Effect of Surface Roughness on Dyne Testing

Question: We have a customer that uses your test markers to measure the surface energy of aluminum before bonding it to a composite. They find the test works great on a smooth surface, but on rougher surfaces they are unsure of results, and don’t think they are accurate. Is there a rule of thumb on how smooth a surface needs to be for surface energy testing?

Answer: I don’t know that you could really come up with a “rule” as far as roughness goes, but there is an obvious problem with textured surfaces: the test fluid will tend to settle into the “valleys” so wetting vs. beading will become more and more difficult to gauge as the roughness increases. This will be as true for plastics, composites, glass, and other materials as it is for metals. It’s important to keep in mind that the dyne test is based on the behavior of a retreating liquid/solid contact line. When this line is on anything other than a horizontal plane, gravity comes into play, either aiding or abetting the retreat of the liquid.

One thing is certain – the testing of this material should be done with bottled test fluids, applied as lightly as possible (in terms of both amount of fluid used and pressure applied) with a cotton swab. Test markers really will not be controllable enough, and will tend to flood the valleys with test fluid. Also, as testing of metals is usually performed to evaluate surface cleanliness, test markers are not a good option, as surface contaminants can affect results (for details on this, please see our discussion here).

Using a strong magnifier will be helpful – I’d look for signs that the fluid is creeping away from even the valleys, and tending to aggregate in micro-puddles rather than coating the entire low area. This discernment may be easiest at the perimeter of the test area. Also, while it may be rather interpretive rather than an absolute indicator, if the high spots on the surface retain a thin film of test fluid, that is a strong suggestion that wetting has been achieved.

Testing PET for the Presence of a Silicone Coating

Question: We convert silicone-coated PET, and test it to make sure we are working with the coated side. At present we’re testing with 30 dyne and 50 dyne inks. Any comments on this, and do you have a test procedure for this application?

Answer: First, the test procedure, which covers virtually all polymers, is available at https://www.accudynetest.com/qctest.html.

Untreated, uncoated PET generally has a surface energy of about 43 dynes/cm. As such, if the PET surface has not been modified, I would expect the 50 dyne/cm test fluid to bead more or less instantaneously when applied to the material. The 30 dyne/cm test fluid would wet out for a long period of time – permanently as likely as not.

For silicone-coated PET, it is obvious the 50 dyne/cm test fluid should bead instantly, as silicone compounds have surface energies ranging in the 20s. In most cases the 30 dyne/cm test fluid would do the same, but some formulations may result in a surface slightly higher than 30 dynes/cm, so some short-lived wetting may be observed.

I would suggest testing at 36 dynes/cm. Print-primed PET has a surface energy of about 38 dynes/cm; virgin PET is at 43, and surface-treated PET should be at 48 dynes/cm or higher. All these surfaces should show wetting for at least 4 seconds, and perhaps even permanently, when tested at 36 dynes/cm. The presence of any silicone compound would cause immediate beading at this dyne level.