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.

Published by

Russ Smith

Russ Smith formed Diversified Enterprises - the first business to focus specifically on applications of the dyne test - in 1986, and has served as President of the company ever since. He has over 30 years of experience in the fields of surface treatment and analysis, and deals with technical inquiries from customers worldwide on a daily basis. Russ is a member of ASTM, the Society of Plastics Engineers, the American Chemical Society, the American Society for Quality, the American Association for the Advancement of Science, and TAPPI.

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