Determination of the surface tension
Test inks can be used to determine the 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 line 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.
Testing of a plastic surface with PINK 38 Jumbo
In this application example, the PINK 38 Jumbo test pen is used to check the surface tension on black PE plastic.
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Testing of a metal surface with PINK 10 ml
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.
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Check metal surfaces for cleanliness.
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.
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Testing of a polyolefin film for 38mN/m
RAPIDTEST 38® serves to check if the treatment of polyolefins (polypropylene, polyethylene, polybutylene) has shown an effect onto the material. A stroke of the pen leaves a full line on thematerial if thematerial’s surface tension has a value of 38mN/m or more. The fluid applied to the surface will dry within seconds, it does not need to be wiped off anymore.
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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.
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.
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.
Test inks for efficient adhesion testing
Test inks are used to record the surface tension of solids, which can be flat or molded. Surface tension is tested using inks whose image can be read within a few seconds of being applied to the surfaces.
The test ink test produces an image that appears as the wettability of the surface. This can be used as an indicator for the adhesive strength of coatings. It can also be used to test the cleanliness of surfaces.
If the ink contracts within the observation time, e.g. 2 seconds, and forms individual dots, the surface tension of the object is below that of the test ink.
If the ink remains as applied or begins to flow apart, the surface tension is at or above that of the test ink.
The value of the test ink to be used is determined on the basis of experience. As a guide value for the surface tension, which enables adhesion, 38 mN/m can be assumed. This guide value may be correct for 2-component systems as an application medium, for water-based coatings higher guide values, e.g. 48 mN/m, must certainly be used.
Test inks can be produced in a wide range, such as 18 mN/m to 105 mN/m. However, it has been shown that the range from 30 mN/m to around 70 mN/m is very suitable for testing. The inks are available as markers or in bottles for application to surfaces with brushes or cotton buds or from spray bottles for larger areas.
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Purpose-orientated ink composition
Purpose-orientated ink composition
The inks are prepared in different compositions, depending on the intended use.
Inks according to DIN 53364 or ISO 8296 have been used for more than 50 years. They contain small quantities of components that are currently classified as toxic, but are harmless when used properly.
In the meantime, non-toxic inks have been produced, but these must be labelled as harmful to health. The latest developments offer label-free inks. The inks must be cleaned off the surfaces after measurement.
Special inks, which dry in seconds after application, can be left on or must be wiped off with solvent. They are mainly used in the film sector to determine the pre-treated side. These inks can also be used for archiving in quality assurance.
quality assurance.
The usability of the inks depends on their use. They can become inaccurate or unusable due to the entry of substances from contaminated surfaces.
Unopened inks can be stored for many months and then still be used. As the inks usually contain solvents, sensitive surfaces should be checked for this. In principle, all solids such as metal, plastic, glass and ceramics can be tested. The inks can be returned for disposal if they are no longer used.
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