Question: I know that ideally dyne testing should be done under standard laboratory conditions, but as we test in our shop, which is not air conditioned, that is not possible for us – we sometimes reach temperatures as high as 40° Celsius, at high humidity. How do we deal with this?
Answer: Ideally, we do recommend testing at about 20° to 25° Celsius, with humidity in the range of 40% to 60% RH, Some variation from these conditions is not likely to affect results meaningfully, as the surface tension of liquids and the surface energy of solids are similarly affected by temperature changes, as shown in the following table. But with more seriously elevated temperatures, caution should be employed.
|Change per °C
|40 dyne/cm test fluid
Surface tension, and change per degree Celsius are shown in dynes/cm (equivalent to mJ/m2).
(a) Critical surface tension in dynes/cm at 20° to 25° C, generally determined by the Zisman method (regression of the cosine of the contact angle), or by the wetting tension method, using solutions of 2-ethoxyethanol and formamide per ASTM Std. D-2578. A more complete list of polymers is available here, and a more complete list of liquids is available here.
The largest numeric effect would be seen when testing at very high treat levels, where the test solutions are formulated from formamide and water. In the most extreme case – polyethylene treated to be water-wettable at 72 dynes/cm – the net drift works out to 0.153 dyne/cm per degree Celsius. This translates to an effect on test results of roughly -3 dynes/cm at 40° C, as the substrate surface energy would be reduced by only 1.14 dynes/cm, whereas the test solution (100% reagent grade water) would be reduced by 4.2 dynes/cm. It can be argued that even in this extreme case, an error of 3 dynes/cm at a treat level of around 70 dynes/cm would not be critical in most instances. But even at a more typical treat level of 40 dynes/cm, the net drift is still 0.083 dyne/cm per degree Celsius, resulting in an effect of about 1.7 dynes/cm, which could be significant.
So much for the hard data. Please note that this analysis assumes that both the test solution and the substrate have been stabilized at the elevated ambient temperature. If this is not ensured, then one component or the other will obviously be affected more by the temperature change, and test results could deviate from those that would be obtained under laboratory conditions in a less predictable fashion.
However, elevated temperatures pose their own problems. Surface tension test fluids will tend to degrade faster at elevated temperatures, and, while open, bottles will be far more prone to preferential evaporation of 2-ethoxyethanol, which has a higher evaporation rate. When testing with ACCU DYNE TESTTM Marker Pens, the test fluid may pass somewhat more readily through the spring-loaded valve tip – this can have an effect due to a tendency towards gravitational wetting when an excess of test fluid is applied. Also, the rate of evaporation of the test fluids once applied to the surface will increase, so the two second timeframe on which the test is based could come into question. Finally, solubility parameters are affected by temperature as well, so the chemical affinity of the fluids to the surface may be changed.
With regard to the substrate, the rate of crosslinking may be affected, which could have an impact on the surface energy when tested vs. its level over time. Finally, elevated temperature (as well as humidity) levels will tend to accelerate treat loss of the substrate. The mobility of surface-blooming additives will be enhanced, and transfer of the treatment from the treated to untreated side of a film once it is wound will also be more pronounced. This will not only drop the dyne level, but can also lead to blocking of the film when it is paid off from the roll during further processing operations. The latter effects are perhaps the best arguments for limiting product exposure to extreme environments to whatever extent is possible.
As to RH, it is best to avoid excessive humidity, as it can cause higher variability in test results. Also, if there is any moisture on the surface, it will absorb into the test solution, changing its surface tension and invalidating the test. Nylons especially may be prone to this, as they absorb water vapor far mote readily than most other polymers.
Given the range of physicochemical effects which can have an impact on the accuracy of test results produced at elevated temperature and or humidity, if product end-use requirements are rigorous, such as in the automotive industry and for many medical applications, it would be prudent to set up an experimental study to correlate dyne testing results obtained in the shop vs. those obtained under laboratory conditions. You can randomly split your samples into two groups. On one set, test at the machine as usual. For the other set, remove to the laboratory, allow the material to stabilize under that environment, then perform the same dyne test (with a separate set of test fluids or test markers which are to be kept in the lab!). Results can be compared readily. If there is a meaningful difference, I would also suggest looking closely at which set of results best predicts end-use performance or adherence to customer specs. Considering all the factors discussed above, you might even find that the results from the elevated temperature testing are the better predictor of product suitability. I wouldn’t want to bet against it a priori, even though such an outcome does seem rather counter-intuitive.