PAINTING POLYPROPYLENE

We are working with flame pretreatment but are seeing inconsistencies. Adding to our confusion is the apparent lack of consistency from dyne solutions—we see wetting out of the dyne solutions from 35–70, but then the coating, which is a 38 dyne, doesn’t stick consistently.

Q. I am the manager in charge of finishing and decorating operations at our plant. We are having an adhesion issue with one of our products. I read a few of your responses on PFOnline (www.pfonline.com) and have been impressed.

I would appreciate your thoughts on the following: We are working to achieve a variegated appearance on mineral-filled polypropylene exterior siding, the substrate being injected molded. Our objective is to use a single-part, water-based coating. Our issue is adhesion of the coating on the multifaceted surfaces. We are working with flame pretreatment but are seeing inconsistencies. Adding to our confusion is the apparent lack of consistency from dyne solutions—we see wetting out of the dyne solutions from 35–70, but then the coating, which is a 38 dyne, doesn’t stick consistently.

We were concerned about contamination, so in our tests we have been wiping the polypropylene parts down with isopropyl alcohol. Obviously, there is a great deal more background to this issue, but this is the simple overview.
Thanks for any help you can give us. M.B.

A. In flame treating, the flame must impinge on the surface to be pretreated. I can’t imagine that parts of the multifaceted surface of your product (exterior siding) are hidden from the flame (i.e., the flame can’t reach or touch them), but it may happen. Perhaps the isopropyl alcohol wipe is not helping. Instead, it may be adding to the problem by introducing and spreading contaminants. The other thing about isopropyl alcohol is that it evaporates quickly and may not wet the surface long enough to clean. For solvent wiping, I prefer using mineral spirits, changing wipers and solvent often. Furthermore, I suggest you read the comments by R.B. in “Plastic Painting Issues” elsewhere in this column for other possible solutions to this problem.