Surface tension and surface energy.

I have blamed surface tension for many of the coatings defects that I have discussed in this column, including craters (Figure 1), poor wetting/dewetting (Figure 2), convection flows, telegraphing, and fat edges. So, I thought that I should stop and discuss this property of materials in more detail. I also will say a little about how to measure it and what to do when it needs to be changed to help solve a problem.

The terms surface tension and surface energy are often used interchangeably. They are the same numerically, although they have different units. They are measures of an effect that arises from unbalanced molecular cohesive forces at a surface that cause the surface to contract and behave like a film or membrane (which is what allows a water strider to skate around on water). The surface molecules have higher free energy than those in the bulk material. The excess free energy is what we call surface energy, which has units of energy/unit area such as Joules/[cm.sup.2]. However, what we measure is surface tension, the force necessary to break a film of a given length, usually in dynes/cm or Newtons/m. High surface tension liquids include (values in dynes/cm = mJ/[m.sup.2]) mercury (484), water (72.8), and ethylene glycol (50). Low surface tension liquids include ethyl ether (17), n-hexane (18.4), ethanol (22.4), MEK (26), and xylene (30).

[FIGURE 1 OMITTED]

How do we measure surface tension? With wet paints, resin solutions, and other liquids, we can measure the force exerted by the liquid as we detach a platinum ring or plate (a microscope cover slip also works well) from its surface or determine the pressure needed to form a bubble in the liquid. Details of these techniques are described in many publications and can be found on the Internet. Solids also have surface energies/tensions, but the measurement of solid surface tensions is more difficult than that of liquids. Polymer surface tensions have been determined by a variety of methods, including extrapolating values for polymer solutions to 100% polymer and extrapolating results on molten polymers to room temperature. However, most paint substrates cannot be dissolved or melted, so other techniques must be used. Most methods involve measurement of contact angles with two or more liquids of known surface tension on the surface of interest, followed by substitution of values into an expression such as the Owens-Wendt-Kaelble equation (see the JOURNAL OF COATINGS TECHNOLOGY, 71, No. 888, 56 (1999), and other publications).

For reasons of paintability, recoatability, good adhesion and prevention of surface defects, it is important for paint and substrate surface tensions to have similar values, with the paint having a lower surface tension than the substrate to insure wetting (see the October 2004 "Coatings Clinic" for a discussion of wetting). In an ideal world, paints would have low surface tensions so that they could wet any substrate, pigment, or contaminant, then dry so that their films had high surface tensions so that they could be easily wet by the next layer of paint. Unfortunately, this scenario is difficult to produce, so we often end up somewhere in the middle. A much worse problem, however, is that of contamination--dirt, overspray, oils, silicones, etc. Such contaminants usually produce small low surface tension points or spots scattered over the wet paint or substrate. For example, a very small quantity of mineral oil or silicone fluid in the form of a vapor or aerosol can cause a large number of craters. Contaminants are difficult to identify and remove because they usually are present in small quantities and rarely affect results of surface tension measurements. Sometimes contamination occurs at the paint plant or in transporting of the paint, but it is more likely to happen in the field or on a customer's paint line. It usually is not suspected until a problem occurs.

[FIGURE 2 OMITTED]

Prevention of surface tension related problems depends on cleanliness at the work site or on the paint line, good surface preparation, viscosity control, as well as on paint properties. Often, low surface tension solvents and/or surfactants must be added in order to prevent defects. However, take care with adding surfactants, as many impart water sensitivity and others (especially silicones) hurt recoatability. We usually screen additives by measuring their ability to reduce the surface tension of the paint in the can. We also can measure what the additives do to the surface tension after the paint has dried or been baked. Unfortunately, there is not yet a practical method for measuring surface tension during drying and baking, which is when defects occur.

"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences and/or photos of coatings defects to Cliff Schoff, c/o. "Coatings Clinic," Coatings Tech, 492 Norristown Rd., Blue Bell, PA 19422; or email [e-mail]