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 TEST^TM 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 TEST^TM 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 Autofill^TM 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.