ZINC-RICH PRIMER

 Q. I am the chief chemist at a paint plant. One of our customers wants a finish system using a zinc-rich primer. He also wants to use his existing TGIC-free topcoat. This question is in regards to issues we face with a zinc-rich primer and TGIC-free top coat. The coating thickness is 80 to 100 µm. After applying both the coats the painted test panels are failing mechanical properties like the mandrel bend test. It is also failing the falling ball impact test at 40 kg-cm. We are trying to resolve this issue. I would appreciate it if you could help with some solutions. Thanking you in advance. V.M.

A. I am not surprised that the mechanical properties of your finish system using a zinc-rich primer are so poor. You didn’t indicate the type of zinc-rich primer used in your finish system.

Looking at your test results, I suspect it is an inorganic zinc silicate, which is far less flexible than the organic type. Using this primer, I would expect your impact values to be less than 20 and perhaps as low as 16. Substituting an organic zinc-rich primer for the inorganic would probably increase the flexibility of your finish at least system two-fold.