Mis-Steps to Avoid When Performing a Dyne Test

Published: February 03, 2025

Certain Precautions Increase Likelihood of Reliable Results

By Russ Smith, President, Diversified Enterprises

The dyne test offers a quick, inexpensive way to determine the wettability of polymers, foils, coated paper, composites and other materials. Developed in the early 1960s, it has been widely employed ever since the original publication of ASTM Standard D-2578 in 1967. It is commonly used to evaluate corona, flame or plasma treatment levels.

The test is relatively simple, but the test fluids are sensitive and the interpretation of results is somewhat subjective. As such, a variety of mistakes may be made that can affect or invalidate results. This article will address those, but first a brief description of the test method is in order.

Dyne testing is based on the behavior of a liquid spread out on a solid surface. If the liquid has a surface tension (dyne level) lower than the critical surface tension of the test sample, it will tend to stay wetted out. Conversely, if the liquid has a higher dyne level than the surface, it will shrink back, and bead up rapidly. The tester starts with a low surface tension test fluid, and works upwards until the dyne level is reached that wets for only a couple of seconds. This transition point indicates that the two phases are in balance, and is the measure of the sample's wettability in dynes/cm.

The two most common missteps are closely related: Starting at a high dyne level and working downwards, or continuing the test at dyne levels higher than the wetting/beading transition point. In both cases, one can encounter spurious wetting. For example, it is not unusual for a test fluid of 50 dynes/ cm to wet out corona activated film that actually has been treated to only 38 dynes/cm. I have never found an explanation for this, but suspect it is related to liquid-solid solubility issues. The takeaway, however, is simple: Start at a low dyne level and work upwards until the transition point is determined and stop! You have completed the test and found the answer you are seeking.

Using too much test fluid can also affect results. Wetting of a liquid on a solid can be imagined as a balance between surface forces and gravity. If you pour enough water on a teflon sheet, gravity will force it to wet the surface for some time. But a thin film of water will bead instantly, as surface forces will dictate the interaction. Use as little test fluid as possible. If you use swabs to apply the fluid, apply only a few drops of test fluid to the tip of the swab. If you are using a valve-tip test marker be sure to evaluate only the final pass, which applies the minimal amount of test liquid.

The pressure of the applicator - whether from a swab or a test marker - is an issue in and of itself. Too much pressure can physically change the surface as you test it, especially with treated films or contaminated surfaces. Always use a light touch.

Dyne solutions are very sensitive reagents, and their surface tension is readily altered. Specific things to avoid include the following:

• Leaving bottles open, which allows evaporation and/or introduction of airborne contaminants, both of which will change the surface tension.

• Re-using an applicator swab, which can introduce surface contaminants or sur face-blooming additives into the dyne solution.
• Similarly, failing to properly flush the tip of a valve-tip test marker to feed fresh, uncontaminated test fluid to its surface.
• Applying dyne solutions with brush-cap bottles. This not only introduces contaminants from the sample surface to the dyne solution, it also transfers too much fluid to the surface.
• Using dyne solutions which have passed their expiration date. Chemical reactions will cause the surface tension of these reagents to drift over time, even if kept securely sealed under laboratory conditions.
• Using dyne solutions which are not at ambient temperature. Some users refrigerate the reagents in an effort to extend their shelf life. This may be beneficial if they are stored away for weeks and allowed to stabilize before use, but if they are applied cold, their surface tension has increased and test results will be skewed accordingly.

Temperature also affects the surface energy of the sample. If it must be tested at an elevated temperature, care must be taken in interpreting results, as the surface energy of solids decreases as temperature increases. In a shop which is not climate-controlled, as long as the test fluid and sample are both at the elevated ambient temperature, results will usually not be affected materially, as both the liquid and the solid will have experienced similar changes and remain in balance for the purpose of this test.

The sample surface to be tested must be representative of the lot from which it is drawn. To avoid contamination, the area to be tested must not be touched. Skin oils are of low surface tension, and can radically affect results. In the same vein, the surface should not be cleaned in any manner prior to testing - this will alter the surface chemistry, and the sample will no longer reflect the entire lot. The difference can be notable. In fact, the dyne test is often used to assess the cleanliness of metals based on this principle.

When testing webs, as a minimum the test should be performed at three locations across the web. Testing at only one location can give an unrealistic measure if the treat level is not uniform.

The area to be tested must be relatively flat, and oriented such that it is horizontal. Otherwise, gravitational wetting or de-wetting will affect test results. An example of this would be testing a web on-machine in a location where the treated side is on the bottom.

There are other mis-steps that can be taken which are less com-mon. I'll list those briefly.

Never retest the same spot on a sample. Once the test fluid has been applied, the surface has been altered. The dyne test is applicable only to materials which are non-absorptive, and are not soluble in the chemicals used in the reagents.

Do not assume that the surface energy of the material will stay constant, notably in the case of corona treated films, which can lose treatment surprisingly fast, especially if they contain slip agents. Do not assume that the results of the dyne test will correlate perfectly with contact angle measurements - the two tests are based on different principles and use different probe liquids. The same caveat applies to results obtained with reagents formulated per ASTM D-2578 vs. alternative formulations.

Finally, when communicating results with others, take care that any pictures shared have been taken at the 2 second timeframe specified by the test standard. A photo taken 15 seconds later no longer documents the relevant test result.

This discussion has covered the overwhelming majority of errors in technique or interpretation that I have encountered over the years. Keep these cautions in mind, and the dyne test should provide useful, reliable and replicable results.

About the Author

Russ Smith formed Diversified Enterprises - the first business to focus specifically on applications of the dyne test - in 1986, and has served as President of the company ever since. He has over 40 years of experience in the fields of surface treatment and analysis, and deals with technical inquiries from customers worldwide on a daily basis. Russ is a member of ASTM, the Society of Plastics Engineers, the American Chemical Society, the American Society for Quality, the American Association for the Advancement of Science and TAPPI.