Shelf Life of Surface Tension Test Fluids

Question: How do you establish the shelf life of your products, and what influences how quickly they degrade?

Answer: This is one of the most difficult questions that we hear – and a frequent one, to boot.

I do not believe there is anyone on earth who truly understands the myriad of variables – let alone their inter-relationships – that affect the degradation of surface tension test fluids. So, the answer to the first half of the question is that, based on feedback from endusers and standard industry practice over time, we have de facto established shelf lives of five and six months, respectively, for ACCU DYNE TESTTM surface tension test fluids and Marker Pens.

These shelf lives reflect our best estimate of a reasonable time frame through which we can guarantee that our product will not lose accuracy without some specific identifiable external cause that explains a change in performance. The extra month on the test markers is due to their sealed environment, compared to bottled test fluids, which must be opened for use(1). It’s helpful to look at shelf life as a risk/reward decision, with the time frame set at the point where risk starts to appreciably increase.

In general, without use and kept sealed and protected from intense light, heat, etc., there is little degradation in accuracy for as long as 18 months or more. The problem is that the onset point and rate of degradation are not predictable, so the assurance level regarding accuracy drops progressively, even for shelved sets of test fluids (or test markers).

The second half of the question, which is the key to the most realistic predictable shelf life in real world use, is of greater practical interest. The change in properties is based on age; frequency of use; environmental conditions (elevated temperature and, less notably, humidity levels tend to accelerate aging); and exposure to evaporation or contamination, including airborne dust and aerosols, as well as what exists on the surface of samples to be tested. Evaporation is an issue because 2-ethoxyethanol evaporates at a faster rate than formamide, meaning that an unsealed container of dyne solution will increase in surface tension due to the change in the ratio of constituents. Contaminants not only tend to reduce the surface tension of the test fluids, they can also accelerate the aging process.

For ACCU DYNE TESTTM Marker Pens, which use the same applicator tip from use to use and are sealed units, contamination is the primary concern, as long as care is taken to keep the caps tightly secured at all times when not in use. High slip films are especially likely to cause contamination problems, as the low surface energy slip agents bloom to the surface and will be more than happy to take residence in the tips of your test markers. To a lesser degree, the same is true for residual mold release on molded and formed parts. Flushing these compounds from the tip is the primary reason for flooding the tip before testing, and only reading results from the final test swath. Procedural details are available here.

As discussed extensively here, machine oils and other processing aids used in the metals industries are simply too aggressive for test markers; for these applications, the test should be performed only with bottled solutions, applied with swabs.

For bottled test fluids, evaporation, introduction of airborne contaminants, and water adsorption – a form of contamination – are the greatest threats. Obviously, the more often the bottles are used, the greater the chance that these processes will reach a level that has an impact on test results. Never re-use an applicator swab, even at the same dyne level, as doing so is a perfect way to introduce surface contaminants into the bottles of dyne solution.

It is more common for dyne solutions to wet more readily (produce a higher dyne level reading) as they age, but this effect is not universal, especially if evaporation has occurred.

Finally, for any enterprise that is ISO or similarly certified, to remain in compliance, test supplies must not be used after their approved shelf life. Ensuring regular deliveries of fresh product is probably the main advantage of our AutofillTM replenishment system, which ensures automatic and timely re-supply. Even for customers that are not certified, I strongly recommend replacing dyne testing supplies at least every eight months. We have a number of testers who purchase on an annual basis, but I feel that is pushing things too far, even under the best of conditions. And, for plants that test frequently on an ongoing basis, a replenishment schedule of three months or even less is a reasonable precaution.

I trust these comments have been helpful – I’d like to offer more precise guidelines, but uncertainty is the nature of the beast, and there doesn’t seem to be much we can do to change that.

(1) Dropper (dispenser) bottles are essentially exempt from the environmental exposure consideration, as the tips needn’t be removed for use. However, since they are made from LDPE, rather than the more stable HDPE narrow and wide mouth bottles, we are still more comfortable with a conservative shelf life assignment.

Using Surface Tension Test Fluids to Calibrate a Tensiometer

Question: Can your dyne solutions be used to verify tensiometer readings? I’d think that cross-checking against these standards would help validate the calibration procedure.

Answer: In a nutshell, the answer is “yes.” But, like most surface tension- and surface energy-related issues it of course is not quite as simple as that. First we’ll go through a basic calibration and verification procedure for setting up a tensiometer, and offer a caveat regarding this application of dyne solutions, which is: If the tensiometer is properly calibrated and produces accurate results when testing both a low- and high-surface tension liquid, it is probably not worth trying to evaluate its performance at intermediate levels of surface tension − the most likely reason for inconsistent tensiometer readings is contamination of the liquid or the test vessel, or physical damage to the ring, plate, or other test probe.

The first step in calibrating any tensiometer is to follow the User’s Manual instructions. In the case of ring tensiometers, such as the DuNouy model we offer, this involves using a known mass to exert force on the ring, and balancing the torsion wire. The procedure is shown on pages 5 – 7 in the User’s Manual. We like to see results that are within +/- 0.2 dynes/cm of the theoretical result, though published specs allow an error margin of up to +/- 0.5 dynes/cm.

Once the torsion wire (or similar adjustment mechanism in tensiometers with different designs) is correctly adjusted, it is good practice to test reagent grade water and a low surface tension liquid − we use 2-ethoxyethanol, as it is a constituent of our dyne solutions and has been used extensively in dyne testing for decades. Any other low surface tension liquid could be used instead.

It is absolutely critical that the test fluids not be contaminated. We recommend directly pouring the test fluid from its pre-packaged container into a petri dish which has been rigorously cleaned. We clean with 99% isopropyl alcohol, then rinse twice in reagent grade water, and air-dry upside down. Even a trace of contamination, moisture, or residual cleaning agent can impart a significant effect on the surface tension of the test solution.

The readings obtained should agree with literature values, the most common of which are shown here. Be sure to adjust for specific density per the equation provided in the user’s manual. Specific densities are shown here. At 25C, reagent grade water has a surface tension of 72.7 dynes/cm and a specific density of 0.999; 2-ethoxyethanol has a surface tension of 28.8 dynes/cm and a specific density of 0.925.

Finally, an adjustment must be made for liquid temperature; liquids vary in surface tension as a function of temperature. Data for this is available here in column 6. Reagent grade water has a change of -0.21 dynes/cm per degree Celsius; for 2-ethoxyethanol the rate of change is -0.13 dynes/cm per degree Celsius.

Assuming these initial steps have been made successfully, test solutions of intermediate surface tensions can be used to compare tensiometer results vs. the known surface tensions of standard dyne solutions over a broad spectrum of dyne levels. To do this successfully, you will need to know the specific densities and actual (as opposed to nominal) surface tensions of the various formulations.

The following table shows nominal dyne level, formulation data, specific density, actual surface tension as measured in our lab, and estimated surface tension change per degree Celsius for a number of dyne solutions, mixed in strict accord with ASTM Std. D2578(1).

Nominal Dyne Level Specific Density(a) Measured Surface Tension(b) Change per °C(c) %2-ethoxyethanol(d) %Formamide(d)
30 0.929 28.6 -0.13 100.0 0.0
34 0.982 32.6 -0.13 73.5 26.5
38 1.037 37.8 -0.14 46.0 54.0
42 1.072 42.1 -0.14 28.5 71.5
46 1.095 46.0 -0.15 17.2 82.8
50 1.110 49.9 -0.15 9.3 90.7
56 1.127 56.9 -0.15 1.0 99.0

Dyne level, measured surface tension, and change per degree Celsius all shown in dynes/cm (equivalent to mJ/m2).

(a) Measured in g/ml at 25°C; derived from data at http://www.accudynetest.com/visc_table.html.
(b) Measured at 72°F (22°C); adjusted and corrected tensiometer results from Diversified Enterprises production lots.
(c) Derived from data at http://www.accudynetest.com/solubilitytable.html.
(d) ASTM Std. D2578-09: Standard test method for wetting tension of polyethylene and polypropylene films.

Please keep in mind the importance of avoiding any contamination, or environmental degradation, of the test liquids. All vessels must be scrupulously cleaned; the tensiometer ring (or similar device) must be free of any damage, as well as properly cleaned and dried before re-use; test fluid bottles need to kept securely closed to avoid evaporation or adsorption of water; etc. Any effects from these potential problems will skew results, casting doubt on your measurement device, whereas the real problem would more likely be in the audit process.

Reference:

  1. ASTM Std. D2578-09: Standard test method for wetting tension of polyethylene and polypropylene films.

Subsequent Processing of Dyne Tested Parts

Question: The parts we test are of high value, and we need to re-introduce them into our manufacturing operation for continued processing. How do we clean off the test solution?

Answer: In general, the dyne test is not intended to be used on material which will continue through the manufacturing process. When necessary, our best suggestion is to wipe the test area clean with 99% isopropyl alcohol. In some cases, this will still leave a stain on the surface, and other solvents, including acetone or MEK, can be investigated. Be sure that the cleaning agent is not soluble with the substrate − any melting or swelling of the surface indicates solubility, which will permanently and significantly alter the surface.

Once an appropriate cleaning agent has been determined, the next consideration is whether the dyne testing or cleaning has altered the surface in any way that will be detrimental to downstream operations. Short chain polymer molecules, volatiles, etc. will be removed to some degree by the dyne test, and even more actively during the cleaning process.

To determine whether there is any deleterious effect, comparisons should be made of those pieces which have been tested and cleaned vs. those that have not. All downstream and end-use quality control tests should be checked to make sure that performance has not been affected.

Finally, please keep in mind that if your products are either medical or food grade, you will need to research any relevant restrictions regarding contact with the constituents of surface tension test fluids.

Corona Treater Output vs. Increase in Dyne Level

Question: Can you offer any general guidelines on the relationship between corona treater power output and dyne level increase?

Answer: The most basic measurement used to address this question is called watt density (Wd). It is measured in kW per ft2 (or m2) per minute. The equation is

(1) Wd = PS/(EW x LS x NST),

where Wd = Watt density; PS = power supply output in kilowatts; EW = electrode width in feet or meters; LS = line speed in feet or meters per second; and NST = number of sides treated.

Other things equal, higher watt densities result in greater increases in the substrate’s surface energy (dyne level). However, the relationship is neither linear nor simple — watt density alone cannot predict dyne level. A myriad of other factors will have an impact on results.

The type of plastic (of the outer layer on coextruded or coated films) is probably the single most important consideration. Whereas some materials, such as polyester, accept treatment readily, others are less susceptible. For example, polyethylene tends to be moderately treatable, whereas polypropylene will require a considerably higher watt density to achieve the same improvement in surface energy.

Film gage will likely have an effect, especially if the substrate includes slip agents, anti-stat additives, or other constituents which tend to bloom to the surface during and after corona treatment. These all tend to decrease the effectiveness of the treatment, especially over time. Film age — especially if it was treated at extrusion — will therefore obviously also have an effect. Films which were corona treated when extruded (a very good practice, as polymer surfaces are more easily modified at higher temperatures, and prior to “setting” their molecular structure), and being re-treated (“bump-treated”) in line for printing, coating, laminating, etc., have a stronger dyne level increase at a given watt density than will films that have not been pre-treated.

During the primary treatment, at extrusion, there will be differences in efficacy between cast and blown films, as well as between films that are oriented or biaxially stretched vs. those that forego these processes. These variations are due to molecular structure and orientation, film temperature, and the proximity of the treater to the extrusion die — closer is better! For example, with cast film, the treatment may be on the cold side, which has been exposed directly to the chiller roll, or on the the hot side. The quench gap and quench tank temperature will have an effect, as both these factors influence molecular structure.

When treating a single side of a film, keep in mind that any back treatment will sap energy from the treater, resulting in a lower dyne level per Wd relationship. Along with back treatment’s potential to cause blocking, this is a good reason to routinely test for this unwanted phenomenon.

Finally, electrode type and gap, humidity, and possibly other effects such as static buildup downline from the treater and the film’s exposure to idler rolls may also have an effect on the relationship between dyne level increase and watt density applied to the surface.

Under any set of conditions, expect the relationship to be non-linear; the shape of the curve relating the two variables will be based on a combination of all factors discussed above.

Having put all these caveats on the table, we can still draw some very general conclusions as to appropriate watt densities for various processes, as follows:

For treatment at extrusion with cast PE film, treat at Wd 2.0 kW/ft2/min cold side; 1.8 kW/ft2/min warm side (no orientation); 2.2 kW/ft2/min for oriented film. With blown PE film, treat at Wd 1.6 kW/ft2/min at top of tower; 2.0 kW/ft2/min halfway down tower; 2.0 kW/ft2/min at winder.(1)

For coating and laminating pre-treated PE film, bump treat at Wd 1.2 – 1.4 kW/ft2/min for solvent coatings; 1.3 – 3.3 kW/ft2/min for water based adhesives; 2.0 – 3.0 kW/ft2/min for UV coatings; 1.0 – 1.5 kW/ft2/min for 100% solids adhesives.(2)

The following data, from Enercon Industries, show typical Wd values, in kW/ft2/min, for printing, coating, and laminating, as well as suggested watt densities to achieve appropriate dyne levels for several materials.(3)

Typical Watt Densities for Printing, Coating, Laminating
Solvent Water UV Solventless
Pretreated LDPE 1.5 – 2.0 2.0 – 2.5 2.0 – 2.5 1.0 – 1.3
Pretreated LLDPE 1.5 – 2.0 2.0 – 2.5 2.0 – 2.5 1.0 – 1.3
PET 1.0 – 1.5 1.0 – 1.5 1.0 – 1.5 1.0 – 1.3
Pretreated BOPP 2.0 – 2.5 2.5 – 3.0 2.5 – 3.0 1.0 – 1.3
Note: Variations in resin blend, additives or process will affect values.

 

Typical Treat Levels & Watt Densities
Incoming Level Desired Level Watt Density
Treated BOPP 34 – 36 40 – 42 2.5 – 3.5
Treated BOPET 40 – 42 54 – 56 0.9 – 1.5
Treated LDPE, high slip 34 – 36 40 – 42 2.5 – 3.5
Cast PP, no slip 38 – 40 40 – 42 1.5 – 2.5
Untreated LDPE, low slip 30 – 31 no data no data
Note: Variations in resin blend, additives or process will affect values.

The following figure shows results published by Kasuga Denki.(4) Note that one square meter = 10.75 square feet, so this includes watt densities of as high as 11 kW/ft2/min for the 10% EVA. This is an unusually high — and probably in most cases unachievable — watt density, as most corona treating systems are sized for a maximum Wd of 4.0 kW/ft2/min or less. The higher watt density data points were probably produced at low line speeds.

corona_10.gif

References:

1) D.A. Markgraf, “Determining the size of a corona treating system,” TAPPI J.72, (Sep 1989), 173-178.

2) no author cited, “Position of corona treating station,” Faustel, http://www.faustel.com/position-of-corona-treating-station/.

3) T.J. Gilbertson, “Using watt density to predict dyne levels,” Enercon Industries, http://www.enerconind.com/treating/library/technical-articles/using-watt-density-to-predict-dyne-levels.aspx.

4) no author cited, “Wettability (wetting tension) and watt density, Kasuga Denki, http://www.ekasuga.co.jp/en/product/185/00235.shtml.

Additional reading:

T.J. Gilbertson, “Blame the corona treater:  the truth about watt density, dyne levels, and adhesion,” Converting Quarterly, 4, (Quarter 2, 2014), 82-84.

no author cited, “Corona treating watt density,” Faustel, http://www.faustel.com/corona-treating-watt-density/.

no author cited, “Watt density: What is the formula to calculate watt density?,” Pillar Technologies, http://www.pillartech.com/Surface-Treatment/Service-info/Troubleshooting-Guides/Watt-Density.

Discrepant Results From One Test Marker Compared to Others at the Same Dyne Level

Question: We purchased several ACCU DYNE TESTTM 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.

Sample Orientation for Dyne Testing

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.

Polymer Surface Energy vs. Coefficient of Friction (COF)

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.

Why Not to Use Brush Applicator Caps for Bottled Dyne Solutions

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!

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.