Should Surface Tension Test Fluids Be Stored at Reduced Temperature to Achieve Maximum Shelf Life?

Question: If chemical reactions affect the accuracy of dyne fluids, wouldn’t it be a good idea to store them in a refrigerator?

Answer: There’s no question that storing dyne solutions at a reduced temperature will slow down any chemical reactions, but that is only one factor that affects their aging or performance. There are a number of things to consider, as I’ll get to below. But first, please keep in mind that 55 to 60 dynes/cm test fluids will freeze at about +3°C (39°F), and we strongly recommend that a change of state be avoided (more information on that topic is available here). Also, in many cases low temperature storage ends up being at relatively high humidity. This can be detrimental, as there may be some degree of water vapor transmission over time from the storage environment into the bottles of test fluid (or the barrels of test markers).

For end-users of ACCU DYNE TESTTM Marker Pens and surface tension test fluids, the most critical considerations are ensuring that the test supplies and material samples are both at ambient temperature when the test is performed, and avoiding repeated temperature cycling from refrigerated to ambient. Temperature (and humidity) cycling are used in accelerated aging studies, which is enough to be said regarding that.

We’ll consider three different scenarios, and discuss the benefits and risks of refrigerated test fluid storage for each.

Some shops need to keep supplies on hand, but only use them for specific short run jobs. In this case, there may be periods of weeks or even a few months when they are not used at all, followed by short periods of relatively intense usage. In this situation, as long as it is ensured that the test markers or dyne fluids are removed from refrigeration long enough in advance – 24 hours is a nice conditioning period – then storing them refrigerated would probably be a prudent idea, especially if there is not an alternate storage area with good environmental control.

By contrast, in many industries – film extrusion, printing and converting for example – dyne testing is an ongoing requirement. In this case, the only reason to refrigerate would be if large “master” bottles (8 ounces or larger) are used to periodically replenish small bottles used for testing, or if ACCU DYNE TESTTM Marker Pens are bought in multiple sets for release to manufacturing as required. Generally speaking, we do not recommend refrigeration of inventory under these conditions, as the chance of using test supplies before they are adequately conditioned, along with the possibility of freezing and exposure to high humidity, combine to create more risk than I would care to take with sensitive reagents.

The last example would be for distributors who purchase to hold inventory for resale. In this case, it is clear that ideally it would be best to store refrigerated in a humidity-controlled environment. The most serious consideration is to make sure that the time your inventory is out of cold storage to fill orders is minimized, so its temperature stays as constant as possible.

So – what’s the quick and easy answer? Of course there is none – this is dyne testing after all, and every situation has its own unique twists – but if I had to make a blanket recommendation, I would say just store your product well sealed under normally controlled laboratory conditions in its original packaging. If your test supplies must be kept in the shop at elevated temperature or humidity, consider increasing the frequency of replenishment. Other than that, don’t worry yourself overmuch about the finer details of thermodynamics.

The Effects of Freezing Dyne Testing Solutions

Question: What happens when your test fluids freeze and then re-thaw, and why are you so concerned with avoiding this when you ship in the winter?

Answer: With regard to how freezing may affect product performance, this is a difficult question to answer, as it comes down to the inability to prove a negative. To start, let me make it clear I am not a chemist by any stretch of the imagination; my background is experimental design and quality control. So, if there are any readers out there who can comment on the chemistry of the freeze/thaw cycle on binary (plus dye) mixtures, I would love to hear from you!

I have heard a number of anecdotal reports of changes in the reactions of surface tension test fluids after they have been frozen, but cannot personally remember ever seeing an effect myself. Nevertheless, subtle changes in the mixtures could have a meaningful impact under some circumstances: Water vapor adsorption could be accelerated; leaching of polymer from the bottle at the liquid/solid interface could be increased; the dispersion of the dye in the 2-ethoxyethanol/formamide mixture could be altered. Undoubtedly a good number of other possibilities exist as well, including the potential for shortened shelf life.

The biggest problem with determining any impact on performance is that to run a comprehensive study on the effect of freezing on test accuracy, you would need to test at least a dozen different dyne levels on an almost literally limitless variety of substrates – a Herculean task at best.

So, in the interest of caution and keeping the anecdotes in mind, I feel it is best to avoid freezing dyne solutions.

But, the most pressing issue with regard to freezing is damage in transit. ACCU DYNE TESTTM Marker Pens will sometimes lose their tip seals and leak during shipping once they have been frozen. I believe that the reason for this is that shrinkage of the plastic spring that controls the release of test fluid from the tip allows seepage of test fluid during the thaw cycle. Sometimes the seal between the pen barrel and the tip leaks for similar reasons. Rarely, the same problem can manifest with bottled test fluids – especially with dropper bottles.

The short and the long of it is that, based on brand stewardship considerations and replacement costs, as well as the potential for effects on measurement accuracy, we feel it is very important to avoid allowing ACCU DYNE TESTTM Marker Pens and surface tension test fluids to freeze.

Determining the Accuracy of Dyne Solutions

Question: We have some dynes that are aging, but still appear to be fine. How can we assure that their performance will still be like new?

Answer: First, it is important to realize that there are three primary reasons why dyne solutions lose accuracy: contamination, evaporation, and aging, during which chemical reactions take place among the constituents. Recommended shelf life is discussed here.

Second, if there is any noticeable change in either the hue or the color density of the test fluids and they are near or past their expiration date, it is probably best to simply replace them.

As to qualification of test solutions, their most critical attribute by far is surface tension, and the most reliable measurement method is with a tensiometer. Please keep in mind that, especially for the lower dyne levels, the nominal surface tension value stated on the bottle is not exactly the same as its true surface tension. The discrepancies derive from the empirical nature of the test, which is based on wetting vs. de-wetting after two seconds of exposure to air. During that brief time frame, evaporation comes into play, altering the balance of test fluid constituents, especially at the periphery, where the liquid/solid interface is evaluated. The chart below shows the relationship of nominal vs. measured surface tension, based on actual production lot testing in our quality lab.

Be certain that the tensiometer is properly calibrated, and make any required adjustments to raw data to derive the correct adjusted surface tensions. Be sure to follow all instructions in your instrument’s owner’s manual. More detailed information on tensiometer calibration, use, and data adjustment is available here. It is critical that all test vessels and apparatus are entirely free of contamination.

As long as your results are all within about +/- 0.5 dynes/cm of the measured surface tensions shown below, the test fluids can be considered accurate with regard to wettability.

Nominal Dyne Level Measured Surface Tension(b) Specific Density(a) Volumetric %2-ethoxyethanol(c) Volumetric %Formamide(c)
30 28.6 0.929 100.0 0.0
32  30.1  0.950 89.5 10.5
34 32.6 0.982 73.5 26.5
36  35.5  1.014 57.5 42.5
38 37.8 1.037 46.0 54.0
40 39.9  1.056 36.5 63.5
42 42.1 1.072 28.5 71.5
44  44.2  1.085 22.0 78.0
46 46.0 1.095 17.2 82.8
48  48.0  1.103 13.0 87.0
50 49.9 1.110 9.3 90.7
52  51.9  1.116 6.3 93.7
54  54.1  1.122 3.5 96.5
56 56.9 1.127 1.0 99.0

Nominal dyne level and measured surface tension are 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) ASTM Std. D2578-09: Standard test method for wetting tension of polyethylene and polypropylene films.

An alternate qualification technique would be to make contact angle measurements of the dyne solutions, when first purchased, on a known low surface energy material such as untreated virgin polyethylene, paraffin, or PTFE. If there is a doubt about the wettability of a given bottle of test fluid at a later date, a comparison of measured vs. expected results will identify any change in wettability. Either static (also known as Young’s) contact angles or retreating contact angles should be measured – not advancing contact angles. Be sure to record which method was used for future comparison. Also be sure that retains of the substrate used for these measurements are kept well sealed, free from contamination, and stored under laboratory conditions.

Even with these precautions, the polymer’s surface energy may change slightly over time, so this would be a more appropriate qualification method if only one or two dyne levels is suspect, rather than a whole batch. The key is to identify whether or not the suspect dyne levels appear as outliers in the curve describing dyne level vs. contact angle. For example, if your initial contact angles for 34, 38, and 42 dynes/cm test fluids, measured on HDPE, had been 20°, 32°, and 42°, and your retest showed contact angles of 22°, 26°, and 44°, that would be a clear signal that the 38 dyne/cm test fluid had changed meaningfully – its contact angle decreased by 6°, whereas the other two increased by 2° each.

A third method of qualification would be to compare the results of the questionable dyne solutions vs. those obtained with a fresh, unused set. This is probably the ideal verification: Whereas surface tension per se is the dominant determinant of accuracy, other factors can affect results to some degree. These include changes in pH or solubility, and the chance that a balance of evaporation and adsorption of contaminants has changed the test fluid’s chemical constituency without affecting its surface tension. At the liquid/solid interface where the dyne test takes place, these subtle changes can sometimes have a significant effect.

Under no circumstances should reagent grade surface tension test fluids be “validated” via a comparison to results from dyne pens of any kind, least of all go/no go permanent markers. Even ACCU DYNE TESTTM Marker Pens, which are designed to minimize the effect of surface contaminants on test results, are not appropriate for qualifying bottled solutions. On the other hand, keeping a master set of bottled test fluids in the Quality Lab – as a standard to which test markers can be compared if questions arise – is good practice, as the master batches will have been better protected during storage, and will be far less likely to have suffered from contamination or evaporation.

Neither do we recommend qualifying dyne solutions by comparing your dyne test results to contact angles produced with water as a probe fluid unless you maintain quality records from both tests on a continuing basis. In that case, a divergence from the expected correlation would certainly alert the tester that both methods need to be verified! If, however, you do not routinely do contact angle measurements, relying on tables or graphs that show a”conversion” from contact angle to dyne level is not a sound policy: The actual relationship between the data sets will often vary by up to a few dynes/cm, and sometimes even more, depending on the material you are testing.

Finally, in some cases, test fluid labels may become illegible or be removed. In that case, identification of correct dyne level would be the objective. Consider a case in which bottles of 34, 40, and 44 dyne/cm test fluids were in doubt. If you do not have access to a tensiometer, but do have a way to measure specific density, the three dyne levels can be readily discerned due to the significant change in specific density vs. dyne level, as shown in the chart above. I would not recommend this method for dyne levels with specific densities that are too similar; trying to sort an entire set of all levels in this fashion would be quite a puzzle indeed!

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.