Question: We purchased several ACCU DYNE TEST^TM Marker Pens from you recently, all at 30 dynes/cm. One test marker seems to be getting a significantly higher amount of “failure” results. Can you provide any insight on this?

Answer: The first thing to check is whether all the test markers have the same lot number, meaning they were all produced from the same master batch. It is extremely unlikely that any significant variation in actual surface tension from lot to lot would occur, but it is not impossible. If the suspect marker is the only one with a non-uniform lot number, we would want to know immediately.

Generally speaking, if one test marker reacts differently from others at the same dyne level, it is due to one of three causes: Either evaporation of test fluid from the pen’s barrel, contamination of the tip (typically by airborne silicone or residual oil from a previous processing stage), or absorption of water from extreme humidity or accidental immersion.

Evaporation will generally increase the surface tension of the test fluid, as 2-ethoxyethanol evaporates faster than formamide. However, the 30 dyne/cm formulation is 100% 2-ethoxyethanol, so any evaporation that does occur should not affect the surface tension of the test fluid.

Contaminants are usually of lower surface tension than the test fluids, but this may not be true at this low a dyne level — the suspect unit may have picked up a contaminant of higher surface tension, raising its dyne level and decreasing its wettability. This would cause false failures. Any absorption of water would also increase the surface tension of the test liquid, with the same result.

Another possibility is that the suspect unit is allowing a smaller amount of liquid to flow through its tip, which could result in a thinner film of liquid being applied. Thicker fluid films will wet somewhat more readily, due to gravitational spreading from the mass of the liquid. Taking care to saturate and then flush the tip in accord with the test procedure so that all test markers apply a similar volume of fluid on the final pass will help minimize this effect.

If you need a more concrete answer, the best thing to do is send us the suspect test marker and one that reads as expected, along with some of your material samples, and we will evaluate the issue in our test lab.

Question: I would like to know if the dyne test can be used effectively on a vertical surface.

Answer: This is a good question. The dyne test is based on wetting (spreading) vs. beading (shrinking) on a flat, horizontal surface. You can imagine it as a balance between the gravitational force tending to spread the liquid over the surface vs. the resistance to spreading due to the surface tension of the liquid. We have had a couple of customers who test their web upside down on line while their machine is stopped — in this case, both forces work together to keep the liquid from spreading; wetting is clearly impeded, and some sort of data adjustment will be necessary.

In a vertical configuration, the bottom of the liquid swath will be strongly drawn downwards away from itself by gravity, whereas the top of the liquid swath will be drawn downwards into itself by the same force.

In brief, this configuration is far from ideal. The reaction of the fluid at the left and right sides of the liquid swath could be evaluated, but there is still an anomaly involved: the dyne test is based on receding contact angles, meaning that the behavior of a liquid on an already wetted surface is what is analyzed. In the case of a vertically positioned sample, the liquid is likely to run down the surface to an area which has not been pre-wetted. In this case, the advancing contact angle of the liquid/solid interface — rather than the receding one — comes into play. At least with regard to polymer testing, this could be a significant source of systematic error.

In summary, I do not advise this orientation. However, if no other option is available, and if you can do direct A:B testing on two surfaces that you know have identical surface energies — one test on a horizontal surface and one on a vertical one —  you may be able to devise a method and a data adjustment which will provide meaningful results. But please keep in mind that, even with the data adjustment, the odds are you will not derive the same dyne level in this manner that another tester might come up with while testing in the traditional horizontal orientation.

Question: Our customer is concerned with controlling their finished product’s COF, but we are using the dyne test to monitor the printing of their plastic parts. What is the relationship between the two? Can one measurement be used to predict the other?

Answer: There is a relationship between surface energy and coefficient of friction (COF): higher COF, like higher surface energy (greater wettability), tends to correlate positively with better adhesion⁽¹⁾. Similarly, higher COF tends to correlate to higher surface energy, as shown in the following link:

se-vs-cof ⁽²⁾

Regression line shown in chart: Surface energy = 34.8 + 65.6 x COF;
or, COF = 0.26 + .011 x Surface energy

The correlation coefficient between these two variables is a very respectable +0.84. Nevertheless, for the two data points furthest from the regression line (PP and PVC), if we were to use COF to predict surface energy, we’d be off by more than 10 and 8 dynes/cm, respectively. A similar analysis of five polymers tested for COF at varying loads showed similar correlation coefficients with surface energy of approximately +0.70 to +0.75⁽³⁾.

Despite these strong correlations with surface energy, COF is generally more dependent on tribological factors (e.g., consider the greater sliding resistance of coarsely sanded surfaces vs. finely sanded ones, despite the fact that the surface energies of the two will be very similar). And, the wettability of most polymers is determined more by the degree of polarity at the surface — this is a primary reason why corona, flame, and plasma treatments are all effective.

Increasing the polarity of the surface will, by itself, increase COF, as there will be greater interfacial attraction between surfaces of higher polarity. The classic example of this is blocking in rolls of corona-treated film, where the release resistance of the web increases with higher treat levels. In this case, blocking is likely caused primarily by the polar attraction between the surfaces, and secondarily by static charges.

But corona treatment also changes the morphology of the film’s surface, so it also has a tribological effect that would contribute to changes in sliding resistance, which is a typical way to measure COF. Depending on the molecular chain structure of the polymer, and how it is altered by surface treatment, this effect could be either positive or negative. I would guess that variations in treater gap, temperature, ozone concentration, polymer type, and other factors could all contribute to this rather complicated scenario. A smoothly stippled treated surface may have a lower COF, whereas one with sharper “edges” may have a higher COF.

Two predominant methods are employed to combat blocking and other film-handling problems — anti-blocking agents and slip additives. The two methods are functionally quite different: anti-blocking agents protrude slightly from the surface of the film, drastically reducing the actual interfacial area. This results in a commensurate reduction in COF. Conversely, slip additives, as they migrate to the surface, produce a lubricious film which reduces COF. In terms of surface energy, slip additives cause a significant (and sometimes problematic!) decrease, whereas anti-blocking agents exhibit a far less pronounced effect. Other types of surface-blooming additives will likely have effects somewhere in between.

Based on the number of variables involved, I would not recommend trying to use either measurement as a predictor of the other, even though overall a strong correlation undoubtedly prevails. Introducing additives, various surface treatments, and other process variables will only increase the scatter, and decrease the degree of correlation.

By way of a summary, years ago a European business developed an online surface treatment monitoring device that used COF as a proxy for treatment level. The concept was that if a baseline COF could be measured upstream from the treater, the change in COF from that baseline to a downstream post-treat measurement would provide an excellent proxy for surface energy change. Unfortunately, the concept apparently fell short in real-world testing: I don’t believe that a single unit was sold. This was probably due to a combination of high capital cost and the litany of process and material interactions this article has briefly discussed.

References:

1) N. Maeda, N. Chen, M. Tirrell, and J.N. Israelachvili, “Adhesion and friction mechanisms of polymer-on-polymer surfaces,” Science, 297, (2002), 379-382.

2) Based on data from V.R. Sastri, Plastics in Medical Devices: Properties, Requirements, and Applications, Elsevier, 2010, p. 57.

3) L.-H. Lee, “Effect of surface energies on polymer friction and wear,” in Advances in Polymer Friction and Wear, Vol. 1, Plenum Press, 1974.

Question: Can you supply dyne fluids with brush-caps?

Answer: We do not feel that using brush-caps is an appropriate application method for surface tension test fluids. This packaging has gained some popularity because historically, these dyne solutions were often sold by treater manufacturers in this format. Unfortunately, this is fundamentally and theoretically about the worst possible way to apply the test fluids.

First, as the brush applicator is used to spread out the test fluid, it will pick up any surface additives or contaminants present on the surface of the sample. This could include surfactants, slip or anti-static agents, anti-blocking compounds, etc. Worse yet, if testing metal for cleanliness, the residual oil on the sample’s surface will be absorbed into the dyne solution, and the brush’s fibers. These contaminants will then be re-introduced into the supposedly reagent grade dyne solution when the bottle is re-capped. This will permanently alter the test fluid, making it essentially useless.

Second, brush applicators apply far too much test fluid. As surface energy is a two-dimensional attribute, you need to use as thin a film of test fluid as is possible. An excessively thick application of test fluid will affect results, as gravitational spreading will become a factor at the liquid—solid interface.

In summary, using brush-caps as applicators for dyne solutions is simply not a good idea at all!

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 TEST^TM 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.

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.

Question: It would be helpful to have one consolidated list Harmonized Codes for all the products you sell.

Answer: I agree. Please keep in mind that despite being called the “Harmonized Tariff Schedule” – which implies uniformity over different countries – there will be differences, notably in the suffixes, in some countries. Anyway, here goes:

ACCU DYNE TEST Marker Pens: 9027.80.8090 (Other instruments and apparatus); 2942.00.5000 (Other organic compounds); or 9608.20.0000 (Felt tipped and other porous-tipped pens and markers).
ACCU DYNE TEST Surface Tension Test Fluids: 2942.00.5000 (Other organic compounds).
ACCU COAT Applicators: 9027.80.8090 (Other instruments and apparatus).
Books: 4901.99.0050 (Technical, scientific, and professional books).
Dispenser bottles: 3923.30.0000 (Carboys, bottles, flasks, and similar articles).
Packaging bottles: 3923.30.0000 (Carboys, bottles, flasks, and similar articles).
Cotton swabs: 5601.21.9000 (Wadding of textile materials and articles thereof).
Drawdown platforms: 9027.80.8090 (Other instruments and apparatus).
Durometers: 9024.80.0000 (Other machines and appliances for testing hardness).
Metering rods: 9027.80.8090 (Other instruments and apparatus).
Spare parts, stroboscopes: 9029.90.6000 (Stroboscope parts and accessories).
Spare parts, tachometers: 9029.90.8040 (Parts and accessories of tachometers).
Spare parts, tensiometers: 9027.80.8090 (Diagnostic instruments and apparatus).
Stopwatches: 9027.80.8090 (Other instruments and apparatus).
Stroboscopes: 9029.20.6000 (Stroboscopes).
Tachometers: 9029.20.4080 (Other speedometers and tachometers).
Tensiometers: 9027.80.8090 (Other instruments and apparatus).
Treater sleeves: 3917.32.0050 (Tubes, pipes, and hoses, other); or 4009.11.0000 (Tubes, pipes and hoses, of vulcanized rubber, except hard rubber, not reinforced/otherwise combined with other materials, without fittings).
Universal blade applicators: 9027.80.8090 (Other instruments and apparatus).
Viscosity cups: 9027.80.8090 (Other instruments and apparatus).
Viscosity oils: 3822.00.0002 (Diagnostic or laboratory reagents).
Wet film gauges: 9027.80.8090 (Other instruments and apparatus).

Question: What are the HMIS ratings and NFPA hazmat data for the dyne solutions and test markers you sell?

Answer: The HMIS ratings for ACCU DYNE TEST Marker Pens and surface tension test fluids are Health – 2; Flammability – 2; Reactivity – 0. The NFPA hazmat data are the same.

Question: My customer has been cleaning their treater sleeves using water and a clean cloth, and they have noted that after cleaning they need to re-adjust many of their operating parameters, including power and treater gap. Is there a problem with this cleaning method? Are there better ones?

Answer: While silicone is inherently very hydrophobic, I can see that after service in a corona treater it may become more hydrophilic, leading to water adsorption into the elastomer. This could affect the dielectric properties of the silicone sleeve, at least until the adsorbed moisture is driven out by the heat generated in the treater. This could certainly create a need to adjust machine settings.

Our recommended cleaning method is to use Lysol Basin Tub & Tile Cleaner rather than water. Be sure any residue from the cleaner is thoroughly removed from the sleeve’s surface, or there may be an effect on machine performance.

An alternate method would be to use 70% or 99% isopropyl alcohol (2-propanol). Based on data from a Balseal Engineering report⁽¹⁾, chemical compatibility between silicone elastomers and isopropyl alcohol is good, though it is possible some color fading may occur⁽²⁾.

References:

1) http://www.balseal.com:8080/sites/default/files/tr60d_020707133101.pdf.

2) Xu, Z.W., J. Jiang, X.X. Zhang, G.B. Liang, and Y. Li, PubMed, 46 (2011), 300-303.

Question: We recently did a dyne test on epoxy-coated steel wire. The lowest level test marker we had was 34 dynes/cm, and it beaded up instantly. Your polymer charts state that epoxies should run between about 42 and 52 dynes/cm. What gives?

Answer: The polymer chart for epoxies (http://www.accudynetest.com/polymer_surface_data/epoxy.pdf) shows data for unmodified epoxy resins. In many cases, these resins are modified to increase hydrophobic properties, making them more weather-resistant. These epoxy blends, once cured, will have lower surface energies than unmodified epoxies. There is a very good chance that the resin you are using is modified with a low energy additive. I’ll briefly discuss two examples pulled from the literature.

a) Epoxy modified with fluorinated poly (aryl ether ketone) (PETK): PETK, like other fluorinated polymers, has a very low surface energy. A study published in 2010⁽¹⁾ demonstrated that higher concentrations of PETK did, in fact, significantly reduce the surface energy of the epoxy blend. One interesting result from this study was that resins cured at low temperatures (30° C) demonstrated markedly greater reductions in surface energy than did those cured at higher temperatures (80° C). The reason for this is that at the higher cure temperature, phase separation is less pronounced because cross-linking occurred more rapidly, impeding the ability of the PETK to bloom to the surface.

b) Epoxy modified with polysiloxane showed similar results⁽²⁾. For example, for material processed in a silicone mold, unmodified epoxy showed a contact angle with water of 68° (roughly 39 dynes/cm), whereas blends containing polysiloxane showed contact angles of up to nearly 100° (roughly 28 dynes/cm). Undoubtedly the cure in a silicone mold also influenced both readings, as surface-to-surface transfer within the mold would result in traces of silicone – another low energy solid – on the surface of the cured resin.

Please keep in mind that, in general, lower surface energy components will tend to move towards the surface during curing, so the decrease in surface energy can be quite dramatic even at low concentrations of additives.

References:

1) W. Brostow, M. Dutta, and P. Rusek, “Modified epoxy coatings on mild steel: Tribology and surface energy,” European Polymer J., 46 (2010), 2181-2189.

2) Z.-x. Huang, Y. Huang, and Y.-z. Yu, “Modification of epoxy resin with polysiloxane bearing pendant quarternary ammonium groups,” Chinese J. of Polymer Science, 20 (2002), 537-541.