Testing Metals for Cleanliness

Question: Almost all of the material on your site regards polymers. Can you offer guidelines for using the dyne test to evaluate the cleanliness of metals?

Answer: You are right. For quite some time, people in the metal-processing industry have understood that the surface energy of a metal is a good proxy for the efficacy of the cleaning process. The reason for this is simple: Contaminants are of lower surface energy than the underlying metal surface. Therefore, contaminated surfaces wet poorly, and the degree of contamination can be determined by the dyne test. This application of the test has become quite widespread over the past several years.

If all you need is a test procedure, it may be best to go directly to this page. If you have an interest in a somewhat deeper understanding of the issue, please read on.

Published data for various metals document very high surface energies — typically over 100 dynes/cm. Yet, most specifications for actual manufacturing processes are in the range of 34 to 50 dynes/cm. The reason for this is that the theoretical dyne levels are achievable only in a vacuum. In an oxygen-containing environment such as air, the metal’s surface oxidizes in an instant. Thus, in the real world, the values we are measuring are the surface energies of the oxides that naturally form on the metal’s surface.

A good way to look at the surface energy of metals is to consider what I’ll call the “practical limit of cleanliness” (measured in dynes/cm). Ideally, this should equate to the dyne level of the metal oxides that have formed on the surface. In practice, it will generally be a bit lower than that, as it is essentially impossible to remove all traces of contamination. (Please note that this discussion does not address “cleaning” methods such as plasma treatment or aggressive etching with acids or bases, as these methods can significantly change the molecular constituency at the surface — it is based on more traditional methods such as the use of surfactants or solvents which will solubilize and lift off contaminants without interacting with the underlying surface.)

If you are operating the cleaning line for your process, rather than purchasing materials which are cleaned off-site, finding this practical limit of cleanliness can be achieved empirically by running a designed experiment: Vary the process conditions and cleaning agents, and evaluate the effectiveness of each combination by means of the dyne test. As long as the cleaning line is functioning properly, the highest recorded dyne level is a reasonable estimate of the practical limit.

Reaching the practical limit of cleanliness will usually increase costs and environmental impact. Fortunately, in most cases this is not necessary. Most adhesives, sealants, coatings, inks, and other liquids used in industry are formulated to solubilize and incorporate a small amount of organic material present at the liquid—solid interface without adverse effects. So, the key is to find the optimal degree of cleanliness. This equates to the lowest dyne level which will meet end-use criteria. In some cases this could allow a considerable residue of contamination; in others, the demands placed on the final product may dictate that the surface energy be very close to its practical upper limit. Finding this optimum dyne level can be readily accomplished by correlating dyne test results vs. results from final product performance evaluations.

As you can imagine by now, the range of required dyne levels will vary considerably from operation to operation and from alloy to alloy. However, in most cases, a surface energy, as determined by the dyne test, of about 35 to 45 dynes/cm is generally acceptable for most processes. As an example, for stainless steel, a surface energy of 32 to 36 dynes/cm likely reflects a poorly cleaned surface, adequately cleaned surfaces will fall in the range of 38 to 42 dynes/cm, and well-cleaned material will fall in the 42 to 46 dyne/cm range. But please do keep in mind that empirical testing is the only way to find the optimal level for your specific material/process requirements.

So, what are the “nuts and bolts” of dyne testing metal surfaces? A detailed test procedure is available here. Briefly, the following points are all important:

  • Cleanliness testing of metals should always be done with bottled test solutions, applied with cotton applicator swabs. We recommend dropper bottles, which can apply a standardized amount of test fluid to each swab. The smallest amount of test solution that can practically be applied is ideal: surface energy is a two-dimensional property. Applying too thick a film of test fluid can cause gravitational spreading, which will result in higher dyne level readings.
  • Do not touch or in any way contaminate the surface to be tested.
  • Do not use contaminated test fluid; dispose of it immediately and use a fresh batch.
  • Dyne pens should not be used for cleanliness testing: Even ACCU DYNE TESTTM Marker Pens will end up with contaminated tips and produce errant results if used on oily or silicone-contaminated surfaces.
  • A fresh, unused swab must be used for each application of test fluid.
  • Each test must be performed on a previously untested part of the sample.
  • The test should always start from a low dyne level — one that you expect should fully wet the surface. As long as that dyne solution stays wetted out, and does not shrink in and form beads within two seconds, the test should be repeated at the next higher dyne level (if the test fluid beads immediately, drop down at least four dynes/cm, and start the test over).
  • Always keep bottles securely capped when not in use. Evaporation of the two test fluid constituents will not be equal; the 2-ethoxyethanol will evaporate preferentially, increasing the surface tension of the test fluid.
  • Dyne level testing for cleanliness should always be done at relatively standardized sample, ambient, and test solution temperatures. We recommend testing within a range of 15°C (59°F) and 30°C (86°F), and 35% to 70% relative humidity. Ideally, the range would be 20°C to 25°C and 40% to 60% RH. We will post another entry in this blog that will address this issue in more detail as soon as possible.
  • If the part that is tested must be retained for subsequent processing (as in testing expensive aerospace components, e.g.), the test area should be cleaned with isopropyl alcohol immediately, then sent back through the cleaning process for a second pass. We also recommend re-testing this location vis a vis a contiguous location on the same part to ensure that the dyne test did not permanently alter the surface.
  • For large-scale test programs, one master trainer should directly train all testers to ensure uniformity of technique.

Note: We strongly recommend testing over a range of increasing surface tensions, but it is possible to use the dyne test for metal cleanliness as a simple go/no go evaluation, as long as all protocol is rigorously followed. The advantage of testing over several dyne levels is that you will see a progression towards wetting as you approach the actual dyne level of the sample. This provides a cross-corroboration of the accuracy of each individual dyne solution. For example, if you were to see time to beading of 12 seconds at 36 dynes/cm, 6 seconds at 38 dynes/cm, and just two seconds at 40 dynes/cm, this would be a reasonable progression for a surface of 40 dynes/cm.

If, on the other hand, the time to beading went from 12 to 20 to 2 seconds over the same range of dyne levels, it would be a warning that the 38 dyne/cm test fluid has become contaminated, thus reducing its actual surface tension. If a retest on a fresh sample showed the same counterintuitive progression, it would clearly be time for a fresh bottle of 38 dyne/cm surface tension test fluid.

In some rare instances, the discontinuity of the time to beading progression could be reversed — consider the following case:

 Dyne Level of Test Fluid  Time to Beading
 34  20 seconds
 36  4 seconds
 38  7 seconds
 40  2 seconds

Clearly, the 36 dyne/cm test is not in line with the expected progression. While it is possible that the dyne solution is inaccurate (too high a surface tension in this instance), it is more likely that the location on the sample that was tested with this dyne level was more contaminated, i.e., dirtier, than the surrounding areas which were tested at 34, 38, and 40 dynes/cm, all of which showed results adhering to the expected progression of time to wetting. This can be a flag that, while the cleaning process may be mostly effective, it may not be removing contaminants uniformly.

It is also possible that skin oil or some other contaminant was introduced to the surface between removal from the cleaning line and testing, or that the 36 dyne/cm test fluid was left uncapped for some time, resulting in an evaporation-based increase in surface tension. Again, re-testing on a fresh sample should shed light on the underlying cause of the aberrant results.

Hopefully this overview, along with the associated test procedure, will clear up a number of questions on evaluating the cleanliness of metal surfaces via the dyne test. We welcome any new inquiries regarding specific applications or unusual results.

Polarity of Corona-Treated Polymer Film

Question: We have had UV-cured ink adhesion problems on some print jobs, and our ink supplier was concerned with the low polarity of our substrate. How is polarity related to the dyne level of the material that we are printing, and how is it affected by corona treatment?

Answer: Most polymers are relatively non-polar, as shown in our polymer surface energy table at http://www.accudynetest.com/polytable_02.html. The polar component of the surface energy is shown in column 5 (8.0 dynes/cm for ABS, for example). Untreated polyethylene has a polarity of only about 1.4 dynes/cm. Corona, plasma, and flame treatments all increase the polar component, which is what increases the total surface energy, providing the improvement in wetting and adhesion.

The polar component is basically a measure of free electrons (or free radicals) available on the material’s surface for bonding, whereas the dispersive component of surface energy (also known as dispersion forces, London forces, or van der Waals forces) is based on general atomic-level forces involving the entire structure of the polymer molecules on the surface of the substrate.

Dispersion forces tend to be greater for larger molecules, and the surface energy of most untreated polymer surfaces is primarily determined by them. Unless you are a physical or surface chemist or have similar training, they are not easy to visualize. The polar component is more intuitive: an extra (or missing) electron is available for bonding at an atomic level with an oppositely charged surface.

Fortunately (from the point of view of understanding, at least), the dispersion forces are relatively unimportant when it comes to the adhesion of a printing ink. Inks are designed to find polar sites which will anchor and attract them until they are fully cured or dried. Surface treatment provides such polarity, and increases the overall dyne level (which is comprised of both polar and dispersion forces) by doing so. Thus, the change in dyne level between untreated and treated polymer surfaces is essentially a measure of the increase in polarity on that surface.