People treat coating substrates so the coating will wet and adhere to the solid substrate. To achieve a stable coating, the surface energy of the substrate needs to be higher than the surface energy of the coating. But what is the surface energy of a solid? There is no definitive way to measure surface energy, but there are numerous ways to estimate it.
Dyne pens for surface energy measurement
To quote the surface energy of a film or foil (e.g., on a COA), you need a standardized method that works for your system. There are no perfect measurements of surface energy; however, there are practical ways to estimate it for different systems. Why care about surface energy? It determines if the liquid coating wets the substrate – and is used as a surrogate to verify the dry coating will adhere to the substrate.
The most common method uses Dyne pens (markers filled with blends of formamide and 2-ethoxy ethanol specified in ASTM D2578, “Standard Test Method for Wetting Tension of Polyethylene and Polypropylene” plus a dye to aid visualization). The standard method is to draw three stripes at three points across the web and see how long it takes for the stripes to break up. Ignoring the first stripe (to eliminate any effects of contamination), the ink that takes between one and three seconds to break up gives the wetting tension. Why three seconds? Because 2-ethoxy ethanol is 98 times more volatile than formamide, so the surface tension of the ink goes up quickly (the rate depends on the wind speed in the room).
Then there is the contamination issue: Although the standard method ignores the first stripe because it could be contaminated, there is no guarantee that the contamination will be gone after one stripe (badly contaminated pens should be discarded). The ASTM method involves dropping the pure liquid blend onto the substrate to see if it wets or not. Also, this method is intended for polyolefins – not polyesters, metal foils, etc.
Introduce polar groups
Polyolefins are non-polar, but surface treatment introduces polar groups on the surface. For polar substrates, the common approach is to break the surface energy into polar and non-polar components using the Fowkes correlation, per ASTM D7490. This has been automated with both laboratory and production-line instruments. If you are coating polar substrates or are optimizing surface treatment, the polar component should be tracked.
Dyne pens cannot determine the polar surface energy, but they may be fine for quality control (if you get the same number you always get, then expect the same adhesion you always get).
But, there are complications
Bumps on the surface can pin the liquid, preventing dewetting and confounding either ASTM method. When coating rough substrates, your surface-energy QC may be measuring variations in surface texture.
Some substrates have thin layers of material that may dissolve as you are trying to measure wetting (e.g., lubricants on metal foils and exudate on plastic film). In these cases, all you can do is detect changes, which opens up simpler approaches.
There are many solvents with different surface tensions and solubility for contaminants. If a bead of pure solvent with a surface tension of 5-6 Dynes/cm lower than the coating just wets the web, that is a fairly robust (time-dependent only if you are dissolving something) indication that this system will lay down well and adhere like normal.
E.J. (Ted) Lightfoot, Ph.D.
716-449-4455; TedLightfootLLC@gmail.com
ARC Member; ARC TV Presenter;
ARC R2R Presenter