coatings or rethink things on an engineering level. Some simple in-shop fixes should do the trick.
Eliminate Premix Contamination from Acid Activation
A large number of PTFE adhesion problems can be traced back to this source. This issue is especially pertinent to coaters who use premixed primers such as those from DuPont, but it applies to everyone.
The problem has its root in a common practice: after spraying, coaters will often pour unused coating from their manual gun or from the line back into the container with the rest of the original coating. Even if the gun uses acid-resistant parts (such as plastic bottles and hoses), it is almost inevitable that a metal part somewhere within the gun will make contact with the coating. The metal activates the acid in the primer, and when this activated primer is poured back in with the rest of the unused coating – that whole contaminated batch of coating is much more likely to have an adhesion problem when it finally is applied.
There are two ways to fix this problem. The first one is never pour unused coating from a gun back into a container of virgin unused coating. Period.
The second way to fix this problem is not use premixed coating, and to mix a new batch of coating for each application. This means the coater can make exactly the amount of coating that is needed, and not worry so much about waste or keeping track of separate containers of unused coating from a gun.
Problems with the acid activator are likely to be at the source of many PTFE delamination or adhesion issues, especially if you are using pure PTFE. Without binding agents, the acid is necessary to etch the PTFE into the surface of the metal.
Redesign the Surface Preparation Process
At AIC, we are sticklers for providing the highest-quality surface preparation for every part. But some shops may reduce costs by using simple (and less thorough) surface preparations, which may have been totally adequate for some of the classic PTFE formulations.
Although many of the new PTFE coatings use the same binding agents as the old ones – PPS or phenolics, for example – they may not be as tolerant of surface preparation issues in some cases. It is much more cost effective to look at the surface preparation process than to start right off the bat with switching the coating used.
Recalibrate the Surface Profile
Many parts require grit blasting to create a profile on the surface of the metal that is favorable to adhesion. Testing and recalibrating the profile depth, coverage or uniformity may be needed for the new PFOA-free formulation.
There are a number of other in-shop issues that contribute to adhesion problems, that can be resolved without having to change coatings. Call us if you would like to discuss your part (even if you want to keep your current coater). We’re always happy to share information that can help.