Determination of the surface tension

Test inks can be used to determine the surface energy / surface tension of solids made of plastics – metals – glass – ceramics. The adhesion possibility of the surfaces in particular for printing, bonding and painting should also be determined based on the wetting pattern .

The surface tension is determined by applying a stroke of ink measuring just a few centimetres in length to the surfaces to be evaluated and observing the behaviour of the stroke of ink.

If the stroke contracts within 2 or 4 seconds – depending on the ink specification – the surface tension of the test area is lower than that of the test ink.

Conversely, if the stroke spreads, this would show that the surface tension of the applied ink is lower than that of the surface.

If the stroke remains unchanged during the observation period, the value of the surface tension has been reached exactly or is slightly higher.

Determination of the surface tension

In this application example, is tested with test ink PINK from 10 ml bottles with cotton swabs. Take a new cotton swab after each use.

In this application example, the PINK 38 Jumbo test pen is used to check the surface tension on black PE plastic.

Cleanliness of materials

The terms cleanliness and purity of materials need to be defined in more detail. As the materials, whether as moulded parts or film, are present in their surface, the term cleanliness may be appropriate here because purity of materials may refer to their structure, i.e. including the internal framework of an aggregate object.

Contaminations of materials may have multiple causes and may present themselves in various ways. You need to consider particle contaminations as well as film-type contaminations. Particle contamination may be present as individual irregular or intermittent dots.

Film-type contamination may, however, completely or partially cover surfaces. These may, if clean, also be described as pure, which would have to be subject to a relevant definition, e.g. of the surface tension / surface energy.

These definitions cannot refer to the internal framework of a workpiece, which, if anything, can only be described by purity. As it is the surfaces that will be further processed, we must also define the cleanliness states; this is generally possible using the term surface tension.

Two types of testing procedures complement each other

Processing may be printing, painting or gluing. The procedures used are simple solutions such as to test surfaces using test inks and measure contact angles. The first procedure is straightforward and therefore hands-on because ink can be easily applied to testing surfaces during production processes. The latter procedure requires a machine capable of displaying the surface tension components as polar and dispersive ones.

The test ink procedure only shows the sum of both values, which is mostly sufficient for assessing surfaces. Neither procedure is contactless or continuously applicable. The latter means that for manufacturing processes, in particular those using film, measurements can only be performed when systems are idle, i.e. webs are not moving.

In exceptional cases, a measurement may even require a very slow speed. The terms cleanliness and purity can be used optionally, but cleanliness can only be applied to surfaces. Try not to mix the terms to avoid confusion.

Pre-treatment at increased surface tension

The term pre-treatment refers to cleaning surfaces using mechanical means, most commonly washing with or without a solvent. For a few decades, pre-treatment has also included physical treatment using corona, plasma and flames, where electrical influence modifies the surfaces, boosts their polar ratio, increases surface tension and significantly improves adhesion.

For example, plastics from polyolefins, whose natural surface tension values are 30mN/m, can be relatively easily increased to values exceeding 45mN/m, delivering very good adhesive values for printing, gluing and painting. There is no way you can achieve these values using traditional cleaning procedures, but we can see that mechanical pre-cleaning is required in most cases to enable viable pre-treatment using physical procedures.

This is especially true for metal surfaces contaminated with, e.g. oils used for manufacturing film or moulded parts.