A material’s surface energy can be measured by using standard wetting tension solution or dyne pens. Under the guidance of ASTM D2578, the standard dyne testing gives quick results.
Wet the tip of the applicator with the test solution from the calibrated bottle. A minimum amount of solution should be used since excess can distort the reading. Spread the solution over approximately one square inch of the material to be tested.
For maximum accuracy when testing materials, a wetting tension solution from the middle of the range should first be applied (i.e. 38 dynes/cm). If the solution wets the surface within two (2) seconds without forming globules or beading, the treatment level of the material is either higher than or exactly that of the liquid. Do not repeat the test on the same area of material.
A second test using a solution of the next higher value (in this case 40 dynes/cm) should then be performed. The process should be repeated using solutions of increasing values until the solution beads within two (2) seconds of application.
However, should the first application of solution have beaded within two (2) seconds, then the same test should be repeated but using the next lower value.
In this way, one is able to pinpoint the treatment level measurement through two (2) tests. For example, it can be established that the level of treatment of the tested material is between the values of two solutions (i.e. 36-38 dynes/cm). With a certain amount of practice, it can be accurately estimated whether the level lies closer to 36 or 38 dynes/cm.
These test solutions are designed to assist personnel with routine testing. The tests give good insight on whether the material is being treated enough or not.
Since the solutions are made up of liquids with varying surface tensions and are also hygroscopic, it is imperative that the lids be firmly replaced immediately after use.
The surface tension is a definite criteria for the adhesions of ink onto PE and PP. There are, however, other factors such as migration of slip additives that influence the adhesion of inks quite negatively. These other factors do not necessarily register on surface tension testing. Consequently, even though good surface tension results were found, the ink adhesion can result negatively. It is also possible that polymer plastics with the same surface tension can give varying degrees of print adhesion.
In most cases, however, one can disregard these exceptions and get best possible results of adhesion using the above-described procedures. Too low of a surface tension value almost always results in poor adhesion.