WETTING SOLUTIONS: SURFACE TREATING EQUIPMENT

WETTING TENSION SOLUTION FOR THE
EXAMINATION OF POLYMER PLASTIC SURFACE ENERGY

For successful printing and adhesion to polyolefin materials, it is necessary to treat the desired area so the surface energy rises to a defined point. Surface energy is measured in dynes/cm (mN/m).

Untreated PE and PP have a surface tension of about 30 dynes/cm. After treatment for printing with solvent-based ink, PE and PP should have a value of approximately 38-40 dynes/cm. For printing with water-based ink, the surface should have a value of approximately 42-44 dynes/cm.

Polymer plastics with a surface tension of less than 37 dynes/cm have binding difficulties. When performing the “Tape Test”, negative results are proof of these binding problems.

For many coating and adhesion applications, however, a surface tension of between 48 to 56 dynes/cm is often required.

In general, the ASTM procedure D2578 covers the measurement of surface tension of various olefin substrates using specific testing solutions in the presence of air.

The value of treatment is tested, by brushing the solution onto the desired surface. The test solutions are graduated from 30 dynes/cm to 72 dynes/cm.

Both dyne test solution and dyne test pens are available from 3DT. The level of accuracy is plus or minus 1 dynes/cm.

Standard Test Ink Set

The standard test ink set is blue and consists of 10 bottles of 30 ml each in a protective box covering the following ranges of surface tension: 34-36-38-40-42-44-46-48-52-56 dynes/cm (Part #53410-04).

Test ink is also available in single 30 ml bottles with the following ranges of surface tension: 30-32-34-36-38-39-40-42-44-46-48-50-52-54-56-60-64-68-70-72

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38 Dyne Test Pen

The 38 dynes/cm test pen is a quick method of determining pretreated surfaces. The test pen is used as any other felt tip pen. If the line drawn on the surface to be tested is continuous, the material has been pretreated at no less than 38 dynes/cm and can be printed. If the line is interrupted, the surface has been poorly treated or not treated at all. The substances in the test pens are classified as “non-toxic.” Test pens are available individually (Part #53410-02), or in a pack of ten (Part #53410-03).

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Instructions For Use

A material's surface energy can be measured by using surface treating equipment such as 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 an 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 procedures described above. Too low of a surface tension value almost always results in poor adhesion.