Pinholes and pinholing.

Pinholes constitute a common defect with many possible causes. Pinholes occur in waterborne and solventborne coatings, air dry and baked coatings. They all have small diameters or they would not be called pinholes. Many of them are deep; some of them are shallow (see Figure 1). Some of them are small solvent pops; others are small craters. Many of them originate in the coating where they appear. whereas others stem from an underlying coating or even the substrate.

Possible pinhole mechanisms include:

* Poor wetting of the substrate on application, poor flow, or both, Sprayed waterborne coatings that do not coalesce properly and give pinholes or incomplete coverage are said to suffer poor knitting. The thinner the coating, the more likely this is to happen.

* Air entrapment or foam bubbles that break, but do not flow out (see Figure 2).

* Solvent that forces its way out of a coating late in the bake.

* Volatiles from amino resin cure reactions, moisture-curable polyurethane crosslinking, and 2K waterborne urethane cure, especially if the reactions are rapid [see "More on Popping," JCT CoatingsTech, 3 (11), 120 (Nov./Dec. 2006)].

* Gassing from the substrate. Examples include rupture (hydrogen evolution) in cationic ED coatings (Figure 3), galvanized gassing (hydrogen, methane) from electrogalvanized steel, and volatiles (air, solvents) from plastics.

* Waterborne basecoats that are dehydrated too much or too little. If the former occurs, the basecoat is likely to be porous, will absorb solvent from the clearcoat, and then blow it out through the semi-solid coating. If too much moisture remains in the basecoat, it will blow out during the final bake and make holes in the film.

* Thick spots or bands due to irregular spraying or flow from edges, protrusions, or style lines may pop or pinhole on baking.

With all these possible mechanisms, how do we identify the root cause? That is often difficult, but there are things that allow us to narrow down the choices. The first step always is to look at the defects at low power (10-50X) with an optical microscope, preferably a stereo microscope. You should be able to tell whether they are deep or shallow and their approximate diameter. If they look deep, the next step usually is to cross section a few pinholes and look at them under a higher power microscope (100-400X). That will enable you to see how far down they go and whether there are defects below them in the undercoat or substrate. Because the defects are small, it may take a little practice to avoid losing them by over-sanding or polishing. Measure the film thickness in the pinhole area via the cross sections or by other means. Apply the paint over a range of thicknesses (wedge panel). Is pinholing worse where the film is thin? If so, insufficient flow and/or wetting may be the problem. If the paint is waterborne, try adding a surfactant or a little low surface tension solvent, reapply, and see if the pinholes have gone away. If the thicker films pinhole (which is what usually happens), then prevention depends on applying thin films. Keeping the surface open with slow tail-end solvents also should help.

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To determine whether the paint has a tendency to foam or trap air (mainly waterbornes), stir it, shake it, or put it in a blender, then pour it into a graduated cylinder and see if the bubbles dissipate rapidly, slowly, or not at all. If there is a problem, adding a defoamer may help and reformulating with low foam surfactants should be tried. Volatiles from cure reactions can be controlled by applying thin films, keeping the film open, and slowing down the cure reaction. Keeping relative humidity below 70% helps keep carbon dioxide evolution from waterborne 2K urethanes from causing problems.

For basecoat/clearcoat systems, observe how the base/ clear combination responds to lowering the temperature of the dehydration bake and also raising it. The results may point to a possible means of reducing or preventing the problem. If certain spots or areas are too thick, work with the customer to better control the application.

By Clifford K. Schoff, Schoff Associates