Plasticizers and extrusion agents.... the scourge of hydrophilic coatings.
Every hydrophilic coating has a certain set of substrates that it coats well, and other substrates that are not so good. Exactly which substrates a given hydrophilic coating sticks to varies from company to company because it all really depends on the ingredients in the coating, including the solvent and other additives in there, not to mention the chemical makeup of the substrate. Since every company out there has a different formulation that interacts differently with various surfaces, results can be different. One company's coating may coat polyurethanes really well, whereas another company's hydrophilic coating might not stick so well to that particular polyurethane.
Ok, so be it. When searching for a coating, you should be aware of this fact. However, it can be more complicated.
In a previous post about hydrophilic coatings on PVC, I also mentioned that plasticizers can be a problem. This is true of any other material, not just PVC. Pretty much all commercially available plastics out there contain plasticizers, especially if they have a prescribed durometer. Who knows what these plasticizers are? Many of them are proprietary, like the ones found in the various types of Dow's Pellethane, which is usually a polyurethane-based material. Like the case with PVC, these plasticizers can bloom to the surface while your coating is curing or while your substrate is soaking in a solvent.... or they could just simply be present at the surface to begin with, without any prompting. Also, like the case with PVC, these plasticizers can knock the coating off the surface, i.e. drastically reduce adhesion.
Thus, Polyurethane A may be chemically identical to Polyurethane B, but the "B" polymer might contain disruptive plasticizers that cause problems for the coating, whereas "A" may not have any that interfere. Moreover, when you go from one hydrophilic coating vendor to another, you might find that "B" coats fine and "A" is the problem now.
Extrusion agents, which sometimes include certain waxes, are equally troublesome. They can cause the same problems as plasticizers. The processing of the plastic can drastically alter the distribution of extrusion agents and/or plasticizers in the material causing weird things to happen. The other day, I saw a piece of polyurethane tubing which was extruded in a metal mold, and then had a lumen created in the center with forced air. The outside of the tube coated beautifully! The inside of the tube was an absolute pain in the rear to coat consistently, even when controlling for un-related factors that occur with inner diameters! It was like night and day. The best explanation for that was that since the two surfaces were formed differently (OD vs. ID), they had different distributions of extrusion wax in them, and the ID side had too much for getting a good coating. I was amazed.
So, if you are developing a product and you run across this phenomena, you have two choices:
1) Seek another material for your device
2) Seek another coating vendor that may not have that problem with that particular material.
Taking the second option may be tough, however, because it is a general rule of thumb that these plasticizers will cause problems. Of course, changing your device material mid-stream is bad too, which is another reason why you should always select your coating vendor as early as possible in the design process.