Conventional wisdom has it that PVC is not paintable. Is that a reasonable statement? Had Westeck not already been painting uPVC for ten years we’d say the same thing. So: Can PVC be painted?
The fact is that uPVC and cPVC (PVC from here on) are difficult to paint. The reason is, is that PVC has a low surface energy. All materials and liquids have a “surface energy”. The scientific unit of surface energy is Dynes/Centimeter. For purposes of discussion we will talk about the surface energy, SE, of water and how it relates to PVC.
Water has a surface energy of 72DYNE (DYNE is short for Dynes/Centimeter) and PVC has a surface energy of 41DYNE (list of the surface energy of various polymers). Since water has a higher surface energy (SE) than PVC, it will bead up and roll off like: “water-off-a-ducks-back”. In order for the water to “stick” or “wet out” onto PVC, PVC has to be treated to rise the SE of the PVC above the SE of water. Can this be done? By the average consumer? ...well…no…but don’t give up yet, and read on… The average consumer can do two things to increase the SE of PVC, the first of which is to sand the surface lightly. This will remove the surface waxes to the degree and create more area for the water to wet out. We recommend you use Green Scotch Brite because sand paper will load up very quickly with the surface waxes on the PVC, this raises the SE. The second way to raise the SE is to wipe the PVC with Acetone. Acetone is detrimental to PVC. If one were to immerse PVC in Acetone, it would break down the molecular structure over time and destroy the PVC. This, however, can be a benefit when preparing PVC for painting. The Acetone does two things: (1) it swells the surface making it slightly more porous and (2) it disperses the surface waxes that are inherent on the surface from the extrusion process when it is manufactured. So, Acetone will also raise the SE of the PVC.
There is another piece of the equation that has to be considered. Even if a consumer prepares the surface as described, the SE of PVC will never reach the SE of water (with these techniques) which is 72DYNE and with all the best preparation, the SE of the PVC will be variable due to the waxes that are external and internal to the PVC. To combat this, paints made of a blend of acrylic and polyurethane, (i.e. special latex paints) have been formulated with additives to reduce the SE of the liquid paint. These additives reduce the SE of PVC paints down to 34DYNE which is lower than the untreated SE of PVC.
In theory one would not have to treat the PVC if the paint has a surface energy below the PVC; but there’s a problem with theory – reality. Empirical results suggest a combination of good surface preparation and a paint coating with a low SE can result in good adhesion of paint to PVC.
There is a lot more to it than that: the paint formulation is actually a cacophony of chemistry. Every micro ingredient and every pigment change can change the adhesion of the liquid paint to the PVC substrate but that’s beyond the scope of this article. Suffice it to say that the paints developed for PVC niche have been tested for adhesion on PVC each and every time a change is made to the formulation.