The standard “ASTM D5946-17 Standard test method for corona-treated polymer films using water contact angle measurements” developed by the American Society for Testing and Materials (ASTM) describes a test method for measuring the contact angle of water droplets on corona-treated polymer film surfaces. This standard is essentially the same as the standard “ISO 15989 Plastics - Film and sheets - Measurement of water contact angle of corona-treated films” published by the International Organization for Standardization (ISO).
For the purposes of this standard, the static contact angle is the angle between the tangent line drawn to the surface of the droplet at three phase points and the substrate surface when a liquid drop is at rest on a flat solid surface. The contact angle values can be determined as follows:
The first method may be biased due to the subjective nature of visually finding a tangent to the droplet image at the three-phase point. If this method is to be used, the bias of each operator's measurements must be determined. Since the second method uses the measured dimensions of the droplet, it is expected to have less bias. Comparative analyses using these various procedures have not been conducted. Therefore, caution is advised when comparing data obtained from these different methods. The second method may not be applicable in special cases where the contact angle is greater than 90 degrees.
In the ASTM D5946-17 standard test method, water drops are placed on the surface of a film sample and the contact angle values are measured and averaged.
The ability of polymer films to retain inks, coatings, adhesives, and the like depends primarily on the character of their surface and can be improved by one of several surface treatment techniques. Electrical discharge treatments, such as corona treatment, have been found to increase the wetting voltage of a polymer film. The stronger the treatment, the more actively the surface reacts with the different polar interfaces. Therefore, if the ink, coating, or adhesive contains polar functions, it is possible to relate the contact angle of a polymer film surface to its ability to accept and retain inks, coatings, or adhesives. The contact angle by itself is not a completely acceptable measure of ink, coating, or adhesive adhesion.
The wetting stress of a polymer film belongs to a group of physical parameters for which no standard of accuracy exists. The wetting stress of a polymer cannot be measured directly because solids do not deform measurably in response to surface energy. Many indirect methods have been proposed. Different test methods tend to produce different results on the same samples. Practical determination of the surface energy of a solid exploits this interaction of the solid with the test liquids.
Although the surface finish level of polymer films has traditionally been defined in industry in terms of dynes/cm, these values are derived from a subjective interpretation of observed test fluid behavior.
The ranges of water contact angle values given in the standard can be used as a guide to define the surface finish level of polyolefins and many other polymer films having initially low surface energies. The suitability of the test for specification acceptance, production control and end use of polymer films should be determined through capability studies for each particular film and process.
In almost all materials, there are changes in contact angle as you move from one point to another. Uneven processing of the film with corona treatment devices can also add variability to the results. Therefore, multiple measurements are necessary to reflect changes in processing and surface roughness.
The wetting tension of a polymer film in contact with a liquid drop in the presence of air is a function of the surface energies of both the air-film and film-liquid interfaces. Any trace of surface-active contamination in the test liquid or film may affect the results. It is therefore important that no part of the film surface to be tested is touched or rubbed, that all equipment is scrupulously clean, and that water purity is carefully maintained. Glass apparatus in particular is likely to be contaminated with detergents having very low surface tension unless special precautions are taken to ensure its absence, such as cleaning with chromic sulphuric acid and rinsing with distilled water.
The presence of anti-slip, anti-static or anti-blocking and other additives can alter the surface of the film due to the leaching of these additives into the surface over time. Thin embossed films introduce more variability into the measurements.
Polymer films often generate static charges during processing. Corona treatment often makes matters worse. For surface measurement, the film sample may carry the static charge generated during processing or may gain a charge through friction as it slides over the sample holder surface.
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