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)
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!