How to Ensure the Accuracy of the Dyne Pen Test

Problem: We have some aged dynes that still look good. How can we guarantee they will still perform like new?

answer:

First, it's important to recognize the three main reasons that a reachin solution loses its accuracy: contamination, evaporation, and aging, during which chemical reactions between components occur. The recommended shelf life is discussed here.

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

As for the quality of the test solution, by far the key property is surface tension, which is reliably measured using a tensiometer. Keep in mind that, especially with lower dyne levels, the nominal surface tension value stated on the bottle is not exactly the same as its true surface tension. The difference stems from the empirical nature of the test, which is based on wetting versus dewetting after two seconds exposure to air. In that brief time frame, evaporation comes into play, changing the balance of the test fluid composition, especially at the periphery of the liquid/solid interface being evaluated. The graph below shows the nominal surface tension versus the measured surface tension, based on actual production batch testing in our quality laboratory.

Ensure that the tensiometer is properly calibrated and make any necessary adjustments to the raw data to yield the correctly adjusted surface tension. Be sure to follow all instructions in the instrument user manual. More details on tensiometer calibration, use, and data adjustment are available here. Important: All test vessels and equipment are completely free from contamination.

As long as your results are within approximately +/- 0.5 dynes/cm of the measured surface tension shown below, the wettability of the test fluid can be considered accurate.

Nominal dyne levels and measured surface tensions are shown in dynes/cm (equivalent to mJ/m2).

(a) Measured in g/ml at 25°C; derived from data available online.

(b) Measured at 72°F (22°C); Adjusted and calibrated tensiometer results from production batches of a diversified enterprise.

(c) ASTM Standards. D2578-09: Standard Test Method for Wetting Tension of Polyethylene and Polypropylene Films.

Another qualification technique is to measure the contact angle of a dyne solution on a known low surface energy material such as untreated virgin polyethylene, paraffin, or PTFE at the time of first purchase. If there is later doubt about the wettability of a given bottle of test liquid, a comparison of the measured results with the expected results will identify any changes in wettability. The static (also called Young's) or receding contact angle should be measured - not the advancing contact angle. Be sure to document which method was used for future comparisons. Also ensure that the substrates used for these measurements are kept well sealed, free from contamination, and stored under laboratory conditions.

Even with these precautions, the surface energy of the polymer may change slightly over time, so if only one or two dyne levels are suspected, rather than the entire batch, it would be a more appropriate identification method. The key is to determine whether suspicious dyne levels appear as outliers in the curves depicting dyne levels versus contact angle. For example, if your initial contact angles for 34, 38, and 42 dyne/cm test fluids measured on HDPE are 20°, 32°, and 42°, and your retests show contact angles of 22°, 26°, and 44°, which would be a clear signal that the 38 dyne/cm test fluid has undergone a meaningful change - its contact angle has decreased by 6°, while the other two have each increased by 2°.

A third method of identification is to compare the results of the dyne solution in question with those obtained using a new, unused set. This might be good validation: while surface tension itself is the main determinant of accuracy, other factors can also influence the results to some extent. These include changes in pH or solubility, and the potential for evaporation and adsorption equilibrium of contaminants to alter the chemical composition of the test fluid without affecting its surface tension. At the liquid/solid interface where Dyne tests are performed, these small changes can sometimes have dramatic effects.

Under no circumstances should a reagent-grade surface tension test fluid, especially a go/fail irreversible marker, be "verified" by comparison with the results of any type of dyne pen. Even marker pens, which are designed to minimize the impact of surface contamination on test results, are not suitable for qualified bottled solutions. On the other hand, it is good practice to keep a set of bottled test liquid masterbatches in the quality lab—as a standard against which test markers can be compared in case of problems—because the masterbatches will get lost during storage. Better protection and much less chance of contamination or evaporation.

We also do not recommend comparing your dyne test results to the contact angle produced with water as the probe fluid to determine the quality of your dyne solution unless you maintain ongoing quality records for both tests. In this case, a deviation from the expected correlation would certainly alert the Tester that both methods need validation! However, if you do not make contact angle measurements on a regular basis, relying on tables or graphs showing the "conversion" from contact angle to dyne level is not a sound strategy: the actual relationship between data sets will often vary by as much as a few dynes/cm, Sometimes even more, depending on the material you're testing.

Finally, in some cases, test fluid labels may become illegible or be removed. In this case, the goal is to determine the correct level of dyne. Consider a case where 34, 40 and 44 dyne/cm test fluid bottles are in doubt. If you do not have access to a tensiometer, but have a way to measure specific density, the three dyne levels can be easily discerned due to the significant variation in specific density versus dyne levels, as shown in the image above. I don't recommend this approach for dyne levels with too similar specific densities. Trying to sort the entire collection of all levels in this way is indeed a puzzle!